Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1998-12-10
2003-06-03
Mullis, Jeffrey (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S064000, C525S191000, C525S192000, C525S218000, C525S221000, C525S225000, C525S227000
Reexamination Certificate
active
06573330
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processes for preparing a water-absorbent resin and, more particularly, to processes for preparing a water-absorbent resin by polymerization of a water-soluble ethylenic unsaturated monomer.
BACKGROUND OF THE INVENTION
Water-absorbent resins have found wide application in the fields of sanitary materials such as menstrual articles and disposable diapers, agricultural materials such as water retention agents and soil conditioning agents, and industrial materials such as cutoff materials and anti-dewing agents. Particularly, the water-absorbent resins are advantageously used in the field of the sanitary materials. Generally known as the water-absorbent resins for use in the aforesaid fields are lightly crosslinked polymeric resins. Examples thereof include starch type water-absorbent resins such as a hydrolyzate of a graft copolymer of starch and acrylonitrile (Japanese Examined Patent Publication No. 49-43395 (1974)) and a neutralized graft copolymer of starch and acrylic acid (Japanese Unexamined Patent Publication No. 51-125468 (1976)), a saponified copolymer of vinyl acetate and acrylic acid ester (Japanese Unexamined Patent Publication No. 52-14689 (1977)), and a partially neutralized polyacrylic acid (Japanese Unexamined Patent Publications No.62-172006 (1987), No. 57-158209 (1982) and No. 57-21405 (1982)).
Properties required for a water-absorbent resin, particularly, in the field of the sanitary materials include a high water-absorbency, a high water-absorbing rate, a high gel strength after water absorption, a low content of fine particles, and a sharp distribution of particle size. Where the water-absorbent resin is compounded with a pulp material to form a water-absorbent material for the sanitary materials, the water-absorbing rate is an important factor which influences the properties of the water-absorbent material, i.e., the reduction in the release amount of once absorbed liquid from the water-absorbent material and the increase in the diffusibility of the absorbed liquid into interior of the water-absorbent material.
Where a water-absorbent resin having a relatively low water-absorbing rate is employed, for example, the resulting water-absorbing material exhibits an excellent liquid diffusibility. However, when the water-absorbent material is pressed (e.g., when a baby wearing a disposable diaper comprised of such a water-absorbent material sits immediately after urination), the release amount of liquid is increased because the water-absorbent resin requires much time for the liquid absorption. In addition, if the liquid supply rate to the water-absorbent material is higher than the water-absorbing rate of the water-absorbent resin, excess liquid may leak from the water-absorbent material.
Where a water-absorbent resin having a relatively high water-absorbing rate is employed for the water-absorbing material, on the other hand, liquid absorption by the water-absorbent resin locally occurs in a portion of the water-absorbent material to which liquid is supplied, and the water-absorbent resin in that portion swells and gelatinizes to cause blocking. In such a case, the liquid diffusion into the water-absorbent material is blocked by the gelatinous water-absorbent resin, so that the liquid is prevented from spreading throughout the water-absorbent material. As a result, the release amount of the absorbed liquid is increased.
Therefore, a water-absorbent resin exhibiting initially a high water-absorbing rate and thereafter a moderate water-absorbing rate is desirable for a water-absorbent material in order to reduce the liquid release amount and increase the liquid diffusibility. A mixture of a water-absorbent resin component having a high water-absorbing rate and a water-absorbent resin component having a low water-absorbing rate may afford a water-absorbent resin having such properties. The water-absorbent resin mixture, however, is less homogeneous because it is prepared by mixing a plurality of kinds of water-absorbent resins having different particle diameters and different bulk densities. Hence, no practical solution to the aforesaid problem has been found yet.
It is, therefore, an object of the present invention to prepare a water-absorbent resin ensuring the reduction in the liquid release amount and the increase in the liquid diffusibility.
It is another object of the present invention to prepare a water-absorbent resin ensuring the reduction in the liquid release amount and the increase in the liquid diffusibility, particularly, for use in sanitary materials.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a process for preparing a water-absorbent resin by polymerization of a water-soluble ethylenic unsaturated monomer, which comprises the step of polymerizing the water-soluble ethylenic unsaturated monomer in the presence of a water-absorbent resin having a water-absorbing rate different from that of a water-absorbent resin resulting from polymerization of the water-soluble ethylenic unsaturated monomer.
In a preferred mode of the present invention, the polymerization is performed by a reversed-phase suspension polymerization method.
In accordance with another aspect of the present invention, there is provided a process for preparing a water-absorbent resin by polymerization of a water-soluble ethylenic unsaturated monomer, which comprises the steps of: preparing an aqueous solution containing the water-soluble ethylenic unsaturated monomer dissolved therein and a water-absorbent resin having a water-absorbing rate different from that of a water-absorbent resin resulting from polymerization of the water-soluble ethylenic unsaturated monomer; adding the aqueous solution to a hydrocarbon solvent and dispersing the aqueous solution therein; and heating the hydrocarbon solvent containing the aqueous solution dispersed therein to polymerize the water-soluble ethylenic unsaturated monomer contained in the aqueous solution.
In accordance with further another aspect of the present invention, there is provided a process for preparing a water-absorbent resin by polymerization of a water-soluble ethylenic unsaturated monomer, which comprises the steps of: adding an aqueous solution containing the water-soluble ethylenic unsaturated monomer to a hydrocarbon solvent and dispersing the aqueous solution therein; adding a water-absorbent resin having a water-absorbing rate different from that of a water-absorbent resin resulting from polymerization of the water-soluble ethylenic unsaturated monomer to the hydrocarbon solvent which contains the aqueous solution dispersed therein; and heating the hydrocarbon solvent containing the aqueous solution and the added water-absorbent resin to polymerize the water-soluble ethylenic unsaturated monomer contained in the aqueous solution.
In accordance with still another aspect of the present invention, there is provided a process for preparing a water-absorbent resin by polymerization of a water-soluble ethylenic unsaturated monomer, which comprises the steps of: polymerizing the water-soluble ethylenic unsaturated monomer to afford a slurry; and further adding the water-soluble ethylenic unsaturated monomer to the slurry, and polymerizing the added water-soluble ethylenic unsaturated monomer. In at least one of the aforesaid steps, the polymerization is allowed to take place in the presence of a water-absorbent resin having a water-absorbing rate different from that of a water-absorbent resin resulting from the polymerization of the water-soluble ethylenic unsaturated monomer.
In accordance with yet another aspect of the present invention, there is provided a process for preparing a water-absorbent resin by polymerization of a water-soluble ethylenic unsaturated monomer, which comprises the steps of: polymerizing the water-soluble ethylenic unsaturated monomer to afford a slurry; and adding to the slurry a water-absorbent resin having a water-absorbing rate different from that of a water-absorbent resin resulting from the pol
Fujikake Masato
Nakamura Morio
Nawata Yasuhiro
Shirakawa Magoto
Yamamori Masakazu
Connolly Bove & Lodge & Hutz LLP
Mullis Jeffrey
Sumitomo Seika Chemicals Co. Ltd.
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