R. Ragheb, K. Park, U. Nobbmann, J. Casola
Malvern Panalytical,
United States
Keywords: asphalt emulsions, laser diffraction, dynamic light scattering, electrophoretic light scattering, size, zeta potential
Summary:
Asphalt binder emulsions have proven to be an environmentally friendly, energy efficient, and cost-effective alternative to hot asphalt binder mixtures [1]. They are typically used for roadway resurfacing where they are employed as an adhesive for post-applied aggregate chip. They are typically composed of bitumen, water, and an emulsifying agent or surfactant and must give the required stability and application characteristics for optimum performance. For example, the structural layers must have adequate mechanical properties to make them less susceptible to early-life damage. Malvern Panalytical has several tools that can provide valuable metrics for production and characterization of asphalt emulsions. Two tools highlighted here are the Mastersizer and the Zetasizer. The Mastersizer 3000 uses the technique of laser diffraction to determine the particle size distribution by measuring the angular intensity of light scattered as a laser beam passes through a particle dispersion - large particles scatter light at small angles and small particles scatter light at large angles. L aser diffraction provides a droplet size distribution, showing the size population distribution of millions of droplets. This plot can show if the emulsion is becoming finer or coarser which has several effects on the coating process of the emulsions. Finer particles and narrower distributions set and cure faster, while width of the distribution can affect the flow-behavior and stability of the emulsion so must be precisely controlled. Figure 1 shows the results of 4 different asphalt emulsion samples measured on the Masterszier 3000. Per ISO 13320:2009, laser diffraction data is to report 3 size parameters: D10, D50 and D90. These D-values (D10, D50 & D90) are the intercept for 10%, 50% and 90% respectively of the cumulative volume. The other statistical number that is commonly used for laser diffraction is SPAN which is calculated by (D90-D10)/D50. SPAN will increase as particle size distribution width broadens. The Mastersizer 3000 was able to clearly differentiate 4 asphalt emulsion samples even if these samples have similar particle size range and median size. The Zetasizer can be used to measure particle size through dynamic light scattering (DLS) and zeta potential through electrophoretic light scattering (ELS) and complements the Mastersizer’s analytical range. As shown in Figure 2, the Zetasizer can be used to measure the average size and degrees of polydispersity of asphalt emulsions using the Z-average and Polydispersity Index (PDI). With ELS or zeta potential analysis, an electric field is applied to a dispersion of particles which move with a velocity related to their surface charge. Zeta potential is another common metric for stability and formulation applications. Emulsion droplets must be stabilized through a steric or charge repulsion mechanism. In the latter case zeta potential is a key metric, with high positive or negative Zeta potentials (usually > 30 mV) required to achieve a stable system. Figure 3 shows the Zeta potential distributions for several formulations exhibit a higher absolute value for zeta potential. References: [1] www.asphaltmagazine.com