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.
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.
Diglycidyl Ether of Bisphenol-A (DGEBA):
Diglycidyl ether of bisphenol-A (DGEBA) is a typical commercial epoxy resin and is synthesised by reacting bisphenol-A with epichlorohydrin in presence of a basic catalyst.
structure of DGEBA
The properties of the DGEBA resins depend on the value of n, which is the number of repeating units commonly known as degree of polymerisation The number of repeating units depend on the stoichiometry of synthesis reaction. Typically, n ranges from 0 to 25 in many commercial products.
Novolac Epoxy Resins:
Novolac epoxy resins are glycidyl ethers of phenolic novolac resins. Phenols are reacted in excess, with formaldehyde in presence of acidic catalyst to produce phenolic novolac resin. Novolac epoxy resins are synthesised by reacting phenolic novolac resin with epichlorohydrin in presence of sodium hydroxide as a catalyst.
Structure of novolac epoxy resin
Novolac epoxy resins generally contain multiple epoxide groups. The number of epoxide groups per molecule depends upon the number of phenolic hydroxyl groups in the starting phenolic novolac resin, the extent to which they reacted and the degree of low molecular species being polymerised during synthesis. The multiple epoxide groups allow these resins to achieve high cross-link density resulting in excellent temperature, chemical and solvent resistance. Novolac epoxy resins are widely used to formulate the moulding compounds for microelectronics packaging because of their superior performance at elevated temperature, excellent mouldability, and mechanical properties, superior electrical properties, and heat and humidity resistance.
Curing of Epoxy Resins
The curing process is a chemical reaction in which the epoxide groups in epoxy resin reacts with a curing agent (hardener) to form a highly crosslinked, three-dimensional network. In order to convert epoxy resins into a hard, infusible, and rigid material, it is necessary to cure the resin with hardener. Epoxy resins cure quickly and easily at practically any temperature from 5-150oC depending on the choice of curing agent.
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 and polymercaptans. 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. Figure 2.4 shows cure reaction of epoxy resin with phenolic hardener.
Rubber Toughening of Epoxy Resins
The usefulness of epoxy resins in many engineering applications is often limited by their brittle nature and poor thermal conductivity. The term toughness is a measure of material's resistance to failure i.e. the total amount of energy required to cause failure.
There are several approaches to enhance the toughness of epoxy resins which includes: (i) chemical modification of the epoxy backbone to make it more flexible structure, (ii) increasing the molecular weight of epoxy, (iii) lowering the cross-link density of matrix, (iv) incorporation of dispersed toughener phase in the cured polymer matrix, and (v) incorporation of inorganic fillers into the neat resin.
Amongst these approaches, toughening via dispersed toughener (flexibiliser) phase has been shown to be most effective. The flexibilisers can be reactive or non-reactive rubber.
Various types of thermoplastic polymers as well as reactive rubbers are employed to enhance toughness of epoxy resin. Thermoplastic polymers, such as polyetherimide, polysulphone, polyethersulphone, and polycarbonate have been studied to modify epoxy resins. These studies show significant improvement in the toughness of epoxy resins.
The reactive rubbers used for toughening epoxy resins include, liquid acrylonitrile-butadiene copolymers with various terminal groups, polysiloxanes, polyepichlorohydrin, and polyurethanes.
Although liquid acrylonitrile-butadiene copolymers with carboxyl- (CTBN) and amine- (ATBN) terminated groups have been widely used for epoxy toughening, the relatively high glass transition temperature of the copolymer limits their low-temperature applications. In addition, these copolymers also increase the CTE value of the moulding compound. Also the presence of unsaturated structure of butadiene system is prone to thermal instability and thus unsuitable for long term use at higher temperatures.
Polysiloxanes have excellent thermal stability, moisture resistance, good electrical properties, low stress and lower Tg values. However polysiloxanes are not compatible with epoxy resins. Addition of compatibilisers such as, methylphenylsiloxane enhances the compatibility but at the same time raises the Tg of polysiloxane modifier restricting its low temperature applications.
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