2.2. Composite Preparation

FS stems were stripped of seeds. Both WF and FS were dried in air-circulating oven LOIP LF-120/300-VS2 (Saint Petersburg, Russia) at 105 °C for 3 h. Then, they were crushed by a rotary mill Vilitek DM-6 (Moscow, Russia) and filtered through a sieve set Matest A059-02KIT (Arcore, Italy) and laboratory sieves with a mesh of 80, 140, and 200 microns. Four fractions of each filler were collected from the sieves for the further work (0–80, 80–140, 140–200, and 0–200 µm).

The mixing of the polymers with the fillers was performed with 70% wt. of LDPE and 30 wt.% of filler (WF or FS). Plasti Brabender Plasti-Corder Lab-Station (Duisburg, Germany) with twin screw cam mixer was employed for making composites. The mixing temperature was 140 °C, and screw speed was 30 rpm. Then, the obtained pieces of composites were pressed for 3 min by a hydraulic press VNIR PRG-1–10 (Moscow, Russia) under the pressure of 7 kN with the plates heated to 135 °C. In order to form a fine crystalline structure, the obtained hot films were immediately immersed to the cool (10 °C) water. As a result, round films with the diameter of 70 mm and thickness of 150–200 µm were obtained to be used in biodegradation test.