WO2010000408A1

WO2010000408A1 - Thinned starch, preparation and use thereof

Info

Publication number: WO2010000408A1
Application number: PCT/EP2009/004514
Authority: WO
Inventors: Asko Olavi Karppi
Original assignee: Basf Se
Priority date: 2008-06-30
Filing date: 2009-06-23
Publication date: 2010-01-07
Also published as: EP2310419A1

Abstract

The present invention relates to thinned starch, especially to preparation and use thereof. The starch is a reaction product of granular starch, acid and stabilizing agent. The starch is useful e.g. as binder in coating composition for paper and paperboard.

Description

Thinned starch, preparation and use thereof

BACKGROUND OF THE INVENTION

This invention relates to thinned starch, especially to preparation and use thereof.

Thinned starches are industrially important and widely used starch products. They are used for example in paper industry in the coating colours and as surface sizes. Original molecular size of native starch is too high at such applications and thus it has to lowered i.e. starch has to be "thinned". There are many known chemical reactions and processes in literature to lower the molecular size of starch. Starch thinning is described for example in the book: Modified starches: Properties and uses (O. B. Wurzburg, CRC Press 1986) at chapter 2. Starch is typically thinned by acids, oxidants, enzymes or thermally.

The most of the starch products in paper making applications are sold in granular form. This is beneficial in transport and storage, but also microbe point of view. It is thus important that starch stays in granular form during the process steps and not for example swell or dissolve in water, or even gelatinize.

Starches are used in the paper making typically to increase dry strength characteristics. Starches are dissolved in water before use by cooking, typically by jet-cooking with steam. Unthinned starches are used in the wet end applications, e.g. as cationic wet end starch. Thinned starches are used in the surface applications in the paper making, e.g. as coating starch and surface size starch. Especially at surface size application typical starch load is so high, that native or unthinned starch can't be used due to too high viscosity at the required starch concentration. At coating colour applications starch is used typically at concentration ab.ove 25 - 30 % to keep the dry substance content of the coating colour at sufficient level. If starch is too diluted, the water of the starch solution decreases dry substance content of the coating colour. In addition viscosity of native or unthinned starch will impact also too much to viscosity of the coating colour. Thinned starches are thus preferred at coating colour and surface size applications. Typical starch concentrations are 25 - 30 % in the making of coating colours and 10 - 20 % in surface sizing.

Starch solutions overall are not necessarily stable liquids, but they tend to more or less retrograde during storage. Retrogradation is a typical phenomenon for starch solutions, especially at high concentrations and low temperatures. It is a complex process, but the main reason is because of aggregates formed by amylose. Retrograted starch at concentration over 10 % is not a fluid liquid anymore, but a gel. Starch in gel form has not much potential in the surface applications in the paper making. Hardness of non stabilized retrograded starch increases in function of starch concentration. Thus for example retrograded non-stabilized coating starch is like thick marmalade and can't be even transferred with pumps. Such retrograded starch would cause serious problems at the paper mill. Retrogradation of starch is described for example in the book: Starch Conversion Technology (edited by G.M.A. van Beynum and J.A. Roels, Marcel Dekker 1985) in pages 40 - 43.

Solution stability of the cooked starches is an important parameter in the coating colour and surface size applications in the paper making. Starch solutions must stay at suitable viscosity level at the target concentrations at temperatures around 30 - 40 ⁰C, but occasionally even lower e.g. 20 ⁰C. If starch solution at concentration above 10 % retrogrades, it will most probably worsen the quality of the paper. Retrograded starch may also stick in the pipes and tanks. It is thus very important, that solution stability of starch solution is adequate at the concentrations, which are required for the application.

Solution stability of starch solution can be determined for example using a special S-index. S-index is value in which viscosity value at low temperature, e.g. 20 ⁰C is divided with viscosity value at high temperature, such as 70 ⁰C or higher. It is believed that viscosity value at 70 ⁰C or above it is affected mainly due to impact of starch's molecular weight and that impact of retrogradation to viscosity is not remarkable. At 20 ⁰C retrogradation may impact dominantly to viscosity value. Gels or soft marmalade type states are typical at 10 % concentration, if solution stability is not sufficient. At concentration of 30 % and above thick marmalade type states are observed with starches with poor solution stability.

