Cellulose ethers with delayed solubility and having a reduced glyoxal content

Abstract

The invention relates to a method for preparing cellulose derivatives which are reversibly crosslinked with glyoxal and thus display delayed water solubility. In the methods, a water-wetted cellulose derivative is mixed at a temperature of between 20 to 70° C. with an aqueous solution containing glyoxal, a monovalent or polyvalent organic acid, and at least one alkaline earth salt and/or alkali salt of phosphoric acid as a buffer substance to reversibly crosslink the cellulose derivative. The amount of glyoxal is 0.010 to 0.050 mol, in each case in relation to 1 mol of anhydroglucose units of the cellulose derivative, and the molar ratio of monovalent or polyvalent organic acid to glyoxal is in the range of 1:1 to 1:12. The cellulose derivative is then dried and milled, and the drying and milling may also be combined. The invention also relates to cellulose derivatives produced according to the method.

Claims

1. A process for producing cellulose derivatives reversibly crosslinked with glyoxal and thus having delayed water-solubility, comprising the steps of: a) providing a water-moist cellulose derivative, b) providing an aqueous solution comprising glyoxal, one or more alkaline earth metal salts and/or alkali metal salts of phosphoric acid as buffer substance, c) mixing the aqueous solution from b) with the cellulose derivative from a) at a temperature between 20 to 70° C. to achieve reversible crosslinking of the cellulose derivative, d) drying the reversibly crosslinked cellulose derivative and e) grinding the cellulose derivative, where steps d) and e) may be combined into a grinding-drying operation, wherein the aqueous solution as per b) comprises an organic acid having a basicity of one or more, where a monobasic, dibasic, tribasic or polybasic organic acid is present in an amount of 0.002 to 0.015 mol and the glyoxal is present in an amount of 0.010 to 0.050 mol, in each case per mole of anhydroglucose units of the cellulose derivative, and the monobasic organic acid to glyoxal molar ratio is in the range from 1:1 to 1:6, the dibasic organic acid to glyoxal molar ratio is in the range from 1:1 to 1:10, and the tribasic organic acid to glyoxal molar ratio is in the range from 1:2 to 1:12.

2. The process as claimed in claim 1, wherein the cellulose derivative is a nonionic cellulose ether.

3. The process as claimed in claim 1, wherein the cellulose derivative is a cellulose ether which, after a hot-water wash, is a filter cake having a dry-matter content of 30 to 70%.

4. The process as claimed in claim 3, wherein the process further comprises adding and kneading the filter cake with a crosslinker solution consisting of glyoxal, salts of phosphoric acid, an organic acid and water with continuous mixing and subsequently drying or grinding or being subjected to a grinding-drying operation.

5. The process as claimed in claim 4, wherein the process further comprises admixing the glyoxal with the cellulose ether in a fraction of not more than 0.035 mol per mole of anhydroglucose units of the cellulose ether.

6. The process as claimed in claim 1, wherein the process further comprises admixing the organic acid having a basicity of one or more into the cellulose ether in a fraction of 0.004 to 0.010 mol per mole of anhydroglucose unit of the cellulose ether.

7. The process as claimed in claim 1, wherein the organic acid having a basicity of one or more used is an aliphatic, aromatic and/or heterocyclic, saturated or unsaturated carboxyolic acid having 1 to 7 carbon atoms and one to three carboxyl group(s), where the organic acid may contain functional groups.

8. The process as claimed in claim 7, wherein the organic acid having a basicity of one or more is acetic acid, lactic acid, salicylic acid, adipic acid, citric acid, tartaric acid.

9. The process as claimed in claim 1, wherein the alkali metal salts of phosphoric acid are sodium dihydrogenphosphate and disodium hydrogenphosphate.

10. The process as claimed in claim 1, wherein the buffer/crosslinker solution has a pH of between 3 and 8.

11. A cellulose derivative reversibly crosslinked with glyoxal comprising an organic acid having a basicity of one or more in a fraction of 0.002 to 0.015 mol and glyoxal in a fraction of 0.010 to 0.050 mol, in each case per mole of anhydroglucose units of the cellulose derivative, and also one or more alkaline earth metal salts and/or alkali metal salts of phosphoric acid as buffer substance, and the molar ratio of organic acid having a basicity of one or more to glyoxal is in the range from 1:1 to 1:12.

