Cleaning evaluation method and electrode for cleaning evaluation

Abstract

To provide a new method capable of accurately evaluating the state of a cleaning surface (the kind, presence, etc., of a residue).SOLUTION: The present invention is a cleaning evaluation method including a measurement step of bringing at least a pair of flexible electrodes containing an electrolytic solution into close contact with a cleaning surface of a substrate and applying an alternating current between the electrodes to obtain an AC characteristic of the cleaning surface, and a determination step of determining cleanliness or a residue on the cleaning surface based on the AC characteristic. The electrode is made of, for example, a porous base material such as a water-absorbing sponge. When the surface to be cleaned is washed with water and the residue is a hydrophobic oil film, an aqueous solution of an inorganic salt may be used as the electrolyte. Examples of the AC characteristics include impedance, relaxation time, dielectric constant, and dielectric loss tangent. The AC characteristics are obtained from, for example, an impedance spectrum or a relaxation time distribution function.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating a cleaning surface. [Background technology]

[0002] The surface of a member (substrate) to be subjected to surface treatment, painting, etc. is cleaned beforehand. Cleaning methods include water washing (sometimes referred to as "water washing"), acid washing, alkali washing, etc., and are often used to remove oils and grease adhering to the surface (degreasing). If the surface is not cleaned sufficiently, unintended reaction products may be generated on the surface, and defects such as unevenness and poor adhesion may occur in the layer or film that is formed.

[0003] For this reason, it is important to evaluate the condition of the cleaned surface (cleaned surface) (presence or absence of residue, cleanliness, etc.). However, it is difficult to stably visually inspect the extremely thin oil film remaining on the surface after cleaning on a production line, etc. For this reason, a method for accurately evaluating the condition of the cleaned surface has been desired. Relevant disclosures can be found in the following patent documents, for example: [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 58-35086 [Patent Document 2] Patent Publication No. 5-118989 [Patent Document 3] Patent Publication No. 2011-107029 [Patent Document 4] Patent Publication No. 7-243970 [Patent Document 5] Patent Publication No. 2005-262234 [Patent Document 6] Patent Publication No. 2015-45514 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, the presence or absence of plating applied to the surface of a steel sheet is determined based on the contact resistance obtained by passing a direct current through the steel sheet.

[0006] Patent Documents 2 and 3 evaluate the degreasing state (cleaning level) based on the intensity (distribution) of fluorescence obtained by irradiating excitation light. Patent Document 4 measures the amount of oil applied to a metal surface based on the spectral distribution of reflected light, including fluorescence, obtained by irradiating excitation light. Measurements using reflected light are easily affected by the measurement environment (light leakage, etc.), making them difficult to introduce into production lines.

[0007] Patent Document 5 adjusts the oil film on the surface of a metal strip based on the wiping temperature, while Patent Document 6 determines the oil film thickness on the cylinder liner surface based on the amplitude of the reflected ultrasonic wave.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a method for evaluating the condition of a cleaning surface using a technique different from conventional methods. [Means for solving the problem]

[0009] As a result of extensive research, the inventors have discovered that it is possible to accurately evaluate the condition of a surface to be cleaned (presence or absence of residue, type of residue, etc.) based on the AC characteristics of the surface to be cleaned measured using a specific electrode. By expanding on this finding, the inventors have completed the present invention, which will be described below.

[0010] <Evaluation method for phosphorus oxide film formation> The present invention provides a cleaning evaluation method comprising a measurement step of bringing at least a pair of flexible electrodes containing an electrolyte into close contact with the cleaning surface of a substrate and passing an alternating current between the electrodes to determine the AC characteristics of the cleaning surface, and a determination step of determining the cleanliness or residue of the cleaning surface based on the AC characteristics.

[0011] According to the cleaning evaluation method of the present invention (also simply referred to as the "evaluation method"), the cleanliness or residue on the substrate surface (cleaned surface) after cleaning can be accurately determined. The mechanism behind this is presumed to be as follows.

