This is a continuation of application Ser. No. 07/537,829 filed Jun. 14, 1990, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for bleaching a cloth dyed with an indigo dye and more particularly, to a method for bleaching a cloth dyed with an indigo dye, such as a denim, by using an aqueous solution comprising a dichloroisocyanuric acid alone or as a main component. 
     BACKGROUND OF THE INVENTION 
     In recent years, cloths dyed with an indigo dye, particularly a bleached denim comprising a blue denim having been subjected to bleaching processing, have become popular. 
     Bleaching of a blue denim includes a case where a blue denim is relatively uniformly bleached and a case where a blue denim is non-uniformly bleached. Usually, in the case where a blue denim is relatively uniformly bleached, dip bleaching is carried out by using sodium hypochlorite. In the case where a blue denim is non-uniformly bleached, bleaching is carried out by impregnating a pumice or the like with a sodium hypochlorite solution and, after drying, revolving and stirring a blue denim in a cleaning device. In any of these cases, sodium hypochlorite is mainly used as a bleaching agent. As other bleaching agents than sodium hypochlorite, U.S. Pat. No. 4,218,220 proposes use of trichloroisocyanuric acid as a bleaching agent. 
     In any of these methods, a step for removing chlorine by using a reducing agent such as sodium thiosulfate is introduced after the bleaching processing step. 
     The above-described case of using sodium hypochlorite as a bleaching agent involves such a defect that cotton fibers are deteriorated and, hence, when the degree of bleaching is further controlled, a method for controlling an available chlorine concentration of or processing time with a processing solution is employed. However, in this case, the bleaching power of sodium hypochlorite is so strong that it is not easy to control the degree of bleaching. Further, as a method for controlling the degree of bleaching, a method for changing a pH of or processing time with a processing solution could be considered, but this method is not desired because the pH of the sodium hypochlorite solution is lowered, or if the temperature of the processing solution is increased, a chlorine gas is volatilized. 
     In order to solve this problem, U.S. Pat. No. 4,218,220 proposes use of trichloroisocyanuric acid as a bleaching agent. However, trichloroisocyanuric acid is considerably acidic such that a 0.1% aqueous solution thereof has a pH of from 1 to 2, and its bleaching power is too high. Further, in the case that it is aimed to suppress the bleaching power of trichloroisocyanuric acid by adding an alkaline agent thereto, there are still such problems that not only is it dangerous because decomposition of trichloroisocyanuric acid causes generation of nitrogen chloride, but a blue denim is likely yellowed. 
     Bleaching of a blue denim is likely delicately influenced by not only the type of a bleaching agent used but the processing condition employed. Therefore, undesired phenomena such as deterioration of fibers by reduction in the tear strength and yellowing of the blue denim sometimes occur. 
     SUMMARY OF THE INVENTION 
     The present inventors have found that the above-described problems can be solved by using dichloroisocyanuric acid and then accomplished the present invention. 
     An object of the present invention is to provide a method for bleaching a cloth dyed with an indigo dye, in which the degree of bleaching can be easily controlled and, after bleaching, neither yellowing nor deterioration of fibers occurs. 
     That is, the present invention relates to a method for bleaching a cloth dyed with an indigo dye, which comprises using, as a processing solution, an aqueous solution of a dichloroisocyanuric acid salt which is optionally compounded with a basic compound and/or a surfactant. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The cloth dyed with an indigo dye, which is used in the method of the present invention, is not particularly limited, but the method of the present invention is particularly effective for a denim. Besides, the method of the present invention is also applicable to traditional products such as knitwear, yukata, and pongee as well as handicraft products such as shop curtain and tablecloth. 
     As the dichloroisocyanuric acid salt, sodium dichloroisocyanurate and/or potassium dichloroisocyanurate is preferred. A suitable concentration of the dichloroisocyanuric acid salt in the processing solution is from 0.05 to 1.5% by weight, preferably from 0.1 to 1.0% by weight, in terms of the available chlorine concentration. 
     In the range of the available chlorine concentration of 0.5% by weight or more, since a processing solution comprising an aqueous solution containing only a dichloroisocyanuric acid salt tends to cause problems such as yellowing of fibers after bleaching processing, in order to prevent such problems, it is preferred to compound therewith at least one member selected from basic compounds, anionic surfactants, and nonionic surfactants. 
