Well, that white plastic headrest slider/regulator is definitely either polyethylene or nylon, because the thin pieces of plastic I epoxied to the sides of the top head part peeled right off. I ended up buying this special Loctite glue/prep kit that works specifically with polyethylene and polypropylene. I had purchased a used slider from the wrecker, but it already had a hair-line crack in the usual place on the top. So, to reinforce it, I scored both sides of the plastics being mated together and applied the activator and then the glue, then pressed the pieces together. Seems to be holding now and the head rest is now installed and working once again. I am mindful to mention to people sitting in the car not to lower the head rest all the way down because that's what seems to make them crack due to the pressure.