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Role of Molecular Weight and Molecular Weight Distribution on Weldability and Joint Strength of Polypropylene by Hot Plate Welding

Azhdari, Moein | 2014

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 46285 (07)
  4. University: Sharif University of Technology
  5. Department: Material Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Bagheri, Reza
  7. Abstract:
  8. In this study, role of molecular weight and Molecular Weight Distribution (MWD) on joint strength of polypropylene was investigated by hot plate welding. Standard tensile specimens of three different co-polypropylene with low, medium and high Melt Flow Index (MFI) values (MR230C, 440L and 548R respectively) were injected and welded after cutting. Hot plate welding device was used to join specimens. Melt Flow Index and dynamic rheology tests were employed in order to study the rheological behavior of polymers melt. Furthermore tensile test was done to investigate joint strength of specimens at 150, 175 and 200 ͦ C. It was observed that in constant temperature, increasing viscosity (typically molecular weight) causes the increase in number of molecular bridges per chain through the interface and effectively strong entanglements. Thus, joint strength was increased. In addition, with increasing temperature, joint strength of interface increases. It’s because of higher mobility, center of mass diffusion distance and more molecular entanglements at the interface. In order to investigate role of MWD, granules of 548R and MR230C with weight percent of 40-60, 30-70 and 20-80 respectively, were mixed, injected, cut and after preparation, welded at 175 ͦ C. It was revealed that adding low amounts of 548R to MR230C due to its plasticizing effect, causes the increase in joint strength. Moreover, melt of 440L and 70-30wt% specimens have identical rheological behavior. It was observes that in all temperatures, 70-30wt% specimens have higher joint strength than 440L. It’s due to positive effect of 548R short chains in re-entanglement of themselves, restoration of the entanglement network of MR230C longer chains and facilitate of long chains (MR230C) diffusion after faster diffusion of shorter chains because of the vacuity created around them
  9. Keywords:
  10. Polypropylene ; Bond Strength ; Molecular Weight ; Molecular Weight Distribution (HWD) ; Hot Plate Welding

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