     Even in the range of the available chlorine concentration of less than 0.5% by weight, it is, as a matter of course, possible to use the above-described basic compounds and/or surfactants, use of which is rather preferred. 
     Examples of basic compounds which can be used in the present invention include basic inorganic compounds such as sodium carbonate, sodium metasilicate, and trisodium phosphate and basic organic salt compounds such as sodium citrate. 
     A suitable amount of the basic compound added is adjusted such that the processing solution has a pH of from 5 to 11 and preferably from 6 to 10. If the pH exceeds 11, not only the bleaching effect is lowered, but reduction in the strength of fiber occurs. On the other hand, if the pH is less than 5, yellowing likely occurs. 
     Examples of anionic surfactants which can be used in the present invention include alkylbenzenesulfonic acid salts, alkanesulfonic acid salts, α-olefin-sulfonic acid salts, and alkyl sulfate polyoxyethylene salts. 
     Examples of nonionic surfactants which can be used in the present invention include alkyl polyoxyethylene ethers and alkylphenyl polyoxyethylene ethers. 
     A suitable amount of the surfactant compounded is from 0.01 to 0.2% by weight in the processing solution. If the amount is less than 0.01% by weight, the yellowing-preventing effect is not satisfactory, whereas if it exceeds 0.2% by weight, a rinsing operation in the subsequent step becomes likely insufficient. 
     In the method of the present invention, the dichloroisocyanuric acid salt can be used in combination with a cationic surfactant. In this case, a concentration of the dichloroisocyanuric acid salt in the aqueous solution is from 0.05 to 0.4% by weight, preferably from 0.1 to 0.2% by weight, in terms of the available chlorine concentration. 
     As will be clear from the Examples as described later, in the case that a cloth is treated with a processing solution containing sodium dichloroisocyanurate in an effective chlorine concentration of about 0.2% by weight and 0.02% by weight of lauryldimethylbenzylammonium chloride as a cationic surfactant, a bleaching effect of the cloth which is substantially equal to that attained in the case that a cloth is treated with a processing solution containing only sodium dichloroisocyanurate in an effective chlorine concentration of about 0.4% by weight can be attained. That is, the cationic surfactant functions as a filler for the bleaching agent. 
     A suitable amount of the cationic surfactant is from 0.005 to 0.5% by weight, preferably from 0.01 to 0.05% by weight, in the aqueous solution. If the amount exceeds 0.5% by weight, a rinsing operation in the subsequent step becomes likely insufficient. 
     The bleaching effect of a cloth increases in substantial proportion to the amounts of the dichloroisocyanuric acid salt and cationic surfactants added. Accordingly, though it is possible to control these amounts, if the effective chlorine concentration is less than 0.05% by weight, desired results cannot be obtained even though the amount of the cationic surfactant added is increased. Further, if the effective chlorine concentration exceeds 0.4% by weight, the bleaching by combined use with the cationic surfactant markedly proceeds, whereby yellowing of the cloth so-called as &#34;chlorine yellowing&#34; is likely generated. 
     Moreover, though the cationic surfactant may be used in combination with the above-described basic compound and nonionic surfactant, it is not preferred to use the cationic surfactant in combination with the anionic surfactant. 
     A suitable temperature of the bleaching processing is not higher than 70° C., preferably from 30° to 70° C., and more preferably from 50° to 65° C. If the temperature is lower than 30° C., it is not efficient because it takes a long period for achieving the bleaching. If it exceeds 70° C., decomposition of chlorine is vigorous, and the bleaching is likely non-uniform. A suitable weight ratio (i.e., bath ratio) of the cloth to the processing solution is from 1:10 to 1:50 and preferably from 1:20 to 1:40. If the bath ratio exceeds 1:10, the bleaching is likely non-uniform due to twisting between the fibers. Further, a bath ratio of less than 1:50 could be employed, but it is of no efficiency. A processing time is usually from 10 to 30 minutes though it varies depending on the temperature and bath ratio. 
     After the bleaching processing by the method of the present invention, reduction processing, rinsing and drying steps which are carried out in the conventional bleaching step are employable. Further, a method in which the above-described bleaching solution is impregnated into a pumice or the like and local bleaching is performed is also applicable. 
