I just found the answer to my own question in an issue of
HomePower magazine.
Quote:
The buffer breakdown temperature of generic propylene occurs at about 285°F. A few years ago, The Dow Chemical Co. introduced DowFrost HD, which has a maximum temperature of 325°F.
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So, it seems you can get an antifreeze system REALLY hot before the glycol is a problem. I think other plastics in the system will become a problem before the antifreeze starts to break down.