**3.5 Fillers used in PVA/starch biobased films**

Filled PVA/starch biobased films are the high-potential class of hybrid materials composed of filler incorporated into a biobased matrix [37]. With the aim to attain synergic effects, such a collaboration between environmental biopolymers and fillers is one of the most impressive ways to improve the features of this bioblends [38]. Because of the nature and the geometry of the filler, the properties of biobased films such as gas barrier, mechanical stiffness, transparency, and thermal stability have been enhanced [37, 39].

In a study, the use of silica as a filler has increased the TS of biobased films [40]. With the increase of silica amounts in blend film, the water absorption and water vapor transmission of starch have been decreased. This was due to the complex structure designed by links between silica and hydroxyl groups of starch and PVA. This phenomenon prevented the water molecules from dissolving and developed the water resistance of the biofilm. Furthermore, silica has also improved the compatibility between PVA and starch and formed a rigid structure. Even, according to SEM results, the low amount of silica has provided excellent diffusion and interaction between starch and PVA. On the other hand, filler silica has shown less effect on the biodegradability of the films because of decreasing microorganism penetration rate [9].

Nano-calcitine was preferred as a filler for PVA/starch film because of its positive effects on the physicochemical properties of blend films. As an example, the addition of nano-calcitine into blend film reduced the crystallinity, water solubility, biodegradability, and oxygen permeability. At the same time, it increased TS, limiting oxygen index, decomposition temperature, and water absorption [41]. Simultaneously, addition of nano-SiO2/TiO2/CaCO3 into PVA/starch blends increased the TS of biofilms enhancing the interfacial adhesion through inter- and intramolecular interactions. With nano-TiO2, an increase in clearness of biofilm was noticeably observed. However, water vapor permeability of biofilms containing nano-SiO2 was lower than that of biofilms containing nano-TiO2/CaCO3 [4, 42]. Therewithal, TS and Young's modulus of biobased films were also increased with filler TiO2 unlike E% [43].

Zirconium phosphate as another filling material had an attractive effect in the biobased films because of composing new hydrogen links. The addition of zirconium phosphate decreased the moisture uptake, while the degradation temperatures of biobased films increased [44]. The filler clay had an important effect on biobased films due to its hydrophilicity. The use of clay in biofilm increased TS and heat resistance, enhanced the barrier properties to water vapor, and lowered glass transition temperature [24].

In PVA/starch biohybrid film, natural raw materials were also added as fillers such as cellulose nanofibers, chitosan, and feather keratin. In investigations, cellulose nanofibers blocked the recrystallization of starch by decreasing the mobility of polymer chains. Hence, the physicomechanical properties and crystal structure of blend film were significantly enhanced. In relation to this, storage conditions of biobased films improved. Cellulose nanofibers significantly enriched also the stiffness and strength of blend films by the storage conditions [34]. Similarly, the storage conditions of biobased films in natural weathering could be also enhanced by added graphene into PVA and starch [45]. Moreover, when the chitosan known as a natural filler was added to biofilms, their physicochemical properties such as TS, E%, water vapor permeability, and oxygen transmission rate improved. Also, water vapor and oxygen permeability, water uptake, and hydrophobic character of the chitosan-added bioblend film were better than biofilm without chitosan due to its incorporation [46]. As different inorganic salts are used in the biobased films, their crystalline [47], thermal, water vapor barrier, and mechanical properties can be significantly affected via strong hydrogen bonds. For this reason, Jiang et al. have reported that LiCl, MgCl2,6H2O, CaCl2, and AlCl3,6H2O salts have provided a good compatibility with PVA and starch [48, 49]. Moreover, the ZnO added biobased films have shown good dispersion, homogeneity, mechanical properties, and water resistance [49]. Another filler salt, AlCl3.6H2O, can show compatibility with PVA and starch. Hence, these salts have presented great destroying effect on the crystalline and good mechanical properties [50].
