| Dispersion stabilizer for suspension polymerization of vinyl compound and method of producing the same -> Monitor Keywords |
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Dispersion stabilizer for suspension polymerization of vinyl compound and method of producing the sameRelated Patent Categories: Plastic And Nonmetallic Article Shaping Or Treating: Processes, MiscellaneousDispersion stabilizer for suspension polymerization of vinyl compound and method of producing the same description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060180956, Dispersion stabilizer for suspension polymerization of vinyl compound and method of producing the same. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a divisional of U.S. patent application Ser. No. 10/732,524, filed Dec. 11, 2003, now allowed, which claims priority to Japanese patent application No. 2002-359605, filed Dec. 11, 2002, and Japanese patent application No. 2002-359606, filed Dec. 11, 2002, all of which are incorporated by reference in their entirety. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates generally to a dispersion stabilizer for use in suspension polymerization of a vinyl compound and a method of producing the same. [0004] 2. Related Background Art [0005] Industrially, vinyl polymers such as vinyl chloride resin are produced by suspension polymerization. In this suspension polymerization, a vinyl compound such as vinyl chloride is dispersed in an aqueous medium containing a dispersion stabilizer, and polymerization is conducted using an oil-soluble catalyst. Generally, the factors governing the quality of vinyl polymers include, for example, the degrees of conversion, the water/monomer ratio, the polymerization temperature, the type and amount of catalyst, the type of polymerization vessel, the stirring rate, and the type of dispersion stabilizer. Among these factors, the type of dispersion stabilizer has a considerable effect. [0006] The following properties are important for a dispersion stabilizer used for suspension polymerization of a vinyl compound. (1) Addition of a small amount functions to allow vinyl polymer particles produced therewith to have a narrow particle size distribution. (2) It functions to make vinyl polymer particles produced therewith as uniform as possible and making them porous. This property is required for obtaining, for example, a vinyl polymer that can be processed easily and has a high rate of plasticizer absorption, a vinyl polymer that facilitates the removal of monomers such as vinyl chloride remaining in polymer particles, and a vinyl polymer that can prevent fish eyes from being formed in molded articles. (3) It functions to form polymer particles having a high bulk density. (4) It functions to suppress the generation of wet foam and dry foam and thereby prevent any decrease in productivity. (5) It has no harmful effect on the hue of a vinyl polymer obtained therewith. (6) It does not make an aqueous solution cloudy during charging for polymerization. [0007] Conventionally, as a dispersion stabilizer for suspension polymerization of a vinyl compound, for instance, cellulose derivatives such as methylcellulose and carboxymethylcellulose, and partially saponified polyvinyl alcohol have been used either alone or in appropriate combinations. Conventional dispersion stabilizers, however, do not always satisfactorily meet the performance criteria described in (1) to (6) above. [0008] For instance, polyvinyl alcohol having a polymerization degree of 2000 and a saponification degree of 80 mol % and polyvinyl alcohol having a polymerization degree of 700 to 800 and a saponification degree of 70 mol % are disclosed as dispersion stabilizers for suspension polymerization of vinyl chloride (Poval, Koubunshi Kankoukai, pp. 413-414, issued in 1984). These dispersion stabilizers, however, do not meet satisfactorily the performance criteria described in (1) to (3) above. [0009] Furthermore, JP5(1993)-88251B describes a dispersion stabilizer that is used for suspension polymerization of a vinyl compound and is made of polyvinyl alcohol. This polyvinyl alcohol has a mean degree of polymerization of at least 500 and a ratio Pw/Pn, between a weight average degree of polymerization Pw and a number average degree of polymerization Pn, of 3.0 or lower. This polyvinyl alcohol includes a carbonyl group and a vinylene group adjacent thereto. Furthermore, a 0.1-% aqueous solution of this polyvinyl alcohol has absorbances of at least 0.3 and at least 0.15 at wavelengths of 280 nm and 320 nm in the ultraviolet absorption spectrum, respectively, and a ratio (b)/(a) of the absorbance (b) at a wavelength of 320 nm to the absorbance (a) at a wavelength of 280 nm of at least 0.30. [0010] JP5(1993)-105702A describes a dispersion stabilizer that is used for suspension polymerization of vinyl chloride and is made of polyvinyl alcohol. This polyvinyl alcohol has a saponification degree of 75 to 85 mol % and contains 0.01 to 0.15 mol % carboxyl groups. Its 0.1-wt. % aqueous solution has an absorbance of at least 0.1 at a wavelength of 280 nm and a clouding point of at least 50.degree. C. The above-mentioned two stabilizers made of polyvinyl alcohol, however, also do not always meet the performance criteria described in (1) to (5). [0011] Moreover, JP8(1996)-208724 discloses a dispersant used for suspension polymerization of monomers having an ethylenically unsaturated double bond. This dispersant is made of a vinyl alcohol polymer. Its 1-wt. % aqueous solution has an absorbance of at least 2.5 at a wavelength of 280 nm. The vinyl alcohol polymer has a mean degree of polymerization of at least 500, a saponification degree of 60 to 90 mol %, a ratio Mw/Mn of weight-average molecular weight Mw to number-average molecular weight Mn of 2.5 or lower, a block character with respect to the saponification degree of 0.45 or lower, and a methanol-soluble portion of 10 wt. % or less. This dispersant exhibits properties that are balanced relatively well with respect to the requirements described in (1) to (3). However, in some cases, it does not perform satisfactorily