Ask the Chemist

By Tim Atkinson — Senior Chemist 

Epoxy user Tom Inston II, Theodore, Ala. asked us some excellent questions about using epoxy’s compatibility with other chemical and composite materials. Gougeon Chemist Tim Atkinson provided answers.

Q: We all love to experiment with recipes for food and beverages, but chemical recipes (formulae) are another matter. Is there such a thing as the “Chemical Commandments” out there?

A: Experimenting with chemical formulae brings up the question of chemical compatibility. Material Safety Data Sheets (MSDS) should be available for any chemical product that is sold to the public. Most employers are required to keep a file of MSDS for all of the chemical products that are used in the workplace. The MSDS contain information such as health risks, first aid procedures, and storage and handling procedures. There is also a section on the MSDS that gives information about chemical incompatibilities, so a user won’t accidentally mix two chemicals that could react violently or produce hazardous byproducts.

Q: The chemical industry has introduced many new and marvelous products and information concerning their applications. What is missing is the information on non-compatibility with other composites. Am I going to see a reaction between substances immediately, tomorrow or next year?

A: Epoxies and most other polymers are chemically inert when they are fully cured. Once the polymer is fully cured, you shouldn’t have to worry about any reaction taking place between materials. This is not to say you may not see a failure <R>at a future date. Many polymers are gradually degraded by UV radiation (sunlight). Exposure to solvents or other harsh chemicals can also affect the polymer.

Q: Is there such a list of “do not use on” certain generic or brand name materials?

A: Unfortunately, I do not know of any single source where you could find information about using two products together. The two main criteria used to judge whether two products work well together are adhesion and chemical compatibility. By chemical compatibility, I mean that the presence of one substance will not interfere with the cure mechanisms of the other. For instance, some single-part polyurethanes will not cure over an epoxy that has amine blush.

Many of these potential problems can be overcome by changes to the fabrication process. Allowing enough time for a complete cure, or applying heat to accelerate the cure before applying a different material; surface preparation, such as washing or abrasion; and the order in which the materials are applied, can all affect the final outcome of the project. Primers, tie coats or special surface treatments are sometimes required in order to obtain good adhesion between materials. A tie coat is usually applied between a polyester gel coat and epoxy laminate when building in a female mold. Epoxy will not usually bond to polyethylene, but if the surface of polyethylene is flame treated before applying the epoxy, you will get a very good bond.

Some materials cannot be bonded with epoxy (nylon comes to mind) and require special adhesives.