EP2109697A2

EP2109697A2 - A printing substrate

Info

Publication number: EP2109697A2
Application number: EP08737606A
Authority: EP
Inventors: Olli Hakkila; Paul-Heinz Daehling
Original assignee: Stora Enso Oyj
Current assignee: Stora Enso Oyj
Priority date: 2007-02-05
Filing date: 2008-02-04
Publication date: 2009-10-21
Also published as: CN101680188A; EP2109697A4; FI20070635A0; WO2008096274A8; WO2008096274A2; WO2008096274A3

Abstract

The invention relates to a printing substrate for gravure and offset printing, which substrate exhibits a Cobb(w15) water absorption value below 15 g/m², preferably below 12 g/m², a Cobb(o,C10) oil absorption value below 1.2 g/m², preferably below 1 g/m², and an Ink setting below 0.5, preferably below 0.3. The low water and oil absorption in combination with the quite fast ink setting of the substrate according to the invention gives rise to a printed substrate with improved printing qualities.

Description

A printing substrate

TECHNICAL FIELD

The present invention relates to a wood fiber containing printing substrate. It additionally relates to a printing method comprising said printing substrate.

BACKGROUND OF THE INVENTION

One of the most commonly used printing methods of today is offset printing, a printing process that uses an intermediate blanket cylinder to transfer an image from an image carrier (the plate) to the substrate (usually paper) . An offset printing unit comprises a plate cylinder, ink and dampening rollers, a blanket cylinder and an impression cylinder. The printing plate comprises hydrophilic, water receptive parts and hydrophobic, ink receptive parts. The water receptive parts of the plate form non-image areas whereas the ink receptive parts form image areas. The dampening rollers apply chemically modified water, so called fountain solution, to the non-image areas of the plate whereafter ink is applied to the image areas of the plate. The ink and the fountain solution are transferred to the printing substrate from the printing plate via a rotating blanket cylinder which is covered with a smooth offset blanket.

In the heatset web offset printing method, the substrate is continuously printed in one or more printing units and subsequently dried in a hot air dryer in order to dry the ink printed on the surface. Finally, the ink is hardened on chilling rolls. Both sides of the moving web are printed simultaneously in a printing nip formed by the blanket cylinders of an upper and a bottom printing unit. In conventional 4-color printing, eight printing units are needed, four on both sides of the web. Ink from all printing units is transferred onto the substrate before the drying. This means that ink from the first printing unit is printed on a dry paper surface and that ink from the last printing unit is transferred on a substrate that has been wetted with the fountain solution by the preceding printing units.

The printing ink mainly consists of colorant pigment, binder to fasten the pigment on the substrate surface and solvent to provide appropriate flow properties. In the dryer, most of the ink solvent is evaporated quickly in hot air. The ink film on the substrate surface gets its' final hardness in the cooling stage on the chilling rolls. Thereafter, the printed web usually enters a folding unit, where it is folded and cut into printed signatures.

Fountain solution is needed to separate the image area and the non-image area on the offset printing plate. However, the fountain solution may cause several problems after the ink is transferred onto the printing substrate. Fountain solution transferred to a coated printing substrate may decrease the strength of the coating layer. In multicolor heatset web-offset printing of coated substrates, the substrate is wetted in the first printing unit. In the subsequent printing units, the ink is transferred on an already wetted substrate surface. When tacky ink is transferred on a coating layer with decreased strength, picking of loosened coating particles might occur. These particles have a tendency to accumulate on the printing blanket in a cumulative way causing a need to clean the blankets.

It has previously been assumed that the printing substrate, e.g. a printing paper, has to be highly porous and hydrophilic in order to absorb the fountain solution transferred to the substrate. However, the penetration of fountain solution into a porous, fibrous material may cause swelling of wood containing fibers. The swelling of fibers causes an increase in the paper dimensions so that an image printed in the first pair of printing units does not perfectly match the image printed in the fourth pair of printing units. This is especially a problem at the edges of wide webs. This so called register or fan-out problem is seen as blurred color images on the printed surface. Dot doubling which cause unwanted tones in a printed image may also result from dimensional changes in the substrate between the printing units.

