MXPA02002357A - Low viscous cellulose ethers that flocculate in hot water, method for the production thereof and their use. - Google Patents

Abstract

The invention relates to low viscous cellulose ethers that flocculate in hot water and that have a high degree of purity and whiteness. The invention also relates to a method of producing the same by acidic catalyzed hydrolytic depolymerization in the presence of an oxidation agent, as well as to the use thereof.

Description

EFFECTS OF LOW VISCOSITY CELLULOSE THAT FLOCCULATE IN HOT WATER. METHOD FOR THE PRODUCTION OF THEM AND THEIR USE • 5 DESCRIPTIVE MEMORY The present invention relates to cellulose ethers of low viscosity of high purity and high whiteness coagulable in hot water, to a process for the preparation of these cellulose ethers by depolymerization by means of acid catalyzed hydrolytic degradation, where it is suitable in the presence of an oxidation agent, as well as the use thereof. The degradation of cellulose ethers with high degrees of polymerization has been known for a long time and can be achieved from different forms. In particular, the degradation to very low viscosity products has attracted considerable attention, because these products can be advantageously used inter alia as a coating material for seeds or active pharmaceutical ingredients, but also, for example, in suspension polymerization. The ethers of • 20 cellulose whose HOPPER viscosity, measured as a 2.0% solution (absolutely dry) in water at 20 ° C, is not higher than 50 mPas, are referred to below as very low viscosity products.

... A », The procedures employed to degrade cellulose ethers include, in addition to acid catalyzed hydrolytic cleavage of the acetal bond, oxidative degradation inter alia and degradation by high energy radiation or microorganisms / enzymes. The reduction of the viscosity of cellulose ethers by irradiation with high energy radiation is described, for example, in DE-A-44 34 280 and US-A-2 895 891. Salt-free cellulose ethers are also they are linked instead of degraded, if appropriate reaction conditions are chosen. The disadvantages of the process are essentially the production of inhomogeneous materials (depends on the degree of polymerization of the thickness of the irradiated layer, the penetration depth of the radiation, the intensity of the radiation), the impossibility of eliminating by-products from the mixture. of the reaction, as well as the radiation treatment that is generally exposed to public criticism. The methods for the oxidative degradation of cellulose ethers are described inter alia in US-A-2 912 431, US-A-4 316 982, CH-B-461 455, DE-A-20 16 203 and GB -B-953 944. US-A-2 912 431 discloses a process in which hypohalites, peroxides or periodates degrade carboxymethylcelluloses in a mixture with aqueous alcohol of 40 to 80 ° C with simultaneous bleaching. The degradation of cellulose ethers that are wetted with water and have a dry content of 40 to 75% using ozone / air / oxygen mixtures of 0 to 60 ° C, is described in US-A-4 316 .. __, ._. 982. CH-B-461 455 describes a process in which the cellulose ether with a maximum water content of 75% is mixed with aqueous solution of hydrogen peroxide of 0.1 to 10% concentration. The resulting mixture is then oxidatively degraded and dried at a temperature of 100 to 250 ° C until H202 is consumed. DE-A-20 16 203 discloses a process for the degradation of cellulose ethers in which a very substantially dry powder with a maximum water content of 5%, is mixed with hydrogen peroxide solution and is degraded to a temperature between 50 and 150 ° C. In GB-B-953 944, the viscosity of the water-soluble nonionic cellulose ethers is reduced in the dry or wet state, by reaction with H2? 2 at elevated temperatures. The oxidative degradation of cellulose ethers generally leads to the formation of numerous residues. If the degradation occurs in the finished product or moistened without a subsequent purification step in order to optimize the result, the residues that have formed are no longer removed from the depolymerized product. The very low viscosity cellulose ethers prepared by oxidative degradation are usually colored, due to the drastic reaction conditions. Simple methods of hydrolytic degradation with inorganic or organic acids are described, for example, in US-A-1 679 943, US-A-1 943 461, EP-B-0 497 985 and EP-A-0 210 917. In US-A-1 943 461, the pre-ground cellulose ethers are degraded with dilute acids or mixtures of the __. __.__ ,, ___ _ ,. ___ fa_í. _ ,,. «*." _.__. "^" ", _ ,,« _fcj_, s < _. _ ». *, ..« «atesa», v. . »" _ »... . ", F ___ M M The same (concentration of 0.5 to 5%, a multiple of the weight of the cellulose ether to be degraded) in a closed pressure vessel under a pressure of 0.7 to 5.2 bar and at temperatures between 115 and 160 ° C for 20 to 60 minutes. US-A-1 679 943 describes the degradation of cellulose ethers with different acid mixtures, without the need for either a pressure vessel or an elevated temperature. However, especially at room temperature, this results in unacceptably long reaction times, which could be around several days. In EP-B-0497 985, pulps with a low copper number, that is, a high content of α-cellulose, are washed, dried, milled and mixed with 0.5% by weight of aqueous HCl solution to a temperature of approximately 70 ° C. The water content of the cellulose ether plays an important role in this process. On the other hand, should not fall below 1% during drying, because excessive drying leads to hardening and yellowing of the products but, on the other hand, should not be greater than 5% during the degradation, because considerable amounts of water they favor the formation of gel with low molecular weight cellulose ethers. The resulting cellulose ethers have very low viscosities (< 20 mPas, 2.0% solution at 20 ° C). A similar procedure is described in EP-A-0 210 917. In this case, a cellulose ether powder containing from 3 to 8% water is degraded with from 0.1 to 1% by weight of an aqueous solution. of HCl at a temperature of 40 to 85 ° C.

