Low density polyethylene is the most widely used in packaging printing plastics. It is a member of the polyolefin family. Olefin, which means oil-forming is an old synonym for alkenes, and was, originally, the name given to ethylene. Alkenes are hydrocarbons containing carbon-carbon double bonds, such as ethylene and propylene. In the plastic industry, olefin is a common term that refers to the family of plastics based on ethylene and propylene. The term polyolefin strictly applies to polymers made of alkenes, whether homopolymers or copolymers. It includes the family of polyethylene, and the family of polypropylene.
Polyethylene (PE) is a family of addition polymers based on ethylene. Polyethylene can be linear or branched, homopolymer or copolymer. In the case of a copolymer, the other comonomer can be an alkene such as propene, butane, hexane or octane; or a compound having a polar functional group such as vinyl acetate (VA), acrylic acid (AA), ethyl acrylate (EA), or methyl acrylate (MA). If the molar precent of the comonomer is less than 10%, the polymer can be classified as either a copolymer or homopolymer.
Polyethylene was the first olefinic polymer to find use in food packaging. Introduced in the 1950s, it became a common material by 1960, used in film, molded containers, and closures. Since low density polyethylene was first introduced in 1940, strength, toughness, thermal and heat sealing properties, optical transparency, and processing conditions have been much improved. Today there are a number of polyethylene grades of relevance to packaging.
Low density polyethylene has a branched structure. The family of branched polyethylenes included homopolymers and copolymers of ethylene, which are non-linear, thermoplastic and partially crystalline. They are fabricated polymerization of ethylene under these conditions produceds a branched polymer that is actually a mixture of large molecules with different backbone lengths, various side chain lengths and with various degrees of side-chain branching.
While linear PE, because of its stereoregularity and the small size of its pendant groups, has a high percent crystallinity, from 70% to 90%, the presence of branches in the backbone chain acts to limit the formation of polyethylene crystals by introducing irregularities in the structure. Therefore, linear PE produces a highly crystalline polymer that has a relatively high density and is known as high density polyethylene (HDPE). Branched PE has lower crystallinity and consequently lower density, so is known as low density PE (LDPE). LDPE typically has a crystallinity of 40% to 60%, with a density of 0.910g to 0.940g per cube center meters. In contrast, HDPE has a density of about 0.940g to 0.970g cube center meters. Comonomers such as propylene and hexane are commonly used in the reaction to help control molecular weight. A wide variety of branched polyethylenes are commercially available, with properties dependent on the reaction conditions and on the types and amount of comonomer.
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