Hi all – Modern self-adhesive stamps use "Pressure Sensitive Adhesive" (PSA). I'm stumped by what PSA is, fundamentally, since calling it "pressure sensitive" doesn't differentiate it from any other adhesives from a lay perspective. All adhesives require contact pressure to bond, or at least contact, and I'm not sure how contact can be distinct from "pressure" in this context.
What is it about PSA that makes it different from any other adhesive category? Is it more pressure sensitive than others in some sense? I'm not sure what that would mean, physically, scientifically, or tribologically, so any explanation would be most appreciated.
My readings so far don't help me. Are PSAs a different set of compounds than the epoxies, PVA, polyurethanes, superglue, etc? Why aren't the others considered "pressure sensitive"?
It seems like "self-adhesive" applications always use "pressure sensitive" adhesive, e.g. stamps and envelopes. "Self-adhesive" always takes the form of a peel-off strip. So the adhesive has to selectively bond with most substrates, typically paper, but not with the peel-off strip – does this characteristic have any connection to it being a pressure sensitive adhesive?
Thoughts? Does anyone know more about PSA and why it's used in stamps? Could they not use other adhesives and still have peel-off strips? Bonus question: What adhesives are used to manufacture envelopes? I mean in the seams, the pre-sealed parts, not the flap, which is either gum arabic or PSA.