9 Coatings and other applications of epoxy resins

X.M. CHEN and B. ELLIS

9.1 Introduction

The current use pattern for epoxy resins is similar to that suggested by Lee
and Neville in 1967 who estimated consumption as coatings 55%, composite
matrices 20%, castings 10%, adhesives 5% and miscellaneous 10%. These
figures may be compared with the 1990 and 1991 estimates given in chapter
1, Table 1.1, even though the categories are not identical. Lee and Neville
(1967a) pointed out that there are difficulties in obtaining exact
consumption estimates. However, the main difference is that coatings
have a slightly lower share of the epoxy market. Epoxy resins are usually
more expensive than their rivals, such as phenolic resins for coatings or
laminates. Therefore, epoxy resins find application because of their superior
properties, which include both processing and those of the cured resin. The
processing is convenient since it is possible to formulate compositions with
the required rheological properties, such as low viscosity, and there is also a
wide choice of hardeners so that it is possible to cure at ambient as well as
elevated temperatures. Because epoxy resins can be cross-linked without
the formation of low molecular weight products, volatiles are not evolved
during cure, and the resins have only a relatively low shrinkage during
curing. Their mechanical and electrical properties are superior to other
resins and they have good heat and chemical resistance.
There is a large range of epoxy resins available commercially and some of
the more important types are listed in Table 9.1. Exact specifications can be
obtained from resin suppliers and many mixtures are offered by
compounders. Selection of a resin-hardener system can be made by a critical
assessment of processing and end-use requirements, e.g. the electrical
properties, or resistance to UV radiation. It is possible to enhance a desired
parameter simply by altering the cure schedule without a change in the initial
composition of the resin-hardener system. Thus, it is possible to increase
the glass transition temperature (Tg) of the cured resin by post-curing.
Such effects can be appreciated by study of the time - temperature-
transformation diagram, TIT, given in Figure 1.2 and the detailed
discussion of cure given in chapter 3. Extensive data compilations of the
properties of epoxy resins are available and Table 9.2 is a selection of the
'tensile' mechanical properties and also an estimate of the heat distortion
temperature. The Young's modulus of epoxy resins is somewhat lower than
Table 9.1 Summary of commercially available epoxy resins a

E.e.w. Viscosity Applications

(250°C) cp

1. Bisphenol A-epichlorohydrin resins

a. Liquid resins
i. Wide ranging characteristics 95-450 5000-30000 Table 9.3
ii. Chain extension 185-200 2000-7000 Contain specific catalysts for conversion of these resins
iii. Lower viscosity resins Contain reactive diluents, e.g. glycidyl ethers of iso-octanol or
butane-l,4-diol
b. Solid resins
i. So. pt. 60-180°C 500-6000 Coatings, especially powder coatings
Manufacture of epoxy resin esters for coatings
With amino- or phenolic-resins; high chemical resistance for
tube coating primers and stoving enamels
Castings
Prepregs
ii. Mixtures and solutions Solvents include butoxy( ethoxy )ethanol, xylene, acetone,
methyl isobutyl ketone and combinations
2. Epoxy-novolacs
Derived from bisphenol A-epichlorohydrin resins
i. Phenol, liquid 170-190 30000-90000 Coatings, encapsulators, laminating, moulding, adhesives
ii. Phenol, solid (So. pt. 90-100) 190-220 Moulding powder
(So. pt. 90-95) 275-330 Adhesives, coatings
iii. Cresol, solid (So. pt. 35-90) 200-240 Adhesives, coatings, mouldings
3. Polyhydric phenols
i. Bisphenol F
Diglycidyl ether of bisphenol F 158-175 5000-7000 Additive for BPA resins in coating and flooring, e.g.
CH, Araldite 07281

- - O-CH,-CH-CH,
"(~rn-cHoJ9 ~ /\
 ii. Others include:
Tetraphenol ethane derivatives Powder coating
Polyglycidyl ether of tetraphenol ethane: Shell 1031

"O~ )§l0"
5§5rn-rn~
HO OH

4. Amines
Reaction products of epichlorohydrin with
amines such as:

~
r$) NH2
C$JNH2
~ NH2

DDM Aniline p-amino-phenol

Triglycidyl p-amino phenol Union Carbide ERL-0500

5. Halogen-containing epoxides Flame retardancy applications: general purpose laminating,
moulding
6. Cyclo-aliphatic resins
Range of resins lower viscosities than BPA Good UV light stability, electrical properties: general purpose
resins casting resin, filament winding. Acid scavenger, plasticizer
3 ,4-epoxycyclohexylmethyI3 ,4-epoxy- 131-143 350--450 Union Carbide ERL-4221
cyclohexane carboxylate
o
°D~-O-CH2-o0
So. pt. = softening point, 0c. a For a fuller list see Tanaka (1988).
306 CHEMISTRY AND TECHNOLOGY OF EPOXY RESINS

Table 9.2 Mechanical properties and range of heat distortion temperatures of cured epoxy
resins

Mechanical properties

Young's Tensile Elongation Heat distortion

modulus strength at break temperature
(GN. m- 2 ) (MN .m- 2 ) (%) (0C)

Badge type:
Flexible 0.1-1 10-70 5-10
Rigid 1.5-3.5 25-90 2.5-4 100--200
Epoxy-novolac 1.5-3.5 70 2 150-250
Cycloaliphatic epoxy 3.5 90 2-10' Upto200
Phenol-formaldehyde 2-4 20-60 1.5 150-170
Unsaturated polyesters 1.5-3.5 40-90 <2 Up to 200

, With up to 30 phr of a polyol flexibilizer, elongations at break may be up to 30%.

