Polyethylene, types, characteristics, properties and production.
Polyethylene – a thermoplastic polymer of ethylene. Is an organic compound and has long molecules.
Description and features of polyethylene
Physical, chemical and other properties of polyethylene
Physical properties of polyethylene (table)
Description and features of polyethylene:
Polyethylene – a thermoplastic polymer of ethylene, belongs to the class of polyolefins. Also called polythene.
Polyethylene is an organic compound and has long molecules …—CH2—CH2—CH2—CH2—…, where “—” represents covalent bonds between atoms of carbon. Thus, a molecule of polyethylene have a simple chemical structure and represents a chain of atoms of carbon, each of which are attached two molecules of hydrogen.
Chemical formula of polyethylene (С2Н4)n. Molecular weight – up to 500 000 g/mol.
The chemical formula of ethylene from which the polyethylene C2H4. The rational formula is ethylene CH2=CH2.
In turn, the polyolefins are a class of macromolecular compounds (polymers) obtained from low molecular weight substances – olefins (monomers) – unsaturated hydrocarbons (ethylene, propylene, butylene, and other alpha-olefins). They are produced from oil or natural gas by polymerizing the same (homopolymerization) or different (copolymerization) of monomers in the presence of a catalyst.
Polyethylene it is a solid mass of white color (thin sheets of transparent and colorless).
There are two modifications of polyethylene: linear and nonlinear polyethylene. They differ from each other in structure and thus the properties. The first is a linear form, the monomers are linked in linear chains with the degree of polymerization is usually 5000 or more. They do not have side branches of the main chain. Another non-linear form, there are many side branches of monomers, which is attached to the main chain in a random way.
Polyethylene exhibits different properties. A variety of properties of polyethylene can be attributed to its molecular structure, molecular weight and degree of crystallinity, which in turn depends on the molecular weight and degree of branching monomers. The less branched polymer chains and the smaller the molecular weight, the higher the crystallinity of the polyethylene and the more it is tight. Thus, there is a linear relationship between density and crystallinity.
Polyethylene is the most common of polymers. Every year it produces more than 100 million tons, accounting for 34 % of the total production of all plastics.
Physical, chemical and other properties of polyethylene:
pure polyethylene is white in color, opaque in thick layers, thin sheets are transparent and colorless,
– kristallizuetsya in the temperature range from -60 °C to minus 369 °C,
– has a low weight and a different density, which depends on the variety and method of producing a certain type of polyethylene,
– not sensitive to shock, is a shock absorber
– has an extremely low adhesion,
– has a low coefficient of friction,
– characterized by absolute water-resistance. It is not wetted by water and does not absorb it. However, short-term treatment of polyethylene with acid or oxidizing agents leading to oxidation of the surface and wetting it with water, polar liquids, and adhesives. In this case, from polyethylene can be bonded,
– when heated to 80-120°C is softened. Polyethylene is not able to withstand high temperatures, which makes it possible to use it in extreme conditions,
– is characterized by resistance to frost. Polyethylene can be used at temperatures from -70°C to 100 °C. Some types of polyethylene retain their useful properties at temperatures below -120°C. Frost resistance of polyethylene depends on the variety and method of producing a certain type of polyethylene,
– polyethylene in the form of thin films has a high flexibility and transparency, and in the form of sheets becomes hard and opaque,
– is a dielectric,
– resistant to water,
– has excellent vapor barrier and waterproofing. But permeable to oxygen and carbon dioxide,
under the action of sunlight becomes brittle. As a Supplement stabilizer against ultraviolet radiation used carbon soot,
– is a chemically stable substance
– does not react with alkalis of any concentration, with solutions of neutral, acidic and basic salts, organic and inorganic acids, even with concentrated sulfuric acid. But collapses under the action of 50% nitric acid at room temperature and exposed to liquid and gaseous chlorine and fluorine. At temperatures above 60 °C sulphuric and nitric acids also rapidly destroy it.
at room temperature is not soluble in organic solvents. At temperatures above 80 °C, first swells and then dissolves in aromatic hydrocarbons and their halogenated derivatives,
– burns with a bluish flame, with a weak light and a yellow tip, while emits a smell of paraffin, that is the same as comes from a burning candle. The material continues to burn on removal of the flame source and produces streaks,
– because of its chemical stability in the natural environment decompose for about 500 years, which significantly degrades the environment. So to counter pollution of plastic bags, about 40 countries have imposed a ban or restriction on the sale and (or) production of plastic bags. However, if the polyethylene to enter the special additives since the update time of decomposition in the natural environment amounts to 1.5-3 years. By the addition-biodegradation, it is decomposed into elementary components: water, carbon dioxide and biomass
– biologically inert.
Physical properties of polyethylene (table):
|Name of the parameter:||Value:|
|Molecular weight, g/mol||up to 500,000*|
|Density, g/cm3||from 0.88 to 0.96*|
|Melting point, oC||from 115 to 180*|
* depends on the type of polyethylene.
Types of polyethylene:
The classification of polythene is laid its density. Polyethylene is classified into the following types:
– low density polyethylene (high pressure). Denoted LDPE, LDPE, LDPE, LDPE (Low Density Polyethylene).
– high-density polyethylene (low pressure), HDPE, HDPE, HDPE, HDPE (High Density Polyethylene),
– medium-pressure polyethylene (high density), pest,
– linear medium density polyethylene, PESP, MDPE or PEMD,
– linear low density polyethylene, and LLDPE, LLDPE or PELLD,
– polyethylene of very low density, VLDPE,
– polyethylene low density, ULDPE,
– metallocene linear low density polyethylene, MPE,
– cross-linked polyethylene, PEX or XLPE, XPE,
– high molecular weight polyethylene, VMPE, HMWPE or PEHMW or VHMWPE,
– ultra-high molecular weight polyethylene, UHMWPE, UHMWPE.
Polyethylene is produced by polymerization of ethylene. Initially, the polyethylene is produced in granules of 2 to 5 mm, the final form it acquires in the course of thermal processing on special equipment.
Each type of polyethylene is obtained in various ways. Consider the example of low-density polyethylene (high pressure), the medium-pressure polyethylene and high density polyethylene (low pressure).
Low density polyethylene (a high pressure LDPE, LDPE, LDPE, LDPE) is formed in autoclave or tubular reactors at:
the temperature of 200-260 °C,
– a pressure of 150-300 MPa,
in the presence of initiator (oxygen or organic peroxide).
The reaction occurs by a radical mechanism in the melt. The liquid product is subsequently granulated. Obtained in this method, the polyethylene has a molecular weight of 80 000 – 500 000 g/mol and a crystallinity of 50-60 %.
Medium-pressure polyethylene (high density of pecd) is formed by:
a temperature of 100-120 °C,
– pressure of 3-4 MPa,
in the presence of a catalyst (e.g., catalysts, Ziegler-Natta).
The reaction product falls out of solution in the form of flakes. Obtained in this method, the polyethylene has a molecular weight of 300 000 – 400 000 g/mol and a crystallinity of 80-90 %.
High-density polyethylene (low pressure HDPE, HDPE, HDPE, HDPE) is formed by:
a temperature of 120-150 °C,
– pressure below 0.1-2 MPa,
in the presence of a catalyst (e.g., catalysts, Ziegler-Natta).
Polymerization occurs in the slurry for ion-coordination mechanism. Obtained in this method, the polyethylene has a molecular weight of 80 000 – 300 000 g/mol and a crystallinity of 75% to 85%.
In the polymerization process of polyethylene, the latter can be chemically modified. To produce new species. For example, metallocene linear low density polyethylene, biodegradable polyethylene, polyethylene quantum.
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