Research On Polypropylene/Graphite Electrically Conductive Nanocomposites

Polypropylene (PP)/ maleic anhydride grafted polypropylene (gPP) /expanded graphite (EG) electrically conductive nanocomposites were prepared by solution intercalation (SI) and master-batch melt mixing (MMM) methods. Electric volume resistivity (Pv) measurement, TEM, SEM, OM observations and XRD, DSC, DMTA analyses were used to examine the influences of preparation methods, processing conditions, EG weight or volume fractions (fw or ), gPP weight content (Cg) or gPP/EG weight ratio (Rw) and PP/gPP weight ratio (Rw) on structure or morphology and electrical properties and their dependence to temperature of the nanocomposites, as compared with PP/gPP/EG, PP/EG composites prepared by direct melt mixing (DMM) method. The main results obtained are as follow:(1) The structure and electrical properties of the composites depended strongly on the preparation methods. The percolation thresholds on weight (fw. c) of the composites with Rw= 1.5 prepared via IS, MMM and DMM and the PP/EG controls were determined to be 7.05,10.50,13.22 and 13.56%, respectively. Correspondingly, the size and aspect ratio of EG particles within the particles in the composites increased in sequence of DMM-, MMM- and Si-composites, and were smallest in the PP/EG control.(2) The increment of Cg could enhance conductibility of the composites. The fwcs of the composites with Rw = 1 prepared via MM and DMM were 7.96 and 11.91% respectively, which were lower than those of the composites with Rw=1.5 prepared by corresponding methods( 10.50 and 13.22%). The composites with Rw=1.5 prepared by same method. This could be attributed to the increment of size and sapect ratio of EG particles with risen Cg in the compsotes.(3) The percolation theory could be employed to describe the conducting behavior of these materials. The percolation thresholds on volume (c) of the composites with Rw= 1.5 prepared via SI, MMM and DMM and the PP/EG controls were determined to be 2.91, 4.51, 5.63 and 5.83%, respectively, and the critical exponents (u) of SI, DMM-composites and PP/EG controls were 5.13, 3.93 and 4.23 respectively. There existed some correlation between the characteristic percolation parameters and the structural characteristics of the composites.(4) The processing parameters played important role in determining the structure and electrical properties of the composites prepared by same method. The. c s of the composites with Rw=1.5 prepared via DMM under optimized and non-optimized kneading parameters were determined to be 9.49 and 13.22% respectively. This could be attributed to the fact that the diminishing of shearing extent exerted on materials and breakup extent of EG particles during melt mixing led to the increment of size and aspect ratio of EG particles.(5) The resistivity-temperature characteristics of the composites depended on preparation methods and structure of materials. The Ps of the composites with Rw= 1.5 and , fw. c prepared via SI and optimized DMM depended all on temperature (T). When T approached to melt point of PP (Tm 170 ), the composites exhibited the PTC effect and after exceeding Tm, the NTC effect The PTC intensity increased with decreased (fw -fw. c), the maximum PTC intensity of the Si-composites were lower than that of the optimized DMM-composites. This could be attributed to the fact that the size and aspect ratio of EG particles in the former were larger than those in the latter; the formed conducting path network in the former was more difficult to be broken than that in the latter as the crystalline phase melted and the volume expanded.