Method for coating dry finish paperboard

The present patent application relates to cardboard coating methods, and in particular to methods for coating cardboard with dry varnish, which results in a smooth cardboard structure.

BACKGROUND OF THE INVENTION [0002] Paper or cardboard substrates used for printing and packaging are usually required to have good optical properties, excellent smoothness and excellent printability. In addition, such strength and stiffness are required that the substrates can pass smoothly through high speed printing and processing machines without tearing or pulling. High stiffness is necessary to maintain the structural integrity of cardboard products during filling and subsequent use.

[0003] Stiffness is closely related to the basis weight and density of the substrate. For a given clamp (thickness) the general tendency is that the stiffness increases with the increase of the basis weight. However, if the grammage is increased to improve stiffness, a larger number of fibers should be used, increasing cost and weight.

[0004] In addition to the mechanical properties of stiffness and strength, the paper or cardboard substrates to be printed must have the desired level of gloss and smoothness. One of the main means to obtain smoothness in the substrate is calendering the substrate during production. Calendering results in a reduction of the clamp, which usually results in a corresponding reduction in stiffness. This is particularly the case for calandering a wet pile. Calendering a wet pile requires re-moistening the sheet that has been pre-dried to about 5 percent moisture or less. Now the re-moistened sheet is passed through a calendering device having two or more rolls. The fiber network is compressed as a result of the pressure exerted by the rollers. The re-wetting of the substrate makes the surface fibers easier to compress and enables an aggressive development of smoothness. However, the compression densifies the sheet so that the product produced using the "wet lacquer coating" process can have up to 25% higher density after passing through the calendering of the wet stack.

[0005] Alternatively, the manufacturers have attempted to smooth the cardboard surface by covering the entire cardboard surface with a primer containing various pigments, such as, for example, clay, calcium carbonate and titanium dioxide, and then coating this primer with a second, and sometimes even a third coat, usually referred to as a topcoat. Typically, the more pigment (in the form of pigmented coatings) applied to the surface, the better the resulting smoothness. However, the use of relatively large amounts of pigments usually increases the cost and weight of paper or cardboard.

[0006] The relationship between stiffness and smoothness is usually inversely proportional to the amount of fibers per unit area. It would be desirable to be able to produce finished paper or cardboard with a smooth surface that has been developed without the need for compaction, thus maintaining maximum thickness with minimal use of cellulose fibers.

VP / 3981 / AW

[0007] WO2007 / 084806 discloses a process for the production of coated paper with reduced gloss spots, wherein the cellulose fiber web with basis weight e.g. 0.18 kg / m² (109 lb / 3000 ft²) or more is first calandered at high pressure, then coated with a coating composition, and then calendered at low pressure. The product obtained has a Parker Print Surf smoothness of no more than 1.2 μm. Coating compositions comprising pigments, such as, for example, calcium carbonate and clay, are disclosed. However, there is no disclosure of any void volume range of the pigment deposit and it is contemplated that the moisture of the base layer during the first calendering step is not particularly limited, while the moisture during the second calendering step falls within the conventional process.

SUMMARY [0008] According to one aspect of the invention for which protection is sought, the invention provides a method for producing coated cardboard, comprising the following steps:

producing a cellulose fiber network, said network having a basis weight of at least about 85 lb / 3000 ft² (about 0.138 kg / m²) of the network mentioned;

calendering said network at least once to form a cardboard substrate, each of said calendering steps being carried out without substantially introducing moisture into said network;

applying an undercoat to at least one surface of said cardboard substrate to form a coated cardboard, said undercoat comprising a pigment component comprising at least one pigment, said pigment component having a volume of sludge voids of at least 45 percent as measured by the dilution method pigment mixture with water to 50% by weight of solid particles, sample centrifugation of 70 g of the resulting suspension at 8000 g for 90 minutes and calculation of the amount of water remaining in the empty voids of the sediment after casting and weighing the liquid supernatant; and applying a topcoat to said undercoat of said coated board to form a board with a topcoat having the outermost surface with a topcoat,

[0009] Optionally, said cardboard substrate is a cardboard substrate with solid bleached sulphate.

[0010] Optionally, said primer is applied to at least one surface of said cardboard substrate in a coating weight of at most 9 lb on each side.

3000 ft² (about 14.6 g / m²) cardboard substrate.

[0011] Optionally, said primer is applied to said substrate surface ₂ cardboard with a coating weight of at most 8 lb / 3000 ft on each side² (about 13.0 ₂ g / m²) cardboard substrate.

[0012] Optionally, said pigment component comprises clay with a hyper-lattice having a aspect ratio of at least 40: 1.

VP / 3981 / AW

[0013] Optionally, said cardboard substrate defines a plurality of depressions in said at least one surface, and wherein said step of applying a primer comprises applying said primer such that said primer is substantially taken inside said plurality of said recesses without substantially covering completely. said at least one surface.

Preferably, said undercoat forms a discontinuous film on said at least one face of said cardboard substrate.

[0015] Optionally the undercoat is applied in the form of a slurry.

[0016] Optionally, said pigment component comprises clay with a hyper-lattice having a aspect ratio of at least about 40: 1 and an additional inorganic pigment.

[0017] Optionally, said pigment component comprises ground calcium carbonate having a coarser particle size distribution such that 60% or less of calcium carbonate particles have a diameter of less than 2 micrometers.

[0018] Optionally, said pigment component has a volume of sludge voids of at least 47 percent.

[0019] Optionally, said pigment component has a volume of sludge voids of at least 50 percent.

[0020] Optionally, said network is not subjected to a wet calendering process. Alternatively, said network is subjected to a calendering process of the wet stack in the absence of any water tank.

[0021] In a further aspect, the invention provides a primer for use in a method for producing coated boardboard according to any of the preceding preceding paragraphs, wherein the undercoat comprises a pigment component comprising at least one pigment, said pigment component having a volume of sludge volume of at least 45 percent, measured using a technique of diluting the pigment mixture with water to 50% by weight of solid particles, centrifuging the sample 70 g of the resulting suspension at 8000 g for 90 minutes and calculating the amount of water remaining in the empty sediment after casting and weighing the liquid supernatant.

[0022] Other aspects of the disclosed method for coating cardboard will become apparent from the following description, accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS [0023]

Fig. 1 is a photograph of an uncoated face of an exemplary cardboard substrate (i.e., raw pile);

Fig. 2 shows a photographic comparison of the surface of a cardboard substrate coated with different amounts (in pounds per 3,000 ft²; 1 lb / 3000 ft² is about 1.628 g / m²) coarse calcium carbonate according to the prior art;

Fig. 3 shows a photographic comparison of the surface of a paperboard substrate coated with 2 2 2 in different amounts (in pounds per 3000 ft²; 1 lb / 3000 ft² is about 1.628 g / m²) the undercoat disclosed;

VP / 3981 / AW

EP 2 514 868 B1

Fig. 4 is a graphical illustration of the percentage volume of sludge voids depending on the percentage of clay component for various pigment mixtures developed with very coarse calcium carbonate;

Fig. 5 is a graphical illustration of the percentage volume of sludge voids depending on the clay content percentage for different pigment blends formulated with coarse calcium carbonate;

Fig. 6 is a graphical illustration of the percentage volume of sludge voids depending on the clay content percentage for different pigment blends formulated with fine calcium carbonate;

Fig. 7 shows a first graphical comparison of the Parker Print Surface smoothness depending on the coating weight for cardboard only with primer and dry varnish;

Fig. 8 shows a second graphical comparison of the Parker Print Surface smoothness depending on the coating weight for different pigment systems;

Fig. 9 is a side cross-sectional view of a cardboard substrate coated with the disclosed undercoat according to the disclosed method;

Fig. 10 is a side cross-sectional view of the cardboard substrate of Fig. 9 shown at a second, higher magnification;

Fig. 11 is a schematic representation of one aspect of a method for producing a cardboard substrate coated with a dry varnish; and Fig. 12 is a schematic representation of one aspect of a method for producing a cardboard substrate coated with a dry varnish formed by the method shown in Fig. 11.

