Materials 2022 , 15 , 4542
17of 27
Although the density of raw materials plays a significant role in defining the final density of the panels, the panel density can also be adjusted by other manufacturing parameters, i.e., compression ratio, water content, press temperature, pressing schedule, or adhesive load [175], which may increase the cost of the final product. The pressing temperature is an essential factor that influences the performance of the panels by providing the thermal energy for curing the adhesive and mechanical compres- sion force to consolidate the mat [176]. The effect of pressing temperature varies with the density of raw material and panel type, as the higher density panels have higher maximum core temperatures due to their capability to build higher internal gas pressure [177]. Binder- less particleboards prepared with oil palm trees showed higher mechanical properties by increasing the press temperature [130]. The MOE, IB, and TS of MDF panels made from corn stalks and 10% UF adhesive were improved by increasing the press temperatures from 170 to 180 ◦ C, while a negative effect was observed with further increasing the pressing temperature to 190 ◦ C. MDF panels produced with cotton stalks and 10% UF adhesive demonstrated higher bending properties and lower TS values with increasing pressing temperatures from 170 to 190 ◦ C, while opposite results were obtained for IB strength [118]. Nogueira et al. [178] tested three-layer particleboards from sugarcane bagasse and waste plastic bags. A reduction in TS and WA could be observed as the press temperature increased from 160 to 220 ◦ C. 5. Material Processing and Pretreatment In addition to the type of raw materials and manufacturing parameters, the processing and pretreatment affect the properties of the final panel. The raw materials from non-wood lignocellulosic and agricultural sources have been mainly processed mechanically and some were chemically pretreated to create evenly sized particles or to improve their performance in final panels [72,126,139]. The mechanical processing of NWLMs and ARs, such as canola stalks, with a hammermill, has often been used to prepare the raw material [122]. Ndazi et al. [179] produced single-layer particleboards with ground (8 mm sieve) and untreated rice husks and 15% of PF. The results showed that the mechanical properties of the panels decreased by grinding. Many factors can be varied in fiber production for fiberboard raw material. Zawawi et al. [180] investigated the influence of refining conditions, pressure and temperature on oil palm fruit husk’s fiber and fiberboard properties. It was found that higher pressure and a higher temperature in the refiner ultimately led to increasing MOE, MOR, IB and reduced water absorption. Overly aggressive refining conditions, however, produced shorter fiber lengths and consequently reduced the fiberboards’ physical and mechanical properties. Chemical pretreatments of raw materials are performed to optimize the bonding capability of the particles and the adhesive. Due to the increase in reactive hydroxide (OH) groups during an alkali treatment, the binding of the raw material and the adhesive improves [181,182]. The hydrophilic nature of raw materials can also reduce mechanical properties due to water absorption and a reduction of the water resistance. Acetylation can increase the hydrophobicity of a raw material, which leads to less thickness swelling and improved mechanical strength. [183]. Cotton stalks pretreated with 1–5% sodium hydroxide (NaOH) were used to prepare single-layer particleboards in combination with 10% UF adhesive [126]. The NaOH decomposes the lignin and reduces the content of hemicelluloses and extractives. In relative terms, since cellulose is more resistant to NaOH, the proportion increases. In addition, the surface roughened by the NaOH treatment offers better bonding between raw material and adhesive [184]. The results showed that the static bending properties of the panels (MOE and MOR) improved by 1% NaOH treatment, while pretreatment of the cotton stalks at 3% and 5% of NaOH resulted in the strength reduction of the panels. Treatment at a higher concentration degrades the cell wall components stronger and deforms the particle structure, resulting in a reduction in mechanical strength. Figure 4 shows NaOH treated canola stalks with rougher surface than untreated ones. Mo et al. [139] bleached wheat stalks with 3% sodium
Made with FlippingBook - Online magazine maker