11 EPOXY RESINS AND CURING

AGENTS
A. Adomenas, K. Curran and M. Falconer-Flint

11.1 INTRODUCTION
In the plastics industry epoxy resins are classified as thermosetting resins, and they are used
in the paint industry as convertible coatings. Epoxy resins are converted to a thermoset state
by chemical reaction between the resin and a curing agent. Depending on the curing agent
this reaction can take place at elevated temperatures or at room temperature. The cured
resins are not soluble in solvents and cannot be melted by heating. This property is in direct
contrast to thermoplastic products such as polyvinyl chloride (PVC) or polystyrene, or
non-convertible coatings such as chlorinated rubber, acrylic or nitrocellulose lacquers, which
remain soluble in solvents and can be remelted by heating.
Epoxy resins became commercially available in Australia in the early 1950s, and since that
time have become firmly established in many industries. Most commonly used types are
based on epichlorhydrin and diphenylol propane (Bisphenol A) and are available in a range
of molecular weights. The low molecular weight resins are liquid; the high molecular weight
resins are solid.
What are the main properties of the epoxy resin-based systems that influence the choice
of epoxy resins from a wide range of plastics and resins available at present? The important
properties are listed below.
(a) The chemical structure of epoxy resins gives them high chemical resistance against a
wide range of severe corrosive conditions. These properties are derived from the
aromatic nature of the backbone and good chemical stability of the phenolic ether
linkage.
(b) Epoxy resins have good adhesion to a wide range of materials, including metals, wood,
concrete, glass, ceramic and many plastics. This is due to the presence of polar hydroxyl
and ether groups in the resin.
(c) Low shrinkage during curing results in good dimensional accuracy in construction of
structural items and enables manufacture of high-strength adhesives with a glue line of
low residual stress.
(d) Complicated shapes can be reproduced easily using liquid epoxy resins systems, which
can be cured at room temperatures.
(e) Good physical properties such as toughness, flexibility and abrasion resistance can be
obtained.
(f) Although there are temperature limitations, epoxy resins generally perform better than
most thermoplastics at elevated temperatures.
One of the main users of epoxy resins is the paint industry, which produces special types
of surface coatings (discussed in further detail later). The electrical industry was one of the

179
P. Parsons et al. (eds.), Surface Coatings
© Chapman & Hall 1993
180 SURFACE COATINGS

first industries in which epoxy resins became established during the early stages of
commercial production. Epoxy resins are convenient to use in solvent-free liquid form,
which sets into a hard infusible solid after the addition of curing agent. They are used
extensively-for example, for potting, embedding or encapsulation of electrical components,
for cable joining to make waterproof joints, and for manufacture of Telecom terminal
pillars.
Another industry to recognize the value of liquid epoxy resins was the engineering
industry. Low shrinkage and good dimensional stability in service are important properties
utilized for manufacture of foundry patterns, vacuum-forming moulds, press tools for
prototype or short runs, drilling jigs, and checking fixtures. Adhesives are used in many
applications in place of soldering, bolts or rivets, particularly in small-parts assembly and in
aircraft construction. Fibreglass-reinforced plastics are manufactured from epoxy resins for
applications where chemical resistance and good physical strength properties are the main
requirements-for example, piping and storage vessels in the chemical industry. In the civil
engineering industry the use of epoxy resins has also become established as a standard
practice. Epoxy adhesives cure more rapidly than cement, and good adhesion to new and
old concrete enables more rapid construction and repair of concrete structures.

