Polytetrafluoroethylene granulated powder and its manufacturing method

Abstract

To provide granulated powder having good handleability and causing little desorption of a filler during production or use.SOLUTION: The present invention is a method for producing polytetrafluoroethylene granulated powder, including stirring a mixture containing a filler, polytetrafluoroethylene powder, and a multi-hydrolyzable group-type silane coupling agent in mixed liquid of water and water-insoluble organic liquid to perform mixed granulation.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a granulated polytetrafluoroethylene powder that exhibits minimal detachment of fillers during production and use, and to a method for producing the same. [Background technology]

[0002] Polytetrafluoroethylene (PTFE) has a low coefficient of friction and excellent heat and chemical resistance, but is prone to wear and creep deformation. Therefore, compositions containing various fillers to improve wear resistance and creep resistance are widely used as sliding and sealing components. PTFE compositions are obtained by mixing PTFE powder with fillers, and granulated powders with high apparent density are produced to improve powder handling and powder flowability. Granulation is also performed on PTFE powder alone to improve its handleability.

[0003] One known method for producing granulated powders is to wet the powder with an organic solvent to form a slurry, which is then rolled to form granules (Patent Document 1). However, this method is not suitable for continuous production because it is difficult to completely discharge the slurry for each batch. Another method, called underwater granulation, involves stirring a mixture of powder and a water-insoluble organic solvent in water (Patent Document 2), which is also used due to its high productivity. This method is called underwater granulation. However, this method has the problem that when filler particles with hydrophilic surfaces are used, the filler is detached into the water. As a result, the detached filler aggregates during the process and is mixed into the product, resulting in defective products. Furthermore, the loss of filler material can result in a lower content ratio than the target value.

[0004] Therefore, methods have been proposed, such as treating a hydrophilic filler with an amino group-containing silane (aminosilane) or a silicone resin (Patent Document 3), treating a hydrophilic filler with an amino group-containing silicone resin (Patent Document 4), and using a specific fluorine-containing solvent as the solvent (Patent Document 5). However, when metal powders or metal compounds (metal salts, metal oxides, etc.) with a highly hydrophilic surface and a high specific gravity are used as fillers, especially when fillers with small particle sizes are used, detachment occurs even when the above methods are used. Small particle size fillers are preferred because they can achieve the desired effect even in small amounts, but they are prone to detachment, which is an issue.

[0005] Furthermore, although PTFE has excellent surface non-stickiness, there is also the problem of fillers easily detaching from the resulting granulated powder. This problem occurs not only when a filler with a hydrophilic surface is used, but also when a filler with a hydrophobic surface is used, and it is therefore necessary to improve the adhesion between PTFE and fillers with hydrophobic surfaces. Furthermore, when a filler with a large particle size is used, detachment during granulation is unlikely to occur, but there is also the problem that it is relatively easy to detach from the resulting granulated powder. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 44-22620 [Patent Document 2] Special Publication No. 60-21694 [Patent Document 3] Japanese Patent Application Publication No. 51-549 [Patent Document 4] Japanese Patent Application Laid-Open No. 2001-220482 [Patent Document 5] Japanese Patent Application Laid-Open No. 2002-201287 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a granulated powder that is easy to handle and in which the filler is hardly detached during production (granulation) or use. [Means for solving the problem]

[0008] The present inventors have discovered that by using a specific silane coupling agent when mixing PTFE powder with a filler, particularly a filler with a hydrophilic particle surface, the detachment of the filler is reduced and further the detachment of the filler from the resulting granulated powder can be suppressed, thereby completing the present invention.

[0009] That is, one embodiment of the present invention is a method for producing polytetrafluoroethylene granulated powder, characterized in that a mixture containing a filler, polytetrafluoroethylene powder, and a polyhydrolyzable group-type silane coupling agent is stirred in a mixture of water and a water-insoluble organic liquid to perform mixing and granulation.

[0010] The order in which the filler, polytetrafluoroethylene powder, and polyhydrolyzable coupling agent are mixed is not particularly limited. The polyhydrolyzable coupling agent may be added to the filler, and then the polytetrafluoroethylene powder may be added, or the filler, polytetrafluoroethylene powder, and polyhydrolyzable coupling agent may be mixed simultaneously.

[0011] The polyhydrolyzable group-type silane coupling agent is preferably a bis-silane coupling agent represented by the following formula (1) or a polymer-type silane coupling agent containing a repeating monomer unit represented by the following formula (2). X-R1-X (1)

[0012] [X is a hydrolyzable silyl group (-Si-(OR2) n (R3) 3-n ) (n=1 to 3), and R2 and R3 are each —CH3 or —C2H5; R1 is a straight-chain or branched-chain hydrocarbon having 2 to 12 carbon atoms, which may contain a substituent and in which hydrogen may be substituted with a halogen.]

[0013] [ka] (2)

[0014] [X is a hydrolyzable silyl group (-Si-(OR2) n (R3) 3-n ) (n=1 to 3), and R2 and R3 are each —CH3 or —C2H5; A is —CH— or N; R a ,R b are each independently a hydrocarbon having 1 to 10 carbon atoms which may contain a substituent and in which hydrogen may be substituted with halogen, R c is a hydrocarbon having 1 to 5 carbon atoms in which hydrogen may be substituted with a halogen, p=3~30.

[0015] When the polyhydrolyzable group-type silane coupling agent is a bis-silane coupling agent represented by formula (1), it is preferable that R1 does not contain a substituent or contains an imino group (-NH-) as a substituent. When the polyhydrolyzable group-type silane coupling agent is a polymeric silane coupling agent containing a repeating monomer unit represented by formula (2), R a and R b It is preferred that the group does not contain a substituent or contains an imino group (—NH—) as a substituent.

