Cellulose ether with delayed dissolution and reduced glyoxal content

Abstract

The present invention relates to a method for preparing a cellulose derivative that is reversibly crosslinked with glyoxal and thus exhibits delayed water solubility. In this method, a water-wetted cellulose derivative is mixed at a temperature of 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 for reversibly crosslinking the cellulose derivative. The amount of glyoxal is in each case 0.010 to 0.050 mol per 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 ground. Drying and grinding may be combined. The present invention also relates to a cellulose derivative produced according to this method.

Description

[Technical Field]

[0001] The present invention relates to a method for producing cellulose derivatives that are reversibly crosslinked with glyoxal and therefore have delayed water-solubility. [Background technology]

[0002] Due to their excellent properties, such as swelling ability, gel formation and dissolution behavior, cellulose ethers are used in a variety of applications, for example as thickeners, adhesives, binders and dispersants, humectants, protective colloids, stabilizers, and also as suspending and emulsifying agents and film formers.

[0003] These applications require cellulose ethers to be emulsified, dispersed, or dissolved without clumping. However, when dry powder cellulose ethers are dissolved in water, they often gel on the surface and form clumps. This problem can sometimes be solved by treating them with sufficient amounts of dialdehydes, particularly glyoxal. The formation of hemiacetals under acidic catalysis results in reversible crosslinking, which does not substantially affect solubility but improves dispersibility, slows dissolution in water, and prevents clump formation. The dissolution delay can be reduced by increasing the pH ("Cellulose Ethers," Chapter 2.1, Ullmann's Encyclopedia of Industrial Chemistry, 2006, Wiley-VCH Verlag GmbH, Weinheim, Germany).

[0004] International Patent Publication No. 2017 / 064164A1 discloses a cellulose ether temporarily crosslinked with a glyoxalic acid (C1-C4) alkyl ester mono(C1-C4) alkyl acetal. The proportion of the crosslinker is preferably 0.01 to 10 parts by weight per 100 parts by weight of the cellulose ether. European Patent Application Publication No. 2 177 538 A1 relates to a cellulose ether containing alkyl and hydroxyethyl groups that is mixed with a crosslinker and then subjected to milling and drying. The crosslinker is preferably a mono- or dialdehyde, more preferably glyoxal. According to US Patent Application Publication No. 2019 / 0062458A1, the crosslinked water-insoluble cellulose is used to produce membranes or hydrogels. The crosslinker can be an aldehyde, organic chloride, ether, polyfunctional carboxylic acid, glycerol, urea derivative, and / or glycidyl ether.

[0005] DE 10 75 773 discloses the cross-linking of cellulose ethers with glyoxal to improve solubility and / or prevent aggregation during dissolution, where the cellulose ether was suspended in acetone and an aqueous solution of glyoxal was added.

[0006] U.S. Patent No. 3,072,635 describes a method for preparing water-dispersible cellulose derivatives by treatment with 0.001 to 0.2 moles of glyoxal per mole of anhydroglucose units of the cellulose ether. The glyoxal is dissolved in alcohol, and the cellulose ether preferably contains 1.25 to greater than 5.0% by weight of glyoxal.

[0007] Part of the disclosure in DE 12 39 672 is the possibility of adjusting the degree of crosslinking and thus delaying dissolution via the amount of glyoxal. This is evident from the examples given, in which water-soluble methylcellulose is homogenized with 0.001 to 0.2 moles of glyoxal per mole of anhydroglucose units of the methylcellulose at pH 2 to 7, dried, and ground. It is shown that 0.48% by weight of glyoxal in the cellulose derivative at pH 2 (established with phosphoric acid) results in only a 15-minute solvation delay. The amount of unbound glyoxal is not given. There is also no mention of crosslinking at higher pH values ​​or the associated solvation delay.

[0008] EP 0 597 364 discloses a method in which sodium dihydrogen phosphate is added to a glyoxal solution.

[0009] EP 1 316 563 A1 describes a method for producing glyoxal-crosslinked cellulose ethers without the addition of acidic catalysts. In this case, the viscosity reduction decreases with increasing pH of the cellulose ether. It should be noted that increasing pH results in a lower solvation delay. The delay time of methylhydroxyethylcellulose MHEC containing 1.7% by weight of glyoxal and having a pH of 6.2 in demineralized water is only 20 minutes, whereas the delay time of crosslinked MHEC containing 2.0% by weight of glyoxal and having a pH of 4.7 is 60 minutes.

