Abstract:
An inkjet printing method, comprising the steps of: 
     A) providing an ink jet printer that is responsive to digital data signals; 
     B) loading the printer with an ink jet recording element comprising a support having thereon an image-receiving layer comprising at least about 70% by weight of porous polymeric particles in a polymeric binder, the porous polymeric particles having a core/shell structure comprising a porous polymeric core covered with a shell of a water-soluble polymer; 
     C) loading the printer with an ink jet ink composition; and 
     D) printing on the ink jet recording element using the ink jet ink in response to the digital data signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly assigned, co-pending U.S. patent application Serial Numbers: 
     Ser. No. 09/608,527 by Missell et al., Jun. 30, 2000, entitled “Ink Jet Printing Method”; and 
     Ser. No. 09/608,842 by Miisell et al., Jun. 30, 2000, entitled “Ink Jet Printing Method”; 
     Ser. No. 09/607,416 by Missell et al., filed of Jun. 30, 2000, entitled “Ink Jet Printing Method”; 
     Ser. No. 09/608,969 by Kapusniak et al., Jun. 30, 2000, entitled “Ink Jet Recording Element ”; 
     Ser. No. 09/607,417 by Kapusniak et al., Jun. 30, 2000, entitled “Ink Jet Recording Element”; 
     Ser. No. 09/607,419 by Kapusniak et al., filed Jun. 30, 2000, entitled “Ink Jet Recording Element”; and 
     Ser. No. 09/608,466 by Kapusniak et al., filed Jun. 30, 2000, entitled “Ink Jet Recording Element”; 
     the disclosures of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an ink jet printing method. More particularly, this invention relates to an ink jet printing method using a recording element containing porous polyneric particles. 
     BACKGROUND OF THE INVENTION 
     In a typical ink jet recording or printing system, ink droplets are ejected from a nozzle at high speed towards a recording element or medium to produce an image on the medium. The ink droplets, or recording liquid, generally comprise a recording agent, such as a dye or pigment, and a large amount of solvent. The solvent, or carrier liquid, typically is made up of water, an organic material such as a monohydric alcohol, a polyhydric alcohol or mixtures thereof. 
     An ink jet recording element typically comprises a support having on at least one surface thereof an ink-receiving or image-forming layer, and includes those intended for reflection viewing, which have an opaque support, and those intended for viewing by transmitted light, which have a transparent support. 
     While a wide variety of different types of image-recording elements for use with ink jet devices have been proposed heretofore, there are many unsolved problems in the art and many deficiencies in the known products which have limited their commercial usefulness. 
     It is well known that in order to achieve and maintain photographic-quality images on such an image-recording element, an ink jet recording element must: 
     Be readily wetted so there is no puddling, i.e., coalescence of adjacent ink dots, which leads to non-uniform density 
     Exhibit no image bleeding 
     Absorb high concentrations of ink and dry quickly to avoid elements blocking together when stacked against subsequent prints or other surfaces 
     Exhibit no discontinuities or defects due to interactions between the support and/or layer(s), such as cracking, repellencies, comb lines and the like 
     Not allow unabsorbed dyes to aggregate at the free surface causing dye crystallization, which results in bloom or bronzing effects in the imaged areas 
     Have an optimized image fastness to avoid fade from contact with water or radiation by daylight, tungsten light, or fluorescent light 
     An ink jet recording element that simultaneously provides an almost instantaneous ink dry time and good image quality is desirable. However, given the wide range of ink compositions and ink volumes that a recording element needs to accommodate, these requirements of ink jet recording media are difficult to achieve simultaneously. 
     Ink jet recording elements are known that employ porous or non-porous single layer or multilayer coatings that act as suitable image-receiving layers on one or both sides of a porous or non-porous support. Recording elements that use non-porous coatings typically have good image quality but exhibit poor ink dry time. Recording elements that use porous coatings exhibit superior dry times, but typically have poorer image quality and are prone to cracking. U.S. Pat. Nos. 5,027,131 and 5,194,317 relate to an ink jet recording medium containing polymeric particles in an ink recording layer. However, there is no mention of porous core/shell particles. 
     Japanese Kokai Hei 7[1995]-172037, 2[1990]-127447 and 2[1990]-55185 relate to an ink jet recording sheet containing porous resin particles in an ink recording layer. Again, however, there is no mention of porous core/shell particles. 
     It is an object of this invention to provide an ink jet printing method using an ink jet recording element that has a fast ink dry time with minimal puddling. It is another object of this invention to provide an ink jet printing method using an ink jet recording element that is free from cracking. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved in accordance with the invention which comprises an ink jet printing method, comprising the steps of: 
