In this activity, you will be able to make glue out of milk! The main ingredient is a protein in milk called casein that you will turn into your own glue, and then test against a commercial glue.
- ½ cup (118 mL) of skim (or non-fat) milk
- 1 tbsp. (15 mL) of vinegar (acetic acid solution)
- 1½ tsp. (7 mL) of baking soda (sodium bicarbonate)
- 1 tbsp. (15 mL) of water
- stirring spoon
- plastic funnel
- paper coffee filter, cheesecloth, or paper towels
- 1-cup (236 mL) microwave-safe dish
- small plastic cup
- 16-oz. (473 mL) disposable plastic cup
- potholder or insulated kitchen glove
- Popsicle sticks
- single-hole punch
- binder clip
- 12-inch (30 cm) piece of string
- 3–4 lbs. (1.4–1.8 kg) of dry sand
- store-bought glue for comparison
- Safety goggles required
- Protective clothing suggested
- Caution: hot liquids!
- Do not eat or drink any of the materials used in this activity
- Gloves should be used if you are allergic to casein or other milk proteins
- Thoroughly wash hands after this activity
- Have a parent or adult help you
Disposal: There are no hazardous materials used in this activity, and everything can be disposed of in the trash. Containers should be cleaned for reuse.
Note: Follow Milli’s Safety Tips found in this issue of Celebrating Chemistry.
- With the supervision of an adult, warm up (30 seconds in the microwave works!) ½ cup (118 mL) of skim milk in the dish. The temperature should be about 165° F (74° C). Do not overheat the milk.
- Using a potholder or insulated kitchen glove, remove the dish of milk from the microwave.
- Stirring constantly, add the vinegar to the milk. Continue stirring until no more lumps form.
- Let it settle for 5 minutes. You should observe the separation of the solids (which sink to the bottom) from the liquid (which stays on top).
- Put a paper coffee filter (or cheesecloth or paper towel) in the funnel. Place a plastic cup under the funnel.
- Pour the liquid first, and then the solids into the funnel.
- Pour the filtered liquid down the drain, and save the solid.
- Using the spoon, gently squeeze off any excess liquid left in the solid. Transfer the solid from the paper filter into a plastic cup.
- Add one tbsp. (15 mL) of fresh water to the solid in the plastic cup. Mix it well with a spoon.
- Slowly add the baking soda, mixing it well. Repeat the process until no more gas bubbles are formed. (Slightly more than ¼ tsp. (1.2 mL) of baking soda may be needed — but don’t use too much.)
- Your glue is ready! This glue will be usable for about one day, and will then spoil. The spoiled glue can be disposed of in the trash.
- Before you continue with the rest of the experiment, clean up your working area. Soap and water can be used for cleaning the containers. The coffee filter (or cheesecloth or paper towel) can be disposed of in the trash.
Where’s the chemistry?
The first part of this experiment uses vinegar to make the protein casein clump together and separate from the liquid part of the milk. The protein solids are also called ‘curds’ and the liquid part is called ‘whey.’ Just like Little Miss Muffet, we are working with curds and whey!
The glue is formed because the chemical reaction between the milk and vinegar promotes the linkage of many molecules of casein. These links form tangled chains, and these tangled chains are what make glue sticky! It is important to use milk without fat in it; fat gets in the way of the reaction and prevents the chains of the polymer from sticking together. The baking soda (sodium bicarbonate) neutralizes the excess of vinegar (acetic acid solution); the bubbles produced in the reaction are made of carbon dioxide gas.
How good is your glue?
Now, let’s compare the strength of your glue with store-bought glue.
- Use the glue you made to stick together two Popsicle sticks. Place them so that only one inch (2.5 cm) of the sticks overlap each other and stick together. Make a couple of these test samples.
- Repeat the same procedure using the commercial glue. Label the Popsicle sticks clearly.
- Wait several hours for the glue to dry.
Before testing, which glue do you think will be stronger? Why do you think that?
To get a general idea of how strong the glues are, try to separate the sticks in one of the samples by hand. What do you observe? How much force does it take to break them apart? Was each one easy or hard? Next, we will test them more carefully.
Single-Lap-Joint Shear Test for Popsicle Sticks
In the chemical industry, chemists and chemical engineers agree on standard testing methods. That way, everyone around the world can do the same test on products for characteristics like water content, aging, and strength. The Single-Lap-Joint Shear Test is a standard way to test the strength of glue to hold together two pieces of metal when they are being pulled apart. Here, we’ll do a version using two Popsicle sticks instead of metal to test both glues’ ability to keep the sticks from being pulled apart by the weight of sand in a cup.
- Have an adult help you use a single-hole punch to make two holes near the top rim of a plastic cup.
- Loop the string through the binder clip and both holes of the cup. Knot the string at both ends, so the knots are next to the holes in the cup.
- Pick up the two sticks you glued together with store-bought glue. Attach a binder clip to the bottom stick.
- Holding the other end of the stick assembly, add sand slowly to the cup until the glue can no longer hold them together. Record the amount of sand used by estimating how full the cup was (such as ¼ or 1/3 full). Repeat with your homemade glue.
Single Lap Shear Test
Amount of Sand Added Before Glue Failed
Which glue was able to carry the most weight? How does this compare with your hypothesis?
What do you think would happen if you tested different ingredients? What if you tried a milk with higher fat content (2% milk, for example)?
Dr. Alexsa Silva is the Director of Instruction and Outreach in the Department of Chemistry at Binghamton University in Binghamton, NY.
Dr. Miranda J. Gallagher is a Postdoctoral Research Associate in the Department of Chemistry at Rice University in Houston, TX.