FIELD OF THE INVENTION

The present invention is in the field of paper manufacturing, especially in the field of paper coatings and paper coating materials. The invention relates to a process for coating a fiber matrix using dry coating methods.

BACKGROUND OF THE INVENTION

Conventional paper coatings are applied on a paper substrate as water containing coating colors. These coating formulations are prepared by mixing solids of suitable properties, such as inorganic pigments, binders and additives, with water.

Because the conventional coating mixes are applied on the surface of the paper web as water-based slurry, the water carried over to the web by the coating mix must be removed and therefore effective drying is needed after coating. The higher the web speed, the higher drying capacity is needed and the longer becomes the dryer section. Drying is also an energy-intensive process and the investment costs of a dryer section are high.

The above described problem with the water containing coatings can be avoided by a dry surface treatment process in which dry coating powder is applied on a web. For example WO 2004/044323 (Metso Paper Inc.) discloses a method for coating a web of paper by a dry coating powder. The method comprises the step of applying the coating powder to at least one side of the moving web. The coating powders include inorganic materials and polymeric binder material.

The dry surface treatment technology is still under development. Especially the coating materials used need to be improved. Many different kinds of coating compositions are known from the prior art. US 2005/0006041 (Omya Ag) discloses a composite composition of co-structured or co-adsorbed fillers containing at least two different types of mineral or organic pigments and their use in papermaking for the coating. This composition contains at least one pigment having a surface with at least one hydrophilic site and at least one pigment having at least one organophilic site.

One of the problems related to the coating by the dry surface treatment process is that these known coating powders results in coatings having a quality that is unacceptable for high speed printing. Pick is the name commonly given to damage to the paper surface occurring during the printing operation. When the printing form is lifted from the paper, the ink exerts on the paper a force, which increases with increasing viscosity and tack of the ink and with increasing printing speed. When this force exceeds a critical value, which depends on the paper, the surface of the paper is damaged. The minimum printing speed at which pick occurs is a measure of the pick resistance of the paper. Pick velocity [m/s] is the velocity at which pick of the surface on the printed paper begins under the conditions defined in the International standard ISO 3783 (IGT type tester). This pick resistance measurement can be used to describe the surface strength of the paper.

Typically the pick resistance problem has been tried to solve by modifying the binder material used in the coating composition. In the book series of Papermaking Science and Technology, Book 1 1 "Pigment Coating and Surface Sizing of Paper" discloses on page 210 that it is well known that dry pick strength increases with decreasing particle size of latexes and increasing latex carboxylation.

Multilayered coatings have been used for many reasons. For example EP1092479 teaches a method to coat heat sensitive material with a low- temperature curing powder and a second high temperature coating powder. The low temperature curing powder protects the heat sensitive substrate.

US6387500B relates to a method to decrease the amount of a white pigment present in a coating in a paper and to a method to increase opacity. For this purpose a multi-layered coating containing at least two layers for paper or board is disclosed. The first layer contains at least one dark colorant, pigment and at least one binder. The second layer contains another pigment than dark one and at least one pigment and at least binder. The slurries and dry mixtures containing these ingredients are also disclosed.

US2009136692 teaches production of ink-receiving coating structure by layering a finely agglomerated pigment on the layer of coarse-grained pigment, which is disposed on the support.

JP2005330609A by Daio paper Corp. discloses a method for improving the smoothness and gloss of the paper. The top layer is composed of delaminated clay and the undercoating average particle size below 1 .5 μιτι.

None of the above mentioned multilayered coatings refer to the improvement on the surface strength of the coating. Especially, the amount of binder has not been considered relevant in these applications, and for example US 6387500 is content with disclosing that the binder should be present in an amount sufficient to bind the pigments to the substrate, i.e. the basic purpose of using a binder. The general believe is, however, that increased binder amount improves the strength of the coating. For economical reasons it has not been reasonable to raise the surface strength only by increasing binder dosage. Thus, there is a need for the development of new dry coating powders and dry coating methods for improving the surface strength.

BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail by means of preferred embodiments. In Example 2 reference is made to the attached drawings, in which the following scanning electron microscopy pictures are shown:

Figure 1 , SEM picture of test-point 1 , Reference Figure 2, SEM picture of test-point 2 Figure 3, SEM picture of test point 3 Figure 4, SEM picture of test point 4 DESCRIPTION OF THE INVENTION

An object of the present invention is thus to provide a process for coating a fiber matrix using dry coating methods so as to alleviate the above disadvantages. The objects of the invention are achieved by a dry coating composition, which is applied on the fiber matrix using a process, which is characterized by what is stated in the claim 1 . The preferred embodiments of the invention are disclosed in other claims.

In general it was considered that the amount of binder in the coating layer has influence on the strength of the layer and would therefore be an important factor for preventing pick. Against the general knowledge in the field of the art, it was surprisingly found out that decreasing the amount of binder on the top-layer of the coating gave better surface strength. Thus, the present invention is based on the realization that the distribution of the binder content in the coating layer is one of the determinant factors of the final surface strength of the coating. As a first aspect the present invention provides a process for coating a fiber matrix, wherein the process comprises applying coating composition comprising binder and inorganic particles on the fiber matrix using dry coating method to form a coating. The process is characterized in that the binder content in the coating is not homogenous i.e. the inhomogeneous binder content in the coating is spread so that the binder content on top of the coating is lower than the binder content close to the fiber matrix, and on top of the coating the binder content is between 1 and 20 % by weight of the coating composition.

The application of pigment only on the top of the coating is very challenging because of poor fluidity of dry pigment particles. Thus, a small amount (at least 1 %) of binder in the top of the coating is needed to apply the coating and to fix the pigment to the coating. Preferably the binder content on top of the coating is between 3 and 7 % by weight of the coating composition.

The expression "close to the fiber matrix" means that this region includes the part of the coating that is in contact with the fiber matrix, but does not include the uppermost layer, which is on top of the coating. The expression "on top of the coating" means the uppermost layer of the coating, which is on top of the coating, and it may also include some part of the region in the middle of the coating.

In an embodiment of the invention the fiber matrix is paper or board. Paper is distinguished from board or paperboard primarily on basis weight. The term "paper" is generally applied to sheets having basis weights of 8 to 170 g/m ² while the term "board" or "paperboard" refers to basis weights of 130 to 450 g/m ² . As there is some overlap in basis weights, a further distinction is based on stiffness and end uses. At equivalent basis weights, a board will be stiffer and more rigid than paper. Board or paperboard is typically used for packaging applications. One of the objectives in coating paper or board is to improve the appearance and smoothness of the surface before printing.

The inventors of the present invention found out that more binder is needed in the powder coating composition, which is applied on the fiber matrix interface, than in the powder coating composition, which is applied as a top coating layer. High binder loading between the top coating layer and the fiber matrix also causes closure of the fiber matrix against vapour and liquid penetration, without causing tackiness and printing problems on the top layer.

In an embodiment of the present invention the coating is formed of one or more separate layers. The one or more layers of coating powder may be applied in a "dry on dry" process and for example two layers having different binder content may be partly fused and cured together to form a single layer having inhomogeneous binder content.

In an embodiment of the present invention the coating comprises one layer, which is formed by applying on the fiber matrix a first coating composition and then applying at least one other coating composition having lower binder content than the first coating composition, wherein the at least one other coating composition is applied on top of the first coating composition before the first coating composition is hardened allowing a partial mixing of the different coating compositions and formation of a single coating layer having an inhomogeneous binder content. In another embodiment, the coating comprises a single layer having inhomogeneous binder content, wherein the amount of the binder increases towards the fiber matrix interface. At the fiber matrix interface there is about 10 to 30 parts of binder per 100 parts of pigment, preferably about 20 to 25 parts of binder per 100 parts of pigment. At the top of the coating there is less binder than at the fiber matrix interface, preferably about 1 to 20 parts of binder per 100 parts of pigment, more preferably about 3 to 7 parts of binder per 100 parts of pigment.

In another embodiment the coating is a multilayered coating, which comprises two or more homogeneous, but separate layers. Against the fiber matrix interface there is a first layer with higher binder content, about 10 to 30 parts of binder per 100 parts of pigment, preferably about 20 to 25 parts of binder per 100 parts of pigment. On the top of the first layer there are one or more layers from which at least the uppermost layer contains less binder than the first layer. The uppermost layer, which is also called top layer, contains about 1 to 20 parts of binder per 100 parts of pigment, preferably 3 to 7 parts of binder per 100 parts of pigment.

In another embodiment the coating comprises one layer having a first region and a second region. The first region is close to the fiber matrix and the second region includes the top of the coating. Depending on the application the height of the first region and second region can vary. The binder content in the first region is between 10-50 % by weight of the coating composition. Using coating with binder content of more than 50 % in the first region causes difficulties due to the restrictions of manufacturing of dry coating. In wet coating methods using coatings that have binder content of 100 % is normal. In dry coating however, pulverizing binder is difficult without a small amount of pigment.

In an embodiment the amount of the binder in the second region is from 1 to 20 % by weight of the coating composition. In some embodiments the amount of the binder in the second region is from 2 to 1 1 % or more specifically from 3 to 7 % by weight of the coating composition. In a coating having one layer comprising two regions, the second region contains always less binder than the first region. In one specific embodiment the coating comprises at least two separate layers, from which the first layer facing the fiber matrix is a pure binder layer i.e. the fiber matrix is first coated with pure binder and the other layers can be as described above. An additional advantage of the present invention is that the properties of the coating can be varied not only by varying the amount of the binder in different layers but also by selecting different binders and pigments in different layers. For example, in different regions of the coating different binders with preferred properties can be used. One such preferred property might be the glass transition temperature of the binder (T _g ) i.e. the hardness of the binder.

Hard binder should be used when the binder content of the coating composition is high and soft binder when the binder content is low. Thus, in some applications it is advantageous to have a hard binder in the layer against the fiber matrix, where the binder content is higher, and a soft binder at the top layer, where the binder content is lower. This type of layered structure improves powder fixing on the fiber matrix. It also diminishes powder sticking on hot fixing calender and/or plates. A soft binder is a binder that can be molded with a smaller amount of energy i.e. a binder having a glass transition temperature (T _g ) < 35 °C. A hard binder has a T _g > 35 °C. By layering it is also possible to use different pigments in the top layer and in the other layers. Pigments that are more desired for printing can be used in the top layer, and for example pigments with high opacity or lower price can be used in the other layers.

The thickness of the layers depends on targeted use of manufactured paper and properties that are needed for that use such as printability, porosity or surface strength. The thickness of the layers depends also on the components (pigments and binders) used in the coating as well as on the smoothness of the base paper. Typically the thickness of the coating is from 5 to 20 μιτι.

In order to optimize the optical and printing properties of the coated paper or board, especially gloss and opacity as well as surface strength, the coating of the present invention may contain up to 75% of pigment particles having a particle size D90 below 2 μηη before mixing with the binder. If in the coating a layer contains two pigments having different particle size distributions, then the mixture has a bimodal particle size distribution. Such a bimodal particle size distribution is a preferred embodiment of the invention. In a process for preparing a coating composition suitable for the process of the present invention, pigment and binder are mixed together. This may be done by mixing a needed amount of binder with the pigment, and pulverizing the obtained mixture. The binder dosage is typically between 5-30 parts per 100 parts inorganic particles. The mixing can be performed with or without additional water. After mixing the obtained mixture is dried by a convenient manner such as spray drying or freeze drying. Dry mixture is then ground for example with a jet mill and then coating composition is recovered in dry powder form. Typically the dry coating composition according to the present invention is pulverized to a particle size D90 from 2 μιτι to 15 μιτι. The obtained coating powder can be used for coating a paper substrate in a dry coating method. The dry coating composition may also comprise other additives, such as optical clarifier or antiblocking agent.

In the present invention the pigment used can be any inorganic pigment or mixture of pigments typically used in paper coatings, such as ground calcium carbonate (GCC). Other possible pigments include natural calcium carbonate, for example chalk, calcite, marble or any other form of calcium carbonate, natural or synthetic precipitated silicates, calcium sulfate, titanium dioxide, talcs, micas, clays, calcined clays, zinc oxide, other metal oxides, hydroxides, sulfates and carbonates such as satin white, crystalline and amorphous aluminum hydroxides. The pigments used may be different in different layers/regions of the coating.

By using a specific polymeric binder composition in the coating composition the final coating quality, namely the surface strength, may be further improved. Suitable binders to be used in the present invention include synthetic polymers and natural polymers, especially thermoplastic polymers. The thermoplastic polymer is preferably obtainable by free radical emulsion polymerization of ethylenically unsaturated monomers. Examples of a preferred binder material are styrene butadiene copolymer and styrene/butyl acrylate/acrylic acid copolymer.

EXAMPLES Example 1

In the performed experiments 5, 10, 15 or 25 parts of binders were mixed with 100 parts pure pigments or with pigment blends in water state. The pigment used in the examples was ground calcium carbonate (GCC). There were two different GCC having different particle sizes. Particle size properties referred to in the present application are measured in a well-known manner by sedimentation of the particle material in a fully dispersed condition in an aqueous medium using a SEDIGRAPH 5100 machine as supplied by Micromeretics Corporation. The measurements were made in water at the standard temperature of 35 °C. The binders used were Binder 1 (styrene butyl acrylate acrylic acid copolymer, T _g = 47 °C). Binder 2 (styrene butadiene copolymer, T _g = 22 °C) and the mixtures were freeze dried and micronized in fluidized bed jet mill of the type Alpine AFG 100. Powders thus obtained having a particle size D90 3-13 μιτι were applied on paper substrate (base paper 37 g/m ² ) using Nordson semiautomatic system with Sure Coat spray gun.

The fixing of the coating powder on a base paper was done by thermo- mechanical treatment using laboratory calender (DT Paper Science; heated steel roll, paper roll). In fixing each sample was run trough four calender nips with following condition. The first three nips: the calandering speed 4 m/min, temperature of heated roll was 150 °C and the nip load 64 kN/m. Siliconised baking paper was used to prevent sticking onto the hot calender roll. The last fixing nip was done without siliconised baking paper in the calendering speed 4 m/min, temperature of heated roll was 150 °C and the nip load 164 kN/m. Pick velocity [m/s] is the velocity at which pick of the surface on the printed paper begins under the conditions defined in the International standard ISO 3783. In the Examples the pick velocity was measured using IGT type tester.

Coating tests Coatings of 15 g/m ² /side were applied on base papers. In the reference sample the coating was homogenous coating formed of a coating powder containing 15 parts of Binder 1 . The test samples were done using the same total amount of coating powder, but the coating contained in the lower layer 50 wt-% of the total coating powder a first coating powder, which contained 25 parts of binder per 100 parts of pigment, and in the upper layer 50 wt-% of the total coating powder a second coating powder, which contained 5 parts of binder per 100 parts of pigment, as shown in Table 1 .

Adhesiveness to the roll during the calandering process was evaluated visually and classified to categories: no adhesion (n); very slight adhesion (vs); slight adhesion (s) and strong adhesion (A). In the IGT measurements "N.P." means that measurement was not possible.

The results clearly show that the test points, where the adhesion to the calender roll was strong (A) or even slight (s), the surface strength was so weak that the IGT measurement could not be done. The layered coating has clear positive effect to the surface strength compared to the reference, which contains the same total amount of binder, but is homogenously distributed in the coating.

Table 1 , IGT results and adhesiveness of powder to the calender

The results in Table 1 also show that higher binder content on top of the coating than close to the fiber matrix results in higher surface strength comparing to the reference with uniform binder content. Table 2, IGT results and adhesiveness of powder to the calender

The results in Table 2 show that using hard binder close to the fiber matrix and soft binder on top of the coating result in higher surface strength when no silicon paper is used.

In Tables 1 and 2 "with silicon paper" means calendering 3 times with silicon paper and once without silicon paper, and "without silicon paper" means calendering only once without silicon paper. Example 2

The coating powders were otherwise made using binders as in the Example 1 , but an additional test coating powder was made and it contained 10 parts of Binder 1 per 100 parts of pigment. The layered coatings were done according to the Table 3.

Table 3, Binder distribution in coating layers SEM studies

Scanning electron microscopy studies were done of the cross-sections of the coated paper.

Figure 1 , SEM picture of test-point 1 , Reference

Figure 2, SEM picture of test-point 2

Figure 3, SEM picture of test point 3

Figure 4, SEM picture of test point 4

The scanning electron microscopy pictures clearly show that coating, in which the binder is homogenously distributed in the coating layer (white part in the SEM picture), is more loosely bound to the fiber matrix (the grayish part) compared to the layered coating structure, in which the powder containing 25 parts of binder is located in the layer against the fiber matrix and the powder containing 5 parts of binder per 100 parts of pigment is a top layer. On the other hand when the lower layer contained less binder than the top layer, the coating was almost completely detached from the fiber matrix, which caused poor surface strength of the paper. Example 3

A coating powder containing 5 parts of Binder 1 per 100 parts of pigment was prepared as in Example 1 . Sheets of paper were surface pretreated with Binder 1 using dosages according to Table 4. The binder was diluted to different dry contents and the mixtures were applied on the paper sheets on a drawdown laboratory coater (RK K coater), using rod NrO. After the pretreatment the paper sheets were dried with infrared dryer and the paper sheets were coated with coating powder containing 5 parts of Binder 1 as in example 1 . The surface strength was measured using IGT method and the results are shown in Table 4. Table 3, Surface strength results

(d.s. = dry solids)

The results clearly show that the increase of binder in the interface between fiber matrix and coating layer increases IGT value and strength of the coating.

Example 4 A coating powder was made of gypsum according the same method as that was used for calcium carbonate in Example 1 . Binder 1 was used in an amount of 5 parts per 100 parts of pigment. The amounts of Binder 1 in calcium carbonate are presented in Table 5 and the surface strengths are shown respectively. Coating powder amount was 10 g/m ² and it was divided so that the lower layer contained 50 wt-% and top layer 50 wt-% of total coating powder. Table 5, Surface strength results

The results show that different pigments behave in the same manner as calcium carbonate in Example 1 . The tests 1 and 2 in Table 5 show that different results can be obtained when using in the coating composition the same components in the same amounts, but divided differently between the coating layers. In test 2, where there was more binder in the coating layer facing the base paper, the IGT result was twice as good as in test 1 , where the coating layer was homogeneous. Test 3 shows that it is possible to further improve the surface strength by changing the pigment used in the coating layer(s). Example 5

The coating powder was prepared according to the Example 1 using calcium carbonate as pigment. The coating of paper sheets were done as is presented in Table 6. The porosity and roughness of paper sheets were measured using Bendtsen method (using Andersson & Sorensen equipment).

Table 6, Porosity and roughness results

The results show that porosity of the paper increased significantly when using more binder in the lower coating layer and that porosity can be controlled also by temperature of calender. The significant increase in porosity could be done without diluting the roughness results, which stayed at low levels.