The use Method of Silane Coupling Agent and its Specific Application in the Adhesive Industry

Dec 24, 2021

Coupling agent is an important treatment agent with a wide range of applications, mainly used as an auxiliary agent for polymer composite materials. The biggest feature of the molecular structure of the coupling agent is that the molecule contains two groups with different chemical properties. Synthetic resins or other polymers undergo chemical reactions or generate hydrogen bonds to dissolve them. Therefore, coupling agents are called "molecular bridges" to improve the interface between inorganics and organics, thereby greatly improving the properties of composite materials, such as physical properties, electrical properties, thermal properties, and optical properties. Coupling agents are used in the rubber industry to improve the wear resistance and aging resistance of tires, rubber sheets, hoses, rubber shoes and other products, and can reduce the amount of NR, thereby reducing costs.There are many types of coupling agents, mainly including silane coupling agent, titanate coupling agent, aluminate coupling agent, bimetal coupling agent, phosphate coupling agent, borate coupling agent, chromium complex The most widely used coupling agents are silane coupling agents and titanate coupling agents.

The specific applications of silane coupling agents in the adhesive industry are as follows:
①In the structural adhesive, the metal and non-metal bonding, if a silane-based tackifier is used, it can condense with the metal oxide or with another silanol, so that the silicon atoms are in close contact with the surface of the object. For example, adding silane as a tackifier in the nitrile phenolic structural adhesive can significantly improve the bonding strength. 
② Silane has been widely used as a treatment agent in bonding glass fibers at home and abroad. It can chemically react with the interface to improve the bonding strength. For example, if the neoprene adhesive bonding does not use silane as the treatment agent, the bonding peel strength is 1.07 kg/cm2, and if the aminosilane is used as the treatment agent, the bonding peel strength is 8.7 kg/cm2. [5]
③In the bonding of rubber and other materials, silane tackifier has a special function. It obviously improves the bonding strength of various rubbers and other materials. For example, when glass and polyurethane rubber are glued, if silane is not used as a treatment agent, the peel strength of the glue is 0.224 kg/cm2, and if silane is added, the peel strength is 7.26 kg/cm2. [5]
④ Bonding problems that cannot be solved with general adhesives can sometimes be solved with silane coupling agents. For example, aluminum and polyethylene, silicon rubber and metal, silicon rubber and organic glass, can be based on the chemical bond theory, select the corresponding silane coupling agent, and get a satisfactory solution. For example, vinyl triperoxide tert-butyl silane (Y-4310) can be used to bond polyethylene to aluminum foil; butadienyl triethoxy silane can make the pull-off strength of silicone rubber and metal reach 21.6～ 22.4 kg/cm2. General adhesives or resins combined with coupling agents can not only increase the adhesive strength, but also increase the water resistance and durability of the adhesive force. For example, polyurethane and epoxy resin have high adhesion to many materials, but the durability and water resistance of the adhesion are not ideal; after adding the silane coupling agent, the performance in this area can be significantly improved . ⑤ It can be used for organic treatment in the production process of electrolytic copper foil. That is, the silane coupling agent is uniformly sprayed on the surface of the copper foil to form an organic film, which further improves the oxidation resistance and solder resistance, and also helps to improve the bonding force between the copper foil and the substrate. 
Other applications of silane coupling agents include:
①Attach the immobilized enzyme to the surface of the glass substrate,
②Sand control during oil well drilling,
③Make the masonry surface hydrophobic,
④The fluorescent lamp coating has a higher surface resistance by preventing moisture absorption;
⑤Improve the moisture absorption performance of the organic relative to the glass surface in the liquid chromatography column.
How to use edit Broadcast
Surface pretreatment method

