Patent Publication Number: US-2005136220-A1

Title: Methods of producing recording sheets having reduced curl

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to recording sheets. More specifically, the invention relates to recording sheets for use in image-forming apparatuses and methods of producing the recording sheets.  
     BACKGROUND OF THE INVENTION  
      The use of digital image-forming apparatuses such as, for example, thermal ink-jet printers, large-format plotters, piezo-electric printers, large form plotters, laser printers, silver halide grade photo imaging apparatuses, and others has grown in recent years. The growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today&#39;s image-forming apparatuses offer acceptable print quality for many commercial, business and household applications at costs lower than those offered in the past.  
      Media products for receiving printed images are used in conjunction with these image-forming apparatuses. Known imaging and printing media often include a base substrate, i.e., a type of paper, coated with a single or multi-layer functional polymer coating. The polymeric coating may enhance the deposition of the ink onto the media, prevent smearing of the image formed on the media or protect the media from abrasion, spills, or other image-degradation effects. However, if the media includes two or more individual polymeric layers and the polymeric layers possess different thermal, hygroscopic or other environmental properties, any environmental change may generate a resultant force between the polymeric layers and, thus, cause the media to curl.  
      The intensity or the radius of the curl depends on the modulus of the individual coating layers, the differences in dimensional changes of the polymeric layers and the stiffness of the materials used in the substrate. The media typically curls more in a weaker direction. Curl is an important quality criteria for printing and imaging media since curling of the media may cause feeding failure of the media into the image-forming apparatus, crushing of print-heads used by the image-forming apparatus, or mis-registering of the media into the image-forming apparatus.  
      Conventional print media has a lower modulus in the cross machine direction (CD) than the machine direction (MD) due to the nature of cellulosic fibrous composites, polymer films, and the manufacturing conditions used to manufacture the print media. The ratio of MD to CD stiffness may be two or higher. Accordingly, the axis of curl that typically affects the print media occurs along the CD direction (i.e., the print media curls in the CD direction). Known print media are characterized by MD and CD stiffness parameters that are coupled together, wherein the enhancement of CD stiffness also enhances and, thus, is limited by the MD stiffness. If the MD stiffness becomes too high, printer feeding failure may result.  
     BRIEF SUMMARY OF THE INVENTION  
      In one embodiment, a method for producing a print medium is disclosed. The method includes extruding a polymer to form a polymeric layer and attaching the polymeric layer to a first surface of a substrate. A three-dimensional pattern is formed on the polymeric layer.  
      In another embodiment, a recording sheet having reduced curl is described. The recording sheet includes a substrate having a first surface and an opposing, second surface. A means for reducing curl of the recording is attached to a first surface of the substrate.  
      A method for reducing curl of a recording sheet is further disclosed. The method includes providing a recording sheet having a substrate and at least one embossed, polymeric layer attached to at least one surface of the substrate. The method further includes balancing the stiffness of the machine direction and the cross machine direction to reduce curl of the printing system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of the invention may be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which:  
       FIG. 1  illustrates a top view of one embodiment of a recording sheet of the present invention;  
       FIG. 2  is a cross-section of the recording sheet of  FIG. 1 ;  
       FIG. 3  is a schematic diagram of one embodiment of an extrusion system of the present invention;  
       FIG. 4  illustrates a schematic diagram of one embodiment of a cast film extrusion system of the present invention;  
       FIG. 5  depicts one embodiment of an extrusion coating system of the present invention;  
       FIG. 6  is a schematic diagram of one embodiment of an extrusion lamination system of the present invention;  
       FIG. 7  is a schematic diagram of another embodiment of an extrusion lamination system of the present invention  
       FIG. 8  is a perspective view of another embodiment of a recording sheet of the present invention;  
       FIG. 9  is a cross-section of another embodiment of a recording sheet of the present invention; and  