that of phenol formaldehyde (P/F) and unsaturated polyester resins but the
tensile strengths are comparable and may be higher. This is because the
extensibility, that is the fracture strain, of the epoxy resin is much higher
than that of the 'brittle' resins. It is possible to select from a range of
flexibilizers (see chapter 4) to 'tailor' the mechanical properties to suit the
application requirements. For instance the 'tensile strength' can be
significantly affected by cure treatment and the fracture behaviour of epoxy
resins should always be considered when an application demands optimum
strength. There is a detailed discussion of fracture behaviour in chapter 5
which is also relevant to the adhesive (chapter 7) and composite (chapter 8)
applications of epoxy resins.
With standard BPA-EpiCI resins the heat distortion temperatures are
similar to those of PIF and polyester resins, but with suitable hardeners and
a post-cure at elevated temperature it is possible to attain higher glass
transition temperatures and hence higher use temperatures. For instance a
BADGE-type resin cured with trimellitic anhydride and cured for 1 hour at
120°C followed by 2 hours at lS0°C may have a heat distortion temperature
of up to 2S0°C.
Cyclo-aliphatic resins have very good outdoor ageing properties. This is
because they do not contain aromatic rings which absorb the UV radiation
which leads to degradation processes. Also, the chlorine, and other halogen,
content of the cyclo-aliphatic resins is essentially zero and hence these resins
have very good electrical properties.
Data compilations such as that given in Table 9.2 are indicative of the
properties that may be achieved with epoxy resins. However, by comparison
with the PIF and unsaturated polyester resins it would not be possible to
assess the superior technical properties of the epoxy resins. Their processing
properties offer considerable advantages. The range of hardeners available
is a major asset since it is possible to formulate a curing system to suit almost
Table 9.3 Some typical curing agents and applications for BADGE-type resins

Hardener' Physical state Concentration Typical cure schedule Approximate heat Suggested applications 3
at ambient (phr)2 distortion temperature
COC)
1. Aliphatic amine
Triethylene tetramine Liquid 13 a. 25°C, 7 days a. - General purpose
b. 25°C, 1 day plus post- b. Up to 120 Co, A, Ca, M, F, Sand
cure at elevated T FW
leffamine D-400 Liquid 55 1l0-120°C, 30 min -30 Flexible resin Co, A,
Ca,FandS
2. Aromatic amine
DDM Solid 27 1 hat 150°C Upto 150 Glass cloth laminate
+3hat 180°C
DDS (orDAS) Solid 25 5 h at 125°C Upto175 A, Ca, M, E, Sand FW
+ 1 hat200°C
3. Polyamides Liquid Depends on type a. Ambient temperature Up to 100 a. Co,A,Ca,F,M
7 days andS
b. 1 h at 100°C Up to 100 b. CaandA
4. Anhydrides
Methyltetrahydro- Liquid 80 2hat20°C 130 Liquid hardener
phthalic anhydride +2hat 150°C processing
(MTHPA) A,Ca,M,SandFW
Trimellitic anhydride Solid 35 1 h at 120°C 250 High heat distortion
(TMA) +2hat 150°C temperature
A,Ca,M,SandFW
5. Miscellaneous
Dicyandiamide (DICY) Solid 4 1 h at 180°C 150 A, Ca, M, E, Sand FW
BF3-Monoethylamine Solid 3 1 hat 120°C 170 A, Ca, M, E, Sand FW
(BF3-MEA) +2hat 170°C

, See chapter 2 for fuller details. Many hardeners are mixtures and/or derivatives and are not included in this list. Many other hardeners may be used. Shell
list 30 curing agents for Epon 828. This table is only illustrative.
2 phr = parts per hundred of resin by weight.
3 Code: Co = coatings; A = adhesives; Ca = castings; E = electrical laminates; F = flooring; M = mouldings, S = structural laminates; FW =
filament winding.
310 CHEMISTRY AND TECHNOLOGY OF EPOXY RESINS

which the film former becomes cross-linked either by reaction with oxygen
or suitable curing agents or hardeners. A general text on surface coatings
is that of Paul (1986).
There are many different coating methods which can be adopted for
specific applications, depending on the coating thickness, coating speed and
type of finish required. A review of 25 methods with outline descriptions is
given by Scharenberg (1985), spray coating is discussed by Coeling (1985)
and powder coating by Richart (1982, 1985). A listing of the polymer
systems used for specific industrial coatings is given in Table 9.4. Epoxy
resin coatings are discussed in detail by Somerville and Smith (1973), Potter
(1970), Lee and Neville (1967) and Tess (1988a). Resin manufacturers offer
advice on the use of epoxy resins for the fomulation of coating compositions,
Ciba-Geigy (1988) and Shell (1991) offer a range of epoxy resins for surface
coatings, including Epon 828, with an epoxy equivalent-weight of 185-192.
This resin and also the Ciba-Geigy equivalent GY-250 have been
formulated as a heat-cured coating for strengthening glass (Chen et al.,
1992). Epoxy resins can be formulated to have much shorter cure times (1 to
30 minutes
at 240-135°C) than equivalent polyurethanes or polyesters (10 minutes
at 200°C) and have excellent adhesion to most substrates, provided the
surface has had an appropriate pre-treatment.

