technical paper | Carbon black
are important in conductive applications. For a more porous material, there are an increased number of aggregates per unit weight providing a more conductive material at lower carbon black loadings.
Figure 2: Distinction between a carbon black particle and aggregate
Surface activity is a generic term and describes the interaction of a carbon black with its surroundings. The mechanisms of interaction, both physical and chemical, may vary considerably depending on the specifi c system in which the carbon black will be used. Carbon blacks as produced tend to be hydrophobic in nature and have a neutral surface. Some applications prefer a post- treated product, where the surface of a product can be made more hydrophilic. There are advantages to be gleaned from better compatibility between the carbon black and the matrix leading to improved dispersion and improved performance.
The distinction between carbon black particles and aggregates is shown in Figure 2. This is important to understand since the primary unit is the aggregate. However, the particle size plays a dominant role in a number of performance parameters.
2) Dispersion of carbon black Figure 3: Relationship between fundamental properties and ease of dispersion
carbon blacks providing higher jetness materials. Surface area (assessed using iodine and nitrogen methods) as well as tinting strength are measures of particle size. The term “structure” is commonly used to refer to the chain or grape-like formations in carbon black aggregates. The carbon black particles within these units are chemically bound together by surface growth phenomena. Mean aggregate sizes typically range from 50 nm to 300 nm – low to high degrees of aggregation – which yields differing shape complexities. For coloration, low structure products are preferred, whereas for conductive needs higher structure products are chosen. Oil absorption is used to measure the structure of a carbon black. Pore size distribution describes the amount of
porosity in a carbon black. Porosity in carbon black may range from mild surface roughening to extreme hollowing. The degree of porosity in a carbon black is depicted by the difference in its total surface area (NSA) and external surface area (STSA). Porous carbon blacks
38 COMPOUNDING WORLD | November 2013
The quality of almost every carbon black containing system is strongly dependent on uniform carbon black dispersion through the matrix. However, there are many different connotations of the word dispersion. In general, dispersion can be divided into two main categories; macro-dispersion and micro-dispersion. Macro- dispersion refers to the extent of dispersion down to a size range of a few microns. Finer and/or lower structure carbon blacks are more diffi cult to disperse than coarser/higher structure carbon blacks. The effect of carbon black morphology on dispersion is depicted in Figure 3.
Measures of macro-dispersion include blown fi lm
and fi lter pressure values. Figure 4 shows the disper- sion analysis of a 25 nm carbon black at 2.5% loading in HDPE prepared different dispersion levels. Automated image analysis of the microtomed samples shows calculated Dispersion Indices (DI) ranging from 72 to 96. Clearly as the dispersion index gets higher, which indicates better dispersion, smaller and fewer areas of undispersed carbon black are seen. Filter pressure values on these materials, as expected, show an increased pressure rise for the lowest DI material and the lowest rise for the high DI sample (Figure 5). The degree and hardness of carbon black aggregates, as well as the level of shear used during mixing, play a key role in determining the macro-dispersion of a carbon black in any given system. Micro-dispersion is a major infl uence in the
development of colour characteristics. This is disper-
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