This invention relates to improved suspension polymerization of vinyl chloride. The invention more specifically relates to the production of vinyl chloride resin having an excellent porous granular structure and high workability or processability and containing little residual monomer. The expression "polymerization of vinyl chloride" herein denotes homopolymerization of vinyl chloride and copolymerization of vinyl chloride and another ethylenically unsaturated monomer copolymerizable therewith, vinyl chloride being predominant. The expressions "vinyl chloride resin" and "vinyl chloride polymer" herein denote homopolymer and copolymers of vinyl chloride, vinyl chloride being predominant.
Suspension polymerized vinyl chloride resins have been widely used as materials in the form of rigid or non-or little plasticized resins as well as of soft or plasticized resins to be molded into various articles.
However, the polymers and copolymers thus obtained have been accompanied by problems such as low absorption rates of stabilizers and plasticizers in the polymer or copolymer and, moreover, differences in these absorption rates within each grain and between grains, whereby the workability of these polymers and copolymers deteriorate, even causing fish eyes. Furthermore, these problems are aggravated when a low degree of polymerization is selected in order to obtain high workability. In other words, the grains of a polymer obtained by suspension polymerizing at an elevated temperature are less porous and frequently are very nearly transparent. Such vinyl chloride polymers of a low degree of polymerization are used for blow molding because of their high workability as rigid resins and are useful in the production of packaging containers for commodities such as wines, cooking oils, sauces, and the like.
However, from the view-point of food sanitation, a slight content of residual vinyl chloride monomer or chain-transfer agents of the order of p.p.m (part per million) of the thus polymerized polymer has recently become a problem. Although the residual content of the monomer depends on the drying condition of the polymer, and also with the compounding conditions of the polymer, the content is further affected by the granular structure of the polymer beads and by microporosity which constitutes an important factor for evaporating the monomer.
With respect to the suspension polymerization of vinyl chloride, there have been proposed various procedures which are directed to producing polymer beads or grains which have granular homogeneity or improved porosity in beads thereby to improve the workability of the polymer and also to minimizing the quantity of scales which are frequently deposited in a polymerization vessel.
In these procedures, there is ordinarily used a suspending agent such as polyvinyl alcohol, a copolymer of vinyl acetate and maleic anhydride, a cellulose derivative, a polyethyrene oxide, gelatine, and also various combinations of two or more of these materials. The suspending agent may further be combined with a nonionic surfactant or with an anionic surfactant plus a water soluble inorganic polyvalent salt.
However, for evaluating these polymerizing procedures from the viewpoint of elevating the average porosity of the thus polymerized resin, the bulk density of the resin, quantity of plasticizer retention in the resin, rate of absorption of a plasticiser in the resin, feasibility of gelation, and the like have been used as reference criteria, but since these reference criteria are not necessarily suitable for nonuniformity between the beads of the polymer thus produced, the aforementioned procedures cannot be said to be satisfactory in reducing the residual monomers and eliminating fish eyes. This is particularly true when the monomers are polymerized at an elevated temperature for reducing the degree of polymerization, and therefore it has been difficult to obtain a polymer which has sufficient porosity and minimal quantity of a residual monomer.