Plastics History of Plastics and Polymers Plastics are polymers. What is a polymer? The simplest definition of a polymer is something made of many units. Think of a polymer as a chain. Each link of the chain is the "mer" or basic unit that is made of carbon, hydrogen, oxygen, and/or silicon. To make the chain, many links or "mers" are hooked or polymerized together. Polymerization can be demonstrated by linking strips of construction paper together to make paper garlands or hooking together hundreds of paper clips to form chains. Polymers have been with us since the beginning of time. Natural polymers include such things as tar and shellac, tortoise shell and horns, as well as tree saps that produce amber and latex. These polymers were processed with heat and pressure into useful articles like hair ornaments and jewelry. Natural polymers began to be chemically modified during the 1800s to produce many materials. The most famous of these were vulcanized rubber, gun cotton and celluloid. The first semi-synthetic polymer produced was Bakelite in 1909 and was soon followed by the first synthetic fiber, rayon, which was developed in 1911. Even with these developments, it was not until World Did you know? War II that significant changes took place in the polymer industry. Prior to World War II, natural substances were Polyethylene (there are two generally available; therefore, synthetics that were being types--high density developed were not a necessity. Once the world went to polyethylene or HDPE, and war, our natural sources of latex, wool, silk and other low density polyethylene or materials were cut off, making the use of synthetics LDPE) played a key critical. During this time period, we saw the use of nylon, supporting role during acrylic, neoprene, SBR, polyethylene and many more World War II as a critical polymers take the place of natural materials that were no material which insulated longer available. Since then, the polymer industry has radar electronics. continued to grow and has evolved into one of the fastest growing industries in the U.S. and in the world.
The Structure of Polymers Many common classes of polymers are composed of hydrocarbons. These polymers are specifically made of small units bonded into long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are bonded along the backbone. Below is a diagram of polyethylene, the simplest polymer structure.
1 There are polymers that contain only carbon and hydrogen. Polypropylene, polybutylene, polystyrene, and polymethylpentene are examples of these. Even though the basic makeup of many polymers is carbon and hydrogen, other elements can also be involved. Oxygen, chlorine, fluorine, nitrogen, silicon, phosphorous and sulfur are other elements that are found in the molecular makeup of polymers. Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen and oxygen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen. Vulcanized rubber and thiokol contain sulfur. There are also some polymers that, instead of having a carbon backbone, have a silicon or silicon-oxygen backbone. These are considered inorganic polymers. One of the most famous silicon-based polymers is Silly PuttyTM.
Molecular Arrangement of Polymers Think of how spaghetti noodles look on a plate. This is similar to how polymers can be arranged if they are amorphous. An amorphous arrangement of molecules has no long-range order or form in which the polymer chains arrange themselves. Amorphous polymers are generally transparent. This is an important characteristic for many applications such as food wrap, PlexiglasTM, headlights and contact lenses. Controlling and quenching the polymerization process can result in amorphous organization. Obviously, not all polymers are transparent. The polymer chains in objects that are translucent and opaque are in a more crystalline arrangement. By definition a crystalline arrangement has atoms, ions, or in this case, molecules in a distinct pattern. You generally think of crystalline structures in salt and gemstones, but not in plastics. Just as quenching can produce amorphous arrangements, processing can control the degree of crystallinity. The higher the degree of crystallinity, the less light can pass through the polymer. Therefore, the degree of translucence or opaqueness of the polymer is directly affected by its crystallinity. Engineers are always producing better materials by manipulating the molecular structure that affects the final polymer produced. Manufacturers and processors introduce various fillers, reinforcements, and additives into the base polymers to expand product possibilities.
