Polymer particles and method for producing same

Abstract

The present invention provides polymer particles wherein the degree of yellowing during heating is suppressed. With respect to the polymer particles according to the present invention, since the proportion of the cumulative number of particles that have a particle diameter of 60% or less of the median diameter in the number-based particle size distribution is set to 17% or less and the number of small-diameter particles is set to a specific proportion or less, yellowing of the polymer particles due to heating is reduced. As a result, a product that contains the polymer particles can be prevented from the occurrence of coloring due to yellowing of the polymer particles, and the polymer particles are suitable for use in the production of articles for optical applications.

Description

Polymer particles and their manufacturing method

[0001] The present invention relates to polymer particles and a method for producing the same.

[0002] Polymer particles having a volume average particle size of 1 to 100 μm have conventionally been widely used in applications such as matting agents for paints and inks, additives for adhesives, shrinkage reducing agents for artificial marble, paper treatment agents, fillers for external preparations such as cosmetics, column fillers used in chromatography, additives for toners used in electrostatic image development, antiblocking agents for films, and light diffusing agents for optical sheets such as light diffusing sheets, antiglare sheets, and light diffusing sheets.

[0003] In particular, when polymer particles are used as a light diffusing agent, they are dispersed in a light diffusing sheet, and in order to prevent the light diffusing sheet from yellowing, it is necessary for the polymer particles themselves to have a small degree of yellowing at high temperatures. If yellowing occurs during molding of the light diffusing sheet, the total light transmittance decreases, and the desired properties may not be obtained.

[0004] The polymer particles are generally produced by polymerizing a monomer using a general-purpose polymerization method such as suspension polymerization, seed polymerization, emulsion polymerization, etc. In these polymerization methods, a surfactant is usually used to stably carry out the polymerization reaction and to suppress the generation of coarse particles.

[0005] For example, Patent Document 1 discloses a light diffusing agent that consists of resin microparticles obtained by polymerizing a vinyl monomer in a medium containing a surfactant, and in which the amount of surfactant remaining in the resin microparticles is 0.05 parts by weight or less per 100 parts by weight of the resin microparticles.

[0006] Japanese Patent Application Laid-Open No. 2006-233055

[0007] However, although the light diffusing agent prevents yellowing during heating by limiting the amount of surfactant remaining in the resin particles to a certain level, it does not necessarily sufficiently prevent yellowing. For example, when heated in an oven at 180°C for 3 hours, the degree of yellowing may increase, and the prevention of yellowing of the resin particles is not sufficient.

[0008] Furthermore, if the amount of surfactant used during the production of resin microparticles is reduced in order to reduce the amount of surfactant remaining in the resin microparticles, another problem arises in that the growing particles aggregate during polymerization, making it difficult to produce the resin microparticles.

[0009] The present invention provides polymer particles with reduced yellowing upon heating and a method for producing the same.

[0010] The polymer particles of the present invention are characterized in that the cumulative number ratio of particles having a particle size of 60% or less of the median size in the particle size distribution on a number basis is 17% or less.

[0011] The method for producing polymer particles of the present invention is a method for producing polymer particles in which raw material monomers are polymerized in a dispersed state in an aqueous medium, and is characterized in that a surfactant is supplied to the aqueous medium when the polymerization rate of the raw material monomers is less than 94% and when the polymerization rate is 94% or more.

[0012] As described above, in the polymer particles of the present invention, the cumulative number ratio of particles having a particle diameter that is 60% or less of the median diameter in the number-based particle size distribution is 17% or less, and the number of small-diameter particles is a predetermined ratio or less, thereby reducing yellowing of the polymer particles due to heating.

[0013] Therefore, it is possible to prevent the occurrence of coloring in products containing polymer particles due to yellowing of the polymer particles, and the composition can be suitably used in the production of products for optical applications.

[0014] In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in an example or a value that can be unambiguously derived from an example. In this specification, a numerical value connected with "to" means a numerical range that includes the numbers before and after "to" as the lower and upper limits.

[0015] [Polymer Particles] The polymer particles of the present invention have a cumulative number ratio of particles having a median diameter of 60% or less in the number-based particle size distribution of 17% or less.

[0016] In the particle size distribution based on the number of polymer particles, the horizontal axis represents particle size (particle diameter) and the vertical axis represents number (%), and the cumulative number ratio of particles having a particle size of 60% or less of the median size (small particle size) is set to 17% or less, thereby reducing the content of small particle size. Note that the particle size on the horizontal axis in the particle size distribution based on the number of particles is the spherical equivalent diameter.

[0017] The polymer particles are used by being contained in a synthetic resin. To produce a product containing the polymer particles, for example, the synthetic resin and the polymer particles are supplied to a known kneading device such as an extruder and melt-kneaded in the kneading device, or the polymer particles are dispersed in a binder resin of a paint and applied, and if necessary, heated for drying.

[0018] It has long been recognized that heating during the manufacturing process of a product containing polymer particles can cause the polymer particles to yellow, and as mentioned above, it has been disclosed that surfactants are the cause. However, the inventors have investigated this issue and found that the main cause is the small diameter particles contained in the polymer particles, rather than the amount of surfactant contained in the polymer particles.

[0019] As described above, polymer particles are generally heated in the process of producing a product containing polymer particles. If small-diameter particles are included in the polymer particles, they are likely to aggregate due to the influence of large-diameter particles (hereinafter sometimes referred to as "main particles") excluding the small-diameter particles during melt-kneading of the synthetic resin or when dispersed in the binder resin. Furthermore, due to their small particle size, the small-diameter particles are easily deteriorated as a whole by heating. Therefore, the small-diameter particles are likely to turn yellow as a whole due to heat deterioration, and the yellowness becomes more noticeable due to aggregation and aggregation.

