Polymers

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POLYMERS

Polymers are substances whose molecules have high molar masses and are composed of a large number of repeating units. There are both naturally occurring and synthetic polymers. Among naturally occurring polymers are proteins, starches, cellulose, and latex. Synthetic polymers are produced commercially on a very large scale and have a wide range of properties and uses. The materials commonly called plastics are all synthetic polymers.

Polymers are formed by chemical reactions in which a large number of molecules called monomers are joined sequentially, forming a chain. In many polymers, only one monomer is used. In others, two or three different monomers may be combined. Polymers are classified by the characteristics of the reactions by which they are formed. If all atoms in the monomers are incorporated into the polymer, the polymer is called an addition polymer. If some of the atoms of the monomers are released into small molecules, such as water, the polymer is called a condensation polymer. Most addition polymers are made from monomers containing a double bond between carbon atoms. Such monomers are called olefins, and most commercial addition polymers are polyolefins. Condensation polymers are made from monomers that have two different groups of atoms which can join together to form, for example, ester or amide links. Polyesters are an important class of commercial polymers, as are polyamides (nylon).

POLYETHYLENE

Polyethylene is perhaps the simplest polymer, composed of chains of repeating –CH₂– units. It is produced by the addition polymerization of ethylene, CH₂=CH₂ (ethene). The properties of polyethylene depend on the manner in which ethylene is polymerized. When catalyzed by organometallic com pounds at moderate pressure (15 to 30 atm), the product is high density polyethylene, HDPE. Under these conditions, the polymer chains grow to very great length, and molar masses average many hundred thousands. HDPE is hard, tough, and resilient. Most HDPE is used in the manufacture of containers, such as milk bottles and laundry detergent jugs. When ethylene is polymerized at high pressure (1000–2000 atm), elevated temperatures (190–210°C), and catalyzed by peroxides, the product is low density polyethylene, LDPE. This form of polyethylene has molar masses of 20,000 to 40,000 grams. LDPE is relatively soft, and most of it is used in the production of plastic films, such as those used in sandwich bags.

POLYPROPYLENE

This polymer is produced by the addition polymerization of propylene, CH₂=CHCH₃ (propene). Its molecular structure is similar to that of polyethylene, but has a methyl group (–CH₃) on alternate carbon atoms of the chain. Its molar masses falls in the range 50,000 to 200,000 grams. Polypropylene (PP) is slightly more brittle than polyethylene, but softens at a temperature about 40°C higher. Polypropylene is used extensively in the automotive industry for interior trim, such as instrument panels, and in food packaging, such as yogurt containers. It is formed into fibers of very low absorbance and high stain resistance, used in clothing and home furnishings, especially carpeting.

POLYSTYRENE

Styrene, CH₂=CH–C₆H₅, polymerizes readily to form polystyrene (PS), a hard, highly transparent polymer. The molecular structure is similar to that of polypropylene, but with the methyl groups of polypropylene replaced by phenyl groups (–C₆H₅). A large portion of production goes into packaging. The thin, rigid, transparent containers in which fresh foods, such as salads, are packaged are made from polystyrene. Polystyrene is readily foamed or formed into beads. These foams and beads are excellent thermal insulators and are used to produce home insulation and containers for hot foods. Styrofoam is a trade name for foamed polystyrene. When rubber is dissolved in styrene before it is polymerized, the polystyrene produced is much more impact resistant. This type of polystyrene is used extensively in home appliances, such as the interior of refrigerators and air conditioner housing. [For more information about this polymer, see Chemical Demonstrations: A Handbook for Teachers of Chemistry, by Bassam Z. Shakhashiri, Volume 1 (1983), page 241.]

POLYVINYL CHLORIDE

Polymerization of vinyl chloride, CH₂=CHCl (chloroethene), produces a polymer similar to polyethylene, but having chlorine atoms at alternate carbon atoms on the chain. Polyvinyl chloride (PVC) is rigid and somewhat brittle. About two-thirds of the PVC produced annually is used in the manufacture of pipe. It is also used in the production of “vinyl” siding for houses and clear plastic bottles. When it is blended with a plasticizer such as a phthalate ester, PVC becomes pliable and is used to form flexible articles such as raincoats and shower curtains.

