Extenders

(or fillers), materials that are added to plastics, rubber, adhesives, paints and varnishes, and other polymeric materials to facilitate their processing and impart to them the required properties (strength or electrical and frictional properties), as well as to reduce costs. The most widely used extenders for polymeric materials are finely dispersed solids, such as carbon black, silica, chalk, kaolin, talc, mica, and graphite. Glass, asbestos, and synthetic fibers, as well as monocrystalline fibers of some metals (“whiskers”), are also used. Sheet materials (paper or fabrics) are used in the production of laminated plastics; gases (CO ₂, N₂), volatile hydrocarbons, or water are used for foam plastics.

Extenders for polymeric materials that improve the performance of products are usually called active (reinforcing) extenders; those that have no such effect are called inactive (inert) extenders. Fiber and sheet extenders are also called reinforcing extenders. The most important active extender is carbon black, which is used in types of rubber derived from most synthetic elastomers to produce strong and wear-resistant products. The extenders are combined with polymers in a variety of ways, such as mixing in a roller mill or impregnation with polymer solutions or melts. The content of extenders in polymeric materials varies greatly; in highly extended compositions it may even exceed the polymer content.