[0020] On the other hand, since the main particles have a larger particle diameter than the small diameter particles, even when heated in the same manner as the small diameter particles, the main particles are less likely to be heated as a whole. Therefore, thermal degradation in the main particles occurs in a dispersed manner throughout the particles, and the yellowing caused by thermal degradation is dispersed, making the yellowing less likely to appear than in the small diameter particles.

[0021] As described above, the polymer particles contain a reduced number of small-diameter particles to reduce the occurrence of yellowing, and by reducing the number of small-diameter particles, the particle size of the entire polymer particles is made uniform, and by maintaining a uniformly dispersed state in the synthetic resin, aggregation is prevented, reducing an increase in thermal conductivity, and reducing yellowing of the entire polymer particles due to thermal deterioration.

[0022] The number-based particle size distribution of polymer particles is measured as follows. When the arithmetic (number) average particle diameter in the number-based particle size distribution of polymer particles is 1 μm or more, it is measured as follows. The arithmetic (number) average particle diameter in the number-based particle size distribution of polymer particles is measured using a Coulter Multisizer™ 4e (a measuring device manufactured by Beckman Coulter, Inc.). The measurement is carried out using an aperture calibrated in accordance with the Multisizer 3 User's Manual published by Beckman Coulter, Inc.

[0023] The aperture used for the measurement is appropriately selected depending on the size of the polymer particles to be measured. Current (aperture current) and Gain are appropriately set depending on the size of the selected aperture. Table 1 shows the volume average particle diameter of the polymer particles and the aperture diameter corresponding to this volume average particle diameter.

[0024]

[0025] The measurement sample is a dispersion prepared by dispersing 0.1 g of polymer particles in 10 mL of a 0.1% by mass aqueous solution of a nonionic surfactant using a touch mixer (trade name "TOUCHMIXER MT-31" manufactured by Yamato Scientific Co., Ltd.) and an ultrasonic cleaner (trade name "ULTRASONIC CLEANER VS-150" manufactured by Vervoclear Co., Ltd.). During the measurement, the contents of the beaker are gently stirred to an extent that no air bubbles are introduced, and the measurement is terminated when 100,000 polymer particles have been measured. The particle diameter of the polymer particles is the equivalent sphere diameter. In other words, the particle diameter of the polymer particles is the diameter of a perfect sphere having the same volume as the polymer particles. The arithmetic mean particle diameter in the number-based particle size distribution of the polymer particles is the arithmetic mean value in the number-based particle size distribution of 100,000 particles. The median diameter in the number-based particle size distribution of polymer particles refers to the diameter D50 at which the cumulative frequency (number) is 50% in the number-based particle size distribution of 100,000 particles.

[0026] In the number-based particle size distribution of polymer particles, the coefficient of variation (CV value) of particle size of polymer particles is calculated by the following formula: Coefficient of variation (CV value) of particle size of polymer particles = 100 × (standard deviation of particle sizes in the number-based particle size distribution of polymer particles) / [arithmetic (number) average particle size in the number-based particle size distribution of polymer particles]

[0027] When the arithmetic mean particle diameter in the number-based particle size distribution of polymer particles is less than 1 μm, it is measured as follows. The arithmetic mean particle diameter in the number-based particle size distribution of polymer particles is measured using a laser diffraction / scattering particle size analyzer (model "LS230" manufactured by Beckman Coulter, Inc.). Specifically, 0.1 g of polymer particles (equivalent to 0.1 g in the case of a dispersion) and 20 mL of a 2% by mass anionic surfactant solution are placed in a test tube. Thereafter, the polymer particles are dispersed for 5 minutes using a test tube mixer (manufactured by AS ONE Corporation, "Test Tube Mixer TRIO HM-1N") and an ultrasonic cleaner (manufactured by AS ONE Corporation, "ULTRASONIC CLEANER VS-150") to prepare a dispersion. The arithmetic mean particle diameter (number) of the polymer particles in the dispersion was measured using the laser diffraction / scattering particle size analyzer while irradiating the resulting dispersion with ultrasound. The optical model used during measurement was adjusted to the refractive index of the produced polymer particles. When one type of monomer is used to produce the polymer particles, the refractive index of the homopolymer of that monomer is used as the refractive index of the polymer particles. When multiple types of monomers are used to produce the polymer particles, the average value obtained by weighting the refractive index of the homopolymer of each monomer with the amount of each monomer used is used as the refractive index of the polymer particles. The coefficient of variation (CV value) of the particle size of the polymer particles is calculated in the same manner as when the arithmetic (number) average particle size in the number-based particle size distribution of the polymer particles is 1 μm or more. <Measurement conditions for laser diffraction scattering particle size distribution analyzer> Medium = water Refractive index of medium = 1.333 Refractive index of solid = refractive index of polymer particles Relative concentration of PIDS: 40 to 55%

[0028] The volume average particle size in the volume-based particle size distribution of polymer particles refers to an arithmetic average value obtained based on the volume-based particle size distribution obtained by measuring in the same manner as the arithmetic (number) average particle size in the number-based particle size distribution of polymer particles described above.

[0029] The median diameter of the polymer particles in the number-based particle size distribution is preferably 0.1 μm or more, more preferably 0.2 μm or more, more preferably 0.5 μm or more, and more preferably 1 μm or more. The median diameter of the polymer particles in the number-based particle size distribution is preferably 50 μm or less, more preferably 45 μm or less, more preferably 40 μm or less, and more preferably 35 μm or less. When the median diameter of the polymer particles in the number-based particle size distribution is within the above range, the influence of yellowing due to thermal degradation of small-diameter particles of the polymer particles can be effectively reduced, and the thermal yellowing of the polymer particles can be effectively reduced.

[0030] The cumulative number ratio of the polymer particles, i.e., small diameter particles, which are 60% or less of the median diameter in the particle size distribution based on the number of particles, is 17% or less, and preferably 16% or less. As described above, particles having a particle size of 60% or less of the median diameter are likely to aggregate due to the influence of the main particles, and aggregation improves thermal conductivity, making the small diameter particles more likely to yellow. However, as described above, the cumulative number ratio of the small diameter particles is set to a predetermined amount or less, so that the polymer particles are less likely to yellow when heated.

