Contents

1. PRINCIPLES OF FILM FORMATION
Cohesive and Adhesive Forces
1. Mechanical Forces
2. Molecular Forces
Evaporation of Solvent from a Solution
1. Typical Materials
2. Properties of Materials
3. Effects of Evaporation
Evaporation of One of the Phases of an Emulsion
Evaporation of Solvent Plus Polymerisation
1. Oxygen Induced Mechanisms
2. Heat Induced Polymerisations
3. Use of Water as a Curing Agent
4. Systems Using Catalysts
Systems Employing Substantial Amounts of Curing
Agents
Systems Employing the Solvent as a Film Former
2. CHEMISTRY AND PROPERTIES OF DRYING AND
OTHER OILS
Vegetable Oils
1. Origin
2. Production of Oils
3. Composition of Crude Oils
4. Refining
Fatty Acids
1. Saturated Acids
2. Monoethenoid Acids
3. Polyethenoid Acids
4. Substituted Acids
Glyceride Structure and Film Formation
1. Fatty Acid Composition
2. Fatty Acid Distribution
Chemical Reactions of Glycerides
1. Ester Reactions
Industrial Applications of Ester Reactions
1. Synthetic Oils
2. Fat Splitting
3. Alcoholysis
Reactions Associated with Unsaturation
1. Oxidation
2. Polymerisation
3. Isomerisation
4. Hydrogenation
5. Reaction with Sulphur
6. Reaction with Maleic Anhydride
Specific Reactions
1. Castor Oil Reactions
2. Dehydrated Castor Oil
Film Properties
1. Oily Media
2. Varnish Media
3. Alkyd Media
Synthetic Drying Oils
1. Hydrocarbon Drying Oils
2. Fatty Acid Condensation Products
3. CHEMISTRY OF RESIN FORMATION AND ITS
PROPERTIES
Introduction
Fundamentals of Polymer Formation
1. Functions or Reactive Groups
2. Classification of Polymers
Formation of Polymers
1. Condensation Reactions
2. Addition Polymerisation
Types of Polymers
1. Polyesters
2. Polyamides
3. Phenolic Resins
4. Amino Resins
5. Epoxide Resins
6. Vinyl Polymers
7. Acrylic Polymers
8. Silicones
The Size of Polymer Molecules
1. Estimation of Molecular Weight
2. Measurement of Mn
3. Measurement of Mw
4. Viscosity Relationship
Physical Properties of Polymers
1. Factors Affecting Tensile Strength
2. Cohesive Energy
3. Influence of Molecular Order
4. Intermolecular Attraction
5. Crystallinity
6. Achievement of Flexibility
Chemical Properties of Polymers
1. Effect of Molecular Weight on Solubility
2. Effect of Polymer Structure
Selection and Design of Polymers
1. Addition-Condensation Polymers
2. Designing for Water Solubility
3. Use of Inorganic Ingredients
4. Advent of Truly Synthetic Polymers
4. PROCESSING OF OIL AND RESIN
General Requirements for Processing Equipment
Materials of Construction
Design of Reaction Kettles
1. The Kettle Body
2. Branches and Connections
3. Stirring Equipment
Fume Disposal and Scrubbing
1. Disposal Systems for General Use
2. Water Scrubbing of Anhydride Vapours
3. Packed Scrubbers
Condensing and Refluxing
1. Condensers for P.F., V.F. and M.F. Resins
2. Condensers for Alkyd and Polyester Type Resins
Ancillary Equipment
1. Thinning and Blending Tanks
2. Instruments
3. Vacuum Equipment
4. Valves and Fittings
5. Inert Gas Pipes
6. Pressure and Flow Indication
7. Fume Extraction
8. Lagging
9. Miscellaneous
Heating and Cooling
1. Criteria for Selection of Heating and Cooling Systems
2. Heating of Low Temperature Products
3. Heating at Higher Temperatures
4. Fluid Heat Transmission
5. Heating by Electricity
6. Heating of Pipework and Ancillaries
5. INORGANIC PIGMENTS
Introduction
Origins of Pigments
1. Comparison of Natural and Synthetic Pigments
2. Problems in Producing Natural Pigments
3. Pigment Classification
Pigmentary Properties
1. Particle Size and Particle Size Distribution
2. Particle Shape
3. Colour
4. Refractive Index
Chemical Engineering Processes of Manufacture
1. Precipitation
2. Vapour Phase Oxidation
3. Heterogeneous Surface Reaction (Corrodibility and
    Corrosion)
4. Solid Phase at Elevated Temperature
Important Groups of Pigments
1. Titanium Dioxide Group
2. Lead Group
3. Zinc Group
4. Antimony Group
5. Lead Chrome Group
6. Chrome Green Group
7. Iron Oxide Group
8. Iron Blue Group
9. Ultramarine Group
10. Cadmium Yellow and Red Group
6. ORGANIC PIGMENTS
Important Properties of Organic Pigments
1. Light Fastness
2. Fastness to Solvents
3. Heat Fastness
4. Chemical Fastness
Types of Organic Pigments
1. General Classification
2. Classification by Chemical Constitution
Azo Pigments
1. Monoazo Pigments
2. Disazo Pigments
Non-azo Pigments
1. Miscellaneous Products
2. Phthalocyanine Pigments
3. Vat Pigments
4. Miscellaneous Heterocyclic Compounds
Factors Governing Choice of Organic Pigments
1. Hiding Power
2. Dispersion
3. Stability of Pigmented Systems
Organic Pigments in Architectural (Decorative)
Finishes
1. Solvent-Based Paints
2. Water-Based Paints
Organic Pigments in Industrial Finishes
1. Air-Drying Industrial Finishes
2. Finishes Drying by Solvent Evaporation
3. Heat-Cured Industrial Finishes
4. Chemically Cured Finishes
7. EXTENDERS
Introduction
1. Production and Manufacture
2. Opacity
3. Chemical Constitution and Composition
Oxides
Silicas
Hydroxides
Alumina
Carbonates
1. Calcium Carbonate
2. Magnesium Carbonate
3. Calcium-Magnesium Carbonate
4. Barium Carbonate
Silicates
1. Aluminium Silicates
2. Calcium Silicates
3. Magnesium Silicates
4. Asbestos
Sulphates
1. Barium Sulphate
2. Calcium Sulphate
8. SOLVENTS
Introduction
Characteristics of Solvent Groups
1. The Terpenes
2. Hydrocarbon Solvents
3. Ketones
4. Esters
5. Glycol Monoethers
6. Ethers
7. Alcohols
8. Halogenated Compounds
9. Nitroparaffins
Evaluation and Selection of Solvents
1. Solvency
2. Tolerance for Non-solvents
3. Viscosity of Resin Solutions
4. Drying Time
5. Final Properties of the Film
6. General Conclusions
Solvent Requirements of Specific Resins—Convertible
Systems
1. Oil Varnishes
2. Alkyd and Alkyd/Amino Resin Composition
3. Silicones
4. Acrylic Resins
5. Urethanes
6. Phenolic Resins
7. Epoxy Resins
8. Polyester Resins
Solvent Requirements of Specific Resins-Non-
Convertible Systems
1. Cellulose Compositions
2. Vinyl Resins
3. Acrylic Resins
4. Shellac and Other Spirit-Soluble Resins
5. Rubber Resins and Derivatives
9. PROPERTIES OF SOLVENTS
10. PLASTICIZERS
Introduction
Molecular Structure of Polymer-Plasticiser Systems
1. Effect of Molecular Size
2. Types of Polymers
3. Identification of Polymer Types
Criteria of Plasticiser Efficiency and Compatibility
of Polymers
1. The Second-Order Transition Temperature
2. Tests to Show Whether A Given Polymer System Can Be
    Plasticised
3. Properties of Concentrated Polymer Solutions
4. Compatibility of Resin and Plasticiser
5. Vapour Pressure of Plasticisers
Properties of Plasticised Polymers
1. Exudation Phenomena and Exudate Composition
2. Migration of Plasticisers
3. Tensile Strength
4. Viscosity of Plasticisers and Its Effects
5. Inflammability
The Chemical Types of Plasticisers
1. Hydrocarbons
2. Esters
3. Epoxidised Vegetable Oils
4. Polyesters
Toxicity of Plasticisers
1. Hydrocarbons
2. Halogenated Hydrocarbons
3. Alcohols
4. Glycols
5. Ketones
6. Esters-organic
7. Esters-Inorganic
11. SURFACE ACTIVE AGENTS
Introduction
Types of Surfactants
1. Anion Active
2. Cation Active
3. Ampholytic
4. Non-ionic
5. Miscellaneous
Properties
1. Compatibilities
2. Chemical Stability
3. Physico-Chemical Characteristics
4. Surface and Interfacial Tension
Suspension, Sedimentation and Flocculation
1. Factors Governing Sedimentation Rate
2. Emulsions
Choice of Surfactant
1. Effect of Chain Length
2. Hydrophile/Lipophile Balance
3. Foaming and Anti Foaming
Pigment Treatment
1. Surfactants as Additives in Grinding and Dispersion
2. Pigment Pretreatment
3. Pigment Flushing
Specific Uses in Paints
1. Oil-Bound Water Paints
2. Emulsion (Polymerised) Paints
3. Adhesion of Paints
4. Rheological Properties
5. Speciality Paints
6. Miscellaneous Allied Applications
12. OPTICAL PROPERTIES
Introduction
1. Factors Affecting the Appearance of Coatings
2. Application of Optical Data
Light Transmission, Absorption and Reflection
Correlation of Light Beam Phenomena
Scattering
Opacity
Types of Transparent Coatings
Methods of Measuring Clarity
Scattering Materials
Effects of Pigment Properties
Reflectance Measurement
Gloss
Gloss Measurement Techniques
Colour
Spectrophotometry
Colorimetry
Alternative Methods of Colour Measurement
Appearance of Coatings
Fluorescence
Fading
Lightfastness Tests
External Influences on Lightfastness
Standards of Lightfastness
13. RHEOLOGICAL CHARACTERISTICS
Introduction
Rheological Behaviour In Liquids
1. Newtonian Flow
2. Non-Newtonian Flow
Theories of Viscosity
Eyring’s Theory
Einstein’s Equation
Molecular Complications
Relaxation Mechanisms
Rheological Measurements
1. Coaxial Cylinder Viscometer
2. Cone-and-Plate Viscometer
3. Capillary Flow Viscometers
4. Falling Sphere Viscometers
5. Efflux Viscometers
Practical Applications
1. Brushing Properties
2. Sagging and Flow
14. EMULSIONS AND OTHER AQUEOUS MEDIA
Introduction
Emulsion Media
Emulsion Polymerisation
Polymerisation
Copolymerisation
Formation of Polymer Emulsions
Particle Charge in Polymer Emulsions
Surface Coating Emulsions
Polyvinyl Acetate and Its Copolymers
Polystyrene
Butadiene/Methyl Methacrylate Copolymers
Emulsified Resins and Oils
Coacervate Emulsions
Emulsion Paints
Film Formation
Composition and Rheology
Solution Media
Proteins
Synthetic Water-Soluble Polymers
Maleinised Oils
Silicates and Siliconates
Solid Cementitious Binders
15. CORROSION
Corrosion of Metals
Electrochemical Basis of Corrosion
Electronic Permeability of the Oxide Film
Permeability of the Oxide Film to Metal Cations
Electrolytic Resistance of the Solution
Effect of an Applied E.M.F.
Protective Action of Organic Coatings
Permeability of Organic Coatings to Oxygen and Water
Permeability of the Oxide Film to Metal Cations
Resistance Inhibition
Metallic Pigments
16. FILM PROPERTIES AND DEFECTS
Properties
1. Adhesion
2. Hardness
3. Flexibility
4. Film Strength or Cohesion
5. Abrasion Resistance
6. Water Absorption
7. Water Permeability
8. Chemical Resistance
9. Solvent Resistance
10. Heat Resistance
11. Colour Retention
12. Fungus Resistance
13. Durability
Defects
1. Black Spotting
2. Blistering
3. Bloom
4. Blushing
5. Bronzing
6. Chalking
7. Cracking
8. Cratering
9. Flaking
10. Floating and Flooding
11. Gas-Checking and Frosting
12. Orange Peel
13. Ropiness or Ropy Finish
14. Seediness
15. Sheariness
16. Silking
17. Sleepiness
18. Sulphide Staining
19. Sweating
20. Wrinkling or Rivelling
17. SURFACE PREPARATIONS
Metal Surfaces
1. Iron and Steel
2. Aluminium
3. Cadmium
4. Copper and Brass
5. Lead
6. Magnesium
7. Stainless Steels, Nickel and Chromium
8. Tin
9. Zinc
10. Pretreatment Primer for Metallic Surfaces
Wood
1. Characteristic Properties
2. Preparation for Painting
3. Preparation for Varnishing and Lacquering
Plaster and Cement Surfaces
1. Drying and Priming
2. Treatment of Efflorescence
3. Control of Drying Out Process
4. General Principles
5. Asbestos Cement
Masonry and Building Boards
1. Brickwork
2. Stone Masonry
3. Miscellaneous Building Boards
Preparation for Repainting
1. Removing Old Paint
2. Dealing with Contaminated Surfaces
3. Schedules of Painting
18. APPLICATION TECHNIQUES
Introduction
Brush and Roller Application
Use and Maintenance of Brushes
Roller Application—Hand
Roller Application—Machine
Spray Application
Compressed Air
Spray Guns and Accessories
Metering Spray Equipment
Spray Booths
Hot Spraying
Steam Spraying
Petroleum Solvent Spraying
Cold Hydraulic Spraying
Hot Hydraulic Spraying
Electrostatic Spraying
Dip Application
Slipper Dip
Trichloroethylene Dip
Controlled Extraction
Flood Coating
Flow Coating
Curtain Coating
Barrelling and Centrifugal Application
Stoving
Operation of Stoving Ovens
Convection Ovens
Radiant Heat Ovens
19. MODERN METHODS OF ANALYSIS
I—Absorption spectroscopy
Introduction
General Features
Wavelength
Intensity
Quantitative Analysis
Ultra-Violet Spectroscopy
Principle
Instruments and Technique
Analytical Applications
Infra-Red Spectroscopy
Principle
Instruments and Technique
Analytical Application
II—Gas chromatography
Introduction
Basis of System
Injection System
Detector
Applications
Solvent Analysis
Plasticiser Analysis
Hydrocarbon Analysis
Fatty Acid Analysis
Phenol Analysis
Resin and Polymer Analysis
Recent Developments

Sample Chapters

Principles of Film Formation

The Production of a surface coating by any method depends primarily on two factors the cohesion between the film forming substances and the adhesion between the film and the substrate. These two factors are antipathetic. If the cohesion is at a maximum then adhesion will be nil and (although difficult to visualise) vice versa. The intermolecular forces involved in each case are fundamentally the same but on the one hand the reactions occur within a liquid phase or one derived there from and on the other between two phases.

The forces holding the film together are of the same type as those involved in the adhesion of the film and it is therefore convenient to discuss them side by side.

Cohesive and Adhesive Forces

Mechanical Forces

Granted that the surfaces involved do not yield areas of repellency as in the painting of wet timber  a considerable strength arises by interlocking of molecules or polymers. A typical case is that of timber and an oil based primer the oil penetrates into the cellular structure and dries therein and the strength of the bond is determined by the cohesive forces existing within the film. It should be noted that the depth of penetration has no bearing on the strength of the joint once interlocking has occurred.

Similar effects occur on zinc sprayed surfaces  phosphate coatings  roughened metal  and sanded primers.

It is not illogical to assume a similar mechanical attachment between pigment particles and the medium.

An oil medium itself must also be subject to a mechanical cohesion by interlocking of polymer chains of the simple glyceride molecules.

Molecular Forces

In any system the tendency to randomness that is the entropy of the system  will tend to increase. Any process that may occur to oppose this will therefore result in stability because energy will be required to overcome this stability and so allow the random process to continue.

These forces will operate in the formation of surface coatings and influence the cohesion adhesion balance.

Electrostatic effects

In coating a metal  for instance iron with a varnish  it is often overlooked that in point of fact the varnish is actually being applied to an iron oxide film. All metals are coated with such an oxide  the coating ranging from thin acid resistant adherent transparent layers on the precious metals through oxides of high tensile strength such as that of aluminium  to the water soluble oxide layers of the alkali metals. With a completely dry fresh film  therefore  adhesion should occur between the oxide film and the medium. This may be assumed to occur via polar groups.

However  in actual practice the solid oxide is coated with adsorbed gases (air and water vapour). Employing Langmuir  s suggestions  the surface of the substrate may be considered as a mosaic of oxide crystals containing elementary spaces from which residual valency forces extend to a distance of approximately 10–8 cm. These valency forces yield a   chemical union of elementary spaces with molecules of the adjacent gas  in a manner that depends upon the specific forces involved  the relative sizes of the elementary spaces and the adsorbed molecules  and the orientation of the latter  . The films so formed have been assumed to be mono molecular  but there is no reason to assume that other layers may not be built up by molecular forces such as hydrogen bonding once a water layer has been adsorbed on the surface.

A similar adsorption will occur at the surface of pigment particles  hence the cohesion of the pigmented film will depend on how far the medium may be adsorbed on the crystals faces of the pigments.

Hydrogen bonding occurs as a result of the intense unshielded field of the positive nucleus. The effcct of hydrogen bonding on solvents is seen in the elevation of the boiling point and an increase in the density in comparison with similar substances. This can be seen in the following list of boiling points and densities (in each case the more highly hydrogen bonded substance is placed first)

This means that whenever a hydrogen bonded solvent is employed  considerable energy in the form of latent heat is required to effect evaporation  and this is important where the abstraction of the latent heat occurs from the ambient atmosphere  hence causing deposition of water in the film.

