How do high-density polyethylene and low-density polyethylene differ?

High-density polyethylene (HDPE) and low-density polyethylene (LDPE) are both forms of polyethylene, but they differ significantly in their molecular structure, manufacturing processes, and physical properties.

Molecular Structure and Manufacturing

• LDPE: Produced under very high pressures and temperatures, LDPE features a polymer structure with both long and short branches. These branches prevent the molecules from packing closely together, resulting in a material that is only partly crystalline.12
• HDPE: Manufactured at low temperatures and pressures using specific catalysts (such as Ziegler-Natta or chromium oxide), HDPE has a linear structure. The lack of branches allows the polymer chains to pack closely together, creating a highly crystalline material.31

Physical Properties and Uses

• Flexibility vs. Strength: Due to its branched structure, LDPE is a very flexible material, commonly used for packaging film, trash bags, and squeeze bottles. In contrast, the dense, crystalline structure of HDPE provides high strength and moderate stiffness, making it suitable for blow-molded bottles, pails, and appliance housings.123
• Thermal Resistance: HDPE has a melting point more than 20 °C (36 °F) higher than LDPE, allowing it to withstand temperatures up to 120 °C (250 °F) and making it suitable for sterilization.31