Silk: a unique natural fiber candidate
Generally, there is a growing interest in more sustainable material technologies. Composite materials are ubiquitous, finding applications across various sectors. However, their limited end-of-life options and complexities (or in some cases infeasibility) in recycling are discouraging their continued use in sectors which are subject to specific and general government legislations on material reuse, recycle, and waste management (such as the EU End-of-life Vehicle Directive [Directive 2000/ 53/EC], EU Directive on Waste Incineration [Directive 2000/76/EC], and EU Directive on Landfill of Waste [Directive 99/31/EC]). Consequently, there is a tremendous drive in developing a feasible composite recycling industry, but also substantial interest in incorporating more bio-based constituents in polymer composites as substitutes to both conventional, petrochemical-derived polymers and their synthetic reinforcements.
For example, bio-based composites reinforced with plant fibers such as flax, jute, and hemp have been widely investigated as eco-friendly alternatives to conventional glass fiber-reinforced composites [1—3]. These plant-based biocomposites have penetrated various markets, including the automotive industry (for interior panels), building and construction industry (for decking), and sporting equipment industry (e.g., for surfboards and bicycles) . Today, plant-based biocomposites have captured around 10—15% (by volume) of the EU fiber-reinforced composites
Natural Fiber-Reinforced Biodegradable and Bioresorbable Polymer Composites.
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market, making them several (five to eight) times larger than the carbon fiber composites market [1,5].
Strikingly, silk, the only natural fiber to exist as a continuous filament, has had no commercial applications and only limited scientific investigations as a reinforcement for engineering composites.
On the other hand, there has been significant scientific progress and breakthrough in the development of novel, silk-based biocomposites for biomedical applications over the past decades. The biocompatibility and bioresorbable properties of the proteinaceous silks, their unique combination of high strength and toughness, and their easy processability into various useful “regenerated” morphologies (including aqueous solutions, films, hydrogels, sponges, fibers and cords, and nonwoven mats) make them ideal for a wide range of clinical applications: from braided suture threads for surgical options, to porous, all-silk composite scaffolds for cartilage and bone repair. Authoritative literature reviews on these are presented in [6—9].
The question arises: is there a case for silks as suitable polymer reinforcements in engineering composites? More specifically, what advantages do silks and their composites offer in comparison to conventional materials? To address these questions, in this two-part chapter, we fabricate and characterize two forms of silk composites: (1) novel syntactic foams, where silk cocoons are employed as volume- occupying, structural particulate reinforcements in polymer foams; and (2) laminate composites, where silk fiber nonwoven mats and woven textiles are employed as fiber reinforcements in polymer resins. In light of the achieved properties, the potential applications of these new silk composite material technologies are briefly explored. Finally, we present recent findings on life cycle assessment studies on silk to evaluate the sustainability of silk composites.
We note that while spider silks may offer a wider range of properties including (in the case of dragline silk) being intrinsically stronger and tougher than their silkworm counterparts, as silkworm silks are more readily available in large quantities at competitive prices with good mechanical properties, they are appropriate first candidate silks for use in complex composites.