World Library  
Flag as Inappropriate
Email this Article
 

Polyethylene

Polyethylene
Skeletal formula of a polyethylene monomer
Spacefill model of polyethylene
Sample of granulated polyethylene
Names
IUPAC name
Polyethene or Poly(methylene)
Other names
Polyethene
Identifiers
 YesY
Abbreviations PE
ChemSpider
KEGG  YesY
MeSH
Properties
(C2H4)n
Density 0.91–0.96 g/cm3[1]
Melting point 115–135 °C (239–275 °F; 388–408 K)[1] (239–275 °F)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
The repeating unit of polyethylene in the most stable staggered conformation.

Polyethylene (abbreviated PE) or polyethene (organic compounds that differ in terms of the value of n.

Contents

  • Properties 1
    • Physical properties 1.1
    • Chemical properties 1.2
  • Process 2
    • Monomer 2.1
    • Polymerization 2.2
  • Classification 3
    • Ultra-high-molecular-weight polyethylene (UHMWPE) 3.1
    • High-density polyethylene (HDPE) 3.2
    • Cross-linked polyethylene (PEX or XLPE) 3.3
    • Medium-density polyethylene (MDPE) 3.4
    • Linear low-density polyethylene (LLDPE) 3.5
    • Low-density polyethylene (LDPE) 3.6
    • Very-low-density polyethylene (VLDPE) 3.7
    • Copolymers 3.8
  • History 4
  • Environmental issues 5
    • Biodegrading plastics 5.1
    • Bio-based polyethylene 5.2
  • Joining 6
    • Nomenclature and general description of the process 6.1
  • References 7
  • Bibliography 8
  • External links 9

Properties

Physical properties

Polyethylene is a thermoplastic polymer consisting of long hydrocarbon chains. Depending on the crystallinity and molecular weight, a melting point and glass transition may or may not be observable. The temperature at which these occur varies strongly with the type of polyethylene. For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range 120 to 180 °C (248 to 356 °F). The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C (221 to 239 °F).

Chemical properties

Most LDPE, MDPE, and HDPE grades have excellent chemical resistance, meaning they are not attacked by strong acids or strong bases, and are resistant to gentle oxidants and reducing agents. Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin. The material continues burning on removal of the flame source and produces a drip.[3] Crystalline samples do not dissolve at room temperature. Polyethylene (other than cross-linked polyethylene) usually can be dissolved at elevated temperatures in aromatic hydrocarbons such as toluene or xylene, or in chlorinated solvents such as trichloroethane or trichlorobenzene.[4]

Process

Monomer

ethylene skeleton
ethylene 3D model

The ingredient or monomer is ethylene (IUPAC name ethene), a gaseous hydrocarbon with the formula C2H4, which can be viewed as a pair of methylene groups (=CH
2
) connected to each other. Because the compound is highly reactive, the ethylene must be of high purity. Typical specifications are <5 ppm for water, oxygen, and other alkenes. Acceptable contaminants include N2, ethane (common precursor to ethylene), and methane. Ethylene is usually produced from petrochemical sources, but also is generated by dehydration of ethanol.[4]

Polymerization

Ethylene is a rather stable molecule that polymerizes only upon contact with catalysts. The conversion is highly exothermic. Coordination polymerization is the most pervasive technology, which means that metal chlorides or metal oxides are used. The most common catalysts consist of titanium(III) chloride, the so-called Ziegler-Natta catalysts. Another common catalyst is the Phillips catalyst, prepared by depositing chromium(VI) oxide on silica.[4] Ethylene can be produced through radical polymerization, but this route has only limited utility and typically requires high-pressure apparatus.

Classification

Polyethylene is classified into several different categories based mostly on its density and branching. Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight. With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE, and LDPE.

Ultra-high-molecular-weight polyethylene (UHMWPE)

Stainless steel and ultra high molecular weight polythene hip replacement

UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.1 and 5.67 million. The high molecular weight makes it a very tough material, but results in less efficient packing of the chains into the crystal structure as evidenced by densities of less than high density polyethylene (for example, 0.930–0.935 g/cm3). UHMWPE can be made through any catalyst technology, although Ziegler catalysts are most common. Because of its outstanding toughness and its cut, wear, and excellent chemical resistance, UHMWPE is used in a diverse range of applications. These include can- and bottle-handling machine parts, moving parts on weaving machines, bearings, gears, artificial joints, edge protection on ice rinks, and butchers' chopping boards. It is commonly used for the construction of articular portions of implants used for hip and knee replacements. As fiber, it competes with aramid in bulletproof vests.

