Technical Field of the Invention

The present invention relates to a method for the production of a coating color, containing a highly concentrated starch solution for the coating of paper and board, to the use of said highly concentrated starch solution as a binder in a coating color, and to a coating color obtainable by said method. Background Art

The interest in the paper industry to strive in the green direction with graphical papers has increased over the last years. The markets focus on carbon foot print and life cycle analysis has made the paper producers more eager to be in front with new green products for the market. This trend in combination with high oil prices and therefore increased cost for latex as binder in coating colors has speeded up the process in the paper industry to do real breakthrough achievements in using biopolymers/starches in full scale in exchange for latex.

Coating colors, also called coating slips, are used for coating of both paper and board products, e.g. for coatings that conventionally contain pigments, binders, and other additives, such as thickeners, foam retardants, dispersing agents, viscosity regulating agents, and dyestuffs. Examples of pigments are kaolin and calcium carbonate. The main function of the binders is to bind the pigments to the paper and board surface and to keep the pigment particles as such together. Normally, the binders are divided in two groups, i.e. water soluble binders and latex binders. The main function of the water soluble binders is to act as pigment binder and to provide water retention. More precisely, the water is not allowed to penetrate into the paper and board too fast when the coating color has been applied on the surface thereof. The latex is comprised of polymers dispersed in water and is normally used in combination with a water soluble biopolymeric binder, such as starch, starch derivatives, or CMC (carboxy methyl cellulose). One of the most commonly used biopolymeric binder in coating colors with high dry substance contents have been dextrin. A dextrin has the advantage of being cold water soluble directly in the pigment solution and therefore not bringing any extra water into the coating color. However, there is a drawback with dextrin, i.e. the small contribution this biopolymer gives to the water retention of the coat- ing color. Less contribution to the strength of the coating due to the lower molecular weight of a dextrin is another disadvantage compared to a modified starch with higher molecular weight. Another disadvantage of some dextrins is that the whiteness of the coating can be negatively influenced. Thus, there is also a need of a binder not having said drawbacks of dextrin.

U.S. Patent Application No. 2010/00159104 discloses a procedure to gelatinize and dissolve starch for use in an adhesive composition, ending with a higher concentration than otherwise possible with conventional technique, i.e. a starch concentration of more than 35%. With this technique a starch solution essentially free from granules is obtained. Due to the higher starch concentration in the slurry preparation obtained the achieved gelatinized product creates a higher viscosity which in turn generates a need of higher steam pressure than in a conventional cooking procedure with a view to obtaining a solution.

U.S. Patent Application No. 2010/0058953 discloses the use of legume based dextrin for coating applications, e.g. for laying paper or flat carton. The legume dextrin products are specified with specific molecular weights and stability parameters, and starch concentrations in the interval of 35-75% is disclosed.

Summary of the Invention

An object of the present invention is to eliminate the drawbacks with conventional coating colors art disclosed above. This object is fulfilled with a method according to the present invention having the features disclosed in subsequent independent claim 1 and in the dependent claims. More precisely, the method according to the present invention refers to a method for the production of a coating color containing a highly concentrated starch solution for the coating of paper and board, wherein said method comprises the steps of cooking a starch solution, evaporation of the cooked starch solution for the production of a highly concentrated starch solution having a starch dry matter content of 40-70%, and mixing said highly concentrated starch solution with other conventional components present in a coating color.

In such a way, a cooked starch solution, intended as a component in a coating color for coating applications, such as for top coatings, having a high- er starch content compared to known starch solutions for the same purpose is obtained with the method according to the present invention. This gives the opportunity to exchange more latex in high dry substance coating colors. It is not known before to subject a conventionally cooked starch solution, intended for use in a coating color for coating applications, to an evaporation step, thereby increasing the starch dry content of the starch to a desired high level as defined in the present invention.

The present invention also refers to use of the highly concentrated starch solution having a starch dry matter content of 40-70% produced in the evaporation step in the method according to the present invention as a binder in a coating color for the coating of paper and board.

Further, the present invention refers to a coating color obtainable by the method according to the present invention.

Brief Description of the Drawings

Figure 1 discloses a plant for the production of a highly concentrated starch solution by use of the method according to the present invention.

Figure 2 shows a graph illustrating the Brookfield viscosity development during the evaporation step comprised in the method according to the present invention in a laboratory experiment.

Figure 3 shows a graph illustrating the Brookfield viscosity develop- ment during the evaporation step comprised in the method according to the present invention in an experiment in production scale.

Detailed Description of Preferred Embodiments

As the starch source or raw material for the starch involved in the present invention roots and tubers, such as potato and tapioca; grains and cereals, such as wheat, barley, corn, and rice; and legumes, such as peas and beans, may be used. The raw material also includes other varieties of these starches, e.g. high amylose or high amylopectin starch, as well combinations or blends thereof. The starch involved in the present invention may also be a modified starch. Examples of modified starches are starch esters, such as acetates, phosphates, sulfates, sulphosuccinated, succinated, etc; and starch ethers, such as cationic and anionic starch ethers, hydroxy- propylated starches etc; crosslinked starches, such as sodium trimetaphos- phate reacted starches, epichlorohydrine reacted starches, etc. Other useful starches are degraded starches, e.g. oxidized starches, such as hypochlorite oxidized, and hydrogen peroxide oxidized etc; acid modified starches; persulphate-degraded starches; enzymatically degraded starches; thermally acid degraded starches, such as dextrins, and combinations of one or more there- of. However, legume based dextrins are specifically excluded from the scope of the present invention.

The method according to the present invention can easily be applied on all native and modified starches suitable for conventional cooking. By cooking starch with the conventional technique and at standard slurry con- centrations of 35-45%, a totally dissolved starch solution free from traces of granules is obtained in a first preparation step. It is a pre-request with a totally dissolved starch solution to be able to use the starch solution in a coating color for paper and board coating. In the first preparation step a starch chosen from the starch source disclosed above is provided. In the case the modified starch is warm water soluble it includes a dry powder and/or an intermediate starch, such as a cake or a slurry. In the case a warm water soluble starch is gelatinized and dissolved with a process known in the art, batch or continuous cookers, e.g. a jet cooker, are used, wherein the cooked starch normally has a solid concentration of more than 20%, preferably 30- 45%.

In a second preparation step an evaporation is performed, and the solution is thereby concentrated to levels of starch concentrations of higher than 40% in dry matter content, preferably 40-70%, and most preferably between 45 and 65%. This results in a product having the same molecular weight and the same other properties, such as being "free from traces of granules", compared to a non-evaporated starch solution. Such an evaporation step has previously not been performed in a method for the production of a coating color. The expressions "starch dry content" and "starch dry matter content" used throughout the present patent application text are both intended to mean the solid starch content when completely dried.

The present invention will now be disclosed with reference to Figure 1 , which shows a general flow diagram illustrating the equipment involved in the method according to the invention. Referring to Figure 1 , a starch solution source 1 is cooked with a conventional cooker, not illustrated in the figure. The cooked starch solution is then fed via a feed pump 2 to a heat exchanger 3, in which the starch solution is preheated. However, in one embodiment the cooked starch solution is not preheated. The heat exchanger 3 is heated by a heating agent source 8, e.g. steam. Used heating agent 9 is discharged from the heat exchanger 3. The preheated starch solution is then fed to a forced circulation evaporation (FCE) unit 16 via a valve 7 regulating the volume of the starch solution in the FCE unit 16. The FCE unit 16 comprises an evapo- ration device (evaporator) 4, a collection vessel 5, and a circulation pump 6. The evaporation device 4 is heated by a heating agent source 8 ^' . Used heating agent 9 ^' is discharged from the evaporation device 4. Suitable evaporation devices 4 useful in the method according to the present invention are e.g. a forced circulation evaporator, a natural circulation evaporator, a plate eva- porator, a thin film evaporator, a flash evaporator, a rising film evaporator, a falling film evaporator, or the like. The temperature in the FCE unit is 60- 130°C, preferably 70-125°C, and the pressure is 20-270 kPa, preferably 35- 200 kPa. Due to the evaporation process the concentration of the starch solution may be increased up to 70% starch dry matter content, preferably 40- 70%, most preferably 45-65%.

The starch solution having been subjected to the evaporation is then fed from the evaporation device 4 to the collection vessel 5, from which the starch solution is recirculated back to the evaporation device 4 by use of the circulation pump 6 as long as the desired starch concentration in the starch solution exiting the evaporation device 4 not yet has been reached. The starch density, corresponding to a specific starch concentration, in the collection vessel 5 is continuously measured with a control unit (not shown in Figure 1 ). The recirculated starch solution is optionally connected with the flow of pre-heated (cooked) starch solution, fed between the heat exchanger 3 and the evaporation device 4, before said recirculated starch solution reaches the evaporation device 4. Vapor exiting the collection vessel 5 is fed to a condenser 10, from which a condensate 1 1 is taken out. The condenser 10 is cooled with a cooling agent source 12, e.g. cold water. Used cooling agent 13 is discharged from the condenser 10. The condenser 10 is also connected to a pressure control unit 14. When the starch dry matter content in the starch solution which has been subjected to evaporation has reached a desired high level, as measured in the collection vessel 5, a highly concen- trated starch solution 15 is discharged from the collection vessel 5. Said highly concentrated starch solution having a concentration higher than 40% may then be mixed with conventional components in coating colors, thereby arriving at a coating color having an advantageously high dry matter content and being useful for coating, preferably for top coating, of paper and board surfaces. The coating obtained on said paper or board surface has the following advantages compared to corresponding known coatings: It has a higher dry matter content compared to a coating color containing a conventionally cooked starch, wherein less water has to be removed for obtaining the required total dry content interval of approximately 65-72% for a conventional coating color. Further, the coating color produced with the method according to the present invention gives a coating having a better water retention and a higher surface strength compared to a coating color containing dextrin as the biopolymer binder. The higher dry content in the starch solution gives the possibility to lower the latex content, thereby reducing the costs and being a more environmental-friendly alternative, without decreasing the dry content of the latex based coating color.

Example 1

In a laboratory experiment the dry matter content and the viscosity of starch solutions produced according to the present invention was determined. As raw material different modified starches were used, more precisely oxidized (OX) potato starch, hydroxypropylated and oxidized (HP + OX) potato starch, potato dextrin, and acid modified waxy barley starch. For the experiment the chemically modified potato starches and the potato dextrin were received from Emsland Starke GmbH. The acid modified waxy barley was received from Lyckeby Culinar. The starch dry matter content of the starch powders were analysed according to test A using a moisture analyzer of the type HB43-S from Mettler Toledo. The drying temperature was 135°C. The starches were then dispersed in room temperate water with a mixer (Eurostar digital, type Model Euro ST-D from IKA-Werke GmbH) to a starch slurry with a starch dry matter content corresponding to 30-45% for the different starches. The starch slurries were then gelatinized and dissolved with a continuous jet cooker made by Lyckeby Starch AB. The cooking temperature was set to 125°C with a steam injector.

The dry matters of the cooked starches were determined according to test A disclosed above. Each starch solution was then transferred to a rotary evaporator (Rotavapor from Buchi Labortechnik AG equipped with a vacuum pump and a vacuum control unit from Vacuubrand GmbH). The starch solution was then heated via a heating water bath (70-85°C) under vacuum conditions (200-500 mbar) with a view to enhancing the efficiency of the process. During the evaporation samples were taken out and the starch dry matter content and the viscosity value was determined according to test A and test B, respectively, until the starch solutions started to crystallize. The point of crystallization is illustrated in Table 1 . The viscosity determination was made by using a Brookfield viscometer (RVT) from Brookfield engineering laboratories at 100 rpm and a temperature of 60°C. A suitable spindle was chosen depending on the viscosity level.

Table 1

Product Dry matter content (%) prior to crystallisation

Potato dextrin 64

Potato (OX) 60

Potato (HP + OX) 63

Waxy Barley 69

(Acid modified) Figure 2 illustrates the Brookfield viscosity development during the evaporation process in Example 1 . To conclude, this experiment shows the possibility for these three differently modified starches to reach concentrations much above what is possible with standard cooking equipment, i.e. more than 35-40%.

Example 2

In an experiment in production scale the viscosity development at increasing dry matter content of a starch solution produced with the method according to the present invention was determined. As raw material hydroxypropylated and oxidized (HP + OX) potato starch was used. The plant used for the experiment is disclosed below with reference to Figure 1 , wherein some of the components of the plant are specified in detail.

A starch solution source 1 containing hydroxypropylated and oxidized potato starch solution was received from Emsland Starke GmbH. The dry matter of the initial starch solution was determined to 17,5% according to a test C by use of a moisture analyzer (MRS-120-3) from Kern & Sohn GmbH and a drying temperature of 140°C. The starch solution was then preheated with the heat exchanger 3 and pumped via the valve 7 into a one stage forced circulation evaporator unit (FCE) 16 from GIG Karasek GmbH. The starch solution which had been subjected to evaporation was then collected in the collection vessel 5, from which it was recirculated to the evaporation device 4 via the circulation pump 6. The starch solution volume in the collection vessel 5 was set to 30% throughout the experiment and was automatically adjusted via the valve 7. The heat exchanger 3 and the evaporation device 4 were heated with steam as the heating agent source 8, and 8 ^' , respectively. The vapor from the collection vessel 5 was then condensed via the condenser 10, and the condensate 1 1 was removed. The condenser 10 was cooled with a cooling agent, e.g. cold water, from the cooling agent source 12.

The evaporated starch solution was circulated and concentrated at a speed of 52 ton/h in the FCE unit 16. The pressure in the FCE unit 16 was controlled by a pressure control unit 14. During the evaporation step samples were taken out from the bottom discharge of the collection vessel 5 and the dry matter (DM) content and the viscosity value was determined according to test C disclosed above and a test D at 100 rpm and at temperatures of 60°C, 70°C and 80°C until a dry matter value of 50,5% was reached. In test D a Brookfield viscometer (LVDV) from Brookfield engineering laboratories was used, and a suitable spindle was chosen depending on the viscosity level. The highly concentrated starch solution 15 was discharged from the bottom of the collection vessel 5. The process parameters and the concentration development are illustrated in table 2 below, and the viscosity development graphs are shown in Figure 3. This example illustrates the possibility for the modified starch to reach concentrations much above what is possible with standard cooking equipment, i.e. more than 30-45%.

Table 2: Process parameters

Example 3

Example 3 discloses a laboratory experiment showing the satisfactory properties of the highly concentrated starch solution produced with the method according to the present invention and used in a coating color for a top coating. A reference coating color and 3 different test coating colors, A-C are prepared wherein each of A-C is represented by two different test solu- tions having different latex and starch concentrations, respectively. The different coating colors are evaluated in view of water retention and the surface strength is evaluated on coated papers.

The coating colors used are prepared according to a typical top coating recipe. Both the reference coating color and the test coating colors contain 100% calcium carbonate, HC 90, as pigment. The reference coating color contains 7 parts of latex as a binder and 0.3 parts of a synthetic thickener. The test coating colors contain 5 and 3 parts, respectively, of latex as a binder, 0.1 part of a synthetic thickener, and 2 and 4 parts, respectively, of a starch as a biopolymer binder. The total dry content of the coating colors is set to 72% and the viscosity target for the colors is 1500 mPa-s.

According to the standard procedure for the preparation of the coating colors, the pigments are first dispersed for 15 minutes, and then the latex is added during 5 minutes. In the test coating colors the addition of latex is followed by addition of the starch during 20 minutes. Finally, the synthetic thickener is added, and each coating color is then dispersed during another 20 minutes. Directly after the preparation of the coating colors the water retention and the viscosity is measured.

The water retention of the coating colors is measured with an Abo Akademi Gravimetric meter, AA-GWR. The viscosity is measured with a Nitec viscosimeter Rl:1 :m, RVT at 100 rpm at 27°C.

Then the coating colors are applied with 10 g/m ² as a top coat to a paper in a SUMIT lab coater CU 5/200. The paper has before been surface sized. The surface strength of the top coated paper is evaluated according to standard procedures at a speed of 0.5 m/s. The results obtained show that a highly concentrated starch solution prepared and used according to the present invention gives a better result in view of water retention and surface strength compared to a conventional binder. The results are illustrated in Table 3 below. Table 3

Coating Latex Starch Water Surface color (parts) (parts) retention strength

Reference ¹ 7 — 0 0

A ² 5/3 2/4 +/+ +/+

B ³ 5/3 2/4 ++/++ +/+

C ⁴ 5/3 2/4 ++/++ ++/++

¹ Coating color without starch;

² Coating color containing highly concentrated potato dextrin

3Coating color containing highly concentrated potato (OX);

⁴ Coating color containing highly concentrated potato (HP + OX);

(0 in the table = reference point)

(+ and ++ = improved result in relation to reference point) As to the cost aspect, the method according to the present invention exemplified in Example 3 allows a maintained or increased coating color dry content, which in relation to the conventional recipe with a starch dry content of less than 40% results in cost reductions due to a reduced energy consumption (KJ/ton paper) for evaporation of water from the coating odor and due to an increased paper production (ton paper/h) involving an increased utilization. Further, as stated above, the method according to the present invention also allows a reduction of the amount of latex in the coating color and thereby also a cost reduction. The cost reductions disclosed above more than well compensate for the initial energy cost for the increase of the starch dry matter content from what is possible to obtain with conventional techniques up to starch dry matter contents of more than 40%.