Quantifying densification effects on the morphology of nano-sized carbon black aggregates

Carbon black (CB) powders have a variety of industrial applications because of their fine sizes, large surface area, and fractal nature. Their properties can easily be modified by exposure to different environments. We report that physical agitation increases particle effective density ( ρ eff ) by more than 10 % and decreases intra-aggregate voids for two commercial CB samples. This study further investigates the underlying changes in physical properties at the primary particle and single aggregate level of the densified CB samples. The ρ eff and dynamic shape factor (χ) of size-selected CB aggregates were determined as a function of mobility diameter. In addition to a standard method from the American Society for Testing and Materials for calculating the primary particle diameter ( d pp ) of CB using its external surface area, four other methods were used to calculate 2D projected d pp based on transmission electron microscopy (TEM) images, enabling intercomparison and cross-validation of the results obtained. The morphological descriptors of both primary particles and aggregates were statistically quantified, including aspect ratio, roundness, convexity, circularity and sphericity. The specific surface area, d pp , and primary particle morphological descriptors of fresh and densified CB derived from a given method are within the measurement uncertainty range. In addition, CB aggregates with and without densification show similar fractal dimension. However, densified CB aggregates show decreased elongation, larger χ by >10 %, increased fractal proportionality constant by >0.19 depending on the overlapping coefficient, and increased similarity to convex hulls and spheres. The increased primary particle packing of a densified CB sample indicated by its increased fractal proportionality constant, caused by physical agitation only, is demonstrated to be responsible for the observed morphology changes. • Carbon black aggregates are densified by physical agitation, showing >10 % higher effective density. • Primary particle diameter is measured by five different methods. • Densified carbon black aggregates show increased dynamic shape factor but their fractal dimension does not change. • Increased primary particle packing explains the changes in morphological parameters of densified aggregates.