Aggregate Size and Shape and Carbon Black Structure
Aggregates play a critical role in most carbon black polymer applications, with their size and shape both being important. The term “structure” is frequently used to describe it. These days, the term structure occurs in two distinctly different ways when referring to carbon black. One relates to the size and shape of the aggregates and agglomerates and is often referred to as permanent structure, while the other refers to carbon black network built up between carbon black particles when they are in the polymer and is regarded as transient. Here, we only discuss the permanent structure, while the transient one is discussed later.
The carbon black aggregates arise because especially small primary particles formed during the early stages of carbon black production do not survive in isolated form but fuse together to form discrete, rigid colloidal entities of various sizes and shapes (Hess and Herd 1993; Taylor 1997). These are described as carbon black aggregates and are the smallest dispersible units composed of the extensively coalesced, nondiscrete primary particles which are separable from the aggregate only by rupture. Continuous carbon layers coat the coalesced primary particle units and in this manner connect them by strong covalent bonds to form rigid aggregates. The size and shape of the aggregates can vary from individual spheroidal particles to clusters of a few or a multitude of primary particles with a more or less irregular three-dimensional, chain-like, fibrous, or grape-like morphology. Usually, the tendency to form aggregates decreases with increasing primary particle size. Carbon black grades with large primary particles above 100 nm usually show low structure, and even isolated primary particles can occur (Figs. 5 and 7).
Fig. 5 TEM images of carbon black grades indicating the relationship between carbon black structure and primary particle size (adapted from Lei et al. 2014)
Fig. 6 TEM image of agglomerated carbon black aggregates of ENSACO® 250G indicating a high carbon black structure
Usually, the aggregates go on to form agglomerates which are physically held together. The TEM image of Fig. 6 indicates agglomerated carbon black aggregates of high-structure ENSACO® 250G. Hence, the principal problems to characterize aggregate size and shape are: (1) dispersion, (2) measurement in two dimensions,
(3) conversion to three dimensions, (4) choice of the parameters to measure and compute, and (5) application to a sufficient amount of aggregates for statistical reliability (Medalia 1982). Most of the studies have been based on TEM techniques resulting in two-dimensional images that have to be transferred to three-dimensional images by the appropriate stereological method. Nonmicroscopic methods to measure the aggregate size are based on aerosol and liquid measurements (Bart and Sun 1985). Hereby, the disk centrifuge photosedimentometry (DCP) is the most common method. DCP measures the distribution of an aggregate diameter, the equivalent Stokes diameter being the diameter of a sphere of equal specific gravity which settles in a centrifugal field at the same rate in the same fluid as the aggregate.
Like other morphological properties, the aggregate shape is distributive in nature and can be classified by four aggregate shape categories (Hess and Herd 1993): (1) spheroidal (discrete particles rather than aggregates), (2) ellipsoidal, (3) linear, and
(4) branched. Carbon black products show different weight fractions of all four categories. Highly structured carbon black grades typically contain a high weight fraction of branched aggregates. With decreasing carbon black structure, the fraction of spheroidal, ellipsoidal, and linear aggregates increases. TEM pictures of a low- and high-structure carbon black are shown in Fig. 7a, b; a schematic illustration is given in Fig. 8.
The complexity of the arrangements of the carbon black particles, aggregates, and agglomerates results in a void volume, and this can be used as a convenient way to compare the structure of different carbon black materials. The void volume depends on the size and shape of the aggregate, the aggregate agglomeration, and the porosity on the primary particles. Therefore, the carbon black structure can be considered as the sum of a number of accessible voids by unit weight which is composed of: (i) the interaggregate space, (ii) the interstices within the aggregates, and (iii) the porosity of elementary particles. The higher is the structure level of the aggregate, the higher is the volume of the voids.
Fig. 7 TEM images of aggregates of high structure N-472 with branched primary particle aggregates (a) and low structure N-990 with isolated spherical primary particles (b)
Fig. 8 Schematic representation of low- and high-structure carbon black particles
Oil absorption is a simple method used to determine void volume and hence carbon black structure. In principle, many oils can be used, but the common ones are dibutyl phthalate (DBP) or a special paraffin oil (ASTM D2414). The DBP absorption (in mL (100 g carbon)-1) is higher the more complex the structure of the carbon black aggregates is.
The carbon black structure is very sensitive to the state of compression of the carbon black. Important stages during carbon black production and processing that can cover a broad range of compression states are: (1) carbon black powder after the gas separation step in the manufacturing process, (2) densified carbon black flakes or pelletized carbon black, and (3) the polymer compound. The compressed oil absorption number (COAN) measured by ASTM D3493 at a given compression state is attributed to the difference in sensitiveness of the carbon black structure toward compression observed for different carbon black grades. A similar concept takes into account the mechanical resistance of carbon black to compression by measuring the decrease of void volume with increasing compaction pressure at a given weight (ASTM D6086).