Category: plastics

How are the paints made that artists (like Rembrandt and Monet) used in the past? — SB, Oedenrode, The Nederlands

These paints consisted principally of a pigment and a drying oil binder. The pigment was usually a colored powder that didn’t dissolve in the oil. Historically, these pigments were materials collected from nature. The drying oil binder was usually linseed oil, obtained from the seed of the flax plant and a byproduct of the linen industry. Like most organic oils, linseed oil is a triglyceride—it consists of a glycerin molecule with three fatty acid chains attached to it. But while in typical animal or tropical plant oils the carbon atom chains of the fatty acids are completely decorated with hydrogen atoms (saturated fats) or almost completely decorated (monounsaturated fats), the carbon atom chains in linseed oil are missing a significant number of hydrogen atoms (polyunsaturated fats). The polyunsaturated character of linseed oil makes it vulnerable to a chemical reaction in which the chains stick permanently to one another—a reaction call polymerization. With time and exposure to air, the molecules in linseed oil bind together forever to form a real plastic! This “drying” process takes weeks, months, or years, depending on the chemicals present in the paint. It can be accelerated by the addition of catalysts—chemicals that assist the polymerization process but that don’t become part of the final molecular structure of the plastic.

What are gas permeable contact lenses made from and what do they use to pigment them? — TG, Tulsa, OK

A gas permeable contact lens is one that allows oxygen to diffuse through it to the cornea of the wear’s eye. While conventional hard lenses were made almost entirely of a plastic known as poly(methyl methacrylate) or PMMA, commonly known as Plexiglas or Lucite, gas permeable hard or semirigid lenses are copolymers containing both methacrylate and siloxane molecular units. The polymers used in soft lenses are made only of siloxane molecular units and are commonly known as silicon rubbers. The molecules in silicon rubbers are mobile at remarkably low temperatures, giving silicon rubber its flexibility. In fact, these molecules are so mobile that they must be linked together or “vulcanized” to keep them from flowing as a liquid at room temperature. Even when they have been linked together, portions of these molecules are very mobile, so that gas atoms and molecules can diffuse easily through them. I’m not sure what chemicals are used to color contact lenses, but I expect that the dye molecules are permanently linked to the polymer molecules to keep them in place.

How does hair spray work? — KC, IL

While I don’t know exactly what chemicals are used in hairspray, the main constituents are almost certainly polymer molecules—otherwise known as plastics. In the container, these polymer molecules are dissolved in a volatile solvent such as an alcohol or water, and pressurized with a chemical such as propane or a hydrofluorocarbon. When you spray the mixture onto your hair, the solvent evaporates and leaves the polymer molecules clinging to the hairs. These molecules are very long chains of atoms that form a stiff web around each hair and stiffen it. In general, the characteristics of polymers change with temperature and chemical environment. The polymer used in hairspray should be in the “glassy” regime, meaning that its atoms and molecules are essentially immobile at room temperature. Once the solvent is gone, the web of polymer molecules on the hairs is stiff and keeps the hairs from changing shape. Before you panic at the idea of spraying plastic onto your hair, consider that starch is also a polymer, as is hair itself. So putting hairspray on your hair is no different from putting starch on clothes.

How does Styrofoam work?

Styrofoam is a rigid foam consisting of gas trapped in the closed bubbles of polystyrene. Polystyrene itself is a clear plastic that’s used in many disposable food containers. It’s a stiff, amorphous solid at temperatures below 100° C, where amorphous means that it has none of the long-range order associated with crystalline solids. The long, chain-like polystyrene molecules are arranged like a tangled bowl of spaghetti noodles. Amorphous plastics tend to be clear because they’re very homogeneous (uniform) internally and let light passes through them without being deflected or reflected. Plastics that are partially crystalline tend to be white. I think that items bearing the #5 recycling label are made of polystyrene.

But when air or another gas is injected into melted polystyrene and the mixture is beaten to a froth, it forms a stiff white solid when it cools. The whiteness comes about because of inhomogenieties—the gas spoils the uniformity of the plastic so that light is deflected and reflected as it passes through the material. The Styrofoam retains the rigidity of the polystyrene plastic below 100° C, so that it’s suitable for beverage containers for liquids that are no hotter than boiling water. At one time, one of the gases used to make polystyrene foams was Freon, but I believe that Freon is no longer used for this purpose.

How is powder coating done?

Powder coating is done by combining the components of the coating (the binder—a polymer having giant chain-like molecules, the pigments, and the additives) to form a uniform solid, which is then pulverized to a dry powder and sprayed onto the surface to be coated. This coating is then baked to form a continuous film. There are two main classes of powder coatings: thermosetting and thermoplastic coatings. In a thermosetting film, crosslinking occurs between the molecules in the powder during baking. This crosslinking turns the baked film into a single giant molecule that can’t melt or flow. In a thermoplastic film, thermal energy makes the binder molecules mobile enough to become entangled so that a continuous film forms and this film hardens upon cooling. While a thermoplastic film can still melt or flow, it can do that only at elevated temperatures. The powders are often given electric charges during spraying so that electrostatic forces will hold them in place until they’re baked on.

Please explain pectin and why sugar and acid are needed when making jelly.

The molecules of pectin contain enormous chains of atoms, often hundreds or even thousands of atoms long.. Such chains are also found in cellulose and starch, and are used by plants to give them strength and structure. These chain-like molecules are naturally occurring polymers or plastics. The giant molecules in pectin are based on small molecular units of D-galacturonic acid that have joined together like strings of paper dolls. The presence of acid groups on the pectin molecules help to make pectins very water soluble and also sensitive to the acid-base balance of their environment. I am not an expert in the exact structure and chemistry of pectin, or in the proper pH needed for jellymaking, so I can’t give you an exact explanation for how to control the jelling process with acids. But the jell forms because these giant molecules spread out in the viscous solution of sugar and fruit juice, and form a tangled network of filaments that span the entire container. At high temperatures, there is enough mobility in the molecular chains to allow the mixture to flow, but at room temperature, the tangle of molecular filaments prevents flow. In the language of polymer or plastic science, the mixture goes from a liquid flow regime at high temperature to an elastic plateau regime at low temperature. When you deform cold jelly, you are pulling the filaments tight but they can’t disentangle themselves enough to allow the jelly to actually flow. When you deform the cold jelly too far, the filaments begin to break and the jelly tears into fragments. However, when you warm the jelly, thermal energy allows the filaments to move past one another and the jelly begins to flow like a thick (or viscous) liquid.

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.

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 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.

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.