Mix It Up! Cross Linking Polymers
Of all the iterations we see of science lessons preparing “Silly Putty” from borax and glue, we’d like to see students looking carefully at each aspect of this classic chemical reaction, including the solution chemistry and the simple math involved preparing each reactant.
- Elmer's Glue
Sodium tetraborate (Borax is also termed Granubarmd), the common laundry detergent additive, lends itself well to this experiment because it is quite easy and safe to handle in the laboratory. It is a white crystalline solid with a chemical formula of Na2B4O7*5H2O (the *5H2O indicates that the compound is in the pentahydrate form, as were our copper sulfate crystals in our Zinc/Copper redox segment). It is sometimes used in farming to reach the correct boron concentration in soil, yet more commonly used as a laundry detergent supplement. We use a 4% solution in our segment that is “scaled up” to produce a 2 liter sample, yet in a classroom situation, you may elect to have students add 4 grams of the solute to 96 ml of distilled water to the same effect. You may also lower the concentration of borax to 3% (3 grams solute to 97 ml solvent) if you find (as we did) that the solution saturates at 4%.
Alternatively, you may elect to warm the distilled water to drive the full 4 gram sample of solute into solution, or perhaps introduce a variable into the experiment by asking six teams to prepare samples at increasing concentrations (Team 1 – 1%, Team 2- 2%, etc.) and then gather the class to evaluate the six product samples to judge which concentration was optimal based on the stability of the final product.
In dissolving the glue by 80%, we model adding the glue to the distilled water in a wide mouth Nalgenetm graduate or pitcher.
In preparing a classroom (student) version of the experiment, we would scale the sample down to 50 ml of glue diluted with 40 ml of distilled water. As the glue is nontoxic and water soluble, we find it manageable to work with in a science lab, even with smaller children. In lieu of a glass or Pyrex stirring rod, we can recommend a wood splint or popsicle stick if teachers are concerned with breakage as students stir the reacted polymers (quite thick and viscous, potentially causing the glass rod to “snap” if overeager students stir the reaction vigorously. Student will note the good solubility of the glue in the distilled water, and you may take a moment to comment on the pigmentation of the glue (titanium dioxide is in fact the same material that is added to skim milk to make it appear “whiter”, richer and more appetizing).