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Investigation of Effective Microstructural Parameters on the Stress Whitening in Polypropylene Blends

Farmahini Farahani, Mohammad | 2022

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  1. Type of Document: Ph.D. Dissertation
  2. Language: English
  3. Document No: 55759 (57)
  4. University: Sharif University of Technology, International Campus, Kish Island
  5. Department: Science and Engineering
  6. Advisor(s): Bagheri, Reza
  7. Abstract:
  8. Stress whitening is a type of discoloration that can occur in variety of polymers used in automotive and household industries. The current research tries to investigate the effective microstructural parameters such as strain rate, rubber second phase, and branched polypropylene (PP) on the stress whitening in polypropylene in order to propose toughed PP blends with limited stress whitening. It is shown that increasing the strain rate in tensile test results in formation of more visible whitened area within the plastically deformed zone. This is the consequence of further growth of micro-voids at higher strain rates. The results are in accordance with the Mie scattering concepts. Besides, the current research examines the pattern of stress whitening occurrence including the onset and the progress in PP by conducting gray scale, blush measurements and microscopic studies during tensile tests. Histograms of digital image processing in deformed zone, showed that stress whitening intensification occurs gradually and generally through the deformation region. Microscopic evaluations also showed that the origin of stress whitening from two distinct resources, (a) defects including micro-voids and voids (b) orientations of fibrillar. Moreover, the current study has examined the influence of strain rate and thermal softening on yield stress and post yielding behavior of an isotactic PP. A two-process Ree and Eyring model and the heat equation in adiabatic deformation were coupled with a physical parameter described as the temperature sensitivity of the flow stress at yielding, to simulate the effect of gradual temperature rise in adiabatic heating on yielding behavior. Two transition points in the yield stress versus strain rate diagram were noticed. The first one occurring at the strain rate of about 7*10-5 s-1 is associated with a shift in deformation mechanism from Process I, i.e. side chain motions, to main chain motions, named Process II. The other transition point, equal to 3*10-2 s-1, is the strain rate value at which adiabatic heating surpasses strain hardening. This change from isothermal to adiabatic deformation, causing approximately 2 ̊C increase in temperature, is associated with the decline of the yield stress and also variations in the post yielding behavior of PP. Microscopic results showed that there is a transition strain rate (̊T) at which both craze extension and stress whitening reach their maximum value. As the number and extension of craze increased, scattering entities in the specimen increased, the specimen became increasingly white and, consequently, there was an increase in whitening intensity up to about 0.03 s-1. By further enhancing the strain rate beyond 0.03 s-1, craze extension decreased and stress whitening became smaller due to the thermal softening mechanism activated in the adiabatic deformation. Then, pure IPP and its compounds with different types of rubber phase entitled as ENGAGE, LC670, and Vistamaxx were used to investigate the stress whitening resistance in the tensile and impact process. Results show that the compounds of IPP with LC670 and IPP with Vistamaxx show better whitening resistance. Whereas, the compounds with LC670 depicted better mechanical properties in compare to Vistamaxx compounds. Compounds were then examined via transmission optical microscopy. Results illustrated that while incorporation of a propylene-based rubber does not affect spherulitic microstructure of PP that much, ethylene-octane elastomers act as nucleating agents and refine the microstructure significantly. Moreover, it was observed that compatibility of the propylene-based rubber particles with the matrix favors crazing in the PP. Then the effect of adding branched PP to the PP matrix as an entanglement density factor was investigated. The results show that by adding branched PP, the craze structure reduces and in return the whitening intensity decreases. Finally, the best compound showing better impact and whitening resistance was proposed. The optimal compounds designed with a combination of 90%w of PP, 7.5%w of LC670, 2.5%w of branched PP.
  9. Keywords:
  10. Polypropylene ; Strain Rate ; Yield Stress ; Rubber Phase ; Branched Polypropylene ; Digital Image Processing ; Post-Yield Softening ; Isothermal to Adiabatic Deformation ; Mie Theory ; Post Yield Behavior ; Stress Whitening ; Thermal Softening

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