Thesis title: Bio-based surface engineering of natural reinforcements for sustainable, durable, and fire-resistant biocomposites
This doctoral dissertation investigates two critical limitations that currently restrict the large-scale deployment of sustainable biocomposites: in-service durability and fire resistance. The objective is to demonstrate that surface engineering, i.e. the targeted modification of the surface of natural fibers or bio-derived fillers before their incorporation into the polymer matrix, can significantly improve environmental stability, mechanical properties, and flame resistance, while avoiding the need for additives of toxicological or environmental concern. Across the different case studies, the common design principle is the localization of bio-based functionality at the reinforcement/filler–matrix interphase, addressing complementary performance bottlenecks (durability, fire behavior, and processing stability).
A bio-based strategy is proposed to enhance the durability of composites reinforced with natural fibers. Wool fabrics, an underutilized low-grade by-product of the sheep industry that falls outside textile specifications, were functionalized with a lignin-based coating polymerized through an enzymatic (laccase-mediated) process under mild conditions and without toxic solvents. This coating acts as a natural sizing at the fiber/matrix interface and reduces the chemical heterogeneity and intrinsic hydrophilicity of wool, improving compatibility with the matrix and limiting moisture uptake, swelling, and hydrolytic degradation. At the textile level, the lignin-based treatment increased the limiting oxygen index (LOI) from 25.9% to 28.2% and reduced moisture uptake from 29.2% to 25.1%, supporting an improved moisture-barrier behavior. In epoxy laminates reinforced with the coated wool, improved load transfer, higher thermo-mechanical stability, and better retention of properties under humid conditions were observed, indicating increased service durability without compromising processability. In particular, flexural strength and modulus increased by approximately ~21% and ~31%, respectively, and a higher property retention after moisture exposure (immersion–drying cycle) was observed, indicating a more durable interphase. In addition, the lignin-based coating proved to be effective for the development of multifunctional wool textiles, in which UV protection, partial flame retardancy, antibacterial activity, and moisture-barrier effects are directly imparted to the fiber, without the need for different types of synthetic finishes.
Regarding flame resistance, a bio-inspired, halogen-free flame-retardant coating was developed and immobilized on the surface of natural fibers. This strategy avoids the conventional use of flame retardants dispersed in the polymer matrix, which often suffer from leaching and poor interfacial compatibility, thereby reducing durability and mechanical performance. Specifically, through a bio-inspired process, flax and basalt fibers were functionalized by anchoring a hybrid organic–inorganic gallic acid / iron phenyl phosphonate (GA-Fe-P) flame-retardant system onto their surface, and were then used as reinforcement in the production of PLA-based composite materials. This interfacial layer promotes the formation of a cohesive char, reduces the candlewick effect, lowers the release of combustible volatiles and smoke, and improves thermal stability under flame exposure, without requiring high additive loadings in the PLA matrix and without impairing the mechanical properties. In flax fiber reinforced PLA composites, this approach enabled a strong smoke suppression (TSR ~ −87% and SEA ~ −68%) along with a reduction in pHRR (~ 5%). In basalt fiber reinforced PLA composites, pHRR decreased by ~ 8%, consistent with improved condensed-phase protection.
In addition, the thesis explores the functionalization of bio-derived carbonaceous fillers with the dual objective of improving durability and enhancing the flame resistance of PLA-based composites. Specifically, hydrochar (HC) obtained via hydrothermal carbonization of brewers’ spent grain was surface-modified with phytic acid, a renewable phosphorus source. In its untreated state, the hydrochar surface catalyzes PLA chain scission during melt processing, leading to molecular weight reduction, rheological instability, and accelerated thermal degradation, which negatively affects long-term material stability. Phytic acid passivates these reactive sites and, at the same time, introduces phosphorus confined at the filler/matrix interface. This dual effect enhances processing stability and limits polymer degradation, thereby contributing to improved durability, while also enabling condensed-phase and gas-phase flame-retardant mechanisms that result in higher thermo-mechanical stability and a reduction in heat release under fire conditions. Quantitatively, phytic-acid functionalization led to a reduction in pHRR of ~15% and a decrease in smoke release of ~12%, while improving melt-processing stability by mitigating HC-induced chain scission.
Overall, the findings indicate that bio-based surface engineering is a viable strategy to address both durability and flame resistance in sustainable composite systems. The results suggest that controlling chemistry and functionality at the fiber/filler-matrix interface can improve interfacial compatibility, reduce moisture uptake, stabilize processing behavior, enhance thermo-mechanical response, and contribute to flame suppression, while limiting the use of conventional halogenated or high-load additives. Although the flame-retardant performance achieved through fiber-localized coatings does not yet match that typically obtained with high-load bulk additives, the approach offers a lower-impact route that avoids issues of leaching and mechanical deterioration. Overall, the strategies developed here are still at a materials-development stage and require further validation under application-relevant conditions, but they outline a technically plausible direction for the development of biocomposites with improved performance while maintaining their sustainable character. From a broader perspective, the results support the implication that interphase-targeted, bio-based modifications can deliver multi-property gains (durability, fire performance, and processing stability) while reducing reliance on conventional high-load or potentially hazardous additives. Two experimental chapters are reproduced in the thesis as Author-Accepted Manuscripts (AAMs).