Method for measuring varnish layer thickness

Abstract

The present invention relates to a method for estimating the thickness of a varnish coating of a moving steel substrate provided with a varnish coating, the varnish coating having a thickness of 0.5-5 μm, comprising the following steps: i. illuminating said moving coated steel substrate with an illumination source forming an angle of incidence of 51°-61° with respect to the normal of said steel substrate, ii. p-polarizing the light after reflection on said moving steel substrate and measuring the intensity of the light after reflection on said moving steel substrate, iii. evaluating the absorbance spectrum of said varnish coating in said wavelength range, iv. measuring the absorbance spectrum A MEAS and v. estimating the varnish thickness using the area under the curve and a function relating the varnish thickness to the area under the curve of the absorbance spectrum of the coating.

Description

[Technical Field]

[0001] The present invention relates to a method for estimating the thickness of a varnish coating having a thickness of 0.5 to 5 μm on a moving steel substrate.

[0002] The present invention is particularly directed to estimating the thickness of varnish coatings on electrical steel strip. [Background technology]

[0003] After the annealing process, electrical steel is typically coated with a varnish to insulate the steel from electrical currents and reduce eddy currents. This varnish is typically applied as a wet film and then cured to obtain a dry film with a net-like thickness of 0.8 to 5.0 μm.

[0004] After hardening, the varnish thickness is measured to control the quality of the coated steel strip and to adapt the main coating process parameters. Summary of the Invention [Problem to be solved by the invention]

[0005] The object of the present invention is to provide a method that makes it possible to estimate the thickness of a coated varnish layer on a moving steel substrate. [Means for solving the problem]

[0006] This is achieved by providing a method according to any one of claims 1-10.

[0007] Other features and advantages will become apparent from the following description of the invention.

[0008] The present invention relates to a method for estimating the thickness of a varnish coating having a thickness of 0.5 to 5 μm on a moving steel substrate 1 provided with a varnish coating 2, comprising the following steps: i. illuminating the moving coated steel substrate with an illumination source L comprising wavelengths between 2.7 and 3.7 μm and forming an angle of incidence between 51° and 61° relative to the normal of the steel substrate; ii. p-polarizing the light after reflection on the moving steel substrate and polarizing the light in the wavelength range W of at least 2.7 to 3.7 μm MEAS measuring the intensity of light at iii. Using the reference spectrum and the intensities measured in step ii, MEAS The absorbance spectrum of the varnish coating in A MEAS determining a iv. At least the wavelength range W MEAS The absorbance spectrum A MEAS determining the area under the curve of v. estimating the varnish thickness using the area under the curve and a reference value relating the area under the curve of the absorbance spectrum of the coating to the varnish coating thickness; Includes: [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 illustrates a configuration for performing the claimed process steps. [Figure 2] FIG. 10 illustrates the third step of the process. [Figure 3] FIG. 1 shows the relationship between the area under the curve of the absorbance spectrum of a varnish coating and the thickness of the varnish coating when illuminated at an angle of 56° from the surface normal, with a polarizer in front of the camera oriented so that only p-polarized light is captured. [Figure 4] FIG. 1 shows the relationship between the area under the curve of the absorbance spectrum of a varnish coating and the thickness of the varnish coating when light is projected at an angle of 56° from the surface normal. DETAILED DESCRIPTION OF THE INVENTION

[0010] This method is applied after varnishing, which may comprise two steps: a coating step in which a wet film is deposited on the steel substrate, and a drying step in which the wet film is dried and reticulated.

[0011] In step i, the objective is to illuminate the coated, moving steel substrate with a light source emitting a known spectrum, so that at least a portion of the light passes through the varnish coating and is reflected on the steel substrate.

[0012] As shown in FIG. 1, a light source 3 illuminates a moving steel substrate 1 coated with a layer of varnish 2 .

[0013] Figure 1 shows the spectrum S SOURCE As indicated by , the light emitted by the light source 3 is preferably broadband light, meaning that the light source produces a broad continuous spectrum of frequencies from at least 2.7 to 3.7 μm.

[0014] Preferably, in step i, the illumination is performed by a spectrally neutral illumination source.

[0015] Preferably, the illumination source L and the moving coated steel substrate are positioned at a distance of 20 cm to 60 cm, meaning that the light travels 20 cm to 60 cm before being reflected by the moving coated steel substrate. Preferably, the illumination source L is configured to illuminate an area having a width equal to the strip width and a length of at least 20 mm, even more preferably 30 mm.

