The Complete Book on Adhesives, Glues & Resins Technology (with Process & Formulations) 2nd Revised Edition

Contents

          ADHESIVES
1. Glues of Animal Origin
Properties
Methods of Manufacture
Commercial Grades and Specifications
Methods of Analysis
Sampling
Procedure
Identification
Physical Measurements
Determination of Other Constituents
2. Fish Glues
Introduction
Manufacturing Process
Properties
Applications & Formulations
Rubber-to-Steel
Strawboard-to-Steel
Rubber-or Cork-to-Plywood
Paper-to-Steel
Straight Line Gluing
3. Animal Glues
Introduction
Chemical Composition
Manufacture of Animal Glues
Properties
Liquid Animal Glues
Formulation & Applications
Methods of Application
    4. Casein Glues and Adhesives
Introduction
Properties
Casein Blend Glues
Lime free Casein Adhesives
Applications
Casein Adhesives for Bonding Paper
Casein Adhesive for Binding Dissimilar Materials
5. Blood Albumen Glues
Introduction
Solubility Categories
Properties
Blood-Soybean Flour Combinations
Mold Resistance
Application
6. Amino Resin Adhesives
Introduction
Manufacturing Technology
Urea Adhesive for Plywood
Urea Adhesive for Particle Board
Spray Dried Melamine-formaldehyde Resins
Foundry Resin
Aniline-Formaldehyde Resin
Ø Represents benzene ring
Sulfonamide-Formaldehyde Resins
Applications
Adhesives for Hardwood Plywood
Sand Core Binder
Water Proof Corrugated Board
Compounding and Formulation
7. Cyanoacrylate Adhesives
Introduction
Bonding with Cyanoacrylates
Adhesive Properties
Applications
8. Epoxy Resin Adhesives
Introduction
    Chemistry
Epoxy Novolac Resins
Flexible Epoxy Resins
Epoxidized Olefins
Speciality Epoxy Resins & Derivatives
Epoxy Esters of Rosin
Epoxy Esters of Styrenated Rosin
Epoxy Esters of Disproportionated Rosin
Epoxy Novolac Esters
Epoxy Ester of Maleopimaric Acid
Compounding
Curing Agents
Diluents
Modifiers
Flexibilizers
Fillers
Accelerators
Speciality Additives
Manufacture of Adhesives
9. Phenolic Resin Adhesives
Introduction
Resole resin
Novalac Resins
Manufacture
Applications and Formulations
Contact Adhesives
Adhesive Compounding
Nitrile/Phenolic Contact Adhesives
Structural Adhesives
Vinyl/Phenolic
Epoxy/Phenolic
Hot Melt Adhesives
Hot Melt Vinyl Film to Wood Laminating Adhesives
Pressure Sensitive Adhesives (PSA)
10. Polychloroprene Resin Adhesives
Introduction
Types of Polychloroprene
          Applications and Formulations
Applications

11. Polyester Resin Adhesives
Introduction
Linear Polycarbonates
Polymerized Oils
Alkyd Resins
Unsaturated Polyester Adhesives
Adhesives for Flexible Printed Circuit
Allyl Ester Adhesives
12. Polyethyleneimine in Adhesives
Introduction
Applications
General Adhesives
Tie Coat Adhesives
13. Polysulfide Sealants and Adhesives
Introduction
Polysulfide Sealants
Chemistry
Compounding
Curing Agent
Retarder
Reinforcement
Adhesion Additives
Primers
Improved Heat Resistance
Applications
Adhesives from Polysulfide Liquid Polymer
Epoxy Resin Reactions
14. Resorcinolic Adhesives
Introduction
Resorcinol-Phenol Formaldehyde Resins
Modified Resorcinol Resins
Aspects of Adhesion Mechanism
Formulation of Glue Mixtures
Laminating
    15. Ethylene Copolymer Hot Melt Adhesives
Introduction
Crystallinity
Compatibility
Pressure Sensitive Tack
Hot Melt Adhesive Formulating
Book Binding Adhesives
Carton and Case Sealing Adhesives
Carpet Application
Shoe Adhesives
Pressure Sensitive Adhesives (PSA)
Furniture Adhesives
16. Furan Resin Adhesives
Introduction
17. Isocyanate Adhesives
Introduction
Advantages of Isocyanate Adhesives
Disadvantages of Isocyanates
Applications
Types and uses of Isocyanate based Adhesive System
18. Lignin Adhesives
Introduction
Formulations
19. Polyamide Adhesives
Introduction
          Class I: Thermoset Adhesives Containing Liquid                    
          Polyamide Curing Adhesives
Class II: Nylon-Epoxy Resins
Class III: Thermoplastic Hot Melt Polyamide Adhesives
Class IV: Thermoplastic-Thermoset Adhesives
20. Polyimide Adhesives
Introduction
Adhesive and Bonding Technology
Foam System
    21. Rosin Adhesives
Introduction
Applications
Formulations
Solvent Adhesives
Emulsion Adhesives
Hot Melt Adhesives
Methods of manufacture
22. Silicone Adhesives and Sealants
Introduction
Chemistry
Oxime silane
Properties
Rheological Characteristics
Thermal Stability
Weathering Characteristics
Adhesion Characteristics
Applications
Industrial
Construction
23. Tannin Adhesives
Introduction
Formulation
24. Terpene Based Adhesives
Introduction
Chemistry
Beta-pinene resins
Dipentene resins
Alpha-pinene resins
Physical characteristics of resins
Pressure sensitive adhesives
Hot melt adhesives
Analytical methods
Commercial resins and their uses
Commercial production
Applications in pressure sensitive adhesives
Applications in hot melt adhesives
    25. Starch Adhesives
Introduction
Unmodified Starches
High Strength Adhesive
Cheap Diluted Adhesive
Non-weather Proof Corrugated Board Adhesive
Water Resistant Corrugated Paper Box Adhesive
Final Mixture
Acid Modified or Thin Boiling Starch Adhesive
Oxidised Starch Adhesives
Dextrin Based Adhesives
Properties
26. Acrylic Adhesives and Sealants
Polymerization
Solution Polymerization
Properties of the product
Emulsion polymerization
Properties of the dispersion
Properties
Formulations and Applications
Adhesives to paper coated with PVDC
Delayed tack adhesive
Adhesives for Laminating
Laminating Plasticized PVC film to textiles
Laminating PVC film to particle board
Laminating plasticized PVC film to split leather
High temperature &pressure lamination
Flocking Adhesives
Building Adhesives
Adhesives for plasticized PVC floor tiles
Adhesives for ceramic tiles
Pressure-Sensitive Adhesives
Flame Resistant & Pressure Sensitive Adhesive
Acrylic Sealants
Aqueous Acrylic Sealants
Solvent-Based Acrylic Sealants
27. Pressure Sensitive Adhesives
Adhesive Strip for Antomotive Trim
          Eva-Trialkyl Cyanurate Copolymer Adhesive
Carboxylate Polymer Based Adhesives
Fumaric Diester Vinyl Acetate Polymer
28. Hot melt Adhesives
Introduction
Advantages
Disadvantage
Formulations
Ethylene-vinyl Acetate
Amorphous polypropylene and Petroleum Resin
Isopropenyltoluene Copolymers as Tackifiers
  Chlorinated Polyphenyl, Chlorinated
Polyisoprene and Nitroso Compound
Carpet Backing Formulation
Other Polyolefin Compositions
Amorphous Polyolefin and Styrene Butadiene
  Block Copolymers
a-Methylstyrene Tert Butyl Styreneolefin terpolymers
Alkoxystyrene-Acrylonitrile, Copolymers
Boric Acid as Viscosity Stabiliser in Ethylene-
Propylene Adhesives
Thermoplastic Polymer and Chelate of Aminoacetic
Acid
Coal Tar Pitch and Ethylene-Acrylic-Acid Copolymer
Water-Moistenable Vinyl Pyrrolidone-Vinylacetate
Product
          RESINS
1. Alkyd Resins
Introduction
Classification
Synthesis
Etherification
Addition reactions of unsaturated monobasic
fatty acids
          Addition reactions with other unsaturated alkyd      
          ingredients
Reactions during coating formation with drying
          alkyds
Reactions during coating formation in alkyd blends
Raw materials
Manufacture
Health and Safety
Quality Control and Specifications
Analysis
Calculations
Uses
Use of Alkyds in Trade-Sales Finishes
Methods of Analysis
Determination of Composition
Chemical Methods
Determination of Properties and Impurities
2. Acrylic Modified Alkyd Resins
Traffic paints
Industrial applications
Conclusion
3. Alkyd-Amino Combinations Based on Neem Oil
Aim of present investigation
Uses of oils in surface coatings
Neem oil
Alkyd resins
Amino resins
Experiments & Results
Preparation of alkyd resin
Alkyd resin preparation
Preparation of amino resin
Testing of performances of resin samples
Discussion
Analysis of neem oil
Preparation of alkyd from neem oil
Preparation of urea formaldehyde resin
Preparation of thiourea formaldehyde resin
Preparation of various samples (mixtures)
Performances of various resin samples
Scratch hardness
Conclusion
      4. Amino Resins
Introduction
Raw materials
Chemistry of resin formation
Typical resin formulations and techniques
Urea formaldehyde resins
High solids urea-formaldehyde adhesive resin
Protective coating resin with high mineral spirits
tolerance
Methylated urea formaldehyde textile resins
Urea-formaldehyde particle board adhesive
Melamine-formaldehyde resins
Butylated melamine protective coating resin
Chlorine resistant melamine resin
Trimethoxymethyl melamine
Hexamethoxymethyl melamine
Melamine resin molding powder
Melamine resin acid colloid
Control of the extent of the reaction
Free formaldehyde estimation
Viscosity tests
Solubility tests
Cure tests
Urea versus melamine resins
Package stability
Competitive product analysis
Chemical modification for water soluble products
Chemical modification for oil soluble products
Ethyleneurea
Methylated uron textile resins
Uron resins
Glyoxal resins
Miscellaneous resins
Amino resins in the paper industry
Formulations for regular and HE colloids
Toxicity
Methods of Analysis
Competitive Product Analysis
    5. Carbohydrate Modified Phenol-formaldehyde
Resins
Introduction
Research on Carbohydrate Modified Resins
Carbohydrate-Modified Base-Catalyzed PF resins
Bonding Veneer Panels
Bonding Flakeboard Panels
Carbohydrate-Modified PF Resins Cured at
  Neutral Conditions
Bonding Veneer Panels
Color of Bondline
Conclusions
6. Epoxy Resins
Introduction
Synthesis of Resin Intermediates
Cycloaliphatic epoxies
Epoxidized polyolefins
Epoxidised oils and fatty acid esters
Aliphatic-cycloaliphatic glycidyl type resins
Epoxy novolac resins
Resins from phenols other than bisphenol A
Resins from aliphatic polyols
Resins from long chain acids
Fluorinated epoxy resins
Epoxy resins from methylepichlorohydrin
Miscellaneous epoxy resins
Epoxy esters
Water borne epoxy resins and derivatives
Diluents and modifiers
Epoxide reactions and curing mechanisms
Curing of epoxy esters
7. Photographs of Plant & Machinery with Supplier’s Contact Details

