Some years ago at work I installed some schedule 80 (thick wall) PVC piping as a conduit for thermocouple wire in a wood drying kiln. Temps get up to about 160 to 180 F. over the next few weeks the PVC piping lengthen and sagged and looked like crap. But its still there and it stopped softening in the heat after a month or so. And yes it did get brittle. I figured that I baked out the plyability (sp?).
And I know that there are two types of ABS piping. One type come on a roll and has very thin walls. Other type is a cast that you see more for the couplings. much thinker walls. I haven't tried to bake these yet. hehe