Preparation of Silicone Elastic Weatherable Anticorrosive Coatings and Its Application in Steel Structure

Fan Bo, Chen Shilong, Zhang Chunhui, Zhang Juming (Zhejiang Liniz Advanced Materials Co., Ltd., Lin’an, Zhejiang 311305, China

Abstract : As viewed from molecular structure of polymer, α,ω-dihydroxy polydimethylsiloxane,hydroxyl-terminated polyphenylmethylsiloxane and modified polysiloxne are selected as film-forming materials, and further mixed with functional pigments/fillers and functional additives to prepare a reactive environmental friendly coating composition having good anticorrosive, weatherable and elastic properties. The properties of the coating have been characterized by acid/alkali resistance test, UV accelerating ageing test and tension test, etc. The results show that the prepared coating has good salt fog resistance above 2 500 h, good acid (5%H2SO4) and alkali (5% NaOH) resistance above 6 600 h respectively, no chalking and peeling after 1 000 h UV test, meanwhile, exhibits high tensile strength and good elongation at break. The prepared coating composition has good application potentials.

0 Introduction
Coating steel structures with anticorrosive coatings has always been One of the most economical and effective metal protection methods in modern industry. In recent years, due to the increasingly harsh environment of steel structures, strong acid and alkali environment, high salt mist environment, high temperature difference environment and high humidity environment have all put severe tests on the use effect and life of traditional anticorrosive coatings. Especially in high latitudes and coastal areas, where the temperature difference between day and night and the temperature difference between the four seasons is large, the steel structure will continue to expand and deform under the effect of repeated temperature gradients, which will cause the anticorrosive coating on the metal surface to be forced to deform. . Traditional anticorrosive coatings will inevitably crack under such repeated effects, and the corrosion of the protected metal will also occur.

There are many forms of coating failure, but cracking and peeling failure will cause the coating to completely lose its protective effect on the substrate. In the presence of a temperature gradient, the steel will expand or contract to a certain degree. The expansion coefficient of the steel made of Q235-B is 1.2 × 10-5 m / ℃, which is 1.7 times that of concrete. Ordinary alkyd, acrylic and epoxy anticorrosive coatings have a small linear expansion coefficient, which will inevitably lead to the formation of coating cracks under the deformation of the substrate due to long-term thermal expansion and contraction. Cracking of the coating will cause the failure of the anticorrosive effect.

Figure 1 shows the corrosion phenomenon caused by cracking of the anticorrosive coating on the surface of the oil storage tank in an oil depot. The corrosion of the oil tank in the entire oil depot area is almost the damage of the substrate caused by the cracking of the coating. The depth of the severe corrosion has reached several millimeters. According to the staff of the oil depot, the paint used in the plant area is refurbished, repaired and repainted every six months or one year, resulting in very large economic costs.

Corrosion caused by coating cracking

In the face of such serious cracks leading to the failure of anti-corrosion, the most commonly used method on the market is to add a certain amount of toughening agent to conventional coatings to improve the toughness of the coating, thereby preventing the coating from cracking. This method inhibits the cracking of the coating to a certain extent, but it cannot withstand the impact of higher temperature gradients.

1 Experiment
1.1 Experimental raw materials
α, ω-dihydroxy polydimethylsiloxane, hydroxyl-terminated polyphenylmethylsiloxane: Solvent; fluorine modified polysiloxane, polyether changed to polysiloxane, fluorine-containing coupling agent: Self-made; rutile titanium dioxide: DuPont R906; aluminum tripolyphosphate, zinc phosphate: Changzhou Tianxiang; fumed silica, composite antirust pigments and additives: commercially available.