     A) providing an ink jet printer that is responsive to digital data signals; 
     B) loading the printer with an ink jet recording element comprising a support having thereon an image-receiving layer comprising at least about 70% by weight of porous polymeric particles in a polymeric binder, the porous polymeric particles having a core/shell structure comprising a porous polymeric core covered with a shell of a water-soluble polymer; 
     C) loading the printer with an ink jet ink composition; and 
     D) printing on the ink jet recording element using the ink jet ink in response to the digital data signals. 
     Using the ink jet printing method of the invention, an ink jet recording element is obtained which has less cracking than prior art elements while providing good image quality and fast ink dry times with minimal puddling. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The support used in the ink jet recording element employed in the invention may be opaque, translucent, or transparent. There may be used, for example, plain papers, resin-coated papers, various plastics including a polyester resin such as poly(ethylene terephthalate), poly(ethylene naphthalate) and poly(ester diacetate), a polycarbonate resin, a fluorine resin such as poly(tetrafluoro ethylene), metal foil, various glass materials, and the like. In a preferred embodiment, the support is opaque. The thickness of the support employed in the invention can be from about 12 to about 500 μm, preferably from about 75 to about 300 μm. 
     The porous polymeric particles which are used in the invention comprise a porous polymeric core covered with a shell of a water-soluble polymer. The porous polymeric core of the porous polymeric particles are in the form of porous beads, or irregularly shaped particles. 
     Polymers which can be used as a core for the core/shell particles used in the invention comprise, for example, acrylic resins, styrenic resins, or cellulose derivatives, such as cellulose acetate, cellulose acetate butyrate, cellulose propionate, cellulose acetate propionate, and ethyl cellulose; polyvinyl resins such as polyvinyl chloride, copolymers of vinyl chloride and vinyl acetate and polyvinyl butyral, polyvinyl acetal, ethylene-vinyl acetate copolymers, ethylene-vinyl alcohol copolymers, and ethylene-allyl copolymers such as ethylene-allyl alcohol copolymers, ethylene-allyl acetone copolymers, ethylene-allyl benzene copolymers, ethylene-allyl ether copolymers, ethylene acrylic copolymers and polyoxy-methylene; polycondensation polymers, such as, polyesters, including polyethylene terephthalate, polybutylene terephthalate, polyurethanes and polycarbonates. 
     In a preferred embodiment of the invention, the porous polymeric core is made from a styrenic or an acrylic monomer. Any suitable ethylenically unsaturated monomer or mixture of monomers may be used in making such styrenic or acrylic polymer. There may be used, for example, styrenic compounds, such as styrene, vinyl toluene, p-chlorostyrene, vinylbenzylchloride or vinyl naphthalene; or acrylic compounds, such as methyl acrylate, ethyl acrylate, n-butyl acrylate, n-octyl acrylate, 2-chloroethyl acrylate, phenyl acrylate, methyl- α-chloroacrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate; and mixtures thereof. In another preferred embodiment, methyl methacrylate is used. 
     A suitable crosslinking monomer may be used in forming the porous polymeric core in order to modify the porous polymeric particle to produce particularly desired properties. Typical crosslinking monomers are aromatic divinyl compounds such as divinylbenzene, divinylnaphthalene or derivatives thereof; diethylene carboxylate esters and amides such as ethylene glycol dimethacrylate, diethylene glycol diacrylate, and other divinyl compounds such as divinyl sulfide or divinyl sulfone compounds. Divinylbenzene and ethylene glycol dimethacrylate are especially preferred. While the crosslinking monomer may be used in any amount, at least about 27 mole % is preferred. 
     The porous polymeric particles used in this invention have a porous polymeric core that can be prepared, for example, by pulverizing and classification of porous organic compounds, by emulsion, suspension, and dispersion polymerization of organic monomers, by spray drying of a solution containing organic compounds, or by a polymer suspension technique which consists of dissolving an organic material in a water immiscible solvent, dispersing the solution as fine liquid droplets in aqueous solution, and removing the solvent by evaporation or other suitable techniques. The bulk, emulsion, dispersion, and suspension polymerization procedures are well known to those skilled in the polymer art and are taught in such textbooks as G. Odian in “Principles of Polymerization”, 2nd Ed. Wiley (1981), and W. P. Sorenson and T. W. Campbell in “Preparation Method of Polymer Chemistry”, 2nd Ed, Wiley (1968). 
     Techniques to synthesize porous polymer particles are taught, for example, in U.S. Pat. Nos. 5,840,293; 5,993,805; 5,403,870; and 5,599,889, and Japanese Kokai Hei 5[1993]-222108, the disclosures of which are hereby incorporated by reference. For example, an inert fluid or porogen may be mixed with the monomers used in making the core. After polymerization is complete, the resulting polymeric particles are, at this point, substantially porous because the polymer formed around the porogen thereby forming the pore network. This technique is described more fully in U.S. Pat. No. 5,840,293 referred to above. 