DETAILED DESCRIPTION [0024] A method of coating a cardboard substrate with a coating is disclosed. The coating may comprise a primer and optionally one or more intermediate coatings and one or more topcoats.

[0025] As used herein, the term "cardboard substrate" broadly refers to any cardboard material that can be covered with a primer. It will be obvious to those skilled in the art that the cardboard substrate can be bleached or ungrounded, with a basis weight of about 85 lb / 3000 ft² (about 0.138 kg / m²) or more if not coated. Examples of suitable cardboard substrates include a flat corrugated board, a corrugating agent and solid bleached sulphate (SBS).

[0026] In one aspect, the cardboard substrate can be produced in a continuous production process that uses a dry pile calendaring. In other words cardboard cardboard substrate can be produced without using a wet pile calender.

[0027] Referring to Fig. 11, one aspect of the method for producing a dry-suited cardboard substrate 37 may start in a headbox 22 that may drain a pulp suspension (with such additives as are necessary to improve the integrity and functional properties of the substrate) for a Fourdrinier machine 24, which may include a movable screen with very fine mesh to form a network 26. The network 26 may go through one or

VP / 3981 / AW

As an option, the coating press 32 may be used to add functional properties and to potentially reduce the thickness of the nip clamp 26, while the dryer 34 may then dry the network 26. Finally, the net 26 may pass through a dry-planner calender 36 to form a finished cardboard substrate 37. The calender rolls may be heated by steam. The gripping loads and the number of calender handles can be significantly reduced to minimize or prevent the thickness of the caliper to decrease.

[0028] Therefore, without a cardboard substrate re-moistened in the calender 36, the fibrous substrate is only minimally thickened in the calender pile. Therefore, the calendering activity does not have a significant impact on a significant part of the cardboard substrate, while the losses in the clamp due to the compaction are minimal before coating.

[0029] In one aspect, the disclosed primer may contain a pigment or pigment blend designed to provide relatively high percentage voids of sludge voids (i.e., greater packing of particles) and high smoothness at relatively low coating weights. This high volume of sludge voids may be obtained by using components having relatively high aspect ratios and / or a relatively large average particle size. For example, a volume of sludge voids greater than 45 percent may be desirable, while a volume of sludge voids greater than 47 may be even more desirable, while a volume of sludge voids greater than 50 may be even more desirable.

[0030] In one particular aspect, the disclosed primer may comprise a pigment mixture with a high content of clay and calcium carbonate. The pigment mixture may be dispersed in a carrier, such as, for example, water, to facilitate application of a primer to a suitable substrate, such as, for example, a cardboard substrate. Additional ingredients such as, for example, binders, stabilizing agents, dispersing agents and additional pigments may be combined with the pigment mixture to form a finished undercoat without departing from the scope of the present invention.

[0031] The clayey component of the pigment mixture of the disclosed primer may be any flat clay having a relatively high aspect ratio or aspect ratio (i.e., a hyper clay). As used herein, the terms "aspect ratio" and "aspect ratio" refer to the geometry of individual clay particles, especially the comparison of the first dimension of a clay molecule (e.g., the diameter or length of a clay molecule) with the second dimension of the clay molecule (e.g., clay thickness or width). The terms "hyper-flat", "high aspect ratio" and "relatively high aspect ratio" refer to ratios generally exceeding 40: 1, such as, for example, 50: 1 or more, in particular 70: 1 or more, and preferably 90: 1 or greater. .

