JP2007229630A - White coated steel sheet having high reflectance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a white coated steel sheet which is manufactured by 2-coat 2-bake coating of base coating and top coating and in which the coating defects caused by bumping evaporation of coating are prevented, and whose L-value is very high. <P>SOLUTION: In the white coated steel sheet, the film thickness of the top coating is larger than 20 μm and the film thickness of the whole coating is ≤30 μm. The top coating film containing 1-5 mass% silica having 1-10 μm average particle size in addition to titania (TiO<SB>2</SB>) of a pigment. Even when the thickness of the top coating film is increased to about 40 μm, occurrence of the coating defects due to the foaming is prevented. It is preferable that the kind of a resin for the film is a polyester based resin and the content of titania is 50-65 mass%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a white coated steel sheet having a high reflectivity capable of thick-coating a top coating without causing a coating defect, and is useful for improving the efficiency of a lighting fixture.

  Conventionally, there is a need for coated steel sheets with high light reflectivity, and when such coated steel sheets are used as reflectors or covers for proof instruments, the efficiency of lighting equipment can be improved, which is effective for energy saving. Attention has been paid. Such a coated steel sheet is painted white because the light reflectivity is highest in white.

  Japanese Patent Application Laid-Open No. 2001-243819 proposes a coated steel sheet having a light reflectivity with an L value of more than 90 according to JIS Z8722 and a glossiness of more than 80 according to the 60-degree specular gloss method and excellent workability. .

Japanese Patent Application Laid-Open No. 2003-236981 proposes that the reflectivity of the coating film can be improved by controlling the shape (swell) of the coating film surface.
JP 2003-236981 A Japanese Patent Laid-Open No. 2001-243819

In a white coated steel sheet having high light reflectivity, titania (TiO ₂ ) is used as a white pigment. In that case, if the density | concentration of the white pigment (titania) in a coating film is the same, a coating film will become white and the reflectivity will become high, so that a coating film is thick.

  On the other hand, if the thickness of the coating film is the same, the higher the white pigment (titania) concentration in the coating film, the higher the whiteness (reflectivity) of the coating film. However, in practice, the whiteness is improved until the pigment concentration reaches about 50 to 65% by mass, but even if the pigment concentration is increased further, the whiteness is not improved. This is thought to be because the light beam does not reflect effectively or the uniform dispersion of the pigment becomes difficult and the apparent particle size increases.

  From the above, in order to obtain a white coated steel sheet with high whiteness (reflectivity), it is concluded that the pigment concentration in the coating film should be 50 to 65% by mass and the thickness thereof should be increased. However, when a thick film coating is performed using a white paint containing titania so as to form a thick coating film by a single coating, a coating defect caused by a phenomenon called “Waki” occurs.

  Waki is a phenomenon in which when a painted paint is dried in an oven, the solvent contained in the paint does not escape smoothly from the coating film that has started to harden, but evaporates suddenly. As shown in FIG. 1, when the coating film thickness is increased and exceeds a certain value, a crack generation region occurs, and a coating defect (burst) due to the crack occurs.

  The armpit is a phenomenon commonly seen regardless of the type of pigment or resin when the coating film thickness is increased, but in the case of a white coated steel plate, the hiding power of the pigment is relatively small. To increase the thickness, it is necessary to make the coating thickness very large. For this reason, there was a situation that the armpit was particularly likely to occur.

  In order to prevent this wrinkle, in the production of the conventional white coated steel sheet, the amount of the top coating applied at one time is kept low, and the coating is divided into several times (that is, once or twice between the undercoating and the top coating). It was necessary to perform intermediate coating), and it took time and effort to paint, which was a factor in increasing the cost of white-coated steel sheets. In Patent Documents 1 and 2 described above, no measures are taken to prevent the occurrence of cracks. Therefore, the coating film thickness of the top coat film is suppressed to a relatively small thickness, and sufficient whiteness is not obtained.

  The present invention makes it possible to apply a thick film at one time without generating any cracks, so that a white coated steel sheet having a high reflectance (whiteness) can be produced by simply applying an undercoat and an overcoat without intermediate coating. It is an object of the present invention to provide a technology that can be manufactured while suppressing the above.