It must be realised that hydrogen bonding between solvent molecules is a dynamic equilibrium as the molecules are in a constant movement  unlike that existing between polymers where a static state arises.

Hydrogen bonding between polymer chains  as  for instance  in Nylon 66  leads to orientation of the atoms regularly 

If a number of segments of chains are similarly orientated  then crystallinity arises in the system.

Now these crystalline sections  which may occur more than once on portions of the same chain  will remain associated unless solvents having groups with a greater hydrogen bonding tendency are employed  so breaking the existing hydrogen bonds and producing solution of the polymer.

An interesting case where polar linkages afford adhesion is the use of an intermediate to bond a surface coating to glass. Methyl acrylato chromic chloride  made from chromyl chloride and methacrylic acid  is hydrolysed and dehydrated at the glass surface. Ether linkages are produced and the remaining hydrogen atoms attach themselves to the free oxygen ends of the silica tetrohedra network.

The methy acrylic residues are left at the surface to present an oleophilic interface.

Van der Waals   and London forces

When molecules approach each other  attraction occurs between them. Normally this gives rise to liquid cohesion  that is  viscosity. If the chains in a polymer are free to undergo rotation  then the polymer is said to be a liquid. If energy is now withdrawn by cooling the substance  the chains cease  either completely or partially to move  and ultimately the viscosity becomes so high that the substance is said to be a solid. If the chains on cooling orient themselves in such a way that regularity occurs by attraction between molecules such as then a crystalline condition  as occurs with hydrogen bonding  will arise. The forces holding the chains together will  however  be smaller.

Metallic Forces

In a metal  the electrons in the outer orbits are not confined to anyone atom  and hence metals will conduct electricity. Bonding of the metallic type does not occur in organic surface coatings  but does form the basis of deposited metallic coatings. It has been suggested that one function of paint films on metals is to prevcnt the flow of the electrons in metals and so act as corrosion inhibitors.

Ionic Forces

The crystal lattice of salts is built up from thcir ions. This type of structure does not occur generally in organic coatings  but does occur in such materials as phosphate and thick oxide films.

In each of these cases  solution plays an important part by reducing the viscosity of the film forming medium. Except in the case where natural oils are employed  as in linseed oil and red lead mixtures  most media consist of a high proportion of polymerised material. It is  therefore  of value to consider the physical chemistry of the solution of polymers. This problem has been dealt with by a number of authors.

The employment of the correct solvents is important because a poor solvent will mean that a thin film will be produced  giving low obliteration. The whole economics of cheap poor solvents  high viscosity media and pigments with high hiding power can only be worked out experimentally  until the rheology of paint systems becomes better understood. For instance  differential evaporation may leave a poor solvent in the mixture so that the film medium is no longer soluble.

Dispersion of polymers by solvent occurs in two ways. In the first case  crystalline areas are separated  and in the second the random parts of a chain are separated from neighbouring chains by the solvent molecules. Disrupting of crystalline areas formed by hydrogen bonding involves the use of energy either in the form of heat (see below under   Transition Temperature  ) or electrostatic attraction of the hydrogen bonded solvent for the individual atoms in the polymer chains. The crystalline areas formed by van der Waals   forces may be disrupted by the entry of solvent molecules. The action of solvents parallels that of heat. When a polymer is heated  the chains of molecules remain unaffected until a temperature is reached at which   random   parts of the chains are free to rotate. This is known variously as the second order transition temperature  the glass temperature  and the brittle temperature.

If there are no crystalline areas  then a change will occur from a glassy solid to a viscous liquid or a rubber. Linear segments become free to rotate  further heat causing fresh segments to   thaw out  . The temperature  therefore  remains stationary as additions of heat are absorbed as rotational energy  until the movement of chains across each other is possible when the substance flows. The glass temperature is exceedingly important in surface coating technology  as it will affect resistance to cold shock.

Crystalline segments of the chains will require more energy to overcome van der Waals   forces or hydrogen bonding  and in these cases a considerable rise in temperature will occur before these areas are disorientated. Within this temperature range considerable flexibility will arise due to the random portions being free to slide and rotate while cohesion is maintained by the crystalline portions.

The term S  the change in entropy  merits some discussion. It is particularly important in relation to surface coating polymers as these  in many cases  are in solution  or externally plasticised  which amounts to the same thing. The entropy of a system is a measure of disorder or randomness. In a gas there is complete disorder  in a crystal practically none. Randomness in a polymer system  as already pointed out  depends on the temperature.

If a wholly amorphous polymer is treated with solvent of the correct type  it will swell by absorption of the solvent. This absorption occurs because as the polymer chains rotate  spaces are left which the solvent molecules can fill. As more solvent is added  the chains are further separated until a viscous dispersion occurs. Further additions allow chains to slide past each other and the mixture is a liquid.

The presence of crystallinity complicates the system because in addition to the simple free energy considerations  there are also the energies required to overcome bonding forces.

Light cross linking will also affect the entropy as randomness will only occur in segments of the chains. Complete cross linking reduces the change in entropy to a minimum. This explains why a small amount of cross linking  as in an incompletely converted undercoat  allows attack by solvents. On the other hand  highly cross linked systems  such as stoved melamine formaldehyde finishes  are resistant to solvent attack.

The solubility factors have also been investigated for alkyd resins.

It is difficult to see how any quantitative ideas can be developed without taking into account the molecular weight distribution in any polymeric system. Two polymer solutions of the same viscosity could be prepared  one containing high molecular weight material dissolved in monomer  and the other of lower molecular weight but substantially free of monomer. Addition of a poor solvent will result in precipitation in the first case  but not necessarily in the second.

An important type of non volatile solvent is the plasticiser. This operates as does any other solvent by separating the chains  but the solvent remains in the film indefinitely. The presence of a plasticiser is important as most paint films are subject to stresses  which result in the stretching of the chains to take up new configurations and the rotation of the random parts of the chains  so increasing the chance of producing crystallinity. This state of affairs is reversible on the removal of stress. On the other hand  if too much plasticiser is present  the chains become free to slide over each other and the film will flow irreversibly  as removal of stress will not cause the chains to slide back again.

The presence of a plasticiser in a cross linked film is necessary to allow some movement of chains across each other  to relieve stresses  and Jordan has  in fact  suggested that non drying oils perform such a function in oxidised films.

Internal plasticisation takes place if large groups are introduced as side chains in polymers. These serve the same purpose as external plasticisers in keeping chains apart  they also allow increased solubility by acting as centres for their own   solution.

Turning now to actual film formation  it will be convenient to examine the systems already enumerated.

Evaporation of Solvent from a Solution

If a solid is dispersed in a solvent and the solvent evaporated  then the film left will adhere to a substrate providing its adhesive force is high  but it will not form a continuous film unless the cohesive force is also considerable. These two forces must also balance.

Some doubt has been thrown on this idea by a suggestion that lack of adhesion is due to the presence of   anti adhesive   substances. While this is known practically  for example  with silicone contamination  critical opinion will wait for further experimental proof of this thesis.

From what has been said already it will be seen that the ability of a polymer to form a film will depend on its glass temperature. This can be seen by examination of a typical glassy substance  rosin. If this be dissolved in alcohol and the solvent evaporated from the film  a friable powder is obtained. However  if the temperature be raised  there comes a point at which adhesion improves. The same result can be obtained by the use of a solvent of high boiling point  i.e. a plasticiser  which results in a lowering of the glass temperature.

Highly crystalline materials will have high cohesive forces and increase the strength of the film but they will have in consequence low adhesion.

In any film in this class  therefore  it is necessary to have at least a resinous polymer  a plasticiser and probably a crystalline polymer.

Typical Materials

The following are typical of materials used to achieve film formation as discussed in the foregoing.

Bitumen Solutions. These are complicated mixtures consisting of glasses often plasticised naturally or by added fluxes. It is worth noting that those most resistant to atmospheric attack  for example  the stearine pitches and the blown bitumens  both contain oxygen and are therefore likely to be hydrogen bonded. The coal tar pitches also contain phenolic bodies known to be highly polar solvents.

Chemistry and Properties of Drying and Other Oils

The Early development of the surface coatings industry was founded upon the use of unsaturated vegetable oils  which have the property of absorbing oxygen from the air when exposed in thin films  to form elastic solvent resistant coatings. This property  which is possessed by a number of oils of varying importance  and especially by linseed oil  has also been utilized in the production of printing inks  linoleum  oilskins  and in a number of other industries. Their growth was fostered  not by the application of scientific knowledge of the structure of oils nor of the chemical reactions that make film formation possible  but by a process of trial and error typical of crafts generally. The vegetable oils came to be classified according to their performance into three groups the drying  semi drying  and non drying oils. Of these  only the drying oils could be used to prepare paints and varnishes of suitable quality  and semi drying oils were regarded as extenders or adulterants. Non drying oils could not be used at all.

More recent developments  which were begun by the introduction of oil modified alkyd resins in 1929  and were followed by a considerable increase in scientific knowledge  have changed the picture completely. It is now true to say that it is possible to produce some form of surface coating from almost any of the known vegetable oils. The statement might even be extended to include animal fats  but they are not generally such promising raw materials  and only selected marine oils have actually been used. However  any modern account of the oils for surface coatings must cover a very wide field  and be based upon scientific principles even although some details are still not completely understood. Space does not permit a full description of the many different types of oil occurring in nature to be given  and reference should be made to one or more of the excellent treatises on the subject that are now available.

Vegetable Oils

Origin

Oils are found in a variety of the organs of living plants  but the fruits or seeds provide the only sources rich enough to supply commercial quantities. A large number of plants ranging from annuals to trees secrete oil in their seeds to feed the embryo in the early stages of its development  and some also produce oil in the fruit coat enveloping them. In the olive  oil from the seed closely resembles that from the fruit coat in composition  but this is exceptional  there being in general no similarity between them. In stillingia  for example  the fruit coat contains a solid fat and the seed a drying oil  and the oil palm has a red semi liquid oil in the fruit coat (palm oil) and a white solid fat in the seed (palm kernel oil). On the other hand  many oleaginous seeds are surrounded by tissues containing very little oil  and it is advantageous to remove them before milling or extraction process known as decortication.

The main sources of oleaginous seeds are areas of cultivation in many parts of the world  although certain wild crops are still harvested in parts of Africa and the Far East. There is an increasing tendency to install oil mills near the plantations  and this is sometimes desirable to eliminate deterioration during transit  but the majority of oilseeds can be shipped to any part of the world without risk of damage provided that the moisture content is not too high. Where lipolytic enzymes are present  as for example in conophor seeds  the development of acidity in transit has been prevented by a heat treatment before despatch.

Production of Oils

Commercial oilseeds contain anything from 20 to over 50 per cent of their weight of oil  according to variety  and vary in size from tiny seeds like tobacco which require 13 000 to the gramme  to large fruits like coconuts. Both these factors have a bearing upon the type of treatment employed  and this in turn has an effect upon the quality of the oil produced. Two main processes are employed in practice  viz. oil milling and solvent extraction.

Milling

Until fairly recently milling was carried out by means of hydraulic presses  but these have now been almost entirely replaced by continuous expellers  in which increasing pressure is applied between a rotating worm and a barrel housing. Before feeding into this machine  the seeds are broken up if necessary  further reduced by passing through a set of rollers  and the oil cells are then disrupted by cooking with steam. The pre treatment has a profound effect upon both the yield of oil obtained and its quality  mild conditions favouring the latter at the expense of the former. In some instances  the oil may be recovered in two or more stages  as for example with castor beans  where a medicinal quality is first obtained by low temperature pressing  followed by a   firsts   quality by hot pressing  and   seconds   by solvent extraction. This  however  is an exception and requires a plant of special design. Where expellers are used in two stage processes  the cake from a low pressure machine may be passed either to a high pressure expeller or to a solvent extractor.

Solvent Extraction

Many devices have been proposed for the production of vegetable oils by extraction with solvents by batch or continuous processes  and a number of them are in current use. Most employ the counter current principle  the seed being washed successively by an oil solvent mixture or miscella of progressively decreasing strength so that the final wash is with pure solvent  and the strong miscella is removed at the other end of the system where fresh seed is introduced. The oil is recovered from the miscella by distillation  and the meal after steaming to remove solvent contains less than 1 per cent of oil. A number of solvents  both inflammable and non inflammable  are suitable for the process  but the one most commonly used is a petroleum fraction boiling between 60° and 80°C  rich in normal hexane. Solvent extraction is generally applied to seeds with a relatively low oil content that are easily flaked  or to cakes produced by low pressure expelling.

Composition of Crude Oils

Vegetable oils in general consist of mixed triglycerides of the higher fatty acids  and the identity and composition of the mixture of acids present  together with their distribution amongst the glyceride molecules  determines the nature and properties of the various individuals. Although the composition of any given type of oil is subject to variations which may be due to the climate prevailing where the seed was grown  to differences in botanical species  or adulteration due to careless harvesting  the crude oils of commerce are remarkably constant in their properties. Identification  or the detection of admixture  can generally be made on the basis of the usual constants of oil analysis.

Apart from the glycerides forming their main constituents  crude oils contain minor amounts of a number of impurities which include free fatty acids  colouring matter  phosphatides  carbohydrates  protein fragments  sterols  tocopherols  hydrocarbons  waxes  flavours  odours  etc.  in quantities depending upon their nature and method of production. The phosphatides and various resinous and mucilaginous materials of uncertain identity are precipitated as spawn when the oil is heated  and are gradually deposited in the form of   foots   when it is stored under moist conditions. In order to avoid this  raw linseed and soya bean oils are sometimes subjected to a   de gumming   process in which the precipitation is accelerated by the addition of water. Although the non glyceride components are present in minor amounts  and some have little or no effect upon the technical properties of the oil  others act as antioxidants which serve the purpose of protecting it against deterioration  and give rise to the   induction period   observed during the autoxidation of drying oils. These materials have been studied in some detail  but their nature and mode of action are still imperfectly understood. It is believed that the principal agents are the tocopherols  and that their action may be reinforced by phosphatides and other antioxidants of the phenolic type.

Refining

The purposes of oil refining are to remove those impurities the presence of which is objectionable in the particular application for which it is to be used. In the production of media for surface coatings  the main object is to remove phosphatides and other   break   materials  as well as waxes which may give rise to film defects  and sometimes to reduce the colour without destroying the capacity for further bleaching on heating. Both acid and alkali treatments are practised  but alkali refining is now by far the more important. It may be carried out either by batch or continuous processes  and methods of refining in solution (miscella) have also been proposed. Many variations are employed to deal with oils of different kinds  but the basic principle is to treat with a caustic soda solution of suitable strength in an amount slightly greater than would be required to neutralise the free fatty acids present  and to wash out the resulting soap with water. The soapstock is treated with mineral acid to give a mixture of fatty acids and neutral oil known commercially as acid oil. The oil after neutralisation is dried under vacuum  and bleached if necessary by heating with a small quantity of activated fuller  s earth. It is then filtered  and freed from wax by chilling to near the freezing point  i.e. 40° 50° F and filtering off the deposit.

Refining with alkali removes phosphatides  protein fragments  carbohydrates  resinous and mucilaginous materials and free fatty acids. The content of sterols  tocopherols  hydrocarbons  etc.  which make up the unsaponifiable portion of the crude oil are little affected.

Fatty Acids

The fatty acid mixtures obtained by the hydrolysis of glycerides occurring in natural fats are usually complex  and contain a variety of both saturated and unsaturated individuals. Where one particular acid predominates  it is often of unusual structure and is typical of the oil concerned  as for example ricinoleic acid in castor oil  elaeostearic acid in tung oil or licanic acid in oiticica oil. The identity of the vast majority of oils is not  however  determined by the presence of one typical acid  but by the proportions in which a number of common acids are present  and the manner in which they are distributed amongst the glyceride molecules.

Fatty acids with chain lengths of from six to twenty four carbon atoms have been isolated in appreciable quantities from natural glycerides  and when it is considered that they may be saturated or contain up to six double bonds per molecule  the possibilities for complexity are very great indeed. There are  however  a number of circumstances that reduce the possibilities to some extent. For example  it is observed that  almost without exception  natural fatty acids have straight chains and contain an even number of carbon atoms. This is thought to be due to the biological process by which they are formed from carbohydrates. Unsaturated acids with less than sixteen carbon atoms are unimportant  and but few of the large number of possible isomers actually exist in nature. Most of the polyunsaturated acids found in the drying oils have eighteen carbon atoms  and those with more than four double bonds are encountered only in marine oils.

Saturated Acids

The saturated fatty acids have the general formula CnH2n02 and form a homologous series commencing with the lower members which are liquid at ordinary temperatures  and progressing to solids with increasing melting points. Table 1 shows the acids occurring most commonly in natural glycerides  together with some of their characteristics. The first three  caproic  caprylic and capric  are never present in more than minor amounts  and their principal source is in coconut and palm kernel oils. Lauric acid is the first member of the series to be widely distributed in nature and occurs in practically all of the seed fats of the laurel family  from which it derives its name. Coconut and palm kernel oils are  however  the principal sources from which it is obtained commercially. Myristic acid is found in small quantities in most animal and vegetable fats and forms only a minor component of the oils used in surface coatings. Palmitic acid has been reported to be present in practically every animal and vegetable fat that has been examined  but usually in minor quantities. It may  however  comprise as much as 10 per cent of the total fatty acids present in groundnut  soya bean and maize oils  20 per cent in cottonseed  and over 35 per cent in palm oil. Stearic acid is also of considerable importance  although it is not so widely distributed as palmitic acid. It forms a major component of the saturated acids present in drying oils. The saturated acids with more than eighteen carbon atoms  arachidic  behenic and lignoceric have comparatively little importance  occurring only in minor quantities.

