JP5003633B2 - Method for forming fluororesin coating film - Google Patents

Description

  The present invention relates to a method for forming a fluororesin coating film.

Aqueous fluororesin dispersions containing fluororesin fine particles are widely used in applications that take advantage of the characteristics of fluororesins such as heat resistance, chemical resistance, non-adhesiveness, self-lubricity, weather resistance, and water repellency.
Generally, an aqueous fluororesin dispersion is produced by an emulsion polymerization method. For example, in the case of polytetrafluoroethylene (hereinafter referred to as PTFE), an aqueous dispersion is produced by the following method.
Tetrafluoroethylene (hereinafter referred to as TFE) is polymerized alone while stirring a mixture of pure water, a peroxide polymerization initiator, an anionic fluorosurfactant, and a paraffin wax as a polymerization stabilizer. Alternatively, a PTFE aqueous emulsion in which PTFE fine particles having an average particle diameter of 0.1 to 0.5 μm are dispersed in the mixture is obtained by copolymerization with a small amount of a comonomer (see Non-Patent Document 1).

Since the PTFE aqueous emulsion easily aggregates and is unstable, conventionally, the PTFE aqueous emulsion is added to C ₈ H ₁₇ C ₆ H ₄ O (C ₂ H ₄ O) ₁₀ H (Triton X-100 manufactured by Dow Chemical Company). ) And other polyoxyethylene alkylphenyl ether type nonionic surfactants are added to stabilize the product. Then, the stabilized PTFE aqueous emulsion is concentrated using a method such as electrophoresis or phase separation to obtain a high concentration PTFE aqueous dispersion, and further to the high concentration PTFE aqueous dispersion as necessary. Water, ammonia, a surfactant and other components are added to obtain an aqueous PTFE dispersion.
The PTFE aqueous dispersion obtained above is applied to a heat-resistant substrate, and after a PTFE fine particle coating layer is formed on the heat-resistant substrate, the PTFE fine particle coating layer together with the heat-resistant substrate exceeds the melting point of PTFE. By heating, the PTFE fine particles are fired, fused, and used in the form of a heat-resistant substrate with a PTFE coating. As a specific example of such a heat-resistant substrate having a PTFE coating film, a heat-resistant base material having a PTFE coating film formed on a non-adhesive electric kettle, pan, glass fiber cloth, etc. having a fluororesin coating film formed on a metal plate A belt etc. are mentioned.

However, the method for forming a fluororesin coating film in the conventional example has the following problems.
When the coating layer is formed by coating the fluororesin aqueous dispersion with a film thickness of a certain level or more, cracks and pinholes are generated in the coated film after baking, and the coating film performance and durability are impaired. As a result, the quality of the coating film is degraded. The thickness at which cracks start to occur is called the crack limit film thickness (Critical Film Thickness, hereinafter referred to as CFT). In the case of PTFE, the CFT is generally 10 to 12 μm. For this reason, the PTFE aqueous dispersion is usually applied onto the heat-resistant substrate with a coating thickness such that the coating thickness is CFT or less. However, depending on the application, a considerably thick film may be required to ensure sufficient durability of the fluororesin coating film. For example, a fluororesin coating film having a thickness of 100 μm may be required, but in order to obtain such a film thickness, it has been necessary to repeat application and baking several times or more until a predetermined thickness is obtained. For this reason, there has been a problem that the number of coatings is remarkably increased and the processing cost is increased.

  As a method for addressing the above problems, it has been proposed to increase the surfactant content in the PTFE aqueous dispersion and increase the CFT (Patent Document 1, Patent Document 2), but a significant improvement effect is obtained. It is not done. On the other hand, there are problems that the cost for increasing the amount of the surfactant increases, the viscosity of the aqueous dispersion increases, and the odor accompanying the generation of pyrolytic gas of the surfactant increases.

As another method, there is an attempt to increase CFT by blending PTFE fine particles having different particle size distributions (Patent Document 3), but a great improvement effect has not been obtained.
JP 2001-89624 A JP 2006-117900 A JP 2001-64466 A Fluoropolymer Handbook, 28 pages, edited by Takaomi Satokawa, published by Nikkan Kogyo Shimbun, 1990

  The present invention has been made in view of the above-described conventional problems, and even if it is applied thickly, the quality of the coating film does not deteriorate, and the number of overcoating can be reduced. It aims at providing the formation method of the fluororesin coating film which can obtain the fluororesin coating film excellent in durability.

  As a result of repeated research to overcome the above-mentioned problems, the present inventor uses a specific fluororesin aqueous dispersion containing a nonionic surfactant and heat-treats it at a specific temperature after application to a heat-resistant substrate. After reducing the amount of nonionic surfactant in the coating layer, pressurize the coating layer using a pressurizing means and calcinate it to form a fluororesin coating film free of cracks and pinholes beyond a certain level. The present invention has been completed by finding that it is possible to form the film with a large thickness, and that all of the above-mentioned problems can be solved.

That is, this invention provides the formation method of the fluororesin coating film which has the following structures.
[1] Fluororesin fine particles having an average particle diameter of 0.1 to 0.5 μm and a melting point exceeding 200 ° C. are 20 to 70% by mass with respect to the total amount of the dispersion, and the nonionic surfactant is the fluorine. A coating layer in which a fluororesin aqueous dispersion containing 2 to 12% by mass with respect to the total mass of the resin fine particles is applied to a heat resistant substrate, and a coating layer of the fluororesin aqueous dispersion is formed on the heat resistant substrate. The coating layer after the forming step and the coating layer forming step is heat-treated at a temperature of 200 ° C. to less than the melting point of the fluororesin, and the content of the nonionic surfactant in the coating layer is determined by the fluororesin fine particles. A heating step for reducing the total mass of the coating layer to less than 2% by mass, a pressurizing step for pressurizing the coating layer after the heating step with a pressure of 0.1 to 100 MPa using a pressurizing means, The coating layer after the pressing step is melted with the fluororesin. Heating at a temperature of ˜420 ° C., firing the fluororesin fine particles in the coating layer, and forming a coating film of the fluororesin on the heat-resistant substrate. Method for forming a film.

[2] The method for forming a fluororesin coating film according to the above [1], wherein the nonionic surfactant is a compound represented by the following formula (1).
R ¹ —O—A—H (1)
(Wherein R ¹ is a saturated alkyl group having 6 to 18 carbon atoms and 10% or less of hydrogen atoms may be substituted with a halogen atom. O is an oxygen atom. A is 5 to 20) And a polyoxyalkylene chain composed of 0 to 3 oxyethylene groups, 0 to 3 oxypropylene groups, and 0 to 3 oxybutylene groups.)
[3] The method for forming a fluororesin coating film according to the above [1] or [2], wherein the fluororesin is polytetrafluoroethylene or a tetrafluoroethylene copolymer.

[4] The viscosity of the aqueous fluororesin dispersion is in the range of 1 to 1000 mPa · s when measured with a Brookfield viscometer using a # 1 spindle at a liquid temperature of 23 ° C. and 60 rpm. The method for forming a fluororesin coating film according to any one of [1] to [3].
[5] The method for forming a fluororesin coating film according to any one of the above [1] to [4], wherein the coating film has a thickness of 1 to 1000 μm.

According to the method for forming a fluororesin coating film of the present invention, it is possible to suppress the occurrence of cracks and pinholes even if the fluororesin aqueous dispersion is applied thickly and baked, and the desired thickness can be reduced with a small number of applications. The fluororesin coating film can be formed, and there is an economic advantage that the processing cost is reduced.
Moreover, the pressurization process which the formation method of the fluororesin coating film of this invention has contributed to the smoothness of the surface of a coating film, and the improvement of an external appearance. Furthermore, according to the method for forming a fluororesin coating film of the present invention, the frequency at which fluororesin agglomerates adhere to form a foreign defect is reduced during application, and the product yield can be improved.
Furthermore, in the method for forming a fluororesin coating film according to the present invention, the nonionic surfactant present in the fluororesin aqueous dispersion coating layer is thermally decomposed and removed, and then the pressure treatment is performed. It is considered that the void volume between the fluororesin fine particles in the inside can be reduced, and it can contribute to an increase in the fusion rate of the fluororesin during firing.

Embodiments of the present invention will be described below.
The method for forming a fluororesin coating film of the present invention includes a coating layer forming step of forming a coating layer of a specific fluororesin aqueous dispersion on a heat-resistant substrate, a heating step of heating the coating layer under specific conditions, It has in order a pressurization process which pressurizes the application layer after heating on specific conditions, and a baking process which bakes the application layer after pressurization on specific conditions.
First, the said fluororesin aqueous dispersion used for this invention is demonstrated below.
The aqueous fluororesin dispersion used in the present invention has an average particle size of 0.1 to 0.5 μm, and a fluororesin fine particle having a melting point of over 200 ° C. The ionic surfactant is contained in an amount of 2 to 12% by mass with respect to the total mass of the fluororesin fine particles.

