Abstract
Fiber processing is an important factor that affects the physical and mechanical properties of long kenaf fiber. The physical-mechanical properties, such as color, tensile strength, and moisture regain (MR), are the main characteristics that influence the processing and performance on the final products from kenaf fiber. The objective of this research was to evaluate the physical and mechanical properties of kenaf fiber at the week of planting, with different processing methods that were planted in a selected location in Malaysia. Kenaf fiber was separated by the water retting method, where the combination of water retting and mechanical retting was based on the available facilities and proficiency at the sites. The tensile strength and chemical composition of kenaf fiber showed large variability for every location collected and for each processing technique used. Visual observations and color testing indicated that kenaf fiber with extended water immersion exhibited higher lightness and smoother fiber. A slight reduction in moisture regains was observed with increased crystallinity index. The differences among properties of the kenaf fiber that were influenced by harvesting week and processing methods could give some references and overview for the planters, suppliers, industries, and researchers to identify the suitable fiber quality for their targeted final products.
Download PDF
Full Article
Effect of Harvesting Time and Water Retting Fiber Processing Methods on the Physico-mechanical Properties of Kenaf Fiber
Hazwani Husna Abdullah,a Sarani Zakaria,a,* Noor Intan Saffinaz Anuar,a,b Kushairi Mohd Salleh,a,* and Sharifah Nabihah Syed Jaafar a
Fiber processing is an important factor that affects the physical and mechanical properties of long kenaf fiber. The physical-mechanical properties, such as color, tensile strength, and moisture regain (MR), are the main characteristics that influence the processing and performance on the final products from kenaf fiber. The objective of this research was to evaluate the physical and mechanical properties of kenaf fiber at the week of planting, with different processing methods that were planted in a selected location in Malaysia. Kenaf fiber was separated by the water retting method, where the combination of water retting and mechanical retting was based on the available facilities and proficiency at the sites. The tensile strength and chemical composition of kenaf fiber showed large variability for every location collected and for each processing technique used. Visual observations and color testing indicated that kenaf fiber with extended water immersion exhibited higher lightness and smoother fiber. A slight reduction in moisture regains was observed with increased crystallinity index. The differences among properties of the kenaf fiber that were influenced by harvesting week and processing methods could give some references and overview for the planters, suppliers, industries, and researchers to identify the suitable fiber quality for their targeted final products.
Keywords: Chemical composition; Color testing; Moisture regain; Mechanical separation
Contact information: a: Bioresources and Biorefinery Laboratory, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia; b: Fiber and Biocomposite Centre, Malaysian Timber Industry Board, Banting 42700, Selangor, Malaysia;
* Corresponding authors: szakaria@ukm.edu.my, kushairisalleh@gmail.com
INTRODUCTION
In Malaysia, kenaf was introduced as an alternative crop to farmers while the government slowly tried to reduce tobacco plantations due to environmental awareness and economic consideration (Izran et al. 2014; Misnon et al. 2014). The National Tobacco and Kenaf Board (NKTB) is the national agency that has been established to coordinate the implementation of policies, programs development, and commercialization of kenaf and tobacco. Universities, research institutes, and industries also have become progressively involved in the research on kenaf, covering the downstream, midstream, and upstream area. The understanding of the fiber properties is essential to ensure they meet the industry concerns and provide acceptable requirements and standards to the end products. Moreover, this knowledge could also be used as a guideline in the selection of fiber treatment. There have been extensive studies and developments conducted and reported on kenaf (Hibiscus cannabinus L.) (H’ng et al. 2009). Kenaf has been recognized as a crop with great potential after a few established commodity plant periods due to a shortage of wood fiber from virgin forests, and because of its excellent mechanical properties.
Kenaf can be harvested within 3 to 6 months as green or dry material and it consists of 25 to 40% bast and 60 to 75% core. In terms of mechanical properties, kenaf exhibits almost similar properties to jute fiber with more lustrousness and brighter color (Anuar et al. 2018). Superior strength of kenaf bast fiber makes it suitable for high performance products, such as reinforced composites, while the core material should be infrequently utilized due to its moderate properties (Chan et al. 2013). Kenaf harvesting is usually practiced either by using machines, such as a combination of mower conditioners and baling equipment, a sugarcane-type harvester, and 4GL-180II machines (Pari et al. 2015), or manually by hand. For the harvesting process, some plantation sites still use manual techniques that are labor-intensive, making them less profitable (Dauda et al. 2013). During the harvesting process, kenaf stems were cut near the plant base approximately 3 to 5 cm above the soil, where it can maximize the fiber yield and fiber length (Singh 2010). The beginning of the flowering stage is the best time for kenaf harvesting for good fiber quality and high yield production. Iii et al. (2002) mentioned that the protein content, the kenaf leaf, and stalk percentage, are related to the age of kenaf during harvesting time. After the harvesting process, kenaf stem or stalk was biologically or chemically retted to extract the bast part. Retted fibers were manually stripped from the stem. Mechanical separation can be conducted prior to the retting process, depending on the equipment and machines available. Then, kenaf fiber was washed and dried well before being transported for further processing (Grubben 2004).
Different fiber separation techniques produce fibers with differences in physical, mechanical, and morphological properties, which subsequently influence the properties of the final product. The separation between the bast and core parts is conducted with various methods such as mechanical separation and retting process. During mechanical separation, fluted rollers are used to break and crush the woody core, while the remaining bast fibers are converted into ribbon form. In contrast, the retting process is used to remove the gum component, such as lignin, hemicelluloses, pectin, and other impurities, while keeping the cellulose in good condition (Yu and Yu 2007). Traditionally, kenaf stems are kept on the field (dew retting) or immersed in the pond/tank/river (water retting) for two or three weeks. Microbial activities in the retting process break down the fiber parts by degrading and softening the pectins that glue the plant tissues together and release individual fibers (Md. Tahir et al. 2011). Dhulina et al. (2019) summarized that long kenaf fiber could be produced by dew, enzymatic, chemical, and water retting, while mechanical retting provided short or twisted fiber. A combination of mixing processes, such as chemical and enzymatic retting, was introduced to improve the fiber quality, mechanism, and environmental issues of the traditional retting process. However, the recent alteration on the retting process showed multiple advantages and disadvantages in terms of cost, energy used, and fiber quality itself. The final product from the fiber produced should be taken into consideration in the selection of the retting method.
To date, research on the suitability of each location, soil condition, climate, fiber processing, and machines used are still ongoing to obtain the best results for fiber quality and yield. In terms of fiber processing, each location is practicing different methods based on the local infrastructure, logistics, and guidelines provided by NKTB. The information on the physical and mechanical properties of Malaysia cultivated kenaf has gone through different processing techniques, as it is important for the planters, suppliers, industries, and researchers to obtain the good quality of the fiber. In this study, kenaf fibers were collected from selected locations throughout Malaysia and were evaluated. This procedure was carried out to generate information on the mechanical, physical, and optical properties of kenaf fiber at different harvesting periods and fiber processing methods on selected processing locations in Malaysia.
EXPERIMENTAL
Kenaf Fiber Preparation
For the sample collection of fiber, kenaf were harvested at 12 and 16 weeks after planting (WAP) and was obtained from five states in Malaysia: Pahang (PH-1), Johor (JH-2), Perlis (PR-3), Terengganu (TG-4), and Kelantan (KN-5). The fibers were air-dried until the moisture content reached below 12% before testing. The samples were kept in a closed dark container to minimize degradation and color changing. Water retting was conducted at local fiber processing sites where the duration (days), type of water, type of container, pre-treatment process, and drying period were conducted differently according to the local fiber processing site practice. The processing of the samples is simplified in Table 1.
Table 1. Kenaf Processing Stages by States in Malaysia
Characterization
Morphology of kenaf fiber
The morphological observations of the kenaf fiber were investigated using a scanning electron microscope (SEM) LEO 1450VP (Carl Zeiss Microscopy GmbH, Jena, Germany. Prior to scanning, the samples were sputter-coated with gold (Polaron SC7680; Quorum Technologies Ltd., East Sussex, United Kingdom). The diameters of the fiber were measured during the SEM observation at 200X magnification. A Dino-Lite Pro microscope AM-413ZT (Dino-Lite, Torrance, CA, USA) was used to capture surface images of kenaf fiber strands at 70X magnification.
Chemical analysis
The composition of cellulose, hemicelluloses, lignin, ash, and extractives of kenaf fiber was measured according to TAPPI Test Methods: TAPPI T203 cm-99 (1999), TAPPI T222 (1999), TAPPI T244 (1998), and ASTM D1107-96 (2001), respectively. Hot-water solubility of kenaf fibers was conducted to measure extraneous components such as sugars, starches, and others. Samples were ground using a Microfine Grind Drive (IKA-Werke GmBH, Staufen, Germany) and placed in the sieve machine. Samples that passed through at BS 40-mesh sieve and retained at BS 60-mesh were collected for further processing.
Moisture regains
Moisture regains (MR) for kenaf fiber were conducted according to ASTM D2495-07 (2007). Approximately 10 g of ground kenaf fiber was placed in the oven at 105 °C and dried for 24 h. The fibers were weighed and then moved into the humidity chamber with the relative humidity set at 65% and at a temperature of 20 °C. After 8 h, the weights of the fibers were measured. The process of weighing, drying, and cooling the fibers was repeated until the change in mass between two successive weighings was less than 0.1% of the specimen mass.
X-ray diffraction (XRD).
The crystallinity percentage of the kenaf fibers was determined by using a Bruker AXS X-ray diffractometer (AXS GmbH, Karlsruhe, Germany). The Cu Kα radiation and anode voltage were at (λ = 0.154 nm) and 40 kV, respectively. The percentage of the crystallinity index was automatically measured using the instrument software (Bruker AXS GmbH, V.1.2, Staufen, Germany).
