There is nothing more satisfying to me than a genuine reference question: a question with an obscure, but findable, and even interesting, answer.
LM at our Northeast Branch, a veteran of many years in the Reference Department at the main library, called to ask if I knew whether there was a grammatical term for brand names that have "gone generic", like Xerox, Saran Wrap, Scotch Tape, and so on.
I thought, "L., you're asking me? I'm not worthy!" When I started in reference, she could find answers more quickly in our print reference collection than I could find them online. But the branches don't have extensive reference collections.
I was dazed with my fall allergy attack when she called last week. To my shame, I didn't get around to searching until today.
They are called genericized trademarks or proprietary eponyms. According to Randall E. Krause, to whose Database of American Proprietary Eponyms I originally linked, which link is now dead, they are not nouns, but are in fact rare, proper adjectives.