EPOXIDE RESINS

Definition
Introduction
Bisphenol A based resins
Glycidyl ester resins
Glycidyl amine resins
Glycidyl ethers of novolac resins-
Brominated resins
Diluents
Reactive diluents
Non-reactive diluents.
Curing Agents for Epoxide Resins.
DEFINITION
Epoxy resin is defined as a molecule containing more than one epoxide
groups. The epoxide group also termed as, oxirane or ethoxyline group, is
shown below
These resins are thermosetting polymers and are used as adhesives, high
performance coatings and potting and encapsulating materials. These
resins have excellent electrical properties, low shrinkage, good adhesion to
many metals and resistance to moisture, thermal and mechanical shock.
Viscosity, epoxide equivalent weight and molecular weight are the
important properties of epoxy resins.
INTRODUCTION
Epoxide or epoxy resins are materials which contain two or more epoxide
groups or more generally glycidyl groups per molecule.
The same terms are, however, also used to describe the cured materials.
The uncured resins range from free flowing liquids to high melting solids,
which can be cured or cross-linked into hard infusible materials by reaction
with an appropriate curing agent.

epoxide groups glycidyl groups

The curing reaction is brought about by the addition of a suitable di- or
poly-functional curing agent to the resin.
Typical curing agents include primary and secondary amines,
polyamides and organic anhydrides.
These are generally used in roughly stoichiometric proportions with the
resin and may require heat to effect cure.
Other curing agents used are the catalytic curing agents, such as the
boron trifluoride complexes.
These are used in catalytic amounts and may require either ambient
temperature or heat cure. No by-products are evolved during cure.
The resultant cured resins are generally hard thermoset materials with
excellent mechanical, chemical and electrical properties.
Epoxide resins are widely used in coating, adhesive, flooring,
laminating and casting applications.
One of the main advantages which epoxide resins have over polyester
resins is their low shrinkage during cure. This is generally between 1
and 2% but can be reduced to virtually zero by the addition of fillers.
Epoxide resins are usually classified according to their epoxide
equivalent weight or Epoxy Molar Mass (EMM).
This is the equivalent weight in grams of resin containing one
epoxide group. Alternatively this may be expressed as epoxide
content, which is the number of equivalents per kilogram of resin.
Resins used for laminating applications fall into two broad classes:
(a) the liquid resins-used mostly for wet lay-up and site applications,
although some are used for prepregs, and
(b) the low molecular weight solid resins-used in solution for making
prepregs.
Types of Epoxy Resins:
There are two main categories of epoxy resins, namely the glycidyl epoxy,
and non-glycidyl epoxy resins.
The glycidyl epoxies are further classified as glycidyl-ether, glycidyl-ester
and glycidyl-amine.
The non-glycidyl epoxies are either aliphatic or cycloaliphatic epoxy
resins.
Glycidyl epoxies are prepared via a condensation reaction of appropriate
dihydroxy compound, dibasic acid or a diamine and epichlorohydrin.
While, non-glycidyl epoxies are formed by peroxidation of olefinic double
bond.
Glycidyl-ether epoxies such as, diglycidyl ether of bisphenol-A (DGEBA)
and novolac epoxy resins are most commonly used epoxies.
MONOMERS
Epoxy resins are typically formed by the reaction of compounds
containing at least two active hydrogen atoms (polyphenolic
compounds, diamines, amino phenols, heterocyclic imides and
amides, aliphatic diols, etc.) and epichlorohydrin.
THE RESINS

These can be conveniently divided into six classes of resins:

1. bisphenol A based;
2. glycidyl esters;
3. glycidyl amines;
4. novolacs;
5. brominated resins;
6. cycloaliphatic and other resins.
BISPENOL A BASED RESINS
They range from medium viscosity liquids through to high melting
solids and are all prepared by the reaction between epichlorohydrin
and bisphenol A.
With some ten times excess epichlorohydrin the product is virtually
pure monomer, i.e. two molecules of epichlorohydrin plus one
molecule of bisphenol A.
As the proportion of epichlorohydrin is reduced, so the molecular
weight of the resin is increased.
All the resins produced in this way consist of mixtures of different
molecular weight species and it is this molecular weight distribution
which governs the viscosity/melting point of the resin.
The synthesis of diglycidyl ether of bisphenol A (DGEBA), the most widely used epoxy
resin monomer, is:

