Water-absorbable polyacrylic acid resin powder, and process for production thereof
A process according to the present invention for production of water-absorbable polyacrylic acid resin powder, including: (a) polymerizing an acrylic acid-based monomer aqueous solution containing bubbles, and (b) drying a hydrogel crosslinked polymer thus obtained in the step of polymerizing, further includes (c) lowering solubility of dissolved gas in a monomer aqueous solution in the presence of a surfactant and/or a dispersing agent, so as to generate the bubbles in the acrylic acid-based monomer aqueous solution. With this arrangement, it is possible to provide white water-absorbable resin with better water absorbing rate, keeping or without significantly losing the other properties (permeability potential, bulk specific gravity, surface tension, absorbency against pressure, impact resistance, etc.)
1. Water-absorbable resin powder being water-absorbable polyacrylic acid resin powder in which particles having a particle diameter of 850 μm to 150 μm accounts for 95 wt % or more, and having an internal cell rate of 2.8% to 6.6%, the internal cell rate being defined by the following equation:
(internal cell rate [%]={(Real Density [g/cm 3 ])−(Apparent Density [g/cm 3 ])}/(Real Density [g/cm 3 ])×100,
the water-absorbable resin powder having a degree of whiteness of 68 or more [WB value] as measured with use of a spectral colorimeter.
2. The water-absorbable resin powder as set forth in claim 1 , containing a surfactant and/or a dispersing agent inside thereof, wherein:
the water-absorbable resin powder has a surface tension of 60 [mN/m] or greater, and a particle surface being coated with a/the surfactant.
3. The water-absorbable resin powder as set forth in claim 1 , wherein a saline flow conductivity thereof is 20 [×10 −7 ·cm 3 ·sec·g −1 ] or greater, and a water absorbing rate (FSR) thereof is 0.25 [g/g/sec] or greater.
4. The water-absorbable resin powder as set forth in claim 1 , wherein an absorbency without pressure (CRC) thereof is 25 [g/g] or more, and an absorbency against pressure (AAP) under load of 50 [g/cm 2 ] thereof is 15 [g/g] or more.
5. The water-absorbable resin powder as set forth in claim 1 , wherein an absorbency without pressure (CRC) thereof is 25 [g/g] or more, and a saline flow conductivity thereof is 20 [×10 −7 ·cm 3 ·sec·g −1 ] or greater.
6. The water-absorbable resin powder as set forth in claim 1 , further comprising:
one or more kinds of permeability improving agents selected from the group consisting of multivalent metal cations, polyamine polymers and water insoluble fine particles.
7. The water-absorbable resin powder as set forth in claim 1 , further comprising a surfactant.
8. The water-absorbable resin powder as set forth in claim 1 , further comprising p-methoxy phenol by 5 ppm to 60 ppm.
9. The water-absorbable resin powder as set forth in claim 1 , further comprising one or more kinds of additives selected from the group consisting of chelating agents, α-hydroxy carboxylic acids, and inorganic or organic reducing agents.
10. The water-absorbable resin powder as set forth in claim 1 , wherein
a water absorbing rate index there of is 90 or more, water absorbing rate index being defined by the following equation:
(Water Absorbing Rate Index)=(FSR [g/g/sec])×(Bulk Specific Gravity [g/cm 3 ])×(Weight Average Particle Diameter [μm]),
where FSR is a water absorbing rate for saline so as to swell 20-fold, and
a bulk specific gravity thereof is in a range of 0.6 to 0.8 [g/cm 3 ].
11. The water-absorbable resin powder as set forth in claim 1 , wherein a surface tension thereof is 60 [mN/m] or greater.
12. The water-absorbable resin powder as set forth in claim 1 , wherein a bulk specific gravity thereof is in a range of 0.58 to 0.80 [g/cm 3 ].