Aqueous dispersions of a variety of polymers are widely used as paper sizing agents. The published European Patent Application EP 276 770 A2 and the International Patent Applications WO 99/42490 and WO 00/46264 disclose aqueous dispersions of starch-based polymers and their use as paper sizing agents.

European Patent Application EP 276 770 A2 discloses an aqueous polymer dispersion of fine particle size. The dispersion is produced by reacting (a) 20 to 65 weight percent of (meth) acrylonitrile, (b) 35 to 80 weight percent of one or more acrylic acid esters of monofunctional, saturated C3 g-alcohols and (c) 0 to 10 weight percent of another ethylenically unsaturated copolymerizable monomer by emulsion polymerization in the presence of a degraded starch. The dispersion has a solids content of from 35 to 63 percent.

WO 00/46264 discloses a polymer dispersion that consists of the following components: (a) 5 to 50 weight percent of starch, 50 to 95 weight percent of a monomer mixture comprising at least one vinyl monomer, and c) water. The monomers can be styrene, alpha-methylstyrene, acrylates, acrylonitrile, or vinyl acetate. The dispersion has a solids content of from 10 to 60 percent.

WO'99/42490 relates to fine-particle aqueous polymer dispersions for paper sizing based on styrene/ (meth) acrylic acid ester copolymers. The dispersions are obtained by radically initiated emulsion polymerization of (a) 30 to 60 weight percent of an optionally substituted styrene, (b) 60 to 30 weight percent of a (meth) acrylic acid ester Cl 4 alkyl ester and (c) 0 to 10 weight percent of another ethylenically unsaturated copolymerizable monomer and (d) 10 to 40 weight percent of degraded starch with a molar weight Mn of 500 to 10,000 in the presence of a graft-active, water-soluble redox system. The dispersion has a solids content of from 10 to 40 percent.

Unfortunately, polymer dispersions of a solid content disclosed in the above- mentioned prior art when used as a paper sizing agent tend to cause deposition on the paper sizing machines thus often resulting in a non-uniform sizing of the paper and in an insufficient ink jet printability and/or toner adhesion. Another disadvantage of dispersion polymers is that their shelf-life, that means their storage stability, is limited and the

dispersions tend to break after some time which makes the polymers useless for paper sizing.

One object of the present invention is to find aqueous compositions comprising new starch-based polymers with an improved shelf-life.

Another object of the present invention is to find aqueous compositions comprising new starch-based polymers which are useful as paper sizing compositions and which do not have a great tendency to deposit on paper sizing machines.

One aspect of the present invention is a starch-based polymer which comprises a) from 35 to 65 weight percent of structural units derived from an ethylenically unsaturated monomer free of carboxyl groups, b) from 35 to 65 weight percent of structural units derived from an ethylenically unsaturated mono-carboxylic acid, a salt thereof, an ethylenically unsaturated dicarboxylic acid or a salt thereof, and c) from 0 to 15 weight percent of structural units derived from another ethylenically unsaturated copolymerizable monomer, wherein the structural units a), b) and c) are grafted on the starch and the percentages of a), b) and c) are based on the total weight of a) and b).

The structural units a), b) and c) are generally grafted on the starch in a copolymerized form.

Another aspect of the present invention is a process for preparing the above- described starch-based polymer wherein A) from 35 to 65 weight percent of an ethylenically unsaturated monomer free of carboxyl groups, B) from 35 to 65 weight percent of an ethylenically unsaturated mono-carboxylic acid, a salt thereof, an ethylenically unsaturated dicarboxylic acid or a salt thereof and C) from 0 to 15 weight percent of another ethylenically unsaturated copolymerizable monomer, based on the total weight of A) and B), are polymerized in the presence of a starch.

Yet another aspect of the present invention is an aqueous composition comprising the above-mentioned starch-based polymer.

Yet another aspect of the present invention is a surface sizing agent for paper, paperboard or cardboard which comprises the above-mentioned starch-based polymer.

Yet another aspect of the present invention is the use of the above-mentioned starch- based polymer as a surface sizing agent for paper, paperboard or cardboard.

It has been found that the starch-based polymers of the present invention have a high water solubility if they are in a carboxylate form or if their free acid form is transformed into a salt form by reaction with a base after polymerization. Due to their high water solubility surface sizing agents can be produced in the form of an aqueous solution instead on a dispersion. The surface sizing agents in the form of an aqueous solution have a long shelf life and do not tend to deposit on paper coating machines.

Starches which are useful as a starting material for preparing the starch-based polymer of the present invention are known in the art. The starches may be native starches, oxidized native starches, starch ethers, starch esters, oxidized starch ethers, oxidized starch esters, cationically modified starches or amphoteric starches.

Useful natural starches are potato, maize, rice, wheat tapioca or waxy maize starches. The most preferred kind of starch is potato starch. Little foaming has been observed with the starch-based polymers of the present invention when potato starch, preferably degraded potato starch, is used as a starting material for the polymer of the present invention.

Preferred are degraded starches which have a lower viscosity than natural starches.

Exemplary thereof are dextrins, such as white or yellow dextrins or maltodextrins; acid- modified starches, oxidized starches or enzymatically modified starches. The degraded starches preferably have an intrinsic viscosity in the range from 0.1 to 15 dl/g, more preferably from 0. 3 to 12 dl/g, most preferably from 0. 5 to 10 dl/g. The intrinsic viscosity can be determined as described in"Methods of Carbohydrate Chemistry" ; Volume IV, Academic Press New York and Frankfurt, 1964, page 127." An enzymatically modified starch can be produced in a known manner by treating starch with an alpha-aniylase.

Alternatively, degradation of natural starches can be achieved by treatment with an acid, preferably hydrochloric acid or sulphuric acid.

Oxidized starches are the preferred degraded starches. Preferred oxidizing agents are hypochlorites, such as sodium hypochlorite or calcium hypochlorite. During such oxidation carboxyl groups and carboxylate groups are introduced into the starch while at the same time depolymerization occurs. The oxidation of starches is described in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998. The resulting degree of substitution D. S. with carboxylic acid groups is preferably from preferably from 0.0001

to 1.0, more preferably from 0.001 to 0.3, most preferably from 0.02 to 0.2. The degree of substitution is the average number of carboxylic groups per anhydroglucose unit.

A preferred type of degraded and oxidized potato starch is Perfectamyl A 4692, commercially available from Avebe, the Netherlands, having a degree of substitution D. S. with carboxylic acid groups of from 0.04 to 0.048. The use of oxidized potato starch is particularly preferred for obtaining a starch-based polymer of high water solubility as described further below.