12. The cellulose derivative reversibly crosslinked with glyoxal as claimed in claim 11, wherein said cellulose derivative is a nonionic cellulose ether.

13. The cellulose derivative reversibly crosslinked with glyoxal as claimed in claim 11, wherein the acid having a basicity of one or more is acetic acid, lactic acid, salicylic acid, adipic acid, citric acid, tartaric acid and/or malic acid.

14. The cellulose derivative reversibly crosslinked with glyoxal as claimed in claim 11, wherein the alkali metal salts of phosphoric acid are sodium dihydrogenphosphate and disodium hydrogenphosphate and the sodium dihydrogenphosphate and disodium hydrogenphosphate ar contained therein in an amount in each case gf 0.002 to 0.015 mol per mole of anhydroglucose units of the cellulose ether.

15. The cellulose derivative reversibly crosslinked with glyoxal as claimed in claim 11, wherein said cellulose derivative contains less than 1000 ppm of free glyoxal.

16. The process as claimed in claim 2, wherein the cellulose derivative is a methylhydroxyethylcellulose, methylhydroxypropylcellulose, methylcellulose, or a mixture thereof.

17. The process as claimed in claim 5, wherein 0.01 to 0.03 mol of glyoxal is admixed to the cellulose ether per mole of anhydroglucose units of the cellulose ether.

18. The process as claimed in claim 7, wherein the organic acid having a basicity of one or more is an aliphatic saturated or unsaturated carboxyolic acid and the functional groups are hydroxyl groups and/or amino groups.

19. The process as claimed in claim 8, wherein the organic acid having a basicity of one or more is lactic acid or adipic acid.

20. The process as claimed in claim 9, wherein the sodium dihydrogenphosphate and disodium hydrogenphosphate are admixed to the cellulose ether each in an amount of 0.003 to 0.015 mol per mole of anhydroglucose units of the cellulose ether.

21. The process as claimed in claim 9, wherein the sodium dihydrogenphosphate and disodium hydrogenphosphate are admixed to the cellulose ether preferably each in an amount of 0.005 to 0.009 mol per mole of anhydroglucose units of the cellulose ether.

22. The process as claimed in claim 10, wherein the buffer/crosslinker solution has a pH of between 4 and 6.5.

23. The cellulose derivative reversibly crosslinked with glyoxal as claimed in claim 12, wherein said cellulose derivative is methylhydroxyethylcellulose, methylhydroxypropylcellulose, methylcellulose, or a mixture thereof.

24. The cellulose derivative reversibly crosslinked with glyoxal as claimed in claim 13, wherein the acid having a basicity of one or more is lactic acid or adipic acid.

25. The cellulose derivative reversibly crosslinked with glyoxal as claimed in claim 14, wherein the amount of sodium dihydrogenphosphate and disodium hydrogenphosphate contained therein is in each case 0.005 to 0.009 mol per mole of anhydroglucose units of the cellulose ether.

Description

EXAMPLES

[0021] In the tables set out below and in the examples, the percentage fractions are reported in percentages by weight.

[0022] For verifying the storage stability of the cellulose ethers produced in the invention, the aging of the samples is simulated with an accelerated test in accordance with ASTM D6819. For this test, the air-dry cellulose ethers are transferred into glass vessels having temperature-stable and leakproof closure caps. It should be ensured that the ratio of the mass of the sample to be aged to the volume of the glass vessel is always identical (2.76 g of absolutely dry (bone dry) cellulose ether in 100 ml). The sample is subsequently adjusted to a constant moisture content of 10% by weight and stored at 100° C. for 6 h.

[0023] The solvation delays reported were measured using a Brabender Viscograph (single-speed 75 rpm, load cell 250 cmg, measuring pot and measuring sensor with pins) in aqueous solution (with mains water, at 20° C.). Solvation time ST is the time in minutes between the addition of the product and the attainment of a viscosity of 100 Brabender Units (BU)=65 mPa.Math.s. Solvation end time SET is the time in minutes after which there is no longer any increase in viscosity. For the individual viscosity stages, the concentration of the measurement solution, the initial mass of substance and the amount of dissolution water are evident from the table below.

