Polyethylene carbon black composites

The PTC phenomenon, in effect, provides the means of making a thermal switch, operated either by an external change in temperature or by an increase in current that generates heat internally, see Fig. 8.20. A common form of self-regulating heater is made by extruding a strip with the cross-section shown in Fig. 8.21, where the conductive component consists of polyethylene/carbon-black composite that is radiation-crosslinked after fabrication. The strip may be extruded in long lengths... 

Manocha L M, Valand J and Manocha S (2007) Development of composites incorporating carbon iianofibers and iiaiiotubes, J Nanosci Nanotechnol 7 1845-1850. Traiiia M, Pegoretti A and Penati A (2007) Time-temperature dependence of the electrical resistivity of high density polyethylene - carbon black composites, J Appl Polym Sci 106 2065-2074. 

Cao Q, Song Y, Tan Y, Zheng Q (2009) Thtamal-induced pracolation in high-density polyethylene/carbon black composites. Polymra 50 6350... 

Fig. 8.12 The dependence of the conductivity of a polyethylene furnace black composite on carbon-black content. Reprinted from Hindermann-Bischoff and Ehrburger-Dolle (2001). Copyright 2001, with permission from Elsevier.

Carbon composites have been developed as alternative materials for carbon paste electrodes because of the limited utility of the latter in most organic solvents. These composites include polyethylene/carbon black [49], Kel-F/graphite [50], carbon black immobilized in cross-hnked polyethylene [51], and epoxy/graphite [52]. A collection of for these materials is available [20]. Thus, for platinum = 0.24 cm/s, for pyrolytic graphites 0.002 <... 

Yui, H., Wu, G., Sano, H., Sumita, M., and Kino, K. 2006. Morphology and electrical conductivity of injection-molded polypropylene/carbon black composites with addition of high-density polyethylene. Polymer 47 3599-3608. 

Yin et al. [68] investigated the critical resistivity, dispersivity, and percolation threshold of low-density polyethylene carbon black. Li et al. [69] investigated the electrical properties and crystallization behavior of four different kinds of carbon black-filled polypropylene composites, prepared by the melt mixing method. All showed typical characteristics of percolation, but noticeably different percolation thresholds. When using carbon black with a higher structure, smaller particle diameter, and larger surface area, the composite showed better electrical conductivity and a lower percolation threshold. 

A composite consisting of a mixture of carbon particles (e.g., carbon black or graphite) and a polymer binder such as polyethylene or polypropylene with a surface layer of a carbon-black or carbon-felt... 

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

Fig.36. Variation in electrical conductivity (o) with molecular weight for polyethylene composites filled with 4% by volume carbon black, demonstrating the effects of orientation (I), degradation (II) and flow-induced segregation of carbon black aggregates (III). ( ) injection moulded (O) compression moulded (unoriented) [181]...

Fig. 8.14 Comparison of the experimental and theoretical dependences of resistivity on carbon-black content iaa polyethylene composite. Reproduced with permission of the Institute of Physics from Scarisbrick (1973).

One way which is often used to impart an artificial structure to the dispersion of a carbon black in a composite is to coat the particles of a moulding powder, e.g. 1 mm diameter particles of polyethylene, with a conductive carbon black by mixing them together in a ball-mill. If subsequent moulding does not shear the mixture too much (rotational casting is ideal) the black remains concentrated in a honeycomb-like network, and the overall conductivity is greatly enhanced over that of a uniform dispersion of the same proportion of black. 

The first column in Table II shows the composition of the pyrolysis products from an input of polyethylene. Beside the gaseous products, i.e. mainly methane, ethane, ethylene and pro-pene, the liquid products consist of up to 95 % benzene, toluene, styrene and naphthalene. Only a small amount (1.0 wt-%) of carbon black is produced. 

A large diffusion may be found also for composite materials, carbon, or metal based. In the first case different types of polymeric resins (thermoplastics, such as polypropylene, polyethylene, and PVDF, or thermosettings, such as epoxies and phenolics) are filled with carbonaceous powders (graphite or carbon blacks), to provide a material characterized by very high chemical stability in the fuel cell environment and satisfactory properties of electrical conductivity, but which cannot offer sufficient robustness at thickness lower than 2 mm. The metal composite plates are essentially based on combinations (sandwiches of different layers) of stainless steel, porous graphite, and polycarbonates, with the aim to exploit the characteristics of different materials. Their fabrication can be more complex but this is compensated by the possibility to incorporate other functional components, such as manifolds, seals, and cooling layers. 

C.M. Chan, C. Chi-Leung, M.F.Y. Matthew, Electrical properties of polymer composites prepared by sintering a mixture of carbon black and ultra-high molecular weight polyethylene powder. Polym. Eng. Sci. 37, 1127-1136 (1997)... 

A typical application of TGA is its use in compositional analysis. For example, a particular polyethylene part contained carbon black and a mineral filler. The electrical properties were important in the use of this product and could be affected by the carbon black content. TGA was used to determine the carbon black content and mineral-filler content for various lots, which were considered either acceptable or unacceptable. The samples were heated in nitrogen to volatilize the PE, leaving carbon black and a mineral-filler residue. The carbon content was then determined by switching to an... 

Although in situ infrared spectroscopy has been applied widely in terms of the systems studied, the reflective electrodes employed have been predominantly polished metal or graphite, and so an important advance has been the study of electrochemical processes at more representative electrodes such as Pt/Ru on carbon [107,122,157], a carbon black/polyethylene composite employed in cathodic protection systems [158] and sol-gel Ti02 electrodes [159]. Recently, Fan and coworkers [160] took this concept one step further, and reported preliminary in situ FTIR data on the electro-oxidation of humidified methanol vapor at a Pt/Ru particulate electrode deposited directly onto the Nafion membrane of a solid polymer electrolyte fuel cell that was mounted within the sample holder of a diffuse reflectance attachment. As well as features attributable to methanol, a number of bands between 2200 and 1700 cm were observed in the spectra, taken under shortoperating conditions, the importance of which has already been clearly demonstrated [107]. 

Yamauchi et al. investigated the structures of NR/high density polyethylene (HDPE) thermoplastic elastomers (TPEs) and their composites with carbon black using SANS. The extremely low contrast between crystalline and amorphous HDPE for SANS enabled us to measure the interface thickness between NR and HDPE in the TPE (5 mn) as well as that between NR and carbon black in the composite (2.4 mn). The interface thickness and fractal dimension of the blends and composites were earefully analysed using SANS even though both polymers were not deuterated. It was revealed that NR and HDPE are immiscible blends. 

Paul A et al (1997) Electrical properties of natural fiber reinforced low-density polyethylene composites a comparison with carbon black and glass fiber filled low-density polyethylene composites. J Appl Polym Sci 63 247-266... 

Zhou and co-workers [28] measured the electrical conductivity/resistivity of carbon black filled linear low-density polyethylene (LLDPE) and blends of LLDPE with ethylene-methylacrylate (EMA). The percolation threshold of the blended polymer composite was significantly lower than that of the LLDPE composite, although in an EMA composite the threshold is higher. This effect was due to preferential absorption of the carbon black into the LLDPE due to phase separation and immiscibility in low-density polyethylene (LDPE)/EMA blends. The viscosity of polymers in the blend... 

Electrical conductivity measurements have been reported on a wide range of polymers including carbon nanofibre reinforced HOPE [52], carbon black filled LDPE-ethylene methyl acrylate composites [28], carbon black filled HDPE [53], carbon black reinforced PP [27], talc filled PP [54], copper particle modified epoxy resins [55], epoxy and epoxy-haematite nanorod composites [56], polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA) blends [57], polyacrylonitrile based carbon fibre/PC composites [58], PC/MnCli composite films [59], titanocene polyester derivatives of terephthalic acid [60], lithium trifluoromethane sulfonamide doped PS-block-polyethylene oxide (PEO) copolymers [61], boron containing PVA derived ceramic organic semiconductors [62], sodium lanthanum tetrafluoride complexed with PEO [63], PC, acrylonitrile butadiene [64], blends of polyethylene dioxythiophene/ polystyrene sulfonate, PVC and PEO [65], EVA copolymer/carbon fibre conductive composites [66], carbon nanofibre modified thermotropic liquid crystalline polymers [67], PPY [68], PPY/PP/montmorillonite composites [69], carbon fibre reinforced PDMS-PPY composites [29], PANI [70], epoxy resin/PANI dodecylbenzene sulfonic acid blends [71], PANI/PA 6,6 composites [72], carbon fibre EVA composites [66], HDPE carbon fibre nanocomposites [52] and PPS [73]. 

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