**3.4 Cross-linkers**

Cross-linking modification method is an effective and frequently applied approach to enhance the physicochemical and mechanical properties of PVA and starch [33]. Cross-linking can be carried out via treatment of granular starch using functional or multifunctional materials, which generated stable ether (R-O-R) or ester (R-CO-OR) linkages with the hydroxyl groups (-OH) in starch [9, 34]. Some of these multifunctional compounds are monosodium phosphate, sodium trimetaphosphate, sodium tripolyphosphate, epichlorohydrin, phosphoryl chloride, a mixture of adipic and acetic anhydrides, and a mixture of succinic anhydride and vinyl acetate. Cross-linker starch showed better compatibility and interaction with PVA than those of unmodified starch; such as, water absorption and TS of starch cross-linked films with sodium trimetaphosphate were higher than those of uncross-linked starch films, unlike E%. Moreover, weight loss in the soil of uncrosslinked starch films was higher than that of the cross-linked starch films. Since the weight loss of starch under the soil is related to the amount of moisture, the use of cross-linked starch improves the water resistance of the biobased films [9].

When epichlorohydrin was used for cross-linking, the TS and %E of starch/ PVA blend films increased. Thermal degradation of biofilms has been diminished by the cross-linker epichlorohydrin [35]. If sodium carbonate and sodium hexametaphosphate as the other cross-linkers are used, the equilibrium moisture content of the biofilms is significantly reduced by lowering their hydrophilic characteristic. Furthermore, these modifications increase the TS and modulus of elasticity of biofilms, unlike elongation at break [33].

In a study, the usage of sodium trimetaphosphate and sodium tripolyphosphate as the cross-linker enhanced the physicochemical properties such as swelling behavior compared to the uncross-linked starch [8]. Likewise, the swelling of the

**9**

*The Effects of Novel Additives Used in PVA/Starch Biohybrid Films*

biofilms was intensely reduced after utilization of cross-linker epoxidized natural rubber owing to the interaction between the mixtures. Thus, the hydrophilicity of the blend film decreases due to the reduction of the number of free hydroxyl groups in PVA and starch molecule. And, %E of film improves with the addition of cross-linker in blend polymer [36]. Singha and Kapoor have reported that the TS of PVA/starch cross-linked with glutaraldehyde has shown improvement. Moreover, modification with glutaraldehyde also improved the thermal stability of films. Moreover, their antibacterial activities against Gram-positive bacteria compared to Gram-negative bacteria indicated good resistance [15]. Additionally, borax can also be used as a cross-linker for starch and PVA. The enthalpy and crystallinity slightly decreased with increasing concentration of borax due to increasing cross-linking. Also, it improved the TS and %E of biobased films compared to the biobased films without cross-linker. Citric acid as another cross-linker can also be preferred for biobased films. This cross-linker decreases the water absorption of biobased films. So, citric acid also acts not only as a plasticizer but also as a cross-linker [29].

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

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

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

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

*DOI: http://dx.doi.org/10.5772/intechopen.81727*

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

have been enhanced [37, 39].

penetration rate [9].

filler TiO2 unlike E% [43].
