Kenaf Fiber: Properties, Cultivation, Production, Uses and Advantages

Introduction
There is an increasing demand for natural fibers worldwide due to their renewable and biodegradable nature. The textile industry uses a variety of fibers as its raw materials. Some of these fibers were known and used in the years before civilization, as well as in the modern era, and kenaf fiber is one of them. Kenaf fiber is one of the famous natural fibers used as reinforcement in polymer matrix composites. Kenaf comes from the plant Hibiscus cannabinus, which has been used as a source of fiber for making cordage and coarse fabrics. It is used mainly as a jute substitute. The fibers in kenaf are found in the bast and core. Kenaf fiber is gaining attention as an alternative reinforcement for composite products due to low cost, reduced environmental impact, and attractive mechanical properties.

Here is a detailed discussion of kenaf fiber.

History of Kenaf
Kenaf is an annual spring crop cultivated for a long time. Kenaf was first domesticated and used in northern Africa. In Africa this fiber is sometimes known as Guinea hemp. Although it was first used in Africa but it has long been around the world. The Egyptian people cultivated kenaf 3000 years ago. They also used to eat kenaf leaves. It was also being used as animal feeding. India has produced and used kenaf for the last 200 years, while Russia started producing kenaf in 1902 and introduced the crop to China in 1935. The kenaf crop was introduced into southern Europe in early 1900s. It is mainly grown in India, China, United States of America, Mexico and Senegal.

Kenaf was unknown in the West until late in the 18th century, when cordage and sacking made from the fiber were brought to Europe. It remained one of the less important bagging materials until World War II, when shortages of jute and other bagging fibers led to a new interest that continued after the war, as supplies of established materials remained insufficient and prices increased. Production of kenaf is fairly constant throughout recent years although many producers experienced a substantial decline in 1999. India, Bangladesh, China, Myanmar, Nepal and Thailand are at present the major producers of Kenaf. India, Bangladesh and China are the large producers.

Chemical Composition of Kenaf Fiber 

Chemical Composition [ wt % ]
Cellulose 45 – 57
Lignin 21.5
Hemicellulose 8 – 13
Pectin 0.6
Wax 0.8
Moisture 6.2 – 12
Ash 2 – 5

Properties of Kenaf Fiber

1. Physical properties of kenaf fiber
Kenaf fiber is strong and also soft handle. Kenaf has a breaking strength similar to that of low –grade jute, and it is slightly weakened only when wet. Kenaf fibers have a long staple, meaning very fine and strong yarn can be spun. It can be either lightweight or heavyweight. Kenaf fiber has a very little elasticity. It is fragile in nature. This fiber is capable of being chemically degraded. In addition to being the most absorbent natural fiber on the planet, kenaf is also hydrophobic (it does not absorb water). Kenaf fibre looks aesthetically pleasing and it looks like Linen. This fiber can be lustrous. Being environmentally friendly is another important benefit of kenaf fiber.

untreated kenaf fiber
Fig: Untreated kenaf fiber

2. Mechanical properties of kenaf fiber

Mechanical properties Characteristics
Density (g/cm3) 1.2
Breaking Strength (MPa) 100.64
Elastic Modulus (GPa) 23
Yarn Breaking load (N) 79
Tensile Strength (MPa) 283-800
Elongation (%) 17.3
Moisture Absorption (%) 8.3

Cultivation of Kenaf Fibre
Kenaf is a soft bast fiber from the kenaf plant. The fiber is long, light yellow to gray, and harder and more lustrous than jute. Kenaf could be blended with cotton and made into yarn & woven into fabrics. Kenaf is a warm-season annual row crop in the same plant family as okra and cotton (Malvaceae). Kenaf plants are capable of growing to a height of 20 feet under favorable conditions, may or may not develop branches, it’s depending on the cultivar, seed spacing and growing conditions. However, heights generally average 8 to 14 feet in a growing season of 4 to 5 months and produce 6 to 10 tons of dry matter per acre within five to eight months after planting.

kenaf plant
Fig: Kenaf plant

The stalks consist of two kinds of fiber: an outer fiber (bast) and an inner fiber (core). The bast is comparable to softwood tree fibers, while the core is comparable to hardwood fibers. After harvest, the plant is processed to separate these fibers for various products. Kenaf is well adapted to a wide range of soil types; however, best yields occur on well-drained fertile sites. While abundant soil moisture is necessary, waterlogged or poorly drained soils are a detriment to growth. Kenaf’s sensitivity to frost is an important consideration when selecting a suitable site. Seeds are broadcast or drilled after all danger of frost has passed. It is important to plant into a fine, well-prepared seedbed to provide good seed-to-soil contact for germination. This crop is grown in dense stands to limit branching and promote the development of long fibers in the main stem.

