Materials 2022 , 15 , 4542
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4. Panel Manufacturing Parameters The performance of the panels prepared from alternative furnish raw materials are highly influenced by production parameters, including adhesive type and ratio, panel density, and pressing factors (speed and press temperature). Previous studies have shown a direct relation between the panel’s adhesive type and ratio and mechanical properties [91,162]. Papadopoulos et al. [163] revealed that the mechanical properties of bamboo par- ticleboards increased with increasing the UF adhesive loads from 10% to 14%. Similar results were reported for UF-bonded particleboards made with cotton stalks [125]. UF, phenol-formaldehyde (PF), and melamine urea-formaldehyde (MUF) adhesives at 10% and 8% load were used for the manufacturing of three-layer particleboards with hazelnut husks, and the results illustrated identical mechanical properties for panels bonded within UF and PF, and lower MOR and IB values for those with MUF [146]. Barbu et al. [164] compared single-layer particleboards from walnut and hazelnut husks bonded with 10% MUF or PUR adhesive. Both panels with PUR adhesive illustrated higher bending properties (MOR and MOE) and lowered TS values than the MUF-bonded panels. The compatibility of various alternative furnish materials with conventional adhesive systems is rather challenging. For instance, the curing behavior of a standard UF adhesive in the hot press depends not only on the hardener type and the pressing temperature but also on the pH value of the raw material [165]. The presence of a high amount of wax and silica in stalks and husks cause poor interactions at the interfaces between the adhesive and the substrates. It also hampers the proper poly-condensation of the MUF adhesive, which results in weak bond lines [33]. Apart from the common UF and MUF adhesives, other adhesive types were also investigated for manufacturing panels from NWLM and ARs, such as pMDI [33], PF [166], bio-based systems [167], natural rubber [168] and soybean flour [139]. When using pMDI, the panel requirements are met in almost all studies (Tables 5, 7 and 8). Pan et al. [169] evaluated the performance of the single-layer particleboards made with rice stalks and a 4% adhesive mixture of pMDI and rice bran. The authors suggested that 20% of the adhesive can be replaced by rice bran while achieving a comparable mechanical strength to the control panel. Single-layer rice stalk particleboards with UF and corn starch as adhesive were compared by Hussein et al. [170]. With 10% adhesive load in each case, MOR and IB were significantly lower with corn starch bonded panels than with UF. Methylene diphenyl diisocyanate (MDI), UF, soybean protein isolate (SPI), and defatted soybean flour (SF) based adhesive systems were compared in single-layer wheat stalk particleboards [139]. The mechanical properties (MOR, MOE, and IB) of the panels prepared by 8% UF, 10% SPI, and 15% SF were identical or inferior to the ones manufactured with 4% MDI. Single-layer particleboards prepared with corn stover and 10% soy-based adhesive reached the minimum requirements for the bending properties (MOR, MOE) according to American National Standards Institute (ANSI) but not for IB [145]. Battegazzore et al. [167] evaluated the bending properties of fiberboards made with hemp fibers and particleboards with rice husks. Both panel types were bonded with corn starch (37.5% for hemp and 50% for rice husks) and were formed through a wet process. The results showed that both panel types achieved the minimum requirements for the MOR (EN 312:2003). It is well known that the mechanical properties of wood-based panels are directly related to their density [171,172]. The density of a wood-based panel usually correlates linearly with its mechanical and physical properties. One reason is the increased contact area of the particles or fibers covered by the adhesive. A higher density also allows the adhesive to spread more widely but compromises heat transfer during the pressing process [173]. The property can also be transferred to panels from NWLM and ARs [120]. Since the materials generally have a lower density than wood (Table 4), the density of the final panels can also be lower. Previous studies on panels with alternative materials reported density values ranging from 400 [124] to 780 kgm − 3 [128,138], depending on the material type used. Many studies have though focused on panel densities of about 700 kgm- 3 [110,125,136,174].
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