Relationship Between Powder Surface Characteristics and Viscoelastic Properties of Powder-Filled Semisolids

Abstract

The viscoelastic properties of dispersions of powdered zinc oxide in anhydrous lanolin and colloidal sulfur in anhydrous lanolin were characterized by dynamic mechanical testing. The elastic shear modulus, G″, viscous shear modulus, G″, and loss tangent (damping), tan δ, were determined as a function of shear frequency, v, temperature, T, and volume fraction of powder, Φ2. A priori, it might be expected that zinc oxide and colloidal sulfur would elicit different viscoelastic properties due to their contrasting surface characteristics; zinc oxide has a hydrophilic surface and colloidal sulfur has a hydrophobic surface. Even though constitutive mathematical models, derived to predict the mechanical behavior of solid-filled polymeric materials, were not designed to account for differences in surface characteristics of the filler, the findings of these experiments show that these models are useful in explaining the differences in viscoelastic behavior of powder-filled semisolids due to surface characteristics of the filler. One model of particular value was the Kerner equation. With it, mechanisms were postulated for zinc oxide–zinc oxide interactions, sulfur–sulfur interactions, zinc oxide-anhydrous Ianolin interactions, and sulfur–anhydrous Ianolin interactions, within dispersions as a function of v, T, and Φ2. In addition, damping was used to further identify the influence of temperature. Data obtained from three temperatures, where anhydrous Ianolin exists in three different structural states, shows that the influence of the powder on damping is not only determined by the surface characteristics of the powder but also the structural state of anhydrous Ianolin.