Sample Chapters

 GLUES OF ANIMAL ORIGIN

Animal glues are essentially natural high-polymer proteins.

These organic colloids are derived from collagen which is the

protein constituent of animal hides, connective tissues, and

bones. There are two principal types of animal glues, hide and

bone, differing in the type of raw materials used. In both cases,

animal glue is obtained by hydrolysis of the collagen in the raw

material.

Animal glues find application in a wide range of industrial

uses. They are used in woodworking for such applications as

assembly, edge gluing, and laminating. In the paper industry,

they are used as sizing materials and as binders in paper coating,

and also for paper creping. Animal glues find wide use during

paper manufacture for the retention and recovery of paper fibers

and pigments.

The coated abrasive industry uses animal glues in the

manufacture of abrasive paper and cloth. Closely allied with the

coated abrasives is the use of animal glue in preparation of

compounds for coating wheels, discs, belts, etc.

Animal glues are widely used in the manufacture of gummed

papers and tapes and in paper and paperboard converting.

Animal glues and glue-based compounded products are used

in paper containers—set up and folding boxes, spiral and

convolute tube winding, and laminating. Applications in

bookbinding, magazine and catalogue production, and allied fields

include binding, casemaking, padding, looseleaf binders, and

various luggage and case covering applications.

Animal glues are employed as warp sizing, throwing, and dyeleveling

agents in the textile industry. They are used in the

match industry for match-head compositions. Other uses include

paper gaskets, cork compositions, rubber compounding,

compositions for printing, coating and graining rollers, mining,

ore refining, and metal plating.

Glue molecules consist of amino acids connected through

polypeptide linkages to form long-chain polymers of varying

molecular weights. In hot aqueous solution the glue molecules

take up random configurations of essentially linear form. A wide

range of molecular weights, varying from 20,000 to 250,000 have

been reported. Acidic and basic sites on the amino acid side

chains and terminal groups affect the interactions among the

protein molecules and water, and are believed to be responsible

for the gelation and rheological properties of animal glues.

Because of the presence of both acidic and basic functional

groups in the protein molecule, the molecules are amphoteric

and can bear either a positive or negative charge. Animal glues

can act either as acids or bases depending upon the pH in water

solution. In acidic solution, the protein molecules have an

overall positive charge and function as cations, in alkaline

solution the molecules are negatively charged and behave as

anions. The point where the net charge on the protein is zero

is known as the isoelectric point (IEP). The isoelectric point of

animal glues usually lies in the pH range of 4.5-5.6. Glues in

solution at pH values lower than their IEP have cationic

characteristics while they have anionic characteristics at pH

values above their IEP. Many properties of glue solutions, such

as viscosity, solubility, gel strength, and optical clarity, pass

through a maximum or minimum at this point.

Commercial animal glues are dry, hard, odorless materials

available in granular or pulverized form which vary in color from

light amber to brown. They may be stored indefinitely in the dry

form.

The density of dry animal glue is approximately 1.27 g/ml.

