JPH11241912A - Film thickness measurement method - Google Patents

Abstract

(57) [Summary] [Technical Problem] Technology for measuring the wet film thickness immediately after coating to improve the responsiveness of film thickness change, enabling dry film thickness to be measured in a wet state, and ensuring film thickness over the entire length of a coated product. Develop. SOLUTION: In a method of measuring a film thickness by infrared irradiation, a wavelength at which infrared energy is absorbed only by a paint and no infrared energy is absorbed by a solvent and / or a wavelength at which infrared energy is absorbed by both a paint and a solvent. Irradiate the wet coated film using infrared rays in the wavelength range including

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of measuring the thickness of a coated film in a coating line of a coated steel plate in a wet state using infrared rays.

[0002]

2. Description of the Related Art In the production of a coated steel sheet having an organic coating film, the thickness of the coating film (hereinafter simply referred to as the film thickness) is dependent on the product performance such as corrosion resistance and weldability. This is a very important management item from a viewpoint. For this reason, various developments and proposals have conventionally been made as a method for measuring the film thickness, but a measurement method using infrared absorption is generally used as an online film thickness measurement method. When the coating film is irradiated with infrared light of a specific wavelength, the irradiated infrared light passes through the coating film and reaches / reflects the surface on which the coating film is formed, and then the reflected infrared light passes through the coating film again to the outside. , A part of infrared rays is absorbed by an organic substance such as a paint contained in the coating film. The above-described method is based on the Lambert-Beer's formula shown in the following formula (1) between the ratio of the energy of the reflected infrared light (I) to the energy of the irradiated infrared light (I ₀ ) and the thickness (t) of the coating film. This is based on the fact that the law holds.

I / I ₀ = R · e ^−2Kt (1) where R: reflectance on the coating film forming surface K: infrared absorptivity in the coating film When measuring the film thickness using the above rule, There are a two-wavelength method using infrared light of two wavelengths and a three-wavelength method using infrared light of three wavelengths. The three-wavelength method has been devised to improve the measurement accuracy of the two-wavelength method.
It is publicly known in JP 3610.

This three-wavelength method uses a wavelength λ _{a at which} absorption occurs.
When the object to be measured is irradiated with infrared light having _a wavelength of λ _b and λ _C (where 2λ _a = λ _b + λ _c ) where no infrared light is absorbed and the intensity of reflected light of each wavelength other than the infrared absorption is attenuated. Properties of steel sheet surface
(Surface roughness, reflectance, etc.) as much as possible. Further, a method considering the influence of infrared reflection on the coating film surface has been proposed in JP-A-63-235805.

[0005]

However, in all of these conventional methods for measuring the film thickness, the film thickness after baking and drying is measured. In order to adjust the distance, it is necessary to control and correct the distance from the coating position to the measurement position. For this reason, we want to measure the film thickness in the wet state when actually painting.
Simply installing the conventional measuring device immediately after the coating device makes it difficult to measure due to the influence of the coating concentration.

On the other hand, as a method of measuring the film thickness in the wet state, the film thickness before and after the transfer of the coating roll is measured, and the rotation speed of the coating roll and the moving speed of the coating roll are used to determine the thickness of the coating object. A method for calculating the film thickness is disclosed in JP-A-63-2423.
No. 75 discloses this. However, even with this method, a change in the surface properties of the coating roll and an influence on the measured film thickness due to a change in the coating concentration occur, so that it is difficult to perform high-precision measurement.

Here, an object of the present invention is to measure the film thickness in a wet state immediately after coating (hereinafter, referred to as a wet film thickness) to minimize the dead time until the film thickness measurement, and to perform baking and drying. It is possible to guarantee the film thickness over the entire length of the coated product by enabling the subsequent film thickness (hereinafter, referred to as dry film thickness) to be measured in a wet state,
An object of the present invention is to provide a method for accurately measuring a wet film thickness.

