The development of science and technology revolutionized the surface coating industry in the progressive countries of the world. There has been considerable impact in this field. We have completely replaced costly petroleum solvent with water. So we get cheaper finished products with no evaporation loss and tire hazard. So we can say surface coating industry is now eco-friendly. Finally this book will help old as well as new entrepreneurs and technocrats to achieve improved performance on various surface coating raw materials as well as finished products.
Silicone Resins
The silicon element was first studied in 1824 by Bezelius and later by Friedel and Krafts. The first systematic investigation of silicon and silicones was carried out by Kipping between 1899 and 1944. The first commercial silicone products were made in the USA in the early 1940s.
A silicone is a compound that contains the elements silicon and oxygen, and organic groups; the silicon is present in sufficient amount to affect the properties measurabley. Silicones may also be thought of as hybrid polymers, that is, a cross between organic and inorganic substances, and having some of the characteristics of both parent stocks. They have some of the thermal and chemical stability of the silicas and silicates, and also some of the reactivity, solubility and plasticity that is typical of organic materials. They also have their own peculiar chemistry as well, since the Si—C bond so much characteristic of silicones is not found in any other materials.
The skeletal structure of all silicones is related to taht in silica and the silicates. But in the silicones, the silica structure is modified by removing some of the oxygens and replacing them with carbon stoms of organic groups. The organic groups loosen up the hitherto close three-dimensional formation of silica, interrupting the closed network, resulting in the formation of a zig-zagging silicon-oxygen backbone. The larger the organic group attached to the silicon, the greater the organic nature of the molecule. Most commonly, short chain hydrocarbons from methyl to amyl groups, ring structures such as the phenyl, and unsaturated groups sucha s the vinyl are grafted onto the silicon; when long chain hydrocarbons are the organic substitutes, the silicon shows nearly all the reactions normally associated with organic chemicals (oxidation, great change in viscosity with change of temperature).
Thus the basic picture of silicones becomes that of a more or less complex network of alternating Si and O atoms, some of the valences of silicon being satisfied by organic groups, hence the name ‘polyorganosiloxane’.
PREPARATION OF SILICONES
Preparation of Silicon
Preparation of Silanes (Mostly Organochlorosilanes)
Direct Process
Grignard Process
The silicon tetrachloride is made by the following process:
The reaction product is a mixture of chlorosilanes, of which the commerical demand is the largest for dimethyl dichlorosilane. The reaction can be regulated in such a way as to give a pre-ponderance of this material. The crude mixtures of chlorosilanes are separated by fractional distillation.
Preparation of Silanols
The silanol monomers are unstable and condense with each other by the elimination of water. To regulate the polymerization reaction, heat, and acid or alkaline catalysts are used.
Polymerization
By varying the ratio of M, D, T and Q units the ratio of organic radicals to silicon atoms can be varied (R : Si ratio or ‘degree of substitution’); thereby a wide range of products can be made.
Silicone oil: MD×M x [ 2000 D : M ratio effects the viscosity
Silicone gum: MD×M x ] 2000
Silicone resins: MDT (most common) or MDTQ combinations.
Silicone Resins
Silicone resins nave now been used for over thirty years by the surface caoting industry. They were initially used only as heat resistant coating materials for use at high temperatures in such applications as ovens, space heaters and chimneys, where they are still the best materials available for those purposes; in more recent years silicone resins hae been used in decorative coatings for improving the weather resistance. In addition, otehr properties, such as anti-stick effect, water-repellent action and antifoam properties, are often utilized.
Silicone resins used in formulating surface coatings are of three types:
- pure silicone resins, that is, used as sole binder in the final system;
- blending resins, that is, silicones blended with organic resins; and
- reactive intermediates, that is, silicone intermediate copolymerized with organic resins.
Pure Silicone Resins
Pure silicone resins are the hydrolysis products of di- and trifunctional chlorosilanes: R2SiCl2 and RSiCl3, where R is most commonly methyl or phenyl or a mixture of both. (Some other groups might also be present).
For resin production, the chlorosilanes are dissolved in aromatic hydrocarbons (toluene, xylene), and hydrolyzed with water. The mixture separates into two layers: a solvent phase, in which the silicone hydrolyzate is dissolved, and a water phase. Following the separation of the two phases the solvent phase is neutralized, filtered, stripped from part of the solvent, and finally ‘bodied’ to the desired viscosity.
Three main parameters will influence the resin properties: R:Si ratio; methyl: phenyl ratio; and viscosity.
R: Si Ratio
This is determined by the ratio of di- and tri-functional silanes in the starting materials. Increased amounts of tri-functional silanes will lower the R: Si ratio and lesser amounts will increase it. In general, the lower theR : Si ratio, the faster the curing rate of the resin, even to the stage where it will air-dry. But the cured film becomes harder and less flexible.
Commercial silicone resins have an R : Si ratio of 1.28 to 1.67, i.e. a ratio of tri- to di-functionality of 2.5 to 0.5:1 in their starting materials.
Methyl-and phenyl-content
Heat stability, oxidation resistance and package stability, increase with increasing phenyl content, but pure phenyl resins cannot be used as they are brittle fusible solids which form weak films. Increased methyl content improves flexibility, water repellency, chemical resistance, gloss retention, and cure time.
The methyl : phenyl ratio in commercial silicone paint resins is usually 0.5 to 2.5:1.
Viscosity
The resin viscosity (at the same solid contents) depends on the degree of polymerization of the resin.
In resin manufacture, the polymerization (bodying) is stopped at a predetermined viscosity by cooling the mixture and diluting it with solvent. With straight silicone resins the resin viscosity will not influence the final properties of the paint film, because when the already applied resin is heat cured, the reaction that had been previously stopped (during bodying in the kettle) picks up again where it left off, and continues to completion. The mechanism of two-reactions, that is, bodying and cure are identical condensation reactions. However, the resin viscosity affects such properties as package stability; gel time, solubility (all of which decrease with increasing viscosity), and film build-up, air-drying, wetting (which properties improve with increased visosity).
Properties of Pure Silicone Surface Coating Resins
Silicone resins are supplied as 5-0 to 80 per cent solutions in aromatic hydrocarbon sovents, generally xylene acnd toluene; 100 per cent solid resins are also available for special uses (e.g. to paint hot surfaces, or to obtain high viscosity solutions, or when a particular solvent is desired). These solutions are light coloured ad of low viscosity.
Heat resistance. With little exception, heat resistance is directly proportional to the percentage of silicone in the resin. Coloured enamels based on silicone resins will give nearly indefinite service up to about 200°C and properly formulated enamels will withstand several thousand hours of continuous exposure at 250° to 350°C.
Black enamels are serviceable to 550°C and aluminium combinations to 650°C. Formulations containing ceramic frits will operate successfully at 750°C. Above 500°C the resin gradually breaks down but still serves to bind the pigment to the substrate. The disintegration is followed by a Substrate—O—Si—O—Aluminium (or Ceramic Frit) bond formation. This results in a fusing effect yielding an extremely durable and heat-stable finish.
Resistance to weathering. Paints based on silicone resins are virtually non-yellowing and non-chalking, even when chalking type pigments are used. The only deterioration that occurs normally is dirt retention. These excellent weathering properties are largely due to the water-repellency, low water absorption, and ultra violet and radiation resistance of the silicone resins.
Chemical resistance. Silicone resins, by virtue of their water repellency, are resistant to attack or staining by most aqueous chemicals, dilute acids and salts. Solvent resistance is generally poor, but is substantially improved by the addition of selective organic resins. Where a combination of heat and a corrosive atmosphere occurs, use of zinc-rich primer increases the exposure life of the paint. Few organic substances adhere well to cured silicone resins. This anti-stick property is used in mould release coatings.
Adhesion. On steel, chemical treatment (phosphating) or sandblasting is recommended for optimum bonding. However, phosphating can be detrimental to high temperature properties. Adhesion to aluminium, magnesium and tin are good, but adhesion to copper and zinc is poor.
Organic modifications. Silicone resins are inferior to organic resins in solvent resistance, toughness, adhesion, low temperature cure, and they are also more expensive. These deficiencies can be improved with organic modifications. The modification can be effected either by cold blending with compatible organic resins, or by chemical combination of silicone and organic intermediates to form copolymers.
Blending Resins
Silicone resins have limited compatibility with organic film formers. Special silicone resins of high phenyl content have been developed to enhance blending capabilities of silicones. Cold blending of silicones and organics will yield a system with intermediate properties. These properties depend on the ratio of ingredients and the organic resin chose. The organic component generally contributes to quick drying, abrasion resistance, hardness and hot strength. The presence of silicone improves weatherability and heat resistance. The benefits of silicone addition are seldom in evidence below 25 per cent based on total resin solids.
Silicone blending resins have good compatibility with short and medium oil length alkyds, phenolics, urea and melamine formaldehyde resins, low molecular-weight epoxy resins, ethyl celulose, nitrocellulsoe, acrylics, coumarone-indene resins and polyesters. Curing temperature of these systems is usually reduced to 150°C (1 hour).
Typical uses of cold blends are for coating of space heaters, incinerators, dryers, light bulbs, generators and processing equipment.
Silicone Intermediates: Silicone-organic Copolymers
Silicone-organic copolymers differ from cold blends in that the silicone intermediate is reacted with the organic resin in a resin kettle with application of heat. Either fusion or solvent processes are suitable.
The silicone intermediate is a low molecular weight silicone resin containing relatively high percentage (5 f to 20 per cent) of reactive hydroxyl (Si-OH) or alkoxyl (SI-OR) groups attached to silicon. By itself, it is of little use for surface coating, but by copolymerization with organic resins, heat resistance and weatherability will be upgraded. Compared with pure silicone resins, these copolymers are generally superior in cure characteristics (easier cure), solvent resistance, and mechanical properties Commerially available silicone intermediates are cyclic and linear types with hydroxyl or methoxyl functional groups. They will react with any organic resin which has fee hydroxyl groups to form copolymers with C— O—Si bonding. Two commercially available intermediates are:
The silicone content of the silicone-modified organic systems ranges between 20 and 80 per cent, but for most applications it is between 25 to 50 per cent.
Paints made from silicone-modified air-drying alkyd resins and silicone modified polyurethanes are used for maintenance paints of all colours and glosses. Their excellent weatherability, together with good chemical rsistance, is valuable.
The use of silicone-copolymer maintenance paints, although higher priced on the volume basis, reduces the total cost of the paint-maintenance programme because they last longer and, therefore, reduce the frequency of repainting. Ships, road and rail rankers or carriages, road signs, all of which are exposed to severe weathering and corrosion, are typical examples of where silicone copolymers are commonly used.
The largest single use for silicone-organic-copolymer paints is for coated coils mainly for the building industry. Col coating is a relatively new continuous mechanized process for applying a paint coating to one or both sides of a coil of sheet metal. The most common metals are cold-rolled steel, galvanised steel and aluminium. The paint is applied by roller to the unwound and cleaned coil. It is then baked at high temperature for a short period (silicone modified reesins are usually baked at 250° to 300°C for 60 to 90 seconds), cooled and the painted metal recoiled again. Subsequentlythe coated metal is fabricated into the finished product so the film must be tough and yet elastic to suit the fabrication, which usually involves bending.
Industrial buildings, decorative building panels, caravans, Venetian blinds are made from coil-coated metal. Silicone-polyesters and silicone-acrylics are widely used for coil coating. They will retain their good appearance in outdoor exposure, that is, no colour change and no chalking, for a much longer time than conventional organic coatings do.
Preparation and Formulation of Silicone-Resin based Coatings
The formulation of such coatings requires no special equipment or techniques. Pigment dispersions may be prepared with roller mills, pebble mills, sand grindrs (for high gloss enamels) or high speed mixers (for low gloss finishes). Precaution must be taken to maintain the desired pigment: binder ratio. Overloading of silicone resins with pigment can reduce the craze life of the coating.
Silicone resins are not ‘contaminating’, segregation of equipment is not necessary. It is, however, advisable to avoid contact with lead. Equipment that has recently been used to make paints containing lead compounds must be thoroughly cleaned before use; pails and drums with soldered seams must be coated. Even minute quantities of lead compounds catalyze silicone resins and may produce gelation, or an extremely short shelf life.
Cure Catalyst Driers
Coatings based on silicone resins will heat-cure without driers. Where a cure temperature of about 30°C lower is desired, metallic driers may be added. Zinc, manganese and cobalt octoates or napthanates (at concentrations of 0.05 to 0.2 per cent metal) have been found suitable. Iron can affect colour, shelf life and heat resistance. Lead causes gelation and should be avoided. Driers for silicone-modified resins should be selected according to the organic constituent. Formulations using lead should be thoroughly checked for package stability.
Pigments and Dyes
Nearly any colour shade can be obtained in a silicone-resin based coating. Conditions of end-use will dictate the type of pigment or dye that may be used in a particular application:
- only heat-stable inorganic pigments should be used for high temperature coating;
- chemical and/or weather-resistant pigments or dyes should be used in paints exposed to extreme weathering and to corrosive atmosphere; and
- pigments and all other additives should be free of lead.
Metallic pigments commonly used are zinc dust and particularly aluminium. Pigments normally employed are inorganic, with the exception of phthalcyanines. Where temperature conditions are not too severe heat-stable dyes may be used.
Extender fillers, which supply bulk or filling to the formulation, are also used in silicone paints. Here again the end-use conditions must be taken into account. Most commonly, mica, silica and silicate extenders are used.
Thinners
Aromatic hydrocarbons, and blends of aromatic and aliphatic hydrocarbons hydrocarbons (KB Value above 50), ketones, esters, chlorinated hydrocarbons and glycol ethers are suitable. Which thinners may be used with silicone-modified organic resins depends on the organic constituent, for example, silcone-modified long oil-alkyds can be thinned with White Spirit.
Formulations
Some typical formulations will illustrate the various types of silicone surface coating paints presently used.
Application Guides
Application methods used with silicone-based coatings are mostly the same as those used with coatigns based on organic resins. However, the application of silicone coatings often requires more care and attention to details of surface preparation and film thickness than do their organic resin counterparts. This is not because of the nature of the resin, but because of the performance standards that will be required for the coating, and the severe conditions to which it will probably be exposed.
Surface Preparation
Clean surfaces are required. Oil, rust and other, particularly organic contaminants, will cause poor adhesion and early film failure. For steel, sandblsting is recommended, but wire brushing and solvent cleaning may also be satisfactory. With new metals, particularly aluminium, care should be taken to remove factory-applied protective lacquers. Some bonderizing treatments, such as phosphates, should be avoided because of their poor heat stability.
Priming
Primers are used with silicone resin-based coatings for much the same reasons they are used with other types of coatings: to promote betteradhesion, and to give an extra measure of protection under certain environmental condition. Best results are obtained with primers that are based on silicone or silicone-modified resins, and which contain zinc dust.
As long as the coating will be exposed only to climatic temperatures, primers based upon organic resins are satisfactory. High quality oil-based primers give excellent results on wood surface. On metal surfaces, primers based on epoxy ester, chlorinated rubber or alkyd resins can be used. If the coating be exposed to temperatures in the 150° to 250°C range, and conditions may be damp or corrosive, a primer based on a silicone-modified resin should be considered. Silicone-epoxy, silicone-phenolic and silicone-alkyd formulations have all been utilized under such conditions.
Applying the Coating
Silicone-based coatings can usually be applied by any of the conventional means. Most are designed for spraying, but formulations may be adjusted for brushing, dipping and roller coating.
If more than one coat of a bake-on type of paint is to be applied and it is impractical to cure the first coat before applying the second—for instance, on a stack or similar equipment—then it is imperative that the second coat be applied by spraying.
When the operating temperature of the surface will not exceed 150°C application may be as follows:
- apply two coats of a standard primer (total film thickness of 50 to 75 mm); and
- apply two coats (25 rnm each) of silicone-modified coating, allowing 24 hours drying time between coats.
If service temperatures exceed 150°C, the total coating thickness should be reduced to minimize adhesion problems. Thickness of the primer coat should be varied as follows:
- 150 to 250°C 25 mm primer;
- above 250°C 12 mm primer.
Finishing topcoat should be about 25 mm thick; 24 hours drying time should be allowed between each coat, or oven curing—where practical— should be used to accelerate the cure.
Curing
Although silicone resins will air-dry to a tack-free state to achieve optimum film properties, cure at elevated temperatures is required. This is especially true if the coating is to be exposed to extreme temperatures.
In such applications, where oven curing is impossible, for example on chimneys, furnaces or other large equipment, the coating is cured in situ when the unit is put into service. There is more danger of under curing than of overcuring silicone baking enamels. Undercuring causes alms to be relatively soft and have poor adhesion. Many coatings based on silicone-modified resins will develop good fim properties by curing at ordinary atmospheric temperatures.
Curing cycles are, for the most part, dependent upon the silicone content of the resin vehicle. A typical cure for a coating based on 100 per cent silicone resin would be 30 to 60 minutes at 250°C; for a 50 to 80 per cent silicone, a satisfactory cure would be 15 to 30 minutes at about 220°C. For silicone-modified organic copolymers containing 25 or 30 per silicone, cures range from air-dry to 30 minutes at 177°C
Uses
Silicone resins are mainly used whenever heat resistant and/or good weathering coatings are required. They are comparatively expensive materials but their long life compensates for this by extending re-painting cycles and thereby reducing labour costs. The following recommendations can be made for the use of silicone resins for a given temperature range:
- Up to 95°C Modified-silicone resins, for longer paint life.
- 95—300°C Aluminium coatings based on modified-silicone resins, or coloured coatings based on silicone resins and heat-stable pigments.
- 300—400°C Aluminium coatings based on modified-silicone resins, or black or grey coatings based on silicone resins.
