Abstract:
A paperboard including a solid bleached sulfate paperboard substrate and a coating applied to 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 3 microns, the basis weight being at most about Y 1  pounds per 3000 ft 2 , wherein Y 1  is a function of the caliper thickness (X) in points and is calculated as follows: Y 1 =3.79+13.43X−0.1638X 2 .

Description:
PRIORITY 
   The present patent application claims priority from U.S. Ser. No. 61/056,712 filed on May 28, 2008, the entire contents of which are incorporated herein by reference. 

   BACKGROUND 
   The present patent application is directed to low density paperboard and, more particularly, to low density paperboard having a smooth surface. 
   Paperboard is commonly used in various packaging applications. For example, aseptic liquid packaging paperboard is used for packaging beverage containers, boxes and the like. Therefore, customers often prefer paperboard having a generally smooth surface with few imperfections to facilitate the printing of high quality text and graphics, thereby increasing the visual appeal of products packaged in paperboard. 
   Conventionally, paperboard smoothness is achieved by a wet stack calendering process in which the paperboard is rewetted and passed through a calendering device having two or more hard rolls. The wet stack calendering process smoothes the paperboard by compressing the fiber network to eliminate the pits and crevices in the raw stock board. Therefore, smooth paperboard is typically more dense (i.e., less bulky) than less smooth paperboard. 
   For example, in  FIG. 1 , the basis weight in pounds per ream (1 ream 3000 ft 2 ) of certain prior art solid bleached sulfate (SBS) paperboard products is plotted against caliper thickness (1 point=0.001 inch), thereby providing a visual representation of prior art paperboard density (i.e., basis weight divided by caliper thickness). As can be seen, the data points generally fall within a range between curve R 1  and curve R 2 . Lower density paperboard (i.e., paperboard falling below curve R 1 ), particularly low density paperboard having a smooth surface, has not been observed in the prior art. 
   Nonetheless, low density is a desirable quality in many paperboard applications. However, preparing a smooth paperboard using the conventional wet stack calendering process requires substantially increasing the paperboard density. 
   Accordingly, there is a need for a low density paperboard that provides the desired smoothness for high quality printing, while reducing manufacturing cost. 
   SUMMARY 
   In one aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness of at most about 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 1 , wherein Y 1  is a function of the caliper thickness (X) in points and is calculated using Eq. 1 as follows:
 
 Y   1 =3.79+13.43 X− 0.1638 X   2   (Eq. 1)
 
   In another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness of at most about 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 2 , wherein Y 2  is a function of the caliper thickness (X) in points and is calculated using Eq. 2 as follows:
 
 Y   2 =3.71+13.14 X− 0.1602 X   2   (Eq. 2)
 
   In another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness of at most about 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 3 , wherein Y 3  is a function of the caliper thickness (X) in points and is calculated using Eq. 3 as follows:
 
 Y   3 =3.63+12.85 X− 0.1566 X   2   (Eq. 3)
 
   In another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness of at most about 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 4 , wherein Y 4  is a function of the caliper thickness (X) in points and is calculated using Eq. 4 as follows:
 
 Y   4 =3.50+12.41 X− 0.1513 X   2   (Eq. 4)
 
   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 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 5 , wherein Y 5  is a function of the caliper thickness (X) in points and is calculated using Eq. 5 as follows:
 
 Y   5 =3.30+11.68 X− 0.1424 X   2   (Eq. 5)
 
   In yet another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness of at most about 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 1 ′ wherein Y 1 ′ is a function of the caliper thickness (X) in points and is calculated using Eq. 6 as follows:
 
 Y   1 ′=36.26+8.3432 X+ 0.01629 X   2   (Eq. 6)
 
   In yet another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness of at most about 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 2 ′ wherein Y 2 ′ is a function of the caliper thickness (X) in points and is calculated using Eq. 7 as follows:
 
 Y   2 ′=35.55+8.173 X+ 0.01602 X   2   (Eq. 7)
 
   In yet another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness of at most about 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 3 ′ wherein Y 3 ′ is a function of the caliper thickness (X) in points and is calculated using Eq. 8 as follows:
 
 Y   3 ′=34.83+8.010 X+ 0.01570 X   2   (Eq. 8)
 
   In yet another aspect, the disclosed low density paperboard may include a fiber substrate and a coating applied to the fiber substrate to form a coated structure, the coated structure having a Parker Print Surf smoothness of at most about 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 4 ′ wherein Y 4 ′ is a function of the caliper thickness (X) in points and is calculated using Eq. 9 as follows:
 
