Patent Publication Number: US-2003226648-A1

Title: Multiple ply paperboard material having improved oil and grease resistance and stain masking properties and method for forming same

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates in general to paperboard material having an improved oil and grease resistance and, more particularly, to a multiple ply paperboard material having improved oil and grease resistance properties, and improved stain masking properties to minimize the appearance of oil and grease stains in the paperboard material.  
       [0003] 2. Background Art  
       [0004] The treatment of paperboard to impart some degree of oil and grease resistance has been known in the art for many years. Oil and grease resistant paperboard has been particularly advantageous in packaging and shipping applications involving products containing fat, oil and/or grease. In particular, certain products contain liquid-phase oil or grease, which oil and grease can leak or migrate directly into and wick through the fiber matrix of the paperboard packaging. Other products contain solid-phase fat, but are often subjected to high enough temperatures during their packaging, transportation and/or use to transform at least a portion of the fat into oil or grease. The resulting oil or grease often penetrates into uncoated paperboard through the fiber matrix. Either scenario results in paperboard packaging that is greasy and/or oily to the touch, a higher undesirable outcome. Moreover, the absorption by and migration of oil and grease into and through the paperboard often causes the paperboard to turn a darker color, thus staining the paperboard. This unsightliness is likewise undesirable as stains are easily seen by consumers who are either purchasing or using a particular product.  
       [0005] To prevent grease and oil migration and staining, it has been a common practice to employ a grease-resistant liner in conjunction with the paperboard to help prevent and/or minimize the migration of grease and oil into the paperboard fibers. One common prior art method is to place tin foil on the interior of a paperboard package, between the product to be packaged and the inside of the paperboard wall. While the tin foil has been found to be effective in containing grease and oil leaks and minimizing penetration of grease and oil into the fiberboard, it is also prohibitively expensive in many applications, and can present disposal problems.  
       [0006] Plastic films, such as polyethylene films, likewise have been used to line certain packages for housing oily or greasy foods. One such example is microwave popcorn packaging, which is exposed to the melting lard used to cook the popcorn kernels at high temperatures. Once again, while plastic films have worked well, they pose certain disposal challenges, as well as certain cost prohibitions.  
       [0007] Paperboard has also been treated with chemical coatings employing a grease resistant agent. For instance, certain fluorochemicals, in particular certain fluoropolymers, are known to impart grease and oil resistance to paperboard fibers when coated onto paperboard sheets which are used to package oily or fatty products. For example, Fitzgerald, U.S. Pat. No. 5,674,961, Schwartz et al., U.S. Pat. No. 4,579,924, Schwartz et al., U.S. Pat. No. 4,529,658, Fieuws et al., U.S. Pat. No. 5,370,919, Audenaert et al., U.S. Pat. No. 5,466,770, Overcash et al., U.S. Pat. No. 5,603,996, Sandstrom et al., U.S. Pat. No. 5,876,815 and Yamaguchi et al., U.S. Pat. No. 6,293,972 each disclose the use of fluorinated chemicals in treating paperboard for oil and grease resistance, either in the wet end formation of the paperboard material or as a separate coating for paperboard material.  
       [0008] As one particular example, Sandstrom et al. discloses a laminate paperboard product having grease and oil resistance properties, as well as improved glueability and printability properties. The laminate product comprises a cellulosic substrate, a fluorine containing polymer coating on a surface of the substrate and a latex layer on top of the fluorinated polymer coating. The latex layer is disclosed as including a copolymer, such as a styrene-acrylic copolymer, a styrene-butadiene copolymer or a vinyl acetate-acrylic copolymer, a pigment, such as clay or calcium carbonate, and other potential additives, such as dispersants, lubricants or cross-linkers.  
       [0009] While these fluorochemical treatments have likewise worked well in certain applications, they do not always infiltrate the paper fiber matrix in such a manner as to maximize oil and grease resistance. Furthermore, oil and grease may still migrate through these types of coatings, given an ample amount of time, thus leading to the above described problems, including undesirable greasiness to the touch, and grease and oil staining to the eye.  
       [0010] It is also a common practice to coat various paperboard packages with a pigment based coating to provide a finished and printable surface. Typically, many pigment-based coatings employ a clay pigment and a latex or acetate binder. Those coatings may also include a small amount of a whitening or refractive agent such as titanium dioxide or calcium carbonate, to improve the ability of pigment-based coatings to scatter incoming light. For instance, each of Self et al., U.S. Pat. No. 4,391,833, Fujita et al., U.S. Pat. No. 4,543,280, Guez et al., U.S. Pat. No. 5,665,466, Jacobson et al., U.S. Pat. No. 5,851,652, Haskins et al., U.S. Pat. No. 5,861,209, and Dragoon et al., U.S. Pat. No. 3,463,659, each disclose the use of a pigment layer on top of a paperboard material. The pigment based coating typically provides improved printability and/or water impermeability, as well as even some potentially opacifying properties to the coating. Several of these pigment coatings employ titanium dioxide for the purpose of providing an opaque property to the coating.  
