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Carbohydrate Polymer Technologies and Applications 2 (2021) 100050
enhanced the dry tear strength of paper by 14% in bleached wood pulp ( Abdel-Halim, 2014 ; Bülichen, Kainz & Plank, 2012 ; Miura, Nishizawa, Nishimura & Sekigawa, 1972 ; Tang et al., 2013 ).
tensile index of paper made from Eucalyptus bleached pulp. The ten- sile index of the handmade paper from same pulp has been enhanced by ∼ 11% with the action of NCC. The application of NCC ranges from pulp and paper industries to pharmaceuticals and from food to cosmet- ics ( Bai, Holbery & Li, 2009 ; De Jesus Silva, De Almeida, De Oliveira, Da Silva & De Mendonça Neto, 2013 ; Ditzel et al., 2017 ; Liu, Chen, Yue, Chen & Wu, 2011 ; Peng, Dhar, Liu & Tam, 2011 ).
2.4. Nanocellulose
The latest and the most acclaimed area of applied cellulosic sci- ence is the production and application of nanocellulose ( Hubbe, 2019 ; Raghav, Sharma & Kennedy, 2021 ). There are primarily two types of nanocellulose:
2.5. Other cellulose derivatives
Methyl cellulose is another important water-soluble cellulose deriva- tive produced by alkaline activation of cellulose under heat treatment followed by a reaction with methyl sulphate or methyl chloride ( Fig. 1 ). It works as adhesives, thickeners, binders, emulsifiers and stabilizers. Methyl cellulose is widely used as sizing agents in paper industries ( Karrasch, Jäger, Saake, Potthast & Rosenau, 2009 ; Lavanya, Kulkarni, Dixit, Raavi & Krishna, 2011 ). Another derivative, hydroxypropyl cellulose , is produced commer- cially by alkali activation followed by reaction with propylene oxide at high temperature (120–160 °C) ( Miura et al., 1972 ) ( Fig. 1 ). Hydrox- ypropyl cellulose and hydroxypropyl methyl cellulose (HPMC) are used as surface coating and barrier additives. HPMC as coating additive con- trols water absorption capacity, flexibility and mechanical properties of paper ( Khwaldia, 2013 ; Klass, 2011 ; Sothornvit, 2009 ).
(a) Nano-Fibrillated Cellulose (NFC) (b) Nano-Crystalline Cellulose (NCC)
NFC or microfibrillated cellulose differs from NCC or cellulose nanocrystals in the process of manufacturing; the former is produced from mechanical shearing whereas the latter is manufactured purely by chemical hydrolysis ( Arbatan et al., 2012 ; Aulin, Lindström & Strom, 2013 ; Lavoine, Desloges, Dufresne & Bras, 2012 ; Martins et al., 2012 ; Mertaniemi, Laukkanen, Teirfolk, Ikkala & Ras, 2012 ; Pajari, Rautkoski & Moilanen, 2012 ; Syverud & Stenius, 2009 ). The disintegration process for obtaining NFC from cellulosic raw material differs with nature of raw material. Enzymatic and chemi- cal pre-treatment methods of cellulosic mass or delignified pulp prior to mechanical homogenization have also been reported. This process predominantly yields homogeneous fibrils of width ranging between 3 and 20 nm. Since, the composition of fibres is conserved; the fibrils have anionic surface charge. This negative charge is conferred by car- boxyl groups of hemicellulosic origin ( Ahola, Österberg & Laine, 2008 ). This surface to charge ratio is the most important property of NFC that imparts mechanical strength (both wet and dry) to paper sheets ( Ahola et al., 2008 ; Nakagaito, Iwamoto & Yano, 2005 ; Pääkkö et al., 2008 ; Saito, Nishiyama, Putaux, Vignon & Isogai, 2006 ). The adsorption and activity of cationic poly-electrolytes (in very low concentrations) have improved in a bi-layer system by the addition of nano-fibrillated cellulose due to increment of bonding sites provided by NFC. The wet and dry tensile strength has increased upto ∼ 40% by the addition of NFC. The same also indicates that NFC has the potency to replace var- ious chemical strength additives or minimize their concentration used in paper manufacturing ( Ahola et al., 2008 ). Sehaqui et al. (2013) have experimentally established that NFC increases the tensile index and ten- sile energy absorption (TEA) in unbeaten softwood pulp and the results are comparable to the same achieved by mechanical beating of identical pulp. TEA is the work done when a paper specimen is stressed to rup- ture in tension under prescribed conditions as measured by the integral of the tensile strength over the range of tensile strain from zero to max- imum strain. It is expressed as energy per unit area (test span × width) of paper specimen. The strength indices are found to increase by 20–30% as compared to control due to improved inter-fibre stress transfer offered by NFC. This gives a significant perspective that amalgamates improvement of paper properties alongwith the reduction of energy during the production of high density paper sheets ( Sehaqui, 2013 ; Sehaqui et al., 2012 ). The other form of well acclaimed nanocellulose is NCC. It can be pro- duced on a good scale from microcrystalline cellulose obtained from acid hydrolysis of delignified lignocellulosic biomasses. Nanoscale miniatur- ization is commonly done by sulphate hydrolysis and high-pressure ho- mogenization followed by differential centrifugation. NCC is generally needle shaped with dimensions around 60 × 10 nm. The acidic produc- tion of NCC imparts negative charges on its surface ( Ditzel, Prestes, Car- valho, Demiate & Pinheiro, 2017 ). Hence it functions as an excellent binder in paper production besides conferring the same with barrier, magnetic and electronic properties. Another unique feature of NCC is the capability to form chiral nematic (parallel orientated crystal) phase be- yond a critical threshold. Anionic nano-crystalline cellulose has shown synergistic effect with cationic starch (charge ratio 1:1) to increase the
3. Hemicelluloses
Hemicellulose unlike cellulose is a complex branched polymer com- prising pentosans and hexosans found in plant cell walls. Based on origin hemicellulose may be broadly classified in to hardwood hemicellulose (Glucuronoxylans and glucomannan), softwood hemicelluloses (galac- toglucomannan, and arabinoglucuronoxylan) and storage hemicellu- lose (Galactomannans). Glucouranoxylan is composed of 𝛽 - d -(1 → 4) xy- lose units backbone with acetyl substitution at C-2 or C-3 of the xy- lose and side chains of 4-O-methylglucuronic acid units whereas gluco- mannans mainly composed of (1 → 4) linked 𝛽 - d -glucopyranose and 𝛽 - d -mannopyranose units. Galactoglucomannans consist mainly 𝛽 -(1 → 4) backbone of d -glucose and d -mannose with side chains of galactose moi- eties. 𝛽 -(1 → 4) linked d - xylopyranose units with (1 → 2) linked branches of d -glucopyranosyluronic acid is the main feature of arabinoglu- curonoxylan with 𝛼 -(1 → 3) linked side chains of l -arabinofuranose ( Rowell, Pettersen, Han, Rowell & Tshabalala, 2005 ). The interac- tion varies with the degree of galactosylation of the linear xyloglucan backbone ( Buckeridge, Dietrich & de Lima, 2000 ; De Lima & Buck- eridge, 2001 ; Vincken, De Keizer, Beldman & Voragen, 1995 ). Galac- tomannans are chief storage hemicelluloses in leguminous seeds hav- ing linear 𝛽 -(1–4) linked mannose with single galactosyl branch units ( Buckeridge, Pessoa dos Santos & Tiné, 2000 ). Hemicellulose enhances fibrillation in pulp fibres as well as plastic- ity and surface area thereby conferring increased fibre-fibre bonding and subsequently higher tensile strength in paper sheets. Ascribed to its hydrophilic properties, hemicelluloses increase the swelling of fibres, hence resulting in increased fibre flexibility and bonding conformations ( Anjos, Santos & Simoes, 2004 ). So, hemicelluloses can act as binders while imparting improved tensile and burst indices. However, hemicel- lulose as additive has been proven to be more effective than in-situ hemi- celluloses in terms of strength promotion due to the fact that greater part of in-situ hemicelluloses lie deeper into the cell wall and cannot partici- pate in fibre bonding. Hemicellulose content reduces refining energy of pulp, although higher hemicellulose ( ∼ 20%) content has been shown to be detrimental to pulp strength (tear vs. tensile) as compared to lower hemicellulose content when the refining energy is increased (2–5 N/m) ( Mobarak & Fahmy, 1973 ). It has also been found that hemicellulose as wet end additives provide better strength to pulp fibres when added after refining due to enhanced hydrogen bonding between cellulose and hemicellulose ( Anjos et al., 2004 ; Mobarak & Fahmy, 1973 ).
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