a heat - shrinkable , wrap around sleeve is formed of a cross - linked corrugated layer of polyolefin extending between two cross - linked layers of homogeneous plastic material with an elongation at least equal to that of the corrugated polyolefin layer . preferably , the corrugations extend at an angle , which is preferably a right angle , to the longitudinal edges of the sleeve and in the direction of stretching of the sleeve prior to it being heat - shrunk onto the joint .