       [0011] Likewise, still other patents disclose the use of pigments or inks to help mask stains which may infiltrate the paperboard material. For instance, Allen, U.S. Pat. No. 4,521,492, Allen, U.S. Pat. No. 4,568,574, Middleton et al., U.S. Pat. No. 4,147,836 and Overcash et al., U.S. Pat. No. 5,603,996 each disclose the use of a pigment or ink in a top coating layer to help mask a stain which may penetrate into an underlying paperboard material.  
       [0012] However, many of these coatings have been largely incapable of hiding grease and oil stains that result from the penetration of oil and grease through one or more plies of a paperboard package. Moreover, when multiple pigment-based coatings are required, typically only the very top coat is enhanced, thus leaving underlying coats susceptible to the appearance of visible stains.  
       [0013] Thus, it is desirable to provide a multiple ply paperboard material having improved oil and grease resistance properties to hinder penetration of grease and oil into and through the paperboard material from either or both of the top and bottom plies.  
       [0014] It is likewise desirable to provide an oil and grease resistant paperboard treatment to improve the penetration of certain grease resistant fluorochemicals into the fiber matrix, and the adsorption of certain grease resistant fluorochemicals onto the individual paper fibers.  
       [0015] It is also desirable to minimize or mask the appearance of oil and grease stains which result when oil and grease penetrate the inside layer of a multiple ply paperboard package, and migrate through to the outer layer of the paperboard package. It is a goal in the art to disguise or mask the stains, such as by tinting or dyeing one or more layers and/or coatings of the paperboard material, thus leading to a more desirable paperboard package.  
       [0016] It is further desirable to provide a process for forming a multiple ply paperboard material having improved oil and grease resistance properties, in combination with an improved ability to mask oil and grease stains.  
       [0017] To this end, it is a goal to provide a process for applying a combination of a grease resistant coatings, with one or more pigment-based coatings having increased refractive and opacity enhancing capabilities to mask or hide oil and grease stains. It is also a goal to provide a process for tinting or dyeing one of the paperboard layers or coatings to decrease the contrast between a potentially stained portion of the paperboard and an unstained portion of the paperboard.  
       SUMMARY OF THE INVENTION  
       [0018] The present invention is directed to a multiple-ply paperboard material having improved oil and grease resistance properties, and improved stain masking properties to minimize the appearance of oil and grease stains in the paperboard material. In one embodiment, the multiple-ply paperboard material includes a substantially paperboard substrate having at least two plies formed of a substantially paperboard material, including at least a top ply and a bottom ply, and a grease resistant coating. The top ply includes an outer surface and an inner surface, such that the inner surface of the top ply is attached to at least one other ply. Likewise, the bottom ply also includes an outer surface and an inner surface, such that the inner surface of the bottom ply is attached to at least one other ply.  
       [0019] In one preferred embodiment, there is a substantially paperboard core having at least one ply or layer positioned between the top ply and the bottom ply.  
       [0020] The grease resistant coating is applied to at least a portion of the outer surface of the top ply and to at least a portion of the outer surface of the bottom ply. In one embodiment, the grease resistant coating includes a fluorochemical component and a surfactant component to hinder the penetration and migration of grease and oil into said substantially paperboard substrate through both the top ply and the bottom ply to, in turn, increase the overall oil and grease resistance of the multiple-ply paperboard material.  
       [0021] In one embodiment, the fluorochemical component of the grease resistant coating includes a fluorinated salt. In one embodiment, the surfactant component of the grease resistant coating includes isopropyl alcohol.  
       [0022] In another embodiment, the fluorochemical component of the grease resistant coating includes a fluorinated polymer. In this embodiment, a surfactant may not be necessary in the grease resistant coating. Preferably, the grease resistant coating includes a solvent to facilitate application of the fluorochemical component to the top and bottom plies.  
       [0023] The multiple-ply paperboard material preferably further includes at least one pigment-based coating applied to at least a portion of at least one of the top ply and the bottom ply over at least a portion of the grease resistant coating. In a preferred embodiment, the pigment-based coating is applied to the top ply, which typically forms the outside of a package constructed from the multiple-ply paperboard material.  
       [0024] The pigment-based coating includes a pigment component and a binder component. The pigment component preferably includes a clay component and an opacity enhancing component to minimize the appearance of grease and oil stains in the paperboard material. The opacity enhancing component of the pigment component is preferably selected from the group consisting of titanium dioxide and calcium carbonate, although other opacity enhancing components may likewise be used with the pigment-based coating.  
       [0025] In one embodiment, pigment component including approximately 25-70 parts by weight opacity enhancing component, preferably titanium dioxide. Also in an embodiment, the titanium dioxide comprises approximately 50 parts by weight of the pigment component. In an embodiment, the pigment component comprises approximately 80% by weight of the pigment-based coating.  
       [0026] In another embodiment, the multiple-ply paperboard material includes two pigment-based coatings, including a pre-coating and a top coating. The pre-coating is preferably applied to at least a portion of at least one of the outer surface of the top ply and the outer surface of the bottom ply over at least a portion of the grease resistant coating. Again, the pre-coating may be applied only to the top ply, as the bottom ply is often hidden on the inside of a package. The pre-coating preferably includes a pigment component and a binder component  
       [0027] The top coating is applied to at least one of the top ply and the bottom ply over at least a portion of the pre-coating. The top coating likewise includes a pigment component and a binder component. Preferably, the pigment component of both the pre-coating and the top coating includes both a clay component and an opacity enhancing component. The opacity enhancing component minimizes the appearance of grease and oil stains in the paperboard material.  
