LIGNIN AS BINDER IN PARTICLE BOARDS USING HIGH-FREQUENCY HEATING - PROPERTIES AND MODULUS CALCULATIONS

This paper reports the results of a study exploring the possibility of employing unmodified Kraft (sulphate) lignin as a binder in particle boards manufactured using high frequency (27 MHz) heating. The mechanical parameters of the samples varied substantially with their density. For instance, the modulus of elasticity increased from 2.5 to 6.1 GPa when density changed from 700 to 1200 kg/m3. Similar results were obtained when replacing the lignin component with some thermoplastic binders (PVCAC, PVC, LDPE, PA 11, and an acrylic copolymer). The water resistance of the samples was relatively low. For instance, a sample containing 20% lignin and having a density of 700 kg/m3 exhibited after 2 h exposure to water a thickness swelling of 40%. With a similar sample containing PVCAC as binder, the corresponding value was 13%. It was possible to reduce the degree of swelling with hydrophobizing additives or by using acetylated wood flakes. An addition Of 5% of a polyethylene wax thus reduced the swelling of lignin bound samples to below 5%. Similar results were obtained with PVCAC as binder. Supplementary experiments were carried out with adding hollow microspheres (Expancel) to the material. Reduced swelling was recorded also in this case. Predictions of the modulus values based on the Halpin-Tsai concept produced a fair degree of agreement with the experimental data, provided the density and the binder content were sufficiently high.