[0016] In step ii, the objective is to polarize the light that passes through the varnish coating and is reflected by the steel substrate, and that is p-polarized, at least in the wavelength range W MEAS The intensity is measured at the spectrum S MEAS The goal is to obtain this.

[0017] As shown in Figure 1, any intensity recording means such as a camera 4 is used to measure the intensity of the light beam reflected on the steel substrate 1, and the spectrum S of the reflected light beam is obtained. MEAS can be determined.

[0018] Furthermore, the p-polarization can be achieved by any p-polarization means, for example by a polarizer such as a wire grid polarizer.

[0019] Preferably, the moving steel substrate and the intensity recording, e.g., camera, are positioned at a distance of 50 cm to 150 cm, meaning that the reflected light travels 50 cm to 150 cm before entering the intensity recording means.

[0020] Preferably, the distance between the moving steel substrate and said p-polarization means, such as a wire grid polarization means, is between 50 cm and 150 cm, meaning that the reflected light travels between 50 cm and 150 cm before entering the p-polarization means.

[0021] The combination of distances disclosed above allows for stable measurements even when the moving steel substrate is vibrating, and indeed, without being bound by any theory, this makes it possible to reduce the risk of optical misalignment due to vibrations of the moving coated steel strip.

[0022] In step iii, the objective is to obtain a wavelength range W MEAS Absorbance spectrum of varnish coating in A MEAS The purpose is to determine:

[0023] Those skilled in the art will recognize the reference spectrum S REF and the measured spectrum S MEAS , e.g., using the measured intensities of step ii, know how to evaluate the absorbance spectrum of the varnish coating.

[0024] For example, this is the spectrum S of p-polarized light reflected off an uncoated steel substrate, as shown in Figure 2.REF or with a laboratory calibrated reference signal or a calculated reference. An uncoated steel substrate is a steel substrate that is not coated with a varnish layer.

[0025] The use of such spectra allows for a better assessment of the absorbance due to the coating layer alone, as it can take into account intensity variations due to the environment, for example, substrate variations and illumination fluctuations and non-uniformities.

[0026] This can be done using the measured intensities in step ii and computational means having access to a database containing at least one reference spectrum.

[0027] In step iv, the objective is to obtain a wavelength range W MEAS The method further comprises determining the area under the curve of the absorbance spectrum of the varnish coating at 1000 kJ / cm. Those skilled in the art will know how to determine such an area under the curve based on the absorbance spectrum. For example, step iv can include a baseline correction step to separate the absorbance peak from the global absorbance spectrum.

[0028] This is the absorbance spectrum A evaluated in step iii. MEAS This may be done using computational means with access to

[0029] In step v, the objective is to estimate the thickness of the varnish coating.

[0030] To do so, a relationship between the area under the curve and the varnish coating must be established, which can be done by performing steps i to v on a varnish-coated steel strip with a known coating thickness.

[0031] Step v may be performed using calculation means having access to the area under the curve estimated in step iv and a database comprising area under the curve values ​​associated with varnish thickness values.

[0032] For example, in FIG. 3, the area under the curve for 21 different thicknesses has been calculated, and a curve relating the area under the curve to the thickness of the varnish coating can then be plotted.

[0033] Surprisingly, while studying the p-polarized light spectrum, it was found that a quasi-linear variation between the area under the curve in step iv and the varnish thickness could be obtained using the intensity of a light beam forming an angle of incidence between 51° and 61° relative to the normal to the steel substrate. More importantly, this allowed each value of the area under the curve to be associated with a single varnish thickness value.

[0034] Indeed, as shown in FIG. 3, when the light source and the surface of the steel substrate form the angle claimed, the value of the area under the curve corresponds to only one varnish thickness value.

[0035] Conversely, when the light source is set at a common angle of 45° between the light source and the surface of the steel substrate, as shown in Figure 4, a non-linear response is measured, i.e., the area under the curve can correspond to multiple varnish thicknesses. For example, in Figure 4, an area under the curve of 0.65 corresponds to three thickness values: 650 nm, 900 nm, and 1600 nm.

[0036] Apparently, the claimed angle of incidence of the illumination source coupled with the p-polarized light of the measured light has a synergistic effect that makes it possible to avoid or at least significantly reduce interference, thereby making it possible to associate each value of the area under the curve with a single varnish thickness value.

