Preparation and properties of PVC
Get a polyvinyl chloride polymer.zation of vinyl chloride (B.).ur-NIJ for homogenization.Radical Polymerization.However, because PVC is not soluble in water, bulk polymerization of the monomer, as well as in the aquatic environment is the nature of hetero-.Because of the low mobility of the macroradicals in the solid phase is hindered their interaction and, consequently, low rate of the polymer chain breakage;At the same time, the rate constants of initiation and chain growth are the same as in the homogenization.Environment.Therefore, with increasing amounts of polyvinyl chloride and increases the overall rate of polymerization (autocatalytic process).The reaction rate increases to the degree of conversion of monomer 60-70%, then begins to decrease due to exhaustion.Heat of reaction 92.18 kJ / mol, activation energy ca.83.80 kJ / mol.The degree of polymerization in a large, highly dependent on temperature, due to a tendency to chain transfer.Polymerization temperature has some effect on the degree of crystallinity of polyvinyl chloride at temperatures from -10 to 20 ° C are PVC with a high glass transition temperature and sindiotaktichnostyu to 105 ° C.
Industrial production of PVC is carried out in three ways:1) The suspension polymerization of a periodic pattern.A solution containing 0.02-0.05% by weight of an initiator (eg, atsilperoksidy, diazo compounds), stirred vigorously in an aqueous medium containing 0.02-0.05% by weight of protective colloid (for example, methylhydroxypropylcellulose, polyvinyl alcohol).The mixture is heated to 45-65 ° C (depending on the desired molecular weight polyvinyl chloride) and a given temperature is maintained within narrow limits in order to obtain a homogeneous molecular weight polyvinyl chloride polymerization takes place in the droplets during her there is some aggregation of particles;The result is a porous pellet size 100-300 micron PVC.After the fall of pressure in the reactor (the degree of conversion of about 85-90%) remove unreacted monomer, polyvinyl chloride is filtered off, dried in a stream of hot air, sieved through a sieve and packed.Polymerization is carried out in reactors of large volume (200 m3);new fully automated production.Specific consumption of 1,03-1,05 m / m PVC Benefits way:ease of removal of heat of reaction, high efficiency, the relative purity of the PVC, good sovmeschaemost it with the components during processing, ample possibility of modifying the properties of PVC by the introduction of various additives and modify a profile.
2) bulk polymerization of a periodic pattern in two steps.In the first reaction mixture containing 0.02-0.05% by weight of initiator is polymerized with vigorous stirring until a conversion of about 10%.Get a fine suspension of particles ("embryos") in polyvinyl chloride monomer, which is transferred to the second stage reactor;here also introduce additional kolichestvava monomer and initiator and continuing polymerization with slow stirring and the temperature of a given degree of conversion of about 80%.In the second stage is the further growth of particles of polyvinyl chloride and its partial aggregation (new particles are formed).Get porous granules of PVC with dimensions of 100-300 mm, depending on temperature and stirring rate in the first stage.UNCURED monomer is removed, polyvinyl chloride purged with nitrogen and sieved.The powder is dry and easily processed.Advantages over the method by suspension:lack of preparation stages of the aqueous phase, separation and drying of polyvinyl chloride, resulting in reduced capital investment, energy consumption and maintenance costs.Limitations:formed PVC uniform molecular weight, its thermal resistance is lower than that of polyvinyl chloride produced by the first method.
3) Emulsion polimerizatsiyapo periodic and continuous scheme.Use a water-soluble initiators (H2O2, persulfates), as emulsifiers - surfactants (eg., Alkyl-or arilsulfaty, sulfonates).The radicals are generated in the aqueous phase containing up to 0.5% by weight of initiator and emulsifier 3%;then continuing the polymerization in micelles of emulsifier.In the continuous process the reactor receives water phase and monomer.Polymerization occurs at 45-60 ° C and low stirring.The resulting 40-50% ny latex with a particle size of 0.03-0.5 micron polyvinyl chloride is removed from the bottom of the reactor, where there is no mixing;the degree of conversion is 90-95%.For the periodic components of technology - (aqueous phase, monomer, usually a latex kolichestvovo from previous operations, since bare nazsdftvsq latex, and other additives) are charged into the reactor and mixed throughout the volume.The resultant latex after removal of the monomer is dried in a spray chamber and the powder was sieved PVC.Although a continuous process of high-efficiency, the advantage is often given periodically, because they can get the desired particle size distribution of polyvinyl chloride (particle size of 0.5-2 microns in the range), which is essential for its processing.Emulsion polyvinyl chloride significantly contaminated with auxiliary substances, phase polymerization, and therefore it is manufactured only from pasta and plastisols.
suspension polymerization in the world is not less than 80% of polyvinyl chloride, other ways - by about 10%.
PVC recycle all known methods of processing plastics in rigid (vinyl plastic) and in soft or softened (Plasticized), materials and products.