Polymer particles and use thereof

Abstract

To provide polymer particles enabling suitable handling of an acrylic polymer having a low Tg in the form of powder.SOLUTION: Polymer particles comprising an acrylic polymer are provided, the acrylic polymer having a glass transition temperature (Tg) of 0°C or lower. The surface of the polymer particles is coated with inorganic particles. The content of the inorganic particles in the polymer particles is more than 5 pts.wt. per 100 pts.wt. of the acrylic polymer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to polymer particles containing an acrylic polymer, a method for producing the polymer particles, and a method for producing a pressure-sensitive adhesive composition using the polymer particles. [Background technology]

[0002] Generally, adhesives (also referred to as pressure-sensitive adhesives; the same applies hereinafter) are in a soft solid (viscoelastic) state in a temperature range near room temperature and have the property of easily adhering to an adherend by pressure. Utilizing these properties, adhesives are widely used in a variety of fields, for example, in the form of a supported adhesive sheet having an adhesive layer on a support, or in the form of a supportless adhesive sheet having no support. A typical example of an adhesive is an acrylic adhesive based on an acrylic polymer. From the viewpoint of easily realizing an acrylic adhesive that exhibits good tack (adhesion) in the room temperature range (e.g., about 25°C), it is advantageous for the acrylic polymer to have a glass transition temperature (Tg) lower than the room temperature range, and for example, a Tg of 0°C or lower is preferable. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 4520680 Summary of the Invention [Problem to be solved by the invention]

[0004] The acrylic polymer is often produced, stored, transported, mixed with other materials, etc. in the form of a polymer-containing liquid in which the polymer is dispersed or dissolved in a liquid such as water or an organic solvent. This is because acrylic polymers used as the main material of acrylic pressure-sensitive adhesives generally have high viscosity and adhesive properties in a dry state in which they are not dispersed or dissolved in the liquid, making them difficult to handle (for example, transport, storage, transport within a factory, supply to equipment, etc.).

[0005] Patent Document 1 relates to a method for producing ionomer microparticles that have minimal or almost no pressure-sensitive adhesive properties (paragraph 0017), and describes the production of ionomer microparticles by suspension polymerization of a monomer raw material containing isooctyl acrylate or 2-ethylhexyl acrylate as the main component and further containing methacrylic acid in the presence of zinc oxide and a surfactant (Examples 1 to 9). However, Patent Document 1 does not consider the blocking resistance or flowability of the ionomer microparticles, and cannot be said to have taken into consideration practical storage stability, ease of supplying to an apparatus (feedability), etc.

[0006] Therefore, an object of the present invention is to provide polymer particles that can be conveniently handled in powder form, including a low-Tg acrylic polymer. Another related object is to provide a method for producing such polymer particles. Yet another related object is to provide a method for producing a pressure-sensitive adhesive composition using the polymer particles. [Means for solving the problem]

[0007] This specification provides polymer particles containing an acrylic polymer. The glass transition temperature (Tg) of the acrylic polymer is 0°C or lower. The surfaces of the polymer particles are coated with inorganic particles. The content of the inorganic particles in the polymer particles is more than 5 parts by weight per 100 parts by weight of the acrylic polymer. In this way, by making the low-Tg acrylic polymer into polymer particles (composite particles) whose surfaces are coated with inorganic particles and by making the content of the inorganic particles more than 5 parts by weight per 100 parts by weight of the acrylic polymer, the acrylic polymer can be conveniently handled in powder form.

[0008] In some preferred embodiments of the polymer particles, the content of the inorganic particles relative to 100 parts by weight of the acrylic polymer is more than 5 parts by weight and less than 50 parts by weight. By setting the content of the inorganic particles within this range, the structure of the polymer particles can be destroyed (for example, by kneading under appropriate conditions) to improve the tackiness of the resulting PSA.

[0009] The polymer particles according to some embodiments include, as the inorganic particles, first inorganic particles and second inorganic particles having an average particle size smaller than that of the first inorganic particles. By using the first inorganic particles and the second inorganic particles in combination, polymer particles having better powder-like handleability (e.g., one or both of the blocking resistance and the feedability described below) can be obtained. The content of the second inorganic particles in the polymer particles can be, for example, in the range of 5 to 40 parts by weight.

[0010] The first inorganic particles preferably have an average particle size of, for example, 5 μm or more and less than 50 μm. When first inorganic particles of such a particle size are used, polymer particles that exhibit good feedability are likely to be obtained.

[0011] The second inorganic particles preferably have at least one of the following characteristics: (A) an average particle size of 0.1 μm or more and less than 5 μm; and (B) an average major axis of 0.1 μm or more and less than 5 μm. By using second inorganic particles having such a particle size in combination with the first inorganic particles, polymer particles that are easier to handle in powder form can be obtained.

[0012] The polymer particles disclosed herein have a bulk density of 0.4 g / cm 3 More than 1.0g / cm 3 The technology disclosed herein can be implemented in the form of polymer particles having a bulk density within the above range, and can preferably exhibit the desired effects.

[0013] In some embodiments, the polymer particles preferably exhibit a tackiness of 0.10 MPa or more in a probe tack measurement by kneading them under conditions of a shear rate of 15 [ / s] or more. Polymer particles that can exhibit a predetermined level of tackiness (adhesiveness) by kneading them under the above conditions are suitable as raw materials or precursors for pressure-sensitive adhesives.

[0014] The polymer particles disclosed herein can be preferably produced by, for example, a method including obtaining the acrylic polymer by suspension polymerization, which can be carried out, for example, by polymerizing the acrylic polymer in an aqueous solvent in which at least a portion of the inorganic particles are dispersed.

[0015] According to this specification, there is provided a method for producing a pressure-sensitive adhesive composition using any of the polymer particles disclosed herein. The production method includes preparing the polymer particles and destroying the structure of the polymer particles. By destroying the structure of the polymer particles, a mixture can be prepared in which the inorganic particles are dispersed in a continuous phase containing the acrylic polymer. This mixture can be preferably used as a pressure-sensitive adhesive composition or a constituent thereof.

[0016] Appropriate combinations of the elements described in this specification may also be included within the scope of the invention for which patent protection is sought by this patent application. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is an explanatory diagram schematically illustrating a powder made of polymer particles according to one embodiment. [Figure 2] FIG. 1 is an explanatory diagram schematically illustrating the configuration of a polymer particle according to one embodiment. [Figure 3] FIG. 2 is an explanatory diagram schematically illustrating the structure of a mixture obtained from the powder of the polymer particles shown in FIG. [Figure 4] FIG. 4 is an explanatory diagram schematically illustrating the structure of a pressure-sensitive adhesive layer obtained from the mixture shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0018] Preferred embodiments of the present invention are described below. Matters necessary for carrying out the present invention other than those specifically mentioned in this specification can be understood by those skilled in the art based on the teachings for carrying out the invention described in this specification and the common general technical knowledge at the time of filing. The present invention can be carried out based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. Furthermore, in the following drawings, components and parts that perform the same function may be denoted by the same reference numerals, and redundant explanations may be omitted or simplified. Furthermore, the embodiments shown in the drawings are schematic for the purpose of clearly explaining the present invention, and do not necessarily accurately represent the size or scale of the actual product provided.

[0019] In this specification, the term "adhesive" refers to a material that is in a soft solid (viscoelastic) state at temperatures around room temperature and has the property of adhering to an adherend under pressure. The adhesive referred to here is generally a material having a complex tensile modulus E* (1Hz)<10 7 dyne / cm 2 The material may be a material having the properties satisfying the above (typically, a material having the above properties at 25°C).

[0020] In this specification, "(meth)acryloyl" refers to acryloyl and methacryloyl in a comprehensive sense. Similarly, "(meth)acrylate" refers to acrylate and methacrylate in a comprehensive sense, and "(meth)acrylic" refers to acrylic and methacrylic in a comprehensive sense.

[0021] In this specification, the term "acrylic polymer" refers to a polymer containing more than 50% by weight of monomer units derived from an acrylic monomer as the monomer units constituting the polymer. The acrylic monomer refers to a monomer derived from a monomer having at least one (meth)acryloyl group in one molecule.

[0022] <Polymer particles> (acrylic polymer) The polymer particles provided by this specification include an acrylic polymer and inorganic particles. The acrylic polymer is preferably a polymer of a monomer raw material (monomer component) that contains, for example, an alkyl(meth)acrylate as a main monomer and may further contain a secondary monomer copolymerizable with the main monomer. Here, the main monomer refers to a component that accounts for more than 50% by weight of the monomer composition in the monomer raw material.