[0037] Furthermore, measuring the intensity in the wavelength range of 2.7-3.7 μm makes it possible to measure reflected light in the range where the varnish coating has a high absorbance. This is particularly true for electrical steel and / or varnish coatings used in thin layers, e.g., less than 5 μm thick.

[0038] Preferably, the steel substrate is an electrical steel. The electrical steel contains 0 to 6 weight percent silicon. Electrical steels can be divided into two categories: non-oriented steels and oriented steels. Electrical steels are used to manufacture articles of manufacture with specific magnetic properties, such as stators and rotors for electric motors, transformers, and wind turbines.

[0039] Preferably, the varnish coating has a thickness of 0.5 to 6 μm. Preferably, the varnish coating has a thickness of 0.5 to 2 μm.

[0040] Preferably, the varnish coating is an aqueous solution containing 25 to 75 weight percent resin, 5 to 15 weight percent solvent, and the balance water. For example, the varnish coating contains 30 to 50 weight percent dry extract of acrylic resin and phosphate pigment, 5 to 10 weight percent co-solvent (alcohol), and the balance water. In another example, the varnish coating contains 40 to 60 weight percent dry extract of a mixture of polyurethane resin, aluminum, and silicon oxide, 5 to 10 weight percent co-solvent (alcohol), and the balance water.

[0041] Preferably, in step i, the light source L forms an angle of 53° to 59° with respect to the normal to the steel substrate. Obviously, such an angle range allows for a more linear relationship between the area under the curve and the coating thickness. Even more preferably, in step i, the light source forms an angle of 55° to 57° with respect to the normal to the steel substrate.

[0042] Preferably, in step i, the light source L includes a wavelength of 1.0 to 5.0 μm, and the wavelength range W MEAS is at least 1.0-5.0 μm. Such a range allows for a broader range of wavelengths emitted by the light source, which can then be measured in step ii and processed in steps iii and iv, thereby increasing the accuracy of the estimation.

[0043] Preferably, in step i, the entire width of the moving steel substrate is illuminated.

[0044] Preferably, in step ii, the measurements are performed by a hyperspectral camera.

Claims

1. 1. A method for estimating the thickness of a varnish coating having a thickness of 0.5 to 5 μm on a moving steel substrate 1 provided with a varnish coating 2, comprising: i. illuminating the coated moving steel substrate with an illumination source L comprising wavelengths between 2.7 and 3.7 μm and forming an angle of incidence between 51° and 61° relative to the normal of the moving steel substrate; ii. After reflection on the moving steel substrate, the light is p-polarized and is polarized in the wavelength range W of at least 2.7 to 3.7 μm. ＭＥＡＳ measuring the intensity of light at iii. Using the reference spectrum and the intensities measured in step ii, measure at least the wavelength range W ＭＥＡＳ The absorbance spectrum A of the varnish coating ＭＥＡＳ determining a iv. At least the wavelength range W ＭＥＡＳ The absorbance spectrum A ＭＥＡＳ determining the area under the curve of the intensity as a function of wavelength representing the absorbance of v. Estimating the varnish thickness using the area under the curve and a reference value relating the area under the curve of the absorbance spectrum of the varnish coating to the varnish coating thickness; A method comprising:

2. The method of claim 1 , wherein the moving steel substrate is electrical steel.

3. The method according to claim 1 or 2, wherein the varnish coating has a thickness of 0.5 to 6 μm.

4. The method according to claim 1 or 2, wherein the varnish coating has a thickness of 0.5 to 2 μm.

5. 3. The method of claim 1 or 2, wherein the varnish coating is an aqueous solution containing 25 to 75 weight percent resin, 5 to 15 weight percent solvent, and the balance consisting of water.

6. 3. The method according to claim 1, wherein in step i, the illumination source L forms an angle of 53° to 59° with respect to the normal of the moving steel substrate.

7. In step i, the illumination source L includes wavelengths from 1.0 to 5.0 μm, and the wavelength range W ＭＥＡＳ The method according to claim 1 or 2, wherein the thickness is at least 1.0 to 5.0 μm.

8. 3. The method according to claim 1, wherein in step i, the illumination source L and the moving coated steel substrate are spaced apart by a distance of 20 cm to 60 cm.

9. 3. The method according to claim 1 or 2, wherein in step i, the illumination source L is configured to illuminate an area having a width equal to the width of the strip and a length of at least 20 mm.

10. The method of claim 1 or 2, wherein in step ii, the measurements are performed by a hyperspectral camera.

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