Paint and method for painting resin molded products using said paint

Abstract

To prevent a recess 6 from being formed at a position corresponding to a pinhole 4 of a resin molding 1 on a surface of a top coating film 3.SOLUTION: A coating method of a molding comprises the steps of: applying an undercoating material to a resin molding 1 to form an undercoating film 2; applying a topcoating material to a surface of the undercoating film 2 to form a topcoating film 3; heating and curing the topcoating film 3. A coating material containing a base resin with a volume expansion coefficient larger than that of the resin molding 1 and granular fillers with a volume expansion coefficient smaller than that of the base resin and a number average particle diameter of 2 μm or more to 12 μm or less is used as the undercoating material.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a coating material and a method for coating a resin molded product with said coating material. [Background technology]

[0002] It is known that resin molded products formed by injection molding, press molding, internal pressure molding, etc. can develop pinholes on the surface due to air mixed in with the molding material. In particular, CFRP (carbon fiber reinforced plastic) molded products are prone to pinholes due to air between the prepreg layers.

[0003] Patent Document 1 describes how, in the internal pressure molding of FRP hollow molded products, prepregs for forming the surface layer are laid on the inner surfaces of the upper and lower molds, an internal pressure bag wrapped in prepregs is placed between them, and the internal pressure bag is pressurized with air, thereby giving the molded product a beautiful appearance.

[0004] Furthermore, as described in Patent Document 2, it is common practice to apply a base coat (e.g., a primer surfacer) to the surface of the FRP and then apply a top coat on top of that to improve the paint finish. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2008-246675 A [Patent Document 2] Japanese Patent Application Publication No. 02-071877 Summary of the Invention [Problem to be solved by the invention]

[0006] However, even if the surface of the base coating is polished and then topcoated, when the surface of the topcoat is observed, depressions may be found in areas corresponding to pinholes in the resin molded product, and there is a limit to how much the paint appearance of the resin molded product can be improved.

[0007] Therefore, an object of the present invention is to improve the paint appearance of a resin molded product. [Means for solving the problem]

[0008] The present inventors have investigated depressions on the surface of topcoat coating films and have obtained the following findings, which have led to the completion of the present invention. (1) When the surface of the base coating is polished smooth and then a topcoat is applied, the surface of the topcoat must be smooth. (2) However, when the topcoat coating is heated and cured, dents appear on the surface. (3) Dimples on the surface of the topcoat film occur at the locations where pinholes were present on the surface of the resin molded product. In particular, even if the surface of the base coat is smooth, such dents occur on the surface of the topcoat film. (4) The dent occurs when the thermal expansion coefficient of the base coating film is greater than that of the resin molded product. (5) The area of ​​the base coating where the pinholes in the resin molded product have been filled has a locally increased volume. (6) When the topcoat is heated to harden, the area where the volume of the base coat has increased is locally raised because the amount of thermal expansion is greater than that of the surrounding area. On the other hand, because the topcoat is uncured, the protrusion in the corresponding area of ​​the topcoat surface is small (almost nonexistent), and as a result, the thickness of the film in that area is thinner than the surrounding area. (7) After the topcoat film is hardened by heating, the raised areas of the base coat shrink as it cools, and the raised areas disappear. As a result, the thinner areas of the topcoat film sink, causing the above-mentioned depressions.

[0009] In order to solve the above problems, the present invention aims to reduce the difference between the volume expansion coefficient of the undercoat coating film and the volume expansion coefficient of the resin molded product.

[0010] The coating material disclosed herein is a coating material used for surface treatment of a resin molded product, Contains a base resin and a filler, The base resin is a urethane resin or an epoxy resin, The filler is a granular filler having a number average particle size of 2 μm or more and 12 μm or less, and has a volume expansion coefficient smaller than that of the base resin, The volume expansion coefficient of the above filler is 1.5×10 -5 / K or more 9.0×10 -5 / K or less, The filler content is 30% by volume or more and 45% by volume or less, The contact area ratio of the filler to the resin molded product is 10% or more and 35% or less. It is characterized by:

[0011] With such a paint, since the volume expansion coefficient of the filler is smaller than that of the base resin, even if the volume expansion coefficient of the base resin itself is larger than that of the resin molded product, it is possible to form an undercoat film on the surface of the resin molded product whose volume expansion coefficient difference with that of the resin molded product is small. Therefore, even if there is a pinhole on the surface of the resin molded product, the surface of the undercoat film above the pinhole is prevented from being significantly raised, and therefore the occurrence of a dent on the surface of the topcoat film is suppressed.

[0012] Here, if the contact area between the filler of the base coating film and the resin molded product becomes large, in other words, if the adhesive area between the resin part of the base coating film and the resin molded product becomes small, the adhesion between the base coating film and the resin molded product will be correspondingly reduced.

[0013] On the other hand, since the filler is granular, even if a part of the filler comes into contact with the surface of the resin molded product, the contact area between the filler and the resin molded product does not increase, unlike when the filler is flat. Also, if the filler content is the same, the smaller the particle size of the filler, the larger the contact area. However, since the number-average particle size of the filler is 2 μm or more, the increase in the contact area is also suppressed in terms of the particle size. Therefore, even if the filler content is increased to adjust the volume expansion rate, it is easy to ensure the adhesion between the base coating film and the resin molded product.

[0014] In order to ensure the above-mentioned adhesive force (prevention of an increase in the contact area) and to ensure the fluidity (paintability) of the base coating, it is preferable to employ a spherical filler as the filler.

[0015] In addition, when a force is applied to peel the base coating from the resin molded product, stress is concentrated on the edge of the filler in contact with the resin molded product, and the smaller the particle size of the filler, the greater the stress concentration. In contrast, the filler has a number-average particle size of 2 μm or more, which suppresses stress concentration and is therefore advantageous in ensuring the adhesion of the base coating to the resin molded product. However, since the larger the particle size of the filler, the lower the fluidity of the paint, it is preferable that the number-average particle size of the filler is 12 μm or less from the viewpoint of ensuring paintability.

[0016] As the number-average particle size, for example, the filler can be observed with a scanning electron microscope, the particle sizes of a plurality of particles (for example, 100 particles) are measured, and the average value thereof can be adopted.

[0017] The use of the resin molded product is not particularly limited, and examples thereof include transportation machinery and equipment, conveying machinery and equipment, office equipment, etc. The resin molded product is not limited to a fiber-reinforced resin molded product such as CFRP, and may be a resin molded product that is not reinforced with fibers.

[0018] In one embodiment, the volume expansion coefficient of the filler is half or less than that of the base resin, which makes it easy to keep the volume expansion coefficient of the undercoat low without excessively increasing the filler content.

[0019] The above configuration In the present invention, the filler content (volume concentration) in the base coating is 30% by volume or more and 45% by volume or less, which makes it easy to bring the volume expansion coefficient of the base coating close to that of the resin molded product.

[0020] The material of the filler is not particularly limited, but for example, alumina filler, silica filler, calcium carbonate filler, glass beads, etc. can be preferably used.

[0021] The above configuration In the above, the base resin of the paint is a urethane resin or an epoxy resin.

[0022] The coating method for a resin molded product disclosed herein comprises the steps of: A step of applying a base paint to a resin molded product to form a base coating film; A step of curing the base coating film; A step of applying a topcoat paint to the surface of the base coating film to form a topcoat coating film; and a step of heating and curing the topcoat coating film. The undercoat paint contains a base resin having a larger volume expansion coefficient than the resin molded product and a filler having a smaller volume expansion coefficient than the base resin, the filler being a granular filler having a number-average particle size of 2 μm or more and 12 μm or less. It is something like the matrix resin of the resin molded product is an epoxy resin, and the base resin of the undercoat paint is a urethane resin; The volume expansion coefficient of the above filler is 1.5×10 -5 / K or more 9.0×10 -5 / K or less, The filler content in the base coating is 30% by volume or more and 45% by volume or less, The contact area ratio of the filler to the resin molded product is 10% or more and 35% or less. It is characterized by:

[0023] According to this, the volume expansion coefficient of the base resin of the primer is larger than that of the resin molded product, but since the primer contains a filler whose volume expansion coefficient is smaller than that of the base resin, the volume expansion coefficient of the primer coating film obtained by the primer can be kept low. In other words, the volume expansion coefficient of the primer coating film can be made close to that of the resin molded product.

