The effects of heat treatment on some technological properties of Scots pine (Pinus sylvestris L.) wood
Introduction
Scots pine has superior technological properties and high usage potential. It is an important tree species in the forest products industry of Turkey, covering over one million ha, making up 5% of the total Turkish forestlands (Anonymous, 2001).
Heat treatment is one of the processes used to modify the properties of wood (Mazela et al., 2004). Heat treatment, as a wood modification method, serves to improve the natural quality properties of the wood, such as dimensional stability and resistance to bio-corrosion and equip the wood material with new properties.
The heat treatment process involves exposing wood to elevated temperatures ranging from 160 to 260 °C (Militz, 2002). The temperature and duration for heat treatment generally vary from 180 to 280 °C and 15 min to 24 h depending on the process, wood species, sample size, moisture content of the sample and the desired mechanical properties, resistance to biological attack, and dimensional stability of the final product (Militz, 2002, Kamdem et al., 2002, Sanderman and Augustin, 1963).
Reports from various studies indicate that the effect of high-temperature heating (particularly above 175 °C) on strength and brittleness of wood varies depending on the chemical and anatomical nature of the wood and the methods of heating (Hillis, 1984). In other words, the extent of the change in timber properties during heat treatment is determined by the method of thermal modification, the wood species and its characteristic properties, the initial moisture content of the wood, the surrounding atmosphere, and the treatment temperature and time. It is reported that temperature has a stronger effect on timber properties than the amount of time the timber is exposed to heat (Mitchell, 1988). Treating wood at lower temperatures for longer time periods does not produce the desired properties. Temperatures over 150 °C alter the physical and chemical properties of wood gradually (Syrjanen and Oy, 2001, Mitchell, 1988). The higher the treatment temperature the better the wood’s biological durability. However, at temperatures over 150 °C the strength properties start to weaken. The wood becomes more brittle, and bending and tension strength decrease by 10–30%. Therefore, the use of heat-treated wood in load-bearing constructions is restricted (Jamsa and Viitaniemi, 2001).
Heat treatment reduces certain mechanical properties, but the dimensional stability and the biological durability of wood increases through heat treatment. In addition, heat treatment results in favorable changes in the physical properties of the wood, such as reduced shrinkage and swelling, low equilibrium moisture content, enhanced weather resistance, a decorative dark color, and better decay resistance (Yildiz, 2002). Therefore, heat-treated wood is an eco-friendly alternative to impregnated wood materials, and heat-treated wood can be used for garden, kitchen and sauna furniture, cladding on wooden buildings, bathroom cabinets, floor material, musical instruments, ceilings, inner and outer bricks, doors and window joinery, and a variety of other outdoor and indoor wood applications (Syrjanen and Oy, 2001).
Industrial-scale heat treatment process was developed at the Technical Research Center of Finland in the early 1990’s. The total production capacity of heat-treated wood in 2002 is estimated approximately to be 265 000 m3 (Syrjanen and Oy, 2001, Rapp, 2001). Recent efforts on thermal treatment of wood have led to the development of several treatment processes, and materials produced through thermal treatments have been introduced to the European market. Some of the products developed by thermal treatment include Thermowood (Stellac) in Finland (Viitaniemi et al., 1994), Torrefaction (Perdure) in France (Weiland and Guyonnet 1997) and PLATO-Wood in the Netherlands (Militz, 2002).
Research on the effects of heat treatment on the technological properties of Turkish native trees is rather limited. This study aims at examining the effects of heat treatment on technological properties of Scots pine (Pinus sylvestris L.) wood, which has not been done before. Scots Pine is one of the most common wood species naturally grown and intensively used in the forest product industry in Turkey. Improving the characteristics of Scots pine through heat treatment would offer the timber product industry many interesting opportunities.
Section snippets
Methods
Scots pine logs (min. diameter 40–50 cm) were obtained from Aladağ-Bolu, Turkey. Lumber from the logs was prepared in the sawmill of Forestry Faculty, Istanbul University, Turkey [TS 4176]. Scots pine lumber was planed with a knife angle of 45° and then small clear specimens were cut for compression strength parallel to grain (2 × 2 × 3 cm), bending strength (2 × 2 × 36 cm), modulus of elasticity in bending (2 × 2 × 36 cm), janka-hardness (5 × 5 × 5 cm), impact bending strength (2 × 2 × 30 cm) and tension strength
Results and Discussion
Table 1 shows the changes in the compression strength parallel to grain, bending strength, modulus of elasticity in bending, janka-hardness, impact bending strength and tension strength perpendicular to grain at varying treatment temperature and durations. The values indicate that heat treatment diminished the technological properties of Scots pine. ANOVA and Duncan’s Multiple Range Tests show that all differences were significant (Table 1). Furthermore, as (Table 2) shows an increase in time
Conclusion
In this study on Scots pine (P. sylvestris L.) wood, it was found that compression strength parallel to grain, bending strength, modulus of elasticity in bending, janka-hardness, impact bending strength and tension strength perpendicular to grain decrease by heat treatment. Furthermore, the results showed that an increase in time and duration resulted in greater decreases in treated samples compared to the control samples. On the other hand, the smallest decreases were observed in the treatment
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