Formulation considerations epoxide resins

The machinability of epoxides varies considerably from filler to filler, and, if ease of machinability is required, the use of talc may provide this property. Talc is also of use in epoxide resin formulations when the thermal expansion coefficient has to be lowered by a large amount, since loadings up to 80 parts per hundred of resin can be tolerated. Loading epoxides with talc has also been shown to increase their lap shear strengths when they are used as adhesives. 

For cold-curing epoxides wide variations in adhesive material properties are possible, with different combinations of resin, hardener, filler, and the multitude of modifiers. Products which cure at ambient temperature cannot achieve the same performance as is obtained by curing at elevated temperature. For products cured at room temperature their TgS, at 40-50 °C initially, are relatively low and may be lowered even further by absorbed water, in liquid or vapour form. This may also be accompanied by a reduction in strength and modulus. Thus the use of materials with a slow and small water uptake is to be preferred, which implies a fairly highly cross-linked formulation. Such considerations do of course depend upon the performance and durability expectations in service. Whilst the environmental durability of joints can often be improved enormously by the surface pretreatment methods employed (see Chapters 3 and 4), the adhesive must be selected carefully to ensure long term durability in consideration of the modes and duration of loading, and the environmental conditions. Ideally the adhesive should be fairly tolerant of poor surface pretreatment procedures. 

In order to obtain an epoxide compound suitable for a particular application, all the above considerations have to be taken into account and the successful formulation can only be made with full knowledge of the intended application. Each of the major components commonly used in epoxy resin formulations is now individually discussed and the influence of physical and chemical properties on the properties of the final formulation described. 

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