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Wikipedia

Polyethylene

January 26, 2023

This article is about the plastic. For the Radiohead song, see Polyethylene (Parts 1 & 2).
"Polythene" redirects here. For the Feeder album, see Polythene (album).

Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic.[5] It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bottles, etc.). As of 2017, over 100 million tonnes of polyethylene resins are being produced annually, accounting for 34% of the total plastics market.[6][7]

Polyethylene
Names
IUPAC name
Polyethene or poly(methylene)[1]
Other names
Polyethene
Polythene
Identifiers
  • 9002-88-4 Y
Abbreviations PE
ChemSpider
  • none
ECHA InfoCard 100.121.698
KEGG
  • C19503 Y
MeSH Polyethylene
  • 6325
UNII
  • 3W404QE89S Y
  • DTXSID8031946
Properties
(C2H4)n
Density 0.88–0.96 g/cm3[2]
Melting point 115–135 °C (239–275 °F; 388–408 K)[2]
Not soluble
log P 1.02620[3]
−9.67×10−6 (HDPE, SI, 22 °C)[4]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
The repeating unit within polyethylene in the most stable staggered conformation

Many kinds of polyethylene are known, with most having the chemical formula (C2H4)n. PE is usually a mixture of similar polymers of ethylene, with various values of n. It can be low-density or high-density: low-density polyethylene is extruded[verification needed] using high pressure (1,000–5,000 atm (100–510 MPa)) and high temperature (520 K (247 °C; 476 °F)), while high-density polyethylene is extruded[verification needed] using low pressure (6–7 atm (610–710 kPa)) and low temperature (333–343 K (60–70 °C; 140–158 °F)). Polyethylene is usually thermoplastic, but it can be modified to become thermosetting instead, for example, in cross-linked polyethylene.

History

Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while investigating diazomethane.[8][a][9][b] 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 again accidentally discovered in 1933 by Eric Fawcett and Reginald Gibson 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 difficult to reproduce at first. 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 low-density polyethylene (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, Du Pont at Sabine River, Texas, and Bakelite Corporation at Charleston, West Virginia, began large-scale commercial production under license from ICI.[12]

The landmark breakthrough in the commercial production of polyethylene began with the development of catalysts that promoted the polymerization at mild temperatures and pressures. The first of these was a catalyst based on chromium trioxide discovered in 1951 by Robert Banks and J. Paul Hogan at Phillips Petroleum.[13] In 1953 the German chemist Karl Ziegler developed a catalytic system based on titanium halides and organoaluminium 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 heavily used industrially. By the end of the 1950s both the Phillips- and Ziegler-type catalysts were being used for high-density polyethylene (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.

Properties

The properties of polyethylene can be divided into mechanical, chemical, electrical, optical, and thermal properties.[14]

Mechanical

Polyethylene is of low strength, hardness and rigidity, but has a high ductility and impact strength as well as low friction. It shows strong creep under persistent force, which can be reduced by addition of short fibers. It feels waxy when touched.

Thermal

The commercial applicability of polyethylene is limited by its low melting point compared to other thermoplastics. For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range 120 to 130 °C (248 to 266 °F). The melting point for average commercial low-density polyethylene is typically 105 to 115 °C (221 to 239 °F). These temperatures vary strongly with the type of polyethylene, but the theoretical upper limit of melting of polyethylene is reported to be 144 to 146 °C (291 to 295 °F). Combustion typically occurs above 349 °C (660 °F).

Chemical

Polyethylene consists of nonpolar, saturated, high-molecular-weight hydrocarbons. Therefore, its chemical behavior is similar to paraffin. The individual macromolecules are not covalently linked. Because of their symmetric molecular structure, they tend to crystallize; overall polyethylene is partially crystalline. Higher crystallinity increases density and mechanical and chemical stability.

The standard method to test plastic density is ISO 1183 part 2 (gradient columns), alternatively ISO 1183 part 1 (MVS2PRO density analyzer).[15]

Most LDPE, MDPE, and HDPE grades have excellent chemical resistance, meaning that they are not attacked by strong acids or strong bases and are resistant to gentle oxidants and reducing agents. 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.[5]

Polyethylene absorbs almost no water; the gas and water vapour permeability (only polar gases) is lower than for most plastics. Oxygen, carbon dioxide and flavorings, on the other hand, can pass it easily.

PE can become brittle when exposed to sunlight, carbon black is usually used as a UV stabilizer.

Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin (similar to candle flame). The material continues burning on removal of the flame source and produces a drip.[16]

Polyethylene cannot be imprinted or bonded with adhesives without pretreatment. High-strength joints are readily achieved with plastic welding.

Electrical

Polyethylene is a good electrical insulator. It offers good electrical treeing resistance; however, it becomes easily electrostatically charged (which can be reduced by additions of graphite, carbon black or antistatic agents). When pure, the dielectric constant is in the range 2.2 to 2.4 depending on the density [17] and the loss tangent is very low, making it a good dielectric for a capacitor

Optical

Depending on thermal history and film thickness, PE can vary between almost clear (transparent), milky-opaque (translucent) and opaque. LDPE has the greatest, LLDPE slightly less, and HDPE the least transparency. Transparency is reduced by crystallites if they are larger than the wavelength of visible light.[18]

Manufacturing process

Monomer

 
 
Ethylene (ethene)

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. Typical specifications for PE purity are <5 ppm for water, oxygen, and other alkenes contents. 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.[5]

Polymerization

Polymerization of ethylene to polyethylene is described by the following chemical equation:

n CH
2=CH
2 (gas) → [−CH
2−CH
2−]
n (solid) ΔH/n = −25.71 ± 0.59 kcal/mol (−107.6 ± 2.5 kJ/mol)[19]

Ethylene is a 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.[5] Polyethylene can be produced through radical polymerization, but this route has only limited utility and typically requires high-pressure apparatus.

Joining

See also: Solvent bonding

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

Pressure-sensitive adhesives (PSA) are feasible if the surface chemistry or charge is modified with plasma activation, flame treatment, or corona treatment.

Classification

Polyethylene is classified by 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. There are several types of polyethylene:

With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE, and LDPE.

Ultra-high-molecular-weight (UHMWPE)

Main article: Ultra-high-molecular-weight polyethylene
 
Stainless steel and ultra high molecular weight polyethylene hip replacement

UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.5 and 7.5 million amu.[21] 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, steel cable replacements on ships, 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 (HDPE)

Main article: High-density polyethylene
 
HDPE pipe on site during installation in outback Western Australia. The white outer layer is co-extruded to provide a reduction of thermal heating, known as Acu-Therm.

