REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. Ser. No. 13/140,237 filed on Jun. 16, 2011 (to issue as U.S. Pat. No. 9,567,709 on Feb. 14, 2017), which is a National stage entry of International application PCT/US10/23290 filed on Feb. 5, 2010, which claims priority from U.S. Ser. No. 61/151,323 filed on Feb. 10, 2009, and the entire contents of each of the above applications are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present patent application is directed to low density paper and paperboard and, more particularly, to low density paper and paperboard having a smooth, coated surface on both sides. 
         [0003]    Paperboard is commonly used in various packaging applications. For example, high end personal care or commercial printing applications and the like. The paperboard often receives a variety of graphic treatments to enhance its visual impact on the shelf. Likewise, quality papers to be utilized as a medium for printing require smooth coated surfaces, with few imperfections to facilitate the printing of high quality text and graphics. 
         [0004]    Conventionally, smoothness is achieved by calendering. Calendering serves to mechanically compress the sheet, providing a surface roughness low enough to produce final coated smoothness acceptable to the industry. However, this compression results in the severe densification of the sheet. Therefore, smooth papers and paperboard are typically more dense (i.e., less bulky) than less smooth paper and paperboard. This effect is magnified when a smooth, coated print surface is required on both sides of the paperboard. 
         [0005]    For example, in  FIG. 1 , the basis weight in pounds per ream (1 ream=3000 ft.sup.2) of certain prior art coated two-side (C2S) solid bleached sulfate (SBS) paperboard products and C2S fine paper products is plotted against caliper thickness (1 point=0.001 inch=1 mil), thereby providing a visual representation of prior art paper and paperboard density (i.e., basis weight divided by caliper thickness). As can be seen, for a given caliper, the sheet will have typically been pressed to a given density range in order for the needed surface smoothness to be developed. 
         [0006]    Nonetheless, low density is a desirable quality in many paper and paperboard applications. However, preparing a smooth surface using the conventional calendering process requires substantially increasing the density of the fiber substrate. 
         [0007]    Accordingly, there is a need for a low density paper and paperboard that provides the desired smoothness on both sides for high quality printing, while reducing raw material cost. 
       SUMMARY 
       [0008]    In one aspect, the disclosed low density paper or paperboard may include a fiber substrate and a coating applied to each side of the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf (PPS 10, soft platen) smoothness on each side of at most about 2 microns, a caliper thickness and a basis weight, the basis weight being less than about Y.sub.1, wherein Y.sub.1 is a function of the caliper thickness (X) in points and is calculated using Eq. 1 as follows: 
         [0000]        Y .sub.1=29.15+11.95 X− 0.07415 X .sup.2   (Eq. 1)
 
         [0009]    In another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to each side of the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness on each side of at most about 2 microns, a caliper thickness and a basis weight, the basis weight being at most about Y.sub.2, wherein Y.sub.2 is a function of the caliper thickness (X) in points and is calculated using Eq. 2 as follows: 
         [0000]        Y .sub.2=28.41+11.73 X− 0.07324 X .sup.2   (Eq. 2)
 
         [0010]    In another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to each side of the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness on each side of at most about 2 microns, a caliper thickness and a basis weight, the basis weight being at most about Y.sub.3, wherein Y.sub.3 is a function of the caliper thickness (X) in points and is calculated using Eq. 3 as follows: 
         [0000]        Y .sub.3=27.78+11.51 X− 0.07207 X .sup.2   (Eq. 3)
 
         [0011]    In another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to each side of the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness on each side of at most about 2 microns, a caliper thickness and a basis weight, the basis weight being at most about Y.sub.4, wherein Y.sub.4 is a function of the caliper thickness (X) in points and is calculated using Eq. 4 as follows: 