TABLE-US-00001 Concentration of the Initial mass Viscosity stage measurement solution of substance Dissolution water [mPa*s] [%] [g] [g] ≤50 5 21.5 409   >50/≤150 4 17.2 413 >150/<600 3 12.9 417  600-1500 2.5 10.75 419 2000-4000 2 8.6 421 >4000-30000 1.5 6.45 424 >30000 1 4.3 426

[0024] The pH of the cellulose ether was determined from a 1% by weight aqueous solution of the absolutely dry cellulose ether by means of a pH meter with combined pH electrode.

[0025] The viscosity of the cellulose ethers was measured using a Brookfield rotational viscometer (model DVI) and using a No. 5 spindle for the 16 000 mPa.Math.s viscosity stage and a spindle 4 for the 5000 mPa.Math.s viscosity stage, with a rotary speed of 20 rpm, from a 1.9% by weight aqueous solution of the absolutely dry cellulose ether at 20° C. The water used to produce the sample solution had a hardness of 20° dH (German hardness).

[0026] The quantitative determination of the methoxy (—OCH.sub.3), hydroxyethyl (—OC.sub.2H.sub.4), and hydroxypropyl (—OC.sub.3H.sub.6) contents and the calculation of DS (average degree of substitution) and MS (molar degree of substitution) were ascertained according to the Zeisel method familiar to the skilled person, by reaction with hydriodic acid and subsequent GC analysis of the resultant alkyl iodides according to Z. Anal. Chem. 286 [1997] 161-190.

[0027] The total glyoxal content was ascertained by colorimetry through the reaction of the dissolved glyoxal with 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) in an acidic medium. For this reason, 0.1 g of air-dry cellulose ether were dissolved in 100 g of distilled water for 24 h. The dissolution procedure here should not be accelerated by alkalification. 2 ml of the cellulose ether solution were pipetted into the reaction vessel and 5 ml of the reagent solution (consisting of 0.2 g of MBTH, 10 g of demineralized water and 40 g of glacial acetic acid) were added thereto via pipette. The reaction vessels were closed, and the reaction mixtures were briefly mixed by shaking and were left to stand in the dark for 2 h. After a reaction time of 2 h, the absorbance of the sample and of the blank sample was measured at 405 nm in a 1 cm cuvette in the Dr. Lange CADAS 100 photometer. The absorbance of the sample had to be between 0.100 and 2.000. In the case of an absorbance below 0.100, the result is to be reported as <400 ppm of glyoxal. If the absorbance was above 2.000, the CE solution was diluted by serial dilution prior to the reaction.

Calculation

[0028] [00001]$\frac{\left(E_P-E_0\right)*4.207*100*F}{E}=\mathrm{ppm}\mathrm{of}\mathrm{glyoxal}$

E.sub.P=Absorbance of sample
E.sub.0=Absorbance of blank sample
F=Dilution factor
E=Initial mass in g
4.207=Slope of the calibration line

[0029] To determine the free, i.e., unbound, glyoxal, 0.2 g of the air-dried cellulose ether were admixed with 10 ml of tetrahydrofuran. This THF-cellulose ether suspension was shaken with a shaker machine for 4 h and then filtered via a fluted filter into a test tube. 2 ml of the extract were admixed with 5 ml of MBTH reactant solution (consisting of 0.2 g of MBTH, 10 g of demineralized water and 40 g of glacial acetic acid), left to stand in the dark for 2 h, and subsequently subjected to colorimetric measurement at 405 nm (CADAS 100 photometer from Dr. Lange) (see operations for determining the total glyoxal content).

Example 1

[0030] Water-moist methylhydroxyethylcellulose MHEC (200 g of dry matter) with an average degree of substitution DS(methyl) of 1.55 (the DS denotes the average number of methyl groups per anhydroglucose unit) and a molar degree of substitution MS(hydroxyethyl) of 0.22 (the MS denotes the average number of hydroxyethyl groups per anhydroglucose unit) and with a viscosity of 16 000 mPa.Math.s was placed in an LK5 laboratory kneader from Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of water, citric acid, sodium hydroxide and glyoxal. The target moisture content for the MHEC of 73% was established using ice. When all of the ingredients had been added, the whole was kneaded for 30 min. The moist product was subsequently predried to a hand-dry state in a fluidized-bed dryer and dry-ground using an Alpine mill (model D 100 UPZ) employing a 180 μm sieve. A target final moisture content was less than 5%.

