Patent Publication Number: US-10760220-B2

Title: Packaging material and method for making the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 15/122,039, filed Aug. 26, 2016, which is itself a 35 U.S.C. 371 national stage filing of International Application S.N. PCT/US2014/035138, filed Apr. 23, 2014, both of which are incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     Packaging materials may be made on a papermaking machine, such as a Fourdrinier Machine. Papermaking generally involves forming a web of fibers on a conveyer belt (often referred to as a wire), pressing the fibers to drain water from the web, and then drying the pressed web. The papermaking process may also include calendering, where a roll is used to smooth the dried web. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of examples of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear. 
         FIG. 1  is a flow diagram depicting several examples of a method for making a packaging material; 
         FIG. 2  is a cross-sectional view depicting an example of the packaging material including an image layer and a strength layer; 
         FIG. 3  is a cross-sectional view depicting another example of the packaging material including the image layer in contact with one opposed surface of the strength layer and a second strength layer in contact with another opposed surface of the strength layer; and 
         FIG. 4  is a cross-sectional view depicting yet another example of the packaging material including the strength layer made up of two sub-strength layers and respective image layers in contact with outer opposed surfaces of the strength layer. 
     
    
    
     DETAILED DESCRIPTION 
     Examples of the packaging material disclosed herein include a multi-layered structure with a di-valent or multi-valent salt present (e.g., distributed) throughout an image layer, which is positioned as at least one of the outermost layers of the structure. Examples of the multi-layered structure also include a strength layer. It is believed that the image layer enhances the print quality characteristics of the multi-layered structure, and the strength layer enhances the durability of the multi-layered structure. 
     The di-valent or multi-valent salt remains in the image layer, at least in part because the strength layer includes softwood fibers of a specific length that form a fiber mat with relatively low porosity that acts as a barrier to the salt. This fiber mat reduces salt migration from the image layer through the strength layer during the papermaking process. The presence of the salt in the outermost image layer(s) is desirable for enhancing the compatibility of the packaging material with inkjet inks subsequently printed thereon. The salt provides the packaging material with an ink fixing characteristic. 
     Additionally, the methods for making the packaging material disclosed herein are streamlined, in part because the salt may be added to the image layer during the forming process. As such, additional offline coating and/or printing processes are not required. The methods disclosed herein enable a traditional papermaking machine (e.g., a paperboard duo Fourdrinier machine having multiple headboxes) to be used, even when the machine does not include a surface sizing station. 
     Referring now to  FIG. 1 , the steps of various examples of the method  100  for making examples of the packaging material are illustrated. Different examples of the method  100  are denoted by the different arrows between the boxes. For example, one example of the method is shown by the bold arrows, and includes steps  102  through  112 . Examples of the resulting packaging materials are shown in  FIG. 2  through  FIG. 4 . The specific layered structures shown in  FIG. 2  through  FIG. 4  will be described throughout the discussion of  FIG. 1 . 
     While not shown in  FIG. 1 , at the outset of any of the examples of the method, suitable pulps stocks are formed for the image layer(s) and the strength layer(s). All of the pulp stocks described in conjunction with the method(s) disclosed herein initially contain water along with at least one type of fiber for making a particular layer. It is to be understood that when the pulp stock is dried, there may be some minimal loss of the fiber(s) in the final layer that is formed. 
     Examples of the pulp stock for the image layer(s) include about 99% water. The remaining components in the image layer(s) pulp stocks are the hardwood fibers, and the salt. In some instances, no other components are added. In other instances, other fibers and/or additives may be included in the image layer pulp stock. 
     The hardwood fibers included in the image layer pulp stock have an average length ranging from about 0.5 mm to about 1.5 mm. These relatively short fibers improve the formation and smoothness of the packaging material. In addition, it is believed that ink applied to an image layer including relatively short fibers may be distributed more precisely. The hardwood fibers are present in an amount ranging from about 70 wt % to about 100 wt % of a total wt % of the solid components (i.e., total solids wt %) of the image layer pulp stock. In an example, suitable hardwood fibers include pulp fibers derived from deciduous trees (angiosperms), such as birch, aspen, oak, beech, maple, and eucalyptus. The hardwood fibers may be bleached or unbleached hardwood fibers. 
     When the image layer pulp stock includes less than 100 wt % of the hardwood fibers previously defined, the pulp stock may also include up to 20 wt % of fibers other than the hardwood fibers. These other fibers may be a different type of fiber from, but have the same length as, the hardwood fiber. The other fibers may be natural fibers, virgin fibers, recycled fibers, non-deinkable fibers, unbleached fibers, synthetic fibers, mechanical fibers, or combinations thereof. One example of the other fibers includes softwood fibers. 
     The hardwood fibers and/or other fibers may be prepared by any known pulping process, such as, for example, chemical pulping processes. Two suitable chemical pulping methods include the kraft process and the sulphite process. The hardwood fibers may also be mechanically pulped, thermomechanically pulped, or chemi-thermomechanically pulped. 
