WO2002042334A1 - Method for purifying non-ionic alkyl cellulose ethers - Google Patents

Abstract

The invention relates to a method for purifying non-ionic alkyl cellulose ethers which is characterized by dissolving the non-ionic alkyl cellulose ether in a suitable solvent or solvent mixture, removing the residues and then isolating the alkyl cellulose ether from the solvent or from the solvent mixture.

Description

description

Process for the purification of nonionic alkyl cellulose ethers

The present invention relates to a process for the purification of nonionic alkyl cellulose ethers as part of their workup, which leads to products via a combination of a solution of the alkyl cellulose ethers in water or water / solvent mixtures, removal of insoluble residues and subsequent removal of the alkyl cellulose ethers from the solvent or solvent mixture come off without residue.

The production of cellulose ethers with uniform or different substituents is known (see, for example, Ullmann's Enzyklopadie der Technischen Chemie, Vol. 9, "Celluloseether", Verlag Chemie, Weinheim, 4th edition, 1975, pp. 192ff; K. Engelskirchen: "Polysaccharide- Derivatives "in Houben Weyl, Vol. E20 / III, 4th edition, Georg Thieme Verlag, Stuttgart, 1987, pp. 2042ff).

For the production of these cellulose ethers, such as methyl cellulose, methyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose or

Ethylhydroxyethylcellulose, the starting material, the pulp, is first ground to enlarge the surface, the particle size generally being less than 2.5 mm, if possible even less than 1 mm. The resulting voluminous pulp powder is converted into "alkali cellulose" by adding base, such as NaOH, KOH, LiOH or ammonium hydroxides, in solid or liquid form. This is followed, with or without isolation of the alkali cellulose, by a single-stage or multi-stage, continuous or discontinuous etherification using the appropriate reagents. The resulting cellulose ethers are purified in a known manner from reaction by-products, dried and ground with water or suitable solvent mixtures and optionally mixed with other components. Solutions of alkyl cellulose ethers produced in this way generally give insoluble residues of between 0.1 and 20% by weight, based on the amount of alkyl cellulose ether used. The absolute residue level is significantly influenced by the type of etherification, the amount of etherification and the uniformity of the substituent distribution along the anhydroglycose backbone.

The insoluble residues mainly consist of cellulose fibers and / or swelling bodies. The pulp fibers are low-etherified constituents of the pulp, the fiber structure of which is practically completely preserved and whose hydroxyl groups are not accessible to the solvent, generally water, since they are integrated into a dense network via solid intra- and / or intermolecular hydrogen bonds are. Swelling bodies occur with sufficient but uneven etherification. Although the polar groups of the cellulose ether are accessible in a wide range for the formation of a hydration shell, some areas of the polymer chains are substituted so low or the fibrillar character is still preserved, so that complete solvation of the polymer molecules is prevented. Swell body residues therefore increasingly occur when the average degree of polymerization of the pulp used is increased.

Essential criteria of the product quality of alkyl cellulose ethers are, among other things, the residue level in aqueous solution and the filter residue that usually correlates with it.

The product quality in the production of the alkyl cellulose ethers can be influenced in particular by the process of disintegrating the pulp by means of a strong base, the so-called alkalization. Great importance must be attached to adequate homogenization of the heterogeneous system. The polymer chains of the pulp used, which are closely linked via hydrogen bonds, are at least partially dissolved by the action of the base and the fibrillar structure is expanded so that the hydroxyl groups of the anhydroglycose backbone are made accessible to the reagents with which they are to be reacted and activated by deprotonation , Depending on their origin, type and pretreatment, pulps have different degrees of crystallinity. The degree of crystallinity describes the relationship between crystalline and amorphous areas of the pulp and can be used as an approximate measure for an estimate of how easily a pulp can be broken down. The hydroxyl groups of Linters pulps, which generally have a high degree of crystallinity, are difficult to access for the alkali. Sufficient digestion takes a comparatively long time or a large amount of alkali compared to the digestion of an amorphous wood pulp.

In addition to the absolute amount of substitution, the uniform distribution of the substituents can be influenced to a certain extent via the reaction conditions, a suitable reaction procedure and the type and amount of mixed etherification. Regardless of the targeted influencing of these parameters, however, it is practically impossible in the production of alkyl cellulose ethers according to the prior art to produce products which dissolve without residues. A small residue remains.

