A superabsorbent polymer (SAP) is a synthetic polymeric material capable of absorbing moisture from about 500 to 1000 times its own weight, and is also called a SAM (super absorbency material), an AGM (absorbent gel material), etc. Since superabsorbent polymers started to be practically applied in sanitary products, they are now being widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for civil engineering and construction, sheets for raising seedlings, fresh-keeping agents for food distribution fields, or the like.
As a preparation process for such superabsorbent polymers, a process of reverse phase suspension polymerization or a process of solution polymerization has been known. Of them, preparation of the superabsorbent polymer by reverse phase suspension polymerization is disclosed in, for example, Japanese Patent Laid-open Publication Nos. S56-161408, S57-158209, S57-198714, etc. Further, preparation of the superabsorbent polymer by the solution polymerization additionally includes a thermal polymerization method in which a water-containing gel polymer is polymerized while being broken and cooled in a kneader equipped with a plurality of shafts, and a photo-polymerization method in which an aqueous solution with a high concentration is irradiated with UV rays onto a belt to be polymerized and dried at the same time.
In the products made of superabsorbent polymers, permeability is an index of determining fluidity of a liquid to be absorbed. That is, a liquid cannot flow readily through a superabsorbent polymer with low permeability. Permeability may differ depending on particle size distribution of crosslinked polymers, particle shape, connectedness of open pores between particles, surface modification of a swollen gel, etc.
As one of methods of increasing permeability of the superabsorbent polymers, there is a method of performing a surface crosslinking reaction after polymer polymerization. In this regard, a technology of adding an inorganic filler together with a surface crosslinking agent, a technology of coating the surface of the superabsorbent polymer with a polymer such as polyamine, etc., has been suggested.
However, permeability of the superabsorbent polymer may be improved by these methods, but there is a limitation in that centrifuge retention capacity (CRC) and absorbency under pressure (AUP) of the superabsorbent polymer become relatively low. Therefore, there is a trade-off between permeability of the superabsorbent polymer and absorption properties such as centrifuge retention capacity, absorbency under pressure, etc. Accordingly, there is an urgent demand for a technology capable of improving these physical properties at the same time.
Further, during preparation and handling of the superabsorbent polymer, the superabsorbent polymer particles are broken or the surface thereof is damaged by collision between superabsorbent polymer particles and collision with an inner wall of a device such as a transfer tube, thereby generating a problem of deteriorating the physical properties of the superabsorbent polymer.