Nanoparticles (EU) are normally defined as those particles that have a dimension of between 1 and 100 nm. The use of nanoparticles in coatings has provided a means to further improve performance such as scratch resistance, hardness, antistatic properties and UV resistance. These performance attributes are derived from the property profiles of nanoparticles.
Nanoparticles provide the inherent properties of the material they are derived from. For example, nano alumina (EU) maintains the properties of alumina, such as hardness and scratch resistance, but only on a nanoscale. Likewise, nano silica (EU) provides hardness, nano titanium dioxide provides a high refractive index and UV stabilization, and nano zinc oxide (EU) remains a UV light absorber (EU), even if the zinc oxide particles are nano-sized. The benefits of these materials are imparted to the coatings that they are used in.
The most pronounced property that is influenced by the particle size is the change in light scattering. For example, nano-sized particles may produce transparent coatings as light-scattering decreases with decreasing particle size. Most objects are visible due to light scattering from their surfaces. Scattering of light depends on the wavelength or frequency of the light being scattered as well as the size, shape and type of particle.
Table I – Particle Size Perspective
Since visible light has a wavelength on the order of micrometers, most particles much smaller than this, such as nano particles, are mostly transparent as their ability to scatter light diminishes with their size. However, light scattering is also dependent on the Refractive Index (RI) and the difference in RI between the interface of the particle and the surrounding medium. For example, if the surrounding medium has an RI similar to that of the RI of the particle, then the mixture of the two materials will be more transparent. To illustrate, silica has an RI of about 1.5 and polymethylmethacrylate (EU) has an RI of about 1.5, so a coating comprised of nano silica and an pMMA will be nearly transparent. The properties of nanoparticles based on their dimension can be quite dramatic.
Schematic I – Nanoparticles and Light Transparency
Ultrafine zinc oxide and ultra fine titanium dioxide as a nanomaterial are engineered to have primary particles less than 100 nm are more transparent to visible light (400 – 700 nm), but are effective UV absorbers and thus used in coatings and in sunscreens. The relationship between nanoparticle composition, coupled with their optical properties as delineated above, provide an avenue to impart unique features and performance to coatings which to a large degree have yet to be fully exploited. For example, nanoparticles can be used to provide the surface performance characteristics of a material such as scratch and abrasion resistance without a major influence on gloss or color.
As the dimension of a particle decreases, the ratio of surface area to volume increases quite dramatically. Higher surface area produces greater interaction of particles and higher attractive forces. The high attractive forces of unstabilized nanoparticles produce large agglomerates that are microsize (> 100 nm) in dimension and thus defeat any advantage that nanoparticles provide to enhance performance.
Schematic II – Nanoparticles and Agglomeration
Accordingly, the advantages of nanoparticles for use in coatings requires that the particles be used in a stabilized deagglomerated state. There are multiple ways that stabilization can be accomplished, as each type of nanoparticle is different from a compositional standpoint, the supplier should be consulted on the best means to achieve a stabilized particle.
Table II – Examples of Nanoparticles and their Application
As illustrated in Table II, the proper use of nanoparticles in coatings can impart multiple beneficial properties. Stabilization of dispersed nanoparticles is essential to ensure that the full benefits of these materials are realized. Secondly, formulations utilizing nanoparticles must be tailored to provide proper acceptance rather than as a drop-in to achieve a desired property. In summary, properly formulated coatings utilizing nanoparticle technology can achieve performance attributes heretofore not obtainable by other means. View all nanoparticle suppliers (EU) on ULProspector.com.
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