However, for certain applications of cellulose ethers, particularly in the pharmaceutical sector (coating of tablets) or in the polymerization (production of transparent films), it is very important that the solutions prepared from the cellulose ethers have the lowest possible insoluble constituents, since fibers in particular form during film formation can lead to an irregular surface or lead to undesirable solid inclusions in transparent films.

Nonionic cellulose ethers with a flocculation point below the boiling point of water are usually cleaned of reaction by-products and salts by treating them with hot water or water-solvent mixtures (see e.g. Ullmann's Encyclopedia of

Technical Chemistry, Vol. 9, "Celluloseether", Verlag Chemie, Weinheim, 4th edition 1975, p. 192ff). The cellulose ethers are at all times during the Processing undissolved or swollen to the maximum, however, does not lose its solid-like consistency.

WO-A-98/31710 describes a process for the production of finely divided polysaccharide derivatives in which the cellulose ethers are swollen or dissolved in a sufficient amount of solvent or solvent mixture, so that superordinate structures are largely eliminated. The pretreated material is then converted into a finely divided powder using a mill-drying device or by dispersing it in a non-solvent surrounding medium.

In JP-A-10/287701, cellulose ethers are mixed with water in order to obtain emulsified liquids of low viscosity, which are then spray-dried. The result is fine cellulose ether powder with a narrow particle size distribution.

A disadvantage of the known processes is that the cellulose ethers are not separated from insoluble, low etherification components similar to cellulose fibers. The result is products whose solutions have residues.

The object of the present invention was therefore to develop a process for the purification of nonionic alkyl cellulose ethers which provides solutions which are virtually free of residues.

The object of the invention is achieved by a process for the purification of nonionic alkyl cellulose ethers, which is characterized in that the nonionic alkyl cellulose ether is dissolved in a suitable solvent or solvent mixture, then the residues are removed and then the alkyl cellulose ether is separated from the solvent or solvent mixture. This results in solid alkyl cellulose ethers, which dissolve without residue. The height of the filter residue can be used as a criterion for the absence of residues. This should be less than 0.1% by weight, preferably less than 0.09% by weight and particularly preferably less than 0.08% by weight, based on the dry cellulose ether, corrected for the moisture. The cellulose ethers purified by the process according to the invention have a filter residue which is reduced by at least 60% compared to the cellulose ethers cleaned according to the prior art.

Preferred nonionic alkyl cellulose ethers are methyl cellulose, methyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose and ethyl hydroxypropyl cellulose.

Water is particularly preferred as the solvent.

Mixtures of water and organic solvents, such as. B. methanol or ethanol, in suitable proportions. In this context, “suitable” means a ratio that is capable of dissolving the corresponding alkyl cellulose ether. A solvent mixture which contains a maximum of 80% methanol or ethanol and at least 20% water is preferred.

Solvent mixtures of methanol with dichloromethane, chloroform, carbon tetrachloride, dichloroethane, trichloroethane or chlorobenzene can also be used in suitable proportions. Solvent mixtures containing 20 to 60% methanol are preferred here. Solvent mixtures of ethanol with dichloromethane, chloroform, carbon tetrachloride, dichloroethane, trichloroethane or chlorobenzene can also be used. Solvent mixtures containing 25 to 55% ethanol are preferred here.

The weight ratio of solvent or solvent mixture to nonionic alkyl cellulose ether is chosen depending on the average degree of polymerization of the alkyl cellulose ether so that the viscosity of the resulting solution, from which the residues are to be separated, does not exceed 20,000 mPas, measured according to Höppler.

In order to simplify the handling of the cellulose ether solution in the course of the work-up, the viscosity of the solution should preferably be below 10000 mPas and particularly preferably below 5000 mPas. The smaller the average polymer chain length of the alkyl cellulose ether, the lower the resulting viscosity of the solution and the lower the weight ratio of solvent or solvent mixture to alkyl cellulose ether can be selected.

The separation of the insoluble residues contained in the solvent or solvent mixture can be carried out using common separation processes such as Filtration, centrifugation or decanting take place.