[0010] Glyoxal is classified as an irritant and even a sensitizer under the CLP Regulation (Regulation (EC) No. 1272 / 2008) and is suspected of having mutagenic effects in humans. Therefore, products containing 0.1% to 1% free glyoxal must be labeled in accordance with the CLP Regulation. The use of glyoxal-crosslinked cellulose ethers in the personal care sector is also permitted, provided that cosmetic manufacturers ensure that the final product does not exceed the glyoxal limit of 100 ppm (SCCP / 0881 / 05). Because the glyoxal used for reversible crosslinking is released again when the cellulose ether is dissolved in an aqueous medium, the CLP classification requires that the total, as well as the unbound, amount of glyoxal in the final product be minimized.

[0011] European Patent No. 1 452 544 B1 describes a method for producing cellulose ethers with reduced unbound glyoxal content by adding water-soluble borate or aluminum salts to a phosphate-buffered crosslinker solution. As is clear, the amount of unbound glyoxal can be reduced, while the addition of sodium tetraborate and the resulting increase in pH reduces the solvation delay. There is no clear comment on storage stability. Furthermore, however, the glyoxal reduction effect is low, and the additional additives used are similarly not unobjectionable from a toxicological point of view.

[0012] WO 2012 / 122153A1 describes a dry blend consisting of a standard commercially available cellulose ether reversibly crosslinked with glyoxal and a partially neutralized, powdered, solid, water-soluble polycarboxylic acid. Preferably, the acid is partially neutralized poly(meth)acrylic acid, polymaleic acid, citric acid, adipic acid, oxalic acid, malonic acid, or glutaric acid. When the crosslinked cellulose ether is stirred in water, the acid lowers the pH, resulting in slower breakage of the hemiacetal bonds and preventing the formation of lumps. WO 2014 / 175903 describes a method for dispersing a dry cellulose ether formulation in an aqueous solution without lumps. The cellulose ether formulation comprises a reversibly crosslinked cellulose ether and a solid, water-soluble acid, preferably citric acid, tartaric acid, oxalic acid, malonic acid, or poly(meth)acrylic acid. The formulation is similarly a dry blend consisting of a standard commercially available crosslinked cellulose ether and an acid.

[0013] The present invention aims to reduce the content of bound and unbound glyoxal in cellulose ethers without changing the properties of the cellulose ethers, particularly their solvation retardation and viscosity, thereby significantly minimizing health risks and avoiding mandatory labeling.

[0014] Surprisingly, it has been found that by adding an acidic organic catalyst, specifically an aromatic organic acid with a basicity of 1 or greater, to a glyoxal solution buffered with an alkali metal salt or alkaline earth metal salt of phosphoric acid, the efficiency of glyoxal crosslinking can be increased, and the amount of glyoxal used can be significantly reduced. The solvation retardation of the cellulose ether produced according to the present invention, which has a reduced glyoxal content, is comparable to that of the cellulose ether produced according to the prior art. Surprisingly, the expected decrease in viscosity due to the decrease in pH did not occur. Summary of the Invention

[0015] The subject of the present invention is therefore a method for producing a cellulose derivative reversibly crosslinked with glyoxal and therefore having retarded water solubility, comprising the following steps: a) providing a water-wettable cellulose derivative; b) providing an aqueous solution comprising glyoxal, one or more alkaline earth metal and / or alkali metal salts of phosphoric acid as buffer substances; c) mixing the aqueous solution of b) with the cellulose derivative of a) at a temperature of 20-70°C to achieve reversible cross-linking of the cellulose derivative; d) drying the reversibly crosslinked cellulose derivative; and e) Grinding the cellulose derivative Including, Steps d) and e) can be integrated into a grinding-drying operation, b) is characterized in that the aqueous solution according to b) comprises an organic acid having a basicity of 1 or more, in which in each case the amount of organic acid having a basicity of 1 or more is 0.002 to 0.015 mol and the amount of glyoxal is 0.010 to 0.030 mol per mole of anhydroglucose unit of the cellulose derivative, the molar ratio of monobasic organic acid to glyoxal is in the range of 1:1 to 1:6, preferably 1:1 to 1:4, the ratio of dibasic organic acid to glyoxal is in the range of 1:1 to 1:10, preferably 1:2 to 1:6, and the ratio of tribasic organic acid to glyoxal is in the range of 1:2 to 1:12, preferably 1:3 to 1:8. It is a method.

