Effect of PA6 fiber length and orientation, and bonding agents on mechanical properties and interface adhesion in NBR/PA6 short fiber composites

Due to limited research on interfacial shear stress (ISS) and critical fiber length (Lc) in rubber-based composites, this study systematically investigates how fiber length, orientation, and bonding agents influence the mechanical properties of NBR/PA6 composites, focusing on evaluating ISS and Lc. Composite samples with fibers of varying lengths (1–12 mm) were produced, incorporating different bonding agents. Preparation involved internal mixing followed by processing on a two-roll mill to induce fiber orientation. Tests, including tensile, fiber pull-out, FTIR analysis, SEM imaging, and dynamic mechanical thermal analysis, were conducted to assess the composites’ mechanical properties and micromechanics. The addition of bonding agents significantly reduced the critical fiber length, a result attributed to increased ISS. This improved fiber-matrix interaction was confirmed by decreased void content and FTIR analysis, which showed covalent and hydrogen bonds between fibers and the matrix. However, excessive bonding agent (beyond 5 phr for epoxy resin) caused phase separation and overplasticization, while high fiber content led to aggregation and hindered interfacial bonding, lowering ISS. Tensile strength increased with fiber length up to 9 mm but decreased afterward due to fiber entanglement and uneven dispersion, reducing reinforcement efficiency. Evaluation of theoretical models showed decreased accuracy at high fiber aspect ratios. These findings suggest that optimizing fiber-reinforced rubber composites involves maintaining fiber lengths above the critical length, using moderate fiber content, and applying suitable interfacial adhesion strategies.