BACKGROUND OF THE INVENTION The predominant part of all paper and paperboard that is produced to be used in copying machines, laser printers, ink jet printers and in different printing methods such as offset printing and flexoprinting e. g. , is surface treated in the papermaking machine by the surface being coated by some sort of composition. This composition may be composed in the main part of starch in a dissolved form, in which case the surface treatment usually is called a surface sizing. If the composition contains some sort of pigment, in the form of solid particles, the surface treating is called pigmenting and the composition is called a coating paste.

The objectives of this surface treating are several. Surface coating with compositions consisting in the main of starch solutions prevents the paper from dusting in printing, copying and optional post treatment e. g. In certain cases, some type of synthetic polymer is added to the surface sizing solution in order to improve printing quality when using an ink jet printer.

Compositions containing pigments, coating pastes, are most often used to improve the printability of the paper. Moreover, many paper properties are improved. Above all, improved printability properties are attained by improvement of the surface smoothness.

The printing ink absorption increases and becomes more even and the representation of images is improved. To these advantages may be added that the paper exhibits an im- proved"feeling".

A conventional coating paste usually contains an inorganic pigment that most often consists of calcium carbonate and/or clay. Other pigments exist too, such as e. g. silicon

dioxide and in rare occasions, organic pigments such as plastic pigments. Besides plastic pigments, the paste also contains a binding agent that can consist of dissolved starch and/or a synthetic binding agent. Examples of common synthetic binding agents are styrene butadiene latex and acrylate latex.

The applying of starch solution or coating paste may take place in different ways. One common method is to apply the starch solution or coating paste, by aid of grooved rods, onto two rolls that form a nip through which the paper web passes. The starch or paste applied onto the rolls is in that way transferred to the paper web, in the nip.

Another method is to transfer the starch or paste from a roll or in direct contact with a fountain, to the paper web in order in the next step to level out the film formed on the paper web, by aid of a blade. In this surface application method, which is called a blade coating, one side of the paper is usually treated first, where after that side is dried before the next side is treated in the same way.

Yet another treatment method has been developed lately. This method is described in a number of patent applications, WO 97/13035 and WO 97/13036 e. g. , and amounts in short in that the starch solution or coating paste is applied on the running paper web by spraying. This contact free application offers many advantages and the technique is expected to be used in an ever increasing extent in the future.

Just before the finished paper is rolled up in the end of the paper making machine, the paper web passes one or more roll nips in the so called calendering machine, in order for the surface coarseness to diminish. The surface coarseness is measured in the unit "ml/min Bendtsen", and the lower the value, the greater the surface smoothness of the paper. The surface coarseness decreases with an increased line pressure, which is positive for the printability properties e. g. , of the paper. The bulk however also decreases with an enhanced line pressure, which on the contrary is not desirable.

Another important property of the paper or paperboard is its opacity, which constitutes a measure of the lack of transparency of the paper. By an increased opacity, the basis weight of the paper (the weight in gram of 1 m2 paper) may be decreased with large economical savings as a result. An increased opacity for a copying paper or a paper intended for offset printing also allows for improved use of both sides of the paper for printing or copying, without the print on the one side being disturbed by the print on the

opposite side. For a paper intended for envelopes e. g. , the importance of having a good opacity is great.

Another important optical property is the whiteness of the paper or paperboard. A high whiteness is achieved by choice of raw material having a high brightness, and by aid of optical whiteners. An efficient use of an optical whitener demands that it functions together with the surface treating composition. Here, the concept of"carriers"has been introduced, meaning that one of the components in the surface treating composition has to be a good carrier for an optical whitener, in order for the whiteness to develop efficiently.

Yet another important property is the thickness of the paper. For one thing, the thickness has a value in it self, giving e. g. a thicker printed paper that gives an expensive im- pression. For another thing, the stiffness of the paper increases with the thickness (the stiffness is a function of the E-modulus times the thickness in cubic). An increased thickness (decreased density) of the paper may also be used in order to decrease the basis weight. Paper makers most often use the concept of"bulk", which is the inverted value of the density. Accordingly, a bulk that is as high as possible is aimed for.

Another important property is the air permeability of the paper. Here, a relatively low value is aimed for, i. e. a high air resistance, in order to among other things avoid double feeding in suction fed printing presses and through-printing. in connection with flexoprinting. A high air resistance is also necessary in the use of machines for automatic insertion in envelopes. Here too, an air resistance that is too low causes double feeding and problems in opening the flap of the envelope.

