Studies on the comprehensive performance of graphite and additives filled high density polyethylene composites

The comprehensive performance of graphite and additives filled high‐density polyethylene (HDPE) composites is studied in this article. Four graphites with different particle diameters are used as conductive fillers in HDPE/graphite. Plasticizer, nucleator, and certain particle diameter graphite are employed to prepare HDPE composite. The behavior of crystallization and the distribution of graphite are also studied by means of SEM. An orthogonal design experiment is taken to optimize the content of the filler. The experimental results indicate that the positive temperature coefficient (PTC) effect is related to the particle diameter of graphite. And the bending strength of HDPE/graphite composite with the plasticizer and nucleator is two times less than that of HDPE‐graphite blends. Meanwhile, the high PTC intensity (the ratio of peak resistivity to room temperature resistivity) is also preserved. An excellent comprehensive performance conductive composite is prepared. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

多壁碳纳米管/高密度聚乙烯复合材料的导电行为研究

采用超声波分散溶液混合法,制备出导电性能优良的多壁碳纳米管(MWNTs)/高密度聚乙烯(HDPE)导电复合材料。研究了不同含量及长径比的MWNTs对HDPE导电性能的影响。结果表明:MWNTs可以显著提高复合材料的导电性,其体积电阻率由1017Ω.m降至107Ω.m;长径比较小的MWNTs分散性较好,并能显著提高材料的PTC(正温度系数效应)强度,当w(MWNTs-60100)=7%(相对于材料总质量而言)时,材料的PTC强度达到2.8。采用差示扫描量热(DSC)法分析了复合材料的结晶行为,证明MWNTs可以成为HDPE的成核剂,并能提高HDPE的成核速率,使晶粒尺寸分布变窄。     

Effect of the carbon black structure on the stability and efficiency of the conductive network in polyethylene composites

Composites of high‐density polyethylene (HDPE) with different kinds of carbon black (CB) were prepared through melt blending. The influence of the CB structure on the stability and efficiency of the conductive network in HDPE/CB composites were mainly investigated. Scanning electron microscopy was used to observe the morphology of the CB primary aggregates. The relationship between the temperature‐resistivity behaviors of the composites and the crystallization behaviors of the matrix were also investigated. High‐structure CB built an effective conductive network at a low filler content compared to the low‐structure one because of its branched morphology. Therefore, the composite containing high‐structure CB revealed a lower percolation threshold. The composite containing low‐structure CB obtained a stronger positive temperature coefficient (PTC) intensity because the cluster network was fragile and easily damaged during matrix melting. The reproducibility of the results of PTC effect of the composite containing high‐structure CB was better than that of the composite containing a low‐structure one. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

HDPE/CB复合体系PTC行为的工艺研究

采用高结构导电炭黑(CB)Vxc—72与半晶聚合物高密度聚乙烯(HDPE)进行熔融共混，制备复合型导电高分子材料。研究了该复合体系中偶联处理、混炼时间、辐射及退火处理等工艺因素对其PTC(电阻—温度效应)性能的影响。结果表明，当辐照剂量为140—160kGy、炭黑非均匀分散且进行退火处理时复合体系具有最佳的PTC性能，但混炼时间过长、偶联处理均会使复合体系PTC强度降低。     

Effect of nanoclay on positive temperature coefficient to resistivity characteristics of high density polyethylene/silver‐coated glass bead composites

