A polyphenylenesulfide composition for powder coating

Abstract: There is disclosed a polyphenylenesulfide composition for powder coating. The composition comprises polyphenylenesulfide resin powder and hydrophobic silica, the hydrophobic silica being added to said resin powder in the form of ultrafine particles by 0.05 to 0.6% by weight, the ultrafine particles having been subjected to a surface hydrophobic treatment by a silylating agent and having average diameter of 0.1 µm or less on primary particles.

Description: FIELD OF THE INVENTION The present invention relates to a polyphenylenesulfide composition for powder coating, and more particularly to a polyphenylenesulfide composition for powder coating that can form a coated film the surface of which is smooth, uniform, and free from cissing. BACKGROUND OF THE INVENTION Since polyphenylenesulfides having as basic units are excellent in properties, such as heat resistance, water resistance, chemical resistance, abrasion resistance, and electrical insulation, coating containing polyphenylenesulfide as a base is used in the fields of rust prevention and corrosion protection of metals and electrical insulation work. The coating containing polyphenylenesulfide as a base is generally applied in such a way that the coating in the form of a powder or slurry is applied to the surface of an object and is heated to melt to form a coated film, or in such a way that the coating in the form of a powder is sprayed onto the surface of a preheated object to form a coated film. This is because polyphenylenesulfides have a property that they do not dissolve in any organic solvent. However, if a powder of polyphenylenesulfide is applied by using, for example, electrostatic spray coating, fluidized bed coating, or flame spray coating, which is a usual method in powder coating, a number of cissing formation phenomena (a phenomenon to form spot uncontinuous parts, or cratering, on the surface of coated film by the repellence of the coating composition) appear on the surface of the film when the powder is heated and melted, and in many cases the cissing remain after curing and cooling, which makes it very difficult to form a uniform and smooth coated film. The phenomena not only injure the appearance of the film hut also cause pinholes, which restricts extraordinarily the application of this coating, and techniques of solving these problems are desired. Thus, despite that the properties of polyphenylenesulfide coatings are excellent, the use of coating containing polyphenylenesulfide as a base is unavoidably under extreme restrictions for powder coating. For preventing the cissing phenomena which occur on the coating film when a polyphenylenesulfide composition is applied, an addition of metal oxide such as titanium dioxide to the composition was disclosed (e.g., Japanese Patent Publication No. 38689/1974 and Japanese Patent Application (OPI) No. 234067/1988). However, according to the inventor's test followed the disclosure the effect for preventing the cissing on the film was insufficient by such polyphenylenesulfide coating. As the cissing preventive agent, for example, acrylate polymers or copolymers, such as polyethyl acrylates, polybutyl acrylates, and poly-2-ethylhexyl acrylates, and fluorine-type surface-active agents, such as fluorinated alkyl esters, are widely used in conventional coatings. For example, Japanese Patent Application (OPI) No. 118468/1981 describes that a certain surface-active agent can be used for the prevention of cissing of epoxy resin powder coatings, and Japanese Patent Application (OPI) No. 193970/1984 describes that the addition of an acrylic oligomer, i.e., Modaflow (trade name, manufactured by Monsato Chemical Company, U.S.A.), to epoxy resin-type powder coatings is effective in the prevention of cissing. In order to develop a polyphenylenesulfide composition for powder coating, the inventors first tried to apply several kinds of polyphenylenesulfide resin powders having different formulations onto metal plates by the usual powder coating technique. As the result, a number of cissing formation occurred on the coating surface. Therefore, to prevent the cissing formation, known prior cissing preventive agents as described above were studied regarding the kind and the amount to be added varied widely. However, it was found that the conventional general-purpose known cissing preventive agents for powder coating could not prevent cissing formation in the case of polyphenylenesulfide resin powders. Such cissing formation occurring on the coating surface at the time of powder coating of a polyphenylenesulfide resin powder are considered, for example, due to the action of cohesion that is characteristic of polyphenylenesulfide resins, which takes place at the time of being heated and melting after the application, and the presence of such cissing affects considerably adversely the long-term durability of the coating. The surface of coating film obtained from a polyphenylenesulfide composition for powder coating becomes sometimes orange-peel-like more or less. Therefore, it is found that the smoothness of the coating film is insufficient when particularly fine smoothness is required. SUMMARY OF THE INVENTION The object of the present invention is to provide a polyphenylenesulfide composition for powder coating which resolves the above problems involved in polyphenylenesulfide used in powder coating, does not cause cissing formation, and can make the formed film smooth and uniform. Other and further objects, features, and advantages of the invention will appear fully from the following description. DETAILED DESCRIPTION OF THE INVENTION In the development of a polyphenylenesulfide composition, the inventors have keenly studied to prevent cissing formation that will occur on the coating surface when a polyphenylenesulfide resin powder is applied to powder coating, and they have found that, the addition of a specified hydrophobic silica, i.e., a hydrophobic silica in the form of ultrafine particles whose surfaces have been made hydrophobic by treatment using a silylating agent, and whose average diameter of the primary particles is 0.1 µm or less, by an amount of 0.05 to 0.6% by weight to a polyphenylenesulfide composition, is quite effective, completing the present invention. That is, the present invention provides: (1) a polyphenylenesulfide composition for powder coating which comprises a polyphenylenesulfide resin powder and hydrophobic silica, the hydrophobic icsilica being added to said resin powder in the form of ultrafine particles by 0.05 to 0.6% by weight, the ultrafine particles having been subjected to a surface hydrophobic treatment by a silylating agent and having average diameter of 0.1 µm or less on primary particles; and (2) a polyphenylenesulfide composition for powder coating stated under (1), characterized in that aluminum oxide in the form of ultrafine particles the average diameter on the primary particles of which is 0.1 µm or less is further added in such a proportion: weight of aluminum oxide weight of hydrophobic silica ≦ 2.0. The polyphenylenesulfide that is a base of the present composition is a compound made up of repeating units represented by which may include partially oxidized, branched, or crosslinked products. Suitably the particle diameter of the polyphenylenesulfide powder is 200 µm or less, preferably 150 µm or less, so that the composition may be suitable for electrostatic spray coating, fluidized bed coating, flame spray coating, etc. Ones having average particle diameter in the range from 30 to 80 µm are more preferable. The second essential component in the present composition is a hydrophobic silica in the form of ultrafine particles selected from a specifically limited group. The hydrophobic silica in the form of ultrafine particles is obtained by subjecting silica ultrafine particles, which the average diameter on the primary particles is 0.1 µm or less, preferably 0.005 to 0.050 µm, to surface treatment with a silylating agent, so that hydrophilic silanol groups present on the surfaces of the silica particles are masked and the silica ultrafine particles are made hydrophobic. In the hydrophobic silica in the form of ultrafine particles, the average diameter on the primary particles is required to be 0.1 µm or less. If the average diameter exceeds 0.1 µm, since the dispersed density of the hydrophobic silica in the composition decreases and therefore the cissing formation preventive effect lowers considerably, the object of the present invention cannot be attained. More preferably, the ultrafine powder has an average diameter on primary particles of 0.005 to 0.050 µm. It is further important that the hydrophobic silica in the form of ultrafine particles is one whose particle surfaces have been made hydrophobic by treatment using a silylating agent. The silylating agent is a silane compound that is used by replacing polar reactive groups (i.e., active hydrogen), such as alcohol groups and amino groups in inorganic compounds or organic compounds, by organosilicon groups, in order to, for example, stabilize the reactivity and make the surfaces hydrophobic, and examples of such silane compounds are chlorosilanes, such as dimethyldichlorosilane, trimethylchlorosilane, and n-butyldimethylchlorosilane; silazanes, such as octamethylcyclotetrasilazane and hexamethyldisilazane; and alkoxysilanes, such as methyltrimethoxysilane and octyltrimethoxysilane; which silylating agents can be suitably used in the manufacture of the above hydrophobic silica in the form of ultrafine particles. As regards hydrophobic silica, for example, Japanese Patent Application (OPI) No. 230820/1987 describes that, in order to improve the humidity resistance and water resistance of liquid epoxy resin coatings, the addition of a hydrophobic silica having a particle diameter of 13 to 40 µm is effective, and Japanese Patent Application (OPI) No. 57858/1981 describes that, in order to improve the fluidity of a powder of epoxy resin powder coatings, the addition of a hydrophobic silica is effective. However, the composition of the present invention differs in resin and also in actions and effects. It is very surprising that the above specified hydrophobic silica is quite effective in solving the difficult problem of preventing cissing formation that will occur when a polyphenylenesulfide resin powder is applied by powder coating, which was not prevented by using cissing preventive agents conventionally known in the field of powder coatings. As the hydrophobic silica that is utilized in the present invention in the form of ultrafine particles that have been made hydrophobic by treatment with a silylating agent, those commercially available, for example, Aerosil R-805 (trade name, manufactured by Degussa Co.), which is a hydrophobic silica in the form of ultrafine particles that have been made hydrophobic by treatment with a functional silylating agent containing an octyl group, Aerosil R-974 and Aerosil R-972 (trade names, manufactured by Nippon Aerosil Co., Ltd.), which are hydrophobic silicas in the form of ultrafine particles that have been made hydrophobic by treatment with a functional silylating agent containing a dimethyl group, and Aerosil R-812 (trade name, manufactured by Degussa Co.), which is a hydrophobic silica in the form of ultrafine particles that have been made hydrophobic by treatment with a functional silylating agent containing a trimethyl group can be used. The hydrophobic silica is added by an amount of 0.05 to 0.6% by weight based on the whole composition. If the amount to be added is smaller than 0.05% by weight, the effect of suppressing cissing formation at the time of application is considerably lowered, whereas if the amount to he added is over 0.6% by weight, the smoothness of the coating formed on an object is injured. A preferable amount to be added is 0.1 to 0.5% by weight. When a coating film having an excellent smoothness is needed in the polyphenylenesulfide composition for powder coating, in addition to the above hydrophobic silica, aluminum oxide in the form of ultrafine particles whose average diameter on the primary particles is 0.1 µm or less, preferably 0.005 to 0.05 µm, is used. As the aluminum oxide in the form of ultrafine particles, for example, Aluminum Oxide C (trade name, manufactured by Degussa Co.) can be mentioned. The amount of the aluminum oxide in the form of ultrafine particles to be added in addition to the hydrophobic silica is desirably such that: Weight of aluminum oxide Weight of hydrophobic silica ≦ 2.0. When the amount of the aluminum oxide to be added is over the above proportion, it is not preferable because the effect of the addition of the hydrophobic silica decreases and cissing formation is liable to occur on the coating surface. A more preferable proportion is Weight of aluminum oxide Weight of hydrophobic silica ≦ 1.8. The present composition can be manufactured easily by mixing uniformly using a stirring apparatus, such as a Henschel mixer. At that time, if necessary, other additives, for example a color pigment such as titanium dioxide, red iron oxide, and carbon black; an inorganic filler such as glass beads, glass powder, glass short fibers, glass flakes, talc, calcium carbonate, and barium sulfate; an ultraviolet stabilizer; and a rust preventive agent may be added in suitable amounts to be mixed with the composition; and also other polymers, such as polyamides, polycarbonates, polysulfones, polyethersulfones, polyphenyleneoxides, polyimides, polyamidoimides, and epoxy resins may be mixed uniformly. In the present invention, conventional constituents for powder coating can be also added arbitrary. The present composition is applied onto the surface of an object, for example, of a metal, glass, or ceramic by the usual powder coating method and is then heated at a temperature of 300 to 400°C for a prescribed period to be formed into a film. At that time, prior to the powder coating, a suitable primer may be applied to the surface of the object. When the present composition is used for powder coating, cissing formation do not occur at the time of application and a smooth coating can be formed. Further, since the base is polyphenylenesulfide and the film is excellent in heat resistance, water resistance, chemical resistance, abrasion resistance, and electrical insulation, the formation of the coating onto places where these properties are required is useful. In particular, application of the composition, for example, to electrical insulating coatings on heat resistant/chemical resistant precoated plates and electric and electronic components, and to heat resistant/chemical resistant/corrosion preventive coatings of auto parts, chemical plants, hot water supply pipes, hot spring pipings, pipings of ships and sea water plants, and pipes for oil fields, is useful. Now the present invention will be described in more detail in accordance with examples, but the invention is not limited by them. Examples 1 to 13 and Comparative Examples 1 to 11 Components whose compositions are shown in Table 1 were mixed well uniformly by a Henschel mixer to prepare polyphenylenesulfide compositions of Examples 1 to 13. For comparison, polyphenylenesulfide compositions having compositions as shown in Table 2 were prepared, and they were referred to as Comparative Examples 1 to 11. The surfaces of stainless steel plates (SUS 304) having 0.8 mm of thickness, 150 mm of length, and 70 mm of width were degreased with trichloroethylene and were coated with the above-mentioned 24 respective compositions shown in Tables 1 and 2 by electrostatic powder coating, to form respective coatings having a thickness of 70 µm, and the coatings were baked at a temperature of 340°C for 30 min. Hot-rolled steel plates having 5 mm of thickness, 150 mm of length, and 70 mm of width were subjected to grit blast treatment to Sa 2 1/2 and were preheated to 340°C, and their surfaces were coated with the compositions of Examples 2 and 4 and Comparative Example 3 by electrostatic powder coating, to form respective coatings having a thickness of 200 µm, and they were baked at a temperature of 340°C for 30 min. For these thus obtained coatings, the presence or absence of cissing and the smoothness were observed visually. The results are shown in Table 3. In Table 3, "cissing present many" means that cissings of 6 of more per a plate present, and "cissing present a little" means that cissings of 1 to 5 per a plate present. From the results in Table 3, the following is apparent: In comparative Examples 1 to 11, either the preventing of cissing phenomena or the smoothness was not good, and thus the desired coating film being excellent in preventing of cissing formation and in smoothness could not obtained. Among them, for example, Comparative Example 2 containing a hydrophobic silica, but the effect for preventing of cissing formation was deficient because the amount of the hydrophobic silica was insufficient. In Comparative Example 5, the smoothness of coating film was not good because of excess amount of the hydrophobic silica added. Further, in Comparative Examples 6 and 7, the preventing of cissing formation did not exhibit at all, since an ordinary silica powder was added. Further, in Comparative Example 10, the cissing formation became apparent owing to no addition of hydrophobic silica. In Comparative Example 11, hydrophobic silica in a prescribed amount and aluminum oxide were added, but the cissing formation could not prevented sufficiently because of too large amounts of aluminum oxide being added. On the contrary, in Examples 1 to 13, both the preventing of cissing formation and the smoothness of coating film were attained. Having described our invention as related to the present embodiments, it is our intention that the invention not be limited by any of the details of the description, unless otherwise specified, but rather be construed broadly within its spirit and scope as set out in the accompanying claims.

Claims: A polyphenylenesulfide composition for powder coating which comprises a polyphenylenesulfide resin powder and hydrophobic silica, the hydrophobic silica being added to said resin powder in the form of ultrafine particles by 0.05 to 0.6% by weight, the ultrafine particles having been subjected to a surface hydrophobic treatment by a silylating agent and having average diameter of 0.1 µm or less on primary particles. The polyphenylenesulfide composition for powder coating as claimed in claim 1, wherein aluminum oxide in the form of ultrafine particles the average diameter on the primary particles of which is 0.1 µm or less is further added in such a proportion: weight of aluminum oxide weight of hydrophobic silica ≦ 2.0. The polyphenylenesulfide composition for powder coating as claimed in claim 1, wherein the average particle diameter of polyphenylenesulfide is 200 µm or below. The polyphenylenesulfide composition for powder coating as claimed in claim 1, wherein the average particle diameter of polyphenylenesulfide is 150 µm or below. The polyphenylenesulfide composition for powder coating as claimed in claim 1, wherein the average diameter on primary particles of hydrophobic silica is 0.005 to 0.1 µm. The polyphenylenesulfide composition for powder coating as claimed in claim 1, wherein the average diameter on primary particles of hydrophobic silica is 0.005 to 0.050 µm. The polyphenylenesulfide composition for powder coating as claimed in claim 1, wherein the silylating agent is selected from the group consisting of dimethyldichlorosilane, trimethylchlorosilane, n-butyldimethylchlorosilane, octamethylcyclotetrasilazane, hexamethyldisilazane, methyltrimethoxysilane, and octyltrimethoxysilane. The polyphenylenesulfide composition for powder coating as claimed in claim 1, wherein the hydrophobic silica is added by an amount of 0.1 to 0.5 wt.% based on the total of the composition. The polyphenylenesulfide composition for powder coating as claimed in claim 2, wherein the average diameter on the primary particles of aluminum oxide is 0.005 to 0.1 µm. The polyphenylenesulfide composition for powder coating as claimed in claim 2, wherein the average diameter on the primary particles of aluminum oxide is 0.005 to 0.05 µm. The polyphenylenesulfide composition for powder coating as claimed in claim 2, wherein the aluminum ultrafine particles is added in a proportion of weight of aluminum oxide weight of hydrophobic silica ≦ 1.8.

Inventors: MATSUMURA TOYOHIRO, MORITA KAZUYUKI, MATSUMURA, TOYOHIRO, MORITA, KAZUYUKI
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1991
Main classification: C09D181
IPC codes: C09D181, C09D5