Characteristics of Polymers Polymers are divided into two distinct groups: thermoplastics and thermosets. The majority of polymers are thermoplastic, meaning that once the polymer is formed it can be heated and reformed over and over again. This property allows for easy processing and recycling. The other group, the thermosets, cannot be remelted. Once these polymers are formed, reheating will cause the material to scorch. Every polymer has very distinct characteristics, but most polymers have the following general attributes. 1. Polymers can be very resistant to chemicals. Consider all the cleaning fluids in your home that are packaged in plastic. Reading the warning labels that describe what happens when the chemical comes in contact with skin or eyes or is ingested will emphasize the chemical resistance of these materials. 2. Polymers can be both thermal and electrical insulators. A walk through your house will reinforce this concept, as you consider all the appliances, cords, electrical outlets and wiring that are made or covered with polymeric materials. Thermal resistance is evident in the kitchen with pot and pan handles made of polymers, the coffee pot handle, the foam core of refrigerators and freezers, insulated cups, coolers,
2 and microwave cookware. The thermal underwear that many skiers wear is made of polypropylene and the fiberfill in winter jackets is acrylic. 3. Generally, polymers are very light in mass with varying degrees of strength. Consider the range of applications, from dime store toys to the frame structure of space stations, or from delicate nylon fiber in pantyhose to KevlarTM, which is used in bulletproof vests. 4. Polymers can be processed in various ways to produce thin fibers or very intricate parts. Plastics can be molded into bottles or the body of a car or be mixed with solvents to become an adhesive or a paint. Elastomers and some plastics stretch and are very flexible. Other polymers can be foamed like polystyrene (StyrofoamTM) and urethane. Polymers are materials with a seemingly limitless range of Did you know? characteristics and colors. Polymers have many inherent properties that can be further enhanced The plastics we all know as by a wide range of additives to broaden their Silly Putty were invented by uses and applications. an engineer in the 1940s In addressing positive attributes of polymers, it is -- he originally called it equally important to discuss some of the difficulties Nutty Putty because of its associated with the material. Plastics deteriorate but ability to stretch to many never decompose completely. This is why recycling of times its original size plastics is so important. Plastics make up 9.9 percent of our trash by weight. In 1997, Americans produced 217 million tons of trash so approximately 22 million tons of plastic was trashed instead of being recycled. Applications for recycled plastics are growing every day. Plastics can be blended with virgin plastic (plastic that has not been processed before) to reduce cost without sacrificing properties. Recycled plastics are used to make polymeric timbers for use in picnic tables, fences, and outdoor toys, thus saving natural lumber. Plastic from 2-liter bottles is even being spun into fiber for the production of carpet and clothing. A solution for plastics that are not recycled, especially those that are soiled, such as used microwave food wrap or diapers, can be a waste-to-energy system (WTE). Incineration of polymers produces heat energy. The heat energy produced by the burning plastics not only can be converted to electrical energy but helps burn the wet trash that is present. Polymers affect every day of our life. These materials have so many varied characteristics and applications that their usefulness can only be measured by our imagination. Polymers are the materials of past, present, and future generations. How Are Plastics formed? Plastics are formed in three main ways. Plastic formation includes extrusion, injection and stretch blow molding. In Extrusion Blow Molding (EBM), plastic is melted and extruded into a hollow tube. The tube is then put into a cooled metal mold. Air is blown into the
3 tube, inflating it into the shape of the mold to form things such as hollow bottles, containers or parts. After the plastic has cooled, the mold is opened and the part is ejected. The process of Injection Blow Molding (IBM) is used for the production of hollow plastic objects in large quantities. In the IBM process, the polymer is injection molded onto a core pin; then the core pin is rotated to a blow molding station to be inflated and cooled. This is the least- used of the three blow molding processes and is typically used to make small medical and single serve bottles. The injection blow molding process is divided into three steps: injection, blowing and ejection. The injection blow molding machine is based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mold. The preform mold forms the external shape and is clamped around the core rod which forms the internal shape of the preform. The preform consists of a fully formed bottle or jar neck with a thick tube of polymer attached, which will form the body. The preform mold opens and the core rod is rotated and clamped into the hollow, chilled blow mold. The core rod opens and allows compressed air into the preform, which inflates it to the finished shape. After a cooling period the blow mold opens and the finished article is removed from the core rod and leak-tested prior to packing. In the Stretch Blow Molding (SBM) process, the plastic is first molded into a "preform" using the Injection Molded Process. These preforms are produced with the necks of the bottles, including threads on one end. These preforms are packaged, and fed later into an EBM blow molding machine. In the SBM process, the preforms are heated then blown using high pressure air into bottles using metal blow molds. Usually the preform is stretched with a core rod as part of the process. The stretching of some polymers, such as PET (polyethylene terephthalate) results in strain hardening which allows the bottles to resist deforming under the pressures formed by carbonated beverages, which typically approach 60 psi. The main applications of stretch blow molding are bottles, jars and other containers. The Injection blow molding process produces bottles of superior quality compared to extrusion blow molding. The process is ideal for both narrow and wide-mouthed containers and produces them fully finished with no flash (seam). A sign of injection blow molding is the seam where the two halves of the mold meet. This picture shows what happens inside the blow mold. The preform is first stretched mechanically with a stretch rod. As the rod travels down low- pressure air of 70 to 350 psi is introduced blowing a 'bubble'. Once the stretch rod is fully extended, high-pressure air of up to 580 psi blows the expanded bubble into the shape of the blow mold.
