An overview of challenges in processing of sustainable plant fiber composites

Traditional composites often rely on synthetic fibers, raising environmental concerns due to their nonbiodegradability
and resource-intensive production. Consequently, sustainable composites have emerged as a
potential substitute for these composites owing to their renewability, eco-friendliness, and biodegradability.
Plant fibers are the key choice as reinforcement material for fabricating these composites. Researchers have
explored the use of plant fibers to reinforce polymers, as a potential replacement for synthetic fibers, owing to the
minimal environmental impact and contribution to carbon neutrality, lower greenhouse gas emissions, reduced
energy consumption, and less dependence on fossil fuels. Flax, hemp, jute, ramie, etc., are some common examples
of plant fibers with the potential to be used in various applications. However, several critical challenges
persist in the processing of plant fiber composites, hindering widespread industrial adoption. The paper at hand
provides a comprehensive overview and critically discusses challenges inherent to processing plant fiber-based
polymer composites. It begins with an overview of common composite fabrication techniques, provides
insight into the plant fiber structure, alongwith their mechanical properties and architectural configurations as
reinforcements. A major challenge with these composites is the inherent hydrophilicity of plant fibers, leading to
moisture absorption and swelling. The review explores absorption kinetics, including Fickian and non-Fickian
models such as the Dual-Stage Fick’s Law and the Carter–Kibler two-phase (Langmuir) model, and also discusses
the corresponding swelling kinetics. Another critical focus of this review is on the compaction and
impregnation behavior of plant fiber preforms, highlighting issues such as compressibility, non-uniform resin
flow, and mold filling inconsistencies. Thermal stability during processing is also discussed, particularly focusing
on the thermal degradation thresholds of plant fibers. Each section concludes with a focused discussion on the
underlying mechanisms, current mitigation strategies, and knowledge gaps. By consolidating insights across
these domains, this review provides a foundational understanding of the interplay between plant fiber characteristics
and processing phenomena.