A method for detecting adhesion between coating layers of color-coated products

Abstract

The present invention discloses a method for detecting the adhesion between coating layers of color-coated products, comprising the following steps: 1) bending and flattening the front and back of the color-coated plate to be tested so that the wave shape of the color-coated plate is less than 1 mm; 2) boiling; adding the flattened color-coated plate to boiling water; 3) removing the boiled color-coated plate and air-drying or blotting the surface; setting a scribe line of not less than 50 mm in length on the surface of the color-coated plate, and the scribe line must cut through the coating to the substrate; 4) performing a pencil hardness test in a direction perpendicular to the scribe line from the scribe line. The method of the present invention can realize efficient detection of the adhesion between layers of color-coated products, effectively discovering the risk of peeling between coating layers during user-end processing and later use; and the test method involved is simple and easy to operate, suitable for promotion and application.

Description

Technical Field

[0001] The invention belongs to the technical field of performance detection of coated steel plates, and in particular relates to a method for detecting the adhesion between coating layers of a color-coated product. Background Art

[0002] As an environmentally friendly, energy-saving, and easy-to-process and shape new material, color-coated steel sheets have been widely used in construction, home appliances, and interior decoration. With the increasing demand for the use and quality of color-coated products, lightweight steel buildings, housing structures, high-rise curtain walls, and other applications have begun to use color-coated products directly on a large scale. These products offer advantages such as a one-step molding process, a pre-painted color coating, and the elimination of chemical corrosion treatment or spraying processes. They effectively address VOC emissions and the discharge of heavy metal-containing wastewater and residues associated with painting. Metallic paint color-coated products, in particular, have seen increasing market adoption in recent years due to their unique and aesthetically pleasing appearance.

[0003] The coating structure of conventional color-coated products usually includes a primer and a topcoat that are sequentially set on the front and back of the metal substrate respectively. In recent years, with the increasing requirements of users for the film thickness of special coatings, some color-coated products no longer use the conventional coating structure of primer + topcoat, and need to use a special coating structure. For example, for coating products with a front surface greater than 45 microns, three coatings are required (one layer of primer + two layers of topcoat), specifically including: primer and topcoat set on the back of the metal base plate, and primer, first layer of topcoat and second layer of topcoat set on the front of the metal substrate. For another example, for a 12-micron epoxy coating on the back, the epoxy primer needs to be applied twice, specifically including: the first layer of primer and the second layer of primer set on the back of the metal substrate, as well as the primer and topcoat involved on the front of the metal substrate.

[0004] Currently, the testing method for the bending performance of color-coated products generally uses the "bending test," "reverse impact test," "scratch test," and "cupping test" in "GB / T13448-2019 Test Methods for Color-Coated Steel Sheets and Strips." These tests can detect the adhesion between the coating and the metal substrate when the steel sheet is deformed during processing. When adjacent coatings use the same type of paint, the adhesion between them varies from the traditional primer + topcoat model due to the coating's characteristics. Even with products that meet traditional adhesion standards, peeling of the coating layers may still occur during subsequent processing at the user's site. Further exploring new and efficient testing methods that more comprehensively and accurately evaluate inter-coating adhesion can effectively prevent peeling of the coating layers during user processing, which is of great significance for promotion and application. Summary of the Invention

[0005] The main purpose of the present invention is to address the problems and shortcomings of the existing technology and provide a method for efficiently detecting the adhesion between layers of color-coated products, which can effectively detect the risk of peeling between coatings during user-end processing and later use; and the test method involved is simple and easy to operate, and is suitable for promotion and application.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for detecting the adhesion between coating layers of a color-coated product comprises the following steps:

[0008] 1) Bend and flatten the front and back of the color-coated steel plate to be tested so that the wave shape of the steel plate is less than 1mm;

[0009] 2) Boiling: Add the smoothed color-coated sheet to boiling water and boil it;

[0010] 3) Take out the boiled color-coated board and air-dry or absorb the surface; set a line of not less than 50mm in length on the surface of the color-coated board (using a marking pen, etc.), and the marking line must cut through the coating to the substrate;

[0011] 4) Perform a pencil hardness test from the drawn line in a direction perpendicular to the drawn line.