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.[22]

Cross-linked (PEX or XLPE)

Main article: Cross-linked polyethylene

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 (MDPE)

Main article: Medium-density polyethylene

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 (LLDPE)

Main article: Linear low-density polyethylene

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 they are 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.[23]

Low-density (LDPE)

Main article: Low-density polyethylene

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.[24]

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 (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.

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 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 in cement production.

Types of polyethylenes

The particular material properties of "polyethylene" depend on its molecular structure. Molecular weight and crystallinity are the most significant factors; crystallinity in turn depends on molecular weight and degree of branching. The less the polymer chains are branched, and the lower the molecular weight, the higher the crystallinity of polyethylene. Crystallinity ranges from 35% (PE-LD/PE-LLD) to 80% (PE-HD). Polyethylene has a density of 1.0 g/cm3 in crystalline regions and 0.86 g/cm3 in amorphous regions. An almost linear relationship exists between density and crystallinity.[14]

The degree of branching of the different types of polyethylene can be schematically represented as follows:[14]

PE-HD  
PE-LLD

 

PE-LD  

The figure shows polyethylene backbones, short-chain branches and side-chain branches. The polymer chains are represented linearly.

Chain branches

The properties of polyethylene are highly dependent on type and number of chain branches. The chain branches in turn depend on the process used: either the high-pressure process (only PE-LD) or the low-pressure process (all other PE grades). Low-density polyethylene is produced by the high-pressure process by radical polymerization, thereby numerous short chain branches as well as long chain branches are formed. Short chain branches are formed by intramolecular chain transfer reactions, they are always butyl or ethyl chain branches because the reaction proceeds after the following mechanism:

 

Environmental issues

 
A recyclable bag manufactured from polyethylene, resin identification code
 
Say no to polythene. Sign. Nako, Himachal Pradesh, India.
See also: Plastic pollution

Polyethylene is produced from ethylene, and although ethylene can be produced from renewable resources, it is mainly obtained from petroleum or natural gas.

Moreover, the widespread usage of polyethylene poses difficulties for waste management if it is not recycled. Polyethylene, like other synthetic plastics, is not readily biodegradable, and thus accumulates in landfills. Recycling is made easier if marked with a recycling code or resin identification code. This can read "PE" or "02" ("plastic number 2") for PE-HD and "04" ("plastic number 4") for PE-LD.

In Japan, getting rid of plastics in an environmentally friendly way was the major problem discussed until the Fukushima nuclear disaster in 2011 became a larger issue. It was listed as a US$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.[25]

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.[26]

Biodegradability

Polyethylene, like other synthetic plastics, is not readily biodegradable, and thus accumulates in landfills. However, there are a number of species of bacteria and animals that are able to degrade polyethylene.

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 within six weeks. He later guessed that it would be gone after six more weeks.[27]

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.[28]

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.[29]

In 2014, a Chinese researcher named Jun Yang discovered that Indian mealmoth larvae could metabolize polyethylene from observing that plastic bags at his home had small holes in them. Deducing that the hungry larvae must have digested the plastic somehow, he and his team analyzed their gut bacteria and found a few that could use plastic as their only carbon source. Not only could the bacteria from the guts of the Plodia interpunctella moth larvae metabolize polyethylene, they degraded it significantly, dropping its tensile strength by 50%, its mass by 10% and the molecular weights of its polymeric chains by 13%.[30][31]

In 2017, researchers reported that the caterpillar of Galleria mellonella eats plastic garbage such as polyethylene.[32][33]

Climate change

When exposed to ambient solar radiation the plastic produces two greenhouse gases, methane and ethylene. Of particular concern is the plastic type which releases gases at the highest rate: low-density polyethylene (or LDPE). Due to its low density properties it breaks down more easily over time, leading to higher surface areas. The production of these trace gases from virgin LDPE increase with surface area/time, with rates at the end of a 212-day incubation of 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 gases at rates ~2 times and ~76 times higher in comparison to water for methane and ethylene, respectively.[34]

Chemically modified polyethylene

Polyethylene may either be modified in the polymerization by polar or non-polar comonomers or after polymerization through polymer-analogous reactions. Common polymer-analogous reactions are in case of polyethylene crosslinking, chlorination and sulfochlorination.

Non-polar ethylene copolymers

α-olefins

In the low pressure process α-olefins (e.g. 1-butene or 1-hexene) may be added, which are incorporated in the polymer chain during polymerization. These copolymers introduce short side chains, thus crystallinity and density are reduced. As explained above, mechanical and thermal properties are changed thereby. In particular, PE-LLD is produced this way.

Metallocene polyethylene (PE-MC)

Metallocene polyethylene (PE-M) is prepared by means of metallocene catalysts, usually including copolymers (z. B. ethene / hexene). Metallocene polyethylene has a relatively narrow molecular weight distribution, exceptionally high toughness, excellent optical properties and a uniform comonomer content. Because of the narrow molecular weight distribution it behaves less pseudoplastic (especially under larger shear rates). Metallocene polyethylene has a low proportion of low molecular weight (extractable) components and a low welding and sealing temperature. Thus, it is particularly suitable for the food industry.[14]: 238 [35]: 19 

Polyethylene with multimodal molecular weight distribution

Polyethylene with multimodal molecular weight distribution consists of several polymer fractions, which are homogeneously mixed. Such polyethylene types offer extremely high stiffness, toughness, strength, stress crack resistance and an increased crack propagation resistance. They consist of equal proportions higher and lower molecular polymer fractions. The lower molecular weight units crystallize easier and relax faster. The higher molecular weight fractions form linking molecules between crystallites, thereby increasing toughness and stress crack resistance. Polyethylene with multimodal molecular weight distribution can be prepared either in two-stage reactors, by catalysts with two active centers on a carrier or by blending in extruders.[14]: 238 

Cyclic olefin copolymers (COC)

Cyclic olefin copolymers are prepared by copolymerization of ethene and cycloolefins (usually norbornene) produced by using metallocene catalysts. The resulting polymers are amorphous polymers and particularly transparent and heat resistant.[14]: 239 [35]: 27 

Polar ethylene copolymers

The basic compounds used as polar comonomers are vinyl alcohol (Ethenol, an unsaturated alcohol), acrylic acid (propenoic acid, an unsaturated acid) and esters containing one of the two compounds.

