The peeling of the sheet material 10 away from flange 3 is usually accompanied by a tearing in sealant substructure 14 which comprises a cohesive failure in sealant substructure 14 in the area of the seal 7 at flange 3, especially where a modifying material such as HDPE is used as a component of at least one of the layers in sealant substructure 14.