Moreover, porous, water absorbing printing substrates need to be dried at high temperatures. The web temperature can exceed 100 ⁰C after 0,2 s in the dryer and web exit temperatures can reach 150°C. Drying at such high temperatures causes fiber roughening, fluting and blistering. This is especially a problem in the printing of paper comprising mechanical fibers. One reason for the occurrence of fiber rising is the fast moisture evaporation from the base sheet of a porous, coated printing substrate as vapor pressure opens fiber bonds. Fluting is the permanent formation of wave patterns in the print. The mechanism of fluting is not thoroughly understood, but it is a result of uneven moisture escape from the substrate due to high temperature drying. Blistering due to high temperature drying can sometimes be so severe that the base sheet delaminates. Blister bubbles can also lead to web breaks in the printing. The heatset web-offset printing method of today is also very energy consuming. The air temperatures in the dryer can typically reach 250 ⁰C.

Thus, there is a desire to provide a printing substrate that overcomes these problems.

In the prior art, there have been attempts to overcome these problems by providing a printing substrate with a non- porous, oleophilic surface. WO2004030917 relates to a coated printing substrate with an oleophilic surface, which substrate has a Gurley-Hill permeance value bigger than 5000s/100ml, and an IGT ink set-off value bigger than 0, 6 print density units at 30s delay time. By oleophilic surface it is meant that the surface repels the fountain solution, whereby it is apparent that the contact angle with water of the surface is more than 90 ° .

However, vapor originating from moisture in the base paper can not evaporate readily from a closed, non-porous substrate, such as the one described in WO2004030917. This gives rise to an enhanced vapor pressure inside the substrate causing severe print quality problems like fiber rising and blistering. These problems may take place even if the web exit temperature from the dryer is under 115 ⁰C as mentioned in WO2004003293, since the maximum web temperature is normally not in the web exit but somewhere inside the dryer.

Furthermore, some oleophilic surfaces, such as the one described in WO2004030917, also tends to be quite hydrophobic. If the non-porous surface is too hydrophobic the continuous flow of fountain solution from the printing plate via the blanket to the paper is disturbed. Fountain water concentrates in the printing unit which can be seen as print defects in certain image areas. On the other hand, if the surface is too hydrophilic, the forces between the substrate and the water are higher than the forces between the substrate and the oil. Consequently, water on the surface of the substrate repels ink which can result in lower color strength and uneven print.

The substrate disclosed in WO2004003293 is coated with plate like pigments, such as kaolin and talc. Kaolin and talc are relatively expensive pigments compared to for example calcium carbonate. Moreover, plate like pigments, such as kaolin and talc, gives rise to printing papers with a lower brightness compared to, e.g., calcium carbonate. SUMMARY OF THE INVENTION

One object of the present invention is to provide a printing substrate that do not involve the problems of the prior art.

A further object of the present invention is to provide a printing substrate for heatset web-offset printing, which has excellent dimension stability and allows a low energy drying.

Yet another object of the present invention is to provide a printing substrate for heatset web-offset printing which gives rise to low fluting and low fiber roughening and improved cracking resistance.

Yet another object of the invention is to provide a printing substrate with an improved print quality and runnability.

The above-mentioned objects, as well as other advantages, is attained by providing a printing substrate according to claim 1. The printing substrate can, e.g., be a fiber containing substrate, preferably a cellulose fiber containing substrate such as paper or paperboard.

The printing substrate of the invention exhibits a Cobb(wl5) water absorption value below 15 g/m², preferably below 12 g/m², a Cobb(o,C10) oil absorption value below 1.2 g/m², preferably below 1 g/m², and an Ink setting below 0.5, preferably below 0.3. Preferably, the Cobb (wl5) water absorption value within the range of 1 - 15 g/m², most preferably within the range of 2 - 15 g/m², and the Cobb (o,C10) is within the range of 0.2 - 1.2 g/m2, most preferably within the range of 0.3 - 1 g/m². The low water and oil absorption in combination with the quite fast ink setting of the substrate according to the invention gives rise to a printed substrate with improved printing qualities. The substrate of the invention exhibits low water absorption but, nevertheless, it allows vapor to penetrate through the structure, whereby problem like blistering is avoided. Moreover, the fountain solution transferred onto the substrate is prevented from penetrating into the substrate structure due to the low water absorption. In this way, no harmful changes in substrate dimensions due to fiber swelling take place and misregister and dot doubling can be avoided. Good dimension stability also allows the use of wide webs in printing with sharp pictures (good register) and good tone reproduction, which affects the development of wider heatset web offset presses. Low water absorption also gives rise to an improved productivity and less blanket washes are needed since the surface of the paper maintains its' good wet strength. Furthermore, other wet web problems such as web tension problems and web breaks, are avoided as the fountain solution does not penetrate into the fibrous inner structure and cause changes in fiber bonding. Thereby the runnability of the offset printing is improved. The consumption of the fountain solution decreases due to the low water absorption which is economically beneficial.