A moisture content range of 10-14% is considered normal for

the commercially dried product. Inorganic ash content, consisting

mainly of calcium salts, may vary from 2% to 6%. Hide glues

are generally neutral in water solution with a usual pH range of

6.5-7.5, and bone glues are slightly acidic with values in the pH

range of 5.5-6.5.

Animal glues are soluble only in water. They are insoluble

in oils, greases, alcohols, and other organic solvents. When

placed in cold water, the glue particles absorb water and swell

to form a spongy gel. When heated the particles dissolve to form

a solution. When the solution is cooled the glue forms an elastic

gel. This property is thermally reversible, and upon application

of heat the gel liquifies. The gelling or melting point of an animal

glue solution will vary from below room temperature to over 120ºF,

depending upon glue grade, concentration, and the presence of

modifiers.

Viscosity in solution and the gel-forming characteristic when

cooled are important properties of animal glues, especially in

adhesive and sizing or coating applications. These properties vary

with the degree of hydrolysis of the collagen precursor and have

a marked bearing on working properties. Animal glues are graded

as to viscosity (fluidity) and gel strength (stiffness of gel

formation) under standard conditions and are available in a wide

range of viscosities and gel strengths.

Animal glues are compatible with and may be modified by

such water soluble materials as glycerin, sorbitol, glycols, sugars,

syrups, and sulfonated oils to act as plasticizers and modify the

working properties of the glue. A degree of moisture resistance

and increase in the solution melting point of animal glues may

be imparted by the proper use of such materials as aldehyde

donors and metal salts.

Since they possess amphoteric properties, animal glues are

highly effective with suitable modification as colloidal flocculants

or suspending agents.

Methods of Manufacture

Both major types of animal glues are prepared by the

hydrolysis of collagen and differ mainly in the type of raw material

used and the manufacturing processes employed.

Hide glues are prepared by initially washing the raw material

with water, followed by curing in a calcium hydroxide (lime)

solution which conditions the collagen for subsequent glue

extraction by hydrolysis. The cured stock is then washed, treated

with dilute mineral acid, such as sulfuric, sulfurous, or

hydrochloric, for pH adjustment, followed by a water rinse. The

stock is then transferred to extraction kettles or tanks and is

heated with water to extract the glue. Several hot water

extractions are made until the glue is completely removed from

the stock.

Dilute glue solutions are filtered, concentrated by vacuum

evaporation and dried. The dry product is ground to the desired

particle size.

Bone glues are made from the collagen occurring in animal

bones. Green bone glues are prepared from fresh bones and

extracted bone glues from bones which have been degreased

prior to processing for glue.

Both types of bone glues are initially conditioned by cleansing

with water and/or dilute acid solutions. The glue is extracted

in pressure tanks with a series of steam and hot water

applications. The dilute glue solutions are filtered or centrifuged

to remove suspended particles and free grease, followed by

vacuum evaporation, drying, and grinding.

Animal glues contain preservatives added during

manufacture to provide adequate protection under conditions of

normal usage and may contain foam control agents, depending

upon the end use.

Commercial Grades and Specifications

Animal glues are graded according to standard methods

developed and adopted by the National Association of Glue

Manufacturers (NAGM). Grades are based on gel strength and

viscosity values.

It is common to market animal glue under brand names or

grade designations identified by the midpoint gram values shown

in Table 1 or by National Association of Glue Manufacturers’

grade number.

Table 2 lists the typical properties of hide and bone types

of glues.

Viscosity of animal glue solutions vary over a wide range,

depending upon grade, concentration, and temperature. Table 3

lists typical viscosity values at 140ºF for a range of dry glue grades

at various concentrations.

CASEIN GLUES AND ADHESIVES

Introduction

Casein is milk protein, obtained from skimmed milk by

precipitation with sulphuric, hydrochloric or lactic acid, to a pH

of about 4.5.100 kgs. of milk usually yields about 3 kgs. of casein.

The precipitated casein is filtered, washed thoroughly, ground

and screened to get 20 mesh or finer product for glue

manufacture. Commercial casein contains about 80-90 per cent

of protein, 1-4 per cent ash, 0-1-3 per cent of butter fat, 7-10

per cent moisture, 0-4 per cent lactose and 0-3 per cent acids

expressed as lactic acid. The composition and amounts of

impurities depend on, among other factors, the method of

manufacture. Reunet casein is not, as it is, suitable for glue

manufacture due to high ash content.

Properties

Casein is an off white powder. Its molecular weight is about

13000-19,000. It is insoluble in water at its isolectric point pH

4.6; the solubility increasing acidity or alkalinity, in the latter

it is more readily soluble. Fixed alkalies like sodium hydroxide,

remain in the glue line as sodium caseinate, a soluble salt while

calcium hydroxide on which water-resistant wood glues are

based, forms insoluble calcium caseinate in the glue line.

Similar insoluble caseinates are formed by zinc, chromium and

aluminium salts etc. Casein powder has a shelf life of above 1

year at 20ºC.

Casein adhesives are unsuitable for outdoor use although

they are more resistant to temperature changes and moisture than

other water-based adhesives. Resistance to dry heat up to 70ºC

is good, but under damp conditions the adhesives lose strength

and are subject to biodeterioration. Their resistance to organic

solvents is generally good. Casein adhesives are often compounded

with materials such as latex and dialdehyde starch to improve

durability. Strong alkaline nature of mixed casein adhesives often

affects the bonding of timber with high resin or oil content by virtue

of a saponification action on poorly wetting surface contamination.

Resultant bonds may be stronger than those obtained from

synthetic resins. Hard woods are subject to staining. Gap-filling

properties are good. Alkaline nature of casein glues precludes the

use of copper or aluminium mixing vessels.

Like animal glues, casein glues have fairly good bond strength

and those containing sodium hydroxide have even better

resistance to water than animal glues; also they recover their

original strength on redyeing. The fact that casein glues do not

gelatinise makes them much easier to handle. The most serious

drawback of casein for use in adhesive is the presence of fatty

matter which has a very adverse effect on the tensile strength

of joint made.

Classification of casein glues and adhesives:

Water-resistant Casein-lime Glue

Water-resistant casein glue sets to a gel as the result of a

slow chemical reaction, sodium caseinate gradually converted to

calcium caseinate. Some of the calcium hydroxide in the formula

has produced sodium hydroxide from a sodium salt also in the

formula, dissolving the casein. The chemicals are dry mixed with

casein as a ready mix powder and shipped to the user as a

complete prepared glue for dispersing in water.

Calcium hydroxide when present in excess, shortens the

working life but increases the water resistance of the glue line.

Addition of sodium silicate (silica: soda ratio 3:1) increases the

working life of the glue, at all levels of alkalinity. A simple

formula using lime and alkali is given below. It has good water

resistance, good working life about 7 hours.

Casein Blend Glues

Casein blends with blood are dry powder glues for cold

pressing plywood. The blood constituent in casein glue

contributes quick setting, thus reducing clamp time, and both

dry and wet strengths are improved. These glues are used in

the construction of flush doors, boxes, furniture and other wood

assembly work where dark colour imparted by the blood is

accepted. Blend glues may compromise mixtures of casein with

soyabean meal. This type of blend is a way to the reduction of

the cost.

