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Electrical conductivity and positive temperature coefficient (PTC) behavior of carbon black (CB) filled incompatible polyblends of ethylene‐vinyl acetate copolymer/low density polyethylene (EVA/LDPE) were investigated. In comparison with single polymer systems, more possibilities for tailoring composite performance were brought about with the employment of polymer blends as matrix resins in conductive composites. Based on the concepts of double percolation and two‐step percolation, PTC‐type composites with balanced performance, improved processability, and reproducibility can be made. Thermodynamical and kinetic factors including interfacial energy, melt viscosity, blending ratio, melt mixing time, sequence of blending as well as CB concentration were shown to be closely related to the ultimate properties obtained.

Heat treatment of polymer-based composites is critical for the enhancement of both stability and long-term service life, especially when the materials function under an inconstant temperature environment. The present article discusses the effect of heat-treatment conditions on the electrically conductive properties of carbon black (CB)-filled low-density polyethylene (LDPE) and ethylene-vinyl acetate copolymer (EVA) composites, which are candidates for positive temperature coefficient (PTC) materials. It was found that the dispersion mode of CB particles changes as a function of the matrix morphology. When the composites are irradiated to form crosslinked networks in the matrix for the elimination of negative temperature coefficient (NTC) behavior, some of the produced free radicals are also entrapped for quite a long time after the irradiation treatment. These residual radicals further enhance the interaction between CB and the matrix and further induce the crosslinking of the matrix so that the composites' conductivity changes with time as a result of the continuous variation in the contacts between the conductive fillers. To improve the quality of the conduction paths in the composites, appropriate post-heat treatment should be carried out, which speeds up the formation of the above-mentioned two kinds of crosslinked structures within a limited time. Annealing at 75 o C for more than 10 h is believed to be an effective way. After the treatment, a balanced performance characterized by reduced roomtemperature resistivity and improved PTC intensity was obtained. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2768 -2775, 2002

Polyblends prove to be able to provide more possibilities for tailoring conductive polymer composites in comparison with individual polymer systems. Accordingly, ethylene-vinyl acetate-low-density polyethylene (EVA-LDPE) filled with carbon black (CB) was prepared in this study as a candidate for positive temperature coefficient (PTC) material. In consideration of the fact that CB distribution plays the leading role in controlling a composite's conduction behavior, chemical treatment of CB was applied to reveal its influence on percolation and the PTC effect. It was found that titanate coupling agent treatment facilitated sufficient distribution of CB in LDPE phase, leading to lower resistivity and a squarer PTC curve. Composites filled with nitric-acid-treated CB exhibited specific temperature dependence of resistivity as a result of the heterogeneous dispersion of CB at the interface of EVA-LDPE, which might provide the materials with a new function.

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 ethylenevinyl 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 roomtemperature 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.

For the production of polymer-based conducting composites serving as positive temperature coefficient (PTC) materials with lower room-temperature resistivity and sufficiently high PTC intensity, carbon black has been pretreated with acrylic acid and some initiator and then melt-mixed with low-density polyethylene. Because of the in situ formation of covalent bonding at the filler/matrix interface, the distribution status and thermally induced displacement habit of the conductive fillers have changed accordingly. As a result, the electrical performance of the composites can be tailored as desired. The amount of acrylic acid and the treatment sequence of carbon black exert an important influence on the effectiveness of the modification.

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In the course of long‐term service, electrically conductive polymer composites acting as positive temperature coefficient (PTC) materials are faced with performance decay characterized by gradually increased room temperature resistivity and decreased PTC intensity. To reveal the influencing factors and to find appropriate ways for solving the problems, thermal‐cold cycling experiments (which simulate the extreme operating conditions of PTC type materials in a laboratory environment) and electrification tests are carried out in the current work. The results demonstrate that irreversible damage of partial conductive networks and, in particular, oxidation degradation induced crystallizability deterioration of the matrix polymer are responsible for the electrical performance decay. Additionally, an increase in the contact resistance formed at the metallic electrode/composite contacts exerts a negative influence on the service life of the composites. Polym. Compos. 25:270–279, 2004. © 2004 Society of Plastics Engineers.