[0012] Residues on the cleaned surface (oil film, etc.) are considered to be a type of dielectric. Therefore, if there are residues on the cleaned surface, AC characteristics corresponding to the type and shape of the residues should be obtained.

[0013] However, even when a thin plate or foil metal electrode was placed on the surface to be cleaned and an AC current was applied, no effective AC characteristics reflecting the condition of the surface could be obtained. The residue on the surface to be cleaned is usually a thin film or layer that is difficult to see with the naked eye, so it is thought that this is because a solid electrode with a certain rigidity does not adhere stably to the residue or comes into contact with the object being cleaned regardless of whether or not there is residue. In other words, it is thought that information about the residue cannot be obtained because substances other than the residue (such as air) are present between the metal electrode surface and the surface to be cleaned, or the solid electrode comes into direct contact with the object without any dirt in between.

[0014] On the other hand, the electrode of the present invention is made of a flexible substrate containing an electrolyte, and therefore stably contacts (adheres) with the cleaning surface or residue thereon. When an alternating current is applied between such electrodes, AC characteristics are obtained that reflect the condition of the cleaning surface or residue with which the electrode surface is in contact, thereby enabling accurate understanding of the condition of the cleaning surface. For example, if the residue is composed of hydrophobic oil, a stable interface with the aqueous solution is formed, making it easy to obtain a stable output.

[0015] The evaluation method of the present invention can be carried out simply by placing (contacting) electrodes on the cleaning surface to be inspected and passing an alternating current through them, so it is possible to quickly grasp (estimate) the condition of the cleaning surface of a product (component) extracted from a production line where a cleaning process, surface treatment process, painting process, etc. is carried out on-site.

[0016] 《Electrode / device》 The present invention can also be understood as an electrode or device used in the above-mentioned evaluation. For example, the present invention may be an electrode for evaluating cleaning, which has a flexible porous substrate and an electrolyte solution impregnated into the substrate, and is used in the above-mentioned evaluation method.

[0017] The present invention may also be a cleaning evaluation device comprising at least a pair of flexible electrodes containing an electrolyte, a measuring means (and further an analyzing means) for determining AC characteristics by passing a current between the electrodes in close contact with the cleaning surface of a substrate, and a determining means for comparing the AC characteristics with a reference characteristic prepared in advance to determine the cleanliness of the cleaning surface or the presence of residues.

[0018] "others" (1) In this specification, the terms "means" and "steps" can be interpreted interchangeably, and thus the components of a "product (apparatus)" and those of a "method" can be interchangeable. All or part of the "means" or "steps" can be realized, for example, by executing a program on a computer.

[0019] (2) Unless otherwise specified, "x to y" in this specification includes a lower limit of x and an upper limit of y. Any numerical value included in the various numerical values ​​or ranges described in this specification may be used as a new lower limit or upper limit to create a new range, such as "a to b." [Brief explanation of the drawings]

[0020] [Figure 1] 1 is an overview of a measurement system (example). [Figure 2] 1 is an example of an impedance spectrum obtained by measuring a cleaning surface. [Figure 3] 10 is an example of a relaxation time distribution function spectrum obtained by measuring a cleaned surface. DETAILED DESCRIPTION OF THE INVENTION

[0021] One or more components arbitrarily selected from this specification may be added to the above-described components of the present invention. The contents described in this specification may be applied not only to "products" but also to "methods" as appropriate. Which embodiment is best depends on the target, required performance, etc.

[0022] "subject" The evaluation target is the surface of the substrate after cleaning (cleaned surface). There are various cleaning purposes, cleaning solutions, cleaning processes, etc.

[0023] The substrate may be of any material or form (shape, size). The substrate may be a conductor or a non-conductor. For example, when the substrate is made of a metal (conductor) such as a pure metal, alloy, or composite material, the substrate may be in its raw form after being cast or rolled, or may be machined or surface-treated (plated, anodized, etc.). A typical example of a substrate is a plate material such as a steel plate. The plate material may be dull-finished or plated (e.g., zinc-plated).