9.2.2 Surface preparation and primers

It is a primary requirement that coatings, when cured, should form a
continuous film. This depends not only on the rheological properties of the
epoxy formulation when applied, but also and in many cases more import-
antly, on the interfacial tension of the coating formulation and the surface
that is to be coated. Any surface features which may hinder film formation
must be treated prior to the coating operation. For the surface to be readily
wetted by the epoxy formulation, a critical condition is that the contact angle
between the formulation and the solid surface is low. Simple cleaning of the
surface by appropriate solvents may be sufficient surface preparation when
there is only surface contamination and non-adherent scale is absent. For
example iron or steel should be rust free. On the other hand, if the coating is
designed to do more than just cover the surface, specific treatments will be
required. For example, the application of a strengthening coating to glass
surfaces requires exposure to water vapour prior to coating to optimize
hydrolytic durability (Ellis et al., 1991). Other surface preparation methods
include mechanical abrasion, chemical etching, and flame treatment, to
name but a few. These are treatments used to modify different surfaces in
order to improve the adhesion of coatings. In chapter 8, there is a discussion
of surface preparation to achieve good adhesion and the same principles are
involved here. Misev (1991a) gives details of the surface treatment of
 COATINGS AND OTHER APPLICATIONS 311

aluminium, steel and galvanized steel prior to the application of a powder

coating.
A primer may be used to improve the 'wettability' of the surface and also
to protect a cleaned surface so that it will not become contaminated before
application of the coating. The primer has to adhere well to the substrate
surface and offer good wet ability and bonding of the coating. Silane and
other coupling agents can be used to improve bonding, especially to
hydrated surfaces. Typical coupling agents have two types of reactive
groups, one reacts readily with inorganic substrates and the other with
organic groups. Thus, coupling agents are often used in applications where
inorganic surfaces are to be bonded to an epoxy resin, and they may be
incorporated either into a primer or the coating. Coupling agents are
discussed in chapters 8 and 9.

9.2.3 Solution coatings

An important class of epoxy coatings are those in which the rheological
properties of the composition are adapted for the specific applications by the
use of a suitable solvent or solvent mixture. The rheological properties will
depend on the resin, its type and molecular weight, the pigment and its
concentration and the solvents. The apparent viscosities are normally within
the range of 0.05-1.0 Pas (0.5-10 poise), but it should be noted that
viscosities are highly temperature-dependent. Factors which determine the
selection of solvent include its vapour pressure and hence its rate of
evaporation and flash point, its toxicity as well as its effect on the flow
properties of the composition. However, the paramount condition for
selection of solvent is its miscibility with the epoxy resin together with the
hardener and any other additives present. Solutions should be stable for the
required shelf-life of coating composition and during application.
Selection of a solvent or solvent mixture which is miscible with epoxy
resins is aided by matching the Hidebrand's solubility parameters, <5 (see
Barton, 1983a), of the polymer and solvent. This is defined as
112
<5 = C 1I2 = ( - ~) = (ilf U/V)1I2

where C is the cohesive pressure, or cohesive energy density (cohesive

energy per unit volume), U the molar internal energy, that is the molar
potential energy of the material relative to the ideal vapour pressure at the
same temperature, V the volume, the ilf U is molar vaporization energy,
that is the energy required to vaporize a mole of the liquid to its saturated
vapour. For volatile liquids the solubility parameter (Grulke, 1989) is more
readily calculated from the molar enthalpy of vaporization, il TH,
<5 = [(Llf H - RT)/v]112
312 CHEMISTRY AND TECHNOLOGY OF EPOXY RESINS

where R is the gas constant and T the absolute temperature. For the use of
this equation the vapour phase must obey the ideal gas law. For epoxy resins
the value may be determined from swelling measurements or by calculation
useing additive group cohesive energy constants (see Barton, 1983).
The simple matching of solubility parameters is only suitable for an initial
selection of a solvent because other factors such as polar attraction force and
hydrogen bonding complicate solvent-solute interactions. Tess (1988b)
gives useful diagrams for solvent selection and Barton's (1983) solubility
maps for epoxy resins can also be consulted. Solvents for epoxy resins
include xylene, methyl ethyl ketone (MEK) , methyl isobutyl ketone
(MIBK) and proprietary solvents, such as oxitol. Mixed solvents are often
used so that not only solubility but the rate of evaporation and also the
rheological properties of the solution are optimized for the specific
application. Solvent mixtures include toluene with acetone, MEK, MIBK
and also xylene, MIBK and 2-ethoxyethanol (Lee and Neville, 1967).
After the selection of suitable solvents and subsequent formulation of a
coating solution, other factors need to be considered carefully to ensure
successful coating application. These include the condition of the surface
prior to coating, concentration ofthe solution, curing technique, pot-life of
the solution, etc. It should be noted that the coating solution is a dynamic
system within which chemical reaction (cure) progresses with time. There-
fore care should be taken in its storage. Also, the rate of evaporation of the
solvent in the system depends not only on the vapour pressure of the solvent,
but also the curing method used. Evaporation will be faster at higher
temperatures, but then the rate of cure is higher and the film may harden
before all solvent molecules diffuse out of the coating. The trapped solvent
molecules will affect the properties of the film which may be impaired and
may lead to poor adhesion. Thus, it is always important to consider all the
factors in order to establish a set of optimized application conditions.
The presence of pigment will have a considerable effect on the flow
properties of a coating solution. Nielsen (1977) discusses the effect of fillers
on viscosity and suggests a generalized relationship for the viscosity of a
dispersion, related to that of the fluid 'I,
/ _ 1 +AB¢2
'I '10 -1 - B1jJ¢2

where A = kE - 1; kE is the Einstein coefficient, B = 1 for rigid fillers,

¢2 is the volume fraction of pigment and ¢m is the maximum packing

fraction,
¢m = (True volume of the filler)/(apparent volume of the filler).
 COATINGS AND OTHER APPLICATIONS 313

However, the flow properties of dispersion systems are often very

complicated and require measurements over an appropriate range of rates
of shear (Utracki, 1988). They may be Bingham bodies with a yield stress,
pseudoplastics or thixotropic.

