HEAT-SEALABLE PAPERBOARD STRUCTURES AND METHODS

Abstract

A heat-sealable paperboard structure including a paperboard substrate having a first major side and a second major side opposed from the first major side, a heat-sealable barrier coating on the first major side of the paperboard substrate, and a top coat positioned over the heat-sealable barrier coating, wherein the top coat forms a discontinuous layer over the heat-sealable barrier coating.

Claims

1. A heat-sealable paperboard structure comprising: a paperboard substrate comprising a first major side and a second major side opposed from the first major side; a heat-sealable barrier coating on the first major side of the paperboard substrate; and a top coat positioned over the heat-sealable barrier coating, wherein the top coat forms a discontinuous layer over the heat-sealable barrier coating.

2. The heat-sealable paperboard structure of claim 1 wherein the top coat comprises a binder and a pigment.

3. The heat-sealable paperboard structure of claim 2 wherein a ratio of the pigment to the binder in the top coat is at least about 1:1, by weight.

4-6. (canceled)

7. The heat-sealable paperboard structure of claim 2 wherein the binder comprises a latex.

8. (canceled)

9. The heat-sealable paperboard structure of claim 2 wherein the pigment comprises calcium carbonate.

10. (canceled)

11. The heat-sealable paperboard structure of claim 1 wherein the discontinuous layer has a coat weight ranging from about 0.1 lb/3000 ft.sup.2 to about 4.0 lb/3000 ft.sup.2.

12-14. (canceled)

15. The heat-sealable paperboard structure of claim 1 wherein the heat-sealable barrier coating comprises binder and pigment.

16. The heat-sealable paperboard structure of claim 15 wherein a ratio of the pigment to the binder in the heat-sealable barrier coating is at most 1:1 by weight.

17-25. (canceled)

26. The heat-sealable paperboard structure of claim 25 wherein the platy clay has an average aspect ratio of at least about 40:1.

27. The heat-sealable paperboard structure of claim 1 wherein the heat-sealable barrier coating has a coat weight ranging from about 4 lb/3000 ft.sup.2 to about 20 lb/3000 ft.sup.2.

28-29. (canceled)

30. The heat-sealable paperboard structure of claim 1 wherein the paperboard substrate comprises solid bleached sulfate.

31. The heat-sealable paperboard structure of claim 1 wherein the paperboard substrate has a basis weight ranging from about 80 lb/3000 ft.sup.2 to about 300 lb/3000 ft.sup.2.

32-34. (canceled)

35. The heat-sealable paperboard structure of claim 1 further comprising one or more coating layers positioned between the paperboard substrate and the heat-sealable barrier coating.

36. The heat-sealable paperboard structure of claim 1 further comprising one or more coating layers on the second major side.

37. The heat-sealable paperboard structure of claim 1 having a Parker Print Surface (PPS-10S) smoothness of at most about 3.1 μm.

38-39. (canceled)

40. The heat-sealable paperboard structure of claim 1 having a 30-minute-water-Cobb rating of at most about 10 g/m.sup.2.

41-45. (canceled)

46. The heat-sealable paperboard structure of claim 1 having a blocking rating of less than 2 at 50° C. and at 60 psi in a 24-hour period.

47. (canceled)

49. The heat-sealable paperboard structure of claim 1 having a heat-sealability (% fiber tear) of at least about 95%.

50. A container comprising the heat-sealable paperboard structure of claim 1.

51. A method for manufacturing a heat-sealable paperboard structure comprising: preparing a heat-sealable barrier coating formulation comprising a binder and a pigment; applying the heat-sealable barrier coating formulation to a first major side of a paperboard substrate to form a heat-sealable barrier coating; preparing a top coat formulation comprising a binder and a pigment; and applying the top coat formulation over the heat-sealable barrier coating to form a discontinuous layer of top coat over the heat-sealable barrier coating.

52-55. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is an elevational view, in section, of one example of a container (e.g., a cup) that can be manufactured using the disclosed heat-sealable paperboard structures;

[0013] FIG. 2 is a top plan view of the container of FIG. 1;

[0014] FIG. 3 is a plan view a die-cut blank that may be wrapped around a mandrel to form the side wall of the container of FIG. 1;

[0015] FIG. 4 is a schematic cross-sectional view of one example of the disclosed heat-sealable paperboard structure;

[0016] FIG. 5 is a schematic cross-sectional view of another example of the disclosed heat-sealable paperboard structure;

[0017] FIG. 6 is a top view of an example heat-sealable paperboard structure taken using a scanning electron microscope at 200× magnification;

[0018] FIG. 7 is a cross-sectional view of an example heat-sealable paperboard structure taken using a scanning electron microscope at 1000× magnification; and

[0019] FIG. 8 is an illustration of a device for testing blocking of coated paperboard samples.

