After placing a dry solid block of nonporous plastic material in the freezer for a while, I was surprised to find out when I retrieved it that it had expanded globally by a few inches. Which type of plastic must this be and do plastics expand at colder temperatures?
Materials with a negative thermal expansion (NTE) coefficient will increase in size as they are cooled down. Not all plastics have a NTE, but it is not a property seen only in one specific type of plastic. I don't think the plastic can be identified only knowing that it has a NTE.