JP3685279B2 - Method for producing aqueous dispersion of synthetic resin - Google Patents

Description

【００４５】
【発明の効果】
本発明はイオン性あるいはイオン性基に変化可能な官能基を有する合成樹脂と水および必要により有機溶剤よりなる水系分散体の製造において、二軸押出機を使用することにより、合成樹脂が溶融する程の高温の条件下にありながら、その混練・分散時間が非常に短い為、加水分解等の影響を最も受け易いポリエステル樹脂でさえ、劣化を抑制することができ、かつ製造設備に高負荷がかからず、水分散化に要する時間が短く、低コストで種々の合成樹脂の水系分散体を連続法により提供することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing an aqueous dispersion containing a synthetic resin such as a polyester resin, a polyamide resin, a polyolefin resin, or a vinyl resin as an active ingredient. More specifically, in the production of synthetic resin aqueous dispersions that have excellent processability and adhesion properties and water resistance, the production equipment does not take a heavy load, and production time is significantly shorter than before, The present invention also relates to a manufacturing method that can be manufactured at low cost.
[0002]
[Prior art]
Many synthetic resins, including polyester resins, are made into aqueous dispersions in applications such as paints, inks, coatings, and adhesives by utilizing their excellent coating processability and adhesion to various substrates. May be widely used. However, in order to disperse in water, special equipment is required, complicated manufacturing processes are required, or the resin must be changed so that it can be easily dispersed. It is currently used in a form dissolved in a solvent.
[0003]
For example, in the case of a polyester resin, as an aqueous method by graft modification of a low molecular weight polyester, as in US Pat. No. 3,634,351, Japanese Patent Publication No. 57-57065, and US Pat. No. 4,517,322, Although many techniques are already known, since they are low molecular weight polyesters, the excellent processability and adhesion properties originally required for polyesters are deteriorated. Also, methods for making high molecular weight unsaturated copolyester water-based can be found in JP-A-59-223374, JP-A-62-225510, JP-B-61-57874, and JP-A-3-294322. In actual industrial production, productivity is low due to reactivity problems such as gelation and reaction condition limitations.
[0004]
On the other hand, as a method for dispersing or making a high molecular weight copolymer polyester resin in water, a method in which a hydrophilic raw material is copolymerized with the copolymer polyester resin is known. For example, a raw material containing a sulfonic acid metal base, a polyalkylene glycol, an aliphatic dicarboxylic acid, or the like is used alone or in combination. A dispersion can be obtained by dispersing these polyester resins all together in a solvent containing water, or by dissolving the polyester resin in an organic solvent in advance and adding water in a subsequent step.
[0005]
However, the method of dispersing such a polyester resin in a lump with a solvent containing water leads to an increase in cost because it takes a long time to dissolve. Therefore, if a reduction in time is desired, more organic solvent is required, which is insufficient for the purpose of reducing or eliminating the original organic solvent. In addition, the method of obtaining a dispersion by adding water to a polyester resin-dissolved product has a long time for dissolution, and when a large amount of resin is dissolved in a small amount of organic solvent, it is like a dissolution tank having a stirrer. There was a demerit that an unusually high load was applied to the manufacturing equipment.
[0006]
As described above, in the conventional technology, a synthetic resin such as a high molecular weight copolyester resin having characteristics such as processability, adhesion, and water resistance is dispersed in water or made into an aqueous dispersion to form a stable aqueous dispersion. However, the production conditions in which unusually high loads are applied to the production equipment and the problem of cost increase due to long-time dissolution have not been solved.
[0007]
[Problems to be solved by the invention]
The present invention produces an aqueous dispersion of a synthetic resin that has the characteristics that do not impair the original properties of the synthetic resin, such as excellent processability, adhesion, and water resistance, while solving the above-mentioned problems. The purpose is to do.
[0008]
[Means for Solving the Problems]
In view of the above-mentioned problems, the present inventors have dispersed a synthetic resin having excellent processability, adhesiveness, water resistance, and the like in water, or made it water-soluble to produce a stable aqueous dispersion or aqueous solution. As a result of intensive investigations into a production method that does not impose high loads on production equipment and has a short production time in production, synthetic resin having functional groups that can be changed to ionic or ionic groups, water, and, if necessary, organic solvents Was found to be able to achieve the intended purpose by dispersing the synthetic resin in an aqueous medium.