A preferred group of starches are anionic starches which comprise carboxyl, phosphate or sulfate groups or their alkali metal or ammonium salts. The degree of substitution D. S. with carboxylic acid groups is preferably from 0.0001 to 1.0, more preferably from 0.001 to 0.3, most preferably from 0.02 to 0.2. The carboxyl, phosphate or sulfate groups or their alkali metal or ammonium salts can be produced by the above- mentioned acidic or oxidative treatments which lead to degraded starches, however the present invention is not limited to these procedures. The mentioned groups can also be introduced into starch by other methods known in the art, for example by the anionic modification disclosed in the published International Patent Application WO 00/60167 on page 5, lines 13 to 35. For example, starch phosphate esters can be produced by reacting starch with phosphoric acid or a water-soluble salt of ortho-, pyro-or tripolyphosphoric acid. A preferred representative of a starch monophosphate is Nylgum A 85, which is an anionic phosphate ester of hydrolyzed potato starch and commercially available from Avebe, the Netherlands.

Other useful starches are known non-ionic starches, for example starch acetates, starch ethers, such as hydroxyethyl starch or hydroxypropyl starch, or cyanoethyl starch.

Cationic or amphoteric starch ethers may also be used as a starting material for the starch-based polymer of the present invention. Representative of cationic starch ethers are quaternary ammonium alkyl ethers, such as hydroxypropyltrimethyl ammonium starch.

Preferred examples are Amylofax 10 or Amylofax 15 which are commercially available from Avebe, the Netherlands. However, cationic or amphoteric starches are generally less preferred. It has been found that aqueous solutions containing starch-based polymers produced from such starches generally are slightly turbid.

The starch-based polymer of the present invention comprises the structural units a), b), and c) described further below in polymerized form.

The starch-based polymer comprises a) from 35 to 65 weight percent, preferably from 40 to 60 weight percent, more preferably from 45 to 55 weight percent of structural units derived from an ethylenically unsaturated monomer free of carboxyl groups, b) from 35 to 65 weight percent, preferably from 40 to 60 weight percent, more preferably from 45 to 55 weight percent, of structural units derived from an ethylenically unsaturated mono- carboxylic acid, a salt thereof, an ethylenically unsaturated dicarboxylic acid or a salt thereof, and c) from 0 to 15 weight percent, preferably from 0 to 12 weight percent, more preferably from 0 to 8, most preferably from 2 to 8 weight percent, of structural units derived from another ethylenically unsaturated copolymerizable monomer. The starch- based polymer may comprise more than one type of units a), more than one type of units b) and/or more than one type of units c), provided that the total amount of the units a), b) and c) is within the ranges indicated above.

Preferred ethylenically unsaturated monomers free of carboxyl groups from which the structural units a) are derived are aromatic vinyl compounds, such as styrene, vinyltoluene or alpha-methylstyrene; vinyl ethers, nitrogen-containing vinyl compounds, such as acrylonitrile, or aliphatic olefins, such as diisobutylene.

The structural units a) preferably correspond to the Formula I : wherein R4 is selected from Cl-C8 alkyl and substituted and non-substituted aryl and R5, R6 and R are independently at each occurrence selected from hydrogen and Cl-C4 alkyl.

The selection of the specific ethylenically unsaturated monomer free of carboxyl groups is not critical. But for reasons of ease of production and availability of starting materials, styrene and diisobutylene are the preferred monomers. The most preferred monomer from which the structural units a) are derived is styrene.

Preferred examples of the structural units b) are derived from an ethylenically unsaturated mono-carboxylic acid, such as acrylic acid, methacrylic acid or crotonic acid or from an ethylenically unsaturated dicarboxylic acid, such as itaconic acid, fumaric acid or maleic acid. Methacrylic acid is the most preferred ethylenically unsaturated carboxylic

acid. Their salts, such as the ammonium, alkali metal salts or alkaline earth metal salts are also useful. The most preferred salt is the sodium metal salt.

Useful a structural units c) can be derived from a wide range of monomers, such as vinylpyrrolidone, vinylformamide, vinyl sulphonic or phosphonic acid, acrylamide or a lower aliphatic acrylate with an aliphatic side chain of less than 14 carbon atoms, more preferably less than 4 carbon atoms, such as methylmethacrylate or butylacrylate, half-esters of maleic anhydride with alcohols having less than 14 carbon atoms, more preferably less than 4 carbon atoms, a vinylester such as vinylacetate, nitrogen-containing acrylates in their non-quaternized and non-protonated form, such as 2-acrylamido-2-methyl-propane sulfonic acid or 2-(dimethylamino) ethylmethacrylate, (meth) acrylates produced from (meth) acrylic acid and monomethyl-ethyleneoxideglycolethers comprising from 1 to 100, preferably from 5 to 40, ethylenoxid units or the mentioned (meth) acrylates wherein up to 50 weight percent, preferably up to 20 weight percent, and more preferably up to 10 weight percent of ethyleneoxide units are replaced by propyleneoxide units. The monomer is preferably water-soluble.

The weight percentage of the starch is generally from 1 to 75 percent, preferably from 5 to 60 percent, more preferably from 15 to 35 percent, most preferably about 30 percent, based on the total weight of starch and the monomers from which the structural units a), b) and c) are derived. The amount of starch which remains unreacted in the subsequently described polymerization reaction, that means onto which the monomers have not been grafted, is generally up to 50 percent, depending on the grafting/polymerization conditions described below. But even in the case of a lower degree of grafting, the polymer composition which is obtained after polymerization and, if appropriate, conversion of free carboxyl groups into carboxylate groups with a base and which comprises non-reacted starch, non-grafted copolymers comprising structural units a), b) and optionally c) and starch-based grafted polymers comprising structural units a), b) and optionally c) are still substantially water-soluble. However, if optimized conditions are applied, a grafting of more than 80 percent is usually achieved while leaving less than 20 percent of the starch non-reacted.

The starch-based polymers of the present invention comprise copolymers derived from structural units a), b) and c) which are grafted onto the starch polymer backbone.

Surprisingly, the starch-based polymers of the present invention are substantially water-

soluble when the structural units b) are derived from a carboxylate salt or when the produced starch-based polymers are treated with a base and converted into their carboxylate-salt form after the polymerization. The term"water-soluble"as used herein means that generally at least 10 weight percent, preferably at least 20 weight percent, more preferably at least 30 weight percent of the starch-based polymer is soluble in water, based on the total weight of polymer and water. The term"substantially water-soluble"as used herein means that at least 95 weight percent, generally at least 98 weight percent, preferably at least 99.5 weight percent of the sum of the monomers are converted into water-soluble polymer and only up to 5 weight percent, generally only up to 2 weight percent, preferably only up to 0.5 weight percent of non-water soluble particles are produced. Depending on the selected type of starch, monomer ratios, their ratios to starch and the polymerization conditions as described below in detail, the conversion of monomer into water-soluble polymer is usually even higher than 99.9 weight percent with less than 0.1 weight percent non-water-soluble particles. It was analyzed by IR-spectroscopy that the non-water-soluble particles, if formed, consist of polymerized structural units a), such as polystyrene.