     In addition to hardwood fibers, the image layer pulp stock further includes a water soluble di-valent or multi-valent salt. The di-valent or multi-valent salt is present in an amount ranging from about 5 lb per ton of the amount of total fiber(s) in the image layer pulp stock to about 50 lb per ton of the amount of total fiber(s) in the image layer pulp stock. Some examples of the di-valent or multi-valent salt may include a salt of any metals of Group I, Group II, and Group III of the Periodic Table of Elements, as well as a salt of any of the transition metals. Some examples of metal cations include calcium ions, copper ions, nickel ions, magnesium ions, zinc ions, barium ions, iron ions, aluminum ions, and chromium ions; and some examples of anions for forming the metal salt include chloride ions, iodide ions, bromide ions, nitrate ions, phosphate ions, chlorate ions, acetate ions, propionates, formates, oxalates, and/or combinations thereof. 
     In an example, the di-valent or multi-valent salt may be chosen from calcium chloride (CaCl 2 ), magnesium chloride (MgCl 2 ), aluminum chloride (AlCl 3 ), magnesium sulfate (MgSO 4 ), calcium acetate (Ca(CH 3 COO) 2 ), calcium propionate (Ca(C 2 H 5 COO) 2 ), calcium lactate (C 6 H 10 CaO 6 ), calcium nitrate (Ca(NO 3 ) 2 ), magnesium acetate (Mg(CH 3 COO) 2 ), magnesium propionate (Mg(C 2 H 5 COO) 2 ), and combinations thereof. 
     When the image layer pulp stock includes less than 100 wt % of the hardwood fibers, the image layer pulp stock may also contain up to 10 wt % (with respect to total solids) of an additive. Suitable additives may be selected from the group consisting of a dry strength additive, wet strength additive, a filler, a retention aid, a dye, an optical brightening agent (i.e., optical brightener), a surfactant, a sizing agent, a biocide, a defoamer, and a combination thereof. 
     Examples of dry strength additives that may be added include anionic polyacrylamides, cationic polyacrylamides, amphoteric polyacrylamides, polyvinyl alcohol, cationized starch, vegetable galactomannan, and/or combinations thereof. Wet strength additives may be added, such as polyaminepolyamide epichlorohydrin resins. 
     Suitable fillers that may be added include carbonates (e.g., ground calcium carbonate and precipitated calcium carbonate), titanium dioxide, clays (e.g., kaolin clay), silicates, oxides, zeolites, talc, and combinations thereof. 
     Any suitable dye may be added, an example of which IRGALITE® Blue Dye (BASF Corp.). 
     Some suitable retention aids include polyacrylamide-based systems (such as PERCOL® polyacrylamides (BASF Corp.) and the Eka PL Series (Eka Chemicals, AkzoNobel Corp.), and solutions of particles and charged polymers (such as COMPOZIL® Select and Eka NP (Eka Chemicals, AkzoNobel Corp.). 
     Example optical brighteners include TINOPAL® ABP-A (BASF Corp.), and examples of suitable defoamers include AC-22 available from Performance Process, Inc., and ANTISPUMIN® 7100 available from Evonik-Degussa GmbH. 
     Some suitable surfactants include those of the Eka DPC Series, available from Eka Chemicals, AkzoNobel Corp. 
     Suitable sizing agents that may be added include fatty acids, metal salts of fatty acids, alkyl ketene dimer emulsification products, epoxidized higher fatty acid amides, alkenyl acid anhydride emulsification products and rosin derivatives, alkylsuccinic acid anhydride emulsification products and rosin derivatives, and/or combinations thereof. 
     Examples of suitable biocides include AQUATREAT® DNM 30 (AkzoNobel Corp), SPECTRUM™ XD3899 (Ashland, Inc.), and MYACIDE® AS and Protector) DZ (BASF Corp.). 
     The image layer pulp stock may be made by incorporating at least the hardwood fibers into a suitable amount of water to form a slurry. As an example, the slurry may contain 99% water and 1% fibers, where 100% of the fibers are the hardwood fibers disclosed herein. If the other fibers are included, they may be added into the slurry. 
     The slurry may be refined. In an example, a double disk refiner is used. The double disk refiner is a refining mechanism, which uses a free rotating disk rotor between two non-rotating disks. The rotating disk and the two non-rotating disks are each fit with a refining plate on each side thereof. The rotating disk, and associated refining plates rotate between the two non-rotating disks fit with refining plates. The refiner applies mechanical and hydraulic forces to alter the fibers within the slurry. For example, the refining process may cause one or more of the following: removal of the primary walls, formation of fiber debris, internal and external fibrillation, fiber shortening, and increased fiber flexibility within the slurry. Refining may be accomplished to achieve a desired freeness of pulp (e.g., targeting a certain number according to the Canadian Standard Method (CSF)). As an example, refining of the image layer pulp stock may be accomplished in a manner sufficient to target a CSF ranging from about 400 to about 450 for the hardwood fibers. 
     The salt (e.g., in solution form) and any additives can either be added to the slurry before or after refining. 
     After refining, the slurry may also be passed through a screen, which removes the larger debris but allows the fibers (and the additives and salt) to pass through the screen. The smaller unwanted particles that remain after the screening are removed by a centrifugal cleaner, which uses centrifugal force and fluid shear to remove the smaller unwanted particles. The smaller particles can be removed using this process, in part because the slurry components separate based on the particles weight and particle shape. This slurry (i.e., the image layer pulp stock) may be used in any examples of the method  100  shown in  FIG. 1  to form examples of the image layer. 
     Examples of the pulp stock for the strength layer(s) include about 99% water. The remaining component in the strength layer(s) pulp stock is the softwood fibers. In some instances, no other components are added. In other instances, other fibers and/or additives may be included in the strength layer pulp stock. 
