Natural fibers exhibit a wide range of physical and mechanical properties depending on their botanical source, microstructure, and chemical composition. In addition to plant-based fibers, animal- and mineral-based natural fibers exhibit distinct physical and mechanical behaviors due to their protein-based and inorganic compositions, respectively. Table 2 shows the physical and mechanical properties of various natural fibers collected from different literature sources. The density of these fibers typically ranges between 0.55 and 1.60 g cm⁻³, which is significantly lower than that of synthetic fibers, making them attractive for lightweight composite applications. Animal fibers such as wool and silk also exhibit relatively low densities and favorable strength-to-weight ratios, whereas mineral fibers generally show higher density and lower elongation characteristics.⁵⁴

Abaca fiber generally possesses a density of about 1.50 g cm⁻³, showing elongation between 3 and 12%, tensile strength reaching up to 980 MPa, and a modulus of elasticity around 12–72 GPa, indicating high stiffness and strength as reported by Biagiotti et al.⁷⁹ Bagasse fiber, on the other hand, shows much lower density, ranging from 0.55 to 1.25 g cm⁻³, with tensile strength values varying from 20 to 350 MPa and modulus between 0.5 and 27 GPa. The relatively lower strength of bagasse compared to abaca is attributed to its higher lignin content and porous cell structure as indicated by Yan et al.⁸⁰

Bamboo fiber demonstrates an intermediate behavior, with density values between 0.60 and 1.50 g cm⁻³, elongation varying from 1.3 to 7%, tensile strength in the range of 140–800 MPa, and modulus between 11 and 36 GPa. These values suggest that bamboo fiber offers a favorable balance between flexibility and strength, a property emphasized in the works of Sathishkumar et al.⁵⁶ and Sujaritjun et al.⁸¹ Similarly, banana fiber exhibits densities between 0.75 and 1.35 g cm⁻³, elongation at break ranging from 1–9%, tensile strength from 54–914 MPa, and modulus between 7.7 and 32 GPa. The moderate stiffness and strength of banana fiber are linked to its semicrystalline cellulose regions and the presence of natural microvoids.⁸²

Coconut fiber, being highly lignified, exhibits a comparatively low density ranging from 0.38 to 1.10 g cm⁻³, tensile strength between 83 and 222 MPa, and modulus values close to 12–32 GPa. Despite the low tensile properties, its high elongation makes it suitable for energy-absorbing and cushioning applications, as highlighted by Danso et al.⁸³ Coir fiber, another lignocellulosic fiber from the same family, possesses densities around 1.15–1.25 g cm⁻³, an extremely high elongation of 15–51%, tensile strength from 95 to 304 MPa, and modulus values up to 6 GPa. The high extensibility and ductile nature of coir fiber are consequences of its elevated lignin content.^84,85

Cotton fiber is comparatively dense, around 1.50–1.60 g cm⁻³, with elongation between 3 and 10%, tensile strength ranging from 200 to 800 MPa, and modulus values within 5.5–13 GPa. Its moderate stiffness and consistent strength have been widely reported in studies by Wambua et al.⁸⁶ and Saba et al.⁸⁷ Flax fiber, one of the strongest bast fibers, shows a density near 1.50 g cm⁻³, tensile strength from 345 to 2000 MPa, elongation between 1 and 4%, and modulus as high as 103 GPa. The superior strength and stiffness of flax are associated with its high cellulose content and low microfibrillar angle.^88,89

Zhang et al.⁹⁰ and Pickering et al.⁹¹ demonstrated that hemp fiber shows similar performance, with density between 1.40 and 1.50 g cm⁻³, tensile strength in the range of 270–1110 MPa, elongation from 1 to 4%, and modulus values up to 90 GPa. The high mechanical properties of hemp are the result of its high cellulose crystallinity and strong fibrillar alignment. Jute fiber, another commonly used bast fiber, exhibits density values from 1.30 to 1.50 g cm⁻³, elongation between 1.5 and 3%, tensile strength of 200–800 MPa, and modulus between 10 and 55 GPa. The variability in jute's tensile strength is due to its nonuniform fibrillar structure and variations in retting quality.^92,93

Pineapple leaf fiber possesses excellent mechanical characteristics with density between 1.07 and 1.56 g cm⁻³, elongation of up to 14.5%, tensile strength reaching 1627 MPa, and modulus between 60 and 82 GPa. These outstanding values are mainly due to its high cellulose content and strong inter-fibrillar bonding, as observed by Abiola et al.⁸² and Saba et al.⁸⁷ Ramie fiber also shows exceptional stiffness, with density around 1.50 g cm⁻³, elongation between 1.2 and 4%, tensile strength from 400 to 938 MPa, and modulus as high as 128 GPa. The strong crystalline regions and highly oriented microfibrils of ramie account for its excellent load-bearing capacity.^19,85

Sisal fiber demonstrates a moderate to high level of mechanical strength, with density between 1.30 and 1.50 g cm⁻³, elongation from 2 to 7%, tensile strength varying between 350 and 855 MPa, and modulus between 9 and 38 GPa. The good balance of stiffness and ductility in sisal is largely due to its moderate cellulose and lignin contents.^93–95

Animal fibers such as wool and silk are primarily protein-based materials. Wool fibers consist mainly of keratin with a helical molecular structure and disulfide bonds, which contribute to their elasticity and resilience.⁷⁶ This structure results in moderate tensile strength, typically around 100–200 MPa, and high elongation at break in the range of 25–35%, as well as good thermal insulation and moisture absorption properties.⁹⁶ Wool fibers have also been investigated as reinforcements in polymer composites, showing improved mechanical performance.^9,10 Silk fibers, composed mainly of fibroin with highly ordered β-sheet structures, exhibit higher tensile strength and stiffness compared to wool, and their mechanical properties can be further enhanced through processing and modification, reaching strengths comparable to spider silk.⁷⁷

In contrast, mineral fibers mainly refer to asbestos, which consists of naturally occurring fibrous silicate minerals. Asbestos fibers exhibit high tensile strength, typically ranging from 200 to 1000 MPa, combined with very low elongation at break, usually below 3%, as well as excellent thermal, chemical, and electrical resistance. Although these properties historically made asbestos valuable in industrial applications, its use has been largely discontinued due to its carcinogenic nature and associated health risks.⁷⁸

In general, fibers such as flax, hemp, ramie, pineapple, and sisal exhibit the highest tensile strength and modulus values, confirming their suitability for high-performance structural composites. In contrast, fibers like coir, bagasse, and coconut, while possessing lower tensile strengths, display higher elongations and energy absorption capabilities, making them better suited for flexible or impact-resistant materials. Animal fibers are particularly suitable for insulation, damping, and bio-composite applications due to their renewability and biodegradability, whereas mineral fibers are characterized by extreme durability but limited applicability because of environmental and health concerns.