Reduced percolation thresholds of immiscible conductive blends

The DC conductivity of polymer blends composed of poly(ethylene‐co‐vinyl acetate) (EVA) and high density polyethylene (HDPE), where a conductive carbon black (CB) had been preferentially blended into the HDPE, were investigated to establish the percolation characteristics. The blends exhibited reduced percolation thresholds and enhanced conductivities above that of the individually carbon filled HDPE and EVA. The percolation threshold of the EVA/HDPE/CB composites was between 3.6 and 4.2 wt % carbon black, where the volume resistivity changed by 8 orders of magnitude. This threshold is at a significantly lower carbon content than the individually filled HDPE or EVA. At a carbon black loading of 4.8 wt %, the EVA/HDPE/CB composite exhibits a volume resistivity which is approximately 14 and 11 orders of magnitude lower than the HDPE/CB and EVA/CB systems, respectively, at the same level of incorporated carbon black. The dielectric response of the ternary composites, at a temperature of 23°C and frequency of 1 kHz, exhibited an abrupt increase of ca. 252% at a carbon concentration of 4.8 wt %, suggesting that the percolation threshold is somewhat higher than the range predicted from DC conductivity measurements. Percolating composites with increasing levels of carbon black exhibit significantly greater relative permittivity and dielectric loss factors, with the composite containing 6 wt % of carbon black having a value of ϵ′ ≈ 79 and ϵ″ ≈ 14. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1899–1910, 1999     

Positive‐temperature‐coefficient/negative‐temperature‐coefficient effect of low‐density polyethylene filled with a mixture of carbon black and carbon fiber

Low‐density polyethylene (LDPE) filled with carbon black (CB) and carbon fiber (CF) composites were prepared by a conventional melt‐mixing method. The effects of a mixture of CB and CF on the positive‐temperature‐coefficient (PTC) effect and the negative‐temperature‐coefficient (NTC) effect, as well as the percolation threshold, were examined in detail. A synergy effect between CB and CF occurred, in that continuous conductive pathways formed within the CB/CF‐filled composite. The percolation threshold was moved to a reduced filler content with the addition of CF to an LDPE/CB composite. A model was proposed to explain the difference in the PTC behavior of composites containing CB and CF and composites containing only CB or CF. In addition, the NTC effect was weakened with a mixture of CB and CF, and a relatively small radiation dose was required to eliminate the NTC phenomenon in LDPE/CB/CF composites. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3094–3101, 2003     

Effects of thermal volume expansion on positive temperature coefficient effect for carbon black filled polymer composites

Polymeric positive temperature coefficient (PTC) materials have been prepared by incorporating carbon black (CB) into two different polymer matrices, crystalline high density polyethylene (HDPE) and amorphous polystyrene (PS). The effects of thermal volume expansion on the electrical properties of conductive polymer composites were studied. The volume fraction of conductive particles behaves like a switch from insulator to conductor in the polymeric PTC composite. Our mathematical model and experimental model have proved that the abrupt resistivity increase at PTC transition range and at the percolation curve close to the critical volume fraction for both polymeric PTC composites have the same conductive mechanism. The thermal expansion is one of the key factors responsible for the PTC effect and can be seen by comparing the PTC transition curves from model predictions and experiment. Furthermore, the model predicts PTC curves of CB/PS composite more successfully than it does for the CB/HDPE composite, and the reasons for this are also discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3078–3083, 2007     

炭黑填充聚甲基乙烯基硅氧烷硫化胶的阻温特性与电阻弛豫

研究了炭黑填充硅橡胶硫化胶的热循环以及热处理过程中的导电行为,发现在热循环中阻温关系曲线逐渐向低电阻方向移动,而在恒温下发生电阻弛豫现象;分析了硫化胶的导电机制,讨论了阻温关系发生移动的原因.     

