Glass microspheres represent a class of additives that offer aesthetic, process control and cost benefits, while providing flexibility in a wide range of potential applications. Prizmalite’s solid glass microspheres are the smallest commercially available and are the first to be approved for incorporation into thin-film automotive paints and coatings.
The size of a glass microsphere is an important factor in the advantages spheres offer. There is an inverse relationship between the size of a microsphere and its surface area: the smaller the size of the sphere, the more spheres can fill a given volume of space and the greater the total surface area. This property is important because the greater surface area means greater compatibility between the spheres and polymers, such as plastics, paints or other materials, and this compatibility significantly expands the spheres’ potential applications. The greater the surface area, the larger the area for a polymer to adhere to, which means the polymer “wets out” more completely and is better dispersed. Increased “wetting out” results in stronger adhesion to the glass spheres themselves and reduced air pockets, which destroy the physical properties of a formulation, such as impact or tear strength. Prizmalite microspheres are not, however, nanoparticles and, as such, do not raise the regulatory and other concerns associated with sub-micron-size materials.
Solid glass imparts visual and material benefits that cannot be replicated when spheres are made of other materials such as ceramics or polymerics, aluminum oxides, or silicas and mineral fillers. Solid glass refracts [bends] and reflects light. Most ceramics [with exceptions] do not transmit light or exhibit specular [mirror-like] reflection due to their internal crystalline structures and surface irregularities. Instead of being reflected back, the light is “trapped” in the structure and emitted as diffuse or scattered reflectance, which is not as strong or direct as light transmitted through glass, which produces mirror-like reflectance. Hollow glass can also possess numerous surface and interior micro irregularities that also diffuse light. Because the thickness of a hollow bead’s wall is inversely proportional to its diameter, however, the larger hollow spheres that might offer some reflective properties have very low crush strengths, which preclude their incorporation into most formulations.
Prizmalite microspheres consist of amorphous glass manufactured in a furnace from silicon dioxide, combined with other oxides categorized as “refractory” chemicals because of their high melting points. Crystalline forms of silica are carcinogenic because like asbestos, they break or shatter into shard-like or unevenly shaped respirable-size particles that can lodge in the lungs. Glass spheres, because of their amorphous chemical structure, round shape and particle size, are not toxic, are easily dislodged if inhaled, and do not accumulate in the body’s organs.
Glass spheres are frequently compared to silica and mineral fillers such as talc, but the benefits of glass spheres are much broader than those of these products. Silica and talc are generally used as matting agents, where the objective is to reduce the gloss of a paint film by scattering the light reflected from the painted surface. While silica is the major material used to enhance the matte effect in paints, it typically offers few benefits other than very low cost.
Most silicas significantly increase a paint’s viscosity and if larger particle sizes are used to reduce undesired thickening, the resulting painted surface can be rough and easily discolored by accidental rubbing. Most silicas are hydrophilic [readily absorbing or dissolving in water], which limits their usability in outdoor paints and necessitates the application of surface coatings to allow the silica to properly disperse in water-based paint systems. Silica offers almost no hiding power when used in paints and both silica and minerals readily “dust” [dust flies into the air due to small particle size and low density of the material] during compounding.
Minerals such as talc and kaolin can be used as matting [or flatting agents], but typically produce a cheap-looking surface appearance and can impart a rough, dull, gray or green tint to paint, requiring the use of additional TiO2 pigment to restore whiteness. Glass spheres can also act as a flatting agent to produce a matte appearance when the load levels of the spheres are increased, typically in a range of between about 10% and 15% when calculated on a dry weight basis. Unlike minerals, however, the addition of glass spheres as a flatting agent does not dull or “muddy” the underlying color or appearance so the color’s depth and tone is maintained even in a matte finish.
Glass spheres do not dust during compounding, have very low oil absorption, and have limited effect on the viscosity or thickness of a formulation. While they do not thicken a formulation, glass spheres will strengthen the surface of materials into which they are incorporated. Because of their spherical shape, glass spheres also promote the flow of paints or coatings, which silica and other minerals do not.
Solid glass spheres can be coated with an almost unlimited range of coatings to achieve specific results when incorporated into formulations. Prizmalite has developed proprietary coatings that:
improve abrasion resistance when applied to glass microspheres that are added to textile or vinyl inks
create a water-resistant barrier on Prizmalite microspheres for applications where hydrophobicity is important
enhance the dispersability of Prizmalite spheres so they can more efficiently distribute pigments and other additives within a formulation
promote the placement of Prizmalite spheres in formulations by, for example, allowing the spheres to "pop up" to the surface of a paint or coating when surface exposure to light is desirable. Alternatively, Prizmalite has developed coatings
that promote the “sinking” of glass spheres when adhesion of the spheres to a substrate is critical.