Ultrasonic Dispersion of Silica (SiO2)

Silica, also known as SiO2, nano-silica or micro-silica is used in toothpaste, cement, synthetic rubber, high-performance polymer or in food products as a thickener, adsorbent, anti-caking agent, or carrier for fragrances and flavors. Below you will learn more about the uses of nanosilica and microsilica and how the sonomechanical effects of ultrasonics can improve process efficiency and final product performance by making better silica suspensions and facilitating silica nanoparticle synthesis.

Advantages of Ultrasonic Dispersion of Nano Silica (SiO2)

Silica is available in a wide range of hydrophilic and hydrophobic forms and has an extremely fine particle size of a few micrometers down to some nanometers. Typically silica is not well dispersed after wetting. It does also add a lot of microbubbles to the product formulation. Ultrasonication is an effective process technology to disperse micro-silica and nano-silica and remove dissolved gas and micro-bubbles from the formulation.

Ultrasonic dispersion is a technique that utilizes high-intensity, low-frequency ultrasound waves to disperse and deagglomerate particles in a liquid medium. When it comes to the dispersion of silica and nano-silica, ultrasonic dispersion offers several advantages:
 

The Importance of Particle Size of Silica

For many applications of nano-size or micro-size silica, a good and uniform dispersion is very important. Often, a mono-disperse silica suspension is required, e.g. for particle size measurement. In particular for use in inks or coatings and polymers to improve scratch resistance, the silica particles need to be small enough not to interfere with the visible light to avoid haze and maintain transparency. For most coatings silica particles need to be smaller than 40nm to fulfill this requirement. For other applications, silica particle agglomeration hinders each individual silica particle to interact with the surrounding media.
Ultrasonic homogenizers are more effective in the dispersing silica than other high-shear mixing methods, such as rotary mixers or tank agitators. The picture below shows a typical result of ultrasonic dispersing of fumed silica in water.

Ultrasonic Dispersion of Fumed Silica in Water

Processing Efficiency in Silica Size Reduction

La dispersione a ultrasuoni della nano-silice è superiore ad altri metodi di miscelazione ad alto coefficiente di taglio, come l'Ultra-Turrax IKA. Gli ultrasuoni producono sospensioni con particelle di silice di dimensioni più piccole e l'ultrasonicazione è la tecnologia più efficiente dal punto di vista energetico. Pohl e Schubert hanno confrontato la riduzione delle dimensioni delle particelle di Aerosil 90 (2% in peso) in acqua utilizzando un Ultra-Turrax (sistema rotore-statore) con quella di un Hielscher UIP1000hd (dispositivo a ultrasuoni da 1 kW). Il grafico sottostante mostra i risultati superiori del processo a ultrasuoni. Come risultato del suo studio, Pohl ha concluso che "A energia specifica costante, gli ultrasuoni EV sono più efficaci del sistema rotore-statore". L'efficienza energetica e l'uniformità delle dimensioni delle particelle di silice sono di estrema importanza nei processi produttivi, dove contano i costi di produzione, la capacità di processo e la qualità del prodotto.

Ultrasonics vs. Ultra-turrax for Silica Dispersion

The pictures below show the results that Pohl obtained by sonicating spray freeze-dried silica granules.

Left: REM-pictures of spray freeze silica granules before ultrasonic deagglomeration
Right: TEM-pictures of ultrasonically dispersed silica fragments
Study and images: Pohl and Schubert, 2004)

High-Performance Ultrasonic Dispersers for High-Quality Silica Formulations

Hielscher Ultrasonics is a German family-owned business specialized in the development, manufacturing and supply of high-performance ultrasound homogenizers for the treatment of liquids, solid-loaded suspensions and pastes. Hielscher ultrasonic homogenizers reliably process silica slurries and other nano-supensions in order to obtain any desired specification. Even product formulations that are highly sensitive, abrasive, or highly viscous can be efficiently dispersed and deagglomeration using ultrasonication. Our advanced ultrasonicators are extremely versatile and offer sophisticated batch and inline treatment possibilities. Reliably high quality standards and reproducible outcomes are key features of ultrasonic silica dispersion.
Hielscher state-of-the-art industrial-grade ultrasonicators feature a smart and user-friendly menu, programmable settings, automatic data protocolling on an integrated SD-card, browser remote control and high robustness.
The amplitude is the most influential parameter when it comes to ultrasonic processing. Amplitude refers to the maximum displacement or peak-to-peak movement of an ultrasonic wave. For ultrasonic dispersion, deagglomeration, and wet-milling often high amplitudes are required in order to apply sufficient energy for particle size reduction. Hielscher industrial ultrasonic processors can deliver exceptionally high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. For even higher amplitudes, customized ultrasonic sonotrodes are available.
From small and mid-size R&D and pilot ultrasonicators to industrial systems for commercial silica manufacturing in continuous mode, Hielscher Ultrasonics has the right ultrasonic processor to cover your requirements for superior silica processing.

Design, Manufacturing and Consulting – Quality Made in Germany

Hielscher ultrasonicators are well-known for their highest quality and design standards. Robustness and easy operation allow the smooth integration of our ultrasonicators into industrial facilities. Rough conditions and demanding environments are easily handled by Hielscher ultrasonicators.

Hielscher Ultrasonics is an ISO certified company and put special emphasis on high-performance ultrasonicators featuring state-of-the-art technology and user-friendliness. Of course, Hielscher ultrasonicators are CE compliant and meet the requirements of UL, CSA and RoHs.

