Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-07-31
2002-12-17
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S557000, C524S503000, C524S803000, C525S056000, C525S059000, C525S532000
Reexamination Certificate
active
06495623
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an aqueous emulsion that comprises a polyvinyl alcohol having at least 1.9 mol % of 1,2-glycol bonds (this will be hereinafter referred to as the first aspect of the invention), and to a dispersant for suspension polymerization that comprises the polyvinyl alcohol (this will be hereinafter referred to as the second aspect of the invention).
According to the invention, the polyvinyl alcohol having at least 1.9 mol % of 1,2-glycol bonds is used as a dispersant for emulsion polymerization or a dispersant for suspension polymerization, and this ensures good polymerization stability even when its amount used is reduced; the aqueous emulsion comprising the polyvinyl alcohol is resistant to water and the viscosity of the aqueous emulsion depends little on ambient temperatures; the vinylic polymers obtained through suspension polymerization in the presence of the polyvinyl alcohol are yellowed little; and the waste water in the suspension polymerization pollutes little the environment.
BACKGROUND ART
<1> Background Art of the First Aspect of the Invention, Aqueous Emulsion
Heretofore, polyvinyl alcohol (hereinafter referred to as PVA) is widely used as protective colloid in emulsion polymerization of an ethylenic unsaturated monomer, especially a vinyl ester monomer such as typically vinyl acetate. Aqueous emulsions of vinyl ester polymers obtained through emulsion polymerization with PVA serving as protective colloid have many applications in various fields of adhesives for paper products, wood products, plastics, etc., binders for dip-coated paper, non-woven fabrics, etc., as well as admixtures, jointing agents, coating compositions, paper processing agents, fiber processing agents, etc.
Controlling the degree of hydrolysis of PVA therein makes it possible to change the physical properties of such aqueous emulsions, some of which will have low viscosity just like Newtonian fluid, and will be relatively highly resistant to water, while some others will have high viscosity relatively independently of ambient temperatures. Depending on such their properties, aqueous emulsions containing PVA have many applications in various fields.
However, some aqueous emulsions containing PVA have drawbacks in that they are not resistant to water and that their viscosity greatly varies depending on ambient temperatures.
PVA serving as a dispersant for emulsion polymerization is generally grouped into “completely hydrolyzed PVA” having a degree of hydrolysis of 98% or so and “partially hydrolyzed PVA” having a degree of hydrolysis of 88 mol % or sol Aqueous emulsions containing the former PVA could have relatively good water resistance but are problematic in that their viscosity greatly varies depending on ambient temperatures. On the other hand, the viscosity of aqueous emulsions containing the later PVA does not so much depend on ambient temperatures but its temperature dependency is not still negligible, and, in addition, the aqueous emulsion is are poor in their water resistance. To solve the problems with these aqueous emulsions containing PVA, using the two types of PVA as combined in aqueous emulsions, or using PVA having an intermediate degree of hydrolysis therein is tried. However, no one has heretofore succeeded in realizing aqueous, PVA-containing emulsions that satisfy the two requirements of good water resistance and temperature independence for their viscosity. In that situation, PVA with ethylene units therein has been proposed for use in aqueous emulsions (Japanese Patent Laid-Open No. 81666/1996). Containing it, aqueous emulsions having improved water resistance and low-temperature storage stability. However, their water resistance is not satisfactory as yet and their temperature independence is not also satisfactory (see Comparative Example 7 given hereinafter). Also known is using a mercapto-terminated PVA as a dispersant for emulsion polymerization, as in Comparative Example 6 given hereinafter (Japanese Patent Laid-Open No. 24481/1991). As in Comparative Example 6, however, this is not satisfactory in point of its water resistance, and its temperature dependence of the viscosity is not satisfactorily lowered.
Given that situation, the first aspect of the invention is to provide an aqueous emulsion having the advantages of improved water resistance and lowered temperature dependence of the viscosity, and to provide a method for producing such an aqueous emulsion having the advantage of good polymerization stability, especially that still having the advantage of good polymerization stability even when the amount of the dispersant added is reduced.
<2> Background Art of the First Aspect of the Invention, Dispersant for Suspension Polymerization
For producing vinylic polymers such as polyvinyl chloride resins and the like on an industrial scale, for example, widely employed is a method of suspension polymerization that comprises dispersing a vinylic compound such as vinyl chloride or the like in an aqueous medium in the presence of a dispersant followed by polymerizing it in the presence of an oil-soluble initiator. In general, the factors that govern the quality of vinylic polymers produced through such suspension polymerization include the polymerization conversion, the ratio of monomer to water, the polymerization temperature, the type and the amount of the initiator, the type of the polymerization reactor, the stirring speed, and the type of the dispersant. Above all, the type of the dispersant has the most significant influence on the polymer quality.
The necessary properties of the dispersant for suspension polymerization of vinylic compounds are that <1> it is effective for narrowing as much as possible the particle size distribution of the vinylic polymer particles obtained, <2> it is effective for making the polymer particles porous in order that the polymer particles can rapidly absorb a plasticizer so as to be readily processed, that the monomer such as vinyl chloride or the like remaining in the polymer particles can be readily removed, and that the shaped articles of the polymer particles are prevented from having fish eyes and other defects therein, <3> it is effective for making the polymer particles have a large bulk density, <4> it ensures stable polymerization even when its amount added is small, and the waste water in the process of polymerization pollutes little the environment, and <5> it does not have any negative influence on the heat resistance of the vinylic polymers obtained.
Vinylic compounds such as vinyl chloride are generally polymerized in a mode of batch-system suspension polymerization. One typical process of batch-system suspension polymerization of vinylic compounds comprises feeding an aqueous medium, a dispersant, a polymerization initiator and a vinylic compound into a polymerization reactor, then optionally adding necessary additives thereto, and heating the reactor to polymerize the monomer therein. For improving the productivity in the process, the recent tendency in the art is toward shortening the time to be taken in one batch polymerization. For this, for example, a reflux condenser or the like is installed in the system of suspension polymerization of vinylic compounds to thereby increase the efficiency of removing polymerization heat from the system; or an aqueous medium having been previously heated is charged into the system to shorten the heating time in the system (hot-charge process). However, conventional dispersants for suspension polymerization of vinylic compounds foam greatly in polymerization reactors to reduce the effective reactor capacity and to lower the polymer productivity. In particular, when such a conventional dispersant is used in a polymerization reactor equipped with a reflux condenser, it interferes with temperature control in the reactor; or when it is used in the hot-charge process, the vinylic polymer particles produced could not be porous. These are fatal defects of conventional dispersants. On the other hand, if a defoa
Fujiwara Naoki
Inomata Naokiyo
Tanimoto Seiji
Cain Edward J.
Kuraray Co. Ltd.
Yeh James T.
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