As described above, the average particle size of the fluororesin fine particles contained in the aqueous fluororesin dispersion is 0.1 to 0.50 μm. In the present invention, the average particle size is 0.15 to 0.40 μm. It is preferably 0.20 to 0.35 μm. When the average particle size is less than 0.1 μm, the molecular weight of the fluororesin is low, and the mechanical properties of the obtained fluororesin product are lowered. When the average particle size is greater than 0.50 μm, the fluororesin fine particles are rapidly precipitated and contained. The storage stability of the aqueous dispersion is poor.
In addition, the average particle diameter of the fluororesin fine particles used in this specification is obtained by photographing fluororesin fine particles at a magnification of 10,000 using a scanning electron microscope, and measuring the length of the major axis and the minor axis of 100 fine particles. The average value of 100 particles was obtained by setting the value obtained by dividing the total length of the long axis and short axis of each fine particle by 2 as the particle diameter of the fine particle.

  The fluororesin constituting the fluororesin fine particles used in the present invention is a fluororesin having a melting point exceeding 200 ° C. Specific examples of such fluororesins include PTFE, TFE / perfluoro (alkyl vinyl ether) (hereinafter referred to as PAVE) copolymer (hereinafter referred to as PFA), and TFE / hexafluoropropylene (hereinafter referred to as HFP). Polymer (hereinafter referred to as FEP), polychlorotrifluoroethylene (hereinafter referred to as chlorotrifluoroethylene as CTFE, polychlorotrifluoroethylene as PCTFE), ethylene / TFE copolymer (hereinafter referred to as ETFE), ethylene / CTFE And a copolymer (hereinafter referred to as ECTFE), a TFE / vinylidene fluoride (hereinafter referred to as VdF) copolymer, and the like.

Examples of the PAVE include perfluoro (methyl vinyl ether), perfluoro (ethyl vinyl ether), perfluoro (propyl vinyl ether) and the like. PAVE may include one or more of these.
Among such fluororesins, PTFE or TFE copolymer is preferable and PTFE is more preferable as the fluororesin used in the method for forming a fluororesin coating film of the present invention. Specific examples of the TFE copolymer include the PFA, FEP, ETFE, TFE / VdF copolymer, and the like.
The PTFE includes not only a TFE homopolymer but also a very small amount of halogenated ethylene such as CTFE, a halogenated propylene such as HFP, fluorovinyl ether such as PAVE, or the like that does not substantially impart melt processability. Based on copolymerizable components that can be copolymerized with TFE, such as fluoro (butenyl vinyl ether), perfluoro (2,2-dimethyl-1,3-dioxole), perfluoro (4-methyl-1,3-dioxole) Also included are so-called modified PTFE containing polymerized units.

  The fluororesin constituting the fluororesin fine particles used in the present invention can be arbitrarily selected with respect to the average molecular weight as long as the melting point and the average particle size in the state of fine particles satisfy the above conditions. In the case of PTFE preferably used in the present invention, the number average molecular weight is preferably in the range of 500,000 to 30 million, more preferably in the range of 1 million to 25 million. If the number average molecular weight is less than 500,000, the mechanical properties of PTFE may be lowered, and PTFE having a molecular weight greater than 30 million is difficult to produce industrially. The number average molecular weight of PTFE used in the present specification is the number average molecular weight obtained from the latent heat amount in differential thermal analysis by the method of Suwa (described in J. Appl. Polym. Sci, 17, 3253 (1973)). Say.

The fluororesin fine particles contained in the aqueous fluororesin dispersion in the present invention have a two-layer structure of an inner layer and an outer layer, and the two layers are composed of fluororesins having different monomer compositions and / or average molecular weights. There may be. Moreover, it has a multilayer structure of two or more layers, and each layer may be composed of a fluororesin having a different monomer composition and / or a different average molecular weight, and the fluororesin constituting these layers is a monomer composition. Alternatively, the average molecular weight may be continuously changed.
For example, the polymerization method is described below. During the polymerization of TFE, the addition pattern of the polymerization initiator is changed to change the outer layer from the inner layer to high molecular weight PTFE or low molecular weight PTFE. Copolymerization is performed later in the polymerization of TFE. Monomers (CTFE, HFP, etc.) are injected, the inner layer is PTFE, the outer layer is a TFE copolymer (TFE / CTFE copolymer, TEF / HFP copolymer), and only in the initial stage of TFE polymerization. Polymerization monomers (CTFE, HFP, etc.) are injected, and then TFE is polymerized, and the inner layer is made of TFE / CTFE copolymer, TFE / HFP copolymer, etc., and the outer layer is made of PTFE. It can be used in the invention.

The fluororesin fine particles having an average particle diameter of 0.1 to 0.5 μm and a melting point exceeding 200 ° C. used in the present invention are obtained by, for example, an emulsion polymerization method which is a general method for producing fluororesin fine particles. It can be produced by polymerizing, as a raw material monomer, a constituent monomer of a fluororesin whose melting point exemplified above exceeds 200 ° C.
In the emulsion polymerization method, a fluorine resin aqueous solution is obtained by homopolymerizing a fluorine-containing monomer having a vinyl group in an aqueous medium or copolymerizing the fluorine-containing monomer and another raw material monomer (which may or may not be fluorine-containing). This is a polymerization method for obtaining an emulsion. In general, emulsion polymerization of a fluorinated monomer is carried out by introducing a raw material monomer into this mixture while stirring a mixture of water, a polymerization initiator, a surfactant, etc. It is carried out by copolymerizing with raw material monomers. For example, as a suitable emulsion polymerization method for TFE, in a pressure-resistant autoclave, while stirring a mixture of water, a polymerization initiator such as a polymerization initiator, an anionic fluorosurfactant, and paraffin wax, TFE is pressurized. The method of superposing | polymerizing by inject | pouring is mentioned.

Examples of the polymerization initiator used in the emulsion polymerization method include persulfates such as ammonium persulfate and potassium persulfate, water-soluble organic peroxides such as disuccinic acid peroxide, diglutaric acid peroxide, and tert-butyl hydroperoxide, and chlorates. One or more of oxidation-reduction polymerization initiators, or the like, which are combinations of bromates, permanganates and reducing agents can be used. Specifically, the amount of the polymerization initiator used for the emulsion polymerization may be 0.001 to 1% by mass with respect to the mass of the finally produced fluororesin.
Examples of the polymerization stabilizer include paraffin wax.

Specific examples of the anionic fluorine-based surfactant used in the emulsion polymerization method include an anionic fluorine-based surfactant represented by the following formula (2).
R ² -COOX (2)
(In the formula, R ² is a polyfluoroalkyl group in which 90 to 100% of the hydrogen atoms in the alkyl group having 4 to 9 carbon atoms which may contain 1 to 2 etheric oxygen atoms are substituted with fluorine atoms) Yes, O is an oxygen atom, and X is an ammonium ion or a hydrogen ion.)
Specific examples of the anionic fluorine-based surfactant represented by the formula (2) include C ₇ F ₁₅ COONH ₄ , HC ₇ F ₁₄ COONH ₄ , C ₆ F ₁₃ COONH ₄ , HC ₆ F ₁₂ COONH ₄ , C _{5 F 11 COONH 4, HC 5} F 10 COONH 4, C 4 F 9 COONH 4, C 8 F 17 COONH 4, C 4 F 9 OC 2 F 4 OCF 2 COONH 4, C 2 F 5 OC 2 F 4 OCF 2 _{COONH 4, C 2 F 5 OC} 2 F 4 OCF 2 COOH, C 3 F 7 OCF (CF 3) CF 2 OCF (CF 3) COONH 4, C 2 F 5 OCF (CF 3) CF 2 OCF (CF 3) _{COONH 4, C 4 F 9 OCF} (CF 3) COONH 4 , and the like. Among these anionic fluorosurfactants, C ₇ F ₁₅ COONH ₄ (ammonium perfluorooctanoate), C ₂ F ₅ OC ₂ F ₄ OCF ₂ COONH _{4 and the} like are used from the viewpoint of stability of the emulsion polymerization process. Are preferably used.

  The amount of the anionic fluorosurfactant used is preferably 0.05 to 1.0% by mass, more preferably fluorine based on the total mass of the fluororesin fine particles finally obtained during the polymerization of the fluorine-containing monomer. It is 0.1-0.5 mass% with respect to the total mass of resin fine particles, Most preferably, it is 0.15-0.3 mass%. When the amount of the surfactant used is less than 0.05% by mass, the fluororesin fine particles are likely to aggregate, and when it is more than 1.0% by mass, the fluororesin is hardly obtained as fine particles. The anionic fluorosurfactant may be used after being dissolved in water before the start of the polymerization reaction, or may be injected as an aqueous solution into the autoclave during the polymerization.

