Materials 2022 , 15 , 4542
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They suggested 25% peanut husks as the optimum level to achieve the standard requirements for boards in interior applications. The MOR and MOE values, in particu- lar, decreased with a higher proportion of peanut husks. The panels with 100% peanut husks, on the other hand, showed lower TS than the ones with 25% husks. High density (>940kgm − 3 ) one-layer particleboards made with 15%, 30%, and 100% oat husks and 10% polyurethane resin reached the acceptable level of the EN 312:2003 standard for general purpose in MOR and IB [161]. Recently, Farag et al. [148] used olive stones together with an unsaturated polyester liquid resin for preparing single-layer particleboard panels, and they found that the panels fulfilled the MOR requirements for general purpose (EN 312:2003) at 20% adhesive load (Figure 3d). However, the maximum permitted values mentioned in the EN 312:2003 standard for the wet condition in TS were slightly exceeded. Single-layer particleboard panels from rice husks and 8% UF were tested by Melo et al. [105]. The rice husk panels showed significantly lower MOR, MOE, IB, and higher TS than the reference panels from industrial wood particles. The authors report that one reason may be the cylindrical and hollow structure of the hole rice husk particles, which could act as a barrier during gluing. Likewise, a lower permeability of the husks for the resin could have a negative effect on an even distribution of the adhesive. Faria et al. [149] investigated three-layer particleboards from Eucalyptus wood, different proportion of soybean husks in the CL and 10% UF. Panels with 100% soybean husks in the core layer did not meet any standard requirements. However, a high MOR was observed with a 1:1 ratio of Eucalyptus wood and soybean husks. The MOR increase was attributed to various factors, such as a higher interaction of the particles due to the increase of the compression ratio and better adhesive distribution on the particles. A combination of raw materials from non-wood lignocellulosic and agricultural sources was also used for panel preparation [160]. Khedari et al. [62] reported particleboards with low thermal conductivity using co- conut coir and durian peel and combinations thereof, bonded with 12% UF resin. They found that a 90:10 mix ratio of coconut coir and durian peel was the optimum to fulfill the minimum requirements for IB values according to the Japanese Industrial Standard (JIS A-5908 Type-8). Nicolao et al. [157] developed particleboard from a combination of rice husks and jute fibers. The three-layer panel consisted of a rice husk core and different numbers of jute fiber surface layers bonded with 10% soybean protein adhesive (Figure 3c). With MOR from 12.6 to 27.9 Nmm − 2 , the bending properties improved with an increased number of jute surface layers. In addition to the classic panel types, fiberboard, and particleboard, investigations were also conducted with plywood or special panels but to a lesser extent. Abdul Khalil et al. [160] tested five-layer plywood with UF or PF from oil palm wood (500 gm − 2 ), as well as five-layer hybrid plywood with two layers consisting of oil palm empty fruit bunch fibers. The hybrid plywood achieved higher MOR and MOE than the oil palm wood plywood. It was attributed to the higher density of the hybrid panel. The studies described in Tables 5–8 show that NWLM and ARs, especially in fiber- boards and particleboards, were extensively tested and the requirements were met in many cases. In particular, NWLM benefit from their long fibers in the panels. Grow care residues have been little studied thus far, and their integration into panels also negatively influences the bending behavior. The much-noticed harvest residues are generally well suited for both fiberboard and particleboard. They perform particularly well in combination with wooden material. Various husk types of process residues could not achieve sufficient bending strength values. Other process residues, such as coconut coir or sugarcane bagasse, appear suitable as raw materials for panels.
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