出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2015/12/03 21:37:36」(JST)
この記事には複数の問題があります。改善やノートページでの議論にご協力ください。
|
ポリエチレン (polyethylene) 略称PEは、エチレンが重合した構造を持つ高分子である。最も単純な構造をもつ高分子であり、容器や包装用フィルムをはじめ、様々な用途に利用されている。
基本的にはメチレン(-CH2-)のくり返しのみで構成されているが、重合法によって平均分子量や分枝数、結晶性に違いが生じ、密度や熱特性、機械特性などもそれに応じて異なる。
一般に酸やアルカリに安定。低分子量のものは炭化水素系溶剤に膨潤するが、高分子量のものは耐薬性に非常に優れる。濡れ性は低い。絶縁性が高く、静電気を帯びやすい。
JIA K 6748によると、ポリエチレンはエチレンの単独重合体、エチレンと5mol%以下のα-オレフィレン単量体との共重合体、およびエチレンと官能基に炭素、酸素、および水素原子だけを持つ1mol%以下の非オレフィン単量体との共重合体と定義されている。-JIA K 6748
ポリエチレンの分類は密度による分類と、製造方法による分類の2種類がある。
組成上は同じポリエチレンであっても、構造によって性質が大きく異なるため、実用上、密度や分子量によって数種類に分類されている。リサイクル識別表示マーク(SPIコード)は高密度ポリエチレンが 2、低密度ポリエチレンが 4 と定められている。
そのほか、ポリエチレンを部分構造として持つコポリマーも多く開発されている。代表的なものとして、酢酸ビニルとの共重合体であるエチレン酢酸ビニルコポリマー (EVA、Ethylene vinyl acetate copolymer) がある。
ポリエチレンは1898年、ドイツのペヒマンがジアゾメタンを熱分解している際に偶然発見された。
1930年代、ICIの研究者によって酸素を開始剤とする高圧合成法が開発され、工業的な合成が始められた。1951年に米フィリップス石油の研究者らによって酸化クロムが、続いて1953年にドイツのツィーグラーがチーグラー・ナッタ触媒として知られるハロゲン化チタン系触媒が開発されると、高性能のポリエチレンが安価に製造されるようになり、世界的にポリエチレン製品が広まっていった。
1976年、カミンスキーがメタロセン骨格を持つ触媒を開発し、ポリエチレンの分子量、分岐数などの制御の他、コポリマーの合成も容易となった。
現在では、用途に応じて様々な合成法が利用されている。エチレン (CH2=CH2) を低圧条件(1 - 6気圧)で存在下、あるいは中圧条件(30 - 40気圧)で酸化クロム系触媒存在下で重合させると HDPE に、1,000気圧以上の高圧条件で重合させると分枝が増えて LDPE となる。過酸化ベンゾイルやアゾビスイソブチロニトリルを開始剤としてラジカル重合によって製造することも可能である。
ポータル ファッション |
ポータル 化学 |
|
この項目は、布や繊維、それを素材とする製品に関連した書きかけの項目です。この項目を加筆・訂正などしてくださる協力者を求めています(P:ファッション)。 |
この項目は、化学に関連した書きかけの項目です。この項目を加筆・訂正などしてくださる協力者を求めています(プロジェクト:化学/Portal:化学)。 |
Names | |
---|---|
IUPAC name
Polyethene or Poly(methylene)
|
|
Other names
Polyethene
|
|
Identifiers | |
CAS Number
|
9002-88-4 Y |
Abbreviations | PE |
KEGG | C19503 Y |
MeSH | Polyethylene |
Properties | |
Chemical formula
|
(C2H4)n |
Density | 0.91–0.96 g/cm3[1] |
Melting point | 115–135 °C (239–275 °F; 388–408 K)[1] (239–275 °F) |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|
|
Infobox references | |
Polyethylene (abbreviated PE) or polyethene (IUPAC name polyethene or poly(methylene)) is the most common plastic. The annual global production is around 80 million tonnes.[2] Its primary use is in packaging (plastic bags, plastic films, geomembranes, containers including bottles, etc.). Many kinds of polyethylene are known, with most having the chemical formula (C2H4)n. Thus, PE is usually a mixture of similar organic compounds that differ in terms of the value of n.
Polyethylene is a thermoplastic polymer consisting of long hydrocarbon chains. Depending on the crystallinity and molecular weight, a melting point and glass transition may or may not be observable. The temperature at which these occur varies strongly with the type of polyethylene. For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range 120 to 180 °C (248 to 356 °F). The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C (221 to 239 °F).
