Desensitizing solution for use in offset printing
FIELD OF THE INVENTION 
     The present invention relates to a cyan-free desensitizing solution for use in offset printing which is entirely free of cyan compounds. 
     BACKGROUND OF THE INVENTION 
     An electrophotographic offset printing plate (hereinafter referred to as the &#34;master&#34;) has a light-sensitive layer comprising fine particles of a photoconductive material (e.g., zinc oxide) dispersed in a resin binder; this plate is subjected to a conventional electrophotographic operation for forming an oleophilic image on the light-sensitive layer. 
     Offset printing generally uses a plate comprising a highly water wettable non-image area (hydrophilic area) and a poorly water wettable image area (oleophilic area). However, the master for use in electrophotographic offset printing has its image area composed of a hydrophobic photoconductive layer, so if the plate is immediately subjected to printing, the printing ink adheres to the non-image area and prevents normal operation. It is therefore necessary to render the non-image area of the master hydrophilic by desensitizing it before printing. Desensitizing solutions (also called &#34;etching solution&#34;) that have been proposed to date are classified in two types, one type being a cyan compound containing solution mainly comprising a ferrocyanate (as described in U.S. Pat. No. 3,001,872) or ferricyanate, and the other type being a cyan free solution mainly comprising an amine cobalt complex (as described in U.S. Pat. No. 4,208,212), phytic acid (as described in U.S. Pat. No. 3,592,640 and Japenese Patent Publication No. 2839/83) or its derivative, or a guanidine derivative. 
     However, none of these processing solutions are completely satisfactory. The first type of solution containing a ferrocyanate or ferricyanate has the advantages of high desensitizing power, ability to form a strong hydrophilic film, and fast film formation. Nevertheless, the ferrocyanide or ferricyanide ion is unstable to heat and light, and when exposed to light, it undergoes either a color forming reaction or a precipitation reaction that weakens the desensitizing power of the solution. Furthermore, the presence of cyanide ions (CN - ) leads to the formation of a detectable free cyanide which is carried into the effluent and pollutes water-courses. 
     The cyan-free desensitizing solution has been proposed to eliminate these defects of the cyan-containing solution. But even this second type of desensitizing solution has been unable to provide a completely satisfactory lithographic printing master. Particularly, the cyan-free solution has a slower rate of hydrophilic film formation than the cyan compound-containing solution, and is unable to form an immediately printable, hydrophilic film of high physical strength by one pass through an etch processor, and as a result, scum often forms on the non-image area, or insufficient edge acuity of halftone dots occurs. 
     Inositol hexaphosphate esters and metal derivatives thereof are known to form chelate compounds with metals, and have already been proposed for use as desensitizers in the processing of the offset printing master as described in U.S. Pat. No. 3,592,640. However, these compounds have such a slow film forming rate that the desired hydrophilic film is not obtainable by a single pass through the processor, and the resulting poor ink separation may often cause scum on the non-image area or insufficient edge acuity of halftone dots. 
     In previously filed Japenese Patent Publication No. 5799/83, use has been proposed of an ion complex of a water-soluble cationic polymer and a mono- or divalent metal salt of an inositol hexaphosphate ester (e.g., sodium phytate, potassium phytate or calcium phytate). This complex has improved water retention but does not achieve a satisfactory etching speed. At the same time, the complex is highly dependent on moisture and is so sensitive to the printing environment that scum easily forms on the non-image area during printing at low humidity. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems discussed above, the present inventors have made extensive studies, and have now discovered that by using at least one member selected from the group of an ammonium salt and amine salt of the inositol hexaphosphate ester, instead of a mono- or divalent metal salt thereof, an ion complex having not only a higher etching speed, but also appreciably improved printing characeristics at low humidity can be produced. 