Besides fibres, paper and paperboard also contain calcium carbonate or clay. The content of such inorganic materials, called fillers, is usually between 15 and 25 %.

Nowadays, the most common filler in the production of printing and copying paper is PCC which is short for Precipitated Calcium Carbonate. PCC does on the one hand give a relatively high bulk, but this filler lowers the air resistance to levels that in many cases are unacceptable.

Another important property of the paper, which is affected by the surface treatment, is its printability properties. Besides the properties already mentioned, such as a higher and more even absorption of printing ink and an improved representations of images, that hitherto has been the object in offset printing above all, more and more work has

been put down in order to achieve better printability properties in the use of so called ink jet printers. A paper with good printability properties for ink jet printers is in the main characterised by the following: A minimal or non existing running of ink outside the printed object, so that the edges of the object will not become uneven and sprawly. The objects, the characters e. g. if it is a text that is printed, are perceived as unclear and blurry if this property is not fulfilled. The running of ink is measured by visual methods or by aid of an image analysis instrument.

A high optical density. This property is most often measured by an optical densitometer.

A minimal through-print. This is not achieved if a penetration of the ink pigment or colouring agent into the paper is too strong, causing the colour to be seen on the back side of the paper. A high through-print often results in a low optical density. The through-print is measured visually or with an optical densitometer.

Low or non existing so called bleeding. If ink is applied on a surface where ink in another colour already has been applied, one of the colours may run into the other. This problem, that is caused by too slow penetration into the paper, is called bleeding and is measured visually or by aid of image analysis.

In order to achieve a paper that has good printability properties for ink jet printers, the technique known hitherto has only offered very expensive, special pigments and in the use of cheaper pigment such as calcium carbonate or clay, these printability properties have not been noticeably affected.

As a conclusion, a surface sized paper has good physical properties such as a low density and a high stiffness that result in good runnability in printing presses, copying machines, printers and converting machines, such as envelope manufacturing machines, but exhibits an impaired printing quality in the use of ink jet printers and different printing presses. A pigmented or coated paper gives a considerably improved printing quality, but has a bad stiffness due to its low thickness and it is most often perceived as "limp". In order to achieve a good quality in more demanding colour prints by aid of ink jet printers, expensive special pigments have however hitherto been required in the paper manufacturing.

In the patent literature, there is described the use of cross-linked starch in connection with paper manufacturing, the cross-linked starch however not being used in surface treating compositions. Examples of such patents are US 4,810, 785 ; US 5,122, 231 ; US 5,368, 690; US 5,523, 339.

From US 5,314, 753, it is known to use non gelatinised starch grains in a coating composition for matt paper in an amount of from 10 to 65 percent by weight, based on the solids content. The remaining part of the solid particle content is calcium carbonate.

From US 5,439, 558, it is known a coating composition that in the main comprises kaolin and calcium carbonate, but also 2-25 % non gelatinised starch grains, counted on the total pigment content.

From DE 26 05 575, it is known to use 0.5-10 parts of non gelatinised starch particles in a latex coating that is specially developed for bank notes, in order to fulfil the very special requirements on bank notes.

SE 421 638 shows a wallpaper that can be embossed and printed, and that can have paper or paperboard as a base material. The wallpaper is coated with a coating agent that consists of non gelatinised starch in an amount of 10-100 % by weight.

ACCOUNT OF THE INVENTION The objective of the invention is to offer a surface treating composition, by aid of which the problems mentioned above are eliminated or limited and/or great economical profits are attained. The invention may beneficially be used for papers of different types, such as paper for ink jet printers, envelope paper, copying paper, paper for laser printers or other types of paper or paperboard.

Accordingly, it is a main objective of the invention to offer a new and improved surface treating composition that can be used in the manufacturing of paper.

Another objective of the invention is to offer a new surface treating composition that gives improved printability properties to the paper in the use of ink jet printers and printing presses, in a manner that is considerably more cost efficient than for known compositions.

Another objective of the invention is to offer a new and improved surface treating composition that gives a higher bulk to the paper.

Another objective of the invention is to offer a new and improved surface treating composition that gives a higher air resistance to the paper.

Another objective of the invention is to offer a new and improved surface treating composition that gives a higher opacity to the paper.

Another objective of the invention is to offer a new and improved surface treating composition that gives a higher brightness and/or that can reduce the requirement of optical whiteners.

Another objective is to offer an improved method of producing paper.

Another objective of the invention is to offer paper with improved properties.

