(373c) Conductive ABS/PC Engineering Plastics Prepared By PC/MWCNT Masterbatch Approach

Carbon-filler-filled immiscible polymer blends have been investigated for more than two decades. High performance conductive materials can be prepared when the conductive carbon filler is selectively localized in one polymer phase or at the interface of a co-continuous structure, or in the continuous phase of a sea-island structure. When the filler is confined in the dispersed phase, the composite is usually non-conductive unless the size of the dispersed phase is very small and their number is large to allow tunneling effect to happen. In general, the localization of the filler depends on many factors such as the affinity of filler to each polymer, blend morphology, mixing sequence, processing parameters, viscosity of each polymer, etc.

Among the various conductive carbon fillers, carbon nanotubes (CNTs) have attracted the attention of many researchers due to its excellent properties such as high respect ratio, high electrical conductivity and low percolation threshold. In this work, conductive ABS/PC engineering plastics are successfully prepared with relatively high rubber content of ABS by pre-compounding MWCNTs into the thermodynamically favored PC phase. A significant number of MWCNTs have migrated to the continuous ABS phase.

All the composites were prepared by melt mixing in a torque rheometer at 250 °C. The rotation rate was 60 rpm and the mixing time was 5 min. The ABS content is 70%. PC/MWCNT masterbatchs containing 8% MWCNTs were used. Three difference PCs (with molecule weights of 25000, 19000 and 15000) and three ABSs (with rubber contents of 20%, 10% and 5%, denoted as ABS-20, ABS-10 and ABS-5) were used.

The electrical resistivities of the samples show that the molecular weight of PC has no much effect, but the rubber content of ABS has dramatic effect on the electrical resistivity of the composite. The low rubber content ABS-5 gives a very high electrical resistivity (Log value=15~16), while the other two ABSs having relatively high rubber contents (ABS-10 and ABS-20) result in much lower electrical resistivities (Log value=7~5 and 4~3) which means a conductive ABS/PC engineering plastic with relatively high rubber content of ABS were successfully prepared by PC/MWCNT masterbatch approach.

FESEM observations of the samples indicate that all the three samples have sea-island structure with ABS as the continuous phase and PC as the dispersed phase, but the localizations of MWCNTs are different. In ABS-5, almost all the PC holes are filled with MWCNT clusters. While many MWCNTs are found in the continuous ABS phase for the other two kinds of ABS, indicating the migration of a significant number of MWCNTs from the thermodynamically favored PC phase to the continuous ABS phase which correspond with the result of the electrical resistivities. TEM photos confirm these results.

The different electrical properties and localization of MWCNTs with the different rubber content ABS indicate that the rubber particles must play an important role in the MWCNT migration. When PC/MWCNT precompound is melt-mixed with PB, many MWCNTs migrate to the PB phase, indicating that PB has better affinity to MWCNTs than PC presumably due to the π-π interaction between the electron-rich MWCNTs and the electron-rich double bonds of PB chains and the flexible nature of PB chains.

The mechanism for the MWCNT migration from the thermodynamically favored PC phase to the unfavored ABS phase at 5 min of mixing can be analyzed through the morphology evlution. Because PC is the minor component and PC/MWCNT masterbatch with a high MWCNT concentration (8%) is used, many MWCNTs may have at least some part or one “end” out of the PC phase during the breakup of the PC cylinders. These MWCNT “ends” not only can have contact with SAN matrix, but also have a great chance to contact with the elongated rubber particles. As a result, those MWCNTs containing these “ends” will be dragged out of the PC phase to the ABS phase by the surrounding rubber particles as MWCNTs have better affinity to PB than to PC. Because MWCNT migration is strongly related to the probability of contact between MWCNTs and the rubber particles, MWCNT localization is different when different rubber content ABS is used.

In conclusion, conductive ABS/PC engineering plastics can be prepared by PC/MWCNT masterbatch approach using high rubber content of ABS with the benefit of the migration of carbon nanotubes. This surprising migration is explained by a combination of the morphology evolution, high rubber content and higher affinity of MWCNTs to PB than to PC.

Acknowledgements

We thank Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University for kindly providing MWCNTs. This work was supported by the National Natural Science Foundation of China (No. 50973053), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20090002110072) and Mitsubishi Engineering-Plastics Corporation (Japan).