[0032] In one aspect, the clayey component of the pigment mixture may comprise a flat clay in which the clay particles on average have a ratio of 40: 1 or greater. In another aspect, the clay component may comprise a flat clay in which the clay particles have a coefficient on average_® 50: 1 ratio or larger. An example of such CONTOUR clay^® 1180 available at Imerys Pigments, Inc. in Roswell, Georgia. In another aspect, the clay component may comprise flat clay,

VP / 3981 / AW

In which the clay particles have an aspect ratio of 90: 1 or greater. An example of such XP-6100 clay is also available at Imerys Pigments, Inc. Additional examples of suitable flat clays are disclosed in US Patent No. 7208039 to Jones et al.

[0033] In another aspect, the clayey component of the pigment mixture may comprise a flat clay having a relatively large average particle diameter. In a first particular aspect, the clay component may have an average particle diameter of about 4 micrometers or more. In a second particular aspect, the clay component may have an average particle diameter of about 10 micrometers or more. In a third particular aspect, the clay ingredient may have an average particle diameter of about 13 micrometers or more.

[0034] The calcium carbonate constituent of the pigment mixture of the disclosed primer may contain calcium carbonate. In one aspect, the calcium carbonate constituent may contain fine grains_® calcium carbonate. CARBITAL is an example of such fine-grained calcium carbonate^® 95 available at Imerys Pigments, Inc. in Roswell, Georgia, where 95 percent of calcium carbonate particles are smaller than about 2 microns in diameter. In another aspect, the calcium carbonate constituent may contain coarse calcium carbonate. An example of such_® coarse calcium carbonate is CARBITAL^® 60 also available at Imerys Pigments, Inc., in which 60 percent of the calcium carbonate particles are smaller than about 2 microns in diameter. In another aspect, the calcium carbonate constituent may contain a very coarse carbonate_® calcium. CARBITAL is an example of such a very coarse calcium carbonate^® Also available from Imerys Pigments, Inc., only about 35 percent of calcium carbonate particles are smaller than about 2 microns in diameter.

[0035] In another aspect, the calcium carbonate constituent of the pigment mixture may have an average particle size of about 1 micron or more, such as, for example, about 1.5 microns, and in particular 3 micrometers or more.

[0036] Without being limited to any particular theory, it is believed that the pigment blends that are formulated to provide a high percentage of sludge voids volume (i.e., larger particle packing) ensure high smoothness at relatively low coating weights, thereby reducing the cost of raw materials . In addition, it is believed that by using a clayey component having a relatively high aspect ratio and / or a relatively large average particle size and a calcium carbonate constituent having a relatively large average particle size, a relatively high and thus desirable volume percentages of the sludge voids are obtained. For example, a volume of sludge voids greater than 45 percent may be desirable,

One suitable technique for measuring the volume of sludge voids includes the production of a pigment or pigment mixture, and then diluting with water up to 50 percent by weight of the particulates to form a slurry. A sample of 70 g of the suspension is placed in a centrifuge tube and centrifuged at approximately 8000 g for 90 minutes. The sample is removed from the centrifuge, while the liquid supernatant is separated and weighed. The sludge is usually packed sufficiently

VP / 3981 / AW

Densely so that the liquid supernatant can easily be poured off. On the basis of the mass of water removed, the amount of water still contained in the empty sediment spaces can be calculated. Then, using particle density, the mass of water in the empty spaces can be transformed into a percentage of the volume of the sludge voids.

[0038] Referring to Figs. 4-6, a percentage volume of sludge voids is provided for the various pigment mixtures in relation to the weight percent of the clay component in _® pigment mixture. Specifically, Figs. 4-6 compare the use of CARBITAL^® 35 (very coarse), CARBITAL^® 60 (coarse) and CARBITAL^® 95 (fine-grained) as _® calcium carbonate constituent and XP-6100 (aspect ratio above 90: 1), CONTOUR^®1180 (aspect ratio 50: 1), CONTOUR^® Xtrm (aspect ratio 45: 1) and KCS (aspect ratio around 10: 1 (not very high clay aspect ratio)) as clayey.