[0024] Therefore, even if there is a pinhole on the surface of a resin molded product, when the topcoat is heated for curing, the portion of the basecoat corresponding to the pinhole is prevented from being significantly raised by thermal expansion, and the portion of the topcoat corresponding to the pinhole is prevented from becoming thin, which prevents the surface of the topcoat from being dented after cooling.

[0025] The filler is a granular filler having a number-average particle size of 2 μm to 12 μm. This prevents the contact area of ​​the filler with the resin molded product from increasing, the stress concentration from increasing when a force is applied to the base coating film to peel it off from the resin molded product, and the fluidity of the base coating from decreasing. This makes it easy to ensure the adhesion of the base coating film to the resin molded product without decreasing the paintability.

[0026] In one embodiment, the resin molded product is a long fiber reinforced resin molded product. In the case of a long fiber reinforced resin molded product, the amount of air mixed into the molding material increases, and relatively large pinholes are likely to occur. Even in such a case, by using a granular filler having a number average particle size of 2 μm to 12 μm and a volume expansion coefficient smaller than that of the above-mentioned base resin as a filler for the base paint, it is possible to suppress the occurrence of dents on the surface of the top coat film due to pinholes in the resin molded product while ensuring the paintability of the base paint and the adhesion of the base coat film to the resin molded product.

[0027] In one embodiment, a step of polishing the surface of the base coating film is provided between the step of curing the base coating film and the step of forming the top coating film. This makes it easy to ensure the surface smoothness of the top coating film. In other words, by adjusting the volume expansion rate of the base coating film with the filler, it is possible to suppress the occurrence of dents on the surface of the top coating film due to pinholes in the resin molded product, so that the effect of smoothing the surface of the base coating film by polishing it can be utilized to improve the surface smoothness of the top coating film.

[0028] In one embodiment, the volume expansion coefficient of the filler is half or less than that of the base resin, which makes it easy to keep the volume expansion coefficient of the undercoat low without excessively increasing the filler content.

[0029] In one embodiment, the filler is a spherical filler, which is advantageous in ensuring the adhesion of the base coating film to the resin molded product (preventing an increase in the contact area) and in ensuring the fluidity (paintability) of the base coating.

[0030] The above configuration In the present invention, the filler content is 30% by volume or more and 45% by volume or less, which makes it easy to bring the volume expansion coefficient of the undercoat film close to the volume expansion coefficient of the resin molded product.

[0031] The above configuration In the above, the matrix resin of the resin molded product is an epoxy resin, and the base resin of the undercoat is a urethane resin. The urethane resin has a larger volume expansion coefficient than the epoxy resin, so that the above-mentioned dents are likely to occur on the surface of the topcoat film. However, the occurrence of the dents can be suppressed by blending the above-mentioned filler in the undercoat.

[0032] The present invention is applicable even if the matrix resin of the resin molded product and the base resin of the undercoat paint are the same, because even if the types of resins are the same, a difference in thermal expansion coefficient may occur between the resin molded product and the undercoat paint depending on the types and amounts of compounding agents and reinforcing fibers in the resin molded product.

[0033] The material of the filler is not particularly limited, but for example, alumina filler, silica filler, calcium carbonate filler, glass beads, etc. can be preferably used.

[0034] In the coating method, the use of the resin molded product is not particularly limited, and examples thereof include transportation or conveyance machinery and equipment and office equipment. The resin molded product is not limited to a fiber-reinforced resin molded product such as CFRP, and may be a resin molded product that is not reinforced with fibers. Effect of the Invention

[0035] According to the present invention, it is possible to suppress the occurrence of dents on the surface of the topcoat coating film due to pinholes in the resin molded product while ensuring the coatability of the base paint and the adhesion of the base coating film to the resin molded product. [Brief description of the drawings]

[0036] [Figure 1] Cross-sectional view of a painted resin molded product. [Diagram 2] FIG. 4 is a cross-sectional view showing a state in which a base coating film has been formed on a resin molded product. [Diagram 3] 1 is a cross-sectional view showing a state in which a base coating film and a top coating film have been formed on a resin molded product, and showing the protrusion of the surface of the base coating film. [Figure 4] 1 is a cross-sectional view showing a state in which a base coat film and a top coat film have been formed on a resin molded product, and a depression in the top coat film. [Diagram 5] Graph showing the change over time in the surface height of the base coating film (with a large volume expansion coefficient) and the top coating film above a pinhole when the top coating film is heated and cured. [Figure 6] Graph showing the change over time in the surface height of the base coating film (with a small volume expansion coefficient) and the top coating film above a pinhole when the top coating film is heated and cured. [Figure 7] FIG. 2 is a cross-sectional view of a resin molded product having a base coating film containing flat fillers. [Figure 8] FIG. 2 is a cross-sectional view of a resin molded product on which a base coating film containing spherical fillers is formed. [Figure 9] FIG. 2 is a graph showing the relationship between the contact area ratio of various filler materials to a resin molded product and the adhesion of the base coating film. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Hereinafter, the embodiments of the present invention will be described with reference to the drawings. The following description of the preferred embodiments is merely illustrative in nature and is not intended to limit the present invention, its application, or its uses.

[0038] As shown in FIG. 1, a base coat serving as a primer or a primer surfacer is applied to the surface of a resin molded product 1 to form a base coat film 2, and a top coat is applied on top of that to form a top coat film 3.

[0039] An example of the resin molded product 1 is a fiber-reinforced resin molded product with a thermosetting resin as a matrix. The reinforcing fibers are, for example, carbon fibers (long fibers). A specific example of the resin molded product (CFRP molded product) 1 is a molded product made by molding a layered fiber-reinforced resin material in which a plurality of prepreg layers are laminated. The prepreg layer is a sheet material containing a reinforcing fiber material and impregnated with a thermosetting resin as a matrix into the reinforcing fiber material. The reinforcing fiber material is in a state in which fiber bundles of carbon fibers are oriented in a predetermined direction by, for example, plain weaving. The resin molded product 1 may be a resin molded product that does not contain reinforcing fibers.

[0040] The resin molded product 1 can be used, for example, for vehicle parts, that is, engine parts such as an engine cover, and vehicle exterior materials such as a bonnet, a rear fender, a roof, a door, a front panel, a rear panel, and a lift gate.

[0041] <Base paint for resin molded products> The paint contains a base resin and a filler having a smaller volume expansion coefficient than the base resin, in order to adjust the volume expansion coefficient of the base coating film 2. A pigment for concealing the base color (the color of the resin molded product 1) may also be contained.

[0042] The base resin may be either a urethane-based resin or an epoxy-based resin.

[0043] There is no particular restriction on the material of the filler, so long as it has a smaller volume expansion coefficient than the base resin, and examples of the filler include alumina, silica, calcium carbonate, glass beads, and the like.

[0044] The volume expansion coefficients of the above-mentioned representative base resins and fillers, as well as the resin molded article 1, are as follows. It is preferable that the volume expansion coefficient of the filler is 1 / 2 or less, 1 / 5 or less, or even 1 / 10 or less of the volume expansion coefficient of the base resin, and is also preferably smaller than the volume expansion coefficient of the resin molded article 1.

[0045] Urethane resin; 30 x 10 -5 / K~60×10 -5 / K Epoxy resin; 13.5 x 10 -5 / K~25.5×10 -5 / K Alumina ;2.0×10 -5 / K~2.4×10 -5 / K Amorphous silica: 0.15 x 10 -5 / K Calcium carbonate; 7.5 x 10 -5 / K~9.0×10 -5 / K Glass beads; 1.5 x 10 -5 / K~2.7×10 -5 / K Resin molded product (CFRP molded product); 19.5×10 -5 / K(Example)

[0046] The filler must be granular, and preferably spherical, from the viewpoint of preventing an increase in the contact area with the resin molded article 1.

[0047] The filler has a number-average particle size of 2 μm or more and 12 μm or less from the viewpoints of preventing an increase in the contact area with the resin molded article 1, preventing stress concentration when a force is applied to peel the base coating film 2 from the resin molded article 1, and the paintability of the base coating.

[0048] The filler content in the base paint is preferably 30% by volume or more and 45% by volume or less from the viewpoint of reducing the volume expansion coefficient of the base coating film 2, i.e., bringing the volume expansion coefficient of the base coating film 2 closer to the volume expansion coefficient of the resin molded product 1, and from the viewpoint of the paintability of the base coating.

