Desizing cotton and cotton-containing fabrics
The present invention relates to a process for desizing cotton or cotton-containing fabrics in strongly alkaline solutions with an alkali metal hypochlorite in the presence of an activator for the hypochlorite, and the use of such acitvators for desizing in stronly alkaline aqueous solutions. 
     In order to be able to weave cotton fabrics and cotton/polyester blends without difficulty, the warp threads are sized. In practice, starch-containing sizing agents are generally used, these frequently also being mixed with semi-synthetic sizing agents, such as carboxymethylcellulose, and synthetic sizing agents, e.g. polyvinyl alcohol or polyacrylates. The fabric is desized by degrading the starch and then washing out the degradation products. The starch can be degraded, for example, enzymatically, or oxidatively with a persulfate or a bromite in an alkaline medium or a medium rendered alkaline with sodium carbonate. 
     The oxidative desizing of cotton and cotton-containing fabrics in the presence of a hypochlorite in alkaline solutions is known in principle. The disadvantage of this process is that the degradation of natural starch with hypochlorite solutions cannot be carried out at room temperature (cf. K. Frinken, Textilbetrieb, 1974, 92 (4), pages 47-49, in particular page 47, left-hand column, first paragraph), and increasing fiber damage results at elevated temperatures. 
     Moreover, it is known that hypochlorite can be used to bleach desized fabric. In this bleaching process, the pH is kept as a rule at from 11 to 12; in order to avoid fiber damage in this pH range, in which the hypochlorite exhibits pH-dependent activity, the residence time has to be kept very short. 
     For this reason, there is to date no satisfactory industrial desizing process which employs an alkali metal hypochlorite. 
     We have found a process for desizing cotton and cotton-containing fabrics in strongly alkaline aqueous solution with an alkali metal hypochlorite, wherein from 0.5 to 8.0 g/l of a tertiary hydroxyalkylamine which is soluble in aqueous alkaline solution, of an oxyethylated and/or oxypropylated ethylenediamine, of an oxyethylated polyethyleneimine, of cyanuric acid, of cyanoguanidine, of oxazolidone, of pyrrolidone, of N-methylpyrrolidone or of N-methylimidazole is added to the aqueous bath, as an activator for the hypochlorite. 
     The novel desizing process is carried out at a strongly alkaline pH, corresponding to a content of from 15 to 150, preferably from 25 to 70, g of sodium hydroxide per liter of water, and at room temperature, advantageously at from 10° to 55° C., using an amount of alkali metal hypochlorite, in particular sodium hypochlorite, which corresponds to from 2 to 8, preferably from 2 to 4, g of active chlorine. 
     The residence time is from 2 to 24 hours and depends of course on the temperature and the amount of active chlorine used. The novel desizing process advantageously also produces a bleaching effect. 
     For the purposes of the present invention, cotton-containing fabrics are blends containing not less than 30% of cotton, in particular cotton with polyesters. 
     Specific examples of suitable activators which are used according to the invention and are soluble in the strongly alkaline aqueous solution are tertiary trishydroxyalkylamines where alkyl is of 2 or 3 carbon atoms, such as triethanolamine, bis-hydroxyethyl-hydroxypropylamine or trishydroxypropylamine, oxyethylated and/or oxypropylated ethylenediamines containing a total of from 4 to 10 ethylene oxide and/or propylene oxide units, such as N,N&#39;-tetrahydroxyethylethylenediamine or N,N&#39;-tetrahydroxypropylethylenediamine, urea derivatives, such as N,N&#39;-tetramethyl urea, N,N&#39;-tetrahydroxyethyl urea, N,N&#39;-dimethylethylene urea, N,N&#39;-bishydroxyethylethylene urea, propylene urea, N,N&#39;-dimethylpropylene urea or N,N&#39;-bis-hydroxyethylpropylene urea, oxyethylated polyethyleneimines which contain from 2 to 50 ethyleneimine units and whose imino and amino groups are alkylated with from 1 to 5 ethylene oxide units, e.g. (pentahydroxyethyl)-diethylenetriamine or (hexahydroxyethyl)-tetraethylenepentamine, and cyanuric acid derivatives, such as trishydroxyethyl- and trishydroxypropylmelamine and 2,4,6-trishydroxylamine-triazine. 
     Particularly preferred activators are cyanuric acid, oxazolid-2-one, N-methylpyrrolid-2-one and N-methylimidazole. 
     The preferred amounts of activator for the hypochlorite are from 3 to 5 g/l. 
     The additives according to the invention act in a surprising and unforeseeable manner as activators for the hypochlorite, which is usually deactivated in the strongly alkaline aqueous medium. These additives permit a relatively short residence time for the fabric during the oxidative action of the hypochlorite. The risk of damage to the fabric is substantially reduced. At the same time the fabric is bleached, and residual seed coats from the cotton are for the most part also removed. 
     Compared with enzymatic desizing, which as a rule is also effected by a dwell method at above 50° C., the novel process can be carried out very effectively at room temperature, i.e. about 20° C., in an energy-saving manner. The bleaching effect and the removal of cotton seed coats without additional heat treatment are the particularly noteworthy advantages compared with the enzymatic desizing method. 
     In oxidative desizing with persulfates, no significant bleaching effect is obtained when the procedure is carried out by a dwell method at room temperature. Removal of the cotton seed coats is insufficient. In practice, therefore, the oxidative desizing with persulfates is carried out as a heat treatment. 
     A further advantage which may be mentioned is that when the novel desizing process is used, the treated fabric only requires mild bleaching with hydrogen peroxide in order to achieve pure whiteness, and only about one third to one fourth of the amount of hydrogen peroxide otherwise usually required need be used. 
     Otherwise, in the novel desizing process, wetting agents, detergents and other conventional assistants, which of course must be stable to hypochlorite, can be added in a conventional manner. The additives are familiar to the skilled worker and serve as a rule to make the course of the desizing process more reliable. This also includes the use of sequestering agents and dispersants when the fabric is being washed. 
     Because of its low stability (the solutions can only be kept at about 0° C.), hypobromite is not used in industry. Disproportionation of the solutions is moderately fast even at room temperature (Cotton/Wilkinson, Anorganische Chemie, Verlag Chemie, Weinheim 1967, pages 531-534). 
    