Most LDPE, MDPE, and HDPE grades have excellent chemical resistance, meaning they are not attacked by strong acids or strong bases, and are resistant to gentle oxidants and reducing agents. Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin. The material continues burning on removal of the flame source and produces a drip.[3] Crystalline samples do not dissolve at room temperature. Polyethylene (other than cross-linked polyethylene) usually can be dissolved at elevated temperatures in aromatic hydrocarbons such as toluene or xylene, or in chlorinated solvents such as trichloroethane or trichlorobenzene.[4]
The ingredient or monomer is ethylene (IUPAC name ethene), a gaseous hydrocarbon with the formula C2H4, which can be viewed as a pair of methylene groups (=CH
2) connected to each other. Because the compound is highly reactive, the ethylene must be of high purity. Typical specifications are <5 ppm for water, oxygen, and other alkenes. Acceptable contaminants include N2, ethane (common precursor to ethylene), and methane. Ethylene is usually produced from petrochemical sources, but also is generated by dehydration of ethanol.[4]
Ethylene is a rather stable molecule that polymerizes only upon contact with catalysts. The conversion is highly exothermic. Coordination polymerization is the most pervasive technology, which means that metal chlorides or metal oxides are used. The most common catalysts consist of titanium(III) chloride, the so-called Ziegler-Natta catalysts. Another common catalyst is the Phillips catalyst, prepared by depositing chromium(VI) oxide on silica.[4] Ethylene can be produced through radical polymerization, but this route has only limited utility and typically requires high-pressure apparatus.
Polyethylene is classified into several different categories based mostly on its density and branching. Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight. With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE, and LDPE.
UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.1 and 5.67 million. The high molecular weight makes it a very tough material, but results in less efficient packing of the chains into the crystal structure as evidenced by densities of less than high density polyethylene (for example, 0.930–0.935 g/cm3). UHMWPE can be made through any catalyst technology, although Ziegler catalysts are most common. Because of its outstanding toughness and its cut, wear, and excellent chemical resistance, UHMWPE is used in a diverse range of applications. These include can- and bottle-handling machine parts, moving parts on weaving machines, bearings, gears, artificial joints, edge protection on ice rinks, and butchers' chopping boards. It is commonly used for the construction of articular portions of implants used for hip and knee replacements. As fiber, it competes with aramid in bulletproof vests.
HDPE is defined by a density of greater or equal to 0.941 g/cm3. HDPE has a low degree of branching. The mostly linear molecules pack together well, so intermolecular forces are stronger than in highly branched polymers. HDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts; by choosing catalysts and reaction conditions, the small amount of branching that does occur can be controlled. These catalysts prefer the formation of free radicals at the ends of the growing polyethylene molecules. They cause new ethylene monomers to add to the ends of the molecules, rather than along the middle, causing the growth of a linear chain.
HDPE has high tensile strength. It is used in products and packaging such as milk jugs, detergent bottles, butter tubs, garbage containers, and water pipes. One-third of all toys are manufactured from HDPE. In 2007, the global HDPE consumption reached a volume of more than 30 million tons.[5]
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.
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 by copolymerization of ethylene with short-chain alpha-olefins (for example, 1-butene, 1-hexene, and 1-octene). LLDPE has higher tensile strength than LDPE, and it exhibits higher impact and puncture resistance than LDPE. Lower thickness (gauge) films can be blown, compared with LDPE, with better environmental stress-cracking resistance, but is not as easy to process. LLDPE is used in packaging, particularly film for bags and sheets. Lower thickness may be used compared to LDPE. It is used for cable coverings, toys, lids, buckets, containers, and pipe. While other applications are available, LLDPE is used predominantly in film applications due to its toughness, flexibility, and relative transparency. Product examples range from agricultural films, Saran wrap, and bubble wrap, to multilayer and composite films. In 2013, the world LLDPE market reached a volume of US$40 billion.[6]
LDPE is defined by a density range of 0.910–0.940 g/cm3. LDPE has a high degree of short- and long-chain branching, which means that the chains do not pack into the crystal structure as well. It has, therefore, less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength and increased ductility. LDPE is created by free-radical polymerization. The high degree of branching with long chains gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap. In 2013, the global LDPE market had a volume of almost US$33 billion.[7]
The radical polymerization process used to make LDPE does not include a catalyst that "supervises" the radical sites on the growing PE chains. (In HDPE synthesis, the radical sites are at the ends of the PE chains, because the catalyst stabilizes their formation at the ends.) Secondary radicals (in the middle of a chain) are more stable than primary radicals (at the end of the chain), and tertiary radicals (at a branch point) are more stable yet. Each time an ethylene monomer is added, it creates a primary radical, but often these will rearrange to form more stable secondary or tertiary radicals. Addition of ethylene monomers to the secondary or tertiary sites creates branching.
VLDPE is defined by a density range of 0.880–0.915 g/cm3. VLDPE is a substantially linear polymer with high levels of short-chain branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (for example, 1-butene, 1-hexene and 1-octene). VLDPE is most commonly produced using metallocene catalysts due to the greater co-monomer incorporation exhibited by these catalysts. VLDPEs are used for hose and tubing, ice and frozen food bags, food packaging and stretch wrap as well as impact modifiers when blended with other polymers.
Recently, much research activity has focused on the nature and distribution of long chain branches in polyethylene. In HDPE, a relatively small number of these branches, perhaps one in 100 or 1,000 branches per backbone carbon, can significantly affect the rheological properties of the polymer.
