JP7798943B2 - Resin composition for impact absorbing sheet, method for producing same, and impact absorbing sheet - Google Patents

Description

This disclosure relates to a resin composition for an impact absorbing sheet, a method for producing the same, and an impact absorbing sheet.

Conventionally, shock-absorbing sheets are known that are incorporated into various electronic devices such as mobile phones and smartphones. Shock-absorbing sheets that absorb shock through a foamed structure are often used. To form such shock-absorbing sheets, a dispersion containing (meth)acrylic resin particles is used because of its excellent foaming properties and the ease with which a foamed structure can be formed.
In recent years, techniques have been developed to impart impact absorption properties to sheets formed using a dispersion containing (meth)acrylic resin particles by methods other than foaming.

For example, Patent Document 1 discloses a polymerizable composition comprising: (meth)acrylic resin A, which contains structural units derived from a monomer having a glass transition temperature of 50°C or higher when made into a homopolymer, and has a glass transition temperature of -40°C or higher and 10°C or lower, and which has a carboxy group; and (meth)acrylic resin B, which contains structural units derived from a (meth)acrylic acid alkyl ester monomer having an alkyl group with 4 to 8 carbon atoms, and has a glass transition temperature of -80°C or higher and -20°C or lower, and which has a carboxy group; and (meth)acrylic resin A, which contains particles The resin composition for an impact-absorbing sheet has a morphology in which the average particle size of the (meth)acrylic resin A particles is 50 nm or more but less than 500 nm, the average particle size of the (meth)acrylic resin B particles is 5 μm or more but less than 100 μm, the glass transition temperature of the (meth)acrylic resin B is lower than the glass transition temperature of the (meth)acrylic resin A, and the ratio of the content of the (meth)acrylic resin A particles to the content of the (meth)acrylic resin B particles is within a range of 90/10 to 40/60 by mass. The resin composition disclosed in Patent Document 1 is designed to form an impact-absorbing sheet using the relatively soft particles of the (meth)acrylic resin B rather than air bubbles.

Patent No. 7036376

In recent years, demand for electric vehicles (EVs) is expected to expand due to environmental concerns, and development of combined batteries for use in EVs is progressing. Fixing each cell in place is important for EV batteries, and maintaining a constant distance between the electrodes can improve the battery's performance and lifespan. Furthermore, because the cells of EV batteries repeatedly expand and contract as they are charged and discharged, shock-absorbing sheets with cushioning properties are used to secure the cells in place to relieve stress. Dispersions containing (meth)acrylic resin particles are used to form shock-absorbing sheets for cell fixation, as this allows for the formation of sheets with greater durability than urethane resins.

Batteries used in EVs may generate heat during charging and discharging, or may be exposed to humid environments, such as when driving in the rain. Therefore, impact-absorbing sheets for cell fixation are required to be able to fully perform as impact-absorbing sheets even in humid and hot environments. However, impact-absorbing sheets formed using dispersions containing (meth)acrylic resin particles tend to easily absorb moisture when exposed to humid and hot environments due to the presence of hydrophilic groups derived from surfactants within the sheet. When such impact-absorbing sheets absorb moisture, their strength significantly decreases and their resilience is impaired. Therefore, there is a need to develop materials that can be used to form impact-absorbing sheets that have excellent resilience even in humid and hot environments.
Furthermore, since an impact absorbing sheet for fixing a cell comes into contact with the substrate of the cell, it is also required to have excellent adhesion to the substrate, for example, in a high-humidity environment. If the adhesion between the impact absorbing sheet and the substrate decreases and the impact absorbing sheet lifts or peels off from the substrate, the impact absorbing sheet cannot relieve the stress from the cell, and the battery performance will be significantly reduced.

The present disclosure has been made in consideration of the above circumstances.
The problem to be solved by the embodiments of the present disclosure is to provide a resin composition for an impact absorbing sheet that can form an impact absorbing sheet that has excellent recovery properties in a humid and hot environment and excellent adhesion to a substrate in a high humidity environment, a method for producing the same, and an impact absorbing sheet.

Specific means for solving the problems include the following aspects.
<1> (Meth)acrylic resin particles comprising: a structural unit (a) derived from a vinyl cyanide monomer; a structural unit (b) derived from a monomer having a carboxy group; and a structural unit (c) derived from a monomer having at least one of an aldehyde group and a ketone group, wherein the content of the structural unit (b) is 1.0% by mass to 4.0% by mass with respect to all structural units; and the content of the structural unit (c) is 1.0% by mass to 2.0% by mass with respect to all structural units;
a hydrazide crosslinking agent;
a crosslinking agent other than the hydrazide-based crosslinking agent;
Including,
The resin composition for impact absorbing sheets has a content of the hydrazide-based crosslinking agent of 0.3 to 1.0 part by mass relative to 100 parts by mass of the (meth)acrylic resin particles.
<2> The resin composition for an impact absorbing sheet according to <1>, wherein the vinyl cyanide monomer is acrylonitrile.
<3> The resin composition for an impact absorbing sheet according to <1> or <2>, wherein the monomer having at least one of an aldehyde group and a ketone group is a (meth)acrylamide derivative.
<4> The resin composition for an impact-absorbing sheet according to any one of <1> to <3>, wherein the monomer having at least one of an aldehyde group and a ketone group is diacetone acrylamide.
<5> The resin composition for an impact absorbing sheet according to any one of <1> to <4>, wherein the hydrazide-based crosslinking agent is an aliphatic hydrazide-based compound.
<6> The resin composition for an impact-absorbing sheet according to any one of <1> to <5>, wherein the hydrazide-based crosslinking agent is adipic acid dihydrazide.
<7> The resin composition for impact absorbing sheets according to any one of <1> to <6>, wherein the crosslinking agent other than the hydrazide-based crosslinking agent includes at least one selected from the group consisting of an oxazoline-based crosslinking agent, an epoxy-based crosslinking agent, and an isocyanate-based crosslinking agent.
<8> The resin composition for an impact absorbing sheet according to any one of <1> to <7>, wherein the average particle size of the (meth)acrylic resin particles is 50 nm to 500 nm.
<9> A method for producing a resin composition for an impact-absorbing sheet, the method comprising: mixing (meth)acrylic resin particles that contain structural units (a) derived from a vinyl cyanide monomer, structural units (b) derived from a monomer having a carboxy group, and structural units (c) derived from a monomer having at least one of an aldehyde group and a ketone group, wherein the content of structural units (b) is 1.0% by mass to 4.0% by mass with respect to all structural units, and the content of structural units (c) is 1.0% by mass to 2.0% by mass with respect to all structural units; a mixture of a hydrazide-based crosslinking agent in an amount of 0.3 parts by mass to 1.0 part by mass with respect to 100 parts by mass of the (meth)acrylic resin particles; and a crosslinking agent other than the hydrazide-based crosslinking agent.
<10> An impact absorbing sheet formed from the resin composition for impact absorbing sheets according to any one of <1> to <8>.

Embodiments of the present disclosure provide a resin composition for an impact absorbing sheet that can form an impact absorbing sheet that has excellent recovery properties in a humid and hot environment and excellent adhesion to a substrate in a high-humidity environment, a method for producing the same, and an impact absorbing sheet.

The resin composition for impact-absorbing sheets and its manufacturing method, as well as the impact-absorbing sheets disclosed herein, are described in detail below. The requirements described below may be based on representative embodiments of the present disclosure, but the present disclosure is not limited to such embodiments and can be implemented with appropriate modifications within the scope of the purpose of the present disclosure.

In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits, respectively.
In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with a value shown in the examples.

In this disclosure, a combination of two or more preferred aspects is a more preferred aspect.

In this disclosure, when the resin composition for impact absorbing sheets contains multiple substances corresponding to each component, the amount of each component in the resin composition for impact absorbing sheets refers to the total amount of the multiple substances present in the resin composition for impact absorbing sheets, unless otherwise specified.

In the present disclosure, the term "(meth)acrylic monomer" means a monomer having a (meth)acryloyl group.
In the present disclosure, the term "(meth)acrylic resin" refers to a resin that contains structural units derived from (meth)acrylic monomers, and in which the content of structural units derived from (meth)acrylic monomers is 50.0 mass % or more of all structural units (i.e., all structural units constituting the (meth)acrylic resin. However, in cases where the (meth)acrylic resin contains structural units derived from reactive surfactants, this excludes the structural units derived from the reactive surfactants).

In this disclosure, "(meth)acrylic" is a term that encompasses both "acrylic" and "methacrylic," "(meth)acrylate" is a term that encompasses both "acrylate" and "methacrylate," and "(meth)acryloyl" is a term that encompasses both "acryloyl" and "methacryloyl."

In this disclosure, "n-" means normal, "i-" means iso, "s-" means secondary, and "t-" means tertiary.

