JP7567841B2 - Metal plate coating paint - Google Patents

Description

The present invention relates to a paint for application to metal plates. In particular, it relates to a paint for application to metal plates to obtain a lubricating film with excellent sliding properties during press forming, even during severe drawing.

Metal sheets such as cold-rolled steel sheets, hot-rolled steel sheets, stainless steel sheets, and aluminum sheets are widely used in a wide range of fields, primarily for automobile body applications, and in such applications, they are generally press-formed before use. In recent years, there has been a demand for the integration of parts to reduce processes and for improved design, making it necessary to enable more complex forming.

If more complex press forming were attempted, the metal sheet could break due to being unable to withstand the forming, or die galling could occur during continuous press forming, which could have a serious negative impact on automotive productivity.

One method for improving the press formability of metal sheets is to perform a surface treatment on the die. Although this method is widely used, it is not possible to adjust the die after the surface treatment. Another problem with this method is that it is expensive. Therefore, there is a strong demand for improving the press formability of the steel sheet itself.

One way to improve press formability without surface treatment of the mold is to use a high viscosity lubricant. However, this method can result in poor degreasing after press forming, which can lead to poor paintability.

As a result, various lubricating surface treatments are being investigated as a technology that allows press molding without the need for surface treatment of dies or high-viscosity lubricants.

Patent Document 1 describes a technology for forming a lubricating film on a zinc-plated steel sheet, the lubricating film being an acrylic resin film containing synthetic resin powder.

Patent document 2 describes a metal plate coated with a lubricating film in which a solid lubricant protrudes 0.01 to 1.5 μm from the surface of the resin film.

Patent document 3 describes a lubricated surface-treated metal product with excellent press formability, which is coated with a 0.5 to 5 μm film of polyurethane resin containing a lubricant.

Patent document 4 describes a technology for forming an alkali-soluble organic film on a steel sheet, in which a lubricant is added to an epoxy resin.

Japanese Patent Application Publication No. 9-170059 Japanese Patent Application Publication No. 10-52881 JP 2000-309747 A JP 2000-167981 A

However, in Patent Documents 1 to 4, although lubricity is achieved through the lubricating effect of the lubricant contained therein, press formability is not necessarily sufficient in complex molding.

The present invention was made in consideration of these circumstances, and aims to provide a paint for applying to metal plates that forms a film that has low sliding resistance in areas at risk of cracking during press forming and has excellent press formability in areas where the surface pressure is high and where mold galling is expected to occur, in metal plates that are subjected to complex forming that is difficult to press form.

The metal plate also needs to be rust-resistant when stored as a coil. Furthermore, when the metal plate is used as an automobile body, it needs to have sufficient film-removing properties in the alkaline degreasing step of the painting process, and it also needs to have excellent adhesion in the assembly process. Furthermore, the paint needs to be stable enough to keep the ingredients uniformly dispersed.

The present inventors have conducted extensive research to solve the above problems. As a result, they have discovered that the above problems can be solved by applying to a metal plate a coating material containing an acrylic resin having a glass transition point (Tg) of 100°C or higher and a ratio R of the acid value to the glass transition point (R = acid value (mg-KOH/g)/Tg (°C)) of 1.50 or higher, and a polyolefin wax having a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or less.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[1] A coating material for applying to metal sheets, comprising an acrylic resin having a glass transition point (Tg) of 100°C or higher and a ratio of acid value to glass transition point R = acid value (mg-KOH/g)/Tg (°C) of 1.50 or higher, and a polyolefin wax having a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or less.
[2] The coating material for applying to metal sheets according to [1], wherein the acid value of the acrylic resin is 180 mg-KOH/g or more and 350 mg-KOH/g or less.
[3] The coating material for applying to metal sheets according to [1] or [2], wherein the ratio R of the acid value to the glass transition point of the acrylic resin is 2.05 or less.
[4] A paint for applying to metal sheets according to any one of [1] to [3], wherein the proportion of the acrylic resin in the total solid content is 30% or more, and the proportion of the wax in the total solid content is 5% or more and 50% or less, in terms of mass proportion when the paint is dried.
[5] The coating material for coating metal sheets according to any one of [1] to [4], wherein the acrylic resin has a mass average molecular weight of 5,000 or more and 30,000 or less.
[6] The coating material for applying to metal sheets according to any one of [1] to [5], wherein the acrylic resin is a styrene-acrylic resin.
[7] A coating material for applying to metal sheets according to any one of [1] to [6], which contains 5% or more and 30% or less of a rust inhibitor based on the total solid content, in terms of mass ratio when the coating material is dried.
[8] The coating material for applying to metal sheets according to any one of [1] to [7], wherein the rust inhibitor is at least one selected from the group consisting of aluminum salts of phosphoric acids, zinc salts, and zinc oxide.
[9] The coating material for coating metal sheets according to any one of [1] to [8], wherein the wax has an average particle size of 0.01 μm or more and 0.5 μm or less.
[10] The coating material for coating metal sheets according to any one of [1] to [9], containing 1% or more and 10% or less of silica in terms of mass ratio when the coating material is dried.

