Polyisobutylene – elastic rubber: production and applications
Polyisobutylene is a synthetic saturated polymer with a carbon chain structure. Production technology of polyisobutylene is a low-temperature copolymerization of isobutylene isoprene or another diolefin. There are two main synthesis methods that differ in the polymerization medium. In the first case, aliphatic hydrocarbons are used, the second technique is focused on alkyl chlorides.
Polymerization in liquid ethylene
Synthesis occurs at low temperatures. This is due to the physical properties of the monomer. The boiling point of ethylene is -104°C. The value is indicated at atmospheric pressure. Accordingly, in order to obtain polyisobutylene with a molecular weight above 100,000, it is necessary to keep the temperature no higher than -85°C. Obviously, the reaction requires constant cooling. The removal of heat during polymerization in liquid ethylene is carried out due to the evaporation of the solvent.
Boron trifluoride is usually chosen as the catalyst. The connection is attractive for a number of reasons. Fluoride is volatile at atmospheric pressure. Consequently, the substance is easily separated from impurities, dosed and removed from the polymer during mixing and heating. The catalyst contains sulfur dioxide in volumes up to 1.5% by weight of boron trifluoride. This has no effect on the molecular weight of the polyisobutylene, but slightly slows down the polymerization. The purification of boron fluoride from sulfur dioxide is based on differences in the boiling and crystallization temperatures of these compounds. Sulfur dioxide condenses at -10.1°C, crystallizes when the index drops to -72.7°C. Therefore after passing technical boron fluoride through a heat exchanger cooled with liquid ethylene, the content of boron dioxide in the catalyst is reduced to 0.7%.
Polymerization of isobutylene is achieved for by the use of a horizontal reactor. The main structural element is a moving steel belt made in the form of a continuous chute. It plays the role of a transporter. Two components are fed onto the tape in separate streams:
- liquid mixture of ethylene and isobutylene containing additives of rate and molecular weight regulators;
- solution of boron fluoride in ethylene.
The temperature of the liquid composition is kept around -100°C. The polymerization rate and molecular weight of polyisobutylene are determined by the isobutylene/ethylene ratio. Reducing the percentage of the second component leads to an increase in temperature. Accordingly, the molecular weight and yield of polyisobutylene are reduced.
The copolymerization process itself takes 15-20 seconds. During the specified time the main amount of ethylene evaporates. With such a rapid polymerization, splashing of the reaction mass occurs. This leads to the formation of a layer of polymer on the walls of the reactor. Therefore, the continuous process is difficult.
Further, in the middle of the tape, an antioxidant solution is introduced into the polymer. This simultaneously deactivates the catalyst. After that the polymer that behaves like a shapeless mass is cut off from the tape and fed to heated rollers. The process is completed by degassing and homogenization of the product. The resulting polyisobutylene is cooled and packaged.
Polymerization in an alkyl chloride medium
Methyl chloride or chloroethane is commonly used. The synthesis is carried out using aluminum chloride as a catalyst. The molecular weight of the polymer is controlled by the introduction of diene hydrocarbons or diisobutylene.
Such a scheme is inapplicable in the production of low molecular weight polyisobutylenes. The inappropriate use of polymer chain length regulators is due to the broad molecular weight distribution. High heterogeneity of MWD adversely affects the technical properties. Therefore, to obtain low molecular weight polyisobutylenes, the polymerization temperature is increased.
Polymerization of isobutylene in an alkyl chloride medium has a number of advantages compared to synthesis in liquid ethylene:
- easier to control the process temperature;
- polymers are characterized by a high level of homogeneity;
- higher productivity.
Another advantage of polymerization in an alkyl chloride medium is its versatility. One equipment can be used for the synthesis of butyl rubber and various grades of polyisobutylenes. It is enough to change the composition of the initial charging material.
Main applications of polyisobutylenes
The fields of application of polyisobutylenes are diverse for a simple reason. Modern industry produces a wide range of different brands of oligo- and polyisobutylenes, which differ mainly in molecular weight:
- on the basis of dimers, trimers and tetramers of isobutylene, high-octane motor fuel (polymer-gasoline) is produced;
- Oligoisobutylenes with M at the level of 200 – 500 are used to obtain highly efficient cutting fluids;
- Ultra-high molecular weight polymer (MW above 1 million) is in demand in the footwear industry.