In this disclosure, "mass %" and "weight %" are synonymous, and "parts by mass" and "parts by weight" are synonymous.

In this disclosure, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

In this disclosure, "resilience" means the ability to return to its original shape after being compressed and deformed.

[Resin composition for impact absorbing sheet]
The resin composition for an impact absorbing sheet of the present disclosure (hereinafter also simply referred to as "resin composition") comprises (meth)acrylic resin particles which include a structural unit (a) derived from a vinyl cyanide monomer, a structural unit (b) derived from a monomer having a carboxy group, and a structural unit (c) derived from a monomer having at least one of an aldehyde group and a ketone group, wherein the content of structural unit (b) is 1.0% by mass to 4.0% by mass with respect to all structural units, and the content of structural unit (c) is 1.0% by mass to 2.0% by mass with respect to all structural units; a hydrazide crosslinking agent; and a crosslinking agent other than the hydrazide crosslinking agent, wherein the content of the hydrazide crosslinking agent is 0.3 parts by mass to 1.0 parts by mass with respect to 100 parts by mass of the (meth)acrylic resin particles.
The resin composition of the present disclosure is a resin composition used to form an impact absorbing sheet.
According to the resin composition of the present disclosure, it is possible to form an impact absorbing sheet that has excellent restoration properties in a humid and hot environment and excellent adhesion to a substrate in a high humidity environment.
The reason why the resin composition of the present disclosure can exhibit such an effect is unclear, but the present inventors speculate as follows: However, the following speculation is not intended to limit the resin composition of the present disclosure, but is described as an example.

The resin composition of the present disclosure contains (meth)acrylic resin particles, a hydrazide-based crosslinking agent, and a crosslinking agent other than the hydrazide-based crosslinking agent. The (meth)acrylic resin particles have carboxy groups derived from the structural unit (b) and aldehyde groups and/or ketone groups derived from the structural unit (c). Therefore, according to the resin composition of the present disclosure, the aldehyde groups and/or ketone groups in the (meth)acrylic resin particles undergo a crosslinking reaction with the hydrazide-based crosslinking agent, and the carboxy groups in the (meth)acrylic resin particles undergo a crosslinking reaction with a crosslinking agent other than the hydrazide-based crosslinking agent, thereby forming a dense crosslinked structure. The formation of a dense crosslinked structure increases the elasticity of the film and suppresses moisture penetration into the impact absorbing sheet when placed in a humid and hot environment. Therefore, it is believed that impact absorbing sheets formed from the resin composition of the present disclosure have excellent resilience in a humid and hot environment. Furthermore, the (meth)acrylic resin particles in the resin composition of the present disclosure contain a structural unit (a) derived from a vinyl cyanide monomer. Because vinyl cyanide monomer is a component with relatively high polarity, impact-absorbing sheets formed from the resin composition of the present disclosure exhibit enhanced interaction with the substrate due to the high polarity of the vinyl cyanide monomer. This is thought to result in excellent adhesion to the substrate even in high-humidity environments.

In the present disclosure, "(meth)acrylic resin particles which include a structural unit (a) derived from a vinyl cyanide monomer, a structural unit (b) derived from a monomer having a carboxy group, and a structural unit (c) derived from a monomer having at least one of an aldehyde group and a ketone group, wherein the content of the structural unit (b) is 1.0% by mass to 4.0% by mass with respect to all structural units, and the content of the structural unit (c) is 1.0% by mass to 2.0% by mass with respect to all structural units" are also referred to as "specific (meth)acrylic resin particles".
In addition, in the present disclosure, "crosslinking agents other than hydrazide-based crosslinking agents" are also referred to as "other crosslinking agents."

The components of the resin composition of the present disclosure are described below.

[Specific (meth)acrylic resin particles]
The resin composition of the present disclosure comprises (meth)acrylic resin particles (i.e., specific (meth)acrylic resin particles) that include a structural unit (a) derived from a vinyl cyanide monomer, a structural unit (b) derived from a monomer having a carboxy group, and a structural unit (c) derived from a monomer having at least one of an aldehyde group and a ketone group, wherein the content of the structural unit (b) is 1.0% by mass to 4.0% by mass with respect to all structural units, and the content of the structural unit (c) is 1.0% by mass to 2.0% by mass with respect to all structural units.
In the resin composition of the present disclosure, the specific (meth)acrylic resin particles are preferably present in a dispersed state in a medium containing water.
The resin composition of the present disclosure may contain only one type of specific (meth)acrylic resin particles, or may contain two or more types.

<Structural Unit (a) Derived from Vinyl Cyanide Monomer>
The specific (meth)acrylic resin particles contain a structural unit (a) derived from a vinyl cyanide monomer. In the present disclosure, the term "structural unit derived from a vinyl cyanide monomer" refers to a structural unit formed by addition polymerization of a vinyl cyanide monomer.

The structural unit (a) contained in the specific (meth)acrylic resin particles contributes to the formation of an impact-absorbing sheet that exhibits excellent adhesion to the substrate even in high-humidity environments. Vinyl cyanide monomers are monomers with relatively high polarity. When the (meth)acrylic resin particles in the resin composition of the present disclosure contain structural unit (a), the formed impact-absorbing sheet is thought to exhibit excellent adhesion to the substrate even in high-humidity environments due to enhanced interaction with the substrate caused by the high polarity of the vinyl cyanide monomer.

The type of vinyl cyanide monomer is not particularly limited.
Specific examples of vinyl cyanide monomers include acrylonitrile, methacrylonitrile, ethacrylonitrile, vinylidene cyanide, fumaronitrile, and 2-chloroacrylonitrile.
The vinyl cyanide monomer is preferably acrylonitrile, for example, in that it is easy to form an impact absorbing sheet having excellent elongation and strength.

The specific (meth)acrylic resin particles may contain only one type of structural unit (a), or may contain two or more types.

The content of the structural unit (a) in the specific (meth)acrylic resin particles is not particularly limited, but, for example, from the viewpoint of the adhesion of the formed impact absorbing sheet to a substrate in a high-humidity environment, it is preferably 5.0% by mass to 30.0% by mass relative to all structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded).
In one embodiment, the content of the structural unit (a) in the specific (meth)acrylic resin particles may be 5.0% by mass to 20.0% by mass, 5.0% by mass to 15.0% by mass, 5.0% by mass to 10.0% by mass, 10.0% by mass to 20.0% by mass, or 10.0% by mass to 15.0% by mass, relative to all structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded).

<Structural Unit (b) Derived from Monomer Having a Carboxy Group>
The specific (meth)acrylic resin particles contain a structural unit (b) derived from a monomer having a carboxy group. In the present disclosure, the term "structural unit derived from a monomer having a carboxy group" refers to a structural unit formed by addition polymerization of a monomer having a carboxy group.
It should be noted that the "monomer having a carboxy group" in the present disclosure does not include a monomer having a carboxy group and at least one of an aldehyde group and a ketone group. That is, in the present disclosure, a monomer having a carboxy group and at least one of an aldehyde group and a ketone group is classified as a "monomer having at least one of an aldehyde group and a ketone group."

The type of the monomer having a carboxy group is not particularly limited.
Examples of the monomer having a carboxy group include a monomer having at least one carboxy group and an ethylenically unsaturated group in one molecule.
Specific examples of the monomer having a carboxy group include (meth)acrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, glutaconic acid, citraconic acid, ω-carboxy-polycaprolactone mono(meth)acrylate [for example, ω-carboxy-polycaprolactone (n≈2) monoacrylate], and succinic acid derivatives (for example, 2-acryloyloxyethyl-succinic acid).
The monomer having a carboxy group is preferably at least one selected from acrylic acid and itaconic acid.

The specific (meth)acrylic resin particles may contain only one type of structural unit (b), or may contain two or more types.

The content of the structural unit (b) in the specific (meth)acrylic resin particles is 1.0% by mass to 4.0% by mass relative to all structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded).
When the content of the structural unit (b) in the specific (meth)acrylic resin particles is 1.0 mass% or more relative to the total structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded), the resulting impact absorbing sheet tends to have excellent recovery properties in a humid and hot environment. The reason for this is presumably that the crosslinking reaction between the carboxyl group of the structural unit (b) and other crosslinking agents is sufficiently carried out, thereby forming a dense crosslinked structure, increasing the elasticity of the impact absorbing sheet and suppressing the penetration of moisture into the impact absorbing sheet when placed in a humid and hot environment.
When the content of the structural unit (b) in the specific (meth)acrylic resin particles is 4.0 mass% or less relative to the total structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded), the formed impact absorbing sheet tends to have excellent adhesion to the substrate in a high-humidity environment. The reason for this is presumably that the crosslinking reaction between the carboxyl group of the structural unit (b) and other crosslinking agents does not occur excessively, and the formed impact absorbing sheet is less likely to become hard and thereby have a reduced adhesion to the substrate.
The content of the structural unit (b) in the specific (meth)acrylic resin particles is preferably 1.5% by mass to 4.0% by mass relative to all structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded).

<Structural Unit (c) Derived from a Monomer Having at Least Either an Aldehyde Group or a Ketone Group>
The specific (meth)acrylic resin particles contain a structural unit (c) derived from a monomer having at least one of an aldehyde group and a ketone group. In the present disclosure, the term "structural unit derived from a monomer having at least one of an aldehyde group and a ketone group" refers to a structural unit formed by addition polymerization of a monomer having at least one of an aldehyde group and a ketone group.
In the present disclosure, the term "monomer having at least one of an aldehyde group and a ketone group" also includes a monomer having a carboxy group and at least one of an aldehyde group and a ketone group.

The type of the monomer having at least one of an aldehyde group and a ketone group is not particularly limited.
The monomer having at least one of an aldehyde group and a ketone group may have only an aldehyde group, only a ketone group, or both an aldehyde group and a ketone group.
Examples of the monomer having at least one of an aldehyde group and a ketone group include a monomer having at least one aldehyde group and/or ketone group and an ethylenically unsaturated group in one molecule.

Specific examples of the monomer having at least one of an aldehyde group and a ketone group include diacetone (meth)acrylamide, acetoacetoxyethyl (meth)acrylate, vinyl methyl ketone, vinyl ethyl ketone, vinyl isobutyl ketone, acrolein, methacrolein, and 4-vinylbenzaldehyde.
The monomer having at least one of an aldehyde group and a ketone group is preferably a (meth)acrylamide derivative, more preferably diacetone acrylamide.
In the present disclosure, the term "(meth)acrylamide derivative" refers to a compound having a substituent other than hydrogen on the nitrogen atom of (meth)acrylamide.

The specific (meth)acrylic resin particles may contain only one type of structural unit (c), or may contain two or more types.

The content of the structural unit (c) in the specific (meth)acrylic resin particles is 1.0% by mass to 2.0% by mass relative to all structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded).
When the content of the structural unit (c) in the specific (meth)acrylic resin particles is 1.0 mass% or more relative to the total structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded), the resulting impact absorbing sheet tends to have excellent restoration properties in a humid and hot environment. The reason for this is presumably that the crosslinking reaction between the aldehyde group and/or ketone group of the structural unit (c) and the hydrazide crosslinking agent is sufficiently carried out, thereby forming a dense crosslinked structure and increasing the elasticity of the impact absorbing sheet.
When the content of the structural unit (c) in the specific (meth)acrylic resin particles is 2.0% by mass or less relative to the total structural units of the specific (meth)acrylic resin particles (excluding the structural unit (e) when the specific (meth)acrylic resin particles contain the structural unit (e) derived from a reactive surfactant), the resulting impact absorbing sheet tends to have excellent recovery properties in a humid and hot environment. This is presumably because the reaction between the aldehyde group and/or ketone group of the structural unit (c) and the hydrazide crosslinking agent is carried out in the correct amount, resulting in the formation of a dense crosslinked structure. Furthermore, when the content of the structural unit (c) in the specific (meth)acrylic resin particles is 2.0% by mass or less relative to the total structural units of the specific (meth)acrylic resin particles (excluding the structural unit (e) when the specific (meth)acrylic resin particles contain the structural unit (e) derived from a reactive surfactant), an impact absorbing sheet with a highly uniform thickness tends to be obtained. The reason for this is presumably that the coating film is less likely to shrink when dried.
The content of the structural unit (c) in the specific (meth)acrylic resin particles is preferably 1.0% by mass to 1.5% by mass relative to all structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded).

<Structural Unit (d) Derived from (Meth)acrylic Acid Alkyl Ester Monomer>
The specific (meth)acrylic resin particles preferably contain a structural unit (d) derived from a (meth)acrylic acid alkyl ester monomer. In the present disclosure, the term "structural unit derived from a (meth)acrylic acid alkyl ester monomer" refers to a structural unit formed by addition polymerization of a (meth)acrylic acid alkyl ester monomer.
The "(meth)acrylic acid alkyl ester monomer" in the present disclosure does not include a monomer having a carboxy group or a monomer having at least one of an aldehyde group and a ketone group. That is, the "(meth)acrylic acid alkyl ester monomer" in the present disclosure refers to a (meth)acrylic acid alkyl ester monomer that does not have any of a carboxy group, an aldehyde group, or a ketone group.

The type of (meth)acrylic acid alkyl ester monomer is not particularly limited.
The (meth)acrylic acid alkyl ester monomer may be an acrylic acid alkyl ester monomer or a methacrylic acid alkyl ester monomer.
The alkyl group contained in the (meth)acrylic acid alkyl ester monomer may be unsubstituted or may have a substituent, but is preferably unsubstituted.
The alkyl group contained in the (meth)acrylic acid alkyl ester monomer may be linear, branched, or cyclic.
The number of carbon atoms in the alkyl group of the (meth)acrylic acid alkyl ester monomer is, for example, preferably 1 to 18, more preferably 1 to 12, even more preferably 1 to 8, and particularly preferably 1 to 4.

Specific examples of the (meth)acrylic acid alkyl ester monomer include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, n-octyl (meth)acrylate, i-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-nonyl (meth)acrylate, i-nonyl (meth)acrylate, n-decyl (meth)acrylate, stearyl (meth)acrylate, lauryl (meth)acrylate, cyclohexyl (meth)acrylate, and isobornyl (meth)acrylate.
The (meth)acrylic acid alkyl ester monomer preferably contains at least one selected from the group consisting of n-butyl acrylate, ethyl acrylate, and methyl methacrylate, more preferably contains at least one selected from n-butyl acrylate and ethyl acrylate, and even more preferably contains n-butyl acrylate.

When the specific (meth)acrylic resin particles contain the structural unit (d), they may contain only one type of structural unit (d), or two or more types.

When the specific (meth)acrylic resin particles contain the structural unit (d), the content of the structural unit (d) is not particularly limited, but for example, it is preferably 50.0% by mass to 93.0% by mass, more preferably 55.0% by mass to 93.0% by mass, even more preferably 60.0% by mass to 93.0% by mass, and particularly preferably 65.0% by mass to 93.0% by mass, relative to all structural units of the specific (meth)acrylic resin particles (however, in cases where the specific (meth)acrylic resin particles contain a structural unit (e) derived from a reactive surfactant, the structural unit (e) is excluded).
The content of the structural unit (d) in the specific (meth)acrylic resin particles being 50.0 mass% or more relative to all structural units of the specific (meth)acrylic resin particles (excluding structural unit (e) in cases where the specific (meth)acrylic resin particles contain structural unit (e) derived from a reactive surfactant), means that the structural unit (d) is contained as a main component of the structural units of the resin that forms the specific (meth)acrylic resin particles.

<Other structural units>
The specific (meth)acrylic resin particles may, as necessary, contain structural units other than the structural unit (a), the structural unit (b), the structural unit (c), and the structural unit (d) (so-called other structural units), as long as the effects of the resin composition of the present disclosure are not impaired.

Another structural unit is, for example, structural unit (e) derived from a reactive surfactant. In this disclosure, a "structural unit derived from a reactive surfactant" refers to a structural unit formed by addition polymerization of a reactive surfactant, and a "reactive surfactant" refers to a surfactant having an ethylenically unsaturated double bond.

The type of reactive surfactant is not particularly limited.
The reactive surfactant may be an anionic reactive surfactant, a nonionic reactive surfactant, or a cationic reactive surfactant.
The reactive surfactant is preferably at least one selected from the group consisting of anionic reactive surfactants and nonionic reactive surfactants.

The reactive surfactant preferably has an oxyalkylene group.
Specific examples of the oxyalkylene group include an oxyethylene group, an oxypropylene group, and an oxybutylene group.
As the oxyalkylene group, for example, an oxyethylene group is preferred from the viewpoint of high reactivity with the monomer.

The average number of moles of oxyalkylene groups added is not particularly limited, but from the viewpoint of the dispersibility of the specific (meth)acrylic resin particles, it is preferably 5 to 50, and more preferably 10 to 30.

The reactive surfactant having an ethylenically unsaturated double bond can be obtained by adding a group having an ethylenically unsaturated double bond to a surfactant.
Specific examples of the group having an ethylenically unsaturated double bond include a (meth)acryloyl group, a vinyl group, an allyl group, an isopropenyl group, a 1-propenyl group, an allyloxy group, and a styryl group.
As the group having an ethylenically unsaturated double bond, for example, a 1-propenyl group or an allyloxy group is preferred from the viewpoint of high reactivity with the monomer.

