“…First of them is related to the modification of the polymer matrix [55,56], while second is focused on size reduction [57], treatment or functionalization of cellulosic filler/fiber. In this subchapter we will focus on the second route related to the surface of filler/fiber, which can be successfully modified by silane treatment [58,59], mercerization [60,61], acetylation [62,63] maleic anhydride treatment [64], esterification and etherification [65,66], isocyanate [67], potassium permanganate [68] and other chemical treatment methods [69][70][71]. Furthermore, different kinds of surfactants and plasticizers applied as physical additives were considered as wetting agents to reduce the tensions in boundary between polymer matrix and reinforcement phase [72][73][74].…”