Polymerization, latex growth

Computerized HDC for Monitoring the Latex Emulsion Polymerization Particle Growth Patterns. 

Water. Latices should be made with deionized water or condensate water. The resistivity of the water should be at least lO Q. Long-term storage of water should be avoided to prevent bacteria growth. If the ionic nature of the water is poor, problems of poor latex stabiUty and failed redox systems can occur. Antifreeze additives are added to the water when polymerization below 0°C is required (37). Low temperature polymerization is used to limit polymer branching, thereby increasing crystallinity. 

In emulsion polymerization the compartmentalization of reaction loci and the location of monomer in polymer particles favor the growth and slow down termination events. The contribution of solution polymerization in the continuous phase is strongly restricted due to the location of monomer in the monomer droplets and/or polymer particles. This gives rise to greatly different characteristics of polymer formation in latex particles from those in bulk or solution polymerization. In emulsion polymerization, where polymer and monomer are mutually soluble, the polymerization locus is the whole particle. If the monomer and polymer are partly mutually soluble, the particle/water interfacial region is the polymerization locus. 

The preparation of a latex by emulsion polymerization comprises two stages (i) particle nucleation (ii) particle growth. For the latex to be monodisperse, the particle nucleation stage must be short relative to the particle growth stage. Despite many investigations, there is disagreement as to the locus of particle nucleation (i) monomer-swollen emulsifier micelles (ii) ad-... 

Continuous emulsion polymerization processes are presently employed for large scale production of synthetic rubber latexes. Owing to the recent growth of the market for polymers in latex form, this process is becoming more and more important also in the production of a number of other synthetic latexes, and hence, the necessity of the knowledge of continuous emulsion polymerization kinetics has recently increased. Nevertheless/ the study of continuous emulsion polymerization kinetics hasf to datef received comparatively scant attention in contrast to batch kinetics/ and very little published work is available at present/ especially as to the reactor optimization of continuous emulsion polymerization processes. For the theoretical optimization of continuous emulsion polymerization reactors/ it is desirable to understand the kinetics of emulsion polymerization as deeply and quantitatively as possible. 

The "onion skin" growth mechanism is supported by filming experiments in which film formation is greatly effected by the nature of the monomer composition added last in the polymerization. In power feed examples, as well as in staged feeds, hard and hydrophobic compositions hinder film formation while softer and more hydrophilic compositions aid film formation. Curiously, in this respect, it was found that the filming characteristics of all-acrylic latexes responded to non-uniform polymerization techniques much more dramatically than did their styrene-acrylic counterparts. 

Polyfvinyl acetate) (PVAc) latexes produced by batch and continuous emulsion polymerization were used in this study. Details for the apparatus and the polymerization procedure can be found in Penlidis et al. (6,12,K3). Samples taken during the reaction were subsequently analyzed to follow conversion- and particle growth-time histories. The batch experimental runs were designed to yield similar conversion-time histories but different particle sizes. Conversion was measured both off-line, by gravimetric analysis, and on-line using an on-line densitometer (a U-tube DPR-YWE model with a Y-mode oscillator with a PTE-98 excitation cell and a DPR-2000 electronic board by Anton Paar, Austria). A number of runs were repeated to check for reproducibility of the results. Four batch runs are described in Table I below and their conversion histories are plotted in Figure 1. 

Both of the above techniques seem to give consistent results in the detection of particle growth for latexes produced by continuous emulsion polymerization, as well. More details and results from this ongoing research will be published shortly. 

Application of High-Speed, Integrated, Computerized, Hydrodynamic Chromatography for Monitoring Particle Growth During Latex Polymerization... 

Information on particle growth during either a seeded polymerization or during the growth stage of an un-seeded polymerization at different degrees of conversion also could enhance the understanding of the kinetics. In earlier work (4,5) the rate of polymerization, for polystyrene latexes primarily, has been related to the latex particle diameter and the total number of particles in the reactor. It would be useful to obtain kinetic data and develop the kinetic relationships for styrene (S)-butadiene (B) latexes. 

In this publication monitoring of different particle growth patterns during latex polymerizations using the high speed computerized HDC will be described for S/B latexes. Kinetic information will not be dealt with in this paper. 

The second particle growth pattern was derived from a latex polymerization that was carried out using a simple electrolyte addition to the reactor. 

In the seeded emulsion polymerization of some monomers —e.g., styrene—it is possible to obtain final latexes with uniform, large particles by adjusting, during polymerization, the quantity of added emulsifier the formation of new particles is prevented by the limited amount of emulsifier. For vinyl chloride, limited emulsifier is not sufficient to prevent the formation of new particles in fact, to obtain a monodispersed latex, the surface of the particles seeded in a given water volume must be controlled. It is assumed that the growth of new nuclei is related either to the rate of formation of primary useful radicals or to the rate that these are taken by the surface of sized particles. 

Few works have appeared on the seeded emulsion polymerization of vinyl chloride (VC). Giskehaug (5) recently used this technique in a kinetic study of the emulsion polymerization of VC, but he has not determined the number and distribution of particles in the final latexes. Kotlyar et al. (6) do not give sufficient experimental data for an exhaustive analysis of the results moreover, most of the growth experiments seem to have been carried out in the presence of free emulsifier. The data reported in some industrial patents (1,9) point out only the impor-... 

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