Basecoat and associated paperboard structure

[0001] The present patent application relates to the coating of cardboard, and more particularly to basecoats and also to smooth cardboard structures formed using the disclosed basecoats. BACKGROUND OF THE INVENTION [0002] Cardboard is used in various packaging applications. For example, aseptic cardboard fluid packaging are used as cardboard packaging for beverages, boxes and the like. Thus, customers often prefer cardboard with a generally smooth surface with few imperfections, which facilitates the printing of text and graphics in high quality, increasing the attractiveness of products wrapped in cardboard.

[0003] By default, the smoothness of the carton is achieved by a wet calendering process in which the carton is wetted and passed through a calendering device having two or more hard rollers. The wet calendering process smooths the carton by squeezing the fiber network to reduce cavities and cracks in the cardboard raw material (see Fig. 1).

[0004] The end result is a smooth cardboard with reduced cardboard thickness and weight and therefore reduced rigidity. However, rigidity is an important requirement in many carton applications, such as aseptic carton for liquid packaging. Thus, the preparation of a smooth, yet rigid carton using standard wet calendering requires an increase in the weight of the carton, which substantially increases the cost of raw material.

[0005] Alternatively, manufacturers are trying to smooth the cardboard surface by coating the entire surface of the cardboard with a base coat containing various pigments, such as clay, calcium carbonate, TiO₂ and the like, followed by coating this base with a second and sometimes a third coating which is generally referred to as topcoat. It has been found that high amounts of relatively fine pigment particles applied to the surface of the cardboard provide a smoother surface without the need for wet calendering, which allows weight to be retained. For example, as shown in Fig. 2, it has been found that relatively high amounts (e.g., 17.2 g / m2)² (10.6 pounds at 3000 ft²) or more) relatively finely ground calcium carbonate, such as CARBITAL® 95 (Imerys Pigments, Inc. Roswell, Georgia), applied to the rough surface of the cardboard provided the greatest smoothness. It should be understood that the more pigment applied to the surface of the cardboard, the greater the smoothness. However, the use of relatively high amounts of pigments substantially increases the cost of preparing a smooth and highly printable carton.

[0006] Accordingly, there is a need for a base coating and related carton structure that maintains the weight of the carton that provides the desired smoothness for high-quality printing while reducing manufacturing cost.

[0007] US 2003085012A discloses a paper base coating comprising a mixture of pigments including, e.g., a ground calcium carbonate component and a hyper-platelet kaolin component, wherein the hyper-platelet kaolin has an aspect ratio of at least about 70: 1 (aspect ratio

- at least 100: 1 is also disclosed.) Cardboard structures are also revealed. The calcium carbonate particle size and calcium carbonate content of the pigment blend are not disclosed.

SUMMARY [0008] In one aspect, the invention provides a base coating comprising a pigment mix comprising a milled calcium carbonate component and a hyperplated clay component, wherein said milled calcium carbonate component is at least 10 weight percent of said pigment mix and at most 60 percent of said milled carbonate component calcium has a particle size less than 2 microns, and wherein said hyperplated clay component has an aspect ratio of at least 40: 1.

[0009] Preferably, said average aspect ratio of said hyperplastic clay component is at least 70: 1, and more preferably at least 90: 1.

[0010] Preferably, at least 35 percent of said ground calcium carbonate component has a particle size less than 2 micrometers.

[0011] Preferably, said pigment mixture consists essentially of said hyperplasia clay component and said ground calcium carbonate component.

[0012] Preferably, said base coating also comprises a carrier, wherein said pigment blend is suspended in said carrier forming a suspension.

[0013] The invention also provides a cardboard structure comprising a cardboard substrate coated with a base coating as defined above, wherein said cardboard substrate has a basis weight of at least 0.138kg / m² (85 pounds / 3,000 ft²) and the weight of the coating, on each side, of said base coating at a maximum of 14.6 g / m2² (9 lb / 3,000 ft²), wherein said average aspect ratio of said hyperplated clay component is at least 70: 1.

[0014] In one embodiment, said base coating forms a discontinuous film on the surface of said cardboard structure.

