Self-protecting of wood-plastic composites against microorganisms and UV radiation

In this work, polypropylene composites filled with wood from the invasive species Prunus serotina Ehrh., modified with glycerol and citric acid (GlyCA), were investigated with respect to biological resistance and durability under accelerated outdoor weathering simulated in a weathering chamber. The main novelty and achievement of this research is the description of a self-protecting mechanism against selected strains of Listeria and Trichoderma, including the pathogenic Listeria monocytogenes and Trichoderma viride, as well as the determination of the influence of wood on the weathering resistance of wood–plastic composites (WPCs). The self-protecting mechanism is based on the inhibition of microorganisms sensitive to pH changes, resulting from the hydrolysis of the GlyCA modifier and the subsequent release of citric acid. The citric acid migrated to the WPCs surface as a result of polyglycerol citrate hydrolysis and also the breaking of hydrogen bonds between citric acid and wood components and/or glycerol. For the first time, a direct relation between the supermolecular structure of the polymer matrix and the biological activity of a bio-based material was demonstrated, originating from the release of a hybrid, bioactive GlyCA modifier. An increase in the β-phase content promoted citric acid release and consequently enhanced the self-protecting properties of the composites. In addition, the presence of wood significantly improved the weathering resistance of WPCs, including resistance to UV exposure. This effect was reflected in carbonyl index values up to 20 % lower for WPCs compared to neat polypropylene, as well as in a slower deterioration of mechanical properties after 250 and 500 h of weathering, as determined by static tensile and flexural tests.