Waste-Tea-Leaf-Derived Microcrystalline Cellulose as a Reinforcement for FDM-Printed PLA: Extraction, Filament Fabrication, Mechanical Performance, and Acoustic Absorption

Upcycling agro-industrial residues into functional reinforcements supports the circular economy while addressing the brittleness and limited stiffness of poly(lactic acid) (PLA) in fused deposition modeling (FDM). In this study, industrial waste tea leaves were converted into microcrystalline cellulose (WTL-MCC) and used to produce multifunctional PLA composite filaments. WTL-MCC was extracted by dewaxing, alkali treatment, bleaching, and mild acid hydrolysis and characterized by FTIR, XRD, TGA–DTG, DSC, UV–Vis, BET, SEM–EDAX, and AFM. It showed a crystallinity index of 48%, a crystallite size of 2.8 nm and thermal stability to 250 °C (Tmax = 322 °C). Filaments containing 1, 3, 5, and 7 wt.% WTL-MCC were extruded (1.75 ± 0.05 mm; >97% yield) and printed into ASTM specimens for mechanical (D638/D790/D695) and acoustic (E1050) testing. The properties peaked at 3 wt.% WTL-MCC; beyond this loading, strength declined owing to particle agglomeration and aligned inter-raster voids, confirmed by fractography. The optimized PLA/3MCC composite exhibited mid-to-high-frequency sound absorption (αmax = 0.76 at 4000 Hz; NRC 0.385) arising from micro-voids and cellulose surface roughness. Waste-tea-derived MCC, therefore, yields dimensionally consistent printable PLA filaments and multifunctional printed parts, combining an enhanced load-bearing capacity with useful acoustic damping, with 3 wt.% identified as the optimum loading.