Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-02-27
2002-09-03
Wilson, Donald R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C526S077000, C526S317100, C526S318500
Reexamination Certificate
active
06444744
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Technical Field
The present invention relates to a hydrophilic resin, an absorbent article, and acrylic acid for polymerization.
B. Background Art
In recent years, water-absorbent resins, having water-absorbency to the high degree, has been developed as some of hydrophilic resins and are practically used as absorbent articles, such as disposable diapers and sanitary napkins, in combination with fibrous base materials such as cotton, pulp, paper, and sponge. As to the water-absorbent resin, acrylic polymers such as crosslinked partially neutralized polyacrylic acids are industrially most commonly used because of their high water-absorbency, which acrylic polymers are obtained using either one or both of acrylic acid and its salt as the monomer (e.g. JP-A-62-054751).
The above conventional water-absorbent resins have problems in that when preserved for a long time (4 months or longer), they color or discolor to brown or yellow even at room temperature, so the absorbent articles using such water-absorbent resins greatly lose their goods values during preservation.
Concerned with such problems, JP-A-05-086251 regards it as a factor that a very small amount of transition metal in the water-absorbent resin generates radical species to cause unpreferable reactions such as decomposition of the water-absorbent resin and the cleavage of polymer chains, and thus the above document proposed an art in which the coloring with time is prevented by scavenging the transition metal using organic phosphoric acid compounds or their salts.
However, the above prior improving art as disclosed in JP-A-05-086251 has problems as follows.
According to the above prior improving art, as to the water-absorbent resin powder standing alone, its coloring degree (YI) is around 7.8 in the initial stage, but increases up to 35.0~37.2 when the resin powder is left at 70° C. under 65% RH in the closed system for 1 week (Comparative Examples 1~3 of the '251 publication), whereas when the organic phosphoric acid compound is added in a ratio of 0.1~0.63 weight % to the resin powder, the coloring degree (YI) is 20.2~20.8 after 1 week, so it is assumed in the '251 publication that the change of the coloring degree (&Dgr;YI) could be suppressed to 12.4~13.0 (Examples 1~3 of the '251 publication). Indeed the above prior improving art may suppress the coloring with time to some extent, but the addition of the organic phosphoric acid compound not only does complicate the process, but also is not necessarily favorable in view, for example, of the safety. In addition, even Example 2 that provides the most excellent results among the examples of preferred embodiments, as set forth in the above prior improving art, merely results in YI=12.2 and &Dgr;YI=about 4.4 when the resin is left at 70° C. under 65% RH for 1 week, and, naturally, problems of great coloring or discoloring occur when the resin is preserved for a still longer time.
Water-absorbent resins or their products (absorbent articles such as diapers) are internationally traded and, in many cases, preserved for a long time or under high humidity. Therefore, the problems of the coloring often occur. problems and sufficiently bears being practically used. On the basis of such observation results, the present inventors devised the below-mentioned new coloring evaluation method.
Incidentally, some conventional resins, as obtained by reversed-phase suspension polymerization, display the coloring degree (YI) of a little more than 20, but a large amount of hydrophobic organic solvent is used for the reversed-phase suspension polymerization, so the resultant resin has problems on the safety due to the residue of the organic solvent and is therefore not fit for sanitary materials, and further has problems on the cost for the use of the organic solvent. In addition, there are further problems in that the resultant water-absorbent resin comprises spherical fine particles and is therefore difficult to mix with or bind to pulp. and further. is generally insufficient with regard to the crosslinking degree of the surface neighborhood. As a result, the absorption capacity under a load or the liquid permeability is low.
In the above prior improving art, the coloring evaluation is carried out to water-absorbent resins as obtained by polymerization, drying, and pulverization, but there are also problems in that no sufficient study is made about influences to the coloring of factors, such as surface neighborhood crosslinking (as carried out after polymerization and drying) of water-absorbent resin particles, or particle size, shape, or water content of the water-absorbent resin. Generally, examples of the properties which water-absorbent resins should have are as follows: upon contact with aqueous liquids such as body fluids, excellent water absorption amount or speed, the liquid permeability, the gel strength of the swollen gel, the smallness of water-soluble content or monomer residue, the suction power to suck up water from a base material
In recent years, the amount of water-absorbent resin, as used for absorbent articles, tends to increase. The above prior improving art can prevent the coloring if the amount of water-absorbent resin as used is small, but the above prior improving art cannot respond to a large amount of water-absorbent resin. Thus, it is desired to improve the water-absorbent resin itself, namely, to develop a water-absorbent resin that displays extremely little coloring.
The above prior improving art has further problems on the coloring evaluation for the following reason. That is to say, the coloring evaluation in the prior improving art is carried out in the closed system. However, sanitary materials including water-absorbent resins are usually preserved not in an entirely sealed state, but in the at least partially open system (for example, sanitary materials have sewing machine stitches to open their packages), so the coloring evaluation needs to be carried out in the open system.
In the process of diligent study to achieve the below-mentioned objects, the present inventors tried to evaluate the coloring in the open system with regard to water-absorbent resins now on the market. As a result, the coloring degree (YI) was 40~50 or more. That is to say, the conventional water-absorbent resins displayed the great coloring degree in the open system. Thus, the present inventors confirmed that the coloring evaluation in the closed system according to the above prior improving art had a tendency to display a lower value than that in the open system (for example, coloring degree (YI)=12.2 in Example 2 of the above prior improving art increases to about 23~about 30 in the open system evaluation), and further that if YI=20 or less in the open system coloring evaluation, such a water-absorbent resin can solve the above containing aqueous liquids. Among these properties, the absorption actions under a load (e.g. absorption capacity under a load or liquid permeability under a load) are made much of as the fundamental properties which water-absorbent resins should have. There are known methods in which the surface of water-absorbent resins is crosslinked with surface-crosslinking agents for the purpose of obtaining water-absorbent resins of high absorption capacity under a load (EP 668080, U.S. Pat. No. 5,597,873, U.S. Pat. No. 5,422,405, U.S. Pat. No. 5,409,771). In production processes for water-absorbent resins, not only the above surface-crosslinking step but also the drying step of the water-absorbent resin involves heating of the water-absorbent resin. In the above surface-crosslinking step, the polyhydric alcohol is preferable in view of the safety or the resulting properties and is therefore often used, but the polyhydric alcohol makes crosslinking by dehydration and is therefore low reactive. Thus, the surface-crosslinking that involves the use of the polyhydric alcohol needs relatively high temperature or a long time. If the above property improvement or productivity is made much of, the surfa
Fujimaru Hirotama
Harada Nobuyuki
Ishizaki Kunihiko
Nakahara Sei
Haugen Law Firm PLLP
Nippon Shokubai Co. , Ltd.
Wilson Donald R.
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