JP7743169B2 - Resin particles for conductive fine particles - Google Patents

Description

The present invention relates to resin particles for use as conductive microparticles.

Conventionally, when assembling electronic devices, a connection method using anisotropic conductive materials has been adopted to establish electrical connections between numerous opposing electrodes and wiring. Anisotropic conductive materials are materials in which conductive particles are mixed with a binder resin or the like, and examples include anisotropic conductive paste (ACP), anisotropic conductive film (ACF), anisotropic conductive ink, and anisotropic conductive sheet.

Conductive microparticles used in anisotropic conductive connections generally have a core (substrate) of resin particles, with a conductive metal layer such as nickel formed on the surface of the core, due to their excellent compressive deformation properties.

In recent years, in particular, as electronic components have become smaller and more densely mounted, there has been a trend toward thinner electrodes, which are the connected media, and the substrates and films that hold the electrodes. Applying excessive pressure during mounting can result in damage to the substrate, etc. Therefore, there is a demand for anisotropic conductive materials that can be used for low-pressure mounting, and various attempts have been made to obtain conductive particles that can be easily deformed without the application of excessive pressure.

For example, Patent Document 1 discloses that when the compressive modulus of elasticity of the resin particles is set to 100 to 3000 N/ ^mm2 when the diameter of the resin particles is displaced by 10%, and the recovery rate after compression of the resin particles is measured in a range of 10 to 60% of the diameter, there exists a compression rate at which the recovery rate after compression is 0.0%, thereby obtaining conductive particles that can achieve low resistance even when pressurized and connected at low pressure.

Patent Document 2 discloses that conductive microparticles having excellent softness and reduced inter-particle variation in mechanical properties can be obtained by using resin particles, which are base particles, as core-shell particles, setting the glass transition temperature (Tg _core ) of the non-crosslinkable vinyl monomer A, which is a constituent unit of the core, to less than 40°C, setting the glass transition temperature (Tg _shell ) of the non-crosslinkable vinyl monomer B, which is a constituent unit of the shell, to be higher than the glass transition temperature (Tg _core ), and controlling the degree of crosslinking of the shell to be the same as or higher than the degree of crosslinking of the core.

Patent document 3 discloses that by controlling the compressive modulus (10% K value) when the polymer microparticles are displaced by 10% of their particle diameter to 10 to 250 kgf/ ^mm2 , the compressive deformation recovery rate to 30% or more, and the breaking strain to 30% or more, conductive microparticles with flexibility that will not damage substrates and their wiring can be obtained.

Patent Document 4 discloses that conductive fine particles that are flexible yet resistant to aggregation and coalescence can be obtained by controlling the Tg of the shell portion of the fine particles so that it changes continuously or discontinuously from near the core portion toward the outermost layer of the shell portion, so that the Tg near the core portion is 0°C or less and the Tg near the outermost layer of the shell portion is 0° ^C or ^more , and the K value at 10% compressive deformation is 1.47 x 109 N/m2 or less.

JP 2014-96329 A JP 2014-63673 A Japanese Patent Application Laid-Open No. 2000-319309 Japanese Patent Application Laid-Open No. 2002-363205

However, while softened resin particles such as those described in Patent Documents 1 and 2 can ensure low resistance even when the pressure during pressure connection is relatively low, there are cases where the initial connection resistance cannot be kept sufficiently low when the pressure during pressure connection is low, for example, 2.5 MPa or less. Furthermore, Patent Documents 3 and 4 do not consider connection resistance at all.

The present invention was made in consideration of the above circumstances, and its purpose is to provide resin particles for conductive microparticles that can achieve low resistance even when connected under pressure at low pressure.

As a result of intensive research conducted by the present inventors to solve the above problems, they found that if the resin particles are core-shell particles having a core and a shell covering the core, and the resin particles are controlled so that the compressive deformation recovery rate is 25% or less, the compressive modulus (10% K value) when the diameter is displaced by ¹⁰ % is 1500 N/mm2 or less, and the coefficient of variation (CV) of the particle diameter is 10% or less, the resin particles have excellent dispersibility and suppressed aggregation, and furthermore, they can easily deform even at low pressure, ensuring a large contact area with the connected medium, thereby realizing low resistance even when pressure-connected at low pressure, and thus completed the present invention.

That is, the resin particles according to the present invention are resin particles for use as conductive microparticles, and are core-shell particles having a core and a shell covering the core, characterized in that the compressive deformation recovery rate of the resin particles is 25% or less, the compressive elastic modulus (10% K value) when the diameter of the resin particles is displaced by 10% is 1500 N/mm2 or ^less , and the coefficient of variation (CV) of the particle diameter of the resin particles is 10% or less.

In the resin particles of the present invention, the core is preferably a copolymer containing, as monomer units, a silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) above 0°C and a silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) of 0°C or lower.

In the resin particles of the present invention, the content of the silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) of 0°C or less is preferably 15% by mass or more and 50% by mass or less, based on 100% by mass of the total amount of monomers forming the core.

In the resin particles of the present invention, the content of the silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) above 0°C is preferably 35% by mass or more and 70% by mass or less, based on 100% by mass of the total amount of monomers forming the core.

In the resin particles of the present invention, it is preferable that the glass transition temperature (Tgcore) of the core calculated based on the FOX equation is 0°C or lower.

In the resin particles of the present invention, the core preferably has a degree of crosslinking represented by the following formula of 30% or less.
Degree of crosslinking (%) = (mass of crosslinkable monomer/mass of total monomer) x 100

In the resin particles of the present invention, it is preferable that the average thickness of the shell be 0.05 μm or more and 1 μm or less.

In the resin particles of the present invention, the ratio of the average shell thickness to the number-average particle diameter of the resin particles is preferably 0.01 or more and 0.05 or less.

In the resin particles of the present invention, the dispersion ratio calculated by the following measurement method is preferably 90% or more.
(Method for measuring dispersion ratio)
(1) 2.5 g of resin particles and 100 g of a 1% aqueous solution of polyoxyethylene alkyl ether sulfate ester ammonium salt, which is an emulsifier, are placed in a 200 mL beaker and ultrasonically dispersed for 15 minutes, and then passed through a sieve with openings 1.6 μm times the number average particle diameter.
(2) Dry the sieve at 120°C for 2 hours.
(3) The total weight (g) of the sieve and the polymer particles remaining on the sieve is measured, and the weight (g) of the sieve is subtracted to determine the weight (g) of the polymer particles remaining on the sieve.
(4) The weight (g) of the polymer particles remaining on the sieve is applied to the following formula to calculate the dispersion rate (%).
Dispersion rate (%)=100−((weight of polymer particles remaining on the sieve (g)/2.5 (g))×100)

According to the present invention, the resin particles are core-shell particles having a core and a shell covering the core, and the resin particles are controlled so that the compressive deformation recovery rate is 25% or less, the compressive modulus (10% K value) when the diameter is displaced by 10% is 1500 N/mm2 or ^less , and the coefficient of variation (CV) of the particle diameter is 10% or less.Therefore, by using these resin particles as a base material for conductive microparticles, it is possible to achieve low resistance even when the particles are connected under pressure at low pressure.

1. Resin particles (base material)
The resin particles for conductive fine particles of the present invention have a core and a shell that covers the core.

The resin particles have a compressive deformation recovery rate (hereinafter sometimes simply referred to as "recovery rate") of 25% or less. Here, the compressive deformation recovery rate is determined by applying the maximum displacement L1 (μm) when the resin particles are compressed to the maximum load (49 mN) and the recovered displacement L2 (μm) when the load is reduced from the maximum load to the minimum load (0.49 mN) to the following formula, and the detailed measurement method will be described in the Examples.
Recovery rate (%) = (L2/L1) x 100

By setting the recovery rate to 25% or less, the flexibility of the resin particles is improved, making them more susceptible to deformation even at low pressures of, for example, 2.5 MPa or less. This increases the contact area with the connected medium, thereby reducing resistance even when pressure-connected at low pressures. The recovery rate of the resin particles is preferably 23% or less, more preferably 21% or less, and even more preferably 18% or less. On the other hand, if the recovery rate is too low, for example, when conductive microparticles based on resin particles are used as an anisotropic conductive material, the repulsive force in the connected state may be too small, resulting in an increase in the initial resistance value. Therefore, the recovery rate of the resin particles is preferably 1% or more, more preferably 2% or more, and even more preferably 3% or more.

The resin particles have a compressive modulus of elasticity (10% K value) of 1500 N/mm ² or less when the diameter is displaced by 10%. By making the 10% K value of the resin particles 1500 N/mm ² or less, the softness of the resin particles is improved, making the resin particles more likely to deform even at low pressure, and increasing the contact area with the connected medium, thereby reducing resistance even when pressure-connected at low pressure.

When the number-average particle diameter of the resin particles is 10.0 μm or more, the 10% K value of the resin particles is preferably 600 N/ ^mm2 or less. This makes it possible to easily deform the resin particles at a low pressure of 2.5 MPa or less. It is more preferably 450 N/mm2 or ^less , and even more preferably 350 N/ ^mm2 or less. On the other hand, if the 10% K value of the resin particles is too low, the particles become too soft and it becomes difficult to maintain their spherical shape during plating processing, etc. Therefore, the 10% K value of resin particles having a number-average particle diameter of 10.0 μm or more is preferably 10 N/mm2 or ^more , more preferably 50 N/ ^mm2 or more, and even more preferably 100 N/ ^mm2 or more.