[0008]

In general, when coating, it is common practice to dilute the coating with a solvent from the viewpoints of workability, surface appearance, and film thickness controllability. Therefore, the present inventors, the thickness of the coating film in the wet state is a state where the paint and the solvent coexist, therefore, what behavior the paint and the solvent take during infrared spectroscopic analysis The following experiment was conducted by paying attention to.

FIG. 1 schematically shows a liquid sample cell 10 in an infrared spectrophotometer, in which a window material 12 accommodates a sample 16 supported by a copper foil 14, which is used as a light source of the spectrometer. It is placed in the path, and the intensity of the transmitted light at that time is measured by a detector (not shown). In this way, the transmitted light was measured by an infrared spectrophotometer separately using the unprepared coating stock solution and the solvent.

FIG. 2 is a graph showing the spectral characteristics (transmittance) of each of the paint and the solvent obtained by this method. The paint at this time was a commercially available paint used for a normal coated steel sheet, and the solvent was also a commercially available thinner. As can be seen from the spectral characteristics shown in FIG. 2, the wavelength at which infrared energy is absorbed by both the paint and the solvent (λ ₂ ) and the wavelength at which infrared energy is absorbed by the paint and the infrared energy is not absorbed by the solvent (λ ₁ ). As in this example, in the case of using the thinner is the coating and solvent used in the conventional coated steel sheet is, lambda ₁ is 6.0 mu
m and λ ₂ were 3.4 μm.

Therefore, the present inventors have further studied these two wavelengths. As a result, the absorbance at the wavelength (λ ₁ ) showing the absorption characteristic only by the paint is changed by changing the ratio of the paint to the solvent and the wet film thickness. No change was observed, and it was confirmed that it corresponded exclusively to the paint components in the coating film. Therefore,
If the relationship with only the paint components in the wet state coating film is obtained, the film thickness after drying (and further the film thickness in the wet state) can be measured based on the relationship, and the paint concentration and the wavelength can be measured. If the relationship of the absorbance at (λ ₁ ) is obtained, the paint concentration can be measured, and the wavelength at which the concentration is affected
By performing the density correction at (λ ₂ ), the actual wet film thickness and dry film thickness can be measured.

It is to be noted that such a difference in the absorption characteristics occurs because the solvent and the paint are first absorbed at λ ₂ because the components in the solvent and the solvent contained in the paint are the same. absorption occurs, whereas, the absorption at lambda ₁ paint only occurs coating components to form the dry film, believed to be due to absorption of the pigment.

Here, the present invention relates to (1) measuring the film thickness of an organic coating film formed on the surface of a metal base material, when measuring the film thickness of the coating film in a wet state, In the film thickness measuring method of irradiating infrared rays having a wavelength at which absorption occurs to the coating film and measuring the thickness of the coating coating film from the intensity of the reflected light, infrared energy absorption by only the coating material contained in the coating coating film is observed. Detecting the intensity of reflected light when irradiating infrared rays in a wavelength range including a wavelength at which infrared energy absorption by the solvent does not occur to the coating film in a wet state, and calculating a detection formula from the detection value in advance. (2) measuring the thickness of the organic coating film formed on the surface of the metal base material, wherein the dry film thickness td of the coating film is calculated based on the thickness of the coating film. Measure film thickness in wet condition In doing so, the coating film is irradiated with infrared light having a wavelength at which absorption occurs, and in a film thickness measuring method for measuring the thickness of the coating film from the intensity of the reflected light, only the paint contained in the coating film is used. Infrared energy absorption is seen, the wavelength in which the infrared energy absorption by the solvent does not occur, and the infrared ray in the wavelength range including the wavelength where the infrared energy absorption occurs by both the paint and the solvent, when irradiating the coating film in the wet state The intensity of each reflected light is detected, and the wet film thickness tw and the dry film thickness of the coating film are determined based on the detection values based on a preset calculation formula.
This is a film thickness measurement method characterized by simultaneously calculating td.

