JP2024058393A - Modified butadiene-isoprene random copolymer and its manufacturing method - Google Patents

Abstract

[Problem] To provide a material used in a rubber composition containing natural rubber, butadiene rubber, and silica, which improves the compatibility between the natural rubber and the butadiene rubber and the dispersibility of silica. [Solution] A random copolymer of butadiene and isoprene having at its terminal at least one functional group A selected from the group consisting of a hydroxyl group, a carboxy group, and an alkoxysilyl group, wherein the proportion X of repeating units having a 1,4-cis structure among the repeating units derived from the butadiene is 90 mol % or more, and the proportion Y of repeating units having a 1,4-cis structure among the repeating units derived from the isoprene is 90 mol % or more. [Selected Figure] None

Description

The present invention relates to a modified butadiene-isoprene random copolymer and a method for producing the same.

A rubber composition containing natural rubber, butadiene rubber, and silica is known as a rubber composition used in tires, etc. (for example, Patent Document 1).

JP 2015-218237 A

In this situation, the present inventors evaluated the rubber composition of Patent Document 1 and found that further improvement of the properties (toughness, etc.) is desirable in consideration of the expected increasing level of requirements in the future. The results of the inventors' investigations revealed that the properties (toughness, etc.) can be improved by improving the compatibility between natural rubber and butadiene rubber and the dispersibility of silica.

In view of the above, the present invention aims to provide a material for use in a rubber composition containing natural rubber, butadiene rubber, and silica, which improves the compatibility of natural rubber and butadiene rubber and the dispersibility of silica.

As a result of intensive research into the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by a modified butadiene-isoprene random copolymer having a specific functional group at its terminal and a specific microstructure, and have arrived at the present invention.
That is, the present inventors have found that the above problems can be solved by the following configuration.

(1) A random copolymer of butadiene and isoprene having at least one functional group A selected from the group consisting of a hydroxyl group, a carboxyl group, and an alkoxysilyl group at its terminal,
A modified butadiene-isoprene random copolymer, in which a proportion X of repeating units having a 1,4-cis structure among the repeating units derived from butadiene is 90 mol % or more, and a proportion Y of repeating units having a 1,4-cis structure among the repeating units derived from isoprene is 90 mol % or more.
(2) The modified butadiene-isoprene random copolymer according to (1) above, having a weight average molecular weight of 1,000 to 10,000,000.
(3) The modified butadiene-isoprene random copolymer according to (1) or (2) above, wherein the proportion X and the proportion Y are 92 to 99 mol %.
(4) The modified butadiene-isoprene random copolymer according to any one of (1) to (3) above, having a glass transition temperature of −60° C. or lower.
(5) A method for producing the modified butadiene-isoprene random copolymer according to any one of (1) to (4) above, comprising the steps of:
Copolymerizing butadiene and isoprene using a rare earth catalyst and an aluminoxane;
Thereafter, the polymerization is terminated using a modifying agent to obtain the modified butadiene-isoprene random copolymer.
(6) The method according to (5) above, wherein the rare earth catalyst is Sc, Gd or Nd.
(7) The method according to (5) or (6) above, wherein the modifying agent is at least one selected from the group consisting of epoxides, lactones having a carboxy group or an alkoxysilyl group, and epoxides having a carboxy group or an alkoxysilyl group.

As described below, the present invention provides a material that is used in a rubber composition containing natural rubber, butadiene rubber, and silica, and that improves the compatibility of the natural rubber and butadiene rubber and the dispersibility of the silica.

The modified butadiene-isoprene random copolymer of the present invention will now be described.
In this specification, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits.
In the present specification, each component may be used alone or in combination of two or more. When two or more components are used in combination, the amount of each component refers to the total amount unless otherwise specified.
In addition, in this specification, when the modified butadiene-isoprene random copolymer of the present invention is used in a rubber composition containing natural rubber, butadiene rubber, and silica, the compatibility between the natural rubber and butadiene rubber and the dispersibility of silica are improved, and when the rubber composition containing the modified butadiene-isoprene random copolymer of the present invention has excellent silica dispersibility, low heat build-up, abrasion resistance, and toughness, this is also referred to as "the effects, etc. of the present invention are excellent."