Synthesis of Epoxy Monomer from Bisphenol A and Epichlorohydrin

The oxirane group of an epoxy monomer reacts with different curing
agents such as aliphatic amines, aromatic amines, phenols, thiols,
polyamides, amidoamines, anhydrides, thiols, acids and other
suitable ring opening compounds; forming rigid thermosetting
products.
The cured epoxies are brittle in nature due to the high degree of
cross-linking, and they contribute to weakening epoxy impact
strength and other relevant properties.

Hence, modification of epoxy monomers is necessary to improve

their flexibility and toughness as well as thermal properties.
The three primary classes of epoxies used in composite applications
are:
Phenolic glycidyl ethers
Aromatic glycidyl amines and
Cycloaliphatics
The commercially available liquid resins (EMM range 180-196) have a
tendency to crystallise during storage.
This is due to their relatively high purity. Most contain in excess of 90%
of the pure diglycidyl ether of bisphenol A, which is a colourless
crystalline material with a melting point of 44· 5-46°C and an EMM
(epoxy molar mass) of 170 (molecular weight 340). These resins can
therefore be considered to be supercooled liquids.
GLYCIDYL ESTER RESINS
Whilst numerous resins of this type can be made from all types of diacids,
few are in commercial production.
A resin such as the one shown offers considerable advantages over the
bisphenol A based resins in terms of viscosity, outdoor weathering and
tracking resistance.
Such resins are used for high voltage electrical insulators and heat cured
laminates. In general, resins of this type are used with anhydride curing
agents, to give systems with viscosities and reactivities ideally suited to
vacuum impregnation, laminating and casting applications.
GLYCIDYL AMINE RESINS
Such resins can be made from most diamines and may be liquids or solids.
One such resin is the tetraglycidyl amine of 4,4' -diaminodiphenylmethane.
Other resins may be based on secondary amines such as hydantoin, where a
diglycidyl amine resin results.
Few resins of this type are commercially available. Those which are
available are mostly cured with anhydrides to give products with
good high temperature strength retention and good resistance to
nuclear radiation.
These systems are used for wet lay-up laminating and filament
winding.
GLYCIDYL ETHERS OF NOVOLAC RESINS

Novolac resins are produced when phenol and formaldehyde react together
in acid solution. The simplest novolac is bisphenol F.
These resins are then reacted with epichlorohydrin to form glycidyl ether
novolacs. The bisphenol F resin is a liquid while the higher molecular weight
resins are solids.
Such resins, when cured, offer high temperature stability and good chemical
resistance but have the disadvantage, with certain curing agents, of having a
low strain at break (1-1'5%). Thus they can fail by interlaminar shear if used
in 90° cross-plied carbon fibre composites, due to the strain induced by the
negative coefficient of thermal expansion of the fibre along its length.
 BROMINATED RESINS
Various commercial brominated resins are available with different bromine
contents. They may consist of resins prepared from tetrabromobisphenol A alone
or blends with normal bisphenol A resins.
The structure of the diglycidyl ether of tetrabromobisphenol A is:

