This is designed to lend a better understanding concerning how plastics are made, the different types of plastic and their numerous properties and applications.
What Is Plastic?
A plastic is a type of synthetic or man-made polymer; similar in many ways to natural resins found in trees and other plants. Webster's Dictionary defines polymers as: any of various complex organic compounds produced by polymerization, capable of being molded, extruded, cast into various shapes and films, or drawn into filaments and then used as textile fibers.
A Little History
The history of manufactured plastics goes back more than 100 years; however, when compared to other materials, plastics are relatively modern. Their usage over the past century has enabled society to make huge technological advances. Although plastics are thought of as a modern invention, there have always been "natural polymers" such as amber, tortoise shells and animal horns. These materials behaved very much like today's manufactured plastics and were often used similar to the way manufactured plastics are currently applied. For example, before the sixteenth century, animal horns, which become transparent and pale yellow when heated, were sometimes used to replace glass.
Alexander Parkes unveiled the first man-made plastic at the 1862 Great International Exhibition in London. This material—which was dubbed Parkesine, now called celluloid—was an organic material derived from cellulose that once heated could be molded but retained its shape when cooled. Parkes claimed that this new material could do anything that rubber was capable of, yet at a lower price. He had discovered a material that could be transparent as well as carved into thousands of different shapes.
In 1907, chemist Leo Hendrik Baekland, while striving to produce a synthetic varnish, stumbled upon the formula for a new synthetic polymer originating from coal tar. He subsequently named the new substance "Bakelite." Bakelite, once formed, could not be melted. Because of its properties as an electrical insulator, Bakelite was used in the production of high-tech objects including cameras and telephones. It was also used in the production of ashtrays and as a substitute for jade, marble and amber. By 1909, Baekland had coined "plastics" as the term to describe this completely new category of materials.
The first patent for polyvinyl chloride (PVC), a substance now used widely in vinyl siding and water pipes, was registered in 1914. Cellophane was also discovered during this period.
Plastics did not really take off until after the First World War, with the use of petroleum, a substance easier to process than coal into raw materials. Plastics served as substitutes for wood, glass and metal during the hardship times of World War’s I & II. After World War II, newer plastics, such as polyurethane, polyester, silicones, polypropylene, and polycarbonate joined polymethyl methacrylate and polystyrene and PVC in widespread applications. Many more would follow and by the 1960s, plastics were within everyone's reach due to their inexpensive cost. Plastics had thus come to be considered 'common'—a symbol of the consumer society.
Since the 1970s, we have witnessed the advent of 'high-tech' plastics used in demanding fields such as health and technology. New types and forms of plastics with new or improved performance characteristics continue to be developed.
From daily tasks to our most unusual needs, plastics have increasingly provided the performance characteristics that fulfill consumer needs at all levels. Plastics are used in such a wide range of applications because they are uniquely capable of offering many different properties that offer consumer benefits unsurpassed by other materials. They are also unique in that their properties may be customized for each individual end use application.
Oil and natural gas are the major raw materials used to manufacture plastics. The plastics production process often begins by treating components of crude oil or natural gas in a "cracking process." This process results in the conversion of these components into hydrocarbon monomers such as ethylene and propylene. Further processing leads to a wider range of monomers such as styrene, vinyl chloride, ethylene glycol, terephthalic acid and many others. These monomers are then chemically bonded into chains called polymers. The different combinations of monomers yield plastics with a wide range of properties and characteristics.
Many common plastics are made from hydrocarbon monomers. These plastics are made by linking many monomers together into long chains to form a polymer backbone. Polyethylene, polypropylene and polystyrene are the most common examples of these. Below is a diagram of polyethylene, the simplest plastic structure.
Even though the basic makeup of many plastics is carbon and hydrogen, other elements can also be involved. Oxygen, chlorine, fluorine and nitrogen are also found in the molecular makeup of many plastics. Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen.
Characteristics of Plastics
Plastics are divided into two distinct groups: thermoplastics and thermosets. The majority of plastics are thermoplastic, meaning that once the plastic is formed it can be heated and reformed repeatedly. Celluloid is a thermoplastic. This property allows for easy processing and facilitates recycling. The other group, the thermosets, can not be remelted. Once these plastics are formed, reheating will cause the material to decompose rather than melt. Bakelite, poly phenol formaldehyde, is a thermoset.
