Adhesion of polyethylene blends to polypropylene

The effect of chain microstructure on adhesion of ethylene copolymers to polypropylene (PP) was studied using coextruded microlayers. Adhesion was measured by delamination toughness G using the T-peel test, and interfacial morphology was examined by atomic force microscopy. Good adhesion to PP was achieved with homogeneous metallocene catalyzed copolymers (mPE) with density 0.90 g cm−3 or less. Good adhesion was attributed to entanglement bridges. In contrast, a heterogeneous Ziegler-Natta catalyzed copolymer (ZNPE) of density 0.925 g cm−3 exhibited poor adhesion to PP due to an amorphous interfacial layer of low molecular weight, highly branched fractions that prevented effective interaction of ZNPE bulk chains with PP. Blending mPE with ZNPE eliminated the amorphous interfacial layer and resulted in epitaxial crystallization of ZNPE bulk chains with some increase in G. Increasing the mPE content of the blend past the amount required to completely resolve the amorphous interfacial layer of ZNPE resulted in a steady, almost linear, increase in G. Phase separation of mPE and ZNPE during crystallization produced an interface with regions of epitaxially crystallized ZNPE bulk chains and other regions of entangled mPE chains. Entanglement bridges imparted much better adhesion than did epitaxially crystallized lamellae.