Improving the hail resistance of wood coatings through reinforcement with nanofibrillated cellulose
WHFF Project 2016.01
Projectmanagement: Dipl. Ing. Tina Künniger
The most important facts in brief
- Different wood coatings for exterior use reinforced with 0%, 2.5%, 3.5% and 5% nanofibrillated cellulose (NFC) were investigated.
- NFC increased the impact tensile strength as well as the relaxation modulus of the coatings.
- While only relatively shallow indentation depths were measured at the impact points of the ice spheres on new surfaces, these indentations were much deeper on the pre-weathered specimens.
- The impact, even with very low kinetic energy, resulted in dents, hairline cracks, larger cracks or peeling and flaking of the coating. In most cases, the minor damages were only visible under the microscope.
- The reinforcement of the coating by NFC contributed to the reduction of the maximum indentation depth and to a lower intensity of cracks.
- After 9 months of natural weathering, 99% of all hail impacts were clearly visible due to cracks and the growth of microorganism within the crack lines.
- Despite an improvement in hail resistance, none of the tested variants met the requirements for hail resistance class 2.
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 correlation between material type and hail resistance. In the synthesis report of the Prevention Foundation of the Cantonal Building Insurers from 2007, it is mentioned that an increased use of wood products 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, wood products coated with a thick-layer varnish or lacquer (layer thickness > 30 μm) are NOT classified so far due to their insufficient performance. In contrast to the untreated wood or wood coated with thin-layer varnish, not only the visual appearance of the surface must be evaluated after hailing, but also the waterproofness must be tested.
Four water-borne, white-pigmented wood coatings for exterior use based on different binder systems 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 standing films. Subsequently, the coatings applied to planed spruce panels in new and pre-weathered state were shot with ice spheres (Ø 20 mm) to simulate hail. Two intensities of hail 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 the hail impact and again after 28 days storage and after 9 months of natural weathering.
NFC as reinforcement resulted in increased impact tensile strength as well as increased relaxation modulus of the coatings. The NFC network reduced the plastic deformation of the coatings.
The impact, even with very low kinetic energy, damaged the coating on all specimens in the form of dents, hairline cracks, larger cracks or flaking. However, these were very often only visible under the microscope. While only relatively small indentation depths were measured at the impact points of the ice spheres on as-new surfaces, the indentations were much deeper on the pre-weathered specimens. At very low shot velocities, the measured indentation depths correlated with the intensity of the cracks formed. At greater shot velocities, cracks were generally more severe, regardless of the measured indentation depth.
NFC could contribute to the reduction of the maximum indentation depths and to a lower intensity of cracks. However, the current approach (mixing NFC into an existing coating) did not result in a sufficient improvement in hail resistance to at least meet the requirements for hail resistance class HR 2. After 9 months of natural weathering, 99% of all hail impacts were clearly visible through cracks and the microorganism growth that occurred within the cracks.
The visual assessment of the samples from a distance of 50 cm and 5 m, which is usual for the assessment of the hail resistance, was not sufficient for a correct assessment of the hail resistance class of coated wooden components. For a realistic assessment of the hail resistance class, aged surfaces must be tested and the final assessment should only be made after 6-month natural weathering.
The project was supported by the Forest and Wood Research Promotion Switzerland WHFF-CH of the Federal Office for the Environment FOEN and the Conference for Forests, Wildlife and Landscape KWL of the cantons.
Click here to read the report about the project on ARAMISDownload