Constitutive Analysis of Rubber Compounds with Varying Molecular Structure

The numerical dimensioning of injection heads and tools for the extrusion of rubber profiles requires both, the experimental investigation and numerical consideration of the viscoelastic material behavior. Usage of finite element software allows drastic reduction of a time-consuming empiric adaptation process for the design of extrusion tools. In this context, viscoelasticity covers three different aspects: • shear-thinning: The dynamic viscosity decreases with increasing shear strain rate, thus the material is called non-Newtonian. Experimental characterization of this behavior is commonly based on capillary viscometry (see Figure 1(a)). In such tests, a piston presses (at controlled velocity v) the heated material out of a reservoir into and through a capillary die (with a specific ratio of length to diameter L/D). After reaching a steady flow state within the die, the pressure p of the melted material is measured in the reservoir, in the immediate vicinity of the capillary entry. One capillary experiment allows various piston velocities, depending on available melt in the reservoir. Evaluation of experiments consists of transforming measured pressure-velocity curves into a relationship between viscosity η and shear strain rate γ& [1]. • die swell phenomenon: Heated rubber melts expand upon exiting from capillary tubes. This swelling of the extrudate on emerging from a die is typical for nonNewtonian viscoelastic liquids. Investigation of this behavior is done during a capillary experiment under usage of a swell value measuring unit (see Figure 1(b)), giving access to a cross-sectional area ratio χ of strand diameter d and diameter of employed die D. After measurement of the melt pressure p, the piston stops. Then, for each piston velocity v, the strand diameter is measured under consideration of the relaxation of the material [2]. • elasticity: Performance of tests with a rubber process analyzer allows the separation of the stored energy in the melt, representing the elastic portion, and the energy dissipated as heat, representing the viscous portion. This type of torsional rheometer strains a sample in shear by oscillating the lower die sinusoidally (see Figure 1(c)). The upper die is fixed and connected with a measuring device, which records the applied bending moment S*. The latter is split into an elastic component S’ and a viscous component S’’ by means of a Fourier transformation. Afterwards, by applying a form factor the storage modulus G’ and the loss modulus G’’ for all investigated angular frequencies ω are provided [3].