Polyethylene-High Density is a polymer that can be used to form a plastic bag and its zip closure. It is part of the Polyethylene (PE) family, which is the most widely used plastic. Polyethylene-High Density, also known as HDPE, is commonly used in packaging such as milk jugs and detergent bottles, and is the key ingredient in many plastic toys. We use a mix of different polyethylene densities to get the ideal strength and flexibility for different bag types. It is approved for use in products that come into direct contact with food. Polyethylene-High Density 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.