Effect of cryo-milling on PLA/CMC and PLA/ CaCO3 composites: A comparison study

This study examines a new approach for improving the dispersion of hydrophilic fillers into a hydrophobic polymer matrix, a long-standing challenge that often limits the thermal and mechanical performance of biocomposites. Two different fillers, natural cellulose microcrystals (CMC) and mineral calcium carbonate (CaCO 3 ), were incorporated into polylactic acid (PLA) at five different weight fractions. Biocomposite films were prepared using physical mixing and cryo-milling, followed by hot pressing. Cryo-milling was explored as a strategy to enhance filler distribution within the matrix. Thermal properties, crystallization behavior, and mechanical performance were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and tensile testing, with statistical significance evaluated using t-scores. CMC addition lowered the glass transition temperature (T g ) by up to 7 °C (5 %-PHY), whereas CaCO 3 increased T g by up to 5 °C (40 %-PHY). Both fillers exhibited nucleating activity, with the largest crystallization temperature (T c ) reduction observed in CMC/PLA (−6 °C at 5 %-CRYO). Thermal degradation temperature (T d ) improved by up to 10 °C with CMC (10 %-CRYO) but decreased by as much as 74 °C with CaCO 3 (40 %-CRYO). Mechanically, tensile strength and strain decreased with filler addition, but CMC composites, particularly those prepared by cryo-milling, retained greater ductility than CaCO 3 composites. Modulus increased in all cases, with the highest gain for CaCO 3 /PLA (up to +79 %, 30 %-PHY). Rule of Mixtures, inverse ROM, and Halpin–Tsai model predictions indicated that PHY CaCO 3 composites matched upper-bound reinforcement trends, while CRYO samples approached the lower-bound inverse ROM, reflecting weaker stress transfer from smaller particles and limited interfacial bonding. Although cryo-milling improved dispersion, good dispersion alone was insufficient to maximize mechanical performance. Enhancing interfacial adhesion, such as through surface modification, remains essential for achieving the full reinforcing potential of these fillers in sustainable PLA composites. • This study explored the impact of adding CNCs and CaCO 3 to polylactic acid using physical mixing and cryo-milling. • CNC lowered T g , while CaCO 3 raised T g . Both fillers increased crystallization, with CaCO 3 having a stronger effect. • Both fillers reduced tensile strength and strain, but CNC composites retained more strength than CaCO 3 composites. • Future studies will focus on improving interfacial bonding through surface modification using, for example, PEG.