technical paper | Carbon black
•2.5% carbon black (<25nm) in HDPE prepared with
differing dispersion levels
•Automated Dispersion Index of ten microtomed fi elds viewed at 100x magnifi cation
degrees of colour, UV protection and conductivity. Each of these will be described briefl y in this section.
Colour: Figure 4: Dispersion analysis
sion as measured on a size similar to the wavelength of visible light. It can be quantitatively assessed with electron microscopy (Figure 6). These micrographs show a jet/blue dispersion when the aggregates are fully dispersed and uniformly spaced/separated throughout the matrix. However, a poor jetness and brown tone dispersion was seen when the carbon black is poorly dispersed and areas of agglomerated carbon black can clearly be seen. Colour properties are indirect measures of micro-
dispersion, with blue tone and wet tinting strength being sensitive indicators. While surface area and structure contribute strongly, good compatibility between the carbon black surface and the matrix is essential for adequate micro-dispersion.
3) Performance needs in plastics
When used in plastics carbon black is more than just a functional colorant. It can contribute to differing
Two main mechanisms govern the interaction of light with carbon black – absorption and scattering. Carbon blacks and the matrix containing them, by nature, strongly absorb all wavelengths of light and therefore are not commonly viewed in transmitted light. However, carbon blacks are not perfect absorbers. Smaller particle size carbon blacks exhibit overall higher extinction than larger particle size carbon blacks and therefore develop more jetness. The relative absorptivity of carbon blacks is known to increase from red-blue violet. In other words, the undertone or “colour” that a carbon black fi lled plastic part would exhibit is a brown tone. This is due to the carbon black’s preferential absorbance of short wavelengths at the blue end of the spectrum. For thick fi lm and part applications, fi ner carbon blacks yield a bluer tone relative to their coarser counterparts.
The relationship between Hunter L and b values and
STSA (external surface area) is shown in Figure 7. This clearly shows the decreasing L value (increasing jetness) as a function of increasing surface area, and the trend towards bluer tone for high surface area carbon blacks.
UV Protection: Carbon black is an effective UV stabilizer for outdoor applications, inhibiting the physical and chemical process of UV induced degradation. Carbon black absorbs the UV energy and converts it into thermal energy, which can be dissipated by the compound. Finer grades with external surface areas greater than 100 m2
/g provide optimum UV protection (Figure 8). Above this value there is limited increase in UV absorption. A high coeffi cient of absorptivity (COA) is indicative of
Figure 5: Dispersion analysis using fi lterability data 40 COMPOUNDING WORLD | November 2013
Figure 6: TEM micrographs of good and poor dispersions
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