Supplementary fibre supplies for the manufacture of MDF and particleboard. Part VI. Process development for the manufacture of particleboard using steam explosion treated wheat straw and combination resin system
Experimental work was carried out for the manufacture of UF resin bonded wheat straw particleboards with various steam explosion treatments and a combination resin system. Panels with different combinations of wood and straw particles were also made. The effect of different treatment conditions (i.e., steam temperature and retention time), resin combination and mixing proportion of wood and straw materials on the properties of boards was investigated. To clarify the mechanism of bondability improvement, chemical characterization of wheat straws was conducted to identify the acidity, wettability, and silicon content of the raw material before and after treatment. Two panels were produced for each condition, and a total of 30 boards were made. The properties of panels were tested in accordance with ASTM D1037.
The acidity of the wheat straw was low and wettability was very poor before the treatment. Both acidity and wetttability were increased after steam explosion treatment. The ash and silicon contents of the wheat straw before the treatment were much higher than those of wood material. Part of ash and silicon can be removed or washed out by steam when the steam was released after the treatment, therefore resulted in the decreased silicon content. The increased acidity and wettability as well as the decreased silicon content contributed to the bondability improvement between straw particles and UF resin.
The results show that the steam explosion treatment and/or its combination with combination resin system can be a feasible approach to improve the bonding strength between wheat straw materials and adhesive binders. Generally, panel properties in terms of bending strength (MOR and MOE) and internal bond (IB) strength increased, thickness swelling (TS) and water absorption (WA) decreased for the panels with the steam explosion treated straws. This indicates that steam explosion treatment improved all the panel properties. The better performance of the boards with treated straws are attributed to the improved bondability between straw particles and UF resin due to increased acidity and wettability as well as the decreased silicon content after treatments.
The effect of the combination of resins and treatments on the panel properties shows that the MOR and MOE values for the panels from the straws after 12 h water soaking treatment were significantly improved at the UF/MDI combination of 10%/2%. The IB strength dramatically increased with the use of combination resins. The TS and WA showed the decreased values for all the panels made with the combination of UF and MDI resins. The results indicate that the combination of dual resin system and steam explosion treatment can significantly improve the panel performances of straw boards.
The properties of mixed straw and wood particleboards indicate that there was a decreasing trend of the MOR, MOE, and IB with increase of straw addition contents while a increasing trend in TS and WA. Pure wheat straw board had the lowest bending and IB values, and highest TS and WA. The inferior strength and dimensional stability properties of pure straw boards were due to the poor bonding between the straw particles caused by the outer waxy layer and high silica content of straw materials. The use of wood particles to replace part of straw materials can help improve the panel properties.