JP4468251B2 - Method for producing polymer - Google Patents

Description

  The present invention relates to a method for producing a copolymer.

  Copolymers are used in fields such as dispersants and thickeners. When polymerizing a copolymer by a copolymerization reaction, the reaction rate of each monomer varies depending on its structure. Therefore, a copolymer having the desired molecular weight can be obtained by controlling the concentration of each monomer during polymerization. I can do it.

  In batch production, when the next batch is performed without washing in the same tank, the polymerization initiator may remain in the residue of the previous batch, and the polymerization starts before the polymerization initiator is added. As a result, there was a problem that a polymer having a target molecular weight could not be obtained.

  Patent Document 1 discloses a method of controlling the polymerization rate during curing by adding a polymerization inhibitor in advance in the production of bioactive cement. However, this polymerization inhibitor is added not for completely stopping the polymerization reaction but for controlling the polymerization rate.

In Patent Document 2, after adding a small amount of a polymerization initiator, detoxifying a polymerization inhibitor contained in a raw material such as a monomer, etc., after confirming a polymerization start point at which polymerization is possible by increasing the temperature in the polymerization tank Further, a method for producing a polymer having a target molecular weight by further adding a polymerization initiator or the like is disclosed. However, this method is a method in which the polymerization inhibitor contained in the raw material is detoxified and then polymerized. When a polymerization initiator is present in the reaction system, the temperature rises before the addition of the polymerization initiator. The polymerization may not be controlled. When the polymerization control cannot be sufficiently performed, a polymer having a target molecular weight may not be obtained, or turbidity may occur in the polymer aqueous solution after polymerization.
JP 2000-279506 A JP 2003-268010 A

  An object of the present invention is to provide a method capable of efficiently producing a copolymer having a target molecular weight with good reproducibility without performing washing in a continuous batch reaction in the same tank.

The present invention provides a method for producing a copolymer in a batch process using a polymerization initiator, and a method for producing a copolymer that satisfies the following condition 1.
Condition 1: The reaction is carried out after adding a polymerization inhibitor in an amount of 0.9 to 50 equivalents of the polymerization initiator contained in the reaction product of the previous batch remaining in the reaction vessel.
The present invention further provides a method for producing the above copolymer, which satisfies the following condition 2.
Condition 2: The reaction vessel is not washed after the reaction product of the previous batch is withdrawn from the reaction vessel.

  In the continuous batch reaction in the same tank by the method of the present invention, the polymerization reaction before the addition of the polymerization initiator is suppressed by adding a polymerization inhibitor in advance and detoxifying the polymerization initiator existing in the adhesion residue of the previous batch. In addition, by starting the reaction immediately after the addition of the polymerization initiator, it is possible to efficiently produce a copolymer having a target molecular weight with good reproducibility without washing between batches.

  The type of copolymer produced by the method of the present invention is not particularly limited, and the monomer used is not particularly limited. However, the method of the present invention is effective for hydrophobic groups and hydrophilic groups. It can use suitably for manufacture of the amphiphilic polymer which has both.

  The amphiphilic polymer can be produced by, for example, a method of polymerizing a monomer component containing a hydrophilic monomer and a hydrophobic monomer, or a method of copolymerizing a reactive surfactant and a hydrophilic monomer.

  When the monomer component including the hydrophilic monomer and the hydrophobic monomer is polymerized, an ionic hydrophilic monomer and a nonionic hydrophilic monomer can be used as the hydrophilic monomer. As the ionic hydrophilic monomer, a monomer having a cationic or anionic hydrophilic group in the molecule can be used.

  Cationic hydrophilic monomers include dimethylaminoethyl (meth) acrylate, diethylaminoethyl (meth) acrylate, allylamine or inorganic acids such as hydrochloric acid, sulfuric acid, acetic acid, phosphoric acid, salts of organic acids, or methyl halide ( Contains quaternary ammonium salts obtained by reaction with quaternizing agents such as chloride, bromide, etc.), ethyl halide (chloride, bromide, etc.), dialkyl (methyl, ethyl, etc.) sulfuric acid, dialkyl (methyl, ethyl, etc.) carbonate, etc. Vinyl monomers can be used.

