Effect of fiber extraction methods on some properties of kenaf bast fiber

Abstract

The objective of this study was to investigate the effect of different extraction methods on the fiber morphology, density, chemical composition and tensile strength of kenaf bast fiber. Kena fbast fibers were extracted using different methods (crude, decorticated, water retted, 5% sodium hydroxide retted and 5% benzoate retted) and their physicochemical characteristics were investigated. The morphological characteristics showed a significantly reduced lumen diameter and increased cell wall thickness after treated with NaOH at 5%. On the other hand, increased lumen diameter and a decreased cell wall thickness were observed with the decorticated and water retted bast fibers. A slight increase in fiber density was observed for NaOH and benzoate retted bast fibers indicating cell wall densification. Interestingly, the extraction methods used in this study produced bast fibers with high cellulose content and low sugar and starch due to the removal of wax, oil, pectin and hemicelluloses. Regarding the tensile strength, the water retted bast fibers showed highest tensile strength (426.05 MPa), while there was one no difference between decorticated and NaOH retted fibers (386.83 and 393.03 MPa, respectively).

Introduction

Cellulosic fibers produced from plant stems, such as kenaf, sisal, hemp, flax, jute, and ramie, are a viable reinforcement in composite materials, and there has been tremendous increase in a research interests regarding their applications over the past few years, possibly due to their biodegradability and low cost (Mohan and Kanny, 2012). Currently, natural fibers are used mainly in low performance applications such as construction and automotive industries (Placet et al., 2012). Before fibers can be used as reinforcement in high performance applications, it is a prerequisite to accurately understand their micro-mechanical behavior. This requires application of advanced and sophisticated experimental techniques, as well as a development of theoretical tools, in order to correctly relate the microstructure and complex organization of such fibers to their mechanical properties (Placet et al., 2012).

Among these natural fibers kenaf (Hibiscus cannabinus L.) belongs to the Malvaceae family and grows annually in tropical and sub-tropical areas (Kaldor et al., 1990). Kenaf has a long history of cultivation for its fiber especially in United States, India, Bangladesh, Thailand, and to a small extent in Southeast Europe, parts of Africa as well as in Brazil where it is cultivated throughout the year (Shi et al., 2011). There are two distinct types of fiber in kenaf stem, namely bast fiber and inner core which constitutes about 30–40% and 60% of the total dry weight of the stalk, respectively (Abdul Khalil et al., 2010). Kenaf bast fiber is a lignocellulosic fiber and has been used for production of fiber board and particle board, textiles, a fuel and as a reinforcement material for composites (Aleksandra et al., 2007, Abdul Khalil et al., 2009). Ligno-cellulosic fibers have a complicated structure. Kenaf bast fiber is made up from elementary fibers which are glued together by a pectin interface, to form technical fiber bundles. These bundles are separated from one another through partial decomposition of the cell wall, induced by bacteria or mechanical processes.

Many studies reported that methods of retting are important in determining fiber properties (Paridah and Khalina, 2009, Kawahara et al., 2005). Retting or so-called as degumming is a process for removing non-cellulosic material attached to the fibers to release the individual cellulosic fibers (Sur, 2005, Zhang et al., 2005). During the retting process phloem-derived fiber bundles are loosened from other stem tissue composed of hemicelluloses, lignin and pectin. Recently, some mechanical methods for separating the kenaf fiber have been described and practically performed (Abdolreza et al., 1997). However, in some developing countries due to cost effective wages, kenaf fibers are separated manually rather than mechanically. Even though, some researchers have also reported that the mechanical decortications is a fast and simple process compared to water and chemical retting, and produces a big quantity of fiber with high quality (Liu, 2005, Zhang, 2003, Abdolreza et al., 1997). From SEM analysis it was also reported that 3% NaOH was ineffective concentration for removing the impurities from kenf bast fibers surface, while 6% NaOH was the optimum concentration for the chemical treatment to remove the impurities (Mwaikambo and Ansell, 2002, Edeerozey et al., 2007). Jinshu et al. (2011), Keshk et al. (2006), Song and Obendorf (2006) have reported that high cellulose content can be obtained from kenaf bast fiber using water and NaOH retting processes. It was also reported that the cross-sectional shape of the kenaf fiber varied widely from noncircular shapes such as a kidney bean shape for cotton to the reasonably circular one for wool (Goswani et al., 2004).

However, there have been many studies on kenaf and very limited information is available on the effect of extraction methods on the properties of kenaf bast fiber. Therefore, the purpose of the current research is to investigate the effect of different extraction methods, i.e. water retting, chemical retting and mechanical decortications on the characteristics of kenaf bast fibers. These characteristics include changes in fibers morphology such as fiber length, diameter, lumen diameter, and cell wall thickness, density, chemical composition and tensile strength.