Polyethylene-Low 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-Low Density, also known as LDPE, is commonly used in rigid plastic containers. 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-Low 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.