Study On PTC Effect In Polymer-Based Composites

Conductive polymer composites not only have the electrical conductivity of inorganic fillers and flexibility of polymers,but also have some new properties that a single phase doesn't possess.Certain conductive polymer composites have been paid more attention as PTC material because of the advantages of low cost,excellent formability, flexibility and lightweight over the conventional inorganic ceramic PTC materials.The PTC properties of the composites,which consist of polymer matrix,such as PVDF,PP and HDPE,and organic fillers with different shape and properties were studied.The following are the contents in detail.(1)Effects of surface modification of CB by employing titanate coupling agent(NDZ-102),silane coupling agent(KH550)and dilute nitric acid based CB-PVDF/PP nanocomposites were investigated,and the mechanisms of the modification were also analyzed.The results are discussed in terms of the dispersion behaviors of CB and its relational electrical and dielectric properties near the percolation threshold of the composite.Results suggest that the coupling agent can act as a bridge to link CB and.polymers tightly together.The interaction is more remarkable for NDZ-102 than for KH550 because of the flexile single interface layer of NDZ-102 between CB and polymer matrix.It is essentially important for composites to have significant dielectric constant and great PTC intensity as well as good stability.(2)PTC composites manufactured by mixing CB and various polymers,HDPE and PP,were investigated with regard to the PTC behavior and the correlation between the conductivity and the thermal behavior of the composite system.The percolation threshold of CB/PP composite is lower than the value of CB/HDPE composite.The PTC intensity of CB filled HDPE is higher than that of PP might due to the higher crystallinity of HDPE(67.55%)than PP(47.59%).(3)The mechanism of the special conductive network formation in the binary-polymer matrix composites were investigated through the system(CB-HDPE)/PP composites.It is confirmed in the system that the percolation threshold is much lower than those of the individual polymers,and the lowest vales and the highest PTC intensity all appear at 1/1 volume ratio of two polymers.The novel low percolation threshold can be attributed to the selective localization of CB in one polymer phase and the double-percolation effect of both polymer co-continuous phases when the volume ratio of two polymers is very close to 1/1.The cause of remarkable PTC effect can be identified as the double-percolation effect and the consecutive volume expansion of both continuous polymers.It is worth mentioning that the binary-polymer composites with ultra-low percolation threshold,enhanced PTC effect,as well as the significant dielectric permittivity are promising a potential application.(4)Addition of CF to CB/HDPE and CB/PP composites,respectively, the effects of the secondary inorganic filler on the network structure, electrical and thermal properties have been examined and correlated to establish structure-property relationship.Results confirm that the conductivity of CB/HDPE and CB/PP composites can be improved upon addition of a reasonably small amount of CF at the same filler concentration due to bridging of CB particles by CF.The better PTC effect,especially for(CB-CF)/PP composites is achieved than for a single CB blends,and the PTC effect is dependent on the CF content in the fillers.