As the reactive surfactant, commercially available products can be used.
Examples of commercially available reactive surfactants include "Aqualon (registered trademark) KH-10" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd. [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether sulfate ester ammonium [type of oxyalkylene group: oxyethylene group, average number of moles added: 10], active ingredient concentration: 99% by mass, anionic reactive surfactant], and "ADEKA REASOAP (registered trademark) SR-10" manufactured by ADEKA Corporation [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether sulfate ester ammonium [type of oxyalkylene group: oxyethylene group, average number of moles added: 10], active ingredient concentration: 100% by mass, anionic reactive surfactant] and "ADEKA REASOAP (registered trademark) ER-10" [active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [type of oxyalkylene group: oxyethylene group, average number of moles added: 10], active ingredient concentration: 100% by mass, nonionic reactive surfactant].

When the specific (meth)acrylic resin particles contain the structural unit (e), they may contain only one type of structural unit (e), or two or more types.

When the specific (meth)acrylic resin particles contain the structural unit (e), the content of the structural unit (e) is not particularly limited, but is preferably 0.1 to 10.0 parts by mass, and more preferably 2.0 to 6.0 parts by mass, per 100 parts by mass of all structural units of the specific (meth)acrylic resin particles (excluding the structural unit (e)).

<<Average particle size of specific (meth)acrylic resin particles>>
The average particle size of the specific (meth)acrylic resin particles is not particularly limited.
In the present disclosure, the term "average particle size" refers to the average primary particle size.
The average particle size of the specific (meth)acrylic resin particles is, for example, preferably 50 nm to 500 nm, more preferably 100 nm to 400 nm, and even more preferably 150 nm to 300 nm.
When the average particle size of the specific (meth)acrylic resin particles is 50 nm or more, the resulting impact sheet tends to have improved recovery in a humid and hot environment, presumably because a relatively high molecular weight (meth)acrylic resin is more likely to be formed inside the particles, and a crosslinking reaction is more likely to occur inside the particles.
When the average particle size of the specific (meth)acrylic resin particles is 500 nm or less, the resulting impact absorbing sheet tends to have improved recovery in a humid and hot environment. This is presumably because the increased contact area between the particles facilitates the progress of the crosslinking reaction between the particles, resulting in a dense crosslinked structure.

The average particle size of the specific (meth)acrylic resin particles can be adjusted to the desired value by adjusting, for example, the amount of surfactant used in polymerizing the (meth)acrylic resin, the polymerization reaction conditions (e.g., stirring speed, reaction temperature, and dropping rate), etc.

In the present disclosure, the "average particle size of resin particles" is a value measured by the dynamic light scattering method described in "New Experimental Chemistry Lectures 4, Basic Technology 3, Light (II)," edited by the Chemical Society of Japan, pp. 725-741 (published by Maruzen Co., Ltd. on July 20, 1976).
The specific method of dynamic light scattering is as follows: 5 mL of an aqueous dispersion of resin particles is collected in a 10 mm square glass cell using a Pasteur pipette, and this is set in a dynamic light scattering photometer (for example, Zetasizer 1000HS (trade name) from Sysmex Corporation). The attenuation factor (Attenuator) setting is set to x16 (16 times), and the concentration of the aqueous dispersion of resin particles is adjusted so that the attenuation factor Count Rate is 150 kCps to 200 kCps. After that, the results of measurements under conditions of a measurement temperature of 25°C ± 1°C and a light scattering angle of 90° are processed by computer to determine the average particle size of the resin particles in the aqueous dispersion. The Z-average value is used as the average particle size value.

<<Content of specific (meth)acrylic resin particles>>
The content of the specific (meth)acrylic resin particles in the resin composition of the present disclosure is not particularly limited, but is, for example, preferably 50.0 mass% to 99.8 mass%, more preferably 80.0 mass% to 99.0 mass%, and even more preferably 90.0 mass% to 98.0 mass%, relative to the total solid content in the resin composition.

In this disclosure, "total solid content in a resin composition" refers to the mass of the residue remaining after removing the solvent from the resin composition. In this disclosure, "solvent" refers to water and organic solvent. For example, if the only solvent contained in the resin composition is water, the solid content refers to the components other than water contained in the resin composition. If the solvent contained in the resin composition is water and an organic solvent, the solid content refers to the components other than water and the organic solvent contained in the resin composition.

[Method for producing specific (meth)acrylic resin particles]
The method for producing the specific (meth)acrylic resin particles is not particularly limited as long as it is possible to produce the above-described specific (meth)acrylic resin particles.
The specific (meth)acrylic resin particles can be produced by polymerizing the above-mentioned monomers.
The method for polymerizing the monomers is not particularly limited and can be appropriately selected from commonly used polymerization methods.
The specific (meth)acrylic resin particles are preferably produced by emulsion polymerization.

Examples of emulsion polymerization methods for producing specific (meth)acrylic resin particles include methods [1] to [3] shown below. Hereinafter, the monomers that form the structural units of the specific (meth)acrylic resin particles are also referred to as "monomer components."

[1] A method in which a monomer component, a reactive surfactant and/or a non-reactive surfactant, and water are charged into a reactor equipped with a thermometer, a stirrer, a raw material inlet pipe, a reflux condenser, a nitrogen inlet pipe, etc., and the temperature is raised while stirring under a nitrogen stream, and then a polymerization initiator, a reducing agent, etc. are appropriately added to allow the emulsion polymerization reaction to proceed (so-called bulk charging method).
[2] A method in which at least a reactive surfactant and/or a non-reactive surfactant and water are charged into a reactor equipped with a thermometer, a stirrer, a raw material inlet pipe, a reflux condenser, a nitrogen inlet pipe, etc., and the temperature is raised while stirring under a nitrogen stream, and then a monomer component is added dropwise, and a polymerization initiator, a reducing agent, etc. are added as appropriate to allow the emulsion polymerization reaction to proceed (so-called monomer dropping method).
[3] A reactive surfactant and/or non-reactive surfactant and water are charged into a reactor equipped with a thermometer, a stirrer, a raw material inlet pipe, a reflux condenser, a nitrogen inlet pipe, etc., and the temperature is raised while stirring under a nitrogen gas flow. Then, a polymerization initiator, a reducing agent, etc. are added as appropriate. Meanwhile, in a separate vessel, a monomer component is pre-emulsified using at least a reactive surfactant and/or non-reactive surfactant and water to prepare an emulsion of a mixture containing the monomer component and the reactive surfactant and/or non-reactive surfactant. The emulsion of the mixture containing the monomer component and the reactive surfactant and/or non-reactive surfactant is then added dropwise to the reactor, and a polymerization initiator, a reducing agent, etc. are added as appropriate to allow the emulsion polymerization reaction to proceed (the so-called emulsion monomer dropping method).
Among these, the emulsion polymerization method for producing the specific (meth)acrylic resin particles is preferably the emulsion monomer dropping method [3] above from the viewpoint of industrial productivity.

The specific (meth)acrylic resin particles obtained by the above emulsion polymerization method are obtained in the form of a dispersion in a medium containing at least water.

In the method for producing the specific (meth)acrylic resin particles, as the surfactant, only a reactive surfactant may be used, only a non-reactive surfactant may be used, or both a reactive surfactant and a non-reactive surfactant may be used.
Details of the reactive surfactant are as described above.

In this disclosure, "non-reactive surfactant" refers to a surfactant that does not have an ethylenically unsaturated double bond.
The type of non-reactive surfactant is not particularly limited.
The non-reactive surfactant is preferably at least one selected from the group consisting of anionic non-reactive surfactants and non-ionic non-reactive surfactants, and more preferably a combination of anionic non-reactive surfactants and non-ionic non-reactive surfactants.

Examples of anionic non-reactive surfactants include polyoxyalkylene polycyclic phenyl ether sulfate salts typified by polyoxyethylene distyrenated phenyl ether ammonium sulfate, polyoxyalkylene polycyclic phenyl ether sulfate salts typified by polyoxyethylene distyrenated phenyl ether ammonium sulfate, polyoxyalkylene alkyl phenyl ether sulfate salts typified by polyoxyethylene nonylphenyl ether sodium sulfate, polyoxyalkylene alkyl ether sulfate salts typified by polyoxyethylene lauryl ether sodium sulfate, polyoxyalkylene alkyl ether sulfate salts, and alkyl phosphate salts.
Examples of nonionic non-reactive surfactants include polyoxyalkylene alkyl ethers typified by polyoxyethylene oleyl ether and polyoxyethylene lauryl ether, and polyoxyalkylene styrenated phenyl ethers typified by polyoxyethylene styrenated phenyl ether.

