Indigo (Dye)

The start size has been checked by optical microscopy. Majority of particles use to be < 15 µm. Only a few particles with up to 50 µm length could be found. It is plausible that these are just harder agglomerated particles.

For this start size, small 1 mm Ø grinding balls should be used to achieve an optimum grinding result (usually 20x bigger as the largest particles). The 1 mm Ø grinding balls use to be accelerated fast enough to achieve good grinding results with a classic line planetary mill. The usage of smaller grinding balls is not recommended for a classic line planetary mill.

Abrasion on grinding bowl or balls will always been shown when grinding. We recommend using zirconium oxide grinding balls to avoid a discoloration of sample. With grinding balls made of metal, abrasion will lead to a gray discoloration of sample. If that kind of discoloration can be tolerated, also grinding bowls and balls made of steel can be used as requested.

To avoid over pressure, we ground the sample in steps of 5 minutes, followed by a programmed pausing time of 10 minutes. After several cycles, the outside temperature of the bowl should be checked (remain below 75°C); grinding time or programmed pausing time might be readjusted afterwards.

With 15 grams of sample, the maximum volume of the 80 ml grinding bowl use to be reached. Viscosity of ground sample use to be quite thick after 30 minutes. We recommend grinding with a maximum sample amount of 10 g per 80 ml bowl. When bigger bowls like e.g. 500 ml volume will be used, it is plausible that the machine will not reach maximum speed.

After 15 minutes of grinding, a bit of sample has been taken to determine present particle distribution with Laser Particle Sizer ANALYSETTE 22 NanoTec plus. Already d50 < 1,1 µm has been achieved. After additional 15 minutes of grinding, d50 < 1,02 µm has been reached. Longer grinding time might be able to reduce the fineness a little further. But this use to be a typical range a classic line planetary mill is able to reach.

Smaller grinding balls will not been accelerated enough with a classic line planetary mill to improve the grinding result significantly. It is plausible that a Planetary Micro Mill PULVERISETTE 7 premium line with up to g-force 95 g will be capable to grind even finer (see result 2).

For our second trial, we used isopropyl alcohol (IPA) instead of ordinary ethanol. Our isopropyl alcohol use shows a higher purity to avoid any kind of solving effects when particles will be ground with smallest possible grinding balls.

To be on the save side, 5 mm Ø grinding balls have been used for pre grinding. These grinding balls can be changed rapidly (sieving out) and will be suitable to achieve a fineness of particles < 5 µm or finer. Then the pre ground sample can be refilled into the bowl with smaller grinding balls again (e.g. 0,1 mm Ø grinding balls).

To avoid overpressure, we recommend to program 3x longer pausing times (e.g. 3 minutes of grinding and 9 minutes of programmed pausing time - keep in mind to oberserve the bowls temperature).

After 5 minutes of grinding, only a very small amount of particles larger than 10 µm use to be present. We proceeded grinding for a total of 15 minutes. Now the balls need to be changed to 0,1 mm Ø for a grinding into nano range (see result 3).

After we changed the grinding balls to 0,1 mm Ø, we needed to add ~ 10 ml of isopropyl alcohol to maintain a motor oil like viscosity again (for an optimum grinding result).

We proceeded grinding for 30 minutes at maximum speed (1100 rpm).

With the maximum g-force of 95 g, the used 0,1 mm Ø grinding balls comminuted the sample to d50 < 100 nm. Because sample use to be < 1 µm, we recommend using Mie parameters to achieve a more precise result. We found a refractive index of 1,7 and recalculated the result with that parameter. This might be a more reliable result with d50 < 150 nm.

Nevertheless, d50 result is still way finer as requested. Probably 15 minutes might be fulfilling to achieve the desired fineness of d50 < 250 nm.

It might be still possible to proceed grinding to achieve a better fineness. Then a bit more solvent should be added in order to achieve motor oil like viscosity again. Else, about 10 g of sample might be used per 80 ml grinding bowl.