An alkaline etching modification method of epoxy-carbon-based infrared light-absorbing coating

Abstract

The application relates to a caustic etching modification method of an epoxy-carbon-based infrared light absorption coating, belongs to the technical field of surface modification of infrared optical materials, and particularly relates to a post-treatment modification process of an epoxy-carbon-based blackening coating for refrigeration type infrared detectors, optical light-shielding cavities, diaphragms, inner walls of instruments and other parts, and is suitable for fine processing scenes of middle and short wave infrared stray light suppression and low reflection functional coating. The method of the application is that a composite alkaline etching liquid is brushed on the surface of the blackening coating of a solidified workpiece, then constant temperature reaction, neutralization cleaning and drying steps are carried out to realize modification. The method of the application utilizes the selective etching effect of the alkaline system on the surface layer of the epoxy resin, only remolds the surface microstructure, does not corrode the carbon black filler, does not fall off and does not peel, and significantly reduces the reflectivity of the surface of the workpiece.

Description

Technical Field

[0001] This invention belongs to the field of infrared optical material surface modification technology, specifically involving the post-processing modification of epoxy-carbon-based blackening coatings for components such as cooled infrared detectors, optical shielding cavities, apertures, and instrument inner walls. It is applicable to the fine processing scenarios of mid- and short-wave infrared stray light suppression and low-reflection functional coatings. Background Technology

[0002] To reduce specular reflections from internal structures, suppress stray light interference, and ensure detection accuracy, infrared optical systems typically require an infrared-absorbing blackening coating on their surface. Epoxy-carbon composite coatings are currently the most widely used functional matting coating material in the field of infrared optics, but this coating has significant limitations, such as:

[0003] Patent publication number CN111909618A discloses an anti-internal reflection black paint and optical element. It uses epoxy resin as the film-forming matrix and combines it with carbon black functional filler to prepare a matte coating. It is mainly applied to the inner wall of optical structure. It relies on the optical absorption characteristics of carbon black filler to suppress stray light. The overall process of this technology is coating + curing and shaping, without subsequent chemical treatment process. The microstructure of the coating surface is fixed and the optical performance cannot be adjusted a second time.

[0004] Patent publication number CN111393988A discloses a graphene composite ultra-black matte coating and its preparation method. It employs a resin matrix composite with various carbon-based fillers to optimize the coating's broad-spectrum matte performance, making it suitable for low-reflection optical environments. This approach still uses a pre-coating filler blending modification method, only incorporating a conventional surface activation process, and does not perform deep post-processing operations such as chemical etching or surface microstructure reshaping on the fully cross-linked and cured epoxy coating.

[0005] The patent with publication number CN109837580A discloses a special anti-stray light coating for the inner wall of optical instruments. It optimizes the formula and construction process for use in sealed cavities and precision optical components. The physical and chemical properties of the coating are stable after the coating is formed. However, the overall technical solution is a one-time fixed preparation and does not have a technical path for subsequent surface performance optimization.

[0006] Existing technologies have the following shortcomings:

[0007] 1. Existing epoxy-carbon-based infrared coatings rely on early formulation doping and component ratio control to achieve basic light absorption performance. After the coating is cured and crosslinked, the surface structure and infrared reflection parameters are completely fixed and cannot be optimized or adjusted in the second way.

[0008] 2. Conventional blackening coatings have a dense and smooth surface after curing, with a high specular reflection ratio. However, they lack sufficient extinction capability in the core working band of 2.5-9.5μm and have limited stray light suppression effect, making it difficult to meet the requirements of high-precision infrared detection equipment.

[0009] 3. Existing post-coating treatment methods are scarce. Conventional surface treatments are mostly mechanical processing, which easily generates dust and excess material and damages precision substrates. They are not suitable for special structures such as closed cavities, irregular curved surfaces, and narrow slits.

[0010] 4. Existing chemical treatment methods are rarely used to cure epoxy-carbon based coatings, and there is no gentle, selective surface corrosion process for epoxy groups. Summary of the Invention

[0011] The purpose of this invention is to overcome the shortcomings of existing epoxy-carbon-based infrared absorbing coatings, such as fixed properties after curing, high mid-wave infrared reflectance, and insufficient stray light suppression. A method for alkaline etching modification of epoxy-carbon-based infrared absorbing coatings is proposed.

[0012] The present invention discloses an alkaline etching modification method for epoxy-carbon-based infrared light-absorbing coatings, characterized in that the method involves brushing a composite alkaline etching solution onto the surface of the blackened coating of the workpiece after curing, followed by constant temperature reaction, neutralization cleaning and drying steps to achieve modification.

[0013] The composite alkaline etching solution is prepared by mixing 15-17% sodium hydroxide aqueous solution and ethylene glycol solution in a volume ratio of 4:1-6:1.

[0014] The temperature of the constant temperature reaction is 75-200℃, preferably 175-200℃; the holding time for the reaction is 10-120 min, preferably 15-30 min.

[0015] The neutralization and cleaning step involves rinsing with deionized water until all residual alkaline etching solution is completely removed, and the rinsing solution on the workpiece surface is neutral.

[0016] The drying step employs hot air drying or constant temperature baking, drying thoroughly at 60°C, and then cooling to room temperature to complete the overall modification process.

[0017] In this invention, the concentration of sodium hydroxide aqueous solution refers to the mass fraction, that is, the percentage of solute mass contained in a unit mass of solution.

[0018] This invention utilizes a fully cross-linked and cured epoxy-carbon-based composite coating as the treatment target to achieve secondary modification and upgrading of the finished coating. It employs a sodium hydroxide-ethylene glycol-water ternary composite alkaline system, without polyol blending or additional additives, resulting in a simple and highly selective formulation. Brush application replaces the traditional soaking process, enabling flexible application to localized modifications, irregular structures, and inner walls of cavities. By utilizing the selective etching effect of the alkaline system on the surface epoxy resin, only the surface microstructure is reshaped, without corroding the carbon black filler, and without powdering or peeling. Attached Figure Description

[0019] Figure 1 This is a comparison chart of the total reflectance test results of the coating integrating sphere before and after modification treatment in Example 1.

[0020] Figure 2 This is a comparison chart of the total reflectance test results of the coating integrating sphere before and after modification treatment in Example 2.

[0021] Figure 3 This is a comparison chart of the total reflectance test results of the coating integrating sphere before and after modification treatment in Example 3.

[0022] Figure 4 This is a comparison chart of the total reflectance test results of the coating integrating sphere before and after modification treatment in Example 4.

[0023] Figure 5 Comparison of coating morphology characterization results before modification (a), after modification in Example 1 (b), after modification in Example 2 (c), after modification in Example 3 (d), and after modification in Example 4 (e).

[0024] Figure 6 This is a comparison chart of the total reflectance test results of the coating integrating sphere before and after modification treatment in Comparative Example 1.

[0025] Figure 7 This is a comparison chart of the total reflectance test results of the coating integrating sphere before and after modification treatment in Comparative Example 2.

[0026] Figure 8 This is a comparison chart of the total reflectance test results of the coating integrating sphere before and after modification treatment in Comparative Example 3. Detailed Implementation

[0027] Example 1: In this example, an epoxy-carbon based infrared absorbing coating is modified by alkaline etching at room temperature. The specific steps include:

[0028] Step S1, Prepare the composite alkaline etching solution:

[0029] Step S1-1, Preparation of sodium hydroxide solution: Add solid sodium hydroxide to deionized water and stir until dissolved to obtain a 17% sodium hydroxide aqueous solution;

[0030] Step S1-2, Preparation of the mixed solution: Take the sodium hydroxide aqueous solution and ethylene glycol solution prepared in step S1-1, and stir and mix them evenly at a volume ratio of 4:1 to prepare the composite alkaline etching solution;

[0031] Step S2, brush etching:

[0032] Step S2-1, coating: The composite alkaline etching solution is evenly and completely coated onto the surface of the cured workpiece blackened coating using a brush coating process. Two orthogonal brush coatings are applied to ensure that the coating surface is completely wetted, with no missed areas or local accumulation.

[0033] Step S2-2, constant temperature reaction: Place the workpiece in a constant temperature environment of 200℃ and keep it at that temperature for 20 min to react.

[0034] Step S2-3, Neutralization and Cleaning: After the reaction is complete, rinse the coating surface repeatedly with deionized water to thoroughly remove any residual alkaline etching solution until the surface rinsing solution is neutral.

[0035] Step S2-4, Drying: After cleaning, use hot air drying or constant temperature baking to dry thoroughly at 60℃, and then cool to room temperature to complete the overall modification process.

[0036] The workpiece modified by the method of this embodiment was subjected to reflectance performance testing using a Fourier transform infrared spectroscopy (FTIR) instrument. Figure 1 , Figure 5 As shown in (b), the average integrated sphere total reflectance of the modified workpiece coating in the 2.5-9.5μm infrared band is 4.13%, which is significantly lower than the 6.17% of the unmodified blackened coating. The total reflectance is reduced by 33.04%. The microstructure of the coating surface shows obvious micropores and a large surface roughness with Ra of 4.73μm. There is no peeling or powdering.

