Process for the production of water-absorbing compositions

Details Process for the production of water-absorbing compositions Process for the production of water-absorbing compositions

1998-09-28

2001-11-13

Lipman, Bernard (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S271000, C526S272000

Reexamination Certificate

active

06316554

ABSTRACT:

The present invention relates to a process for producing water-absorbing compositions of the kind sometimes known as superabsorbent polymers.
Water-absorbing compositions are widely used in the manufacture of products which require high absorption capability; for example, surgical and dental sponges, tampons, sanitary napkins and pads, pant liners, disposable diapers, adult incontinence pads, coverstock for feminine hygiene products, training pads, bandages, wound covers, patient underpads (for example pads of the type described in U.S. Pat. Nos. 3,814,101 4,342,314 and EP 0 052 403 which are incorporated herein by reference), mortuary pads, casket liners, forensic examination pads, meat trays, soaker pads for food use, medical tray pads, fenestration drapes, other medical or surgical related articles, seed germination pads, capillary mats, baby bibs, desiccant strips for anti-rust use, bath mats, packaging, sorbents, clothing, breast pads, underarm pads, industrial wipes, domestic wipes, wipes, filters, cable wrap, food preservation articles, roofing materials, automotive trim, furniture, gasket sealants, pond liners, bedding, cement, household pet litter, soil modifiers, and the like. Water-absorbing compositions are also used for the modification of soil to improve water retention and increase air capacity and for a host of other applications. It will be understood that some of the articles referred to are not per se absorbent but may be rendered absorbent by means of the presence in their structure, or at their surface of absorbent materials.
Water-absorbent compositions suitable for these and other uses may be in any suitable form including powders, fibers, filaments, films and coatings on fibers, filaments, sheets and fabrics.
As used herein, the term “water” when used in the phrases “water-absorbing”, “water-absorbent”, “water-swellable” and the like is understood to mean not only water but also aqueous media such as, in particular, electrolyte solutions such as body fluids and other polar organic liquids.
Water-absorbent compositions may be produced from a variety of polymers. One class of water-absorbent compositions are those produced from a copolymer of an
60
,&bgr; unsaturated monomer having at least one pendant unit selected from a carboxylic acid group and derivatives thereof and a copolymerizable monomer. A proportion of the pendant units are present in the final copolymer as the free acid and a proportion as the salt of the acid. These copolymers are capable of being cross-linked, either internally or with a variety of cross-linking agents, to form the water-swellable composition. Examples of water-absorbent compositions of this type can be found in U.S. Pat. Nos. 4,616,063, 4,705,773, 4,731,067, 4,743,244, 4,788,237, 4,813,945, 4,880,868 and 4,892,533 and European Patent Nos. 0 272 074 and 0 264 208, European Published Application Nos. 0 436 514, 0 269 393, 0 397 410, and 0 232 121 and United Kingdom Patent No. 1 288 563. Other water-absorbent compositions are described in WO93/12275 and WO93/17066. Each of these documents are incorporated herein by reference.
Suitable derivatives of carboxylic acid groups include carboxylic acid salt groups, carboxylic acid amide groups, carboxylic acid imide groups, carboxylic acid anhydride groups and carboxylic acid ester groups.
Suitable &agr;,&bgr; unsaturated monomers having at least one pendant unit selected from a carboxylic acid group and derivatives thereof include maleic anhydride; crotonic acid; fumaric acid; mesaconic acid; the sodium salt of maleic acid; the sodium salt of 2-methyl, 2-butene dicarboxylic acid; the sodium salt of itaconic acid; maleamic acid; maleamide; N-phenyl maleimide; maleimide; maleic anhydride; fumaric anhydride; itaconic anhydride; citraconic anhydride; mesaconic anhydride; methyl itaconic anhydride; ethyl maleic anhydride; diethylmaleate; methylmaleate and their mixtures.
Any suitable copolymerizable comonomer can be employed. Suitable copolymerizable comonomers include ethylene, propylene, isobutylene, C
1
to C
4
alkyl methacrylates, vinyl acetate, methyl vinyl ether, isobutyl vinyl ether, and styrenic compounds having the formula:
wherein R represents hydrogen or an alkyl group having from 1 to 6 carbon atoms and wherein the benzene ring may be substituted. Suitable substituents include low molecular weight alkyl or hydroxy groups.
Suitable C
1
to C
4
alkyl acrylates include methyl acrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n-butyl acrylate and the like and their mixtures.
Suitable C
1
to C
4
alkyl methacrylates include methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-propylmethacrylate, n-butyl methacrylate, and the like, and their mixtures.
Suitable styrenic compounds include styrene, &agr;-methylstyrene, p-methylstyrene, t-butyl styrene and their mixtures.
The pendant units on the &agr;, &bgr;-unsaturated monomer will determine what, if any, additional reactions must be carried out to obtain a copolymer in which a proportion of the pendant units are present as the free acid and a proportion are present as the salt of the acid.
In general, if the &agr;,&bgr;-unsaturated monomer bears only carboxylic acid amide, carboxylic acid imide, carboxylic acid anhydride, carboxylic acid ester groups, or mixtures thereof, it is necessary to convert at least a portion of these carboxylic acid derivative groups to carboxylic acid groups by, for example, an hydrolysis reaction. If the &agr;,&bgr;-unsaturated monomer bears only carboxylic acid salt groups, partial acidification to form carboxylic acid groups will be necessary.
One copolymer that is particularly suitable for use in the production of water-absorbing composition is a copolymer of maleic anhydride and isobutylene. Another suitable copolymer is that of maleic anhydride and styrene copolymer. Suitable copolymers will preferably have peak molecular weights of from about 5,000 to about 500,000 or more.
Suitable copolymers of maleic anhydride and isobutylene can be prepared using any suitable conventional method. Such copolymers are also commercially available from Kuraray Isoprenc Chemical Company, Ltd., Tokyo, Japan, under the trade mark ISOBAM. ISOBAM copolymers are available in several grades which are differentiated by viscosity molecular weight: ISOBAM-10, 160,000 to 170,000; ISOBAM-06, 80,000 to 90,000; ISOBAM-04, 55,000 to 65,000; and ISOBAM-600, 6,000 to 10,000.
The copolymer of the &agr;,&bgr;-unsaturated monomer described above and the copolymerisable monomer may be combined with a cross-linking agent having two or more groups reactive with functional groups on the polymer chain, for example hydroxyl or heterocyclic carbonate groups. In one alternative, it may crosslink intramolecularly in the absence of an external cross-linking agent by means of covalent or hydrogen bonds.
The water-absorbent composition preferably comprises from about 20 to about 80 wt % pendant carboxylic acid groups and from about 80 to about 20 wt % pendant carboxylate salts and a suitable amount of a cross-linker if required.
Any suitable free radical polymerization catalyst can be used to catalyze the polymerization reaction. Suitable catalysts include benzoyl peroxide, such as that sold under the trade name PERKADOX and the VAZO azobisisobutyronitrile catalysts which are commercially available from E. I. DuPont de Nemours. VAZO-64 and VAZO-67 catalysts are suitable for use in this invention at amounts of from about 0.2 to about 2 mole percent or greater based on the amount of &agr;,&bgr; unsaturated monomer such as maleic anhydride.
The water-absorbing composition of this invention can be prepared using any suitable blending method. After the water-absorbing composition is prepared, but typically before it is cured, but in some instances as it is curing, it is processed into any desired form using conventional methods of fabrication. For example, the water-absorbing composition can be subjected to casting; spray drying; air-assisted spray drying; air attenuation; or wet, dry or flash spinning. In addition,

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