Many kinds of polyethylene are known, with most having thechemical formula (C2H4)n. PE is usually a mixture of similarpolymers ofethylene, with various values ofn. It can below-density orhigh-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.[10] Being ahydrocarbon, polyethylene is colorless to opaque (without impurities or colorants) and combustible.[11]
Polyethylene was first synthesized by the German chemistHans von Pechmann, who prepared it by accident in 1898 while investigatingdiazomethane.[12][a][13][b] When his colleaguesEugen Bamberger and Friedrich Tschirner characterized the white, waxy substance that he had created, they recognized that it contained long −CH2− chains and termed itpolymethylene.[14]
Apill 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 syntheses) was again accidentally discovered in 1933 by Eric Fawcett and Reginald Gibson at theImperial Chemical Industries (ICI) works inNorthwich,England.[15] Upon applying extremely high pressure (several hundredatmospheres) to a mixture of ethylene andbenzaldehyde they again produced a white, waxy material. Because the reaction had been initiated by traceoxygen 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 SHFcoaxial cables ofradar sets. During World War II, further research was done on the ICI process and in 1944,DuPont at Sabine River, Texas, andUnion Carbide Corporation at South Charleston, West Virginia, began large-scale commercial production under license from ICI.[16][17]
The landmark breakthrough in the commercial production of polyethylene began with the development ofcatalysts that promoted thepolymerization at mild temperatures and pressures. The first of these was a catalyst based onchromium trioxide discovered in 1951 byRobert Banks andJ. Paul Hogan atPhillips Petroleum.[18] In 1953 the German chemistKarl Ziegler developed a catalytic system based ontitaniumhalides 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- andZiegler-type catalysts were being used for high-density polyethylene (HDPE) production. In the 1970s, the Ziegler system was improved by the incorporation ofmagnesium chloride. Catalytic systems based on soluble catalysts, themetallocenes, were reported in 1976 byWalter Kaminsky andHansjörg Sinn. The Ziegler- and metallocene-based catalysts families have proven to be very flexible at copolymerizing ethylene with otherolefins and have become the basis for the wide range of polyethyleneresins available today, includingvery-low-density polyethylene andlinear low-density polyethylene. Such resins, in the form ofUHMWPE fibers, have (as of 2005) begun to replacearamids in many high-strength applications.
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.[7][19] LDPE is softer and more transparent than HDPE. For 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).
MostLDPE,MDPE, andHDPE 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 inaromatic hydrocarbons such astoluene orxylene, or in chlorinated solvents such astrichloroethane ortrichlorobenzene.[7]
Polyethylene absorbs almost nowater. The permeability for water vapor and polar gases of is lower than for most plastics. On the other hand, non-polar gases such asOxygen,carbon dioxide, andflavorings can pass it easily.
Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin (similar tocandle flame). The material continues burning on removal of the flame source and produces a drip.[20]
Polyethylene cannot be imprinted or bonded with adhesives without pretreatment. High-strength joints are readily achieved withplastic welding.
Depending on thermal history and film thickness, PE can vary between almost clear (transparent), milky-opaque (translucent) andopaque. LDPE has the greatest, LLDPE slightly less, and HDPE the least transparency. Transparency is reduced bycrystallites if they are larger than the wavelength of visible light.[22]
The ingredient ormonomer isethylene (IUPAC name ethene), agaseoushydrocarbon with the formula C2H4, which can be viewed as a pair ofmethylene groups (−CH 2−) connected to each other. Typical specifications for PE purity are <5 ppm for water, oxygen, and otheralkenes contents. Acceptable contaminants include N2, ethane (common precursor to ethylene), and methane. Ethylene is usually produced frompetrochemical sources, but is also generated bydehydration of ethanol.[7]
Ethylene is a stable molecule that polymerizes only upon contact with catalysts. The conversion is highlyexothermic.Coordination polymerization is the most pervasive technology, which means that metal chlorides or metal oxides are used. The most common catalysts consist oftitanium(III) chloride, the so-calledZiegler–Natta catalysts. Another common catalyst is thePhillips catalyst, prepared by depositingchromium(VI) oxide on silica.[7] Polyethylene can be produced throughradical polymerization, but this route has only limited utility and typically requires high-pressure apparatus.
Polyethylene is classified by itsdensity andbranching. Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure, and themolecular weight. There are several types of 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 millionamu.[25] The high molecular weight makes it a verytough material, but results in less efficient packing of the chains into thecrystal 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- andbottle-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 ofimplants used forhip andknee replacements. Asfiber, it competes witharamid inbulletproof vests.
HDPE pipe on site during installation in outback Western Australia. The white outer layer, Acu-Therm, is co-extruded to provide a reduction of thermal heating.
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 bychromium/silica catalysts,Ziegler–Natta catalysts ormetallocene catalysts; by choosing catalysts and reaction conditions, the small amount of branching that does occur can be controlled. These catalysts prefer the formation offree 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, andwater pipes.
PEX is a medium- to high-density polyethylene containingcross-link bonds introduced into the polymer structure, changing the thermoplastic into athermoset. 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.
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.
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 bycopolymerization of ethylene with short-chainalpha-olefins (for example,1-butene,1-hexene, and1-octene). LLDPE has higher tensile strength than LDPE, and it exhibits higher impact andpuncture resistance than LDPE. Lower-thickness (gauge) films can be blown, compared with LDPE, with betterenvironmental 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.
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 thecrystal structure as well. It has, therefore, less strong intermolecular forces as theinstantaneous-dipole induced-dipole attraction is less. This results in a lowertensile strength and increasedductility. LDPE is created byfree-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.) Secondaryradicals (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.
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 therheological properties of the polymer.
In addition tocopolymerization with alpha-olefins, ethylene can be copolymerized with a wide range of other monomers and ionic composition that creates ionized free radicals. Common examples includevinyl acetate (the resulting product isethylene-vinyl acetatecopolymer, or EVA, widely used in athletic-shoe sole foams) and a variety ofacrylates. Applications ofacrylic copolymer include packaging and sporting goods, andsuperplasticizer, used in cement production.
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.[19]
The degree of branching of the different types of polyethylene can be schematically represented as follows:[19]
PE-HD
PE-LLD
PE-LD
The figure shows polyethylene backbones, short-chain branches and side-chain branches. The polymer chains are represented linearly.
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 byintramolecularchain transfer reactions, they are alwaysbutyl orethyl chain branches because the reaction proceeds after the following mechanism:
The widespread usage of polyethylene poses potential difficulties forwaste 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$90billion market.[26]
It is possible to rapidly convert polyethylene to hydrogen andgraphene by heating. The energy needed is much less than for producing hydrogen by electrolysis.[27][28]
Several experiments have been conducted aimed at discovering anenzyme or organisms that will degrade polyethylene. Several plastics - such as polyesters, polycarbonates, and 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 degradation experiments are difficult because yields and rates are very slow.[29] Further confusing the situation, even preliminary successes are greeted with enthusiasm by the popular press.[30][31][32] Some technical challenges in this area include the failure to identify enzymes responsible for the proposed degradation. Another issue is that organisms are incapable of importing hydrocarbons of molecular weight greater than 500.[29]
TheIndian 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 thePlodia 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%.[33][34]
The caterpillar ofGalleria mellonella is claimed to consume polyethylene. The caterpillar is able to digest polyethylene due to a combination of itsgut microbiota[35] and its saliva containingenzymes that oxidise and depolymerise the plastic.[36]
When exposed to ambient solar radiation the plastic produces trace amounts of twogreenhouse gases,methane andethylene. The plastic type which releases gases at the highest rate islow-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.[37]
Polyethylene may either be modified in the polymerization bypolar or non-polarcomonomers or after polymerization through polymer-analogous reactions. Common polymer-analogous reactions are in case of polyethylenecrosslinking,chlorination andsulfochlorination.
In the low pressure processα-olefins (e.g.1-butene or1-hexene) may be added, which are incorporated in the polymer chain during polymerization. These copolymers introduce short side chains, thuscrystallinity anddensity are reduced. As explained above, mechanical and thermal properties are changed thereby. In particular, PE-LLD is produced this way.
Metallocene polyethylene (PE-M) is prepared by means ofmetallocene catalysts, usually including copolymers (z. B. ethene / hexene). Metallocene polyethylene has a relatively narrowmolecular 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.[19]: 238 [38]: 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.[19]: 238
Cyclic olefin copolymers are prepared by copolymerization of ethene andcycloolefins (usuallynorbornene) produced by using metallocene catalysts. The resulting polymers are amorphous polymers and particularly transparent and heat resistant.[19]: 239 [38]: 27
The basic compounds used as polar comonomers are vinyl alcohol (Ethenol, an unsaturated alcohol), acrylic acid (propenoic acid, an unsaturated acid) andesters containing one of the two compounds.
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.[39]: 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.[39]: 239
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.[40] However, they are more sensitive to heat and oxidation than ethylene homopolymers. Ethylene/acrylic acid copolymers are used asadhesion promoters.[19]
If salts of an unsaturated carboxylic acid are present in the polymer, thermo-reversible ion networks are formed, they are calledionomers. Ionomers are highly transparent thermoplastics which are characterized by high adhesion to metals, high abrasion resistance and high water absorption.[19]
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 ofpolyvinyl acetate (1.17 g/cm3).[39]: 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):[19]: 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).[39]: 235
Ethylene-ethyl acrylate copolymers behave similarly to ethylene-vinyl acetate copolymers.[19]: 240
A basic distinction is made between peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), electron beam crosslinking (PE-Xc) and azo crosslinking (PE-Xd).[41]
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 usingperoxides (e. g.dicumyl ordi-tert-butyl peroxide) is still of major importance. In the so-calledEngel process, a mixture of HDPE and 2%[42] peroxide is at first mixed at low temperatures in an extruder and then crosslinked at high temperatures (between 200 and 250 °C).[41] The peroxidedecomposes to peroxide radicals (RO•), which abstract (remove) hydrogen atoms from the polymer chain, leading toradicals. When these combine, a crosslinked network is formed.[43] The resulting polymer network is uniform, of low tension and high flexibility, whereby it is softer and tougher than (the irradiated) PE-Xc.[41]
Silane crosslinking (PE-Xb): In the presence ofsilanes (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 awater bath byhydrolysis, which condense then and crosslink the PE by the formation of Si-O-Si bridges. [16]Catalysts such asdibutyltin dilaurate may accelerate the reaction.[42]
Irradiation crosslinking (PE-Xc): The crosslinking of polyethylene is also possible by a downstream radiation source (usually anelectron accelerator, occasionally anisotopic radiator). PE products are crosslinked below the crystalline melting point by splitting offhydrogen atoms.β-radiation possesses apenetration depth of 10mm,ɣ-radiation 100 mm. Thereby the interior or specific areas can be excluded from the crosslinking.[41] However, due to high capital and operating costs radiation crosslinking plays only a minor role compared with the peroxide crosslinking.[39] In contrast to peroxide crosslinking, the process is carried out in thesolid state. Thereby, the cross-linking takes place primarily in the amorphous regions, while the crystallinity remains largely intact.[42]
Azo crosslinking (PE-Xd): In the so-calledLubonyl process polyethylene is crosslinked preaddedazo compounds after extrusion in a hot salt bath.[39][41]
Chlorinated Polyethylene (PE-C) is an inexpensive material having a chlorine content from 34 to 44%. It is used in blends withPVC because the soft, rubbery chloropolyethylene is embedded in the PVC matrix, thereby increasing theimpact 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.[39] When chlorinated polyethylene is added to other polyolefins, it reduces the flammability.[19]: 245
Chlorosulfonated PE (CSM) is used as starting material for ozone-resistantsynthetic rubber.[44]
Braskem andToyota Tsusho Corporation started joint marketing activities to produce polyethylene fromsugarcane. Braskem will build a new facility at their existing industrial unit inTriunfo, 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 frombioethanol derived from sugarcane.[45]
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 namepolyethene is systematically derived from the scientific name of the monomer.[46][47] The alkene monomer converts to a long, sometimesvery long, alkane in the polymerization process.[47] In certain circumstances it is useful to use a structure-based nomenclature; in such casesIUPAC recommends poly(methylene) (poly(methanediyl) is a non-preferred alternative).[46] The difference in names between the two systems is due to theopening up of the monomer's double bond upon polymerization.[48] The name is abbreviated toPE. In a similar mannerpolypropylene andpolystyrene are shortened to PP and PS, respectively. In the United Kingdom and India the polymer is commonly calledpolythene, from the ICItrade name, although this is not recognized scientifically.
^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; ...].[12]: 2643
^Die Abscheidung weisser Flocken aus Diazomethanlösungen erwähnt auch v. Pechmann (diese Berichte31, 2643);[12] 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)[13]: footnote 3 on page 956 [Von Pechmann (theseReports,31, 2643)[12] 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)].
^abcdeWhiteley, Kenneth S.; Heggs, T. Geoffrey; Koch, Hartmut; Mawer, Ralph L.; Immel, Wolfgang (2000). "Polyolefins".Ullmann's Encyclopedia of Industrial Chemistry.doi:10.1002/14356007.a21_487.ISBN3-527-30673-0.
^Bamberger, Eugen; Tschirner, Friedrich (1900)."Ueber die Einwirkung von Diazomethan auf β-Arylhydroxylamine" [On the effect of diazomethane on β-arylhydroxylamine].Berichte der Deutschen Chemischen Gesellschaft zu Berlin.33:955–959.doi:10.1002/cber.190003301166.[page 956]:Eine theilweise – übrigens immer nur minimale – Umwandlung des Diazomethans in Stickstoff und Polymethylen vollzieht sich auch bei ganz andersartigen Reactionen; ... [A partial – incidentally, always only minimal – conversion of diazomethane into nitrogen and polymethylene takes place also during quite different reactions; ...]
^"Poly – the all-star plastic".Popular Mechanics. Vol. 91, no. 1. Hearst Magazines. July 1949. pp. 125–129. Retrieved20 February 2014 – via Google Books.
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^abcdefghijkKaiser, Wolfgang (2011).Kunststoffchemie für Ingenieure von der Synthese bis zur Anwendung (3. ed.). München: Hanser.ISBN978-3-446-43047-1.
^abPlastics Design Library (1997).Handbook of Plastics Joining: A Practical Guide. Norwich, New York: Plastics Design Library. p. 326.ISBN1-884207-17-0.
^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.ISBN978-0-323-35435-6.
^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 & Technology.48 (23):13776–84.Bibcode:2014EnST...4813776Y.doi:10.1021/es504038a.PMID25384056.
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