     Thus, according to the present invention, (a) at least one member selected from the group of an ammonium or amine salt of an inositol hexaphosphate ester, (b) a low molecular weight electrolyte, and (c) a water-soluble cationic polymer are used as the effective components of a cyan-free desensitizing solution, and an ion complex formed between the water-soluble cationic polymer and the ammonium or amine salt of the inositol hexaphosphate ester is effectively used in the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The ammonium and amine salts of the inositol hexaphosphate ester may be used either independently or as a mixture of the two salts, in any desired proportions. The resulting ion complex has the ability to form a strong, hydrophilic film and is adsorbed strongly to a chelate compound with metal ions, thereby improving significantly the hydrophilicity and film-forming properties of said compound. As a result, the desensitizing solution of the present invention provides a printing master that can be processed at high speed without causing scum on the non-image area or insufficient edge acuity in halftone dots. 
     Suitable examples of the water-soluble cationic polymer used in the desensitizing solution are water-soluble compounds having molecular weights in the range of from about 500 to 100,000 and having an amino group, imino group, tertiary amine group, quaternary ammonium salt group, or a hydrazine group in the molecule thereof. Specific examples include a melamine-formaldehyde resin, acetoguanamine-formaldehyde resin, benzoguanamine-formaldehyde resin, polyethyleneimine, polyamidepolyamine epichlorohydrin, aniline resin hydrochloride, polythiourea hydrochloride, cationized amino resin, polyvinylpyridine hydrochloride, cationically modified product of polyacrylamide (e.g., vinylamine polymer produced from polyacrylamide by the Hofmann degradation, polyacrylamide subjected to the Mannich reaction with formalin and secondary amine, or such polyacrylamide which is further converted to a quaternary ammonium salt with dimethyl sulfate), polyvinylbenzyl chloride converted to a quaternary ammonium salt with a tertiary amine, poly(N-vinyl-2-methylimidazoliummethyl sulfate), dimethylaminoethyl(meth)acrylate polymer or a quaternary ammonium salt thereof, amino group containing (meth)-acrylate polymer [diethylaminoethyl(meth)acrylate polymer or a quaternary ammonium salt thereof], a polyalkylimine compound of the type shown in U.S. Pat. No. 3,617,266 which is prepared from a polymer containing an aliphatic amino group in the repeating unit by converting all or part of the amino groups to either a salt or quaternized form, an amine adduct of epoxy resin, an amine adduct of a maleic polymer, an amino group containing polyamide resin, polyamide-epoxy resin and a polyamide-epichlorohydrin resin, preferably polyamide-polyamine epichlorohydrin, a polyamide-epichlorohydrin resin, polyethyleneimine converted to a quaternary ammonium salt with dimethyl sulfate, a melamine-formaldehyde resin and polyvinylbenzyl chloride converted to a quaternary ammonium salt with a tertiary amine. 
     Examples of the low molecular weight electrolyte also used in the desensitizing solution of the present invention include salts of inorganic acids such as sulfuric acid, hydrochloric acid, HBr, HI, HF, nitric acid, perchloric acid and HPF 6 , as well as salts of organic acids such as organic sulfonic acid (e.g., methanesulfonic acid), amidosulfonic acid, organic phosphonic acid, oxalic acid, formic acid, trichloroacetic acid and picric acid. Compounds which are alkali metal salts, alkaline earth metal salts or ammonium salts of these acids and which have a water solubility of at least about 10 wt% at room temperature are used with advantage, preferably alkali metal salts or ammonium salts of nitric acid, sulfuric acid, carboxylic acid and sulfonic acid. Specific examples include NaCl, NaBr, KCl, KBr, LiCl, LiBr, NH 4  Cl, NaNO 3 , KNO 3 , NH 4  NO 3 , Ca(NO 3 ) 2 , Mg(NO 3 ) 2 , Na 2  SO 4 , K 2  SO 4  , MgSO 4 , (NH 4 ) 2  SO 4 , HCOONa, CH 3  COONa, (COONa) 2 , Cl 3  CCOONa, NH 4  F, KPF 6 , ##STR1## and NH 4  SO 3  NH 2 , preferably NH 4  NO 3 , Na 2  SO 4 , K 2  SO 4 , (NH 4 ) 2  SO 4 , HCOONa, ##STR2## and sodium malonate. 
     The ammonium salt and amine salt of the inositol hexaphosphate ester are available as a commercial product, or they may be easily synthesized by known method as described, for example, in U.S. Pat. Nos. 2,691,035, 2,718,523, 2,750,400, 2,815,360 and 3,016,398, or synthesized by skilled worker in accordance with these methods. The ammonium salt of the inositol hexaphosphate ester is more preferred. 