TABLE-US-00002 Batch No. 1 (comparative) Buffer NaOH/AGU(MHEC) (mol/mol) 0.015 Glyoxal/AGU(MHEC) (mol/mol) 0.046 Acid/AGU(MHEC) (mol/mol) 0.008 NaH.sub.2PO.sub.4*H.sub.2O/AGU(MHEC) (mol/mol) 0 Na.sub.2HPO.sub.4/AGU(MHEC) (mol/mol) 0 Acid Citric acid crosslinked MHEC pH 5.7 Viscosity (mPas) 12140 Total glyoxal (ppm) 13885 Free glyoxal (ppm) 1279 ST (min) 29 SET (min) 43 aged crosslinked MHEC pH 4.8 Viscosity (mPas) 9580 ST (min) 42 SET (min) 57 Postcrosslinking 45 [aged ST*100/ST-100] (%)

Example 2

[0031] Water-moist MHEC (200 g of dry matter) with an average degree of substitution DS (methyl) of 1.55 and a molar degree of substitution MS (hydroxyethyl) of 0.22 and with a viscosity of 16 000 mPa-s was placed in an LK5 laboratory kneader from Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of water, sodium dihydrogenphosphate.H.sub.2O, disodiumn hydrogenphosphate and glyoxal. The target moisture content for the MHEC of 73% was established using ice. When all of the ingredients had been added, the whole was kneaded for 30 min. The moist product was subsequently predried to a hand-dry state in a fluidized-bed dryer and dry-ground using an Alpine mill (model D 100 UPZ) employing a 180 μm sieve. A final moisture content of less than 5% was achieved.

TABLE-US-00003 Batch No. 1 2-1 2-2 (comparative) (comparative) (comparative) Buffer NaOH/AGU(MHEC) (m ol/mol) 0.015 0 0 Glyoxal/AGU(MHEC) (mol/mol) 0.046 0.046 0.023 Acid/AGU(MHEC) (mol/mol) 0.008 0 0 NaH.sub.2PO.sub.4*H.sub.2O/AGU(MHEC) (mol/mol) 0 0.008 0.008 Na.sub.2HPO.sub.4/AGU(MHEC) (mol/mol) 0 0.008 0.008 Acid Citric acid — — crosslinked MHEC PRODUCT PROPERTIES pH 5.7 6.9 7.0 Viscosity (mPas) 12140 12200 12400 Total glyoxal (ppm) 13885 15167 6796 Free glyoxal (ppm) 1279 1700 669 ST (min) 29 22 14 SET (min) 43 39 26 aged crosslinked MHEC pH 4.8 4.3 5.0 Viscosity (mPas) 9580 10980 10140 ST (min) 42 42 18 SET (min) 57 71 28 Postcrosslinking 45 91 29 [aged ST*100/ST-100] (%)

[0032] It becomes apparent that with use of the phosphate buffer (examples 2-1 and 2-2) in comparison to the cellulose ether (example 1), produced according to the prior art, the pH of the cellulose ether is raised and the viscosity prior to aging is identical. After aging of the samples, the viscosity dropped to a similar level. On the basis of the raised pH of the cellulose ether using the phosphate buffer in the glyoxal crosslinking, the solvation delay ST is lower. The postcrosslinking, captured through the simulation of aging, was increased, however, for the cellulose ethers using the phosphate buffer (example 2-1).

Example 3

[0033] Water-moist MHEC (200 g of dry matter) with an average degree of substitution DS (methyl) of 1.55 and a molar degree of substitution MS (hydroxyethyl) of 0.22 and with a viscosity of 16 000 mPa.Math.s was placed in an LK5 laboratory kneader from Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of sodium dihydrogenphosphate.H.sub.2O, disodium hydrogenphosphate, citric acid, water and glyoxal. The target moisture content for the MHEC of 73% was established using ice. When all of the ingredients had been added, the whole was kneaded for 30 min. The moist product was subsequently predried to a hand-dry state in a fluidized-bed dryer and dry-ground using an Alpine mill (model D 100 UPZ) employing a 180 μm sieve. A final moisture content of less than 5% was achieved.

