Question

Why are physical properties important for bioplastics and what are they

Answer 1

Important properties of common bioplastics are generally mechanical, optical and barrier properties. Mechanical properties include tensile elasticity, tensile stress and breaking elongation, optical properties refer to transparency, haze, etc., and barrier properties involve water vapor transmission rate and gas transmission rate. These characteristics are mainly based on the physical properties such as crystallinity, molecular weight, etc. of bioplastics. Tensile elasticity of petroleum-based low-density PE (LDPE, thickness: 30 μm), bio-based LDPE (30 μm) and PLA (25 μm) were 223.5, 248.4 and 2555.6 MPa, respectively. This means that PLA possessed highest stiffness among the plastics determined. Tensile elasticity of bioplastics decreased as thickness decreased and as temperature increased, which meant flexibility of bioplastics increased when temperature increased. As temperature decreased, the effect on mechanical properties of plastics increased. Moreover, machine and transverse direction of a plastic sample also affected its mechanical properties. Bio-based LDPE indicated similar mechanical properties to petroleum-based LDPE. As thickness of petroleum-based PE increased, transparency decreased. For the same thicknesses of LDPE samples, petroleum-based LDPE with lower crystallinity showed higher transmittance of visible light than that of bio-based LDPE representing in higher transparency. The transmittance of visible light of PLA was the highest among the plastics analyzed in this study. At 23°C and 50% relative humidity, water vapor transmission rate of PLA (25 μm) was 54.4 g/m2 ·d, in contrast, the water vapor transmission rates of petroleum-based and bio-based LDPE (30 μm) were 7.3 and 3.7 g/m2 ·d. The plastic film provided a resistance to water vapor transfer across it, thus the molecular structure of plastics could be a factor, which affects water vapor transferring through plastic material. Compared with PE, hydrophilic molecular structure in PLA material indicated higher water vapor transmission. Due to these physico-chemical properties, bio-based LDPE could be applied as an alternative to petroleum-based LDPE, while PLA, which was bio-based and biodegradable, was brittle and transparent so that it can be applied as food packaging containers.