Abstract:
The toner image receiver sheet of the invention, which is useful for digital offset printing, comprises a resin-coated paper substrate on which is disposed an image-receiving layer comprising an imino-functionalized polymer and a polyurethane. In a process for making the receiver sheet, a composition, preferably an aqueous solution containing the imino-functionalized polymer and polyurethane, is applied to a resin-coated surface of a paper substrate, thereby forming an image-receiving layer on the substrate. The image-receiving layer provides the receiver sheet of the invention with a glossy, non-tacky surface having excellent toner transfer and adhesion characteristics.

Description:
FIELD OF THE INVENTION 
     This invention relates to toner-receptive media for use in digital offset color printing. More specifically, this invention relates to a glossy receiver sheet for receiving images of electrically charged toners using a digital offset color printing press. 
     BACKGROUND OF INVENTION 
     Currently available receiver media for digital offset printing presses consist of a variety of papers and films having a clay coating or clay base. The papers have a very low gloss, with only the film-based receivers exhibiting glossy surfaces. 
     In a digital offset printing press, the toner &#34;ink&#34; is not absorbed into the substrate but rather resides on the surface of the media. The problem of adhesion and transfer at this toner-media interface, especially on a high gloss surface, requires that the media have a special surface treatment to promote total toner transfer and adhesion to the surface. 
     One type of surface treatment of paper known in the prior art does not produce a distinct layer on the paper, but rather causes the surface to become basic or alkaline. Such a process is not applicable to glossy films or resin-coated papers because it would reduce gloss, make the surface very tacky to the touch, degrade toner transfer and adhesion, and cause paper feeding and paper stacking problems resulting from the tacky surface. 
     PCT International Application WO 96/06383 discloses a printing method to form an image of colored polymer toner particles on a paper substrate whose surface has been treated with a compound having a basic functionality, preferably a polymeric imine compound. The method is purported to improve adhesion on many but not all of the papers tested. 
     European Patent Application EP 0 637 603 Al discloses a polymeric film substrate coated with a polyamido-polyethyleneimine. Although the primary purpose of the applied coating is to render the coated film receptive to direct extrusion overcoating with other polymers, improved ink adhesion is also recited as an object. 
     UK Patent Application GB 2 212 741 A discloses a polyolefin film coated with an aqueous solution of a modified polyethyleneimine compound. The coated film is intended for use as receiver for UV-curable inks. 
     There is a continuing need for a receiver sheet for images from digital offset printing presses that has a glossy surface with excellent toner transfer and adhesion properties. The present invention meets this need. 
     SUMMARY OF INVENTION 
     The present invention provides glossy, toner-receptive media comprising resin-coated paper for the transfer and adhesion of an &#34;ink&#34; comprising pigmented toner particles in a carrier of hydrocarbon liquids. The receiver sheet of the invention has a non-tacky surface and yields high quality images with good transfer and excellent adhesion. 
     The toner image receiver sheet of the invention, which is useful for digital offset printing, comprises a resin-coated paper substrate on which is disposed an image-receiving layer comprising an imino-functionalized polymer and a polyurethane. 
     Further in accordance with the invention is a process for making the toner image receiver sheet by applying a composition, preferably aqueous, comprising the imino-functionalized polymer and polyurethane to a resin-coated surface of a paper substrate, thereby forming an image-receiving layer on the substrate. 
     The image-receiving layer provides the receiver sheet of the invention with a glossy, non-tacky surface that exhibits excellent toner transfer and adhesion characteristics. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     In a digital offset printing process, a latent image is projected digitally to a drum and, using an electrostatic process, is &#34;inked&#34; with electrically charged liquid toner particles. The resulting toner particle image is transferred to a blanket drum using electrostatic and pressure forces, where it is heated to form a thin polymer film. This film is then transferred to the image-receiving layer on the receiver substrate. A digital offset color press especially suitable for use with the receiver sheet of the present invention is the Indigo E-Print 1000 press, available from Indigo N.V. 
     The liquid toner composition comprises a hydrocarbon carrier liquid such as Isopar™ L and M with pigmented toner particles, as described in the aforementioned PCT W096/06384, the disclosure of which is incorporated herein by reference. 
     Preferred substrates are resin-coated papers ranging in thickness from about 5 mils (125 μ) to 10 mils (250 μ). The resin may be, for example, polyethylene, polypropylene, polyesters, or mixtures thereof, applied on either or both sides of the paper by any methods known in the art such as, for example, solvent coating, melt extrusion, or lamination. 
     An image-receiving layer (IRL) of the invention is applied to a resin-coated side of the substrate by any of the known coating arts. The IRL, which preferably is applied using an aqueous solution of the IRL components, has a dry thickness of, preferably, about 0.5 μ to 20 μ, more preferably, about 1 μ to 7 μ. The IRL comprises a blend of a polymer having an imino functionality and a polyurethane and preferably exhibits a basic pH, more preferably, from about 9 to 11. 
     In preferred embodiments, the imino-functionalized polymer includes the moiety --NH--, at least a portion of which may be alkylated by, for example, ethylene oxide, yielding an N-hydroxyethylsubstituted material. Preferred polyimines include, for example, polyethylenimine (50% water solution), polyethylenimine (80% ethoxylated), and polyethylenimine, epichlorohydrin modified, all available from Aldrich Chemical Co. Alternatively, other polymeric materials having an imino functionality may be employed. 
     Preferred polyurethanes are water-dispersible polyurethane-ureas such as those described in U.S. Pat. No. 4,501,852, the disclosure of which is incorporated herein by reference. Preferred commercially available aqueous dispersions of polyurethane-ureas derived from bis(4-isocyanatocyclohexyl)methane include Bayhydrol 140Q, Bayhydrol-123, Bayhydrol-121, and Bayhydrol-110, manufactured by Bayer Corp. Preferred ratios of polyurethane:polyimine are from about 60:40 to 90:10, the preferred ratio being about 70:30. 
     The following examples further illustrate the invention: 
     Example 1--Preparation of receiver sheets 
     The following procedure is exemplary: To a 12-15 weight percent aqueous solution of a mixture of Bayhydrol-110 and polyethylenimine (80% ethoxylated) in a 70:30 weight ratio was added 0.05 weight percent of Olin 10G surfactant. The resulting solution was coated on a corona discharge-treated, resin-coated (single side) paper of 7.5-mil (190-μ) thickness at a dry coverage of 0.40 g/ft 2 . 
     Similar receiver sheets in accordance with the invention were prepared by substantially the same procedure and with the same IRL component weight ratio, substituting 9.25-mil (230-μ) thick resin-coated paper, polyethylenimine, Bayhydrol-121, and Bayhydrol-123 as appropriate. 
     Control coatings that included the polyurethane but omitted the polyimine component were also prepared. A coating in which polyethylenimine was present but the polyurethane was omitted was also prepared, but it could not be tested because of its extremely tacky surface. Also included as a control for gloss measurements was a commercial clay-coated paper, Lustro Indigo Gloss Cover, manufactured by Warren Co., whose surface treatment was similar to that described in PCT WO 96/06384. The constitution of the prepared control coatings and receiver sheets of the invention are given in TABLE 1. 
     