Ethylene copolymers with unsaturated alcohols

Ethylene/vinyl alcohol copolymer (EVOH) is (formally) a copolymer of PE and vinyl alcohol (ethenol), which is prepared by (partial) hydrolysis of ethylene-vinyl acetate copolymer (as vinyl alcohol itself is not stable). However, typically EVOH has a higher comonomer content than the VAC commonly used.[36]: 239 

EVOH is used in multilayer films for packaging as a barrier layer (barrier plastic). As EVOH is hygroscopic (water-attracting), it absorbs water from the environment, whereby it loses its barrier effect. Therefore, it must be used as a core layer surrounded by other plastics (like LDPE, PP, PA or PET). EVOH is also used as a coating agent against corrosion at street lights, traffic light poles and noise protection walls.[36]: 239 

Ethylene/acrylic acid copolymers (EAA)

Copolymer of ethylene and unsaturated carboxylic acids (such as acrylic acid) are characterized by good adhesion to diverse materials, by resistance to stress cracking and high flexibility.[37] However, they are more sensitive to heat and oxidation than ethylene homopolymers. Ethylene/acrylic acid copolymers are used as adhesion promoters.[14]

If salts of an unsaturated carboxylic acid are present in the polymer, thermo-reversible ion networks are formed, they are called ionomers. Ionomers are highly transparent thermoplastics which are characterized by high adhesion to metals, high abrasion resistance and high water absorption.[14]

Ethylene copolymers with unsaturated esters

If unsaturated esters are copolymerized with ethylene, either the alcohol moiety may be in the polymer backbone (as it is the case in ethylene-vinyl acetate copolymer) or of the acid moiety (e. g. in ethylene-ethyl acrylate copolymer). Ethylene-vinyl acetate copolymers are prepared similarly to LD-PE by high pressure polymerization. The proportion of comonomer has a decisive influence on the behaviour of the polymer.

The density decreases up to a comonomer share of 10% because of the disturbed crystal formation. With higher proportions it approaches to the one of polyvinyl acetate (1.17 g/cm3).[36]: 235  Due to decreasing crystallinity ethylene vinyl acetate copolymers are getting softer with increasing comonomer content. The polar side groups change the chemical properties significantly (compared to polyethylene):[14]: 224  weather resistance, adhesiveness and weldability rise with comonomer content, while the chemical resistance decreases. Also mechanical properties are changed: stress cracking resistance and toughness in the cold rise, whereas yield stress and heat resistance decrease. With a very high proportion of comonomers (about 50%) rubbery thermoplastics are produced (thermoplastic elastomers).[36]: 235 

Ethylene-ethyl acrylate copolymers behave similarly to ethylene-vinyl acetate copolymers.[14]: 240 

Crosslinking

Main article: Cross-linked polyethylene

A basic distinction is made between peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), electron beam crosslinking (PE-Xc) and azo crosslinking (PE-Xd).[38]

 

Shown are the peroxide, the silane and irradiation crosslinking. In each method, a radical is generated in the polyethylene chain (top center), either by radiation (h·ν) or by peroxides (R-O-O-R). Then, two radical chains can either directly crosslink (bottom left) or indirectly by silane compounds (bottom right).

  • Peroxide crosslinking (PE-Xa): The crosslinking of polyethylene using peroxides (e. g. dicumyl or di-tert-butyl peroxide) is still of major importance. In the so-called Engel process, a mixture of HDPE and 2%[39] peroxide is at first mixed at low temperatures in an extruder and then crosslinked at high temperatures (between 200 and 250 °C).[38] The peroxide decomposes to peroxide radicals (RO•), which abstract (remove) hydrogen atoms from the polymer chain, leading to radicals. When these combine, a crosslinked network is formed.[40] The resulting polymer network is uniform, of low tension and high flexibility, whereby it is softer and tougher than (the irradiated) PE-Xc.[38]
  • Silane crosslinking (PE-Xb): In the presence of silanes (e.g. trimethoxyvinylsilane) polyethylene can initially be Si-functionalized by irradiation or by a small amount of a peroxide. Later Si-OH groups can be formed in a water bath by hydrolysis, which condense then and crosslink the PE by the formation of Si-O-Si bridges. [16] Catalysts such as dibutyltin dilaurate may accelerate the reaction.[39]
  • Irradiation crosslinking (PE-Xc): The crosslinking of polyethylene is also possible by a downstream radiation source (usually an electron accelerator, occasionally an isotopic radiator). PE products are crosslinked below the crystalline melting point by splitting off hydrogen atoms. β-radiation possesses a penetration depth of 10 mm, ɣ-radiation 100 mm. Thereby the interior or specific areas can be excluded from the crosslinking.[38] However, due to high capital and operating costs radiation crosslinking plays only a minor role compared with the peroxide crosslinking.[36] In contrast to peroxide crosslinking, the process is carried out in the solid state. Thereby, the cross-linking takes place primarily in the amorphous regions, while the crystallinity remains largely intact.[39]
  • Azo crosslinking (PE-Xd): In the so-called Lubonyl process polyethylene is crosslinked preadded azo compounds after extrusion in a hot salt bath.[36][38]

Chlorination and sulfochlorination

Chlorinated Polyethylene (PE-C) is an inexpensive material having a chlorine content from 34 to 44%. It is used in blends with PVC because the soft, rubbery chloropolyethylene is embedded in the PVC matrix, thereby increasing the impact resistance. It also increases the weather resistance. Furthermore, it is used for softening PVC foils, without risking the migrate of plasticizers. Chlorinated polyethylene can be crosslinked peroxidically to form an elastomer which is used in cable and rubber industry.[36] When chlorinated polyethylene is added to other polyolefins, it reduces the flammability.[14]: 245 

Chlorosulfonated PE (CSM) is used as starting material for ozone-resistant synthetic rubber.[41]

Bio-based polyethylene

Main articles: Bioplastics and Renewable 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, Rio Grande do Sul, 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.[42]

Polyethylene can also be made from other feedstocks, including wheat grain and sugar beet. 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.

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.[43][44] The alkene monomer converts to a long, sometimes very long, alkane in the polymerization process.[44] 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).[43] The difference in names between the two systems is due to the opening up of the monomer's double bond upon polymerization.[45] The name is abbreviated to PE. In a similar manner polypropylene and polystyrene are shortened to PP and PS, respectively. In the United Kingdom and India the polymer is commonly called polythene, from the ICI trade name, although this is not recognized scientifically.