[0009]
According to the present invention, a synthetic resin having an ionic group or a functional group that can be changed to an ionic group is continuously charged into a twin-screw extruder, and heated and melted in a cylinder while water and as required. Add and knead the organic solvent. This makes it possible to produce a low-cost aqueous dispersion with a short production time without imposing any high load on the production equipment.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The synthetic resin used in the present invention has an ionic or functional group that can be changed to an ionic group, and has properties such as workability, adhesiveness, adhesion, and water resistance when it is made into a coating film or film. a synthetic resin which exhibits, polyester-based resins.
[0011]
In the present invention, the ionic group or the functional group that can be changed to an ionic group is a hydrophilic group such as a sulfonic acid, phosphoric acid, phosphonic acid, carboxylic acid or a salt thereof, an amino group, an amide group, a hydroxyl group, and a sulfobetaine group. It is a polar group.
Of these, sulfonate groups and carboxylate groups are preferred because they are easy to introduce and have excellent stability.
The salt is preferably a metal salt, and examples thereof include alkali metals such as Na and K, and alkaline earth metals such as Ca and Mg.
[0012]
Hereinafter, as an example of the synthetic resin used in the present invention, a case of a polyester resin that is most susceptible to hydrolysis and the like and is most difficult to produce will be described, but the present invention is not limited thereto.
In the polyester resin, preferably, 50 mol% or more of the total acid component is aromatic dicarboxylic acid, and the number average molecular weight is 5000 or more and 100,000 or less. Thereby, the low-cost manufacturing method of the water dispersion which has the characteristics, such as the outstanding workability required for high molecular weight polyester, adhesiveness, and water resistance, can be provided.
[0013]
Among the acid components, the polyester resin used in the present invention includes aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyl-p, p′-dicarboxylic acid, diphenyl- m, m′-dicarboxylic acid, diphenylmethane-p, p′-dicarboxylic acid, 2,2′-bis (4-carboxyphenyl) propane, indene dicarboxylic acid, and the like, and one or more of these are combined. Used. These aromatic dicarboxylic acids can be arbitrarily selected from the physical properties and economics of the coating, but terephthalic acid and isophthalic acid are used in combination from the viewpoint of coating physical properties, mechanical properties, and water dispersibility. It is preferable.
Trifunctional or higher carboxylic acids can also be used to increase the toughness of the coating film. For example, trimellitic acid or pyromellitic acid may be used at a concentration of 10 mol% or less.
[0014]
The polyester resin used in the present invention is preferably an aliphatic dicarboxylic acid such as succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, and dimer acid, preferably in the range of less than 50 mol% of the acid component. Alternatively, alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid, tetrahydrophthalic acid, and hexahydroisophthalic acid can be used. In particular, as the aliphatic dicarboxylic acid used in combination with the aromatic dicarboxylic acid, adipic acid and sebacic acid are preferable from the viewpoints of adhesion to the substrate, economy, and physical properties of the coating film. However, caution is required because the presence of an aliphatic dicarboxylic acid or alicyclic dicarboxylic acid decreases the storage stability of the aqueous dispersion.
[0015]
The polyol component of the polyester resin used in the present invention includes ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 3-methyl-1,5-pentanediol, 2-ethyl-2- Butylpropanediol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, dimethylolheptane, dimethylolpentane, diethylene glycol, triethylene glycol, cyclohexanedimethanol, TCD glycol, polyethylene glycol, polypropylene glycol, poly Tetramethylene glycol, ethylene oxide or propylene oxide derivatives of bisphenol A, ethylene oxide or propylene oxide derivatives of bisphenol S, bisphenol Ethylene oxide or propylene oxide derivatives of Nord F can be mentioned.
Further, trifunctional or higher functional polyols such as glycerin, trimethylolpropane and pentaerythritol may be used in the range of 10 mol% or less.