Color testing
The color examination of kenaf fiber was carried out using a portable CR-410 Chroma Meter (Fig. 1) (Konica Minolta Inc., Osaka, Japan), which is designed to evaluate the color of the objects and the texture and surface conditions. The diameter of the measurement or illumination area of this Chroma meter is 50 mm (Kassim et al. 2012). In the color space defined by the International Commission on Illumination (CIE), L* indicates the lightness of the object, while a* and b* are the chromaticity coordinates. In the color space diagram, the a* and b* indicate the color direction: +a* is the red direction, –a* is the green direction, +b* is the yellow direction, and –b* is the blue direction. Five bundles of kenaf fiber were randomly chosen and placed parallel and close-together in five metal plates. The measurements taken were firmly covered by glass to ensure that the entire gap was covered by kenaf fiber. For each plate, four measurement spots were chosen. The calorimeter automatically showed the L*, a*, and b* results on the screen. The average readings from all the samples were taken for further analysis.
Fig. 1. Measurement taken using Chroma meter 410
Single fiber tensile strength
The tensile strength testing for single fiber was performed using a universal testing machine Shimadzu AG-X Plus (Shimadzu Corporation, Kyoto, Japan). The tensile strength test was conducted based on ASTM D3822-01 (2001). Bundles of fiber were manually separated one-by-one and temporarily placed on a glass slide. Double-sided tape was used to attach the fiber to the glass slide. A total of 40 single fiber diameter was measured using an Olympus BX43 Biological Microscope (Olympus Corporation, Tokyo, Japan) at 10X magnification. Finally, a single fiber was directly clamped between two blocks on both ends. The gauge length was set at 10 mm. A load cell of 10 N was used to measure the force. For each sample, 40 readings were calculated based on the load (N) per area of the fiber.
RESULTS AND DISCUSSION
Morphology Properties of Kenaf fiber
Samples were collected at different periods (12 and 16 WAP) based on the schedule provided by NKTB. Except for the sample from PH-1, the other states underwent mechanical separation (decorticator) before water retting. The variations in the samples collection in terms of color, physical appearance, MR, and others were due to several factors such as the easiness of processing operation, the cost of fiber transportation, water consumption, and the storage space required. Different physical appearances and colors can be observed for all collected kenaf fibers, as shown in Fig. 2.
The different sequences in processing activities contributed to the difference in the kenaf fibers’ appearances. It was expected to be a small difference in terms of fiber surface roughness according to the retting process at selected locations. As shown in Fig. 2e, visually kenaf fibers PH-1 showed smoother and lighter color. At the processing site of PH-1, kenaf stem was completely submerged in a big pond with water collected from rain, stream, and underground water. Leaves, shoots, and flowers from the kenaf stem were naturally retted during the water retting process. After 14 days, the kenaf stems were taken out from the pond, followed by the manual separation of bast and core by hand. The fibrous bark was completely removed, and the core part successfully separated from the bast manually. Md. Tahir et al. (2011) reported the existence of pectynolytic bacteria in the decomposition of pectineus substances during water retting produced a high-quality fiber with greater uniformity compared to the mechanical extraction and dew retting. Furthermore, it was reported that the conditions for bacterial growth and activities were found to be more consistent in water retting, which produced the uniform colonization of bacteria and resulted in better fiber quality (Nair et al. 2013).
Fig. 2. Kenaf fiber sample collected: a) KN-5, b) JH-2, c) PR-3, d) TG-4, and e) PH-1
As shown in the green circle in the figures, there were lots of cores left, more deteriorations, and unused substances attached to the kenaf fiber of JH-2, PR-3, and TG-4 (Figs. 3a, b, and e). The circle is shown in Fig. 3e, which represents kenaf fibers of TG-4, shows unclean kenaf fibers with fibrous bark and cortical cell due to the short immersion days observed. The fibers of TG-4 were also coarser and possessed lower uniformity on the surface. The spot of deterioration exhibited in Fig. 3b of the PR-3 sample was spotted, and incomplete separation of fiber with unused substances can be seen in Fig. 3a and c representing samples from JH-3 and KN-5, respectively. Meanwhile, more broken core and kenaf fibrous bark skin were found in the kenaf fiber of JH-2, KN-5, and TG-4 as observed in Fig. 3a, c, and e, respectively, due to the mechanical extraction activity conducted before the water retting process. Mechanical extraction-water retting technique is also known as ribbon retting and has been conducted widely on bast fiber, such as kenaf and jute, in Bangladesh. The requirement of water is almost half in volume, and the duration time of mechanical extraction can be reduced to 4 or 5 days (Sarkar and Sengupta 2015). Md. Tahir et al. (2011) stated that even though the mechanical extraction could produce a massive volume of fiber, the quality of the fiber tends to be lower and produced shorter fibers. Furthermore, by conducting mechanical extraction, the removal of cementing compounds in fibers, such as waxes, hemicellulose, and lignin, are not complete (Hulle et al. 2015).
Scanning electron microscope analysis was conducted to observe the fiber structure surface after the retting process. Figure 4 shows the longitudinal images of kenaf fibers surface. In general, kenaf fiber from all locations at 12 and 16 WAP showed straight, rough, and clear physical appearances with less fibrillation or pulled out effect because there was no chemical or mechanical treatment conducted.