  In the case of a white coating film containing 50 to 65% by mass of titania, the L value is 93 or more white unless the coating film thickness is 30 μm or more. Here, white means that the absolute value of both the a value and the b value is 3 or less.

  If the coating film thickness is further increased, the L value is further increased. For example, when the coating film thickness is 40 μm, the L value is around 95, and at 50 μm, the L value exceeds 96. However, if an attempt is made to obtain such a film thickness only by the undercoating and the overcoating, the film thickness of the overcoating must be increased, and a crack occurs. For this reason, it has been difficult to produce a white coated steel sheet having an L value higher than 95 in the past because of the occurrence of cracks in the two-coat two-bake coating with only the undercoat and the topcoat.

  According to the present invention, in addition to the white pigment (usually titania), the top coat film contains colorless inorganic particles having a larger particle size than the white pigment such as silica, thereby reducing the thickness of the region where the crack occurs. It can be shifted to a higher film thickness side. As a result, the maximum coating film thickness of the top coating film that does not generate any cracks is larger than before, and it is possible to produce a white coated steel sheet having high reflectivity at a low cost by 2-coat 2-bake coating.

  Here, the present invention is a white coated steel sheet having a top coating film thickness of more than 20 μm and a total coating film thickness of 30 μm or more, wherein the top coating film has an average particle diameter of 1 to 10 μm in addition to the white pigment. The white-coated steel sheet is characterized in that it contains colorless inorganic particles, thereby eliminating or suppressing the occurrence of cracks in the top coat film.

  In another aspect, the present invention provides a method for producing a white coated steel sheet by coating a steel sheet with a base paint and a top paint each containing a white pigment, wherein the top paint has an average particle size of 1 in addition to the white pigment. A method for producing a white-coated steel sheet, comprising 10 to 10 μm of colorless inorganic particles, thereby increasing a minimum coating film thickness that can be applied without generating a crack.

  The reason why the total coating film thickness is set to 30 μm or more in the white coated steel sheet of the present invention is that the coating film thickness is necessary for the white coated steel sheet having an L value of 93 or more as described above. In general, since the thickness of the undercoat coating is as thin as 10 μm or less, in order to obtain a total coating thickness of 30 μm or more by two coats and two bake coating of the undercoat and the top coat, it is necessary to make the top coat thickness greater than 20 μm. However, when the coating film thickness of the top coat film is larger than 20 μm, it becomes easy to generate a crack as illustrated later.

  As shown in FIG. 2, when a thick film is applied by a conventional method, when the solvent in the paint volatilizes when the coating film is dried and seized, small bubbles generated by evaporation of the solvent gather. Since it becomes a large bubble and it becomes difficult to come off from the coating film, a phenomenon called “Waki” occurs, and a coating defect resulting therefrom occurs. This coating defect causes irregular depressions (bumps) that can be seen with the naked eye on the surface of the coating film, thereby reducing the smoothness of the surface and remarkably deteriorating the appearance of the coating. In addition, large bubbles remain as voids inside, which adversely affects the performance of the coating film. Therefore, since the coated steel sheet in which the coating defects are remarkably generated becomes a defective product and is discarded, the thickness of one coating is preferably limited to a range where no cracking occurs or a range where cracking is slight. There is a need.

  In the present invention, by adding colorless inorganic particles (e.g., inorganic oxides such as silica) having a significantly larger particle diameter than the white pigment to the top coat film, as shown in FIG. The small bubbles generated by the can move smoothly along the periphery of the inorganic particles, so they do not collect into large bubbles but move to the surface of the coating film while being dispersed in the small bubbles and diffused outside the coating film. The In particular, a large effect was obtained when the average particle diameter of the colorless inorganic particles was 1 to 10 μm. If the average particle diameter of the colorless inorganic particles is smaller than 1 μm, the colorless inorganic particles aggregate in the coating film, so that a uniform coating film cannot be obtained. When the average particle diameter of the colorless inorganic particles is larger than 10 μm, the processability and corrosion resistance of the coating film are adversely affected.