 Y   4 ′=33.79+7.769 X+ 0.01524 X   2   (Eq. 9)
 
   In yet 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 3 microns, a caliper thickness and a basis weight, the basis weight being at most about Y 5 ′, wherein Y 5 ′ is a function of the caliper thickness (X) in points and is calculated using Eq. 10 as follows:
 
 Y   5 ′=32.77+7.537 X+ 0.01475 X   2   (Eq. 10)
 
   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 
       FIG. 1  is a graphical representation of basis weight versus caliper thickness of certain prior art paperboard materials; 
       FIG. 2  is a cross-sectional view of one aspect of the disclosed low density paperboard; 
       FIG. 3  is a graphical representation of basis weight versus caliper thickness of various exemplary aspects of the disclosed low density paperboard; 
       FIG. 4  is a schematic illustration of a first aspect of a process for preparing the disclosed low density paperboard; 
       FIG. 5  is a schematic illustration of a second aspect of a process for preparing the disclosed low density paperboard; 
       FIG. 6  is a graphical representation of density versus caliper thickness of various exemplary aspects of the disclosed low density paperboard; 
       FIG. 7  is a graphical representation of density versus Parker Print Surf smoothness of an exemplary aspect of the disclosed low density paperboard having a caliper thickness of about 10 points; 
       FIG. 8  is a graphical representation of density versus Parker Print Surf smoothness of various exemplary aspects of the disclosed low density paperboard having a caliper thickness of about 12 points; 
       FIG. 9  is a graphical representation of density versus Parker Print Surf smoothness of various exemplary aspects of the disclosed low density paperboard having a caliper thickness of about 18 points; and 
       FIG. 10  is another graphical representation of basis weight versus caliper thickness of the various exemplary aspects shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 2 , one aspect of the disclosed low density paperboard, generally designated  10 , may include a fiber substrate  12 , a basecoat  14  and a topcoat  16 . The paperboard  10  may have a caliper thickness T and an upper surface S upon which text or graphics may be printed. Additional layers may be used without departing from the scope of the present disclosure. 
   In one aspect, the fiber substrate  12  may be a paperboard substrate. As used herein, “paperboard substrate” broadly refers to any 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 paperboard substrate may be bleached or unbleached, and typically is thicker and more rigid than paper. Generally, a paperboard substrate has an uncoated basis weight of about 85 pounds per 3000 ft 2  or more. Examples of appropriate paperboard substrates include corrugating medium, 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. 
   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 for increasing bulk, such as hollow plastic pigments or expandable microspheres, or other chemical bulking agents. Still furthermore, the fiber substrate  12  may be substantially free of ground wood particles. 
   The topcoat  16  is an optional layer and may be any appropriate topcoat. For example, the topcoat  16  may include calcium carbonate, clay and various other components and may be applied to the basecoat  14  as a slurry. Topcoats are well known by those skilled in the art and any conventional or non-conventional topcoat  16  may be used without departing from the scope of the present disclosure. 
   The basecoat  14  and topcoat  16  may be any coating that improves the smoothness of the surface S 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 3.0 microns) and low density paperboard. Those skilled in the art will appreciate that the basecoat  14 , as well as the techniques (discussed below) for applying the basecoat  14  to the fiber substrate  12 , may be significant factors in maintaining a low density product. 
   In a first aspect, the basecoat  14  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 . 
   The ground calcium carbonate component may be a coarse ground calcium carbonate, such as CARBITAL® 60 available from Imerys Pigments, Inc. of Roswell, Ga., or an extra coarse ground calcium carbonate, such as CARBITAL® 35, also available from Imerys Pigments, Inc. The platy clay component may be a high aspect ratio clay having an aspect ratio (i.e., the ratio of the clay particle length or diameter to the thickness), on average, of about 50:1, such as CONTOUR® 1180 available from Imerys Pigments, Inc., or a very high aspect ratio clay having an aspect ratio, on average, of about 90:1, such as XP-6100 (also known as BARRISURF X) also available from Imerys Pigments, Inc. 
   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. 61/038,579 filed on Mar. 21, 2008, the entire contents of which are incorporated herein by reference. 
   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 . The first coater  40  may be a blade coater or the like and may apply the carbonate/clay basecoat  14  onto the web  26 . An optional dryer  42  may dry, at least partially, the carbonate/clay basecoat  14  prior to application of the optional topcoat  16  at the second coater  44 . Another optional dryer  46  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 . 
   In a second aspect, the basecoat  14  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. 
   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. 
   Accordingly, in another aspect, a low density 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 . The first coater  84  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  may dry, at least partially, the basecoat prior to application of an optional topcoat at the second coater  88 . Another optional dryer  90  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. 