High-density polyethylene (HDPE)

HDPE pipe installation in storm drain project in Mexico.

HDPE is defined by a density of greater or equal to 0.941 g/cm3. HDPE has a low degree of branching. The mostly linear molecules pack together well, so intermolecular forces are stronger than in highly branched polymers. HDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts; by choosing catalysts and reaction conditions, the small amount of branching that does occur can be controlled. These catalysts prefer the formation of free radicals at the ends of the growing polyethylene molecules. They cause new ethylene monomers to add to the ends of the molecules, rather than along the middle, causing the growth of a linear chain.

HDPE has high tensile strength. It is used in products and packaging such as milk jugs, detergent bottles, butter tubs, garbage containers, and water pipes. One-third of all toys are manufactured from HDPE. In 2007, the global HDPE consumption reached a volume of more than 30 million tons.[5]

Cross-linked polyethylene (PEX or XLPE)

PEX is a medium- to high-density polyethylene containing cross-link bonds introduced into the polymer structure, changing the thermoplastic into a thermoset. 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 because 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.

Medium-density polyethylene (MDPE)

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, and screw closures.

Linear low-density polyethylene (LLDPE)

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 (for example, 1-butene, 1-hexene, and 1-octene). LLDPE has higher tensile strength than LDPE, and it exhibits higher impact and puncture resistance than LDPE. Lower thickness (gauge) films can be blown, compared with LDPE, with better environmental stress-cracking resistance, but is not as easy to process. LLDPE is used in packaging, particularly film for bags and sheets. Lower thickness may be used compared to LDPE. It is used for cable coverings, toys, lids, buckets, containers, and pipe. While other applications are available, LLDPE is used predominantly in film applications due to its toughness, flexibility, and relative transparency. Product examples range from agricultural films, Saran wrap, and bubble wrap, to multilayer and composite films. In 2013, the world LLDPE market reached a volume of US$40 billion.[6]

Low-density polyethylene (LDPE)

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 crystal structure as well. It has, therefore, less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength and increased ductility. LDPE is created by free-radical polymerization. The high degree of branching 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. In 2013, the global LDPE market had a volume of almost US$33 billion.[7]

The radical polymerization process used to make LDPE does not include a catalyst that "supervises" the radical sites on the growing PE chains. (In HDPE synthesis, the radical sites are at the ends of the PE chains, because the catalyst stabilizes their formation at the ends.) Secondary radicals (in the middle of a chain) are more stable than primary radicals (at the end of the chain), and tertiary radicals (at a branch point) are more stable yet. Each time an ethylene monomer is added, it creates a primary radical, but often these will rearrange to form more stable secondary or tertiary radicals. Addition of ethylene monomers to the secondary or tertiary sites creates branching.

Very-low-density polyethylene (VLDPE)

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 (for example, 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. VLDPEs 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 one in 100 or 1,000 branches per backbone carbon, can significantly affect the rheological properties of the polymer.

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 (the resulting product is ethylene-vinyl acetate copolymer, or EVA, widely used in athletic-shoe sole foams) and a variety of acrylates. Applications of acrylic copolymer include packaging and sporting goods, and superplasticizer, used for cement production.

History

Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while investigating diazomethane.[8][9] When his colleagues Eugen Bamberger and Friedrich Tschirner characterized the white, waxy substance that he had created, they recognized that it contained long -CH2- chains and termed it polymethylene.[10]

A pill box presented to a technician at ICI in 1936 made from the first pound of polyethylene

