Basecoat and associated paperboard structure

ES 2 397 590 T3

DESCRIPTION

Base layer and associated cardboard structure.

Field

The present patent application relates to coatings for cardboard and, more particularly, to base layers, as well as to smooth cardboard structures, formed with the base layers described.

Background

Cardboard is used in various packaging applications. For example, aseptic liquid carton for packaging is used for packaging beverage cartons, boxes in general, and the like. Therefore, customers often prefer a carton that has a generally smooth surface, with few imperfections, to facilitate high-quality printing of text and graphics, thereby increasing the visual appeal of the products packed in the carton.

Conventionally, the flatness of the board is achieved by a wet stack calendering process, in which the board is re-wetted and passed through a calendering device having two or more hard rollers. The wet stack calendering process smooths the board by compressing the fiber network, in order to reduce pits and cracks in the virgin board (see Figure 1).

The result is a smooth cardboard, which has a reduced thickness and volume, which is why its rigidity is also lower. However, stiffness is an important requirement for many carton applications, such as liquid and aseptic packaging carton. Therefore, preparing a smooth and rigid board at the same time, using a conventional wet stack calendering process, requires increasing the basis weight of the board, thus increasing the cost of the raw material substantially.

Alternatively, manufacturers have tried to smooth the surface of the cardboard by coating the entire surface of the cardboard with a base coat comprising various pigments, such as clay, calcium carbonate, TiO2 and the like, to then place on this base a second and in Sometimes a third coatings, which are usually called the protective layer. It has been found that by applying large amounts of relatively fine pigment particles to the surface of the board, a smoother surface is obtained, without requiring wet stack calendering, thus maintaining bulk. For example, as shown in Figure 2, it was found that optimal smoothness was obtained by applying relatively large amounts (e.g. 17.2 g / m²(10.6 pounds per 3000 square feet) or more) of relatively finely ground calcium carbonate — such as CARBITAL® 95 (Imerys Pigments, Inc. of Roswell, Georgia) —to the rough surface of the cardboard. By the way, it has been understood that the greater the amount of pigment applied to the surface of the cardboard, the better the resulting smoothness. However, the use of relatively high amounts of pigments substantially increases the cost of preparing a high quality, smooth board for printing.

Accordingly, there is a need to find a base layer and associated cardboard structure that maintain the bulk of the cardboard and offer the desired smoothness for high quality printing, while reducing manufacturing costs.

US patent document US 2003085012A describes a base coat for paper, comprising a mixture of pigments including, e.g. eg, a ground calcium carbonate component and a hyperlaminated kaolin (clay) component, where the hyperlaminated kaolin has an aspect ratio of at least about 70: 1 (also described, a aspect ratio of at least 100: 1). Cardboard structures are also described. Description of both the particle size of the calcium carbonate and the calcium carbonate content of the pigment mixture is omitted.

Compendium

In one aspect, the invention provides a base coat comprising a pigment mixture, which includes a ground calcium carbonate component and a hyperlaminated clay component, wherein said ground calcium carbonate component constitutes at least 10%. by weight of said mixture of pigments, and at most, 60% of said ground calcium carbonate component has a particle size of less than 2 microns, and wherein said hyperlaminated clay component has an average aspect ratio of at least 40: 1.

Preferably, said average aspect ratio of said hyperlaminated clay component is at least 70: 1, more preferably at least 90: 1.

ES 2 397 590 T3

Preferably, at most 35% of said ground calcium carbonate component has a particle size of less than 2 microns.

Preferably, said pigment mixture consists essentially of said hyperlaminated clay component and said ground calcium carbonate component.

Preferably, said base coat also comprises a carrier, wherein said mixture of pigments is dispersed in said carrier to form a suspension.

The invention also provides a cardboard structure comprising a cardboard substrate coated with a base layer as defined above, wherein said cardboard substrate has a basis weight of at least 0.138 kg / m² (85 lbs./3000 sq. Ft.) And the coating weight of that base coat, per side, is 14.6 g / m maximum.² (9 lbs. / 3000 sq. Ft.), Wherein said average aspect ratio of said hyperlaminated clay component is at least 70: 1.

