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Water based crosslinking improves the corrosion resistance of coatings

water based crosslinking improves the corrosion resistance of coatings

August 19, 2019

the shadow caused by global environmental problems over the application of some mature metal surface pretreatment processes (such as chromate treatment and phosphating treatment) is becoming heavier and heavier, and the market demand for environment-friendly corrosion protection systems is stronger than ever before. Among them, aqueous silane technology is a promising solution to solve this global environmental regulatory problem. It can provide a substitute without heavy metals and volatile organic compounds (VOC) to provide protection for metal corrosion prevention. This anti-corrosion mechanism can be explained by passivating the metal surface with aqueous silane paint film, which can well isolate water, salt and other corrosive materials and coatings in the surrounding environment. It is worth noting that the waterborne silane technology studied in this paper can be regarded as a conversion coating or pretreatment of metal surface, rather than a conventional waterborne coating or primer. The control accuracy of the former often fails to meet the requirements, while the latter is generally used in the constant pressure system, with higher power consumption. The aqueous silane technology requires high-temperature curing process to achieve the best effect, which is difficult to achieve in some application fields or industrial production. Adding double ended silane to the system can reduce the conditions of high temperature curing process. In this work, we proved that the introduction of double ended silane into the aqueous silane system can improve the passivation of the metal surface, enhance the hydrophobicity of the system, improve the crosslinking density of the system, and significantly improve the corrosion resistance of the aqueous silane system


around us, whether bridges, tunnels, automobiles, electrical appliances, or buildings, anti-corrosion technology is one of the key technologies to maintain its integrity and durability. There are many methods to protect metals from corrosion, including chromate treatment and phosphating. These methods have been widely used in corrosion prevention in the past decades. Although these anti-corrosion schemes are cheap and well-known, the corresponding government regulations are gradually becoming stringent, and people's overall awareness of the harm brought by these schemes is also improving. Hexavalent chromium, in particular, has been used as a key material for chromate anti-corrosion treatment in the past 90 years, and has recently been severely restricted by new regulations. Since the European Union listed hexavalent chromium as a carcinogen and a substance that induces mutation in organisms in 2013, European chemical registration, evaluation, authorization and restriction (reach) regulations have forced hexavalent chromium to be phased out in most industrial applications throughout Europe. Although most industries must stop using hexavalent chromium by January 2019, some industries, such as the aerospace industry, have been approved to continue using hexavalent chromium until 2026. However, the anti-corrosion technology for aerospace applications requires several years of research, development and identification, which is why there is an urgent need to study chromium free corrosion technology

silane technology is a feasible alternative to these harmful anti-corrosion systems. However, functional organosilane has been widely used as adhesion promoter for decades, but the real application of these materials in anti-corrosion coatings has only begun in recent years. The functional organosilane coating can form a protective barrier on the metal substrate after proper preparation and application, so as to protect the metal from corrosion for a long time. Previous research results show that only 0 The active silane content of 0 wt% solids can effectively improve the adhesion of the coating. Therefore, adding functional silicone to the coating system can improve the adhesion or corrosion resistance of the coating without significantly increasing the VOC content in the system. This is one of the reasons why aqueous silane technology provides excellent corrosion resistance without the need for dangerous goods pretreatment, volatile solvents or heavy metals

before studying the corrosion resistance of aqueous silane, it is an important process to study the mechanism of adhesion between functional organic silicon and metal surface. In the past few decades, functional organosilanes have been used as coupling agents for organic and inorganic materials in many different industries. The mechanism is that functional organosilicon contains hydrolyzable alkoxysilane (Si or) functional groups, which can be combined with the surface of inorganic materials. In this paper, the organosilane studied contains silicon functional groups composed of alkoxy groups, especially methoxy and ethoxy groups. Functional organosilicon also contains organic functional groups that can react with organics. The simultaneous reaction of silicon functional groups and organic functional groups makes silicone perfectly act as an adhesion promoter between inorganic and organic materials

in order to combine the coating system containing functional silicone with inorganic substrate, alkoxy group must be hydrolyzed to form silanol group. When the hydrolyzed organosilicon contacts the surface of the inorganic material, the silanol group begins to form a hydrogen bond with the hydroxyl group on the surface of the inorganic material. After evaporation of water, these hydrogen bonds will form siloxane bonds between the surfaces of organosilicon and inorganic materials. It is well known that siloxane bonds formed by these functional organosilicon can provide strong adhesion properties. Through appropriate surface treatment and material selection, coatings containing silicone can form siloxane bonds at multiple parts of the inorganic surface, and form a three-dimensional complex structure of siloxane in the whole film-forming process (as shown in Figure 1). The organosilicon coating can passivate the surface of the metal substrate and provide a barrier to prevent water and salt from contacting the metal surface. In addition, organic functional groups can provide additional hydrophobicity and adhesion for organic topcoats applied on the surface that can be used for further corrosion prevention

