Aqueous compositions of solvent-soluble dyestuffs
This is a continuation of application Ser. No. 08/123,958, filed Sep. 20, 1993, abandoned. 
    
    
     This invention relates to dyestuffs and more particularly to solutions of dyestuffs and to a method of preparing such solutions. 
     The handling of dyestuffs in powder form is difficult because of the generation of dust and the resulting possible toxicity and explosion hazards. Hygienic, environmental and safety considerations make it desirable to use solutions of solvent-soluble dyestuffs, particularly solutions of potentially dangerous heavy metal types of dyestuff, such as chromium and cobalt premetallised dyestuffs. 
     However, a problem that is commonly encountered with many solvent dyestuffs is the difficulty in dissolving sufficient quantity of dyestuff. This problem has been exacerbated by increasing environmental awareness which has made it mandatory that a high level of dyestuff solubilisation is achieved. 
     The present methods of dyestuff solution preparation are frequently poor in this respect. 
     It has now been found that an unusually high rate of dissolution of dyestuff can be achieved by a process according to the invention. 
     According to the invention, there is provided a process for the preparation of a liquid composition comprising, 
     i) mixing an aqueous presscake of a solvent soluble dyestuff into a water miscible liquid (e.g. solvent); and 
     ii) distilling off the water until a water content of 15% or less by weight of the mixture is attained. 
     This process will generally allow dissolved dyestuff concentrations of up to 50% (preferably 20-50%) by weight of the composition to be achieved. More preferably the composition comprises 25-50% of dissolved dyestuff by weight of the total composition. 
     Any solvent-soluble dyestuff known in the art can be used. The invention works particularly well when the dyestuff used is a 1,2-metal complex dyestuff, more particularly when the metal is selected from chromium, cobalt, copper or aluminium or mixtures thereof. Preferably such dyes are of the azo series or of the copper phthalocyanine series. 
     Non metallised solvent dyestuffs may also be used in the present invention, preferably of the anthraquinone series. 
     Typical of the dyestuffs that can be used are: 
     C.I. Solvent Black 45 
     C.I. Solvent Blue 44 
     C.I. Solvent Brown 28 
     C.I. Acid Violet 66 
     C.I. Solvent Red 8 
     C.I. Solvent Red 91 
     C.I. Solvent Red 92 
     C.I. Solvent Red 127 
     C.I. Solvent Red 124 
     C.I. Solvent Red 89 
     C.I. Solvent Red 90:1 
     C.I. Solvent Orange 41 
     C.I. Solvent Orange 62 
     C.I. Solvent Yellow 83 
     C.I. Solvent Yellow 62 
     C.I. Solvent Yellow 48 
     C.I. Solvent Yellow 79 
     C.I. Solvent Yellow 83:1 and 
     C.I. Solvent Blue 45 (non metallised) 
     Aluminium solvent dyestuffs are also suitable for use in the present invention, as exemplified in British Patent Publication No. 2 259 305A; the contents of which are incorporated herein by reference. 
     More preferred dyestuffs used in the invention are: 
     C.I. Solvent Black 45 
     C.I. Solvent Blue 44 
     C.I. Solvent Brown 28 
     C.I. Acid Violet 66 
     C.I. Solvent Orange 41 
     C.I. Solvent Red 8 
     C.I. Solvent Red 91 
     C.I. Solvent Red 92 
     C.I. Solvent Yellow 62; and 
     C.I. Solvent Yellow 83 
     Any water miscible solvent known in the art may be used in the present invention. Preferably the solvent comprises one or more solvents that form an azeotropic mixture with water. Blends of solvents may also be used. Typical classes of solvent that can be used in the present invention are selected from aprotic solvents (like dimethylformamide and dimethylsulphoxide) alkanols (eg glycols and alcohols), ethers and ketones. Examples of suitable solvents include: propylene glycols (especially 1-methoxy-2-propanol), tetrahydrofurfuryl alcohol, dimethylformamide and dimethylsulphoxide. 
     An especially preferred class of solvent is the dialkylene glycol monoalkyl ethers, most particularly propylene glycol monomethyl ether and dipropylene glycol monomethyl ether. 
     Further according to the invention, there is provided a liquid dyestuff composition comprising a solvent-soluble dyestuff and a water miscible organic solvent, the composition comprising over 20, (advantageously over 25% and preferably over 30%) by weight of dissolved dyestuff. 
     Preferably in such a composition at least 2%, preferably 2-15%, more preferably 2-10, most preferably 4-10% by weight of water is present. 
     Preferably the dyestuff concentration in a liquid dyestuff composition according to the invention is 20 to 50%. More preferably the composition comprises 25-50% of dissolved dyestuff by weight of the total composition. 
     Further according to the invention there is provided a process for preparing a liquid composition of a solvent dyestuff comprising: 
     a) mixing an aqueous presscake of the dyestuff into a solvent (this may simply be carried out by breaking the presscake into suitably sized fragments and stirring them into the solvent); and 
     b) raising the temperature of the mixture to a temperature at which the water and some solvent readily distils off until the water content of the mixture drops below 15%: (the optimum temperature will vary depending on the solvent and the amount of water present, but a typical temperature is 98°-102° C. (where the solvent is dipropylene glycol monomethyl ether). 
     A typical aqueous presscake has a water content of between 30 and 60% by weight. It is preferred that the final water content is from 2-10%, more preferably 5-10%. It is undesirable to reduce the water content below 2% because the dyestuff solubility drops (it is essential to have more than 0%). 
     The resulting liquid composition of solvent-soluble dyestuffs is one that is easy and convenient to handle and in which the dissolved dyestuff content is significantly higher than those of previous liquid compositions of solvent dyestuffs. This means that dyeings of a particular depth can be carried out using a lesser amount of a liquid composition according to the present invention than that of an art recognised liquid solvent-soluble dyestuff composition. Volatile Organic Content (VOC) requirements can therefore be much more easily met. 
     The compositions according to the invention are particularly useful for dyeing leather and polyester, cellulose acetates (e.g. 2 1/2 and 3) and regenerated cellulose (particularly, cellulose acetates (e.g. 2 1/2 and 3) and regenerated cellulose dyed in the (solubilised) mass) and lacquers such as cellulose lacquers. 
     The invention will now be illustrated by the following Examples in which all parts and percentages are by weight unless indicated to the contrary. 
    
