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MNL17-EB/Jun
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Bauer, 1 E d w a r d J
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Watkins 2

tain one or more epoxy (oxirane) groups per molecule
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1
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As n increases, the epoxy equivalent weight (EEW) increases, as does the number of hydroxyl groups
...

Resins having n values less than 1 are viscous liquids; they
are used mainly in ambient-temperature cure coatings, electrical castings, flooring, electrical laminates, and fiber-reinforced composites
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The
higher n value resins, particularly those above 3000 molecular weight, are normally used in solution and find their greatest application in heat-cured coatings
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Table 1 displays a range of typical epoxy resins
that are commercially available along with their properties
and applications
...
Almost since their
commercial introduction in 1950, epoxy resin systems have
been used in protective coatings
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Although in recent years the noncoating applications of epoxy resin have been growing, coatings still represent about
half of the annual epoxy resin usage
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They are used extensively in coatings for refineries,
chemical plants, and marine equipment such as offshore
drilling platforms and merchant ships
...
Epoxy resins are
the predominant thermosetting resin used for the interior
linings of beer and beverage cans, cans for hard-to-contain
food products such as sauerkraut, tomato juice, and meat
products, and for chemical-resistant linings of pails and
drums
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The principal components of any epoxy resin coating system are the epoxy resin and the curing agent or hardener
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Curing agents function by
reacting with specific groups in the epoxy resin molecule to
give a three-dimensional, infusible polymer network
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CURING AGENTS
Epoxy resins are reactive intermediates composed of mixtures of oligomeric materials containing one or more epoxy
groups per molecule
...
Cross-linking agents, or
curing agents as they are generally called, function by reaction with or cause the reaction of epoxide or hydroxyl groups
in the epoxy resin
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Selection of the curing agent is as important as that of the
base resin; it is dependent on the performance requirements
of the film and the constraints dictated by the specific method
of application
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The principal amine-functional curing
agents used in ambient-cure coatings are aliphatic amine adducts of epoxy resins, polyamides, and ketimines
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, Westhollow Research Center, P
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Box 1380, Houston, TX
77521-1380
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astm
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I-Idealized structure of a bisphenol epoxy resin
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Average
Molecular Wt

Average
EEW~

Approximate
Average
Value of n

Viscosity,
P, 25~

Softenin~
Point, ~ v

350

182


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-

380
600
900
1400
2900

188
310
475
900
1850

0
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40
70
100
130

3750

3050

13

Solid

150

Applications
Solventless and solvent-borne ambient cure
coatings, electrical encapsulation, flooring, and
filament winding
Solvent-borne, ambient cure coatings
Powder coatings
Heat cure, solvent-borne coatings for cans, drums,
primers, etc
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e
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Alsoreferred to as WPE (weight per epoxide) and EMM
(epoxy molar mass)
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bSoftening point by Durran's mercury method [ASTMSpecification for Epoxy Resins (D 1763)]
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Aminoand phenoplast cross-linking resins are generally etherified
urea-formaldehyde, melamine-formaldehyde, and phenolformaldehyde condensates
...