Positive temperature coefficient to resistivity characteristics of high density polyethylene (HDPE)/silver (Ag)‐coated glass bead (45 wt%) composites, without and with nanoclay, has been investigated with reference to HDPE/carbon black (CB) (10 wt%) composites. Plot of resistivity versus temperature of HDPE/CB (10 wt%) composites showed a sudden rise in resistivity (PTC trip) at ≈128°C, close to the melting temperature (T_m) of HDPE. However, for HDPE/Ag coated glass bead (45 wt%) composites, the PTC trip temperature (≈88°C) appeared well below the T_m of HDPE. Addition of 1 phr clay in the composites resulted in an increase in PTC trip temperature of HDPE/Ag‐coated glass bead (45 wt%) composites, whereas no significant effect of clay on PTC trip temperature was evident in HDPE/CB/clay composites. We proposed that the PTC trip temperature in HDPE/Ag‐coated glass bead composites was governed by the difference in coefficient of thermal expansion of HDPE and Ag‐coated glass beads. The room temperature resistivity and PTC trip temperature of HDPE/Ag‐coated glass bead (45 wt%) composites were found to be very stable on thermal cycling. Dynamic mechanical analyzer results showed higher storage modulus of HDPE/Ag‐coated glass bead (45 wt%) composites compared with the HDPE/CB (10 wt%) composites. Thermal stability of HDPE/Ag‐coated glass bead (45 wt%) composites was also improved compared with that of HDPE/CB (10 wt%) composites. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Factors influencing the resistivity–temperature behavior of carbon black filled isotactic polypropylene/high density polyethylene composites

The relationship between morphology and resistivity–temperature behavior of carbon black (CB) filled isotactic polypropylene/high density polyethylene (iPP/HDPE) composites was investigated. The positive temperature coefficient intensity for all composites studied in this paper was lower than one and the negative temperature coefficient (NTC) effect was obvious. The factors influencing resistivity–temperature behavior include the CB contents, types of the polymer matrices and their composition, which determine the phase morphology and thus the conductive network. The types of iPP and HDPE influenced the NTC effect, while the morphology of the composites mainly influenced the initial volume resistivity of the composites.     

Economical conductive graphite-filled polymer composites via adjustable segregated structures: Construction,low percolation threshold,and positive temperature coefficient effect

The economical graphite-filled thermoplastic urethane/ultra-high molecular weight polyethylene (TPU/UHMWPE) composites with the segregated structure were constructed by the combination of mechanical crushing and melt blending method. The low percolation threshold of 1.89 wt% graphite in the adjustable segregated composites was obtained and high electrical conductivity was about 10⁻¹ S m⁻¹ at 10 wt% graphite loadings owing to the formation of three-dimensional conductive networks. Moreover, when the graphite loadings were over the percolation threshold, the remarkable positive temperature coefficient (PTC) effect of electrical resistivity for TPU/UHMWPE-Graphite composites were achieved, originating from the combined thermal motion of TPU and UHMWPE. Meanwhile, the outstanding repeatability of PTC effects was obtained after 5-time cycles. Therefore, economical conductive polymer composites were still the promising field in the practical application of PTC materials.     

Surface resistivity and mechanical properties of rotationally molded polyethylene/graphite composites

Antistatic polymers are required to dissipate static charges safely from component surfaces. Our overall objective has been to develop cost‐effective flame‐retarded and antistatic polyethylene compounds suitable for rotomolding. This communication considers the surface resistivity and mechanical properties of rotationally molded linear low‐density polyethylene (LLDPE)/graphite composites containing natural Zimbabwean graphite, expandable graphite, or expanded graphite. Dry blending and melt compounding were employed to obtain antistatic composites at the lowest graphite contents. Dry blending was found to be an effective mixing method for rotomolding antistatic LLDPE/graphite composites, thereby eliminating an expensive compounding step. Dry‐blended Zimbabwean graphite composites showed the lowest surface resistivity at all graphite contents, with a surface resistivity of 10⁵ Ω/square at 10 wt% loading. Although rotomolded powders obtained following the melt compounding of Zimbabwean graphite exhibited higher resistivity values, the variability was much lower. Injection molding resulted in surface resistivity values above 10¹⁴ Ω/square for all compositions used. The rotomolded composites exhibited poor mechanical properties, in contrast to injection‐molded composites. The Halpin‐Tsai model showed good fits to the tensile modulus data for injection‐molded Zimbabwean and expandable graphite. J. VINYL ADDIT. TECHNOL., 19:258–270, 2013. © 2013 Society of Plastics Engineers     