    
     EXAMPLES AND COMPARATIVE EXAMPLES @ 
     The activating effect of the additives on a strongly alkaline hypochlorite solution was determined from the decrease in the active chlorine content of the solutions as a function of time. The particular content of active chlorine was determined titrimetrically with sodium thiosulfate. 
     
                                           TABLE                                   
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Activating effect                                                         
Amount of           Active chlorine content as a per-                     
activator added     centage of the initial value after                    
in g/l              1 hour                                                
                        3 hours                                           
                            5 hours                                       
                                24 hours                                  
__________________________________________________________________________
Without an additive 99  98  95  93                                        
Triethanolamine   1 86  73  65  57                                        
Propylene urea    3 20  10  --  --                                        
Malonic acid      5 91  84  76  10                                        
Cyanuric acid     3 94  91  82  76                                        
Oxazolidone       3 90  73  64  10                                        
Cyanoguanidine    5 85  71  54  21                                        
1-Methylpyrrolidone                                                       
                  3 91  85  80  55                                        
1-Methylimidazole 3 88  75  70  45                                        
NH--(CH.sub.2 --CH.sub.2 --NH).sub.35 --C.sub.3 H.sub.6 OH                
                  2 67  48  27   5                                        
C.sub.3 H.sub.6 OH                                                        
Trishydroxyethyl- 3 54  41  39  38                                        
melamine                                                                  
Trishydroxypropyl-                                                        
                  3 46  31  29  25                                        
melamine                                                                  
2,4,6-Trishydroxyl-                                                       
                  3 78  75  70  47                                        
amine-triazine                                                            
__________________________________________________________________________
 Test conditions: 20° C., 5 g/l of active chlorine, 40 g/l of NaOH.
 
    
     The table shows that the residual active chlorine content decreases as the intensity of activation increases. 
     The alkaline hypochlorite solutions were used for desizing polyester/cotton blends. The specimens were padded with desizing liquor, squeezed off to a liquor pick-up of 100% and allowed a dwell period. After the treatment, all fabrics were washed in the same manner. The whiteness, the extent of desizing (according to the Tegewa scale) and the mean degree of polymerization (DP value) of the cellulose were determined for these specimens. 
     EXAMPLE 1 
     Fabric: 65:35 polyester/cotton blend 
     Size: modified starch 
     Whiteness of the loomstate goods: 67.8 
     Seed coats per 100 cm 2  : 52 
     DP value: 2190 
     Compositions of the desizing liquor: 40 g/l of NaOH, 5 g/l of active chlorine, 3 g/l of nonylphenol oxyethylate and 2 g/l of wetting agent (phosphoric acid ester). 
     Residence temperature: 45° C. 
     1. A. 
     
