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I have some old computer case parts made of ABS plastic. Two decades of storage has embrittled the plastic to the point where they easily crack or shatter with the slightest bend.
What is going on chemically that's causing the plastic to become brittle. Is there anything that can be done to reverse it?
Unfortunately, there is not too much to do. The original ABS substance is a copolymer made of acrylonitrile, butadiene and styrene. It is rather hard and brittle. To make it softer, some proportion of a plasticizer has to be added when it's hot and malleable. In the last century, the plasticizer was in general dinonylphthalate. It works well, but it is a bit volatile. So after a couple of years, the plasticizer gets evaporated, and the polymer becomes brittle again. This "illness" can only be cured by remelting the plastic and adding some new dinonylphthalate. Today better plasticizers have been discovered that are not so volatile.