     The softwood fibers included in the strength layer pulp stock(s) have an average length ranging from about 1.5 mm to about 3.0 mm. The softwood fibers are present in an amount ranging from about 70 wt % to about 100 wt % of the solid components of the strength layer pulp stock. In an example, suitable softwood fibers include pulp fibers derived from coniferous trees (gymnosperms), such as varieties of fir, spruce, and pine (e.g., loblolly pine, slash pine, Colorado spruce, balsam fir, and Douglas fir). 
     When the strength layer pulp stock includes less than 100 wt % of the softwood fibers previously defined, the pulp stock may also include up to 30 wt % of other fibers other than the softwood fibers. These other fibers may be a different type of fiber as, but have the same length as, the softwood fiber. The other fibers may be natural fibers, virgin fibers, recycled fibers, non-deinkable fibers, unbleached fibers, synthetic fibers, mechanical fibers, or combinations thereof. In an example, the strength layer pulp stock may include a bulk of softwood fibers with a low level of hardwood, recycled, or other types of fibers, such as cellulose fibers. 
     The softwood fibers and/or other fibers may be prepared via any known pulping process, such as, for example, chemical pulping processes. Two suitable chemical pulping methods include the kraft process and the sulphite process. The softwood fibers may also be mechanically pulped, thermomechanically pulped, or chemi-thermomechanically pulped. 
     When the strength layer pulp stock includes less than 100 wt % of the softwood fibers, the image layer pulp stock may also contain up to 10 wt % (with respect to total solids) of an additive. In some instances, suitable additives for the strength layer pulp stock may include the dry strength additive, the wet strength additive, the filler, or a combination thereof. Any of the examples previously described may be used. In other instances, any of the additives (and amounts thereof) previously described for the image layer pulp stock may be used in the strength layer pulp stock. 
     The strength layer pulp stock may be made by incorporating at least the softwood fibers into a suitable amount of water to form a slurry. If the other fibers are included, they may be added into the slurry. As an example, the slurry may contain 99% water and 1% fibers, where 99% of the fibers are the softwood fibers disclosed herein and 1% of the fibers are other fibers. 
     In an example, the slurry may be refined. In another example, the slurry may not be refined. If the slurry is refined, the same process as previously described for the image layer slurry may be used. When the strength layer pulp stock is refined, the refining may be accomplished to achieve the desired freeness of pulp as described above (i.e., targeting a certain number according to the Canadian Standard Method (CSF)). As an example, refining of the strength layer pulp stock may be accomplished in a manner sufficient to target a CSF ranging from about 300 to about 500 for the softwood fibers. 
     Any other desirable additives may be added to the refined or unrefined slurry. In another example, the other additive(s) may be added as the slurry is refined. The strength layer slurry may also undergo the same screening and cleaning process previously described for the image layer slurry. This slurry (i.e., the strength layer pulp stock) may be used in any examples of the method  100  shown in  FIG. 1  to form examples of the strength layer. 
     In the examples of the method  100  shown in  FIG. 1 , the strength and image layer pulp stocks are jetted from respective headboxes of a traditional papermaking machine. Prior to jetting, the respective pulp stocks are introduced into respective headboxes in a suitable manner. 
     In one example of the method  100 , a packaging material with an image layer and a strength layer is formed. An example of this packaging material  20  is shown in  FIG. 2 . As depicted, the packaging material  20  includes the strength layer  24  having two opposed surfaces S 1 , S 2  and the image layer  22  in contact with one of the opposed surfaces S 1  of the strength layer  24 . 
     To make this example of the packaging material  20 , the method  100  includes the step of jetting, from a first headbox, the strength layer pulp stock (shown as “first” pulp stock in  FIG. 1 ) onto a wire to form a strength layer precursor. This is shown at step  102 . The strength layer precursor is a wet web of at least the softwood fibers. 
     The method  100  also includes, at step  104 , jetting, from a second headbox, the image layer pulp stock (shown as “second” pulp stock in  FIG. 1 ) onto a second wire to form an image layer precursor. It is to be understood the wire upon which the strength layer precursor is formed is different than the second wire upon which the image layer precursor is formed. The image layer precursor is a wet web of at least the hardwood fibers and the salt. 
     Once the strength layer precursor and image layer precursor are formed, in the next step  126  of the method  100 , the image layer precursor and strength layer precursor are placed into contact with each other. It is desirable that when the precursors are in contact, the image layer precursor should overlie the strength layer precursor. Placing the precursors in contact may be accomplished by moving the respective wires so that respective surfaces of the image layer precursor and the strength layer precursor are adjacent to one another and touch. 
     In some instances, it may be desirable to slightly dry (i.e., remove some of the water from) the strength layer precursor prior to placing the image layer precursor and the strength layer precursor in contact. In an example, water removal may be passive, where water is allowed to drain, filter, etc. from the strength layer precursor prior to applying the image layer precursor. Water removal may be accomplished so that the consistency (or concentration) is increased to a desirable level. 
     Consistency is defined as the weight in grams of oven-dry fiber in 100 grams of pulp-water mixture (i.e., pulp stock). To determine the consistency, TAPPI Test method TAPPI/ANSI T 240 entitled “Consistency (concentration) of pulp suspensions” may be used. 
