PUR binding.

PUR binding is perfect binding with a polyurethane reactive hot melt, an isocyanate-terminated adhesive that sets as it cools and then keeps curing by reacting with moisture from the air and the paper. That second stage is what separates it from EVA hot melt: EVA is thermoplastic and only ever cools back to what it was, while PUR crosslinks into a thermoset that holds through heat, cold, and the coated and digitally printed stocks EVA struggles to grip. It also goes on in a much thinner film than EVA, which is why PUR books lie flat instead of fighting the reader. The catch is the clock, because a book is not at full strength coming off the binder, and trimming or boxing it before the cure has run gives you pages pulling out of a spine that looked perfect on the line.