The Remarkable Power of Colloidal Silica

What Is Colloidal Silica?

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 electrostatically 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 specifications. Silica particle sizes typically vary from 5 nm to 40 nm, and particle size distribution can vary from narrow to wide, depending on the manufacturing process.

Most grades of colloidal silica are stable at pH 8–10.5 and carry an anionic surface charge that is stabilized in the dispersion with sodium or ammonium ions. 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. 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

The stability of colloidal silica grades in a broad pH range allows it to be used in a variety of aqueous formulations. In modern times, colloidal silica is used in applications ranging from building and construction to coatings to inorganic catalysts. However, colloidal silica has not always been the versatile problem solver 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 was not until the production of LUDOX® in the late 1940s that the applications for colloidal silica began to expand. One of the earliest applications for colloidal silica was in anti-slip coatings for floors, as shown in the video below—a DuPont advertisement from the 1950s explaining how LUDOX® is used in floor wax.

Today, LUDOX® colloidal silica manufactured by W.R. Grace, can be found in many products, such as latex paints, technical ceramics, and flexible packaging, just to name a few. LUDOX® is a monodisperse colloidal silica with discrete, spherical silica particles produced to tight specifications. It functions in commercial applications in four major ways: as a binder, surface modifier, pozzolanic additive, and flocculant.

Binder Applications

LUDOX® colloidal silica forms strong ionic and hydrogen bonds with metal oxides and fibrous materials. These bonds are thermally stable and chemically inert, making them especially useful binders for the following products.

 

• Investment casting slurries
• Vacuum-formed refractories
• Organic catalysts

Surface Modification Application

The silica particles in LUDOX® also adhere well to many surfaces and bond well to functionalized polymers via ionic and hydrogen bonding. Once applied to a surface, the silica particles provide an increased coefficient of friction and introduce antiblocking properties that are useful for the applications listed below.

 

• Paper and cardboard manufacturing
• Flooring
• Polishing slurries
• Pressure-sensitive adhesives
• Packaging coatings
• Architectural coatings

Pozzolanic Additive Applications

Silica particles functional as pozzolanic additives to strengthen cementitious materials, such as concrete. This is achieved effectively using colloidal silica, where the small silica particles penetrate deeply within the pores of concrete to react with free lime, making it ideal for concrete densifier formulations.

Flocculant Applications

As mentioned, 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 the following applications.

 

• Paper retention and drainage
• Beverage fining
• Insulation board

 

The characteristics of LUDOX® colloidal silica and its usefulness as a binder and surface modifier are further highlighted in the short video below.

Selecting a Colloidal Silica Product

Choosing the appropriate grade of colloidal silica is heavily dependent on the end-use application and the desired functionality. In general, colloidal silicas with smaller particle sizes perform better as binders, while grades with larger particle sizes are more effective surface modifiers. Other important factors to consider are the pH, density, viscosity, and ionic character of the end formulation, as well as 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 by clicking the link below to discuss your application with a product expert and request a sample. For more information about silica products, such as fumed silica and precipitated silica, check out this article.