  As the anionic hydrophilic monomer, an unsaturated carboxylic acid, an organic sulfonic acid having an unsaturated double bond, a neutralized salt thereof, or the like can be used. Examples of the unsaturated carboxylic acid include (meth) acrylic acid, crotonic acid, maleic acid, itaconic acid and the like. Examples of the organic sulfonic acid having an unsaturated double bond include vinyl sulfonic acid, styrene sulfonic acid, and 2- (meth) acrylamide-2-methylpropane sulfonic acid.

  When these anionic hydrophilic monomers are used as neutralized salts, sodium hydroxide, potassium hydroxide, ammonia, or the like can be used as a neutralizing base.

  As the nonionic hydrophilic monomer, one or more compounds selected from the group consisting of unsaturated carboxylic acid amide compounds, other hydrophilic vinyl compounds, and hydrophilic allyl compounds can be used. Examples of unsaturated carboxylic acid amide compounds include (meth) acrylamide, diacetone (meth) acrylamide, N-methyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, N-methylol (meth) acrylamide and the like. be able to. Other nonionic hydrophilic vinyl compounds include N-vinyl-2-pyrrolidone, N-vinyloxazolidone, N-vinyl-5-methyloxazolidone, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meta ) Acrylate and the like. Examples of the hydrophilic allyl compound include allyl alcohol and methallyl alcohol.

  In the present specification, (meth) acrylate means acrylate or methacrylate, (meth) acrylic acid means acrylic acid or methacrylic acid, and (meth) acrylamide means acrylamide or methacrylamide. means.

  In the production of amphiphilic polymers, hydrophobic monomers can be used at the same time to strengthen the hydrophobicity in the polymer chain, such as aromatic vinyl compounds, vinyl cyanide compounds, diene compounds, vinyl carboxylic acid esters. One or more compounds selected from the group consisting of compounds, alkyl vinyl ether compounds, other vinyl compounds, and hydrophobic allyl compounds can be mentioned.

  Examples of the aromatic vinyl compound include styrene, α-methylstyrene, α-chlorostyrene, p-tert-butylstyrene, p-methylstyrene, p-chlorostyrene, o-chlorostyrene, 2,5-dichlorostyrene, 3 , 4-dichlorostyrene, divinylbenzene and the like. Examples of the vinyl cyanide compound include acrylonitrile and methacrylonitrile. Examples of the diene compound include diolefin compounds such as allene, butadiene, and isoprene, and chloroprene. Examples of vinyl carboxylic acid ester compounds include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, and benzyl (meth). Acrylate, glycidyl (meth) acrylate, ethylene glycol di (meth) acrylate, 1,4-butanediol di (meth) acrylate, diethylene glycol di (meth) acrylate, triethylene glycol di (meth) acrylate, tetraethylene glycol di (meth) ) Acrylate, phenoxypolyethylene glycol (meth) acrylate, and epoxy (meth) acrylates, urethane (meth) acrylates, divinyl compounds, etc. That.

  The reactive surfactant used when synthesizing the amphiphilic polymer by the method of copolymerizing the reactive surfactant and the hydrophilic monomer is a polymerizable unsaturated double bond or ionic in the molecule. It is a substance having a hydrophilic group and a hydrophobic group. For example, a known reactive surfactant such as succinic acid polyoxyethylene alkyl ester, which is an amphiphilic compound having a vinyl group or an allyl group in the molecule and at least one ionic hydrophilic group, is used. can do.

  As the hydrophilic monomer to be copolymerized with the reactive surfactant, a polymerizable monomer having any one of cationic, anionic and nonionic hydrophilic groups in the molecule can be used. As the hydrophilic monomer, the above-mentioned cationic, anionic or nonionic hydrophilic monomer can be used.

  When an amphiphilic polymer is produced using a monomer component including a hydrophilic (meth) acrylamide monomer and a hydrophobic (meth) acrylate monomer, the turbidity of the polymer solution obtained from the raw material monomer ratio at the initial reaction is obtained. Has an important impact on the degree. From the viewpoint of suppressing turbidity of the amphiphilic polymer, the concentration of the raw material hydrophobic (meth) acrylate monomer at the initial reaction represented by the following formula (I) is preferably 0.5 to 10 mol%, and 1 to 5 mol%. Is more preferable.

