How Plastics Work

Before the invention of plastic, the only substances that could be molded were clays (pottery) and glass. Hardened clay and glass were used for storage, but they were heavy and brittle. Some natural substances, like tree gums and rubber, were sticky and moldable. Rubber wasn't very useful for storage because it eventually lost its ability to bounce back into shape and became sticky when heated.

In 1839, Charles Goodyear accidentally discovered a process in which sulfur reacted with crude rubber when heated and then cooled. The rubber became resilient upon cooling -- it could stretch, but it snapped back to its original shape. It also retained its resilience when heated. We now know that the sulfur forms chemical bonds between adjacent rubber polymer strands. The bonds cross-link the polymer strands, allowing them to "snap back" when stretched. Charles Goodyear had discovered the process now known as vulcanization, which made rubber more durable.

In 1846, Charles Schonbein, a Swiss chemist, accidentally discovered another polymer when he spilled a nitric acid-sulfuric acid mixture on some cotton. A chemical reaction occurred in which the hydroxyl groups of the cellulose fibers in the cotton were converted to nitrate groups catalyzed by the sulfur. The resultant polymer, nitrocellulose, could burst into a smokeless flame and was used by the military in place of gunpowder. In 1870, chemist John Hyatt reacted nitrocellulose with camphor to make celluloid, a plastic polymer that was used in photographic film, billiard balls, dental plates and Ping-Pong balls.

In 1909, a chemist named Leo Baekeland synthesized Bakelite, the first truly synthetic polymer, from a mixture of phenol and formaldehyde. The condensation reaction between these monomers allows the formaldehyde to bind the phenol rings into rigid three-dimensional polymers. So, Bakelite can be molded when hot and solidified into a hard plastic that can be used for handles, phones, auto parts, furniture and even jewelry. Bakelite is hard, resistant to heat and electricity, and can't be easily melted or scorched once cooled. The invention of Bakelite led to a whole class of plastics with similar properties, known as phenolic resins.

In the 1930s, a Dupont chemist named Wallace Carruthers invented a plastic polymer made from the condensation of adipic acid and a certain type of diaminohexane monomers that could be drawn out into strong fibers, like silk. This plastic became known as nylon. Nylon is lightweight, strong and durable and became the basis of many types of clothing, coverings (tents), luggage, bags and ropes.

The use of these early polymers became widespread following World War II and continues today. They lead to the creation of many other plastics, like Dacron, Styrofoam, polystyrene, polyethylene and vinyl.

In the next two sections, we'll learn about the chemistry of plastic.

All plastics are polymers, but not all polymers are plastics. Some familiar nonplastic polymers include starches (polymers of sugars), proteins (polymers of amino acids) and DNA (polymers of nucleotides -- see How DNA Works). The simplified diagram below shows the relationship between monomers and polymers. Identical monomers can combine with each other to form homopolymers, which can be straight or branched chains. Different monomers may combine together to form copolymers, which also may be branched or straight.

The chemical properties of a polymer depend on:

The monomers that are found in many plastics include organic compounds like ethylene, propylene, styrene, phenol, formaldehyde, ethylene glycol, vinyl chloride and acetonitrile (we'll examine many of these as we discuss various plastics). Because there are so many different monomers that can combine in many different ways, we can make many kinds of plastics.

There are a few ways that monomers combine to form the polymers of plastics. One method is a type of chemical reaction called a condensation reaction. In a condensation reaction, two molecules combine with the loss of a smaller molecule, usually water, an alcohol or an acid. To understand condensation reactions, let's look at another hypothetical polymer reaction.

Monomers 1 and 2 both have hydrogen (H) and hydroxyl groups (OH) attached to them. When they come together with an appropriate catalyst (an atom or a molecule that speeds up the chemical reaction without being used up in it), one monomer loses a hydrogen while the other loses a hydroxyl group. The hydrogen and hydroxyl groups combine to form water (H2O), and the remaining electrons form a covalent chemical bond between the monomers. The resulting compound is the basic subunit of copolymers 1 and 2. This reaction occurs over and over again until you get a long chain of copolymers 1 and 2.