Lime free Casein Adhesives

Casein solution not involving lime may be used as adhesives

for adherends other than wood. These are prepared by dissolving

casein with sodium salts, which provide a sufficiently medium

alkali to dissolve the casein. Commonly used sodium salts are

borax soda ash, trisodium phosphate, and others. Casein in

solution in strong alkalies, such as sodium hydroxide, and

ammonium hydroxide, also have the adhesive value. Organic

amines dissolve casein and there is some small use of alkyl

amines, ethanolamines and morpholine as the solvent for casein

adhesives. To give limefree casein adhesives some measure of

water resistance, formaldehyde or formaldehyde donor in the

form of resin or hexamethylene tetramine may be added. More

commonly, an oxide or salt of zinc, aluminium or chromium are

added to improve water resistance. A formulation is given below

used for plywood.

 

AMINO RESIN ADHESIVES

Introduction

Amino resins are the condensation products of amino

compound with aldehydes. The most common and widely used

amino compounds are urea and melamine where as formaldehyde

is almost always the aldehyde.

In a poly condensation reaction, a reactant of functionality

greater than two leads to branching and crosslinking. The

resultant three dimensional network can attain greater size

indefinitely, becoming insoluble and infusible. The over all

reaction of amino resin can be described in three stages. The

first stage is the reaction of amino compound and formaldehyde

to a form a methylol derivatives

RNH2+HCHO RNHCH2OH

Urea is tetrafunctional and melamine is hexafunctional.

Theoritically therefore, the initial reaction can lead to the

formation of a tetramethylol derivatives of urea or a hexamethylol

derivatives of melamine, of the ratio of formaldehyde to ammonia

is high enough for urea, formation of a methyl group slows

formation of another. These methylol derivatives are condensed

with the evolution of water of formaldehyde.

The properties of the adhesive intermediates are very much

dependent on the reaction condition. Molecular weight may vary

from a few hundred to a few thousand, with a wide distribution

of molecular size. Characteristic of commercial products are

solubility, viscosity, pH and concentration. The products are

available either in dry form or in aqueous solutions. Urea resin

adhesives are usually marketed in aqueous solution whereas

melamine resin adhesives are available in powder form.

Manufacturing Technology

Fig. 25.1 is a flow chart for the manufacture of amino resin

adhesives. All the commercial processes are batch type. The unit

operations are reflux and condensation, filtration and spray

drying. Because of the corrosiveness of formaldehyde and its

formic acid content, the reaction is usually carried out in a

stainless steel vessel. The reaction vessel is equipped with a

turbine agitator and reflux condenser and jacketed for heating

or cooling.

The order of addition to the reactor is formaldehyde, boric

acid and urea. When all the components are added to the reactor.

pH is adjusted to 7.0-7.8 and the charge is heated to 120ºC.

Disappearance of urea causes the pH to drop to about 4.0. The

reaction mixture is refluxed at atmospheric pressure for 2 hr.

Vacuum is applied and distillation is carried out under vacuum

of 28-29 in. Of mercury, until approximately 33 parts by weight

of water is removed. Then the system is shifted to total reflux

and cooled to about 30ºC. The pH is adjusted to 7.2-7.4 with

sodium hydroxide. The molar ratio: formaldehyde, urea, is

usually 1.75:1 to 2:1 for plywood adhesives.

board applications, to avoid the smell of formaldehyde in the final

product. General practice is about 1.3-1.5 moles of formaldehyde

per mole of urea. The pH is adjusted to 8.5 to 9.0 and the mixture

is heated to the boiling point under agitation. The solution is

refluxed for 40 min and cooled. The pH is then adjusted to

7.0-8.0 with a saturated solution of trisodium phosphate.

Adhesives for Hardwood Plywood

Plywood is an assembly of an odd number of layers of wood

(Veneer) joined together by means of an adhesive. The difference

between hard wood, plywood and soft wood plywood is that the

former has a ply of wood from the broad-leaf tree, e.g. oak,

walnut, maple etc. where as the veneers for softwood plywood

comes from coniferous trees. Hardwood plywood is generally used

for decorative purposes, softwood plywood for structural purposes.

The adhesive is applied by means of rubber covered rollers in

the glue spreaders. The coated veneers are alternated with the

uncoated veneer in the final assembly. Then the assembled

veneers are pressed in a hot press at approximately 90ºC and

150-300 psi pressure. Press time is about 5-7 min.

Sand Core Binder

Cores are projections of sand in the mould cavity for the

purpose of marking holes in the casting. After casting, the cores

are surrounded by metal and should be removed without

damaging the casting. Urea resins are capable for forming

mechanically strong cores.

PHENOLIC RESIN ADHESIVES

Introduction

Phenolic resins are the reaction products of phenol or

substituted phenols with formaldehyde. An unlimited variety of

resins are possible depending on (1) the choice of phenol (2) the

phenol: formaldehyde molar ratio (3) the type and amount of

catalyst used (4) the time and temperature of the reaction.

Resole resin

The active positions on the phenol molecule are the two

ortho and one para positions. When there is more than one mole

of phenol in the presence of an alkaline catalyst, resole is

formed. The amount of heat determines the final form of product,

e.g., whether the resin is of low viscosity, water soluble liquid

or a grindable solid. If the reaction is carried too far, the resole

can gel. Therefore the reaction is always conducted under

carefully controlled conditions of time, temperature, pH and mole

ratio of formaldehyde to phenol.

Novolac Resins

The reaction of one mole of phenol with less than one mole

of formaldehyde, under acid conditions, results in a novolac

resin. Novolac resin contains methylene links and are phenol

terminated. Methylol and methylene ester groups that are

present in resole resins are absent in novolac. Therefore, this

type of resin is incapable of further reaction without the addition

of more formaldehyde. This is accomplished by the addition of

hexamethylene tetramine, which is known as “hexa”. Hexa

makes the non-heat-reactive thermoplastic novolac capable of

reacting under heat to a cross linked advantage that novolac

resins have over resoles is that no water of reaction is evolved

during cure with hexa. Molecular weight of phenolic novolacs are

in the 500-900 range.

Manufacture

A typical phenolic resin is made by a batch process in a

jacketed stainless steel reaction kettle, equipped with anchor

type agitator and condenser. Molten phenol and formaldehyde

(37-40%) are charged into the kettle and agitation begin. For a

novolac, an acid catalyst is added and steam is introduced into

the jacket to heat the batch with atmospheric reflux. The reaction

is continued for 3-6 hrs at 100ºC. The reaction time is dependent

upon pH and phenol: formaldehyde mole ratio. Following the

reaction period, the batch is dehydrated under atmospheric

pressure and than vacuum. If the resin is to be solid in solution,

the solvent is slowly added to the molten resin in the still,

cooled by refluxing and discharged into drums. Most of the solid

resins discharged into pans are pulverized and blended with

hexa before packaging.