[0016] A further subject of the present invention is a reversibly crosslinked cellulose derivative produced by the aforementioned method, i.e., a cellulose derivative reversibly crosslinked with glyoxal, characterized in that in each case, per mole of anhydroglucose unit of the cellulose derivative, the proportion of organic acid having a basicity of 1 or greater is 0.002 to 0.015 mol, the amount of glyoxal is 0.010 to 0.050 mol, and the molar ratio of organic acid having a basicity of 1 or greater to glyoxal is in the range of 1:2 to 1:12. Preferably, the reversibly crosslinked cellulose derivative of the present invention contains less than 1000 ppm of free glyoxal. DETAILED DESCRIPTION OF THE INVENTION

[0017] The organic acids are aliphatic, aromatic and / or heterocyclic, preferably C1-C7 carboxylic acids having 1 to 3 carboxyl groups, which may also contain functional groups such as hydroxyl and / or amino groups. Aromatic and heterocyclic acids may further have nitrogen, oxygen or sulfur as heteroatoms. Aliphatic acids may similarly have at least one double bond. More preferably used acids are those employed in the pharmaceutical and / or food industries. A pK of 2.5 or more and less than 5 is preferred. a Of the monocarboxylic acids having a pK range of 2.5 to 5, these are preferably acetic acid, lactic acid and salicylic acid. a1 Range and pK below 6 a2 Among the di- or tricarboxylic acids having the formula (I), these are preferably adipic, tartaric, or malic acid. Preference is generally given to carboxylic acids that are solid under standard conditions. They are also easier to measure. Acetic acid is less preferred due to its strong odor and relatively high volatility. The amount of acid is in each case 0.002 to 0.015 mol, more preferably 0.004 to 0.010 mol, per mole of anhydroglucose unit of the cellulose ether.

[0018] Preferably, glyoxal is used in the form of an approximately 40% by weight aqueous solution. The amount of glyoxal is in each case 0.010 to 0.050 mol, more preferably 0.015 to 0.025 mol, per mole of anhydroglucose unit of the cellulose ether.

[0019] The buffering agent used is an alkali metal or alkaline earth metal salt of phosphoric acid, more preferably an alkali metal salt of phosphoric acid, and very preferably sodium dihydrogen phosphate monohydrate and disodium hydrogen phosphate. The amount of sodium dihydrogen phosphate and disodium hydrogen phosphate is preferably 0.003 to 0.015 mol in each case, more preferably 0.005 to 0.009 mol in each case, per mole of anhydroglucose units of the cellulose ether.

[0020] The cellulose derivative used is preferably a water-wettable cellulose ether with a water content of 30 to 70% by weight, obtained after hot water washing. This water-wettable cellulose ether is kneaded with a solution, preferably an aqueous solution, of the acid, phosphate, and glyoxal used, followed by drying and grinding, or a grinding-drying operation. This solution is used to establish the pH and increase the efficiency of glyoxal crosslinking. As a result, the amount of glyoxal used can be significantly reduced, while still achieving the same solvation retardation as that of cellulose ethers prepared according to the prior art. [Example]

[0021] In the tables and examples below, percentage fractions are reported as weight percent.

[0022] To verify the storage stability of the cellulose ether prepared in this invention, an accelerated test according to ASTM D6819 is used to simulate sample aging. In this test, air-dried cellulose ether is transferred to a glass container with a temperature-stable, leak-proof closure cap. It is important to ensure that the ratio of the mass of the sample to the volume of the glass container is always the same (2.76 g of bone-dry cellulose ether per 100 ml). The sample is then adjusted to a constant moisture content (10 wt%) and stored at 100°C for 6 hours.

