BACKGROUND OF THE INVENTION To improve printing properties and functionality, paper or paperboards are coated with various coating materials. Conventionally, aqueous suspension containing coating colour is applied onto one or both sides of the paper or paperboard web and the grammage of the coating is controlled using either blade, roller, or airbrush. The coated paper web is then subject to drying followed by subsequent treatments, for instance, calendering. As the conventional techniques all require direct contact of the coating device with the paper or paperboard web, a high strength of the web is necessary for machine runnability.

Web breakage in the coating section due to strength failure is the most common reason for production stops, especially when coating is an on-line operation in conjunction to the paper or paperboard machine. For this reason, coating has been mainly limited to paper or paperboard grades with relatively strong pulp furnish, typically so-called woodfree grades having chemical pulp as the major component. In case of paper or paperboard grades based on mechanical pulp, for instance LWC (Low Weight Coated), MWC (Medium Weight Coated) or MFC (Machine Finished Coated), a high percentage of the pulp furnish, usually 50%, has to be constituted with so-called reinforcement pulp which is a chemical pulp much stronger than mechanical pulp both in dry and wet strength, however much more expensive.

As is well known, mechanical pulp is not only more economical in production and more environmentally friendly, but also superior in printing properties compared to chemical pulp. There is therefore a great incentive to maximize the use of mechanical pulp in coated grades, provided obstacles such as breakage problems can be handled properly.

In the conventional coating process, moisture content of the base paper or base paper- board is a key parameter. Strength of the paper web decreases rapidly with moisture content. If the moisture content is too high entering the coating unit, or the moisture content is drastically increased upon contact with the aqueous coating colour, breaking of the web may occur thus jeopardising the coating operation. In addition to interme- diate drying of the base paper or base paperboard, the mill has to find a compromise between, on one hand, as high as possible a solid content of the coating colour and, on the other hand, an appropriate viscosity of the coating colour. Too high a viscosity at increasing solid content leads to inferior coating quality and also increases the risk of worn-out of the coating device.

Disclosed in US 4,944,960 and later in US 5,633,044, a spray coating technique provides new means for the coating of paper and paperboard. The technique is characte- rized by non-contacting application of an aerosol of coating material on the web surface, generated in arrayed nozzles at high pressure in a closed chamber. Despite the disclosu- res there has been no industrial implementation of the spray technique so far, neither has there been any report on the possibility of applying this technique to obtain mainly mechanical pulp based coated paper or paperboard.

DESCRIPTION OF THE INVENTION The present invention utilises the spray coating technique according to US 4,944,960 and US 5,633,044 in the production of coated paper and paperboard comprising a base paper or paperboard which to more than 50% dry weight consists of mechanical pulp as defined in present claim 1. By mechanical pulp is meant either SGW (stone ground- wood pulp), PGW (Pressurized stone groundwood pulp), SPGW (Super-pressurized stone groundwood pulp), RMP (refiner mechanical pulp), TMP (thermo mechanical pulp), CTMP (chemi-thermo mechanical pulp), or recycled mechanical pulp of any sort, or a combination of the said mechanical pulps. According to one aspect of the invention, said base paper or paperboard to more than 60% dry weight, preferably more than 70% dry weight and even more preferred to more than 80% dry weight, consists of mecha- nical pulp. The cellulose raw material used is preferably hardwood and/or softwood, although other raw materials are conceivable.

The objective of the present invention is to produce coated paper or paperboard mainly based on mechanical pulp with a minimum use of reinforcement chemical pulp, thus achieving better production economy and at the same time improving the product properties such as printability that are favoured by the use of mechanical pulps.

Another objective of the present invention is to reduce the demand of drying either of the base paper prior to the coating operation or of the coated paper after the coating operation. Yet another objective of the present invention is to find a more economical recipe/composition for the coating colour, as conventional techniques require high solid content in the coating colour, which both limits the choice of coating pigments and demands special treatment of the coating mixture with for instance dispersing agents.

By the present invention it has most surprisingly been shown that a paper web comp- rising as low as down to 10% reinforcement chemical pulp can be successfully coated with the spray coating technique. The strength property of the coated products is fully acceptable according to pilot trials in respect to runnability and is comparable to similar products produced with conventional techniques but having 50% reinforcement pulp. In production of paperboard, the term paper web means uncoated base paperboard.

By the present invention is has also most surprisingly been shown that a base paper or paperboard having moisture contents of as high as up to 25%, preferably up to 15%, can be coated without causing runnability problems in the coating operation because of the non-contacting coating operation. The high moisture content does not adversely affect the coating quality after drying and supercalendering.

Hydrophobic treatment with sizing agents such as alkenyl succinic acid anhydride (ASA) or alkyl keten dimer (AKD), starch or other polymer based sizing agents, in amounts of 0.1-3 %, preferably 0.1-1 % and even more preferred 0.1-0.5 % of total furnish weight for the paper or paperboard gives further improvement of the results.

Other additives such as wet strength agents for instance can also be added to the pulp furnish to further improve machine runnability and product properties.

