Taguchi-Based Optimization of Puncture and Impact Resistance in Alkali-Treated Natural Luffa/Polyester Composites

Natural fiber-reinforced polymer composites offer sustainable alternatives to synthetic materials, but limited puncture and impact resistance restricts structural applications. This study optimized the quasi-static puncture and Charpy impact performance of alkali-treated Luffa cylindrica/unsaturated polyester composites using the Taguchi method. Four processing parameters, including NaOH concentration (4–12 wt.%), treatment temperature (90–120 °C), treatment duration (1–5 h), and fiber loading (5–15 wt.%), were investigated through an L9 orthogonal array. Quasi-static puncture and Charpy impact tests were conducted on longitudinal and transverse specimens, and parameter significance was evaluated by ANOVA. Treatment temperature and duration dominated puncture energy, whereas treatment duration and temperature primarily controlled the transverse and longitudinal Charpy responses, respectively. The optimum treatment (4–8 wt.% NaOH, 90°C, 1 h, and 5 wt.% fiber) increased puncture energy to maximum load by 183% and Charpy impact energy by 100–124% compared with untreated composites. Confirmation experiments validated the predicted optimum with errors below 5%. SEM, FTIR, and XRD analyses revealed partial removal of amorphous constituents, increased cellulose crystallinity (45–68%), and stronger fiber–matrix interfacial bonding, enhancing stress transfer and energy absorption. These findings establish a statistically validated strategy for developing lightweight Luffa-based natural fiber composites with enhanced puncture and impact resistance.