with respect to the requirement described in (6). SUMMARY OF THE INVENTION [0012] With the foregoing in mind, the present invention intends to provide a new dispersion stabilizer that satisfies, at a high level, the performance criteria required for a dispersion stabilizer for use in suspension polymerization. The present inventors studied assiduously to solve the above-mentioned problems and as a result, achieved the present invention described bellow. [0013] A production method of the present invention is a method of producing a dispersion stabilizer that is suitable for use in suspension polymerization of a vinyl compound and that contains a vinyl alcohol polymer (A). This method comprises producing the vinyl alcohol polymer (A) by heat-treating a vinyl alcohol polymer (B) at a temperature of 90 to 180.degree. C. for 0.5 to 20 hours in an atmosphere having an oxygen concentration of 8000 ppm or lower. The vinyl alcohol polymer (B) has a saponification degree of at least 60 mol %, and contains residual acetic acid groups whose block character is in a range of 0.3 to 0.6. [0014] A dispersion stabilizer of the present invention is a dispersion stabilizer that is suitable for use in suspension polymerization of a vinyl compound and contains a vinyl alcohol polymer (A'). In a 0.1-wt. % aqueous solution of the vinyl alcohol polymer (A'), a ratio (b)/(a) between absorbance (a) at a wavelength of 280 nm and absorbance (b) at a wavelength of 320 nm is at least 0.7. The 0.1-wt. % aqueous solution has a transmittance of at least 80% at a wavelength of 500 nm, at 30.degree. C. Furthermore, a 1-wt. % aqueous solution of the vinyl alcohol polymer (A') has a yellow index (YI) of 40 or lower. [0015] The dispersion stabilizer according to the present invention provides effects such as, for instance, less polymer scale adhesion to the inner wall of a polymerization vessel, as compared to conventional dispersion stabilizers. Hence, the use of the dispersion stabilizer of the present invention allows suspension polymerization to be carried out more stably. Furthermore, foaming caused by polyvinyl alcohol can be suppressed by using this dispersion stabilizer. Moreover, polymer particles with a high bulk density can be produced through the suspension polymerization of a vinyl compound using the dispersion stabilizer. The polymer particles thus formed are industrially very useful since they have gelling characteristics and a high plasticizer absorptivity and are excellent in processability. DETAILED DESCRIPTION OF THE INVENTION [0016] Hereinafter, preferred embodiments of the present invention are described. The present invention relates to a dispersion stabilizer comprising a vinyl alcohol polymer (hereinafter also referred to as a vinyl alcohol polymer (A)) and a method of producing the same. The dispersion stabilizer of the present invention may contain elements other than the vinyl alcohol polymer (A) as long as the advantageous of the present invention are maintained. The dispersion stabilizer of the present invention can be used as a stabilizer for suspension polymerization of a vinyl compound. [0017] The production method of the present invention includes a step of producing the vinyl alcohol polymer (A) by the heat treatment of a vinyl alcohol polymer (hereinafter also referred to as a vinyl alcohol polymer (B)) at a temperature of 90 to 180.degree. C. for 0.5 to 20 hours in an atmosphere having an oxygen concentration of 8000 ppm or lower. The vinyl alcohol polymer (B) has a saponification degree of at least 60 mol % and contains residual acetic acid groups whose block character is 0.3 to 0.6. Furthermore, dispersion stabilizers containing the vinyl alcohol polymer (A) produced by this method form an another aspect of the invention. [0018] In a preferred example of the present invention, the difference in block character of residual acetic acid groups between the vinyl alcohol polymer (B) that has not been heat-treated and the vinyl alcohol polymer (A) that has been heat-treated is at least 0.02 and the difference in surface tension between their 0.4-wt. % aqueous solutions is 0.5 mN/m or less. [0019] In a preferred example of the present invention, a difference in viscosity between 4-wt. % aqueous solutions of the vinyl alcohol polymer (B) that has not been heat-treated and the vinyl alcohol polymer (A) that has been heat-treated is 0.5 mPas or less. [0020] The block character of the residual acetic acid groups of the vinyl alcohol polymer (B) that has not been heat-treated is 0.3 to 0.6, preferably 0.32 to 0.58, and further preferably 0.35 to 0.55. In this specification, the term "residual acetic acid groups (i.e. residual acetoxyl groups)" denotes acetic acid groups (acetoxyl groups) that have not been saponified in the production of the vinyl alcohol polymer carried out by saponification of a vinyl ester polymer. When the block character of the residual acetic acid groups of the vinyl alcohol polymer (B) is less than 0.3, aqueous solutions of the vinyl alcohol polymer (A) that has been heat-treated have a inferior handling properties. On the other hand, when the block character of the residual acetic acid groups of the vinyl alcohol polymer (B) exceeds 0.6, a vinyl polymer obtained by suspension polymerization using the vinyl alcohol polymer (A) has decreased plasticizer absorptivity. In this connection, with respect to the block character of the residual acetic acid groups, details including its measurement and the like are described in Poval (Koubunshi Kankoukai, issued in 1984, pp. 246-249) and Macromolecules, 10, 532 (1977). Continue reading about Dispersion stabilizer for suspension polymerization of vinyl compound and method of producing the same... Full patent description for Dispersion stabilizer for suspension polymerization of vinyl compound and method of producing the same Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Dispersion stabilizer for suspension polymerization of vinyl compound and method of producing the same patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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