[0014]

Next, embodiments of the present invention will be described more specifically with reference to the accompanying drawings. FIG.
1 is a schematic explanatory view of a film thickness measuring device 20 for performing a film thickness measuring method according to the present invention.

In the figure, the wavelengths λ ₁ and λ radiated from the light source 21 are shown.
Infrared rays including the infrared rays ₂ pass through the chopper as they pass through the polarizer 22 or as infrared rays having wavelengths λ ₁ and λ ₂ respectively selected by a filter (not shown) disposed in the chopper 23. The first optical means 24 such as a mirror irradiates the measurement target 25 such as a coated steel plate in a different direction.
The infrared light reflected on the coating film forming surface 28 of the measurement object 25 is then collected by the second optical unit 26, and the intensity thereof is detected by the light intensity detector 27. Is input to the film thickness calculator 28 as a signal.

The film thickness calculator 30 calculates the wavelength λ ₁ or the wavelength λ
_1, lambda ₂ in the range intensity detection signals of the reflected light when irradiated with infrared rays, including, on the basis of the calculation formula which is set in advance, and outputs a dry film thickness and wet film thickness, respectively.

Hereinafter, the specific operation will be described.
The formula for calculating the dry film thickness td is shown in FIG. 4 by the absorbance at the wavelength λ ₁ (absorption is observed only by the paint) and the film thickness (μ) of the paint alone.
m), it can be obtained as follows.

First, assuming that I ₀ : light quantity without a coating film and I ₁ : light quantity with a coating film, the following equations (2) and (3) are obtained. Transmittance = I ₁ / I ₀ ... (2) Absorbance A = log ₁₀ (I ₁ / I ₀ )... (3) On the other hand, from FIG. Is represented by the following equation (4). _{tg = B 1 · A + B} 2 ··· (4) Here, B _1, B ₂ are constants.

The dry film thickness td after drying is obtained by multiplying the film thickness tg of the paint stock solution by the non-volatile residue N ₁ of the paint stock solution, that is, the ratio of components remaining after drying and baking of the stock solution before dilution with the solvent. Therefore, it can be expressed by a linear equation as shown in the following equation (5). _{td = tg · N 1 = C} 1 · A + C 2 ··· (5) Here, C _1, C ₂ are constants.

Next, another embodiment of the present invention will be described as follows. That is, the relationship between the paint concentration Nd at the reference film thickness and the absorbance at the wavelength λ ₁ is as shown in FIG. 5, and the relationship between the paint concentration Nd and the absorbance at this time is expressed by the following equation.
It is expressed by a linear equation as shown in (6). In addition, paint concentration (%)
Is the ratio of the stock solution of the paint to the paint after dilution with the solvent. _{Nd = D 1 · A + D} 2 ··· (6) Here, D _1, D ₂ are constants.

The relationship between the paint concentration Nd and the actual non-volatile residue Nv of the wet coating film can be expressed by the following equation (7). _{Nv = N 1 · Nd = E} 1 · A + E 2 ··· (7) Here, E _1, E ₂ are constants.

Here, when the actual wet film thickness tw is obtained,
Since the wet film thickness is obtained by dividing the dry film thickness by the actual non-volatile residue Nv, it is expressed by equation (8). tw = td / Nv = tg / Nd ··· (8) That is, since a tw = td / Nv, this equation and Nv = N ₁ · Nd
From the relational expression, tw = (td / N ₁ ) (1 / Nd), and (td / N ₁ ) = tg
Therefore, by substituting this, the latter half of the above equation (8) can be obtained. When the equations (4) and (6) are introduced into the equation (8), the wet film thickness tw is simplified and represented by the absorbance A.

[0023]

(Equation 1)

Therefore, according to this embodiment of the present invention, the wet film thickness is calculated by the equation (9) by measuring the absorbance A at the infrared wavelength λ ₁ by the film thickness measuring device of FIG. The film thickness can be easily obtained.