According to the present invention, the coefficient of friction with dies and the like is significantly reduced, and a metal plate with excellent press formability is obtained. As a result, metal plates that are subjected to complex forming have stable and excellent press formability. In addition, the stability of the paint is ensured, and the metal plate with the coating has good rust prevention properties. Furthermore, since the adhesion is good, adhesives can be used in the same way as with conventional metal plates, and since the coating is easily removed by alkaline degreasing, it does not interfere with the painting process.

FIG. 2 is a schematic front view showing a friction coefficient measuring device. FIG. 2 is a schematic perspective view showing the shape and dimensions of the bead in FIG. 1 .

The following describes an embodiment of the present invention.

The coating material for metal sheets of the present invention contains an acrylic resin with a glass transition point (Tg) of 100°C or higher, and a ratio of acid value to glass transition point R = acid value (mg-KOH/g)/Tg (°C) of 1.50 or higher, and a polyolefin wax with a melting point of 100°C or higher and 145°C or lower, and an average particle size of 3.0 μm or less.

Hereinafter, the ratio of acid value to glass transition point, R = acid value (mg-KOH/g)/Tg (°C), will be expressed as R = acid value/Tg.

The glass transition point of the acrylic resin in the coating material for application to metal sheets of the present invention is set to 100°C or higher in order to obtain good lubricity. If the glass transition point is less than 100°C, the resin will soften during sliding, reducing the retention of the wax and reducing the ability to prevent direct contact between the metal sheet and the die, making it difficult to obtain good sliding properties. It is preferably 110°C or higher and 150°C or lower. If it exceeds 150°C, the resin will tend to become too hard and brittle during sliding, making it difficult to obtain good lubricity.

Here, the glass transition point is the intermediate glass transition temperature measured based on JIS K 7121 "Method for measuring glass transition temperature of plastics."

The ratio R of the acid value and the glass transition point of the acrylic resin (acid value/Tg) is 1.50 or more. Even if the glass transition point is 100°C or more, if the acid value is low (R<1.50), excellent lubricity cannot be obtained. Although the cause of this is not clear, it is thought that the carboxyl group in the acrylic resin has a high affinity with the mold, and the wax in the film is transferred to the mold during sliding. When the acrylic resin component containing wax is transferred to the mold during sliding, the mold surface is protected by the wax, and the effect of preventing direct contact with the metal plate is enhanced, improving the sliding property. Therefore, when the acid value is low (R<1.50), the sliding property is poor due to a lack of carboxyl groups. When the glass transition point of the acrylic resin increases, the resin becomes less likely to soften due to sliding, making it difficult to transfer to the mold. Therefore, in order to obtain excellent sliding property when the glass transition point increases, the acid value must also be increased. In other words, the ratio R of the acid value and the glass transition point (acid value/Tg) must be 1.50 or more. It is preferably 1.80 or more. Although there is no particular upper limit for R, it is preferable to set it to 2.05 or less. The reason for this is that if R exceeds 2.05, the rust prevention properties may deteriorate.