When the number-average particle diameter of the resin particles is less than 10.0 μm, the 10% K value of the resin particles is preferably 1350 N/ ^mm2 or less. This makes it possible to easily deform the resin particles at a low pressure of 1 MPa or less. It is more preferably 1250 N/ ^mm2 or less, and even more preferably 1000 N/mm2 or ^less . On the other hand, if the 10% K value of the resin particles is too low, the particles become too soft and it becomes difficult to maintain their spherical shape during plating processing, etc. Therefore, the 10% K value of resin particles having a number-average particle diameter of less than 10.0 μm is preferably 20 N/ ^mm2 or more, more preferably 150 N/mm2 or ^more , even more preferably 300 N/ ^mm2 or more, and particularly preferably 500 N/ ^mm2 or more.

The 10% K value of resin particles can be measured by a compression test using a known micro-compression tester. For example, a known micro-compression tester (such as the Shimadzu MCT-W500) is used to apply a load toward the center of the particle at room temperature at a loading rate of 0.445 mN/sec. The compression load and compression displacement are measured when the particle is deformed until the particle diameter is displaced by 10%, and the 10% K value can be calculated using the following formula.

(where E is the compressive elastic modulus (N/mm ² ), F is the compressive load (N), S is the compressive displacement (mm), and R is the particle radius (mm).)

The number-average particle diameter of the resin particles is preferably 2 μm or more and 50 μm or less. Resin particles with a number-average particle diameter within this range can be suitably used for electrical connections of electrodes and wiring in semiconductor packaging, such as for touch panels and LEDs. An example of a touch panel application is the connection between a touch panel lead-out circuit and a flexible printed circuit board (FPC). The number-average particle diameter of the resin particles is more preferably 4 μm or more, even more preferably 6 μm or more, and more preferably 35 μm or less, even more preferably 25 μm or less.

The number-based coefficient of variation (CV value) of the particle diameter of the resin particles is 10% or less. This suppresses variations in mechanical properties when the resin particles are used as conductive microparticles, improving connection reliability. It is preferably 8.0% or less, more preferably 5.0% or less, even more preferably 4.0% or less, and even more preferably 3.0% or less. There is no particular lower limit to the number-based coefficient of variation (CV value) of the particle diameter of the resin particles, but it is preferably 0.5% or more, more preferably 1% or more, and even more preferably 1.5% or more.

The number-average particle size and coefficient of variation of particle size of the resin particles referred to in the present invention are values obtained by measurement using the Coulter Counter method, and the measurement method is described in detail in the Examples.

The resin particles preferably have a dispersion rate of 90% or more. A dispersion rate of 90% or more allows conductive fine particles with suppressed aggregation to be obtained, making it easier to achieve low resistance. The dispersion rate of the resin particles is more preferably 93% or more, even more preferably 95% or more, even more preferably 97% or more, and most preferably 100%. The dispersion rate is measured as follows.
(Method for measuring dispersion ratio)
(1) 2.5 g of resin particles and 100 g of a 1% aqueous solution of polyoxyethylene alkyl ether sulfate ester ammonium salt, which is an emulsifier, are placed in a 200 mL beaker and ultrasonically dispersed for 15 minutes, and then passed through a sieve with openings 1.6 μm times the number average particle diameter.
(2) Dry the sieve at 120°C for 2 hours.
(3) The total weight (g) of the sieve and the polymer particles remaining on the sieve is measured, and the weight (g) of the sieve is subtracted to determine the weight (g) of the polymer particles remaining on the sieve.
(4) The weight (g) of the polymer particles remaining on the sieve is applied to the following formula to calculate the dispersion rate (%).
Dispersion rate (%)=100−((weight of polymer particles remaining on the sieve (g)/2.5 (g))×100)

When measuring the dispersion rate of resin particles, if a sieve with a mesh size 1.6 times the number average particle size is not available, the type and mesh size of the sieve may be determined based on the following criteria. For the electroformed sieve described below, reference may be made to JP-A-2001-252588.
Number average particle diameter less than 6 μm: electroformed sieve with 8 μm openings Number average particle diameter 6 μm or more and less than 9 μm: electroformed sieve with 12 μm openings Number average particle diameter 9 μm or more and less than 13 μm: electroformed sieve with 16 μm openings Number average particle diameter 13 μm or more and less than 17 μm: stainless steel sieve with 25 μm openings Number average particle diameter 17 μm or more and less than 24 μm: stainless steel sieve with 32 μm openings Stainless steel sieve - Number average particle diameter 24 μm or more and less than 40 μm: 45 μm mesh stainless steel sieve - Number average particle diameter 40 μm or more and less than 70 μm: 75 μm mesh stainless steel sieve - Number average particle diameter 70 μm or more and less than 140 μm: 150 μm mesh stainless steel sieve - Number average particle diameter 140 μm or more and less than 200 μm: 250 μm mesh stainless steel sieve

The degree of crosslinking of the resin particles is preferably 50% or less, more preferably 40% or less, and even more preferably 30% or less, and is preferably 5% or more, more preferably 10% or more, and even more preferably 20% or more. When the degree of crosslinking of the resin particles is in the above range, softness and flexibility are easily exhibited.
The degree of crosslinking in the resin particles can be evaluated by the following formula: In the formula, the crosslinkable monomer includes vinyl-based crosslinkable monomers and silane-based crosslinkable monomers, which will be described later.
Degree of crosslinking (%) = (mass of crosslinkable monomer/mass of total monomer) x 100

The shape of the resin particles (substrate) is not particularly limited and may be, for example, spherical, spheroidal, confetti-like, thin plate-like, needle-like, or cocoon-like, but spherical is preferred, and true spheres are particularly preferred.

1-1. Core In the present invention, the core is preferably a copolymer containing, as monomer units, a silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) above 0°C (hereinafter sometimes simply referred to as a "high Tg silicon-free monofunctional (meth)acrylate") and a silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) of 0°C or lower (hereinafter sometimes simply referred to as a "low Tg silicon-free monofunctional (meth)acrylate"). A core containing these two types of monomers as constituent monomers has excellent softness and flexibility.

The homopolymer glass transition temperature (Tg) can be determined, for example, from the values listed in POLYMER HANDBOOK FOURTH EDITION, Coatings and Paints (Paint Publishing, vol. 10 (No. 358), 1982). For those not listed here, they can be determined by differential scanning calorimetry (DSC) as shown in the examples.

Below, we will first explain the general monomers that can be used as the core-constituting monomers (meaning the monomers that serve as raw materials for forming the core), and then provide a detailed description of high Tg silicon-free monofunctional (meth)acrylates and low Tg silicon-free monofunctional (meth)acrylates selected from among these.

1-1-1. Core Monomers The monomers constituting the core may be so-called vinyl monomers alone, or so-called silane monomers may be used in combination with vinyl monomers. When vinyl monomers are used, vinyl groups are polymerized to form an organic skeleton, which can exhibit excellent elastic deformation during pressure connection. On the other hand, when silane monomers are used, siloxane bonds are generated by the hydrolysis and condensation reaction of the silane monomers to form a polysiloxane skeleton, which can exhibit high contact pressure against the connected body during pressure connection. Vinyl monomers are divided into vinyl crosslinkable monomers and vinyl non-crosslinkable monomers, and silane monomers are divided into silane crosslinkable monomers and silane non-crosslinkable monomers.

A vinyl-based crosslinkable monomer is one that has two or more crosslinkable groups, including at least a vinyl group, in its molecule. Specific examples include a monomer (monomer (1)) that has two or more vinyl groups in one molecule, or a monomer (monomer (2)) that has one vinyl group and one functional group other than the vinyl group (a protic hydrogen-containing group such as a carboxy group or a hydroxy group, or a terminal functional group such as an alkoxy group) in one molecule. However, in the case of monomer (2), to form a crosslinked structure as a vinyl-based crosslinkable monomer, the reactive (bonding) partner group, such as a carboxy group, hydroxy group, or alkoxy group, of monomer (2) must be present in another monomer.

In the present invention, the term "vinyl group" refers not only to a carbon-carbon double bond, but also to substituents having a polymerizable carbon-carbon double bond, such as a (meth)acryloyl group, allyl group, isopropenyl group, vinylphenyl group, and isopropenylphenyl group. Furthermore, in this specification, "(meth)acryloyl group," "(meth)acrylate," and "(meth)acrylic" refer to "acryloyl group and/or methacryloyl group," "acrylate and/or methacrylate," and "acrylic and/or methacrylic," respectively.

Examples of the monomer (1) include allyl (meth)acrylates such as allyl (meth)acrylate;
Alkanediol di(meth)acrylates such as ethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, 1,10-decanediol di(meth)acrylate, and 1,3-butanediol di(meth)acrylate; di(meth)acrylates such as polyalkylene glycol di(meth)acrylates such as diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, decaethylene glycol di(meth)acrylate, pentadecaethylene glycol di(meth)acrylate, pentacontahexaethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate;
tri(meth)acrylates such as trimethylolpropane tri(meth)acrylate;
tetra(meth)acrylates such as pentaerythritol tetra(meth)acrylate;
hexa(meth)acrylates such as dipentaerythritol hexa(meth)acrylate;
Aromatic hydrocarbon-based crosslinking agents such as divinylbenzene, divinylnaphthalene, and derivatives thereof (preferably styrene-based polyfunctional monomers such as divinylbenzene);
heteroatom-containing crosslinking agents such as N,N-divinylaniline, divinyl ether, divinyl sulfide, and divinyl sulfonic acid;
The monomer (1) may be used alone or in combination of two or more kinds.

