Patent Publication Number: US-6209998-B1

Title: Treatment of image printed with pigmented ink to enhance gloss

Description:
TECHNICAL FIELD 
     The present invention relates generally to printing glossy images, and more particularly to a process for increasing the gloss of an image printed with a liquid ink. 
     BACKGROUND ART 
     Photographic and other high definition images are conventionally reproduced with a uniform, high gloss surface that enhances the clarity and sharpness of the image. Pigmented inks are desirable for inkjet printers because they can be used on a variety of print media to produce saturated, opaque colors, and are typically more permanent and have greater resistance to ultra-violet light than dye-based inks. However, especially when applied to a print medium with a smooth (glossy) surface, pigmented inks tend to produce an image whose inked portions have a relatively low gloss in comparison with either the surrounding print medium or the corresponding portions of an image printed with dye-based inks. This low gloss is believed to be the result of the ink being deposited on the smooth surface of the glossy paper as loosely packed clumps of discrete pigmented particles held together by a polymeric binder, thereby resulting in a rough (low gloss) surface texture in the inked portions the image. This clumping effect is exacerbated in certain types of aqueous ink systems in which ionic forces normally disperse the individual particles of pigment in a liquid suspension medium, but cause certain combinations of pigmented particles to combine into even larger precipitates. 
     It is known to apply heat to the liquid ink before and during the printing process to thereby speed up the rate at which the suspension medium can be evaporated or absorbed and the ink thereby becomes sufficiently dry that the printed image will not be subject to smearing when it comes into contact with the printer mechanism or with previously printed media. However, a rapid loss of solvent or other suspension medium encourages the individual particles of pigment to flocculate into distinct clumps rather than remaining dispersed within a more or less homogenous layer. 
     Conventional print-zone heating not only exacerbates the undesirable clumping of the pigmented particles, but can also cause glossy paper to form undesirable wrinkles at all but the lowest heat settings, particularly when the heat source is a heating element below the print medium. 
    
    
     DISCLOSURE OF INVENTION 
     A high gloss pigmented surface is formed by first applying a liquid pigmented ink an aqueous or other a liquid suspension medium containing individual particles of pigment and an appropriate binder onto a smooth surface of a sheet of print medium to thereby form a printed image, then drying (fixing) the printed image by evaporating or absorbing the suspension medium until the pigment has adhered to the smooth surface of the print medium as a loss ieted lyer having a rough top surface comprising loosely packed clumps of discrete pigmented particles held together by a binder, and then heating the rough top surface of the pigmented layer until at least some of the particles at respective outer surfaces of the clumps of pigment at the top of the pigmented layer have softened and flowed together to form a high gloss pigmented layer having a smooth upper surface. 
     Preferably, a relatively low level of heat is applied for a relatively long time to the liquid ink and the print medium to thereby speed up the evaporating or absorption of the suspension medium, and then a relatively high level of heat is applied for a relative short period of time to heat the surface of the already fixed image without burning or distorting the underlying print media, thereby providing the image with the desired high gloss finish. 
     In one practical embodiment, the ink is applied by an inkjet printing mechanism which applies and fixes the printed image as a sequence of individual horizontal image swaths, using unheated air to dry and fix each individual horizontal swath of the image as the ink is being applied, and the high heat level is produced by a jet of heated air directed against the upper surface of the print medium as a separate step after the entire image has been printed and fixed. 
     The invention may also be applicable to pigmented inks without binders, and to other types of printed images having an upper surface that smooths out when heated. 
     PREFERRED MODE FOR PRACTICING THE INVENTION 
     In an experiment intended to simulate the invention and to provide a quantitative measurement of its efficacy, a number of color squares each approximately 3×3 cm in area were printed with a conventional ink-jet printer on a letter size sheet of commercially available (HP brand) glossy paper using a 4-pass unidirectional print mode (in which each forward traverse of the carriage causes ink to be applied to every other pixel on every other line). The tests were made with two different proprietary formulations of ink in which the cyan component exhibited pronounced surface roughness when used in conventional print processes, which was even more pronounced in secondary colors in which the cyan was mixed with other primary colors. One formulation contained a linear binder and the other contained a latex binder. The ink was applied with 100% coverage (one drop of ink per pixel) for primary colors (eg, cyan) and 200% coverage (one drop of each of two colors of ink) for secondary colors (eg, blue=cyan+magenta and green=cyan+yellow). 
     The printed image was air dried under standard laboratory conditions (approx 23° C. and 55% humidity); no print-zone or preheater was used. A conventional 1000 watt hair dryer was used to provide a jet of heated air that was applied to the surface of the printed image to simulate a post-heater stage for heating the surface after the entire image was printed and fixed. Heating was done for 10-15 seconds. 
     The following gloss readings were taken at a 60° angle, using a BYK Gardner Micro-TRI-gloss meter. The indicated measurements are based on a relative scale in which 0 corresponds a highly diffusive surface that completely scatters a collimated beam of light, and 100 corresponds to a highly reflective surface in which a collimated beam of light is reflected completely as specular light. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                 Ink with 
                 Ink with 
                 Ink with 
                 Ink with 
               
               
                   
                 Linear 
                 Linear 
                 Latex 
                 Latex 
               
               
                   
                 Binder, without 
                 Binder, with 
                 Binder, without 
                 Binder, with 
               
               
                 Gloss 
                 Post-Heat 
                 Post-Heat 
                 Post-Heat 
                 Post-Heat 
               
               
                   
               
             
            
               
                 Cyan 
                 58 
                 70 
                 67 
                 74 
               
               
                 Green 
                 36 
                 42 
                 42 
                 45 
               
               
                 Blue 
                 36 
                 40 
                 43 
                 47 
               
               
                   
               
            
           
         
       
     
     From the above test measurements, it can be seen that post-heating increases the apparent gloss of both primary and secondary colors by at least about 10%, for both formulations of ink, but that the improved gloss is especially noticeable in the inks with the linear binder. 
     The brief heating of the image surface caused by exposure to the hot (50° C.-150° C.) jet of air is believed to temporarily soften or reduce the viscosity (or modulus) of the upper portion of the pigmented layer, thereby allowing the intermolecular forces (surface tension) at the surface to produce a smooth, high gloss surface. 
     Particularly notable is that the present invention provides the means to increase gloss of printed images even without changes in ink formulation. Moreover, since the heat is preferably applied from above and only the upper portion of the ink layer needs to be softened, the underlying print medium is not adversely affected. 
     In a practical embodiment, it is contemplated that the ink will be applied by the inkjet printing mechanism as a sequence of individual horizontal image swaths, using only unheated air to dry and fix each individual horizontal swath of the image as the ink is being applied, and the high heat level will be produced by a jet of heated air directed against the upper surface of the print medium in the output tray after the entire image has been printed and fixed. Alternatively, the hot air could be applied to the upper surface while the print medium was still inside the printer and/or the upper surface could be softened by radiant heat rather than by convection. It should be understood that the foregoing embodiments are merely exemplary, and that the principles of the present invention are believed to be applicable to other ink systems, such as pigmented inks without binders and dye-based inks on media having a water soluble top coat. In either case, the application of heat to the top surface of the image after the image has been fixed is expected to result in a smoother and therefore more glossy surface.