The present invention relates to a process for the preparation of low-viscosity cellulose ethers, for example, hydroxyalkylcelluloses, carboxymethylcelluloses, mixed ethers and in particular methylcelluloses having viscosities below about 100 mPa.s, measured in aqueous solution, with the use of chlorite solutions.
Cellulose ethers of different properties are required for various fields of application. In addition to their solubility properties or their degree of substitution, one of the important parameters for characterizing the cellulose ethers is their viscosity. Essentially three process methods are described in the literature for the preparation of low-viscosity water-soluble cellulose ethers, but all these have the main aim of breaking down the molecule chains, i.e., to produce intermediates or end products of a lower molecular weight from starting materials of a relatively higher molecular weight. This aim is achieved either by degradation of the cellulose molecule before or during etherification or by degradation of the cellulose ether molecule. In the first two process methods, an essential disadvantage of the intermediates obtained by these methods is that, even before the usually applied purification stages in the processes for the preparation of these cellulose ethers, they are obtained as relatively short-chain molecules, so that they either swell excessively in the purification fluid (consisting in most cases of water or aqueous organic solvents) or dissolve therein to an undue extent, along with the impurities.
For carrying out the third process variant there have been disclosed, for example, degradation with chemical oxidizing agents--such as hypochlorite or H.sub.2 O.sub.2 --or with high-energy radiation.
German Patent No. 667,864 describes a process for lowering the viscosity of aqueous solutions of cellulose ethers, wherein microorganisms or enzymes are allowed to act on aqueous cellulose ether solutions. The viscosity is lowered to about 50% of the original value.
In the process for lowering the viscosity of cellulose ethers by means of hydrogen peroxide according to German Auslegeschrift No. 2,016,203 (corresponding to U.S. Pat. No. 3,728,331), a substantially dry, free-flowing cellulose ether of small particles is mixed with 10 to 50 % strength hydrogen peroxide solution, and this mixture is heated to a temperature of 50.degree. to 150.degree. C. until the cellulose ether has a lower viscosity. Depending on the quantity of H.sub.2 O.sub.2, the reaction temperature and duration, as well as the magnitude of the initial viscosity, the viscosity is lowered to about 1% of the initial value.
Swiss Patent No. 461,455 discloses a process for the preparation of low-viscosity water-soluble cellulose ethers by oxidative degradation of higher-viscosity ethers with H.sub.2 O.sub.2, wherein the water-moist cellulose ether is mixed with an aqueous solution of H.sub.2 O.sub.2, a certain mixing ratio of ether, H.sub.2 O.sub.2 and water content being adjusted, and the mixture is then dried at 100.degree. to 250.degree. C. until the H.sub.2 O.sub.2 has been fully consumed. The viscosity is lowered to about 0.5% of the initial value.
British Patent No. 953,944 describes a process for lowering the viscosity of water-soluble, non-ionic cellulose ethers, wherein the ethers are reacted with an aqueous solution of H.sub.2 O.sub.2 at temperatures of about 70.degree. to 100.degree. C. in aqueous suspension or by spraying. The viscosity is lowered to about 0.5% of the initial value.
In the process according to U.S. Pat. No. 2,512,338, a mixture of H.sub.2 O.sub.2 or a metal peroxide and a metal salt with an Mn, Co, Fe or Pb ion is added to the reaction mixture of alkali cellulose and an etherifying agent, in order to influence the viscosity. The viscosity is lowered to about 0.3% of the initial value.
U.S. Pat. No. 2,895,891 discloses irradiation of cellulose ethers with ionizing radiation, which leads to a reduction of viscosity of non-ionic and ionic cellulose ethers in the dry state; however, a decrease in viscosity is observed in the (water)-moist state only in the case of ionic cellulose ethers, but not in the case of non-ionic cellulose ethers, and the latter are crosslinked under these conditions.
In U.S. Pat. No. 2,912,431, a salt having an alkali metal, alkaline earth metal or ammonium cation and a hypohalite, peroxide or periodate anion is added to a mixture of a fibrous carboxyalkylcellulose and an aqueous alcohol for bleaching, for lowering the viscosity and for compaction. The viscosity is lowered to about 1% of the initial value, and the reaction temperature is between 40.degree. and 80.degree. C.
In the process according to U.S. Pat. No. 4,061,859, for the purpose of lowering the viscosity with simultaneous reduction of the molecular weight, a dry cellulose ether powder is reacted with a) gaseous hydrogen halide at up to about 80.degree. C. and then b) with gaseous sulfur dioxide at room temperature. The additional process step b) prevents the discoloration (essentially yellowing) of the cellulose ether, which occurs when step a) alone is carried out. The viscosity is lowered to about 0.05% of the initial value, and the moisture content in the cellulose ether powder which is to be treated should not exceed a maximum value of about 5%.
