Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic.[ It is a polymer, primarily used for packaging (, , geomembranes and containers including , , , etc.). , over 100 million of polyethylene are being produced annually, accounting for 34% of the total plastics market.]
Many kinds of polyethylene are known, with most having the chemical formula (C2H4) n. PE is usually a mixture of similar of ethylene, with various values of n. It can be low-density or high-density and many variations thereof. Its properties can be modified further by crosslinking or copolymerization. All forms are nontoxic as well as chemically resilient, contributing to polyethylene's popularity as a multi-use plastic. However, polyethylene's chemical resilience also makes it a long-lived and decomposition-resistant pollutant when disposed of improperly.
History
Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while investigating
diazomethane.
When his colleagues
Eugen Bamberger and Friedrich Tschirner characterized the white, waxy substance that he had created, they recognized that it contained long −CH
2− chains and termed it
polymethylene.
The first industrially practical polyethylene synthesis (diazomethane is a notoriously unstable substance that is generally avoided in industrial syntheses) was again accidentally discovered in 1933 by Eric Fawcett and Reginald Gibson at the Imperial Chemical Industries (ICI) works in Northwich, England.
The landmark breakthrough in the commercial production of polyethylene began with the development of 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.
In 1953 the German chemist
Karl Ziegler developed a catalytic system based on
titanium 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
, 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
and have become the basis for the wide range of polyethylene
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
in many high-strength applications.
Properties
The properties of polyethylene depend strongly on type. The molecular weight, crosslinking, and presence of comonomers all strongly affect its properties. It is for this structure-property relation that intense effort has been invested into diverse kinds of PE.
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.
Polyethylene absorbs almost no water. The permeability for water vapor and polar gases of is lower than for most plastics. On the other hand, non-polar gases such as Oxygen, carbon dioxide, and flavorings can pass it easily.
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.
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 ).
When pure, the dielectric constant is in the range 2.2 to 2.4 depending on the density
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 if they are larger than the wavelength of visible light.[Chung, C. I. (2010) Extrusion of Polymers: Theory and Practice. 2nd ed.. Hanser: Munich.]
Manufacturing process
Monomer
The ingredient or monomer is ethylene (IUPAC name ethene), a hydrocarbon with the formula C2H4, which can be viewed as a pair of (−−) connected to each other. Typical specifications for PE purity are <5 ppm for water, oxygen, and other contents. Acceptable contaminants include N2, ethane (common precursor to ethylene), and methane. Ethylene is usually produced from petrochemical sources, but is also generated by dehydration of ethanol.[
]
Polymerization
Polymerization of ethylene to polyethylene is described by the following chemical equation:
- (gas) → − (solid)Δ/ =
[Victor Ostrovskii et al. Ethylene Polymerization Heat (abstract) in Doklady Chemistry 184(1):103–104. January 1969.]
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.[ Polyethylene can be produced through radical polymerization, but this route has only limited utility and typically requires high-pressure apparatus.
]
Joining
Commonly used methods for joining polyethylene parts together include:[Plastics Design Library (1997). Handbook of Plastics Joining: A Practical Guide. Norwich, New York: Plastics Design Library. p. 326. .]
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:
-
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)
UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.5 and 7.5 million amu.
High-density (HDPE)
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 HDPE pipe.
Cross-linked (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 (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 (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 (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.
Low-density (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.
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 free radical (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 rheology 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 . Applications of Acrylic resin 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.[
]
The degree of branching of the different types of polyethylene can be schematically represented as follows:[
]
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 group or ethyl group chain branches because the reaction proceeds after the following mechanism:
Environmental issues
The widespread usage of polyethylene poses potential difficulties for waste management because it is not readily biodegradable. Since 2008, Japan has increased plastic recycling, but still has a large amount of plastic wrapping which goes to waste. Plastic recycling in Japan is a potential US$90 billion market.
It is possible to rapidly convert polyethylene to hydrogen and graphene by heating. The energy needed is much less than for producing hydrogen by electrolysis.
