Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2001-08-03
2002-02-19
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S103000, C162S108000, C162S157600, C604S368000, C604S374000, C604S375000, C210S500210, C210S500290
Reexamination Certificate
active
06348133
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a wet-laid nonwoven structure containing fibers and water insoluble, water-swellable, superabsorbent, ion sensitive polymer particles (SAP) and process for making an absorbent structure. The structure is intended for use in absorbent hygiene products such as diapers, incontinence pads, sanitary napkins and tampons and in wiping materials for mopping up spills of fluids. A wet-laid nonwoven fabric is a fabric comprising fibers which have been deposited from an aqueous suspension onto a moving foraminous support.
BACKGROUND OF THE INVENTION
Fibrous, non-woven, superabsorbent, ion sensitive polymer-impregnated structures are known.
See generally, U.S. Pat. Nos. 5,167,764, 5,607,550, 5,516,585 and European Publication No. 437,816. Additionally, the following references disclose previously attempted methods of handling superabsorbent or hydrogel ion sensitive polymers to obtain superabsorbent structures. See U.S. Pat. No. 3,669,103; 4,610,678; 4,986,882; 5,049,235; 5,137,600; 5,160,789; 5,443,899; 5,531,728; and 5,547,745. See U.S. Pat. No. 5,516,585 teaching the well known binder polymers suggested for use with air laid nonwoven structures containing SAP.
EP-A-437816 discloses a nonwoven wet-laid superabsorbent material produced by the process of blending superabsorbent, ion sensitive polymer particles with a liquid to form a slurry, mixing particles with that slurry, filtering that slurry/fibre mixture to remove a portion of the liquid and drying the superabsorbent slurry/fiber mixture to form a nonwoven wet-laid superabsorbent material.
EP-A-359615 discloses a method for the manufacture of a superabsorbent fibrous structure in which a dry solid absorbent is applied directly to a wet-laid web of cellulosic fibers prior to drying the wet web.
EP-A-273075 discloses a high water-absorbency paper made by sheeting a mixture of wood pulp fiber, water-soluble resin and high water-absorbency resin.
A number of techniques for applying binders to webs of fibers are known. For example, U.S. Pat. No. 4,600,462 of Watt describes a process in which an adhesive binder is sprayed onto one or both surfaces of an air laid cellulose fiber web. Submersion of the web in the adhesive binder is another method disclosed in this patent of applying the binder. Individual binder coated fibers for mixing with other fibers are not produced by this process. A hydrophile solution is also all applied to the web. As another example, U.S. Pat. Nos. 4,425,126 and 4,129,132 of Butterworth, et al. describe a fiberous material formed by combining thermoplastic fibers and wood pulp, heat fusing the combined fibers, and thereafter depositing a binder on the heat fused web. Because the fibers are heat fused prior to adding the binder, individual binder coated fibers for mixing with other fibers are not produced by this process.
Absorbent products such as diapers which include particles of a superabsorbent ion sensitive polymer such as crosslinked sodium polyacrylate disposed between layers of wood pulp are known for example from EP-A-257951.
The use of fibers of water-swellable water-insoluble superabsorbent, ion sensitive polymer is disclosed in U.S. Pat. No. 5,607,550, wherein it is taught that incorporation of superabsorbent, ion sensitive polymers in particulate form in the fiber web has significant disadvantages in many respects. The prior art teaches that superabsorbent, ion sensitive polymer particles are less securely retained both during formation of the wet-laid nonwoven structure and when the structure is in further processed during incorporation into an absorbent product. Moreover the art indicates a relatively less uniform dispersion of superabsorbent, ion sensitive polymer particles in the web occurs as opposed to the dispersion of the SAP fiber in the web. It is also taught conventionally that with superabsorbent, ion sensitive polymer particle-impregnated structures, the particles become loosely attached to the fibrous structure of the nonwoven fabric.
In order to provide sufficient absorbency performance necessary for utilization in an absorbent article, it has been found that the loading of superabsorbent in a web must be at least about 50% by weight of the entire structure. However, significant loadings of particles in the fiber structure (such as above about 50% SAP particles on the total weight of the web) require sufficient strength of the wet web in the process
Whereas the cost associated with forming fibers of superabsorbent, ion sensitive polymer is inherently higher than that of the particulate SAP, it would be desirable to overcome the aforementioned drawbacks in the use of particles of SAP. Composite structures of fibers impregnated with superabsorbent, ion sensitive polymer particles could greatly reduce the manufacturing cost of end use products such as those aforementioned, however when forming such a structure with 50% or higher loadings of particles of SAP, the web thus formed has a rough texture which can be felt in a disposable article having a top sheet. The texture is telegraphed through the article and may provide undesirable comfort for the user.
Accordingly, there is an unmet need to develop improved superabsorbent, ion sensitive polymer particle-impregnated structures having improved texture for their intended uses. An improved superabsorbent, ion sensitive polymer particle-impregnated structure has been isolated and found to exhibit unexpected smooth texture and surprisingly good absorbency.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a non-woven, wet-laid, superabsorbent, ion sensitive polymer particle-impregnated fibrous structure having smooth surface texture and which is free of binder polymer. All percentages specified herein are weight percentages. Specifically, the structure comprises from 50% to 80% of ion sensitive SAP having a particle size of less than 250 microns, wood pulp fibers and cellulose acetate fibers, each in preferred specified amounts indicated below. The superabsorbent, ion sensitive polymer particle-impregnated fibrous structure is made from a aqueous, wet-lay process in which an aqueous furnish comprises )lids of fibers and superabsorbent, ion sensitive polymer. The aqueous furnish contains 0.2% to 5% or more dissolved salt, which is preferredly Na
2
SO
4
. The furnish is passed over a moving foraminous support, such as a Fourdrinier wire, and a wet web structure is formed. The wet web structure is conveyed to an in-line washing zone with a sufficient volume of water, preferably impinging the wet web in a continuous curtain flow, and the washed wet web structure is dried to form the non-woven, wet-laid, superabsorbent, ion sensitive polymer particle-impregnated structure (web).
The wet-laid, superabsorbent, ion sensitive polymer particle-impregnated structure, on a dry weight basis, comprises about 50% to about 80% water insoluble, water swellable ion sensitive polymer (SAP) and about 20% to about 50% fibers (fibrous portion). The fibrous portion of the web comprises 5% to 50% cellulose acetate fibers and 50% to 95% pulp fibers. Preferably the fibrous portion comprises 10% to 50% cellulose acetate fibers and 50% to 90% wood pulp fibers. More preferredly the fiberous portion comprises 10% to 40% cellulose acetate fibers and 60% to 90% wood pulp fibers. Most preferredly, the fibrous portion comprises 5% to 20% cellulose acetate fibers and 80% to 95% wood pulp fibers.
The wet-laid, superabsorbent, ion sensitive polymer particle-impregnated structure contains from 0% to 35% residual salt in the dried web, preferably from 5% to 35% such residual salt remains, and most preferably 10% to 16% residual salt. The level of residual salt on the total weight of the dried web has been found to correlate inversely with the AUL absorbency performance, described below. The minimum AUL absorbency of the wet-laid, superabsorbent, ion sensitive polymer particle-impregnated web which contains from 5% to 35% residual salt is greater than or equal to about 13 g/g. AUL on the basis of SAP polymer
Banchik David T.
BASF Corporation
Fortuna Jose
LandOfFree
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