[0023] As the alkyl(meth)acrylate, for example, a compound represented by the following formula (1) can be suitably used. CH2=C(R 1 )COOR 2 (1) Here, R in the above formula (1) 1 is a hydrogen atom or a methyl group. 2 is a chain alkyl group having 1 to 20 carbon atoms (hereinafter, this range of carbon atoms is referred to as "C 1-20From the viewpoint of making it easier to obtain acrylic polymers with properties suitable for forming adhesives, R 2 C 1-14 alkyl(meth)acrylates, which are chain alkyl groups of the formula R 2 C 1-10 More preferred is alkyl(meth)acrylate, which is a chain alkyl group represented by the formula R 2 Alkyl (meth)acrylates in which is an n-butyl group or a 2-ethylhexyl group are particularly preferred.

[0024] R 2 C 1-20 Examples of alkyl(meth)acrylates, which are chain alkyl groups, include methyl(meth)acrylate, ethyl(meth)acrylate, propyl(meth)acrylate, isopropyl(meth)acrylate, n-butyl(meth)acrylate, isobutyl(meth)acrylate, s-butyl(meth)acrylate, pentyl(meth)acrylate, isopentyl(meth)acrylate, hexyl(meth)acrylate, heptyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, octyl(meth)acrylate, and isooctyl(meth)acrylate. Examples of alkyl (meth)acrylates include butyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate. These alkyl (meth)acrylates can be used alone or in combination of two or more. Preferred alkyl (meth)acrylates include n-butyl acrylate (BA), n-heptyl acrylate, and 2-ethylhexyl acrylate (2EHA).

[0025] The technology disclosed herein is a method for preparing a polymerizable compound in which the monomer component is R 2 C4-10 Among the alkyl(meth)acrylates contained in the monomer component, R 2 C 4-10 Alkyl (meth)acrylate (C 4-10 This can be preferably implemented in an embodiment in which the total amount of the chain alkyl (meth)acrylates (typically the total amount of BA and 2EHA) is 70% by weight or more (typically 80% by weight or more).

[0026] In some embodiments, the monomer component comprises at least C 4-5 It is preferable that the monomer component contains a chain alkyl acrylate (e.g., BA). 4-10 Chain alkyl (meth)acrylate, C 4-5 The proportion of the chain alkyl acrylate may be, for example, 5% by weight or more, 15% by weight or more, 30% by weight or more, 40% by weight or more, 50% by weight or more or more than 50% by weight, 70% by weight or more, 80% by weight or more, 90% by weight or more, 95% by weight or more, or 100% by weight. In some embodiments, the monomer component contains at least C 6-10 It is preferable that the monomer component contains a chain alkyl acrylate (e.g., 2EHA). 4-10 Chain alkyl (meth)acrylate, C 6-10 The proportion of the chain alkyl acrylate may be, for example, 5% by weight or more, 15% by weight or more, 30% by weight or more, 40% by weight or more, 50% by weight or more or more than 50% by weight, 70% by weight or more, 80% by weight or more, 90% by weight or more, 95% by weight or more, or 100% by weight. 4-5 Chain alkyl acrylate (e.g., BA) and C 6-10 When both a chain alkyl acrylate (e.g., 2EHA) and a chain alkyl acrylate are included, the weight ratio (C 4-5Chain alkyl acrylate / C 6-10 The weight ratio (C) is not particularly limited, and may be, for example, 5 / 95 to 95 / 5, 10 / 90 to 90 / 10, or 30 / 70 to 70 / 30. From the viewpoint of improving the blocking resistance of the polymer particles disclosed herein, in some embodiments, 4-5 Chain alkyl acrylate / C 6-10 The ratio of the linear alkyl acrylate is suitably 40 / 60 or more, advantageously 50 / 50 or more (e.g., greater than 50 / 50), preferably 60 / 40 or more, more preferably 70 / 30 or more, may be 80 / 20 or more, 90 / 10 or more, 95 / 5 or more, or may be 100 / 0.

[0027] The alkyl (meth)acrylate is C 4-10 When a chain alkyl (meth)acrylate (typically at least one of BA and 2EHA) is contained, the alkyl (meth)acrylate may be a chain alkyl (meth)acrylate other than the alkyl (meth)acrylate (R 2 is less than C4 or C 10 The alkyl (meth)acrylate may or may not further contain a chain alkyl group of more than C. 4-10 In an embodiment containing a chain alkyl (meth)acrylate and another alkyl (meth)acrylate, the total amount of the other alkyl (meth)acrylate is preferably about 30% by weight or less (e.g., 20% by weight or less, typically 15% by weight or less) of the monomer components constituting the acrylic polymer. In addition, from the viewpoint of obtaining the effect of the other alkyl (meth)acrylate, the total amount is preferably about 1% by weight or more (e.g., 5% by weight or more, typically 10% by weight or more) of the monomer components. In some embodiments, the other alkyl (meth)acrylate is preferably a compound represented by the formula (1) R 2 C 1-3Alkyl (meth)acrylates, which are chain alkyl groups, are preferably used. Specific examples thereof include methyl acrylate, methyl methacrylate, and ethyl acrylate.

[0028] The secondary monomer copolymerizable with the alkyl (meth)acrylate main monomer can be useful for introducing crosslinking points into the acrylic polymer and for increasing the cohesive strength of the acrylic polymer. As the secondary monomer, for example, the following functional group-containing monomer components can be used alone or in combination of two or more: Carboxy group-containing monomers: for example, ethylenically unsaturated monocarboxylic acids such as acrylic acid (AA), methacrylic acid (MAA), and crotonic acid; ethylenically unsaturated dicarboxylic acids such as maleic acid, itaconic acid, and citraconic acid, and their anhydrides (maleic anhydride, itaconic anhydride, etc.). Hydroxyl group-containing monomers: for example, hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 2-hydroxybutyl (meth)acrylate; unsaturated alcohols such as vinyl alcohol and allyl alcohol. Amide group-containing monomers: for example, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N-butyl(meth)acrylamide, N-methylol(meth)acrylamide, N-methylolpropane(meth)acrylamide, N-methoxymethyl(meth)acrylamide, N-butoxymethyl(meth)acrylamide. Amino group-containing monomers: for example, aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, t-butylaminoethyl (meth)acrylate. Monomers having an epoxy group: for example, glycidyl (meth)acrylate, methyl glycidyl (meth)acrylate, allyl glycidyl ether. Cyano group-containing monomers: for example, acrylonitrile, methacrylonitrile. Keto group-containing monomers: for example, diacetone (meth)acrylamide, diacetone (meth)acrylate, vinyl methyl ketone, vinyl ethyl ketone, allyl acetoacetate, vinyl acetoacetate. Monomers having a nitrogen atom-containing ring: for example, N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole, N-vinylmorpholine, N-vinylcaprolactam, and N-(meth)acryloylmorpholine. Alkoxysilyl group-containing monomers: for example, 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, 3-(meth)acryloxypropylmethyldiethoxysilane.

[0029] The functional group-containing monomers can be used alone or in combination of two or more. Carboxy group-containing monomers, hydroxyl group-containing monomers, and cyano group-containing monomers are preferred because they can effectively introduce crosslinking points and improve cohesive strength, and carboxyl group-containing monomers and hydroxyl group-containing monomers are more preferred. Suitable examples of carboxyl group-containing monomers include AA and MAA. In some embodiments, MAA can be preferably used from the viewpoint of anti-blocking properties, etc. AA and MAA may be used in combination. Suitable examples of hydroxyl group-containing monomers include 2-hydroxyethyl acrylate, 4-hydroxybutyl acrylate, etc.

[0030] In embodiments using a functional group-containing monomer, the proportion of the functional group-containing monomer in the total monomer components constituting the acrylic polymer is not particularly limited. Generally, from the viewpoint of achieving a good balance between cohesive strength and adhesiveness, the proportion of the functional group-containing monomer is preferably about 0.1% by weight or more (e.g., 0.5% by weight or more, typically 1% by weight or more). Furthermore, taking into account the adhesive effect of alkyl (meth)acrylate, in some embodiments, the proportion of the functional group-containing monomer in the total monomer components is preferably about 40% by weight or less (e.g., 30% by weight or less, typically 20% by weight or less), and may be 10% by weight or less, 5% by weight or less, 3% by weight or less, or 1% by weight or less.

[0031] In some embodiments, the monomer components constituting the acrylic polymer preferably contain at least a carboxyl group-containing monomer as the functional group-containing monomer, from the viewpoints of the cohesion properties of a PSA containing the acrylic polymer and adhesion to polar surfaces. The proportion of the carboxyl group-containing monomer in the total monomer components constituting the acrylic polymer may typically be, for example, 0.1 wt% or more. From the viewpoint of achieving higher usage effects, it is appropriate to set it to 0.5 wt% or more, preferably 1 wt% or more, more preferably 2 wt% or more, even more preferably 3 wt% or more, 3.5 wt% or more, 4 wt% or more, 4.5 wt% or more, or even 4.8 wt% or more. On the other hand, from the viewpoints of the flexibility (e.g., impact absorption, surface conformability) and low-temperature properties of a PSA formed from the polymer particles disclosed herein, in some embodiments, it is appropriate to set the proportion of the carboxyl group-containing monomer in the total monomer components to 15 wt% or less, for example, 10 wt% or less, 8 wt% or less, or 6 wt% or less.

