HOT BEVERAGE BAG
To avoid a burn, beverage heating time should not exceed 6 minutes.
Use caution when handling HOT beverage bag.
1. Open beverage bag. Fill with water to appropriate fill line. Add beverage powder.
2. Remove air from bag, then secure closure.
3. Place beverage bag in FRH as you would an MRE entrÃƒÂ©e.
4. Activate FRH.
5. Load FRH/beverage bag into an MRE carton.
6. Heat 4 to 6 minutes. Remove HOT beverage bag. Discard FRH. Save carton.
7. Place beverage bag back in carton. Carton insulates hot beverage and protects hands from heat.
8. Open beverage bag. Drink beverage.
As for the what the bag is made of of, it says "high density polyethylene film (HDPE) of a minimum 0.002 inch thickness...constructed with 1/8 inch wide heat sealed sides and a folded bottom."
Here's a picture of it from the PCR (click to see a larger version):