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: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>P</mml:mi> <mml:mn>1</mml:mn> </mml:math> (fixed epoxy-to-curing-agent ratio) and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>P</mml:mi> <mml:mn>2</mml:mn> </mml:math> (fixed curing-agent proportion). Composites containing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mn>10</mml:mn> <mml:mtext>–</mml:mtext> <mml:mn>40</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>w</mml:mi> <mml:mi>t</mml:mi> <mml:mtext>%</mml:mtext> </mml:math> rosin were fabricated and characterized using Fourier Transform Infrared Spectroscopy ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>F</mml:mi> <mml:mi>T</mml:mi> <mml:mi>I</mml:mi> <mml:mi>R</mml:mi> </mml:math> ), <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>X</mml:mi> <mml:mo>−</mml:mo> <mml:mi>r</mml:mi> <mml:mi>a</mml:mi> <mml:mi>y</mml:mi> </mml:math> Diffraction ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>X</mml:mi> <mml:mi>R</mml:mi> <mml:mi>D</mml:mi> </mml:math> ), Thermogravimetric Analysis ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>T</mml:mi> <mml:mi>G</mml:mi> <mml:mi>A</mml:mi> </mml:math> ), Scanning Electron Microscopy ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>S</mml:mi> <mml:mi>E</mml:mi> <mml:mi>M</mml:mi> </mml:math> ), tensile and flexural tests. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>F</mml:mi> <mml:mi>T</mml:mi> <mml:mi>I</mml:mi> <mml:mi>R</mml:mi> </mml:math> confirmed hydrogen-bonding interactions between rosin and epoxy without new chemical bonds, while <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>X</mml:mi> <mml:mi>R</mml:mi> <mml:mi>D</mml:mi> </mml:math> patterns revealed an increase in amorphous character. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>T</mml:mi> <mml:mi>G</mml:mi> <mml:mi>A</mml:mi> </mml:math> indicated reduced thermal stability compared to neat epoxy, attributed to the low-molecular-weight fractions of Pine rosin. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>S</mml:mi> <mml:mi>E</mml:mi> <mml:mi>M</mml:mi> </mml:math> showed more uniform particle dispersion and stronger interfacial adhesion in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>P</mml:mi> <mml:mn>2</mml:mn> </mml:math> , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mn>10</mml:mn> <mml:mtext>–</mml:mtext> <mml:mn>20</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>w</mml:mi> <mml:mi>t</mml:mi> <mml:mtext>%</mml:mtext> </mml:math> rosin under <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mspace width="0.25em"/> <mml:mi>P</mml:mi> <mml:mn>2</mml:mn> </mml:math> , maintaining about <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mn>42</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>M</mml:mi> <mml:mi>P</mml:mi> <mml:mi>a</mml:mi> </mml:math> tensile and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mn>44</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>M</mml:mi> <mml:mi>P</mml:mi> <mml:mi>a</mml:mi> <mml:mspace width="0.25em"/> </mml:math> flexural strength. At higher loadings <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mo>≥</mml:mo> <mml:mn>30</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>w</mml:mi> <mml:mi>t</mml:mi> <mml:mtext>%</mml:mtext> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> </mml:math> , 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.