Coat of many properties

Dry coating is a relatively new technology which is used to create new generation materials by attaching submicron-sized guest particles to large host powders without using any solvents or producing waste.

Previously, coating depended on wet methods, or was achieved by mechanical means such as pan coaters or through deposition or interfacial polymerisation.

Coating is a versatile technique, which can form barriers or protective layers, control the release of active ingredients, and mask taste. Changes in the properties of the original particles can be observed including electrical conductivity, flowability, solubility, wetability and shape. However, the resultant waste streams, often VOCs, add expense and environmental concerns.

Pioneered by Hosokawa Micron in Japan through the development of its Angmill, Mechanofusion combines high shear and compression. Applying mechanical force to a mixture of fine and coarse particles forms an ordered mixture where guest particles are small enough to be held to the surface by van der Waals forces. Further mechanical action can cause these particles to generate a continuous coating in the form of a non-porous film or porous layer.

The principle of operation, shown above, starts with a quantity of each of the powders measured into the chamber. The bowl rotates forcing powder to circulate and be compressed between the stationary compression head and the sidewall. The intense shear and compression forces cause enough local heat to fuse materials together with very strong physical and chemical bonds. A scraper increases circulation of the materials. The wall can be cooled, preventing the continuouly circulating bulk material from overheating.

Among the effects achievable with Mechanofusion is improvement in flowability. PMMA particles in the size range 5-15um show a remarkable change in flow properties when Mechanofused with a coating of TiO2 particles (15-50nm). This is indicated by the angle of repose, measured using a powder tester.

The angle of repose is closely related to shell thickness. When the shell of TiO2 is less than 13nm thick, the angle is seen to increase. As the thickness increases beyond this point, the angle of repose falls. Once the thickness of the shell coating is greater than the diameter of the TiO2 particles, the increase in flowability occurs.

This indicates that simple mixing or even loose coating cannot give this effect.

Coatings can be classified in a number of ways; embedding, encapsulation, filming, surface covering and loose coating. It is also possible to perform a number of coating operations and produce distinct layers on a particle or modify particle shape.

The nature of coating, and whether a dry coating is achievable through mechanical action, is relatively hard to predict and has generally been determined empirically. Modelling has not yet been effective on the macro or micro scale. However, the nature of coating formed depends strongly on the properties of the powders, such as the ratio of host and guest particle size, melting points, fracture toughness and ductility.

Whether improving flow properties of powders for fire suppression systems, modifying electrical and thermal properties for electrical power applications, changing the particle shape of battery materials or embedding active pharmaceutical powders into carrier materials, dry powder coating and Mechanofusion are opening up new possibilities for composite materials.