MASS POLYMERISATION

Continuous mass polymerisation units are extensively used for making polystyrene. Great care is necessary to prevent the heat of reaction accelerating the polymerisation to such an extent that the reaction gets out of control. The problem is made particularly difficult by the fact that heat can only be taken away from the points of higher temperature by conduction because of the very high 

That such a process is today commercially important is a measure of the success of chemical engineers in overcoming heat transfer problems involved with masses incapable of being stirred. An idea of the extent of the problem can be gauged from the fact that it takes six hours to cool a sample of polystyrene from 160°C using a cooling medium at 15°c when the heat transfer distance is two inches. 

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For both suspension and mass polymerisations at less than 2% conversion, PVC precipitates from its monomer as stable primary particles, slightly below 1-p.m dia (4,10—12). These primary particles are stabilised by a negative chloride charge (4,13). Above 2% conversion, these primary particles agglomerate. Sectioning the PVC grains of either suspension or mass resins readily shows the skins primary particles at 1-p.m dia, and agglomerates of primary particles at 3—10-pm dia (4,7,8,14). 

Such effects are especially observable when polymerisation occurs in the bulk (mass polymerisation). In this case, another effect can be observed at high... 

There are three main techniques of polymerisation bulk, solution and emulsion. In bulk or mass polymerisation, the catalyst is added directly to the monomer and heat may be applied to start the reaction. In solution polymerisation, the monomer is dissolved in an organic solvent. In emulsion polymerisation, the monomer or monomers are stirred up with water and an emulsifying agent to form a stable emulsion. Control of the reaction is obviously much easier with either solution or emulsion polymerisation than with bulk polymerisation. 

Unlike bulk plastics and paper where unwanted substances can be removed by vacuum stripping (e.g. vinylchloride monomer from polyvinylchloride, styrene from polystyrene) or by washing (e.g. organic and metallic residues in mass-polymerised plastics), adhesives by their gummy nature are difficult to clean-up. Residues of incomplete polymerisation and reaction by-products could be effectively retained and may subsequently migrate. On the other hand, adhesives are generally not used in direct contact with the packaged foods. Rather, they are applied at seams and pack ends and any contact with the food is likely to be incidental and limited in area. 

Slush powders were prepared from mass polymerised and suspension polymerised vinyl chloride polymers and the absorption of plasticisers into the polymers was investigated using the Haake rheomix procedure. The fusion behaviour of the powders was also investigated and the morphology of the particles analysed by scanning electron microscopy. The properties of the two different powders are compared. 6 refs. 

Polystyrene can be polymerised by foam processes, mass polymerisation, solution polymerisation, suspension polymerisation and emulsion polymerisation. Each contributes to the final characteristics of the plastics and the likely residues. A limit for styrene monomer may be imposed. Styrene can be readily analysed down to a 0.01% level. Polystyrene exhibits low shrinkage, hence is an excellent moulding material. 

To overcome the major drawbacks of the mass polymerisation technique, monomers or reactants are dissolved in a suitable inert solvent or mixture of solvents, where the polymers are also expected to be soluble. The localised heat accumulation or gel effect is not observed in this case so heat dissipation is not a problem. However, the choice of solvent is critical in obtaining a high molecular weight with a controlled polymer structure. Desirable characteristics of solvents include non-interaction with the components of the medium, moderate volatility, non-toxicity and good solvating power, both for monomer and polymer. 

Taylor-Couette High heat and mass Polymerisation ... 

These catalysts were developed from the generation. At low temperatures (below 100 °C) the active violet y or 5 form of the brown P-TiCls is formed. Through the smaller size of the primary crystallites, the surface area and activity of the catalyst was increased. The and generation catalysts (unsupported catalysts) were used in suspension processes with hexane as a solvent, in mass polymerisation processes (Rexene, Phillips), in the BASF gas phase process (vertical agitation) and in the solution process (Eastman). 

Masse-Polymerisation/ Substanzpolymerisation bulk polymerization, mass polymerization... 

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