Because of its expertise in inorganic chemistry (rare earths, silicas, zirconium, titanium), Rhodia has regularly developed nanoparticle systems (with particle diameters typically between 5 and 100 nanometers) for more than 10 years. Fields of application vary from tires and cosmetics to plastics and coatings or catalysis/filtration. This knowledge has been adapted to coatings technologies, especially for organic transparent coatings, such as those used in wood coatings.
There are advantages brought by cerium oxide nanoparticles for meeting the performance needs of the wood coating industry, particularly for the high-performance field.
As commonly known, we use the name “nanomaterial” to describe a material or a composition built from nanoparticles, which are active as such in the application. The typical size of these nanoparticles is lower than one hundred nanometers (100 nm).
A nanomaterial, under the definition used in this paper, is a dispersion of nanoparticles in a host matrix (e.g., polymer, coating, cosmetic formulation, etc.). Then it is possible to describe three different systems:
• nano primary particles;
• nano secondary aggregates composed of several primary particles; and
• nano tertiary aggregates composed of several secondary particles.
Cerium Oxide Nanoparticles
Today, cerium oxide is largely used in the catalysis field (mainly for diesel engines), and in chemical and mechanical polishing (CMP). However, cerium oxide is also well known for its optical properties and ability to filter ultraviolet (UV) rays. Moreover, Rhodia expertise ensures good size control from 5-nm diameter up to 100 nanometers. We are able to obtain stable sols of cerium oxide nanoparticles with diameters of 10 nm. These sols appear as a clear liquid, since the particles are small enough to be totally transparent. For instance, at the same solid concentration (1g/l) and for similar particle size, a titanium dioxide sol appears milky (Figure 1).
Despite their small size, cerium oxide nanoparticles are very effective in term of UV filtration (Figure 2). According to theory, cerium oxide shows a UV cut-off threshold at around 370 nm, similar to that of nano titanium oxide. Cerium oxide and titanium oxide are both semi-conductors (with a band gap around 3.0 - 3.2 eV) and present the same classical UV absorption mechanism: under UV-light illumination, the absorption of a photon with a higher energy than the band gap creates an electron-hole pair.
In the case of titanium oxide, these holes and electrons migrate to the surface of the particles (rather than recombining together inside the particles). When holes and electrons join the surface, they can react with oxygen, water or hydroxyls to form free radicals. This process is currently named “photocatalysis” (Figure 3).
These free radicals are oxidant entities and can cause the degradation of organic molecules, in particular polymers, which can be an important issue for protective coatings.
In contrast, cerium oxide absorbs UV without being photoactive. Indeed, cerium oxide has a localized electron (4 f orbital) while titanium oxide has less localized electrons than cerium oxide (3 d orbital). So the cerium-oxygen bonding is more ionic than the titanium-oxygen bonding, and logically, the charge carriers (holes and electrons centres) creation is lower than in the case of titanium oxide. Moreover, cerium oxide shows a very fast recombination of charge carriers before they can migrate to the surface (because of crystal defects, oxido-reduction reaction), so, there is no further creation of free radicals. Due to the combination of these two phenomena, cerium oxide does not show any photocatalytic effect.
In other respects, one can see in Figure 2 that cerium oxide presents properties of transparency in the visible spectrum from 400 up to 800 nm higher than those of titanium oxide. This result is in good agreement with the refractive index values of these two materials (2.1-2.2 for cerium oxide and 2.5-2.7 for titanium oxide).
Thus far, one can consider cerium oxide as an excellent candidate for meeting the needs of the protective coating field. However, even if we have now defined a satisfactory chemical composition and size, with proper UV-filtration and transparency but without photocatalytic effect, it is still necessary to reach a good state of compatibility of these nanoparticles within waterborne and solventborne wood stain formulations.
To overcome this issue, Rhodia has developed a set of proprietary treatments that can be used to obtain alkaline aqueous cerium oxide sols (from 100 up to 300 g/l) and totally stable organic sols up to 300 g/l, both perfectly stable on the nano scale and easy to handle and formulate in wood stains.
Both aqueous and solvent-based nanometric cerium oxide systems are especially adapted to wood coating technologies and named RhodigardTM nanotechnology.
Application to Coatings Technologies
Wood is a living material and requires care and protection. In order to meet this requirement, especially in terms of aesthetics and durability, the coatings industry uses high-quality products. Cerium oxide nanoparticles, properly dispersed in coating formulations using the specific chemistry described previously, combine the advantages of organic ultraviolet (UV) absorbers with those of mineral additives. The cerium oxide nanoparticles ensure the durability of the UV absorption function whilst improving the hardness and strengthening the organic binders currently used in wood technology. Since the nanoparticles do not scatter light, the coating remains transparent. The transparency (i.e. no coloration, no whitening) is an important requirement for the wood coating industry; since wood is a natural material, the coating must be as neutral as possible. When the durability is targeted, coloured pigments are often added to help in this way, but this negatively impacts the aesthetics of the end product. Organic UV absorbers are also efficient, but their actions are limited because of progressive destruction of active molecules (migration, leaching, photochemical activity).
Hardness and Abrasion
Hardness testing, performed according to the Persoz method, corresponds to the measurement of the energy absorption by the film when submitted to repeated loadings. The result (expressed in seconds) is the number of oscillations of a pendulum in contact with the tested film when it is inclined 12° at the beginning and 4° at the end of the test. This is a fairly complex experiment involving hardness, friction and mechanical losses. In other words, it characterizes the visco-elasto plasticity behaviour of the film. The higher the result, the better the film properties.
Abrasion resistance is a key property for coatings. One can simulate a very practical situation using a simple testing device, where an abrasive material is moved at a controlled velocity under a constant pressure. The measurement of the surface gloss, before and after the test, is a good indication of the abrasion resistance of the coating.
In order to characterize the mechanical properties of the modified coating, a free film is formed through the deposition of a coating layer onto a non-sticky surface (glass). Dog bone specimens are cut and tested on a tensile machine. The yield stress, tensile strength and elongation are recorded at the breaking point.