Use of Silane Coupling Agents (Complete Part)
I. General Principles for Selecting Silane Coupling Agents
It is known that the hydrolysis rate of silane coupling agent is determined by the silicon group Si-X, while the reactivity with organic polymer is determined by the carbon functional group C-Y. Therefore, it is very important to select suitable silane coupling agents for different substrates or treatment objects. The method of selection is mainly through pre-selection by experiment, and should be based on existing experience or rules. For example, in general, unsaturated polyesters use silane coupling agents containing CH2=CMeCOO, Vi and CH2-CHOCH_2-; epoxy resins mostly use coupling agents containing CH2-CHCH_2O and H2N-silane; phenolic resins mostly use coupling agents containing H2N-and H2NCONH-silane; polyolefins use vinyl silane; rubber vulcanized with sulfur yellow mostly uses alkyl silane and so on. The bonding degree of heterogeneous materials is affected by a series of factors, such as wettability, surface energy, interface layer and polar adsorption, acid-base interaction, interpenetrating network and covalent bond reaction. Therefore, it is sometimes not precise enough to rely solely on the experimental pre-selection, and the composition of materials and their sensitivity to the reaction of silane coupling agents need to be considered comprehensively. In order to improve the hydrolysis stability and reduce the cost of modification, silane coupling agent can be mixed with trialkyl silane, and for refractory materials, silane coupling agent can be used as a cross-linking polymer. When silane coupling agents are used as tackifiers, they are mainly realized by forming chemical bonds and hydrogen bonds with polymers, wetting and surface energy effects, improving the crystallinity of polymers, acid-base reactions and the formation of interpenetrating polymer networks. Viscosity enhancement mainly revolves around three systems: (1) inorganic material to organic material; (2) inorganic material to inorganic material; (3) organic material to organic material. For the first kind of bonding, inorganic materials are usually required to be bonded to polymers, so the reactivity of Y in silane coupling agent with functional groups in polymers should be given priority; the latter two belong to the bonding between the same types of materials, so the anti-hydrophilic polymer of silane coupling agent itself and the silane coupling agent selected when inorganic materials require tackifying.


II. Usage
As mentioned above, one of the main applications of silane coupling agents is to treat inorganic fillers used in organic polymers. The latter can transform its hydrophilic surface into a hydrophilic organic surface by silane coupling agent, which can not only avoid the aggregation of particles and the rapid thickening of polymers in the system, but also improve the wettability of organic polymers to reinforcing fillers. Carbon functional silane can also make reinforcing fillers bond firmly with polymers. However, the use effect of silane coupling agent is also related to the type and dosage of silane coupling agent, the characteristics of substrates, the properties of resins or polymers, and the application occasions, methods and conditions. This section focuses on two methods of using silane coupling agent, i. e. surface treatment method and whole blending method. The former is to treat the surface of the matrix with dilute solution of silane coupling agent, while the latter is to add the original solution or solution of silane coupling agent directly into the mixture composed of polymer and filler, so it is especially suitable for the material system requiring mixing.