     In an example, the consistency of the initial strength layer pulp stock is around 1% (e.g., including about 99% water and 1% solids). After jetting to form the strength layer precursor, the water begins to drain from the pulp stock, thereby increasing the consistency. It may be desirable to remove (e.g., by draining) a certain amount of the water from the strength layer precursor prior to bringing the image layer precursor in contact therewith. As such, the strength layer precursor may be exposed to drying (e.g., filtering, draining, etc.) in order to obtain a consistency ranging from about 5% to about 30%. Some specific examples of desirable strength layer precursor consistency levels (prior to image layer precursor application) include 5%, 10%, 15%, or 20° A. A higher consistency (i.e., less water in the strength layer precursor) may contribute to improving the salt retention in the image layer after the two precursors are put into contact. Since some water does remain in the strength layer precursor, it is still considered a wet web. 
     Due to the fact that the image layer precursor and the strength layer precursor are wet webs, this is a wet-on-wet process. This wet-on-wet process is advantageous, in part because subsequent papermaking steps (e.g., removing water, drying, etc.) do not have to be performed separately for each layer of the multi-layered structure. In addition to improving the efficiency of the method, the wet-on-wet process improves the adhesion between the layers by increasing bonding strength due to hydrogen bonding. 
     After the strength layer precursor and image layer precursor are placed in contact, the remaining water is removed from the image layer precursor and strength layer precursor (as shown at step  128 ). Some remaining water may be removed from the precursors in a press section of the papermaking machine. In an example, water removal is accomplished using rollers under high pressure. The precursors are passed between the rollers to squeeze out as much water as possible. Water removal may also be accomplished using a filtration process. It is to be understood that some water may remain in the precursors after the removal process takes place. 
     The orientation of the precursors during water removal is such that the image layer precursor overlies the strength layer precursor. This is desirable because the water drains generally in a direction toward the surface S 2  of the strength layer precursor. As the water is drained, salt from the image layer precursor may have a tendency to migrate with the water. However, the strength layer precursor aids in keeping most if not all of the salt from moving with the water. This is due, at least in part, to the strength layer low porosity fiber mat creating a barrier layer. The fiber mat enables at least the bulk of the salt to be maintained within the image layer precursor. As discussed above, increased dryness/consistency of the strength layer precursor before coming in contact with image layer precursor will also increase the salt retention. As such, salt retention may be at least partially controlled by controlling the consistency of the strength layer precursor. 
     Even though the bulk of the salt remains in the image layer precursor, some of the salt may still migrate through to the strength layer precursor. As such, the strength layer  24  that is ultimately formed may also contain some of the di-valent or multi-valent salt that migrated from the image layer precursor. However, it is to be understood that the di-valent or multi-valent salt present in the final image layer  22  is at least five times the amount of the di-valent or multi-valent salt present in the final strength layer  24 . 
     The final step  130  of this example of method  100  includes drying the strength layer precursor and image layer precursor to form the packaging material, which includes the image layer  22  and the strength layer  24 . Drying may be accomplished in any suitable manner. In an example, a series of steam heated drying cylinders are utilized, and the pressed precursors are passed around these cylinders. Drying removes excess water from the packaging material  20  that is formed; although it is to be understood that some water may still remain in the respective layers  22 ,  24 . 
     While not shown in  FIG. 1 , the packaging material  20  may also be exposed to a calendering. Calendering may be performed in a typical manner, e.g., using heavy steel rollers. The rollers apply pressure to the passing packaging material  20  to smooth and/or enhance the gloss of the packaging material  20 . One or more nips may be used in the calendering process. 
     While also not shown in  FIG. 1 , the packaging material  20  may also be exposed to a reeling process. In the reeling process, a reel is used to wind the packaging material  20  to form a roll. 
     As mentioned above, the process involving steps  102 ,  104 , and  126 - 130  of  FIG. 1  forms the packaging material  20  shown in  FIG. 2 . An example of image layer  22  may be formed from the image layer pulp stock including water, 100 wt % (with respect to solids in the pulp stock) unbleached hardwood fibers having the length within the range provided herein, CaCl 2  as the salt in an amount of 12 lb per ton of the total fiber in the image layer pulp stock, cationic starch as an additive in an amount of 20 lb per ton of the total fiber in the image layer pulp stock, and AKD (alkyl ketene dimer) as another additive in the amount of 5 lb per ton of the total fiber in the image layer pulp stock. 
     Another example of the image layer  22  may be formed from the image layer pulp stock including water, 70 wt % unbleached hardwood fibers having the length within the range provided herein, 30 wt % unbleached softwood fibers, CaCl 2  as the salt in an amount of 12 lb per ton of the total fiber in the image layer pulp stock, cationic starch as an additive in an amount of 20 lb per ton of the total fiber in the image layer pulp stock, and AKD as another additive in the amount of 5 lb per ton of the total fiber in the image layer pulp stock). An example of strength layer  24  of the packaging material  20  is formed from the strength layer pulp stock including water and 100 wt % unbleached softwood fibers having the length within the range provided herein. 