The particles can be easily separated by filtration, since they are of large size, usually of an average particle size of more than 20 microns or even more than 50 microns.

In contrast to the polymer dispersions disclosed in the published patent applications EP-A-276770, WO 99/42490 and WO/0046264, the substantially water-soluble starch- based polymers of the present invention can be dissolved in water to produce aqueous solutions which have a high degree of transparency. The transmission of an aqueous solution comprising 15 weight percent starch-based polymer of the present invention polymer, based on the total weight of polymer and water, is preferably at least 30 percent, more preferably from 50 to 95 percent and most preferably from 70 to 99 percent.

A transmission of higher than 75 percent can usually be achieved when using oxidized potato starch, particularly starch oxidized with a hypochlorite, for producing the polymer of the present invention. The above-mentioned transmission values are measured at a wave length of 650 tun by using a UV-Shimadzu 2100 photometer in transmission against distilled water which shows at this wave length a transmission of 100 percent. The above- mentioned transmission of an aqueous solution of a starch-based polymer of the present invention is even more surprisingly high when taking into consideration that starch solutions themselves do not exhibit a clarity of 100 percent.

The observed transmission of the polymer solutions of the present invention are in contrast to the transmission rates of the polymer dispersions disclosed by EP-A-276770 and WO/0046264. These polymer particles are insoluble in water and the transmission of the dispersions having a polymer content of 15 weight percent or less is 1 percent or less.

Not even a small portion of these polymer particles are colloidally dissolved in the water- phase because of their hydrophobic nature.

In the starch-based polymers of the present invention substantially all structural units a), b) and c) are copolymerized and grafted onto a starch as explained above. The starch- based polymers are substantially water-soluble as explained above and have a good balance of hydrophobic and hydrophilic moieties, which renders them very useful as surface sizing agents for paper, paperboard or cardboard.

It has been found that the foam level of the starched-based polymers of the present invention is surprisingly low. The foam level has been found to be significantly lower than the copolymers disclosed in the published European patent application EP-A-516885 which discloses the copolymerization of styrene and methacrylic acid in the absence of a starch using an alcohol/water mixture as a polymerization medium.

It has been found'that the starched-based polymers of the present invention are less surface active and entrain much less air into the sizing solutions than those disclosed in EP-A-516885 . Low air entrainment is important for achieving a homogeneous sizing of the paper, paperboard or cardboard. Furthermore the starched-based polymers impart paper a low Cobb value and an excellent ink jet printability and toner adhesion for laser printers when sized onto its surface.

Without wanting to be bound to theory, it is believed that the reasons for the reductions in air entrainment and foaming are due to alterations in the copolymerization reactivity parameters of the monomers when copolymerized in the presence of a starch and grafted onto the starch polymer backbone thus resulting in different chemical microstructures, that means different monomer sequence distributions and different molecular weight distributions, compared to copolymers produced from monomers a), b) and optionally c) in the absence of a starch.

The starch-based polymer of the present invention is produced by polymerizing A) from 35 to 65 weight percent, preferably from 40 to 60 weight percent, more preferably

from 45 to 55 weight percent of structural units derived from an above-mentioned ethylenically unsaturated monomer free of carboxyl groups, B) from 35 to 65 weight percent, preferably from 40 to 60 weight percent, more preferably from 45 to 55 weight percent, of structural units derived from an above-mentioned ethylenically unsaturated mono-carboxylic acid, a salt thereof, an ethylenically unsaturated dicarboxylic acid or a salt thereof, and C) from 0 to-15 weight percent, preferably from 0 to 12 weight percent, more preferably from 0 to 8 weight percent, most preferably from 2 to 8 weight percent, of structural units derived from an above-mentioned other ethylenically unsaturated copolymerizable monomer, in the presence of an above-mentioned starch. The polymerization is a free-radical polymerization.

It has been found that the polymerization conditions, i. e. the manner and the sequence of adding the monomers and the initiator are generally important in order to control their grafting onto starch and to obtain substantially water-soluble polymers.

The weight percentage of the starch that is used in the polymerization reaction is preferably from 1 to 75 percent, more preferably from 5 to 60 percent, most preferably from 15 to 35 percent, based on the total weight of starch and the monomers from which the structural units a), b) and c) are derived.

The polymerization is generally carried out in a liquid diluent, such as an alcohol/water mixture, but most preferably water alone. Preferably, the polymerization is controlled and regulated by means of an initiator, optionally a co-catalyst and optionally a chain transfer agent as described further below. The polymerization can be carried out under acidic or alkaline conditions, preferably at a pH value of from 5 to 9, more preferably from 6.5 to 8, most preferably from 7 to 7.8.

The polymerization can be conducted in the presence of an acid, such as phosphoric acid and/or in the'presence of its salts. In this case preferably natural starch is used as a starting material and the degradation of natural starch and the polymerization and grafting of the monomers a), b) and optionally c) can be simultaneously achieved.

Suitable initiators are preferably inorganic peroxides such as ammonium, sodium or potassium peroxodisulfate, or a hydrogen peroxide. Organic peroxides, such as

dicumylperoxide or azo compounds can also be used. The initiator is generally used in an amount of from 0.05 to 5 weight percent, more preferably from 0.1 to 3 weight percent and most preferably from 0.25 to 2.5 weigh percent, based on the total weight of the monomers a),. b) and optionally c).

The initiator, such as a peroxide, can be used in combination with a co-catalyst, preferably a redox co-catalyst which accelerates the decomposition of the initiator.

Preferred redox-co-catalysts are inorganic Fe2+ salts, preferably FeS04, or other agents which act as reducing agents for peroxides, such as dithionites, preferably Na2S205, L-ascorbic acid, phosphites, hyphosphites, sulfides or hydrogensulfides. A preferred molar ratio between the co-catalyst and the initiator is from 1: 10 to 10: 1, most preferably about 1: 1. The use of a co-catalyst is particularly useful if hydrogen peroxide is used as an initiator. The initiator and optionally the co-catalyst are preferably used as aqueous solutions in the polymerization reaction described below.

Preferably, a chain-transfer agent is used in combination with an initiator.

The chain-transfer agent is preferably used in a weight range of from 0.05 to 5 weight percent, more preferably from 0.1 to 3 weight percent and most preferably from 0.5 to 2. 5 weight percent, based on the total weight of the monomers a), b) and optionally c).

A preferred molar ratio between the chain-transfer agent and the initiator is from 1: 10 to 10: 1, most preferably about 1: 1. Although various molar ratios of initiator to chain transfer agents are useful, most preferably an equimolar ratio of initiator to chain-transfer agent is used. Particularly suitable chain-transfer agent are mercaptanes, such as mercapto- propionic acid or thioglycols. Mercaptanes with longer aliphatic chains, such as a dodecylmercaptane, are less preferred because of their lower water-solubility and lower reactivity unless they are used in combination with beta-cyclodextrines as described in Macromol. Chem. Phys. , 2000,201, pages 2455 to 2457. Other useful chain transfer agents are olefinic dithioethers, such as 2-methylenepropane-1, 3-thiol as described in Macromol. Chem. Phys. 200, pages 58-64 (1999). Further useful chain-transfer agents are the ones which are generally known in the production of polystyrene or poly (meth) acrylic acid or poly (meth) acrylates or, if modifying monomers c) are used in the olymerization, chain- transfer agents which are known in the state of the art for the polymers of these modifying monomers. Examples are terpinols and/or unsaturated alicyclic hydrocarbons, as described in Winnacker-Kuechler, Chemische Technologie, volume 6, on pages 374 and 381,

Carl Hanser Verlag Muenchen Wien, 1982. Unsaturated alicyclic hydrocarbons may be used in combination with cyclodextrines as complexing agents/phase transfer agents in order to both enhance their water-solubility or water-dispersability and their reactivity.

Aldeydes, Acetals or halogenated methanes such as tri-or tetrachloro-methanes are also useful chain-transfer agents, although the latter are less preferred.

It has been found that the use of a chain transfer agent in combination with an initiator is useful for controlling the molecular weight of the starch-based polymer of the present invention. Aqueous solutions of starch-based polymers which have been prepared using an initiator in combination with a chain transfer agent generally have a Brookfield viscosity of from 30 to 3000 mPa's (cps), preferably from 100 to 500 mPa s (cps) at a polymer concentration of 15 weight percent, measured at 25 degree Celsius. Aqueous solutions with these viscosities have the most practical use as a paper surface sizing agent.

In the absence of any chain transfer agent typically still clear, transparent aqueous solutions with higher viscosities are obtained. Depending on the polymerization conditions, sometimes more water-insoluble particles are produced in the absence of any chain transfer agent, in particular if a high level of peroxide is used as an initiator.

Preferably, the monomers, an initiator, and an optional co-catalyst and/or chain- transfer agent are added to an aqueous solution of starch as described further below.

The concentration of the starch solution is preferably from 0.5 to 50 weight percent, more preferably from 5 to 30 weight percent and most preferably from 10 to 20 weight percent, depending on the kind of starches used and depending on its viscosity. The starch is preferably dissolved prior to the addition and polymerization of the monomers according to known procedures. For example, the powdered starch can be added into water and heated, for example to a temperature of from 80 to 100 degree Celsius, preferably from 85 to 95'degree Celsius, for several minutes, for example between 10 and 60 minutes, until the solution becomes clear. Although of advantage, a complete dissolution is not necessarily required for the production of water-soluble starch-based graft polymers of the present invention. The monomers and initiator and an optional co-catalyst and/or chain-transfer agent may also be added into turbid solutions and still substantially water-soluble starch- based graft polymers may be obtained.

According to a first preferred embodiment of the process of the present invention, hereafter designated as procedure I, the monomers, the initiator and an optional co-catalyst

and/or chain-transfer agent are added simultaneous into the aqueous starch solution.

The monomers are preferably mixed and added simultaneously with, but separately from the aqueous initiator solution to the aqueous starch solution. Alternatively, the monomers are not premixed and added simultaneously, but separately to the aqueous starch solution.

Less preferred is the addition of an aqueous solution of the initiator into an aqueous solution of a water-soluble monomer b) and optionally c) and addition of this aqueous solution to the aqueous starch solution simultaneously with but separately from the monomer (s) a) and optionally c).

The monomers and the initiator and an optional co-catalyst and/or chain-transfer agent can be added continuously or in portions. Continuous addition is preferred.

The monomers and the initiator and an optional co-catalyst and/or chain-transfer agent are generally added over a period of from 0.1 to 10 hours, more preferably from 0.5 to 6 hours and most preferably from 1 to 4 hours. However, the addition of the monomers and the initiator should generally be carried out in such a manner that the initiator is not in contact with the starch solution without the presence of the monomers a), b) and optionally c) when the polymerization is started.

According to a second and third preferred embodiment of the process of the present invention, hereafter designated as procedures II and III, the monomers, optionally a co- catalyst and preferably a chain transfer agent are added into the starch solution prior to polymerization. Procedures II and III are even more preferred than procedure I.

According to procedure II an initiator is continuously added into the reaction mixture comprising the monomers, optionally a co-catalyst and preferably a chain transfer agent.

Alternatively, a chain transfer agent and a co-catalyst also added in a continuous manner, but generally each is added simultaneously with but separately from with the initiator solution during the course of the polymerization. The addition time of the initiator, the chain transfer agent and a redox-co-catalyst generally is from 0.1 to 10 hours, preferably from 1 to 4 hours.

Procedure III differs from procedure II in that an initiator is added in portions into the reaction mixture comprising the monomers, optionally a co-catalyst and preferably a chain transfer agent. By addition in portions is understood that a portion of the initiator is added at the beginning of the polymerization reaction in such a manner that this portion is

sufficient to initiate the free-radical polymerization and further portions are added in order to complete the polymerization.

The polymerization temperature generally is in the range from 10 to 130 degree Celsius, preferably from 20 to 100 degree Celsius, more preferably from 40 to 90 degree Celsius, most preferably from 60 to 85 degree Celsius. A post polymerization is usually allowed for 0.1 to 5 hours by which the polymerization is allowed to proceed to completion at the corresponding temperature. The starch-based graft polymers generally start precipitating from the reaction mixture 2 to 15 minutes after the polymerization has started.

If the structural units b) comprise free carboxylic acid groups, a white precipitate is generally formed which does not dissolve until the end of the polymerization or post polymerization if the latter has been allowed. Then the reaction mixture is generally cooled, preferably to a temperature of from 20 to 80 degree Celsius, more preferably from 30 to 70 degree Celsius, most preferably from 45 to 65 degree Celsius.

'The polymerization of the monomers proceeds with a high conversion, the residual content of the monomers, such as styrene, is generally below 500 ppm, typically below 200 ppm according to procedure I and generally below 100 ppm, typically 60 ppm according to procedures II and III as determined by gas chromatography measurements. The content of the residual monomers can be further reduced to significantly below 10 ppm by allowing a prolonged post polymerization in combination with an additional amount of initiator.

Preferably from 0. 5 to 15 weight percent, more preferably from 1 to 5 weight percent of initiator is used, based on the residual, total weight of non-reacted monomers.

Alternatively, other conventional methods of the art can be applied, such as stripping, hot water steam distillation or by passing an air or nitrogen stream through the reaction mixture before, during or after the neutralization described below. Preferably, these methods are applied directly after the polymerization or post polymerization.

Preferably, carboxylic acid groups in the produced starch-based polymer of the present invention are neutralized with a base such as an aqueous solution of an alkali metal hydroxide, preferably sodium hydroxide or potassium hydroxide; or of an alkaline earth metal hydroxide, preferably calcium hydroxide; or of ammonia. Organic amines or amides such as primary, secondary or tertiary amines i. e. butylamine, diethanol or triethanolamines or urea may be added in combination with the bases as described above. Preferred are aqueous solutions of an alkali metal hydroxide or ammonia, the concentrations of which are

not critical or mixtures thereof. Most preferred is an aqueous ammonia solution having a concentration of about 25 weight-percent. The neutralization can also be carried out at ambient temperature, although higher temperatures are preferred, since the polymer dissolves more quickly. The reaction mixture may be further diluted with water before or during the neutralization. Preferably, more than 80 percent of the free carboxyl groups, most preferably substantially all free carboxyl groups are neutralized to achieve dissolution of the starch-based polymer of the present invention in water.

Known additives can be added to the aqueous solutions of the starch-based polymers of the present invention, such as UV-stabilizers, heat stabilizers and/or defoamers.

It has been analyzed by gel permeation chromatography (GPC) measurements that starch-based grafted polymers prepared according to procedures I and II generally have at least bimodale molecular weight distributions. For example, if oxidized potato starch is used as a starting material, preferably Perfectamyl A 4692 which is commercially available from Avebe, Netherlands, the starch-based polymers of the present invention have a main polymer fraction having the most frequent molecular weights (peak maximum 1) in the range of from 2000 to 60,000 g/mole, more preferably from 5000 to 50,000 g/mole and most preferably from 10, 000 to 20,000 g/mole and a second polymer fraction with higher molecular weights having the most frequent molecular weights in the range of from 500,000 to 1,500, 000 g/mole (peak maximum 2). Polyacrylic acid sodium salt standards of known molecular weights have been used for calibration. It has been shown that the higher molecular weight polymer fraction is enriched with structural units a), such as styrene. which are generally hydrophobic, as is explained in more detail further below with respect to procedure III. The weight portion of the main polymer fraction (peak 1) is typically from 95 to 99.99 percent, preferably from 97.5 to 99.5 percent and the weight portion of the second polymer fraction (peak 2) is typically from 0.001 to 5 percent, preferably from 0.5 to 2.5 percent.

It has been found that particularly clear and transparent solutions are obtained after neutralization and dissolution of the polymer precipitate if starch-based grafted polymers are prepared according to procedure II or III, and the monomers are grafted onto an anionic -starch, preferably an oxidized potato starch, for example a potato starch which is commercially available from Avebe, Netherlands under the designation Perfectamyl A 4296, or a phosphate ester of a potato starch, for example an anionic phosphate ester of

hydrolyzed potato starch which is commercially available from Avebe, the Netherlands under the designation Nylgum A 85. Generally less than 0.1 weight percent, in many cases even less than 0.01 weight percent, of water-insoluble polymer particles, based on the total monomer weight are formed or none at all. The aqueous polymer solutions can be used without filtration or may be filtrated using a 2 or 5 micrometer filter, depending on the level of water-insoluble particles and on the type of end-use as described below.

Surprisingly, starch-based grafted polymers prepared according to procedure III, which is. characterized by a portion-wise addition of an initiator, generally have at least trimodale molecular weight distributions. Typically they have a polymer fraction having the most frequent molecular weights (peak maximum 1) in the range of from 2000 to 60,000 g/mole, a second polymer fraction having the most frequent molecular weights (peak maximum 2) in the range of from 120,000 to 500,000 and a third polymer fraction having the most frequent molecular weights (peak maximum 3) in the range of 700,000 to 1,500, 000 g/mole. If oxidized potato starch is used as a starting material, the weight portion of the first polymer fraction (peak 1) is typically from 45 to 97.99 percent, preferably from 65 to 94.5 percent, the weight portion of the second polymer fraction (peak 2) is typically from 2 to 50 percent, preferably from 5 to 32.5 percent and the weight portion of the third polymer fraction is generally from 0. 001 to 5 percent, more preferably from 0.5 to 2. 5 percent.

It has been shown by Gel Permeation Chromatography (GPC) measurements that the highest polymer fraction is enriched with structural units a) which are generally hydrophobic, such as styrene. The intensity of the signals in the case of the high molecular weight fraction (peak maximum) is about 3 times higher when detected at 260 nm (absorption maximum for styrene) compared to the other fractions (peak maximum at lower molecular weights) and in comparison to a detection at 205 nm. For the determination of the mass weight portions of all peak maxima the detection is performed at 205 nm.

Without wanting to be bound to any theory, it is believed that the observed, advantageously low foaming level of the starch-based polymers of the present invention is caused by the presence of the high molecular weight polymer fraction which is enriched with structural units a). It has been found that aqueous solutions of the starch-based polymers of the present invention exhibit significantly lower foam levels than polymer

solutions prepared from the same monomer ratios in the absence of a starch according to processes disclosed by EP-A-516885.

According to a further procedure IV, a mixture of water and alcohol can be used as a polymerization medium instead of water. Apart from the different polymerization medium procedure IV can be carried out as described in procedures I to III above. The amount of alcohol is generally from 0.001 to 17.5 weight percent, preferably from 0.01 to 10 weight percent and most preferably from 0.1 to 5 weight percent, based on the total weight of the monomers. Alcohols having a boiling point of up to 100 degree Celsius at atmospheric pressure, such as methanol, ethanol, isopropanol or mixtures thereof, are preferred.

The most preferred lower boiling alcohol is isopropanol. It has been found that generally no substantial amounts of insoluble polymer particles are formed. Furthermore, aqueous solutions of the starch-based polymers prepared according to procedure IV generally exhibit a low foam level. The solutions usually exhibit a clarity of more than 85percent when measured at 650 nm. The solubility of hydrophobic modifying monomers c), such as longer chain carbon atoms containing acrylates and half-esters can be enhanced using an alcohol in addition to water as a polymerization medium. The alcohols may be removed and recycled after polymerization, but generally it is preferred to further dilute the solutions after the polymerization and neutralization of the carboxylic acid containing polymer moieties and to use the solutions as such with their residual alcohol.

A starch-based grafted polymer of the present invention is preferably used in the form of an aqueous composition comprising it. Preferably, the aqueous composition comprises from 0.01 to 40 weight percent, more preferably from 1 to 25 weight percent of the starch-based polymer, based on the total weight of the starch-based polymer and water.

The starch-based grafted polymers of the present invention are particularly useful as a surface sizing agent for paper, paperboard or cardboard. They are optionally used in combination with a starch solution and optionally in combination with antifoaming agents, defy-amers or/or optical brighteners. If the starch-based grafted polymer solutions of this invention may be used in combination with known sizing agents and/or in combination with hydrophobic additives, such as waxes, octadecylamine, alkenyl succinic anhydride, alkyl ketene dimer or cationic polymeric microparticles, rosin sizing soaps and optionally in combination with calcium carbonate and/or talk fillers. The starch-based polymers of the present invention may be used in combination with known aqueous polymer dispersions,

such as styrene (acrylonitrile) /acrylate polymers or with acrylonitrile/acrylate polymers which are known surface sizing agents.

The starch-based grafted polymers of the present invention are also useful as dispersants for minerals and inorganic particles such as clay, talcum, gypsum, CaC03 or Ti02, as superpl'asticizers for mortar, cement and concrete.

Furthermore, they are useful as dispersing aids for inks, organic pigments, dyes or colours.

The starch-based polymers of the present invention are also useful as dispersants for pesticides in order to disperse them in aqueous solutions.

The starch-based polymers of the present invention are also useful as rheology modifiers for water-soluble polymers and latices.

Furthermore, they are useful as anti-clumping agents in gypsum plaster applications for construction.

The starch-based polymers of the present invention are also useful as emulsifiers for emulsion and dispersion polymerizations in order to avoid agglomerations of the polymer particles during the polymerization. The starch-based grafted polymer solutions of this invention may be applied in combination with other emulsifiers and/or with additional rheology modifiers. Such other emulsifiers, preferably anionic and/or non-ionic emulsifiers may already be added into the starch solution prior to the polymerization during the grafting polymerization as described in procedures I, II, III or IV above.

In most of the above-mentioned uses the starch-based polymers of the present invention are preferably used as aqueous solutions.

The present invention is further illustrated by the following examples which should not be construed to limit the scope of the present invention. Unless otherwise indicated, all parts and percentages are by weight. The properties listed in the examples are measured as follows.

Measurement of the Foam Level of the Sizing Compositions : From the copolymer compositions prepared according to the examples below paper surface sizing compositions are prepared by adding 2.2 grams of the 15 weight-percent copolymer composition of each sample to 300 ml of a 6 weight-percent maize starch

solution (Cerestar 05070 from Cerestar, Benelux NV). The examples are not restricted to the use of maize starch, instead potato starch can be used at the same or comparative levels.

Each prepared solution is filled into a 2000 ml measuring cylinder placed in a thermostated water bath which keeps the temperature of the solution at 60 degree Celsius.

A gear pump circulates the solution at a rate of 3 1/min through a nozzle. Thereby air is sucked into the solution by the vacuum generated by the solution pumped through the nozzle. After having passed the nozzle the solution drops from the top of the measuring cylinder onto the surface. The total volume (liquid and foam) is measured after 30 minutes.

The foam volume is the difference between the total volume and the initial volume of 300 ml non-foamed sizing composition.

Sizing And Cobb-Test: Unsized base paper (60 g/m2) made from virgin fibers is sized with each sizing composition on a laboratory size press consisting of two horizontally parallel rubber covered rotation reels which are pressed against each other with a defined pressure. The surface sizing solution is applied as a pond from above the nip and the paper in form of a DIN A4 sheet is let through the pond and nip. The surface sizing solution is thus applied to the surface and partly also pressed into the sheet. The sheet is then dried with one side in contact with a heated surface.

The Cobb test measures the hydrophobicity by measuring how many grams water is absorbed by the paper per m2 in 1 minute of contact time. The Measurement is done in accordance with DIN 53132. The value is reported as g/m2.

Determination of Molecular Weights @ Molecular weights are determined by aqueous Gel Permeation Chromatography (GPC), also known as High Pressure Size Exclusion Chromatography (HPSEC) with aqueous NaC104 and NaHP04 containing methanol eluants.

The equipment used is a HP 1100 with UV-detection at 205 and 260 nm using an autosampler. A'set of 2 columns is used (Hema Bio 100 and 1000) for good separation.

The system is calibrated with polyacrylic acid sodium salt standard in the range between 855 and 1, 100,000.

Transmission: The clarity of the aqueous polymer solutions is measured by using a UV-Shimadzu 2100 photometer in transmission at 650 nm against distilled water which has a transmission of 100 percent at this wave length.

The Brookfield-viscosity is measured at 25 degree Celsius.

Example 1 : 33 Grams of Perfectamyl A 4692 (oxidized potato starch, commercially available from Avebe, having a viscosity of 80 to 115 mPa s is added into 300 grams of water.

The mixture is heated to 90 degree Celsius and this temperature is kept for 20 minutes.

51.2 grams of styrene and 62,4 grams of methacrylic acid and 0.9 grams of mercapto- propionic acid are added, hence the weight ratio of the monomers is 45: 55.

The temperature is then adjusted to 85 degree Celsius.

2, 86 grams of ammonium peroxodisulfate are dissolved in 70,3 grams of water.

The initiator solution is added portion-wise into the solution at 85 degree Celsius in the following manner : 15 grams are added at once. A white precipitate is formed after a couple of minutes. 7.5 Grams are added after 45 minutes, another 7.5 grams after 90 minutes, another 7. 5 grams after 120 minutes, another 7.5 grams after 150 minutes, another 7.5 grams after 180 minutes, 9.75 grams after 240 minutes and 10.9 grams after 270 minutes.

The viscous reaction mixture containing a white precipitate is allowed to stir for another 30 minutes so that the total polymerization time is 300 minutes.

The mixture is allowed to cool to 65 degree Celsius. 56 Grams of a 25 weight percent aqueous ammonia solution is added within 20 minutes. The copolymer grafted onto starch starts dissolving. The reaction mixture is further heated at 70 degree Celsius for one hour in order to complete the dissolution. Then the solution is diluted with water to a non-volatile content of 15 weight percent. Its Brookfield-viscosity is 210 cps (mPa's) and its pH is 9.5.

The solution is yellow to brown, but almost clear and transparent with substantially no unsolved particles visibly. The transmission is 79.7 percent at 650 nm.

The foam level of a sizing composition with starch as described above is determined to be 360 ml after 30 minutes according to the above-described foam test. The Cobb value of a sized base paper is 24.0 g/m2.

The grafted polymer can be completely. precipitated from the aqueous solution by adding an acid such as hydrogenchloride and adjusting the pH to a value of 1.

IR-spectroscopy measurements of the dried, solid polymer (I (Br-technique) illustrate the existence of starch as a major component of the solid polymer by a strong IR absorption band for ethers. Therefore, styrene-methacrylic acid copolymers grafted onto starch, such as Perfectamyl A 4692, produced according to this invention characteristically differ from precipitated styrene-methacrylic acid-copolymers, which are known from the published European patent application EP-A-516885, by IR-spectroscopy in the solid state in the region between 980cm~l and 1210 cri 1. Redissolution of the polymer in aqueous ammonia at a pH of 9. 5 yield the same'GPC-chromatograms with substantially the same molecular weight distribution. This illustrates that a degradation of the copolymer, in particular a degradation of starch, does not take place during the precipitation of the polymer with an acid if performed at ambient temperature.

The foam level of a'corresponding sizing composition containing, instead of the starch-based polymer of Example 1, as a comparison a styrene-methacrylic acid copolymer prepared according to example 1 of EP-A-516885 is 1420 ml after 30 minutes. The Cobb value of a sized base paper is 23.8 g/m2. The transmission of the aqueous copolymer solution prepared according to Example 1 of EP-A-516885 is 82.2percent at 650 mu.,- Comparative Example 1: Example 1 is repeated, however without any starch present during the polymerization. Then the reaction mixture is treated with 60 grams of a 25 weight percent aqueous ammonia solution at 65 degree Celsius. The styrene-methacrylic acid (salt) - copolymer does not dissolve at all, the mixture remains white and the particles agglomerate to larger lumps upon cooling to ambient temperature. The lumps do not dissolve if the mixture is further diluted with water. pH of the solid/liquid reaction mixture is 9.5.

This mixture is not useful'all for surface paper sizing.

This example illustrates that a water-soluble styrene-methacrylic acid salt copolymer made from a styrene/methacrylic acid weight ratio of 45: 55 cannot be produced if the copolymer is not grafted onto a starch according to the processes of this invention or, alternatively, if the copolymerization is not carried out using an alcohol/water mixture at a

weight ratio of from 1 : 1 to 4: 1 as disclosed in European Patent Application EP-51688-A1 or at a weight ratio of from 1: 4 to 5: 2 as disclosed in European Patent Application EP-744417-AI.

Example 2: Example 1 is exactly repeated, however instead of 33 grams Perfectamyl A 4692 and 0. 9 grams ofmercaptopropionic acid, 40 grams of Perfectamyl A 4692 and 1. 0 grams of mercaptopropionic acid are used respectively. After treatment with 55 grams of a 25 weight percent aqueous ammonia solution and dilution with water to 15 weight percent, the pH is 9.34 and the Brookfield-viscosity is 100 cps (mPas). The transmission is 55.2 percent at 650 nni.

Molecular weight (GPC): Maximum peak 1: 11 200 g/mole ; maximum peak 2: 210 000 g/mol ; maximum peak 3: 1 204 000 g/mole (peak height of 3percent related to the one of maximum peak 1 (=100percent).

The foam level of a sizing composition with starch is 420 ml after 30 minutes.

The Cobb value of a base sized paper is 24.3 g/m2.

Example 3 : 40 Grams of Perfectamyl A 4692 are added into 300 grams of water. This mixture is heated to 90 degree Celsius. This temperature is kept for 20 minutes in order to completely dissolve the starch in water. Then the temperature is allowed to lower to 85 degree Celsius.

51,2 grams of styrene, 62,4 grams of methacrylic acid and 0.9 grams of mercaptopropionic acid are added at once.

An initiator solution of 2,86 grams of ammonium peroxodisulfate dissolved in 70 grams of distilled water is added in a continuous manner over the course of 2 hours. Then the reaction mixture containing the precipitated grafted copolymer is allowed to stir for another 3 hours at 90 degree Celsius in order to complete the polymerization. The solution is allowed to cool to 65 degree Celsius.

58 Grams of a 25 weight percent aqueous ammonia solution is added within 20 minutes.

The mixture is stirred for another hour. The solution is diluted to a non-volatile content of 15 weight percent. A yellow to brown, but almost clear and transparent solution with no visibly unsolved particles is obtained. The transmission is 75.1 percent at 650 nm.

Molecular weight (GPC): Maximum peak 1: 4 300 g/mole; maximum peak 2: 1 050 000 g/mole. The Brookfield-viscosity is 70 cps (mPa's). The pH is 9.4.

The foam level of a sizing solution with starch is only 300 ml after 30 minutes. The Cobb value of a base sized paper is 25.3 g/m2.

Example 4: 40 Grams of Perfectamyl A 4692 are added into 300 grams of water and heated to 90 degree Celsius for 20 minutes. A solution of 51,2 grams of styrene, 62,4 grams of methacrylic acid and 1 gram of mercaptopropionic acid is added during the course of 2 hours continuously but separately together with a solution of 2,86 grams of ammonium peroxodisulfae in 70 grams of water at 85 degree Celsius. After about 10 minutes the copolymer grafted onto starch starts precipitating from solution.

The reaction mixture is allowed to stir for another 3 hours at 85 degree Celsius after all of the monomer and initiator solution has been added in order to complete the polymerization.

The temperature is decreased to 60 degree Celsius. A solution of 6 grams of a 50 weight- percent aqueous caustic solution diluted with 16 grams of water is slowly added within 10 minutes followed by the addition of 51 grams of a 25 weight percent aqueous ammonia solution which is added within 20 minutes. The solution starts dissolving. The solution is diluted with water to a non-volatile content of 15 weight percent Molecular weight (GPC): Maximum peak 1: 7100 g/mole; maximum peak 2 : 1085000 g/mole with a height of about 1 percent related to the height of peak 1 (=100 percent).

The Brookfield-viscosity'is'100 cps (mPas). The pH is 9.4.

The foam level of a sizing composition with starch is 520 ml after 30 minutes.

The Cobb value of a base sized paper is 27.0 g/m2'.

This solutions contains few non-dissolved particles, the average particle size of which is i09 microns. 4.6 Grams of the aqueous solution containing 15 weight-percent non-volatiles is filtered by using a microfilter with a pore size of 2 micrometers. The residue on the filter is dried and weighed to be 0.0073 grams. Hence, only 0.3 weight percent of the copolymer remain non-dissolved. The water-insoluble particles consist of polystyrene as shown by

IR-spectroscopy. The remaining solution is almost clear and transparent. No unsolved particles are visible. The transmission is 78.4 percent at 650 mn.

Example 5: The chain transfer agent in this example is 1-dodecanethiol in combination with methyl-beta-cyclodextrin W7 M1, 8.

Example 1 is repeated, but instead of 33gram Perfectamyl A 4296,35 grams of Perfectamyl A 4296 are used in combination with 5 grams of methyl-beta-cyclodextrin (me-beta-CD) from Wacker Chemie GmbH Burghausen, Germany with a degree of methylation of about 1. 8 per anhydroglucose unit. Instead of 1 gram of mercapto propionic acid, 2 grams of 1-dodecanthiol are dissolved in the aqueous starch solution and stirred at 85 degree Celsius for 20 minutes. Then, the monomers (51.2 grams of styrene and 62.4 grams of methacrylic acid) are added at once. The initiator is added portion-wise as described by example 1.

The neutralization of the grafted copolymer is achieved by using 60 grams of a 25 weight percent aqueous ammonia solution. The solution is diluted with water to a non-volatile content of 15 weight percent. This solution is clear and transparent.

Transmission : 57. 2 percent at 650 nm. pH : 9. 5.

Brookfield viscosity: 50 cps (mPa's), measured at 25 degree Celsius at 20 rpm, spindle 3.

The foam volume is 520 ml after 30 minutes.

When Example 5 is repeated with 1. 8 grams of 1-dodecanthiol only and without beta-cyclodextrin as a phase transfer catalyst yielded a gel-like product is achieved having a viscosity of 23750 cps (mPa's), measured at 25 degree Celsius at a non-volatiles content of 15 weight percent. The pH is 9.5.

Example 6: Example 2 is repeated, however 40 grams of Perfectamyl A 4692 are dissolved in a mixture of 5 grams of isopropanol and 295 grams of water instead of 300 grams of water only. The weight level of isopropanol to the total sum of monomers is 4.4 percent.

The neutralization of the grafted copolymer is carried out with 60 grams of a 25 weight

percent aqueous ammonia solution and diluted with water to a non-volatile content of 15 weight percent. The solution is slightly yellow, but completely clear and transparent.

Transmission : 9.5 percent at 650 nm ; pH: 9.5.

Brookfield viscosity: 300 cps (mPa's) at 25 degree Celsius.

Foam volume : 480 ml after 30 minutes.

Example 7 : Example 1 is repeated, however instead of 33 grams of Perfectamyl A 4296 40 grams of an anionic phosphate ester of hydrolyzed potato starch which is commercially available from Avebe, the Netherlands under the designation Nylgum A 85 are used.

The neutralisation of the grafted copolymer is carried out with 60 grams of a 25 weight percent aqueous ammonia solution and further diluted with water to a non-volatile content of 15 weight percent. The solution is almost clear and of high transparency with no non- dissolved particles.

Transmission : 88. 6 percent at 650 nm; pH: 9.5.

'. X3rookfield-viscosity. 450 cps' (20 rpm, spindle 3,25 degree Celsius) Foam Volume: 560 ml after 30 minutes.

Example 8 : Example 8 exemplifies the use of cationic starch, commercially available as Amylofax 15'from Avebe.

Example 1 is repeated, however instead of 40grams of Perfectamyl A 4296, 40 grams of Amylofax 15 are used and 5 grams of isopropanol are added to 295 grams of water once the starch has dissolved. 1.0 Grams of mercapto propionic acid is used.

The grafted polymer is dissolved by adding 62 grams of an aqueous 25 weight percent ammonia solution. The solution is yellow, almost clear and transparent with no non- dissolved particles visible after dilution to 15 weight percent.

Brookfield viscosity: 275 cps (mPas) at 25 degree Celsius; pH: 9.5.

Transmission at 650 nm : 785percent.

Foam volume after 30 minutes: 420 ml.

Example 9: Example 8 exemplifies the use of cationic starch and the use of poly (ethylene- glycol) methylether-methacrylate having a molecular weight of 1100 as a modifying monomer. Example 8 is repeated, however Amylofax 15 is dissolved in 500 grams of water containing no isopropanol. 0.9 Grams of mercaptopropionic acid is used and additionally, 5 grams of poly- (ethyleneglycol) methylether-nlethacrylate as a modifying monomer at an amount of 4.4 weight percent related to the total weight of monomers.

62 Grams of a 25 weight percent aqueous ammonia solution is used for neutralizing and dissolution of the grafted polymer. A yellow, brownish but completely clear and transparent solution is obtained which is diluted to 15 weight percent.

Brookfield viscosity: 200 cps (mPas) at 25 degree Celsius; pH: 9.5.

Transmission at 650 nm : 87percent Foam volume after 30 minutes: 500 ml.

Example 10: Example 10 exemplifies the use of Perfectamyl A4692 which has been further enzymatically degraded with alpha-Amylase.

40 grams of Perfectamyl A4692 is brought into 300 grams of water which has been heated to 85 degree Celsius. This temperature is kept for half an hour. Then 100 mg of alpha-amylase is added and the solution is heated at 85 degree Celsius for another 20 minutes.

The grafting reaction of styrene and methacrylic acid is performed as described by Example 2. The polymer is dissolved by adding further 200 grams of water and a solution of 62 grams of a 25 weight percent aqueous ammonia solution. A transparent, yellow to brown solution is obtained. The average molecular weight Mw is 20 300. A bimodale distribution with the most frequent molecular weights of 11, 900 (peak 1) and 122 000 (peak 2) is obtained. The height of peak 2 is 13 percent related to the one of peak 1 (=100 percent).

Viscosity : 100 cps (mPå : s), measured at 25 degree Celsius at a non-volatile content of 15 weight percent ; pH: 9,5 ; Transmission at 650 nm : 89. 95 percent ; Foam Volume after 30 minutes: 360 ml

Example 11: Example 11 exemplifies the use of natural starch and polyethylene glycol monomethylether methacrylate as a modifying agent.

Example 2 is repeated, however instead of Perfectamyl A 4692,30 grams of the natural maize starch Cerestar 07311 and in addition 5 grams of polyethylene glycol monomethylether methacrylate is used as a modifying agent, that means component c) in the starch-based polymer of the present invention.

The grafted polymer is dissolved by adding further 200 grams of water and 63 grams of a 25 weight percent aqueous ammonia solution. The solution is diluted to a non-volatile content of 15 weight percent. The solution is yellow to brown and somewhat turbid.

Brookfield-viscosity : 175 cps (mPas) at 25 degree Celsius; pH: 9,5.

Transmission at 650 nm: 34.5 percent; foam volume at 30 minutes: 480 ml.