Polyethylene or polythene (IUPAC name polyethene or poly(methylene)) is the most widely ud plastic, with an annual production of approximately 80 million metric tons.[1] Its primary u is within packaging (notably the plastic shopping bag).
Contents [hide]新年活动 ∙ 1 Description ∙ 2 Classification ∙ 3 Ethylene copolymers ∙ 4 History ∙ 5 Physical properties ∙ 6 Environmental issue ∙ 7 Biopolyethylene ∙ 8 Joining ∙ 9 References ∙ 10 Bibliography ∙ 11 External links |
|
[edit] Description
Polyethylene is a thermoplastic polymer consisting of long chains of the monomer ethylene (IUPAC name ethene). The recommended scientific name polyethene is systematically derived from the scientific name of the monomer.[2][3] In certain circumstances it is uful to u a structure-bad nomenclature; in such cas IUPAC recommends poly(methylene)[3] (poly(methanediyl) is an non-preferred alternative[4][5]). The difference in names between the two systems is due to the opening up of the monomer's double bond upon polymerization.
高考试卷The name is abbreviated to PE in a manner similar to that by which other polymers like polypropylene and polystyrene are shortened to PP and PS respectively. In the United Kingdom the polymer is commonly called polythene, although this is not recognized scientifically.
The ethene molecule (known almost universally by its common name ethylene) C2H4 is CH2=CH2, Two CH2 groups connected by a double bond, thus:
Polyethylene contains the chemical elements carbon中医三字经 and hydrogen.
Polyethylene is created through polymerization of ethene. It can be produced through radical polymerization言语道断, anionic addition polymerization, ion coordination polymerization or cationic addition polymerization. This is becau ethene does not have any substituent groups that influence the stability of the propagation head of the polymer. Each of the methods results in a different type of polyethylene.
[edit] Classification
Polyethylene is classified into veral different categories bad mostly on its density and branching. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure and the molecular weight. With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE and LDPE.
∙ Ultra high molecular weight polyethylene (UHMWPE)
∙ 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)
c调音阶∙ Very low density polyethylene (VLDPE)
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/cm粗犷3). UHMWPE can be made through any catalyst technology, although Ziegler catalysts are most common. Becau of its outstanding toughness and its cut, wear and excellent chemical resistance, UHMWPE is ud in a diver range of applications. The 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 competes with Aramid in bulletproof vests, under the tradenames Spectra and Dyneema, and is commonly ud for the construction of articular portions of implants ud for hip and knee replacements.
HDPE is defined by a density of greater or equal to 0.941 g/cm3. HDPE has a low degree of branching and thus stronger intermolecular forces and tensile strength. HDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts. The lack of branching is ensured by an appropriate choice of catalyst (for example, chro
mium catalysts or Ziegler-Natta catalysts) and reaction conditions. HDPE is ud in products and packaging such as milk jugs, detergent bottles, margarine 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.[6]
PEX is a medium- to high-density polyethylene containing cross-link bonds introduced into the polymer structure, changing the thermoplast into an elastomer. The high-temperature properties of the polymer are improved, its flow is reduced and its chemical resistance is enhanced. PEX is ud in some potable-water plumbing systems becau 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/cm自来也图片3quiet的副词. 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 nsitive than HDPE, stress cracking resistance is better than HDPE. MDPE is typically ud in gas pipes and f
ittings, 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, 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 ud in packaging, particularly film for bags and sheets. Lower thickness may be ud compared to LDPE. Cable covering, toys, lids, buckets, containers and pipe. While other applications are available, LLDPE is ud 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 2009 the world LLDPE market reached a volume of almost 24 billion US-dollars (17 billion Euro).[7]