[0032] Furthermore, for the purpose of improving the cohesive strength of the acrylic polymer or adjusting the Tg, copolymerization components other than the functional group-containing monomers described above can be used as needed. Examples of such copolymerization components include vinyl ester monomers such as vinyl acetate and vinyl propionate; aromatic vinyl compounds such as styrene, substituted styrenes (α-methylstyrene, etc.) and vinyltoluene; cycloalkyl(meth)acrylates such as cyclohexyl(meth)acrylate, cyclopentyl(meth)acrylate and isobornyl(meth)acrylate; aryl(meth)acrylates (e.g., phenyl(meth)acrylate), aryloxyalkyl(meth)acrylates (e.g., phenoxyethyl(meth)acrylate), arylalkyl(meth)acrylates, and the like. Examples of the monomer include aromatic ring-containing (meth)acrylates such as (meth)acrylates (e.g., benzyl (meth)acrylate); olefin-based monomers such as ethylene, propylene, isoprene, butadiene, and isobutylene; chlorine-containing monomers such as vinyl chloride and vinylidene chloride; isocyanate group-containing monomers such as 2-(meth)acryloyloxyethyl isocyanate; alkoxy group-containing monomers such as methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate; and vinyl ether-based monomers such as methyl vinyl ether and ethyl vinyl ether.

[0033] The amount of copolymerization components other than the functional group-containing monomer is not particularly limited and may be appropriately selected depending on the purpose and application, but for example, it is preferably 10% by weight or less of the monomer composition of the acrylic polymer, may be 3% by weight or less, or may be less than 1% by weight (for example, 0% by weight or more and less than 1% by weight).

[0034] In some embodiments of the polymer particles disclosed herein, a monofunctional monomer is preferably used as the monomer component constituting the acrylic polymer from the viewpoint of ease of kneading, which will be described later. The monofunctional monomer refers to a compound having only one radically polymerizable functional group (typically an ethylenically unsaturated group) in one molecule. In some embodiments, a monomer having two or more radically polymerizable functional groups in one molecule (a polyfunctional monomer) may be used as the monomer component constituting the acrylic polymer, for the purpose of imparting appropriate cohesiveness to the polymer particles. Examples of such polyfunctional monomers include 1,6-hexanediol di(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, glycerin di(meth)acrylate, epoxy acrylate, polyester acrylate, urethane acrylate, divinylbenzene, butyl di(meth)acrylate, hexyl di(meth)acrylate, and the like. The amount of polyfunctional monomer used is suitably 1% by weight or less of the total monomer components, and may be 0.5% by weight or less or 0.1% by weight or less. Polyfunctional monomers may not be used.

[0035] In the technology disclosed herein, the acrylic polymer contained in the polymer particles is suitably designed to have a Tg of 0°C or less. An acrylic polymer having a Tg of 0°C or less makes it easier to obtain desired adhesive properties in an acrylic pressure-sensitive adhesive containing the acrylic polymer as a base polymer. The Tg of the acrylic polymer can be adjusted by the type and amount ratio of monomers used in synthesizing the polymer.

[0036] Here, the Tg of an acrylic polymer refers to the Tg calculated by the Fox equation based on the composition of the monomer components used in the synthesis of the polymer. The Fox equation, as shown below, is a relational expression between the Tg of a copolymer and the glass transition temperature Tgi of a homopolymer obtained by homopolymerizing each of the monomers constituting the copolymer. 1 / Tg=Σ(Wi / Tgi) In the above Fox formula, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi represents the weight fraction of monomer i in the copolymer (copolymerization ratio by weight), and Tgi represents the glass transition temperature (unit: K) of the homopolymer of monomer i.

[0037] The glass transition temperature of the homopolymer used to calculate Tg is determined based on the value described in the publicly available literature. For example, for the following monomers, the following values ​​are used as the glass transition temperatures of the homopolymers of the monomers: 2-Ethylhexyl acrylate -70℃ n-Butyl acrylate -55℃ Methyl methacrylate 105℃ Methyl acrylate 8℃ Vinyl acetate 32℃ Acrylic acid 106℃ Methacrylic acid 228℃

[0038] For the glass transition temperatures of homopolymers of monomers other than those listed above, the values ​​given in "Polymer Handbook" (3rd Edition, John Wiley & Sons, Inc., 1989) shall be used. If multiple values ​​are given in this document, the highest value shall be used.

[0039] For monomers for which the glass transition temperature of the homopolymer is not listed in the Polymer Handbook, the value obtained by the following measurement method will be used (see JP 2007-51271 A). Specifically, 100 parts by weight of the monomer, 0.2 parts by weight of azobisisobutyronitrile, and 200 parts by weight of ethyl acetate as the polymerization solvent were charged into a reactor equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, and the mixture was stirred for 1 hour while passing nitrogen gas through. After removing oxygen from the polymerization system in this manner, the temperature was raised to 63°C and the reaction was continued for 10 hours. The mixture was then cooled to room temperature to obtain a homopolymer solution with a solids concentration of 33% by weight. This homopolymer solution was then cast onto a release liner and dried to prepare a test sample (sheet-like homopolymer) approximately 2 mm thick. This test sample was punched out into a disk with a diameter of 7.9 mm, sandwiched between parallel plates, and subjected to a shear strain of 1 Hz using a viscoelasticity tester (ARES, manufactured by Rheometrics). Viscoelasticity was measured in shear mode at a temperature range of -70 to 150°C and a heating rate of 5°C / min. The peak top temperature of tan δ was taken as the Tg of the homopolymer.

[0040] From the viewpoint of easily obtaining better adhesiveness (such as tack) and flexibility (for example, impact absorption, surface conformability, etc.), in some embodiments, the Tg of the acrylic polymer is preferably −10° C. or lower (for example, −20° C. or lower), more preferably −30° C. or lower, and may be −40° C. or lower, or may be −45° C. or lower. From the viewpoint of forming a pressure-sensitive adhesive having higher flexibility, in some embodiments, the Tg of the acrylic polymer may be −50° C. or lower, −60° C. or lower, or −65° C. or lower. Furthermore, the Tg of the acrylic polymer may be, for example, −80° C. or higher, −70° C. or higher, or −65° C. or higher. From the viewpoint of the blocking resistance and fluidity (e.g., feedability) of the polymer particles, in some embodiments, the Tg of the acrylic polymer is advantageously −60° C. or higher (e.g., greater than −60° C.), preferably −58° C. or higher or −57° C. or higher, more preferably −56° C. or higher, and may be −55° C. or higher, −53° C. or higher, or −50° C. or higher (e.g., −49° C. or higher).

[0041] (Inorganic particles) The polymer particles disclosed herein contain more than 5 parts by weight of inorganic particles relative to 100 parts by weight of the acrylic polymer. The polymer particles whose surfaces are coated with the inorganic particles allow the low-Tg acrylic polymer to be conveniently handled in powder form.

[0042] Various inorganic particles that can help prevent fusion of polymer particles can be used as the inorganic particles. Examples of inorganic materials constituting the inorganic particles include, but are not limited to, tricalcium phosphate, magnesium phosphate, aluminum phosphate, zinc phosphate, calcium carbonate, magnesium carbonate, barium carbonate, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, ferric hydroxide, calcium metasilicate, calcium silicate, aluminum silicate, magnesium silicate, sodium magnesium silicate, calcium sulfate, barium sulfate, talc, kaolin, mica, bentonite, silica, alumina, titania, and zirconia. Examples of inorganic materials constituting the inorganic particles include various inorganic materials that can be used as dispersants for suspension polymerization. Examples of preferred inorganic particles include tricalcium phosphate, calcium carbonate, silica, and talc. The calcium carbonate may be light calcium carbonate or heavy calcium carbonate. In some embodiments, light calcium carbonate is preferred.

[0043] The size of the inorganic particles is not particularly limited and can be selected to obtain suitable properties depending on the form and purpose of use. When the size of the inorganic particles is expressed as an average particle diameter, in some embodiments, the average particle diameter is suitably approximately 250 μm or less, advantageously 100 μm or less, preferably 70 μm or less, more preferably 50 μm or less (e.g., less than 50 μm), and may be 40 μm or less, 30 μm or less, 20 μm or less, or 15 μm or less, from the viewpoint of the blocking resistance of the polymer particles, for example. In some embodiments, the average particle diameter of the inorganic particles may be, for example, 0.01 μm or more, and from the viewpoint of the blocking resistance of the polymer particles, preferably 0.03 μm or more, may be 0.05 μm or more, or may be 0.07 μm or more. The value of the average particle diameter is the volume-based cumulative 50% particle diameter (D50) based on a laser diffraction / scattering method. If a nominal value of the average particle size is provided by a manufacturer, etc., that nominal value may be used. The same applies to the average particle sizes of the first and second inorganic particles described below.

