In the most general terms colloidal silica is a dispersion of amorphous silicon dioxide (silica) particles in water. These amorphous silica particles are produced by polymerizing silica nuclei from silicate solutions under alkaline conditions to form nanometer sized silica sols with high surface area. A charge is then induced on the silica nanoparticle surface that allows the silica particles to repel one another and form a stable dispersion, or colloid.
Colloidal Silica Characteristics
Colloidal silicas are produced in a variety of grades that range in a number of factors. Particle size typically varies from 5nm to 40nm, and particle size distribution can vary from narrow to wide depending on the manufacturing process. Standard colloidal silica is stable at a pH from 8 - 10.5 and carries an anionic surface charge that is stabilized with sodium or ammonium. In certain grades, some of the silicon atoms in the silica particle are replaced with aluminate ions to allow for enhanced stability in a wider pH range, usually 3.5 - 10.5.
Alkaline Stable Colloidal Silica
pH Stable Colloidal Silica
Colloidal silica can also be produced to carry a positive surface charge that is stable in the acidic pH range. This is accomplished by modifying the surface of the particle with aluminum and charge stabilizing the particle with a chloride anion.
Applications for Colloidal Silica
Colloidal silica hasn't always been the versatile problem solver that it is today. In fact, early colloidal silicas were not commercially useful because they were too unstable and contained only low levels of silica. It wasn't until the production of LUDOX™ in the late 1940's that the applications for colloidal silica began to expand. One of the earliest applications for colloidal silica was in anti-slip coatings for floors. The 1950's Dupont advertisement below explains how LUDOX ™ is used in floor wax.
Today, LUDOX® colloidal silica is used in applications ranging from a high temperature binder for the production of aerospace parts to a fining agent used to produce beer and wine. LUDOX® functions in commercial applications in three major ways; as a binder, surface modifier, and flocculant.
LUDOX® colloidal silica forms strong ionic and hydrogen bonds with granular and fibrous material. These bonds are thermally stable and chemically inert making them especially useful for:
Investment Casting Slurries
Vacuum Formed Refractories
Surface Modification Application
The silica particles in LUDOX® adhere well to many surfaces via ionic and hydrogen bonding. Once applied to a surface the silica particles provide an increased coefficient of friction that is useful for:
Paper & cardboard
Industrial and architectural coatings
LUDOX® colloidal silica is available in grades with anionic sol charge as well as cationic sol charge. When added to solutions LUDOX® particles bond ionically with particles of the opposite charge and fall out of suspension. This makes LUDOX® an excellent flocculant for:
Paper retention and drainage
Beverage fining (beer and wine)
The characteristics of LUDOX® Colloidal Silica and it's usefulness as a binder and surface modifier are further highlighted in the short video below.
Selecting a Colloidal Silica
Selection of the appropriate grade of colloidal silica is heavily dependent on the end use application and the functionality you want the silica to achieve. In general, colloidal silicas with a smaller particle size perform better as binders, while grades with higher particle size are more effective surface modifiers. Other important factors to consider are the pH and ionic character of the end formulation as well the storage stability required. We have created an easy to use selector guide to help you find the right grade of colloidal silica for your application. You can also contact us to discuss your application with a product expert.