9.2.4 Dip coats

As probably the oldest and easiest method to apply coatings, dipping has
intrinsic advantages over other coating techniques. For example, compared
with spraying, dipping produces minimum material waste and provides, in a
single dip, a coating layer covering all the immersed parts. Furthermore, in
the dipping process, objects to be coated do not need to be spun to ensure
full coverage, which is inevitable for many spraying operations. The minus
side of dipping is also obvious. Firstly, since the object has to be dipped into
the fluid epoxy coating solution or emulsion, the size of the container (dip
tank) has to be large enough to accommodate a reservoir of material plus the
volume of the immersed part. This means that larger objects which can be
sprayed with ease would be more difficult to coat by dipping. Secondly, since
the container has to be constantly open during continuous operation,
evaporation of the carrier (solvent or water) will cause a change in the
concentration of the system in the dipping tank. Some overflow
arrangement may be installed in order to overcome such problem as well as
providing agitation.
In the actual coating process, the following points deserve careful and
combined consideration for successful applications.
• It is important to minimize the evaporation surface of the liquid.
This requires well thought out tank design.
• The material with which the tank is made must not react with the
coating solution or emulsion.
• Anti-contamination measures should be considered, depending on
the type of application.
• Degree of agitation should be determined to suit the application. For
example, continuous agitation may be required for some emulsion
systems which are likely to agglomerate.
• When the formulation is light sensitive, such as in the case of epoxy
acrylate-based UV curable systems, a light shield should be installed
to prevent unnecessary initiation of cure of the system in the tank.
• Dipping and withdrawal speed are the factors which need careful
monitoring for control of the coating thickness. The concentration of
the coating solution and the temperature difference between the
liquid and to-be-coated objects are also factors which affect coating
thickness.
314 CHEMISTRY AND TECHNOLOGY OF EPOXY RESINS

9.2.5 Epoxy emulsions and other water-based coatings

With the ever growing environmental concern over pollution by chemical
wastes it is expected that, in the not too distant future, tougher
environmental legislation will be introduced. This will certainly change
current industrial practice. As a direct consequence, many industrial
operations that are common today will have to be up-graded when chemical
wastes require further treatment before discharge into the environment, and
some may be banned all together. In either case, substantial capital invest-
ment will be inevitable to ensure on-going industrial production. Volatile
solvents are prime candidates for legislation, but at present the rate of
change is uncertain. Installation of solvent extraction and/or recovery
systems will involve additional costs and could mean that the operation loses
its commercial viability. The long-term solution may lie in changing the
carrier from organic solvents to water wherever possible. Research has been
carried out recently to extend the application of such water-based coating
systems.
To use water instead of solvent as carrier, the immediate difficulty is that
most epoxy resins do not form stable solutions in water. Thus, unlike organic
solvents, the system formulated will be heterogeneous at the colloidal level
when the resin is dispersed or emulsified with water as the continuous phase.
However, for many coating operations heterogeneity up to a certain scale
will not affect the properties of coatings. Water-based epoxy emulsion type
coatings are now readily available and also water soluble epoxy resins have
been produced by the introduction of hydrophilic groups.
Epoxy resin emulsions are produced by mechanical shearing to reduce the
particle size of the resin and addition of a suitable surface active agent to
stabilize the emulsion. One example, of the many of the emulsifiers
available that can be used to stabilize epoxy emulsions is

'O""-@-°frn,-rn,+ ]0

The hydrocarbon 'end' is soluble in the resin and the polar part of the
molecule is hydrophilic where the length of the ethylene oxide chain can be
chosen to suit the application. There are very many such emulsifying agents
available commercially, and mixtures may be used to ensure that an
emulsion is stable (Ash and Ash, 1980-1983). It is essential that the particle
or droplet size of the emulsion is less that one micro metre to ensure that the
emulsion does not settle out. Also, the emulsifying agent should be chosen
to ensure that the droplets do not aggregate or even worse, coalesce. The
stability of an emulsion is affected by shearing and hence the surface active
agent should form a protective layer around each droplet. Changes in
temperature also affect the hydrophile-lyophile balance (HLB) of the
surfactant (Shinoda and Friberg, 1986).
 COATINGS AND OTHER APPLICATIONS 315
To recapitulate from this discussion it can be easily appreciated that two
basic criteria for the selection of emulsifiers must be satisfied.
• Reduce the interfacial tension between water and resin
• Form protective films around the resin droplets.
Although analytical methods can assist in the selection of emulsifier, a
direct screening of the available emulsifiers is essential in practice to ensure
that all requirements are satisifed.
There are three sets of conditions that an epoxy coating emulsion must
comply with:
1. Preparation, stability and shelf-life
2. Application of the coating
3. Film formation.
The essential factors for the first set have already been outlined. For the
second set the rheological properties of the emulsion must be such that the
emulsion can be applied either by dipping, spraying or electrodeposition.
With dipping the film thickness is determined by 'drainage' of the emulsion
as the coated product is removed from the emulsion. The 'quality' of the film
formed depends on even drainage, such quality involves control of the
coating thickness as well as the avoidance of surface blemishes. It is
particularly important that there is even coverage, which is difficult to
achieve by spray application at places adjacent to corners, which should be
avoided if possible by alternative product design. The drip that forms in the
region of the 'neck' as the product leaves the coating bath can be trouble-
some. With spraying it is essential that fine droplets are formed since when
large they splash where they hit which can lead to a 'more or less grainy
surface' which eventually leads to a film with imperfect specular reflection.
The rheology of the emulsion will affect size reduction in the spray
equipment and also the flow of the emulsion on the coated surface which can
lead to the same type of defects that occur with dip coating, except for a
'neck' blemish. Design of equipment for the support of the product during
spraying is required so that the uncoated region adjacent to suspension
hooks or grips is unimportant or can be subsequently treated appropriately.
Film formation involves the evaporation of water which requires more
heat than for the removal of an organic solvent. The rate of diffusion of
solvent or water through a polymer film is also important since it is essential
that all of the low molecular weight species are removed prior to curing.
With evaporation of water, a stage will be reached when the emulsion
inverts and the epoxy resin becomes the continuous phase. Such a phase
inversion will depend on the concentration of water present in the film and
the temperature as well as other factors. The phase inversion temperature
(PIT) is discussed by Shinoda and Friberg (1986). Evenness and film quality
may be improved by the use of an organic solvent to reduce the viscosity of
the resin prior to cure. The flow properties of the uncured film will depend
316 CHEMISTRY AND TECHNOLOGY OF EPOXY RESINS

