Adhesion to Untreated Polyethylene and Polypropylene by Needle-like Polyolefin Crystals

Polyolefins, such as polyethylene and polypropylene, are globally the most widely used plastics and are considered to be essential materials. Because they lack polar groups, polyolefins are poorly adhesive, which poses serious recycling challenges. Here, we describe a novel adhesion system involving a polyurethane adhesive and an untreated polyolefin, in which the interface is tightly bound by needle-like polyolefin crystals in the same way that nails adhere to wood. The polyurethane and polyolefin inter-diffuse at high temperatures to form a mixed interfacial layer, after which the polyolefin recrystallizes in the layer upon cooling. Polyurethanes with optimally balanced non-polar and polar blocks exhibit superior adhesion due to their larger miscibility windows, polyolefin compatibilities, and the resulting long needle-like polyolefin crystals.