MATERIALS AND METHODS

Sisal production and yield: The production of sisal in Kenya, Tanzania, Brazil and China were studied from 1961 to 2009 from the commodity data website [http://data.mongabay.com/commodities/category/1-Production/1-Crops/789-Sisal], which records the production and export of products worldwide. Sisal fiber production and yield were considered.

Characterization of Kenyan sisal fiber: Sisal fiber samples were collected from Kenyan sisal factories and characterized. The fiber characteristics considered included the physical, mechanical and chemical properties of the fiber.

Chemical ingredients of Kenyan sisal: The chemical composition of the Kenyan sisal fiber was investigated using Chinese standards for the determination of the chemical ingredients of bast fibers (GB 5889-1986, P. R. of China).

Tensile properties: The mechanical properties were measured using a single fiber tensile machine, which had a speed of 5 mm min^-1 and a gauge length of 10 cm.

Cross-sectional and longitudinal morphology: In order to evaluate the surface morphology of the fiber the sisal fibers were investigated using a Scanning Electron Microscopy (SEM), Hitachi TM1000. The excitation energy for the general procedure was 15 Kev. The fiber samples were sputtered with gold before examination to ensure good conductivity. The cross-sectional and longitudinal morphology were investigated.

Chemical groups in sisal: The chemical groups in sisal fiber were investigated using Fourier transform infrared spectrophotometry (FT-IR) Nicolet 8700. FT-IR spectra of each sample studied using a range of 4000 to 400 cm^-1. Spectra outputs were recorded in the absorbance mode as a function of wave number.

Crystallinity of sisal: The sisal fiber crystalline index was obtained from an X-ray Diffraction (XRD) patterns of the sisal fiber obtained from a D/MAX-2550PC X-ray detector diffraction system. The machine was set at voltage of 18 kW, current of 30 mA and scan rate of 2° min^-1.

RESULTS AND DISCUSSION

The trend of sisal production and yield: The production and yield of sisal in Kenya and other selected countries (Brazil, Tanzania and China) is given in Fig. 1 and 2. Corresponding values for three other countries (Brazil, Tanzania and China) have been included for comparison purposes. The figures indicated that the production of sisal in Kenya exhibited a declining trend in the last four decade. The production of sisal in Tanzania has shown a drastic decrease while Brazil has experienced an erratic production howbeit with marginal increase.

The Kenyan agricultural sector enjoyed government protection up to 1982, when agricultural subsidies were removed. During the years of government protection, sisal production was robust, with higher production level being realized in 1970. The major decline in production recorded in the late 1970’s can be attributed to high level of corruption in government departments. When the government liberalized the agricultural sector, the small scale farmers were unable to grow sisal, due to low sisal prices and high cost of inputs. In 2008, for example eight large scale companies produced over 90% of the Kenyan sisal (Kiome, 2010). The production from small scale farmers was a merge 420 tons. The Kenyan government needs to look into ways and means of encouraging the small scale sisal farmers to boast sisal production. In Brazil, the bulk of the sisal is produced by the small scale farmers. It is a high time, Kenya tried to study the Brazilian sisal growing industry with an aim of encouraging small scale sisal farmers. Going by reported findings available in the public domain, the decline in sisal production in Tanzania was caused by poor husbandry, propagation of the wrong varieties and a decrease in the length of sisal leaves (Hartemink and Wienk, 1995). Similar factors could also be responsible for the decrease of sisal yield in Kenya, since the two are neighboring countries. The phenomena increase in sisal yield in China is hereby noted and should be considered a research subject by the Kenyan sisal industry.

The characterization of the sisal fiber
Cross-sectional and longitudinal morphology of sisal: The morphology of the Kenyan sisal fiber for the longitudinal and cross-section views obtained using the SEM are given in Fig. 3. These SEM images are similar to those obtained by Mukherjee and Satyanarayana (1984) for Indian sisal and Martins et al. (2004) for Brazilian sisal. The surface irregularities appearing on the surface of the fiber will lead to variations of sisal fiber properties.

Fig. 1:	Sisal production from 1961 to 2009

Fig. 2:	Sisal yield from 1961 to 2009

Fig. 3(a-b):	The SEM of the Kenyan sisal fiber, (a) Longitudinal view and (b) Cross sectional view

The cross-section view attest to the fact that sisal fiber is made up of several fiber cells (Filho et al., 2009). These fiber cells are held together by resin like material called lignin.

