Why does old rubber become hard and brittle? Why does old rubber become sticky and gooey? I’ll answer both questions here.
First of all, to reiterate, synthetic rubbers become brittle because the plasticizers leak out into the atmosphere, which then wrecks other havoc on nature, most significantly our own air we breathe. In particular, UV light exposure speeds up this process. Rubber becomes brittle in the the cold, deformations in that temperature can easily crack and crumble it. Rubber is also generally a thermoset plastic, rather than a thermoplastic, so temperature extremes and wide temperature range cycling can age it faster, making it become brittle and crack. Oxidation, simply exposure to oxygen and environmental conditions that speed up chemical reactions such as high temperature, is also another cause of rubber becoming brittle. Oxidation affects both natural and synthetic rubbers. These phenomenon are generally known as “dry rot.”
Now, what about the other end, when rubber turns into goop? As it happens, all rubbers, synthetic or natural, start out as goop, and the curing process is what turns them into the commercial variants we know and love. Essentially, when commercial rubber turns back into goop, this is a “reversion” of the manufacturing process. For recycling rubber, this is a good thing, as it allows you to reuse the material just the same as you would commercially.