Sisal fiber is a hard fiber extracted from the leaves of the sisal plant (Agave sisalana). Though native to tropical and sub-tropical North and South America, sisal plant is now widely grown in tropical countries of Africa, the West Indies, and the Far East. Sisal fibers are extracted from the leaves.
Sisal fiber is one of the most widely used natural fibers and is very easily cultivated. It has short renewal times and grows wild in the edges of fields. Tanzania and Brazil are the two main producing countries.
A sisal plant produces about 200-250 leaves and each leaf contains 1000-1200 fiber bundles which are composed of 4% fiber, 0.75% cuticle, 8% dry matter and 87.25% water. So normally a leaf weighing about 600 g will yield about 3% by weight of fiber with each leaf containing about 1000 fibers.
The sisal leaf contains three types of Fibres, namely (1) mechanical, (2) ribbon, and (3) xylem.
- The mechanical fibers are mostly extracted from the periphery of the leaf. They have a roughly thickened-horseshoe shape and seldom divide during the extraction processes. They are the most commercially useful of the sisal fiber.
- Ribbon fibers occur in association with the conducting tissues in the median line of the leaf. They are the longest fibers and compared with mechanical fibers they can be easily split longitudinally during processing.
- Xylem fibers have an irregular shape and occur opposite the ribbon fibers through the connection of vascular bundles They are composed of thin-walled cells and are therefore easily broken up and lost during the extraction process.
Sisal Fiber Processing
The processing methods for extracting sisal fibers include (1) retting followed by scraping and (2) mechanical means using
decorticators. the mechanical process yields about 2-4% fiber (15 kg per 8 h) with good quality having a lustrous color while the retting process yields a large number of poor quality fibers. After extraction, the fibers are washed thoroughly with plenty of clean water to remove the surplus wastes such as chlorophyll, leaf juices, and adhesive solids.
Properties of Sisal Fiber
The length of sisal fiber is between 1.0 and 1.5 m and the diameter is about 100-300 mm. The fiber is actually a bundle of hollow sub-fibers. Their cell walls are reinforced with spirally oriented cellulose in a hemicellulose and lignin matrix. So, the cell wall is a composite structure of lignocellulosic material reinforced by helical microfibrillar bands of cellulose.
The composition of the external surface of the cell wall is a layer of ligneous material and waxy substances which bond the cell to its adjacent neighbors. Hence, this surface will not form a strong bond with a polymer matrix. Also, cellulose is a hydrophilic glucan polymer consisting of a linear chain of 1, 4-b-bonded an-hydro glucose units and this large amount of hydroxyl groups will give sisal fiber hydrophilic properties. This will lead to a very poor interface between sisal fiber and the hydrophobic matrix and very poor moisture absorption resistance.
Sisal and synthetic hybrid-fiber composites
Reinforcement by two or more fibers in a single matrix leads to hybrid composites with a great diversity of material properties. It appears that the behavior of hybrid composites is simply a weighted sum of the individual components so that there is a more favorable balance of properties in the resultant composite material. Sisal and glass fibers are one good example of
hybrid composites possessing very good combined properties.
Sisal fiber is an effective reinforcement of polymer, rubber, gypsum and cement matrices. This has created a range of technological applications beyond its traditional usage as ropes, carpets, mats, etc.
The mechanical and physical properties of sisal fiber not only depend on its source, position, and age which will affect the structure and properties but also depend on the experimental conditions, such as fiber diameter, gauge length, strain rate and test temperature.
Sisal and glass fibers can be combined to produce hybrid composites which take full advantage of the best properties of the constituents. Almost all the mechanical properties show `positive’ hybrid effects.