**2. Experimental**

Black spruce fibers were hammer milled with a Wiley Laboratory Mill mounted with a 2 mm opening sieve. Fibers were classified with Ro-tap Laboratory Sieve Shaker to obtain fibers with size in the class of 200–60 mesh (75–250 μm) and ovendried to reach 3% moisture content. Polymers used in this study are high-density polyethylene (HDPE) (DOW DMDA-8907 NT7, Dow Chemical) and polypropylene (PP 4150H, Pinnacle polymers, USA). The HDPE is a semi-crystalline material (typically 70–80%) has a 0.95 density, 9.0 g/10 min melt index, and a 135°C melting point. The PP has a density of 0.90 g/cm3 and a melt flow index of 55 g/10 min at 230°C.

Composite pellets were processed by a counter-rotating, intermeshing, conical twin-screw extruder (Thermo Scientific HAAKE PolyLab OS Rheodrive 7 with Rheomex OS extruding module). The screw speed was 50 rpm, and the barrel and die temperature was 165°C. The wood fibers proportion in the composites was maintained constant at 35 wt %. Composites were cooled in a water bath and cut up in pellets of 3 mm size. Injection molding with Minijet Haake injection machine produced test samples for bending and tensile tests using a mold temperature of 80°C, and barrel and nozzle temperature of 175°C.

Three-point bending properties were measured on 1 mm thick, 10 mm wide, and 60 mm long test samples and at a test speed of 1.4 mm/min according to the ASTM D 790 standard. Tensile properties were measured on dog-borne shaped samples of 2 mm thickness, 4 mm width, and 75 mm length at a 5 mm/min speed according to the ASTM D 638.

WPC water uptake was assessed according to ASTM 1037 on bending type specimens in triplicates. Samples were soaked in distilled water, and their weight was measured periodically up to 45 days immersion using a laboratory balance with an accuracy of ±0.001 g.

A Shimadzu IR Tracer-100 (Kyoto, Japan) served for FTIR spectroscopy. The analyzed spectrum was 400–4000 cm−1 with a resolution of 1 cm−1. We used the software lab solutions IR de Shimadzu with 50 scans for each measurement. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the WPC samples were conducted in NETZCH leading thermal analyzer using a heating rate of 10°C/min under helium with 25 mL/min flow rate from room temperature to 900°C.

Scanning electron microscopy served to analyze fractured surfaces of WPC tensile specimens. Micrographs were generated at 20 keV at a 5 mm working distance with Hitachi S3500 (Tokyo-Japan) electron microscope. Samples were prepared and gold-coated before observations.

The mechanical properties were subjected to an analysis of variance using the ANOVA procedure of the IBM SPSS statistics software. Polymer variability was the studied factor. Effects and differences between means were considered statistically significant at p < 0.05. ANOVA assumptions were verified using graphical diagnostics and Levene test for equal variances.