     In an example, one of the previously described image layer pulp stocks and strength layer pulp stock are jetted separately and put into contact. After the image layer pulp stocks and strength layer pulp stock are placed in contact, they are exposed to water removal, dried, and in some instances calendered/reeled as previously described to form the packaging material  20  having the layers  22 ,  24  adhered to one another. In another example, one of the previously described image layer pulp stocks and strength layer pulp stock are jetted separately and put into contact once the strength layer consistency (dryness) has reached a desirable level, e.g., 20%. After the image layer pulp stocks and strength layer pulp stock are placed in contact, they are exposed to further water removal through filtration, pressing and drying, and in some instances calendered/reeled as previously described to form the packaging material  20  having the layers  22 ,  24  adhered to one another. Alternatively, the image layer pulp stock may be jetted directly onto the strength layer pulp stock, and then the pulp stocks are exposed to water removal, drying, etc. 
     It is to be understood that the layers  22 ,  24  that are formed have approximately the same amount of the fibers, and in some instances salt and/or additives, which are used in the respective pulp stocks, taking into account minor loss due to the water removal process. 
     In another example of the method  100 , another example of the packaging material is formed, with an image layer and two strength layers. An example of this packaging material  20 ′ is shown in  FIG. 3 . As depicted, the packaging material  20 ′ includes the strength layer  24  having the two opposed surfaces S 1 , S 2 , a second strength layer  24 ′ in contact with one of the opposed surfaces S 1 , and the image layer  22  in contact with the second strength layer  24 ′. 
     To make this example of the packaging material  20 ′, step  102  may be performed, which forms the strength layer precursor (in this example, the precursor to strength layer  24 ). This step may be performed in the manner previously described. 
     The method  100  also includes step  106 , where a strength layer pulp stock (referred to as the third pulp stock in box  106  in  FIG. 1 ) is jetted from another (e.g., third) headbox onto another (e.g., third) wire to form a second strength layer precursor. This second strength layer precursor ultimately forms the second strength layer  24 ′ shown in  FIG. 3  (i.e., the middle layer of the multi-layered packaging material  20 ′). 
     The second strength layer pulp stock (i.e., third pulp stock in  FIG. 1 ) may include any of the components previously described for the strength layer pulp stock (used in step  102 ) and may be made by the same process. However, the second strength layer pulp stock may include from about 50 wt % to about 100 wt % of the softwood fibers having the length ranging from about 1.5 mm to about 3.0 mm. In an example, the strength layer pulp stock used to form strength layer  24  may be the same as the second strength layer pulp stock used to form the second strength layer  24 ′. For example, each of the strength layer pulp stocks may include the same type and amount of softwood fibers, with or without the same amount and type of additive(s). In another example, the strength layer pulp stock used to form strength layer  24  may be different than the second strength layer pulp stock used to form the second strength layer  24 ′. For example, the second strength layer pulp stock may include a different type and length of softwood fibers than are present in the strength layer pulp stock. 
     This example of the method  100  also includes step  104 , which forms the image layer precursor (in this example, the precursor to image layer  22 ). This step may also be performed in the manner previously described. 
     The strength layer precursor, the second strength layer precursor, and the image layer precursor are then placed into contact with each other (as shown at step  108 ). It is to be understood that water from one or both of the strength layer precursors may be allowed to drain so that the precursor(s) have a desired consistency before being placed into contact with the image layer precursor. 
     It is desirable that when the precursors are in contact, the image layer precursor should overlie the strength and second strength layer precursors. Placing the precursors in contact may be accomplished by moving the respective wires so that respective surfaces of the image layer precursor and the second strength layer precursor are adjacent to one another and touch, and such that respective surfaces of the second strength layer precursor and the strength layer precursor are adjacent to one another and touch. In an example, the second strength layer precursor and the strength layer precursor may be placed into contact first by moving the corresponding wires into an appropriate position. Then the image layer precursor may be placed into contact with the exposed surface of the second strength layer precursor by moving at least the wire upon which the image layer precursor is formed adjacent to the exposed surface. The layering of the precursors is a wet-on-wet process. 
     The layered precursors form a stack, which includes the image layer precursor positioned as one of the outermost layers of the stack. 
     Step  110  of this example of the method  100  includes removing the water from the stack. Water removal may accomplished by any suitable process, including the use of high pressure and roller or filtration. In this example of the method  100 , the orientation of the precursors during water removal is such that the image layer precursor overlies both the second strength layer precursor and the strength layer precursor. This is desirable because, as described above, the water drains generally in a direction toward the opposed surface S 2  of the strength layer precursor. As the water is drained, salt from the image layer precursor may have a tendency to migrate with the water. However, the softwood fibers, porosity, and consistency of the strength and second strength layer precursors keep most, if not all, of the salt from moving with the water by forming the fiber mat previously discussed. 
