A method for determining the minimum film thickness of intermediate coating

Abstract

This application discloses a method for determining the minimum thickness of an intermediate coating, relating to the technical field of coating thickness determination. The method comprises: preparing paint film samples with different intermediate coating thicknesses, measuring the transmittance of each paint film sample at a preset wavelength of ultraviolet and visible light; obtaining the transmittance at a set wavelength and the average transmittance of each wavelength interval; obtaining the average of the transmittance at the set wavelength and the average transmittance for each paint film sample, constructing an intermediate coating thickness model, and obtaining model parameters; and obtaining the minimum film thickness corresponding to the set wavelength and each wavelength interval based on the maximum transmittance limit at the set wavelength, the maximum transmittance limit for each wavelength interval, and the model parameters. The product of the maximum value of the minimum film thicknesses and a safety factor is used as the minimum film thickness of the intermediate coating. This application not only simplifies the minimum film thickness determination process, but also greatly reduces the determination time and saves determination costs.

Description

Technical Field

[0001] The present application relates to the technical field of coating thickness determination, and in particular to a method for determining the minimum film thickness of an intermediate coating. Background Art

[0002] At present, the automotive intermediate coating is a coating used between the automotive primer and topcoat. It can firmly adhere to the surface of the primer and can easily combine with the topcoat coating above it, playing a connecting role and further isolating the contact between the external environment and the vehicle body substrate.

[0003] In the prior art, the thickness of the intermediate coating layer is determined by UV aging testing of a complete paint coating. However, artificial UV aging tests typically require 500-1000 hours, and require repeated preparation of samples with various intermediate coating thicknesses. Furthermore, the UV aging test costs approximately 8 yuan per hour for a single sample. This not only complicates sample preparation but also takes a long time and incurs high testing costs, making the determination of intermediate coating thickness time-consuming and costly. Summary of the Invention

[0004] In response to one of the defects in the prior art, the purpose of this application is to provide a method for determining the minimum film thickness of the intermediate coating, so as to solve the problem that the process of determining the film thickness of the intermediate coating in the related art is time-consuming and costly.

[0005] To achieve the above objectives, the present application provides a method for determining the minimum film thickness of an intermediate coating, which comprises the following steps:

[0006] Prepare paint film samples with different intermediate coating thicknesses and measure the transmittance of ultraviolet and visible light in a preset wavelength band for each paint film sample;

[0007] Selecting a set wavelength and a plurality of non-connected wavelength intervals from the preset wavelength band, and obtaining the transmittance of the set wavelength and the average transmittance of each wavelength interval;

[0008] Obtain the transmittance at a set wavelength and the average of the average transmittances for each paint film sample, use the logarithm of the average value as the independent variable, and the intermediate coating thickness as the dependent variable to construct an intermediate coating thickness model and obtain the model parameters;

[0009] According to the maximum transmittance limit of the set wavelength, the maximum transmittance limit of each wavelength range, and the model parameters, the minimum film thickness corresponding to the set wavelength and each wavelength range is obtained respectively, and the product of the maximum value of each minimum film thickness and the safety factor is used as the minimum film thickness of the intermediate coating.

[0010] In some embodiments, the intermediate coating thickness model is:

[0011]

[0012] Wherein, D is the thickness of the intermediate coating; is the logarithm of the mean; a and b are model parameters.

[0013] In some embodiments, constructing an intermediate coating thickness model specifically includes:

[0014] A curve of the logarithm of the intermediate coating film thickness and the average value is constructed, and the model parameters of the intermediate coating film thickness model are determined by the least squares method to obtain the intermediate coating film thickness model.

[0015] In some embodiments, obtaining the minimum film thickness corresponding to a set wavelength or any wavelength range specifically includes:

[0016] Multiply the maximum transmittance limit of the set wavelength or the maximum transmittance limit of any wavelength range by 100, subtract the result from b, and take the quotient of the difference and a as the minimum film thickness corresponding to the set wavelength or the wavelength range.

[0017] In some embodiments, preparing a paint film sample specifically includes:

[0018] Fix the transparent polytetrafluoroethylene sheet on the steel plate and mask the edge of the transparent polytetrafluoroethylene sheet with high-temperature resistant tape;

[0019] After spraying the intermediate coating and the topcoat in sequence, the paint film is dried to obtain the paint film, and the paint film is peeled off from the transparent polytetrafluoroethylene sheet in one piece to obtain a paint film sample.

