Diluents, Flexibilisers and other Additives

For a number of purposes the unmodified epoxide resins may be considered to have certain disadvantages. These disadvantages include high viscosity, high cost and too great a rigidity for specific applications. The resins are therefore often modified by incorporation of diluents, fillers, and flexibilisers and sometimes, particularly for surface coating applications, blended with other resins. 

The diluents tend to have an adverse effect on physical properties and also tend to retard cure. Many are also skin irritants and must be used with care. For this reason they are seldom used in amounts exceeding 10 phr. 

Fillers are used in tooling and casting application. Not only do they reduce cost but in diluting the resin content they also reduce curing shrinkage, lower the coefficient of expansion, reduce exotherms and may increase thermal conductivity. Sand is frequently used in inner cores whereas metal powders and metal oxide fillers are used in surface layers. Wire wool and asbestos are sometimes used to improve impact strength. 

In order to increase the flexibility, and usually, in consequence, the toughness of the resins, plasticisers and flexibilisers may be added. Non-reactive plasticisers such as the conventional phthalates and phosphates have proved unsuccessful. Monofunctional materials, which in some cases also act as reactive diluents, have been used but are not of great importance. 

More interest has been shown in polymeric flexibilisers, particularly the low molecular weight polyamides from dimer acid (see Chapter 18), the low molecular weight polysulphides (Chapter 19), polyamines and the polyglycol diepoxides. 

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Diluents, Flexibilisers and other Additives 771 The amine flexibilisers may be used in two ways ... 

The wide range of properties available from different combinations of epoxy resins and their hardeners can be further extended by the use of other additives. These include anti-oxidants, diluents, flexibilisers, stabilisers, tougheners, fillers, surfactants and adhesion promoters. 

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