on the pigment, its type, concentration and particle size distribution as well
as the presence of other additives. The flexibility of the coating can be
improved by the addition of alkyd, polyester or acrylic resins, polymeric
amines and polyamides may be used as curing agents. A can coating has
been formulated from a higher molecular weight epoxy resin (M. wt -15-
20 000) which was initially dissolved in solvent and then dispersed in water
and cured with either a melamine or urea formaldehyde resin. The baked
films were 'flexible' and resistant to the pasteurization conditions required
for beer cans. Water-based epoxy resin coatings can be formulated for
electrodeposition, both anodic and cathodic (Brewer, 1985). These coatings
were developed for the applicance and automobile industries with cathodic
systems being replaced by the anodic coatings because higher corrosion
resistance can be produced. With electrodeposition (ED) the loss of paint
due to overspray is eliminated, so that the process offers advantages
compared with spraying since with ED it is possible to obtain more uniform
coatings even in recessed regions. Very many formulations have been
proposed for water-based coatings for dipping, spraying or electro-
deposition.
It is possible to build hydrophilicity into an epoxy resin and an example is
the reaction product of an epoxy resin with dibasic acids or anhydrides, and
many different combinations have been evaluated. The synthesis of
hydrophilic epoxy resins may be illustrated by the following reactions with
both epoxy and hydroxyl groups,

oII
C-OH
R'"
'C-OH
II
o

oII 0
II
OH
O-C-R'-C-OH
+ *-CH 2-tH-CH 2-*
- *-CH 2-CH-CH
I 2- *

The carboxyl acid groups are, when neutralized, hydrophilic and the higher
the acid content the more readily dispersible these resins are in water
containing amines or inorganic bases. The degree of hydrophilicity IS
controlled by the extent of neutralization of the pendant acid groups.

9.2.6 Powder coatings

Powder coatings with 100% solids content do not contain a solvent or water
carrier. They are blends of liquid and solid resins with curing agents plus
 COATlNGSAND OTHER APPLICATIONS 317
other additives. Thus pollution, fire and explosive hazards are avoided in the
coating plant. Also thick coatings can be applied in a single operation, with
high application rates. The technology of powder coating is the subject of a
recent text (Misev, 1991). There are many varied formulations and several
different methods of application may be used. To formulate powder coating
systems, high molecular weight (usually over 1000) epoxy resins are
compounded with hardeners and other additives such as a flow-control
agent, pigments, fillers and catalysts. The exact combination depends on
requirements of the specific application. Fusion and dry blending are
commonly used techniques in producing appropriate powder formulations.
It should be noted that some dry-blending in a Z-blade mixer may require
heating the components to a relatively 'high' temperature, thus formulations
with reactive hardeners must be avoided. Latent hardeners may be used or
two part mixes, with the hardener and catalyst kept separately until the
coating is applied. The compounded resin is ground in a ball-mill or other
comminuting machine so that it is fine enough to pass a 45 mesh, with a
particle size less than 350 {lm. It may be finer for specific applications. The
hardeners that may be used include dicyandiamide, amine adducts,
polyesters and phenolic resins. Very fast cures of 30 seconds with tempera-
tures up to 250°C can be obtained with acid anhydrides catalysed with
stannic octoate. The anhydride and catalyst can be mixed into separate
portions of resin to avoid premature cure. Also, this type of system is
sensitive to water, which will be absorbed during storage unless suitable
precautions are taken. Water absorption reduces the hardener efficiency
which leads to poorly cured coatings.
Methods of application which are frequently used include fluidized-bed,
electrostatic spray, flock spray, and electrostatic fluidized-bed (Richart,
1982, 1985).
Powder coatings have excellent electrical resistance and thermal stability.
These are properties attractive to electrical and electronic industries as
coatings for vital components which are solvent sensitive and where contact
with water must be avoided. Coating underground pipes is another
important application due to the good resistance to corrosive soil and
cathodic disbonding of such coatings. It can also be applied to many general
objects as protective and decorative coatings. The composition of powder
coatings can be varied to suit the requirements of specific applications.
The adoption of powder coating may involve expensive investment
especially when changing from another type of coating line, colour matching
may also be difficult. However, powder coatings have many advantages over
other coating methods which include, little pollution or fire hazard
problems, readily automated process technology, wide range of coating
thickness possible in one application with exceptional film quality.
318 CHEMISTRY AND TECHNOLOGY OF EPOXY RESINS