The polymerization temperature for producing the fluororesin fine particles used in the present invention by emulsion polymerization is not particularly limited, but is preferably from 30 to 100 ° C, particularly preferably from 50 to 90 ° C.
As described above, fluororesin fine particles are obtained by emulsion polymerization as a fluororesin aqueous emulsion in a state of being dispersed in a mixture of various substances used in the polymerization. In order to adjust the average particle size of the fluororesin fine particles to 0.1 to 0.5 μm used in the present invention, although it depends on the type of raw material monomer, the amount of anionic fluorosurfactant used and the adjustment of the polymerization time are adjusted. The polymerization conditions such as the above may be adjusted as appropriate.
The fluororesin aqueous emulsion thus obtained is used as it is for the preparation of an aqueous fluororesin dispersion. However, the content of the fluororesin fine particles in the aqueous fluororesin emulsion is lower than the content of the fluororesin fine particles required for the aqueous fluororesin dispersion used in the present invention (20 to 70 mass% with respect to the total amount of the dispersion). In this case, the fluororesin aqueous emulsion is preferably concentrated by an appropriate method so that the content of the fluororesin fine particles becomes a content that can be used for preparing the fluororesin aqueous dispersion.

  In addition, an anionic fluorosurfactant represented by the anionic fluorosurfactant represented by the formula (2) is generally difficult to be decomposed in nature. Therefore, the fluororesin aqueous dispersion used in the present invention It is desirable to keep the content inside low. As a method for keeping the content of the anionic fluorosurfactant low, emulsion polymerization is carried out with as little use amount as possible, and the anionic fluorosurfactant is removed from the supernatant after concentration after emulsion polymerization. Known methods such as a method (international publication WO 03/078379 pamphlet), a method of adsorbing with an anion exchange resin (international publication WO 00/35971 pamphlet), and a method of removing by ultrafiltration (Japanese Patent Laid-Open No. 55-120630) Is mentioned.

  The fluororesin aqueous dispersion used in the present invention is obtained by the fluororesin fine particles having an average particle diameter of 0.1 to 0.5 μm and a melting point of more than 200 ° C. in the total amount of the dispersion. On the other hand, it is contained in an amount of 20 to 70% by mass. The content of the fluororesin fine particles is preferably 35 to 67% by mass, and more preferably 50 to 65% by mass. If the content of the fluororesin fine particles is lower than 20% by mass, the viscosity of the fluororesin aqueous dispersion is low and the storage stability is not sufficient. Moreover, manufacture is not easy when fluororesin content is higher than 70 mass%.

  The aqueous fluororesin dispersion used in the present invention further contains 2 to 12% by mass of a nonionic surfactant with respect to the total mass of the fluororesin fine particles. By containing a nonionic surfactant, the fluororesin aqueous dispersion can maintain a stable dispersion of fluororesin fine particles. As the nonionic surfactant, specifically, polyoxyethylene alkyl ether, polyoxyethylene fatty acid ester, sorbitan fatty acid ester, alkyl polyglucoside, fatty acid diethanolamide, alkyl monoglyceryl ether, polyoxyethylene alkyl phenyl ether, Fatty acid monoglyceride etc. are mentioned. In the present invention, one of these can be used alone, or two or more can be used as a mixture. In the present invention, it is preferable to use a nonionic surfactant having a thermal decomposition temperature lower than the melting point of the fluororesin contained in the fluororesin aqueous dispersion for the following reasons.

  In the fluororesin coating film forming method of the present invention to be described later, the concentration of the nonionic surfactant from the coating layer of the fluororesin aqueous dispersion is less than 2% by mass with respect to the total mass of the fluororesin fine particles before the pressure treatment. However, if the thermal decomposition temperature of the nonionic surfactant is higher than the melting point of the fluororesin, the fluororesin will exceed the melting point in order to thermally decompose the nonionic surfactant. It will be necessary to heat. When the fluororesin fine particles are fused by this heat treatment, the coating layer becomes a relatively hard resin layer, and the effect of the subsequent pressure treatment, that is, by pressurizing the coating layer containing the fluororesin in a relatively soft state. In general, it is difficult to obtain an effect of erasing defects such as cracks already generated in the previous stage. In addition, even if the surface of the coating layer containing the nonionic surfactant is not thermally decomposed and more than the above amount is dried and moisture is removed, the pressure is released in the pressurizing process. The loss of the fluororesin-containing coating layer occurs, and the uniformity of the finally obtained coating film is impaired.

Among such nonionic surfactants, a nonionic surfactant represented by the following formula (1) can be preferably used in the present invention.
R ¹ —O—A—H (1)
(Wherein R ¹ is a saturated alkyl group having 6 to 18 carbon atoms and 10% or less of hydrogen atoms may be substituted with a halogen atom. O is an oxygen atom. A is 5 to 20) And a polyoxyalkylene chain composed of 0 to 3 oxyethylene groups, 0 to 3 oxypropylene groups, and 0 to 3 oxybutylene groups.)

In the formula (1), the alkyl group represented by R ¹ is a saturated alkyl group having no double bond or benzene ring in the structure. The alkyl group represented by R ¹ may be ^{one in} which 10% or less of the hydrogen atoms in the alkyl group are replaced with halogen atoms such as fluorine, chlorine, bromine and the like. The number of carbon atoms of an alkyl group (hereinafter simply referred to as “alkyl group”) in which 10% or less of hydrogen atoms may be substituted with a halogen atom is suitable for the present invention, preferably 8 to 16, more preferably 10-14. When the carbon number of the alkyl group is less than 6, the surface tension of the fluororesin aqueous dispersion may increase and the wettability may decrease. Conversely, when the alkyl group has more than 18 carbon atoms, the dispersion may be left standing. The storage stability of the fluororesin aqueous dispersion may be impaired. When the carbon number of the alkyl group is within the above range, the wettability is good and the storage stability is also good.

When the alkyl group represented by R ¹ has a branched structure, wettability is better and a suitable aqueous fluororesin dispersion is obtained, which is preferable. The branched carbon atom may be a secondary carbon atom or a tertiary carbon atom, preferably a secondary carbon atom. Preferable specific examples of the alkyl group having a branched _{structure, C 10 H 21 CH (CH} 3) CH 2 -, C 9 H 19 CH (C 3 H 7) -, C 6 H 13 CH (C 6 H 13 _{) -, CH 3 CH (CH} 3) CH 2 CH (CH 2 CH (CH 3) CH 2 CH (CH 3) 2) - , and the like.

A in Formula (1) is a polyoxyalkylene chain composed of 5 to 20 oxyethylene groups, 0 to 3 oxypropylene groups, and 0 to 3 oxybutylene groups. The number of oxyethylene groups is preferably 6 to 15, particularly preferably 7 to 12. The number of oxypropylene groups is preferably 0 to 2, particularly preferably 0 to 1.5. The number of oxybutylene groups is preferably 0 to 2, particularly preferably 0 to 1.5. When the number of oxyethylene groups is 20 and the number of oxypropylene groups and oxypropylene groups is more than 3, respectively, the viscosity of the fluororesin aqueous dispersion may be lowered or the stability may be lowered, and the number of oxyethylene groups is less than 5. And the fluororesin aqueous dispersion composition may have poor defoaming properties, wettability, and viscosity characteristics. When the number of each group is within the above range, properties such as viscosity, stability, defoaming property and wettability are good, which is preferable.
The oxypropylene group or oxybutylene group may be branched or linear, but is preferably branched.
In general, a nonionic surfactant is a mixture of a plurality of molecules in which a certain chain length distribution and isomers are mixed, and the chain length in the formula (1) is an average chain length in a plurality of molecules. Represent. Each numerical value is not limited to an integer.

The nonionic surfactant represented by the formula (1) preferably has a number average molecular weight of 450 to 800, more preferably 500 to 750, and particularly preferably 550 to 700. If the number average molecular weight is greater than 800, the nonionic surfactant may be difficult to handle due to low fluidity. If it is less than 450, the permeability and wettability of the aqueous fluororesin dispersion will be low. This is not preferable.
The number average molecular weight of the nonionic surfactant depends on the number of moles of raw materials charged during the synthesis of the surfactant, but can be measured by GPC (permeation chromatography) method or ultracentrifugation after preparing an aqueous solution. it can.

Specifically, as the nonionic surfactant represented by the formula (1),
_{C 13 H 27 - (OC 2} H 4) 10 -OH,
_{C 13 H 27 - (OC 2} H 4) 9 -OH,
_{C 13 H 27 - (OC 2} H 4) 8 -OC 3 H 6 -OH,
_{C 12-14 H 25-29 - (OC 2} H 4) 9 -OH,
_{C 12 H 25 - (OC 2} H 4) 10 -OH,
_{C 10 H 21 CH (CH 3} ) CH 2 - (OC 2 H 4) 9 -OH,
_{C 10 H 21 CH (CH 3} ) CH 2 - (OC 2 H 4) 8 -OC 3 H 6 -OH,
_{C 10 H 21 CH (CH 3} ) CH 2 - (OC 2 H 4) 8 -OCH (CH 3) CH 2 -OH,
C ₁₆ H _33- (OC ₂ H ₄ ) ₁₀ -OH,
_{HC (C 5 H 11) (} C 7 H 15) - (OC 2 H 4) 9 -OH,
_{C 13 H 27 -OCH (C 2} H 5) CH 2 - (OC 2 H 4) 9 -OH,
_{C 13 H 27 -OCH (C 2} H 5) CH 2 - (OC 2 H 4) 8 -OH,
_{CH 3 CH (CH 3) CH} 2 CH (CH 2 CH (CH 3) CH 2 CH (CH 3) 2) - (OC 2 H 4) 9 -OH,
Etc. Examples of commercially available products include Taditol (registered trademark) 15S series manufactured by Dow Chemical Company, Lionol (registered trademark) TD series manufactured by Lion Corporation, and New Coal Series manufactured by Nippon Emulsifier Co., Ltd.

