Polyethylene

Description

Polyethylene is a monomer ethylene (IUPAC name ethene). The recommended scientific name 'polyethene' is systematically derived from the scientific name of the monomer.^[1][2] In certain circumstances it is useful to use a structure–based nomenclature. In such cases IUPAC recommends poly(methylene).^[2] The difference is due to the 'opening up' of the monomer's double bond upon polymerisation.

In the polymer industry the name is sometimes shortened to PE, in a manner similar to that by which other polymers like polystyrene are shortened to PP and PS, respectively. In the United Kingdom the polymer is commonly called polythene, although this is not recognised scientifically.

The ethene molecule (known almost universally by its common name ethylene), C₂H₄ is CH₂=CH₂, Two CH₂ groups connected by a double bond, thus:

Polyethylene is created through ion coordination polymerization or cationic addition polymerization. This is because ethene does not have any substituent groups that influence the stability of the propagation head of the polymer. Each of these methods results in a different type of polyethylene.

Classification

Polyethylene is classified into several different categories based mostly on its molecular weight.

• Ultra high molecular weight polyethylene (UHMWPE)
• Ultra low molecular weight polyethylene (ULMWPE - PE-WAX)
• High molecular weight polyethylene (HMWPE)
• High density polyethylene (HDPE)
• High density cross-linked polyethylene (HDXLPE)
• Cross-linked polyethylene (PEX)
• Medium density polyethylene (MDPE)
• Low density polyethylene (LDPE)
• Linear low density polyethylene (LLDPE)
• Very low density polyethylene (VLDPE)

UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.1 and 5.67 million. The high molecular weight results in less efficient packing of the chains into the Aramid in bulletproof vests as Spectra (or Dyneema) fibers.

HDPE is defined by a density of greater or equal to 0.941 g/cm³. HDPE has a low degree of branching and thus stronger intermolecular forces and tensile strength. HDPE can be produced by chromium/silica catalysts, chromium catalysts or Ziegler-Natta catalysts) and reaction conditions. HDPE is used in products and packaging such as milk jugs, detergent bottles, margarine tubs, garbage containers and water pipes.

PEX is a medium- to high-density polyethylene containing elastomer. The high-temperature properties of the polymer are improved, its flow is reduced and its chemical resistance is enhanced. PEX is used in some potable water plumbing systems, as tubes made of the material can be expanded to fit over a metal nipple, and it will slowly return to its original shape, forming a permanent, water-tight connection.

MDPE is defined by a density range of 0.926–0.940 g/cm³. MDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts.MDPE has good shock and drop resistance properties. It also is less notch sensitive than HDPE, stress cracking resistance is better than HDPE. MDPE is typically used in gas pipes and fittings, sacks, shrink film, packaging film, carrier bags, screw closures.

LLDPE is defined by a density range of 0.915–0.925 g/cm³. LLDPE is a substantially linear polymer, with significant numbers of short branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (e.g. 1-octene). LLDPE has higher tensile strength than LDPE. Exhibits higher impact and puncture resistance than LDPE. Lower thickness (gauge) films can be blown compared to LDPE, with better environmental stress cracking resistance compared to LDPE but is not as easy to process. LLDPE is used in packaging, particularly film for bags and sheets. Lower thickness (gauge) may be used compared to LDPE. Cable covering, toys, lids, buckets and containers, pipe. While other applications are available, LLDPE is used predominantly in film applications due to its toughness, flexibility, and relative transparency.

LDPE is defined by a density range of 0.910–0.940 g/cm³. LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the free radical polymerization. The high degree of branches with long chains gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap.

VLDPE is defined by a density range of 0.880–0.915 g/cm³. VLDPE is a substantially linear polymer, with high levels of short chain branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (e.g. 1-butene, 1-hexene, and 1-octene). VLDPE is most commonly produced using metallocene catalysts due to the greater co-monomer incorporation exhibited by these catalysts. VLDPE’s are used for hose and tubing, ice and frozen food bags, food packaging and stretch wrap, as well as impact modifiers when blended with other polymers.

Recently, much research activity has focused on the nature and distribution of long chain branches in polyethylene. In HDPE, a relatively small number of these branches, perhaps 1 in 100 or 1,000 branches per backbone carbon, can significantly affect the rheological properties of the polymer.

Ethylene copolymers

In addition to copolymerization with alpha-olefins, ethylene can also be copolymerized with a wide range of other monomers and ionic composition that creates ionized free radicals. Common examples include vinyl acetate (resulting product is ethylene-vinyl acetate copolymer, or EVA, widely used in athletic shoe sole foams), and a variety of acrylates (applications include packaging and sporting goods).

History

Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while heating diazomethane. When his colleagues Eugen Bamberger and Friedrich Tschirner characterized the white, waxy substance he had created, they recognized that it contained long -CH₂- chains and termed it polymethylene.

The first industrially practical polyethylene synthesis was discovered (again by accident) in 1933 by Eric Fawcett and Reginald Gibson at the Michael Perrin, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial LDPE production beginning in 1939.

Subsequent landmarks in polyethylene synthesis have revolved around the development of several types of halides and organoaluminum compounds that worked at even milder conditions than the Phillips catalyst. The Phillips catalyst is less expensive and easier to work with, however, and both methods are used in industrial practice.

By the end of the 1950s both the Phillips and Ziegler type catalysts were being used for HDPE production. Phillips' initially had difficulties producing a HDPE product of uniform quality, and filled warehouses with off-specification plastic. However, financial ruin was unexpectedly averted in 1957, when the hula hoop, a toy consisting of a circular polyethylene tube, became a fad among youth in the United States.

A third type of catalytic system, one based on aramids in many high-strength applications.

Until recently, the metallocenes were the most active single-site catalysts for ethylene polymerisation known — new catalysts are typically compared to zirconocene dichloride. Much effort is currently being exerted on developing new single-site (so-called Group 4 metals show substantially higher activity than the metallocenes.

Physical properties

Depending on the xylene, or chlorinated solvents, such as trichloroethane or trichlorobenzene.

References