Examples of polymers useful in this invention are polyethylene (ethylene homopolymer), polypropylene, poly-4-methyl-1-pentene, copolymers of 99.5 to 85 weight % ethylene and 0.5 to 15 weight % of another monoolefin, preferably an aliphatic monoolefin having 4 to 10 carbon atoms and in particular of those having 6 to 10 carbon atoms.