   At this point, those skilled in the art will appreciate that the basecoats  14 , topcoats  16  and associated application techniques disclosed above may substantially increase the smoothness of the resulting paperboard  10  while essentially maintaining the caliper thickness of the fiber substrate  12  throughout the coating process. 
   EXAMPLES 
   Specific examples of smooth, low density paperboard prepared in accordance with the present disclosure are presented below. 
   Example 1 
   A low density uncoated solid bleached sulfate (SBS) board having a basis weight of about 120 lbs/3000 ft 2  was prepared using a full-scale production process. 
   A high-bulk, carbonate/clay basecoat was prepared having the following composition: (1) 50 parts high aspect ratio clay from Imerys Pigments, Inc., (2) 50 parts PG-3 from Omya (an extra coarse ground calcium carbonate), (3) 19 parts of a polyvinyl acetate latex (a binder), and (4) an alkali-swellable synthetic thickener in a quantity sufficient to raise the viscosity of the blend to 2500 centipoise, at 20 rpm, on a Brookfield viscometer. 
   A topcoat was prepared having the following composition: 50 parts fine carbonate; 50 parts fine clay; 17 parts polyvinyl acetate; and minor amounts of coating lubricant, plastic pigment, protein, dispersant, synthetic viscosity modifier, defoamer and dye. 
   The basecoat was applied to the uncoated board using a trailing bent blade applicator. 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 6.0 lbs/3000 ft 2 . The topcoat was applied over the basecoat to further improve the surface smoothness. The topcoat was applied at a coat weight of about 5.4 lbs/3000 ft 2 . 
   The resulting coated structure had a total basis weight of about 130.0 lbs/3000 ft 2 , a caliper of about 0.012 inches (12 points) and a Parker Print Surf (PPS 10S) smoothness of about 1.5 microns. 
   Example 2 
   A low density uncoated board having a basis weight of about 186.8 lbs/3000 ft 2  was prepared using a pilot production process. 
   A first basecoat was prepared as a 17 percent solids slurry including, by weight, 97 percent low molecular weight ethylated starch and 3 percent soybean oil-based release agent. The slurry was applied to the surface of the uncoated board at a coat weight of about 2.7 lbs/3000 ft 2 . The treated board was then contacted with a polished drum at a temperature of about 430° F. and a pressure of about 200 pounds per lineal inch, thereby boiling the starch and shaping the surface of the board to replicate the drum surface. The resulting coated structure had a basis weight of 189.5 lbs/3000 ft 2 , a caliper thickness of about 18.2 points and a PPS 10S smoothness of about 2.95 microns. 
   A second basecoat was prepared as a mixture of 100 parts ground calcium carbonate with 16 parts polyvinyl acetate latex as a binder and about 1.5 parts of a low molecular weight polyvinyl alcohol as a thickener. The second basecoat was applied to the coated board at a coat weight of about 2.5 lbs/3000 ft 2 . The resulting coated structure had a basis weight of 191.8 lbs/3000 ft 2 , a caliper thickness of about 18.1 points and a PPS 10S smoothness of about 2.28 microns. 
   A topcoat was prepared as a pigment blend of 70 parts fine clay, 30 parts fine ground calcium carbonate, 20 parts of a styrene-acrylic latex (a binder) and about 1.5 parts of a low molecular weight polyvinyl alcohol (a thickener). The topcoat was applied over the second basecoat at a coat weight of about 1.9 lbs/3000 ft 2 . The resulting coated structure had a total basis weight of about 193.7 lbs/3000 ft 2 , a caliper thickness of about 18.2 points, and a PPS 10S smoothness of about 1.26 microns. 
   Example 3 
   An uncoated board having a basis weight of about 185 lbs/3000 ft was coated with about 2.7 lbs/3000 ft 2  of starch using the first basecoat process described above in Example 2. The resulting coated structure had a total basis weight of about 187.7 lbs/3000 ft 2 , a caliper thickness of about 17.9 points and a PPS 10S smoothness of about 2.40 microns. 
   Example 4 
   A low density uncoated board having a basis weight of about 112 lbs/3000 ft 2  was prepared using a full-scale production process. The basecoat of Example 2 was applied in the described manner at a coat weight of about 3.8 lbs/3000 ft 2 . 
   A topcoat formulation was prepared as an 85/15 blend of a fine ground calcium carbonate and a fine coating clay, with 14 parts polyvinyl acetate latex and 2 part carboxymethyl cellulose (a water soluble thickener). The topcoat was applied over the basecoat using a typical topcoat application technique at a coat weight of about 6.6 lbs/3000 ft 2 . 
   The resulting coated structure had a total basis weight of about 118.5 lbs/3000 ft 2 , a caliper thickness of about 10 points and a PPS 10S smoothness of about 2.35 microns. 
   Example 5 
   Using the processes described in Example 2, a coated paperboard sample was prepared by applying a starch slurry at a coat weight of about 3 lbs/3000 ft 2  and a topcoat at a coat weight of about 6 lbs/3000 ft 2 . The resulting coated structure had a basis weight of about 141.8 lbs/3000 ft 2 , a caliper thickness of about 12.8 points and PPS 10S smoothness of about 2.20 microns. 
   Example 6 
   A low density uncoated board having a basis weight of about 119 lbs/3000 ft 2  was prepared using a full-scale production process. The uncoated board was coated with a starch slurry at a coat weight of about 3 lbs/3000 ft 2  using the first basecoat formulation and associated process described in Example 2. Samples 1 and 2 were collected without a topcoat. Samples 3 and 4 received a topcoat having the topcoat formulation described in Example 2 at a coat weight of about 8-9 lbs/3000 ft 2 . Sample 4 also underwent a typical gloss calendering process. The resulting data is presented in Table 1: 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
               Caliper 
               Basis Weight 
               PPS 10S 
             