Electrical properties and morphology of carbon black‐filled HDPE/EVA composites

The sensitive effect of weight ratio of the high‐density polyethylene (HDPE)/ethylene‐vinylacetate copolymer (EVA) on the electrical properties of HDPE/EVA/carbon black (CB) composites was investigated. With the EVA content increasing from 0 wt % to 100 wt %, an obvious change of positive temperature coefficient (PTC) curve was observed, and a U‐shaped insulator‐conductor‐insulator transition in HDPE/EVA/CB composites with a CB concentration nearby the percolation threshold was found. The selective location of CB particles in HDPE/EVA blend was analyzed by means of theoretical method and scanning electron micrograph (SEM) in order to explain the U‐shaped insulator‐conductor‐insulator transition, a phenomenon different from double percolation in this composite. The first significant change of the resistivity, an insulator‐conductor transition, occurred when the conductive networks diffused into the whole matrix due to the forming of the conductive networks and the continuous EVA phase. The second time significant change of the resistivity, a conductor‐insulator transition, appeared when the amorphous phase is too large for CB particles to form the conductive networks throughout the whole matrix. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of coefficient of thermal expansion on positive temperature coefficient of resistivity behavior of HDPE–Cu composites

Positive temperature coefficient of resistivity (PTCR) characteristics of (high density polyethylene) HDPE–Cu composites has been investigated with reference to the conventional HDPE–CB (carbon black) composites. Plot of resistivity against temperature of HDPE–CB composites showed a sudden rise in resistivity (PTC trip) at 127°C, close to the melting temperature of HDPE. However, the PTC trip temperature (98°C) for HDPE–Cu composites was appeared well below the melting temperature of HDPE. Addition of 1 phr nanoclay in the composites resulted in an increase in PTC trip temperature of HDPE–Cu composites, whereas no significant effect of nanoclay on PTC trip temperature was evident in case of HDPE–CB–clay composites. We proposed that the PTC trip temperature in HDPE–Cu composites was governed by the difference in coefficient of thermal expansion (CTE) of HDPE and Cu. The room temperature resistivity and PTC trip temperature of HDPE–Cu composites were very much stable upon thermal cycling. DMA results showed higher storage modulus of HDPE–Cu composites than the HDPE–CB composites. Thermal stability of HDPE–Cu composites was also improved compared to that of HDPE–CB composites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Thermally induced double‐positive temperature coefficients of electrical resistivity in combined conductive filler–doped polymer composites

Polymer composites of low‐density polyethylene/polypropylene/graphite/vanadium dioxide (LDPE_0.8/PP_0.2/Gr_0.4/VO₂) are prepared by classical melt‐mixing technology and show a notable double positive temperature coefficient of electric resistivity (PTC), which originates from the combined effect of highly conductive Gr and VO₂ with a thermal phase transition. When the weight ratio of VO₂ is 8 wt %, the positive temperature coefficient intensity (PTCI) for the composites reaches 3.85 orders of magnitude. The model system demonstrates the reason for the improvement in the PTC performance of the polymer composites by analyzing the construction of the conductive networks. Therefore, the addition of phase‐transition compounds may be a promising path to improving PTC materials. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44876.     

Co‐crystallization in ternary polyethylene blends: tie crystal formation and mechanical properties improvement

Understanding the co‐crystallization behavior of ternary polyethylene (PE) blends is a challenging task. Herein, in addition to co‐crystallization behavior, the rheological and mechanical properties of melt compounded high density polyethylene (HDPE)/low density polyethylene (LDPE)/Zeigler ? Natta linear low density polyethylene (ZN‐LLDPE) blends have been studied in detail. The HDPE content of the blends was kept constant at 40 wt% and the LDPE/ZN‐LLDPE ratio was varied from 0.5 to 2. Rheological measurements confirmed the melt miscibility of the entire blends. Study of the crystalline structure of the blends using DSC, wide angle X‐ray scattering, small angle X‐ray scattering and field emission SEM techniques revealed the formation of two distinct co‐crystals in the blends. Fine LDPE/ZN‐LLDPE co‐crystals, named tie crystals, dispersed within the amorphous gallery between the coarse HDPE/ZN‐LLDPE co‐crystals were characterized for the first time in this study. It is shown that the tie crystals strengthen the amorphous gallery and play a major role in the mechanical performance of the blend.© 2016 Society of Chemical Industry     