INVESTIGATION OF MICROSTRUCTURE AND CONDUCTIVE MECHANISM OF HIGH DENSITY POLYETHYLENE/CARBON BLACK PARTICLE COMPOSITE BY POSITRON ANNIHILATION LIFETIME SPECTROSCOPY

The microstructure and conductive mechanism of high density polyethylene/carbon black (HDPE/CB) compositewere investigated by positron annihilation lifetime spectroscopy (PALS). The PALS were measured in two series of samples,one with various CB contents in the composites and the other with various γ-irradiation doses in HDPE/CB compositecontaining 20 wt% CB. It was found that CB particles distribute in the amorphous regions, the CB critical content value inHDPE/CB composite is about 16.7 wtO/ and the suitable γ-irradiation dose for improving the conductive behavior ofHDPE/CB composite is about 20 Mrad. T'he result observed for the second set of samples suggests that γ-irradiation causesnot only cross-linking in amorphous regions but also destruction of the partial crystalline structure. Therefore, a suitableirradiation dose, about 20 Mrad, can induce sufficient cross-linking in the amorphous regions without enhancing thedecomposition of crystalline structure, so that the positive temperature coefficient (PTC) effect remains while the negativetemperature coefficient (NTC) effect is suppressed. A new interpretation of the conductive mechanism, which might providea more detailed explanation of the PTC effect and the NTC effect has been proposed.     

Resistivity control in the semiconductive region for carbon-black-filled polymer composites

It is known that the electrical volume resistivity of insulating polymers filled with conductive fillers, such as metal particles and/or carbon black (CB) particles, suddenly decreases at a certain content of the filler. Therefore, it is very difficult to control the resistivity in the semiconductive region for the CB-filled composites. We examined two effects to control the electrical volume resistivity in the semiconductive region for CB-filled polymer composites. One is the effect of fluorination of the CB surface on the percolation behavior using surface-fluorinated CB particles as a filler. The other is the effect of copolymerization of polyethylene (PE) with a vinyl acetate (VA) functional group on the percolation behavior using poly(ethylene-co-VA) (EVA) as a matrix. By immersion heat measurements, it was found that the London dispersive component turned out to be the predominant factor of the surface energy of fluorinated CBs. The London dispersive component of the surface energy significantly decreased, while the polar component slightly increased on increasing the fluorine content. The resistivity of fluorinated a CB-filled low-density PE composite showed that the percolation threshold increased, and the transition from the insulating state to the conductive state became sluggish, on increasing the fluorine content. In the case of using EVA as a matrix, on the other hand, the percolation curve was moderated with the increase in the VA content. Therefore, copolymerization of PE with VA is also suitable for the design of a semiconductive polymer composite as well as for fluorination of the CB surface. The total surface area per unit mass of dispersed CB particles in the EVA matrix estimated from small-angle X-ray scattering decreased with increasing CB content. Further, the decrease in the surface area is moderated with an increase in VA content. It was found that the difference in the percolation curve is due to the difference in the dispersive state of CB particles.     

Relationship between electrical resistivity and particle dispersion state for carbon black filled poly (ethylene-<Emphasis Type="Italic">co</Emphasis>-vinyl acetate)/poly (<Emphasis Type="SmallCaps">L</Emphasis>-lactic acid) blend

It is known that the electrical volume resistivity of insulating polymers filled with conductive fillers suddenly decreases at a certain content of filler. This phenomenon is called percolation. Therefore, it is known that controlling resistivity in the semi-conductive region for carbon black (CB) filled composites is very difficult. When poly (ethylene-co-vinyl acetate) (EVA) is used as a matrix, the percolation curve becomes gradual because CB particles disperse well in EVA. In this study, the relationship between the dispersion state of CB particles and electrical resistivity for EVA/poly (L-lactic acid) (PLLA) filled with CB composite was investigated. The apparent phase separation was seen in the SEM photograph. It was predicted that the CB particles located into the EVA phase in the light of thermodynamical consideration, which was estimated from the wetting coefficient between polymer matrix and CB particles. The total surface area per unit mass of dispersed CB particles in the polymer blend matrix was estimated from small-angle X-ray scattering and the volume resistivity decreased with increasing CB content. The values of the surface area of CB particles in CB filled EVA/PLLA (25/75 wt%) and EVA/PLLA (50/50 wt%) polymer blends showed a value similar to that of the CB filled EVA single polymer matrix. In electrical volume resistivity measurement, moreover, the slopes of percolation curves of EVA/PLLA (25/75 wt%) and EVA/PLLA (50/50 wt%) filled with CB composite are similar to that of EVA single polymer filled with CB composite. As a result, it was found that CB particles selectively locate in the EVA phase, and then the particle forms conductive networks similar to the networks in the case of EVA single polymer used as a matrix.     

