*The Effects of Novel Additives Used in PVA/Starch Biohybrid Films DOI: http://dx.doi.org/10.5772/intechopen.81727*

*Fillers - Synthesis, Characterization and Industrial Application*

**3.3 Effect of plasticizers added simultaneously into biobased films**

When the plasticizers having both hydroxyl and carboxyl groups were simultaneously added into the biobased films, their physicomechanical properties were better than those of plasticizers with only hydroxyl group-containing agents. Yoon et al. reported that as the additives containing both hydroxyl and carboxyl groups were simultaneously added into the formulation, the TS and %E were enhanced compared to the glycerol, having only hydroxyl groups. For instance, %E of the glycerol-succinic acid–added films increased, while only succinic acid–added film showed inadequate potential. The usage of the plasticizer composed of both hydroxyl and carboxyl groups could enhance the flexibility and strength. Moreover, the degree of swelling and solubility values of the comalic acid-/tartaric acid-added films were higher than those of coglycerol-/succinic acid-added films. [6]. This was because the hydroxyl (-OH) group number of malic acid-tartaric acid (7 hydroxyl number) was higher (i.e., having a more hydrophilic character) than glycerolsuccinic acid mixture (5 hydroxyl number). %E of the glycerol or sorbitol-succinic acid–added films increased, while %E of succinic acid–added film decreased with increasing amount of additives. Nevertheless, TS of glycerol or sorbitol-succinic acid–added films decreased, while TS of succinic acid–added film diminished with increasing content of additive. The results of TS and %E mentioned above showed that using cohydroxyl/carboxyl group as a functional group increased physicochemical and mechanical properties of films [30]. Eventually, when the plasticizers having both hydroxyl and carboxyl groups were used simultaneously, TS and %E of the films were found to be better than the films containing plasticizer having only

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].

biofilms, unlike elongation at break [33].

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

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

**8**

the hydroxyl group [7].

**3.4 Cross-linkers**

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].