Kenaf is the most sustainable fiber, due to its growth rate, and ability to replenish. It does not require much water for its cultivation and processing. Almost no fertilizer or pesticides used in the cultivation processes. 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 colour 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. Increased kenaf commercialization does not seem to be limited by either agronomic production or the availability of suitable harvesting and processing systems, but understanding the harvesting and production systems in relationship to kenaf production and products will enhance the management of this crop as it continues to compete in the marketplace.

Production Process of Kenaf Fiber

Harvesting:
The harvest method depends on the production location, the equipment availability, processing method, and final product use. Kenaf crop can be harvested with both hand and machine harvesting. Plant are harvested for their stalk from which the fiber is extracted. Mainly, there are two methods of harvesting fibers, which are hand harvesting, and using the whole stalk harvesters. In hand harvesting, the process starts from harvesting manually by hand. The plant will be cut near the ground level with a sickle or a curved blade. This harvesting process has been used since 6000 years ago, and therefore it is the most traditional way of harvesting. This method requires human labour and time-consuming. The other method of harvesting and extraction is by using Kenaf Whole Stalk Harvesters. In this method, harvesting could be summarized into three steps which starts with pulling and windrowing, followed by retting, and finally balling parallel stalks.

Currently, there are two type of harvesters available in the market which are the Sugarcane-type harvesters and forage-type harvesters. Once the kenaf stalks are harvested and the outer fiber is separated it goes through a refinement or combing process making the fiber finer and finer. It is an economic advantage to use presently available commercial harvesting equipment, if possible rather than investing in the development and production of kenaf specific equipment.

Retting:
After harvesting, it will undergoes retting process to extract the fibers. Currently, there are four types of retting process that has been used which are biological retting (dew retting, water retting, enzyme retting), mechanical retting (using decorticator), chemical retting, and physical retting.

1. Biological Retting: In biological retting, the dew retting were carried out by spreading out the stalks on the field and expose them to the rain, sun and dew for several weeks until the stalks begin to separate from the fiber naturally. Due to the long exposure to the natural conditions, fibers tends to lose its original colour and turns brown. Furthermore, this method reduces the fibers strength and quality. Water retting involves tying several bundles of stalks and immersed them in streams or water of at least 60 cm in depth. This process usually took around 2 weeks until the bast fibers naturally separates from the core fibers. These bundles were then taken out of the water and left to dry. Finally, enzyme retting the safest and fastest process of retting, where the core fibers can be taken out from the bast fibers after several hours. Some examples of enzymes that has been used are pectinase, and xylanase.

2. Mechanical Retting: The second type of retting process is mechanical retting which uses Kenaf decorticators. A decorticator is basically a machine which could separate the bast and core fibers for further processing. It could ensure that the fiber will not be damage while being extracted. Some of the component of this machines are the frame (to hold all the components), knife (to cut the stalks), knife plate (to hold the knife in place), feeding mouth and security cover (to place the stalks into the machine with safety), and much more. The decorticator machine works by feeding the stalks into feeding mouth, then the stalks will be sent to the beaters to be crushed. The crushed stalk that contain both bast fiber and core fiber will be pushed into the delivery plate. Finally, the bast fiber will be separated from the crushed core fiber manually.

3. Chemical Retting: The third process which is chemical retting is a process where the stalks were treated with acid and alkalis to remove the core fibers easily from the bast fibers. In these method, the stalks are boiled with chemicals such as sodium hydroxide, sodium benzoate, and hydrogen peroxide, at a specific temperature for several hours, and then washed with clean water. The fibers obtained from this method usually felt coarser and rough compared to other methods.

4. Physical Retting: The last process for fiber extraction is Physical retting. This process involves two steps which starts with modifying the surface/volume ratio of fibers through chemical pre-treatment. Then, the stalks will be subjected to steam explosion where the fibers will be blown apart and separated. In this method, the water will be vaporized completely and increased in volume and it requires small amount of processing time. However, this method could only produce short fibers.

