There are at least three different classes of hybrid adhesives and within each class are a significant number of variations. The main classes of hybrid adhesives are the following:
- A hybrid adhesive joint. This is a joint that is made via two or more different processes such as welding-and-bonding or bolting-and-bonding. The number of different types of hybrid adhesive joints that have been evaluated over the last several decades, especially in the automotive industry, is astounding. Applications include both thermosetting adhesives with various mechanical fasteners as well as pressure sensitive adhesives with mechanical fasteners.1 Entire textbooks have been written on this type of hybrid.2
- A hybrid adhesive within a joint. This type of hybrid can also have several variations.
- Two different adhesive formulations with different properties (e.g., modulus) that are used at specific locations within the same joint to optimize joint properties.3,4
- A double backed pressure sensitive tape where one side of the tape has a different adhesive coating than the other side. Silicone - acrylic PSA tapes is a commercial example of this. The silicone side is used to bond to a low surface energy substrate and the acrylic side is used to bond to a more structural, higher energy substrate.
- An adhesive that is formulated with a hybrid polymer. These represent single adhesive formulations that are made from two or more distinctly different chemical species. This type of hybrid adhesive probably is the closest to Websters definition. But these systems can also have several variations:
- An adhesive that is a simple mixture of two different base polymer resins. A simple mixture of polyurethane and epoxy resins cured by an amine is one example of this type of variation.
- An adhesive where a single base polymer is made from raw materials originating from different chemical species. One example of this is an epoxy resin molecule where a polyurethane group is attached to the molecular structure, and another example could be silylated polyurethane adhesive. Perhaps, the most common examples of this variation are not even called hybrids (gratefully) but are referred to as copolymers (e.g., vinyl acetate ethylene).
I have preferred definitions for the above types of adhesive systems. owever, I will gladly surrender to more public discussion of the topic. It is apparent that one of my criticisms is that the term "hybrid" is used too loosely (something like "nano") and a simpler, more direct definition would be preferable.
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References
- Geiss, P.L., et. al., "Hybrid Joining with Pressure Sensitive Adhesives", Pressure Sensitive Tape Council, TECH 33 Papers, 2010.
- Silva, L.F.M., et. al., eds., Hybrid Adhesive Joints, Advanced Structural Materials Series, Vol. 6, Springer Publishing, 2011.
- Fitton, M.D. and Broughton, J.G., "Variable Modulus Adhesives: An Approach to Optimized Joint Performance", Int. J. of Adhesion and Adhesives, Vol. 25, 2005, pp. 329-336.
- daSilva, L.F.M. and Adams, R.D., "Adhesive Joints at High and Low Temperatures Using Similar and Dissimilar Adherends and Dual Adhesives", Int. J. of Adhesion and Adhesives, Vol. 27, 2007, pp. 216-226.
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