Low S-index value indicates good solution stability of starch solution and high S- index value poor stability. Starch concentration impacts also to S-index value. S-index value of the same starch increases when concentration increases. If the same starch is analyzed e.g. at 10 % concentration and 30 % concentration, S- index at 30 % concentration is higher.

Starch is conventionally modified with two main processing techniques, wet and dry processes. Final product with both of the processes is typically dry powder, with moisture content of 10 - 20 %. In wet processes (also called slurry processes) starch is handled as aqueous slurry through the process steps. Maximum starch concentration of slurry is around 44 %, typically between 39 - 42 %. Wet thinning processes have some disadvantages. In the wet thinning processes a part of starch will dissolve in water, which causes loss of starch. Wet processes create waste water, which must be handled. In addition, processed starch must be often dried, which requires energy. All these disadvantages are costly. On the other hand product purity of wet processed starches is typically very good. The dissolved material, which contains oligosaccharides, even sugars and hydrolyzed proteins etc. is not beneficial in starch performance point of view, i.e. starch give better strength to paper than sugars. In addition, solution stability of some wet thinned starches such as sodium hypochlorite oxidized starches, are typically very good. Starch solutions of hypochlorite oxidized starches can be good at concentrations up to 35 - 40 %.

Processes, in which starch and reaction mixtures of starch, are in powder or moist powder form during the process steps, are called dry processes. Typically moisture of dry processes is then less than 25 %. Dry processes are economically beneficial due to many reasons, such as savings at drying energy and waste water handling.

United States patent no. 6482267 concerns dry thinning of starch. It describes a method of simultaneous thinning and stabilization in dry phase. Starch is treated with acid and aldehyde compound in dry phase. Acid can be organic acid or mineral acid. Aldehyde compound can be non-polymerized aldehyde such as formaldehyde, acetaldehyde, isobutyraldehyde, crotonaldehyde, furfural, acrolein or glyoxal. Aldehyde can also be polymerized aldehyde in cyclic or non- cylcic form, such as paraformaldehyde. The most advantageously aldehyde compound according to US 6482267 is a condensate of aldehyde and a nitrogenous compound. Such condensates are for example urea-formaldehyde resins and melamine-formaldehyde resins, which are also used in the experiments of US 6482267.

Example 2 in US 6482267 describes simultaneous thinning and stabilization of native maize starch at dry matrix. Acid in the example is hydrochloric with quantity of 0.13 % of dry starch. Stabilizing agent is paraformaldehyde with quantity of 0.1 % of dry starch. Moisture of the reaction mixture is 15 %.

Thinning rate is measured using water fluidity value, which is 73. Jet-cooked starch, which is diluted to 15 % dry solids content, has viscosity of 27 mPas at 80 ⁰C and 120 mPas at 20 ⁰C. Stabilizing index, S-index, is defined to be value of viscosity value at 80 ⁰C divided with viscosity value at 20 ⁰C. S-index of example 2 product is 4.4. According to example 2, water fluidity value of 73 is the same as Brookfield viscosity of 27 mPas at 80 ⁰C at 15 % dry solids content. Example 3 describes simultaneous thinning and stabilization with different acids, stabilizing agents, reaction temperatures, reaction times, starches, moisture and reactor. Also reference test without any stabilizing agent, is included. Only 2 of 9 trial products has significantly high S-index values. One is reference product, without any aldehyde compound, and the other one is a product stabilized with melamine-formaldehyde resin. Water fluidity values of the starches are between 59 - 99.

However, even though the products made with the process described in US 6482267 are cost efficient, there are limitations in their industrial use. Thinned starches are widely used at paper production e.g. as coating starches and as surface size starches. Coating starches are used in the coating colour or coating composition, which is added on the base paper with a coater. Surface size starches are also dosed on the surface of the paper. Most of the added starch and other dry material retain on the paper, only volatile substances will evaporate in the drying period. Therefore only safe starch products are preferred in such surface applications.

According to US 6482267, the most preferred stabilizing agents are condensates of an aldehyde compound and a nitrogenous compound. In US 6482267, these agents are urea-formaldehyde and melamine-formaldehyde resins, which are also used in example 3. Also aldehydes are mentioned to be suitable stabilizing agents. Paraformaldehyde is used to stabilize starch in the example 2. Technical performance of other aldehydes than formaldehyde as stabilizing agent is not described and there are no examples of their uses. Thus, according to US 6482267 the preferred aldehyde is formaldehyde or paraformaldehyde.

A problem with starches treated with formaldehyde or formaldehyde-resins is the hazard of formaldehyde. Formaldehyde is a classified chemical. Unreacted formaldehyde residues may cause safety issues. Formaldehyde resins are not totally stable products and they contain free formaldehyde. Formaldehyde resins can also liberate formaldehyde. Formaldehyde containing starches or potential formaldehyde liberating starches have limited market in the paper industry. Such starches are not typically accepted in paper manufacturing and especially not to the surface applications, such as coating starches or surface size starches. There is thus a need for formaldehyde free economically beneficial thinned starch.

In addition, concerning coated papers, the technique has been improved during time. Solids contents of the coating compositions are increasing, being up to 70 % or even over. Starch must be able to cook to higher dry solids content level, so that the water in starch doesn't decrease the dry solids content of the coating colour. Practically this means that cooked starch must be stable at concentration up to 30 - 40 % and sometimes even up to 50 - 55 %. Such high concentration requires maximum solution stability of the cooked starch. Good solution stability of starch solution at 15 % dry solids content does not mean that the starch has also good solution stability at concentration of 30 - 40 % or even higher, e.g. 50 - 55 %. Solution stability at concentrations of 30 % or more require improved stabilizing effect, which practically means more substitution or more efficient substituted groups or cross-linking, compared to solution stability requirement at dry solids content of 15 %. It can be summarized that the higher the starch concentration is the more difficult is to get the adequate solution stability. There is no indication that dry thinned and stabilized starches according to US 6482267 meet the solution stability criteria at concentration of 30 % or more. Especially there is no indication that dry thinned and stabilized formaldehyde free starches have adequate solution stability at concentration of 30 % or more and thus meet the criteria in the paper making applications, especially at surface applications. DESCRIPTION OF THE INVENTION

It has been observed that formaldehyde free thinned starches with good solution stability at concentration of 30 % or more can be produced by treating starch simultaneously with acid and dialdehyde compound with 4 - 8 carbons at moisture between 3 - 25 % and at temperature between 60 - 170 ⁰C for 0.1 - 20 h. Process according to invention consists of creating a reagent mixture of acid, dialdehyde with 4 -8 carbons, water and additionally penetration agent, introducing the reagent mixture homogenously with starch and reacting it at moisture between 3 - 25 % at 60 - 170 ⁰C and when adequate thinning is achieved, thinning reaction is stopped or slowed down.

Acid can be inorganic or organic acid or hydrochloride of amino acid. Preferably acid is inorganic mineral acid or hydrochloride of amino acid. Preferable acid compounds are for example hydrochloric acid, sulphuric acid and betaine hydrochloride. Suitable acid concentrations are 0.1 - 0.5 % sulphuric acid, 0.05 - 0.25 % hydrochloric acid and 0.5 - 1.5 % of betaine hydrochloride. All concentrations are calculated as absolute compound of dry starch.

Suitable stabilizing agents are linear or branched, aliphatic or aromatic dialdehydes with 4 - 8 carbons. Suitable linear aldehydes are for example succinic dialdehyde and glutaraldehyde. Suitable branched dialdehydes are for example 3-methylglutaraldehyde or 3-ethylglutaraldehyde. Suitable aromatic aldehyde is for example orto-phthaldialdehyde. Preferably dialdehyde with 4 - 8 carbons is glutaraldehyde. Preferable concentration of dialdehyde with 4 - 8 carbons is 0.1 - 2.0 % of dry starch, more preferably 0.2 - 0.8 %.

Starch according to invention can be practically any starch, native or modified, non-ionic or ionic, anionic or cationic. Preferably starch is native. Starch origin can be potato, wheat, tapioca, barley, maize, waxy maize, amylopectin potato, amylose potato and any other starch. Moisture during reaction stage is a critical parameter. It can be generalized that, if moisture level is too high, above 25 %, solution stability of the final product is not good. Moisture in reaction mixture at start can be higher for example up to 18 % with grain starches, such as maize, waxy-maize, wheat and barley starch and up 23 % with tuber starches, such as potato and tapioca starch. In the cases that start moisture is higher than 15 %, a part of the moisture must be evaporated during the heating stage. Suitable moisture level in the reaction mixture during reaction stage is 2 - 20 %, more preferably 3 - 15 % and most preferably 5 - 10 %.

Temperature during reaction impacts on reaction speed of the thinning reaction. Basically thinning reaction takes place at any temperature, but realistic commercial process requires temperature over 60 ⁰C. Below it reaction speed is too low. On the other hand if temperature is very high, e.g. over 150 ⁰C, reaction speed is too high and controlling of the thinning rate is difficult. Preferable temperature is thus between 60 - 150 ⁰C, more preferably 70 - 130 ⁰C and most preferably 80 - 110 ⁰C.

Reaction time depends on acid concentration of the reaction mixture, reaction temperature and also dialdehyde concentration of the reaction mixture. Higher acid concentration and temperature speeds up the thinning reaction but dialdehydes slows down the viscosity decrease of cooked starch. The impact of dialdehydes is assumed to be due to at least partial cross-linking reaction between dialdehyde and starch. Cross-linking between dialdehydes and starch backbone increases the molecular weight of starch. In order to achieve the same molecular weight compared to thinned starch, which is thinned with no dialdehydes or less amount of dialdehydes, longer reaction time is required. Preferred reaction time using preferred acid concentration and aldehyde concentration at temperature of 60 - 150 ^CC is between 0.1 - 20 h. Important issue of the invention is that the reaction mixture is homogenous. If reaction mixture is non-homogenous, reaction time to target thinning level varies, but also solution stability is non-adequate or poor. The probable reason for variation in reaction time as well as poor solution stability is the existence of starch granules without acid and dialdehyde and starch in those granules is little substituted or cross-linked or even not substituted or cross-linked at all. Such starch worsens the solution stability but also increases reaction time. Reaction mixtures made with high shear mixers are preferable for production of thinned starches according to invention. There are many suitable high speed mixers in the market. Concerning batch mixers suitable mixers are for examples are Cyclomix, produced by Hosokawa Micron and plough mixer produced by Loedige. Concerning continuous mixers, suitable mixers are for example Flexomix, produced by Hosokawa Micron and Corimix produced by Loedige. The main issue concerning the mixers is that starch granules and liquid reagents form as homogenous mixture as possible.

In order to improve the diffusion of acid and dialdehyde inside the starch granules, it is possible to use penetration agents in the reagent mixture. Preferred penetration agents are low volatile compounds with bound oxygen. Such agents are for example as glycerol, ethylene glycol or PEG 200

(polyethylene glycol with MW average of 200 Dalton). The function of them is that they can diffuse inside starch granules. The penetration agents help acid and dialdehyde to diffuse evenly into starch granules and thus impact positively to the homogeneity of the reaction mixture. Suitable quantity of penetration agents is between 0.5 - 3 % of dry starch.

EXAMPLES

The following examples clarify the invention.

Example 1 Production of thinned starch

1000 g dried native wheat starch powder (s.c. 94.5 %) was loaded into a 5 liter Morton-Lόdige plough reactor, which was equipped with heated jacket and a spraying unit. Starch was agitated with speed of 290 rpm. Reagent mixture of water, glycerol, glutaraldehyde solution and sulphuric acid was made in ice and sprayed into the starch powder through 0.3 mm atomizing nozzle with 2.5 bar pressure. Reaction mixture was heated to the target temperature and kept at the temperature until thinning rate was adequate.

Thinned starch was slurried in water. Concentration of the slurry was 43 %. Slurry was neutralized with sodium hydroxide to pH 6.5 and then cooked with jet-cooker. Temperature was 150 ^CC and pressure 4.2 bar. Concentration of cooked starch was about 40 %. Cooked starch was diluted with water to target concentration for viscosity determinations. Viscosities were measured with Brookfield RVT DV Il -viscometer, manufactured by Brookfield Engineering Laboratories inc., at different temperatures.

Viscosity results show the improved solution stability of dialdehyde containing starch (1B) compared to reference starch (1 A). In addition, the results show that S₈0/20 -index increases when starch concentration increases.

Example 2.

Production of thinned starch

1000 g dried native wheat starch powder (s.c. 94.3 %) was loaded into a 5 liter Morton-Lόdige plough reactor, which was equipped with heated jacket and a spraying unit. Starch was agitated with speed of 290 rpm. Reagent mixture of glycerol, aqueous glutaraldehyde and aqueous betaine hydrochloride was made in ice bath and sprayed into the starch powder through 0.3 mm atomizing nozzle with 2.5 bar pressure. Reaction mixture was heated to the target temperature and kept at the temperature until thinning rate was adequate.

Thinned starch was slurried in water, neutralized with sodium hydroxide to pH 6.5 and then cooked with jet-cooker. Cooking temperature was 150 ⁰C and pressure 4.2 bar. Cooked starch was diluted with water to target concentration for viscosity determinations. Viscosities were measured at different temperatures.

The results show that thinned starch with good solution stability at concentration up to 34 % can be produced with betaine hydrochloride as acid and glutaraldehyde as stabilization agent.

Example 3.

Production of thinned starch 1000 g dried native wheat starch powder (s.c. 94.5 %) was loaded into a 5 liter Morton-Lόdige plough reactor, which was equipped with heated jacket and a spraying unit. Starch was agitated with speed of 290 rpm. Reagent mixture of hydrochloric acid, glycerol, aqueous glyoxal and water was made in ice bath and sprayed into the starch powder through 0.3 mm atomizing nozzle with 2.5 bar pressure. Reaction mixture was heated to the target temperature and kept at the temperature until thinning rate was adequate.

Thinned starch was slurried in water, neutralized with sodium hydroxide to pH 6.5 and then cooked with jet-cooker. Temperature was 150 °C and pressure 4.2 bar. Cooked starch was diluted with water to target concentration for viscosity determinations. Viscosities were measured at different temperatures.

The results show that thinned starch which is stabilized with glyoxal has good solution stability at 10 %. Sβo_/2o-index value is low. At concentration of 30 % and above that, solution stability is poor. Glyoxal does not give sufficient solution stability for thinned starch.

Example 4.

Production of thinned starch

1000 g dried native wheat starch powder (s.c. 94.5 %) was loaded into a 5 liter Morton-Lόdige plough reactor, which was equipped with heated jacket and a spraying unit. Starch was agitated with speed of 290 rpm. Reagent mixture of sulphuric acid, ethylene glycol and aqueous glyoxal was made in ice bath and sprayed into the starch powder through 0.3 mm atomizing nozzle with 2.5 bar pressure. Reaction mixture was heated to the target temperature and kept at the temperature until thinning rate was adequate.

Thinned starch was slurried in water, neutralized with sodium hydroxide to pH 6.5 and then cooked with jet-cooker. Temperature was 150 ⁰C and pressure 4.2 bar. Cooked starch was diluted with water to target concentration for viscosity determinations. Viscosities were measured at different temperatures.

The results show that glyoxal is insufficient for stabilization of acid thinned starch. Solution stability at 38 % concentration is poor, even though dosage of glyoxal was very high, 2 % of dry starch.

Example 5

Production of thinned starches

150 kg wheat starch (s.c. 88.9 %) was loaded into a 600 liter Lόdige plough reactor, which was equipped with a jacket and additional chopper agitators, which were located underneath the spray nozzles. The liquid chemicals were pre-mixed and sprayed into starch with 2.5 bar pressure. Plough agitator and chopper agitator were on during the dosage of liquid chemicals. The reaction mixture was then dried with air flow at 35 °C to moisture level about 7 %. The dried reaction mixture was then heated to 98 ⁰C. The mixture was kept at the temperature for desired reaction time. When the reaction time was completed, a sample of 5 kg was taken and it was immediately slurried in water and neutralized with sodium carbonate to pH 7.

The starch slurries were cooked with a jet-cooker. The cooking temperature was 150 ⁰C. Viscosities were measured at concentration of 30 % at different temperatures.

The results show that the thinning reaction speed is faster when penetration agent is used in the reaction mixture. The Sβo/2o -values are low in all of the samples, which indicate very good solution stability of the cooked starches. Example 6

Production of thinned starch

The production procedure was the same as in the example 5, but 150 kg barley starch (89.2 %) was used as a raw material starch.

The starch slurry was cooked with a jet-cooker. The cooking temperature was 150 ⁰C. Viscosities were measured at concentration of 30 % at different temperatures.

The results show that thinned barley starch with good solution stability can be processed with the method according to invention.

Example 7

Coating colour application test

Performance of thinned starches according to invention was evaluated with coating colour application test, which covers the making of the coating colour, rheology analyses, laboratory scale paper coating and paper analyses.

Coating colour was made at first. Pigment slurries were mixed properly with IKA Euro-ST P CV - laboratory mixer, manufacrued by IKA-Werke, before they were added. The chemicals were added step wise. After the addition the mixture was mixed 1 min with IKA mixer, before the next chemical was added. The chemicals were added according to recipe, the first chemical in the recipe was added first and so on. When all chemicals were added, the mixture was agitated for 5 min, after which the coating colour was ready. The following characteristics were analyzed from the coating colour: dry solids content, pH, temperature, low shear viscosity, middle range shear viscosity, high shear viscosity and static water retention. Dry solids content was measured with M2 Moisture Analyzer, manufactured by Denver Instrument. Low shear viscosity was measured with Brookfield RVT DV Il -viscometer, manufactured by Brookfield Engineering Laboratories. Middle range shear viscosity was measured with Rheostress 1 - viscometer (HS25 sensor system), manufactured by Thermo Electron

Corporation. High shear viscosity was measured with ACAV A2 -capillary viscometer (diameter of the capillary 0.5 mm and length 50 mm), manufactured by ACA Systems. Static water retention was measured with AA-GWR, manufactured by DT Paper Science. Thinned starches Exp 5 A1 and Exp 5 A2 form the example 5 were used in the coating colour test. Commercial wet processed, oxidized coating starch was used as a reference starch. Viscosity figures of the reference starch are in the table below:

Recipe of the clay rich coating colour for LWC paper is in the table below:

FWA: fluorescence whitening agent

The rheological properties of the coating colours:

The rheological properties of CC2 are in the similar level compared to ref in static water retention, low and middle shear viscosity, while for high shear viscosity level is slightly higher. For CC3 static water retention is slightly higher level, while low and middle shear viscosities is slightly lower. High shear viscosity is in the same range. Overall the thinned starches according to invention perform in equal manner compared to reference coating starch in the examined coating colour.

The laboratory scale coating test

In order to analyze the strength and optical properties of the above mentioned coating colours, they were applicated on the base paper with K-Control Coater, model nb 202 (bar 1 , speed 7), manufactured by RK Print Coat Instruments Ltd. The base paper was 43 g/m² commercial wood containing base paper for LWC paper. Target coat weight was 10 g/m². The coated sheets were dried with Dow Sheet Coater and Dryer D150 (infra dryer), manufactured by Enz Technik A.G. The drying speed was 5.2 m/min and each sheet was dried once. The dried sheets were calendered with DT laboratory calendar, manufactured by DT paper science. Each sheet was run twice though the nip. Cylinder temperature was 40 ⁰C and pressure 15 bar. The following analyses were measured from the calendered sheets: coat weight, gloss, optical properties, IGT and wet pick resistance. Gloss was measured with Zehnther ZLR 1050 (75° Tappi), manufactured by Zenhtner. Optical properties were analyzed with L&W Elrepho, manufactured Lorentzen &Wettre. Surface strength properties were evaluated by measuring picking resistance with IGT AIC 2-5 printability tester, manufactured IGT Testing Systems. Wet pick resistance (ink transfer) was measured with Pruefbau Multipurpose Printability testing Machine MZ2, manufactured by Pruefbau. The measured strength values are in the table below.

Loπlleux Ink 3801

Dampening solution 20 μl 10 % IPA Delay after moistening 1 s Pressure agaist the sample. 800 N Speed 1 m/s

The results show that though the gloss values of uncalandered papers are at the same level, after calandering the gloss of trial point 3 (Strach Exp 5 A2) is improved. Gloss of trial points 1 and 2 are at the same level. Optical properties of all test points are equal.

Concerning coating starches, the strength properties are the most important features. The results show that both IGT values and wet pick resistance values can be considered equal with the reference starch. Example 8

Coating colour application test

The performance of thinned starches according to invention in the high solids coating colour was examined. The coating colour was made in the same manner as in example 7. Thinned starches Exp 5 A2 and Exp 6 were used in the test. Reference starch was the same as in the example 7.

Recipe of the high solids coarse carbonate containing precoating colour formulation is below:

Analysis results of the coating colour:

The results show that the movability of the thinned starches according to invention is on higher level. This is reflected to low, middle and high shear viscosity values, which are all at lower level compared to the reference starch. For static water retention it can be observed that the value is slightly higher. The rheology values overall indicate good runnability in the high solids carbonate containing precoat application.

Claims

1. A reaction product of granular starch, acid and dialdehyde compound with A- 8 carbons.

2. The reaction product according to claim 1 , characterized in that the starch is native or modified, non-ionic or ionic, anionic or cationic, preferably native.

3. The reaction product according to claim 1 or 2, characterized in that the starch origin is any of potato, wheat, tapioca, barley, maize, waxy maize, amylopectin potato and amylose potato.

4. The reaction product according to any of claims 1 to 3, characterized in that the acid is inorganic or organic acid or hydrochloride of amino acid, such as hydrochloric acid, sulphuric acid and betaine hydrochloride.

5. The reaction product according to any of claims 1 to 4, characterized in that the dialdehyde is linear such as succinic dialdehyde and glutaraldehyde or branched such as 3-methylglutaraldehyde and 3-ethylglutaraldehyde, aliphatic or aromatic such as orto-phthaldialdehyde, preferably glutaraldehyde.

6. A process for preparing thinned starch, characterized in that a mixture containing granular starch, acid and dialdehyde compound with 4-8 carbons is reacted at 60 - 170 ⁰C, preferably 60 - 150 ⁰C, more preferably 70 - 130 ⁰C and most preferably 80 - 110 ⁰C .

7. The process according to claim 6, characterized in that moisture content of the mixture is 2 - 25 %, preferably 2 - 20 %, more preferably 3 - 15 % and most preferably 5 - 10 % .

8. The process according to claims 6 or 7, characterized in that the mixture is reacted 0.1 - 2O h.

9. The process according to any of claims 6 to 8, characterized in that the mixture contains penetration agents, such as low volatile compounds with bound oxygen, for example glycerol, ethylene glycol and polyethylene glycol.

10. The process according to any of claims 6 to 9, characterized in that the reaction mixture is made with high shear mixer.

11. Use of the reaction product according to any of claims 1 to 5 or the starch obtainable according to any of claims 5 to 10 in the production of paper or paperboard.

12. The use according to claim 11 , characterized in that the paper or paperboard is coated at least once on at least one side with a coating composition containing the reaction product or the starch.

13. The use according to claims 11 or 12, characterized in that solids content of the reaction product or the starch is 30 - 55 %.

14. A paper or paperboard web or sheet with at least three coating layers comprising binder and pigment on at least one side thereof, characterized in that at least the outermost coating layer (topcoat) contains as binder the reaction product according to any of claims 1 to 5 or the starch obtainable according to any of claims 5 to 10.

15. The paper or paperboard web or sheet according to claim 14, characterized in that the reaction product or the starch forms 20 - 50 % by dry weight of the binder in the outermost coating layer.