[0023] The reported solvation delays were measured in aqueous solution (mains water, 20°C) using a Brabender Viscograph (single speed 75 rpm, load cell 250 cmg, measuring pot and measuring sensor with pin). The solvation time ST is the time in minutes from addition of the product until a viscosity of 100 Brabender units (BU) = 65 mPa sec is reached. The solvation end time SET is the time in minutes until no further increase in viscosity is observed. For the individual viscosity stages, the concentration of the measuring solution, the initial mass of the substance and the amount of dissolved water can be seen from the table below.

[0024] [Table 1]

[0025] The pH of the cellulose ether was measured from a 1 wt % aqueous solution of bone-dried cellulose ether using a pH meter equipped with a composite pH electrode.

[0026] The viscosity of the cellulose ether was measured at 20°C from a 1.9 wt% aqueous solution of bone-dried cellulose ether using a Brookfield rotational viscometer (DVI model) at 20 rpm with a No. 5 spindle for the 16000 mPa·s viscosity step and a No. 4 spindle for the 5000 mPa·s viscosity step. The water used to prepare the sample solution had a hardness of 20° dH (German hardness).

[0027] Quantification of methoxy (-OCH), hydroxyethyl (-OCH), and hydroxypropyl (-OCH) content and calculation of DS (average degree of substitution) and MS (molar degree of substitution) were confirmed by reaction with hydroiodic acid according to the Zeisel method, well known to those skilled in the art, followed by GC analysis of the resulting alkyl iodides according to Z. Anal. Chem. 286

[1997] 161-190.

[0028] Total glyoxal content was determined by colorimetric determination based on the reaction of dissolved glyoxal with 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH) in an acidic medium. For this purpose, 0.1 g of air-dried cellulose ether was dissolved in 100 g of distilled water for 24 hours. This dissolution procedure should not be accelerated by alkalinization. 2 ml of the cellulose ether solution was pipetted into a reaction vessel, to which 5 ml of reagent solution (0.2 g MBTH, 10 g demineralized water, and 40 g glacial acetic acid) was added. The reaction vessel was closed, and the reaction mixture was briefly mixed by shaking and allowed to stand in the dark for 2 hours. After the 2-hour reaction period, the absorbance of the sample and blank sample was measured at 405 nm in a 1 cm cuvette in a Dr. Lange CADAS100 photometer. The absorbance of the sample should be between 0.100 and 2,000 nm. For absorbances below 0.100, the results should be reported as glyoxal < 400 ppm. For absorbances above 2.000, the CE solution was diluted by serial dilution before reaction. calculation

[0029]

number

[0030] E P = absorbance of sample E0 = absorbance of blank sample F = dilution ratio E = initial mass in g 4.207 = slope of calibration line

[0031] To measure free, or unbound, glyoxal, 0.2 g of air-dried cellulose ether was mixed with 10 ml of tetrahydrofuran. The THF-cellulose ether suspension was shaken on a shaker for 4 hours and then filtered through a fluted filter into a test tube. 2 ml of the extract was mixed with 5 ml of MBTH reaction solution (0.2 g MBTH, 10 g demineralized water, and 40 g glacial acetic acid), allowed to stand in the dark for 2 hours, and then subjected to colorimetric measurement at 405 nm (Dr. Lange CADAS 100 photometer) (see procedure for measuring total glyoxal content).

[0032] Example 1 Water-wettable methylhydroxyethyl cellulose MHEC (200 g dry matter) with an average degree of substitution (DS) of 1.55 (DS indicates the average number of methyl groups per anhydroglucose unit) and a molar degree of substitution (MS) of 0.22 (MS indicates the average number of hydroxyethyl groups per anhydroglucose unit), and a viscosity of 16,000 mPa·s, was placed in an LK5 laboratory kneader manufactured by Erweka Apparatebau GmbH and mixed with an aqueous solution of water, citric acid, sodium hydroxide, and glyoxal. A target moisture content of 73% was established for the MHEC using ice. Once all ingredients were added, the mixture was kneaded for 30 minutes. The wet product was then pre-dried to a hand-dry state in a fluidized-bed dryer and dry-milled using an Alpine mill (model D 100 UPZ) employing a 180 μm sieve. The target final moisture content was less than 5%.