Saturated fatty acids may be regarded as undesirable constituents of drying oils  but owing to their distribution in the glycerides it is not an easy matter to remove them.

Monoethenoid Acids

It is widely believed that in the biosynthesis of vegetable fats the products are liquid at the prevailing temperature. If the component glycerides were derived only from saturated acids of the chain lengths normally encountered  they would have high melting points  and one of the ways of preventing this is by the introduction of unsaturation. In the study of natural fats it is found that unsaturated acids are invariably present  and as a general rule the proportion found in a particular species is greater if the plant is cultivated in a cold climate than in a warm one.

The first series of unsaturated acids having one double bond is represented by the empirical formula CnH(2n 2)O2 and the more important members are shown in Table 2. It will be seen that the series is by no means complete  being not only confined to compounds with an even number of carbon atoms from ten to twenty six  but very few of the large number of possible isomers have been identified. It has often been stressed that the majority of monoethenoid fatty acids have the double bond between the ninth and tenth carbon atoms. Too much importance should not be attached to this rule  which would be equally true if it were the convention to begin counting from the other end of the chain instead of from the carboxyl group. In other words  the tendency appears to be for unsaturation to occur near the centre of the fatty acid molecules. Unsaturated acids are also subject to cis trans isomerisation  as shown In Fig. 1 and much has been learnt about this since the development of infra red absorption analysis. With monoethenoid acids the cis form  having the lower melting point  is most commonly found in nature.

With the exception of crotonic acid (C4)  which is present in croton oil  no naturally occurring monoethenoid acid with a chain length below C10 is known. Although a number of acids with from ten to fourteen carbon atoms have been identified  the first member of the series of any importance is palmitoleic acid  which is found in appreciable quantities in marine oils and the fats of cold blooded animals generally. It also occurs in small quantities in vegetable oils from various sources  sometimes sufficient to be detected by classical methods  and sometimes in traces that become evident only in analysis by gas liquid chromatography. Of the sixteen possible isomers of cis octadecenoic acid  only three have been characterised with certainty viz. oleic  petroselenic and vaccenic acids  of which oleic acid is by far the most important. It has been found in every plant and animal fat to the extent of at least 10 per cent  and frequently more than 50 per cent of the total fatty acids. Other isomers are produced during the hydrogenation of fats containing polyunsaturated acids  or may be made synthetically. Trans isomers are formed from oleic acid by elaidinisation in the presence of suitable reagents such as oxides of nitrogen  sulphur  selenium  etc.  and during fat hydrogenation. Apart from erucic acid  which is an important constituent of rapeseed and a few other oils  the higher members of the series are confined to marine oils.

Monoethenoid acids may be oxidised  but are not thought to take part in reactions leading to film formation under the conditions generally employed. For example  olive oil  which has an iodine value of 85 and may contain over 80 per cent of oleic acid  is almost completely lacking in drying properties or any tendency to become gummy when exposed in thin films.

Polyethenoid Acids

Although the introduction of a plurality of double bonds in the carbon chain increases the possibility of positional isomerism  actually fewer isomeric polyethenoid than monoethenoid acids have been detected in natural fats. The principal members of the series are given in Table 3. The diethenoid linoleic acid is the most widely distributed in the plant kingdom  and is characteristic of the semidrying oils of commerce  including soya bean  niger seed  safflower  sunflower  tobaccoseed  poppyseed  etc. It also occurs in major quantities in liquid nondrying oils such as groundnut  maize and cottonseed  and is associated with more unsaturated acids in the conventional drying oils. It is believed to be essential to the maintenance of health and normal reproduction of animals  at least some species of which are incapable of synthesising it  and must obtain the requisite quantities by ingestion from other sources. Linolenic acid is not so widely distributed in nature as linoleic acid  but is characteristic of linseed oil and a major constituent of other drying oils such as conophor  perilla  stillingia and hempseed. Owing to the presence of three double bonds  it absorbs oxygen more rapidly  and the presence of relatively small amounts greatly increases the drying potential of oils rich in linoleic acid. The conjugated isomer  elaeostearic acid  is not widely distributed  but is of considerable importance owing to its occurrence as the characteristic acid of tung oil. The different arrangement of doubble bonds gives rise to marked differences in physical characteristics and chemical properties  including refractive index  absorption spectra  addition of halogens  mechanism of oxidation  and especially in the rate of polymerisation. Natural or  elaeostearic acid has a cis  trans  trans configuration  but it readily changes to the high melting trans  trans  trans form. Attempts to produce it by isomerisation of linolenic acid appear to have given rise to a third form known as pseudo elaeostearic acid  which differs from the natural material in many of its properties. The other polyethenoid acids are of considerably less importance  but are not without interest. The 2 3 4 5 decadienoic acid found by Hilditch in stillingia oil is believed to account for the fact that it dries more rapidly than would be expected from the iodine value. Parinaric acid is the only authentic acid containing four double bonds that has been isolated from seed fats  and they are believed to occupy the conjugated position. Fatty acids with up to twenty six carbon atoms and six double bonds have been isolated from oils of marine origin. Two acids are also known which contain a double bond and a triple bond  one of which occurs in ongokea oil.

Substituted Acids

In addition to the members of the simple homologous series of fatty acids mentioned above  a number of substituted acids of more unusual structure occur  which are often characteristic of the oils produced by plants of a particular species. Only those that are of interest in surface coatings will be considered here.

Ricinoleic acid  which resembles oleic acid in structure but has an hydroxyl group on the twelfth carbon atom  is characteristic of castor oil  and is responsible for its unusual and useful properties. The presence of the hydroxyl group enables it to take part in a number of chemical reactions  including dehydration  which gives rise to a mixture of conjugated and non conjugated isomers of linoleic acid. A less familiar hydroxy acid   hydroxy elaeostearic or kamlolenic acid  has more recently been shown to be the major constituent of kamala oil. Licanic acid  or 4 keto elaeostearic acid  was first isolated from oiticica oil  but is now known to occur in po yoak oil and in number of others. The naturally occurring acid is solid at room temperature  melting at 74° 75°C  and is readily converted into an isomeric  form melting at 99.5°C. It resembles elaeostearic acid in many of its properties  but the presence of the keto group provides additional reactivity.

Glyceride Structure and Film Formation

Fatty acids comprise between 94 and 96 per cent of the glycerides occurring in natural fats  and therefore occupy a most important position in the chemistry of these compounds. As has already been indicated  each fat contains a number of individuals  and the analysis of natural fatty acid mixtures has been the subject of a vast amount of work for a great number of years. Fats containing only oleic and saturated acids were the first to yield results of a reasonable degree of accuracy  the presence of polyunsaturated acids introducing a complication that was not easily resolved. The thiocyanogen method of Kaufmann  followed by the ultra violet absorption technique first proposed by Mitchell  have become valuable analytical tools  and the introduction of gas liquid chromatography has recently made it possible to check the results obtained by these and other methods. The means are now at hand to analyse almost any natural fatty acid mixture  but so wide is the field to be covered that accurate results have not yet been obtained with all known fats  and published compositions should never be accepted without scrutinising the methods by which they were determined.

Fatty Acid Composition

Table 5 shows the fatty acid composition and other characteristics of a typical specimen of most of the natural oils that have been used to a greater or less extent in surface coatings. It should be borne in mind that the figures are subject to fluctuation  but for purposes of clarity it is preferable to give a single set of figures all relating to the same sample  rather than inserting the upper and lower limits. Following a procedure adopted elsewhere  the oils have been classified into four sections 1st  linoleic oils  2nd  linolenic  3rd  conjugated oils  and 4th  miscellaneous oils.

Linoleic Oils

The linoleic oils are derived from a variety of sources and form an important group  certain members of which are produced in very large quantities for edible purposes. The linoleic content varies widely between the different individuals  e.g. groundnut oil contains roughly 30 per cent  maize 40 per cent  cottonseed 45 per cent  soya bean 50 per cent  poppyseed and sunflower 65 per cent  grapeseed and niger 70 per cent and safflower and tobaccoseed 75 per cent. Soya bean oil also contains 7 15 per cent of linolenic acid  and is sometimes classified with the linolenic oils  although for technical purposes it is employed as a semi drying oil. Together with hempseed and rubberseed (20 per cent linolenic acid)  and candlenut (30 per cent linolenic acid) it might be placed in an intermediate group.

The linoleic oils are sometimes up graded by a solvent segregation process  using furfural or liquid propane  and it has been claimed that the products resemble linseed oil in their drying properties. This is not correct  the presence of the third double bond in linolenic acid conferring a considerable increase of reactivity in oxidation and polymerisation reactions. It is only when the acids are combined with a complex structure that is capable of film formation through cross linking  as for example in alkyd resins  that roughly comparable results may be obtained.

Although a number of oils in the linoleic group are used for technical purposes  soya pean oil is the most important owing to the large quantities produced from seed grown in China  Manchuria and the U.S.A. The other members that contain over 50 per cent of linoleic acid  and are therefore most suitable for use in surface coatings  are less readily available  although there has recently been an increase in safflower cultivation. Soya bean oil is obtained from the seeds of a leguminous plant which contain rather less than 20 per cent of oil but give a meal of high protein content that is extensively used in animal feeding stuffs. The principal outlet for the oil is in edible products  only a relatively small amount being used for technical purposes. The iodine value is generally within the range 125 140  and the variations are due mainly to the linolenic acid content  most specimens containing about 50 per cent of linoleic acid. Other members that have been used to a minor extent in alkyd resins include tobaccoseed  safflower  niger  groundnut  cottonseed and maize oils. The fatty acid mixture obtained by the fractionation of tall oil has a composition very similar to that occurring in oils of the linoleic group  and is sometimes used as an alternative in alkyds.

Linolenic Oils

A group that is comparatively small in numbers  but of considerable importance technically  is that of the linolenic oils. The only members to have been used outside the countries of origin are linseed  perilla and stillingia oils. Others known to contain more than 45 per cent of linolenic acid include chia  conophor and lallemantia  and many more contain lesser quantities.

Linseed oil is obtained from the seeds of the flax plant  extensively cultivated in many parts of the world including the America  Russia  India and the Far East. The principal exporting countries are Argentina  Canada  India and the U.S.A. The composition of the oil varies somewhat according to the country of origin  and climate appears to exercise the greatest influence  the highest iodine values being recorded with oils produced from seed grown under cold conditions. Although it is possible to obtain samples with values below 130 or above 200  the oil produced commercially is generally within the range 175 195. Painter have analysed a large number of oils and shown that the linolenic acid content varies from 20.5 per cent at iodine value 128 to 61.8 per cent at iodine value 203. Commercial samples generally contain between 48 and 57 per cent. Perilla and stillingia oils have been available for limited periods only  and are acceptable replacements for linseed oil in surface coatings.

Conjugated Oils

The naturally occurring conjugated oils all contain major quantities of acids with three double bonds in the conjugated position  such as  elaeostearic acid or a substituted form like licanic acid. Conjugated isomers of linoleic acid exist only in modified oils  and are produced by the dehydration of castor oil  or by isomerisation. The principal sources of the natural conjugated oils are certain plants of the Rosaceae and Euphorbiaceae families  and more particularly of the genera Licania  Parinarium  Aleurites  Garcia and Ricinodendron. The only members to have been produced commercially are tung and oiticica oils.

Although highly unsaturated  the natural conjugated oils cannot be used directly as media for surface coatings because they give frosted films on drying. Their structure gives rise to rapid polymerisation on heating  and the oils gel in a few minutes at 280ºC. The principal application is in the manufacture of varnishes using either pure or reduced phenolic resins  which are outstanding as regards water and chemical resistance. Tung oil is produced from nuts of two species of the genus Aleurites  both of which are indigenous to China. Aleurites Fordii is generally preferred on account of the fact that it gives an oil with a higher elaeostearic content and greater reactivity than that produced from the montana species. Apart from China  the principal sources of supply are the U.S.A.  Argentina and Brazil. The montana oil comes from Nyasaland. Tung oil is free from phosphatides  and is invariably used in the raw state  refining with alkali being extremely difficult owing to the tendency to form emulsions. Oiticica oil is produced from the kernels obtained from the fruits of a slow growing tree found wild in Brazil. When freshly expressed  it sets to a soft buttery mass that is somewhat sensitive to oxidation  but it may be stabilised in the liquid condition by a mild heat treatment.

Miscellaneous Oils

Under this heading may be discussed a number of oils of widely different composition that have found some applications in surface coatings. Apart from the unsaturated oils of marine origin  they may be classed as non drying oils suitable for the production of alkyds of the plasticising type. Castor oil is by far the most important member owing to its unusual versatility. Not only can it be used as such  or in the fully hydrogenated form  in non drying alkyds  but it can be converted into a drying oil by dehydration and take part in a number of other reactions associated with free hydroxyl groups. The castor plant grows chiefly in India  Manchuria  Russia and Brazil  and the seeds contain between 40 and 45 per cent of oil  which is used entirely for medicinal and technical purposes. Certain marine oils obtained mainly from sardine  menhaden  whale  herring and pilchard  or fractions segregated from them by solvent extraction  are also used as drying oils.

The whole oils contain a much wider range of glycerides than is normally encountered in vegetable fats  including significant amounts of saturated glycerides that may be separated by a   winterisation process  . The highly unsaturated glycerides  containing fatty acids with over twenty carbon atoms and four or more double bonds are also objectionable in surface coatings  and segregation processes are generally aimed at producing a fraction of intermediate unsaturation.

Fatty Acid Distribution

The analysis of glycerides  or the determination of the distribution of the individual fatty acids amongst the glyceride molecules  is a matter of considerable difficulty  and the results obtained to date are of doubtful accuracy. The methods used include fractional crystallisation  chromatography  and the Craig counter current distribution technique. As a result of his own extensive studies  Hilditch formulated the principle of even distribution  which states that the fatty acids are distributed evenly and widely amongst the glyceride molecules. This principle has been challenged by a number of workers  and other types of distribution  such as the random theory of Dutton and the restricted random theory of Kartha have been postulated. Evidence that conflicts with the random theory is provided by the fact that the properties of many oils are changed by interesterification  which is known to give mixtures of the random type. It is also known that in certain fats like cocoabutter  which contain a high proportion of glycerides with one oleic and two saturated groups  the oleic acid invariably occupies the  position. An attempt to resolve the anomalous situation created by these conflicting views has recently been made by Youngs.

The glyceride composition of drying oils is of particular importance because only polyunsaturated acids are capable of being linked together by oxidation or polymerisation  and the observation that the acids separated from polymerised glycerides contain little polymeric material higher than dimers. Individual glyceride molecules may  therefore  be regarded as non functional  or mono   di  or trifunctional according to the number of polyunsaturated radicals present. In a polymerisation reaction  non functional glycerides are non reactive  mono functional glycerides can act only as chain terminators  di functional glycerides may form long chains  and tri functional glycerides can produce cross linked systems. In a mixture of the different types  a solvent resistant coating can be obtained only if sufficient tri functional glycerides are present to introduce the requisite cross linking. In the absence of reliable analytical results  no certain forecast can yet be made of the drying potential of an oil on the basis of the fatty acid composition alone. It should be noted  however  that a random distribution would give a greater amount of trifunctional glycerides than the other current theories of glyceride structure  and the figures given by way of illustration in Table 6 can be regarded as showing the situation most favourable for film formation at any given polyunsaturated acid content. They were calculated using the equations for random distribution given by Feuge  Kraemer and Bailey. All of the natural drying and semi drying oils have more than 50 per cent of polyunsaturated acids  and it will be seen that this is near to the limit below which glycerides capable of cross linking do not occur.

Chemistry of Resin Formation and Its Properties

Introduction

Although synthetic resins have been used in surface coatings for half a century  it was not until the 1930  s that scientific method was introduced into their study. The turning point can be identified with the publication in 1929 of papers by Staudinger  Kienle and Carothers stating the fundamental concepts of polymer formation and polymer character from which modern polymer science has grown.

Fundamentals of Polymer Formation

Staudinger recognised polymers as very large molecules built up of repeating units derived from the starting materials or monomers. Thus the rubber molecule was seen to be composed of many isoprene units  and they are now known to be joined together by a typical 1 4 addition of the conjugated system.

It was realised that no special forces were involved in polymer formation and nothing beyond the ordinary operation of chemical valency and reactivity need be invoked to explain the existence of these molecules.

Functions or Reactive Groups

The reacting groups in the molecules are called functional groups or functions  following Kienle  and a compound is said to be mono   di  or tri functional  depending on the presence of 1  2 or 3 such groups in the molecule. The term poly functional is used generally to describe molecules having more than one functional group  so polyfunctional molecules are the fundamental building blocks from which polymers are made.

Familiar reactive groups or functions are

Consequently it can be seen that the following materials are bifunctional

are tri  and tetra functional respectively.

Bifunctional Reactants

It should be noted that the double bond and the epoxy group each yield two reactive points at which union to other molecules can occur

Each of these groups must therefore be regarded as two functions. Ethylene CH2=CH2 and ethylene oxide CH2 CH2 each have a functionality of two  whilst butadiene has a functionality of two or four according to whether one or both of its double bonds react under the experimental conditions.

