Question: We are having problems bonding aluminum components with a two-part epoxy adhesive because of the difference in thermal expansion coefficients of the metal and the adhesive. We need to match the expansion coefficients as closely as possible, but more importantly need to get an adhesive that has a constant expansion coefficient from batch to batch.
Answer: The difference in thermal expansion coefficients between substrates and adhesives can often cause serious problems. Stresses in the bond-line can cause failure of bonds or movement of parts. I have seen glass crack when bonded to metals with an epoxy adhesive. The most common way of mitigating these problems with aluminum is to use an aluminum-filled adhesive, preferably one that is rubber-toughened. This combination of filler and toughener will minimize the differences in thermal expansion coefficients and the effects of stress on the bonded assembly. With regard to obtaining a constant expansion coefficient from batch to batch, you first need to ensure that your adhesive supplier maintains the appropriate quality standards in manufacturing the adhesive consistently. Secondly, you must ensure that the two adhesive components are mixed accurately. Finally, you must ensure that your cure schedule is consistent. If the degree of cure is different from batch to batch, then the thermal expansion coefficient will also differ.
Question: My adhesive supplier keeps talking about "structural adhesives" and "non-structural adhesives." Could you explain the difference?
Answer: This question is often asked by engineers but is not always easy to answer because there are many factors to be taken into account. Many people consider an adhesive to be "structural" if the substrates being bonded break before the adhesive fails on testing. The problem with this definition is that it is much too general. For example, one can bond paper with a white glue or glue stick and tear the paper, but we would not normally consider these adhesives to be "structural."
Furthermore, a high-strength structural epoxy adhesive will give outstanding performance in a metal bonding situation, but you won't get the metal to fail before the adhesive bond (though in bonding thin metals you can bend or distort the metal and still maintain a good bond).
Another factor that must always be taken into account is the area of the bond. If you are bonding large areas, such as in the manufacture of panels, then an adhesive with a shear strength of few hundred psi will often be sufficient to handle the weight of the panels. However, in bonding small areas in a high-load situation, you may need an adhesive with 3000-4000 psi shear strength.
Here's an appropriate definition for structural adhesives: "Structural adhesives are distinguished from other adhesives by being high-strength materials that are designed to support loads, often substantial loads. These adhesives are also often subjected to cycling high and low temperatures and aggressive fluids, or the weather. In general, they are used for the bonding of rigid structures, although some degree of flexibility or toughness is often desirable in the adhesives to counter the effects of movement, impact or vibration."1
The most important structural adhesives are based on anaerobics, epoxies, reactive acrylics polyurethanes, reactive hot melt polyurethanes and some special formulations of cyanoacrylates.
Questions for publication should be directed to Dr. Dave Dunn at 242 Trails End, Aurora OH 44202; phone (330) 562-2930; fax (253) 681-8460; e-mail DrDave242@att.net ; or visit http://www.fldenterprises.com .