[0012] Furthermore, the color-coated plate to be tested is placed in an environment with a temperature of 21-25° C. and a relative humidity of 45-55% for more than 24 hours before use.

[0013] In the above scheme, the control requirements of the boiling time with the coating thickness d are: d≤25μm 55~65min; 25μm <d≤30μm 85~95min;30μm<d≤35μm 100~110min;35μm<d≤40μm 115~125min。

[0014] Furthermore, all the color-coated plate samples were immersed in boiling water, and the water temperature was maintained during the boiling period and the liquid level did not drop significantly.

[0015] Furthermore, the size of the color-coated plate sample is 100-120 mm × 230-250 mm, and the sample should be flat, with no oil stains, damage, or burrs on the surface.

[0016] Furthermore, the line is a straight line.

[0017] In the above scheme, the marking step includes: using a pencil sharpener to sharpen the pencil until a 3-5 mm cylindrical lead is exposed (the lead should not be loose or cut), holding the pencil perpendicular to the surface of 400# sandpaper, and grinding on the sandpaper until a flat end surface and a sharp edge are obtained (the edge should not be broken or notched).

[0018] In the above scheme, the pencil hardness test steps include: 1) placing the sample horizontally on a workbench, holding a sharpened pencil at an angle of 30-50° to the coating, and applying sufficient and uniform pressure perpendicular to the stroke, starting from the scribe line, to a degree that breaks the pencil edge or plows through the coating; 2) the pencil is advanced for a stroke of 5-8 mm; after each use, the pencil should be rotated 180° before reuse or re-sharpened; 3) starting with the hardest pencil (usually 3H), plowing is defined as separation of more than 0.5 mm between the topcoat and the next layer. If the coating is plowed through once out of three attempts, a softer pencil is used, and the process continues until a pencil is found that fails to plow through the coating layer three times in a row. The hardness of this pencil is the pencil hardness of the interlayer adhesion being measured.

[0019] Furthermore, the pencils are labeled as follows: 5H, 4H, 3H, 2H, H, F, HB, B, 2B; 5H is the hardest, 2B is the softest, and the hardness decreases from 5H to 2B.

[0020] Furthermore, a pencil is placed on the drawn line with the pencil projection line forming a 90-degree angle with the drawn line to test the pencil hardness.

[0021] Furthermore, the pencil is a special pencil for 505 coating testing.

[0022] In the above solution, the pencil hardness test result is less than or equal to grade B, which is judged as unqualified.

[0023] Furthermore, if the coating topcoat is a fluorocarbon coating, the pencil hardness test result is less than or equal to level 2B and is judged to be unqualified.

[0024] Furthermore, if the steel plate thickness is greater than 0.8 mm or the yield strength is greater than 350 MPa, and the pencil hardness test result is less than or equal to HB, it is judged as unqualified.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1) The present invention first boils and marks the color-coated plate in sequence, and then performs a pencil hardness test perpendicular to the marked area. This testing method can effectively identify products that may have the risk of delamination between coatings during user-end processing and later use, and effectively ensure the accuracy of the test results;

[0027] 2) The detection method of the present invention is relatively simple and can be used with conventional detection equipment and materials, and is suitable for promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 In Application Example 1 of the present invention, (a) the sample morphology of the color-coated steel coil after the coating interlayer adhesion test and (b) the construction effect diagram;

[0029] Figure 2 The application effect diagram of the qualified color-coated steel coil described in Comparative Example 1. DETAILED DESCRIPTION

[0030] The present invention will be further described in detail below with reference to the embodiments for a clearer understanding of the present invention, but they do not limit the present invention.

[0031] The following examples use the following equipment and materials: Zhonghua 505 coating testing pencils, designated 5H, 4H, 3H, 2H, H, F, HB, B, and 2B (5H being the hardest and 2B being the softest, with hardness decreasing from 5H to 2B); 400-grit sandpaper; a pencil sharpener; a marking pen designed for marking salt spray specimens; and a ruler.

