Study On Toughening Of Electrically Conductive Polypropylene Nanocomposites

At present, adding conductive fillers through melt compounding, in-situ compounding or solution compounding were widely used to enhance electrical conductivity of insulating polymers. However, high loading of conductive fillers is usually needed when the electrical conductivity is required, which not only increases the final cost of the composites, but also often impairs the mechanical properties especially the toughness. Therefore, endowing polymer satisfactory conductivity as well as high toughness is of important scientific significance and practical value.In this thesis, polypropylene (PP) and carbon nanotube (CNT) were chosen as matrix and conductive filler, and polyethylene-octene copolymer (POE), polyethylene-octene copolymer grafted with maleic anhydride (POE-g-MA) and ultrahigh molecular weight polyethylene (UHMWPE) were chosen as toughening agent to prepare PP/CNT/POE, PP/CNT/POE-g-MA and PP/CNT/UHMWPE ternary nanocomposites through melt compounding. The relationship between microstructure and electrical conductivity and notched impact strength was studied. Moreover, properties of PP/CNT/POE,PP/CNT/POE-g-MA ternary nanocomposites were studied by changing compounding sequences. Following is the main work:1. PP/CNT nanocomposites were prepared by melt compounding and electrical conductivity and toughness (notched Izod impact strength) of the nanocomposite were studied. The percolation threshold was observed between 2-3 wt% of CNT and impact strength decreased with increasing the loading of CNTs.2. PP/CNT/POE, PP/CNT/POE-g-MA and PP/CNT/UHMWPE ternary nanocomposites were prepared through two-step compounding sequence, which means PP was firstly compounded with CNT for 10 min and then the PP/CNT nanocomposite was blended with one of the second polymer components POE, POE-g-MA or UHMWPE for 5 min later. Electrical conductivity and toughness (notched Izod impact strength) of the ternary nanocomposites were studied. The insulator-semiconductor transition of the ternary nanocomposites with UHMWPE took place between 2-3 wt% of CNT, the same as the binary nanocomposites. However, the incorporation of 30 wt% POE clearly decreased the conductivity and delayed the transition of the PP/CNT binary nanocomposites. Surprisingly,30 wt% of POE-g-MA severely reduced the conductivity of PP/CNT binary nanocomposites, Their conductivities are rather low even at high CNT loadings of 5 to 10 wt%. The reason why the conductivity of ternary composites was lower than binary composites at the same CNT loading is the selective location of CNTs and the reduced number of CNTs in the continuous PP matrix. POE and POE-g-MA were effective in toughening the PP/CNT nanocomposites. The notched impact strength reached 55.5 kJ/m2 for the nanocomposite with 30 wt% of POE and 47.0 kJ/m2 for the nanocomposite with 30 wt% POE-g-MA. On the contrary, the toughening efficiency of UHMWPE was very limited.3. Two other compounding sequences including one-step method and the other two-step method were adopted to prepare PP/CNT/POE and PP/CNT/POE-g-MA ternary nanocomposites. One-step method means the three components were compounded together for 15min. The other two-step method means PP was blended first with one of the second polymer components for 10min and then the blend was compounded with CNT for 5min. Electrical conductivity and toughness (notched Izod impact strength) of the ternary nanocomposites prepared by different compounding sequences were compared. Compared with one-step method, PP/CNT/POE ternary nanocomposites prepared with two-step method own better conductivity and similar toughness. The conductivity of PP/5% CNT/30% POE ternary nanocomposites prepared by the other two-step method is 2-order lower than the two-step method, whereas the toughness is similar. Compared with the two-step method, the conductivity of PP/5% CNT/30% POE-g-MA ternary nanocomposites prepared by the other two-step method is similar, whereas the toughness was only half.