    
     EXAMPLE 1 
     A solution of CI Solvent Yellow 62 in dipropylene glycol monomethyl ether can be prepared as follows. The solution contains about 40% by weight dyestuff. 
     A reactor equipped with a stirrer, heater and distillation column is charged with 1500 parts of dipropylene glycol monomethyl ether (hereinafter referred to as the glycol solvent) and the glycol solvent is heated to 60° C. 1718 parts of an aqueous presscake of CI Solvent Yellow 62 (58.2% by weight of dyestuff) is gradually added with stirring to the glycol solvent. 
     This mixture is heated to 100°-110° C. while stirring and the mixture is distilled at 98°-102° C. to remove water, a total of 734 parts being removed. The distillate is about 90% water and 10% solvent. A dark brown solution remains in the reactor containing 40% dyestuff, 56% solvent and 4% water. 
     EXAMPLES 2 TO 6 
     Example 1 is repeated, replacing CI Solvent Yellow 62 with the same weight of the following dyestuffs: 
     
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Example 2        CI Solvent Yellow 83                                     
Example 3        CI Solvent Orange 41                                     
Example 4        CI Solvent Red 92                                        
Example 5        CI Solvent Red 91                                        
Example 6        CI Solvent Blue 45                                       
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     In each case the result is a dark coloured solution of the dyestuff with a dye content of 40% or more. 
     EXAMPLE 7 
     Example 1 is repeated, replacing CI Solvent Yellow 62 with the same weight of a dyestuff of the formula (a compound according to Example 5 of British Patent Publication No. 2 259 305): ##STR1## 
     The result is a dark coloured solution of the dyestuff with a dye content of 40% or more. 
     EXAMPLES 8 TO 10 
     Example 7 is repeated, replacing the dyestuff of Example 5 of British Patent Publication No. 2 259 305 with the same weight of the following dyestuffs: 
     EXAMPLE 8 ##STR2## 
     EXAMPLE 9 ##STR3## 
     EXAMPLE 10 ##STR4## 
     Each of Examples 1 to 10 can be repeated by replacing the solvent, dipropylene glycol monomethyl ether, by 1500 parts of propylene glycol monomethyl ether. 
     APPLICATION EXAMPLE 
     18 parts of the dyestuff concentrate of Example 1 are stirred into 100 parts of a nitrocellulose lacquer prepared from: 
     18.8% of Nitrocellulose A15 (in the form of white flocs wetted with 35% isopropanol) 
     6.3% of an acrylic acid butyl ester polymer, a softener resin (Acronal 4F from BASF) 
     3.3% of diphenyloctylphosphate, a softener (Santiser 141 from Monsanto) 
     10.0% of methoxy propanol (Dowanol PM from Dow Chemical) 
     10.0% ethoxypropanol 
     41.6% ethanol and 
     10.0% ethyl acetate. 
     The solubility is good. 
     With a 25 mm blade, a wet film of this lacquer composition is placed on aluminium foil and the lacquer is dried for 5 hours at 130° C. A reddish yellow lacquer coating results with good light and migration fastness properties. 
     The Application Example is repeated using 10 to 25 parts (especially 15 parts) of any one of the dyestuff concentrations of Examples 2 to 10 in place of that of Example 1. 
     The text of UK 2,259,305A follows: ##SPC1##