[0044] Furthermore, when the size of inorganic particles is expressed by the average major axis, the average major axis is suitably approximately 500 μm or less, advantageously 200 μm or less, preferably 140 μm or less, more preferably 100 μm or less, and may be 70 μm or less, or may be 50 μm or less. Furthermore, in some embodiments, the average major axis of inorganic particles may be, for example, 0.01 μm or more, suitably 0.03 μm or more, advantageously 0.05 μm or more, preferably 0.08 μm or more, and more preferably 0.10 μm or more. The average major axis can be determined by analyzing scanning electron microscope (SEM) images. More specifically, the average major axis of the inorganic particles can be determined by acquiring SEM images of the inorganic particles to be measured, drawing the smallest rectangle circumscribing each particle image, and arithmetically averaging the lengths of the long sides of those rectangles. If a nominal value for the average major axis is provided by a manufacturer or the like, that nominal value may be used. The average minor axis of the inorganic particles can be determined by arithmetically averaging the lengths of the short sides of the rectangles. If a nominal value for the average minor axis is provided by a manufacturer, this nominal value may be used. The same applies to the average major axis and average minor axis of the second inorganic particles described below.

[0045] One type of inorganic particle may be used alone, or two or more types of inorganic particles differing in material and / or size may be used in combination. Some preferred embodiments of the polymer particles include first inorganic particles and second inorganic particles having an average particle size smaller than that of the first inorganic particles. Here, the term "the average particle size of the second inorganic particles is smaller than that of the first inorganic particles" means that at least one of the following conditions is satisfied: (1) the average particle size R2 [μm] of the second inorganic particles is smaller than the average particle size R1 [μm] of the first inorganic particles, and (2) the average major axis L2 [μm] of the second inorganic particles is smaller than the average particle size R1 [μm] of the first inorganic particles. By using a combination of first and second inorganic particles that satisfy this average particle size relationship, the second inorganic particles can fill the gaps between the first inorganic particles on the polymer particle surface, making it easier to obtain polymer particles that are easier to handle in powder form, such as polymer particles with improved blocking resistance and / or flowability (e.g., feedability). The ranges that R1, R2, and L2 can take can be selected from the examples of the upper and lower limits of the average particle size and average major axis of the inorganic particles described above so as to satisfy at least one of R1>R2 and R1>L2.

[0046] In some embodiments, the average particle size (D50)R1 of the first inorganic particles is suitably 1 μm or more, preferably 3 μm or more, more preferably 5 μm or more, and may be 6 μm or more, 7 μm or more, or even 9 μm or more. It is advantageous from the viewpoint of the blocking resistance of the polymer particles that the average particle size R1 is not too small. Furthermore, in some embodiments, the average particle size R1 of the first inorganic particles may be, for example, approximately 250 μm or less, advantageously 100 μm or less, preferably 70 μm or less, more preferably 50 μm or less (e.g., less than 50 μm), and may be 40 μm or less, 30 μm or less, 20 μm or less, or 15 μm or less. It is preferable that the average particle size R1 is not too large from the viewpoint of ease of production of the polymer particles and ease of preparation of a pressure-sensitive adhesive composition using the polymer particles, etc.

[0047] In some embodiments, the average particle size (D50)R2 of the second inorganic particles is smaller than the average particle size R1 of the first inorganic particles and is preferably less than 5 μm, more preferably 3 μm or less, and may be 2 μm or less, or may be 1 μm or less. It is preferable that the average particle size R2 is not too large in order to efficiently fill the gaps between the first inorganic particles and obtain good blocking resistance. Furthermore, in some embodiments, the average particle size R2 of the second inorganic particles is suitably 0.01 μm or more, preferably 0.03 μm or more, and may be 0.05 μm or more, or may be 0.07 μm or more. Having an average particle size R2 that is not too small can be advantageous in terms of preventing the second inorganic particles from being embedded inside the polymer particles due to external pressure, movement of the acrylic polymer, or the like, and in terms of stably maintaining good handleability of the polymer particles disclosed herein. From the viewpoint of favorably achieving the effect of using the first inorganic particles and the second inorganic particles in combination, in some embodiments, the value of R1 / R2 may be, for example, 50 or less, preferably 20 or less, and more preferably 10 or less. Moreover, the value of R1 / R2 may be greater than 1, for example, 1.1 or more, 1.5 or more, 2 or more, or 3 or more.

[0048] In some embodiments, the average major axis L2 of the second inorganic particles is smaller than the average particle diameter R1 of the first inorganic particles and is suitably 10 μm or less, preferably 7 μm or less, more preferably less than 5 μm, and may be 4 μm or less, or may be 3 μm or less. It is preferable that the average major axis L2 is not too large in order to efficiently fill the gaps between the first inorganic particles and obtain good blocking resistance. Furthermore, in some embodiments, the average major axis L2 of the second inorganic particles is suitably 0.01 μm or more, preferably 0.03 μm or more, may be 0.05 μm or more, 0.07 μm or more, 0.1 μm or more, 0.5 μm or more, 0.8 μm or more, 1 μm or more, or 1.5 μm or more. Having an average major axis L2 that is not too small can be advantageous in terms of preventing the second inorganic particles from being embedded inside the polymer particles due to external pressure, movement of the acrylic polymer, etc., and stably maintaining good handleability of the polymer particles disclosed herein in powder form. To facilitate optimal use of the effects of using the first inorganic particles and the second inorganic particles in combination, in some embodiments, the value of R1 / L2 may be, for example, 50 or less, preferably 20 or less, and more preferably 10 or less. Furthermore, the value of R1 / L2 may be greater than 1, for example, 1.1 or more, 1.5 or more, 2 or more, or 3 or more.

[0049] In some embodiments, the average minor axis S2 of the second inorganic particles is suitably less than 5 μm, preferably 3 μm or less, more preferably 2 μm or less, and may be 1 μm or less, 0.7 μm or less, or 0.5 μm or less. In some embodiments, the average minor axis S2 of the second inorganic particles is suitably 0.005 μm or more, preferably 0.01 μm or more, and may be 0.05 μm or more, 0.07 μm or more, 0.1 μm or more, 0.2 μm or more, or 0.3 μm or more. In some embodiments, the value of R1 / S2 may be, for example, 200 or less, preferably 100 or less, and more preferably 50 or less. In addition, the value of R1 / S2 may be greater than 1, for example, 2 or more, 5 or more, 10 or more, or 15 or more. In some embodiments, the average aspect ratio of the second inorganic particles is, in principle, 1 or greater, and may be, for example, greater than 1, such as 1.1 or greater, 1.5 or greater, 2 or greater, or 3 or greater. In terms of the strength and fluidity of the second inorganic particles, in some embodiments, the average aspect ratio of the second inorganic particles is suitably 30 or less, preferably 20 or less, or may be 15 or less, 10 or less, or 7 or less. The average aspect ratio of the second inorganic particles is calculated as the ratio of the average major axis L2 to the average minor axis S2.

[0050] As described above, the content of inorganic particles in the polymer particles disclosed herein (in embodiments containing multiple types of inorganic particles, the total content of these particles) is greater than 5 parts by weight relative to 100 parts by weight of the acrylic polymer contained in the polymer particles. In some embodiments, the content of inorganic particles relative to 100 parts by weight of the acrylic polymer is preferably 10 parts by weight or more (e.g., greater than 10 parts by weight) or 12 parts by weight or more, more preferably 15 parts by weight or more or 17 parts by weight or more, and even more preferably 20 parts by weight or more. Increasing the content of inorganic particles tends to improve the blocking resistance and flowability (e.g., feedability) of the polymer particles. The content of inorganic particles relative to 100 parts by weight of the acrylic polymer may be, for example, less than 100 parts by weight, 80 parts by weight or less, or 60 parts by weight or less. Not having too much inorganic particle content in the polymer particles can be advantageous from the standpoint of improving the tackiness of a pressure-sensitive adhesive formed using the polymer particles and improving the flexibility (shock absorption, surface conformability, etc.) of the pressure-sensitive adhesive. In some embodiments, the content of inorganic particles per 100 parts by weight of the acrylic polymer is preferably 50 parts by weight or less (e.g., less than 50 parts by weight), more preferably 48 parts by weight or less or 45 parts by weight or less, and may be 40 parts by weight or less, or may be 35 parts by weight or less.

[0051] In embodiments in which the polymer particles disclosed herein contain the first inorganic particles and the second inorganic particles, the content of the first inorganic particles relative to 100 parts by weight of the acrylic polymer contained in the polymer particles may be greater than 0 parts by weight, for example, 1 part by weight or more, preferably 3 parts by weight or more, more preferably 5 parts by weight or more or more than 5 parts by weight, and may be 7 parts by weight or more, or may be 9 parts by weight or more. Furthermore, the content of the first inorganic particles relative to 100 parts by weight of the acrylic polymer may be, for example, 40 parts by weight or less, 35 parts by weight or less, or 30 parts by weight or less, and in some embodiments, preferably 25 parts by weight or less, more preferably 20 parts by weight or less (e.g., less than 20 parts by weight), may be 15 parts by weight or less, or may be 10 parts by weight or less. By setting the content of the first inorganic particles within the range of any combination of the upper and lower limits described above, the effects of using the first and second inorganic particles in combination can be suitably exhibited.

