JPH0452310B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a fluororesin composition for coating, and more specifically, it has a thermal fluidity that does not cause cracking of the coating film during application and drying, and provides a smooth coating film surface after baking. This invention relates to an aqueous dispersion of fluororesin. (Prior art and its problems) Thermofluidic fluoroplastics such as tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer or tetrafluoroethylene/hexafluoropropylene copolymer have chemical resistance unique to fluoroplastics. It has excellent properties such as low coefficient of friction, high non-adhesive properties, and low melt viscosity compared to other fluorocarbon resins, making it possible to melt mold. In recent years, an aqueous dispersion of heat-flowable fluororesin has been developed, and products coated with fluororesin can be easily obtained by applying it onto a substrate using a spray gun, but it is still difficult to obtain a uniform coating. It may be difficult to apply a suitable coating. That is, when it is applied and dried, cracks may occur in the coating film, resulting in a cracked coating film, and when it is baked, unevenness may occur on the coating film surface, making it impossible to obtain a smooth surface. Such cracks in the coating film and lack of surface smoothness are undesirable because they directly impair the value of the product, and prevention thereof is desired. Japanese Patent Publication No. 47-21688 discloses that water-soluble methylcellulose or ethylcellulose and an oil-like or grease-like trifluorochloroethylene low polymer are dissolved or dispersed in an aqueous dispersion of a tetrafluoroethylene/hexafluoropropylene copolymer. Coating compositions are described. The present inventors have conducted extensive research to convert aqueous dispersions of heat-flowable fluororesins, such as tetrafluoroethylene/perfluoroalkyl vinyl ether copolymers, into enamels with excellent surface smoothness. It has been found that adding hydroxypropyl methylcellulose at a specific thermal gelling temperature to the dispersion can prevent this coating from cracking when drying and significantly improve the surface smoothness after firing. (Means for Solving the Problems) According to the present invention, an aqueous dispersion of a thermofluid fluororesin such as a tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer has a thermal gelation temperature higher than 60°C. There is provided a fluororesin composition for coating, characterized in that it contains 2 to 10% by weight of hydroxypropyl methyl cellulose having a high temperature and a temperature of 70°C or less per resin. (Function) In the present invention, a thermofluid fluororesin is used among fluororesins because it has excellent chemical resistance unique to fluororesins, has a small friction coefficient, and is highly non-adhesive. This is because the melt viscosity is also lower than that of other fluororesins. According to the present invention, among various water-soluble cellulose derivatives, the thermal gelation temperature is higher than 60°C and
By selecting hydroxypropyl methylcellulose with a temperature below 70℃ and blending it with the aqueous dispersion of thermofluid fluororesin, cracks in the coating film are prevented during drying, and the surface of the coating film is significantly smoothed during baking. can do. The effect of the present invention of significantly smoothing the surface of the heat-flowable fluororesin coating film upon firing is something that is not found in ordinary water-soluble binders or ordinary water-soluble cellulose derivatives, and is unique to hydroxypropyl methylcellulose. This is a unique action and effect. This fact is clear by referring to Table 1 below. Further, in the present invention, the thermal gelation temperature of hydroxypropyl methylcellulose is higher than 60°C and
It is also important that the temperature is 70°C or less in terms of the smoothness of the fired fluororesin coating film. As shown in Table 2 below, when the thermal gelation temperature is below 60℃ or when the thermal gelation temperature exceeds 70℃, the smoothness of the fired PFA coating film is lower than that of the present invention. and decreases. In the present invention, the effect that hydroxypropyl methylcellulose at a specific thermal gelation temperature significantly smoothes the fluororesin coating film during firing is as follows:
This phenomenon was discovered as a result of numerous experiments, and although the reason is still not fully clear,
This seems to be because in the combination of the present invention, the heat flow temperature of the heat flowable fluororesin and the decomposition temperature of the compounding agent (binder) are close to each other. (Preferred Embodiment) In the present invention, it is preferable to use tetrafluoroethylene/perfluoroalkyl vinyl ether (PFA) alone as the heat-flowable fluororesin, but other heat-flowable fluororesins, such as tetrafluoroethylene/hexane Fluoropropylene copolymer (FEP), polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), tetrafluoroethylene/ethylene copolymer (ETFE), etc. may also be used. . Among these,
