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Plastics

Plastics and Polymers

Plastics have changed the world

First published 28 August 2007

Look around you, and chances are high that a variety of the things you can see are made of plastics. There are hard plastics and soft plastics, clear ones and colorful ones, and plastics that look like leather, wood, or metal. Developed during the twentieth century, plastics have changed the world.

All plastics were soft and moldable during their production - that's why they're called plastics. The Greek word plasticós means "to mold." You can form nearly any object out of plastics from bristles on toothbrushes to bulletproof vests to fibers for making textiles for clothes. Soon, tiny plastic projectiles may be used as carriers of vaccine, making it possible to swallow the vaccine instead of getting an injection!

Plastic objects

PLASTICS = synthetic polymers

monomer - "one part" (mono = one, mer = part)

polymer - "many parts" (poly = many)

organic material - carbon compounds


organic chemistry - the study of compounds that contain carbon

 

What are plastics?

Plastics are synthetic materials, which means that they are artificial, or manufactured. Synthesis means that "something is put together," and synthetic materials are made of building blocks that are put together in factories.

The building blocks for making plastics are small organic molecules - molecules that contain carbon along with other substances. They generally come from oil (petroleum) or natural gas, but they can also come from other organic materials such as wood fibers, corn, or banana peels! Each of these small molecules is known as a monomer ("one part") because it's capable of joining with other monomers to form very long molecule chains called polymers ("many parts") during a chemical reaction called polymerization. To visualize this, think of a single paper clip as a monomer, and all the paper clips in a box chained together as a polymer.

Process

1. Crude oil, the unprocessed oil that comes out of the ground, contains hundreds of different hydrocarbons, as well as small amounts of other materials. The job of an oil refinery is to separate these materials and also to break down (or "crack) large hydrocarbons into smaller ones.

2. A petrochemical plant receives refined oil containing the small monomers they need and creates polymers through chemical reactions.

3. A plastics factory buys the end products of a petrochemical plant - polymers in the form of resins - introduces additives to modify or obtain desirable properties, then molds or otherwise forms the final plastic products.

 

How to make plastic: polymerization in detail

First, find a suitable molecule. One such molecule is the ethylene monomer, the starting point for a variety of plastics. Ethylene is a small hydrocarbon consisting of four hydrogen atoms and two carbon atoms.

Polymerization is often started by combining the monomers through the use of a catalyst - a substance that aids a chemical reaction without undergoing any permanent chemical change itself. During the chemical reaction, hundreds or thousands of monomers combine to form a polymer chain, and millions of polymer chains are formed at the same time. The mass of polymers that results is known as a resin. Resins are sold to plastics factories, usually in the form of powder, tiny granules, or pellets. The plastics manufacturer adds coloring agents and other additives that modify the properties of the material for the intended product. Finally, the resin is formed into the body of a cell phone, fibers for a sweater, or one of a myriad of other plastic products.

When you polymerize ethylene you get a polyethylene resin. There are a number of polyethylene resins families that differ by such properties as density and molecular weight, and they can be made into a huge variety of plastic products. One of the most common is the plastic grocery bag.

Polyethylene is made from just ethylene monomers - but it's also possible to create polymers from two or more different monomers. You can make hundreds of different polymers depending on which monomers and catalysts you use.

Ethylene Polyethylene Polyeten
A monomer from oil - this one is the hydrocarbon ethylene. A polymer - polyethylene - made of ethylene monomers. These pellets, or resins, are chains of polymers if you look at them on a molecular level.

 

Polymers are everywhere

Plastics are polymers, but polymers don't have to be plastics. The way plastics are made is actually a way of imitating nature, which has created a huge number of polymers. Cellulose, the basic component of plant cell walls is a polymer, and so are all the proteins produced in your body and the proteins you eat. Another famous example of a polymer is DNA - the long molecule in the nuclei of your cells that carries all the genetic information about you.

People have been using natural polymers, including silk, wool, cotton, wood, and leather for centuries. These products inspired chemists to try to create synthetic counterparts, which they have done with amazing success.

 

Thermoplastics and thermosets

Plastics are classified into two categories according to what happens to them when they're heated to high temperatures. Thermoplastics keep their plastic properties: They melt when heated, then harden again when cooled. Thermosets, on the other hand, are permanently "set" once they're initially formed and can't be melted. If they're exposed to enough heat, they'll crack or become charred.

80% of the plastics produced are thermoplastics and of these Polyethylene, Polypropylene, Polystyrene and Polyvinylchoride (PVC) are the most commonly used (70%).

 

Thermoplastics

Plastics that can be reshaped

Thermosets

Plastics that can't be reshaped
Analogies Melted ice

When ice is heated, it melts. When a thermoplastic object is heated, it melts as well.

