Unit of Chemistry Teaching Education University of Helsinki Science Education Centre LUMA Centre Finland

PLASTIC FROM MILK

FOCUS GROUP: This experiment can be done with all age groups and the theory can be fitted on the knowledge level. When making the plastic with primary school students, you could talk about networking and the networked structure of plastic in general. With them the theory doesn’t go too deep and the milk should be warmed up by the teacher. With high school students, you could talk about polymerization and how you can make a polymer from monomers. With upper secondary level students, you could talk about how casein crosslinks and forms polymers. It can be done in molecular and bond level. LENGTH: 20 - 45 minutes MOTIVATION: You want to make a present for your friend, but you don’t know what to make them. Here is a guide how to make jewelry or a refrigerator magnet from milk! OBJECTIVE: To understand everyday chemistry phenomena and to understand industrial processes on a small scale. Understanding the chemical structure of plastics. TEYWORDS: Proteins – Denaturation – Polymers – Acidity – Everyday chemistry

Finland’s first plastic factory started operating in 1921 in Tampere. This factory produced plastic from milk. The plastic was used to making buttons. Its strength was a low fire sensitivity, but the challenge with it was that it had a low moisture resistance. Eventually, the casein plastic was replaced by synthetic thermoplastics that came on the market in the 1950s and 1960s.

BACKROUND INFORMATION

Plastic has gradually become an indispensable assistant for out everyday lives. It is lightweight, inexpensive, pleasant to touch and possible to use for almost any purpose.

Plastics are mainly polymers. Polymers are long chained molecules in which the same base structure is repeated many times. One molecule may be composed of 1 000 to 100 000 smaller building blocks so called monomers. Polymers can be divided into natural polymers and synthetic polymers. Starch, keratin and DNA are examples of natural polymers. Synthetic polymers are industrially manufactured polymers such as polyethylene. Polyethylene is the most commonly used synthetic polymer in the world and is used e.g. in plastic bags.

In this experiment the casein plastic is made from proteins in milk. Proteins are polymers consisting of amino acids. The proteins in milk can be modified e.g. with heat and acids. Heat modifies, denatures, the structure of whey proteins in milk and acid causes the casein proteins to attach to each another with whey protein bridges. This creates a protein structure which, when dried, hardens into casein plastic.

Unit of Chemistry Teaching Education University of Helsinki Science Education Centre LUMA Centre Finland

Now you will learn how to make plastic at home!

Did you know: When a protein is denatured, its three-dimensional structure changes and the protein loses its biological activity. A familiar reaction of this is when you boil an egg. The heat modifies the structure of the proteins in the egg so that the initially liquid egg becomes solid.

PREMILITARY QUESTIONS FOR STUDENTS

What objects are made out of plastic?

Why are the products you mentioned made out of plastic?

Describe what kind of material is plastic.

What raw materials are used by industries when making plastic?

SUPPLIES

A saucepan or other heat SAFETY AND WASTE MANAGEMENT resistant container/dish. A spoon Use a lab coat and goggles. A graduated measuring cylinder Vinegar is an acid, corrosive substance, rinse A thermometer spatter with plenty of water. A hotplate

REAGENTS

Fresh nonfat milk Vinegar Food coloring

Unit of Chemistry Teaching Education University of Helsinki Science Education Centre LUMA Centre Finland

INSTRUCTIONS

Measure 200 ml of milk and pour it into the saucepan.

If you want colorful plastic, add a few drops of food coloring.

Heat the milk carefully up to 50-60 °C (if the temperature rises above 60 degrees, milk will quickly burn to the bottom!).

When the milk is warm, take the saucepan out from the hotplate and add 10 ml of vinegar to the milk and stir.

WHAT CAN YOU OBSERVE?

Lift the plastic with a spoon onto a paper towel and shape it to the shape you want. You can glue a magnet to the back of your plastic to make a great refrigerator magnet.

QUESTIONS AFTER THE WORK

When does a similar type of reaction occur in milk, where lumps are formed? As the milk ages, lactic acid begins to accumulate in the milk. Lactic acid bacteria break down lactose and lactic acid is formed. This makes the pH level lower and makes the proteins lose their negative charge, making them more soluble to water.

Why does acid change the water solubility of casein micelles? Acid neutralizes the negative charge of casein micelles so that they no longer interact with water. This makes the proteins separate from the milk into its own phase.

What happens to human proteins if the body temperature rises too high? High fever is dangerous to humans. At a temperature above 42 degrees human proteins begin to denature, lose their functional three-dimensional shape.

Unit of Chemistry Teaching Education University of Helsinki Science Education Centre LUMA Centre Finland

CHEMISTRY BEHIND THIS EXPERIMENT

In recent years the theory of this plastic has been the subject of many debates in the scientific community. According to recent research, the theory behind this experiment is the following:

Milk contains two types of protein: casein (about 80 %) and whey (about 20 %). Proteins are long chains of amino acids that may contain 100 to 100 000 amino acids attached to each other. We know that there are 20 different amino acids.

There are four different types of casein protein: αS1 -casein, αS2 -casein, β-casein and κ-casein.

Caseins are organized into micelles, so that the inner parts of the micelle are

predominantly hydrophobic (lipophilic) like αS1 -caseins, αS2 -caseins and β-caseins. On the outer part of the micelle are the hydrophilic (water soluble) κ-caseins. The micelle maintains its form because of calcium phosphate clusters in the middle.

Unit of Chemistry Teaching Education University of Helsinki Science Education Centre LUMA Centre Finland

κ-caseins have a negatively charged tail part, which is ‘’hang’’ outside the micelles. These tail parts look like hairs and they make the casein micelles water soluble. As a result of the negative charge, the casein micelles repel each other. In milk, the micelles are surrounded by water molecules.

Milk also contains whey proteins: beta-lactoglobulin, alpha-lactalbumin and immunoglobulins. In addition, milk contains also various enzymes. When milk is heated, whey proteins are denatured, meaning that their three-dimensional shape is broken. Denaturation breaks the hydrogen bonds that hold the three-dimensional structure of the protein and opens the disulfide bridges.

When acid is added to heated milk, the positively charged protons of the acid first neutralize the negative charges on the ‘’hairs’’ of casein micelles, making them less soluble in water. This enables the casein micelles to get even closer to one another. Under the influence of acid, calcium phosphate clusters that strengthen the structure of the micelle are released from the inside of the micelle. Unit of Chemistry Teaching Education University of Helsinki Science Education Centre LUMA Centre Finland

The broken disulfide bridges from the denatured whey proteins are starting to look for a new pair to form a new disulfide bridge. Whey proteins bind with disulfide bridges onto the casein micelle’s surface, forming a protein network.

The more whey proteins bind between the casein micelles, the stronger casein plastic is formed. Cheese is made of a similar protein network. In fact, plastic form milk is cheese!