Formation of Linear Polymers

Bifunctional reactants can only yield polymers in which the monomer units are joined in a straight chain the so called linear polymers. Thus

Cross linked Polymers

When a reactant of greater functionality than two is present  branching and crosslinking can occur leading to three dimensional networks which can attain indefinitely great size and become immobile  infusible and insoluble. This condition is known as gelation. The transition from an open chain to a cross linked structure is fundamental to the curing of convertible coatings and thermosetting resins generally.

As an example we may consider the reaction of glycerol with phthalic acid or its anhydride. This is basically a simple esterification  distinguished only in that reaction occurs at several points in each molecule  thus

Representing the glycerol residue by G and the phthalic acid residue by P we can depict the following as typical parts of a glyceryl phthalate macromolecule

In such cases  no regular structure can be assigned to the macromolecules for  in accordance with Kienle  s second postulate  reaction between functions occurs in a random manner and a whole range of molecular sizes and isomeric structures is built up as reaction proceeds. Gelation in such systems will occur at an early stage of the reaction if a large proportion of the reacting molecules have high functionalities. On the other hand  the presence of monofunctional reactants serves to limit the growth of the polymer molecules and retard gelation  a principle which finds wide application in the manufacture of resins for surface coatings which are generally required to retain their solubility at least until some much later stage of their existence.

Classification of Polymers

Polymers may be classified in several ways to facilitate their study  and the utility of the system adopted depends on a personal point of view. The terms rubber  resin  and plastic  frequently used to distinguish polymeric materials  are associated with certain physical characteristics  but with the possible exception of the first have tended to lose through indiscriminate use any precise significance they may have had.

A traditional classification is that into thermosetting and thermoplastic  or convertible and non convertible types  according to the resin  s ability to cure or dry to form insoluble or infusible cross linked products. This is a convenient division when considering surface coating materials. Purely utilitarian in origin  it is now seen to be founded on the functionality concept. It should be realised  however  that whilst polymers which are classified as convertible will dry by chemical reaction in the air or cure when stoved  other materials  non convertible under these conditions  can sometimes be made to cure in special circumstances. Thus polyvinyl chloride  which is normally regarded as a thermoplastic  can be cross linked by heating in the presence of polyamines.

Perhaps the most generally useful classification is based on the type of chemical reaction utilised in building up the backbone of the polymer molecule. It leads directly to a consideration of the types of linkages uniting the polymer segments and permits a number of practical generalisations concerning the chemical nature of polymers  their mode of formation and the properties associated with given types.

Formation of Polymers

The two main divisions of polymer forming reactions are designated condensation polymerisation (or polycondensation) and addition polymerisation (or simply polymerisation). Condensation and addition reactions have exactly the same meaning in polymer science as in classical organic chemistry.

Condensation Reactions

Typical condensation reactions are the formation of an ester from an acid and an alcohol  and of an amide from an acid and an amine. By definition  a by product of low molecular weight is eliminated  in these cases water.

Polycondensation

Application of such reactions to polyfunctional reactants  already exemplified in aminocaproic acid  glycerol and phthalic acid  yields condensation polymers. The mechanism and kinetics are the same for the polycondensation reactions as for the simple condensations  the only distinguishing feature  chemically  is the functionality. The reaction rate depends on temperature  and on the concentration of reactants and products at any given stage in the reaction. The last is very important when  as in the formation of polyesters and polyamides  the reaction is reversible  for  in accordance with the Law of Mass Action  the forward reaction will only proceed to completion if the reverse reaction is prevented by separation of polycondensate from by product. This is usually accomplished by distilling off the byproduct leaving the polycondensate in the reaction vessel.

When as is usual the by product is water its removal can be facilitated by azeotropic distillation. The best known example of this is provided by the solvent process for making alkyds. Provided the raw materials be not too volatile efficiency of by product removal can be facilitated by applying vacuum to the reaction vessel as is done in the later stages of polyamide manufacture.

Some polycondensations  such as the reactions between phenol or urea and formaldehyde  are not reversible in the sense that the polymers are not broken down to the starting materials by hydrolysis. As will be seen  however  water and other by products are evolved in the later stages of the reaction  and here also the application of vacuum assists in the attainment of high molecular weights.

Interfacial Polycondensation

Recent work in the laboratories of the du Pont company has revealed an interesting new technique known as interfacial polycondensation. Polyamides of very high molecular weight are formed rapidly at the interface between an aqueous solution of a diamine and a solution of a diacyl halide in a water immiscible solvent. The technique has been extended to the preparation of polyurethanes and materials difficult to prepare by conventional polycondensation methods  for example  polyphenyl esters and polysulphonamides.

Sequence of Reactions

In all polycondensation reactions the greater part of the starting materials react through one or more of their functions at an early stage to form products of low molecular weight. These then react with one another and with remaining monomers so that mean molecular weight increases as reaction proceeds  the precise relationship between degree of reaction and molecular size depending on the mean functionality of the initial reactants.

Addition Polymerisation

The principal form of addition polymerisation is often referred to as vinyl or ethenoid polymerisation and comprises the union of unsaturated monomer molecules by the opening of and interaction between their double bonds. This is a chain reaction of either free radical or ionic mechanism. The former is the more important in the production of surface coating polymers and is similar kinetically to the formation of hydrogen chloride from gaseous hydrogen and chlorine.

Three reaction steps can be distinguished. First  free radicals are generated by the thermal dissociation of an added substance known as an initiator  such as benzoyl peroxide  and these react with monomer molecules to form an active species which initiates the polymerisation reaction. Many unsaturated monomers have structures of the type CH2=CH  i.e. they are monosubstituted ethylenes.

Writing the initiating free radical as R.  the initiation reaction may then be expressed

The resulting active species is another free radical and this now reacts successively with further monomer molecules in the second step  called propagation.

Many thousands of monomer molecules may be combined in this way before the third step  termination  destroys the active species by elimination of the free radical  and the final fully grown polymer molecule results.

Termination may occur by combination or by disproportionation of two growing chains  thus 

Copolymer Formation

If a mixture of monomers is present they enter the growing chain in a more or less regular manner depending on their reactivities  and the product is then known as a copolymer. Styrene and maleic anhydride  for example  exhibit a strong tendency to regular alternation  thus

Rate of Polymerisation

These polymer forming reactions are exceedingly rapid but  in the conversion of a batch of monomer to polymer they are only occurring in a very small fraction of the total molecules present at anyone time. Thus  although the time taken to form one polymer molecule of some thousands of monomer units may be only a few seconds  the conversion of all the monomer present may take many hours. The overall polymerisation rate and the molecular weight of the polymer depend on the relative rates of the initiation  propagation and termination steps  and the relative concentrations of monomer and initiator. The most important feature to remember is that polymer of high molecular weight is present from the very commencement of the process. Molecular weight does not increase progressively with time as in a polycondensation reaction all that happens is the conversion of more monomer into polymer.

Addition Polymerisation in Practice

The practical methods of conducting these polymerisations include polymerisation in bulk  in an aqueous suspension of droplets of monomer  in solution and in emulsion. Polymers for coatings are usually made in solution or in emulsion. Emulsion polymers have very much higher molecular weights than polymers prepared in solution.

Chain Transfer Reactions

Two of the more recent developments in the chemistry of addition polymers have been the discovery and application of chain transfer and stereospecific polymerisation. The most interesting type of chain transfer occurs when a free radical on the end of a growing polymer chain withdraws a reactive atom from a   dead   polymer molecule  thereby creating a growing centre on the latter. Branching can thus occur in  for example  polyvinyl acetate  which would otherwise be a linear molecule. The phenomenon can moreover be deliberately applied to graft branches of one type on to a backbone of another. Thus methyl methacrylate polymerised in the presence of polystyrene yields a proportion of   graft copolymer   molecules such as

Materials of this sort may possess a balance of properties different from those of a simple copolymer and it is sometimes possible to prepare graft copolymers of which no conventional copolymer analogue exists. Commercial exploitation of these materials is  however  still in its infancy.

Stereospecific Polymerisation

In stereospecific polymerisation  the reaction is induced by a complex initiator  which  unlike those of prior art  exercises a directive influence on the way the monomer molecules enter the polymer chain. One result is an almost total absence of branching by chain transfer. The other  only relevant when the repeat unit contains an asymmetric carbon atom  is the introduction of successive segments in the same or a regularly alternating sense in space. If propylene is polymerised by conventional means the dextro and laevo configurations arising from the presence of an asymmetric carbon atom in each segment  are disposed at random along the polymer chain. The polymer is said to be atactic. Stereospecific polymerisation  however  can yield either a   syndiotactic   polymer with regular alternation of dextro and laevo configurations  or an   isotactic   form in which every segment has the same configuration.

Stereospecific polymers are harder  more crystalline and less soluble than their atactic counterparts. They are finding app1ication as structural materials and fibres  but whether in view of their reduced solubility they will make an impact on surface coating formulation remains to be seen.

Polyaddition Reactions

Other addition reactions leading to polymer formation are known which do not depend on the foregoing mechanism. Examples are the polymerisation of ethylene oxide in the presence of a trace of water and the reaction between di isocyanates and dihydric alcohols.

These reactions are referred to as polyadditions to distinguish them from addition polymerisations of the ethenoid type. They differ from polycondensations in being unaccompanied by the liberation of by products  but in most other respects they are more akin to polycondensation than to ethenoid polymerisation.

Types of Polymers

Polyesters

Of the condensation polymers used in surface coatings  pride of place is taken by the alkyds. These are polyesters  the molecules of which contain fatty acid side chains  which have the effect of limiting molecular weight  imparting solubility in hydrocarbons and otherwise modifying the physical and chemical properties of the material. The traditional polyester modified in this way is glycerol phthalate and a typical structure would be

where L = fatty acid residue.

Formulation of Alkyds

Such materials can be made by simple interaction of a mixture of glycerol  phthalic anhydride and fatty acids. Alternatively a fatty oil may be used as starting material when it becomes necessary first to convert this into a   monoglyceride   by alcoholysis with glycerol before adding phthalic anhydride and esterifying. Formulating variables are manifold. The proportion of fatty acid to phthalic may be varied between wide limits to yield products lying almost anywhere between oil and unmodified glycerol phthalate in solubility and other physical characteristics. The fatty acids or oil may be fully saturated or predominately unsaturated  the former yielding plasticising alkyds of maximum colour and gloss retention and the latter convertible coatings which  by virtue of their high functionality in terms of unsaturated centres cross link and dry rapidly by oxidation to yield tough durable films. Glycerol may be replaced by pentaerythritol or other polyols. Pentaerythritol is particularly valuable as a component of drying alkyds at the longer oil lengths because the increased molecular complexity resulting from its use leads to increased speed of drying and film durability. Phthalic anhydride can be replaced by other polybasic acids or anhydrides. Adipic acid yields more flexible products  whilst isophthalic acid yields somewhat harder materials  halogenated acids may be introduced to render the resins more or less fire resistant.

Structures Analogous to Alkyds

It can easily be seen that the older oleoresinous varnishes fit into the same pattern of structure and behaviour as the alkyds. Linseed oil molecules  the glycerol esters of the unsaturated acids linoleic and linolenic  on heating form stand oils due to the formation of dimeric and trimeric fatty acids by interaction of double bonds.

Here we have an   alkyd   in which phthalic acid is replaced by more flexible aliphatic structures and some of the unsaturation of the fatty acids is lost. By comparison with a phthalic alkyd of similar degree of polymerisation  therefore  the product dries more slowly to yield softer films.

In the same way it will be appreciated that many hard resins used in varnish manufacture react with oils by a process of ester interchange to yield structures similar in pattern to the alkyds. Copal resin  for example  contains polybasic resin acids  whilst the rosin maleic adducts are semi synthetic materials produced by Diels Alder addition of maleic anhydride to the conjugated alicyclic acids present in rosin and hence contain tricarboxylic molecules.

Saturated and Unsaturated Polyesters

In the surface coating industry the name   polyester   is rarely applied to oil modified alkyds and is reserved for types containing no oil or fatty acid modification. These polyesters are of two kinds. The best known are the unsaturated polyesters commonly derived from diols such as propylene glycol or diethylene glycol and unsaturated dicarboxylic acids such as maleic and fumaric. In some types unsaturation is introduced in other ways such as by the incorporation of glycerol monoallyl ether. These polyesters are essentially linear in structure and in use are mixed with an unsaturated monomer  commonly styrene  which  under the influence of a peroxide initiator  copolymerises with the unsaturated centres  thereby cross linking the system to a hard  glossy film.

The degree of cross linking is controlled by using a proportion of phthalic or other non polymerisable acid in the polyester.

The other type  the saturated polyester  is derived from saturated or aromatic dicarboxylic acids and mixtures of polyhydric alcohols having an average hydroxyl functionality greater than 2. These polyesters are produced with free hydroxyl groups and are subsequently reacted with polyisocyanates to effect cross linking through urethane groups. For use in coatings  it is important to have polyesters substantially free from acidity because carboxyl groups liberate CO2 on reaction with isocyanates and so cause foaming.

Polyamides

The best known polyamides are the   nylons   derived from  aminocaproic acid or from hexamethylene diamine and adipic or sebacic acid. These are high melting materials of low solubility and their main use as surface coatings is in wire enamels. Many attempts have been made to prepare more soluble types  but the major success in this direction was only achieved in 1944  when it was shown that polymerised fatty acids containing predominantly dilinoleic acid could be produced economically and converted into polyamides. Two classes of such polyamides have been developed. The first is a more or less linear type derived from a simple diamine such as ethylenediamine. These resins are essentially   non convertible  . The other type is derived from polyamines such as diethylene triamine  the molecules are branched and are deliberately made to contain free amino groups. These find their major outlet as curing agents for epoxy resins.

An important recent development has been the incorporation of the ethylenediamine dilinoleic acid type into alkyd resins. When heated together  ester amide and other interchange reactions occur  leading to a complex resin which yields highly thixotropic solutions in hydrocarbons. Such materials form the basis of   jelly   or   dripless   paints and are finding increasing use as formulating tools to control paint structure.

It is easily seen that phenol  meta cresol and other materials of functionality greater than 2 can yield cross linked products by these means.

Under acid conditions  phenol yields only a bifunctional derivative and the inter unit linkages in this case are essentially methylene groups.

Unless subjected to high temperatures or treated with alkali and formaldehyde  these so called novolaks are non convertible.

A wide variety of phenolic resins can be made by varying the nature of the phenol  the ratio of phenol to formaldehyde and the conditions of preparation. The use of phenols containing higher alkyl groups promotes solubility in oils and hydrocarbons  whilst substitution in the para position by a tertiary carbon atom prevents the formation of quinonoid structures on oxidation and renders the resins substantially non yellowing. It may also be noted that the methylol compounds can be reacted with a variety of other materials. An important example is the reaction of methylol phenols with rosin to yield   reduced phenolic resins  .

Cashew Nut Shell Liquid Resins. A novel resin in the phenolic series is that derived from cashew nut shell oil. This oil  obtained from the shell of the cashew nut  on refining and steam distillation contains essentially a mixture of substituted mono  and dihydric phenols.

The substituent hydrocarbon radical is unsaturated.

The mixture of phenols is capable of polymerising by a variety of routes  either by condensation as a phenol with formaldehyde or furfuraldehyde  or by acid catalysed polymerisation of the unsaturated residue.

The main use of this type of resin is in the insulating field  as the fully cured resin has good flexibility. It is  therefore  used as a wire enamel  and also for laminated board which is later punched or stamped.

Processing of Oil and Resin

The Processing of most synthetic resins  stand oils and similar products is carried out under batch working conditions and the major proportion of existing reaction units are designed for this purpose. The comments and recommendations dealt with in this section apply to this class of equipment generally.

Some work has been done on the design of continuously operating units  but apart from the dehydration of some formaldehyde types  mainly on large scale  continuous production units are generally only suitable for one particular product  and as most manufacturers require a wide range  the more flexible batch unit has been and is preferred.

General Requirements for Processing Equipment

Although the basic requirements to be achieved in the design of individual process units vary between different classes of resins and stand oils  etc.  there are certain main factors common to nearly all. The materials being reacted must be combined in a suitable vessel  with agitation if necessary  heated to a predetermined temperature schedule until the required reaction is completed  and then cooled and subjected to after treatment. Also it is usually necessary to supply some sort of vapour condensation system for refluxing condensate  or separating part of the condensate and refluxing the residue  or removal of fumes and vapour entirely. All these stages must be under control and clearly indicated both for quality of product and safety conditions.

The basic designs for the three main types of process units are shown in the simplified flow diagrams. Fig. 1 shows a typical stand oil unit  Fig. 2 typical P.F. or U.F. units  and Fig. 3 typical alkyd (polyester) units. There are  of course  many modifications  which can be made  and these are discussed later.

Materials of Construction

The choice of material for the construction of the equipment is of prime importance  and both economic and product quality factors must be considered. It is generally accepted that an installation should be dealt with on the basis of at least a ten year life  and a correctly designed unit  which has facilities  provided to suit possible modifications can be economically used beyond this life span.

The constructional materials most commonly used in Britain at the present time are listed in Table l  although other materials may be employed in specific cases.

Mild steel has not been generally classified as  although many installations have been made in this material  its life is often comparatively short and many products are precluded. It is  however  extensively used in those positions in plant where contact with the product is avoided.

Design of Reaction Kettles

The Kettle Body

The first consideration should be given to the shape of the reaction vessel for a particular process and apart from the limitations which may be imposed by the siting of the unit with regard to height  etc.  the ratios of free surface area and heating surface area to volume and to each other must be realistic. Also  to a less extent  the use of standard sheet sizes of material is preferred  often giving increased capacity at little or no extra cost.

