Ultrasonic Wet-Milling and Micro-Grinding of Particles in High Performance Coatings

Ultrasonication is an efficient means for the wet-milling and micro-grinding of particles. Besides dispersing and deagglomerating, the wet milling is an important application of Hielscher ultrasonic devices.

In particular for the manufacturing of superfine-size slurries, ultrasound has many advantages, when compared with common size reduction equipment, such as: colloid mills (e.g. ball mills, bead mills), disc mills, jet mills, rotor-stator mixers (ultra turrax) or high-pressure homogenizers. Ultrasonication allows for the processing of high-concentration and high-viscosity slurries – therefore reducing the volume to be processed. Ultrasonic milling is especially suited to process micron-size and nano-size materials, such as ceramics, alumina trihydrate, barium sulphate, calcium carbonate and metal oxides. The tables below show microscopic images of the milling of alumina trihydrate (from 150 micron down to 10 micron), ceramics (from 30 micron down to 2 micron) and sodium carbonate (from 70 micron down to 3 micron).

Ultrasonic devices are very easy to install and to operate. There are two parts only in contact with the material to be milled: the titanium sonotrode and the stainless steel flow cell. Due to the simple design of the ultrasonic flow cell, the units can be cleaned quickly. As Hielscher ultrasonic devices have a very high efficiency in the conversion of electrical into mechanical energy generally less power is needed for the ultrasonic milling than for conventional milling equipment.

The particle milling effect is based on intense ultrasonic cavitation. When sonicating liquids at high intensities, the sound waves that propagate into the liquid media result in alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency. During the low pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high pressure cycle. This phenomenon is termed cavitation. The implosion of the cavitation bubbles results in micro-turbulences and micro-jets of up to 1000km/hr. Large particles are subject to surface erosion (via cavitation collapse in the surrounding liquid) or particle size reduction (due to fission through inter-particle collision or the collapse of cavitation bubbles formed on the surface). This leads to sharp acceleration of diffusion, mass-transfer processes and solid phase reactions due to crystallite size and structure.

Ultrasonic processors and flow cells for dispersing and for the wet-milling of powders are available for laboratory and production level. The industrial systems can easily be retrofitted to work inline.

For further information, please visit http://www.hielscher.com