                       TABLE 1______________________________________Receiver             Substrate                         Toner  TonerSheet  IRL Components                Thickness                         Transfer                                Adhesion______________________________________Invention1      Bayhydrol-110/EPEI                7.5 mil  Excellent                                Excellent2      Bayhydrol-110/EPEI                9.25 mil Excellent                                Excellent3      Bayhydrol-121/EPEI                7.50 mil Excellent                                Excellent4      Bayhydrol-121/EPEI                9.25 mil Excellent                                Excellent5      Bayhydrol-123/EPEI                7.5 mil  Excellent                                Excellent6      Bayhydrol-123/EPEI                9.25 mil Excellent                                Excellent7      Bayhydrol-123/PEI                7.5 mil  Excellent                                ExcellentControl8      Bayhydrol-110 7.5 mil  Poor   Poor9      Bayhydrol-121 7.5 mil  Poor   Poor10     Bayhydrol-123 7.5 mil  Poor   Poor11     Bayhydrol-110 9.25 mil Poor   Poor12     Bayhydrol-121 9.25 mil Poor   Poor13     Bayhydrol-123 9.25 mil Poor   Poor______________________________________ EPEI  Polyethylenimine (80% ethoxylated) PEI  Polyethylenimine 
    
     Example 2--Evaluation of toner transfer from the blanket to the IRLs 
     The evaluation of toner transfer from the blanket drum of an Indigo E-Print 1000 press to an IRL was based on the detection of toner remaining on the blanket surface following transfer. The blanket surface was wiped with a clean cloth wetted with Isopar™ L hydrocarbon liquid. The cloth was then examined to see whether it had picked up any untransferred toner. The qualitative results, summarized in TABLE 1, show that substantially complete transfer had occurred with receiver sheets 1-7 of the invention, essentially no toner being detectable on the cloth. However, control receiver sheets 8-13, whose image-receiving layers included only the polyurethane component, exhibited poor toner transfer, based on the large amounts of toner observed on the cloth in each instance. 
     As previously mentioned, a control sheet whose image-receiving layer contained only polyethylenimine had an extremely tacky surface, which precluded evaluation of its toner transfer and adhesion properties. 
     Example 3--Evaluation of toner adhesion to the IRLs 
     A test of toner adhesion to the IRL surfaces consisted of applying by hand 3M Scotch™ Magic Tape over the imaged area immediately after printing. The tape was then pulled off the IRL surface to see whether any of the toner image was removed in the process. The qualitative results, also summarized in TABLE 1, show that excellent toner adhesion, i.e., no image removal, was observed with sheets 1-7 of the invention, while control receivers 8-13 showed poor adhesion, i.e., substantial image removal under the test conditions. 
     Example 4--Gloss measurements of the IRLs 
     Gloss measurements were measured with a Gardner Micro TRI gloss meter, model 4520, at a setting of 60 degrees. The measurements were taken in the background, or D-min, areas of the imaged IRLs. The results, summarized in TABLE 2, show that receiver sheets 1-6 of the invention yielded gloss values in the 70s and 80s, while the Lustro clay-coated control receiver had a gloss of only 29. 
     
                       TABLE 2______________________________________                  SubstrateReceiver Sheet    IRL Components                  Thickness                           60 Degree Gloss______________________________________Invention1        Bayhydrol 110/EPEI                   7.5 mil 752        Bayhydrol 110/EPEI                  9.25 mil 763        Bayhydrol 121/EPEI                   7.5 mil 754        Bayhydrol 123/EPEI                  9.25 mil 815        Bayhydrol 123/EPEI                   7.5 mil 806        Bayhydrol 123/EPEI                  9.25 mil 83Control  Lustro clay-coated                   7.5 mil 29______________________________________ EPEI  Polyethylenimine (80% ethoxylated) 
    
     The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.