Nucap and Nulok are high quality, very clean, water-washed kaolin clays surface-modified with a coupling agent. The coupling agent modification produces a filler which chemically bonds to the elastomer enhancing many physical and processing properties. Typical coupling agents utilized are of the mercapto, polysulfide, amino and vinyl types. Some clays beneficiated by the water-washed process undergo an additional calcination (heat treating) process. Calcined clays can also be chemically modified with coupling agents to enhance their performance in elastomers. The treated calcined grades available are Polyfil WC, Polyfil WC-426 and Nylok 171. Clays are used as low cost non-black reinforcing, stiffening or extending fillers in natural and synthetic rubbers. Typical loadings are from 20 phr to 200 phr. Chemically modified clays are utilized to increase physical properties including modulus, compression set, abrasion and tear resistance while reducing die swell and heat buildup (hysteresis).