Molecular Structure Engineering of Flow Modifiers for Controlling the Melt Flowability of Recycled Polyamide 66

Abstract

Recycled polyamide 66 (rPA66) is a promising engineering thermoplastic for sustainable material applications. However, its high melt viscosity and limited processability remain major challenges during the melt compounding process. In this study, a series of stearyl diamide‐based flow modifiers with different molecular center structures were synthesized to investigate the relationship between molecular architecture and flow‐modifying performance in rPA66. The modifiers were designed with long stearyl chains for internal lubrication and amide groups for compatibility with the rPA66 matrix, while the center structures were varied from flexible aliphatic and ether‐containing units to rigid aromatic structures. Thermal and structural analyses revealed that the molecular center strongly affected melting/crystallization behavior, thermal stability, hydrogen bonding, and molecular packing. Rheological analysis showed that all modifiers reduced the complex viscosity of rPA66, but their efficiency depended strongly on center structure and loading content. Flexible centers, particularly Hex‐St and Diox‐St, effectively enhanced melt flowability by promoting chain mobility and internal lubrication, whereas aromatic‐centered modifiers provided higher thermal stability and structure‐dependent flow behavior. Mechanical analysis indicated that optimized modifier loading could maintain or improve mechanical properties, whereas excessive loading led to property deterioration, probably due to excessive plasticization, weakened intermolecular interactions, or increased phase heterogeneity. These results demonstrate that center‐structure engineering of stearyl diamide flow modifiers is an effective strategy for balancing melt processability and mechanical performance in rPA66. This study provides useful design guidelines for developing new flow modifiers for recycled engineering plastics.