In particular, degradation to products of very low viscosity of high purity can not be achieved using HCl as a gas. Methods of this type are described, for example, in US-A-3 391 135 and US-A-4 061 859. US-A-3 391 135 describes a process for preparing cellulose ethers with viscosities of solution of less than 10 mPas (2.0% solution at 20 ° C) from high viscosity cellulose ether powders with water contents of less than 5% at a temperature of 30 to 80 ° C. The excess HCl gas is removed and the cellulose ether is then neutralized by mixing a weak base. In accordance with US-A-4 061 859, the cellulose ethers are degraded as dry powders with a water content of 0.01 to 5% by weight, using hydrogen halide at a temperature of 15 to 80 ° C, and then they are neutralized by mixing sodium bicarbonate or passing to ammonia gas. The whitening of the obtained material is achieved with sulfur dioxide gas, with which the degraded material is contacted after the depolymerization step. With this procedure it is possible to degrade cellulose ethers to products of very low viscosity whose initial viscosity was several hundred thousand mPas. A whitening step following depolymerization is possible to rinse the products. The hydrolytic degradation is moderate and neutral with respect to the functional groups and can be used to prepare products of very high concentration. > Aja viscosity. However, if the cellulose ether to be degraded is present in a relatively considerable dilution in aqueous medium, it is hardly possible to avoid yield losses through the partial development of the material. If, on the other hand, completely dried, ground and prepared cellulose ethers are degraded with a little aqueous or gaseous HCl, neutralization by mixing with weak bases is necessary. This increases the salt content of the finished product and the residues of the reaction are not removed from the product. If the water content during degradation is too high, only with difficulty is it possible avoid partial dissolution and adhesion of the material. The homogenous distribution of small amounts of acid and a maximum uniform degree of polymerization are then equally difficult to achieve. If the reaction is • performed under particularly moderate conditions, it is only possible to reduce, but not suppress, yellowing. Although the subsequent whitening of the products increase the whiteness of the material, this supposes an additional step in the procedure and does not lead to the elimination of the formed by-products. Therefore, one purpose of the present invention was to provide a process for the depolymerization of cellulose ethers without the • 20 mentioned disadvantages of the prior art. In particular, possible ways of preparing cellulose ethers of very low viscosity were looked for which, in addition to maximally uniform polymerization grades and small waste constituents, have a very low salt content and a high whiteness and are intended to provide transparent solutions with high transmittances, so that they can be used advantageously, in particular in the areas of coating polymerization of suspensions, drugs (tablets, dragees, capsules), cosmetics and food. This purpose is achieved through a process for the depolymerization of coagulable cellulose ethers in hot water through the hydrolytic degradation by means of acids, which is characterized in that the degradation is carried out at a temperature above the dark point of the ether of cellulose as a concentrated aqueous suspension. In this regard, it is possible to use as cellulose ethers all the cellulose ethers known to be coagulable in hot water. Preference is given to alkylcelluloses, such as, for example, methyl, ethyl and propylcellulose and the mixed ethers thereof, such as, for example, hydroxyethylmethyl, hydroxypropylmethyl, ethylhydroxyethyl and ethylmethylcellulose. The preparation and processing of the cellulose ethers used for degradation are not absolutely restricted in any way. These can be prepared and processed by all procedures known to the person skilled in the art (Ullmann's Enzyklopadie der Technischen Chemie, Volume 9, "Celluloseether", Verlag Chemie, Weinheim, 4th edition 1975, pp. 192 ff). In the same way, the degree of polymerization and the viscosity of the cellulose ethers to be used are not absolutely restricted in any way. However, the cellulose ether used for the degradation should, preferably, have a degree of polymerization not too far from the degree of polymerization that is intended to be achieved through depolymerization. In a particularly preferred embodiment, the method of * 5 The invention is used to prepare very low viscosity cellulose ethers having HOPPLER viscosities, measured as a 2.0% solution (absolutely dry) in water at 20 ° C, from < 50 mPas. The acids suitable for hydrolytic degradation are mineral acids, but also strong organic acids, and mixtures thereof.