Each plastic has very distinct characteristics, but most plastics have the following general attributes.
Plastics can be very resistant to chemicals. Consider all the cleaning fluids in your home that are packaged in plastic. The warning labels describing what happens when the chemical comes into contact with skin or eyes or is ingested, emphasizes the chemical resistance of these materials. While solvents easily dissolve some plastics, other plastics provide safe, non-breakable packages for aggressive solvents.
Plastics can be both thermal and electrical insulators. A walk through your house will reinforce this concept. Consider all the electrical appliances, cords, outlets and wiring that are made or covered with plastics. Thermal resistance is evident in the kitchen with plastic pot and pan handles, coffee pot handles, the foam core of refrigerators and freezers, insulated cups, coolers and microwave cookware. The thermal underwear that many skiers wear is made of polypropylene and the fiberfill in many winter jackets is acrylic or polyester.
Generally, plastics are very light in weight with varying degrees of strength. Consider the range of applications, from toys to the frame structure of space stations, or from delicate nylon fiber in pantyhose to Kevlar®, which is used in bulletproof vests. Some polymers float in water while others sink. But, compared to the density of stone, concrete, steel, copper, or aluminum, all plastics are lightweight materials.
Plastics can be processed in various ways to produce thin fibers or very intricate parts. Plastics can be molded into bottles or components of cars, such as dashboards and fenders. Some plastics stretch and are very flexible. Other plastics, such as polyethylene, polystyrene (Styrofoam™) and polyurethane, can be foamed. Plastics can be molded into drums or be mixed with solvents to become adhesives or paints. Elastomers and some plastics stretch and are very flexible.
Polymers are materials with a seemingly limitless range of characteristics and colors. Polymers have many inherent properties that can be further enhanced by a wide range of additives to broaden their uses and applications. Polymers can be made to mimic cotton, silk, and wool fibers; porcelain and marble; and aluminum and zinc. Polymers can also make possible products that do not readily come from the natural world, such as clear sheets, foamed insulation board, and flexible films. Plastics may be molded or formed to produce many kinds of products with application in many major markets.
Polymers are usually made of petroleum, but not always. Many polymers are made of repeat units derived from natural gas or coal or crude oil. But building block repeat units can sometimes be made from renewable materials such as polylactic acid from corn or cellulosics from cotton linters. Some plastics have always been made from renewable materials such as cellulose acetate used for screwdriver handles and gift ribbon. When the building blocks can be made more economically from renewable materials than from fossil fuels, either old plastics find new raw materials or new plastics are introduced.
Many plastics are blended with additives as they are processed into finished products. The additives are incorporated into plastics to alter and improve their basic mechanical, physical, or chemical properties. Additives are used to protect plastics from the degrading effects of light, heat, or bacteria; to change such plastic properties, such as melt flow; to provide color; to provide foamed structure; to provide flame retardancy; and to provide special characteristics such as improved surface appearance or reduced tack/friction.
Plasticizers are materials incorporated into certain plastics to increase flexibility and workability. Plasticizers are found in many plastic film wraps and in flexible plastic tubing, both of which are commonly used in food packaging or processing. All plastics used in food contact, including the additives and plasticizers, are regulated by the U.S. Food and Drug Administration (FDA) to ensure that these materials are safe.
There are several different processing methods used to make plastic products. Below are the four main methods in which plastics are processed to form the products that consumers use, such as plastic film, bottles, bags and other containers.
Extrusion—Plastic pellets or granules are first loaded into a hopper, then fed into an extruder, which is a long heated chamber, through which it is moved by the action of a continuously revolving screw. The plastic is melted by a combination of heat from the mechanical work done and by the hot sidewall metal. At the end of the extruder, the molten plastic is forced out through a small opening or die to shape the finished product. As the plastic product extrudes from the die, it is cooled by air or water. Plastic films and bags are made by extrusion processing.