TABLE-US-00004 Batch No. 2-1 2-2 (comparative) (comparative) 3-1 3-2 Buffer NaOH/AGU(MHEC) (mol/mol) 0 0 0 0 Glyoxal/AGU(MHEC) (mol/mol) 0.046 0.023 0.046 0.023 Acid/AGU(MHEC) (mol/mol) 0 0 0.004 0.004 NaH.sub.2PO.sub.4*H.sub.2O/AGU(MHEC) (mol/mol) 0.008 0.008 0.008 0.008 Na.sub.2HPO.sub.4/AGU(MHEC) (mol/mol) 0.008 0.008 0.008 0.008 Acid — — Citric acid Citric acid crosslinked MHEC PRODUCT PROPERTIES pH 6.9 7.0 5.7 6.0 Viscosity (mPas) 12200 12400 10980 11900 Total glyoxal (ppm) 15167 6796 14760 7957 Free glyoxal (ppm) 1700 669 2166 1028 ST (min) 22 14 34 37 SET (min) 39 26 48 51 aged crosslinked MHEC pH 4.3 50 4.3 5.0 Viscosity (mPas) 10980 10140 8800 7220 ST (min) 42 18 44 37 SET (min) 71 28 58 50 Postcrosslinking 91 29 29 0 [aged ST*100/ST-100] (%)

[0034] Through the addition of citric acid as catalyst for the glyoxal crosslinking to the phosphate buffer, the ST was significantly increased. However, the products were not storage-stable, since the viscosity after aging decreased much more greatly than that of an MHEC without addition of acid in the glyoxal solution. The postcrosslinking after aging had fallen as a result of the addition of citric acid. The glyoxal crosslinking accordingly was more efficient; it was catalyzed by the acid.

Example 4

[0035] Water-moist MHEC (200 g of dry matter) with an average degree of substitution DS (methyl) of 1.55 and a molar degree of substitution MS (hydroxyethyl) of 0.22 and with a viscosity of 16 000 mPa.Math.s was placed in an LK5 laboratory kneader from Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of sodium dihydrogenphosphate.H.sub.2O, disodium hydrogenphosphate, lactic acid, water and glyoxal. The target moisture content for the MHEC of 73% was established using ice. When all of the ingredients had been added, the whole was kneaded for 30 min. The moist product was subsequently predried to a hand-dry state in a fluidized-bed dryer and dry-ground using an Alpine mill (model D 100 UPZ) employing a 180 μm sieve. A final moisture content of less than 5% was achieved in this way.

TABLE-US-00005 Batch No. 1 2-1 (comparative) (comparative) 4-1 4-2 Buffer NaOH/AGU(MHEC) (mol/mol) 0.015 0 0 0 Glyoxal/AGU(MHEC) (mol/mol) 0.046 0.046 0.046 0.023 Acid/AGU(MHEC) (mol/mol) 0.008 0 0.009 0.009 NaH.sub.2PO.sub.4*H.sub.2O/AGU(MHEC) (mol/mol) 0 0.008 0.008 0.008 Na.sub.2HPO.sub.4/AGU(MHEC) (mol/mol) 0 0.008 0.008 0.008 Acid Citric acid — Lactic acid Lactic acid crosslinked MHEC pH 5.7 6.9 8.5 6.4 Viscosity (mPas) 12140 12200 11560 11920 Total glyoxal (ppm) 13885 15167 15529 7373 Free glyoxal (ppm) 1279 1700 1527 527 ST (min) 29 22 39 25 SET (min) 43 39 58 37 aged crosslinked MHEC pH 4.8 4.3 4.2 44 Viscosity (mPas) 9580 10980 11220 10320 ST (min) 42 42 50 30 SET (min) 57 71 72 42 Postcrosslinking 45 91 28 20 [aged ST*100/ST-100] (%)

[0036] Through the addition of lactic acid as catalyst for the glyoxal crosslinking to the phosphate buffer, the ST was significantly increased. Accordingly, it was possible to reduce the amount of glyoxal used by half. Consequently, the total glyoxal content and the free glyoxal content in the MHEC were also considerably reduced. The product was no longer liable to labeling requirements. The loss in viscosity after aging was small and similar to that when using a pure phosphate buffer. As a result of the addition of lactic acid, the postcrosslinking after aging was lower than in the case of the MHEC without addition of acid in the phosphate buffer.