[0052] In an embodiment in which the polymer particles disclosed herein contain the first inorganic particles and the second inorganic particles, the content of the second inorganic particles relative to 100 parts by weight of the acrylic polymer contained in the polymer particles may be greater than 0 parts by weight, for example, 1 part by weight or more, preferably 3 parts by weight or more, more preferably 5 parts by weight or more or more than 5 parts by weight, 7 parts by weight or more, 9 parts by weight or more, 10 parts by weight or more or more than 10 parts by weight, or 12 parts by weight or more. Furthermore, the content of the second inorganic particles relative to 100 parts by weight of the acrylic polymer may be, for example, 50 parts by weight or less, 40 parts by weight or less, 45 parts by weight or less, 30 parts by weight or less or less than 30 parts by weight, or 25 parts by weight or less. By setting the content of the second inorganic particles within the range of any combination of the upper and lower limits described above, the effects of using the first and second inorganic particles in combination can be optimally exhibited.

[0053] The ratio (by weight) of the content of the second inorganic particles to the content of the first inorganic particles is not particularly limited and may be, for example, 0.2 or more, 0.5 or more, or 0.8 or more. From the viewpoint of more suitably achieving the effects of using the first and second inorganic particles in combination, in some embodiments, the ratio (content of the second inorganic particles / content of the first inorganic particles) is preferably 1 or more (e.g., greater than 1), may be 1.2 or more, or may be 1.4 or more. Furthermore, the ratio (content of the second inorganic particles / content of the first inorganic particles) may be, for example, 10 or less, and in some embodiments, preferably 5 or less, more preferably 4 or less, may be 3 or less, may be 2.5 or less, or may be 2 or less.

[0054] The material constituting the first inorganic particles can be appropriately selected from the above-mentioned examples of inorganic materials. In some embodiments, suitable examples of the first inorganic particles include tricalcium phosphate and calcium carbonate (e.g., precipitated calcium carbonate, heavy calcium carbonate, etc.).

[0055] The material constituting the second inorganic particles can be appropriately selected from the above-mentioned examples of inorganic materials. In some embodiments, suitable examples of the second inorganic particles include calcium carbonate (e.g., light calcium carbonate, heavy calcium carbonate, etc.), talc, and silica.

[0056] (Method of producing polymer particles) The polymer particles disclosed herein can be produced, for example, by a method including polymerizing the above-mentioned monomer components (monomer raw materials) to form an acrylic polymer. The method for polymerizing the monomer components is not particularly limited. A preferred polymerization method is, for example, suspension polymerization.

[0057] The initiator used for polymerization can be appropriately selected from conventionally known polymerization initiators, such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylpropionamidine) disulfate, 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] hydrate, 2,2'-azobis(N,N'-dimethyleneisobutylamidine), and 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride. Examples of suitable initiators include, but are not limited to, azo initiators such as potassium persulfate and ammonium persulfate; peroxide initiators such as benzoyl peroxide, t-butyl hydroperoxide, and hydrogen peroxide; substituted ethane initiators such as phenyl-substituted ethane; carbonyl initiators such as aromatic carbonyl compounds; and redox initiators such as a combination of a persulfate and sodium hydrogen sulfite, or a combination of a peroxide and sodium ascorbate. These polymerization initiators can be used alone or in combination of two or more.

[0058] The amount of the polymerization initiator used is not particularly limited as long as it is a normal amount, and can be selected, for example, from the range of about 0.005 parts by weight or more (preferably 0.01 parts by weight or more) to about 1 part by weight or less (preferably 0.8 parts by weight or less) relative to 100 parts by weight of the total monomer components.

[0059] During polymerization, a chain transfer agent (which may also be understood as a molecular weight regulator or polymerization degree regulator) may be used as needed. Examples of chain transfer agents include mercaptans such as dodecyl mercaptan (also known as dodecanethiol or lauryl mercaptan), glycidyl mercaptan, 2-mercaptoethanol, mercaptoacetic acid, 2-ethylhexyl thioglycolate, and 2,3-dimercapto-1-propanol, as well as α-methylstyrene dimer. These chain transfer agents may be used alone or in combination of two or more.

[0060] The amount of the chain transfer agent used can be about 0.001 parts by weight or more (typically about 0.005 parts by weight or more, for example, about 0.01 parts by weight or more) relative to 100 parts by weight of the monomer components. In some preferred embodiments, the amount of the chain transfer agent used relative to 100 parts by weight of the monomer components can be, for example, 0.005 parts by weight or more, 0.01 parts by weight or more, 0.02 parts by weight or more, or 0.03 parts by weight or more. The amount of the chain transfer agent used relative to 100 parts by weight of the monomer components can be, for example, about 5 parts by weight or less (typically about 2 parts by weight or less, for example, about 1 part by weight or less), and can be 0.5 parts by weight or less, 0.2 parts by weight or less, or 0.1 parts by weight or less.

[0061] In some preferred embodiments, the acrylic polymer is synthesized by suspension polymerization of the monomer components in an aqueous solvent in which at least a portion of the inorganic particles constituting the polymer particles disclosed herein are dispersed. The inorganic particles may function as a suspension stabilizer during suspension polymerization. The polymer particles disclosed herein can be preferably produced, for example, by a method comprising carrying out suspension polymerization in the presence of inorganic particles in an amount advantageously at least 10 wt %, preferably at least 20 wt % and more preferably at least 30 wt % of the total amount of inorganic particles constituting the polymer particles from the viewpoint of suspension polymerization stability. The amount of inorganic particles present during suspension polymerization may be 35 wt % or more, 40 wt % or more, 50 wt % or more, 70 wt % or more, 90 wt % or more, or even 100 wt % of the total amount of inorganic particles constituting the polymer particles.

[0062] In some embodiments, the amount of carboxyl group-containing monomer that can be contained in the monomer component is suitably equal to or less than the amount of inorganic particles present during suspension polymerization of the monomer component, preferably less than the same amount, on a weight basis. For example, the weight of the carboxyl group-containing monomer that can be contained in the monomer component is preferably 0.9 times or less, more preferably 0.8 times or less, 0.7 times or less, or 0.6 times or less, relative to the weight of the inorganic particles present during suspension polymerization. This tends to improve the stability of the suspension polymerization and favorably produce polymer particles that are easy to handle in powder form. Note that when the monomer component does not contain a carboxyl group-containing monomer, the weight of the carboxyl group-containing monomer that can be contained in the monomer component is 0 times the weight of the inorganic particles present during suspension polymerization. The technology disclosed herein can also be implemented in an embodiment in which the monomer component does not contain a carboxyl group-containing monomer.

[0063] When producing polymer particles containing first and second inorganic particles, in some embodiments, it is preferable to use the first inorganic particles as the inorganic particles present during suspension polymerization, and add some or all of the second inorganic particles after suspension polymerization. By adding at least some of the second inorganic particles, which have a relatively smaller particle size than the first inorganic particles, after suspension polymerization, the second inorganic particles can efficiently fill the gaps on the surface coated with the first inorganic particles, making it easier to obtain polymer particles that are easy to handle in powder form. It is preferable to select the first inorganic particles present during suspension polymerization that can appropriately function as a suspension stabilizer. Suitable examples include tricalcium phosphate and calcium carbonate. For example, in embodiments where the monomer component includes a monomer having an acid group (e.g., a carboxyl group-containing monomer), tricalcium phosphate can be preferably used as the inorganic particles present during suspension polymerization.

[0064] By carrying out suspension polymerization in an aqueous solvent in which inorganic particles are dispersed, composite particles are formed with the inorganic particles attached to their surfaces. If the amount of inorganic particles present during suspension polymerization is less than 100% by weight of the total inorganic particles, the remaining inorganic particles can be added to the polymerization reaction solution obtained by suspension polymerization and mixed to further attach the remaining inorganic particles to the composite particles. By drying the particles thus produced, polymer particles containing an acrylic polymer and having their surfaces covered with inorganic particles can be obtained, preferably in powder form.

[0065] In the suspension polymerization, a dispersant other than inorganic powder (typically an organic dispersant) can be used as needed. Such dispersants are not particularly limited, but water-soluble polymers such as polyvinyl alcohol, polyacrylic acid, gelatin, starch, cellulose ethers (e.g., carboxymethyl cellulose, hydroxyethyl cellulose, etc.), sodium polyacrylate, and polyvinylpyrrolidone are preferably used. The organic dispersants can be used alone or in combination of two or more. The amount of organic dispersant used is typically preferably 5 parts by weight or less, but may also be 4 parts by weight or less, 3 parts by weight or less, or 2 parts by weight or less, per 100 parts by weight of the monomer components. In some embodiments, the amount of organic dispersant used per 100 parts by weight of the monomer components is suitably 1 part by weight or less or less than 1 part by weight, and may also be 0.5 parts by weight or less, or 0.1 parts by weight or less. An organic dispersant need not be used.