[0024] The surface to be cleaned may be, for example, a surface that has been cleaned (eg, degreased) with a solution containing a surfactant or the like or an organic solvent, or may be a surface that has been washed with water (eg, pure water) (water-washed surface).

[0025] The components and form of residues on the cleaned surface vary depending on the cleaning solution (especially the type and components of the cleaning solution used immediately before) and the cleaning method, etc. In the case of water washing, a typical example is an oil film (including an oil layer) remaining on the cleaned surface.

[0026] For the sake of convenience, the present specification will use as a representative example the case where an oil film remains on a surface washed with water. The term "oil (film)" as used herein refers to a hydrophobic or water-insoluble (slightly soluble, insoluble, etc.) substance (film, layer). Typical examples of oil include organic compounds with 5 or more carbon atoms, or even 11 or more carbon atoms.

[0027] "electrode" The electrodes may be of any type as long as they can hold the electrolyte and allow AC current to flow through the electrolyte to the surface to be cleaned. Of course, the electrodes should be detachable from the surface to be cleaned.

[0028] The type and components of the electrolyte may be selected depending on the surface to be cleaned or the residue to be evaluated. The electrolyte may be an aqueous or non-aqueous electrolyte. Examples of aqueous electrolytes include aqueous solutions of inorganic salts (NaCl, KCl, Na2SO4, etc.).

[0029] The electrodes that hold (or are impregnated with) the electrolyte may be made of, for example, a porous substrate with liquid permeability. The porous substrate may be, for example, a resin (including rubber, elastomer, etc.) sponge (foam, sponge, etc.) or a filter (including paper, cloth, etc.). The sponge may be made of, for example, polyurethane (PU), polyvinyl alcohol (PVA), melamine (MF), etc.

[0030] The power input section to the electrode may be provided with a conductive material (terminal, lead piece, etc.) that has little effect on conductivity and AC characteristics. Examples of such conductive materials include precious metals (Pt, Au, Ag), stainless steel, Ti substrates, and pnictide conductive materials (Ti3P, FeTiP, XTiP (X: metal element), etc.). The form of such conductive materials is not limited, and may be, for example, porous (e.g., punched metal), mesh, thin plate, foil, film, etc.

[0031] The form (size and shape) of the electrode is not limited. For example, the electrode may be square or circular with a side or diameter of about 1 to 30 mm or 5 to 15 mm. The thickness may be, for example, about 0.1 to 10 mm or 0.5 to 5 mm.

[0032] The electrodes are held to the surface to be cleaned using, for example, adhesive tape or a jig. If the substrate of the surface to be inspected is magnetic (e.g., ferritic steel plate), the electrodes may be held (magnetically attached) to the surface to be cleaned by the magnetic force of a permanent magnet.

[0033] The electrodes are spaced apart enough to prevent short-circuiting. The spacing (the distance between adjacent electrode edges) is, for example, about 1 to 30 mm, 3 to 15 mm, or 5 to 10 mm. The AC characteristics to be compared are preferably measured by passing an AC current between electrodes spaced the same apart.

[0034] Three or more electrodes (electrode group) may be arranged, and a current may be applied between a selected pair of electrodes to measure AC characteristics. The multiple electrodes (including a pair) may be arranged in-plane on the surface to be cleaned. The form and method of wiring from the electrodes to the external circuit are not important.

[0035] Evaluation Method (1) Measurement Electricity is passed through the electrodes that are in close contact with the surface to be cleaned, and AC characteristics (values) related to the surface to be cleaned are measured, such as impedance (spectrum), relaxation time (spectrum), dielectric constant (dielectric spectrum), dielectric loss tangent (tan δ), conductance, capacitance, and (apparent) electric capacity.

