Science of Plastics

Definition

Plastics are a group of materials, either synthetic or naturally occurring, that may be shaped when soft and then hardened to retain the given shape. Plastics are polymers. A polymer is a substance made of many repeating units. The word polymer comes from two Greek words: poly, meaning many, and meros, meaning parts or units. A polymer can be thought of as a chain in which each link is the “mer,” or monomer (single unit). The chain is made by joining, or polymerizing, at least 1,000 links together. Polymerization can be demonstrated by making a chain using paper clips or by linking many strips of paper together to form a paper garland.

Examples

Naturally occurring polymers include tar, shellac, tortoiseshell, animal horn, cellulose, amber, and latex from tree sap. Synthetic polymers include polyethylene (used in plastic bags); polystyrene (used to make Styrofoam cups); polypropylene (used for fibers and bottles); polyvinyl chloride (used for food wrap, bottles, and drain pipe); and polytetrafluoroethylene, or Teflon (used for nonstick surfaces). Although many polymers are hydrocarbons that contain only carbon and hydrogen, other polymers may also contain oxygen, chlorine, fluorine, nitrogen, silicon, phosphorus, and sulfur.

Natural polymers, such as cellulose and latex, were first chemically modified in the 19th century to form celluloid and vulcanized rubber. The first totally synthetic polymer, Bakelite, was produced in 1907. The first semisynthetic fiber, rayon, was developed from cellulose in 1911. However, it was not until the global disruption caused by World War II, when natural sources of latex, wool, silk, and other materials became difficult to obtain, that synthetics were mass produced. Synthetic rubber was needed for tires, and nylon was needed as a replacement for silk for parachutes. Today synthetic polymers in the form of plastics are in wide use, and the plastics industry is one of the fastest growing in the United States and around the world. The industry produces approximately 150 kilograms of polymers per person annually in the United States.

Structure

Monomers can be chemically joined together in two ways: addition polymerization or condensation polymerization. Addition polymerization has three basic steps: initiation, propagation, and termination. In this type of polymerization the monomers join by adding on to the end of the last “mer” in the chain, just like making a chain of paper clips. Polyethylene, polystyrene, and acrylic are examples of plastics formed by addition polymerization. These polymers are often thermoplastic in nature: they can be heated and made soft and then hardened when cooled. They are easily processed, reprocessed, or recycled. See the attached tables, Some Addition Polymers and Some Condensation Polymers, for examples of each type.

During condensation polymerization a small molecule is eliminated as the monomers join together. Nylons, some polyesters, and urethanes are examples of condensation polymers. These polymers can be thermoplastic or thermosetting. Although all plastics are in a liquid state at some point in processing and are solid in the finished state, once a thermoset polymer is formed, it cannot be melted and reformed.

The monomers in a polymer may be arranged in a variety of ways. For example, the monomers may have a linear arrangement like a long chain of paper clips, although the tetrahedral carbon bonds actually give the chain a zigzag configuration. Polyethylene is the simplest example of a linear polymer.

Polyethylene Zigzag Structure

conf_in_chem-polyethzigzag.gif

If the monomers not only form straight chains but also form long side chains off the main backbone, the resulting polymer is described as branched and may look like a tree branch or the stems of a bunch of grapes. Another arrangement occurs when the long chains are chemically linked together, forming a mesh-like structure known as a crosslinked configuration. Vulcanized rubber, which is formed by reacting natural rubber (isoprene) with sulfur, is an example of a crosslinked polymer.

The polyvinyl alcohol is cross-linked using borax, Na2B4O7x10H2O (sodium tetraborate).

conf_in_chem-crosslink.jpg

The polymer molecules can also have different arrangements. If the arrangement has no particular order or form, like the arrangement of spaghetti on a plate, the polymer is said to be amorphous (having no shape). Amorphous polymers are often transparent and, therefore, are used as food wrap, headlights, and contact lenses. These materials also tend to have lower melting points. If the arrangement is in a distinct pattern, the polymer is said to be crystalline. The higher the degree of crystallinity, the less light passes through. Such materials are either translucent or opaque. This quality depends on the degree of crystallization and the presence of additives. Crystalline polymers have greater strength and tend to have higher melting points.

Characteristics of Polymers

Polymers seem to have a limitless range of characteristics along with properties that allow them to be dyed in an endless array of colors. Their properties can be enhanced by additives. Being able to design or engineer polymers for specific applications makes plastics unique materials. Although each polymer has unique characteristics, most polymers have some general properties:

  1. They are resistant to chemicals.
  2. They are insulators of heat and electricity.
  3. They are light in mass and have varying degrees of strength.
  4. They can be processed in various ways to produce fibers, sheets, foams, or intricate molded parts.

The raw material for manufacturing plastic products is called a resin. Some of the most common resins are polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene (PS). These resins are often used in packaging. The Recycling Code chart in the linked PDF shows the recycling code for these resins.

Some Additional Polymers

Polymer name Monomer(s) Polymer Use
 

Polyethylene

 

 

CH2=CH2

(ethene)

 

-CH-CH2

 

Most common polymer. Used in bags, wire insulation, and squeeze bottles

 

Polypropylene CH2=CH

½

CH3

(1-propene)

 

-CH2-CH-

½

CH3

Fibers, indoor-outdoor carpets, bottles

polystyrene1.png

polystyrene2.png

Polystyrene

CH2=CH

½

 

 

 

(styrene)

 

-CH2-CH-

½

 

 

 

 

Styrofoam, molded objects such as tableware (forks, knives, and spoons), trays, videocassette cases
Polyvinyl chloride

(PVC)

CH2=CH

½

Cl

(vinyl chloride)

 

-CH2-CH-

½

Cl

Clear food wrap, bottles, floor covering, synthetic leather, water and drain pipe
Polytetrafluoroethylene

(Teflon)

CF2=CF2

(tetraflouroethene)

 

-CF2-CF2 Nonstick surfaces, plumbing tape, chemical-resistant containers and films

 

Polymethyl methacrylate

(Lucite, Plexiglas)

 CO2CH3

½

CH2=C

½

CH3

(methyl methacrylate)

 

 CO2CH3

½

-CH2-C-

½

CH3

 

Glass replacement, paints, and household products
Polyacrylonitrile

(Acrilan, Orlon, Creslan)

CH2=CH

½

CN

(acrylonitrile)

 

-CH2-CH-

½

CN

Fibers used in knit shirts, sweaters, blankets, and carpets

 

 

Polyvinyl acetate

(PVA)

CH2=CH

½

OOCCH3

(vinyl acetate)

 

-CH2-CH-

½

OOCCH3

 

 

Adhesives (Elmer’s glue), paints, textile coatings, and chewing gum
Natural rubber  CH3

½

CH2=C-CH=CH2

(2-methyl-1,3-butadiene)

 

 CH3

½

-CH2-C=CH-CH2

Rubber bands, gloves, tires, conveyor belts, and household materials
Polychloroprene

(neoprene rubber)

 Cl

½

CH2=C-CH=CH2

(2-methyl-1,3-butadiene)

 

 Cl

½

-CH2-C=CH-CH2

 

 

Oil- and gasoline-resistant rubber

Styrene butadiene rubber1.png

Styrene butadiene rubber1.png

Styrene butadiene rubber

(SBR)

CH2=CH

½

 

 

 

 

CH2=CH-CH=CH2

 

-CH2-CH-CH2-CH-CH-CH2

½

 

Non-bounce rubber used in tires

Some Condensation Polymers

Polymer name Monomers Polymer Use
 

Polyamides

(nylon)

 

case-plastics-monomer1.png

 

conflicts-plastics-polymer1.png

 

Fibers, molded objects
Polyesters

(Dacron, Mylar, Fortrel)

monomer-polyester-dacron.png

 

polymer-polyester-dacron.png

Linear polyesters, fibers, recording tape
Polyesters

(Glyptal resin)

monomer-polyester-glyptal.png

 

polymer-polyester-glyptal.png

Cross-linked polyester, paints
Polyesters

(Casting resin)

monomer-polyester-resin.png

 

polymer-polyester-resin.png

Cross-linked with styrene and benzoyl peroxide, fiberglass boat resin, casting resin
Phenol-formaldehyde