- 400-650°C Aluminium coatings based on silicone resins
- Above 650°C Ceramic coatings.
Toxicity
Silicone resins alone are physiologically inert and present no toxicity problems. However, other ingreidnts in the formulated protective coatings, such as solvents, driers, certain pigments and dyes and some of the organic resins blended with silicones, are toxic or irritating to humans. Therefore, precautions related to these materials should be taken to prevent accidents.
Other Silicone Resin Applications
Electrical Varnishes
Their excellent dielectric properties in combination with heat resistance and water repellency, make the silicones useful as electrical insulators for coils, motors, transistors and other electrical and electronic comonents. Silicones can be used in Class H insulating systems.
Release Resins
Semi-permanent release agent for most organic materials.
Masonry Water Repellents
Used on exterior above-ground masonry surfaces to prevent water absorption.
Other Silicones for Surface Coatings
Silicones are also used in the surface coating industry in the form of additives. Very small amounts of low viscosity silicone fluids can inhibit pigment separation, improve levelling qualities, increase slip and improve mar resistance. High viscositv ruids and gums are added to suitably formulated paints to produce hammertone finishes. Silicone anti-foams are used during organic resin production. Organo-functional silanes are effective adhesion promoters to metal and glass surfaces.
Inorganic Pigments
‘Colourants’ (materials used to impart colour) may either be pigments or dyestuffs. A pigments is and remains insoluble when used in a surface coating, whereas a dyestuff at some stage is soluble either in the solvents or the binder or both. This solubility or insolubility is the reason a coating coloured with an insoluble pigment is to a greater or lesser degree opaque. A dyestuff, on the other hand, may impart a great depth of colour (their tinting strengths are usually very high) to the coating, but in a thin enough section the coating will be coloured, but transparent. Dyestuffs are used only to a limited extent in surface coatings because they are transparent and generally speaking their light fastness is fairly poor. Typical examples of their uses are in flexographic printing inks, some metal decorating inks, foil coatings, and in timber stains. Where maximum light and weather fastness is required with transparency (e.g. in metallic automotive finishes) a high light-fast pigment having opacity is used.
Nearly all colourants, irrespective of whether they are pigments or dyestuffs and irrespective of the purpose for which they are used (e.g. colouring surface coatings, foodstuffs, textile fibres) are listed in a valuable reference work called the Colour Index. This is available in five volumes and is jointly published by the British Society of Dyers and Colourists and the American Association Textile Chemists and Colorists. The Colour Index is constinually updated to accommodate new colourants as they become commercially available and to keep abreast of other changes in colourant manufacturing. It classifies all colourants in a number of ways including.
- Chemical constitution (each colourant is allotted a constitution number);
- Colour Index generic name, e.g. C.I. Pigment Red 112, C.I. Acid Blue 2, and C.I. Direct Redf 16; and
- commercial name; it also identifies the manufacturer of the commerical material.
Using the generic name of the constitution number from the Colour Index means that there is no possibility of confusion about the particular colourant intended, particularly those known by their trade or commercial name.
The Functions of a Pigment
Pigments are used in surface coatings to fulfin one or more of three possible functions:
- Optical function. This isperhaps the most important characteristics. Teh wagelength dependent optical data of both the pigment and the binder determines the optical properties of the coating (e.g. colour, opacity, gloss).
- Protective function, Factors such as weather stability, surface hardness, flexibility, corrosion prevention, adhesion are all considered to be part of ‘protection’.
- Reinforcing function. This can also be considered to be a ‘protective function’ for the binder Correct selection of pigment type and level of pigmentation can increase the film’s cohesiveness and increase elaticity, hardness, and abrasion without necessarily impairing impact strength.
In a given pigment/binder system the performance of a pigment will be dependent on:
- the optical data of the components (e.g. refractive indices);
- the particle shape of the pigment;
- the particle size distribution of the pigment; and
- the efficiency of the pigment dispersion in the binder.
These also are affected by how well the binder has ‘wet’ the pigment. It is determined by the dispersing energy, or work done, during formulatin/ manufacture of the coating.
PROPERTIES OF PIGMENTS
Optical Data
The colour and opacity of a pigment are its main optical functions. Pigment colour depends on which wavelengths of incident light re absorbed and which are reflected by the pigment. A red pigment appears red if it selectively absorbs most of the wavelengths of the incident light and reflects red wavelengths. A white pigment appears white because it reflects all of the wavelengths of incident daylight. Titanium dioxide pigments may have a yellow or blue tone; thie means that they reflect the yellow or blue wavelengths slightly more efficiency than the rest of the spectrum. A black pigment absorbs all the wavelenths present in the incident light.
When discussing the opacity of the pigment it is necessary firstly to understand refractance. When light of a given wvelength passes from one medium to another it is refracted (the path of the light is bent) at the interface. The amount of refractance is dependent upon the wavelength of the light and the optical density of each medium. This amount of refractance for a given substance (light going from air into that substance) is referred to as the refractive index.
Fig. 1. The refractive index is the ratio of the sine of the angle of incident lifht to the sine of the angle of refracted light at the interface.
If air is replaced by another medium, for example water, then the ratio of the sines of the two angles in the example above would be different. In a paint film the refractive index of the binder interacts with the refractive index of the pigment. The greater the refractive index of the ore opaque will it appear to be in the film; that is, greater the difference in refractive index the more opaque the film. An extender pigment may have a refractive index of 1.5 and it appears quite white and opaque as a dry powder in air. When wetted out with, for example, linseed oil, which has a refractive index of about the same value, the extender would be almost transparent. Compare this to titanium dioxide, which has a high degree of opacity with a refractive inded of 2.7.
Particle Shape
The particle shape, size, and distribution of a pigment influence the rheological properties, shade, matting effect (gloss), weathering characteristics, and ease of dispersion. The particle shape of a pigment does not necessarily reflect the ccystalline structure of the pigment.
Pigment particles can be considered in thee different classes: primary particles, aggregtes and agglometrates. A primary particle is a single ‘piece’ of pigment which can be identified as an individual by microscopic examination. Te primary particles reflect the cystalline shape of the compound under consideration. Aggregates are primary particles that are firmly ‘cemented’ together at crystalline areas. Agglomerates are comparatively Isely bound primary particles and aggregates that are joined at crystal corners and edges. In general, ‘particle size’ refers to primary particle size. It is these three types of particles or lumps that affect the dispersion characteristics of the pigment. (The surface character of the pigment also influences its dispersion charateristics.)
Dispersibility
An efficient pigment dispersion, in the binder or vehicle, is essential, not only from the standpoint of economy of pigment usage, but aso to ensure that the coating will give maximum durability in service and to ensure that its storage stability and colour constancy are optimized. When the pigment is dispersed the vehicle ‘wets’ it in several ways. Firstly a certain amount of vehicle is not in actual contact with the pigment particles; it is distributed in the spaces between the pigment particles. It is also distributed in the crevices and cracks on the surface of the pigments particle, and over the actual surface of the pigment particles it is closely bound or adsorbed. This adsorption is important for the pigment’s rheological properties. As stated before, particle shape and size affects rheology. The more convoluted the shape and the smaller the particle size, the greater the surface area for a given weight of pigment. The greater the surface area of a pigment the more vehicle is required to wet it- hence the effect on rheology. A pigment contributing to poor flow will usually give poor gloss in the coating because of its high vehicle requirement, althoughin gloss paints large pigment particles (lower surface area), which protrude above the binder level on the surface of the film, induce a matted effect.
The oil absorption of a pigment (the minimum amount of linseed oil required to just produce a coherent paste when worked into a given mass of pigment) is useful information when formulating. Low oil requirements generally indicate good flow (of partricular importance for gravure inks) and easy dispersion.
The dispersion process reachese on optimum degree when each particle of pigment is separately and completely wetted with vehicle. This requires the displacement of air or moisture or both from the surface of the ‘dry’ pigment, that is, it must overcome the surface forces holding the unwanted material to the pigment surface so that it can be replaced by vehicle. The case with which a pigment becomes perfectly wetted is dependent on the surface characteristics of the pigment and the nature of the vehicle. Surfactants are often added to both aqueous and non-queous vehicles to improve their pigment wetting characteristics.
As the average particle size of pigments in a fim is about 1 mm or less and this is comparable to the wavelength of visible light (0.4 mm to 0.7 mm) the particle size distribution affects light scattering (this, together with refractive index affects opacity) and the colour of the reflected light. This can be demonstrated very simply by taking a piece of brown glass from a beer bottle. If the piece of glass is powdered, as the particles become smaller they gradually lose their brownness and become white, a siilar effect is seen in a series of synthetic red iron oxides: those with small particles give a yellowish shade, and as the particle size increases, the shade moves more to the blue end of the spectrum, so that the grade withthe largest particles is no longer a terractota shade, but is quite purplish.
The Classification of Pigments
Pigments have been classified in a variety of ways but here the approach has been to initially classify according to ‘organic’ and ‘inorganic’. This main classification was adopted not only because the title of the chapter reflects a class of pigments, but also some general statements can be made on the properties of pigments classed in the way. It should be remembered that therse properties are generalized: there is always an exception here on there.
Organic pigments are considered and extender pigments. The true inorganic pigments will be discussed here, with the exception of titanium dioxide, which is so important that is warrants a chapter on its own. Further subgroups of these pigments are shown in figure 3.
Each of the pigments shown in figure 3 will be disccussed in turn in respect of their properties and uses. Their manufacture will not be dealt with in detail except to indicate in general terms how they are produced, or where the method of manufacture influences their properties.
PROPERTIES OF INORGANIC PIGMENTS
Iron Oxides
As may be seen from figure 3, iron oxides suitable for use in surface coatings are found naturally or may be produced synthetically. Natural iron oxides are probably the oldest pigments in use and they were certainly ued in cave paintings by prehistoric man. In general, iron oxides are excellent pigments of outstanding light and weather resistance and of very moderate cost. They have excellent chemical resistance and are insoluble and unreactive in virtually all coatings systems. They are available in yellow, red, brown and black and some of the natural grades possess a strong greenish cast. Their main disadvantage is the dirtiness of their shades. Brilliant, clean shades cannot be produced.
Natural Iron Oxides
The shade and quality of natural iron oxides varied with the geography of the deposit and in fact many of the names reflect their origin. Raw sienna is a natural yellow iron oxide with varying clay content. It can be calcined to give burnt sienna, a bright brownish red with characteristics tinting properties. Umbers are hydrated ferric oxides that contain manganese dioxide. They also contain clay as an impurity. Raw umber is a greenish brown (the green shade is particularly noticeable when extended with white pigment and when calcined produce burnt umber a deep rich brown.
As may be imagined, the colour of the natural iron oxides varies even from the same ore body as do the content of impurities. For this reason, irrespective of the care taken to produce a uniform product, the shade and tinting strength of a natural iron oxide will be subject to wider variation than is the case with synthetic iron oxide.
To produce a suitable pigment, the crude ore is dried and ground to the fineness required. In addition purification steps may be undertaken, for example to remove water-soluble salts (by washing) that would interfere with the performance of an anti-corrosion coating, or to remove hard, gritty impurities by settling the dispersed material in water. As mentioned before, calcination may be undertaken to increase the red component of the colour. Finally, micronization may be undertaken, which effectively removes oversize agglomerates of pigment to allow for easier dispersion on a high speed disperser. The shade is not altered by this process (except certain yellows) but the tinting strength an be increased by up to 10 per cent.
Natural red oxides include a type known as Persian Gulf reds. As the name implies, they are mined in the Persian Gulf. The content of Fe2O2 is only around 70 to 75 per cent, but they possess a bright shade and are primarily uses a colour pigment rather than a primer pigment, as their solule salt content can be high. Spanish reds usually have a higher Fe2O3 content (up to 90 per cent) and the better qualities have low soluble-salt content. For this reason they are used extensively in metal primers. Their shade is somewhat duller than those from the Persian Gulf.
Natural yellow oxiees or ochres are essentially hydrated ferric oxide containing varying amounts of impurties. Frnch ochrese have a reasonably bright shade but a low tint strength. South African ochre is probably still the most widely used natural yellow pigment and contains about 60 per cent Fe2O3 corresponding to about 70 per cent hydrate iron oxide. Silicates are the main impurities.
Natural brown oxides are typified by the siennas and umbers already mentioned. Good quality raw sienna is quite transparent and has a clean tint but comparatively low strength. Burnt sienna is a redder mahogany shade with high transparency like the uncalcined material. Umbers are mined in Cyprus and may contain up to 15 to 20 per cent manganese dioxide. These too, are quite transparent.
Micaceous iron oxide is an unusual form of natural iron oxide. It does not contain mica, but it possesses lamellar (plate-like) particles which, when viewed under a microscope at low power, are quite thin in comparison to their area, much like mica. The platelets give a charateristically bright red colour when viewed with transmitted light (i.e. with light shining from below the microscope stage and into the objective). The material is black or dark grey and produces a scintillating effect in paint. The platelets orientate themselves parallel to the substrate to give an armoured effect to the film which not only physically reinforces the film itself, but also acts as an impediment to the diffusion of oxygen and moistue through the film. It is used widely as a finishing coat for structural steel wrk (e.g. Sydney Harbour Bridge).
Micaceous iron oxide deposits are fairly widespread but the most famous in terms of qality and high purity is mined in Austria. Some modification of the dark grey shiny colour is possible by adding, for example aluminum flake, phthalocyanine blue, to give more decorative effects. Care must be taken whne incorporating micaceous iron oxide into the vehicle: it is easy to overgrind, which results in a reddening of colour and breaking up of the platelet structure, defeating the purpose of its use. It has a great ability to absorb ultraviolet radiation which affords protection to the polymer in the binder and assists durability.
Fig. 4. Micaceous iron oxide platelets.
Synthetic Iron Oxides
Synthetic iron oxides were first produced as a byproduct of aniline production (for the manufacture of coal tar dyestuffs) about seventy years ago. Todayprobably less than 25 per cent of iron oxide pigments used in the world are of natural origin.
Aniline was produced by reducing nitrobenzene in acid solution with iron filings: the iron oxide so produced was a dirty grey colour but, by carefully controlling the reaction with the addition of aluminium or ferrous aalts, slurries of black or yellow iron oxides, suitable for use as pigments, can be obtained.
When black or yellow iron oxides are calcined at 700° to 800°C (both are hydrated oxides), they lose water of hydration and convert to iron oxide red. Yellow and black iron oxides are only stable up to about 180°C. At that temperature they start to oxidize and go red. Brown iron oxides are mixtures of red, yellow and black oxides and consequently have the same limitation of heat stability. (There are special iron oxide blacks and browns with a non-hydrated chemical constitution and a spinel crystal structure which are stable up to 1000°C) Red iron oxide is stable to 1200°C.
Another method sometimes used for the manufacture of iron oxide yellow is by hydrolyzing ferrous sulphate at an elevated temperature in the presence of iron and oxygen. The reaction mass is ‘seeded’ with finely divided hydrated iron oxide particles which grow to the required size in a few days. Pigment obtained in this manner is extremely soft in textue and disperses very easily. Calcining yellow oxide obtained by this method gives red oxide also of good texture. A dry method of manufacture requires heating ferrous sulphate or chloride in air with the presence of calcium or magnesium oxide or carbonate to temperatures up to 700°C. The alkaline earth is used to react with the liberated acid when the red iron oxide is formed.
As previously mentioned, the light-scattering effect of a pigment is dependent upon refractive index and particle size. Normal iron oxides hae particle sizes ranging from 0.1 to 1.0 mm and the scattering constant is at a maximum within this range speciality iron oxide pigments are manufactured with a particle size down to 0.01 mm and, as may be predicted, the opacity of these pigments in an organic binder is extremely low. They are of definite commercial interest and are known as ‘transparent iron oxides’. They are quite difficult to disperse properly and their full transparency can only be obtained with very intensive grinding techniques (ideally on a three roll mill) because of the very high surface area/unit mass. Like all iron oxides, they cannot be compared with organic pigments for cleanliness of shade, but they are used wherever the transpaency of a dyestuff is required with the absolute fastness properties of the iron oxides (for example, the ‘colour’ in metallic automotive lacquers).
Let us now consider other changes in the properties of synthetic iron oxide with changing particle size. Take a uniform iron oxide red of the Bayferrox Red 100 series (table 2).
These red pigment have a spherical particle shape. The iron oxide yellow have an acicular (needle-like) shape and, as more it can be accommodated between their particles, the yellows have a much higher oil number than the reds. The needle structure of the yellows imparts to them a certain sensitivity to grinding; they can be overground resulting in particles breaking and the shade becoming duller. Micronization also changing the shades of the yellows towards red; micronized reds, with their spherical particles, show no drift in shade when micronized.
Finally, a word about iron oxide pigments in primers, in which the majority of this class of pigment is used. Shade here is of secondary importance; they are used in primers as inert pigments which play a valuable physical reinforcing role. Generally iron oxides are judged as to suitability as primer pigments solely on their content of water-soluble salts. Metals corrode more rapidly in the presene of ions (especially chloride ions) but hydroxyl and chromate ions passivate. However, judgement on the basis of water-soluble salt content alone is incorrect. The wettability of the pigment is also of importance. For example, calcined reds are not so easily wetted out as those that have been merely dried and are not as good in anti-corrosive primer performance. In addition, the primary particle size is also important. Pigments with larger particle sizes give better results particularly where wettability is poor. On the other hand, large agglmetrates can approach the film thickness of the primer; micronized grades ensure that film defects due to pigment occlusions do not occur.
Chromates
The chromates are an important class of inortanic pigments. They are used in two ways: firstly, for their pigments properties they are used to impart colour (and opacity) to the surface coating, and secondly, as an anti-corrosive pigment in anti-corrosive primers for steel. The two main types of chromate pigments are those of zinc and lead. Zinc chromates can perform both functions mentioned above, whereas lead chromates are used only for their colour.
