Silane coupling agents disperse PU molecules in water to form a stable dispersion through the hydrophilicity of hydrophilic groups. Polyamine groups react with residual isocyanates to expand the chain, while the silicon oxygen groups of silane coupling agents undergo hydrolysis and condensation to crosslink the system, achieving internal crosslinking while expanding the chain, without affecting the dispersion of prepolymers. Table 1 summarizes the basic properties of the synthesized silicone modified waterborne polyurethane lotion. With the increase of the mass fraction of silane coupling agent, the lotion gradually turns white from almost transparent liquid, and the average particle size of lotion gradually increases. At the same time, the lotion kept good dispersion stability except for the silane coupling agent with 8.9% mass fraction.
TEM photos of the morphology and structure of the latex particles of the modified waterborne polyurethane lotion synthesized under different silane coupling agent mass fractions, with magnification of 100000 times. The latex particles in all lotion are irregular in shape. Without silane coupling agent, the particle size of latex particles is very small (particle size meter measurement result is 47nm), and agglomeration is obvious; The situation where the mass fraction of silane coupling agent is 1.9% is similar, and the particle size of latex particles is not large (57nm), and there is also agglomeration phenomenon; When the mass fraction of silane coupling agent is 7.6%, the number of latex particles that are basically spherical increases, and the particle size also increases significantly.
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/square centimeter. If amino silane is used as a treatment agent, the bonding peel strength is 8.7 kg/square centimeter.
3. Silane coupling agents 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/square centimeter. If silane is added, the peel strength is 7.26 kg/square centimeter.
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.
