Natural Cellulose Fibers from Renewable Resources

Keywords

Coconut husk • Retting • Decorticating • Leaf • Fiber properties • Microfibrillar angle • High elongation

About 62 million tons of coconuts are grown in about 92 countries across the world. Coconut trees or palms and the husks of the coconut fruit have extensively been used as sources for fibers. Fibers obtained from the husks (Fig. 9.1) of coconuts are generally termed “coir fibers” and are used for a variety of applications. Each coconut or copra yields about 80-90 g of husk fibers in Asia, whereas coconuts grown in the Caribbean contain thick husks and could yield up to 150 g of fiber. Each husk is composed of about 70 % pith and 30 % fiber and consists of 60 % long (150-350 mm), 30 % medium, and 10 % short fibers (<50 mm). About 5-6 million tons per year of brown coir and 125,000 tons of white fiber, mostly in India, are produced every year [13Van]. Fibers are obtained from the husks using conven­tional retting and chemical and biological means. In a conventional process, the husks are retted in brackish water for 3-6 months or in saltwater for 10-12 months to soften the fibers. Later, the fibers are separated by decorticating and beating and hackled and washed. This traditional processing yields the finest and whitest fibers. Alternative to traditional retting, mechanical processes to defibrillate or decorticate the husk have been developed. These machines and processes can process husks that have been treated for 5 days in water, but the quality of the fibers is heavily dependent on the processing conditions and severity of treatments. Recently, enzymatic processes have also been developed that are cleaner and milder and produce fibers with better quality.

Similar to the fibers obtained from palm trees and fruits, coir fibers are relatively coarser and have low tensile strength but high elongation as seen in Table 9.1. Also, properties of the coir fibers vary considerably with the changes in gauge length and strain rate as seen in Tables 9.2 and 9.3. Increasing gauge length decreased strength and elongation due to the increase in the higher number of weak spots that make the

Fig. 9.1 Digital image of a sliced coconut showing the outer husk (brown) that is used for fiber extraction and the inner (white) edible part

fibers more susceptible to breakage. At lower strain rates, the applied load is mostly shared by the amorphous regions, and therefore, the fibers have lower strength at lower strain rates [07Tom]. It was also reported that coir fibers had a crystallinity index of 57 and a microfibrillar angle of 51°, higher than other reports on coir fibers [07Tom].

In Sri Lanka, one of the major producers of coconuts, three major varieties of coconut trees are grown, and fibers obtained from them are graded by length as long (>150 mm), medium (100-150 mm), short (<100 mm), and very short (<50 mm). Although the chemical composition of the different grades and varieties of fibers was similar, considerable variations in tensile properties were observed. Tensile strength was found to vary from about 0.9 to 1.3 g/den and elongation varied between 20 to 28 % [05Nan].

In addition to the husk, other parts of the coconut tree such as the petiole shown in Fig. 9.2 have also been used for fiber production. However, considerable variations in properties have been observed for the fibers from the various parts as seen in Table 9.4 [82Sat].

Up to six times variation in diameter, 50 % difference in density, and 30° variation in microfibrillar angle are seen between fibers obtained from different parts of the coconut plant. A distinguishing feature of all the fibers obtained from the coconut trees is the considerably high microfibrillar angle that is responsible for the high elongation, compared to the fibers obtained from plant basts or stems.

Fibers obtained from the petiole exhibited the highest strength and modulus but considerably low elongation which should be related to the function of the different parts. Since the petiole forms the base of the leaves and is attached to the stem of the coconut tree, it is necessary for the petiole to be strong to withstand the forces of nature. Therefore, fibers obtained from the petiole are considerably stronger.

References

[82Sat] Satyanarayana, K. G., Pillai, C. K.S., Sukumaran, K., Pillai, S. G.K., Rohatgi, P. K., Vijayan, K.: J. Mater. Sci. 17, 2453 (1982)

[05Nan] Nanayakkara, N. H.A. S.Y., Ismail, M. G.M., Wijesundara, R. L.C.: J. Nat. Fibers 2, 69 (2005)

[07Tom] Tomczak, F., Sydenstricker, T. H.D., Satyanarayana, K. G.: Comp. Part A 38, 1710 (2007)

[09Kal] Kalia, S., Kaith, B. S., Kaur, I.: Polym. Eng. Sci 49, 1253 (2009)

[13Bua] Buana, S. A.S., Pasbaskhsh, P., Goh, K. L., Bateni, F., Haris, M. R.H. M.: Fiber Polym. 14(4), 623 (2013)

[13Suj] Sujaritjun, W., Uawongsuwan, P., Pivsa-art, W., Hamada, H.: Energy Procedia 34, 664 (2013)

[13Van] Van Dam, J. E.G., Common fund for commodities. Technical paper no. 6. ftp://ftp. fao. org/docrep/fao/004/y3612e/y3612e00.pdf. Accessed October 2013.