Abstract
A study on the fracture characteristics of unmodified and chemically modified Scots pine (Pinus sylvestris) is presented. The investigated material consisted of small-dimension sawn timber originating from young logs (thinnings), aged 30–40 years. The modified samples were acetylated with acetic anhydride in an industrial scale process without the use of any catalyst, reaching an acetyl content of approximately 20%. Clear wood specimens, consisting of either heartwood or sapwood, were extracted and conditioned to equilibrium at a relative humidity of 60% and a temperature of 20 °C. The fracture energy for mode I loading in tension perpendicular to the grain was determined using single edge notched beam (SENB) specimens, subjected to three-point bending. Additionally, the modulus of elasticity along the grain and the tensile strength perpendicular to the grain were determined for sapwood specimens. The findings demonstrated a significant decrease (between 36 and 50%) in the fracture energy for the acetylated specimens, compared to the unmodified specimens. No significant effect of the acetylation process on the modulus of elasticity, nor on the tensile strength could be concluded. This study indicates that the acetylation process used results in an increased brittleness for Scots pine. Further studies are needed to analyse why the fracture energy is impaired, and to examine whether and how current timber engineering design provisions can or should be revised to account for the increased brittleness of acetylated Scots pine.
Highlights
Softwoods demonstrate low durability and poor dimensional stability when exposed to changes in moisture content, resulting in, for example, crack initiation caused by differential swelling, or loss of strength due to biological degradation
The measured moisture contents are slightly lower than values reported for acetylated Scots pine in previous studies (Epmeier and Kliger 2005)
Modified samples had an acetyl content of approximately 20%, and all specimens were conditioned until equilibrium at a relative humidity (RH) of 60% and a temperature of 20 °C
Summary
Softwoods demonstrate low durability and poor dimensional stability when exposed to changes in moisture content, resulting in, for example, crack initiation caused by differential swelling, or loss of strength due to biological degradation. To overcome these drawbacks, without the use of toxic preservatives, different modification methods have been developed. Division of Structural Mechanics, Faculty of Engineering LTH, Lund University, P.O. Box 118, 221 00 Lund, Sweden. Division of Solid Mechanics, Faculty of Engineering LTH, Lund University, P.O. Box 118, 221 00 Lund, Sweden proven to be successful in limiting the hygroscopic characteristics of wood (Rowell 2006). As there is a change in chemistry of the cell wall polymers, there is an impact on the physical properties of the wood (Rowell 1996)
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