Design of Experiments to Investigate the Effects of Additives on the Properties of a Poly(hydroxyalkanoate)/Poly(lactic acid) Blend

Poly(hydroxyalkanoates) (PHAs) are among sustainable, bio-based polymers with desirable properties that include biodegradability, non-toxicity, and piezoelectric characteristics. The choice of feedstock used for producing PHAs could help with attaining carbon neutrality. Furthermore, PHA-based packaging materials have contributed to the ongoing efforts to reduce plastic waste and microplastics. Nevertheless, the widespread use of PHAs has been limited due to their high cost, together with the need to improve the mechanical and thermal properties of PHAs. Blending with other bio-based polymers, suitable fillers, and foaming agents has been used to manipulate the properties while reducing the overall cost of PHA-based products. This study examined the effects of calcium carbonate (CC) and boron nitride (BN), as fillers, and azodicarbonamide (AZ), as a foaming agent, on the properties of an amorphous poly(hydroxyalkanoate) (PHA) blended with poly(lactic acid) (PLA). A twin-screw micro-compounder was used to prepare formulations containing 1 - 3 wt% of CC and BN at a temperature of 185°C. Then, a design of experiments (DoE) using JMP^® software enabled us to investigate the combined effects of the selected filler (BN) and the foaming agent (AZ) on the properties of the blend at a compounding temperature of 205°C (the average thermal decomposition temperature of AZ). The formulations were characterized via density measurements, tensile testing, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The response surface analysis was used to predict an optimal design based on the target mechanical properties (<i>i.e.</i>, modulus, tensile strength, strain at break, and toughness). To validate the design, the proposed optimal formulation (0.25 wt% AZ and 1 wt% BN) was prepared using the micro-compounder and characterized via tensile testing, DSC, TGA, and melt flow index (MFI) measurements. This study demonstrated that the overall material cost could be reduced via density reduction (20 - 21% for the optimal formulation). Furthermore, the addition of AZ significantly influenced the mechanical properties and crystallinity of the PHA/PLA blend. Reducing the concentration of AZ via DoE (I-optimal design) could alleviate the toxicity concerns of AZ for food packaging. This study also explored the effect of cold rolling on the toughness and crystallinity of PHA/PLA. Our results indicated a 3-fold increase in toughness (p &lt; 0.05) and an increase of 20% in crystallinity upon cold rolling.