A water absorbent resin (SAP/Super Absorbent Polymer) is a water-swellable and water-insoluble polymer gelling agent, and is frequently used primarily in disposable applications as water absorbent articles such as paper diapers and sanitary napkins, as well as water retention agents for agricultural and horticultural use, industrial water stopping materials and the like. As to such a water absorbent resin, many monomers and hydrophilic polymers have been proposed as raw materials. However, a polyacrylic acid (salt)-type water absorbent resin which uses acrylic acid and/or salts thereof as monomers is in particular industrially most frequently used due to their high water absorption performance.
The water absorbent resin is produced through various steps such as a polymerization step, a drying step, a pulverization step, a classification step, and a surface crosslinking step (Patent Literature 1 to 3). Furthermore, along with a performance improvement and thickness reduction of a paper diaper that is its principal application, there is a demand for an increase in use amount of water absorbent resin (for example, 50% by weight or more), and for many physical properties (functions). Examples thereof include absorption capacity, gel strength, extractables (Patent Literature 4), absorption speed, absorption capacity under load (Patent Literature 5), liquid permeability (Patent Literature 6 to 9), particle size distribution, urine resistance, antibacterial property, impact resistance, powder fluidity, deodorization property, coloration resistance, low dust property, and the like. Among these physical properties, liquid permeability is a more important factor, and there have been suggested many methods or technologies for improving liquid permeability under load such as SFC (Saline Flow Conductivity; Patent Literature 6) and GBP (Gel Bed Permeability; Patent Literature 7 to 9) and liquid permeability without load.
Furthermore, in connection with the physical properties described above, many technologies combining plural parameters have also been suggested. For example, a technology for defining impact resistance (FI) (Patent Literature 10), a technology for defining absorption speed (FSR/Vortex) or the like (Patent Literature 11), and a technology for defining the product of liquid diffusion performance and amount of core absorption after 60 minutes (DA60) (Patent Literature 12) are known.
Furthermore, as methods for enhancing liquid permeability (SFC, GBP or the like), there are known a technology of adding gypsum before polymerization or during polymerization (Patent Literature 13), a technology of adding a spacer (Patent Literature 14), a technology of using a nitrogen-containing polymer having a nitrogen atom of 5 to 17 [mol/kg] which is capable of protonation (Patent Literature 15), a technology of using a polyamine and a polyvalent metal ion or a polyvalent anion (Patent Literature 16), a technology of coating a water absorbent resin having pH of less than 6 with a polyamine (Patent Literature 17), and a technology of using polyammonium carbonate (Patent Literature 18).
In addition to these, a technology of using a polyamine at extractables portion of 3% by weight or more, and a technology of defining the suction index (WI) or gel strength (Patent Literature 19 to 21) are known. Furthermore, in order to improve coloration and liquid permeability, a technology of controlling methoxyphenol which is a polymerization inhibitor used at the time of polymerization and then using a polyvalent metal salt (Patent Literatures 22 and 23) is also known.