Suitable polymers of ethylene (3) are selected from an ethylene homopolymer (such as HDPE) and poly(ethylene-co-C3-C10??-olefin) having the above-mentioned intrinsic viscosity measured in tetrahydronaphthaline at 135??? C. The said copolymer contains a minor amount of comonomeric recurring units, such as from 0.5 to 20% by weight.