In one embodiment, said base layer forms a discontinuous film on a surface of said cardboard substrate.

Preferably, said cardboard substrate is formed as a fabric of fibers, which define a plurality of holes in one of its surfaces, and wherein said base layer is housed substantially within said plurality of said holes without substantially covering said surface by full.

Preferably said base layer is applied to said cardboard substrate in a coating weight, per side, maximum of 14.6 g / m², more preferably, at most 13.0 g / m², most preferably, at most, of

11.4 g / m² of said cardboard substrate.

In one embodiment, the cardboard frame has a basis weight of at least 0.138 kg / m² (85 pounds per 3,000 square feet).

Brief description of the drawings

Figure 1 is a photograph of an uncoated surface of an exemplary cardboard substrate (ie, virgin material).

Figure 2 is a photographic comparison of the surface of a cardboard substrate coated with various amounts (in pounds per 3000 square feet) of fine ground calcium carbonate according to the prior art.

Figure 3 is a photographic comparison of the surface area of ​​a cardboard substrate coated with various amounts (in pounds per 3000 sq ft; 1 lb / 3000 sq ft = 1.63 g / m²) of the base layer described.

Figure 4 is a graphical illustration of percent void volume of sediment versus percent clay component for various pigment mixtures formulated with extra coarse ground calcium carbonate.

Figure 5 is a graphical illustration of percent void volume of sediment versus percent clay component for various pigment mixtures formulated with a coarse ground calcium carbonate.

Figure 6 is a graphical illustration of percent void volume of sediment versus percent clay component for various pigment mixtures formulated with a fine ground calcium carbonate.

Figure 7 is a first graphical comparison of Parker Print surface smoothness versus coating weight (lbs per 3000 sq ft; 1 lb / 3000 sq ft = 1.63 g / m²).

Figure 8 is a second graphical comparison of Parker Print surface smoothness versus coating weight (lbs per 3000 sq ft; 1 lb / 3000 sq ft = 1.63 g / m²).

Figure 9 is a side cross-sectional view of a cardboard substrate coated with the described base layer according to the described method.

And Figure 10 is a side cross-sectional view of the cardboard substrate of Figure 9 shown in a second larger magnification.

ES 2 397 590 T3

Detailed description

As indicated above, the base coat of the invention comprises a specific mixture of pigments, with a high aspect ratio between clay (average aspect ratio at least 40: 1) and calcium carbonate. The pigment mixture can be dispersed in a vehicle, such as water, to facilitate application of the base coat to an appropriate substrate, eg, a cardboard substrate. Additional components — such as binders, stabilizers, dispersing agents, and additional pigments — may be combined with the pigment blend to form the final basecoat, without departing from the scope of the present disclosure.

As used herein, the phrase "cardboard substrate" refers, in general terms, to any cardboard material that is capable of being coated with the described base layer. Those skilled in the art will appreciate that the cardboard substrate can be bleached or unbleached and is generally thicker and stiffer than paper. Generally, a cardboard substrate has an uncoated basis weight of about 0.138 kg / m²(85 pounds per 3,000 square feet) or more. Examples of suitable board substrates include a corrugated medium, linerboard, and solid bleached sulfate (SBS) board.

As used herein, the terms "aspect ratio" and "shape factor" refer to the geometry of individual clay particles, specifically, a comparison between a first dimension of a clay particle (eg ., the diameter or length of the clay particle) and a second dimension of the clay particle (eg, the thickness or width of the clay particle). The terms "hyperlaminate", "high aspect ratio" and "relatively high aspect ratio" refer to aspect ratios that are generally greater than 40: 1, such as 50: 1 or more, in particular 70: 1 or more, and preferably 90: 1 or more.

In a preferred embodiment, the clay component may include a rolled clay where, on average, the clay particles have a aspect ratio of about 50: 1 or more. An example of such a clay is CONTOUR® 1180, available from Imerys Pigments, Inc. of Roswell, Georgia. In another preferred embodiment, the clay component may include a rolled clay wherein, on average, the clay particles have a aspect ratio of about 90: 1 or more. An example of such a clay is XP-6100, also available from Imerys Pigments, Inc. Other examples of rolled clays from origin are described in US Patent No. 7,208,039 to Jones et al.

