No abstract

Rice straw, a waste agriculture material grown and harvested in Willows, CA, was used as a raw material in the production of thin medium-and high-density fiberboards (MDFs and HDFs). The rice straw was cleaned, size-reduced, and soaked in water before being refined. Defibration was performed in a pressurized pilotplant single-disk refiner, OHP 20". The fiber production capacity reached a level of 63 kg/h, and the proper fiber quality for MDF/HDF production was established. Analysis of the produced fiber showed an average fiber length of approximately 0.9 mm, an average fiber width of 31 µm, a shive weight of below 24%, and a dust content of less than 30%. Production of fiberboards was performed by addition of 3%, 4%, and 5% methylene diphenyl diisocyanate (MDI). The flexural properties, internal bond strength, and thickness swelling of the produced fiberboards were evaluated according to ASTM methods. The finished fiberboards based on rice straw and MDI resin showed excellent properties. The internal bond (IB) reached levels of 2.6 MPa, and the modulus of rupture (MOR) and modulus of elasticity (MOE) showed levels comparable to those of woodbased fiberboards and were acceptable according to the requirements of medium-density fiberboard (MDF) for interior applications (American National Standards Institute, ANSI A208.2-2002). The water-repelling properties of the 3-mm rice-straw fiberboards were encouraging; the thickness swelling (TS) was in the range of 15-30%. Two different methods to avoid adhesion between the press plates and the resinated fiber material during hot pressing were investigated: protective paper sheets were placed between the fiber mat and press plates, or a press-release agent was sprayed on steel plates that were then placed in the press before pressing. Satisfactory results were obtained with both methods, and no adhesion was observed between the fiberboard and the steel plates. The method of using press-release agent during pressing had no notable negative effects on the fiberboard properties.

Wheat straw was used to produce medium-density fiberboard (MDF). The chemical and physical characteristics of fractionated size-reduced wheat straw were investigated. The pH, pH-buffering capacity, ash, and silicon content increased as wheat straw particle size decreased. Ash of the finest straw, <0.2 mm, had high ash (15%) and silicon (18%) contents. The outer and inner parts of size-reduced straw were analyzed using scanning electron microscopy (SEM). The SEM micrographs revealed a complex ultrastructure containing a notable portion of thin-walled cells approximately 1 µm thick. Pressurized defibration of size-reduced wheat straw produced lignocellulosic fibers nearly 1.0 mm long combined with approximately 24% of small particles and dust. The high water uptake of straw-based MDF was significantly reduced using melamine-modified urea-formaldehyde (UF) resin and removing wheat straw particles and dust by screening. UF resin was added at levels of 12.5%, 13.1%, and 14%. In terms of water resistance, 12-mm-thick straw MDF displayed thickness swelling below 10%, acceptable according to the EN 622-5 MDF standards. It was concluded that manufacturing wheat straw MDF entails straw size reduction (hammer-milling), removing small particles and dust, and adding melamine-modified UF resin to attain necessary MDF quality standards.