PSI - Issue 14

Ramesh Babu Adusumalli et al. / Procedia Structural Integrity 14 (2019) 150–157 R.B. Adusumalli / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Natural cellulose fibres such as flax and jute are used widely in making ropes, composites and textiles (e.g. linen fabrics). Pulp extracted from wood is normally used to make paper, but it is considered as natural cellulose fibre. These natural cellulose fibres are 0.4-5 mm in length if separated from the bundle. Regenerated or man made cellulose fibres are made from natural cellulose after dissolving it in solvents such as N-Methylmorpholine N-oxide (NMMO) followed by spinning and stretching. It should be noted here that degree of polymerization and crystallinity of naturally grown cellulose can’t be retained by dissolving and stretching even with a high draw ratio. So mechanical properties of man made cellulose fibres are always lower than naturally grown cellulose fibres and these fibres are cut to staple length of 38mm in order to blend with cotton and wool to make textile fabrics. Wool and human hair fibres are used in textiles and wig industries respectively and both are grouped as alpha keratin fibres. On the other hand, Kevlar and E-glass fibres are used as reinforcements in making composites. Single fibre mechanical properties such as strength and modulus are of interest in composite, textile and allied industries because these fibres are not homogeneous in terms of ultrastructure and composition. Both flax and hair fibres have composite structure wherein microfibrils act as reinforcement and lignin acts as matrix (in case of hair cystine amino acid acts as matrix). Traditionally, single fibre tensile tests until fracture and post fracture scanning electron microscopy (SEM) are carried out to understand the structural integrity of the fibre, because some fibres are composite in nature and are highly anisotropic due to arrangement of microfibrils in fibre direction or due to strong covalent bonding in fibre direction. Researchers have recently started using nanoindentation of fibre cross sections as reported by Samanta et al. (2016) to study the structural integrity of the small area of the fibre. In this study, modulus, strength and elongation from tensile experiment and modulus from nanoindentation test were obtained and structure-property correlations were compared and discussed for six different fibres. Though mechanical properties are not a concern in traditional textiles, quantifying strength and modulus of fibres is the need of the hour in composite industries involved in designing and processing woven or knitted preforms made up of single fibres of 8-100 µm diameter. Flax and pulp are considered as natural cellulose fibres and they consist of cellulose, hemicelluloses and lignin and their cell wall structure resembles fibre reinforced composite as explained by Adusumalli et al. (2010). Flax is bast fibre extracted from the outer layer of the flax stem through retting process and bundle of such fibres is shown in Fig. 1. (b). Pulp fibres are separated from wood through cooking and bleaching. Both flax and pulp fibres are complex anisotropic materials and are composed of cell wall and lumen as shown in Fig. 1. (a) and (b). The S2 wall, is thickest among all cell wall layers, and almost half of it is made up of partly crystalline cellulose microfibrils wound in a spiral fashion around the stem axis and embedded in a matrix of lignin as explained by Adusumalli et al. (2010). The angle of the cellulose micro fibrils with respect to fibre’s axis is known as micro fibril angle (MFA) and it influences mechanical properties of the fibre. This MFA increases from flax (5-10 0 ) to pulp (>10 0 ) due to cooking, bleaching and refining involved in pulp fibre extraction. Flax has slightly higher cellulose content and more crystallinity than pulp. Even though it is widely accepted that flax and pulp fibres vary in their mechanical properties, not enough studies have been carried out due to their short lengths (1-4 mm) and limited cell wall thickness (2-5 µm). Viscose and lyocell are considered as man made cellulose fibres and they are manufactured from fully delignified pulp. Viscose or rayon fibre is produced by using cellulose xanthate (CS 2 ) process and lyocell is produced by using NMMO solvent as reported by Adusumalli et al. (2009). In lyocell process, highly viscous dope is obtained and this dope is spun using platinum bushings followed by removal of solvent in coagulation bath. Different draw ratios and rearrangement of cellulose microfibrils give slightly higher crystallinity for lyocell fibres compared to viscose fibres. Both fibres are entirely made up of cellulose and their diameters vary between 9 µm to 34 µm. It should be noted that staple fibres have 50 % less modulus and low degree of preferred orientation compared to long filaments as given in Adusumalli et al. (2009). Normally viscose fibre has lobed cross section and lyocell has circular cross section as shown in Fig. 1. Human hair and wool are considered as natural keratin fibres. From the cross sectional view shown in Fig. 1. (c), hair fibre is composed of three parts, cuticle, cortex and medulla as explained in Mishra et al. (2016). Cortex resembles S2 wall and medulla resembles lumen present in pulp fibres. Cortex is the thickest region and contributes mostly to the hair’s mechanical properties and it is made from alpha keratin protein based cortical cells. The alpha keratin protein has microfibrils aligned in fibre axis as reinforcement and amorphous cystine (rich in S-S linkages) as a matrix. In the