How Adhesive Tape Works

The Science of Stickiness
Pressure-sensitive adhesives make packing tape stick to the box on contact.
Pressure-sensitive adhesives make packing tape stick to the box on contact.
© DreamPictures/Shannon Faulk/Blend Images/Corbis

Adhesion certainly didn't begin with tape. Woodworkers in ancient Egypt used natural glue made from animal collagen. Natural, viscous substances like beeswax and resin have always held materials together effectively [source: THToC].

In modern times before tape, glues and epoxies did most of the sticky work. But they had serious drawbacks, especially in household use. Messiness, permanence and drying to a hard finish all made traditional glues, which typically bond by chemical means, less-than-great for small, quick everyday jobs.

The adhesives used in tape work differently. They're called pressure-sensitive adhesives (PSA) and include materials like silicones, acrylics and rubbers -- all polymers to which a tacky resin is added to increase stickiness [source: ThomasNet]. These PSAs rely on physical reactions, not chemical ones, to adhere. This contributes to qualities like removability and flexibility -- some of the stuff we love most about tape.

In a pressure-sensitive bond, there are two main processes at work: wetting and van der Waal's forces. The former establishes adhesion. The latter makes it stronger [source: Hyperphysics].

Wetting is pretty simple. In this case, it refers to the way a solid adhesive penetrates a substrate (the material the tape is applied to). The solid adhesive used in tape is good at wetting because it has a low surface energy, meaning its surface molecules are moving around a lot, or are energized, causing looser bonds. This trait allows the molecules of the adhesive to flow relatively easily, even though it's in solid form, into the pores of the substrate material. All it takes is a little pressure. And the better the ability to flow into the substrate, the stronger the physical bond.

Some adhesives will form stronger bonds over time as molecules seep deeper into the substrate material. From the start, though, a different physical phenomenon adds to the strength of pressure-sensitive adhesion. Molecular attractions called van der Waal's forces take effect even before pressure is applied to the tape.

Van der Waal's forces are weak attractions between molecules that may not normally have positive or negative charges. Some primarily neutral molecules whose protons and electrons aren't evenly distributed throughout can at times exhibit charges, known as dipole moments [source: Hyperphysics]. These charges, or polarities, allow them to form physical bonds with other charged molecules; they can, in fact, induce charges in other mostly neutral molecules by mere proximity [source: Hyperphysics].

Molecules in a pressure-sensitive adhesive can exhibit dipole moments, and they induce corresponding dipole moments as they near the surface molecules of the substrate. The oppositely charged molecules of the adhesive and the substrate, on contact, form physical bonds, augmenting the strength of the wetting-based adhesion.

Adhesion is only part of the equation, though. The beauty of adhesive tape lies as much in its design as in its bond.

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