Silane coupling agent: Silane coupling agent is obtained by the addition of silicon chloroform (HSiCl3) and unsaturated olefins with reactive groups catalyzed by platinum chloric acid, followed by alcoholysis. Silane coupling agents are essentially a type of silane with organic functional groups, which have both reactive groups in their molecules that can chemically bond with inorganic materials (such as glass, silica sand, metals, etc.) and reactive groups that can chemically bond with organic materials (synthetic resins, etc.). It can be represented by the general formula Y (CH2) nSiX3, where n=0-3; X - a hydrolyzable group; Y is an organic functional group that can react with resins. X is usually a chlorine group, methoxy group, ethoxy group, methoxyethoxy group, acetoxy group, etc. These groups hydrolyze to form silanol (Si (OH) 3), which combines with inorganic substances to form siloxane. Y is vinyl, amino, epoxy, methacryloyloxy, mercapto, or urea. These reactive groups can react with organic substances and combine.
Therefore, by using silane coupling agents, a "molecular bridge" can be built between the interface of inorganic and organic substances, connecting two materials with vastly different properties together, thereby improving the performance of composite materials and increasing adhesive strength. This characteristic of silane coupling agent was first applied to glass fiber reinforced plastics (GRP) as a surface treatment agent for glass fiber, which greatly improved the mechanical properties, electrical properties and anti-aging properties of GRP. Its importance in the GRP industry has long been recognized.
At present, the use of silane coupling agents has expanded from glass fiber reinforced plastics (FRP) to glass fiber surface treatment agents for glass fiber reinforced thermoplastic plastics (FRTP), surface treatment agents for inorganic fillers, sealants, resin concrete, water crosslinked polyethylene, resin encapsulation materials, shell molding, tires, belts, coatings, adhesives, grinding materials (grindstones), and other surface treatment agents.
Among the two types of functional groups with different properties in silane coupling agents, the Y group is the most important and has a significant impact on the performance of the product, playing a decisive role in determining the performance of the coupling agent. Only when the Y group can react with the corresponding resin can the strength of the composite material be improved. Generally, it is required that the Y group be compatible with the resin and capable of coupling reaction. So, certain resins need to choose silane coupling agents containing appropriate Y groups. When Y is a non reactive alkyl or aryl group, it does not work on polar resins, but can be used in the bonding of non-polar resins such as silicone rubber, polystyrene, etc. When Y contains reactive functional groups, attention should be paid to its reactivity and compatibility with the resin used. When Y contains amino groups, it belongs to catalytic properties and can act as a catalyst in the polymerization of phenolic, urea formaldehyde, melamine formaldehyde, as well as a curing agent for epoxy and polyurethane resins. At this time, the coupling agent fully participates in the reaction and forms new bonds. Aminosilane coupling agents are universal and can almost couple with various resins, except for polyester resins. The type of X group has no effect on the coupling effect.
Therefore, based on the types of reactive groups in the Y group, silane coupling agents are also referred to as vinyl silane, amino silane, epoxy silane, mercapto silane, and methacryloyloxy silane, among others. These organic functional silanes are the most commonly used silane coupling agents.
The application of silane coupling agents can be roughly summarized into three aspects:
（1） Used for surface treatment of glass fibers, it can improve the bonding performance between glass fibers and resins, greatly enhancing the strength, electrical, water resistance, weather resistance, and other properties of glass fiber-reinforced composite materials. Even in wet conditions, its effect on improving the mechanical properties of composite materials is significant.
At present, the use of silane coupling agents in glass fibers is quite common, accounting for about 50% of its total consumption. Among them, the most commonly used varieties are vinylsilane, aminosilane, methacryloyloxysilane, etc.
（2） Used for filling plastics with inorganic fillers. The filler can be surface treated in advance or directly added to the resin. It can improve the dispersibility and adhesion of fillers in resins, enhance process performance, and improve the mechanical, electrical, and weather resistance properties of filled plastics (including rubber).
（3） Used as a thickening agent for sealants, adhesives, and coatings, it can improve their adhesive strength, water resistance, weather resistance, and other properties. Silane coupling agents can often solve the problem of some materials being unable to bond for a long time.
The principle of silane coupling agent as a viscosity enhancer is that it has two types of functional groups; A functional group can bond with the adhered skeleton material; And another type of functional group can bind with polymer materials or adhesives, forming strong chemical bonds at the bonding interface, greatly improving the bonding strength. There are generally three methods for the application of silane coupling agents: one is as a surface treatment agent for the skeleton material; The second is to add it to the adhesive, and the third is to directly add it to the polymer material. From the perspective of fully utilizing its effectiveness and reducing costs, the first two methods are better.
The specific applications of silane coupling agents in the adhesive industry are as follows:
① In the bonding of metal and non-metal in structural adhesives, if silane based tackifiers are used, they can condense with metal oxides or with another silanol, thereby tightly contacting the silicon atoms with the surface of the adhesive. Adding silane as a tackifier to nitrile phenolic structural adhesive can significantly improve the bonding strength.
② Silane has been widely used as a treatment agent for bonding glass fibers both domestically and internationally. It can undergo chemical reactions with the interface, thereby improving the bonding strength. For example, if silane is not used as a treatment agent for chloroprene adhesive bonding, the bonding peel strength is 1.07 kg/cm2. If amino silane is used as a treatment agent, the bonding peel strength is 8.7 kg/cm2.
③ Silane thickeners have special functions in the bonding of rubber with other materials. It significantly improves the bonding strength between various rubbers and other materials. For example, when bonding glass and polyurethane rubber, if silane is not used as a treatment agent, the peel strength of the adhesive is 0.224 kg/cm2. If silane is added, the peel strength is 7.26 kg/cm2.
④ Adhesive problems that cannot be solved with regular adhesives can sometimes be solved with silane coupling agents. Aluminum and polyethylene, silicone rubber and metal, silicone rubber and organic glass can all be solved satisfactorily by selecting corresponding silane coupling agents based on chemical bond theory. For example, ethylene based tert butyl silane can be used to bond polyethylene with aluminum foil; The use of butadiene based triethoxysilane can achieve a pull-out strength of 21.6-22.4 kg/cm2 between silicone rubber and metal.
The combination of general adhesives or resins with coupling agents can not only improve the bonding strength, but more importantly, increase the water resistance and durability of the bonding force. Although polyurethane and epoxy resin have high adhesion to many materials, their durability and water resistance are not ideal; After adding silane coupling agent, this performance can be significantly improved.
Other applications of silane coupling agents include:
① Attach the immobilized enzyme to the surface of the glass substrate,
② Sand control in oil well drilling,
③ Make the surface of bricks and stones hydrophobic,
④ By preventing moisture absorption, the fluorescent lamp coating has a high surface resistance;
⑤ Improve the moisture absorption performance of organic phases on glass surfaces in liquid chromatography columns.