11.2 EPOXY RESIN MANUFACTURE AND CHARACTERIZATION

By far the most important class of epoxy resins used at present commercially on a large scale
is that based upon the reaction between diphenylol propane (DPP) and epichlorhydrin (ECH)
in the presence of alkali. The basic reactions are shown below:

o CH3

2Cl-CH,-cH -.::cH, + HO-©--f-©-oH~ (a)

CH3

OH CH3 OR
Cl-CH,-Ci'i -CH,-o--©--{--@-o-CH,-bI-CH,Cl + 2NaOH~ (bl
CHa

(c)

The basic reaction is formation under alkaline conditions of a chlorhydrin ether of DPP (b)
followed by dehydrochlorination of the chlorhydrin group by alkali to form an epoxy group,
thus giving the diglycidyl ether of DPP (c). An excess of ECH will favour a high proportion
of the simple diglycidyl ether of DPP; higher DPP ratios will give higher molecular weight
polymers. Commercial grades of resin can be represented by the following formula:
 EPOXY RESINS AND CURING AGENTS 181

The pure diglycidyl ether of DPP with n = 0 is a crystalline solid. The commercial
low-viscosity liquid resins are rich in this compound, and the average n is approximately 0.2.
Viscosity can be further reduced by addition of monoepoxide compounds such as aliphatic
glycidyl ethers. In low melting point solid resins, average n is approximately 2. The common
commercial high molecular weight resins have average n up to 13. There are also very high
molecular weight resins with molecular weights up to 200 000; these resins have mainly
hydroxy groups and practically no epoxy groups and are used as thermoplastic resins for
non-convertible coatings.
The commercial epoxy resins are characterized by specifying their main properties, as in
sections 11.2.1 to 11.2.3.
11.2.1 Epoxide group content (EGC), epoxy equivalent weight (EEW) or epoxy molar
mass (EMM)
EGC is express~d as millimoles of epoxide groups per 1 kilogram of resin (m mol/kg). It
can also be expressed as epoxy molar mass (EMM), in which case the unit is grams per mole
of epoxide (g/mol). Conversion to EGC from EMM can be done by the following formula:

1000 x 1000
EGC(m moVkg) = EMM(glmol)
EMM is numerically identical to EEW (epoxy equivalent weight).
11.2.2 Hydroxyl content
This is expressed as millimoles of hydroxyl groups per 1 kilogram of resin (m mol/kg). For
esterification purposes, one epoxy group is equal to two hydroxyl groups, and this must be
taken into account when calculating esterification equivalents.
11.2.3 Viscosity
The commonly used metric viscosity unit is the pascal second (Pa.s), which has replaced the
poise unit (1 poise = 0.1 Pa.s). Viscosity is normally measured at 25 C on liquid low-viscosity
0

resins without dilution. A 70 percent solution is used for high-viscosity liquid resins, and a
40 percent solution is commonly used for solid resins in solvents such as glycol ethers.
Properties of a typical range of epoxy resins are shown in table 11.1.
Selection of the resin grade depends on the application. Low molecular weight resins are
rich in epoxy groups and are used for applications where the crosslinking reaction is through
epoxy groups. These are usually two-component systems which cure at room temperature.
In high molecular weight resins, hydroxyl groups are predominant, and reactions involving
both hydroxyl and epoxy groups are selected for further polymerization and crosslinking.
These systems require elevated temperatures for crosslinking.

11.3 EPOXY RESIN-BASED SURFACE COATINGS

Coatings consume 45-50 percent of the epoxy resin produced throughout the world. Epoxy
resins are reactive with a number of different types of curing agents and yield a wide variety
of products with different cure requirements and end-use performance. These coatings can
be classified as follows:
(a) two-pack room-temperature-cured marine and maintenance coatings;
(b) epoxy fatty acid esters which are used as single-pack room-temperature-cured coatings
or as baking primers and enamels;
(c) high-temperature-cured container coatings;
182 SURFACE COATINGS