     Step  112  includes drying the stack of the image layer precursor, the second strength layer precursor, and the strength layer precursor to form the packaging material  20 ′. 
     While not shown in  FIG. 1 , the packaging material  20 ′ may also be exposed to a calendering or reeling process, as described above. 
     As mentioned above, the process involving steps  102 ,  106 ,  104 , and  108 - 112  of  FIG. 1  forms the packaging material  20 ′ shown in  FIG. 3 . An example of image layer  22  may be formed from the image layer pulp stock including water, 100 wt % bleached hardwood fibers having the length within the range provided herein, CaCl 2  as the salt in an amount of 12 lb per ton of the total fiber in the image layer pulp stock, cationic starch as an additive in an amount of 20 lb per ton of the total fiber in the image layer pulp stock, and AKD (alkyl ketene dimer) as another additive in the amount of 5 lb per ton of the total fiber in the image layer pulp stock. 
     An example of strength layer  24  of the packaging material  20 ′ is formed from the strength layer pulp stock including water and 100 wt % unbleached softwood fibers having the length within the range provided herein. An example of strength layer  24 ′ of the packaging material  20 ′ is formed from the second strength layer pulp stock including water, 50 wt % recycled fibers, 50% bleached chemi-thermomechanical fibers. In another example, the second strength layer pulp stock may include the previously listed components as well as a dry strength additive, and may be formed without any refining. 
     In an example, the previously described image layer pulp stock, strength layer pulp stock, and one of the second strength layer pulp stocks are jetted separately and the precursors are put into contact (with or without altering the consistency of the strength layer precursor(s)), exposed to water removal, dried, and in some instances calendered/reeled as previously described to form the packaging material  20 ′ having the layers  22 ,  24 ′,  24  adhered to one another. 
     It is to be understood that the layers  22 ,  24 ′,  24  that are formed have approximately the same amount of the fibers, and in some instances salt and/or additives, which are used in the respective pulp stocks, taking into account minor loss due to the water removal process. 
     In yet another example of the method  100  shown in  FIG. 1 , another example of the packaging material is formed, with an image layer, a strength layer (composed of two strength sub-layers in contact with one another), and a second image layer. An example of this packaging material  20 ″ is shown in  FIG. 4 . As depicted, the packaging material  20 ″ includes two strength sub-layers  24   A ,  24   B  forming the strength layer  24 , and respective image layers  22 ,  22 ′ on opposed surfaces S 1 , S 2  of the strength layer  24 . 
     To make this example of the packaging material  20 ″, two separate bi-layer structures are formed and then placed into contact with one another. One of the bi-layer structures is formed in steps  102 ,  104 ,  118 , and  122 ; and another of the bi-layer structures is formed in steps  114 ,  116 ,  120 , and  124 . 
     To form one of the bi-layer structures, step  102  may be performed. This step may be performed as previously described. Step  102  forms the strength layer precursor, which in this example is a precursor to strength sub-layer  24   A  and a portion of strength layer  24 . Step  104  may then be performed as previously described to generate the image layer precursor, which is a precursor to the image layer  22 . 
     This example of the method  100  continues with step  118 , where the image layer precursor (formed in step  104 ) and the strength layer precursor (formed in step  102 ) are placed into contact to form the bi-precursor structure. Prior to placing them in contact, the strength layer precursor (formed in step  102 ) may have its consistency increased in the manner previously described herein. Placing these precursors into contact may be accomplished as previously described in reference to step  126  of the first example of the method  100 . 
     At step  122 , water is removed from the bi-precursor structure in any suitable manner, such as those previously described in reference to step  130  of the first example of the method  100  disclosed herein. Orientation of the bi-precursor structure during water removal is such that the image layer precursor overlies the strength layer precursor. As previously described, this is desirable because the softwood fibers in the strength layer precursor keep most if not all of the salt from moving with the water out of the image layer precursor. 
     To form the other of the bi-layer structures, step  114  may be performed to generate a second strength layer precursor, which in this example is a precursor to strength sub-layer  24   B  and to another portion of strength layer  24 . Since the strength sub-layer  24   B  and the strength sub-layer  24   A  make up portions of the same strength layer  24 , it is desirable that the second strength layer pulp stock (i.e., third pulp stock in box  114  of  FIG. 1 ) have the same composition as the strength layer pulp stock used to form the precursor to the strength sub-layer  24   A  (formed in step  102 ). As such, the second strength layer pulp stock (i.e., third pulp stock in box  114  of  FIG. 1 ) may include any of the components previously described for the strength layer pulp stock (used in step  102 ) and may be made by the same process. It is to be understood however, that the second strength layer pulp stock may, in some instances, have a different type of softwood fiber than the strength layer pulp stock used to form the precursor to the strength sub-layer  24   A . 