[0016] In another embodiment of the present invention, a filler of 3 to 40% by volume is A granulated powder comprising 60 to 97% by volume of polytetrafluoroethylene powder, The polytetrafluoroethylene granulated powder contains 1% by weight or less of a polyhydrolyzable group-type silane coupling agent relative to the total amount of the granulated powder.

[0017] The polyhydrolyzable group-type silane coupling agent is preferably a bis-silane coupling agent represented by formula (1) or a polymeric silane coupling agent containing a repeating monomer unit represented by formula (2). X-R1-X (1)

[0018] [X is a hydrolyzable silyl group (-Si-(OR2) n (R3) 3-n ) (n=1 to 3), and R2 and R3 are each —CH3 or —C2H5; R1 is a straight-chain or branched-chain hydrocarbon having 2 to 12 carbon atoms, which may contain a substituent and in which hydrogen may be substituted with a halogen.]

[0019] [ka] (2)

[0020] [X is a hydrolyzable silyl group (-Si-(OR2) n (R3) 3-n ) (n=1 to 3), and R2 and R3 are each —CH3 or —C2H5; A is —CH— or N; R a ,R b are each independently a hydrocarbon having 1 to 10 carbon atoms which may contain a substituent and in which hydrogen may be substituted with halogen, R c is a hydrocarbon having 1 to 5 carbon atoms in which hydrogen may be substituted with a halogen, p=3~30.

[0021] The polyhydrolyzable group-type silane coupling agent is preferably a bis-silane coupling agent represented by formula (1). The polyhydrolyzable group-type silane coupling agent is preferably a polymeric silane coupling agent containing a repeating monomer unit represented by formula (2).

[0022] The average particle size of the granulated powder is preferably 200 to 800 μm. [Effects of the Invention]

[0023] The PTFE granulated powder of the present invention can be produced by a highly productive underwater granulation method. Furthermore, the method for producing the PTFE granulated powder of the present invention enables granulation in which the filler is uniformly dispersed without being released into the aqueous phase, even when a small-particle-sized hydrophilic filler is used. The resulting granulated powder is easy to handle and the filler is less likely to be released, making it easy to produce molded products by compression molding, and it can be used for a variety of applications, such as sliding members and sealing materials. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a photograph of the wastewater from Example 7 listed in Table 5. [Figure 2] 1 is a photograph of the wastewater of Comparative Example 8 shown in Table 5. DETAILED DESCRIPTION OF THE INVENTION

[0025] The PTFE granulated powder of the present invention is produced from a filler, a polytetrafluoroethylene powder, and a specific silane coupling agent. First, each component will be described below.

[0026] (1) Polytetrafluoroethylene (PTFE) The PTFE powder used in the present invention may be a homopolymer of tetrafluoroethylene (TFE) (homo-PTFE), a TFE copolymer (modified PTFE) containing 1% by weight or less of a monomer copolymerizable with TFE, or a mixture thereof.

[0027] The monomer (comonomer) copolymerizable with TFE contained in the modified PTFE can be a monomer containing an unsaturated bond and capable of radical polymerization. To maintain the excellent performance of PTFE, such as heat resistance and chemical resistance, it is preferable to use a fluorine-containing monomer as the comonomer. Specific examples of comonomers include perfluoroalkenes having 3 or more carbon atoms, preferably 3-6 carbon atoms, perfluoro(alkyl vinyl ethers) having 1-6 carbon atoms, and chlorotrifluoroethylene. Among these, hexafluoropropylene (HFP), perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether) (PEVE), perfluoro(propyl vinyl ether) (PPVE), perfluoro(butyl vinyl ether) (PBVE), and chlorotrifluoroethylene are preferred.

[0028] When modified PTFE is used, the presence of comonomers makes it difficult for molecular chains to slide against each other, increasing the strength and elastic modulus of the resin and improving creep resistance. However, if the amount of comonomer exceeds 1% by weight, the sliding properties of PTFE decrease and it becomes fluid at temperatures above its melting point, making it unsuitable for use at high temperatures. Furthermore, it becomes difficult to produce molded products using the free-baking method, in which a compression-molded composition is heated and baked above its melting point. For this reason, the comonomer content is preferably in the range of 0.001 to 1% by weight of comonomer units.

[0029] As a method for polymerizing polytetrafluoroethylene, known polymerization methods such as suspension polymerization and emulsion polymerization can be used. Powdered PTFE (molding powder) obtained by suspension polymerization is preferably used in the resin composition of the present invention for the following reasons. Compared with polytetrafluoroethylene obtained by emulsion polymerization, polytetrafluoroethylene obtained by suspension polymerization is less likely to undergo fibrous formation due to shear stress, and therefore can be dry-mixed at room temperature. Furthermore, suspension polymerization is easy to handle because the powder does not solidify during the injection process during molding, and is also advantageous in terms of low cost.

[0030] Polytetrafluoroethylene can be used as a molding material if it has a molecular weight that allows it to be molded by means of compression molding, etc. It is known that the melting point of PTFE correlates with the molecular weight, and polymers with a melting point of approximately 327°C are suitable for use as molding resins for sliding parts, as they have good mechanical strength and heat resistance.

[0031] Furthermore, this polytetrafluoroethylene is usually used in a powder state to mold various products. Its average particle size is 100 μm or less, preferably in the range of 5 to 100 μm, and more preferably in the range of 10 to 50 μm. Polytetrafluoroethylene with an average particle size in this range is excellent in uniform mixing with various fillers. Such polytetrafluoroethylene can be directly produced in powder form by suspension polymerization. Furthermore, commercially available molding powders can also be used as long as they have this average particle size.