[0049] Furthermore, the above-mentioned undercoat coating may contain a leveling agent, a sagging prevention agent, a stabilizer, a conductivity imparting agent, etc., as required.

[0050] <How to paint plastic molded products> - Formation of base coating film - As shown in Fig. 2, the above-mentioned primer coating is applied to the surface of the resin molded product 1 that needs to be coated to form a primer coating film 2. The method for applying the primer coating is not particularly limited, but air spray coating, airless spray coating, brush coating, etc. can be used. The formation of this primer coating film 2 serves to fill pinholes 4 in the resin molded product 1.

[0051] The thickness (thickness after curing) of the base coating film 2 in flat portions of the resin molded product 1 other than the pinholes 4 may be, for example, approximately 10 μm or more and 100 μm or less, and it is preferable that it be 15 μm or more and 80 μm or less, and even more preferably 20 μm or more and 60 μm or less.

[0052] - Hardening of the base coating - After the base paint is applied, the base coating film 2 is cured by drying in the air at room temperature for, for example, about 20 to 60 minutes. The base coating film 2 can also be cured by heating.

[0053] -Polishing the base coating- The surface of the cured undercoat coating film 2 is polished as necessary. This polishing can be performed using sandpaper, buff cloth, etc., although there is no particular limitation. In addition, dry polishing or wet polishing such as water polishing may be used.

[0054] From the viewpoint of improving the surface smoothness of the base coating film 2, it is preferable that the film thickness of the base coating film 2 be 80% or more and 99% or less of the film thickness before polishing, and it is more preferable that the film thickness be 85% or more and 98% or less, and further preferably 90% or more and 97% or less.

[0055] - Formation of topcoat film - As shown in Figure 3, a topcoat film 3 is formed by topcoat painting (application of topcoat paint) on a hardened base coat film 2. There is no particular limitation on the method for applying the topcoat paint, but air spray painting, airless spray painting, brush painting, rotary atomization painting, etc. can be used. Various painting methods can be used for topcoat painting, such as two coats of a solid coat and a clear coat, or three coats of a metallic coat, a color clear coat, and a clear clear coat.

[0056] The thickness (thickness after curing) of the topcoat coating film 3 can be set so that the colored layer excluding the clear layer is, for example, from 10 μm to 40 μm, and the clear layer is, for example, from 20 μm to 50 μm.

[0057] - Heat curing of topcoat film - The resin molded article 1 on which the topcoat coating film 3 has been formed is carried into a heating furnace and held at a temperature of, for example, 100° C. to 130° C. for 10 minutes to 60 minutes, thereby hardening the topcoat coating film 3.

[0058] When the undercoat film 2 does not contain the above-mentioned filler, the volume expansion coefficient of the undercoat film 2 is the volume expansion coefficient of the base resin (30×10 in the case of a urethane resin). -5 / K~60×10 -5 / K), and the volume expansion coefficient of the resin molded product 1 (for example, 19.5×10 -5 / K). Therefore, when the topcoat coating film 3 is heated to cure, the area in the base coating film 2 where the pinhole 4 has been filled and the volume has increased will undergo significant thermal expansion, resulting in a protuberance 5, as shown by the dashed line in Figure 3. On the other hand, since the topcoat coating film 3 is uncured, there is almost no protuberance at the area corresponding to the pinhole 4. As a result, the thickness of the topcoat coating film 3 at that area will be thinner than the surrounding area.

[0059] Thereafter, the portion of the base coat 2 that caused the protuberance 5 shrinks as it cools, and the protuberance disappears. As a result, the portion of the top coat 3 that has become thinner sinks, forming a depression 6, as shown by the dotted line in Figure 4.

[0060] In contrast, in the case of this embodiment, since the base coating contains the above-mentioned filler, the volume expansion coefficient of the base coating film 2 is close to that of the resin molded product 1. Therefore, when the top coating film 3 is heated for hardening, the pinholes 4 in the base coating film 2 are filled and the volume is increased, and the occurrence of a bulge is suppressed. This prevents the film thickness of the top coating film 3 in the area corresponding to the pinhole from becoming thin, i.e., prevents the occurrence of a dent.

[0061] <Confirmation of the effect of filler in reducing dents> In order to confirm this effect, two types of test pieces with different volume expansion coefficients of the base coating were created using aluminum plates instead of resin molded products in the following manner.