Another way that monomers can combine to form polymers is through addition reactions. Addition reactions involve rearranging electrons of the double bonds within a monomer to form single bonds with other molecules. Imagine that two people (each a monomer) stand close together and each person has his/her arms folded (double bond). Then they unfold their arms and hold hands (single bond). The two people now make a polymer, and the process can be repeated.

Various polymer chains can interact and cross-link by forming strong or weak bonds between monomers on different polymer chains. This interaction between polymer chains contributes to the properties of specific plastics (soft/hard, stretchy/rigid, clear/opaque, chemically inert).

Now we'll learn about the different types of plastic.

Plastics can be divided into two major categories:

1. Thermoset or thermosetting plastics. Once cooled and hardened, these plastics retain their shapes and cannot return to their original form. They are hard and durable. Thermosets can be used for auto parts, aircraft parts and tires. Examples include polyurethanes, polyesters, epoxy resins  and phenolic resins.

2. Thermoplastics. Less rigid than thermosets, thermoplastics can soften upon heating and return to their original form. They are easily molded and extruded into films, fibers and packaging. Examples include polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC).

Let's look at some common plastics.

Polyethylene terephthalate (PET or PETE): John Rex Whinfield invented a new polymer in 1941 when he condensed ethylene glycol with terephthalic acid. The condensate was polyethylene terephthalate (PET or PETE). PET is a thermoplastic that can be drawn into fibers (like Dacron) and films (like Mylar). It's the main plastic in ziplock food storage bags.

Polystyrene (Styrofoam): Polystyrene is formed by styrene molecules. The double bond between the CH2 and CH parts of the molecule rearranges to form a bond with adjacent styrene molecules, thereby producing polystyrene. It can form a hard impact-resistant plastic for furniture, cabinets (for computer monitors and TVs), glasses and utensils. When polystyrene is heated and air blown through the mixture, it forms Styrofoam. Styrofoam is lightweight, moldable and an excellent insulator.

Polyvinyl Chloride (PVC): PVC is a thermoplastic that is formed when vinyl chloride (CH2=CH-Cl) polymerizes. When made, it's brittle, so manufacturers add a plasticizer liquid to make it soft and moldable. PVC is commonly used for pipes and plumbing because it's durable, can't be corroded and is cheaper than metal pipes. Over long periods of time, however, the plasticizer may leach out of it, rendering it brittle and breakable.

Polytetrafluoroethylene (Teflon): Teflon was made in 1938 by DuPont. It's created by polymerization of tetrafluoroethylene molecules (CF2=CF2). The polymer is stable, heat-resistant, strong, resistant to many chemicals and has a nearly frictionless surface. Teflon is used in plumbing tape, cookware, tubing, waterproof coatings, films and bearings.

Polyvinylidine Chloride (Saran): Dow makes Saran resins, which are synthesized by polymerization of vinylidine chloride molecules (CH2=CCl2). The polymer can be drawn into films and wraps that are impermeable to food odors. Saran wrap is a popular plastic for packaging foods.

Polyethylene, LDPE and HDPE: The most common polymer in plastics is polyethylene, which is made from ethylene monomers (CH2=CH2). The first polyethylene was made in 1934. Today, we call it low-density polyethylene (LDPE) because it will float in a mixture of alcohol and water. In LDPE, the polymer strands are entangled and loosely organized, so it's soft and flexible. It was first used to insulate electrical wires, but today it's used in films, wraps, bottles, disposable gloves and garbage bags.

In the 1950s, Karl Ziegler polymerized ethylene in the presence of various metals. The resulting polyethylene polymer was composed of mostly linear polymers. This linear form produced tighter, denser, more organized structures and is now called high-density polyethylene (HDPE). HDPE is a harder plastic with a higher melting point than LDPE, and it sinks in an alcohol-water mixture. HDPE was first introduced in the hula hoop, but today it's mostly used in containers.

Polypropylene (PP): In 1953, Karl Ziegler and Giulio Natta, working independently, prepared polypropylene from propylene monomers (CH2=CHCH3) and received the Nobel Prize in Chemistry in 1963. The various forms of polypropylene have different melting points and hardnesses. Polypropylene is used in car trim, battery cases, bottles, tubes, filaments and bags.