       [0028] In one embodiment, the pre-coating includes approximately 80% by weight of the pigment component, and the top coating includes approximately 70%-80% by weight of the pigment component. The opacity enhancing component of both the pre-coating and the top coating preferably includes titanium dioxide. In one embodiment, the pigment component of the pre-coating including approximately 10-50 parts by weight titanium dioxide, and the pigment component of the top coating including approximately 25-70 parts by weight titanium dioxide. In the same embodiment, the pigment component of the pre-coating preferably includes approximately 10 parts by weight titanium dioxide and the pigment component of the top coating preferably includes approximately 50 parts by weight titanium dioxide.  
       [0029] In another embodiment, at least one of the pre-coating and the top coating further includes a fluorochemical component to enhance the grease resistance of the multiple-ply paperboard material.  
       [0030] In still another embodiment, the grease resistant coating further includes a masking agent to minimize the appearance of grease and oil stains in the multiple-ply paperboard material. In one embodiment, the grease resistant coating further includes a polyvinyl alcohol component, and a masking agent, such as a tint or a dye, is mixed with the polyvinyl alcohol component to darken the appearance of the outer surface of at least one of the top ply and the bottom ply to, in turn, minimize the appearance of grease and oil stains in the multiple-ply paperboard material. In another embodiment, at least one of the pigment based coatings includes a masking agent such as a tint or a dye to minimize the appearance of grease and oil stains in the paperboard material.  
       [0031] In another embodiment, the multiple-ply paperboard material includes a substantially paperboard substrate having at least two plies, including at least a top ply and a bottom ply, a grease resistant coating and at least one pigment-based coating. The top ply and bottom ply are both formed of a substantially paperboard material. The top ply includes an outer surface and an inner surface, such that the inner surface of the top ply is attached to at least one other ply. Likewise, the bottom ply also includes an outer surface and an inner surface, such that the inner surface of the bottom ply is attached to at least one other ply. The grease resistant coating is applied to at least a portion of the outer surface of the top ply and to at least a portion of the outer surface of the bottom ply, and includes a fluorochemical component to hinder the penetration and migration of grease and oil into said substantially paperboard substrate through both the top ply and the bottom ply to, in turn, increase the overall oil and grease resistance of the multiple-ply paperboard material. In one embodiment, the fluorochemical component comprises a fluorinated polymer.  
       [0032] The at least one pigment-based coating is applied to at least a portion of at least one of the outer surface of the top ply and the outer surface of the bottom ply over at least a portion of the grease resistant coating. The pigment-based coating preferably includes a pigment component and a binder component. The pigment component includes a clay component and an opacity enhancing component to minimize the appearance of grease and oil stains in the paperboard material. In one embodiment, the opacity enhancing component includes titanium dioxide, and the pigment component of the pigment-based coating includes approximately 25-70 parts by weight titanium dioxide.  
       [0033] In another related embodiment, the multiple-ply paperboard material includes two pigment-based coatings, including a pre-coating and a top coating. The pre-coating is applied to at least a portion of the outer surface of the top ply over at least a portion of the grease resistant coating, and includes a pigment component and a binder component. The top coating is applied over at least a portion of the pre-coating, and includes a pigment component and a binder component. The pigment component of both the pre-coating and the top coating includes both a clay component and an opacity enhancing component, wherein the opacity enhancing component preferably comprises titanium dioxide.  
       [0034] In a specific form of the embodiment, the pigment component of the pre-coating including approximately 10-50 parts by weight titanium dioxide and the pigment component of the top coating including approximately 25-70 parts by weight titanium dioxide. In another specific embodiment, at least one of the pre-coating and the top coating includes a masking agent, such as a tint or a dye, to minimize the appearance of grease and oil stains in the paperboard material.  
       [0035] In anther embodiment, the multiple-ply paperboard material includes a top ply and a bottom ply which have been formed with a fluorochemical additive to impart oil and grease resistance to the paperboard material. In particular, the top ply and the bottom ply each include a fluorochemical component integrally bonded therewithin to hinder penetration of grease and oil into said substantially paperboard substrate through both the top ply and the bottom ply to, in turn, increase the overall oil and grease resistance of the multi-ply paperboard. The multiple-ply paperboard further includes at least one pigment-based coating applied to at least a portion of the outer surface of the top ply over at least a portion of the grease resistant coating, which pigment-based coating includes a pigment component and a binder component. The pigment component includes a clay component and an opacity enhancing component to minimize the appearance of grease and oil stains in the paperboard material.  
       [0036] The present invention is also directed to a process for improving the resistance of multiple-ply paperboard material to the penetration and migration of oil and grease to, in turn, minimize the appearance of oil and grease staining in the paperboard material. The process includes the steps of forming a substantially paperboard substrate having at least two plies, including at least a top ply and a bottom ply; attaching the inner surface of the top ply to at least one other ply, thereby leaving the outer surface of the top ply exposed; attaching the inner surface of the bottom ply to at least one other ply, thereby leaving the outer surface of the bottom ply exposed; and applying a grease-resistant coating to at least a portion of the outer surface of the top ply and to at least a portion of the outer surface of the bottom ply.  