Biodegradability
Several experiments have been conducted aimed at discovering enzyme or organisms that will degrade polyethylene. Several plastics - polyesters, polycarbonates, polyamides - degrade either by hydrolysis or air oxidation. In some cases the degradation is increased by bacteria or various enzyme cocktails. The situation is very different with polymers where the backbone consists solely of C-C bonds. These polymers include polyethylene, but also polypropylene, polystyrene and acrylates. At best, these polymers degrade very slowly, but these experiments are difficult because yields and rates are very slow.[
]
Bacteria and insect case studies
The Indian mealmoth larvae are claimed to metabolize polyethylene based on observing that plastic bags at a researcher's 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%.
The caterpillar of Galleria mellonella is claimed to consume polyethylene. The caterpillar is able to digest polyethylene due to a combination of its gut microbiota
Climate change
When exposed to ambient solar radiation the plastic produces trace amounts of two , methane and ethylene. The plastic type which releases gases at the highest rate is low-density polyethylene (LDPE). Due to its low density it breaks down more easily over time, leading to higher surface areas. When incubated in air, LDPE emits gases at rates ~2 times and ~76 times higher in comparison to incubation in water for methane and ethylene, respectively. However, based on the rates measured in the study methane production by plastics is presently an insignificant component of the global methane budget.
Chemically modified polyethylene
Polyethylene may either be modified in the polymerization by polar or non-polar or after polymerization through polymer-analogous reactions. Common polymer-analogous reactions are in case of polyethylene Cross-link, chlorination and sulfochlorination.
Non-polar ethylene copolymers
α-olefins
In the low pressure process Alpha-olefin (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.
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.[
]
Cyclic olefin copolymers (COC)
Cyclic olefin copolymers are prepared by copolymerization of ethene and (usually norbornene) produced by using metallocene catalysts. The resulting polymers are amorphous polymers and particularly transparent and heat resistant.[
]
Polar ethylene copolymers
The basic compounds used as polar comonomers are vinyl alcohol (Vinyl alcohol, an unsaturated alcohol), acrylic acid (propenoic acid, an unsaturated acid) and 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.
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.[
]
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.[
]
If salts of an unsaturated carboxylic acid are present in the polymer, thermo-reversible ion networks are formed, they are called . Ionomers are highly transparent thermoplastics which are characterized by high adhesion to metals, high abrasion resistance and high water absorption.[
]
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).[ 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):][ 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).][
]
Ethylene-ethyl acrylate copolymers behave similarly to ethylene-vinyl acetate copolymers.[
]
Crosslinking
A basic distinction is made between peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), electron beam crosslinking (PE-Xc) and azo crosslinking (PE-Xd).[ Vorschau auf kunststoffe.de]
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).
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Peroxide crosslinking (PE-Xa): The crosslinking of polyethylene using (e. g. dicumyl peroxide or di-tert-butyl peroxide) is still of major importance. In the so-called Engel process, a mixture of HDPE and 2%
[ 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.][
]
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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 such as dibutyltin dilaurate may accelerate the reaction.
[
]
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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 Millimetre, Gamma ray 100 mm. Thereby the interior or specific areas can be excluded from the crosslinking.
[ However, due to high capital and operating costs radiation crosslinking plays only a minor role compared with the peroxide crosslinking.][ 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.][
]
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Azo crosslinking (PE-Xd): In the so-called Lubonyl process polyethylene is crosslinked preadded after extrusion in a hot salt bath.
[
]
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.[ When chlorinated polyethylene is added to other polyolefins, it reduces the flammability.][
]
Chlorosulfonated PE (CSM) is used as starting material for ozone-resistant synthetic rubber.[ Chlorsulfoniertes Polyethylen (CSM). ChemgaPedia.de]
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, Rio Grande do Sul, Brazil with an annual production capacity of , and will produce high-density and low-density polyethylene from bioethanol derived from sugarcane.
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.[ 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).]
Footnotes
Bibliography
External links