Examples of the monomer (2) include monomers having a carboxy group, such as (meth)acrylic acid, carboxymethyl (meth)acrylate, carboxyethyl (meth)acrylate, carboxypropyl (meth)acrylate, carboxybutyl (meth)acrylate, and carboxypentyl (meth)acrylate;
Monomers having a hydroxy group, such as hydroxy group-containing (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 2-hydroxybutyl (meth)acrylate, and hydroxy group-containing styrenes such as p-hydroxystyrene;
Monomers having an amino group, such as aminoalkyl (meth)acrylates such as aminomethyl (meth)acrylate, aminoethyl (meth)acrylate, aminopropyl (meth)acrylate, aminobutyl (meth)acrylate, and aminopentyl (meth)acrylate; alkylaminoalkyl (meth)acrylates such as methylaminomethyl (meth)acrylate, ethylaminomethyl (meth)acrylate, methylaminoethyl (meth)acrylate, ethylaminoethyl (meth)acrylate, methylaminopropyl (meth)acrylate, and ethylaminopropyl (meth)acrylate; and dialkylaminoalkyl (meth)acrylates such as dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, and diethylaminopropyl (meth)acrylate;
Monomers having an alkoxy group, such as alkoxy group-containing (meth)acrylates such as 2-methoxyethyl (meth)acrylate, 3-methoxybutyl (meth)acrylate, and 2-butoxyethyl (meth)acrylate, and alkoxystyrenes such as p-methoxystyrene;
The monomer (2) may be used alone or in combination of two or more kinds.

Examples of vinyl-based non-crosslinkable monomers include a monomer (monomer (3)) having one vinyl group per molecule, or monomer (2) in which the monomer components do not contain other monomers having groups that react with functional groups other than the vinyl group possessed by monomer (2).

Examples of the monomer (3) include alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate;
cycloalkyl(meth)acrylates such as cyclopropyl(meth)acrylate, cyclopentyl(meth)acrylate, cyclohexyl(meth)acrylate, cyclooctyl(meth)acrylate, cycloundecyl(meth)acrylate, cyclododecyl(meth)acrylate, isobornyl(meth)acrylate, and 4-t-butylcyclohexyl(meth)acrylate;
aralkyl (meth)acrylates such as phenyl (meth)acrylate, benzyl (meth)acrylate, tolyl (meth)acrylate, and phenethyl (meth)acrylate;
Alkylstyrenes such as styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, α-methylstyrene, and pt-butylstyrene;
alkylene glycol monoalkyl ether (meth)acrylates such as ethylene glycol monobutyl ether (meth)acrylate, diethylene glycol monobutyl ether (meth)acrylate, diethylene glycol monomethyl ether (meth)acrylate, diethylene glycol monoethyl ether (meth)acrylate, and triethylene glycol monomethyl ether (meth)acrylate;
alkylene glycol monoaryl ether (meth)acrylates such as diethylene glycol monophenyl ether (meth)acrylate, ethylene glycol monophenyl ether (meth)acrylate, and ethylene glycol monotolyl ether (meth)acrylate;
styrene-based monofunctional monomers such as halogen group-containing styrenes such as o-chlorostyrene, m-chlorostyrene, and p-chlorostyrene;
The monomer (3) may be used alone or in combination of two or more kinds.

Silane-based crosslinking monomers have two or more crosslinkable groups (alkoxy groups, vinyl groups, etc.) within the molecule and can be divided into those that form a crosslinked structure (first type) between organic polymer skeletons (e.g., vinyl polymer skeletons) and organic polymer skeletons; those that form a crosslinked structure (second type) between polysiloxane skeletons; and those that form a crosslinked structure (third type) between organic polymer skeletons and polysiloxane skeletons. Of these, silane-based crosslinking monomers that can form the third type of crosslinked structure are preferred.

Examples of silane-based crosslinkable monomers that can form the first form include dimethyldivinylsilane, methyltrivinylsilane, and tetravinylsilane. These silane-based crosslinkable monomers may be used alone or in combination of two or more.

Examples of silane-based crosslinkable monomers that can form the second form include tetrafunctional silane-based monomers such as tetramethoxysilane, tetraethoxysilane, tetraisopropoxysilane, and tetrabutoxysilane; and trifunctional silane-based monomers such as methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, hexyltrimethoxysilane, and hexyltriethoxysilane. These silane-based crosslinkable monomers may be used alone or in combination of two or more.

Examples of silane-based crosslinkable monomers that can form the third form include those having a (meth)acryloyl group, such as 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-acryloxypropyltriethoxysilane, and 3-methacryloxyethoxypropyltrimethoxysilane;
those having a vinyl group, such as vinyltrimethoxysilane, vinyltriethoxysilane, and p-styryltrimethoxysilane;
those having an epoxy group, such as 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane;
those having an amino group, such as 3-aminopropyltrimethoxysilane and 3-aminopropyltriethoxysilane;
The silane-based crosslinkable monomers may be used alone or in combination of two or more.

Examples of silane-based non-crosslinkable monomers include bifunctional silane-based monomers such as dialkylsilanes, such as dimethyldimethoxysilane and dimethyldiethoxysilane; and monofunctional silane-based monomers, such as trialkylsilanes, such as trimethylmethoxysilane and trimethylethoxysilane. These silane-based non-crosslinkable monomers may be used alone or in combination of two or more.

1-1-2. High Tg Silicon-Free Monofunctional (Meth)acrylates High Tg silicon-free monofunctional (meth)acrylates are monomers that have a homopolymer glass transition temperature (Tg) above 0°C, contain no silicon, and contain one (meth)acryloyl group per molecule. By including a high Tg silicon-free monofunctional (meth)acrylate as a monomer unit in the core, the recovery rate of the resin particles can be reduced, and flexibility can be improved. The high Tg silicon-free monofunctional (meth)acrylate preferably has a homopolymer glass transition temperature (Tg) of 5°C or higher, more preferably 10°C or higher, and even more preferably 15°C or higher, and preferably 300°C or lower, more preferably 200°C or lower, and even more preferably 150°C or lower.

Of the total amount of monomers forming the core (100% by mass), the content of high-Tg silicon-free monofunctional (meth)acrylate is preferably 35% by mass or more and 70% by mass or less. A content of high-Tg silicon-free monofunctional (meth)acrylate of 35% by mass or more reduces the recovery rate of resin particles, making it easier to improve flexibility. Therefore, the content of high-Tg silicon-free monofunctional (meth)acrylate is preferably 35% by mass or more, more preferably 40% by mass or more, and even more preferably 45% by mass or more. On the other hand, if the content of high-Tg silicon-free monofunctional (meth)acrylate is too high, the content of low-Tg silicon-free monofunctional (meth)acrylate will be low, making it difficult to improve softness. Therefore, the content of high-Tg silicon-free monofunctional (meth)acrylate is preferably 70% by mass or less, more preferably 65% by mass or less, and even more preferably 60% by mass or less.

Examples of high-Tg silicon-free monofunctional (meth)acrylates include the above-mentioned monomer (2) and (meth)acrylates of the above-mentioned monomer (3) whose homopolymer glass transition temperature (Tg) is above 0°C.

Monomer (2) is preferably at least one monomer selected from the group consisting of monomers having a carboxy group and monomers having a hydroxy group. As the monomer having a carboxy group, (meth)acrylic acid is preferred. As the monomer having a hydroxy group, hydroxyalkyl (meth)acrylate is preferred, hydroxyalkyl methacrylate is more preferred, hydroxyalkyl methacrylate in which the alkyl group has 1 to 4 carbon atoms is even more preferred, and hydroxyalkyl methacrylate in which the alkyl group has 2 to 3 carbon atoms is even more preferred.

Monomer (3) is preferably at least one monomer selected from the group consisting of alkyl (meth)acrylates, cycloalkyl (meth)acrylates, and aralkyl (meth)acrylates, more preferably at least one monomer selected from the group consisting of alkyl (meth)acrylates and cycloalkyl (meth)acrylates, and even more preferably alkyl (meth)acrylates.

As the alkyl (meth)acrylate, at least one monomer selected from the group consisting of methyl acrylic acid and alkyl methacrylic acid having an alkyl group with 1 to 5 carbon atoms is preferred, alkyl methacrylic acid having an alkyl group with 3 to 4 carbon atoms is more preferred, and alkyl methacrylic acid having an alkyl group with 4 carbon atoms is even more preferred.

Cycloalkyl(meth)acrylates are preferably those in which the cycloalkyl group has 5 to 30 carbon atoms, more preferably those in which the cycloalkyl group has 5 to 10 carbon atoms, even more preferably those in which the cycloalkyl group has 5 to 7 carbon atoms, and even more preferably those in which the cycloalkyl group has 6 carbon atoms.

As aralkyl (meth)acrylates, aralkyl (meth)acrylates containing an aralkyl group having 7 to 12 carbon atoms are preferred, aralkyl (meth)acrylates containing an aralkyl group having 7 to 8 carbon atoms are more preferred, and aralkyl (meth)acrylates containing an aralkyl group having 7 carbon atoms are even more preferred.