POLYETHYLENE TEREPHTHALATE

Polyethylene terephthalate (PET), or polyethylene terephthalic ester (PETE), is a condensation polymer produced from the monomers ethylene glycol, HOCH₂CH₂OH, a dialcohol, and dimethyl terephthalate, CH₃O₂C–C₆H₄–CO₂CH₃, a diester. By the process of transesterification, these monomers form ester linkages between them, yielding a polyester. PETE fibers are manufactured under the trade names of Dacron and Fortrel. Pleats and creases can be permanently heat set in fabrics containing polyester fibers, so-called permanent press fabrics. PETE can also be formed into transparent sheets and castings. Mylar is a trade name for a PETE film. Transparent 2-liter carbonated beverage bottles are made from PETE. (The opaque base on some bottles is generally made of HDPE.) One form of PETE is the hardest known polymer and is used in eyeglass lenses.

POLYTETRAFLUOROETHYLENE

Teflon is a trade name of polytetrafluoroethylene, PTFE. It is formed by the addition polymerization of tetrafluoroethylene, CF₂=CF₂ (tetrafluoroethene). PTFE is distinguished by its complete resistance to attack by virtually all chemicals and by its slippery surface. It maintains its physical properties over a large temperature range, -270° to 385°C. These properties make it especially useful for components that must operate under harsh chemical conditions and at temperature extremes. Its most familiar household use is as a coating on cooking utensils.

POLYURETHANE

This important class of polymers is formed by the addition polymerization of an diisocyanate (whose molecules contain two –NCO groups) and a dialcohol (two –OH groups). The polymer chain is linked by urethane groups (–O–CO–NH–). The –NH– portion of the urethane group can react similarly to an –OH group, producing cross-linking between polymer chains. Polyurethane is spun into elastic fibers, called spandex, and sold under the trade name Lycra. Polyurethane can also be foamed. Soft polyurethane foams are used in upholstery, and hard foams are used structurally in light aircraft wings and sail boards. The formation of some polyurethane (and polystyrene) foams exploits the exothermic nature of the polymerization reaction. A liquid with a low boiling point, called a blowing agent, is added to the monomers before the polymerization starts. As the polymerization proceeds, it releases enough heat to boil the liquid. The boiling liquid produces bubbles that create a foam. In the past, the most commonly used low-boiling liquids were chlorofluorocarbons. However, the damaging effect of chlorofluorocarbons on the stratospheric ozone layer has eliminated their use. Other low-boiling liquids have other disadvantages, such as flammability. Therefore, most polyurethane and polystyrene foams are manufactured by forcing a pressurized gas, such as nitrogen or carbon dioxide, into the polymerizing mixture. [For more information about this polymer, see Ibid., Volume 1, page 216.]

POLYAMIDE

Polyamides are a group of condensation polymers commonly known as nylon. Nylon is made from two monomers, one a dichloride and the other a diamine. One particular nylon is made from 1,6-diaminohexane, NH₂(CH₂)₆NH₂ and sebacoyl chloride, ClCO(CH₂)₈COCl. When these polymerize, the resulting molecules contain repeating units of –NH(CH₂)₆NH–CO(CH₂)₈CO–. Molecules of HCl are released during the polymerization. This particular polymer is called nylon 6-10 because it contains alternating chains of 6 and 10 carbon atoms between nitrogen atoms. Nylon can be readily formed into fibers that are strong and long wearing, making them well suited for use in carpeting, upholstery fabric, tire cords, brushes, and turf for athletic fields. Nylon is also formed into rods, bars, and sheets that are easily formed and machined. In this form, nylon is used for gears and for automobile fuel tanks. [For more information about this polymer, see Ibid., Volume 1, page 213.]

