Big targets for small particles

Anyone who produces sophisticated, complex or expensive media wants to keep an eye on all the properties that are crucial for quality, efficacy and safety during production. Measuring the particle size of products with nanoparticles has so far posed challenges – because it is only possible offline. A research project aims to change this.

Written by Marius Schaub

A wooden mill with freshly ground flour, next to it an open sack of grain — Coarse or fine? The particle size has a major influence on the properties of the product.

What do printing inks, catalysts and medicines have in common? Their effectiveness and functionality depend largely on the nature of the particles bound in them. If the size of the coloring particles deviates from the requirements, inks can clog print heads, for example. In medicine, particles that are too large or too small can even endanger the patient’s health.

Numerous manufacturing industries are therefore interested in knowing the particle size of the media in their production processes as precisely as possible. Until now, however, this has usually only been possible offline. This means that a sample is taken from the running process and analyzed. Size determination is possible in various ways: by sieve analysis, by static or dynamic image analysis, by means of laser light scattering or laser diffraction and with dynamic light scattering.

The methods are established but have disadvantages: if the test shows that the particles have already been ground too small, the batch is lost. Even with products that may contain particles with a wide particle size distribution, grinding for too long leads to unnecessary energy consumption. In addition, the random sampling procedure is time-consuming and expensive.

Research institutes and suppliers of measurement and analysis technology are therefore looking for ways to measure the particle size distribution in running processes, i.e. inline. Parsum and Alexanderwerk, for example, offer an inline solution for measuring the particle size of continuously produced granulate in roller presses, which was developed in collaboration with the Hamburg University of Technology (TUHH). Manufacturers such as Microtrac and PS Prozesstechnik also offer analysis systems for the online measurement of particle size and shape.

A metal probe with integrated laser for measuring particle size distribution

Inline measurement of the particle size of nanoparticles

However, nanoparticles still pose a challenge for online or inline measurement during the grinding process. Ingredients of this size, typically between 1 and 100 nanometers, are being used in more and more products, as they often offer many advantages over larger particles. Against this background, a European research project has been looking for ways to measure the particle size distribution of nanoparticles in ongoing grinding processes.

PAT4Nano (Process Analytical Technology Tools for Realtime Physical and Chemical Characterization of Nanosuspensions) is the name of the consortium from industry and research that has been researching practicable approaches for inline measurements of the smallest ingredients over the past four years. The manufacturing companies Janssen Pharmaceutica, Agfa-Gevaert and Johnson Matthey were involved. They cover the areas of pharmaceuticals, inks and pigments as well as materials for catalysis, batteries and glass production. Malvern and In-Process-LSP were involved as technology providers. The research was driven by several institutes: the University of Limerick, the Netherlands Organization for Applied Scientific Research (TNO), the Nanoscale Biophotonics Laboratory (NBL) from Ireland and the Fraunhofer Institute for Laser Technology (Fraunhofer ILT) from Germany.

As part of the research project, In-Process-LSP has developed a PAT instrument for the inline and online analysis of nanoparticles that combines spatially resolved optical technology with dynamic light scattering. The Fraunhofer ILT has taken a different approach: The research team has developed a novel laser-based technology for particle analysis. “We have developed our method on the basis of dynamic light scattering,” explains Dr. Christoph Janzen, who conducts research in the field of bioanalytics at the Fraunhofer ILT. The measuring principle is based on so-called Brownian motion: In the liquid medium, the suspended nanoparticles are excited by collisions with molecules of the solvent and are in constant motion. The smaller the particles, the faster this movement takes place. This is where the laser measurement method comes in: “We focus a laser into the solution and analyze the scattered light or its temporary fluctuation,” says Janzen. The particle size can be derived from the fluctuation using mathematical methods. Ultimately, this makes it possible to determine the moment during the grinding process when the desired particle size is reached.

A metal impeller with three blades for analyzing sample liquids

Probe head with impeller and laser measurement technology

In addition to a suitable method, however, it was also necessary to develop an instrument that could be used during the ongoing process. The inline measurement cannot be carried out directly inside the ball mill because undisturbed diffusion is required to determine the particle sizes by means of dynamic light scattering. As ground material is continuously mixed in a running ball mill, the particles do not diffuse freely in the liquid medium. Sampling with a cuvette, on the other hand, would not fulfill the requirement of continuous process monitoring.

An immersion probe was therefore designed, which is located in a housing with a rotating impeller. The wheel transports part of the sample liquid between its blades. When it stops, the gaps are closed and decoupled from the flow. This allows the particles in the housing to diffuse freely, enabling undisturbed measurement. For this purpose, an integrated laser is focused into the solution in the measuring chamber. The impeller then starts up again, the sample liquid under investigation is replaced and a new sample can be taken. “The advantage of this method is that the measurements are taken under the same conditions that prevail in the grinding process,” explains Janzen from Fraunhofer ILT.

The fact that the particle concentration in the immersion probe differs from that in the ball mill is still a challenge for the project partners involved. Using 3D cross-correlation, two DLS measurements are therefore to be carried out at the same location, allowing fluctuating signal intensities to be compared with each other. Even though this approach is very complex, Janzen is optimistic: “The method is not yet robust enough, but the results with the high-precision SLE-manufactured holder are promising,” says the researcher. The process is therefore to be pursued further with users and measuring device manufacturers – until it is ready for series production.

Author

Marius Schaub