In the case of the manufacture of stand oils  dehydrated castor oils and similar products  convenient ratios of depth of charge to diameter of vessel are approximately 1.1   1 for standard methods and 1.5   1 for the Sommer and similar systems. For alkyd or polyester resin types a convenient ratio is approximately 1   1  and for most formaldehyde types approximately 1   1.25. These are for medium sizes of equipment and typical dimensions are given in Table 2 covering a range of sizes.

The thickness of the vessel walls and dishings can normally be calculated using the standard formulae given in B.S. 1500  and specifications for test pressures of finished vessels should be at least twice the greatest pressure that is to be encountered. It is also advisable to include a safety allowance on the thickness of any reaction vessel to cater for any friction  erosion or particular condition which may arise. In most equipment this allowance would be satisfied by approximately 10 per cent of thickness and usually only implies rounding off the thickness calculated to the next standard sheet thickness.

For example  the requirements of a reaction vessel for the production of alkyd resins by the solvent or azeotropic method  and having a working capacity of 500 gallons  can be calculated. The gross capacity allowing 50 per cent freeboard would be 750 gallons  and dimensions of 4 ft 6 in. dia. and 7 ft 3 in. deep on the straight side are convenient as dishings are standard for top and bottom. The material chosen is stainless steel of F.M.B. grade and full vacuum is to be catered for as well as 15 lb/sq. in. pressure.

Branches and Connections

The necessary fittings and branches for the reaction kettle will depend upon its use  but it is as well to remember two main points. Firstly  when a vessel is being fabricated it is not expensive to have extra branches or bosses provided and blanked off  but additions can only be made later at considerably enhanced cost. Secondly  an excessive number of connections on a small unit can only lead to difficulties in manufacture and operation  thus multi purpose fittings should be used. A good example of a kettle top cover is shown in Fig. 4  all the fittings are accessible and sufficient space is allowed to permit a neat layout of incoming connections to be made. Table 3 gives recommended numbers of branches  etc.  which can be achieved without difficulty.

Types of Agitators

The combination anchor  paddle and foam breaker unit is very commonly used  and is particularly useful where vessels with heating and cooling jackets  induction heating or external electric jackets are employed. It is suitable for mixing light solids into liquids and gives a gentle agitation to the mass of the charge with emphasis on maintaining a steady movement across the sides and bottom of the vessel. It is usually employed at speeds ranging between 25 and 60 r.p.m.  based on a nominal peripheral speed of 300/700 ft/min. and not a standard rate of revolutions.

The turbo type agitator which gives a centrifugal pumping action is more effective for mixing finely divided or flake type solids into liquids  but does not give the movement over the outside walls and the bottom of the vessel achieved with the anchor type. It is  however  of considerable assistance where immersion heaters are employed such as electric elements  combustion tubes or flue gas induction tube stacks. The speeds are 80 200 r .p.m. again based on peripheral speed considerations.

More recently the marine type propeller has been employed and for the best effect these are arranged to force the liquid down into a cruciform baffle formed on the bottom of the vessel. These have great advantages in that the volume of liquid moved is greatly enhanced without increase of motive power  and also that agitation is achieved by turning the charge over without the normal stirring action  the flow pattern being shown in Fig. 5. This type of stirrer is particularly suitable where solids are to be melted as adequate clearances are available  and the design lends itself to a robust unit which does not clog  Speeds may vary between 90 and 300 r.p.m. depending upon viscosities  etc.  and are related to quantities to be moved  or more correctly to pumping speeds.

Sealing

Where the shaft of the stirrer enters the kettle it is necessary to provide a suitable gland which must be capable of being repacked quickly and efficiently and lubricated continuously  and must be designed to suit the particular conditions. It is as well to be generous in the dimensions of the gland at the expense of some extra power losses in the drive  as this will be amply repaid by the added security given. Glands and packings can vary widely  but a simple and effective type is shown in Fig. 6.

In the case of small kettles it is often possible to rely on the bearings provided in the reduction gearbox unit  with the steadying effect of the gland section  but for large vessels it is often preferable to employ a steady bearing at the bottom. This should be supported by a straddle across the outlet  and generally the most satisfactory bearing consists of a replaceable section on the end of the stirrer shaft rotating in a renewable split bush in the bearing housing. Dissimilar materials should be used and a fair clearance allowed 0.020/0.030 in. (Fig. 7).

Drive Units

The selection of suitable drive units for stirring gear is very much a question of preference of individual users and either turret types or pulley drives are employed. The turret type is normally slightly the less expensive in first cost and consists of a reduction gearbox mounted on a support stool and supporting the drive motor.

It is economical in space but is not convenient where variable speed is a requirement or when maintenance of the gearbox is necessary.

If the pulley and belt drive system is employed  then a simple and robust system of speed control can be achieved by the use of expanding pulleys. Many other methods of speed control can be used and information is freely available.

The types of drive motors employed may vary  but the totally enclosed and fan cooled units are most often employed  and these should be specified for high starting torque. When stirrers are first started  there may be residues from preceding batches in bearings and glands which will cause extra load  and thus if a 170 or 200 per cent starting torque is specified  the extra torque available will normally overcome the   drag  . This also applies to pumps dealing with viscous or sticky materials.

Fume Disposal and Scrubbing

For some types of stand oil and synthetic resin production units  vapours evolved during the process can be regarded as waste products  and as such the only problem is the disposal of these fumes or vapours. Unfortunately  they are often objectionable either in odour or because they have unpleasant toxic or irritant effects. Normally they must be disposed of without allowing the ingress of free air and several methods of achieving this are possible.

Disposal Systems for General Use

The simplest system of fume disposal where the fumes are readily soluble in water is the water sealed type stack pipe. This is commonly fabricated in cast iron or mild steel  and as its construction is cheap it is common to allow fairly large diameters  8 in. being considered a convenient diameter for a reaction kettle of capacity 2 tons.

An alternative scrubbing system  which can be usefully employed in disposing of fumes  which are not so readily soluble  is shown in Fig. 8. In this type of scrubber the fumes are passed through a constant mist of fine droplets of liquid  which is held in the base of the scrubbing unit. Additions may be made to the liquid in use to counter acidity or particular components of the fume  and a further advantage is that the pressure drop through the system is very small. Depending upon the fume conditions to be met  the complete unit can be fabricated in stainless steel  which is expensive  or alternatively in mild steel and galvanised  or metal sprayed internally to minimize corrosion. A mild steel unit can have a good life as the surfaces are continuously washed.

Water Scrubbing of Anhydride Vapours

In the manufacture of alkyd resins by the fusion process  a quantity of the particular anhydride being employed will be vaporised and will pass from the reaction kettle with other unwanted materials  the fumes being very unpleasant. Some designs have been made of   dry   condensers to recover the anhydride sublimed for re use  but in general it is preferable to use a water scrubbing system for their removal.

Design Considerations

This type of scrubbing unit is very effective  but the design and construction for a particular capacity is important  for if the unit is too large the water consumption is uneconomic  and if too small the pressure loss is excessive besides not giving complete condensation.

The vapour duct should be constructed in stainless steel and should be kept clear of obstructions. It should also be provided with some form of heating to maintain it at a controlled temperature between 140ºC and 160ºC  depending upon the particular conditions  which can only be decided by trial in operation. Typical methods of heating are by use of low voltage transformers  Pyrotenax heating cable loops  steam jacketing  or glass woven electric heating tapes. Pyrotenax cable is most convenient as it can easily be rendered flameproof in dual condensing system equipments  and also its outer sheath can be effectively earthed for safety.

The sprays used are normally of the double ended type constructed from stainless steel  and the sizes of the jets should be such that the spray just reaches the wall of the tower  this giving effectively several curtains of water for the fumes to pass through. This also washes the walls of the tower  which can be fabricated  in mild steel and still have a good life.

Sludge Handling Equipment

The sludge receiver is normally constructed in mild steel  and the most important point is that its capacity should be adequate for the quantity of water used  the recommended quantities being 300 400 g.p.h. for a typical reaction kettle of 2 tons working capacity.

The sludge pump rating is normally adjusted to the full flow of water from the spray jets  and to eliminate any possibility of flooding if the sludge pump should fail  it is advisable to fit a level control in the sludge receiver to cut off the water delivery to the jets automatically. The pump is normally constructed in cast iron and gunmetal  and must be capable of working against vacuum where the production schedule requires this.

It is not advisable to use isolating valves in fume ducts where this type of process is being carried on as it is almost impossible to adjust temperatures correctly and cleaning operations will be a constant necessity. A simple blocking plate assembly is often used and offers a clean arrangement.

Packed Scrubbers

Alternative scrubbing units can be constructed employing packings such as coke or carbon lightly irrigated with water. These are very effective but tend to clog quickly and are expensive to operate. They are often used  however  where the simple scrubbers described above do not completely condense or absorb the fumes  and where conditions by local authorities  etc.  render it essential.

Condensing and Refluxing

The correct design of the condensing equipment is one of the most important aspects of synthetic resin process plant and great care must be taken that all the main considerations as below are fully taken into account.

For synthetic resins and allied products it is normally an essential function of maintenance that the condenser must be regularly cleaned. For this reason the standard condenser used is of the tube in shell type with the vapours passing through the tubes  and deviations from this standard are rare.

Condensers for P.F.  U.F. and M.F. Resins

During the processing of the formaldehyde types of resins  the condenser has two main functions  first refluxing during the initial stages when it assists in the control of the exothermic reaction which  depending upon the mix  can be extremely violent  and second during the final stages when the excess water is being stripped off. The first duty is normally the greater and the design is based on this premise  assuming a nominal rate of 20 lb/min condensate per ton of charge  for medium sized batches.

During the exothermic reaction the condensate can be returned to the charge at a relatively high temperature as the latent heat absorbed by the evolution of vapour is the major factor in heat release  but when stripping off the water at the end of the run the condensate must be sufficiently cooled to remain liquid under vacuum conditions. To this end it is essential that the entry of the vapour and outlet for the condensate are at opposite ends of the condensing unit  and also that sufficient length of path is available to permit adequate heat transfer to be effected.

Design and Layout of Tubes

Precise calculation of surface areas and cross sectional areas are extremely difficult as the factors to be allowed for  dirt coefficients (fouling factors) and similar unknown quantities vary widely between different formulations and also at different stages in the process  but convenient ratios between diameter and length of individual tubes vary from 1  80 to 1  120. These ratios must be used with discretion  as under normal circumstances the minimum convenient bore is ¾ in. and the working space available must also be taken into consideration.

A typical condenser layout is shown in Fig. 2 and alternative arrangements are noted for the tubes  and to suit differing site conditions. Average condensing surface areas for standard formulations are given in Table 4.

During the main period of exothermic reaction refluxing of the condensate occurs  the condensate being returned to the reaction kettle through a three way cock  and subsequently when dehydration of the product is required  the position of the three way cock is reversed to permit the draining of the condensate to waste.

It is essential that the reflux lines should be of ample size for their duty  and in order to minimize any possibility of blockages  etc.  slow bends should be employed. If a sight glass is fitted in the main reflux return feed  the operator is able to detect quickly any unusual occurrence and also this can  to some extent  indicate the state of the process.

Waste Receiver

The waste or dump receiver must be of a suitable size for the particular batch and formulation  and the normal system is to allow for the receiver to be two thirds full after dehydration is complete. Plate type sight glasses can be employed to check the final quantities involved.

When designing the receiver and condenser it should be noted that they must be suitable for both vacuum and pressure conditions  although normally if the vessels are designed to suit vacuum it will be found that this represents the more arduous duty.

Inorganic Pigments

Introduction

The Conversion of the   Rocket   type of locomotive into the present day   Royal Scot   is an object lesson in evolution. Step by step some features have been modified and adapted  others have been discarded. Some entirely new characteristics have appeared while certain basic structures have been maintained relatively unchanged throughout.

Although knowledge of this history is perhaps not vital to the users of such transport  it certainly helps to explain and understand the particular pattern  which has evolved  and it demonstrates the extreme changes which may occur by small adaptations.

The inorganic pigments used for surface coatings have in the same way slowly evolved over a series of years by small changes in characteristics  until ultimately extreme changes in properties have occurred although the basic structures have been maintained unchanged.

Many inorganic pigments have been evolved from intractable parent material over a period of fifty to a hundred years by gradual modification.

The users of transport have the change in name from the   Rocket   type of engine to the   Royal Scot   locomotive to indicate changes of characteristics and progress in design  but often confusion is caused by using the same pigment name over a period of years without reference to other changes that have taken place.

For this reason a short evolutionary history is given of all the important pigments described  to enable a comparison to be made of both the uses and properties of pigments given the same name at different periods of their history.

Origins of Pigments

Comparison of Natural and Synthetic Pigments

Several of the parent bodies of the inorganic pigments are found in naturally occurring deposits in the earth  s crust. The corresponding pigmentary materials are often made synthetically. In general  the only characteristic in common between the natural and the synthetic pigments is the chemical compound upon which the pigments are based  otherwise their characteristics are quite different  and the reason is not far to seek.

The naturally occurring materials are usually stable corrosion products  which occur in a macrocrystalline form often associated with silica. In order to be used as a pigment the ore found in the earth  s crust is mined  crushed  washed and classified into particle sizes of different ranges. During the crushing process the crystalline material is broken into fragments  the crystals cleaving by way of the cleavage plane. In the case of the disorientated material  crushing and grinding breaks the material into fragments  and the distribution of the size range in both cases is solely determined by chance. The shape of the particle is also determined by chance  even when a crystalline ore fractures mainly along a plane of cleavage.

A synthetically prepared pigment  on the other hand  can be designed to have the most suitable crystal lattice  crystal habit  and  to a considerable extent  particle size and particle size range  and to maintain its chemical constitution within any desired limits.

If these two types of material are used as pigments for surface coatings the differences between the natural and the synthetic pigments are greater than the similarities  and the natural and synthetic pigments used under such circumstances are distinctly different pigments.

Problems in Producing Natural Pigments

The natural pigments create one problem of which there is no simple solution. After crushing  washing  classification and drying  a very small quantity of oversized particles are entrained in the pigment particle size range. The removal of these few   boulders   from the main bulk of pigment can be achieved by further grinding which widens the range of particle sizes introducing other undesirable properties. Reclassification will remove these   boulders    but only by processes giving uneconomical yields.

The presence of even small quantities of oversize pigment particles reduces considerably the output of pigmented pastes and a satisfactory method of removing these large particles has not yet been devised.

For this reason an account is given of the basic chemical engineering process used in pigment manufacture.

Pigment Classification

Since  in the same way  the physical properties of the inorganic pigments depend upon a series of physical characteristics which can be measured  no attempt has been made to give a certain pigment name specific surface coating properties but to outline the physical basis upon which such properties depend  and to consider each pigment modification as an entirely different pigment belonging to a specific class.

In the beginning of the search for suitable pigments for the surface coating industry any coloured material  from whatever origin  was considered and tried. This has resulted in a very large number of materials called   Inorganic Pigments   and a list of such materials is an extremely long one. As the basic physical use of such materials has become known  so has the number of pigments in use been reduced  and this trend will continue with the search for new pigments directed along specific avenues defined by physical properties.

Pigmentary Properties

An inorganic pigment is a finely divided inorganic substance  usually a stable solid  which is added to the vehicle to produce a stable surface coating.

These properties are related to particle size and particle size distribution  particle shape  colour  and refractive index at all wavelengths. These four physical properties  which are discussed in turn below  can be measured and controlled and used to give suitable properties for the pigmented system.

Particle Size and Particle Size Distribution

The particle size and refractive index of an inorganic pigment closely determine the scattering coefficient of the pigment. The refractive index will be discussed later and is of greater importance than particle size  but since for anyone material the refractive index is fixed and particle size and particle size distribution can be controlled in manufacture  the influence of this latter factor will be determined.

The high brightness of a white surface coating material is obtained by having a large number of pigment vehicle interfaces each of which returns a quantity of the light towards the surface of the paint. It can be shown for any one particle that if d is the diameter  and the wavelength  then as d diminishes the light reflected increases to a maximum and then decreases. It then increases to another maximum before decreasing finally into the colloidal range.

In practice we never achieve a uniform particle size. Instead a white pigment is a mixture of particle sizes grouped around the size of greatest frequency. Thus  the wave point of reflected light is the combined point from the various particle sizes. Nevertheless  it is possible to define the range of size most desirable. They should be not smaller in diameter than the short wavelength (blue end of the visible spectrum) and not larger than the long wavelength (red end). Unfortunately only in certain cases has sufficient data been collected showing the relation between particle size  particle size distribution and opacity  but the basic physical quantitative conditions are now known.

Particle Shape

Practically all synthetic inorganic pigments are crystalline  showing a well defined crystal lattice  but the manner in which the basic unit lattices are built into a crystal may differ during crystal formation. Very small traces of impurities may cause changes in crystal habit without change in crystal lattice. Such changes may be superficially considerable a cubic habit may change into an acicular one  or be a partly disorientated (spheroidal) one  which is often unstable  reverting to a more stable crystal form after ageing.

Influence on Pigment Properties

The geometry of the shape of a particle is of importance for two reasons. The surface area of any shape approaching that of a sphere is a maximum  the edges of an acicular shape often offer areas of high surface activity and by their alignment in the surface relieve and reduce surface strain. For example  the weathered surface of an acicular zinc oxide surface shows microcracking  whilst  under the same conditions  the other form of zinc oxide shows macrocracking.

Since several pigments show more than one form of crystal habit and more than one form of crystal lattice  the physical form and properties of certain inorganic compounds can be widely different. Iron oxide has a minimum of three crystal lattices and three crystal habits  offering the possibilities of nine different physical forms.

