Coupling mechanism of silane coupling agent and its application in sealant

Silane coupling agents are widely used in sealants. The main application methods are primer coating on the surface of the adhered material, surface modification of fillers, and blending with basic polymers (base glue) as additives.

2.1 Silane coupling agent for primer

The sealant with its own high strength may not achieve ideal bonding strength when applied to all substrates. This may be due to the lack of hydroxyl groups on the surface of the substrate, or it may be related to the active groups on the surface of the sealant. One of the effective means to improve the bonding strength is to prepare the primer solution with silane coupling agent as the active ingredient, and coating it on the surface of the substrate.

Youngman et al. [16] applied a primer containing KSA (-NCO-based siloxane compound) on a glass substrate, and studied its effect on the bonding properties of polyurethane sealants. The content is

When the coating time interval between primer solution and sealant is 15min, the peel strength reaches 9.5N/mm, and it is cohesive failure. After soaking in water for 7d, the peel strength only decreases by 32%. Xiang Kai et al. [17] pointed out that for

For the same type of building substrate, different primers must be used to achieve the desired effect. It is worth noting that the excessive amount of primer solution (the concentration of silane coupling agent at the interface is too high) will lead to the decrease of adhesive strength [12,18]. This may be because only a few molecular layers of the coupling agent act at the interface, and too much coupling agent will form a loose "weak interface layer" at the interface through physical adsorption or other effects, weakening the silane coupling. The "bond bridge" effect of the joint agent.

2.2 Silane coupling agent used for surface modification of filler particles

Silica, calcium carbonate, titanium dioxide, etc., as fillers for sealants, play an important role in the reinforcement and increment of sealants. Research [19] proved that the smaller the particle size of CaCO 3 filler particles and the narrower the particle size distribution width, the better the reinforcement effect. However, even nano-scale filler particles will exhibit strong polarity and strong hydrophilicity due to the large number of hydroxyl groups on the surface of the particles, thereby forming network aggregates, which are difficult to fully disperse in the organic phase, and thus cannot give full play to their performance. Nano effect [20-22]. Hu Wenqian et al. [23] studied the influence of the specific surface area and surface characteristics of fumed silica on the reinforcing effect of silicone rubber. The results show that hydrophobic modification of silica can effectively improve the dispersion of filler particles in the organic phase, and obtain Higher crosslink density for enhanced reinforcement. In recent years, many studies have proved that silane coupling agents are effective agents for surface modification and modification of filler particles. Wang Yuxuan et al[24] used KH550, KH560 and KH570 three silane coupling agents to coat the surface of fumed silica, and studied the effect of three modified silicas on the properties of polyurethane sealants. The white carbon black can be used to prepare the polyurethane sealant with the best comprehensive performance. The research of Wu Wei et al. [25] showed that the molecular structure of the silane coupling agent has an important influence on the surface modification effect of SiO2: Compared with KH858, KH570 is due to the electron-withdrawing effect of ester groups in its structure, and forms The π44 delocalized bond can weaken the double bond strength and is easy to further modify the particle surface by polymerization. Therefore, selecting a suitable silane coupling agent not only plays an important role in improving the performance of the sealant, but also is expected to realize the fine design and even multiple modification of the filler surface. In addition, the amount of silane coupling agent will also have a certain impact on the morphology of modified SiO2[13]: too little amount of coupling agent KH560 will cause a small amount of physically adsorbed water on the surface of SiO2 particles; SiO 2 can have good dispersibility, but it is difficult to form a good interface; further increasing the amount of KH560 will cause the formation of organic films on the surface of SiO 2 .

2.3 Silane coupling agent blended with base polymer as auxiliary

Silane coupling agent is one of the indispensable components in the preparation process of sealant.

Blend [26 ~ 28]. At this time, the silane coupling agent must migrate from the interior of the polymer to the interface to function. In addition, the silane coupling agent with special structure also has the functions of water scavenger and co-catalyst. Studies have shown that some structural silane coupling agents (such as vinyltrimethoxysilane) can be used as water scavengers to remove the water contained in the rubber compound, which improves the storage stability and product quality of the sealant [29]. However, some silane coupling agents with double amino structure also have a catalytic effect, in which the primary amino group mainly affects the surface drying time of the sealant, and the secondary amino group mainly affects the curing speed of the sealant [30]. The design of a silane coupling agent with a special structure can control the diffusion rate of moisture to a certain extent and thus affect the cross-linking of the sealant.

Joint curing speed [31].