US 3065040 A
Description (OCR text may contain errors)
States This invention relates to textile bleaching processes using chlorites.
The use of chlorites for bleaching textiles, particularly in processes combining padding and steaming, is known in the art. Several operational methods are known, according to the oldest of which acidic solutions of chlorite are padded onto the goods to be bleached, which are subsequently steamed. This process has the disadvantage that the impregnating baths are strongly active, that is they evolve chlorine dioxide gas on decomposition. To overcome this disadvantage, it has been suggested that the goods being treated be impregnated with alkaline chlorite solutions and that the chlorite incorporated in the treated goods subsequently be activated with mixtures of acid and steam. In this process, a particularly heavy evolution of chlorine dioxide occurs in the steaming chamber, and the mixture of acid, steam, and chlorine dioxide is considerably corrosive to the apparatus used.
In further developments of the padding and steaming process, the advantages of weakly alkaline impregnation baths have been retained while avoiding the disadvantage of acid activation by adding organic esters to the impregnation baths. During subsequent steaming, these esters release acid and activate the chlorite present in the goods treated. Although this latter method is considerably superior to those earlier described, the high cost of the esters considerably increases the cost of the bleaching process.
More recently, the addition of salts of strong acids and weak bases (e.g. ammonium salts of strong acids) to the impregnation baths has been proposed. These salts replace the esters as substances forming acid during the steaming process. These salts, which are hydrolyzed only at temperatures above 60 0., form acid and cause an activation of the chlorite on the fiber treated. Although these acid-forming substances are relatively cheap compounds, their use is impaired by the fact that the acids formed injure the goods being treated.
It has now been found that the detrimental effect on textiles bleached with water-soluble chlorites using padding and steaming methods and impregnation baths containing salts of strong acids and weak bases as acid-forming substances can be avoided by additionally including weak acids and/ or salts of weak acids and strong bases in the impregnation baths.
The addition of salts of strong bases and weak acids prevents the textile fiber from being damaged. However, the complete elimination of seed husks requires an increased bleaching period since activation of the chlorite is delayed by the buffer action of the salt added. it is of special advantage to add weak acids, rather than weak acid salts, since the buffering salts which inhibit acid corrosion and are subsequently activated by heat are formed between the added weak acid and the alkali of the chlorite solution. Using this method, the bleaching reaction is not delayed, which is of utmost importance since padding and steaming are usually performed as continuous processes and bleaching periods as short as possible give the highest yields.
A similar eifect is obtained by adding to the impregnation baths mixtures of salts of strong bases and weak acids together with weak acids difierent from those acids from which the salts are derived. When using such mixtures, bleaching is delayed only shortly due to the higher quantity of buffer substances present.
It has proved advantageous to fix the pH of the impregnation baths of the invention at a value lower than the pH (about 8-9) of alkaline chlorite baths consisting of sodium chlorite and salts of strong acids and weak bases. According to the invention, it is most favorable to operate at a pH between about 6 and 7.5, preferably between 6.5 and 7.5.v The amount of weak acid or of weak acid salt depends on the pH desired.
Suitable salts of weak acids and strong bases are, for example: sodium formate or the corresponding acetate, propionate, citrate, lactate, tartrate, and benzoate. Naturally, the corresponding potassium salts can be used instead of sodium salts, as can the salts of the alkaline earth metals if sutliciently soluble.
As weak acid, the organic acids are especially suitable, such as formic acid, acetic acid, propionic acid, lactic acid, tartaric acid, citric acid, benzoic acid and the like. The first dissociation constants of these acids all lie between about 1 10- and about 1.4 l0- which are the first dissociation constants of tartaric and propionic acids, respectively.
The following examples illustrate the invention but are not intended to limit it:
Example 1.Raw desized cotton fabric was impregmated with a solution (pH=8.8) containing 15 grams/ liter of 100% sodium chlorite, 8 grams/ liter of ammonium sulphate and 5 grams/ liter of a chlorite resistant wetting agent.