The acid value of the acrylic resin is preferably 180 mg-KOH/g or more and 350 mg-KOH/g or less. If it is less than 180 mg-KOH/g, the film may not be easily removable by alkali, and the adhesive may not provide sufficient adhesive strength. If it exceeds 350 mg-KOH/g, the rust prevention properties may deteriorate.

Here, the acid value refers to the number of milligrams of potassium hydroxide required to neutralize the carboxyl groups contained in 1 g of resin, and is measured based on JIS K 0070 "Testing methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products." In this specification, the unit is shown as mg-KOH/g.

The wax used in the present invention may be a polyolefin wax having a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or less.

The reason why polyolefin wax is used as the wax is that it has low surface energy and self-lubricating properties, which provides good lubrication. In addition, it is relatively easy to adjust the melting point of polyolefin to between 100°C and 145°C by controlling the density and molecular weight.

When the melting point is 100°C or higher and 145°C or lower, in addition to the self-lubricating properties of the polyolefin wax itself, the wax becomes semi-molten due to sliding during press molding, and the lubricating film components mixed with the organic resin can cover the mold surface, and excellent lubrication effects can be obtained by suppressing direct contact between the mold and the steel sheet. When the melting point is less than 100°C, the wax completely melts due to frictional heat caused by sliding during press molding, and the wax itself does not provide sufficient lubrication effect, and the aforementioned mold coating effect cannot be obtained. Also, when the melting point exceeds 145°C, it does not melt during sliding, and sufficient lubrication effect cannot be obtained, and the mold coating effect cannot be obtained. Furthermore, it is preferable that the melting point is 120°C or higher and 140°C or lower.

Here, the melting point of the wax is the melting temperature measured based on JIS K 7121 "Method for measuring transition temperature of plastics."

If the average particle size of the wax exceeds 3.0 μm, it becomes difficult to mix with the organic resin during sliding, and the aforementioned coating effect of the mold cannot be obtained, resulting in insufficient lubrication. The average particle size of the wax is preferably 0.5 μm or less, and even more preferably 0.3 μm or less.

The average particle size of the wax is preferably 0.01 μm or more. If it is less than 0.01 μm, it will be easily dissolved in the lubricating oil during sliding, and may not exhibit sufficient lubrication improvement effects. It also tends to aggregate in the paint used to form the film, resulting in low paint stability. More preferably, it is 0.03 μm or more. Taking into consideration the mixability with the above acrylic resin, the average particle size is preferably 0.01 μm or more and 0.5 μm or less.

The average particle size is the median diameter of the volume average diameter, and is determined by a laser diffraction/scattering method. For example, it can be determined by measuring a sample diluted with pure water using a laser diffraction/scattering type particle size distribution measuring device Partica LA-960V2 (manufactured by Horiba, Ltd.).

Among polyolefin waxes, polyethylene wax provides the best lubrication effect, so it is preferable to use polyethylene wax.

It is preferable that the proportion of the acrylic resin in the total solid content of the coating material for applying to metal sheets of the present invention when the coating material is dried is 30% or more by mass, and the proportion of the wax in the total solid content is 5% or more and 50% or less by mass.

When the proportion of acrylic resin in the total solids is 30% or more, the properties influenced by the physical properties of the acrylic resin component, such as the effect of improving lubricity due to transfer to the mold during sliding, film removal properties, and adhesion, can be fully obtained. If it is less than 30%, the influence of other components becomes large, and the target performance may not be obtained.

If the mass ratio of wax in the total solids is less than 5%, sufficient lubrication effect may not be obtained. If it is 10% or more, a particularly good lubrication effect can be obtained. In addition, the mass ratio of wax in the total solids is preferably 50% or less. If it exceeds 50%, the wax is likely to fall off due to a lack of base resin components, adhesion to the metal plate may be poor, and it may not exist stably as a film, resulting in poor adhesion. In addition, when used as an automobile body, sufficient degreasing may not be obtained in the alkaline degreasing process of the painting process, and the film may not be sufficiently removed in the alkaline degreasing process, resulting in a film remaining, which may deteriorate the paintability. More preferably, it is 30% or less.

Here, the mass ratio of wax to the total solids is the ratio of the mass of wax solids to the mass of the total solids in the paint.

