I've been working with silicone adhesives lately - mostly the 1-part acetoxysilicones. [*] Silicones are good examples of an adhesive with lackluster adhesive strength but high cohesive strength so that the overall adhesion is high.
Confused? Let me explain. Cohesive strength is the strength of the bulk material, which is pretty high for a silicone adhesive because of the crosslinking. The crosslinking is not enough to make it brittle; there is still a good deal of flexibility which I'll talk more about in a minute.
Adhesive strength is how well the adhesive sticks to the substrate. As I mentioned above, it really isn't all that great. I've you ever tried to remove excess silicone adhesive, you'll notice that once you get it started, you can peel it off pretty easily and cleanly.
The disclaimer however, is the phrase "once you get it started". That can get to be quite a challenge because of the flexibility and the cohesive strength. All that flexibility adds to the "toughness" of the adhesive, toughness being the area under the stress-strain curve. By being flexible rather than brittle, the adhesive can deform quite a bit and all the deformation represents additional work that you have to put into the adhesive before you can begin to put work into the adhesive interface.
You can see the same situation with foamed and foam-backed adhesives. You can make a pretty measly adhesive look a lot better by coating it onto a soft foam and then sticking it to a substrate.
[*] I'm faced with constant complaints about the acetic acid smell. Last year, I had constant complaints about the mercaptains that I was working with (thiol-ene chemistry). My Xmas wish for next year is that Famous Dave's Bar-B-Que wants me to look at the rheology of pork ribs. Then we'll see who's complaining.
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