I have a Polypropylene coolant overflow catch tank on my boat's engine that doesn't deform, and the windshield washer fluid tank on my car is also polypropylene (I think the car's engine compartment can get much hotter than the boat's).

The following quote is from http://en.wikipedia.org/wiki/Polypropylene

"The melting of polypropylene occurs as a range, so a melting point is determined by finding the highest temperature of a differential scanning calorimetry chart. Perfectly isotactic PP has a melting point of 171 °C (340 °F). Commercial isotactic PP has a melting point that ranges from 160 to 166 °C (320 to 331 °F), depending on atactic material and crystallinity. Syndiotactic PP with a crystallinity of 30% has a melting point of 130 °C (266 °F).[2]"