Wood-Composite Boards with a Low Coefficient of Linear Thermal Expansion

Abstract

A number of industries require materials with a low coefficient of linear thermal expansion (CLTE), in particular, in the production of satellite spherical antennas. The latter are formed from composites containing carbon fibers and synthetic resins. The composition is cured by heating up to 180 °C. This leads to a thermal expansion of the mold and a change in the geometric characteristics of the product. Therefore, specific requirements are imposed on the materials for making molds. The high cost of special materials used for molds determines the need to search for other materials with a low CLTE. Wood is a possible solution to this problem. Its CLTE along the fibers is less than that of the vast majority of materials, and is approximately 3 ‧ 10-6 K–1, which is comparable to special materials. However, the expansion of wood across the fibers is much higher than the longitudinal one, which excludes the use of solid wood. Anisotropy can be reduced by creating a composite in which the fibers are uniformly oriented in all structural directions, bringing the value of wood expansion across the fibers closer to the value of expansion along the fibers. The traditional approach to producing wood composites, based on the use of synthetic adhesives, fails to achieve a noticeable reduction in thermal expansion due to the high CLTE of adhesives The use of boards made of hydrodynamically activated wood particles without binders is promising. Three series of experiments have been carried out: with varying the density of the boards, preliminary thermal modification of the original wood and the use of alkali during hydrodynamic processing. The thermal expansion study has been carried out using the NETZSCH DIL-402 C induction dilatometer in dynamic mode with a heating rate of 2 K/min. It has been established that thermal expansion increases with increasing density.The average CLTE at a density of 950 kg/m3 is 12 ‧10–6 K–1 and at a density of 1,100 kg/m3 it is 17‧10–6 K–1. At a comparable density, the thermal expansion of boards without binders is significantly lower than that of fiberboards (MDF). Preliminary thermal modification of wood does not significantly affect the CLTE of the boards. The use of alkali in the hydrodynamic treatment also has no effect.