Adhesives

Close your eyes, say the word "glue" and thoughts of your early school days might come to mind. A small bottle of glue remains a key part of most elementary students' back-to-school kits.

Glue has been part of the human tool kit for thousands of years. There's evidence that Neanderthals mixed glue with the paint they used for cave paintings in Lascaux, France . That allowed the works to last by resisting the moisture of the cave walls.

Archaeologists studying burial sites of prehistoric tribes have found broken pottery vessels that had been repaired with sticky resins from tree sap. Digs at sites of old Babylonian temples have uncovered statues with ivory eyeballs glued into eye sockets. The tar-like glue has held for almost 6,000 years.

Travel to Greece or Italy and you'll see examples of adhesives holding up for more than 2,000 years. Artisans in both countries were big on mosaics in floors and tiled walls — many of which are still intact.

Traditionally, glue was made from fish or animal byproducts. Ancient peoples discovered that bones, hides, skin, sinew, and other connective tissues from animals could be processed to remove collagen, the protein in these tissues. The collagen was sticky — ideal for holding things together.

Animal-based glues are still used — mainly in corrugated cardboard, envelopes, bottle labels, book bindings, cartons, furniture, and laminated film and foils.

But most glue sold today is made from chemical-based ingredients. The switch to adhesives like all-purpose white glue, began around 1947. That's when Elmer's — at the time a division of the Borden Company — came out with a product called Cascorez Glue. It was packaged in glass bottles and came with wooden sticks attached to the bottle with rubber bands.

The company continues to command a significant chunk of the estimated $21 billion a year global adhesives market.

Any substance that is capable of holding materials together by surface attachment that resists separation is considered an adhesive. Among the most common ones are:

• Drying adhesives, which include glues such white glue and rubber cements. They are applied to surfaces which are then bonded together as the adhesive dries. The surfaces glued together are sometimes held in place by clamps — or tiny hands holding together popsicle sticks.
• Contact adhesives are applied to both surfaces but are allowed some time to dry before the two surfaces are pushed together. The bond forms very quickly once the surfaces are held together, so there is usually no need to use clamps.
• Hot adhesives — also know as thermoplastic adhesives — are very popular among people who do crafts. They are applied hot in a soft form and harden as they cool. If you use a glue gun, you've used a thermoplastic adhesive.
• Reactive adhesives work by chemical bonding with the surface material. They are applied in thin films. They are usually used for industrial purposes, such as to prevent bolts and screws from becoming loose in rapidly moving assemblies, like car engines. Tighter bolts and screws mean engines run much more quietly than they used to.
• Pressure-sensitive adhesives hold two surfaces together under pressure that is applied briefly at room temperature. Think Scotch tape and duct tape. This type of adhesive also has medical uses, such as holding a wound dressing in place.

But adhesives also have temporary uses. What office doesn't have a steady supply of those little yellow Post-it Notes? They came to be in the mid-1970s, several years after Dr. Spencer Silver, a scientist at the 3M company, discovered an adhesive that wasn't very sticky.

In 1974, Art Fry, another 3M scientist, got tired of losing his place in the hymnal while singing in his church choir. He longed for a bookmark that would stick to a page without ripping it. He remembered Silver's adhesive and came up with those little yellow pieces of paper that you can stick just about anywhere.

By 1981, Post-It Notes were on sale in the United States, Canada and across Europe.

Scientists have been trying to develop strong but temporary adhesives for decades. In the 1940s, Swiss inventor George de Mestral came up with what became Velcro after noticing burrs on his dog after a walk.

Today, scientists are looking to geckos, spiders and mussels as they search for stronger temporary adhesives. Two scientists at Northwestern University have created an adhesive material they call "geckel," which combines elements of the adhesive abilities of geckos and mussels. Geckos can scurry up smooth surfaces thanks to the flat pads on their feet that are covered with very fine hairs that are split at the ends. But their adhesive ability drops dramatically in water.

The geckel adhesive created by the scientists uses arrays of silicone pillars that are flexible enough to adapt to rough surfaces. The researchers coated the pillars with a thin layer of a synthetic polymer that mimics the wet adhesive mussel proteins. The adhesive works just as well on dry or wet surfaces. The scientists say the adhesive could be used in place of sutures to close wounds or as water-resistant bandages.

Meanwhile, Italian scientists say they're on the verge of figuring out a way to make clothing that mimics the natural stickiness of geckos and spiders. The scientists say they've calculated how much stickiness would be needed to support the weight of an adult.

Professor Nicola Pugno at the Polytechnic of Turin said microscopic hollow fibres called carbon nanotubes could be used to make a suit that would do the job. He added that the nanotubes could have applications from space exploration to designing gloves and work boots for window cleaners.

But Pugno adds there would still be obstacles to creating a suit that would allow a person to climb up a building like a gecko. For one thing, gecko muscles are much different than human muscles. Sticking to a wall for any length of time would take a lot more out of a human than a gecko.