In the aviation industry, corrosion protection is an important issue, often using multi-layer coatings to protect the metal substrates of aircraft. A typical system containing hexavalent chromium or chromate provides excellent corrosion protection and consists of three coatings coated on the treated surface. However, hexavalent chromium is a highly toxic, carcinogenic, mutation-causing, and environmentally harmful compound. Starting in 2017, the use of chromate will be authorized by the REACH Directive. Therefore, the replacement of chromium compounds is an urgent and important issue for the aviation industry. To date, Airbus Group companies, including Airbus, have successfully replaced chromates during the anode treatment and etching phase (chemical acid treatment for deoxygenation and treatment of metal surfaces). In addition, different alternatives to inorganic and organic inhibitors, low temperature plasma deposits or sol-gel coatings1 were studied. To date, none of these methods has been effective for chromate coatings.
Within the framework of the UV-cured sol-gel hybrid coating (MHYRCEA) for aeronautical substrates belonging to the French National Research Agency (ANR) research project, we combine UV curing technology with the chemical machinery of sol-gel hybridization to develop a single-step chromate-free coating. The UV curing steps comply with new environmental regulations and the requirements for shorter production cycle times in the aviation industry. In addition, it has been proved that this sol-gel hybrid material, because they have good adhesion to the metal, and can form a dense shield to prevent the penetration of corrosion trigger 2-4, so that the metal substrate has good passive corrosion resistance. This one-step process consists of a light-induced sol-gel process and an organic light polymerization step of cation (or free-based) 5,6. Through the catalytic action of photoic acid-producing agents, organic groups carried in organic alkydsilane can be photopolysed with organic resins, and inorganic networks 7,8 are formed due to inorganic polymerization of alkyl methylsilline alkyl groups. Organic resins provide a better and thicker coating than pure inorganic sol-gel layers.
focus of this study is to localize these new UV-cured sol-gel hybrid coatings and to understand their protection. The first part lists a series of performance characteristics of hybrid coatings prepared with epoxy resins and positive alkyl trimethylsilane with different chain lengths. The second part discusses the corrosion protection, solvent resistance, UV stability and adhesion of UV-gel hybrid coatings on different substrates, including aeronautical substrates and composites.
Materials and Coatings Preparation
Different positive alkyl trimoxysilane prefabses used in synthetic sol-gel hybrid coatings include orthotinyl trimoxysilane (C4TMS), orthoethylene trimethylenesilane (C10TMS), positive octane tropoxysilane (C8TMS) and positive hexane (C16TMS).
Irgacure250, expressed as I250, is 75% of acrylic carbonate ((4-methyl phenyl) 4-(2-methyl propylene) benzene-based iodine hexafluorophosphate solution, used as photoreceptic acid production agent (PAG). Darocur1173, expressed as D1173, is (2-hydrogen-2-methyl-1-benzene-based-1-acetone) and is used as a source of free fundamentals. The cation resin used is epoxy resin (expressed as ER) and the free-based resin is the methyl acrylates resin.
two UV-cured sol-gel hybrid coatings were compared with the three DTM coatings cured at room temperature. Table 1 shows the main characteristics of these formulations.
coating is applied to the substrate (AA2024-T3 or steel plate or fluorinated crucifixion ingotide) with an automatic coater equipped with a winding wire rod. The aluminum plate needs to be specially treated with alkaline degreasing solution and acid etching before coating. Light polymerization of UV-cured sol-gel hybridization formulations in ultraviolet light transmission systems equipped with 10m/min belt speed and molten lamps (H-bulbs). Each paint film is lighted 5 times in a row (total light dose is 7.3 J/cm2). DTM formulations are aggregated at room temperature for more than 7 days without UV exposure. After curing, the average thickness of each coating is measured with a vortex thickness gauge.
test to assess the corrosion resistance of metal materials. Tested in accordance with ISO9227 standards, the corrosion protection of different coatings in man-made atmospheres is compared. Place the painted plate on a shelf in a box with a temperature of (35±2) degrees C at an angle of 20±5 degrees vertically. It was exposed to artificial salt mist obtained from a solution of sodium chloride (50±5) g/L; At 25 degrees C± 5 degrees C. Regularly check the appearance of corrosion on the plate and report any signs of corrosion (i.e. uniform corrosion, pits, etc.).
