Water-absorbent resins have been used in the field of sanitation as menstrual articles, diaper, disposable house-cloth and the like, as well as in the field of agriculture and horticulture as water retentive materials and soil improvers. Further, they are useful in other various fields such as coagulation of sludges, prevention of dew condensation on construction materials, dehydration of oils and so on. They have been widely used particularly in the field of sanitation as menstrual articles, diaper, disposable house-cloth and the like. In this case, since they come in direct contact with human body, great importance has come to be placed on the safety of water absorbent-resins. In general, requirements for physical properties of water-absorbent resins include water absorbency, water absorption rate, gel strength after water absorption, shape and compatibility with other materials used together. Good water absorbent resins are those satisfying not only these physical properties but also safety.
As such water absorbent-resins, there are known high molecular resins which are crosslinked slightly, and their examples include crosslinked carboxymethyl cellulose, crosslinked polyethylene oxide, crosslinked hydrolyzate of starch-acrylonitrile graft copolymer, crosslinked polyacrylic acid salt and crosslinked vinyl alcohol-acrylic acid salt copolymer. Of these water-absorbent resins, crosslinked hydrolyzate of starch-acrylonitrile graft copolymer has a relatively high water absorbency. However, since it contains starch which is a natural high molecular substance and causes putrefactive decomposition, its storage over a long period of time is difficult. Moreover, its production process is complicated. With respect to crosslinked carboxymethyl cellulose and crosslinked polyethylene oxide, no product is available yet which has a satisfactory water absorbency. Crosslinked vinyl alcoholacrylic acid salt copolymer has a relatively high water absorbency. However, its production process is complicated incurring a high cost. In contrast, crosslinked acrylic acid salt polymer has a high water absorbency and can be prepared from a starting material, acrylic acid which is readily available commercially, can be produced at a uniform quality and inexpensively, and causes no putrefaction; thus, crosslinked acrylic acid salt polymer has many advantages and is said to be the most desirable water-absorbent resin.
As a process for polymerizing acrylic acid or an alkali metal acrylate, there are known bulk polymerization process, aqueous solution polymerization process, inverse emulsion polymerization process, inverse suspension polymerization process, etc. In processes other than inverse emulsion polymerization process and inverse suspension polymerization process, polymerization in an ordinary polymerization reactor is difficult because removal of the heat generated by polymerization is difficult and the polymerization mixture has an extremely high viscosity, and further it is difficult to obtain a product in powder state.
A process for producing a powdery polymer from an .alpha.,.beta.-unsaturated carboxylic acid in accordance with inverse emulsion polymerization process or inverse suspension polymerization process is known in Japanese Patent Publication No. 10644/1959, Japanese Patent Publication No. 30710/1979, Japanese Patent Application Kokai (Laid-Open) No. 26909/1981, etc. In order to carry out this process, selection of an appropriate protective colloid agent is necessary. This protective colloid agent is very important with regard to stabilization of W/O type suspension and control of the particle size of the polymer.
As said protective colloid agent, there are known sorbitan fatty acid esters (e.g. sorbitan monostearate and sorbitan monopalmitate), sorbitol fatty acid esters (e.g. sorbitol monostearate) and so forth. When inverse emulsion polymerization or inverse suspension polymerization is conducted using such a protective colloid agent, the polymer obtained becomes very fine powders having particle diameters as small as 100 to 10 .mu.m. This necessitates setting up of a countermeasure for dust due to handling of the powders.
Such very fine powders are liable to form unswollen powder lumps when the powders absorb a liquid to be absorbed, resulting in insufficient absorption. Moreover, when this water-absorbent polymer is mixed with or attached to a pulverized pulp, a nonwoven cloth or the like, the polymer is liable to come off from the pulp or cloth.
When inverse suspension polymerization is conducted using a protective colloid agent having a HLB of 8 to 12 as described in Japanese Patent Application Kokai (Laid-Open) No. 131608/1981, such as sorbitan monolaurate, the polymer obtained becomes granular powders having particle diameters of 100 to 500 .mu.m. This solves the above mentioned drawbacks; however, a large amount of the polymer adheres to the inside wall of a polymerization reactor during polymerization and it is not satisfactory in view of stable operation.
As other inverse suspension polymerization processes, there are known processes disclosed in Japanese Patent Application Kokai (Laid-Open) Nos. 158209/1982 and 158210/1982, etc. High molecular protective colloid agents used in these processes melt at the time of drying the formed polymer in an ordinary manner and induce partial or entire conversion of the polymer into lumps or adhesion of the polymer to the wall of a drier.
Protective colloid agents used in accordance with the processes described in Japanese Patent Application Kokai (Laid-Open) Nos. 98512/1982 and 98513/1982, etc. are not easily available commercially and the water-absorbent resins produced therewith are not sufficient in safety.
All the water-absorbent resin products obtained by the above mentioned processes have had a drawback that, when used in the field of sanitation, particularly in a diaper, even if such a water-absorbent resin is uniformly applied in the diaper, some of the resin powders that have absorbed a liquid to be absorbed tend to form lumps and the other cannot be effectively utilized.