Optionally, the clay component of the pigment mixture can include a laminated clay having a relatively high particle size. In one embodiment, the clay component can have an average particle size of about 4 microns or more. In a second embodiment, the clay component can have an average particle size of about 1.0 microns or more. In a third particular embodiment, the clay component can have an average particle size of about 13 microns or more.

The calcium carbonate component in one embodiment can include a coarsely ground calcium carbonate. An example of such a coarse ground calcium carbonate is CARBITAL® 60, also available from Imerys Pigments, Inc., where about 60% of the calcium carbonate particles are less than about 2 microns in diameter. In another embodiment, the calcium carbonate component may include an extra coarse ground calcium carbonate. An example of such an extra coarse ground calcium carbonate is CARBITAL® 35, also available from Imerys Pigments, Inc., where only about 35% of the calcium carbonate particles are less than about 2 microns in diameter.

In another embodiment, the calcium carbonate component of the pigment mixture can have an average particle size of about 1 micron or more; such as about 1.5 microns, and more particularly 3 microns or more.

Far from being limited to any particular theory, it is believed that pigment mixtures that are formulated to provide a relatively high percentage of void volumes of sediment (i.e., more compressed condensation of the particles) offer a high level of smoothness to Relatively low coating weights, thereby reducing raw material costs. On the other hand, it is believed that by using a clay component having a relatively high aspect ratio and / or a relatively high average particle size and a calcium carbonate component having a relatively high average particle size, percentages are obtained. relatively high — and therefore desirable — void volume of sediment. For instance,

ES 2 397 590 T3 those volumes under vacuum of sediment that exceed 45% may be desirable, while volumes under vacuum of sediment that exceed a value of 47.5% may be even more desirable and even more so, volumes sediment vacuum that exceed 50%.

An appropriate technique for measuring the void volume percent of sediment includes preparing a sample of the pigment mixture having the desired weight percent of the clay component to the calcium carbonate component. The pigment mixture sample is then diluted with water, 50% by weight of the solids, to obtain a suspension. A 70 gram sample of the suspension is placed in a centrifuge tube and centrifuged at approximately 8000 g for a period of about 90 minutes. The sample is then removed from the centrifuge and the clear supernatant is separated and weighed. The sediment is generally compressed with sufficient density that the supernatant liquid can be easily poured out. Based on the weight of the water removed, it is possible to calculate the weight of the water that is still lodged in the sediment voids. Then, using the densities of the particles, the weight of the water in the voids can be converted to the void volume percent of sediment.

Referring to Figures 4 to 6, the percentage of sediment void volume for various pigment mixtures is provided versus the weight percentage of the clay component in the pigment mixture. Specifically, Figures 4 to 6 compare the use of CARBITAL ® 35 (extra coarse), CARBITAL® 60 (coarse) and CARBITAL® 95 (fine) as the components of calcium carbonate and XP-6100 (aspect ratio greater than 90: 1), CONTOUR® 1180 (aspect ratio close to 50: 1), CONTOUR® Xtrm (aspect ratio close to 45: 1) and KCS (aspect ratio close to 10: 1 (not a high aspect ratio clay)) such as clay component.

Figures 4 to 6 indicate that coarse ground calcium carbonate (Figures 4 and 5), in particular extra coarse ground calcium carbonate (Figure 4), and clays of high aspect ratio, in particular clays having a proportion dimensions greater than 70: 1, more particularly, greater than 90: 1 (XP-6100 clay), are those that contribute the maximum percentage of volume to the sediment void.

Likewise, the concave shape of the curves observed in Figures 4 to 6, in particular the curves associated with XP-6100 clay, indicates that the maximum percentage of sediment void volume is reached when the clay component is mixed with the calcium carbonate component. For example, referring to Figure 4, when extra coarse ground calcium carbonate and XP-6100 are used, the maximum void volume percent of sediment is obtained between about 60 and about 90% by weight of the clay component.