Example 5

[0037] Water-moist MHEC (200 g of dry matter) with an average degree of substitution DS (methyl) of 1.55 and a molar degree of substitution MS (hydroxyethyl) of 0.22 and with a viscosity of 16 000 mPa.Math.s was placed in an LK5 laboratory kneader from Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of sodium dihydrogenphosphate.H.sub.2O, disodium hydrogenphosphate, acetic acid, water and glyoxal. The target moisture content for the MHEC of 73% was established using ice. When all of the ingredients had been added, the whole was kneaded for 30 min. The moist product was subsequently predried to a hand-dry state in a fluidized-bed dryer and dry-ground using an Alpine mill (model D 100 UPZ) employing a 180 μm sieve. A final moisture content of less than 5% was achieved.

TABLE-US-00006 Batch No. 1 2-1 2-2 (comparative) (comparative) (comparative) 5-1 Buffer NaOH/AGU(MHEC) (mol/mol) 0.015 0 0 0 Glyoxal/AGU(MHEC) (mol/mol) 0.046 0.046 0.023 0.023 Acid/AGU(MHEC) (mol/mol) 0.008 0 0 0.008 NaH.sub.2PO.sub.4*H.sub.2O/AGU(MHEC) (mol/mol) 0 0.008 0.008 0.008 Na.sub.2HPO.sub.4/AGU(MHEC) (mol/mol) 0 0.008 0.008 0.008 Acid Citric acid — — Acetic acid crosslinked MHEC pH 5.7 6.9 7.0 6.4 Viscosity (mPas) 12140 12200 12400 12260 Total glyoxal (ppm) 13885 15167 8796 7835 Free glyoxal (ppm) 1279 1700 669 691 ST (min) 29 22 14 27 SET (min) 43 39 26 42 aged crosslinked MHEC pH 4.8 4.3 8.0 4.8 Viscosity (mPas) 9580 10980 10140 11080 ST (min) 42 42 18 28 SET (min) 57 71 28 40 Postcrosslinking 45 91 29 4 [aged ST*100/ST-100] (%)

[0038] As a result of the addition of acetic acid as catalyst for the glyoxal crosslinking to the phosphate buffer, it was possible to retain the ST even with halving of the amount of glyoxal used. Accordingly, the values for total glyoxal and unbound/free glyoxal in the MHEC had fallen significantly as well. The product was no longer liable to labeling requirements. The loss in viscosity after aging was small and similar to that when using a pure phosphate buffer. As a result of the addition of acetic acid, the postcrosslinking after aging was lower than in the case of the MHEC without addition of acid in the phosphate buffer.

Example 6

[0039] Water-moist MHEC (200 g of dry matter) with an average degree of substitution DS (methyl) of 1.55 and a molar degree of substitution MS (hydroxyethyl) of 0.22 and with a viscosity of 16 000 m.Math.Pas was placed in an LK5 laboratory kneader from Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of sodium dihydrogenphosphate.H.sub.2O, disodium hydrogenphosphate, adipic acid, water and glyoxal. The target moisture content for the MHEC of 73% was established using ice. When all of the ingredients had been added, the whole was kneaded for 30 min. The moist product was subsequently predried to a hand-dry state in a fluidized-bed dryer and dry-ground using an Alpine mill (model D 100 UPZ) employing a 180 μm sieve. A final moisture content of less than 5% was achieved.

