Classification of Natural Fibres

The use of natural fibres from plant, animal, and other sources as reinforcing or laminating material in the making of composite reduces concerns about environmental sustainability. A lot of natural fibres have a hollow space at their core called the lumen; this is made up of randomly arranged nodes that divide the fibre into distinct cells. The surface of natural fibres is rough, and these surface irregularities create good bonds to the matrix in a composite structure. The mechanical properties specific to individual natural fibres are important criteria in determining the end uses of composites. Tenacity and elongation up to the breaking point of natural fibres, especially flax, hemp, and ramie fibres, compete with E-glass fibres. The tensile strength and Young’s modulus of the fibres increase with an increase percentage of cellulose.⁹ As compared to synthetic fibres such as glass and aramid, natural fibres have low density, making them a suitable alternative for low weight applications where the weight of the material is a major problem.¹⁰

In the early 1990s, fourth-generation composites were made from fibre reinforcement of polymer to produce hybrid composites." The presence of lignin content in natural fibres would be used to develop sustainable multifunctional composites along with matrix polymer strength. The stiffness of natural fibres depends on the percentage of cellulose contents and the arrangement of microfibrils.¹² A fibre’s impact strength and load-bearing strength depend solely upon the alignment of the micrifi- brils.¹³’¹⁴ Fibres act as a load-bearing element, and the lignin, as a matrix load transfer medium, holds the fibre together and allows it to be oriented in a desired direction.¹⁵ Agro-based bast fibres such as jute and flax are generally preferred by the composite industries because of their good structural and reinforcement performance. The cellulose fibrils in bast fibres hold the lignin and hemicellulose together.¹⁶ The classification of natural fibres is shown in Figure 1.1.

Characterization of Fibres

Protein Fibre

In protein fibres, numerous reactive functional groups are present, with amino acids interconnected by peptide bonds. Protein contents are oriented parallel to the fibre axis. Protein fibre is commonly known for its lustre and softness. Alpaca goat fibre has good insulation properties. Angora fibre is very fine and warm to the touch. It is generally blended with wool fibre to reduce its cost. Camel hair has good sheen and insulation properties. Cashmere goat fibres feel soft and luxurious and have good absorbency.

Anti-parallel beta-sheet crystals form silk fibroins into high fibres with high tensile strength and toughness fibre.^17-19 The low-density silk fibres inherently possess good elongation and flame-resistant properties. Its mechanical properties are better than those of plant fibres.^20-23 Honey-bee silk fibre has good toughness and a stretch- ability of more than 200%.²⁴ Silk fibre blending is an efficient way to produce bio materials exhibits the combination features from its components.²⁵ Chicken-feather fibre shows good flexural modulus and noise reduction coefficient at its highest contribution percentage in the production of composite materials.^26-28 Human hair is also a protein fibre; its core constituent is keratin, and it is tough, intricate, and incredibly strong. A single strand of hair can bear a load of 100-150 grams.²⁹

Mineral Fibre

Asbestos occurs naturally as fibre. A synthetic mineral fibre known as rock wool or slag wool is produced by blowing air or steam through molten rock or slag. It is soft and flexible and good insulator of electricity, heat, and corrosion. Mineral fibres are used as fillers in fireproofing and thermal insulation materials. Basalt is naturally available worldwide. It is eco-friendly in nature; its fibres are produced by the process of drawing and winding fibres from the melt. It has good fire resistance, is chemically inert and can tolerate impact load.³⁰

Brucite is the mineral form of magnesium hydroxide; it has good anti alkaline properties. It is more stable in an alkaline medium than glass fibre. The moderate strength of this fibre can reach up to 900 MPa.³¹

Cellulose Fibre

Leaf, bast, seed, fruit, cane, grass, reed, and stalk fibres have high cellulose content. The content of renewable cellulose fibre and its percentage weight of various fibres are shown in Table 1.1.

FIGURE 1.1 Classification of natural fibres.

TABLE 1.1

Cellulose content in various kinds of natural fibres³²-³⁷

Cellulose-based fibres are subjected to a mercerization process, which leads to fibrillation, in which the fibre bundle is broken into smaller fibres, in turn decreasing the fibre diameter. As part of this process, the roughness of the surface topography increases in line with increases in the aspect ratio and mechanical properties of composites.³⁸-⁴¹ Reaction sites can be improved in high cellulose content fibres by alkali treatment, which increases the mechanical bonding of fibres during the composite manufacturing process.^42,43 The fraction of the volume of cellulose and the degree of cellulose crystallinity determine the fibre’s longitudinal Young’s modulus⁴⁴ After alkali treatment, cellulose fibre shows more aligned microfibrillar angle and thus has increased the load-bearing capacity ⁴⁵ The effectiveness of industrial composite material depends upon the fibre type and its cellulose content; adding a small quantity of matrix results in considerable changes in its physical, chemical and mechanical properties (e.g. tensile strength, Young’s modulus)^46,47