[0033] [Table 2]

[0034] Example 2 Water-wettable MHEC (200 g dry material) with an average degree of substitution (DS) of 1.55 (methyl) and a molar substitution (MS) of 0.22 (hydroxyethyl), and a viscosity of 16,000 mPa·s, was placed in an LK5 laboratory mixer from Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of water, sodium dihydrogen phosphate·H2O, disodium hydrogen phosphate, and glyoxal. A target moisture content of 73% was established for the MHEC using ice. Once all ingredients were added, the mixture was mixed for 30 minutes. The wet product was then pre-dried to air-dryness 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.

[0035] [Table 3]

[0036] Compared with the cellulose ether prepared according to the prior art (Example 1), the use of phosphate buffer (Examples 2-1 and 2-2) increases the pH of the cellulose ether, revealing identical viscosity before aging. After aging, the viscosity decreased to the same extent. The solvation delay ST was lower due to the increase in pH of the cellulose ether prepared using phosphate buffer with glyoxal crosslinking. However, the cellulose ether prepared using phosphate buffer showed increased postcrosslinking during aging, as observed in the simulation (Example 2-1).

[0037] Example 3 Water-wettable MHEC (200 g dry material) with an average degree of substitution (DS) of 1.55 (methyl) and a molar substitution (MS) of 0.22 (hydroxyethyl), and a viscosity of 16,000 mPa·s, was placed in an LK5 laboratory mixer from Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of sodium dihydrogen phosphate·H2O, disodium hydrogen phosphate, citric acid, water, and glyoxal. A target moisture content of 73% was established for the MHEC using ice. Once all ingredients were added, the mixture was mixed for 30 minutes. The wet product was then pre-dried to air-dryness 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.

[0038] [Table 4]

[0039] The addition of citric acid to the phosphate buffer as a catalyst for glyoxal crosslinking significantly increased the ST. However, the product was not stable on storage, as the viscosity decreased significantly after aging compared with the MHEC product without the addition of acid to the glyoxal solution. Post-crosslinking after aging was reduced as a result of the addition of citric acid. Thus, glyoxal crosslinking was more efficient and acid-catalyzed.

[0040] Example 4 Water-wettable MHEC (200 g dry material) with an average degree of substitution (DS) of 1.55 (methyl) and a molar substitution (MS) of 0.22 (hydroxyethyl) and a viscosity of 16,000 mPa·s was placed in an LK5 laboratory kneader manufactured by Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of sodium dihydrogen phosphate·H2O, disodium hydrogen phosphate, lactic acid, water, and glyoxal. A target moisture content of 73% was established using ice. Once all ingredients were added, the mixture was kneaded for 30 minutes. The wet product was then pre-dried to air-dryness 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.

[0041] [Table 5]

[0042] The addition of lactic acid to the phosphate buffer as a catalyst for glyoxal crosslinking significantly increased the ST. Consequently, the amount of glyoxal used could be halved. Consequently, the total and free glyoxal content in MHEC was also significantly reduced. The product is no longer subject to labeling requirements. The decrease in viscosity after aging was small and similar to that observed with pure phosphate buffer. The addition of lactic acid resulted in lower post-crosslinking after aging than with MHEC without added acid in the phosphate buffer.

[0043] Example 5 Water-wettable MHEC (200 g dry material) with an average degree of substitution (DS) of 1.55 (methyl) and a molar substitution (MS) of 0.22 (hydroxyethyl), and 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 sodium dihydrogen phosphate·H2O, disodium hydrogen phosphate, acetic acid, water, and glyoxal. A target moisture content of 73% was established for the MHEC using ice. Once all ingredients were added, the mixture was kneaded for 30 minutes. The wet product was then pre-dried to air-dryness in a fluidized-bed dryer and dry-milled using an Alpine mill (Model D 100 UPZ) employing a 180 μm sieve. A final moisture content of less than 5% was achieved.

[0044] [Table 6]

[0045] The addition of acetic acid to the phosphate buffer as a catalyst for glyoxal crosslinking allowed the ST to be maintained even when the amount of glyoxal was halved. Accordingly, the total glyoxal and unbound / free glyoxal values ​​of MHEC were significantly reduced. The product is no longer subject to labeling requirements. The decrease in viscosity after aging was small and similar to that observed with pure phosphate buffer. The addition of acetic acid resulted in lower post-crosslinking after aging than MHEC without added acid in the phosphate buffer.