     The respective components of the desensitizing solution according to the present invention are preferably used in the following amounts, with the total amount of the desensitizing solution being taken as 1,000 parts by weight: (a) at least one member selected from the group of the ammonium salt and amine salt of the inositol hexaphosphate ester, from 10 to 200 parts by weight, and more preferably from 40 to 75 parts by weight; (b) the low molecular weight electrolyte, from 20 to 150 parts by weight, and more preferably from 40 to 100 parts by weight; (c) the water-soluble cationic polymer, from 0.2 to 20 parts by weight, and more preferably from 1 to 10 parts by weight; and the remainder is substantially water. 
     These components are dissolved in ion-exchanged or tap water to make a sample of the desensitizing solution of the present invention. The order of addition of the respective components is not critical, but preferably, the ammonium salt or amine salt of the inositol hexaphosphate ester is first dissolved in water, then the low molecular weight electrolyte is added, followed by addition of an aqueous solution of the cationic polymer. The desensitizing solution may contain various additives such as a pH modifier (e.g., organic or inorganic acids, or basic hydroxide such as potassium hydroxide and sodium hydroxide), a pH buffer (e.g., phosphates), a wetting agent (e.g., ethylene glycol, sorbitol, glycerin or gum arabic), an antiseptic (e.g., salicylic acid, phenol, butylparabenzoate, or sodium dehydroacetate), and a rust inhibitor (e.g., EDTA or amines). The desensitizing solution of the present invention is preferably used at a pH in the range of from 3 to 6. It may be diluted with water for use as dampening water. 
     Thus, the cyan-free desensitizing solution of the present invention has the following advantages: it does not contain ferrocyanide or ferricyanide that is an environmental hazard and which is deteriorated by light or heat; it can be stored for an extended period without causing discoloration or precipitation; it is less sensitive to the printing environment than the prior art cyanfree solution; and it has such an improved film forming speed that it can be subjected to fast etching for making an offset printing master having no scum on the non-image area or which is free from insufficient edge acuity of halftone dots. Further, the cyan-free desensitizing solution of the present invention is also useful as the etching solution or dampening solution for the lithographic plate prepared from the conventional presensitized lithographic plate, i.e., the lithographic printing plate comprising the aluminum suppot having a lithographically suitable light-sensitive layer applied thereon. 
     According to a preferred embodiment the electrophotographic recording layer is rendered waterreceptive at the areas to be hydrophilized after the printing master has been mounted on the press, thus obviating any separate immersion treatment. The hydrophilizing treatment of said layer may be carried out by means of an absorbent pad impregnated with a desensitizing solution of this invention. 
     Electrophotographic recording materials, which are especially suited to be used in the preparation of a planographic printing plate, are described, e.g., in the United Kingdom Patent Specifications Nos. 1,125,580 and 1,125,579, and U.S. Pat. No. 4,456,670. 
     Any known process for forming the electrostatic latent image and hydrophobic image may be applied. 
     According to a common technique the hydrophobic image is formed by the consecutive steps of producing an electrostatic image on a photoconductive zinc oxide/hydrophobic binder layer by integrally electrostatically charging that layer, subsequently imagewise exposing and developing the latter with a hydrophobic developer powder, which is fixed to the recording layer, e.g., by heating. 
     The powder image can be formed by the known dry &#34;carrier-toner development&#34; or by a liquid development based on electrophoresis wherein charged hydrophobic particles are attracted from an electrically insulating liquid to the charged areas of the recording layer. Such development technique is described, e.g., in the U.K. Patent Specification No. 755,486. 
     The present invention is hereunder described in greater detail by reference to working examples and comparative examples. 
    
