Effect of active thinner structure on corrosion resistance of UV-cured metal coatings.

Abstract
traditional metal protection coatings to solvent-based, a large number of solvent volatilization easy to lead to environmental pollution, photo-cured coatings have high efficiency, environmental protection, low energy consumption characteristics, so the development of green-friendly photo-cured metal coatings have important social and economic significance. The anti-corrosion properties of organic coatings are closely related to their chemical structure, and active thinners are one of the main film-forming substances of photo-cured coatings. In this paper, acrylics of different structures are used as active thinners of bisphenol A epoxy acrylates, and the absorbent rate and anti-corrosion properties of the photo-curing coating obtained are systematically

. From the chemical structure analysis of the causes of the difference in water absorption rate and corrosion resistance of photo-curing coating, the study found that UV-cured coatings with higher cross-link density and hydrophobic structure showed lower water absorption and higher corrosion resistance. However, there is no inevitability between corrosion resistance and water absorption, and cross-link density and adhesion to substrates must be considered at the same time.
Keywords: photo-cured coatings; coating structures; water absorption rates
and corrosion resistance
Metallic materials have been playing an important role in the development and progress of human society, yet the global economic losses caused by metal corrosion amount to hundreds of billions of dollars each year. Among the many metal anti-corrosion means, anti-corrosion coating has always been an efficient and economical means. However, the increasingly serious environmental problems have prompted the coating industry to develop in an environment-friendly direction, and traditional solvent-based coatings have been criticized for their high energy consumption and high VOC emissions.
curing coatings, because of the high curing rate, low VOC emissions and "5E" and other advantages, has gradually become a research hot spot in recent years, by the paint industry's attention. Today, photo-curing coatings have been widely used in wood, paper and 3C electronics and other fields. However, most of its applications in the field of metals are confined to cans, warning signs and metal ornaments, and there are few light-curing coatings directly used for metal corrosion protection.
to apply photo-cured coatings to the field of metal corrosion protection, we need to understand the corrosion principle and failure mechanism of organic coatings. Organic coatings isolate metals from corrosive media by forming a physical barrier on the metal surface, however, as early as the 1960s and 1970s Mayne et al. have found that organic coatings are far less dense than they are completely resistant to corrosive media. There are a large number of micro-pores and free volumes in the coating, water molecules penetrate into the coating, and fill these micro-pores and free volumes, thus forming the permeation channel of the corrosive medium, so studying the absorption behavior of the coating can help to evaluate the barrier and corrosion resistance of the organic coating. Kamimura et al. point out that the water diffusion process in coatings has a great influence on the diffusion of corrosive ions in coatings. In some cases, it can even be considered a decisive step in the entire coating failure process. Yu et al. studied the absorbent behavior of epoxy amine curing coatings of different structures through water absorption test, positive electron indestruction life spectrum (PALS) and attenuation full-reflection Fourier Transformation Infrared Spectrum (ATR-IR), and the results showed that the free volume fraction in the coating structure was not the deciding factor affecting the balance of water content of the coating, and the polarity of the coating structure was the controlling factor of the coating absorbent behavior. Zhang et al. have conducted similar studies that have shown that EPA coatings containing ester bonds have a greater free volume than EP-coated crosslinking networks containing hydroxyl, but have only half the water absorption rate of the latter. In many previous studies, it has been pointed out that the polar group in the coating will form a water penetration channel in the cross-linking system, promote the absorption of water in the coating and reduce the anti-corrosion properties of the coating, for example, Adamson et al. through the study that the water molecules will bind to the hydro-water part of the polymerization network and cause the coating to dissolve, and eventually into the dense cross-linking network. Mijovic et al. suggest that more than 95% of the water molecules in epoxy resin form hydrogen bonds with polar groups, and that electrolyte ions need to form an ionized hydration layer if they want to penetrate into an organic coating. In addition, the crosslink density of the coating is also an important factor affecting the corrosion resistance of the coating. Coatings with high crosslink density tend to provide higher barrier properties, and the higher the crosslinking density of the coating, the more difficult it is for water and electrolyte ions to spread into the coating, so the better the barrier performance of the corrosive medium.
, active thinners account for a higher proportion in light-cured coating formulations. As one of the main film-forming substances, its chemical structure will affect the crosslink density and hydrophobicity of the coating, and ultimately affect the barrier properties of the coating. Therefore, it is necessary to study the effect of the structure of active thinner in photocortified coating on the absorption rate and corrosion resistance of the coating, and there are few systematic studies in this field.
this topic aims to study the effect of the chemical structure of active thinners in photo-cured coatings on the barrier and corrosion-resistant properties of UV photo-cured coatings. A series of active thinners of different structures were selected and photo-cured coatings were prepared, the absorption rate of the coating was tested by weight gain and thermal weight analysis (TGA), and the corrosion resistance of the coating was indicated by electrochemical impedance spectrum (EIS) and neutral salt spray testing.