1.2 Experimental equipment
Vacuum mixer (NHZ-5): Rugao Xingye Machinery Factory; High-speed disperser (JFS-550): Guangmei Precision Instrument; Three-roller grinder (S150): Longxin Chemical Machinery Co., Ltd .; Electronic tensile test Machine (DXLL-1000): Shanghai Dengjie Machinery Equipment Co., Ltd .; Digital Viscometer (SNB-3): Shanghai Jingtian Electronic Instrument Co., Ltd .; Salt Fog Machine (YQ-0250), High and Low Temperature Humidity and Heat Test Chamber ( GDW / SJ-0300): Oswitt Technology Co., Ltd .; Scraper Fineness Meter (QXD): Tianjin Jingke Material Testing Machine Factory; Constant Temperature and Humidity Box (LHS-250HC-I): Shanghai Yiheng Scientific Instrument Ltd.

1.3 Preparation process:
first add resin and filler into vacuum mixer, and mix thoroughly under heating and vacuum for 2 ~ 3 hours; after the mixture is cooled to room temperature, discharge it, and grind it with three rollers to a fineness below 35 μm; After passing the test, the mixed materials, pigments, diluents and auxiliaries are added to the high-speed disperser, and dispersed at room temperature for 30 minutes. After adjusting to the appropriate viscosity, filter and package.

1.4 Plate making
According to the standard testing requirements, air spraying is used to prepare samples that meet the testing requirements, and the prepared samples are cured according to the standard requirements. After the curing period, various properties are tested.

2 Experimental results
2.1 Product properties
See Table 1 for properties of silicone elastic anticorrosive coatings.

Performance of silicone elastic anticorrosive coating

2.2 Filler surface treatment
Due to the high oil absorption value of some fillers, the surface of unmodified fillers contains a large number of hydroxyl groups. The presence of these hydroxyl groups on the one hand causes the filler particles to agglomerate themselves through hydrogen bonding, and on the other hand, the hydroxyl groups cause the filler to show hydrophilic properties. The compatibility of a polysiloxane having a continuous phase and having hydrophobicity is deteriorated. Therefore, adding a small amount will cause a significant increase in the viscosity of the resin. At this time, it is difficult to continue adding other functional fillers, so the surface of these fillers needs to be treated with hydrophobicity. In this study, phenyl silicone monomer, hexamethyldisilazane, silane coupling agent and other treatment agents were used to modify the surface of the filler.

2.3 Enhancement of hydrophobic properties
Anticorrosive coatings exposed to natural conditions require excellent hydrophobic properties. Good hydrophobicity can effectively prevent moisture from entering the protected metal material, and it is also conducive to water droplets to coat the The dust is taken away and plays a self-cleaning role. The main film-forming material of silicone elastomer coatings has weak polarity and low surface energy, and has good hydrophobic effect itself, which is also the advantage of silicone coatings. However, in the application of anticorrosive coatings, due to the harsh environment, not only the natural wind, sun, and rain, but also the influence of corrosive gases dissolved in water on the coating, so higher hydrophobicity of the coating Claim.
In order to further improve the hydrophobic performance, in this study, a hydroxyl-terminated polysiloxane containing a trifluoropropyl side chain and a fluorine-containing coupling agent were introduced into the cross-linking system, which could be introduced into the elastic anticorrosive coating through a cross-linking reaction during the curing stage. The introduction of these fluorine-containing groups can further reduce the surface energy of the coating, thereby further improving the hydrophobic performance of the coating. At the same time, the introduction of fluorine-containing groups can also improve the oleophobic and oil-resistant properties of the coating, and can provide more protection for the steel structure to be protected.

