by W. Kern Hendricks
Clamping pressure with epoxy adhesives
Precision joinery is one of the marks of a competent woodworker. But an obsession to achieve nearly invisible glue joints can be a liability when working with epoxies. The common adhesives that woodworkers are accustomed to working with (urea formaldehyde and resorcinol, for example) generally require tight joints, adequate clamping pressure, and warm temperatures to perform properly. These same bonding techniques are naturally used when a woodworker first starts to experiment with epoxy resin adhesives (old habits and obsessions die hard). The results are unexpected and disappointing when the resultant bond fails under light loading, and the epoxy is blamed for the problem. Most often, it's the woodworker's techniques that need to be examined.
Epoxy resin adhesives have been used with great success in wooden boat construction for the past 25 years. In fact, they have largely supplanted plastic resin and resorcinol glues for two main reasons:
1) Epoxy adhesives are far more forgiving when used in less than perfect working conditions.
2) They do not require tight fitting joints to successfully bond two pieces of wood. In fact, the joint strength will usually be higher if the joints are machined to a loose fit!
Epoxies are also used in boat building to coat and protect wood from moisture. When a coat of epoxy is applied, it actually seeps into the wood pores, providing a strong bond. This ability to "saturate" the wood fibers applies, of course, when two pieces of wood are bonded. If you plan to stain prior to applying finish, be sure to apply the stain in the bonding area before gluing. Otherwise, the epoxy "squeeze out" will coat and saturate the surrounding wood, effectively preventing stain penetration.
A No-Pressure Situation
Why does a woodworker often experience failures when using epoxy adhesives? They use clamping pressure (as opposed to simply closing the joint) and squeeze out most of the epoxy, leaving only a thin film that is absorbed by the wood. A starved joint results.
When bonding with epoxies, clamp to close, and no more. If the glue-up will be subjected to stress when unclamped, then allow the epoxy to cure for several days before removing the clamps - epoxies continue to gain strength for a week or longer. The addition of a filler such as fine sanding dust to the epoxy will act to fill gaps. When using a filled epoxy, be sure to coat both sides of the joint with the mixed epoxy before adding the filler.
Weather resistance of expoxy adhesives
You need more than a strong glue joint when a project will be exposed to constant wetting and drying. Glued-up woodworking assemblies must also be protected from the deep moisture cycles that occur in exterior applications.
Epoxy is the natural choice when constructing projects that will be placed in outdoor environments. Epoxy resin adhesives have been used by modern wooden boat builders for over twenty years, and woodworkers assume that epoxy adhesives are suitable for other projects that will be used in hostile environments. Often the results prove embarrassing when joints open up and fail. Why do the epoxies survive the harsh marine environment, yet fail in more benign land-based uses?
The key is recognizing that all suppliers of marine epoxy adhesives expect that the user will also use the resin to encapsulate the bonded wood. Indeed, it is common for an entire boat to be coated with epoxy, or sheathed using fiberglass cloth and epoxy resin.
This approach is often impractical and, at times, impossible for certain projects. But an understanding of why encapsulation works so well will help woodworkers produce products that will survive in exterior applications.
Epoxy resins are very effective moisture barriers. Wood encapsulated with epoxy has a very stable moisture content that hovers around the equilibrium moisture content for the environmental exposure. Encapsulated wood does not experience the dimensional changes that are normally associated with changes in the atmospheric moisture content. When unprotected woodwork is exposed to exterior environments, the glue joints can be stressed to the point of failure.
Woodworkers using epoxy resins must recognize this and strive to protect the wood from deep moisture cycling. Various finish oils, water repellents, coatings and varnishes have proven effective in protecting the epoxy glue joints. But many of these protective coatings wear off quickly, and frequent re-application is necessary for continued protection. Woodworkers should make sure their customers understand the importance of maintaining these protective coatings and finishes.
The Forest Products Laboratory is completing some work on the use of "primers" that can be used with epoxy resins to improve the situation. Early tests show that epoxies used with these primers are as effective against shrink/swell moisture cycling as a resorcinol adhesive. Long term testing by epoxy suppliers will be needed before guarantees can be made.
W. Kern Hendricks works for System Three Resins, Inc., a supplier of epoxy resin adhesives to owner/builders of boats and airplanes. He has a degree in chemical engineering and has worked with epoxy resins in wood applications for the past 20 years.