Particulate Fillers in Thermoset Plastics

Abstract

Although thermoset polymers are significant users of particulate fillers, the scientific literature is much scarcer than for other polymer types. This is because they are usually used in combination with glass fibers, which then dominate the properties of the composite.

The processing of thermosets is different to that for the other two polymer types, with the filler being 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, there is less damage to the filler particles, which means that it is easier 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. Because they are polymerized during molding, thermosets can also exhibit higher mold shrinkage and controlling this is an important role of particulate fillers. Fillers are also important in reducing polymerization exotherms, which can otherwise cause problems.

The main general purpose filler used in thermosets is calcium carbonate in various forms. This is mainly employed for cost reduction, shrinkage, and exotherm control. Large quantities of aluminum hydroxides are also used for low smoke and fume flame-retardant and aesthetic purposes. Epoxy printed circuit boards use fillers such as alumina to impart high thermal conductivity while retaining low electrical conductivity. Thermosets make more use of hard fillers such as crystalline silicas than other polymer composites. These are used to improve abrasion resistance in flooring and solid surface applications.

Most thermosets are polar, and this 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.