[0032] Example 1

[0033] The method for detecting the adhesion between coating layers of a color-coated product according to the present invention comprises the following steps:

[0034] (1) Two samples 1# and 2# of the edge of polyester 0.5mm TDC51D+AM with a yield strength of 280MPa with a total film thickness of about 22 microns were bent and flattened on the front and back sides respectively. The resulting sample size was 100mm×240mm. The sample was flat, with no oil stains, no damage, and no burrs on the edge. Then, the sample was placed in an environment with a temperature of 23℃±2℃ and a relative humidity of 50±5% for at least 24 hours.

[0035] (2) Boiling: Add an appropriate amount of water to a suitable container, heat it to boiling, and immerse the entire sample obtained in step 1) in it; heat it for 1 hour, maintaining the water temperature and ensuring that the liquid level does not drop significantly; remove the sample and immediately dry it with cold air or use absorbent paper to dry the surface of the color-coated plate;

[0036] (3) Use a pencil sharpener to sharpen the pencil until a 3-5mm cylindrical lead is exposed (the lead should not be loose or cut), hold the pencil perpendicular to the 400# sandpaper surface, and scratch it on the sandpaper until the end surface is flat and the edge is sharp (the edge should not be broken or notched); use a marking pen to draw a 60mm straight line along the ruler, and scratch the coating to the substrate (do not use too much force to avoid scratching the substrate);

[0037] (4) Place a special Chinese pencil sharpened on sandpaper on the scribe line at a 90-degree angle to the scribe line to test the pencil hardness. The specific steps include: 1) Place the sample horizontally on the operating table, hold the sharpened pencil at a 45-degree angle to the coating, and apply sufficient and uniform pressure downward and forward from the scribe line, with the force being appropriate to break the edge of the pencil or plow through the coating; 2) The pencil advances for about 5-8 mm. After using the pencil once, rotate it 180° before using it again or re-sharpen it; 3) Start with the hardest pencil (usually starting with 3H). If the topcoat and the next layer of coating are separated by more than 0.5 mm, it is defined as plowing. If the coating can be plowed through once in three times, switch to a softer pencil until a pencil that cannot plow through the coating layer for three consecutive times is found. The hardness of this pencil is the pencil hardness of the interlayer adhesion being tested.

[0038] If the pencil adhesion test result is less than or equal to grade B, it is judged as unqualified.

[0039] Application Example 1

[0040] The above method was used to test the interlayer adhesion of 0.7mm thick TDC51D+AM, 280MPa, ocean blue polyester color-coated steel coils produced by the color-coating mill at Wuhan Iron and Steel's cold-rolling mill. The result was F. This batch of color-coated steel coils was used in a large warehouse and logistics plant in Xuchang, Henan Province (January 2024), where 840-watt tile pressing was performed on site. The color-coated steel coils performed well in processing and use.

[0041] Application Example 2

[0042] The above method is used to test the interlayer adhesion of the coating to H, which is produced by the color coating unit of the cold rolling mill with a thickness of 0.7mm and a yield strength of 390MPa white-gray polyester color-coated steel coil.

[0043] This batch of color-coated steel coils was used in a food supply plant in Shandong (March 2024). Flat composite panels were used on site, and the color-coated panels were in good condition.

[0044] Comparative Example 1

[0045] For 0.8mm thick, TS350GD+AM, ocean blue, fluorocarbon-coated steel coils with a yield strength of 385 MPa, produced by the color-coating mill at Wuhan Iron and Steel's cold-rolling mill, surface hardness testing using only the existing methods in GB / T 12754 and GB / T 13448 was conducted. The coating surface hardness H passed the test, along with other standard properties such as adhesion. However, this batch of color-coated steel coils was used in a large factory in Baoding, Hebei Province, where they were used for 750-watt tile pressing, resulting in coating scratches.

[0046] In response to the above application problems, samples of this batch of products were retrieved and further tested according to the test method described in Example 1. The results showed that the interlayer adhesion grade of this batch of products was 2B, which was judged to be unqualified, which matched the actual application effect.

