Suitable materials for the core layer comprise polyolefins such as low density polyethylene (LDPE), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), copolymers of ethylene with at least one C3 -C10 ??-olefin, ionomer, and ethylene polymers having at least one functional moiety selected from the group of esters, anhydrides and carboxy