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When you grabbed that plastic grocery bag, I bet you didn’t know it contained the compound polyethylene. Let’s learn more about polyethylene, including its properties and uses.

What Is Polyethylene?

Plastic containers, plastic bottles, plastic bags, and plastic toys; everywhere we look, we see plastic items. Did you know that in 2012 alone, we managed to generate more than 32 million tons of plastic waste? Now, that is what most people would call a lot of plastic! Although plastic is composed of several different organic molecules, one in particular is called polyethylene. Polyethylene is an organic polymer made of several monomer subunits, and it is one popular compound.

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Before we address its popularity in industry, let’s talk more about its structure.

The Structure of Polyethylene

As you can see here, polyethylene contains a lot of carbon and hydrogen atoms and serves as a great illustration of a polymer.

Polymers are gigantic molecules that have many repeating molecules, or subunits, bound together by bonds.

Polyethylene is composed of several monomers called ethylene molecules. This is what one monomer of ethylene would look like.

You may be wondering what the n represents in the first image. In chemistry, n acts as a placeholder for a number.

N gives you some idea about the potential length of the chain. As polyethylene molecules can be very long, this type of information can be very helpful, in that a typical polyethylene molecule can contain more than 500 ethylene subunits!Polyethylene is a thermoplastic, and as such, plays a distinct role in the manufacturing of plastic products. A thermoplastic is any polymer that can be shaped and molded as a liquid and remain in that shape as a solid. Polyethylene performs this task quite well. Let’s learn about some of the physical properties of polyethylene that allow it to perform this task.

Properties and Types of Polyethylene

The two most common types of polyethylene compounds are high density polyethylene (HDPE) and low density polyethylene (LDPE).

Both compounds have very different physical properties. For example, while LDPE compounds have a melting point of 115º C, HDPE compounds melt at 135º C. LDPE is more flexible than HDPE, but when it comes to strength, HDPE is the winner. Although we know what a general polyethylene looks like, both HDPE and LDPE have their own unique structures, as shown here.

However, just remember that both are still polymers, and both contain ethylene subunits.

Also, pay close attention to the fact that LDPE chains are branched, and HDPE chains are linear.

Popularity of Polyethylene

This is the part of the lesson we’ve been patiently waiting for: why is polyethylene so popular in the plastics industry? Some of the reasons include its awesome physical properties and versatility. For example, LDPE compounds are found in the products we use on a daily basis, such as plastic bottles and liners, cling wrap, and sandwich bags. Although HDPE compounds are used to make your freezer bags and the plastic water pipes in your basement, you’ll be less likely to find it in everyday products. If you’re wondering why, think back to what we said in the previous section about the structure of LDPE and HDPE.

More of your plastic products contain LDPE-related compounds, as opposed to HDPE compounds, because they have branch chains. In industry, branch chains equal cheap and fast production. However, the linear chains found in HDPE make for a stronger plastic that can be produced in a more cost effective way.

Lesson Summary

Polyethylene is a type of polymer built from monomer subunits called ethylene molecules. Polyethylene chains can range in length. Because of its ability to shape as a liquid and retain that shape as a solid, it is classified as a thermoplastic.

There are many types of polyethylene chains, but the two common ones include LDPE and HDPE compounds, which have differing physical properties. For example, LDPE chains are branched, and HDPE chains are linear. LDPE is more commonly used by industry to make plastic products because it is easier to work with.

Key Terms

  • Polyethylene: organic polymer made of several monomer subunits, commercial and popular compound
  • Polymers: gigantic bonded together molecules that have many repeating molecules, or subunits
  • Thermoplastic: any polymer that can be shaped and molded as a liquid and keep that shape as a solid
  • High density polyethylene (HDPE): common type of compound, its chains are linear and stronger and have higher melting point
  • Low density polyethylene (LDPE): common polyethylene, chains are branched and more flexible with lower melting point

Learning Outcomes

Completing this lesson should help students do the following:

  • Describe polyethylene
  • Explain how polyethylene is made
  • Identify the commercial uses of polyethylene

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