[0062] A pseudo-pinhole (a truncated cone-shaped recess with a bottom diameter of 20 μm) with a diameter of 527 μm and a depth of 218 μm was formed on the smooth surface of an aluminum plate. The volume expansion coefficient of the aluminum plate was 7.5×10 -5 / K.

[0063] As the undercoat paint, a urethane-based resin paint containing no filler and a urethane-based resin paint containing a filler were prepared. Each undercoat paint was applied to the surface of the above-mentioned aluminum plate to form a undercoat film. The surface of each undercoat film was polished to be smooth, and a clear paint was applied thereon as a topcoat paint to form a topcoat film.

[0064] The volume expansion rate of the base coat film using a base coat that does not contain fillers is 40.5 x 10 -5 / K (The difference in the volume expansion coefficient Δα between the base coating and the aluminum plate is 33.0×10 -5 / K). The volume expansion rate of the base coat film due to the base coat containing filler is 27.0×10 -5 / K (The difference in the volume expansion coefficient between the base coating and the aluminum plate, Δα, is 19.5×10 -5 / K).

[0065] Both test pieces were heated from room temperature to 100°C, then slowly cooled, and the change over time in the surface height above the artificial pinholes of the undercoat coating and topcoat coating was measured.

[0066] Figure 5 shows the case without filler (Δα = 33.0 × 10 -5 / K), and Fig. 6 shows the test results for the case containing a filler (Δα = 19.5 × 10 -5 / K) test results are shown.

[0067] First, looking at the former (Fig. 5), which has a large difference in the coefficient of volume expansion, the surface of the base coat rises significantly as the temperature rises, and after rising to a height of about 6 μm, the surface sinks as the temperature slowly cools, and finally the amount of rise becomes zero. On the other hand, the surface of the top coat rises gently as the temperature rises, but the amount of rise is much less than that of the base coat. This means that the thickness of the top coat is thinner above the pseudo-pinhole. Then, as the surface is slowly cooled after that, the surface height of the top coat falls in the same manner as the surface of the base coat. In other words, the depression progresses, and eventually a depression of about 6 μm in depth, comparable to the above-mentioned height of the rise, appears on the surface of the top coat.

[0068] In contrast, in the latter case (Figure 6), where the difference in volumetric expansion coefficients is smaller, the surface of the base coat rises as the temperature rises, but the amount of rise is small. The top coat only shows a slight rise on its surface. Therefore, the top coat above the pseudo-pinhole is slightly thinner. With the subsequent gradual cooling, the surface of the base coat drops, and ultimately the amount of rise becomes zero. A decrease in the surface height of the top coat is also observed, but the final indentation of the top coat surface is small, at around 1 μm.

[0069] From the above, it can be seen that by making the volume expansion coefficient of the base coating film closer to the volume expansion coefficient of the resin molded product, dents in the top coating film can be suppressed.

[0070] In FIG. 3, assuming that the diameter of the pinhole 4 is constant in the depth direction, its depth is L (μm), the temperature rise during hardening of the topcoat coating is ΔT (K), and the difference in linear expansion coefficient between the basecoat coating and the resin molded product is Δγ, the protrusion height of the basecoat coating (depth of the recess in the topcoat coating) ΔL (μm) can be expressed as follows:

[0071] ΔL=3×(Δγ×L×ΔT) ……(1)

[0072] The reason why ΔL is set to three times (Δγ×L×ΔT) is because it is assumed that the resin inside the pinhole 4 expands only in the upward direction. In the example of Figure 5, L = 218 μm, ΔT = 80 K, Δγ = 11 × 10 -5 / K, the protuberance height (dent depth) ΔL is approximately 5.8 μm, a value close to the result in Figure 5. When a relatively large dent occurs on the surface of the topcoat film like this, it can be said that the ideas about the causes of dent occurrence explained in Figures 3 and 4 are directly applicable to the actual occurrence of dents.

[0073] <Adhesion of base coating to resin molded products> When flat filler is blended into the base coating to reduce the volume expansion rate of the base coating, as shown in Fig. 7, some of the flat filler 7 in the base coating 2 will lie flat and come into contact with the surface of the resin molded product 1, which tends to increase the contact area. In the area where the filler 7 comes into contact with the surface of the resin molded product 1, the adhesion at the interface between the base coating 2 and the resin molded product 1 will be poor. In other words, in the case of the flat filler 7, the contact area with the resin molded product 1 tends to be large, which reduces the adhesion of the base coating 2 to the resin molded product 1.