In still another embodiment of the present invention, not only the wavelength λ ₁ at which infrared absorption is caused by the paint alone but also the wavelength λ _{2 at which} infrared absorption is observed by both the paint and the solvent.
Irradiation in the wavelength range including According to this embodiment, the infrared ray having the wavelength (λ ₁ ) is applied to the coating liquid having a constant depth by the wavelength (λ ₂ ).
Is applied to the wet coating film. By determining the concentration of the coating solution from the measurement of the absorbance in the infrared lambda ₁ According to this, it is possible to correct the measured wavelength of lambda ₂ affected by the concentration of the coating solution, it is because it is possible to obtain the same measurement results . Next, the operation and effect of the present invention will be described more specifically with reference to examples.

[0026]

EXAMPLE This example is an example in which the film thickness measuring method according to the present invention is applied to a two-roll coating type coating equipment.

FIG. 6 (a) shows a measuring apparatus and a peripheral configuration when the method according to the first embodiment of the present invention is performed. As a method of coating, the paint 34 of the paint pan 32 is lifted by the pickup roll 36, the paint is squeezed between the applicator roll 38, and the paint on the applicator roll is transferred to the surface of the steel sheet S on the backup roll 40. The film thickness is adjusted by adjusting the peripheral speed of each roll, the amount of pressing between the rolls, and the like.

In such a configuration, in the present invention, the backup roll immediately after the applicator roll 38 is used.
Above 40, a film thickness measuring device 42 is installed and a specific wavelength λ of infrared light is set.
Measure the amount of infrared light of the selected wavelength by absorption and reflection of ₁ .
The signal is output to the film thickness calculator 44. In this film thickness calculating device 44, the dry film thickness td is obtained by the above equation (5), or the dry film thickness td and the wet film thickness td are obtained by the equations (8) and (9).
w is calculated.

According to another embodiment, as shown in FIG. 6 (b), the paint supplied to the paint pan 32 is partially supplied to an FT-IR device 46, which is a Fourier transform type absorptiometer, and a specific wavelength is supplied. measuring the concentration of the coating solution from the infrared absorption of lambda _1, and outputs the signal to the film thickness calculating unit 44.

On the other hand, the film thickness calculating device 44 includes a film thickness measuring device.
42 (infrared wavelength λ ₂ )
Appropriately corrected from the paint density data obtained using the infrared wavelength lambda ₁ from T-IR apparatus 46 calculates the wet film thickness and dry film thickness.

In this example, the infrared absorption by the paint and the solvent was measured, and the wet film thickness was measured. Film thickness measuring device
The specific wavelength of 42 was 3.4 μm, and a P-deflection Brewster angle method was adopted to reduce the influence of surface reflected light, and the Brewster angle was 56 °. In addition, to reduce the effect of fluctuations in the steel plate surface reflectivity, a reference wavelength of 3.2
, And a three-wavelength system using 3.6 μm infrared rays.

Here, "P deflection Brewster angle system"
Is a method of irradiating an object by deflecting infrared light by a certain angle with a polarizer, and setting the Brewster angle to prevent the angle of incidence on the measurement object from being reflected by the coating film surface It is.

As shown in FIG. 7, when the wavelength in the 3.4 μm band is measured as the same film thickness by the total reflection absorption method, the absorbance varies with the paint concentration, so that the absorbance of the paint concentration needs to be corrected.

In the present invention, the paint is branched from the pipe on the paint supply side to the paint pan and circulated to the FT-IR device 46.
The FT-IR device 46 measures the absorbance by irradiating the paint with infrared light having a specific wavelength of 6.0 μm, at which the paint alone absorbs infrared light, and measures the concentration of the paint. The results are shown in FIG.

The measurement of the absorbance in the FT-IR device 46 was performed by using the total reflection absorption method so as not to be affected by the thickness of the paint. That is, the angle at which the infrared rays enter the coating film is determined by using a prism having an angle θ at which the infrared rays are totally reflected in the coating liquid, and the reflection depth in the coating liquid is always kept constant. As a result, if the reflection depth in the coating liquid is constant, the coating concentration can be accurately measured by the aforementioned equation (6).

