Extraction and tensile properties of natural fibers: Vakka, date and bamboo
Introduction
Vegetable fiber is one of the varieties of natural fibers obtained from stems, leaves, roots, fruits and seeds of plants. Vegetation is exploited for its ability to yield fibers directly from wild or natural forms. However, from commercial and technological points of view, cotton, kenaf, sisal, flax, palm, coir, arecanut and banana fibers acquire utmost significance, since reinforced plastics, strings, cords, cables, ropes, mats, brushes, hats, baskets and fancy articles such as bags are manufactured with those fibers.
All the ligno-cellulosic based natural fibers consist of cellulose micro-fibrils in an amorphous matrix of lignin and hemi-cellulose. These fibers consist of several fibrils, which run all along the length of the fiber: each fibril exhibits a complex layered structure made up of a thin primary wall encircling a thicker secondary layer and is similar to that of a single wood fiber. The fibers considered in the present analysis are treated as single units at the macro level as shown in Fig. 1.
Roystonea regia (Oreodoxa regia), popularly known as Royal palm which is locally called vakka, a native of West Indies and neighbouring parts of tropical America, has been cultivated in India [1]. An attempt to explore this tree for its fiber has not been taken up yet. In the present investigation, its fibers have been identified and found to be a potential reinforcement.
The date, botanically known as Phoenix sylvestris [2], the new name of which is Arecaceae, belongs to Palmaceae family. The fiber is picked from the dried leaves called leaf stalk fibers referred to hereafter as date (L) or from the netted structure called amplexicaul fiber referred to hereafter as date (A). The netted structure is the sheathing leaf base, which surrounds the stem.
As far as the characteristics are concerned, bamboos are tall, perennial, arborescent grasses, belonging to the Bambusae, a tribe under Graminae. Earlier, the fracture properties of bamboo culms and nodes have been studied [3]. Bamboo is a typical natural composite material, and the fibers are distributed densely in the outer surface region, and sparsely in the inner surface region. It is evident that the fracture toughness of the bamboo culm depends on the volume fraction of fibers. The bamboo has multi-nodes and functionally gradient structure, macroscopically as well as microscopically [4], [5], [6], [7], [8], [9], [10]. The effect of the absorption of water on mechanical properties of bamboo has also been studied [11].
Okubo et al. [12] undertook an in-depth analysis into the mechanical properties of polypropylene composites using bamboo fiber, extracted by steam explosion technique. The tensile strength and modulus of the polypropylene based composites increase about 15% and 30%, respectively due to better impregnation and reduction of the number of voids, compared to those of fibers that were mechanically extracted.
Thwe and Liao [13] examined the effect of fiber content, fiber length, bamboo to glass fiber ratio, and coupling agent (maleic anhydride polypropylene) on tensile and flexural properties of bamboo fiber reinforced polypropylene (BFRP) and bamboo-glass fiber reinforced polypropylene hybrid composite (BGRP). It was shown that hybridization with synthetic fibers is a viable approach for enhancing the mechanical properties and durability of natural fiber composites.
Thwe and Liao [14] carried out an extensive survey on the durability of BFRP and BGRP subjected to hygrothermal ageing and fatigue behaviour under cyclic tensile load. The authors concluded that BGRP shows a better resistance to environmental ageing than BFRP. An unreinforced polypropylene has a longer fatigue life than BFRP and BGRP composites at the specified cyclic load levels. In comparison to BFRP, the BGRP that is a hybrid composite presents a better fatigue resistance.
An analysis by Ismail et al. [15] on the effect of a silane coupling agent (Si69) on curing characteristics and mechanical properties of bamboo fiber filled natural rubber composites highlighted various aspects of this phenomenon. It was concluded that the presence of a silane coupling agent, Si69 improves the adhesion between the fiber and rubber matrix and consequently enhances the tensile strength, tear strength, hardness and tensile modulus.
Ismail et al. [16] examined the curing characteristics and mechanical properties of bamboo fiber reinforced natural rubber composites, as a function of fiber loading, and phenol formaldehyde and amethylenetetramine bonding agents. It was concluded that adhesion between the bamboo fiber and natural rubber can be enhanced by the use of bonding agents. As a result, the tensile modulus and hardness of composites increase with increasing filler loading and the presence of bonding agents.
Yao and Li [17] carried out a thorough investigation into the preparation and flexural properties of bamboo fiber reinforced mortar laminates. The laminate was a sandwich plate combined with reinforced bamboo plate and extruded PVA fiber reinforced mortar sheet. The results of the investigation show that the flexural strength values can be improved to greater than 90 MPa for laminates with reformed bamboo plate on the bottom, which formed a tension layer and the fiber-reinforced mortar sheet on the top that acts as compressive layer.
Among the well-known natural fibers (jute, coir, straw, banana, etc.), bamboo has low density and high mechanical strength. The specific tensile strength and specific gravity of bamboo are considerably less than those of glass fibers. However, cost considerations make bamboo an attractive fiber for reinforcement.
The present work provides extraction procedures for vakka, date and bamboo fibers. Date fiber is extracted from leaf stalks and amplexicaul. The bamboo fiber is extracted by retting and manual extraction, and chemical extraction procedures. The chemical and mechanical extraction procedures of bamboo are referred to here as bamboo (C) and bamboo (M), respectively. The cross-sectional shapes of the fibers are investigated and circular cross-section fibers are identified for tensile testing of fibers. The picnometric procedure has been employed for determining the densities of various fibers. The tensile properties of vakka, date and bamboo are experimentally determined along with a reinvestigation of sisal, banana, palm and coconut fibers. The tensile properties of vakka, date and bamboo fibers are compared with those of the established fibers like sisal, banana, palm and coconut.
Section snippets
Fiber extraction
Sisal, banana, coconut and palm fibers are collected from various local sources, and vakka, date (L), date (A), bamboo (C) and bamboo (M) fibers are extracted.
Fiber testing
An oven of size 450 × 450 × 450 mm (model—CIC-12) is used to dry the fibers. The oven has an automatic temperature control unit with an operating range 50–300 °C. An electronic weighing machine (0.0001 g accuracy) is used to weigh the fibers.
The percentage of moisture present per unit weight of each variety of fiber is evaluated. The fiber density is measured by the picnometric procedure. The experimental results for various fibers are enumerated in Table 1.
The diameter of the fiber was determined by
Extraction of fibers
The vakka fiber was extracted using a simple water retting process. It was observed that foreign matter (lignin, gums, etc.) was dissolved/separated in water within 15 days. Hand washing process results in complete separation of fibers from the foreign matter. The leaf sheath contains about 65% of fiber and 35% foreign matter. The process is simple and results in an excellent quality of fiber, on par with any other exploited fibers. Since, the fiber grows in the sheath longitudinal direction, a
Conclusions
Vegetation associated with agriculture and forestry is a large source for extracting fibers, which has been largely under utilised. Fibers that can be extracted from the vegetation with water retting process are inexpensive.
The process of extraction of vakka fiber is simple and results in an excellent quality of fiber, comparable to any of the presently explored fibers. Large polymeric composite components could be made of these fibers, since the fiber is long enough and uniform. Low density
Acknowledgements
The authors gratefully acknowledge the financial support extended by the All India Council for Technical Education, IG Sports Complex, IP Estate, New Delhi, India, (F. No. 8017/RDII/BOR/TMAT 049/Rec. 416) under Thrust Area Programme in Technical Education to carryout the Research project.
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