Mechanical Characterization and Optical Microscopy of Homemade Slime and the Effect of Some Common Household Products

Abstract

Commercial slime is popular among young children for fun and educational purposes, and it can also be useful in showcasing school students how mixing different substances can change the chemical nature and thus the viscoelastic properties of materials. It can thus be a vital technique in developing analytical thinking in children with regards to the chemical bonding of materials at the microscale and how it is directly interlinked with the mechanical properties of a material (macroscale). The simplest slime is synthesized by mixing polyvinyl acetate (PVA) glue with sodium tetraborate (borax), which is a salt of boric acid. The reaction between PVA and borax forms cross-links between polymer chains due to the creation of weak bonds to the OH groups of PVA [1]. The three-dimensional network polymers formed as a result lead to the viscoelastic nature of the fluid that gives slime its specific texture. The sodium tetraborate interlinks with the PVA through hydrogen bonds[2] or reversible covalent bonds [3] to form “di-diol” complexes, yielding slime. In this work, we analyze homemade slime using optical imaging and mechanical characterization techniques the effect of adding some common household materials.The slime was prepared in four ways: baseline (A), baseline with shaving cream (B), baseline with clay (C) and baseline with shaving cream and clay (D). The baseline was made by combining a mixture of 1 cup polyvinyl acetate (PVA), C4H6O2 (glue) and 1 cup water with a mixture of 10 ml sodium tetraborate, Na₂[B₄O₅(OH)₄]·8H₂O (borax) with 800 ml water. Both mixtures were kneaded well until the slime was formed. To the baseline, 53 ml of shaving cream (mainly water, stearic acid and lanolin) was added to make mixture B. Mixture C was prepared by adding 23 g of clay to the baseline while mixture D was made by adding shaving cream and clay to the baseline in the same amounts as in mixture B and C. The slimes were tested by scanning electron microscopy and optical microscopy to observe the internal structure of the specimens and mechanical testing to evaluate the elastic modulus. The images showed samples C and D (with added clay) to have larger bubbles which may be due to high water-absorbing and moisture-retention capabilities of clay. Mechanical testing results proved the high elasticity of the slime samples: specimen A had an elastic modulus of 93 MPa while adding shaving cream (specimen B) increased the modulus to 194 MPa. As is known, a higher elastic modulus signifies a stiffer material, i.e. it stretches less when pulled and vice versa, thus the addition of shaving cream decreases the elasticity of the slime samples. Similarly, incorporating clay with into the baseline (specimen B) also decreased the flexibility of the slime (increasing the Young Modulus by more than twice to 224 MPa), making it more manageable and keep its shape after deforming. Consequently, adding both clay and shaving cream also resulted in an overall stiffer material with an elastic modulus of 229 MPa. Finally, the results show an interesting link between microscopic bubble size and the effect on the mechanical properties.[1] N. Isokawa, K. Fueda, K. Miyagawa and K. Kanno, “Demonstration of the Coagulation and Diffusion of Homemade Slime Prepared Under Acidic Conditions without Borate”, J. of Chem. Edu., 2015, 92, 11, 1886–1888.[2] Casassa, E. Z.; Sarquis, A. M.; Van Dyke, C. H. “The gelation of polyvinyl alcohol with borax: A novel class participation experiment involving the preparation and properties of a slime”, J. Chem. Educ., 1986, 63, 57−60.[3] Shibayama, M.; Sato, M.; Kimura, Y.; Fujiwara, H.; Nomura, S. B n.m.r. “Study on the reaction of poly(vinyl alcohol) with boric acid”, Polymer, 1988, 29, 336−340. Figure 1