The paint concentration Nd is calculated based on the absorbance measured by the FT-IR device 46. This is obtained by previously measuring data when a known concentration is changed, and using the data obtained thereby. The relational expression of FIG. 8 is derived, and the following expression (10) is obtained from this relation.

Nd = a ₁ A _6.0 + b ₁ (10) A _6.0 : absorbance at a wavelength of 6.0 μm Here, a ₁ and b ₁ are constants.

The wet film thickness tw can be determined by the following equation (11) obtained from FIG. tw = a ₂ A _3.4 + b ₂ (11) A _3.4 : Absorbance in the 3.4 μm band after three-wavelength correction Here, a ₂ and b ₂ are constants.

Equation (11) uses a wavelength in the 3.4 μm band where the absorbance changes due to the effect of the paint concentration, and needs to be corrected. However, since the concentration is not corrected, the absorbance of this equation is Assuming that the target of the paint concentration is N _{0 in} FIG. 7, the absorbance A at this time is A = a ₃ N ₀ + b ₃ (a ₃ and b ₃ are constants), and the concentration at the time of actual measurement is A = a ₃
As Nd + b _3, a density correction to the _{tw = a 2 A 3.4 × k} + b 2 ··· (12) However, K = (3.4 μm band absorbance at actual concentration) /
(Absorbance at 3.4 μm band at target concentration) = (a ₃ Nd + b ₃ ) / (a ₃ N ₀ + b
₃ ) where K is a variable determined by the concentration.

By simplifying the equation (12) by substituting the equation (10), the following equation (13) for the wet film thickness is obtained. _{tw = c 1 A 3.4 A 6.0} + c 2 A 3.4 + b 2 ··· (13) The dry film thickness td is multiplied by the actual paint nonvolatile content Nv wet thickness tw td = tw · Nv ···· (14) Here, Nv is the product of the paint concentration Nd and the non-volatile residue N ₁ (constant) of the paint stock solution, and is expressed by absorbance as in the following equation (15). ^{_{td = αA 3.4 A 6.0 2 +}} βA 3.4 A 6.0 + γA 3.4 + δA 6.0 + ε ··· (15) Here, α, β, γ, δ , is both epsilon constant.

FIG. 10 shows a case where the wet film thickness is not corrected by the paint concentration, and FIG. 11 shows a case where the wet film thickness is corrected. As can be seen from these results, when the correction is performed, the accuracy is ± 20%, but when the correction is performed, the accuracy is ± 10%.
The following precision wet film thickness can be measured, and it can be confirmed that the wet film thickness measurement is highly accurate.

FIG. 12 shows a comparison of the dry film thickness measurement result according to the present invention with the film thickness measured by X-ray fluorescence. The measurement result of the dry film thickness was about ± 10% in the present invention, and a good result was obtained.

The calibration curve for online wet film thickness calculation is prepared with an accuracy of about 5%, but the overall equipment accuracy is ± 10% as described above. Improvement ・ Further accuracy can be expected depending on the countermeasures against vibration in the line.

In the present invention, the method of measuring the film thickness has been described. However, from the result of the film thickness measurement of the present invention, the signal output to a film thickness control device or the like can be applied to various automatic film thickness control systems which have been developed. Needless to say, this is considered to contribute to the improvement of the accuracy of the automatic film thickness control system. In addition, since the paint concentration is also detected, the paint concentration is stabilized by outputting a signal to the control system of the paint supplied to the coating equipment.

[0045]

As described above, according to the present invention, the dry film thickness after drying can be accurately measured from the film thickness in the wet state immediately after the coating roll, and the film thickness in the wet state can also be accurately measured. The following effects can be obtained.