Roles of work of adhesion between carbon blacks and thermoplastic polymers on electrical properties of composites

The effect of the work of adhesion between carbon blacks and different thermoplastic polymers on the positive temperature coefficient (PTC) of composites was investigated. Thermoplastic polymers, such as EVA, LDPE, LLDPE, HDPE, and PP, were used with the addition of 30 wt% of carbon blacks. The work of adhesion based on the surface free energy of a composite was studied in the context of two-liquid contact angle measurements using deionized water and diiodomethane. It was observed that the resistivity on PTC behavior was greatly increased near the crystalline melting temperature, due to the thermal expansion of polymeric matrix. It was shown that the PTC intensity defined as the ratio of the maximum resistivity (rho(max)) to the resistivity at room temperature (rho(RT)) had the largest value on CB/HDPE composites. From the experimental results, the decrease in the work of adhesion induced by interactions between carbon black surfaces and polymer chains is an important factor in the fabrication of a PTC composite.     

Effects of the reduction of PAni-coated oxidized multiwall carbon nanotubes on the positive temperature coefficient behaviors of their carbon black/high density polyethylene composites

Polyaniline (PAni)-coated reduced multiwall carbon nanotubes (PRMWNTs) and carbon black (CB)-filled high-density polyethylene (HDPE) composites (PRMWNTs/CB/HDPE) were prepared through a melt mixing method. Oxidized MWNTs (OMWNTs) were prepared by treating pristine multiwall carbon nanotubes (MWNTs) with an acid mixture (HNO₃:H₂SO₄), and PAni-coated OMWNTs (POMWNTs) were synthesized via in-situ polymerization of aniline monomer in the presence of OMWNTs. POMWNTs were further reduced using hydrazine monohydrate to obtain the PRMWNTs. Fourier transform infrared (FT-IR) spectra and thermogravimetric analysis (TGA) confirmed the formation of PRMWNTs. PRMWNTs showed significantly improved thermal stability and electrical conductivity comparing to POMWNTs. The positive temperature coefficient (PTC) behavior of PRMWNTs/CB/HDPE composites revealed enhanced PTC intensity and electrical conductivity at room temperature compared to POMWNTs/CB/HDPE composites. The PRMWNTs-10/CB/HDPE composite showed high peak resistivity (301.99 MΩ-cm) and low room temperature resistivity compared to the POMWNTs/CB/HDPE composite, and thus showed the highest PTC intensity value of 6.693 as well as very excellent cyclic stability.     

PE/CB复合材料的辐照效应

研究了两种炭黑(CB)对PE的影响及PTC功能材料挤出后的特性，发现挤出后粒子和聚合物取向对材料电性能都有较大影响。经γ射线辐照后HDPE／CB功能复合材料稳定性大为提高，初步探讨了辐射对PTC功能材料稳定性的影响。结合辐射交联等方法提高材料的稳定性。用扫描电镜(SEM)观测了一系列PE／CB的形态、CB的分布、链段的分子运动，并结合Fisher的toy model对PE／CB机制做了较系统的解释。     