As the non-reactive surfactant, commercially available products can be used.
Examples of commercially available anionic non-reactive surfactants include "Neopelex G-65" manufactured by Kao Corporation, "Hitenol NF-13" and "Hitenol NF-17" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., and "Newcol 707-SF" manufactured by Nippon Nyukazai Co., Ltd.
Examples of commercially available nonionic non-reactive surfactants include "EMULGEN 1135S-70" manufactured by Kao Corporation and "NOIGEN DKS NL-600F" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.
The above "Neopelex,""Hitenol,""Newcol,""Emulgen," and "Noigen" are all registered trademarks.

When a non-reactive surfactant is used in the method for producing a resin composition disclosed herein, only one type of non-reactive surfactant may be used, or two or more types may be used.

When a non-reactive surfactant is used in the method for producing a resin composition of the present disclosure, the amount of the non-reactive surfactant used is not particularly limited.
When a non-reactive surfactant is used, the lower limit of the amount of the non-reactive surfactant used is, for example, preferably 0.1 parts by mass or more, and more preferably 2 parts by mass or more, per 100 parts by mass of the total of the monomer components.
When a non-reactive surfactant is used, the upper limit of the amount of the non-reactive surfactant used is, for example, preferably 10 parts by mass or less, and more preferably 6 parts by mass or less, per 100 parts by mass of the total of the monomer components.

The total amount of the reactive surfactant and the non-reactive surfactant used is not particularly limited.
The lower limit of the total amount of reactive surfactant and non-reactive surfactant used is, for example, preferably 0.1 parts by mass or more, and more preferably 2 parts by mass or more, per 100 parts by mass of the total of the monomer components.
The upper limit of the total amount of reactive surfactant and non-reactive surfactant used is, for example, preferably 10 parts by mass or less, and more preferably 6 parts by mass or less, per 100 parts by mass of the total of the monomer components.

The polymerization initiator is not particularly limited as long as it can be used in ordinary emulsion polymerization.
Examples of the polymerization initiator include persulfates, organic peroxides, and azo compounds.
Specific examples of persulfates include ammonium persulfate, sodium persulfate, and potassium persulfate. Specific examples of organic peroxides include t-butyl hydroperoxide, benzoyl peroxide, lauroyl peroxide, caproyl peroxide, di-i-propyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and t-butyl peroxypivalate. Specific examples of azo compounds include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), and 2,2'-azobis(isobutyrate)dimethyl.

In the emulsion polymerization method for producing specific (meth)acrylic resin particles, only one type of polymerization initiator may be used, or two or more types may be used.

The polymerization initiator is used in an amount that is normally used.
The amount of the polymerization initiator used is preferably, for example, 0.1 to 2 parts by mass per 100 parts by mass of the total of the monomer components.

The reducing agent is not particularly limited as long as it can be used in ordinary emulsion polymerization.
Examples of reducing agents include sodium metabisulfite, sodium sulfite, sodium hydrogensulfite, sodium pyrosulfite (also called "sodium disulfite"), sodium hydroxymethanesulfinate, sodium pyrophosphate, thioglycolic acid, sodium thiosulfate, thiourea dioxide, L-ascorbic acid, tartaric acid, citric acid, and glucose.

In the emulsion polymerization method for producing specific (meth)acrylic resin particles, only one type of reducing agent may be used, or two or more types may be used.

The reducing agent is used in an amount normally used.
The amount of the reducing agent used is preferably, for example, 0.1 to 2 parts by mass per 100 parts by mass of the total of the monomer components.

The polymerization temperature is preferably, for example, 50°C to 70°C.
The polymerization time is preferably, for example, 4 to 8 hours.

In the above, emulsion polymerization was cited as an example of a method for producing specific (meth)acrylic resin particles, but the method for producing specific (meth)acrylic resin particles in the present disclosure is not limited to the above-mentioned emulsion polymerization, and for example, seed polymerization can also be used.

[Hydrazide-based crosslinking agents]
The resin composition of the present disclosure contains a hydrazide-based crosslinking agent.
In the present disclosure, the term "hydrazide crosslinking agent" refers to a compound having two or more hydrazide groups in one molecule.
The hydrazide-based crosslinking agent is a crosslinking agent that undergoes a crosslinking reaction with the aldehyde group and/or ketone group contained in the specific (meth)acrylic resin particles.

The type of hydrazide crosslinking agent is not particularly limited.
Specific examples of hydrazide crosslinking agents include adipic acid dihydrazide, oxalyl dihydrazide, glutaric acid dihydrazide, carbodihydrazide, malonic acid dihydrazide, sebacic acid dihydrazide, dodecanedioic acid dihydrazide, isophthalic acid dihydrazide, and succinic acid dihydrazide.
The hydrazide crosslinking agent is preferably an aliphatic hydrazide compound such as adipic acid dihydrazide, and more preferably adipic acid dihydrazide.

As the hydrazide crosslinking agent, commercially available products can be used.
Examples of commercially available hydrazide crosslinking agents include "ADH (adipic acid dihydrazide)" and "CDH (carbodihydrazide)" manufactured by Japan Finechem Co., Ltd.

The resin composition of the present disclosure may contain only one type of hydrazide-based crosslinking agent, or may contain two or more types.

The content of the hydrazide-based crosslinking agent in the resin composition of the present disclosure is 0.3 parts by mass to 1.0 part by mass per 100 parts by mass of the specific (meth)acrylic resin particles.
When the content of the hydrazide crosslinking agent in the resin composition of the present disclosure is 0.3 parts by mass or more relative to 100 parts by mass of the specific (meth)acrylic resin particles, the formed impact absorbing sheet tends to have excellent recovery properties in a humid and hot environment. The reason for this is presumably that the crosslinking reaction between the hydrazide crosslinking agent and the aldehyde groups and/or ketone groups of the specific (meth)acrylic resin particles is sufficiently carried out, thereby forming a dense crosslinked structure and increasing the elasticity of the impact absorbing sheet.
When the content of the hydrazide-based crosslinking agent in the resin composition of the present disclosure is 1.0 part by mass or less relative to 100 parts by mass of the specific (meth)acrylic resin particles, an impact-absorbing sheet with a highly uniform thickness tends to be obtained. The reason for this is presumably that shrinkage of the coating film during drying, which is caused by the inclusion of an excessive amount of the hydrazide-based crosslinking agent, is suppressed.

[Other crosslinking agents]
The resin composition of the present disclosure contains a crosslinking agent other than a hydrazide-based crosslinking agent (i.e., another crosslinking agent). In the resin composition of the present disclosure, the other crosslinking agent contributes to the formation of an impact-absorbing sheet that has excellent recovery properties in a humid and hot environment.

The other crosslinking agent is not particularly limited, but is preferably a crosslinking agent that undergoes a crosslinking reaction with carboxy groups. A crosslinking agent that undergoes a crosslinking reaction with carboxy groups forms a crosslinked structure by crosslinking with the carboxy groups of the specific (meth)acrylic resin particles, which tends to increase the elasticity of the impact absorbing sheet and improve its recovery in humid and hot environments.

Other crosslinking agents include, for example, oxazoline-based crosslinking agents, epoxy-based crosslinking agents, isocyanate-based crosslinking agents, and carbodiimide-based crosslinking agents.
All of these crosslinking agents undergo a crosslinking reaction with a carboxy group.
In the present disclosure, the term "oxazoline-based crosslinking agent" refers to a compound having two or more oxazoline groups in one molecule, the term "epoxy-based crosslinking agent" refers to a compound having two or more epoxy groups in one molecule, the term "isocyanate-based crosslinking agent" refers to a compound having two or more isocyanate groups in one molecule, and the term "carbodiimide-based crosslinking agent" refers to a compound having two or more carbodiimide groups or cyanamide groups in one molecule. The cyanamide group is a group that is tautomeric with the carbodiimide group.

The other crosslinking agent preferably includes at least one selected from the group consisting of oxazoline-based crosslinking agents, epoxy-based crosslinking agents, and isocyanate-based crosslinking agents, and more preferably an oxazoline-based crosslinking agent.

The oxazoline group contained in the oxazoline-based crosslinking agent may be any of a 2-oxazoline group, a 3-oxazoline group, and a 4-oxazoline group.
Examples of the oxazoline crosslinking agent include polymer compositions containing structural units derived from compounds such as 2-vinyl-2-oxazoline, 5-methyl-2-vinyl-2-oxazoline, 4,4-dimethyl-2-vinyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 4,4-dimethyl-2-isopropenyl-2-oxazoline, 4-acryloyloxymethyl-2,4-dimethyl-2-oxazoline, 4-methacryloyloxymethyl-2,4-dimethyl-2-oxazoline, 4-methacryloyloxymethyl-2-phenyl-4-methyl-2-oxazoline, 2-(4-vinylphenyl)-4,4-dimethyl-2-oxazoline, 4-ethyl-4-hydroxymethyl-2-isopropenyl-2-oxazoline, and 4-ethyl-4-carbethoxymethyl-2-isopropenyl-2-oxazoline.

Commercially available examples of oxazoline-based crosslinking agents include "Epocross WS-300," "Epocross WS-500," and "Epocross WS-700" manufactured by Nippon Shokubai Co., Ltd. "Epocross" is a registered trademark.

Examples of epoxy crosslinking agents include ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, tripropylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol diglycidyl ether, polytetramethylene glycol diglycidyl ether, glycerol diglycidyl ether, glycerol triglycidyl ether, diglycerol polyglycidyl ether, and polyglycerol. Examples of compounds include glycerol polyglycidyl ether, resorcinol diglycidyl ether, 2,2-dibromoneopentyl glycol diglycidyl ether, trimethylolpropane triglycidyl ether, pentaerythritol polyglycidyl ether, sorbitol polyglycidyl ether, adipic acid diglycidyl ester, phthalic acid diglycidyl ester, tris(glycidyl)isocyanurate, tris(glycidoxyethyl)isocyanurate, 1,3-bis(N,N-glycidylaminomethyl)cyclohexane, and N,N,N',N'-tetraglycidyl-1,3-benzenedi(methanamine).

As the epoxy-based crosslinking agent, commercially available products can be used.
An example of a commercially available epoxy crosslinking agent is "Denacol EX-810" manufactured by Nagase ChemteX Corp. "Denacol" is a registered trademark.

Examples of the isocyanate crosslinking agent include aliphatic polyisocyanate compounds, alicyclic polyisocyanate compounds, and aromatic polyisocyanate compounds.
"Aliphatic polyisocyanate compounds" include, for example, aliphatic polyisocyanate compounds, polymers of aliphatic polyisocyanate compounds, adducts of aliphatic polyisocyanate compounds and polyol compounds (e.g., trimethylolpropane (TMP); the same applies hereinafter), and biuret compounds of aliphatic polyisocyanate compounds. Specific examples of aliphatic polyisocyanate compounds include hexamethylene diisocyanate (HMDI), pentamethylene diisocyanate (PDI), tetramethylene diisocyanate, trimethylhexamethylene diisocyanate, and lysine diisocyanate.
The "alicyclic polyisocyanate compound" includes, for example, an alicyclic polyisocyanate compound, a polymer of an alicyclic polyisocyanate compound, an adduct of an alicyclic polyisocyanate compound and a polyol compound, and a biuret of an alicyclic polyisocyanate compound. Specific examples of the alicyclic polyisocyanate compound include isophorone diisocyanate (IPDI), hydrogenated tolylene diisocyanate, hydrogenated xylene diisocyanate, hydrogenated 4,4'-diphenylmethane diisocyanate, and 4,4'-dicyclohexylmethane diisocyanate.
The "aromatic polyisocyanate compound" includes, for example, an aromatic polyisocyanate compound, a polymer of an aromatic polyisocyanate compound, an adduct of an aromatic polyisocyanate compound and a polyol compound, and a biuret of an aromatic polyisocyanate compound. Specific examples of the aromatic polyisocyanate compound include tolylene diisocyanate (TDI), xylylene diisocyanate (XDI), and 4,4'-diphenylmethane diisocyanate.

An example of a commercially available isocyanate-based crosslinking agent is "Takenate WD-725" manufactured by Mitsui Chemicals, Inc. "Takenate" is a registered trademark.

The resin composition of the present disclosure may contain only one type of other crosslinking agent, or may contain two or more types.

The content of the other crosslinking agent in the resin composition of the present disclosure is not particularly limited, but is preferably 0.5 to 10 parts by mass, more preferably 2 to 8 parts by mass, and even more preferably 2 to 6 parts by mass, per 100 parts by mass of the specific (meth)acrylic resin particles.

[Aqueous medium]
The resin composition of the present disclosure may contain an aqueous medium.
The aqueous medium is not particularly limited and can be appropriately selected depending on the purpose.
Examples of aqueous media include water and water-miscible organic solvents.
Examples of water-miscible organic solvents include monohydric alcohol compounds such as methanol, ethanol, propanol, and isopropanol; polyhydric alcohol compounds such as glycerin, ethylene glycol, diethylene glycol, and propylene glycol; and glycol derivatives such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, and dipropylene glycol monomethyl ether.
The aqueous medium is preferably water from the viewpoint of productivity, for example.

When the resin composition of the present disclosure contains an aqueous medium, the content of the aqueous medium is not particularly limited.
The content of the aqueous medium in the resin composition of the present disclosure is preferably 20% by mass or more, and more preferably 30% by mass or more, relative to the total mass of the resin composition, for example, from the viewpoint of coatability.
The content of the aqueous medium in the resin composition of the present disclosure is preferably 80 mass% or less, and more preferably 70 mass% or less, relative to the total mass of the resin composition, for example, from the viewpoint of the drying properties of the coating film and the coatability.
In one embodiment, the content of the aqueous medium in the resin composition of the present disclosure may be, for example, 20% by mass to 80% by mass, or 30% by mass to 70% by mass, relative to the total mass of the resin composition.

[Other ingredients]
The resin composition of the present disclosure may contain components other than the components described above (so-called other components) as needed, as long as the effects of the invention are not impaired.
Examples of other components include various additives such as non-reactive surfactants, antioxidants, antistatic agents, pH adjusters, antifoaming agents, and preservatives.

[Method of producing resin composition]
The method for producing the resin composition of the present disclosure is not particularly limited as long as it can produce the resin composition of the present disclosure described above. The resin composition of the present disclosure can be produced, for example, by mixing (meth)acrylic resin particles (i.e., specific (meth)acrylic resin particles) that contain a structural unit (a) derived from a vinyl cyanide monomer, a structural unit (b) derived from a monomer having a carboxy group, and a structural unit (c) derived from a monomer having at least one of an aldehyde group and a ketone group, the content of the structural unit (b) being 1.0% by mass to 4.0% by mass of all structural units, and the content of the structural unit (c) being 1.0% by mass to 2.0% by mass of all structural units, with 0.3 parts by mass to 1.0 parts by mass of a hydrazide crosslinking agent and another crosslinking agent per 100 parts by mass of the (meth)acrylic resin particles.
The method for producing the specific (meth)acrylic resin particles is as described above.

The method for producing a resin composition according to the present disclosure includes a step of mixing specific (meth)acrylic resin particles, a hydrazide crosslinking agent, and another crosslinking agent (also referred to as a "mixing step"). The mixing step may be a step of mixing a dispersion of the specific (meth)acrylic resin particles, a hydrazide crosslinking agent, and another crosslinking agent.
The mixing step may be a step of mixing specific (meth)acrylic resin particles, a hydrazide crosslinking agent, and another crosslinking agent all at once, a step of mixing a mixture of specific (meth)acrylic resin particles and a hydrazide crosslinking agent with another crosslinking agent, or a step of mixing specific (meth)acrylic resin particles with a mixture of a hydrazide crosslinking agent and another crosslinking agent. In the present disclosure, the "mixture of specific (meth)acrylic resin particles and a hydrazide crosslinking agent" and the "mixture of a hydrazide crosslinking agent and another crosslinking agent" may each independently be in a state where the mixed substances exist without reacting with each other, or in a state where some of the mixed substances have reacted, or in a state where all of the mixed substances have reacted.
From the viewpoint of pot life, the mixing step is preferably a step of mixing a mixture of the specific (meth)acrylic resin particles and the hydrazide crosslinking agent with another crosslinking agent.

The mixing method is not particularly limited, and any known mixing method can be used.
The mixing method may be, for example, a method of mixing by stirring.

[Shock absorbing sheet]
The impact absorbing sheet of the present disclosure is a sheet formed from the resin composition of the present disclosure.
The impact absorbing sheet of the present disclosure is a sheet formed from the resin composition of the present disclosure, and therefore has excellent restoration properties in a humid and hot environment and excellent adhesion to a substrate in a high humidity environment.

The thickness of the impact absorbing sheet of the present disclosure is not particularly limited, but is preferably 50 μm to 300 μm, more preferably 70 μm to 200 μm, and even more preferably 80 μm to 150 μm.

In the present disclosure, the "thickness of the impact absorbing sheet" means the average thickness of the impact absorbing sheet.
The average thickness of the impact absorbing sheet is a value determined by the following method.
The thickness of the impact absorbing sheet is measured at 10 randomly selected locations in the thickness direction using a film thickness meter. The arithmetic mean value of the measured values is calculated and used as the average thickness of the impact absorbing sheet.

The method for producing the impact absorbing sheet of the present disclosure is not particularly limited, and a commonly used method can be adopted. The impact absorbing sheet of the present disclosure can be produced, for example, by the method shown below.
The resin composition of the present disclosure is applied to a release sheet to form a coating film on the release sheet. The formed coating film is then dried to produce an impact absorbing sheet with a release sheet. Next, another release sheet is attached to the exposed surface of the impact absorbing sheet.
The release sheet protects both sides (front and back) of the impact absorbing sheet until the impact absorbing sheet is put into practical use, and is peeled off when the impact absorbing sheet is used, and does not constitute the impact absorbing sheet of the present disclosure.

The release sheet is not particularly limited as long as it can be easily peeled off from the impact absorbing sheet.
Examples of release sheets include resin films, paper, synthetic paper, and composite sheets made by laminating two or more of these, each of which has been surface-treated with a release agent on one or both sides (so-called easy-release treatment).
Examples of release agents include silicone-based release agents (e.g., silicone), wax-based release agents (e.g., paraffin wax), and fluorine-based release agents (e.g., fluorine-based resins).
Examples of resin films include polyester films such as polyethylene terephthalate (PET) films.
Examples of paper include fine paper and coated paper.
The thickness of the release sheet is not particularly limited, but is generally 20 μm to 180 μm.

The method for applying the resin composition is not particularly limited.
Examples of methods for applying the resin composition include known methods using a doctor blade, a wire bar, or the like.
The amount of the resin composition to be applied is not particularly limited, and is set appropriately depending on, for example, the thickness of the impact absorbing sheet to be formed.

The method for drying the coating film is not particularly limited.
Examples of methods for drying the coating film include natural drying, heat drying, hot air drying, and vacuum drying.
The drying temperature and drying time of the coating film are not particularly limited, and are set appropriately depending on the thickness of the coating film, the amount of aqueous solvent in the coating film, and the like.
An example of the drying conditions is drying at 80° C. for 10 minutes using a hot air circulation dryer.

The resin composition of the present disclosure will be described in more detail below with reference to examples. The present disclosure is not limited to the following examples as long as it does not deviate from the gist of the disclosure.
In the examples, the average particle size of the (meth)acrylic resin particles was measured by the dynamic light scattering method described above, using the same measuring device as that described in the examples.

[Production of (meth)acrylic resin particles]
[Manufacture example A-1]
In a stainless steel container, 14.3 parts by mass of ion-exchanged water, 1.0 parts by mass (active ingredient equivalent) of an anionic non-reactive surfactant [trade name: Neopelex G-65, manufactured by Kao Corporation], and 4.0 parts by mass (active ingredient equivalent) of a non-ionic non-reactive surfactant [trade name: Emulgen 1135S-70, manufactured by Kao Corporation] were mixed to prepare a non-reactive surfactant solution. In a separate stainless steel container, 42.0 parts by mass of ethyl acrylate (EA), 41.9 parts by mass of n-butyl acrylate (n-BA), 8.6 parts by mass of methyl methacrylate (MMA), 5.0 parts by mass of acrylonitrile (AN), 1.5 parts by mass of acrylic acid (AA), and 1.0 part by mass of diacetone acrylamide (DAAM) were mixed to prepare a monomer mixture. The monomer mixture prepared above was gradually added to the non-reactive surfactant solution prepared above while stirring with a stirrer, thereby preparing an emulsion containing the monomer components.
Next, a flask equipped with a reflux condenser, a stirrer, a nitrogen gas inlet pipe, a pump for dropping the emulsion (product name: Hicera Pump V-10, manufactured by Iwaki Corporation), and a pump for dropping the polymerization initiator (product name: Metering Pump MP-2000, manufactured by Tokyo Rikakikai Co., Ltd.) was prepared as a polymerization reaction apparatus.
26.9 parts by mass of ion-exchanged water was placed in a flask of a polymerization reaction apparatus. Nitrogen was then blown into the flask at a flow rate of 300 mL/min, and the ion-exchanged water was stirred at a stirring speed of 240 revolutions per minute (rpm). Under a nitrogen stream, 3.0% by mass of the emulsion prepared above (also referred to as the "first emulsion"), 2.0 parts by mass of a 5.0% by mass aqueous solution of ammonium persulfate (polymerization initiator), and 0.8 parts by mass of a 10.0% by mass aqueous solution of sodium metabisulfite (reducing agent) were added to the stirred ion-exchanged water. The internal temperature of the flask was then raised to 60°C. After the internal temperature of the flask stabilized at 60°C, the nitrogen flow rate was changed to 50 mL/min, and the remaining entire amount of the emulsion prepared above (also referred to as the "second emulsion") was added dropwise to the flask over 5 hours. In parallel with the dropwise addition of the second emulsion, 8.0 parts by mass of a 2.4% by mass aqueous solution of ammonium persulfate (polymerization initiator) and 8.0 parts by mass of a 2.0% by mass aqueous solution of sodium metabisulfite (reducing agent) were added dropwise over 5.5 hours. The liquid obtained after the dropwise addition was cooled 2.5 hours after the completion of the dropwise addition of the polymerization initiator and the reducing agent, thereby completing the polymerization reaction. After the completion of the polymerization reaction, an aqueous dispersion of acrylic resin particles A-1 was obtained.

[Production Examples A-2 to A-12 and A-16 to A-21]
In Production Examples A-2 to A-12 and A-16 to A-21, the same operation as in Production Example A-1 was carried out except that the composition of the (meth)acrylic resin particles was changed to the composition shown in Table 1, thereby obtaining aqueous dispersions of (meth)acrylic resin particles A-2 to A-12 and A-16 to A-21.

[Manufacture example A-13]
In Production Example A-13, the same operation as in Production Example A-1 was performed, except that the composition of the (meth)acrylic resin particles was changed to the composition shown in Table 1 and the non-reactive surfactant shown in Table 1 was not used, to obtain an aqueous dispersion of (meth)acrylic resin particles A-13.
In Production Example A-13, a reactive surfactant was used in place of a non-reactive surfactant in Production Example 1.

[Production Examples A-14 and A-15]
In Production Examples A-14 and A-15, the composition of the (meth)acrylic resin particles was changed to the composition shown in Table 1, and the type and amount of the reactive surfactant was changed to the type and amount shown in Table 1. The same operation as in Production Example A-1 was performed to obtain aqueous dispersions of (meth)acrylic resin particles A-14 and A-15.
In Production Examples A-14 and A-15, a reactive surfactant was used in the same manner as a non-reactive surfactant in Production Example 1.

The compositions (unit: parts by mass) of (meth)acrylic resin particles A-1 to A-21 and the average particle diameters (unit: nm) of (meth)acrylic resin particles A-1 to A-21 are shown in Table 1. In addition, if a non-reactive surfactant was used in producing the (meth)acrylic resin particles, the type and amount of the non-reactive surfactant used are also shown in Table 1.

Of the (meth)acrylic resin particles A-1 to A-21 obtained above, (meth)acrylic resin particles A-1 to A-6, A-8 to A-10, and A-12 to A-17 correspond to the specific (meth)acrylic resin particles of the present disclosure.

Details of the components listed in Table 1 are as follows:
In Table 1, for the sake of convenience, "vinyl cyanide monomer" is represented as "(a),""monomer having a carboxy group" is represented as "(b),""monomer having at least one of an aldehyde group and a ketone group" is represented as "(c),""(meth)acrylic acid alkyl ester monomer" is represented as "(d)," and "reactive surfactant" is represented as "(e)."

(a) vinyl cyanide monomer "AN": acrylonitrile (b) monomer having a carboxy group "AA": acrylic acid "IA": itaconic acid (c) monomer having at least one of an aldehyde group and a ketone group "DAAM": diacetone acrylamide (d) (meth)acrylic acid alkyl ester monomer "EA": ethyl acrylate "n-BA": n-butyl acrylate "MMA": methyl methacrylate (e) reactive surfactant - anionic reactive surfactant -
"KH-10" [trade name: Aqualon (registered trademark) KH-10, active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether sulfate ester ammonium [type of oxyalkylene group: oxyethylene group, average number of moles added: 10], active ingredient concentration: 99% by mass, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.]
"SR-10" [Product name: ADEKA REASOAP (registered trademark) SR-10, active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether sulfate ester ammonium [type of oxyalkylene group: oxyethylene group, average number of moles added: 10], active ingredient concentration: 100% by mass, manufactured by ADEKA Corporation]
-Nonionic reactive surfactant-
"ER-10" [Product name: ADEKA REASOAP (registered trademark) ER-10, active ingredient: polyoxyethylene-1-(allyloxymethyl) alkyl ether [type of oxyalkylene group: oxyethylene group, average number of moles added: 10], active ingredient concentration: 100% by mass, manufactured by ADEKA Corporation]

<Non-reactive surfactant>
-Anionic non-reactive surfactant-
"G-65" [product name: Neopelex (registered trademark) G-65, active ingredient: sodium dodecylbenzenesulfonate, active ingredient concentration: 65% by mass, manufactured by Kao Corporation]
-Nonionic non-reactive surfactant-
"1135S-70" [product name: Emulgen (registered trademark) 1135S-70, active ingredient: polyoxyethylene alkyl ether, active ingredient concentration: 70% by mass, manufactured by Kao Corporation]

The amounts of the non-reactive surfactant and the reactive surfactant shown in Table 1 are both values calculated as active ingredients.
In Table 1, "-" means that the component in that column was not used.

[Preparation of Resin Composition]
Example 1
To 100 parts by mass of the (meth)acrylic resin particles A-1, 0.5 parts by mass (solid content equivalent) of a hydrazide crosslinking agent [trade name: ADH, manufactured by Nippon Finechem Co., Ltd.] and 2.1 parts by mass (solid content equivalent) of an oxazoline crosslinking agent [trade name: EPOCROS WS-500, manufactured by Nippon Shokubai Co., Ltd.] were added, and the mixture was mixed using a disper at a rotation speed of 500 rpm for 10 minutes to obtain the resin composition of Example 1.

[Examples 2 to 19 and Comparative Examples 1 to 8]
In Examples 2 to 19 and Comparative Examples 1 to 8, the resin compositions of Examples 2 to 19 and Comparative Examples 1 to 8 were obtained by the same operation as in Example 1, except that the composition of the resin composition was changed to the composition shown in Table 2.

[Measurement and Evaluation]
1. Restorability in a humid and hot environment The resin composition prepared above was applied to one side of a release sheet using a doctor blade so that the thickness of the dried film (i.e., impact absorbing sheet) would be approximately 100 μm, forming a coating film. The formed coating film was then dried using a hot air circulation dryer at a drying temperature of 80° C. for 10 minutes to produce an impact absorbing sheet with a release sheet. Eight impact absorbing sheets with a release sheet were prepared for each resin composition.
Seven impact absorbing sheets from which the release sheets had been removed were placed on the impact absorbing sheet surface of one impact absorbing sheet with a release sheet, and the resultant was cut into a circle with a diameter of 25 mm to prepare a test piece.
The test specimen was placed in a thermo-hygrostat with an ambient temperature of 60°C and humidity of 90% RH and allowed to stand for 90 hours. The test specimen was removed from the thermo-hygrostat, and after 30 minutes, the thickness of the test specimen was measured using a film thickness meter (product name: Dual Type Film Thickness Gauge LZ-990, manufactured by Kett Electric Laboratory Co., Ltd.). After the thickness measurement, the test specimen was placed in a tacking tester (product name: TAC1000, manufactured by Rhesca Co., Ltd.), and a probe (diameter: 5 mm) was pressed against the impact-absorbing sheet surface of the test specimen at a speed of 0.1 mm/sec until a load of 4 kg was reached. After the load reached 4 kg, the probe was retracted at a speed of 0.01 mm/sec, and the dent depth (so-called recovery distance) when the load reached 0 kg was measured and recorded. The recovery rate (unit: %) was calculated from the obtained recovery distance and the thickness of the test specimen based on the following formula. The recovery rate values are shown in Table 2.
In this evaluation test, if the recovery rate of the test piece was 35% or less, it was determined that the impact absorbing sheet had excellent recovery properties in a humid and hot environment.

Recovery rate (%) = recovery distance (μm) ÷ specimen thickness (μm) × 100

2. Adhesion to Substrate in High Humidity Environment The resin composition prepared above was applied to one side of a 100 mm x 150 mm stainless steel plate (SUS plate) using a doctor blade so that the thickness of the dried film (i.e., impact-absorbing sheet) would be approximately 100 μm, forming a coating film. The formed coating film was then dried using a hot air circulation dryer at a drying temperature of 80°C for 10 minutes. The resulting SUS plate with impact-absorbing sheet was used as an evaluation sample. The evaluation sample was immersed in water at 30°C for 1 day and then removed from the water. The appearance of the evaluation sample after removal was visually observed and evaluated according to the following evaluation criteria. The evaluation results are shown in Table 2. If the evaluation result was "A," it was determined that the impact-absorbing sheet had excellent adhesion to the substrate in a high-humidity environment.

-Evaluation criteria-
A: No lifting or peeling of the impact absorbing sheet from the SUS plate was observed.
B: Lifting and/or peeling of the impact absorbing sheet from the SUS plate was observed.

Details of the crosslinking agents listed in Table 2 are as follows:
<Hydrazide-based crosslinking agents>
"ADH" (trade name, chemical name: adipic acid dihydrazide, manufactured by Nippon Finechem Co., Ltd.)
<Other crosslinking agents>
"WS-500" (product name: Epocross (registered trademark) WS-500, oxazoline-based crosslinking agent, manufactured by Nippon Shokubai Co., Ltd.)
"EX-810" (trade name: Denacol (registered trademark) EX-810, epoxy crosslinking agent, manufactured by Nagase ChemteX Corporation)
"WD-725" (trade name: Takenate (registered trademark) WD-725, isocyanate-based crosslinking agent, manufactured by Mitsui Chemicals, Inc.)

In Table 2, a "-" in the resin composition column means that the component in that column is not used.

As shown in Table 2, it was confirmed that the impact absorbing sheets formed from the resin compositions of Examples 1 to 19 have excellent recovery properties in a humid and hot environment and excellent adhesion to the substrate in a high humidity environment.
On the other hand, it was confirmed that the impact absorbing sheets formed from the resin compositions of Comparative Examples 1 to 8 were inferior to the impact absorbing sheets formed from the resin compositions of the Examples in at least one of their resilience in a humid and hot environment and their adhesion to the substrate in a high humidity environment.

Claims (10)

(meth)acrylic resin particles comprising a structural unit (a) derived from a vinyl cyanide monomer, a structural unit (b) derived from a monomer having a carboxy group, and a structural unit (c) derived from a monomer having at least one of an aldehyde group and a ketone group, wherein the content of the structural unit (a) is 5.0% by mass to 30.0% by mass, the content of the structural unit (b) is 1.0% by mass to 4.0% by mass, and the content of the structural unit (c) is 1.0% by mass to 2.0% by mass, based on all structural units;
a hydrazide crosslinking agent;
a crosslinking agent other than the hydrazide-based crosslinking agent;
Including,
The resin composition for impact absorbing sheets has a content of the hydrazide-based crosslinking agent of 0.3 parts by mass to 1.0 parts by mass relative to 100 parts by mass of the (meth)acrylic resin particles. The resin composition for impact absorbing sheets according to claim 1, wherein the vinyl cyanide monomer is acrylonitrile. The resin composition for impact absorbing sheets according to claim 1, wherein the monomer having at least one of an aldehyde group and a ketone group is a (meth)acrylamide derivative. The resin composition for impact absorbing sheets according to claim 1, wherein the monomer having at least one of an aldehyde group and a ketone group is diacetone acrylamide. The resin composition for impact absorbing sheets according to claim 1, wherein the hydrazide-based crosslinking agent is an aliphatic hydrazide-based compound. The resin composition for impact absorbing sheets according to claim 1, wherein the hydrazide-based crosslinking agent is adipic acid dihydrazide. The resin composition for impact absorbing sheets according to claim 1, wherein the crosslinking agent other than a hydrazide-based crosslinking agent includes at least one selected from the group consisting of an oxazoline-based crosslinking agent, an epoxy-based crosslinking agent, and an isocyanate-based crosslinking agent. The resin composition for impact absorbing sheets according to claim 1, wherein the average particle diameter of the (meth)acrylic resin particles is 50 nm to 500 nm. A method for producing a resin composition for an impact-absorbing sheet, the method comprising: mixing (meth)acrylic resin particles which contain a structural unit (a) derived from a vinyl cyanide monomer, a structural unit (b) derived from a monomer having a carboxy group, and a structural unit (c) derived from a monomer having at least one of an aldehyde group and a ketone group, the content of said structural unit (a) being 5.0% by mass to 30.0% by mass , the content of said structural unit (b) being 1.0% by mass to 4.0% by mass, and the content of said structural unit (c) being 1.0% by mass to 2.0% by mass, with a mixture of a hydrazide-based crosslinking agent in an amount of 0.3 parts by mass to 1.0 parts by mass per 100 parts by mass of the (meth)acrylic resin particles; and a crosslinking agent other than the hydrazide-based crosslinking agent. An impact absorbing sheet formed from the resin composition for impact absorbing sheets described in any one of claims 1 to 8.