To make resole resin, an alkaline catalyst such as sodium

hydroxide is added to the phenol and formaldehyde before

heating the batch to 80-100ºC. Reaction times are generally 1-

3 hr. Since resole resin is capable of gelling in the still,

dehydration temperature is kept below 105ºC. By the application

of vacuum solid resoles are discharged into resin coolers. The

low molecular weight, water soluble resins are finished at as

low a temperature as possible, usually about 50ºC, whereas the

less reactive para-substituted resoles can be finished at

temperature as high as 120ºC.

Adhesive Compounding

There are two methods in general used for compounding

polychloroprene/phenolic adhesives. The first method involves

masticating the rubber on a two roll mill to reduce crystallinity

and improve solubility. The time of milling and degree of shear

are frequently used to control adhesive viscosity. The magnesium

oxide and zinc oxide are compounded into the rubber on an

unheated mill. The magnesium oxide is always added before zinc

oxide to preclude premature curing of the rubber. The antioxidant

is also added. The compounded rubber is then dissolved with

the resin in the solvent blend in a cement tub.

Compounders who do not have milling equipment use the

slurry method for adhesive preparation. This method consists

of simply adding of resin, pigments and antioxidants together

with the unmilled rubber to the solvent blend in the cement tub.

Adhesives made from unmilled rubber will be more viscous and

therefore, are usually produced at lower solid content. They will

also have higher initial cohesive strength. Adhesives from milled

polymer are however uniform and retain their uniformity upon

ageing.

Vinyl/Phenolic

Vinyl formal, vinyl acetal, and vinyl butyral may be combined

with phenolic resin to produce tough structural adhesives with

good impact strength, resistance to oil and aromatic fuels and

good salt spray and weathering resistance. The presence of

hydroxyl groups on the vinyl chain makes it likely that

crosslinking occurs between the phenolic resole resin and

hydroxyl groups during elevated temperature cure.

 

ACRYLIC ADHESIVES AND SEALANTS

POLYMERIZATION

All industrial polymerization processes are carried out at an

elevated temperature in the presence of an initiator.

Polymerization can be carried out in bulk, solution, suspension,

or emulsion. The most important processes for producing acrylics

for adhesives-solution and emulsion polymerization are dealt

with here.

SOLUTION POLYMERIZATION

In solution polymerization, the monomer or monomer

mixture is dissolved in a solvent which is relatively inert to free

radicals, e.g., ethyl or butyl acetate, benzene, toluene, petroleum

solvent of ketones; then polymerization is effected at elevated

temperatures in the presence of an initiator such as an organic

peroxide or an azo compound which is soluble in the solvent.

Properties of the product

The type of solvent used has a great influence on the reaction

speed and the molecular weight of the resulting polymer in

solution, because of the different chain transfer activities of the

various solvents. Thus, the viscosity of a polymethyl acrylate

solution, and the molecular weight of the polymer, decreases in

the following order: benzene, ethyl acetate, ethylene dicholoride,

butyl acetate, methyl isobutyl ketone, and toluene. The solvent

with the highest chain transfer activity gives polymers of lowest

molecular weight. The molecular weights of solution polymers

are normally lower than those of emulsion polymers. In selecting

the solvent to be used, consideration must also be given to the

economic and safety aspects.

Emulsion polymerization

The emulsion polymerization, process for homo-and

copolymerization of acrylic compounds is of greater significant

than the solution polymerization process. It is effected in the

presence of emulsifiers and initiators (e.g. alkali persulfaces),

normally in water as the external phase. Suitable emulsifiers

are, for example, alkali salts of longchain aliphatic carboxylic or

sulfonic acids, of sulfated ethylene oxide adducts.

PROPERTIES

At room temperature, the homopolymers of methacrylic and

acrylic acid as well as those of the lower methacrylic acid esters

are hard, nontacky products which are suitable only for special

applications in the adhesives field. Homopolymers of acrylic acid

esters from alcohols with at least 2 carbon atoms, which are

elastic, soft, and partially highly tacky products, are used for a

much larger range of adhesive applications.

Further physical properties of the most important acrylic

homopolymers in comparison to those of polyvinyl acetate are

given in Table 1 (properties increasing in direction of arrow).

FORMULATIONS AND APPLICATIONS

Adhesives for paper converting

The requirements imposed on adhesives for bonding paperto

paper are normally not too severe. Because the absorptivity

and surface condition of paper, animal and vegetable adhesives

can attain satisfactory wetting and encourage and thus yield

adequate bonding strength.

However, since the requirements imposed on the bonding

speed-have increased steadily, they can no longer be met with

adhesives based on natural products. Thus, it was not possible

to design and use modern automatic paper converting machines

until it was discovered that polymer dispersions based on

polyvinyl acetate are suitable for this application. By modifying

these homopolymeric polyvinyl acetate dispersions by adding, for

instance, plasticizers, solvents and resins, it was even possible

to render these adhesives suitable for bonding coated and

lacquered paper and to a certain extent also for bonding paper

to polymer films. The range of application for dispersions

modified in this way is, however, limited by the fact that

plasticizer migration may adversely affect the adhesion and/or

bonded materials. The demand for raw materials and adhesives

with improved specific adhesion therefore increased with the

improvements of the materials used in the paper converting

industry, such as printing, lacquering, application of water vapor

impermeable coatings on paper and board, and the use of polymer

films.

Because of their high specific adhesion to a great variety of

surfaces, polyacrylic acid esters in the form of aqueous

dispersions and organic solutions were found suitable for the

production of adhesives for surfaces which are difficult to bond.

The products in question are either copolymers of acrylic acid

esters with one another, or, especially in the case of packaging

adhesives, copolymers of acrylic acid esters with vinyl propionate

or vinyl acetate i.e. copolymers of vinyl acetate with acrylic acid

esters. Terpolymers produced from acrylic acid, acrylic acid ester,

and vinyl acetate have increased adhesion to metal foils and

various plastics and are therefore used for producing adhesives

for this field of application. Terpolymers of this type are also

very resistant to plasticizer migration, e.g., from plasticized PVC

film.

Flame Resistant & Pressure Sensitive Adhesive

For some applications it may be necessary to use a flame

resistant pressure-sensitive adhesive. Acrylics can be rendered

flame resistant.

Pressure-sensitive acrylic adhesives can normally be applied

by the conventional methods, e.g., direct or reverse roll coating

or by air-knife coating. The adhesive compound is applied either

directly to the final substrate or a release paper. In the latter

case the adhesive on the carrier is dried or crosslinked before

it is transferred to the final substrate. This transfer method is

necessary in those case where the backing material would

deteriorate during the drying or crosslinking process. The transfer

method is, for instance, commonly used for producing decorative

films.

For protecting the adhesive coat of pressure sensitive

adhesive materials during transport and storage, the adhesive

coat except in the case of adhesive tapes in rolls, is usually

covered by a release paper or a release foil, Silicone treated

paper, polyethylene or PVC films are, for instance, suitable for

this purpose. Long-chained acrylates exhibit good release effects.

The release material must not have any adverse effect on the

pressure-sensitive adhesive coating. Undesirable effects can be

obtained when unsuitable silicones are used or when the

silicones are not properly processed.