9.3 Industrial and related applications

9.3.1 Tooling
Epoxy resin tooling was initially used in the aircraft and car industries but
has now been extended to mould and tool making for many different end-
products, such as ship and boat building, domestic equipment and pottery.
A combination of techniques similar to those used for casting and laminating
are used for the production of moulds and foundary patterns. The resins are
usually formulated with a liquid resin cured with a room temperature
hardener. The properties are modified by the use of additives which not only
improve specific properties but also usually reduce materials' costs. Tools
can usually be fabricated in a fraction of the time required to machine metal
and the costs of epoxy tooling are much lower than conventional metal tools.
Epoxy resins are especially advantageous for the construction of prototypes
and tooling for short runs. Lee and Neville (1967) have a chapter on epoxy
resin tooling in which much detailed information is given which is still
relevant. Special resins for tooling applications can be formulated and
details are available from resin manufacturers. Some of the more important
tooling applications of epoxy resins include:
1. Moulds for various purposes such as form master moulds for vacuum
forming and injection moulding.
2. Jigs and fixtures to assist accurate positioning and checking shapes
and dimension of components for such operations as drilling, cutting,
and welding.
3. Metal-forming tools to replace the much more costly steel tools for
prototype and short run production. Examples of such tools are
stretch blocks, press tools, drop-hammer punches and dies. A rubbery
resilient facing on one of the tools of the set may be required, e.g. for
a drop hammer.
4. Foundry patterns ranging from master moulds from the original
pattern, duplicate patterns made from the master mould to core
boxes.
The benefits of using epoxy resins instead of traditional steel or aluminium
alloy as tooling materials are numerous. In addition to facile forming of
complex shapes the resins have low cure shrinkage and close tolerance can
be maintained. Also, the inert nature of epoxy resin to both heat, when
operational temperature is below its glass transition temperature, and
chemical attack, which may corrode steel, ensures shape stability. Other
important advantages include lower costs than for metals, lighter weight,
also the shape of the tool can be readily adjusted, and lead-times are shorter.
Epoxy resin formulations for tooling applications are normally room
temperature curable systems made up with liquid BADGE-type resins with
 COATINGS AND OTHER APPLICATIONS 319
polyamines (normally aliphatic) as hardeners. Depending upon specific
application, diluents, plasticizers, fillers can also be introduced into the
system for the purpose of reducing cost and improving required physical and
chemical properties. In cases where special toughness is required, laminates
of epoxy resin with various fibres can also be used. Open mesh 'tooling
fabrics' are available and higher stiffness can be attained by the use of carbon
fibre reinforcement (chapter 8).

9.3.2 Civil engineering

In the field of civil engineering the consumption of conventional materials
such as concrete, wood, metal and glass is astronomical. Epoxy resins
exhibit excellent adhesion to these materials and therefore have been used
widely for surfacing, coatings and for repairs (Maslow, 1979). The most
frequent application of epoxy resins in civil engineering include:
1. Flooring
2. Road and bridge coatings
3. Concrete bonding and repair.

9.3.2.1 Flooring Epoxy flooring can be formulated to be 100% non-

volatile and 100% reactive so that one inch thick coatings may be applied in
a single application. Floors of industrial premises such as chemical plants
and warehouses are subject to constant and severe abuse of various forms.
Typical examples are chemical spillage, harsh detergent cleaning, and heavy
frictional wear by vehicles such as forklifts etc. Since floors of such industrial
buildings are usually constructed with materials such as concrete, which
under adverse service conditions are likely to gradually and then rapidly
disintegrate, epoxy resins have been found to be very useful in aiding the
maintenance of such floors, due to their chemical resistance, high
mechanical strength and facile application.
In formulating a flooring system, the epoxy resin is usually mixed with
hardeners, fillers and pigments according to service requirements.
Colouring materials can also be added to suit different purposes. Prior to
application of the resin the surface has to be thoroughly cleaned and pre-
treated with an epoxy or other primer. Depending on the viscosity of the
formulated system, it can be applied either by brush, roller, squeegee or
trowel. For high viscosity formulations such as those primarily for wear
resistance where large quantities of solid filler are added, the material has to
be screeded by trowelling. On the other hand, low viscosity formulations for
covering continuous floors with normal wear conditions can be applied with
a squeegee or plastic comb. Self-levelling epoxy flooring compositions are
formulated so that their viscosity is low. Epoxy flooring may be classified
into three types, seamless flooring, industrial or epoxy concrete and terrazzo
320 CHEMISTRY AND TECHNOLOGY OF EPOXY RESINS

flooring. Seamless flooring has a 'low' viscosity due to the use of a reactive
diluent and can be applied in thickness of 10--60 mm. They can be used to
coat Portland cement floors, wood or metal after a specific surface
preparation before application of the resin. Industrial epoxy flooring or
epoxy concrete contains 75-85% particulate filler, usually sand (10--100
mesh). With high filler loadings the viscosity is high and the composition is
applied by trowelling with thickness of 3-25 mm CI8 to 1 inch). These
compositions have low shrinkage and may be used to repair cement floors.
Epoxy terrazzo floors contain coloured stone or marble chips and a fine
particulate filler, such as calcium carbonate, and are trowelled to give 6-
12 mm (I;cI;2 inch) thick coatings. After setting overnight they are wet-
ground to a polished terrazzo surface which is readily maintained.
The advantages of epoxy flooring may be summarized as follows:
1. Ease of seamless application, room temperature cure with low
shrinkage
2. Excellent adhesion to many substrates
3. Chemical resistant and durable
4. Tough, flexible and abrasion-resistant
5. Dust-free, skid-resistant and readily maintained.