The subsequent separation of the alkyl cellulose ether from the solvent or solvent mixture can be carried out, for example, by heating the solution to a temperature above the flocculation point of the cellulose ether with flocculation and subsequent separation from the solvent or solvent mixture using suitable units, e.g. Filters, centrifuges or decanters are made.

It is also possible to spray dry the solution. Likewise, the solvent or solvent mixture can be separated from the cellulose ether by at least partial distillation.

Surprisingly, it has been found that the separation of the nonionic alkyl cellulose ethers, in particular the flocculation from heated or boiling, aqueous solution, can be carried out particularly well in the presence of special salts. The presence of the salts causes an agglomeration of the finely divided precipitation particles to form larger groups that are easier to separate. If these salts are completely absent, a very finely divided flocculation product precipitates, the separation of which is much more difficult. Preferably If sulfates, carbonates and / or chlorides and particularly preferably sodium sulfate, sodium carbonate and / or sodium chloride are used as flocculation aids and facilitate the precipitation of the alkyl cellulose ether from a heated solution.

It is also possible to use compounds other than the salts mentioned which facilitate flocculation of the corresponding cellulose ethers and in particular reduce the flocculation temperature. Flocculation can also be carried out by adjusting the aqueous solutions containing alkyl cellulose ethers to an alkaline pH by means of bases. Preferred bases are e.g. Lithium hydroxide, sodium hydroxide, potassium hydroxide and / or ammonia solution. It is also conceivable to add special polyacrylamides or other flocculants.

The alkyl cellulose ethers are preferably dissolved in the solvent or solvent mixture as a crude product which is not or only partially purified, as is obtained after the conversion from cellulose to cellulose ether. This means that in particular reaction by-products and salts such as sodium chloride are still present. The salt content of the cellulose ethers is preferably between 0.5 and 300% by weight, and particularly preferably between 1 and 60% by weight, based on the cellulose ether.

The salts are removed particularly effectively from the product in the course of the work-up process if polar solvents such as water or water-containing solvent mixtures are used to dissolve the alkyl cellulose ether. Due to their positive temperature coefficient of solubility, the salts remain in solution, while the alkyl cellulose ether is flocculated by coagulation in a hot medium.

The product produced according to the invention has a morphological structure, which is characterized in that the fiber structure resulting from the fibrillar character of the pulp is completely destroyed. If organic solvents or solvent mixtures, such as, for example, methanol / water or ethanol / water mixtures, are used, the cellulose ether can also be separated from the solvent or solvent mixture by removing the volatile constituents by distillation, leaving the cellulose ether behind. In this case, it makes sense to use purified cellulose ethers with a residual salt content of less than 5% by weight as the starting products for the separation of the alkyl cellulose ethers from insoluble constituents, since salts can only be removed by additional process steps.

The same applies to the separation of the cellulose ether from the solvent or solvent mixture by spray drying.

The method according to the invention is described in more detail below on the basis of exemplary embodiments, but without being restricted thereby.

General remarks on the analytical measurement methods used:

The viscosities of the end products are 2.0% aqueous

Solutions, based on% by weight alkyl cellulose ether, corrected for moisture, measured with a Höppler falling ball viscometer at 20 ° C.

The molar degree of substitution, for example with ethylene oxide or propylene oxide (MS (EO), MS (PO)), and the average degree of substitution, for example with methyl chloride (DS), are calculated from the percentage contents determined experimentally using the Zeisel digestion% OC ₂ H,% OC3Hδ and% OCH3 according to:

MS (EO) = [% OC ₂ H ₄ x 3.68] / [100 - (% OCH ₃ x 0.452) -% OC ₂ H ₄ ] MS (PO) = [% OC ₃ H ₆ x 2.79] / [100 - (% OCH ₃ x 0.452) -% OC ₃ H ₆ ] DS = [% OCH ₃ x 5.226] / [100 - (% OCH ₃ x 0.452) -% OC _{2 (3} ) H ₄ (6) ] The residue level is determined by dissolving 2 to 5 g of alkyl cellulose ether (weight corrected for residual salt content and moisture) in 150 to 500 g of water at 20 ° C. and spinning with a centrifuge at 8000 rpm for 20 minutes. The supernatant solution is carefully sucked off from the residue and this is shaken 6 to 10 times with 400 g of water and spun off again. The remaining material is transferred quantitatively into a dish and dried to constant weight for at least 8 hours at 105 ° C., weighed and the percentage of the remaining residue in the total weight is determined.