Zinc Chromate
This materials is manufactured by suspending zinc oxide in water and adding potassium dichromate and chromic acid.
Basic zinc potassium chromate is the commercial ‘zinc chromate’ pigment. It is slightly soluble in water and when manufactured by this method is not washed before drying and milling.
A second method of preparation is to use sulphuric or hydrochloric acid with potassium dichromate instead of chromic acid:
Potassium sulfate is also formed in this process: it must be washed out before filtration, drying and milling.
Another type of zincchromate corresponds to the formula ZnCrO4— 4Zn(OH)2 and is known as zinc tetroxychromate. It is produced by reacting zinc oxide with chromic acid:
Basic zinc potassium chromates are lemon yellow and are useful as colour pigments although they are more transparent and have a low tinting strength compared to lead chromates. Their good light fastness makes them suitable for combination (dry co-grinding) with Prussian blue to produce ‘zinc green’. Because they are basic in character they can cause thickening of a paint containg high acidity media. Where unreacted zinc oxide is persent in the pigment this thickening effect is particularly bad. Zinc tetroxychromate has a very low water solubility and is particularly transparent.
Because the anti-corrosive properties of these pigments are so important, it may be of advantage to briefly outline the corrosion proess. This will be in described in simplified terms.
The corrosion of metals is an electrochemical process. Iron requires moisture and oxygen to corrode and the process may be simplified.
Equation 1 shows iron going into solution at the anode to form ferrous ions and liberating electrons. The electrons go through the metal; to the cathode where they react with oxygen and water to form hydroxyl ions (Equation II). Equation III combines I and II and equation IV shows the final oxidation to rust. These equations assume the existence of a potential difference between different parts of the steel micro-structure. This is caused by differences in composition or configuration or both. The presence of electrolytes in the water accelerates the corrosion reactions. This is why chromate pigments being used as corrosion inhibitors must contain low minimum contents of chloride, nitrate and sulphate ions. Where aluminium and light alloys are being painted, lead and iron must also be absent.
Basic zinc potassium chromate must contain at least 43 per cent CrO3 to conform to specification. It can contribute in several ways to inhibiting corrosion. Because it has a slight solubility in water, it releases chromate ions, which can react directly with the iron to form an impervious film and thus retard reaction 1. This mechanism is considered as the absorption of chromate ions onto the metal surface, followed by the oxidation of the ions to an impervious layer of Fe2O3 and the reduction of the chromate ion. The ferric oxide formed is in close contact with the metal as distinct from the formation of rust, which is a spongy mass. And, because of its basic nature, zinc potassium chromate retards the reaction represented by equation II and promotes the neutralization of free acid. It can be used as a sole pigment or in combination with others, for example in zinc chromate/red iron oxide primers.
Zinc tetroxychromate is used exclusively in the manufacture of each primers. These were developed as a substitute for the difficult phosphating pre-treatment of metal used in shipbuilding. The etch primer consists of an alcoholic solution of phosphoric acid and a dispersion of zin tetroxychromate in alcoholic polyvinyl solution. When an etch primer is applied to steel, a zinc phosphate coating with tightly adhering ferric oxide is formed. Chromate ion is available for passivating the surface at the anode. A chromic phosphate complex is formed with the polyvinyl butyral, which insolubilizes the resin by cross-linking and effectively seals the surface. The triple action of phosphating, passivating and sealing explains the excellent results achieved in protecting the surface and preparing it for further coating. The presence of heavy metals must be avoided at the surface and preparting it for further coating. The presence of heavy metals must be avoided at all costs (iron contamination from steel ball milling) as they interfere with the complex butyral formation.
Lead Chromate
Lead Chromate pigments range in shade from pale primrose yellow through to orange and scarlet, and they have good opacity and tinting strength. The shade obtained depends upon the crystalline form, which can be controlled during and after precipiation. Conditions affecting the crystalline form include solution strength of the reactants, reaction temperature, proportion of reactants, pH, and rate of reaction. In addition, the after-treatment influences the properties of the pigment.
The colour is primarily due to the presence of lead chromate PbCrO4 either alone or in admixture with lead sulphate and/or lead molybdate (table 25.3).
Lead sulphate occurs with lead chromate not simply as a mixture but in ‘mixed crystals’. The mixed crystals, formed when the two substances are coprecipitated, are actually ‘solutions’ of one solid in another. Coprecipitation is achieved, for example, by dissolving sodium chromate and sodium sulphate in water together and adding a solution of lead nitrate.
Coprecipitation of lead molydate also gives a solid solution or mixed crystal of scarlet or molybdate orange:
All lead chromates, with the exception of orange chromes, are very sensitive to alkali; alkai makes their colour redder. They all darken somewhat when exposed to tight (primrose is the worst affected), but technology has largely oercome this through small additions of titanium dioxide, alumina or silica. Molybdate oranges are stronger than orange chromes; with some loss of brilliancy in shade, they have been rendered sufficiently light-fast to be used in combination with high-fastness organic maroons and violets to produce bright red automotive topcoats.
Lead chromates have been recently criticized for their toxicity on two counts: they contain both lead ad hexavalent chromium. Technically, it is not easy to use alternative pigments and obtain the same property profile, even leaving economic considerations aside. Low-dusting lead chromates have become more popular and some of the latest technology involves the use of polytetraflurethylene fibres to reduce dusting in the dry pigment.
Chrome Greens
These are sometimes known as Brunswick green and are mixtures of lead chromate with Prussian blue. As a mixture, this class of pigment is notorious for flotation. They can be produced by:
- precipitating lead chromate in the presence of Prussian blue in suspension;
- by mechanically mixing pastes of the two pigments; or
- by dry blending.
The coprecipitation method is said to give the most uniform product of the brightest shade. Bright shades of green are prepared from the primrose or lemon chromes, whereas the redder chrome yellows give dirtier olive greens.
‘Flotation’ or ‘flooding’ are terms used to described change in shade of the surface of paint after it has been applied. Flooding is usually used to describe an uneven resultant shade (streaks or circular pattern) and flooding usually applies to a uniform change of shade. These effects are caused by non-uniform wetting of the pigment mixture. Two pigment mixtures are notorious for this viz., pale blue shades produced by adding phthalocyanine blue to titanium dioxide, and chrome greens. Chrome greens usually go blue due to the Prussian blue rising to the surface of the still wet film.
Apart from their toxicity (lead chromate content), chrome greens have two disadvantages. They tend to evidence flotation as mentioned above and they also ‘ble off quite frequently on exposure, particularly when exposed to an industrial atmosphere. It is thought that this is due to sulphur dioxide attacking the yellow lead chromate and converting it to white lead sulphate.
Chrome greens are comparatively cheap, high opacity pigments that can be obtained in very clean, bright shades. Care should be taken when handling the dry colour, because Prussina blue is combustible and lead chromate can supply the oxygen for combustion. Chrome green once ignited can, therefore, continue to burn in the absence of atmospheric oxygen.
Prussian Blue
This material, which is chemically ferric ferrocyanide/potassium ferroferricyanide is also known as iron bronze. Hamburg, milori, mineral or Chinese blue. Some Prussian blues have the potassium atom replaced by the ammonium group. It also contains water of hydration, which is only completely removed at about 250°C with decomposition.
A Prussian blue is termed a bronze blue if it exhibits a high degree of bronzing, reddish golden sheen which appears quite metallic; this characteristics is sometimes required, for example, in the manufacture of printing inks. The bronzing is determined by the method of manufacture rather than the composition of the pigment. Pigments showing moderate bronzing characteristics are often termed milori blues and they are the main types used to produce lead chrome greens as they produce the brightest green shades. The so-called non-bronzing Prussian blues are those favoured for the manufacture of paint, but it must be clearly understood that all Prussian blues rend to bronze to a greater or lesser degree. This bronzing effect is often able to be seen as a ‘furry’ apperance whe the surface is viewed through polarizing sunglasses in strong bright sunshine.
Improved resistance to alkali attack can be achieved by partly replacing the potassium atom in Prussian blues with nickel, manganese or especialy cobalt. Alkalis at quite low concentrations attack normal Prussian blues, converting them to brown ferric hydroxide. They are are quite stable to acids and have a high tinting strength but poor opacity. Their light-fastness is very good.
Prussian blues can undergo ‘can fading’ on storage and this hs nothing to do with loss of tinting strength due to crystallization or re-agglomeration. It is thought to be a reduction of the compound because it is often reversed by applying the ‘faded’ paint to a surface when oxidation results in a resoration of the colour.
Ultramarine Blue
This is the first blue pigment used by man and was produced by the ancient Egyptians by powdering the blue stone azurite. The natural product is no longer used as a pigment and it is synthetically manufactured by calcining a mixture of kaolin, sodium carbonate, sulphur and carbon at above 800°C. It is not a simple compound and is said to have the general formula Na7Al6SiO24S2. Variation in shade of ultramarine is obtained by varying the particle size after suitable washing, milling and separation has been carried out. The larger the particle size the deeper and darker the blue, but the lower the tinting strength. Conversely, smaller particle size gives paler blues of higher tinting strength. If ultramarine blue is subjected to hydrogen chloride gas at elevated temperature, sulfur and sodium are removed from the crystal lattice and ultramarine vilet and pink result.
Ultramarine blue shows extreme hydrophilic properties and is thus easy to disperse in water. This led to its once popular use as a laundry blue, which neutralized a yellowish shade in white fabric to make the wash appear brighter and whiter. Today a similar effect is achieved by including organic optical brightening agents in laundry detergents. These fluoresce in ultraviolet light and so the fabric appears brighter in daylight.
Ultramarine has good light-fastness, but because of its extreme sensitivity to acids, paint films pigmented with it when exposed to industrial type atmosphere fade rapidly, especially in tints.
Chrome Oxide Pigments
One must always differentiate between ‘chrome green’ which is a mixture of Prussian bue and lead chromate and ‘chrome oxide green’ which is pure chromium sesqui-oxide (Cr2O3) manufactured by reducing hexavaient chromium compounds (sodium or potassium dichromate). The reducing agent is normally sulfur. Chromium oxide green has excellent fastness to light, weather, alkali and acids and is thus often used to pigment cement-based roducts. It also has a high thermal stability, up to 1000°C, above which there is a slow change in shade due to particle growth. As with other pigments already discussed, the shade of chrome oxide green depends on the particle size distribution: the larger the particles, the bluer the shade and the weaker the tinting strengh. Chromium oxide green, however, is not nearly as bright and clean in shade as chrome green or phthalocyanine green.
Chromium oxide has an unusual reflectance curve in the near infrared region, and in combination with iron oxides and a special organic black, if forms the pigmentation required for infra-red reflecting camouflage paints.
As distinct from the chrome yellows and greens, in which chromium is present in a hexavaient state, chrome oxide green can be produced in a very pure form in which the chromium is entirely in the trivalent state. Very little chromium can be dissolved from it by 0.1N hydrochloric acid, and for this reason chrome oxide green is considered non-toxic. It is allowed in Europe for pigmenting materials used in the packaging of foodstuffs.
Hydrated chrome oxide green contains only about 80 per cent Cr2O3 and the remainder is both physically and chemically bound water. It has a purer, bluer shade than the normal chrome oxide green. Its particle size, however, is very small (about 0.01 mm), which results in a high oil absorption and very poor opacity. Because of its water content, it is not thermally stable and starts to lose its water at around 95°C. It can tolerate higher temperatures (up to 140°C) for very short periods oly. Its other fastness properties, like those of chrome oxide green, are excellent.
Cadmium Pigments
Cadmium pigments are available from comparatively greenish shaded yellows through orange and red to deep bordeaux. They are available as ‘pure’ pigments or as extended products produced by coprecipitation with barium sulphate; the latter group are also known as cadmium lithopones.
The pure cadmium pigments are essentially cadmium solfide, CdS, which crytallizes in a Wurtizite lattice, which is isomorphous with zinc sulfide. This is used to change the shade of the golden yellow cadmium sulfide to one with a much greener shade. By substituting zinc for the cadmium in cadmium sulfide, the shade of the resultant pigment goes increasingly greener with rising zinc content. In the opposite direction, by substituting selentium for the sulfur in the cadmium sulfide, the resulting pigment goes increasingly redder and deeper in shade until, with the cadmium borderux, selenium makes up about 25 per cent of the total weight of the pigment.
Cadmium sufide and sulfoselenide are precipitated from a solution of cadmium salts by the addition of sodium sulfide, in the case of yellows, or a mixture of sodium sufide and selenide, in the case of reds. The precipitate is washed and filtered and has little or no pigmentary properties unit it has been calcined at 500 to 700°C which, by causing the particles to grow, produces the final optical properties of shade, opacity, and colour intensity desired.
Cadmium pigments have brilliant clean shades similar to organic pigments, but their thermal stability (up to 600°C) far outstrips that of the organic pigments. For this reason their application is in the plastics proceessing industry. They have a high opacity but, compared to many other inorganic pigments, they are very expensive. They have very good light-fastness in mass tone and on reduction with titanium dioxide, but their weather resistance is suspect. Cadmium sulfide is especailly affected by water and oxidizes to cadmium sulfate, with a corresponding loss in colour intensity. The speed with which this occurs depends on the type of cadmium pigment and the binder used. Cadmium yellows are not as weather resistant as reds and bordeaux, which are stabilized against oxidation by the presence of selenium. The waters absorbancy of the binder has a significant influence on the durability of the pigments. In the surface coatings industry, only cadmium reds in low water-absorbing media can perform adequately in respect of weather resistanc: paints based on air drying alkyds, for example, are unsuitable. Cadmium pigments have been used to a considerable extent in the printing industry because of the softness of their texture: inks formulated from them cause little abrasion.
Care should be taken when using cadmium pigments over copper or its alloys. Darkening of the pigment can be significant.
Cadmium pigments are suspect in many countries owing to the toxicity of soluble cadmium. Many manufactures, however, maintain that leaching of cadmium from materials coloured with cadmium pigments is minimal.
Red Lead
This pigment has been used for many years. It is essentially lead tetroxide Pb3O4 or PbO2.2PbO. It is also known as minium, mineral oragne,or Paris red. It is manufactured by heating litharge (PbO) in air. When heated by itself it liberates oxygen at about 500C.
It has rarely used as a colour pigment; its main use has been as a metal primer pigments in combination with linseed oil with which it reacts to form tough, elastic films that have excellent adhesion to metal. The reactivity depends on the content of Pb3O4 and, if this is high enough, the dispersion is usable for about two weeks. The article of commerce contains about 90 per cent Pb3O4; the remainder is PbO. These are known as non-setting red lads. Lower Pb3O4 content in the pigment gives very rapid thickening with linseed oil.
Red lead primers are corrosion inhibitive owing to the basicity of the pigment.
White Lead
The naturally occurring lead carbonate named ‘cerrusite’ was used as a pigment well over 2000 years ago. The white lead of commerce is a synthetic mixture of basic carbonates of lead approximating for formula 2PbCO3. Pb(OH)2. It is still sometimes known as ‘cerussa.’ It also goes under the name of ‘flake lead’ ‘Flake white’ in artists’ colours).
Metallic lead in a physical form with a large surface area (e.g. ribbon, spart erings) is held in towers; lead acetate solution is allowed in trickle over it so that air can easily gain access to the lead surface. This causes oxidation of the lead to lead oxide, which is soluble in lead acetate solution. This solution is then carbonated by bubbling carbon dioxide through it. White lead is precipitated and filtered off leaving a solution of lead acetate, which is returned to the lead towers to take up lead oxide again. The reactions involved, in simplified form, are:
The filter cake may be fed directly into kneaders without drying off the water. Linseed oil is run in during the kneading process and the oil preferentially wets the white lead, displacing the water from the cake. The water is decanted and heat and vacuum applied to the kneader to remove the last traces of moisture. The result is a finely dispersed system of white lead in linseed oil. This process is known as ‘flushing’ and is used with many pigments’ (organic and inorganic) aqueous presscakes to replace the water with the non-aqueous medium required. Since the pigment does not go through a drying process which would cause the particles to form aggregates, the pigment in a flushed preparation is almost entirely in its primary particle form. This is particularly valuable for the manufacture of printing inks, in which it happens that white lead is not used.
White lead has a fair opacity and until the advent of titanium dioxide was the major white pigment used in surface coatings. It darkens badly when exposed to sulfur or sulfides because of the formation of black lead sulfide. A significant property is its ability to produce tough, elastic films with linseed oil, by forming fatty acid soaps. It was used extensively in primers for timber, plaster and the like, either alone or combined with some red lead, to give the famous ‘pink primer’. It was also extensively used in undercoats and finish coats. Although still used, the past tense is emphasized above because of the embargo on the used of highly toxic lead (either as red lead or white lead) in decorative finishes.
Zinc Oxide
The pure oxide exhibits a strikigly brilliant white shade that can be dulled by the presence of impurities. Commerical zinc oxide is classified for colour by means of seals. White, green, and red seals are typical of commerical classification, in decreasing order of shade purity. It is a versatile pigment in the paint industry. It is almost completely opaque to ultraviolet light; this property explains the protective action imparted to exterior paints containng zinc oxide. When combined with oleo-resinous media, it forms zinc soaps, which markedly increase the abrasion resistance of the film. Zinc oxide is also fungistatic and inhibits the growth of mildew on paint surfaces although it has little or no distant action. It it also used in anti-fouling paints.
Because of the ease with which zinc oxide reacts with acidic media, care must be exercised when using binders of high acidity because severe thickening may ensue. Zinc soaps can improve brushability and certainly have a beneficial effect on pigment dispersion and dispersion stabilization.
