That's probably because you're a male. Kotex is sold in Australia and New Zealand - it is a feminine hygiene product. Back in the old days when there were only two or three companies that sold these types of products, "Kotex" became synonymous with "sanitary napkin." Of course today there are a dozen different competing brands of this product line and "Kotex" no longer has that instant recognition factor.
Anyway, the technical term for the phraseology OP is looking for is "
proprietary eponym." Some of the more common examples in the US are:
plastic wrap/cling film - Saran Wrap
facial tissue - Kleenex
photocopy - Xerox
cola-flavored soda pop - Coke
transparent adhesive tape - Scotch Tape
hook-and-loop fabric fastener - Velcro
adhesive bandage - Band-Aid
flavored drink powder - Kool-Aid
digital video recorder - TiVo (often used as a verb, as in "Don't tell me how the ballgame ended, I TiVo'd it and haven't watched it yet.")
petroleum jelly - Vaseline