Polymer Technology

Suspension polymerization is essentially a bulk polymerization process carried out in millions of small reactors (droplets). Liquid vinyl chloride under pressure is dispersed in water by vigorous stirring in a reactor of 25-150 m3 capacity, fitted with a jacket and/or condenser for heat removal and baffles for optimum mixing. This results in the formation of droplets of average diameter 30-40 ~lm which are stabilized against coalescence by protective colloids. The other essential ingredient is a monomer-soluble free radical initiator.

Suspension PVC plant
a) Reactor; b) Blowdown vessel; c) VCM recovery plant; d) Stripping column; e) Heat excbanger; f) Centrifuge; g) Driers

The formulations charged to the reactor are normally referred to as recipes. A basic recipe for suspension processes can be simply water, VCM, initiator, and suspension agent, for example:
VCM 100 parts
Water 90-130 parts
Protective colloid 0.05-0.08 part
Initiator 0.03-0.08 parts

[ad#aryshi1]
 
These quantities vary depending on the PVC grade, reactor Size, plant type, etc.
After charging, the reactor contents are heated to the reaction temperature of 45°C-70°C. Reactors are 80-90% full at this stage Heat causes some of the initiator to decompose into free radicals, and the monomer in the droplets begins to polymerize. The strongly exothermic reaction is controlled by removing heat via jacket and/or by boil-off into a condenser .
Progress of the reaction can be followed by monitoring the water temperature in the jacket since the flow rate is constant and a constant batch temperature is maintained by progressively reducing the cooling temperature. As the conversion increases towards 70% and beyond, the pressure in the reactor starts to fall. The reaction is terminated at a predetermined pressure by either addition chain terminator and/or venting off the unreacted monomer to the recovery.
The batch is discharged from the reactor to a feed vessel and then fed continuously through a stripper column. The unreacted monomer is recovered. After passing through a heat exchanger the slurry is fed to a continuous centrifuge to give a wet-cake with 20-30% moisture content. The remaining water is then removed by conventional flash or fluid-bed drying to give a dry, free flowing powder with a residual VCM content below 1 ppm.
Water used in the process should be demineralized since IOniC species, especially sodium ions, can affect the performance of other additives and influence final polymer properties. Protective colloids used today are cellulose ether derivatives and partially hydrolyzed polyvinyl acetates or polyvinyl alcohols. Diacetyl peroxides, peroxydicarbonates, and alkyl peroxyesters are the preferred initiators.
PVC is unique in its relationship between reaction temperature and Kvalue. With PVC the length of the polymer chain (degree of polymerization, DP) is determined by the ratio of the rate of chain propagation to chain transfer Both the rate constants kp and ktr depend only on tempertature therefore the molecular mass of PVC is controlled by the reaction temperature. The general K-value range of commercial resins is K55 to K75. K-value is determined from the solution viscosity measurement in specific solvent at 25°C.