DETAILED DESCRIPTION

[0020] It has now been discovered that a heat-sealable paperboard-based structure having a first major surface with high water barrier properties and minimal to no blocking tendencies can be achieved by positioning the heat-sealable barrier coating layer on the first major side of the underlying paperboard substrate, which has traditionally formed the first major surface of the structure, beneath a lower-binder, calenderable top coat applied as a discontinuous layer over (e.g., directly to) the barrier coating layer such that the heat-sealable barrier coating is positioned between the paperboard substrate and the top coat. Heat-sealability is provided by a heat-sealable barrier coating. Such a structure may be particularly well-suited for holding cold beverages (e.g., iced soft-drinks), cold foodstuffs (e.g., ice cream), hot beverages (e.g., coffee) and hot foodstuffs (e.g., soup).

[0021] Referring to FIGS. 1 and 2, one example of a disclosed paperboard-based container, generally designated 10, may include a side wall 12 having an upper end portion 14 and a lower end portion 16, and a bottom wall 18 connected (e.g., heat-sealed) to the lower end portion 16 of the side wall 12, thereby defining an internal volume 20 within the container 10. The upper end portion 14 of the side wall 12 may define an opening 22 into the internal volume 20. Optionally, the upper end portion 14 of the side wall 12 may additionally include a lip 24 (e.g., a rolled lip), such as for securing a lid (not shown) or the like to the container 10.

[0022] While the container 10 is shown in FIG. 1 as a tall cup (e.g., a 12-ounce, 16-ounce, 21-ounce or 24-ounce disposable take-out cup) having a frustoconical side wall 12, those skilled in the art will appreciate that the disclosed container 10 may be formed in various shapes, sizes and configurations, and may be formed with fewer or more walls than the side and bottom walls 12, 18 discussed above, without departing from the scope of the present disclosure.

[0023] As shown in FIG. 2, the side wall 12 of the container 10 may be assembled from a blank 30 (FIG. 3) that has been cut to the desired silhouette and then wrapped around a mandrel (not shown). While the blank 30 is wrapped around the mandrel, the first end 32 of the blank 30 overlaps a second end 34 of the blank 30, and the overlapping ends 32, 34 may be connected (e.g., by heat-sealing), thereby defining a seam 36 that extends from the upper end portion 14 to the lower end portion 16 of the side wall 12. Once the side wall 12 has been assembled, the bottom wall 18 may be connected (e.g., heat-sealed) to the lower end portion 16 of the side wall 12, thereby yielding the container 10.

[0024] Referring to FIG. 4, the side wall 12 of the container 10 may be formed from a paperboard structure 40 having a first major surface 42 and a second major surface 44. The first major surface 42 of the paperboard structure 40 may correspond to the interior surface 28 of the container 10. The second major surface 44 of the paperboard structure 40 may correspond to the exterior surface 26 of the container 10.

[0025] The paperboard structure 40 may be a layered structure that includes a paperboard substrate 46 having a first major side 48 and a second major side 50. A heat-sealable barrier coating 52 and a top coat 54 may be applied to the first major side 48 of the paperboard substrate 46 such that the top coat 54 forms a discontinuous layer 56 over (e.g., directly adjacent) the heat-sealable barrier coating 52. The heat-sealable barrier coating 52 may be positioned between the top coat 54 and the paperboard substrate 46. The discontinuous layer 56 of top coat 54 may define, at least partially, the first major surface 42 of the paperboard structure 40 and, thus, the interior surface 28 of the container 10.

[0026] At this point, those skilled in the art will appreciate that various additional layers, barrier or non-barrier, may be incorporated into the paperboard structure 40 between the paperboard substrate 46 and the discontinuous layer 56 or on top of the second major side 50 without departing from the scope of the present disclosure. In one variation, as shown in FIG. 5, the paperboard structure 40′ may include a basecoat 45 between the paperboard substrate 46′ and the heat-sealable barrier coating 52′. In another variation, as shown in FIG. 5, the heat-sealable paperboard structure 40′ may include a basecoat 47 on the second major side 50 of the paperboard substrate 46′. In yet another variation, as shown in FIG. 5, the paperboard structure 40′ may include a first basecoat 45 between the paperboard substrate 46′ and the heat-sealable barrier coating 52′ and a second basecoat 47 on the second major side 50 of the paperboard substrate 46′.

