Carbon black dispersion measurement via interferometric microscopy.

Characterization of the filler dispersion within a rubber compound has been of documented interest for nearly a century due to the need for compound mixing optimization. Over-mixing increases the compound cost, while poor filler dispersion decreases product lifetimes and can cause catastrophic failure in extreme cases. One of the oldest and most fundamental techniques for characterizing filler dispersion is analysis of microtomed thin sections with light microscopy. The use of this technique is limited, however, by the required microtoming, which is labor and time intensive and is not suitable for routine, quick analyses. Faster and relatively inexpensive methods for characterizing filler dispersion rely on surface roughness created when samples are razor cut. The main advantage of these techniques is the ease of sample preparation; however, it is often difficult to quantify the resulting surface roughness of the sample.

New innovations

To combine the quantitative results from light microscopy with the speed of the surface roughness techniques, the method of interferometric microscopy (IFM) has been applied to carbon black dispersion characterization in vulcanized elastomeric systems by Columbian Chemicals (ref. t), and the resulting technique has been licensed to Ambios Technology.

The Ambios Xi-100 IFM uses white light interference to generate a three-dimensional topographic map of the razor cut rubber surface (figure 1). The peaks and valleys observed in this figure correspond to the undispersed carbon black agglomerates within the sample. Based on this surface topography, data analysis techniques are utilized to characterize the surface roughness and carbon black dispersion. IFM results from carbon black compounds correlate extremely well (R2 = 0.954) with those obtained via light microscopy, according to the study by Columbian (ref. 1). Although the initial correlation was performed only for carbon black samples, this technique can be applied to other fillers besides carbon black.

[FIGURE 1 OMITTED]

Interferometric microscope characterization

Once the Xi-100 topographic map of the sample is obtained, the analysis software is used to isolate, define and characterize the peaks and valleys found on the sample surface. The acquired topographic maps often have long-range variation of the sample surface, which interferes with identification of the peaks associated with the carbon black agglomerates. To remove this long-range variation, the as-acquired data are processed using a band-pass Fast Fourier Transform filter. and the height scale origin is set to the mean height of the image.

The next step is to isolate the surface features and quantify them. Since the base surface of the data has been flattened and set to zero, setting a minimum height displacement from the image mean plane can identify the peaks (positive height) and valleys (negative height) present in the data (figure 2). Parameters to describe the carbon black agglomeration, such as the number of peaks, the peak area, the aspect ratio of each peak and the height of each peak, as well as others, are obtained and cataloged by the program. The ability of the Xi-100 to provide true three-dimensional topographic information is invaluable for carbon black dispersion characterization.

[FIGURE 2 OMITTED]

Sample preparation and processing

A key factor to this technique is a measurement time of only minutes. A rubber sample is placed in a guillotine sample cutter and cut with a razor blade, exposing the peaks and valleys due to the undispersed carbon black agglomerates. The fresh-cut surface is then placed under the Xi-100 objective and quickly scanned. The customizable software enables the user to automate the acquisition of several measurements using a motorized stage. The topographic data from each measurement location are then automatically processed, and the surface roughness characterized. Finally, a printable report is generated displaying 3D images of the sample, the dispersion index, roughness and many other key statistics.

Reference

(1.) A.P Smith, T.L. Aybar, R.W. Magee and C.R. Herd, "Carbon black dispersion measurement in rubber vulicanizates via interferometric microscopy, " Rubber Chem and Tech., Vol. 77, No. 4, p. 691, Sept.-Oct. 2004.