Why Does the Adhesive of Some Tape and Envelopes Have a Purple Fluorescent Effect When You Pull Them Apart?

The fluorescent effect of some adhesives is well known, but there is insufficient energy in the sparks to ignite gas such as methane.

The colored glow is a form of chemiluminescence. Separating the gummed surfaces requires energy that breaks the attractive forces between the molecules of gum.

Presumably, the act of pulling apart the surfaces supplies excess energy to the gum molecules that lifts them into an excited state.

As they decay back to their normal state the energy is released in the form of visible light. The difference in energy between the excited and ground states defines the wavelength and hence the color of the light produced; in this case purple.

This phenomenon is different from fluorescence, where light, often ultraviolet, is absorbed and then re-emitted at a longer wavelength, in the visible spectrum.

Fluorescence gives rise to Day-Glo colors and the blue glow you might observe while drinking tonic water near one of the ultraviolet lamps often found in nightclubs.

A similar effect can be seen when stripping off a length of electrical insulating tape.

We first noticed this about 30 years ago and the discovery came, by coincidence, shortly after an explosion in a coal mine. The last people to go down the mine before the explosion had been a crew of electricians.

More recently, there has been an explosion attributed to essentially this cause, or at least to peeling off an adhesive label.

A heavy-duty lead-acid battery exploded when an operator peeled an adhesive label from it. Investigation showed that this could generate >8 kV potential. Discharge through the hydrogen/oxygen headspace consequent upon recharging batteries caused the explosion.

Many people have observed vivid discharges when opening Royal Society of Chemistry self-adhesive envelopes.