[1] Modified butadiene-isoprene random copolymer The modified butadiene-isoprene random copolymer of the present invention (hereinafter also simply referred to as the "copolymer of the present invention") is
A random copolymer of butadiene and isoprene having at least one functional group A selected from the group consisting of a hydroxyl group, a carboxyl group, and an alkoxysilyl group at its terminal,
In the modified butadiene-isoprene random copolymer, a proportion X of repeating units having a 1,4-cis structure among the repeating units derived from butadiene is 90 mol % or more, and a proportion Y of repeating units having a 1,4-cis structure among the repeating units derived from isoprene is 90 mol % or more.

The copolymer of the present invention has such a structure, and is therefore believed to be able to solve the above-mentioned problems of the present invention. The reason for this is not clear, but it is believed that the butadiene-derived repeating units and isoprene-derived repeating units of the copolymer of the present invention interact with the butadiene rubber and natural rubber in the rubber composition, respectively, and that the specific functional groups at the terminals interact with the silica.

[Skeleton]
The skeleton (main chain structure) of the copolymer of the present invention is a random copolymer of butadiene and isoprene.
The fact that the copolymer is a random copolymer can be confirmed by the fact that it has only one glass transition temperature.

[Butadiene unit content]
In the random copolymer, the proportion of the repeating units derived from butadiene (hereinafter, the "repeating units derived from butadiene" are also referred to as "butadiene units") among all repeating units (hereinafter, also referred to as "butadiene unit content") is not particularly limited, but is preferably 5 to 90 mol%, and more preferably 10 to 60 mol%, for reasons such as better effects of the present invention.

[Isoprene unit content]
In the random copolymer, the proportion of repeating units derived from isoprene (hereinafter, "repeating units derived from isoprene" will also be referred to as "isoprene units") among all repeating units (hereinafter, also referred to as "isoprene unit content") is not particularly limited, but is preferably 10 to 95 mol%, and more preferably 40 to 90 mol%, for reasons such as better effects of the present invention.

[Ratio X]
In the random copolymer, the proportion X of repeating units having a 1,4-cis structure among the repeating units derived from butadiene (butadiene units) (hereinafter, simply referred to as "proportion X") is 90 mol % or more. Note that the butadiene units have three types of microstructures: 1,4-cis structure, 1,4-trans structure, and vinyl structure.
The proportion X is preferably 92 to 99 mol % because the effects of the present invention are more excellent.

[Ratio Y]
In the random copolymer, the proportion Y of repeating units having a 1,4-cis structure among the repeating units derived from isoprene (isoprene units) is 90 mol % or more (hereinafter, simply referred to as "proportion Y"). The isoprene units have three types of microstructures: a 1,4-cis structure, a 1,4-trans structure, and a vinyl structure.
The ratio Y is preferably 92 to 99 mol % because the effects of the present invention are more excellent.

[End]
The copolymer of the present invention has at least one functional group A (hereinafter, simply referred to as "functional group A") selected from the group consisting of a hydroxyl group (-OH), a carboxyl group (-COOH), and an alkoxysilyl group at its terminal. The functional group A may be bonded to the skeleton (main chain structure) via a divalent linking group.
The copolymer of the present invention may have the functional group A at least at one end, and preferably has the functional group A at only one end because the effects of the present invention are more excellent.
The functional group A is preferably an alkoxysilyl group because the effects of the present invention are more excellent.

[Alkoxysilyl group]
The alkoxysilyl group is a group represented by ≡SiOR (where R is a hydrocarbon group).

The alkoxysilyl group is preferably a group represented by --Si(R ¹ ) _m (R ² ) _n , because this provides better effects and the like of the present invention.
Here, R ¹ represents an alkoxy group, R ² represents a hydrogen atom or a substituent, m represents an integer of 1 to 3, n represents an integer of 0 to 2, and m+n is 3.
The above R ¹ is preferably an alkoxy group having 1 to 10 carbon atoms, and more preferably an alkoxy group having 1 to 5 carbon atoms, because the effects of the present invention are more excellent.
The substituent of ^R2 is preferably a hydrocarbon group. Examples of the hydrocarbon group include an aliphatic hydrocarbon group, an aromatic hydrocarbon group, and a combination thereof. The aliphatic hydrocarbon group may be linear, branched, or cyclic. Specific examples of the aliphatic hydrocarbon group include linear or branched alkyl groups (particularly, having 1 to 30 carbon atoms), linear or branched alkenyl groups (particularly, having 2 to 30 carbon atoms), and linear or branched alkynyl groups (particularly, having 2 to 30 carbon atoms).
The above m is preferably an integer of 2 to 3, and more preferably 3, because the effects of the present invention are more excellent.
The above n is preferably an integer of 0 to 1, and more preferably 0, because the effects of the present invention are more excellent.

[Molecular weight]
The weight average molecular weight (Mw) of the copolymer of the present invention is preferably from 1,000 to 10,000,000, more preferably from 100,000 to 5,000,000, and even more preferably from 1,000,000 to 3,000,000, because the effects of the present invention are more excellent.

The number average molecular weight (Mn) of the copolymer of the present invention is preferably 1,000 to 10,000,000, more preferably 50,000 to 3,000,000, and even more preferably 500,000 to 1,000,000, because the effects of the present invention are more excellent.

In this specification, the weight average molecular weight (Mw) and number average molecular weight (Mn) are values calculated as standard polystyrene obtained by gel permeation chromatography (GPC) measurement under the following conditions.
Solvent: Tetrahydrofuran Detector: RI detector

[Glass-transition temperature]
The glass transition temperature (Tg) of the copolymer of the present invention is preferably −60° C. or lower, and more preferably −70° C. or lower, because the effects of the present invention are more excellent. The lower limit of the Tg is not particularly limited, but is preferably −120° C. or higher, and more preferably −100° C. or higher, because the effects of the present invention are more excellent.

In this specification, the glass transition temperature (Tg) is measured using a differential scanning calorimeter (DSC) at a heating rate of 20°C/min and calculated using the midpoint method.

[Production method]
The method for producing the copolymer of the present invention is not particularly limited. However, because the effects of the present invention on the obtained copolymer are more excellent, a production method including the following (1) and (2) (hereinafter also referred to as the "production method of the present invention") is preferred.
In the following description, when the effect of the present invention is excellent with respect to the obtained copolymer, it will be simply referred to as "the effect of the present invention is excellent."

(1) Polymerization step: A step of copolymerizing butadiene and isoprene using a rare earth catalyst and an aluminoxane. (2) Terminal modification step: A step of obtaining the modified butadiene-isoprene random copolymer by terminating the polymerization using a modifier.

Each step is explained below.

[Polymerization step]
The polymerization step is a step of copolymerizing butadiene and isoprene using a rare earth catalyst and an aluminoxane. It is believed that the polymerization using a rare earth catalyst and an aluminoxane can produce a copolymer having a high ratio X and a high ratio Y.

<Monomer>
The polymerization process uses butadiene and isoprene as monomers.

(proportion of each monomer)
The ratio of the amount of butadiene used to the amount of all monomers used is not particularly limited, but the preferred range and the reasons therefor are the same as those for the butadiene unit content described above.
The ratio of the amount of isoprene used to the amount of all monomers used is not particularly limited, but the preferred range and the reasons for it are the same as those for the isoprene unit content described above.

<Rare earth catalyst>
The rare earth element catalyst includes, for example, a rare earth element or a compound containing a rare earth element.
Rare earth elements include scandium; yttrium; and the lanthanides (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium lanthanides).
Examples of compounds containing rare earth elements include halides of rare earth elements, complexes of rare earth elements, and alloys of rare earth elements and metals.

The rare earth element catalyst is preferably Sc (scandium), Gd (gadolinium) or Nd (neodymium) because it provides better effects of the present invention, and is more preferably Nd.

(amount to use)
The amount of the rare earth catalyst used is not particularly limited, but in order to obtain better effects of the present invention, the amount is preferably 0.00001 to 10 mol %, and more preferably 0.0001 to 1 mol %, based on the amount of all monomers (butadiene and isoprene) used.

<Aluminoxane>
Aluminoxanes are products of the reaction of trialkylaluminum with water, and are believed to act as co-catalysts.
Examples of the aluminoxane include methylaluminoxane, ethylaluminoxane, propylaluminoxane, butylaluminoxane, isobutylaluminoxane, methylethylaluminoxane, methylbutylaluminoxane, and methylisobutylaluminoxane. Among them, modified methylaluminoxanes (MMAO) such as methylethylaluminoxane, methylbutylaluminoxane, and methylisobutylaluminoxane are preferred. These aluminoxanes can be used alone or in combination of two or more.

(amount to use)
The amount of the aluminoxane used is not particularly limited, but in order to obtain better effects and the like of the present invention, the molar ratio of aluminum atoms in the aluminoxane to the rare earth element-based catalyst is preferably 20 to 10,000, more preferably 50 to 2,000, and even more preferably 100 to 1,000.

[Terminal modification step]
The terminal modification step is a step of obtaining the copolymer of the present invention by terminating the polymerization using a modifying agent after the polymerization step. By terminating the polymerization using a modifying agent, the modifying agent is bonded to the growing terminal of the random copolymer obtained in the polymerization step.
From the viewpoint of completely terminating the polymerization, an alcohol (e.g., methanol) or the like may be added in addition to the denaturant.

<Modifier>
The modifier is not particularly limited as long as it is a modifier that forms the above-mentioned functional group A by reacting with the growing end of the random copolymer obtained in the polymerization step.

The modifier is preferably at least one selected from the group consisting of epoxides, lactones having a carboxy group or an alkoxysilyl group, and epoxides having a carboxy group or an alkoxysilyl group, more preferably epoxides having a carboxy group or an alkoxysilyl group, and even more preferably epoxides having an alkoxysilyl group (e.g., 3-glycidoxypropyltrimethoxysilane), because this provides better effects and the like of the present invention.

(epoxide)
The epoxide is not particularly limited as long as it is a compound having an oxacyclopropane (oxirane) structure. The epoxide bonds to the growing end of the random copolymer by ring-opening of the oxirane. At this time, a hydroxyl group is formed. Therefore, when an epoxide is used as a modifier, a random copolymer having a hydroxyl group at the end as the functional group A is obtained.

Specific examples of epoxides include ethylene oxide, propylene oxide, butylene oxide, cyclohexene oxide, styrene oxide, 1-phenylpropylene oxide, methyl glycidyl ether, ethyl glycidyl ether, glycidyl isopropyl ether, butyl glycidyl ether, 1-methoxy-2-methylpropylene oxide, allyl glycidyl ether, 2-ethyloxyl glycidyl ether, phenyl glycidyl ether, p-tert-butylphenyl glycidyl ether, lauryl alcohol glycidyl ether, stearyl glycidyl ether, palmityl glycidyl ether, myristyl glycidyl ether, lauryl glycidyl ether, capryl glycidyl ether, and caproyl glycidyl ether.

(Lactones having a carboxy group or an alkoxysilyl group)
A lactone having a carboxy group or an alkoxysilyl group bonds to the growing end of a random copolymer by ring-opening of the lactone, and therefore, when a lactone having a carboxy group or an alkoxysilyl group is used as a modifying agent, a random copolymer having a carboxy group or an alkoxy group as a functional group A at the end is obtained.

(Epoxides having a carboxy group or an alkoxysilyl group)
An epoxide having a carboxy group or an alkoxysilyl group is bonded to the growing end of a random copolymer by ring-opening of the epoxide. At this time, a hydroxyl group is formed. Therefore, when an epoxide having a carboxy group or an alkoxysilyl group is used as a modifier, a random copolymer having a hydroxyl group and a carboxy group or an alkoxy group as a functional group A at the terminal is obtained.

(amount to use)
The amount of the modifier used is not particularly limited, but in order to obtain better effects of the present invention, the molar ratio to the rare earth catalyst is preferably 0.1 to 10, and more preferably 1 to 5.

[2] Rubber Composition The rubber composition of the present invention (hereinafter also referred to as "the composition of the present invention") is a rubber composition containing the above-mentioned copolymer of the present invention as a rubber component.

[Rubber component]
The rubber component contains the copolymer of the present invention. The rubber component may contain a rubber component other than the copolymer of the present invention (another rubber component).

[Copolymer of the present invention]
The copolymer of the present invention is as described above.

<Content>
The content of the copolymer of the present invention in the rubber component is not particularly limited, but in order to obtain better effects of the present invention, the content is preferably 2 to 50 mass%, more preferably 4 to 40 mass%, even more preferably 6 to 30 mass%, and particularly preferably 8 to 20 mass%.

[Other rubber components]
The other rubber components are not particularly limited, but specific examples include natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene butadiene rubber (SBR), acrylonitrile-butadiene copolymer rubber (NBR), butyl rubber (IIR), halogenated butyl rubber (Br-IIR, Cl-IIR), chloroprene rubber (CR), etc. Among these, natural rubber and butadiene rubber are preferred because they provide better effects of the present invention.

＜Molecular weight＞

(Weight average molecular weight)
The weight average molecular weight (Mw) of the other rubber components is not particularly limited, but for reasons such as better effects of the present invention, it is preferably more than 50,000, more preferably 100,000 to 10,000,000, and even more preferably 200,000 to 2,000,000.

(Number average molecular weight)
The number average molecular weight (Mn) of the other rubber is not particularly limited, but for reasons such as better effects of the present invention, it is preferably 50,000 to 5,000,000, and more preferably 100,000 to 1,000,000.

<Content>
When the rubber component contains other rubber components, the content of the other rubber components in the rubber component is not particularly limited, but for reasons such as better effects of the present invention, the content is preferably 50 to 98 mass%, more preferably 60 to 96 mass%, even more preferably 70 to 94 mass%, and particularly preferably 80 to 92 mass%.

[Preferred embodiment]
The rubber component preferably contains natural rubber and butadiene rubber in addition to the copolymer of the present invention, because this provides better effects of the present invention.

<Content>
In this case, the preferred range of the content of the copolymer of the present invention in the rubber component is the same as the preferred range described above.
Furthermore, the contents of the natural rubber and butadiene rubber in the rubber component are each preferably 25 to 49 mass%, more preferably 30 to 48 mass%, even more preferably 35 to 47 mass%, and particularly preferably 40 to 46 mass%, for reasons such as better effects of the present invention.

[silica]
The composition of the present invention preferably contains silica because the effects of the present invention are more excellent.
The silica is not particularly limited, and any conventionally known silica can be used.
Examples of the silica include wet silica, dry silica, fumed silica, diatomaceous earth, etc. The silica may be used alone or in combination of two or more kinds.

〔Content〕
In the composition of the present invention, the content of silica is preferably 10 to 200 parts by mass, and more preferably 50 to 150 parts by mass, per 100 parts by mass of the rubber component, because the effects of the present invention are more excellent.

[Optional ingredients]
The composition of the present invention may contain components (optional components) other than the above-mentioned components, as necessary.
Examples of such components include various additives commonly used in rubber compositions, such as fillers other than silica (preferably carbon black), silane coupling agents, terpene resins (preferably aromatic modified terpene resins), thermally expandable microcapsules, zinc oxide (zinc white), stearic acid, antioxidants, waxes, processing aids, process oils, liquid polymers, thermosetting resins, vulcanizing agents (e.g., sulfur), vulcanization accelerators (accelerators), and vulcanization activators.

[Application]
The composition of the present invention is suitably used for, for example, tires, conveyor belts, hoses, vibration-proof materials, rubber rolls, outer covers of railway cars, etc. In particular, it is suitably used for tires (particularly treads).

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these.

[Production of modified butadiene-isoprene random copolymer]
Each copolymer was produced as follows. All of the copolymers correspond to the copolymer of the present invention described above.

[Copolymer 1]

<Polymerization step>
Nd (0.4 g), MMAO (10 mL), butadiene (15% by mass toluene solution) (50 g), and 37.9 g of isoprene were added to a two-neck flask (500 mL) and stirred (room temperature, 1.5 hours) to copolymerize butadiene and isoprene.

<Terminal modification step>
Thereafter, KBE-403 (3-glycidoxypropyltrimethoxysilane) (0.6 g) was added and stirred (room temperature, 2.0 hours). Furthermore, a hydrochloric acid methanol solution (7.5 mL) was added and stirred (room temperature, 1.0 hour). Next, purification was performed twice using toluene as a good solvent and methanol as a poor solvent. After that, vacuum drying (60°C, 5.0 hours) was performed. In this manner, a copolymer was obtained. The obtained copolymer is designated as Copolymer 1.

Copolymer 1 is a modified butadiene-isoprene random copolymer having an alkoxysilyl group at its terminal (butadiene unit content: 20 mol %, isoprene unit content: 80 mol %), in which the proportion X is 95 mol % and the proportion Y is 98 mol %.
Copolymer 1 has a Mw of 1,297,000 and a Tg of -71°C.

[Copolymer 2]

<Polymerization step>
Nd (0.27 g), MMAO (6.8 mL), butadiene (15% by mass solution in toluene) (50 g), and 22.1 g of isoprene were added to a two-neck flask (500 mL), and the mixture was stirred (room temperature, 1.5 hours) to copolymerize butadiene and isoprene.

<Terminal modification step>
Thereafter, KBE-403 (3-glycidoxypropyltrimethoxysilane) (0.6 g) was added and stirred (room temperature, 2.0 hours). Furthermore, a hydrochloric acid methanol solution (7.5 mL) was added and stirred (room temperature, 1.0 hour). Next, purification was performed twice using toluene as a good solvent and methanol as a poor solvent. After that, vacuum drying (60°C, 5.0 hours) was performed. In this manner, a copolymer was obtained. The obtained copolymer is designated as Copolymer 2.

Copolymer 2 is a random copolymer of butadiene and isoprene (butadiene unit content: 30 mol%, isoprene unit content: 70 mol%) having an alkoxysilyl group at the end, and is a modified butadiene-isoprene random copolymer in which the proportion X is 95 mol% and the proportion Y is 98 mol%.
Copolymer 2 has a Mw of 2,300,000 and a Tg of -84°C.

[Copolymer 3]

<Polymerization step>
Nd (0.16 g), MMAO (4.0 mL), butadiene (15% by mass solution in toluene) (50 g), and 9.5 g of isoprene were added to a two-neck flask (500 mL), and the mixture was stirred (room temperature, 1.5 hours) to copolymerize butadiene and isoprene.

<Terminal modification step>
Thereafter, KBE-403 (3-glycidoxypropyltrimethoxysilane) (0.6 g) was added and stirred (room temperature, 2.0 hours). Furthermore, a hydrochloric acid methanol solution (7.5 mL) was added and stirred (room temperature, 1.0 hour). Next, purification was performed twice using toluene as a good solvent and methanol as a poor solvent. After that, vacuum drying (60°C, 5.0 hours) was performed. In this manner, a copolymer was obtained. The obtained copolymer is designated as Copolymer 3.

Copolymer 3 is a random copolymer of butadiene and isoprene (butadiene unit content: 50 mol%, isoprene unit content: 50 mol%) having an alkoxysilyl group at the end, and is a modified butadiene-isoprene random copolymer in which the proportion X is 95 mol% and the proportion Y is 98 mol%.
Copolymer 3 has a Mw of 1,230,000 and a Tg of -89°C.

[Summary of the structure and properties of each copolymer]
The structures and physical properties of each copolymer are summarized in Table 1 below.

Claims (7)

A random copolymer of butadiene and isoprene having at least one functional group A selected from the group consisting of a hydroxyl group, a carboxyl group, and an alkoxysilyl group at its terminal,
A modified butadiene-isoprene random copolymer, wherein a proportion X of repeating units having a 1,4-cis structure among the repeating units derived from the butadiene is 90 mol % or more, and a proportion Y of repeating units having a 1,4-cis structure among the repeating units derived from the isoprene is 90 mol % or more. The modified butadiene-isoprene random copolymer according to claim 1, having a weight average molecular weight of 1,000 to 10,000,000. The modified butadiene-isoprene random copolymer according to claim 1, wherein the proportions X and Y are 92 to 99 mol%. The modified butadiene-isoprene random copolymer according to claim 1, which has a glass transition temperature of -60°C or lower. A method for producing the modified butadiene-isoprene random copolymer according to any one of claims 1 to 4, comprising the steps of:
Copolymerizing butadiene and isoprene using a rare earth catalyst and an aluminoxane;
Thereafter, the polymerization is terminated using a modifying agent to obtain the modified butadiene-isoprene random copolymer. The method according to claim 5, wherein the rare earth element catalyst is Sc, Gd or Nd. The method according to claim 5, wherein the modifying agent is at least one selected from the group consisting of epoxides, lactones having a carboxy group or an alkoxysilyl group, and epoxides having a carboxy group or an alkoxysilyl group.