Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1998-11-20
2001-01-16
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S317100, C526S318430, C526S318500, C422S138000
Reexamination Certificate
active
06174978
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Technical Field
The present invention relates to a production process of a water-absorbent resin, more particularly, a production process of a water-absorbent resin by polymerizing statically an aqueous solution including a hydrophilic monomer.
B. Background Art
In recent years, in the fields of sanitary materials and so on such as paper diapers, physiological napkins and so-called incontinent pads, a water-absorbent resin is widely used as the object of absorbing body fluids.
As to the above water-absorbent resin, the following materials are, for example, known: crosslinked products of partially neutralized polyacrylic acid, hydrolysis products of starch-acrylonitrile graft polymer, neutralized products of starch-acrylic acid graft polymer, saponified products of vinyl acetate-acrylic acid ester copolymer, hydrolysis products of acrylonitrile copolymer or acrylamide copolymer or their crosslinked products, crosslinked products of cationic monomer.
As a production process of these water-absorbent resins, a process of carrying out an aqueous solution polymerization while stirring an aqueous solution including a hydrophilic monomer of which the main component is acrylic acid or its salt is generally employed.
As for the stirring polymerization, because the polymerization can be carried out while cutting to little pieces a hydrogel polymer generated with the progress of the polymerization, this is excellent in that polymerization in which the polymerization peak temperature is controlled to some extent can be carried out by removing the polymerization heat with a relatively compact apparatus. However, because molecular chains are cut due to the shear force of the stirring, there are such problems that the molecular weight is hardly raised and that the network of the crosslinking structure is apt to fall into disorder. In comparison with this process, by a process of polymerizing statically an aqueous monomer solution without stirring, as is suggested in JP-A-62-156102, JP-A-01-126310, JP-A-03-174414, JP-A-04-175319, JP-A-04-236203 and so on, the water-absorbent resin is obtained without problems mentioned above.
However, in the conventional static aqueous solution polymerization, because the polymerization temperature cannot be controlled, the maximum attained temperature of the polymerization system exceeds 110° C. Therefore, the water-soluble content, which is undesirable for the water-absorbent resin, is increased, so only the water-absorbent resin of low physical properties can be obtained. Or, to control the maximum attained temperature, it is necessary to lower the concentration of the aqueous monomer solution, so it is unavoidable to employ production processes of low productivity.
SUMMARY OF THE INVENTION
A. Object of the Invention
It is an object of the present invention to solve the above-mentioned problems and to provide a production process by which the polymerization temperature can be controlled in the static aqueous solution polymerization, and a water-absorbent resin of excellent quality can be obtained with high productivity.
B. Disclosure of the Invention
The present inventors studied with encouragement to themselves and great efforts the cause why the polymerization temperature cannot be controlled in the static aqueous solution polymerization, considering that the stirring polymerization does not have such a problem. As a result, it was found that in the stirring polymerization, because the surface area of the water-absorbent resin becomes large due to stirring, much polymerization heat is taken away by the latent heat of vaporization, so the polymerization temperature is controlled naturally. Then the present inventors thought that, similarly in the static polymerization, the removal of the polymerization heat should be sufficiently carried out using together the cooling due to the heat conduction transfer from a contact face with the polymerization system and the cooling due to the latent heat of vaporization which is caused by evaporation of a solvent from the polymerization system. “Heat conduction transfer” means the transfer of heat from polymerization through a conducting medium without perceptible motion of the medium itself Cooling due to heat conduction transfer means cooling by keeping the surface of a cooling apparatus in contact with a polymerization system at a temperature in the range of 0 to 30° C. “Latent heat of vaporization” means the quantity of heat released from polymerization by an aqueous solution undergoing a change of state from liquid to vapor. Cooling due to the latent heat of vaporization means facilitating vaporization of solvent from the polymerization system by removing vapor collected in the upper space of a polymerization system. Facilitating vaporization of solvent means facilitating a transformation of energy where 1) heat energy generated in the polymerization reaction is changed into 2) energy used for phase transition of the solvent, which may also be referred to as the energy of the latent heat of vaporization. As a result of cooling due to latent heat of vaporization, an increase in temperature by an exothermic reaction of the reaction system can be suppressed. Thus, the inventors achieved the present invention.
To solve the above problems, the present invention has the below constitution.
(1) A production process of a water-absorbent resin, comprising the step of polymerizing statically an aqueous solution with a thickness of 10~50 mm including a hydrophilic monomer, characterized in that the maximum attained temperature of the polymerization system is controlled in the range of 60~95° C. in the way to remove the polymerization heat by using both cooling due to heat conduction transfer and cooling due to the latent heat of vaporization.
(2) A production process of a water-absorbent resin, comprising the step of polymerizing statically an aqueous solution with a thickness of 10~50 mm including a hydrophilic monomer, characterized in that the increase in solid content of from the aqueous solution including the hydrophilic monomer to a hydrogel polymer that will form after going through the maximum attained temperature of the polymerization system is controlled in the range of 0.2 to 10 weight % in the way to remove the polymerization heat by using both cooling due to the heat conduction transfer and cooling due to the latent heat of vaporization.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.
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Hatsuda Takumi
Motono Yoshihiro
Namba Takashi
Harlan R.
Nippon Shokubai Co. , Ltd.
Wu David W.
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