Performance stabilization of conductive polymer composites

Conductive polymer composites used as candidates for positive temperature coefficient (PTC) materials are faced with performance decay characterized by gradually increased room‐temperature resistivity and decreased PTC intensity. Considering that deterioration of the properties is mainly related to the capability of conductive networks established by conductive fillers to recover from the effect of repeated expansion/contraction in a timely manner, the present work introduces chemical bonding into the filler/matrix interphase. The experimental results indicate that in the composites consisting of conductive carbon black (CB), low‐density polyethylene (LDPE), and ethylene–vinyl acetate copolymer, CB particles can be covalently connected with LDPE through melt grafting of acrylic acid. As a result, the composites are provided with reduced room‐temperature resistivity and significantly increased PTC intensity. Compared with the composites filled with untreated CB, the present composites possess reproducible PTC behavior and demonstrate stable electrothermal output in association with negligible contact resistance at the composites/metallic electrodes contacts. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2438–2445, 2003     

Positive temperature coefficient effect and interaction based on low‐density polyethylene/graphite powder composites

In this article, the positive temperature coefficient (PTC) and interaction based on low‐density polyethylene (LDPE) filled with the loading of graphite (G) powder have been investigated. The dependence of the room temperature resistivity on filler content showed the significant decrease. The PTC behavior enhanced with increasing graphite content but this was not always the case. The maximum PTC effect was observed in LDPE/G composites (G, 45 wt %) with the relatively low room temperature resistivity. The thermal behavior was measured by differential scanning calorimetry (DSC). The structure characteristic for LDPE/G composites was examined by X‐ray diffraction (XRD), field‐emission scanning electron microscopy (SEM), and stress–strain test. The fact was revealed that the slight interaction between LDPE matrix and graphite may lead to change the thermal‐electric properties of the PTC materials. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Resistivity‐temperature behavior and morphology of low density polyethylene/graphite powder/graphene composites

In this work, the positive‐temperature‐coefficient (PTC) effect of resistivity of low density polyethylene/graphite powder (45%) composites (LDPE/GP) in the presence of graphene before and after crosslinked was comparatively investigated by differential scanning calorimetry (DSC), X‐ray diffraction (XRD), scanning electron microscopy, Raman spectrum, and resistivity‐temperature test. The composites showed the repeatability of the PTC effect with heating cycles and a certain improvement in the room temperature resistivity. After crosslinked, the composites presented a higher PTC trip temperature at about 140°C than pure LDPE (T_m = 112°C), and stronger PTC intensity than room temperature resistivity (over 5 orders of magnitude). The results from DSC, XRD, and Raman spectrum indicated that the addition of graphene resulted in the gradual enhancement in the crystallization of LDPE matrix, which was the origin of the improvement of the PTC behavior of the composites. As a result, we could conclude that the additional conducting filler could improve the PTC effect of the conducting composite system. POLYM. COMPOS., 35:1453–1459, 2014. © 2013 Society of Plastics Engineers     

Analysis of electrical,dielectric, and electromagnetic interference shielding behavior of graphite filled high density polyethylene composites

Conducting polymer composites were prepared by hot compression molding of high‐density polyethylene (HDPE) prelocalized with graphite (Gr). The variation of room temperature electrical conductivity, dielectric constant, and Shore‐D hardness as a function of graphite content were studied below and above percolation threshold. The percolation threshold at which insulator‐conductor transition takes place is estimated to be 0.029 volume fraction of graphite. The observed increase in dielectric constant with addition of graphite also found to be percolative in nature. Above percolation threshold, the dielectric constant is found to decrease sharply with increase in frequency. Electromagnetic shielding effectiveness of HDPE/Gr composites was evaluated in the X‐band frequency range. The composites were found to possess both positive and negative temperature coefficient of resistance. Reproducibility of electrical resistivity with temperature was observed only below the processing temperature. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

