Atmospheric Plasma-Treated Recycled Polyethylene (rPE) as a Compatibilizer in rPE/Oil Palm Empty Fruit Bunch Fiber Composites

Composites based on rPE and OPEFB waste are considered sustainable materials. However, their performance is limited by hydrophobic–hydrophilic incompatibility, which weakens interfacial adhesion. This study investigated the atmospheric-pressure air plasma treatment of rPE to enhance its compatibility with OPEFB fibers and evaluated its role as a compatibilizer. Atmospheric plasma treatment for 120 s introduced oxygen-containing groups onto the rPE surface, as evidenced by C-O and OH peaks in the FTIR spectra and the higher O/C ratio in the XPS analysis. Consequently, the water contact angle decreased, reducing the difference in surface tension between rPE and OPEFB from 45.61% to 7.80%. Composites containing 20 wt.% OPEFB were fabricated by varying the proportion of untreated rPE with p-rPE. All p-rPE-based composites exhibited fewer interfacial voids than untreated rPE composites, indicating improved fiber–matrix adhesion. The tensile strength and elastic modulus increased with the p-rPE content, whereas the elongation at break remained higher than that of the untreated composite. Therefore, p-rPE shows potential as a compatibilizer, enabling agricultural and plastic waste value enhancement.