[0020] In some embodiments, the thickness of the intermediate coating is in the range of 6-20 microns.

[0021] In some embodiments, the preset wavelength range is 290-600 nanometers.

[0022] In some embodiments, among the multiple non-connected wavelength intervals, the maximum value of at least one wavelength interval is smaller than the set wavelength.

[0023] In some embodiments, the set wavelength is 400 nanometers, and the multiple non-connected wavelength intervals are 301-320 nanometers, 401-430 nanometers, 431-450 nanometers, and 451-470 nanometers.

[0024] In some embodiments, measuring the transmittance of ultraviolet and visible light of any paint film sample in a predetermined wavelength band specifically includes:

[0025] The transmittance of the paint film sample in the ultraviolet / visible light band of 290-600 nm was measured using an ultraviolet-visible spectrophotometer, with measurement taken every 2 nm, an integration time of 3 seconds, and a slit width of 4 nm.

[0026] The beneficial effects of the technical solution provided by this application include:

[0027] The method for determining the minimum film thickness of the intermediate coating of the present application is as follows: by preparing paint film samples with different intermediate coating film thicknesses and measuring the transmittance of ultraviolet and visible light of each paint film sample in a preset band, a set wavelength and multiple non-connected wavelength intervals can be selected from the preset band, and the transmittance of the set wavelength and the average transmittance of each wavelength interval can be obtained; then, the transmittance of the set wavelength and the average value of each average transmittance are obtained for each paint film sample, and the logarithm of the average value is used as the independent variable and the intermediate coating film thickness is used as the dependent variable to construct an intermediate coating film thickness model and obtain the model parameters; based on the maximum transmittance limit of the set wavelength, the maximum transmittance limit of each wavelength interval, and the model parameters, the minimum film thickness corresponding to the set wavelength and each wavelength interval can be obtained, and the product of the maximum value of each minimum film thickness and the safety factor is used as the minimum film thickness of the intermediate coating. Since the transmittance of ultraviolet and visible light of the paint film sample is directly measured and the set wavelength and the minimum film thickness corresponding to each wavelength range are determined by the model parameters of the constructed intermediate coating film thickness model, the maximum value of each minimum film thickness can be selected and multiplied by the safety factor to obtain the minimum film thickness of the intermediate coating. This not only makes the determination process simple and feasible, but also greatly reduces the determination time and saves the determination cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0029] Figure 1 Flowchart of the method for determining the minimum film thickness of the intermediate coating in the embodiment of the present application. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of this application more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0031] The embodiment of the present application provides a method for determining the minimum film thickness of an intermediate coating, which can solve the problem in the related art that the process of determining the film thickness of the intermediate coating is time-consuming and costly.

[0032] like Figure 1As shown, the method for determining the minimum film thickness of the intermediate coating of this embodiment specifically includes the steps of:

[0033] S1. Prepare paint film samples with different intermediate coating thicknesses and measure the transmittance of ultraviolet and visible light in a preset wavelength band for each paint film sample.

[0034] The number of samples of the paint film is at least 3, and the thickness of the intermediate coating layer ranges from 6 to 20 microns.

[0035] S2. Select a set wavelength and multiple non-connected wavelength intervals from the above-mentioned preset wavelength band, and obtain the transmittance of the above-mentioned set wavelength and the average transmittance of each wavelength interval.

[0036] In this embodiment, the transmittance of each paint film sample at a set wavelength and the average transmittance in each wavelength range need to be obtained respectively.

[0037] S3. Obtain the transmittance at a set wavelength and the average value of each average transmittance for each paint film sample, use the logarithm of the above average value as the independent variable, and use the intermediate coating thickness as the dependent variable to construct an intermediate coating thickness model and obtain the model parameters.

[0038] S4. According to the maximum transmittance limit of the set wavelength, the maximum transmittance limit of each wavelength range, and the model parameters, the minimum film thickness corresponding to the set wavelength and each wavelength range is obtained respectively, and the product of the maximum value of each minimum film thickness and the safety factor is used as the minimum film thickness of the intermediate coating.

