Low-density polyethylene (LDPE) is athermoplastic made from themonomerethylene. It was the first grade ofpolyethylene, produced in 1933 by John C. Swallow and M.W Perrin who were working forImperial Chemical Industries (ICI) using a high pressure process viafree radical polymerization.[1] Its manufacture employs the same method today. TheEPA 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 asNo. 1 (polyethylene terephthalate) or2 plastics (high-density polyethylene).[4][5]
LDPE is defined by a density range of 917–930 kg/m3.[6] Atroom temperature it is not reactive, except to strong oxidizers; somesolvents 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 morebranching (on about 2% of the carbon atoms) thanHDPE, so itsintermolecular forces (instantaneous-dipole induced-dipole attraction) are weaker, itstensile strength is lower, and itsresilience is higher. The side branches mean that its molecules are less tightly packed and less crystalline, and therefore itsdensity is lower.
When exposed to consistent sunlight, the plastic produces significant amounts of twogreenhouse gases:methane andethylene. 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]
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Polyolefins (LDPE, HDPE, PP) are a major type of thermoplastic.[10] LDPE is widely used for manufacturing various containers, dispensing bottles, wash bottles, tubing, plastic parts for computer components, and variousmolded laboratory equipment. Its most common use is inplastic bags. Other products made from it include:
This article incorporates text by Royer et al available under theCC BY 4.0 license.
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