The Construction And PTC Properties Of PVDF/CB Conductive Composites

Positive Temperature Coefficient(PTC)material refers to a kind of material which is responsive to temperature.Its resistivity increases with increasing temperature and rapidly increases by 3-8 orders of magnitude near the characteristic transition point temperature of the material.In practical applications,PTC materials not only need high PTC strength,but also have low percolation value and high repeatability of their resistance with temperature.Polyvinylidene fluoride(PVDF)is an ideal matrix for the preparation of polymer-based PTC materials due to its high melting temperature,high mechanical strength and good chemical resistance.However,the percolation value of PVDF-based PTC materials is high,and the PTC strength is low.In this paper,PVDF is used as matrix,and the PVDF-based conductive composites were designed from the perspective of the selection of conductive filler,construction of segregated and segregated-double percolation structure.The percolation behavior and PTC properties of the composites were studied.The main contents include the following aspects:(1)The PVDF/CB composites with PVDF as the matrix,carbon black with different specific surface areas(VXC-72 and Acetylene CB)as conductive fillers were prepared by the melt blending and compression molding.The effect of specific surface area of carbon black on the percolation behavior and PTC performance of PVDF-based conductive composites was investigated.The results show that increasing the specific surface area of conductive carbon black is beneficial to reduce the percolation value and improve the PTC performance of the PVDF-based conductive composites.The effect of carbon black content on the PTC strength in composites was further analyzed,and it is found that the maximum PTC strength can be obtained when the content of carbon black is slightly higher than the percolation value.According to the above research,VXC-72 with large specific surface area was determined as the raw material for the follow-up work in this paper.Morphological structure,rheological properties and mechanical properties of PVDF/VXC-72 conductive composites were studied.(2)Composites with significantly decreased electrical percolation threshold were obtained by constructing segregated carbon black conductive network in PVDF.Firstly,the PVDF particles and carbon black particles were mixed by mechanical blending,and the carbon black particles were distributed on the surface of the PVDF particles to prepare PVDF/CB mixture.Then,the segregated structure PVDF/CB conductive composites with low percolation value were prepared by hot pressing process.The effects of particle size of PVDF on percolation behavior,thermal properties and PTC properties of the composites were studied.The results show that increasing the particle size of PVDF is beneficial to obtain lower percolation value and higher PTC strength.In addition,the crystallization characteristics and microscopic results of the prepared materials were studied.The results show that due to the high crystallinity of large size PVDF particles,and conductive carbon black only distributes in the continuous amorphous region,so the content of carbon black needed to form continuous conductive network is low and the percolation value is low.Moreover,the volume expansion caused by melting of large particle size PVDF is larger than that caused by small particle size,and the destructiveness to the conductive network is large.Compared with the small particle size PVDF matrix conductive composites,it has higher PTC strength.(3)In order to further reduce percolation value and increase PTC strength,based on the difference between the melting point of poly(vinylidene fluoride)(PVDF)and linear low density polyethylene(LLDPE),conductive PVDF/LLDPE/CB composites with segregated and double-percolated structure were prepared by simply melted mixing.Firstly,LLDPE was used as the percolating phase and was blended with CB to get LLDPE/CB masterbatch.Then,LLDPE/CB masterbatch and PVDF were melt blended at135?,which is higher than the melting temperature of LLDPE and lower than that of PVDF.The surface of PVDF particles were wrapped by LLDPE/CB.Thus prepared PVDF/LLDPE/CB composites have low percolation threshold(2.299 wt%)and obvious positive-temperature-coefficient(PTC)effect of resistivity.A fast transition from conductor to insulator was observed when the temperature was close to the melting point of LLDPE matrix,the resistivity went up by four orders of magnitude,and the composites keep good reproducibility after heating-cooling cycles.The PVDF/LLDPE/CB composites with segregated and double-percolated structure can be used as heat sensitive resistor,and have the great potential application in temperature overload protection.