[0066] The polymer particles disclosed herein preferably have a limited surfactant content. In some embodiments, the surfactant content is suitably less than 3 parts by weight, preferably less than 1 part by weight, more preferably less than 0.5 parts by weight, and may even be less than 0.1 parts by weight, or even 0 parts by weight (i.e., no surfactant) relative to 100 parts by weight of the acrylic polymer contained in the polymer particles. By limiting the surfactant content in this way, it is possible to prevent the surfactant from seeping onto the polymer particle surface and improve the blocking resistance of the polymer particles.

[0067] The polymer particles disclosed herein preferably contain substantially no organic solvent. Here, "polymer particles substantially contain substantially no organic solvent (also referred to as solventless)" means that the amount of organic solvent contained in the polymer particles is less than 1% by weight (e.g., less than 0.1% by weight). Such solventless polymer particles are preferred from the viewpoint of environmental hygiene, etc. Polymer particles that contain substantially neither water nor organic solvent are more preferred. Since such polymer particles are in the form of a dry powder, they can be preferably used, for example, in the preparation of a solventless pressure-sensitive adhesive composition.

[0068] <Structure and Usage of Polymer Particles> The polymer particles disclosed herein are, for example, as schematically shown in FIG. 1, polymer particles 10 containing an acrylic polymer 12, the surfaces of which are coated with inorganic particles 14. Here, the glass transition temperature (Tg) of the acrylic polymer 12 is 0°C or lower, and the content of the inorganic particles 14 in the polymer 10 is greater than 5 parts by weight per 100 parts by weight of the acrylic polymer 12. When the polymer particles 10 are in a form in which the surfaces are coated with inorganic particles 14, the low-Tg acrylic polymer 12 can be preferably handled in the form of a powder (dry powder) as shown in FIG. 1.

[0069] In some embodiments, the polymer particles 10 may include, as inorganic particles 14, first inorganic particles 142 and second inorganic particles 144 having an average particle size smaller than that of the first inorganic particles, as shown in FIG. 2, for example.

[0070] 1 and 2, for ease of understanding, inorganic particles 14 are disposed only on the surface of polymer particles 10, but this is not intended to limit the morphology of the polymer particles disclosed herein. In other words, the polymer particles disclosed herein being coated with inorganic particles means that the majority (e.g., 70% by weight or more) of the inorganic particles contained in the polymer particles are at least partially exposed on the surface, and does not exclude the presence of a relatively small amount of inorganic particles that are not exposed on the surface (e.g., inorganic particles buried in the acrylic polymer). The effects of the technology disclosed herein can be appropriately achieved when the majority of the inorganic particles contained in the polymer particles are at least partially exposed on the surface of the polymer particles.

[0071] The polymer particles disclosed herein can exhibit adhesiveness by disrupting and homogenizing their structure. The method for homogenizing the polymer particle structure is not particularly limited, and examples include (a) a method of kneading polymer particles, (b) a method of dissolving polymer particles in an organic solvent, (c) a method of mixing polymer particles with a liquid monomer (which may be a polyfunctional monomer), and intermediate or combined methods thereof. From the viewpoints of ease of operation and reducing environmental impact, in some embodiments, a method of kneading polymer particles (e.g., kneading while heating) can be preferably employed. Kneading can be performed using a common kneading device (e.g., a twin-screw extruder). By homogenizing the polymer particle structure, a mixture 20 can be obtained in which inorganic particles 14 are dispersed in a continuous phase containing an acrylic polymer 12, as schematically shown in FIG. 3 . This mixture 20 can be a pressure-sensitive adhesive composition containing the acrylic polymer 12. The mixture (kneaded product) obtained by the above method (a) can be preferably used, for example, as a hot-melt pressure-sensitive adhesive composition or a component thereof. The mixture obtained by the above method (b) can be used, for example, as a solvent-based pressure-sensitive adhesive composition or a component thereof. The mixture obtained by the above method (c) can be used, for example, by further blending a photoinitiator, if necessary, as an active energy ray (e.g., ultraviolet ray)-curable pressure-sensitive adhesive composition or a component thereof. The pressure-sensitive adhesive composition (which may be the mixture 20 shown in FIG. 3) can be used to form, for example, a pressure-sensitive adhesive layer 30 as shown in FIG. 4.

[0072] When preparing the pressure-sensitive adhesive composition, various materials that can be used as components of pressure-sensitive adhesives, such as tackifying resins and crosslinking agents, can be added as needed. Examples of tackifying resins include rosin-based tackifying resins (including rosin derivative tackifying resins), petroleum-based tackifying resins, terpene-based tackifying resins, phenol-based tackifying resins, and ketone-based tackifying resins. These can be used alone or in combination of two or more. Examples of crosslinking agents that can be used include isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, melamine-based crosslinking agents, peroxide-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, carbodiimide-based crosslinking agents, hydrazine-based crosslinking agents, and amine-based crosslinking agents. Both oil-soluble and water-soluble crosslinking agents can be used. These crosslinking agents can be used alone or in combination of two or more. Examples of other materials that can be used in preparing the pressure-sensitive adhesive composition include crosslinking aids, release modifiers, plasticizers, softeners, fillers, colorants (pigments, dyes, etc.), antistatic agents, antioxidants, UV absorbers, antioxidants, light stabilizers, preservatives, photoinitiators, etc. As for such various additives, conventionally known ones can be used in the usual manner, and they do not particularly characterize the present invention, so detailed explanations will be omitted.

[0073] As a means for forming a pressure-sensitive adhesive layer from a pressure-sensitive adhesive composition, various known means such as coating, drying, polymerization, crosslinking, and irradiation with active energy rays (e.g., ultraviolet rays) can be used alone or in appropriate combination depending on the composition of the pressure-sensitive adhesive composition. For example, a conventional coater (e.g., a gravure roll coater, a reverse roll coater, a kiss roll coater, a dip roll coater, a bar coater, a knife coater, a spray coater, etc.) can be used to coat the pressure-sensitive adhesive composition. In the case of a hot-melt pressure-sensitive adhesive composition, the pressure-sensitive adhesive layer can be formed by coating the pressure-sensitive adhesive composition in a heated and molten state using a hot-melt coater, followed by cooling and curing. Kneading of polymer particles and hot-melt coating may be performed continuously.

[0074] The thickness of the pressure-sensitive adhesive layer is not particularly limited and may be, for example, in the range of 5 μm to 500 μm. From the viewpoint of easily exhibiting better tack, in some embodiments, the thickness of the pressure-sensitive adhesive layer is suitably 10 μm or more, preferably 15 μm or more, and may be 20 μm or more, 30 μm or more, 50 μm or more, 70 μm or more, or even 100 μm or more. As the thickness of the pressure-sensitive adhesive layer increases, the impact absorption properties of the pressure-sensitive adhesive layer tend to improve. Furthermore, in some embodiments, from the viewpoint of suppressing cohesive failure, the thickness of the pressure-sensitive adhesive layer may be, for example, 400 μm or less, 300 μm or less, 250 μm or less, 200 μm or less, or 150 μm or less.

[0075] The PSA layer may be a PSA sheet or a component thereof. That is, this specification provides a PSA sheet having the PSA layer. The PSA layer may be formed from a PSA composition prepared using any of the polymer particles disclosed herein. The PSA sheet may be in the form of a PSA sheet (substrate-less double-sided PSA sheet) comprising a PSA layer, a single-sided PSA sheet having a PSA layer on one side of a substrate (support substrate), or a double-sided PSA sheet having PSA layers on both sides of a substrate. Various sheet-like substrates can be used as the substrate, including, for example, resin films, paper, cloth (woven fabrics, nonwoven fabrics, etc.), rubber sheets, elastomer sheets, foam sheets, metal foils, and composites thereof. The total thickness of the PSA sheet having the substrate (i.e., the total thickness of the substrate and the PSA layers formed on one or both sides thereof) may be, for example, 20 μm or more, 50 μm or more, 100 μm or more, 150 μm or more, or 200 μm or more. The total thickness of the pressure-sensitive adhesive sheet having the substrate may be, for example, 1000 μm or less, 800 μm or less, 500 μm or less, 400 μm or less, or 300 μm or less.

[0076] <Characteristics of polymer particles> In some embodiments of the polymer particles disclosed herein, the polymer particles have a bulk density of 0.4 g / cm as measured by the method described in the Examples below. 3 It is preferable that the bulk density is 0.4 g / cm or more. 3 If the bulk density is less than 0.4 g / cm, the particle size of the polymer particles will be too large, which may cause problems such as the structure of the polymer particles collapsing due to pressure during storage or transportation, reducing blocking resistance, or making production by suspension polymerization difficult. 3 If the bulk density is less than 0.45 g / cm, the particle size of the polymer particles will be too small, resulting in a large specific surface area, which may make it difficult to balance blocking resistance and adhesive performance. 3 More preferably, it is 0.5 g / cm or more. 3 or more (e.g., 0.55 g / cm 3 In some embodiments, the bulk density of the polymer particles is 1.0 g / cm or more. 3 It is appropriate that the concentration is less than 0.9 g / cm 3 Preferably, it is 0.8 g / cm or less. 3 or less (e.g., 0.7 g / cm 3 It is more preferable that the bulk density of the polymer particles is not too high. For example, it can be advantageous from the viewpoint of improving the feedability of the polymer particles.

[0077] The average particle size of the polymer particles disclosed herein can be determined, for example, by microscopic observation. In some embodiments, the average particle size of the polymer particles is suitably approximately 100 μm or more, advantageously 200 μm or more, preferably 300 μm or more, and more preferably 400 μm or more. In some embodiments, the average particle size of the polymer particles is suitably approximately 1000 μm or less, advantageously 900 μm or less, preferably 800 μm or less, and more preferably 700 μm or less. Polymer particles having such an average particle size are preferred because they tend to be easy to handle in powder form. The average particle size of the polymer particles of Examples 1 to 19 prepared in the Examples described below was all approximately 600 μm.

[0078] The polymer particles disclosed herein can be kneaded under appropriate conditions to develop adhesiveness. For example, by kneading under conditions of a shear rate of 15 [ / s] or higher (e.g., conditions of 20 [ / s] or higher, conditions of 30 [ / s] or higher, etc.), it is preferable that a tackiness of 0.10 MPa or higher is developed in a probe tackiness measurement. More specifically, for example, in the adhesiveness development test described in the Examples below, it is preferable that a tackiness of 0.10 MPa or higher is developed. The tackiness is, for example, preferably 0.15 MPa or higher, more preferably 0.20 MPa or higher, and may be 0.25 MPa or higher, or may be 0.30 MPa or higher. The upper limit of the tackiness is not particularly limited. In some embodiments, taking into consideration the balance with the ease of handling of the polymer particles in powder form, the tackiness may be, for example, 1.0 MPa or lower, 0.8 MPa or lower, 0.7 MPa or lower, 0.6 MPa or lower, or 0.5 MPa or lower.

[0079] <Application> The uses of the polymer particles disclosed herein and the PSA compositions produced using the polymer particles are not particularly limited. The polymer particles disclosed herein can be preferably used, for example, in the production of hot-melt PSA compositions, and hot-melt PSA compositions have the advantage of being able to produce relatively thick PSA layers with good productivity. Relatively thick PSA layers generally have good impact absorption. Therefore, the PSA sheets disclosed herein can be preferably used, for example, in various products requiring impact resistance, or in a form attached to components constituting the products, for applications such as fixing, joining, molding, decorating, protecting, and supporting the products or components. In particular, they can be preferably used to fix the products or components. For example, portable electronic devices are at risk of being dropped due to their usage, and PSA sheets used for portable electronic devices may be required to have impact resistance. The double-sided PSA sheets disclosed herein are suitable for such portable electronic devices.

[0080] Non-limiting examples of the portable electronic devices include mobile phones, smartphones, tablet computers, notebook computers, various wearable devices (for example, wristwear devices worn on the wrist like a wristwatch, modular devices worn on a part of the body with a clip or strap, eyewear devices including eyeglasses (monocular and binocular, including head-mounted devices), clothing devices attached to shirts, socks, hats, etc. as accessories, earwear devices attached to the ears like earphones, etc.), digital cameras, digital video cameras, audio equipment (portable music players, IC recorders, etc.), calculators (calculators, etc.), portable game devices, electronic dictionaries, electronic organizers, e-books, in-car information devices, portable radios, portable televisions, portable printers, portable scanners, portable modems, etc. In this specification, "portable" does not simply mean that the device is portable, but rather means that the device has a level of portability that allows an individual (average adult) to carry it relatively easily.

[0081] The matters disclosed by this specification include the following: [1] Polymer particles containing an acrylic polymer, The acrylic polymer has a glass transition temperature of 0°C or lower, the surfaces of the polymer particles are coated with inorganic particles, The polymer particles have a content of the inorganic particles of more than 5 parts by weight based on 100 parts by weight of the acrylic polymer. [2] The polymer particles according to [1] above, wherein the content of the inorganic particles is more than 5 parts by weight and less than 50 parts by weight per 100 parts by weight of the acrylic polymer. [3] The inorganic particles include first inorganic particles and second inorganic particles, The polymer particles according to [1] or [2] above, wherein the second inorganic particles have an average particle size smaller than the average particle size of the first inorganic particles. [4] The polymer particles according to [3], wherein the first inorganic particles have an average particle size of 5 μm or more and less than 50 μm. [5] The second inorganic particles are (A) the average particle size is 0.1 μm or more and less than 5 μm, and (B) The average major axis is 0.1 μm or more and less than 5 μm. The polymer particles according to the above [3] or [4], which satisfy at least one of the above. [6] The polymer particles according to any one of [3] to [5] above, wherein the content of the second inorganic particles is 5 parts by weight or more and 40 parts by weight or less relative to 100 parts by weight of the acrylic polymer. [7] Bulk density is 0.4 g / cm 3 More than 1.0g / cm 3 The polymer particles according to any one of the above [1] to [6], which are: [8] The polymer particles according to any one of [1] to [7], which exhibit a tackiness of 0.10 MPa or more in a probe tackiness measurement by kneading under conditions of a shear rate of 15 [ / s] or more. [9] A method for producing the polymer particles according to any one of [1] to [8] above, comprising obtaining the acrylic polymer by suspension polymerization.

[10] Preparing the polymer particles according to any one of [1] to [8] above; Destroying the structure of the polymer particles; A method for producing a pressure-sensitive adhesive composition, comprising: [Example]

[0082] Several examples of the present invention will be described below, but it is not intended that the present invention be limited to those shown in these examples. In the following description, "parts" and "%" are by weight unless otherwise specified. In addition, "-" in Tables 1 and 2 indicates that the component was not used, and that the evaluation was not measurable or was not evaluated.

[0083] <Production of Acrylic Polymer Particles> (Example 1) A mixed solution of 100 parts of a monomer component consisting of 95 parts of n-butyl acrylate (BA) and 5 parts of methacrylic acid (MAA), 0.10 parts of a polymerization initiator (2,2'-azobisisobutyronitrile), and 0.05 parts of a chain transfer agent (t-lauryl mercaptan) was prepared. A reaction vessel equipped with a thermometer, stirrer, nitrogen inlet tube, and reflux condenser was charged with 400 parts of distilled water and 10 parts of tricalcium phosphate (manufactured by Yoneyama Chemical, average particle size 10.2 μm (nominal value)), and the above mixed solution was added and suspended while stirring at 300 rpm. While continuing the stirring, nitrogen gas was introduced and nitrogen substitution was carried out for 1 hour, and the system was then heated to 60°C and suspension polymerization was carried out for 3 hours, followed by an aging reaction at 75°C for 3 hours. After the system was cooled to room temperature (approximately 25°C), 15 parts of light calcium carbonate (manufactured by Maruo Calcium, major axis 2.0 μm, minor axis 0.4 μm (catalog values)) was added and stirred at 100 rpm. The contents of the reaction vessel were subjected to solid-liquid separation using an 80-mesh metal mesh, and the solid matter remaining on the mesh was washed with water and then removed using a hot air dryer. In this way, powdery acrylic polymer particles according to this example were obtained. These acrylic polymer particles contained an acrylic polymer P1 having a copolymer composition corresponding to the composition of the above-mentioned monomer components.

[0084] (Examples 2-5, 11-19) Powdery acrylic polymer particles according to each example were obtained in the same manner as in Example 1, except that the amounts of tricalcium phosphate and precipitated calcium carbonate used were as shown in Tables 1 and 2. In Examples 4 and 19, precipitated calcium carbonate was not used.

[0085] (Example 6) Powdery acrylic polymer particles were obtained in the same manner as in Example 1, except that a monomer component consisting of 66.5 parts of BA, 28.5 parts of 2-ethylhexyl acrylate (2EHA), and 5 parts of MAA was used. The acrylic polymer particles contain an acrylic polymer P2 having a copolymer composition corresponding to the composition of the above-mentioned monomer components.

[0086] (Example 7) Powdery acrylic polymer particles were obtained in the same manner as in Example 1, except that a monomer component consisting of 47.5 parts of BA, 47.5 parts of 2EHA, and 5 parts of MAA was used. The acrylic polymer particles contained acrylic polymer P3 having a copolymer composition corresponding to the composition of the above monomer components.

[0087] (Example 8) Powdery acrylic polymer particles were obtained in the same manner as in Example 7, except that the amount of precipitated calcium carbonate used was as shown in Table 1.

[0088] (Example 9) Powdered acrylic polymer particles were obtained in the same manner as in Example 1, except that silica (Silsic SA-2 manufactured by Yamamori Tsuchimoto Kogyosho, average particle size 2.0 μm (nominal value)) was used instead of precipitated calcium carbonate.

[0089] (Example 10) Powdered acrylic polymer particles were obtained in the same manner as in Example 1, except that a monomer component consisting of 100 parts of BA was used and heavy calcium carbonate (manufactured by Maruo Calcium, average particle size 6.3 μm (nominal value)) was used instead of tricalcium phosphate. The acrylic polymer particles contain an acrylic polymer P4 having a copolymer composition corresponding to the composition of the above monomer components.

[0090] (Example 20) In Example 1, when neither tricalcium phosphate nor precipitated calcium carbonate was used, the particles aggregated during suspension polymerization, making it impossible to obtain a powdery acrylic polymer. Therefore, the following measurements and evaluations were not performed.

[0091] <Measurement and Evaluation> (bulk density) The acrylic polymer particles according to each example were filled into a sample bottle of a fixed volume (50 mL) at room temperature (about 25°C), and the bulk density was calculated from the volume of the sample bottle and the weight of the filled acrylic polymer particles. The results are shown in the corresponding columns in Tables 1 and 2.

[0092] (Feed test) Two types of stainless steel conical funnels were prepared. The shapes of each funnel are shown below. 10mm diameter funnel: Upper inner diameter 27mm, lower inner diameter 10mm, height 50mm 6mm diameter funnel: Upper inner diameter 27mm, lower inner diameter 6mm, height 50mm Using these funnels, a feed test was conducted at room temperature (approximately 25°C) according to the following procedure. That is, the hole of a 10 mm diameter funnel was blocked with a blocking plate, 7 g of acrylic polymer particles according to each example was placed in the funnel, the blocking plate was then removed, and the time until all the acrylic polymer particles in the funnel had fallen (feed time) was measured. If the required time was less than 3 seconds, the feed time was similarly measured using a funnel with a 6 mm hole diameter. Based on the results, the feedability was evaluated according to the following four levels. The results are shown in the corresponding columns in Tables 1 and 2. The higher the score, the better the feedability. 4 points: Feed time using a 6mm diameter funnel is less than 3 seconds 3 points: Feed time using a 10mm diameter funnel is less than 3 seconds 2 points: Feed time using a funnel with a hole diameter of 10 mm is 3 seconds or more 1 point: The contents do not fall completely through a 10mm diameter funnel (the funnel gets clogged)

[0093] (Blocking test) At room temperature (approximately 25°C), 10 g of acrylic polymer particles according to each example was placed in a glass bottle, and a load was applied so that a pressure of 1.7 MPa was applied to the acrylic polymer particles in the glass bottle. After 24 hours, the load was removed, and the presence or absence of adhesion (blocking) of the acrylic polymer particles was confirmed by visual observation. If no blocking had occurred, the applied pressure was changed to 4.3 MPa, and the presence or absence of blocking after 24 hours was similarly confirmed. Based on the results, blocking resistance was evaluated using the following three levels. The results are shown in the corresponding columns in Tables 1 and 2. The higher the score, the better the blocking resistance. 3 points: No blocking under the condition of applied pressure of 4.3 MPa 2 points: No blocking under the condition of applied pressure of 1.7 MPa 1 point: Blocking occurs under the condition of an applied pressure of 1.7 MPa

[0094] The applied pressure in the blocking test was 1.7 MPa, which is a test condition that simulates the pressure (calculated from the bulk density of the polymer particles and the shape of the pail) that would be exerted on the polymer particles near the bottom of a pail when the polymer particles of each example are stored in the pail. The applied pressure in the blocking test was 4.3 MPa, which is a test condition that simulates the pressure (calculated from the bulk density of the polymer particles and the shape of the paper bag) that would be exerted on the polymer particles near the bottom of a typical 25 kg paper bag (length 838 mm, body width 443 mm, gusset 102 mm) in the longitudinal direction when the polymer particles of each example are stored in the paper bag. A rating of 2 or higher in the blocking test is considered to have a practical level of blocking resistance suitable for storage and distribution in powder form.

[0095] (Adhesion development test) The acrylic polymer particles according to each example were placed in a kneader (a tabletop kneader manufactured by Xplore Instrument, model number "MC15HT") and kneaded at 120°C and 100 rpm (corresponding to a shear rate of 33 [ / s]) for 5 minutes to obtain a kneaded product of the acrylic polymer particles. The above kneaded material was sandwiched between release liners and press-molded using a press (a heated press manufactured by Tester Sangyo, model "SA-501") at 120°C and 5 MPa to obtain a viscoelastic sheet with a thickness of 100 μm. The resulting viscoelastic sheet was backed with a glass slide to prepare a test sample. The probe tack of the viscoelastic sheet was measured using this test sample under the following conditions. The measurement was performed five times, and the arithmetic mean of the obtained values ​​is shown in Tables 1 and 2 as the tack [MPa] after kneading. [Probe tack measurement conditions] Equipment used: RHESCA tacking tester, model number "TAC-2" Probe: Stainless steel (SUS) probe, tip diameter 5 mm Test speed: 120mm / min Load: 10gf Press time: 0.1 seconds

[0096] [Table 1]

[0097] [Table 2]

[0098] As shown in Tables 1 and 2, the acrylic polymer particles of Examples 1 to 18 were non-tacky particles suitable for handling as a powder. Specifically, the acrylic polymer particles of Examples 1 to 18 exhibited anti-blocking properties at or above a practical level and also had good feedability. Furthermore, although the acrylic polymer particles of Examples 1 to 18 were non-tacky particles in powder form, they could be kneaded to homogenize their structure to form viscoelastic sheets (pressure-sensitive adhesive sheets) exhibiting adhesive properties with a tack value of 0.10 MPa or more. The acrylic polymer particles of Examples 1 to 4, 6 to 9, and 11 to 12 formed pressure-sensitive adhesive sheets exhibiting particularly good adhesive properties. In particular, the acrylic polymer particles of Examples 1 to 3, 6, and 12 exhibited a good balance between good anti-blocking properties and feedability in powder form and good adhesive properties after structural homogenization. Furthermore, a comparison of Examples 1, 11 to 14 shows that adjusting the amount of the first inorganic particles used is effective in improving the feedability, and a comparison of Examples 1, 15 to 18 shows that adjusting the amount of the second inorganic particles used is effective in improving the anti-blocking properties.

[0099] On the other hand, the acrylic polymer particles of Example 19, which contained a small amount of inorganic particles, had poor blocking resistance and were inferior in feedability to the acrylic polymer particles of Examples 1 to 18. When neither tricalcium phosphate nor precipitated calcium carbonate was used as the inorganic particles in Example 1, a powdery acrylic polymer could not be obtained (Example 20).

[0100] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. [Explanation of symbols]

[0101] 10 Acrylic polymer particles (composite particles) 12 Acrylic polymers 14 Inorganic particles 142 First inorganic particle 144 Second inorganic particle 20. Pressure-sensitive adhesive composition 30 adhesive layer

Claims

1. Polymer particles comprising an acrylic polymer, The acrylic polymer has a glass transition temperature of 0°C or lower, the surfaces of the polymer particles are coated with inorganic particles, The polymer particles have a content of the inorganic particles of more than 5 parts by weight based on 100 parts by weight of the acrylic polymer.

2. The polymer particles according to claim 1, wherein the content of the inorganic particles is more than 5 parts by weight and less than 50 parts by weight based on 100 parts by weight of the acrylic polymer.

3. the inorganic particles include first inorganic particles and second inorganic particles; The polymer particles according to claim 1 , wherein the second inorganic particles have an average particle size smaller than the average particle size of the first inorganic particles.

4. The polymer particles according to claim 3 , wherein the first inorganic particles have an average particle size of 5 μm or more and less than 50 μm.

5. The second inorganic particles are (A) the average particle size is 0.1 μm or more and less than 5 μm, and (B) The average major axis is 0.1 μm or more and less than 5 μm; The polymer particles according to claim 4, which satisfy at least one of the above.

6. The polymer particles according to claim 3 , wherein the content of the second inorganic particles is 5 parts by weight or more and 40 parts by weight or less relative to 100 parts by weight of the acrylic polymer.

7. Bulk density is 0.4 g / cm ３ 1.0g / cm or more ３ 7. The polymer particles according to claim 1, wherein:

8. The polymer particles according to any one of claims 1 to 6, which exhibit a tackiness of 0.10 MPa or more in a probe tackiness measurement by kneading at a shear rate of 15 [ / s] or more.

9. A method for producing the polymer particles according to any one of claims 1 to 6, comprising obtaining the acrylic polymer by suspension polymerization.

10. Providing polymer particles according to any one of claims 1 to 6; disrupting the structure of the polymer particles; A method for producing a pressure-sensitive adhesive composition, comprising:

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