Minijet 2

To study the surface properties, WPC samples with dimensions of 60 × 10 × 1 mm³ were prepared using HAAKE MiniLab II and MiniJet II (Thermo Fisher Scientific, Karlsruhe, Germany). Before testing, the samples were conditioned at 60 °C in a drying cabinet for 24 h and then stored for 1 h at room temperature in a desiccator. The total surface free energy (SFE) and its dispersive (Lifshitz–Van der Waals interactions) and polar (Lewis acid-base interactions) components were calculated using the Owens–Wendt–Rabel–Kaelble method [53 ], for which the contact angle values with different liquids, namely water, DMSO and diiodomethane, were measured. The contact angles were measured with a Kruss tensiometer K 100 M (Kruss GmbH, Hamburg, Germany) using the Wilhelmy method. For the calculation of SFE, a special software program (Advance Software Assurance, version 3.2), mounted in the tensiometer, was applied. Five replicates were tested for each sample, and the standard deviation for each value was found.

Before fabricating the WPC samples, the powdered rPP with the mixture of the activated birch wood dust and the WWB microparticles was mixed for 5 min at room temperature with a vibratory micromill PULVERISETTE 0 (Frisch GmbH, Idar-Oberstein, Germany). The samples for tensile and bending tests were prepared by the extrusion and molding methods using HAAKE MiniLab II and MiniJet II (Thermo Fisher Scientific, Karlsruhe, Germany). The processing conditions were as follows: a chamber temperature of 175 °C, screw rotation speed of 130 rpm, circulation time in the extruder of 5 min, injection pressure of 60 MPa and mold temperature of 120 °C. The WPC samples for tensile and bending tests were fabricated according to ASTM D638 (2007) and ISO 178 (2010), respectively. In a three-point bending test, samples with a length of 80.0 mm, width of 10.0 mm and thickness of 4.0 mm were used. “Dogbone” samples for a tensile test have the following sizes: overall length of 63.0 mm, overall width of 9.4 mm, length of narrow section of 9.4 mm, width of narrow section of 3.15 and thickness of 3.15 mm. The filling degree in the WPC samples was 30 %. The compositions of the WPC samples are given in Table 2.