The influence of the different physical forms upon the properties of the surface coating material  although not known quantitatively  is often considerable  changing both flow properties and colour of the product. Such changes will be considered under the appropriate pigment name.

Colour

The colour of a pigment is an important property  indeed  certain pigments are selected for that purpose alone and often a certain type is referred to by a colour name  e.g.   yellow iron oxide    since iron oxides have a wide range of colours. It is not possible to record a colour by words alone and the only way is to plot the spectrophotometric curve of the colour over the visual spectrum under standardised conditions. The spectrophotometric curve is the record of the reflection of light plotted  wavelength by wavelength  continuously over the visual spectrum.

If the method of presenting the coloured material to the light source is standardised  then the spectrophotometric curve will uniquely define the colour. Typical curves are given as representing typical white pigments (Fig. 1).

Refractive Index

In section 1 the importance of the refractive index has already been stressed. Probably due to the fact that the refractive index of a particular pigment cannot be changed  the subject has received little attention from the systematic workers.

Indications of Opacity

Little data is available of the refractive index of any pigment over a wide range of wavelengths  so it is not possible to use this data for purposes of calculation  but isolated measurements are readily available and are often used as a guide for opacity. Since the measurements are usually made at a wavelength in the yellow region of the spectrum  the measurements give some indication of opacity  but for a complete understanding the change in the refractive index over the whole range of the visible spectrum must be known.

The refractive index of the pigment in relation to that of the medium determines the opacity of the surface coating layer. The smaller the change of refractive index at an interface  the greater the intensity of light transmitted.

Influence of Wavelength

Although little systematic work has been done upon change in refractive index with wavelength it is known that the index varies near an absorption band  falling rapidly on the short wavelength side and rising rapidly on the long wavelength side. The blue and green hue pigments  having absorption bands in the long wave region  possess relatively low refractive indices for the effective light they reflect. Iron blue has a R.I. of 1.56 for the blue end of the spectrum  and since the medium used has a R.I. of 1.5  this pigment will be relatively transparent in such a medium. The iron oxides reflect in the red and yellow region and absorb in the green and blue region. The R.I. of iron oxide  in the wavelength region of high reflection  has a R.I. of 2.8 3.1  and such a pigment will be highly opaque in a medium of R.I. 1.5. As a general rule the blue and green hue pigments have a low R.I. and the yellow and red hue pigments a high R.I.1 for the effective light they reflect.

Control of Opacity

It is desirable to have control over the opacity (and transparency) of pigments over the whole hue range. This is usually achieved by adding a non selective reflecting pigment of high R.I. to the blue and green pigments  to increase opacity  and decreasing particle size in the yellow and red pigments  to increase transparency. Examples of this are the addition of titanium dioxide to iron blue to increase opacity  and decrease of the particle size of the iron oxides to decrease opacity.

In the past  the desire to have available pigments of varying hues and opacity has led to an extensive search for inorganic pigments of a wide range of hues and with varying degrees of opacity. With an understanding of the principles of the optical properties given above it is now known that the desired hue and degree of opacity can be achieved by mixing relatively few pigments. The ultimate effect will be to concentrate development upon these few pigments and only search for new products along well defined avenues.

Following a brief outline of unit processes used in pigment manufacture  the remainder of this chapter will be devoted to an account of the important classes of inorganic pigments.

Chemical Engineering Processes of Manufacture

The four main unit processes used for the preparation of pigments are discussed in the following.

Precipitation

Such processes presuppose the formation of the pigment in the presence of electrolytes. By choice of radicals or addition of electrolytes  and conditions of concentration of soluble compounds and temperature  the crystal form  habit and size can be controlled. If in the first precipitation an intermediate compound is produced  then similar conditions apply and control of both form and size of the intermediate can be obtained.

Vapour Phase Oxidation

Many elementary metals melt at low temperature and may be oxidized to a controlled degree  which is dependent upon the injection of a stream of metal vapour into a stream of air or oxygen.

If the oxygen is in excess  then oxidation is complete  and by varying the concentration of the element vapour crystal habit can be changed from growth under starvation conditions (resulting usually in acicular habit) to growth under conditions of excess metallic oxide (often producing a disorientated mass which after ageing gives the most stable crystalline form). It is possible  then  by changing the metallic vapour/oxygen ratio  to change the crystal habit from one extreme form to another  often without change of crystal lattice.

The consequent changes in physical properties are also wide  since many such properties are dependent upon crystal habit and particle size.

Heterogeneous Surface Reaction (Corrodibility and Corrosion)

This is the historically basic process for the preparation of all pigments. Naturally occurring pigments are all the ultimate corrosion products naturally produced by the exposure of elements to natural favourable conditions. Although chemically they may appear to be complex  they are in all cases the most permanent compounds  which can exist under such an environment. They are  therefore  the most permanent of pigmentary materials  showing stable crystal lattice and crystal form.

The processes of natural exposure are often reproduced artificially in order to produce suitable pigments. Sheets of metal are exposed to the natural components of our climate water vapour and carbon dioxide (sometimes reinforced by additions from other sources) and the corrosion processes allowed to proceed until a condition of equilibrium is established  thus producing  under the conditions employed  the most stable compound. The process is allowed to proceed until corrosion is complete and no quantity of unchanged metal remains. Chemically the products are nearly always coordinate compounds of the metal  i.e. a molecular structure with a central metal atom surrounded by other groupings  usually carbonate and hydroxyl  coordinately linked. Rarely are other elements involved.

Solid Phase at Elevated Temperature

The reactions that take place at the interface of solid materials are often used in preparing inorganic pigmentary materials. Such reactions depend upon surface area temperature and so are often very slow  demanding many hours at high temperature for completion.

The products are in all cases complex in structure and usually based upon a skeleton of silica. They are often very stable to all other physical changes.

The unit processes used for filtering  washing  drying  disintegration and size classification are those used normally for such purposes.

Organic Pigments

Surface coatings were originally used to impart protection to the objects on which they were applied. The inclusion of coloured pigments gave decorative effects also  but this was of secondary importance. Over a period of time  the emphasis has changed so that the decorative effect obtained by the application of a surface coating is of equal importance  and in some cases of greater importance  than the protective action. In consequence  demand has changed from the dull functional shades obtainable with inorganic pigments to the brighter  more varied hues only provided by the use of organic types  and particularly in the post war period  consumers have been encouraged to expect surface coatings to provide a complete range of hues  comparable to the gamut obtainable on textile materials for a long time.

This change can be seen in many aspects of colour usage  in the field of house paints  the once popular full shade greens  browns and creams have been replaced by brighter  more varied pastel shades  accompanied by a range of strong shades of clear  pure tone. A comparison of British Standard 38IC (1948) shades with B.S. 2660 (1955) or the   House and Garden   range of colours will illustrate this point. A second example appears in car finishes  where the once predominant black together with a few strong shades have been replaced by a wide variety of colours. Organic pigments are being used in increasing quantities to fulfil these demands and in some cases have been specially developed for the purpose.

Important Properties of Organic Pigments

The type of surface coating  the method of application and the intended use of the finish all impose conditions  varying in severity  which must be withstood by the coloured pigments present if the coloured effect is to be retained during application and subsequently. The ideal pigment would withstand all treatments  and would be usable in all types of surface coating irrespective of the severity of the conditions  but such ideal products are rare and compromise is frequently necessary.

Four properties of organic pigments are of major importance in determining their ultimate use  namely  fastness to light  solvents  heat and chemicals.

Light Fastness

A coloured finish is required to maintain its coloured effect during its useful life.

Inorganic pigments such as iron oxides or carbon black give effectively permanent coloration  but organic pigments need not approach this performance except where resistance to outside exposure is involved since for interior use a lower level of light resistance is acceptable.

The light fastness and durability of coloured finishes is dependent to some extent on the medium used  but the controlling factors are type of pigment used and its concentration. Although the light fastness and durability of a finish containing an organic pigment alone  e.g. a full shade  may be satisfactory  if the same pigment is used with opaque white to give a reduced shade  the performance may not be adequate and in fact the tint produced may be quite fugitive. The type of opaque white used will be a factor in determining the resistance of tints to exposure  but it is primarily the type of organic pigment which governs the performance over a wide range of depths of shade. Because the behaviour of organic pigments varies more widely in pastel than in full shades  the result of tests in pale shades is a more valuable yardstick to use in assessing the resistance of a pigment to light and weathering. In addition  a fastness rating based on pale shades is of direct value because of the predominance of pastel shades in surface coatings  and because it enables a prediction to be made of the performance of a pigment when used as a component of mixtures. Only a product of high light fastness should be used as the minor component of any mixture  otherwise rapid change of hue can occur on exposure caused by loss of the fugitive component.

Fastness to Solvents

A wide range of solvents is employed in the various types of modern finishes  to produce satisfactory coloured finishes  organic pigments must be capable of resisting the effects of solvents in order to avoid defects such as hazing  blooming  bronzing and bleeding into adjacent white or pale coloured finishes. The degree of solubilisation of a pigment is dependent on both the type of solvent  and temperature  and different results can be obtained between tests on pigment powders and on pigmented compositions. However  the examination of the resistance of pigment powders to a range of solvents gives an overall indication of the performance to be expected in use.

Heat Fastness

Where the finish is cured by the application of heat  the coloured pigment must withstand the conditions of stoving which are applied. This may be a relatively low temperature for a long time  e.g. 1 hour at 100ºC  or a much shorter time at higher temperatures  e.g. 8 minutes at 180°C  or with infra red stoving temperatures around 300ºC may be obtained. Repeated stoving schedules may also be experienced in  for instance  two  or three tone finishes  or in tin printing. It is essential to consider heat fastness as a function of the whole system  as different fastness can be obtained depending on the medium used.

Chemical Fastness

With the extension of the use of decorative effects to industrial equipment  pigmented finishes are in some cases required to withstand chemical attack  and to maintain their colour in the presence of acids  alkalis and other reagents. Tests on the dry pigment can indicate stability or otherwise according to the degree of colour change occurring  but the medium used in a chemically resistant finish will afford a degree of protection to sensitive pigments  some of which can be used in very resistant media. In order to make use of the widest possible range of pigments  selection on a basis of tests on pigment powders should be supported by trials on pigmented finishes.

Tinctorial Strength

A further property of pigments which is of importance when appraising products is tinctorial strength. When pastel or reduced shades are made in paint from organic pigments and opaque white such as titanium dioxide  it is found that different organic pigments give different depths of shade  even though the proportion of coloured pigment to white is the same in each case. This visual difference is caused when the organic pigments differ in tinctorial strength. The relative amounts of two coloured pigments which  with the same quantity of opaque white  give pale shades of equal visual depth is a measure of their relative tinctorial strengths  the stronger pigment is that which requires the smaller amount to give the visual match. The cost of making a pastel shade is  therefore  dependent on the tinctorial strength of the pigment used as well as its cost per pound.

In the discussions which follow on the properties of organic pigments and their selection for particular uses  verbal ratings are employed to describe the fastness of a pigment to light  solvents  heat and chemicals  and to indicate its tinctorial strength. A five step scale  P = poor  F = fair  M = moderate  G = good  and E = excellent is used. The use of a verbal scale simplifies the task of comparing the products of different manufacturers  since their own methods of assessment differ widely. The term   excellent   implies that the pigment is virtually unaffected by the particular treatments. In the case of heat fastness  the selected treatment is in a stoving medium for 10 minutes at 180ºC. A rating of   excellent   given for heat fastness does not guarantee no change at temperatures higher than this  e.g. in silicone resins at temperatures appreciably greater than 200ºC.

Types of Organic Pigments

The group of organic pigments used in surface coatings is a large and expanding one and includes a wide variety of chemical types. In comparison with the natural and synthetic inorganic pigments  the main advantages of the organic group are the availability of a wider range of hues  brighter shades and higher tinctorial strengths. Their basic fastness properties to light  solvents  heat and chemicals vary widely  necessitating careful selection for any particular end use.

General Classification

Organic pigments may be divided into three main groups according to their composition Pigment Dyestuffs  Toners  and Lakes.

Pigment Dyestuffs

Pigment dyestuffs are wholly organic water insoluble compounds  produced by a wide variety of chemical reactions. This group is easily the largest and includes the azo  phthalocyanine and vat pigments.

Toners

This description is applied to water insoluble products made by the precipitation of water soluble dyes  the final product being free from inorganic base (e.g. alumina) or extenders (e.g. barytes). Precipitation is effected either by heavy metal salts such as calcium  barium or manganese  with anionic (sulphonated) azo dyes  or by complex mixtures of phosphoric  tungstic and molybdic acids with cationic (basic) dyes. Usage of the latter group is primarily in printing inks.

Lakes

These are similar in constitution to the toners  i.e. they are derived from watersoluble organic compounds by precipitation  but in the presence of a base such as alumina or a co precipitate of alumina and blanc fixe  where the base is an essential and integral part of the product. The presence of the base ensures complete insolubilisation of the dye and a satisfactory degree of water resistance in the lake. The type of base used will depend on the final application of the lake  e.g. alumina/blanc fixe for surface coatings  alumina for printing inks. Water soluble dyes which do not give water insoluble toners can be converted to lakes.

While commercially available toners are usually prepared from organic compounds made specially for the purpose  lakes can be prepared from any watersoluble dye and potentially the entire range of anionic water soluble textile dyes is available for use. The fastness and tinctorial properties of lakes  however  are in general inferior to those of pigment dyestuffs and the group is of declining importance.

Extended or reduced pigments can be made from products in all the three previous groups by mixing pigments with extenders such as blanc fixe. These derivatives are frequently erroneously classed as lakes. They are  however  merely intimate physical mixtures of the parent product and extender  and although extended pigments frequently show some advantage in ease of dispersion over their concentrated counterparts  the fastness properties of the latter remain unaffected.

Usage of the terms   toner   and   lake   differs in the United States  there    toner   implies a concentrated organic pigment  i.e. it includes both the pigment dyestuffs and toners of the present classification    lake   implies the presence of extender or base and hence in the U.S.   lakes   comprise both extended pigments and the true precipitated lakes defined earlier.

General Properties of Organic Pigments

The three groups of organic pigments differ in the range of properties which they possess these are summarised in the following table.

It will be noted that although in resistance to such agencies as heat and solvents  toners and lakes compare well with pigment dyestuffs  it is only in the latter group that the highest general resistance occurs.

Classification by Chemical Constitution

As pointed out earlier  many chemically different organic compounds are used as pigments. The second edition of the   Colour Index   lists and numbers dyes and pigments in a systematic manner based on chemical constitution. In the present treatment the pigments will be examined under the two main headings of azo  and non azo  and within each group individual pigments will be arranged in order of C.I. numbers.

Azo Pigments

Azo compounds are characterised by the presence of the azo chromophore   N=N  joining together two aromatic nuclei and the basic reaction has been known for about a hundred years. Aromatic primary amines when treated at low temperatures with nitrous acid form an extremely reactive diazonium salt which is capable of combining with other aromatic compounds to form the azo linkage. The reaction producing the diazonium salt is termed diazotisation  the combination of diazonium salts with other aromatic products (second components) is the coupling stage.

If either the first component (diazotised amine) or second component contains sulphonic acid groups  the resulting azo compound will possess water solubility and can be isolated as a sodium salt by   salting out    where such groups are absent the resultant product will be water insoluble but may be somewhat soluble in oils or solvents. The presence of other substituent groups in either part of the molecule will affect tinctorial properties and non aqueous solubility.

Since the above reactions are typical of those used in the manufacture of azo pigments a brief description of the manufacturing cycle of one important product  Toluidine Red  will suffice to illustrate the processes used for the whole group.

Typical Manufacturing Process for a Monoazo Pigment

Meta nitro para toluidine is made from toluene by the following series of reactions

The second component   naphthol  is derived from naphthalene as follows

A solution of meta nitro para toluidine in dilute hydrochloric acid at 10° 15°C is stirred and sodium nitrite solution run in so that an excess of nitrous acid is maintained. The diazo compound formed is filtered and excess nitrous acid removed.  Naphthol is dissolved in caustic soda solution  sodium acetate is added as a buffer salt  and hydrochloric acid is added to precipitate the naphthol in a finely divided state. The  diazo solution is then run in over a period with stirring and a red insoluble pigment is formed which is maintained as a slurry. When coupling is complete as judged by the presence of excess diazo component  the slurry is filtered  the pigment washed to remove impurities and the filter cake dried in a stove. The dried lump is then ground to powder.

Since tinctorial properties vary from batch to batch because of slight variations in processing conditions or variations in purity of the coupling components used  selective blending of batches is carried out to maintain manufacture within accepted limits of a standard.

Formation of Disazo Pigments

Toluidine red is a monoazo pigment  i.e. only one azo linkage is present in the molecule. If an aromatic primary diamine is diazotised on each amino group  coupling with two molecules of second component is possible and a disazo pigment is formed

Disazo pigments can also be prepared by coupling two molecules of a diazonium compound with the same second component.

Monoazo Pigments

Couplings with Acetoacetarylamides

These monoazo arylamide pigments are yellow to orange in hue  and have the general characteristic of poor resistance to heat and solvents. Their light fastness varies from good to moderate  and Yellow 1OG is outstanding in this property. Although their tinctorial strengths are in the group   poor   to   moderate    there are important variations between individuals. For example  Arylamide Yellow 1OG is the weakest of the series  a property most unfortunately associated with high light fastness.