     The present invention is described in more detail with reference to the following Examples and Comparative Examples. 
     EXAMPLES 1 TO 12 AND COMPARATIVE EXAMPLES 1 TO 14 
     Bleaching was carried out in the manner described below under the conditions as shown in Table 1, followed by evaluation. The results obtained are also shown in Table 1. For reference, the physical properties of a denim which had been subjected to a bleaching processing step with only water are shown in Table 1, too. 
     [Bleaching Method] 
     Into a 500 ml beaker, 500 ml of distilled water was charged, and a bleaching agent was added thereto under the conditions as shown in Table 1, followed by keeping the mixture at 50° C. Two blue denim cloth pieces (15 cm×8 cm in size) were dipped therein and bleached for 10 minutes by means of a detergency tester (&#34;Tergoto Meter Model 7243&#34; manufactured by U.S. Testing Co., Ltd.) at 100 rpm, followed by adding thereto 0.5% by weight of sodium thiosulfate to effect chlorine-removal treatment. Thereafter, the resulting cloth pieces were air dried at room temperature for 24 hours, ironed, and then evaluated with respect to the following items. 
     [Bleaching Effect] 
     The color tone of the bleached denim cloth piece was measured in terms of lightness (L) and hue (a, b) by means of a differential colorimeter (manufactured by Tokyo Denshoku K.K.), and suitability of the bleaching effect as well as degree of yellowing were visually observed. The results are shown in Table 1. 
     The bleaching effect is expressed by three ratings, &#34;suitable&#34;, &#34;excessive&#34;, and &#34;insufficient&#34;; and the yellowing is expressed by the following symbols. 
     A: not yellowed 
     B: slightly yellowed 
     C: yellowed 
     D: markedly yellowed 
     [Tear Strength] 
     The bleached denim cloth piece was measured with respect to tear strength under the conditions according to the single tongue method as defined in JIS L1004 by means of a Tensilon (a trade name of Toyo Boldwing Co., Ltd.). 
     The denim used was 14 oz., and the chemicals used are shown below. 
     [Bleaching Agent] 
     (1) sodium hypochlorite solution 
     (2) trichloroisocyanuric acid powder 
     (3) sodium dichloroisocyanurate powder 
     (4) sodium dichloroisocyanurate dihydrate powder 
     (5) potassium dichloroisocyanurate powder 
     (6) high test hypochlorite 
     [Basic Substance] 
     (1) sodium carbonate powder 
     (2) sodium metasilicate powder 
     (3) trisodium phosphate powder 
     (4) sodium citrate 
     [Surfactant] 
     Anionic Surfactant 
     (1) sodium salt of alkyl sulfate-decaethylene oxide adduct (&#34;Persoft-EL&#34;, a trade name of Nippon Oil and Fats Co., Ltd.) 
     (2) sodium alkylbenzenesulfonate (&#34;Newrex Paste H&#34;, a trade name of Nippon Oil and Fats Co., Ltd.) 
     (3) sodium α-olefin-sulfonate (&#34;Nikkol OS-14&#34;, a trade name of Nikko Chemical Co., Ltd.) 
     Nonionic Surfactant 
     (1) alkyl polyoxyethylene ether (&#34;Nonion E-215, a trade name of Nippon Oil and Fats Co., Ltd.) 
     (2) alkylphenyl polyoxyethylene ether (&#34;Nonion NS-202&#34;, a trade name of Nippon Oil and Fats Co., Ltd.) 
