Abstract: Polymeric material is plastified in an extruder and is then divided into a major part and a minor part. Either the major part or (more often) the minor part is modified by a chemical reaction, such as grafting (any necessary ingredients being added, if not already present). The major and minor parts are then recombined in an extrusion head so that the modified and unmodified materials form distinct parts of the product. In this way, a single extruder can be used to make, for instance, a product with surface characteristics modified by grafting, wire-covering with only a surface layer crosslinked, or heat-sealable crosslinked packaging film.

Abstract: A flame-retardant cross-linkable polymer composition for extrusion which is capable of producing non-porous products comprises: at least one flame-retardant filler of the class that evolve substantial amounts of bound water on heating at temperatures above a value T.sub.d that lies in the range 120.degree.-250.degree. C.; a compatible ethyelene copolymer base in which the filler is dispersed; a hydrolysable unsaturated silane; and a free-radical grafting initiator having a half-life of less than 10 minutes at a temperature 25.degree. C. below T.sub.d.