TABLE-US-00007 Batch No. 1 2-1 2-2 (comparative) (comparative) (comparative) 6-1 6-2 6-3 Buffer NaOH/AGU(MHEC) (mol/mol) 0.015 0 0 0 0 0 Glyoxal/AGU(MHEC) (mol/mol) 0.04 0.04 0.023 0.04 0.023 0.016 Acid/AGU(MHEC) (mol/mol) 0.00 0 0 0.008 0.004 0.008 NaH.sub.2PO.sub.4*H.sub.2O/AGU(MHEC) (mol/mol) 0 0.008 0.008 0.008 0.008 0.008 Na.sub.2HPO.sub.4/AGU(MHEC) (mol/mol) 0 0.008 0.008 0.008 0.008 0.008 Acid Citric acid — — Adipic acid Adipic actd Adipic acid crosslinked MHEC pH 5.7 6.9 7.0 5.0 5.8 5.0 Viscosity (mPas) 12140 12200 12400 12200 12280 11820 Total glyoxal (ppm) 13885 151 7 6796 1 033 7202 7243 Free glyoxal (ppm) 1279 1700 9 1121 43 <400 ST (min) 29 22 14 55 37 34 SET (min) 43 39 26 77 50 52 aged crosslinked MHEC pH 4.8 4.3 5.0 4.1 4.3 4.5 Viscosity (mPas) 95 0 10980 10140 10880 10220 101 0 ST (min) 42 42 18 67 37 27 SET (min) 57 71 28 92 51 40 Postcrosslinking [aged ST*100/ST − 100] (%) 45 1 29 22 0 0 indicates data missing or illegible when filed

[0040] As a result of the addition of adipic acid as catalyst for the glyoxal crosslinking to the phosphate buffer, the ST was significantly increased. Accordingly, it was possible to reduce the amount of glyoxal used by two thirds. Correspondingly, there were considerable reductions in the total glyoxal and the free glyoxal in the MHEC as well. The loss in viscosity after aging was low and similar to that when using a pure phosphate buffer. The postcrosslinking after aging was marginal or could no longer be found, owing to the addition of adipic acid. The product was therefore not liable to labeling requirements. The ST and the storage stability were the same as those of a cellulose ether produced according to the prior art.

Example 7

[0041] Water-moist MHEC (200 g of dry matter) with an average degree of substitution DS (methyl) of 1.62 and a molar degree of substitution MS (hydroxyethyl) of 0.21 and with a viscosity of 5000 mPa.Math.s was placed in an LK5 laboratory kneader from Erweka Apparatebau GmbH. There the water-moist cellulose ether was mixed either with an aqueous solution of glyoxal, water, citric acid and sodium hydroxide (Batch No. 7-1, as a comparative) or with an aqueous solution of sodium dihydrogenphosphate.H.sub.2O, disodium hydrogenphosphate, water and glyoxal (Batch No. 7-2, as a comparative) and mixed with the acid indicated in the table (Batch No. 7-3 to 7-6). The target moisture content for the MHEC of 73% was established using ice. When all of the ingredients had been added, the whole was kneaded for 30 min. The moist product was subsequently predried to a hand-dry state in a fluidized-bed dryer and dry-ground using an Alpine mill (model D 100 UPZ) employing a 180 μm sieve. A final moisture content of less than 5% was achieved in this way.

TABLE-US-00008 Batch No. 7-1 7-2 (comparative) (comparative) 7-3 7-4 7-5 7-6 Suffer NaOH/AGU(MHEC) (mol/mol) 0.015 0 0 0 0 0 Glyoxal/AGU(MHEC) (mol/mol) 0.046 0.023 0.023 0.023 0.023 0.023 Acid/AGU(MHEC) (mol/mol) 0.004 0 0.008 0.008 0.008 0.008 NaH.sub.2PO.sub.4*O/AGU(MHEC) (mol/mol) 0 0.008 0.008 0.008 0.008 0.008 Na.sub.2HPO.sub.4/AGU(MHEC) (mol/mol) 0 0.008 0.008 0.008 0.008 0.008 Acid Citric acid — Lactic acid Salicylic acid Acetic acid Adipic acid crosslinked MHEC pH 5.7 7.0 6.2 5.9 6.5 5.1 Viscosity (mPas) 4830 5010 4960 5050 4530 5030 Total glyoxal (ppm) 14683 7371 7621 7664 8234 6896 Free glyoxal (ppm) 1085 596 612 735 320 324 ST (min) 31 14 27 34 29 38 SET (min) 48 21 38 49 41 54 aged crosslinked MHEC pH 4.6 4.9 4.2 4.4 4.5 4.4 Viscosity (mPas) 3970 4400 4170 4130 4390 4690 ST (min) 46 16 24 31 26 31 SET (min) 61 24 32 42 34 42

[0042] From the table it is apparent that for different initial viscosities of the MHEC as well, it was possible to achieve the positive effects with the new crosslinker/buffer solutions.