Couplings with 2 Naphthol

These examples of couplings with 2 naphthol range in hue from orange to bluish red  Para Red being the bluest in shade. As is evident from the table  the fastness properties  with the exception of chemical resistance  are very variable. Only Dinitraniline Orange and Parachlor Red possess any measure of light fastness in tints  the latter contrasting sharply in this property with the inferior performance of its isomeric compound  Chlorinated Para Red. Part of the increased light fastness of Parachlor Red may be an ancillary effect of its lower tinctorial strength. Some degree of increased colour retention on exposure is to be expected since fading is produced by a photochemical reaction and the degree of fade will depend on the amount of light and the amount of coloured pigment  in addition to the rate of degradation of the coloured pigment under constant conditions of illumination.

High tinctorial strength with only fair light fastness in tints shown by Para Red is a property of little value to the paint industry  and inadequate durability in strong shades together with insufficient heat fastness are the reasons for the declining importance of the pigment.

The most important naphthol pigment  Toluidine Red  owes its position to its excellent heat fastness and very good colour retention in full shades. It is capable of producing bright  strong red shades with a reasonable degree of hiding power although the solvent resistance is poor.

Couplings with 3 Hydroxy 2 naphthanilides

A comparison of Tables 3 and 4 shows immediately that pigments based on naphthanilides offer a higher level of fastness properties than is available in the naphthol series. The hue of the naphthanilide pigments is bluer than that of the naphthol derivatives  there is no true orange in the products tabulated and the bluest gives bordeaux shades.

The light fastness in tint shown by naphthanilide couplings is superior to that of all naphthol pigments except Parachlor Red and Dinitraniline Orange  and the former series has the added advantage of higher tinctorial strength.

The solvent fastness of the products listed in Table 4 is in general only equal to that of the naphthol pigments and here lies the main disadvantage in terms of current requirements. Where the molecular weight is increased by greater structural complexity in both components  as in Carmine FB  the solvent fastness rises to moderate. This level of performance is sufficient to give freedom from bloom in stoving finishes but not sufficient to prevent bleed into white stoving overspray enamels.

The naphthanilide pigments were developed to give bright shades with high light fastness in tint  properties which were accompanied by high tinctorial strength. Their shortcomings in solvent fastness and the fact that no light fastness rating of   excellent   can be given  led to the search for other products  pigments resulting from this search are discussed later.

Couplings with Heterocyclic Hydroxy Compounds

Yellow R gives very similar fastness properties to Arylamide Yellows G and 5G. Its main interest lies in its redder shade produced by coupling with a pyrazolone instead of an acetoacet arylamide (d. Benzidine Orange and Benzidine Yellow).

Nickel Azo Yellow on the other hand is a unique product. Although duller in shade than Arylamide Yellow 1OG  but of comparable strength and light fastness  the major deficiencies of Yellow 10G  poor heat and solvent fastness  are absent. The resistance to solvents is sufficiently high to give freedom from bloom in stoving enamels  but the product will not give enamels or lacquers which are non bleeding when oversprayed. The performance under these conditions is  however  satisfactory for all but the most critical applications. The high level of fastness exhibited by Nickel Azo Yellow is very largely due to the presence of ionic nickel as a coordination complex  since the free dye base is of poor resistance to solvents and light. The pigment is  however  not fast to acids  because demetallisation occurs with liberation of the dye base.

Couplings containing Sulphonic Acid Groups

The toners are listed in Table 6. It will be seen that their principal virtues are high heat resistance  high strength and good solvent resistance  but that there are exceptions to this generalisation in the case of each fastness property. Their chemical fastness is poor  the principal reason being that alkalies are capable of destroying the metal salt present  liberating the parent water soluble azo coupling.

The light fastness of toners in reduced shades is inadequate for them to be used in most surface coatings  their full shade light fastness is of a higher order ranging from good to very good. In durability  however  even in full shades the products do not give very satisfactory performance for under the combined action of light and weather  colour fading and pigment solubilisation occur simultaneously. In weathering resistance  the manganese toners show better performance than the corresponding calcium and barium types even though all these products give the same order of light fastness.

Bordeaux BL is a toner of very high hiding power  and although below the average of the remainder of the toners in resistance to solvents gives lacquers which are non bleeding when oversprayed.

Pigment Scarlet 3B is an example of a dye lake  giving similar fastness properties to the toners. Its tinctorial strength  however  is low and this is due in part to the diluting effect of the alumina base present.

Disazo Pigments

The properties of the disazo pigments can be compared with those of the arylamide yellows listed in Table 2  which contain similar second components and the contrast is immediately apparent. The disazo pigments possess much superior resistance to heat and solvents and are of much higher tinctorial strength. Unlike the arylamide yellows  therefore  the disazo pigments can be used in industrial stoving finishes  their light fastness  however  does not in general equal that of arylamide yellows. Disazo products such as Yellow GR  2G and NCG have  however  sufficient light fastness in full shade and when used as the major component of mixtures with opaque white to make them useful in medium quality stoving enamels.

Non azo Pigments

The group of azo pigments was extended more rapidly in the early stages of the development of organic pigments than were the non azo types. Several non azo pigments have been known for a long period and are still in use  and because of their historical importance they will be discussed first.

Miscellaneous Products

The three products listed in Table 8 as being lakes on alumina show quite typical behaviour  very good fastness to heat and solvents  moderate light fastness  lack of resistance to acids and alkalis  and poor tinctorial strength. They do  however  still find use in strong shades because of good colour retention on exposure coupled with economy.

Aniline Black has the general inertness to be expected from a highly oxidised product  its chief virtue lies in intensity of shade compared with carbon blacks  i.e. a deeper black is produced.

Pigment Green B is an organo/iron coordination complex and shows typical acid instability. The pigment is very dull in shade  being much duller and weaker than Phthalocyanine Green. However  it offers an economic means of making olive greens  particularly in water paints.

The demand for pigments of higher all round fastness than are obtainable with the azo group has been met in the modern additions to the non azo pigment group. It is not possible to classify these non azo pigments in the formal manner of chemical constitution as was used for the azo series. For convenience  the compounds studied  which are of extremely diverse constitution  will be divided into three groups  i.e. Phthalocyanines  Vat Pigments  and Miscellaneous Heterocyclic Compounds.

Phthalocyanine Pigments

The first pigment in this group  Phthalocyanine Blue B  was marketed over twenty years ago. The products have been continuously improved in brightness of shade  tinctorial strength and ease of dispersion  and in addition a large number of specialised forms have appeared  each suited to a specific application. Phthalocyanine blues are produced by a condensation reaction from relatively simple intermediates. Phthalic anhydride  urea and cupric chloride are fused together  or alternatively heated together in a suitable high boiling solvent to give the crude blue compound which has the structure.

The crude blue compound produced by the condensation reaction requires further treatment before it can be used most effectively as a pigment. For example  by dissolving the crude in concentrated sulphuric acid and reprecipitating by controlled dilution with water  a product of useful pigmentary form is obtained. Compared with the crude material  the pigmentary form gives much brighter shades  is stronger tinctorially and possesses enhanced ease of dispersion. This need for a separate pigment conversion stage following the formation of the required chemical compound is typical of the group of non azo pigments being discussed  and contrasts sharply with the formation of azo pigments where the useful pigmentary form is obtained directly in the aqueous coupling suspension.

Copper Phthalocyanine

Copper phthalocyanine can exist in two crystalline modifications   and the latter being the more stable. Pigmentary Phthalocyanine Blue B (the form) in the presence of certain solvents  particularly aromatic hydrocarbons  will recrystallise into the form  of large crystal size. In a paint or lacquer this change results in loss of tinctorial strength and brightness. The use of unmodified form copper phthalocyanine pigments is  therefore  restricted to those uses where crystallising solvents are not present  e.g. water paints. A slightly chlorinated copper phthalocyanine does not crystallise in solvents and is the product used in paints  lacquers and enamels.

By special processing in the presence of solvents it is possible to prepare copper phthalocyanine in the modification with small crystal size. This product  which is not further affected by solvents  is greener  brighter and somewhat weaker tinctorially than the form  and is of interest mainly because of its superior brightness of shade.

The copper atom is so strongly coordinated with the organic molecule that it cannot be removed by acid treatment. Other metal phthalocyanines can also be produced  but they are not commercially of value  giving either inferior tinctorial properties or unstable products from which the metallic nucleus can be removed quite simply.

Copper phthalocyanine can be chlorinated so that substitution of chlorine occurs in the peripheral benzene rings of the molecule and 14 15 chlorine atoms can be introduced. The product is Phthalocyanine Green  a bright bluish green pigment.

Metal free Phthalocyanine

It is also possible to obtain a metal free phthalocyanine which is much greener in hue than the copper compound. It also tends to crystallise in strong solvents and a modified form is necessary to give adequate stability in paint media. The metalfree phthalocyanine is considerably more expensive than the copper derivative.

From their tabulated fastness properties  the virtues of phthalocyanine pigments are obvious and their properties are much superior to the azo pigments listed earlier. The restriction of available hues to the blue and green portions of the spectrum led to a search for reds and yellows of comparable fastness properties.

Vat Pigments

Vat dyestuffs widely used in textile dyeing and printing are water insoluble compounds capable of being solubilised by alkaline reduction (vatting)  and when reconverted to the oxidised form on a fabric give a wide range of shades of excellent fastness. Though of varied chemical constitution  they are polynuclear compounds comparable in complexity to the phthalocyanines. Because they are chemically inert  except to reduction  and of very low solvent solubility  efforts were made to exploit these properties in the pigment field.

Vat dyes as used on textiles are not directly suitable for use as pigments  and physical treatments  similar to those employed on phthalocyanines  are necessary to give a powder product of adequate ease of dispersion in paint media. In addition  purification is frequently required to remove by products of the manufacturing processes  which otherwise could lead to bleeding in solvents.

The selection of vat dyes for use as pigments has been reviewed by Vesce  it is apparent that not all vat dyes can be converted to give highly durable pigments  and among those now in general use there are important variations in performance.

Extenders

Introduction

It is not easy to devise a clear simple definition  but extenders  known also by the alternative name of fillers  are essentially inorganic compounds  usually with limited opacity in non aqueous media  and limited staining or colouring properties. There are exceptions  more particularly in respect of colour  where the extender may have too much self colour to permit its use in whites or delicate pastel shades.

The limited opacity of the extenders is in line with their refractive indices  which are invariably notably lower than those of the regular pigments  and low refractive index may be regarded perhaps as a feature of extenders  which distinguishes them from the pigments as such.

Bradley (Jun.) suggests that extenders would be found in the refractive index range of 1.45 1.70  by contrast with the opaque pigments from 1.94 to 2.70  the former range of figures being very near the refractive index of oleoresinous media. In aqueous media  by contrast with the performance in non aqueous media  some extenders demonstrate a useful if not an entirely satisfactory degree of opacity  and a good example of this is the almost traditional use of whiting in water paints and distempers  where it often constitutes a large proportion of the total pigment extender.

Many extenders are natural minerals  suitably pulverised  and are used as such  or they may be subjected to processing to provide more generally acceptable products  or to enhance certain properties. These natural extenders have been greatly augmented in recent years by types  which have been specially synthesised.

Whereas it might have been true in the past that extenders were used to provide weight or bulk to a paint  the myth that weight alone has any merit as been largely dissipated  and the exploitation of extenders to provide bulk only has become much less important. The selection of extenders has become much more discriminating.

The practice of selling paint on a weight basis has almost disappeared  and most paints are sold in gallons or other unit of volume  and the area covered by a unit volume (gallon) is often quoted. This leaves no doubt in the mind of the purchaser of the true value of the paint  or of the amount he will need to cover a given area  after making due allowances for the nature of the surface involved.

Production and Manufacture

Extenders may be derived from the natural mineral  which is ground in different types of mills  and the powdered products may be classified by dry or wet means into finer or coarser particles  the latter being reground as necessary. Dry classification may be by screening or air floatation  wet classification may involve settling tanks  the finer particles being carried farther through the systems  and the coarser particles being settled out at an earlier stage.

Micronising of extenders  using superheated steam  is becoming a popular way of producing extenders in the finer ranges of particle size  and increasing numbers of extenders are being supplied in micronised form.

Some natural minerals as mined are not sufficiently white in colour for general use in all types of paints  and the agent responsible for the discoloration  often an iron compound  is washed out by acid treatment. The efficiency of this process depends upon whether the iron or other coloured agent is present as free oxide which may be conveniently soluble in dilute acid  or whether it is in chemical union as part of a complex compound. When the iron or coloured agent is part of the complex compound itself  this may remain fixed and will control the improvement of colour which can be achieved.

Opacity

This subject is dealt with more extensively in other Chapter  but it is perhaps of interest to compare the refractive indices of extenders and pigments as in Tables 1 and These figures show clearly the distinctive groupings of the two types of material.

Amongst the higher refractive index extenders would be included barytes and blanc fixe  barium carbonate  some calcium and magnesium carbonates  calcined china clay (aluminium silicate)  calcium metasilicate (wollastonite)  and perhaps asbestine. The lower refractive index types include the synthetic silicas  diatomaceous silicas  some being reported as low as 1.40  and some precipitated calcium and aluminium silicates. From the lower refractive index types would be found extenders suitable for producing mattness  in clear lacquers  with minimum effect on transparency  for grain fillers which are not readily distinguishable  and for pigmented products where minimum colour change is required.

Natural and Synthetic Extenders

The extenders which are mined as natural minerals are likely to contain more insoluble impurities than the synthesised types  where sometimes the main possibility is the existence of water soluble salts retained as the result of a precipitation process. It must not be concluded from this that the natural mineral extenders are free from soluble impurities  for small amounts of soluble impurities may be found in them also a fact which is recognised in official specifications.

It is obvious that the amount of water soluble matter in both natural and synthesised extenders will depend on the degree of washing which they receive before they are delivered to the consumer.

Surface Coating Agents

It is becoming very popular to provide natural and synthetic extenders with a surface coating consisting of fatty acid  fatty acid soap or resinous matter  in order to facilitate wetting or dispersion  especially in oleoresinous media. This means that grinding or milling times are reduced a most important factor in processing costs  and in production from a given milling unit. The presence of a surface coating may also be helpful in providing the required compatibility with certain types of synthetic resin  especially when the proportion of extender used is high. The actual bond between particles of extender and the synthetic resin (or other type of medium) may have a very important effect on the general physical properties of the applied coating.

The amount of surface coating agent used is usually less than about 3 per cent of the weight of the extender  but it could be somewhat higher. Generally speaking  it is desirable that the amount of surface coating agent does not exceed that necessary to provide a more or less continuous layer of active material on the surface of the individual particles at the minimum thickness. Larger amounts may begin to have other effects on the properties of the medium which are undesirable.

Oxides

Silicas

By far the most important member of this group is silica or silicon dioxide which is found in several forms  naturally as quartz  crypto  crystalline quartz  and diatomaceous silica (diatomite  kieselguhr  infusorial earth)  or it is also produced synthetically  the latter often being distinguished by its extremely finely divided condition.

The differences between the chemical compositions of the various forms of silica (natural and synthetic) are shown in Table 3.

Where several grades of an extender are available from one source the analytical and physical figures quoted may apply to the range as a whole and not to an individual member. This method of recording applies throughout this chapter.

Analysis of Silicas

Consideration of the analytical figures for the various types of silica indicates that natural quartz silica and some of the synthetic types may be very rich in silicon dioxide (SiO2) indeed  the figure being in excess of 99 per cent for the dried material.

The diatomaceous silicas may contain notable amounts of other metallic impurities such as compounds of iron  aluminium  calcium and magnesium  which may be significantly absent from synthetic silicas.

The amount of water soluble salts in the synthetic silicas is likely to vary appreciably according to their method of preparation  degree of washing  and ease or difficulty of removal  and whilst for some applications their presence may be acceptable there are others  for example  in anti corrosive primers  where it is usually advisable that they should be at the minimum possible level.

Chemically held water is also likely to vary  depending upon the source or method of preparation of the silica and whether or not it has been calcined.

The pH values of the aqueous extracts from the various forms of silica vary appreciably. The synthetic silicas yield pH figures from 9 to 2.4 and appear to be more often on the acid side of neutrality. By contrast  a group of proprietary diatomaceous silicas all fall within the range 7.0 9.9.

Physical Properties

Perhaps the most distinctive feature of the whole group of silicas is the extremely fine particle size of some of the synthetic varieties  which is reflected in considerable surface areas per unit weight  and in very high oil absorptions.

It is clear that apart from breaking down the loose agglomerations of the particles  no real grinding is necessary with some of the synthetic silicas. The high oil absorption (adsorption) figures of both the diatomaceous and synthetic silicas suggest their value as matting agents  and their potential influence on the rheological characteristics of media  and these are fields in which they are of special interest.

As matting agents even the addition of only 1 per cent by weight of a fine synthetic silica has a significant effect on gloss  5 per cent may produce a semi gloss  and 12 15 per cent practically a dead matt result  depending upon the nature of the medium.

The surface areas per unit weight are not necessarily in direct proportion to their oil absorptions  which suggests that some of the surface area presented may be inaccessible to medium. This may indicate the possibility of arranging for voids in the pigmented coating  which could allow for   breathing   of the coating  if desired.

Applications

When particles of infinitely irregular shapes are required the diatomaceous silicas are a good choice because they represent the siliceous skeletons of aqueous diatoms or tiny plants  of which many thousands of varieties are possible. The fine synthetic silicas are very useful agents for inducing thixotropy in coatings  especially where these are required to remain on vertical surfaces without curtaining  and one special application in this category is for polyester finishes where it is necessary to apply the equivalent of several normal thicknesses of coating in a single application  and to achieve this without affecting transparency. They are also useful in the production of one coat decorative finishes where controlled thixotropy is essential  and in paint removers  where the object is to retain a substantial thickness of coating on vertical surfaces long enough for the active agents to penetrate and soften the old coatings. The very small particle size and large surface areas indicate that these might be used as pigment suspending agents.

