Kathrin Hielscher of Hielscher Ultrasonics has published a white paper focusing on ultrasonic milling and dispersing in the nano-technology sector.
The dispersing and deagglomeration of solids into liquids is an important application of ultrasonic devices.
If powders are wetted, the individual particles build agglomerates and are held together by attraction forces of various physical and chemical natures, including van der Waals forces and liquid surface tension.
This effect is stronger for higher-viscosity liquids, such as polymers or resins.
The attraction forces must be overcome in order to deagglomerate and disperse the particles into liquid media.
An even dispersion and deagglomeration is important to use the full potential of the particles.
Nano particles offer extraordinary characteristics, which can only be exploited in an evenly dispersed state.
The application of mechanical stress, such as generated by ultrasonic cavitation, breaks the particle agglomerates apart.
Also, liquid is pressed between the particles.
Different technologies are commonly used for the dispersing of powders into liquids.
This includes high-pressure homogenisers, agitator bead mills, impinging jet mills and rotor-stator-mixers.
High-intensity ultrasonication is an interesting alternative to these technologies and particularly for the particle treatment in the nano-size range the only effectual method to achieve the required results.
By high-power/low-frequency ultrasound, high amplitudes can be generated.
Thereby, high-power/low-frequency ultrasound can be used for the processing of liquids such as mixing, emulsifying, dispersing and deagglomeration, or milling.
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.
Cavitation is the formation, growth, and implosive collapse of bubbles in a liquid.
Cavitational collapse produces intense local heating (5,000K), high pressures (1,000atm), enormous heating and cooling rates (>109K/sec) and liquid jet streams (400km/h).
There are different means to create cavitation, such as by high-pressure nozzles, rotor-stator mixers, or ultrasonic processors.
In all those systems, the input energy is transformed into friction, turbulences, waves and cavitation.
The fraction of the input energy that is transformed into cavitation depends on several factors describing the movement of the cavitation-generating equipment in the liquid.
The intensity of acceleration is one of the most important factors influencing the efficient transformation of energy into cavitation.
Higher acceleration creates higher-pressure differences.
This in turn increases the probability of the creation of vacuum bubbles, instead of the creation of waves propagating through the liquid.
Thus, the higher the acceleration the higher is the fraction of the energy that is transformed into cavitation.
In case of an ultrasonic transducer, the amplitude of oscillation describes the intensity of acceleration.
Higher amplitudes result in a more effective creation of cavitation.
In addition to the intensity, the liquid should be accelerated in a way to create minimal losses in terms of turbulences, friction and wave generation.
For this, the optimal way is a unilateral direction of movement.
This makes ultrasound an effective means for the dispersing and deagglomeration but also for the milling and fine grinding of micron-size and sub micron-size particles.
In addition to its outstanding power conversion, ultrasonication offers full control over the parameters of amplitude, pressure, temperature, viscosity and concentration.
This offers the possibility to adjust all these parameters with the objective to find the ideal processing parameters for each specific material.
This results in higher effectiveness and optimised efficiency.
Industrial Implementation of Ultrasound Ultrasonic processing of particles allows processing all particles evenly.
Hielscher's industrial ultrasonic processors are commonly used for inline-sonication.
Therefore, the suspension is pumped into the ultrasonic reactor vessel.
There it is exposed to ultrasonic cavitation at a controlled intensity.
The exposure time is a result of the reactor volume and the material feed rate.
Inline sonication eliminates bypassing because all particles pass the reactor chamber following a defined path.
As all particles are exposed to identical sonication parameters for the same time during each cycle, ultrasonication typically shifts the distribution curve rather than widening it.
Generally, 'right tailing' cannot be observed at sonicated samples.
The option of repeated ultrasonic processing by a loop setup enables the perfect sonication to be found for every pigment and every ink formulation.
Such treated pigment particles result in better ink quality and show higher stability, an increased shelf life (also at elevated temperatures), freeze-thaw stability, reduced flocculation stable rheology and lower viscosity at higher particle loading.
High-power equipment uses more electricity.
Considering rising energy prices, this affects the costs of processing.
For this reason, it is important, that the equipment does not lose much energy in the conversion of electricity into mechanical output.
Regarding energy consumption, ultrasound is to name as very energy efficient.
Hielscher ultrasonic processors are claimed to have efficiency of >85 per cent.
This helps to reduce electricity costs and gives you more processing performance.
Kusters et al sum up in their study that ultrasonic fragmentation is equally efficient as conventional grinding.
In another study, Pohl et al compared the processing efficiency of ultrasonic dispersion of silica with other high-shear mixing methods, such as with an IKA Ultra-Turrax (rotor-stator-system).
Pohl et al compared the particle size reduction of Aerosil 90 (2%wt) in water using an Ultra-Turrax (rotor-stator-system) at various settings with that of a Hielscher UIP1000hd ultrasonic device in continuous mode.
The study of Pohl et al concludes that 'at constant specific energy EV ultrasound is more effective than the rotor-stator-system' and that 'the applied ultrasound frequency in the range from 20kHz up to 30kHz has no major effect on the dispersion process'.
The break up of the agglomerate structures in aqueous and non-aqueous suspensions allows utilising the full potential of nanosize materials.
Investigations at various dispersions of nanoparticulate agglomerates with a variable solid content have demonstrated the considerable advantage of ultrasound when compared with other technologies, such as rotor stator mixers, piston homogenisers or wet milling methods, such as bead mills or colloid mills.
Hielscher ultrasonic systems can be run at fairly high solids concentrations.
For example, for silica the breakage rate was found to be independent of the solid concentration up to 50 per cent by weight.
Ultrasound can be applied for the dispersing of high-concentration master-batches, processing low and high viscosity liquids.
Conclusion: Dispersing and wet milling by ultrasonic cavitation is a proven technique to achieve evenly distributed dispersions at nano range as well as particle size reduction down to micron- and nano-size.
Full control over the parameters of amplitude, pressure, temperature, viscosity and concentration allows us to find the right process adjustment regarding particle characteristics and aimed size.
With ultrasonic industrial processors in the power range between 500W up to 16kW per device it becomes possible to develop specific process setups to fulfil specific requirements.
With the wide device range, all steps of development - from first testing to process optimisation and final production - are covered.
The advantages shown above turn ultrasonic dispersing and milling into a potential technology for industrial processing in various sectors, such as for the production of paints and coatings, ink and inkjet, cement and concrete, or cosmetics.
