I am doing some experiments about the cross-link ability of polyurethane. First I cut a PU square into 2 halves. Then I stack half of each cut part on each other, apply some pressure on it by a 500g weight then heated all of them at 120oC for 20 minutes but after the process, they did not connect together. What did I do wrong? How to properly initiate the cross-link process? Thank you.