[0047] Comparative Example 2

[0048] For 0.7mm thick, TDC51D+AM, 318MPa yield strength, tornado white polyester color-coated steel coils produced by the color-coating mill at Wuhan Iron and Steel's cold-rolling mill, surface hardness testing using only traditional methods according to national standards GB / T 12754 and GB / T 13448 yielded satisfactory results for the coating's surface hardness H, as well as other standard properties such as coating adhesion. However, when these color-coated steel coils were used on the roof of Terminal 3 at Urumqi International Airport in Xinjiang, the coating was scratched at the external corner bends during on-site use of 680-watt pressed tiles.

[0049] In response to the above application problems, samples of this batch of products were retrieved and further tested according to the test method described in Example 1. The results showed that the interlayer adhesion grade of this batch of products was 2B, which was judged to be unqualified, which matched the actual application effect.

[0050] Comparative Example 3

[0051] For the 0.5mm thick, TDC51D+AM, 325MPa yield strength, tornado white polyester color-coated steel coils produced by the color-coating unit of Wuhan Iron and Steel's cold rolling mill, only the traditional methods of national standards GB / T 12754 and GB / T 13448 were used to test the surface hardness. The coating surface hardness H was qualified, and other conventional properties such as coating adhesion were also qualified. However, when this batch of color-coated steel coils were used for the wall panels of the new terminal building of Hohhot International Airport in Inner Mongolia, 840-tile pressed tiles were used on site, and the coating was scratched at the corners and bends.

[0052] Further testing was conducted using a method similar to that of the present invention. The testing method was essentially the same as in Example 1, except that the boiling time was 0.5 hours. The final hardness grade was F, indicating passing. However, when this batch of color-coated steel coils was used for wall panels at the new terminal building of Hohhot International Airport in Inner Mongolia, 840-tile pressed tiles were used on-site, and the coating was scratched at the corners and bends.

[0053] In response to the above application problems, samples of this batch of products were retrieved and the test method described in Example 1 was further used. The results showed that the interlayer adhesion grade of this batch of products was 2B, which was judged to be unqualified, which matched the actual application effect.

[0054] The above embodiments are merely examples for illustrative purposes only and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications may be made based on the above descriptions. It is not necessary and impossible to enumerate all implementation methods here. Therefore, any obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for detecting the adhesion between coating layers of a color-coated product, characterized in that: The steps include: 1) Bend and flatten the front and back of the color-coated sheet to be tested so that the wave shape of the color-coated sheet is less than 1mm; 2) Boiling: Add the smoothed color-coated sheet into boiling water and boil it; 3) Take out the boiled color-coated board and air-dry or vacuum-dry the surface; set a line of not less than 50mm in length on the surface of the color-coated board, and the line must cut through the coating to the substrate; the line is a straight line; 4) Conduct a pencil hardness test perpendicular to the scribe line. The pencil hardness is the pencil hardness of the interlayer adhesion being tested. The control requirements of the boiling time according to the coating thickness d are: d≤25μm 55~65min; 25μm <d≤30μm 85~95min;30μm<d≤35μm 100~110min;35μm<d≤40μm 115~125min; The pencil numbers are: 5H, 4H, 3H, 2H, H, F, HB, B, 2B; The pencil hardness test result is less than or equal to grade B, which is judged to be unqualified; If the coating topcoat is a fluorocarbon coating, the pencil hardness test result is less than or equal to 2B, which is considered unqualified; If the steel plate thickness is greater than 0.8 mm or the yield strength is greater than 350 MPa, and the pencil hardness test result is less than or equal to HB, it is judged as unqualified.

2. The method according to claim 1, characterized in that The size of the color-coated plate is 100-120 mm×230-250 mm.

3. The method according to claim 1, characterized in that The pencil hardness test procedure includes the following steps: 1) placing the specimen horizontally on a workbench, holding a sharpened pencil at an angle of 30-50° to the coating, and applying sufficient, uniform pressure perpendicular to the stroke to break the pencil edge or plow through the coating; 2) advancing the pencil over a distance of 5-8 mm; 3) starting with the hardest pencil, plowing is defined as separation of more than 0.5 mm between the topcoat and the next layer of coating. If the coating is plowed through once out of three attempts, a softer pencil is used until a pencil is found that fails to plow through the coating layer three times in a row.

4. The method according to claim 1, wherein Among the pencil grades, 5H is the hardest and 2B is the softest, and the hardness decreases from 5H to 2B.

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