SEMI-BATCH POLYMERISATION

The value of P for the free running operation of the system is obtained from the batch model equations and deviations from the desired value are used to calculate the required monomer feed volume flow-rate for the semi-batch polymerisation process. 

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. 

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. 

Zeaiter, J. Romagnoli, J.A. Gomes, V.G. Gilbert, R.G. Operation of semi-batch emulsion polymerisation reactors modelling, validation, and effect of operating conditions. Chem. Eng. Sci. 2002, 57, 2955-2969. 

Batch polymerisations are often performed in screening experiments on the laboratory-scale level. However, batch polymerisations are used less often in large-scale, commercial produchon processes than semi-continuous polymerisations because of the inherent limitations in heat transfer and copolymer composition control. 

The preparation of aqueous acryUc-polyurethane hybrid emulsions by semi-batch pulsion polymerisation of a... 

SEMI-BATCH SURFACTANT-FREE EMULSION POLYMERISATION OF BUTYL ACRYLATE IN THE PRESENCE OF CARBOXYLIC MONOMERS... 

A study was made of the impact of incorporation of a small amount of carboxylic monomers (acrylic acid or methacrylic acid) into the latex particles in the limited flocculation process, often encountered in the semi-batch surfactant-free emulsion polymerisation of pure butyl acrylate. The possibility of producing carboxylated polybutyl acrylate latices with a smaller particle size was evaluated. The resultant latex was characterised to gain a better understanding of the effect of the surfactant-free technique on their physical properties, e.g. zeta potential, distribution of acrylic acid or methacrylic acid in the particles, and stability towards the added salt, compared with the conventional emulsion polymerisation system stabiUsed by surfactants. 35 refs. 

As stated above, the polymerisation reactions were carried out at different pressures and temperatures in a stirred semi-batch reactor with continuous in-flow of ethylene the catalyst was used in suspension in toluene. 

Crowley TJ, Meadows ES, Kostoulas A, Doyle III FJ. Control of particle size distribution described by a population balance model of semi-batch emulsion polymerisation. J Process Control 2000 10 419-432. 

Both one-phase and two-phase polymerisation systems lend themselves to continuous polymerisation processes in which all the reactants are fed to the process continually and polymer is removed continually. Continuous processes are particularly useful for the manufacture of high volume products and, although initial capitalisation can be more expensive, operating costs are reduced in comparison to batch or semi-batch processes. 

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. 

During the Intervals 1 and 11 of a batch emulsion polymerisation, monomers are divided, that is, partitioned, over the monomer droplets, the aqueous phase and the polymer particles. The monomer that is consumed by polymerisation in the polymer particles is replaced by monomer that is transferred from the monomer droplets through the aqueous phase into the particle phase. In Interval 111, there are no droplets and the monomer is mosdy located in the polymer particles. In the semi-batch processes, monomers are continuously fed into the reactor, usually under starved conditions, namely, at high instantaneous conversions, for example, polymer/monomer ratios close to 90/10 on weight bases. Under these circumstances, only the newly fed monomer droplets are present in the reactor and the life-time of these droplets is short because the monomers are transferred through the aqueous phase to the polymer particles where they are consumed by polymerisation. 

In this chapter batch and semi-batch operation modes to produce copolymers in emulsion polymerisation will be discussed. 

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.

Graft polyether polyols are synthesised by in situ radical polymerisation of vinylic monomers in liquid polyethers, by batch, semi-continuous or continuous processes. The solid fraction varies between 10-50%, more frequently being between 10-40% [1-10]. 

Sugar latexes based on saccharide derivatives, such as 3-MDG, 1- or 3-MDG and IDTF are synthesised in batch and semi-continuous emulsion polymerisation. The polymerisations are carried out at 60 or 70 deg.C initiated by potassium peroxodisulphate (KPS), in the presence of... 

Polymerisations may be categorised by both the polymerisation mechanism (e.g. radical polymerisation, anionic polymerisation etc.) and by the polymerisation technique (e.g. solution polymerisation, emulsion polymerisation etc.). A third factor is how the reactor is operated in batch mode, or by adding monomers during the process (semi-continuous) or by continuous operation. Mechanism, technique and process strategies (mode of operation) all have an influence on rates of polymerisation and characteristics of the formed polymer. In this chapter, we will focus on the special characteristics that can be distinguished in an emulsion polymerisation related to rate, development of molar mass and chemical composition. In Chapter 4 the effects of the process strategy will be discussed. 

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