The chemical composition of Kenyan sisal: As stated earlier the chemical contents of sisal fiber were determined using Chinese standards for bast fibers. The composition of the Kenyan sisal fiber was 63-69% cellulose, 10-13% lignin, 18% hemicellulose, 0.8-1.6% pectin, 1.5% wax and 0.6-1% water soluble matter. As stated earlier the chemical contents of a bast fiber has been proven to affect the mechanical properties of the fiber (Mukherjee and Satyanarayana, 1984). Sisal fiber like other bast fibers is considered to be a natural composite material where cellulose is the main weight bearing component and cellulose and lignin are the matrix (the joining gum). Typically a fiber cell will consist of fibrillae in a matrix of lignin (De Andrade Silva and Filho, 2007). The fibrillae is also another composite consisting of cellulose in a matrix of hemicelluloses. As shown in Table 1 and 2 different researchers have reported varying values. The difference in the results could be attributed to the fact that sisal is a natural fiber and its properties are bound to be affected by the growing conditions. Referring to Table 2, which is a summary of the chemical contents of sisal as reported by several researchers, the cellulosic contents of the Kenyan sisal can be adjudged as average.

Table 1:	Ingredients of sisal

Table 2:	Mechanical characteristics of sisal

Apart from the reports of Favaro et al. (2010), Rowell et al. (2000) and Salazar et al. (2011), the cellulose contents for the Kenyan sisal can be adjudged to be low. The lower percentage of cellulose, accompanied by higher percentage of hemicellulose may lead to some changes in the physical and mechanical characteristics of the fiber. Hemicellulose has lower degree of polymerization, is attacked by both acids and alkalis, while cellulose has very high resistance to alkali attacks, but is attacked by acids. Lignin binds the cellulose and hemicellulose together. Levels of 4-8% should be optimum. Higher percentages of lignin will have adverse effect on the mechanical strength.

Chemical groups in Kenyan sisal: The FT-IR spectroscopy was used to study the functional groups present in the sisal fiber. Fig. 4 gives the graph of the absorbency. The peaks at 3340.8 and 1035 cm^-1 could be attributed to the O-H stretching and bending groups, respectively. There are a series of peaks in the 1720 to 1610 cm^-1 range which could be attributed to the carbonyl stretching (C = O) for acetyl groups in hemicelluloses and for aldehydic groups present in lignin (Favaro et al., 2010; Abdul Khalil et al., 2010) which is one of the ingredients found in bast fibers. The presence of an aldehyde could be confirmed by a peak that occurs at 2840 cm^-1. The vibration peak at 1244.4 cm^-1 could be attributed to C-O stretching vibrations. Other minor peaks at 2922.9 and 1426.5 cm^-1, could be attributed to C-H₂ stretching and bending vibrations, respectively.

Crystallinity of sisal: The XRD diffraction curve for the Kenyan sisal fiber is given in Fig. 5. There are two main peaks; one sharp peak at 22.1° and a broad peak at around 15°. Other minor’s peaks were noticed at 34.8 and 44.5°. The Crystallinity of the sisal fiber was 51.15%. These results are comparable to the values reported by Yi et al. (2010) for Chinese sisal, albeit on the lower side. As noted earlier the lower percentage of cellulose, coupled with a higher percentage of hemicellulose and lignin could have lead to lower crystallinity.

Fig. 4:	The FT-IR for Kenyan sisal fiber

Fig. 5:	X-ray diffraction for Kenyan sisal

Tensile properties: The tensile strength for the Kenyan sisal was 450 MPA (410 -570) while the elongation was 4.76% (3.9 to 5.17%). A summary of the mechanical properties of sisal given in Table 2, indicated that the tensile properties of the Kenyan sisal was comparative lower than those reported by other researchers. Considering reports from Joseph et al. (1999), Mohanty et al. (2000) and Taj et al. (2007), higher cellulose contents accompanied with lower lignin contents gives higher tensile strength. This could be attributed to the fact that cellulose is the main strength bearing element in the sisal fiber structure and lignin and hemicellulose are the matrix member of the component.

The lower tensile strength reported for the Kenyan sisal could therefore be attributed to the lower percentage of cellulose coupled with higher percentages of lignin.

CONCLUSION

A study of the production and characterization of the Kenyan sisal was undertaken. According to the data obtained, it can be concluded that Kenyan sisal production and yield has been declining in the last four decades. This is the trend in the other sisal producing nations, except Brazil whose production has been rising and China whose yield has been increasing. The Characterization of the Kenyan sisal indicated that the Kenyan sisal has a higher percentage of lignin and hemicellulose. The tensile strength of the Kenyan sisal was comparable to the lower spectrum of sisal tensile strength for sisal from other regions (countries).

ACKNOWLEDGMENTS

We wish to thank the Chinese government which supported the project through the Sino-Africa 20+20 program and the China Scholarship Council.