Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2001-01-19
2002-09-10
Henderson, Christopher (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
C528S503000
Reexamination Certificate
active
06448372
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing a vinyl chloride polymer.
2. Description of the Prior Art
Vinyl chloride polymers are commonly produced through the following steps. A vinyl chloride monomer alone or a mixture of a vinyl chloride monomer and a vinyl monomer copolymerizable therewith, deionized water, a polymerization initiator and a dispersant are introduced into a polymerization vessel. Maintaining these at a stated temperature (usually from 30 to 75° C.) with stirring, polymerization is carried out until it comes to a conversion of 60 to 98%. At a point of time where the conversion has reached a stated value, a polymerization inhibitor is introduced to stop the polymerization. After the polymerization has been completed, an unreacted monomer or monomers remain(s) in a large quantity in the polymerization vessel and in the resultant vinyl chloride polymer slurry. Accordingly, after the polymerization has been stopped, the unreacted monomer is recovered out of the polymerization vessel by evacuation. Thereafter, the polymer slurry obtained by the polymerization reaction is withdrawn out of the polymerization vessel into a blow-down tank. The polymer slurry is further fed into a monomer stripping column (hereinafter “stripper”), where the polymer slurry is subjected to stripping to remove the unreacted monomer remaining in the slurry. Then, the polymer slurry from which the residual unreacted monomer has been reduced is fed into a decanter, followed by dehydration, and thereafter the dehydrate is fed to a drying step to effect drying. Through the foregoing steps, a dried vinyl chloride polymer product is manufactured.
In recent years, in the manufacture of vinyl chloride polymers, it has become common to lower the weight ratio of water/monomer per one batch of polymerization to 1.50 or below to charge the monomer in a larger quantity in order to improve productivity. However, polymerization carried out at the water/monomer weight ratio lowered to 1.50 or below makes the unreacted monomer remain in the resultant polymer slurry in a larger quantity.
Meanwhile, where the residual unreacted monomer is in a large quantity in the polymer slurry fed to the step of dehydration, almost all the monomer harmful to human beings is discharged to the atmosphere in the step of dehydration and the subsequent step of drying. Also, the residual monomer may remain in the dried vinyl chloride polymer product in a larger quantity. Thus, this is very unfavorable from the standpoint of safety and sanitation and environmental problems. Accordingly, the residual unreacted monomer in the polymer slurry obtained in the step of polymerization must sufficiently be reduced by means of the stripper. As a method by which the residual unreacted monomer in the polymer slurry is more reduced, a method of feeding steam to the stripper in a large quantity is known in the art. This method, however, is undesirable from the viewpoint of energy saving and cost reduction, and also may cause burnt matter of vinyl chloride polymers in the stripper because of the steam. Such burnt matter may remain in final products as foreign matter to cause a problem. Accordingly, it has been sought to provide a method by which the residual unreacted monomer in the polymer slurry can efficiently be removed using steam in a smaller quantity in the stripper.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for producing a vinyl chloride polymer at a high productivity, in the stripping step of which the residual unreacted monomer in the polymer slurry can efficiently be removed using steam in a smaller quantity.
As a result of extensive researches and studies on the above problems the prior art has had, the present inventors have discovered that, when the proportion of water/monomer is lowered to a weight ratio of 1.50 or below to charge the monomer in a larger quantity, the polymer slurry may be fed into the stripper after the viscosity (20° C.) of the resultant vinyl chloride polymer slurry has been adjusted to a specific range by further adding water to carry out stripping, whereby the residual unreacted monomer in the polymer slurry can effectively be reduced to bring about an improvement in productivity per one batch of polymerization. Thus, they have accomplished the present invention.
More specifically, the present invention provides a process for producing a vinyl chloride polymer, comprising the steps of:
(a) subjecting a vinyl chloride monomer alone or a mixture of a vinyl chloride monomer and a vinyl monomer copolymerizable therewith, to suspension polymerization in water in the presence of a polymerization initiator and a dispersant;
(b) subjecting the resultant vinyl chloride polymer slurry (hereinafter often “polymer slurry A”) to stripping to remove an unreacted monomer remaining in the slurry; and
(c) dehydrating the polymer slurry having been subjected to stripping;
wherein, in the polymerization step (a), the proportion of water/monomer is set in a weight ratio of from 0.80 to 1.50, and the viscosity at 20° C. of the polymer slurry A to be fed to the stripping step is previously kept adjusted to 0.30 Pa·s (300 cP) or lower.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described below in detail.
(a) Polymerization Step
The polymerization step is the step of subjecting a vinyl chloride monomer alone or a mixture of a vinyl chloride monomer and a vinyl monomer copolymerizable therewith (hereinafter often “monomer mixture”), to suspension polymerization in water in the presence of a polymerization initiator and a dispersant to obtain a vinyl chloride polymer slurry. In the process of the present invention, it is essential that the proportion of water/monomer is set in a weight ratio ranging from 0.80 to 1.50.
This polymerization step is described below in greater detail.
A vinyl chloride monomer or monomer mixture, deionized water, a polymerization initiator and a dispersant are introduced into a polymerization vessel, and polymerization is carried out maintaining a stated temperature (from 30 to 75° C.) with stirring. At a point of time where the conversion has reached a stated value (60 to 98%), a polymerization inhibitor is introduced to stop the polymerization.
As the monomer, a vinyl chloride monomer or a monomer mixture of a vinyl chloride monomer and a vinyl monomer copolymerizable with the vinyl chloride monomer (usually containing 50% by weight, and preferably 70% by weight or more, of the vinyl chloride monomer) is used. The copolymerizable vinyl monomer in the monomer mixture may include monomers 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, maleic anhydride, vinyl acetate and vinyl propionate, vinyl ethers such as lauryl vinyl ether and isobutyl vinyl ether, vinylidene chloride, acrylic acid, styrene, &agr;-methylstyrene, and acrylonitrile. Any of these may be used alone or in combination of two or more types.
The polymerization initiator may include those exemplified by diacyl organoperoxides such as isobutyryl peroxide, 3,5,5-trimethylhexanolyl peroxide and lauroyl peroxide, peroxyester organoperoxides such as cumyl peroxyneodecanoate, t-butyl peroxyneoheptanoate, t-butyl peroxyneodecanoate and t-hexyl peroxyneodecanoate, and peroxydicarbonate organoperoxides such as diallyl peroxydicarbonate and bis(2-ethylhexyl) peroxydicarbonate. Any of these may be used alone or in combination of two or more types. Any of these organoperoxide type polymerization initiators may usually be used in an amount of 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 whole monomer to be charged.
The dispersant may include those exemplified by cellulose derivatives such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and hydroxypropyl methyl cellulose, water-solub
Amano Tadashi
Hara Ichiro
Kobayashi Takashi
Nakahara Yoshinori
Saito Ryuichi
Henderson Christopher
Shin-Etsu Chemical Co. , Ltd.
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