[0015] Said cardboard substrate is preferably formed of a network of fibers defining a plurality of depressions in its surface, and wherein said base coating is substantially received within said plurality of said depressions without substantially coating said surface.

[0016] Preferably, said base coating is applied to said cardboard substrate, on each side, in an amount of at most 14.6 g / m²and more preferably at most 13.0 g / m2²and most preferably at most 11.4 g / m2² said cardboard substrate.

[0017] In one embodiment, the cardboard structure has a basis weight of at least 0.138kg / m² (85 pounds per 3000 square feet).

BRIEF DESCRIPTION OF THE DRAWINGS [0018] Fig. 1 is a photograph of the uncoated surface of an exemplary cardboard substrate (i.e., raw material).

[0019] Fig. 2 shows a photographic comparison of the surface of a cardboard substrate coated with various amounts (in pounds at 3000 ft²) a finely ground calcium carbonate component according to prior art;

[0020] Fig. 3 shows a photographic comparison of the surface of a cardboard substrate coated with various amounts (in pounds at 3000 ft²; 11 b / 3,000ft² = 1.63 g / m²) the disclosed base coat;

[0021] Fig. 4 is a graphic illustration of the percentage intergranular volume of sediment as a function of the percentage of clay component for various mixtures of pigments formulated with very coarsely ground calcium carbonate;

[0022] Fig. 5 is a graphic illustration of the percentage intergranular volume of sludge depending on the clay component for various mixtures of pigments formulated with coarsely ground calcium carbonate;

[0023] Fig. 6 is a graphic illustration of the percentage intergranular volume of sediment as a function of the percentage of clay component for various mixtures of pigments formulated with finely ground calcium carbonate;

[0024] Fig. 7 is a first graphical comparison of the smoothness of Parker Print Surface depending on the weight of the coating (in pounds at 3000 ft²; 1lb / 3,000ft² = 1.63 g / m²);

[0025] Fig. 8 is a second graphical comparison of the smoothness of Parker Print Surface depending on the weight of the coating (in pounds at 3000 ft²; 1 lb / 3,000ft² = 1.63 g / m²);

[0026] Fig. 9 is a side cross-sectional view of a cardboard substrate coated with the disclosed base coat according to the disclosed method; and [0027] Fig. 10 is a side cross-sectional view of the cardboard substrate of Fig. 9, shown in a second, larger, enlargement.

DETAILED DESCRIPTION [0028] As noted above, the base coating according to the invention may contain a specific blend of pigments with high aspect ratio clays (average aspect ratio at least 40: 1) and calcium carbonate. The pigment mix may be suspended in a carrier, such as water, to facilitate the application of the base coat to a suitable substrate, such as a cardboard substrate. Additional components such as binders, stabilizers, suspending agents and additional pigments may be combined with the pigment blend to form a final base coat without departing from the scope of the present disclosure.

[0029] The term "cardboard substrate" as used herein generally refers to any cardboard material suitable for coating with the disclosed base coat. It will be obvious to those skilled in the art that the cardboard substrate may be bleached or unbleached, and is usually thicker and stiffer than paper. Basically, the cardboard substrate has an uncoated basis weight of about 0.138kg / m2² (85 pounds at 3000 ft²) or greater. Suitable examples of cardboard substrates include corrugated medium, flat cardboard and cellulose bleached solid cardboard (SBS).

[0030] The terms "aspect ratio" and "shape factor" as used herein refer to the geometry of individual clay particles, and in particular to comparing the first dimension of the clay particle (e.g., diameter or length of the clay particle) to the second dimension of the clay particle (e.g., thickness or width of the clay particle ). The terms "hyper plate", "high aspect ratio" and "relatively high aspect ratio" refer to aspect ratios typically exceeding 40: 1, such as 50: 1, in particular 70: 1 or higher, and preferably 90: 1 or higher.

[0031] In a preferred embodiment, the clay component may comprise platelet clay, wherein the clay particles have an average aspect ratio of about 50: 1 or higher. An example of such clay is

CONTOUR® 1180 available from Imerys Pigments, Inc. from Roswell, Georgie. In another preferred embodiment, the clay component may include lamellar clay where the clay particles have an average aspect ratio of about 90: 1 or higher. An example of such clay is ΧΡ-6100, also

- available from Imerys Pigments, Inc. Additional examples of suitable platelet clays are disclosed in US Patent No. 7,208,039 to Jones et al.

