Sustainable Bio-Based Rosin–Epoxy Composites: From Pine Forests to Polymers via Processing–Structure–Property Optimization

Abstract

This study investigates the effect of processing strategy on the structure–property relationships of bio-based epoxy composites reinforced with pine rosin powder extracted from Pinus halepensis trees in Algeria's Green Dam. Two processing protocols were employed: $P1$ (fixed epoxy-to-curing-agent ratio) and $P2$ (fixed curing-agent proportion). Composites containing $10\text{–}40wt\text{\%}$ rosin were fabricated and characterized using Fourier Transform Infrared Spectroscopy ( $FTIR$ ), $X-ray$ Diffraction ( $XRD$ ), Thermogravimetric Analysis ( $TGA$ ), Scanning Electron Microscopy ( $SEM$ ), tensile and flexural tests. $FTIR$ confirmed hydrogen-bonding interactions between rosin and epoxy without new chemical bonds, while $XRD$ patterns revealed an increase in amorphous character. $TGA$ indicated reduced thermal stability compared to neat epoxy, attributed to the low-molecular-weight fractions of Pine rosin. $SEM$ showed more uniform particle dispersion and stronger interfacial adhesion in $P2$ , correlating with improved mechanical retention. Tensile and flexural tests revealed that rosin acts as a natural plasticizer: strength and modulus decreased gradually with increasing rosin content, while ductility improved. Optimal performance was achieved at $10\text{–}20wt\text{\%}$ rosin under $P2$ , maintaining about $42MPa$ tensile and $44MPa$ flexural strength. At higher loadings $(\ge 30wt\text{\%})$ , agglomeration caused embrittlement. These findings demonstrate that the processing route critically governs the microstructure and mechanical behavior of bio-based epoxy composites. Algerian pine rosin thus provides a sustainable and cost-effective bio-filler for developing flexible, partially bio-based epoxy materials suited for coatings, adhesives, and structural applications.