Aliphatic amines such as ethylene diamine (EDA), diethylene triamine (DETA), and triethylene tetramine (TETA) have
always been popular curing agents for epoxy resins because
of their ability to readily cure at r o o m temperature
...
Epoxy/
amine adducts, amine-terminated polyamides, ketimines,
and other modified polyamines are less hazardous derivatives
of aliphatic amines and often provide performance and handling advantages
...
Since curing agent types have been divided into
ambient-cure and heat-cure, for convenience the types of
epoxy coatings can also be classified similarly
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Ambientcure coatings are based on low-molecular-weight epoxy resins having high epoxy group content and are generally cured
through reaction of the epoxy group
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Historically ambient-cure coatings have been applied out of
solvent, typically at about 40% weight solids
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Heat-cure coatings are used in industrial finishes, automotive primers, appliance primers, pipe coatings, and coatings for beer and beverage cans, as well as cured coatings for
pails and drums
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Powder coatings, however,
are generally manufactured from intermediate molecular
weight solid resins and are cured t h r o u g h the epoxy group
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TWO-PACKAGE, AMBIENT-CURE
COATINGS
By far the largest volume of ambient-cure epoxy resin coatings are the "two-package" type, which are typically manufactured from liquid or low-molecular-weight solid epoxy resins
cured with a polyamine, amine adduct, or polyamide
...
Two-package epoxy coatings are mixed just prior to application and are characterized
by a limited working life or pot life after the resin and curing
agent components are mixed
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Two-package epoxy resin coatings include a broad range of
products and properties
...
Resins
differ primarily in molecular weight
...
However, cross-link density decreases with increasing molecular
weight, resulting in reduced solvent and chemical resistance
as well as nonvolatile content of the paint at application
viscosity
...
Among the advantages provided by aliphatic
amine adducts are:
9 Milder odor
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9 Less tendency to produce blush when coatings are applied
under conditions of high humidity
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9 Less tendency to corrode metal containers
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e
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The performance properties of amine-cured coatings are not
significantly different from those of aliphatic polyamine
cured systems
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Initial flexibility and impact resistance are excellent, and retention of these properties is adequate for most
uses over rigid or semi-rigid substrates
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Polyamides are obtained from the condensation of dimerized and/or trimerized fatty acids with aliphatic polyamines such as diethylene triamine and triethylene tetramine to give an amine-terminated polyamide
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Like epoxy resins, polyamides are also mixtures of oligomers
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Polyamide cured coatings exhibit
somewhat better retention of flexibility and impact resistance
on aging than polyamine adducts
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Ketones add reversibly to primary amines with the loss of
water to give ketimines
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Ketimine curing

agents can be considered blocked polyamines, which in the
presence of water hydrolyze to produce a ketone and a polyamine
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Atmospheric moisture, which is absorbed during and following application of the coating,
serves as the source of water required to activate the curing
agent
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Typical applications of two-package coatings of the above
type are in heavy-duty maintenance and marine coatings,
tank linings, aircraft primers, internal pipe coatings, for gas
transmission lines, coatings for plastic products, and highperformance architectural coatings
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Epoxy esters are usually prepared from solid epoxy resins having EEWs in the range of
900
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Subsequently, these hydroxyl groups and those already present in the resin are esterified at higher temperatures with the aid of esterification catalysts and with azeotropic removal of water
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Like alkyd resins, epoxy resin esters may be made in long,
medium, and short oil lengths
...
By proper selection of acids and adjustment of reaction
ratios, long, medium, or short oil esters may be prepared with
drying, semidrying, or nondrying characteristics
...
Air-dry epoxy resin ester coatings are "one-package" or
one-component systems, since they cross-link or cure only on
exposure to air
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They do not, however, possess the
outstanding chemical resistance of amine-cured epoxy coatings, but they are superior to alkyd paints
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HEAT-CURED SOLVENT-BORNE EPOXY
R E S I N COATINGS
Conventional solvent-borne heat-cured or "baked" epoxy
resin coatings are based on high-molecular-weight epoxy resins, that is, resins with EEWs around 1750 or greater
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Thermosetting resins such as urea-formaldehyde,
melamine-formaldehyde, and phenol-formaldehyde resins
are used as cross-linkers for the coatings of this type
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Thus, these systems are supplied as "one-component" systems, i
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, the resin, curing
agent, and accelerator are packaged together
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They are typically
used in linings for beer and beverage containers and as clear
coatings for brass and jewelry
...
Phenoplast
(phenol-formaldehyde) cured coatings are more chemically
resistant, and they find application in beer and beverage
containers (particularly in Europe), drum and pail linings,
internal coatings for pipe, wire coatings, and appliance
primers
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HEAT-CURED WATERBORNE EPOXY
COATINGS
The earliest epoxy resin coatings for beer and beverage
containers were solvent-borne amino- or phenoplast cured
systems, the particular systems used being dependent on the
application
...
These coating systems are based on high-molecularweight epoxy resins onto which are grafted terpolymers of,
for example, styrene/methacrylic acid/ethyl acrylate
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The dispersed resin can then be cured with an aminoplast resin to give coatings with properties that make them
suitable for beer and beverage containers
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The part to be coated is dipped into the
electrodeposition bath, and an appropriate electrical charge
is applied, causing the coating to deposit onto the part
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In the United States, epoxy-based electrodeposition coatings account for over 92% of all electrodeposition
coatings
...
Over 40 million
pounds of epoxy resin are used in the United States in CED
automotive primers, making this one of the largest single end
uses for epoxy resins in coatings
...
CED primers are used because they afford