POLYACRYLAMIDE

Polyacrylamide is a condensation polymer with an unusual and useful property. The structure of polyacrylamide is similar to that of polyethylene, but having a hydrogen on every other carbon replace by an amide group, –CONH₂. The molecule is composed of repeating –CH₂–CH(CONH₂)– units. The amide groups allow for linking between polymer strands. The –CONH₂ group from one molecule can react with the same group of another molecule, forming a link between them with the structure –CONHCO–. This produces a network of polymer chains, rather like a tiny sponge. The free, unlinked amide groups, because they contain –NH₂ groups, can form hydrogen bonds with water. This gives the tiny cross linked sponges a great affinity for water. Polyacrylamide can absorb many times its mass in water. This property is useful in a variety of applications, such as in diapers and in potting soil. The polyacrylamide will release the absorbed water if a substance that interferes with hydrogen bonding is added. Ionic substances, such as salt, cause polyacrylamide to release its absorbed water. [For more information about this polymer, see Ibid., Volume 3 (1989), page 368.]

Some polymers have the ability to interact with polarized light. This interaction shows itself when the polymer is placed between polarizing filters whose axes are at a 90-degree angle to each other. In this arrangement, very little light passes though both filters. When a substance that interacts with polarized light is placed between the filters, light will pass through the filters. Materials that interact with polarized light have either unsymmetrical molecules or an orderly but unsymmetrical arrangement of molecules. Certain polymers allow light to pass through crossed polarizing filters. Polymers that do this have molecules in a regular but unsymmetrical arrangement. Such polymers are called crystalline polymers. In other polymers the arrangement of the molecules is random, with the molecules tangled around each other. These are called amorphous polymers, and they do not interact with polarized light. However, an amorphous polymer can sometimes be made to interact with polarized light by stretching or compressing it. Stretching or compressing an amorphous polymer can cause the molecules in some areas to become aligned. This increases the crystallinity of the polymer and increases its interaction with polarized light. This technique is exploited in the manufacture of polarizing sunglasses. A thin film of polyvinyl alcohol is stretched to align its molecules and then laminated between protective plastic sheets.

OTHER PLASTICS

Over the past few decades, the use of polymers in disposable consumer goods has grown tremendously. This growth is proving to be taxing on the waste disposal system, consuming a large fraction of available landfill space. Furthermore, the raw materials for these polymers are obtained from petroleum, a limited, non-renewable resource. To reduce the demand for landfill space and the consumption of limited petroleum reserves, the recycling of polymers has become a subject of concern. One of the problems faced in recycling polymers is the great variety of polymers in use. To help sort wastes by type of polymer, most disposable polymeric goods are labeled with a recycling code: three arrows around a number above the polymer's acronym. These are intended to help consumers separate the waste polymers according to type before disposing of them. (In the city of Madison, currently only type 1 (PETE) and type 2 (HDPE) polymers are being recycled. These account for over 90% of all waste plastic containers. Other plastics comprise less than 1% of household waste.)

The recycling of polymers is not a closed loop, where a material is reformed into new products repeatedly, such as is the case with aluminum. Most polymeric materials are recycled only once, and the product made of recycled polymer is discarded after use. To obtain the maximum benefit from recycled polymer, the products made from it generally are intended to have a relatively long useful life. Recycled polymers are used in such products as cafeteria trays, large plastic toys, impact absorbing highway pylons, and carpeting. Some recycled PETE is now used in 2-liter soft-drink containers. In general, products made from recycled polymers are more expensive than those made from virgin plastic. This is the case because current manufacturing facilities are geared to production from new materials. As more factories capable of using recycled plastics are constructed, the costs of using recycled polymers will decline. More such factories will be built soon only if we are willing to pay "up front" the costs of recycling (as higher prices of consumer goods), rather than delaying costs of disposal (in the form of increased taxes). These high, up-front costs have caused a number of politicians and journalists to question the wisdom of recycling (see, for example, "What a Waste" in the June 30, 1996 issue of the New York Times Magazine). Such questioning is good, for it is only after all aspects of this issue are considered that appropriate action can be taken. Not only the costs of recycling must be considered, but also the costs of producing new materials, including the depletion of non-renewable resources, and the costs of disposal, both now and in the future, must be weighed to determine the best course of action.

EXERCISE: Discover the local recycling guidelines for your area and report on them. What items can be recycled and which ones cannot? Report your source of information.

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