 

Acrylic Sealants

Linseed oil and bitumen were for a long time the commonest

base materials for building sealants. Further developments in

building construction and the steadily increasing demands on

quality resulted in the development of a number of synthetic

polymers for sealants. The first work on acrylics for building

sealants was carried out in response to the technical and

economic success which was achieved by this product class in

the last 10-15 years in the field of surface coatings, such as

paints. The first serviceable acrylic compounds were placed on

the market in about 1960. In the meantime the acrylics,

particularly the aqueous acrylic dispersions, gained considerable

significance in the production of sealants because of their

outstanding aging resistance and adhesion properties as well

as their favourable price.

It is expected that the increase of the consumption of

aqueous acrylic sealants will be above average in the next few

years. The total consumption of sealants will increase only be

approximately 20% during the same period.

The acrylics used nowadays for sealants are tailor-made

copolymers of acrylic and/or methacrylic acid esters and other

monomers.

Usually several monomers are present in order to achieve

the desired properties, such as elasticity, adhesion, resistance

to UV radiation, resistance to chemicals, and hardness, suitable

polymers are linear polymers, most of which are thermoplastic,

as well as polymers which can be rendered adequately elastic

by cold vulcanization, oxidation, or by the effect of alkaline

substance, such as caustic soda solution, cement, or lime.

Acrylics which can be crosslinked with the aid of oxidative

catalysts or epoxy resins are also well known. The dominating

raw materials for acrylic sealants are based on aqueous acrylic

dispersions.

Solvent-containing products and solvent-free products are

also available on the market. The solvent-free products have

been on the market only for a short time and the experience

gained with them is still inadequate. The solvent-containing

products have been on the market for a long time but they gained

considerable less significance than the acrylic dispersions.

Solvent-containing products are usually 80-90% solutions in

xylene. In the initial stage aqueous dispersions were available

with a solids content of only 50-55%. Dispersions with a higher

solids content, were obtained by improving the polymerization

technique. All acrylics must be modified with fillers and other

aids in order to achieve optimum properties.

Aqueous Acrylic Sealants

Aqueous acrylic sealants are employed mainly for those

applications for which compounds bases on linseed oil, butyl

rubber, or polyisobutylene have been used hitherto, i.e., the

sealing of joints which are subject to little elongation. Viz joints

between curtain walls and door and window frame joints.

In view of the experience gained hitherto, it appears that

the aqueous acrylic sealants are also suitable for joints between

prefabricated concrete building components with a practical

elongation of approximately 10-15%.

Soft acrylic sealants with a high degree of elongation have

already been successfully used for many years for joints between

small building components and special applications, e.g., aerated

concrete.

Even the results obtained hitherto in the trials for expansion

joints, which were commenced some time ago, have been positive

until now. Practice will show whether aqueous acrylic sealants

are in fact suitable for this application.

Harder compounds are mainly used for do-it-yourself

application and for sanitary equipment, e.g., bathtub and

washbasins.

AMINO RESINS

Introduction

Amino resin are manufactured throughout the industrialised

world to provide a wide variety of useful products. Adhesives (qv),

representing the largest single market, are used to make plywood,

chipboard, and sawdust board. Other types are used to make

laminated wood beams, parquet flooring, and for furniture

assembly. Some amino resins are used as additives to modify

the properties of other materials. For example, a small amount

of amino resin added to textile fabric imparts the familiar washand-

wear qualities to shirts and dresses. Automobile tires are

strengthened by amino resins which improve the adhesion of

rubber to tire cord. A racing sailboat may have a better change

to win because the sails of polyester have been treated with an

amino resin. Amino resins can improve the strength of paper

even when it is wet. Molding compounds based on amino resins

are used for parts of electrical devices, bottle and jar caps,

molded plastic dinnerware, and buttons.

Amino resins are also often used for the cure of other

resins such as alkyds and reactive acrylic polymers. These

polymer systems may contain 5-50% of the amino resin and are

commonly used in the flexible backings found on carpets and

draperies, as well as in protective surface coatings, particularly

the durable baked enamels of appliances, automobiles, etc. The

term amino resin is usually applied to the board class of

materials regardless of application, whereas the term aminoplast

or sometimes amino plastic is more commonly applied to

thermosetting molding compounds based on amino resins. Amino

plastics and resins have been in use for the past fifty years.

Compared to other segments of the plastics industry, they are

mature products, and their growth rate is now only about half

of that of the plastics industry as a whole.

Most amino resins are based on the reaction of formaldehyde

with urea or melamine. Although formadehyde combines with

many other amines, amides, or amino triazines to give useful

products, only a few have found commercial utility, and they are

of minor importance compared to the major products based on

urea and melamine. Benzoyuanamine, e.g., is used is amino

resins for coatings because it provides excellent resistance to

laundry detergent, a definite advantage in coatings for automatic

washing machines, dihydroxyethyleneurea is used for making

amino resins that provide wash-and-wear properties to clothing.

Aniline-formaldehyde resins were formerly important because of

their excellent electrical properties, but have been supplanted

by newer thermoplastics. Nevertheless, some aniline resins are

still used as modifiers for other resins. Acrylamide occupies a

unique position in the amino resin field since it not only contains

a formaldehyde-reactive site but also a polymerisable double

bond. Thus it forms a bridge between the formaldehyde

condensation polymers and the versatile vinyl polymers and

copolymers.

Formaldehyde links two molecules together and is hence

diffunctional. Each amino group has two replaceable hydrogens

that can react with formaldehyde and thus is also difunctional.

Since urea and melamine, the amino compounds commonly used

for making amino resins, contain two and three amino groups,

they react polyfunctionally with formaldehyde to form threedimensional,

cross-linked polymers. Compounds with a single

amino group, such as aniline or toluenesulfonamide, can react

with formaldehyde to form only linear polymer chains.

This is true under mild conditions, but in the presence of

an acid catalyst a higher temperatures, the aromatic ring of

aniline, e.g., may react with formaldehyde to produce a crosslinked

polymer. The use of thiourea improved gloss and water

resistance, but stained the steel molds. As amino resins

technology progressed the amount of thiourea the formulation

could be reduced and finally eliminated altogether.

Melamine resins were introduced about ten years after

molding compound. They were very similar to those based on

urea but had superior qualities. Melamine resins rapidly

supplanted urea resins and were soon used in molding,

laminating, and bonding formulations, as well as for textile and

paper treatments. The remarkable stability of the symmetrical

triazine ring made these products resistant to chemical change

once the resin had been cured to the insoluble, crosslinked

state. Future markets for amino resins and plastics appear to

be secure because they provide unusual qualities. New

developments will probably occur in the areas of more highly

specialine materials for treating textiles, paper, etc, and for use

with other resins in the formulation of surface coatings where

a small amount of an amino resin can significantly increase the

value of the basic material. Looking further into the future, the

fact that amino resins are largely based on nitrogen may put

them into a position to compete with other plastics as raw

materials based on fossil fuels become more costly.