However, mineral acids are preferred. The mineral acids used are, preferably, hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid. However, it is also possible to use mixtures thereof. The strong organic acids used are preferably trifluoroacetic acid, acetic acid, formic acid, oxalic acid, phthalic acid, maleic acid and benzoic acid. However, it is also possible to use mixtures thereof. The acid-catalyzed hydrolytic degradation is carried out, according to the invention, above the dark point of the cellulose ether. A temperature scale of 70 to 105 ° C is preferred. The process of the invention is further characterized in that the degradation is carried out as a concentrated suspension. The proportion of water to cellulose ether preferably does not exceed 10: 1, * • * _. _. The higher preference does not exceed 7: 1 and, more preferably, does not exceed 5: 1, by weight. The use of minimal amounts of water as a suspending agent leads to very small losses of performance, which are generally caused by the dissolution of low viscosity constituents. In a more preferred embodiment, oxidation agents are added to the concentrated aqueous suspension before, during and / or after depolymerization in acidic or neutral medium. The oxidizing agents used are, preferably, hydrogen peroxide and salts thereof, other peroxo compounds such as, for example, sodium peroxosulfate, perborates (also in combination with activators such as TAED), sodium chlorite, halogens. , halogen oxides and other compounds used for bleaching. Hydrogen peroxide (H202) is particularly preferred. The oxidizing agents are generally used in this respect in amounts of 0.01 to 20% by weight, preferably 0.01 to 10% by weight, and more preferably 0.01 to 5% by weight, based on the cellulose ether. Surprisingly, it has been found that the addition of oxidation agents even in small amounts of clearly less than one percent, preferably during hydrolytic degradation, in addition to reducing the viscosity, leads to residues of the reaction which are usually so less partially absorbed in the cellulose ether and causes a coloration thereof, being converted by oxidation to a form with a better water solubility. This leads to an improved removal of the residues of the depolymerized cellulose ether. Accordingly, the use of oxidation agents such as, for example, H2O2, leads to an improvement in the whiteness of the products, with a simultaneous increase in the degree of uniformity. In addition, the addition of oxidation agents ensures a further reduction of the final viscosity under conditions that are otherwise identical. Therefore, it is possible to ensure a defined target viscosity to reduce the reaction time and / or decrease the amount of acid, as compared to carrying out the process without the addition of oxidizing agents. The addition of the oxidation agent is, in principle, also conceivable before or after the hydrolytic degradation by means of acids, but addition during the polymerization is preferred. Further degradation and rinsing through oxidation and / or elimination of by-products can be performed either in one step or in succession, specifically both in the acidic medium and after (partial) neutralization has occurred. An additional possibility is to carry out the degradation both in the already formulated product and in the raw products moistened, with the usual moisture content. Since a subsequent drying and grinding is necessary to formulate the product, it is preferable that the raw products moistened, as they result in the production processes, be used for the degradation.