       [0037] In one embodiment, the grease-resistant coating includes a fluorochemical component and a surfactant component to hinder the penetration and migration of grease and oil into said substantially paperboard substrate through both the top ply and the bottom ply to, in turn, increase the overall oil and grease resistance of the multi-ply paperboard.  
       [0038] In another embodiment, the process further includes the step of applying a pigment-based coating having a pigment component and a binder component to at least a portion of the outer surface of the top ply over at least a portion of the grease resistant coating. The pigment component preferably includes a clay component and an opacity enhancing component to minimize the appearance of grease and oil stains in the paperboard material.  
       [0039] In another related embodiment, the process further including the steps of applying a pre-coating, including a pigment component and a binder component, to at least a portion of the outer surface of the top ply over at least a portion of the grease resistant coating, and applying a top coating, including a pigment component and a binder component, over at least a portion of the pre-coating. In one specific embodiment, the step of applying the pre-coating includes applying the pre-coating in a ratio of approximately 80% by weight of the pigment component to approximately 20% by weight of the binder component, and the step of applying the top coating includes the step of applying the top coating in a ratio of approximately 70%-80% by weight of the pigment component to approximately 20-30% by weight of the binder component.  
       [0040] In this embodiment, the pigment component of both the pre-coating and the top coating preferably includes both a clay component and an opacity enhancing component to minimize the appearance of grease and oil stains in the paperboard material. Further, the opacity enhancing component preferably includes titanium dioxide, and the pigment component of the pre-coating preferably includes approximately 10-50 parts by weight titanium dioxide, while the pigment component of the top coating preferably includes approximately 25-70 parts by weight titanium dioxide.  
       [0041] In another embodiment, the process further includes the step of mixing a fluorochemical component with the pre-coating before applying the pre-coating to at least a portion of the top ply over at least a portion of the grease resistant coating.  
       [0042] In yet another embodiment, the process includes the step of mixing a masking agent, such as a tint or a dye, with the grease-resistant coating before applying the grease resistant coating to the top and bottom plies of the paperboard material to minimize the appearance of grease and oil stains in the multiple-ply paperboard material. In a related embodiment, the process includes the step of coating the outer surface of the top ply with a masking agent before the step of applying the grease-resistant coating to at least a portion of the outer surface of the top ply, to minimize the appearance of grease and oil stains in the multiple-ply paperboard material. In another related embodiment, the process includes the step of mixing a masking agent, such as a tint or a dye, to at least one of the pre-coating and the top coating to minimize the appearance of grease and oil stains in the multiple-ply paperboard material.  
       [0043] In another embodiment, the process includes the steps of forming a substantially paperboard substrate having at least two plies, including at least a top ply and a bottom ply; forming at least one of the top and bottom plies with a fluorochemical component integrally bonded therewithin to hinder the penetration and migration of grease and oil into said substantially paperboard substrate through both the top ply and the bottom ply to, in turn, increase the overall oil and grease resistance of the multiple-ply paperboard; attaching the inner surface of the top ply to at least one other ply, thereby leaving the outer surface of the top ply exposed; attaching the inner surface of the bottom ply to at least one other ply, thereby leaving the outer surface of the bottom ply exposed; applying a pre-coating, including a pigment component and a binder component, to at least a portion of the outer surface of the top ply over at least a portion of the grease resistant coating, wherein the pigment component includes approximately 50-90 parts by weight of a clay component and approximately 10-50 parts by weight of an opacity enhancing component; and applying a top coating, including a pigment component and a binder component, over at least a portion of the pre-coating, wherein the pigment component includes approximately 30-75 parts by weight of a clay component and approximately 25-75 parts by weight of an opacity enhancing component to minimize the appearance of grease and oil stains in the multiple-ply paperboard material.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0044]FIG. 1 is a front elevational view in cross section of a grease resistant paperboard material according to one embodiment of the present invention;  
     [0045]FIG. 2 is a front elevational view in cross section of a grease resistant paperboard material having a masking agent according to another embodiment of the present invention;  
     [0046]FIG. 3 is a front elevational view in cross section of a grease resistant paperboard material according to yet another embodiment of the invention; and  
     [0047]FIG. 4 is a schematic of a process for improving the grease resistance of multiple-ply paperboard material, and minimizing the appearance of oil and grease stains in the multiple-ply paperboard material.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0048] While this invention is susceptible of embodiment in many different forms, there are shown in the drawings, and will be described in detail, several specific embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principals of the invention and is not intended to limit the invention to the embodiments illustrated.  
     [0049] Multiple-ply paperboard  20  is shown in FIG. 1 as including core  22 , top liner  24 , bottom liner  26 , top grease resistant coating  28 , bottom grease resistant coating  30 , pigment based pre-coating  32  and pigment based top coating  34 . At the outset, it must be noted that the multiple-ply paperboard can be used in any number of paperboard applications, including packaging, shipping or displaying. Indeed, the grease resistant multiple-ply paperboard is not limited to box applications, but can also be used in bag or envelope type applications as well. Additionally, while the grease resistant multiple-ply paperboard material that is the subject of the present invention is particularly useful in packaging products having fat, oil or grease as part of their composition, those with ordinary skill in the art with the present disclosure before them will appreciate that the multiple-ply paperboard can be used in combination with other materials for packaging a wide variety of items.  