The water absorbing types of fine silica have been used to remove free water from aluminium paints and thus prevent development of pressure and loss of leafing properties.

The diatomaceous silicas are also used as matting agents in flat finishes  which do not gloss up when rubbed  and their effect is claimed to be due to their capacity for causing light to diffuse in all directions  and it is to this feature that their opacifying power is attributed. Some of the mattness obtained from diatomaceous silicas is achieved because some of the odd shaped particles in heavily pigmented systems are believed to project through the upper surface of the coating.

Their heterogeneous forms prevent tight packing which allows greater freedom of movement of moisture in coatings  and this reduces blistering and peeling  more particularly on absorbent surfaces. Similarly  better solvent release is obtained in highly pigmented sealers and undercoats  and to their unique multivarious structures is attributed their reinforcing action on coatings which results in better toughness  flexibility and resistance to cracking when exposed to weather influences.

In emulsion paints the diatomaceous silicas are claimed to give better washability. The presence of voids in the individual particles makes them of interest in fireproofing paints where heat insulation is of importance.

Improved adhesion  especially on soft surfaces such as wood  is provided by diatomaceous silicas which tend   to anchor   to them.

Ordinary natural powdered silica is used in grain fillers for wood because of its transparency  which preserves the natural colour of the wood and avoids muddiness  for conferring adhesion it is often used in undercoats to provide a key both to the primer and for the finishing coat  and in finishes where abrasion resistance is important  such as in road line paints  paints for airplane runways  and floor paints.

Coarse silica or sand is a good constituent in deck paints for reducing slip  and in sound deadening coatings or compounds for motor cars and metal domestic equipment.

Hydroxides

Alumina

Hydrate of alumina  Al2O3.3H2O  or in its calcined form Al2O3  together with grades containing intermediate amounts of combined water  are available in very pure state having less than one per cent of impurities. Some grades are colloidally dispersible in water.

The specific gravity varies from 2.45 to 3.9 according to the combined water content.

The aluminas have not yet found many direct applications in surface coatings  although they are used as a basis on which dyes are precipitated. Some grades have a marked livering tendency with fatty acids such as may be present in paint media  and this has limited their possibilities  but there are types of coating where a measure of thickening is desirable and they are worth consideration for these specific purposes.

Solvents

Introduction

The Primary function of solvents in surface coatings can be stated very simply. They are incorporated in order to make it possible to apply the film forming material to the required surface by anyone of a number of methods so as to obtain a uniform film of specified thickness. Once the coating has been applied  the solvents must evaporate as completely as possible. The solvents  in fact  may be said to   convey   the film former to the surface.

Simple though this function may appear in principle  there are many factors to be considered when selecting a suitable solvent mixture for a particular composition. This is because the function is not simple in practice  the solvents being called upon to contribute much more than is implied by their primary function.

As in all broad classifications  there are exceptions  and they arise in this case because of certain convertible resins which by their nature are soluble only in a much narrower range of solvents and in which the solvents play some part during film formation. Notable examples of such systems are the amine cured epoxy resin coatings and the urethanes where the curing reaction is rapid and is taking place during the evaporation of the solvents.

Characteristics of Solvent Groups

This section deals with groups of solvents from the aspect of their suitability for the various types of resins  consideration being given to those characteristics which are appropriate in each particular case. For instance  when choosing hydrocarbon solvents  aromatic content is an important factor. In the case of oxygenated solvents  their polarity gives a clue to their suitability as solvents for a particular resin since  in general  resins of high polarity require highly polar solvents  whereas those of low polarity are dissolved by solvents of low polarity. This matter of polarity and the other factors which must be considered in choosing the most suitable solvents for a particular type of coating are discussed in detail later.

The Terpenes

These consist mainly of hydrocarbons  obtained largely directly from the pine tree by distillation or extraction  or as by products of paper pulp manufacture  the most widely used solvents in this range being turpentine  dipentene and the pine oils.

There are four grades of turpentine commercially available  three of which are obtained from various species of pine trees by steam or destructive distillation and the fourth from sulphite liquor  a by product of the paper industry. Two grades are covered by British Standards Specifications  Type I B.S.S. 244 and Type 2 B.S.S. 290  whilst all four grades are included in the American Society for Testing Materials   specification ASTM D.13 51. The three grades obtained from the pine tree consist essentially of  pinene and boil in the 150º 170ºC range  that obtained from sulphite liquor has a variable composition and a somewhat wider boiling range.

Until the advent of petroleum hydrocarbon solvents of the white spirit type  turpentine was the principal solvent used in paints and varnishes. Dipentene has similar solvency characteristics to turpentine  the commercial product consisting mainly of dl limonene and boiling in the 170 190ºC range. It has an appreciably lower evaporation rate than turpentine and is normally used in minor proportions only. It improves flow and brushing characteristics and also acts as an anti skinning agent.

The pine oils are complex mixtures  largely of terpene alcohols  boiling in the 195º 225ºC range. They are useful constituents of brushing compositions  where  they improve flow and brushing characteristics but are used only in small quantities  because their evaporation rate is very low.

Hydrocarbon Solvents

The hydrocarbons  as a class  are of low polarity and strongly hydrophobic  and are solvents for non polar or weakly polar materials  amongst which are oils  bitumen  coal tar and many of the resis used in surface coatings. The aromatic hydrocarbons are solvents for a wider range of these materials than the paraffins or the naphthenes. The hydrocarbons are not solvents for nitrocellulose  but some of the cellulose ethers are soluble in aromatic hydrocarbons.

There is a very wide range of hydrocarbon solvents available to the surface coatings industry  derived from petroleum or coal tar  and these solvents range from purely aliphatic to wholly aromatic materials and cover an extensive boiling range. It is not easy to classify them in a logical manner  and in practice the technologist will select from a list of properties those which meet his requirements in such respects as boiling range  flash point and aromatic content. There is  however  an accepted terminology in the trade in this country  which makes it possible to put most of the solvents into groups  and the following descriptions deal with them on this basis. At the same time  American equivalents have been included  and reference is also made to some solvents which are available in various parts of the world but not necessarily in the United Kingdom.

White Spirits

These are petroleum fractions boiling in the 150º 210ºC range and are the most widely used solvents in the industry. They are also sometimes referred to as petroleum spirits or mineral turpentine  or   turps substitute    and are known in the United States as mineral spirits. The grade used on the largest scale has an aromatic content of about 15 per cent  and it is this material which is referred to simply as White Spirit in this country and as Mineral Spirits Regular in the United States. In some countries a high aromatic white spirit is also available with an aromatic content in the region of 45 per cent. High flash white spirits are also available which resemble ordinary white spirit except that the lower boiling fraction is not present.

In addition  there are the low odour and odourless white spirits with special characteristics. The low odour white spirits are distillation products from which practically all the aromatics have been removed  whilst the odourless white spirits are produced synthetically by an alkylation process. The reduced odour of the low odour grades and the virtual absence of odour in the odourless grades are real advantages in paints used for interior decoration. The virtual absence of aromatics poses certain problems on solubility of alkyds  but these can be overcome by modification of the resins. At the same time  there are certain advantages in physical characteristics which can be obtained by using these purely aliphatic solvents  such as reduction of   sinking   of flat wall paints.

Special Boiling point Spirits

These are petroleum fractions of lower boiling ranges than the white spirits  frequently closer cut and with aromatic contents for the most part in the 2 15 per cent range. These fractions are referred to in many parts of the world as SBP solvents  with a suffix to indicate type or boiling range. In the United States  the solvents in this range are known as VM & P naphthas or just as naphthas. There are a few exceptions to this generalisation  a limited number of   cuts   with higher aromatic contents between 35 per cent and 60 per cent being available.

Aromatic Solvents

Under this heading are included all hydrocarbon solvents with aromatic contents of 80 per cent and over  which is a fairly well recognised definition in the paint trade. Whereas all these solvents originally came from coal tar  in recent years the petroleum industry has become a major producer of these materials. From coal tar they are obtained by distillation  but this is not the case with petroleum derived aromatics. Petroleum products are predominantly aliphatic or naphthenic  and various ingenious chemical processes have had to be devised to reform the molecules. One such process called   hydroforming   dehydrogenates the naphthenes to aromatics and cyclises some of the paraffins.

Aromatic hydrocarbon solvents are available  either as   cuts   or as single hydrocarbons of a high degree of purity  both from the petroleum and the coal tar industry. The petroleum   cuts    which are normally used in surface coatings rather than the pure hydrocarbons  are sold under various proprietary names and are specified by boiling range and aromatic content. The coal tar solvent   cuts   are referred to in various ways such as 90  s toluene  5° solvent xylene  solvent naphtha and heavy naphtha.

Ketones

The aliphatic ketones with the general structure R CO R are intermediate between the hydrocarbons and alcohols in polarity. The lower members of the series are strongly hydrophilic  but they lose this characteristic rapidly as the size of the hydrocarbon groups increases. Thus  whereas acetone is miscible with water in all proportions  methyl isobutyl ketone is soluble in water only to the extent of about 2 per cent. This rapid change in the hydrophilic nature of ascending members of the series is reflected in solubility characteristics  but not to as great an extent as might be expected from this factor alone. The ketones are solvents for a wide range of resins  including many for which few solvents exist. In addition to their widespread use in nitrocellulose lacquers  they are the principal active solvents used in lacquers based on cellulose acetate  and on copolymers of vinyl chloride/vinyl acetate  vinyl chloride/vinylidene chloride and vinylidene chloride/acrylonitrile. They are very stable under normal conditions and cover a wide range in evaporation rate from acetone to isophorone. Acetone has too great an evaporation rate to be used to a major extent in nitrocellulose lacquers of the normal type  but methyl ethyl ketone and methyl isobutyl ketone figure prominently in a very large proportion of nitrocellulose and vinyl resin lacquers. Of the cyclic ketones  the most widely used are cyclohexanone and methyl cyclohexanone  which are similar to the aliphatic ketones in their solubility characteristics.

Esters

The esters  with their typical R CO OR structure  are similar to the ketones in polarity  but are less hydrophilic. Methyl acetate is soluble in water to the extent of 24.5 per cent  whilst the figure for butyl acetate falls to 0.68 per cent at 20ºC.

With the exception of ethyl lactate  the esters used as solvents are the acetates. The lower molecular weight esters  the formates  are unsuitable because of instability  being readily hydrolysed by water  and are too volatile. Whilst the acetate esters are less stable than the ketones  they are quite satisfactory in this respect for all normal lacquer applications and are  in fact  used widely  especially in the nitro cellulose lacquer field. The members of the series used most extensively are ethyl acetate  isopropyl acetate and the butyl acetates. In the higher boiling range  ethyl lactate and the acetate of ethylene glycol monoethyl ether figure prominently  the latter particularly in the epoxy resin field.

Glycol Monoethers

These solvents possess interesting structures in that they have both the strongly polar hydroxyl group and the less polar ether group in the same molecule.

Of the glycol ethers  the methyl derivative is the only one which will dissolve cellulose acetate  whereas both the methyl and ethyl ethers of diglycol are solvents. It would appear that the influence of the larger non polar hydrocarbon group has been counteracted in the diglycol ethyl ether by the presence of the second ether group.

These solvents form an extremely useful series of solvents for many resins  including nitrocellulose  alkyds and shellac. They are all completely miscible with water at normal temperatures and with most hydrocarbons and have a very mild odour. They are in the medium to high boiling range  but are much slower in evaporation than would be anticipated from their boiling points compared with most of the other solvents  and the proportion which can be incorporated in air drying compositions is therefore limited to some extent.

Ethers

The aliphatic ethers such as ethyl and isopropyl are solvents for relatively few of the resins used in coatings  although in conjunction with ethyl alcohol they dissolve nitrocellulose. In fact  one of the earliest nitrocellulose solutions known as   collodion   cotton  still used for certain specialised applications  makes use of an ether/alcohol solvent mixture. The cyclic ethers such as 1 4 dioxane and tetrahydrofuran  in contrast to the aliphatic ethers  are solvents for a very wide range of resins  in which respect they are similar to the ketones. Both are solvents for cellulose acetate and the various vinyl copolymers  whilst tetrahydrofuran is also a solvent for polyvinyl chloride. With the exception of 1 4 dioxane  these ethers are too volatile for use in coatings and are prone to peroxide formation.

Alcohols

The hydroxyl group renders the alcohols highly polar and strongly hydrophilic  methyl alcohol  ethyl alcohol and the propyl alcohols all being completely miscible with water. The hydrocarbon portion of the molecule makes its presence felt sharply in the higher alcohols  for normal butyl alcohol is soluble to the extent of only 7.7 per cent in water at 20ºC. This marked reduction in the hydrophilic nature of the molecule with increasing molecular weight is reflected in solubility characteristics. Thus  whilst linseed oil is only partially miscible with methyl alcohol  it is completely miscible with butyl alcohol. Castor oil with its hydroxyl groups is  however  completely miscible with methyl alcohol.

The alcohols as a class are very stable compounds and are to be found in many types of surface coatings. The higher members of the series such as the butyl alcohols improve flow and inhibit blushing tendencies in nitrocellulose lacquers and spirit varnishes  and impart stability and good flow to alkyd/amino resin compositions. Certain resins  such as the higher molecular weight epoxy resins where there are polar and non polar groups in the same molecule  can be brought into solution in mixtures of the highly polar alcohols and the non polar aromatic hydrocarbons  although they are not soluble in either of these solvents alone.

Halogenated Compounds

In spite of their reduced inflammability  the chlorinated compounds do not find widespread use as solvents in surface coatings. Whilst they are not solvents for nitrocellulose  they will dissolve many of the resins used in coatings  including a number of the other cellulose derivatives. However  they suffer from a number of disadvantages  being less stable  more toxic and generally more costly on a volume basis than many of the other solvents. One field where they figure prominently is in paint removers  methylene chloride frequently being the major constituent.

The only other halogen compounds of interest in the coatings field are the chloro fluoro compounds used as propellants in aerosol paint packs which are becoming increasingly popular. These have similar solvency characteristics to the chlorinated compounds.

Nitroparaffins

The commercially available nitro paraffins  i.e. nitro methane  nitroethane  1 nitropropane and 2 nitropropane  are solvents for a wide range of resins  including nitrocellulose. Cellulose acetate is soluble in nitromethane and nitroethane  whilst the vinyl chloride/acetate copolymers are soluble in nitroethane and the nitropropanes. They are all similar in evaporation rate and fall in the medium boiling range close to butyl acetate. They are stable compounds with a mild odour and have proved useful in cellulose acetate and vinyl copolymer lacquer formulations.

Evaluation and Selection of Solvents

The paint formulator is faced with a large number of commercial solvents and an even larger number of commercial resins  each with its own solvent requirements  and solvents must be selected carefully if good results are to be obtained at minimum cost. Selection is far from simple and the general problems involved will now be discussed.

The viscosity of a resin solution is therefore of great importance or  to put the problem in a more realistic way  it is imperative to know what proportion of solvent must be used to obtain the best results.

The above five headings cover all the important factors involved in solvent selection and these factors and the test methods employed in their evaluation will now be described in more detail. One general point to be noted is that these methods of evaluation should be considered as sorting tests only and are no substitute for practical evaluation under specific conditions of use.

It must also be noted that many of the test methods for solvents were worked out in the sphere of nitrocellulose lacquers. This was because solvents represent a relatively high proportion of the total cost of these lacquers and also because they greatly affect their performance. Results thus obtained for nitrocellulose lacquers cannot be assumed necessarily to have any significance for other resin systems  but the basic principles of the test methods used can often be applied.

Although the primary object is to consider the scientific background of use of solvents  it would be unrealistic not to recognise that the problem is to choose suitable solvents at the lowest possible cost. In a clear lacquer based on high viscosity nitrocellulose the cost of solvents may be over half the total cost and  whilst this is admittedly exceptional  solvent cost is significant in all cases and there is always an incentive to reduce it. Psychologically  the fact that solvents are not present in the final film reinforces the wish to reduce their cost. This must not be taken too far  however  because of the essential function  which they perform in film formation.

Solvency

In the first place  it is essential to remember that there is a fundamental difference between resin solutions and solutions of simple crystalline solids such as common salt. Resins do not give saturated solutions except in one or two special cases  usually as more resin is added  it becomes progressively more difficult to dissolve it owing to increasing viscosity  but there is no critical point at which no more resin will dissolve. Similarly  during film formation  as solvent is lost the viscosity rises progressively  but there is no critical point at which the resin crystallises. The final film can be regarded as a highly concentrated solution of resin in solvent  and indeed its properties are those of a liquid  a supercooled liquid  and not those of a crystalline solid.

In fact  so far from forming saturated solutions corresponding to an upper limit of solubility  some resin solutions show the opposite behaviour  that is  they show a lower limit of solubility below which the solution goes cloudy. On standing  the cloudy solution forms two layers  the upper of which is solvent alone. A well known example of this is the solubility of solid grade epoxy resins in many of the aliphatic ketones and esters where solutions can be obtained at higher concentrations  but are precipitated on further addition of the same solvent. However  solubility limits (minimum solids contents) can be determined. An alternative way of expressing the same effect is the   white spirit tolerance   of short oil varnishes. This depends on the maximum number of volumes of white spirit  which may be added to one volume of varnish without precipitation or cloudiness.