[0046] Example 6 Water-wettable MHEC (200 g dry material) with an average degree of substitution (DS) of 1.55 (methyl) and a molar substitution (MS) of 0.22 (hydroxyethyl), and a viscosity of 16,000 mPa·s, was placed in an LK5 laboratory mixer from Erweka Apparatebau GmbH, where it was mixed with an aqueous solution of sodium dihydrogen phosphate·H2O, disodium hydrogen phosphate, adipic acid, water, and glyoxal. A target moisture content of 73% was established for the MHEC using ice. Once all ingredients were added, the mixture was mixed for 30 minutes. The wet product was then pre-dried to air-dryness 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.

[0047] [Table 7]

[0048] The addition of adipic acid to the phosphate buffer as a catalyst for glyoxal crosslinking resulted in a significant increase in ST. Consequently, the amount of glyoxal used could be reduced by two-thirds. Accordingly, both total and free glyoxal in MHEC were significantly reduced. The decrease in viscosity after aging was small and similar to that observed when pure phosphate buffer was used. With the addition of adipic acid, crosslinking after aging was slight or no longer observed. Therefore, the product was not subject to labeling obligations. The ST and storage stability were the same as those of cellulose ethers prepared according to the prior art.

[0049] Example 7 Water-wettable MHEC (200 g dry weight) with an average degree of substitution (DS) of 1.62 (methyl) and a molar substitution (MS) of 0.21 (hydroxyethyl), and a viscosity of 5000 mPa·s, was placed in an LK5 laboratory kneader manufactured by Erweka Apparatebau GmbH. There, the water-wettable cellulose ether was mixed with either an aqueous solution of glyoxal, water, citric acid, and sodium hydroxide (Batch No. 7-1, for comparison) or an aqueous solution of sodium dihydrogen phosphate·H2O, disodium hydrogen phosphate, water, and glyoxal (Batch No. 7-2, for comparison), and with the acids indicated in the table (Batch Nos. 7-3 to 7-6). A target water content of 73% was established for the MHEC using ice. Once all ingredients were added, the mixture was kneaded for 30 minutes. The wet product was then pre-dried in a fluidized bed dryer to air-dry condition and dry-ground using an Alpine mill (model D 100 UPZ) employing a 180 μm sieve, thus achieving a final moisture content of less than 5%.

[0050] [Table 8]

