polymers - are compounds that have a high molecular weight of several thousand units.The polymerization reaction is the basis of modern materials receiving different function and properties.They are the low density have a high durability, capable of softening when heated and easy to molding, which allows to obtain articles of different designs and sizes.The polymers are inert to corrosive environments have electrical insulating properties and does not corrode.Due to its unique properties, which can be easily adjusted at the stage of the synthesis, the use of modern polymeric materials is constantly expanding.
By heating and cooling the products of chemical industry in two ways behave.Some
soften when heated and solidify again upon cooling.Such materials include products, which is based on the preparation, for example, the polymerization reaction of alkenes, i.e., polyethylene and polypropylene.They are called thermoplastic materials.It has similar properties as polyvinyl chloride and polystyrene.
polymers of different type can be heated only once, because after cooling, they harden and more do not soften when heated.These materials are called thermosetting, they include phenol-formaldehyde or urea-formaldehyde resins.Thermoplastics and thermosets are its advantages.First produced in granular form.Of these, after heating and softening obtained products of any shape, but the operation can not be heated.The latter are produced as a gummy mass.
ethylene polymerization reaction can be written as follows: CH2 = CH2 → (-CH2-CH2-) n.Under certain conditions, in the presence of an initiator (they favor oxygen gas or a solution of organic peroxide in oil) takes place between carbon atoms gap π-tie (otherwise a double bond) and a connection between the n-th number of formed free radicals.The polymerization reaction occurs by a radical chain mechanism.The molecular weight of the polymer material is directly dependent on the number n, with its increasing it grows.By adjusting the conditions of the polymerization reaction, the operator achieves the synthesis of polyethylene receiving material with desired properties: fluidity (or melt index), strength, density, dielectric loss tangent, dielectric constant, and others.
Synthesis of high-pressure polyethylene or the polymerization reaction is carried out in autoclave or tubular reactors at temperatures up to 300 ° C and a pressure of 1000 to 3000 atm.This releases an enormous amount of heat.It is given hot water that is fed into the reactor jacket.The degree of purity to be supplied for heat removal of water depends largely as a resin material, and process safety.If water is poorly purified and contains many impurities (e.g., hardness salts in the form of calcium and magnesium cations, anions, silicic acid, chlorine, etc.), the reactor jacket formed deposits or metal begins to corrode.Due to changes in the thickness of the walls of the reactor heat removal across the surface becomes uneven, and the temperature conditions of the polymerization may become unmanageable.With a sharp rise in temperature may occur polymer oxidation or decomposition of the destruction of the reactor.
polymerization reaction, which is formed as a result of polyethylene, can occur at lower pressures and temperatures.But this requires a catalyst.If high pressure polyethylene from the reactor as a melt exits containing unreacted ethylene, which is then separated and the polymer was pelletized, the polyethylene produced under low pressure and exits the reactor as a powder, or rather a suspension in a hydrocarbon solvent.The powder was separated from the solvent and washed free of catalyst impurities and then granulated and special equipment called extruder.
Thus, the polymerization reaction of ethylene in the industry used for the synthesis of polyethylene.According to GOST 16338-85 produce low-pressure polyethylene slurry and gas-phase marks, according to GOST 16337-77 produce high-density polyethylene as the autoclave and tubular brands.