Zinc Sulfide Lithopone
Zinc sulfide is used as such in a relatively pure form or in a form coprecipitated with barium sulfate (Blanc Fixe) known as lithopone. Two types of lithopone are commonly used; one containing 30 per cent zinc sulfide and one containing 60 per cent zinc sufide. Coprecipitation is achieved by reacting an aqueous solution of zinc sulphate with barium sulphide:
Pure zinc sulfide is obtained by reacting barium sulfide with zinc chloride.
Zinc sulfide can exist in two crystalline forms: cubic Sphalerite or hexagonal Wurtizite. Wurtzite is unstable under the influence of ultraviolet radiation and photochemically produces zinc metal. This was the cause of the early zinc sulfides darkening on exposure. Today zinc sulfide pigments are stabilized by the addition of traces of cobalt or manganese.
After titanium dioxide, zinc sulfide is the strongest white pigment available to the paint chemist. The l;ithopones are corespondingly weaker, depending on their zinc sulfide content. As distinct from zinc oxide, zinc sulfide is considered a neutral pigment as it will not react with fatty or resinous acids in the medium. It is remarkably unreactive and will not cause darkening even if used in combination with lead pigments It is extremely easy to disperse and has a comparatively low oil requirement.
Although the zinc sulfide pigments are fast to light and have good opacity, they are unsuitable for use in exterior finish coats as their chalking resistance is very poor. Chalking is so intensive that the coating is soon completely eroded by the weather. This effect is not so noticeable in aqueous polymer dispersion systems. In non-aqueous media the chalking rate can also be improved by the addition of zinc oxide.
Zinc sulfides are excellent for the formulation of interior paints because of the purity of tints obtainable with coloured pigments.
Zinc Phosphate
The material, known commercially by this name and showing superior corrosion inhibitive properties, is actually zinc orthophosphate dihydrate of a definite chemical structure:
Until recently, the nature of the mechanism by which zinc phosphate behaves as an active corrosion inhibting pigment had not been understood. That it is corrosion inhibiting, there is no doubt; steel primed with zinc phosphate primer followed by intermediate and finish coats when exposed shows little tendency to corrode even after the paint system has been stripped and the metal re-exposed again for several weeks. It would seem, however, that the ability of zinc phosphate to render ammonium sulfate comparatively harmless as a corrosion initiator and propagator is of prime importance. Ammonium sulfate is always deposited in industrial atmosphere where sulfur dioxide is present. It has also been stated that it can form complex hetero-acids to inhibit corrosion. This is supported by the fact that the amount of water of hydratin is extremely important in practical trials carried out on its anticorrosive properties. More or less than 2 moles of water show quite inferior anti-corrosive properties. The great advantages of zinc phosphate as an anticorrosion primer are that:
- it is non-toxic;
- it is white and of poor tinting strength, so that colours ‘made to order’ are facilitated;
- it has good weathering characteristics in its own right;
- it imparts excellent intercoat adhesion;
- it is very easy to disperse in the medium; and
- it produces primers of excellent flow and brushout characteristics.
Cacium Plumbate
This anticorrosive pigment is analogous in strcuture to red lead if we consider its structure, not as the empirical formula Ca2PbO4, but as 2CaO.PbO2 (cf 2PbO.PbO2). It is made by heating lime with lead oxide in a stream of air at above 700°C.
Calcium plumbate is insouble in water although extraction with water gives an alkaline reaction. It reacts with acidic media in the same way as red lead and, by forming both lead and calcium soaps, excellent adhesion (particularly to zinc and galvanized iron) and toughness of the film are obtained. Apart from its alkaline reaction, its anti-corrosive effect is due to the formation of a film of calcium carbonate at the cathodes.
Carbon blacks
There are so many carbon/blacks available commercially that a full description of this class of pigment would run to a work comparable in size to the whole of this book. Of necessity, the topic will here be treated superficially with only the most important differences highlighted.
Many of the carbon blacks used as pigments have an extremely high specific surface area per unit mass. This means that the oil requirement is very high, but secondary problems can arise: some carbons have a very high surface activity which ca give rise to inefficient wetting out, flotation and flooding, adsorption of components of the formulation (e.g. driers, antiskinning agents, flow promotes). A measure of the surface activity of a carbon black is the DPG index. This indicates the amount of diphenyl guanidine (DPG) absorbed by the pigment when a given mass of it is shaken with a solution of DPG in benzene.
Although carbon blacks are all black as the name implies, the degree of blackness must be taken into account when selecting the material as a pigment. This degree of blackness has several components: tinting strength, jetness, and the shade of the undertoe.
Impingement or channel blacks are made by burning methane from natural gas with an air supply insufficient for complete combustion. The quality of the black produced depends upon the composition of the gas used as fuel, the amount of air admitted, distance of impingement etc. Chanel blacks have a very high surface area, a high tint strength, and good jetness.
Furnace, acetylene and thermal blacks are of inferior quality for use as pigments although they are purer than channel blacks as they have a lower content of volatile matter. They are important in the rubber industry, which uses enormous quantities of carbon black.
Lamp black or vegetable black, is so named because of the vegetable origin of lamp oils (rape seed oil) used in lamps. Today the fuel used is very high hoiling high aromatic oils derived from coal tar. The oil is burt in shallow troughs and the smoke led through chambers containing baffles that trap the soot. A weaker black is wood or vien black, which is charcoal produced from any woody matter eg grapevine stems, nut shells, twigs).
Bone, ivory and drop blacks are generally known as animal blacks. They are made from bones (or at one time, ivory). They tend to contain a fairly high proportion of calcium phosphate. Drop black gets its name because it was originally made by wet grinding charred bones and sold in drop like shapes from the drying trays.
Mixed Phase Pigments
These pigments are sometimes known as ceramic pigments because they were first developed in the colouring of ceramic glazes. Further development of their pigmentary properties has now made available to the surface coatings chemist a range of pigments with rather unique characteristics. They are all based on metal oxides and, by incororating foregin ions inc certain crystal lattices, special colour effects can be achieved. The two stable crytsal lattices chosen are the rutile and Spinel structures. By partial or complete substitution of metal ions, such as nickel, chromium, cobalt, iron or copper, in the TiO, (rutile) or MaGl2O4 (spinel), structures a variety of colours can be obtained. The formation of the mixed phase is a solid state reaction and requires extremely high temperatures. This accounts for the outstanding thermal stability of these pigments which can withstand temperatures of 1000°C
They also have outstanding light and weather resistance in mass tone or extended with titanium dioxide and are capable of withstanding attack by a great variety of chemicals, and comparateively high concentrations of acids and alkalis. Some of them have quite a brilliant, pure shade approaching that of organicc pigments. In the inorganic ranges such a brilliancy of shade is probably only approached by the cadmium pigments. Alone or in combination with high quality organic yellows, a range of yellow shades with excellents properties cna be obtained to substitute for lead pigments but at higher cost.
They suffer from the disadvantage of poor opacity which with inorganic pigments is usually very high and have low tinting strength. At high PVCs the degree of gloss is often unsatisfactor, varying from type to type in this class. Nevertheless, because of their other outstanding properties, application in coil coatings, high quality decorative topcoats and automotive lacquers is common. Especially because of their heat resistance, non-bleeding and migration resistance, they are much used in the colouring of plastics.
Although the tinting strength of the black ceramic pigment is lower than that of other inorganic blacks,it is frequently used to make grey with titanium dioxide because it exhibits far less tendency to flocculation and floatation.
Some of the shades of pigments obtainable together with their constitution are shows in Table 4.
Brown vary in shade from reddish brown (almost terracotta) to a deep chocolate bronw with a variety of metal combinations in spinel structures: manganese, zinc, aluminium and chromium; chromium; iron and zinc; manganese, chromium, zinc and aluminium
Tin, silicon, magnesium, and tungsten also find their way into the crystal lattices of this pigment class to produce special colour effects.
Bronze Powders
These are, like aluminium powders, actually fine flakes. They have limited application in paints and are used to obtain copper and gold metallic effects. They are largely used by printing ink manufacturers, especially for label and packaging printing, where their good metallic colour can be used to much greater effect in advertising and for eye appeal. They are available in various shades (pale gold, rich gold, green gold, antique gold), and finenesses and thickness chosen will depend upon their ultimate use in rotogravure or offset applications. The brightness of the bronze depends upon the surface polish of the flake and the strict control of a narrow range in particle size distribution.
Stainless Steel Flake
This has a limited application in the surface coatings industry. It is used in place of aluminium wherever absolute chemical inertness is required, but care must be exercised in the presence of chloride ions. Its greatest application is in imparting impermeability to coatings applied to the interior of food storage tanks and bins.
Titanium Dioxide Pigments
In 1791 William Gregor discovered titanium dioxide in black magnetic sand in Cornwall and in 1795 M.H. Klaproth isolated the oxide from mineral rutile in Hungary, but the first commercial production of titanium dioxide pigment did not take place until the 1920s. The first pigments produced were acheived by reacting ilmenite (FeTiO3) with sulfuric acid, followed by a hydrolysis procedure, which tincorporated calcium or barium sulfate. This produced the first composite pigments in which the titanium dioxide was present in the anatase crystal form. The first rutile titanium dioxide pigments appeared in the 1940s, still utilizing the sulfuric acid reaction as the first step and finally, in the late 1950s, rutile pigments produced by the chloride route were introduced commercially.
Whichever route is adopted for manufacture, which is governed by a complex equation of availability and price of feedstock, together with effluent disposal legislation, the process requires that the titanium dioxide be produced chemically pure and with a predetermined and exact crystal size distribution. Although modifiers may be added to control rutilization and crystal growth in either the sulfate or chloride process, the rigorous exclusion of impourities, particularly those of the transition metals, is first necessary. The final surface of titanium pigments is also interesting in that the majority of pigments sold do not have a TiO2 surface but rather one that is convered with oxides of silica and alumina in a fashion that renders the pigments most suitable for the intended end use.
The high refractive index, and the chemical and physical stability, together with the control of crystal size distribution and the final surface modifications, produce a wide range of stable, high opacity pigments whose properties have been tailored for the consumer.
There are two major routes employed in the manufacture of titanium dioxide pigments. The first uses the hydrolysis of aqueous solutions and is known as the sulfate route; the second involves the vapour phase oxidation of titanium tetrachloride and is known as the chloride route.
The Sulfate Process
This process requires the dissolution of asuitable ore, usually ilmenite, in sulfuric acid. The insoluble impurities are removed and, following hydrolysis at boiling point, the precipitated titanium hydrate is washed and lecached free of soluble impurities. Controlled calcination of the pulp produces pigmentary titanium dioxide of the correct crystal size distribution; this material is then subjected to a finishing coating treatment and milling. Over twenty separate operations are involved, some batch and some continuous, occupying two weeks of processing between ore dyring and final product. Following the process flow sheet (figure 1) the main operations as identified are listed below.
Drying and grinding. Ilmenite ore is dried and then ground in a ball mill to a fineness of less than 45 mm.
Digestion. The ground ore is ‘digested’ by addition of concentrated sulfuric acid, forming salts of titanium and iron. These salts are carefully dissolved in water and recycled acid.
Reduction. Metallic iron (scarp metal) is introduced at this point to reduce ferric salts to the ferrous state. A small amount of titanium is also reduced to prevent re-oxidation of the iron.
Clarification. Siliceous insolubles and small quantities of unreacted ilmenite are removed from the liquor by setting. This process is considerably enhanced by the use of flocculants and the flocculated shudge is drawn off at the bottom of the clarifier and washed free of liquor which is then recyled. The overflow liquor is sent forward for crystallization.
Crystallization. The liquor is cooled and a portion of the ferrous sulfate crystallizes out as copperas. The copperas is removed by using centrifuges and the liquor goes forward for a final clarification.
Liquor filtration. The final small amount of insoluble residue is removed by pressure filtration of the liquor using pre-coated filters.
Concentration and hydrolysis. The liquor is concentrated by evaporation at reduced pressure and subjected to a boiling hydrolysis in the presene of titania seeds, which control the hydrilysis of the titanium salts in the liqour.
Washing and leaping. The precipitated titanium hydrate pulp is filtered and washed, subjected to a ‘reducing leach,’ and washed again. The final traces of iron, chromium, vanadium, and other undesirable contaminants are removed by these procedures.
Calcination. Various conditioning agents to promote the desired pigmentary characteristics are added adn calcination in rotary kilns takes place. The pulp is first dehydrated, then the solid phase growth of the titanium dioxide crystal takes place. Dependent on previous additives, the rutile or anatase modification is produced.
Milling. After cooling, the kiln discharge is milled to break down the aggregates. Some pigment may be packed at this stage, but the majority of material phases forward for further processing.
Wet treatment. The pigment is dispersed in water and subjected to further milling and classification. Coating reagents incorporating alumina, silica, and other proprietary treatments are the empolyed by controlled precipitation from suitable solutions to give different grades of pigment dependent on the desired end use properties.
The coated pigment is washed free of soluble salts, dried and micronized. During this final milling, an organic topcoating may be added to aid incorporation in the customer’s media.
The Chloride Process
The process uses gaseous chlorination of an ore of high titanium content, such as rutile, followed by distillatin and finally vapour phase oxidation of the titanium tetrachloride. The profess is a cyclic one to this point, but the remaining wet treatment operations are similar to the sulfate process. The process flow sheet in figure 2 illustrates these points.
Chlorination. Dry titanium ore is mixed with dry coke and fed to a reaction vessel. Chlorine (mostly recycled) is introduced and a high temperature (900°C) reactin produces titanium tetrachloride, carbon monoxiide, and carbon dioxide.
Purification. Titanium tetrachloride is condensed and impurities removed from the liquid by settling and chemical treatment. The crude titanium tetrachloride is then fractionally distilled.
Oxidation. Pure titanium tetrachloride is vapourized, superheated, and ‘burnt’ in a reactor in the presence of oxygen. Modifying agents are also added to the reactor and pigmentary titanium dioxide forms in milliseconds with chlorine as a by-product. After cooling, the pigment is filtered off in baghouse filters and the chlorine recycled to the chlorination state.
Wet treatment. From this point the process is similar to the sulfate process.
Applications of Titanium Pigments
The surface coating industry is the major consumer of titanium pigments, which in 1979 accounted for about 60 per cent of total titanium pigment usage world-wide. The other major markets for titanium pigments are the plastics and paper industries, and to a lesser degree the rubber, ceramic, textile, masonry product and cosmetic industries. Present world capacity is about 2.2 million tonnes and this is expected to increase it approximately 3 per cent a year.
Each of the many grades of titanium pigment has particular properties, but it has generally been accepted that these grades can be classified by four broad pigment type (table 1).
Surface Coatings Industry
The heading for this section covers a very broad spectrum of applications, including some of the better known segments of paints, printing inks, and specialized coatings.
Titanium pigment is usually considered only as an opacifier; that is only one facet of its many properties. Titanium pigments also offer polymer protection by the absorption of harmful ultraviolet rays present in the sun’s light.
Chemical treatment of the titanium pigment by pigment manufacturers imparts to the pigment a combination of properties by variations in the inorganic/organic coating types and levels referred to earlier. The obvious desirable properties are:
- high degree of dispersion in a range of media;
- opacity (hiding power);
- brightness;
- tinting strength;
- durability;
- gloss development; and
- end product stability
The properties listed are by no means complete and are not in any specific order of importance. For example, the manufacturer of an interior flat latex paint is not really interested in durability and high gloss; opacity (hiding power) and lack of gloss are more important. Hence, these two specific properties can be built into a particular titanium pigment (type IV in table 1).
a Measured by Method 1 B.S. 3484 1962; results quoted to nearest 0.05
bMeasured by the Strohlein Area-meter
cPalete Knife Method based on B.s. 3483 1962; expressed in grams oil per 100 g of pigment
dRelative to white lead at 100.
Dispersion of Titanium Pigments
The ability to achieve the desired properties of a system (opacity or durability or tint-reducing strength) is dependent on the ability to fully disperse the titanium pigment down to its ultimate particle size and for those discrete particles to be held in a deflocculated state. Much has been written on the topic of dispersion and there are many methods of achieving dispersion, but with rising costs of energy and manpower, the surface coatings industry is demanding rapid wetting and easy-dipsersing pigments, so that ultimate particle size is achieved in a minimum time with minimum power costs. These challenges have been met by the titanium pigment manufacturers and there are now many grades of titanium pigments on the market which will diperse rapidly under high speed impeller dispersion (types III and IV in table 1).
The optimum titanium pigments crystal size to achieve maximum high scattering and, hence, opacity in films normally used in surface coatings, is about 0.23 mm; the processing carried out be the titanium pigment manufacturer ensures that the crytals are closely controlled to about this size. Subsequent processing is undertaken to minimize the spread of particle size to ensure maximum opacity and ease of dispersion. In a bag of premium quality titanium pigment there will be very few individual particles; the pigment tends to form loosely held agglomerates.
Agglomerates should not be confused with aggregates, which consist of sintered crystals. These agglomerates will be readily broken down under normal paint milling conditions.
The importance of thorough pigment dispersion cannot be overemphasized since opacity, tinting strength, gloss development, brightness, durability, and economic usage of titanium pigments are all influenced by pigment dispersion. Thorough dispersion is as important to the optical and protective properties of a coating as the correct selection and proportions of the constituent raw materials.
Opacity
The use of titantium pigment in teh coating industry revolves around its ability to scatter light which has been discussed. Fresnel first propounded a theory based on refractive inded ratios, which simply says that the greater the refractive index difference between particle and medium, the greater the amount of light reflected from the surface. This is how opacity is achieved in a paint film by using titanium pigment. Table 2 gives some refractive indices (R.I.s) of well known pigments and extenders and it is clear that, in a paint medium based on, for example, a long oil alkyd of RI 1.53, the high RI in air of rutile titanium pigment of 2.71 will give a greater ratio than that of, say, barytes in the same medium.
