Using Urban Residue in Polylactic Acid Composites Part I: Effect of Castor Oil on the Crystallization Kinetics

This work aimed analyzing the chemical and thermal aspects of Polylactic Acid (PLA) composites with Coconut Fiber (CF) and irradiated Coconut Fiber (CFI), upon castor oil (CO) addition. Influence of treated and untreated CF, at 10 and 20 wt.%, on the crystallization kinetics of PLA/CO composites is also investigated. At first, CF is chemically treated through mercerization under NaOH solution with subsequent gamma irradiation. Chemical changes include absence of bands ≈3300 cm⁻¹ and the presence of vibrations at 3000–2851 cm⁻¹ in composites with treated fibers, as evidenced by fourier transform infrared spectroscopy (FTIR). Thermal behavior, as well as crystallization kinetics, are investigated by differential scanning calorimetry (DSC) at different heating rates (5, 10, 15, and 20 °C min⁻¹), using Pseudo-Avrami (PA), Kissinger-Akahira-Sunose (KAS), and Ozawa-Flynn-Wall (OFW) models. Activation energy (E_a) is calculated using OFW, KAS, Friedman (FR), and Vyazovkin (VZ) methods. Results showed changes in the FTIR spectra for PLA/5%CO/10%CF and PLA/5%CO/20%CFI, indicating absence of the carbonyl group (1594 cm⁻¹) present in the ester group of PLA. Insertion of CFs increased the crystallization temperature. Presence of CFI provided an inversion in E_a, when compared to PLA/CO, suggesting that irradiation induces PLA stronger interactions. Gathered results highlight the potential of modified coconut fibers for the development of sustainable composite materials.