W = B / (A + B) × 100 (I)
(Where, W: raw material hydrophobic (meth) acrylate monomer concentration [mol%] at the initial reaction, A: nonionic hydrophilic (meth) acrylamide monomer amount [mol] at the initial reaction, B: initial (The amount of the hydrophobic (meth) acrylate monomer [mol] during the reaction is shown.)

  Examples of the polymerization initiator used in the present invention include persulfates such as ammonium persulfate, sodium persulfate and potassium persulfate, azo compounds such as azobis-2-methylpropionamidine hydrochloride and azoisobutyronitrile; benzoyl peroxide And peroxides such as lauroyl peroxide, and the like, and azo compounds are preferred.

  As the polymerization inhibitor used in the present invention, quinone polymerization inhibitors such as parabenzoquinone (PBQ), methoquinone (MeHQ), hydroquinone (HQ), 3,5-ditertiarybutyl-4-hydroxytoluene (BHT), Although phenothiazine (PHEN) can be mentioned, a quinone polymerization inhibitor is preferable.

  In the present invention, one or more monomers as described above are used and polymerized by suspension polymerization, solution polymerization or the like in a batch system.

  In the present invention, the organic solvent used in producing the polymer is not particularly limited, and can be selected depending on the solubility of the monomer used or the polymer to be synthesized. Examples thereof include alcohols such as methanol, ethanol and isopropyl alcohol, hydrocarbons such as benzene, toluene, cyclohexane and n-heptane, esters such as ethyl acetate, and ketones such as acetone and methyl ethyl ketone. These organic solvents can be used alone, but a mixture of two or more of them can be used to show the solubility of the monomer mixture.

  The average molecular weight of the polymer obtained by the method of the present invention is preferably 5000 to 100,000 in terms of weight average, more preferably 10,000 to 500,000, and particularly preferably 20000 to 200,000.

  The method of the present invention satisfies the above-mentioned condition 1, preferably conditions 1 and 2. However, in order to carry out the method of the present invention, it is necessary to grasp the weight of the reactant in the previous batch remaining in the reaction vessel. desirable. The residual weight may be calculated from the material balance (difference between the charged amount and the extracted amount), and once the actual measurement is performed by, for example, solvent cleaning as a test, the value can be used from the next time.

  The amount of the polymerization initiator contained in the reaction product of the previous batch remaining in the reaction vessel can be calculated. However, since a part of the reaction was deactivated in the previous batch reaction, the activity should be measured once. That value can be used from the next time.

  In condition 1, the amount of the polymerization inhibitor added to the reaction vessel is 0.9 equivalent times or more of the polymerization initiator remaining in the reaction vessel from the viewpoint of suppressing the start of the polymerization reaction before the start of the polymerization reaction. And 1.0 equivalent times or more is preferable. Further, from the viewpoint of economy and the reaction after the start of polymerization without delay, it is 50 equivalent times or less of the polymerization initiator remaining in the reaction tank, more preferably 20 equivalent times or less, still more preferably 10 equivalent times or less, 5 equivalent times or less is particularly preferable.

  In addition, when the polymerization inhibitor is contained in the raw material monomer charged into the reaction vessel before the start of the reaction, the total amount of the polymerization inhibitor present in the reaction vessel is preferably within the above range.

  In the method of the present invention, it is desirable to add the polymerization inhibitor to the reaction vessel and then mix the reaction vessel with stirring or the like. In a reaction tank having an external circulation line, it is desirable to mix it including the external circulation line. The addition of the polymerization inhibitor is preferably performed before charging the raw material monomer into the reaction vessel from the viewpoint of suppressing the polymerization of the monomer.

  The material of the raw material dropping tank and the polymerization reaction tank is not particularly limited, and glass, iron, stainless steel (SUS304, SUS316, SUS316L, etc.) can be used. When iron or stainless steel is used, the trace metal from the tank is used. Due to the elution, a polymer having a target molecular weight may not be obtained, or turbidity may be generated in the polymer aqueous solution after polymerization. Therefore, when using a reaction vessel of such a material, it is preferable to carry out the reaction by adding a chelating agent.

  As the chelating agent, a phosphonic chelating agent, a carboxylic acid chelating agent, a glycerine oxime chelating agent and the like can be used, and a phosphonic chelating agent is preferred. As the phosphonic chelating agent, phosphonic acids represented by the following general formulas (1) to (4) or salts thereof are preferable.

[Wherein n is a number from 1 to 8, and X is a formula (5)

The phosphonic acid group or phosphonic acid group represented by these is shown. M and M ′ each independently represent a hydrogen atom, an alkali metal, an alkaline earth metal, ammonium, or a mono, di, or trialkyl (preferably having 2 to 6 carbon atoms) ammonium that may be substituted with a hydroxyl group. ]

Among these phosphonic chelating agents, those represented by general formula (1), (3) or (4) are preferred, those represented by general formula (3) or (4) are preferred, and general formulas are particularly preferred. In (3) and (4), n = 2 is preferable. Moreover, as for M and M '(counter ion) in the phosphonic acid (salt) group represented by Formula (5), a hydrogen atom, sodium, and potassium are preferable. These compounds represented by the general formulas (1) to (4) are, for example, the “DEQUEST” series of Solutia Europe SA / NV, specifically, the Dequest 2006 [aminotri (methylene Phosphonic acid) 5Na salt], Dequest 2010 (1-hydroxyethylidene-1,1-diphosphonic acid), Dequest 2041 [ethylenediaminetetra (methylenephosphonic acid)], Dequest 2066 [Diethylenetriaminepenta (methylenephosphonic acid) 7Na salt ] Or the like.
The amount of the chelating agent used is preferably from 100 to 3000 ppm, more preferably from 200 to 2000 ppm, particularly preferably from 300 to 1500 ppm, based on the weight of the raw material monomer, from the viewpoint of suppressing the turbidity of the polymer solution.

  In the following examples, “%” is based on weight unless otherwise specified. Moreover, the weight average molecular weight and turbidity of the polymer were measured by the method shown below.

<Measurement method of weight average molecular weight>
A column in which two TSKgel α-Ms manufactured by Tosoh Corporation are connected in series is used. As an eluent, LiBr 50 mmol / L and acetic acid 1% dissolved in ethanol / water mixed at a volume ratio of 30:70 are allowed to flow at a flow rate of 1 ml / min (column temperature 40 ° C.). The polymer concentration is adjusted to 2 μg / ml using this eluent, and then 100 μl is injected into the column for measurement. The detector uses RI. The molecular weight is converted to the value of polyethylene glycol measured under the same conditions.

<Measurement method and evaluation of turbidity>
Turbidity is measured by the method of JIS K 0101 9.4.
The measured turbidity (mg / L) was evaluated according to the following criteria.
○: Less than 10 turbidity and visually transparent.
Δ: Turbidity of 10 to less than 20, with slight turbidity visually observed.
X: Turbidity is 20 or more, and turbidity is observed visually.

Synthesis example 1
Initial charge: In a 2 liter glass reaction vessel, 203.1 g of water, 135.3 g of isopropyl alcohol (IPA), 35.3 g of monomer A (acrylamide), 2.2 g of monomer B (butyl methacrylate), 90% monomer C ( An aqueous solution of 3.6 g of methacryloyloxyethylenedimethylethylaminoethyl sulfate) and 0.7 g of 85% phosphoric acid were charged, and the temperature was raised to 62 ° C. in a nitrogen atmosphere.

  Polymerization reaction: A mixed solution of 153.4 g of water, 153.4 g of IPA, 98.5 g of monomer A, 57.8 g of monomer B, 83.1 g of 90% monomer C aqueous solution, and 4.1 g of 85% phosphoric acid was charged in a glass dropping container, In a separate glass dropping vessel, 69.5 g of a 2% aqueous solution of V-50 (manufactured by Wako Pure Chemical Industries, Ltd., polymerization initiator, molecular weight 271.2) was charged into a reaction vessel maintained at 62 ° C. from these two dropping vessels. It was dripped at the same time for 5 hours. Subsequently, it was aged at 62 ° C. for 2 hours to obtain a polymer solution. The weight average molecular weight of the obtained polymer was 27700, and the turbidity was 2.

The polymer solution obtained by polymerization was extracted from the reaction vessel, but 10.0 g remained attached to the reaction vessel. In the following Examples 1 to 5 and Comparative Examples 1 and 2, this residual amount was used as the contamination amount of the next batch. The residue contained 0.014 g of a polymerization initiator (in terms of the pure amount of polymerization initiator) as calculated from the material balance, but its activity as a polymerization initiator was reduced by 23%. It corresponds to 0.011 g in terms of conversion.
The monomer used for the reaction contained 98 ppm of the polymerization inhibitor methoquinone (MeHQ, molecular weight 124.1).

Example 1
Initial charge: 203.1 g of water and 135.3 g of IPA were the same as in Synthetic Example 1, except that the reaction vessel in which 10.0 g of the reaction product of Synthesis Example 1 remained was used as it was, and 0.00813 g of the polymerization inhibitor methoquinone (MeHQ) was charged. After charging 35.3 g of monomer A, 2.2 g of monomer B, 3.6 g of 90% monomer C aqueous solution and 0.7 g of 85% phosphoric acid, the temperature was raised to 62 ° C. in a nitrogen atmosphere. After the temperature increase, the solution in the tank was sampled and analyzed, and no polymerization occurred.
At this time, the raw material hydrophobic (meth) acrylate monomer concentration W represented by the formula (I) is 3.0 mol% (A: 0.5 mol, B: 0.015 mol in the formula (I)). It is.

  Polymerization reaction: Thereafter, a polymer was synthesized by the same procedure as in Synthesis Example 1. 30 minutes after the start of dropping, the solution in the tank was sampled and analyzed. As a result, the polymerization reaction proceeded, and no delay in the polymerization reaction was observed. Further, the polymer obtained after the completion of the reaction was not turbid or separated.

Example 2
A polymer was obtained in the same manner as in Example 1 except that the amount of the polymerization inhibitor (methoquinone) added in Example 1 was changed to 0.0122 g to obtain a polymer.

Example 3
A polymer was obtained in the same manner as in Example 1 except that the amount of polymerization inhibitor (methoquinone) added in Example 1 was changed to 0.0163 g to obtain a polymer.

Example 4
A polymer was obtained by carrying out a polymerization reaction in the same manner as in Example 1 except that the amount of polymerization inhibitor (methoquinone) added in Example 1 was changed to 0.0203 g.

Example 5
A polymer was obtained in the same manner as in Example 1 except that the amount of the polymerization inhibitor (methoquinone) added in Example 1 was changed to 0.122 g to obtain a polymer. When the solution in the tank was sampled and analyzed over time after the start of dropping, the polymerization reaction did not proceed for 30 minutes. The weight average molecular weight of the obtained polymer was 33100, the turbidity was 3, and no layer separation was confirmed.

Comparative Example 1
A polymer was obtained in the same manner as in Example 1 except that no polymerization inhibitor (methoquinone) was added in Example 1 to obtain a polymer. The weight average molecular weight of the obtained polymer was 21,300, the polymer solution was cloudy and the turbidity was 100 or more. When the initial charge liquid was sampled and heated at 62 ° C. for 2 hours, 57% of the initial charge monomer was polymerized.

Comparative Example 2
A polymer was obtained in the same manner as in Example 1 except that the amount of methoquinone added in Example 1 was changed to 0.00407 g. The resulting polymer solution was cloudy. Further, when the initial charge liquid was sampled and heated at 62 ° C. for 2 hours, 28% of the initial charge monomer was polymerized.

  The reaction conditions and results of Examples 1 to 5 and Comparative Examples 1 and 2 are summarized in Table 1.

* 1: Abundance of polymerization initiator (V-50) in the reaction vessel [ppm vs. monomer in the reaction vessel]
* 2: Abundance of polymerization inhibitor (MeHQ) derived from raw materials in reaction tank [ppm monomer in reaction tank]
* 3: Amount of added polymerization inhibitor (MeHQ) in reaction tank [ppm monomer in reaction tank] * 4: Total amount of polymerization inhibitor (MeHQ) in reaction tank [ppm monomer in reaction tank ]
* 5: Initial charge, monomer polymerization rate in 2 hours after temperature rise [wt% vs initial charge monomer]

Synthesis example 2
In Synthesis Example 1, the reaction was performed in the same manner as in Synthesis Example 1 except that the reaction tank at the time of initial charging was replaced with a 3 liter SUS304 reaction tank. The polymer solution obtained by polymerization was extracted from the reaction vessel, but 10.0 g remained attached to the reaction vessel. In the following Examples 6 to 9, this residual amount was used as the contamination amount of the next batch. The residue contained 0.014 g of a polymerization initiator (in terms of the pure amount of polymerization initiator) as calculated from the material balance, but its activity as a polymerization initiator was reduced by 23%. It corresponds to 0.011 g in terms of conversion.

Example 6
Initial charge: Using the reaction vessel in which 10.0 g of the reaction product of Synthesis Example 2 remained, the polymerization inhibitor methoquinone (MeHQ) 0.00813 g and a chelating agent (Dequest 2066, manufactured by Solutia Europe SA / NV) ) Except for charging 0.0242 g, 203.1 g of water, 135.3 g of IPA, 35.3 g of monomer A, 2.2 g of monomer B, 3.6 g of 90% monomer C aqueous solution, 0.7 g of 85% phosphoric acid were used. After the preparation, the temperature was raised to 62 ° C. in a nitrogen atmosphere.

  Polymerization reaction: 0.137 g of chelating agent (Diquest 2066), 153.4 g of water, 153.4 g of IPA, 98.5 g of monomer A, 57.8 g of monomer B, 83.1 g of 90% aqueous solution of monomer C, 85 A mixed solution of 4.1 g of% phosphoric acid was charged, and 69.5 g of a 2% aqueous solution of V-50 (manufactured by Wako Pure Chemical Industries, Ltd.) was charged into another glass dripping container 2, and 62 ° C. from these two dripping containers. Was simultaneously added dropwise to the reaction vessel maintained at 5 hours. Subsequently, it was aged at 62 ° C. for 2 hours to obtain a polymer solution. Turbidity and layer separation were not confirmed in the polymer obtained after completion of the reaction. Moreover, the weight average molecular weight of the obtained polymer was 28900, and turbidity was 5.

Example 7
A polymer was obtained in the same manner as in Example 6 except that the amount of the chelating agent in Example 6 was changed to 0.0403 g of the reaction vessel and 0.228 g of the dropping container 1. Turbidity and layer separation were not confirmed in the polymer obtained after completion of the reaction. Moreover, the weight average molecular weight of the obtained polymer was 26600, and the turbidity was 4.

Example 8
Initial preparation: The same procedure as in Example 6 was performed except that the amount of the chelating agent added in Example 6 was changed to 0.0403 g in the reaction vessel.

  Polymerization reaction: A polymer was obtained by carrying out a polymerization reaction in the same manner as in Example 6 except that the glass dropping container 1 of Example 6 was changed to a dropping container made of SUS304 and the addition amount of the chelating agent was changed to 0.228 g. . Turbidity and layer separation were not confirmed in the polymer obtained after completion of the reaction. Moreover, the weight average molecular weight of the obtained polymer was 26800, and the turbidity was 8.

Example 9
A polymer was obtained in the same manner as in Example 6 except that the amount of the chelating agent added in Example 6 was changed to 0.0121 g in the reaction vessel and 0.0685 g in the dropping container 1. The weight average molecular weight of the obtained polymer was 27600, the polymer solution was slightly cloudy and the turbidity was 17.

* 1 to * 5: Indicates the same meaning as in Table 1.
* 6: Amount of added chelating agent in reaction tank and dripping tank [ppm monomer in tank]

Claims (6)

A method for producing a copolymer in batch mode using a polymerization initiator, which satisfies the following condition 1.
Condition 1: The reaction is carried out after adding a polymerization inhibitor in an amount of 0.9 to 50 equivalents of the polymerization initiator contained in the reaction product of the previous batch remaining in the reaction vessel. The manufacturing method according to claim 1, further satisfying the following condition 2.
Condition 2: The reaction vessel is not washed after the reaction product of the previous batch is withdrawn from the reaction vessel. The production method according to claim 1 or 2 , wherein the polymerization initiator is an azo compound. Polymerization inhibitor is a quinone-based polymerization inhibitor, the production method according to any one of claims 1-3. The production method according to any one of claims 1 to 4 , wherein the reaction is carried out using 100 to 3000 ppm of the chelating agent with respect to the weight of the raw material monomer. The production method according to claim 5 , wherein the chelating agent is a phosphonic chelating agent.