TABLE 11.1
Properties of a typical range of epoxy resins

Approx.
EEW Approx. Typical
Approx. or EMM EGC viscosity Solids*
Resin grade n g/mol m mOl/kg range Pa.s % mass
1. Low-viscosity liquid resins 0.2 200 5000 0.6-1.8 100
modified with monoepoxide
diluents
2. Low-viscosity basic resins 0.2 200 5000 8.0-16.0 100
3. High-viscosity resins 0.5 250 4000 0.4-1.0 70
4. Solid resins:
Durrans melting point approx. 70 C
0
2 500 2000 0.1-0.2 40
Durrans melting point approx. 100 C0
4 900 1100 0.5-1.0 40
Durrans melting point approx. 130 C0
9 1700 600 2.0-3.5 40
Durrans melting point approx. 150 C0
13 2700 370 4.0-12.0 40
* in diethylene glycol mono butyl ether

(d) powder coatings; and

(e) water-based epoxy coatings (see chapter 12).

11.3.1 Two-pack marine and maintenance coatings

Marine and maintenance coatings are two-component systems based on low molecular liquid
(EEW 190 to 210) and solid (EEW 350 to 500) epoxy resins. These versatile systems take
advantage of the ability of epoxy resins to react with a variety of curing agents and cure
at or somewhat below room temperature to provide strongly adhering, solvent and
corrosion-resistant coatings.
The choice of epoxy resin (liquid or low molecular weight solid) will depend upon the
application. Where high build, high solids, and the ultimate in chemical or solvent resistance
is desired, the liquid epoxy resin will be used. Where greater flexibility and faster tack free
times are required, low molecular weight solid epoxy resins will be used. About 85 percent
of maintenance coatings are based on a low molecular weight solid epoxy resin, such as Dow
Epoxy Resin D.E.R. * 660 or 661, because the short tack-free time reduces dirt pickup
during the curing cycle and allows earlier recoatability.

11.3.2 Curing agents for two-pack coatings

The reaction for crosslinking epoxy resins can be between epoxy resin molecules with the
aid of a catalyst. However, a majority of uses of epoxy resins employ a reactive hardener,
such as amines, acid anhydrides and phenolic resins, which combine with epoxy or hydroxyl
groups in the resin, to form a thermoset product.
The most commonly used curing agents for room-tern perature-cured systems are
polyfunctional amines and polyamide resins. As shown below, the active hydrogen atoms of
primary and secondary amine groups react with epoxy groups.
A primary monoamine has a functionality of 2 for epoxy reactions, whilst the epoxy group
has a functionality of 1. Thus a diepoxide reacted with a monoamine would give a linear

*Trademark of Dow Chemical Company.

 EPOXY RESINS AND CURING AGENTS 183

OH OR
I I
-CH-CHs ", /,CHI-CH-

-CH -CH / '

I
N-R-N

I
CH -CH- '" II I
OH OH

polymer. Consequently the useful curing agents are polyamines, such as:
Functionality Molar mass
Ethylene diamine (EDA) 4 60
Diethylene triamine (DET) 5 103
Triethylene tetramine (TETA) 6 146
The reaction mechanism suggests that the curing agent should be used in stoichiometric
proportions. For example, with epoxy resin having an EMM of 190 the amount of DET
would be:

1~3 = 20.6 per 190 g of resin

This is normally expressed as parts by mass of curing agent for 100 grams of resin (phr),
so for our example it would be 11 phr of DET. The use of incorrect proportions can lead
to water sensitivity, reduced chemical resistance and poor physical properties if an excessive
amount of amine is used. Even a small excess of amine will adversely affect the water
sensitivity, whilst low curing agent content will result in poor solvent resistance and poor
physical properties. When cured at ambient temperatures epoxy/polyamine coatings
develop maximum physical and chemical resistance in about seven days.
Disadvantages of aliphatic polyamine curing agents are their high volatility, toxicity,
tendency to blush, and inconvenient low mix ratios.