The first industrially practical polyethylene synthesis (diazomethane is a notoriously unstable substance that is generally avoided in industrial application) was discovered in 1933 by Eric Fawcett and Reginald Gibson, again by accident, at the Imperial Chemical Industries (ICI) works in Northwich, England.[11] Upon applying extremely high pressure (several hundred atmospheres) to a mixture of ethylene and benzaldehyde they again produced a white, waxy, material. Because the reaction had been initiated by trace oxygen contamination in their apparatus, the experiment was, at first, difficult to reproduce. It was not until 1935 that another ICI chemist, Michael Perrin, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial LDPE production beginning in 1939. Because polyethylene was found to have very low-loss properties at very high frequency radio waves, commercial distribution in Britain was suspended on the outbreak of World War II, secrecy imposed and the new process was used to produce insulation for UHF and SHF coaxial cables of radar sets. During World War II, further research was done on the ICI process and in 1944 Bakelite Corporation at Sabine, Texas and Du Pont at Charleston, West Virginia, began large scale commercial production under license from ICI.[12]

The breakthrough landmark in the commercial production of polyethylene began with the development of Ziegler-type catalysts were being used for HDPE production. In the 1970s, the Ziegler system was improved by the incorporation of magnesium chloride. Catalytic systems based on soluble catalysts, the metallocenes, were reported in 1976 by Walter Kaminsky and Hansjörg Sinn. The Ziegler- and metallocene-based catalysts families have proven to be very flexible at copolymerizing ethylene with other olefins and have become the basis for the wide range of polyethylene resins available today, including very low density polyethylene and linear low-density polyethylene. Such resins, in the form of UHMWPE fibers, have (as of 2005) begun to replace aramids in many high-strength applications.

Environmental issues

A bag manufactured from polyethylene.

Although ethylene can be produced from renewables, polyethylene is mainly made from petroleum or natural gas.

Biodegrading plastics

One of the main problems of polyethylene is that without special treatment it is not readily biodegradable, and thus accumulates. In Japan, getting rid of plastics in an environmentally friendly way was the major problem discussed until the Fukushima disaster in 2011. It was listed as a $90 billion market for solutions. Since 2008, Japan has rapidly increased the recycling of plastics, but still has a large amount of plastic wrapping which goes to waste.[14]

In May 2008, Daniel Burd, a 16-year-old Canadian, won the Canada-Wide Science Fair in Ottawa after discovering that Pseudomonas fluorescens, with the help of Sphingomonas, can degrade over 40% of the weight of plastic bags in less than three months.[15]

The thermophilic bacterium Brevibacillus borstelensis (strain 707) was isolated from a soil sample and found to use low-density polyethylene as a sole carbon source; when incubated together at 50°C. Biodegradation increased with time exposed to ultraviolet radiation.[16]

In 2010, a Japanese researcher, Akinori Ito, released the prototype of a machine which creates oil from polyethylene using a small, self-contained vapor distillation process.[17]

Acinetobacter sp. 351 can degrade lower molecular-weight PE oligomers. When PE is subjected to thermo- and photo-oxidization, products including alkanes, alkenes, ketones, aldehydes, alcohols, carboxylic acid, keto-acids, dicarboxylic acids, lactones, and esters are released.[18]

Bio-based polyethylene

Braskem and Toyota Tsusho Corporation started joint marketing activities to produce polyethylene from sugarcane. Braskem will build a new facility at their existing industrial unit in Triunfo, RS, Brazil with an annual production capacity of 200,000 short tons (180,000,000 kg), and will produce high-density and low-density polyethylene from bioethanol derived from sugarcane.[19]

Polyethylene can also be made from other feedstocks, including wheat grain and sugar beet. Retrieved from cane sugar, i.e. plant biomass renewable feedstock, Brazil is the first country to develop the product. These developments are using renewable resources rather than fossil fuel, although the issue of plastic source is currently negligible in the wake of plastic waste and in particular polyethylene waste as shown above.

Joining

Commonly used methods for joining polyethylene parts together include:[20]

Adhesives and solvents are rarely used because polyethylene is nonpolar and has a high resistance to solvents. Pressure-sensitive adhesives (PSA) are feasible if the surface is flame treated or corona treated. Commonly used adhesives include:[20]

Nomenclature and general description of the process

The name polyethylene comes from the ingredient and not the resulting chemical compound, which contains no double bonds. The scientific name polyethene is systematically derived from the scientific name of the monomer.[21][22] The alkene monomer converts to a long, sometimes very long, alkane in the polymerization process.[22] In certain circumstances it is useful to use a structure-based nomenclature; in such cases IUPAC recommends poly(methylene) (poly(methanediyl) is a non-preferred alternative).[21] The difference in names between the two systems is due to the opening up of the monomer's double bond upon polymerization.[23] The name is abbreviated to PE. In a similar manner polypropylene and polystyrene are shortened to PP and PS, respectively. In the United Kingdom the polymer is commonly called polythene, from the ICI trade name, although this is not recognized scientifically.