Furthermore, the concave shape of the curves indicates that certain mixtures of the clay component and the calcium carbonate component provide a void volume percentage of sediment that is similar, if not higher, than that obtained when using a 100% clay. percent aspect ratio. Thus, the curves indicate that combining a less expensive calcium carbonate with a more expensive high aspect ratio clay can yield a similar, if not superior, coating material in terms of void volume percent of sediment. By the way, when comparing figure 4 with figure 6 for example, the curves indicate that the thicker the calcium carbonate, the lower the high aspect ratio of the clay to be used must be if a higher percentage of calcium is to be achieved. sediment void volume. For example, referring to Figure 4, when extra-coarse ground calcium carbonate is mixed with XP-6100 clay, a 45:55 mix of the clay component to the calcium carbonate component provides the same volume percent to the sediment void than 100% of the high aspect ratio clay.

Referring to Figures 7 and 8, Parker Print Surface ("PPS", Parker Print Surface) smoothness values ​​of cardboard coated with various base layers in a pilot coater are presented with respect to the weight of the layer coating. base in pounds per ream (3000 square feet). 1 lb / ream = 1.63 g / m². Those skilled in the art will appreciate that PPS smoothness values ​​taken from samples prepared with a pilot coater are generally greater than PPS smoothness values ​​obtained from samples prepared on a full scale grinder. However, the PPS smoothness values ​​taken using a pilot coater are indicative of the improvement provided by the base coats described over prior art coatings. For reference, when using a pilot coater, PPS smoothness values ​​of about 7.0 microns or less are generally desirable. PPS smoothness values ​​of about 6.5 microns or less are preferred, and even more, PPS smoothness values ​​of about 6.0 microns or less are preferred.

Of particular interest, as shown in Figure 7, base coats that include coarse or extra coarse ground calcium carbonate and high aspect ratio clay, in particular XP-6100 clay, provide a relatively high percentage of volume. sediment vacuum and exhibit PPS smoothness values ​​typically below about 7 microns, by coating weight of about 14.6 g / m² (9 pounds per ream) or below a cardboard substrate. By the way, as represented by the

On the positive slope of the curves in Figure 7, the best smoothness (ie, a lower PPS smoothness value) of the resulting board is directly correlated with the lowest coating weights. These data are contrary to the expectations of those skilled in the art, who would expect to obtain higher smoothness values ​​with high coating weights.

Certainly, when a full scale crusher was used, a base coat that included a 50:50 blend of CARBITAL® 35 pigments (olid calcium carbonate) and XP-6100 (high aspect ratio clay and large average particle size ) provided a PPS smoothness value of approximately 2 microns, at a relatively low coating weight of 9.8 g / m² (6 pounds per ream).

Consequently, the substrates of a coating, such as a board with base layers comprising ground calcium carbonate - in particular, coarse or extra-coarse ground calcium carbonate - and high aspect ratio clay - in particular, clay having a high aspect ratio size greater than about 70: 1, more particularly high aspect ratio clay having a relatively high average particle size - provide a smooth cardboard structure, without sacrificing bulk, as well as reducing manufacturing cost by combining a less expensive rolled clay with a less expensive calcium carbonate, while requiring surprisingly low coating weights to achieve the desired smoothness.

Likewise, those skilled in the art will appreciate that the type of high aspect ratio clay selected and the type of ground calcium carbonate chosen, as well as the ratio of the clay component to the calcium carbonate component, may be governed by considerations of costs, in view of the desired smoothness.

The base layers described can be applied to the surface of a substrate, such as cardboard (eg liquid and aseptic packaging cardboard), in an amount sufficient to fill the holes and cracks in the substrate, without having to cover the entire surface of it. Therefore, the described basecoat can be used in conjunction with the described method of applying the basecoat, in order to obtain a high surface smoothness, with a relatively small amount of basecoat. Indeed, as discussed above, a great surface smoothness can be achieved with an unexpectedly small amount of the base coat described.