The filter residue is determined using the following method:

5.0 g of alkyl cellulose ether (initial weight corrected for residual salt content and moisture) are dissolved in 1000 g of water at 20 ° C. with stirring. It is then filtered through a water jet vacuum using a nylon filter (mesh size: 0.02 mm), the initial weight of which has been determined in advance, and dried at 105 ° C. for 60 minutes. After complete filtration, the residue on the filter is washed with 1000 g of deionized water and then the filter is dried on a watch glass in a drying cabinet at 105 ° C. for 60 minutes. After cooling in the desiccator, the filter is weighed on the analytical balance, the weight difference compared to the pure filter weight is calculated and the percentage residue level, based on the weight of alkyl cellulose ether, is calculated.

Production example 1:

A wood pulp 1 is combined with propylene oxide and methyl chloride according to a common production process, as described in EP-A-0 117 490

Alkyl cellulose ether with a MS (PO) of 0.20 and a DS of 1.9 implemented. The crude product obtained after the reaction and distillation of the solvent contains, in addition to approx. 30% alkyl cellulose ether and reaction by-products, approx. 50% salt (NaCl) and 20% moisture (water and residual solvent).

Workup of the raw product Comparative Example 1a: 930 g of the crude product mixture from Preparation Example 1 are whipped in 5 liters of boiling water, adjusted to pH 6.5 to 7.5 with 15% hydrochloric acid and after 5 to 10 minutes of boiling over a glass frit in a water jet vacuum from the filtrate, which, in addition to salt, contains part which contains reaction by-products and solvent residues, separated.

The filter cake is again opened in 5 liters of boiling water and, after 5 to 10 minutes of boiling, separated by renewed filtration through a glass frit.

The product thus purified from salt and reaction by-products and solvent residues is dried at 70 ° C. to a residual moisture content of less than 5%. The result is a product which has a solution viscosity of 180 mPas in 2.0% aqueous solution.

Example 1b: 500 g of the crude product mixture from preparation example 1 are poured into 2.5 liters of boiling water, adjusted to pH 6.5 to 7.5 with 15% hydrochloric acid and, after 5 to 10 minutes of boiling, separated off in a water jet vacuum using a glass frit. The product pre-cleaned in this way consists of approx. 40% alkyl cellulose ether, 5% salt and 55% moisture. 190 g of this pre-cleaned mixture are completely dissolved in one liter of water at 20 ° C., the homogeneous solution is distributed over 4 polypropylene centrifuge tubes and spun in a centrifuge at 8000 rpm for 20 minutes. It is decanted from the residue in a beaker and the aqueous solution is slowly heated to boiling while stirring. The coagulated product is suction filtered through a glass frit, the filter cake is washed with 800 g of boiling water and the resulting product is dried in a drying cabinet at 70 ° C. to a residual moisture content of less than 5%. The result is a product which has a solution viscosity of 180 mPas in 2.0% aqueous solution.

The residue level and filter residue of the products of comparative example 1a and example 1b are determined as described. The results are summarized in Table 1. Production example 2:

A wood pulp 2 is converted according to a conventional production process, as described in EP-A-0 117 490, with propylene oxide and methyl chloride to an alkyl cellulose ether with an MS (PO) of 0.74 and a DS of 1.9. The crude product obtained after the reaction and distillation of the solvent contains, in addition to approx. 30% alkyl cellulose ether and reaction by-products, approx. 50% salt (NaCl) and 20% moisture (water and residual solvent).

Processing of the raw product

Comparative Example 2a:

It is worked up as described for Comparative Example 1 a. The result is a

Product that has a solution viscosity of 480 mPas in 2.0% aqueous solution.

Example 2b

It is worked up as described for Example 1b. The result is a product which has a solution viscosity of 480 mPas in 2.0% aqueous solution.

The residue level and filter residue of the products of comparative example 2a and example 2b are determined as described. The results are summarized in Table 1.

Production example 3: A wood pulp 1 is produced according to a common production process, as in the

EP-A-0 117 490 described, with ethylene oxide and methyl chloride to one

Alkyl cellulose ether with a MS (EO) of 0.17 and a DS of 1.8 implemented.

