Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-02-22
2003-06-10
Zalukaeva, Tatyana (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S330300, C525S384000, C521S064000, C428S402000
Reexamination Certificate
active
06576713
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Technical Field
The present invention relates to a water-absorbent resin powder and a production process therefor. More specifically, the present invention relates to a production process for a water-absorbent resin powder which is excellent in particle diameter distribution and properties, wherein the production process not only inhibits adhesion and aggregation in the production steps, but also is excellent in energy efficiency, drying efficiency, and productivity.
Furthermore, the present invention relates to a process for obtaining a water-absorbent resin powder as modified by adding an aqueous liquid to the resultant powder, which process is to more improve properties of the water-absorbent resin powder by bettering the uniform mixability of the aqueous liquid even if a special mixer or organic solvent is not especially used.
B. Background Art
In recent years, water-absorbent resins are widely used as components of sanitary materials, such as disposable diapers, sanitary napkins and incontinent pads, for the purpose of causing the water-absorbent resins to absorb much water. In addition, the water-absorbent resins are widely used not only for the sanitary materials, but also for the purpose of absorbing or retaining water, like water-retaining agents for soil and drip sheets for foods.
As to the above water-absorbent resins, the following are known as their examples: partially-neutralized and crosslinked poly(acrylic acids); hydrolyzed copolymers of starch-acrylonitrile; neutralized graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid ester; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; crosslinked polymers of carboxymethyl cellulose; crosslinked copolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS); crosslinked poly(ethylene oxide); crosslinked polyallylamine; and crosslinked polyethylenimine. Many of them are used as powders.
The mainstream of production processes for these water-absorbent resins is a process comprising the steps of: polymerizing an aqueous monomer solution (containing a crosslinking agent, if necessary); and drying the resultant crosslinked hydrogel polymer; and further, if necessary, pulverizing the resultant dry polymer; thus obtaining a water-absorbent resin as a powder. However, because of high absorbency, tackiness, and adhesion, and inferior heat resistance, the crosslinked hydrogel polymer of the water-absorbent resin has disadvantages in that: drying and pulverizing thereafter are very difficult, the productivity is low, and the properties of the resultant water-absorbent resin and the energy efficiency are very bad.
Known examples of methods for drying such a crosslinked hydrogel polymer of the water-absorbent resin include: drying with a drum dryer (JP-A-053165/1979); a method in which a dry powdery acrylic acid polymer and a hydrogel are mixed together and then dried while being stirred (JP-A-117551/1982); azeotropic dehydration (JP-A-198714/1982); drying at a specific dew point (JP-A-026604/1989/U.S. Pat. No. 4,920,202); freeze-drying (JP-A-304127/1989, JP-A-304128/1989); a method which involves stir-drying in a cylindrical dryer (JP-A-240112/1990/U.S. Pat. No. 5,005,771); a method in which a gel is extruded through specific apertures and then dried (U.S. Pat. No. 5,275,773); microwave drying (JP-A-209010/1993/U.S. Pat. No. 5,075,344); a method which involves the use of a specific gel cutter followed by hot-wind drying (JP-A-230124/1993); a method which involves hot-wind drying while measuring the differential pressure (JP-A-073518/1996); a method which involves stir-drying after adding a surfactant (JP-A-134134/1996); and a method which involves static drying, and then pulverizing, and then stirring or fluidize-drying (JP-A-240914/1999/EP 0926162).
In addition, a drying method in which the crosslinked hydrogel polymer is dried in a state laminated on a punching metal or a metal gauze is known. However, after being dried, the resultant dry polymer displays bad releasability to adhere to or clog the metal gauze or the apertures, therefore particularly the hot-wind drying etc. have problems of much deterioration of the drying efficiency involved by deterioration of permeability. Thus, a method is also known in which method a special conveyer having pins is used for the purpose of inhibiting such adhesion and clogging (JP-A-270070/1995/DE 19511769).
Furthermore, besides properties (such as water absorption capacity, water-extractable content, and liquid permeability), the particle diameter distribution is important for the water-absorbent resin, and the importance of water-absorbent resins having specific narrow particle diameter distributions is also known (JP-A-132802/1989/U.S. Pat. No. 5,061,259, JP-A-196802/1990/U.S. Pat. No. 5,244,735, JP-A-191604/1990/U.S. Pat. No. 4,973,632, JP-A-507564/1994/U.S. Pat. No. 5,419,956, and EP 0629411). In addition, a water-absorbent resin having a plurality of particle diameter distributions is also known (EP 0845272 and JP-A-130978/1999).
Thus, efficient classification is demanded as a classification method for such an aimed particle diameter adjustment, and the following methods are also known as methods for classifying water-absorbent resins after drying them: a method which involves the use of a heated or heat-retained sieve (JP-A-202187/1998/EP 0855232); and a method which involves the use of a classifying gauze as coated with such as Teflon (JP-A-156299/1999). Also known is a method in which an undried product from the drying step is classified before or during the pulverization step (JP-A-292919/1989/EP 0948997).
However, even in these methods, because of high absorbency and tackiness and inferior heat resistance, the water-absorbent resin and its crosslinked hydrogel polymer have disadvantages in that: drying and pulverizing thereafter are very difficult, and the deterioration of properties and particle diameter distribution of the resultant water-absorbent resin are seen, and further, the energy efficiency and the productivity are very bad. In addition, there are problems in that aggregation of powder in the production steps is seen also after the pulverization or classification step, so that the production efficiency and the quality are deteriorated.
Furthermore, there is a known method in which the particle diameter distribution of the water-absorbent resin powder or its absorption properties under a load are improved by adding water only or an aqueous liquid containing an additive to the resultant polymer powder. Such a method is often used for such as granulation (U.S. Pat. No. 5,369,148), surface-crosslinking (U.S. Pat. No. 5,409,771, U.S. Pat. No. 5,422,405, and U.S. Pat. No. 5,597,873), and decrease of residual epoxy compounds (U.S. Pat. No. 5,981,070).
However, the water-absorbent resin absorbs water in a moment and thereby displays tackiness. Therefore, hitherto, the improvement of properties of the water-absorbent resin is insufficient because of nonuniform mixing of the aqueous liquid, and further, according to circumstance, continuous operation itself might be difficult because the water-absorbent resin, as aggregated due to the nonuniform mixing, adheres to the mixer. In addition, a polyhydric alcohol is favorable as the crosslinking agent or a solvent therefor also in respect to properties and safety, but it is difficult to uniformly mix the water-absorbent resin with aqueous liquids, particularly, an aqueous polyhydric alcohol solution because of its high hydrophilicity and viscosity.
Thus, there are known arts in which, when modifying a water-absorbent resin powder by adding thereto an aqueous liquid, a special mixer is used (EP 0450923, EP 0812873, etc.), or an inorganic powder is used (U.S. Pat. No. 4,587,308), or an organic solvent is also used by adding it into an aqueous liquid (U.S. Pat. No. 4,734,478). However, the use of organic solvents, particularly, volatile ones, involves problems of causing the deterioration of properties in add
Fujino Shin-ichi
Harada Nobuyuki
Hatsuda Takumi
Ishizaki Kunihiko
Kanto Teruyuki
Nippon Shokubai Co. , Ltd.
Roylance Abrams Berdo & Goodman L.L.P.
Zalukaeva Tatyana
LandOfFree
Water-absorbent resin powder and production process therefor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Water-absorbent resin powder and production process therefor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Water-absorbent resin powder and production process therefor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3161244