11.3.2.1 Aliphatic amine adducts

An aliphatic amine adduct is a reaction product of an epoxy resin with an excess of
polyamine, resulting in a polymer with amine functionality. Amine adducts may contain the
excess unreacted aliphatic amine or can be purified by stripping unreacted amine from the
product to produce so-called 'isolated adduct'.
The advantages of aliphatic amine adducts over aliphatic polyamines are their reduced
tendency to blush, less critical and more convenient mix ratios, shorter induction times, and
reduced volatility. Amine adducts can be formed with liquid or solid epoxy resins. Amine
184 SURFACE COATINGS

adducts formed by reaction with solid epoxy resins are used in conventional solvent-based
industrial coatings. For higher solids or solventless coatings, lower molecular weight liquid
epoxy resins are used.
11.3.2.2 Polyamides
Polyamides used in marine and maintenance coatings are amine terminated condensation
products of polyamines and dimer fatty acids. A typical polyamide structure would be:

H2N - C 2H4 - f- C2H 4 - ~- (rr - C34H 68 - rr - ~ - C2H 4 - ~ - ~H4 - N)n - H

H H 0 0 H H

By varying the dimer fatty acid and/or the polyamine functionality, the viscosity and
concentration of active amine on the polyamide may be tailored for specific application.
Polyamide manufacturers offer a wide range of products.
Disadvantages of polyamides include dark colour, high viscosity and reduced chemical
resistance relative to aliphatic polyamines and poor high-temperature properties.
Advantages of polyamide curing agents are non-critical mix ratios, low exotherm, low
toxicity and volatility, and excellent water resistance when cured. Polyamides are widely used
in zinc-rich primers, as their reactivity with zinc is negligible. Epoxy/polyamides are also
used as 'splash zone' coatings for ships, oil-drilling rigs, and other structures subjected to
wet/dry cycling and abrasion.
11.3.2.3 Amidoamines
The use of amidoamines is newer technology; they are the condensation products of
monobasic fatty acids and a polyamine. The structure of a typical amidoamine is given below.

where R = long chain fatty acid.

Upon heating, the amidoamine can cyclize to form an aminoethylimidazoline:

When compared to aliphatic polyamines, amidoamines offer more convenient (higher) mix
ratios and improved flexibility and moisture resistance. They are lighter in colour than
polyamides. The properties of amidoamines are dependent upon the level of imidazoline
present. Their reactivity varies with composition but is generally slower than polyamines and
faster than polyamides.
11.3.2.4 Cycloaliphatic amines
When compared to aliphatic amines, cycloaliphatic amines display improved thermal
properties and toughness. However, they are less reactive and may require the use of an
accelerator or heat treatment to achieve reasonable cure times. Cycloaliphatic amines may
 EPOXY RESINS AND CURING AGENTS 185

be adducted with epoxy resins. Structures for two popular cycloaliphatic amine curing agents
are given below:

1,3 - diamino cyc10hexane isophorone diamine

11.3.2.5 Mannich bases

As reaction products of phenol, formaldehyde, and polyamine, Mannich bases have the
following structure:
OH
o-CH,NH(R)"NH, where R + CH,. etc

This class of epoxy curing agent offers low temperature cure, freedom from blushing, and
good chemical and moisture resistance.
11.3.2.6 Aromatic amines
Aromatic amines are characterized by excellent thermal and chemical properties, which may
be attributed to the presence of the aromatic ring. When used with epoxy resins, heat is
normally required for complete cure. However, suitably accelerated solutions of aromatic
amines in non-volatile solvents are capable of curing epoxy systems at ambient temperatures.
Aromatic amines are often solid in form, making them less convenient to formulate when
compared to liquid aliphatic amines. The structure for methylene dianiline, a commonly
used aromatic amine, is given below:

H,N -0- {-o- NH,

H
methylene dianiline

11.3.3 Other components of marine and maintenance coatings

11.3.3.1 Pigments and fillers
Pigment and fillers are used in marine and maintenance coatings to enhance cured film
properties, to contribute hiding power, and decrease the overall cost. When formulating
two-pack epoxy coatings, the pigments may be added to the resin and/or curing agent
portions and can be dispersed using standard techniques. In epoxy/polyamine systems, the
pigment is usually dispersed in the epoxy resin. In epoxy/polyamide formulations, the
pigment is preferentially dispersed in the polyamide because of its significantly better wetting
properties. Pigment-dispersing agents and thixotropic agents are used to improve pigment
dispersion and prevent any setting tendency.
186 SURFACE COATINGS

11.3.3.2 Solvents
Solvents serve to solubilize the epoxy resin and curing agent during formulation and
application. Properly selected solvents enhance the flow and levelling, rheology, potlife,
tack-free time and recoatability. Conversely, poor solvent balance can result in film defects
such as pin holes and orange peel appearance.
Most ketones, esters, ethers and particularly glycol ethers are 'true' solvents for epoxy
resins and solvate them over a broad range of concentration. True solvents are those that
are volatile under normal drying conditions and in which the binder is completely soluble.
Aromatic solvents, alcohols, and other latent solvents may be used with ketones and glycol
ethers in amounts up to 50 percent of the total volatiles.
Toluene and xylene are widely used as diluents in marine and maintenance coatings.
Aliphatic hydrocarbons are poor solvents for epoxy resins and are not normally used in
epoxy coatings. .
Solvent selection is based upon the desired viscosity, curing agent type, and planned
method of application.
11.3.3.3 Additives
Flow-control agents, such as urea formaldehyde and silicone resins can be used to control
'crawling' and 'cratering' of two-pack marine and maintenance coatings.
Anti-foaming agents and defoamers are used to reduce air entrapment and increase
pigment-grinding efficiency.
11.3.4 Formulations
The following are typical two-pack marine and maintenance coating formulations.
11.3.4.1 General-purpose white gloss epoxy enamel
A general-purpose epoxy-polyamide gloss enamel with excellent water resistance, this
formulation is suitable for brush and roller application and may be thinned for spray
application.
 EPOXY RESINS AND CURING AGENTS 187

Parts by weight
Part A
Dow Epoxy Resin D.E.R. 671 x 74 1 30
Methyl isobutyl ketone (MIBK) 9
Xylene 14.5
Part B
Versamid 115 x 70 2 12.5
Titanium dioxide, Ti-Pure R-960 3 20
Calcium carbonate 11.5
Chrome green oxide 2.5
Bentone 34 4 0.001
Cab-O-Sil TS 720 5 0.001
BYK P 1045 0.6
Total 100.00
Properties
Weight/litre: 1.26 kg/litre
% non-volatile (weight): 65.34%
% non-volatile (volume): 51.94%
1 Dow Chemical Company
2Henkel Corporation
3E.I. DuPont
4NL Industries
5Cabot Corporation

11.3.4.2 100 percent solids coating formulation

The formulation below is a liquid epoxy resin-cycloaliphatic amine coating system for a
well-prepared metal substrate. Applied coating will be tack-free in approximately eight hours
and develop maximum properties in five to seven days.
Parts by weight
Part A
Dow Epoxy Resin D.E.R. 331 1 40
Titanium dioxide 12
Barytes 20
Talc 7
Aerosil 200 2 0.2
Flow control aid, BYK 300 3 0.004
Antifoam, BYK 052 0.004
Part B
Ancamine 1693 4 20
100
Properties
Weight/litre: 1.54 kg/litre
Pigment volume concentration: 16%
% non-volatile: 100%
1 Dow Chemical Company
2Degussa
3BYK Chemie
4Pacific Anchor Corporation
188 SURFACE COATINGS