         [0000]        Y .sub.4=26.89+11.17 X− 0.07034 X .sup.2   (Eq. 4)
 
         [0012]    In another aspect, the disclosed low density paperboard may include a fiber substrate, a topcoat, and a coating positioned between the fiber substrate and the topcoat, the fiber substrate, the basecoat and the topcoat forming a coated structure, wherein the coated structure has a Parker Print Surf smoothness of at most about 2 microns, a caliper thickness and a basis weight, the basis weight being between about Y.sub.1 and about Y.sub.5, wherein Y.sub.1 and Y.sub.5 are functions of the caliper thickness (X) in points and are calculated used Eq. 1 above and Eq. 5 as follows: 
         [0000]        Y .sub.5=26.15+10.83 X− 0.06815 X .sup.2   (Eq. 5)
 
         [0013]    Other aspects of the disclosed low density paperboard will become apparent from the following description, the accompanying drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a graphical comparison of density versus caliper thickness of certain prior art paper and paperboard materials to paper and paperboard according to the present disclosure; 
           [0015]      FIG. 2  is a cross-sectional view of one aspect of the disclosed low density paper or paperboard; 
           [0016]      FIG. 3  is a graphical representation of basis weight versus caliper thickness of various exemplary aspects of the disclosed low density paperboard; 
           [0017]      FIG. 4  is a schematic illustration of a first aspect of a process for preparing the disclosed low density paperboard; 
           [0018]      FIG. 5  is a schematic illustration of a second aspect of a process for preparing the disclosed low density paperboard; 
           [0019]      FIG. 6  is a graphical representation of density versus smoothness (Parker Print Surf) of certain prior art 10 point (caliper) products; and 
           [0020]      FIG. 7  is a graphical representation of density versus smoothness (Parker Print Surf) values of certain prior art 12 point (caliper) products. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring to  FIG. 2 , one aspect of the disclosed low density paperboard, generally designated  10 , may include a fiber substrate  12 , a basecoat  14   a ,  14   b  and an optional topcoat  16   a ,  16   b . The coating formulations may differ from side-to-side in formulation as well as in amount applied. Additionally, one side may have only a base coating, while the other side could be both base and top coated. The paperboard  10  may have a caliper thickness T and layers of coating on each side on which graphics may be printed. Additional layers may be used without departing from the scope of the present disclosure. 
         [0022]    In one aspect, the fiber substrate  12  may be a paper or paperboard substrate. As used herein, “fiber substrate” broadly refers to any paper or paperboard material that is capable of being coated with a basecoat, and may be a single-ply substrate or a multi-ply substrate. Those skilled in the art will appreciate that the fiber substrate may be bleached or unbleached. Generally, the fiber substrates noted herein have uncoated basis weights of about 65 pounds per 3000 ft.sup.2 or more. Examples of appropriate substrates include paper cover stock, linerboard and solid bleached sulfate (SBS). In one particular aspect, the fiber substrate  12  may include a substantially chemically (rather than mechanically) treated fiber, such as an essentially 100 percent chemically treated fiber. Examples of appropriate chemically treated fiber substrates  12  include solid bleached sulfate paperboard or solid unbleached sulfate paperboard. 
         [0023]    Additional components, such as binders, fillers, pigments and the like, may be added to the fiber substrate  12  without departing from the scope of the present disclosure. Furthermore, the fiber substrate  12  may be substantially free of plastic pigments or other chemical bulking agents for increasing bulk, such as hollow plastic pigments or expandable microspheres, Still furthermore, the fiber substrate  12  may be substantially free of ground wood particles. 
         [0024]    The topcoat  16   a ,  16   b  is an optional layer and may be any appropriate topcoat. For example, the topcoat  16   a ,  16   b  may include calcium carbonate, clay and various other components and may be applied to the basecoat  14   a ,  14   b  as a slurry. Topcoats are well known by those skilled in the art and any conventional or non-conventional topcoat  16   a ,  16   b  may be used without departing from the scope of the present disclosure. 
