Sisal - Natural Cellulose Leaf Fibres from Plants or Vegetables

Sisal fibre is a hard fibre 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 fibres are extracted from the leaves.

Sisal fibre is one of the most widely used natural fibres and is very easily cultivated. It has short renewal times and grows wild in the hedges 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 fibre bundles which is composed of 4% fibre, 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 fibre with each leaf containing about 1000 fibres.

The sisal leaf contains three types of fibres: (1) mechanical, (2) ribbon and (3) xylem.

1. The mechanical fibres 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 fibre.
2. Ribbon fibres occur in association with the conducting tissues in the median line of the leaf. They are the longest fibres and compared with mechanical fibres they can be easily split longitudinally during processing.
3. Xylem fibres have an irregular shape and occur opposite the ribbon fibres 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.

The processing methods for extracting sisal fibres include (1) retting followed by scraping and (2) mechanical means using decorticators. mechanical process yields about 2-4% fibre (15 kg per 8 h) with good quality having a lustrous colour while the retting process yields a large quantity of poor quality fibres. After extraction, the fibres are washed thoroughly in plenty of clean water to remove the surplus wastes such as chlorophyll, leaf juices and adhesive solids.

The length of sisal fibre is between 1.0 and 1.5 m and the diameter is about 100-300 mm. The fibre is actually a bundle of hollow sub-fibres. Their cell walls are reinforced with spirally oriented cellulose in a hemi-cellulose 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 lignaceous material and waxy substances which bond the cell to its adjacent neighbours. 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 anhydroglucose units and this large amount of hydroxyl groups will give sisal fibre hydrophilic properties. This will lead to a very poor interface between sisal fibre and the hydrophobic matrix and very poor moisture absorption resistance.

Reinforcement by two or more fibres in a single matrix leads to hybrid composites with a great diversity of material properties. It appears that the behaviour of hybrid composites is simply a weighted sum of the individual components so that there is a more favourable balance of properties in the resultant composite material. Sisal and glass fibres are one good example of hybrid composites possessing very good combined properties.

Sisal fibre 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 fibre 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 fibre diameter, gauge length, strain rate and test temperature.

Sisal and glass fibres 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.