[0032] Optionally, the clay component of the pigment mix can consist of lamellar clay with a relatively high average particle size. In one embodiment, the clay component may have an average particle size of about 4 microns or more. In a second embodiment, the clay component may have an average particle size of about 1.0 microns or more. In a third particular embodiment, the clay component may have an average particle size of about 13 microns or more.

[0033] The calcium carbonate component in one embodiment may contain coarsely ground calcium carbonate. An example of such a coarsely ground calcium carbonate is CARBITAL® 60, also available from Imerys Pigments, Inc., where about 60 percent of the calcium carbonate particles have a diameter less than 2 micrometers. In another embodiment, the calcium carbonate component may contain very coarsely ground calcium carbonate. An example of such extra coarsely ground calcium carbonate is CARBITAL® 35, also available from Imerys Pigments, Inc., where only about 35 percent of the calcium carbonate particles have a diameter less than 2 micrometers.

[0034] In another embodiment, the calcium carbonate component of the pigment mix may have an average particle size of about 1 micron or larger, such as about 1.5 micron, and in particular, 3 microns or larger.

[0035] Without being limited to any particular theory, it is believed that pigment mixtures that are formulated to provide a relatively high percentage of inter-grain volume of sludge (i.e. denser particle packing) provide greater smoothness at relatively low coating weights, which reduces costs raw materials. In addition, it is believed that the use of a clay component having a relatively high aspect ratio and / or a relatively high average particle size and a calcium carbonate component with relatively high average particle sizes leads to a relatively high, and thus desirable, percentage of inter-grain size sludge. For example, inter-grain volumes exceeding 45 percent may be desirable, while inter-grain volumes of sludge exceeding 47.5 percent may be more desirable, and inter-grain volumes of sludge exceeding 50 percent may be even more desirable.

[0036] One suitable technique for measuring the intergranular volume of sludge is to prepare a sample of a pigment mix with the desired content in percent by weight of the clay component relative to the calcium carbonate component. The sample of the pigment mix is ​​then diluted with water to a solids content of 50 weight percent to provide a slurry. 70 grams of suspension is placed in a centrifuge tube and centrifuged at approximately 8000g for approximately 90 minutes. The sample is then removed from the centrifuge and the transparent liquid supernatant is separated and weighed. The pellet is usually densely packed enough for the liquid supernatant to be easily drained off. Based on the weight of the water removed, the water content remaining in the inter-grain volume of the sludge can be calculated. Then, based on the density of the particles, the water weight in the gaps can be converted into a percentage of the inter-grain volume of the sludge.

[0037] Referring to Figs. 4-6, a percentage inter-grain volume of sludge is provided for various pigment mixtures depending on the weight percentage of the clay component in the pigment mixture. In particular, Figs. 4-6 compare the use of CARBITAL® 35 (very thick), CARBITAL® 60 (thick) and CARBITAL® 95 (fine) in terms of the carbonate component

-5 calcium ΧΡ-6100 (aspect ratio greater than 90: 1), CONTOUR® 1180 (aspect ratio greater than 50: 1), CONTOURU® Xtrm (aspect ratio greater than 45: 1) and KCS (aspect ratio approximately 10: 1 ( clay with a non-high aspect ratio)) in terms of the clay component.

[0038] Figs. 4-6 indicate that coarsely ground calcium carbonate (Figs. 4 and 5), in particular very coarsely ground calcium carbonate (Fig. 4) and clays with a high aspect ratio, in particular clays with an aspect ratio above 70 : 1, especially above 90: 1 (clay 61-6100), provide the highest percentage of inter-grain volume of sludge.

