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Theoretical and Experimental Investigation of Die Swell Phenomenon for Polymer Nano-Composites

Khodadadi Yazdi, Mohsen | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41413 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Ramezani Saadat Abadi, Ahmad
  7. Abstract:
  8. This thesis was conducted both numerically and experimentally. In the experimental investigations aqueous solutions of high molecular weight Carboxymethylcellulose (CMC) was used. The experimental set-up was composed of a glass syringe, and a piston which can easily move through the syringe. Different die with aspect ratio ranging from 5.25 to 28.8 can be attached to the syringe. The syringe then is filled with the CMC solution with different weight fractions. Then using a force exerting on the piston, the solution comes out of the die and exiting velocity was calculated from weight of CMC solution that exit from die in a definite time. High quality photographs were taken from extrudate using a camera mounted on a board which can move upward and downward. Analyzing photographs enable us to calculate the swell ratio at any distance from the die. It is worth mentioning that rheological parameters were obtained using a plate-plate Rheometer; this parameter is then used in the numerical simulation section.In the numerical simulation section, three different approaches were used. In the first approach, continuity, momentum, and a constitutive equation were solved in the contraction geometry similar to experimental set-up so that velocity, pressure, and stress profiles in the domain was obtained and using shear and normal stresses at the die wall the swell ratio was calculated. Results show a good agreement with experimental data.In the second approach, the surface capturing method was used in order to predict the die swell phenomenon. Different types of constitutive equations including Giesekus, PTT, and Oldroyd-B were used in the simulation. This approach is so sensitive to time step and mesh so that it usually takes a lot of time to solve the PDE set through the domain. Indeed, the method is so time consuming and the calculation cost is high. However, the calculation was lessen using a graded mesh rather than a uniform one.The third approach uses the microstructure of polymer chains in order to estimate the swell magnitude. When the polymer goes through the die, stresses cause the flexible chain to be stretched along flow direction so that if one assume a polymer chain to be confined in an ellipsoid, the ellipsoid get more stretched in the die compared to when polymer is at rest; this leads to a increase in ellipsoid big diameter and a resultant decrease in other diameter which is perpendicular to the flow direction. After emerging from die, the polymer chain tends to recoil which causes the ellipsoid to recover its previous shape which is close to a more spherical shape. This leads a gradual increase in extrudate diameter and shrinkage in longitudinal direction and this is the key idea in estimating swell ratio.Then suspension of micro and nano-fiber dispersed in a polymeric matrix was studied and it was concluded that dispersing fiber in the base fluid leads the normal stresses to enhance. Normal stress intensification results in vortex growth in the contraction. Under these circumstances, the tanner relation predicts the swell ratio to increase which apparently contradict experimental results. The fiber-matrix interaction was taken into account using the conformational rheological models which predict a decrement in swell magnitude. The shift factor method for prediction of swell ratio was then presented which seems a good approach for swell in composite materials
  9. Keywords:
  10. Inflation ; Nanocomposite ; Viscoelastic Fluids ; Molding ; Die Swell

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