         [0025]    The basecoat  14   a ,  14   b  may be any coating that improves the smoothness of the surface of the paperboard  10  without substantially reducing the caliper thickness T of the paperboard  10 , thereby yielding a smooth (e.g., Parker Print Surf smoothness below about 2.0 microns) and low density paper or paperboard. Those skilled in the art will appreciate that the basecoat  14   a ,  14   b  as well as the techniques (discussed below) for applying the basecoat  14   c ,  14   b  to the fiber substrate  12 , may be significant factors in maintaining a low density product. 
         [0026]    In a first aspect, the basecoat  14   a ,  14   b  may be a carbonate/clay basecoat. The carbonate/clay basecoat may include a ground calcium carbonate component, a platy clay component and various optional components, such as latex binders, thickening agents and the like. The carbonate/clay basecoat may be dispersed in water such that it may be applied to the fiber substrate  12  as a slurry using, for example, a blade coater such that the carbonate/clay basecoat substantially fills the pits and crevices in the fiber substrate  12  without substantially coating the entire surface of the fiber substrate  12 . 
         [0027]    Specific examples of appropriate carbonate/clay basecoats, as well as techniques for applying such basecoats to a fiber substrate  12 , are disclosed in U.S. Ser. No. 12/326,430 filed on Dec. 2, 2008, the entire contents of which are incorporated herein by reference. 
         [0028]    Accordingly, in one aspect, a low density paperboard  10  may be prepared by the process  20  illustrated in  FIG. 4 . The process  20  may begin at the head box  22  which may discharge a fiber slurry onto a Fourdrinier  24  to form a web  26 . The web  26  may pass through one or more wet presses  28  and, optionally, through one or more dryers  30 . A size press  32  may be used and may slightly reduce the caliper thickness of the web  26  and an optional dryer  34  may additionally dry the web  26 . In one aspect, the web  26  may pass through a calender  36  with the nip loads substantially reduced to minimize or avoid reduction in caliper thickness. Preferably, the calender  36  would be run as a dry calender. In another aspect, the calender  36  may be omitted or bypassed. Then, the web  26  may pass through another optional dryer  38  and to the first coater  40   a . The first coater  40   a  may be a blade coater or the like and may apply the carbonate/clay basecoat  14   a  onto the web  26 . An optional dryer  42   a  may dry, at least partially, the carbonate/clay basecoat  14   a  prior to application of the optional topcoat  16   a  at the second coater  44   a . Optional dryer  46   a  may dry the topcoat  16   a . Likewise coating will be applied to the opposite side of the sheet by passing through a coater  40   b  which may be a blade coater or the like and may apply a basecoat  14   b  onto the web  26 . An optional dryer  42   b  may at least partially dry the basecoat  14   b  prior to application of the optional topcoat  16   b  at coater  44   b . Another optional dryer  46   b  may finish the drying process before the web  26  proceeds to the optional gloss calender  48  and the web  26  is rolled onto a reel  50 . 
         [0029]    In a second aspect, the basecoat  14   a ,  14   b  may be a film-forming polymer solution applied to the fiber substrate  12  and then brought into contact with a heated surface in a nip, causing the solution to boil and create voids in the film which remain after the film is dried, resulting in a smooth surface. The film forming polymer may be a starch and the heated surface may be a heated roll. 
         [0030]    Specific examples of appropriate film-forming polymers, as well as techniques for applying such polymers to a fiber substrate, are disclosed in PCT/US07/04742 filed on Feb. 22, 2007, the entire contents of which are incorporated herein by reference, in U.S. Ser. No. 60/957,478 filed on Aug. 23, 2007, the entire contents of which are incorporated herein by reference, and in PCT/US07/19917 filed on Sep. 13, 2007, the entire contents of which are incorporated herein by reference. 