11.4 EPOXY RESIN ESTERS

Epoxy resin esters are the reaction product of epoxy resins and vegetable oil fatty acids.
They are widely used as surface coating materials, replacing alkyds, polyesters, and
oleoresinous vehicles in many applications.
The advantages of epoxy esters are: excellent flexibility, better chemical resistance than
alkyds, excellent durability, very good adhesion, ease of handling, rapid air dry or baking
cures when formulated with formaldehyde resins, and very good film toughness.
11.4.1 Fatty acid choice
A wide variety of fatty acids is available to react with epoxy resins. They provide a range
of properties, depending on end-use requirements and intended cure conditions. The most
commonly used drying and semi-drying acids are tall, soybean, linseed, safflower, and
dehydrated castor. Non-drying fatty acids sometimes used are coconut and castor. The type
and amount of unsaturation in the fatty acid molecule determines its drying characteristics.
The type and percentage of fatty acids in an epoxy ester determine to a large extent the
properties of an epoxy ester. It also determines the oil length classification of the epoxy ester.
Short oil (30-50 percent equivalents) generally retain epoxy resin properties and are used
for air-drying spraying systems as well as for amine resin-modified baking finishes. The long
oil epoxy esters (70-90 percent equivalents) are soluble in aliphatic solvents, have better
flexibility and water resistance, and are used for air-drying brush-applied coatings.
11.4.2 Choice of resin
A solid bisphenol-based epoxy resin with an epoxide equivalent weight of 875-975 and a
theoretical esterification equivalent weight of 198 is the standard esterification grade of
epoxy resin. A typical resin in this classification is Dow Epoxy Resin D.E.R. 664, which is
supplied with an esterification catalyst already added. Esters prepared from this resin will
have consistent solution viscosity and acid value.
The method used to calculate the required amounts of epoxy resin (DER 664) and of fatty
acids (dehydrated castor) to produce an ester of a particular degree of esterification and of
medium oil length (60% conversion) is:
Equivalent Equivalents Parts by
weight required weight Weight %
DER 664 198 x 1 198 53.75
DCO 284 x 0.6 170.4 46.25
11.4.3 Epoxy ester preparation
The epoxy resin, fatty acids, modifiers and catalyst are reacted in a closed kettle equipped
with good agitation, inert gas blanketing, temperature control and condenser with water
trap. All the raw materials are charged to the reactor and put under an inert atmosphere.
Heat is applied, and when the resin starts to melt, agitation is started. When a clear solution
is obtained the temperature is raised to 240·-260· C and held for 4-6 hours, until the
desired acid number is obtained.
11.4.4 Solvents
Short oil esters require aromatic solvents, such as xylene and toluene. Medium oil esters are
soluble in mineral spirits/aromatic solvent mixtures. Long oil epoxy esters are normally
soluble in mineral spirits.
11.4.5 Curing epoxy esters
Epoxy esters cure by oxidation of the double bonds present in unsaturated vegetable fatty
acids. This mechanism consists of two steps:
 EPOXY RESINS AND CURING AGENTS 189

(a) oxidation at the film surface; and (b) polymerization throughout the depth of the film.
A metallic drier that will promote the oxidation mechanism is required. The driers most
used are cobalt naphthenate or octoate, and a manganese salt. The driers are used at
0.005-0.1 percent metal based on resin solids.
Ambient-temperature-cured films will reach maximum properties in approximately seven
days. Alternatively can be cured at 120· -150· C for 20-30 minutes.
The performance of epoxy resin esters may be improved by cross linking with a melamine
or a urea formaldehyde resin. Typical ratio is about 70:30 epoxy ester to amino resin. Cure
schedule for these systems is about 30 minutes at 150· C.