[0032] Examples of such PTFE molding powder include Teflon (registered trademark) PTFE 7-J (homo PTFE, average particle size: 50 μm) manufactured by Mitsui-Chemours Fluoro Products Co., Ltd., and Teflon (registered trademark) PTFE manufactured by Mitsui-Chemours Fluoro Products Co., Ltd. Examples of suitable PTFE include Teflon (registered trademark) PTFE 7A-J (homo PTFE, average particle size: 30 μm) manufactured by Mitsui-Chemours Fluoroproducts Co., Ltd., and Teflon (registered trademark) PTFE 70-J (modified PTFE, average particle size: 35 μm) manufactured by Mitsui-Chemours Fluoroproducts Co., Ltd.

[0033] (2) Filling material In the present invention, the term "filler" refers to a powdery substance used to improve various physical properties of molded articles. Examples of fillers used in the present invention include various organic and inorganic substances mixed with PTFE powder for molding. Examples of organic substances include engineering plastics such as polyphenylene sulfide, polyethersulfone, polyphenylsulfone, polyamide, polyimide, phenolic resin, urea resin, epoxy resin, urethane resin, melamine resin, polyester resin, polyether resin, acrylic resin, acrylic silicone resin, silicone resin, and silicone polyester resin. Examples of inorganic substances include metal powder, metal oxides (e.g., aluminum oxide, zinc oxide, tin oxide, and titanium oxide (titanate)), glass, ceramics, silicon carbide, silicon oxide, calcium fluoride, carbon black, carbon fiber, graphite, coke, mica, talc, barium sulfate, and molybdenum disulfide. These substances can also be used in combination as needed.

[0034] The present invention also makes it possible to use fillers with hydrophilic particle surfaces, which have traditionally been difficult to handle in underwater granulation. Specific examples of fillers with hydrophilic particle surfaces include metals (bronze, aluminum, etc.), metal compounds (metal oxides, metal salts, metal halides), inorganic salts (silicon carbide, silicon oxide, boron nitride, calcium fluoride, etc.), glass, and silica. The use of a specific silane coupling agent in the present invention can prevent detachment of fillers with hydrophilic particle surfaces due to migration of the filler into the aqueous phase during underwater granulation, even for fillers with hydrophilic particle surfaces. Furthermore, detachment can be suppressed even for fillers with high specific gravities, such as metals and metal compounds, which tend to migrate to the aqueous phase during underwater granulation.

[0035] Alternatively, particles of engineering plastics with hydrophobic surfaces, such as wholly aromatic polyester resins, or carbon-based fillers with graphene structures, such as carbon black, carbon fiber, graphite, and coke, may be used. The use of a specific silane coupling agent allows the filler to adhere to the granulated powder with sufficient strength, preventing the filler from detaching due to vibrations during post-granulation processes or when the granulated powder is used. Furthermore, residual imino groups also provide the effect of preventing static electricity from building up in the granulated powder.

[0036] The particle shape of the filler can be various, such as particulate, fibrous, or flake-like, and any shape can be used. A small particle size can improve the physical properties of the filler even in small amounts, but there is a problem that the filler is prone to migrate into the water during underwater granulation. In the present invention, the use of a specific silane coupling agent can suppress the detachment of small particle size fillers with hydrophilic surfaces, so that small particle size fillers of 0.1 μm to 100 μm can also be used.

[0037] However, from the viewpoint of efficiently producing high-quality granulated particles, the average particle size of the filler is preferably 1 μm to 500 μm, and more preferably 3 μm to 300 μm. The average particle size of the filler means the particle size at an integrated value of 50% (volume basis) in the particle size distribution measured by a laser diffraction / scattering method.

[0038] (3) Silane coupling agents (polyhydrolyzable silane coupling agents) Generally, a silane coupling agent is a compound having a hydrolyzable silyl group, such as the one shown below, which reacts with an inorganic material to form a covalent bond. R-Si-(OR2) n (R3) 3-n (n=1-3)(3)

[0039] R2 and R3 are each independently a methyl group or an ethyl group, and R is called an organic substituent, which is a saturated or unsaturated alkyl group containing one or more reactive groups such as a vinyl group, epoxy group (alicyclic epoxy group, glycidyl group), methacryl group, acrylic group, styryl group, amino group, imino group, sulfide group, disulfide group, aryl group, diamino group, mercapto group, ureido group, or isocyanate group, depending on the desired properties.

[0040] The silane coupling agent used in the present invention has a hydrolyzable silyl group: -Si-(OR2) n (R3) 3-n ) (n=1 to 3). Specific examples of the polyhydrolyzable group silane coupling agent include (A) bis-silane coupling agents having a structure in which hydrolyzable silyl groups are present at both ends of an organic substituent, and (B) polymeric silane coupling agents having a structure in which a polymer has a repeating organic substituent structure and has hydrolyzable silyl groups in its side chains, and it is preferable to use these.

[0041] (A) Bis-silane coupling agent The bissilane coupling agent (A) used in the present invention is a bissilane represented by the following formula (1) having two hydrolyzable silyl groups in the molecule. X-R1-X (1)

[0042] X is a hydrolyzable silyl group [-Si-(OR2) n (R3) 3-n ] (n=1 to 3), preferably a trialkoxysilyl group: -Si-(OR2)3 or a dialkoxysilyl group: -Si-R3(OR2)2. R2 and R3 each represent an alkyl group, preferably -CH3 or -C2H5. X is more preferably a trimethoxysilyl group.