     
         TABLE 1  Bleaching Condition  Bleaching Agent &amp;  Evaluation Result Available Chlorine Basic  pH of Processing Tear Color Tone Bleaching  Concentration  (%) Substance (%) Surfactant (%) Solution Strength (kg) L a b Effect Yellowing   Example 1 DCCNa (0.3) -- -- 5.8 4.4 34.2 -0.9  -7.8 suitable A Example 2 DCCNa (0.3) sodium carbonate -- 9.2 4.5 43.0 -3.1  -8.5 suitable A (0.3) Example 3 DCCNa (0.3) sodium m-silicate -- 8.7 4.6 31.5 -0.7 -13.3 suitable A   (0.3) Example 4 DCCNa (0.3) sodium citrate -- 7.4 4.4 35.4 -1.6 -10.2 suitable A   (0.3) Example 5 DCCNa (0.3) -- α OS--Na (0.1) 6.1 4.5 32.9 -0.4 -14.4 suitable A Example 6 DCCNa (0.3) sodium carbonate α OS--Na (0.1) 9.0 4.5 36.4 -0.8 -13.3 suitable A   (0.3) Example 7 DCCNa (0.3) -- AS--Na (0.1) 6.3 4.7 35.5 -2.0 -10.9 suitable A Example 8 DCCNa (0.5) -- POE (0.2) 6.1 4.5 46.9 -4.1  -9.9 suitable A Example 9 DCCNa (0.5) sodium carbonate POE (0.2) 9.2 4.5 54.3 -7.6  -0.6 suitable A (0.5) Example 10 DCCNa.2H.sub.2 O -- ABS--Na (0.2) 6.2 4.6 33.1 -0.7 -15.5 suitable A  (0.3) Example 11 DCCK (0.3) -- -- 6.0 4.5 33.6 -1.4 -8.1 suitable A Example 12 DCCK (0.3) -- ABS--Na (0.2) 6.3 4.7 34.0 -1.8 -11.8 suitable A Comparative NaClO (0.1) -- -- 7.6 4.3 22.5  1.2 -14.9 insufficient A Example 1 Comparative NaClO (0.3) -- -- 8.1 3.3 27.7  0.3 -17.9 suitable B Example 2 Comparative NaClO (0.5) -- -- 8.6 2.7 42.2 -2.5 -16.1 suitable C Example 3 Comparative NaClO (2.0) -- -- 9.7 1.6 69.5 -3.7   4.3 excessive C Example 4 Comparative high test -- -- 8.3 3.2 25.4  0.1 -13.3 suitable C Example 5 hypochlorite  (0.3) Comparative high test -- -- 8.8 2.5 43.3 -1.8 -19.3 suitable C Example 6 hypochlorite   (0.5) Comparative  TCCA (0.05) -- -- 5.5 4.5 29.1 -1.9 -15.5 insufficient A Example 7 Comparative TCCA (0.3) -- -- 4.3 4.4 39.0 -0.4  -7.7 suitable D Example 8 Comparative NaClO (0.3) sodium carbonate -- 8.7 2.1 25.1  0.2 -17.0 suitable C Example 9  (0.3) Comparative NaClO (0.1) -- ABS--Na (0.1) 7.9 4.0 20.9  1.4 -15.3 insufficient A Example 10 Comparative NaClO (0.3) -- ABS--Na (0.1) 8.3 3.4 27.1  0.4 -13.3 suitable C Example 11 Comparative high test -- POE (0.2) 8.2 2.6 23.1 -0.2 -15.8 suitable A Example 12 hypochlorite  (0.3) Comparative TCCA (0.3) trisodium -- 5.7 4.0 36.1 -0.3 -10.9 suitable B Example 13  phosphate (0.3) Comparative TCCA (0.3) trisodium ABS--Na (0.1) 5.7 4.0 34.8 -0.2 -13.3 suitable B Example 14 phosphate (0.3) Reference -- -- -- 7.4 4.5 18.3 -8.2 -10.2 -- -- DCCNa: sodium dichloroisocyanurate  DCCNa.2H.sub.2 O: sodium dichloroisocyanurate dihydrate DCCK: potassium dichloroisocyanurate OS--Na: sodium olefin-sulfonate AS--Na: alkyl sulfate 10EONa salt POE: polyoxyethylene alkyl ether ABS--Na: sodium alkylbenzenesulfonate TCCA: trichloroisocyanuric acid 
    
     EXAMPLES 13 TO 29 AND COMPARATIVE EXAMPLES 15 TO 18 
     Bleaching was carried out in the manner described below under the conditions as shown in Table 2, followed by evaluation. The results obtained are also shown in Table 2. 