1-1-3. Low Tg Silicon-Free Monofunctional (Meth)acrylates Low Tg silicon-free monofunctional (meth)acrylates are monomers that have a homopolymer glass transition temperature (Tg) of 0°C or lower, contain no silicon, and contain one (meth)acryloyl group per molecule. By including a low Tg silicon-free monofunctional (meth)acrylate as a monomer unit in the core, the 10% K value of the resin particles can be reduced, improving softness. The low Tg silicon-free monofunctional (meth)acrylate preferably has a homopolymer glass transition temperature (Tg) of -2°C or lower, more preferably -5°C or lower, and even more preferably -20°C or lower, and preferably -150°C or higher, more preferably -120°C or higher, and even more preferably -100°C or higher.

Of the total amount of monomers forming the core (100% by mass), the content of low Tg non-silicon-containing monofunctional (meth)acrylate is preferably 15% by mass or more and 50% by mass or less. A low Tg non-silicon-containing monofunctional (meth)acrylate content of 15% by mass or more reduces the 10% K value of the resin particles and improves softness. Therefore, the content of low Tg non-silicon-containing monofunctional (meth)acrylate is preferably 15% by mass or more, more preferably 25% by mass or more, and even more preferably 30% by mass or more. On the other hand, if the content of low Tg non-silicon-containing monofunctional (meth)acrylate is too high, the content of high Tg non-silicon-containing monofunctional acrylate will be low, making it difficult to reduce the recovery rate. Therefore, the content of low Tg non-silicon-containing monofunctional (meth)acrylate is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less.

The ratio of the content of low Tg silicon-free monofunctional (meth)acrylate to the content of high Tg silicon-free monofunctional (meth)acrylate, based on 100% by mass of the total amount of monomers forming the core, is preferably 0.2 or more, more preferably 0.4 or more, and preferably 1.2 or less, more preferably 0.9 or less. This makes it easier to achieve both flexibility and softness.

Examples of low-Tg silicon-free monofunctional (meth)acrylates include (meth)acrylates of the above-mentioned monomer (2) and monomer (3) having a homopolymer glass transition temperature (Tg) of 0°C or lower.

Monomer (2) is preferably a monomer having a hydroxy group. As a monomer having a hydroxy group, hydroxyalkyl (meth)acrylate is preferred, hydroxyalkyl acrylate is more preferred, hydroxyalkyl acrylate in which the alkyl group has 1 to 4 carbon atoms is even more preferred, and hydroxyalkyl acrylate in which the alkyl group has 2 to 3 carbon atoms is even more preferred.

Monomer (3) is preferably at least one monomer selected from the group consisting of alkyl (meth)acrylates, alkylene glycol monoalkyl ether (meth)acrylates, and alkylene glycol monoaryl ether (meth)acrylates, and more preferably alkyl (meth)acrylates.

Examples of alkyl (meth)acrylates include alkyl acrylate and alkyl methacrylate. As alkyl acrylates, alkyl acrylates in which the alkyl group has 2 to 10 carbon atoms are preferred, and alkyl acrylates in which the alkyl group has 2 to 6 carbon atoms are more preferred. As alkyl methacrylates, alkyl methacrylates in which the alkyl group has 8 to 30 carbon atoms are preferred, and alkyl methacrylates in which the alkyl group has 10 to 20 carbon atoms are more preferred.

As alkylene glycol monoalkyl ether (meth)acrylates, alkylene glycol monoalkyl ether (meth)acrylates in which the number of repeating units (n) of the alkylene glycol structure is 1 to 5 and the alkyl group has 1 to 5 carbon atoms are preferred, and alkylene glycol monoalkyl ether (meth)acrylates in which the number of repeating units (n) of the alkylene glycol structure is 1 to 3 and the alkyl group has 1 to 4 carbon atoms are more preferred.

As alkylene glycol monoaryl ether (meth)acrylates, alkylene glycol monoaryl ether (meth)acrylates in which the number of repeating units (n) of the alkylene glycol structure is 1 to 5 are preferred, and alkylene glycol monoaryl ether (meth)acrylates in which the number of repeating units (n) of the alkylene glycol structure is 1 to 2 are more preferred.

As a combination of monomer components constituting the core as described above, a combination of a high Tg silicon-free monofunctional (meth)acrylate and a low Tg silicon-free monofunctional (meth)acrylate is preferred, and a combination of a silane-based crosslinkable monomer, a high Tg silicon-free monofunctional (meth)acrylate, a low Tg silicon-free monofunctional (meth)acrylate, and a di(meth)acrylate of monomer (1) is more preferred.

1-1-4. Physical Properties of the Core The glass transition temperature (Tgcore) of the core calculated based on the FOX equation is preferably 0°C or lower. Having a core glass transition temperature (Tgcore) of 0°C or lower makes it easier to reduce the 10% K value of the resin particles. The core glass transition temperature (Tgcore) is more preferably -3°C or lower, even more preferably -5°C or lower, and even more preferably -10°C or lower. There is no particular restriction on the lower limit of the core glass transition temperature (Tgcore), but it is preferably -50°C or higher, more preferably -30°C or higher, and even more preferably -20°C or higher.

The FOX formula is a formula for calculating the glass transition temperature (Tg) of a copolymer based on the glass transition temperatures (Tg) of the homopolymers of each monomer that makes up the copolymer. Details of the formula are described in Bulletin of the American Physical Society, Series 2, Vol. 1, No. 3, p. 123 (1956). Specifically, the glass transition temperature (Tg) calculated using the FOX formula is calculated using the following formula:

[In the formula,
Tgcore represents the glass transition temperature (unit: absolute temperature) of a copolymer having a non-crosslinkable monomer as a monomer unit, _Wi represents the mass fraction of non-crosslinkable monomer i, and _Tgi represents the glass transition temperature (unit: absolute temperature) of a homopolymer formed from non-crosslinkable monomer i.
The glass transition temperature Tgcore (unit: absolute temperature) calculated by the above formula is converted from absolute temperature (K) to Celsius temperature (°C).

The glass transition temperatures (Tg) of homopolymers of various monomers used for calculations using the FOX formula can be, for example, those listed in POLYMER HANDBOOK FOURTH EDITION, Coatings and Paints (Paint Publishing, vol. 10 (No. 358), 1982). For those not listed here, they can be determined by differential scanning calorimetry (DSC), as shown in the examples.

In the core of the present invention, the degree of crosslinking is preferably 30% or less, more preferably 20% or less, and even more preferably 15% or less, and is preferably 5% or more, more preferably 7% or more, and even more preferably 10% or more. When the degree of crosslinking of the core is within the above range, it is easy to exhibit appropriate softness and flexibility.
The degree of crosslinking in the core can be evaluated by the following formula: In the formula, the crosslinkable monomer includes the above-mentioned vinyl-based crosslinkable monomer and the above-mentioned silane-based crosslinkable monomer.
Degree of crosslinking (%) = (mass of crosslinkable monomer/mass of total monomer) x 100

The number-average particle diameter of the core is preferably 2 μm or more and 50 μm or less. Cores with a number-average particle diameter within this range can be suitably used for electrical connections of electrodes and wiring in semiconductor packaging, for example, for touch panels and LEDs. An example of a touch panel application is the connection between a touch panel lead-out circuit and a flexible printed circuit board (FPC). The number-average particle diameter of the core is more preferably 4 μm or more, even more preferably 6 μm or more, and more preferably 40 μm or less, even more preferably 35 μm or less.

The coefficient of variation (CV value) of the core particle diameter is preferably 10% or less, more preferably 8.0% or less, even more preferably 5.0% or less, and even more preferably 4.0% or less. By reducing the coefficient of variation of the core particle diameter, it is possible to suppress variation in the mechanical properties when the conductive microparticles are formed. There is no particular lower limit for the coefficient of variation of the core particle diameter, but it is preferably 0.5% or more, more preferably 1% or more, and even more preferably 1.5% or more.

1-2. Shell The monomer component constituting the shell may be the vinyl monomer or silane monomer described above, or a combination of these may be used. The vinyl monomer described above is preferred as the monomer component constituting the shell, with the vinyl monomer described above being preferred as the vinyl monomer, and di(meth)acrylates being preferred as the monomer (1). Using these monomers as the monomer component constituting the shell improves the swelling resistance of the resin particles and further improves dispersibility. Polyalkylene glycol di(meth)acrylates are preferred as the di(meth)acrylates. Polyalkylene glycol di(meth)acrylates having a number (n) of repeating units of the alkylene glycol structure of 2 to 8 are preferred as the polyalkylene glycol di(meth)acrylates. This facilitates the development of flexibility in the core. Polyalkylene glycol di(meth)acrylates having a number (n) of repeating units of the alkylene glycol structure of 3 to 5 are more preferred, and polyalkylene glycol di(meth)acrylates having a number (n) of repeating units of the alkylene glycol structure of 3 are even more preferred.

The content of monomer (1) is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, particularly preferably 97% by mass or more, and most preferably 100% by mass, based on 100% by mass of the monomer components forming the shell.

The higher the degree of crosslinking of the shell, the more improved the swelling resistance and dispersibility of the resin particles, and therefore the degree of crosslinking of the shell is preferably 70% or more, more preferably 80% or more, even more preferably 90% or more, still more preferably 95% or more, and most preferably 100%.
The degree of crosslinking in the shell can be evaluated by the following formula: In the formula, the crosslinkable monomer includes the above vinyl-based crosslinkable monomer and the above silane-based crosslinkable monomer.
Degree of crosslinking (%) = (mass of crosslinkable monomer/mass of total monomer) x 100

The average thickness of the shell is preferably 0.05 μm or more and 1 μm or less, more preferably 0.1 μm or more and 0.8 μm or less, and even more preferably 0.2 μm or more and 0.7 μm or less. This makes it easier for the shell to exhibit its functions such as swelling resistance. The average thickness of the shell referred to in the present invention can be calculated using the Coulter counter method according to the following formula.
Average shell thickness (μm)=(number average particle size A (μm) of resin particles−number average particle size B (μm) of cores before shell coating)/2

The ratio of the average shell thickness (μm) to the number-average particle diameter (μm) of the resin particles is preferably 0.01 or more and 0.05 or less, and more preferably 0.02 or more and 0.04 or less. This allows the core to exhibit its flexibility and softness while making it easier for the shell to exhibit its functions such as swelling resistance.

2. Conductive Fine Particles The present invention also includes conductive fine particles in which at least one conductive metal layer is formed on the surface of the resin particles (base material). The conductive fine particles will be described in detail below.

2-1. Conductive Metal Layer The metal constituting the conductive metal layer is not particularly limited, but examples thereof include metals and metal compounds such as gold, silver, copper, platinum, iron, lead, aluminum, chromium, palladium, nickel, rhodium, ruthenium, antimony, bismuth, germanium, tin, cobalt, indium, nickel-phosphorus, nickel-boron, and the like, as well as alloys thereof. Among these, gold, nickel, palladium, silver, copper, and tin are preferred because they form conductive fine particles with excellent conductivity. In addition, in terms of cost, nickel and nickel alloys (Ni—Au, Ni—Pd, Ni—Pd—Au, Ni—Ag, Ni—P, Ni—B, Ni—Zn, Ni—Sn, Ni—W, Ni—Co, Ni—W, Ni—Ti); copper and copper alloys (alloys of Cu and at least one metal element selected from the group consisting of Fe, Co, Ni, Zn, Sn, In, Ga, Tl, Zr, W, Mo, Rh, Ru, Ir, Ag, Au, Bi, Al, Mn, Mg, P, and B, preferably Ag, Ni, Sn, Z, and alloys with Ag and at least one metal element selected from the group consisting of Fe, Co, Ni, Zn, Sn, In, Ga, Tl, Zr, W, Mo, Rh, Ru, Ir, Au, Bi, Al, Mn, Mg, P, and B, preferably Ag—Ni, Ag—Sn, and Ag—Zn); and tin and tin alloys (for example, Sn—Ag, Sn—Cu, Sn—Cu—Ag, Sn—Zn, Sn—Sb, Sn—Bi—Ag, Sn—Bi—In, Sn—Au, and Sn—Pb). Of these, nickel and nickel alloys are preferred. The conductive metal layer may be a single layer or a multilayer. In the case of a multilayer, preferred combinations include nickel-gold, nickel-palladium, nickel-palladium-gold, and nickel-silver.

The thickness of the conductive metal layer is preferably 0.010 μm or more, more preferably 0.030 μm or more, even more preferably 0.050 μm or more, and is preferably 0.30 μm or less, more preferably 0.25 μm or less, even more preferably 0.20 μm or less, and even more preferably 0.15 μm or less. If the thickness of the conductive metal layer is within the above range, stable electrical connection can be maintained when the conductive fine particles are used as an anisotropic conductive material. The thickness of the conductive metal layer can be measured, for example, by the method described below in the Examples.

The conductive metal layer only needs to cover at least a portion of the resin particle surface, but it is preferable that the surface of the conductive metal layer is substantially free of cracks and surfaces on which the conductive metal layer is not formed. Here, "substantial cracks and surfaces on which the conductive metal layer is not formed" means that when the surfaces of any 10,000 conductive microparticles are observed using a scanning electron microscope (magnification 1000x), cracks in the conductive metal layer and exposed resin particle surfaces are substantially not visible to the naked eye.

The number average particle diameter of the conductive fine particles of the present invention is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 4 μm or more, and is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 25 μm or less. If the number average particle diameter is within this range, the conductive fine particles can be suitably used for electrical connections of miniaturized and narrowed electrodes and wiring.
As the number-average particle diameter of the conductive fine particles, it is preferable to use the number-based average particle diameter of 3,000 particles determined using a flow particle image analyzer (Sysmex Corporation's "FPIA (registered trademark)-3000").

The conductive fine particles of the present invention may also have an insulating resin layer on at least a portion of their surface. That is, an insulating resin layer may be further provided on the surface of a conductive metal layer. By laminating an insulating resin layer on the conductive metal layer on the surface in this way, lateral conduction, which is likely to occur when forming high-density circuits or connecting terminals, can be prevented.

The insulating resin layer is not particularly limited as long as it ensures insulation between the conductive microparticles and can be easily broken down or peeled off by a certain amount of pressure and/or heat. Examples include thermoplastic resins and crosslinked products thereof, such as polyolefins such as polyethylene; (meth)acrylate polymers and copolymers such as polymethyl (meth)acrylate; polystyrene; thermosetting resins such as epoxy resins, phenolic resins, and amino resins (e.g., melamine resins); water-soluble resins such as polyvinyl alcohol, and mixtures thereof. However, if the insulating resin layer is too hard compared to the base particles, there is a risk that the base particles themselves will break down before the insulating resin layer breaks down. Therefore, it is preferable to use an uncrosslinked or relatively low-crosslinked resin for the insulating resin layer.

The insulating resin layer may be a single layer or may consist of multiple layers. For example, it may be a single or multiple film-like layers, or a layer in which insulating particles in the form of granules, spheres, chunks, scales, or other shapes are attached to the surface of a conductive metal layer, or a layer formed by chemically modifying the surface of a conductive metal layer, or a combination of these. The thickness of the insulating resin layer is preferably 0.01 μm or more and 1 μm or less, more preferably 0.02 μm or more and 0.5 μm or less, and even more preferably 0.03 μm or more and 0.4 μm or less. If the thickness of the insulating resin layer is within the above range, the electrical insulation between particles is good while maintaining the conductivity of the conductive particles.

3. Manufacturing Method First, a manufacturing method of the resin particles will be described.
The resin particles are produced by forming a core and then forming a shell so as to cover the core.

3-1. Core Manufacturing Method The core manufacturing method is not particularly limited as long as it polymerizes the core monomers described above, and any conventionally known method can be used, such as emulsion polymerization, suspension polymerization, dispersion polymerization, first seed polymerization, or sol-gel seed polymerization. For example, a method of synthesizing a core by the first seed polymerization method is preferably used, since it is easy to control the particle size of the core and to easily obtain a core with a narrow particle size distribution.

The first seed polymerization method is a method of obtaining cores through a seed particle preparation step, an absorption step in which core monomers are absorbed into the seed particles, and a polymerization step in which the core monomers absorbed into the seed particles are polymerized. The techniques and conditions for each step are not particularly limited and can be any known seed polymerization technique that can be appropriately adopted. However, the following techniques are preferably used, for example:

In the seed particle preparation process, when synthesizing resin particles composed solely of organic materials, seed particles can be prepared using the vinyl monomers described above by methods such as soap-free emulsion polymerization or dispersion polymerization. In this case, it is preferable to use a styrene-based monofunctional monomer such as styrene as the vinyl monomer. On the other hand, when synthesizing particles composed of organic materials and materials having a polysiloxane skeleton, seed particles (polysiloxane particles) can be prepared using the silane monomers described above by hydrolysis and condensation polymerization in a solvent containing water (e.g., a mixed solvent of water and an organic solvent such as alcohols, ketones, esters, (cyclo)paraffins, or aromatic hydrocarbons). In this case, it is preferable to use the silane-based crosslinkable monomers described above as the silane monomers to form polymerizable polysiloxane particles. For the hydrolysis and condensation polymerization, a basic catalyst such as ammonia, urea, ethanolamine, tetramethylammonium hydroxide, alkali metal hydroxide, or alkaline earth metal hydroxide can be preferably used as a catalyst, and if necessary, an anionic, cationic, or nonionic surfactant, or a polymer dispersant such as polyvinyl alcohol or polyvinylpyrrolidone can also be used in combination.

There are no particular limitations on the method for absorbing the core monomer into the seed particles in the absorption process. For example, the core monomer may be added to a seed particle dispersion in which seed particles are previously dispersed in a solvent, or the seed particles may be added to a solvent containing the core monomer. However, in the former method, using the reaction solution obtained by polymerization, hydrolysis, and condensation as the seed particle dispersion is particularly preferred from the standpoints of process simplification and productivity. The core monomer may be added alone or as a solution dissolved in a solvent. However, to ensure efficient absorption into the seed particles, it is preferable to emulsify and disperse the core monomer in water or an aqueous medium (e.g., a water-soluble organic solvent such as an alcohol, ketone, or ester, or a mixture of these with water) using an emulsifier before adding the emulsion. It is preferable to use at least the high-Tg silicon-free monofunctional (meth)acrylate and the low-Tg silicon-free monofunctional (meth)acrylate described above as the core monomer to be absorbed.