     [0050] Core  22  is preferably formed of four or more plies of raw filler material. It is contemplated that the plies may all be of like raw material, or that the ply compositions may vary, depending on the particular application. Further, while six plies are preferred, core  22  may comprise more or less than six plies of filler material.  
     [0051] Top liner  24  is preferably attached to the top most ply of core  22 , while bottom liner  26  is preferably attached to the bottom most ply of core  22 . Top liner  24  and bottom liner  26  are preferably formed from types of solid bleached sulfate paper and newsprint raw material fiber mix. That composition can be the same as one or more plies of core  22 ; however, top liner  24  and bottom liner  26  may also be different sets of fibers or raw materials than are found in core  22 . Further, top liner  24  may differ in composition from bottom liner  26 , again depending on the particular paperboard applications. Of course, top liner  24  and bottom liner  26  may also be formed of other materials as well. For instance, top liner  24  and bottom liner  26  may be formed from unprinted paper, or ledger paper, as would be known by those with ordinary skill in the art with the present disclosure before them. Moreover, as will be discussed below in reference to FIG. 3, the top and bottom liners may be formed with a fluorochemical additive, such that the fluorochemical component integrally bonds to the paper fibers, thus imparting a grease-resistant property to the top and bottom liners before they are attached to core  22 .  
     [0052] Top grease resistant coating  28  and bottom grease resistant coating  30  are preferably applied to both top liner  24  and bottom liner  26 , respectively. This provides two separate barriers to grease and oil which attempt to infiltrate the fiber matrix. Typically, the bottom liner forms the inside of a package contemplated for direct contact with a product containing fat, oil or grease. Thus, oil and/or grease must breach two grease resistant coatings to reach the top surface of a package manufactured from multiple-ply paperboard material  20 . However, it is also contemplated that the grease resistant coating may be applied to only one of the top and bottom liners, preferably the bottom liner forming the inside of a package contemplated for direct contact with a fat, oil or grease containing product.  
     [0053] Top grease resistant coating  28  and bottom grease resistant  30  preferably comprise substantially the same coating composition, which may take a number of different forms. In one embodiment, shown in FIG. 1, top and bottom grease resistant coatings  28  and  30  include a solvent component, a fluorochemical component and a surfactant component. The coating preferably comprises approximately 5% v/v fluorochemical component (5% of the total solution by volume is the fluorochemical), although other volumetric solutions may be used in conjunction with the present invention. The solvent is preferably a polyvinyl alcohol, although other solvents are likewise contemplated for use in the top and bottom grease resistant coatings, as would be known by those with ordinary skill in the art with the present disclosure before them.  
     [0054] Moreover, the solvent may also be modified with a masking agent, such as a tint or a dye, to darken the grease resistant coatings. A coating containing the masking agent pre-darkens the area beneath the top surface of the multiple-ply paperboard, thus minimizing the ability of a consumer to see stains which may penetrate and infiltrate through bottom liner  26 , core  22  and top liner  24 . The tint or dye is believed to decrease the contrast between portions of the underlying paperboard which may become stained by the penetration of oil or grease, and those portions which are not stained. Indeed, the masking agent could take the form of a black or otherwise dark dye, a viscous liquid to stain the top liner or other materials to turn the top grease resistant coating dark. Further, if a masking agent is employed in the multiple-ply paperboard material, it may be only desirable to use the masking agent in the top grease resistant coating  28 , as the consumer or user of a product made with the multiple-ply paperboard typically views only the top of the paperboard. Bottom grease resistant coating  30  typically forms the inside portion of a container, and is thus not seen until the container is opened.  
     [0055] In one embodiment, the fluorochemical component is preferably a fluorinated salt which penetrates the fiber matrix of top liner  24  and bottom liner  26  to disperse therein. A suitable fluorinated salt includes Lodyne 2000, manufactured by Ciba Specialty Chemicals Corporation of High Point, N.C. Of course other commercially available fluorinated salts are likewise contemplated for use with the present invention, as would be known by those with ordinary skill in the art with the present disclosure before them.  
     [0056] The surfactant is added to the top and bottom grease resistant coatings  28  and  30  to allow better penetration, dispersion and adsorption of the fluorinated salt molecules onto the surface of the paper fibers. The combination of the surfactant and the fluorochemical component lowers the surface tension of the top and bottom liners. Lowering the paperboard&#39;s surface tension delays or inhibits the penetration of non-polar fat, oil or grease molecules, which are exposed to the paperboard material. One suitable surfactant is isopropyl alcohol, commercially available as a commodity chemical. The surfactant can also be selected from the class of ethoxylated diols. A suitable surfactant is Surfynol 465, commercially available Air Products and Chemicals, Inc. of Allentown, Pa.  