       FIGS. 10-14  illustrate computer models of various embodiments of recording sheets of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A recording sheet, or print medium, and method of producing recording sheets suitable for use in an image-forming apparatus such as, for example, an inkjet-printer, a large-format plotter, a piezo-electric desktop printer, a large-form plotter, a laser printer, a silver halide grade photo imaging apparatus, or any other image-forming apparatus are described.  
      Referring to  FIG. 1 , there is shown a top view of one embodiment of a recording sheet of the present invention generally at  10 . A cross machine direction (CD) of the sheet  10  is illustrated with arrow  12  and a machine direction (MD) is illustrated with arrow  14 . It will be apparent by those of ordinary skill in the art, that although  FIG. 1  illustrated the CD direction with arrow  12  and the MD direction with arrow  14 , the CD direction and the MD direction may be interchangeable depending on the method of converting, i.e., the method of separating individual recording sheets  10  from a larger roll of recording sheet material.  
       FIG. 2  illustrates a cross-section of the recording sheet  10  of  FIG. 1 . The recording sheet  10  includes a substrate  16  and a means for reducing curl of the recording sheet, such as, for example, an embossed layer  18  having a three-dimensional pattern formed thereon, wherein the embossed layer  18  enables the recording sheet  10  to resist curling. By producing the embossed layer  18  and balancing axial stiffness of the recording sheet  10 , MD and CD stiffness may be independently controlled and decoupled, thus reducing curl of the recording sheet  10 . The embossed layer  18  enables CD stiffness (which is typically the curl-generating direction) to be enhanced without influencing the MD stiffness. Accordingly, the CD curl of the recording sheet  10  may be reduced without effectuating printer feeding failure.  
      The recording sheet  10  also includes at least one ink absorbent layer  20  for receiving ink. In another embodiment, the recording sheet  10  may have an ink absorbent layer on each side, i.e., a double sided recording sheet. In another embodiment, the embossing may be done in a direction on the recording sheet  12  to enable MD stiffness to be enhanced, while the CD stiffness is not influenced.  
      The substrate  16  of the recording sheet  10  may be porous throughout, it may be nonporous throughout, or it may comprise both porous and nonporous regions. Further, the substrate  16  may be substantially transparent, substantially opaque or the substrate  16  may be of intermediate transparency.  
      Examples of porous substrate materials that may be used to form the substrate  16  include, without limitation, paper, paperboard, synthetic fiber composite, wood, cloth, nonwoven fabric, felt, unglazed ceramic material, polymeric membranes, porous foam, microporous foam or any combinations thereof. The porous substrate material may be coated or laminated to render one or more surfaces substantially nonporous, thus, providing substrates having at least one substantially nonporous surface.  
      Examples of substantially nonporous substrates that may be used to form the substrate  16  include, but are not limited to, sheets or films of organic polymer such as, for example, poly(ethylene terephthalate), polyethylene, polypropylene, cellulose acetate, poly(vinyl chloride), thermoset organic polyers, copolymers such as saran, or any combinations thereof. The recording sheet  10  may be metallized or unmetallized. Examples of metallized sheets include metal substrates, including, without limitation, aluminum foil and copper foil. Other examples of nonporous substrates include porous or microporous foams comprising a thermoplastic organic polymer, wherein the foam has been compressed to such an extent that the resulting deformed material is substantially nonporous. Yet another example is a substrate including glass.  
      The ink absorbent layer  20  of the recording sheet  10  may also include or be coated with materials that increase the adhesion of inkjet dyes or pigments to the recording sheet  10 , optimize image quality, increase resistance to scratches, increase resistance to air fading, increase resistance to moisture, increase resistance to UV light, and/or provide a matte finish, a textured finish, or a glossy finish. Such materials include, but are not limited to, gelatin, alumina, silica, calcium carbonate, clay, polyvinyl pyrrolidone, cellulosic polymers, methylhydroxyl propyl cellulose, polyvinyl alcohol, polyesters, polystyrenes, polystyrene-acrylic, polymethyl methacrylate, polyvinyl acetate, polyolefins, poly(vinylethylene-co-acetate), polyethylene-co-acrylics, amorphous polypropylene and copolymers, graft copolymers of polypropylene, or any combinations thereof.  
