A Useful Tool for Measuring Carbon Black: Disc Centrifuge Photosedimentometry (DCP)

Summary

An industrial carbon black sample was used for the purpose of demonstrating the efficacy of the Brookhaven Instruments DCP. Due to the intrinsically differential nature of the results, it is possible to identify the existence of multiple species even from raw, uncorrected results. The basis of this technique is sedimentation, meaning that larger particles will sediment first, followed by smaller particles. Selecting an optimal disc speed allows for detection of the dominant species present, determined for this sample to be on the order of 100 nm.

Introduction

Carbon Black is a near universal material that has broad industrial application. It is used as an abrasion-resistant filler in the manufacture of tires and other rubber materials. Carbon black is also used as a pigment in coatings and lacquers, plastics, printing inks and tinting blacks. Since the particle size distribution (PSD) of aggregates of carbon black is strongly correlated with the thermal and mechanical properties of dispersions, the measurement of a carbon black PSD is very useful for quality control. The typical size range for non-agglomerated carbon black is from 10 nm to 500 nm. While a sample can be unimodal, the typical carbon black PSD is broad.

Given the small sizes involved and the breadth of a typical distribution, a disc centrifuge photosedimentometer (DCP) has been the instrument of choice for more than 40 years. It is a rugged instrument, the theory of which is well developed. The accuracy and high-resolution capability make this a valuable industrial tool.

Sample Preparation & Experimental Setup:

The sample was prepared for analysis as follows:

100 mgs of finely milled industrial carbon black was dispersed in 20 mL of ethanol and subsequently sonicated and then placed into an ice bath. Following this, 60 mL of 0.1% v/v Triton X-100 was added to the mixture, which was intermittently probe sonicated for a duration of 10 minutes. The disc was prefilled with a gradient consisting of ethanol, Triton X-100, and dodecane. This mixture, referred to as the spin fluid, was allowed to equilibrate at 8000 rpm for 10 minutes prior to sample injection.

The disc centrifuge induces sedimentation of particles in order of size, physically separating mixtures into different components. Planning a successful DCP experiment involves selecting an appropriate gradient and disc speed to achieve the desired level of separation in a reasonable measurement window, in this case a disc speed of 8000 rpm allows us to cover a size range spanning from d = 0.029 to 0.656 μm over the course of a 60 min run. The DCP modeling utility (calculator) allows us to estimate this prior to performing an actual measurement.

chemical parameters
All the parameters used to calculate a size distribution are either measured, user-controlled, or easily available from chemical reference handbooks.

Results:

Uncorrected, or Raw, DCP data. Note axes are Time (sec) and Raw Signal (volts).
Uncorrected, or Raw, DCP data. Note axes are Time (sec) and Raw Signal (volts).

Raw DCP data are transformed in a single step into an easily readable particle size distribution using a series of known values (parameters describing the dimensions of the disc, speed, fluid density, viscosity, etc.).  The most conventional DCP measurement is made in line start mode, meaning that a small amount of sample is injected at start and allowed to migrate towards the edge of the disc, separating each component of the mixture into discrete bands when present. Even without correction (above), it should be apparent that the DCP produces an intrinsically differential distribution when operated in this way.

Differential & Cumulative size distributions. Note the orientation of the size distributions is mirrored relative to Raw.
Differential & Cumulative size distributions. Note the orientation of the size distributions is mirrored relative to Raw.

The differential distribution (above) shows a nominal size of a little under 0.1 μm, coexisting with a continuum of increasingly small numbers of larger particles up until around 0.3 μm, although 90 percent of detectable sizes are below 0.13 μm.

The mean diameter is calculated as 0.096 μm (96 nm)
The mean diameter is calculated as 0.096 μm (96 nm)

Recap:

The DCP is tool that is perfectly suited to the analysis of industrial carbon black samples:

  • Operating in line-start mode makes it trivial to distinguish monomodal from multimodal samples even without data correction.
  • The DCP modeling utility makes it possible to select the appropriate combination of disc speed and measurement duration to obtain a useful separation window.

See also:

Applications: Carbon BlackParticle Sizing
Instruments: BI-DCP
Posted on: Feb 16, 2021