Thesis-2012-Al-Lafi.pdf (7.77 MB)
Performance of polycarbonate nanocomposites at high strain rates
thesisposted on 2012-04-19, 12:03 authored by Waleed Al-Lafi
This project was conducted in order to develop high-performance (Polycarbonate (PC) based nanocomposite) materials at high strain rate, for potential applications in military, sports and personal protection. In this project, a series of PC nanocomposites were successfully fabricated by melt compounding method. The two kinds of nanofillers namely Na + montmorillonite clay and chemically modified multiwalled carbon nanotubes with hydroxyl group (MWCNTs-OH), respectively, were employed to fabricate polymer nanocomposites. A number of techniques including wide angle X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), modulated differential scanning calorimetry (DSC), were employed to study microstructure and morphology of these nanocomposites. Mechanical properties at low and high strain rates of these nanocomposites were mainly investigated. The SEM images revealed that uniformly dispersion and distribution of CNTs in nanoscale have been achieved in PC or high density polyethylene (HDPE) matrix. By means of DSC studies, the “nanoeffects” on the glass transition temperature of PC and crystallinity of HDPE were examined and discussed. A simple method to detect state of dispersion of nanofiller in polymer matrix by the comparison of the values of bulk density determined theoretically and experimentally, respectively has been established. The dynamic mechanical behaviours of PC and its nanocomposites with MWCNT and clay nanofillers were analysed with temperature and frequency. The addition of the nanofillers has significant influence on the dynamical response of the PC to temperature and frequency. The results revealed that MWCNT and clay nanofillers can enhance the storage modulus and decrease the damping property of the PC. Abstract ii The addition of MWCNTs has significant influence on the impact performance of PC and HDPE at a quasi-static rate revealed by an instrumented falling weight impact test (IFWIT). The results indicated that the maximum load of PC is significantly improved by the filler. The PC specimen containing 1 wt% MWCNTs showed the highest peak load value of approximately 884N, much higher than 209N of the pure PC. The PC nanocomposites are able to sustain much higher external force before fracture, and the behaviour contributes to greater deflection. The increased filler content leads to higher impact force due to the particle interface react and form a tortuous fracture path. The incorporation of MWCNTs causes a significant improvement of impact failure energy of the PC. Incorporation of 1 wt% MWCNTs caused significant improvement of 500% in impact failure energy. The performance of the PC and HDPE nanocomposites were examined by means of Split Hopkinson Pressure Bars Apparatus. The strain rates from 102 to 104 s -1 were used. For PC/clay nanocomposites, a slight enhancement in yield stress was observed. Yield stress decreases with increasing strain at a certain range of strain rates. In addition to increasing the strain rate, the process of strain hardening dominates the plastic deformation and then thermal softening upon reaching stress collapse. The region of thermal softening was increased with the increase of strain rate. Similar conclusions were drawn for the PC/MWCNT nanocomposites. Yield stress decreases with increasing strain at a certain level of strain rate. Moreover, with an increase in MWCNT content, the temperature effect on the performance of the PC appeared. For HDPE/MWCNT nanocomposites, it is clear that the high strain rate and high MWCNT contents have a significant influence on the performance of the HDPE. The rapid decrease in yield stress was observed due to the temperature effect. From our results, we concluded that polymer nanocomposites could be used for the minimisation of the trauma injury at certain impact rates as results of significant improvement on the performance of PC matrix by adding nanofiller. At very high impact rates, the function of nanofillers could vanish due to temperature effect.
- Aeronautical, Automotive, Chemical and Materials Engineering
Publisher© Waleed Al-Lafi
NotesA Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.