Sisal fibre: an overview

                                                                                         

Sisal fibres are made from sisal plant leaves. The word sisal means cold water. Sisal fibre occupies 6th place among fibre plants, which represent 2% of the world’s production of plant fibres (plant fibres provide 65% of the world’s fibres).

Sisal Plant

The sisal plant produces approximately 200 - 250 leaves throughout its productive period. The life span of sisal plant is 7-10 years. The shape of sisal leaves is like sword and is about 1.5 to 2 meters tall. Young leaves may have a few minute teeth along their margins, but lose them as they mature.

A good sisal plant yields about 200 commercial used leaves with each leaf having a mass composition of 4% fibre, 0.75% cuticle, 8% other dry matter and 87.25% moisture. Thus a normal sisal leaf weighing about 600g yields about 3% by weight of fibre with each leaf containing about 1000 fibres. The fibre is extracted from the leaf either by retting, by scraping or by retting followed by scraping or by mechanical means using decorticators. Diameter of the fibre varies from 100mm to 300mm (Mukherjee & Satyanarayana, 1984).

Manufacturing Process of Sisal Fibre:

Sisal fibre made from the process of Decortication, leaves are crushed and beaten by a rotating wheel set with blunt knives, so that only fibres remain. The other parts of the leaf are washed away by water. Decorticated fibres are washed before drying the sun or by hot air. The fibre quality depends upon moisture content so proper drying is important. To get better grades of fibre artificial drying has been found in place of sun drying. Dry fibres are machine combed and sorted into various grades, largely on the basis of the previous in-field separation of leaves into size groups.

A sisal fibre in cross-section is built up of about 100 fibre cells. Figure 1 shows a schematic sketch of a fibre cell. The cell walls consist of several layers of fibrillar. In the primary wall, the fibrillae have a reticulated structure. In the outer secondary wall (S1), which is located inside the primary wall, the fibrillae are arranged in spirals with a spiral angle of 40° (for sisal fibre) in relation to the longitudinal axis of the cell. The fibrillae in the inner secondary wall (S2) of sisal fibres have a sharper slope, 18° to 25°. The thin, innermost, tertiary wall has a parallel fibrillar structure and encloses the lumen. The fibrillae are, in turn, built up of micro-fibrillae with a thickness of about 20µm. The microfibrillae are composed of cellulose molecular chains with a thickness of 0.7µm and a length of a few µm.

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