[0043] R1 is a linear or branched hydrocarbon having 2 to 12 carbon atoms, which may contain a substituent and in which hydrogen may be substituted with a halogen. The hydrocarbon preferably has 5 to 10 carbon atoms, and the hydrocarbon is preferably linear. The hydrocarbon with a larger carbon number is preferred because it increases hydrophobicity and improves affinity with PTFE. R1 without a substituent is preferred because it is believed that the increased hydrophobicity improves affinity with PTFE and also enhances the effect of suppressing the detachment of the filler into the aqueous phase. If a substituent is present, the substituent may be present between carbon atoms of the hydrocarbon as, for example, -NH-, -S-, -SS-, -CH-, etc., and preferably contains an -NH- (imino group). The number of imino groups in R1 is preferably one. The presence of an imino group in the molecule contributes to antistatic properties, resulting in improved water solubility of the molecule. Since a small amount is sufficient for many fillers, mixing during granulation is improved. The hydrogen of the hydrocarbon may be substituted with a halogen, for example, chlorine.

[0044] Specific examples of the bis-silane coupling agent include, but are not limited to, 1,2-bis(triethoxysilyl)ethane, 1,2-bis(triethoxysilyl)ethylene, 1,6-bis(trimethoxysilyl)hexane, 1,8-bis(trimethoxysilyl)octane, 1,8-bis(triethoxysilyl)octane, bis[3-tri(methoxysilyl)propyl]amine, bis[3-tri(ethoxysilyl)propyl]amine, N,N'-bis[3-tri(methoxysilyl)propyl]ethylenediamine, 1,4-bis(trimethoxysilylethyl)benzene, 1,3-bis(trimethoxysilylpropyl)benzene, 1,6-bis(trimethoxysilyl)-2,5-dimethylhexane, bis(triethoxysilylpropyl)disulfide, and mixtures of two or more selected from these. Among these, 1,6-bis(trimethoxysilyl)hexane, 1,8-bis(trimethoxysilyl)octane, or bis[3-tri(methoxysilyl)propyl]amine is preferred, and 1,8-bis(trimethoxysilyl)octane is more preferred. The bis-silane coupling agent used in the present invention has two or more hydrolyzable silyl groups, and is therefore thought to bond with the filler to be mixed therewith, thereby preventing its separation.

[0045] Such bis-silane coupling agents can be produced by the methods described in JP-A-5-194551, JP-A-2005-509683, and US Pat. No. 6,242,627. Alternatively, commercially available bis-silane coupling agents such as SIB1824.0 and SIB1832.7 manufactured by Gelest, Inc., Dynasilane 1124 manufactured by Evonik, and KBM-3086 manufactured by Shin-Etsu Chemical Co., Ltd. can also be used.

[0046] (B) Polymeric silane coupling agent The polymeric silane coupling agent is a silane coupling agent containing a repeating monomer unit of the following formula (2).

[0047] [ka] (2)

[0048] X is a hydrolyzable silyl group (-Si-(OR2) n (R3) 3-n ) (n=1 to 3), preferably a trialkoxysilyl group: -Si-(OR2)3 or a dialkoxysilyl group: -Si-R3(OR)2. R2 and R3 each represent an alkyl group, preferably -CH3 or -C2H5. X is more preferably a trimethoxysilyl group. A is -CH- or N.

[0049] R a ,R b R are each independently a hydrocarbon having 1 to 10 carbon atoms, which may contain a substituent and in which hydrogen may be substituted with a halogen, preferably having 2 to 8 carbon atoms, and more preferably having 2 to 6 carbon atoms. If the hydrocarbon has 2 or more carbon atoms, it is preferred because it is believed that the hydrophobicity will be strong and the affinity with PTFE will be improved. a ,R b R having no substituent is preferred because it is thought that this will increase hydrophobicity and improve affinity with PTFE, and is also preferred because it will be more effective in suppressing the detachment of the filler into the aqueous phase. a ,R b R may exist as, for example, -NH-, -S-, -SS-, -C6H4-, etc., and is preferably one containing -NH- (imino group). a or R b If an imino group is contained within the molecule, it is preferable that each imino group has a repeating structure with two or more carbon atoms in the main chain. Having an imino group within the molecule contributes to antistatic properties, which in turn improves the water solubility of the molecule, and since a small amount is sufficient to achieve the desired effect with many fillers, it improves mixing during granulation. c is a hydrocarbon having 1 to 5 carbon atoms in which hydrogen may be substituted with halogen, and is preferably a straight-chain hydrocarbon.

[0050] The molecular weight of the polymeric silane coupling agent depends on the molecular structure of the basic skeleton, but is, for example, 500 to 5000, preferably 900 to 3000, and more preferably 1200 to 2000. The degree of polymerization p of the polymeric silane coupling agent depends on the molecular structure of the basic unit, but is p=3 to 30, preferably 3 to 20, and more preferably 3 to 10.

[0051] Among the polymeric silane coupling agents (B) used in the present invention, it is preferred that A is N, that is, a hetero-bonded nitrogen atom, for example, a polyethyleneimine structure.

[0052] A specific example of a polymeric silane coupling agent is trimethoxysilylpropyl-modified polyethyleneimine, and it is preferable to use a modified polyethyleneimine polymer having the basic unit (4) below.

[0053] [ka] (4)

[0054] The amine moiety in the polymer unit above becomes an ammonium ion and can be substituted with any anion (e.g., Cl - ) may form a salt. [p is 3 to 30.]

[0055] Examples of such polymeric silane coupling agents having a hydrolyzable silyl group side chain include X-12-1048 and X-12-972F manufactured by Shin-Etsu Chemical Co., Ltd., and SSP-060, SSP-065, and SSP-050 manufactured by Gelest, Inc.

[0056] The polymeric silane coupling agent used in the present invention has multiple hydrolyzable silyl groups, and therefore acts to bond with the mixed filler and / or PTFE, thereby preventing their separation. Furthermore, since it has a hydrophobic polymer backbone, it is thought that detachment of the filler due to its migration into the aqueous phase during the granulation process is prevented.

[0057] The silane coupling agent used in the present invention, especially one having an imino group, is thought to not only strengthen the adhesion between the PTFE and the filler but also to provide antistatic properties to the granulated particles, which is thought to make the granulated particles less likely to clog the sieve.

[0058] The silane coupling agent used in the present invention is preferably used as a solution dissolved in a solvent in order to be uniformly dispersed in the mixing step.The solvent is not particularly limited, but examples thereof include various organic solvents such as toluene, isopropyl alcohol, 1,3-dichloro-1,1,2,2,3-pentafluoropropane, and 1-chloro-2,3,3-trifluoropropene, and for example, 1-chloro-2,3,3-trifluoropropene can be used.

[0059] (4) Granulated powder of the present invention and its manufacturing method The granulated powder of the present invention is produced from a mixture containing a filler, polytetrafluoroethylene (PTFE) powder, and a polyhydrolyzable silane coupling agent. The PTFE:filler composition ratio, by volume, is 99-20% by volume:1-80% by volume, preferably 97-60% by volume:3-40% by volume, and more preferably 94-75% by volume:6-25% by volume. If the filler content in this mixture is low, it becomes difficult to obtain the effect of the filler in improving physical properties, while if the filler content is too high, the physical properties, such as elongation, of the molded product produced from the resulting granulated powder will be reduced.

[0060] The content of the silane coupling agent used in the present invention is 0.001 to 1.0 wt % of the mixture (total amount of granulated powder), preferably 0.01 to 0.3 wt %, and more preferably 0.01 to 0.15 wt %. Using an amount of silane coupling agent within this range can suppress detachment of the filler. Other additives can also be added to the mixture as optional components. Examples of additives include one or more of solid lubricants, oxidation stabilizers, heat stabilizers, wear-resistant materials, weather stabilizers, flame retardants, and pigments. These additives may be added within a range that does not affect the physical properties of the granulated particles (e.g., 5 wt % or less).

[0061] The method for mixing a filler, polytetrafluoroethylene (PTFE) powder, and a polyhydrolyzable silane coupling agent can be produced by mixing the PTFE powder and the filler to form a mixed powder and then treating the mixed powder with a solution containing the silane coupling agent. Alternatively, the filler may be mixed with a solution containing the silane coupling agent to perform surface treatment, and then mixed with the PTFE powder. Alternatively, the PTFE powder, the filler, and the silane coupling agent solution may be simultaneously mixed to produce the composite.

[0062] The granulated powder of the present invention can be produced from the above mixture by any of various known granulation methods, but is preferably produced by underwater granulation, which involves adding a mixture containing a filler, polytetrafluoroethylene powder, and a polyhydrolyzable silane coupling agent to a mixture of a water-insoluble organic liquid and water, followed by stirring.

[0063] Alternatively, the mixture may be mixed with a water-insoluble organic liquid to obtain a slurry, which may then be granulated by stirring the slurry at high speed with a large amount of water. The granulated powder may then be removed from the water and dried to obtain a granulated powder. A surfactant may also be added to the water during high-speed stirring to produce the granulated powder.

[0064] The water-insoluble organic liquid used in the present invention can be any organic solvent that has low surface energy and is incompatible with water, and examples thereof include hydrocarbons such as hexane, kerosene, cyclohexane, benzene, toluene, and xylene, ethers such as diethyl ether and dipropyl ether, halogenated hydrocarbons such as methylene chloride, dichloroethylene, trichloroethylene, tetrachloroethylene, chlorobenzene, dichlorobenzene, fluorotrichloromethane, fluorodichloroethane, dichlorotrifluoroethane, trichlorotrifluoroethane, difluorotetrachloroethane, dichloropentafluoropropane, and decafluoropentane, and fluoroethers such as perfluoropropyl methyl ether, perfluorobutyl methyl ether, perfluorobutyl ethyl ether, perfluoropentyl methyl ether, and perfluoropentyl ethyl ether. Two or more of these can also be used in combination.

[0065] The average particle size of the granulated powder of the present invention is preferably 200 μm to 800 μm. If the particle size is too large, voids will remain when the powder is compression molded, deteriorating the tensile and compressive properties. If the particle size is too small, the filler particles that have fallen off the granulated powder particles will aggregate, easily causing defects. [Example]

[0066] The present invention will be specifically explained below with reference to examples. The raw materials used in the examples and comparative examples and the methods for evaluating the resulting PTFE molding powders are as follows.

[0067] 1.Raw materials (a) PTFE raw material powder: Teflon (registered trademark) PTFE 7-J (homo-PTFE, average particle size: 50 μm) manufactured by Mitsui-Chemours Fluoro Products Co., Ltd. (b) Filler: Potassium titanate particle powder: Terraces (registered trademark) JP (manufactured by Otsuka Chemical Co., Ltd.) (average particle size 4 to 12 μm) Coke powder (Oriental Industries Co., Ltd., product name At-No. 5C) (average particle size 50μm) Bronze powder (average particle size 15 μm) Solvent for mixing silane coupling agent: 1-chloro-2,3,3-trifluoropropene (AMOLEA® AS-300, manufactured by AGC Corporation) (c) Silane coupling agent:

[0068] [Table 1] (d) Water-insoluble organic liquid: tetrachloroethylene

[0069] 2. Physical property evaluation method (a) Transmittance measurement Turbidity: The granulation wastewater was diluted 10 times and the turbidity was measured using a HACH portable turbidity meter, Model 2100P. When resin and filler are mixed and granulated using the underwater granulation method, the filler that does not bond with the resin and separates will migrate from the water-insoluble organic liquid into the aqueous phase. Therefore, by measuring the turbidity of the granulation wastewater, the degree of detachment due to migration of the filler into the aqueous phase can be indirectly evaluated. (b) Average particle size: Measured using RPS-02 model manufactured by Seishin Enterprise Co., Ltd.

[0070] Eight standard sieves, each measuring 14, 16, 20, 28, 35, 48, 70, and 100 mesh in order from top to bottom, were stacked and the weight of the powder remaining on each sieve was determined. Based on these weights, the particle sizes of 16%, 50%, and 84% by weight were determined on a logarithmic probability chart, which was used as a criterion for determining the width of the particle size distribution.

[0071] Example 1 A 3.5 kg composition consisting of 85 wt% (89.9 vol%) PTFE molding powder (Teflon® 7-J, melting point 327°C) and 15 wt% (10.1 vol%) potassium titanate (Terases® JP, Otsuka Chemical Co., Ltd.) was added to a Henschel mixer and mixed for 5 minutes. Next, a solution of 3.5 g of bis[3-(trimethoxysilyl)propyl]amine (bissilane compound A) dissolved in 20 g of 1-chloro-2,3,3-trifluoropropene (AMOLEA® AS-300, AGC Corporation) as a silane coupling agent was added and stirred for 15 minutes. After stirring for 40 minutes, 20 g of purified water was added and stirred to obtain a homogeneous mixture.

[0072] 20 L of water was added to a 50 L sealed vessel equipped with a stirrer and kept at 70°C. The mixture obtained above was added, and 50% by weight of tetrachloroethylene was added relative to 100% by weight of the mixture. The mixture was then stirred at 630 rpm for 10 minutes, then the rotation speed was reduced to 236 rpm, and the mixture was stirred for an additional 40 minutes and sized. The water was separated using a 150 mesh sieve, and the mixture was dried at 170°C for 5 hours to obtain a granulated powder.

[0073] Example 2 A granulated powder was obtained in the same manner as in Example 1, except that 1,8-bis(trimethoxysilyl)octane (bissilane compound B) was used as the silane coupling agent instead of bis[3-(trimethoxysilyl)propyl]amine (bissilane compound A).

[0074] Example 3 A granulated powder was obtained in the same manner as in Example 1, except that trimethoxysilylpropyl-modified polyethyleneimine (polymeric silane compound C) was used instead of bissilane A as the silane coupling agent.

[0075] (Comparative Example 1) A granulated powder was obtained in the same manner as in Example 1, except that 3-glycidoxypropyltrimethoxysilane (silane compound D) was used instead of bissilane A as the silane coupling agent.

[0076] (Comparative Example 2) A granulated powder was obtained in the same manner as in Example 1, except that 3-methacryloxypropyltrimethoxysilane (silane compound E) was used instead of bissilane A as the silane coupling agent.

[0077] (Comparative Example 3) A granulated powder was obtained in the same manner as in Example 1, except that 3-acryloxypropyltrimethoxysilane (silane compound F) was used instead of bissilane A as the silane coupling agent.

[0078] The compositions and results of Examples 1 to 3 and Comparative Examples 1 to 3 are summarized in Table 2 below. In Comparative Examples 1 to 3, in which silane compounds D, E, and F were used, the average turbidity of the granulation wastewater was 150 or more. On the other hand, it decreased to 110.3 in Example 1, in which bissilane A was used, 62.7 in Example 2, in which bissilane B was used, and 15.9 in Example 3, in which polymeric silane C was used. From these results, it was confirmed that the use of a bissilane coupling agent and a polymeric silane coupling agent suppresses the detachment of the filler into the aqueous phase.

[0079] [Table 2]

[0080] Example 4 A 3.5 kg composition consisting of 90 wt% (93.4 vol%) PTFE molding powder (Teflon® 7-J, melting point 327°C) and 10 wt% (6.6 vol%) potassium titanate (Terases® JP, manufactured by Otsuka Chemical Co., Ltd.) was added to a Henschel mixer and mixed for 5 minutes. Next, 20 g of pure water was added and stirred for 15 minutes. Then, a solution of 3.5 g of trimethoxysilylpropyl-modified polyethyleneimine (polymeric silane compound C) dissolved in 20 g of 1-chloro-2,3,3-trifluoropropene (AMOLEA® AS-300, manufactured by AGC Corporation) as a silane coupling agent was added and stirred for 40 minutes to obtain a uniform mixture.

[0081] 20 L of water was added to a 50 L sealed container equipped with a stirrer and kept at 70°C. The mixture obtained above was added, and 50% by weight of tetrachloroethylene was added relative to 100% by weight of the mixture. The mixture was then stirred at 630 rpm for 10 minutes, then the rotation speed was reduced to 236 rpm, and the mixture was stirred for an additional 40 minutes and sized. The water was separated using a 150 mesh sieve, and the mixture was dried at 170°C for 5 hours to obtain a granulated powder.

[0082] Example 5 A granulated powder was obtained in the same manner as in Example 4, except that 1,8-bis(trimethoxysilyl)octane (bissilane compound B) was used as the silane coupling agent instead of trimethoxysilylpropyl-modified polyethyleneimine (polymeric silane compound C).

[0083] Comparative Example 4 A granulated powder was obtained in the same manner as in Example 4, except that N-2-(aminoethyl)-3-aminopropyltrimethoxysilane (silane compound G) was used as the silane coupling agent instead of polymeric silane compound C.

[0084] (Comparative Example 5) A granulated powder was obtained in the same manner as in Example 4, except that 3-glycidoxypropyltrimethoxysilane (silane compound D) was used as the silane coupling agent instead of polymeric silane compound C.

[0085] (Comparative Example 6) A granulated powder was obtained in the same manner as in Example 4, except that 3-acryloxypropyltrimethoxysilane (silane compound F) was used as the silane coupling agent instead of polymeric silane compound C.

[0086] The compositions and results of Examples 4 to 5 and Comparative Examples 4 to 6 are summarized in Table 3 below. First, looking at the particle size distribution (d16, d50, and d84) of the granulated powder, Examples 4 and 5 and Comparative Examples 4 to 6 all produced powders with particle sizes within the specified distribution range. Meanwhile, comparing the average turbidity of the granulation wastewater, Comparative Examples 4 to 6, which used silane compounds G, D, and F, all had an average turbidity of 100 or more. Meanwhile, Example 4, which used polymeric silane compound C, had a turbidity of 9.6, and Example 5, which used bissilane B, had a turbidity of 16.1, demonstrating that detachment of the filler into the aqueous phase was significantly suppressed.

[0087] [Table 3]

[0088] Example 6 A 4.0 kg composition consisting of 75% by weight (75.3% by volume) of PTFE molding powder (Teflon® 7-J, melting point 327°C) and 25% by weight (24.7% by volume) of coke powder (Oriental Sangyo Co., Ltd., At-No. 5C) was added to a Henschel mixer and mixed for 5 minutes. Next, 20 g of pure water was added and stirred for 15 minutes. After stirring, a silane coupling agent solution containing 4.0 g of trimethoxysilylpropyl-modified polyethyleneimine (polymeric silane compound C) dissolved in 20 g of 1-chloro-2,3,3-trifluoropropene (AMOLEA® AS-300, manufactured by AGC Corporation) was added and stirred for 40 minutes to obtain a uniform mixture.

[0089] 20 L of water was added to a 50 L sealed container equipped with a stirrer and kept at 70°C. The mixture obtained above was added, and 50% by weight of tetrachloroethylene was added relative to 100% by weight of the mixture. The mixture was then stirred at 630 rpm for 10 minutes, then the rotation speed was reduced to 236 rpm, and the mixture was stirred for an additional 40 minutes and sized. The water was separated using a 150 mesh sieve, and the mixture was dried at 170°C for 5 hours to obtain a granulated powder.

[0090] (Comparative Example 7) A granulated powder was obtained in the same manner as in Example 6, except that N-2-(aminoethyl)-3-aminopropyltrimethoxysilane (silane compound G) was used as the silane coupling agent instead of trimethoxysilylpropyl-modified polyethyleneimine (polymeric silane compound C).

[0091] The compositions and results of Example 6 and Comparative Example 7 are shown in Table 4 below. Comparing the average turbidity of the granulation wastewater, when coke with a hydrophobic surface was used as the filler, the turbidity of Example 6 and Comparative Example 7 was low at 2.6 and 4.0, respectively. However, Example 6, which used polymeric silane C, achieved slightly improved desorption of the filler into the aqueous phase compared to Comparative Example 7, which used silane G. In addition to measuring the turbidity of the granulation wastewater, the ratio of fine granulated powder that passed through a 70-mesh sieve to the detached filler (coke) was simply measured for the produced granulated particles. The ratio was 39.2% for Example 6, which used polymeric silane C, and 50.3% for Comparative Example 7, which used silane G.

[0092] [Table 4]

[0093] Example 7 A 4.0 kg composition consisting of 60 wt% (85.7 vol%) PTFE molding powder (Teflon® 7-J, melting point 327°C) and 40 wt% (14.3 vol%) bronze powder was added to a Henschel mixer and mixed for 5 minutes. Next, a silane coupling agent solution consisting of 4.0 g of bis[3-(trimethoxysilyl)propyl]amine (bissilane compound A) dissolved in 20 g of 1,3-dichloro-1,1,2,2,3-pentafluoropropane (HCFC-225cb) was added and stirred for 15 minutes. After stirring for 40 minutes, 20 g of pure water was added and the mixture was stirred to obtain a uniform mixture.

[0094] 15 L of water was added to a 50 L sealed container equipped with a stirrer and kept at 70°C. The mixture obtained above was added, and 50% by weight of tetrachloroethylene was added to 100% by weight of the mixture. The mixture was then stirred at 565 rpm for 10 minutes, then the rotation speed was reduced to 236 rpm, and the mixture was stirred for an additional 40 minutes and sized. The water was separated using a 150 mesh sieve, and the mixture was dried at 170°C for 5 hours to obtain a granulated powder.

[0095] (Comparative Example 8) A granulated powder was obtained in the same manner as in Example 7, except that 3-acryloxypropyltrimethoxysilane (silane compound F) was used as the silane coupling agent instead of bis[3-(trimethoxysilyl)propyl]amine (bissilane compound A).

[0096] In Example 7 and Comparative Example 8, bronze powder was used as a filler. However, due to its high specific gravity, bronze quickly settled, making turbidity measurements impossible. Therefore, the granulation wastewater was evaluated visually (see Figures 1 and 2). In Comparative Example 8, in which Silane F was used, bronze powder settled across the entire bottom of the container (white plastic bucket) containing the granulation wastewater (Figure 2). On the other hand, in Example 7, in which Bissilane A was used, the bottom of the container (white plastic bucket) containing the granulation wastewater was visible (Figure 1) (the detached and settled bronze powder was concentrated in the center of the bottom). Comparison of Example 7 and Comparative Example 8 revealed that the use of Bissilane A instead of Silane F reduced the detachment of bronze powder into the aqueous phase.

[0097] Example 8 Granulated powder was obtained in the same manner as in Example 7, except that 0.03 wt. % of polymeric silane C was used as the silane coupling agent instead of 0.1 wt. % of bissilane A. Visual inspection of the granulation wastewater in Example 8, as in Example 7 and Comparative Example 8, revealed that the amount of settled bronze powder was less than in Example 7, confirming that the detachment of bronze powder due to migration to the aqueous phase was reduced not only in Comparative Example 8 but also in Example 7. These results confirm that polymeric silane C is highly effective in suppressing detachment of filler due to migration to the aqueous phase, even at lower concentrations than bissilanes.

[0098] Example 9 Granulated powder was obtained in the same manner as in Example 8, except that the amount of silane coupling agent (polymeric silane C) was changed to 0.02 wt%. The granulation wastewater in Example 9 was in the same state as in Example 8, and it was confirmed that the detachment of the bronze powder due to migration to the aqueous phase had decreased. From the results of Examples 8 and 9, it was confirmed that even if the content of the silane coupling agent was reduced, the detachment of the filler due to migration to the aqueous phase was suppressed. [Industrial Applicability]

[0099] The method for producing granulated powder of the present invention can reduce the detachment of filler from the granulated powder. Because the method for producing granulated powder of the present invention is an underwater granulation method, granulated powder can be produced efficiently with high productivity. The PTFE granulated powder of the present invention can be easily compressed to produce molded products, and can be used for various applications such as sliding members and sealing materials.

Claims

1. A method for producing polytetrafluoroethylene granulated powder, characterized in that a mixture containing a filler, polytetrafluoroethylene powder, and a polyhydrolyzable group-type silane coupling agent is stirred in a mixed liquid of water and a water-insoluble organic liquid to perform mixing and granulation.

2. 2. The method for producing a granulated powder according to claim 1, wherein the polyhydrolyzable group-type silane coupling agent is a bis-silane coupling agent represented by the following formula (1) or a polymeric silane coupling agent containing a repeating monomer unit represented by the following formula (2): X-R 1 -X (1) [X is a hydrolyzable silyl group (—Si—(OR 2 ) n (R 3 ) 3-n ) (n=1 to 3), and R 2 and R 3 are -CH 3 Or, -C 2 H 5 and R 1 is a straight-chain or branched-chain hydrocarbon having 2 to 12 carbon atoms, which may contain a substituent and in which hydrogen may be substituted with a halogen. 【Chemistry 1】 （２） [X is a hydrolyzable silyl group (—Si—(OR 2 ) n (R 3 ) 3-n ) (n=1 to 3), and R 2 and R 3 are -CH 3 Or, -C 2 H 5 and A is —CH— or N; R a , R b are each independently a hydrocarbon having 1 to 10 carbon atoms, which may contain a substituent and in which hydrogen may be substituted with a halogen, R c is a hydrocarbon having 1 to 5 carbon atoms in which hydrogen may be substituted with a halogen, p = 3 to 30.

3. 3. The method for producing a granulated powder according to claim 2, wherein the polyhydrolyzable group-type silane coupling agent is a bis-silane coupling agent represented by formula (1).

4. R 1 The method according to claim 3, wherein either does not contain a substituent or contains an imino group (-NH-) as a substituent.

5. 3. The method for producing a granulated powder according to claim 2, wherein the polyhydrolyzable group-type silane coupling agent is a polymer-type silane coupling agent containing a repeating monomer unit of formula (2).

6. R a and R b The method according to claim 5, wherein either does not contain a substituent or contains an imino group (-NH-) as a substituent.

7. 3 to 40 volume percent of a filler; 60 to 97% by volume of polytetrafluoroethylene powder, A polytetrafluoroethylene granulated powder containing 1% by weight or less of a polyhydrolyzable group-type silane coupling agent relative to the total amount of the granulated powder, The granulated powder, wherein the polyhydrolyzable group-type silane coupling agent is a bis-silane coupling agent represented by formula (1) or a polymer-type silane coupling agent containing a repeating monomer unit represented by formula (2). X-R 1 -X (1) [X is a hydrolyzable silyl group (—Si—(OR 2 ) n (R 3 ) 3-n ) (n=1 to 3), R 2 and R 3 are each —CH 3 or —C 2 H 5 , R 1 is a straight-chain or branched-chain hydrocarbon having 2 to 12 carbon atoms, which may contain a substituent and in which hydrogen may be substituted with a halogen. 【Chemistry 2】 （２） [X is a hydrolyzable silyl group (—Si—(OR 2 ) n (R 3 ) 3-n ) (n=1 to 3), R 2 and R 3 are each —CH 3 or —C 2 H 5 , A is —CH— or N; R a and R b each independently represent a hydrocarbon having 1 to 10 carbon atoms, which may contain a substituent and in which hydrogen may be substituted with halogen; R c is a hydrocarbon having 1 to 5 carbon atoms in which hydrogen may be substituted with halogen; p = 3 to 30.

8. 8. The polytetrafluoroethylene granulated powder according to claim 7, wherein the polyhydrolyzable group-type silane coupling agent is a bis-silane coupling agent represented by formula (1):

9. 8. The polytetrafluoroethylene granulated powder according to claim 7, wherein the polyhydrolyzable group-type silane coupling agent is a polymer-type silane coupling agent containing a repeating monomer unit of formula (2).

10. The granulated powder according to any one of claims 7 to 9, having an average particle size of 200 to 800 µm.

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