[0016]
Furthermore, in the polyester resin used in the present invention, it is preferable that the functional group that can be changed into an ionic group or an ionic group is at least one of a sulfonate, a carboxylic acid, or a salt thereof, thereby achieving high water dispersion stability. Sex is obtained. Particularly preferred are 0.01 to 0.5 meq / g sulfonate and / or 0.03 to 0.8 meq / g carboxylic acid or salt thereof. The above hydrophilic functional groups are necessary for the purpose of stably maintaining an aqueous dispersion produced from the polyester resin. However, introduction of excessive hydrophilic functional groups is not preferable because of adverse effects on water resistance. Further, when the concentration is lower than the above range, good dispersibility in water cannot be obtained.
[0017]
In introducing the sulfonate into the polyester resin of the present invention, a dicarboxylic acid or glycol containing a sulfonic acid metal salt can be used. Examples of the dicarboxylic acid containing a sulfonic acid metal salt include metal salts such as sulfoterephthalic acid, 5-sulfoisophthalic acid, 4-sulfonaphthalene-2,7-dicarboxylic acid, and 5 [4-sulfophenoxy] isophthalic acid. it can. Examples of the glycol containing a sulfonic acid metal base include metal salts such as 2-sulfo-1,4-butanediol and 2,5-dimethyl-3-sulfo-2,5-hexanediol. Examples of the metal salt include salts of Li, Na, K, Mg, Ca, Cu, Fe and the like. Of these, 5-sodium sulfoisophthalic acid is particularly preferable.
[0018]
In introducing the carboxylate into the polyester resin of the present invention, a polyfunctional carboxylic acid can be used. Examples of the polyfunctional carboxylic acid include trimellitic anhydride, pyromellitic dianhydride, acid anhydrides such as butanetetracarboxylic acid, and polyvalent carboxylic acids. The above acid anhydride or polyvalent carboxylic acid is subjected to an addition reaction at 200 to 230 ° C. under normal pressure after completion of polyester polymerization to obtain a polyester having a predetermined amount of carboxylic acid. By neutralizing these carboxylic acids with water in which a neutralizing agent is dissolved and, if necessary, an organic solvent, in a twin screw extruder or a container having a stirring facility after the extruder, a carboxylate is added to the polyester resin. Can be introduced. Without neutralization, the carboxyl group does not dissociate and does not contribute to good water dispersibility production. As the base for neutralization, aqueous ammonia, methylethanolamine, dimethylethanolamine, diethylethanolamine, N-methyl-diethanolamine, bis-hydroxypropyl-methylamine, tri-n-butylamine, triethylamine, bis-2-hydroxypropyl Amine, N-methyl-ethanolamine, aminomethylpropanol, 3-amino-1-propanolamine, isopropanolamine, 2-amino-2-hydroxymethyl-1,3-propanediol, aminomethyl-propanediol, cyclohexylamine, Examples thereof include amines such as t-butylamine, salts of weak acids and strong bases such as sodium carbonate and potassium carbonate, and hydroxides of alkali metals such as sodium hydroxide and potassium hydroxide. Of these bases for neutralization, amines such as ammonia water, triethylamine, and dimethylethanolamine that are not volatilized and remain when the coating film is dried are more preferable from the viewpoint of water resistance.
[0019]
In order to further improve the water dispersion stability, it is possible to introduce an ether group into the polyester resin of the present invention. In introducing the ether group, a glycol having an ether group in the main chain can be used. Specific examples include diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol and the like.
[0020]
The water used in the present invention is ion-exchanged water, distilled water, tap water or the like, and is not particularly limited, but ion-exchanged water / distilled water is preferable from the viewpoint of stable production of the aqueous dispersion.
[0021]
The organic solvent used in the present invention is not particularly limited as long as it dissolves the synthetic resin. Toluene, MEK, cyclohexanone, Solvesso 100, Solvesso 150, MIBK, n-butyl cellosolve, t-butyl cellosolve, THF, n-propanol, isopropanol, particularly It is not limited. Among them, n-butyl cellosolve and t-butyl cellosolve are preferable from the viewpoint of coating film characteristics and stability as an aqueous dispersion.
[0022]
The method of using the twin-screw extruder according to the present invention is not limited as long as the synthetic resin is put into the twin-screw extruder and water and an organic solvent are put in if necessary while heating and melting in the cylinder. I do not receive it.
