Apparent interfacial shear strength of thermosetting resins reinforced with fique fibres using microbond test

The use of natural fibres as reinforcement in composite materials has gained increasing interest due to their low cost, renewability, and environmental advantages. However, effective fibre-matrix adhesion remains a key challenge due to the inherent incompatibility between hydrophilic natural fibres and hydrophobic polymeric matrices. To address this issue, surface treatments are frequently applied. This study evaluates the apparent interfacial shear strength (IFSS) of fique fibres embedded in polyester, vinylester, and epoxy matrices using the microbond test, while also assessing the effect of a polyvinyl alcohol (PVA) coating as a surface treatment. Furthermore, a perimeter-based calculation method was adopted to minimize the geometric estimation errors that are inherent to the non-circular cross-section of natural fibres. Results indicated that epoxy resin exhibits the highest IFSS for bare fique fibres. Conversely, PVA-coated fibres show a significant decrease in IFSS when embedded in epoxy, suggesting poor chemical compatibility under the tested conditions. In contrast, PVA-coated fibres in polyester resin achieved IFSS values comparable to those of vinylester and epoxy matrices, highlighting this combination as a promising, cost-effective alternative. Critical length calculations further confirmed that PVA-coated fibres in epoxy require greater lengths to act as effective reinforcement. These findings provide valuable insights for optimizing the interfacial performance of natural fibre-reinforced composites through low-cost surface modifications and accurate micromechanical characterization.