Footnotes

  1. ^ 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 precipitate, which can be crystallized with chloroform; ...].[8]: 2643 
  2. ^ Die Abscheidung weisser Flocken aus Diazomethanlösungen erwähnt auch v. Pechmann (diese Berichte 31, 2643);[8] 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)[9]: footnote 3 on page 956  [Von Pechmann (these Reports, 31, 2643)[8] 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)].

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External links

 
Look up polyethylene in Wiktionary, the free dictionary.
 
Wikimedia Commons has media related to Polyethylene.
  • Polythene's story: The accidental birth of plastic bags
  • Polythene Technical Properties & Applications
  • Article describing the discovery of Sphingomonas as a biodegrader of plastic bags 5 November 2012 at the Wayback Machine Kawawada, Karen, Waterloo Region Record (22 May 2008).

January 26, 2023
polyethylene, this, article, about, plastic, radiohead, song, parts, polythene, redirects, here, feeder, album, polythene, album, polythene, abbreviated, iupac, name, polyethene, poly, methylene, most, commonly, produced, plastic, polymer, primarily, used, pac. This article is about the plastic For the Radiohead song see Polyethylene Parts 1 amp 2 Polythene redirects here For the Feeder album see Polythene album Polyethylene or polythene abbreviated PE IUPAC name polyethene or poly methylene is the most commonly produced plastic 5 It is a polymer primarily used for packaging plastic bags plastic films geomembranes and containers including bottles etc As of 2017 update over 100 million tonnes of polyethylene resins are being produced annually accounting for 34 of the total plastics market 6 7 Polyethylene NamesIUPAC name Polyethene or poly methylene 1 Other names PolyethenePolytheneIdentifiersCAS Number 9002 88 4 YAbbreviations PEChemSpider noneECHA InfoCard 100 121 698KEGG C19503 YMeSH PolyethylenePubChem CID 6325UNII 3W404QE89S YCompTox Dashboard EPA DTXSID8031946PropertiesChemical formula C2H4 nDensity 0 88 0 96 g cm3 2 Melting point 115 135 C 239 275 F 388 408 K 2 Solubility in water Not solublelog P 1 02620 3 Magnetic susceptibility x 9 67 10 6 HDPE SI 22 C 4 Except where otherwise noted data are given for materials in their standard state at 25 C 77 F 100 kPa Infobox references The repeating unit within polyethylene in the most stable staggered conformation Many kinds of polyethylene are known with most having the chemical formula C2H4 n PE is usually a mixture of similar polymers of ethylene with various values of n It can be low density or high density low density polyethylene is extruded verification needed using high pressure 1 000 5 000 atm 100 510 MPa and high temperature 520 K 247 C 476 F while high density polyethylene is extruded verification needed using low pressure 6 7 atm 610 710 kPa and low temperature 333 343 K 60 70 C 140 158 F Polyethylene is usually thermoplastic but it can be modified to become thermosetting instead for example in cross linked polyethylene Contents 1 History 2 Properties 2 1 Mechanical 2 2 Thermal 2 3 Chemical 2 4 Electrical 2 5 Optical 3 Manufacturing process 3 1 Monomer 3 2 Polymerization 4 Joining 5 Classification 5 1 Ultra high molecular weight UHMWPE 5 2 High density HDPE 5 3 Cross linked PEX or XLPE 5 4 Medium density MDPE 5 5 Linear low density LLDPE 5 6 Low density LDPE 5 7 Very low density VLDPE 5 8 Copolymers 5 9 Types of polyethylenes 5 10 Chain branches 6 Environmental issues 6 1 Biodegradability 6 2 Climate change 7 Chemically modified polyethylene 7 1 Non polar ethylene copolymers 7 1 1 a olefins 7 1 2 Metallocene polyethylene PE MC 7 1 3 Polyethylene with multimodal molecular weight distribution 7 1 4 Cyclic olefin copolymers COC 7 2 Polar ethylene copolymers 7 2 1 Ethylene copolymers with unsaturated alcohols 7 2 2 Ethylene acrylic acid copolymers EAA 7 3 Ethylene copolymers with unsaturated esters 7 4 Crosslinking 7 5 Chlorination and sulfochlorination 7 6 Bio based polyethylene 8 Nomenclature and general description of the process 9 Footnotes 10 References 11 Bibliography 12 External linksHistory EditPolyethylene was first synthesized by the German chemist Hans von Pechmann who prepared it by accident in 1898 while investigating diazomethane 8 a 9 b 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 again accidentally discovered in 1933 by Eric Fawcett and Reginald Gibson 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 difficult to reproduce at first 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 low density polyethylene 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 Du Pont at Sabine River Texas and Bakelite Corporation at Charleston West Virginia began large scale commercial production under license from ICI 12 The landmark breakthrough in the commercial production of polyethylene began with the development of catalysts that promoted the polymerization at mild temperatures and pressures The first of these was a catalyst based on chromium trioxide discovered in 1951 by Robert Banks and J Paul Hogan at Phillips Petroleum 13 In 1953 the German chemist Karl Ziegler developed a catalytic system based on titanium halides and organoaluminium 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 heavily used industrially By the end of the 1950s both the Phillips and Ziegler type catalysts were being used for high density polyethylene 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 Hansjorg 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 Properties EditThe properties of polyethylene can be divided into mechanical chemical electrical optical and thermal properties 14 Mechanical Edit Polyethylene is of low strength hardness and rigidity but has a high ductility and impact strength as well as low friction It shows strong creep under persistent force which can be reduced by addition of short fibers It feels waxy when touched Thermal Edit The commercial applicability of polyethylene is limited by its low melting point compared to other thermoplastics For common commercial grades of medium and high density polyethylene the melting point is typically in the range 120 to 130 C 248 to 266 F The melting point for average commercial low density polyethylene is typically 105 to 115 C 221 to 239 F These temperatures vary strongly with the type of polyethylene but the theoretical upper limit of melting of polyethylene is reported to be 144 to 146 C 291 to 295 F Combustion typically occurs above 349 C 660 F Chemical Edit Polyethylene consists of nonpolar saturated high molecular weight hydrocarbons Therefore its chemical behavior is similar to paraffin The individual macromolecules are not covalently linked Because of their symmetric molecular structure they tend to crystallize overall polyethylene is partially crystalline Higher crystallinity increases density and mechanical and chemical stability The standard method to test plastic density is ISO 1183 part 2 gradient columns alternatively ISO 1183 part 1 MVS2PRO density analyzer 15 Most LDPE MDPE and HDPE grades have excellent