The degradation can, for example, be carried out in place of the last washing step, when the salt content has been partially reduced, since the excess salt of the reaction to give the cellulose ether is likewise eliminated by the aqueous suspension. The resulting products have an extremely low salt content. The aqueous solutions of the degraded cellulose ethers generally have slightly acidic pH values, due to the generation of acidic groups in the basic structure of the cellulose ether. The pH of these solutions can be adjusted to a substantially neutral pH of 5.5 to 8.0 using, after depolymerization, not water but an aqueous solution of at least one basic salt (such as, for example, sodium carbonate, sodium bicarbonate). , sodium sulfate, sodium bisulfate) to wash the degraded cellulose ether, at a temperature above the dark point of the degraded cellulose ether, to make the byproducts of the reaction disappear. An additional mixing step, as described in other methods to adjust a target pH, is unnecessary. The base is evenly distributed and the cellulose ether can, in the normal procedure, be dried and milled in one or more steps. The yields of the process described are generally between about 80% and 96%, depending on the final viscosity required and the level of the dark point of the cellulose ether to be degraded. Since in the aqueous preparation of cellulose ethers generally a wash loss of between 3 and 8% is produced, iá? ÁákJinim-Ai ali --ii. depending on the viscosity, and that the degradation can be performed in place of a washing step, the yield losses are minimized. It is possible to increase the performance, in particular with products having a high dark point, by increasing the degradation temperature when carried out in a pressure apparatus. The viscosity of the resulting products can be adjusted essentially, as desired, through the amount of acid employed, the reaction time and, when appropriate, the amount of additional oxidation agent, and is highly reproducible. Due to the uniform degradation associated with good mixing and distribution of the 10 reagents, the products have virtually uniform polymerization grades. Through the process of the invention, it is possible to prepare cellulose ethers of high purity and high whiteness. Particularly good results are achieved in the preparation of methylhydroxypropylcelluloses. Therefore, the present invention is also related to methylhydroxypropylcelluloses with a Hoopler viscosity, measured in a 2.0% (absolutely dry) solution in water at 20 ° C, preferably from < 50 mPas and, more preferably in particular, < 5 mPas. The whiteness of the methylhydroxypropyl cellulose having a viscosity on the scale of 5 to 50 mPas, is preferably greater than 60%, and in the methylhydroxypropyl cellulose having a viscosity of < 5 mPas is preferably greater than 50% (base: DIN 5033 standard). Since the whiteness depends inter alia on the particle size distribution of the cellulose ether, the - EÉ_, £ _ £ • established values are related to products whose proportion of particles with a size of < 125 μm does not exceed 50%, but is preferably less than 10%. The salt content of the prepared cellulose ethers is preferably less than 0.4% by weight, more preferably less than 0.2% by weight and, even more preferably, less than 0.1% by weight. The most preferred methylhydroxypropylcelluloses have a • content of methoxy groups on the scale of 28 to 32% by weight and a content of hydroxypropyl groups on the scale of 5 to 9% by weight. Due to their purity and high whiteness, the described cellulose ethers are especially suitable for the coating of pharmaceutical products and seeds, as well as for use in cosmetics, food or • in suspension polymerization. The invention is described in detail below by means of 15 examples of modalities which, however, are not restricted thereto. Viscosities were measured in the examples, unless otherwise indicated, 2.0% concentration (absolutely dry), in aqueous solution, using a Hoppler ball drop viscometer distributed by Haake. Unless otherwise indicated, the stated amounts of acid mean percentages by weight of concentrated HCl (37% concentration), based on the amount of cellulose ether used.