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exceptional corrosion protection and because they are deposited uniformly to all areas of the automobile, even in areas
which would be inaccessible to other coating application
methods such as spray
...

The preparation of CED coatings generally begins by reacting a bisphenol A based liquid epoxy with bisphenol A to give
an epoxy resin with an epoxy equivalent weight in the range
of 500 to 1000
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This diol can be an aliphatic diol or a polyether
diol
...
These primary hydroxyls are reacted with the epoxy groups in the presence of a suitable
catalyst (e
...
, a tertiary amine) to form ether linkages between the epoxy and the flexibilizing diol
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The remaining epoxy functionality is then reacted with amines
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One favored method to
accomplish this is to use a diketimine of diethylene triamine
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These primary amines are fairly basic, resulting in stable dispersions at a relatively high bath pH
(pH > 6)
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In practice, specialized CED resins are used to make
the pigment grind pastes
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Curing agents used are generally blocked isocyanates
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An example of such a curing agent would be the reaction
product of 3 mol of toluene diisocyanate with 1 tool of trimethylolpropane
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Catalysts such as
tin or lead salts are generally used to facilitate unblocking
and coating cure
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A low-molecular-weight organic acid, such as
lactic or acetic acid, is then added to the mixture to make
a m m o n i u m salts with the amine groups in the resin
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Solvents may be required in the preparation of the CED resin
or other components
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7 lb/gal
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Electrolysis of water occurs, forming hydroxide ions in the immediate
vicinity of the automobile surface
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In this way, a uniform
film is applied to the entire conductive surface of the automobile
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PAINT AND COATING TESTING MANUAL

E P O X Y R E S I N P O W D E R COATINGS
Powder coatings are produced by melt blending homogenous dispersions of nonvolatile solid resins, curing agents,
pigments, fillers, and various additives
...
The resultant powder
is normally electrostatically deposited onto grounded substrates and, through the application of heat, converted into
very high performance thermoset films
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The 1970's volatiles regulations and energy concerns raised
interest in powder coating technology
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The advantages
for the use of powder coatings can best be summed up in the
"Four E's," used by The Powder Coating Institute: (1) excellence of finish, (2) economy in use, (3) energy efficiency, and
(4) environmental acceptability
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Powder coatings is the fastest growing
area of coatings technology
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The unique characteristics of solid epoxy resins account for
their choice by formulators for use in powder coatings applications
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They have relatively low melt viscosity and high reactivity via the terminal oxirane functionality
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The
primary limitations for bisphenol-A based epoxy resins in
powder coatings are yellowing and loss of gloss that occur
when these coatings are exposed to exterior weathering conditions
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Functional coatings are normally
applied at film thicknesses greater than about 3 mi] and are
expected to withstand some rather severe service
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Although decorative
powder coatings are functional, these are normally used at a
film thickness of 3 mil or less and are not expected to perform
significantly better than baked films derived from "wet" coatings
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REFERENCES
[1] Lee, H
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, Handbook of Epoxy Resins, McGraw-Hill
Book Co
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[2] May, C
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, Epoxy Resins Chemistry and Technology,
Marcel Dekker, Inc
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[3] Bauer, R
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, Epoxy Resin Chemistry, ACS Symposium Series 114,
American Chemical Society, Washington, DC, 1979

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