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Thus, pressing the mats formed from low-density chips requires altered process parameters ( i.e. , pressure and temperature regimes) (Moslemi 1974). An effectively modified pressing scheme (temperature, pressure, and time) should provide a proper cross-sectional density profile and expected mechanical performance, while overall panel density is decreased. It is recognized that reduced pressure imposes a prolonged pressing time and yields more uniform density profile. On the other hand, an increased pressure results in a higher compression ratio for face layers and a lower one for the core layers of a panel (Keylwerth 1958; Plath 1971; Wong et al. 1999; Dunky 2001). Phenomena occurring during pressing of particleboards play a crucial role in their manufacturing, as the applied regimes determine the final properties of the product. Thus, better recognition and description of variables makes easier optimization of processes possible and, as a result, minimizes production costs, so that implementation of new technologies or products becomes easier. It is commonly agreed that pressing is necessary for the compression of a mat to the target thickness and for the proper development of adhesive interactions between wood particles. The compression is primarily affected by (1) the pressure and amount of binder, which determine the contact area between chips; and (2) the temperature governing the curing of the binder and development of bondlines (Moslemi 1974). Other factors affecting mat compression process include time, press closing rate, target density of the product, type and amount of binder, as well as characteristics of the material subjected to bonding: chip dimensions, density, and moisture content. Although there are numerous reports regarding the effect of the factors mentioned above on the compression process during manufacturing of wood-based composites (Kelly 1977; Steffen et al. 1999; Miyamoto et al. 2002; Dai et al. 2004; Nemli et al. 2007; Cai et al. 2009), none of them considers the compression of low-density particleboards. Therefore, in this paper, some aspects of low-density mat compression are discussed. Particleboards made of softwood, hardwood, and their mixtures are compared. EXPERIMENTAL A total of 15 three-layer particleboards of density 500 kg/m 3 with dimensions of 320 x 320 x 18 mm 3 were made. 5 panels were prepared in each series: (1) industrial poplar chips, (2) industrial pine chips, and (3) both poplar (face layers) and pine (core layer) chips. The moisture contents were as follows: poplar 3.5% (face layer) and 2.9% (core layer); pine 5.0% (face layer) and 4.5% (core layer). Wood densities were 450 and 520 kg/m 3 , respectively, for poplar and pine. A commercial urea-formaldehyde (UF) resin was used as a binder, hardened with 10% aqueous ammonium sulfate solution (3% for face layers, 4% for core layer (based on resin solids). Glue rates were as follows: face layer 12% and core layer 8%. A paraffin emulsion was used as a hydrophobic agent (1% based on dry wood). All the mats were cold pre-pressed at 0.5 MPa pressure for 30 s. The difference in bulk density of pine and poplar chips, resulted in variations in the final thickness of the mats subjected to hot-pressing. In order to achieve comparable board density, the weight of the respective mats was constant. Hot pressing parameters were adopted from industrial conditions and from the literature (Moslemi 1974). Maximum unit pressure upon board pressing was set at 2.5 MPa, however the true momentary pressure was computer-controlled as follows: (1) increased until target thickness is achieved and then
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Boruszewski et al . (2016 ). “Particleboard density ,” B io R esources 11(3), 6909-6919.
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