PVC suspension resin is a polymer made of vinyl chloride monomer by the production method of suspension polymerization process. Not only the suspension method is the most common method for producing PVC resin, but also most PVC resin for sale in the market are PVC suspension resin.
In general, the production method of PVC suspension resin includes three main processes. Firstly, use water as the suspending medium. Secondly, the liquid vinyl chloride monomer is dispersed into droplets under stirring. Thirdly, the polymerization process of vinyl chloride droplets and PVC particles. Initiator dissolved in the vinyl chloride monomer decomposes into free redicals at the polymerization temperature and initiates the polymerization reaction. Meanwhile, dispersant dissolved in water can protect the adhesion of PVC-VCM swollen particles after the VCM droplets reach a certain conversion rate.
Acetylene method and acetylene oxychlorination method are the two main methods to prepare vinyl chloride. The purity of VCM used for suspension polymerization method is above 99.9%. Production raw materials are very important to the quality of PVC suspension resin.
Main Factors Affecting The Particle Morphology of PVC Suspension Resin
The main factors are dispersant and stirring, and other relatively minor factors are conversion rate, polymerization temperature, water-oil ratio, initiator, pH value, etc.
Due to the different types of dispersants used, polyvinyl chloride resin can be divided into loose type and compact type. Compact resins are basically eliminated, and the preferred dispersant for producing loose resins is polyvinyl alcohol.
In order to obtain PVC resin with loose and uniform particles, narrow particle size distribution and appropriate apparent density, it is difficult for a single dispersant system to meet the above requirements. Two or more dispersant composite systems are often used. Using the dispersion system for suspension polymerization (using polyvinyl alcohol with a high degree of alcoholysis as the main dispersant, and polyvinyl alcohol with a low degree of alcoholysis as a secondary dispersant) is relatively easy to prepare PVC suspension resin that meets the particle shape requirements.
The amount of suitable dispersant also has a great influence on the shape of polyvinyl chloride resin particles. The amount of dispersant should be selected according to the stirring state, water ratio and product requirements.
The effect of stirring on the morphology of polyvinyl chloride resin particles is mainly manifested in affecting the particle size, particle size distribution, and porosity of PVC suspension resin. The effect of stirring and the effect of dispersant interact and complement each other.
The relationship curve between the average particle size of polyvinyl chloride resin and the stirring speed is saddle-shaped. When the stirring speed is lower than the critical speed, the particle size decreases with the increase of the speed, and the dispersion plays a major role. When the stirring speed is higher than the critical speed, the particle size increases with the increase of the stirring speed. Stirring intensity also affects the particle microstructure level. With the increase of stirring speed, the primary particles of polyvinyl chloride resin become thinner, the porosity increases, and the oil absorption rate increases.
In order to obtain a polyvinyl chloride resin with a loose structure, the conversion rate must be properly controlled. When the conversion rate is low, there is a layer of dispersant film on the surface of the droplet. As the polymerization progresses, the dispersion and protective film gradually become a graft copolymer, and the film adhesion becomes stronger and stronger. When the conversion rate is 5%~15%, the droplets tend to coalesce and are in an unstable state. If the conversion rate is >30%, the strength of the film increases, coalescence decreases, and it gradually becomes stable.
The influence of polymerization temperature on the morphology of polyvinyl chloride resin particles mainly affects the porosity. The porosity is inversely proportional to the polymerization temperature. The polymerization temperature is low, and the resin structure formed is relatively loose. The porosity of the polyvinyl chloride resin obtained by polymerization at high temperature is low. This is because as the polymerization temperature increases, the primary particles become smaller, the degree of fusion deepens, and the particles are spherical. When the polymerization temperature is low, it is easy to form irregular aggregates, thereby increasing the porosity.
Water to Oil Ratio
When feeding, the ratio of water to the weight of vinyl chloride monomer is called the water-oil ratio. The porosity of resin particles is directly proportional to the water ratio, and the apparent density is inversely proportional to the water ratio. Under the action of stirring, the vinyl chloride monomer is dispersed into droplets of 30-150 μm. When the water-oil ratio is 1:1, there is enough free fluid, and the viscosity of the system is low, which ensures the flow and heat transfer effect.
However, after polymerizing into loose particles, a considerable amount of water is adsorbed on the inner and outer pores and particle surfaces, which reduces the free fluid, strengthens the viscosity of the system, and makes heat transfer difficult. This will make the particle size distribution of the produced polyvinyl chloride resin worse, and the particle shape and Representation density is affected. Therefore, the water-oil ratio should not be too small in actual production. When producing loose resin, the water-oil ratio should be controlled above 1.0~1.6.
The activity and amount of suitable initiators also have a great influence on the morphology of PVC suspension resin particles. Initiator is an important auxiliary agent to adjust the polymerization rate, and thus has a certain influence on the shape of polyvinyl chloride particles. The choice of the amount of initiator should be determined according to the polymerization temperature, water ratio and product requirements.
If the pH value of the suspension is too high or too low, the dispersant and colloid retention ability of the dispersant will be reduced, which will eventually lead to thicker PVC resin particles. Experiments have proved that the pH value of the suspension is maintained at 6.5~8.5, which has the best particle distribution.
In addition, factors Affecting the shape of polyvinyl chloride particles include water quality, vinyl chloride monomer quality, instrument measurement, etc. All these factors are included in the entire suspension reaction system and form a system force, which has a comprehensive effect on the shape of resin particles.