In 1930 food-packing companies were enthralled with the relatively new and improved film called cellophane, a transparent polymer made from cellulose. Cellophane wrappers could help keep packaged food fresh yet would still allow customers a view of the contents. Sealing cellophane packages satisfactorily was a problem, however, until the 3M Company invented and trademarked Scotch tape—a name that the public nonetheless widely uses for all adhesive-backed cellophane tapes. (The analogous product Sellotape, introduced seven years later in Europe, has the same problems with generic use of its name.)
Engineers call the glue in Scotch tape a pressure-sensitive adhesive. It does not stick by forming chemical bonds with the material it is placed on, says Alphonsus Pocius, a scientist at the 3M Corporate Research Materials Laboratory in St. Paul, Minn. Instead applied pressure forces the glue to penetrate the tiniest microscopic irregularities on the material’s surface. Once there, it will resist coming back out, thus keeping the tape stuck in place. The glue “has to be halfway between liquid and solid,” Pocius explains: fluid enough to spread under pressure but viscous enough to resist flowing.
Concocting the right kind of glue is only part of the invention, however. The typical adhesive tape contains not just two materials (glue and backing, which can be cellophane or some other plastic) but four. A layer of primer helps the glue stick to the plastic, while on the other side a “release agent” makes sure that the glue does not stick to the top. Otherwise, Scotch tape would be impossible to unroll.
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Adhesive tape recently caught the attention of physicists. Researchers showed that unrolling tape in a vacuum chamber releases x-rays, and they used those x-rays to image the bones in their fingers as a demonstration. The discovery could lead to cheap, portable (and even muscle-powered) radiography machines. The unrolling creates electrostatic charges, and electrons jumping across the gap between tape and roll produce x-rays. In the presence of air the electrons are much slower and produce no x-rays. But try unrolling tape in a completely dark room, and you will notice a faint glow.