    
     EXAMPLE 1 
     
         ______________________________________                                    
                    Amount                                                
Components          (parts by weight)                                     
______________________________________                                    
Water               759                                                   
Ammonium salt of inositol hexa-                                           
                    76                                                    
phosphate ester (50% aq. soln.)                                           
CH.sub.2 (OH)COOH   23                                                    
NaCl                61                                                    
Polyamide-epichlorohydrin resin                                           
                     5                                                    
(30% aq. soln.)                                                           
Glycerin            76                                                    
______________________________________                                    
 
    
     EXAMPLE 2 
     
         ______________________________________                                    
                    Amount                                                
Components          (parts by weight)                                     
______________________________________                                    
Water               713                                                   
Ammonium salt of inositol hexa-                                           
                    100                                                   
phosphate ester (50% aq. soln.)                                           
CH.sub.2 (OH)COOH    30                                                   
NH.sub.4 Br          50                                                   
Polyethyleneimine quaternized                                             
                     7                                                    
with dimethylsulfuric acid                                                
Glycerin            100                                                   
______________________________________                                    
 
    
     EXAMPLE 3 
     
         ______________________________________                                    
                    Amount                                                
Components          (parts by weight)                                     
______________________________________                                    
Water               608                                                   
Amine salt of inositol hexa-                                              
                    80                                                    
phosphate ester (50% aq. soln.)                                           
CH.sub.2 (COOH).sub.2                                                     
                    25                                                    
K.sub.2 SO.sub.4    80                                                    
Melamine-formaldehyde resin                                               
                     7                                                    
(30% aq. soln.)                                                           
Sorbitol            200                                                   
______________________________________                                    
 
    
     COMPARATIVE EXAMPLE 1 
     
         ______________________________________                                    
                    Amount                                                
Components          (parts by weight)                                     
______________________________________                                    
Water               770                                                   
Ammonium salt of inositol hexa-                                           
                    100                                                   
phosphate ester (50% aq. soln.)                                           
CH.sub.2 (OH)COOH    30                                                   
Glycerin            100                                                   
______________________________________                                    
 
    
     COMPARATIVE EXAMPLE 2 
     
         ______________________________________                                    
                    Amount                                                
Components          (parts by weight)                                     
______________________________________                                    
Water               770                                                   
Amine salt of inositol hexa-                                              
                    100                                                   
phosphate ester (50% aq. soln.)                                           
CH.sub.2 (OH)COOH    30                                                   
Glycerin            100                                                   
______________________________________                                    
 
    
     COMPARATIVE EXAMPLE 3 
     
         ______________________________________                                    
                    Amount                                                
Components          (parts by weight)                                     
______________________________________                                    
Water               865                                                   
CH.sub.2 (OH)COOH    30                                                   
Polyamide-epichlorohydrin resin                                           
                     5                                                    
(30% aq. soln.)                                                           
Glycerin            100                                                   
______________________________________                                    
 
    
     COMPARATIVE EXAMPLE 4 
     
         ______________________________________                                    
                    Amount                                                
Components          (parts by weight)                                     
______________________________________                                    
Water               763                                                   
Ammonium salt of inositol hexa-                                           
                    100                                                   
phosphate ester (50% aq. soln.)                                           
CH.sub.2 (OH)COOH    30                                                   
Polyamide-epichlorohydrin resin                                           
                     7                                                    
(30% aq. soln.)                                                           
Glycerin            100                                                   
______________________________________                                    
 
    
     COMPARATIVE EXAMPLE 5 
     
         ______________________________________                                    
                    Amount                                                
Components          (parts by weight)                                     
______________________________________                                    
Water               745                                                   
Sodium salt of inositol hexa-                                             
                    100                                                   
phosphate ester                                                           
NH.sub.4 Br          50                                                   
Polyamide-epichlorohydrin resin                                           
                     5                                                    
(30% aq. soln.)                                                           
Glycerin            100                                                   
______________________________________                                    
 
    
     An intermediate layer having the compositions shown below was coated on a paper base (103 g/m 2 ) laminated with an aluminum foil (10 μthick) to a thickness of 4 μby a conventional method. 
     
         ______________________________________                                    
                   parts by weight                                        
______________________________________                                    
Colloidal alumina (&#34;Alumina Sol                                           
                      50                                                  
200&#34;, a product of Nissan Chemical                                        
Industries, Ltd.)                                                         
Polyvinyl acetate (M--w = 80,000)                                         
                     100                                                  
Methanol             100                                                  
______________________________________                                    
 
    
     A photosensitive layer having the following compositions was coated on the surface of the intermediate layer to a dry thickness of 25 g/m 2 . 
     