      In the embodiment of  FIG. 2 , the embossed layer  18  has the pattern formed on an upper surface  22  of the recording sheet  10 . In one embodiment, the pattern includes upper areas  24  and lower areas  26 , i.e., peaks and valleys, wherein the upper areas  24  and the lower areas  26  are substantially planar. The upper areas  24  are substantially parallel to the lower areas  26  and extend across a substantial width in the CD direction  12  of the recording sheet  10 . Although the embodiment of  FIG. 2  has been described with an exemplary pattern, the exemplary pattern is illustrative and it will be apparent by those of ordinary skill in the art that the pattern may comprise any pattern that decouples the CD  12  stiffness from the MD  14  stiffness.  
      The embossed layer  18  may be produced using an extrusion process, a hot melt process, or any combination thereof. Referring now to  FIG. 3 , there is illustrated one embodiment of an extrusion system, such as a sheet or flat film extrusion line that may be used to form the embossed layer  18  of the recording sheet  10  of  FIG. 2 , shown generally at  30 . The extrusion system  30  includes a hopper  32  for holding a material used to form the embossed layer  18 . In one embodiment, the material used to form the embossed layer  18  comprises polyolefin. In other embodiments, the material used to form the embossed layer  18  may comprise any other polymer including, without limitation, cellulose based polymers and polyethylene, polystyrene, polypropylene, ethylene/vinyl acetate copolymer, ethylene acrylic acid, ethylene methyl acrylic acid, acid copolymer, ethylene vinyl alcohol copolymer, polyester, polyamides, polycarbonate, polyurethane, any extrusible materials. The extrusion system  30  further includes a reducer  34  that feeds the polyolefin into a feed throat  36 . The polyolefin in transported to a barrel  38  wherein the polyolefin is conditioned to appropriate conditions for extruding the polyolefin through a die  40 . The die  40  extrudes the polyolefin to form the extruded polyolefin. The extrusion system  30  is powered by a motor  42 .  
      The extruded polyolefin is transferred from the die  40  to a three roll stack  44  that transports the extruded polyolefin to a cooling section  46  such that the extruded polyolefin may be cooled and the appropriate pattern set into the extruded polyolefin. The pattern may be imparted on the extruded polyolefin with the die  40  or with one of the three rollers of the three roll stack  44 . The cooled, extruded polyolefin passes through nip rolls  48  and is wound on a winder  50 , wherein the cooled, extruded polyolefin may be attached to a substrate, such as by laminating, to form the embossed layer  18 .  
      Referring now to  FIG. 4 , there is illustrated another embodiment of a system  60 , such as a cast film extrusion line, that may be used to form the embossed layer  18 . The system  60  includes a hopper  62  for holding the material used to form the embossed layer  18 . In one embodiment, the material comprises polyolefin, but in other embodiments may comprise any other known polymer including, but not limited to, cellulose based polymers and polyethylene. The system  60  further includes an extruder  64  for preparing the polyolefin for passage through a film die  66 . Once the extruded polyolefin exits the film die  66 , it passes an air knife  68  that directs the extruded polyolefin around a chill roll  70 . An outer surface  72  of the chill roll  70  is embossed with a pattern and used to form a pattern on one side of the extruded polyolefin. The patterned, extruded polyolefin passes over a stripping roll  74  and through an edge trim slitter  76  where the patterned, extruded polyolefin is sized and shaped.  
      A thickness of the patterned, extruded polyolefin is monitored with a thickness scanning system  78 . The patterned, extruded polyolefin passes through a surface treatment element  80  that treats the patterned surface of the extruded polyolefin for subsequent attachment to a substrate. The patterned, extruded polyolefin is then wound with a winder  82  for subsequent incorporation into a recording sheet.  
      In other embodiments, the pattern may be formed on the embossed layer  18  with any mechanical, chemical or optical patterning process known in the art, including, without limitation, etching and laser ablation. The mechanical, chemical or optical patterning methods may be used to form the pattern in the embossed layer  18  after an extrusion coating process or a film extrusion/lamination process.  
      The recording sheet  10  may be produced with an extrusion coating process, an extrusion/lamination process, or a combination thereof. In one embodiment, the recording sheet  10  is produced with the extrusion coating process where a polymer layer is attached to a substrate to form the recording sheet. Referring now to  FIG. 5 , there is illustrated one embodiment of an extrusion coating system  90  that includes a hopper  92  for holding material used to form the embossed layer  18  of the recording sheet  10 . In one embodiment, the material is polyolefin, but may comprise other polymeric substances including, without limitation, polyethylene and cellulose based polymers. The extrusion coating system  90  also includes an extruder  94  for preparing the polyolefin material for passage through a film die  96 .  
