With approximately 30% share of the total amount of polyethylene is the plastic most commonly used. Polyethylene is a partially crystalline and non-polar thermoplastic. By choosing the polymerization conditions can be molecular weight, molecular weight distribution, adjust average chain length and degree of branching.
Types
Due to the different density, a distinction into four main types, the short names are not always used consistently:
High density polyethylene (high density), HDPE or HDPE
Medium density polyethylene (medium density), PE-MD or MDPE
Low density polyethylene (low density), LD-PE or LDPE
linear low density polyethylene (linear, low density), LLDPE or LLDPE
Other important types are
Polyethylene ultra high molecular weight (ultra-high molecular weight), PE-UHMW or UHMWPE
Polyethylene, very low density (very low density), PE-VLD or VLDPE.
characteristics
The density of PE-VLD is from 0.90 to 0.93 g / cm3; in HDPE from 0.94 to 0.97 g / cm3.
The low density of LDPE grades resulting from the high proportion of short- and long-chain branches, which prevents a close aggregation of the polymer chains. Therefore have LDPE grades with about 35% and a lower degree of crystallinity than HDPE types (about 80%). They also have a higher light transmission, which can range in thin films to transparency. The higher degree of crystallization is also the cause of the higher melting temperature of PE-HD.
Strength, hardness and stiffness of PE are lower than in most other thermoplastics, however, has PE high extensibility and low-temperature impact strength and good sliding friction behavior. For injection molded moldings types are used with high molecular weight, wherein HDPE-UHMW is no longer thermoplastic. PE can be stretched to very strong reinforcing fibers, the strength is based on a generated by the processing method extremely high crystallinity. The maximum continuous operating temperature is depending on the type at about 60 to 85 ° C, briefly, 80 to 120 ° C. possible (about 150 ° C in HDPE-UHMW).
PE has good electrical insulation properties and has good chemical resistance to a variety of acids, bases, oils and greases. While LDPE very limited to hydrocarbons is only resistant, HDPE can also be used for fuel tanks. Often such containers are additionally equipped with barrier films or a plasma coating, since PE has a high gas permeability (permeation). Strong oxidizing agents such as highly concentrated inorganic acids and halogens attack PE. PE is flammable and not weather-resistant, so as additives are flame retardants, and UV absorbers required.
Application Areas
The main use of PE are films and packaging, such as produced by blow molding containers, simple injection-molded parts and pipes and cable insulation. PE powder used for coating of textiles or paper.
Technical importance are the following special types and copolymers:
chlorinated polyethylene (PE-C): to increase the flame retardancy of polyolefins or to increase the impact strength of PVC, cross-linked as a weather-resistant and easy colorable elastomer for cable jackets
chlorosulfonated polyethylene (CSM) rubber-elastic after crosslinking for cable insulation, coated fabrics, roofing membranes, hose covers and inflatables
Ethylene-vinyl acetate copolymers (EVAC): up to 10% vinyl acetate for greenhouse sheeting, pipes, bellows with high resilience; to 30% vinyl acetate thermoplastic cable sheathing; to about 40% vinyl acetate adhesives; about 40% vinyl acetate crosslinked as elastomer (EVM) with a very high filler content almost exclusively for cable insulation (FRNC cables)
Ethylene-vinyl alcohol copolymers (EVOH) with 53-68% vinyl alcohol have minimal gas permeability and can be used as so-called barrier films
cross-linked polyethylene (PE-X): elastomer, pipes for hot water and underfloor heating
PE compound with about 6% starch for biodegradable products (tote bags, garbage bags). Only the strength of the enveloping PE matrix However, it is degraded, not. The already present in PE sensitivity to UV-light can also be increased by incorporating specific, UV-sensitive molecular groups further, so that a photo-degradation is possible.
Cyclic olefin copolymers (COC) can be prepared, for example, by copolymerizing ethylene with cyclic olefins such as norbornene. Semi-crystalline types have a high heat resistance and good chemical resistance.
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