Mechanical Recovery of Damaged Glass Fiber Reinforced Polypropylene via Repetitive Sub-Melting Thermal Annealing: A Molecular and Structural Study

This study clarifies the mechanical recovery and structural retention of damaged glass fiber-reinforced polypropylene (PP/GF) composites subjected to repetitive sub-melting thermal annealing at 160 °C. Initial thermal annealing increased matrix crystallinity from 0.30 to 0.37 (+23.3%), thereby elevating flexural strength from 103 MPa to 122 MPa (+18.4%). Following one repair cycle, flexural strength was completely restored to 103–105 MPa (100% recovery rate). However, after three repair cycles, short-duration annealing (120 s) resulted in severe mechanical degradation down to 30 MPa (−70.9%), whereas long-duration annealing (1800 s) retained a strength of 59 MPa (−42.7%, preserving 57.3% of initial strength). Concurrently, the crystallinity index increased to 0.49 (+63.3%) for 1800 s. These results establish that extended dwell time at sub-melting temperatures is essential for promoting molecular chain re-entanglement while suppressing void growth. These findings present an engineering guideline for low-energy, structure-preserving repair of thermoplastic composites.