Resin Identification Code
4 The Society of the Plastics Industry, Inc. (SPI) introduced its voluntary resin identification coding system at the urging of recyclers around the country. A growing number of communities were implementing recycling programs in an effort to decrease the volume of waste subject to rising tipping fees at landfills. In some cases, test programs were driven by state-level recycling mandates. The resin code was developed to meet recyclers' needs while providing manufacturers a consistent, uniform system that could apply nationwide. Because municipal recycling programs traditionally have targeted packaging - primarily containers - the resin coding system offered a means of identifying the resin content of bottles and containers commonly found in the residential waste stream. Recycling firms have varying standards for the plastics they accept. Some firms may require that the plastics be sorted by type and separated from other recyclables; some may specify that mixed plastics are acceptable if they are separated from other recyclables; while others may accept all material mixed together. Not all types of plastics are generally recycled, and recycling facilities may not be available in some areas.
Where does all the recycled plastic go? . 56% of recycled PET finds a market in the manufacture of fiber (carpet and clothing).
. Other large markets for recycled PET are for strapping (13%) and new containers (14% -food and non-food).
.29% of recycled HDPE bottles go into making new bottles.
.The plastic pipe industry consumes 18% of the recycled HDPE.
.Other strong markets for HDPE are for lawn and garden products (such as edging), plastic lumber (decks, benches, picnic tables), film and sheet, and a variety of injection molding products (buckets, crates and automobile parts).
Recycling Facts:
.The U.S. plastics recycling industry employs more than 53,000 persons.
5 . More than 20,000 communities, 63 percent of the nation's total, are estimated to have access to a community recycling program (curbside or drop-off) that collects plastics.
. PET bottles (soda, water) and HDPE bottles (milk, laundry detergent) are by far the most commonly collected plastic materials in community recycling programs.
. Post-consumer plastic bottle recycling has increased dramatically over the last ten years, from 234 million pounds in 1989 to over 1.5 billion pounds in 1999.
. 95% of all plastic bottles in the United States market are manufactured from PET or HDPE (48% and 47% respectively).
. HDPE and PET bottles showed the highest recycling rates of any plastic bottles types, at 23.8 and 22.8 percent respectively.
Answer the following questions in your notebook!
Matching: 1. Polymer A. PS 2. Resin Identification codes B. LDPE 3. Thermoplastic C. HDPE 4. Thermosets D. cannot be remelted 5. Polypropylene E. PVC 6. Polystyrene F. a way of identifying plastics 7. Polyvinyl Chloride G. PP 8. High Density Polyethylene H. something made of many units 9. Polyethylene Terephthalate I. can be heated and reformed over and over again 10. Low Density Polyethylene J. not one of the six or a combination of resins 11. Other K. PET
True (T)/False (F): 12. HDPE and PET are the most commonly recycled plastics. 13. Silly Putty is a polymer 14. The basic makeup of many polymers is carbon and hydrogen. 15. Plastics make up 20 percent of our trash by weight. 16. Oxygen makes up the backbone of hydrocarbons. 17. Plastic from 2-liter bottles can be spun into fiber for the production of carpet.
Short Answer: 18. What is a polymer? 19. Why are polymers important? 20. What polymers have you used today? (List at least 5) 21. Even though plastics do not deteriorate completely, why can it be said that they are environmentally friendly?
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