  As a result, the maximum coating film thickness that does not cause any cracking (in other words, the minimum coating film thickness in the area where the cracking occurs) is significantly larger than before, so the coating film thickness can be increased during top coating. By two-coat two-bake coating with only the undercoat and the topcoat, it is possible to obtain a thick-film white-coated steel sheet having a sufficiently high L value and excellent reflectivity. Since these inorganic particles are colorless, there is no adverse effect on the color of the white coated steel sheet.

  The white pigment is not shown in FIGS. 2 and 3 because the white pigment such as titania has a very small particle size of 0.2 to 0.3 μm. Such ultrafine particles, even if they are inorganic oxides, cannot perform the above-described colorless inorganic particles during drying of the coating film.

  According to the present invention, when the top coating material contains colorless inorganic particles having a remarkably large particle size as compared with the white pigment, the region where the crack occurs is remarkably shifted to the higher film thickness side, so that the film thickness of the top coating film is 20 μm. Even if it is made larger, the occurrence of cracks can be prevented completely, or even when the film thickness becomes very large, the cracks can be significantly reduced.

  In the conventional 2-coat 2-bake coating, the L value of the white coated steel sheet has been suppressed to about 94 at the maximum due to the limitation of the top coat film thickness to prevent the occurrence of cracks. On the other hand, in the present invention, the thickness of the top coating film where no crack is generated is increased by at least 10 μm, and for example, a 40 μm top coating film can be coated without generating a crack. Thereby, a white coated steel sheet having a high reflectivity with an L value of 95 or more can be produced by two coats and two bake. Therefore, according to the present invention, it is possible to produce a high-quality white coated steel sheet at a low cost.

The white coated steel sheet of the present invention has at least one layer of a white coating film, that is, a coating film colored with a white pigment, typically titania (TiO ₂ ), on at least one surface of the steel sheet. The coating film is usually two layers of an undercoating film and an overcoating film. In this case, a coated steel sheet can be produced by 2-coat 2-bake coating.

  As described above, the present invention has the effect that a white coated steel sheet having a very high L value can be obtained by this 2-coat 2-bake coating, and in order to take advantage of this effect, It is preferable to have only two layers, an undercoat film and a topcoat film. However, it is also possible to interpose one or more intermediate coating films between the undercoating film and the top coating film.

The base steel plate may be a bare steel plate (cold rolled steel plate or hot rolled steel plate) or a plated steel plate. An alloy steel plate such as a stainless steel plate can also be used. A preferable base material in terms of corrosion resistance is a zinc-based plated steel sheet including zinc plating and zinc alloy plating. The plating method may be any one of hot dipping, electroplating, and vapor phase plating. Examples of the zinc alloy plating include, but are not limited to, Zn—Ni alloy plating, Zn—Al alloy plating (Al content 5 or 55 mass%), and Zn—Fe alloy plating. From the viewpoint of cost, a hot dip galvanized steel sheet is preferred. The plating coverage may be in the normal range, but will be in the range of 20-100 g / m ² per side, for example.

  The base steel plate may have been subjected to an appropriate base treatment before painting. When the base material is a galvanized steel sheet, the conventional typical base treatment was chromate treatment. However, because of the harmfulness of hexavalent chromium, chemical conversion treatment that does not use hexavalent chromium can be used for the base treatment. preferable. For example, a zinc phosphate treatment, a chemical conversion treatment containing trivalent chromium, or various chromium-free non-chromium base treatments using silica, silicate, phosphate, or the like can be used.

  The resin type of the coating film is not particularly limited, and a polyurethane resin, a melamine resin, and particularly an epoxy resin can be used for the undercoat, but at least the resin type of the top coat film may be a polyester resin. preferable. More preferably, the resin type of all the coating films is a polyester resin. This is because the polyester resin coating film is balanced in various physical properties such as processability and strength.

  This polyester-based resin is a bake-curing type polyester-based resin commonly used in paints, and is obtained by blending a small amount of a curing agent with a polyester resin. As the curing agent, melamine resin, blocked isocyanate and the like can be used. The blending amount of the curing agent is suitably about 20 to 35 parts by mass with respect to 100 parts by mass of the polyester resin.