[0031] The number-based arithmetic mean particle diameter Dp of the polymer particles is preferably 0.1 μm or more, more preferably 0.2 μm or more, more preferably 0.5 μm or more, preferably 1 μm or more, more preferably 2 μm or more, and more preferably 3 μm or more. The number-based arithmetic mean particle diameter Dp of the polymer particles is preferably 50 μm or less, more preferably 45 μm or less, more preferably 40 μm or less, and more preferably 35 μm or less. When the number-based arithmetic mean particle diameter Dp of the polymer particles is in the above range, the influence of yellowing due to thermal degradation of small diameter particles of the polymer particles can be effectively reduced, and the thermal yellowing of the polymer particles can be effectively reduced.

[0032] In the particle size distribution based on the number of polymer particles, the CV value of the particle diameter of the polymer particles is preferably 70% or less, more preferably 69% or less. When the CV value of the particle diameter of the polymer particles is 70% or less, the influence of yellowing due to thermal deterioration of small-sized polymer particles can be effectively reduced, and the thermal yellowing of the polymer particles can be effectively reduced.

[0033] Even if the polymer particles contain a relatively large amount of surfactant, the number of small-diameter particles is kept below a predetermined ratio, thereby reducing yellowing during heating. The amount of surfactant adhering to the surface of the polymer particles is preferably 0.2 g or less per 100 g of polymer particles. When the amount of surfactant adhering to the surface of the polymer particles is within the above range, yellowing of the polymer particles due to heating can be reduced.

[0034] The amount of surfactant (g / 100g) adhering to the surface of polymer particles refers to a value calculated as follows. Approximately 0.10g of polymer particles are accurately weighed into a centrifuge tube as a sample, and 5mL of methanol is added as an extractant using a whole pipette. The polymer particles and the extractant are thoroughly mixed. Ultrasonic extraction is performed for 15 minutes, followed by centrifugation at 3500 rpm for 15 minutes. The resulting supernatant is used as the test solution. The surfactant concentration in this test solution is measured. The amount of surfactant (g) adhering to the surface of 100g of polymer particles is calculated using the following formula from the measured surfactant concentration (g / mL) in the test solution, the mass of the polymer particles used as the sample [sample mass (g)], and the amount of extractant (extraction volume (mL)]. The volume of extractant is 5mL. Amount of surfactant (g / 100g) = 100 x [surfactant concentration in test solution (g / mL) x extraction volume (mL)] / sample mass (g).

[0035] The synthetic resin constituting the polymer particles is not particularly limited, and examples thereof include acrylic resins, styrene resins, etc. The synthetic resins may be used alone or in combination of two or more.

[0036] The acrylic resin is obtained by polymerizing raw material monomers including an acrylic monomer, and is preferably obtained by polymerizing raw material monomers including an acrylic monomer and a polyfunctional monomer. The acrylic resin is preferably crosslinked with a polyfunctional monomer. The acrylic monomer is not particularly limited, and examples thereof include acrylic acid, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, dodecyl acrylate, stearyl acrylate, 2-ethylhexyl acrylate, tetrahydrofurfuryl acrylate, methacrylic acid, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, n-octyl methacrylate, dodecyl methacrylate, 2-ethylhexyl methacrylate, and stearyl methacrylate.

[0037] Examples of polyfunctional monomers include acrylic polyfunctional monomers such as trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, decaethylene glycol di(meth)acrylate, pentadecaethylene glycol di(meth)acrylate, pentacontahexaethylene glycol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, 1,3-butylene di(meth)acrylate, and allyl (meth)acrylate; and aromatic divinyl compounds such as divinylbenzene, divinylnaphthalene, and derivatives thereof. Note that (meth)acrylate means acrylate or methacrylate.

[0038] The styrene-based resin is not particularly limited, and examples thereof include homopolymers of styrene-based monomers such as styrene, α-methylstyrene, vinyltoluene, chlorostyrene, ethylstyrene, i-propylstyrene, dimethylstyrene, and bromostyrene, and copolymers thereof.

[0039] The styrene-based resin may also be a copolymer of the styrene-based monomer and a vinyl monomer copolymerizable with the styrene-based monomer, the copolymer containing the styrene-based monomer component as the main component. Examples of such vinyl monomers include polyfunctional monomers such as divinylbenzenes (e.g., o-divinylbenzene, m-divinylbenzene, p-divinylbenzene), alkylene glycol di(meth)acrylates (e.g., ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate), (meth)acrylonitrile, methyl (meth)acrylate, etc., with polyfunctional monomers being preferred, and ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and divinylbenzene being more preferred, with divinylbenzene and ethylene glycol di(meth)acrylate being particularly preferred. The monomers copolymerizable with styrene may be used alone or in combination.

[0040] The yellowing index of the polymer particles when heated at 180° C. for 3 hours is preferably 3.0 or less, more preferably 2.8 or less, and still more preferably 2.6 or less. When the yellowing index of the polymer particles when heated at 180° C. for 3 hours is 3.0 or less, the yellowing of the polymer particles due to heating can be reduced.

[0041] The yellowing degree of polymer particles when heated at 180°C for 3 hours is measured as follows: The b* value of polymer particles (crosslinked polystyrene particles) is measured by measuring color in the L*a*b* color system in accordance with JIS Z8729.

[0042] Specifically, 3.0 g of polymer particles are filled into a measurement container. The b* value of the filled polymer particles is measured using a colorimeter. The b* value of the obtained polymer particles is defined as the initial b* value.

[0043] Next, the polymer particles are placed in an oven maintained at 180°C and heated for 3 hours. The polymer particles are removed from the oven and left to stand in an atmosphere of 25°C for 1 hour. Thereafter, the b* value of the polymer particles is measured in the same manner as above, and this is taken as the b* value after heating. The yellowing index is calculated based on the following formula: Yellowing index = b* value after heating / initial b* value

[0044] [Method for Producing Polymer Particles] Next, a method for producing polymer particles will be described. The method for producing polymer particles is not particularly limited, and the polymer particles can be produced by polymerizing raw material monomers in the presence of a polymerization initiator as needed in a general manner.

