Nylon is a synthetic polymer with many, many uses. It was originally intended as a replacement for silk. Today it's used in many different ways from clothing to rope to strings for musical instruments. We use nylon in the zip closure of plastic bags. It is approved for use in products that come into direct contact with food. Nylon 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.