WHFF Project 2016.01
Projektleitung: Dipl. Ing. Tina Künniger
The short video about the project on Youtube can be watched under the following link (only available in German): https://youtu.be/W9XVAMuSfbE
The most important facts in brief
Different coatings for exterior wood reinforced with 0%, 2.5%, 3.5% and 5% nanofibrillated cellulose (NFC) were investigated.
The exposure, even with very low kinetic energy, resulted in damage to the coating in the form of dents, hairline cracks, larger cracks and spalling on all samples.
While only relatively shallow indentation depths were measured at the impact points of the ice balls on "as-new" surfaces, these dents were much deeper on the pre-weathered specimens.
NFC was able to contribute to the reduction of the maximum indentation depth and to a lower intensity of the cracks on the pre-weathered specimens
After 9 months of outdoor weathering, 99% of all hailstorms were clearly visible through cracks and the microorganism growth that occurred within them.
None of the tested variants met the requirements for hail resistance class 2.
Project description
Wood has been a popular construction material for various parts of the building envelope for many centuries. To ensure functionality and long durability as well as due to aesthetic requirements, windows and doors, but also components such as facades are usually provided with coatings.
Due to their position in the building envelope, these components are exposed to mechanical influences such as hail to varying degrees. Particularly today's popular construction methods without protective canopies mean that facades, but also window frames and sashes, for example, are directly exposed to the weather and, in the event of a hailstorm, are subjected to severe stress and possibly damaged.
There is a direct dependence between material type and hail resistance. In the 2007 synthesis report of the Prevention Foundation of the Cantonal Building Insurers, it is explicitly mentioned that the increased use of wood did not lead to an increased vulnerability of the building envelope.
However, the current classification of wood in the current hail register shows the existing problem. Natural planed wood as well as wood with a hydrophobic coating or thin-layer glaze (layer thickness < 30 μm) are classified in hail resistance class HR 3 (HR 1 = very weak hail resistance; HR 5 = very high hail resistance). However, woods coated with a thick-film varnish or lacquer (layer thickness > 30 μm) are NOT classified so far due to their poor performance!
Here, in contrast to the untreated wood or wood coated with thin-film varnish, not only the appearance must be evaluated after hailing, but also the watertightness must be tested.
Four water-thinnable, white-pigmented coatings for exterior wood based on different binders were selected for the tests. Important properties of the coating systems, reinforced with 0%, 2.5%, 3.5% and 5% nanofibrillated cellulose (NFC), were characterized in the liquid state and as free films. Subsequently, the coatings applied to planed spruce lamellae in new and pre-weathered condition were shot with ice balls (Ø 20 mm) to simulate hail. Two hail intensities were selected, with the higher one meeting the requirements of hail resistance class 2. The impact points were examined, documented and evaluated with respect to damage such as dents, cracks, etc. directly after hail impact, after 28 days and after 9 months of outdoor weathering.
Conclusions
The exposure, even with very low kinetic energy, resulted in damage to the coating in the form of dents, hairline cracks, larger cracks and spalling on all specimens. Significant differences were observed on as-new and pre-weathered surfaces. While only relatively small indentation depths were measured at the impact points of the ice balls on "as-new" surfaces, these dents were much deeper on the pre-weathered samples.
At very low shot velocities, the measured indentation depths correlated with the intensity of the resulting cracks. At higher shot velocities, cracks were generally more severe, regardless of the measured indentation depth.
NFC was able to contribute to the reduction of the maximum indentation depth and to a lower intensity of the cracks on the pre-weathered specimens.
After 9 months of outdoor weathering, 99% of all hailstorms were clearly visible through cracks and the microorganism growth that occurred within them. None of the tested variants met the requirements for hail resistance class 2.
Download the full report:
You can find more information about the project on ARAMIS.
The project was supported by the Swiss Forest and Wood Research Funding Switzerland (FOWO-CH) of the Federal Office for the Environment FOEN and the Conference for Forests, Wildlife and Landscape KWL of the cantons.