9.3.2.2 Road and bridge coatings. Epoxy resin coatings have been used
to protect and repair road and bridge surfaces. Spalling is a commonly
encountered problem on the surfaces of roads and bridges. It is caused by
the combination and accumulation of many different factors such as freeze-
thaw cycles, spillage by vehicles, road gritting and de-icing operations.
Epoxy resins that are used for the treatment of these conditions are
usually liquids cured by amine hardeners at ambient temperatures. Coal tar
is used as extender and often in high proportions in their formulation. When
coated onto the structure and cured, the resin formulation forms a film
which prevents the spalling of concretes on the road surface and bridge
structure. Furthermore, the use of such formulations to seal cracks can
significantly slow down their growth by bonding the damaged sections into
an integral part. It also limits water penetration into the structure of the
bridge. Ingress of water to steel reinforcement or framework causes
corrosion. Hence epoxy resin coatings can give substantial protection
against such damage. Some other applications of epoxy resin road and
bridge coatings include:
1. Repair of scaling concrete roads with 'thin' coatings
2. Repair of 'slippery' but otherwise satisfactory asphalt or concrete
road surfaces
3. Fuel spillage protection
4. Lightweight surfacing for bridge decks
 COATINGS AND OTHER APPLICATIONS 321

5. Waterproof membrane between concrete and an asphalt topping

6. Upgrading additive for bituminous surfacing compositions
7. Road marking and coloured surfaces.

9.3.2.3 Concrete bonding and repair. Use of epoxy resin formulations as

concrete bonding and repairing materials finds application over much wider
areas than just bridges and roads (Allen and Edwards, 1987). Virtually, any
structure with concrete as its primary construction material will develop
cracks with age. Epoxy formulations can be coated onto the surface to
prevent the initiation of such cracks and also to repair existing cracks.
Structural defects occurring in other materials such as wood, brick and metal
can also be repaired by epoxy formulations. Special formulations are
available from resin suppliers.

9.3.3 Moulding compounds

Consideration of the application of epoxy resin moulding powders requires
comparison with both alternative thermosetting resins and also with casting
which is an alternative method of processing liquid epoxy resins. Epoxy
resins can be moulded (Lee, 1988) by either compression or transfer
moulding and also liquid-injection moulding. Reaction injection moulding
also offers processing advantages (Becker, 1979; Manzione, 1989).
The superiority of epoxy resins compared with conventional phenol, urea
and melamine formaldehyde resins is that, during cure, volatile products are
not evolved and hence their moulding shrinkage is much lower. Also, they
can be formulated so that at the moulding temperature they have lower
viscosities so that much lower moulding pressures are required. For con-
ventional P/F, U/F or M/F resins the moulding pressure is 1000 to 5000 psi
(7-20 MPa) whilst for epoxy resin moulding powders the pressure can be
very low, always less than 1000 psi. Thus presses and auxilary equipment can
be simpler, and the moulding tools will also have to withstand lower stresses.
However, because of the low viscosity of the epoxy resin at moulding
temperatures there may be a problem due to excessive flash and 'tight'
fitting moulds are essential. Some of the advantages of epoxy moulding
powders compared with other thermosetting resins may be listed as follows.
1. Better dimensional tolerances and stability
2. Better electrical and mechanical properties
3. Higher chemical and heat resistance
4. Lower water absorption (depends on hardener and filler).
The disadvantages as well as higher cost include the need for closer mould
temperature control, mould release and ejection problems. Mould release
can be difficult because of the adhesive properties of the epoxy resin, which
322 CHEMISTRY AND TECHNOLOGY OF EPOXY RESINS

of course is often an advantage since the epoxy resins bond well to metal
inserts. At the moulding temperature the cured product is flexible and
ejector pins can cause damage to the moulding. This can be avoided by
suitable design so that excessive local stresses are avoided during ejection of
the moulded part.
Compared with casting the moulding of epoxy resins offers the advantages
of simpler handling, higher filler loadings and also fibrous reinforcement. Its
disadvantages include higher capital costs and storage problems with the
uncured moulding powder. Their shelf-life may be limited due to incipient
cure if the storage temperature is not low, for example it may be only several
weeks at 22°C but much longer in sealed packages refrigerated at 7°e.
Epoxy moulding powders are formulated with a mixture of solid or liquid
BPA resins which may be blended with epoxy novolacs. The latter improves
the flow properties and has higher functionality and hence decreases gel and
cure time. The curing agents are often diaminodiphenol methane (DDM) or
acid anhydrides (PA or THPA), but others may be used for specific
purposes, for instance chlorine-containing curing agents have been
suggested for improving flame resistance. The moulding powders contain
suitable particulate fillers, such as silica flour, and can be reinforced with
fibres, 1.5-25 mm (116 to 1 inch) long. Zinc stearate is often included as a
release agent, to assist ejection of the cured product from the mould.
Thermal conductivity can be increased by use of metal powder as fillers, but
should be avoided when the electrical properties are important. The density
of the filled resins ranges from about 1.6 to 2.2. Lower densities, 0.75-1.00,
can be obtained by the incorporation of microballons, composed of either
clay, U/F or P/F resins or glass (Lee and Neville, 1967a). A major
application of moulding powders is the encapsulation of electrical
equipment and electronic devices. It is essential that during moulding the
resin flows so that the component is not damaged or displaced. Typical
moulded parts include deposited carbon resistors; chokes; ceramic,
polyester and glass capacitors; metallized thin-film resistors; pulse trans-
formers; toroid, solenoid and relay coils; printed circuits, solid-state circuits
and many more products (Lee, 1986, 1988). Epoxy resin moulding powders
also have applications in chemical plant, such as valves and filter plates
where their chemical resistance offers major advantages. Structural
components with good thermal and chemical resistance coupled with good
mechanical properties and light weight are also moulded from epoxy resins.

