Acetylation of European hornbeam (Carpinus betulus L.) wood for outdoor applications

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Absztrakt (kivonat)

Many European wood species have low natural durability which makes them unable to be used for exterior applications without additional protection. In this work, European hornbeam wood (Carpinus betulus L.) was industrially acetylated in order to improve its properties and widen its usage. Thorough literature research was made on the availability and future of European hornbeam wood, from forestry and wood industry point of view. Then, the current state of research was explained regarding the modification of this species. Previous work of my master thesis on acetylated hornbeam discussed improved dimensional stability, mechanical properties, durability against fungi in laboratory conditions, and aesthetical color. In order to understand the changes in product-related properties after acetylation, chemical analysis was carried out to determine the chemical components, pH, buffering capacity, and content of organic extractives. These findings were evaluated by Fourier-transform infrared spectroscopy (FTIR) and high-performance liquid chromatography (HPLC). After acetylation, changes in microscopic structure were examined by bright-field microscopy and scanning electron microscopy. In a six-year-long test in soil, acetylated hornbeam showed no signs of fungal decay or insect damage, except for one stake, that was locally attacked by brown rot after 1.5 years. The decayed area was less acetylated compared to the good performing stakes, proven by FTIR analysis. Acetylated hornbeam is more hydrophobic, having increased contact angle and lower surface energy. This influences its bonding and coating properties. Compared to untreated hornbeam, the shear strength was reduced in dry conditions, but showed enhanced strength when soaked in cold or boiling water. The same was experienced in the bonding strength results with polyvinyl acetate and polyurethane adhesives. When exposed to ultraviolet light, acetylated hornbeam fades and grays similarly to untreated hornbeam, based on test results of natural irradiation (2 years) and artificial (200 hours) irradiation. Besides getting a patina, it remains intact, without cracking or warping, like untreated hornbeam. For the case when photostability is desired, various coatings were tested, among which three dark pigmented stains showed the best results. According to these results, use classes, product groups, market share of acetylated hornbeam were determined, supplemented by SWOT analysis and related literature, in order to give a full picture of it as a product.

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