PFA is most preferred, followed by FEP. In order to obtain these aqueous dispersions, for example, when powder or granules are used, they can be obtained by pulverizing them with a ball mill or the like, if necessary, and then dispersing them in water. In addition, aqueous dispersions or organosol emulsions in water obtained by emulsion polymerization can be used as they are. These aqueous dispersions may also contain other organic liquids, but it is not preferable to mix them in large amounts from the viewpoint of the working environment during coating. After hydroxypropyl methylcellulose is mixed with an aqueous dispersion of thermofluid fluororesin, the viscosity of a 2% aqueous solution is required to ensure ease of handling, i.e., ease of application with a spray gun and maintenance of uniformity of the coating film. It is preferably 400 cps or less at 20°C. The amount of hydroxypropyl methylcellulose to be blended is difficult to define, depending on the concentration of the aqueous dispersion of heat-flowable fluororesin, the type of hydroxypropyl methylcellulose, the desired coating thickness, etc. % to 10% by weight, especially 4% to 6% by weight. The amount is 2
If it is less than 10% by weight, the surface smoothness of the coating film after firing cannot be obtained, and if it exceeds 10% by weight, the decomposed product of hydroxypropyl methylcellulose tends to remain in the coating film even after firing, causing pinholes or non-adhesion. It may damage your sexuality. Any known method may be used to blend hydroxypropyl methylcellulose into an aqueous dispersion of a heat-flowable fluororesin, such as adding it directly to the aqueous dispersion while stirring, or adding it directly to the aqueous dispersion while stirring; Examples include a method of mixing with a dispersion liquid. Furthermore, in the present invention, the thickening effect of hydroxypropyl methylcellulose can be utilized to prevent dripping during application and to prevent repelling during overcoating. Sagging during coating occurs when the aqueous dispersion applied to a vertical surface flows, causing uneven streaks or puddles on the coating film, and repellency during multiple coatings occurs when a coating is applied once using an aqueous dispersion. This method is not preferred because a coating film of sufficient thickness cannot be obtained and multiple coats are often required. Prevention of such dripping and repelling is necessary, for example, if the goal is to have a coating thickness of 10 to 30μ. The viscosity of the aqueous dispersion of thermofluid fluororesin
By adding hydroxypropyl methylcellulose to 290 cps or more, dripping can be prevented, and by adding 350 cps or more, splashing can also be prevented. Of course, even if a known thickener other than hydroxypropyl methylcellulose is used, dripping and repelling can be prevented as long as the viscosity is within the above range. The viscosity here is determined by a B-type viscometer at 27
This is a value measured under the conditions of ℃ and 60 rpm. The fluororesin composition for coating of the present invention may also contain various other known additives, such as fatty acids, sulfuric acid esters of higher alcohols, alkylbenzenesulfonic acids, alkylnaphthalenesulfonic acids, polynaphthalenesulfonic acids, and Interfaces of these salts, carboxylic acids having fluoroalkyl groups, sulfonic acids, phosphoric acids and their salts, esters, polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene acyl esters, sorbitan fatty acid esters, etc. The activator can be used as a dispersant or leveling agent, and higher alcohols such as octyl alcohol and cyclohexanol, ethylene glycol, polymethylsiloxane, etc. can be blended as antifoaming agents. In addition, pigments, dyes, inorganic and organic fillers, etc. may also be blended. Examples will be shown next, but the present invention is not limited to these examples unless the purpose of the invention is impaired. Example 1 and Comparative Examples 1 to 7 The formulations shown in Table 1 were added to an aqueous PFA dispersion containing 54% by weight of PFA having a melt viscosity of 3×10 ⁵ ps at 372°C. The resulting PFA aqueous dispersion composition was coated with a bar coater on an iron plate roughened with sandpaper so that the dry coating thickness was 20 to 25μ.
After drying at 80°C for 10 minutes, it was baked at 380°C for 10 minutes.
Table 1 shows the results of visual judgment of the degree of cracking of the coating film during drying and the smoothness of the coating film surface after baking (◎...Very good crack prevention or surface smoothness, ○...Good, △ ...Minor cracks or slightly good smoothness, ×...Cracks or no smoothness). The following commercially available formulations were used. Hydroxypropyl methylcellulose; Metrose 65SH50 Shin-Etsu Chemical Silica; Mizukasil P-527 Mizusawa Chemical Calcium carbonate; Hakuenka PZ Shiraishi Kogyo Acrylic emulsion; Primal TR-534 Rohm & Haas Japan Polyvinyl alcohol; Gohsenol GH-17 Nippon Gohsei Carboxymethyl cellulose; Celogen PR Daiichi Kogyo Seiyaku Hydroxyethylcellulose; Natrosol 250LR Hercules

【table】

[Table] Example 2 and Comparative Examples 8 to 10 The same PFA aqueous dispersion as in Example 1 was blended with the cellulose derivatives shown in Table 2, and the same tests as in Example 1 were conducted. The results are shown in Table 2. The cellulose derivative used is a commercially available product listed below. Methylcellulose; Metrose SM100 Shin-Etsu Chemical Hydroxypropyl methylcellulose; Metrose 60SH50, 65SH400, 90SH100, Shin-Etsu Chemical

[Table] Example 3 A fluorine-based surfactant and a silicone-based antifoaming agent were added to the same aqueous dispersion of PFA and hydroxypropyl methylcellulose as in Example 1.
It was mixed with 100ppm and adjusted by adding water so that PFA was 35% by weight. The resulting PFA aqueous dispersion was heated to 27°C.
The viscosity measured with a B-type viscometer at 60 rpm was 950 cps.
When this aqueous dispersion was applied to an iron plate using a spray gun, there was no dripping during application, cracking of the coating during drying, or repellency during overcoating, and the surface of the coating after firing was excellent. was. Example 4 The same hydroxypropyl methylcellulose as in Example 1 was added to the same PFA aqueous dispersion as in Example 1.
In addition, 2 parts by weight of carboxymethyl cellulose (Celogen PR) and 100 ppm of a silicone antifoaming agent were added. The viscosity of the resulting PFA aqueous dispersion using a B-type viscometer at 27°C and 60 rpm is
It was 760cps. When this aqueous dispersion was applied to an aluminum plate, there was no sagging during application, cracking of the coating during drying, or repellency during overcoating, and the surface of the coating after firing was excellent. . Example 5 53% by weight FEP with melt viscosity 8×10 ⁴ ps at 372°C
5 parts by weight of hydroxypropyl methyl cellulose (Metrose ^R 65SH50) and the same surfactant and antifoaming agent as in Example 3 were added to the dispersed PFA aqueous dispersion.
They were mixed in 100 ppm increments and adjusted by adding water so that the FEP was 35% by weight. The resulting FEP aqueous dispersion is
The viscosity measured using a Type B viscometer at 27°C and 60 rpm was 680 cps. When applied to an iron plate in the same manner as in Example 3,
There was no sagging during application, no cracking of the coating film during drying, no repellency during overcoating, and the surface smoothness of the coating film after baking was also excellent. (Effects of the Invention) According to the present invention, hydroxypropyl methyl cellulose having a thermal gelation temperature within a specific range is selected, and this is added to a thermofluidic material such as tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer (PFA). By blending with base resin,
It is possible to enamel a heat-flowable fluororesin aqueous dispersion by preventing cracks in the heat-flowable fluororesin coating during drying and significantly smoothing the heat-flowable fluororesin coating during firing. It became.

Claims (1)

[Claims] 1. Hydroxypropyl methylcellulose having a thermal gelation temperature higher than 60°C and lower than 70°C is added to an aqueous dispersion of a thermofluid fluororesin in an amount of 2 to 10% per resin.
A fluororesin composition for painting, characterized in that it contains % by weight. 2. The composition according to claim 1, wherein the thermofluid fluororesin is a tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer. 3. The composition according to claim 1, wherein the thermofluid fluororesin is a tetrafluoro-hexafluoropropylene copolymer.