Problem definition and objectives: Wood, as a natural and widely used material in various industries, possesses diverse physical and chemical properties that affect its applications. One of the important properties of wood is its thermal conductivity, which is influenced by multiple factors. In this study, the effect of the amount of extractives, ash content, and density on the thermal conductivity of five wood species—including fir (Abies alba), alder (Alnus glutinosa), silk tree (Albizia julibrissin), plane tree (Platanus orientalis), and walnut (Juglans regia)—was investigated. The main objective of this study is to gain a more precise understanding of the relationship between the physical and chemical properties of wood and its thermal conductivity to enhance industrial applications.
Methodology: For this purpose, two sound and knot-free samples of each species, measuring 2×20×20 centimeters, were prepared in an intermediate direction between radial and tangential planes from trees grown in the forests of Nowshahr. The samples were placed for two weeks in a conditioning room with a relative humidity of 60% and a temperature of 20°C to reach an equilibrium moisture content of 12%. Thermal conductivity measurements were conducted using a device according to the ASTM C177 standard. Additionally, density measurements were performed based on the ISO 3131 standard, and the determination of extractives and ash content was carried out according to TAPPI standards.
Results: The results showed that walnut had the highest density at 0.66 g/cm³, while fir had the lowest at 0.42 g/cm³. Plane tree exhibited the highest ash content at 3.09%, whereas walnut had the lowest at 1.33%. Regarding extractives, silk tree showed the highest amount at 3.98%, and alder had the lowest at 0.77%. Statistical analysis indicated that there were significant differences in density, ash content, and extractives among the species at the 95% confidence level. Furthermore, the results demonstrated that species with higher density and higher extractives content also had higher thermal conductivity. For example, silk tree had the highest thermal conductivity (0.253 W/m·°C), and fir had the lowest value (0.129 W/m·°C). This can be attributed to the reduction of porosity and the replacement of air in the wood's voids with extractives, leading to increased heat transfer. Ash content and the presence of metal ions such as calcium, potassium, magnesium, iron, copper, zinc, and nickel can also affect thermal conductivity, but they are not solely determining factors. Increases in extractives content and density have a greater effect on thermal conductivity than ash content. The findings of this study highlight the importance of the physical and chemical properties of wood in determining its thermal conductivity.
Conclusion: This information can be utilized in various industries such as construction, interior design, wood product manufacturing, and wood drying planning. A more precise understanding of these properties aids in selecting appropriate wood species for specific applications, enhancing the efficiency and performance of products. In general, this research shows that, in addition to density, extractives play a significant role in the thermal conductivity of wood. Therefore, considering the chemical composition of wood alongside its physical properties is of great importance for improving industrial processes and developing new applications.