chemical resistance meaning that they are not attacked by strong acids or strong bases and are resistant to gentle oxidants and reducing agents 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 5 Polyethylene absorbs almost no water the gas and water vapour permeability only polar gases is lower than for most plastics Oxygen carbon dioxide and flavorings on the other hand can pass it easily PE can become brittle when exposed to sunlight carbon black is usually used as a UV stabilizer Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin similar to candle flame The material continues burning on removal of the flame source and produces a drip 16 Polyethylene cannot be imprinted or bonded with adhesives without pretreatment High strength joints are readily achieved with plastic welding Electrical Edit Polyethylene is a good electrical insulator It offers good electrical treeing resistance however it becomes easily electrostatically charged which can be reduced by additions of graphite carbon black or antistatic agents When pure the dielectric constant is in the range 2 2 to 2 4 depending on the density 17 and the loss tangent is very low making it a good dielectric for a capacitor Optical Edit Depending on thermal history and film thickness PE can vary between almost clear transparent milky opaque translucent and opaque LDPE has the greatest LLDPE slightly less and HDPE the least transparency Transparency is reduced by crystallites if they are larger than the wavelength of visible light 18 Manufacturing process EditMonomer Edit Ethylene ethene 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 CH2 connected to each other Typical specifications for PE purity are lt 5 ppm for water oxygen and other alkenes contents 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 5 Polymerization Edit Polymerization of ethylene to polyethylene is described by the following chemical equation n CH2 CH2 gas CH2 CH2 n solid DH n 25 71 0 59 kcal mol 107 6 2 5 kJ mol 19 Ethylene is a 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 5 Polyethylene can be produced through radical polymerization but this route has only limited utility and typically requires high pressure apparatus Joining EditSee also Solvent bonding Commonly used methods for joining polyethylene parts together include 20 Welding Hot gas welding Infrared welding Laser welding Ultrasonic welding Heat sealing Heat fusion Fastening Adhesives 20 Pressure sensitive adhesive PSAs Dispersion of solvent type PSAs Polyurethane contact adhesives Two part polyurethane Epoxy adhesives Hot melt adhesives Solvent bonding Adhesives and solvents are rarely used as solvent bonding because polyethylene is nonpolar and has a high resistance to solvents Pressure sensitive adhesives PSA are feasible if the surface chemistry or charge is modified with plasma activation flame treatment or corona treatment Classification EditPolyethylene is classified by 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 There are several types of polyethylene Ultra high molecular weight polyethylene UHMWPE Ultra low molecular weight polyethylene ULMWPE or PE WAX High molecular weight polyethylene HMWPE High density polyethylene HDPE High density cross linked polyethylene HDXLPE Cross linked polyethylene PEX or XLPE Medium density polyethylene MDPE Linear low density polyethylene LLDPE Low density polyethylene LDPE Very low density polyethylene VLDPE Chlorinated polyethylene CPE With regard to sold volumes the most important polyethylene grades are HDPE LLDPE and LDPE Ultra high molecular weight UHMWPE Edit Main article Ultra high molecular weight polyethylene Stainless steel and ultra high molecular weight polyethylene hip replacement UHMWPE is polyethylene with a molecular weight numbering in the millions usually between 3 5 and 7 5 million amu 21 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 steel cable replacements on ships 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 HDPE Edit Main article High density polyethylene HDPE pipe on site during installation in outback Western Australia The white outer layer is co extruded to provide a reduction of thermal heating known as Acu Therm 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 22 Cross linked PEX or XLPE Edit Main article Cross linked polyethylene 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 MDPE Edit Main article Medium density polyethylene 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 LLDPE Edit Main article Linear low density polyethylene 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 they are 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 23 Low density LDPE Edit Main article Low density polyethylene 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 24 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 VLDPE Edit 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 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 Edit In addition to copolymerization with alpha olefins ethylene can 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 in cement production Types of polyethylenes Edit The particular material properties of polyethylene depend on its molecular structure Molecular weight and crystallinity are the most significant factors crystallinity in turn depends on molecular weight and degree of branching The less the polymer chains are branched and the lower the molecular weight the higher the crystallinity of polyethylene Crystallinity ranges from 35 PE LD PE LLD to 80 PE HD Polyethylene has a density of 1 0 g cm3 in crystalline regions and 0 86 g cm3 in amorphous regions An almost linear relationship exists between density and crystallinity 14 The degree of branching of the different types of polyethylene can be schematically represented as follows 14 PE HD PE LLD PE LD The figure shows polyethylene backbones short chain branches and side chain branches The polymer chains are represented linearly Chain branches Edit The properties of polyethylene are highly dependent on type and number of chain branches The chain branches in turn depend on the process used either the high pressure process only PE LD or the low pressure process all other PE grades Low density polyethylene is produced by the high pressure process by radical polymerization thereby numerous short chain branches as well as long chain branches are formed Short chain branches are formed by intramolecular chain transfer reactions they are always butyl or ethyl chain branches because the reaction proceeds after the following mechanism Environmental issues Edit A recyclable bag manufactured from polyethylene resin identification code Say no to polythene Sign Nako Himachal Pradesh India See also Plastic pollution Polyethylene is produced from ethylene and although ethylene can be produced from renewable resources it is mainly obtained from petroleum or natural gas Moreover the widespread usage of polyethylene poses difficulties for waste management if it is not recycled Polyethylene like other synthetic plastics is not readily biodegradable and thus accumulates in