| A physical method was used to modify wheat straw material to solve its inherent bonding problem which is caused by its higher ash and extractive contents and a waxy substance on the surface of wheat straw. Specifically, the mechanism of bondability improvement through steam explosion treatment and their effects on panel properties were discussed.To investigate the mechanism of bondability improvement, the effects of steam temperature and retention time on morphology, acidity, wettability, chemical composition, and thermal properties of wheat straws were studied. The results showed that the proportion of coarse particles decreased while the small particles and fiber bundles increased with increasing severity of steam explosion treatment. Higher steam temperature and longer treatment time resulted in more homogeneous fiber-like materials. SEM image revealed that the waxy outer layer on straw was destroyed, resulting in more porous structure and a better permeability on straw surface. The pH value of the untreated wheat straw fiber was nearly 7 and the pH values and acid buffer capacities of straws were greatly reduced after steam explosion treatments. This indicated that the acidity of straws increased after steam explosion treatments. The dynamic contact angle of the straws before the treatment was nearly 90 degree, indicating that the straw materials without treatment are more hydrophobic. After steam explosion treatments, the contact angle of straws was significantly reduced, showing that the surface wettability of the treated straws was improved. The ash and silicon contents of straws were also significantly reduced by steam explosion treatments. The improved acidity and wettabillity as well as decreased silicon content would contribute to the improved bondability between straw particles and water-soluble adhesive binders.Particleboards bonded with urea formaldehyde (UF) and a combination of UF and MDI resins were manufactured using wheat straws modified through different steam explosion treatments. The effect of steam explosion treatment, resin system, and mixing ratios of wood and straw materials on the panel properties was investigated. Generally, with steam explosion treatment of straw fibers, panel properties were significantly improved. The internal bond (IB) value of the particleboard made from the straws pre-treated at steam temperature of 190°C and retention time of 3 min was 10 times higher than that of control. The boards with the straws modified at 190°C and 2 min had the highest modulus of rupture (MOR) and modulus of elasticity (MOE), which are, respectively, more than and nearly 2 times higher than those of the control. Thickness swelling (TS) was 1/4 of the value of control. The IB strength dramatically increased with the use of dual resins at all conditions. The IB value of the board with both dual resins and steam explosion modification was more than 17 times higher than that of the control. This indicates that dual resin system combined with steam explosion modification can significantly enhance the bonding strength between straw particles and UF resin. The dual resin system also contributed in improving the dimensional stability of straw board. The mechanical properties of mixed straw and wood particleboards increased with more wood loading levels. The use of wood particles to replace part of straw materials can help improve the panel properties.
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