Impact Modifiers for Thermosetting Resins

A wide variety of chemicals can be used to modify specific properties of the resin, such as resistance to mechanical or thermal shock, increased elongation and 

To improve impact strength, there are two routes (which may well be best used in combination) to modify the resin, or to improve the interface bond between the resin and reinforcement. 

There is a wide variety of chemicals that can be used to modify specific properties of the resin, such as resistance to mechanical or thermal shock, increase elongation, and higher impact strength and flexibility. Usually this will involve a trade-off of some other property, such as physical strength, electrical properties, resistance to chemicals or solvents, and/or performance at elevated temperatures. For epoxies, there are DER-type flexible resins and monofunctional epoxide compounds, such as epoxidized cashew nut oil flexibilizer. Typically 30% or less by weight of these can be used, they can also be used at ratios of 1 1 to obtain a flexible and rubbery cured composition. The additives are also shelf-stable when blended with the resin. 

Modifiers that may also be reactive as curing agents (such as polysulphides, triphenyl phosphite, and some polyamides) can be used. Polysulphide polymers used on their own will react slowly with expoxies. One to three parts of an active catalytic amine or amine salt can be used to accelerate cure. Triphenyl phosphite reduces viscosity and lowers cost. Ratios up to 2 5 phr appear to have no severe effect on physical properties at room temperature. Although reactive with epoxy, triphenyl phosphite is not an effective curing agent by itself it needs also a polyfunctional amine. About 75% of the normal stoichiometric amount of amine gives optimum results when 2 5 phr triphenyl phosphite is used with a resin such asDER331. 

Non-reactive modifiers such as dibutyl phthalate, pine oil, and glycol ethers are not used widely because they can cause severe reduction in cured resin properties. Their primary function is to lower cost and the chief requisites are that they are compatible with the resin both before and after cure, do not vaporize or foam during cure, and do not migrate excessively from the cured composition. 

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