      After the extruded polyolefin passes through the film die  96 , the extruded polyolefin is laminated to an uncoated substrate  100  with a pressure roll  102 . The uncoated substrate  100  may comprise any porous or non-porous substrate as previously described herein, such as, for example, a cellulose fiber network composite. The pressure roll  102  applies pressure to the extruded polyolefin and the uncoated substrate  100  between the pressure roll  102  and a chill roll  104 . The chill roll  104  has a pattern embossed on an outer surface  106  of the chill roll  104  such that a pattern may be imparted to the extruded polyolefin as the extruded polyolefin is laminated to the uncoated substrate  100 . A coated substrate  108  comprising the substrate and the extruded polyolefin is released from the chill roll  104  and is wound on a wind-up roll  110  for subsequent processing. The coated substrate  108  may be further processed, dimensioned and cut into sheets, such as the recording sheet  10  of  FIG. 1 .  
       FIG. 6  illustrates one embodiment of an extrusion lamination system  120  that includes an extruder  122  for preparing a polymeric material, such as polyolefin, for extrusion through a die  124 . As extruded polyolefin  126  exits the die  124 , the extruded polyolefin  126  is laminated to a first layer  128  and a second layer  130 . The first layer  128  is unwound from a first roll  132  and the second layer  130  is unwound from a second roll  134 . An upper nip roll  136  and a lower nip roll  138  provide pressure for the lamination process. The first layer  128  and the second layer  130  may be any type of conventional layer used to form recording sheets, including, but not limited to, imaging layers, ink receiving layers, polymeric layers, substrates, anti-curl layers, stacking layers, or any combinations thereof.  
       FIG. 7  illustrates another embodiment of an extrusion lamination system  120  that is substantially similar to the extrusion lamination system of  FIG. 6 . In the extrusion system  120  of  FIG. 7 , the extruded polyolefin  126  may be pre-formed and supplied on a supply roll  127 , wherein the extruded polyolefin  126  is directed between the upper nip roll  136  and the lower nip roll  138  by a positioning or tension roller  125 .  
      Referring now to  FIG. 8 , a perspective view of a media used to model different embossing patterns and showing x, y and z coordinates is illustrated at  140 . Symmetry boundary conditions in the y direction are indicated at  142  and symmetry boundary conditions in the x direction are indicated at  144 . The y-component variations and the x-components are fixed.  
       FIG. 9  illustrates a cross-section of another embodiment of a recording sheet  150  produced using the methods of the present invention. The recording sheet  150  includes a substrate layer  152 , a non-embossed polymer layer  154 , an embossed polymer layer  156  and an image receiving layer  158 . The substrate layer  152  can be porous throughout, nonporous throughout or any combination thereof as previously described herein with regard to the recording sheet  10  of  FIG. 2 . The non-embossed polymer layer  154  and the embossed polymer layer  156  can be polyolefin, but may alternatively comprise any other known polymers including, for example, cellulose based polymers and polyethylene. The image receiving layer  158  can be gelatin, but may alternatively comprise any other ink receiving material as described herein with regard to the recording sheet  10  of  FIG. 2 .  
      A finite element based computational tool is used to simulate the curl behavior of recording sheets produced using methods of the present invention. By varying the size and spacing of a pattern of the embossed polymer layer  156 , dimensions of the pattern formed on the embossed polymer layer  156  are optimized in order to reduce curl of the recording sheet  150 . The finite element based computational tool is a conventional method to analyze static and dynamic structural analysis of the recording sheet  150 .  
      The properties of the polymer layers  154  and  156  in combination with the substrate layer  152  used in the computer modeling are listed in Table 1. The properties of the ink receiving layer  158  used in the computer modeling are depicted in Table 2.  
                                                       TABLE 1                                                           Shear   Shear   Shear           Young&#39;s   Young&#39;s   Young&#39;s   Poisson   Poisson   Poisson   Modulus,   Modulus,   Modulus,           Modulus, x   Modulus, y   Modulus, x   Ratio, υ xy     Ratio, υ yz     Ratio, υ zx     G xy     G yz     G zx                                                                              Polymer   14000000000   1000000000   7000000000   0.3   0.3   0.3   10000000   10000000   1000000       Layer/   Pa   Pa   Pa               Pa   Pa   Pa       Substrate       Composite                  
 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                 Normalized 
                   