         ______________________________________                                    
Residence                                                                 
        Amount of                 Residual                                
time in activator          Tegewa seed   DP                               
hours   added in g/l                                                      
                  Whiteness                                               
                           value  coats  value                            
______________________________________                                    
3       Without an                                                        
                  82.9     8      20     1310                             
        additive                                                          
        Cyanuric  82.5     9      23     1710                             
        acid 3                                                            
        Cyanuric  82.6     9      21     1750                             
        acid 5                                                            
5       Without an                                                        
                  82.8     9      11     1291                             
        additive                                                          
        Cyanuric  82.8     9      13     1796                             
        acid 3                                                            
        Cyanuric  83.0     9      10     1744                             
        acid 5                                                            
______________________________________                                    
 
    
     1. B. 
     The fabric was the same as that described in 1.A., and the active chlorine content of the desizing liquor was reduced to 3 g/l. 
     Residence time: 5 hours 
     Residence temperature: 45° C. 
     
         ______________________________________                                    
Amount of                       Residual                                  
activator              Tegewa   seed   DP                                 
added in g/l Whiteness value    coats  value                              
______________________________________                                    
Without an additive                                                       
             81.9      8         9     1356                               
Oxazolidone 3                                                             
             81.7      9        10     1796                               
Oxazolidone 5                                                             
             81.5      9        12     1944                               
______________________________________                                    
 
    
     The results of 1.A. and 1.B. show that the cotton is substantially damaged in the absence of an activator, and fabrics with such low DP values are normally useless. 
     EXAMPLE 2 
     Fabric: Loomstate cotton plain-weave sized with starch; whiteness 56.8; seed coats per 100 cm 2  : 158; DP value: 2350 
     Composition of the desizing liquor: 40 g/l of NaOH, 3.2 g/l of active chlorine, 3 g/l of nonylphenol oxyethylate and 1 g/l of wetting agent (phosphoric acid ester). 
     Residence time: 5 hours 
     Residence temperature: 45° C. 
     
         ______________________________________                                    
                                Residual                                  
Amount of activator    Tegewa   seed   DP                                 
added in g/l Whiteness value    coats  value                              
______________________________________                                    
Without an additive                                                       
             70.1      5        62     1856                               
Oxazolidone 3                                                             
             67.8      6        80     2296                               
Oxazolidone 5                                                             
             67.0      7        78     2310                               
______________________________________                                    
 
    
     The numerical results confirm the particularly high activity of oxazolidone. 
     For comparison, an experiment was carried out as described in Example 2, but without the addition of hypochlorite and without a catalyst. 
     
         ______________________________________                                    
Whiteness                                                                 
         Tegewa value                                                     
                     Residual seed coats                                  
                                   DP value                               
______________________________________                                    
63.1     4-5         110           2305                                   
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     The numerical values show that, with regard to whiteness and the removel of seed coats, alkali alone gives unsatisfactory results, so that a stronger peroxide bleach is required. 
     EXAMPLE 3 
     Fabric as described in Example 2. 
     Composition of the desizing liquor: 60 g/l of NaOH, 4 g/l of active chlorine, 3 g/l of nonylphenol oxyethylate and 2 g/l of wetting agent. 
     Residence time: 16 hours 
     Residence temperature: 22° C. 
     
         ______________________________________                                    
                                Residual                                  
Amount of activator    Tegewa   seed   DP                                 
added in g/l Whiteness value    coats  value                              
______________________________________                                    
Without an additive                                                       
             66.7      6-7      90     2056                               
1:1 mixture of                                                            
             66.5      6-7      67     2191                               
triethanolamine and                                                       
cyanuric acid 2                                                           
______________________________________                                    
 
    
     EXAMPLE 4 
     The fabric and the test conditions employed were as described in Example 3. 2 g/l of a sequestering agent (a carboxyl-containing copolymer) were added in the washing procedure. 
     
         ______________________________________                                    
                                Residual                                  
Amount of activator    Tegewa   seed   DP                                 
added in g/l Whiteness value    coats  value                              
______________________________________                                    
Without an additive                                                       
             70.5      6-7      68     2005                               
1-Methylpyrrolidone 3                                                     
             71.8      6-7      72     2196                               
1-Methylimidazole 3                                                       
             72.4      6-7      78     2104                               
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     The specimens were then bleached with hydrogen peroxide. 
     Bleach recipe: 5 g/l of stabilizer, 10 g/l of NaOH, 5 g/l of wetting detergent and 5 ml/l of aqueous 35% strength H 2  O 2  solution. 
     Liquor pick-up: 100% 
     Steaming time: 10 minutes 
     Bleaching temperature: 100°-102° C. 
     