These and other objectives are attained according to the invention by the offering of a surface treating composition that comprises at least one cross-linked starch which not at all or only partly has been dissolved and which thereby also exists in the form of solid starch particles (pigment), in an amount of more than 65 % at room temperature and counted on the content of solid substance in the composition. At least 10 % of the starch content of the composition exists moreover as solid particles in the entire temperature range of 20-50 °C. The latter feature contributes to the effect of the invention since the pigment, already at the manufacturing, is exposed to an increased temperature. This takes place as the starch grains are mechanically treated (by extrusion or grinding e. g.), in order to obtain a suitable particle size, preferably 1-5 urn and even more preferred 1- 25 um. The composition is moreover heated before and in connection with the applying on the paper or paperboard web, temperatures of up to about 50 °C being common.

Finally, the composition is heated as it dries on the paper or paperboard web in the paper or paperboard machine.

According to one aspect of the invention, the content of starch particles is more than 70 %, preferably more than 75 % and even more preferred at least 80 % at room tempera- ture and counted on the content of solids in the composition.

It is preferred that the just given ranges for the content of starch particles in the composition are valid at 50 °C too.

Cross-linked starch is obtained by reaction with bi-or polyfunctional reagents. They react with several hydroxyl groups in the starch and result thereby in the building of bridges between the starch molecules. At an increased degree of cross-linking, the starch grain becomes more and more resistant to gelatinising.

The cross-linked starch that is used in this invention may include starch that is treated with one or more of the compounds described in"Starch Derivates ; Production and Uses"by Morton W. Rutenberg and D. Solarek and"Starch ; Chemistry and Technology", Chapter X, pp. 324-332,1984, and the patents referred to above. Such cross-linking compounds may consist of methylamine compounds, polyvalent acids, polyvalent acid esters, polyvalent acid halides, polyvalent acid anhydrides, polyaldehydes, polyepoxides, polyisocyanates, divinyl compounds, phosphoryl chloride, polyamine polyepoxide resin, 1,4-butane diolglycidyl ether, epichlorohydrin, trimetaphosphates, mixtures of anhydrides of acetic acid and two and three protonic acids, sodium hypochlorite, phosphor oxy trichloride.

At none or a low degree of cross-linking, the starch dissolves if a starch slurry is heated.

At an increased degree of cross-linking, the disposition of the starch to go into dissolved form, decreases. The starch used in the invention may have very different degrees of cross-linking. From a relatively low degree of cross-linking, in which about 10 % of the starch exist as solid particles at a temperature of about 20 to 50 °C, and the rest of the starch constitutes a binder in dissolved form, to a high degree of cross-linking, where about 100 % of the starch exist as solid particles in the temperature interval given above. It is preferred that at least 15 %, preferably at least 20 % of the starch content of the composition at about 20 to 50 °C is constituted by starch particles according to the invention. It should be noted that what has just been stated is not contradictory to the limitation that more than 65 % of the solids content should be starch particles, given that the solid pigments of the composition are essentially only composed of these starch particles or only at very small amounts comprise other types of pigments. Independent of the cross-linking degree, the composition may also contain other types of soluble starch or some synthetic polymer, in order to obtain an adequate amount of binder and further improved printability properties. Examples of such polymers are styrene- acrylate, styrene maleic acid anhydride, polyvinylalcohol, polyvinylpyrrolidon, polyvinylformamide.

In order to achieve some of the paper properties that have been mentioned above, as good printability properties when using ink jet printers e. g. , the starch should be additionally modified. The starch may be modified to act as cation active, anion active, amphoteric or hydrophobic.

In the production of cation active starches, the starch is substituted by cation active groups, ammonium compounds most often being used, that preferably are quaternary, but that also may be primary or tertiary. Such a cationising process for starch is well known and is described among other places in US 4, 088, 600; US 2,876, 217 and US 4,840, 705. In the production of anion active starches, the starch is substituted with anion active groups that most often consist of carboxylic, carboxylate, sulphonic, phosphonic or phosphate groups. Such methods are well described in the literature, among other places in the references mentioned above, by Morton W. Rutenberg and D. Solarek, and STARCH: Chemistry and Technology by Roy L. Whistler, James N. Bemiller and Eugene F. Pachall, respectively. In the production of amphoteric starch, the starch is substituted both by cation active and anion active groups.

Preferably, the starch exhibits a degree of substitution of 0. 01-0. 30 and even more preferred 0.02-0. 20. Uncharged starch too may however work according to the invention.

In the manufacturing of hydrophobic starches, several reagents may be used, whereof benzyl chloride, styrene oxide, phenyl isocyanate and butyl glycidyl ether may be mentioned.

By the starch being brought to react with for example the chlorohydrin of a quaternary ammonium compound, cationic hydrophobic groups are incorporated in the starch molecule.

According to one aspect of the invention, the surface treating composition may also comprise a minor amount of other pigments in the form of solid particles. Such pigments may e. g. be constituted by calcium carbonate, clay, silicon dioxide and/or aluminium oxide. It is however also conceivable, and in many cases preferable, that the solid content of the composition is essentially only constituted by the inventive starch particles.

Surprisingly, it has been shown that effects are obtained by the composition mentioned above, that significantly reduce or eliminate many of the shortcomings that can be connected with prior art. Among other things, the printability properties, brightness, opacity, bulk, surface smoothness and air resistance of the paper are improved. More specifically, the printability properties of the paper are improved when ink jet printers are used, whereby the running of ink decreases, the optical density increases, the print- through decreases and the bleeding decreases or is eliminated. In addition, the print- ability properties of the paper are improved when printing presses based on offset or flexoprinting technique are used. In colour copying, the image quality is e. g. improved.

Double feeding in suction fed printing presses is avoided and envelopes manufactured from paper that has been surface treated with said compositions work excellent in automatic document insertion machines.

One theory is that the positive, surprising effects of the invention depend on that the particles of the surface treating composition are amorphous and that they thereby have a much larger specific surface than conventional pigments. The high specific surface of the filler composition is probably also the reason for the improved printing properties of the paper when ink jet printers are used as the absorption of ink into the paper is im- proved due to an increased number capillaries having a smaller radius. This large surface also advantageously absorbs printing ink in offset printing. Furthermore, it has been observed that the starch particles, even if being non gelatinised, exhibit an advantageously sticky surface.

In the printing by aid of flexo-technique, the denser surface may be the explanation for the improved printing result as the flexo-ink remains in the surface of the paper. The large surface of the starch pigments at the same time prevents slow drying by its good absorption ability. The plastic character of the particles makes them more easily calendered, whereby only a relatively low line pressure has to be used in the calendering machine in order to obtain the desired surface smoothness. This low line pressure may, together with the, as compared to calcium carbonate and clay, significantly lower density of the starch particles, explain the high bulk that is obtained compared to conventionally surface sized or coated paper. The lack of transparency of the starch particles also increases the opacity of the paper as compared to the effect of the transparent starch solution that is used in surface sizing.

EXAMPLE

A non surface treated paper having a basis weight of 80 g/m2 was coated in a pilot test with a composition consisting only of cross-linked starch in water, at room temperature.

The starch was delivered by Emsland-Starke Gmbh and had the trade name F 6270. The starch was cationic, with a nitrogen content of 0.35 %. The average particle size had been brought down to about 10, um by mechanical treatment. Of the starch, about 15 % was dissolved and accordingly, about 85 % of the starch existed as solid particles. The coating was calculated to 1.5 gram per square meter and side. The applying on the paper surface took place by aid of a SymSizer from Metso Oy. This paper was called A.

As a reference, there was used a paper that had been surface sized by starch solution in a conventional way, i. e. all starch was in dissolved form. The coating on this paper was also calculated to 1.5 gram per square meter and side and the nitrogen content was 0.15 %. This paper was called B.

The physical paper properties are shown in Table 1.

Table 1 Property Paper A Paper B Basis weight 84 gram/m2 84 gram/m2 Air resistance (Gurley). 29 seconds 14 seconds Opacity93. 4% 91. 1 % Bulk 1.42 cm3/gram 1.34 cm3/gram Whiteness 159 % CIE 152 % CIE Surface roughness 192 ml/min Bendtsen 208 ml/min Bendtsen The optical density was measured in printing with an ink jet printer of the type Hewlett Packard 950 C and the ink bleeding was measured in printing with an ink jet printer of the type Epson 980 C. These printers are very common and constitute excellent tools for evaluating the printing quality of different papers. The optical densities measured are shown in Table 2.

Table 2 Property Paper A Paper B Density-black 1. 50 1. 48 Density-cyan 1. 39 1. 32 Density-magenta 1. 29 1. 20 Density-yellow 1.14 1.08 Bleeding 2.0 2.0

In the visual evaluation of print-through, paper A exhibited significantly less penetration into the paper as compared to paper B. Both papers exhibited a minimal running of ink.

In printing with other types of ink jet printers, the same improvement was achieved, especially concerning the optical density. The difference between paper A and B was very clear in visual evaluation (blind test) of tests printed by ink jet printers.

The invention is not limited to the described embodiments, but may be varied within the scope of the claims.