Laminates of polyethylene foam with an anisotropic pore structure

A process for producing a laminate comprising a plurality of crosslinked polyethylene foam layers having an anisotropic pore structure in at least one internal layer, said process comprising the steps of arranging at least three polyethylene sheets or layers which contain equal quantities of a peroxidic crosslinking agent and differing quantities of a chemical blowing agent which decomposes at a temperature higher than the temperature at which the peroxidic crosslinking agent functions, as a multi-layer structure with each of the two external layers containing a smaller proportion of blowing agent than contained in the internal layer, heating the multi-layer structure to a temperature at which each of the sheets or layers is crosslinked and at the same time welded together to form a laminate and subsequently heating the laminate at a higher temperature which causes the chemical blowing agent to decompose and foam the laminate structure.