Thought I’d share some responses that I received on another forum:
“They are actually a mylar type material, very durable…”
“They are made with tyvec. They are very durable…”
“Are you staying onsite? If so, I believe you can have your tickets transferred to your roomkey if you’re concerned about the durability of the tix.”
“However, if you do have them transferred to your room key be prepared for problems with getting FastPasses. I overheard a CM at Kilimanjaro Safaris in March '06 telling a guest that 80% of their problems with the FastPass machines are with guests using the plastic tickets.
The “cardboard” tickets are remarkably durable. If they weren’t then one would wonder why Disney would use that same material for Annual Passes which get far more use than the MYW tickets.”
"The Tyvec tickets are actually better than the hard plastic cards. The hard plastic cards tend to demagnetize more often than the Tyvec, and the plastic cards can also crack easily, all of which would require you to get the card replaced.
The Tyvec tickets are as tough as the hard plastic, but more flexible. Their magnetic strips demagnetize less often than the plastic cards, too.
Those puppies are tough as nails."
If the cards are actually Tyvek instead of cardboard, then they are plastic! (Tyvek is made from 100% polyethylene fibers.)