Controlled feed polymerisations

Examples of Controlled Feed Polymerisations 8.1 Methyl Methacrylate with TGA. (Cx = 0.63)... 

Table 13. Predicted Data for Controlled Feed Polymerisation of MMA...

Table 14. Expected Data for a Controlled Feed Polymerisation of Styrene...

As the feed material gets passed the temperature gradient, polymerisation occurs and the fully, polymerised material emerges from the base of the tower. The reaction process gets controlled by a complex array of heating and cooling jackets and coils. 

All polymerisations were carried out in nitrogen purged xylene solutions in a thermostatically controlled one litre glass reactor. Semi-batch processes were carried out in a similar reactor which was provided with calibrated peristaltic pumps (computer controlled when necessary) for delivering the monomer feeds. Typically, experiments were carried out at 80°C with monomer concentrations which gave solids contents in the range 10 - 60% at 100% conversion. 

The major limitations of the feed forward control strategy presented here are that (i) it is only as good as the fundamental data which are used in the models and (ii) it can only be used for systems which conform to the conventionally accepted mode of behaviour of free radical chain polymerisation in solution. However, the same approach can be used with the appropriate models for any copolymerisation process. The range of application can be increased by making an arbitary assessment of the parameters necessary for the control models and/or by introducing a feedback loop which incorporates some state measurement device, e.g., an in-line gas chromatograph for measurement of residual monomers concentrations. Such a scheme is shown in Figure 21. 

This process has many advantages, as thermal control is excellent in water and the viscosity of the medium remains low and constant. Each droplet of monomer is converted directly into a polymer bead. Provided that the size of the droplets is well controlled, polymer beads of defined size, e.g. in the range 10-1000 pm, are obtained at the end of conversion. This permits easy storage and feeding of moulding machines for transformation into objects. An example of an industrial process for suspension polymerisation is presented in Figure 3.9. 

The size, stmcture, and composition of latex particles depend not only upon the components used in the emulsion recipe, but also upon how the components were combined and polymerised. The emulsion polymerisation process can be modified by controlling the feed strategy to produce latex particles with specific features. Each feed strategy has its advantages and disadvantages, and the optimal one must be selected based upon individual circumstances. The major feed strategies are listed next and are illustrated in Figure 3. 

In the feeding stage, additional monomer (often in the form of an emulsion to promote mass transfer and reduce the possibility of monomer pooling) is continuously pumped to the reactor to supply monomer to the polymerising particles. By controlling the rate of monomer addition, the rate of heat generation may be controlled. Initiator may be continually added over the course of the polymerisation to control the radical flux, polymerisation rate, and the rate of heat... 

A few ppm of blue dye are added to GPPS to control the colour of the polymer. Dyes are generally dissolved in styrene during the feeding preparation and fed to the polymerisation train. 

Polymerisation is carried out via a semi-batch (semi-continuous) process that allows reactants to be added during the polymerisation. It is usual for at least one of the redox initiators to be added as a metered feed over the course of the reaction. Other reactants can be added in order to control desired properties such as molecular weight distribution. Temperature control over the course of the polymerisation is also possible, particularly as the oil phase helps to dissipate the heat of polymerisation. Overall the semi-batch polymerisation technique is more versatile than the batch process and, as a consequence, it is possible to better manipulate polymer properties via this route. 

Reaction conditions in commercial polymerisations are selected to favour rapid controllable conversion of the monomer and this, together with a timed, uniform feed rate, restricts the build-up of unreacted monomer. 

Random copolymers that show inter-chain variations but uniform composition within a chain. This situation is possible with emulsion polymerisation but not with solution polymerisation (unless resulting from blends of different batches). It occurs where reactivity ratios are different and where no allowance is made to control the monomer feed ratio. 

Random copolymers which show variations between chains and along the chain. This is likely to occur with solution polymerisation where the reactivity ratios differ widely and where no monomer feed control is exercised. 

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