In one preferred embodiment, the printing substrate further exhibits a contact angle with water below 90°. In this way, water from the printing blanket is transferred to the hydrophilic printing substrate whereby a continuous flow of fountain solution from the printing unit onto the paper is enabled. Most preferably, the contact angle with water is below 90° but above 60°. This prevents the substrate from being too hydrophilic. Thereby, problems with low color strength and uneven print are avoided. In another embodiment of the invention, the printing substrate is a fiber containing substrate, such as paper or paperboard, coated with a coating composition, which composition comprises a pigment composition.

Preferably, the pigment composition comprises calcium carbonate. The use of calcium carbonate as a pigment is economical beneficial since calcium carbonate is a relatively inexpensive coating pigment. Moreover, calcium carbonate has a potential to improve paper brightness. Most, preferably, the pigment composition comprises calcium carbonate to an amount of more than 10 %, most preferably to an amount of more than 20% of the pigment composition. This enhances the optical properties of the substrate even further.

In one preferred embodiment, the pigment composition comprises fine particulate talc with a spherical diameter of micrometer size, calcium carbonate particles of nanometer size and a binder comprising a copolymer including as monomer at least one dicarbon acid and at least one monomer chosen from the group of diamine, triamine dialcanolamine or trialcanolamine. This specific pigment composition makes it possible to control the water absorption properties and the porosity of the paper, since the separation of the calcium carbonate particles from the talc particles is avoided. Moreover, the minor water and solvent that is absorbed by the substrate according to the invention is trapped by the nano size pigments in the structure of the coating layer and prevented from migrating into the base sheet of the substrate .

Preferably, the pigment composition comprises talc and calcium carbonate in a ratio within the range of about 3:1 to about 1:3, most preferably in a ratio of 2:1. The talcum content improves the water repellency of the surface and keeps the ink setting on an appropriate level. The substrate of the invention gives rise to good dry and wet strength which, in combination with the low water absorption of the substrate, further prevents the tendency of piling of coated material on the offset blanket. This allows the use of a substrate with fast ink setting and an easily drying low tack ink, which makes it possible to dry the printed substrate at low temperatures. Thereby, problems related to high temperature drying, such as fiber rising and waviness, are minimized. The specific surface properties of the inventive substrate allow the use of an easily drying ink, e.g., like the one disclosed in EP 1602696 A 1. This makes it possible to dry the print at temperatures chosen so that the maximum web temperature obtained in the drying is kept below 130 °C, preferably below 100 ⁰C. The drying of the printed substrate of the invention at such low temperatures results in a printed substrate with a higher moisture content. A printed substrate with high moisture content has better resistance against cracking when folded in a finishing stage and does not exhibit static electricity problems in handling of printed signatures. Cracking resistance is especially needed in wire-stitching stage in magazine finishing line as wire stables are pushed through the fold of printed signatures from different print runs and then closed underside. Moreover, the moisture remains even in base sheet which decreases the fluting of the printed signature. The problem of inside papers swelling out of covers is also prevented due to the high moisture content of the printed substrate. High vapor pressures in the substrate are further minimized due to the low temperature drying. This improves the print quality since fiber rising is minimized and it enables the use of a base sheet with a decrease of z-strength as the risk for blistering does not exist.

The invention further relates to a printed substrate comprising said printing substrate and heatset offset ink printed on the substrate and to a web heatset offset printing method comprising the steps of printing said printing substrate in at least one web heatset offset printing unit and drying said substrate in at least one drying unit. Preferably, the drying is performed at a temperature chosen so that the maximum web temperature obtained in the drying is below 130 ⁰C, preferably below 100 0C.

The printing substrate of the invention is most suitable for heatset offset printing, but can for example also advantageously be printed in the sheet fed offset and gravure printing methods. Although the above discussion regarding the advantageous of the printing substrate is primary associated with heat set web offset printing, it is understood that the low water absorption of the printed substrate in sheet fed offset printing also gives rise to a good dimension stability and water proof properties that are especially advantageous for products like maps or food packaging materials. The use of the printing substrate in gravure printing allows the use of water based inks since the low water absorption generates the good dimension stability needed in wide webs.

DETAILED DESCRIPTION

It has been discovered that an improved printing quality can be achieved by using a printing substrate with low water absorption but which, nevertheless, allows vapor to penetrate through the substrate. In accordance with the present invention, such a substrate is achieved by providing a base paper and a coating formulation designed to provide low Cobb water and Cobb oil values and a fast ink setting.

In one embodiment of the invention, such a substrate is achieved by providing a base paper with a coating composition comprising a pigment composition, which pigment composition includes inorganic or organic microparticles, inorganic nanoparticles and a binder. Preferably, the binder in the pigment composition is a copolymer comprising as a monomer at least one dicarbonacid and at least one monomer from the group consisting of diamine, triamine, dialkanolamine or trialkanolamine. In this way, the microparticles are covered by the nanoparticles and the segregation between the nanoparticles and the microparticles is avoided whereby the water absorption and the pore structure can be controlled.

In this context, microparticles mean that the particles' spherical diameter is of micrometer size, i.e. within the range of 0.3 - 100 μm, most preferably between 1 - 25 μm. Preferably, the microparticles are particulate talc, most preferably plate-like talc, since talc improves the water repellency of the surface and keep the ink setting at appropriate levels.

With nanoparticles it is meant that the particles' spherical diameter is of nanometer size, i.e. less than 200 nm. Preferably, the nanoparticles are particulate calcium carbonate. Calcium carbonate is an inexpensive pigment and gives rise to good optical properties. The calcium carbonate can be precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC) .

In one embodiment of the invention, the nano calcium carbonate is ground calcium carbonate (GCC) that is grinded in the presence of a hydrophobation agent, e.g ethylene- acrylic acid copolymer (EAA) . The hydrophobation agent reduces the water absorption of the nano calcium carbonate, and thus reduces the water absorption of the printing substrate even further. Other pigments, such as calcined clay, TiO2 and plastic pigments could alternatively or additionally be used in the pigment composition.

The pigment binder that prevents the separation between the nanoparticles and the microparticles may, for e.g. be a copolymer from adipic acid with N-2-aminoethyl) -1, 2- ethandiamin and epichlorhydrin. The binder is preferably present in the pigment composition at an amount of about 2% of the total amount as calculated on the microparticles.

The coating composition may further comprise one or more binder to bind the pigment composition to the surface of the substrate. This binder may be natural or synthetic and may include, but are not limited to, starch, protein and latex.

Other additives, such as lubricants, may also be added to the composition.

The base paper can be any kind of base paper. The coating composition may be applied at one or both sides of the base paper at a wide range of coat weights, e.g. in an amount of 5-40 gsm per side, preferably in an amount of 10 - 20 gsm per side. The coating can be applied on the base paper using any kind of coating technique, e.g. a size press, by roll or jet application, by jet coating, by blade coating or by curtain coating. The base paper can be multilayer coated, e.g. double coated, whereby one of the coating layers is the coating composition described herein and the other is a conventional coating composition. For example, the printing paper of the invention, i.e. the base paper coated with the coating composition that is designed to provide low Cobb values and a fast ink setting, can be further coated with an additional, conventional, coating composition. Alternatively, the base paper can first be coated with a conventional coating composition whereupon the coating composition designed to provide the low Cobb values and fast ink setting forms a top layer on top of the first, conventional coating. In these ways, the optical properties of the printing substrate can be further improved. The coated paper may be calendered, e.g. in a supercalender.

The invention is described in more details with reference to some examples below. It is to be understood that the invention is not limited to the particular process steps and materials disclosed herein.

Measurement and evaluation methods

Cobb Unger (o, CUlO) is measured in accordance with SCAN 37:77, CU 10. This method determines the oil absorbency of the paper or paperboard. According to this method, the mass of castor oil absorbed per unit area by one side of a paper or board during 10 s time under specified conditions is measured.

Cobb Unger (wl5) is measured using ISO 535:1991(E). In accordance with this method, the mass of water absorbed in a specified time by Im² of paper or board during 15 s time under specified conditions is measured. The conditioning atmosphere is according to ISO 187 (23°/50%RH) .

Ink setting is determined in accordance with Prϋfung von Druckpapieren, Merkblatt V/32/99. The ink set-off is a measure of the speed of ink setting on the substrate. The printed substrate is brought in temporary contact with a counter printing paper in a printing unit under defined line pressure and at fixed time intervals. Thereafter, the coloring resulting from the ink transfer to the counter paper is measured with an optical densitometer. The measurement is performed according to Zellcheming standard V/31793 with an amount of ink on the paper surface of 1,5 g/m². The smaller the ink setting value is the faster is the ink setting. In the example below, the ink used is Wegschlagtestfarbe Michael Huber Mϋnchen 520068 06.09.06, 0.100. The counter paper used is Scheufelen APCO II/II. Air conditioning is according to ISO 187 (23°C/50%RH) . The test direction is the print direction, parallel to the machine direction. The time intervals are: 15s; 30 s; 60s; 120s; 300s. The ink setting value is given at 60s time. The printing machine used is Prϋfbau Probeandruckgerat . In the printing of the substrate, endless vulcanized rubber with 65"shore hardness is used as printing form. In the printing of the counter paper a printing form in aluminum is used. The pressure substrate/counterform is 200 N/cm.

Porosity is measured using a micromeritics automatic porosimeter, Auto Pore III 9405. The evaluation range is 0,01 - 0,001 μm, and the measurement is conducted according to the pressure intervals shown in appendix I. According to the mercury Porosimetry method, the porosity is characterized by applying various levels of pressure to a sample immersed in mercury. As pressure increases during an analysis, pore size is calculated for each pressure point, and the corresponding volume of mercury required to fill these pores is measured.

Contact angle is measured using SCAN P 18:66. According to this method, the shape of a water droplet over the substrate surface is measured. If the substrate repels water, the drop will become ball shaped. If the water wet the substrate, the drop will be more planar. The water repelling or water loving properties can be found out by measuring the angle whose other leg is the base of the drop on substrate surface and the other leg is the tangent of the drop in the immediate nearness of the surface. In the example shown below, the angle is measured after 0.1 s and the conditioning atmosphere is according to ISO 187 (23°/50%RH) . K&N ink absorbency is measured in accordance with SCAN-P 70:95 using a special ink provided by K&N to evaluate the absorption of ink by the substrate surface. The K&N ink absorbency is measured by the percentage surface brightness drop when K&N ink is applied for 2 minutes and thereafter removed. The smaller the number for ink absorbency, the more nonporous the coating is and, consequently, a lesser degree of ink penetrates into the coating.

Fluting is measured in with a Lehmannprofilometer, AS300. This instrument works with a laser to record the wave contour on the printed surface. The fluting index is calculated via the square of the slope each wave, averaging several waves in different printed areas. The lower the fluting index is, the lower is the fluting.

Printing ink tack is measured with a Tack-O-Scope device in accordance with ISO 12634. The test condition is as follows: ink amount 0.4 ml, stabilizing time 30 s, stabilizing speed 50 m/min, testing speed 150 m/min, temperature 30 °C. Maximum ink tack and time for the maximum ink tack were taken from the tack curve to describe drying characteristics of the ink. The shorter the time for maximum tack is, the faster is the ink drying propensity. The tack value characterizes the force needed to split an ink film in ink transfer between the substrate and the printing blanket. A lower tack value means a lower splitting force and lower strength demand on the paper.

Passes to fail is measured with a Prufbaϋ Deltack instrument. In this method, ink is transferred onto a paper substrate whereupon the formed ink layer is split every third second. The printing ink tack increases during the test as ink sets on the paper. The operator will visually notice how many times the ink splitting can take place before the paper surface breaks. The number of ink splitting times is called Passes to Fail. The higher the number is, the better is the surface strength of the printing substrate. The ink used in this test is Huber Wegschlagtestfarbe ink 520068. The preset printing impression (unit A) is 800 N, the printing impression (unit B) 250 N, the printing speed is 0.5 m/, the temperature 18 0C and the ink amount 190 μL.

Picking is defined as pulling off individual particles from the paper surface i.e. tearing off the paper or board surface during the printing process.

The dry pick is measured with a Multipurpose Printability Tester produced by Prϋfbau, Munich with the following settings :

Ink unit temperature: 23 ⁰C Application pressure: 150 N/cm Rubber print form 4 cm width Short sample print carrier Printing speed 0-3.0 m/s rising

In accordance with this test, the paper is printed at an accelerated speed with a Prϋfbau Probeandruck instrument. The ink splitting force increases as ink transfer speed on paper increases. At a certain ink transfer speed, ink splitting force exceeds the paper surface strength which can be seen as pulling off individual particles from the paper. In this test the speed at which the picking starts is given in units m/s. The higher the paper surface strength the higher is the speed where picking starts.

In the example below, the following inks from Michael Huber, Munich are used:

Nr. 2 (408002) normal setting *(Inko 14.8) Nr. 3 (408003) high tack values *(Inko 19.5) The wet picking is measured with a Multipurpose printability tester with wetting unit delivered by Prϋfbau, Munich with the following settings:

Ink unit temperature: 23⁰C

Application pressure: 15 kp/cm

Rubber print form 4cm in width

Long print sample carrier

Printing speed 0.5-4.0 m/s (constant) in steps of 0.5 m/s

Time interval between wetting and printing units = Is

In the wet picking test, the paper is first wetted. After 1 s, the wetted paper is printed at constant speed with a Prϋfbau Probeandruck instrument. At a certain ink transfer speed, the ink splitting force exceeds the paper surface' s wet strength, which can be seen as pulling off coating layer particles from the print. In this test the speed at which the wet picking first is notices is given in units m/s. The higher the paper surface wet strength is, the higher the speed where picking occurs is. In the example below, the following testing inks from Michael Huber, Munich, are used:

Nr. 2 (408002) normal tack values *(Inko 14.8) Nr. 3 (408003) high tack values *(Inko 19.5)

Example 1

The base paper used in the example below was a NeoPress G uncoated base paper, 36 gsm, comprising 50% of northern bleached softwood kraft (NBSK) pulp and 50% of mechanical pulp.

Three samples of the base paper was coated with coating compositions comprising pigment compositions including talc particles and ground calcium carbonate particles of different ratios as presented in table 1. table 1

Pl P2 P3

Talc :CaCO3 25: 75 33: 66 50: 50

The talc particles used in the pigment compositions have spherical diameters in the micrometer size, the ground calcium carbonate particles have spherical diameters in the nanometer size. The pigment compositions further comprises a copolymer of adipic acid with N- (2-aminoethyl) -1, 2- ethandiamine and epichlorhydrin as a binder.

The pigment compositions were prepared according example Ia and Ib below. The examples show the preparation of Pl and P3. The pigment composition P2 was prepared in the same way as P3, but with different ratios of calcium carbonate and talc. In the preparation of P2, the additional additives and reaction components were recalculated to the same proportions as in the preparation of P3.

Example Ia, preparation of Pl

Particulate talc 1:

The particulate talc used in the example Ia is Finntalc P 05 Pulver, MONDO Minerals, Finland.

Binder 1:

The Binder is a 15 % aqueous solution of a copolymer of adipic acid with N-2-aminoethyl) -1, 2-ethandiamin and epochlorhydrin with the following characteristics:

total chlorine content: 1,5 w% Organic chorine content: < 0.5 w% MW>1000 g/mol Brookfield viscosity of the aquous solution: 80 mPa*s +/- 30 mPa*s. pH 3.0

Said binder may for example be produced by producing an intermediate product (1) by the reaction between diethylentriamine, monoethanolamine and adipic acid in distillated water. The resulting intermediate product (1) is reacted with epicholorhydrin using sulfphuric acid and calciumsorbat as catalysator. Thereafter, the solid content is adjusted to 12-20 w% by the addition of water and the pH is adjusted to pH 3 by the addition of sulphuric acid.

Nano particulate calcium carbonate 1:

The nano particulate calcium carbonate is produced by continuously grinding Norwegien Marmor particles of an equivalent diameter of 45 μm. The grinding is performed in the presence of 0.85 w% sodium/magnesium-polyacrylat, calculated on the total dry weight of the composite, with a MW of 6000 g/mol, as grinding additive, and 1 w% polyethylene-polyacrylacid-copolymer-sodium salt, calculated on the total dry weight of the composite. The nano-calcium carbonate is grinded to a dry content of 72 w% with the particle size as shown in table 2

Table 2

Production of the pigment composition Pl:

400 kg of the Particulate talc 1 is grinded for 10 minutes in a KFM 2000 D Batch Mixer/Dryer, in the presence of 53,3 kg aqueous solution of the binder 1. Thereafter, the mixture is homogenized for an additional 10 minutes, whereby an intermediate product (2) is achieved.

77.5 kg of the nano particulate calcium carbonate 1 is mixed with 17.5 kg water. Thereafter, 180 g of a 42 w% aqueous solution of a polyacrylic acid sodium salt (MW: 4000g/mol, pH: 8.5) is added to the solution and homogenized for 2 minutes. The slurry is mixed with the intermediate product (2) for 30 minutes. Therafter, the mixture is filtered through a 104 μm sieve.

The resulting composite has a Brookfield viscosity (after 5 days) of 108/109/112 mPa*s, a pH of 8.86 and a solid content of 64.76 w% .

In the resulting composite, the nano particulate calcium carbonate is not segregated from the micro particulate talc.

Example Ib, preparation of P3

Particulate talc 1:

The particulate talc used in the example is Finntalc P 05 Pulver, MONDO Minerals, Finland.

Binder 1:

The Binder is a 15 % aqueous solution of a copolymer of adipic acid with N-2-aminoethyl) -1, 2-ethandiamin and epichlorhydrin described in example Ia. Nano particulate calcium carbonate 2:

The nano particulate calcium carbonate is produced by continuously grinding Norwegien Marmor particles of an equivalent diameter of 45 μm. The grinding is performed in the presence of 0.85 w% sodium/magnesium-polyacrylat, calculated on the total dry weight of the composite, with a MW of 6000 g/mol, as grinding additive, calculated on the total dry weight of the composite. The nano-calcium carbonate is grinded to a dry content of 72 w% with the particle size as shown in table 3:

Table 3

Production of the pigment composition Pl:

400 kg of the Particulate talc 1 is grinded for 10 minutes in a KFM 2000 D Batch Mixer/Dryer, in the presence of 53,3 kg aqueous solution of the binder 1. Thereafter, the mixture is homogenized for an additional 10 minutes, whereby an intermediate product (3) is achieved.

522.6 kg of the nano particulate calcium carbonate 2 is mixed with 388 kg water. Thereafter, 8,9 kg of a 42 w% aqueous solution of a polyacrylic acid sodium salt and 3 kg NaOH(IO w%) is added to the solution. The slurry is mixed with the intermediate product (3) . Thereafter, the mixture is filtered through a 104 μm sieve. The resulting composite has Brookfield viscosity (after 5 days) of 76/75/77 mPa (measured after 5/60/170 min) , a pH of 8.65 and a solid content of 58.6 w%

Example Ib, preparation of the printing substrate

Three different coating compositions were prepared using the pigment compositions Pl, P2 and P3.

The coating compositions were prepared as presented in table 4 below.

Table 4

The thereby produced printing substrates were printed in a heatset web offset printing press. The printing press used was a Albert-Frankenthal A 101 S heatset web-offset press with four blanket-to-blanket printing units and a four module 8 m long MEG Sigma hot air dryer. The printing speed was 50 000 cpls/h (= 6,2 m/s), printed web exit temperature right after the dryer was 90 ⁰C and after the chilling rolls 18,1 ⁰C. The fountain water used in the printing consisted of 3 % Huber Hit Redufix-R and 5 % IPA with a temperature of 10 ⁰C, a conductivity of 740 μS/cm and a pH of 5,6. The printing blankets were Day Durazone 5000. The printing ink used was Huber Group 25 H series ink with the following properties measured with a Tack-O-Scope and in comparison is another commercial ink (legend ink A is 29 H 3800 Rollo- Therm series by Huber Group) . As can be seen below, the ink used in the printing is a very fast drying type with short time to maximum tack and the ink tack is low.

Time for maximum tack Maximum tack

25 H Ink A 25 H Ink A

Black 251 s 961 S 152 192

Cyan 158 s 444 S 114 152

Magenta 124 s 769 S 94 169

Yellow 143 s 761 S 127 209 Substrate properties, i.e. ink setting, Cobb (wlδ) , Cobb (o,C10), contact angle with water, surface pore size and K&N absorbency, were measured for all the substrates Sl, S2, S3 in accordance with the measuring and evaluation methods described above. For the sake of comparison, the same measurements were performed on a reference substrate, NovaPress (Rl), 70 gsm, which is a commercially available MWC offset paper, produced by Stora Enso Publication Paper, Veitsiluoto mill. The NovaPress is coated with a coating composition comprising calcium carbonate and clay and is calendered on a Supercalender with 12 rolls in stack.

The properties of the printing substrates, Sl, S2 and S3, according to the invention and the reference, Rl, are shown in table 5.

Table 5

The contact angle was determined to 86,1° after 0,1 s for

S3. The print quality was evaluated by measuring the fluting for the printing substrates Sl, S3 and Rl according to the method described above and by visual characterization. For the paper substrates Sl, S2 and S3, also the wet and dry pick was determined. For the paper substrates S2 and Rl, the passes to fail were determined in accordance with the method described above. The results from the fluting, picking and passes to fail measurements are shown in table 6.

Table 6

*) Sl and S3 are measured with ink n. 2 and S3 with ink n 3. described in the method descriptions of wet and dry picking tests respectively.

As can be seen in table 5, the fluting tendency was much lower for the paper according to the invention compared to the prior art reference. The paper according to the invention further showed a high dry and wet picking resistance and a better surface strength (passes to fail) , which in combination with the low water absorption allows the use of a fast setting ink. Moreover, the printed surface of Sl, S2 and S3 was much smoother and less roughened as compared to Rl . While specific embodiments and examples of the products and methods of the invention have been shown and described, it will be appreciated by those skilled in the art that changes and modifications may be made thereto without departing from the invention in its broader aspects and as set forth in the following claims.

Appendix I

Pressure Pressure Pressure

Number [MPa] Number [MPa] Number [MPa]

1 0,0351 41 1,9991 81 13,9311

2 0,0406 42 2,1967 82 14.2148

3 0,0604 43 2,3592 83 14,5709

4 0,0803 44 2,5239 84 14,8596

5 0,1003 45 2,6554 85 15,2299

6 0,1252 46 2,8544 86 15,5721

7 0,1501 47 3,0295 87 16,3823

8 0,185 . 48 3,2629 88 16,9829

9 0,2099 49 3,4937 89 17,5186

10 0,2413 50 4.0492 90 18,1955

11 0,2768 51 4,5916 91 18,8334

12 0,2873 52 5,4959 92 20,1199

13 0,3008 53 5,7921 93 21,1192

14 0,3186 54 6,1855 94 23,9643

15 0,3492 55 6,6149 95 24,4351

16 0,4055 56 6,983 96 25,0901

17 0,4546 57 7,6079 97 28,9628

18 0,5019 58 7.9799 98 32,2049

19 0,553 59 8,3988 99 34,2826

20 0,5977 60 8,7862 100 36,7433

21 0,6507 61 9,0198 101 39,0274

22 0,6989 62 9,3084 102 41,4373

23 0,7509 63 9,6878 103 46,1483

24 0,7974 64 10,0456 104 48,4397

25 0,85 65 10.3432 105 50,852

26 0,9012 66 10,509 106 53,2377

27 0,9509 67 10,724 107 55,6331

28 0,9992 68 10,9551 108 58,1834

29 1,0475 69 11,158 109 60,4795

30 1,1267 70 11,3517 110 65,1204

31 1,2053 71 11.5598 111 67,6779

32 1,2508 72 11.7114 112 69,8604

33 1.2727 73 11,8947

34 1,3025 74 12,0943

35 1,4483 75 12,3066

36 1,574 76 12,4522

37 1,6299 77 12,6887

38 1,7063 78 12,9838

39 1,8493 79 13,3224

40 1.9533 80 13,5873

Claims

Patent claims

1. Printing substrate for gravure and offset printing, which substrate exhibits;

(i) a Cobb(wl5) value below 15 g/m2;

(ii) a Cobb(o,C10) value below 1.2 g/m2, preferably below 1 g/m2; and

(iii) an Ink setting below 0.5, preferably below 0.3.

2. A printing substrate according to claim 1, wherein the substrate further exhibits a contact angle with water no greater than 90°.

3. A printing substrate according to any one of the preceding claims, wherein the substrate is coated with a coating composition comprising a pigment composition.

4. A printing substrate according to claim 4, wherein the pigment composition comprises calcium carbonate.

5. A printing substrate according to claim 5, wherein the pigment composition comprises calcium carbonate to an amount of at least 10%, most preferably to an amount of at least 20%, of the pigment composition.

6. A printing substrate according to any one of the preceding claims, wherein the pigment composition comprises fine particulate talcum with a spherical diameter of micrometer size, calcium carbonate particles of nanometer size and a binder comprising a copolymer, which copolymer including as monomer at least one dicarbon acid and at least one monomer chosen from the group of diamine, triamine dialcanolamine or trialcanolamine.

7. A printing substrate according to anyone of the claims 3- 6, wherein the pigment composition comprises calcium carbonate and talc within the range of about 3:1 to about 1:3.

8. A printed substrate comprising; a printed substrate according to any of the claims 1 - 7; and heatset web offset ink printed on the substrate.

9. A heatset web offset printing method comprising the steps of; printing a substrate according to any of the claims 1 - 7, in at least one heatset offset printing unit; and drying said substrate.

10. A heatset web offset printing method according to claim 9, wherein the drying is performed at a temperature so that the maximum web temperature obtained in the drying is below 130 ⁰C, preferably below 100 °C.

11. A sheet fed offset printing method comprising the steps

printing a substrate according to any of the claims 1-7.