Low-density polyethylene


Low-density polyethylene is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries using a high pressure process via free radical polymerization. Its manufacture employs the same method today. The EPA estimates 5.7% of LDPE is recycled in the United States. 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.

Properties

LDPE is defined by a density range of 0.917–0.930 g/cm3. It is not reactive at room temperatures, except by strong oxidizing agents, and some solvents cause swelling. It can withstand temperatures of 80 °C continuously and for a short time. Made in translucent or opaque variations, it is quite flexible and tough.
LDPE has more branching than HDPE, so its intermolecular forces are weaker, its tensile strength is lower, and its resilience is higher. Also, because its molecules are less tightly packed and less crystalline due to the side branches, its density is lower.
When exposed to ambient solar radiation the plastic produces two greenhouse gases, methane and ethylene. Due to its low density properties it breaks down more easily over time, leading to higher surface areas. The production of these trace gases from virgin LDPE increase with surface area/time, with rates at the end of a 212-day incubation of 5.8 nmol g-1 d-1 of methane, 14.5 nmol g-1 d-1 of ethylene, 3.9 nmol g-1 d-1 of ethane and 9.7 nmol g-1 d-1 of propylene. When incubated in air, LDPE emits gases at rates ~2 times and ~76 times higher in comparison to water for methane and ethylene, respectively.

Chemical resistance

LDPE is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic parts for computer components, and various molded laboratory equipment. Its most common use is in plastic bags. Other products made from it include: