A METHOD FOR COATING OF PAPER OR PAPERBOARD USING A SPRAY COATING TECHNIQUE The invention relates to the coating of paper and paperboard using spray coating technique and is especially directed to a method wherein, on the basis of the surface energy of the paper or paperboard to be coated, the surface energy of the coating colour to be used is suitably adjusted in order to achieve a good coating result, par- ticularly good smoothness and covering. The invention is also directed to a method wherein the surface to be coated is adjusted to be hydrophobic and is coated using spray coating technique.

Background of the invention The purpose of coating is to improve the surface properties of paper or paperboard, such as gloss, smoothness and printability. For coating, various techniques have been developed, of which the nowadays most commonly used is a blade coating method. However, a blade coating process requires certain strength of the base pa- per to be coated. The requirement of strength has put requirements on the composi- tion of the base paper. The proportion of strength-giving pulp fibre in the fibre ma- terial of the base paper must be high. For example such an LWC base paper is suit- able for blade coating, wherein the proportion of chemical pulp of the fibre material is typically at least 40%, as well as a fine paper grade, wherein almost or exactly 100% of fibre material is chemical pulp.

Base papers containing a lot of chemical pulp fibre are more expensive than wood- containing paper or paperboard grades. For this reason, it would be advantageous to be able to coat base paper grades of a lower strength level, wherein the proportion of chemical pulp fibre is small or which contain a lot of recycled fibre. It is eco- nomically profitable to use a coating method, which is less strenuous for the base paper. The spray coating process, which has been developed during the last few years and wherein there is no contact between the paper web and the surface that

apportions coating, is well suitable for this purpose. The process is disclosed for example in U. S. patent No. 6,106, 902.

In a spray coating process, the coating colour is forced through several adjacent nozzles under a high pressure onto a paper or paperboard web. The row of nozzles is located across the web so that the coating colour is sprayed evenly in small drops across the entire web area. In order to get the coating colour into small droplets, a suitable pressure is generated using a pump.

It has proven troublesome to obtain an even coating on a paper or paperboard sur- face using a spray coating technique. The coating colour coming from the nozzles under a high pressure is poorly spread on the surface of absorptive paper or paper- board. In order to achieve the required covering, the spray nozzles are placed so that the edges of the coating colour spraying fans overlap each other.

During coating, the edges of the coating colour sprays tend to dry and form visible stripes of thicker coating. Coating colours having higher dry matter content more easily form stripes or streaks in the coating. Decreasing the dry matter content of the pigment coating colour, i. e. increasing the amount of water, helps to decrease the streaking of the coating. However, it is not advantageous to bring a lot of water into the coating, as it has to be dried from the web. The low dry-matter content of the coating colour can also cause plenty of water to penetrate from the coating colour into the base web, which may for example stretch the web, thus weakening its run- nability and impairing the quality of paper.

Coating with a spray technique sets requirements on the smoothness of the paper surface. In the spray coating process, the coating is spread over the entire surface of the base paper, and it follows the roughness of the base paper, resulting in an un- even coated surface. In blade coating on the contrary, the physical treatment of the surface produces an even surface despite of variations in the base paper surface.

Coating colour compositions suitable for blade coating have proven to be unsatis-

factory in spray coating technique, in particular with regard to the smoothness of the coating.

Above all, hydrophilic base paper or paperboard is difficult to coat evenly with spray technique, which sets certain requirements on the coating colour used in the coating. The surface of a hydrophilic paper absorbs the coating colour liquid very quickly and thus the coating colour should be such that the absorption of water into the paper is as slow as possible in order to obtain an even coating. In particular, large pigment particles in a spray coating colour quickly form a steric hindrance in the drop. This effectively prevents spreading of the coating colour and the smooth- ness of the coating remains poor.

Summary of the invention According to the invention it has now been found that when the base paper or pa- perboard is coated with a spray coating technique, in order to obtain an acceptable final result especially with regard to the smoothness of the coating, one or the other, either the base (i. e. paper or paperboard) or the coating colour has to be hydropho- bic in nature. Consequently, a spray coating colour for a hydrophilic base paper or paperboard has to be adjusted to have a hydrophobic character, and a spray coating colour for a hydrophobic base paper or paperboard has to be adjusted to have a hy- drophilic or slightly hydrophobic, preferably hydrophilic, character.

The invention is thus directed to a method for the coating of paper or paperboard on at least one side using a spray coating technique, wherein when the dispersive com- ponent of the surface energy of the surface to be coated is (i) at most 20 mN/m, preferably at most 10 mN/m, the dynamic surface ten- sion of the coating colour is adjusted to at least 65 mN/m, preferably to at least 70 mN/m;

(ii) over 20 mN/m, preferably 30-60 mN/m, the dynamic surface tension of the coating colour is adjusted to at most 60 mN/m, preferably to at most 55 mN/m ; and then the surface in question is coated with a coating color thus obtained, using a spray coating technique.

The method according to the invention is preferably used for the coating of paper- board or paper, which contains plenty of mechanical pulp or recycled fibres, such as newspaper or LWC base paper, wherein the proportion of chemical pulp is 20-40%.

Detailed description of the invention Hydrophilic base With a hydrophilic paper or paperboard is meant a paper or paperboard, wherein the polar component of the surface energy is high and the dispersive component almost zero. The polar component of the surface energy in a hydrophilic paper or paper- board is thus about 30-70 mN/m, preferably 40-60 mN/m, and the dispersive component below 20 mN/m, more preferably below 10 mN/m.

A hydrophilic base paper may for example be a mechanical base paper or paper- board, produced without adhesive additives, paper or paperboard, where starch or other hydrophilic polymer has been added to the pulp or as surface size, or a paper or paperboard pre-coated with a hydrophilic pre-coating colour.

A mechanical base paper, produced without adhesive additives, is made of hydro- philic fibres. The paper structure is formed of capillaries of varying size and shape.

Due to the porous structure and the tendency of the fibres to absorb water, paper is very absorptive and hydrophilic.

Due to its hydroxyl groups, starch is hydrophilic in nature. Adding starch into paper pulp or on a surface increases the polar component of the surface energy of paper.

As a surface size it changes the surface structure of paper but the hydrophilicity of the paper increases.

Paper or paperboard may also be coated with a hydrophilic pre-coating containing hydroxyl groups. The binder of the pre-coating may contain hydroxyl groups or the pigment may for example be a hydrophilic kaolin. In the preparation of the pre- coating colour, it may be necessary to use plenty of dispersing agent that lowers hydrophobicity. Dispersing agents affect surface energy by lowering the dispersive component.

In the method according to the invention, a hydrophilic base paper or paperboard is spray coated with a coating colour, which is hydrophobic in nature. With a hydro- phobic coating colour is herein meant a coating colour, which has a dynamic sur- face tension of at least 65 mN/m, preferably at least 70 mN/m, measured with a bubble pressure-method at 10 milliseconds.

A hydrophobic spray coating colour is obtained by adding into the coating colour chemicals, which raise the dispersive component of the surface energy of the coat- ing colour. Such chemicals are styrene acrylate based polymers, alkenyl succinic acid anhydride compounds, styrene maleic anhydride based polymers, alkyl ketene dimer compounds, other waxes, other hydrophobic agents used (e. g. resin), or vari- ous combinations of these. Further, the binder can be chosen so that it does not con- tain reactive hydroxyl groups which lower hydrophobicity. The pigment in the coat- ing colour may also be hydrophobic.

In order to improve covering, it has to be aimed at building a coating colour composition, which forms as little steric hindrance as possible. We have found that the best result on a hydrophilic surface is given by small, under 1 am in size, round or needle-like pigment particles in a hydrophobic coating colour.

In addition, the water retention of the coating colour should be good, in order for the coating colour to hold water as long as possible. The water retention of a coat- ing colour can be improved with different water-binding chemicals. These are starch, soya, carboxy methyl cellulose, polymeric thickeners and associative thick- eners. In spray coating the coating colour must not have a high viscosity due to technical functionality. Thus the use of these chemicals is strictly restricted. The Haake viscosity of a coating colour to be used in spray coating should be under 11, preferably under 10, at a shear rate of 25 000 rpm, when carbonate pigments are used, and under 7, preferably under 6, when kaolin pigments are used.

In blade coating the dry matter content of the coating colour may be even 65-70% when carbonate pigment is used, whereas 55-60% is a usual level in spray coating.

Hydrophobic base With a hydrophobic paper or paperboard is meant a paper or paperboard wherein the dispersive component of surface energy is high and the polar component almost zero. The dispersive component of the surface energy of a hydrophobic paper or paperboard is thus over 20 mN/m, preferably 30-60 mN/m, and the polar compo- nent below 15 mN/m, more preferably below 10 mN/m. The dispersive component is thus clearly higher than the polar one.

A hydrophobic base paper or paperboard may for example be a base paper or paper- board, where a hydrophobic chemical has been added to the pulp or as surface size, or a base paper or paperboard coated with a hydrophobic pre-coating.

Usually the hydrophobicity of the paper to be coated is not adjusted. However, if desired, the hydrophobicity of the paper may be increased by adding a hydrophobic adhesive compound to the pulp, whereby at the same time the runnability of paper is improved as the tensile strength increases. These hydrophobic agents are for example styrene acrylate based polymers, alkenyl succinic acid anhydride

ample styrene acrylate based polymers, alkenyl succinic acid anhydride compounds, styrene maleic anhydride based polymers, alkyl ketene dimer compounds, other waxes, other hydrophobic agents used (e. g. resin), or various combinations of these.

In the surface sizing of paper or paperboard, chemicals of similar type are used in admixture with the surface size starch to give hydrophobicity. From the point of view of the invention, especially advantageous among the above-mentioned hydro- phobic agents are styrene maleic anhydride based polymers, as they provide the paper surface with a more even particle size distribution. Contrary to for example alkyl ketene dimer compounds, styrene maleic anhydride based polymers are well suitable for use also when the coating machine has been integrated with the paper machine (online-coating), as the hydrophobicity is achieved already at the paper machine and not after maturing.

A hydrophobic pre-coating contains chemicals, which raise the dispersive compo- nent of the paper's surface energy clearly higher than the polar one. Among these chemicals are the above-mentioned hydrophobic agents. Further, the binder is se- lected so that it does not contain reactive hydroxyl groups, which lower hydropho- bicity. Also the pigment of the coating colour may be hydrophobic.

A physically compressed base paper or paperboard behaves in spray coating like a hydrophobic base paper or paperboard. Due to the smaller pore size, the paper sur- face absorbs less water than an untreated base paper. Such a physically compressed base paper or paperboard may for example be an uncoated base paper, which has been compressed with hot calendering, or a pre-coated paper or paperboard, which has been calendered.

With hot calendering the surface of an uncoated paper is made smoother and at the same time the pores of the paper become denser. Heating raises possible adhesives from the lower layers to the surface of paper. The adhesives can be hydrophobic agents added to the pulp or lignins from the original wood material. Due to their

effect, the dispersive component of surface energy becomes higher than the polar one or, in other words, the surface becomes hydrophobic.

In the process according to the invention a hydrophobic base paper or paperboard is spray coated with a hydrophilic coating colour. However, a hydrophobic base paper or paperboard is not as demanding regarding the spray coating as a hydrophilic one.

In some cases, the coating colour may be slightly hydrophobic (dynamic surface tension below 70 mN/m, preferably below 65 mN/m). However, it has been found that, when a spray coating technique is used, the best surface on a hydrophobic or on a physically compressed base paper is obtained by using a hydrophilic spray coating colour, preferably a hydrophilic coating colour mostly containing small, round or needle-like pigment particles below 1 llm in size. With a hydrophilic coat- ing colour is herein meant a coating colour with a dynamic surface tension of at most 60 mN/m, preferably at most 55 mN/m, measured with a bubble pressure- method at 10 milliseconds.

A hydrophilic coating colour is obtained for example by using a hydrophilic binder, which contains reactive hydroxyl groups. An example of such a hydrophilic coating colour component is starch. However, starch increases viscosity, and therefore it is more advantageous to use surface active agents which increase the coating colour's hydrophilicity. Of these may be mentioned alkyl phenol etoxylates, alkyl phenol sulphonates, alcohol etoxylates, polymaleinates, alkylene oxide block copolymers and other polymeric surface active agents and combinations of these.

Good water retention is an important factor also in hydrophilic coating colours. The water retention of a hydrophilic coating colour can be improved with the same wa- ter-binding chemicals, which were mentioned above in connection with a hydro- phobic coating colour.

In both hydrophobic and hydrophilic coating colours the rheology of the coating colour can be adjusted by removing the air included in the coating colour during

mixing stage as thoroughly as possible. The coating colour viscosity is lowered when the air mixed in the coating colour is reduced. In spray coating the evenness of the spray is improved when the coating colour material coming from the nozzles is homogeneous without air bubbles. Suitable chemicals for removing air from the coating colour are organic silane compound, hydrocarbons, vegetable oil or mineral oil based fatty acid esters or mixtures of a fatty acid and a fatty acid ester.

The rheology of the coating colour can be affected also with dispersing agents. With the aid of these, the viscosity of the coating colour is changed at the shear rates used in spray coating. Generally known dispersing agents are for example polymeric acrylic acid salts, fatty alcohol etoxylates, propylene oxide ethylene oxide block copolymers, polycarbonic acid salts, as well as derivatives or naphthalene sulphonic acid and phenol sulphonic acid.

Agents which affect spreading of the coating colour are so called lubricants, which do not have chemical interactions with pigment or binder. Stearate does not func- tion as a lubricant meant herein, as it has chemical interactions in the coating colour and it raises viscosity. Suitable lubricants are for example polyalkylene glycols, fatty acid salts and wax emulsions.

The coating method according to the invention, which is based on a spray technique enables achieving an even coating result with good coating coverage and a good quality on base papers and paperboards with different hydrophobic- ity/hydrophilicity. The method is suitable for use in the coating of several different wood-containing paper qualities, as well as in the coating of papers containing mainly chemical pulp fibre.

The invention is also directed to a method for the coating of paper or paperboard with a spray coating technique on at least one side, wherein the surface to be coated is adjusted to have a hydrophobic character (the dispersive component of surface energy over 20 mN/m), whereafter it is coated with a coating colour using a spray

coating technique. The paper or paperboard to be coated can be adjusted to have a hydrophobic character for example by internal sizing of paper or paperboard with a hydrophobic chemical.

The invention is illustrated with the aid of the following example.

The hydrophobic (base paper 1) or hydrophilic (base paper 5) base paper with a basis weight of 70 g/m2 in Table 2 was spray coated with the coating colours listed in Table 1, using a coat weight of 10 g/m2.

Table 1. Characteristics of coating colours used in spray coating Coating colour 1 10 23 Pigment GCC PCC Kaolin Solids content 60. 0 60. 0 59. 1 pH 9. 2 9. 1 9. 1 Brookfield (mpas) 102 126 154 Haake (mpas) 10. 2 6. 3 9. 7 Water retention (g/m2) 337 461 131 dyn. surface tension lOms 68 55 52 (mN/m) Table 2. Characteristics of base papers used in spray coating Base paper 1 5 total surface energy 35.53 56.72 (mN/m) pol. comp. (mN/m) 10.2 47.3 disp. comp. (mN/m) 25. 33 9.42 Roughness (nm) 87.2 55.6 After drying the spray coated papers were photographed and the enlarged copies are shown in the enclosed Figures.

Figure 1 shows the situation when hydrophobic base paper 1 has been spray coated with a hydrophobic coating colour 1, wherein the pigment particles were mostly small and round (GCC, ground calcium carbonate).

Figure 2 shows the situation when hydrophobic base paper 1 has been spray coated with a hydrophobic coating colour 10, wherein the pigment particles were mostly small and needle-like (PCC, precipitated calcium carbonate).

Figure 3 shows the situation when hydrophobic base paper 1 has been spray coated with a hydrophobic coating colour 23, wherein the pigment particles were mostly large and plate-like (kaolin).

Figure 4 shows the situation when hydrophilic base paper 5 has been spray coated with hydrophobic coating colour 1, wherein the pigment particles were mostly small and round (GCC).

Figure 5 shows the situation when hydrophilic base paper 5 has been spray coated with hydrophilic coating colour 10, wherein the pigment particles were mostly small and needle-like (PCC).

Figure 6 shows the situation when hydrophilic base paper 5 has been spray coated with hydrophilic coating colour 23, wherein the pigment particles were mostly large and plate-like (kaolin).

From Figures 1-3 it can be concluded that the best result in spray coating of a hy- drophobic base paper is, from the point of view of the smoothness of the coating, obtained by using a hydrophilic coating colour, wherein the pigment particles are mostly small and needle-like.

From Figures 4-6 it can be seen that in the spray coating of a hydrophilic base paper it is essential from the point of view of the smoothness of the coating, that the coating colour is hydrophobic. A large and plate-like pigment particle is clearly a worse alternative than a small and needle-like or round one.

Based on the results, it seems that from the point of view of the smoothness of the coating, spray coating is less critical on a hydrophobic than on a hydrophilic base paper or paperboard.