They are extremely scratch-proof, environmentally friendly, easy to apply and versatile in their application because they can be applied equally as well to parquet floors as to metals and plastics. These are the advantages promised by acrylic paints containing the new siloxane-encapsulated nanoparticles developed by researchers at Dusseldorf-based Degussa AG. In comparison to conventional acrylic systems, the wear resistance of these coatings is ten times higher, made possible by the exact mix of inorganic nanoparticles, based on the pyrogenic silica Aerosil, and the organo-functional Sivento silanes. These are in turn suspended in a liquid acrylic-based UV-cross-linkable binder.
Combining Scratch Resistance and Easy Processability
The material was developed by the Aerosil and Silanes Business Unit of Degussa AG, who collaborated with the Institute of Surface Modification in Leipzig, and together succeeded in combining two normally mutually exclusive effects - namely those of scratch resistance and easy processability.
The scratch resistance of the coating is improved by adding a high percentage of silicon dioxide (SiO2) to the mix. However, owing to the polarity on the surface of the individual particles, such an increase increases the viscosity, in turn making it harder to process. To overcome this, the researchers encapsulate the nanoparticles directly in the binder with a thick, soft polysiloxane coating of organo-functional silanes. In this way the polarity of the particle surface is changed, which drastically reduces the viscosity of the product without impairing the scratch resistant properties of the particles. Via this process, the particles are transformed into nanocapsules with a very hard silicon dioxide core and a soft siloxane capsule that can be directly synthesised in the UV-cross-linkable binder, or in the paint itself without the need for additional solvent.
New Hybrid Coating
The result is a completely new inorganic/organic acrylic-based hybrid coating that can contain more than 30% by weight of nanocapsules. This high concentration is responsible for the extremely good scratch and wear resistance of the coating, which is ten times higher than conventional acrylic systems. Thanks to the incorporation of silicon-oxygen bonds, the coating is also UV, thermally and mechanically stable.
Bjorn Borup, a member of the development team, explains further, ‘The silane, by itself, has a part that reacts with hydrophilic groups - basically the SiOR groups react under hydrolysis conditions with hydrophilic groups, especially with SiOH groups on the surface of the aerosil. The silane also has a hydrophobic part and it is this that makes the hydrophilic aerosil hydrophobic. This is the crux of the matter, I would say, the changing of the polarity, or rather the nature of the aerosil particle, by putting a substance on it that can react with both hydrophilic groups and has a hydrophobic bond.’
Due to their hybrid nature, the UV- and radiation-curing coatings combine the positive properties of inorganic materials, such as hardness, mechanical stability, durability, non-flammability and LTV resistance, with the advantages of organic polymers, such as elasticity and cross-linkability. Furthermore, they are highly reactive as they cure in seconds, they are easy to apply as no special equipment is needed, they are environmentally friendly because they contain no solvents, and finally, because of the small particle size, they are transparent. This last attribute ensures that they will have many applications, especially as varnishes and barrier coatings.