(Bakelite)

 

monomer-bakelite.png

 

 

polymer-bakelite.png

Mixed with fillers, molded electrical cases, adhesives, laminates, varnishes
Cellulose acetate

(cellulose is a polymer of

glucose)

monomer-cellulose.png

 

polymer-cellulose.png

 

Photographic film
Silicones

 

monomer-silicone.png

polymer-silicone.png

Water-repellent coatings, temperature-resistant fluids and rubber
Polyurethanes

 

monomer-polyurethane.png

 

polymer-silicone.png

 

Foams, rigid and flexible, fibers

Recycling Codes for Plastic Resins

Recycling code Polymer and structure Uses

recycle1.png

recycle-polymer-1.png

Poly(ethylene terephthlate) (PET)

Bottles for soft drinks and other beverages

recycle2.png

recycle-polymer-2.png

High-density polyethylene

Containers for milk and other beverages, squeeze bottles

recycle3.png

recycle-polymer-3.png

Vinyl/polyvinyl chloride

Bottles for cleaning materials, some shampoo bottles

recycle4.png

recycle-polymer-4.png

Low-density polyethylene

May have some branches

Plastic bags, some plastic wraps

recycle5.png

recycle-polymer-5.png

Polypropylene

Heavy-duty microwavable containers

recycle6.png

recycle-polymer-6.png

Polystyrene

Beverage/foam cups, toys, window in envelopes

recycle7.png

All other resins, layered multimaterials, some containers Some ketchup bottles, snack packs, mixture where top differs from bottom

 

 

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The History and Future of Plastics (7)-The Future of Plastics

Despite growing mistrust, plastics are critical to modern life. Plastics made possible the development of computers, cell phones, and most of the lifesaving advances of modern medicine. Lightweight and good for insulation, plastics help save fossil fuels used in heating and in transportation. Perhaps most important, inexpensive plastics raised the standard of living and made material abundance more readily available. Without plastics many possessions that we take for granted might be out of reach for all but the richest Americans. Replacing natural materials with plastic has made many of our possessions cheaper, lighter, safer, and stronger.

Since it’s clear that plastics have a valuable place in our lives, some scientists are attempting to make plastics safer and more sustainable. Some innovators are developing bioplastics, which are made from plant crops instead of fossil fuels, to create substances that are more environmentally friendly than conventional plastics. Others are working to make plastics that are truly biodegradable. Some innovators are searching for ways to make recycling more efficient, and they even hope to perfect a process that converts plastics back into the fossil fuels from which they were derived. All of these innovators recognize that plastics are not perfect but that they are an important and necessary part of our future.

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The History and Future of Plastics (6)-Plastic Problems: Waste and Health

Plastic’s reputation fell further in the 1970s and 1980s as anxiety about waste increased. Plastic became a special target because, while so many plastic products are disposable, plastic lasts forever in the environment. It was the plastics industry that offered recycling as a solution. In the 1980s the plastics industry led an influential drive encouraging municipalities to collect and process recyclable materials as part of their waste-management systems. However, recycling is far from perfect, and most plastics still end up in landfills or in the environment. Grocery-store plastic bags have become a target for activists looking to ban one-use, disposable plastics, and several American cities have already passed bag bans. The ultimate symbol of the problem of plastic waste is the Great Pacific Garbage Patch, which has often been described as a swirl of plastic garbage the size of Texas floating in the Pacific Ocean.

The reputation of plastics has suffered further thanks to a growing concern about the potential threat they pose to human health. These concerns focus on the additives (such as the much-discussed bisphenol A [BPA] and a class of chemicals called phthalates) that go into plastics during the manufacturing process, making them more flexible, durable, and transparent. Some scientists and members of the public are concerned about evidence that these chemicals leach out of plastics and into our food, water, and bodies. In very high doses these chemicals can disrupt the endocrine (or hormonal) system. Researchers worry particularly about the effects of these chemicals on children and what continued accumulation means for future generations.

