Nanoglaze for rainy days
Nanoscale additives have been applied to improve the weather-protection and antifungal action of 2-in-1 waterbased glazing systems used in outdoor wood applications.
The latest EU regulations for volatile organic compounds have forced many producers to switch to water-based systems.
According to Dr Mario Beyer of the Institute of Wood Technology in Dresden (IHD), Germany, the protection efficiency of these coating systems, however, is still low. ‘The somewhat different film formation processes of the water-based coatings may cause some mechanical weakness or a certain predisposition to staining or fouling if the raw materials are not thoroughly selected.’
To address this problem, Beyer is part of an IHD team that has undertaken ‘a screening of stateof- the-art coating systems in combination with usual light protective and fungicidal additives’. To do this, they have applied ‘a combination of natural and artificial weathering test procedures, as well as testing the film protection ability of the most common fungicidal substances’.
They have also investigated nanoscale additives from a variety of producers. He outlines that they ‘varied concentration and tried to find optimum particle size distributions. The testing was done mostly by artificial methods since we got results in comparatively short time spans. The most promising variants were then optimised’.
Beyer underlines that ‘the quality of nanoscale metal oxides depends strongly on their production technology. For a good performance, you need narrow size distributions in the lower particle size range, otherwise we observe haze’.
Titanium dioxide (TiO2) and zinc oxide (ZnO) provide the best light protection, with microionised iron oxide formulations also performing well. ‘Their (TiO2 and ZnO) UV absorption quality is quite similar to that of organic UV absorbers. Their advantage is their high photochemical stability, which leads to extended lifetimes when compared to organic substances,’ he explains.
However, Beyer cautions that ‘one has to pay attention to the potential photocatalytic activity of TiO2 or ZnO, which can lead to photooxidative damage of the binder if the nanoparticles are not deactivated’. While they are relatively expensive, he says, ‘nanoscale UV absorbers can be applied in smaller concentrations than the organic ones and last longer’.
The researchers have also found that ‘organic fungicides are highly efficient but suffer a strong UV-sensitivity. Therefore, these substances must be additionally UV-protected. We investigated nanoformulations of conventional fungicides. They showed a somewhat better performance than the dissolved fungicides. Their effect, however, was not fully sufficient to justify the use of more expensive additives instead of the conventional ones.’
The team has tested novel compounds with fungicidal properties, yet these have not shown sufficient activity and are not market-ready.
On the effect of the nanoadditives on binding properties, he says, ‘we did not observe additional decay, chalking or weakening of the mechanical properties exceeding that of conventional systems’.
Overall, Beyer claims the project has shown that ‘the weather protection performance of transparent thin lacquers can be improved by the investigated additives’, adding that the raw materials tested are currently available in the market. However, he admits that ‘water-borne ones are still behind the systems containing certain solvent-based fractions’.
The group is pursuing several research avenues, including the light protection of wood and wood materials, techniques for printing on wood materials, powder coating and the irradiative curing of coatings for interior and exterior applications.