Why do stickers have varying adhesive strengths?

Stickers & labels have varying strengths of adhesive because we all use them in different conditions.

The two factors that mostly define how strong the adhesive needs to be is; surface energy & how smooth the surface is.

A smooth surface is easier to stick to simply because more of the glue on the back of your sticker touches the surface.

Surface energy (aka surface tension) can be a complex subject, but basically, it's how sticky a surface is. 

The higher the surface energy, the easier a glue will stick. This image shows how surface energy changes between materials:

Source

If you're ever in doubt either ask your suppler of stickers & labels or go for a permanent adhesive (aka medium strength). It's the best all-rounder.

 

Want to find out more about surface energy? Read on.

Surface energy in technical speak is; the degree to which a material surface will repel or attract.

Imagine a freshly waxed or brand new car. The water beads and runs off easily. The wax creates a low surface-energy, so the water cannot stick to it.

The technical term is called 'wetting-out'. The easier it is for the adhesive to 'wet-out', the better the adhesion.

High surface energy values attract, whereas low surface energy values repel.

Materials with higher values bond better because good wet-out occurs of the adhesive.

Wet-out is an industry term to describe the flow of an adhesive over a material surface. Good wet-out ensures maximum contact and helps achieve a strong bond.

We measure surface energy in dynes per centimetre (Dyn/cm).

To get even deeper into this subject, Adhesives.org is a useful resource.