Is it food grade plastic? If it is, don't sweat it.
I used to teach brewing classes. We used something called a Bru-Bin for those. It was essentially a food grade plastic bucket with a heating element. It would take mild boil for over an hour, no problem.
I used plastic coolers as my mash tuns. I liked to do decoction mashes. (You're adding differing amounts of boiling water over 90 minute period to control your mash temps.) Coolers would last about 15 brewing sessions before they were shot. If I used coolers for a single step mash, where the temps are around 175 degrees at mash-in and 150 at mash-out, coolers worked great and were long lasting.