Adhesion to untreated polyethylene by diffusion: Effect of polyurethane adhesive molecular weight on polyethylene penetration

Polyethylene is the most consumed plastic globally; however, its poor adhesiveness poses serious challenges, particularly in the fabrication of composites and the recycling of mixed plastic waste. While these unfavorable properties are conventionally improved by pretreating the polyethylene surface, the treated surface frequently reverts to its original state over time, which is an unresolved drawback. Here, we describe a novel adhesion system; a polyurethane adhesive penetrates an amorphous part of untreated polyethylene and they form a mixing layer at the adhesive interface. The interfacial mixing layer forms upon heat treatment, even below the melting point of polyethylene and the polymers bind tightly at the interface via entanglement. The peel strength increases with increasing the thickness of the mixing layer. While decreasing the molecular weight of polyurethane promotes inter-diffusion, it also causes a decline in the mechanical strength of the polyurethane adhesive layer. Polyurethanes with optimally balanced molecular weight exhibit superior adhesion due to balance diffusivity and mechanical strength.