Structure and mechanical properties of SRP/HDPE/POE (EPR or EPDM) composites

The effects of dicumyl peroxide (DCP) and ethylene-octylene copolymer (POE), ethylene-propylene copolymer (EPR) or ethylene-propylene-diene terpolymer (EPDM) on the structure and properties of scrap rubber powder (SRP)/high-density polyethylene (HDPE) composites were studied. Torque and gel content results show that DCP promotes crosslinking of the SRP/HDPE/POE (EPR or EPDM) composites. The SRP/HDPE/POE (EPR or EPDM) composites containing a small amount of DCP have better mechanical properties than their corresponding composites without DCP. Highest tensile strength and elongation at break are found in the dynamically vulcanized SRP/HDPE/POE composite. Scanning electron microscopy (SEM) results show that good bonding between SRP and matrix is formed in the SRP/HDPE/POE (EPR or EPDM) composites with DCP.     

纤维状填料/HDPE复合体系的导电渗流与流变渗流行为的关系

研究了纤维状导电材料不锈钢纤维(SSF)填充高密度聚乙烯(HDPE)导电复合体系的导电渗流与流变渗流行为之间的关系,并与颗粒状导电颗粒炭黑(CB)/HDPE导电复合体系进行了比较.发现当SSF含量极低(0.3vol%)时,SSF/HDPE体系即发生导电渗流现象,且导电渗流转变区域极窄;而仅当SSF含量达到4.8vol%时,该复合体系才表现出流变渗流现象,这一结果与CB/HDPE体系及纳米级导电纤维填充体系截然不同.此外,通过正温度系数效应的研究发现SSF形成的导电通路稳定性高于CB/HDPE体系.我们认为,SSF/HDPE体系呈现的这些特点均与SSF较大的直径及长径比且其导电通路及流变渗流网络的形成机理不同有关.     

LDPE／CPE／炭黑三相复合导电体系的电学性能

研究了ＬＤＰＥ／ＣＰＥ／炭黑三相复合导电体系的导性能和电流－电压特性。实验结果表明复合体系具有明显的导电渗滤效应，在共混比ＬＤＰＥ／ＣＰＥ＞５０／５０时，导电性能随ＣＰＥ含量增加而显著提高，这主要是由于炭黑粒子在ＬＤＰＥ和ＣＰＥ两组分分中的分布不均匀性所致。     

多壁碳纳米管填充高密度聚乙烯复合材料的导电特性

自20世纪90年代Iijima发现碳纳米管(Carbon nanotubes,CNTs)以来,由于其独特的力学性能、电学性能和极高的纵横比,使CNTs在纳米电子器件、催化剂载体、电极材料、储氢材料等方面的应用取得了引人注目的进展,迄今,CNTs填充高分子复合材料的研究主要集中在力学增强以及光学性能的改进方面,其制备方法主要是溶液法和原位聚合方法,     

Effect of compounding procedure on morphology and crystallization behavior of isotactic polypropylene/high‐density polyethylene/carbon black ternary composites

The effect of compounding procedure on morphology and crystallization behavior of isotactic polypropylene/high‐density polyethylene/carbon black (iPP/HDPE/CB) composite was investigated. iPP/HDPE/CB composites were prepared by four compounding procedures (A: iPP + HDPE + CB; B: iPP/HDPE + CB; C: HDPE/CB + iPP; D: iPP/CB + HDPE). Scanning electron microscopy observation showed that CB particles are mainly distributed in HDPE in all composites, and the phase morphology of composites was obviously affected by a compounding procedure. The size of the HDPE/CB domains in the composites prepared by procedures A and D decreased with the increase in CB content, whereas that of HDPE/CB in the composites prepared by procedures B and C rarely changed with the increase in CB content. The crystallization behaviors of the composites were significantly affected by their phase morphology, which resulted from the variation of compounding procedure. The isothermal crystallization rate of iPP in the composites prepared by procedures A and D was obviously increased, which may originate from the small HDPE/CB droplets dispersed in the iPP phase. The non‐isothermal crystallization curves of composites prepared by procedure D represented two peaks because the iPP component in these composites had the fastest crystallization rate, whereas the curves of composites prepared by other compounding sequences only exhibited one peak. Moreover, the crystallinity of HDPE almost increased by one time with the incorporation of only 1 phr CB because the CB particles selectively located in the HDPE phase, and the crystallinity of HDPE decreased with the further increase of CB content because of the strong restriction of CB on the HDPE chains. Copyright © 2011 John Wiley & Sons, Ltd.     

