Particle structure of suspension poly(vinyl chloride) and its origin in the polymerization process

Abstract

SynopsisThe initial stage of the suspension polymerization of poly(viny1 chloride) (PVC) is characterized I>y the formation of colloidally stahle micron-sized grains of PVC inside the polymerizing ca. 150 pm vinyl chloride droplets. The fate of these micron-sized PVC grains depends upon the agitation conditions. If no agitation is employed, they serve as growth centers for further polymerimtion to give a final particle possessing a uniform internal head morphology. In agitated systems, these grains coagulate early in the conversion to give a more irregular structure in the interior of the PVC particle. The formation of these stable growth centers appears to he unique to PVC. The polymerization of' acrylonitrile. also insoluhle in its monomer. is characterized hy rapid agglomeration of t.he precipitated Fblymer throughout the polymerization. In PVC. the colloidal stability of the polymerizing grains is demonstrated to he electrical in nature. A pericellular membrane or skin formed by polymerization in hot.h the water and vinyl phase completely surrounds the polymerizing droplet after nhorit (I-.'L)% conversion. This skin is responsil)le for the charge retention of the PVC grains inside the polymerizing monomer droplets.

Colloidal stability of PVC primary particles in vinyl chloride

“…The electrophoretic mobility of colloidally stable primary particles has been reported as -1.2 x 104 m 2 V -1 s -1 [2] and -1.5 x 104 m 2 V -1 s -1 [3], corresponding to a potential of-80 and -100 mV, respectively. In the first study, the particle radius was 0.15 ~ [2].…”

“…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].…”

Mathematical modeling of the porosity of suspension poly(vinyl chloride)

“…Suspending agents and porosizers are also responsible for an important morphological feature of S-PVC, that is, a membrane that wraps S-PVC grains, usually called pericellular membrane. In addition to controlling the coalescence of the drops, suspending agents copolymerize in the interface of the drop, with VCM forming the pericellular membrane (Davidson and Witenhafer, 1980). This structure becomes stronger and more coherent as conversion increases by adsorbing more PVA from the aqueous phase, as discussed by Allsopp (1981) and observed experimentally by Zerfa and Brooks (1996).…”

Suspension Polymerization Processes

“…PVC porosity can also be increased by using non-ionic surfactants as secondary stabilizers [17]. PVA can become grafted onto polymer that is formed inside the drops so that a "skin" forms on the final particle surface [18,19]. Formation of that skin, which is difficult to remove, can affect the final polymer properties.…”

“…But, in vinyl chloride polymerization, phase separation is inherent because VCM and PVC are almost immiscible and polymer structure is affected by coagulation of primary polymer particles inside the drops [70]. In that case, particle porosity facilitates the subsequent uptake of plasticizers by the PVC.…”