**2. Natural fibers**

Natural fibers are biosourced materials extracted from plants (lignocellulosic) or animals [7]. Lignocellulosic fibers are produced by plants for which, on a dry basis, the cell walls are mainly composed of cellulose, with hemicelluloses, lignins, pectins and extractives in lower amounts. Chemical composition and distribution mostly depend on fiber source and varies within different parts even of the same type or family [7, 8]. According to their source, lignocellulosic fibers can be classified as bast fibers, leaf fibers, fruits‐seeds fibers, grass‐reed fibers and wood fibers [7, 9–12]. **Table 1** presents some examples of each category [13].


**Table 1.** Lignocellulosic fibers classification [13].

Due to natural fibers' strength, stiffness, availability, low cost, biodegradability and lower density (1.2–1.5 g/cm3 ) compared to synthetic fillers such as talc (2.5 g/cm3 ) and glass fiber (2.5 g/cm3 ) [14–16], they can be effectively used in lightweight composites production [8, 9, 17].
