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Processing of Thermoset Composites
Processing of thermosets is completely different to that for the other two polymer types, with the filler having to be added to the pre-polymerized monomer, rather than the polymer itself. This means they are added to a relatively low viscosity, often liquid, phase, where the high shear that helps dispersion in other polymer types is missing. On the other hand, the processing causes less damage to the filler particles, which means that thermosets are more able to use fillers such as mica, wollastonite, and glass fibers. It is also easier to incorporate hard fillers, such as crystalline silicas, and temperature-sensitive ones like cellulosics, than it is with other polymers. Thermoset polymers are also able to tolerate larger particle size fillers and higher addition levels.
In addition, most thermosets are quite polar, which means that they can wet and interact well with many types of fillers, especially minerals like carbonates. This reduces the need for surface-modifying species, but dispersants and coupling agents may still be utilized, especially with siliceous fillers. Coupling agents are also often used to help property retention under adverse environmental conditions rather than to improve initial properties.
The addition of particulates can significantly increase pre-cure resin viscosity making molding difficult and special methods have to be adopted when very high
Table 1 The main filled thermoset composites in commercial use
loadings are desired. High filler addition levels are often required when using aluminum hydroxide for flame-retardant purposes, with 150-300 phr (parts per hundred resin) often being required. Two approaches are used to control viscosity: use of special dispersing agents and particle size manipulation to maximize packing.
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