The Nature Of Pressure Sensitive Adhesive (Psa)

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.

Pressure sensitive adhesive and self adhesive are the same thing. They do not require moisture. You are trying to over analyze te words used in naming them.

But what is it though? How is it different from other adhesives? I think the naming is supposed to be significant, and it's clearly treated as a distinct category, which is strange given that all adhesives should be pressure sensitive.

Self adhesives, or pressure sensitives are sticky materials which are attached to a substrate. Prior to their invention (at 3M Corp., I believe) adhesives were just separate materials, generally glues, which bond things together through drying.

The sticky material of self adhesives is chemically unstable, and as it stabilizes over time it will liquify, then crystallize. It may or may not separate from its substrate, but it will migrate into porous material such as paper (stamps), thus making them worthless.

The defining feature of a pressure-sensitive adhesive (PSA) isn't simply that you press on it—it's that it forms a bond with light pressure alone, without requiring activation by water, heat, or a chemical reaction.

Most other adhesive systems work differently:

PVA (white glue) bonds as water evaporates.
Epoxies cure through a chemical reaction between two components.
Cyanoacrylates (super glue) polymerize in the presence of moisture.
Hot-melt adhesives require heat to become fluid before bonding.

A PSA, by contrast, is already in its adhesive state. Applying pressure simply increases the intimate contact between the adhesive and the surface by allowing it to "wet" the microscopic peaks and valleys. The bond comes primarily from intermolecular forces (such as van der Waals interactions), not from curing or drying.

This is also why PSAs work so well for self-adhesive stamps and labels. They remain permanently tacky, but are protected by a silicone-coated release liner (or release paper) that has such low surface energy that the adhesive won't permanently bond to it. Once the liner is peeled away, the adhesive readily bonds to paper, cardboard, and many other materials with only finger pressure.

Traditional stamp gum, by comparison, is a water-activated adhesive that remains dry and non-tacky until moistened.

Regarding envelope construction, the side and bottom seams are typically made with fast-setting water-based adhesives (often dextrin or PVA formulations), while the sealing flap is either coated with water-activated gum or, on self-sealing envelopes, a pressure-sensitive adhesive protected by a release strip.

In short, "pressure-sensitive" is a genuine technical classification in adhesive science. The pressure isn't what creates the adhesive—it simply provides enough contact for an adhesive that is already tacky to develop its bond.

Pressure Sensitive Adhesive = Adhesive Sensitive to Pressure