Experimental Investigation and Identification of Material Parameters for Rubber Blends

Capillary rheometry simulates polymer extrusion in a simplified way. It allows the characterisation of polymers by means of determination of the viscosity function and extrudate swell. As regards viscous properties, there is still a lack of constitutive characterisation of rubber blends. In this contribution new concepts are presented how the viscous properties are computed without using common correction methods. The die swell is the determinant criterion for the production of rubber profiles by means of extrusion. Therefore its experimental investigation and numerical treatment is of high interest. This was one of the motivations for starting a research project in the field of rubber blend technologies. The consideration of the die swell for the production of rubber profiles is necessary. Thus, the final goal of this project is the numerical prognosis concerning tools for the extrusion of rubber. The task of the present material characterisation is to determine all required state variables which describe the flow situation of a die through a circular capillary. For the description of the pseudo-plastic material behaviour of rubber blends the power law by OSTWALD [3] is used. Its application to the investigated rubber blends is possible for a common interval of the shear strain rate γ . The coupled state variables are: − material parameters of the power law, − pressure loss according to viscoelastic properties, − shear stress at the wall of the capillary, − shear strain rate at the wall of the capillary and − wall slippage velocity along the capillary. If these state variables are known the determination of the shear rate dependent viscosity η is possible. Various types of rubber blends are used for the material characterisation, with Ethylene propylene diene monomer (EPDM) as the main component. In order to determine viscous properties of rubber blends experiments with a capillary-viscometer are necessary. Due to the nonlinear coupling of the viscosity and the shear strain rate, correction methods are used to determine these material properties. First, Newtonian material properties are assumed. The resulting physical values are called apparent and marked with a subscript “ap”. Second, the apparent values will be transformed into true values using correction methods. 2. Experimental Investigation