Compositions – Humidostatic – water removive – bindive – or emissive
Reexamination Certificate
2001-04-06
2003-05-20
Lovering, Richard D. (Department: 1712)
Compositions
Humidostatic, water removive, bindive, or emissive
C264S177170, C516S108000, C523S312000, C524S801000, C524S832000, C524S833000, C524S916000, C526S932000, C528S930000
Reexamination Certificate
active
06565768
ABSTRACT:
This application is a 371 of PCT/EP 99/07176, filed Sep. 28, 1999.
The present invention relates to a process for preparing water-swellable hydrophilic polymers, to the polymers obtained thereby and to the use of these polymers.
Hydrophilic hydrogels are obtainable by polymerization of unsaturated acids, for example acrylic acid, methacrylic acid, acrylamidopropanesulfonic acid, etc., in the presence of small amounts of multiply olefinically unsaturated compounds are already known as superabsorbent polymers.
Also known are hydrophilic hydrogels obtainable by graft copolymerization of the olefinically unsaturated acids onto different matrices, for example polysaccharides, polyalkylene oxides and derivatives thereof.
The hydrogels mentioned are notable for high absorbency for water and aqueous solutions and are therefore widely used as absorbants in hygiene articles.
Such water-swellable hydrophilic polymers are generally prepared by free-radical polymerization in an aqueous solution which contains the monomers with or without a grafting base and crosslinker.
The water-swellable hydrophilic polymers produced for use in the hygiene and sanitary sector have a degree of neutralization in the range from 60 to 85 mol % based on the polymerized acid-functional monomer units, so that the hydrogels formed in use are pH neutral with regard to the skin.
The degree of neutralization is generally set prior to the polymerization, since this avoids the technically difficult neutralization of an acidic hydrogel of high viscosity. However, the polymerization of, for example, acrylic acid in the neutral pH range is slower, and leads to lower molecular weights, than the polymerization in the acidic range. This is explained by the electrostatic repellency between the most recently incorporated monomer unit and the next monomer unit to be incorporated, which repellency arises only minimally, if at all, in the case of a polymerization in the acidic pH range, since the monomer units are present in the uncharged, acidic form.
The trend toward ever thinner diaper constructions requires water-swellable hydrophilic polymers providing better and better performance characteristics with regard to absorption capacity, gel strength, gel permeability and residual extractables.
The desired combination of high absorption, high gel strength, high gel permeability and low residual extractables can only be provided by a polymerization where very high molecular weights are obtained for the primary polymer chains. The preferred way to provide such products is therefore a polymerization in aqueous solution where the acid-functional monomer units present in the monomer solution are only partly preneutralized, if at all. The degree of neutralization of the acid-functional monomers is preferably in the range from 0 to 40 mol %, particularly preferably in the range from 0 to 25 mol %.
Polymerization and subsequent coarse comminution provides acidic hydrogel particles which have to be adjusted to the desired ultimate degree of neutralization of 60-85 mol % based on acid-functional monomer units by neutralizing these acid-functional monomer units. This neutralization is a process which is technically difficult to carry out and which has to meet particular requirements. First, the gel must not be excessively sheared during the contacting with the neutralizing agent so as to avoid increasing the extractables content, which would have an adverse effect on the properties of the end product and accordingly is undesirable. Secondly, neutralization has to be completely homogeneous in order that sufficiently good drying characteristics may be obtained for the gel particles. This is because acidic hydrogel particles having a low degree of neutralization are very tacky and are incapable in the subsequent belt drying of forming the loose assembly that is needed if high drying rates are to be obtained.
The subsequent neutralization of acidic hydrogels is known in principle.
DE-A-26 12 846 discloses a process for preparing a water-absorbing resin by polymerizing at least one starch and/or cellulose with at least one water-soluble monomer having a polymerizable double bond and with a crosslinker. The polymers obtained are neutralized with bases, although the method of neutralization is not more particularly specified.
According to EP-A-0 205 674, acidic polymers are prepared at from 0 to 100° C., preferably from 5 to 40° C., and their pH is adjusted by subsequent partial neutralization of the hydrogel. Neutralization is effected here by adding the gel to a very dilute sodium hydroxide solution. This method is disadvantageous, since large amounts of water have to be evaporated at the drying stage owing to the very dilute nature of the sodium hydroxide solution.
EP-A-0 303 440 describes the production of a hydrated crosslinked gel polymer which has 10 to 50 mol % of the acid-functional monomers neutralized and which is adjusted to the desired ultimate degree of neutralization by adding a neutralizing agent in a reaction vessel having a plurality of rotary shafts each fitted with stirring blades. True, this process provides homogeneous neutralization, since new surfaces are constantly being generated for the gel particles, but the shearing force on the gel is too high and leads to an undesirable increase in extractables.
EP-A-0 238 050 claims a process for the batchwise production of finely divided crosslinked water-absorbing polymers by conducting the polymerization in a kneader and having a degree of neutralization for the (meth)acrylic acid in the range from 0 to 100 mol %. The polymerization batch is neutralized to the desired ultimate pH in the kneader used for the polymerization, either during the polymerization or subsequently thereto. This again provides homogeneous neutralization, but the shearing forces applied are too high, so that an undesirable increase in the extractables content occurs.
In U.S. Pat. No. 5 453 323 and EP-A-0 530 438, acrylic acid is used together with water-soluble hydroxyl-containing polymers to prepare under adiabatic conditions and without neutralization of the monomers polymer gels which are subsequently comminuted in an unspecified meat grinder. The neutralizing agent is added to this comminuted gel and the mixture is again chopped. The postcrosslinker is then added and the gel is again chopped three times in order that all the reactants may be incorporated in the gel in a homogeneous manner. This repeated chopping of the gel exerts an undesirable shearing stress on the gel, elevating the level of extractables.
EP-A-0 629 411 describes the polymerization of acrylic acid with crosslinkers. The gel obtained is subsequently partially neutralized with an alkali metal salt and further crosslinked by addition of a crosslinker. The method of neutralization is not further specified in the reference; one example mentions kneading the gel with the neutralizing agent in an extruder.
DE-A-195 29 348 describes preparing superabsorbent polymers by polymerizing a partially preneutralized monomer solution under adiabatic conditions. The degree of preneutralization of the acid-functional monomers is in the range from 5 to 30 mol %. The acidic gel is neutralized after its comminution in simple mixing assemblies such as a rotating drum or in a Drais mixer, the aqueous solution of the bases being introduced via nozzles or spray injectors, for example. True, this avoids any mechanical damage to the polymer gel, but cannot provide homogeneous neutralization, since the gel is not destructured in the course of the mixing with the neutralizing agent. The pH inhomogeneities of the gel in turn lead to inferior drying, which is undesirable for economic reasons.
It is an object of the present invention to provide a process for postneutralizing acidic hydrogels homogeneously and with minimal shear stress on the gel to avoid an undesirable increase in the extractable fractions.
We have found that this object is achieved by the process for preparing water-swellable hydrophilic polymers by neutralization of the acidic hydrogel having a
Dentler Joachim
Engelhardt Fritz
Herfert Norbert
Schliwa Rudolf
Stüven Uwe
BASF - Aktiengesellschaft
Lovering Richard D.
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