How Everything Works
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Plastics
Page 1 of 3 (27 Questions and Answers)

443. How do covalent bonds work?
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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.

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

445. How does the process of retreading a tire work?
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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.

446. If a racquetball is one long strand of molecules, if you made a cut in the ball, wouldn't the whole ball fall apart?
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A racquetball is made of vulcanized rubber. Rubber consists of countless molecules, each one of which is principally a long chain of carbon atoms, decorated with hydrogen and other atoms. It resembles of bowl of tiny spaghetti strands though each rubber molecule is much, much longer than it is thick. But simple rubber melts rather easily and becomes gooey when warm. To make it more durable, it must be vulcanized. During vulcanization, the individual rubber molecules are cross-linked to form a permanent network of coupled strands. They can't move relative to one another, which is why the racquetball can't melt. It can only burn when you heat it. So the whole racquetball is one giant molecule. If you cut it in half, you are slicing the molecule in half. It doesn't crumble, it just has many of its bonds broken. That's not a problem because bonds break and remake all the time in the molecules around us.

447. If nothing sticks to Teflon, then how does Teflon stick to a pan?
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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. Teflon is attached to surfaces by sintering it from a powder. At a high enough temperature, the molecular chains begin to move about somewhat so that they bind together into a continuous material. They also enter 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.

448. If rubber cannot melt, how is it molded (vulcanized?) into tires, o-rings, gaskets, and such? You answered this later in the lecture; sulfur is added to the rubber and then the things are molded, right?
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Yes. Vulcanization is done with the object in its final form. The plastic is assembled while it is still thermoplastic; without the cross-links that render it unmeltable. It is then vulcanized into a single giant molecule; a thermoset. This vulcanization may be done with sulfur, as in car tires, or it may be some other reaction. In silicone rubber (e.g. bathtub chalking), the vulcanization occurs spontaneously in air. The polydimethyl siloxane molecules are treated at their ends so that they vulcanize in air, releasing acetic acid (the vinegar smell). The resulting thermoset silicone rubber is one giant molecule and cannot melt any more.

449. Is it true that milk stored in plastic is not as healthy as milk in cardboard containers due to radiation?
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Probably. HDPE (high density polyethylene) allows blue and ultraviolet light to strike the milk, degrading some of its nutrient molecules. It isn't radiation from the plastic but rather the sunlight that the plastic doesn't keep out of the milk. Adding an absorbing chemical to the plastic would help, but it would create an amber plastic (like amber medicine bottles; which are colored for this same reasons). If we could get used to having amber plastic, we would probably be better off. However, people seem to tolerate amber orange juice jugs but not amber milk jugs.

450. What chemical reactions cause the basic atoms to form different molecules and, therefore, different polymers?
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Covalent bonds are very strong and very directional (meaning that they tend to arrange the atoms at specific angles with respect to one another). Once a molecule has formed, the covalent bonds usually prevent it from rearranging at all but the highest temperatures. Much of the field of organic chemistry is devoted to the problems of controlling the formation of covalent bonds. Very subtle reactions are used to replace one atom with another or with a specific group of atoms. The only real control that the organic chemist has is energetics, dynamics, and statistics. By energetics, I mean that objects tend to follow paths that reduce their potential energies as quickly as possible so that molecules will undergo reactions that reduce the overall potential energies as quickly as possible. If you chose the right chemicals, you can use this energetic control to determine the final molecules. By dynamics, I mean that the reaction pathways are also influenced by issues of motion (inertia, momentum, etc.) so that some energetically favorable reactions may not form because inertia and momentum makes it hard for them to occur. By statistics, I mean that reactions that increase the order of the molecules tend to be rather rare. Nature is always becoming more disordered so that a reaction that brings more order to the universe is unlikely to occur. When you mix chemicals together, they are unlikely to react to form a complete Faberge Egg, complete with a miniature winter scene inside. These different reaction issues can be used together or separately to manipulate atoms into a specific molecule. Usually some of the molecules produced in a synthesis are imperfect and must be separated from the desired molecules. So most organic synthesis projects involve many reaction and purification steps.

451. What is plastic explosives made of?
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I don't know for sure, but I suspect that they are plasticized materials (polymer molecules and softening chemicals) in which either the polymer molecule or the plasticizer or both are explosives. Actually, I just looked it up and found that it is based on RDX (a nitrated form of hexamethylenetetramine). The RDX is mixed with oils, waxes, and plasticizers to make a stiff putty. That being the case, it isn't really based on polymer molecules so that the name "plastic" refers more to its ability to assume different shapes at will.

452. Why do some glues dry faster than others?
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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.

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