Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2000-08-18
2002-10-29
Wilson, D. R. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S329900, C525S375000, C524S556000, C524S916000, C442S417000, C604S367000
Reexamination Certificate
active
06472478
ABSTRACT:
The present invention relates to a process for the gel or surface postcrosslinking of water-absorbing hydrogels by copolymerization with 2-oxazolidinones, to the polymers obtainable in this way and to their use in hygiene articles, packaging materials and nonwovens.
Hydrophilic highly swellable hydrogels are, in particular, polymers composed of (co)polymerized hydrophilic monomers, or are graft (co)polymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose ethers or crosslinked starch ethers, crosslinked carboxymethylcellulose, partially crosslinked polyalkylene oxide, or natural products that are swellable in aqueous liquids: 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, hydrbphilic 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 and 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, since it necessitates the taking of special protective measures in the production plant in order to avoid unwanted side effects. In addition, the abovementioned crosslinkers possess skin-irritant properties, which appears problematic in their use in hygiene articles.
Polyfunctional alcohols are also known crosslinkers. For example, EP-A-0 372 981, U.S. Pat. No. 4,666,983 and U.S. Pat. No. 5,385,983 teach the use of hydrophilic polyalcohols and 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 slow even at such temperatures.
The object was therefore, using compounds which are relatively slow to react yet are reactive with carboxyl groups, to achieve just as good if not better gel or surface postcrosslinking. This object is to be achieved with a very short reaction time and a very low reaction temperature. Ideally, the prevailing reaction conditions should be the same as those obtaining when highly reactive epoxides are used.
It has surprisingly now been found that this object can be achieved to outstanding effect with 2-oxazolidinones as crosslinkers. In particular, the moderate 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 gel or 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 and dried by means of an increase in temperature, if the rosslinker is a compound of the formula
in which R
1
and R
2
independently of one another are H, hydroxyl, phenyl or C
1
-C
6
-alkyl, R
3
is hydrogen, C
1
-C
12
-alkyl, N-hydroxy-(C
2
-C
6
)-alkyl, C
1
-C
12
-alkenyl or C
6
-C
12
-aryl and R
4
and R
5
independently of one another are C
1
-C
12
-alkyl, C
1
-C
12
-alkenyl, C
6
-C
12
-aryl, hydroxyl, C
1
-C
12
-alkoxy or hydrogen, dissolved in an inert solvent. Examples of preferred and suitable crosslinkers of this type are 2-oxazolidinone, N-methyl-2-oxazolidinone and N-hydroxyethyl-2-oxazolidinone.
The preferred temperature range for postcrosslinking and drying is that between 50 and 250° C., in particular 50-200° C. and, with particular preference, the range between 100-180° C. The surface postcrosslinking solution is preferably applied to the polymer by spraying in suitable spray mixers. 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 or mixing and drying systems, such as 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 preferred residence time at this temperature in the reaction mixer or dryer is less than 30 minutes, with particular preference less than 10 minutes.
In one preferred embodiment of the invention the reaction is accelerated by adding an acidic catalyst to the surface postcrosslinking solution. Catalysts which can be used in the process of the invention are all inorganic acids, their anhydrides, and organic acids. Examples are boric, sulfuric, hydroiodic, phosphoric, tartaric, acetic and toluenesulfonic acid. Also suitable in particular are their polymeric forms, anhydrides, and the acid salts of the polybasic acids. Examples thereof are boron oxide, sulfur trioxide, diphosphorus pentoxide, and ammonium dihydrogen phosphate.
The crosslinker is dissolved in inert solvents. The crosslinker is used in an amount of from 0.01-1.0% by weight based on the polymer employed. As an inert solvent, preference is given to water and to mixtures of water with monohydric or polyhydric alcohols. It is, however, possible to employ any organic solvent of unlimited miscibility with water which is not itself reactive under the process conditions. Where an alcohol/water mixture is employed the alcohol content of this solution is, for example, 10-90% by weight, preferably 30-70% by weight, 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). The alcohol mixtures may comprise the alcohols in any desired mixing ratio. 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 also 1,3-propanediol.
In another preferred embodiment of the invention the surface postcrosslinking solution is employed in a proportion of 1-20% by weight based on the mass of the polymer. Particular preference is given to an amount of solution of 2.5-15% by weight based on the polymer.
The invention additionally provides crosslinked water-absorbing polymers which are obtainable by the process of the invention.
The hydrophilic highly swellable hydrogels to be employed in the process of the invention are in particular, polymers composed of (co)polymerized hydrophilic monomers, or are graft (co)polymers of one or more hydrophilic monomers on a suitable graft base, crosslinked cellulose ethers or crosslinked starch ethers, or natural products which are swellable in aqueous liquids: guar derivatives, for example. These hydrogels are known to the person skilled in the art and are described, for example, in U.S. Pat. No. 4,286,082, DE-C-27 06 135, U.S. Pat. No. 4,340,706, DE-C-37 13 601, DE-C-28 40 010,
Daniel Thomas
Engelhardt Fritz
Frenz Volker
Funk Rüdiger
Riegel Ulrich
BASF - Aktiengesellschaft
Wilson D. R.
LandOfFree
Process for crosslinking hydrogels with bis- and poly-2-... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for crosslinking hydrogels with bis- and poly-2-..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for crosslinking hydrogels with bis- and poly-2-... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2967369