What material does a dentist/dental lab use to make tooth whitening trays?

I understand it is some sort of polymer plastic, but would like to know which polymer plastic in particular?
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It is usually Thermo Plastic & individual doses of peroxide in syringe form. To create custom-fit mouth trays, your dentist will take an impression of your teeth and then have exact molds made of your tooth arches (upper, lower or both). At home, you'll fill these trays with the specified peroxide solution in the manner your dentist directs - usually minutes or hours every day for up to two weeks.
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