RE: Polyurethene glue crumbles?
Your glue may be bad but I doubt it. The situation in which your comparison is based is bad. Without diving into a chemistry seminar I'll just state that the poly glue left to cure in an unrestrained state. (on a benchtop) will expand TOO large and the air bubbles will be so big that surface thickness will be thin, therefore weak. Left to cure in a restrained sate (between 2 pieces of wood) keeps the density high therefore creating a strong bond.