     [Bleaching Method and Evaluation Method] 
     Five liters of warm water at 50° C. was charged in a small-sized electric washing machine, and a bleaching agent was added thereto under the conditions as shown in Table 2. One blue denim cloth piece (30 cm×45 cm in size) was thrown into this processing solution and bleached for 30 minutes, followed by subjecting to a chlorine-removal treatment with 5 liters of warm water having 20 g of sodium thiosulfate added thereto for 15 minutes. The resulting cloth piece was rinsed with tap water for 15 minutes, air dried at room temperature for 24 hours, and then ironed. The color tone of the bleached denim cloth piece was measured in terms of lightness (L) and hue (a, b) by means of the same differential colorimeter as used in Example 1. 
     The denim used was the same type as in Example 1, and the cationic surfactant used is as follows. 
     (1) hexadecyltrimethylammonium chloride 
     (2) stearyltrimethylammonium chloride 
     (3) lauryldimethylbenzylammonium chloride 
     (4) stearyldimethylbenzylammonium chloride 
     
                                           TABLE 2__________________________________________________________________________  Bleaching Condition  Bleaching Agent &amp;    Evaluation Result  Available Chlorine             Cationic  Color Tone  Concentration (%)             Surfactant (%)                       L  a   b__________________________________________________________________________Example 13  DCCNa (0.06)             lauryldimethyl-                       19.7                           1.9                              -13.7             benzylammonium             chloride (0.01)Example 14  DCCNa (0.06)             lauryldimethyl-                       21.5                           1.9                              -15.7             benzylammonium             chloride (0.02)Example 15  DCCNa (0.06)             lauryldimethyl-                       28.9                           0.4                              -17.0             benzylammonium             chloride (0.03)Example 16  DCCNa (0.18)             lauryldimethyl-                       57.5                          -2.6                               -8.7             benzylammonium             chloride (0.01)Example 17  DCCNa (0.18)             lauryldimethyl-                       64.8                          -2.6                               -3.2             benzylammonium             chloride (0.02)Example 18  DCCNa (0.18)             lauryldimethyl-                       75.2                          -2.1                                4.0             benzylammonium             chloride (0.03)Example 19  DCCNa (0.27)             lauryldimethyl-                       66.9                          -2.6                               -1.2             benzylammonium             chloride (0.01)Example 20  DCCNa (0.27)             lauryldimethyl-                       70.2                          -2.7                               -1.9             benzylammonium             chloride (0.02)Example 21  DCCNa (0.27)             lauryldimethyl-                       71.5                          -2.6                                1.0             benzylammonium             chloride (0.03)Example 22  DCCNa (0.36)             lauryldimethyl-                       75.1                          -2.3                                2.9             benzylammonium             chloride (0.01)Example 23  DCCNa (0.36)             lauryldimethyl-                       78.7                          -2.1                                4.8             benzylammonium             chloride (0.02)Example 24  DCCNa (0.36)             lauryldimethyl-                       74.9                          -2.2                                4.7             benzylammonium             chloride (0.03)Example 25  DCCK (0.18)             lauryldimethyl-                       63.3                          -2.4                               -3.1             benzylammonium             chloride (0.02)Example 26  DCCNa (0.06)             hexadecyltrimethyl-                       68.6                          -2.6                               -3.9             ammonium chloride             (0.02)Example 27  DCCNa (0.06)             stearyltrimethyl-                       56.5                          -3.1                               -5.1             ammonium chloride             (0.02)Example 28  DCCNa (0.06)             lauryldimethyl-                       67.3                          -2.4                               -1.7             benzylammonium             chloride (0.02)Example 29  DCCNa (0.06)             stearyldimethyl-                       52.3                          -2.7                               -9.0             benzylammonium             chloride (0.02)Comparative  DCCNa (0.18)             --        42.0                          -1.3                              -17.2Example 15Comparative  DCCNa (0.27)             --        55.2                          -2.6                              -12.9Example 16Comparative  DCCNa (0.36)             --        65.9                          -2.9                               -6.4Example 17Comparative  DCCK (0.18)             --        39.8                          -1.3                              -17.7Example 18__________________________________________________________________________ 
    
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.
A method for bleaching a cloth dyed with an indigo dye by using a processing solution containing a bleaching agent, wherein said processing solution is an aqueous solution of a dichloroisocyanuric acid salt, is disclosed. According to the invention, the degree of bleaching can be easily controlled and, after the bleaching, neither yellowing nor deterioration of fibers occurs.