Solvent selection in epoxy adhesive formulations

Solvents and diluents are used to reduce the viscosity of the epoxy system, to better mix the resin and other components, or to make the adhesive better applied to the surface of the substrate. Solvents and diluents are small quantities of substances that are compatible with epoxy resins and their curing agents both chemically and physically. Their main difference is vapor pressure.

The vapor pressure of the solvent is relatively high and will evaporate under specific environmental conditions. Some solvents can quickly evaporate at ambient temperature and pressure, while others may require high temperatures and subvolumes to volatilize.

Diluent vapor pressure is very low, generally does not volatilize at room temperature. The main role of the diluent in the epoxy formulation system is to reduce the viscosity, improve the mixing of the resin and filler, increase the amount of filler, or improve the application performance. Active and non-reactive epoxy diluents have been discussed in a previous article by SpecialChem.

The focus of this article is to discuss the solvents commonly used in epoxy adhesive formulations. Epoxy adhesive formulations require a variety of solvents with different vapor pressure ranges, dissolution strengths, and dispersion capabilities. These different requirements are due to (1) a wide variety of epoxy resins, curing agents, and organic additives that can be used. (2) Different methods are used to apply epoxy to substrates (brush, spray, trowel, etc.).

When choosing a solvent, consider its solubility in the resin, the required viscosity of the final product, drying time, and wetting effect. All of these properties affect the application properties and bonding of the adhesive product.

Function of Solvents

The solvent is used to temporarily reduce the viscosity of the epoxy system, making the resin molecules in the formulation more mobile. The use of solvents is mainly due to one or more of the following reasons:

Helps disperse, mix and wet the resin system during formulation.

The viscosity of the adhesive system is reduced, the various components are easily mixed, and the use phase is better dispersed.

The solid resin or curing agent is liquefied so that it can be used on a supporting carrier or dried on a substrate to form a film.

Solvents in adhesive formulations generally require the dissolution of more than one component (eg, resins, curing agents, polymer additives, etc.). It is often necessary to achieve the required solubility or to meet certain process conditions (such as drying time) by mixing different solvents. The solvents used in synthetic resins and elastomers are generally organic, and generally require mixed solvents to achieve the desired properties. Polar resins require polar solvents, and non-polar resins require non-polar solvents. Solvent-based adhesives are typically solids at 10-30% and the viscosity varies from 1000-30000 cps, depending on the specific application.

In epoxy adhesive systems, solvents are generally used to reduce the viscosity of the system to meet the process conditions of the application. After the adhesive is applied to the surface of the substrate, the solvent volatilizes before the curing reaction. Otherwise, bubbles or vapor pockets may occur between the bonding surfaces, resulting in a decrease in the adhesion between the epoxy and the substrate. The solvent used in the adhesive formulation cannot cause corrosion or other adverse effects on the substrate being applied. Plastics, elastomers, polymer foams, and the like are particularly sensitive to certain solvents in epoxy adhesive systems. Before making the formula, it is necessary to test whether the used solvent will cause corrosion, degradation and other adverse effects on the substrate.

In some water-soluble resins, water is sometimes used as a solvent. Some adhesives can also be used in aqueous emulsion or latex form. During the oil crisis of the early 1970s, water-based adhesives were once considered a possible alternative to solvent-based adhesives. However, water-based adhesives can never meet the demanding requirements of solvent-based adhesives, because the removal of moisture from the adhesive surface takes a long time, consumes large amounts of energy, and the moisture that is dried in the oven causes corrosion, and the moisture resistance of the cured aqueous adhesive. Poor.

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