Injection molding—Injection molding, plastic pellets or granules are fed from a hopper into a heating chamber. An extrusion screw pushes the plastic through the heating chamber, where the material is softened into a fluid state. Again, mechanical work and hot sidewalls melt the plastic. At the end of this chamber, the resin is forced at high pressure into a cooled, closed mold. Once the plastic cools to a solid state, the mold opens and the finished part is ejected. This process is used to make products such as butter tubs, yogurt containers, closures and fittings.
Blow molding—Blow molding is a process used in conjunction with extrusion or injection molding. In one form, extrusion blow molding, the die forms a continuous semi-molten tube of thermoplastic material. A chilled mold is clamped around the tube and compressed air is then blown into the tube to conform the tube to the interior of the mold and to solidify the stretched tube. Overall, the goal is to produce a uniform melt, form it into a tube with the desired cross section and blow it into the exact shape of the product. This process is used to manufacture hollow plastic products and its principal advantage is its ability to produce hollow shapes without having to join two or more separately injection molded parts. This method is used to make items such as commercial drums and milk bottles. Another blow molding technique is to injection mold an intermediate shape called a preform and then to heat the preform and blow the heat-softened plastic into the final shape in a chilled mold. This is the process to make carbonated soft drink bottles.
Rotational Molding—Rotational molding consists of a closed mold mounted on a machine capable of rotation on two axes simultaneously. Plastic granules are placed in the mold, which is then heated in an oven to melt the plastic Rotation around both axes distributes the molten plastic into a uniform coating on the inside of the mold until the part is set by cooling. This process is used to make hollow products, for example large toys or kayaks.
Durables vs. Non-Durables
All types of plastic products are classified within the plastic industry as being either a durable or non-durable plastic good. These classifications are used to refer to a product's expected life.
Products with a useful life of three years or more are referred to as durables. They include appliances, furniture, consumer electronics, automobiles, and building and construction materials.
Products with a useful life of less than three years are generally referred to as non-durables. Common applications include packaging, trash bags, cups, eating utensils, sporting and recreational equipment, toys, medical devices and disposable diapers.
7 Major Plastics
Polyethylene Terephthalate (PET or PETE) is clear, tough and has good gas and moisture barrier properties making it ideal for carbonated beverage applications and other food containers. The fact that it has high use temperature allows it to be used in applications such as heatable pre-prepared food trays. Its heat resistance and microwave transparency make it an ideal heatable film. It also finds applications in such diverse end uses as fibers for clothing and carpets, bottles, food containers, strapping, and engineering plastics for precision-molded parts.
High Density Polyethylene (HDPE) is used for many packaging applications because it provides excellent moisture barrier properties and chemical resistance. However, HDPE, like all types of polyethylene, is limited to those food packaging applications that do not require an oxygen or CO2 barrier. In film form, HDPE is used in snack food packages and cereal box liners; in blow-molded bottle form, for milk and non-carbonated beverage bottles; and in injection-molded tub form, for packaging margarine, whipped toppings and deli foods. Because HDPE has good chemical resistance, it is used for packaging many household as well as industrial chemicals such as detergents, bleach and acids. General uses of HDPE include injection-molded beverage cases, bread trays as well as films for grocery sacks and bottles for beverages and household chemicals.
Polyvinyl Chloride (PVC) has excellent transparency, chemical resistance, long term stability, good weatherability and stable electrical properties. Vinyl products can be broadly divided into rigid and flexible materials. Rigid applications are concentrated in construction markets, which includes pipe and fittings, siding, rigid flooring and windows. PVC's success in pipe and fittings can be attributed to its resistance to most chemicals, imperviousness to attack by bacteria or micro-organisms, corrosion resistance and strength. Flexible vinyl is used in wire and cable sheathing, insulation, film and sheet, flexible floor coverings, synthetic leather products, coatings, blood bags, and medical tubing.
Low Density Polyethylene (LDPE) is predominantly used in film applications due to its toughness, flexibility and transparency. LDPE has a low melting point making it popular for use in applications where heat sealing is necessary. Typically, LDPE is used to manufacture flexible films such as those used for dry cleaned garment bags and produce bags. LDPE is also used to manufacture some flexible lids and bottles, and it is widely used in wire and cable applications for its stable electrical properties and processing characteristics.