Real-Time Fourier Transform Infrared Spectroscopy (RT-FTIR)
monitors epoxy consumption and hydrolysis of methyl methylsilosilane using Fourier Transform Infrared Spectroscopy (RT-FTIR) in transmission mode. In these measurements, the formula coating is applied to the crystals of the fluorinated radon and then exposed to both ultraviolet and infrared analysis beams at room temperature. The infrared spectrum is recorded using a BrukerVertex 70 spectro-photonometer equipped with a MCT detector. The resolution of the infrared spectrum is 4cm-1. The irradiation unit is a Hamamastu L8252He-Xe equipped with a light reflector and flexible light catheter that reflects light at 365nm. The light intensity is 200mw/cm2. During ultraviolet light exposure, the spectral band (CH telescopic vibration of epoxy) that appears at 3050cm-1 is selected to measure the consumption of epoxy groups. The disappearance of the vibration spectrum (CH3 symmetrical stretch vibration) at 2840cm-1 evaluates the hydrolytic dynamics of the methoxygenic group.
Magnetic Resonance Spectroscopy (NMR)
performed a 29Si cross-polar angle rotation (CPMAS) solid-state nuclear magnetic resonance spectroscopy (NMR) experiment using a Bruker dual-channel 7mm probe on Bruker's Avance II400 spectrometer. In this case, the recirculation delay time for the use of the radon roor at 79.48 MHz is 5 seconds, the rotation frequency is 4kHz, and the contact time is 4 milliseconds.
the contact angle of water is determined using KLUS DSA100 material equipped with a camera. Measurements are made at room temperature using a fixed sessile drop-technology method. Take ten measurements per sample and calculate the average.
is resistant to ultraviolet light and chemical
QuV accelerated aging test can simulate outdoor weather resistance and reproduce the damage caused by sun and rain. The performance of UV-resistant light is evaluated according to Cycle 1 of the ASTM G154 standard. The gloss hold rate at an angle of 60 degrees is determined in accordance with iso2813. Chemical resistance is determined in accordance with ISO2812. Expose the painted plate to chemical media at room temperature by contact with droplets or by fully soaking it.
to assess the performance of the coating from the substrate, according to the ISO2409 standard for the rowing test. The adhesion of the coating was measured before the test (t-0) and after 14 days of immersion in water at room temperature (T-14).
results and discussion of
UV-cured sol-gel hybrid coating protection
The starting point of this study is the use of a new UV-cured sol-gel process to synthesize AA2024T-3-based corrosion-resistant coatings9. A potential photogenic acid-producing agent (PAG) is used to cure the coating film based on positive alkyl trimethylsilane and epoxy resins10. Hydrolysalization and shrinking of the inorganic part of the hybridized organic alkaloxysilane prelude can be achieved by the photogenic action of Bronster superic acid after PAG decomposition. The convergence of cation light in the epoxy group occurs at the same time. The combination of organic and inorganic light polymerization, through further UV curing processes, has led to the formation of new coatings with interesting corrosion protection properties, as only a few corrosion pits have been observed after 400 hours of salt spray exposure (Figure 1). When the length of the alkyl chain rises from C4 to C8, corrosion protection is significantly enhanced. However, from C8 to C16, corrosion protection is reduced.
study, other performance tests were conducted to understand the protective properties of these coatings and to determine which parameters or combinations of parameters could lead to high corrosion protection.
synergetization of organic-inorganic light polymerization
using RT-FTIR to monitor the hydrolysing and epoxy group consumption of methyl methyl silica. Figure 2 shows that both reactions occur at the same time period. Simultaneous organic and inorganic polymerization reactions have a beneficial effect on the generation of homogenous impurities, with little or no phase separation. The two network structures can run through each other on a molecular scale. The consumption of epoxy groups is only slightly affected by the length of the alkyl chain, while the hydrolysing is slower with longer alkyl chains. These results are very consistent with the effect of spatial bit resistance, which dominates in sol-gel chemistry, on hydrolytic reaction dynamics1.
density of inor
network structure can be understood from 29Si solid state NMR. For different alkyl trimethylsilane precongenerations, the environment of silicon atoms is determined (Figure 3). Fix the mass ratio of the inoror general motor at 50%. The absence of resonance at T0 means that all Si atoms are involved in some degree of Si-O-Si crosslinking, demonstrating the reaction efficiency of the sol-gel. The length of the alkyl chain affects the density of the inorgetable network structure, as the T1 species increases with the length of the alkyl chain.
the hydrophobicity of the membrane surface can be assessed by measuring the water contact angle. Figure 4 shows the longest alkyl chain length that results in the most hydrophobic surface. In addition, the addition of only 5% (mass ratio) of the alkyl trimethylene silane pre-drive experience increased the water contact angle from 76 to 101 degrees. Therefore, hydrophobicity is considered to hinder the penetration of water and corrosion triggers, thus delaying the hydrolysis of the Si-O-Al bond, while corrosion occurs.
