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.

Features and Property of Sisal Fibre:

Sisal fibers are smooth, straight and yellow in colour.

Sisal is fairly coarse and inflexible so the sisal fibre can be long or short.

Sisal is valued for cordage use because of its strength, durability, ability to stretch, affinity for certain dyestuffs, and resistance to deterioration in saltwater

In natural fibres, the flaws or weak links are irregularly spaced in the fibre, the strength will depend on the length of the fibre used for the tensile test (McLaughlin, 1980)

In the case of sisal fibre tensile strength and percentage elongation decrease with increase in test length and Young’s Modulus and Average Modulus increase with test length. When speed of testing increases the Young’s Modulus and tensile strength increases.(See Table 1 and 2)

The comparison of sisal fibre polyester composites with other natural fibre is shown in Table 3 by using Charpy test with pendulum impact-testing machine using a pendulum load of 0.4 kgs. It can be seen from table 3 that sisal fibre composites have the maximum work of fracture followed by pineapple fibre composite. Banana and coir fibre composite have comparatively low work of fracture. It is a generally accepted fact that the toughness of a fibre reinforced composite is mainly depending on the fibre stress-strain behavior. Strong fibres with high failure strain impart high work of fracture on the composites. From the table it is interesting to note that, among sisal, pineapple and banana fibre reinforced polymer composites, sisal fibre-polyester composites is likely to give high work of fracture because of the high toughness of sisal fibre which is found in agreement with the experimental results. However, the large difference observed between banana and pineapple fibres is not explained by taking into account of their comparative mechanical properties. Similarly, very low toughness cannot be expected for coir composites because of the high toughness of the fibre.

Figure 2 shows the influence of microfibrillar angle of the fibre on the work of fracture values of different natural fibre reinforced polymer composites. It can seen that microfibrillar angle in plant fibres plays an important role in determining the impact behavior of these composites and this effect should be taken into account along with the other parameters while predicting the impact properties of natural fibre composites

Sisal fibre-low density polyethylene (LDPE) composites showed a better reinforcing effect due to the high matrix ductility and high strength/modulus ratio of sisal fibres as compared to that of LDPE matrix.

The CTDIC (cardanol derivative of toluene diisocyanate) treatment reduced the hydrophilic nature of the sisal fibre and thereby enhanced the tensile properties of the sisal-LDPE composites. They found that peroxide treated composites showed an enhancement in tensile properties due to the peroxide induced grafting. It was also found that permanganate treated composites also showed a similar trend.

The effects of various chemical treatments on the tensile properties of sisal-polyethylene composites are presented in Table 4. Treatments using chemicals such as sodium hydroxide, isocyanate, permanganate and peroxide were carried out to improve the bonding at the fibre-polymer interface.

Table 5 shows the tensile properties of solution mixed sisal fibre reinforced polypropylene (PP), low density polyethylene and polystyrene composites (Joseph et al., 1999). It is clear from the table that in the case both PP-sisal and LDPE-sisal composites, the tensile strength and modulus go on increasing as the percentage of fibre content increases from 0 to 30%, whereas, the values change in an irregular manner in the case of sisal – polystyrene composites. Since PP is more crystalline than LDPE, the increase in tensile strength by the addition of sisal fibre is less in the case of PP compared to LDPE. But the strength of the composite formed by the addition of fibre is more in the case of PP compared to LDPE. In the case of polystyrene at 10% fibre loading, the tensile strength is decreased by 40% but in the case of PP, it is increased by 3%. However, at high fibre loading, the tensile strength values are comparable for both PP and polystyrene. Thus, PP is found to be a good matrix for sisal polyolefin composites.

Application of Sisal Fibre:

There are three grades of sisal fibre. The lower grade fibre has high content of cellulose and hemicelluloses so mainly used in paper industry. The medium grade fiber is used in the cordage industry for making: ropes, baler and binders twine. Ropes and twines are widely employed for marine, agricultural, and general industrial use. The higher-grade fiber after treatment is converted into yarns and used by the carpet industry

Products made from sisal are being developed rapidly, such as furniture and wall tiles made of resonated sisal.

The sisal reinforced composites are used in the internal linings of vehicles, the sides of car doors, package holders, panels, ceilings, wheel wells, consoles, skid plates etc. to reduce weight of vehicle to reduce fuel consumption.

Recent year’s sisal has been utilized as a strengthening agent to replace asbestos and fiberglass as well as an environmentally friendly component in the automobile industry

Sisal Fibre Can Be Used As A Replacement For Silk Fibre:
The process of turning sisal fibres into silky fabric involves a high degree of beating and pulping. The result is a fabric that is light enough to be worn in the hottest weather. It is able to be woven into nearly invisible sheers and is used as a replacement for silk. Because of the amount of work to process the sisal into this sheer fabric its value is very high.

The use of sisal fibre as reinforcing agent in polymer based composites were reviewed from viewpoints of status and future expectations of natural fibres in general, structure and properties of sisal fibre, fibre surface modifications, and physical and mechanical properties of sisal fibre based polymer composites.

Sisal fibres have good potential as reinforcements in polymer (thermoplastics, thermosets and rubbers) composites. Due to the low density and high specific properties of sisal fibres, composites based on these fibres may have very good implications in the automotive and transportation industry.

Sisal fibre reduced equipment abrasion and subsequent reduction of re-tooling costs will make these composites more attractive.

The fibres are also used as a source of raw material in plastic industry.

Sisal fibre used as packaging, tying and gardening.

The waste produced by decorticate like sisal juice, particles of crushed parenchymatose tissue and fragments of leaves and fibres sizes used as fertilizer or animal feed.

Sisal fibre is used in geotextiles as sandwiching the natural component between layers of polypropylene or polyester.

In construction industry sisal fibres are used as reinforcement. In matrices of cement, plaster and concrete the main contribution of sisal fibres is to hold the cracked areas of the matrix together. At that point, the load is transferred to the fibre, which begins to control the behaviour of the composite according to its characteristics, such as elasticity, length, direction, volume etc.

Other products developed from sisal fiber include spa products, cat scratching posts, lumbar support belts, rugs, slippers, cloths, disc buffers, hand bags, jewel boxes, hot pads, pet toys etc.

References:

1.www.fao.org
2.www.sciencedirect.com