Coupled Effects of Reprocessing, TiO2, and Fused Filament Fabrication on Polylactic Acid Weathering Durability: A Critical Review

Titanium dioxide (TiO2) is incorporated into polylactic acid (PLA) to improve stiffness, promote crystallization, and provide ultraviolet (UV) shielding. However, its influence on durability is complex, as photocatalytic activity may accelerate degradation under environmental exposure. This review examines how TiO2 characteristics, melt processing history, fused filament fabrication (FFF) architecture, and exposure conditions collectively influence PLA durability. Primary studies and previous reviews are critically compared based on material formulation, processing conditions, specimen geometry, environmental exposure, and measured properties. Evidence indicates that reprocessing generally reduces molar mass and melt viscosity, while crystallization and printing conditions may partially preserve mechanical performance. TiO2 can enhance selected properties, although its effects depend on particle characteristics, dispersion, and surface accessibility. Furthermore, interlayer interfaces and connected porosity influence moisture transport and damage localization, limiting the applicability of findings from films and moulded specimens to FFF components. While existing reviews have addressed PLA–TiO2 composites and their properties, the combined influence of reprocessing, TiO2 incorporation, FFF architecture, and tropical weathering has received limited attention. Unlike previous reviews that primarily examine these factors separately, this review integrates their potential interactions to provide a processing-history-dependent perspective on PLA durability. It also identifies research priorities involving controlled processing comparisons, multiscale characterization, and complementary natural and accelerated weathering tests to support more reliable assessments of outdoor performance.