Methodical approach to interface design: Role of sizing in the micro-mechanical performance and durability of basalt fibers-based composites

Basalt fibers are renowned for their outstanding mechanical performance and promising end-of-life recyclability, while maintaining adequate performance compared to glass fibers. While their use is expanding, a deep comprehension of the fiber surface chemistry and particularly sizing is crucial. This thin layer, mainly made of film formers, organosilanes, among other components, governs the interface properties and thus the global performance of fiber-reinforced polymer composites. This study investigates the surface properties of basalt fibers treated with a range of film former, employing advanced surface characterization techniques. These including XPS, TGA, SEM-EDX, wettability, and five-parameter Weibull statistical analysis. The results show that the film former, polymeric versus oligomeric, strongly influences surface morphology, coverage, and flaw distribution. Oligomeric sizing, such as epoxy-based, achieves a coating with a narrower interfacial shear strength distribution. Polymeric film formers, generate thicker and more heterogeneous coatings, exhibiting dynamic rearrangements under hygrothermal conditions, showing a time-dependent healing effect. Its status modifies the surface key properties for further adhesion with polymer matrices. Beyond the insights into sizing-dependent surface performance, this work proposes a structured framework for characterizing and interpreting fiber surface to understand composite performance. By mapping different properties, the study encourages the transition from empirical, trial-and-error material development, still prevalent in industrial settings, towards more predictive, data-informed composite design. Our findings aim to contribute to a collaborative and continuous effort concerning interphase engineering in the growing field of basalt fiber composites. By positioning on fiber surface understanding, this work looks for unlocking the full potential of polymer-reinforced materials.