Laser sensitive lithographic printing plate having specific photopolymer composition

Abstract

High speed violet or ultraviolet laser sensitive lithographic printing plate comprising on a hydrophilic substrate a specific photosensitive composition is described. The photosensitive layer comprises a polymeric binder, a multifunctional (meth)acrylate monomer, a free-radical initiator, and a sensitizing dye, with specific weight ratio of the monomer to the polymer. Combination of multifunctional urethane (meth)acrylate monomer and multifunctional non-urethane (meth)acrylate monomer can be advantageously used. The plate is imagewise exposed with a violet or ultraviolet laser at a dosage of less than 300 μJ/cm 2 , and then developed with an aqueous developer or with ink and/or fountain solution.

1 . A method of processing a lithographic printing plate comprising in order: 
 (a) providing a lithographic printing plate comprising (i) a hydrophilic substrate, (ii) an oleophilic photosensitive layer comprising an alkaline soluble polymeric binder, a urethane (meth)acrylate monomer having at least 4 (meth)acrylate groups, a free radical initiator, and a sensitizing dye, and (iii) a water soluble or dispersible overcoat; wherein the weight ratio of all the urethane monomers having at least 4 (meth)acrylate groups to all the alkaline soluble polymeric binders is at least 0.9;    (b) exposing said plate with a laser having a wavelength of from 250 to 430 nm at a dosage of less than 300 μJ/cm 2  according to digital imaging information to cause hardening of the photosensitive layer in the exposed areas; and    (c) contacting said exposed plate with an aqueous alkaline developer having a pH of 9.0 to 13.5 to remove the non-exposed areas of the photosensitive layer.    
     
     
         2 . The method of  claim 1  wherein said weight ratio is from 1.2 to 6.0.  
     
     
         3 . The method of  claim 1  wherein said weight ratio is from 1.8 to 5.0.  
     
     
         4 . The method of  claim 1  wherein said weight ratio is from 2.4 to 4.0.  
     
     
         5 . The method of  claim 1  wherein said urethane monomer has at least 5 (meth)acrylate groups.  
     
     
         6 . The method of  claim 1  wherein said urethane monomer has at least 6 (meth)acrylate groups.  
     
     
         7 . The method of  claim 1  wherein said urethane monomer has at least 6 (meth)acrylate groups, and the weight ratio of all the urethane monomers having at least 6 (meth)acrylate groups to all the alkaline soluble polymeric binders is at least 0.9.  
     
     
         8 . The method of  claim 1  wherein said monomer is an acrylate monomer.  
     
     
         9 . The method of  claim 1  wherein said photosensitive layer further comprises a non-urethane (meth)acrylate monomer.  
     
     
         10 . The method of  claim 1  wherein said free radical initiator is a hexaarylbiimidazole or titanocene compound.  
     
     
         11 . The method of  claim 1  wherein said free radical initiator is a hexaarylbiimidazole compound.  
     
     
         12 . The method of  claim 1  wherein said sensitizing dye is a dialkylaminobenzophenone compound.  
     
     
         13 . The method of  claim 12  wherein said dialkylaminobenzaphenone compound is a 4,4′-bis(dialkylamino)benzophenone compound.  
     
     
         14 . The method of  claim 1  wherein said photosensitive layer further comprises a hydrogen donor compound.  
     
     
         15 . The method of  claim 14  wherein said hydrogen donor compound is selected from the group consisting of 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, 2-mercaptobenzothiazole, 3-mercapto-1,2,4-triazole, N,N-dialkyl benzoic alkyl ester, N-phenylglycine ethyl ester, and N-phenylglycine benzyl ester.  
     
     
         16 . The method of  claim 1  wherein said free radical initiator is a hexaarylbiimidazole compound, said sensitizing dye is a 4,4′-bis(dialkylamino)benzophenone compound, and said photosensitive layer further includes a hydrogen donor compound.  
     
     
         17 . The method of  claim 1  wherein said photosensitive layer is semisolid at 25° C.  
     
     
         18 . The method of  claim 1  wherein said substrate is a grained and anodized aluminum sheet (with or without further hydrophilic treatment) having a reflection optical density of from 0.30 to 0.50.  
     
     
         19 . The method of  claim 1  wherein said developer has a pH of from 10.0 to 13.0.  
     
     
         20 . The method of  claim 1  wherein said plate is heated to an elevated temperature of 50 to 200° C. for 1 to 600 seconds after said laser exposure (step b) and before said development (step c).