Ultrasonic Dispersion of Silica (SiO2)
Silica Dispersion / Silica Suspension / 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.
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.
Processing Efficiency in Silica Size Reduction
Ultrasonic dispersion of nano-silica is superior to other high-shear mixing methods, such as an IKA Ultra-Turrax. Ultrasonics produces suspensions of a smaller silica particle size and ultrasonication is the more energy efficient technology. Pohl and Schubert compared the particle size reduction of Aerosil 90 (2%wt) in water using an Ultra-Turrax (rotor-stator-system) with that of a Hielscher UIP1000hd (1kW ultrasonic device). The graphic below shows the superior results of the ultrasonic process. As a result of his study Pohl concluded, that “At constant specific energy EV ultrasound is more effective than the rotor-stator-system.” Energy-effiency and silica particle size uniformity are of utmost importance in production processes, where manufacturing cost, process capacity and product quality matter.
The pictures below show the results that Pohl obtained by sonicating silica spray freeze granules. (Click at the images to enlarge!)
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 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!