Month: December 1969

How does the process of retreading a tire work?

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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 unvulcanized rubber onto the outside of the tire. The tire is then placed in a mold and heated. This heating causes a chemical reaction known as vulcanization to occur in the new tread rubber. This vulcanization bonds all the rubber molecules together and also binds them to the original tire. If done correctly, the entire tire, old and new, becomes a single gigantic molecule and the chances of losing the tread while driving should be minimal. Furthermore, the mold forms a tread on the surface of the new rubber so that the tire is structurally very much like a new tire. However, poor retreading work or accumulated damage due to many retreading operations can produce a weak tire and allow the tread to tear away from the tire body. This separation usually occurs while the tire is spinning rapidly and the tension forces within the tire are maximized. Such separation accounts for the huge strips of tread material you often see on highways.

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

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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.

How do covalent bonds work?

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How do covalent bonds work?

When two atoms form a covalent bond, their total energy is reduced by their proximity. It thus takes energy to separate them. If that energy isn’t available, they will cling to one another indefinitely. The two ways in which they lower their total energy by being close are (1) electrostatic attraction and repulsion and (2) lower kinetic energy. Two atoms experience both attractive and repulsive forces as they approach one another. Their positively charged nuclei repel one another, their negatively charged electrons repel one another, but their nuclei attract their electrons. The nuclei never get very close and the electrons manage to stay relatively far apart, too. The dominant effect is an attraction between the electrons and the two nuclei. The result is a net attraction. The nearby atoms are pulled toward one another by these electric forces. The lower kinetic energy comes about because of quantum effects. The electrons travel about the nuclei as waves. When the atoms are far apart, the electrons must orbit their individual atoms. Because they are then confined to small domains, they must have short wavelengths. These waves must be short enough to fit properly into their small confines. Short wavelength objects have high kinetic energies (e.g. short wavelength light is x-rays and gamma rays). But when the atoms are touching, the electrons can spread out between both atoms. Their wavelengths increase and their kinetic energies diminish. These two effects (lowered electrostatic potential energy and lowered kinetic energy) reduce the total energy when the two atoms touch. The result is the covalent bond.

I thought that glass could move, that is supposed to be a check for antique glas…

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I thought that glass could move, that is supposed to be a check for antique glass (It has flowed downward). Does glass move over hundreds of years?

People used to think so. They thought that glass was simply a very, very viscous liquid. However, it now appears that something happens below the glass transition temperature that stops all flow. In effect, the viscosity goes to infinity. While it might be a liquid in principle, it simply doesn’t flow, even in terms of geological time scales. Antique glasses have non-uniform thicknesses because of how they were made. The earlier techniques involved stretching blown glass and tended to make sheets with irregular thicknesses. Antique glass exhibits these irregularities.

Why is stainless steel a sterile material? Why is it used for surgical tools and…

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Why is stainless steel a sterile material? Why is it used for surgical tools and to pierce ears?

Stainless steel is not inherently sterile but it can be made sterile and its lack of corrosion provides no hidden cavities that might harbor germs. A stainless steel surface can be made relatively flat and it will remain that way indefinitely. In contrast, a rusting steel surface has a complicated surface that is constantly changing. That surface is harder to keep clean than a flat stainless surface. Although stainless steel seems ideal for medical purposes, it is not hypoallergenic. Many people react badly to nickel, which is present it high quantities in surgical stainless. It also turns some people’s skin green.

Why is silver used so often for tableware?

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Why is silver used so often for tableware?

Silver is used in tableware because of its whitish luster and preciousness. It is not really the most practical metal for cutlery. It tarnishes slowly by reacting with sulfur pollutants, which are present in the air in trace amounts. Pure silver is also very soft because it allows slippage to occur easily. To harden tableware, silver is alloyed with about 5% copper. The resulting material is much harder than either of the pure metals. Jewelry silver has even more copper; up to about 20%.

What makes stainless steel stainless?

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What makes stainless steel stainless?

Stainless steel resists corrosion because one of the metals (iron, nickel, and chromium) or one of their oxides is bound to be stable in almost any chemical environment. Corrosion stops at the grain boundaries around the stable materials so that they form a protective layer above the other materials beneath them. — Thanks to David Ingham for this answer

Is it true that striking two hammers together will release little splinters? If …

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Is it true that striking two hammers together will release little splinters? If so, why?

The head of a hammer is made of very tough steel. Depending on the type of hammer, that head may even hardened tool steel. In that case, the head will not yield, except to the most incredible forces. It will instead deform elastically and then return to its original shape. However, if you smack two hardened hammerheads against one another, the forces that they exert on one another may become so great that the heads will shatter. The symptom will probably not be the release of a few tiny splinters but rather large chunks of hard steel flying off in all directions.

How is the strength of a clipping device such as Caribeener, hook, or chain link…

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How is the strength of a clipping device such as Caribeener, hook, or chain link calculated? I think it is measured in kilo-newtons. What elements are taken into consideration when that strength is measured?

One of the most critical measures of a clip-ping device is the maximum tension that it can tolerate without failing. I would expect a tester to measure that failure tension by putting the clipping device in a simulated working environment and exposing it to greater and greater tension until it fails. For example, a chain link would be put between two sturdy hooks and then the hooks would be pulled apart until the chain link broke or deformed permanently. Since tension is a force, it’s natural to measure it in newtons or kilo-newtons (1000 newtons). (There are 4.4482 newtons in 1 pound of force.) But what constitutes failure is complicated since anything that is exposed to tension deforms somewhat. However, if the tension is less than a certain threshold, the deformation will be purely elastic—meaning that the device will return to its original shape once the tension is released. But if the tension exceeds that threshold, the deformation will be plastic—meaning that it will be permanent and the device will not return to its original shape once the tension is released. I would expect the rated strength of a clipping device to be a reasonable fraction (probably about 50%) of the tension required to cause plastic de-formation of that device.

How do steak knives differ in structure from the “super” cut-through-anything …

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How do steak knives differ in structure from the “super” cut-through-anything non-damageable knives?

A good knife is distinguished both by its cutting edge and the backbone that supports that edge. The ideal knife has a very hard cutting edge (one that never undergoes plastic deformation and thus never becomes dull) and a very tough backbone (one that can absorb an enormous amount of energy before breaking). The backbone can experience plastic deformation when necessary, in order to absorb energy. Cheap steak knives are made of only one steel: a moderately hard and moderately tough material. They gradually dull because of plastic deformation in their edges but they never break because their backbone is flexible. A great knife is made of several steels, which can be formed by proper heat treatment of a single piece of metal: a very hard edge and a very tough backbone. It never gets dull because its cutting edge never yields and it never breaks because it bends before breaking.