2.4 Improvement of self-cleaning
The main organic substance of the silicone elastomer is a crosslinked network of polysiloxane. Because the polarity of polydimethylsiloxane is weak and the intermolecular force is very small, the incompletely crosslinked polysiloxane molecules or segments have a strong ability to move, which can migrate to the surface of the coating. As a result, the adsorption of paint on dust is enhanced. After the coating is used for a period of time, a layer of dust will accumulate on the surface, and serious dust accumulation will not only make the color of the coating dim, but long-term surface pollution will also cause the growth of mold and moss, which will seriously affect the appearance and durability of the coating. Therefore, the self-cleaning performance of the coating must be improved to ensure the long-term effect of the coating.
In order to improve the self-cleaning performance, this study uses a polyfunctional silicone resin to increase the degree of cross-linking of the cross-linking network, reduce the activity of molecular chains and segments in the cross-linking network, reduce the ability of the coating surface to adsorb dust, and promote Dust rolling and falling. Through these improvement measures, the self-cleaning performance of the coating has been greatly improved.

3 Application Cases
After the successful research and development of silicone elastic weather-resistant anticorrosive coatings, it has been successively applied to more than a dozen projects, large and small, and has been recognized by users, and has been affirmed by users in terms of anti-corrosion and weather resistance. Only a few representative projects are shared below.

3.1 Solar Bracket Anticorrosion Project of a Sewage Treatment Plant in Taizhou
The plant is located in the southeast coastal zone and belongs to the marine climate affected area. The salinity of the air is high, the relative humidity is large, and the temperature difference between day and night is large. In addition, the factory is located in the chemical industry park, the gas composition is complex and highly corrosive. The steel structure of the factory is very corroded, as shown in Figure 2.

Corrosion of steel structure in a sewage treatment plant in Taizhou

The harsh environmental conditions in this area make coatings must have excellent acid and alkali resistance, salt spray resistance, weather resistance and water resistance. The plant manager decided to use an elastic anticorrosive coating after many comparisons. In April 2015, on-site construction guidance was performed by specialized construction personnel. It has been running for more than 2 years, and the anti-corrosion effect is good, as shown in Figure 3.

Operation effect of anti-corrosive paint applied to a sewage treatment plant in Taizhou

3.2 A Luzhou Chemical Co., Ltd.
The company is surrounded by chemical plants and complex industrial gases. Rainfall is plentiful in southern Zhejiang. Corrosive gases and rainwater directly cause damage to the coating. The high relative humidity in the environment accelerates the corrosion of steel structures. The plant’s steel structure storage tanks, guardrails, brackets, etc. all showed severe corrosion. In 2016, our company carried out trial coating on one of the storage tanks. After a trial run of about 1 year, this year began to purchase a large number of elastic anticorrosive coatings for the coating projects of the remaining storage tanks.

3.3 Storage tank and pipeline anticorrosion project of
an oil depot in Zhejiang This oil depot is located near a river in Zhejiang province. The air humidity is relatively high, and the temperature difference between seasons is obvious. The steel structure storage tanks and pipelines in the factory area can see severe corrosion caused by coating cracking (Figure 4). According to the person in charge of the plant, the storage tanks are repaired at least once a year, and in severe cases, they need to be repainted. Nevertheless, a large area of ​​corrosion still occurs, and most of them are caused by cracking of the coating, as shown in Figure 5.

Before and after construction of anti-corrosion coating for storage tanks of a certain chemical company in Luzhou
Corrosion caused by cracking of the coating of an oil storage tank in Zhejiang

After investigating and inspecting the company’s field, the responsible team of the oil depot decided to use silicone elastic weatherproof anticorrosive coatings, and began construction in 2017. The coating tanks are currently in trial operation.

4 Conclusions
The silicone elastic weatherproof anticorrosive coating prepared by this institute has excellent corrosion resistance, weather resistance and good elasticity. The salt spray resistance time is more than 2 500 hours, the acid and alkali resistance time is 3 600 hours, and the coating is not chalked or peeled off under 1000 hours of UV light. The tensile strength is> 4.0 MPa, and the elongation at break reaches 230%. Above, the coating has excellent anti-corrosion and weather resistance, which can not only ensure good anti-corrosion performance but also prolong the service life of the coating, save material costs and labor costs, and provide a new idea for the development of metal anti-corrosive coatings.

Leave a Reply

Your email address will not be published. Required fields are marked *