Submitted by Athousandlipsticks t3_10bvhgz in askscience
I’m trying to melt those white plastic bags and using a heat gun for this, however strangely enough the plastic just warps and then solidifies into a very hard substance rather than melting first. Can anyone explain to me why?
PD_31 t1_j4d2i98 wrote
Usually polymers go through a glass transition at a certain temperature (called, surprise, surprise, the glass transition temperature). Below that they are hard and rigid (like glass), above that they become rubbery. Smaller molecules, called plasticisers, can be added which will artificially decrease the glass transition temperature (e.g. PVC tubing compared to uPVC used in windows and doors). Since these are small molecules they are more volatile and once gone, the polymer will become glassy (those PVC tubes stiffening with age). So my guess would be that the plasticisers in the bag have evaporated, causing the polymer to transition from rubber to glass despite the increase in temperature.