Tinting Strength
The tinting strength of coloured pigments is a measure of the comparative amounts of two coloured pigments that need to be blended with the same amount of the same white pigment in the same medium to produce a tint of equal strength in each case. With this definition in mind, a sereis of white pigments when evaluated for tinting strength in a standard blue pigment gave the following results presented in table 3.
These results show that the tinting strength of rutile titanium pigment far exceeds that of all other listed pigments and is one of the reasons rutile pigments are used throughout the surface coatings industry.
It is important to remember when choosing a particular grade of titanium pigment that a variatin in the tinting strength of two similar types of titanium pigment may result in different depths of tints in surface coatings.
Durability
Durability, in the surface coatings context, is the degree to which paints and paint materials withstand the destructive effect of the conditions of which they are subjected.
Most polymers are attacked by ultraviolet radiation in the sun’s rays. Some polymers are more resistant to these harmful ultraviolet rays than others and thus a paint film containing a resin of high resistance to ultraviolet degradation should be more durable than a point film containing a resin of low resistance to ultraviolet degradation.
Two principal factors which affect the durability of paint are:
- the chemical and physical nature of the paint film, and
- the environment that the paint film has to withstand.
The ways in which these two factors interact determine the durability of the film.
The short wavelength radiation in the sun’s rays is the determining influence on polymer degradation; if an effective ultraviolet absorber can be built into the paint system for durability of the paint film can be prolonged. Rutile titanium dioxide pigment is a good ultraviolet absorber. Absorption coefficients for typical clear films vary between 0.1 and 2.1 mm-3. The equivalent absorption coefficient for titanium pigment is 16000 mm-1 and hence the inclusioni of this pigment into paint fims has a profound influence upon that film’s resistance to ultraviolet attack.
Although titanium pigment is a good absorber of ultraviolet light, it should be pointed out that the influence of titanium pigment is both protective and destructive. The pigment protects the film by absorbing ultraviolet radiation to such an extent that there is little radiation left to damage the binder directly. However, the rutile or anatase surface is ‘photo-active’ and, as a result, can initiate reaction leading to polymer breakdown. This protective/destructive balance is largely governed by the titanium pigment manufacturer’s ability to mask this photo-activity by the inclusion of coatings of oxides of aluminium and/or silicon on to the titanium dioxide crystal.
Gloss Development
Gloss is ‘the degree to which a surface simulates a perfect mirror in its capacity to reflect incident light’. With this definition as a starting point, it becomes obvious that the film with the least defects at its surface, and the one with the one with the most ability to form a glass like surface, will give the highest gloss. Any lack of gloss in the paint film can be ascribed to discontinuities at the surface, the for suh discontinuities scatter much of the incidental light and thus reduce the amount of specular reflection. These discontinuities at the surface may arise from the substrate, from the paint, and from the method of application. The minimize these surface imperfections, the titanium pigment manufacturer will have in his range a grade of pigment which will be easily dispersed, is resistant to flocculation, and has a narrow range of particle size distribution.
Storage Stability
Once the optical properties required of the paint have been achieved by careful selectin and dispersion of the correct grade of titanium pigment, thought must be given to stability of the finisher product. A well formulated paint will have a good balance of ingredients, and without this balance, instability in the can may result.
Titanium pigment is not normally the culrit in can instability; but the incorrect choice of pigment can cause instability, such as bodying or syneresis or flocculation. Again, the norganic coatings on tehtitanium pigment d help to overcome instability in the can, and it is not normal these days to use an uncoated, unrefined grade of titanium pigment in prenium quality paints.
In all the properties described above, the underlying requirement of best utilization of a particular property has been dependent upon good pigment dispersion and it cannot be stressed enough that dispersing the titanium pigment to its ultimate particle size is the key to obtaining best optical and physical properties of the paint.
OTHER INDUSTRIES
Plastics Industry
Unlike the surface coatings industry, which uses relatively thin films, the plastics industry uses relatively thick profiles. The pigment loadings tend to be lower than in paint products; this is a generalization because thermoplastic polymers such as ABS, which has a pronounced yellow undertone, rquire relatively high pigmentation to mask this polymer colour.
The titanium pigment requirement for thermoplastic products is easy dispersion under low shear conditions. This is assisted by use of titanium pigments, which are hydrophobic in nature. This is achieved by milling with an organic coating, which renders the pigment hydrophobic. Surface coatings pigments tend to be hydrophylic.) A small quality of surface moisture on the titanum pigment (less than 1.0 per cent) is usually acceptable to the surface coatings industry but can be detrimental to the thermoplastics industry, where high processing temperatures volatilize this moisture. Where extruder or compounder barrels are not vented to allow escape of volatiles, surface imperfections in the finished product result.
Methods of incorporation of titanium pigments into plastics are varied, but the major methods are:
- dry blender,
- ribbon blender;
- high speed mixer;
- internal mixer; and
- continuous compounder.
The last three mixers are al used where high pigment loadings—up to 70 phr (parts per hundred of resin or rubber)—are necessary;.
The mixers mentioned above are in general those utilized by injection moulders adn extruders. However, other plastics and, in particular, PVC plastisols require a moistue-free easy-dispersing pigment, but in this case it is usual to disperse the pigment by means of a Cowles type disperser. The specialized plastics grade pigments are developed for ease of dispersion by Cowles for this end use.
The largest potential user of titanium pigment in the thermoplastics industry is the rigid PVC industry, where plumbing and household fittings are a fast growing application in which both opacity and polymer protection are essential properties of the titanium pigment.
Rubber Industry
Applications in rubber are of relatively small volume and the dispersion equipment is of a ery high shear nature, that is internal mixers, and here the titanium pigment and used is an uncoated anatase or rutile. Anatase is used where chalking is required, and rutile where non-chalking is required. For specialized, the coated and refined plastics grade rutiles are normally preferred.
Paper Industry
Historically, anatase has been used for paper production. However, a shift to rutile was made in Australia a number of years ago, and considerable reduction in titanium pigment usage was made by this change. The two large areas of rutile pigment usage in the paper industry are:
- beter addition to wood pulp; and
- board coating.
In both areas opacity, dispersion, and brigtne are essential, and in Australia a coated and refined rutile pigment is preferred.
Ceramics Industry
Vitreous enamels account for the bulk of titanium pigment usage in ceramics. Requirements are for easy and complete solubility into vitreous frit with good colour reproduction in the final enamel. This application requires special production techniques to provide the required properties, and only few titanium pigment manufacturers supply these highly specialized products. The special vitreous enamel grades are used for first manufacture, whilst small quantities standard anatase are usually used for mill additions to the slip.
Other Applications
In all other applications, for example, cosmetis, cement products, food, and leather, some form of dispersion of the titanium pigment will be necessary. Where titanium pigments are to be used in textile fibres, they are usually referred to as delusterants and are normally anatase pigments, whose abrasivity is not so great as rutile: this prevents wear of the spirmerettes used when spinning synthetic fibres such as nylon and rayon, titanium pigments in industry are also used in artifical leather, artists colours, asbestos cement sheeting, bitumen floorings, cement products, ceramics, cosmetics, floorcovering (vinyl and linoleum), fibers, glass, laminating paper, leathers, mouldings, paints, papers, paper board, pharmaceuticals, plastics of all kinds, printing inks, putty, road marking compositions, rubber, shoe creams and polishes, soap, synthetic rubber, and a host of others.
Paint Driersh1/p]
Driers are materials that promote or accelerate the curing or hardening of film formers that contain oxidizable or drying oil components. Air drying is the formation of a solid film at ambient temperature by oxidation processes from an applied liquid coating. Such coatings are convertible coatings in that the film, after drying, does not re-dissolve in the original carrier solvent. The air-drying process can be described as autoxidation, as it takes place automatically after the coating has been spread. Autoxidation is a chemical process that is greatly affected by temperature and the presence of catalysts. Humidity also affects autoxidation of conventional paints and the presence of light is helptul.
The autoxidation of unsaturated drying and semi-drying oils does not proceed rapidly enough to be commercially acceptable; for instance inseed oil with conjugated unsaturation requires over 24 hours to become non-tacky. The so-called ‘non-yellowing’ drying oils with non-conjugated unsaturation autoxidize at even slower rates. The conversion of these oils into higher molecular weight polymers (alkyds) with greater propensity to oxidize still does not give commercially acceptable drying times unless autoxidation catalysts or added.
Driers are a long established yet rather specialized group of additivies. Because of their specific effects on oxidizing film formers, driers do not normally form part of formulations in which film formation in by other mechanisms. Thus lacquers and latex paints that dry by evaporation of solvent or water, and epoxies, urethanes, and baking enamels that form films by chemical reaction do not contain drier additives as oxidation catalysts. The chemical compounds forming the majority of driers are metallo-organics (metal carboxylates) formed from a metal base and an organic acid. An older term is ‘heavy metal soap.’ Many other organic compounds have been assessed for ‘drying’ capabilities but only one compound, 1:10 phenanthroline has achieved commercial acceptance and its use is synergistic to metallo-organics. Metallo-organics may be classified by their activity into primary and secondary types.
Primary driers (or active driers) are compounds of cobalt and manganese, which have the highest catalytic activity and most pronounced accelerating effect on film formation. Primary driers are known as ‘surface’ or ‘skin’ driers since in oxidizable vehicles, cobalt or manganese, when used alone, will cause the surface of the film to rapidly set to a near solid white the underlying film does not reach this advanced state of oxidisation. This uneven hardening will, in thick films, cause the defect known as ‘wrinkling’.
Secondary driers (or auxiliary driers) are compounds of lead, calcium, zinc or zirconium. While they possess a much lower level of catalyric activity, these metals, when used in combination with cobalt and/or manganese, give films that do not surface-dry as rapidly and thus harden through more uniformly. Such films are said to ‘through harden’ and secondary driers are known as ‘through driers’. Lead compounds are the major drier of this type. For a more detailed description of the processes of oxidation film formation.
Types of Driers and Manufacturing Methods
Metallo-organics, rated in terms of decreasing volume usage in Australia, are compounds of:
- lead
- calcium
- cobalt,
- manganese,
- zirconium
- zinc, iron.
- ate earth, aluminium
Three routes are used commercially for manufacture of driers. All three processes are carried out in the presence of hydrocarbon solvent. They are completed by filtration, analysis for metal content, and adjustment with additional solvent to the required metal content.
Direct Fusion Process
A metal oxide, carbonate or hydroxide is heated with the appropriate organic acid (R = acid radical); for example:
This reaction forms the neutral soap.
For some metals an important step is further reaction to form a basic soap of higher metal content and greater solubility.
Only fusion processes allow the production of basic soaps.
Precipitation Process
An organic acid is made into an alkali metal (sodium) soap in aqueous solution and an aqeuous solution of the metal salt is added. The reaction is carried in the presence of solvent and the metal carboxylate precipitated by the double decomposition reaction is dissolved. The water layer is then separated and the solvent solution is washed free of electrolytes and dired, for example:
Direct Metal Reaction (DMR) Process
US patent 2,584,051 describes the direct reaction of fine cobalt metal powder with organic acids. The process requires the presence of oxygen and water and is believed to proceed through the in situ production of cobalt hydrate (hydroxide).
The Organic Radical of Metallo-Organic Driers
The stoichiometric amount of metal that can combine with an organic carboxylic acid to give a metal soap is determined by the acid value of the organic component, the requirements of drier radicals are formation of metal soaps of complete solubility in all paint solvents, high metal concentration at low solution viscosities, and products of satisfactory stability.
Driers based on three types of carboxylates are used in the paint industry in Australia:
- refinery by-product base-naphthenic acid (naphthenates).
- oxo-process base—2-ethyl-hexoic acid (octoates).
- Koch-process—neo-decanoic acid (neo decanoates).
Note that, although linoleates (based on linseed oil) are used in printing inks, use in paints is not recommended.
A comparison of the acid values of commercial drier acids is shown in table 1. It follows that octoates (from 2-ethyl hexoic acid) can be made at higher metal concentrations thatn naphthenates. The descending order of availability of driers of high metal concentrations with low viscosity is octoates: neo-decanoates; nahthenates; and linoleates.
Naphthenates
Crude oils from Romania, the Caribbean and some states of the USA contain higher proportions of saturated alicylic hydrocarbons and are called ‘naphthenic base crudes.’ Still residues from these crudes contain considerable acid fractions, which can be removed by caustic refining: subsequent acidulation yields crude commerical naphthenic acid, vacuum distillation then yields refined naphthenic aid of a quality suitable for drier manufacture. By its source naphthenic acid is of variable composition, as it is a mixture of chemical species. Typical components of naphthenic acid are:
saturated monocrylic acid of general formula CnH2n-1 COOH with the carbon ring invariably five membered; a typical acid is:
saturated cyclic carboxylic acids of the form:
aliphatic carboxylic acids of the general form:
Commercial refined naphthenic acid contains an unsaponifiable fraction, essentially mineral oil, which carries through into naphthenate drier. The presence of this minor quantity of mineral oil, the variable composition of naphthenic acid, and its fluctuating availability gave impetus to the development of driers based on synthetic acids.
Synthetic acids have the advantages of less variable asay, improved availability, and chemically pure composition. The advantages expected of synthetic based driers for the above reasons have not yet been realized. Naphthenates persist, and recent pressure on the availability of naphthenic acid has caused naphthenate prices to command a premium relative to synthetics. The term ‘chemically pure naphthenate’ occasionally occurs; the meaning is ‘made only from naphthenic aid’, that is free from adulteration with linoleates and the like. By reasons of its lower acid value compared to synthetics the maximum metal content of a solid naphthenate (no solvent content) is lower than synthetic based driers.
The maximum metal contents for 100 per cent neutralized naphthenates made from a commercial 230 acid value, high assay, naphthenic acid are shown in table 2. Since the figures in the table are for 100 per cent solid materials it is obvious that a low viscosity solution of, say, lead of 32 per cent metal content contains materials other than naphthenate.
Octoates
The metal compounds of 2-ethyl hexoic acid are termd octoates, and they were the first of the synthetic-based driers developed. Because of the high acids value, this base allows high metal contents at low viscosity 2-ethyl hexoic acid is a water-white liquid of high purity which allows production of very pale coloured lead, calcium, zinc and zirconium driers. This advantage obviously does not apply to cobalt and manganese, where the metals contribute strong colours.
Neo-Decanoates
Where R1R2R3, are straight chain alkyl groups.
A typical product is Versatic acid from Shell Company. While commerically significant in the U.S.A. neo-decanoates have not achieved great popularity in Australia. Driers based on naphthenic—neo decanoic acid mixes are also sold in the USA, mainly for commercial rather than technical reasons.
Performance of Synthetic Based Driers Versus Naphthenates
Several technical studies of the efficiency of naphthenate driers compared to synthetic acid equivalent have concluded that the effect of the metal far outweights minor differences brought about by the organic base, but that some formulations do show better response for synthetic-based driers (particularly cobalt). It must be emphasized that this is a general conclusion. The paint technologist’s brief is to discern small differences, isolate their origin, and apply the findings. Note that differences may be found in specific formulations with regard to performance and stability.
Drier Recommendations
The technology of the use of driers is still an empirical art rather than a science, since different oils (and resins containing oil) respond to the catalytic activity of metals in different ways. Any rules that apply must be broad. After considering these, invidual metals will be examined.
Mixed Drier Systems are Normal for Air-dry Finishes
The primary driers (cob; It and manganese) functions as true catalysts with lowering of the energy of activation of the autoxidative process. The secondary driers (lead, calcium and zinc) function by modification of the polymerization processes of autoxidation accelerated by cobalt and manganese. A combination of drier metais is usually employed, for example:
- lead with cobalt and manganese;
- lead with cobalt, or
- calcium with cobalt and zinc.
Temperature has Considerable Effect on Drier Metal Activity
Some high temperature baking systems use single metal driers. Although iron in oleo-resinous can-coating is satisfactory, it has no value in room temperature drying. At temperatures below 2°C air drying times are much prolonged.
The Sequence of Drier Addition should be Considered
Coloured complexes of varying permanence can be formed when driers and some anti-skinning agents are mixed as will be seen below. This can be minimized if driers are fully incorporated before anti-skin is added. A new drier mix should be assessed in a clear vehicle as well as in the pigmented product.
Drier Requirements vary with Pigmentation
A lead chromate yellow enamel, for instance, does not generally have the same drier mix as a white or black version.
PRIMARY DRIERS
Cobalt
Cobalt has the highest level of activity in promoting autoxidation. It is referred to as a skin drier, top drier, on surface drier, since if used alone, cobalt will give rise to rapid surface drying with little or no through-dry. This effect and subsequent volume changes is the cause of ‘wrinkling’, a serious defect in normal air-dry enamels. Cobalt is normally used in conjunction with other drier metals of known through-drying properties. Such combinations give rise to films that harden through uniformly and hence do not suffer from a wrinkling defect even when applied as a heavy (thick) film.
Because of its high catalytic activity, cobalt is used at levels of only 0.02 to 0.08 per cent by mass of metal based on the vehicle solids, the formlator selects an optimum by experiment.
Cobalt drier has a strong red-violet colour and when used at the above levels, contributes a little colour to a clear varnish. However, if cobalt drier is mixed with gum turpentine, dipentene, or similar solvents prone to take up oxygen on storage, a strong green colour develops. This colour usually disappears from the dried coating, but will be more persistent if calcium is used with cobalt.
Cobalt is a transition metal, which can exist in two velence states and is known for its ability to from co-ordination complexes. If cobalt drier is added to some undiluted resins, profound changes in viscosity may occur, although this does not happen when cobalt drier is added to the prediluted resin. This is usually restricted to resins containing high molecular weight fractions, such as very high viscosity alkyds and copolymers. A similar effect occurs with calcium and zinc driers.
