"I'm an analytical chemist specializing in polymer characterization.  I've never performed ASTM D6691 but it appears that it's measuring the amount of carbon dioxide produced vs a reference standard in the same conditions.  I think it means that at the end of 97 days it had produced 88.1% of the CO2 as the standard did.  Structurally. I don't see why it wouldn't break down eventually so it just takes a little longer. "