At least one artists’ materials manufacturer uses ‘nanospheres’ as an ingredient in some of their oil paint. What are these scrumptious-sounding little globes, and what do they do to the paint?
The manufacturer claims that paint formulated with nanospheres has “much-improved resistance to cracking.” They also advertise a white paint “ground with hollow optically pure silicon nanospheres.
This has been a recent breakthrough in paint making, with companies like Mercedes Benz applying the new nano-technology to produce paints brighter, deeper, richer, and more resistant to surface damage.”
Nanospheres is not a term used in the paint industry (at least that I could find) to describe an additive used in paint. The confusion surrounding some additives, especially in artists’ paint, is often acerbated by marketing descriptions. According to my understanding of these additives, the material described as ‘nanospheres’ is fumed or pyrogenic silica.* The term ‘nanospheres’ is not typically used in the industry to designate fumed silica but appears to be a term coined by some companies to obscure the actual ingredients. Indeed, the term sounds more enticing than fumed silica and plays well with the idea of the optical properties of paint.
As you may know, fumed silica consists of highly-dispersed, amorphous, pulverulent synthetic silica. Different production processes result in silicon dioxide products with different technical properties and applications. We can divide the types of silica products into various groups, which can also be further subdivided based on the differentiation between untreated and chemically-after-treated silicon dioxide products:
Synthetic Silicon Dioxide Products
- Thermal, pyrogenic, or fumed silica
- Silica by flame hydrolysis
- Plasma silica
- Arc silica
- Wet process silica
- Precipitated silica
- Silica gel
- Vitreous silica
The chemical formula for fumed silica is SiO2. However, in reality, there are no isolated SiO2 molecules present. Instead, the silicon atoms develop single covalent bonds with four directly adjacent oxygen atoms. The SiO4 tetrahedrons serve as the fundamental building blocks for the structure of the macromolecular network. In principle, the SiO4 tetrahedrons could be arranged regularly, or they could be entirely at random.
Visually, fumed silica is identified as a loose, bluish-white powder. Fumed silica consists of about 98% by volume of air. With the eye, it is possible to recognize microscopic fumed silica particles as well as larger, loose networks that collapse when touched even lightly. The smallest particles of some fumed silica grades measure in the nanometers—a human hair is 3,000 times thicker. To illustrate the size of these particles, let’s imagine blowing up a soccer ball (or football outside the U.S.) to the size of the earth; a primary particle of fumed silica, in this illustration, would be about the size of a soccer ball.
Fumed silica consists of spherical primary particles. The primary particles form a loose network. The smaller the primary particles, the more strongly pronounced the formation of these networks of particles, forming aggregates and agglomerates. In transmission electron microscopy (abbreviated TEM), fumed silica primary particles often appear to “line up” with each other, forming irregular chains. The individual types of fumed silica differ distinctly according to their primary particle size: the average primary particle size ranges from seven to 40 nanometers. The primary particles are spherical, but they appear as flat discs in TEM images. Whether these primary particles are hollow spheres, I have not been able to determine from the technical information available from the manufacturers of fumed silica products.
The primary particles of fumed silica form aggregates that appear much like “snowballs” of about 100 nanometers. These snowballs are pretty compact and, for example, cannot be broken down into smaller particles during dispersion in paint. The aggregates of fumed silica particles form much larger units known as agglomerates: micrographs of fumed silica particles show that agglomerates of about 10 to 200 μm form.
If fumed silica is dispersed in a liquid, the surface silanol groups interact with each other via the molecules in the liquid. This results in temporary, three-dimensional lattice structures becoming “visible” as thickening. The structure is broken down with mechanical stress, for example, from stirring or shaking. The system becomes more fluid, and its viscosity drops. When the liquid is static, the fumed silica particles join again to form lattice structures, and the liquid regains its original viscosity value. This behavior is known as ‘thixotropy.’
It is used in paint primarily to stabilize pigment particle dispersion, reduce pigment settling, control rheology (such as prevent paint from sagging during application), fix special effects, enhance corrosion resistance, improve optical properties, develop viscoelastic properties (such as adhesion and elasticity), and increase the abrasion resistance of coatings.
There are several companies producing silica products; perhaps the better-known ones are marketed under the trade names of Cab-O-Sil by Cabot and Aerosil by Evonik (formerly Degussa). There are articles in trade journals about nanospheres, but they are unrelated to their use in coatings.
Resistance to Cracking
Marketing brochures for fumed silica claim that paint formulated with it provides improved resistance to wear and scuffing. However, crack resistance is usually provided in latex and other water-solved paints and specific epoxy formulations.
In regards to improving the chroma of oil paint, it would make the paint more transparent, so in that regard, it may improve the light transmittance quality and transparency of the paint. If this is a mixing white, it would be a more transparent white for mixing with other colors.
* Note that nanoparticles are being claimed by one company, PPG, to form superior automotive coatings. The nanoparticles, covered under U.S. Patent Applications 2003/0162876 and 2003/0162015 (August 2003, U.S. Patent & Trademark Office), are complex silicon oxides and are reportedly successfully incorporating these nanoparticles in coatings. They also report that NanoProducts’ nanoparticle-based coatings show excellent chip resistance, excellent scratch resistance (and retained scratch resistance after simulated weathering), outstanding appearance, sag resistance, superior sandability, and resistance to water spotting. Whether this relates equally well to artists’ paints remains to be tested.
For more information, follow this link: Nanotechnology Enables Breakthrough Coatings With Improved Resistance to Chipping, Scratching, and Water Spotting.
Also, find literature on fumed silica at this link: Industry Brochure - AEROSIL® for Paints & Coatings.