Raw materials

Urea

Urea (carbamide) is the most important building block for

amino resins because urea-formaldehyde is the largest selling

amino resins, and urea is the raw material for melamine, the

amino compound used in the next largest selling type of amino

resin. Urea is also used to make a variety of other amino

compounds, such as ethyleneureas, and other cyclic derivatives

used for amino resins for treating textiles. They are discussed

below:

Urea is soluble in water, and the crystalline solid is some

what hygroscopic, tending to cake when exposed to a humid

atmosphere. For this reason , urea is frequently pelletised or

prilled (formed into little beads) to avoid caking and making it

easy to handle. Only about 10% of the total ureas production

is used for amino resins, which thus appear to have a secure

source of low-cost raw materials. Urea is made by the reaction

of carbon dioxide and ammonia at high temperature and

pressure to yield a mixture of urea and ammonium carbamate;

the latter is recycled.

CO2 + 2HN3 → NH2CONH2 + H2O = H2NCOONH4

Melamine

Melamine (cyanurrotriamide, 2,4,6-triamino-s-triazine) is a

white crystalline solid, melting at approximately 350ºC with

vaporisation, only slightly soluble in water, commercial product,

recrystallised grade, is at least 99% pure. Melamine was

systhesised early in the development of organic chemistry, but

it remained of theoretical interest until it was found to be a

useful constituent of amino resins. Melamine was first made

commercially from dicyandiamine but is now made from urea, a

much cheaper starting material. The urea is dehydrated to

cyanamide which trimerises to melamine in an atmosphere of

ammonia to suppress the formation of deamination products.

The ammonium carbamate also formed in recycled and converted

urea. For this reason the manufacture of melamine is usually

integrated with much larger facilities with much larger facilities

making ammonia and urea. Since melamine resins are derived

from urea, they are more costly and are therefore restricted to

applications requiring superior performance. Essentially all of

the melamine produced is used for making amino resins and

plastics.

Formaldehyde

Pure formaldehyde is a colorless, pungent smelling reactive

gas. The commercial product is handled either as solid polymer

paraformaldehyde, or in aqueous or alcoholic solutions. Marketed

under the trade name Formcel, solution is methanol, n-butanol,

and isobutanol, are widely used for making alcohol-modified urea

and melamine resins for surface coatings and treating textiles.

Aqueous formaldehyde, known as formalin, is usually 37 wt %

formaldehyde, though more concentrated solutions are available.

Formalin is the general-purpose formaldehyde of commerce

supplied unstabilised or methonol-stabilised. The latter may be

stored at room temperature without precipitation of solid

formaldehyde polymers because it contains 5-10% of methyl

alcohol. The uninhibited type must be maintained at a

temperature of at least 32ºC to prevent the separation of solid

formaldehyde polymers. Large quantities are often supplied in

more concentrated solutions. Formalin at 44, 50, or even 56%

may be used to reduce shipping costs and improve manufacturing

efficiency. Heated storage tanks must be used. For example,

formalin containing 50% formaldehyde must be kept at a

temperature of 55ºC to avoid precipitaton. Formaldehyde

solutions stabilised with urea are used and various other

stabilisers have been proposed. With urea-stabilised

formaldehyde the user only adjust the U/F (urea/formaldehyde)

ratio by adding more urea to produce a urea resin solution ready

for use.

Paraformaldehyde is a mixture of polyoxymethylene glycols,

HO (CH2O)n H, with n from 8 to as much as 100. It is

commercially available as a powder (95%) and a flake (91%). The

remainder is a mixture of water and methanol. Paraformaldehyde

is an unstable polymer that easily regenerates form-aldehyde

in solution. Under alkaline conditions, the chains depolymerize

from the ends, whereas in acid solution the chains are randomly

cleaved. Paraformaldehyde is often used when the presence of

a large amount of water should be avoided as in the preparation

of alkylated amino resins for coatings. Formaldehyde may also

exist in the form of the cyclic trimer trioxane. This is a fairly

stable compound that does not easily release formaldehyde,

hence it is not used as a source of formaldehyde for making

amino resins. Approximately 25% of the formaldehyde produced

in India is used in the manufacture of amino resins and plastics.

Other materials

Benzoguanamine and acetoguanamine may be used in place

of melamine to achieve greater solubility inorganic solvents and

greater chemical resistance. Aniline and toluenesulfonamide

react with formaldehyde to form thermoplastic resins. They are

not used alone, but rather as plasticizers for other resins

including melamine and urea-formaldehyde. The plasticizer may

be made separately or formed in situ during preparation of the

primary resins.

Water borne epoxy resins and derivatives

Electrodeposition is an important new technique for coating

metals, and water-borne epoxy ester-based vehicles are among

the leading coating systems thus applied. The coatings are used

as corrosion-resistant primers for automobiles, appliances, and

electrical parts. An early approach involved maleinising epoxylinsed

fatty acid ester (effecting a Diels-Alder condensation,

between maleic anhydride and a fourcarbon conjugated

unsaturated segment of the fatty acid), then dissolving in butyl

cellosolve and subsequently neutralising 80% of the composition

by ammonia solution or tertiary amine.

R-COOH + NH3 → R-COO- + NH4

+

Maleated resin Resin anion

The resin anion is deposited on an anode under an applied

dc voltage of 100 volts or more. The coating is then cured by

baking at elevated temperatures. The epoxy ester can be made

water-soluble by esterifying unreacted hydroxyl groups with

phthalic acid and thus preparing a phthalic acid half ester of

specific acid number. Other approaches include the use of dimer

acid and versatic acid. This is still an expanding field, and further

advances are expected.

Emulsions of epoxy resins themselves are generating

interest because of their ecological advantages. Coatings applied

from water-based epoxies reduce hazards from fire, toxicity,

pollution etc. A bisphenol resin modified with a reactive diluent,

and containing emulsifier, is easily emulsified just before use

in a high-speed agitator with gradual addition of water. The

coatings based on this resin are cured with polyamide emulsions.

Among suitable emulsifying agents for a bis-epi resins are the

derivatives of nonylphenol and ethylene oxide.

Diluents and modifiers

Many applications have requirements for viscosity, flexibility,

impact resistance, adhesion, pot-life, cost etc., which can be met

by the use of diluents and various modifiers.

Diluents

These liquids are used primarily to reduce the viscosity of

the epoxy resin system. They may be nonreactive. The

nonreactive diluents may be volatile organic solvents or

nonvolatile plasticizers. Solvents are used to obtain deep

penetration in such applications as prepreg laminating and

filament winding. Ketones, esters, and glycol ethers are true

solvents for epoxy resins; but aromatic hydrocarbons and

alcohols are sufficiently compatible to function as diluents. Some

of the commercial medium-viscosity (2000-4000 cps) resins

contain dibutyl phthalates as a nonreactive diluent. Viscosity

reduction of 70-80% is obtained by the use of about 15 phr of

dibutyl phthalate. The products obtained from such resins are

generally softer, less brittle and have less solvent resistant than

products based on unmodified or 100% reactive resins. Pine oil

was suggested as a nonreactive diluent.

Reactive diluents are those that may take part in the curing

reaction and become an integral part of the crosslinked system.

The reactive sites on these may be either epoxides or other

functional groups. Monoepoxide diluents include butyl glycidyl

ether, cresyl glycidyl ether, phenyl glycidyl ether, styrene oxide.