11.4.6 Formulations
The following formulations are for three epoxy esters of different oil length. There is a
comparison of physical properties of films cured at ambient temperature for three days and
those baked for 20 minutes at 150· C.
Esters of Dow Epoxy Resin D.E.R. 664 and DCO fatty acids
Formulation (% by weight) Short oil Medium oil Long oil
D.E.R. 664 resin 60 50 45
DCO fatty acids 40 50 55
Solution properties
Acid number (solids) 2.2 8.0 14.7
Solvent Xylene Xylene/MS, 1: 1 Mineral spirits
Non-volatiles % (wt) 50 50 50
Viscosity, Gardner-Holdt W V W
Colour, Gardner 5 5 5
Cooking time at 260· C 3.5 h 4h 4 h (250· C)
Film properties l
Dry to touch 15 min 18 min 23 min
Dry foil free 2 h 2.5 h 4h
Sward hardness (24 hours) 27 19 15
(7 days) 43 30 25
Air dry (3 days)
Knoop hardness 8.7 7.1 6.5
Flexibility, I/S" mandrel Passes Passes Passes
Adhesion Excellent Excellent Excellent
Reverse impact ~nch/pounds) >30 >30 >30
Water resistance vsh-cr sh-cr h-pr
Caustic resistance 3 Excellent Good Good
Solvent resistance 4 Excellent Excellent Excellent
Baked (20 min at 150· C)
Knoop hardness 13.1 12.1 10.1
Water resistance 2 Clear Clear Clear
Caustic resistance 3 Excellent Excellent Good
IDrier 0.04% cobalt, 25-38 J.lm films.
25 days water immersion at 25° C.
vsh-cr = very slight haze, complete recovery, 30 minutes.
Sh-cr = slight haze, complete recovery, 30 minutes
h-pr = haze, partial recovery, 30 minutes
3Immersion in NaOH at room temperature, two weeks.
4} hour immersion in mineral spirits.
190 SURFACE COATINGS

11.5 HIGH-TEMPERATURE CURED COATINGS

This classification includes container and coil coatings that require cure temperatures of
approximately 200· C. Higher molecular weight solid epoxy resins with an 'n' value between
5 and 30, such as Dow Epoxy Resins D.E.R. 664, D.E.R. 667, D.E.R. 668 and D.E.R. 669E,
are used in container and coil applications because they provide a high degree of flexibility
and yield formulation viscosities that can be applied by roller or spray techniques. As the
molecular weight of an epoxy resin increases, the molecule continues to be difunctional with
epoxy groups on the two ends of the molecule. At the same time, the value of 'n' is
increasing, and secondary hydroxyls are being added to the backbone. The hydroxyl groups
become the more important crosslinking site, and epoxide content decreases to 1-2 percent
by weight.

II.5.1 Crosslinkers
Crosslinkers that react with the secondary hydroxyl groups under heat are used for can and
coil coatings. The classes of curing agents mainly used are:
(a) phenol formaldehyde resins;
(b) melamine formaldehyde resins;
(c) urea formaldehyde resins and
(d) polyisocyanates (exterior coatings).
Coatings based on phenol formaldehyde crosslinked epoxy resins are characterized by
excellent chemical resistance, moisture and salt spray resistance, abrasion resistance, and
superior elevated temperature performance. This type of coating is used in three-piece food
cans where sterilization involves harsh conditions with temperatures up to 150· C. Other
applications include linings for drums, pails and lids, and appliance primers.
Epoxy resins thermally cured with amino formaldehyde (urea or melamine) resins are
characterized by clarity, absence of colour and taste, good flexibility, and solvent resistance.
Physical properties can be optimized by proper selection of type and amount of amino
formaldehyde crosslinker.
Melamine formaldehyde resins are used when maximum resistance to solvent, water,
detergent, or stain is required. They are used at a 5-35 percent level, based on resin solids.
Melamine formaldehyde resins are widely used in linings for beer and beverage cans,
closures and in appliance primers.
Urea formaldehyde resins are used to reduce costs and lower cure temperatures. These
crosslinkers impart excellent flexibility and adhesion to epoxy resin coatings.