             
                 
               Sample 
               (points) 
               (lbs/ream) 
               (microns) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               1 
               11.1 
               121.7 
               2.31 
             
             
                 
               2 
               11 
               120.8 
               2.5 
             
             
                 
               3 
               11.5 
               130 
               2.29 
             
             
                 
               4 
               11.6 
               130 
               1.38 
             
             
                 
                 
             
           
        
       
     
   
   The density (i.e., basis weight divided by caliper) versus caliper data from Examples 1-6, together with density versus caliper data for prior art paperboard, is plotted in  FIG. 6 . Those skilled in the art will appreciate that significantly lower densities are achieved when paperboard is prepared in accordance with the present disclosure. Those skilled in the art will also appreciate that density is a function of caliper, so one should compare individual calipers separately when evaluating PPS. 
     FIG. 7  illustrates density versus Parker Print Surf smoothness for a 10 point board (Example 4) in accordance with the present disclosure, plotted against density versus Parker Print Surf smoothness of prior art 10 point board.  FIG. 8  illustrates density versus Parker Print Surf smoothness of 12 point board (taken from Examples 1-6), plotted against density versus Parker Print Surf smoothness of prior art 12 point board.  FIG. 9  illustrates density versus Parker Print Surf smoothness of 18 point board (taken from Examples 1-6), plotted against density versus Parker Print Surf smoothness of prior art 18 point board. Those skilled in the art will appreciate that the paperboard of the present disclosure presents significantly lower densities relative to the prior art, while maintaining smoothness (i.e., low Parker Print Surf smoothness values). 
   The basis weight versus caliper data from Examples 1-6, together with basis weight versus caliper data for prior art paperboard ( FIG. 1 ), is plotted in  FIG. 3 . All of the data points from Examples 1-6 fall below curve Y 1 , which is a plot of Eq. 1, while all of the prior art data is found above curve Y 1 . Furthermore, eight of the data points from the disclosed Examples fall below curve Y 2 , which is a plot of Eq. 2, six of the data points fall below curve Y 3 , which is a plot of Eq. 3, and two of the data points fall below curve Y 4 , which is a plot of Eq. 4. 
   Similarly, basis weight versus caliper data of paperboard prepared in accordance with the present disclose, together with basis weight versus caliper data for prior art paperboard, is plotted in  FIG. 10 . All of the data points from fall below curve Y 1 ′, which is a plot of Eq. 6, while all of the prior art data is found above curve Y 1 ′. Furthermore, several data points fall below curve Y 2 ′, which is a plot of Eq. 7, with several more data points falling below curve Y 3 ′, which is a plot of Eq. 8, and others falling below curve Y 4 ′, which is a plot of Eq. 9. 
   While basis weight data is currently presented in  FIGS. 3 and 10  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, 11, 12, 13 and 18 point calipers, it is to be expected that similar low densities may be achieved at other caliper thicknesses. 
   Thus, the paperboard of the present disclosure provides desired smoothness (e.g., PPS 10S smoothness below 3 microns, and even below 1.5 microns), while maintaining low board density (e.g., basis weight below the disclosed thresholds as a function of caliper thickness). While such paperboard has been desired, it is believed that it has not yet been achievable in the prior art. 
   Although various aspects of the disclosed low density paperboard 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.