Tag: plastics

If a racquetball is one long strand of molecules, if you made a cut in the ball,…

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If a racquetball is one long strand of molecules, if you made a cut in the ball, wouldn’t the whole ball fall apart?

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.

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

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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. 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.

If rubber cannot melt, how is it molded (vulcanized?) into tires, o-rings, gaske…

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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?

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.

Is it true that milk stored in plastic is not as healthy as milk in cardboard co…

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Is it true that milk stored in plastic is not as healthy as milk in cardboard containers due to radiation?

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.

What chemical reactions cause the basic atoms to form different molecules and, t…

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What chemical reactions cause the basic atoms to form different molecules and, therefore, different polymers?

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.

What is plastic explosives made of?

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What is plastic explosives made of?

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.

Why do some glues dry faster than others?

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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.

Why is it so expensive to recycle plastic?

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Why is it so expensive to recycle plastic?

Different plastics are handled differently for recycling. Thermosets, such as rubber in tires, cannot be melted and cannot be recycled. Only thermoplastics can be melted for true reuse. There are 6 common thermoplastics that are recycled. These are numbered 1 through 6 on their bottoms. Objects made from one of these plastics can be collected together, melted, and then reformed into new useful objects. Unfortunately, the melted and reformed plastic isn’t as pure as the original. The plastics manufacturers would rather clean up petroleum into petrochemicals and then make pure plastics than start with plastic objects, clean them, and reuse them. Because the recycler can’t control what was in the plastic objects, these objects cannot be used for critical applications such as food containers or plumbing. Thus most recycled plastic is used for less profitable applications. If the recycler could be absolutely sure that the plastic hadn’t been contaminated, some of it could be reused very easily. Plastic milk jugs could be reformed into plastic milk jugs over and over again.