The wrinkling effect with coblat is turned to advantage in the production of alkyd-based baking wrinking enamels.
Manganese
In the above system of describing drier action, manganse is said to have surface drying activity with some through-drying properties. It has been proposed by Berry and Mueler that manganese favours a breakdown of oxygenated products, while cobalt promotes hydroperoxide formation. Further it is claimed taht manganese gives more rapid drying when the oil or polymer has passed through the induction stage of drying.
In air-dry coatings, manganese is used at 0.02 to 0.06 per cent metal content based on the vehicle solids. Manganese gives better low-temperature drying performance than cobalt, and films containing manganese do not suffer from wrinkling under high humidity conditions, as do films with cobalt alone. It is generally preferred to formulate white enamels with no manganese or very low manganese content; a pink or pink-yellow colour develops if higher manganese contents are used. Manganese also has the advantage that it does not cause baked films to over-harden or embrittle, as is the case if cobalt alone is used. Also in systems prone to skinning, such as urethane oils, manganese can be used with good results. Manganese drier is seldom used alone: cobalt is the usual primary drier; manganese is a very useful modifier.
Lead
Lead is known for its ‘through-dry’ properties and in air-dry coatings in is used at the rate of 0.5 to 1.0 per cent based on the vehicle solids. This is far in excess of that required if catalytic activity were the only mechanism for the drying action of lead. The ‘through-dry’ properties are ascribed to an ability to cause polymerization after the induction period of the drying process has begun. The different mechanisms proposed for various metals may explain synergistic drier activity.
Recent replacement of lead driers in the USA has given a slightly down-graded performance. In Australia, where ‘soluble’ lead was much earlier banned from house paint, lead driers are considered ‘insoluble’ and hence of low toxicity. For these reasons, the continued use of lead drier in air-dry enamels appears probable.
Lead drier in many alkyd systems causes a strong haze or even precipitation of lead phthalate. Incorporation of calcium drier in the mix at the rate of one-quarter of lead cocentration can control this problem. Although some authors claim this hazing reduces the gloss of enamel films, it apears to be of consequence only in clear enamels. Lead drier should be avoided in silver enamels pigmented with aluminium powder:-it causes leafing grades to rapidly loose leafing properties and non-leafing grades to floculate. White architectural enamesl formualted with lead drier may show discolration in atmospheres containing hydrogen sulphide. Notwithstanding the apparent limitations on the use of lead as a drier, it remains the major volume drier.
Calcium
Calcium is used as a lead substitute where lead-cobalt-manganese mixtures are replaced by calcium-cobalt-zincconium. Calcium is also used to control the blooming tendency of long-oil pentaerythritol alkyds under adverse drying conditions, although this may reduce the water spot resistance of the dried film. Calcium is used at 0.025 to 0.15 per cent metal on resin solids content. Calcium drier is sometimes used as a pigment grinding aid as it displays good pigments wetting properties. Calcium drier is also used to control lead precipitation with some alkyds.
Zinc
Zinc is a secondary drier, and its main function is to promote even through-hardening of cobalt-containing coatings. Zinc is often used in low-temperature baking enamles: normal drier levels give severe wrinkling unless zinc drier is added. Typically an addition of 2 per cent by volume of 4 per cent zinc drier to an air-dry refinish enamel converts it to low bake use.
Zinc is an excellent pigment grinding aid and it is much favoured for the grinding of carbon blacks. If incorporated in the grinding charge, it can control drier adsorption problems that may occur in black enamels. Zinc drier has a low order of fungicidal properties but may be described as fugistatic. This property is of value in timber treatments (particularly clears, where the pale colour is estermed).
Zirconium
Zirconium is a secondary drier that achieved prominence as a substitute for lead in lead-free drier mixes. It was previously claimed that zinconium was a lead substitute at metal concentrations as law as 0.02 per cent. The consensus now is that this over-stated prformance and metal contents of 0.06 to 0.15 per cent on resin solids show reasonable results; cobalt-zirconium-calcium mixes nearly equal cobalt-lead performance.
Iron
Iron has no function in air-drying finishes and it is used in oleo-resinous baking can linings and dark coloured baking enamels. Generally a baking temperature above 130°C is equired for iron to possess any catalytic activity. Iron drier has a very dark colour.
Rare Earth and Cerium
A mixture of hafnium and lanthanum octotags has been promoted as ‘rare earth’ drier. This product and cerium have no activity in air-dry coatings but have found some use in semi-oxidizing alkyd baking enamels. They have the advantage that in very thin films, such as can-coatings, their moderate catalytic activity allows the film to dry without undue yellowing. (The same formulations would show severe yellowing and embrittlement if a high activity catalyst such as cobalt were used.
Aluminium
A report by Love draws attention to an aluminium complex which improved film hardness and reduced the tendency to yellowing in the dark. Prior to this aluminium octoate was used as a thickener or ‘puffing agent’ in solvent systems, and as a flow control agent.
Vanadium
Vanadium drier has a blue colour and has been suggested for white roller coatings and clear overprint varnishes to reduce yellowing. The catalytic activity is much less than cobalt (although higher than cerium) and commerical acceptance appears limited.
Stability of Drying Performance on Storage
Naphthenate and octoate driers, with no tendency to autoxidize, overcame many of the defects of the early linolcate driers, but loss of drying properties (in enamesl pigmented with small particle size, high surface area organic pigments) can present a problem: carbon black is the most common cause of concern. The problem is caused by adsorption, and study of the adsorption isotherm indicates that the process is temperature dependent. The apperance of the problem in winter and loss of drying on low temperature storage is predictable. To minimize the problem an enamel grade of carbon black should be selected and attempts to induce jetness by over-pigmentation avoided. Incorporation of zinc or iron drier into the grinding stage in amounts related to the pigment surface area (with the aim of satisfying the surface adsorption before the oxidative catalysts are added) is usually successful. This technique applies to other organic pigments also.
Although adding zinc, calcium or iron drier as dispersants at the grinding stage is acceptable practice, the addition of cobalt or manganese primary drier to a grind is not recommended since viscosity increase or even gelation can occur (particularly if heat is developed). As an added safeguard against adsorption the so called ‘feeder driers’ can be used. These are highly basic soaps of lead or cobalt, which are largely insoluble in the enamel initially but slowly react with the acidity available from the enamel resin and convert to a soluble drier. Thus, in theory, the loss of drier through adsorption is balanced by the generation of additional drier. With careful formulation very good practical resutls can be obtained. Typical feeder driers are:
- lead vanadium hydroxy naphthenate: 1:25 to 2.5 per cent (mass/ mass) on resin solids,
- cobalt hydroxy naphthenate: 0.3 to 0.7 per cent (mass/mass) on resin solids, and
- cobalt hydroxy neo-decanoate: 0.2 to 0.5 per cent (mass/mass) on resin solids.
These are available under a variety of trade names.
A radioisotope tracer technique of measuring adsorption of paint driers by pigments has been developed by E.S. Matsui, who found that the adsorption rate of the various driers differed from one generic type of paint to another.
DRIER RELATED PAINT FILM DEFECTS
Durability and Colour Retention
The formation of a solid film from a liquid applied coating is a result of autoxidation. Continued oxidation of the solid film after drying is part of the process that leads of breakdown and failure of the film with time. Theoretically, it follows that excessive quantities of primary drier would favour continued oxidisation with premature film breakdown. In practice, film that contain double the normal recommended amount of primary drier do show enly slightly earlier onset of surface chalking with latter embrittlement (lessened flexibility). Similarly the yellowing of a paint film on ageing is related to the processes of oxidation, which is another reason for avoiding excess primary drier. Conversely, too low drier levels may cause loss of drying through adsorption after only short periods of storage. Raw material cost must also be considered.
Hazing and Blooming
‘hazing’ is the dulling or loss of gl;oss caused by microscopic surface irregulaties, and it is permanent. ‘Blooming’ is apparently similar but may be removed by washing. The temporary nature of blooming has been ascribed to the presence, in some instances, of minute crystals of ammonium sulfate forming on the coating surface. For not fully explained reasons, certain long-oil pentaerythritol alkyds suffer considerable ‘hazing’ and ‘blooming’. The surface irregularity of hazing has been associated with loss of volatiles from the film in the period when the surface has almost solidified in a rapidly drying film. As cobalt induces rapid surface dry, it has been blamed for the defect. Bloom can be eliminated by addition of zinc or calcium driers, which retard surface dry. This may allow more complete escape of volatiles prior to teh film solidifying and the film may then dry to a fully smooth, defect-free glossy surface.
Driers for use in Water based Systems
Oxidizing and semi-oxidizing oil-modified alkyd emulsions have been in limited use for over thirty years, the driers are dispersed in the solid alkyds prior to the emulsification process. More recently drying alkyds that become water-soluble on neutralization with an amine base have been used in industrial enamels, which require modified driers taht disperse fully in water. Such driers are essentially synthetic acid-based and are dissolved in a suitable cosolvent rather than a hydro-carbon. These driers are commerically available: usually cobalt and manganese only are offered, as lead and other metals do not have the required hydrolytic stability. Titanium chelates are also recommended as pH sensitive substitutes for conventional driers.
The Future of Drier Additives
Oxidizing and semi-oxidizing oils are obtained from the renewable resource of oil-seed crops, so they are independent of fossil hydrocarbon resource restrictions. This may ultimately justify continued use of drying oil based paints with a corresponding use of drier additives. Thus a water-soluble oil-based alkyd enmel represents a system with a reduced demand on hydrocarbon resources relative to some latex systems. Driers appeart to have a future, provided food and fuel demands for vegetable oil are also wet.
The search for an improved drier system continues as recent reports on metal acetylacetonates indicate. It is hapoed that future research will more clearly define the chemistry and mechanisms involved in the complex processes of catalyzed autoxidation.
Paint Additives
The major components of paint are:
- the prime pigment (white and/or coloured), which contributes to the opacity and colour of the coating and the extender pigment or pigments, which are selected by the paint formulator for many reasons, including the extension (or filling) of the film to control the texture of the coating.
- the binder of hold the pigment particles together, to effect adhesion of the coating to the substrate, and to control the desired film properties of the paint; and
- the solvent, which is the vilatile portion of the coating; it privides mobility of the system during manufacture and to control the rheology of the paint for optimum application properties. The solvent in standard decorative coatings is either water or hydrocarbon in nature; the binder is solvent-thinnable or water-thinnable.
In addition to these three major componets, numerous ingredients are used in the paint in relatively small quantities. These are called paint additives. The purpose or function of this minor but significant group of chemicals is to aid in the manufacture of the paint, to contribute to its stability in the package and to aid in the application, the quality, and appearance of the applied film. It is not unusual to have up to ten additives per formulation, each of which has a very definite function. Since most suppliers of additives do not disclose their chemical composition, addivites are generally definied by function rather than chemical type; throughout this chapter additives will be discussed by function. Where possible, however, chemical composition will be referred to in preference to trade names.
It should also be borne in mind that all additives, without exception, have an optimum use level and experimentation should be carried out to arrive at this level. And, because of the chemical nature of most additives and the reactivity of the components of the carefully engineered paint, additives generally have secondary effects. As such, they should be evaluated with care.
Additives in Solvent-Thinned Paints
Since paint driers from the largest single group of additives in this area, they have been treated seprately. The principal group of other additives is discussed below.
Wetting and Dispersing Agents
There two main purposes for wetting agents: improvement of the dispersion and its stabilization. All pigments, by their very nature, are difficult to dispense. The hydrophility of the pigments reduces the case of dispersibility in hydrophobic systems.
Owing to storage and transport, humidity forms a thin water skin around inorganic pigments. The molecular structure of organic pigments causes them to be generally hydrophobic. But they have a large specific surface, and consequently absorb water, and they also change to a hydrophilic character. During dispersion with a wetting agent, the water skin is removed allowing the polar groups to react with the pigment surface; the hydrophobic ‘tail’ is compatible with teh hydrophobic resin.
Suitable wetting agents for solvent thinned paints are classified as follows:
(a) anionic wetting agents
for example fatty alcohol sulfonates or sulfonated oils and alkyl solfonates; fatty, benzoic, crotonic and naphthenic acids, and their salts;
(b) cationc wetting agents
for example, salts of fatty amines and quaternary ammonium salts;
(c) electro-neutral wetting agents
These compounds have organic groups of about the same size as the anion and cation, for example, oleyl amino-oleate.
As mentioned, the correct proportion of wetting agent will stabilize the system and overcome pigment sedimentation and flocculation.
Sedimentation. Owing to different densities and particle sixese, some pigments and tend to settle, which in some cases results in very hand sediments that are difficult to redisperse. Low viscosity paints tend to settle more than high viscosity systems. Sedimentation is overcome by introducing a controlled degree of flocculation.
Flocculation. This is a loose accumulation of primary particle that can be readily destroyed by light sheat forces. Flocculation results in agglomerates of pigment particles. Undesired flocculation (as opposed to the controlled level required to overcome sedimenation) results in bigger agglomerates of pigment particles, which results in turn in decrease in gloss, syneresis, and decrease in hiding power.
For the proper functioning of the wetting agent, optimum quantities are required to achieve adequate dispersion and overcome sedimentation and undesired flocculation. Wetting agents should be evaluated to obtain this optimum level (usually 0.5 to 2 per cent based on pigment). If too much wetting agent is used, the non-polar components may come together to form a double layer which reverses the attempt to make the pigment hydrophobic, for example, it becomes hydrophilic once more. Such an event can lead to flocculation, which is generally recognized by a clear vehicle separation. A paint formualtion without a wetting agent can produce the following problems:
- high viscosity in the mill base;
- long dispersion times;
- flocculation of the paint;
- sedimentation;
- floating when pigment blends or universal stainers are used;
- loss of gloss in the film; and
- poor tinting strength.
Anti-settling, Anti-sag and Bodying Agents
These additives are also described as flow control thixotropes. To understand their function more easily figure 1 traces the posssible paths of viscosity change that can take place in a paint before, during, and after application.
Figure 1. Possible viscosity paths
Viscosity is the of shear stress to the rate of shear. In Newtonina fluids, viscosity is constant, despite variations in shear stress; however, these characteristics are usually exhibited only by pure compounds, which excludes paints.
If a fluid is stirred, brushed, or otherwise worked and becomes less viscous, it is referred to as thixotropic. A dilatnet fluid would exhibit the opposite effect: that is, the more vigorous the stirring, the stiffer the fluid becomes.
During storage, a drift in viscosity (generally upward) usually occurs; this tends to taper off with extended storage to some equilibrium value. Just before application, the paint is strongly agitated or stressed (as on a paint shaker) to achieve a uniform product and then stressed again when it is applied to the surface to be painted. After application, two fators affect the viscosity; the natural drying or setting action of the paint, which acts to convert the paint from a viscous liquid to a solid: and the recovery of the thixotropic viscosity which was largely lost owing to the imposition of strong shearing stresses during application. Only the first factor affects the flow properties if the paint is strictly Newtonian; when the paint is thixotropic, the factor of thixotropic recovery must be considered as a significant secondary effect.
Inspection of the graph indicates that recovery of thixotropic viscosity cna take many paths. If the initial recovery is rapid, the paint is said to have ‘falset body’, a special case of thixotropic behaviour. On the other hand, recovery can also be relatively slow. The paint chemist can choose the type of recovery he wants. Slow recovery is good for levelling and gloss, in which case sagging or flooding may become a problem. Conversely, rapid recovery immobilizes the flow so that running and flooding no longer pose difficulties, but brush markes may then become too pronounced and gloss reduction excessive. To resolve this dilemma, the paint chemist usually effects a compromise between the two extremes. Modern flow control additives yield thixotropic paints characterized by:
- immobilization of the pigment/ vehicle system when at rest, which provides pigment suspension (non-settling) and prevents flooding and floating; most importantly, uniform can appearance enhances product ‘saleability1, and the uniform consistency ensures proper application;
- ready flow during application for easy and enhanced brushability; and
- correct thixotropic recovery for the balanced control of sag with levelling so that gloss is not unduly affected.
Bearing in mind that settling and sagging are affected by the force of gravity (which is a very low shear stress), it is feasible that controlled pigment flocculation will create resistance to such by building up to loosely structured network, thus overcoming settling and sagging at a risk of poorer hiding power or of pigment sedimentation.
Functional additives for achieving structure can be classified as follows.
Aluminium Soaps
The use of aluminium soaps, particularly aluminium stearate, was a popular means of building structure into the paint without causing pigment flocculation. These soaps will ‘gel’ hydrocarbon solvents and a wide range of gel strengths is possible, depending on the solvent and the grade of aluminium sterate. Low gel strength stearates are characterized by aluminium contents of about 6 per cent while high gel strength stearates hae aluminium content greater than 9 per cent. The low gel types form gls with mineral spirits of a soft, slightly stringy character, whereas the high gel types form stiff, crumbly gels. The high gel strength tyeps usually result in paints with poor levelling properties. Care must be exercised when formulating with aluminium soaps to stabilize the resultant gels with a polar solvent such as glycol ether, otherwise the viscosity may rise on storage.
Hydrogenated Castor Oil (Triglyceride of 12-hydroxy Stearic Acid)
This product has a typical three-dimensional triglyceride structure and owes most of its functionality to the presence of the hydroxyl group on the fatty acid chian. When the particles are properly dispersed and solvated, and at sufficient concentration, the polar bonding between hydroxy groups provides the desired structural network in the paint. Thse products show sensitivity to heat in that seeding may develop in the paint if processing or storage temperatures approach 54°C. If this should happen, it can be overcome by redispersing the paint under lower temperature conditions.
Heat Stable Polyol Esters
These are fatty acid esters of a complex polyol, which are less effective than the hydrogenated castor oil products, but they are not subject to seeding.