[0039] Figs. 4-6 show coarse calcium carbonate (Figures 4 and 5), in particular very coarse calcium carbonate (Fig. 4), and clays with a high ratio, in particular clays having a ratio of over 70 : 1, especially above 90: 1 (XP-6100 clay), provide the highest percentage of sludge voids volume.

[0040] Furthermore, the concave shape of the curves in Figs. 4-6, and in particular the curves associated with clay XP 6100, indicates that the maximum percentage volume of sludge voids is obtained when the clayey component is mixed with the calcium carbonate constituent. For example, referring to Fig. 4, when using very coarse calcium carbonate and XP 6100, the maximum percent volume of voids in the sludge is between about 60 and about 90 percent by weight of the clay component.

[0041] Still further, the concave shape of the curve indicates that certain blends of the clay component and the calcium carbonate constituent provide a volume percentage of voids of sludge that is similar, if not larger, as when using clay with a 100 percent aspect ratio. Therefore, the curves indicate that mixing cheaper calcium carbonate with a more expensive clay with a high ratio of proportions yields an equivalent, if not better, coating material in terms of the percentage of empty sludge volume. Indeed, by comparing, for example, Fig. 4 of Fig. 6, the curves indicate that a coarser calcium carbonate and a clay with a lower aspect ratio should be used to achieve a higher percentage of sludge voids volume. For example, referring to Fig. 4,

[0042] Referring to Fig. 12, one aspect of a dry carton coating board method 60 for coating paper may start in an optional dryer 38. The dry-cardboard cardboard substrate 37 may then pass to the first coater 40. The first coater 40 may be a coarse blender or the like and may apply the disclosed undercoat to the dry-powder cardboard substrate 37. The optional dryer 42 may dry, at least partially, the undercoat before applying the optional topcoat onto the second coater 44. Other optional dryer

VP / 3981 / AW

It may complete the drying process before the dry-sour cardboard substrate 47 passes to the optional gloss setting post 48, while the coated dry-liner cardboard substrate 47 will be wound onto a reel 50.

[0043] Referring to Figs. 7 and 8, the Parker Print Surface ("PPS") values ​​of the cardboard coated with different primers on the pilot coat are shown with respect to the weight of the primer coating in pounds per ream (3000 ft)²; 1 lb / 3000 ft² it's about 1,1628 g / m²). It will be obvious to those skilled in the art that the PPS smoothness values ​​obtained from the samples produced in the pilot coat are generally higher than the PPS smoothness values ​​obtained from the samples made and the full-size mill. Nevertheless, the PPS smoothness values ​​obtained using a pilot coater are indicative of the improvement provided by the advantage of the disclosed primers over coatings known in the art. For reference, when a pilot coater is used, PPS smoothness values ​​of about 7.0 micrometers or less are generally desirable, PPS smoothness values ​​of about 6.5 micrometres or less are preferred, while PPS smoothness values ​​of about 6.0 micrometers or less are preferred.

Particular interest, as shown in Fig. 7, are foundations containing coarse or very coarse calcium carbonate and clay with a high aspect ratio, in particular XP-6100 clay, which provide a relatively high percentage of sludge voids volume and current smoothness values. PPS generally less than about 7 micrometres with a coating weight of about 9 lb per ream (about 14.6 g / m2) or smaller in a cardboard substrate. Indeed, as can be seen from the positive slope of the curves in Fig. 7, the improved fluidity (i.e., the lower PPS smoothness value) of the resulting board is directly correlated with the lower coating weights. These data are against the expectations of specialists in this field, who would expect higher values ​​of smoothness at high coating masses.

[0045] Indeed, when a full-size mill was used, a primer containing a 50:50 CARBITAL pigment blend^® 35 (very coarse calcium carbonate) and XP-6100 (clays with a high aspect ratio and high average particle size), a PPS smoothness value of less than about 3 micrometers was obtained with the topcoat, in particular about ₂ micrometer, with a relatively low weight of 6 pounds underlay on a ream (about 9.7 g / m)²).

[0046] Accordingly, coating substrates, such as, for example, a carton with primers containing ground calcium carbonate, in particular coarse or very coarse calcium carbonate, and a clay with a high ratio, in particular clay having a aspect ratio greater than about 70: 1 , in particular clay with a high aspect ratio, having a relatively high average particle size, allows a smooth cardboard structure to be obtained without loss of slack and reduce production costs by combining more expensive flat clay with cheaper ground calcium carbonate, while requiring surprisingly low coatings to achieve the desired smoothness .

[0047] Furthermore, it will be obvious to those skilled in the art that the selected type of clay with a high aspect ratio and the selected type of ground calcium carbonate, as well as the ratio of clay content to the calcium carbonate constituent content can be dictated by cost considerations in the perspective of desired smoothness.

VP / 3981 / AW

[0048] The disclosed foundations may be applied to the surface of the substrate, such as, for example, cardboard (ie cardboard for packages for aseptic and liquid products), in an amount sufficient to fill the recesses and gaps in the substrate without the need to cover the entire surface. substrate. Therefore, the disclosed primer together with the disclosed method of applying a primer can be used to achieve high surface smoothness with a relatively small amount of undercoat. Indeed, as discussed above, high surface smoothness can be achieved with an unexpectedly small amount of the undercoat disclosed.

[0049] In one aspect, the primer is applied to the substrate by means of a knife coater so that the coater will force the primer to penetrate into the recesses and gaps in the substrate while removing the primer from the substrate surface. In particular, as shown in Figures 9 and 10, the undercoat may be applied in a manner that is more similar to sealing, in which substantially all of the undercoat is located rather in recesses and slits in the surface of the substrate than on the surface of the substrate.

At this point, it will be apparent to those skilled in the art that when the disclosed undercoat is used in a coater coating, the spacing between the moving substrate and the coater blade can be minimized to facilitate filling of the pits and crevices in the surface substantially without depositing the primer on the surface substrate (i.e., forming a discontinuous film on the surface of the substrate). In other words, the coating blade can be placed sufficiently close to the surface of the moving substrate so that the coating blade forces the primer to penetrate into the recesses and gaps in the substrate while removing the excess primer from the substrate surface.

EXAMPLE 1 [0051] A first pigment mixture prepared according to one of the aspects of the present _® The invention contains 50 weight percent CARBITAL^® 35 (very coarse calcium carbonate) and 50 weight percent XP-6100 (hyper-fine clay). In a stationary mixer, the coating composition is prepared by combining a 50:50 pigment mixture with water, latex binders and a thickening agent. Water is added in an amount sufficient to form a slurry. Using a blade coater as described above, the coating composition is applied to a pile of green cardboard having a basis weight of about 126 lb / 3000 ft.² (about 0.205 kg / m²) in the following coatings: 6.7, 7.9, 8.9 and 11.3 lb / 3000 ft² (about 10.9, 12.9, 14.5 and 18.4 g / m²). The photographic results are shown in Fig. 3, while the PPS smoothness values ​​are shown in Fig. 7 (data points marked with circles).

[0052] Therefore, as shown in Fig. 3, the disclosed primer and associated method provide optimum smoothness at relatively low coating weights. (Compare Fig. 2 of Fig. 3) W₂ in particular, the highest smoothness is achieved with a coating weight of 6.7 lb / 3000 ft² (around 10.9

2 2 g / m²), good smoothness is achieved at 7.9 lb / 3000 ft² (about 12.9 g / m²), less smoothness at 8.9 lb / 3000 ft² (about 14.5 kg / m²), while even lower smoothness at 11.3 lb / 3000 ft² (about

18.4 g / m²).