In one embodiment, the base coat is applied to the substrate using a knife coater, because the knife coater pushes the base coat into the holes and fissures found in the substrate, while removing the base coat from the surface of the substrate. substratum. Specifically, as shown in Figures 9 and 10, the basecoat can be applied in a manner that is more akin to caulking, where substantially all of the basecoat resides in the pits and cracks in the surface of the substrate, rather than on the surface of the substrate.

By now, those skilled in the art will appreciate that when the described basecoat is used in a knife coater, the gap between the moving substrate and the coater knife can be minimized to facilitate hole filling. and surface cracks, without substantially depositing the base layer on the surface of the substrate (ie, forming a discontinuous film on the surface of the substrate). In other words, the coater blade can be located close enough to the surface of the moving substrate such that the coater blade pushes the base coat into pits and cracks in the substrate surface, removing the same time the surplus of the base layer from the surface of the substrate.

Example 1

A first pigment blend prepared according to one aspect of the present disclosure includes 50% by weight of CARBITAL® 35 (coarse ground calcium carbonate) and 50% by weight of XP-6100 (hyperlaminated clay). In a stationary mixer, a coating formulation is prepared by combining the 50:50 pigment mix with water, latex binders, and a thickening agent. Water is added in sufficient quantity to form a suspension. Using a blade coater in the manner described above, the coating formulation is applied to the paperboard stock, which has a basis weight of approximately 205 g / m2.² (126 pounds per 3000 sq. Ft.) At the following coating weights: 10.9; 12.9;

14.5 and 18.4 g / m² (6.7, 7.9, 8.9, and 11.3 pounds per 3000 sq. Ft.). The photographic results are shown in Figure 3 and the PPS smoothness values ​​are presented in Figure 7 (data points are circled).

Thus, as shown in Figure 3, the described base coat and associated method provide optimum smoothness at relatively low coating weights. (Compare figure 2 with figure 3.) Specifically, the greatest smoothness is achieved at a coating weight of 10.9 g / m² (6.7 lbs. Per 3000 sq. Ft.), Achieving good smoothness at 12.9 g / m² (7.9 lbs. Per 3000 sq. Ft.), With less smoothness at 14.5 g / m² (8.9 lbs. Per 3000 sq. Ft., And even less smoothness at 18.4 g / m² (11.3 pounds per 3000 square feet).

Example 2

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A second pigment blend prepared according to one aspect of the present disclosure includes 50% by weight of OMYA HYDROCARB® 60 (coarse ground calcium carbonate, available from Omya AG of Oftringen, Switzerland) and 50% by weight of XP-6170 (clay hyperlaminate available from Imerys Pigments, Inc.). In a stationary mixer, a coating formulation is prepared by combining the 50:50 pigment mix with water, latex and starch binders and a thickening agent. Water is added in sufficient quantity to form a suspension. Using a blade coater in the manner described above, the coating formulation is applied to the paperboard stock, which has a basis weight of approximately 173 g / m2.² (106 pounds per 3,000 square feet), at the following coating weights:

9.5 and 11.1 g / m² (5.8 and 6.8 lbs. Per 3000 sq. Ft.), Thus providing cardboard structures with improved smoothness at relatively low coating weights.

Accordingly, at this point those skilled in the art will appreciate that basecoats formulated in accordance with the present disclosure to include coarse ground calcium carbonate — in particular, extra coarse ground calcium carbonate — and hyperlaminated clay — in particular, hyperlaminated clays. - whose dimensional ratios exceed approximately 70: 1 and, more particularly, Clays of high dimensional ratios having a relatively high average particle size (e.g., close to 10 microns or more), provide greater surface smoothness at relatively low coating weights, particularly when applied to the substrate using the described method.

While the pigment mixtures discussed above include rolled clay and ground calcium carbonate, in particular coarse ground calcium carbonate, those skilled in the art will appreciate that it is possible to use alternative pigment mixtures, without departing from the scope of the present disclosure. For example, the described basecoat pigment mixture may include a laminated clay and one or more additional inorganic pigment, other than ground calcium carbonate, such as precipitated calcium carbonate, talc, or kaolin clay.