[0036] The AC characteristics are measured using, for example, an LCR meter (so-called impedance analyzer). The relaxation time distribution function spectrum is obtained by performing a relaxation time distribution analysis (DRT) on the impedance spectrum (frequency characteristics of impedance). From this, the relaxation time (spectrum) corresponding to the residue and cleanliness of the cleaning surface can be obtained. The relaxation time is mainly the delay time of the movement of permanent dipoles, and is thought to be due to the dielectric relaxation phenomenon caused by the polarization of the residue (electronic polarization, ionic polarization, or orientation polarization).

[0037] The AC characteristics are measured by applying a voltage of, for example, 0.001 to 0.05 V or 0.005 to 0.025 V. The frequency range is, for example, 10 -3 ~10 7 Hz, 10 -2 ~10 5 Hz or 10 -1 ~10 3 Hz.

[0038] (2) Judgment The condition of the cleaning surface is determined, for example, by comparing the measured AC characteristics with a previously prepared reference characteristic (predetermined value). The determination includes, for example, the presence or absence of residue, the type (e.g., oil type) and amount (e.g., film thickness) of residue, and cleanliness. The cleanliness may be, for example, whether or not the surface is clean, or it may be the extent (area, etc.) or amount (film thickness, etc.) of residue (e.g., oil film) relative to a reference state (a surface to be cleaned without residue), or an index value (area %, mass %, etc.) normalized thereto.

[0039] Examples of AC characteristics include impedance (specific frequency) and impedance spectrum (predetermined frequency range). In the case of a clean metal surface with no residue, the absolute value of impedance |Z| is small not only at a specific frequency but also in a predetermined frequency range. Conversely, if there is residue on the surface, the absolute value of impedance |Z| and the spectrum tend to increase. Specifically, for example, 3 ~10 4 |Z| in Hz is 10 2 Ω or more, 10 3 Ω or more, even 10 4 The upper limit is 10 7 Ω or 10 6 It is Omega.

[0040] The AC characteristics may be a relaxation time distribution function or the relaxation time at which the value reaches a maximum or maximum. In the case of a clean cleaning surface without residue, the relaxation time distribution function does not show a sharp peak (maximum or maximum). On the other hand, in the case of a cleaning surface with residue, the relaxation time distribution function shows a peak relaxation time, for example, between 0.01 and 1000 seconds or between 0.1 and 100 seconds.

[0041] In addition, the state of the cleaning surface may be determined using AC characteristics such as dielectric constant, dielectric loss tangent, and dielectric spectrum.

[0042] 《Application》 The cleaning evaluation is performed on a treated surface (base surface) that is to be painted (including electrodeposition coating) or surface treated (including phosphate treatment), for example. The evaluation method of the present invention may be used for sampling inspection or 100% inspection on-site or on a production line. [Example]

[0043] Various oil films were formed on the surface of a cleaned steel plate (substrate), and their AC characteristics were measured. The present invention will be described in more detail based on these specific examples.

[0044] Subject A dull-finished cold-rolled steel sheet (SPCC t0.7 mm / substrate) in the shape of a strip (70 mm × 150 mm) was prepared, as shown in Figure 1. The dull finish of steel sheet (JIS G3141) is a surface treatment applied to improve the adhesion of paint films and plating, and its surface roughness is approximately Ra 1.6 to 6.3 μm.

[0045] The surface of each steel plate was degreased with a parts cleaner and then thoroughly rinsed with pure water.

[0046] After water rinsing, several types of industrial oils (simply referred to as "oil") used in factories were applied to the steel sheet surface (water-washed surface: sample C0) (samples 1 to 8). Specifically, 4 to 5 drops of oil were applied to the water-washed surface, and the oil was spread evenly and thinly with a scraper.

[0047] "electrode" A sponge (porous substrate: φ10 mm × height 1 mm / manufactured by Toei Light Co., Ltd.) made of highly hydrophilic polyvinyl alcohol resin (PVA) was impregnated with a sodium chloride aqueous solution (1 M), which is an aqueous electrolyte. A ribbon wire (terminal, wiring) made of gold (Au) was attached to the top surface of the electrode.