     A preferred method of preparing the porous polymeric particles having a core/shell structure used in this invention includes forming a suspension or dispersion of ethylenically unsaturated monomer droplets containing a crosslinking monomer and a porogen in an aqueous medium, where the aqueous medium contains an amount of the desired water-soluble polymer, polymerizing the monomer to form solid, porous polymer particles having a core/shell structure, and optionally removing the porogen by vacuum stripping. The water-soluble polymer can also be added to the aqueous media subsequent to the formation of the droplets and before the commencement of the polymerization reaction. 
     The shell which covers the porous polymeric core described above can be formed using a variety of techniques known in the art. The water-soluble polymer shell of the core/shell particles generally cannot be formed on the porous polymeric core merely by contacting a pre-formed core with the water-soluble polymer. Instead, conditions need to be established where the water-soluble polymer chemically reacts with the core surface or is strongly adsorbed thereto. Such conditions are known to one skilled in the art and can be achieved using a chemically reactive core surface and binder polymer. The porous polymeric particles may also comprise a core which is prepared in the presence of the water-soluble polymer so that the shell is formed during core formation rather than after core formation. Examples of techniques which can be used in making core/shell particles may be found, for example, in U.S. Pat. Nos. 5,872,189; 5,185,387 and 5,990,202, the disclosures of which are hereby incorporated by reference. 
     The water-soluble polymer used for the shell of the polymeric particles used in the invention can be any naturally occurring or synthetic polymer which is soluble in water. For example, the water-soluble polymer may be a poly(vinyl alcohol), a gelatin, a cellulose ether, polyvinylpyrrolidone, poly(ethylene oxide), etc. In a preferred embodiment, the water-soluble polymer is a poly(vinyl alcohol) or a gelatin. In general, the shell material comprises up to about 5% by weight of the core/shell particle. 
     In addition to the water-soluble polymer shell, the porous polymeric core surface may be covered with a layer of colloidal inorganic particles as described in U.S. Pat. Nos. 5,288,598; 5,378,577; 5,563,226 and 5,750,378, the disclosures of which are incorporated herein by reference. The porous polymeric core may also be covered with a layer of colloidal polymer latex particles as described in U.S. Pat. No. 5,279,934, the disclosure of which is incorporated herein by reference. 
     The porous polymeric particles used in this invention will usually have a median diameter of less than about 10.0 μm, preferably less than about 1.0 μm. 
     As noted above, the polymeric particles used in the invention are porous. By porous is meant a particle which either has voids or is permeable to liquids. These particles can have either a smooth or a rough surface. 
     The polymeric binder used in the invention may comprise the same materials listed above for the shell materials. For example, the binder may be a poly(vinyl alcohol), a gelatin, a cellulose ether, polyvinylpyrrolidone, poly(ethylene oxide), etc. The image-receiving layer may also contain additives such as pH-modifiers like nitric acid, cross-linkers, rheology modifiers, surfactants, UV-absorbers, biocides, lubricants, water-dispersible latexes, mordants, dyes, optical brighteners etc. 
     The image-receiving layer may be applied to one or both substrate surfaces through conventional pre-metered or post-metered coating methods such as blade, air knife, rod, roll, slot die, curtain, slide, etc. The choice of coating process would be determined from the economics of the operation and in turn, would determine the formulation specifications such as coating solids, coating viscosity, and coating speed. 
     The image-receiving layer thickness may range from about 5 to about 100 μm, preferably from about 10 to about 50 μm. The coating thickness required is determined through the need for the coating to act as a sump for absorption of ink solvent. 
     Ink jet inks used to image the recording elements employed in the present invention are well-known in the art. The ink compositions used in ink jet printing typically are liquid compositions comprising a solvent or carrier liquid, dyes or pigments, humectants, organic solvents, detergents, thickeners, preservatives, and the like. The solvent or carrier liquid can be solely water or can be water mixed with other water-miscible solvents such as polyhydric alcohols. Inks in which organic materials such as polyhydric alcohols are the predominant carrier or solvent liquid may also be used. Particularly useful are mixed solvents of water and polyhydric alcohols. The dyes used in such compositions are typically water-soluble direct or acid type dyes. Such liquid compositions have been described extensively in the prior art including, for example, U.S. Pat. Nos. 4,381,946; 4,239,543 and 4,781,758, the disclosures of which are hereby incorporated by reference. 
    
