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  • Writer's pictureJenna Granger

What is Plastic?

Updated: Dec 22, 2020

Make your own plastic from milk and explore the science of polymers!

Grade Level:



Physical Science:

  • Structure and Properties of Matter

  • Molecules: Structure & Movement

Earth Science:

  • Human Impacts on Earth’s Systems

Tools & Materials

  • Measuring cup

  • 1 cup of milk (more fat the better)

  • Oven and pan (or microwave and microwavable container)

  • White vinegar

  • Measuring spoon

  • Strainer (mesh or small holes preferred)

  • Spoon

  • Paper towels

  • Optional: food coloring & cookie cutter

To Do & Notice

1. Question: Where does plastic come from and how is it made?

2. Make a Claim:

  • Does plastic come from nature or is it man made?

  • What is plastic made out of?

  • Why do we use plastic? Why is it so popular?

3. Exploration:

  • Make a plastic-like structure out of milk and vinegar that demonstrates how plastic is made. Watch this instructional video Make Plastic From Milk for easy to follow steps or follow the steps outlined below.

  1. With parent supervision heat up milk: on the stovetop or in the microwave until it is steaming (not over boiling). You want the milk about as hot as you would make it for hot chocolate.

  2. Combine ingredients: Add 4 teaspoons of vinegar to the milk and stir with a spoon. Stir for a few minutes and watch the milk start to curdle.

  3. Get out excess liquid: Once milk is curdled, pour the milk mixture into a strainer that is sitting on a bowl to catch excess liquid. Hint: to remove as much moisture as possible you may line the strainer with paper towels. Use a spoon to smush out the liquid. Then place mixture onto 2 paper towels and squeeze mixture (over a bowl) to get out even more moisture. Do this until really dry. Mixture will become crumbly.

  4. Knead your mixture and add color if you have food coloring. Spend a few minutes working the mixture.

  5. Mold your mixture into your desired shape: heart, star, ball or flatten and use a cookie cutter to make your shape.

  6. Let Dry. Let it sit on a sunny window sill or anywhere out of the way for at least 24 hours.

  7. Once dry, observe your shape.

4. Analyze & Interpret Data:

  • How is the end product different from the starting material?

  • How do you think the milk turned from a liquid to a solid?

  • How is this demonstration similar to the production of petroleum (oil) based plastic?

5. Communicate Findings:

  • Share your results with SWEP by sending us photos or videos of your experiment. Post your photos on social media and tag us @sweptahoe on Instagram and/or to @swep4 on Facebook. Be sure to hashtag and follow #SWEPsnippets. If you cannot post directly yourself, send your photos or video to SWEP (

What’s Going On

How did the milk turn from liquid to solid?

  • When milk is heated and combined with an acid, such as vinegar, the casein molecules (a protein found in milk) unfold and reorganize into long chains that connect together to make a solid. This long chain is a natural polymer. Natural because it is made from natural ingredients: milk and vinegar.

  • This process of making plastic is called polymerization. In this reaction, a plastic is generated by forming very long chains of molecules. Each chain consists of thousands of repeated units of single molecular units called monomers.

What is plastic?

  • Today’s plastic polymers are created from fossil fuels harvested from the earth and use man-made chemicals to create the long-chain polymers. (Read more about the history of plastic below to learn how milk plastic or “casein plastic” was once more popular.)

Why do we use plastic?

  • Plastic is easily molded and shaped, it lasts a very long time, it is water resistant and has many functions.

Are There Different Types of Plastic?

  • Polymer chains can be made from different molecular structures which will create various types of plastics. How different plastics behave and which products they are used for depends on the molecules they are made of.

  • How can you tell what kind of plastic an item is? There is often a number on a plastic item inside a recycling symbol, this will tell you which kind of plastic it is and gives clues as to the source of the polymer chains. Important Note: just because there is a recycle symbol doesn’t mean that type of plastic is recyclable in our region. In fact, only #1 & #2 plastics are recyclable in our region.

History of Plastic:

  • The first man-made 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. He was able to create a nature-based plastic by combining cellulose from cotton with camphor.

  • In 1907 Leo Baekeland invented Bakelite, the first fully synthetic plastic, meaning it contained no molecules found in nature.

  • From the early 1900s until about 1945, milk was commonly used to make many different plastic ornaments. This included buttons, decorative buckles, beads and other jewelry, fountain pens, the backings for hand-held mirrors, and fancy comb and brush sets. Milk plastic (usually called casein plastic) was even used to make jewelry for Queen Mary of England!

  • During World War II fully synthetic plastic production in the United States increased by 300%. Nylon for rope & parachutes and plexiglass to replace glass on airplanes and ships were some of the newest plastic materials created by the chemical industry.

The Problem with Plastic:

  • Currently, we produce 300 million tons of plastic each year worldwide, half of which is single-use items (like straws, cutlery, bottles & bags). That’s nearly equivalent to the weight of the entire human population!

  • Plastic production contributes to planet-warming greenhouse gas emissions.

  • Left alone, plastics don’t really break down; they just break up. Over time, sun and heat slowly turn plastics into smaller and smaller pieces until they eventually become what are known as microplastics.

  • Microplastics are found in our streams, waterways and even in our snowpack.

  • Plastics (and microplastics) are harmful to wildlife who ingest plastics mistaking it for food. Plastics also threaten wildlife who become entangled in it.

  • It is estimated we will have more plastic in the ocean than fish by 2050.


  • This Plastics 101 by National Geographic video tells the story of plastic, including new developments in the future of plastic.

  • What happens to the plastic that you throw away? This video goes through three options.

  • You are never too young to fix a major problem. Read about this 17-year-old who discovered a new way of making plastic out of natural materials that will biodegrade in 33 days!!!

Going Further

Plastic Scavenger Hunt

  • Can you find all 7 types of plastic in your house?

  • Make a table like the one below in your science notebook or journal. Write down what you find for each category:

Thank you to our many partners for supporting student's science, sustainability and outdoor learning experience.

Please review SWEP's Terms of Use prior to using this resource.

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