Solution in excess of that giving a humidity of 100% was removed by squeezing the fabric, which was then heated to C. with steam and then rolled up. The goods remained for one hour at this same temperature while being slowly rotated in the steaming chamber. The goods were then washed.
The average degree of polymerization of the fabric after bleaching was 1820, as compared with an average degree of polymerization in the raw material of 2960. The bleached fabric had a degree of whiteness of 84.3%. Seed husks were completely destroyed.
Example 2.By proceeding as in Example 1, but ad justing the solution to a pH of 7 by the addition of acetic acid to the chlorite bath, a bleached fabric with a degree of whiteness of 84.2% was obtained. The average degree of polymerization in the bleached fibers was 2760, as compared with 2960 in the raw material. Thus, the fiber has not been damaged, although the seed husks are completely destroyed.
Example 3.-Proceeding as in Example 1, but adding 2.5 grams/liter of sodium acetate to the chlorite bath (pH=8.9), a fabric having a degree of whiteness of 79.7% was obtained. The average degree of polymerization in the bleached material was 2810, as compared with a value of 2960 in the raw material. Seed husk residues still remained after treatment. The reduced degree of bleaching obtained and the incomplete destruction of seed husks, despite a bleaching time equal to that of the earlier examples, are due in this process to the delay occurring in reaching the bleaching stage. If the bleaching time is increased to 1% hours, the seed husks are completely destroyed and the degree of whiteness of the fabric is raised to about 83.9% without significant reduction in the average degree of polymerization. Thus, in this case also, the goods bleached were not damaged. A prolonged bleaching period is required by the delayed activation of the chlorite, caused by the presence of the buffer substances added.
Example 4.-Proceeding as in Example 3 above, but with the addition to the chlorite impregnation bath of such an amount of acetic acid as is necessary to bring the bath to a pH of 6.5, complete bleaching is attained within 75 minutes with destruction of all seed husks. The degree of whiteness was 84.1%. The average degree of polymerization of the bleached material was 2780 as compared with 2960 in the raw material. The use of mixtures of bufifers salts and weak acids in bleaching has the advantage that the pH is least reduced during the bleaching process, since more butler substance is available. In comparison with Example 3, the addition of acid to the solution (Example 4) reduces the bleaching period to a value comparable with that in Example 2.
i I claim:
1. In the method of bleaching textiles by padding said textiles with an impregnation bath containing chlorites and salts of strong acids and weak bases and then steaming said padded textiles, the improvement of buttering said bath at a pH between 6 and 9 by adding to said bath a member selected from the group consisting of weak acids having a first dissociation constant between 1 10 and 1.4 10 the alkali metal and alkaline earth salts of said acids, and mixtures of said salts and said acids.
2. The method of claim 1 wherein said bath is bufiered at a pH between 6.5 and 7.5.
3. The method of claim 1 wherein a weak acid is added to said bath.
4. The method of claim 3 wherein said weak acid is a low molecular weight aliphatic carboxylic acid.
5. The method of claim 4 wherein said weak acid is acetic acid.
6. The method of claim 1 wherein a salt of a Weak acid is added to said bath.
7. The method of claim 6 wherein said salt is sodium acetate.
8. The method of claim 1 wherein a mixture of a weak acid and a salt of a weak acid is added to said bath.
9. The method of claim 8 wherein said mixture is a mixture of acetic acid and sodium acetate.
References Cited in the file of this patent UNITED STATES PATENTS 2,521,340 Carr Sept. 5, 1950 2,739,032 Wilson Mar. 20, 1956 2,810,717 Lamborn Oct. 22, 1957 2,947,700 Waibel Aug. 2, 1960 FOREIGN PATENTS 723,566 Great Britain Feb. 9, 1955 OTHER REFERENCES Journal Textile Inst, 47:3, pp. A-l25, March 1956.