Mix the silane coupling agent into a dilute solution with a concentration of 0.5 to 1%, and only need to apply a thin layer on the clean surface to be adhered during use, and the glue can be applied after drying. Most of the solvents used are water, alcohol (method for methoxysilane, ethanol for ethoxysilane), or a mixture of water and alcohol, and water without fluoride ions and cheap and non-toxic ethanol and isopropanol are preferred. Except aminohydrocarbyl silane, all solutions prepared by other silane coupling agents need to add acetic acid as a hydrolysis catalyst, and adjust the pH value to 3.5-5.5. Long-chain alkyl and phenyl silanes are not suitable for use in aqueous solution due to their poor stability. Chlorosilane and ethoxysilane are accompanied by serious condensation reactions during the hydrolysis process, and they are not suitable for use as aqueous or hydroalcoholic solutions, but are mostly used as alcoholic solutions. For silane coupling agents with poor water solubility, 0.1-0.2% (mass fraction) of non-ionic surfactants can be added first, and then water can be added to process them into aqueous emulsions for use. 

Migration method
The silane coupling agent is directly added to the adhesive component, and the general addition amount is 1 to 5% of the amount of the matrix resin. Relying on the diffusion of molecules after the glue is applied, the coupling agent molecules migrate to the bonding interface to produce coupling. For the adhesive that needs to be cured, it needs to be placed for a period of time after the glue is applied and then cured, so that the coupling agent can complete the migration process in order to obtain better results.
In actual use, the coupling agent often forms a deposition layer on the surface, but only the monomolecular layer is really effective. Therefore, the amount of the coupling agent does not need to be excessive.

The use methods of silane coupling agent mainly include surface pretreatment method and direct addition method. The former is to treat the surface of the filler with a diluted coupling agent, and the latter is to add the original solution of the coupling agent when the resin and the filler are premixed.

The silane coupling agent is formulated into a solution, which is conducive to the dispersion of the silane coupling agent on the surface of the material. The solvent is a solution made of water and alcohol. The solution is generally silane (20%), alcohol (72%), and water (8%) Alcohols are generally ethanol (p-ethoxysilane) methanol (p-methoxysilane) and isopropanol (for silane that is not easily soluble in ethanol and methanol). The hydrolysis rate of silane is related to the PH value, and the neutrality is the slowest. Acid and partial alkali are fast, so it is generally necessary to adjust the pH value of the solution. In addition to aminosilane, a small amount of acetic acid can be added to adjust the pH value to 4 to 5 for other silanes. Since aminosilane is alkaline, it is not necessary to adjust. After the silane is hydrolyzed, it cannot be stored for a long time, it is best to prepare it for immediate use, and it is best to use it up within an hour
The following are some specific applications for user reference:
Pretreatment filler method

Put the filler into a solid mixer (high-speed solid mixer HENSHEL or V-type solid mixer, etc.), and spray the above-mentioned silane solution directly on the filler and stir. The higher the speed, the better the dispersion effect. Generally, stir for 10 to 30 minutes (the slower the speed, the longer the time), and the filler should be dried at 120 degrees Celsius (2 hours) after treatment.

Aqueous solution
(Glass fiber surface treatment agent): Glass fiber surface treatment agent often contains: film forming agent, antistatic agent, surfactant, coupling agent, water. The amount of coupling agent is generally 0.3% to 2% of the total amount of glass fiber surface treatment agent. The 5 times aqueous solution is first adjusted to a certain value with organic acid or salt. Under full stirring, add silane until it is transparent, and then add For the remaining components, for the insoluble silane, isopropanol can be used to help dissolve. During the drawing process, spray the glass fiber surface treatment agent on the glass fiber to dry, and remove the solvent and water.
Bottom surface method

Treat the surface of the substrate with a solution of 5%-20% silane coupling agent as described above by coating, brushing, spraying, and immersing the surface of the substrate, taking it out to dry at room temperature for 24 hours, preferably baking at 120°C for 15 minutes.

Direct join method
Silane can also be directly added to the filler/resin mixture. When the resin and filler are mixed, the silane can be sprayed directly into the mixture. The amount of coupling agent is generally 0.1% to 2% of the amount of filler (determined according to the diameter of the filler). The silane-added resin/filler is then molded (extrusion, compression molding, coating, etc.).