Carbon black in polyethylene

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. 

Comparison of this result with experimental values obtained for a dispersion of carbon black in polyethylene is shown in Fig. 8.14, where the agreement is seen to be fairly good, the discrepancy being at most a factor of about three. 

FIGURE 31-4 TTtefmogravirnetilc determination ot carbon black in polyethylene. (From J. Gibbons. Amer. Lab.. 

Methods for plastics can be found in British standards BS 2782, Methods 823 A and B [70]. which cover dispersion of carbon black in polyethylene, although they could be adapted for other materials. A thin section is c.xamined under lOOx magnification and comparison made with reference photographs. In method A the section is formed by melting and pressing and in method B by microtoming. 

MEASUREMENT OF THE DEGREE OF DISPERSION OF CARBON BLACK IN POLYETHYLENE USING ABSORPTION OF LIGHT. 

Presence of fillers in material. These can greatly change the properties of the base resin which they occur in. For example, the presence of carbon black in polyethylene can turn a resin, which is normally nonhygroscopic, into one which is very moisture sensitive. The addition of glass fillers in the range of 30 to 40% turns nylon from a nonabrasive to an extremely abrasive material, thus requiring very careful selection of equipment and system elements. 

Figure 10.5 also shows another problem encountered in all processes poor mixing of ingredients, in this case carbon black in polyethylene. To achieve the best product strength, such fillers must be mixed to make a uniform material, combining both high dispersion and an even distribution of particles. 

In the field of polymer/additive analysis various validated procedures (after interlaboratory tests) do exist. Various such procedures have been given in the present text (cfr also Chp. 8.3 for CRM development). Here we just mention the ASTM Standard Method of Test for Carbon-Black in Polyethylene Plastics (E 1603) and the ASTM Test Method for Compositional Analysis by Thermogravimetry (ASTM Standard Method E 1131) [89], which outlines a general technique for analysis of... 

As discussed in the previous chapters, single screw extruders are not good dispersive mixers, and therefore reliance is placed on suppliers to provide well dispersed additives in compounds and masterbatches. When it is considered necessary to test incoming materials or compare samples from different suppliers, which may, in particular, be required for carbon black masterbatch, there are a number of quality standards. These are mainly for carbon black in polyethylenes used in water pipes and cables, but can also be used for coloured pipes and cables where agglomerates can cause electrical failures. The tests can be divided between those that examine thin samples, and those which use extrusion filtering. 

But there is another method — the use of heterogeneous blends of polymers [45, 46], To this end, electrical properties and distribution of the filler (carbon black) in the mixtures of polyethylene and thermodynamically incompatible polymers were investigated. 

Fig. 7.17 Unmixed polyethylene superconcentrate (50% carbon black) in a mixture of superconcentrate and unfilled polyethylene as a function of viscosity ratio. [Reprinted by permission from V. W. Uhl and J. B. Gray, Mixing Theory and Practice, Vol. II, Academic Press, New York, 1967.]...

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. 

Fig. 6 Comparison between the number average molar mass and the relative amount of butanedioic acid in photo-oxidised polyethylenes containing different photoinitiators and stabilisers. The used additives were iron dimethyldithiocarbamate in SGI, iron dimethyldithiocarbamate and 0.8% carbon black in SG2 and iron dimethyldithiocarbamate and nickel dibutyldithiocarbamate in SG3...

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. 

Zhou et al. [67] investigated the effect on electrical properties of incorporating carbon black in a low-density polyethylene composite and low-density polyethylene ethylene methyl acrylate blends. Electrical conductivity/resistivity measurements have shown that the percolation threshold of ethylene-methylacrylate blend polymer composites was significantly lower than that of the low-density polyethylene composite, although in an ethylene-methyl acrylate composite the threshold is higher. The effect was due to preferential absorption of the carbon black into low-density polyethylene due to phase separation and immiscibility in low-density polyethylene-ethylene-methyl acrylate blends. Viscosity of polymers in the blend appeared to determine distribution on the carbon black, indicating that choice of polymer viscosity could be used to control carbon black distribution. 

Pourabas and Peyghambardoost [70] prepared positive temperature coefficient composites by using metal-modified and unmodified carbon black in a matrix of high-density polyethylene. Modification with metallic particles led to properties that were related to changing the surface properties of the carbon black. The intrinsic electrical conduction of carbon black also changed after modification. These changes in properties endowed some desirable characteristics to the positive temperature coefficients. 

Conductive multiphase composites have been obtained by dilution of polyoxy-methylene/carbon black in the polyethylene-based phase (Lipatov et al. 1983). In this manner, the filler is localized at the interface between the polymeric components. Though a reinforcement agent was excluded to the interface of the polyoxymeth-ylene phase as it crystallized, a small percent remained dispersed in the component The conductive network at the polyethylene/polyoxymethylene interface enhanced the conductivity level at lower filler amounts. 

When the concentration of carbon black at room temperature is above the percolation threshold, the composite is conducting. However, at higher current loading, the system heats and expands the PE matrix, and, when this approaches the percolation threshold, it becomes highly resistive [205]. This results in a lower current and the device cools to its original state, so a mixture of carbon black and polyethylene acts as a resettable fuse [206]. 

Techniques based on TG analysis have made it possible to readily and accurately measure the carbon-black content in commercial polymer formulations, such as in rubbers, at levels as far apart as 0.1% and 30%. The typical procedure is shown in Fig. 2.15 (sensitivity of the TG scan is 100 wt.% full scale) for a polyethylene masterbatch formulation, which was initially heated in N2 at a rate of 160°C/min. to about 550°C. Pyrolytic decomposition to gaseous products resulted in a 75% weight loss. After changing to O2 atmosphere the carbon-black is then oxidised [151]. The precision of the determination in the PE/CB masterbatch formulation is about 0.05 to 0.1% carbon (absolute). The TG method is fast, le. 6 min at 160°C/min, as compared to 2 h for ASTM D 1063 [266] without TG, thus providing substantial time savings. The compositional analysis (polymer and CB content) of LDPE has been reported [85] Affolter et al [155] have determined the content of carbon-black in polyolefins (2-3% CB) by TG following ISO 9924-1 and have noticed an inhomogeneous distribution in commercial... 

Feedstocks are usually either cat cracker bottoms from a petroleum cracking unit or tars from steam cracking for polyethylene production. Coal tars are commonly used as feedstocks to produce carbon black in China. 

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