[0048] This pair of electrodes was placed on a steel plate, with the bottom surface of the electrode in contact with the cleaned surface of the steel plate. A ribbon wire was placed on the top surface of the electrode, and a permanent magnet (pin-type magnet) was placed on top of it. In this way, the pair of electrodes was magnetically attached to the steel plate and held in place. As shown in Figure 1, a fixed distance was provided between the pair of electrodes. This distance was the same for all samples.

[0049] "measurement" The pair of electrodes was connected to a measuring device. An impedance analyzer (MFIA / measuring means manufactured by Zurich Instruments) was used as the measuring device. AC frequency range: 10 -2 ~5×10 7 Measurements were taken at Hz, applied voltage (amplitude): 10 mV, and measurement interval: 10 points / dec (measurement step).

[0050] The frequency characteristics (impedance spectrum) of the impedance |Z| obtained by the measurements are summarized in Figure 2. In addition, the impedance spectrum was subjected to relaxation time distribution analysis to obtain the relaxation time distribution function spectrum (AC characteristics) that shows the relationship between the relaxation time (τ) and its distribution γ. The results are summarized in Figure 3. The relaxation time distribution analysis (DRT: Distribution of Relaxation Time) was performed using open source software (DRT tools.m / MIT).

[0051] "evaluation" As is clear from Figure 2, the impedance |Z| of Samples 1 to 8, which had an oil film, increased significantly compared to the cleaned surface without an oil film (Sample C0). Therefore, it was found that the presence or absence of an oil film (cleanliness) can be determined by checking the increase in |Z| compared to the reference (Sample C0).

[0052] As is clear from Figure 3, Samples 1 to 8 on which an oil film was formed each showed a unique relaxation time distribution function spectrum. By focusing on the relaxation time when the relaxation time distribution function spectrum shows a peak, it was also found that not only the presence or absence of an oil film but also the type of oil film on the cleaning surface can be determined.

[0053] Incidentally, by using the relationship between impedance, the relaxation time distribution function at a specific relaxation time, or the apparent capacitance and the weight of the oil film, it is possible to quantify the cleanliness of the surface to be cleaned (for example, by converting it into an index value relating to the area or thickness of the adhering (residual) oil film).

[0054] Thus, it was confirmed that the present invention makes it possible to evaluate the condition of the cleaned surface (presence or absence, type, etc. of residue).

Claims

1. a measuring step of bringing at least a pair of flexible electrodes containing an electrolyte into close contact with the surface to be cleaned of the substrate, and passing an alternating current between the electrodes to determine the AC characteristics of the surface to be cleaned; determining the cleanliness or residue of the cleaning surface based on the AC characteristics; A cleaning evaluation method comprising:

2. the electrolyte solution is an aqueous electrolyte solution, The cleaning evaluation method according to claim 1 , wherein the residue is an oil film.

3. The cleaning evaluation method according to claim 2 , wherein the aqueous electrolyte is an aqueous solution of an inorganic salt.

4. The cleaning evaluation method according to claim 1 , wherein the electrode is made of a porous substrate.

5. The cleaning evaluation method according to claim 1 , wherein the surface to be cleaned is a surface to be washed with water.

6. The cleaning evaluation method according to claim 1 , wherein the substrate is made of a metal.

7. The cleaning evaluation method according to claim 1 , wherein the AC characteristic is impedance, relaxation time, dielectric constant, or dielectric loss tangent.

8. The cleaning evaluation method according to claim 1 , wherein the AC characteristics are obtained from an impedance spectrum or a relaxation time distribution function.

9. A flexible porous substrate and an electrolyte solution impregnated in the porous substrate, A cleaning evaluation electrode used in the cleaning evaluation method according to any one of claims 1 to 8.

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