[0039] In addition, the concave shape of the curves in Figs. 4-6, in particular the curves associated with the Yap-6100 clay, indicates that the maximum percent intergranular volume of the sediment is achieved when the clay component is mixed with the calcium carbonate component. For example, with reference to Fig. 4, when very coarsely ground calcium carbonate and ΧΡ-6100 are used, the maximum percentage of intergranular volume of sediment occurs between about 60 and about 90 weight percent of the clay component. [0040] In addition, the concave shape of the curves indicates that certain blends of the clay component and the calcium carbonate component parts provide a percent intergranular volume of sludge that is similar, if not larger, using clay with a 100% high aspect ratio. Thus, the curves indicate that mixing cheaper calcium carbonates with more expensive high form factor clays can provide even, if not better, coating material in terms of percentage of intergranular sludge volume. In fact, when comparing Fig. 4 to Fig. 6, for example, the curves indicate that thicker calcium carbonate, clay with a lower high aspect ratio, must be used to obtain a higher percentage of intergranular sludge. For example, with reference to Fig. 4, when very coarsely ground calcium carbonate is mixed with 61-6100 clay, a 45:55 mix of the clay component to the calcium carbonate component provides the same percentage of inter-grain volume of sludge as 100 percent high-shape clay.

[0041] Referring to Figs. 7 and 8, Parker Print Surface ("PPS") smoothness values ​​of cardboard coated with various basecoats on a coating machine are shown with reference to the weight of the basecoat in pounds per ream (3000 ft)²), 11b / ream = 1.63 g / m². It will be obvious to those skilled in the art that PPS smoothness values ​​taken from samples prepared on a coating machine will be substantially higher than the PPS smoothness values ​​obtained from samples prepared on a full-size mill. However, the PPS smoothness values ​​taken from the coating machine are an indicator of the improvement provided by the disclosed basecoats over prior art coatings. For comparison, when a coating machine is used, generally PPS smoothness values ​​of about 7.0 micrometers or less are required, PPS smoothness values ​​of about 6.5 micrometers or less are preferred, and PPS smoothness values ​​of about 6.0 micrometers or more are preferred smaller.

[0042] In particular, as shown in Fig. 7, basecoats comprising thick or very thick calcium carbonate and clays with a high aspect ratio, in particular ΧΡ-6100 clay, provide relatively high percent grain interstitial volumes and exhibit PPS smoothness values ​​substantially less than about 7 microns, with coating weights of about 14.6 (9 pounds per ream) or less on a cardboard substrate. In fact, as it results from the positive slope of the curves in Fig. 7, the better smoothness (i.e. lower PPS smoothness value) of the resulting cardboard is

-6 directly related to the lower weight of the coating. These data are contrary to the expectations of experts, who would expect higher smoothness values ​​at higher coating weights.

[0043] In fact, when a full-size mill was used, the base coating comprising a 50:50 mix of CARBITAL® 35 (ground calcium carbonate) and ΧΡ-6100 pigments (clays with a high shape index and high average particle size) produced a PPS smoothness value of about 2 micrometers at relatively low coating weight of 9.8 g / m2² (6 pounds per ream).

[0044] Accordingly, the coating of substrates, such as cardboard, with basecoats containing ground calcium carbonate, in particular coarse or very coarsely ground calcium carbonate, clays with a high aspect ratio, in particular clays with an aspect ratio exceeding about 70: 1, and in particular clays with a high aspect ratio having a relatively high average particle size, creates a smooth cardboard structure without losing weight and reduces production costs by combining more expensive lamella clay with cheaper ground calcium carbonate, requiring surprisingly low coating weights to achieve the required smoothness.

[0045] In addition, it will be obvious to those skilled in the art that the type of clay selected with a high aspect ratio and the type of ground calcium carbonate selected, as well as the ratio of the clay component to the calcium carbonate component, may be dictated by cost considerations, taking into account the required smoothness.

[0046] The disclosed basecoats can be applied to a substrate surface, such as a cardboard (e.g., aseptic cardboard packaging for a fluid), in an amount sufficient to fill cavities and cracks in the substrate without having to coat the entire surface of the substrate. Thus, the disclosed base coat together with the disclosed base coat method can be used to obtain a high surface smoothness with a relatively small amount of base coat. In fact, as disclosed above, high surface smoothness can be achieved with an unexpectedly low amount of the base coat disclosed.

