PTC/NTC Effects Of Composite Conductive Polymeric Materials

The dispersion of conductive filler, such as carbon black and carbon fiber as wellas metal powder, into an insulative polymer matrix can yield a conductive polymercomposite. among which, the positive temperature coefficient (PTC) materials filledwith carbon fillers are very often concerned because they are widely used asself-regulating heaters, switching materials and circuit protection devices, etc. ThePTC effect refers to a large resistivity jump of the composite as a result of thebreakage of conductive pathways due to the polymer thermal expansion when it isheated to melting temperature (Tm) of the polymer. However, above Tm, a negativetemperature coefficient (NTC) phenomenon appears because of the rearrangement ofcarbon fillers. NTC effect refers to the decrease of the resistivity with increasingtemperature. A desired PTC performance includes a low room temperature resistivity,high PTC intensity, low NTC intensity, high electrical reproducibility and high PTCtransition temperature, together with low conductive filler content. The percolationthreshold achieved is related to the filler types. If CB serves as a conductive particle,aconsiderable content is needed to achieve the percolation threshold, which probablyresults in many difficulties in preparing polymer/CB composites with simple mixingtechniques. However, when CF is used as fillers instead of CB, a relatively smallamount is required. Usually, a composite filled with a fiber has a lower percolationthreshold than one filled with spherical conductive particle. Fillers with high aspectratio may also increase the tendency to form continuous networks in polymer matrices.In general, these PTC materials show poor reproducibility of resistivity duringa longperiod of time or when undergoing thermal cycles, which is due to the varyingdispersion of carbon fillers in the composites. The presence of NTC effect and theinstability of electrical conductivity have an adverse influence on the application ofPTC materials. Therefore, it is important that a strategy should be found to eliminatethe NTC phenomenon and improve the stability of conductivity.Increasing the PTC intensity of composites materials to eliminate NTCphenomenon by γ-ray and electron beam irradiation crosslinking method have beenstudied by many researchers , but little interest has been focused on surface treatmentfor the conductivity fillers to eliminate NTC phenomenon, there is no report that themethod of eliminating NTC phenomenon using Carbon Fiber oxidized by KMnO4 andthe double-PTC effect in the composites of CF/UHMWPE/EVA.In the first work, the effect of the interaction between polymers and fillers on theelectrical property of the PTC is composites studied by surface treating theconductivity fillers. After CB,CF is treated by coupling agent, we found it is a validmethod for improving the PTC intensity of CB/EVA composite materials. To treatedCB can be increasing the composites resistivity and PTC intensity and the diffusionbehavior of CB particles in the matrix and PTC reproducibility by coupling agent. Theroom-temperature resistivity decline obviously(10?) and PTC intensity stabilityimproved for the composite materials which filled with CF treated by coupling agent.Althouth this is not eliminate NTC phenomenon effectively, but improve well, andPTC intensity reduced slightly. Titanate coupling agent treated CB,CF filled withcomposite materials PTC effect index is much perfected than Silane coupling agent.The interaction of philic-inorganic function group in the coupling agent molecularwith CF is infirmed than CB, lead to CF is poor than CB treated by coupling agent.CF was chemically oxidized by KMnO4 and the EVA/CF composites wereprepared by conventional melt-mixing technique. Many researchers used SEMobservation or tensile stress-strain, oscillatory shear and dynamic mechanicalmeasurements to examine the polymer-filler interactions in the non-crosslinkedpolymer-filler compounds. In our work, we attempted to adopt the solvent extractionmethod for evaluation of the polymer-filler interactions.It is found that the chemicalinteraction between EVA and CF in the oxidized CF-filled composite was enhanceddue to many gel structures is formed. Although the composite with oxidized CFshows a slightly higher resistivity than the composite with original CF at the samefiller concentration at the room temperature, the enhanced interaction of EVA with CFin the oxidized CF-filled composite, due to which conductive network structure wasinvariably stabilized even if during a long time or undergoing several thermal cycles,effectively eliminate NTC phenomenon and improve the electrical reproducibility.The second part of our studies is to eliminate NTC phenomenon of thecomposites by filled the high viscosity polymer. We found that the change ofUHMWPE/EVA weight ratio in the CF/UHMWPE/EVA composites play animportant part in the PTC/NTC effect of the composites. When the UHMWPE/EVAweight ratio is (1/8-1/4), NTC effect improve obviously with the increasing contentsof UHMWPE. We also found an phenomena in our studies, for CF /UHMWPE/EVAcomposites , when the UHMWPE/EVA weight ratio is (1/2-2/1), the double-PTCeffect appeared along with the elimination of the NTC phenomenon due to there aretwo types of conductive pathways. The first type is the conductive paths in the EVAmatrix that are not in any way connected to the CF located at the interface betweenthe EVA matrix and the UHMWPE particles. We refer to this type of conductivepaths as type M (matrix). The second type is the conductive paths in the EVA matrixthat are connected to the CF located at the interface between the EVA matrix and theUHMWPE particles. We refer to this type of conductive paths as type I +M (interface+ matrix). When the content of UHMWPE increased continuesly, as a result of highcontent UHMWPE, when the temperature is at the Tm of UHMWPE, appear PTCeffect.But we found that in these systems there are some problems such as the highpercolation threshold, the poor PTC intensity and the high resistivity at roomtemperature. In the third part, we try to improve the PTC performance of thecomposites by the compound of CF and PVDF which has higher dielectric constant.Good results was obtained: the percolation threshold decreases obviously (8%wt);thePTC intensity enhances(8.16) and the room temperature resistivity falls down(9?).After the CF/PVDF composite was quenched, due to the insufficient crystallization,the volume expansion of composite matrix was restricted. As a result, the PTCtransition temperature goes down, the PTC intensity becomes weak and the PTCtransition range broadens. Besides, the heating rate can influence the PTC effect of thecomposites. It is seen that the higher the heating rate, the higher the startingtemperature of the PTC effects. This is due to the fact that when temperature risingrate increases, the PTC intensity increases since particles are of less agglomeration atthe same temperature. The phenomenon shows that the agglomeration of particlesexists not only in NTC process but also over the entire temperature rising range atdifferent degree. The resistivity–temperature curves shows temperature dependence ofresistivity of the blend under heating and cooling processes, the rapid increase andrapid decrease in electrical conductivity happen at the different temperature, and havemuch diversities.However, the temperature area of the drastic resistivity increase andthe drastic resistivity decrease is as much as the crystalline melting range of thecomposites.