粘接性树脂在PTC自限温伴热带中应用

研究了以乙炔炭黑为导电粒子,ＨＤＰＥ、ＬＬＤＰＥ及粘接性树脂（接枝高密度聚乙烯（ｇ ＨＤＰＥ））为基体的导电复合物ＰＴＣ行为,电致发热特性及电压循环冲击试验。结果发现,ｇ ＨＤＰＥ／ＣＢ导电复合材料、ＬＬＤＰＥ／ｇ ＨＤＰＥ／ＣＢ复合物的ＰＴＣ复演性及电阻稳定性均优于ＬＬＤＰＥ／ＣＢ及ＨＤＰＥ／ＣＢ导电体系     

High‐density polyethylene/carbon black conductive composites. II. Effect of electron beam irradiation on relationship between resistivity–temperature behavior and volume expansion

A study on the contribution of thermal volume expansion to electrical properties is carried out for high‐density polyethylene (HDPE)/carbon black (CB) composites irradiated by an electron beam. The results show that the volume expansion obviously generates the positive temperature coefficient (PTC) characteristic of resistivity for unirradiated HDPE/CB composites, but the contribution of volume expansion is decreased for crosslinked HDPE in the composites by electron beam irradiation. A higher degree of crosslinking produced by irradiation in the molten state limits the movability of HDPE chains and CB particles so effectively that it decreases the PTC intensity, which is compared with that irradiated at room temperature. It is suggested that the differences in the resistivity–temperature behavior are not explained satisfactorily on only the basis of the thermal volume expansion, and the decreased movability of HDPE chains and CB particles are believed to be the most fatal factors in lowering the PTC effect. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 3117–3122, 2002; DOI 10.1002/app.10050     

Study of the morphology and temperature-resistivity effect of injection-molded iPP/HDPE/CB composites

The relationship between morphology and temperature-resistivity effect of injection-molded isotactic polypropylene/high density polyethylene/carbon black (iPP/HDPE/CB) composites with special orientation structure is investigated in detail. The morphological variation induced by melting, disorientation, crystallization and movement of CB particles is responsible for the change of electrical conductivity of the iPP/HDPE/CB composites during the heating and cooling. The room temperature volume resistivity of the composites reduces markedly after a round of heating and cooling because the network is improved through morphological changes and movement of particles during annealing. The continuity of HDPE/CB phase and the effective concentration of the CB particles in HDPE simultaneously determine the temperature-resistivity effects of the composites. Samples with iPP/HDPE mass ratio of 50/50 achieve a better balance of the two factors, which results in more stable conductive properties varying with temperature.     

Thermal characteristics and temperature profile changes of structurally different polyethylenes with peroxide modifications

Crosslinking and processing characteristics of polyethylenes (PEs) with different molecular architectures, namely high‐density polyethylene (HDPE), linear low‐density polyethylene (LLDPE), and low‐density polyethylene (LDPE), were studied with regard to the effects of peroxide modifications and coolant flow rates. Dicumyl peroxide (DCP) and di‐tert‐butyl peroxide (DTBP) were used as free‐radical inducers for crosslinking the PEs. The characteristics of interest included normalized gel content, real‐time temperature profiles and their cooling rates, exothermic period, crystallinity level, crystallization temperature, and heat distortion temperature. The experiments showed that LDPE exhibited the highest normalized gel content. The real‐time cooling rates, taken from the temperature profiles for all PEs before the crystallization region, were greater than those after the crystallization region. The cooling rate of the PEs increased with the presence of DCP, whereas the crystallization temperature of the PEs was lowered. The HDPE appeared to show the longest exothermic period as compared with those of the LLDPE and LDPE. The exothermic period showed an increase with increasing coolant flow rate, but it was decreased by the use of DCP. As for the effect of peroxide type, the gel content and cooling rate of the PE crosslinked by DCP were higher than those for the PE crosslinked by DTBP. The DTBP was the more effective peroxide for introducing crosslinks and simultaneously maintaining the crystallization behavior of the PE. J. VINYL ADDIT. TECHNOL., 20:80‐90, 2014. © 2014 Society of Plastics Engineers