[0074] In contrast, as shown in Fig. 8, in the case of granular filler, particularly in the case of spherical filler 8, the contact area between the filler 8 and the resin molded article 1 is not large. Therefore, even if a relatively large amount of filler 8 is blended into the base coating film 2, a significant decrease in the adhesion of the base coating film 2 to the resin molded article 1 can be avoided. In other words, the adhesion of the base coating film 2 to the resin molded article 1 can be ensured.

[0075] In order to examine the effect of the shape of the filler contained in the undercoat film 2 on the size of the adhesion area of ​​the filler to the resin molded article 1, the following test was carried out.

[0076] The test method was to apply a primer to a PP plate to form a primer film, peel off the primer film, and perform SEM analysis of the contact surface of the primer film that was in contact with the PP plate. The SEM image was then binarized to determine the ratio (area ratio) of the filler contact surface to the total contact area of ​​the primer film. The base resin of the primer was an epoxy resin, and the filler content was 32.3% by volume. The test was performed using flat alumina with particle sizes of 2.0 μm, 5.0 μm, and 9.0 μm, and spherical alumina with particle sizes of 2.5 μm, 4.5 μm, and 10.5 μm as fillers. The results are shown in Table 1.

[0077] [Table 1]

[0078] According to Table 1, even if the filler content is the same, the contact area is smaller with spherical alumina than with flat alumina, and the contact area is smaller with larger particle size than with smaller particle size.

[0079] In addition, when a force is applied to peel the base coating film 2 from the resin molded product 1, stress is concentrated on the edge of the filler in contact with the resin molded product, but the spherical filler 8 has a smaller stress concentration than the flat filler 7, which is advantageous in ensuring the above-mentioned adhesion. Furthermore, the spherical filler 8 also prevents the fluidity of the base coating from decreasing.

[0080] In addition, the adhesion between the undercoat 2 and the resin molded product 1 was evaluated by a peeling test of the undercoat when flat alumina with particle sizes of 2.0 μm and 5.0 μm, spherical alumina with particle sizes of 2.5 μm, 4.5 μm, and 10.5 μm, and flat talc with particle sizes of 2.5 μm and 7 μm were used as fillers. The base resin of the undercoat is an epoxy resin. The results are shown in Figure 9. In the figure, the note "μm" for each plot indicates the particle size of the filler, and "Vol%" indicates the blending ratio of the filler in the undercoat. The vertical axis of Figure 9 indicates the grade of adhesion, with the higher the number, the better the adhesion.

[0081] Figure 9 shows that the smaller the contact area ratio of the filler to the resin molded product, the better the adhesion. This contact area ratio is preferably 35% or less in order to obtain adhesion of grade 3 or higher, which is practically acceptable. The lower limit of the contact area ratio is set at about 10%, as a certain amount of filler needs to be blended to adjust the volume expansion rate of the base coating film.

[0082] (Volume expansion rate and blending ratio of filler) Here, the volume expansion coefficient βall of the base coating film is expressed as follows, assuming that the volume of the base resin is Vp, the volume expansion coefficient is βp, the volume of the filler is Vf, the volume expansion coefficient is βf, and the volume of the base coating film is Vall.

[0083] βall=(Vp×βp+Vf×βf) / Vall ……(2)

[0084] If the filler content in the base coating is λ=Vf / Vall, then Vp / Vall=1-λ, and therefore equation (2) can be transformed into equation (3).

[0085] βall=βp×(1-λ)+βf×λ ……(3)

[0086] When the volume expansion coefficient βall of the base coating film is equal to the volume expansion coefficient βb of the resin molded product, the filler content λ is expressed by equation (4) when βall in equation (3) above is set to βb.

[0087] λ=(βp-βb) / (βp-βf) ……(4)

[0088] In other words, if the volume expansion coefficient βf and the blending ratio λ of the filler are set so as to satisfy equation (4), the volume expansion coefficient βall of the base coating film becomes equal to the volume expansion coefficient βb of the resin molded product, and even if there is a pinhole in the resin molded product, the occurrence of a dent in the surface of the top coating film is prevented.

