Schematic of LDPE branching structure
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 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.
LDPE is defined by a density range of 917-930 kg/m3. 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 90 °C (194 °F) for a short time. Made in translucent or opaque variations, it is quite flexible and tough.
LDPE has more branching (on about 2% of the carbon atoms) than HDPE, so its intermolecular forces (instantaneous-dipole induced-dipole attraction) 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 (consistent sunlight) the plastic produces significant amounts of two greenhouse gases - methane and ethylene. Due to its low density properties (high branching) it breaks down more easily over time compared to other plastics, leading to higher surface areas. Furthermore, production of these trace gases from virgin plastics increase with surface area/time, thus LDPE emits greenhouse gases at a more unsustainable rate compared to other plastics. At the end of a 212-day incubation, emissions have been recorded at 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 methane and ethylene at rates ~2 times and ~76 times, respectively, more than in water.
A GEECO bowl, c.1950, still used in 2014.
A Ziploc bag made from LDPE
Facial wash gel bottle made of LDPE
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: