Plant fibers are by far the main cellulose source for industry nowadays. They are the product of aggregation of cellulose chains into larger structures and found on plant cell walls (Figure 1-5).
1.3.2.1. Wood fibers:
Wood fibers are the most extensively used nowadays by industry for pulp manufacturing. They are obtained from the trees trunks after debarking. From the paper industry perspective, wood fibers are classified into two large groups (García-Hortal, 2007):
• Softwood fibers: They are long and resistant and obtained from conifers. These kinds of trees have a large proportion of fibers in wood, as other elements are basically inexistent. Examples of softwoods would be trees such as pine, spruce, or firs.
• Hardwood fibers: Obtained from homonym plants, which in evolutive terms are newer organisms than conifers. Fibers are shorter than softwoods and are containers of other elements besides fibers which play important roles on plant vital functions, such as vessels. Examples of hardwoods would be trees such as beech, eucalyptus or birch.
1.3.2.2. Non-wood fibers:
Under the non-wood fiber denomination a heterogeneous range of plants with widely differing characteristics is included. The main sources of non-woody raw materials are agricultural residues from monocotyledons, including cereal straw and bagasse, or plants grown specifically for the fiber, such as bamboo, reeds, and some other high-fiber content plants such as flax, hemp, kenaf, jute, sisal, or abaca (Marques et al., 2010). Cellulose fibers obtained from non-wood plants (herbaceous or shrubby) account only for the 9% of the total amount of fibers used worldwide for papermaking (Leponiemi, 2008). However, in some countries with insufficient forest supplies and
non-wood plants abundance such as India and China, these fibers represent the main source for this application (López et al., 2004). Also, other causes such as a growing environmental pressure, restrictions on forest use, the increasing world demand for paper and increases in wood and recycled fibers cost are making manufacturers to have a renewed interest in non-woods (Kissinger et al., 2007).
Interest in non-woods is due to a range of advantages presented by these fibers.
Firstly, their possibility of growing in annual cycles in which they develop full fiber potential (i.e. renewability) compared to the typical long periods of woody plants.
Secondly, they present comparatively smaller amounts of lignin, allowing the use of milder chemical conditions for pulping or bleaching than wood sources (Paavilainen, 1998; Madakadze et al., 2010). Lastly, non-food uses of crops could provide farmers additional incomes derived from food harvests or cattle raising (Kissinger et al., 2007).
However, the characteristic high silica content of non-wood fibers appears as a drawback conditioning their applicability (Andrade and Colodette, 2014). Other disadvantages of these fibers include their seasonal availability (rather than all year) and inconvenient handling due to their high volume and low density. In the developed world, non-wood fibers are typically used for the manufacture of specialty papers, which could not be easily obtained from wood fibers.
Among non-wood fibers, sisal and cotton linters attracted a special interest for the studies performed in this thesis, as will be discussed in following chapters.
• Sisal provides fibers that are isolated from the leaves of Agave sisalana andhave traditionally been of the most used natural fibers for applications such as sacking, cordage, carpets backing or twines. Its annual worldwide production was around 241,000 tons in 2011 (FAO, 2012). Regarding its potential as a raw material for pulp and paper industry, sisal pulp presents some positive features including a high tear resistance, alpha cellulose content, porosity, bulk, absorbency and folding endurance, making it excellent for a variety of specialty papers (Aracri and Vidal, 2012).
• Cotton linters are an important by-product of the textile industry, being the short fiber that cannot be used in the textile process (Morais et al., 2013). They are obtained from cotton plant (Gossypium sp.), an annual shrub harvested for their high industrial interest. When the regular cotton fibers are extracted in the ginning process, the linters remain attached to the seed coat and an
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additional process is required for their removal. The amount of linters produced worldwide is around 2.5 million metric tons, considering the 42 million metric tons of cotton produced in 2010 (Morais et al., 2013). Cotton linters consist on high-quality cellulose fibers presenting very high-cellulose content (98-99%) (Sczostak, 2009). They are typically used on special applications such as the production of cellulose derivatives, cellulose regeneration, or the manufacture of high-added value papers (Sczostak, 2009).
1.3.2.3. Other components present in cellulose fibers
a) Hemicelluloses
Hemicelluloses represent the second polysaccharide in plant biomass, in mass terms, after cellulose. Together with lignin they form an amorphous matrix in which cellulose microfibrils are immersed. Hemicelluloses differentiate from celluloses in that they are formed by 5-6 carbon unit monosaccharaides, are branched, are easier to dissolve and their structure is amorphous. Hemicelluloses are generally hydrophilic and play an important role in fibers water uptake during paper fabrication. Therefore, they participate in fiber interaction increasing their flexibility and binding capacity.
Also, their degree of polymerization is low compared to that of cellulose, with values between 50 and 300 monosaccharide residues. Their chemical composition varies depending of the analyzed plant type. Among the different fiber sources, softwoods contain a higher value of hexosans (glucomanans, galactoglucomannans and arabinoglucoronoxylans) while hardwoods hemicelluloses are mainly composed by pentosans (glucoronoxylans), but glucomannans could be also present (Sjöström and Westermark, 1999). Regarding non-wood plants, composition of their hemicelluloses varies widely depending on the analyzed species. Plants such as kenaf or sisal have hemicelluloses mainly composed by xylose, while others such as flax or hemp have mannose and galactose as main sugars on their hemicelluloses (Marques et al., 2010).
a.1) Hexenuronic acids (HexA)
HexA are formed during alkaline cooking by a β-elimination of methylglucoronic acid, which is randomly distributed among side-chains of xylans, with an alkaline pH and at temperatures between 110 and 150ºC. HexA presence in pulps is deleterious due to some negative effects it has on further processes and final quality. They contribute to kappa number, as they are also oxidized by potassium permanganate during its determination. This fact difficulties the normal estimation of lignin content of pulps (Costa and Colodette, 2007). Also, they consume reactants during pulp bleaching, increasing process cost. Furthermore, a chelating action on metallic ions was observed, increasing pulp metal content. And above all, they reduce the material durability, as they play an important role in brightness reversion on bleached pulps (Cadena et al., 2010a).
b) Lignin
Lignin is produced by plant cells as they go through a maturing process and it is placed between fibrous walls and mainly, in intercellular regions (middle lamella), producing as a result an stiff and cohesive structure. Chemically, is an aromatic heteropolymer with an irregular 3D structure formed by different combinations of three phenylpropane units: syringyl, guaiacyl, and p-hydroxyphenyl. Lignin presents very high hydrophobicity level and because of this it hinders water uptake and fiber swelling when present. On the other hand, it presents some thermoplastic behavior, very important for mechanical pulping. Importantly, lignin is a chromophore being the main target of bleaching processes through its removal or modification.