Study On Polymer-based PTC Conductive Composites Prepared By High-speed Mixing Method

Positive temperature coefficient (PTC) effect of polymer-based conductivecomposites is the nonlinear response to temperature change of its resistivity, i.e.,when temperature rising to the vicinity of the melting point of polymer matrix, theresistivity increases rapidly and jumps by102-108order of magnitude. This smartswitch behavior gives it important applications in the field of heating materials,overheating or overcurrent protection devices and temperature sensors etc.. However,problems of big resistivity at room temperature, inferior thermal stability, shortservice life and NTC effect hampered its application, and how to further improvePTC intensity, and weaken or even eliminate NTC effect is also important.Starting from the viewpoint of reducing the room-temperature resistivity,high-speed mechanical mixing method was introduced to prepare polymer-basedPTC conductive composites. How the method contributing to reducing thepercolation threshold and dosage of conductive filler was firstly studiedsystematically. Concentrated nitric acid oxidation modification, usage of fibrousfiller, and double-base composites were used to improve the PTC performance.Particulate carbon black (CB) and high-aspect-ratio carbon fiber (CF) and graphitefiber (GF) were chosen as electrically conductive fillers, as well as the mostcommonly used high-density polyethylene (HDPE) and super-environmental-resistance polyvinylidene fluoride (PVDF) were chosen as polymer matrix.Percolation behavior of room-temperature resistivity and PTC effect wereinvestigated of two types of binary composites of CB/HDPE, CB/PVDF, threemethods modified-CB/PVDF and CF/PVDF, GF/PVDF and two types of ternarycomplex of double-matrix CB/PVDF/HDPE and double-packing GF/CB/PVDF, respectively. The main contents and innovations were as follows:1) A simple food-mixer with high-speed mechanical mixing function wasintroduced to mix filler and matrix raw material. The percolation threshold ofCB/HDPE and CB/PVDF were2wt.%and3wt.%respectively, which were muchlower than those prepared by traditional melt blending and some solution methods.The percolation threshold of0.3wt.%, got in fibrous filler CF or GF filledPVDF-based composites, was also more significantly decreased compared with thatprepared by melt blending, and that in granular CB filled CB/PVDF composites.Consequently, high-speed mechanical mixing method was not only easy to operate,mixing-efficient, economic and energy saving, but also effective in reducing thepercolation threshold, which was helpful to lower room-temperature resistivity andreduce the dosage of filler.2) Results of filler modifying and thermal cycling treatment showed that,thermal cycle treatment could effectively reduce room-temperature resistivity andmake composites performance more stable, which was of similar effect with the twocoupling agent of (3-Aminopropyl) triethoxysilane (AA-75) and Titaniumdiisopropoxide bis(acetylacetonate)(A-151) modifying CB. The PTC intensity ofconcentrated nitric acid strongly oxidized CB filled PVDF composites nearlydoubled that of the unmodified, and the NTC effect was almost eliminated.3) CF/PVDF and GF/PVDF composites with0.5wt.%filler loading were ofhigh PTC intensity of5.255and4.466magnitude respectively, which were biggerthan that of CB/PVDF composites. Moreover,0.5wt.%CF/PVDF composites wasalmost of no NTC effect. Thermal cycle test results showed that GF/PVDF had agood thermal stability.4) A novel PTC effect, which exhibited a lower transition temperature than themelting point of polymer matrix, was firstly discovered in CF/PVDF and GF/PVDFcomposites with a percolation threshold filler loading (0.3.wt%) prepared bymechanical mixing, The onset transition temperature of CF/PVDF and GF/PVDFcomposites were respectively60oC and70°C, which were much different from those of traditional PTC effect. In addition, its NTC effect was weak too. We infered thatthis was originated from the newly formed weak conductive network throughout thecomposites being easier to be destroied by the thermal diffusion movement ofconductive filler in heating process.5) When the two matrix volume ratio in CB/PVDF/HDPE composites was closeto1:1, the conductive paths formed by the CB alternatively located in HDPE phaseand therefore conductive CB-HDPE around PVDF phase collaboratively constituteddouble percolation network throughout the composites, which most effectivelyformed large number of conductive paths and induced lower resistivity comparedwith that of other composites with different matrix ratios. Furthermore, theCB/PVDF/HDPE composites with matrix volume ratio of1:1had a more excellentPTC performance than that of single based CB/HDPE and CB/PVDF, as well asweakened NTC effect, which was due to the selected location of CB in the twomatrix. In addition, the room temperature resistivity of CB/PVDF/HDPE had strongthermal stability and excellent repeatability.6) Adding CB to GF/PVDF got CB agglomeration on the surface of GF whichincreased the overlap contact area of adjacent GF to effectively formed perfectconductive paths, making the composites more conductive.0.1wt.%CB and0.4wt.%GF filled PVDF composites could effectively eliminate its NTC effect. Adding0.2wt.%CB to0.3wt.%(percolation threshold) GF/PVDF composites increased PTCcritical transition temperature of GF/PVDF, firstly reporting a method to effectivelyregulate the PTC switching temperature by controlling the addition of CB to thecomposites.