Ultrasonic Devices to Disperse Nanomaterials
Nanomaterials have become an integral component of products as diverse as sunscreens, performance coatings, or plastic composites. Ultrasonic cavitation is used to disperse nano-size particles into liquids, such as water, oil, solvents or resins.
The application of ultrasonics to nanomaterials has manifold effects. The most obvious is the dispersing of materials in liquids in order to break particle agglomerates. Another process is the application of ultrasound during particle synthesis or precipitation. Generally, this leads to smaller particles and increased size uniformity. Ultrasonic cavitation improves the material transfer at particle surfaces, too. This effect can be used to improve surface functionalization of materials having a high specific surface area.
Nanomaterials, e.g. metal oxides, nanoclays or carbon nanotubes tend to be agglomerated when mixed into a liquid. Effective means of deagglomerating and dispersing are needed to overcome the bonding forces after wettening the powder. The ultrasonic breakup of the agglomerate structures in aqueous and non-aqueous suspensions allows utilizing 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 (e.g. ultra turrax), piston homogenizers, or wet milling methods, e.g. 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% by weight. Ultrasound can be applied for the dispersing of high concentration master-batches – processing low and high viscosity liquids. This makes ultrasound good processing solution for paints and coatings, based on different media, such as water, resin or oil.
Dispersion and deagglomeration by ultrasonication are a result of ultrasonic cavitation. When exposing liquids to ultrasound the sound waves that propagate into the liquid result in alternating high-pressure and low-pressure cycles. This applies mechanical stress on the attracting forces between the individual particles. Ultrasonic cavitation in liquids causes high speed liquid jets of up to 1000km/hr (approx. 600mph). Such jets press liquid at high pressure between the particles and separate them from each other. Smaller particles are accelerated with the liquid jets and collide at high speeds. This makes ultrasound an effective means for the dispersing but also for the milling of micron-size and sub micron-size particles.
Nanoparticles can be generated bottom-up by synthesis or precipitation. Sonochemistry is one of the earliest techniques used to prepare nanosize compounds. Suslick in his original work, sonicated Fe(CO)5 either as a neat liquid or in a deaclin solution and obtained 10-20nm size amorphous iron nanoparticles. Generally, a supersaturated mixture starts forming solid particles out of a highly concentrated material. Ultrasonication improves the mixing of the pre-cursors and increases the mass-transfer at the particle surface. This leads to smaller particle size and higher uniformity.
Many nanomaterials, like metal oxides, inkjet ink and toner pigments, or fillers for performance coatings, require surface functionalization. In order to functionalize the complete surface of each individual particle, a good dispersion method is required. When dispersed, particles are typically surrounded by a boundary layer of molecules attracted to the particle surface. In order for new functional groups to get to the particle surface, this boundary layer needs to be broken up or removed. The liquid jets resulting from ultrasonic cavitation can reach speeds of up to 1000km/hr. This stress helps to overcome the attracting forces and carries the functional molecules to the particle surface. In sonochemistry, this effect is used to improve the performance of dispersed catalysts.
Ultrasonication of samples improves the accuracy of your particle size or morphology measurement. The new SonoStep combines ultrasound, stirring and pumping of samples in a compact design. It is easy to operate and can be used to deliver sonicated samples to analytic devices, such as particle size analyzers. The intense sonication helps to disperse agglomerated particles leading to more consistent results. Click here to read more!
Ultrasonic processors and flow cells for deagglomeration and dispersion are available for laboratory and production level. The industrial systems can easily be retrofitted to work inline. For the research and process development we recommend using the UIP1000hd (1,000 watts).
Hielscher offers a broad range of ultrasonic devices and accessories for the efficient dispersing of nanomaterials, e.g. in paints, inks and coatings.
- Compact laboratory devices of up to 400 watts power
These devices are mainly used for sample preparation or initial feasibility studies and are available for rental.
- 500 and 1,000 and 2,000 watts ultrasonic processors like the UIP1000hd set with flow cell and various booster horns and sonotrodes can process larger volume streams.
Devices like this are used in the optimization of the parameters (like: amplitude, operational pressure, flow rate etc.) in bench-top or pilot plant scale.
- Ultrasonic processors of 2, 4, 10 and 16kW and larger clusters of several such units can process production volume streams at almost any level.
Bench top equipment is available for rental at good conditions to run process trials. Results of such trials can be scaled linear to production level – reducing the risk and costs involved in the process development. We will be glad to assist you online, on the phone or personally. Please find our addresses here, or use the form below.
Aharon Gedanken (2004): Using sonochemistry for the fabrication of nanomaterials, Ultrasonic Sonochemistry Invited Contributions, 2004 Elsevier B.V.
Nanomaterials – Background Information
Nanomaterials are materials of less than 100nm in size. They are quickly progressing into the formulations of paints, inks and coatings. Nanomaterials fall into three broad categories: metal oxides, nanoclays, and carbon nanotubes. Metal-oxide nanoparticles, include nanoscale zinc oxide, titanium oxide, iron oxide, cerium oxide and zirconium oxide, as well as mixed-metal compounds such as indium-tin oxide and zirconium and titanium, as well as mixed-metal compounds such as indium-tin oxide. This small matter has an impact on many disciplines, such as physics, chemistry and biology. In paint and coatings nanomaterials fulfill decorative needs (e.g. color and gloss), functional purposes (e.g. conductivity, microbial inactivation) and improve protection (e.g. scratch resistance, UV stability) of paints and coatings. In particular nano-size metal-oxides, such as TiO2 and ZnO or Alumina, Ceria and Silica and nano-size pigments find application in new paint and coating formulations.
When matter is reduced in size it changes its characteristics, such as color and interaction with other matter such as chemical reactivity. The change in the characteristics is caused by the change of the electronic properties. By the particle size reduction, the surface area of the material is increased. Due to this, a higher percentage of the atoms can interact with other matter, e.g. with the matrix of resins.
Surface activity is a key aspect of nanomaterials. Agglomeration and aggregation blocks surface area from contact with other matter. Only well dispersed or single-dispersed particles allow to utilize the full beneficial potential of the matter. In result good dispersing reduces the quantity of nanomaterials needed to achieve the same effects. As most nanomaterials are still fairly expensive, this aspect is of high importance for the commercialization of product formulations containing nanomaterials. Today, many nanomaterials are produced in a dry process. As a result, the particles need to be mixed into liquid formulations. This is where most nanoparticles form agglomerates during the wetting. Especially carbon nanotubes are very cohesive making it difficult to disperse them into liquids, such as water, ethanol, oil, polymer or epoxy resin. Conventional processing devices, e.g. high-shear or rotor-stator mixers, high-pressure homogenizers or colloid and disk mills fall short in separating the nanoparticles into discrete particles. In particular for small matter from several nanometers to couple of microns, ultrasonic cavitation is very effective in breaking agglomerates, aggregates and even primaries. When ultrasound is being used for the milling of high concentration batches, the liquid jets streams resulting from ultrasonic cavitation, make the particles collide with each other at velocities of up to 1000km/h. This breaks van der Waals forces in agglomerates and even primary particles.