Pur-binding and perfect-binding are styles of binding where the pages are glued into a flat spine (pur-binding is basically stronger version of perfect binding).
What is the minimum number of pages required for this kind of binding?
Ok, just found an answer to my own question, from my printer:
"Generally speaking about 25 sheets (50pp) is recommended but we have done as little as 15 sheets (30pp) and because it is PUR it stays together. Anything less should be saddle stitched."
This is referring to around 100-120gsm stock.