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Vinyl Chloride polymerization

Vinyl chloride monomer (VCM) is polymerized under pressure between 30 and 80°C over 3 to 10 hours. There are mainly four processes used. Suspension, emulsion, micro suspension and mass process. Usually the suspension process is used. In suspension process vinyl chloride monomer (VCM) is dispersed in water with the aid of suspending agents (e.g. polyvinyl alcohol) VCM and organic peroxide initiators are added and polymerization starts in the VCM droplets. As soon as polymerization starts, formed chains began to precipitate out in the VCM droplet and micron sized poly vinyl chloride (PVC) particles began to agglomerate. This feature creates the unique interparticle morphology of PVC grain. The shape and inside structure of grains are important in processing of PVC. Therefore for PVC the suspending agent type quantity, reactor geometry, agitation speed is important the molecular weight and morphology of PVC is also important for the final performance of PVC.

Effect of particle size

The particle size and its distribution is an important property of PVC. The particle size is affected by the type and quantity of suspending agent, reactor geometry and agitation. Each PVC license has its own characteristics particle size distribution.

Effect of PVC grain morphology

PVC is usually mixed with plastifiers during processing and the characteristics of plasticizer absorption are important during PVC processing. PVC particles are categorized according to their porosity.

» Particles with min porosity are classified as clear particles. These particles are seen like glass spheres under microscope and they do not easily melt. These types of particles can create fish eyes. » Particles which are porous inside but covered with a less permeable outer layer. » Porous particles without any outer layer.

Each PVC particle has its own advantages and draw backs. The target in PVC production is to produce homogeneous shaped porous particles. But for some rigid applications particles with less permeable shell is preferred.

Effect of molecular weight

The molecular weight and density can be regulated by temperature. The molecular weight of PVC is simply detected by a solution viscosity, defined as K value. K value increases as temperature decreases and MW of PVC increases. Temperature is constant during polymerization for a given grade, for example K 65 (most produced grade of S-PVC) is manufactured at approximately 60°C. If polymerization is carried out at lower temperatures high K value PVC with higher MW is obtained.

The bulk density of PVC

Bulk density is a measure of PVC particle size, particle size distribution, particle shape and porosity of grain. Therefore it is very difficult to link bulk density with polymerization conditions. For the particles with similar porosity, the bulk density increases when particle size decreases..

Thermal stability of PVC

Thermal stability of PVC is affected with content of impurities and unhomogenity in general. The rigid and plasticized PVC processing has its own degradation problems. Heavy metal content increases the potential to degrade. Also the very small particles less then 5 micron and very irregular shaped porous particles inside the PVC powder are susceptible degradation initiators. The formation of similar porosity, round shaped particles with normal particle size distribution and MW distribution is desired for processing with min degradation problems. In plasticized PVC production, the homogeneity of plasticizer absorption is also important.

When PVC is degraded initially its color changes from yellow to dark brown –black. During this the PVC chains cross links and the viscosity of melted mass increases enormously. The processing temperature is normally selected according to K value of PVC. K value+100 C could be selected as a starting point for processing and temperature is fine tuned according to machine and PVC properties. The degradation of PVC releases HCl therefore it can be corrosive for the processing equipments.

Color of PVC

The initial color of PVC is also affected with impurities in levels of ppm. The chemicals used during production which are attached to the end of PVC chains of mixed in PVC forms the initial color of PVC. Optical brighteners could be added to improve initial whiteness. During service sun light degrades PVC and yellowness occurs. To inhibit light degradation additives are added to PVC which absorbs the light energy.

Effect of Organic peroxides

In PVC polymerization not only the half life temperature but also the MW and structure of organic peroxide initiators is important. Because polymerization takes place in heterogeneous phase and the molecular diffusion of peroxide during polymerization is important. Organic peroxides are selected so that the rate of polymerization is constant throughout the polymerization. This is advantageous to control the exothermic heat release .Enough peroxide initiator must be present until the end of polymerization in order to minimize the unreacted monomer in the PVC particles. For this purpose usually lauroylperoxide is added.

Fatty acid derived peroxides are preferred for PVC because the decomposition products of these peroxides link to the PVC chains and acts as lubricants. Nowadays high molecular weight PVC is desired and initiators giving free radicals at lower temperatures are developed. In these processes polymerization is carried out at 30-40C and several initiators are used together to increase reactor efficiency.

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