[0039] In this embodiment, before the above step S4, the following steps are further included: obtaining the maximum transmittance limit of the set wavelength and the maximum transmittance limit of each wavelength range.

[0040] Optionally, the maximum transmittance limit of the set wavelength and the maximum transmittance limit of each wavelength range can be set according to actual needs.

[0041] In this embodiment, the safety factor is greater than or equal to 1. Preferably, the safety factor is 1.2.

[0042] The determination method of this embodiment is to prepare paint film samples with different intermediate coating film thicknesses, and measure the transmittance of ultraviolet and visible light of each paint film sample in a preset band, so as to select a set wavelength and multiple non-connected wavelength intervals from the preset band, and obtain the transmittance of the set wavelength and the average transmittance of each wavelength interval; then, obtain the transmittance of the set wavelength and the average value of each average transmittance under each paint film sample, and construct an intermediate coating film thickness model with the logarithm of the average value as the independent variable and the intermediate coating film thickness as the dependent variable, and obtain the model parameters; then, according to the maximum transmittance limit of the set wavelength, the maximum transmittance limit of each wavelength interval, and the model parameters, the minimum film thickness corresponding to the set wavelength and each wavelength interval can be obtained, and the product of the maximum value of each minimum film thickness and the safety factor is used as the minimum film thickness of the intermediate coating. Since the transmittance of ultraviolet and visible light of the paint film sample is directly measured and the set wavelength and the minimum film thickness corresponding to each wavelength range are determined by the model parameters of the constructed intermediate coating film thickness model, the maximum value of each minimum film thickness can be selected and multiplied by the safety factor to obtain the minimum film thickness of the intermediate coating. This not only makes the determination process simple and feasible, but also greatly reduces the determination time and saves the determination cost.

[0043] Based on the first embodiment, in this embodiment, the intermediate coating thickness model is:

[0044]

[0045] Wherein, D is the thickness of the intermediate coating; is the average value of the transmittance at the set wavelength and each average transmittance; is the logarithm of the mean; a and b are model parameters. Preferably, the logarithm of the mean is the logarithm with base 10, i.e.

[0046] On the basis of the above embodiment, in this embodiment, the intermediate coating thickness model is constructed, which specifically includes the following steps:

[0047] A curve of the logarithm of the intermediate coating film thickness and the average value is constructed, and the model parameters of the intermediate coating film thickness model are determined by the least squares method to obtain the intermediate coating film thickness model.

[0048] Based on the above embodiment, in this embodiment, in the above step S4, obtaining the minimum film thickness corresponding to the set wavelength or any wavelength range specifically includes the following steps:

[0049] Multiply the maximum transmittance limit of the set wavelength or the maximum transmittance limit of any wavelength range by 100, subtract it from b, and take the quotient of the difference and a as the minimum film thickness corresponding to the set wavelength or the wavelength range. That is, the minimum film thickness corresponding to the set wavelength or each wavelength range is:

[0050] (mb) / a

[0051] Here, m is the maximum transmittance limit of the set wavelength or the product of the maximum transmittance limit of each wavelength range and 100.

[0052] In this embodiment, based on the model parameters of the above model and the transmittance of the set wavelength or wavelength range, the minimum film thickness requirement corresponding to the set wavelength or each wavelength range can be determined, and the optimal minimum film thickness can be quickly found.

[0053] Specifically, when obtaining the minimum film thickness corresponding to the set wavelength, the maximum transmittance limit of the set wavelength is multiplied by 100 and then subtracted from b, and the obtained difference is divided by a to obtain the minimum film thickness corresponding to the set wavelength.

[0054] Specifically, to obtain the minimum film thickness corresponding to any wavelength range, multiply the maximum transmittance limit for that wavelength range by 100, subtract the result from b, and divide the resulting difference by a to obtain the minimum film thickness corresponding to that wavelength range. Similarly, the minimum film thickness corresponding to each other wavelength range can be obtained in sequence.

[0055] Based on the above embodiment, in this embodiment, in the above step S1, preparing the paint film sample specifically includes the following steps:

[0056] First, a transparent polytetrafluoroethylene sheet was fixed on the steel plate, and the edge of the transparent polytetrafluoroethylene sheet was masked with a high-temperature resistant tape.