     In step  114 , the second strength layer pulp stock (i.e., third pulp stock in box  114  of  FIG. 1 ) is jetted from another (e.g., third) headbox onto another (e.g., third) wire to form the second strength layer precursor. 
     Step  116  may then be performed to generate another image layer precursor. In step  116 , the second image layer pulp stock (i.e., fourth pulp stock in box  116  of  FIG. 1 ) is jetted from another (e.g., fourth) headbox onto another (e.g., fourth) wire to form the second image layer precursor. 
     This image layer precursor ultimately forms image layer  22 ′. The image layer precursor formed at step  116  may be made using any example of the image layer pulp stock described herein. If it is desirable the final image layer  22 ′ have the same composition as the final image layer  22 , then the image layer pulp stock used in step  104  may be the same as the image layer pulp stock (i.e., fourth pulp stock in box  116  of  FIG. 1 ) used in step  116 . However, if it is desirable that the final image layer  22 ′ have a different composition than the final image layer  22 , then the image layer pulp stock used in step  104  may be different than the image layer pulp stock (i.e., fourth pulp stock in box  116  of  FIG. 1 ) used in step  116 . 
     Generally, the image layer pulp stock (i.e., fourth pulp stock in box  116  of  FIG. 1 ) used to form the second image layer precursor in this example of the method  100 , may include water, hardwood fibers (having a length ranging from about 0.5 mm to about 1.5 mm) present in an amount ranging from about 70 wt % to about 100 wt % of total solids, salt, and if desirable, other fibers and/or additives. Suitable amounts for any of these components are previously described in reference to step  104 . 
     This example of the method  100  continues with step  120 , where the second image layer precursor (formed in step  116 ) and the second strength layer precursor (formed in step  114 ) are placed into contact to form the other bi-precursor structure. Prior to placing them in contact, the second strength layer precursor (formed in step  116 ) may have its consistency increased in the manner previously described herein. Placing these precursors into contact may be accomplished as previously described in reference to step  126  of the first example of the method  100 . 
     At step  124 , water is removed from the other bi-precursor structure in any suitable manner, such as those previously described in reference to step  130  of the first example of the method  100 . Orientation of the bi-precursor structure during water removal is such that the second image layer precursor overlies the second strength layer precursor. As previously described, this is desirable because the softwood fibers in the second strength layer precursor keep most if not all of the salt from moving with the water out of the second image layer precursor. 
     Step  132  involves placing the bi-precursor structures in contact so that a stack is formed. The stack has the two strength layer precursors positioned so that they are adjacent to and touch one another. When the two strength layer precursors are in contact, precursors to the sub-layers  24   A  and  24   B  are formed. The precursors may be compressed together using rollers under high pressure and the resulting packaging material  20 ″ may be considered to have a single strength layer  24 . This process is similar to processing using a wet press. 
     Since the two strength layer precursors are in contact in the stack, each of the image layer precursors of the respective bi-precursor structure faces outward. This is desirable when dual-sided printing on the packaging material  20 ″ is to be performed. 
     Step  134  includes drying the stack of the image layer precursor, the strength layer precursors, and the second image layer precursor to form the packaging material  20 ″. 
     While not shown in  FIG. 1 , the packaging material  20 ″ may also be exposed to a calendering or reeling process, as described above. 
     As mentioned above, the process involving steps  102 ,  104 ,  114 - 124 ,  132 , and  134  of  FIG. 1  forms the packaging material  20 ″ shown in  FIG. 4 . An example of image layers  22 ,  22 ′ may be formed from image layer pulp stocks including water, 100 wt % bleached hardwood fibers having the length within the range provided herein, CaCl 2  as the salt in an amount of 12 lb per ton of the total fiber in the image layer pulp stock, cationic starch as an additive in an amount of 20 lb per ton of the total fiber in the image layer pulp stock, and AKD (alkyl ketene dimer) as another additive in the amount of 5 lb per ton of the total fiber in the image layer pulp stock. The sub-layers  24   A ,  24   B  of the strength layer  24  may be formed from a single strength layer pulp stock including water and 100 wt % unbleached softwood fibers having the length within the range provided herein. 
     In the examples of the method  100  disclosed herein, the number and types of pulp stocks that will be used will depend, at least in part, on which layers are desired in the final packaging material  20 ,  20 ′,  20 ″. In an example, the packaging material  20 ,  20 ′,  20 ″ will include at least the strength layer  24  and the image layer  22 . 
     To further illustrate the present disclosure, an example is given herein. It is to be understood that this example is provided for illustrative purposes and is not to be construed as limiting the scope of the present disclosure. 
     EXAMPLE 
     Two samples of the packaging material disclosed herein were prepared along with a control sample. The control (sample 1), was Mottle White #3 media, which is a commercially available 2 layer packaging paper that does not include any salt. No salt was added to sample 1. 
     Samples 2 and 3 used the same Mottle White #3 media as the packaging material, except that CaCl 2  was added to one of the layers. The amount salt present in teach of samples 1-3 is shown in Table 1 below. 
     All three samples were tested for optical density after a 100 μL Fugu ink drawdown was performed using a Mayer Rod #8. Table 1 below shows the optical density results. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Black Optical Density 
               