  The coating film thickness of the undercoat film is usually 10 μm or less, but it can also be made thicker (eg, 20 μm or less, in some cases more than 20 μm). The resin type of the undercoating film is preferably the same as that of the overcoating film. For example, a resin having high adhesion to a steel plate such as an epoxy resin and having a high crosslinking density can also be used. The undercoating film can contain other components such as a rust preventive pigment in addition to the white pigment, preferably titania. However, if the coating film thickness of the top coat film is 30 μm or more, it is not always necessary to include a white pigment in the undercoat film.

  The top coat film contains, in addition to a white pigment (preferably titania), colorless inorganic particles having an average particle diameter of 1 to 10 μm, preferably inorganic oxide particles. As a result, the thickness of the coating film on which the armpit is generated can be shifted to the side of the high coating film thickness, and a thicker top coat film can be applied without generating the armpit. A small amount of other additive components may be further contained.

  As the colorless inorganic particles, inorganic oxide particles are preferable, and silica is particularly preferable. In the present invention, silica means powdery amorphous silica, that is, silica glass powder. Silica is inexpensive and hardly causes hue shift (change in hue) when it is present in a coating film together with titania. Silica also exhibits a gloss adjusting action by forming fine irregularities on the top coat film. Examples of materials other than silica that can be used as colorless inorganic particles in the present invention include colorless metal oxides such as alumina and zinc oxide that do not exhibit specific absorption in visible light.

  The content of the colorless inorganic particles in the top coat film may be set so that the shift to the high coating film thickness side of the armature generation region, which is the object of the present invention, is achieved. When the colorless inorganic particles are silica, the content of silica in the top coat film is preferably 1 to 5% by mass, more preferably 2 to 4% by mass. If the content of the colorless inorganic particles is too small, the effect is hardly obtained, and if it is too large, the workability of the coated steel sheet is adversely affected.

  The film thickness of the top coating film is greater than 20 μm, and the total coating film thickness of all the coating films is 30 μm or more. When the film thickness of the top coat film is 20 μm or less, the occurrence of cracks is often not observed even if colorless inorganic particles are not included according to the present invention. The upper limit of the film thickness of the top coat film may be set so that the occurrence of cracks does not occur remarkably, and this varies depending on the content of colorless inorganic particles and white pigment in the film.

  When the white pigment in the top coat film is titania and the content thereof is 50 to 60% by mass where the whiteness of the film reaches the upper limit, when the coating film does not contain silica, the film thickness is 20 μm. If it exceeds the limit, a crack will be generated. If the film thickness is 30 μm, the crack will become noticeable and the commercial value will be significantly impaired. On the other hand, when silica is contained in the top coat film according to the present invention, when the silica content is 3% by mass, the occurrence of cracks does not occur until the film thickness is 40 μm, and the cracks are slight even when the film thickness is 45 μm. It is after the film thickness reaches 50 μm that the cracks become noticeable. Therefore, the film thickness in this case is 45 μm or less, preferably 25 to 40 μm or less. If the content of the colorless inorganic particles (silica) is large, the film thickness can be further increased. Conversely, if the content is small, the film thickness needs to be further reduced.

  The undercoating film is generally thinner than the topcoating film, and the volatilization of the solvent is relatively easy. Even if a defect occurs on the surface, the undercoating film is covered with the topcoating film. Therefore, it is not necessary for the undercoat coating film to contain the colorless inorganic particles.

The content of the white pigment in the top coat film is preferably set to an amount that maximizes the whiteness of the coat film. Therefore, in the case of titania (TiO ₂ ), which is a preferred white pigment, the content in the top coat film is preferably 50% by mass or more. If the amount is too large, no further increase in the L value can be obtained, so the preferred content of titania is 50 to 65% by mass. About undercoat coating film, in order to ensure coating-film adhesiveness, it is good also considering content of titania as a smaller quantity (for example, 30-50 mass%).

