SE1250693A1 - Process for preparing a coating smear containing a highly concentrated starch solution for coating paper and paperboard - Google Patents

Abstract

A method for the production of a coating color containing a highly concentrated starch solution for the coating of paper and board is disclosed, 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, as well as use of said highly concentrated starch solution as a binder in a coating color, and a coating color obtainable by said method.

Description

A METHOD FOR THE PRODUCTION OF A COATING COLORCONTAINING A HIGHLY CONCENTRATED STARCH SOLUTION FOR THECOATING OF PAPER AND BOARD Technical Field of the lnventionThe present invention relates to a method for the production of a coating color, containing a highly concentrated starch solution for the coatingof paper and board, to the use of said highly concentrated starch solution as abinder in a coating color, and to a coating color containing said highlyconcentrated starch solution.

Background ArtThe interest in the paper industry to strive in the green direction with graphical papers has increased over the last years. The markets focus oncarbon foot print and life cycle analysis has made the paper producers moreeager to be in front with new green products for the market. This trend incombination with high oil prices and therefore increased cost for latex asbinder in paper coatings has speeded up the process in the industry to do realbreakthrough achievements in using biopolymers/starches in full scale inexchange for latex.

Coating colors, also called coating slips, are used for coating of bothpaper and board products, e.g. for coatings that conventionally containpigments, binders, and other additives, such as thickeners, foam retardants,dispersing agents, viscosity regulating agents, and dyestuffs. Examples ofpigments are kaolin and calcium carbonate. The main function of the bindersis to bind the pigments to the paper and board surface and to keep thepigment particles as such together. Normally, the binders are divided in twogroups, i.e. water soluble binders and latex binders. The main function of thewater soluble binders is to act as pigment binder and to provide waterretention. More precisely, the water is not allowed to penetrate into the paperand board too fast when the coating color has been applied on the surfacethereof. The latex is comprised of polymers dispersed in water and isnormally used in combination with a water soluble biopolymeric binder, such as starch, starch derivatives, or CMC (carboxy methyl cellulose). One of themost commonly used biopolymeric binder in coating colors with high drySubstance contents have been dextrin. A dextrin has the advantage of beingcold water soluble directly in the pigment solution and therefore not bringingany extra water into the coating color. However, there is a drawback withdextrin, i.e. the small contribution this biopolymer gives to the water retentionof the coating color. Less contribution to the strength of the coating due to thelower molecular weight of a dextrin is another disadvantage compared to amodified starch with higher molecular weight. Another disadvantage of somedextrins 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 togelatinize and dissolve starch for use in an adhesive composition, ending witha higher concentration than other\Nise possible with conventional technique,i.e. a starch concentration of more than 35%. With this technique a starchsolution essentially free from granules is obtained. Due to the higher starchconcentration in the slurry preparation obtained the achieved gelatinizedproduct creates a higher viscosity which in turn generates a need of highersteam pressure than in a conventional cooking procedure with a view toobtaining a solution.

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

Summary of the lnvention An object of the present invention is to eliminate the drawbacks withconventional coating colors art disclosed above. This object is fulfilled with amethod according to the present invention having the features disclosed insubsequent independent claim 1 and in the dependent claims. Moreprecisely, the method according to the present invention refers to a methodfor the production of a coating color containing a highly concentrated starch solution for the coating of paper and board, wherein said method comprisesthe steps of cooking a starch solution, evaporation of the cooked starchsolution for the production of a highly concentrated starch solution having astarch dry matter content of 40-70%, and mixing said highly concentratedstarch solution with other conventional components present in a coating color. ln such a way, a cooked starch solution, intended as a component in acoating color for coating applications, such as for top coatings, having ahigher starch content compared to known starch solutions for the samepurpose is obtained with the method according to the present invention. Thisgives the opportunity to exchange more latex in high dry substance coatingcolors. lt is not known before to subject a conventionally cooked starchsolution, intended for use in a coating color for coating applications, to anevaporation step, thereby increasing the starch dry content of the starch to adesired high level as defined in the present invention.

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

Further, the present invention refers to a coating color for the coating ofa paper and board, wherein it comprises a highly concentrated starch solutionhaving a starch dry matter content of 40-70%.

Brief Description of the Drawinqs Figure 1 discloses a plant for the production of a highly concentratedstarch solution by use of the method according to the present invention.

Figure 2 shows a graph illustrating the Brookfield viscositydevelopment during the evaporation step comprised in the method accordingto the present invention in a laboratory experiment.