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The History and Future of Plastics (5)-Growing Concerns about Plastics

The unblemished optimism about plastics didn’t last. In the postwar years there was a shift in American perceptions as plastics were no longer seen as unambiguously positive. Plastic debris in the oceans was first observed in the 1960s, a decade in which Americans became increasingly aware of environmental problems. Rachel Carson’s 1962 book, Silent Spring, exposed the dangers of chemical pesticides. In 1969 a major oil spill occurred off the California coast and the polluted Cuyahoga River in Ohio caught fire, raising concerns about pollution. As awareness about environmental issues spread, the persistence of plastic waste began to trouble observers.

Plastic also gradually became a word used to describe that which was cheap, flimsy, or fake. In The Graduate, one of the top movies of 1968, Dustin Hoffman’s character was urged by an older acquaintance to make a career in plastics. Audiences cringed along with Hoffman at what they saw as misplaced enthusiasm for an industry that, rather than being full of possibilities, was a symbol of cheap conformity and superficiality.

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The History and Future of Plastics (4)-Plastics Come of Age

World War II necessitated a great expansion of the plastics industry in the United States, as industrial might proved as important to victory as military success. The need to preserve scarce natural resources made the production of synthetic alternatives a priority. Plastics provided those substitutes. Nylon, invented by Wallace Carothers in 1935 as a synthetic silk, was used during the war for parachutes, ropes, body armor, helmet liners, and more. Plexiglas provided an alternative to glass for aircraft windows. A Time magazine article noted that because of the war, “plastics have been turned to new uses and the adaptability of plastics demonstrated all over again.”[1] During World War II plastic production in the United States increased by 300%.

The surge in plastic production continued after the war ended. After experiencing the Great Depression and then World War II, Americans were ready to spend again, and much of what they bought was made of plastic. According to author Susan Freinkel, “In product after product, market after market, plastics challenged traditional materials and won, taking the place of steel in cars, paper and glass in packaging, and wood in furniture.”[2] The possibilities of plastics gave some observers an almost utopian vision of a future with abundant material wealth thanks to an inexpensive, safe, sanitary substance that could be shaped by humans to their every whim.

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The History and Future of Plastics (3)-The Development of New Plastics

In 1907 Leo Baekeland invented Bakelite, the first fully synthetic plastic, meaning it contained no molecules found in nature. Baekeland had been searching for a synthetic substitute for shellac, a natural electrical insulator, to meet the needs of the rapidly electrifying United States. Bakelite was not only a good insulator; it was also durable, heat resistant, and, unlike celluloid, ideally suited for mechanical mass production. Marketed as “the material of a thousand uses,” Bakelite could be shaped or molded into almost anything, providing endless possibilities.

Hyatt’s and Baekeland’s successes led major chemical companies to invest in the research and development of new polymers, and new plastics soon joined celluloid and Bakelite. While Hyatt and Baekeland had been searching for materials with specific properties, the new research programs sought new plastics for their own sake and worried about finding uses for them later.

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The History and Future of Plastics (2)-The First Synthetic Plastic

The first synthetic polymer was invented in 1869 by John Wesley Hyatt, who was inspired by a New York firm’s offer of $10,000 for anyone who could provide a substitute for ivory. The growing popularity of billiards had put a strain on the supply of natural ivory, obtained through the slaughter of wild elephants. By treating cellulose, derived from cotton fiber, with camphor, Hyatt discovered a plastic that could be crafted into a variety of shapes and made to imitate natural substances like tortoiseshell, horn, linen, and ivory.

This discovery was revolutionary. For the first time human manufacturing was not constrained by the limits of nature. Nature only supplied so much wood, metal, stone, bone, tusk, and horn. But now humans could create new materials. This development helped not only people but also the environment. Advertisements praised celluloid as the savior of the elephant and the tortoise. Plastics could protect the natural world from the destructive forces of human need.

The creation of new materials also helped free people from the social and economic constraints imposed by the scarcity of natural resources. Inexpensive celluloid made material wealth more widespread and obtainable. And the plastics revolution was only getting started.

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The History and Future of Plastics (1)-What Are Plastics, and Where Do They Come From?