The mass average molecular weight of the acrylic resin is preferably 5,000 or more and 30,000 or less. If it is less than 5,000, the rust prevention properties may be poor, and if it exceeds 30,000, the adhesive properties may be deteriorated.

Here, the mass average molecular weight is the mass average molecular weight measured based on JIS K 7252 "Plastics - Determination of average molecular weight and molecular weight distribution of polymers by size exclusion chromatography."

Furthermore, it is preferable that the acrylic resin is a styrene-acrylic resin. By including styrene in the resin monomer, water resistance is improved, resulting in good rust prevention properties. Furthermore, there is also an effect of obtaining better sliding properties compared to when styrene is not included.

The paint for application to metal plates of the present invention preferably contains a rust inhibitor in a mass ratio of 5% to 30% of the total solid content. Even if the paint does not contain a rust inhibitor, rust will not occur under normal storage conditions. However, if the rust inhibitor content is less than 5%, rust prevention will be insufficient under unfavorable storage conditions, and rust may occur when a film is formed on the metal plate. In particular, if the metal strip is stored in a coiled state under unfavorable storage conditions, it may absorb moisture and cause rust.

If the ratio of the rust inhibitor exceeds 30%, the adhesion may deteriorate, and the rust inhibitor may precipitate in the paint, deteriorating the paint stability. It is preferable to use at least one rust inhibitor selected from the group consisting of aluminum salts of phosphoric acids, zinc salts, and zinc oxide. Here, phosphoric acids include orthophosphoric acid as well as condensed phosphoric acids such as pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, and metaphosphoric acid. By using these rust inhibitors, sufficient rust prevention effect can be achieved, and there is also little deterioration in paint stability.

Furthermore, the paint for coating metal sheets of the present invention preferably contains silica in a mass ratio of 1% to 10% of the total solid content. The inclusion of silica makes it possible to suppress precipitation of the rust inhibitor, improving paint stability. In addition, the water repellency of the film is increased, resulting in sufficient rust prevention. If the silica content is less than 1%, it is difficult to obtain the above-mentioned effects, and if it exceeds 10%, adhesion may deteriorate. It is preferable to use colloidal silica with a particle size of 5 nm to 200 nm.

In the present invention, components other than the acrylic resin, wax, rust inhibitor, and silica may include surface conditioners, defoamers, dispersants, and the like that are generally added to paints.

Next, the metal plate of the present invention will be described in detail. In the present invention, a lubricated metal plate is provided in which any of the above-mentioned paints is applied to at least one side of the metal plate and then dried to form a lubricating film. Water or an organic solvent can be used as the solvent for the paint, but water is preferably used. The concentration of the total solids in the paint is preferably 1 to 30% by mass.

If the coating is less than 1% or more than 30%, uneven coating may occur. The coating method is not particularly limited, but examples include a method using a roll coater or a bar coater, or a coating method using a spray, immersion, or a brush. The steel sheet after coating can be dried by a general method. For example, drying with hot air, drying with an IH heater, or infrared heating may be used. It is preferable to coat the metal sheet so that the amount of coating attached per side is 0.3 to 2.5 g/m ² in terms of dry mass. If the coating is less than 0.3 g/m ² , sufficient sliding properties may not be obtained, and if the coating is more than 2.5 g/m ^2, the coating may be removed by alkali or the adhesiveness may be deteriorated. The maximum temperature of the steel sheet during drying is preferably 60°C or higher and lower than the melting point of the wax used. If the coating is less than 60°C, it may take a long time to dry and the rust prevention properties may be poor. If the coating exceeds the melting point of the wax used, the wax may melt and combine, and the particle size may become coarse, resulting in deterioration of lubricity.

There are no particular limitations on the metal plate. Any metal plate can be used, including hot-rolled steel plate, cold-rolled steel plate, stainless steel plate, galvanized steel plate, aluminum plate, and titanium plate.

The present invention will be described below with reference to examples. Note that the present invention is not limited to the following examples.

