Designer droplets – why the size and stability of particles in inkjet inks matter

Inks used in inkjet heads often consist of a solid material dispersed in a liquid phase. The size of these solids is critical to the final product. They are normally less than 200 nm in size: large particles will lead to blocking of the inkjet head and visible imperfections on the printed product, and obviously these need to be avoided.

The fine particles required are normally produced by milling of larger particles until the correct size is achieved. Milling is expensive, and therefore milling longer than necessary is not recommended. There is another potential issue with milling too fine which will be covered later.

Laser diffraction has long been used as a way of monitoring the size reduction of the pigment to optimise the milling process. The technique involves the passage of a diluted sample through a measurement cell where the particles encounter a laser beam. The laser light is scattered by the particles and collected on a series of detectors. Fine particles scatter light at large angles while large particles scatter light at smaller angles. By having detectors at a wide range of angles, we can measure the scattered light and turn this into a particle size distribution which covers a wide range of sizes. The other advantage that laser diffraction has is that it provides a volume-based particle size distribution as particles with larger volumes scatter more light than small particles (a one hundred micron particle has the same volume as a million one micron particles), so if there are only a few large particles laser diffraction is likely to see them (provided the initial sampling has been adequate). This is useful for detecting potential printhead-blocking particles.

An alternate sizing technology that is also used for ink assessment is dynamic light scattering. This will measure much smaller particles (down to less than 1 nm) and can measure higher concentrations, but the maximum measured size is less than laser diffraction, so the most suitable technique will depend on the size of particles that are to be measured.

Large particles are often, as mentioned above, particles that have not been fully milled, but they could also be agglomerates of smaller particles. If the small particles are agglomerating, what causes this? One of the most popular measurements for indicating the stability of a formulation is the measurement of the zeta potential of the particles. This is a value which is indicative of the charge on the particles (at a point just beyond the particle surface called the “slipping plane”). A stable suspension is normally one in which the particle have a charge less than -30 mV or greater than +30 mV. If the particle charge is between these values the formulation may be unstable and aggregate over time, so whilst the particle charge at the time of milling might have been ok, that at the time of usage could be much larger due to agglomerate formation. Measuring the zeta potential at the time of formulation creation can point to potential instability (and product failure) at the time of product use which could be months or even years later. This gives a chance to adjust the formulation to make it more stable. This is often done by changing the pH or salt concentration (for water-based formulations) or the surfactant / stabiliser concentration (for water and non aqueous formulations). This is also why you can get agglomeration if you mill too fine, the resulting particle size distribution has a finer surface area than perhaps expected and there may not be enough stabiliser present to stabilise it (unless present in excess), so the resulting finer product may be more prone to aggregation.

You can hear more from me on this topic at the IMI Europe Inkjet Summer School in Ghent, Belgium, 14-15 June as part of a course on Inkjet Ink Characterisation.

Dr Steve Ward-Smith, Key Account Technical Specialist, Malvern Instruments, UK