Plastic is a word that originally meant “pliable and easily shaped.” It only recently became a name for a category of materials called polymers. The word polymer means “of many parts,” and polymers are made of long chains of molecules. Polymers abound in nature. Cellulose, the material that makes up the cell walls of plants, is a very common natural polymer.

Over the last century and a half humans have learned how to make synthetic polymers, sometimes using natural substances like cellulose, but more often using the plentiful carbon atoms provided by petroleum and other fossil fuels. Synthetic polymers are made up of long chains of atoms, arranged in repeating units, often much longer than those found in nature. It is the length of these chains, and the patterns in which they are arrayed, that make polymers strong, lightweight, and flexible. In other words, it’s what makes them so plastic.

These properties make synthetic polymers exceptionally useful, and since we learned how to create and manipulate them, polymers have become an essential part of our lives. Especially over the last 50 years plastics have saturated our world and changed the way that we live.

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How Plastics Are Made(5)-Thermoplastic and Thermoset Processing Methods

There are a variety of different processing methods used to convert polymers into finished products. Some include:

Extrusion – This continuous process is used to produce films, sheet, profiles, tubes, and pipes. Plastic material as granules, pellets, or powder, is first loaded into a hopper and then fed into a long heated chamber through which it is moved by the action of a continuously revolving screw. The chamber is a cylinder and is referred to as an extruder. Extruders can have one or two revolving screws. The plastic is melted by the mechanical work of the screw and the heat from the extruder wall. At the end of the heated chamber, the molten plastic is forced out through a small opening called a die to form the shape of the finished product. As the plastic is extruded from the die, it is fed onto a conveyor belt for cooling or onto rollers for cooling or by immersion in water for cooling. The operation’s principle is the same as that of a meat mincer but with added heaters in the wall of the extruder and cooling of the product. Examples of extruded products include lawn edging, pipe, film, coated paper, insulation on electrical wires, gutter and down spouting, plastic lumber, and window trim. Thermoplastics are processed by continuous extrusion. Thermoset elastomer can be extruded into weatherstripping by adding catalysts to the rubber material as it is fed into the extruder.

Calendering – This continuous process is an extension of film extrusion. The still warm extrudate is chilled on polished, cold rolls to create sheet from 0.005 inches thick to 0.500 inches thick. The thickness is well maintained and surface made smooth by the polished rollers.  Calendering is used for high output and the ability to deal with low melt strength. Heavy polyethylene films used for construction vapor and liquid barriers are calendered. High volume PVC films are typically made using calendars.

Film Blowing – This process continuously extrudes vertically a ring of semi-molten polymer in an upward direction, like a fountain. A bubble of air is maintained that stretches the plastic axially and radially into a tube many times the diameter of the ring. The diameter of the tube depends on the plastic being processed and the processing conditions. The tube is cooled by air and is nipped and wound continuously as a flattened tube. The tube can be processed to form saleable bags or slit to form rolls of film with thicknesses of 0.0003 to 0.005 inches thick.  Multiple layers of different resins can be used to make the tube.

Injection Molding – This process can produce intricate three-dimensional parts of high quality and great reproducibility. It is predominately used for thermoplastics but some thermosets and elastomers are also processed by injection molding. In injection molding plastic material is fed into a hopper, which feeds into an extruder. An extruder screw pushes the plastic through the heating chamber in which the material is then melted. At the end of the extruder the molten plastic is forced at high pressure into a closed cold mold. The high pressure is needed to be sure the mold is completely filled. Once the plastic cools to a solid, the mold opens and the finished product is ejected. This process is used to make such items as butter tubs, yogurt containers, bottle caps, toys, fittings, and lawn chairs.  Special catalysts can be added to create the thermoset plastic products during the processing, such as cured silicone rubber parts. Injection molding is a discontinuous process as the parts are formed in molds and must be cooled or cured before being removed. The economics are determined by how many parts can be made per cycle and how short the cycles can be.

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.

