Formation of primary particles in vinyl chloride polymerization

Abstract

SynopsisThe formation of primary particles in agitated bulk polymerizations of vinyl chloride was found to proceed in two stages. The first stage occurred at the very beginning of the polymerization; the second stage started as the initially nucleated particles began to agglomerate, and continued up to at least 7% conversion. Upon formation, the primary particles were stable and did not agglomerate until reaching a limiting size, which was found to be lower at higher stirring speeds. The number of particles formed in the first stage was independent of agitation and other polymerization parameters. The rate of particle formation during the second stage was equal to the rate of particle agglomeration. Thus the total number of primary particles formed was determined almost exclusively by the rate of particle agglomeration. Addition of the surfactant Span 20 caused an increase in the total number of primary particles. Also, this addition increased the tendency of the particles to fuse together after agglomeration. These effects can be understood to be a consequence of particle destabilization by the surfactant. When a small amount of a high molecular weight PMMA was dissolved in the monomer, the polymerization behaved quite differently. In this case, the primary particles were prevented from agglomeration, reaching a limiting size independent of stirring speed.

Colloidal stability of PVC primary particles in vinyl chloride

“…The conversion was calculated from rate data obtained in parallel experiments carried out in a stirred reactor [12]. The samples, taken at 0.5 % conversion, were treated and analysed in the same way as described previously [5,12].…”

“…The particle charge was assumed to originate from specific sorption of a limited number of charged species, present as impurities in the monomer. More recent investigations have shown that new primary particles are formed up to at least 7 % conversion, and that the particles formed at these high conversions have the same size and degree of stability as those formed initially [5].…”

“…These particles have an initial size of about 0.1 ~rn [1], and they are stabilized by electrostatic forces during a certain growth period after their formation [2,3]. The size and number of particles formed are closely connected with the particle stability [4,5]. A better knowledge of the stability of the primary particles would be of practical significance, as it might suggest means to control PVC resin morphology.…”

“…After formation, the primary particles grow in size but their stability is limited and decreases with particle growth. Hence, at a certain size the partitles agglomerate [5,9]. Previously, it has been suggested that the number of charges per particle was constant during primary particle growth, leading to a decrease in the particle surface potential, and hence a decrease in stability with particle growth [2].…”

“…Thus, the Debye length must be considerably shorter than previously estimated. This paper will report calculations of the primary particle stability, based on previous reports of the primary particle electrophoretic mobility [2,5], and Debye lengths in the range suggested by the conductivity measurements. The calculations are partly based on the DLVO theory, although the validity of this theory has not been proven for systems with such a low ion concentration as that of the VCM/PVC system.…”