In optimisation of the coating recipe, which comprises coating pigment, binder and other chemicals, the present invention further shows that the coating pigment, the major component in the composition, can be totally or partially substituted by low-cost fillers consisting of calcium carbonate. The total content of calcium carbonate, filler grade or coating grade, can amount to at least 80%, preferably at least 90%, and up to 100% of the total pigment used for coating. The high amount of calcium carbonate results in higher brightness of the coated products, as compared to coating recipes with large amount of kaolin.

With the non-contacting spray technique according to the present invention, the penetration of coating colour into the base paper substrate is lower than with conven- tional contacting technique, the water retention by the base paper is reduced which allows for lower solid content of the coating colour. According to the invention is has been shown that a solid content of 40-60%, preferably 45-55% can be used without causing any operation problems in coating, or mottling problems in the coated products.

Furthermore, the low viscosity at relatively low solid content allows for the addition of, synthetic thickeners, preferably in an amount of 0.5-2% based on the coating colour weight, with the purpose of increasing the pore volume and hence the bulkiness of the coating layer of the coated surface, without risking poorer runnability due to too high a viscosity caused by the additives. This, in other words, provides more room for modifi- cation of the coating composition. According to another aspect, the coating composition may comprise protein, preferably soy protein, in an amount of 0.5-3% based on the coating colour weight.

Also, because of the less stringent requirement on coating pigment, recycled pigment can be added to the coating colour thus giving considerable benefits both economically and environmentally.

To further improve the machine runnability and product properties, carbon dioxide gas can be mixed and dissolved into the coating colour suspension. This has the effects of improving the evenness of the coating layer on the paper surface, as well as stabilizing the coating pigment that to a large extent consists of calcium carbonate. The coating layer has hence better stability and tenacity in the press room upon contact with acidic dampening water used in printing on the press machine.

According to another aspect of the invention, the coating is applied to provide coat weights with a grammage lower than 10 g/m2, preferably lower than 8 g/m2 and even more preferred lower than 6 g/m2, but at least 2 g/m2, on one or both sides of the base paper or paperboard.

According to yet another aspect of the invention, the coating is performed by use of one or a number of nozzle zones inside the closed coating chamber. In case of two or more nozzle zones being used, different coating colour compositions may beneficially be used in the different nozzle zones, in order to optimise the coating. Also, the coating may be performed, simultaneously or in different steps, on one or both sides of the paper or paperboard, as single or multi-ply coating. Moreover, the coating stage may be per- formed in a paper or paperboard machine operated in a so-called on-line mode, or it may be performed in a so-called off-line coating which is physically separated from the paper or paperboard machine.

EXAMPLES In the following the invention will be exemplified by examples. The results discussion is referring to the drawings, of which: Fig. 1 is showing tensile index for different base papers used in the trial series 1, Fig. 2 is showing tear index for the different base papers, Fig. 3 is showing tensile index for the different coated papers, Fig. 4 is showing tear index for the different coated papers, Fig. 5 is showing opacity for the different base papers and coated papers, Fig. 6 is showing brightness for the different base papers and coated papers, Fig. 7 is showing print gloss for the different coated papers, Fig. 8 is showing opacity for the coated papers of trial series 2, Fig. 9 is showing brightness for the coated papers of trial series 2. grial series 1 : Base paper with digerent pulpMurnish Shown in Table 1 are the base paper furnish and coating colour composition for seven different samples in trial series 1. The pigment used in this trial series was a coating grade calcium carbonate. The coat weight was about 10 g/m2 on each side of the base paper. As commercial reference, a blade-coated paper with a base paper furnish of 50% chemical pulp and 50% groundwood pulp was chosen, sample 7. The coating pigment for the reference consisted of 70% kaolin and 30% calcium carbonate, both of coating grade. Because of the poor strength property, only spray coating was performed on the mechanical pulp based samples 1-6 (inventive). Note that there was a small amount of fillers in the mechanical pulp based base papers. This presumably further weakened the base paper. The spray coating operation was performed on a pilot spray-coating machine.

