April 19, 1995

One Minute Papers - Questions and Answers

How does glue get objects to stick to it? Do molecules in the objects bind with molecules in the glue?

Ideally, the glue would form strong covalent bonds with the material and then form countless strong bridges from one object to another. Unfortunately, getting the glue to form such strong bonds with a surface is rarely possible. Instead, the glue forms weaker hydrogen bonds or van der Waals with the surface and is not so firmly attached. The glue's polymer molecules may also extend into the surface, in cracks and fissures to form a more sturdy attachment. Clearly, surface preparation can help the gluing process. Glue will bind more effectively to a porous, rough surface than to a very smooth, impermeable one.

When you did your experiments, you added a lot of water. Why doesn't the water make it more soggy (like a liquid)? Why does it still harden with so much water?

The water was there to separate the polymer molecules and allow them to move around freely. Once I added the sodium borate or borax (sodium tetraborate) solution, the polymer molecules began to cling to one another and formed a clump within the water. The water just doesn't matter because the lump appears within the water. Some number of water molecules are caught up in the lump, but the total amount of water present in the container doesn't affect the lump itself. It's like cooking spaghetti: you can boil the strands in a big pot of water or a little pot of water. When you pour the mixture through a strainer, only the wet spaghetti strands remain.

How does the process of retreading a tire work?

Since a tire cannot be melted, it can't simply be reformed into a new tire. Moreover, it contains lots of belting materials that would have to be removed and reinstalled in the new tire. So the only recycling technique available for tires is to replace the tread itself. They shave away the outside of the tire to remove any remaining tread (working carefully, so as not to damage the belts), and glue a new layer of rubber onto the outside of the tire. This new layer of rubber has the tire tread already formed into it (it is already vulcanized). I suspect that the gluing process involves vulcanizing a new thin layer of rubber between the tire and the tread so that the result is almost as sturdy as the original tire. If the vulcanization is do properly, the new rubber layer will grab onto the outside of the shaved tire and the inside of the tread and form strong covalent bonds with them. The tire will again be one giant molecule.

Why do some glues dry faster than others?

Some glues literally "dry," since they contain a plasticizer chemical that evaporates to leave a firmer plastic. Other glues polymerize directly during the gluing process. For the glues that dry by evaporating plasticizer, the choice of plasticizer is critical. Water leaves relatively slowly compared to volatile organic solvents such as toluene or acetone. That is why water-based white glue dries more slowly than organic-based plastic cement. But the glues that polymerize during the gluing process (they "cure" rather than "dry") have a broad range of speeds. Some of those glues polymerize very rapidly (e.g. superglues and 3-minute epoxies) and some go much slower (normal epoxies). In general, slower glues produce stronger materials because they contain long polymer molecules. The fast curing glues form too many short polymer molecules and are not as tough.

If nothing sticks to Teflon, then how does Teflon stick to a pan?

Working with Teflon is difficult in any case. The molecular chains are extremely long, typically 100,000 carbon atoms long. It does not melt easily (it is used for high temperature applications) and is a very viscous liquid even when it does melt. I could not find specific information about how Teflon is attached to surfaces, but I suspect that melted Teflon enters pores and crevices in the surface and becomes wedged inside when it cools. With enough of its chains extending into the pan surface, the whole Teflon sheet is permanently attached to the pan.