Particle Size Reduction of Titanium Dioxide by Ultrasonic Milling

By AZoNano

Table of Contents

Introduction
Technologies for Powder Dispersion
Parameters
Pressure
Amplitude
Viscosity and Concentration
Temperatures
Energy versus Power and Intensity
Material Examples for Ultrasonic Milling and Dispersing Effects
     Titanium Dioxide (TiO2)
     Ultrasonic Milling of Pigments for Inks
Conclusion
About Hielscher

Introduction

Ultrasonic devices are used for deagglomerating and dispersing of solids into liquids. When powders are wetted, the particles develop agglomerates and are held together by means of various attraction forces, including liquid surface tension and van der Waals forces. For higher viscosity liquids like resin and polymers, this effect is particularly stronger.

In order to deagglomerate and disperse the particles into liquid media, these attraction forces had to be overcome. Moreover, uniform deagglomeration and dispersion is essential to leverage the full potential of the particles. Nanoparticles, in particular, exhibit unique properties that can only be manipulated in uniform dispersed state.

Technologies for Powder Dispersion

Different types of technologies are employed for dispersing powders into liquids. They comprise impinging jet mills, high pressure homogenizers, rotor- stator-mixers, and agitator bead mills. High intensity ultrasonication offers a better alternative to these technologies and proves especially useful for particle treatment in the nano range.

Parameters

Ultrasonic liquid processing is characterized by various parameters such as temperature, pressure, amplitude, concentration, and viscosity. The process result for a given parameter configuration is a function of the energy per processed volume. The function varies with modifications in individual parameters.

Also, the precise power output per surface area of the ultrasonic unit’s sonotrode depends on the parameters. Surface intensity is the power output per surface area of the sonotrode. This surface intensity depends on pressure, temperature, amplitude, reactor volume and viscosity, among others.

Pressure

As a rule, a liquid’s boiling point depends on the pressure. This means when the pressure is higher, the boiling point is also higher and vice versa. High pressure enables cavitation at temperatures near to or above the boiling point, and also boosts the intensity of the implosion. A constant-pressure pump is generally preferred since the ultrasonic power and intensity alters rapidly with variations in pressure.

While transferring liquid to a flow-cell, the pump should be able to manage the specific liquid flow at suitable pressures. Hose or squeeze pumps, membrane pumps, piston or plunger pump s, or peristaltic pumps will generate pressure fluctuations. Hence, pumps like spiral pumps, centrifugal pumps, and gear pumps that supply the liquid to be sonicated at a stable pressure are generally preferred.

Amplitude

The greater the amplitude, the higher is the rate at which the pressure increases and reduces at each stroke. Moreover, the volume displacement of each stroke expands, which results in a larger cavitation volume. When this is applied to dispersions, higher amplitudes demonstrate a higher destructiveness to solid particles. The table given below displays the general values for some ultrasonic processes.

Table 1. General Recommendations for Amplitudes

Process Amplitude
Dispersing/Deagglomeration 10 to 30 micron
Emulsifying 20 to 60 micron
Primary Particle Reduction 40 to 120 micron

Viscosity and Concentration

Ultrasonic milling and dispersing are some types of liquid processes. The particles need to be in a suspension, for instance in water, resins, solvents and oil. With the help of ultrasonic flow-through systems, viscous and pasty materials can be sonicated. High-power ultrasonic processor can be operated at high solids concentrations. Such high concentrations offer efficient ultrasonic processing.

Temperatures

When a liquid is sonicated, power is transmitted into the medium. Ultrasonically generated oscillation tends to cause friction and turbulence, and as a result the sonicated liquid will heat up. However, high temperatures of the processed medium can harm the material and reduce the effectiveness of ultrasonic cavitation.

New ultrasonic flow cells feature a cooling jacket through which a precise control of the material's temperature during ultrasonic processing can be achieved. Similarly, an ice bath for heat dissipation is suitable for the beaker sonication of smaller volumes.

Figure 1. Ultrasonic transducer UIP1000hd (1000 watts) with flow cell equipped with cooling jacket.

Energy versus Power and Intensity

In order to make the sonication process scalable and reproducible, the time of exposure at specific intensity and the sonicated sample volume have to be considered. For a given parameter configuration the process result, for instance chemical conversion or particle size, will depend on the energy per volume.

Modifications in the parameter configuration will alter the result function, and this will modify the amount of energy needed for a given sample value to acquire a particular result value.

Owing to this reason, it is not suitable to apply a certain power of ultrasound to a process in order to obtain a result. Therefore, an advanced technique is needed to find out the required power and the parameter configuration at which the power must be deployed into the process material.

Material Examples for Ultrasonic Milling and Dispersing Effects

Titanium Dioxide (TiO2)

Using ultrasonics for particle size reduction of TiO2 is an effective technique. The red curve in the below table indicates the particle size distribution prior to sonication; the green curve displays the TiO2 particles after sonication with the intensity of 250 Ws/ml; and the yellow curve after sonication with the intensity of 500 Ws/ml.

From the table, it is evident that the ultrasonic processing shifts the curve to the left side. In addition, the curves become narrower and the right tailing, which can be seen at the red curve, vanishes.

Figure 2. Ultrasonic milling of TiO2

Ultrasonic Milling of Pigments for Inks

Figure 3. Before ultrasonication: carbon black pigments, ultrasonically dispersed in UV ink (resolution 100x)

Figure 4. After ultrasonication

The images given above show the result of ultrasonic dispersion of carbon black pigment in UV ink. They clearly display the uniform dispersion grade and the particle size reduction.

Conclusion

Using ultrasonic cavitation for dispersing and wet-milling is ideal for achieving particle size reduction at micron- and nano-size as well as for acquiring uniform distributed dispersions at nano range. The full control over key parameters, such as temperature, pressure, amplitude, concentration and viscosity, helps in identifying the right process adjustment with respect to particle size and characteristics.

About Hielscher

Hielscher Ultrasonics is a family business, located in Teltow near Berlin (Germany). The main emphasis of its activities is the conception, development and production of ultrasonic devices for the use in laboratory and industrial applications. Technological innovations together with the realization of new ultrasound based processes substantiated the company growth and its market acceptance.

Today, ultrasonic devices made by Hielscher Ultrasonics are being used in laboratories and production plants on all continents across the world. More than 70% of the total sales is based on export. Almost every second device is supplied to customers outside Europe. Hielscher Ultrasonics integrates the ultrasonic devices into complex ultrasonic systems, such as wire cleaning systems, too. The systems are produced to meet the customers requirements in terms of power, extended range of accessories and steady state proof equipment.

Hielscher USA, Inc. is the representative for Hielscher ultrasonic equipment in the North American market. It is located in Ringwood, NJ.

This information has been sourced, reviewed and adapted from materials provided by Hielscher.

For more information on this source, please visit Hielscher.

Date Added: Mar 5, 2013 | Updated: Jun 11, 2013
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