A cold-rolled steel sheet SPCD (JIS G 3141) having a thickness of 0.7 mm and a tensile strength of 270 MPa was used, and a coating material having the composition shown in Table 1 was adjusted with pure water to a solid content concentration of 15 mass%, applied with a bar coater, and dried with an IH heater so that the maximum temperature of the steel sheet reached was 80°C, thereby obtaining a lubricated steel sheet. The coating amount was adjusted so that the coating amount per side was 1.0 g/ ^m2 . Note that colloidal silica with a volume average particle size of 9 nm was used as the silica.

The coating weight was calculated by removing the coating from the steel plate after coating application, and dividing the difference in mass of the steel plate before and after coating removal by the area.

(1) Method for Evaluating Press Moldability (Sliding Characteristics) In order to evaluate press moldability, the friction coefficient of each test material was measured as follows.

Figure 1 is a schematic front view of a friction coefficient measuring device. As shown in the figure, a friction coefficient measuring sample 1 taken from a test material is fixed to a sample stage 2, which is fixed to the upper surface of a horizontally movable slide table 3. A vertically movable slide table support 5 having a roller 4 in contact with the slide table 3 is provided on the lower surface of the slide table 3, and a first load cell 7 is attached to the slide table support 5 to measure the pressing load N applied by the bead 6 to the friction coefficient measuring sample 1 by pushing it up. A second load cell 8 is attached to one end of the slide table 3 to measure the sliding resistance force F for moving the slide table 3 horizontally while the pressing force is applied. The test was performed by applying press cleaning oil Pleton R352L manufactured by Sugimura Chemical Industry Co., Ltd. to the surface of the sample 1 as a lubricant.

Figure 2 is a schematic perspective view showing the shape and dimensions of the bead used. The underside of the bead 6 slides while being pressed against the surface of the sample 1. The shape of the bead 6 shown in Figure 2 is 10 mm wide, 59 mm long in the sliding direction of the sample, and the lower parts of both ends in the sliding direction are curved surfaces with a curvature of 4.5 mmR, and the underside of the bead against which the sample is pressed has a flat surface with a width of 10 mm and a length of 50 mm in the sliding direction.

The friction coefficient measurement test was performed using the bead shown in Fig. 2, with a pressing load N of 400 kgf and a sample pull-out speed (horizontal movement speed of the slide table 3) of 20 cm/min. The friction coefficient μ between the test material and the bead was calculated by the formula: μ = F/N.
The sliding properties were evaluated as follows: when the friction coefficient was 0.119 or less, it was evaluated as being particularly excellent sliding properties, ⊚; when it was more than 0.119 and 0.130 or less, it was evaluated as being good sliding properties, ◯; and when it was more than 0.130, it was evaluated as being insufficient, and x.

(2) Method for evaluating film removal property Assuming that the metal sheet according to the present invention is used in automobile applications, the film removal property during degreasing was evaluated. In order to determine the film removal property of the film, first, each test piece was degreased with an alkaline degreaser Fine Cleaner E6403 (manufactured by Nihon Parkerizing Co., Ltd.). The degreasing treatment was performed by immersing the test piece in a degreasing solution with a degreaser concentration of 20 g/L and a temperature of 40° C. for a predetermined period of time and washing with tap water. The surface carbon intensity of the test piece after the degreasing treatment was measured using a fluorescent X-ray analyzer, and the film peeling rate was calculated by the following formula using the measured value, the previously measured surface carbon intensity before degreasing, and the measured value of the surface carbon intensity of the untreated metal sheet.

Film peeling rate (%)=[(carbon strength before degreasing−carbon strength after degreasing)/(carbon strength before degreasing−carbon strength of untreated steel plate)]×100
The removability of the coating was evaluated according to the following criteria, based on the immersion time in the alkaline degreasing solution at which the coating peeling rate was 98% or more. A time of 120 seconds or less was evaluated as good removability and rated as ◯, and a time of more than 120 seconds was evaluated as insufficient removability and Δ.

