Fibre morphology in non-wood plants used in papermaking

Morphological characteristics, such as fibre length and width, are important in estimating pulp quality of fibres (Wood 1981). In fibres suitable for paper production, the ratio of fibre length to width is about 100:1, whereas in tex-tile fibres the ratio is more than 1000:1. In co-niferous trees this ratio is 60–100:1, and in de-ciduous trees 2–60:1 (Hurter 1988, Hunsigi 1989, McDougall et al. 1993). Fibre length and width of non-woody species vary depending on plant species and the plant part from which the fibre is derived (Ilvessalo-Pfäffli 1995). The average fibre length ranges from 1 mm to 30 mm, being shortest in grasses and longest in cotton.

The average ratios of fibre length to diameter range from 50:1 to 1500:1 in non-wood species (Table 2) (Hurter 1988). Lumen size and cell wall thickness affect the rigidity and strength of the papers made from the fibres. Fibres with a large

lumen and thin walls tend to flatten to ribbons during pulping and papermaking, giving good contact between the fibres and consequently hav-ing good strength characteristics (Wood 1981).

Softwood fibres from coniferous trees are ideal for papermaking since their long, flexible struc-ture allows the fibres to pack and reinforce the sheets. Hardwoods from deciduous trees have

shorter, thinner and flexible fibres that pack tightly together and thus produce smooth and dense paper (Hurter 1988, Fengel and Wegener 1989, McDougall et al. 1993).

Non-wood plant fibres can be divided into several groups depending on the location of the fibres in the plant. Ilvessalo-Pfäffli (1995) has described four fibre types: grass fibres, bast fi-bres, leaf fibres and fruit fibres. Grass fibres are also termed stalk or culm fibres (Hurter 1988, Judt 1993) (Table 2).

Grass fibres

Grass fibres currently used for papermaking are obtained mainly from cereal straw, sugarcane, reeds and bamboo (Atchison 1988). The fibre material of these species originates from the xylem in the vascular bundles of stems and leaves. It also occurs in separate fibre strands, which are situated on the outer sides of the vas-cular bundles or form strands or layers that ap-pear to be independent of the vascular tissues (Esau 1960, McDougall et al. 1993, Ilvessalo-Pfäffli 1995). Vascular bundles can be distribut-ed in two rings as in cereal straw and in most temperate grasses, with a continuous cylinder of sclerenchyma close to the periphery. The bun-dles can also be scattered throughout the stem section as in corn (Zea mays L.), bamboo and sugarcane (Esau 1960). The average length of grass fibres is 1–3 mm (Robson and Hague 1993, Ilvessalo-Pfäffli 1995) and the ratio of fibre length to width varies from 75:1 to 230:1 (Table 2) (Hurter 1988).

Wheat (Triticum aestivum L.) is the mono-cotyledon that is used most in commercial pulp-ing. However, fibres from rye (Secale cereale L.), barley (Hordeum vulgare L.) and oat (Avena sati-va L.) are similar to those of wheat (Ilvessalo-Pfäffli 1995) and they could also be used in pa-permaking. Rice straw (Oryza sativa L.) is used in Asia and Egypt. Bagasse is one of the most important agricultural residues used for pulp manufacture. Bagasse pulp is used for all grades of papers (Atchison 1987b). Some reeds (Phrag-mites communis Trin., Arundo donax L.) are collected and used in mixtures with other fibres Fig. 4. Schematic representation of a) the location of fibres

in stem and leaves of monocotyledonous plants (McDou-gal et al. 1993), reprinted with kind permission of John Wi-ley & Sons Ltd and b) primary and secondary cell walls (Taiz and Zeiger 1991).

