The Rheological Properties of Concentrated Suspensions. IV. The Viscoelastic Properties of Concentrated Dispersions in Molten Polymers

Abstract

Abstract The dynamic viscoelastic properties of molten polyethylene with various fillers have been measured in nitrogen gas at frequencies from 0.01 to 0.75 c. p. s. The dynamic viscosity and dynamic modulus have been measured at the temperatures of 150∼230 °C. Glass spheres, calcium carbonate powder and barium sulfate powder have been used as fillers. The average diameter of the particles covers the range of 0.04∼91 μ. The polyethylene used was one of a low density, “ Sumikathene F-70-6,” whose number average molecular weight is 28000. The dynamic viscosity and modulus of molten polyethylene are increased by the addition of fillers. The viscoelastic properties at low frequencies depend differently on the concentration in each suspension. In suspensions of glass spheres of the larger particle sizes (particle diameter : 24.0, 32.3, 49.5 and 91.1 μ), the concentration dependence of the dynamic viscosity, η′, follows a generalized Robinson’s formula : ln(η′⁄η′0)=Kφ⁄(1−S′φ) where η′0 is the dynamic viscosity of polyethylene ; φ, the volume fraction of the suspended particles, and K and S′, 2.5 and 1/0.74 respectively, regardless of the particle size. In suspensions of calcium carbonate of the intermediate particle size (particle diameter : 2.4 μ), the dependence of the dynamic viscosity on the concentration and on the amplitude of oscillation is observed more remarkably than that in suspensions of glass spheres. In suspensions of calcium carbonate and barium sulfate of the smaller particle sizes (particle diameter: 0.04, 0.08 and 0.27 μ), the dynamic viscosity and modulus increase appreciably above a critical concentration of the filler, a point which depends on the particle size. Beyond this critical concentration, the particles seemed to be linked with each other by single polymer molecules. The dynamic viscosity and modulus at lower frequencies increase remarkably as the mean distance between the surfaces of particles becomes less than the molecular dimension of the polymer. In suspensions of the larger particles of glass spheres, this critical concentration can not be observed up to the highest concentration. The temperature dependence of the dynamic viscosity at lower frequencies approximately agrees with that of unfilled polymers. The dependence of dynamic properties on the time and on the amplitude of oscillations is observed in suspensions of calcium carbonate and in those of barium sulfate of comparatively small particles. In the barium sulfate suspensions of the finest particles, the dynamic viscosity and modulus increase with time under periodic strains and decrease with the increase in the amplitude of oscillations. The amplitude dependence of the dynamic modulus is more remarkable than that of the dynamic viscosity generally. In such thixotropic suspensions, the dynamic properties were measured with as small a strain as possible.