Characteristics of silane coupling agent
Silane coupling agent is a kind of organosilicon compound containing two different chemical groups in the molecular structure. It can be expressed by general formula ynsix4-n, in which y is a group that can not be hydrolyzed, such as alkenyl (vinyl), and hydrocarbon with groups such as Cl, NH2, SH, epoxy, (methyl) acryloxy at the end, in which X is a group that can be hydrolyzed and generate Si oh, including Cl, ome, OET, etc.
X and y are two kinds of active groups with different reaction characteristics. Among them, x group is more likely to produce good binding force with inorganic substances, minerals, metals, etc., while Y group is more likely to produce good binding force with resins, Rubbers, etc. in organic matters. It is precisely because silane coupling agent has the properties of simultaneously affinity organic matter and affinity inorganic matter, it can be used as an intermediate medium to make these two kinds of differences For example, KH570 can increase the binding force between organic coatings and inorganic substrates such as iron.
Fig. 1 x group and Y group in silane coupling agent KH570
Application of silane coupling agent in the field of conversion membrane
Exposed metal substrate is easy to be corroded under natural conditions. Coating a layer of coating on the surface of metal substrate is the most common and simple way to alleviate the corrosion of metal substrate. However, due to the different chemical properties between the metal substrate and the coating, the binding force between the metal substrate and the coating is very limited, and the coating is easy to fall off, which makes the metal substrate re exposed to the corrosive environment.
In industry, conversion coating process is often used to increase the binding force of metal substrate and coating. The conversion coating process is simple in operation and stable in performance, such as phosphating process (that is, the workpiece is immersed in the phosphating solution to form a porous, uniform and dense phosphating film, and the coating will penetrate into the pores of the phosphating film to increase the adhesion of the coating). However, the products widely used in the market (phosphating, passivation, etc.) usually contain heavy metals, volatile acids, phosphorus and other pollutants, and their use is increasingly limited. As a kind of environmental friendly organic material, silane coupling agent has the ability to improve the adhesion between coating and metal substrate. The environmental friendly conversion film products using silane coupling agent to improve the adhesion between coating and metal substrate came into being.
The significance of hydrolysis of silane coupling agent
The conversion film product with silane coupling agent alone has thin conversion film and poor corrosion resistance. Although it can improve the adhesion between the coating and the metal substrate, the metal substrate is easy to rust before coating, and the conversion film is difficult to meet the rust prevention requirements between processes. Therefore, it is necessary to add other film-forming materials for compounding to improve all aspects of the performance of the conversion film, The film-forming materials that can be added include fluorozirconate, borate, molybdate, etc. Because these substances are mostly water-soluble inorganic substances, they can not be directly mixed with silane coupling agent, so water is needed as the carrier of coexistence of silane coupling agent and inorganic substances, which requires adding oil-soluble silane coupling agent to water for hydrolysis. Without hydrolysis, the silane coupling agent will float in the water solution like a small oil bead, and cannot be evenly dispersed in the water solution, which will lead to a great discount of the role of silane coupling agent.
Hydrolysis of silane coupling agent
Silane coupling agent is difficult to hydrolyze in water without additives, and the hydrolysis cycle is very long. The hydrolysis of silane coupling agent will be accompanied by the side reaction of polycondensation, and the polycondensation products will precipitate at the bottom of the aqueous solution, affecting the use effect of the product. In order to ensure the normal hydrolysis, we usually use the following methods:
First of all, weak acid and weak alkaline aqueous solutions can promote the hydrolysis of silane coupling agents. Some silane coupling agents with acid group (KH560) or basic group (KH550) are relatively easy to hydrolyze, because their own Y group will affect the pH value of the aqueous solution, making silane coupling agents easier to hydrolyze. Silanes (such as A151) with weak influence of their own groups on the pH value of the aqueous solution can adjust the pH value of the aqueous solution by adding substances such as acetic acid and ammonia water, so that the silane coupling agent is easier to hydrolyze. For example, after adding acetic acid to adjust the pH value to weak acid, the hydrolysis of the silane coupling agent A151 can be carried out, and the hydrolysis speed has been significantly improved.
Secondly, when the silane coupling agent is hydrolyzed, a certain amount of methanol, ethanol and other solvents that can be freely miscible with water will be produced, which is determined by the X Group in the silane structure. If the solvent produced by the hydrolysis of silane coupling agent is added in advance in the aqueous solution, the silane coupling agent will be more fully dispersed in the aqueous solution and the hydrolysate will be more stable. If a small amount of ethanol is added to the aqueous solution before hydrolysis of vinyl silane A151, the oil bead like silane coupling agent can be mixed with the aqueous solution faster and is not easy to polycondensate and precipitate. In addition, when the silane coupling agent is hydrolyzed, it needs to be stirred sufficiently to make the silane coupling agent contact with water more fully and reduce the condensation reaction between the silane coupling agent molecules due to contact. Once the silane coupling agent polycondensates, it is difficult to hydrolyze again.
Is the silane coupling agent really hydrolyzed?
Whether or not the silane coupling agent is hydrolyzed and the degree of hydrolysis seriously affect the adhesion of the coating, so we need to judge whether or not the silane coupling agent is hydrolyzed and the degree of hydrolysis. We can refer to the chemical properties of the silane coupling agent and use large-scale instruments to help us judge whether the silane coupling agent is hydrolyzed and confirm the degree of hydrolysis of the silane coupling agent. Our common methods are:
① NMR NMR spectrometer: in a strong magnetic field, the nuclear energy level splits. When absorbing the external electromagnetic radiation, the nuclear energy level transition will occur, that is, the so-called NMR phenomenon. NMR is one of the most powerful tools for qualitative analysis of organic compounds. Firstly, the silane coupling agent is hydrolyzed in water, and then the hydrolysate is tested by NMR. If the characteristic peak of silane coupling agent can be detected in the hydrolysate, the hydrolysis of silane coupling agent can be judged. If the characteristic peak is quantified by NMR internal standard, the degree of hydrolysis of silane coupling agent can be confirmed.
▲ Figure 2 NMR spectrometer
3 HNMR spectrum after hydrolysis of silane coupling agent kh792
② GC-MS: gas chromatography-mass spectrometry (GC-MS) is a method to identify different substances in samples by combining the characteristics of gas chromatography and mass spectrometry. It is an important tool for qualitative analysis of volatile matter. The silane coupling agent without hydrolysis in aqueous solution can be separated by organic solvent extraction. The separated silane coupling agent will volatilize at a certain temperature. The complete monomer of the silane coupling agent can be detected in GC-MS. The silane coupling agent with complete hydrolysis will not peak in GCMs, so if the monomer of silane coupling agent and the hydrolysate of silane coupling agent cannot be tested in GC-MS test, it means that silane coupling agent has been completely hydrolyzed.
Fig. 4 gas mass spectrometer
5. Total ion outflow of silane hydrolysate
Figure 6 GCMS matching diagram of silane coupling agent KH560
With the upgrading and development of the industry, silane coupling agents will be more widely used in the field of conversion membrane,
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