MXPA00010298A - Use of polymer dispersions for paper mill color removal - Google Patents

Abstract

A hydrophilic dispersion polymer which is useful in the removal of color in paper mill waste water. The polymer is preferably polymerized from diallyl-N, N-disubstituted ammonium halide and (meth) acrylamide.

Description

USE OF POLYMER DISPERSIONS FOR THE REMOVAL OF COLOR IN PAPER MANUFACTURES FIELD OF THE INVENTION A hydrophilic dispersion polymer which is useful in removing the color of waste water in paper mills. The polymer is preferably polymerized from diallyl-N, N-disubstituted ammonium halide and (meth) acrylamide.

BACKGROUND OF THE INVENTION The removal of color from effluent streams from pulp mills continues to be a problem within the paper and pulp industry. It is necessary that these wastewater downstream be treated in color removal before disposal. The production capacity of wood pulp in the United States is approximately 60 million tons per year. Since the average content of wood pulp is about 40%, 150 million tons of wood are needed to produce these 60 tons of pulp. The difference between these two numbers represents it Ref. 124392 the lignin and the hemicellulose which must be removed or separated in the pulping process in order to release the cellulose fibers. The pulping process, however, does not remove 100% of the lignin present in the wood, with approximately 5% remaining after either sulphite pulping or kraft (for mechanical pulping the amount is considerably higher). If a high grade paper is the desired final product, then this 5% residual lignin must be removed in the bleached pulp. Since more than 35% of the pulp produced in the United States is bleached, there are about one million tons of lignin removed every year in the bleaching plants, and most of it is in the caustic extraction stage. This number is significant because the residual lignin is solubilized. This solubilized lignin is a strong absorber of the visible radiation resulting from the conjugation of the unsaturated and quinodal radicals formed during the oxidation step in the bleaching plant. Consequently, the effluent from the bleaching plant is highly colorful. Although there are other sources of color in the waste effluent from paper mills, it is readily apparent that where the bleaching is performed, the effluent produced can be expected to be the main contributor of residual color. Indeed, in Kraft pulp, in the bleaching factories the effluent from the first caustic extraction station accounts for at least 70% of the residual color. The objective of pulping and bleaching operations is the removal of lignin and hemicellulose from cellulose fiber in wood. The 95% that is removed by pulping is sometimes burned as fuel in the process of recovering the inorganic chemicals present in the black liquor. In the bleaching operation, 5% of the residual lignin is separated from the fibers by degradation and solubilization and ending in the waste water. Chemical removal can, therefore, only be done by reducing solubility, which has proven to be a difficult task.

Therefore, the primary source of color in the pulp is lignin. It has also been suggested that Kraft color is due to ketoenolas produced by carbohydrates during the Kraft preparation stage in the papermaking process. The chlorination of the pulp during the bleaching operation results in the formation of colored bodies. The limes are leached from the pulp by caustic alkali solutions. In this way, the effluents of caustic extracts contain a greater proportion of the colored bodies and the other organic materials which have to be disposed during the wastewater treatment. The process of color removal of the effluent stream is further complicated by the presence of lime, solid particulate matter such as pulp, clay, dispersant / active surface materials and polymers used during several stages in the papermaking process. Solid particulate matter is commonly referred to as anionic waste. Most government regulations pertaining to color removal of effluent streams from a papermaking process are directed to the actual color, ie the color at a pH of 7.6 after filtering through a micrometer filter paper 0.8 and expressed as Pt Co color units (ie, Cobalt platinum color using a DR200 spectrophotometer). However, there is an increase in pressure in paper and pulp factories to lower the apparent color of the effluent water since it is the color visible to the naked eye. There are times when the actual color of a system that has undergone the treatment is low, but the corresponding apparent color is high. This problem is commonly caused by the presence of suspended particulate matter that causes an increase in the turbidity of the system. Therefore, it is important that any new treatment for color removal should not only remove the actual color of the effluent, but should also lower the apparent color as well. It has been shown that by-products are soluble in water, and that a significant amount is produced. This throws several demands on chemicals to be used for color removal. There are already available techniques, however, that can remove more than 90% of color either from the total effluent of the factory or from the isolated waste streams, such as the caustic extraction stages of the bleaching plant. These techniques include chemical (e.g., alum, ferric, lime, or polyelectrolyte), biological (e.g. white decomposed fungi), and physical (e.g., ultrafiltration, ion exchange, and carbon absorption) processes. Nobody enjoys extended use due to the unfavorable economic situation. The demands on a product used in a color removal application are completely severe, that is, the product must be able to react with the colored bodies in a manner which results in it becoming insoluble and, due to its extremely large amount produced, the color removal product must work at very low proportions of weight relative to the organic that is removed or its use will be avoided due to prohibitive costs. Among conventional treatments for color removal include the use of ferrous sulfate and a water-soluble cationic amino polymer as described in U.S. Pat. No. 5,200,089; the use of hydrophobic polyelectrolytes in U.S. Pat. Nos. 5,338,816 and 5,314,627; and the use of hydrophobic dispersion polymers in U.S. Pat. No. 5,435,922.

A common problem associated with conventional chemical treatment methods, such as epichlorohydrin / dimethylamine (Epi / DMA), is the fact that these polymers can not reduce the color of a system, below a certain value beyond which they too they tend to redisperse the color. This problem is commonly referred to as "overdose." The present inventors have discovered through extensive experimentation that dispersion copolymers are excellent agents for the removal of both "apparent" and "real" color in the wastewater of the paper and pulp mill. The characteristics of the color removal of acrylamide (AcAm) is significantly improved by imparting a certain degree of hydrophilicity. The modification is carried out by the copolymerization of the AcAm with a hydrophilic monomer selected to form a hydrophilic polyelectrolyte. These hydrophilic polyelectrolytes exhibit excellent replacement rates while avoiding the problem of "overdose" which often arises when conventional polymers are used for color removal. These polyelectrolytes have a unique mode of action which can lead to an organic treatment for the removal of residual water color in paper and pulp mills. The present inventors have discovered that a low molecular weight, water soluble cationic polymer dispersion can be used to successfully remove the color of wastewater effluents from paper and pulp. This single color removal agent is prepared by the polymerization of at least one hydrophilic monomer, for example, the acrylamide and / or the quaternary methyl dimethylaminoethyl (meth) acrylate chloride or ammonium diallyldimethyl chloride in the presence of water, a chain transfer agent, a precipitation aid and an initiator. The present invention also provides many additional advantages which should become apparent as described below.

BRIEF DESCRIPTION OF THE INVENTION A hydrophilic dispersion polymer that is useful in removing the color of wastewater from paper mills. The polymer is preferably polymerized from diallyl-N, N-disubstituted ammonium halide and (meth) acrylamide.

DESCRIPTION OF THE INVENTION The hydrophilic dispersion polymer of the invention is a polymer of the diallyl-N, disubstituted ammonium halide cationic monomer and (meth) acrylamide. A preferred copolymer is formed from ammonium diallyldimethyl chloride (DADMAC) and acrylamide (AcAm). It has been found that the polymer described above confers advantages for use in the paper waste water treatment process. Specifically, the hydrophilic dispersion polymers of the invention show improved or equal activity with respect to the performance of color removal without the undesired addition of oils and surfactants as compared to conventional cationic latex polymers. In addition, these polymers do not require the inverter system and can be induced to the papermaking process using simple feeding equipment. Another advantage concerns the mode of addition of the dispersion polymers. In most cases, conventional water-soluble polymers are now commercially available in the form of powders. Before use, the polymeric powder must be dissolved in an aqueous medium for its current application. The polymer is inflated in an aqueous medium, and the dispersed particles flocculate. It is typically very difficult to dissolve conventional polymers in an aqueous medium. In contrast, the dispersion polymers of this invention, by their nature, avoid problems related to dissolution. In addition, the dispersion copolymers formed of DADMAC and AcAm have the advantage of flexibility in that they can be used either as the unique polymeric treatment, or as a component in a conventional dual polymer program which requires both a conventional coagulant and a flocculant. The polymers of this invention can be used in conjunction with other treatment agents such as alum, ferrous sulfate or other coagulants. THE MONOMERS Example 1 summarizes the process for the preparation of the copolymer at various proportions of the monomer components in the range of about 1:99 to about 99: 1 of the monomer of the acrylamide type with the ammonium halide diallyl-N, N-disubstituted. Each of the two types of monomers used to form the dispersion polymers of this invention will be described later in greater detail. As concerns the diallyl-N, N-disubstituted ammonium halide, the monomer disubstituents may be C 1 -C 2 alkyl groups, aryl groups, alkylaryl groups or arylalkyl groups. In addition each of the disubstituyentes can be a different group. For example, a proposed halide is the ammonium chloride N-methyl-N-ethyl-N, N-diallyl. A specific example of an applicable halide is DADMAC. Preferably the amount of DADMAC present in the copolymer is from about 1 mole percent to about 50 mole percent. The diallyl-N, N-disubstituted ammonium halides, especially DADMAC, are well known and commercially available from a variety of sources. In addition to chloride, the counter ion can also be bromide, phosphate, monohydrogen phosphate and nitrate, among others. A method for the preparation of DADMAC is detailed in U.S. Pat. No. 4,151,202, where the description is incorporated herein by reference. As concerns the acrylamide type monomers, the substituted (meth) acrylamide monomers can have either straight branched or straight chain alkyl groups. Applicable monomers include, but are not limited to, hexyl hexyl (meth) acrylamide, diethylaminopropyl methacrylamide, dimethylaminohydroxypropyl methacrylamide, N-isopropyl methacrylamide, N-tert-butyl methacrylamide, N-alkyl acrylamide Ci-Cio, N-alkyl methacrylamide Ci- Cio, N-aryl acrylamide, N-aryl methacrylamide, N-arylalkyl acrylamide, N-isopropyl methacrylamide, N, N-dimethylacrylamide methacrylamide, N, N-dialkyl Ci-Cio acrylamide, N, N-dialkyl Ci-Cio methacrylamide, N , N-diaryl acrylamide, N, N-diaryl methacrylamide, N, N-diallylalkyl acrylamide, and N, N-diarylalkyl methacrylamide. As used herein, the term "arylalkyl" means comprising the benzyl groups and the phenethyl groups. "Slope Amina" refers to the NH2 group which is attached to the main polymer chain.

MULTI-ANIONIC ANALYSIS SALTS A polyvalent anionic salt is incorporated into an aqueous solution. According to the present invention, the polyvalent anionic salt is suitably a sulfate, a phosphate or a mixture thereof. Preferred salts include ammonium sulfate, sodium sulfate, magnesium sulfate, aluminum sulfate, hydrogen ammonium phosphate, sodium hydrogen phosphate, phosphate and potassium hydrogen phosphate. In the present invention these salts can each be used as an aqueous solution thereof, having a concentration of 15% or more.

THE DISPERSANT A dispersant polymer is present in the aqueous anionic salt solution where the polymerization of the above monomers occurs. The dispersant polymer is a water-soluble high molecular weight cationic polymer and is preferably soluble in the aforementioned aqueous salt solutions. It is preferred that the dispersing polymer be used in an amount of about 1 to 10% by weight based on the total weight of the hydrophilic dispersion polymer.

The dispersant polymer is composed of 20% by mol or more units of the disubstituted dialkyl ammonium halide cationic monomer or, N, N-dialkyl aminoethyl (meth) acrylates and their quaternary salts. Preferably, the% mol residual is the AcAm or (meth) AcAm. The performance of the dispersant is not greatly affected by molecular weight. However, the molecular weight of the dispersant is preferably in the range of about 10,000 to ,000,000. Preferred dispersants include homopolymers of ammonium diallyldimethyl chloride, a quaternary salt of methyl chloride dimethyl amino ethylacrylate and a quaternary salt of methyl dimethyl-aminoethyl methacrylate chloride. According to one aspect of the present invention, a multifunctional alcohol such as a glycerin or polyethylene glycol are co-existing in the polymerization system. The deposition of the fine particles is delicately carried out in the presence of these alcohols.

DISPERSION POLYMERS For polymerizations, a usual water-soluble radical-forming agent can be employed, but preferably water-soluble azo compounds such as 2, 2'-azobis (2-amidinopropane) hydrochloride and hydrochloride 2, 2 * -azobis (N, N-dimethyleneisobutylamine) are used. According to one aspect of the invention, a seed polymer is added before the start of the polymerization of the above monomers for the purpose of obtaining a fine dispersion. The seed polymer is a cationic polymer soluble in water and insoluble in the aqueous solution of the polyvalent anionic salt. The seed polymer is preferably a polymer prepared from the mixture of the above monomer by the process described herein. However, the monomer composition of the seed polymer does not always need to be the same as the water-soluble cationic polymer formed during the polymerization. However, as the water-soluble polymer formed during the polymerization, the seed polymer may contain at least 5 mole percent of the cationic monomer units of the diallyl dimethyl ammonium halide. According to one aspect of the present invention, the seed polymer used in a polymerization reaction is the water soluble polymer prepared in a previous reaction, which uses the same monomer mixture.

THE METHOD One aspect of this invention is a method for the removal of color bodies from waste effluent from paper mills, which comprises the steps of: a) adding an effective amount for the color removal of a dispersion polymer hydrophilic, wherein said polymer results from the polymerization of: i. a cationic diallyl-N, N-disubstituted ammonium halide monomer wherein the substituents of said disubstituted ammonium halide are selected from the group consisting of C 1 -C 20 alkyl groups, alkylaryl groups and arylalkyl groups, and ii. a second monomer of the formula O li wherein R x and R 2 are selected from the group consisting of hydrogen, C 1 -C 0 alkyl groups, aryl groups and alkylaryl groups; R3 is selected from the group consisting of hydrogen and methyl groups and R4 and R5 are selected from the group consisting of straight or branched C1-C10 open-chain alkylene groups and hydrogen, in an aqueous solution of a polyvalent anionic salt wherein said polymerization it is carried out in the presence of a dispersant; and, b) the removal of flocculated colored bodies from said effluent.

The cationic monomer can be ammonium diallyldimethyl chloride and the second monomer can be acrylamide. The hydrophilic dispersion polymer can have a cationic charge of about 1 mol% to about 50 mol%. the hydrophilic dispersion polymer can have an intrinsic viscosity of about 0.5 to about 10 deciliters per gram measured in 1 molar sodium nitrate for a 0.045% polymer solution. Preferably, the hydrophilic dispersion polymer can have an intrinsic viscosity of from about 1 to about 8.5 deciliters per gram measured in 1 molar sodium nitrate for a 0.045% polymer solution. More preferably, the hydrophilic dispersion polymer can have an intrinsic viscosity of about 2.5 to about 7.5 deciliters per gram as calculated in 1 molar sodium nitrate for a 0.045% polymer solution. The dispersion polymer can be added in an amount of about 1 to about 100 ppm.

In the conventional process of manufacturing pulp paper, the effluent stream of the process contains a large number of color bodies. These colored bodies are generally lignins, the degradation products of lignins or humic acid. These colored bodies impart a dark color to the effluent stream. This color is expressed in Pt-Co units and is referred to as its Real Color. The method to calculate the Real Color is standardized by the National Council of Air and Stream Improvement (NCASI) of the Paper and Pulp Industry. This method for determining the Real Color is used here to demonstrate the effectiveness of the present invention. The method is fully described in An Investigation of Improvement Procedure for Measurement of Milk Effluent and Receiving Wa ter Color, NCASI Technical Bulletin No. 2538, December 1971 incorporated herein by reference. The NCASI method for calculating the Real Color is as follows. A sample of the effluent stream is obtained and the pH of the stream is adjusted to a pH of 7.6. The sample is later filtered through a 0.8 micron membrane to remove suspended or flocculated solids. The absorbance of this sample is then determined at 465 nm in a spectrophotometer. This absorption is described as a calibrated curve which is expressed in Pt-Co units. The Real Color of this sample is recorded from the absorbance curve as Pt-Co units.

The color of the effluent stream can also be expressed as "apparent color". The apparent color is generally determined without treating the sample, as is required in the evaluation of Real Color. For purposes of the invention, the apparent color is a function of the turbidity of the effluent stream at an unadjusted pH. Turbidity is typically calculated in FTUs (Turbidity Formazine Units) by the Hach absorptometric method.

This method calculates the extinction of light at 450 nanometers in a spectrophotometer. The polymer can be added undiluted to the effluent before a solid / liquid separation step. The dose will depend on the particular system to be treated, but it is generally in the range of about 1 to about 100 ppm.

The following examples are presented to describe the preferred aspects and utilities of the invention and are not intended to limit the invention unless otherwise indicated in the claims appended thereto.

EXAMPLE 1 A dispersion copolymer of diallyldimethiamonium chloride and acrylamide in a molar ratio 30/70 is synthesized as follows, 25,667 grams of a 49.0% acrylamide solution (0.1769 moles), 161.29 grams of a 62.0% DADMAC solution (0.6192 moles), 200 grams of ammonium sulfate, 40 grams of sodium sulfate, 303.85 grams of deionized water, 0.38 grams of sodium formate, 45 grams of a solution of poly (DMAEA. MCQ) at 20% (quaternary salt of methyl dimethylaminoethylacrylate chloride, IV = 2.0 dl / gm) and 0.2 grams of EDTA to a reactor Two-liter resin equipped with an agitator, a temperature controller, and a water cooling condenser, the mixture is heated to 48 ° C and 2.50 grams of a 4% solution of 2,2-azobis dihydrochloride are added. (2-amidinopropane) and 2.50 grams of a 4% solution of 2,2-azobis dihydrochloride (N, N-dimethylene isobutyramide). The resulting solution is sprayed with 1000 cc / min of nitrogen. After 15 minutes, the polymerization begins and the solution becomes viscous. Over the next 4 hours, the temperature is maintained at 50 ° C and a solution containing 178.42 grams of 49.0% AcAm (1.230 moles) and 0.2 grams of EDTA is pumped into the reactor using a syringe pump. The dispersion of the resulting polymer has a Brookfield viscosity of 4200 cps. The dispersion is then made to react for 2.5 hours at a temperature of 55 ° C. The dispersion of the resulting polymer has a Brookfield viscosity of 3300 cps. 10 grams of 99% adipic acid, 10 grams of ammonium sulfate and 12.5 grams of a 60% aqueous solution of ammonium thiosulfate are added to the polymer dispersion. The resulting dispersion has a Brookfield viscosity of 1312.5 cps and contains 20% of a 50% by weight copolymer of DADMAC and AcAm with an intrinsic viscosity of 6.32 dl / gm as calculated in 1.0 molar NaN03 for a 0.045% polymer solution. .

EXAMPLE 2 The hydrophilic dispersions synthesized according to the procedure described in Example 1 are evaluated for their color removal abilities in wastewater obtained from a paper mill. Each sample of residual water is characterized by each of the following analyzes: Real Color calculated by the NCASI method using a Hach DR2000 spectrophotometer and Turbiedad (Apparent Color) calculated in FTUs (Turbidity Units) Formazine) using the Hach Absorptometric method at a light extinction wavelength of 450 nm. Ferrous sulfate heptahydrate, FeS0 * 7H20, 20% Fe, dry powder, commercial grade is obtained from Van Waters and Rogers Chemical Company. The hydrated lime, Ca (OH) 2 is obtained from nalco Chemical Company.

The tests are all conducted as follows. A sample of 1200 ml of wastewater from an effluent stream from a pulp paper mill is placed in a 1500 ml beaker. A two-inch Teflon coated stir bar is placed in the beaker. The sample is stirred by a magnetic stirring plate. The pH of the sample solution is continuously monitored. The dry ferrous sumate is weighed in a weighted weight boat. The ferrous sulphate is emptied into the vortex of the mixed wastewater. The dissolution is fast. A typical pH depression with 660 ppm of FeS04 »7H20 is 1.5-2.0 pH units. An appropriate amount of hydrated lime as a 2% slurry is added to the wastewater to adjust the pH to 9-10. The typical lime used for wastewater with a pH of 10.2 is treated with 500 ppm of FeS04 «7H20 is 220 ppm as Ca (OH) 2. The treatment to be tested is then added in several doses to the wastewater. The stirring speed is increased by 15 seconds of rapid mixing and then decreased by 15 minutes of slow mixing. At the end of the slow mixing time, the agitation speed is high so that the flocose mass is well mixed. 60 ml of fluid is withdrawn with a long syringe; This sample is used to measure total suspended solids. Another 140 ml are removed and transferred to a small beaker. The Real color and the Apparent Color of supernatant is calculated after 10 minutes of sedimentation. For each of these characteristics, a low number indicates a great efficiency in the removal of the color body.

All tested treatments are available from Nalco Chemical Co., of Naperville, IL. The inventive dispersion polymer D is compared with other polymers of hydrophobic dispersion A-C and also with other polymers of solution E-J. As is evident from the data in Tables 1 and 2, the hydrophilic dispersion polymers are superior. It is noteworthy that in order to obtain much better or comparable color removals with the solution polymers, much larger doses are required. Therefore, D polymers are more efficient since good results are obtained in lower doses than with polymer treatments of conventional solutions.

Table 1 Comparison of Apparent Color A = poly dispersion polymer (DMAEA, BCQ / acrylamide) molar ratio 10/90. B = poly dispersion polymer (DMAEA. MCQ / DMAEA.BCQ / Acrylamide molar ratio /25/65. C = poly dispersion polymer (DMAEA. MCQ / DMAEA.BCQ / Acrylamide molar ratio /50/20 D = poly dispersion polymer (DADMAC / acrylamide), molar ratio 30/70. E = epichlorohydrin / dimethylamine (epi-DMA) homopolymer solution (linear), molecular weight of 20,000. F = homopolymer solution epi-DMA (crosslinked), molecular weight 75,000-100,000 G = poly solution (DADMAC), molecular weight 100,000 H = poly (DADMAC) solution, molecular weight 150,000 I = ethylene dichloride / polymer solution ammonium (EDC / NH4), molecular weight 60,000.

Table 2 Comparison of Actual Color A = poly dispersion polymer (DMAEA, BCQ / acrylamide), molar ratio 10/90. B = Poly dispersion polymer (DMAEA, MCQ / DMAEA, BCQ / Acrylamide), molar ratio /25/65. C = Poly dispersion polymer (DMAEA, MCQ / DMAEA, BCQ / Acrylated), molar ratio /50/20. D = poly dispersion polymer (DADMAC / acrylamide), proportion of the molar percentage 30/70. E = homopolymer solution epi-DMA (linear), molecular weight 20,000. F = homopolymer solution epi-DMA (cross-linked), molecular weight 75,000-100,000. F = poly solution (DADMAC), molecular weight 100,000. H = poly solution (DADMAC), molecular weight 150,000. J = homopolymer epi-DMA solution (cross-linked), molecular weight 60,000.

The changes are made in the composition, operation and arrangements of the method in the present invention described herein without departing from the concept and scope of the invention as defined in the following claims: It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (7)

1. A method for the removal of colored bodies from a waste effluent of a paper mill characterized in that it comprises the steps of: a) the addition of an effective amount for the color removal of a hydrophilic dispersion polymer, in wherein said polymer results from the polymerization of: i. a cationic diallyl-N, N-disubstituted ammonium halide monomer wherein the substituents of said disubstituted ammonium halide are selected from the group consisting of C 1 -C 20 alkyl groups, alkylaryl groups and arylalkyl groups and aryl groups ii. a second monomer of the formula OR II wherein Ri and R2 are selected from the group consisting of hydrogen, Ci-Cio alkyl groups, aryl groups and alkylaryl groups; R3 is selected from the group consisting of hydrogen and methyl groups and R4 and R5 are selected from the group consisting of straight or branched C1-C10 alkylene groups and hydrogen, in an aqueous solution of a polyvalent anionic salt wherein said polymerization is carried out in the presence of a dispersant; and, b) the removal of flocculated colored bodies from said effluent.

2. The method according to claim 1, characterized in that the cationic monomer is diallyldimethyl ammonium chloride and the second monomer is acrylamide.

3. The method according to claim 1, characterized in that the hydrophilic dispersion polymer has a cationic charge of about 1 mol% to about 50 mol%.

4. The method according to claim 1, characterized in that the hydrophilic dispersion polymer has an intrinsic viscosity of about 0.5 to about 10 deciliters per gram as calculated in 1 molar sodium nitrate for a 0.045% polymer solution.

5. The method according to claim 1, characterized in that the hydrophilic dispersion polymer has an intrinsic viscosity of about 1.5 to about 8. 5 deciliters per gram as calculated in 1 molar sodium nitrate for a 0.045% polymer solution.

6. The method according to claim 1, characterized in that the hydrophilic dispersion polymer has an intrinsic viscosity of about 2.5 to about 7.5 deciliters per gram as calculated in 1 molar sodium nitrate for a 0.045% polymer solution.

7. The method according to claim 1, characterized in that the dispersion polymer is added in an amount of about 1 to about 100 ppm.