[0057] Then, after spraying the intermediate coating and the topcoat in sequence, the paint film is dried and peeled off from the transparent polytetrafluoroethylene sheet to obtain a paint film sample.

[0058] In this embodiment, the intermediate coating and topcoat can be sprayed according to the normal painting process, and dried according to the normal process requirements to form a paint film.

[0059] Among them, the topcoat, also known as the final coat, is the last layer of paint applied in a multi-layer coating. The topcoat should have good resistance to external conditions, must have the necessary color and decorative properties, and protect the base coat. Topcoats used outdoors should be selected from coatings with excellent weather resistance. The decorative effect and weather resistance of the topcoat depend not only on the paint base used, but also on the pigment used and the formulation process. In the present embodiment, the thickness of the above-mentioned intermediate coating ranges from 6 to 20 microns.

[0060] Based on actual needs, preferably, the above preset band is 290-600 nanometers.

[0061] Furthermore, among the multiple non-connected wavelength intervals, the maximum value of at least one wavelength interval is smaller than the set wavelength.

[0062] Preferably, the set wavelength is 400 nanometers, and the multiple non-connected wavelength intervals are 301-320 nanometers, 401-430 nanometers, 431-450 nanometers, and 451-470 nanometers.

[0063] Optionally, the set wavelength is 400 nanometers, and the multiple non-connected wavelength intervals are 301-320 nanometers, 351-370 nanometers, 431-450 nanometers, and 451-470 nanometers.

[0064] In other embodiments, the set wavelength is 400 nanometers, and the multiple non-connected wavelength intervals may be 301-320 nanometers, 401-430 nanometers, 431-450 nanometers, and 471-490 nanometers.

[0065] Based on the above embodiment, in this embodiment, in the above step S1, measuring the transmittance of ultraviolet and visible light of any paint film sample in a preset wavelength band specifically includes:

[0066] The transmittance of the paint film sample in the ultraviolet / visible light band of 290-600 nm was measured using an ultraviolet-visible spectrophotometer, with measurement taken every 2 nm, an integration time of 3 seconds, and a slit width of 4 nm. The transmittance values ​​A1-A155 of the paint film sample were obtained.

[0067] In this embodiment, an ultraviolet-visible spectrophotometer is used to measure the transmittance of ultraviolet and visible light through the paint mid-coat, and a mathematical model is constructed to analyze the measurement results and calculate the minimum film thickness of the paint mid-coat, thereby realizing a short process for determining the optimal film thickness of the mid-coat.

[0068] A UV-visible spectrophotometer consists of a power supply, monochromator, absorption cell, detector, and signal processor. The light source is responsible for providing a stable, continuous spectrum of sufficient intensity. Hydrogen or deuterium lamps are typically used in the UV region, while tungsten or halogen lamps are typically used in the visible region. The monochromator decomposes the composite light emitted by the light source and separates monochromatic light of the desired wavelength. Measurements in the visible region are performed using a glass absorption cell, while measurements in the UV region require a quartz absorption cell. The detector detects the intensity of the transmitted light using a photoelectric conversion element, converting the optical signal into an electrical signal.

[0069] Visible light is the part of the electromagnetic spectrum that can be perceived by the human eye, and the visible spectrum does not have a precise range. The wavelength of electromagnetic waves that can be perceived by the average human eye is between 400 and 760nm, and some people can perceive electromagnetic waves with wavelengths between approximately 380 and 780nm.

[0070] Ultraviolet (UV) is the general term for radiation in the electromagnetic spectrum with a frequency of 750 THz to 30 PHz, corresponding to a wavelength of 400 to 10 nm in a vacuum. UV light cannot cause vision and is invisible light with a higher frequency than blue-violet light. Ultraviolet light can be divided into four types: UVA, UVB, UVC, and EUV. UVA is low-frequency, long-wave ultraviolet light with a wavelength of 400 to 320 nm; UVB is medium-frequency, medium-wave ultraviolet light with a wavelength of 320 to 280 nm; UVC is high-frequency, short-wave ultraviolet light with a wavelength of 280 to 100 nm; and EUV is ultra-high-frequency ultraviolet light with a wavelength of 100 to 10 nm.

