stress balls -- memory foam?

hey everyone, im looking for some information about the material used in those stress balls that died with the 90’s. Not the grainy filled ones, but the foamy kind. Is there a technical term for the foam used? And if anyone has any information about similiar materials and/or the processes used with them that’d be great.

thanks yall.

I was just checking this the other day, maybe it’s of some use to you:

Stress balls are alive and kicking I’m sad to say.

Most are made out of polyurethane foam