High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a polyethylene thermoplastic made from petroleum. It is sometimes called "alkathene" or "polythene" when used for pipes. With a high strength-to-density ratio, HDPE is used in the production of plastic bottles, corrosion-resistant piping, geomembranes, and plastic lumber. HDPE is commonly recycled, and has the number "2" as its 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. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its manufacture employs the same method today. The EPA estimates 5.7% of LDPE (recycling number 4) is recycled.Despite competition from more modern polymers, LDPE continues to be an important plastic grade. In 2013 the worldwide LDPE market reached a volume of about US$33 billion.
While there are many types of PE, two most-common ones are low-density polyethylene (LDPE) and high-density polyethylene (HDPE). Just as PVC and CPVC are kissing cousins in the polymer world, LDPE and HDPE have a lot in common — and many differences, too.
Produced through free-radical polymerization, LDPE has the most long- and short-chain branching of any form of PE, resulting in its lower density. The branching keeps the molecular chains from packing tightly in its crystaline form, so LDPE has less tensile strength but greater ductility. That exceptional “formability” makes LDPE particularly useful for a range of applications, from rigid products like plastic bottles, buckets and bowls to filmy ones like plastic grocery bags and plastic cling-wrap. Did you have cereal for breakfast? The lining of the milk carton and the plastic bag inside the cereal box were likely 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 those nicely linear molecules pack together well during crystallization, making HDPE much denser and rigid. That added tensile strength means HDPE is the PE of choice for applications that require a bit more “backbone,” such as milk and detergent jugs, garbage cans, water pipes and children’s toys. It’s also one of the reasons why HDPE has largely replaced cardboard as the tube material of choice in making fireworks. An HDPE tube is less prone to shattering if a firework malfunctions, and once the boom inside the tube is exhausted, the HDPE tube is recyclable.