EXAMPLE 2

VP / 3981 / AW

[0053] The second pigment mixture prepared according to one of the aspects of the present invention comprises 50 weight percent of OMYA HYDROCARBu^® 60 (coarse calcium carbonate available from Omya AG in Oftringen in Switzerland) and 50 weight percent XP-6170 (hyper clay available at Imerys Pigments, Inc.). In a stationary mixer, the coating composition is prepared by combining a 50:50 pigment mix with water, latex and starch binders, and a thickening agent. Water is added in an amount sufficient to form a slurry. Using a blade coater as described above, the composition₂ the coating is applied to a pile of raw cardboard having a grammage of about 106 lb / 3000 ft² (about 2 2 2

172 g / m²) in coatings from 5.8 to 6.8 lb / 3000 ft² (about 9.4 and 11.1 g / m²), thus providing a cardboard structure with increased smoothness with relatively low coating weights.

EXAMPLE 3 [0054] A low-density unpeated whiteboard (SBS) cardboard having a low density of about 120 lb / 3000 ft2 (about 195 g / m2) was made using a full-scale production process. The full-scale production process did not include the process of calandering the wet pile.

[0055] The highly loose carbonate-clay substrate was made in the following composition: (1) 50 parts clay high ratio with Imerys Pigments, Inc., (2) 50 parts PG-3 from Omya (very coarse calcium carbonate), (3 ) 19 parts of polyvinyl acetate latex (binder) and (4) an essentially swelling synthetic thickening agent in an amount sufficient to raise the viscosity of the mixture to 2,500 centipoise (2.5 Pa ^ s) at 20 rpm in a Brookfield viscometer.

[0056] The topcoat was prepared in the following composition: 50 parts fine grained carbonate; 50 parts of fine-grained clay; 17 parts of polyvinyl acetate; and small amounts of a coating lubricant, a plastic pigment, a protein, a dispersing agent, a synthetic viscosity modifier, a defoamer and a dye. [0057] The primer was applied to the uncoated cardboard using a bent applicator of a bent blade applicator. The backing was applied so that the minimum amount of primer required to fill the voids in the roughness of the sheet remained on the sheet, while the excess primer was scraped off the sheet leaving a minimum amount of primer above the surface of the fibrous surface. The primer was applied in a coating weight of approximately 6.0 lb / 3000 ft² (about 9.8 g / m²). The topcoat was applied to the undercoat to further improve the surface's smoothness. The top coat was applied to the primer in a coating weight of approximately 5.4 lb / 3000 ft² (about 8.8 g / m²). [0058] The obtained coated structure had a total grammage of approximately 130.0 lb / 3000 ft² (about 212 g / m²), a thickness of about 0.012 inch (12 points, about 0.30 mm) and a Parker Print Surf (PPS 10S) of about 1.5 micrometers.

Accordingly, it will now be apparent to those skilled in the art that the undercoats formulated in accordance with the present invention contain coarse calcium carbonate, in particular very coarse calcium carbonate, and hyper-lattice clay, in particular hyper-lattice clay having ratios of proportions exceeding about 70: 1, especially clays having a high aspect ratio having a relatively large average particle size (i.e., about 10

VP / 3981 / AW

Micrometers or more), provide greater surface smoothness at relatively low coating weights, especially when applied to the substrate using the disclosed method.

[0060] While the pigment blends discussed above contain flat clay and ground calcium carbonate, and in particular very coarse calcium carbonate, it will be obvious to those skilled in the art that alternative pigment blends can be used without departing from the scope of the present invention. For example, the pigment mixture of the disclosed primer may contain flat clay and one or more additional inorganic pigments other than ground calcium carbonate, such as, for example, precipitated calcium carbonate, talc or kaolin clay.

VP / 3981 / AW

EP 2 514 868 B1