landfills Recycling is made easier if marked with a recycling code or resin identification code This can read PE or 02 plastic number 2 for PE HD and 04 plastic number 4 for PE LD In Japan getting rid of plastics in an environmentally friendly way was the major problem discussed until the Fukushima nuclear disaster in 2011 became a larger issue It was listed as a US 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 25 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 26 Biodegradability Edit Polyethylene like other synthetic plastics is not readily biodegradable and thus accumulates in landfills However there are a number of species of bacteria and animals that are able to degrade polyethylene 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 within six weeks He later guessed that it would be gone after six more weeks 27 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 28 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 29 In 2014 a Chinese researcher named Jun Yang discovered that Indian mealmoth larvae could metabolize polyethylene from observing that plastic bags at his home had small holes in them Deducing that the hungry larvae must have digested the plastic somehow he and his team analyzed their gut bacteria and found a few that could use plastic as their only carbon source Not only could the bacteria from the guts of the Plodia interpunctella moth larvae metabolize polyethylene they degraded it significantly dropping its tensile strength by 50 its mass by 10 and the molecular weights of its polymeric chains by 13 30 31 In 2017 researchers reported that the caterpillar of Galleria mellonella eats plastic garbage such as polyethylene 32 33 Climate change Edit When exposed to ambient solar radiation the plastic produces two greenhouse gases methane and ethylene Of particular concern is the plastic type which releases gases at the highest rate low density polyethylene or LDPE Due to its low density properties it breaks down more easily over time leading to higher surface areas The production of these trace gases from virgin LDPE increase with surface area time with rates at the end of a 212 day incubation of 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 gases at rates 2 times and 76 times higher in comparison to water for methane and ethylene respectively 34 Chemically modified polyethylene EditPolyethylene may either be modified in the polymerization by polar or non polar comonomers or after polymerization through polymer analogous reactions Common polymer analogous reactions are in case of polyethylene crosslinking chlorination and sulfochlorination Non polar ethylene copolymers Edit a olefins Edit In the low pressure process a olefins e g 1 butene or 1 hexene may be added which are incorporated in the polymer chain during polymerization These copolymers introduce short side chains thus crystallinity and density are reduced As explained above mechanical and thermal properties are changed thereby In particular PE LLD is produced this way Metallocene polyethylene PE MC Edit Metallocene polyethylene PE M is prepared by means of metallocene catalysts usually including copolymers z B ethene hexene Metallocene polyethylene has a relatively narrow molecular weight distribution exceptionally high toughness excellent optical properties and a uniform comonomer content Because of the narrow molecular weight distribution it behaves less pseudoplastic especially under larger shear rates Metallocene polyethylene has a low proportion of low molecular weight extractable components and a low welding and sealing temperature Thus it is particularly suitable for the food industry 14 238 35 19 Polyethylene with multimodal molecular weight distribution Edit Polyethylene with multimodal molecular weight distribution consists of several polymer fractions which are homogeneously mixed Such polyethylene types offer extremely high stiffness toughness strength stress crack resistance and an increased crack propagation resistance They consist of equal proportions higher and lower molecular polymer fractions The lower molecular weight units crystallize easier and relax faster The higher molecular weight fractions form linking molecules between crystallites thereby increasing toughness and stress crack resistance Polyethylene with multimodal molecular weight distribution can be prepared either in two stage reactors by catalysts with two active centers on a carrier or by blending in extruders 14 238 Cyclic olefin copolymers COC Edit Cyclic olefin copolymers are prepared by copolymerization of ethene and cycloolefins usually norbornene produced by using metallocene catalysts The resulting polymers are amorphous polymers and particularly transparent and heat resistant 14 239 35 27 Polar ethylene copolymers Edit The basic compounds used as polar comonomers are vinyl alcohol Ethenol an unsaturated alcohol acrylic acid propenoic acid an unsaturated acid and esters containing one of the two compounds Ethylene copolymers with unsaturated alcohols Edit Ethylene vinyl alcohol copolymer EVOH is formally a copolymer of PE and vinyl alcohol ethenol which is prepared by partial hydrolysis of ethylene vinyl acetate copolymer as vinyl alcohol itself is not stable However typically EVOH has a higher comonomer content than the VAC commonly used 36 239 EVOH is used in multilayer films for packaging as a barrier layer barrier plastic As EVOH is hygroscopic water attracting it absorbs water from the environment whereby it loses its barrier effect Therefore it must be used as a core layer surrounded by other plastics like LDPE PP PA or PET EVOH is also used as a coating agent against corrosion at street lights traffic light poles and noise protection walls 36 239 Ethylene acrylic acid copolymers EAA Edit Copolymer of ethylene and unsaturated carboxylic acids such as acrylic acid are characterized by good adhesion to diverse materials by resistance to stress cracking and high flexibility 37 However they are more sensitive to heat and oxidation than ethylene homopolymers Ethylene acrylic acid copolymers are used as adhesion promoters 14 If salts of an unsaturated carboxylic acid are present in the polymer thermo reversible ion networks are formed they are called ionomers Ionomers are highly transparent thermoplastics which are characterized by high adhesion to metals high abrasion resistance and high water absorption 14 Ethylene copolymers with unsaturated esters Edit If unsaturated esters are copolymerized with ethylene either the alcohol moiety may be in the polymer backbone as it is the case in ethylene vinyl acetate copolymer or of the acid moiety e g in ethylene ethyl acrylate copolymer Ethylene vinyl acetate copolymers are prepared similarly to LD PE by high pressure polymerization The proportion of comonomer has a decisive influence on the behaviour of the polymer The density decreases up to a comonomer share of 10 because of the disturbed crystal formation With higher proportions it approaches to the one of polyvinyl acetate 1 17 g cm3 36 235 Due to decreasing crystallinity ethylene