[0071] The method for determining the minimum film thickness of the intermediate coating layer of this embodiment can be applied to determining the minimum film thickness of the intermediate coating layer on the paint surface of an automobile.

[0072] Specifically, taking the case where the thickness of the intermediate coating is in the range of 6-20 microns, the preset wavelength is in the range of 290-600 nanometers, and the safety factor is 1.2 as an example, the method for determining the minimum thickness of the intermediate coating in this embodiment includes the following steps:

[0073] First, prepare a paint film sample: a 50mm*40mm*0.2mm transparent PTFE sheet is fixed to a steel plate. The edge of the transparent PTFE sheet is masked by approximately 10mm with high-temperature resistant tape. An intermediate coat and topcoat are sprayed sequentially according to the normal painting process. The basecoat and clearcoat are sprayed to the required film thickness. The paint film is then dried according to the normal process to form a paint film. After drying, the paint film is peeled off the transparent PTFE sheet in one piece. Five samples with different intermediate coat film thicknesses are prepared.

[0074] Each paint film sample was then tested for transmittance: a UV-visible spectrophotometer was used to measure the transmittance of each sample in the 290-600 nm band, with measurements taken every 2 nm, an integration time of 3 seconds, and a slit width of 4 nm. Transmittance values ​​A1-A155 for each paint film sample were obtained. For each paint film sample, the transmittance at 400 nm, the average transmittance at 301-320 nm, the average transmittance at 401-430 nm, the average transmittance at 431-450 nm, and the average transmittance at 451-470 nm were obtained.

[0075] The average transmittance from 301 to 320 nanometers is the average of the transmittances at 302, 304, 306, 308, 310, 312, 314, 316, 318, and 320 nanometers. The average transmittance from 401 to 430 nanometers is the average of the transmittances at 402, 404, 406, 408, 410, 412, 414, 416, 418, and 420 nanometers.

[0076] By analogy, the average transmittance between 431 and 450 nanometers and the average transmittance between 451 and 470 nanometers are obtained.

[0077] In this embodiment, the maximum transmittance of the intermediate coating at 400 nanometers is limited to 0.025%, the maximum transmittance at 301-320 nanometers is limited to 0.025%, the maximum transmittance at 401-430 nanometers is limited to 0.15%, the maximum transmittance at 431-450 nanometers is limited to 0.142, and the maximum transmittance at 451-470 nanometers is limited to 0.221%.

[0078] Finally, analysis and calculation were performed: for each paint film sample, the average transmittance at 400 nm, the average transmittance at 301-320 nm, the average transmittance at 401-430 nm, the average transmittance at 431-450 nm, and the average transmittance at 451-470 nm were calculated, and the logarithm of the average was calculated to create a curve of the intermediate coating thickness versus the logarithm. A model for the intermediate coating thickness was constructed to obtain the model parameters a and b, and the minimum film thickness requirements were calculated. The minimum film thickness requirement at 400 nm is (0.025-b) / a; the minimum film thickness requirement at 301-320 nm is (0.025-b) / a; the minimum film thickness requirement at 401-430 nm is (0.15-b) / a; the minimum film thickness requirement at 431-450 nm is (0.142-b) / a; and the minimum film thickness requirement at 451-470 nm is (0.221-b) / a. Take the maximum value of the above five minimum film thickness requirements and multiply it by 1.2 to obtain the minimum film thickness requirement for the intermediate coating of the paint.

[0079] The method for determining minimum film thickness in this embodiment is simple and practical. By preparing paint film samples with varying intermediate coating thicknesses and measuring the transmittance of each paint film sample at a predetermined wavelength, the average of the transmittance at a predetermined wavelength and the average transmittance across each wavelength range is determined for each paint film sample. A model for intermediate coating thickness is constructed using the logarithm of the average as the independent variable and the intermediate coating thickness as the dependent variable, and the model parameters are obtained. Subsequently, based on the maximum transmittance limit at the predetermined wavelength, the maximum transmittance limit for each wavelength range, and the model parameters, the minimum film thickness corresponding to the predetermined wavelength and each wavelength range is determined. The product of the maximum value of these minimum film thicknesses and a safety factor is used as the minimum film thickness of the intermediate coating. By directly measuring the transmittance of visible light and ultraviolet light through the paint coating to test the paint coating's resistance to visible light and ultraviolet light, and establishing a mathematical model for intermediate coating thickness, the method allows for a short-term determination of the required minimum film thickness for the paint intermediate coating. The entire determination process can be controlled within eight hours, significantly reducing the time and cost of determining the minimum film thickness.