Polypropylene (PP) has excellent chemical resistance and is commonly used in packaging. It has a high melting point, making it ideal for hot fill liquids. Polypropylene is found in everything from flexible and rigid packaging to fibers for fabrics and carpets and large molded parts for automotive and consumer products. Like other plastics, polypropylene has excellent resistance to water and to salt and acid solutions that are destructive to metals. Typical applications include ketchup bottles, yogurt containers, medicine bottles, pancake syrup bottles and automobile battery casings.
Polystyrene (PS) is a versatile plastic that can be rigid or foamed. General purpose polystyrene is clear, hard and brittle. Its clarity allows it to be used when transparency is important, as in medical and food packaging, in laboratory ware, and in certain electronic uses. Expandable Polystyrene (EPS) is commonly extruded into sheet for thermoforming into trays for meats, fish and cheeses and into containers such as egg crates. EPS is also directly formed into cups and tubs for dry foods such as dehydrated soups. Both foamed sheet and molded tubs are used extensively in take-out restaurants for their lightweight, stiffness and excellent thermal insulation.
There are many other plastics beyond the most common ones described above, for example nylon, ABS copolymers, polyurethanes, and polymethyl methacrylate.
Whether you are aware of it or not, plastics play an important part in your life. Plastics' versatility allow them to be used in everything from car parts to doll parts, from soft drink bottles to the refrigerators they are stored in. From the car you drive to work in to the television you watch at home, plastics help make your life easier and better. So how is it that plastics have become so widely used? How did plastics become the material of choice for so many varied applications?
The simple answer is that plastics can provide the things consumers want and need at economical costs. Plastics have the unique capability to be manufactured to meet very specific functional needs for consumers. So maybe there's another question that's relevant: What do I want? Regardless of how you answer this question, plastics can probably satisfy your needs.
If a product is made of plastic, there's a reason. And chances are the reason has everything to do with helping you, the consumer, get what you want: Health. Safety. Performance. and Value.
Plastics Make It Possible®.
Just consider the changes we've seen in the grocery store in recent years: plastic wrap helps keep meat fresh while protecting it from the poking and prodding fingers of your fellow shoppers; plastic bottles mean you can actually lift an economy-size bottle of juice and should you accidentally drop that bottle, it is shatter-resistant. In each case, plastics help make your life easier, healthier and safer.
Grocery Cart vs. Dent-Resistant Body Panel
Plastics also help you get maximum value from some of the big-ticket items you buy. Plastics help make portable phones and computers that really are portable. They help major appliances—like refrigerators or dishwashers—resist corrosion, last longer and operate more efficiently. Plastic car fenders and body panels resist dings, so you can cruise the grocery store parking lot with confidence.
Modern packaging—such as heat-sealed plastic pouches and wraps—helps keep food fresh and free of contamination. That means the resources that went into producing that food aren't wasted. It's the same thing once you get the food home: plastic wraps and resealable containers keep your leftovers protected—much to the chagrin of kids everywhere. In fact, packaging experts have estimated that each pound of plastic packaging can reduce food waste by up to 1.7 pounds.
Plastics can also help you bring home more product with less packaging. For example, just 2 pounds of plastic can deliver 1,300 ounces—roughly 10 gallons—of a beverage such as juice, soda or water. You'd need 3 pounds of aluminum to bring home the same amount of product, 8 pounds of steel or over 40 pounds of glass. Not only do plastic bags require less total energy to produce than paper bags, they conserve fuel in shipping. It takes seven trucks to carry the same number of paper bags as fits in one truckload of plastic bags. Plastics make packaging more efficient, which ultimately conserves resources.
Plastics engineers are always working to do even more with less material. Since 1977, the 2-liter plastic soft drink bottle has gone from weighing 68 grams to just 47 grams today, representing a 31 percent reduction per bottle. That saved more than 180 million pounds of packaging in 2006 for just 2-liter soft drink bottles. The 1-gallon plastic milk jug has undergone a similar reduction, weighing 30 percent less than what it did 20 years ago.