Now that we have discussed the various types of plastics, let's look at how plastics are made.

To make plastics, chemists and chemical engineers must do the following on an industrial scale:

First, they must start with various raw materials that make up the monomers. Ethylene and propylene, for example, come from crude oil, which contains the hydrocarbons that make up the monomers. The hydrocarbon raw materials are obtained from the "cracking process" used in refining oil and natural gas (see How Oil Refining Works). Once various hydrocarbons are obtained from cracking, they are chemically processed to make hydrocarbon monomers and other carbon monomers (like styrene, vinyl chloride, acrylonitrile) used in plastics.

Next, the monomers carry out polymerization reactions in large polymerization plants. The reactions produce polymer resins, which are collected and further processed. Processing can include the addition of plasticizers, dyes and flame-retardant chemicals. The final polymer resins are usually in the forms of pellets or beads.

Finally, the polymer resins are processed into final plastic products. Generally, they are heated, molded and allowed to cool. There are several processes involved in this stage, depending upon the type of product.

Extrusion: Pellets are heated and mechanically mixed in a long chamber, forced through a small opening and cooled with air or water. This method is used to make plastic films.

Injection molding: The resin pellets are heated and mechanically mixed in a chamber and then forced under high pressure into a cooled mold. This process is used for containers like butter and yogurt tubs. (Custompart.net has a great lesson on injection molding.)

Blow molding: This technique is used in conjunction with extrusion or injection molding. The resin pellets are heated and compressed into a liquid tube, like toothpaste. The resin goes into the chilled mold, and compressed air gets blown into the resin tube. The air expands the resin against the walls of the mold. This process is used to make plastic bottles.

Rotational molding: The resin pellets are heated and cooled in a mold that can be rotated in three dimensions. The rotation evenly distributes the plastic along the walls of the mold. This technique is used to make large, hollow plastic items (toys, furniture, sporting equipment, septic tanks, garbage cans and kayaks).

On the next page we'll learn about new innovations in plastics and how they're recycled.

As we mentioned earlier, there are other polymers besides plastics. Naturally occurring polymers, such as starches, cellulose, soy protein, vegetable oil, triglycerides and bacterial polyesters, can be extracted from crops and bacteria. Furthermore, plants and microorganisms can produce substances like lactic acid, which can be polymerized into bioplastics (polylactic acid, for example). There are two strategies for producing bioplastics.

Fermentation: Bacteria or other microorganisms mass-produce the biopolymers in bioreactors (fermentation tanks). The biopolymers (lactic acid, polyesters) are extracted from the bioreactors and chemically processed into plastics.

Genetic engineering plants as bioreactors: Biotechnologists introduce bacterial genes into plants. These genes code for the enzymes to make bacterial plastics. The plants are grown and harvested, and the plastics are extracted from the plant material.

In 1997, Cargille Dow made a clear plastic (polylactide) from corn. The polylactide fibers were woven into sports apparel, upholstery fabrics and bioplastic wraps.

Bioplastics have the advantage of being produced from renewable resources (bacteria, plants) rather than nonrenewable resources (oil, natural gas). Furthermore, bioplastics are biodegradable -- they can break down in the environment (see How Landfills Work). Bioplastics is a potentially important industry. With current technology, bioplastics might be more expensive to produce, but biotechnology is rapidly advancing and production may become more economical in the future.

Recycling Plastics

Oil-based plastics don't degrade, but many types (including PP, LDPE, HDPE, PET, and PVC) can be recycled. Each type has a code and identifying number, but some plastics aren't as economically feasible to recycle. So it's important to check with your recycler or municipality about which types of plastics will be accepted.

Once collected, plastics go through the following steps

The discovery of plastics revolutionized our society by introducing a huge variety of lightweight, strong, flexible products with many uses. Although plastics do pose disposal problems, recycling is always a possibility. Furthermore, new research into biopolymers may produce new bioplastic products from renewable resources that are biodegradable and easier on our environment.

To learn more about plastics, check out the links on the next page.