German Offenlegungsschrift No. 1,468,835 (corresponding to U.S. Pat. No. 3,394,127) discloses a process for the preparation of alkali metal salts of carboxymethylcellulose, wherein, after etherification, a bleaching agent is added in order to increase the whiteness of the product. When hydrogen peroxide is used, the viscosities reached are stated to be in the range from about 20 to 50 mPa.s, whereas when sodium chlorite is used as a bleaching agent, only viscosities above 1000 mPa.s are obtained, and the desired whiteness is in the same range as in the case of using hydrogen peroxide.
U.S. Pat. No. 2,749,336 describes a process mainly for the preparation of carboxymethylcellulose, wherein a sodium hypochlorite solution is added to the sodium hydroxide solution containing isopropanol before the addition of monochloroacetic acid.
Disadvantages in the known processes with the use of hydrogen peroxide or hypochlorite solutions as the oxidizing agents are, on the one hand, the necessity of carrying out the reaction at a temperature in the range from 40.degree. to 150.degree. C. and, on the other hand, the relatively easy decomposition of such solutions due to the influence of temperature and light, so that the content of active ingredient can vary widely. As a result, the viscosity can differ widely from batch to batch, which is highly undesirable.
A further disadvantage is the instability of the viscosity of the products obtained by these processes, manifested by the fact that the viscosity frequently falls further during storage of the products. The reduction in viscosity of the cellulose ethers should be carried out, as far as possible, with the already washed, still moist products. Therefore, the degradation with high-energy radiation is disadvantageous, since it can then not be carried out with the non-ionic cellulose ether types without interfering side reactions (crosslinking), and a degradation with HCl gas and/or SO.sub.2 gas is also disadvantageous, since in that case the moisture must already have been removed to a very large extent. Moreover, serious corrosion problems arise when the last-mentioned gases are used.
Reactions of cellulose or cellulose ether derivatives with ozone have also already been disclosed. For example, U.S. Pat. No. 3,138,564 discloses a process wherein polymerizable monomers containing vinylidene groups are grafted onto a polysaccharide which was treated beforehand in aqueous dispersion or solution with ozone or an ozonecontaining gas. The excess ozone is blown out of the reaction mixture by means of an inert gas. Among many others, carboxymethylcellulose is also mentioned as a polysaccharide starting material. This reaction between a polysaccharide and ozone serves to generate free-radical intermediates for the grafting which is subsequently to be carried out. A controlled degradation reaction is evidently not intended.
According to the paper "Untersuchungen ueber die Pfropfung von Carboxymethylcellulose mit Acrylnitril [Investigations of the grafting of carboxymethylcellulose with acrylonitrile]" by C. Simionescu, D. Feldman and C. Vasiliu in: Faserforschung und Textiltechnik 13, 1962, No. 2, pages 70 to 79, the grafting can be initiated in the presence of Ce.sup.4+ ions by ultrasonics, UV radiation, a magnetic field or by ozonization. A certain decrease in viscosity is here also pointed out, which can lead to a reduction of the molecular weight by one half. In addition to carboxymethylcellulose, methylcellulose is also employed as a starting material. The treatment of the cellulose ethers present in the form of films is carried out in an aqueous, weakly acidic solution in the simultaneous presence of Ce.sup.4+ ions.
In the paper "Mechanism of ozone attack on .alpha.-methyl glucoside and cellulosic materials" by A.A. Katai and C. Schuerch in: Journal of Polymer Science, volume 4, 1966, pages 2683 to 2703, the interaction of ozone with methylcellulose of a degree of substitution of 1.5 is described. With increasing quantity of ozone converted, the molecular weight of the methylcellulose decreases and the number of carbonyl or carboxyl groups on the molecules increases, these changes taking place in the case of ozonization both in an oxygen atmosphere and in a nitrogen atmosphere. The ozonization process is carried out in a 1.6% by weight aqueous methylcellulose solution.
A disadvantage of these processes of reacting water-soluble cellulose ethers with ozone is that the reaction is carried out in aqueous solution since, as a result, an additional dissolution stage would have to be used in large-scale plants for the production of cellulose ethers, and this would lead, for example, to large capacity vessels (highly concentrated cellulose ether solutions can hardly be prepared) for dissolution, on the one hand, and to energy consumption for removing the solvent again, on the other hand.