Influence of thermo-pressing conditions on the mechanical properties of biodegradable fiberboards made from a deoiled sunflower cake

The objective of this study was to manufacture new biodegradable fiberboards by thermo-pressing. The starting material was deoiled cake (only 0.9% oil content), generated during the biorefinery of sunflower (Helianthus annuus L.) whole plant in a co-rotating twin-screw extruder. All fiberboards were cohesive mixtures of proteins and lignocellulosic fibers, acting respectively as binder and reinforcing fillers. The molding experiments were conducted using a 400 ton capacity heated hydraulic press. The influence of molding conditions on board density, mechanical and thermo-mechanical properties, thickness swelling, and water absorption was examined. Molding conditions included pressure applied (24.5-49.0 MPa), molding time (60-300 s), and mold temperature (156-204 °C), and these greatly affected board density and thus the mechanical and thermo-mechanical properties. Board density increased with increasingly extreme molding conditions, rising from 1162 to 1324 kg/m3. The flexural properties increased at the same time (from 12.2 to 27.7 MPa for flexural strength at break, and from 2183 to 5244 MPa for elastic modulus) and also Shore D surface hardness (from 69.6 to 79.0°). Conversely, Charpy impact strength was low and quite independent of thermo-pressing conditions. Statistical analysis of the Doehlert's experimental design was conducted to determine optimal thermo-pressing conditions for flexural properties, giving 49.0 MPa pressure applied, 300 s molding time, and 204 °C mold temperature. Density of boards molded under these conditions was 1267 kg/m3. Flexural strength at break, elastic modulus and Shore D surface hardness were 30.3 MPa, 5946 MPa, and 81.5°, respectively, and these corresponded to the highest values for the entire study. Such boards largely complied with French standard NF EN 312, type P4 (i.e., load bearing boards for use in dry conditions) for flexural properties. However, thickness swelling (30%) needs to be slightly reduced to achieve the 21% recommended standard value.