                   
                 Thermal 
               
               
                   
                 Environmental 
                   
                 Poisson 
                 Expansion 
               
               
                   
                 Condition 
                 Young&#39;s Modulus 
                 Ratio 
                 Coefficient 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 0 
                 5500000000 Pa 
                 0.3 
                 0.0015 
               
               
                   
                 0.25 
                 4800000000 Pa 
                 0.3 
                 0.0015 
               
               
                   
                 0.5 
                 4500000000 Pa 
                 0.3 
                 0.0015 
               
               
                   
                 0.75 
                 3500000000 Pa 
                 0.3 
                 0.0015 
               
               
                   
                 1 
                 1400000000 Pa 
                 0.3 
                 0.0015 
               
               
                   
                   
               
            
           
         
       
     
       FIGS. 10-14  illustrate modeled displacements in the z direction of various recording sheets using the finite element based computational tool.  FIG. 10  represents a model of a control recording sheet that does not include an embossed layer.  FIGS. 11-14  represent models of recording sheets that each includes a means for reducing curl such as, for example, an embossed, patterned layer having a pattern, wherein the shape and spacing of the pattern is varied as illustrated along a lower edge  160  of the computer model. The MD direction is illustrated by arrow  162  and the CD direction is illustrated by arrow  164 .  
      Each of  FIGS. 10-14  depicts a recording sheet  170  at time  0  and a stressed recording sheet  172  at time  1 . 000  that is subjected to varying environmental conditions, including, but not limited to, temperature changes or humidity changes. The stressed recording sheet  172  is illustrated with varying shades of gray, which represent varying distances (indicated with scale  166 ) that areas of the stressed recording sheet  172  move in relation to the recording sheet  170  being subjected to varying environmental conditions. The changing environmental conditions cause resultant forces of various layers of the stressed recording sheet  172  to change and, thus, to “curl.” A maximum distance that the stressed recording sheet  172  moves in relation to the recording sheet  170  is represented at  174  and a minimum distance is represented at  176 .  
      Simple experimentation will enable one of ordinary skill in the art to determine the appropriate combination and thickness of the various layers of recording sheets and of the type and the pattern of the embossed layers to minimize displacements of the recording sheet  170  in relation to a stressed recording sheet  172 , as depicted in  FIGS. 10-14 . Thus, an appropriate combination of the depth and width between peaks and valleys of the pattern can be modeled as described herein in order to reduce curl of the recording sheet such that printing errors and printing abnormalities may be avoided.  
      Although the present invention has been shown and described with respect to various embodiments, various additions, deletions and modifications that are obvious to a person of ordinary skill in the art to which the invention pertains, even if not shown or specifically described herein, are deemed to lie within the scope of the invention as encompassed by the following claims.