Polymers are compounds that have a highmolecular mass reaching several thousand units. The polymerization reaction underlies the production of modern materials for various purposes and properties. They are of high strength at low density, they are softened by heating and can be easily molded, which allows to obtain products of various designs and sizes. Polymers are inert in corrosive environments, have electrical insulating properties and are not susceptible to corrosion. Due to unique properties that are easily regulated at the synthesis stage, the field of application of modern polymer materials is constantly expanding.
When heated and cooled, these products of chemical production behave in a twofold manner.
Some heat up when heated, and whencooling again solidify. Such materials include products based on, for example, the polymerization reaction of alkenes, i.e., polyethylene and polypropylene. They are called thermoplastic materials. Polyvinyl chloride and polystyrene have similar properties.
Polymers of another type can only be heated by onetimes, since after cooling they harden and do not soften more when heated. These materials are called thermosetting materials, including phenol-formaldehyde or urea-formaldehyde resins. Thermoplastics and thermosets have their advantages. The first release in granular form. Of them, after heating and softening, products of any shape are obtained, but they can not be heated during operation. The latter are available in the form of a resinous mass.
The ethylene polymerization reaction can be recordedin the following form: CH2 = CH2 → (-CH2-CH2-) n. Under certain conditions, in the presence of an initiator (they are gaseous oxygen or a solution of organic peroxide in oil), a π-bond gap (otherwise a double bond) occurs between the carbon atoms and a compound of a n-th amount of free radicals formed. The polymerization reaction proceeds by a radical-chain mechanism. The molecular weight of the polymer material directly depends on the number n, with its increase it grows. Regulating the conditions of the polymerization reaction, the polyethylene synthesis operator achieves the production of a material with the desired properties: fluidity (or melt flow index), strength, density, dielectric loss tangent, dielectric constant, and others.
Synthesis of high-density polyethylene or reactionpolymerization is carried out in autoclave or tubular reactors at temperatures up to 300 ° C and a pressure of 1000 to 3000 atm. At the same time, an enormous amount of heat is released. It is removed by hot water, which is fed into the reactor shirts. From the degree of purity of the water supplied for the removal of heat, water largely depends on both the quality of the polymer material and the safety of the process. If the water is poorly purified and contains a lot of impurities (for example, hardness salts in the form of calcium and magnesium cations, silicic acid anions, chlorine, etc.), then in the reactor jacket deposits are formed or the metal starts to corrode. Due to changes in the thickness of the walls of the reactor, heat removal throughout its surface becomes uneven, and the temperature conditions of the polymerization can become uncontrollable. With a sudden increase in temperature, oxidation of the polymer or its decomposition can occur with the destruction of the reactor.
The polymerization reaction, as a result of whichpolyethylene is formed, it can also proceed at a lower pressure and temperature. But this requires a catalyst. If the high pressure polyethylene from the reactor exits as a melt containing unreacted ethylene, which is then separated and the polymer is granulated, the polyethylene produced at low pressure exits 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 a special equipment called an extruder.
Thus, the polymerization reaction of ethylene inThe industry is used for the synthesis of polyethylene. According to GOST 16338-85 low-pressure polyethylene of suspension and gas-phase grades is produced, according to GOST 16337-77 high-pressure polyethylene is produced both of autoclave and tubular grades.
