High-density polyethylene (HDPE) or high-density polyethylene (PEHD) is a thermoplastic polyethylene made from petroleum. For pipes it is sometimes called “alkaline” or “polyethylene”. High-density HDPE is used in the production of plastic bottles, corrosion-resistant pipes, geomembranes and plastic lumber. HDPE is usually recycled and has the number “2” as the resin identification code.In 2007, the global HDPE market reached a volume of more than 30 million tons.
Low density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. This was the first grade of polyethylene, which was produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its production uses the same method today. EPA estimates that 5.7% of LDPE (recycling #4) is recycled. Despite competition from more modern polymers, LDPE remains an important plastic grade. In 2013, the worldwide LDPE market reached about $33 billion.
While there are many different types of polyethylene, the two most common are low density polyethylene (LDPE) and high density polyethylene (HDPE). Just as PVC and CPVC kissed their cousins in the polymer world, LDPE and HDPE share a lot in common and there are many differences.
LDPE is produced by free radical polymerization, which forms longer and shorter chain branches of any type of polyethylene and reduces its density. The branching keeps the molecular chains from packing tightly in the crystalline form, so LDPE has lower tensile strength but greater flexibility. This exceptional formulation uses LDPE for a wide range of applications from rigid products such as plastic bottles, buckets and bowls to films such as plastic grocery bags and plastic packaging. Do you have cereal for breakfast? The milk carton cover and the plastic bags inside the cereal box are probably made with LDPE.
On the opposite side of the polymer chain, we have HDPE, which is characterized by minimal branching of the polymer chain. Less branching means that the linear molecules are tightly packed together during crystallization, making HDPE very sticky and tough. This added tensile strength means that HDPE PE is the choice for applications that require a bit more backbone, such as milk and detergent jugs, garbage cans, water pipes and children’s toys. This is also one of the reasons why HDPE has largely replaced cardboard as the tube material of choice in fireworks manufacturing. HDPE pipe is less prone to collapse if a fire engine is working and when the boom inside the pipe is exhausted, HDPE pipe is recyclable.