  The coated steel sheet of the present invention can be produced by painting and baking according to a conventional method. Normally, a 2-coat 2-bake coating method is employed, but depending on the resin type, it is possible to omit the dry baking after the undercoat and make 2-coat 1-bake. Of course, when performing the intermediate coating, the painting process increases. The baking conditions may be set according to the resin type and the curing agent. The coating method is not particularly limited, but industrially, it is usually performed by roll coating.

A hot-dip galvanized steel sheet (Zn adhesion amount on one side: 60 g / m ² , steel sheet thickness: 0.5 mm) was used as a base material, and silica-based coating-type non-chromium base treatment (manufactured by Nippon Paint Co., Ltd.) was performed on both sides of the base material. Surf coat EC2000) was applied so that the adhesion amount was 300 mg / m ² .

  On one side of the base material subjected to the ground treatment, a baking-type polyester-based paint containing 53% by mass of titania based on the total solid content (KP color 8602 manufactured by Kansai Paint Co., Ltd.) is applied with a bar coater as an undercoat, and 200 ° C. ( The coating was dried by baking with PMT). The formed undercoat coating film thickness was fixed at 5 μm in all cases.

  Next, a baking type high molecular weight polyester-based paint (KP Color 1510, manufactured by Kansai Paint Co., Ltd.) containing 59% by mass of titania based on the total solid content was applied as a top coat with a bar coater on the undercoat. (PMT) was baked and dried to prepare a white coated steel sheet.

  To the top coat used, silica having an average particle size of 5 μm as colorless inorganic particles is added while changing at a pitch of 0.5% by mass within a range of 0 to 5% by mass based on the total solid content in the paint. The relationship between the processability of the coating film and the coating defects (defects generated by the armpit) was investigated. Moreover, about the coating film thickness of top coat film, the relationship with a coating external appearance (boiled condition) was investigated, changing by the pitch of 5 micrometers in the range of 15 micrometers-50 micrometers.

As for the appearance of paint, the appearance of the white coated steel sheet was visually observed and the number of coating defects (spots) generated by the armpit was counted. evaluated:
5: Good overall, no paint defects due to armpits;
4: 1 to 2 bumps are observed in the sample;
3: 3-10 bumps are observed in the sample;
2: 11 or more spots are observed in the sample;
1: Spots are observed on almost the entire surface.

  Regarding the workability, the sample of the white coated steel sheet obtained was subjected to 2T folding using a vise at room temperature (20 ° C), and the bent portion was observed with a 10-fold loupe. It was evaluated in five stages.

5: No cracks are seen;
4: Cracks are observed in 1 to 2 places;
3: Partial cracks are observed at three or more locations;
2: Small cracks are observed on the entire surface;
1: Large cracks are observed on the entire surface.

  The test results are shown in Tables 1 to 3 and FIGS. In addition, the number of samples was 2 for both the coating appearance and workability, and the results were displayed as average values. In any case, the score 5 is the best, the score 4 is acceptable, but the score of 3 or less is an unacceptable level as a product.

  Table 1 and FIG. 4 show the occurrence of top coating film thickness and coating defects (wax) when the silica addition amount to the top coating film is 0 mass% (comparative example) and 3 mass% (invention example). The relationship is shown. Table 2 and FIG. 5 show the relationship between the amount of silica added and the coating defect (wax) when the top coat film thickness is 30 μm and 40 μm. Table 3 and FIG. 6 show the relationship between the amount of silica added and processability when the top coat film thickness is 30 μm and 40 μm.

  As can be seen from Table 1 and FIG. 4, when the top coating film does not contain silica, which is a colorless inorganic particle, the top coating film begins to generate cracks when the coating film thickness is 25 μm or more, and the coating film thickness is 30 μm or more. Then, the occurrence of cracks is slightly conspicuous (score 4). In particular, the crack becomes more significant as the commercial value is lost at 40 μm or more (score 2 or less).

  On the other hand, when silica is contained in the top coat film in an amount of 3% by mass according to the present invention, no cracking occurs until the coating film thickness is 40 μm (rating 5), but cracking begins to occur at 45 μm. The rating is 4 up to a film thickness of 50 μm, and the occurrence of cracking is remarkable when the film thickness is 55 μm or more. Therefore, compared with the case where no silica is contained, the film thickness at which no cracking occurs can be increased by about 20 μm.