01
Experimental Part
Experimental Raw Materials and Instruments
1,6-hexyl glycol glycerate (HDDA), trihydroxymethyl propane tripropyl acrylates (TMPTA), 2-phenylethylene acrylates (PHEA), phthalates ethyl acrylates (OPPEA), ethyl ethyl ethyl acrylates (EOEOEA), Changxing Materials Industry Co., Ltd.; Oleic acid ethyl esters (n-HA), acrylic positive acrylic esters (n-OA), China Pharmaceutical Group Chemical Reagents Co., Ltd.; 101, Jiangsu Kaiphos Ruiyang Chemical Co., Ltd.; hydroxyethyl methyl acrylates (PM-2), Japan Chemical Co., Ltd.; 2-hydroxy-2-methyl-1-benzene-1-acetone (1173), German Good Manufacturing Chemistry (China) Co., Ltd., low-carbon steel plate, Standard Gerda Precision Instruments (Guangzhou) Co., Ltd.
BYK box coater, BICK Chemical, Germany; F300 track-type photo-curing machine, Fusion, UNITED States; QFH-type film-coated scribe, Tianjin Secully Materials Tester Factory; Qnix1500 coating thickness measuring instrument, ATOMATION, Germany; T GA/DSC1/1100SF thermal heavy analyzer, METLER-Tolido International Trade (Shanghai) Co., Ltd.; CHI600E electrochemical workstation, Shanghai Chenhua Instruments Co., Ltd.; Q-FOG SSP-600 cycle corrosion salt mist tank, Q-Lab Company.
Metallic coating formulation and coating preparation
the lysogene resin, active thinner, adhesion promoter and light trigger agent in accordance with a certain proportion of the uniform mix preparation of photo-curing coating formula, as shown in Table 1.
soaked the low-carbon steel plate in acetone and ultrasonic treatment for 10 minutes to remove surface oil, using the BYK box coating device on the surface of the steel plate to prepare a wet film thickness of 90 m coating, at 50 oC after 30 minutes of low-flow flat UV curing machine exposure 4 times, track speed of 5.4m/min, total irradiated energy of 640mJ/cm2.
coating water absorption test
adds about 0.5 grams of coating formula to the Teflon mold, levels at 50 degrees C, then takes them out of the mold after exposure and cures them by the UV curing machine and places them in the darkroom for 24 hours to stabilize the performance, and then places them in a vacuum oven at 70 degrees C for 24 hours to get a dry film. Soak the dry film in deionized water at 25 degrees C, remove it for a fixed time, use absorbent paper to absorb surface moisture and weigh and record it, and calculate the suction rate of the coating by formula (1).
W0 is the dry film mass, Wt is the wet film quality of the t-moment, and Mt is the water absorption rate of the coating.
coating electrochemical impedance test
a low-carbon steel plate coated with a photocurable coating as a working electrode, the internal diameter of 36mm PMMA tube with glass adhesive glue on the surface of the coating as an electrolytic cell, platinum electrode as a pair of electrodes, glymeric electrode as a matching electrode.
coating is resistant to neutral salt spray
the back and edge parts of the low-carbon steel plate coated with a photo-curing coating are sealed with waterproof tape. Using Q-FOG SSP-600 type circulating corrosion salt mist tank for salt spray resistance test, the test temperature is 35 degrees C, the salt solution concentration is 5wt, pH is 6.5 to 7.2, the experiment conforms to the standard ASTM B117.
02
results and discussion of the effect of the official energy of
active thinners on coating water absorption and corrosion resistance
the official energy of active thinners in photo-cured coatings can directly affect the crosslink density of the coating. Therefore, we selected three active thinners with different functions, namely, single-officer acrylic (n-HA), hexyl glycol dipropylate (HDDA) and trihydroxymethyl propane tripropylene esters (TMPTA) to study the relationship between crosslink density of photocortified coatings and coating water absorption and corrosion resistance.
From Table 2, it can be found that as the active diluent's official energy increases the gel content of the coating also increases, because the photo-curing coating containing the high-level energy activity thinner has a denser cross-linking network, and the denser the cross-linking network of the coating, the less likely the small molecules are to be washed out, so the higher the gel content. The coating water absorption data show that with the increase of active thinner's performance, the absorption rate of the coating decreases gradually. The absorption rate of the coating is influenced by the free volume in the coating cross-linking network, and the cross-linking network formed by the active thinner of the three officials is denser than that of the single official, and the free volume between the networks is smaller, so the water absorption rate is lower.
