Hi Bill Hamm,
I looked up a particular type of nylon (ballistic) - I understand that some polyester could do too. Both needs a coating for making the fabric watertight. That's a new line of thinking for me, one with a lot of possibilities. Yes, the folding is at odds with a smooth looking kayak.
I didn't know that aluminium was poisonous (or rather in excess in the body) .. I don't think that it would match at all the toxicity in the possible organic compounds that could come in play. The tubes could well cover itself in a very stable Al2O3, couldn't it? Anyway, thanks for the input.