The mechanical and strain recovery properties of silk fibroin-reinforced rice starch/polyvinyl alcohol composite films for promising eco-friendly packaging materials

The use of natural polymers to obtain more natural and biodegradable packaging materials has the growing significance and leads to much research involving different properties such as mechanical and thermal. In addition to improve and modify especially the mechanical characteristics of the packaging films, it is quite important to understand the materials response under a fixed applied deformation to which the packaging material will be exposed in different applications and the strain recovery properties following the removal of the stretching or deforming effect. The present study mainly aimed to investigate the uniaxial tensile and strain recovery properties of rice starch (RS)/polyvinyl alcohol (PVA) composites reinforced with silk fibroin (SF) as well as thermal, spectroscopic, and structural properties to obtain eco-friendly and more natural packaging materials with better characteristics. In order to examine these properties, tensile testing, stress-relaxation and strain recovery, FT-IR/ATR and UV–vis spectroscopy, DSC, XRD, SEM and determination of the degree of swelling (DS) and water solubility (WS) methods were used. For the contribution of RS to mechanical properties of RS/PVA films, it was revealed that with increasing RS ratio, the Young’s modulus (E_y) and stiffness of the films increased considerably unlike the decrease in the strain at break value (ε_b). The SF contribution enhanced the ultimate tensile strength (σ_u), E_y and rigidity, but reduced the extensibility of RS/PVA composite films at different mass ratios. The optimal RS/PVA mass ratio was considered as 20/80 for the case of SF addition. It was also observed that at each constant RS/PVA mass ratio, as SF ratio increased, like the decrease in the water swelling, the solubility of the SF/RS/PVA composite films in water decreased that indicated the SF improved the hydrophobicity and the resistance of the SF/RS/PVA composite films against water. The results of the strain recovery processes showed that as the preliminary extension level (ε_pe, %) increased from 2 to 10%, the complete recovery of the RS/PVA (20/80) films increased from around 4000–35,000 min. The SF contribution generally enhanced the recovery process compared to that of RS/PVA (20/80) composite films. Effective ratio of SF addition for the recovery process of RS/PVA (20/80) films was seen as 5%.