1 Introduction
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. The foundation of chemical modification methods lies in the possibility to change the properties of wood by changing its chemistry and these methods have 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 also is an impact on the physical properties of the wood (Rowell
1996).
Acetylation is one of the most studied chemical modification methods (Rowell
2006) and was introduced in Germany in 1928 by Fuchs (
1928). The chemical process of acetylation involves a reaction of acetic anhydride with wood polymers, resulting in the esterification of accessible hydroxyl groups in the cell wall, as well as formation of the by-product acetic acid. The acetylation process is a single-addition reaction, meaning that one acetyl group is bound to one hydroxyl group, without any polymerisation (Rowell
1983). The number of free hydroxyl groups that normally bind water is reduced and substituted by hydrophobic acetyl groups. The combination of a lesser number of accessible hydroxyl groups and more hydrophobic fibres decreases the water sorption. This change in hygroscopicity results in a reduced equilibrium moisture content (EMC) and fibre saturation point (FSP) (Rowell
2006). Furthermore, acetylation impacts the wood volume. For acetylated wood with a weight percentage gain of approximately 20%, the oven-dry wood volume equals the original green volume. Hence, as the wood is in a permanently swollen state, acetylated wood exhibits fewer fibres per cross-section area, compared to its unmodified state (Rowell
1996).
Comprehensive studies have shown that acetylated wood exhibits enhanced dimensional stability and improved resistance to biological degradation; for compilations of studies see for example Rowell (
1983), Rowell (
2006) and Brelid (
2013). Changing the chemical constitution of the cell wall polymers may also impact mechanical properties. For modified wood, well-studied mechanical properties are the modulus of elasticity (MOE) and modulus of rupture (MOR), determined through bending tests (see e.g. Dreher et al.
1964; Larsson and Simonson
1994; Bongers and Beckers
2003; Epmeier and Kliger
2005). Findings depend on wood species, climate conditions and utilised acetylation techniques. Previous studies have also reported that acetylated wood demonstrates improved hardness (Dreher et al.
1964; Bongers and Beckers
2003), improved compressive strength parallel and perpendicular to the grain (Dreher et al.
1964; Bongers and Beckers
2003), improved wet compressive strength (Goldstein et al.
1961) and a reduced relative creep, measured under cyclic relative humidity conditions (Epmeier and Kliger
2005). Insignificant effects from acetylation have been concluded regarding the impact strength (Goldstein et al.
1961; Bongers and Beckers
2003), while significant reductions in the shear strength have been demonstrated for various wood species (Dreher et al.
1964).
Whilst the effects of acetylation on dimensional stability, durability and basic mechanical properties have been investigated extensively over the last decades, less research concerning fracture properties has been performed. The occurrence of knots, holes, notches, moisture gradients etc., can induce large tensile stresses perpendicular to grain in structural elements which may lead to crack initiation and propagation (Gustafsson
2003). It is thus important to consider fracture properties when wood is used for structural applications. In the design of mechanical joints, fracture properties related to mode I and II are decisive, for example, when a load is applied at an angle to the grain, or in order to avoid brittle failure modes, such as splitting (Ehlbeck and Görlacher
2017). In particular, dowel-type joints have been researched in terms of brittle failure modes, for which both mode I and mode II fracture energy are of importance for the load bearing capacity (Sjödin and Serrano
2008; Jensen and Quenneville
2011; Cabrero et al.
2019). A study conducted by Reiterer and Sinn (
2002) indicates a reduction in the mode I fracture energy with approximately 20% for acetylated spruce. The study also includes fracture characteristics of heat-treated spruce specimens, indicating even larger reductions in the fracture energy, in the order of 50%–80%.
The aim of this study is to further investigate the effects of acetylation on the fracture characteristics of wood, namely Scots pine. In the current paper, the term “fracture characteristics” refers to those material parameters that influence the brittleness of the material, i.e. strength, stiffness and fracture energy. The work includes studies on the fracture energy for mode I loading in tension perpendicular to the grain, modulus of elasticity along the grain and tensile strength perpendicular to the grain for modified and unmodified specimens. The fracture energy is determined for specimens consisting of either sapwood or heartwood, whereas the stiffness and strength are only determined for specimens consisting of sapwood. The investigated material consists of wood from young logs from thinnings. Such wood is rarely used for structural purposes due to its poor durability and poor dimensional stability. By acetylation, the aim is to enhance both durability and dimensional stability. If it is possible to do so, without impairing the mechanical properties, it will enable the use of young Scots pine in outdoor load-bearing applications.
4 Conclusion
In this study, unmodified and modified samples of Scots pine were examined. Modified samples had an acetyl content of approximately 20%, and all specimens were conditioned until equilibrium at a RH of 60% and a temperature of 20 °C. Acetylated samples demonstrated a significantly lower moisture content than unmodified samples. Significant differences were also observed regarding the fracture energy, where the mean value decreased with 36% and 50% for acetylated heartwood and sapwood, respectively. No significant effects of the acetylation regarding tensile strength perpendicular to the grain, nor modulus of elasticity parallel to the grain, could be concluded. The observations demonstrate an increased brittleness for acetylated Scots pine. This fact is important to regard in the design of mechanical joints as well as in structural elements where tensile stresses perpendicular to grain appear. Based on the knowledge gained, further studies will be conducted regarding structural applications to determine whether current design codes have to be revised, in order to account for the increased brittleness of acetylated Scots pine.
To further investigate the cause of the decreased fracture energy, both modified and unmodified specimens conditioned at a range of moisture contents should be examined. By doing so, the effect of MC on the fracture energy can be separated from other effects, such as the changed chemistry and the amount of wood fibres. Furthermore, it would be of interest to investigate the microstructure of the modified wood, to study the presence of micro-cracks and determine whether there is a degradation of the cell wall polymers.
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