vinyl acetate copolymers are getting softer with increasing comonomer content The polar side groups change the chemical properties significantly compared to polyethylene 14 224 weather resistance adhesiveness and weldability rise with comonomer content while the chemical resistance decreases Also mechanical properties are changed stress cracking resistance and toughness in the cold rise whereas yield stress and heat resistance decrease With a very high proportion of comonomers about 50 rubbery thermoplastics are produced thermoplastic elastomers 36 235 Ethylene ethyl acrylate copolymers behave similarly to ethylene vinyl acetate copolymers 14 240 Crosslinking Edit Main article Cross linked polyethylene A basic distinction is made between peroxide crosslinking PE Xa silane crosslinking PE Xb electron beam crosslinking PE Xc and azo crosslinking PE Xd 38 Shown are the peroxide the silane and irradiation crosslinking In each method a radical is generated in the polyethylene chain top center either by radiation h n or by peroxides R O O R Then two radical chains can either directly crosslink bottom left or indirectly by silane compounds bottom right Peroxide crosslinking PE Xa The crosslinking of polyethylene using peroxides e g dicumyl or di tert butyl peroxide is still of major importance In the so called Engel process a mixture of HDPE and 2 39 peroxide is at first mixed at low temperatures in an extruder and then crosslinked at high temperatures between 200 and 250 C 38 The peroxide decomposes to peroxide radicals RO which abstract remove hydrogen atoms from the polymer chain leading to radicals When these combine a crosslinked network is formed 40 The resulting polymer network is uniform of low tension and high flexibility whereby it is softer and tougher than the irradiated PE Xc 38 Silane crosslinking PE Xb In the presence of silanes e g trimethoxyvinylsilane polyethylene can initially be Si functionalized by irradiation or by a small amount of a peroxide Later Si OH groups can be formed in a water bath by hydrolysis which condense then and crosslink the PE by the formation of Si O Si bridges 16 Catalysts such as dibutyltin dilaurate may accelerate the reaction 39 Irradiation crosslinking PE Xc The crosslinking of polyethylene is also possible by a downstream radiation source usually an electron accelerator occasionally an isotopic radiator PE products are crosslinked below the crystalline melting point by splitting off hydrogen atoms b radiation possesses a penetration depth of 10 mm ɣ radiation 100 mm Thereby the interior or specific areas can be excluded from the crosslinking 38 However due to high capital and operating costs radiation crosslinking plays only a minor role compared with the peroxide crosslinking 36 In contrast to peroxide crosslinking the process is carried out in the solid state Thereby the cross linking takes place primarily in the amorphous regions while the crystallinity remains largely intact 39 Azo crosslinking PE Xd In the so called Lubonyl process polyethylene is crosslinked preadded azo compounds after extrusion in a hot salt bath 36 38 Chlorination and sulfochlorination Edit Chlorinated Polyethylene PE C is an inexpensive material having a chlorine content from 34 to 44 It is used in blends with PVC because the soft rubbery chloropolyethylene is embedded in the PVC matrix thereby increasing the impact resistance It also increases the weather resistance Furthermore it is used for softening PVC foils without risking the migrate of plasticizers Chlorinated polyethylene can be crosslinked peroxidically to form an elastomer which is used in cable and rubber industry 36 When chlorinated polyethylene is added to other polyolefins it reduces the flammability 14 245 Chlorosulfonated PE CSM is used as starting material for ozone resistant synthetic rubber 41 Bio based polyethylene Edit Main articles Bioplastics and Renewable 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 Rio Grande do Sul 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 42 Polyethylene can also be made from other feedstocks including wheat grain and sugar beet 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 Nomenclature and general description of the process EditThe 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 43 44 The alkene monomer converts to a long sometimes very long alkane in the polymerization process 44 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 43 The difference in names between the two systems is due to the opening up of the monomer s double bond upon polymerization 45 The name is abbreviated to PE In a similar manner polypropylene and polystyrene are shortened to PP and PS respectively In the United Kingdom and India the polymer is commonly called polythene from the ICI trade name although this is not recognized scientifically Footnotes Edit Erwahnt sei noch dass aus einer atherischen Diazomethanlosung sich beim Stehen manchmal minimale Quantitaten eines weissen flockigen aus Chloroform krystallisirenden Korpers abscheiden It should be mentioned that from an ether solution of diazomethane upon standing sometimes small quantities of a white flakey substance precipitate which can be crystallized with chloroform 8 2643 Die Abscheidung weisser Flocken aus Diazomethanlosungen erwahnt auch v Pechmann diese Berichte 31 2643 8 er hat sie aber wegen Substanzmangel nicht untersucht Ich hatte ubrigens Hrn v Pechmann schon einige Zeit vor Erscheinen seiner Publication mitgetheilt dass aus Diazomethan ein fester weisser Korper entstehe der sich bei der Analyse als CH2 x erwiesen habe worauf mir Hr v Pechmann schrieb dass er den weissen Korper ebensfalls beobachtet aber nicht untersucht habe Zuerst erwahnt ist derselbe in der Dissertation meines Schulers Hindermann Zurich 1897 S 120 9 footnote 3 on page 956 Von Pechmann theseReports 31 2643 8 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 Zurich 1897 p 120 References Edit Compendium of Polymer Terminology and Nomenclature IUPAC Recommendations 2008 PDF Retrieved 28 August 2018 a b Batra Kamal 2014 Role of Additives in Linear Low Density Polyethylene LLDPE Films p 9 Retrieved 16 September 2014 poly ethylene ChemSrc Wapler M C Leupold J Dragonu I von Elverfeldt D Zaitsev M Wallrabe U 2014 Magnetic properties of materials for MR engineering micro MR and beyond JMR 242 233 242 arXiv 1403 4760 Bibcode 2014JMagR 242 233W doi 10 1016 j jmr 2014 02 005 PMID 24705364 S2CID 11545416 a b c d Whiteley Kenneth S Heggs T Geoffrey Koch Hartmut Mawer Ralph L and Immel Wolfgang 2005 Polyolefins in Ullmann s Encyclopedia of Industrial Chemistry Wiley VCH Weinheim doi 10 1002 14356007 a21 487 Geyer Roland Jambeck Jenna R Law Kara Lavender 1 July 2017 Production use and fate of all plastics ever made Science Advances 3 7 e1700782 Bibcode 2017SciA 3E0782G doi 10 1126 sciadv 1700782 PMC 5517107 PMID 28776036 Plastics The Facts PDF Plastics Europe Archived from the original PDF on 4 February 2018 Retrieved 29 August 2018 a b c d von Pechmann H 1898 Ueber Diazomethan und Nitrosoacylamine Berichte der Deutschen