in Asia and in South America as raw material for writing and printing papers. In the case of esparto (Stipa tenecissima L.), only leaves are used, whereas bamboo pulp is commonly made from the pruned stem and bagasse pulp from sugarcane waste. When grass species are pulped for papermaking, the entire plant is usually used and the pulp contains all the cellular elements of the plant (Ilvessalo-Pfäffli 1995). The propor-tion of fibre cells in commercial grass pulp can be 65 to 70% by weight (Gascoigne 1988, Ilves-salo-Pfäffli 1995). In addition to fibre cells, the grass pulp also contains small particles (fines) from different vessel elements, tracheids, paren-chyma cells, sclereids and epidermis, which make the grass pulp more heterogeneous than wood pulp, in which all the fibres originate from the stem xylem. Most of the fines lower the drainage of the pulp and thus the drainage time in papermaking is longer (Wisur et al. 1993).

However, the amount of fines decreases if the leaf fraction, the main source of the fines, can be restricted to only the straw component of the grass.

Bast fibres

Bast fibres refer to all fibres obtained from the phloem of the vascular tissues of dicotyledons (TAPPI Standard T 259 sp-98 1998). Fibre cells occur in strands termed fibres (Esau 1960, Il-vessalo-Pfäffli 1995). Hemp, kenaf, ramie (Boechmeria nivea L.) and jute (Corchorus cap-sularis L.) fibres are derived from the second-ary phloem located in the outer part of the cam-bium. In flax, fibres are mainly cortical fibres in the inner bark, on the outer periphery of the vas-cular cylinder of the stem (Esau 1960, McDou-gall et al. 1993, Ilvessalo-Pfäffli 1995). In these plants the length of the fibre cells varies from 2 mm (jute) to 120 mm (ramie) (Esau 1960, Ilves-salo-Pfäffli 1995). Flax fibres consist of up to 40 fibres in bundles of 1 m length. Hemp fibres are coarser than those of flax, with up to 40 fi-bres in bundles that can be 2 m in length (Mc-Dougall et al. 1993). Bast fibres must be isolat-ed from the stem by retting whereby micro-or-ganisms release enzymes that digest the pectic

material surrounding the fibre bundles, thus free-ing the fibres. With ramie, boilfree-ing in alkali is required (McDougall et al. 1993). Bast fibres are used as raw material for paper when strength, permanence and other special properties are needed. Examples include lightweight printing and writing papers, currency and cigarette pa-pers (Atchison 1987b, Kilpinen 1991, Ilvessalo-Pfäffli 1995).

Leaf fibres

Leaf fibres are obtained from leaves and leaf sheaths of several monocotyledons, tropical and subtropical species (McDougall et al. 1993, Il-vessalo-Pfäffli 1995). Strong Manila hemp, or acaba, is derived from leaf sheaths of Musa tex-tilis L., and is mainly used in cordage and for making strong but pliable papers. Sisal is pro-duced from vascular bundles of several species in the genus Agave, notably A. sisalana Perrine (true sisal) and A. foucroydes Lemaire (hene-quen) (McDougall et al. 1993). Leaves of espar-to grass produce a fibre used espar-to make soft writ-ing papers (McDougall et al. 1993).

Fruit fibres

Fruit fibres are obtained from unicellular seed or fruit hairs. The most important is cotton fi-bre, formed by the elongation of individual epi-dermal hair cells in seeds of various Gossypium species (McDougall et al. 1993). The longest fi-bres of cotton (lint) are used as raw material for the textile industry, but the shorter ones (linters, 2–7 mm long), as well as textile cuttings and rags, are used as raw material for the best writ-ing and drawwrit-ing papers (Ilvessalo-Pfäffli 1995).

Kapok is a fibre produced from fruit and seed hairs of two members of the family Bombaceae:

Eriodendron anfractuosum DC. (formerly Ceiba pentandra Gaertn.) produces Java kapok and Bombax malabaricum DC. produces Indian ka-pok. Kapok fibres originate from the inner wall of the seed capsule. The cells are relatively long, up to 30 mm, with thin and highly lignified walls and a wide lumen (McDougall et al. 1993).