9.3.4 Embedding
Embedding is used to protect and/or decorate electrical or electronic
devices. Allied and related terms are casting, potting, impregnation and
encapsulation. Moulding has been discussed in the section on moulding
powders. The electrical properties required for electrical applications have
 COATINGS AND OTHER APPLICATIONS 323
been dealt with in chapter 6. Protective functions of the embedding resin
include exclusion of moisture, oxygen, salt spray, solvent, other chemicals
and microorganisms. The encapsulation of a device also protects it from
mechanical shock and vibrations and provides suitable locations for
mechanical handling. Alternative materials to epoxy resins include
silicones, polyurethanes, polyesters, polysulphides and allyl resins (Lee,
1988). The epoxy resins have low viscosities and shrinkage since volatile
compounds are not formed during cure. By suitable formulation it is
possible to obtain a range of properties, from the rigid to flexible,
elongations at break ranging from 3 to 40%. BPA resins and also mixtures
with epoxy novo lacs are used for potting. Cycloaliphatic epoxy resins have
good dielectric properties and excellent weatherability, which is due to the
absence of chlorine and aromatic rings. The cycloaliphatic resins require
acid anhydride hardeners and longer high temperature cures. The pro-
perties of the resin can be modified by the inclusion of fillers and flexibilizers
(chapter 4). Typical particulate fillers include silica, 'talc, clay and calcium
carbonate, mica and chopped glass. Metal powders can be used to increase
the thermal conductivity of the product. Lower density products can be
produced by the inclusion of micro spheres of saran, PIF resin or glass.

9.3.5 Miscellaneous
It is almost an impossible task to list all the existing applications of epoxy
resins since there are currently many new developments. However, it is
worth mentioning several areas in which epoxy resins are used, although on
a small scale, but nevertheless for important specialized applications, such
as textile finishes (Tanaka and Shiozaki, 1988).

9.3.5.1 Oil field applications. Epoxy resins are used in drilling operations
to consolidate poor well formations, especially where the geological
structures are primarily sandy. Resin compositions are adsorbed onto the
well walls and subsequently cross-linked. Such treatment is particularly
effective in minimizing sand production, which could eventually result in the
well being discarded.

9.3.5.2 As reactant to form epoxy-based polymers. Epoxy resins are very

reactive polymers. Their reactions with other chemicals as well as polymers
can result in numerous new ranges of materials so far as these chemicals and
polymers have functionalities smaller than three. For example, reacting
epoxy resin with acrylic acids yields products, the epoxy acrylate resin,
which are curable by UV irradiation.

9.3.5.3 Epoxy resin foams. Blown foams can be formed with epoxy resin
formulations where a blowing agent is present. During the initial stages of
324 CHEMISTRY AND TECHNOLOGY OF EPOXY RESINS

cure such an agent either evolves a gas or evaporates to leave voids in the
cured network. For example, hydrazide derivatives can be mixed into the
epoxy formulation; with the heat generated by cure or provided by heating
elements, they decompose to form nitrogen which is the blowing agent.

9.3.5.4 Epoxy resin stabilizers. Epoxy compounds also find application

as stabilizers and plasticizers for other polymeric materials. For instance
poly (vinyl chloride), PVC, readily dehydrochlorinates especially at
elevated temperatures. The dehydrochlorination reactions are complex and
appear to be autocatalytic (Hawkins, 1989). Thus, for PVC to be processed
it is essential to add stabilizer which 'sequesters' the hydrochloric acid that is
formed. The epoxy group reacts readily with the HCI as it is formed and also
there is a synergistic effect when metal salts are present as well (Stephenson,
1989). Not only are BPA type epoxy resins effective stabilizers for PVC but
also- epoxidized oils, esters and fatty acids will both function as stabilizers
and plasticizers.

References

Allen, R.T.L. and Edwards, S.C. (eds) (1987) The Repair of Concrete Structures, Blackie,
Glasgow, UK.
Ash, M. and Ash, I. (1980-1983) Encyclopedia of Surfactants, Vol. 1-3. Chemical Publishing
Co., New York.
Barton, A.F.M. (1983a) CRC Handbook of Solubility Parameters and Other Cohesion
Parameters, Ch. 1,2. CRC Press, Boca Raton, USA, pp. 1-21. (1983b) Many references to
epoxy resins, see Index.
Becker, W.E. (1979) Reaction Injection MOUlding Van Nostrand Reinhold Co., New York.
Bonneau, M. and Burkhart, A (1991) MEPPE Focus (Jan.), pp. 1-6.
Brewer, G.E.F. (1985) Ency. Polym. Sci. Eng. (Suppl.) Wiley-Interscience, New York,
pp.675-fJ87.
Burkhart, A (1988) Hybrid Circuit Technology, pp. 222-228.
Chen, X.M., Ellis, B. and Seddon, A.B. (1992) Paper prepared for publication but submission
delayed until after patent application.
Ciba-Geigy (1988) Araldite Resins and Hardeners for Surface Coating Systems, Ciba-Geigy
Plastics, Cambridge, UK.
Coeling, K.J. (1985) Ency. Polym. Sci. Eng., Vol. 3, Wiley-Interscience, New York, pp. 567-
575.
Ellis, B., Chen, X.M. and Seddon, AB. (1991) British Patent Application.
Emerson, J.A., Sparapany, J.J., Martin, A.R., Bonneau, M.R. and Burkhart, A (1990)
IEEE, 40th Components and Technology Conference Vol. 1, pp. 6()()....605.
Grulke, E.A (1989) Polymer Handbook, VII 3rd edn ed. Brandrup, J. and Immergat, E.H.
Wiley, New York, pp. 519-559.
Hawkins, W.C. (1989) Enc. Polym. Sci Eng. 2ndedn Vol. 15, Wiley, New York, pp. 560-563.
Lee, H. and Neville, K. (1967) Handbook of Epoxy Resins, McGraw-Hili, New York, pp.I-16.
(1967b) Ch. 23, pp. 1-5, Ch. 24, 7-40.
Lee, S.M. (1986) Ency. Polym. Sci. and Eng. Wiley-Interscience, New York, Vol. 5., pp. 792-
828, see p. 818.
Lee, S.M. (1988) Epoxy Resins, Chemistry and Technology 2nd edn, ed. May, C.A, pp. 799-
810.
Manzione, L.T. (1989) Enc. Polym. Sci. Eng. 2nd edn. Wiley-Interscience, New York,
pp.72-100.