Modified Clays
The clays are of the montmorillonite type, of which bentonite is one of the better known, although hectorite is also used as the base for one product. The clays, recovered and refined, can imbibe water and swell, so that a very small amount can form a solid gel with water. By chemical modification, they can exhibit similar characteristics in solvent systems, wherein they become useful as flow modifiers for paint. The modified clays also require proper wetting, swelling, dispersion and concentration to perform adequately. The manufacturers literature suggests they be pre-wet with alcohol in order to initiate the swelling action.
The modified clays are of particular interest since they are among the few materials which appear to work well in aromatic-or ester-solvent systems. All the modified clays are powders and, therefore, require dispersion in the paint system. They may be pre-dispersed in solvent and may be purchased as pastes in various solvents. These products develop a colloidal network in the paint system that leaves the pigment dispersion unaffected. It is necessary to employ conditions that will properly swell and solvate the material in order to obtain the properties. Since the bonding or joining of the chains is necessary, a minimum concentration is usually required, below which no activity will be evident.
Structure can also be confirmed through controlled pigment flocculation, which creates resistance to flow because the particle groups which are formed tend to build up into a loosely structured network. Pigment flocculation usually results in a lowering in the opacity of the paint system.
Levelling and flow can be evaluated by using Australian Strandards AS 1580 Test Method.
Anti-skinning Agents
Skinning in paints is generally related to the polymerization and oxidation tendencies of the vehicles or binders used in protective coatings that cause them to dry. Skinning is a troublesome phenomenon, that, paradoxically, is a companion to a desired property. Driers are added to ensure drying; in some of these formulations, drying not only occurs throughout the film, but rapid surface dry also occurs forming a superficial skin. A reduction in driers to eliminate this surface formation will only retard the necessary drying time.
All other things being equal, an enamel based on an oil highly susceptible to oxidation (one containing any predeminance of conjugted double bonds, as in tung oil) would be ore incined to skin than one based n a less reative oi, such as soybean. Similarly, highly polymerize3d alkyd would show greater skinning tendencies than one not processed so far. Driers, when used excessively, have a tendency to aggravate skinning. Any decrease in viscosity of a system lessens the skinning tenderncy, as does any reduction in vehicle solids. Among the fators that will produce skinning are:
- surface oxidation in the container;
- gelation by solvent loss; or
- combination of the above two to upset the colloidal balance of the product.
The best way to avoid skinning is by adding antioxidatants that will counteract the harmful oxidation effects without impairing the necessary and desired drying properties. Most tats and fatty oils contain natural antioxidants and skinning is not a problem as a rule. Where they are not present in sufficient amount, additives must be supplied.
An anti-skinning agnet is evaluated on the basis of several important criteria, in addition to its efficiency in preventing skinning; these are:
- degree of retardation of drying time;
- compatibility with paint and varnish systems;
- degree of discoloration of liquid varnish;
- degree of discoloration of dried film; and
- odour.
In addition, an anti-skinning agent should have no adverse effects on the viscosity or otehr rheological properties of the paint, either initially or during storage; nor should it react unfavourably on the gloss and general durability of the film. The relative efficiencies of various anti-skinning agents vary with the type of vehicle in which they are used.
Anti-skinning agents can be divided into two main classes:
- phenols, and
- oximes.
Another class of much less significance is the solvent type that acts as a dispersing agent for highly polymerized roducts and thus keeps the particles from gelling. Examples of commercial phenols are the cersols, guaiacols and ortho-isopropyl phenol. Examples of commercial oximes are butyraldoxime, methyl ethyl ketoxmie, and cyslohexanone oxime. Pine oil is an example of a possible solvent type.
Phenols
Effective antioxidants of this type are of the general formula:
The phenols in general would be too powerful if it were not possible to reduce the activity of the phenol group by chemically shielding it as is done with all of the products in this class that final use as anti-skinning agents. Thus the ortho substitution of the ring with a methoxy group, as in ortho isopropyl phenol, or with alkyl groups, reduces the activity to appractical level for paints and varnishes. Even with the hidering effect, the products are still fairly powerful antioxidants so that their use level msut be quite low, in order to avoid serious impairment of the drying of the paint. Levels of 0.05 to 0.2 per cent on the mass of paint are normally used; the lower level is more common.
The phenl anti-skinning agents find their major use in products subjected to severe conditions in terms of oxidative stability, or where products of low volatility are desirable or can be tolerated. A major use is in printing inks, where higher drier levels in heavy-bodies vegetable-il bases are common, and skinnig cna be severe. However, printing inks are applied in extremely him films compared to paint, which allows for both volatilization of the phenol and rapid air oxidation of the coating. A second major category for use is in coatings applied by a diping operation. In these instances the paint resids in an open tank for considerable lengths of time and is being continuously aerated as parts pass in and out of the tank. A high level of antioxidant property is required as is a minimum of vlatility so that anti-skinning agent is not lost by evaporation. In most instances, produts coated by this method are subjected to force drying or baking so that low volatility of the phenols is not a problem.
In general, the phenol type anti-skinning agnet is not recommended for trade-sales paint. Its high degree of activity and low volatility require a balancing that is a little tooo delicate to assure good drying of the coating on a reproducible batch basis; and a small erro in additio of material produces a profound effect upon the drying properties. A second problem of phenols is that reactive iron will form a complex with phenol that is of a brown colour. Furthermoe, phenols possess a pungent odour, which, in general, would make them commercially unacceptable.
Oximes
The oximes are by far the most commonly used-skinning agents in the paint industry. The structure of the oxime is characterized by the group —C=NOH. Commercial oximes are as follows:
The use level of the oxine anti-skinning agents is in the range of 0.1 to 0.4 per cent on total weight of paint. Their use is not as critical as the phenols and levels substantially above those suggested can be used without significant effect on drying time of the coating. The choice of product is also not critrical. However, the butyraldoxime generally has a slight margin of superior performance in oils and oleoresinous type systems, whereas the methyl ethyl ketoxime generally shows preferred performance in msut alkyds, epoxy esters and oil modified urethanes. The principal virtue of the cyclobexanone oxime is a bland odour, and it is definitely favoured for coatings that require low odour characteristics.
The oximes function as anti-skinning agents by several mechanisms. They are antioxidants and metal-complexing agents, and the two-liquid products are strong solvents. The function as a mild antioxidation need not be explained, but the function as a metal-complexing agent may be of interest. The metal complex formed is sufficiently strong to impair the activity of the drier metal, thus the oximes become anti-driers. However, the complex fomation is a reversible reaction, so that as the oxime evaporates from a paint film, the reaction is reversed and the drier metal is restored to full activity. There is some evidence that this complex formation is mildly beneficial in preventing loss of dry.
Skin formation in many paints starts with surface gels being formed by oxidation. Strong solvents retard the activity of these gels to coalesce and, therefore, retard skin formation. At one time, terpene solvents, such as dipentene and prine oil, were used to provide some anti-skinning benefit of this type. The liquid oximes, because of similar high solvency, have this secondary anti-skinning benefit in the paint.
The metal-complex formation is an essential part of the anti-skinning activity of the oximes, but it also presents complications. The formation of these complexes in certain vehicle systems can lead to colour formation. While the colour is usually figuitive, in that it disappears when the coating dries, it creates customer acceptance problems since most customers find it hard to believe that a deeply coloured varnish can dry to a pale clear finish. The major discolouration occurs withco bait driers.
The reasons for the colour the attributed to the formation of the metal complexes, but why it occurs with one type of vehicle and not another is not known. The methods by which the problem can be overcome or minimized have been arrived at by a combination of applying the theory of what is happening, with trial and error experimentation. A profound diference is colour intensity can be obtained by changing the order of addition of the materials. Much darker colours are obtained if the oxime anti-skinning agent is added before the drier. This observation, plus knowledge of drier mechanism, suggests taht a good rule is always to add driers before the anti skinning agent. Another approach is simply to change from one oxime to another. The fact that one is an oldoxime and the othrs are ketoximes suggests a diference in te nature of the metal complex, which would make a difference in colour. Unfortunately, there is no way to predict which tyep is better as it changes with vehicle formulation. A third approach is to include zinc as a part of the drier combination, which suppresses the colour formation considerably. In all probability the zinc complexes preferentially and forms a colourless complex.
Evaluation of Anti-skinning Agents
These can be effectively evaluated using Australian Standards As 1580 Test Method 203.1, Skin Formation.
Anti-flood and Anti-float Additives
Flooding is the separation of the pigments into layers in the film. It is caused by differences in the rate of settling of the various pigments used in surface coatings. Pigments of large particle size and high density, such as lead, titanium dioxide and iron oxide, tend to settle faster than the fine, low density pigments, for example carbon black and iron blues. This means that the top of the paint film has a different colour from the bottom.
The term ‘floating’ is sometimes used to mean the same thing, but it is more appropriately applied to the separation of pigments in the bulk dispersions or to mottling in a surface film. The latter phenomenon has been associated with the effect of Benard calls. Floating is also obseved as streaks or striations in the film, but these are conisdered to be distored Benard cells.
Flooding and floating occur under a variety of conditions and the various reasons, which are complex. While paint films dry, a strong movement occurs which becomes apparent as vortex-like currents. These are caused by variations in the surface tension and in temperature. Solvents evaporate at the surface of the paint film, leading to viscosity increases and a cooling of the surface zone, so that its density and surface tension rise, causing the denser material to sink into the paint film. The lower, solvent-rich layers of the film, have a lower density and surface tension, so they rise to the surface, where they spread and the solvent evaporates. The whole cycle may be repeated several times during drying. The rising, spreading, and sinking caused by the different specific gravities and surface tensions, produce local vortices which, according to the Helmholtz theory of current distribution, give rise to irregular hexagons, called Benard cells.
Flooding and floating can be influenced by the following:
- the size of the pigment particles as well as the density (e.g. phthalolocyanine blue is approximately one-fifth size of a titanium dioxide particle and has a density of 1.5 compared with 3.0 for titanium dioxide.
- variations in the pigment surfaces (e.g. phthalolocyanine blue is organophilic.);
- the solvent; almost no floatation has been observed in solvents systems; any such floatation has been attributed to gravitational effects; and
- humidity; this can also present itself as moisture content of the pigment.
Methods of Cure
Several methods of curing the problem are possible, but there is no universal cure and trial and error techniques should be adopted. Some suggested methods of cure are listed below.
- Conferring of structure on the system. This inhibits the mobility and can be effected by the incorporation of a suitable extender into the system, such as china caly or coated whiting. They cannot be used in high gloss systems.
- Incorporation of silicone oil. This may lead to ‘cissing’ on subsequent repainting.
- Additions of castor oil fatty acids or soya lecithin are sometimes effective. The level is very delicate and functions by controlled flocculation leading to an inbuilt structure in the system.
Recognizing Flooding and Floating
One of the easiest tests for recognizing these defects is the rub-out test. The paint to be tested is applied to a glass panel. After a short flash-off time, the still-wet paint is rubbed with the finger until it is almost dry. This breaks down the floes (which do not reform), because the viscosity increases progressively during the rubbing, which also mixes the paint thoroughly. If the paint floats, there is a difference in shade between the normal surface and the rubbed part.
Another quick test is to apply a spot of paitn abut 30 mm in diameter to a glass panel. The paint is dried in a horizontal position. If the paint floats, Bengard cells will be seen at the surface; if the paint floods, the top and bottom will be found to have different shades.
Mildew Inhibitors: Fungicides
Mildew, or mould growth, on a paint fim is both unsightly and destructive. It is a major source of customer complaint, not only for exterior house paints, but also for interior industrial maintenance paints, such as the types that might be used in food processing plants or other severe environments. The particular fungal organisms involved vary with environmental conditions, but for the most part, the presence of one or more in any given environment cannot be avoided. Since the components of the paint film, particularly the vehicle, offer a source of food, the spores that are carried to the surface of the film by the air find a fertile place for growth. Factors affecting growth are the components of the film, the substrate on which the film is coated, and the environmental conditions of moisture and temperature.
Cases of mould growth on painted surfaces occur under a wide range of climatic conditions throughout Australia, from the tropical humid araes in the north to the temperate regions in the south. This leads to considerable expense in redecoration, and it is, therefore, important to ensure that coatings used in areas susceptible to the growth of mould have a high degree of resistance to fungal attack.
Two methods are at present in sue for evaluating componds claimed to increase the fungus resistance of paint. The first is the Vicklund Manowitz (WM) test or one of its numerous modifications. After years of comparing the assessments of these laboratory tests with the actual performance in the field, most workers are now convinced that no correlation exists. The method is valuable for showing whether a compound is active as a fungicide or has some effect in inhibiting fungus growth, but even then some care is required, since some paint compositions that do not show fungus resistance in the VM test may exhibit it on exposure in the field, for example, some zinc oxide formulations. The second method is to expose test panels at sites where mould growth occurs; recommendations for fungicides in paint are largely based on the outcome of such tests. This is also an unsatisfactory procedure because in case of failure it does not give any indication of the reason.
It is clear that, if fungicides are to be used in a rational way, it is desirable to determine the stability of these compounds in paint formulations and under the conditions in which the paint will be used. Some years ago it was proposed to combine bio-assay techiniques with chemical analysis and to investigate systematically the stability of fungicides in paint films. The paints are brushed out on sheets of polyethylene terephthalate polyester film, which are then usually exposed under the following conditions:
- outdoors at Highett, Victoria,
- in a fog room at 20°C,
- in a constant temperature room at 20°C and 65 per cent relative humidity (RH),
- in a hot room at 38°C, 30 per cent relative humidity, and
- outdoors at Lae, New Guinea.
At Lae, panels of asbestos cement painted with same coatings are also exposed, and the mould growth is assessed at intervals. Results obtained under these exposure conditions give a good indication of the fungicidal or fungistatic value of the compound under investigation, that is, how long the fungicidal or fungistatic action is likely to persist, and under what conditions it should be used. As a consequence of this work by Hoffman, specific recommendations for various fungicides under a variety of environmental conditions are given, for example, indoors (cold climates); areas of high humidity; indoors (hot humid climates) outdoors (under cover, hot humid climates), and outdoors (exposed, hot humid disappointing for mildew resistance (but found to be satisfactory against spoilage in latex paints). The chlorinated phenols have about the same activity as the mercurials and the same comments apply.
Tetramethyl thiruam dissulfide has been found to be of little value in humid climates with about the same mould resistance as pentachlorphenol, suitable for general indoor application. Salicylanilide was found to be of little value for exterior application but was found to increase the fungus resistance indoors. Para-toluence sulfonamide was found to increase the fungus resistance of paints indoors and in protected areas outside. N-trichloromethyl thio-phthalimide was found to be very effective for both interior and exterior applications. 2, 3, 5, 6-tetrachloro-4-(methylsulfonyl) pyridine was found to be effective in increasing mould resistance in humid climates. It is also considered suitable for interior applications.
Legislation in Australia does not yet climate the use of mercury, despite the above findings it is probably that mercury is the most commonly used fungicide for paints. Commercially at a sufficient leel, mercury is effective in controlling fungal growth under most conditions.
Fungicides need to be evaluated with extreme care and over a very long time to ensure against a variety of potentially undesirable secondary effects.
Identification of Mildew
Several techniques are available. Some of which are complicated and requiring biological essay techniques. The two most common methods are the use of bleaching agent and optical magnification.
Hyprochlorite solution, such as common household bleach, will destroy the fungus and bleach its colour. This allows a convenient differentiation between mildew and dirt since the bleach will not affect the colour of inorganic dust particles. The bleach can remove some chemical stains, so what could be a chemical staining or discoloation problem could be erroneously attributed to mildew; however, optical magnification as low as ten power is usually sufficient to distinguish between mildew, dirt, and chemical staining.
Mildew Control
Fungicidal additives, such as those mentioned, are widely, used to control mildew growth on all types of paint and in various environments. However, the best of fungicides cannot do the expected job if a highly susceptible paint has been formulated. Conversely, a paint formulation based on a choice of raw materials that will produce the least favourable condition for mildew growth can take maximum advantate of the specific property to be contributed by the fungicide, and a highly mildew-resistant surface at minimal cost can be achieved.
Vehicles vary considerably as related to the ability of the fungus to utilize them as sources of food. Among common vehicles, raw vegetable oils, such as raw linseed oil, are much more susceptible than refined or bodies oils. Chemical modification of the oils also increases their resistance to attack, so that alkyds, epoxy, or urethane esters, and other reacted oils or fatty acids can show substantial resistance. Pigments also can influence resistance to mildew growth, and ones which contribute to a neutral or alkaline environment, such as zinc oxide and barium metaborate, are frequently included in formulations strictly for their merit in controlling such growth. Calcium carbonate as an extender pigment, particularly the natural calcium carbonate, can also produce advantages in mildew control. Control of mildew in prone areas is frequently effected by use of a mould resistant paint system. It is common in such cases to use wood primers and undercoats formulated on zinc oxide with enamel topcoats.
Latex Paint Additives
Latex paint systems are characterized by a dispersed phase which consists not only of the pigment, but also of a major part of all of the binder. By contrast, solvent systems have only the pigment, but also of a major part or all of the binder. By contrast, solvent systems have only the pigment in the dispersed phase. In latex paint systems, the pigments are solid materials, as is the binder, but it is susceptible to plastic flow during formation of the film. Film formation and binding of the pigments is, therefore, a physical and chemical process which imposes requirements for additives quite different from those encountered with hydrocarbon solvent systems. Nevertheless, there is a need for additives with functions similar to the preceding, although of different chemical types. The water-thinned paint additives discussed here refer to non-oxidizable binders.
