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Polyurethane is a food-grade adhesive that we use to laminate two pieces of plastic together during the formation of a plastic bag. It is approved for use in products that come into direct contact with food. Polyurethane can withstand both heat and freezing. Check your product label for more information and rest assured that Ziploc® Brand products that are marked as "microwaveable" meet the safety requirements of the U.S. Food and Drug Administration (FDA) for temperatures associated with defrosting and reheating food in microwave ovens, as well as room, refrigerator, and freezer temperatures.