     [0057] In another embodiment, top grease resistant coating  28  and bottom grease resistant coating  30  include the same or a similar solvent, but a different fluorochemical component. In particular, the fluorochemical component comprises a fluorinated polymer, such as Zonyl 9464 manufactured by DuPont Corporation of Wilmington, Del. The fluorinated polymer is long chained and cationically charged. The fluorinated polymer is believed to penetrate the paperboard fiber matrix in top liner  24  and bottom liner  26  to bond with the anionically charged paper fibers. In this particular coating embodiment, a surfactant is unnecessary. However, it is likewise contemplated that a surfactant may be used, if effective.  
     [0058] Pigment based coating  32 , shown in FIG. 1, preferably comprises a pigment component and a binder component. In one particular embodiment, the pigment component accounts for approximately 80% of pre-coating  32 , while the binder component makes up approximately 20% of pre-coating  32 . The binder component may be selected from one of two general classes of coating adhesives, including hydrophilic water soluble colloids such as starch, protein and polyvinyl alcohol, or resin latices and resin emulsions, such as styrene-butadiene, acrylic emulsions, polyvinyl acetates, copolymers of polyvinyl acetate, and acrylates. Additionally, other binders as would be known by those with ordinary skill in the art with the present disclosure before them are likewise contemplated for use with the present invention.  
     [0059] The pigment component preferably includes both a clay component and an opacity enhancing component. The opacity enhancing component may comprise titanium dioxide, calcium carbonate or other opacity enhancing agents and/or whitening agents as would be known by those with ordinary skill in the art with the present disclosure before them. The pigment component of pre-coat  32  preferably comprises approximately 10-50 parts (by weight) titanium dioxide, and approximately 50-90 parts by weight clay. Preferably, the pigment component of pre-coat  32  comprises approximately 10-20 parts titanium dioxide.  
     [0060] In the present invention, titanium dioxide is the preferred opacity enhancing component. The titanium dioxide acts to refract or scatter light that strikes the surface of multiple-ply paperboard  20 , thus enhancing the opaque appearance of the pigment-coated outer surface of the paperboard. This opaque property serves to mask oil stains, which tend to decrease the light scattering ability of the coatings of the multiple-ply paperboard, thus allowing light to pass through the surface of multiple-ply paperboard  20  without interference and, in turn, allowing a viewer to see a dark stain. The addition of titanium dioxide to the pre-coat of multiple-ply paperboard  20  improves light scattering, decreases the amount of light that passes through to the paperboard substrate and increases the refractive index of the paperboard material. Indeed, the titanium dioxide is believed to have one of the greatest refractive indices of all whitening agents, thus helping to hide oil or grease stains as they are viewed from the exterior or outside of the multiple-ply paperboard material.  
     [0061] Pigment based top coating  34  preferably also comprises a pigment component and a binder component. While the binder may likewise comprise from one of two general classes of coating adhesives, including hydrophilic water soluble colloids such as starch, protein and polyvinyl alcohol, or resin latices and resin emulsions, such as styrene-butadiene, acrylic emulsions, polyvinyl acetates, copolymers of polyvinyl acetate, and acrylates or other binder material, the preferred binder in the present invention is an acrylic vinyl acetate copolymer. Like the pigment component of pre-coating  32 , the pigment component of top coating  34  likewise includes a clay component and an opacity enhancing component. Like pre-coating  32 , the opacity enhancing component of top coating  34  is preferably titanium dioxide, given its excellent refractive index.  
     [0062] Top coating  34  preferably comprises approximately 70-80% pigment, and approximately 20-30% binder. The pigment component preferably includes approximately 25-70 parts titanium dioxide, and approximately 30-75 parts clay. In a preferred embodiment, top coating  34  includes approximately 50 parts titanium dioxide, such that approximately half of the pigment component of top coating  34  comprises a light scattering, whitening agent. Once again, this high loading of titanium dioxide into top coat  34  improves the ability of multiple-ply paperboard material  20  to mask or hide oil and grease stains.  
     [0063] It is likewise contemplated that both pre-coating  32  and top coating  34  may also include a grease resistant component, such as a fluorochemical. Those fluorochemicals described above are believed to be suitable additives to provide pre-coating  32  and top coating  34  with oil and grease barrier properties. Of course, those with ordinary skill in the art with the present disclosure before them will recognize other grease resistant additives suitable for use with the pigment-based pre- and top coatings. Moreover, while the addition of a grease resistant component to pre-coating  32  and top coating  34  is preferably in addition to coating top liner  24  and bottom liner  26  with the grease resistant coating, it is likewise contemplated that a pre-coating and top coating having a grease resistant additive may be applied to the top liner without a separate grease resistant coating.  
     [0064] In another embodiment, shown in FIG. 2, multiple-ply paperboard  35  includes core  22 ′, top liner  24 ′, bottom liner  26 ′, top grease resistant coating  28 ′, bottom grease resistant coating  30 ′, pigment based pre-coating  32 ′, pigment based top coating  34 ′ and stain masking coating  36 . While each of the elements with the exception of stain masking coating  36  are the same or similar to the respective elements in multiple-ply paperboard  20  in FIG. 1, stain masking coating  36  is preferably applied to top liner  24 ′ before top grease resistant coating  28 ′. In particular, stain masking coating  36  preferably comprises a coating of black starch applied to top liner  24 ′. However, other masking agents or viscous liquids that pre-stain or darken top liner  24 ′ are likewise contemplated as would be known by those with ordinary skill in the art with the present disclosure before them.  
