Polyurethane VS Rubber for Suspension Parts

John, I bought the material as bar stock from a primary company called Cadillac Plastics here in Australia. I figured they would have known what they were doing.
Talking with another car guy here in Oz he mentioned that a Packard owner he knew had a similar disaster.

Terry,
My son , who does a lot of rubber reproduction stuff, says it could have been that you could have put in too much catalyst when you poured the biscuits. Were you using a scale when you made them up? Just a thought.
Rubber molding is similar, you can get some real odd results if the mixtures aren't just right.

John

About eleven years ago to save some money I made a set of Donut Rings for the long suspension bolts from polyurethane. The suppliers advised me that it was better than rubber and around the same hardness. They were made and fitted and worked ok while the car was on the road. When reassembling the front suspension recently during the complete restoration I noticed some splits in the donuts. No problem I though as I will install the spare set.
Within minutes of fitting the spare set that had been sitting on a shelf for over ten years and loading them up they started to disintegrate before my eyes. I guess polyurethane has some time related stress corrosion cracking properties.
A set of the proper rubber donuts supplied by Stan Gilliland are now fitted.
The supplier had no explanation of why the polyurethane disintegrated.