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Glue Strength Comparison: Homemade Casein Glue and Commercial Glue

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Author: Mike Calhoun

Casein is the principle protein of milk. It accounts for approximately one third of the non-aqueous (liquid) ingredients in cow’s milk, from which commercial casein is obtained. The casein protein can be used to make glue and is also used in the manufacturing of various goods, including fabrics, adhesives, plastics, and protective coatings. 

In order to remove the casein from the milk, the product goes through a process called precipitation. Essentially, the milk is heated and acid (vinegar) is added. As the heating takes place, the casein content begins to shrink and expel moisture, producing clumpy white solid curds and liquid whey. The next step in extracting the casein involves separating the whey from the curd by filtering. After the separation, a baking soda solution is added to neutralize the acid. The curd is then subjected to pressure to further remove the water content. When the moisture content is reduced to an acceptable level, the curd is dried to form a hard, plastic-like protein substance which can act as an adhesive.

Problem

Compare the holding strength of a sample of homemade casein glue to that of several commercial glues.

Research Questions:

  • How is the casein (protein) separated from the milk?
  • Did the casein glue perform as an adhesive?
  • What was the maximum bonding strength for the homemade casein glue?
  • Which commercial glue(s) displayed more adhesive or bonding strength then the casein glue?
  • Which commercial glue(s) displayed less adhesive or bonding strength then the casein glue?
  • Was the holding power of homemade casein glue comparable to that of the commercial glues? 

Materials

  • Skim milk 
  • Vinegar
  • Baking soda
  • Measuring cup
  • Heat proof container
  • Tablespoons
  • Coffee filter
  • Small plastic funnel
  • Popsicle sticks
  • Small paper or plastic cups
  • Masking tape
  • String
  • Large bowl
  • Kitchen food scale or a digital bathroom scale
  • 4 or more commercial white glues: Scotch® Quick Drying Tacky White Glue, Elmer’s All Multipurpose White Glue®, Amos® White Craft Glue, Blick® Multi-Purpose White Glue.
  • Safety goggles
  • Apron
  • Gloves 

Procedure

  1. Heat a pint (half a liter) of skim milk and add six tablespoons (90 ml) of vinegar slowly, stirring constantly.
  2. When it begins to curdle, remove from heat.
  3. Continue stirring until the curdling stops. Let sit until the curds have all settled to the bottom.
  4. Filter the solid (curds) from the liquid (whey) using a coffee filter placed in a funnel.
  5. Gently press the filter paper around the curds to squeeze out the excess liquid (whey).
  6. Add 1/4 cup (60 ml) of water and a tablespoon (15 ml) of baking soda to neutralize the vinegar. When the bubbling stops, the white casein glue is ready for use.
  7. Glue 2 wooden Popsicle sticks together end to end using the casein glue as shown.
popsicle stick
 
  1. Repeat the same procedure using the other glues. Allow the glues to harden over night.
  2. Place one end the glued Popsicle sticks between a set of books or between two chairs.
  3. Pour a small amount of water into several cups corresponding to the number glues to be tested.
  4. Weigh the cups with a small kitchen food scale or a digital bathroom scale and record the weight on a paper.
  5. Using masking tape attach string across the top of each cup.
  6. Suspend the cups with strings attached over ends of Popsicle sticks.
  7. Place a large bowl underneath to catch any water that might spill.
  8. Follow this same procedure for each glued stick increasing the amounts of water until the glue bonds are broken.
  9. Record the results in a table similar to the one shown.
       Name & Type of Glue   

Maximum Weight Supported (grams or ounces)

Casein Homemade Glue

 

Scotch® Quick Drying Tacky White Glue

 

Elmer’s All Multipurpose White Glue®

 

Amos® White Craft Glue

 
Blick® Multi-Purpose White Glue
 
 
 
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