Do Silane Coupling Agents Really Undergo Hydrolysis?

Hydrolysis method of silane coupling agents

Silane coupling agents are often difficult to hydrolyze in water without any additives, and the hydrolysis cycle is usually long. Along with the hydrolysis of silane coupling agents, a secondary reaction called condensation will occur, and the condensation products will precipitate at the bottom of the aqueous solution, affecting the performance of the product. In order to ensure the normal progress of the hydrolysis reaction, the following methods are commonly used:

First, weakly acidic or weakly alkaline aqueous solutions can promote the hydrolysis of silane coupling agents. Some silane coupling agents with acidic groups (such as KH560) or alkaline groups (such as KH550) are relatively easy to hydrolyze because their own Y groups will affect the pH value of the aqueous solution, making the silane coupling agents easier to hydrolyze. Silanes such as A151, which have weak influences on the pH value of the aqueous solution, can adjust the pH value of the aqueous solution by adding substances such as acetic acid and ammonia to facilitate the hydrolysis of the silane coupling agents. For example, adding acetic acid to adjust the pH value to weak acidity before hydrolyzing silane coupling agent A151 can significantly increase the hydrolysis rate.

In addition, when silane coupling agents are hydrolyzed, a certain amount of solvent such as methanol and ethanol, which can be mixed with water at any ratio, will be produced. This is determined by the X group in the silane structure. If the solvent produced during the hydrolysis of the silane coupling agent is added to the aqueous solution in advance, the silane coupling agent will be more fully dispersed in the aqueous solution, making the hydrolysis solution more stable. For example, by adding a small amount of ethanol to the aqueous solution before hydrolyzing vinylsilane A151, the oil droplet-shaped silane coupling agent can be then mixed with the aqueous solution more quickly and less likely to condense and precipitate. Furthermore, during the hydrolysis of silane coupling agents, sufficient stirring is necessary to enable a full contact between the silane coupling agents and water, reduce the condensation reaction caused by the contact between the silane coupling agent molecules, and prevent the silane coupling agents from undergoing further hydrolysis once condensed.

Methods for determining the hydrolysis of silane coupling agents

The hydrolysis and degree of hydrolysis of silane coupling agents severely affect the adhesion of the subsequent coatings, so it is necessary to determine whether a silane coupling agent has undergone hydrolysis and the degree of its hydrolysis. We can refer to the chemical properties of the silane coupling agent and use large instruments to assist us in determining whether a silane coupling agent has undergone hydrolysis and confirming the degree of hydrolysis. Common methods include:

NMR nuclear magnetic resonance spectrometer

The nuclear magnetic resonance spectrometer is one of the most powerful tools for qualitative analysis of organic compound structures, in which the atomic nuclei undergo energy level splitting in a strong magnetic field, and when absorbing external electromagnetic radiation, nuclear energy level transition occurs, producing the so-called NMR phenomenon. First, hydrolyze the silane coupling agent in water, and then test the hydrolysis solution using NMR. If the characteristic peak of the silane coupling agent can be detected in the hydrolysis solution, it indicates that the silane coupling agent has undergone hydrolysis. By quantifying its characteristic peak with nuclear magnetic internal calibration, the degree of the silane coupling agent's hydrolysis can be confirmed.

GC-MS gas chromatography-mass spectrometer

Gas chromatography-mass spectrometry (GC-MS) is a method that combines gas chromatography and mass spectrometry characteristics to identify different substances in a sample. It is an important tool for qualitative analysis of volatile substances. Silane coupling agents that have not undergone hydrolysis in aqueous solution can be extracted and separated by organic solvents. The separated silane coupling agents will volatilize at a certain temperature, and the complete monomers of the silane coupling agents can be detected in the GC-MS. The fully hydrolyzed silane coupling agent will not appear in the GC-MS, so if the monomer and hydrolysis products of the silane coupling agent cannot be tested in GC-MS, it indicates that the silane coupling agent has been completely hydrolyzed.