Analysis of methods for measuring dispersion of fillers in rubber compounds
There are numerous methods to measure filler dispersion in rubber compounds. Kylie Knipp from Akron Rubber Development Laboratory has compared three of them in the current issue of RFP Rubber Fibres Platics 02 2020: Phillips Dispersion test, Dispergrader test, and nSpec 3D topography scan. A simple SBR compound was selected and five different fillers were evaluated. The paper will briefly discuss the advantages and disadvantages of the three different dispersion measurement techniques.
K. Knipp, Akron Rubber Development Laboratory
The dispersion of fillers in a rubber compound plays an important role in the performance of a rubber part or product. Poor dispersion of fillers can result in early failures for a part that sees repeated flexing, like the sidewall of a tire. A change in the level of dispersion may indicate that something has changed during the processing of the rubber compound, such as the mixing cycle time being shortened. This paper will explore three techniques used to measure the dispersion of fillers in rubber compounds and discuss the advantages and disadvantages of the different methods. The methods used in this paper for measuring dispersion are based around the principle that peaks or valleys are formed at the surface of a freshly cut rubber specimen (see figure). The measurements were performed on SBR compounds and the effect of fillers on properties such as dispersion and tensile strength were examined. The fillers that were evaluated included three grades of carbon black and two grades of silica.
The three techniques used for analyzing dispersion were Phillips Dispersion test, Dispergrader test, and nSpec 3D topography scan. The Phillips Dispersion method uses a light optical microscope to examine a sample at 30x magnification and a visual rating of 1 (bad) to 10 (excellent) is assigned based on a Phillips standard dispersion rating chart The Dispergrader method uses a reflected light microscope equipped with autofocus to quickly and consistently analyze the shadows cast by agglomerates in a freshly cut sample. The nSpec 3D method uses an automated, rapid optical microscope, pattern recognition algorithms, and artificial intelligence to provide 3D surface topographies and quantitative roughness measurements.
There are benefits and flaws for each of the techniques for measuring dispersion. The Phillips Dispersion test is a well-known industry method for rating the dispersion of compounds, however the results are very subjective and can vary between technicians. The Dispergrader test is more complex than the Phillips Dispersion test, however the sample preparation is still fairly simple. The quantitative data that is obtained may be affected by the presence of white fillers such as silica since the ASTM method was intended to measure carbon black dispersion.
The nSpec 3D topography scan is the most complicated of the three methods and requires a skilled technician to achieve a level surface, select the appropriate lighting, and select the appropriate start and stop points for the scan. The quantitative data that is generated is not affected by the presence of white fillers like silica, but the data collected should still be used for comparison purposes only due to the complexity of the sample preparation and setup.
K. Knipp, RFP Rubber Fibres Plastics, 02 2020, 84-91.