High-molecular grades of polyisobutylene stand out for a wide range of applications. Polymers in unvulcanized form are used in diverse areas:
- electrical insulation;
- anti-corrosion coatings for pipelines, chemical equipment;
- sealing material;
- component in the production of sealing compositions;
- glue in the manufacture of artificial furs.
Also, high-molecular grades of polyisobutylene are in demand in the production of waterproof and protective fabrics. Polymers are used as a plasticizer for polyolefins, as ingredients in rubber compounds.
At the same time, polyisobutylene is combined with natural and synthetic rubbers, a number of other substances:
- mineral oils.
This is an incomplete list. Polyisobutylene is also combined with various mineral fillers and pigments. Their introduction improves the properties of the polymer – reduces cold flow, increases strength, hardness and light resistance.
High molecular weight polyisobutylene mixed with polyethylene and polystyrene is used as an insulating material for high voltage cables.
In the rubber industry, polyisobutylene is used in conjunction with natural and synthetic rubbers. Polymer-based rubbers are characterized by high physical and mechanical properties. They have increased heat resistance, water and gas impermeability, resistance to ozone, acids. Polyisobutylene-based rubber is used to produce waterproof fabrics, acid-resistant hoses and conveyor belts, and a number of similar products.
Another area of polyisobutylene use is adhesives. On the basis of high-molecular polymers, adhesives are created for various materials – wood, metal, glass, fabrics, paper, etc.
Finally, the presence of C=C terminal bonds in low-molecular-weight polymer products makes it possible to carry out the functionalization of polyisobutylene according to various schemes. It is used in industrial syntheses to obtain various additives for oils and fuels.
Unique qualities of polyisobutylene
Polyisobutylene (PIB) is a copolymerization product of isobutylene and diolefins (usually isoprene). In its normal state, it is an elastic material with an amorphous structure. Crystallization of polyisoprene is observed with significant stretching. The produced polymers are characterized by a number average molecular weight in the range of 300 – 2500 Mn, and a weight average – from 70,000 to 225,000.
Polyisobutylene is characterized by low water absorption rates – less than 0.1% for 24 hours, vapor and gas permeability of 0.0006 g / (sq. m * h). The polymer retains its elastic properties when cooled down to -50 °C. A further decrease in temperature leads to a loss of elasticity, the material becomes brittle. On the contrary, heating the polymer to 100°C leads to an increase in plasticity. A further increase in temperature to 180 – 200 °C allows the formation of PIB.
Other important physical indicators of polyisobutylene:
- density – 0.91 – 0.93 g/cc, relevant for room temperature;
- thermal conductivity coefficient – 0.116 – 0.139 W/(m*K);
- glass transition temperature – -74°C;
- the Martens heat resistance – 65 – 80 °С;
- dielectric strength – 16 – 20 MV/m.
Polyisobutylene is combustible, which is important to consider during storage. But the critical indicators are high. Ignition temperature is 276°С, flash point is 231°С, self-ignition is 405°С.
It should be noted that the physical characteristics of the polymer are strongly tied to the molecular weight. Also, the parameters change with the introduction of fillers and other substances.
Polyisobutylene is a linear polymer containing one (in most cases) C=C terminal bond per macromolecule. This feature determines the chemical properties of the synthetic analogue of rubber. In particular, polyisobutylene is highly chemically inert compared to other polymers. Therefore, PIB is resistant to most aggressive environments:
- hydrogen peroxide, other peroxides;
- salt solutions;
- dilute and concentrated acids – formic (HCOOH), acetic (CH3COOH), sulfuric (H2S04 – 98.8%), nitric (HN03 – 50%), hydrochloric (Hcl – 37%);
- alkalis (NaOH 40%).
Additionally, polyisobutylene has high water resistance. The polymer is resistant to oxygen and ozone. The combination of these properties makes PIB widely used for anti-corrosion protection and as a waterproofing material.
Polyisobutylene is soluble in mineral oils and some hydrocarbons (aromatic, aliphatic and chlorinated). PIB swells in vegetable oils, diethyl ether, fats and compound ethers.
Degradation and depolymerization of polyisobutylene is observed upon thermal exposure above 620K. Decomposition with rupture of macromolecules is also observed under irradiation with ionizing rays. PIB is moderately resistant to ultraviolet radiation. But studies have shown the formation of 6 types of free radicals during long-term (several days) continuous photo-irradiation with a mercury lamp. This behavior indicates the breaking of C-C and C-H bonds.