  From Table 2 and FIG. 5, when the coating film thickness of the top coat film is 30 μm, the occurrence of cracks can be completely prevented when the silica content is 1% by mass or more. In the case of 40 μm, it can be seen that the occurrence of cracking can be suppressed when the silica content is 2% by mass or more, and it can be completely prevented when the content is 3% by mass or more.

  From Table 3 and FIG. 6, the workability decreases when the amount of silica added to the top coat film increases, but if the silica addition amount is 5% by mass or less, even when the coating film thickness of the top coat film is 40 μm. Workability is within an acceptable range. When the film thickness of the top coat film is further increased, necessary workability can be ensured by reducing the silica content to, for example, 4% by mass or less.

In addition, when the titania content of the undercoat film and the topcoat film and the coating film thickness of the undercoat film are the conditions described in this example, the L value of the coated steel sheet according to the coating film thickness of the topcoat film is as follows. Yes (in parentheses are L values):
Coating film thickness of top coat film: 20 μm (90.5), 25 μm (93.0), 30 μm (93.9), 35 μm (94.2), 40 μm (95.2), 45 μm (96.3), 50 μm (96.5).

  Therefore, according to the present invention, a white coated steel sheet with an optimum performance for a reflector or cover of a lighting fixture, which has excellent reflectivity (whiteness) with an L value of 95 or more without generating any cracks, is primed. And it can be manufactured efficiently and at low cost only by overcoating.

Explanatory drawing which shows typically the relationship between the coating film thickness of a white coated steel plate, and whiteness (L value). Explanatory drawing which shows the presumed mechanism in which the coating defect resulting from an armpit occurs during the drying of the top coat film of the conventional white coated steel plate. Explanatory drawing which shows the presumed mechanism by which generation | occurrence | production of a crack is suppressed by making a top coat film contain a colorless inorganic particle in this invention. The graph which shows the relationship between the coating film thickness (coating film thickness) of a top coat film, and the generation | occurrence | production condition of the coating appearance (boil generation | occurrence | production condition). The graph which shows the relationship between the addition amount of silica to a top coat film, and the coating external appearance (situation | occurrence | production condition of a crack). The graph which shows the relationship between the silica addition amount to top coat film, and workability.

Claims (11)

  A white coated steel sheet having a top coating film thickness of more than 20 μm and a total coating film thickness of 30 μm or more, wherein the top coating film contains colorless inorganic particles having an average particle diameter of 1 to 10 μm in addition to the white pigment. A white coated steel sheet characterized by eliminating or suppressing the occurrence of cracks in the top coat film.   The white coated steel sheet according to claim 1, wherein at least the resin type of the top coat film is a polyester resin.   The white coated steel sheet according to claim 1 or 2, wherein the colorless inorganic particles are silica, and the content of silica in the top coat film is 1 to 5% by mass.   The white coated steel sheet according to any one of claims 1 to 3, wherein the coating film comprises two layers of an undercoat and an overcoat.   The white coated steel plate according to any one of claims 1 to 4, wherein the white pigment is titania.   The white coated steel sheet according to claim 5, wherein the titania content in the top coat film is 50 to 65 mass%.   In the method of producing a white coated steel sheet by coating a steel sheet with a base coat and a top coat, the top coat contains colorless inorganic particles having an average particle diameter of 1 to 10 μm in addition to the white pigment, thereby A method for producing a white coated steel sheet, characterized by increasing the minimum paint film thickness that can be painted without causing it to occur.   The method according to claim 7, wherein the coating thickness of the top coating is greater than 20 μm and the total coating thickness is 30 μm or more in a dry film thickness.   The method of Claim 8 that the coating thickness of top coat is 30 micrometers or more.   The method according to any one of claims 7 to 9, wherein the colorless inorganic particles are silica, and the content of silica based on the total solid content of the top coat is 1 to 5% by mass.   The method according to any one of claims 7 to 10, wherein the white pigment contained in the top coating material is titania, and the content thereof is 50 to 65% by mass based on the total solid content of the top coating material.

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