Polymers are compounds that have a high molecular weight reaching several thousand units. The polymerization reaction underlies the production of modern materials for various purposes and properties. At low density they are characterized by high strength, are able to soften when heated and easily give in to molding, which allows to obtain products of various designs and sizes. Polymers are inert in aggressive environments, have electrical insulation properties and are not susceptible to corrosion. Due to the unique properties that are easily regulated at the synthesis stage, the field of application of modern polymeric materials is constantly expanding.
When heated and cooled, these chemical products behave in two ways.
Some soften when heated, and harden again when cooled. Such materials include products based on, for example, the polymerization of alkenes, i.e. polyethylene and polypropylene. They are called thermoplastic materials. Polyvinyl chloride and polystyrene also have similar properties.
Other types of polymers can be heated only once, since after cooling they harden and do not soften more when heated. These materials are called thermosets, they include phenol-formaldehyde or urea-formaldehyde resins. Thermoplastics and thermosets have their advantages. The first are produced in granular form. Of them, after heating and softening, products of any shape are obtained, but during operation they cannot be heated. The second are available in the form of a resinous mass.
The ethylene polymerization reaction can be written as follows: CH2 = CH2 β (βCH2 β CH2β) n. Under certain conditions, in the presence of an initiator (they are gaseous oxygen or a solution of organic peroxide in oil), Ο-bonds (otherwise a double bond) break between carbon atoms and an nth amount of free radicals are formed together. The polymerization reaction proceeds according to a radical chain mechanism. The molecular weight of the polymer material directly depends on the number n, with its increase it grows. By adjusting the conditions of the polymerization reaction, the polyethylene synthesis operator seeks to obtain a material with the desired properties: fluidity (or melt flow index), strength, density, dielectric loss tangent, dielectric constant and others.
The synthesis of high-pressure polyethylene or the polymerization reaction is carried out in autoclave or tubular reactors at temperatures up to 300 Β° C and pressure from 1000 to 3000 atm. A huge amount of heat is released. It is discharged with hot water, which is fed into the jacket of the reactor. In many respects, the quality of the polymeric material and the process safety depend on the degree of purity of the water supplied for heat removal. If the water is poorly purified and contains many impurities (for example, hardness salts in the form of cations of calcium and magnesium, anions of silicic acid, chlorine and others), then deposits form in the reactor jacket or begins to corrode the metal. Due to changes in the thickness of the walls of the reactor, the heat removal over its entire surface becomes uneven, and the polymerization temperature conditions can become uncontrollable. With a sharp increase in temperature, the polymer may oxidize or decompose with destruction of the reactor.
The polymerization reaction, which results in the formation of polyethylene, can proceed at lower pressures and temperatures. But this requires a catalyst. If the high- pressure polyethylene from the reactor leaves in the form of a melt containing unreacted ethylene, which is then separated and the polymer is granulated, the polyethylene obtained at low pressure leaves the reactor in the form of a powder, more precisely, a suspension in a hydrocarbon solvent. The powder is separated from the solvent and washed from the impurities of the catalyst, and then also granulated on special equipment called an extruder.
Thus, the ethylene polymerization reaction in industry is used to synthesize polyethylene. According to GOST 16338-85, low-pressure polyethylene is produced for suspension and gas-phase grades; according to GOST 16337-77, high-pressure polyethylene is produced both for autoclave and tubular grades.