(3) Method for evaluating rust prevention The rust prevention properties of the metal plate according to the present invention in a stacked state were evaluated, assuming that the metal plate according to the present invention was stored in a coiled state as a metal strip. Each test piece was processed to a size of 150 mm x 70 mm, and rust prevention oil was applied to both sides at 1.0 g/ ^m2 per side. Two test pieces were stacked, and a load was applied to the test pieces at a surface pressure of 0.02 kgf/ ^mm2. The test pieces were then tested in an environment of a temperature of 50°C and a humidity of 95% RH. The rust prevention properties were evaluated by checking the stacked inner surface every 7 days, and evaluating the number of days until rust occurred. When the rust prevention properties were 56 days or more, the rust prevention properties were evaluated as particularly good, and when the rust prevention properties were 21 days or more, the rust prevention properties were evaluated as good, and when the rust prevention properties were less than 21 days, the rust prevention properties were evaluated as insufficient, and △.

(4) Method for evaluating adhesiveness Each test piece was processed to a size of 100 x 25.4 mm, immersed in rust-preventive oil, and then stood vertically for 24 hours to remove excess oil. Two pieces were used, and an epoxy adhesive was uniformly applied to the 25.4 mm x 13 mm portion to a thickness of 0.2 mm, then overlapped and clamped with clips, baked at 180 ° C for 20 minutes, and dried and cured. After cooling, a shear tensile test was performed using an autograph tester to measure the shear adhesive strength. When a similar shear tensile test was performed using two metal plates (original plates) without a lubricating film, the shear adhesive strength was 29.0 MPa. Adhesion was evaluated as ◯ for good adhesion when the adhesive strength was 20 MPa or more, and △ for insufficient adhesion when the adhesive strength was less than 20 MPa.

(5) Evaluation method of paint stability 25 g of each paint, which was adjusted with pure water to a solid content concentration of 15 mass %, was placed in a φ18 mm × 180 mm test tube, shaken well, and then left to stand, and the time until precipitation occurred at the bottom of the test tube was evaluated. Paint stability was evaluated as good (○) when no precipitation occurred after 24 hours, and as insufficient (△) when precipitation occurred within 24 hours.

According to Table 2, all of the metal sheets coated with the metal sheet coating material of the present invention have excellent press formability. In contrast, all of the metal sheets of the comparative examples that do not have the technical features of the present invention have poor press formability.

The coating material for metal sheets of the present invention has excellent sliding properties during press forming, alkali removal, rust prevention, adhesion, and coating stability. Lubricated metal sheets coated with this coating material to form a coating have these excellent properties and can be used in a wide range of fields, primarily in automobile body applications.

1 Friction coefficient measurement sample 2 Sample stage 3 Slide table 4 Roller 5 Slide table support 6 Bead 7 First load cell 8 Second load cell 9 Rail

Claims (9)

A coating material for applying to metal sheets contains an acrylic resin having a glass transition point (Tg) of 100°C or higher and a ratio of the acid value to the glass transition point, R=acid value (mg-KOH/g)/Tg(°C), of 1.80 to 2.05 , and a polyolefin wax having a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or less. The paint for coating metal sheets according to claim 1, wherein the acid value of the acrylic resin is 180 mg-KOH/g or more and 350 mg-KOH/g or less. 3. The paint for coating metal sheets according to claim 1 or 2, wherein the proportion of the acrylic resin in the total solid content is 30% or more, and the proportion of the wax in the total solid content is 5% or more and 50% or less, in terms of mass proportion when the paint is dried. The coating material for coating metal sheets according to any one of claims 1 to 3 , wherein the mass average molecular weight of the acrylic resin is 5,000 or more and 30,000 or less. The paint for coating metal sheets according to any one of claims 1 to 4 , wherein the acrylic resin is a styrene-acrylic resin. 6. The paint for coating metal sheets according to claim 1 , which contains a rust inhibitor in an amount of 5% to 30% by mass based on the total solid content of the paint when dried. 7. The paint for coating metal sheets according to claim 6 , wherein the rust inhibitor is at least one selected from the group consisting of aluminum salts, zinc salts, and zinc oxide of phosphoric acids. The paint for coating metal sheets according to any one of claims 1 to 7 , wherein the wax has an average particle size of 0.01 µm or more and 0.5 µm or less. 9. The paint for coating metal sheets according to claim 1, which contains silica in an amount of 1% to 10% by mass based on the total solid content of the paint when dried.

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