Figure 3 shows a graph illustrating the Brookfield viscositydevelopment 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 thepresent invention roots and tubers, such as potato and tapioca; grains andcereals, such as wheat, barley, corn, and rice; and legumes, such as peasand beans, may be used. The raw material also includes other varieties ofthese starches, e.g. high amylose or high amylopectin starch, as wellcombinations or blends thereof. The starch involved in the present inventionmay also be a modified starch. Examples of modified starches are starchesters, such as acetates, phosphates, sulfates, sulphosuccinated, succinated,etc; and starch ethers, such as cationic and anionic starch ethers,hydroxypropylated starches etc; crosslinked starches, such as sodiumtrimetaphosphated reacted starches, epichlorohydrine reacted starches etc.Other useful starches are degraded starches, e.g. oxidized starches, such ashypochlorite oxidized, and hydrogen peroxide oxidized etc; acid modifiedstarches; persulphate-degraded starches; enzymatically degraded starches;thermally acid degraded starches, such as dextrins, and combinations of oneor more thereof. However, legume based dextrins are specifically excludedfrom the scope of the present invention.

The method according to the present invention can easily be appliedon all native and modified starches suitable for conventional cooking. Bycooking starch with the conventional technique and at standard slurryconcentrations of 35-45%, a totally dissolved starch solution free from tracesofgranules is obtained in a first preparation step. lt is a pre-request with atotally dissolved starch solution to be able to use the starch solution in acoating color for paper and board coating. ln the first preparation step astarch chosen from the starch source disclosed above is provided. ln the casethe modified starch is warm water soluble it includes a dry powder and/or anintermediate starch, such as a cake or a slurry. ln the case a warm watersoluble 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 cookedstarch normally has a solid concentration of more than 20%, preferably 30-45%. ln a second preparation step an evaporation is performed, and thesolution is thereby concentrated to levels of starch concentrations of higherthan 40% in dry matter content, preferably 40-70%, and most preferablybetween 45 and 65%. This results in a product having the same molecularweight and the same other properties, such as being “free from traces ofgranules", compared to a non-evaporated starch solution. Such anevaporation step has previously not been performed in a method for theproduction 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 meanthe 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 inmethod according to the invention. Referring to Figure 1, a starch solutionsource 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 exchanger3, in which the starch solution is preheated. However, in one embodiment thecooked starch solution is not preheated. The heat exchanger 3 is heated by aheating agent source 8, e.g. steam. Used heating agent 9 is discharged fromthe heat exchanger 3. The preheated starch solution is then fed to a forcedcirculation evaporation unit (FCE) unit 16, via a valve 7 regulating the volumeof the starch solution in the FCE unit 16. The FCE unit 16 comprises anevaporation device (evaporator) 4, a collection vessel 5, and a circulationpump 6. The evaporation device 4 is heated by a heating agent source 8'.Used heating agent 9' is discharged from the evaporation device 4. Suitableevaporation devices 4 useful in the method according to the present inventionare e.g. a forced circulation evaporator, a natural circulation evaporator, aplate evaporator, a thin film evaporator, a flash evaporator, a falling filmevaporator, 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 maybe increased up to 70% starch dry matter content, preferably 40-70%, mostpreferably 45-65%.

The starch solution having been subjected to the evaporation is thenfed from the evaporation device 4 to the collection vessel 5, from which thestarch solution is recirculated back to the evaporation device 4 by use of thecirculation pump 6 as long as the desired starch concentration in the starchsolution exiting the evaporation device 4 not yet has been reached. Thestarch density, corresponding to a specific starch concentration, in the vesselsource 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 theevaporation device 4, before said recirculated starch solution reaches theevaporation device 4. Vapor exiting the collection vessel 5 is fed to acondenser 10, from which a condensate 11 is taken out. The condenser iscooled with a cooling agent source 12, e.g. cold water. Used cooling agent 13is discharged from the condenser 10. The condenser 10 is also connected toa pressure control unit 14. When the starch dry matter content in the starchsolution which has been subjected to evaporation has reached a desired highlevel, as measured in the collection vessel 5, a highly concentrated starchsolution 15 is discharged from the collection vessel 5. Said highlyconcentrated starch solution having a concentration higher than 40% maythen be mixed with conventional components in coating colors, therebyarriving at a coating color having an advantageously high dry matter contentand being useful for coating, preferably for top coating, of paper and boardsurfaces. The coating obtained on said paper or board surface has thefollowing advantages compared to corresponding known coatings: lt hashigher dry matter content compared to a coating color containing aconventionally cooked starch, wherein less water has to be removed forobtaining the required total dry content interval of approximately 65-72% for aconventional coating color. Further, the coating color produced with themethod according to the present invention gives a coating having a betterwater retention and a higher surface strength compared to a coating colorcontaining dextrin as the biopolymer binder. The higher dry content in thestarch 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 1ln 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 preciselyoxidized (OX) potato starch, hydroxypropylated and oxidized (HP + OX)potato starch, and potato dextrin.

For the experiment the chemically modified potato starches werereceived from Emsland Stärke GmbH. The starch dry matter content of thestarch powders were analysed according to test A using a moisture analyzerof 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 lKA-Werke GmbH) to a starchslurry with a starch dry matter content corresponding to 30-45% for thedifferent starches. The starch slurries were then gelatinized and dissolvedwith a continuous jet cooker made by Lyckeby Starch AB. The cookingtemperature was set to 125°C with a steam injector.

The dry matters of the cooked starches were determined according totest A disclosed above. Each starch solution was then transferred to a rotaryevaporator (Rotavapor from Büchi Labortechnik AG equipped with a vacuumpump and a vacuum control unit from Vacuubrand GmbH). The starchsolution was then heated via a heating water bath (70-85°C) under vacuumconditions (200-500 mbar) with a view to enhancing the efficiency of theprocess. During the evaporation samples were taken out and the starch drymatter content and the viscosity value was determined according to test Aand test B, respectively, until the starch solutions started to crystallize. Theviscosity determination was made by using a Brookfield viscometer (RVT)from Brookfield engineering laboratories at 100 rpm and a temperature of60°C. A suitable spindle was chosen depending on the viscosity level.

Figure 2 illustrates the Brookfield viscosity development during theevaporation process in Example 1. To conclude, this experiment shows thepossibility for these three differently modified starches to reach concentrations much above what is possible with standard cooking equipment, i.e. more than35-40%.

Example 2ln an experiment in production scale the viscosity development at increasing dry matter content of a starch solution produced with the method according tothe present invention was determined. As raw material hydroxypropylated andoxidized (HP + OX) potato starch was used. The plant used for theexperiment is disclosed below with reference to Figure 1, wherein some of thecomponents of the plant are specified in detail.

A starch solution source 1 containing hydroxypropylated and oxidizedpotato starch solution was received from Emsland Stärke GmbH. The drymatter of the initial starch solution was determined to 17,5% according to atest C by use of a moisture analyzer (MRS-120-3) from Kern & Sohn GmbHand a drying temperature of 140°C. The starch solution was then preheatedwith the heat exchanger 3 and pumped via the valve 7 into a one stage forcedcirculation evaporator unit (FCE) 16 from GIG Karasek GmbH. The starchsolution which had been subjected to evaporation was then collected in thecollection vessel 5, from which it was recirculated to the evaporation device 4via the circulation pump 6. The starch solution volume in the collection vessel5 was set to 30% throughout the experiment and was automatically adjustedvia the valve 7. The heat exchanger 3 and the evaporation device 4 wereheated with steam as the heating agent source 8, and 8', respectively. Thevapor from the collection vessel 5 was then condensed via the condenser 10,and the condensate 11 was removed. The condenser 10 was cooled with acooling agent, e.g. cold water, from the cooling agent source 12.

The evaporated starch solution was circulated and concentrated at aspeed of 52 ton/h in the FCE unit 16. The pressure in the FCE unit 16 wascontrolled by a pressure control unit 14. During the evaporation step sampleswere taken out from the bottom discharge of the collection vessel 5 and thedry matter (DM) content and the viscosity value was determined according totest 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. ln test D a Brookfield viscometer (LVDV) from Brookfield engineering laboratories wasused, and a suitable spindle was chosen depending on the viscosity level.The highly concentrated starch solution 15 was discharged from the bottom ofthe collection vessel 5. The process parameters and the concentrationdevelopment are illustrated in table 1 below, and the viscosity developmentgraphs are shown in Figure 3. This example illustrates the possibility for themodified starch to reach concentrations much above what is possible withstandard cooking equipment, i.e. more than 30-45%.

Table 1: Process parameters Sample Time Tpreheat Tevp in Tevp Out Tvap Pevp Psieam Parc DM(min) (°C) (°C) (°C) (°C) (mbaf) (bar) (K9/m3) (%)1 Start 58,4 96,5 96,1 95,2 850,4 1,46 1042,2 17,52 25 52,9 101,1 100,1 100,0 1006,9 2,47 1048,8 19,23 40 55,7 100,2 99,1 99,1 968,4 2,74 1059,9 20,64 55 56,4 100,6 99,2 99,1 968,1 3,14 1073,8 22,25 70 56,7 102,1 100,6 100,5 968,1 3,54 1088,9 27,16 85 56,0 100,7 99,4 99,3 967,8 3,63 1117,1 36,87 115 54,0 100,7 99,5 99,3 967,8 3,63 1137,7 398 145 51,9 100,6 99,7 99,4 968,1 3,56 1150,5 43,49 175 53,8 100,7 99,8 99,3 968,4 3,57 1160,8 44,610 205 53,0 100,8 99,9 99,3 968,7 3,63 1169,4 45,911 235 50,4 100,9 100,0 99,5 968,7 3,63 1177,3 47,412 260 53,8 101,5 100,4 99,1 969,3 3,63 1198,1 50,5Example 3 Example 3 discloses a laboratory experiment showing the satisfactoryproperties of the highly concentrated starch solution produced with themethod according to the present invention and used in a coating color for atop coating. A reference coating color and 3 different test coating colors, A-Care prepared, and the different coating colors are evaluated in view of waterretention. The surface strength and whiteness are evaluated on coatedpapers. The cost performance is also calculated.

The coating colors used are prepared according to a typical top coatingrecipe. Both the reference coating color and the test coating colors contain100% calcium carbonate, HC 90, as pigment. The reference coating colorcontains 7 parts of latex as binder and 0,3 parts of a synthetic thickener. Thetest coating colors contain 3 and 5 parts, respectively, of latex as a binder, 0.1part of a synthetic thickener, and 4 and 2 parts, respectively, of a starch as abiopolymer 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 coatingcolors, the pigments are first dispersed for 15 minutes, and then the latex isadded during 5 minutes. ln the test coating colors the addition of latex isfollowed by addition of the starch during 20 minutes. Finally, the syntheticthickener is added, and each coating color is then dispersed during another20 minutes. Directly after the preparation of the coating colors the waterretention and the viscosity is measured.

The water retention of the coating colors is measured with an Åbo AkademiGravimetric meter, AA-GWR. The viscosity is measured with a Nitecviscosimeter Rl:1:m, RVT at 100 rpm at 27°C.

Then the coating colors are applied with 10 g/m2 as a top coat to apaper in a SUMIT lab coater CU 5/200. The paper has before been surfacesized and precoated with a standard precoating layer. The surface strength ofthe top coated paper is evaluated according to standard procedures at aspeed of 0.5 m/s. The results obtained show that a highly concentrated starchsolution prepared and used according to the present invention gives a betterresult in view of water retention, surface strength and whiteness compared toa conventional binder.

Claims (9)

11 Claims

1. A method for the production of a coating color containing a highlyconcentrated starch solution for the coating of paper and board, wherein saidmethod comprises the steps of cooking a starch solution, evaporation of thecooked starch solution for the production of a highly concentrated starchsolution having a starch dry matter content of 40-70%, and mixing said highlyconcentrated starch solution with other conventional components present in acoating color.

2. The method according claim 1, wherein the highly concentrated starchsolution has a starch dry matter content of 45-65%.

3. The method according to claim 1 or 2, wherein the evaporation isperformed in an evaporator, preferably a forced circulation evaporator, a plateevaporator, a thin film evaporator, a flash evaporator, a rising film evaporator,or a falling film evaporator.

4. The method according to any one of the preceding claims, wherein thestarch has been obtained from roots and tubers, such as potato and tapioca;grains and cereals, such as wheat, barley, corn, and rice; and legumes, suchas peas and beans.

5. The method according to claim 4, wherein the starch is chosen from nativestarches, high amylose or high amylopectin starches, and combinations orblends thereof; modified starches, preferably starch esters, such as acetates,phosphates, sulfates, sulphosuccinated, succinated; and starch ethers,including cationic and anionic starch ethers, hydroxypropylated starches;crosslinked starches, including sodium trimetaphosphated reacted starches,epichlorohydrine reacted starches; degraded starches, including oxidizedstarches, such as hypochlorite oxidized, and hydrogen peroxide oxidizedstarches; acid modified starches; persulphate-degraded starches;enzymatically degraded starches; thermally acid degraded starches, including 12 dextrins, except from legume based dextrins; and combinations of one ormore thereof.

6. The method according to any one of the preceding claims, wherein if thehighly concentrated starch solution has not reached a starch dry mattercontent of higher than 40% after the evaporation it is recirculated back to theevaporator and is subjected to a further evaporation, wherein saidrecirculation is repeated until a starch dry matter content of higher than 40%is reached in the highly concentrated starch solution exiting the evaporator.

7. The method according to any one of the preceding claims, wherein theconventional components present in a coating color comprises one or more of a pigment, a binder, preferably latex, a biopolymer, and a thickener.

8. Use of the highly concentrated starch solution having a starch dry mattercontent of 40-70% produced in one of steps in the method according to claim1 as a binder in a coating color for the coating of paper and board.

9. Coating color for the coating of paper and board, wherein it comprises ahighly concentrated starch solution having a starch dry matter content of 40-70%.