References

  1. ^ a b
  2. ^ Piringer & Baner 2008, p. 32.
  3. ^
  4. ^ a b c Kenneth S. Whiteley, T. Geoffrey Heggs, Hartmut Koch, Ralph L. Mawer, Wolfgang Immel, "Polyolefins" in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a21_487
  5. ^
  6. ^
  7. ^
  8. ^ H. von Pechmann (1898) "Ueber Diazomethan und Nitrosoacylamine," Berichte der Deutschen chemischen Gesellschaft zu Berlin, 31 : 2640–2646; see especially page 2643. From page 2643: "Erwähnt sei noch, dass aus einer ätherischen Diazomethanlösung sich beim Stehen manchmal minimale Quantitäten eines weissen, flockigen, aus Chloroform krystallisirenden Körpers abscheiden; … " (It should be mentioned that from an ether solution of diazomethane, upon standing, sometimes small quantities of a white, flakey substance, which can be crystallized from chloroform, precipitate; … )
  9. ^ Bamberger claimed that one of his students, Hindermann, had noted the formation of polyethylene in 1897. Eug. Bamberger & Fred. Tschirner (1900) "Ueber die Einwirkung von Diazomethan auf β-Arylhydroxylamine" (On the effect of diazomethane on β-arylhydroxylamine), Berichte der Deutschen chemischen Gesellschaft zu Berlin, 33 : 955–959. From footnote 3 on page 956: "Die Abscheidung weisser Flocken aus Diazomethanlösungen erwähnt auch v. Pechmann (diese Berichte 31, 2643); er hat sie aber wegen Substanzmangel nicht untersucht. Ich hatte übrigens Hrn. v. Pechmann schon einige Zeit vor Erscheinen seiner Publication mitgetheilt, dass aus Diazomethan ein fester, weisser Körper entstehe, der sich bei der Analyse als (CH2)x erwiesen habe, worauf mir Hr. v. Pechmann schrieb, dass er den weissen Körper ebensfalls beobachtet, aber nicht untersucht habe. Zuerst erwähnt ist derselbe in der Dissertation meines Schülers Hindermann, Zürich (1897), S. 120." (Von Pechmann (these Reports, 31, 2643) also mentioned the precipitation of white flakes from diazomethane solutions; however, due to a scarcity of the material, he didn't investigate it. Incidentally, some time before the appearance of his publication, I had communicated to Mr. von Pechmann that a solid, white substance arose from diazomethane, which on analysis proved to be (CH2)x, whereupon Mr. von Pechmann wrote me that he had likewise observed the white substance, but not investigated it. It is first mentioned in the dissertation of my student Hindermann, Zürich (1897), p. 120.)
  10. ^ Eug. Bamberger & Fred. Tschirner (1900) "Ueber die Einwirkung von Diazomethan auf β-Arylhydroxylamine" (On the effect of diazomethane on β-arylhydroxylamine), Berichte der Deutschen chemischen Gesellschaft zu Berlin, 33 : 955–959. From page 956: "Eine theilweise — übrigens immer nur minimale — Umwandlung des Diazomethans in Stickstoff und Polymethylen vollzieht sich auch bei ganz andersartigen Reactionen; … " (A partial — incidentally, always only minimal — conversion of diazomethane into nitrogen and polymethylene takes place also during quite different reactions; … )
  11. ^
  12. ^
  13. ^
  14. ^
  15. ^
  16. ^
  17. ^
  18. ^
  19. ^
  20. ^ a b Plastics Design Library, p. 326.
  21. ^ a b
  22. ^ a b
  23. ^

Bibliography

External links

  • Polythene's story: The accidental birth of plastic bags
  • Polythene Technical Properties & Applications
  • as a biodegrader of plastic bagsSphingomonasArticle describing the discovery of Kawawada, Karen, The Record (May 22, 2008).
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.