11.5.2 Solvents
In can and container coatings, solvents are used to solubilize the epoxy resin and crosslinker
during formulation, application and storage. Solvents are necessary to impart good
application properties, flow, and final film appearance.
A blend of solvents is usually required. Ketones, esters, and glycol ethers are true solvents
for epoxy resins. Latent solvents such as alcohols and aromatic solvents, toluene and xylene,
are used to dilute the resin varnish and slow down the rate of evaporation of the drying
film. Solvent selection is based on the type and amount of epoxy and crosslinker used, the
application method and cure cycle. The higher the molecular weight of the epoxy resin, the
greater the proportion of ketone or glycol ether 'true solvents' required in the solvent blend.

11.5.3 Catalysts
Acid catalysts improve the reaction between epoxy reSInS and phenolic crosslinkers.
 EPOXY RESINS AND CURING AGENTS 191

Common catalysts are phosphoric acid, alkyl phosphoric acid and para-toluene sulfonic acid.
In coatings for food and beverage contact, phosphoric acid is usually used.
Epoxy-aminoplast systems usually do not require a catalyst. The reactivity of aminoplast
resins can vary, and the use of catalyst can decrease cure temperature or the length of the
cure cycle.
11.5.4 Additives
Other additives used in epoxy resin-phenolic or epoxy resin-aminoplast can or coil coatings
are:
(a) flow additives;
(b) pigments;
(c) pigment wetting and dispersing agents;
(d) slip agents; and
(e) anti-settling agents.
11.5.5 General-purpose epoxy-phenolic clear coating for metal containers
The formulation shown below, a 70:30 ratio of Dow Epoxy Resin D.E.R. 667 to phenol
formaldehyde crosslinker, is suitable as a starting point for coating steel drums that contain
aqueous materials, concentrated bases, and other chemicals. It can also be used in interior
food can coatings, and is especially useful for protecting the can from oily foods.
Pigments can be incorporated by making a concentrated pigment grind using the pigment
and some of the formulation solvents and resin, and adding it to the remainder of the
formulation components. If roller coating is the application method, the formulation can be
increased in solids content to 35 percent or slightly higher. For spray applications, it may
be desirable to reduce the solids content, using a blend of glycol ether, n-butanol, and
xylene.
Formulation (parts by weight)
D.E.R. * 667 resin 210.0
Methylon t 75108 100.0
Dowanol* EB 183.0
Dowanol* DPM 9l.50
n-Butanol 137.25
Xylene 274.50
Silicone Resin SR-882* 0.75
Phosphoric acid, 85% 3.00
Total 1000.00
'Trademark of the Dow Chemical Company
tTrademark of BTL Speciality Resins Corporation
tTrademark of General Electric Company

11.6 EPOXY POWDER COATINGS

Epoxy resins can be applied to substrates in a solvent-free form as a powder coating. The
powders are manufactured by dispersing pigments, flow control additives and curing agent
in a molten epoxy resin. On cooling, the blend solidifies to a hard, brittle mass, which is
crushed and ground into a powder of the required particle size. Typical curing agents used
for this application are based on dicyandiamide; they are solid and not reactive at room
temperature. Another type of powder coating is based on a combination of epoxy and
polyester resins. Powder coatings require baking at 170°-200° C to form a thermoset finish.
192 SURFACE COATINGS

The powders can be applied to articles by dipping the preheated object into a bath of the
powder which is fluidized (by blowing air through a specially designed porous bottom of the
bath) or by spraying the powder by flock gun. The most convenient method for application
of powders is electrostatic spraying onto cold or preheated objects. Epoxy powder coatings
have two main advantages compared with solvent-based systems:
(a) absence of solvents reducing health and safety hazards; and
(b) for special heavy duty applications, a high film thickness attainable in one application
without danger of solvent retention or film porosity.
Epoxy powder coatings are utilized where severe corrosion and/or abrasive conditions are
encountered. They have become established in interior decorative applications on articles
of complicated shapes, such as tubular steel furniture and expanded metal articles. Their
use in exterior applications such as agricultural equipment and heavy transport vehicles is
extensive. A major use is in the coating of pipelines for the transmission of natural gas and
water.