     [0065] Moreover, as described above, instead of a separate, independent stain masking layer, top grease resistant coating  28 ′ may also be tinted or dyed with a masking agent, such as a black dye, to darken top liner  24 ′ and mask oil and grease stains. In particular, despite the grease resistant component of top and bottom grease resistant coatings  28 ′ and  30 ′, oil or grease may still penetrate paperboard  35 , migrating through bottom grease resistant coating  30 ′, bottom liner  26 ′, core  22 ′ and into top liner  24 ′. Stain masking coating  36 , however, acts to darken top liner′, thus disguising or masking the oil and grease stains. Notably, stain masking coating  36  may also be applied to bottom liner  26 ′, depending upon whether it is important to hide the appearance of stains from the bottom side of multiple-ply paperboard  35  in a particular application. Additionally, a stain masking coating may be used in lieu of the grease resistant coating to mask visible stains which result from oil and/or grease penetration into the paperboard. However, while acting to hide the oil and grease stains, it is preferable to also employ a grease resistant component to minimize or prevent the actual penetration of oil and grease through the paperboard, to prevent an oily and greasy feel.  
     [0066] Additionally, pigment based pre-coating  32 ′ and/or pigment based top coating  34 ′ may also be modified by tinting or dyeing with a stain masking agent to minimize the appearance of oil and grease stains. In fact, it is contemplated that at least one of the pre-coating and the top coating in any of the embodiments of the present invention may include a masking agent such as a tint or dye to hide oil and grease stains.  
     [0067] In another embodiment, shown in FIG. 3, multiple-ply paperboard  99  includes core  100 , top liner  102 , bottom liner  104 , pigment based pre-coating  106  and pigment based top coating  108 . While core  100 , pigment based pre-coating  106  and pigment based top coating  108  are all similar to those described in reference to both to the multiple-ply paperboard shown in FIGS. 1 and 2, top liner  102  and bottom liner  104  are modified with a fluorochemical component to improve the oil and grease resistance properties of top liner  102  and bottom liner  104 . In particular, a fluorochemical, such as a fluorinated polymer, is employed as a liquid additive in the formation of the top and bottom liners  102  and  104 . The fluorochemical is believed to adsorb or bond onto the surface of the paper fibers which form a top liner  102  and bottom liner  104 . Thus, in multiple-ply paperboard material  99  of FIG. 3, the top grease resistant coating and the bottom grease resistant coating may be eliminated.  
     [0068] However, it is likewise contemplated that a top grease resistant coating and a bottom grease resistant coating may be added over the fluorochemically enhanced top liner  102  and bottom liner  104  to increase the overall oil and grease resistance properties of the multiple-ply paperboard material even further. Further, it is also contemplated that a masking agent, such as that disclosed as used in combination with the multiple-ply paperboard shown and described in reference to FIG. 2, may be used either in the form of a separate stain masking coating or a masking agent mixed with the grease resistant coating, to provide multiple-ply paperboard material  99  with an increased ability to mask oil and grease stains.  
     [0069] Of course, inasmuch as a fluorochemical can be added in the manufacture of both top liner  102  and bottom liner  104 , fluorochemical may also be added to enhance the oil and grease resistance of core  100 , including any and all of the individual plies of raw material which make up core  100 . Again, this can be done alone, or in combination with other grease and oil resistant treatments, and with other masking-type treatments.  
     [0070] The present invention likewise includes a process for forming a multiple-ply paperboard material having improved oil and grease resistant properties, and an improved ability to mask the appearance of oil and grease stains which form in the multiple-ply paperboard material. In particular, and as is shown in FIG. 4, process  40  is shown as beginning with the feed of a web  42  of multiple-ply paperboard material. Web  42  preferably includes a core, a top liner and a bottom liner, as described in reference to FIGS.  1 - 3  above. Thus, the core may include any number of different plies of raw filler material, as is desired for a particular application, while the top and bottom liner are preferably adhered to the core. Moreover, while the process shown in FIG. 4 will be described as including the coating of the top and bottom liner with a grease resistance coating, it is likewise contemplated that the top and bottom liner in web  42  enter process  40  already having been treated with a fluorochemical to enhance their oil and grease resistant properties, such as described in reference to FIG. 3 above.  
     [0071] Process preferably includes calendar stack  44 , clay pre-coating station  46 , clay top coating station  48 , drying station  50 , smoothing station  52  and winder roll  54 . Additionally, a masking agent application station  43  is shown in phantom lines in FIG. 4, before web  42  enters calendar stack  44 . As described above, particularly in reference to FIG. 2, the top liner and/or the bottom liner may be initially coated with a masking agent to help minimize the visibility of oil and grease stains in the multiple-ply paperboard material. Alternatively, a masking agent such as a tint or a dye may be added to one of the coating pans  72 ,  74  and  76 , described below.  
     [0072] Web  42  first enters calendar stack  44 , which includes a series of rollers including top roll  60 , coatings rolls  62 ,  64  and  66 , queen roll  68  and king roll  70 . Coating rolls  62 ,  64  and  66  each include coating pans  72 ,  74  and  76 , respectively, which house a grease resistant coating. As discussed above, the grease resistant coating can take a number of different formulas, and may likewise include a masking agent. In particular, the masking agent would preferably be mixed with the grease resistant coating and administered from coating pan  74 , applied to the top side of the web.  