[0027] Referring back to FIG. 4, the paperboard substrate 46 of the paperboard structure 40 may be (or may include) any cellulosic material that is capable of being coated with the heat-sealable barrier coating 52 and the top coat 54. Those skilled in the art will appreciate that the paperboard substrate 46 may be bleached or unbleached. Examples of appropriate paperboard substrates include corrugating medium, linerboard, solid bleached sulfate (SBS) and unbleached kraft.

[0028] The paperboard substrate 46 may have an uncoated basis weight of at least about 50 pounds per 3000 ft.sup.2. In one expression, the paperboard substrate 46 may have an uncoated basis weight ranging from about 60 pounds per 3000 ft.sup.2 to about 400 pounds per 3000 ft.sup.2. In another expression, the paperboard substrate 46 may have an uncoated basis weight ranging from about 80 pounds per 3000 ft.sup.2 to about 300 pounds per 3000 ft.sup.2. In another expression the paperboard substrate 46 may have an uncoated basis weight ranging from about 90 pounds per 3000 ft.sup.2 to about 250 pounds per 3000 ft.sup.2. In yet another expression the paperboard substrate 46 may have an uncoated basis weight ranging from about 100 pounds per 3000 ft.sup.2 to about 200 pounds per 3000 ft.sup.2.

[0029] Furthermore, the paperboard substrate 46 may have a caliper (thickness) ranging, for example, from about 4 points to about 30 points (0.004 inch to 0.030 inch). In one expression, the caliper range is from about 8 points to about 24 points. In another expression, the caliper range is from about 12 points to about 20 points.

[0030] One specific, nonlimiting example of a suitable paperboard substrate 46 is 13-point SBS cupstock manufactured by WestRock Company of Atlanta, Ga. Another specific, nonlimiting example of a suitable paperboard substrate 46 is 16.5-point SBS cupstock manufactured by WestRock Company. Yet another specific, nonlimiting example of a suitable paperboard substrate 46 is 18-point SBS cupstock manufactured by WestRock Company.

[0031] The heat-sealable barrier coating 52 may be applied to the first major side 48 of the paperboard substrate 46 using any suitable method, such as one or more coaters either on the paper machine or as off-machine coater(s). When heated, a heat-seal coating provides an adhesion to other regions of product with which it contacts.

[0032] The heat-sealable barrier coating 52 may be applied to the paperboard substrate 46 at various coat weights. In one expression, the heat-sealable barrier coating 52 may be applied at a coat weight of about 4 to about 20 pounds per 3,000 ft.sup.2, as dried. In another expression, the heat-sealable barrier coating 52 may be applied at a coat weight of about 6 to about 16 pounds per 3,000 ft.sup.2, as dried. In yet another expression, the heat-sealable barrier coating 52 may be applied at a coat weight of about 8 to about 12 pounds per 3,000 ft.sup.2, as dried.

[0033] The heat-sealable barrier coating 52 may include a binder and a pigment. In one expression, the ratio of the pigment to the binder may be at most 1 part (by weight) pigment per 1 part (by weight) binder. In another expression, the ratio of the pigment to the binder may be about 1:1 to about 1:9 by weight. In yet another expression, the ratio of the pigment to the binder can be about 1:2 to about 1:6 by weight. In yet another expression, the ratio of the pigment to the binder can be about 1:3 to about 1:4 by weight.

[0034] In one particular implementation, the binder of the heat-sealable barrier coating 52 may be an aqueous binder. As one general, non-limiting example, the binder may be a latex. As another general, non-limiting example, the binder may be a water based acrylic emulsion polymer. A specific, non-limiting example of a suitable binder is presented in Table 2. Other aqueous binders are also contemplated, such as styrene-butadiene rubber (SBR), ethylene acrylic acid (EAA), polyvinyl acetate (PVAC), polyvinyl acrylic, polyester dispersion, and combinations thereof.

[0035] The pigment component of the heat-sealable barrier coating 52 may be (or may include) various materials. Several non-limiting examples of suitable inorganic pigments are presented in Table 1. Other pigments, such as plastic pigments, titanium dioxide pigment, talc pigment and the like, may be used without departing from the scope of the present disclosure.