The resulting after the reaction and distillation of the solvent

In addition to approx. 35% alkyl cellulose ether and reaction by-products, the crude product contains approx. 35% salt (NaCI) and 30% moisture (water and

Residual solvent). Workup of the raw product Comparative Example 3a:

It is worked up as described for comparative example 1a. The result is a product which has a solution viscosity of 220 mPas in 2.0% aqueous solution.

Example 3b

It is worked up as described for Example 1b. The result is a product which has a solution viscosity of 220 mPas in 2.0% aqueous solution.

The residue level and filter residue of the products of comparative example 3a and example 3b are determined as described. The results are summarized in Table 1.

Production example 4:

The cellulose ether raw product mixture from preparation example 1 is used.

Working up the crude product Comparative Example 4a:

It is worked up as described for comparative example 1a. The result is a product which has a solution viscosity of 180 mPas in 2.0% aqueous solution.

Example 4b

100 g of the product from Comparative Example 4a are suspended in 1600 g of pure ethanol with stirring, and 400 g of water are then slowly added. The mixture is stirred until a homogeneous solution has formed, which is then transferred in equal parts to sealable polypropylene centrifuge glasses and spun down for 20 minutes at 8000 rpm. The residue is decanted off and the decantate is freed from the solvent mixture by distillation by heating while applying a water jet vacuum, so that a yellowish solid results, which is dried to a residual moisture content of less than 5%. The result is a Product that has a solution viscosity of 180 mPas in 2.0% aqueous solution.

The residue level and filter residue of the products of comparative example 4a and example 4b are determined as described. The results are summarized in Table 1.

Table 1

Claims

claims: 1. A process for the purification of nonionic alkyl cellulose ethers, characterized in that the nonionic alkyl cellulose ether is dissolved in a suitable solvent or solvent mixture, then the Residues removed and then the alkyl cellulose ether separated from the solvent or solvent mixture. 2. The method according to claim 1, characterized in that water is used as solvent. 3. The method according to claim 1, characterized in that a mixture of water and an organic solvent is used as the solvent. 4. The method according to claim 1, characterized in that a mixture of methanol or ethanol with dichloromethane, chloroform, carbon tetrachloride, dichloroethane, trichloroethane or chlorobenzene is used as the solvent. 5. The method according to at least one of the preceding claims, characterized in that the weight ratio of solvent or solvent mixture to alkyl cellulose ether is chosen depending on the average degree of polymerization of the alkyl cellulose ether so that the viscosity of the resulting solution does not exceed 20,000 mPas. 6. The method according to at least one of the preceding claims, characterized in that the separation of the insoluble residues contained in the solvent or solvent mixture is carried out by filtration, centrifugation or decanting. 7. The method according to at least one of the preceding claims, characterized in that the separation of the alkyl cellulose ether from Solvent or solvent mixture is carried out by heating the solution to a temperature above the flocculation point of the cellulose ether with flocculation thereof and then separating it off. 8. The method according to at least one of claims 1 to 6, characterized in that the separation of the alkyl cellulose ether from the solvent or solvent mixture is carried out by at least partial distillation with the alkyl cellulose ether remaining. 9. The method according to at least one of claims 1 to 6, characterized in that the separation of the alkyl cellulose ether from the solvent or solvent mixture is carried out by spray drying. 10. The method according to at least one of claims 1 to 7, characterized in that the alkyl cellulose ether-containing solution additionally contains at least one salt in a concentration of 0.5 to 300 wt .-%, based on the alkyl cellulose ether. 11. The method according to claim 10, characterized in that sulfates, carbonates and / or chlorides are used as salts. 12. The method according to at least one of the preceding claims, characterized in that the aqueous cellulose-containing aqueous solutions are adjusted to an alkaline pH by bases. 13. The method according to claim 12, characterized in that the base used is lithium hydroxide, sodium hydroxide, potassium hydroxide and / or ammonia solution. 14. The method according to at least one of the preceding claims, characterized in that the nonionic alkyl cellulose ether is methyl cellulose, methyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose, Ethylhydroxyethyl cellulose and / or ethyl hydroxypropyl cellulose can be used.