, the higher the coating's water absorption rate, the more channels there are for the penetration of the powered electrolyte ions formed, and the worse the corrosion resistance of the coating. Figure 3a is a Bode graph of the coating soaked in a 3.5wt%NaCl solution for 30 minutes, from which it can be seen that the coating n-HA of a single-officer active diluent is less impedant after 30 minutes of immersion than a coating containing double- and triple-active thinners. It is observed from Figure 3b that the electrochemical impedance of the n-HA coating decreased significantly within 0 to 24h of the immersion, while the electrochemical impedance of the HDDA and TMPTA coatings remained at a high level. In the early stage of immersion, the coating has not failed, the metal substrate has not yet occurred corrosion, at this time the decrease of coating impedance is mainly caused by the coating absorbent. The ideal coating can be regarded as an infinite resistance and a very small capacitor in-union, when the coating absorbs water, because the dielecyte constant and conductivity of the water is greater than the dielecyte constant and conductivity of the coating, so it will lead to a decrease in the resistance value, the capacitor block increase, and ultimately the result is a decrease in the impedance of the coating. The higher the absorption rate of the coating, the more significant the decrease in impedance. The n-HA coating has the highest water absorption rate and the lowest impedance, and it is not difficult to find from the salt mist test photos in Figure 3c that n-HA has the worst corrosion resistance, with the most corrosion points on the n-HA coating surface at 100h and the least corrosion points on the TMPTA coating surface. While the corrosion point on the n-BA surface after 400h has been transformed into a large area of corrosion, the HDDA and TMPTA coating surfaces have not transformed the corrosion area without a large area.
Effect of benzene ring structure on coating water absorption rate and corrosion resistance
In order to study the effect of benzene ring structure in active thinner on coating water absorption rate and corrosion resistance, three active thinners were selected, MEA, PHEA and OPPEA, of which MEA does not contain benzene ring, PHEA and OPPEA contain one and two benzene rings, respectively. From the results of the water absorption rate of Table 3, it can be found that with the increase of the number of benzene rings from zero to two in the active thinner, the absorption rate of the coating gradually decreased, presumably due to the hydrophobic properties of benzene rings, benzene rings as hydrophobic groups, its introduction can improve the hydrophobic properties of the coating. This can be confirmed by the water contact angle data of the coating (Table 3), which gradually increases the surface hydrophobicity of the coating as the number of benzene rings in the active thinner increases.
figure 3a shows that the impedance of the MEA coating after 0.5h soaked in 3.5wt% of the NaCl solution is about 108 s, while the two sets of coating impedances containing benzene rings are in the range of 109 s. Figure 3c is the change curve of coating impedance over time, it can be found that the MEA coating impedance without benzene ring drops rapidly within 0 to 24h, the PHEA coating impedance containing a benzene ring gradually decreases within 0 to 240h, and the OPPEA coating impedance containing two benzene rings does not begin to show a significant decrease until 240h. This shows that the introduction of benzene rings can improve the barrier effect of the coating, and with the increase in the number of benzene rings, the better the barrier capacity. The barrier performance of the coating is improved, as is the anti-corrosion performance. This can be verified from the salt mist results in Figure 3c, where the MEA and PHEA coatings without benzene rings and containing 1 benzene ring are the first to show sporadic corrosion points after 100h salt mist.
effects of different lengths of alkyl chains on coating water absorption and corrosion resistance
Therefore, three active thinners EA, n-BA and n-HA containing different length alkyl chains were selected to study the effect of alkyl chain length on coating water absorption and corrosion resistance in active thinners. As can be seen from Table 4, the water contact angle of the coating increases with the length of the alkyl chain, which means that the hydrophobicity of the coating increases with the increase of the alkyl chain, while the absorption rate of the coating decreases with the length of the alkyl chain. Therefore, it can be concluded that the introduction of alkyl chain can improve the hydrophobicity of the photo-curing coating and reduce the water absorption rate of the coating.
figure 4a shows that the impedance of the n-HA coating after 0.5h immersion in a 3.5wt% NaCl solution is lower than that of the n-BA coating and the EA coating. From figure 4b coating impedance over time change curve, after soaking 360h, the EA coating with the highest absorption rate has the highest impedance, the lowest absorption rate n-HA coating impedance is the lowest. And it is not difficult to find from the salt mist photo in Figure 5c that after the 400h neutral salt mist test, the corrosion area of the coating surface increased with the length of the coating active thinner alkyl chain. All these phenomena show that hydrophobic alkyl chains do not improve the corrosion resistance of the coating. It is speculated that this is due to the increase in the length of the alkyl chain resulting in a decrease in the cross-link density of the coating. The longer the molecular weight of the active thinner with the same mass fraction, the lower the molecular weight of the alkyl chain, the lower the number of molecules and the lower the double-bond content, so the lower the crosslink density of the coating formed. And the corrosive medium to.