When emulsifying and dispersing the core monomer with an emulsifier, preferred emulsifiers include anionic surfactants and nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene fatty acid esters, sorbitan fatty acid esters, polyoxysorbitan fatty acid esters, polyoxyethylene alkylamines, glycerin fatty acid esters, and oxyethylene-oxypropylene block polymers, as they can stabilize the dispersion state of the seed particles after they have absorbed the monomer. Furthermore, the amount of water or aqueous medium used during emulsification and dispersion is typically 0.3 to 10 times the mass of the monomer.

In the absorption step, whether the core monomer has been absorbed into the seed particles can be easily determined, for example, by observing the particles with a microscope before adding the monomer and after the absorption step is completed, and confirming that the particle size has increased due to the absorption of the monomer. In order to obtain the resin particles of the present invention, the absorption ratio represented by the following formula is not particularly limited, but is preferably 1.0 times or more and 50 times or less, more preferably 2.0 times or more and 30 times or less, and even more preferably 3.0 times or more and 20 times or less.
Absorption capacity = (total mass of core monomers to be absorbed) / (mass of seed particles)

The polymerization method used in the polymerization step is not particularly limited, and known methods, such as those using radical polymerization initiators (e.g., peroxide initiators, azo initiators, etc.), can be used. The reaction temperature during radical polymerization is preferably 40°C or higher, more preferably 50°C or higher, and 100°C or lower, more preferably 80°C or lower. If the reaction temperature is too low, the degree of polymerization may not increase sufficiently, resulting in insufficient mechanical properties of the composite particles. On the other hand, if the reaction temperature is too high, inter-particle aggregation may occur during polymerization. The reaction time during radical polymerization can be adjusted depending on the type of polymerization initiator used, but is typically 5 minutes or longer, more preferably 10 minutes or longer, and 600 minutes or shorter, more preferably 300 minutes or shorter. If the reaction time is too short, the degree of polymerization may not increase sufficiently. If the reaction time is too long, inter-particle aggregation may occur. In this polymerization process, if the seed particles are polymerizable polysiloxane particles, the absorbed monomer polymerizes with the radically polymerizable groups in the polymerizable polysiloxane skeleton, resulting in the polysiloxane skeleton and vinyl polymer forming a composite.

3-2. Method of Forming Shell By forming a shell on a core so as to cover the core, resin particles that are core-shell particles can be obtained.

The shell can be formed by polymerizing the above-mentioned monomers, using conventional methods such as emulsion polymerization, suspension polymerization, dispersion polymerization, second seed polymerization, and sol-gel seed polymerization. For example, a method of forming the shell by second seed polymerization is preferred, as it allows for easy control of the shell thickness and makes it easier to obtain resin particles with a uniform thickness and a narrow particle size distribution.

The second seed polymerization method is a method of obtaining core-shell particles through a coating process in which the core surface is coated with a shell monomer, and a polymerization process in which the shell monomer is polymerized on the core surface.

In the coating process, the method for coating the core surface with the shell monomer is not particularly limited. For example, the shell monomer may be added to a core dispersion in which the core is previously dispersed in a solvent, or the core may be added to a solvent containing the shell monomer. However, in the former method, using the reaction liquid obtained by core polymerization as the core dispersion is particularly preferred from the perspectives of process simplification and productivity. The shell monomer may be added alone or as a solution dissolved in a solvent. However, to efficiently coat the core surface with the shell monomer, it is preferable to emulsify and disperse the shell monomer in water or an aqueous medium using an emulsifier before adding the emulsion. The aqueous medium may be the same as the aqueous medium used in the first seed polymerization method described above. When the shell monomer is emulsified and dispersed with an emulsifier, the emulsifier may be the same as the emulsifier used in the first seed polymerization method described above.

In the coating step, whether the shell monomer has been dispersed on the core surface can be easily determined by checking that the reaction solution is an emulsion and that no precipitation occurs. In order to obtain the resin particles of the present invention, the coating ratio shown in the following formula is not particularly limited, but is preferably 0.01 times or more and 1.0 times or less, more preferably 0.03 times or more and 0.8 times or less, and even more preferably 0.05 times or more and 0.32 times or less.
Coverage ratio = (total mass of shell monomers to be covered) / (total mass of core)

After synthesis, the resin particles are typically dried and, in some cases, calcined. The drying temperature is not particularly limited, but is typically in the range of 40°C to 250°C.

In this way, the resin particles are prepared so that the number average particle size, particle size coefficient of variation, etc., satisfy the above-mentioned ranges.

3-3. Method for forming a conductive metal layer Next, a conductive metal layer is formed on the resin particles (substrate) obtained as described above, and an insulating resin layer is further formed as needed, thereby obtaining conductive fine particles.

The methods for forming the conductive metal layer and the insulating resin layer are not particularly limited. For example, the conductive metal layer can be formed by plating the substrate surface using electroless plating, electrolytic plating, or the like; or by forming a conductive metal layer on the substrate surface using physical vapor deposition methods such as vacuum deposition, ion plating, and ion sputtering. Of these, electroless plating is particularly preferred because it does not require large-scale equipment and can easily form a conductive metal layer.

4. Anisotropic Conductive Materials The present invention also includes an anisotropic conductive material obtained by dispersing the above-described conductive fine particles in a binder resin. The form of the anisotropic conductive material is not particularly limited, and various forms, such as anisotropic conductive films, anisotropic conductive pastes, anisotropic conductive adhesives, and anisotropic conductive inks, can be used. By providing these anisotropic conductive materials between opposing substrates or between electrode terminals, good electrical connection can be achieved. Anisotropic conductive materials using the conductive fine particles of the present invention also include conductive materials for liquid crystal display elements (conductive spacers and compositions thereof). Suitable applications of the anisotropic conductive material include touch panel input and LED applications, and they are particularly suitable for use in touch panel mounting.

The binder resin is not particularly limited as long as it is an insulating resin, and examples include thermoplastic resins such as acrylic resin, ethylene-vinyl acetate resin, and styrene-butadiene block copolymer; curable resin compositions that cure upon reaction with a curing agent such as a monomer or oligomer having a glycidyl group and an isocyanate; and curable resin compositions that cure with light or heat.

The anisotropic conductive material of the present invention can be obtained by dispersing the conductive fine particles of the present invention in a binder resin and forming them into the desired form. However, for example, the binder resin and the conductive fine particles can be used separately, and the conductive fine particles can be placed together with the binder resin between the substrates or electrode terminals to be connected to form the connection.

The content of conductive fine particles in anisotropic conductive materials can be determined appropriately depending on the application, but is preferably 1% by volume or more, more preferably 2% by volume or more, and even more preferably 5% by volume or more, relative to the total amount of anisotropic conductive material, and is preferably 50% by volume or less, more preferably 30% by volume or less, and even more preferably 20% by volume or less. If the content of conductive fine particles is too low, it may be difficult to achieve sufficient electrical conductivity. On the other hand, if the content of conductive fine particles is too high, the conductive fine particles may come into contact with each other, making it difficult for the material to function as an anisotropic conductive material.

The film thickness of the anisotropic conductive material, the coating thickness of the paste or adhesive, the printing thickness, etc. should preferably be appropriately set taking into consideration the particle diameter of the conductive microparticles of the present invention used and the specifications of the electrodes to be connected, so that the conductive microparticles are sandwiched between the electrodes to be connected and the gaps between the bonded substrates on which the electrodes to be connected are formed are sufficiently filled with the binder resin layer.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the following examples, and modifications can be made within the scope of the spirit described above and below, and all such modifications are within the technical scope of the present invention. In the following, unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass."

1. Physical property measurement methods Various physical properties were measured by the following methods.

<Number average particle diameter and coefficient of variation (CV value) of seed particles, cores, and resin particles>
In the case of cores and resin particles, 20 parts of a 1% aqueous solution of an emulsifier, polyoxyethylene alkyl ether sulfate ester ammonium salt ("Hitenol (registered trademark) N-08" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), was added to 0.1 parts of the cores or resin particles, and the resulting dispersion was ultrasonically dispersed for 10 minutes to prepare a measurement sample. In the case of seed particles, the dispersion obtained by the hydrolysis and condensation reaction was diluted with a 1% aqueous solution of polyoxyethylene alkyl ether sulfate ester ammonium salt ("Hitenol (registered trademark) N-08" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) to prepare a measurement sample. For each measurement sample, the particle diameters (μm) of 30,000 particles were measured using a particle size distribution analyzer ("Coulter Multisizer III" manufactured by Beckman Coulter, Inc.) to determine the number-average particle diameter. For the cores and resin particles, the standard deviation of the particle diameters on a number basis was also determined along with the number-average particle diameter, and the coefficient of variation (CV value) of the particle diameter was calculated according to the following formula:
Coefficient of variation of particles (%) = 100 × (standard deviation of particle diameter / number average particle diameter)

<Film Thickness of Conductive Metal Layer>
Using a flow particle image analyzer (Sysmex Corporation's "FPIA (registered trademark)-3000"), the number average particle diameter X (μm) of 3,000 resin particles and the number average particle diameter Y (μm) of 3,000 conductive fine particles were measured. The measurements were performed after adding 17.5 parts of a 1.4% aqueous solution of an emulsifier, polyoxyethylene oleyl ether (Kao Corporation's "Emulgen (registered trademark) 430"), to 0.25 parts of the particles and dispersing the mixture ultrasonically for 10 minutes. The film thickness of the conductive metal layer was then calculated according to the following formula:
Conductive metal layer thickness (μm) = (Y-X)/2

<10% K value of resin particles>
Using a microcompression tester (Shimadzu Corporation, "MCT-W500"), at room temperature (25°C), a load was applied to a single particle scattered on a sample stage (material: SKS flat plate) using a 50 μm diameter circular flat indenter (material: diamond) in "soft surface detection" mode toward the center of the particle at a constant loading rate (0.445 mN/sec). The load value (mN) when the compressive displacement reached 10% of the particle diameter (10% compressive load value) and the displacement (μm) at that time were measured. Measurements were performed on 10 different particles for each sample, and the average value was used as the measured value. The obtained compressive load value (mN) was converted to compressive load (N), and the obtained displacement (μm) was converted to compressive displacement (mm). The particle radius (mm) was calculated from the number-average particle diameter (μm) of the resin particles, and these values were used to calculate the particle radius according to the following formula. The above measurements were performed in a constant temperature atmosphere of 25°C.

(wherein E is the compressive modulus of elasticity (N/mm ² ), F is the compressive load (N), S is the compressive displacement (mm), and R is the particle radius (mm).)

<Recovery rate of resin particles>
Using a microcompression tester (Shimadzu Corporation "MCT-W500"), at room temperature (25°C), a single particle scattered on a sample stage (material: SKS plate) was compressed toward the center of the particle at a constant loading rate (2.23 mN/sec) to a maximum load (49 mN) using a 50 μm diameter circular flat indenter (material: diamond) in "soft surface detection" mode. The displacement (μm) at this time was measured and designated as the maximum displacement L1. Next, the load was reduced to a minimum load (0.49 mN) at a constant unloading rate (2.23 mN/sec). The displacement (μm) from the maximum load to the minimum load was measured and designated as the recovery displacement L2. The recovery rate was calculated from the maximum displacement L1 and the recovery displacement L2 according to the following formula. Note that measurements were performed on 10 different particles for each sample, and the average value was designated as the measured value.
Recovery rate (%) = (L2/L1) x 100
The above measurements were carried out in a constant temperature atmosphere of 25°C.

<Evaluation of dispersibility of powder (resin particles) in emulsified water>
2.5 g of polymer particles and 100 g of a 1% aqueous solution of an emulsifier, polyoxyethylene alkyl ether sulfate ester ammonium salt ("Hitenol (registered trademark) N-08" manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.), were placed in a 200 mL beaker and ultrasonically dispersed for 15 minutes, and then passed through a sieve with a mesh size 1.6 times the number average particle size (μm) of the resin particles. After drying the sieve and the polymer particles at 120°C for 2 hours, the total weight (g) of the sieve and the polymer particles remaining on the sieve was measured, and the weight (g) of the polymer particles remaining on the sieve was calculated by subtracting the weight (g) of the sieve, and the dispersion rate (%) in emulsified water was calculated using the following formula.
Dispersion rate (%)=100−((weight of polymer particles remaining on the sieve (g)/2.5 (g))×100)

As a sieve having a mesh size 1.6 times the number-average particle diameter (μm) of the resin particles, a stainless steel sieve with an opening size of 32 μm manufactured by AS ONE Corporation was used for resin particles Nos. 1, 2, 5 to 15, and 17, which have a number-average particle diameter of 19.9 to 20.8 μm (1.6-fold μm value = 31.8 to 33.2 μm), based on the following criteria. For resin particles Nos. 3 and 16, which have a number-average particle diameter of 9.7 to 10.7 μm (1.6-fold μm value = 15.5 to 17.1 μm), an electroformed sieve with an opening size of 16 μm manufactured by AS ONE Corporation was used based on the following criteria. For resin particles No. 4, which have a number-average particle diameter of 4.8 μm (1.6-fold μm value = 7.7 μm), an electroformed sieve with an opening size of 8 μm manufactured by AS ONE Corporation was used based on the following criteria.
Number average particle diameter less than 6 μm: electroformed sieve with 8 μm openings Number average particle diameter 6 μm or more and less than 9 μm: electroformed sieve with 12 μm openings Number average particle diameter 9 μm or more and less than 13 μm: electroformed sieve with 16 μm openings Number average particle diameter 13 μm or more and less than 17 μm: stainless steel sieve with 25 μm openings Number average particle diameter 17 μm or more and less than 24 μm: stainless steel sieve with 32 μm openings

<Homopolymer glass transition temperature (Tg)>
For a monomer (isooctyl acrylate (IOA)) whose homopolymer glass transition temperature (Tg) is not listed in the Polymer Handbook Fourth Edition and Coatings and Paints (Paint Publishing Co., 10 (No. 358), 1982), differential scanning calorimetry (DSC) was performed as follows.

A four-neck flask equipped with a condenser, thermometer, and dropping nozzle was charged with 200 parts of a 10% aqueous solution of polyvinyl alcohol (Poval PVA-205, manufactured by Kuraray Co., Ltd.) and maintained at 25°C. While stirring at 300 rpm, a solution containing 27 parts of isooctyl acrylate and 0.7 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (V-65, manufactured by Wako Pure Chemical Industries, Ltd.) was added through the dropping nozzle and held for 2 hours to produce a suspension. With continued stirring, this suspension was heated to 65°C under a nitrogen atmosphere and held at 65°C for 2 hours to carry out radical polymerization. The emulsion after radical polymerization was subjected to solid-liquid separation and vacuum dried at 40°C for 12 hours to obtain isooctyl acrylate homopolymer particles. Next, using a differential scanning calorimeter (BRUKER DSC3100), the resulting homopolymer particles were heated in a nitrogen atmosphere in the range of -100°C to 150°C at a heating rate of 20°C/min, and the midpoint of the peak of the resulting DSC curve was taken as the homopolymer glass transition temperature (Tg).

2-1. Preparation of core particles Production Example 1
A four-neck flask equipped with a condenser, a thermometer, and a dropping port was charged with 384 parts of ion-exchanged water, 1.2 parts of 25% aqueous ammonia, and 236 parts of methanol, and while stirring, 100 parts of 3-methacryloxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., "KBM503") was added through the dropping port to carry out a hydrolysis and condensation reaction of 3-methacryloxypropyltrimethoxysilane, thereby preparing an emulsion of polysiloxane particles (polymerizable polysiloxane particles) having methacryloyl groups. Two hours after the start of the reaction, the resulting emulsion of polysiloxane particles was sampled, and the particle diameter was measured. The number average particle diameter was found to be 7.3 μm.

Next, a solution of 405 parts of n-butyl methacrylate, 405 parts of n-butyl acrylate, 90 parts of triethylene glycol dimethacrylate, 100 parts of 2-hydroxyethyl methacrylate, and 25 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (Wako Pure Chemical Industries, Ltd., V-65) was added to a solution of 25 parts of a 20% aqueous solution of polyoxyethylene styrenated phenyl ether sulfate ester ammonium salt (Dai-ichi Kogyo Seiyaku Co., Ltd., "Hitenol (registered trademark) NF-08") as an emulsifier in 1,000 parts of ion-exchanged water, and the resulting mixture was emulsified and dispersed to prepare an emulsion of the core monomer.

Furthermore, 150 parts of triethylene glycol dimethacrylate was added to a solution prepared by dissolving 3.8 parts of a 20% aqueous solution of polyoxyethylene styrenated phenyl ether sulfate ester ammonium salt (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., "Hitenol (registered trademark) NF-08") as an emulsifier in 150 parts of ion-exchanged water, and emulsified and dispersed to prepare an emulsion of the shell monomer.

The resulting emulsion of core monomer was added to the emulsion of polymerizable polysiloxane particles, followed by further stirring. One hour after adding the emulsion, a sample of the mixture was taken and observed under a microscope, confirming that the polymerizable polysiloxane particles had absorbed the monomer and become enlarged.

Next, 1,250 parts of a 10% aqueous solution of polyvinyl alcohol ("Poval PVA-205" manufactured by Kuraray Co., Ltd.) and 1,005 parts of ion-exchanged water were added, and the reaction solution was heated to 65°C under a nitrogen atmosphere with continued stirring, thereby carrying out radical polymerization of the core monomer.

Polymerization of the core monomer began, and the temperature inside the flask reached its peak temperature. Immediately after the maximum value was reached, the emulsion of the shell monomer was added. After the addition, the mixture was maintained at 65°C for two hours, allowing radical polymerization of the shell monomer to occur. The emulsion after radical polymerization was subjected to solid-liquid separation, and the resulting cake was washed with ion-exchanged water and methanol, then vacuum-dried at 40°C for 12 hours to obtain Resin Particle No. 1.

(Production Examples 2 to 12, 14 to 17)
Resin particles Nos. 2 to 12 and 14 to 17 were obtained in the same manner as in Production Example 1, except that the amounts of ion-exchanged water, methanol, and aqueous ammonia were appropriately changed to prepare polysiloxane particles having number-based average particle sizes as shown in Table 1, and that the types and amounts of core monomers and shell monomers used were changed as shown in Table 1. Note that no shells were formed for resin particles Nos. 9 and 14 to 17.

(Production Example 13)
A four-neck flask equipped with a condenser, a thermometer, and a dropping nozzle was charged with 200 parts of a 10% aqueous solution of polyvinyl alcohol ("Poval PVA-205" manufactured by Kuraray Co., Ltd.) and maintained at 25°C. While stirring at 300 rpm, a solution containing 27 parts of isooctyl acrylate as a core monomer, 3.0 parts of 1,6-hexanediol diacrylate, and 0.7 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) ("V-65" manufactured by Wako Pure Chemical Industries, Ltd.) was added through the dropping nozzle, and the mixture was maintained for 2 hours to prepare a suspension. With continued stirring, the suspension was heated to 65°C under a nitrogen atmosphere to carry out radical polymerization of the core monomer.

Next, in a separate flask, 0.1 parts of a 20% aqueous solution of polyoxyethylene styrenated phenyl ether sulfate ester ammonium salt ("Hitenol (registered trademark) NF-08" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) serving as an emulsifier was dissolved in 3.0 parts of ion-exchanged water, to which 3.0 parts of divinylbenzene was added as a shell monomer, and the mixture was emulsified and dispersed to prepare an emulsion of the shell monomer.

Polymerization of the core monomer began, and the temperature inside the flask reached its peak temperature. Immediately after the maximum value was reached, the emulsion of the shell monomer was added. After the addition, the mixture was maintained at 65°C for two hours to allow radical polymerization of the shell monomer. The emulsion after radical polymerization was subjected to solid-liquid separation, vacuum dried at 40°C for 12 hours, and classified to obtain resin particles No. 13. The physical properties of the resulting resin particles Nos. 1 to 17 were as shown in Table 2.

In Table 1, the monomers are shown by abbreviations, and the compound names corresponding to each abbreviation are as follows: For non-crosslinkable monomers, the homopolymer glass transition temperature (Tg) is also shown.
nBA: n-butyl acrylate (Tg: 219K)
2EHA: 2-ethylhexyl acrylate (Tg: 223K)
IOA: Isooctyl acrylate (Tg: 212K)
nBMA: n-butyl methacrylate (Tg: 293K)
HEMA: 2-hydroxyethyl methacrylate (Tg: 328K)
St: styrene (Tg: 380K)
CHA: cyclohexyl acrylate (Tg: 292K)
MMA: methyl methacrylate (Tg: 378K)
KBM503: 3-methacryloxypropyltrimethoxysilane 16HXA: 1,6-hexanediol diacrylate 16HX: 1,6-hexanediol dimethacrylate 3EG: Triethylene glycol dimethacrylate DVB: Divinylbenzene

2-2. Preparation of conductive fine particles (formation of conductive metal layer)
Example 1
Resin particles No. 1 synthesized in Production Example 1 were subjected to an etching treatment with sodium hydroxide, followed by sensitization by contact with a tin dichloride solution, and then activation by immersion in a palladium dichloride solution (sensitization-activation method) to form palladium nuclei. Next, 2 parts of the resin particles with palladium nuclei formed were added to 400 parts of ion-exchanged water, and ultrasonic dispersion was performed. The resulting resin particle suspension was then heated in a warm bath at 70°C. While the suspension was heated in this manner, 600 parts of an electroless plating solution ("Shumer S680" manufactured by Nippon Kangen Co., Ltd.) separately heated to 70°C was added to induce an electroless nickel plating reaction. After confirming that hydrogen gas generation had ceased, solid-liquid separation was performed, followed by washing with ion-exchanged water and then methanol, and vacuum drying at 100°C for 2 hours to obtain nickel-plated particles. The obtained nickel-plated particles were then added to a displacement gold plating solution containing gold potassium cyanide, and the surface of the nickel layer was further plated with gold to obtain conductive fine particles No. 1. The film thickness of the conductive metal layer in the obtained conductive fine particles was as shown in Table 3.

Examples 2 to 8, Comparative Examples 1 to 9
Conductive fine particles Nos. 2 to 17 were produced in the same manner as in Example 1, except that resin particles Nos. 2 to 17 synthesized in Production Examples 2 to 17 were used as the base material. The film thicknesses of the conductive metal layers in the obtained conductive fine particles were as shown in Table 3.

Using the obtained conductive particles Nos. 1 to 17, anisotropic conductive films were produced by the following method, and the performance thereof was evaluated by the following method.
That is, for each of conductive microparticles No. 1 to 17, 100 parts of an epoxy resin ("JER828" manufactured by Mitsubishi Chemical) as a binder resin, 2 parts of a curing agent ("SAN-AID (registered trademark) SI-150" manufactured by Sanshin Chemical Co., Ltd.), and 100 parts of toluene were added to 1 part of the conductive microparticles, and 50 parts of zirconia beads with a diameter of 1 mm were further added. The mixture was dispersed by stirring at 300 rpm for 10 minutes using two stainless steel stirring blades. The resulting paste-like composition was then applied to a release-treated PET film using a bar coater and dried to obtain an anisotropic conductive film.

The resulting anisotropic conductive films were divided into those containing resin particles with a number average particle diameter of 10.0 μm or more and those containing resin particles with a number average particle diameter of less than 10.0 μm, and the initial resistance value of each was measured under the following conditions.

(Number average particle diameter 10.0 μm or more)
The anisotropic conductive film was sandwiched between an aluminum-deposited glass substrate having lines for resistance measurement and a polyimide film substrate having a copper pattern formed at a pitch of 100 μm, and thermocompression bonded under compression conditions of 2.5 MPa and 150° C. Thereafter, the initial resistance between the electrodes was measured, and the connection resistance was evaluated as "good" when the initial resistance was 5 Ω or less, and as "poor" when the initial resistance was more than 5 Ω.

(Number average particle diameter less than 10.0 μm)
The anisotropic conductive film was sandwiched between a glass substrate with aluminum vapor deposition on the entire surface, which had lines for resistance measurement, and a polyimide film substrate with a copper pattern formed at a 30 μm pitch, and thermocompression bonded under conditions of 1 MPa and 150°C. The initial resistance between the electrodes was then measured, and connection resistance was evaluated as "good" when the initial resistance was 5 Ω or less, and as "poor" when it exceeded 5 Ω. The evaluation results are shown in Table 3.

As is clear from Table 3, Examples 1 to 8, which used resin particles that met the requirements of the present invention, all had low initial resistance and excellent connection reliability.

On the other hand, in Comparative Examples 1 to 9, which used resin particles that did not meet the requirements of the present invention, the initial resistance was high and the connection reliability was poor.

The resin particles of the present invention are suitable for use in anisotropic conductive materials such as anisotropic conductive pastes, anisotropic conductive films, anisotropic conductive adhesives, and anisotropic conductive inks.

Claims (6)

Resin particles for conductive fine particles,
the resin particles are core-shell particles having a core and a shell covering the core, and the ratio of the average thickness of the shell to the number-average particle diameter of the resin particles is 0.01 or more and 0.05 or less;
the core is a copolymer containing, as monomer units, a silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) of 0°C or lower, a silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) of more than 0°C, and a silane-based monomer;
the content of the silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) of 0°C or lower is 15% by mass or more and 50% by mass or less, based on 100% by mass of the total amount of monomers forming the core;
the shell contains, as a monomer unit, a monomer (1) having two or more vinyl groups in one molecule , and the content of the monomer (1) is 50% by mass or more relative to 100% by mass of the monomer components forming the shell;
the resin particles have a compressive deformation recovery rate of 25% or less,
The compressive elastic modulus (10% K value) when the diameter of the resin particles is displaced by 10% is 1500 N/ ^mm2 or less,
Resin particles for conductive fine particles, characterized in that the number-based coefficient of variation (CV) of the particle diameter of said resin particles is 10% or less. Resin particles for conductive microparticles according to claim 1, wherein the content of the silicon-free monofunctional (meth)acrylate having a homopolymer glass transition temperature (Tg) above 0°C is 35% by mass or more and 70% by mass or less out of a total of 100% by mass of the monomers forming the core. Resin particles for conductive microparticles according to claim 1 or 2, wherein the glass transition temperature (Tgcore) of the core calculated based on the FOX equation is 0°C or lower. 4. The resin particle for conductive fine particles according to claim 1, wherein the core has a degree of crosslinking represented by the following formula of 30% or less.
Degree of crosslinking (%) = (mass of crosslinkable monomer/mass of total monomer) x 100 Resin particles for conductive microparticles according to any one of claims 1 to 4, wherein the average thickness of the shell is 0.05 μm or more and 1 μm or less. 6. The resin particles for conductive fine particles according to claim 1, which have a dispersion rate of 90% or more as calculated by the following measurement method.
(Method for measuring dispersion ratio)
(1) 2.5 g of resin particles and 100 g of a 1% aqueous solution of polyoxyethylene alkyl ether sulfate ester ammonium salt, which is an emulsifier, are placed in a 200 mL beaker and ultrasonically dispersed for 15 minutes, and then passed through a sieve with openings 1.6 μm times the number average particle diameter.
(2) Dry the sieve at 120°C for 2 hours.
(3) The total weight (g) of the sieve and the polymer particles remaining on the sieve is measured, and the weight (g) of the sieve is subtracted to determine the weight (g) of the polymer particles remaining on the sieve.
(4) The weight (g) of the polymer particles remaining on the sieve is applied to the following formula to calculate the dispersion rate (%).
Dispersion rate (%)=100−((weight of polymer particles remaining on the sieve (g)/2.5 (g))×100)