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
1999-12-07
2002-08-13
Cain, Edward J. (Department: 1714)
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...
C523S105000, C523S111000, C524S433000, C524S437000, C524S459000, C524S460000, C524S523000, C524S527000
Reexamination Certificate
active
06433058
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to superabsorbent polymer (SAP) compositions and a process for preparing said compositions.
Superabsorbent polymers are well-known materials which commonly are used in personal care articles such as diapers. These polymers are known to absorb several times their weight of, for example, water, saline solution, urine, blood, and serous bodily fluids.
The development of increasingly thinner diapers has led to diapers having a high density absorbent core, the core having an increased superabsorbent polymer fraction in the superabsorbent polymer/cellulose fluff mixture. Accordingly, the need to have homogeneous distribution of liquid in the absorbent core is increasingly important in order to fully utilize the highly swellable polymer's storage capacity. However, the distribution of liquid is negatively affected by a reduction in the amount of cellulose fluff in the absorbent core. Thus, additional requirements are now being placed on the highly swellable, liquid-storing polymers in terms of liquid management. The liquid-storing polymers must allow, or support, the distribution of the liquid within the superabsorbent polymer/fluff mix, even in a superabsorbent polymer/fluff mix with a high fraction of highly swellable polymers.
In diapers having a high superabsorbent polymer/fluff ratio, the low fluff content can result in poorer liquid distribution, since the distribution capacity of the fluff is insufficient to compensate for the high absorption rate and absorption capacity of conventional superabsorbent polymers. Accordingly, a very large proportion of body fluid entering a personal care article is absorbed by the superabsorbent polymer in the immediate vicinity of the point of fluid entry.
The swelling of a conventional superabsorbent polymer occurs in such a way that a very high absorption rate is observed immediately after an aqueous liquid is added. After only a few minutes, a highly swellable polymer based on crosslinked, partially neutralized polyacrylate has achieved approximately 95% of its absorption capacity, under conditions of free swelling. This behavior is a typical property of crosslinked polyacrylate superabsorbent polymers.
The rapid expansion of the polymer particles around the point of entry of the body fluid into the area of the absorbent core causes a closing of the interstitial spaces and pores in the SAP-fluff matrix. Since the transport of liquid by diffusion through a swollen hydrogel is much slower than transport through the interstitial spaces, a sealing effect occurs in the area of fluid entry. This effect is often referred to as “gel blocking.” Subsequent amounts of fluid can no longer penetrate into the absorbent core and tend to flow in an uncontrolled manner across the surface of the already superficially saturated area to its edge, resulting in undesirable leakage from the absorbent core.
In addition, the storage capacity of the absorbent core decreases, since as a result of the gel blocking of the particles near the surface, the highly swellable polymers embedded deeper in the absorbent core can no longer be reached by subsequent body fluid doses, and hence can not contribute to the total storage capacity.
One approach to the gel blocking problem is to alter the absorption velocity properties of the superabsorbent polymers. For example, U.S. Pat. No. 4,548,847 describes hydrogels reversibly crosslinked by means of at least divalent metal cations, such as Ca
+2
or Ba
+2
. The patent teaches that a delay in swelling can be accomplished by application of a so-called “cation removal agent.” Preferred examples of these are water-soluble compounds such as Na
2
HPO
4
, sodium hexameta-phosphate and the disodium salt of ethylenediaminetetraacetic acid. The effect of these substances is that the reversible crosslinking sites formed by at least divalent metal cations are destroyed by the cation removal agent. With reduced crosslinking density, the product is able to swell more. The patent also describes absorbent articles that contain the absorbent polymers with the delayed swelling feature. An absorbent article built in layers (wound dressing) is described, for instance, in which each layer contains the polymers according to the invention.
GB 2,280,115 A describes an absorbent article that contains coated superabsorbent particles in the area in which body fluids are released. The coating of the superabsorbent particles prevents swelling until the coating has dissolved in the test or body fluid or has been penetrated by it. These are superabsorbent particles that exhibit an activation time until swelling begins, which time can be varied by the coating's material and thickness. Some of the coating materials disclosed are non-reactive polysaccharides such as gelatin, microcrystalline cellulose and cellulose derivatives. The activation period to the start of swelling should be at least 5, preferably 15 and more strongly preferred, 60 minutes.
Coated superabsorbent polymers have the disadvantage that even a small initial wetting, without necessarily leading to the swelling of the highly swellable polymers, leads to a destruction of the surface treatment by dissolution, detachment, swelling or decomposition. Once the coating around the polymers has dissolved, such superabsorbent polymers exhibit the high swelling rate of a conventional material without surface treatment. Thus the desired effect of improved liquid management in the absorbent material is lost.
EP 0 631 768 A1 describes an absorbent article that uses superabsorbent polymers with different absorption velocities. The differences in absorption velocities between the various conventional superabsorbent polymers used arise from different particle size distributions (type 1: 600-850 &mgr;m, type 2: <250 &mgr;m) and are correspondingly small.
U.S. Pat. No. 5,115,011 addresses the gel blocking problem by contacting a water absorbent polymer with an aqueous solution of two water soluble salts, the first being a halogen, sulfate, acetate or nitrate of aluminum, calcium or magnesium, and the second being a monovalent metal salt or ammonium salt of at least one kind of an oxyacid selected from sulfurous acid and thiosulfuric acid. A dry blend of 0.6 g aluminum sulfate and 30 g polymer is prepared in Example for Comparison 3 of the patent, and is shown to have a blocking of 70 percent or more after 5 minutes.
U.S. Pat. No. 5,578,318 discloses the preparation of superabsorbent “hydrophobic coated particles” by dry blending materials, such as non-crosslinked polyacrylate salts, with a source of multivalent ions and, optionally, then adding an alcohol, certain wetting agents, and polysiloxane derivatives. The wetted material is dried prior to use. Example XXIII of this patent discloses a blend of 2.61 weight percent AQUALON A-250, 0.21 weight percent aluminum acetate, and 97.18 weight percent water. After drying, the resulting material of this example exhibited relatively poor performance as a superabsorbent.
U.S. Pat. No. 4,090,013 discloses materials prepared from a water-soluble anionic polyelectrolyte and a polyvalent metal cation source. However, the products are characterized in U.S. Pat. No. 5,578,318 as exhibiting gel blocking.
U.S. Pat. No. 4,693,713 discloses an absorbent for blood and serous bodily fluids, the absorbent comprising a physical mixture of certain polymers and certain compounds. The compounds are described as water soluble, present in the form of a pourable powder at ambient temperature, and not harmful to health. The patent teaches that the compound may be added to the polymer by dissolving it in the monomer solution, or that the compound can be added to the polymer preparation process at any time in dry or dissolved form. Dry blends of polymer and compound are prepared in the examples of the patent.
The problem of gel blocking has been particularly noted in multilayer absorbent articles. One means of solving this problem has been to incorporate slow-rate polymers into the articles. Several means of producing the
Weir Joseph L.
Wilson Larry R.
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