This study comprehensively examines recent developments in bio-epoxy resins and their applications in composites. Despite the reliability of traditional epoxy systems, the increasing demand for sustainability has driven researchers and industries to explore new bio-based alternatives. Additionally, natural fibers have the potential to serve as environmentally friendly substitutes for synthetic ones, contributing to the production of lightweight and biodegradable composites. Enhancing the mechanical properties of these bio-composites also involves improving the compatibility between the matrix and fibers. The use of bio-epoxy resins facilitates better adhesion of natural composite constituents, addressing sustainability and environmental concerns. The principles and methods proposed for both available commercial and especially non-commercial bio-epoxy solutions are investigated, with a focus on promising renewable sources like wood, food waste, and vegetable oils. Bio-epoxy systems with a minimum bio-content of 20% are analyzed from a thermomechanical perspective. This review also discusses the effect of incorporating synthetic and natural fibers into bio-epoxy resins both on their own and in hybrid form. A comparative analysis is conducted against traditional epoxy-based references, with the aim of emphasizing viable alternatives. The focus is on addressing their benefits and challenges in applications fields such as aviation and the automotive industry.

Natural fibres, valued for their low density, cost-effectiveness, high strength-to-weight ratio, and efficient energy absorption, are increasingly emerging as alternatives to synthetic materials in green composites. Although they cannot fully replace synthetic counterparts, like carbon, in structural applications due to their inferior mechanical performance, combining them through hybridization presents a potential solution. This approach promotes a balance between environmental benefits and mechanical efficiency. Recently, the transportation sector has shifted its focus towards delivering lightweight and crashworthy composite structures to improve vehicle performance, address safety concerns, and minimise environmental impact through the use of eco-friendly materials. The crashworthiness of energy absorbers, typically thin-walled structures, is influenced by several factors, including their material and geometric design. This paper presents a comprehensive overview of recent studies focused on the crashworthiness of fibre-reinforced, thin-walled composites under axial crushing. It explores different aspects, such as their materials, cross-sections, stacking sequences, triggering or filling mechanisms, and the effect of loading rate speed. Emphasis is placed on natural-fibre-based materials, including a comparative analysis of synthetic ones and their hybridization. The primary objective is to review the progress of solutions using green composites as energy absorbers in the automotive industry, considering their lightweight design, crashworthiness, and environmental sustainability.

The introduction of bio-based matrices in automotive applications would, in principle, increase their sustainability and, in case the use of secondary raw materials is also involved, even result in reduced resource depletion. The bio-based polymer composite matrix that has been mainly brought forward towards industrial application is poly(lactic acid) (PLA), which has often been proposed as the replacement for matrices based on polyolefins in fields such as packaging and short-term commodities since, in general, it matches the needs for conventional thermoplastic production processes. The passage to the automotive sector is not obvious, though: problems affecting durability, the relation with water and the environment, together with the requirement for outstanding mechanical and impact performance appear very stringent. On the other hand, PLA has obtained durable success in additive manufacturing as a competitor for acrylonitrile butadiene styrene (ABS). Also, the perspective for 3D and 4D printing does not appear to be confined to bare prototyping. These contrasting pieces of evidence indicate the necessity to provide more insight into the possible development of PLA use in the automotive industry, also considering the pressure for the combined use of more sustainable reinforcement types in automotive composites, such as natural fibers.