         ______________________________________                                    
                   parts by weight                                        
______________________________________                                    
Photoconductive zinc oxide                                                
                     100                                                  
(&#34;Sazex 2000&#34;, a product of                                               
Sakai Chemical Industry Co., Ltd.)                                        
Silicone resin (&#34;KR-211&#34;, a product                                       
                     35                                                   
of Shinetsu Chemical Industry                                             
Co., Ltd.)                                                                
Rose bengale         0.1                                                  
Fluorescein          0.2                                                  
Methanol             10                                                   
Toluene              150                                                  
______________________________________                                    
 
    
     The resulting photosensitive sheet was allowed to stand in a dark room at 40% RH and 25° C. for 12 hours. 
     The photosensitive layer was charged with a negative corona with a tension of -6,000 v on the corona wires and exposed through the graphic original. Development was carried out by using a commercially available liquid toner for plate-making. The toner image was fixed to the layer by heating. 
     A cotton pad was impregnated with the desensitizing solutions of Examples 1 to 3 and Comparative Examples 1 to 5, so as to make offset masters. In two different environments, printing was conducted on these masters, with five-fold aqueous dilutions of the respective desensitizing solutions being used as dampening water. The results are shown in Tables 1 and 2. 
     
                       TABLE 1                                                     
______________________________________                                    
(25° C. × 60% RH)                                            
           No. of Sheets That                                             
           Could Be Printed                                               
                          Edge                                            
Etching    without Forming                                                
                          Acuity of                                       
Speed      Scum on the Non-                                               
                          Halftone Ink                                    
(sec)      Image Area     Dots     Separation                             
______________________________________                                    
Ex. 1  8       &gt;4,000         sharp  possible                             
                                     in 10 sec                            
Ex. 2  8       &gt;4,000         sharp  possible                             
                                     in 10 sec                            
Ex. 3  8       &gt;4,000         sharp  possible                             
                                     in 10 sec                            
Comp. 18          100         not sharp                                   
                                     impossible                           
Ex. 1                                                                     
Comp. 18          100         not sharp                                   
                                     impossible                           
Ex. 2                                                                     
Comp. 18           1          not sharp                                   
                                     impossible                           
Ex. 3                                                                     
Comp. 18           1          not sharp                                   
                                     impossible                           
Ex. 4                                                                     
Comp. 18       &gt;4,000         sharp  possible                             
Ex. 5                                in 10 sec                            
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
(20° C. × 30% RH)                                            
           No. of Sheets That                                             
           Could Be Printed                                               
                          Edge                                            
Etching    without Forming                                                
                          Acuity of                                       
Speed      Scum on the Non-                                               
                          Halftone Ink                                    
(sec)      Image Area     Dots     Separation                             
______________________________________                                    
Ex. 1  8       &gt;4,000         sharp  possible                             
                                     in 10 sec                            
Ex. 2  8       &gt;4,000         sharp  possible                             
                                     in 10 sec                            
Ex. 3  8       &gt;4,000         sharp  possible                             
                                     in 10 sec                            
Comp. 18           1          not sharp                                   
                                     impossible                           
Ex. 1                                                                     
Comp. 18           1          not sharp                                   
                                     impossible                           
Ex. 2                                                                     
Comp. 18           1          not sharp                                   
                                     impossible                           
Ex. 3                                                                     
Comp. 18           1          not sharp                                   
                                     impossible                           
Ex. 4                                                                     
Comp. 18           1          not sharp                                   
                                     impossible                           
Ex. 5                                                                     
______________________________________                                    
 
    
     As Tables 1 and 2 show, the desensitizing solutions prepared in Examples 1 to 3 could be used as a high-speed etchant, and at least 4,000 sheets could be printed from the resulting masters irrespective of the environment. No scum formed on the non-image area of the masters or the printed sheets, and the latter had a good ink adhesion and sharp edge acuity of halftone dots. 
     However, the desensitizing solutions prepared in Comparative Examples 1 to 4 formed scum on the non-image area of masters even at low etching speed. Furthermore, ink separation was impossible and there was no edge sharpness in halftone dots. The desensitizing solution prepared in Comparative Example 5 was usable when the printing environment was 60% RH (relative humidity) and the etching time was extended; however, printing was impossible at 30% RH in spite of prolonged etching time. 
     As will be understood from the foregoing data, the desensitizing solution of the present invention containing an ammonium or amine salt of inositol hexaphosphate ester, a water-soluble cationic polymer, and a low molecular weight electrolytic compound as the effective components is not moisture dependent and achieves a very high etching speed. 
     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.