Abstract
Wood, as a biological material, is sensitive to environmental conditions and microorganisms; therefore, wood products require protective measures to extend their service life in outdoor applications. Several modification processes are available for the improvement of wood properties, including commercially available solutions. Among the chemical treatments, acetylation by acetic anhydride is one of the most effective methods to induce chemical changes in the constitutive polymers at the cellular wall level. Acetylation reduces wood shrinkage-swelling, increases its durability against biotic agents, improves UV resistance and reduces surface erosion. However, even if the expected service life for external cladding of acetylated wood is estimated to be 60 years, the aesthetics change rapidly during the first years of exposure. Hybrid, or fusion, modification includes processes where the positive effect of a single treatment can be multiplied by merging with additional follow-up modifications. This report presents results of the performance tests of wood samples that, besides the modification by means of acetylation, were additionally protected with seven commercially available coatings. Natural weathering was conducted in Northern Italy for 15 months. Samples were characterized with numerous instruments by measuring samples collected from the stand every three months. Superior performance was observed on samples that merged both treatments. It is due to the combined effect of the wood acetylation and surface coating. Limited shrinkage/swelling of the bulk substrate due to chemical treatment substantially reduced stresses of the coating film. Hybrid process, compared to sole acetylation of wood, assured superior visual performance of the wood surface by preserving its original appearance.
Highlights
Biodegradability, identified as an important advantage of biomaterials from a sustainability perspective, is its biggest weakness when considering materials durability
It is due to the combined effect of the wood acetylation and surface coating
Even if a properly designed structure may not be affected by the moisture-induced shrinkage, the dimensional changes may affect the surface coatings creating excessive stresses followed by surface cracks [7,8]
Summary
Biodegradability, identified as an important advantage of biomaterials from a sustainability perspective, is its biggest weakness when considering materials durability. As well as other hygroscopic materials, is subject to dimensional distortions caused by changes in the moisture content, air humidity and temperature. Even if a properly designed structure may not be affected by the moisture-induced shrinkage, the dimensional changes may affect the surface coatings creating excessive stresses followed by surface cracks [7,8]. It alters the protection function of the coating and may promote even more excessive decay through the moisture bridges. It is essential to implement best practices of the “protection by design” paradigm to project and construct façade details that allow or compensate for dimensional distortions and minimize related treats [9]
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