[0036] In one variation, the pigment component of the heat-sealable barrier coating 52 may be a clay pigment. As one example, the clay pigment may be platy clay, such as a high aspect ratio platy clay (e.g., an average aspect ratio of at least 40:1, such as an average aspect ratio of at least 60:1).

[0037] In another variation, the pigment component of the heat-sealable barrier coating 52 may be a calcium carbonate (CaCO.sub.3) pigment. As one example, the CaCO.sub.3 pigment may be a coarse ground CaCO.sub.3 with a particle size distribution wherein about 60 percent of the particles are less than 2 microns. As another example, the CaCO.sub.3 pigment may be a fine ground CaCO.sub.3 with a particle size distribution wherein about 90 percent of the particles are less than 2 microns.

[0038] In yet another variation, the pigment component of the heat-sealable barrier coating 52 may be a pigment blend that includes both calcium carbonate pigment and clay pigment.

[0039] The top coat 54 is applied to the heat-sealable barrier coating 52 to form a discontinuous layer 56 over (e.g., directly adjacent) the heat-sealable barrier coating 52. Various techniques may be used for forming the discontinuous layer 56 of top coat 54 over the heat-sealable barrier coating 52, such as one or more coaters either on the paper machine or as off-machine coater(s).

[0040] The top coat 54 may be applied to the heat-sealable barrier coating 52 at various coat weights to achieve the discontinuous layer 56 of top coat 54. In one expression, the top coat 54 may be applied at a coat weight of about 0.1 to 4.0 pounds per 3,000 ft.sup.2, as dried. In another expression, the top coat 54 may be applied at a coat weight of about 0.5 to 3.0 pounds per 3,000 ft.sup.2, as dried. In another expression, the top coat 54 may be applied at a coat weight of about 0.5 to 2.5 pounds per 3,000 ft.sup.2, as dried. In yet another expression, the top coat 54 may be applied at a coat weight of about 0.5 to 2.0 pounds per 3,000 ft.sup.2, as dried.

[0041] Referring to FIG. 6, an SEM was used to show a top view of the discontinuous layer 56 of top coat 54 deposited onto the heat-sealable barrier coating 52. The areas with the heat-sealable barrier coating 52 are darker, while the areas with both the heat-sealable barrier coating 52 and the top coat 54 are brighter.

[0042] Referring to FIG. 7, an SEM was used to show a cross-section view of the discontinuous layer 56 of top coat 54 deposited onto the heat-sealable barrier coating 52.

[0043] The top coat 54 may include a binder and a pigment. The pigments and binders useful for the heat-sealable barrier coating 52 may also be used in the top coat 54. However, the pigment-to-binder ratio of the top coat 54 may be significantly different from the pigment-to-binder ratio of the heat-sealable barrier coating 52. In one expression, the ratio of the pigment to the binder in the top coat 54 can be at least about 1 part (by weight) pigment per 1 part (by weight) binder. In another expression, the ratio of the pigment to the binder in the top coat 54 can be about 1:1 to about 10:1 by weight. In another expression, the ratio of the pigment to the binder in the top coat 54 can be about 1:1 to about 5:1 by weight. In yet another expression, the ratio of the pigment to the binder in the top coat 54 can be about 2:1 to about 4:1 by weight.

[0044] In one particular implementation, the binder of the top coat 54 may be an aqueous binder. As one general, non-limiting example, the binder may be a latex. As another general, non-limiting example, the binder may be a water based acrylic emulsion polymer. A specific, non-limiting example of a suitable binder is presented in Table 2. Other aqueous binders are also contemplated, such as styrene-butadiene rubber (SBR), ethylene acrylic acid (EAA), polyvinyl acetate (PVAC), polyvinyl acrylic, polyester dispersion, and combinations thereof.

[0045] The pigment component of the top coat 54 may be (or may include) various materials. Several non-limiting examples of suitable inorganic pigments are presented in Table 1. Other pigments, such as plastic pigments, titanium dioxide pigment, talc pigment and the like, may be used without departing from the scope of the present disclosure.

[0046] In one variation, the pigment component of the top coat 54 may be a clay pigment. As one example, the clay pigment may be platy clay, such as a high aspect ratio platy clay (e.g., aspect ratio of at least 40:1).

[0047] In another variation, the pigment component of the top coat 54 may be a calcium carbonate (CaCO.sub.3) pigment. As one example, the CaCO.sub.3 pigment can be a coarse ground CaCO.sub.3 with a particle size distribution wherein about 60 percent of the particles are less than 2 microns. As another example, the CaCO.sub.3 pigment can be a fine ground CaCO.sub.3 with a particle size distribution wherein about 90 percent of the particles are less than 2 microns.

