MEASURING PIGMENTS AND INKS WITH ARCHIMEDES
The physical properties of pigments are crucial to their performance in final products such as paints, cosmetics, and inkjet inks. Most fundamentally, pigment particle size affects tincture, or color intensity, and must be monitored and controlled carefully during manufacture. In addition, the dispersion of pigments is key to their rheology, adhesion, and wear. In many applications it is important that the primary pigment particles not aggregate, or else they can, for example, produce poor surface finish or clog inkjet nozzles. On a related note, additives for particulate coatings can improve wear and rheological properties, but also must be monitored to ensure proper performance.
Laser diffraction is widely used to assess pigment properties. The very optical properties that make pigments useful, however, also make them a challenge for light scattering analysis. In particular, pigments' differential absorption vs. wavelength, as well as effects of shape and size, makes their refractive index difficult to quantify precisely, and uncertainties in these parameters lead to errors in light scattering results. These instruments are also challenged by the need to distinguish primary pigment particles from aggregates that can form in unstable dispersions. And, it is typically not possible for light-based instruments to examine the pigments in their opaque product state (e.g., a paint or an ink).
ARCHIMEDES excels at measurement of pigments in part because it is immune to any variation in optical properties. The direct and ultra-precise measurement of each pigment particle's mass gives accurate information about the pigment's physical properties, whether expressed as the mass itself or as the diameter of an equivalent sphere. In addition, ARCHIMEDES' mass-based approach can easily distinguish pigment "primaries" from aggregates, and so assess the stability of the dispersion of a pigment in ink, paint, and cosmetic products.
As an example, consider the analysis of military-grade pigments and additives shown at right. The precise values for mass, equivalent sphere diameter, standard deviation, and many other statistical characterizations are available for the sample populations. The figure highlights the mean diameter and also the "D10/D90" cutoffs for:
- yttria-stabilized zirconia, an additive that improves wear and corrosion resistance of pigment dispersions.
- Nickel Oxide (II), a widely used green/black pigment.
- Lanthanum permanganate, a thin-film coating commonly used on stainless steel.
In addition, examining buoyant mass can give information about the pigment or additive suspension. 
The figure at right shows the time plot of ARCHIMEDES' sensor frequency shift for the same LaMnO4 sample dispersed in isopropyl alcohol. The time record shows both downward-going peaks, caused by LaMnO4 particles denser than the solution (5.5 vs 0.8 g/cc); as well as upward-going peaks signifying droplets or bubbles less dense than the isopropanol. The buoyant mass plot indicates the relative masses and populations of the two constituents, and shows that about 12% of the content is positively buoyant ("floating") droplets. A likely source is surfactant and or air trapped in the native pigment mixture, which was released when suspended in the isopropanol. Monitoring this population showed that it dissipated and vanished in about 1 hour. In contrast, the zirconia and nickel oxide samples contained no such low-density bubbles or droplets.
An analysis of an inkjet ink provides another example of how ARCHIMEDES can give information on multiple levels. Four separate samples of the same type of ink were first diluted in ethyl acetate by about 1000:1 so that the suspension was nearly transparent. This allowed the "primary" pigment particles to be measured, with good agreement in size of 130 nm for all four samples (below, left).
The same samples were then measured in their native, undiluted (and totally opaque) state. In this case the primary constituents are so concentrated (> 1011/ml) that several hundred are present in the sensor at any time, and so cannot be resolved individually. ARCHIMEDES was instead set to detect only particles >1 µm to look for aggregates caused by instability in the suspension that causes flocculation. The results show a significant difference in number concentration of these larger aggregates (above, left). Larger aggregate concentrations correlate with longer shelf life, and with ink samples that caused clogging problems with inkjet nozzles (right).
In sum, ARCHIMEDES provides a superior way to measure pigments and their suspensions. Its accurate and high-resolution measurement of the mass and size of individual pigment particles surpasses light-based methods, and gives valuable feedback during pigment milling and on color performance. And analysis of pigment suspensions can reveal instabilities and aggregation that can affect end product quality.