[0023]
As a method for charging the synthetic resin into the twin-screw extruder, for example, if the synthetic resin is in the form of pellets or flakes, a weight-type or volume-type screw feeder, coil feeder, circle feeder, or the like can be used. If the synthetic resin is amorphous and is a sheet or paste having a glass transition temperature of room temperature or lower, it can be heated and melted and fed with a gear pump, screw feeder or the like. Alternatively, the twin screw extruder may be directly connected to the synthetic resin production and removal step, and the melt may be poured into the twin screw extruder as it is at a high temperature. In any case, in order to prevent hydrolysis of the synthetic resin (especially in the case of a polyester resin) due to moisture brought into the synthetic resin, it should preferably be introduced in a nitrogen atmosphere. More preferably, in a melting stage before the synthetic resin is cooled by being kneaded with water and, if necessary, an organic solvent, it is better to remove mixed water from the synthetic resin together with nitrogen as much as possible under reduced pressure. The process may be performed as degassing from the cylinder in a twin-screw extruder, or a synthetic resin as a raw material may be previously treated in a nitrogen atmosphere for a long time.
[0024]
Examples of a method of adding water and, if necessary, an organic solvent to the twin-screw extruder include a method of supplying from a liquid feed tank into a cylinder through a liquid injection pump, an injection valve, and a check valve. Further, as an order of adding water and, if necessary, an organic solvent, there is a method of adding a small amount of an organic solvent after melting the synthetic resin to dissolve the resin, and then adding water with a cylinder closer to the exit of the extruder.
[0025]
Also, as a step before adding water and, if necessary, an organic solvent, the temperature adjustment and screw rotation speed for melting the synthetic resin should be as low as possible in order to minimize degradation of the polyester resin such as hydrolysis. However, the optimum set value is determined in consideration of the melt viscosity characteristics of the synthetic resin to be used. In particular, the cylinder temperature should be 270 ° C. or lower in order to avoid decomposition of the polyester.
[0026]
The twin-screw extruder used in the present invention may be either the same direction rotating type or the different direction rotating type, but is preferably the same direction type having a high kneading ability. For example, a Toshiba Machine-made co-rotating twin screw extruder “TEM series” and “TEX series” manufactured by Nippon Steel Co., Ltd. can be used. The shape and combination of the screw elements are not particularly limited, but the L / D of the twin screw extruder (the total length of all the barrel lengths is set to the cylinder inner cylinder so that the kneading capacity is as high as possible and the residence time is increased. The optimum element combination is selected according to the required production capacity.
[0027]
Hereinafter, the present invention will be specifically illustrated by examples. In the examples, “parts” means “parts by weight”.
[0028]
Each evaluation / measurement item followed the following method.
[0029]
(1) Molecular weight A solid synthetic resin sample was dissolved in a synthetic resin solvent (tetrahydrofuran in the case of a polyester resin) at a concentration of 3 g / l, and the number average molecular weight was calculated in terms of polystyrene by GPC measurement at 30 ° C. In the case of an aqueous dispersion, the aqueous dispersion was applied to a polypropylene film, and a solid sample was obtained by drying at 120 ° C. for 1 hour, and then GPC was measured.
[0030]
(2) Acid value 0.2 g of a sample was precisely weighed and dissolved in 20 ml of chloroform. Subsequently, it titrated with 0.01N potassium hydroxide (ethanol solution). Phenolphthalein was used as an indicator.
[0031]
(3) Using a glass transition temperature and a melting point differential scanning calorimeter (DSC), the temperature was measured at a rate of temperature increase of 20 ° C./min. As a sample, 5 mg of a sample was placed in an aluminum press-lid container and clamped.
[0032]
(4) Composition analysis of synthetic resin The composition was analyzed by NMR (nuclear magnetic resonance method) or the like.
[0033]
(5) Dispersion stability The aqueous dispersion was visually determined after storage at room temperature for 1 month.
[0034]
(6) Measurement of dispersed particle size The aqueous dispersion was diluted to a low concentration, and the dispersed particle size was measured with a COULTER model N4 type manufactured by Nikkaki Co., Ltd. by photon correlation spectroscopy.
[0035]
(7) Synthesis Example of Polyester Resin Synthesis of Polyester Resin (A) A four-necked flask equipped with a stirrer, a thermometer and a Vigreux fractionation tube was added with 255 parts of dimethyl terephthalate, 245 parts of dimethyl isophthalate, and 3,5-dicarbo. 15 parts of sodium methoxybenzenesulfonate, 547 parts of bisphenol A ethylene oxide derivative (“BPE-20” manufactured by Sanyo Chemical Industries Co., Ltd.), 432 parts of diethylene glycol and 0.3 part of tetrabutyl titanate as a catalyst were charged at 180 to 230 ° C. A transesterification reaction was carried out for 5 hours while distilling off the methanol produced in the above. Subsequently, the pressure of the reaction system was gradually reduced to 0.3 mmHg, and a polycondensation reaction was performed at 255 ° C. for 30 minutes. Subsequently, it cooled to 200 degreeC under normal pressure and nitrogen atmosphere, 10 parts trimellitic anhydride was added, and it heated gradually to 230 degreeC, and added the trimellitic acid, and polyester resin (A) was obtained. As a result of analysis such as NMR, the obtained polyester resin was terephthalic acid / isophthalic acid / 5-sodium sulfoisophthalic acid // bisphenol A ethylene oxide derivative / diethylene glycol // trimellitic acid (post-addition) = 50/48/2 / / 65/35 // 2 (molar ratio), a pale yellow transparent resin having a number average molecular weight of 17,000, an acid value of 98 equivalents / ton, and a glass transition temperature of 58 ° C.
[0036]
Synthesis of polyester resin (B) A four-necked flask equipped with a stirrer, thermometer and Vigreux fractionation tube, 285 parts of dimethyl terephthalate, 30 parts of sodium 3,5-dicarbomethoxybenzenesulfonate, 150 parts of ethylene glycol, While charging 300 parts of 1,4-butanediol, 80 parts of polytetramethylene glycol having a molecular weight of 1000, and 0.3 parts of tetrabutyl titanate as a catalyst, the methanol produced at 180 to 230 ° C. was distilled out of the system for 5 hours. A transesterification reaction was carried out. Next, after the temperature was lowered to 170 ° C., 155 parts of adipic acid was charged, and the esterification reaction was carried out for 1 hour while distilling out the water produced at 180 to 230 ° C. out of the system. After confirming the distillation of the calculated amounts of methanol and water, the pressure of the reaction system was gradually reduced to 0.3 mmHg, and a polycondensation reaction was performed at 250 ° C. for 45 minutes to obtain a polyester resin (B). As a result of analysis such as NMR, the obtained polyester resin was terephthalic acid / adipic acid / 5-sodium sulfoisophthalic acid // ethylene glycol / 1,4-butanediol / polytetramethylene glycol = 56/40/4 // 20. This was a white opaque resin having a number average molecular weight of 23,000, an acid value of 15 equivalents / ton, a glass transition temperature of −30 ° C., and a melting point of 110 ° C.
[0037]
(8) Example of water dispersion of polyester resin <Example 1>
Pelletized polyester resin (A) previously treated for 24 hours under a nitrogen stream at 30 parts per hour to a twin-screw extruder (“TEM-35BS” manufactured by Toshiba Machine Co., Ltd. (total number of barrels: 15)) It was charged through a coil feeder and brought into a molten state at a screw rotation speed of 150 rpm and a cylinder temperature of 230 ° C. From a solvent injection line attached to the fifth cylinder from the first charging port, 9 parts per hour of n-butyl cellosolve was charged by a plunger metering pump and kneaded with polyester resin A. Furthermore, from the solvent injection line attached to the 10th cylinder from the first charging port, 49 parts per hour of ion-exchanged water and 0.18 part of 28% ammonia water per hour were introduced, and the water system was continuously fed from the nozzle of the extruder. The dispersion was discharged. When adjustment of the cylinder temperature immediately after kneading with n-butyl cellosolve was 140 ° C., the temperature of the dissolved product was 130 ° C. Further, the temperature of the dispersion was 80 ° C. when the adjustment of the cylinder temperature immediately before the kneading of the ion exchange water was 85 ° C. The obtained aqueous dispersion was stable at room temperature and was good with transparency. Table 1 shows the number average molecular weight, dispersed particle size and dispersion stability of the dispersed polyester resin.
[0038]
<Example 2>
In Example 1, an aqueous dispersion was obtained in the same manner as in Example 1 except that the setting of the cylinder temperature during resin melting was 280 ° C. The evaluation results of this aqueous dispersion are shown in Table 1.
[0039]
<Example 3>
In Example 1, an aqueous dispersion was obtained in the same manner as in Example 1 except that the screw rotation speed was 300 rpm. The evaluation results of this aqueous dispersion are shown in Table 1.
[0040]
<Example 4>
A pellet type polyester resin (B) treated in a nitrogen stream for 24 hours in advance at 27 parts per hour to a twin-screw extruder (“TEM-35BS” manufactured by Toshiba Machine Co., Ltd. (total number of barrels: 15)) It was charged through a coil feeder and brought into a molten state at a screw rotation speed of 150 rpm and a cylinder temperature of 230 ° C. From a solvent injection line attached to the fifth cylinder from the first charging port, 9 parts per hour of n-butyl cellosolve was charged with a plunger-type metering pump, and kneaded with polyester resin B. Further, 54 parts per hour of ion-exchanged water was introduced from a solvent injection line attached to the 10th cylinder from the first charging port, and the aqueous dispersion was continuously discharged from the nozzle of the extruder. When adjustment of the cylinder temperature immediately after kneading with n-butyl cellosolve was 120 ° C., the temperature of the dissolved product was 125 ° C. Further, when the adjustment of the cylinder temperature immediately before the kneading of the ion exchange water was 80 ° C., the temperature of the dispersed product was 75 ° C. The obtained aqueous dispersion was stable at room temperature and was good with transparency. The evaluation results of this aqueous dispersion are shown in Table 1.
[0041]
<Comparative water dispersion example 1>
30 parts of a pelletized polyester resin (A) previously treated in a nitrogen stream for 24 hours was charged into a four-necked flask equipped with a stirrer, thermometer and condenser, and 9 parts of n-butyl cellosolve was added to the resin solution. However, it took 4 hours to complete the solution at 140 ° C. with repeated stirring and stopping. Subsequently, 49 parts of ion-exchanged water and 0.18 part of 28% ammonia water were added and stirred at 80 ° C. for 30 minutes to obtain an aqueous dispersion. The number average molecular weight of the polyester resin in the obtained aqueous dispersion was 14,200, and the dispersed particle size was 83.0 nm.
[0042]
<Comparison water dispersion example 2>
In Comparative Water Dispersion Example 1, an aqueous dispersion was obtained in the same manner as Comparative Water Dispersion Example 1 except that the polyester resin (A) was changed to the polyester resin (B). The number average molecular weight of the polyester resin in the obtained aqueous dispersion was 19,100, and the dispersed particle size was 95.0 nm.
[0043]
In order to confirm the stable production during the continuous operation by the twin screw extruder, the aqueous dispersion produced at each time point was sampled after 0.5 hours, 2 hours and 5 hours from the start of the continuous operation. The evaluation results are shown in Table 1.
[0044]
[Table 1]

[0045]
【The invention's effect】
The present invention melts a synthetic resin by using a twin-screw extruder in the production of an aqueous dispersion composed of a synthetic resin having a functional group that can be changed to an ionic or ionic group, water, and, if necessary, an organic solvent. Even under high-temperature conditions, the kneading and dispersing time is very short, so even polyester resins that are most susceptible to hydrolysis and the like can suppress degradation and have a heavy load on production equipment. However, the time required for water dispersion is short, and various synthetic resin aqueous dispersions can be provided by a continuous method at low cost.

Claims (2)

An aqueous dispersion of a synthetic resin in which a synthetic resin having an ionic or functional group capable of changing to an ionic group, water and, if necessary, an organic solvent are introduced into a biaxial extruder to disperse the synthetic resin in an aqueous medium in the method of manufacturing, method of manufacturing an aqueous dispersion in which the synthetic resin is characterized in that the polyester resins.   The method for producing an aqueous dispersion according to claim 1, wherein the functional group that can be changed into an ionic group or an ionic group is at least one of a sulfonic acid, a carboxylic acid, or a salt thereof.