Mike was right about plastics- HDPE is often used for barrels, and it's going to very slowly start to deform at boiling, just give it time, it's strength is greatly reduced at higher temps. It will just ooze. You will see that buried rotomolded HDPE tanks will rarely be rated for over 130F.
Polypropylene is much stronger, especially at higher temperatures, the buried tanks are often rated to 150F or better.
So if you're bargain shopping for tanks for hot water, look for a PP stamp, though even that isn't going to take 100C for extended periods without some deformation unless it was overbuilt.