Have you tried contacting one of the TPGs? Different types of plastic melt at different temperatures. Given the amount of money TPGs have invested in the technology behind their holders I don't know that they would tell you the type of plastic, but they might give you an approximate melting point. From what I have read on Net54 I would start with SGC because you seem a lot more likely to get an answer from them than you do from PSA.
As you can see here:
http://machinist-materials.com/compa...r_plastics.htm
and here:
http://plastictroubleshooter.com/The...melt_temps.htm
there is a lot of variability in melt point. That said, it seems like most plastics have a melt point that is 325-350 or higher.