SEMI-CONTINUOUS POLYMERISATION

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. 

There are a wide variety of acryhc monomers available, each having specific properties. Styrene is often used as a comonomer in acrylic latexes because of its compatibility and wide availability. Auxiliary monomers can be used in small amounts to impart special properties to toe latex. Although acrylic monomers and styrene may be similar in reactivity, if toe monomer solubilities are significantly different, copolymerisation is less likely, possibly resulting in structured particle morphologies. The copolymer composition can be made more uniform by semi-continuous polymerisation. In some cases, a stmctured morphology is desired, and can be designed into toe process (225, 372). 

Radiation from Metal Rod 518 Relief of a Runaway Polymerisation Reaction 355 Optimized Selectivity in a Semi-Continuous Reactor 362... 

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... 

Surfmers , i.e. surfactants which also acted as copolymerisable monomers, were synthesised from the hemi-ester of a fatty alcohol and maleic anhydride and were then used in the preparation of self-crosslinking dispersions by seeded semi-continuous emulsion polymerisation of acrylate monomers. Water-borne exterior wood stains were prepared from the dispersions and their properties were studied. The use of surfmers as sole emulsifiers in emulsion polymerisation was considered and data were obtained on the effects of surfmers on film formation, water barrier properties, gloss retention and mechanical properties. Environmental aspects of the use of products involving surfmers were examined. 6 refs. 

The preparation of a synthetic latex is shown to be a very complex process that is affected by the monomers selected, surfactants, initiators and the polymerisation process. The semi-continuous process is the one most frequently used as it provides control of the polymerisation heat removal, as well as control of the composition of the copolymers comprising several types of monomer units. Some aspects of copolymerisation in emulsion and particle growth in the case of the semi-continuous process are discussed. The copolymers usually comprise 4 to 5 comonomers, some of them with functional groups. The functional groups serve as loci for crosslinking, improve colloid stability, increase polarity, improve adhesion and cause alkali-solubility and/or alkali swellability. High value polymer latices with special particle morphology, composition and other... 

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. 

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. 

Bulk polymerisation techniques are usually carried out as continuous or semi-continuous processes. Polymerisation is restricted to a low degree of conversion and is stopped while the viscosity of the reaction mass is still low enough to allow effective agitation. Ibe urueacted monomer is removed and recycled. 

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. 

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. 

For instance, this procedure is followed in manypapers dealing with the semi-continuous emulsion copolymerisation of vinyl acetate and butyl acrylate (e.g. El-Aasser et al, 1983). Two main situations can be distinguished with respect to the monomer addition rate, (a) Flooded conditions the addition rate is higher than the polymerisation rate, (b) Starved conditions the monomers are added at a rate lower than the maximum attainable polymerisation rate (if more monomers were to be present). The latter process (starved conditions) is often applied in the preparation of homogeneous copolymers/latex particles. In this case after some time during the reaction, because of the low addition rates, a steady state is attained in which the polymerisation rate of each monomer is equal to its addition rate and a copolymer is made with a chemical composition identical to that of the monomer... 

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. 

In the case of semi-crystalline polymers, reports can be found on a two-step process in which short chains are crystallised after which the polymerisation is continued in the solid state to obtain, e.g., high-molecular-weight PLLA [19]. In this process, an inert gas stream is applied to remove by-products from the surface. The coupling mainly takes place in the amorphous regions of the materials, where the reactive end groups reside. 

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. 

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. 

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