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plastic food containers, trays and food packaging made from polystyrene

plastic food containers, trays and food packaging made from polystyrene (smartboy10, iStockPhoto)

STEM in Context

Polystyrene: The Pros, the Cons, the Chemistry

Seth Gilchrist & Let's Talk Science

Summary

Learn the organic chemistry behind this very useful plastic. Why is recycling polystyrene hard? Why does polystyrene often end up as solid waste?

Here's a riddle for you: What material can be found in the grocery store, under the road and in the hospital? Give up? The answer is polystyrene! Polystyrene is everywhere. In its expanded foam state, polystyrene can be found in coolers, materials used in road construction, and to package everything from computers to food. The most common and recognizable form of polystyrene is called Styrofoam. In its compact form, it is used as a material to make things like bottles, disposable cutlery, toys, car parts, and many more objects that we use every day.

Empty expanded foam polystyrene food containers
Empty expanded foam polystyrene food containers (Source: User2547783c_812 via iStockphoto).

Misconception Alert

Many people refer to the expanded foam state of polystyrene as Styrofoam. However, Styrofoam is a brand name. 

So, what is it that makes polystyrene so useful for so many different things? To answer this question, let’s look at its chemistry. 

What is the chemical structure of polystyrene?

Polystyrene is a plastic. As its name suggests, it is made up of many ("poly") styrenes. Styrene is a small organic compound with the chemical formula C6H5CH=CH2. Styrene is also known as vinylbenzene. That’s because it has a vinyl (-CH=CH2) functional group attached to a benzene ring (C6H6).

Skeletal formula, molecular formula and space filling model of styrene
Skeletal formula, molecular formula and space filling model of styrene (Sources: Sander de Jong via Wikimedia Commons and Ben Mills via Wikimedia Commons).

Did you know?

Styrene is named for storax balsam, which is an oily resin produced by sweetgum trees (Liquidambar styraciflua). 

Styrene is a clear, colourless liquid. When it is heated, the individual molecules (monomers) join together to form long chains. Long chains of repeating monomers are called polymers

Long-form and abbreviated skeletal formulae for polystyrene
Long-form and abbreviated skeletal formulae for polystyrene (Source: Let’s Talk Science using image by Yikrazuul via Wikimedia Commons).

When lots of styrene chains get mixed together, they get tangled up and make a strong, interconnected mesh. This is polystyrene. When you were a kid, did you ever play the game Barrel of Monkeys? It makes a good analogy for polymer chains. Each individual monkey, with its two hooked arms, are like styrene molecules. On their own, they don't do much. But if you hook their arms together, they will form a chain -- similar to a polymer chain. Then, if you toss them back in their barrel, they'll get tangled up and be more difficult to pull apart -- just like polystyrene! 

How the monomer styrene is polymerized to make polystyrene by LSGScience (2:14 min.).

What are the properties of polystyrene?

Polystyrene exists in two main forms: as a solid or as a foam. As a foam, its two forms are expanded polystyrene (EPS) and extruded polystyrene (XPS). Expanded polystyrene is commonly used in packaging. Extruded polystyrene (XPS) is commonly used in architectural models.

Did you know?

Styrofoam was discovered by accident in 1954 by Dow Chemical Co. chemist Ray McIntire while he was trying to find a flexible electrical insulator.

This chemical structure of polystyrene gives it valuable properties, such as low weight and high strength. It is easy to make things with because it is a thermoplastic. Thermoplastics will fully become a liquid when they reach their melting point. When the thermoplastic is cooled, it becomes a solid again. This is a valuable property, as the process can be repeated several times. 

Did you know?

The melting point of polystyrene is between 210°C and 249°C.

Polystyrene is known for another property that is both useful and bothersome. Like many other plastics, polystyrene is very stable. In other words, it hates to react chemically. You could say that it’s happy just the way it is. 

You can leave polystyrene outside. You can expose it to harsh chemicals. It will stay just like the day you made it. As we’ll see, this can be both a good thing, and a bad thing.

Products made from polystyrene can be found in many landfill sites
Products made from polystyrene can be found in many landfill sites (Source: luoman via iStockphoto).

How do you dispose of polystyrene?

Chemical stability is a good thing when making products that you want to last for a long time, like cell phones, food packaging, and medical supplies.

But what about those products we don’t need to last a long time? When we are finished with our polystyrene food containers, syringes, or cutlery, what do we do with them? If we put them in the trash, they become solid waste in a landfill. And remember, polystyrene doesn’t like chemical reactions. That means these items will happily sit in that landfill for hundreds or thousands of years. 

Recycling polystyrene is possible. First, it is shipped to a recycling facility, where it is sorted and cleaned. Next, the polystyrene is shredded into tiny pieces.Then, it is exposed to heat, which causes it to melt into a paste. The paste is compressed and dried into pellets or blocks before it is shipped to another facility. There, it is made into new products.  

Did you know?

The recycling number of polystyrene is 6.

Unfortunately, this process is expensive and difficult. Because of this, recycling polystyrene is uncommon. In particular, the foam form of polystyrene is seldom recycled. It is inexpensive to make. Also, it is so lightweight that shipping companies don’t find it cost-effective to ship to a recycling centre. Plus, when polystyrene is used to store food or drinks, it is often too contaminated to reuse. That’s because the organic particles stick into the foam.

The most common ways of disposing of polystyrenes are to bury or burn it. When done correctly, burning is an effective method of disposal. The only waste byproducts are carbon dioxide, water, and a small amount of soot. These are the same waste products that result from gasoline combusting in a car’s engine. 

Carbon dioxide is also non-toxic and stable. But it also leads to climate change, so it is not something we want to add to our atmosphere. 

There has been much debate about how long polystyrene will last in the environment, and scientists estimate that it could last anywhere from 500 years to possibly even forever. Because of all this, it’s important to reuse all of the polystyrene products that we can. It’s also important to reduce the amount of polystyrene we use to begin with!

Did you know?  

The world produces more than 12.7 billion kg of polystyrene each year. Americans alone throw away around 25 billion polystyrene cups every year.  That works out to 82 cups per person!

Summing up...

So what do we know about polystyrene? It's made of long chains of styrene molecules. Those styrene molecules are all tangled together to make a cheap, stable, non-toxic, durable plastic. Polystyrene has many different uses. It can be a great help to us in many ways. But we must reduce our use of it, reuse it or even recycle it. That way, we’ll help prevent it from damaging or polluting our environment.
 

Starting Points

Connecting and Relating
  • What polymers can you name other than polystyrene? 
  • Identify some of the items you have used today that are made of polystyrene? 
  • Do you recycle polystyrene and other forms of plastic? Have you taken any steps towards reducing your use of polystyrene because of its negative impact on the environment? 
Connecting and Relating
  • What polymers can you name other than polystyrene? 
  • Identify some of the items you have used today that are made of polystyrene? 
  • Do you recycle polystyrene and other forms of plastic? Have you taken any steps towards reducing your use of polystyrene because of its negative impact on the environment? 
Relating Science and Technology to Society and the Environment
  • What changes, if any, do you see happening that are addressing the issues related to polystyrene use and disposal in society? 
  • Should governments put more emphasis on regulating how much polystyrene is manufactured in the first place or more regulations on how it is handled after it is used? Why? 
Relating Science and Technology to Society and the Environment
  • What changes, if any, do you see happening that are addressing the issues related to polystyrene use and disposal in society? 
  • Should governments put more emphasis on regulating how much polystyrene is manufactured in the first place or more regulations on how it is handled after it is used? Why? 
Exploring Concepts
  • Describe the potential disposal methods for polystyrene. What are the advantages and disadvantages of each method? 
  • What makes polystyrene such a versatile material? 
  • What are some of the challenges of recycling plastic wastes, such as polystyrene? 
  • Can polymers like polystyrene be depolymerized and the resulting products be reused? Research the process of depolymerization and the positive and negative issues associated with this process.  
Exploring Concepts
  • Describe the potential disposal methods for polystyrene. What are the advantages and disadvantages of each method? 
  • What makes polystyrene such a versatile material? 
  • What are some of the challenges of recycling plastic wastes, such as polystyrene? 
  • Can polymers like polystyrene be depolymerized and the resulting products be reused? Research the process of depolymerization and the positive and negative issues associated with this process.  
Nature of Science/Nature of Technology
  • What is polymer science? What other fields of science and technology have been impacted as a result of research and development on polymers? 
Nature of Science/Nature of Technology
  • What is polymer science? What other fields of science and technology have been impacted as a result of research and development on polymers? 
Media Literacy
  • We get messages about reducing, recycling and reducing our use of plastics from many sources. Are there sources and/or types of these messages that are more effective than others? Find some different examples of this type of messaging and evaluate their effectiveness at prompting you to make changes.  
  • How many pieces of polystyrene can students identify in the photograph of solid waste above?
     
Media Literacy
  • We get messages about reducing, recycling and reducing our use of plastics from many sources. Are there sources and/or types of these messages that are more effective than others? Find some different examples of this type of messaging and evaluate their effectiveness at prompting you to make changes.  
  • How many pieces of polystyrene can students identify in the photograph of solid waste above?
     
Teaching Suggestions
  • This article and embedded video support Chemistry, Earth & Environment, Engineering & Technology teaching and learning related to polymers, organic chemistry and the recycling of solid waste. Concepts introduced include polymers, monomers, styrenes, polystyrene, thermoplastic, solid waste, photodegrade and climate change.
  • After reading the article and viewing the embedded video, students could use a Concept Definition Web learning strategy to consolidate their understanding of the concept of polystyrene. Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats. 
  • To explore the STSE issues connected with polystyrene, students could complete a Pros & Cons Organizer learning strategy. Ready-to-use Pros & Cons reproducibles are available in [Google doc] and [PDF] formats. 
  • As a hands-on experience, teachers could have students survey the types and amounts of different plastics found in the school recycling boxes. This information could be put into a graphic organizer and/or transferred into an infographic. Students could also collaborate to brainstorm and propose solutions to reducing the amount of single-use plastics that end up in the school recycling boxes. The information and solutions could be shared with the school population. 
Teaching Suggestions
  • This article and embedded video support Chemistry, Earth & Environment, Engineering & Technology teaching and learning related to polymers, organic chemistry and the recycling of solid waste. Concepts introduced include polymers, monomers, styrenes, polystyrene, thermoplastic, solid waste, photodegrade and climate change.
  • After reading the article and viewing the embedded video, students could use a Concept Definition Web learning strategy to consolidate their understanding of the concept of polystyrene. Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats. 
  • To explore the STSE issues connected with polystyrene, students could complete a Pros & Cons Organizer learning strategy. Ready-to-use Pros & Cons reproducibles are available in [Google doc] and [PDF] formats. 
  • As a hands-on experience, teachers could have students survey the types and amounts of different plastics found in the school recycling boxes. This information could be put into a graphic organizer and/or transferred into an infographic. Students could also collaborate to brainstorm and propose solutions to reducing the amount of single-use plastics that end up in the school recycling boxes. The information and solutions could be shared with the school population. 

Learn more

Polymers: Crash Course Chemistry (2014)

This CrashCourse video (10:14 min.) gives a quick history on polymers, types of polymers, and talks about the chemical processes involved in creating and using polymers.

Plastics by the Numbers (2012)

Greg Seaman, EarthEasy This article on EarthEasy by Greg Seaman covers the types of polymer plastics, which ones can be recycled, and some of the products that are made from them.

Measuring Biodegradability (2008)

This article on the Science Learning Hub explains biodegradability and how it's measured, and how different materials degrade over time.

10 Eco-Friendly Alternatives to Styrofoam

From packing materials to cups and containers, this article by Samantha Allen for Do You Yoga covers 10 different alternatives to plastics like Styrofoam.

References

Bleam, Jr., W. (2002). Polymer basics. ASM International.

Chandra, M., Kohn, C., Pawlitz, J., & Powell, G. (2016, November 22). Real cost of styrofoam. The Green Dining Alliance.

Earth Day Network. (2018, April 18). Fact sheet: How much disposable plastic we use.

Kelly, J. How does polystyrene recycling work?. HowStuffWorks.

Macrogalleria. Polystyrene. (2016). Polymer Science Learning Center.

Plasti-Fab Expanded Polystyrene (EPS) Product Solutions (n.d.). Road embankment construction.

RecycleTech Corporation. (n.d.). Styrofoam recycling process.

Rogers, T. (2015, November 5). Everything you need to know about polystyrene. Creative Mechanisms Blog.

Seth Gilchrist

Seth Gilchrist is a PhD candidate at the University of British Columbia in Biomedical Engineering. He obtained his bachelors degree at the University of Wyoming in 2003 and his Masters at the University of British Columbia in 2006. Seth's current research focuses on bone fracture mechanics in hip fractures sustained by osteoporotic individuals. He perform measurements on proximal femurs that are loaded as they would be in a fall inside a high resolution CT machine and also perform video analysis of femurs loaded in a drop tower under physiological loading rates and conditions.