         [0031]    Accordingly, in another aspect, a low density paper or paperboard  10  may be prepared by the process  60  illustrated in  FIG. 5 . The process  60  may begin at the head box  62  which may discharge a fiber slurry onto a Fourdrinier  64  to form a web  66 . The web  66  may pass through one or more wet presses  68  and, optionally, through one or more dryers  70 . A size press  72  may be used, and may slightly reduce the caliper thickness of the web  66  and an optional dryer  74  may additionally dry the web  66 . In one aspect, the web  66  may pass through a calender  76  with the nip loads substantially reduced to minimize or avoid reduction in caliper thickness. If used, the calender  76  may be run as a dry calender. In another aspect, the calender  76  may be omitted or bypassed. Then, the web  66  may pass to an application  78  of the film forming polymer followed by contacting in a nip with a heated roll  80  and a press roll to form a smooth surface with voids in the polymer film. After application and heat/pressure treatment of the film forming polymer, the web  66  may pass through another optional dryer  82  and to the first coater  84   a . The first coater  84   a  may be a blade coater or the like and may apply a conventional basecoat (e.g., as a second basecoat) onto the starch-coated web  66 . An optional dryer  86   a  may dry, at least partially, the basecoat prior to application of an optional topcoat at the second coater  88   a  . Dryer  90   a  may dry the topcoat. The opposite side of the sheet may then be coated via coater  84   b  which may be a blade coater or the like and may apply conventional basecoat onto web  66 . An optional dryer  86   b  may at least partially dry the basecoat prior to application of an optional topcoat at the next coater  88   b . Another optional dryer  90   b  may finish drying before the web  66  proceeds to the optional gloss calender  92  and finished product is rolled onto a reel  94 . The gloss calender  92  may be a soft nip calender, a hard nip calender, or may be omitted or bypassed. 
         [0032]    At this point, those skilled in the art will appreciate that the basecoats  14   a ,  14   b , topcoats  16   a ,  16   b  and associated application techniques disclosed above may substantially increase the smoothness of the resulting paper or paperboard  10  without substantially increasing the density of the paper or paperboard  10  (i.e., the caliper thickness of the fiber substrate  12  may be substantially maintained throughout the coating process). 
         [0033]      FIGS. 6 and 7  demonstrate the typical trend that as a product becomes more dense it can become smoother. It is obvious from the graphs that the products formed in examples 1 and 2 herein described are significantly different in this regard than other products in the ability to maintain low parker print surf values at new low levels of density. 
       EXAMPLES 
       [0034]    Specific examples of smooth, low density paperboard prepared in accordance with the present disclosure are presented below. 
       Example 1 
       [0035]    A low density uncoated solid bleached sulfate (SBS) board having a basis weight of about 125 lbs/3000 ft.sup.2 was prepared using a full-scale production process. 
         [0036]    A high-bulk, carbonate/clay basecoat was prepared having the following composition: (1) 50 parts XP 6170 from Imerys Pigments, Inc. (a high aspect ratio clay), (2) 50 parts Hydracarb 60 from Omya, Inc. (a ground calcium carbonate), (3) 18 parts of a latex binder, and (4) a synthetic thickener in a quantity sufficient to raise the viscosity of the blend to 2000 centipoise, at 20 rpm, on a Brookfield viscometer. 
         [0037]    A topcoat was prepared having the following composition: 70 parts fine carbonate; 30 parts fine clay; 14 latex binder and minor amounts of coating lubricant, dispersant, synthetic viscosity modifier, defoamer and dye. 
         [0038]    The basecoat was applied to the uncoated board using a trailing bent blade applicator. 2-sided coating application was achieved utilizing four coating heads. In this example, the coatings (top and base) on each side of the sheet were identical in composition. The basecoat was applied such that the minimal amount of basecoat needed to fill the voids in the sheet roughness remained on the sheet, while scraping the excess basecoat from the sheet to leave a minimum amount of basecoat above the plane of the fiber surface. The basecoat was applied at a coat weight of about 7 lbs/3000 ft.sup.2. The topcoat was applied over the basecoat to further improve the surface smoothness. The topcoat was applied at a coat weight of about 7 lbs/3000 ft.sup.2. Coat weights were about the same on each side. 