[0089] From the consideration of the experimental results shown in Figures 5 and 6, the height of the raised area of ​​the base coat (the depth of the recess in the top coat) ΔL can be estimated by the formula (1); ΔL = 3 × (Δγ × L × ΔT). Here, Δγ is the difference in the linear expansion coefficient between the base coat and the resin molded product, so by using the difference in the volumetric expansion coefficient Δα, formula (1) can be expressed as follows:

[0090] ΔL=Δα×L×ΔT ……(5)

[0091] In order to suppress the dent depth ΔL of the topcoat film to about 1 μm, if the pinhole depth L is 218 μm and the temperature rise ΔT during heat curing of the topcoat film is 80 K, then from equation (5), Δα should be set to 6 × 10 -5 / K, and to keep ΔL to about 2 μm, Δα should be 12×10 -5 / K.

[0092] For example, βp=40×10 -5 / K, βb = 18 × 10 -5 / K, βf = 2.4 × 10 -5 / K, when the filler content is, for example, λ = 0.3, the volume expansion coefficient βall of the base coating is 28.72 × 10 -5 / K. Therefore, Δα = 10.72 × 10 -5 / K, the recess depth ΔL of the topcoat film is expected to be 2 μm or less.

[0093] If the filler content λ is 0.45, the volume expansion coefficient βall of the base coating is 23.08×10-5 / K, and Δα = 5.08×10 -5 / K, it is expected that the recess depth ΔL of the topcoat film will be kept to 1 μm or less.

[0094] From the viewpoint of suppressing dents in the topcoat coating, the filler should have a volume expansion coefficient βf of 1.5×10 -5 / K or more 9.0×10 -5 It is preferable to use a filler content γ of 0.3 or more and 0.45 or less (30% or more and 45% or less). [Explanation of symbols]

[0095] 1 Resin molded products 2. Base Coating 3 Topcoat 4 Pinhole 5 ridges 6. Dent

Claims

1. A paint used for surface treatment of a resin molded product, Contains a base resin and a filler, The base resin is a urethane resin or an epoxy resin, the filler is a granular filler having a number average particle size of 2 μm or more and 12 μm or less, and has a volume expansion coefficient smaller than that of the base resin; The volume expansion coefficient of the filler is 1.5×10 −5 / K or more and 9.0×10 −5 / K or less, The filler content is 30% by volume or more and 45% by volume or less, A paint characterized in that the contact area ratio of the filler to the resin molded article is 10% or more and 35% or less.

2. In claim 1, A coating material characterized in that the volume expansion coefficient of the filler is half or less than the volume expansion coefficient of the base resin.

3. In claim 1 or 2, A coating material characterized in that the filler is a spherical filler.

4. In any one of claims 1 to 3, The paint is characterized in that the filler is alumina.

5. A step of applying a base paint to a resin molded product to form a base coating film; A step of curing the base coating film; A step of applying a topcoat paint to the surface of the base coating film to form a topcoat coating film; and a step of heating and curing the topcoat coating film. The undercoat paint used herein contains a base resin having a volume expansion coefficient larger than that of the resin molded product and a filler having a volume expansion coefficient smaller than that of the base resin, the filler being a granular filler having a number-average particle diameter of 2 μm or more and 12 μm or less, the matrix resin of the resin molded product is an epoxy resin, and the base resin of the undercoat paint is a urethane resin; The volume expansion coefficient of the filler is 1.5×10 −5 / K or more and 9.0×10 −5 / K or less, The filler content in the base coating is 30% by volume or more and 45% by volume or less, A method for coating a molded product, characterized in that a contact area ratio of the filler to the resin molded product is 10% or more and 35% or less.

6. In claim 5, The method for coating a molded article, wherein the resin molded article is a long-fiber reinforced resin molded article.

7. In claim 5 or 6, A method for coating a molded product, comprising the step of polishing the surface of the base coating film between the step of curing the base coating film and the step of forming the top coating film.

8. In any one of claims 5 to 7, A method for coating a molded product, characterized in that the volume expansion coefficient of the filler is not more than half the volume expansion coefficient of the base resin.

9. In any one of claims 5 to 8, A method for coating a molded product, wherein the filler is a spherical filler.

Images