(1) Since the wet film thickness and the dry film thickness can be measured simultaneously on-line immediately after the coating, the feedback time can be shortened and the film thickness over the entire length of the product can be made uniform. (2) Since the measurement accuracy of the wet film thickness has been improved, the film thickness can be accurately adjusted to the target film thickness when the film thickness is controlled. (3) By making the thickness of the product uniform, it is possible to produce a high-quality organic film-forming metal material that does not cause welding defects.

[Brief description of the drawings]

FIG. 1 is a schematic illustration of a liquid sample cell for infrared spectroscopy.

FIG. 2 is a graph showing spectral characteristics of infrared rays.

FIG. 3 is a schematic view of the inside of a film thickness measuring apparatus according to the present invention.

FIG. 4 is a graph showing the relationship between the absorbance and the film thickness of only the paint.

FIG. 5 is a graph showing the relationship between absorbance and coating amount concentration.

FIGS. 6 (a) and 6 (b) are side views of respective embodiments showing a film thickness measuring device and its peripheral configuration.

FIG. 7 is a graph showing the relationship between the paint concentration and the absorbance at a specific wavelength in an example.

FIG. 8 is a graph showing the relationship between the paint concentration and the absorbance at a specific wavelength in Examples.

FIG. 9 is a graph showing the relationship between absorbance and wet film thickness.

FIG. 10 is a graph showing wet film thickness measurement accuracy when density correction is not performed.

FIG. 11 is a graph showing wet film thickness measurement accuracy when concentration correction is performed.

FIG. 12 is a graph showing a dry film thickness measurement accuracy according to the present invention.

Claims (3)

[Claims] In the measurement of the thickness of an organic coating film formed on the surface of a metal base material, when measuring the film thickness of the coating film in a wet state, the wavelength at which absorption occurs in the coating film is measured. In the film thickness measuring method of irradiating infrared rays and measuring the thickness of the coating film from the intensity of the reflected light, infrared energy absorption is observed only by the paint contained in the coating film, and infrared energy absorption by the solvent occurs. Infrared light (λ ₁ ) in the wavelength range including the wavelengths not to be reflected is detected as the intensity of the reflected light when the coating film in the wet state is irradiated, and the coating value is calculated from the detected value based on a preset calculation formula. A film thickness measuring method comprising calculating a dry film thickness td of a film. 2. In measuring the thickness of an organic coating film formed on the surface of a metal base material, when measuring the film thickness of the coating film in a wet state, the wavelength at which absorption occurs in the coating film is measured. In the film thickness measuring method of irradiating infrared rays and measuring the thickness of the coating film from the intensity of the reflected light, infrared energy absorption is observed only by the paint contained in the coating film, and infrared energy absorption by the solvent occurs. Infrared light (λ ₁ ) in the wavelength range including the wavelengths not to be reflected is detected as the intensity of the reflected light when the coating film in the wet state is irradiated, and the coating value is calculated from the detected value based on a preset calculation formula. Calculate the dry thickness td of the film, and further determine the paint concentration and wavelength (λ ₁ ) at the previously determined reference film thickness.
And calculating a wet film thickness tw and a dry film thickness based on the relationship. 3. When measuring the thickness of an organic coating film formed on the surface of a metal base material, when measuring the film thickness of the coating film in a wet state, the wavelength at which absorption occurs in the coating film is measured. In the film thickness measuring method of irradiating infrared rays and measuring the thickness of the coating film from the intensity of the reflected light, infrared energy absorption is observed only by the paint contained in the coating film, and infrared energy absorption by the solvent occurs. Irradiation of infrared rays in a wavelength range including a wavelength (λ ₁ ) that does not occur and a wavelength (λ ₂ ) at which infrared energy is absorbed by both the paint and the solvent is applied to the coating liquid at a certain depth and the wet-painted coating film, respectively. The intensity of each reflected light at the time of the above is detected, and the wet film thickness tw and the dry film thickness of the coating film are determined from the detected values based on a calculation formula set in advance.
A film thickness measuring method, wherein td is calculated simultaneously.