SEMI-BATCH COPOLYMERISATION

All the methods mentioned above use a mathematical model of the copolymerisation process in one way or another to arrive at a control policy for the production of compositionally homogeneous products. In this work use is made of a dynamic model of the process to control the feed rate of the more reactive monomer to a semi-batch reactor. Feedback from the process comes from an off-line model. The method is a general one and can be readily extended to accomodate feedback loops using on-line measurement devices with an experimental reactor. 

Experimental System The copolymerisation of styrene with methyl acrylate in toluene using azo-bis-iso- butyronitrile (AIBN) was selected as the model experimental system because the overall rate of reaction is relatively fast, copolymer analysis is relatively simple using a variety of techniques and the appropriate kinetic and physical constants are available in the literature. This monomer combination also has suitable reactivity ratios (i = 0.76 and r4 =0.175 at 80 C),(18) making control action essential for many different values if compositionally homogeneous polymers are to be prepared at higher conversions in a semi-batch reactor. 

The ultimate goal of most of the investigations on emulsion copolymerisation is to be able to control the process in such a way as to produce a copolymer product (latex or coagulate) with desired properties. For this purpose the semi-continuous (sometimes called semi-batch) emulsion copolymerisation process is widely used in industry. The main advantages of this process as compared with conventional emulsion batch processes include a convenient control of emulsion polymerisation rate in relation with heat removal and control of chemical composition of the copolymer and particle morphology. These are important features in the preparation of speciality or high performance polymer latexes. 

This chapter focuses on key features to understand the emulsion copolymerisation kinetics and on the influence of operation on the copolymer composition of the final latex products. Focus is on batch and semi-batch or semi-continuous operation, see Figure 4.1. Only the free-radical emulsion copolymerisation of two monomers is considered but the concepts can be directly applied for formulations containing more than two monomers. The reacting monomers usually having different reactivities, polymerise simultaneously. The reactivities and the individual concentrations of the monomers at the locus of polymerisation, that is, the particle phase, govern the built-in ratio into the polymer chains at a certain time. 

Figure 4.11 shows simulated data for two-seeded semi-batch emulsion copolymerisations of BA and MMA carried out with feeding times of 3 and 6h, respectively. Figures 4.11(a) and (b) present the cumulative and instantaneous copolymer compositions. The results in the Figures 4.11(a) and (b) clearly demonstrate that the steady state is achieved in both cases. However, for the addition period of 6 h the fraction copolymer with a composition deviating from the desired composition of 0.5 is smaller than for the addition period of 3 h. Furthermore, the cumulative composition is closer to 0.5 for the 6 h addition period than for the 3 h addition. In comparison with the batch process, the composition drift is almost negligible as displayed in Figure 4.11 (d) which shows a very narrow chemical composition distribution (CCD) centred at 0.5. 

Figure 4.11 Simulated data for the starved feed semi-batch emulsion copolymerisation of BA and MMA. Initial molar ratio of BA and MMA is one. Instantaneous and cumulative copolymer composition for (a) 3 h and (b) 6 h monomer addition, respectively (c) polymerisation rates (d) CCD for 3 h addition.

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