Drying:
After the retting process, the bast fibers are taken out from core fibers and wash them with clean water for drying them. Different methods could be applied for the drying of kenaf to a moisture content which allow storage of the harvested biomass in a stable manner. The different methods which are described are: drying in the field, drying in the storage, and drying in industrial installations. When weather condition has been bad, artificial drying can also be required in order to reach the low moisture content to avoid mold growth and losses during long storage by natural or forced ventilation.

Application / Uses of Kenaf Fiber
Kenaf is a traditional third world crop that is poised to be introduced as a new annually renewable source of industrial fiber in the so-called developed economies. Kenaf Fiber is a very important raw material in the textile industries. Kenaf fiber can be blended very well with cotton and other fiber. It has commercial and healthy applications in making outerwear owing to its natural absorbency, and fire-retardant abilities. Uses of kenaf fiber include engineered wood; insulation; clothing-grade cloth; soil-less potting mixes; animal bedding; packing material; and material that absorbs oil and liquids. But the main uses of kenaf fiber have been rope, twine, coarse cloth (similar to that made from jute), and paper. Kenaf is a source of cellulose fiber for the production of pulp and paper. It is a non-woody annual with a short life circle of 100-130 days. It produces pulp much faster than pine which takes about 15years to mature. It is a natural blend of short and long fibers similar to the hardwood-softwood blends used in most papers. It is therefore the most viable replacement for trees in paper production. It makes ideal paper fiber as well as great source material for burlap, clothing, canvas, particle board and rope.

kenaf fiber composite board
Fig: Kenaf fiber composite board

Some end uses of kenaf fiber are Twine, Sackcloth, Sacking, Cordage, Hessian, Upholstery, and so on.

Advantages of Kenaf Fiber
Kenaf fibers are used for making textiles, for more than thousands of years. Kenaf can be grown in many places all over the world, including throughout the United States, making it a prime candidate for use by local fiber and textile producers. The kenaf plant grows extremely quickly. For us, this means there is far less danger of over-harvesting or driving it to extinction in order to keep up with production demands. When planted in locations that don’t freeze over, it can even be grown all year round. Kenaf has an ability to absorb huge quantities of CO2, the global warming gas, comparatively 3 times more than a tree. It can convert more CO2 than a tropical rain forest during its growth. Further it can also improve the soil structure, while fixing soil nutrients. It is biodegradable, as no chemicals or pesticides are used during its cultivation.

Kenaf fibers find useful applications in making knit and woven textiles. Kenaf textiles are also naturally very absorbent, and even fire-retardant, making it especially ideal for outerwear or shoes. Fibers spun from kenaf are extremely long, making it a good choice for weaving fabrics. The plants stalk contains less than 30% lignin, making its fiber extraction easier, when compared with other natural fibers. Lignin is glue like substance which fills spaces in the plant fibers. Kenaf fibers have a long staple, meaning very fine and strong yarn can be spun. This makes for quality textiles. Kenaf fibers are a good alternative to synthetic fibers. They offer sustainable advantages while also enhancing cost saving amidst increasing fuel prices. Kenaf fibers have good fire retardant abilities. They also have antimicrobial properties, and posses good resistance to mildew and rot. So, the fiber sees positive applications in making active or sports wear with odor control properties.

Conclusion
Kenaf is a fibrous crop adapted to a temperate and tropical environment, not harmful to the environment, biodegradable and Inexpensive. Kenaf can be considered as a more environmentally acceptable crop than other annual energy crops, since it requires fewer inputs (fertilizers, pesticides). The global market of Kenaf is expanding rapidly over the past several years. Currently, industries are manufacturing more products by using Kenaf composites. In the upcoming future, a great achievement can be made in the improvement of kenaf varieties with the development of science and technology.

References

  1. Kenaf Fibers and Composites Edited by S.M. Sapuan, J. Sahari, M.R. Ishak and M.L. Sanyang
  2. Biodegradable and Sustainable Fibres Edited by R. S. Blackburn
  3. Kenaf: A Multi-Purpose Crop for Several Industrial Applications edited by Andrea Monti and Efthimia Alexopoulou
  4. https://en.wikipedia.org/wiki/Kenaf
  5. https://www.textileblog.com/properties-processing-and-uses-of-kenaf-fiber/
  6. https://www.britannica.com/plant/kenaf/
  7. http://kenafplant.blogspot.com/2010/09/advantages-of-kenaf.html

Author of this Article:
Shamima Akter Riya
Textile Engineering College, Noakhali

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