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...
Patent
1995-08-11
1997-05-13
Smith, Jeffrey T.
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...
5253295, 5253297, 5253303, 525367, 524827, C08L 3102
Patent
active
056293771
DESCRIPTION:
BRIEF SUMMARY
This invention relates to water-absorbent particles of crosslinked carboxyl-containing polymers which have both high absorption capacity and high absorption capacity under load, and methods for preparing such particles and absorbent structures incorporating the particles.
Water-absorbent resin particles, also referred to as aqueous fluid absorbent polymers or superabsorbent polymers, are primarily used in personal care products which absorb body fluids, for example, baby diapers, adult incontinence products and feminine hygiene products. In such applications, water-absorbent resin particles are incorporated into absorbent structures which contain synthetic and/or natural fiber or paper based, woven and nonwoven structures, or toughened masses of fibers, such as fluff pads. The materials used in such structures can quickly absorb aqueous fluids and distribute them over the whole absorbent structure. The structures, in the absence of water-absorbent resin particles, have limited absorption capacity, are bulky due to the large amount of material needed to provide acceptable absorption capacity, and do not retain fluid under pressure. A means for improving the absorbency and fluid retention characteristics of such absorbent structures is to incorporate water-absorbent resin particleswhich imbibe fluids to form a swollen hydrogel material.
The water-absorbent resin particles quickly absorb fluids and retain such fluids to prevent leakage and give the absorbent structure a "dry feel" even when wetted. See U.S. Pat. No. 4,610,678 for examples of such resins. See also Brandt U.S. Pat. Nos. 4,654,039 and Re. 32,649, which discloses a process for the preparation of water-absorbent resins and the use of known crosslinking agents for such resins, and also Parks U.S. Pat. No. 4,295,987 and Japan Patent No. 55-82104. A variation of the basic process is taught in GB Patent No. 2,119,384, which discloses a post polymerization surface crosslinking process in which the previously polymerized absorbent resin powder is mixed with crosslinkers, preferably polyalcohols, a solvent and water, to coat the resin surface and heated to temperatures in the range of 90.degree. C. to 300.degree. C. to crosslink the surface. Commonly assigned Great Britain Patent Application No. 9208449.0 discloses water-absorbent resin particles comprising polymers containing carboxyl moieties which are crosslinked using C.sub.2-10 polyhydric hydrocarbons which are ethoxylated with from 2 to 8 ethylene oxide units per hydroxyl moiety of the ethylene oxide chain wherein the hydroxyl moiety at the end of each chain is esterified with a C.sub.2-10 unsaturated carboxylic acid or ester thereof. In a preferred embodiment, the water-absorbent resin particles are subjected to a heat-treatment after drying and sizing the particles.
To reduce the amount of absorbent resin required it is desirable for the absorption capacity of resin to be as high as possible. This is normally accomplished by adjustment of the crosslinker level within the resin particle. An increase in absorption capacity, however, is generally accompanied by a decrease in the ability of the resin particles to absorb liquid under an applied load. This absorption under load (AUL) is important to the performance of the resin, with higher values being preferred. Most commercially available resin particles demonstrate good AUL at low loads, such as 2.1 kPa (0.3 pound per square inch) (psi where 1 psi=6894.76 Pa). Unfortunately, these resins demonstrate a dramatic drop in absorption capacity as the load is increased to higher levels, such as 4.1 kPa (0.6 psi), 5.5 kPa (0.8 psi) or 6.9 kPa (1.0 psi). What is needed is a water-absorbent resin particle which retains most of its absorption capacity as the load increases.
A further problem with commercially available water-absorbent resin particles is the presence of monomers or low molecular weight polymer which may leach out. This monomer or low weight polymer represents additional raw material cost which provides at best a possible marginal benefit, if a
REFERENCES:
patent: Re32649 (1988-04-01), Brandt et al.
patent: 4295987 (1981-10-01), Parks
patent: 4415388 (1983-11-01), Korpman
patent: 4610678 (1986-09-01), Weisman et al.
patent: 4654039 (1987-03-01), Brandt et al.
Derwent Abstract, JP 55082104, Union Carbide Corp., Jun. 20, 1980.
Buchholz Fredric L.
Burgert Josef H.
Christensen Stephen B.
Gartner Herbert A.
Graham Andrew T.
Smith Jeffrey T.
The Dow Chemical Company
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