Table 1. Base paper furnish and coating colour recipe in trial series 1. Sampl 1 Sampl 2 Sampl 3 Sampl 4 Sampl 5 Sampl 6 Sampl 7 Spray-coated Spray-Coated Spray-Coated Spray-Coated Spray-Coated Spray-Coated Blade-Coated Base paper fumish Grammage (g/m2) 42 42 42 45 45 45 45 45 Chemical pulp 20% 20% 20% 11% 11% 11% 50 Filler 10% 10% 10% 6% 6% 6% Groundwood pulp 70% 70% 70% 50% 50% 50% 50 Thermomechanical pulp 33% 33% Sizing agent 0.8% starch 1.2% Raisafob 0.2% Raisafob 0.2% Raisafob 0.2% Raisafob Coating Color Hydrocarb CC (coating grade) 100 100 100 100 100 100 (Kaolin) 70* Hydrocarb HO (filler grade) 30 Latex SB330 (binder) 14 14 14 14 14 14 14 Nopcote C-104 (lubricant) 0.5 0.5 0.5 0.5 0.5 0.5 Bacote 20 (hardener)0.5 0.5 0.5 0.5 0.5 0.5 OBA (brightening agent) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Other additives 2 (thickener:Coatex RT5) 3 (soya protein) pH 8.5 8.5 8.5 8.5 8.5 8.5 Solid Content, % 55 55 55 55 55 55 Coat weight /side, g/m2 11/9 11/9 11/9 10 10 10 10 Note: The base paper had a grammage of about 42 g/m2 for samples 1-e, and 45 g/m2 for samples 4-7. After coating the grammage was about 62 and 65 g/m2,<BR> respectively. The composition gives by custom the dry amounts of various components of coating colour relative to the dry amount of pigment, which is given the<BR> number 100. If the pigment consists of more than one component then the sum must be 100.<BR> <P>*This is a kaolin pigment of coating grade in the commercial reference. The other ingredients are not known. Raisafob and OBA manufactured by Raisio Chemicals,<BR> Hydrocarb CC and Hydrocarb FF are manufactured by Omya AG, Latex SB 330 is manufactured by Latexia SB, Nopcote C-104 is manufactured by Henkel Nopco<BR> Oy, Bacote is manufactured by HCI Nordic Oy.

As shown in Figures 1-2, the tensile and tear strength measured on the base papers differed depending on the pulp furnish. The commercial reference (sample 7) with 50% chemical reinforcement pulp had considerably higher tensile and tear strength than the inventive mechanical pulp based samples 1-6. After coating, the difference in tensile strength became much smaller. For the tensile index in the cross machine direction, the highest tensilestrengtb was obtained with mechanical pulp based samples, Figure 3. As for the tear index, the coating evened out the difference as well, at least in the machine direction, Figure 4.

Comparing Sample 1 and Sample 2, both having the same base paper, it is noticed that an addition of wet strength agent (starch) by 0.8% improved the tensile strength, while addition of sizing agent reduced the tensile strength but increased the tear strength.

The result indicates that spray coating could be applied on base papers with much weaker pulp furnish. The insensitivity of the base paper strength on coating operation enables an easy incorporation of the coating stage in a so-called online mode, which would greatly improve productivity.

Given the relatively high coat weight, 10 g/m2 on each side, the results are considered extremely good. Based on the results, it is fully possible to use a lower coat weight, for instance around 5 g/m2. At the same time, the amount of mechanical pulp could be increased and still the same strength and runnability would be retained. For low-quality product newsprint that is traditionally produced with up to 100% mechanical pulp and sold uncoated due to low strength, the present coating concept provides an efficient way to improve the surface properties and printability. New product grades derived from newsprint are thus possible.

As shown in Figure 5, the opacity of the coated papers, an important parameter in comparing printability, was higher for the spray-coated mechanical pulp based samples.

This is mainly due to the high amount of mechanical pulp in the base paper furnish resulting in higher opacity. Although the brightness and print gloss (Figures 6-7) were lower for the spray-coated mechanical pulp based samples, the increase in brightness after coating was much more pronounced for the mechanical pulp based samples. The difference after coating was thus to a large degree minimized. Despite the much lower base paper brightness, the brightness of coated paper with higher amount of mechanical pulp (samples 4-6) could be increased to the brightness level around 75% ISO. Addition of thickener in the coating colour significantly improved the brightness and printed gloss, Figures 6-7.

Trial series 2 : Moistuf°e content in the base paper and new pigmefat offiller grade Having established, in trial series 1, that a coated paper mainly based on mechanical pulp can be produced with spray coating technique, a new trial series with a filler grade calcium carbonate pigment (Hydrocarb FF, Table 2) was performed to study the in- fluences of moisture content on the coating operation and coating quality. In trial series 2, the base paper consisted of 90% mechanical pulp and 10% chemical pulp.

Table 2. Coating colour composition. Chemical Manufacturer parts Calcium Carbonate pigment CC-75 Omya AG - Calcium Carbonate pigment Hydrocarb FF Omya AG 100 Binder additive Latex SB370 Latexia SB 12 Lubricant additive- Hardeneradditive- Brightening additive OBA Raisio Chem. 0.5 Thickener additive Coatex RT5 Protein Tech. 2 Int. H 8.5 Solids-% 50 Coatweight g/m2 _8 Final moisture of coated Moisture-% 8 paper In Figures 8-9 there is shown the results for spray coated paper according to: sample A denotes spray coated paper at 10% moisture content, sample B at 15% moisture content, sample C at 10% moisture content with super calendering, sample D at 15% moisture content with super calendering, sample E at 10% moisture content with soft calendering, sample F at 15% moisture content with soft calendering. The trial series was successful with moisture content 10% and 15%, which are conside- red high for base paper. The moisture content of 15% had no negative effect compared to 10% moisture content. The coating operation is thus characterized as wet on wet- coating without any intermediate drying.

It is further shown in trial series 2 that a 100% substitution of the coating pigment by a filler grade calcium carbonate (Hydrocarb FF) is fully acceptable.

While the invention has been exemplified by the different embodiments of the exam- ples, it is to be understood that the invention is not limited thereby but can be varied within the scope of the claims.