[0080] In the description of this application, it should be noted that the terms "upper" and "lower" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application. Unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be internal communication between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to the specific circumstances.

[0081] It should be noted that, in this application, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises", "includes" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the presence of other identical elements in the process, method, article or device comprising the element.

[0082] The above are merely specific embodiments of the present application to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but rather is intended to conform to the widest scope consistent with the principles and novel features of the present application.

Claims

1. A method for determining the minimum film thickness of an intermediate coating, characterized in that: It includes the steps of: Prepare paint film samples with different intermediate coating thicknesses and measure the transmittance of ultraviolet and visible light in a preset wavelength band for each paint film sample; Selecting a set wavelength and a plurality of non-connected wavelength intervals from the preset wavelength band, and obtaining the transmittance of the set wavelength and the average transmittance of each wavelength interval; Obtain the transmittance at a set wavelength and the average of the average transmittances for each paint film sample, use the logarithm of the average value as the independent variable, and use the intermediate coating thickness as the dependent variable to construct an intermediate coating thickness model and obtain the model parameters; According to the maximum transmittance limit of the set wavelength, the maximum transmittance limit of each wavelength range, and the model parameters, the minimum film thickness corresponding to the set wavelength and each wavelength range is obtained respectively, and the product of the maximum value of each minimum film thickness and the safety factor is used as the minimum film thickness of the intermediate coating.

2. The method for determining the minimum film thickness of the intermediate coating according to claim 1, wherein: The intermediate coating thickness model is: Wherein, D is the thickness of the intermediate coating; is the logarithm of the mean; a and b are model parameters.

3. The method for determining the minimum film thickness of the intermediate coating according to claim 2, wherein: Constructing the intermediate coating thickness model, including: A curve of the logarithm of the intermediate coating film thickness and the average value is constructed, and the model parameters of the intermediate coating film thickness model are determined by the least squares method to obtain the intermediate coating film thickness model.

4. The method for determining the minimum film thickness of the intermediate coating according to claim 2, wherein: Get the minimum film thickness corresponding to the set wavelength or any wavelength range, including: Multiply the maximum transmittance limit of the set wavelength or the maximum transmittance limit of any wavelength range by 100, subtract the result from b, and take the quotient of the difference and a as the minimum film thickness corresponding to the set wavelength or the wavelength range.

5. The method for determining the minimum film thickness of the intermediate coating according to claim 1, wherein: Make paint film samples, including: Fix the transparent polytetrafluoroethylene sheet on the steel plate and mask the edge of the transparent polytetrafluoroethylene sheet with high-temperature resistant tape; After spraying the intermediate coating and the topcoat in sequence, the paint film is dried to obtain the paint film, and the paint film is peeled off from the transparent polytetrafluoroethylene sheet in one piece to obtain a paint film sample.

6. The method for determining the minimum film thickness of the intermediate coating according to claim 1, wherein: The thickness of the intermediate coating is in the range of 6-20 microns.

7. The method for determining the minimum film thickness of the intermediate coating according to claim 1, wherein: The preset wavelength range is 290-600 nm.

8. The method for determining the minimum film thickness of the intermediate coating according to claim 7, wherein: Among the multiple unconnected wavelength intervals, the maximum value of at least one wavelength interval is smaller than the set wavelength.

9. The method for determining the minimum film thickness of the intermediate coating according to claim 7, wherein: The set wavelength is 400 nanometers, and the multiple unconnected wavelength intervals are 301-320 nanometers, 401-430 nanometers, 431-450 nanometers, and 451-470 nanometers.

10. The method for determining the minimum film thickness of the intermediate coating according to claim 7, wherein: Measure the transmittance of any paint film sample in the preset UV-visible light band, including: The transmittance of the paint film sample in the ultraviolet / visible light band of 290-600 nm was measured using an ultraviolet-visible spectrophotometer, with measurement taken every 2 nm, an integration time of 3 seconds, and a slit width of 4 nm.

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