3. Results and discussions
3.1. Mechanical properties of PBS/thymol films
The tensile and impact properties of PBS/thymol blends, as well as of pure component, are listed in Table 1. The tensile modulus and the tensile strength of neat PBS were 0.72 and 39.98 MPa respectively. Adding thymol to PBS reduced both the tensile modulus and tensile strength by around 10–40% depending on thymol content. Compared to neat PBS, the blends were less stiff due to the plasticization effect derived from thymol. This allowed PBS to be stretched or deformed easier than the un-modified one. With the presence of 10 wt% thymol, the tensile elongation at break increased from 17.46 for neat PBS to 22.58%. Thymol was believed to behave like plasticizer when it was blended to PBS. It penetrated between the PBS chains and reduced the intermolecular forces making them disentangle easier [21] and [22]. This allowed PBS could be elongated at higher rate and longer distance. Numerous literatures have found the plasticization effects of the essential oils on the food packaging films. In polypropylene (PP) film, the plasticization effect was found to decrease the tensile strength and increase the elongation at break [20]. Kavoosi et al. [23] incorporated 8 wt% thymol to gelatin film and found the reduction of the tensile strength from 2.9 to 1.2 N/m2. Furthermore, the tensile elongation at break was raised from 57 to 170%. Low density polyethylene (LDPE) was also less stiff when carvacrol was added as an antimicrobial agent [24]. Due to the plasticization effect, PLA was also softer and tougher when thymol [25] and limonene [26] were incorporated.