Chemischen Gesellschaft zu Berlin 31 2640 2646 a b Bamberger Eug Tschirner Fred 1900 Ueber die Einwirkung von Diazomethan auf b Arylhydroxylamine On the effect of diazomethane on b arylhydroxylamine Berichte der Deutschen Chemischen Gesellschaft zu Berlin 33 955 959 doi 10 1002 cber 190003301166 Bamberger Eugen Tschirner Friedrich 1900 Ueber die Einwirkung von Diazomethan auf b Arylhydroxylamine On the effect of diazomethane on b arylhydroxylamine Berichte der Deutschen Chemischen Gesellschaft zu Berlin 33 955 959 doi 10 1002 cber 190003301166 page 956 Eine theilweise ubrigens 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 Winnington history in the making This is Cheshire 23 August 2006 Archived from the original on 21 January 2010 Retrieved 20 February 2014 Poly the all star plastic Popular Mechanics Vol 91 no 1 Hearst Magazines July 1949 pp 125 129 Retrieved 20 February 2014 via Google Books Hoff Ray Mathers Robert T 2010 Chapter 10 Review of Phillips Chromium Catalyst for Ethylene Polymerization In Hoff Ray Mathers Robert T eds Handbook of Transition Metal Polymerization Catalysts John Wiley amp Sons doi 10 1002 9780470504437 ch10 ISBN 978 0 470 13798 7 a b c d e f g h i j k Kaiser Wolfgang 2011 Kunststoffchemie fur Ingenieure von der Synthese bis zur Anwendung 3 ed Munchen Hanser ISBN 978 3 446 43047 1 PLASTIC DENSITY IN 2 MINUTES www plastic density com Retrieved 18 June 2021 a href Template Cite web html title Template Cite web cite web a CS1 maint url status link How to Identify Plastic Materials Using The Burn Test Boedeker Plastics Retrieved 8 May 2012 Electrical Properties of Plastic Materials PDF professionalplastics com Professional Plastics Archived PDF from the original on 20 September 2022 Retrieved 11 October 2022 Chung C I 2010 Extrusion of Polymers Theory and Practice 2nd ed Hanser Munich Victor Ostrovskii et al Ethylene Polymerization Heat abstract in Doklady Chemistry 184 1 103 104 January 1969 a b Plastics Design Library 1997 Handbook of Plastics Joining A Practical Guide Norwich New York Plastics Design Library p 326 ISBN 1 884207 17 0 Kurtz Steven M 2015 UHMWPE Biomaterials Handbook Ultra High Molecular Weight Polyethylene in Total Joint Replacement and Medical Devices 3rd ed Elsevier p 3 doi 10 1016 C2013 0 16083 7 ISBN 9780323354356 Market Study Polyethylene HDPE Ceresana Research May 2012 Retrieved 8 May 2012 Market Study Polyethylene LLDPE 2nd edition Ceresana November 2014 Retrieved 3 February 2015 Market Study Polyethylene LDPE 2nd edition Ceresana October 2014 Retrieved 3 February 2015 Prideaux Eric 3 November 2007 Plastic incineration rise draws ire The Japan Times Archived from the original on 22 November 2012 Retrieved 8 May 2012 Nguyen Tuan 17 February 2011 New invention turns plastic bags into oil smartplanet com Retrieved 20 February 2014 CanadaWorld WCI student isolates microbe that lunches on plastic bags The Record com Archived from the original on 18 July 2011 Retrieved 20 February 2014 Hadad D Geresh S Sivan A 2005 Biodegradation of polyethylene by the thermophilic bacterium Brevibacillus borstelensis Journal of Applied Microbiology 98 5 1093 1100 doi 10 1111 j 1365 2672 2005 02553 x PMID 15836478 S2CID 2977246 Tokiwa Yutaka Calabia Buenaventurada P Ugwu Charles U Aiba Seiichi September 2009 Biodegradability of Plastics International Journal of Molecular Sciences 10 9 3722 3742 doi 10 3390 ijms10093722 PMC 2769161 PMID 19865515 Balster Lori 27 January 2015 Discovery of plastic eating bacteria may speed waste reduction fondriest com Yang Jun Yang Yu Wu Wei Min Zhao Jiao Jiang Lei 2014 Evidence of Polyethylene Biodegradation by Bacterial Strains from the Guts of Plastic Eating Waxworms Environmental Science amp Technology 48 23 13776 84 Bibcode 2014EnST 4813776Y doi 10 1021 es504038a PMID 25384056 Forscherin entdeckt zufallig Plastik fressende Raupe Der Spiegel in German 24 April 2017 Retrieved 24 April 2017 Briggs Helen Plastic eating caterpillar could munch waste scientists say BBC News Retrieved 24 April 2017 Royer Sarah Jeanne Ferron Sara Wilson Samuel T Karl David M 2018 Production of methane and ethylene from plastic in the environment PLOS ONE 13 8 e0200574 Bibcode 2018PLoSO 1300574R doi 10 1371 journal pone 0200574 PMC 6070199 PMID 30067755 a b Pascu Cornelia Vasile Mihaela 2005 Practical guide to polyethylene Online Ausg ed Shawbury Rapra Technology Ltd ISBN 978 1859574935 a b c d e f g Elsner Peter Eyerer Peter Hirth Thomas 2012 Domininghaus Kunststoffe 8 ed Berlin Heidelberg Springer Verlag p 224 ISBN 978 3 642 16173 5 Elsner Peter Eyerer Peter Hirth Thomas 2012 Kunststoffe Eigenschaften und Anwendungen 8 ed Berlin Heidelberg Springer Berlin Heidelberg ISBN 978 3 642 16173 5 a b c d e Baur Erwin Osswald Tim A October 2013 Saechtling Kunststoff Taschenbuch p 443 ISBN 978 3 446 43729 6 Vorschau auf kunststoffe de a b c Whiteley Kenneth S 2011 Polyethylene Ullmann s Encyclopedia of Industrial Chemistry doi 10 1002 14356007 a21 487 pub2 ISBN 978 3527306732 a href Template Cite book html title Template Cite book cite book a Missing or empty title help Koltzenburg Sebastian Maskos Michael Nuyken Oskar 2014 Polymere Synthese Eigenschaften und Anwendungen 1 ed Springer Spektrum p 406 ISBN 978 3 642 34773 3 Chlorsulfoniertes Polyethylen CSM ChemgaPedia de Braskem amp Toyota Tsusho start joint marketing activities for green polyethylene from sugar cane Press release yourindustrynews com 26 September 2008 Archived from the original on 21 May 2013 Retrieved 20 February 2014 a b A Guide to IUPAC Nomenclature of Organic Compounds Recommendations 1993 IUPAC Commission on Nomenclature of Organic Chemistry Blackwell Scientific Publications 1993 ISBN 978 0632037025 Retrieved 20 February 2014 a b Kahovec J Fox R B Hatada K 2002 Nomenclature of regular single strand organic polymers IUPAC Recommendations 2002 Pure and Applied Chemistry 74 10 1921 doi 10 1351 pac200274101921 IUPAC Provisional Recommendations on the Nomenclature of Organic Chemistry International Union of Pure and Applied Chemistry 27 October 2004 Retrieved 20 February 2014 Bibliography EditPiringer Otto G Baner Albert Lawrence 2008 Plastic Packaging Interactions with Food and Pharmaceuticals 2nd ed Wiley VCH ISBN 978 3 527 31455 3 Retrieved 20 February 2014 Plastics Design Library 1997 Handbook of Plastics Joining A Practical Guide illustrated ed William Andrew ISBN 978 1 884207 17 4 Retrieved 20 February 2014 External links Edit Look up polyethylene in Wiktionary the free dictionary Wikimedia Commons has media related to Polyethylene Polythene s story The accidental birth of plastic bags Polythene Technical Properties amp Applications Article describing the discovery of Sphingomonas as a biodegrader of plastic bags Archived 5 November 2012 at the Wayback Machine Kawawada Karen Waterloo Region Record 22 May 2008 Portals Chemistry Numismatics Electronics Volcanoes North West England Retrieved from https en wikipedia org w index php title Polyethylene amp oldid 1135109022, wikipedia, wiki, book, books, library,

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