The table below gives you an indication of the approximate processing capacity of our ultrasonicators:

What is Silica (SiO2, Silicon Dioxide)?

Silica is a chemical compound composed of silicon and oxygen with the chemical formula SiO2, or silicon dioxide. There are many different forms of silica, such as fused quartz, fumed silica, silica gel, and aerogels. Silica exists as a compound of several minerals and as synthetic product. Silica is most commonly found in nature as quartz and in various living organisms. Silicon dioxide is obtained by mining and purification of quartz. The three main forms of amorphous silica are pyrogenic silica, precipitated silica and silica gel.

Fumed Silica / Pyrogenic Silica

Burning silicon tetrachloride (SiCl4) in an oxygen-rich hydrogen flame produces a smoke of SiO2 – fumed silica. Alternatively, vaporizing quartz sand in a 3000 °C electric arc, produces fumed silica, too. In both processes, the resulting microscopic droplets of amorphous silica fuse into branched, chainlike, three-dimensional secondary particles. These secondary particles then agglomerate into a white powder with extremely low bulk density and very high surface area. Primary particle size of of the non-porous fumed silica is between 5 an 50 nm. Fumed silica has a very strong thickening effect. Hence, fumed silica is used as filler in silicone elastomer and viscosity adjustment in paints, coatings, adhesives, printing inks or unsaturated polyester resins. Fumed Silica can be treated to make it hydrophobic or hydrophilic for either organic liquid or aqueous applications. Hydrophobic silica is an effective defoamer component (anti-foaming agent).
Click here, to read about ultrasonic degassing and defoaming.
Fumed Silica CAS number 112945-52-5

Silica Fume / Microsilica

Silica fume is an ultra-fine, nano-size powder also known as micro-silica. Silica fume is not to be confused with fumed silica. The production process, particle morphology and fields of application of silica fume are all different from those of fumed silica. Silica fume is an amorphous, non-crystalline, polymorph form of SiO2. Silica fume consists of spherical particles with an average particle diameter of 150 nm. The most prominent application of silica fume is as pozzolanic material for high performance concrete. It is added to Portland cement concrete to improve concrete properties, like compressive strength, bond strength, and abrasion resistance. Beyond that, silica fume reduces the permeability of concrete to chloride ions. This protects the reinforcing steel of concrete from corrosion.
To learn more about ultrasonic mixing of cement and silica fume, please click here!
Silica Fume CAS number: 69012-64-2, Silica Fume EINECS number: 273-761-1

Precipitated Silica

Precipitated silica is white powdery synthetic amorphous form of SiO2. Precipitated silica is used as filler, softener or performance improvement in plastics or rubber, e.g. tires. Other uses include cleaning, thickening or polishing agent in toothpastes.
To learn more about the ultrasonic mixing in toothpaste manufacturing, please click here!
Primary particles of fumed silica have a diameter of between 5 and 100 nm, while agglomerate size is up to 40 µm with average pore size is larger than 30 nm. Like pyrogenic silica, precipitated silica is essentially not microporous.
Fumed Silica is produced by precipitation from a solution containing silicate salts. After a reaction of a neutral silicate solution with a mineral acid, sulfuric acid and sodium silicate solutions are added simultaneously with agitation, such as ultrasonic agitation, to water. Silica precipitates in acidic conditions. Besides factors, such as the the duration of precipitation, the addition rate of reactants, temperature and concentration, and pH, the method and intensity of agitation can vary the properties of the silica. Sonomechanical agitation in an ultrasonic reactor chamber is an effective method to produce a consistent and uniform particle size. Ultrasonic agitation at elevated temperatures avoids the formation of a gel stage.
For more information about ultrasonically assisted precipitation of nanomaterials, such as precipitated silica, please click here!
Precipitated Silica CAS number: 7631-86-9

Colloidal Silica / Silica Colloid

Colloidal silica is a suspension of fine nonporous, amorphous, mostly spherical silica particles in a liquid phase.
The most common uses of silica colloids are as drainage aid in papermaking, abrasive for silicon wafer polishing, catalyst in chemical processes, moisture absorbent, additive to abrasion resistent coatings, or surfactant for flocculating, coagulating, dispersing or stabilizing.
To learn more about colloidal silica in abrasion resistent polymer coatings, please click here!

The production of colloidal silica is a multi-step process. Partial neutralization of an alkali-silicate solution leads to the formation of silica nuclei. The subunits of colloidal silica particles are typically in the range between 1 and 5 nm. Depending on the conditions of polymerization these subunits can be joined together. By reducing the pH below 7 or by addition of salt the units tend to fuse together in chains, which are often called silica gels. Else, the subunits stay separated and grow gradually. The resulting products are often called silica sols or precipitated silica. A colloidal silica suspension is stabilized by pH adjustment and then concentrated, e.g. by evaporation.
To learn more about sonomechanical effects in sol-gel processes, please click here!

Silica Health Risk

Dry or airborne crystalline silicone dioxide is human lung carcinogen that can cause serious lung disease, lung cancer or systemic autoimmune diseases. When silica dust is inhaled and enters the lungs it causes the formation of scar tissue and reduces the lungs ability to take in oxygen (Silicosis). Wetting and dispersion of SiO2 into a liquid phase, e.g. by ultrasonic homogenization, eliminates the risk of inhalation. Therefore the risk of a liquid product that contains SiO2 to cause Silicosis is very low. Please use suitable personal protection equipment when handle silica in dry powder form!

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