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(2) reduced. Other hot pressing parameters were as follows: platen temperature of 180 °C, pressing factor 18 s/mm, press closing rate 2 mm/s, time 324 s. The mat compression process was performed using a computer-controlled press. Variables (mat core temperature with accuracy ± 0.01 °C, pressure with accuracy ± 0.01 MPa, and mat thickness with accuracy ± 0.01 mm) were monitored in real time throughout the pressing process for each batch, using the computer controller. Temperature measurement inside the mat was carried out using a Fe-CuNi thermocouple, fixed in the mat core during its formation. Prior to testing, the boards were conditioned at 20 ± 2 °C and 65 ± 5% RH for seven days. The modulus of rupture (MOR) and modulus of elasticity (MOE) were tested according to European standard EN 310 (1993), and the internal bond strength (IB) was determined according to European standard EN 319 (1993). All mechanical tests were conducted using an electromechanical testing machine Instron model 3369 (Instron Corp., Norwood, MA). Ten specimens were tested in each series. Density profiles were measured on an X-ray density analyzer GreCon Da-X (Fagus-Grecon Greten GmbH & Co. KG, Alfeld-Hannover, Germany) at a scanning speed of 0.5 mm/s. Statistical analysis was performed using STATISTICA version-12 software (StatSoft, Inc., Tulsa, OK). Statistical analysis for all stages of the research was performed at a significance level of 0.05. RESULTS AND DISCUSSION As Fig. 1 indicates, the compression processes can be differentiated for the respective series of boards. The temperature (Fig. 1a), mat thickness curves (Fig. 1b), and pressure (Fig. 1c) were varied. Assuming a constant press-closing rate (2 mm/s), the main factor determining the locus and shape of the pressure curve was the mat thickness, which was affected by the properties of the material. The obtained initial uncompressed mat thickness was as follows: 80 mm for poplar (450 kg/m 3 ), 60 mm for pine (520 kg/m 3 ), and 70 mm for the mixed poplar-pine mats. The different initial mat thickness comes from the variable chip bulk density. We assumed manufacturing boards of same density, so in case of a “heavier” material, its amount w as smaller. The shortest time (21 s) and the lowest pressure (1.38 MPa) necessary for the mat to be compressed to the target thickness (18 mm) were observed for pine, while for poplar mat, the respective values were 38 s and 1.57 MPa, while midway values were recorded for the mixed poplar-pine mats, i.e. , 26 s and 1.61 MPa, respectively. Despite the differences in mat compressing (Fig. 1b), their pressure curves were similar (Fig. 1c). It should be noted that at the moment the mat reached the target thickness (18 mm), the initial temperature was still observed in the core layer. The temperature in the core began to increase gradually but not before the 40 th second of the pressing process. Unlike the poplar and poplar-pine boards, the temperature increased above 80 °C in the pine mat core and then slowed, which might be caused by the evaporation of the volatile organic compounds present in pine wood (McDonald et al. 1999a,b). As Fig. 1a indicates, the target temperature of 100 °C in the mat core was achieved after 200, 135, and 143 s, respectively, for poplar, pine, and poplar-pine boards. It was also found that when the mat core temperature reached 100 °C, at which partial plasticizing of the chips occurred, the pressure required for mat thickness control
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Boruszewski et al . (2016 ). “Particleboard density ,” B io R esources 11(3), 6909-6919.
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