UV curing sol-gel hybridization formula and DTM formula
roduration protection performance
the most needed performance of coatings directly applied to metal substrates is corrosion protection. Corrosion protection is evaluated on AA2024-T3 (standard aluminum substrate for the aviation industry) and steel plates.
two sol-gel hybridization formulations that can be cured by ultraviolet light are selected according to the substrate. In fact, to get the best corrosion protection, the composition should be adapted to the substrate.
ultraviolet curing sol-gel hybrid coating (UV HSG2) is the most effective coating for very high corrosion protection on the AA2024-T3 (Figure 5). Corrosion protection is low on steel plates, but UV-cured sol-gel hybrid coatings (UV HSG1) are still the most effective coating compared to other DTM coatings.
resistant to chemical media
coatings are often exposed to different chemical media: alkalis, acids, solvents, etc. Solvent resistance is a key point for its practical use. Table 2 lists the chemical media resistance of UV-cured sol-gel hybrid coatings and DTM coatings. Ultraviolet curing sol-gel hybrid coating is the only coating that is resistant to all chemical media and, more specifically, to special hydraulic working oils (i.e. aeronautical hydraulic fluids).
in outdoor DTM applications, UV stability needs to be evaluated by the preservation rate after UV exposure. The plate should be checked regularly and the preservation rate should be measured. Expect an 80% preservation rate. Figure 6 represents the measured UV curing sol-gel hybrid coating and the 60-degree-angle preservation rate of the three DTM coatings. None of these formulations contain UV stabilizers. After 400 hours, the two DTM coatings are below the required level. The DTM coating, made with acrylic dispersions, is the best coating for UV resistance. The developed UV-curing sol-gel hybrid coating can meet the requirements in terms of photoresist.
adhesion to composites is increasingly
composites in the aerospace industry. In this case, corrosion protection is useless, but the coating is required to have a very strong adhesion to the substrate. To check this performance, the UV-cured sol-gel hybrid coating is applied to carbon fiber-reinforced plastic (CFRP) (carbon fiber-reinforced plastic-epoxy substrate) plates, which are surface-treated by mechanically polished methods. The adhesion of the coating was evaluated by the rowing test after UV curing and after 14 days of immersion in water. First of all, the UV curing sol-gel hybridization formula has excellent wetting. Second, the ADHD curing sol-gel hybrid coating has a very high adhesion because the edges of the indentations are smooth and there is no coating separation in the small squares of the mesh (Figure 7).
A step-by-step light-induced sol-gel and free-based photorecoagulation process has proven to be an effective method for synthesizing sol-gel hybrid coatings with interesting properties, especially high corrosion protection. Indeed, they effectively protect the AA2024-T3 substrate, which is widely used in the aviation industry, as fewer than five corrosion pits were observed after exposure to 3,000 hours of salt spray testing. A combination of key parameters has been identified to illustrate this high corrosion protection, including the relative synergy of organic and inorganic polymerization, surface hydrophobicity, and the high shielding effect provided by organic and inorganic network structures.
our UV-cured sol-gel hybrid coatings are more effective than DTM coatings in terms of corrosion protection, chemical media resistance and UV resistance. In particular, all the solvents tested did not change the hybrid coating. Another important feature of this innovative technology is its ability to be used in a variety of substrates, especially non-metallic composites.
to the French National Research Agency (research project MHYRCEA and EADS IW, project number ANR-08-MAPR-0003, and Airbus S.A.S. as major industrial partners) for their financial support.
this article won the 2013 European Radiation Technology Paul Dufour Award. The traditional award for best conference paper is named after Paul Dufour, one of the founders of European radiation technology, in recognition of his contribution to ultraviolet/electron beam (UV/EB) technology. of the project