Process development for new products with nanomaterials in their formulation

Recent years have witnessed an increase in the market introduction of new products with nanoparticles in their formulation. This is due to the superior product performance attained with nanoparticles compared to their micron sized counter-parts. Examples vary from high performance coatings (for example automotive coatings for low weight vehicles with lower fuel consumption requiring more durable, scratch resistant surfaces), lithium-ion batteries, car tyres, high performance lubricants (with environmentally friendly anti-wear and anti-friction additives) to numerous others including packaging with superior barrier properties, pharmaceutical and nutraceutical products. This is due to the superior product performance compared to their micron sized counter-parts and in many cases because of the specific product properties that cannot be achieved otherwise. Therefore, achieving a fine dispersion from formulation to production scale is of paramount importance. Whilst many such products have successfully been commercialised, it has been at the expense of lengthy trials; in the case of several others, the formulation had to be shelved due to difficulties in manufacturing in large quantities.

The dispersion process typically has two main stages. Following on from the incorporation of the power into the liquid phase, clusters of particles are broken up using energy intensive process devices.

Powder addition is commonly made at the liquid surface using a stirred tank. Both the hydrodynamic conditions resulting from the choice and operation of the impeller, and material properties such as wettability of particles in the liquid phase, particle size, liquid phase viscosity play a role.

Batch or inline rotor-stators are a good alternative as they allow the introduction of the powder directly into the mixer head, i.e. into regions characterised by high levels of turbulence. The incorporation time can be short, incorporation rate can be maintained constant and an initial or even complete breakup can be achieved.

Subsequent processing to achieve a fine dispersion is affected by the properties of the pre-dispersion generated at this stage.

Our research in the area aims to remove, at least partly, the trial-and-error type of approach and provide guidelines allowing optimal operation. Our labs are equipped with stirred tanks of a range of scales: from bench scale up to 200 litres. Large scale equipment also comprises an inline rotor-stator. The considerations for the process include:

• powder incorporation whilst maintaining the energy input low, i.e. at a low power input and short incorporation time or incorporation rate when adding over a period of time;
• minimal accumulation of large clusters of powder at the liquid surface, or blockage of pipes;
• rapid powder incorporation prior to a steep rise in viscosity when using dissolving solids.

Once incorporated into the liquid phase, a hierarchy of structures form varying from loosely bonded agglomerates to aggregates which are essentially fused particles, and even primary particles. The deagglomeration stage requires energy intensive processing to overcome the tensile strength of the agglomerates.

Examples of inline and batch rotor stator heads used at different scales of operation.

A wide selection of process devices can be employed: batch and inline rotor-stators, ultrasonic processors, stirred bead mills, high pressure jets or microfluidic processors.

Our research in the area focusses on the mechanism and kinetics of breakup, how fine a dispersion can potentially be obtained with the specific formulation. The choice of process equipment to employ is dictated by the volume to process, specific power input, physical properties of the dispersion, in particular dispersion rheology, which may change during deagglomeration.

Scale up of these processes is notoriously difficult and has commonly been the main barrier to the commercialisation of new formulation products. It is a multi-disciplinary area requiring a comprehensive knowledge of the flow fields generated in a process device, material properties such as the tensile strength of agglomerates, flow behaviour of the dispersion and appropriate techniques to monitor the process. It is not unusual to have additional phases in the formulation, for example an immiscible liquid (Pickering emulsion) which also needs to be dispersed, adding onto the complexity of the process.

The outcome of our research has provided guidance to the design and scale up of numerous processes for new formulation products. These also include layered structures, such as graphite or nanoclays, which require delamination.