construction materials prepared from recycled polymeric material containing primarily polyolefins such as high density polyethylene , ultra high molecular weight polyethylene , low density polyethylene , linear low density polyethylene , and polypropylene homopolymers and copolymers are useful for profiled extruded forms such as fence posts , door frames , rails , railway sleepers , and other applications where lumber is conventionally employed . the compositions of the invention contain , in addition to the mixed plastic matrix , reactive compatibilizers , impact modifiers , and reinforcing agents . when desired , foaming agents may also be employed . the recycled mixed plastic materials have been subject to at least one high temperature processing step when used in their original application . such plastics differ in physical and chemical composition from virgin material .