Expanded Bead Blowing – This process begins with a measured volume of beads of plastic being placed into a mold. The beads contain a blowing agent or gas, usually pentane, dissolved in the plastic. The closed mold is heated to soften the plastic and the gas expands or blowing agent generates gas. The result is fused closed cell structure of foamed plastic that conforms to a shape, such as expanded polystyrene cups.  Styrofoam™ expanded polystyrene thermal insulation board is made in a continuous extrusion process using expanded bead blowing.

Rotational Molding – Rotational molding consists of a mold mounted on a machine capable of rotating on two axes simultaneously. Solid or liquid resin is placed within the mold and heat is applied. Rotation distributes the plastic into a uniform coating on the inside of the mold then the mold is cooled until the plastic part cools and hardens. This process is used to make hollow configurations. Common rotationally molded products include shipping drums, storage tanks and some consumer furniture and toys.

Compression Molding – This process has a prepared volume of plastic placed into a mold cavity and then a second mold or plug is applied to squeeze the plastic into the desired shape. The plastic can be a semi-cured thermoset, such as an automobile tire, or a thermoplastic or a mat of thermoset resin and long glass fibers, such as for a boat hull. Compression molding can be automated or require considerable hand labor. Transfer molding is a refinement of compression molding. Transfer molding is used to encapsulate parts, such as for semi-conductor manufacturing

The formation of plywood or oriented strand board using thermoset adhesives is a variant of compression molding. The wood veneer or strands are coated with catalyzed thermoset phenol formaldehyde resin and compressed and heated to cause the thermoset plastic to form into a rigid, non-melting adhesive.

Casting – This process is the low pressure, often just pouring, addition of liquid resins to a mold. Catalyzed thermoset plastics can be formed into intricate shapes by casting. Molten polymethyl methacrylate thermoplastic can be cast into slabs to form windows for commercial aquariums. Casting can make thick sheet, 0.500 inches to many inches thick.

Thermoforming – Films of thermoplastic are heated to soften the film, and then the soft film is pulled by vacuum or pushed by pressure to conform to a mold or pressed with a plug into a mold. Parts are thermoformed either from cut pieces for thick sheet, over 0.100 inches, or from rolls of thin sheet. The finished parts are cut from the sheet and the scrap sheet material recycled for manufacture of new sheet. The process can be automated for high volume production of clamshell food containers or can be a simple hand labor process to make individual craft items.

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How Plastics Are Made(4)-The Two Plastic Types, Based on Processing

A Thermoset is a polymer that solidifies or “sets” irreversibly when heated or cured. Similar to the relationship between a raw and a cooked egg, a cooked egg cannot revert back to its original form once heated, and a thermoset polymer can’t be softened once “set”. Thermosets are valued for their durability and strength and are used extensively in automobiles and construction including applications such as adhesives, inks, and coatings. The most common thermoset is the rubber truck and automobile tire.  Some examples of thermoset plastics and their product applications are:

Polyurethanes:
•  Mattresses
•  Cushions
•  Insulation

Unsaturated Polyesters:
•  Boat hulls
•  Bath tubs and shower stalls
•  Furniture

Epoxies:
•  Adhesive glues
•  Coating for electrical devices
•  Helicopter and jet engine blades

Phenol Formaldehyde:
• Oriented strand board
• Plywood
• Electrical appliances
• Electrical circuit boards and switches

A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding, or pressing. Ice cubes are common household items which exemplify the thermoplastic principle. Ice will melt when heated but readily solidifies when cooled. Like a polymer, this process may be repeated numerous times. Thermoplastics offer versatility and a wide range of applications. They are commonly used in food packaging because they can be rapidly and economically formed into any shape needed to fulfill the packaging function. Examples include milk jugs and carbonated soft drink bottles. Other examples of thermoplastics are:

Polyethylene:
•  Packaging
•  Electrical insulation
•  Milk and water bottles
•  Packaging film
•  House wrap
•  Agricultural film

Polypropylene:
•  Carpet fibers
•  Automotive bumpers
•  Microwave containers
•  External prostheses

Polyvinyl Chloride (PVC):
•  Sheathing for electrical cables
•  Floor and wall coverings
•  Siding
•  Automobile instrument panels

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