Graphene is a nanoscale carbon material with special functions such as surface effect, small size effect, quantum size effect and macroscopic quantum effect, and properties such as good corrosion resistance, electrical conductivity and excellent mechanical performance. Graphene has been attracting attention since 2004 when it was found, and such a material has great application prospects in many fields.
Zinc plating and zinc-nickel alloy plating have been widely used as anti-corrosion coatings. In order to improve the corrosion resistance of the coatings, the plating is often required to be subjected to sealing treatment after passivation treatment to meet the technical requirements of high-end products in the manufacturing industry. As China steps from the large manufacturing country into the strength manufacturing country, the existing plating sealants and the properties thereof cannot meet the increasing technical requirement in the manufacturing.
The Chinese invention patent publication No. CN 106318200 A titled “Modified Aqueous Composite Sealing Agents and Preparation Methods thereof” discloses a composite plating sealing agent comprising: 50 to 80 parts of methyl silicone resin, 10 to 30 parts of polytetrafluoroethylene, 50 to 60 parts of silica sol, 3 to 10 parts of graphene, 0.5 to 2 parts of catalyst, 1 to 3 parts of film-forming agent, 1 to 3 parts of lubricant, 3 to 5 parts of accelerator, 5 to 8 parts of metal corrosion inhibitor, 3 to 5 parts of surfactant, 1 to 3 parts of silane coupling agent, 15 to 20 parts of lower carbon alcohol, and 30 to 50 parts of water.
The composite sealing agent was prepared according to the above scheme and then added with water to formulate a sealing solution. It was found in the experiment that all the reduced graphene oxide were sunk into the bottom of the sealing solution after leaving for a period of time, thereby the properties of graphene were lost, and the formulated sealing solution only has an performance of the ordinary sealing solution.
It is well known that the reduced graphene oxide generally contains only a small amount of hydrophilic groups, and the reduced graphene oxide (RGO) prepared by using sodium borohydride as a reducing agent cannot be dissolved in water without an emulsifier. A methyl silicone and a silica sol have a large specific surface area and are capable of adsorbing a large amount of surfactant molecules. Therefore, when the composite sealing agent is formulated, the added hydrophobic surfactant will be mostly adsorbed on a surface of the silica sol and the colloidal methyl silicone particles, and the remaining surfactant will be insufficient to allow the solid reduced graphene oxide particles to form a stable emulsion. Due to the high viscosity of the composite sealing agent, the reduced graphene oxide will not be precipitated from the sealing agent, but after the sealing agent is diluted with water to formulate a sealing solution, the reduced graphene oxide will slowly aggregate and finally is precipitated from the sealing solution. If an excessive amount of surfactant is added to emulsify graphene, the silica sol and the methyl silicone resin particles will be completely surrounded by the surfactant, and cannot be effectively crosslinked together during the film formation process, resulting in reduced performances of the sealing film, such as hardness, abrasion resistance, corrosion resistance and the like.
The above patent application provides no method for using the composite sealing agent. If the composite sealing agent is directly used for coating the plating without being diluted with water, the sealing agent has different meanings from the plating sealant. In the electroplating production line, the plating piece is directly immersed in the composite sealing agent, and flow marks and nodules will occur on the surface of the plating piece after taking out from the sealing bath and drying, creating defective products; in addition, the plating thus formed is thick, which often results in an oversize of the plating piece. Therefore, the composite sealing agent cannot be directly used as a sealant in the electroplating production line.
The method of preparing reduced graphene oxide described in the above patent application comprises: oxidizing the graphene with concentrated sulfuric acid and concentrated nitric acid, and then reducing the graphene oxide with sodium borohydride in an ice bath to obtain a reduced graphene oxide suspension, and formulating the composite sealing agent directly with the reduced graphene oxide suspension. The reduced graphene oxide suspension contains a large amount of nitrate and sulfate radicals. These corrosive substances present in the composite sealing agent will seriously affect the corrosion resistance of the sealed plating, which is one of serious technical shortcomings of the above patent application.
The above patent application does not indicate the content of the graphene in the graphene suspension, what the catalyst, the film-forming agent, the lubricant and the accelerator are, and the specific components of the hydrophobic surfactant and the silane coupling agent. Therefore, the technical solution lacks integrity.
The Chinese invention patent publication No. CN 106085218A titled “Highly Efficient and Environmentally Friendly Composite Sealing Agents” discloses a composite plating sealing agent comprising 50 to 80 parts of methyl silicone resin, 10 to 30 parts of alkyd resin, 50 to 60 parts of alumina sol, 3 to 10 parts of reduced graphene oxide, 0.5 to 2 parts of catalyst, 1 to 3 parts of film-forming agent, 1 to 3 parts of lubricant, 3 to 5 parts of accelerator, 5 to 8 parts of metal corrosion inhibitor, 3 to 5 parts of surfactant, 1 to 3 parts of silane coupling agent, 15 to 20 parts of lower alcohol, and 30 to 50 parts of water. The sealing agent is substantially the same as the composite sealing agent disclosed in the invention patent publication No. CN 106318200 A titled “Modified Aqueous Composite Sealing Agents and Preparation Methods thereof”. A large amount of surfactant is adsorbed on a surface of the colloidal methyl silicon resin particles, a problem that the reduced graphene oxide is precipitated in the sealing solution is also present, the corrosive substances, i.e. nitrate and sulfate radicals, are still present in the reduced graphene oxide, and the content of the graphene in the reduced graphene oxide suspension is also not shown. In the above sealing agent, no indication is given for which substances of the catalyst, the film-forming agent, the lubricant and the accelerator are, and for the specific components of the hydrophobic surfactant or the silane coupling agent.