Low-density polyethylene

Chemical compound

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.[1] Its manufacture employs the same method today. The EPA estimates 5.7% of LDPE (resin identification code 4) is recycled in the United States.[2] 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.[3]

Despite its designation with the recycling symbol, it cannot be as commonly recycled as No. 1 (polyethylene terephthalate) or 2 plastics (high-density polyethylene).[4][5]

Properties

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LDPE is defined by a density range of 917–930 kg/m3.[6] At room temperature it is not reactive, except to strong oxidizers; some solvents cause it to swell. It can withstand temperatures of 65 °C (149 °F) continuously[6] and 90 °C (194 °F) for a short time. Made in translucent and 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. The side branches mean that its molecules are less tightly packed and less crystalline, and therefore its density is lower.

When exposed to consistent sunlight, the plastic produces significant amounts of two greenhouse gases: methane and ethylene. Because of its lower density (high branching), it breaks down more easily than other plastics; as this happens, the surface area increases. Production of these trace gases from virgin plastics increases with surface area and with time, so that LDPE emits greenhouse gases at a more unsustainable rate than other plastics. In a test at the end of 212 days' incubation, emissions recorded were 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 about 2 times and about 76 times, respectively, more than in water.[7]

The standard method to test plastic density is ISO 1183 part 2 (gradient columns), alternatively ISO 1183 part 1 (MVS2PRO density analyzer).[8]

Chemical resistance

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Applications

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A GEECO bowl, c.1950, still used in 2014. A piece of packaging foam made from LDPE A Ziploc bag made from LDPE Facial wash gel bottle made of LDPE

Polyolefins (LDPE, HDPE, PP) are a major type of thermoplastic.[11] 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:

See also

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References

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