In addition to copolymerization with alpha-olefins, ethylene can also be copolymerized with a wide range of other monomers and ionic composition that creates ionized free radicals. Common examples include vinyl acetate (the resulting product is ethylene-vinyl acetate copolymer, or EVA, widely used in athletic-shoe sole foams) and a variety of acrylates. Applications of acrylic copolymer include packaging and sporting goods, and superplasticizer, used for cement production.
Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while investigating diazomethane.[8][9] When his colleagues Eugen Bamberger and Friedrich Tschirner characterized the white, waxy substance that he had created, they recognized that it contained long -CH2- chains and termed it polymethylene.[10]
The first industrially practical polyethylene synthesis (diazomethane is a notoriously unstable substance that is generally avoided in industrial application) was discovered in 1933 by Eric Fawcett and Reginald Gibson, again by accident, at the Imperial Chemical Industries (ICI) works in Northwich, England.[11] Upon applying extremely high pressure (several hundred atmospheres) to a mixture of ethylene and benzaldehyde they again produced a white, waxy, material. Because the reaction had been initiated by trace oxygen contamination in their apparatus, the experiment was, at first, difficult to reproduce. It was not until 1935 that another ICI chemist, Michael Perrin, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial LDPE production beginning in 1939. Because polyethylene was found to have very low-loss properties at very high frequency radio waves, commercial distribution in Britain was suspended on the outbreak of World War II, secrecy imposed and the new process was used to produce insulation for UHF and SHF coaxial cables of radar sets. During World War II, further research was done on the ICI process and in 1944 Bakelite Corporation at Sabine, Texas and Du Pont at Charleston, West Virginia, began large scale commercial production under license from ICI.[12]
The breakthrough landmark in the commercial production of polyethylene began with the development of catalyst that promote the polymerization at mild temperatures and pressures. The first of these was a chromium trioxide–based catalyst 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 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.
This section may require cleanup to meet Wikipedia's quality standards. The specific problem is: unsourced, poorly sourced, and innacurate statements. Please help improve this section if you can. (September 2012) |
Although ethylene can be produced from renewables, polyethylene is mainly made from petroleum or natural gas.
One of the main problems of polyethylene is that without special treatment it is not readily biodegradable, and thus accumulates. In Japan, getting rid of plastics in an environmentally friendly way was the major problem discussed until the Fukushima disaster in 2011. It was listed as a $90 billion market for solutions. Since 2008, Japan has rapidly increased the recycling of plastics, but still has a large amount of plastic wrapping which goes to waste.[14]
In May 2008, Daniel Burd, a 16-year-old Canadian, won the Canada-Wide Science Fair in Ottawa after discovering that Pseudomonas fluorescens, with the help of Sphingomonas, can degrade over 40% of the weight of plastic bags in less than three months.[15]
The thermophilic bacterium Brevibacillus borstelensis (strain 707) was isolated from a soil sample and found to use low-density polyethylene as a sole carbon source; when incubated together at 50°C. Biodegradation increased with time exposed to ultraviolet radiation.[16]
In 2010, a Japanese researcher, Akinori Ito, released the prototype of a machine which creates oil from polyethylene using a small, self-contained vapor distillation process.[17]
Acinetobacter sp. 351 can degrade lower molecular-weight PE oligomers. When PE is subjected to thermo- and photo-oxidization, products including alkanes, alkenes, ketones, aldehydes, alcohols, carboxylic acid, keto-acids, dicarboxylic acids, lactones, and esters are released.[18]
In 2014, A Chinese researchers discovered that the 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%.[19][20]
Braskem and Toyota Tsusho Corporation started joint marketing activities to produce polyethylene from sugarcane. Braskem will build a new facility at their existing industrial unit in Triunfo, RS, Brazil with an annual production capacity of 200,000 short tons (180,000,000 kg), and will produce high-density and low-density polyethylene from bioethanol derived from sugarcane.[21]
Polyethylene can also be made from other feedstocks, including wheat grain and sugar beet. Retrieved from cane sugar, i.e. plant biomass renewable feedstock, Brazil is the first country to develop the product.[citation needed] 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.
Commonly used methods for joining polyethylene parts together include:[22]
Adhesives and solvents are rarely used because polyethylene is nonpolar and has a high resistance to solvents. Pressure-sensitive adhesives (PSA) are feasible if the surface is flame treated or corona treated. Commonly used adhesives include:[22]
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.[23][24] The alkene monomer converts to a long, sometimes very long, alkane in the polymerization process.[24] 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).[23] The difference in names between the two systems is due to the opening up of the monomer's double bond upon polymerization.[25] The name is abbreviated to PE. In a similar manner polypropylene and polystyrene are shortened to PP and PS, respectively. In the United Kingdom the polymer is commonly called polythene, from the ICI trade name, although this is not recognized scientifically.
Look up polyethylene in Wiktionary, the free dictionary. |
Wikimedia Commons has media related to Polyethylene. |
|
|
|
全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.
リンク元 | 「PE」「ポリエチレン」 |
拡張検索 | 「polyethylene terephthalate」「polyethylene glycol ointment」「polyethyleneimine」「polyethylene glycol」 |
[★] ポリエチレンテレフタラート、ポリエチレンテレフタレート
.