Exfoliated Graphite Nanoplatelet-Filled Impact Modified Polypropylene Nanocomposites
Abstract
Exfoliated graphite nanoplatelets (xGnP)-filled polymer composites have demonstrated superior electrical, mechanical, physical and thermal properties and are becoming a major focus for both academic and industrial research and development (R&D) activities. The main objective of this study was to characterize the influence of xGnP particle diameter, filler loading and the addition of coupling agents on the mechanical, rheological and thermal properties of xGnP-filled impact modified polypropylene (IMPP) composites. IMPP is currently being used at the AEWC Advanced Structures and Composites Center in polymer impregnated (pre-preg) fiber reinforced polymer (FRP) tapes consisting of an IMPP matrix polymer and E-glass continuous fibers. These tapes are layered and pressed into blast protection panels currently being used by the U.S. military. This research aims to implement nanotechnology and unique experimental methodology to increase modulus and strength of neat IMPP while either conserving or improving the uniquely tailored impact properties of the existing IMPP used. The nanoparticles used in this research were xGnP with three different sizes: xGnP5 has an average thickness of 10nm, and an average platelet diameter of 5 μm, whereas xGnP15 and xGnP25 have the same thickness but average diameters are 15 and 25 µm, respectively. The coupling agent used in this study was polypropylene-graft-maleic anhydride (PP-g-MA). Mechanical characterization of the composites was completed via American Society for Testing and Materials (ASTM) testing standards for flexure, tension and Izod impact. Test results show that nanocomposites with smaller xGnP diameter exhibited better flexural, tensile and impact properties for both neat and composites containing coupling agent. For composites containing a coupling agent, tensile and flexural modulus and strength increased with the addition of xGnP. In the case of neat composites, both tensile and flexural modulus and strength decreased at higher filler loading levels. Increasing xGnP loading resulted in reduction of elongation at break for both neat and composites containing coupling agent. Similarly, unnotched and notched impact strengths as well as fracture initiation resistance were dramatically deteriorated with the introduction of xGnP. Explanation for this brittle behavior in a nanoplatelet-filled IMPP is presented throughout this thesis using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and melt flow index testing. The thermal behavior of the composites was investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The DSC results indicated that the addition of xGnP slightly increased the melting temperature (Tm) and increased the crystallization temperature (Tc) of IMPP by 2 to 3°C which is attributed to the heterogeneous nucleation of the xGnP. The TGA results indicated that the degradation temperature of IMPP is lowered with the addition of PP-g-MA, indicative of the poor thermal stability of PP-g-MA. However, the thermal stability of the composites increases with xGnP loading because of the high thermal stability of the xGnP and the hypothesized “tortuosity effect” that the graphite nanoplatelets was inhibiting diffusion of oxygen and volatile products throughout the composites during thermal decomposition.