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Exotherm Control and Shrinkage Reduction

The polymerization of many thermoset polymer systems is considerably exothermic and accompanied by significant shrinkage; both of which can cause problems in molding operations. The presence of inert fillers is often used to reduce both these problems, although care must be taken not to reduce the exotherm too far. Isotropic fillers are best for reducing shrinkage, as their effect is uniform, while anisotropic ones can lead to uneven effects and warpage. While most inorganic fillers could be used, the less expensive calcium carbonates are usually employed when this is the main concern.

Stiffness (Modulus)

While particulate fillers do increase stiffness of thermosets, this is frequently not their primary purpose. This is because thermosets are already much stiffer than other polymers and glass fibers (which are outside of the scope of this article), rather than particulate fillers, which are frequently used for this purpose where it is necessary.

Even so, fillers can have a significant effect, which can be well described by semiempirical models (See Jackson et al. 2003). Typical results for a silica-filled methacrylate polymer are presented in Fig. 1. Increasing aspect ratio also improves modulus, at least up to a limiting value, as shown in Fig. 2. Somewhat surprisingly,

Effect of filler volume fraction on the flex modulus of a silica-filled methacrylate polymer (from Jackson et al. 2003, with permission of Smithers Rapra)

Fig. 1 Effect of filler volume fraction on the flex modulus of a silica-filled methacrylate polymer (from Jackson et al. 2003, with permission of Smithers Rapra)

The effect of aspect ratio of mica on the modulus of a filled unsaturated polyester resin

Fig. 2 The effect of aspect ratio of mica on the modulus of a filled unsaturated polyester resin (adapted from Canova 2000) adding coupling agents has little effect on the initial modulus, probably due to shrinkage effects during curing and processing which create compressive bonding.

 
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