[0037] Example 2: In this example, in step S1-2, the sodium hydroxide aqueous solution and ethylene glycol solution are stirred and mixed evenly at a volume ratio of 5:1 to prepare a composite alkaline etching solution; the remaining steps are the same as in Example 1.

[0038] The workpiece modified by the method of this embodiment was subjected to reflectance performance testing using a Fourier transform infrared spectroscopy (FTIR) instrument. Figure 2 , Figure 5As shown in (c), the average total reflectance of the integrated sphere of the modified workpiece coating in the 2.5-9.5μm infrared band is 5.11%, which is lower than the 6.50% of the unmodified blackened coating. The total reflectance is reduced by 21.34%. The microstructure of the coating surface shows obvious micropores, the surface roughness is moderate, Ra is 3.34μm, and there is no peeling or powdering.

[0039] Example 3: In this example, in step S1-2, the sodium hydroxide aqueous solution and ethylene glycol solution are stirred and mixed evenly at a volume ratio of 6:1 to prepare a composite alkaline etching solution; the remaining steps are the same as in Example 1.

[0040] The workpiece modified by the method of this embodiment, after undergoing reflectance performance testing using a Fourier transform infrared spectrometer, shows the following: Figure 3 , Figure 5 As shown in (d), the average total reflectance of the integrated sphere of the modified workpiece coating in the 2.5-9.5μm infrared band is 5.40%, which is lower than the 5.93% of the unmodified blackened coating. The total reflectance is reduced by 8.75%. The microstructure of the coating surface has shallower micropores and lower surface roughness, with Ra of 2.15μm. There is no peeling or powdering.

[0041] Example 4: In this example, in step S1-1, the concentration of the sodium hydroxide solution is 15%; in step S1-2, the sodium hydroxide aqueous solution and ethylene glycol solution are mixed evenly at a volume ratio of 4:1 to prepare a composite alkaline etching solution; the remaining steps are the same as in Example 1.

[0042] The workpiece modified by the method of this embodiment, after undergoing reflectance performance testing using a Fourier transform infrared spectrometer, shows the following: Figure 4 , Figure 5 As shown in (e), the average total reflectance of the integrated sphere of the modified workpiece coating in the 2.5-9.5μm infrared band is 5.42%, which is lower than the 6.20% of the unmodified blackened coating. The total reflectance is reduced by 12.67%. The microstructure of the coating surface has shallower micropores and lower surface roughness, with Ra of 2.26μm. There is no peeling or powdering.

[0043] The above embodiments demonstrate that brush coating modification using a composite alkaline etching solution can reduce the average integrated sphere total reflectance of the coating in the 2.5-9.5μm infrared band from 6.17% to 4.13%, increase the surface roughness from 1.26μm to 4.73μm, reduce the average integrated sphere total reflectance in the core band by 33.1%, increase the roughness by 2.75 times, and significantly improve the light absorption effect through the light trap microstructure. Simultaneously, it enables localized brush coating, adaptation to irregular structures, and shallow, controllable etching modification, overcoming the shortcomings of prior patents that cannot perform post-processing modification, have fixed performance, and significant limitations, demonstrating significant technological advancement.

[0044] Comparative Example 1: In this comparative example, the sodium hydroxide concentration was 5%; the sodium hydroxide aqueous solution and ethylene glycol solution were mixed evenly at a volume ratio of 4:1 to prepare a composite alkaline etching solution; the remaining steps were the same as in Example 1.

[0045] The workpieces modified by the method described in this comparative example were subjected to reflectance performance testing using a Fourier transform infrared spectroscopy (FTIR) instrument. Figure 6 As shown, the results indicate that the coating showed no significant change, the reflectivity was almost unimproved, and the etching was weak. This suggests that under the same sodium hydroxide solution and ethylene glycol ratio, the lower concentration of the sodium hydroxide aqueous solution cannot effectively break the chemical bonds in the epoxy coating, and etching cannot proceed effectively.

[0046] Comparative Example 2: In this comparative example, the sodium hydroxide concentration was 10%; the sodium hydroxide aqueous solution and ethylene glycol solution were mixed evenly at a volume ratio of 4:1 to prepare a composite alkaline etching solution; the remaining steps were the same as in Example 1.

