Structural and thermal characterization of alkaline-extracted cellulose-based biocomposites with epoxy and vinyl ester resins

Abstract This study investigates the synthesis and characterization of polymer matrix composites reinforced with eucalyptus cellulose fibers, using epoxy and vinyl ester resin as matrices. Composites were produced with 5 and 10 wt.% of eucalyptus fibers, denominated EC/EF (Epoxy Composite reinforced with Eucalyptus Fibers) and VEC/EF (Vinyl Ester Composite reinforced with Eucalyptus Fibers), respectively, for each concentration. The materials characterized by Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetry (TG), Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM/EDS), Shore D hardness testing and Tensile Test. FTIR analyses confirmed the preservation of functional groups and highlighted interactions between the fiber and polymer matrices, while TG results demonstrated improved thermal stability in the composites, with epoxy-based systems showing higher degradation temperatures and char residue compared to vinyl ester. DSC analysis revealed a reduction in glass transition temperature with fiber incorporation, attributed to increased chain mobility and possible agglomeration. XRD confirmed composite formation and cellulose phase presence, particularly in 10 wt.% samples. SEM images showed rougher surfaces and better fiber dispersion at higher loadings, especially for EC/10EF, while EDS showed a uniform distribution of carbon and oxygen across all samples. Shore D hardness increased with fiber content, from 73.1 ± 0.2 for neat vinyl ester to 77.8 ± 0.2 for VEC/10EF. Conversely, tensile strength decreased with fiber incorporation in both matrices (e.g., from 23.4 to 15.4 MPa for EC/10EF, corresponding to a 34.2% reduction), which is attributed to fiber agglomeration and increased chain rigidity. However, the Young’s modulus of the epoxy composites showed an enhancement from 464.8 (pure epoxy) to 608.0 MPa (EC/10EF), representing a 30.8% increase. The results highlight the reinforcing potential of eucalyptus fibers in thermosetting matrices and support their application in structural and sustainable materials. Epoxy composites, in particular, exhibited superior thermal resistance and mechanical performance, suggesting greater suitability for applications requiring durability under thermal stress.