And why does pressing down on it make it stick more?
In: Chemistry
Glue.
Specifically, it is known as a pressure-sensative adhesive, meaning the glue forms a bond between two objects as you push down on the tape, effectively solidifying the glue, but also making sure that the most surface area is coated with the adhesive, which makes the bond stronger.
It requires no heat, water, or solvent to stick.
This glue is also susceptible to damage by contaminates, which is why a bandaid placed on a dirty arm will fall off quickly.
Adhesive tape is a “pressure sensitive adhesive,” a viscoelastic material that wets the adherend surface, but does not bond chemically. In bulk, PSA is like a tacky gel. Unlike glue, there are no chemical changes to the PSA during application, bonding, or removal.
The PSA wets the adherend due to surface energy. Ideally, the PSA will have a lower surface energy than the adherend, so that it wets spontaneously. Pressing down on the tape improves adhesion by rapidly putting the adhesive into more intimate contact with the surface. Once in contact, adhesion will continue to build naturally over time (about an hour). Pressure has no effect on the final bond except to increase the area of contact on rough surfaces.
When the tape is peeled, the adhesive stretches and fibrillates before the bond ruptures. Energy is also dissipated as heat due to the viscous nature of the material.
The “tackiness” of a PSA is a function of its adhesive strength and the speed with which it conforms to a surface. Tack is not necessarily proportional to adhesive strength.
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