UBC Theses and Dissertations
Flow instabilities of PP filled with crosslinked EPDM rubber : using rheology to understand and control its occurrence Ghahramani, Nikoo
Thermoplastic vulcanizates (TPVs) are polymer blends that have good processability like their thermoplastic phase and good elasticity like their elastomeric phase. They consist of a high amount of dynamically cured rubber particles which make flow instability such as melt fracture intense and complicated. In this study a comprehensive rheological analysis is performed to gain a deep understanding of TPVs’ flow instabilities and identify the key parameters that control them. First, a thorough linear viscoelastic analysis is performed using several groups of TPVs which are systematically different in curing level, types of polymer components, and cured rubber content. All the TPVs show a non-terminal behavior reaching to an equilibrium modulus at low frequencies/high relaxation times. The equilibrium modulus, Gy, is an indication of the existence of yield stress and the linear modulus, G(t) can be modeled by a simple power-law model in the form of (𝐺(𝑡)−𝐺𝑦)∝𝑡−𝑝, where t is the time. A yield stress is also observed in the extensional tests and the relationship between extensional and shear yield stress is investigated. Due to the presence of oil and high amount of rubber particles, TPVs show wall slip. The multimode integral Kaye-Bernstein-Kearsley-Zapas (KBKZ) constitutive model considering wall slip is applied to study and model the TPVs’ non-linear behavior. To incorporate slip into the model, a new way is used by applying a fraction of imposed nominal strain to match the experimental data. Moreover, it is assumed that the material does not slip in the linear unyielding region i.e., for shear stresses less than the yield stress. Applying these assumptions, the KBKZ captures the experimental data well. Finally, to study processing parameters that affect the melt fracture phenomenon, capillary experiments are performed. It is observed that TPVs slip massively in capillary flow. Surface fracture of the TPVs gets better with shear rate indicating that the origin of melt fracture is different with that of TPV’s pure component. Yield stress controls flow instability of the TPVs and to overcome and/or mitigate melt fracture, high shear rates are needed to cause flow and eliminate unyielded regions in the complex geometries used in real processing.
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