         [0039]    The resulting coated structure had a total basis weight of about 153 lbs/3000 ft.sup.2, a caliper of about 0.012 inches (12 points) and a Parker Print Surf (PPS 10S) smoothness of about 1.10 microns on the wire side and 1.30 microns on the felt side. 
       Example 2 
       [0040]    A low density uncoated board having a basis weight of about 110 lb/3000 ft.sup.2 was prepared using a pilot production process. 
         [0041]    A high-bulk, carbonate/clay basecoat was prepared having the following composition: (1) 50 parts XP 6170 from Imerys Pigments, Inc. (a high aspect ratio clay), (2) 50 parts Hydracarb 60 from Omya, Inc. (a ground calcium carbonate), (3) 18 parts of a latex binder, and (4) a synthetic thickener in a quantity sufficient to raise the viscosity of the blend to 2000 centipoise, at 20 rpm, on a Brookfield viscometer. 
         [0042]    A topcoat was prepared having the following composition: 70 parts fine carbonate; 30 parts fine clay; 14 parts latex binder; and minor amounts of coating lubricant, dispersant, synthetic viscosity modifier, defoamer and dye. 
         [0043]    The basecoat was applied to the uncoated board using a trailing bent blade applicator. 2-sided coating application was achieved utilizing four coating heads. In this example, the coatings (top and base) on each side of the sheet were identical in composition. The basecoat was applied such that the minimal amount of basecoat needed to fill the voids in the sheet roughness remained on the sheet, while scraping the excess basecoat from the sheet to leave a minimum amount of basecoat above the plane of the fiber surface. The basecoat was applied at a coat weight of about 7 lbs/3000 ft.sup.2. The topcoat was applied over the basecoat to further improve the surface smoothness. The topcoat was applied at a coat weight of about 7 lbs/3000 ft.sup.2. Coat weights were about the same on each side. 
         [0044]    The resulting coated structure had a total basis weight of about 134 lbs/3000 ft.sup.2, a caliper of about 0.010 inches (10 points) and a Parker Print Surf (PPS 10S) smoothness of about 1.20 microns on the wire side and 1.30 microns on the felt side. 
         [0045]    The basis weight versus caliper data from Examples 1 and 2 is plotted in  FIG. 3 , together with basis weight versus caliper data for prior art ( FIG. 1 ). The data points from Examples 1 and 2 fall below curve Y.sub.1, which is a plot of Eq. 1, while all of the prior art data is found above curve Y.sub.1. 
         [0046]    While basis weight data is currently only presented in  FIG. 3  for various caliper thickness ranges, those skilled in the art will appreciate that since the disclosed coatings and techniques were capable of achieving surprisingly low densities at about 10 and 12 point calipers, it is to be expected that similar low densities may be achieved at other caliper thicknesses. 
         [0047]    Thus, the coated two-sided paperboard of the present disclosure provides desired smoothness (e.g., PPS 10S smoothness below 2 microns, and even below 1.5 microns), while maintaining low density (e.g., basis weight below the disclosed thresholds as a function of caliper thickness). While such paperboard has been desired, it has not yet been achievable in the prior art. 
         [0048]    Although various aspects of the disclosed low density paper and paperboard with two-sided coating have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The present patent application includes such modifications and is limited only by the scope of the claims.
A paper or paperboard including a cellulose substrate and a coating applied to each side of the paperboard substrate to form a coated structure, the coated structure having a basis weight, a caliper thickness and a Parker Print Surf smoothness, the Parker Print Surf smoothness being at most about 2 microns, the basis weight being less than about Y1 pounds per 3000 ft2, wherein Y1 is a function of the caliper thickness (X) in points and is calculated as follows: 
         Y 1=29.15+11.95 X −0.07415 X 2.