     [0073] As can be seen from FIG. 4, web  42  travels through calendar stack  44  in a serpentine manner, as the calendar stack coats both the top liner and the bottom liner with the fluorochemical-based oil and grease resistant coating. Notably, while calendar stack  44  is shown as including six rolls, those of ordinary skill in the art with the present disclosure before them will appreciate that a calendar stack employing any number of rolls may be employed in process  40 , depending on the specific requirements to coat a particular web of multiple-ply paperboard material, including paperboard composition, paperboard thickness, coating thickness, coating uniformity, etc. Further, while three of the rolls are shown as including coating pans, the coating pans may be attached to as few as two of the rolls for both the top-side and bottom-side treatment. Alternatively, additional coating pans may be employed as would be known by those with ordinary skill in the art with the present disclosure before them.  
     [0074] Fluorochemically treated web  78  is then moved to pigment-based pre-coating station  46 . As can be seen from FIG. 4, pre-coating station  46  includes pre-coating roller  80  and metering rod  82 . The pre-coating roller  80  further includes pan  84 , which holds the pigment-based pre-coating solution. At pre-coating station  46 , pre-coating roller  80  applies the pigment-based pre-coating solution to fluorochemically treated web  78 . Metering rod  82  then knocks off excess coating to create an acceptably smooth and uniform pre-coated web. As discussed above, the actual pigment-based pre-coating make take several different forms. Further, a fluorochemical agent may also be added to the pigment based pre-coating solution to add yet another layer of oil and grease resistance to the multiple-ply paperboard material produced by process  40 . Likewise, a masking agent such as a dye or a tint may also be added to the pigment based precoating to assist in minimizing the appearance of any potential oil and grease stains.  
     [0075] Further, while pre-coating station  46  is shown as including a metering rod  82  to improve coating uniformity and consistency, it is likewise contemplated that the excess coating can be removed and made more uniform in a number of ways, such as by a blade, an air knife or other methods as would be known by those with ordinary skill in the art with the present disclosure before them. Metering rod  82  is just one example of a suitable method.  
     [0076] Pre-coated web  85  then proceeds to pigment-based top coating station  48 . Top coating station  48  includes top coating roller  86 , pan  90  and air knife  88 . Top coating roller  86  applies the pigment-based top coating contained in pan  90  onto web  85  over the pre-coating layer. Air knife  88  performs a function similar to metering rod  82 , removing excess coating and leveling off the pigment-based top coating as it is applied to web  85 . Air knife  88  employs a jet stream of air to perform this leveling function, as would be known by those with ordinary skill in the art with the present disclosure before them. Further, much like pre-coating station  46 , top coating station may include other alternative or additional mechanisms to make the coating substantially uniform and consistent after application to pre-coated web  85 .  
     [0077] Further, as described above, the pigment-based top coating solution may also include a fluorochemical, so that the pigment-based top coating of the multiple-ply paperboard includes oil and grease resistant properties. Likewise, the pigment based top coating may also include a masking agent such as a tint or a dye to minimize the appearance of any potential oil or grease stains in the paperboard. Additionally, while the pre-coating and top coating are shown in the drawings as being applied to only one side of the web, namely the top liner side of the paperboard material, it is likewise contemplated that the pre-coating and top coating may likewise be applied to both sides of the web, depending on the particular application. Of course, in most oil and grease resistant paperboard applications, the inside of the package does not require a pigment-based coating for color, printing and appearance purposes. However, it is contemplated that certain applications may require dual-sided pigment based coatings. In those applications, alternative and/or additional coating equipment, such as backside coaters, may be employed in process  40  to coat both sides of the web. Indeed, while pre-coating station  46  and top coating station  48  are shown as including a single coating roller, those pigment based coating stations may likewise employ calendar stacks or other alternative coating equipment, as would be known by those with ordinary skill in the art with the present disclosure before them.  
     [0078] After the pigment-based top coating is applied, fully coated web  91  then proceeds to drying station  50 . There, fully coated web  91  preferably passes through a dryer to cure the coating onto the paperboard substrate. Of course, other curing mechanisms are also likewise contemplated for use in curing the coating to the paperboard substrates. Likewise, it is also contemplated that the fluorochemical grease resistant coating applied by calendar stack  44  may be dried or cured before the application of any pigment-based coatings.  
     [0079] The cured web  91  then proceeds to smoothing station  52 , which includes rollers  92  and  94 . There, the coating is made substantially uniform to prepare the web for any final manipulation, such as cutting and/or printing. Moreover, while smoothing station  52  is shown as including two separate rollers, the smoothing station may comprise a calendar stack or any other smoothing mechanism as would be known by those with ordinary skill in the art with the present disclosure before them. The smooth, coated web is then wound up on reel  54 , for storage and subsequent use. Of course, the coated web may also proceed directly to a finishing operation such as a cutting and/or printing station, instead of first being rewound, depending on the particular process and application.  
     [0080] The foregoing description and drawings merely explain and illustrate the invention, and the invention is not limited thereto insofar as the appended claims are so limited as those skilled in the art having the present disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention.