JP5379785B2 - Method for forming topcoat film - Google Patents

Description

  The present invention relates to a film forming method for forming a top coat film on a coating object made of a metal material having a base film formed by a trivalent chromate process after a zinc-based plating process.

  Conventionally, various surface treatments have been performed for the purpose of protecting the surface of metal parts. For example, a chemical conversion film treatment in which a metal salt film is formed on a surface by immersing a metal in a certain solution is a surface treatment in which functional effects such as rust prevention, decoration, and sliding are combined. The chemical conversion film treatment is mainly performed after galvanizing or zinc-based alloy plating (hereinafter referred to as zinc-based plating) is performed on an iron-based component.

  As a rust-proofing treatment for galvanized steel, there is a chromate treatment using a treatment liquid containing hexavalent chromium. If the zinc-based plating is used as it is, white rust due to zinc oxidation is likely to occur, and long-term corrosion resistance cannot be obtained. However, the corrosion resistance of the zinc-based plated steel material is dramatically improved by performing the chromate treatment after the zinc-based plating.

  Chromate treatment is a chemical conversion film treatment using a treatment liquid containing various mineral acids and other additives in chromic acid (or dichromic acid), which is a hexavalent chromium compound, and its composition is changed. Thus, a film (chromate film) of four kinds of colors, blue, yellow, green and black can be formed. The hexavalent chromate film has a high moisture content, is relatively soft, and exhibits high corrosion resistance by exhibiting the self-repairing action that is characteristic of the film.

  However, since hexavalent chromium compounds are harmful to the environment, their use has been regulated. Therefore, the anticorrosive treatment of zinc-based plated steel materials is also being switched to the use of a hexavalent chromium-free treatment solution that does not contain a hexavalent chromium compound. Examples of the hexavalent chromium-free treatment solution include an acidic treatment solution using a trivalent chromium compound and an inorganic or organic treatment solution using no chromium compound. The trivalent chromium compound mainly forms a film having excellent corrosion resistance by reacting with phosphate ions in the treatment liquid to form an insoluble phosphate.

  For example, Patent Documents 1 to 3 describe chemical conversion film treatment using a trivalent chromium compound. In such a chemical film treatment, for example, a trivalent chromium compound is used as a main film forming component, and a hexavalent chromium-free black rust preventive film having a corrosion resistance comparable to a chromate film containing hexavalent chromium is applied to a zinc-based plated steel material. Formed on the surface.

A trivalent chromate film formed by performing a chemical conversion film treatment using general trivalent chromium is harder than a hexavalent chromate film. Therefore, for example, if the trivalent chromate film is formed on a bolt or nut that is a fastening member, there is a problem that torque characteristics at the time of fastening are lowered, and in particular, a loose torque is greatly reduced. In this case, there is a possibility that the function guaranteed by the conventional fastening member on which the hexavalent chromate film is formed cannot be satisfied.
Therefore, in some cases, a multilayer structure is formed by adding a coating to the surface of the trivalent chromate film so as to supplement the corrosion resistance inferior to that of the hexavalent chromate film (for example, Patent Documents 1 and 2). That is, a multi-layer structure of the coating object is formed by forming a top coat film using a known coating solution on a coating object on which a base film is formed by trivalent chromate treatment after the zinc-based plating treatment. Can be planned. By performing the top coat treatment in this way, the corrosion resistance can be improved even when a trivalent chromate film is formed.

Japanese Patent No. 3585937 Japanese Patent Laid-Open No. 2005-126797 JP 2006-348314 A

  However, when the top coat treatment described above is performed, there is a problem that the film thickness of the top coat film is not stabilized, that is, the film thickness cannot be made constant. When the coating thickness cannot be made constant, for example, the torque coefficient of each coating object varies. Therefore, when the trivalent chromate film and the topcoat film are formed on bolts and nuts, there is a possibility that the degree of fastening may vary. Further, in a portion where the coating thickness is thin, there are cases where it is weak against contact with other objects or impact upon fastening with, for example, a spectacle wrench.

  Accordingly, an object of the present invention is to provide a top coat that can uniformly form a film thickness of a top coat film on a coating object made of a metal material in which a base film is formed by a trivalent chromate treatment after a zinc-based plating treatment. It is in providing the formation method of a membrane | film | coat.

  In order to achieve the above object, the method of forming a top coat film according to the present invention is to form a top coat film on a coating object made of a metal material having a base film formed by a trivalent chromate treatment after a zinc plating treatment. The top coat film forming method is characterized in that the first characteristic means is that a thermosetting paint is introduced in a state where the object to be coated and the coating medium are mixed and stirred in a mixing container, and is applied to the coating medium. A mixing step of bringing the coating object into a wet state by transferring the adhered thermosetting paint to the coating object, a separation step of separating the coating object and the coating medium, and the coating object And a heat treatment step of curing the thermosetting paint by heating the object in a wet state.

  According to this configuration, a hexavalent chromium compound, which is an environmentally hazardous substance, is not used when a topcoat film is formed on a coating object on which a base film is formed by a trivalent chromate treatment after a zinc-based plating treatment. Since the process is performed using a chromium-free treatment solution, the top coat film can be formed while suppressing environmental burden. In addition, as will be described in the following examples, the top coat film is a film having a rust preventive power equivalent to or higher than that of hexavalent chromium green chromate.

In the mixing step, the thermosetting paint adhering to the coating medium is transferred to the coating object when the coating object and the coating medium come into contact with stirring. Since the mixing step is performed at room temperature, a liquid thermosetting coating film having a desired thickness and a substantially uniform thickness is formed over the entire surface of the object to be coated. During stirring, if another object comes into contact with the coating object to which the thermosetting paint is attached, the thickness of the coating film at the contact location is reduced. However, when the other object is separated from the contact portion, the thin liquid thermosetting paint film immediately recovers to its original thickness due to the effect of surface tension. This is because the thermosetting paint is liquid.
In the present invention, a thermosetting paint is used to form a topcoat film, but the thermosetting paint maintains a liquid wet state at room temperature, so during the mixing process, The thermosetting paint is not cured. Therefore, the coating film of the thermosetting paint formed on the object to be coated has a property of always maintaining the desired thickness.

  In the separation step, the painting object and the painting medium are separated. Since the separation step is also performed at room temperature, the thermosetting paint is not cured during the separation step. Therefore, for example, even when a separating means or the like comes into contact with a coating object to which a thermosetting paint is attached, the desired thickness of the thermosetting paint film formed on the coating object is always maintained. .

  In the heat treatment step, the separated coating object is heated in a wet state to cure the thermosetting paint. The heated thermosetting coating is cured to form a dry top coat film. This topcoat film is a film obtained by curing a coating film in which a liquid thermosetting paint adheres to the entire surface of the object to be coated with a substantially uniform desired thickness. It becomes uniform over the entire surface of the object. Therefore, when the top coat film is formed on, for example, a bolt, a bolt having a stable torque coefficient can be obtained. Moreover, since the film thickness is uniform, it is possible to eliminate a portion that is vulnerable to contact with other objects and impact.

  In addition, in the paint that hardens the coating film component by volatilization and drying of the solvent in the paint, it is necessary to manage the time in consideration of whether the paint is dried or not when performing the mixing step or the separation step. In the method of the present invention, since a thermosetting paint that does not harden and dry at room temperature is used, the time management becomes unnecessary.

  The second characteristic configuration of the method for forming a topcoat film according to the present invention is that the thermosetting paint is a silicon compound-containing paint.

  The silicon compound-containing coating is usually cured by heating at about 120 to 150 ° C. and does not require excessive heating, and thus is excellent in handling. Furthermore, for example, in the case of a silicon compound-containing coating containing a silicon resin as a silicon compound, the silicon resin is excellent in self-lubricating properties. A wet coating film of a thermosetting paint can be formed quickly in the process.

  A third characteristic configuration of the method for forming a topcoat film according to the present invention is that the silicon compound-containing paint contains 10 to 20% by weight of a silicon compound, and the silicon compound-containing paint uses a viscosity cup NK-2. The value measured at 20 ° C. is that it has a viscosity of 19 to 23 seconds (21 ± 2 seconds).

  In the method for forming a topcoat film of the present invention, the topcoat film is formed by curing a coating film of a thermosetting paint by heating. At this time, in order to maintain a substantially uniform torque coefficient throughout the top coat film, it is necessary that the required amount of coating film components be present in the top coat film. For example, when the thermosetting paint is diluted with a diluent, the coating film component is not diluted below the required amount. That is, if the silicon compound-containing coating is diluted to an extent containing at least 10% by weight of a silicon compound, the cured top coat film can maintain a good torque coefficient. Therefore, if a dilution limit point is set in the thermosetting paint to be used, a topcoat film having a necessary amount of coating film components can be formed at all times.

Since the viscosity of the thermosetting paint increases as the concentration of the coating film component increases, the time required for spreading over the entire surface of the object to be coated becomes longer than when the coating film component has a low concentration. For example, when the silicon compound-containing paint contains 20% by weight or more of the silicon compound, it is not preferable because the time required to spread the entire surface of the object to be coated becomes excessive and the mixing process cannot be completed quickly.
Furthermore, in order to quickly attach the thermosetting paint to the surface of the object to be coated, it is necessary to maintain it within a predetermined viscosity range. Therefore, in order to quickly form a thermosetting paint film on the surface of the object to be coated and to form a topcoat film having a suitable torque coefficient, 10 to 20 weight of silicon compound is added to the silicon compound-containing paint. % And the viscosity range is 19 to 23 seconds (viscosity cup NK-2: 20 ° C.). Thus, the film thickness of the topcoat film can be freely controlled by setting the silicon compound and the viscosity range of the thermosetting paint.

  A fourth characteristic configuration of the method for forming a topcoat film according to the present invention is that a trivalent chromium black chromate film is formed by the trivalent chromate treatment.

  According to this structure, since it can carry out using the hexavalent chromium free processing liquid which does not use a hexavalent chromium compound, it can form a base film (chromate film layer), suppressing an environmental load. Moreover, the chromate film layer excellent in corrosion resistance and weather resistance can be formed.

  A fifth characteristic configuration of the method for forming a topcoat film according to the present invention is that the object to be coated is at least one of a bolt and a nut.

  According to this configuration, a top coat film having a uniform film thickness can be formed even with relatively small processed products such as bolts and nuts. Therefore, torque characteristics when fastening the bolt and nut can be made substantially constant, so that the bolt fastening operation can be performed accurately.

It is the schematic of the coating target object (bolt) which formed the topcoat film | membrane. It is a flowchart which shows the process of the formation method of the topcoat film | membrane of this invention. It is the schematic of a mixing process. It is the schematic of a separation process.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The present invention is a method for forming a topcoat film in which a topcoat film b is formed on a coating object 10 made of a metal material having a base film a formed by a trivalent chromate process after a zinc-based plating process (FIG. 1). , 2). In the method for forming the top coat film, a chemical film treatment is performed without using a hexavalent chromium compound, which is an environmental load substance.

  In the method for forming a top coat film of the present invention, the thermosetting paint 30 is added in a state where the object to be coated 10 and the coating medium 20 are mixed, and the mixture is stirred in the mixing container 40 and adhered to the coating medium 20. The thermosetting paint 30 is transferred to the object to be coated 10. That is, in this method, the mixing process A for bringing the painting object 10 into a wet state, the separation process B for separating the painting object 10 and the painting medium 20, and the thermosetting property by heating the painting object 10 in a wet state. A heat treatment step C for curing the coating material 30.

(Zinc-based plating treatment)
As the ground treatment (first ground treatment) of the coating object 10, a zinc-based plating layer a1 is formed by performing a zinc-based plating treatment. The zinc-based plating treatment is mainly a galvanizing or zinc-based alloy plating on the surface of an iron-based component, and is a chemical film treatment in which functional effects such as rust prevention, decoration, and sliding are combined. Examples of the zinc-based alloy plating include Zn—Ni alloy plating, Zn—Fe alloy plating, and Zn—Al alloy plating. The zinc-based plating treatment may be performed by a known method such as a hot dip galvanizing method, an electroplating method, or a vapor phase plating method. What is necessary is just to set the film thickness of the zinc-type plating layer a1 suitably so that the corrosion resistance required for the coating target object 10 can be provided.

(Trivalent chromate treatment)
In the state of only the zinc-based plating layer a1 formed by performing the zinc-based plating treatment, discoloration and corrosion are likely to occur over time. Therefore, trivalent chromate treatment is performed as a post-treatment (second base treatment) for zinc-based plating to form a trivalent chromium compound film (chromate film layer a2) on the surface of the zinc-based plating layer a1, thereby improving corrosion resistance. I am letting. As the trivalent chromium compound, a compound soluble in an acidic aqueous solution may be used. Examples of the trivalent chromium compound include chromium chloride, chromium sulfate, chromium nitrate, chromium phosphate, and chromium acetate. Trivalent chromium may be used after reducing hexavalent chromium such as chromic acid or dichromate to trivalent with a reducing agent. The concentration of trivalent chromium is preferably in the range of 3.0 to 3.5 g / L, for example. Further, the trivalent chromate treatment liquid is preferably adjusted to a pH in the range of 2.0 to 2.3. Thus, this treatment is performed using a hexavalent chromium-free treatment solution that does not use a hexavalent chromium compound.

Trivalent chromate treatment uses a trivalent chromate treatment solution in which various mineral acids and other additives are added to trivalent chromium, thereby providing a chromate film layer with four different colors: blue, yellow, green, and black. a2 can be formed. The color tone of the chromate film layer a <b> 2 may be appropriately selected according to the place where the object to be coated 10 is used and the application. For example, in the trivalent chromate treatment solution, the black chromate film layer a2 can be formed by appropriately setting the mass ratio of phosphate ions and sulfate ions and the pH. For example, a trivalent chromate treatment solution having a trivalent chromium amount of 3.0 to 3.5 g / L and a pH of 2.0 to 2.3 may be immersed in a treatment temperature of 25 to 35 ° C. for about 50 seconds.
In particular, when a trivalent chromium black chromate film is formed by trivalent chromate treatment, a chromate film layer a2 having excellent corrosion resistance and weather resistance can be formed. The film thickness of the chromate film layer a2 may be appropriately set so that the aesthetic appearance and corrosion resistance necessary for the coating object 10 on which the zinc-based plating layer a1 is formed can be imparted.

(Coating object)
As long as the coating object 10 is a metal material that can form the base film a by the trivalent chromate treatment after the zinc-based plating treatment, a primary processed product such as a steel plate, a steel pipe, or a bar material, or a secondary product obtained by processing them. A processed product or a semi-processed product may be used, and is not particularly limited. Examples of the secondary processed product include fastening parts such as bolts, nuts, and washers, press-formed products, stamped products, forged products, and cast products.
Examples of metals include steel materials (SS400 / SPHC / SPCC / SWCH / S45C / SCM435 / SNB7 etc.), stainless steel (XM7 / 304/316 / 316L / 410/430 etc.), brass, copper and copper alloys, aluminum and aluminum. Alloys, titanium, etc. can be used.

(Painting medium)
When the coating medium 20 is mixed with the object to be coated 10 and the thermosetting paint 30 is added and stirred in the mixing container 40, the thermosetting paint 30 adhering to the coating medium 20 is coated. It is transferred to the object 10.

  The size and material of the coating medium 20 can be appropriately selected according to the shape and size of the coating object 10 and the material of the thermosetting paint 30 constituting the film formed on the coating object 10. Moreover, you may mix and use the coating medium 20 from which a size and a material differ, and surface treatment and a surface membrane | film | coat may be given to the coating medium 20. FIG. The shape of the coating medium 20 can be a disc, sphere, ellipse, cylinder, cone, cube, triangular prism, triangular pyramid, quadrangular pyramid, rhombohedral, irregular shape, etc. The medium 20 may be used alone or in combination as appropriate. The size of the coating medium 20 may be, for example, about 3 to 6 mm in particle diameter when the coating medium 20 is disk-shaped. As the material of the coating medium 20, various metals such as iron, carbon steel, alloys, ceramics, glass, and hard plastics can be used.

  The coating medium 20 has a predetermined size and shape according to the shape of the object to be coated 10. If the coating medium 20 is formed smaller than the object to be coated 10, for example, when the surface of the object to be coated 10 has a concavo-convex shape, the coating medium 20 enters the concave portion and the thermosetting paint 30 is easily attached.

(Thermosetting paint)
The thermosetting paint 30 is a paint containing a coating film component that undergoes a polymerization reaction and crosslinks when heat is applied, and the network structure is increased and cured. Examples of such a coating film component include thermosetting resins such as silicon resin, urea resin, phenol resin, melamine resin, and epoxy resin.
Since the thermosetting paint 30 is a paint that does not harden and dry at room temperature, simply applying it to the object to be coated 10 maintains a wet state unless it is heated to a predetermined temperature or higher.

  In the present invention, a silicon compound-containing paint is particularly used. The silicon compound-containing coating is usually cured by heating at about 120 to 150 ° C. The silicon compound-containing paint contains, for example, a silicon silicon compound as a component. Since the silicon resin is excellent in self-lubricating property, if it is a silicon compound-containing paint containing a silicon resin, it easily spreads over the entire surface of the coating object 10 as soon as it adheres to the coating object 10.

  The silicon compound-containing paint preferably contains about 10 to 20% by weight of a silicon compound with respect to the thermosetting paint 30 in a state of being attached to the object to be coated 10 (in a state of being charged into the mixing container 40). What is necessary is just to dilute with a diluent suitably in order to make it the density | concentration and viscosity range of such a silicon compound.

As the other components, the thermosetting paint 30 contains a resin component, a crosslinking agent component, a coloring pigment, a curing catalyst, an anti-settling agent, an anti-flow agent, an ultraviolet stabilizer, a flame retardant, an antifouling agent, and the like. .
Examples of the resin component include an acrylic resin having a crosslinkable functional group such as a hydroxyl group, an epoxy group, a carboxyl group, and an alkoxysilane basis, a urethane resin, a polyester resin, a vinyl resin, and an alkyd resin.
The crosslinking agent component is a component capable of crosslinking and curing the resin component, and examples thereof include alkyl etherified melamine resin, urea resin, guanamine resin, epoxy compound, and carboxyl group-containing compound.

  The thermosetting paint 30 has a viscosity of about 19 to 23 seconds measured at 20 ° C. using a viscosity cup NK-2 (pore diameter 3 mm: manufactured by Anest Iwata Co., Ltd.), which is a liquid viscosity measuring instrument. Good. This viscosity range is a value obtained by measuring the time (seconds) until the predetermined amount of thermosetting paint 30 accommodated in the viscosity cup stops flowing out.

  In order to maintain a substantially uniform torque coefficient throughout the top coat film b, it is necessary that the required amount of coating film components be present in the top coat film b. That is, when diluting the thermosetting paint 30 with a diluent, the coating film component is not diluted below the required amount. Therefore, if the dilution limit point is set in the thermosetting paint 30 to be used, it is possible to always form the top coat film b having a necessary amount of the coating film component.

  Since the viscosity of the thermosetting paint 30 increases as the coating film component becomes higher in concentration, the time required for spreading the entire surface of the coating object 10 becomes longer than in the case where the coating film component has a low concentration. In order to quickly attach the thermosetting paint 30 to the surface of the object 10 to be coated, it is necessary to maintain it within a predetermined viscosity range. Therefore, in order to form the topcoat film b having a suitable torque coefficient on the surface of the object 10 to be coated, the silicon compound containing 10 What is necessary is just to adjust the thermosetting coating material 30 so that it may contain -20weight% and a viscosity range may be 19-23 seconds.

(Mixing container)
The mixing container 40 is a container in which the object to be coated 10, the coating medium 20, and the thermosetting paint 30 can be added and stirred (FIG. 3). The mixing container 40 of the present embodiment includes an inlet 41 through which the object to be coated 10, the coating medium 20 and the thermosetting paint 30 can be charged, and a stirring means 42 capable of stirring the object to be coated 10, the coating medium 20 and the thermosetting paint 30. The separating means 43 capable of separating the coating object 10 and the coating medium 20 is provided.

The input port 41 may input the coating object 10, the coating medium 20, and the thermosetting paint 30 from the same input port, or may be input from differently formed input ports.
As long as the stirring means 42 is an aspect which can stir uniformly the coating object 10, the coating medium 20, and the thermosetting coating 30, any methods, such as rotation stirring and vibration stirring, are applicable.

  The separating means 43 can use, for example, a mesh-like stainless steel screen (mesh) to separate the object to be coated 10 and the coating medium 20. The mesh of the sieve is set, for example, to have a diameter that allows the coating medium 20 to pass therethrough and does not allow the coating object 10 to pass therethrough. During the stirring, it is necessary to uniformly mix the coating object 10 and the coating medium 20, and to reliably transfer the thermosetting paint 30 attached to the coating medium 20 to the coating object 10. The coating object 10 and the coating medium 20 are not separated by the above.

  For example, when the mixing process A and the separation process B are performed in the same space of the mixing container 40, after the mixing process A is completed, only the coating medium 20 is configured to fall by gravity. When the mixing step A and the separation step B are performed in different spaces of the mixing container 40, the mixture of the coating object 10, the coating medium 20, and the thermosetting paint 30 is separated after the mixing step A is completed. What is necessary is just to transfer to the space which performs B by transfer means, such as a belt conveyor.

(Mixing process)
In the mixing step A, the thermosetting paint 30 is introduced in a state where the object to be coated 10 and the coating medium 20 are mixed and stirred in the mixing container 40 (FIG. 3). Mixing step A is performed at room temperature. The application timing of the coating object 10, the coating medium 20, and the thermosetting coating 30 may be the same. Alternatively, the thermosetting paint 30 may be attached to the coating medium 20 in advance and put into the mixing container 40. These are put into the mixing container 40 and then stirred by a technique such as rotary stirring. By applying the stirring, the coating object 10 can be brought into a wet state by transferring the thermosetting paint 30 adhering to the coating medium 20 to the coating object 10. The mixing step A may be performed until the thermosetting paint 30 is uniformly transferred to all the applied objects 10 to be applied.

  The amount of the thermosetting paint 30 to be input is determined after the thickness of the coating film formed on the coating object 10 and the thermosetting paint 30 adhering to the coating medium 20 are transferred to the coating object 10. The total thickness of the coating film remaining on the medium 20 is used. The film thickness can be controlled by the type or viscosity of the thermosetting paint 30.

  The amount of the thermosetting paint 30 to be input is based on the past data of similar processing, the speed of rotation stirring, the rotation time, the size / input amount of the object to be coated 10, the size of the coating medium 20, It may be determined in consideration of the input amount (prototype verification method). At this time, the thermosetting paint 30 may be appropriately diluted to a desired concentration with a diluent.

  The coating medium 20 may be put into the mixing container 40 with the thermosetting paint 30 attached in advance. The amount of the thermosetting paint 30 to be attached to the coating medium 20 in advance is the total amount of the total surface area of the coating medium 20 and the total surface area of the object to be coated 10 and the desired coating film to be attached to the object to be coated 10. It may be calculated from the correlation with the thickness.

  Thus, since the amount of the required thermosetting paint 30 can be grasped by calculation in advance, the amount of useless thermosetting paint 30 can be suppressed.

  The thermosetting paint 30 adhering to the painting medium 20 is transferred to the painting object 10 when the painting object 10 and the painting medium 20 come into contact with stirring. At this time, since the thermosetting coating 30 is in a liquid state, a liquid coating film having a desired thickness that is substantially uniform over the entire surface of the object to be coated 10 is obtained. During stirring, if another object comes into contact with the coating object 10 to which the thermosetting paint 30 is attached, the thickness of the coating film at the contact location is reduced. However, when the other object is separated from the contact portion, the thin film of the liquid thermosetting paint 30 tends to recover to the original thickness due to the effect of surface tension or the like. Since the thermosetting paint 30 maintains a wet state at room temperature, the thermosetting paint 30 is not cured during the mixing step A, and thus the thermosetting property formed on the object to be coated 10. The coating film of the paint 30 has the property of always trying to maintain the desired thickness.

  For example, when rotating agitation, the rotation speed may be electrically controlled by an inverter or the like. The rotation speed is set to be about 50 to 60 times / minute. The rotation time depends on the type, size, and shape of the object 10 to be coated, but may be set as appropriate, for example, 2 minutes for bolts and 3 minutes for nuts.

(Separation process)
In the mixing process A, after the thermosetting paint 30 is uniformly transferred to all the applied objects 10 to be applied, a separation process B for separating the object 10 and the coating medium 20 is performed (FIG. 4). Separation step B is performed at room temperature.

  In the separation process B, for example, the coating object 10 and the coating medium 20 are passed over a mesh formed on the bottom surface of the mixing container 40. By setting the mesh mesh diameter to a size that allows only the coating medium 20 to pass therethrough, the object to be coated 10 and the coating medium 20 can be separated. Since the separation step B is performed at room temperature, the thermosetting paint 30 maintains a wet state. Therefore, even when the coating object 10 to which the thermosetting paint 30 is attached comes into contact with another object such as a mesh, the coating film of the thermosetting paint 30 formed on the coating object 10 always has a desired thickness. Is maintained.

  Note that if a coating film of the thermosetting paint 30 is formed in advance on the inner wall of the mixing container 40 and the separating means 43, even if the coating object 10 to which the thermosetting paint 30 is attached contacts these. The thermosetting paint 30 attached to the coating object 10 is difficult to transfer to the inner wall of the mixing container 40 and the separating means 43. As a result, the coating film of the thermosetting paint 30 is maintained at a desired film thickness in the object 10 to be coated, and a stable topcoat film b can be obtained. Therefore, when calculating the amount of the thermosetting paint 30 to be charged, it is preferable to take into account the amount of the thermosetting paint 30 to be attached to the inner wall of the mixing container 40 and the separating means 43 in advance.

(Heat treatment process)
In the heat treatment process C, the separated coating object 10 is heated in a wet state to cure the thermosetting paint 30.
A coating film of the wet thermosetting paint 30 is formed on the surface of the separated object 10 to be coated. When the object to be coated 10 is separated, it is heated by a heating means at 120 to 150 ° C. for a predetermined time (for example, 20 to 30 minutes) to form a dry and dry top coat film b. The formed top coat film b is a film obtained by curing the coating film in which the liquid thermosetting coating 30 is adhered to the entire surface of the coating object 10 with a substantially uniform desired thickness. The film has a uniform film thickness over the entire surface. Therefore, there is almost no possibility that the torque coefficient of each painting object 10 will vary. The top coat film b has a thickness of about 1 to 2 μm, for example.

  From the above, the top coat film forming method of the present invention is superior in safety because it does not use an environmentally hazardous substance (hexavalent chromium), and has a rust preventive power equivalent to or higher than, for example, hexavalent chromium green chromate. Can be formed. Furthermore, since the top coat film b can be formed with a substantially uniform film thickness over the entire surface, for example, it becomes a film that can achieve stabilization of the torque coefficient during tightening.

  Examples of the present invention will be described below.

[Example 1]
UBS bolt (M10 × P1.25: manufactured by Saga Iron Works Co., Ltd.) as the coating object 10, disk type (UFO type: carbon sputonic) media as the coating medium 20, and die coat BK as the thermosetting paint 30 (Registered trademark: manufactured by Daisho Co., Ltd.) was used. The die coat BK contains 10 to 20% of a silicon compound (silicon silicon resin) as a main component, has a viscosity of 19 seconds (viscosity cup NK-2: 20 ° C.), and is cured when heated to 125 ° C. or higher. In this example, the die coat BK was diluted 20% with a diluted thinner (manufactured by Dolken), and the silicon compound content was adjusted.

The bolts were subjected to galvanization and trivalent chromate treatment as the base treatment.
The galvanization treatment was performed using Hyperzinc 7900 (manufactured by Nippon Surface Chemical Co., Ltd.) under known conditions by an electrogalvanization method. The trivalent chromate treatment was carried out under known conditions using a triner TR-185 (manufactured by Nippon Surface Chemical Co., Ltd.).
Thus, after performing the zinc plating process to the bolt which is the coating object 10, a trivalent chromium black chromate film was formed by the trivalent chromate process.

  The tumbling machine (mixing container 40) was put together with the bolts and media subjected to the ground treatment. Next, a predetermined amount of 20% diluted die coat BK was put into a tumbling machine and stirred under predetermined conditions to form a wet coating film of die coat BK on the entire surface of the bolt (mixing step A).

The wet bolt and media having a die coat BK coating film formed on the surface thereof were sieved with a stainless steel mesh (hole diameter: 8 mm, manufactured by Daishosha Co., Ltd.) as the separating means 43 to separate only the bolt (separation process) B). The wet bolt was subjected to a heat treatment at 150 ° C. for 30 minutes to cure the coating film (heating step C), thereby producing a processed product of the present invention. At this time, the film thickness of the top coat film formed on the treated product of the present invention was about 1.5 to 2 μm throughout.

  As a comparative example, hexavalent chromium was formed by forming a hexavalent chromate film by subjecting a UBS bolt (M10 × P1.25: manufactured by Saga Iron Works Co., Ltd.) that had been galvanized as a base treatment to a hexavalent chromium green chromate treatment. A green chromate-treated product was prepared.

  A salt spray test (SST test) was performed on each of the product treated with the present invention and the product treated with hexavalent chromium green chromate. As a result, no rust was observed after 168 hours. Therefore, it was recognized that the treated product of the present invention has the same corrosion resistance as the hexavalent chromium green chromate treated product.

  Further, as a result of incorporating the bolt of the product of the present invention into an M108 type tractor (manufactured by Kubota Co., Ltd.) and conducting aged evaluation over about 7 months, no rust was found on the bolt.

  Although all results are not shown, the die coat BK used as the thermosetting paint 30 contains 10 to 20% by weight of a silicon compound by dilution, and has a viscosity of 19 to 23 seconds (viscosity cup NK-2). : 20 ° C.), the same results as above were obtained.

[Example 2]
With respect to the processed product of the present invention produced in Example 1, the torque coefficient during tightening was measured. The measurement was carried out according to a standard method using a horizontal fastening tester. As a result, the torque coefficient of the processed product of the present invention was 0.29 to 0.35, and no large variation was recognized.

  The method for forming a top coat film of the present invention can be used when a top coat film is formed on a coating object made of a metal material having a base film formed by trivalent chromate treatment after zinc-based plating treatment.

A Mixing process B Separation process C Heat treatment process b Top coat film 10 Application object 20 Coating medium 30 Thermosetting paint 40 Mixing container

Claims (5)

A method for forming a topcoat film, wherein a topcoat film is formed on a coating object made of a metal material having a base film formed by a trivalent chromate treatment after a zinc-based plating treatment,
A thermosetting paint is added in a state where the coating object and the coating medium are mixed, and the mixture is stirred in a mixing container, and the thermosetting paint adhering to the coating medium is transferred to the coating object. A mixing step of bringing the object to be coated into a wet state,
A separation step of separating the coating object and the coating medium;
And a heat treatment step of curing the thermosetting paint by heating the object to be coated in a wet state.   The method for forming a topcoat film according to claim 1, wherein the thermosetting paint is a silicon compound-containing paint.   The silicon compound-containing paint contains 10 to 20% by weight of a silicon compound, and the silicon compound-containing paint has a viscosity of 19 to 23 seconds measured at 20 ° C using a viscosity cup NK-2. 3. A method for forming a topcoat film according to 2.   The method for forming a topcoat film according to any one of claims 1 to 3, wherein a trivalent chromium black chromate film is formed by the trivalent chromate treatment.   The method for forming a topcoat film according to any one of claims 1 to 4, wherein the object to be coated is at least one of a bolt and a nut.

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