[0048] Referring back to FIG. 1, the bottom wall 18 of the container 10 may be formed from a paperboard structure, such as the heat-sealable paperboard structure 40 shown in FIG. 4 or the heat-sealable paperboard structure 40′ shown in FIG. 5. However, various other paperboard structures may be used to form the bottom wall 18, such as when printability of the bottom wall 18 is of little or no concern.

EXAMPLES

[0049] Experiments were conducted to evaluate the use of a discontinuous layer of top coat over a heat-sealable barrier coating in connection with a paperboard structure. One heat-sealable barrier coating formulation (BC1) and one top coat formulation (TC1) were prepared and used in the experiments. The pigments used in the formulations are presented in Table 1. The binder used in the formulations are presented in Table 2. The heat-sealable barrier coating formulation (BC1) is presented in Table 3. The top coat formulation (TC1) is presented in Table 4.

TABLE-US-00001 TABLE 1 Name Pigment Description CL-1 BARRISURF ™ XP (IMERYS Platy clay with high Kaolin, Georgia) aspect ratio CC-1 HYDROCARB ® 60 (Omya AG Coarse ground CaCO.sub.3 of Oftringen, Switzerland) (particle size 60% <2 micron) CC-2 HYDROCARB ® 90 (Omya AG) Fine ground CaCO.sub.3 (particle size 90% <2 micron)

TABLE-US-00002 TABLE 2 Name Binder Description SA-1 CARTASEAL ® SCR (Archroma, Water based acrylic emulsion Reinach, Switzerland) polymer, Tg of 30° C.

TABLE-US-00003 TABLE 3 Barrier Coating Formulation (in Parts) BC-1 CaCO.sub.3 (CC-1) 65 CaCO.sub.3 (CC-2) Clay (CL-1) 35 Binder (SA-1) 400

TABLE-US-00004 TABLE 4 Top Coat Formulation (in Parts) TC-1 CaCO.sub.3 (CC-1) CaCO.sub.3 (CC-2) 100 Clay (CL-1) Binder (SA-1) 50

[0050] The formulations were applied at various coat weights to 16.5-point solid bleached sulfate cupstock having a basis weight of 175 pounds per 3000 square feet. A blade coater was used to apply the heat-sealable barrier coating formulation to the felt side of the paperboard substrate. A blade coater was again used to apply the top coat formulation to the heat-sealable barrier coating, thereby yielding a two-layer coating on the felt side of the paperboard substrate. The examples and experimental results (Parker Print Surface Smoothness; Water Cobb; Coffee Cobb; blocking rating; and heat-sealablility) are shown in Table 5.

TABLE-US-00005 TABLE 5 Example Control 1 2 3 4 Barrier Coat Weight 9.8 (lb/3000 ft.sup.2) Top Coat Weight 0 0.5 1.0 1.8 3.0 (lb/3000 ft.sup.2) PPS (μm) 3.3 3.0 2.7 2.2 2.0 H.sub.2O Cobb (g/m.sup.2-30 min) 4.2 3.2 3.5 3.3 3.4 Coffee Cobb (g/m.sup.2-30 min) 13.8 10.1 8.5 9.4 8.1 Blocking Rating 2.0 1.9 1.6 1.1 0.8 (50° C./60 psi/24 h) Heat-Sealability (% fiber 100 100 100 95 95 tear)

[0051] Thus, using a discontinuous layer of top coat over the heat-sealable barrier coating of a paperboard structure provides a smooth surface, as evidenced by the Parker Print Surface (PPS-10S) smoothness results measured according to TAPPI standard T555. All examples of the disclosed heat-sealable paperboard structures exhibited PPS smoothness of 3 microns or less.

[0052] In addition to high smoothness, the examples also surprisingly exhibited excellent barrier properties, as evidenced by the 30-minute-water-Cobb results measured according to TAPPI Standard T441 om-04. For most cases, the additional discontinuous layer of the top coat improved or at least maintained the water barrier properties of the underneath heat-sealable barrier coating 52. All examples had 30-minute-water-Cobb ratings of less than 10 g/m.sup.2, with many below 4 g/m.sup.2.