               
                   
                   
                   
                 (KoD) after Fugu Ink 
               
               
                   
                 Sample ID 
                 Formulation 
                 Drawdown 
               
               
                   
                   
               
             
            
               
                   
                 Sample 1 
                   0 lb/T of CaCl 2   
                 0.95 
               
               
                   
                 Sample 2 
                  8.9 lb/T of CaCl 2   
                 1.39 
               
               
                   
                 Sample 3 
                 30.3 lb/T of CaCl 2   
                 1.58 
               
               
                   
                   
               
            
           
         
       
     
     The black optical density (KoD) was determined using an X-Rite densitometer. The higher KoD measurement demonstrates an improved printability on the packaging material. As shown in Table 1, samples 2 and 3 both had an improved printability with the addition of CaCl 2 , as compared to sample 1 without any salt. 
     Reference throughout the specification to “one example”, “another example”, “an example”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the example is included in at least one example described herein, and may or may not be present in other examples. In addition, it is to be understood that the described elements for any example may be combined in any suitable manner in the various examples unless the context clearly dictates otherwise. 
     It is to be understood that the ranges provided herein include the stated range and any value or sub-range within the stated range. For example, a range from about 50 wt % to about 100 wt % should be interpreted to include not only the explicitly recited limits of about 50 wt % to about 100 wt %, but also to include individual values, such as 60 wt %, 75 wt %, 90 wt %, etc., and sub-ranges, such as from about 65.5 wt % to about 95 wt %, from about 55 wt % to about 75 wt %, etc. Furthermore, when “about” is utilized to describe a value, this is meant to encompass minor variations (up to +/−10%) from the stated value. 
     While several examples have been described in detail, it will be apparent to those skilled in the art that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting.