JPH02240105A - Production of aqueous resin particle - Google Patents

Abstract

PURPOSE:To obtain the title particles suited for a microencapsulated toner by an improved emulsion polymerization process comprising polymerizing a composition comprising a polymerizable monomer, a polymer which can dissolve in this monomer and is positively charged, carbon black and a specified polymerization initiator in an acidic aqueous medium. CONSTITUTION:A monomer composition comprising a polymerizable monomer (e.g. styrene), a polymer which can dissolve in this monomer and is positively charged [e.g. a polymer formed by polymerizing a cationic monomer (e.g. dimethylaminoethyl methacrylate) with a polymerizable monomer copolymerizable therewith (e.g. styrene) in the presence of an azoic polymerization initiator (e.g. 2,2'-azobisisobutyronitrile) in an organic solvent (e.g. toluene), carbon black and a polymerization initiator which can dissolve in this monomer but does not dissolve in an aqueous medium (e.g. 2,2'-azobis(2,4- dimethylvaleronitrile)] is polymerized in an aqueous medium under conditions of a pH of 1-6. In this way, aqueous resin particles suitable as resin particles for a microencapsulated toner can be produced without using any usual aids such as an emulsifier, a water-soluble polymerization initiator and a water-soluble polymeric emulsifier.

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing aqueous resin particles that are positively charged in an aqueous medium. More specifically, the present invention relates to a new method for producing aqueous resin particles by an emulsifier-free emulsion polymerization method, so-called soap-free emulsion polymerization method, which does not use a water-soluble polymerization initiator. [Prior Art] Conventionally, methods for producing aqueous resin particles include (1) a method for producing resin particles by emulsion polymerization using a surfactant; (2) A method for producing aqueous resin particles by a soap-free emulsion polymerization method using a water-soluble polymerization initiator. (3) A method for producing aqueous resin particles by a soap-free emulsion polymerization method using a hydrophilic polymerizable monomer and a water-soluble polymerization initiator. (4) Method for producing water-based resin particles using a water-soluble polymer emulsifier. etc. Among these, in (1) and (4), it is difficult to completely remove the surfactant and polymer emulsifier, and their residue poses a problem when using the resulting aqueous resin. In addition, in (2), the usable polymerization initiators are limited to 3 to 4 types, and the conditions for the polymerization reaction are also limited, so particles with desired characteristics may not be obtained. For example, when microcapsule toner particles for electrophotography are manufactured, sufficient charge cannot be imparted to the particles, resulting in problems with image density in electrophotography. In (3), there is a problem in that it is difficult to obtain a uniform polymer by copolymerizing a hydrophilic monomer with a hydrophobic monomer such as styrene or methyl methacrylate. [Problems to be Solved by the Invention] The purpose of the present invention is to improve the emulsion polymerization method. Another object of the present invention is to provide an aqueous polymerization method which is characterized in that it does not use ordinary emulsifiers, does not use water-soluble polymerization initiators, and does not use water-soluble polymer emulsifiers, etc. The purpose of this invention is to provide a method for producing resin particles. Another object of the present invention is to provide a method for producing resin particles for microcapsule toner. Another object of the present invention is to use microcapsules without using ordinary emulsifiers, without using water-soluble polymerization initiators, and without using water-soluble polymer emulsifiers. It is an object of the present invention to provide a method for producing aqueous resin particles that have excellent charging characteristics when used as a toner shell material, have no defective parts, and have excellent durability and heat resistance. [Means and Effects for Solving the Problems 1] As a result of intensive research, the present inventor has discovered that at least a polymerizable monomer and a positively charged polymer dissolved in the monomer (hereinafter referred to as a cationic polymer) , carbon black, and a monomer composition consisting of a polymerization initiator that dissolves in the monomer but does not dissolve in the aqueous medium, is polymerized in the aqueous medium under conditions of pot to 6. It has been found that the above-mentioned object of the present invention can be achieved by the manufacturing method. The present invention will be explained in more detail below. In the following description, "%" and "part" expressing quantitative ratios are based on weight unless otherwise specified. The polymerizable monomers used in the present invention are described below. That is, for example, styrene, 0-methylstyrene, di-methylstyrene, p-methylstyrene, p-methoxystyrene, p-phenylstyrene, p-chlorostyrene, 3,4-dichlorostyrene, p-ethylstyrene, 2
．． 4-dimethylstyrene, p-rope-dimethylstyrene, p
Styrene and its derivatives such as -tert-butylstyrene, pn-hexylstyrene, pn-cutylstyrene, pn-nonylstyrene, pn-decylstyrene, pn-dodecylstyrene, etc. Ethylenically unsaturated monoolefins such as ethylene, brobylene, butylene, and isobutylene; Vinyl halides such as vinyl chloride, vinylidene chloride, vinyl bromide, and vinyl fluoride; Vinyls such as vinyl acetate, vinyl brobionate, and vinyl benzoate. Esters methyl methacrylate, ethyl methacrylate, probyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, phenyl methacrylate,
Methyl acrylate, ethyl acrylate, n-acrylate
Butyl, isobutyl acrylate, probyl acrylate,
Acrylic acid esters such as low octyl acrylate, dodecyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethyl acrylate, and phenyl acrylate; vinyl ethers such as vinyl methyl ether, vinyl ethyl ether, and vinyl isobutyl ether Vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone, and methyl isobrobenyl ketone; Acrylic acid or methacrylic acid derivatives such as vinyl naphthalenes, acrylonitrile, methacrylonitrile, and acrylamide. During polymerization, polymerization is carried out in the presence of the following crosslinking agent,
It may also be a crosslinked polymer. Divinylbenzene, divinylnaphthalene, divinyl ether, divinyl sulfone, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, 1,3-butylene glycol dimethacrylate ,l,
6-hexane glycol dimethacrylate, neobentyl glycol dimethacrylate, dipropylene glycol dimethacrylate, polybrobylene glycol dimethacrylate, 2.2゜bis(4-methacryloxydiethoxyphenyl)broban, 2.2゛bis( 4-Acryloxydiethoxyphenyl)broban, trimethylolbroban trimethacrylate, trimethylolbroban triacrylate, tetramethylolmethanetetraacrylate, dibromneobentylglycol dimethacrylate, and general crosslinking agents can be used as appropriate. the above,
The cationic polymer dissolved in the monomer contains the positively charged polymerizable monomer (hereinafter referred to as cationic monomer) described below. That is, for example, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, vinylpyridine, vinylimidazole, dimethylaminomethylstyrene, etc. In these cationic monomers, methyl group, ethyl group , may be quaternary ammoniumated with a butyl group, benzyl group, etc. Examples of the above-mentioned polymerizable monomer that copolymerizes with the cationic monomer include styrene, 0-methylstyrene, m-
Methylstyrene, p-methylstyrene, p-methoxystyrene, p-phenylstyrene, p-chlorostyrene,
3.4-dichlorostyrene, p-ethylstyrene, 2.
4-dimethylstyrene, p-n-butylstyrene, p
- Styrene and its derivatives such as tert-butylstyrene, pn-hexylstyrene, pn-butylstyrene, pn-nonylstyrene, pn-decylstyrene, pn-dodecylstyrene; Ethylenically unsaturated monoolefins such as ethylene, propylene, butylene, and isobutylene; Vinyl halides such as vinyl chloride, vinylidene chloride, vinyl bromide, and vinyl fluoride; Vinyl esters such as vinyl acetate, vinyl brobionate, and vinyl benzoate Class; methyl methacrylate, ethyl methacrylate, provil methacrylate, loubyl methacrylate,
Isobutyl methacrylate, n-octyl methacrylate,
Dodecyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, phenyl methacrylate, methyl acrylate, ethyl acrylate, acrylic acid n
- Acrylic acid esters such as butyl, isobutyl acrylate, probyl acrylate, n-octyl acrylate, dodecyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethyl acrylate, phenyl acrylate; vinyl methyl ether , vinyl ethyl ether, vinyl isobutyl ether; vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone, methyl isobrobenyl ketone; vinyl naphthalenes: acrylic acid or methacrylic acid derivatives such as acrylonitrile, methacrylic nitrile, acrylamide, etc. and so on. Cationic polymers are produced, for example, by solution polymerization in organic solvents using an azo polymerization initiator. That is, the cationic monomer, a polymerizable monomer that copolymerizes with the cationic monomer, and an azo polymerization initiator are polymerized in an organic solvent. Here, as the azo polymerization initiator, for example, 1.1'-
Azobis (cyclohexane! monocarponitrile),
2.2°-azobisisobutyronitrile, 2,2°-azobis(2.4-dimethylbaretonitrile), etc. are used. Examples of the organic solvent used include aromatics such as toluene and xylene, alcohols, and ketones. Generally, aromatic compounds, particularly toluene, are preferably used. The proportion of the cationic monomer in the cationic polymer is preferably 3 to 80%, more preferably 7 to 70%. This is because if the cationic monomer content is less than 3%, sufficient positive chargeability cannot be imparted;
If the content is above, the solubility in an aqueous medium becomes strong even though sufficient positive chargeability can be imparted, making it impossible to achieve the object of the present invention. In the present invention, it is preferable to mix the cationic polymer and the polymerizable monomer so that the former becomes 5 to 80%, more preferably 1O to 70%. This is because if the content of the cationic polymer is less than 5%, sufficient positive chargeability cannot be imparted to the aqueous resin particles, and if the content is more than 80%, the cationic polymer and the polymerizable monomer The viscosity of the mixture with
It is not possible to obtain water-based resin particles. In the present invention, the molecular weight of the cationic polymer is 2,000 to 1,000,000 as determined by weight average molecular weight measurement by GPC.
, preferably 5,000 to 50,000
This is the case when it becomes 0. If the weight average molecular weight of the cationic polymer is less than 2,000, the solubility in an aqueous medium will be strong and the object of the present invention cannot be achieved;
0,000 or more, it is virtually impossible to dissolve and mix with the polymerizable monomer. As the carbon black used in the present invention, any type of carbon black can be used as long as it does not inhibit the radical polymerization reaction. Further, the carbon black can be pulverized in advance to obtain a good dispersion state in the polymerizable monomer composition, for example, by cross-mixing with the cationic polymer, or by mixing the carbon black with the polymerizable monomer composition. After mixing with the composition, it can be processed by applying external mechanical force to disperse it. Examples of the carbon black used in the present invention include Monarch 1300, Monarch 1100, Monarch 1000, Monarch 900, and Monarch 8.
80, 800, 700, Mogul L1 Legal 4
00R, 660R, 500R, 330R, 30
0R, 99R1 Vulcan XC-728, Elftex 8, Elftex 12 (all manufactured by Cabot),
Raven 5000, 7000, 5750, 525
0, 3500, 2000, 1500, 250
, same 1200, same 1170, same 1040, same 1035,
same! 000, Conductec 40-220, 975,
Examples include the same 900 and the same SC (manufactured by Columbian Carbon Co., Ltd.). The carbon black is preferably contained in an amount of 0.1 to 20 parts based on 100 parts of the polymerizable monomer, and further,
It is preferable that it is contained in an amount of 0.1 to 8 parts. If the content is 20 parts or more, it will have a strong influence on the polymerization reaction and will significantly inhibit the reaction, while if it is less than 0.1 part, it will not be charged when used in the shell material of microcapsule toner, which is one of the objects of the present invention. No improvement or improvement effect on characteristics was observed. Polymerization initiators that are not soluble in the aqueous medium used in the present invention include the following. That is, for example, 1.1'-azobis(cyclohexane-1-carponitrile), 2.2°-azobisisobutyronitrile, 2.2°-azobis(2.4-dimethylvalenitrile), 2.2 Examples of polymerization initiators include ゜-azobis(4-methoxy-2,4-dimethylbaretonitrile), dicumyl peroxide, dilauroyl peroxide, penzoyl peroxide, tolyl peroxide, and chloropenzoyl peroxide. It is preferable to add it to the mixture of the cationic monomer and the cationic polymer in an amount of 0.1 to 20%, more preferably 0.5 to 15%. if,
If the amount of the polymerization initiator added is less than 0.1%, it is difficult to complete the polymerization reaction, and if the amount of the polymerization initiator added is 20% or more, it is difficult to control the polymerization reaction. In the present invention, it is preferable that p}I of the aqueous medium is between 1 and 6 when measured at 20°C, and even under strong acidity where the pH is less than 1 or under weak acidity where the pH is 6. The aqueous resin particles of the present invention cannot be stably produced. Note that the pH of the aqueous medium is preferably adjusted with hydrochloric acid, acetic acid, etc., and a buffer solution may be used to stabilize the pH. The aqueous resin particles of the present invention are 500 μm to 0.0 μm. The particle size can be adjusted to any size between 1 μm and preferably 1 μm.
The particle size is 00 μI11 to 0.01 μm,
More preferably, the particle size is 50 μl and 11 to 0.1 μm. The aqueous resin of the present invention can be prepared at any solid content concentration of 50% or less based on the aqueous medium, but is preferably prepared at a solid content concentration of 5 to 35%. When the solid content concentration exceeds 50%, heat generation due to the polymerization reaction of the polymerizable monomer increases and becomes difficult to control, and furthermore, scale formation due to aggregation between resin particles increases, which becomes a problem. In addition, in the present invention, a polymerization initiator that does not dissolve in an aqueous medium has a solubility in an aqueous medium of 1 when measured at 20°C.
% or less. When the method of the present invention is applied to the production of microcapsule toner, the aqueous resin particles are the core particles 1 of the microcapsules.
0.1 to 50 parts, preferably 1 to 30 parts. If the amount of particles is less than 0 or 1 part, the core particles will not be encapsulated substantially, and if the amount of aqueous resin particles is more than 50 parts, the core particles will agglomerate among themselves during encapsulation, making it unsuitable for practical use as a microcapsule toner. Cannot be used above. If the particle size of the core material particles is 1, the particle size ratio of the wall material particles and the core material particles is 1/1000 ~! /! It is good if it is between, and more preferably! /500 to 1/2. If the particle size ratio of the aqueous resin particles to the core particles is smaller than 1/1000, that is, if the aqueous resin particles are too small, a capsule wall can be formed on the surface of the core particles, but the microcapsule toner However, if the size is larger than 1/1, that is, if the aqueous resin particles are too large, even if the aqueous resin particles can adhere to the surface of the core material particles, the capsule wall will not be dense and strong. This also results in significantly lower durability. The average particle diameter of the core material particles may be in the range of 1 to 30 μm, more preferably in the range of 5 to 20 μm.

【Example】

Hereinafter, the present invention will be explained in more detail with reference to Examples. Synthesis of Cationic Polymer (1) Dimethylamine ethyl methacrylate 10 parts Styrene 90 parts 2.2°-Azobisisobutyronitrile 10 parts Above polymerizable monomer composition 5. 0kg to toluene 12. Mix with OI2, 2
Pour into a 0β container and stir. After confirming that the polymerization initiator has completely dissolved to form a homogeneous solution, gradually heat the solution to 78°C while stirring and flowing nitrogen. Once the temperature reaches 78°C, the polymerization reaction is carried out at that temperature for 6 hours, and then heated until the solvent toluene refluxes and kept in that state for 1 hour. Thereafter, the solvent is distilled off along with the unreacted polymerizable monomer to concentrate. Further, by drying under reduced pressure at 120° C. for 24 hours, a cationic polymer (1) was obtained. As a result of elemental analysis, it was found that 9.7% of dimethylaminoethyl methacrylate was contained. Synthesis of cationic polymer (2) Diethylaminoethyl methacrylate 8 parts Methyl methacrylate 92 parts 2.2°-Azobisisobutyronitrile 5 parts Cationic polymers except for the above polymerizable monomer composition were used A polymer was obtained in the same manner as in (1). This is referred to as the cationic polymer (2) of the present invention. As a result of elemental analysis, it was found that the content of diethylaminoethyl methacrylate was 8.4%. Synthesis of cationic polymer (3) Dimethylaminomethylstyrene 15 parts Styrene 85 parts Benzoyl peroxide 3 parts Above polymerizable monomer composition 5
．． 0kg to toluene 12. Mix with oj2 and stir in a 20ml container. After confirming that the polymerization initiator was completely dissolved and a homogeneous solution was formed, the solution was stirred while flowing nitrogen.
Gradually heat to 80°C. Once the temperature reaches 80°C, the polymerization reaction is carried out at that temperature for 10 hours, and then heated until the solvent toluene refluxes and kept in that state for 1 hour. Thereafter, the solvent is distilled off along with the unreacted polymerizable monomer to concentrate. Further, by drying under reduced pressure at 120° C. for 24 hours, a cationic polymer (3) was obtained. As a result of elemental analysis, it was found that 15.1% of dimethylamine methylstyrene was contained. Synthesis of cationic polymer (4) Vinylimidazole 30 parts Styrene 70 parts t,t'-azobis(cyclohexane 5 parts - 1-carponitrile) Above polymerizable monomer composition 5. 0kg to toluene 15
, Oβ, and stirred in a container at 30°C. After confirming that the polymerization initiator has completely dissolved to form a homogeneous solution, the solution is gradually heated to 95° C. while flowing nitrogen and stirring. Once the temperature reached 95°C, the polymerization reaction was carried out at that temperature for 8 hours, then heated until the solvent toluene refluxed and kept in that state for 1 hour. Thereafter, the solvent is distilled off along with the unreacted polymerizable monomer to concentrate. Further 120℃
By drying under reduced pressure for 24 hours, a cationic polymer (4) was obtained. Furthermore, as a result of elemental analysis, vinylimidazole was found to be 2
It was found that the content was 9.2%. Production of aqueous resin particles (1) Cationic polymer (1) 50 parts Styrene 48 parts Divinylbenzene 2 parts 2.2°-azobis(2
．． 4-dimethyl 5 parts Valeronitrile) Raben 3500 1 part (
(manufactured by Columbia Carbon) Pour 5β of ion-exchanged water adjusted to pH 2.0 with hydrochloric acid into a reaction vessel with a capacity of 1OI2. While stirring at room temperature, the polymerizable monomer solution composition 2. When 0 kg is added dropwise over 20 minutes, a milky white emulsion is obtained. Next, heat to 60°C, hold at that temperature for 8 hours, further heat to 80°C, hold for 1 hour, and then cool to room temperature. The resulting resin particle aqueous solution was passed through a 100 mesh sieve,
A small amount of resin particle aggregates were removed. This is referred to as the aqueous resin particle (1) of the present invention. In addition, the average particle diameter measured by Coulter N4 type is 0.4
It was 0±0.09 μm. In addition, the solid content determined by concentrating and drying a certain amount of aqueous resin particle liquid was 28.1
%, and the polymerization yield was 97.8%. Production of water-based resin particles (2) Cationic polymer (2) 20 parts styrene 70 parts ethyl acrylate 8 parts divinylbenzene
Part 2 Conductec 40-220
0. 1 part (manufactured by Columbia Carbon) The above monomer composition was placed in a sand mill whose surroundings were cooled, and carbon black was mixed and dispersed for 10 hours.
Next, 5 parts of 2,2'-azobisisobutyronitrile, a polymerization initiator, was added and dissolved. Ion-exchange water adjusted to pH 2.0 with hydrochloric acid at 5° C. is charged into a reaction vessel having a capacity of 10β. While stirring at room temperature, the polymerizable monomer solution composition 1. When 0 kg is added dropwise over 20 minutes, a milky white emulsion is obtained. Next, heat to 78°C, hold at that temperature for 8 hours, further heat to 90°C, hold for 1 hour, and then cool to room temperature. The resulting resin particle aqueous solution was passed through a 100 mesh sieve to remove a small amount of resin particle aggregates. This is the aqueous resin particle (2) of the present invention.
Suppose that In addition, the average particle diameter measured by Coulter N4 type is 0.1
It was 5±0.02 μm. In addition, the solid content determined by concentrating and drying a certain amount of aqueous resin particle liquid was 16.1
%, and the polymerization yield was 97.1%. Production of microcapsule toner (1) (application example) Polyester (composition, terephthalic acid: isophthalic acid: ethylene glycol: trimethylolpropane = 45:10:30:15) 1
A mixture consisting of 00 parts magnetic material 60 parts low molecular weight polypropylene 6 parts (Viscol 660P manufactured by Sanyo Chemical Industries, Ltd.) was heated and kneaded with a roll. After allowing it to cool, it was coarsely crushed, and further finely pulverized using a jet mill and classified using an air classifier to obtain core particles (1) having a volume average particle diameter of 11.6 μm. The aqueous resin particles of the present invention (
Convert 1) to solid content! 5 parts were mixed and dispersed. next,
A spray dryer was used to perform spray drying at an inlet temperature of 160° C. and an outlet temperature of 90° C., and a crosslinked resin coating layer mainly composed of the emulsion-constituting copolymer was provided around the core material particles. This is referred to as the microcapsule toner (1) of the present invention. Colloidal Siriyoku (Aerosil R-972) was added to the capsule toner (1) thus obtained, and NP-
3525 (manufactured by Canon) and it was confirmed that the image had sufficient image density and development durability. In addition, in order to examine the thermal fixability in detail, we prepared a separate fixing unit for the copying machine and used this separate fixing unit to fix an unfixed image on a separately prepared transfer paper, and no offset phenomenon occurred over a wide temperature range. The adhesion properties were also excellent. Furthermore, when this capsule toner was left in an atmosphere of 45°C for a long time, no blockage occurred. No caking was observed. Production of microcapsule toner (2) (application example) Styrene/butyl acrylate/divinylbenzene copolymer (composition ratio: 7:22.5: o.s) 1
Using core material particles consisting of 00 parts magnetic material 60 parts low molecular weight polypropylene 8 parts (Viscol 660P manufactured by Sanyo Chemical Industries),
Microcapsule toner (2) was obtained in the same manner as in Example 1 except that aqueous resin (2) was used. Good results were obtained when the same image quality evaluation and blocking test as for microcapsule toner (11) were conducted. Production of microcapsule toner (3) (application example) Core used in the production of microcapsule toner (1) The core material particles and Aerosil #300 were mixed and stirred, and the surface thereof was coated with silica.Slurry 1 containing 1.0 kg of the core material particles
0 kg was placed in a 204 container and stirred at room temperature. Next, the pH of the aqueous phase was adjusted so that the silica dissociated and acted as an anion. The aqueous resin particles (1
) was added under stirring. After continuing stirring for 1 hour, it was heated to 50°C and kept at that temperature for 2 hours. After cooling to room temperature, filter. It was dry. When the surface of the particles was observed using an electron microscope, it was confirmed that the particles were covered with wall material particles. This is called microcapsule toner (3). Good results were obtained when the same image quality evaluation and blocking test as for Microcapsule Toner (1) were conducted. Comparative Example 1 An attempt was made to produce aqueous resin particles in exactly the same manner as in the production example of aqueous resin particles (1) except that the aqueous medium, ion-exchanged water, was adjusted to pH 7.6. The dispersion state of the polymerizable monomer solution composition in the aqueous phase was extremely unstable, and it was not possible to obtain aqueous resin particles. Comparative Application Example Polymerization was carried out in exactly the same manner as in the production example of aqueous resin particles (1) except that 0.5 parts of sodium dodecylbenzenesulfonate was added as an emulsifier and the PLL of the aqueous phase was not adjusted. Aqueous resin particles (2) were obtained. The comparative microcapsule toner was evaluated for yield, image quality, and durability by spray drying in the same manner as the production of microcapsules (1) except that the comparative resin particles (2) were used. was extremely low.

【Effect of the invention】

According to the present invention, water-based resin particles are produced that are characterized in that they do not use ordinary emulsifiers, water-soluble polymerization initiators, or even water-soluble polymer emulsifiers, and are particularly suitable for producing resin particles for microcapsule toners. A method was provided. Patent applicant Canon Inc.

Claims (1)

[Claims] 1) A monomer composition consisting of at least a polymerizable monomer, a positively charged polymer that dissolves in the monomer, carbon black, and a polymerization initiator that dissolves in the monomer but does not dissolve in an aqueous medium. A method for producing aqueous resin particles, which comprises polymerizing in the aqueous medium at a pH of 1 to 6.