[0053] A hot coffee variant of the Cobb test was also utilized to evaluate the water barrier of the examples shown in Table 5. This test was performed by substituting 23° C. water with 90° C. coffee but otherwise complying TAPPI Standard T441 om-04. The coffee used was obtained by brewing 36 g of Starbucks medium house blend ground coffee with 1100 mL of distilled water in a 12 cup Mr. Coffee maker. All of the examples shown in Table 5 had a 90° C. coffee Cobb rating of less than 15 g/m.sup.2 after 30 minutes, with most less than 10 g/m.sup.2 after 30 minutes.

[0054] Heat-sealability of the coated samples of Table 5 were evaluated on a PMC (Paper Machinery Corporation) cup machine, model PMC-1250, by using each of these samples as side wall for the cup and a control bottom stock for all the cups. Cups were all successfully formed, and fiber tear in percentage of the seam area upon tearing apart the heat-sealed side-wall seam was estimated. High fiber tear percentage means better heat-sealability. Samples 1 and 2 all exhibited 100% fiber tear similar as the control samples without a discontinuous layer of top coat, and samples 3 and 4 also showed excellent fiber tear of 95%.

[0055] Lastly, the blocking rating (50° C./60 psi/24 hrs), was less than 3.0 for all samples, indeed less than 2.1, and less than 1.0 for one sample. Table 6 defines the blocking test rating system.

TABLE-US-00006 TABLE 6 Rating Description 0 Samples fall apart without any force applied 1 Samples have a light tackiness but separate without fiber tear 2 Samples have a high tackiness but separate without fiber tear 3 Samples are sticky and up to 25% fiber tear or coat damage (area basis) 4 Samples have more than 25% fiber tear or coat damage (area basis)

[0056] The blocking behavior of the samples was tested by evaluating the adhesion between the barrier coated side and the other uncoated side. A simplified illustration of the blocking test is shown in FIG. 8. The paperboard was cut into 2-inch by 2-inch square samples. Several duplicates were tested for each condition, with each duplicate evaluating the blocking between a pair of samples 252, 254. (For example, if four duplicates were test, four pairs—eight pieces—would be used.) Each pair was positioned with the ‘barrier-coated’ side of one piece 252 contacting the uncoated side of the other piece 254. The pairs were placed into a stack 250 with a spacer 256 between adjacent pairs, the spacer being foil, release paper, or even copy paper. The entire sample stack was placed into the test device 200 illustrated in FIG. 8.

[0057] The test device 200 includes a frame 210. An adjustment knob 212 is attached to a screw 214 which is threaded through the frame top 216. The lower end of screw 214 is attached to a plate 218 which bears upon a heavy coil spring 220. The lower end of the spring 220 bears upon a plate 222 whose lower surface 224 has an area of one square inch. A scale 226 enables the user to read the applied force (which is equal to the pressure applied to the stack of samples through the one-square-inch lower surface 224).

[0058] The stack 250 of samples is placed between lower surface 224 and the frame bottom 228. The knob 212 is tightened until the scale 226 reads the desired force of 100 lbf (100 psi applied to the samples) or 60 lbf (60 psi applied to the samples). High pressure such as 1000 psi is achieved by reducing the lower surface area of 224 contacting the stack 250 of samples to 0.11 square inch, with an applied force of 110 lb. The entire device 200 including samples is then placed in an oven at 50° C. for 24 hours or 2 hours. The device 200 is then removed from the test environment and cooled to room temperature. The pressure is then released, and the samples removed from the device.

[0059] The samples were evaluated for tackiness and blocking by separating each pair of paperboard sheets. Blocking damage is visible as fiber tear, which if present usually occurs with fibers pulling up from the non-barrier surface of samples 254. If the non-barrier surface was coated with a print coating, then blocking might also be evinced by damage to the print coating.

[0060] For example, in as symbolically depicted in FIG. 8, samples 252(0)/254(0) might be representative of a “0” rating (no blocking). The circular shape in the samples indicates an approximate area that was under pressure, for instance about one square inch of the overall sample. Samples 252(3)/254(3) might be representative of a “3” blocking rating, with up to 25% fiber tear in the area that was under pressure, particularly in the uncoated surface of sample 254(3). Samples 252(4)/254(4) might be representative of a “4” blocking rating with more than 25% fiber tear, particularly in the uncoated surface of sample 254(4). The depictions in FIG. 8 are only meant to approximately suggest the percent damage to such test samples, rather than showing a realistic appearance of the samples.

[0061] Although various examples of the disclosed heat-sealable paperboard structures and methods have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The present application includes such modifications and is limited only by the scope of the claims.