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
2000-08-28
2003-01-07
Wilson, D. 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...
C524S916000, C525S107000, C525S327600, C525S329900
Reexamination Certificate
active
06503979
ABSTRACT:
The present invention relates to a process for the gel or surface postcrosslinking of water-absorbing hydrogels by copolymerization of bis- and poly-2-oxazolidinones.
Hydrophilic highly swellable hydrogels are, in particular, polymers composed of (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose ethers or starch ethers, crosslinked carboxymethylcellulose, partially crosslinked polyalkylene oxide, or natural products that are swellable in aqueous liquids, such as guar derivatives, for example. Hydrogels of this kind are used as products for absorbing aqueous solutions in the production of diapers, tampons, sanitary towels and other hygiene articles, and as water retainers in market gardening.
To improve service properties such as diaper rewet and AUL, for example, hydrophilic highly swellable hydrogels are generally subjected to surface or gel postcrosslinking. This postcrosslinking is known to the person skilled in the art and is preferably carried out in the aqueous gel phase or as surface postcrosslinking of the milled and sieved polymer particles.
Crosslinkers suitable for this purpose are compounds comprising at least two groups which are able to form covalent bonds with the carboxyl groups of the hydrophilic polymer. Examples of suitable crosslinkers are diglycidyl or polyglycidyl compounds, such as diglycidyl phosphonate, alkoxysilyl compounds, polyaziridines, polyamines or polyamidoamines, and these compounds can also be used in mixtures with one another (see for example EP-A-0 083 022, EP-A-0 543 303 and EP-A-0 530 438). Polyamidoamines which are suitable as crosslinkers are described in particular in EP-A-0 349 935.
A major disadvantage of these crosslinkers is their high reactivity. Although this is desirable in terms of chemical conversion it harbors a relatively high toxicological potential. The processing of such crosslinkers in the production plant requires special protective measures in order to meet the requirements of the relevant safety provisions and of occupational hygiene. In addition, the use of polymers modified in this way within hygiene articles appears questionable.
Polyfunctional alcohols are also known for use as crosslinkers. For example, EP-A-0 372 981, U.S. Pat. Nos. 4,666,983 and 5,385,983 teach the use of hydrophilic polyalcohols or the use of polyhydroxy surfactants. According to these documents the reaction is carried out at temperatures of 120-250° C. The process has the disadvantage that the esterification reaction which leads to crosslinking is relatively slow even at such temperatures.
The object was therefore, using compounds which are relatively slow to react yet which are reactive with carboxyl groups, to achieve gel or surface postcrosslinking which is as good if not better than that of the prior art, with as short as possible a reaction time and as low as possible a reaction temperature. Ideally, the prevailing reaction conditions should be the same as those obtained when highly reactive epoxides are used.
It has surprisingly now been found that this object can be achieved to outstanding effect with bis- and poly-2-oxazolidinones. In particular, the reactivity of these crosslinkers can be increased by adding organic or inorganic acidic catalysts. Suitable catalysts are the known inorganic mineral acids, their acidic salts with alkali metals or with ammonium, and their anhydrides. Suitable organic catalysts are the known carboxylic, sulfonic and amino acids.
The invention provides a process for the surface postcrosslinking of water-absorbing polymers in which the polymers are treated with a surface postcrosslinking solution and during or after the treatment are postcrosslinked by means of an increase in temperature and are dried, wherein the crosslinker comprises a bis-2-oxazolidinone or a poly-2-oxazolidinone comprising structural units of the formula
in which R
1
is branched or unbranched C
1
-C
18
-alkylene, branched or unbranched C
2
-C
18
-alkenylene, C
5
-C
8
-cycloalkylene, phenylene, naphthylene, anthracenylene, hydrocarbon-substituted phenylene, naphthylene or anthracenylene or another substituted or unsubstituted C
6
-C
18
-arylene radical, R
2
is branched or unbranched C
1
-C
18
-alkylene and n is an integer from 1 to 50 or a mixture of bis-2-oxazolidinones and poly-2-oxazolidinones dissolved in an inert solvent.
Where R
1
is an alkylene or alkenylene radical it is preferably one having a chain length of from 3 to 12, in particular from 5 to 10 carbon atoms. R
2
is preferably an alkylene radical having a chain length of from 3 to 12, in particular from 5 to 10, carbon atoms.
Terminal structural units of the formula 1 are endgroup-capped. The endgroup used can be any radical which can be introduced into the bis-2-oxazolidinones or poly-2-oxazolidinones and is chemically stable on these compounds. Examples of suitable radicals with which the structural units of the formula 1 can be endgroup-capped are hydrogen, branched or unbranched C
1
-C
18
-alkyl, branched or unbranched C
2
-C
18
-alkenyl, phenyl, naphthyl, anthracenyl, hydrocarbon-substituted phenyl, naphthyl or anthracenyl or another substituted or unsubstituted C
6
-C
18
-aryl radical.
The poly-2-oxazolidinones preferably comprise n bis-2-oxazolidinone units.
Preferably n is a number between 1 and 10, with particular preference between 3 and 6. The postcrosslinking temperature is preferably between 50 and 250° C., in particular 50-200° C. and especially 100-180° C.
To accelerate the surface postcrosslinking reaction, an acidic catalyst may be added to the reaction mixture. Catalysts which can be used in the process of the invention are all inorganic acids, their corresponding anhydrides, and/or organic acids and their corresponding anhydrides. Examples are boric, sulfuric, hydroiodic, phosphoric, tartaric, acetic and toluenesulfonic acid. Also suitable in particular are their polymeric forms, anhydrides, and the acid salts as occur, for example, in the case of the polybasic acids. Examples thereof are boron oxide, sulfur trioxide, diphosphorus pentoxide, and ammonium dihydrogen phosphate.
The process of the invention is preferably conducted by spraying a solution of the surface postcrosslinker onto the dry base polymer powder. Following spray application, the polymer powder is dried thermally, it being possible for the crosslinking reaction to take place either before or during drying. Preference is given to the spray application of a solution of the crosslinker in reaction mixers and spray mixers or mixing and drying systems, such as, for example, Lödige mixers, ®BEPEX mixers, ®NAUTA mixers, ®SHUGGI mixers or ®PROCESSALL apparatus. It is, moreover, also possible to employ fluidized-bed dryers. Drying can take place in the mixer itself, by heating the outer casing or by blowing in hot air. Likewise suitable is a downstream dryer, such as a shelf dryer, a rotary dryer or a heatable screw. Alternatively, azeotropic distillation, for example, can be utilized as a drying technique. The residence time at the preferred temperature in the reaction mixer or dryer is from 5 to 90 minutes, preferably less than 30 minutes and, with very particular preference, less than 10 minutes.
As an inert solvent, preference is given to water and to mixtures of water with simple or polyfunctional alcohols. It is, however, possible to employ all organic solvents of unlimited miscibility with water, examples being certain esters and ketones, which are not themselves reactive under the process conditions. Where an alcohol/water mixture is employed the alcohol content of this solution is 10-90% by weight, preferably 30-70% by weight and, in particular, 40-60% by weight. Any alcohol of unlimited miscibility with water can be employed, as can mixtures of two or more alcohols (e.g. methanol+glycerol+water). Particular preference is given to the use of the following alcohols in aqueous solution: methanol, ethanol, isopropanol, ethylene glycol and, with particular preference, 1,2-propanediol and
Daniel Thomas
Engelhardt Fritz
Frenz Volker
Funk Rüdiger
Riegel Ulrich
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wilson D. R.
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