1. Field of the Invention
The present invention relates to a process for producing polymer particles with an irregular shape useful as a water-absorbent resin having a small apparent specific gravity and being excellent in water absorption, gas permeability and liquid permeability and also in gel strength after water absorption.
2. Description of the Prior Art
Water-absorbent resins have found a wide variety of applications in the fields of medical service, such as sanitary materials, food industry and agricultural technology, etc., by taking advantage of their capabilities of water absorption and water retention. In particular, when the absorbent resins are used in sanitary materials, such as sanitary articles, disposable diapers, etc., a large water absorption per unit weight and a high absorption rate are required of them. The water absorption depends upon the molecular structure of the resin, and it is considered that, among the resins having the same weight, the smaller the particle diameter of the resin powder, the larger the specific surface area and the higher the water absorption rate. For this reason, various proposals have been made for processes for producing a water-absorbent resin comprising a resin powder having a small particle diameter and being suitable for a water-absorbent resin.
For example, Japanese Patent Laid-Open No. 167302/1982 proposes an attempt to improve the water absorption rate by conducting polymerization using a particular surfactant as a dispersion stabilizer for polymerization to provide a water-absorbent resin powder having a particle size reduced to 1 to 40 .mu.m. However, mere pulverization of the water-absorbent resin causes the formation of curd during water absorption, so that no sufficient water absorption rate can be attained.
Japanese Patent Laid-Open No. 106902/1987 describes a process for producing a water-absorbent porous polymer having pores in the inside thereof and a high specific surface area, which comprises preparing an O/W/O emulsion of a monomer and polymerizing the monomer. In this process, however, the step of preparing the O/W/O emulsion is troublesome, and the pores of the resultant polymer are not always interconnected with each other, so that no water-absorbent resin having a satisfactory initial water absorption rate can be produced.
On the other hand, Japanese Patent Laid-Open No. 200102/1986 proposes a process for producing water-absorbent resin particles, which comprises initiating W/O type reverse phase suspension polymerization at 0.degree. to 20.degree. C., holding the reaction system at that temperature until the degree of polymerization reaches 30%, and raising the temperature to complete the polymerization. It discloses that this process provides water-absorbent resin particles wherein fine particles having a size of 1 to 40 .mu.m are relatively loosely bonded to each other and the particles have a high void fraction, are porous, and have a high water absorption rate. In this process, it is necessary above all to control the polymerization temperature to be in the range of 0.degree. to 20.degree. C. until the conversion reaches 30%. It is very difficult, however, to control the polymerization temperature by efficiently removing the heat of polymerization at such a low temperature, which renders this process unsuitable for the mass production of water-absorbent resin particles. Further, the process is disadvantageous in that it is poor in productivity because a large amount of the polymer deposits on a polymerization vessel in the course of the reaction.