Doing more with less helps conserve resources in another way. It helps save energy. In fact, plastics can play a significant role in energy conservation. Just look at the decision you're asked to make at the grocery store checkout: "Paper or plastic?" Plastic bag manufacture generates less greenhouse gas and uses less fresh water than does paper bag manufacture. Not only do plastic bags require less total production energy to produce than paper bags, they conserve fuel in shipping. It takes seven trucks to carry the same number of paper bags as fits in one truckload of plastic bags.
Plastics in Home Construction
Plastics also help to conserve energy in your home. Vinyl siding and windows help cut energy consumption and lower heating and cooling bills. Furthermore, the U.S. Department of Energy estimates that use of plastic foam insulation in homes and buildings each year could save over 60 million barrels of oil over other kinds of insulation.
The same principles apply in appliances such as refrigerators and air conditioners. Plastic parts and insulation have helped to improve their energy efficiency by 30 to 50 percent since the early 1970s. Again, this energy savings helps reduce your heating and cooling bills. And appliances run more quietly than earlier designs that used other materials.
Recycling of post-consumer plastics packaging began in the early 1980s as a result of state level bottle deposit programs, which produced a consistent supply of returned PETE bottles. With the addition of HDPE milk jug recycling in the late 1980s, plastics recycling has grown steadily but relative to competing packaging materials.
Roughly 60 percent of the U.S. population—about 148 million people—have access to a plastics recycling program. The two common forms of collection are: curbside collection—where consumers place designated plastics in a special bin to be picked up by a public or private hauling company (approximately 8,550 communities participate in curbside recycling) and drop-off centers—where consumers take their recyclables to a centrally located facility (12,000). Most curbside programs collect more than one type of plastic resin; usually both PETE and HDPE. Once collected, the plastics are delivered to a material recovery facility (MRF) or handler for sorting into single resin streams to increase product value. The sorted plastics are then baled to reduce shipping costs to reclaimers.
Reclamation is the next step where the plastics are chopped into flakes, washed to remove contaminants and sold to end users to manufacture new products such as bottles, containers, clothing, carpet, plastic lumber, etc. The number of companies handling and reclaiming post-consumer plastics today is over five times greater than in 1986, growing from 310 companies to 1,677 in 1999. The number of end uses for recycled plastics continues to grow. The federal and state government as well as many major corporations now support market growth through purchasing preference policies.
Early in the 1990s, concern over the perceived reduction of landfill capacity spurred efforts by legislators to mandate the use of recycled materials. Mandates, as a means of expanding markets, can be troubling. Mandates may fail to take health, safety and performance attributes into account. Mandates distort the economic decisions and can lead to sub optimal financial results. Moreover, they are unable to acknowledge the life cycle benefits of alternatives to the environment, such as the efficient use of energy and natural resources.
Pyrolysis involves heating plastics in the absence or near absence of oxygen to break down the long polymer chains into small molecules. Under mild conditions polyolefins can yield a petroleum-like oil. Special conditions can yield monomers such as ethylene and propylene. Some gasification processes yield syngas (mixtures of hydrogen and carbon monoxide are called synthesis gas, or syngas). In contrast to pyrolysis, combustion is an oxidative process that generates heat, carbon dioxide, and water.
Chemical recycling is a special case where condensation polymers such as PET or nylon are chemically reacted to form starting materials.
Source reduction is gaining more attention as an important resource conservation and solid waste management option. Source reduction, often called "waste prevention" is defined as "activities to reduce the amount of material in products and packaging before that material enters the municipal solid waste management system."
Source reduction activities reduce the consumption of resources at the point of generation. In general, source reduction activities include:
- Redesigning products or packages so as to reduce the quantity of the materials used, by substituting lighter materials for heavier ones or lengthening the life of products to postpone disposal.
- Using packaging that reduces the amount of damage or spoilage to the product.
- Reducing amounts of products or packages used through modification of current practices by processors and consumers.
- Reusing products or packages already manufactured.
- Managing non-product organic wastes (food wastes, yard trimmings) through backyard composting or other on-site alternatives to disposal.