EXAMPLES 1 TO 12 14 kg of water are heated until boiling, in a 30 I glass vessel distributed by QVF, and are agitated with a paddle type stirrer. The appropriate amount of concentrated hydrochloric acid is then added slowly, and 5 kg (absolutely dry) of starting material are dispersed as a product moistened with water (dry content of f about 60%). After this, 50 g of H202 (100%) are introduced as an aqueous solution of any concentration, through a dropping funnel, which corresponds to using an amount of 1% by weight based on the cellulose ether. The mixture is stirred at a temperature of 90 to 100 ° for the time established in Table 1 and is then neutralized to a pH of 6.5 to 7.5, by slow addition of an equimolar amount of dilute sodium hydroxide solution (1 part of Concentrated NaOH (50% concentration) + 3 parts water) over the course of 30 minutes. The resulting product is filtered under suction, through a porous glass filter with suitable pore size, and is washed with a little boiling water, in order to eliminate residues of suspending agent from the product, some of the which have a considerable color. The product 20 is then dried and pulverized in a commercially available mill. The grinding parameters chosen for this are such that the resulting granules have the following particle size distribution: > 500 μm < 5% l_Ut-.e_.i_ i _. . iné .. t..i fcfc.f. ....... a.j? _ * .._ .. ^. j a. . > t & .. ",,. . ! Ír.i, »¡i: 500 - 125 μm > 85% < 125 _tn < 10% The characteristic analytical data summarized in Table 1 are determined for the granules.

TABLE 1 Acid catalyzed degradation of coagulable cellulose ethers in • water (MHPC) with and without additional oxidation agent • 1) A. Lung pulp 1, methylhydroxypropylcellulose, OCH3 = 30.0%, OC3H6 = 7.2%; raw product moistened with water, dry content of approximately 61%, initial viscosity before degradation (1.9% absolutely dry): 340 mPas. B. Blot pulp 2, methylhydroxypropylcellulose, OCH 3 = 29.1%, OC 3 H 6 = 6.5%; raw product moistened with water, dry content of -J.X_ _ _ > _ '.t, ¿_. _....-_. s_aMt¿_, ** approximately 56%, initial viscosity before degradation (1.9% absolutely dry): 50 mPas. C. Lung pulp 3, methylhydroxypropylcellulose, OCH3 = 29.8%, OC3H6 = 6.9%; raw product moistened with water, dry content of about 62%, initial viscosity before degradation (1.9% absolutely dry): 35,000 mPas. 2) percentage by weight of concentrated hydrochloric acid (37% of • concentration) based on cellulose ether (absolutely dry). 3) base: DIN 5033 standard; measured with LF 90 colorimeter (manufactured 10 by Dr. Bruno Lange) against white standard (white enamel standard; reflectance specification = 71.5%) by measuring the reflectance in% at a defined wavelength; Universal UME 3 measurement unit, color sensor • LF 90, measurement geometry of 0 45 °, type of normal C light, glass specimen, blue filter BG12 / 5 (447 nm), protective light cover d = 50 mm. 15 4) NaCl content due to the neutralization of hydrochloric acid with sodium hydroxide solution. 5) Transmission measured in aqueous solution of 1.0% concentration (absolutely dry) at 578 nm and 415 nm against water as standard. 6) with addition of 10 g of H2O2. • 20 7) with addition of 50 g of NaCl02.

EXAMPLES 13 AND 14 The procedure is as described in Examples 1 to 12, with the exception that 25 kg of water are present initially, instead of 14 kg, and 3.75 kg (absolutely dry) of raw product moistened for degradation are used.

TABLE 2 Acid catalyzed degradation of coagulable cellulose ethers in hot water (MHEC) with and without additional oxidation agent 1) D Lint pulp 4, methylhydroxypropylcellulose, OCH3 = 27.8%, OC2H4 = 5.1% (dry air with 1.4% moisture content); raw product moistened with water, dry content of approximately 59%, initial viscosity before degradation (1.9% absolutely dry): 176,000 mPas. • _._____, __, __. < ____________ t _______________.