[0045] The polymerization method is not particularly limited, and general polymerization methods such as suspension polymerization, seed polymerization, bulk polymerization, and solution polymerization can be used, with suspension polymerization being preferred and emulsion polymerization being more preferred. Emulsion polymerization is a polymerization method in which raw material monomers are dispersed in an aqueous medium and polymerized in the presence of a polymerization initiator and an emulsifier, and is included in suspension polymerization. Note that, as the aqueous medium, water or a mixture of water and an organic solvent [e.g., a lower alcohol (alcohol having 5 or less carbon atoms)] can be used, with water being preferred.

[0046] Conventionally, when the dispersion is heated to polymerize the raw material monomer, the entire amount of surfactant required for polymerization of the raw material monomer is supplied to the dispersion before the start of polymerization. However, it has been found that supplying the entire amount of surfactant to the dispersion before the start of polymerization can result in excessive generation of droplets of the raw material monomer in the early stage of polymerization, resulting in the production of excessive small-diameter particles, and also in the later stage of polymerization of the raw material monomer, resulting in the unstable dispersion state of the raw material monomer, resulting in the production of small-diameter particles.

[0047] The following describes the case where polymer particles are produced by emulsion polymerization. In the method for producing polymer particles of the present invention, a surfactant is supplied to an aqueous medium in separate portions when the polymerization rate of the raw material monomer is less than 94% and when the polymerization rate of the raw material monomer is 94% or more. This allows the raw material monomer to be stably dispersed in the aqueous medium throughout the polymerization process, allowing the growing particles to grow stably without agglomeration, thereby reducing the generation of small-diameter particles. The polymerization rate (%) of the raw material monomer refers to the percentage of the "raw material monomer used in polymerization" relative to the "total amount of raw material monomer supplied to the dispersion."

[0048] In the method for producing polymer particles, a surfactant is supplied to an aqueous medium in which raw material monomers are dispersed when the polymerization rate of the raw material monomers is less than 94%, and the raw material monomers are polymerized (Step 1). The timing for supplying the surfactant to the aqueous medium may be either before or after the start of polymerization, as long as the polymerization rate of the raw material monomers is less than 94%. Alternatively, the surfactant may be supplied to the aqueous medium in which the raw material monomers are dispersed in multiple divided portions.

[0049] The surfactant is supplied to the aqueous medium when the polymerization rate of the raw material monomer is less than 94%, but it is preferable to supply the surfactant to the aqueous medium before the raw material monomer starts to polymerize. By supplying the surfactant to the aqueous medium before the start of polymerization of the raw material monomer, the raw material monomer in the dispersion liquid produced by dispersing the raw material monomer in the aqueous medium can be stabilized before polymerization of the raw material monomer can be carried out, and the generation of small-diameter particles can be further reduced.

[0050] When the surfactant is supplied to the aqueous medium before the initiation of polymerization of the raw material monomers, the order in which the raw material monomers and the surfactant are supplied to the aqueous medium does not matter. To produce a dispersion by dispersing the raw material monomers in the aqueous medium, a general-purpose mixing device (for example, a homogenizer, an ultrasonic processor, or a microemulsifier such as Nanomizer (registered trademark)) may be used.

[0051] When a surfactant is supplied to a dispersion in which the polymerization rate of the raw material monomer is less than 94%, it is preferable to supply the surfactant when the polymerization rate of the raw material monomer is 0 to 90%, more preferably when the polymerization rate of the raw material monomer is 0 to 85%, and even more preferably when the polymerization rate of the raw material monomer is 0 to 80%. The raw material monomer can be polymerized after the raw material monomer is stably dispersed in the dispersion in which the raw material monomer is dispersed in an aqueous medium, thereby further reducing the generation of small-diameter particles. The polymerization rate of the raw material monomer of 0% means "before the start of polymerization of the raw material monomer."

[0052] Of the surfactants to be supplied to a dispersion in which the polymerization rate of the raw material monomer is less than 94%, preferably 70% by mass or more, more preferably 80% by mass or more, more preferably 90% by mass or more, more preferably 99% by mass or more, and more preferably 100% by mass is supplied before the start of polymerization of the raw material monomer. By supplying the surfactant to the dispersion in this manner, the raw material monomer in the dispersion can be brought into a stable dispersed state before polymerization of the raw material monomer is carried out, and the generation of small-diameter particles can be further reduced.

[0053] The amount of surfactant supplied to a dispersion having a polymerization rate of less than 94% of the raw material monomer is preferably 0.005 parts by mass or more, more preferably 0.010 parts by mass or more, and more preferably 0.013 parts by mass or more, per 100 parts by mass of the raw material monomer. The amount of surfactant supplied to a dispersion having a polymerization rate of less than 94% of the raw material monomer is preferably 0.15 parts by mass or less, more preferably 0.13 parts by mass or less, and more preferably 0.10 parts by mass or less, per 100 parts by mass of the raw material monomer. When the amount of surfactant is 0.005 parts by mass or more, the raw material monomer can be stably dispersed in an aqueous medium, and the generation of small-sized particles can be reduced. When the amount of surfactant is 0.15 parts by mass or less, the generation of flat particles and irregular-shaped particles can be reduced, and spherical polymer particles can be easily produced.

[0054] The polymerization temperature (temperature of the dispersion) in the first step can be appropriately selected depending on the type of raw material monomer and the type of polymerization initiator used as needed, which will be described later, but is preferably 25 to 110° C., and more preferably 50 to 100° C. When the polymerization temperature in the first step is within the above range, the raw material monomer can be polymerized after the raw material monomer in the dispersion is in a stable dispersed state, and the generation of small-diameter particles can be further reduced.

[0055] A surfactant is supplied to a dispersion in which the polymerization rate of the raw material monomer is less than 94%, and the dispersion is heated as necessary to polymerize the raw material monomer. As the polymerization of the raw material monomer in the dispersion progresses, the number of growing particles increases and the particle size also increases, so the dispersion stability of the growing particles due to the surfactant supplied in the first step decreases.

[0056] Therefore, when the polymerization rate of the raw material monomer reaches 94% or more, a surfactant is supplied to the dispersion and polymerization is carried out (step 2). In this way, by further supplying a surfactant to the dispersion in the later stage of polymerization when the polymerization rate of the raw material monomer reaches 94% or more, the dispersion stability of the grown particles dispersed in the dispersion is ensured and the generation of small-diameter particles is reduced.

[0057] The polymerization temperature (temperature of the dispersion) in the second step is preferably 40 to 60° C. higher, more preferably 45 to 60° C. higher, and even more preferably 50 to 55° C. higher than the polymerization temperature (temperature of the dispersion) in the first step. By adjusting the polymerization temperature in the second step to a temperature higher than the polymerization temperature in the first step and supplying a surfactant to the dispersion, the remaining raw material monomers can be absorbed into the growing particles while reducing the production of small-diameter particles, and polymer particles with a low content of small-diameter particles can be easily produced.

[0058] When a surfactant is supplied to a dispersion in which the polymerization rate of the raw material monomer is 94% or more, the surfactant is preferably supplied when the polymerization rate of the raw material monomer is 94 to 100%, more preferably when the polymerization rate of the raw material monomer is 94 to 99%, even more preferably when the polymerization rate of the raw material monomer is 94 to 98%, and still more preferably when the polymerization rate of the raw material monomer is 94 to 96%. Polymerization of the raw material monomer can be carried out after the raw material monomer in the dispersion is stabilized, and the generation of small-diameter particles can be further reduced.

[0059] Of the surfactants supplied to a dispersion having a polymerization rate of 94% or higher of the raw material monomer, preferably 70% by mass or higher, more preferably 80% by mass or higher, more preferably 90% by mass or higher, more preferably 99% by mass or higher, and more preferably 100% by mass is supplied to a dispersion having a polymerization rate of 94 to 96% of the raw material monomer. By supplying the surfactant to the dispersion in this manner, the growing particles and raw material monomer in the dispersion can be brought into a stable dispersed state, and then the raw material monomer can be polymerized, thereby further reducing the generation of small-diameter particles.

[0060] The amount of surfactant supplied to a dispersion having a polymerization rate of 94% or more of the raw material monomer is preferably 0.10 parts by mass or more, preferably 0.15 parts by mass or more, and preferably 0.25 parts by mass or more, per 100 parts by mass of the raw material monomer. The amount of surfactant supplied to a dispersion having a polymerization rate of 94% or more is preferably 0.80 parts by mass or less, preferably 0.60 parts by mass or less, and preferably 0.40 parts by mass or less, per 100 parts by mass of the raw material monomer. When the amount of surfactant is 0.10 parts by mass or more, the grown particles and the raw material monomer can be stably dispersed in the dispersion, and the generation of small-diameter particles can be reduced. When the amount of surfactant is 0.80 parts by mass or less, the amount of water required for washing can be reduced, which is environmentally friendly.

[0061] As the surfactant used in the production of polymer particles, known surfactants such as ester type, ether type, and ester-ether type surfactants can be used. Examples of surfactants include polyoxyethylene alkyl ethers such as polyoxyethylene tridecyl ether, polyoxyethylene alkylphenyl ethers such as polyoxyethylene octylphenyl ether, polyoxyethylene styrenated phenyl ether, polyoxyethylene fatty acid esters, polyoxyethylene sorbitan fatty acid esters such as polyoxyethylene sorbitan monolaurate, polyoxyethylene alkylamines, oxyethylene-oxypropylene block polymers, fatty acid soaps such as sodium oleate and castor oil potassium soap, alkyl sulfates such as sodium dodecyl sulfate, sodium lauryl sulfate, and ammonium lauryl sulfate, alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate, alkylnaphthalenesulfonates, alkanesulfonates, dialkyl sulfosuccinates such as sodium di(2-ethylhexyl)sulfosuccinate and sodium dioctyl sulfosuccinate, alkenylsuccinates such as sodium (mono- or di-)alkenylsuccinate, alkylphosphate ester salts, and naphthalenesulfonate formalin condensates.

[0062] The surfactant is preferably an alkyl sulfate ester salt such as dodecyl sulfate (e.g., sodium dodecyl sulfate), an alkyl benzene sulfonate salt such as linear alkyl benzene sulfonate (e.g., sodium dodecyl benzene sulfonate), or a phosphate ester such as polyoxyethylene styrenated phenyl ether phosphate ester.

[0063] In the first step, a polymerization initiator may be added, if necessary, to the aqueous medium in which the raw material monomers are dispersed. The polymerization initiator is not particularly limited, but examples thereof include organic peroxides such as benzoyl peroxide, lauroyl peroxide, o-chlorobenzoyl peroxide, o-methoxybenzoyl peroxide, 3,5,5-trimethylhexanoyl peroxide, t-butylperoxy-2-ethylhexanoate, and di-tert-butyl peroxide; 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2,3-dimethylbutyronitrile); Examples of suitable azo compounds include 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2,3,3-trimethylbutyronitrile), 2,2'-azobis(2-isopropylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), (2-carbamoylazo)isobutyronitrile, 4,4'-azobis(4-cyanovaleric acid), and dimethyl-2,2'-azobisisobutyrate. The polymerization initiator is preferably used in an amount of 0.1 to 3.0 parts by mass per 100 parts by mass of the raw material monomers.

[0064] The reaction solution containing the polymer particles obtained by the polymerization as described above is fed to a general-purpose filter, and the polymer particles contained in the dispersion are separated from the aqueous medium. The separated polymer particles are washed with water, and then dried in a general-purpose manner to remove almost all of the water, and if necessary, classified (preferably air classification) to obtain the polymer particles.

[0065] The obtained polymer particles do not need to have an extremely reduced surfactant content in order to reduce yellowing during use, and the surfactant may be washed away to an extent that does not affect the use of the polymer particles. Therefore, not only can the amount of water used to wash the polymer particles separated from the dispersion be reduced, but the washing time for the polymer particles can also be shortened, thereby improving production efficiency.

[0066] [Uses of Polymer Particles] The polymer particles can be suitably used for optical members such as optical films such as antiglare films and light diffusion films, and light diffusers, and are particularly suitable for antiglare members. The polymer particles can also be used as a matting agent for paints, an antiblocking agent, an additive for improving the physical properties of resins, etc.

[0067] A polymer particle composition can be prepared by dispersing polymer particles in a dispersion medium. A coating film containing polymer particles can be prepared on a substrate by applying the polymer particle composition to the substrate and drying it. Examples of the substrate include films and other molded articles. The polymer particle composition may be molded in a conventional manner to produce a molded product such as a sheet. The dispersion medium is not particularly limited, and examples thereof include synthetic resins and paints.

[0068] When dispersing polymer particles in a synthetic resin, the synthetic resin needs to be heated and melted, and heat is also applied to the polymer particles at this time. Heat is also applied to the polymer particles when molding a polymer particle composition. Furthermore, when polymer particles are contained in a paint, the paint may be applied to a substrate made of a synthetic resin or the like and heated when drying, and in this case, heat is also applied to the polymer particles.

[0069] As described above, the polymer particles have a content of small particles reduced to a predetermined level or less, and coloration is generally suppressed even when heat is applied during use. Therefore, the polymer particles can be suitably used in optical component applications where discoloration such as yellowing is a problem.

[0070] The synthetic resin may be appropriately selected depending on the application, etc. Examples of synthetic resins include (meth)acrylic resins; (meth)acrylic-urethane resins; urethane resins; polyvinyl chloride resins; polyvinylidene chloride resins; melamine resins; styrene resins; alkyd resins; phenolic resins; epoxy resins; polyester resins; silicone resins such as alkylpolysiloxane resins; modified silicone resins such as (meth)acrylic-silicone resins, silicone-alkyd resins, silicone-urethane resins, and silicone-polyester resins; and fluorine-based resins such as polyvinylidene fluoride and fluoroolefin vinyl ether polymers. Note that (meth)acrylic means acrylic or methacrylic.

[0071] The amount of the polymer particles in the polymer particle composition is preferably 2 parts by mass or more, more preferably 4 parts by mass or more, and more preferably 6 parts by mass or more, relative to 100 parts by mass of the synthetic resin. The amount of the polymer particles in the polymer particle composition is preferably 300 parts by mass or less, more preferably 200 parts by mass or less, and more preferably 100 parts by mass or less, relative to 100 parts by mass of the synthetic resin.

[0072] The material of the substrate to which the polymer particle composition is applied is not particularly limited, and examples thereof include synthetic resins such as polyester polymers such as polyethylene terephthalate (PET) and polyethylene naphthalate, cellulose polymers such as diacetyl cellulose and triacetyl cellulose (TAC), polycarbonate polymers, and (meth)acrylic polymers such as polymethyl methacrylate, cement, tile, metal, and glass.

[0073] The present invention will be described in more detail below using examples, but the present invention is not limited thereto. Specific numerical values ​​of the blending ratios (content ratios), physical property values, parameters, etc. used in the following description can be replaced with the upper limit values ​​(numeric values ​​defined as "equal to or less than") or lower limit values ​​(numeric values ​​defined as "equal to or greater than") of the corresponding blending ratios (content ratios), physical property values, parameters, etc. described in the "Description of the Invention."

[0074] Example 1 A raw material monomer composition containing raw material monomers including 94 parts by mass of styrene monomer and 6 parts by mass of divinylbenzene as a polyfunctional vinyl monomer, 1 part by mass of 2,2-azobis(2,4-dimethylvaleronitrile) and 0.2 parts by mass of 2,2-azobisisobutyronitrile as polymerization initiators, 2,600 parts by mass of deionized water as an aqueous medium, 3.0 parts by mass of metathetic magnesium pyrophosphate (magnesium pyrophosphate obtained by a metathetic generation method) as a dispersion stabilizer consisting of an acid-soluble, poorly water-soluble inorganic compound, and 0.03 parts by mass of sodium dodecyl sulfate as a first surfactant was supplied to a high-speed emulsifier / disperser (manufactured by PRIMIX Corporation, trade name "Homomixer MARKII 2.5 type"), and the mixture was stirred and mixed at a rotation speed of 6,000 rpm for 20 minutes, and the droplet size was adjusted to about 6 μm to prepare a dispersion.

[0075] The dispersion was fed into a polymerization reactor equipped with a stirrer and a thermometer, and the starting monomers were polymerized at a constant temperature of 60° C. for 4 hours while being stirred with the stirrer (first step).

[0076] Next, when the polymerization rate of the raw material monomer reached 94%, 0.37 parts by mass of linear alkylbenzenesulfonate sodium salt was supplied as a second surfactant to the dispersion, and the mixture was stirred at 100°C for 2.5 hours to carry out a polymerization reaction, thereby obtaining a reaction liquid in which crosslinked polystyrene particles were dispersed in water as polymer particles.

[0077] Hydrochloric acid was added to the reaction solution to dissolve the dispersion stabilizer (metasetized magnesium pyrophosphate). Thereafter, the reaction solution was fed to a centrifugal dehydrator equipped with a filter cloth in an internal basket, and the basket was rotated for 30 minutes at a centrifugal force of 700 G to remove the liquid, yielding a cake containing crosslinked polystyrene particles.

[0078] Next, while rotating the basket of the dehydrator at a centrifugal effect of 700 G, 500 parts by mass of deionized water was supplied into the basket to wash the cake over 30 minutes. The basket of the dehydrator was further rotated at a centrifugal effect of 700 G for 60 minutes to obtain a cake containing crosslinked polystyrene particles. The obtained cake was dried to obtain crosslinked polystyrene particles.

[0079] Example 2 Crosslinked polystyrene particles were obtained in the same manner as in Example 1, except that the amount of metathesis magnesium pyrophosphate (magnesium pyrophosphate obtained by a metathesis generation method) was changed to 2.5 parts by mass, and the rotation speed of a high-speed emulsifier / disperser (manufactured by PRIMIX Corporation, trade name "Homomixer MARK II 2.5 type") was set to 5000 rpm to prepare a dispersion.

[0080] Example 3 Crosslinked polystyrene particles were obtained in the same manner as in Example 1, except that the amount of metathesis magnesium pyrophosphate (magnesium pyrophosphate obtained by a metathesis generation method) was 2.5 parts by mass, and a dispersion was prepared by stirring and mixing for 15 minutes at a rotation speed of 4500 rpm using a high-speed emulsifier / disperser (manufactured by Primix Corporation, trade name "Homomixer MARK II 2.5 type").

[0081] Example 4 Crosslinked polystyrene particles were obtained in the same manner as in Example 1, except that the amount of metathesis magnesium pyrophosphate (magnesium pyrophosphate obtained by a metathesis generation method) was changed to 2.5 parts by mass, and a dispersion was prepared by stirring and mixing for 15 minutes at a rotation speed of 3500 rpm using a high-speed emulsifier / disperser (manufactured by Primix Corporation, trade name "Homomixer MARK II 2.5 type").

[0082] Example 5 Crosslinked polystyrene particles were obtained in the same manner as in Example 1, except that the amount of metathesis magnesium pyrophosphate (magnesium pyrophosphate obtained by a metathesis generation method) was 2.5 parts by mass, linear alkylbenzene sodium sulfonate was 0.3 parts by mass as the second surfactant, and a dispersion was prepared by stirring and mixing for 12 minutes at a rotation speed of 3000 rpm using a high-speed emulsifier / disperser (manufactured by Primix Corporation, trade name "Homomixer MARK II 2.5 type").

[0083] Example 6 Crosslinked polystyrene particles were obtained in the same manner as in Example 1, except that the amount of metathesis magnesium pyrophosphate (magnesium pyrophosphate obtained by a metathesis generation method) was 2.5 parts by mass, the first surfactant was 0.08 parts by mass of polyoxyethylene styrenated phenyl ether phosphate, and instead of using a high-speed emulsifier / disperser, a high-pressure disperser (manufactured by Nanomizer Co., Ltd., product name "Nanomizer (registered trademark) LA-33") equipped with a suspension disperser (manufactured by Nanomizer Co., Ltd., product name "LNP-20 / 300") was used, and the raw material monomer composition was charged into the high-pressure disperser and an impact force was applied under a high pressure of 29.4 MPa to break down the droplets of the raw material monomer composition.

[0084] Comparative Example 1 Crosslinked polystyrene particles were obtained in the same manner as in Example 1, except that the amount of sodium dodecyl sulfate as the first surfactant was 0.4 parts by mass and no second surfactant was supplied to the dispersion liquid.

[0085] Example 7 The crosslinked polystyrene particles produced in Comparative Example 1 were classified using an air classifier (manufactured by Nisshin Engineering Inc., trade name "Turbo Classifier (registered trademark) TC-15") to obtain crosslinked polystyrene particles.

[0086] Comparative Example 2 A raw material monomer composition containing raw material monomers including 98 parts by mass of styrene monomer and 2 parts by mass of divinylbenzene as a polyfunctional vinyl monomer, 1.5 parts by mass of t-butylperoxy-2-ethylhexanoate and 0.5 parts by mass of t-butylperoxy-2-ethylhexyl carbonate as polymerization initiators, 2000 parts by mass of deionized water as an aqueous medium, 1.4 parts by mass of metathetic magnesium pyrophosphate (magnesium pyrophosphate obtained by a metathetic generation method) as a dispersion stabilizer consisting of an acid-soluble, poorly water-soluble inorganic compound, and 0.056 parts by mass of sodium dodecyl sulfate as a first surfactant was supplied to a high-speed emulsifier / disperser (manufactured by PRIMIX Corporation, trade name "Homomixer MARKII 2.5 type"), stirred and mixed for 15 minutes at a rotation speed of 8000 rpm, and adjusted so that the droplet size was about 6 μm to prepare a dispersion.

[0087] The dispersion was supplied to a polymerization reactor equipped with a stirrer and a thermometer, and the raw material monomers were polymerized at a constant temperature of 90°C for 8 hours while stirring with the stirrer, thereby obtaining a reaction liquid in which crosslinked polystyrene particles as polymer particles were dispersed in water.

[0088] Hydrochloric acid was added to the reaction solution to dissolve the dispersion stabilizer (metasetized magnesium pyrophosphate). Thereafter, the reaction solution was fed to a centrifugal dehydrator equipped with a filter cloth in an internal basket, and the basket was rotated for 30 minutes at a centrifugal force of 700 G to remove the liquid, yielding a cake containing crosslinked polystyrene particles.

[0089] Next, while rotating the basket of the dehydrator at a centrifugal effect of 700 G, 500 parts by mass of deionized water was supplied into the basket to wash the cake over 30 minutes. The basket of the dehydrator was further rotated at a centrifugal effect of 700 G for 60 minutes to obtain a cake containing crosslinked polystyrene particles. The obtained cake was dried to obtain crosslinked polystyrene particles.

[0090] Comparative Example 3 A raw material monomer composition containing raw material monomers including 100 parts by mass of styrene monomer and 6 parts by mass of divinylbenzene as a polyfunctional vinyl monomer, 0.4 parts by mass of an antioxidant (ADK STAB PEP-36: manufactured by ADEKA), 1 part by mass of 2,2-azobis(2,4-dimethylvaleronitrile) and 0.2 parts by mass of 2,2-azobisisobutyronitrile as polymerization initiators, 2,600 parts by mass of deionized water as an aqueous medium, 3.0 parts by mass of metathetic magnesium pyrophosphate (magnesium pyrophosphate obtained by metathetic generation method) as a dispersion stabilizer consisting of an acid-soluble, poorly water-soluble inorganic compound, and 0.04 parts by mass of sodium dodecyl sulfate and 0.6 parts by mass of sodium thiosulfate as a first surfactant was emulsified in a high-speed emulsifier / disperser (manufactured by PRIMIX Corporation under the trade name "Homomixer MARK II"). The mixture was fed into a 2.5" type press and treated at a rotation speed of 4500 rpm for 15 minutes, and the droplet diameter was adjusted to about 9 μm to prepare a dispersion.

[0091] The dispersion was fed into a polymerization reactor equipped with a stirrer and a thermometer, and the starting monomers were polymerized at a constant temperature of 60° C. for 4 hours while being stirred with the stirrer (first step).

[0092] Next, when the polymerization rate of the raw material monomer reached 80 to 90%, 0.4 parts by mass of linear alkylbenzenesulfonate sodium salt was supplied as a second surfactant to the dispersion, and the mixture was stirred at 100°C for 2.5 hours to carry out a polymerization reaction, thereby obtaining a reaction liquid in which crosslinked polystyrene particles were dispersed in water as polymer particles.

[0093] Hydrochloric acid was supplied to the reaction solution to dissolve a dispersion stabilizer (metasetized magnesium pyrophosphate). The reaction solution was then supplied to a centrifugal dehydrator equipped with a filter cloth in an internal basket, and the basket was rotated for 30 minutes at a centrifugal force of 700 G to remove the liquid, yielding a cake containing crosslinked polystyrene particles.

[0094] Next, while rotating the basket of the dehydrator at a centrifugal force of 700 G, 500 parts by mass of deionized water was supplied into the basket to wash the cake over 30 minutes. Rotation of the basket of the dehydrator was continued for 60 minutes, yielding a cake containing crosslinked polystyrene particles. The resulting cake was dried to yield crosslinked polystyrene particles.

[0095] For the obtained crosslinked polystyrene particles, the median diameter in the number-based particle size distribution and the cumulative number proportion of particles (small diameter particles) having a particle diameter of 60% or less of the median diameter in the number-based particle size distribution were measured, and the results are shown in the "Median diameter [μm]" and "Cumulative number proportion [%] of small diameter particles" columns under "Number basis" in Table 2, respectively.

[0096] For the obtained crosslinked polystyrene particles, the arithmetic mean particle diameter Dp in the number-based particle size distribution, the volume-based particle diameter in the volume-based particle size distribution, the CV value of the particle diameter in the number-based particle size distribution, and the amount of surfactant adhering to the surface of the polymer particles were measured in the manner described above, and the results are shown in Table 2 as "Arithmetic mean particle diameter [μm]" on "Number basis," "Volume average particle diameter [μm]" on "Volume basis," "CV value [%]" on "Number basis," and "Amount of surfactant [g / 100 g]," respectively.

[0097] The yellowing index of the resulting crosslinked polystyrene particles was measured in the following manner, and the results are shown in Table 2.

[0098] (Yellowing Degree) The b* value of the polymer particles (crosslinked polystyrene particles) was measured by performing colorimetry using the L*a*b* color system in accordance with JIS Z8729. Specifically, 3.0 g of polymer particles were filled into a measurement container (powder cell manufactured by Konica Minolta Sensing Co., Ltd., product name "CR-A50"). The b* value of the filled polymer particles was measured using a colorimeter (manufactured by Konica Minolta Sensing Co., Ltd., product name "CR-300"). The b* value of the obtained polymer particles was defined as the "initial b* value." Note that the larger the b* value is on the positive side, the greater the yellowing degree.

[0099] Next, the polymer particles were placed in an oven maintained at 180°C and heated for 3 hours. The polymer particles were removed from the oven and left in an atmosphere of 25°C for 1 hour. Thereafter, the b* value of the polymer particles was measured in the same manner as above and was taken as the "b* value after heating." The yellowing index was calculated based on the following formula: Yellowing index = b* value after heating / initial b* value

[0100]

[0101] (Cross-reference to related applications) This application claims priority to Japanese Patent Application No. 2024-55910, filed on March 29, 2024, the disclosure of which is incorporated herein by reference in its entirety.

[0102] The polymer particles of the present invention are less likely to yellow when heated, and therefore can be prevented from causing coloration in products containing the polymer particles due to yellowing of the polymer particles, making them suitable for use in the production of products for optical applications.

Claims

1. Polymer particles characterized in that the cumulative number ratio of particles having a particle size of 60% or less of the median diameter in the particle size distribution based on number is 17% or less.

2. Polymer particles according to claim 1, which have a yellowing index of 3.0 or less when heated at 180°C for 3 hours.

3. Polymer particles according to claim 1 or 2, which have an arithmetic mean particle size of 1 to 50 μm and a CV value of particle sizes of 70% or less in a particle size distribution based on number.

4. Polymer particles according to claim 1 or 2, characterized in that the synthetic resin constituting the polymer particles contains an acrylic resin or a styrene resin.

5. Polymer particles according to claim 1 or 2, characterized in that the surfactant comprises dodecyl sulfate or linear alkylbenzene sulfonate.

6. The polymer particles according to claim 1 or 2, which are used for optical components.

7. A method for producing polymer particles by polymerizing raw material monomers in a dispersed state in an aqueous medium, characterized in that a surfactant is supplied to the aqueous medium when the polymerization rate of the raw material monomers is less than 94% and when the polymerization rate is 94% or more.

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