         ______________________________________                                    
                                Residual                                  
Amount of activator    Tegewa   seed   DP                                 
added in g/l Whiteness value    coats  value                              
______________________________________                                    
Without an additive                                                       
             81.7      9        4      1689                               
1-Methylpyrrolidone 3                                                     
             82.7      9        5      1847                               
1-Methylimidazole 3                                                       
             83.1      9        3      1826                               
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     The results show that a relatively small amount of hydrogen peroxide is sufficient 
     EXAMPLE 5 
     Fabric: 50:50 cotton/polyester poplin shirting 
     Whiteness: 65.8 
     Seed coats per 100 cm 2  : 40 
     DP value: 3012 
     Composition of the desizing liquor: 25 g/l of NaOH: 2 g/l of active chlorine, 5 g/l of wetting agent and detergent. 
     Residence time: 16 hours 
     Residence temperature: 20° C. 
     Liquor pick-up: 100% 
     
         ______________________________________                                    
                                Residual                                  
Amount of activator    Tegewa   seed   DP                                 
added in g/l Whiteness value    coats  value                              
______________________________________                                    
Without an additive                                                       
             72.7      5        6      2739                               
Cyanuric acid 3                                                           
             70.2      5        7      2739                               
Methylpyrrolidone 3                                                       
             70.8      5        8      2830                               
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     EXAMPLE 6 
     Fabric: Cotton twill 
     Whiteness: 55.8 
     DP value: 2980 
     Seed coats per 200 cm 2  : 27 
     Composition of the desizing liquor: 140 g/l of NaOH, 5 g/l of active chlorine, 3 g/l of detergent and 5 g/l of caustic treatment/wetting agent. 
     Residence time: 16 hours 
     Residence temperature: 45° C. 
     Liquor pick-up: 80% 
     
         ______________________________________                                    
                                Residual                                  
Amount of activator    Tegewa   seed   DP                                 
added in g/l Whiteness value    coats  value                              
______________________________________                                    
Without an additive                                                       
             68.1      6-7      5      2314                               
N--methylpyrroli-                                                         
             66.8      6-7      4      2490                               
done 3                                                                    
Oxazolidone 3                                                             
             67.1      6-7      5      2510                               
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     COMPARATIVE EXAMPLE I 
     The fabric described in Example 4 was desized enzymatically. 
     Composition of the desizing liquor: 5 g/l of enzymatic desizing agent, 2 g/l of sodium chloride and 3 g/l of nonylphenol oxyethylate. 
     Residence time: 3 hours 
     Residence temperature: 75° C. 
     
         ______________________________________                                    
Whiteness                                                                 
         Tegewa value                                                     
                     Residual seed coats                                  
                                   DP value                               
______________________________________                                    
59.0     5-6         150           2330                                   
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     The fabric was then bleached as described in Example 4, except that four times the amount of hydrogen peroxide was used compared with Example 4. 
     
         ______________________________________                                    
Whiteness                                                                 
         Tegewa value                                                     
                     Residual seed coats                                  
                                   DP value                               
______________________________________                                    
80.5     7           34            1980                                   
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     When the enzymatically desized fabric was furthermore boiled with 40 g/l of NaOH and 1 g/l of wetting agent for 10 minutes at 100° C. and then bleached using the above recipe (with 20 ml/l of 35% strength H 2  O 2 ) the following effect was obtained: 
     
         ______________________________________                                    
Whiteness                                                                 
         Tegewa value                                                     
                     Residual seed coats                                  
                                   DP value                               
______________________________________                                    
83.0     9           7             1890                                   
______________________________________                                    
 
    
     COMPARATIVE EXAMPLE II 
     Oxidative desizing with K 2  S 2  O 8 . 
     The fabric from Example 2 or Example 4 was treated with the following desizing liquor: 40 g/l of NaOH, 2 g/l of K 2  S 2  O 8 , 5 g/l of wetting agent and detergent, and 2 g/l of sequestering agent. 
     Residence time: 10 minutes 
     Residence temperature: 100°-102° C. 
     Liquor pick-up: 100%. 
     After the washing procedure, the following effects were measured: 
     
         ______________________________________                                    
Whiteness                                                                 
         Tegewa value                                                     
                     Residual seed coats                                  
                                   DP value                               
______________________________________                                    
60.3     6-7         105           2120                                   
______________________________________                                    
 
    
     After bleaching with hydrogen peroxide as described in Example I, the following values were measured on the fabric: 
     
         ______________________________________                                    
Whiteness                                                                 
         Tegewa value                                                     
                     Residual seed coats                                  
                                   DP value                               
______________________________________                                    
81.6     9           4             1810                                   
______________________________________                                    
 
    
     The comparative examples show that, when enzymatic desizing is employed, boiling and stronger peroxide bleaching are required in order to obtain a white fabric free of seed coats. In the case of oxidative desizing with persulfate, too, stronger peroxide bleaching is necessary in order to achieve the same effects.