thermoplastics, materials that soften or melt when heated and harden when cooled. Thermoplastic polymers consist of long polymer molecules that are not linked to each other. i.e., have no cross-links. They are often supplied as granules and heated to permit fabrication by methods such as molding or extrusion. Thermoplastics include polyethylene, polypropylene, polystyrene, polyester, polyvinyl chloride, acrylics, nylons, spandex-type polyurethanes, and cellulosics.