o These resins are used to confer flame retardancy to composites. They are
mostly used in the manufacture offire retardant printed circuit boards
DILUENTS
Diluents are added to epoxide resins primarily to lower viscosity and thus
improve handling characteristics.
They also modify the cured properties of the resin. In general terms, the
higher the proportion of diluent added to the resin the lower the viscosity and
the lower the mechanical properties and chemical resistance of the cured
system.
Diluents can be divided into two classes: (a) the reactive diluents such as the
monoglycidyl ethers and (b) the non-reactive diluents. The reactive diluents
are the most widely used, since they form an integral part of the cured
structure and cannot be leached out.
REACTIVE DILUENTS
These are in the main monoglycidyl ethers although some diglycidyl
ethers such as butane-I,4-diol diglycidyl ether are used.
These compounds contain only one epoxide group per molecule, they
can be considered as chain stoppers in that they reduce the
functionality of the system and therefore reduce cross-link density.
They also alter the epoxide equivalent weight of the blend.
NON-REACTIVE DILUENTS
A wide variety of non-reactive diluents or extenders can be added to
epoxide resin compositions either to reduce viscosity or to reduce cost or
both.
These include such materials as liquid coal tars, pine oil, phthalate
plasticisers, benzyl alcohol and furfuryl alcohol.
Most of these materials are used in what can be described as civil
engineering applications, that is flooring, coating, repair mortars and
adhesive applications.
Three, however, may be used in laminating applications. These are dibutyl
phthalate (DBP), and benzyl and furfuryl alcohols. They are generally only
used in room temperature cured systems.
Benzyl alcohol is also used as a diluent in curing agent formulations.
CURING AGENTS (HARDENERS)
A wide variety of curing agent for epoxy resins is available depending on
the process and properties required.
The commonly used curing agents for epoxies include amines, polyamides,
phenolic resins, anhydrides, isocyanates.
The cure kinetics and the Tg of cured system are dependent on the
molecular structure of the hardener.
The choice of resin and hardeners depends on the application, the process
selected, and the properties desired.
The stoichiometry of the epoxy-hardener system also affects the properties
of the cured material. Employing different types and amounts of hardener
which, tend to control cross-link density vary the structure.
The amine and phenolic resin based curing agents, described below, are
widely used for curing of epoxy resins.
Amine based curing agents:
Amines are the most commonly used curing agents for epoxy cure.
Primary and secondary amines are highly reactive with epoxy.
Tertiary amines are generally used as catalysts, commonly known as
accelerators for cure reactions.
Use of excessive amount of catalyst achieves faster curing, but
usually at the expense of working life, and thermal stability. The
catalytic activity of the catalysts affects the physical properties of the
final cured polymer.
Phenolic novolac resins:
Epoxy resins when cured with phenolic hardener, gives excellent
adhesion, strength, chemical and flame resistance.
Phenolic novolac-cured epoxy systems are mainly used for
encapsulation because of their low water absorption, excellent heat
and electrical resistance.
An accelerator is necessary for the complete cure to occur.
PROPERTIES
High strength
Low Shrinkage
Excellent adhesion to various substrates
Effective electrical insulation
Chemical and solvent resistance, and
Low cost and low toxicity
Epoxies are easily cured, and they are also compatible with most substrates.
They tend to wet surfaces easily, making them especially suitable for composite
applications. Epoxy resin is also used to modify several polymers such
as polyurethane or unsaturated polyesters to enhance their physical and chemical
attributes.
The tensile strength ranges from 90 to 120 MPa
A tensile modulus ranging from 3100 to 3800 MPa
Glass transition temperatures (Tg) that range from 150 to 220 ◦C
Aside from the properties mentioned above, epoxy resins have two main
drawbacks which are their brittleness and moisture sensitivity.
APPLICATIONS
Epoxy resins are of particular interest in structural composite
applications because they provide:
Unique balance of chemical and mechanical properties
As well as extreme processing versatility
Some of their most interesting applications are found in the aerospace
and recreation industries where resins and fibers are combined to
produce complex composites structures. Epoxy resins satisfy a
variety of non-metallic composite designs in commercial and
military aerospace applications including flooring panels, ducting,
vertical and horizontal stabilizers, wings etc.
Epoxy composites are also used to produce lightweight parts for
automobiles, rails, bicycle frames, golf clubs, snowboards, racing
cars and musical instruments.
These applications use complex epoxy formulations which will
include multiple epoxy resins with modifiers for toughness or
flexibility, or flame suppression, fillers for strength, pigments for
colors, curing additives that promote curing reactions.
High temperature applications can be improved by the use of higher
functionality resins, which increases crosslink densities and improves
thermal and chemical resistance.
Epoxy resin (VII) based on tris (hydroxyl phenyl) methane is one of
the important epoxy resins used in high performance applications. At
elevated temperatures, this resin shows excellent:
Physical and electrical properties
Moisture resistance
Formulation stability
Reactivity and retention of properties