[0047] In one embodiment, the base coating is applied to the substrate using a scraper coater in such a way that the scraper coaster pushes the base coat into the cavities and cracks in the substrate, removing the base coating from the surface of the substrate. In particular, as shown in Fig. 9 and 10, the base coating may be applied in a manner that more resembles sealing, where substantially all the base coating is in cavities and cracks in the substrate surface instead of on the substrate surface.

[0048] At this stage, it will be obvious to those skilled in the art that when the disclosed base coat is used in a scraper spreader, the distance between the moving substrate and the coater scraper can be minimized allowing filling cavities and cracks in the surface without significantly applying the base coat to the surface of the substrate ( e.g. forming a discontinuous film on the surface of the substrate). In other words, the coater scraper can be positioned close enough to the surface of the moving substrate in such a way that the coater scraper pushes the base coating into the cavities and cracks in the surface of the substrate, removing excess base coating from the surface of the substrate.

EXAMPLE 1 [0049] The first pigment blend prepared according to an aspect of the present disclosure comprises a weight percentage of CARBITAL® 35 (coarsely ground calcium carbonate) and 50 weight percent

ΧΡ-6100 (hyperplasia clay). In a stationary mixer, the coating formulation is prepared by combining a 50:50 mix of pigments with water and latex substances

-7 binders and thickener. The amount of water added is sufficient to make a suspension. Using a scraper coater as described above, the coating formulation is applied to a cardboard raw material with a weight of about 205 g / m2.² (126 pounds at 3000 ft²) with the following coating weights: 10.9, 12.9, 14.5 and 18.4 g / m2² (6.7, 7.9, 8.9 and 11.3 pounds at 3000 ft²). Photographic results are shown in Fig. 3, and PPS smoothness values ​​are provided in Fig. 7 (circled data points).

[0050] Thus, as shown in Fig. 3, the disclosed base coat and associated method provides optimal smoothness at relatively low coating weights. (Compare Fig. 2 with Fig. 3) In particular, the highest smoothness is achieved with a coating weight of 10.9 g / m2² (6.7 pounds at 3000 ft²), good smoothness is achieved at 12.9 g / m2² (7.9 pounds at 3000 ft²), lower smoothness is achieved at 14.5 g / m² (8.9 pounds at 3000 ft²), and even less smoothness at 18.4 g / m2² (11.3 pounds at 3000 ft²).

EXAMPLE 2 [0051] The second pigment mix prepared according to an aspect of the present disclosure includes 50 percent by weight of OMYA HYDROCARBU 60 (coarsely ground calcium carbonate available from Omya AG from Oftringen and 50 percent by weight 50 61-6170 (hyperplant clay available from Imerys Pigments, Inc. ). In a stationary mixer, the coating preparation is prepared by combining a 50:50 pigment mix with latex water and starch binders and a thickener. The amount of water added is sufficient to make a suspension. Using a scraper coater in the manner described above, the coating preparation is applied to a cardboard raw material with a basis weight of about 173 g / m² (106 pounds at 3000 ft²) with coating weights of 9.5 and 11.1 g / m2² (5.8 and 6.8 pounds at 3000 ft²), thereby providing cardboard structures with better smoothness at relatively low coating weights.

[0052] Accordingly, at this stage, it is obvious to those skilled in the art that basecoats formed according to the present disclosure comprising coarsely ground calcium carbonate, in particular very coarsely ground calcium carbonate and hyperplant clay, in particular hyperplant clay clays with a shape ratio exceeding about 70: 1, in particular, high-ratio hyperplasmic clays having a relatively high average particle size (e.g. about 10 microns and larger), provide greater surface smoothness at relatively low coating weights, in particular when applied to a substrate using the disclosed method.

[0053] When the pigment mixtures disclosed above include lamellar clay and ground calcium carbonate, in particular coarsely ground calcium carbonate, it will be obvious to those skilled in the art that alternative pigment mixtures may be used without departing from the scope of the present disclosure. For example, the pigment blend of the disclosed basecoat may include lamellar clay and one or more additional inorganic pigments other than ground calcium carbonate, such as precipitated calcium carbonate, talc or kaolin.