In simple terms, a structural adhesive is one that can be used to produce a load-bearing joint. However, the term is most often applied to those adhesives that are used for engineering applications where joints will typically have lap shear strengths of greater than 1MPa and, more normally, greater than 10MPa.
Adhesives which fall into this class are:
- Epoxies
- Toughened acrylics
- Polyurethanes
- Cyanoacrylates
- Anaerobics
- Phenolics
- Vinyl acetates
Adhesives that do not normally offer such structural function are termed sealants. Often though, the distinction between a sealant and a structural adhesive is blurred as it depends on the loading of the joint and the end use. For example, pressure sensitive adhesives (PSAs), usually in the form of tapes, would not necessarily be classified as structural adhesives due to their tendency to creep under load. However, with careful joint design, creep can be eliminated and there are many examples where PSAs are used successfully in load-bearing applications.
Another example is use of silicone systems. These materials, generally, have a very low modulus, limited adhesive properties and require a gap of greater than 1mm to achieve a consistent cure. For these reasons they are more often termed sealants. Despite such a label, they do adhere extremely well to glass and have found extensive use in the glazing industry and in the construction of fish tanks where use of silicone adhesives has eliminated the need for any other sort of joining technology. With a full tank of water, loads on the glass walls and adhesive can be termed structural, especially from a fish's point of view!
Further information
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