I used to use one, so I was curious. It was hard to find good info by googling, but I eventually came across a presentation which claimed that the Selectric carbon ribbon "consists of a carbon wax coating on a polyethylene base":

https://www.scribd.com/presentation/419638824/typewriters (slide 21)

Searching for those keywords came up with this, which uses the exact same wording:

https://www.google.com/books/edition/Scientific_Examination_...