Abstract

Thermosetting resins, especially epoxy resins, have historically been based on petroleum-derived monomers, posing problems of toxicity, high costs, and dependence on non-renewable resources. This chapter presents the use of natural plant sources to develop more sustainable resin precursors.

The synthesis of bio-epoxy monomers from vegetable oils, polysaccharides, lignin, polyphenols, and natural resins is currently the subject of interest in several research projects and scientific papers, in some cases reaching the level of product commercialization. Vegetable oils, such as linseed and soybean, and the transformation of saccharides into epoxy monomers are examples of explored options. Naturally occurring epoxy monomers derived from polyphenols from various plant sources can also be found, although their epoxidation requires the use of toxic compounds such as epichlorohydrin.

Several natural sources, such as natural rubber, resin acids, and lignin are examined as alternatives to synthesize epoxy resins of natural origin. Leutelin, recently identified in fruits and medicinal herbs, is highlighted as a promising compound to produce bioepoxy monomers.

Although initially the research focused mostly on the development of monomers of natural origin, research also extends to hardeners of natural origin, highlighting the synthesis of amines from vanillin and curing agents based on phenalkamines. The development of hardeners with reversible bonds, such as lignin imines, is also being explored, and the catalytic effect of hemp fibers in the curing of epoxy resins is highlighted.

The combination with traditional monomers or the development of recyclable resins and vitrimers with reversible bonds are current fields of interest for the development of these resins, especially in the context of their competitors of petrochemical origin.

Abstract

The increasing adoption of natural fibres as composite reinforcement is a promising development in materials science. These fibres have a low carbon footprint and are biodegradable, and they also have remarkable properties such as low density and high specific stiffness and strength. However, the mechanical properties of these composites are influenced by various parameters, which can complicate comparisons due to their diverse internal structures. This study focuses on two key normalised parameters: the Tsai modulus, which represents the trace of the stiffness matrix tensor; and the area of the Omni failure envelope in stress space. Our analysis of published data on unidirectional flax, hemp, jute, and kenaf composites shows that trace-normalised longitudinal Young's modulus can effectively facilitate stiffness comparisons between natural and synthetic fibre composites. A new and innovative way of measuring strength is suggested. This is based on the radius of a circle that matches the area of the Omni stress envelopes. This method is both robust and reliable for quantifying and comparing material strength. Although, extensive mechanical data on natural composites is available, it is difficult to establish design criteria for comparing them. Addressing this gap presents a significant opportunity to unlock the full potential of natural fibres in composite applications, paving the way for a more sustainable future in engineering materials.