JP7795083B2 - crosslinked product - Google Patents

Description

The present invention relates to a crosslinked product.

Polymers known as temperature-responsive polymers have been known to become hydrophobic and insoluble in water above a certain temperature, but become hydrophilic and soluble in water below that temperature. The temperature at which these properties are exhibited in temperature-responsive polymers is called the lower critical solution temperature (LCST).
An example of the temperature-responsive polymer is a homopolymer of N-isopropylacrylamide.
Taking advantage of the above properties, temperature-responsive polymers are applied to human medicines and tests, and in such cases, the LCST of the temperature-responsive polymer with water or the like is designed to be, for example, around body temperature or higher.

On the other hand, a rubber composition used in the tread portion of a tire is known in which gel particles of a thermosensitive, water-absorbing polymer are blended with rubber, and the thermosensitive, water-absorbing polymer is polyacrylamide (for example, Patent Document 1).

Japanese Patent Application Laid-Open No. 2001-123018

As safety requirements for automobiles increase, there is a demand for further improvements in tire performance on ice and wet grip.
In this situation, the present inventors evaluated a rubber composition containing gel particles of a thermosensitive, water-absorbing polymer (three-dimensionally crosslinked between polymers) with reference to Patent Document 1, and found that the gel particles of the thermosensitive, water-absorbing polymer may have a poor balance between water absorption and drainage properties in the usage environment of rubber products such as tires.

The present invention therefore aims to provide a temperature-responsive compound that has an excellent balance of water absorption and drainage properties in the environment in which rubber products are used.

After extensive research into the above-mentioned problems, the inventors discovered that the above-mentioned problems could be solved by crosslinking a copolymer having two specific repeating units.

[1] A crosslinked copolymer having a repeating unit A1 represented by the following formula (A1) and a repeating unit B1 represented by the following formula (B1), wherein the repeating unit A1 and the repeating unit B1 are different from each other.
In formula (A1), each R ^A1 independently represents a hydrogen atom or a branched alkyl group having 3 to 10 carbon atoms, and at least one of the R ^A1 is the above-mentioned alkyl group; in formula (B1), each R ^B1 independently represents a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms, and at least one of the R ^B1 is the above-mentioned alkyl group.
[2] The crosslinked product according to [1], wherein the lower critical solution temperature _T of the crosslinked product or the lower critical solution temperature _C of the copolymer is independently −10° C. to +60° C.
[3] The crosslinked product according to [1] or [2], wherein the homopolymer of the repeating unit A1 has a lower critical solution temperature _A and/or the homopolymer of the repeating unit B1 has a lower critical solution temperature _B.
[4] The homopolymer of the repeating unit A1 has a lower critical solution temperature _A , the homopolymer of the repeating unit B1 has a lower critical solution temperature _B , and the lower critical solution temperature _B is lower than the lower critical solution temperature _A , or
The crosslinked product according to any one of [1] to [3], wherein the homopolymer of the repeating unit A1 has a lower critical solution temperature _A , and the homopolymer of the repeating unit B1 does not have a lower critical solution temperature _B.
[5] The crosslinked product according to any one of [1] to [4], wherein in formula (A1), one R ^A1 is a hydrogen atom and the other R ^A1 is a branched alkyl group having 3 to 10 carbon atoms.
[6] The crosslinked product according to any one of [1] to [5], wherein the repeating unit A1 contains a repeating unit of N-isopropylacrylamide.
[7] In formula (B1), one R ^B1 is a hydrogen atom and the other R ^B1 represents a linear or branched alkyl group having 4 to 10 carbon atoms, or
The crosslinked product according to any one of [1] to [6], wherein R ^B1 each independently represents a linear or branched alkyl group having 1 to 10 carbon atoms.
[8] The crosslinked product according to any one of [1] to [7], wherein the repeating unit B1 contains a repeating unit of N-tert-butylacrylamide or N,N-dimethylacrylamide.
[9] The crosslinked product according to any one of [1] to [8], which is a random copolymer.

[10] A compounding agent for rubber containing the crosslinked product described in any one of [1] to [9].

The crosslinked product of the present invention has an excellent balance of water absorption and drainage properties in the environment in which rubber products are used.

The crosslinked product of the present invention will be described below.
In this specification, a numerical range expressed using "to" means a range that includes the numerical values written before and after "to".
In this specification, each component may be used alone or in combination of two or more. When two or more components are used in combination, the content of the components refers to the total content unless otherwise specified.
In this specification, the repeating unit A1 represented by formula (A1) may be simply referred to as "A1." The same applies to the repeating unit B1 represented by formula (B1), etc.

[Crosslinked product]
The crosslinked product of the present invention is a crosslinked product of a copolymer having a repeating unit A1 represented by the following formula (A1) and a repeating unit B1 represented by the following formula (B1), provided that in the present invention, the repeating unit A1 and the repeating unit B1 are different from each other.
In formula (A1), each R ^A1 independently represents a hydrogen atom or a branched alkyl group having 3 to 10 carbon atoms, and at least one of the R ^A1 is the above-mentioned (branched alkyl group having 3 to 10 carbon atoms).
In formula (B1), R ^B1 each independently represents a hydrogen atom or a linear alkyl group having 1 to 10 carbon atoms or a branched alkyl group having 3 to 10 carbon atoms, and at least one of R ^B1 is the above-mentioned alkyl group (linear alkyl group having 1 to 10 carbon atoms or branched alkyl group having 3 to 10 carbon atoms).

It is believed that the crosslinked product of the present invention has such a structure and therefore exhibits the above-mentioned effects. The reason for this is not clear, but is presumed to be as follows.
Typically, tires are used in a variety of locations, including cold and tropical regions, and therefore the temperature of the tire's operating environment also varies. However, if there is a large difference between the temperature of the tire's operating environment and the lower critical solution temperature of the thermosensitive water-absorbing polymer or its crosslinked product contained in the tire, the thermosensitive water-absorbing polymer or its crosslinked product is thought to be unable to exhibit its temperature responsiveness. For example, if a tire used in a cold region below 0°C contains a crosslinked thermosensitive water-absorbing polymer having an LCST close to human body temperature, the crosslinked thermosensitive water-absorbing polymer will absorb water in the above temperature environment, but will not release water unless the operating environment temperature reaches or exceeds the LCST. If water is not released from the crosslinked thermosensitive water-absorbing polymer as described above, it is expected that tire performance, such as ice performance and wet grip performance, will be reduced.
On the other hand, the LCST that the crosslinked product of the present invention can have can be set in various ranges and in detail by combining the repeating units A1 and B1 in the copolymer (main chain) that constitutes the crosslinked product of the present invention.
As described above, the crosslinked product of the present invention is considered to be a temperature-responsive compound having an excellent balance between water absorption and drainage. As a result, with the crosslinked product of the present invention, it is possible to select a crosslinked product suitable for a rubber product such as a tire depending on the environment in which it will be used.

Furthermore, since the copolymer in the crosslinked product of the present invention is crosslinked to form, for example, a network structure, when the crosslinked product of the present invention comes into contact with water at a temperature below the LCST with water, the crosslinked product of the present invention can absorb water and become gel-like. In other words, under the above conditions, the crosslinked product of the present invention within a rubber product dissolves in water and does not flow out of the rubber product with water. Furthermore, when the crosslinked product of the present invention is in a gel-like state as described above and the ambient temperature of use changes to a temperature above the LCST with water, the crosslinked product of the present invention changes from a gel state (a state in which the degree of hydrophilicity/hydrophobicity is more hydrophilic) to a state in which the degree of hydrophilicity/hydrophobicity is more hydrophobic, allowing water to be expelled from the crosslinked product of the present invention. In this way, it is believed that the crosslinked product of the present invention does not flow out of the rubber product even when the rubber product is wet with water, but remains within the rubber product.

In the crosslinked product of the present invention, the copolymer having the repeating unit A1 and the repeating unit B1 is crosslinked via a crosslinking moiety.
In the present invention, the term "copolymer having repeating units A1 and B1" refers to a chain (main chain) having at least repeating units A1 and B1 in the crosslinked product of the present invention. The term "crosslinked product" refers to the entire crosslinked product of the present invention.
A part of the crosslinked portion may be incorporated into the copolymer to form a part of the copolymer. The crosslinked product of the present invention may be a copolymer (as a whole) having repeating units A1 and B1 and a crosslinked portion. An example of the crosslinked product of the present invention is a network structure.
Furthermore, the crosslinked product of the present invention can be temperature responsive and can have an LCST with a liquid (for example, water).
The crosslinked product of the present invention is capable of absorbing a liquid such as water and maintaining that state (also referred to as a swollen state or a gel state) under temperature conditions below the LCST that the crosslinked product of the present invention can have, because the copolymer (having the repeating unit A1 and the repeating unit B1) is crosslinked.

(Lower critical solution temperature _T of crosslinked product)
The lower critical solution temperature _T of the crosslinked product of the present invention is preferably −10° C. to +60° C., more preferably −10° C. to +40° C., and even more preferably −10° C. or higher and lower than +20° C., from the viewpoint of achieving better effects of the present invention.

-Method for measuring the lower critical solution temperature _T of the crosslinked product of the present invention: The crosslinked product of the present invention was mixed with pure water to adjust the concentration of the crosslinked product of the present invention to 0.5% by mass in a mixture of the crosslinked product of the present invention and pure water (total amount of mixture: 100 g), and the mixture was placed in a quartz cell with an optical path length of 1 cm. The temperature of the mixture was increased from 0°C at a rate of 1°C/min, while the transmittance of the mixture at a wavelength of 500 nm was measured with a spectrophotometer (Hitachi UH-5300). From the measurement results, the midpoint method was used to determine the temperature corresponding to the average value between the approximately maximum value of transmittance (e.g., the transmittance immediately before the transmittance suddenly drops) and the approximately minimum value of transmittance, and this temperature was defined as the LCST (lower critical solution temperature _T ) of the crosslinked product of the present invention (with water).
In this specification, the methods for measuring the lower critical solution temperature _C of the (uncrosslinked) copolymer having the repeating unit A1 and the repeating unit B1, the lower critical solution temperature A of the (uncrosslinked) homopolymer of the repeating unit _A1 , and the lower critical solution temperature B of the (uncrosslinked) homopolymer of the repeating unit _B1 are the same as the method for measuring the lower critical solution temperature _T described above.

(Molar ratio of A1 to B1)
The molar ratio (A1:B1) of the repeating units A1 to B1 constituting the crosslinked product of the present invention is preferably 1:99 to 99:1, more preferably 10:90 to 80:20, even more preferably 10:90 to 75:25, and still more preferably 25:75 to 60:40, from the viewpoint of achieving superior effects of the present invention.

(random copolymer)
The crosslinked product of the present invention is preferably a random copolymer, from the viewpoint of easily exhibiting temperature responsiveness and achieving better effects of the present invention.

[Copolymer]
The copolymer (main chain) constituting the crosslinked product of the present invention has a repeating unit A1 and a repeating unit B1, provided that in the present invention, the repeating units A1 and B1 are different from each other.
The repeating unit A1 is not particularly limited as long as it has a structure satisfying the above formula (A1), and may be a single unit or a combination of two or more units. The same applies to the repeating unit B1.

(Lower critical solution temperature _C of copolymer)
The lower critical solution temperature _C of the copolymer in the crosslinked product of the present invention is preferably −10° C. to +60° C., more preferably −10° C. to +40° C., and even more preferably −10° C. or higher and lower than +20° C., from the viewpoint of achieving better effects of the present invention.
In this specification, the lower critical solution temperature _C of a copolymer refers to the lower critical solution temperature of the copolymer alone, that is, the copolymer having a repeating unit A1 represented by formula (A1) and a repeating unit B1 represented by formula (B1) and not crosslinked.

Furthermore, the copolymer contained in the crosslinked product of the present invention is preferably a random copolymer, as this makes it easier for temperature responsiveness to be exhibited and the effects of the present invention are more excellent.

[Crosslinked part]
Since the crosslinked product of the present invention has crosslinked moieties, when the crosslinked product of the present invention comes into contact with water under a temperature condition below the lower critical solution temperature _T of the crosslinked product with water, the crosslinked product of the present invention can absorb water and become a gel or the like.
The crosslinked moiety constituting the crosslinked product of the present invention can crosslink the copolymer between molecules. The copolymer may be crosslinked intramolecularly. The structure of the crosslinked moiety is not particularly limited as long as it is at least divalent. Examples include a divalent or higher hydrocarbon group, an ester bond, an amide bond, an ether bond, or a combination thereof.
The crosslinked portion may be bonded to the repeating unit A1 or B1 via a group (--C--C) resulting from the cleavage of at least two ethylenically unsaturated bonds, thereby constituting a part of the copolymer.
In the crosslinked product of the present invention, the crosslinked portion can be constituted by, for example, a monomer having a plurality of ethylenically unsaturated bonds, as will be described later. However, one of the preferred embodiments of the crosslinked product of the present invention is one in which the crosslinked product has almost no unreacted ethylenically unsaturated bonds derived from the above-mentioned monomers.

The structure of the crosslinked portion may be, for example, a repeating unit C1 represented by the following formula (C1).

In formula (C1), R ^1s each independently represent a hydrogen atom or an alkyl group, X ^1s each independently represent an ester bond or an amide bond, and R ² represents a hydrocarbon group, an oxyalkylene group, or a polyoxyalkylene group.

・^R1
The alkyl group may, for example, be a methyl group.

・^R2
The hydrocarbon group represented by ^R2 is not particularly limited as long as it is divalent. For example, it may be a hydrocarbon group having 1 to 20 carbon atoms. Examples of the hydrocarbon group include an alkylene group and a phenylene group.
Examples of the oxyalkylene group represented by ^R2 include an oxyethylene group and an oxypropylene group.
Examples of the polyoxyalkylene group represented by ^R2 include a polyoxyethylene group and a polyoxypropylene group. The degree of polymerization of the polyoxyalkylene group is, for example, 2 to 10.
^R2 is preferably an alkylene group having 1 to 12 carbon atoms from the viewpoint of achieving better effects of the present invention, and more preferably an alkylene group having 6 to 12 carbon atoms from the above viewpoint and also because of excellent resistance to acid.

In formula (C1), the moiety shown below can constitute a part of the copolymer: In the formula below, * represents the bonding position with ^X1 in formula (C1).

The crosslinked portion is preferably a repeating unit represented by the following formula (c-1), from the viewpoint of achieving better effects of the present invention (particularly absorbency) and suppressing dissolution and/or outflow into water.
In formula (c-1), R ¹ and R ² are the same as R ¹ and R ² in formula (C1), respectively.

(Ratio of cross-linked parts)
The proportion of the crosslinked moiety (for example, the above formula (C1)) constituting the crosslinked product of the present invention is preferably 0.0001 to 0.5 mol, and more preferably 0.01 to 0.3 mol, per mol of the total number of moles of repeating units A1 and B1, from the viewpoints of achieving better effects of the present invention (particularly water absorbency), being able to suppress dissolution and/or outflow into water, and achieving a fast LCST (speed of phase transition).

[Repeating unit A1]
In the present invention, the repeating unit A1 is a repeating unit represented by the above formula (A1).
In formula (A1), each R ^A1 independently represents a hydrogen atom or a branched alkyl group having 3 to 10 carbon atoms, and at least one of the two R ^A1 is a branched alkyl group having 3 to 10 carbon atoms. The branched alkyl group may form a ring structure.

Branched Alkyl Groups Having 3 to 10 Carbon Atoms Examples of branched alkyl groups having 3 to 10 carbon atoms include an isopropyl group, an isobutyl group, a tert-butyl group, and a cyclohexyl group.

Preferred embodiment of A1 In terms of achieving better effects of the present invention, in formula (A1), it is preferable that one R ^A1 is a hydrogen atom and the other R ^A1 is a branched alkyl group having 3 to 10 carbon atoms, and it is more preferable that one R ^A1 is a hydrogen atom and the other R ^A1 is an isopropyl group or a tert-butyl group.

From the viewpoint of achieving better effects of the present invention, it is preferable that repeating unit A1 contains a repeating unit of N-isopropylacrylamide.

Homopolymer of Repeating Unit A1 From the viewpoint of achieving better effects of the present invention, it is preferable that the homopolymer of repeating unit A1 has a lower critical solution temperature T _A .

(Lowest critical solution temperature _A )
The lower critical solution temperature _A of the homopolymer of the repeating unit A1 constituting the copolymer contained in the crosslinked product of the present invention is preferably +5°C or higher, more preferably +10 to +50°C, from the viewpoint of achieving better effects of the present invention.
In this specification, the lower critical solution temperature _A refers to the lower critical solution temperature of a homopolymer of the repeating unit A1 represented by formula (A1).
The lower critical solution temperature _A of the homopolymer of N-isopropylacrylamide is 32°C.

[Repeating unit B1]
In the present invention, the repeating unit B1 is a repeating unit represented by the above formula (B1).
In formula (B1), each R ^B1 independently represents a hydrogen atom or a linear alkyl group having 1 to 10 carbon atoms or a branched alkyl group having 3 to 10 carbon atoms, and at least one of the two R ^B1 is a linear alkyl group having 1 to 10 carbon atoms or a branched alkyl group having 3 to 10 carbon atoms.

Straight-chain alkyl groups having 1 to 10 carbon atoms Examples of straight-chain alkyl groups having 1 to 10 carbon atoms include methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, and decyl groups.

Branched Alkyl Group Examples of the branched alkyl group represented by R ^B1 include the same as the branched alkyl group having 3 to 10 carbon atoms represented by R ^A1 described above.

Combinations of R ^B1 in Formula (B1) From the viewpoint of achieving better effects of the present invention, combinations of R ^B1 in Formula (B1) include combination 1 in which one R ^B1 in Formula (B1) is a hydrogen atom and the other R ^B1 is a linear or branched alkyl group having 4 to 10 carbon atoms, or
Combination 2, in which each R ^B1 independently represents a linear or branched alkyl group having 1 to 10 carbon atoms, is preferred.
An example of a repeating unit that falls under the above combination 1 is a repeating unit of N-tert-butylacrylamide.
An example of a repeating unit that falls under the above combination 2 is a repeating unit of N,N-dimethylacrylamide.

From the perspective of achieving better effects of the present invention, it is preferable that repeating unit B1 contain a repeating unit of N-tert-butylacrylamide or N,N-dimethylacrylamide.

From the viewpoint of achieving better effects of the present invention, the repeating unit B1 preferably contains a repeating unit of the above combination 1, more preferably contains a repeating unit in which one R ^B1 is a hydrogen atom and the other R ^B1 is a branched alkyl group having 4 to 10 carbon atoms, and further preferably contains a repeating unit of N-tert-butylacrylamide.

(Lowest critical solution temperature _B )
When the homopolymer of the repeating unit B1 constituting the copolymer contained in the crosslinked product of the present invention has a lower critical solution temperature _B , the lower critical solution temperature _B is preferably −20 to +25° C. or less, from the viewpoint of achieving better effects of the present invention.
In this specification, the lower critical solution temperature _B refers to the lower critical solution temperature of a homopolymer of the repeating unit B1 represented by formula (B1).
The lower critical solution temperature _B of the homopolymer of N-tert-butylacrylamide is 0° C. or lower.
The homopolymer of N,N-dimethylacrylamide does not have a lower critical solution temperature.

(Relationship between lower critical solution temperature _A and lower critical solution temperature _B )
When the homopolymer of the repeating unit A1 has a lower critical solution temperature _A , the homopolymer of the repeating unit B1 may or may not have a lower critical solution temperature _B.
The relationship between the lower critical solution temperature _A and the lower critical solution temperature _B can be expressed as follows, for example:
The homopolymer of the repeating unit A1 has a lower critical solution temperature _A , the homopolymer of the repeating unit B1 has a lower critical solution temperature _B , and the lower critical solution temperature _B is lower than the lower critical solution temperature _A , or
The homopolymer of the repeating unit A1 has a lower critical solution temperature _A , while the homopolymer of the repeating unit B1 does not have a lower critical solution temperature _B.
When the relationship between the lower critical solution temperature _A and the lower critical solution temperature _B is the former, the lower critical solution temperature _T or the lower critical solution temperature _C can be adjusted, for example, to a temperature range between the lower critical solution temperature _A and the lower critical solution temperature _B. A specific example of the former relationship between the lower critical solution temperature _A and the lower critical solution temperature _B is when the repeating unit A1 is a repeating unit based on NIPAM and the repeating unit B1 is a repeating unit based on NTBAM.
When the relationship between the lower critical solution temperature _A and the lower critical solution temperature _B is the latter, the lower critical solution temperature _T or the lower critical solution temperature _C can be adjusted to be higher than the lower critical solution temperature _A. A specific example of the relationship between the lower critical solution temperature _A and the lower critical solution temperature _B is when the repeating unit A1 is a repeating unit based on NIPAM and the repeating unit B1 is a repeating unit based on DMAAm.

(Manufacturing method)
The crosslinked product of the present invention can be produced, for example, by polymerizing monomers having ethylenically unsaturated bonds corresponding to the repeating units A1, B1, and crosslinked moieties, respectively. The monomers corresponding to the repeating units A1 and B1 are acrylamide compounds. The monomers that can constitute the crosslinked moieties can have multiple ethylenically unsaturated bonds. The monomers corresponding to the repeating units A1 and B1 are different.
The polymerization is preferably radical polymerization, from the viewpoint that the effect of the present invention is more excellent and the lower critical solution temperature _T of the crosslinked product of the present invention is more likely to be expressed.

Monomers that can form repeating unit A1 Examples of monomers that can form repeating unit A1 include N-alkylacrylamides in which one branched alkyl group having 3 to 10 carbon atoms is bonded to a nitrogen atom, such as N-isopropylacrylamide (NIPAM), N-isobutylacrylamide, N-tert-butylacrylamide (N-t-butylacrylamide) (NTBAM), 2-ethylhexylacrylamide, and N-cyclohexylacrylamide;
Examples include N,N-dialkylacrylamides in which two branched alkyl groups having 3 to 10 carbon atoms are bonded to the nitrogen atom, such as N,N-diisopropylacrylamide.
From the viewpoints of achieving better effects of the present invention and making it easier for the crosslinked product of the present invention to exhibit a lower critical solution temperature _T , the monomer that can constitute the repeating unit A1 preferably contains an N-alkylacrylamide in which one branched alkyl group having 3 to 10 carbon atoms is bonded to a nitrogen atom, and more preferably contains N-isopropylacrylamide (NIPAM).

Monomers that can form repeating unit B1 Examples of monomers that can form repeating unit B1 include N-alkylacrylamides in which one linear or branched alkyl group having 4 to 10 carbon atoms is bonded to the nitrogen atom; and N,N-dialkylacrylamides in which each R ^B1 independently represents a linear or branched alkyl group having 1 to 10 carbon atoms.
From the viewpoints of achieving better effects of the present invention and making it easier for the crosslinked product of the present invention to exhibit a lower critical solution temperature _T , it is preferable that the monomer capable of constituting the repeating unit B1 contains the above-mentioned N-alkylacrylamide or N,N-dialkylacrylamide,
More preferably, the compound includes an N-alkylacrylamide or an N,N-dialkylacrylamide in which one branched alkyl group having 4 to 10 carbon atoms is bonded to a nitrogen atom,
More preferably, it contains Nt-butylacrylamide (NTBAM) or N,N-dimethylacrylamide.

Monomers capable of constituting a crosslinked moiety Examples of the monomers capable of constituting a crosslinked moiety include N,N'-alkylenebis(meth)acrylamides such as N,N'-methylenebis(meth)acrylamide;
alkylene diol di(meth)acrylates such as ethylene glycol di(meth)acrylate, monopropylene glycol diacrylate, 1,4-butanediol di(meth)acrylate, and 1,10-bis((meth)acryloyloxy)decane;
Examples thereof include polyoxyalkylene diol di(meth)acrylates such as diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, and polypropylene glycol di(meth)acrylate.

In the present invention, the molar ratio of the monomer capable of constituting the repeating unit A1 to the monomer capable of constituting the repeating unit B1 is reflected in the molar ratio of the repeating units A1 and B1.
Furthermore, the ratio of the monomer capable of forming the crosslinked moiety to the total number of moles of the monomer capable of forming the repeating unit A1 and the monomer capable of forming the repeating unit B1 is reflected in the ratio of the crosslinked moiety.

Examples of methods for producing the crosslinked product of the present invention include a method of radically polymerizing a monomer capable of forming the repeating unit A1, a monomer capable of forming the repeating unit B1, and a monomer capable of forming a crosslinked moiety in an organic solvent such as methyl ethyl ketone in the presence of an initiator such as azobisisobutyronitrile at 40 to 100°C.
The amount of the initiator used is preferably 0.0001 to 0.05 moles, more preferably 0.001 to 0.03 moles, per mole of the total number of moles of repeating units A1, B1 and monomers corresponding to the crosslinking moiety.
After polymerization, the crosslinked product of the present invention can be purified, for example, by adding water to the reaction solution to remove unreacted raw materials and then vacuum drying.

[Application]
The crosslinked product of the present invention is a temperature-responsive compound having excellent water absorption and drainage properties as described above, and is therefore useful, for example, as a compounding agent for rubber (a material that can be used for rubber).
In the present invention, by selecting the repeating units A1 and B1 contained in the crosslinked product of the present invention, and further, if necessary, by selecting, for example, the molar ratio of the repeating units A1 and B1, the lower critical solution temperature _T of the crosslinked product of the present invention can be adjusted according to the environmental temperature at which the rubber product is used.

[Rubber compounding agents]
The compounding agent for rubber of the present invention is a compounding agent for rubber containing the crosslinked product of the present invention.

(Crosslinked product)
The crosslinked product contained in the compounding agent for rubber of the present invention is not particularly limited as long as it is the crosslinked product of the present invention.

(rubber)
There are no particular restrictions on the rubber to which the compounding agent for rubber of the present invention can be applied.
The rubber compounding agent of the present invention may be applied to a rubber composition, such as a rubber composition for tires, specifically a rubber composition for tire treads and a rubber composition for studless tires.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these.

[Production of crosslinked product]
Each crosslinked product was produced as follows.
Here, specific crosslinked products 1 to 8 are crosslinked products of copolymers having two specific repeating units, and therefore correspond to the crosslinked products of the present invention. Details of each crosslinked product are shown in Table 1.
Table 1 shows the mole % of each repeating unit in the total of repeating units A1 and B1, the ratio of repeating unit C1 (crosslinking agent) (the ratio is the number of moles of repeating unit C1 in the crosslinking moiety per mole of the total of repeating units A1 and B1 (*1 in Table 1)), the amount of initiator used (the amount is the number of moles of initiator per mole of the total of repeating units A1, B1 and the monomers corresponding to the crosslinking moiety (*2 in Table 1)), the lower critical solution temperature _T of each obtained crosslinked product (of the entire compound finally obtained), and the lower critical solution temperature _C of the copolymer.
On the other hand, Comparative Compound 1 is a crosslinked product of NIPAM homopolymer and therefore does not fall under the crosslinked product of the present invention. Comparative Compound 2 is a NIPAM homopolymer and is not a crosslinked product, so does not fall under the crosslinked product of the present invention. Comparative Compound 3 is a NTBAM homopolymer and is not a crosslinked product, so does not fall under the crosslinked product of the present invention.

<Specific crosslinked product 1>
In methyl ethyl ketone (MEK), 1.0 g of N-isopropylacrylamide (NIPAM), 0.12 g of N-t-butylacrylamide (NTBAM), and 36 mg of N,N'-methylenebis(acrylamide) (hereinafter abbreviated as MBAAm) as crosslinking agent 1 were polymerized by radical polymerization using 15 mg of azobisisobutyronitrile (AIBN) as an initiator (60°C, 8 hours). The mixing ratio of NIPAM to NTBAM (molar ratio of NIPAM/NTBAM) was 90/10.
After the polymerization, a large amount of water was added to the reaction solution to remove unreacted raw materials, and the mixture was vacuum dried at 80° C. for 24 hours.

As a result, a crosslinked product represented by the following formula X was obtained.
The crosslinked product represented by the following formula X is a copolymer having repeating units a-1 represented by the following formula (a-1) and repeating units b-1 represented by the following formula (b-1) arranged randomly, which are crosslinked with repeating units c-1 represented by the following formula (c-1). In the following formula X, m is the number of repeating units a-1, n is the number of repeating units b-1, and o is the number of repeating units c-1. The * in repeating unit c-1 represents a bond with another repeating unit a-1 or repeating unit b-1, but these are omitted as they are the same as those already shown in formula X. The obtained crosslinked product is designated specific crosslinked product 1.

Specific Crosslinked Products 2 to 5 described below can also be represented by the following formula X. The repeating unit a-1 in Specific Crosslinked Products 1 to 5 is formed from NIPAM, and the repeating unit b-1 is formed from NTBAM.
Furthermore, the copolymers constituting the specific cross-linked materials 1 to 8 described below are random copolymers, and the specific cross-linked materials 1 to 8 themselves are also random copolymers.
As shown in the evaluation of water absorbency described below, when specific crosslinked products 1 to 8 were immersed in a solvent (water), they did not dissolve in water but swelled (gelled). This suggests that a crosslinking agent was introduced into specific crosslinked products 1 to 8, and specific crosslinked products 1 to 8 were crosslinked to form a network structure.

<Specific crosslinked product 2>
A crosslinked product represented by the above formula X was obtained in the same manner as in the above-mentioned specific crosslinked product 1, except that the mixing ratio (molar ratio) of NIPAM and NTBAM was changed to 75/25 and the amount of crosslinking agent 1 was changed. The obtained crosslinked product is designated as specific crosslinked product 2.

<Specific crosslinked product 3>
A crosslinked product represented by the above formula X was obtained in the same manner as in the above-mentioned specific crosslinked product 1, except that the mixing ratio (molar ratio) of NIPAM and NTBAM was changed to 50/50 and the amount of crosslinking agent 1 was changed. The obtained crosslinked product is designated as specific crosslinked product 3.

<Specific crosslinked product 4>
A crosslinked product represented by the above formula X was obtained in the same manner as in the above-mentioned specific crosslinked product 1, except that the mixing ratio (molar ratio) of NIPAM and NTBAM was changed to 10/90 and the amount of crosslinking agent 1 was changed. The obtained crosslinked product is designated as specific crosslinked product 4.

(Comparative Compound 1)
A crosslinked product was obtained in the same manner as in the above-mentioned specific crosslinked product 1, except that NTBAM was not used and the amount of crosslinking agent 1 was changed. The obtained crosslinked product is designated as comparative compound 1.

(Comparative Compound 2)
A homopolymer of NIPAM was obtained in the same manner as in the above-mentioned specific crosslinked product 1, except that NTBAM and MBAAm were not used. The obtained compound is designated as comparative compound 2.

(Comparative Compound 3)
A homopolymer of NTBAM was obtained in the same manner as in the above-mentioned specific crosslinked product 1, except that NIPAM and MBAAm were not used. The obtained compound is designated as comparative compound 3.

<Specific crosslinked product 5>
A crosslinked product was obtained in the same manner as in the above-mentioned specific crosslinked product 1, except that crosslinking agent 1 (MBAAm) was changed to 50 mg of crosslinking agent 2 (ethylene glycol diacrylate). The obtained crosslinked product is designated as specific crosslinked product 5.

<Specific crosslinked product 6>
A crosslinked product was obtained in the same manner as in the above-described specific crosslinked product 2, except that the mixture of NIPAM and NTBAM was replaced with a mixture of NIPAM and N,N-dimethylacrylamide (DMAAm), and the mixing ratio (molar ratio of NIPAM/N,N-dimethylacrylamide) was changed to 80/20. The obtained crosslinked product is designated as specific crosslinked product 6. The structure of specific crosslinked product 6 is the same as that of formula X above, except that the repeating unit b-1 represented by formula (b-1) is a repeating unit derived from N,N-dimethylacrylamide.

<Specific crosslinked product 7>
A crosslinked product was obtained in the same manner as in the above-mentioned specific crosslinked product 6, except that the mixing ratio (molar ratio) of NIPAM and DMAAm was changed to 60/40. The obtained crosslinked product is designated as specific crosslinked product 7. The structure of specific crosslinked product 7 is the same as that of formula X above, except that the repeating unit b-1 represented by formula (b-1) is a repeating unit derived from N,N-dimethylacrylamide.

<Specific crosslinked product 8>
A crosslinked product was obtained in the same manner as in the above-mentioned specific crosslinked product 6, except that the mixing ratio (molar ratio) of NIPAM and DMAAm was changed to 40/60. The obtained crosslinked product is designated as specific crosslinked product 8. The structure of specific crosslinked product 8 is the same as that of formula X above, except that the repeating unit b-1 represented by formula (b-1) is a repeating unit derived from N,N-dimethylacrylamide.

In each example, polymerization was carried out in the same manner as above, except that no crosslinking agent was used, and a copolymer (or homopolymer) was obtained separately from the specific crosslinked product.

[Evaluation of the crosslinked product]
The crosslinked products produced as described above were evaluated as follows. The results are shown in Table 1.
<Water absorption/drainage properties>
Water absorption: 1.0 g of each crosslinked product that had been produced as described above and then vacuum-dried was immersed in 100 g of water and left for 1 hour at +2°C to swell the crosslinked product. After 1 hour, the state of the crosslinked product (whether it had gelled or dissolved) was visually confirmed.

・Water absorption rate (residual water rate)
Next, each of the swollen crosslinked products was filtered, and the weight W _gel (g) of the filtered gel-like crosslinked product (swollen gel) was measured. Next, the swollen gel was again dried in vacuum at 60°C for 24 hours (pressure during vacuum drying: 0.1 MPa), and the weight W _dry (g) of the crosslinked product after drying was measured. The obtained values W _gel and W _dry were substituted into the following formula to calculate the water absorption rate (residual water rate).
Water absorption rate (residual water rate) [g/g] = (W _gel - W _dry )/W _dry

Evaluation criteria for water absorbency and drainage properties In the present invention, when a crosslinked product was left in water at +2°C for 1 hour and the crosslinked product swelled (gelled) and the water absorption rate (residual water rate) was less than 30, the crosslinked product was evaluated to have excellent water absorbency and drainage properties.
In the above cases, when the water absorption rate (residual water rate) was less than 30, the drainage property was evaluated as being better.
On the other hand, if no gelation was observed after placing the crosslinked product (or uncrosslinked product) in water at +2°C for 1 hour (for example, the crosslinked product (or uncrosslinked product) was completely dissolved in water), the water absorbency was evaluated as poor. Note that if the crosslinked product (or uncrosslinked product) was dissolved in water and no gelation was observed, the compound had no water absorbency, and if the compound was dried at a temperature higher than its LCST, the water absorption rate (residual water rate) after drying would be 0.
Furthermore, when the water absorption rate (residual water rate) was 30 or more, the crosslinked product was evaluated as having poor drainage properties.

<Lower critical solution temperature>
The lower critical solution temperature _T of each crosslinked product produced as described above and the lower critical solution temperature _C of the copolymer contained in the crosslinked product are shown in Table 1. The lower critical solution temperature _T and the lower critical solution temperature _C were measured as described above.
The column for lower critical solution temperature _C of Comparative Example 2 shows the lower critical solution temperature of Comparative Compound 2 (homopolymer). The column for lower critical solution temperature _C of Comparative Example 3 is similar.

From the results shown in Table 1, it can be seen that Comparative Compound 1, which is a crosslinked product of NIPAM homopolymer, had poor drainage properties.
Comparative Compound 2, which is a simple NIPAM homopolymer and is not a crosslinked product, did not absorb water.
Comparative Compound 3, which is a simple NTBAM homopolymer and is not a crosslinked product, did not absorb water.

On the other hand, the crosslinked product and rubber compounding agent of the present invention have an excellent balance of water absorption and drainage properties in the usage environment of rubber products such as tires.

Claims (9)

A compounding agent for rubber , comprising a crosslinked product of a copolymer having a repeating unit A1 represented by the following formula (A1) and a repeating unit B1 represented by the following formula (B1), wherein the crosslinked product has a repeating unit C1 represented by the following formula (C1) as a crosslinked moiety, and the crosslinked product has only the repeating unit A1, the repeating unit B1, and the repeating unit C1 as repeating units , provided that the repeating unit A1 and the repeating unit B1 are different.
In formula (A1), each R ^A1 independently represents a hydrogen atom or a branched alkyl group having 3 to 10 carbon atoms, and at least one of the R ^A1 is the alkyl group; in formula (B1), each R ^B1 independently represents a hydrogen atom or a linear or branched alkyl group having 1 to 10 carbon atoms, and at least one of the R ^B1 is the alkyl group.
In formula (C1), R ^1s each independently represent a hydrogen atom or an alkyl group, X ^1s each independently represent an ester bond or an amide bond, and R ² represents a hydrocarbon group, an oxyalkylene group, or a polyoxyalkylene group. 2. The compounding agent for rubber according to claim 1, wherein a lower critical solution temperature _T of the crosslinked product or a lower critical solution temperature _C of the copolymer having only the repeating unit A1 and the repeating unit B1 is independently −10° C. to +60° C. 3. The compounding agent for rubber according to claim 1, wherein the homopolymer of the repeating unit A1 has a lower critical solution temperature _A and/or the homopolymer of the repeating unit B1 has a lower critical solution temperature _B. the homopolymer of the repeating unit A1 has a lower critical solution temperature _A , the homopolymer of the repeating unit B1 has a lower critical solution temperature _B , and the lower critical solution temperature _B is lower than the lower critical solution temperature _A ; or
The compounding agent for rubber according to any one of claims 1 to 3, wherein the homopolymer of the repeating unit A1 has a lower critical solution temperature _A , and the homopolymer of the repeating unit B1 does not have a lower critical solution temperature _B. The compounding agent for rubber according to any one of claims 1 to 4, wherein in formula (A1), one R ^A1 is a hydrogen atom and the other R ^A1 represents a branched alkyl group having 3 to 10 carbon atoms. The compounding agent for rubber according to any one of claims 1 to 5, wherein the repeating unit A1 includes a repeating unit derived from N-isopropylacrylamide. In formula (B1), one R ^B1 is a hydrogen atom and the other R ^B1 is a linear or branched alkyl group having 4 to 10 carbon atoms, or
7. The compounding agent for rubber according to claim 1, wherein each R ^B1 independently represents a linear or branched alkyl group having 1 to 10 carbon atoms. The compounding agent for rubber according to any one of claims 1 to 7, wherein the repeating unit B1 includes a repeating unit derived from N-tert-butylacrylamide or N,N-dimethylacrylamide. The compounding agent for rubber according to any one of claims 1 to 8, wherein the crosslinked product is a random copolymer.