 

Ice

The melted ice can be formed into a new shape, and it will keep that shape when it's cooled. Similarly, a melted thermoplastic object can be formed into a different shape, and it will keep that new shape when it's cooled.

Raw egg

Just as a raw egg has the potential to become a boiled egg, a fried egg, and so on, thermosetting polymers have the potential to become all sorts of different objects.

Boiled egg

Once an egg has been boiled, however, you can't make it into a fried egg. In the same way, once a thermosetting plastic object has been formed, it can't be remade into a different object.

Reasons for the reactions when heated

thermoplastics heated
Thermoplastics have long, linear polymer chains that are only weakly chemically bonded, or connected, to each other. When a thermoplastic object is heated, these bonds are easily broken, which makes the polymers able to glide past each other like strands of freshly cooked spaghetti. That's why thermoplastics can readily be remolded.

The weak bonds between the polymers reform when the plastic object is cooled, which enable it to keep its new shape.

Thermosets heated
The linear chains are crosslinked - strongly chemically bonded. This prevents a thermoplastic object from being melted and reformed.

How plastic objects are formed The most common method for making plastics is molding. To make a thermoplastic object, plastic granules known as resin are forced into a mold under high heat and pressure. When the material has cooled down, the mold is opened and the plastic object is complete. When making plastic fibers, the molten resin is sprayed through a strainer with tiny holes.

Thermosets are produced in two steps:

1. Linear polymers are formed.

2. The linear polymers are forced into a mold where "curing" takes place. This may involve heating, pressure, and the addition of catalysts. During this process, a cross-linked or networked structure forms, creating a permanently hard object that is no longer meltable or moldable.

Uses There is a huge range of uses including plastic wrap, food containers, lighting panels, garden hoses, and the constantly encountered plastic bag. Thermosets are good to use for things that will be warmed up such as spatulas and other kitchen tools. They're also used in glues, varnishes, and in electronic components such as circuit boards.
Recycling Thermoplastics are easy to recycle since they can be melted and reshaped into other products. For example, a plastic bottle that contained a soft drink could be reformed into the fibres of a fleece jacket. Thermosets are hard to recycle, but today there are methods of crushing the objects into a fine powder form for use as fillers in reinforced thermosets.

Better catalysts improve plastics

For most applications, the ideal polymer is a long, straight chain with a highly regular molecular structure. Early synthetic polymers, however, often exhibited odd little branches and other irregularities. In the 1950s, German chemist Karl Ziegler (1898–1973) discovered that an entirely different type of catalyst - a combination of aluminum compounds with other metallic compounds - could solve some of these annoying problems and increase the length of a polymer chain, producing superior plastics. Ziegler became a wealthy man as a result of patents for plastics such as high density polyethylene (HDPE), which is used to manufacture a variety of products such as bottles or pipe.

Polymers often have short side chains, which can occur on either side of the main chain. If side branches occur randomly to the left or right, the polymer has an irregular structure. Italian chemist Giulio Natta (1903–1979) discovered that some Ziegler catalysts led to a uniform structure in which all the side branches are on the same side. This structure results in stiffer and tougher plastics that are also lightweight, which proved to be of significant economic importance, especially for polypropylene. Almost immediately, new and better plastic products were produced.

For their innovative work in the polymerization of plastics, Karl Ziegler and Giulio Natta shared the Nobel Prize in Chemistry in 1963. Today, Ziegler-Natta catalysts are used throughout the world to produce a variety of polymers.

 

Plastics, society, and the environment

Plastics have contributed to our quality of life in countless ways. Many products would be much more expensive - or wouldn't exist - if it weren't for plastics. Yet, as with the use of any material, there are environmental benefits and impacts.

Firstly, there is an environmental impact from plastics production; however the plastics industry has worked hard to reduce energy and water use, as well as waste generation during the manufacturing processes.

Secondly, during their lives, plastic products can save energy and reduce carbon dioxide emissions in a variety of ways. For example, they're lightweight, so transporting them is energy efficient. And plastic parts in cars and airplanes reduce the weight of those vehicles and therefore less energy is needed to operate them and lower emissions are created.

Make our planet greener! Here are a couple of simple things that people can consider to help improve the environment:
• Recycle plastic products when they reach the end of their useful life.
• Carry groceries in reusable bags.

 

Summary

• Plastics are synthetic materials derived from organic (carbon-containing) compounds. The most common sources for carbon compounds are oil (petroleum) and natural gas.

• Plastics consist of polymers - long molecule chains - often mixed with other substances such as coloring agents and softerners.

• The properties of a particular plastic depend on what the polymer chains look like, how they are bonded to each other, and which additives have been introduced.

• There are two groups of plastics:
Thermoplastics, which melt when heated and can be remolded easily.
Thermosets, which can't melt or be remolded. They crack or charr when heated.

• Ziegler-Natta catalysts revolutionized the production of plastics.

 

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