What is PVC and how is it produced?
Polyvinyl chloride (PVC) is commercially produced by the free radical mechanism and in four ways: solution, mass, emulsion, and suspension.
Business type P. V. C is a white amorphous powder and contains nearly 5% of crystals. “Tg-glass transition temperature” in this polymer is between 68-105°C and in most commercial types, between 80-85°C.
The molecular weight of most commercial types of this polymer varies between 50-120 thousand. By increasing the molecular weight and at the same time as the “K-VALUE” increases, the mechanical properties such as tensile strength and tear resistance also increase. In general, the higher the molecular weight of the polymer; Its physical properties are more desirable, its viscosity in the molten state is higher, its softening point is higher, and its solubility in organic solvents is lower. In other words, resin with low molecular weight has lower melt viscosity and this feature improves its processability. Therefore, the molecular weight in P. V. C will give a proportion of the desired features and ease of processability.
This polymer is resistant to weak acids, weak and strong alkalis, alcohols, aliphatic hydrocarbons, and oils, and it swells or dissolves in acetone, esters, benzene, and gasoline. In contact with aromatic hydrocarbons, it swells.
P. V. Si softened (with softener), with less chemical resistance, compared to Pi. V. Si has not softened. Experiments and calculations have shown that the solubility parameter in this polymer is between 9.7-9.48. The most suitable solvents. V. C is cyclohexanone and T.H.F- tetrahydrofuran. Cethanes, halogenated hydrocarbons and aromatic compounds such as nitrobenzene and ethylene dichloride can also cause some damage. V. Solve 3 in yourself.
In this method, the monomer is added to the reactor containing water and after stirring, it turns into small drops. Drops are stabilized with a SUSPENDING AGENT, which is usually a protective colloidal material such as polyvinyl alcohol, gelatin, or methylcellulose. This substance is added to the water before the monomer. A substance that is soluble in the monomer is also used as a polymerization reaction initiator.
The molecular mass as well as the particle size distribution in the obtained polymer can be controlled by the type and amount of suspending agent and the amount of stirring. The size of the particles is usually between 30-250 microns and the shape of the particles depends on the type of suspending agent. If the suspending agent is from the gelatin family, women with smooth surfaces are obtained. In this case, the surface-to-volume ratio is small, and the absorption of the softener at room temperature is negligible; But if vinyl acetate or maleic acid copolymer is used, the polymer will be porous particles with irregular shapes, high surface-to-volume ratio, and high softener absorption at room temperature.
The free fall of the resin depends on its particle size. Powdered resins with medium and coarse particles fall easily.
Suspended resins are mixed with softener and other additives in heat and pressure and are processed in various ways:
In the form of calender film, they are used in the production of thermal hanging curtains, tablecloths and tablecloths, or they are used in the production of profiles or insulation for wires and cables by extrusion method.
The devices that are used in the process of this type of resin (high-speed mixer, barrel, calender, cylindrical roller, and extruder), on the one hand, have sufficient ability to bring the temperature of the mixture to the temperature required by the process, and on the other hand, the power They provide the necessary for shaping the thermoplastic mixture.
This type of pi. V. Si, is known by the symbol “S” and its use in the cable, shoe, hose and foundation sheet industries. V. 30, more than other industries.
In the emulsion method, water, water-soluble emulsifying agent, catalyst and monomer, while mixing with each other, enter into the reaction and the resulting polymer has a particle size between 0.2-2 microns (in some cases , particle size between 1-0.1 micron is reported). Due to the smallness of the particles, some of them connect and create new particles with sizes between 30-100 microns. These particles are easily broken.
Since the dispersion resins are polymerized in the latex environment by emulsion method, in order to use the polymer particles, the resin must be separated from the aqueous phase. For this purpose, the latex is converted into tiny droplets with relatively high pressure in the spray dryer; Then, the water is removed by the hot air flow and the dry resin accumulated in the bottom of the dryer is pushed to the mill along with the air flow. In the mill, the resin mass formed in the latex or dryer is crushed and reduced.
If this resin is stored in an environment with dry air, the humidity of the powder will reach less than 0.1%. The size of the particles in the dispersion type vinyl resins is smaller compared to the suspension type.
It is reminded that the homogenization of the distribution of monomer drops in water before the start of the reaction, as well as the protection of the drops during polymerization, is done by the emulsifier. For this purpose, polar or ionic substances are used as emulsifiers (in this case, soapy compounds are usually used). After the completion of polymerization, the percentage of emulsifier remains in the polymer, the polarity of the remaining particles makes it impossible to use this polymer in electrical applications, including cable coating. For this reason, for recent applications, the suspension type used Placed.
After the degree of polymerization reaches the desired level, the monomers that participate in the reaction