Claims (17)

1. NOVELTY OF THE INVENTION CLAIMS 1. A process for the depolymerization of coagulable cellulose ethers in hot water by hydrolytic degradation by means of acids, characterized in that the degradation is carried out at a temperature "Higher than the dark point of the cellulose ether as a concentrated aqueous suspension, and because, in addition, oxidation agents are added to the 10 concentrated aqueous suspension, before, during and / or after depolymerization in an acidic or neutral medium.
2. 2. The process according to claim 1, further characterized in that the methyl, ethyl, propyl, hydroxyethylmethyl, hydroxypropylmethyl, ethylhydroxyethyl or ethylmethylcellulose is used as the ether of 15 cellulose.
3. 3. The process according to claim 1 or 2, further characterized in that the degraded cellulose ether has a Hoopler viscosity, measured as a 2.0% solution (absolutely dry) in water at 20 ° C, of < 50 mPas.
4. 4. The process according to at least one of the preceding claims, further characterized in that mineral acids and / or organic acids are used as acids. _ '; _, __ __, _ J ._a¿__ .i. TO _-_". . - «_... 5. - The process according to claim 4, further characterized in that hydrochloric, sulfuric, nitric and / or phosphoric acids are used as mineral acids. 6. The method according to at least one of the preceding claims, further characterized in that the ratio of water to cellulose ether does not exceed 10: 1 by weight. 7.- The procedure in accordance with at least one of the • preceding claims, further characterized in that the peroxo compounds, perborates, sodium chlorite, halogens and / or halogen oxides are used as oxidation agents. 8. The process according to claim 7, further characterized in that the hydrogen peroxide is used as • Oxidation agent. 9. The process according to at least one of the preceding claims, further characterized in that the oxidation agent is used in an amount of between 0.01 and 20% by weight, based on the cellulose ether. 10. The method according to at least one of the preceding claims, further characterized in that, after • 20 the depolymerization, the degraded cellulose ether is washed with at least one aqueous solution of a basic salt, at a temperature higher than the dark point of the degraded cellulose ether, in order to adjust the Aqueous cellulose ether degraded to a pH within the range of 5.5 to 8.0. 11. The process according to claim 10, further characterized in that the sodium carbonate, sodium bicarbonate, sodium sulfate and / or sodium bisulfate are used as salt. I. 12.- A methylhydroxypropylcellulose with a Hoopler viscosity, measured as 2.0% solution (absolutely dry) in water at 20 ° C, from < fifty mPas, obtainable through a method as claimed in at least one of the preceding claims. 10 13.- A methylhydroxypropylcellulose with a Hoopler viscosity, measured as 2.0% solution (absolutely dry) in water at 20 ° C, from < fifty mPas, characterized because it has a whiteness, determined by measuring the reflectance in% at 447 nm against a white standard (white enamel standard, reflectance specification = 71.5%), which is greater than 50%, with. 15 a particle size distribution in which the proportion of particles with a size of < 125 μm does not exceed 50%. 14. - The methylhydroxypropylcellulose according to claim 13, with a Hoopler viscosity of 5 to 50 mPas, characterized in addition to the whiteness, determined by measuring the reflectance in% at 447 20 nm against a white standard (white enamel standard, reflectance specification = 71.5%), is greater than 60%. IJ JJ i J., _, __, _.__ t.-._. _. 15. - The methylhydroxypropylcellulose according to claim 13 or 14, further characterized in that it has a salt content of less than 0.4% by weight. 16.- The methylhydroxypropylcellulose in accordance with 5, except one of claims 13 to 15, further characterized in that M has a content of methoxy groups within the range of 28 to 32% by weight and a content of hydroxypropyl groups within the range of 5 to 9% by weight. 17. The use of methylhydroxypropyl celluloses as claimed in at least one of claims 13 to 16 for coating pharmaceutical products or seeds and for use in cosmetics, foods or in suspension polymerization. 9 *. •