Polymerization reaction

Polymers are compounds that have a high molecular weight that reaches 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 with softening, and once they cool, they solidify again. 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 a different type can be heated only once, since after cooling they harden and do not soften more when heated. These materials are called thermosetting, they include phenol-formaldehyde or urea-formaldehyde resins. Thermoplastics and thermosets have their advantages. The first is released 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 written in 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 n-th amount of free radicals formed together. The polymerization reaction proceeds by a radical-chain mechanism. The molecular weight of the polymer material depends directly 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 predetermined properties: fluidity (or melt flow index), strength, density, dielectric loss tangent, dielectric constant, and others.

Synthesis of high-density polyethylene or a polymerization reaction 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.), the reactor jacket develops deposits or starts to corrode the metal. Because of the change 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 that results in the formation of polyethylene can also take place at lower pressures and temperatures. But this requires a catalyst. If the high pressure polyethylene from the reactor exits in the form of a melt containing unreacted ethylene, which is then separated and the polymer is granulated, the polyethylene produced at low pressure exits the reactor as 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 ethylene polymerization reaction in the industry is used for the synthesis of polyethylene. According to GOST 16338-85 low-density polyethylene of suspension and gas-phase grades is produced, according to GOST 16337-77 high-pressure polyethylene of both autoclave and tubular grades is produced.