Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-07-05
2004-07-20
Egwim, Kelechi C. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C524S458000, C524S567000
Reexamination Certificate
active
06765073
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for preparing vinyl chloride paste resins, and more particularly to a micro-suspension polymerization process for the production of vinyl chloride resins suitable for paste technique.
A micro-suspension polymerization process comprises a homogenizing step wherein a vinyl chloride monomer, water, a surface active agent, a polymerization initiator soluble in the monomer and other polymerization assistants are mixed in a separate apparatus from a polymerization apparatus under a high shear to form a homogeneous dispersion, and a polymerization step wherein the dispersion is transferred to the polymerization apparatus and subjected to polymerization with stirring, thereby producing fine particles of a vinyl chloride paste resin having a basic particle size of about 0.2 to about 3 &mgr;m.
The term “vinyl chloride monomer” as used herein means vinyl chloride alone or a mixture of vinyl chloride and other monomer copolymerizable with vinyl chloride.
In the micro-suspension polymerization, liquid droplets homogenized in the homogenizing step are polymerized after transferring to a polymerization apparatus and subsequently elevating the temperature to a predetermined temperature. Volumetric shrinkage of the reaction mixture occurs with the progress of the polymerization, resulting in decrease in effective area of heat removal. Thus, in addition to decrease in the heat removing capacity of the polymerization apparatus, the volumetric shrinkage causes scaling on the wall and agitating vane of the polymerization apparatus since the liquid level lowers than the cooling water passing level of a jacket to expose a part of the cooled portion.
In order to solve this problem, Japanese Patent Publication Kokai No. 5-140205 proposes a process wherein a jacket is divided to portions and cooling is conducted by passing the cooling water through the jacket so as not to expose the water passing level from the reaction liquid level, or a process wherein water, an aqueous solution of a surface active agent, a vinyl chloride monomer, or an aqueous dispersion containing vinyl chloride is additionally charged in the polymerization system in such an amount that the additional charge of the monomer is not more than 30% by weight of the initially charged monomer when the polymerization conversion of the initially charged monomer have reached not less than 40%.
However, if the water passing is stopped, when the reaction liquid level drops to the upper end of the jacket, so as not to expose the cooling water passing level of the divided jacket above the reaction liquid level, that portion of the jacket immediately loses the heat removing ability. This is unfavorable in polymerization control. Further, even if the additional charge is started at the time when the conversion has reached 40%, the liquid level has already dropped at that time to expose a part of the cooled portion. In case of adding water or an aqueous solution of a surface active agent, it increases a load in a post-treatment step, so the drying efficiency is lowered. In case of adding only the monomer, the stability of a latex is lowered.
It is an object of the present invention to provide a process of the micro-suspension polymerization of a vinyl chloride monomer which is improved in polymerization productivity by preventing the heat removal area of a jacket from decreasing during the polymerization, thereby preventing the heat removal capacity of the jacket from lowering and simultaneously increasing the yield of produced polymer per unit volume of a polymerization vessel.
This and other objects of the present invention will become apparent from the description hereinafter.
SUMMARY OF THE INVENTION
The present inventors paid attention to the volumetric shrinkage of a latex which occurred during the polymerization and made an intensive study on a relationship of the conditions for additionally charging a dispersion of a monomer during the polymerization, a change of the liquid level of the reaction mixture in a reactor and the heat removing capacity of a jacket of the reactor, and they have reached the present invention.
In accordance with the present invention, there is provided a process for preparing a vinyl chloride paste resin which comprises the steps of homogenizing a vinyl chloride monomer in an aqueous medium together with an oil-soluble polymerization initiator to form a first aqueous dispersion, subjecting the first dispersion to a micro-suspension polymerization and additionally charging a second aqueous dispersion containing a vinyl chloride monomer and a surface active agent to the reaction system, wherein the additional charging is carried out in such a manner that the additional charging is started after starting the polymerization but on or before the time the polymerization pump of one or two stage pressurization type, a colloid mill, a centrifugal pump, a homogenizer, an agitator of vibration type, high pressure injection through a nozzle or an orifice, and ultrasonic wave.
If the concentration of the vinyl chloride monomer in the additionally charged dispersion is too low, the solid concentration of a latex obtained by the polymerization is low and, therefore, the load in the drying step to obtain the polymer powder from the latex increases. If the monomer concentration is too high, the stability of the additionally charged dispersion lowers and, therefore, phase separation may occur before the dispersion is introduced into the polymerization reactor, thus resulting in lowering of the stability of the reaction mixture since the monomer is introduced thereto in the form of large droplets. In light of the above, preferably the concentration of the monomer in the aqueous dispersion to be additionally charged is from 25 to 70% by weight, especially from 45 to 60% by weight.
The kind of the surface active agent to be included in the additionally charged dispersion is not particularly limited. The surface active agent may be the same as or different from that included in the reaction mixture, namely the surface active agent used in the initially charged dispersion. The amount of the surface active agent in the additionally charged dispersion is not particularly limited, but is usually from 0.1 to 3 parts by weight per 100 parts by weight of the monomer included in the additionally charged dispersion.
If the time of starting the addition of the second dispersion to the polymerization system is too late, the heat removing area of the jacket has already been decreased at that time. If the cooling water passing to the jacket is stopped in order to prevent scaling, the principle conversion of the initially charged monomer reaches 30% and the additional charging is terminated on or before the time the polymerization conversion of the initially charged monomer reaches 95%.
As the vinyl chloride monomer is used vinyl chloride alone or a mixture of vinyl chloride and other monomer copolymerizable therewith.
Preferably, the monomer concentration of the additionally charged second dispersion is from 25 to 70% by weight and the amount of the additionally charged monomer is from 3 to 30 parts by weight per 100 parts by weight of the monomer initially charged.
According to the present invention, exposure of a jacket of a polymerization apparatus due to volumetric shrinkage of the reaction mixture is prevented by adding a dispersion prepared from a vinyl chloride monomer and an aqueous solution of a surface active agent to a polymerization reactor during the polymerization reaction, thereby effectively utilizing the jacket during the polymerization. In addition, the productivity can be improved by causing the produced polymer particles to absorb the additionally charged monomer and converting it into a polymer, thereby increasing the yield of the polymer per unit volume of the reactor.
DETAILED DESCRIPTION
The second dispersion to be additionally charged can be obtained by previously mixing a vinyl chloride monomer and an aqueous solution of a surface active agent to form a
Nakagawa Tomoaki
Uchida Yoichi
Yoshida Tsuyoshi
Arent & Fox PLLC
Egwim Kelechi C.
Kaneka Corporation
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