The Effect of Orientation, and Temperature on Thermal Conductivity in Nettle Fiber/Polyester Composites

Abstract

In this study, the thermal conductivity (k-values) of pure and composite samples obtained from raw materials which these samples came from different parts (stalk, fiber) and different regions (bottom, middle and top) of nettle plants and collected from low and high altitudes (350 and 2100) in the Black Sea regions were investigated depending on orientation and temperature. According to the ASTM C518 standard, the k-value is only given for the temperature value of 10 °C. However, it is also important to determine the thermal conductivity depend on climatic conditions, ambient temperature, and humidity. It is known that the thermal conductivity coefficient changes depend on the density, pore structure and size, humidity, and temperature. However, for materials with a fibrous structure (rock wool, glass wool, VIP), the orientation of the fiber structure depends on the heat transfer direction also significantly affects the thermal conductivity. In the first stage of the study, the pure thermal conductivity of cellulosic materials was determined. The fiber parts of the nettle plant, which have low thermal conductivity (k < 0.040 W/mK), were used in the formation of the composites. In this context, the thermal conductivity of composites formed with fibers obtained from two different altitudes was characterized depend on temperature (0 ºC, 10 ºC, 20 ºC, 30 ºC, 40 ºC) and orientation (0º, 45º, 90º). Ratios such as 2.5 %, 5 %, 7.5 %, and 10 % by volume were used to form the composite, and the effect of orientation and temperature was measured in composites with a 10 % reinforcement ratio. The manual laying method was used in the production of test samples. The physical and chemical properties of the reinforcement material affecting the thermal conductivity of the composites were determined and its effect on the thermal conductivity coefficient was discussed. In the conducted study, it was determined that the altitude difference affects the characteristic features of the plant (such as fiber diameter, crystal structure, and density), and accordingly, the thermal conductivity behaviors show differences. On the other hand, it was determined that orientation significantly affects the thermal conductivity coefficients, but the effect of temperature increase on the thermal conductivity coefficient change in the same composites is low. As a result, it has been concluded that the fibers obtained from the bottom parts of the plants obtained from high altitudes may be a more suitable insulation material than the fibers obtained from the other part.