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The oldest glue in the world is over 8,000 years old and comes from a cave near the Dead Sea. Today, we have enough types of tape and glue to build and repair almost anything. But what gives glue and tape their stickiness? And is one stronger than the other? Elizabeth Cox explores the world of adhesives.
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متن انگلیسی درس
The oldest glue in the world is over 8,000 years old and comes from a cave near the Dead Sea. Ancient people used this glue, made from a mixture of animal bone and plant materials, to waterproof baskets and construct utensils. And for thousands of years after, plants and animals were the glue that held human civilization together. Today, we have enough types of tape and glue to build and repair almost anything. But what gives glue and tape their stickiness? And is one stronger than the other? Adhesives can be made from synthetic molecules or natural proteins and carbohydrates like the vegetable starch dextrin, the milk protein casein, and the terpenes in tree resin. In order to work, glue and tape need both adhesive bonds and cohesive bonds. Adhesive bonds occur between an adhesive’s molecules and the molecules of whatever it’s sticking to. Cohesive bonds happen between a glue or tape’s own molecules, holding it together. Most glues consist of adhesive polymers dissolved in a solvent that prevents them from sticking to the inside of the bottle. The strong smell of many glues comes from the solvent, which evaporates when exposed to air. Some glues use water as a solvent, but others use chemicals that can be harmful to inhale. Glues with two or more components that chemically react instead of just drying can create stronger bonds. Both the adhesive and cohesive bonds of glue are strong, but the drying process makes them irreversible. This is why, if a glued surface is broken after it dries, it can’t be reattached without new glue. By contrast, when tape is applied to a surface, it forms weaker, reversible bonds, so you can peel a piece of tape off a surface and use it again. These weak bonds, called Van der Waals forces, can occur between any two materials, but only if they’re extremely close together, closer than the naked eye can see. Tape usually consists of a backing coated with a combination of a rubber or rubber-like “stretchy” component, and a compound called a tackifier. That’s the “sticky” component. A tape’s stickiness is determined by the proportion of elastic component and tackifier, the thickness of adhesive spread onto the backing, and the type of backing material. No chemical reaction occurs when tape is pressed onto a surface. Instead, the soft adhesive flows into the cracks and grooves of the surface. This ability to slide into cracks and then stay in place is called viscoelasticity. Once the viscoelastic adhesive fills these microscopic crevices, it is close enough to form Van der Waals forces. So what’s the world’s strongest adhesive? Well, there’s no one answer. In terms of absolute strength of adhesive bonds, glue is stronger than tape, but no single adhesive works well in all circumstances. Of the glues, cyanoacrylates, or super glues, may form the strongest bonds, but two-component epoxy glues have much higher resistance to heat and shearing, and are compatible with a wider range of surfaces. So, if you wanted to dangle an anvil in the air, super glue might be your best bet. But if you’re doing so over an active volcano, you’d want an epoxy instead. And in order to work at all, glues need enough real estate where surfaces touch. If for some reason you wanted to make a chain of bowling balls, duct tape would be better. Engineers weigh similar, if less absurd, factors all the time. Choosing the right glue to withstand the heat inside an engine is a matter of life and death. And though the strength of duct tape’s adhesive bonds can’t compete with those of epoxy glues, tape does have the advantage of instantaneous stickiness in an emergency. Glue may be necessary to get a rocket to space, but when it comes to extraterrestrial repairs, stick to duct tape: liquid glues don’t work in zero gravity.
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