    
     The following example further illustrates the invention. 
     EXAMPLE 
     Preparation C1—Synthesis of Control Polymeric Particles 
     To a beaker were added the following ingredients: 1125 g methyl methacrylate, 1125 g ethylene glycol dimethacrylate, 750 g toluene as a porogen, 81.0 g dioctyl ester of sodium sulfosuccinic acid, Aerosol OT-100®, 56.4 g hexadecane and 45.0 g 2,2′-azobis(2,4-dimethylvaleronitrile), Vazo 52® (DuPont Corp.). The ingredients were stirred until all the solids were dissolved. 
     To this solution were added 9460 g distilled water. The mixture was then stirred with a marine prop type agitator for 20 minutes. The mixture was passed through a Crepaco® homogenizer operated at 350 kg/cm 2 . Two 1.6 kg aliquots of the resulting monomer droplet dispersion were taken out for further use as described below. The balance of the mixture was then added to a flask and placed into a constant temperature bath at 52° C. while stirring at 75 rev./min. for 16 hours, then at 70° C. for 2 hours to polymerize the monomer droplets into porous polymeric particles. 
     The product was filtered through a coarse filter to remove coagulum. Next, toluene and some water were distilled off under vacuum at 70° C. to give 28.6% solids. The porous polymeric particles were measured by a particle size analyzer, Horiba LA-920®, and found to be 0.160 μm in median diameter. A dried portion of the dispersion, analyzed by BET multipoint, had a total surface area of 71.10 m 2 /g, and a total pore volume of 0.274 mL/g. 
     Preparation 1—Synthesis of Core/Shell Polymeric Particles (Invention) 
     One of the 1.6 kg aliquots of monomer droplet dispersion from above was placed into a 3 liter flask and 68.6 g of swollen gelatin (35% dry weight gelatin and 65% water), was added. The flask was then placed into a constant temperature bath at 52° C.. and stirred at 75 rev./min. for 16 hours, then at 70° C. for 2 hours to polymerize the monomer droplets into a porous polymeric core, which is surrounded by a shell of gelatin. 
     The product was filtered through a coarse filter to remove coagulum and then toluene and some water were distilled off under vacuum at 70° C. to give 28.6% solids. The core/shell polymeric particles were measured by a article size analyzer, Horiba LA-920®, and found to be 0.170 μm in median diameter. A dried portion of the dispersion, analyzed by BET multipoint, had a total surface area of 36.06 m 2 /g, and a total pore volume of 0.204 mL/g. 
     Preparation 2—Synthesis of Core/Shell Polymeric Particles (Invention) 
     The other 1.6 kg aliquot of monomer droplet dispersion from above was placed into a 3 liter flask and 190.0 g of 10% poly(vinyl alcohol) solution, made from Gohsenol GH 23® (Gohsen Nippon of Japan) was added. The flask was then placed into a constant temperature bath at 52° C. and stirred at 75 rev./min. for 16 hours, then at 70° C. for 2 hours to polymerize the monomer droplets into a porous polymeric core, which is surrounded by a shell of gelatin. 
     The product was filtered through a coarse filter to remove coagulum and then toluene and some water were distilled off under vacuum at 70° C. to give 23.7% solids The core/shell polymeric particles were measured by a particle size analyzer, Horiba LA-920®, and found to be 0.170 μm in median diameter. A dried portion of the dispersion, analyzed by BET multipoint, had a total surface area of 54.321 m 2 /g, and a total pore volume of 0.266 mL/g. 
     Coating of Elements 
     Control Element C-1 
     A coating solution was prepared by mixing together the control polymeric particles of Preparation C-1 with a binder of a 10% gelatin solution, made from pigskin gelatin (SKW Corp)., The resulting coating solution was 15% solids and 85% water. The weight fractions of the total solids in the solution were 0.75 parts from the solids contained in Preparation C-1, and 0.25 parts from the solids contained in the 10% gelatin solution. The solution was stirred at 40° C. for approximately 30 minutes before coating. 
     The solution was then coated on corona discharge-treated, photographic grade, polyethylene-coated paper using a wound wire metering rod and oven dried for 20 minutes at 60° C. This element was coated to a dry thickness of about 25 μm. 
     Control Element C-2 
     This element was prepared the same as Control Element C-1 except that in the coating solution, the ratios of the components were changed so that the weight fractions of the total solids in the solution were 0.80 parts from the solids contained in Preparation C-1, and 0.20 parts from the solids contained in the 10% gelatin solution. The element was coated to a dry thickness of about 25 μm. 
     Control Element C-3 
     This element was prepared the same as Control Element C-1 except that in the coating solution, the ratios of the components were changed so that the weight fractions of the total solids in the solution were 0.90 parts from the solids contained in Preparation C-1, and 0.10 parts from the solids contained in the 10% gelatin solution. 
     Element 1 (Invention) 
     This element was prepared the same as Control Element C-1 except that the coating solution was made with Preparation 1. 
     Element 2 (Invention) 
     This element was prepared the same as Control Element C-2 except that the coating solution was made with Preparation 1. 
     Element 3 (Invention) 
     This element was prepared the same as Control Element C-3 except that the coating solution was made with Preparation 1. 
     Element 4 (Invention) 
     This element was prepared the same as Control Element C-1 except that the coating solution was made with Preparation 2. 
     Element 5 (Invention) 
     This element was prepared the same as Control Element C-2 except that the coating solution was made with Preparation 2. 
     Element 6 (Invention) 
     This element was prepared the same as Control Element C-3 except that the coating solution was made with Preparation 2. 
     Cracking Evaluation 
     For each coating the coated layer was evaluated for cracking and or flaking of the layer. The following scale was used to rate each coating, with a rating of 1, 2, and 3 being acceptable: 
     
