Structure-Property relationship of cellulosic fibres

The mechanical properties of cellulosic fibres such as strength is attributed to the rigidity and high molecular weight of cellulose chains, intermolecular and intramolecular hydrogen bonding and fibrillar and crystalline structure of the fibres (1). The strength and stiffness of fibres have been reported to be dependent on the crystallinity index and micro-fibril angle-(1). Fibres with higher cellulose content have also been found to be stronger than those with low cellulose content as long as their micro-fibril angle is small. For instance, cotton with micro-fibril angle between 20-30 oC and cellulose content of over 90% exhibits inferior mechanical properties to hemp and flax fibres (2), which have micro-fibril angles less than 5 o and cellulose content of over 80%. To study the property trend individual fibres have been alkalised as alkalisation bulks the cell wall and sets reorganisation on its fine structures. In this study hemp, sisal, jute fibres that have been alkalised have been tested in tension.

Figure 1: The strength of hemp fibre bundles with respect to fibre bundle diameter.

The indicated percentages in the figure are concentrations of caustic soda solution used to soak the plant fibres. The high strength of small diameter fibre bundles is understandable since, in the limit, a single unbroken chain of cellulose molecules must be approaching the theoretical tensile strength of bonds between atoms (3). The poor trend observed for jute and sisal fibre bundles is due to the lack of good inter-fibre bond following the removal of lignin by caustic soda treatment, which may result in fibre-fibre shearing. This paper demonstrates the dependence of the mechanical properties of cellulosic fibres on their physical and fine structure characteristics.

Both the tensile strength and Young’s modulus broadly increase with increase in the crystallinity index indicating a good relationship between the fibre structure and their mechanical properties. Gassan et al., (4) demonstrated that there was a strong correlation between the strength of alkalised jute fibre and crystallinity index, which they referred to as the crystallinity ratio. Their findings concluded that changes in mechanical properties could be attributed to changes in crystalline orientation.

Table 1 shows that all the tested fibres, except sisal treated with 0.03%, 0.08% and 0.24% NaOH, have values of elastic modulus (E) of their micro-fibrils determined using the crystallinity index (CI) which are higher than the elastic modulus determined using the cellulose (Cell) content. This indicates that crystallinity index is a better measure of the plant fibre’s stiffness than cellulose content. The stiffness values obtained in Table 1 are close to stiffness values obtained by McLaughlin and Tait, (5) and Kulkarni et al., (6) using bowstring hemp (Sansevieria metallica Géròme and Labroy) and banana (Musa sepintum) fibres respectively.

The physical morphological model of the plant cell wall in cotton and wood fibre represent the arrangements of the micro-fibril. The initial postulated stiffness model of the micro-fibril inclined at an angle ? to the fibre major axis has been presented in another publication by the author (7) The general equation of the stiffness of plant fibres is derived using the rule of mixtures. For the purpose of determining the micro-fibril angle of hemp, sisal and jute fibres equation 1 is modified to obtain equation 2.

Where Ef , E_Z , and E_X , are the elastic moduli of the fibre, cellulose (taken here also as micro-fibrils) and non-cellulose materials (represented mainly by lignin) respectively.  W_Z is the weight fractions of the micro-fibrils?

Table (1): The estimated elastic moduli of the micro-fibrils of hemp, sisal and jute fibre bundles calculated as a function of crystallinity index (CI) or cellulose content (Z)

Table 1: The estimated elastic moduli of the micro-fibrils as a function of crystallinity index (CI) or cellulose content (Z)

Only the micro-fibril angles of untreated fibres were determined and these are shown in Table (2).

Table (2): Estimates of the micro-fibril angles of untreated hemp, sisal and jute fibre bundles as a function of cellulose content (Z)

Table (2) indicates that hemp fibre bundles possess the lowest micro-fibril angle followed by jute and sisal fibre bundles. This implies that hemp fibre bundles will be stiffer followed by jute and sisal fibres bundles respectively. This method of determining the micro-fibril angle offers a new route not reported before in any literature and it is easier and quicker to use.

A key aspect of the research has been the development of a new method for determining the cellulose content of plant fibres and the development of new, quicker and easier techniques for determining the elastic moduli of the micro-fibrils using the cellulose content and crystallinity index.

Dr Leonard Mwaikambo is Associate Professor of Textile and Materials Science. He was external examiner Anna University, Chennai, Tamil Nadu and Calcutta University. He was Investigator of the Vegetable Oil Polymer Network at the University of Warwick United Kingdom. He was a research fellow University of Warwick. He was a researcher at the Institute for Research and Technology of Plastic Materials Italy. He is a PhD graduate in Materials Science and Engineering from University of Bath. He is a co-author of a book on Biopolymers and author of the book on Sustainable Composite Materials: Exploitation of Plant Resourced Materials for Industrial Application. He is Editorial Board Member of the Journal of Cellulose, Open Access Journal of Science and Technology, Bioresource Journal. His is Editorial Board Member of the Current Applied Polymer Science. He is Editorial Board Member of the Current Trend in Polymer Science. He was a regular reviewer of Composite Science and Technology, Applied Polymer Science, Macromolecular, Macromolecular Rapid Communication, Polymers and Polymer Composites, Journal of Materials Science. He is visiting Professor Technical University of Liberec, Czech Republic.