Particles of moulding compositions prepd. from bitumen/polyethylene/polyisobutylene are prevented from adhering together by the use of a polyethylene (I) powder. - (I) has a melt index of pref. 15-30. Copolymers with e.g. (meth)acrylic esters and vinyl esters may also be used. - Granulated moulding material treated with 1.5% wt. of (I) powder of particle size 500 mu (max.), showed excellent flow properties after 6 months storage, without loss of other desirable properties.