as mentioned above, high temperature curing is usually required to remove all moisture from the silicone coating. This thermal curing process is not always feasible, depending on the specific application or industry, which leads to the exploration of alternative methods for curing silicone coatings. With the use of functional double ended organosilane, the increase of crosslinking density in the system may reduce the high temperature required for the curing process. This additional crosslinking density comes from the alkoxy group introduced by the double ended organosilane. Although these additional alkoxy groups may be crosslinked at room temperature, the condensation rate of silanol groups is significantly accelerated at high temperature. In addition, the crosslinking rate depends on several other factors, including pH, the contained solvent and the concentration of silane in the system. Although trialkoxyorganosilane is widely used in various coatings, double ended organosilicon, such as 1,2-bis (triethoxysilane) ethane (Fig. 2), may have six or more alkoxy groups. When these alkoxy groups are hydrolyzed and condensed in the system, the additional siloxane bonds formed can accelerate the curing process of the system

it is worth noting that there are six alkoxy groups of 1,2-bis (triethoxysilyl) ethane on both sides of the connected carbon atom. These alkyl chain pairs make the double ended organosilane hydrophobic. Therefore, 1,2-bis (triethoxysilyl) ethane is often used in solvent based systems. In solvent based systems, the hydrophobicity of this functional double ended organosilane does not affect the solubilization of ethanol systems. Although hydrophobic organosilicon is difficult to show good stability in aqueous systems, raising the pH value of the system to slightly acidic (pH value) can maximize its hydrolysis rate and minimize its condensation rate. In this way, the solubility and hydrolytic stability of double ended organosilicon in aqueous system can be improved

the two aqueous systems proposed in this paper include: an aqueous organosilicon alcohol system containing functional silica sol and an aqueous organosilicon alcohol system without functional silica sol. These two systems do not contain any volatile organic compounds, which is why they are often used in environmentally friendly anti-corrosion technology. Aqueous organosilicon alcohol system with functional silica sol can also be used as sol-gel transparent finish, while aqueous organosilicon alcohol system without functional silica sol can be used as surface modifier of organic materials or thickener in aqueous polymer system. Although these aqueous systems have excellent corrosion resistance, double ended functional organosilane will be explored as a performance enhancement additive for these aqueous systems, hoping to better understand how to improve this new technology

experimental method

raw material

1, 2-bis (triethoxysilyl) ethane (dynasylan ®* Btse), aqueous organosilicon containing functional silica sol. These are the uses and functional characteristics of the anchor fatigue testing machine introduced by our technicians (dynasylan ®- Sivo110), aqueous organosilicon alcohol system without functional silica sol (dynasylan ® HYDROSIL2926):Evonik Industries AG; Sodium hydroxide (99.99% purity) and ethanol (99.5% purity): sigmaaldrich. Bulk Kleen ®* 737g (patented alkaline powder cleaner): bulk chemicals. Deionized water (DI) with water purification system (water pro ®+ Plus):LabConco。 Aluminum 6061T6 ®‡ Plate: act testpanels, LLC

*Dynasylan ® It is the annotation of Evonik Degussa GmbH. what deserves attention is the trademark of the book

*Bulk Kleen ® Is a registered trademark of bulk chemicals Inc

† WaterPro ® Plus is a registered trademark of bulk chemicals Inc

‡ Aluminum 6061T6 ® It is a registered trademark of ACT test panels, LLC


the water-based coating tested in this paper is prepared in a 150ml glass beaker (see Table 1 for detailed components), and is pre mixed for 96 hours before use. This longer mixing time is to allow enough time for the hydrolysis and condensation of silane molecules in the formula in the presence of water. After enough time, the condensation of silanol groups in the formula will have a considerable impact on the viscosity of the coating at the beginning, and eventually lead to the decline of film-forming performance. In the case of pH, the condensation rate of the water-based coating evaluated in this paper is particularly low. After about three weeks, it has sufficient stability before the solution has slight appearance changes. These changes in appearance may include precipitation, turbidity and the increase in viscosity of the system during mixing. 1,2-bis (triethoxysilyl) ethane is 100% active solids, while the aqueous organosilicon alcohol system containing functional silica sol is 36% active solids, and the aqueous organosilicon alcohol system without functional silica sol is 30% active solids. In this paper, the final solid mass fraction (wt%) of the water-based coating formulation is determined by the transparent film that does not cause any negative optical properties on the metal surface. It should also be noted that the optimal silane concentration in the aqueous protective coating directly depends on the surface roughness of the metal substrate

Table 1 » wb1-wb5 water-based coating composition (mass/g)

cleaning and application process

metal surface cleaning process

before applying the above water-based formula, the metal substrate must be properly cleaned to obtain the best surface wettability. The metal substrate is first wiped with a paper towel soaked in ethanol. After wiping with solvent, dry the metal substrate with an air gun and place it in 140 – 150 ° f alkaline detergent for 3min (add 15 grams of bulk Kleen ®* 737g, add a liter of deionized water and stir for a few hours). The aluminum substrate is washed with direct water, dried with air gun, and alkaline washing process is adopted

coating application process

after proper cleaning and full hydrolysis of the water-based coating, the coating will be constructed through the dip coating process. Sheet metal completely at room temperature

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