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

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: P 1 (fixed epoxy-to-curing-agent ratio) and P 2 (fixed curing-agent proportion). Composites containing 10 – 40 w t % rosin were fabricated and characterized using Fourier Transform Infrared Spectroscopy ( F T I R ), X − r a y Diffraction ( X R D ), Thermogravimetric Analysis ( T G A ), Scanning Electron Microscopy ( S E M ), tensile and flexural tests. F T I R confirmed hydrogen-bonding interactions between rosin and epoxy without new chemical bonds, while X R D patterns revealed an increase in amorphous character. T G A indicated reduced thermal stability compared to neat epoxy, attributed to the low-molecular-weight fractions of Pine rosin. S E M showed more uniform particle dispersion and stronger interfacial adhesion in P 2 , 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 – 20 w t % rosin under P 2 , maintaining about 42 M P a tensile and 44 M P a flexural strength. At higher loadings ( ≥ 30 w t % ) , 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.