Conductive Property Of Carbon Black With Different Microstructure Filled Polypropylene And Its Application In Resistance Welding

The conductive property of polymer-based composites depends on the conductivity network. The type, microstructure, dispersion of conductive filler and other factors have an important effect on the conductivity network. Carbon black (CB) is most widely used, because it not only has varieties types and microstructure, but also easily compounded with polymer, and the conductivity of products is lasting stability. In this thesis, the effect of carbon black microstructure on the property of conductive composites was investigated, and the conductive composites in the field of resistance welding application were explored.Firstly, three kinds of carbon black with different microstructure which had similar average particle size filling polypropylene (PP) matrix were prepared. The conductive percolation and rehological percolation of the three composite systems were investigated. Results showed that low surface area carbon black CB-N660, the middle surface area carbon black CB-K300and high surface area carbon black CB-K600filled polypropylene composites all appeared conductive percolation phenomena and rheological percolation, but the required carbon black content was reduced with the surface area increasing. This was due to the particle density of carbon black in the polypropylene matrix, and the greater surface area of carbon black, the easier to form carbon black network. The carbon black network exhibited conductive percolation in response to the electric field and rheological percolation in response to the mechanical stess field, so the percolation thresholds were close.Secondly, volume resistivity-temperature characteristic of the three composite systems was investigated. Results showed that composites filled with high surface area carbon black CB-K600had a special resistivity-temperature characteristic,"NTC-PTC-NTC" characteristic. In addition, the PTC effect of CB-K600/PP composite was weak, and the electrical heating temperature could be higher than the melting temperature of PP matrix, so it could be applied in resistance welding field to join thermoplastic polymers and its composites. Finally, heating element made of polyproylene filled with high surface area carbon black CB-K600was prepared. Taken the form of a simple overlap welding, continuous glass fiber reinforced polypropylene (CGF-PP) composite specimens were resistance welded, and the properties of heating element, welding voltage, welding time, welding pressure and other process parameters on the welding quality were discussed in detail. The welding coefficient of CGF-PP specimens were between0.55-0.70.