Enhancing the conductivity of isotactic polypropylene/polyethylene/carbon black composites by oscillatory shear

The influence of oscillatory shear on the conductivity of the isotactic polypropylene/polyethylene/carbon black composites is studied. It is found that the oscillatory shear under high strain amplitude can enhance the conductivity of the ternary composites with a HDPE/CB concentration in the percolation region. This is related to the fact that the high-strain oscillatory shear can improve the continuity of the conductive HDPE/CB phase in the composites. This finding has not been previously reported, and it may be used in industry to improve the conductivity of the ternary conductive composites with a low filler loading.     

Polymeric positive-temperature-coefficient materials: dynamic curing effect

The object of this work was to prepare high-density polyethylene (HDPE)/ethylene–propylene–diene terpolymer (EPDM)/conductive carbon black (CB) composites by dynamic curing and to characterize the positive-temperature-coefficient (PTC) performances of the composites.EPDM and dicumyl peroxide were preblended in a research mill. The roll-milled strands were blended with HDPE and CB in a Haake mixer. The sheet resistivity and morphology of the HDPE/EPDM/CB composites with or without the dynamic curing process were investigated. It was concluded that the dynamically cured blends exhibit better PTC performance than the simple blends without dynamic curing. The effects of shear intensity and dicumyl peroxide content during the dynamic curing process were discussed for the PTC characteristics of the HDPE/EPDM/CB composites.     

单轴压力对高密度聚乙烯/碳黑导电复合材料电阻特性的影响

研究了高密度聚乙烯/炭黑导电复合材料在单轴压力作用下电阻的变化规律.结果表明,在低压力下,复合材料电阻随压力增加而降低;而在较高压力下则随压力增大而升高,分别呈现出所谓的“电阻负压力系数”和“电阻正压力系数”效应.电阻的压力依赖性以及由压缩引起的电阻不可回复性,被认为与外力作用下导电网络的重组与破坏有关.由此提出了导电复合材料单轴压力作用-电阻相互关系的唯象模型.     

Effects of heat treatment on electrical conductivity of HDPE/CB composites

The influence of heat treatment on the electrical conductive behavior of carbon black (CB) filled high density polyethylene (HDPE) composites was investigated. The results showed that the effects of annealing temperature on the resistivity and the PTC intensity of the HDPE/CB composites were significant; the resistivity and the PTC intensity of the composites varied with increasing number of thermal cycles; while the variation became small after the third thermal cycle. Furthermore, the variation of the resistivity was 1.7 times higher than that of the composites without annealing, and the variation of the PTC intensity of the composites was 0.22, which were smaller than those of the specimens without heat treatment. A suitable annealing heat treatment could reduce the resistivity and enhance the PTC intensity of the composites; it was also helpful to improve the stability of the properties of the composites and the repeatability of the PTC effect.     

聚乙烯/聚并苯复合材料室温电阻率变化规律研究

以新型导电材料聚并苯替代碳黑作为导电填料制备聚乙烯/聚并苯复合物.确定了复合物渗流转变区,并解释渗流转变现象的产生机理.从聚并苯含量和热处理过程,对聚乙烯/聚并苯复合物室温电阻率变化规律进行讨论.结果表明,聚并苯质量分数在20%～40%之间是聚乙烯/聚并苯复合物渗流转变区;热处理有利于聚乙烯晶区完善排列,也有利于导电链形成;以聚并苯作为导电填料所制备的复合物具有较高的PTC强度;辐射交联可以提高聚乙烯/聚并苯复合物PTC强度,抑制NTC效应.