[0051] From the table it is clear that even when the initial viscosity of MHEC was different, a positive effect could be obtained with the new crosslinker / buffer solution. <Additional Notes> The present invention includes the following aspects. <Section 1> Cellulose derivatives that are reversibly crosslinked with glyoxal and therefore have delayed water solubility 1. A method for producing a body, comprising the steps of: a) providing a water-wettable cellulose derivative; b) providing an aqueous solution comprising glyoxal, one or more alkaline earth metal and / or alkali metal salts of phosphoric acid as buffer substances; c) mixing the aqueous solution of b) with the cellulose derivative of a) at a temperature of 20-70°C to achieve reversible cross-linking of the cellulose derivative; d) drying the reversibly crosslinked cellulose derivative; and e) grinding the cellulose derivative Including, Steps d) and e) can be integrated into a grinding-drying operation, b) characterized in that the aqueous solution according to b) comprises an organic acid having a basicity of 1 or more, in which the amount of monobasic, dibasic, tribasic or polybasic organic acid is in the range of 0.002 to 0.015 mol and the amount of glyoxal is in the range of 0.010 to 0.050 mol per mole of anhydroglucose unit of the cellulose derivative in each case, the molar ratio of monobasic organic acid to glyoxal is in the range of 1:1 to 1:6, the ratio of dibasic organic acid to glyoxal is in the range of 1:1 to 1:10 and the ratio of tribasic organic acid to glyoxal is in the range of 1:2 to 1:12, method. <Section 2> Item 1. The method according to item 1, wherein the cellulose derivative is a nonionic cellulose ether, preferably methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, methylcellulose, or a mixture thereof. <Section 3> Item 3. The method according to item 1 or 2, wherein the cellulose derivative used is a cellulose ether in the form of a filter cake having a dry matter content of 30 to 70% after washing with hot water. <Section 4> Item 4. The method according to item 3, characterized in that the filter cake is added and kneaded with continuous mixing with a crosslinker solution consisting of glyoxal, a salt of phosphoric acid, an organic acid and optionally water, followed by drying and grinding or grinding-drying operation. <Section 5> Item 5. The method according to Item 4, wherein glyoxal is added to the cellulose ether in a proportion of 0.035 mol or less per mole of anhydroglucose units of the cellulose ether, more preferably 0.01 to 0.03 mol of glyoxal is mixed with the cellulose ether per mole of anhydroglucose units of the cellulose ether. <Section 6> Item 6. The method according to any one of Items 1 to 5, wherein the organic acid having a basicity of 1 or more is mixed with the cellulose ether in a ratio of 0.004 to 0.010 mol per 1 mol of anhydroglucose units of the cellulose ether. <Section 7> Item 7. The method according to any one of Items 1 to 6, wherein the organic acid having a basicity of 1 or more used is an aliphatic, aromatic and / or heterocyclic, preferably aliphatic, saturated or unsaturated carboxylic acid having 1 to 7 carbon atoms and 1 to 3 carboxyl groups, and the organic acid can contain functional groups, preferably hydroxyl and / or amino groups. <Section 8> Item 8. The method according to item 7, characterized in that the organic acid having a basicity of 1 or more used is acetic acid, lactic acid, salicylic acid, adipic acid, citric acid, tartaric acid and / or malic acid, preferably lactic acid or adipic acid. <Section 9> Item 1. The method according to Item 1, wherein the alkali metal salt of phosphoric acid is sodium dihydrogen phosphate and disodium hydrogen phosphate, and the sodium dihydrogen phosphate and disodium hydrogen phosphate are mixed with the cellulose ether in an amount of preferably 0.003 to 0.015 mol, more preferably 0.005 to 0.009 mol, per mole of anhydroglucose units of the cellulose ether, respectively. <Section 10> Item 10. The method according to item 1, wherein the buffer / crosslinker solution has a pH of 3 to 8, preferably 4 to 6.5. <Section 11> A cellulose derivative reversibly crosslinked with glyoxal, characterized in that it comprises, in each case per mole of anhydroglucose units of the cellulose derivative, 0.002 to 0.015 mol of an organic acid having a basicity of 1 or more, 0.010 to 0.050 mol of glyoxal, and one or more alkaline earth metal salts and / or alkali metal salts of phosphoric acid as buffer substances, and the molar ratio of the organic acid having a basicity of 1 or more to the glyoxal is in the range of 1:1 to 1:12. <Section 12> Item 12. The cellulose derivative reversibly crosslinked with glyoxal according to item 11, characterized in that it is a nonionic cellulose ether, preferably methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, methylcellulose, or a mixture thereof. <Section 13> 13. The cellulose derivative reversibly crosslinked with glyoxal according to claim 11 or 12, characterized in that the acid having a basicity of 1 or more is acetic acid, lactic acid, salicylic acid, adipic acid, citric acid, tartaric acid and / or malic acid, preferably lactic acid or adipic acid. <Section 14> Item 14. The cellulose derivative reversibly crosslinked with glyoxal according to any one of Items 11 to 13, wherein the alkali metal salt of phosphoric acid is sodium dihydrogen phosphate and disodium hydrogen phosphate, and the amount of sodium dihydrogen phosphate and disodium hydrogen phosphate contained therein is 0.002 to 0.015 mol, preferably 0.005 to 0.009 mol, per mol of anhydroglucose units of the cellulose ether in each case. <Section 15> Item 15. The cellulose derivative reversibly crosslinked with glyoxal according to any one of Items 11 to 14, characterized in that it contains less than 1000 ppm of free glyoxal.

