Process for producing vinyl chloride polymer

Details Process for producing vinyl chloride polymer Process for producing vinyl chloride polymer

1999-12-22

2001-06-05

Teskin, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Treating polymer containing material or treating a solid...

C528S50200C, C526S082000, C526S083000, C526S084000, C526S344200

Reexamination Certificate

active

06242562

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing a vinyl chloride polymer by suspension polymerization.
2. Description of the Prior Art
Vinyl chloride polymers are usually produced through the following steps: the steps of suspension polymerizing vinyl chloride or a monomer mixture containing vinyl chloride, in an aqueous medium in a polymerization vessel to obtain a polymer slurry; collecting unreacted monomers present in the polymerization vessel by evacuation and thereafter removing residual unreacted monomers from the polymer slurry by stripping; dehydrating the polymer slurry having passed through the stripping step, to obtain a polymer cake; and drying the polymer cake. This drying is usually carried out by the use of steam. Thus, the vinyl chloride polymer is obtained as a dried product.
In recent years, it is sought to shorten the time of polymerization production in order to improve productivity. As a part thereof, it is sought to shorten drying time in the drying step when the dried product of a vinyl chloride polymer is obtained. For this end, it is important to lower the water content of the polymer cake to be sent to the drying step after the dehydration. As methods of dehydration by which the water content in the polymer cake can effectively be lowered, a method is employed in which a centrifugal separator such as a screen bowl decanter type or solid bowl decanter type centrifugal separator is used as a dehydrator.
However, even when the dehydrator such as the centrifugal separator is used, the resulting polymer cake still has a water content ranging from 24 to 30% by weight. Such a water content is unsatisfactory from the viewpoint of making the drying time shorter, and it has been sought to make the polymer cake have a much lower water content.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a vinyl chloride polymer production process that can shorten the drying time by lowering the water content in the polymer cake obtained after the polymer slurry is dehydrated.
To achieve the above object, the present invention provides a process for producing a vinyl chloride polymer, the process comprising the steps of;
(A) suspension polymerizing vinyl chloride or a monomer mixture containing vinyl chloride, in an aqueous medium to obtain a polymer slurry;
(B) stripping unreacted monomers remaining in the polymer slurry; and
(C) subjecting the polymer slurry having passed through the step (B), to dehydration at a temperature of from 80° C. to 95° C.
According to this process, when dried products of vinyl chloride polymers are produced, the polymer cake obtained after the polymer slurry is dehydrated can be made to have a lower water content. Hence the drying time in the drying step can be shortened to bring about an improvement in productivity and also to achieve a reduction of energy consumption in the drying step and consequently a reduction of production cost.
This process also has an advantage that vinyl chloride polymer dried products having good anti-initial discoloration properties can be obtained when the polymer slurry is subjected to the dehydration within 60 minutes after the stripping is completed.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described below in greater detail.
(A) Suspension polymerization step:
The suspension polymerization may be carried out under conditions usually employed, without any particular limitations. More specifically, first, vinyl chloride or a monomer mixture containing vinyl chloride, an aqueous medium, a polymerization initiator and a dispersant are charged into a polymerization vessel, and a prescribed polymerization temperature (usually from 30 to 75° C.) is maintained with stirring to polymerize the vinyl chloride or the monomer mixture. At the time the polymerization has reached a prescribed degree (usually from 60 to 98%), the polymerization is terminated. The polymerization may be terminated by, e.g., adding to the reaction mixture an antioxidant having a polymerization inhibitory action.
The monomer to be polymerized is vinyl chloride alone and besides a monomer mixture of vinyl chloride and a monomer that is copolymerizable therewith (a comonomer). (In the mixture, the vinyl chloride may usually be in a content of 50% by weight or more, and preferably 70% by weight or more.) The comonomer is exemplified by &agr;-olefins such as ethylene. propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-tridecene and 1-tetradecene; vinyl esters such as maleic acid and vinyl acetate; vinyl ethers such as lauryl vinyl ether and:isobutyl vinyl ether: maleic anhydride; and vinylidene chloride.
The polymerization initiator is exemplified by diacyl type organic peroxides such as isobutyryl peroxide, 3,5,5-trimethylhexanoyl peroxide and lauroyl peroxide; peroxy ester type organic peroxides such as cumyl peroxyneodecanoate, t-butyl peroxyneoheptanoate, t-butyl peroxyneodecanoate and t-hexyl peroxyneodecanoate; and peroxydicarbonate type organic peroxides such as diallyl peroxydicarbonate and di-2-ethylhexyl peroxydicarbonate. This polymerization initiator may be added in an amount ranging from 0.01 to 0. 3 part by weight, and preferably from 0.03 to 0.2 part by weight, based on 100 parts by weight of the total weight of the monomer charged.
The dispersant is exemplified by cellulose derivatives such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and hydroxypropylmethyl cellulose; water-soluble or oil-soluble partially saponified polyvinyl alcohols, acrylic acid polymers, and water-soluble polymers such as gelatin. Any of these may be used alone or in combination of two or more types, and may also be used in combination with at least one of a nonionic emulsifying agent such as sorbitan monolaurate, sorbitan trilaurate, glycerol tristearate or ethylene oxide-propylene oxide block copolymer, and anionic emulsifying agent such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerol oleate, and sodium lauryl sulfate. Any of these dispersants may be added in an amount ranging from 0.02 to 0.2 part by weight, and preferably from 0.05 to 0.15 part by weight, based on 100 parts by weight of the total weight of the monomer charged.
As the aqueous medium, water may be used. Deionized water or pure water may usually be used as the water.
The aqueous medium may be charged in an amount ranging from 1.0 to 3.0 times and preferably form 1.1 to 2.0 times that of the monomer.
The antioxidant having a polymerization inhibitory action may include, but not limited to, e.g., phenol type antioxidants such as 2,6-di-t-butyl-p-cresol (BHT), 3-t-butyl-4-hydroxyanisol (3-BHA), 2-t-butyl-4-hydroxyanisol (2-BHA), 2,2′-methylene-bis(4-methyl-6-t-butylphenol) (MBMBP), 2,2′-methylene-bis(4-methyl-6-t-butylphenol) (MBEBP), 4,4′-butylidene-bis(3-methyl-6-t-butylphenol) (BBMBP), 4,4′-thio-bis(3-methyl-6-t-butylphenol) (SBMBP), styrenated phenol, styrenated p-cresol, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl) butane, tetrakis[methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl) propionate] methane, octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl propionate), 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzy 1)benzene, 2,2′-dihydroxy-3,3′-di(&agr;-methylcyclohexyl)-5,5′-dimethyl-diphenylmethane, 4,4′-methylene-bis(2,6-di-t-butylphenol), tris(3,5-di-t-butyl-4-hydroxyphenyl) isocyanurate, 1,3,5-tris(3′,5-di-t-butyl-4-hydroxybenzoyl) isocyanurate, bis[2-methyl-4-(3-n-alkylthiopropionyloxy)-5-t-butylphenyl] sulfide, 1-hydroxy-3-methyl-isopropylbenzene, 2,5-di-t-butylhydroquinone, 2,2′-methylene-bis(4-methyl-6-nonylphenol), alkylated bisphenols, 2,5-di-t-amylhydroquinone, polybutylated bisphenol A, bisphenol A, 2.6-di-t-butyl-p-ethylphenol, 2,6-bis(2′-hydroxy-3-t-butyl-5′-methylbenzyl)-4-methylphenol, 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl) isocyanurate, terephthaloy

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