       
         
               
               
             
           
               
                   
               
               
                 RATING 
                 CRACKING/FLAKING DEFECTS 
               
               
                   
               
             
             
               
                 1 
                 No cracking or flaking 
               
               
                 2 
                 Cracking visible under 10X magnification 
               
               
                 3 
                 Cracking and flaking slightly visible to naked eye 
               
               
                 4 
                 Severe cracking with cracks 2 mm or less, flaking causes 
               
               
                   
                 coating to rub off 
               
               
                 5 
                 Very severe cracking with cracks 2 mm or greater and flaking 
               
               
                   
                 causes coating to fall off without rubbing 
               
               
                   
               
             
          
         
       
     
     The following results were obtained: 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                 Element 
                 Cracking Rating 
               
               
                   
                   
               
             
             
               
                   
                 Control C-1 
                 4 
               
               
                   
                 Control C-1 
                 5 
               
               
                   
                 Control C-1 
                 5 
               
               
                   
                 1 
                 3 
               
               
                   
                 2 
                 2 
               
               
                   
                 3 
                 2 
               
               
                   
                 4 
                 3 
               
               
                   
                 5 
                 2 
               
               
                   
                 6 
                 2 
               
               
                   
                   
               
             
          
         
       
     
     The above results show that Controls C-1, C-2, and C-3 had unacceptable cracking, while Elements 1 to 6 employed in the invention, having a porous polymeric core covered with a shell of gelatin or poly(vinyl alcohol), all had acceptable cracking. 
     This invention has been described with particular reference to preferred embodiments thereof but it will be understood that modifications can be made within the spirit and scope of the invention.