Adhesives and sealants troubleshooting tips from Dr. Dave Dunn.

Question:We apply our epoxy adhesives automatically and need to ensure that the adhesive has been mixed correctly and that it has actually been applied to the part. How can we accomplish this, other than having someone actually observe it visually?

Answer:Epoxy adhesives, which comprise both the adhesive component and the hardener, are usually mixed with a static mixer. It is possible to use and monitor automatic flow controllers on both components of the adhesive. However, the most common way to determine the correct mix ratio is to periodically take a sample of the mixed adhesive and measure its properties, including density, cure speed, bond strength, etc.

A few years ago, a major adhesives manufacturer collaborated with an auto company to develop a novel method for controlling the mix ratio of two-part epoxy adhesives. They discovered that the mix ratio can be continuously measured via a tagging system that incorporates microscopic stainless steel-coated glass bubbles in one component of the adhesive. The bubbles are detected by two eddy current sensors integrated into the dispensing equipment. This technique enables 100% knowledge and control of the dispense ratio.

A few methods can be used to detect your adhesive on the parts. One standard method is to incorporate a fluorescent agent into the adhesive and use a fluorescence detector to monitor its presence. Photo cells or a camera system can also be effective. If you use a three-dimensional camera system, you can measure the width, height and position of the bead of adhesive.

Question:What would you recommend as a general-purpose wood adhesive?

Answer:There are several types of wood adhesives, though craftsmen tend to refer to these as glues rather than adhesives. The traditional white glues, or carpenters’ glues, usually do a good job, as do tough acrylics or special formulations of cyanoacrylates.

One-component polyurethane adhesives have also become popular in recent years. These adhesives have a single component and depend on moisture for curing. The moisture is sometimes available in the wood or the atmosphere, but it is common practice to dampen the surface before applying the adhesive. One has to be careful to use the right amount of adhesive and to clamp the parts, because the adhesive generates foam on curing and tends to expand.