The first two are highly efficient diluents providing very great

reduction of viscosity in small amounts. About 12 p butyl glycidyl

ether per 100 standard liquid resin brings the viscosity down

from more than 10,000 cps to about 500-700 cps at 25°C. Since

monoepoxides reduce crosslinking, some of the properties of the

cured resin, such as water resistance, flexural strength, and

heat distortion temperature, are somewhat lowered. To overcome

this disadvantage, diepoxides may be used as diluents, e.g., 1,4-

butanediol diglycidyl ether, bis (2,3-epoxy cyclopentyl) ether. The

nonepoxy type reactive diluents include triphenyl phosphite and

butyrolactone. Details of reactive diluents are shown in Table

6.

Flexibilisers

The bis-epi type resins, epoxy nonvolacs, and other epoxy

resins containing aromatic ring structures, when cured with the

usual amines, or anhydrides, give products that are hard and

brittle, with rather low impact resistance and poor elongation.

Flexibilisers are employed to improve the impact resistance and

increase the elongation of the cured products. Some improvement

in these properties may be achieved with plasticizers such as

dibutyl phthalate, but only at the expense of gross reduction in

other properties such as solvent resistance. In epoxy technology

the term flexibiliser generally refers to those compounds that

undergo reaction and impart flexibility to the system by

increasing the distance between the crosslinks, interposing

segments with greater free rotation. Among favoured categories

of reactive flexibilisers are the aliphatic diepoxides, the

polysulfide telomers, and the amido-amine crosslinking agents

discussed later. The flexible aliphatic epoxy resins, when used

alone give soft-cured compositions having low physical strength.

They are best utilised in blends with the bisphenol-A based

epoxy resins. Incorporation of about 10-30% of flexible resins

retains most of the desirable properties of unmodified system

while improving the impact resistance and elongation.

Improvement in the flexibility of the modified system will also

depend on the type and chain length of the flexibilising resin,

the ether type and long-chain resins being more effective than

the ester type and shorter chain resins. Some epoxy are based

on dimerised C18 fatty acid, polyurethane diglycidyl ethers and

cycloaliphatic diglycidyl ethers. The diglycidyl ethers with

urethane linkages provide tough products with excellent impact

resistance at temperatures as low as 6-55°C. The glycidyl groups

of the cycloaliphatic resins react readily with polyamines at

ambient temperatures.

The polysulfides of commercial significance in epoxy resin

technology are Thiokol’s LP liquid polymers, which are essentially

mercaptan terminated poly (ethyl formaldisulfide):

The various grades of polysulfide polymers differ in amount

of branching and in molecular weight, which may range from 600

to 7500. The liquid polymers most often used with epoxy resins

are low molecular weight polymers with approximate molecular

weight of 1000. These polymers flexibilise epoxy resins by

extending chain length.

This reaction proceeds very slowly at room temperature, and

no useful products are obtained unless curing agents such as

amines or anhydrides are used. The polysulfide-epoxy resin

compounds are usually formulated as two component systems

to give good shelf life and permit easy handling. In most

applications, for every 100 parts of epoxy resin, about 75 parts

of polysulfide liquid polymer is used with about 10 parts of

amine curing agent, commonly a tertiary amine such as 2,4,6-

tris (dimethylaminomethyl) phenol. Aliphatic amines are not

compatible with polysulfides and tend to settle out.

Bituminous modifiers

Coal tar-modified epoxy coatings are used for pipes, tanks,

machinery foundations, and boats, because of their outstanding

resistance to acids, alkalies, and brine. A 50/50 mixture of a

low molecular weight epoxy resin and coal tar pitch, incorporating

7.5 parts of diethylene triamine per hundred parts of blends,

cured to a corrosion-resistant, rubbery product within 24 hours.

Such mixtures also can be cured with amine adducts and

polyamides. When added to flexible epoxy resins, coal tar

provides better elongation without reduction of tensile strength.

These resins can be compounded with asphalt; addition of 30

phr aromatic distillate results in elongation in excess of 300%

even at -18°C.

Synthetic polymers as modifiers

Various thermoplastic and thermosetting polymers, including

elastomers, have been incorporated to modify the properties of

the cured epoxy resin products. A nylon soluble in ethanol-water

mixture, is used in epoxy-nylon film adhesives to obtain high

peel strength as well as good heat resistance. The nylon can be

a major or minor component in the blend. Room temperature

peel strength usually increases with increasing amount of

polyamide, but with the sacrifice of high temperature resistance.

Excessive deformation under high temperature curing can be

reduced by blending with high-temperature-melting nylon

particles of uniform fine size. A thermoplastic polyurethanemodified

epoxy resin has been developed which is reported to

give better peel strength at cryogenic temperatures than that

obtained with epoxy-nylon.

Polyvinyl formal and polyvinyl acetals show good compatibility

with epoxy resins and improve peel strength of adhesives

Polyvinyl formal improves impact resistance of powder coatings

remarkably when used at 35-100 phr level with silica and BF3-

amine complex. Tough powder coatings are claimed by blending

with irradiated polyethylene. Among the thermosetting resins,

phenolics have long been blended to obtain heat resistance in

adhesives and chemical resistance in coatings. Xyleneformaldehyde

resins are useful in formulating epoxy casting

systems. Butylated amino resins are used to crosslink high

molecular weight epoxy resins or epoxy esters to obtain light

colored, chemical-resistant coatings. Solid epoxy resins modified

with hydroxy-functional silicone intermediates yield reaction

products with free terminal epoxy groups available for further

reaction with fatty acids or common curing agents. The siliconeepoxy-

based products have good heat stability, chemical and

moisture resistance as well as good electrical properties making

them suitable for protective coatings, laminates, and molding

materials.

Elastomers provide greater elongation and impact strength.

Polysulfides, the most commonly used elastomer to flexibilise

epoxy resins, have been discussed already. Epoxy-chloroprene

(neoprene) rubber blends have been cured with polyphenols or

aromatic amines to give tough, chemically resistant products.

Epoxy-nitrile rubber blends yield high-peel-strength adhesives.

Carboxyl-terminated nitrile rubbers, introduced by Goodrich,

were shown to toughen the cured epoxy resin at only 5 phr

loading. Improvements in impact have been obtained by the

addition of 5-35% by weight of carboxyl terminated nitrile rubber

to cycloaliphatic resins.

Fillers, reinforcements, and other additives

Incorporation of fillers and reinforcements into epoxy

formulations can result in higher viscosity, longer pot life, lower

exotherm and lower shrinkage. Properties of the cured polymers

may be improved. Above all, use of fibrous fillers may lower the

cost of the formulations. For mechanical strength, asbestos,

glass, graphite and boron fibers are used. Glass fibers the most

common reinforcement, not only increases tensile, flexural, and

impact strength, but also raised heat resistance.and reduces

shrinkage and thermal expansion. Graphite and boron fibers, very

high in modulus and thermal tensile strength, are used for high

performance aerospace application where strength-weight

characteristics are critical. Coated graphitised carbon fibers of

391,000 psi have been incorporated in 60 volume % in epoxy

resins.