The invention is directed to a process for the suspension polymerization of vinyl chloride monomer to produce vinyl chloride resin. In the suspension polymerization of vinyl chloride monomer, it is customary to mix a large volume of water together with suitable suspending agents, an initiator and the vinyl chloride monomer in a reactor under suitable agitation. The mixing of the foregoing reaction ingredients results in the formation of droplets of vinyl chloride monomer in the aqueous medium. When no special care is taken as to the order of addition of the various components, the droplets have various concentrations of initiator depending on the random nature in which the initiator distributed itself when the body of vinyl chloride monomer was dispersed in the aqueous medium by the agitator. Under these circumstances, as the polymerization proceeds, various size particles are formed as a result of the agglomeration of the initially formed particles, varying from lumps down to tiny particles usually referred to as "fines". The resulting polymer product upon recovery from the process is a heterogeneous mixture of particles of various sizes and shapes. The particles have a wide particle size distribution and have a relatively low bulk density. The conventional process also results in the formation of polyvinyl chloride resinous deposits or scale on the walls of the reactor to such an extent that it is ordinarily necessary to open the reactor after every one or more production runs so that the reactor scale can be removed from the walls.
U.S. pending application Ser. No. 204,739, filed Nov. 7, 1980, now abandoned, is directed to a novel suspension polymerization process for producing polyvinyl chloride resin. The process is carried out under conditions such that the initial suspension of monomer droplets is formed so that the monomer droplets have equivalent amounts of initiator accessible to the monomer droplets. The level of suspending agent and agitation are chosen to preclude agglomeration of the initially formed droplets to produce a resin whose grain size distribution results directly from the monomer droplet distribution created at the start of the reaction. The process results in formation of nearly uniform spherical particles that have a narrow grain size distribution. The particles are substantially free of sub-grains because agglomeration has not occurred during the process. Resin deposition on the interior surfaces of the reaction vessel is essentially eliminated. The resulting resin particles have a desirable high bulk density which facilitates their use in extrusion processes. The spherical nature of the particles improves their flowability.
The process of copending application Ser. No. 204,739, involves the following steps:
(1) separately mixing (a) a polymerization initiator and other oil soluble additives with a vinyl chloride monomer, and (b) suspending agent and other water soluble additives with the aqueous phase,
(2) introducing the two phases into a polymerization zone,
(3) initiating agitation to disperse the oil phase as droplets of vinyl monomer having a uniform concentration of initiator,
(4) heating the reactor to an elevated temperature while maintaining agitation until the reaction proceeds to a high conversion of monomer,
(5) stripping residual vinyl monomer, and
(6) drying the polyvinyl chloride produced.
In the foregoing process, the level of suspending agent and agitation are chosen to substantially preclude agglomeration of particles throughout the reaction thereby producing a resin wherein the resin grain size distribution results directly from the oil phase droplet distribution created at the start of the reaction. In the process just described, no significant amount of polymerization begins until after the oil phase droplet distribution is established.
While the just described process has many advantages as set forth above, the process produces particles with a pericellular membrane or skin on the surface of the particle which reduces plasticizer absorption. The particles of the process are excellent for applications such as extrusion of the resin to produce shaped articles, but the product is not well suited for applications requiring inclusion of a plasticizer. Accordingly, it is an object of this invention to modify the process of the copending application Ser. No. 204,739, so that it will produce a product having improved plasticizer absorption.