  The nonionic surfactant shown by Formula (1) can be used individually by 1 type or in mixture of 2 or more types.

  In the aqueous fluororesin dispersion used in the present invention, the content of the nonionic surfactant is 2 to 12% by mass as described above with respect to the total mass of the fluororesin fine particles contained in the aqueous fluororesin dispersion. However, Preferably it is 3-9 mass%, More preferably, it is 4-7 mass%. When the content of the nonionic surfactant with respect to the fluororesin fine particles is less than 2% by mass, the storage stability is lowered and the coating film is likely to be repelled. Moreover, when more than 12 mass%, it is not economical. Moreover, it is not easy to reduce content of a nonionic surfactant to less than 2 mass% by the heating process in the fluororesin coating film formation method of this invention mentioned later.

  The aqueous fluororesin dispersion in the present invention contains water as a dispersion medium for the fluororesin fine particles, but this water may be water contained in the aqueous fluororesin emulsion or the aqueous fluororesin emulsion. Water prepared separately from liquid water may be used. Specifically, the amount of water contained in the aqueous fluororesin dispersion is 30 to 80% by mass with respect to the total amount of the aqueous fluororesin dispersion.

The fluororesin aqueous dispersion contains essential fluororesin particles, nonionic surfactant, water and a pH adjuster such as ammonia as necessary, ammonium laurate, triethanolamine laurate, lauryl. One or more kinds of anionic surfactants such as sodium sulfate, ammonium lauryl sulfate, triethanolamine lauryl sulfate, thixotropic agent, silicone wettability improver, fluorine wettability improver, preservative and the like are contained in a trace amount as appropriate. It may be. The fluororesin aqueous dispersion used in the present invention further includes a water-soluble organic solvent, an organic solvent such as toluene and xylene, a pigment such as titanium oxide, iron oxide, carbon black, and cobalt blue, glass powder, hollow glass beads, One or more kinds of colorants such as graphite fine particles, silica fine particles, mica or titanium oxide-coated mica powder may be appropriately mixed in a trace amount.
Further, a polyethylene oxide thickener having an average molecular weight of 100,000 to 2,000,000 or a water-soluble polyurethane-based associative thickener as an optional component is added to the fluororesin aqueous dispersion in an amount of 0. By containing about 1 to 5.0% by mass, the viscosity increases and the coating becomes thick and easy, and the mechanical stability and storage stability of the aqueous fluororesin dispersion can be improved.

  The viscosity of the fluororesin aqueous dispersion used in the present invention is preferably 1 to 1000 mPa · s when measured with a Brookfield viscometer using a # 1 spindle at a liquid temperature of 23 ° C. and 60 rpm. 3 to 500 mPa · s is more preferable, and 5 to 200 mPa · s is particularly preferable. When the viscosity is less than 1 mPa · s, it tends to flow after application, and the coating thickness tends to be uneven. When it is greater than 1000 mPa · s, it may be difficult to handle in operation. In particular, when the fluororesin aqueous dispersion is applied thickly to a heat resistant substrate, a viscosity of 50 to 200 mPa · s is preferable. In addition, the said viscosity can be prepared by mix | blending the said thickener with a fluororesin aqueous dispersion suitably.

  The pH of the aqueous fluororesin dispersion in the present invention is preferably adjusted to 8.0 to 11.0, and more preferably 9.0 to 11.0. The pH can be adjusted by appropriately adding a pH adjusting agent such as ammonia or aqueous ammonia to the fluororesin aqueous dispersion. When the pH of the aqueous fluororesin dispersion is within this range, the viscosity is stable and the storage stability is excellent.

  The surface tension of the aqueous fluororesin dispersion in the present invention is preferably in the range of 24 to 32 mN / m, more preferably 25 to 31 mN / m, and particularly preferably 26 to 30 mN / m. If the surface tension is less than 24 mN / m, the defoaming property may be deteriorated, which is not preferable. If the surface tension is more than 32 mN / m, the film tends to cause repellency or flapping thickness unevenness when applied to the heat-resistant substrate surface. Become. If the surface tension of the fluororesin aqueous dispersion is too high, add a small amount of silicone surfactant (wetting improver) or fluorine surfactant (wetting improver) to reduce the surface tension. It can be used with reduced and less repellency.

Next, a method for forming a fluororesin coating film of the present invention using the above fluororesin aqueous dispersion will be described.
The method for forming a fluororesin coating film according to the present invention includes (1) a coating layer forming step of forming a coating layer of the fluororesin aqueous dispersion on a heat-resistant substrate, and (2) heating the coating layer under specific conditions. A heating step, (3) a pressing step of pressing the heated coating layer under specific conditions, and (4) a firing step of baking the pressurized coating layer under specific conditions. Hereinafter, each process is demonstrated in order.
Here, in the method for forming a fluororesin coating film of the present invention, in the coating layer forming step (1), the fluororesin aqueous dispersion constituting the coating layer formed on the heat-resistant substrate has been baked. Until then, the composition changes due to the heating step of (2), but the fluororesin itself does not change and it is consistent that it is a coating layer comprising this as a main constituent. In the step, the coating layer is generically referred to as a fluororesin coating layer. In addition, a fluororesin coating layer turns into a fluororesin coating film by finishing the baking process of (4).

(1) Coating layer forming step In the coating layer forming step in the present invention, the fluororesin aqueous dispersion is applied to a heat resistant substrate, and the coating layer of the fluororesin aqueous dispersion is formed on the heat resistant substrate. It is a process. In addition, application | coating means making a fluororesin aqueous dispersion adhere to a heat-resistant base material.

  The heat-resistant substrate in the present invention is not particularly limited as long as it is made of a material that can withstand the firing temperature of the fluororesin described later, and a substrate that does not melt or soften at the firing temperature of the fluororesin is preferable. The shape of the heat-resistant substrate is not particularly limited, and for example, a plate shape, a film shape, a roll shape, or a cloth shape can be used in the present invention. Specific examples of heat-resistant substrates include aluminum plates, iron plates, stainless steel plates and other metal plates, glass plates, ceramic plates and other inorganic material plates, polyimide films and other heat-resistant resin substrates, glass fiber cloths, carbon fiber cloths and aramids. Examples thereof include cloth-like substrates such as fiber cloth. In addition, when the surface of the heat-resistant base material to be used is smooth, when the surface is roughened by a known method such as sandblasting or etching, the fluororesin coating film finally formed on the heat-resistant base material Adhesion is improved, which is preferable.

  In the present invention, as a method of applying the fluororesin aqueous dispersion to the heat-resistant substrate, specifically, a spray coating method, a spin coating method, a dip pulling method (dipping method), a gravure coating method, a curtain coating method, Known methods such as a flow coating method, a screen printing method, and a stencil printing method may be used. The fluororesin aqueous dispersion may be applied to only one surface of the heat-resistant substrate, may be applied to both surfaces, or may be partially applied by a printing method or the like. Further, when the heat resistant substrate is a fiber cloth, it is also preferable to apply the fluororesin aqueous dispersion in a state where it is impregnated by a dipping method (dipping method) or the like.

  In the present invention, the thickness of the coating layer of the fluororesin aqueous dispersion is preferably 1 to 1000 μm, more preferably 5 to 300 μm, and most preferably 10 to 100 μm as the thickness of the fluororesin coating film after firing. . The thickness of the coating layer before the heating step for making the thickness of the fluororesin coating film after baking is 200 to 200 times the thickness of the fluororesin coating film because the fluororesin aqueous dispersion contains moisture and a surfactant. 500% is preferred. When the thickness of the coating layer of the fluororesin aqueous dispersion is smaller than this range, the durability of the resulting fluororesin coating film is inferior. Missing may occur in the resin coating layer.

(2) Heating step The heating step in the fluororesin coating film forming method of the present invention is a heat treatment of the fluororesin coating layer after the coating layer forming step at a temperature of 200 ° C. to less than the melting point of the fluororesin in the coating layer. In this step, the content of the nonionic surfactant in the coating layer is reduced to less than 2% by mass with respect to the total mass of the fluororesin fine particles. The content of the nonionic surfactant in the coating layer after the heating step is preferably 1.5% by mass or less, more preferably less than 1% by mass with respect to the total mass of the fluororesin fine particles. When the content of the nonionic surfactant in the fluororesin coating layer is 2% by mass or more with respect to the total mass of the fluororesin fine particles, the fluororesin is applied from the coating layer during the subsequent pressure treatment. Fine particles easily fall off.

  The temperature of the heat treatment in the heating step is a temperature in the range of 200 ° C. to less than the melting point of the fluororesin in the coating layer. For example, when the fluororesin is PTFE having a melting point of 327 ° C., the temperature of the heat treatment in the heating step is preferably in the range of 200 ° C. to less than 327 ° C., more preferably in the range of 230 ° C. to 320 ° C. The range of 250 ° C to 320 ° C is most preferable. When the temperature of the heat treatment is lower than 200 ° C., the thermal decomposition of the nonionic surfactant is not sufficient, and when the temperature is higher than the melting point of the fluororesin, the fluororesin is melted, followed by pressure treatment The effect of can not be obtained. In the present invention, if the heating step is performed at a temperature lower than the melting point of the fluororesin, the fluororesin fine particles can be maintained in a relatively soft state. As a result, the fluororesin coating layer is easily deformed by the pressure treatment after the heating step, and defects such as cracks generated in the fluororesin coating layer before the pressurization step can be eliminated. However, when the fluororesin coating layer is heated to the melting point of the fluororesin or higher at the time of the heating step, the fluororesin fine particles are fused in the fluororesin coating layer, and the fluororesin coating layer becomes relatively hard. Therefore, even if the pressure treatment is subsequently performed, the fluororesin coating layer is hardly deformed, and defects such as cracks generated in the fluororesin coating layer remain. This tendency is particularly remarkable in PTFE, which is why PTFE is suitable for the fluororesin coating film forming method of the present invention.

The time for the heat treatment in the present invention may be any time as long as the content of the nonionic surfactant in the coating layer can be reduced to less than 2% by mass with respect to the total mass of the fluororesin fine particles. Depending on the type of nonionic surfactant, initial content, thickness and area of the coating layer, heating temperature, etc., specifically, 1 to 120 minutes, preferably 2 to 60 minutes, especially Preferably the range for 5 to 30 minutes is mentioned. When the heat treatment time is shorter than 1 minute, the thermal decomposition of the nonionic surfactant may be insufficient, and when it is longer than about 120 minutes, it is not preferable from the viewpoint of production efficiency.
The heat treatment of the coating layer is not a treatment performed only on the coating layer. Specifically, the entire heat-resistant substrate on which the fluororesin coating layer is formed is placed in a temperature-controlled oven or the like by infrared irradiation. It is performed by a method such as heating.

  Moreover, in the fluororesin coating-film formation method of this invention, it is preferable to provide the drying process which dries the said fluororesin coating layer after the said coating layer formation process prior to the said heating process. The purpose of this drying step is to remove moisture and the like from the coating layer so that the fluororesin aqueous dispersion constituting the fluororesin coating layer has no fluidity. Specific examples of the drying method include natural drying, air drying, and heat drying at a temperature of about 100 to 200 ° C. The drying time is set to such a time that the aqueous fluororesin dispersion of the coating layer is not fluid.

(3) Pressurization process The pressurization process in this invention is a process of pressurizing the said fluororesin coating layer after the said heating process with the pressure of 0.1-100 Mpa using a pressurization means. Although the pressure in the said pressurization process is 0.1-100 Mpa, Preferably it is 1-80 Mpa, More preferably, it is 10-50 Mpa. In the fluororesin coating film forming method of the present invention, by performing this pressurization step, defects such as cracks generated in the fluororesin coating layer before the pressurization step can be eliminated, and finally the quality High fluororesin coating film can be formed. When the pressure is lower than 0.1 MPa, it is difficult to eliminate cracks and the like generated in the fluororesin coating layer before the pressurizing step. When the pressure is higher than 100 MPa, the fluororesin coating layer In some cases, a part or the whole of the fluororesin coating layer adheres to the pressure surface of the pressurizing means for pressurizing and the adhered fluororesin coating layer falls off from the heat-resistant base material when the pressure is released.

The total temperature of the fluororesin coating layer or the heat-resistant substrate with a fluororesin coating layer during the pressurizing process in the pressurizing step is preferably 0 to less than 200 ° C, more preferably 10 to 150 ° C, and particularly preferably 20 ~ 100 ° C. When the temperature of the fluororesin coating layer during the pressure treatment is lower than 0 ° C, the pressurizer tends to condense, which may affect the quality of the resulting fluororesin coating film. The fluororesin coating layer on the heat-resistant substrate may be lost during the pressure treatment.
The time for the pressure treatment in the pressurization step is not particularly limited as long as the treatment is performed at the pressure and the defects such as cracks generated in the fluororesin coating layer before the pressurization step can be eliminated. The pressure is appropriately adjusted depending on the pressure, temperature, the thickness of the fluororesin coating layer, and the like. The pressure treatment time is preferably 0.001 to 60 minutes, more preferably 0.005 to 30 minutes, and particularly preferably 0.01 to 10 minutes. When the pressurization time is shorter than 0.001 minutes, the pressurization effect may be insufficient, and when the pressurization time is longer than 60 minutes, it is not preferable from the viewpoint of production efficiency.

Specific examples of the pressurizing means in the pressurizing step in the fluororesin coating film forming method of the present invention include a pressurizing roller and a press device.
In the present invention, an embodiment in which a pressure treatment is performed on the fluororesin coating layer after the heating step using a pressure roller is schematically shown in FIGS. In FIG. 1, a heat-resistant substrate 1 having a fluororesin coating layer 2 on one side after the heating step is continuously passed between two pressure rollers 4 and subjected to pressure treatment. The specific example which obtains the heat-resistant base material 1 which has 3 is shown. FIG. 2 specifically shows the step of pressure treatment using the pressure roller 4 of the heat-resistant substrate 1 having the fluororesin coating layer 2 on both sides after the heating step in the same manner. The pressure roller 4 used for the pressure treatment is usually made of metal such as iron or stainless steel, but the surface of the pressure roller 4 is coated with a material such as silicon rubber, fluorine rubber, or ethylene propylene rubber. Also good. Further, the pressure roller 4 may have a structure in which a heater is incorporated therein. If such a pressure roller is used, the fluororesin coating layer or the heat resistant substrate with the fluororesin coating layer is preferably used. It is possible to perform pressure treatment while heating at a temperature in the range.

  The adjustment of the pressing time can be performed by adjusting the rotation speed of the pressing roller. In addition, as a method for adjusting the pressure, for example, a prescale (registered trademark) manufactured by Fuji Film Co., Ltd. is applied to a pressure roller, the pressure is determined according to the degree of coloring after passing through the pressure roller, and the pressure is adjusted to a predetermined pressure. Etc. are preferable.

  In the present invention, an embodiment in which pressure treatment is performed on the fluororesin coating layer after the heating step using a press device is schematically shown in FIG. FIG. 3A is a view in which a heat-resistant substrate 1 having a fluororesin coating layer 2 after the heating step on one side is inserted between two press plates 5 of a press device (not shown in its entirety). is there. The press device has a mechanism in which two press plates 5 sandwich the heat-resistant substrate 1 with the fluororesin coating layer 2 and apply a predetermined pressure thereto. The pressurization process to the fluororesin coating layer after the heating step is performed by holding the pressurized state for a predetermined time. (B) shows the heat-resistant substrate 1 having the fluororesin coating layer 3 that has been pressure-treated by the press device in this way. The press plate 5 is usually made of a metal such as iron or stainless steel, but its pressure surface may be covered with a material such as silicon rubber, fluorine rubber, or ethylene propylene rubber.

  Alternatively, a sheet of silicon rubber, fluorine rubber, ethylene propylene rubber or the like is laminated on both surfaces of the heat-resistant substrate 1 having the fluororesin coating layer 2 after the heating step, and after the pressure treatment, the sheet is peeled and removed. You may perform a pressurizing process by the method to do. Thus, the method of protecting the pressing surface of the heat-resistant substrate with a fluororesin coating layer with a sheet can also be applied to a pressing process using a pressing roller. Moreover, the thing of the structure where the heater was integrated in the pressurization surface of the press board 5 may be sufficient, and the pressurization process of the said heat resistant base material with a fluororesin coating layer may be performed, heating at the temperature of the said range. A specific example of such a pressing device is a hydraulic press.

(4) Firing step In the firing step of the present invention, the fluororesin coating layer after the pressurizing step is heated at a temperature of the melting point of the fluororesin to 420 ° C., and the fluororesin fine particles in the fluororesin coating layer are removed. It is a step of firing and forming a coating film of the fluororesin on the heat-resistant substrate. Firing refers to fusing the fluororesin fine particles at a temperature equal to or higher than the melting point of the fluororesin.
In the present invention, the firing temperature is in the range of the melting point of the fluororesin to 420 ° C. In the case of PTFE, 350 to 400 ° C. is further preferable, and 360 to 390 ° C. is particularly preferable. When the firing temperature is higher than 420 ° C., the fluororesin is thermally decomposed to deteriorate the performance, and when it is lower than the melting point of the fluororesin, the adhesion to the heat-resistant substrate is insufficient and the performance and durability are deteriorated. Cause a drop.
The firing time in the firing step is not particularly limited as long as the treatment is performed at the firing temperature and the fluororesin fine particles in the fluororesin coating layer can be sufficiently fused, and the thickness of the coating layer and the firing temperature are not limited. It adjusts suitably by conditions, such as. Such firing time is preferably 1 to 120 minutes, more preferably 2 to 60 minutes, and particularly preferably 3 to 30 minutes. When the firing time is shorter than 1 minute, the adhesion to the heat-resistant substrate is insufficient and the performance and durability may be deteriorated. Moreover, when longer than 120 minutes, it is unpreferable from the point of production efficiency.

In the method for forming a fluororesin coating film of the present invention, the cooling after baking may be any method such as natural cooling, air cooling, or water cooling.
In the present invention, the thickness of the fluororesin coating film finally obtained after firing through the steps described above is preferably in the range of 1 to 1000 μm, more preferably 5 to 300 μm, and most preferably 10 to 100 μm. As mentioned. According to the present invention, as long as the thickness of the finally obtained fluororesin coating film is in the range of 1 to 1000 μm, the fluororesin is free from cracks, pinholes, omissions, etc., and ensures durable quality. It is possible to form a coating film on a heat-resistant substrate.
According to the method of the present invention, a fluororesin coating film having a desired thickness is formed on the heat-resistant substrate as described above, and if necessary, on the fluororesin coating film obtained above. It is also possible to form a coating film made of the same type of fluororesin or a different type of fluororesin and laminate the fluororesin coating film so as to have a required coating thickness.

EXAMPLES Hereinafter, although this invention is demonstrated in more detail by Examples 1-7 and Comparative Examples 1-6, these do not limit this invention.
In addition, the nonionic surfactants (A), (B), and (C) used in the following examples and comparative examples correspond to the nonionic surfactants shown in Table 1 below. Moreover, the thermogravimetric analysis (TGA) result measured with the temperature increase rate of 10 degree-C / min using PYRIS1-TGA by Perkin-Elmer of nonionic surfactant (A)-(C) is shown in FIG.

  In addition, the fluororesin fine particles, the fluororesin aqueous dispersion, the fluororesin coating film and the like used in each example and each comparative example are evaluated by each evaluation item (1) to (9) indicating the evaluation method below. did.

<Evaluation of fluororesin fine particles>
(1) Number average molecular weight of fluororesin: PTFE was determined from the amount of latent heat in differential thermal analysis by the method of Suwa (described in J. Appl. Polym. Sci, 17, 3253 (1973)).
(2) Average particle diameter of fluororesin: After drying the fluororesin aqueous emulsion, the photograph was taken at a magnification of 10,000 using a scanning electron microscope, and the lengths of the major axis and minor axis were measured for 100 fine particles. An average value of 100 particles was obtained by setting the value obtained by dividing the total length of the long axis and short axis of the fine particle by 2 as the particle diameter of the fine particle.

<Evaluation of aqueous fluororesin dispersion>
(3) Content of fluororesin and surfactant in fluororesin aqueous dispersion: Approximately 7 g of fluororesin aqueous dispersion is placed in an aluminum dish (mass = W ₀ ) having a diameter of 60 mm and weighed (mass = W ₁ ). ° C. for 1 hour after drying the mass _(W 120), mass after 10 minutes heat treatment at 260 ℃ _(W 260), mass after 10 minutes heat treatment at ₃₀₀ ℃ _(W 300), and after drying for 35 minutes at 380 ° C. It calculated _| required by following Formula from mass ( _W380 ). The surfactant content referred to in the present invention is a numerical value including a nonionic surfactant, an anionic fluorosurfactant and other thermal decomposition components.
・ Fluorine resin content (% by mass) = [(W ₃₈₀ −W ₀ ) / (W ₁ −W ₀ )] × 100
Surfactant content (mass% / fluororesin) = [(W ₁₂₀ −W ₃₈₀ ) / (W ₃₈₀ −W ₀ )] × 100
Residual surfactant content after heat treatment at 260 ° C. for 10 minutes (mass% / fluororesin) = [(W ₂₆₀ −W ₃₈₀ ) / (W ₃₈₀ −W ₀ )] × 100
Residual surfactant content after heat treatment at 300 ° C. for 10 minutes (mass% / fluororesin) = [(W ₃₀₀ −W ₃₈₀ ) / (W ₃₈₀ −W ₀ )] × 100
In the formula, mass% / fluororesin indicates mass% with respect to the total mass of the fluororesin fine particles.

(4) Content of anionic fluorosurfactant: GCMS (gas chromatography with mass spectrometer) No. manufactured by Agilent Measurements were made using 6850 and 5975. The sample is placed in a 20 cc dedicated vial containing 1.5 cc of a boron trifluoride 14% by weight methanol solution (manufactured by GL Sciences Inc.) and 0.2 cc of a sample containing an anionic fluorosurfactant at 80 ° C. After reacting for 30 minutes for methyl esterification, headspace sampler no. It was introduced into GCMS from G1888.
From the signal intensity of the obtained fragment ion having a mass number of 69, the content of the anionic fluorosurfactant was calculated. Prior to the measurement, a calibration curve was created from the signal intensity obtained using an anionic fluorosurfactant with a known content in advance, and used for quantitative analysis. In the case of ammonium perfluorooctanoate, the detection sensitivity by this method is 0.1 ppm with respect to the sample liquid mass.

(5) pH: measured by the glass electrode method.
(6) Viscosity: The viscosity was measured at a liquid temperature of 23 ° C. and 60 rpm using a # 1 spindle with a Brookfield viscometer.

(7) CFT (Crack Limit Film Thickness): Using an applicator in which the thickness of the coating layer changes continuously up to a thickness of 200 μm, a fluororesin aqueous dispersion is applied on an aluminum plate having a thickness of 1 mm and dried at 120 ° C. for 10 minutes. Baked at 380 ° C. for 10 minutes. Cracks occurred in the thick part of the coating layer and disappeared in the thin part. The thickness of the part where the cracks disappeared was measured at five points with an eddy current film thickness meter, and the average value was obtained as CFT.

<Evaluation of fluororesin coating film>
(8) Coating thickness: About the sample which apply | coated to the aluminum plate and baked, the film thickness was measured 5 points | pieces with the eddy current type film thickness meter, and the average value was calculated | required. Moreover, the sample which apply | coated to the glass fiber cloth and baked was calculated by measuring the weight increase per unit area.

(9) Measurement of pinhole current value: An electrolyte solution was prepared by mixing water 89, ethanol 10, and salt 1 in a mass ratio. 8 sheets of non-woven fabric (Bencot (registered trademark) M-3 manufactured by Asahi Kasei Co., Ltd.) are stacked and cut to a size of 2 cm square (thickness: about 3 mm) and placed on the fluororesin coating film surface formed on the aluminum plate to absorb 1 cc of electrolyte. Then, an AC voltage of 10 V was applied between the platinum electrode placed on the upper surface and the back surface of the aluminum plate, and the value of the flowing current was measured. It shows that there exists a defect of a coating film, such as a crack and a pinhole, so that this electric current value is large. When this current value was 1 mA or more, it was judged as defective, and when it was less than 1 mA, it was judged good.

[Example 1]
(Preparation of aqueous PTFE emulsion)
By emulsion polymerization using ammonium perfluorooctanoate and 0.1% by mass disuccinic acid peroxide catalyst as 0.24% by mass anionic fluorosurfactant with respect to PTFE mass obtained after polymerization. TFE was polymerized to obtain a PTFE aqueous emulsion in which the number average molecular weight of PTFE was 3 million, the average particle diameter of PTFE fine particles was 0.25 μm, and the content of PTFE fine particles was 27% by mass.

(Preparation of aqueous PTFE dispersion)
The nonionic surfactant (A) shown in Table 1 is dissolved in the PTFE aqueous emulsion obtained above at a ratio of 3% by mass with respect to the total mass of the PTFE fine particles, and Mitsubishi, which is an anion exchange resin. After adding 3% by mass of Diaion (registered trademark) WA-30 made by chemical to the total mass of PTFE fine particles and stirring for 48 hours, adsorbing ammonium perfluorooctanoate on the anion exchange resin, 100 mesh filter was used. Filtration was performed to remove the anion exchange resin.

By concentrating the obtained filtrate by electrophoresis for 30 hours, the PTFE content is 66.1% by mass, and the surfactant content is 2.1% by mass with respect to the total mass of PTFE fine particles. % PTFE high-concentration aqueous dispersion was obtained. Next, the nonionic surfactant (A) is added to the PTFE high-concentration aqueous dispersion so that the concentration of ammonia is 2.7% by mass with respect to the total mass of PTFE fine particles and the ammonia concentration is 500 ppm with respect to the total mass of water and PTFE fine particles. Ammonia water is dissolved in the aqueous dispersion, the PTFE fine particle content is 60.4% by mass, the surfactant content is 4.8% by mass, and the perfluorooctanoic acid ammonium content is based on the total PTFE fine particle mass. An aqueous PTFE dispersion having an amount of 0.007% by mass with respect to the total mass of the PTFE fine particles was obtained.
This aqueous PTFE dispersion had a pH of 9.5, a viscosity of 18 mP · s, and a CFT of 12 μm. Further, the surfactant content remaining after heat treatment at 260 ° C. for 10 minutes in this PTFE aqueous dispersion is 1.3% by mass with respect to the total mass of PTFE fine particles, and the surfactant content remaining after heat treatment at 300 ° C. for 10 minutes. Was 0.7% by mass relative to the total mass of the PTFE fine particles.

(Formation of PTFE coating)
As the heat-resistant substrate, an aluminum plate having a thickness of 1 mm, a length of 20 cm, and a width of 15 cm with one side being sandblasted was used. The PTFE aqueous dispersion obtained above was placed in a spray gun (Binks-Blows Spray Gun Model 630), sprayed onto an aluminum plate for 5 seconds, and dried in a 120 ° C. oven for 10 minutes. Cracks were observed on the surface of the PTFE resin coating layer.

  Next, the aluminum plate with the PTFE resin coating layer was heat-treated in a 300 ° C. oven for 10 minutes to thermally decompose the surfactant. Thereafter, the aluminum plate with the PTFE resin coating layer was subjected to pressure treatment by passing between pressure rollers whose structures are shown in FIG. The pressure roller had a diameter of 25 mm and a length of 250 mm, and a silicon rubber layer was formed on the surface. The pressure roller was 30 MPa, the temperature was room temperature, the passing speed was 10 cm / min, and the pressing time was about 2 seconds.

  Furthermore, the aluminum plate with the PTFE resin coating layer after the pressure treatment was baked at 380 ° C. for 30 minutes to obtain a PTFE coating on the aluminum plate. The thickness of this PTFE coating film was 15 μm, and cracks, pinholes and the like were hardly observed in the coating film by visual observation. Moreover, the pinhole electric current value of the obtained PTFE coating film was 0.1 mA or less, which was a good result.

[Example 2]
A PTFE coating film was obtained on an aluminum plate in the same manner as in Example 1 except that the heat treatment conditions in the formation of the PTFE coating film were set at 260 ° C. for 10 minutes. The thickness of the PTFE coating film was 15 μm, and almost no cracks, pinholes, etc. were observed in the coating film by visual observation. The pinhole current value of the obtained PTFE coating film was 0.1 mA or less, which was a good result.

[Example 3]
A 28 μm PTFE coating was obtained on the aluminum plate in the same manner as in Example 1 except that the PTFE aqueous dispersion was sprayed onto the aluminum plate for 10 seconds in the formation of the PTFE coating in Example 1 above. Furthermore, the same coating process was repeated once again on this coating film surface to obtain a coating film thickness of 55 μm in total. Visual observation showed almost no cracks, pinholes, etc. in the coating. Moreover, the pinhole electric current value of the obtained PTFE coating film was 0.1 mA or less, which was a good result.

[Example 4]
(Preparation of aqueous PTFE dispersion)
In the same PTFE aqueous emulsion as obtained in Example 1, the nonionic surfactant (B) shown in Table 1 was dissolved at a ratio of 3% by mass with respect to the total mass of the PTFE fine particles. This was passed through a column packed with Diaion (registered trademark) WA-30 manufactured by Mitsubishi Chemical Corporation to adsorb ammonium perfluorooctanoate.

The filtrate after passing through the column is concentrated by electrophoresis, and the PTFE content is 65.6% by mass, and the surfactant content is 2.2% by mass with respect to the total mass of PTFE fine particles. An aqueous dispersion was obtained. To this PTFE high-concentration aqueous dispersion, 2.6% by mass of the nonionic surfactant (B) with respect to the total mass of the PTFE fine particles, and FZ-77 manufactured by Toray Dow Corning Co., Ltd., which is a silicone-based wettability improver, were PTFE. Polyethylene oxide (PEO-3, molecular weight 1 million) manufactured by Sumitomo Seika Co., Ltd., water and PTFE fine particles of 0.5% by mass with respect to the total mass of fine particles and 0.1% by mass with respect to the total mass of PTFE fine particles as a thickener. Ammonia water is dissolved so that the ammonia concentration is 500 ppm with respect to the total mass, the PTFE fine particle content is 60.4 mass% with respect to the total amount of the aqueous dispersion, and the surfactant content is with respect to the total mass of PTFE fine particles. A PTFE aqueous dispersion having 4.8% by mass and an ammonium perfluorooctanoate content of 0.001% by mass with respect to the total mass of PTFE fine particles was obtained.
This aqueous PTFE dispersion had a pH of 9.3, a viscosity of 85 mP · s, and a CFT of 11 μm. The surfactant content after heat treatment at 300 ° C. for 10 minutes in this PTFE aqueous dispersion was 0.9% by mass with respect to the total mass of PTFE fine particles, and the surfactant content after heat treatment at 260 ° C. for 10 minutes was PTFE. It was 1.7% by mass with respect to the total mass of the fine particles.

(Formation of PTFE coating)
A PTFE coating film was formed on an aluminum plate in the same manner as in Example 3 except that this aqueous PTFE dispersion was used as a coating solution. The thickness of this PTFE coating film was 28 μm. Visual observation showed almost no cracks, pinholes, etc. in the coating. Moreover, the pinhole electric current value of the obtained PTFE coating film was 0.1 mA or less, which was a good result.

[Example 5]
In Example 4 above, the same procedure as in Example 4 above was performed, except that the temperature of the entire aluminum plate with the PTFE resin coating layer was set to 80 ° C. during the pressure treatment in the formation of the PTFE coating. A PTFE coating was formed on the aluminum plate. The PTFE coating thickness was 27 μm. Visual observation showed almost no cracks, pinholes, etc. in the coating. Moreover, the pinhole electric current value of the obtained PTFE coating film was 0.1 mA or less, which was a good result.

[Example 6]
A glass fiber cloth (size 20 cm × 15 cm) having a thickness of 220 μm and a unit weight of 200 g / m ² was used as a heat-resistant substrate, and this glass fiber cloth was added to the same PTFE aqueous dispersion as that obtained in Example 4 above. It was soaked for a minute and pulled up. After natural drying, it was heated at 300 ° C. for 10 minutes.

About the glass fiber cloth which has this PTFE resin application layer on both surfaces, it passed between the pressurization rollers set to the pressure of 40 Mpa at a speed | rate of 10 cm / min after cooling, and the pressurization process was performed. Further, baking was performed at 380 ° C. for 10 minutes to obtain a PTFE coating film. The thickness of one side of this PTFE coating film was 55 μm, and cracks, pinholes and the like were hardly recognized in the coating film by visual observation.
Further, the same coating process as described above was further repeated once to obtain a coating film of 102 μm in total per side. Almost no cracks, pinholes, etc. were observed in the coating film.

[Example 7]
In Example 4, the pressure treatment after the heat treatment at 300 ° C. for 10 minutes in the formation of the PTFE coating was performed by placing silicon rubber having a thickness of 1 mm on the PTFE resin coating layer and the same hydraulic pressure as shown in FIG. A PTFE coating film was formed on an aluminum plate in the same manner as in Example 4 except that pressing was performed at a pressure of 30 MPa for 3 minutes and the silicon rubber was peeled off, followed by baking at 380 ° C. for 10 minutes. Formed.
The PTFE coating thickness was 27 μm. Visual observation showed almost no cracks, pinholes, etc. in the coating. Moreover, the pinhole current value of the obtained PTFE coating film was 0.1 mA or less, and good results were obtained.

[Comparative Example 1]
In Example 1, a PTFE coating film was obtained on an aluminum plate in the same manner as in Example 1 except that no pressure treatment was performed in the formation of the PTFE coating film. The thickness of the PTFE coating film was 16 μm, and cracks and surface roughness were recognized in the coating film after firing by visual observation. Moreover, the pinhole current value of the obtained PTFE coating was as large as 4 mA due to cracks.

[Comparative Example 2]
In Example 1, the same operation as in Example 1 was performed except that the heat treatment at 300 ° C. for 10 minutes in the formation of the PTFE coating was performed, but the PTFE resin coating layer was lost during the pressure treatment, A defect remained in the PTFE coating film on the aluminum plate obtained after firing. For this reason, the pinhole electric current value of the obtained PTFE coating film was as large as 16 mA.

[Comparative Example 3]
In Example 1, the same operation as in Example 1 was performed except that the heat treatment in the formation of the PTFE coating film was performed at 350 ° C. exceeding the melting point (327 ° C.) of PTFE for 10 minutes. As in Example 1, cracks occurred in the PTFE resin coating layer after the drying operation, the cracks did not disappear even in the pressurizing step, and the cracks remained in the PTFE coating film obtained on the aluminum plate after firing. . Moreover, the pinhole current value of the obtained PTFE coating was as large as 3 mA.

[Comparative Example 4]
In Example 1, the same operation as in Example 1 was performed except that the pressure of the pressure treatment in the formation of the PTFE coating was reduced to 0.05 MPa. Roughness was observed, and cracks and surface roughness remained on the PTFE coating film obtained on the aluminum plate after firing. Moreover, the pinhole electric current value of the obtained PTFE coating film was as large as 3 mA.

[Comparative Example 5]
In Example 7, the same operation as in Example 7 was performed except that the pressure treatment in forming the PTFE coating was performed at a pressure of 200 MPa for 3 minutes, but the pressure was too high and the pressure was removed. Later, when the silicon rubber was removed, the PTFE resin coating layer was missing, and the PTFE coating film obtained on the aluminum plate after firing remained missing. Moreover, the pinhole electric current value of the obtained PTFE coating film was as large as 12 mA.

[Comparative Example 6]
A PTFE coating film was formed on an aluminum plate in the same manner as in Example 1 except that Asahi Glass Co., Ltd. Fullon (registered trademark) AD1 was used as the aqueous PTFE dispersion.
The PTFE aqueous dispersion contains the same PTFE fine particles as the PTFE fine particles contained in the PTFE aqueous emulsion described in Example 1 as PTFE fine particles, and the content thereof is 60.6% by mass with respect to the total amount of the aqueous dispersion. The polyoxyethylene alkylphenyl ether nonionic surfactant (C) shown in Table 1 is 4.9% by mass relative to the total mass of PTFE fine particles, and the content of ammonium perfluorooctanoate is based on the total mass of PTFE fine particles. On the other hand, it is 0.15% by mass, the pH is 9.6, the viscosity is 20 mP · s, and the CFT is 11 μm. Further, the surfactant content of the aqueous PTFE dispersion after heat treatment at 300 ° C. for 10 minutes was 3.6% by mass relative to the total mass of the PTFE fine particles.

Since the nonionic surfactant (C) contained in this aqueous PTFE dispersion is hardly thermally decomposed, the nonionic surfactant remains in the PTFE resin coating layer even after the heat treatment in the formation of the PTFE coating film. The PTFE resin coating layer was missing during the pressure treatment, and the missing portion remained in the PTFE coating film obtained on the aluminum plate after firing. For this reason, the pinhole current value of the obtained PTFE coating film was as large as 8 mA.
Table 2 shows the characteristics of the fluororesin aqueous dispersion, the production conditions, and the evaluation results of the obtained fluororesin coating film for Tables 1 to 7 and Comparative Examples 1 to 6 in Table 3.

In Tables 2 and 3, (A), (B), and (C) indicated in the column of nonionic surfactant type are the nonionic surfactants (A) and (C) shown in Table 1, respectively. B) and (C) are represented.
The film thickness is shown in the form of “first film thickness / total film thickness of the first time and the second time” in the example in which the coating film was formed twice.
Regarding the content, the fluororesin fine particles indicate mass% with respect to the total amount of the aqueous dispersion, and for the surfactant and the residual surfactant, the mass% with respect to the total amount of fluororesin fine particles contained in the aqueous dispersion (% by mass / fluororesin). Indicates.

  From these results, the fluororesin coating film obtained in the examples according to the method of the present invention has high quality without generation of cracks and pinholes even when the fluororesin aqueous dispersion is applied thicker than CFT and fired. I understand that. In Comparative Examples 1 to 6, the essential steps of the fluororesin coating film forming method of the present invention are omitted or the conditions are carried out outside the scope of the present invention, but the fluororesin coating films obtained by these comparative examples It can be seen that problems such as the occurrence of cracks and missing are occurring.

  Further, Triton X-100 manufactured by Dow Chemical Co., which is a conventional alkylphenol-based nonionic surfactant used in Comparative Example 6, is shown in FIG. 4 (C) as a result of thermogravimetric analysis (TGA). Unless the temperature is higher than the melting point of fluororesin (327 ° C. in the case of PTFE), thermal decomposition is difficult. This difficulty of thermal decomposition is because Triton X-100 has a benzene nucleus in the molecule. In Comparative Example 6, the nonionic surfactant is not thermally decomposed even after the heat treatment step and exceeds the range of the method of the present invention, that is, more than 2% by mass with respect to the total mass of the fluororesin fine particles. It was. In such a state, even if the surface of the fluororesin coating layer is dried and moisture is removed, in the pressure treatment, the coating film is lost when the pressure is released, and the uniformity of the coating film is increased. You can see that

  On the other hand, as shown in FIG. 4 (A) and FIG. 4 (B), respectively, the nonionic surfactants (A) and (B) are easily pyrolyzed at a low temperature. Below the melting point. In the method of the present invention, if such a nonionic surfactant is used, it is easy to control the heating step of removing the nonionic surfactant in the fluororesin coating layer to a predetermined amount by heat treatment, It can be seen that in the next pressurizing step, the fluororesin coating layer is pressurized and then depressurized, so that the fluororesin coating layer is not missing and the pressurization can be performed satisfactorily.

Compared with conventional methods, the fluororesin coating film forming method of the present invention does not deteriorate in quality even when applied thickly, and the number of repeated coatings can be reduced, resulting in a reduction in processing costs, coating film performance and durability. A good fluororesin coating film can be obtained.
For this reason, it can be more preferably used for many applications where a fluororesin aqueous dispersion has been conventionally applied. For example, it can be applied to glass fiber cloths, aramid fiber cloths, and carbon fiber cloths, for use in non-adhesive processing of electric kettles and cakes by coating fluororesin aqueous dispersions alone or pigments and heat-resistant resins and coating them on aluminum plates. Fluorine resin obtained by peeling the fluororesin layer after applying and baking a fluororesin aqueous dispersion on an aluminum plate or stainless steel plate, etc. There are many uses in which the fluororesin aqueous dispersion has been conventionally used, in addition to the use for processing into an ultrathin sheet.

It is a figure which shows typically the one aspect | mode of the pressurization process by the pressurization roller in the fluororesin coating-film formation method of this invention. It is a figure which shows typically another one aspect | mode of the pressurization process by the pressurization roller in the fluororesin coating-film formation method of this invention. It is a figure which shows typically the one aspect | mode of the pressurization process by the press apparatus in the fluororesin coating-film formation method of this invention. It is a figure which shows the TGA analysis result of nonionic surfactant (A), (B), and (C).

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Heat resistant substrate, 2 ... Fluorine resin coating layer after heat treatment (before pressure treatment), 3 ... Fluorine resin coating layer after pressure treatment, 4 ... Pressure roller, 5 ... Press plate

Claims (5)

Fluorine resin fine particles having an average particle diameter of 0.1 to 0.5 μm and a melting point of over 200 ° C. are 20 to 70% by mass with respect to the total amount of the dispersion, and a nonionic surfactant is added to the fluororesin fine particles. A coating layer forming step of applying a fluororesin aqueous dispersion containing 2 to 12% by mass with respect to the total mass to a heat resistant substrate, and forming a coating layer of the fluororesin aqueous dispersion on the heat resistant substrate; ,
The coating layer after the coating layer forming step is heat-treated at a temperature of 200 ° C. to less than the melting point of the fluororesin, and the content of the nonionic surfactant in the coating layer is adjusted to the total mass of the fluororesin fine particles. A heating step that reduces to less than 2% by weight,
A pressurizing step of pressurizing the coating layer after the heating step at a pressure of 0.1 to 100 MPa using a pressurizing means;
The coating layer after the pressurizing step is heated at a temperature of the melting point of the fluororesin to 420 ° C., the fluororesin fine particles in the coating layer are baked, and the fluororesin coating film is formed on the heat-resistant substrate. And a baking step for forming the fluororesin coating film. The method for forming a fluororesin coating film according to claim 1, wherein the nonionic surfactant is a compound represented by the following formula (1).
R ¹ —O—A—H (1)
(Wherein R ¹ is a saturated alkyl group having 6 to 18 carbon atoms and 10% or less of hydrogen atoms may be substituted with a halogen atom. O is an oxygen atom. A is 5 to 20) And a polyoxyalkylene chain composed of 0 to 3 oxyethylene groups, 0 to 3 oxypropylene groups, and 0 to 3 oxybutylene groups.)   The method for forming a fluororesin coating film according to claim 1 or 2, wherein the fluororesin is polytetrafluoroethylene or a tetrafluoroethylene copolymer.   The viscosity of the fluororesin aqueous dispersion is 1-1000 mPa · s when measured with a Brookfield viscometer using a # 1 spindle under the conditions of a liquid temperature of 23 ° C. and 60 rpm. 2. A method for forming a fluororesin coating film according to item 1.   The thickness of the said coating film is 1-1000 micrometers, The formation method of the fluororesin coating film of any one of Claims 1-4.

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