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
2001-05-17
2003-12-02
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...
Reexamination Certificate
active
06657015
ABSTRACT:
DESCRIPTION
The present invention relates to a process for the gel or surface postcrosslinking of water-absorbent hydrogels with 2-oxotetrahydro-1,3-oxazines, the polymers thus obtainable and their use in hygiene articles, packaging materials and nonwovens.
Hydrophilic, highly swellable hydrogels are in particular polymers of (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable grafting base, crosslinked cellulose or starch ethers, crosslinked carboxymethylcellulose, partly crosslinked polyalkylene oxide or natural products that are swellable in aqueous fluids, for example guar derivatives. Such hydrogels are used as products for absorbing aqueous solutions in the manufacture of diapers, tampons, sanitary napkins and other hygiene articles, and as water retainers in market gardening.
To improve application properties, for example diaper rewet and absorbency under load (AUL), hydrophilic, highly swellable hydrogels are generally surface or gel postcrosslinked. This postcrosslinking is preferably carried out in the aqueous gel phase or as surface postcrosslinking of the ground and classified polymer particles.
Useful crosslinkers for this purpose include compounds containing at least two groups capable of entering covalent bonds with the carboxyl groups of the hydrophilic polymer. Useful compounds include for example di- or polyglycidyl compounds, such as diglycidyl phosphonate, alkoxysilyl compounds, polyaziridines, polyamines or polyamidoamines, and these compounds can also be used in mixtures with each other (see for example EP-A-0 083 022, EP-A-0 543 303 and EP-A-0 530 438). Polyamidoamines useful as crosslinkers are described in EP-A-0 349 935 in particular.
A major disadvantage of these crosslinkers is their high reactivity, since it necessitates particular precautions in production to avoid undesirable side effects. Moreover, the aforementioned crosslinkers have skin-irritating properties, which makes their use in hygiene articles problematical.
Known crosslinkers also include polyfunctional alcohols. For instance, 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. The reaction is carried out at 120-250° C. The process has the disadvantage that the esterification which leads to crosslinking is very slow even at such temperatures.
Prior German Patent Application DE-A-19 807 502 describes a process for postcrosslinking with 2-oxazolidinones.
It is an object of the present invention to provide gel or surface postcrosslinking equivalent to or superior to the prior art by using relatively inert compounds capable of reacting with carboxyl groups. This object is to be achieved with a very short reaction time and a very low reaction temperature.
We have found that this object is achieved, surprisingly, when 2-oxotetrahydro-1,3-oxazines are used as crosslinkers. More particularly, the moderate reactivity of the crosslinkers can be boosted with inorganic or organic acidic catalysts. Useful catalysts include known inorganic mineral acids, their acidic salts with alkali metals or ammonium and also their corresponding anhydrides. Useful organic catalysts include known carboxylic acids, sulfonic acids and amino acids.
The invention accordingly provides a process for the gel and/or surface postcrosslinking of water-absorbent polymers by the polymer being treated with a surface postcrosslinking solution and being postcrosslinked and dried during and after the treatment by raising the temperature, wherein the crosslinker comprises a compound of the formula I
where R
1
is hydrogen, C
1
-C
4
-alkyl, C
1
-C
4
-hydroxyalkyl, trialkylsilyl or acetyl and R
2
, R
2′
, R
3
, R
3′
, R
4
, R
4′
are each independently hydrogen, C
1
-C
12
-alkyl, C
1
-C
12
-alkenyl or C
6
-C
12
-aryl, dissolved in an inert solvent.
The postcrosslinking and drying temperature is preferably 50-250° C., especially 50-200° C., most preferably 100-180° C. The surface postcrosslinking solution is preferably sprayed onto the polymer in suitable spray mixers. Following spray application, the polymer powder is dried thermally, and the crosslinking reaction can take place not only before but also during the drying. Preference is given to spray application of a solution of the crosslinker in reaction mixers or mixing and drying systems such as, for example, Lödige mixers, BEPEX® mixers, NAUTA® mixers, SHUGGI® mixers or PROCESSALL®. Moreover, fluidized-bed dryers may also be used.
Drying may take place in the mixer itself, by heating the outer casing or by blowing hot air in. It is similarly possible to use a downstream dryer such as a tray dryer, a rotary tube dryer or a heatable screw. But it is also possible, for example, to use an azeotropic distillation as a drying technique. The preferred residence time at this temperature in the reaction mixer or dryer is less than 60 min, particularly preferably less than 30 min.
In a preferred embodiment of the invention, the reaction is accelerated by adding an acidic catalyst to the surface postcrosslinking solution. Useful catalysts for the process of the invention include all inorganic acids, their corresponding anhydrides, and organic acids. Examples are boric acid, sulfuric acid, hydroiodic acid, phosphoric acid, tartaric acid, acetic acid and toluenesulfonic acid. More particularly their polymeric forms, anhydrides and also the acidic salts of the polybasic acids are also suitable. Examples of these are boron oxide, sulfur trioxide, diphosphorus pentoxide and ammonium dihydrogenphosphate.
The crosslinker is dissolved in inert solvents. The crosslinker is used in an amount of from 0.01 to 5%, preferably 0.01-1.0%, preferably from 0.05 to 0.5%, by weight, based on the polymer used. The preferred inert solvent is water or a mixture of water with mono- or polyhydric alcohols. However, it is also possible to use any unlimitedly water-miscible organic solvent which is not itself reactive under the process conditions. When an alcohol-water mixture is used, the alcohol content of this solution is for example 10-90% by weight, preferably 30-70% by weight, especially 40-60% by weight. Any alcohol of unlimited miscibility with water can be used, as can mixtures of two or more alcohols (eg. methanol+glycerol+water). The alcohol mixtures may contain the alcohols in any desired mixing ratio. However, it is particularly preferable to use the following alcohols in aqueous solution: methanol, ethanol, isopropanol, ethylene glycol and particularly preferably 1,2-propanediol and 1,3-propanediol.
In a further preferred embodiment of the invention, the surface postcrosslinking solution is used in a ratio of 1-20% by weight, based on the mass of the polymer. Particular preference is given to a solution quantity of 0.5-10% by weight, based on the polymer.
The invention further provides crosslinked water-absorbent polymers that are obtainable by the process according to the invention.
The hydrophilic, highly swellable hydrogels to be used in the process of the invention are in particular polymers of (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable grafting base, crosslinked cellulose or starch ethers or natural products swellable in aqueous fluids, for example guar derivatives. Preferably the polymer to be crosslinked is a polymer containing sstructural units derived from acrylic acid or its esters, or obtained by graft copolymerization of acrylic acid or acrylic esters onto a water-soluble polymer matrix. These hydrogels are known to one 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, DE-A-43 44 548, DE-A-40 20 780, DE-A-40 15 085, DE-A-39 17 846, DE-A-38 07 289, DE-A-35 33 337, DE-A-35 03 458, DE-A-42 44 548, DE-A-42 19 607, DE-A-40 21 847, DE-A-38 31 261, DE-A-35 11 086, DE-A-31 18 172, DE-A-30 28 043, DE-A-44 18 881, EP-A-0 801 483, EP-A-0 455 985, EP-A-0 467
Daniel Thomas
Engelhardt Fritz
Frenz Volker
Riegel Ulrich
Weismantel Matthias
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wilson D. R.
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