[0047] The workpieces modified by the method described in this comparative example were subjected to reflectance performance testing using a Fourier transform infrared spectroscopy (FTIR) instrument. Figure 7 As shown, the results indicate that the coating showed no significant change, the reflectivity was almost unimproved, and the etching was weak. This suggests that under the same sodium hydroxide solution and ethylene glycol ratio, temperature, and time parameters, a low concentration of sodium hydroxide aqueous solution cannot effectively break the chemical bonds in the epoxy coating, and etching cannot proceed effectively. This demonstrates that specific concentration and process are key to achieving effective etching, and the choice of etching solution formulation is not obvious.

[0048] Comparative Example 3: In this comparative example, the sodium hydroxide concentration was 17% and no ethylene glycol was added to the etching solution; the remaining steps were the same as in Example 1.

[0049] The workpieces modified by the method described in this comparative example were subjected to reflectance performance testing using a Fourier transform infrared spectroscopy (FTIR) instrument. Figure 8 As shown, the results indicate that the etching of the entire coating surface is uneven, with localized over-etching and whitening. Compared to the unmodified area, the reflectivity of the over-etched areas is significantly reduced, while the reflectivity of the shallowly etched areas is only slightly reduced or even unchanged. This suggests that without ethylene glycol, the etching solution cannot effectively form a film on the coating surface. Furthermore, the rapid evaporation of the aqueous solution during heating prevents uniform and effective surface micro-etching. Ethylene glycol is crucial for effective etching.

[0050] Comparative Example 4: In this comparative example, the sodium hydroxide concentration was 17%; the sodium hydroxide aqueous solution and ethylene glycol solution were mixed evenly at a volume ratio of 2:1 to prepare a composite alkaline etching solution; the remaining steps were the same as in Example 1.

[0051] The workpieces modified by etching using the method described in this comparative example were tested for reflectance using a Fourier transform infrared spectrometer. The results showed that the coating exhibited large-area peeling, powdering, and light leakage, indicating that under the same concentration of sodium hydroxide solution, excessive ethylene glycol would lead to over-etching.

[0052] Comparative Example 5: In this comparative example, the sodium hydroxide concentration was 17%, and it was mixed with glycerol to form a composite alkaline etching solution, instead of using an aqueous sodium hydroxide solution and an ethylene glycol solution. The aqueous sodium hydroxide solution and glycerol were mixed evenly at a volume ratio of 4:1 to prepare the composite alkaline etching solution; the remaining steps were the same as in Example 1.

[0053] The workpieces modified by the method described in this comparative example were subjected to reflectance testing using Fourier transform infrared spectroscopy. The results showed that, under the same volume ratio, when glycerol and sodium hydroxide aqueous solution were mixed, the etching solution easily condensed into small beads on the epoxy coating surface during brushing, making it difficult to spread. In some areas, it even failed to adhere at all, resulting in unsatisfactory etching effects, poor in-plane uniformity, and significant oily residue on the coating surface. This indicates that wettability depends not only on the surface tension value but also on factors such as viscosity, polarity, and interaction with the substrate.

[0054] Although glycerol has a lower surface tension than water, it is much higher than that of ethylene glycol. Combined with its high viscosity, glycerol is neither easy to spread nor easy to form a uniform liquid film due to its viscosity. Therefore, the high viscosity of glycerol results in a weak shear thinning effect during brushing, leading to poor flowability. Even if forcibly spread, it is prone to uneven shrinkage during heating due to viscosity changes.

Claims

1. A method for alkaline etching modification of an epoxy-carbon-based infrared absorbing coating, characterized in that, This method involves brushing a composite alkaline etching solution onto the surface of the cured blackened coating on the workpiece, followed by a constant-temperature reaction, neutralization cleaning, and drying process to achieve modification. The composite alkaline etching solution is prepared by mixing 15-17% sodium hydroxide aqueous solution and ethylene glycol solution in a volume ratio of 4:1-6:

1.

2. The alkaline etching modification method for an epoxy-carbon-based infrared absorbing coating as described in claim 1, characterized in that, The temperature of the isothermal reaction is 75-200℃, preferably 175-200℃; the holding time for the reaction is 10-120 min, preferably 15-30 min.

3. The alkaline etching modification method for an epoxy-carbon-based infrared absorbing coating as described in claim 1, characterized in that, The neutralization and cleaning step involves rinsing with deionized water until all residual alkaline etching solution is completely removed, and the rinsing solution on the workpiece surface is neutral.

4. The alkaline etching modification method for an epoxy-carbon-based infrared absorbing coating as described in claim 1, characterized in that, The drying step employs hot air drying or constant temperature baking, drying thoroughly at 60°C, and then cooling to room temperature to complete the overall modification process.

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