Surface Active Agents
These fall into three general groups, anionic surfactants used primarily for pigment dispersions, non-ionic surfactants for stabilization of the total dispersed systems, and anti-foam agents used to prevent, reduce, or eliminate from formation in the paint during manufacture and application. Cationic surfactants are not commonly used.
Dispersing Agents (Anionic Surfactants)
The anionic surfactant is the major type of surface active agent used as a dispersent for pigments in aqueous solutions. Not only is the anionic essential for good dispersion, but it is also important in latex systems for optimizing hiding power, flow, and levelling. It also contributes to overcoming floating and pigment settling. It is usually the sodium or potassium salt of a compound that contains phosphate, sulfonate or arbonyl groups. Surfactants of this type have both strong hydrophobic and hydrophilic ends, which cause them to orientate readily at the pigment-water interface. By imparting a negative charge to the pigment particle they help maintain dispersions through the mutually repelling nature of like charges on each pigment particle they help maintain dispersions through the mutually repelling nature of like charges on each pigment particle. The amount of surfactant to be used is a function of the total surface area of the pigments used in the particular formula.
Each pigment in a latex system requires a certain amount of anionic surfactant to disperse it. For a given dispersing agent for a particular pigment, there is a range of effective concentrations that will stabilize the system.
At the lower end of this range is the fluidity point, which is defined as the concentration of dispersant needed to achieve fluidity in a mass of pigment dampened with water. The fluidity pint is the minimum amount of dispersant which must be used to deflocculate the pigment in the system. At the other end of the effective dispersant scale is the Concentration-Aggregation (CA) Value. This is the level or concentration of dispersant necessary to protect pigment particles from aggregation or flocculation under conditions promoting severe flocculation. Dispersant is increased until ionic additive fails to cause flocculation (or even gelation); the concentration of dispersant is the CA Value. This procedure has been used to determine directly the minimum and maximum dispersant levels desirable for any bled of pigmnets and extenders.
Excessive quantities of anionics must be avoided since they cause forming and also result in water sensitivity of the formulation, resulting in poor scrub resistance and poor durability. In practice, it has been found that the ideal level of dispersal is approximately 30 to 35 per cent of the CA Value.
Stabilizing Surfactants (Non-ionics)
The non-ionic surfactants are usually of the alkyl phenoxy ethanol types or the alkyd ethers. While these are usually spoken of as stabilizers, for the dispersed system they can function as pigment dispersants and frequently have a significant effect on colour acceptance of tinting colours. However, as dispersants for the white and extender pigments, they are not as effective as the anionics. The non-ionics do not ionize in the aqueous solution but solubilize through hydrogen bonding at the ether oxygens. As with the anionics, the oil-loving portion orients to the pigment particle. The hydrophilic portion is the oxygenated chain.
As a guide for the proper selection of non-ionic, the HLB System has been devised. This stands for Hydrophile-Lipophile Balance. IN this system, each surfactant is assigned a numerical value called its HLB. The HLB offers a measure of the relative size and influence of the hydrophobic portion of the molecule. If a 100 per cent hydrophil;ic non-ionic surfactant existed, it would be expected to have an HLB of 100. Instead, a factor of 1 / 5 is applied to give convenience in handling smaller numbers. Thus, a completely hydrophilic surfactant has an HLB of 20; a completely lipophilic one has a value 0. The HLB of a non-ionic surfactant is calculated by determining the weight percent of the oxyethylene groups and dividing by 5. A surfactant with an HLB of 10 has an equal balance of oil-loving and water-loving groups. Emulsion ingredients are also assigned HLB numbers. Such a number represents the HLB of a surfactant that would produce a stable emulsion when used with the given ingredients.
The fact taht some pigments are easier to disperse than others can be explained by reference to HLB. Each pigment has its own ‘Required HLB’, that is, there is a vaue of HLB in which regin it is most easily wetted.
Many latex paint problems can be traced to flocculation of one or more of the pigments. This can occur if a stainer or universal colourant is added to base white latex paint which is not correctly balanced in HLB.
Anti-form Agents
Foam can be present during filling and application of some solvent thinnable systems, in which case low levels of silicone anti-foam additivies can overcome most if not all of such problems. But foam, in the sense in which the term will be considered here, consists of a system containing gaseous and liquid phases, in which the former is rather initimately dispersed throughout the latter.
Various theories of the mechanics of foam inhibition have been advanced, none of which is universally accepted or applicable in every instance. These theories do indicate that an important relationship exists between the anti-foaming properties of a substance and teh respective surface tensions of the defoamer and foaming solution. Since other factors are involved, such as surface activity, solubility, viscosity and elasticity of the film surrouding the bubbles, it is feasible that these factors create a very complex interrelation, and, because of the complexity of foam control, general industrial practice is to approach each foaming problem by an empirical, trial and error method.
An anti-foam agents must suppress foam during manufacture, filling, tinting and application of the paint. It should not cause pigment flocculation, emulsion instability, or itself have poor compatibility so that ‘fish eyes’ or gel particles are apparent in the wet or dry film. It should not have adverse effect on tint colour acceptance, and it should retain its activity over a long period of time. Rules to aid selection of anti-foam agent are:
- the surface tension of the anti-foaming agent must be lower than that of the foaming solution;
- the solubility of the anti-foaming agent in the foaming solution must be low;
- the anti-foaming agent must be readily dispersible in the foaming solution;
- the anti-foaming agent must not react with the foaming solution;
- the anti-foaming agent should have a high spreading co-efficient;
- the anti-foaming agent must not leave a residue that is detrimental to the end product; and
- the anti-foaming agent must not leave odour or taste where this is important.
Common practice is to put half the anti-foam in during pigment dispersion to prevent foam at this point of manufacture. The remaining portion is usually added during letdown or thinning of the paint to provide better protection during filling of the cans and in final application of the paint. It is not uncommon to use two different products, since one may function best under the severe condition of dispension while another may function best under milder conditions and have greater permanence of action.
The type and quantity of anti-foam agent is not readily generalized since there are variations in performance among products from one paint formulation to another. Manufacturers literature should be consulted for specific recommendations, but generally about 0.5 per cent by volume is the maximum in the finished paint.
Evaluation of anti-foam agents for a particular paint is usually by a method that involves severe agitation and beating of air into the paint, followed by an observation as to how quickly the paint retunrs to original density. This type of test should be performed on freshly prepared paint and also on paint that has been aged, since many of the anti-foam agents lose activity on agening in the paints. Ageing may be accelerated by storing the paints at about 49°C for several months before re-evaluation.
In addition to observations on density, the film properties should be observed before and after ageing. Film defects are more readily seen by drawing down 40 mm wet films on glass and observing the surface appearance as well as pinholes. Paints should be roller coated to ascertain the amount of bubbling under the roller and the permanence of the bubbles of the bubbles formed.
Latex Thickening Agents
Thickening agents are used to provide proper paint consistency, prevent pigment settling, and to assist in applying an adequate thickness of paint. In addition to their use as viscosity control agents, thickeners are useful for good dispersion of pigments and for stabilization of the paint so that pigment coagulation and flocculation are prevented. Consequently they are commonly called protective colloids. All have differing effects on the rheology at high shear and must be evaluated accordingly, particularly with regard to roller spatter which is important in trade paints. The materials in use include: (a) proteins such as casein; (b) cellulose derivatives; (c) water soluble thermoplastic acrylic polymers; (d) alginates; and (e) polysaccharides.
These are all fiarly high molecular weight materials that dissolve by formation of hydration layers along their polymer chains. High water demand extenders and modified clays of the attapulgite and montmorillonite type will contribute to the thickening of the latex paint system, but they are most often used in conjuction with one of the aforementioned organic thickners.
The thickening agents have significant effects on properites other than the consistency of the paint and package stability. Among these are flow and levelling, colour acceptane and colour uniformity, water resistance, stain removal, and ease of application. They affect dispersion and deflocculation of the pigments and contribute t the stability of the system. This last point is most evident in the effect on freezc-thaw stability, which can be seriously impaired if an inadequate thickener, either in type or quantity, is used.
Protein thickeners, such as csein and alpha protein, were quite common in the early latex paints, but are not as comon new. Their advantages are fairly good flow and levelling with good thickening properties, and reasonably good water resistance, particularly in aged films. Their disadvantages are that they must be heated in the presence of alkali, preferably amonia, in order to achieve adequate solubility, their films tend to yellow with age, and they are highly susceptible to spoilage and putrefaction.
Among the water-soluble cellulose derivatives, hydroxyethyl cellulose and methyl cellulose are the products often used as thickening agents for paint. Both types are available at more than one visocisty grade; other properties also vary to some degree, as well as viscosity. The choice between the two chemical types is a matter of individual preference. In general, it is somewhat easier to make aqueou solutions with the hydroxyethyl cellulsoe and paints prepared with it show somewhat better acceptance of tint colours. On the other hand, methyl derivatives of cellulose give slightly better flow and levelling and are less subject to biological degradation.
The water-soluble acrylic polymers are used largely in acrylic latex paint systems. The polymers are available in three forms: the sodium or ammonium salts of polyacryli acids, and acidic emulsions that solubil;ize and thicken when alkaline. In general, these products have less viscosity build in the paint system than those based on cellulsoe derivatives; but provide better flow and levelling properties. The products are supplied in solution or emulsin form.
Practices in the use of thickening agents vary depending upon aspects such as euqipment and plant layout. As stated, the proteins must be pre-dissolved while the acrylates are available in solution form. Cellulosics may be prepared as separate solutions or added during the pigment dispersion phase of the paint manufacturing operation.
Preservatives
The basic nature of a latex paint is conductive to the growth of fungi and bacteria with the resultant hazard of degradation of some of the components of the paint and a loss of paint properties. Degradation of protein thickeners results in strongly offensive odours, while degradation of the cellulosics yields varying degrees of viscosity loss.
The direct causative factor is the enzyme produced by the microorganisms so as to change the materials present into a form useful as food to the organism. The enzyme is chemical material, and it is not readily destroyed once it is present in the paint, so that even if the imcro-organism is killed, the exnyme can persist and continue its destructive action. Despite contradictory opinions, experience shows that enzyme activity bears some quantitatie relationship between amont of enzymes present and amount of thickener degraded, so that it is possible to restore viscosity by adding more thickener, although in some instances the quantity required may be an impratical aswer to the problem. And the enzyme is usually quite specific to a food source, so that if the micro-organism has been killed and annot generate another enzyme, viscosity can be restored by adding a thickener of a different chemical type.
The best solution to the problem is to avoid it. If the contaminating organism is killed before it can generate a significant amount of enzyme, there will be no problem. Once again, experience indicates that the organism does not generate the enzyme until the particular source of food is present. Therefore, protection of the thickening agent solution, or any aqueous material contraining nutrient material is required. This means all aqueous raw materials, including the latex and residual materials of this type in pipe-lines, pumps and bottoms of tanks. Acceptable preservatives are varied in composition. Experience has shown that compounds at conpcentrations producing a complete kill in 4 hours have not resulted in practical commercial problems, whereas a 24-hours period is too long.
An intermediate ground probably exists, but it has not been well defined. Therefore, any product at a concentration that will produce a sterile system in 4 hours should be completely satisfactory. The phenylmercuric compounds and the alkyl mercury compounds can produce this condition at low concentration and low cost. Tributyltin oxide is also quite stasfactory, but ast substantially higher cost. The chlorinted phenols have also been used rather widely. In addition, l-3-chloroallyl)-3, 5, 7, triaza-1-azoniaadamantane chloride, as well as barium metaborate have been found effective in controlling bacterial degradation in latex paints.
The use of an adequate preservative at an effective concentration is a necessity. Production of latex paints should be regarded in a manner similar to food processing, in that every precaution should be taken to prevent contamination and to avoid conditions which would allow growth, or build-up of the micro-organisms. An excessive load of micro-organism contamination can over-whelm an otherwise satisfactorily protected paint and require an uncommonlylarge amount of preservative, with an obviously higher cost for protection.
Coalescing Aids
Film formation in latex paints is described as coalescence, which means to come together, to fuse or to blend. The latex particles do exactly this, through a plastic flow characteristic which is affected by their dispersion in water and by addities in the system. Coalescng aids are, in reality, temporary plasticizers, which promote the coalescence by increasing the amount of plastic flow. The type of materials used are eitehr-alcohols (such as butyl glycol), tributyl phosphate, pine oil, or other strong solvents. All of these have a degree of volatility so that within a period of time, which can be number of days, they are lost from the film so that the latex hardens to its required properties.
Stability of the paint must be tested, particularly when formulating with ether acetates, as with high pH some latex systems tend to hydrolyse liberating acetic acid, adversely affecting stability. A commercial ester-alcohol has been found to provide very effective low temperature coalescne and better colour development than the conventional glycol ethers and glycol ether acetates mentioned above.
The presence of water is also critical during coalescence. The latex particles must remain mobile so that they can move and pack in a manner that will permit a reasonably continuus film, a suddne removal of water will not permit a proximity of particles necessary for coalescence into a good film and many voids would exits. In extreme cases, latex particles could be so greatly separated that no film could form. Therefore, controlled loss of water is important.
Temperature has a significant effect on the film formation of latex paints, it is recommended that latex paints not be applied at temperatures less than 10°C as coalescence will be impaired. Coalescing aids and the recommended level for use with a particular latex should hence be evaluated at 10°C.
Wet-edge Extenders
Water is an intractable material because it is extremely difficult to modify its properties. Evaporation rate, boiling point, and other properties are only slightly modified by adding other materials. By contrast, control of such factors in hydrocarbon solvent systems is not only easy, but wide variations of properties are also possible.
The most critical factors affecting water removal are temperature, humidity, wind and substrate. Rapid removal of water by any of these means leaves the latex particles standed, so that they are unable to approach neighbour particles and merge to form a film. Control tools available to the formulator are the co-solvents such as the glycols, alcohols and ether-alcohols, which reduce the vapour pressure and therefore reduce evaporation rate.
The glycols are considered high boiling point liquids beause of their low vapour pressure, compared with that of water at any given temperature. It is interesting to note that at 20°C the vapour pressure of water is more than 100 times as great as that of propylene glycol—the most volatile of the glycols listed. This low volatility of the glycols lessens their tendency to evaporate and onsequently gives rise to the use of this class of additive as a wet-ede extender. Thickening agents and high water demand extender pigments can also affect this property.
The gyycols do not have sharp freezing points. Under normal conditions, ethylene glycol and the other members of the ethylene series super-cool rather than freese, propylene glycol and its homologs set to glass-like solids rather than freezing. The addition of water to a glycol yields a solution with a freezing point below that of water. Because the glycols, which are poor solvents for the latex particles, are readily soluble in water and retard its desorption from the latex particle, they are control agents for freeze-thaw resistance. Normally, glycol is used at about 10 kilograms per 400 litres of paint.
Latex particles will coalesce if allowed to become physically close to one another. The dispesed latex particle, with its dispesats and adsorbed water layer, remains physically separated from other particles, but if the protective water layer is removed, coalescence cna result. Many experiments have shown that quick freezing, so that the system becomes physicaly rigid, which prevents movement of particles, is less deleterious than slow freezing, which gradually robs the particle of its protective water layer. Modifiers that improve the plastic flow of the particle (i.e. coalescing aids) can aggravate the problem and must be carefully evaluated for this property. Consequently, although both coalescing aids and freeze-thaw stabilizers are slow boilers, the higher the level of coalescing aid required, the higher must also be the level of freeze-thaw stabilizer.
Freeze-thaw Stabilizers
If an emulsion paint freezes, ice crystals foam, causing partial coagulation. The slower the freezing, the larger are the ice crystals and, therefore, the greater is the damage. Generally, the higher the PVC of the paint the less will be the damage from freezing. Some emulsions are easily broken by freezing, while others show only a viscosity increase. The process is generally irreversible, damage ranges from visocisty increase to solidification. Anti-freeze addivities are usually incorpoated in latex paints to ovrcome this tendency since many latex paints are transported, stored, and used in climates that hae temperatures under 10°C over long periods. It is hence important to take formulatin precautions againt freezing if the perforance of the latex is to be maintained. Testing is carried out by comparing paints exposed in cycles of seven days at 0°C with controls stored at 25°C.
The glycols are usually the types of additives used to formulate for freeze-thaw resistance as mentioned above. The glycols, although they depress the freezing point somewhat, do not necessarily prevent freezing, but rather contribute to an orderly redistribution during thawing of the water-soluble materials upon which the dispersio of the system depends. Since the glycols are used as wet-edte exteders this normal use level generally is sufficient for them to function in addition as freeze-thaw additives.
Other Considerations
The major types of paint additives for decorative paints been discussed. This list should not be considered complete since there are many other types. For example, compounds used to protect the metal container against rust (with water-thinned paints) have not been discussed. Anti-rust compounds that may be considere here are sodium benzoate and sodium nitrate.
Many paint additivies are prefixed by ‘anti’, which suggests that many such additivies are problem solvers. Because some addivites are very reactive and there is not much detail available for the paint formulator to warn agaisnt possible problem areas, secondary problems do arise. Trade literature describes a commonly used flow control thixotrope as ‘a multi-purpose flow additive (which) has proven to be amazingly versatile in its applicability to diverse coating systems’. Again a surfactant is described in trade literature as ‘a combination of special purpose chemicals formulated and processed for use with a wide range of non-aqueous systems without undersirable side effects’. And finally, a commonly used anti-foam in the paint industry is described as being ‘based on a synthetic organic chemical with a unique structure taht is designed to give maximum anti-foam activity at low concentration’.
Secondary Effects
Bearing in mind the nature of the above description, it is not surprising that most additives have detrimental effects on other paint properties. Consequently, they need to be evaluated with this in mind and not incorporated beyond their optimum use level.
^ Top
Adhesives, Chemical, Drugs, Gums, Insecticides, Jute, Pe ...