Claims

1. 1. A method for producing a composition comprising a cellulose derivative reversibly crosslinked with glyoxal and thus having delayed water solubility, comprising the steps of: a) providing a water-wettable cellulose derivative; b) providing an aqueous solution comprising glyoxal and one or more alkaline earth metal and / or alkali metal salts of phosphoric acid as buffer substances; c) mixing the aqueous solution of b) with the cellulose derivative of a) at a temperature of 20-70°C to achieve reversible cross-linking of the cellulose derivative; d) drying the reversibly crosslinked cellulose derivative; and e) pulverizing the cellulose derivative Including, Steps d) and e) can be integrated into a grinding-drying operation; the aqueous solution according to b) comprises an organic acid having a basicity of 1 or greater, the organic acid having a basicity of 1 or more is an aliphatic, aromatic and / or heterocyclic carboxylic acid having 1 to 7 carbon atoms and 1 to 3 carboxyl groups, and the organic acid may contain a hydroxyl group and / or an amino group as a functional group; In any case, the amount of monobasic, dibasic, tribasic or polybasic organic acid is 0.002 to 0.015 mol, the amount of glyoxal is 0.010 to 0.050 mol, the molar ratio of monobasic organic acid to glyoxal is in the range of 1:1 to 1:6, the ratio of dibasic organic acid to glyoxal is in the range of 1:1 to 1:10, and the ratio of tribasic organic acid to glyoxal is in the range of 1:2 to 1:12, per mole of anhydroglucose unit of the cellulose derivative; the cellulose derivative is a nonionic cellulose ether, the water-wettable cellulose derivative is a cellulose ether in the form of a filter cake having a dry matter content of 30 to 70% after washing with hot water; method.

2. The method described in claim 1, characterized in that the cellulose derivative is methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, methyl cellulose, or a mixture thereof.

3. The method according to claim 1, characterized in that the filter cake is added and kneaded while continuously mixing with a crosslinker solution consisting of glyoxal, a salt of phosphoric acid, an organic acid and optionally water, followed by drying and grinding or subjecting to a grinding-drying operation.

4. 4. The method of claim 3, wherein glyoxal is mixed with the cellulose ether in a proportion of 0.035 mol or less per mole of anhydroglucose units of the cellulose ether.

5. 5. The method according to claim 4, wherein glyoxal is mixed with the cellulose ether in a proportion of 0.01 to 0.03 mol per mole of anhydroglucose unit of the cellulose ether.

6. 6. The method according to claim 1, wherein the organic acid having a basicity of 1 or more is mixed with the cellulose ether in a ratio of 0.004 to 0.010 mol per mole of anhydroglucose unit of the cellulose ether.

7. 2. The method according to claim 1, characterized in that the organic acid having a basicity of 1 or more used is acetic acid, lactic acid, salicylic acid, adipic acid, citric acid, tartaric acid and / or malic acid.

8. 2. The method of claim 1, wherein the alkali metal salt of phosphoric acid is sodium dihydrogen phosphate and disodium hydrogen phosphate, and the sodium dihydrogen phosphate and disodium hydrogen phosphate are mixed with the cellulose ether.

9. 10. The method of claim 1, wherein the buffer / crosslinker solution has a pH of 3-8.

10. 10. The method of claim 9, wherein the buffer / crosslinker solution has a pH of 4 to 6.

5.

11. 1. A composition comprising a cellulose derivative reversibly crosslinked with glyoxal, in each case per mole of anhydroglucose unit of the cellulose derivative, an organic acid having a basicity of 1 or more in a proportion of 0.002 to 0.015 mol, and glyoxal in a proportion of 0.010 to 0.050 mol, and one or more alkaline earth metal and / or alkali metal salts of phosphoric acid as buffer substances, the molar ratio of organic acid having a basicity of 1 or more to glyoxal being in the range of 1:1 to 1:12, the organic acid having a basicity of 1 or more is an aliphatic, aromatic and / or heterocyclic carboxylic acid having 1 to 7 carbon atoms and 1 to 3 carboxyl groups, and the organic acid may contain a hydroxyl group and / or an amino group as a functional group; A composition wherein the cellulose derivative is a nonionic cellulose ether.

12. The composition of claim 11, characterized in that it is methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, methyl cellulose, or a mixture thereof.

13. 12. The composition according to claim 11, characterized in that the acid having a basicity of 1 or more is acetic acid, lactic acid, salicylic acid, adipic acid, citric acid, tartaric acid and / or malic acid.

14. 14. The composition according to claim 11, wherein the alkali metal salts of phosphoric acid are sodium dihydrogen phosphate and disodium hydrogen phosphate, and the amount of sodium dihydrogen phosphate and disodium hydrogen phosphate contained therein is in each case 0.002 to 0.015 mol per mole of anhydroglucose units of the cellulose ether.

15. Composition according to any one of claims 11 to 14, characterized in that it contains less than 1000 ppm of free glyoxal.

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