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a concentration of 0.5–1.0%) have maximally enhanced the tensile in- dex of paper between ∼ 10–35%. Higher molecular weight guar gum addition corresponds to considerable increase in paper strength. Usu- ally the guar additives are added in a low pulp consistency and neu- tral pH ( Chudzikowski, 1971 ; Dasgupta, 1999 ). A novel blend of guar with, algin and tamarind kernel polysaccharide finds its effective use in paper coating and surface sizing ( Yin & Lewis, 1981 ). Ionic and non- ionic guar gum has successfully increased the strength and retention of fillers in tissue paper manufacturing Wang (2013) . Moreover, guar gum combined with precipitated calcium carbonate and organic titanium en- hances fibre cross-linking and higher retention of fillers (by ∼ 2–8%) in paperboards ( Chudzikowski, 1971 ; Dasgupta, 1999 ; Lee et al., 2005 ; Wang, 2013 ; Xie, Song, Liu & Qian, 2016 ; Yin & Lewis, 1981 ).
6.3. Carrageenan
Carrageenan is a commercially important natural, abundantly avail- able, sulphated polysaccharide (gum) extracted from red algae Gelidium elegans that has promising application in paper science and nanotechnol- ogy besides traditional uses in cosmetics, medicine and food thickening ( Cadirci et al., 2016 ; Chen, Lee, Juan & Phang, 2016 ; Lin, Liang & Chang, 2016 ; Seo, Lee, Lee & You, 2010 ). Carrageenan is a polymer with alter- nate units of galactose and 3.6-anhydrogalactose linked by 𝛽 -(1–4) and 𝛼 (1–3) glycosidic bond with 15–40% sulphated monosaccharide units, hence defining its anionic nature ( Necas & Bartosikova, 2013 ). Com- mercial carrageenans are mainly constituted by three types namely 𝜅 -, 𝜄 -, and 𝜆 -carrageenan. 𝜅 -Carrageenan has one sulfate ester, while 𝜄 -and 𝜆 - carrageenan contain two and three sulfates per repeating dimer, respec- tively. Fig. 4 (c) − 4(e) shows the structural behaviour of three types of carrageenans ( Rhein-Knudsen, Ale & Meyer, 2015 ). Carrageenan due to its sulphate groups participate in extensive hydrogen bonding with cel- lulose fibers in paper by abridging the inter-fibre gaps ( Liu et al., 2017 ). As reported earlier, by virtue of this bonding property, carrageenan as an additive confers superior strength in paper, thereby increasing tensile index by ∼ 25% and burst index by ∼ 15% when compared to sheets with- out carrageenan ( Huq et al., 2012 ; Liu et al., 2017 ; Przybysz, Dubowik, Kucner, Przybysz & Buza ł a, 2016 ; Xie et al., 2016 ) .
6.4. Xanthan gum
Xanthan gum is produced by gram-negative yellow pigmented bacte- ria Xanthomonas campestris . The gum has the property to produce stable aggregates (pseudoplasticity) in aqueous medium owing to the capabil- ity of forming extensive hydrogen bonding. Hence it generates highly viscous aqueous solution even at low concentrations. Xanthan back- bone comprises of pentasaccharide repetitive units having (1–4)- 𝛽 - d - glucopyranose unit along with tri-sachharide lateral branches attached at every C-3 carbon of the alternate sugar residue of the main chain. The lateral chains consist of 𝛽 - d -mannopyranose residue linked to (1– 4) 𝛽 - d -glucuronic acid which is linked with (1–2) 𝛼 - d -mannopyranose residue. Apart from these, pyruvate acetal groups are present at the end of d -mannopyranose (C6-OH position) unit ( Patel, Maji, Moorthy & &Maiti, 2020 ). Fig. 4 (f) represents the complex structure of xanthan gum ( Patel et al., 2020 ). The chemical complexity of xanthan provides its capability of bonding and rheological modifications. Industrially used xanthan gum has 37% glucose, 43.4% mannose, 19.5% glucuronic acid, 4.5% acetate and 4.4% pyruvate ( Lachke, 2004 ). Xanthan gum used alone and in combination with other gums and polyelectrolytes as wet- end additive increases the tensile strength of paper ( Mukherjee et al., 2014 ). Taggart, Schuster and Schellhamer (1992) showed that xanthan has promising capabilities as both retention aid and strength enhancer when used with starch on kraft-softwood mixed pulp. In the study, reten- tion of starch was reported to increase by 60–70% by separately adding xanthan. The same also increased the tensile index by ∼ 10% and burst index by ∼ 35%. Besides these, xanthan has been effectively used as a
Fig. 4. Structures of common gums used as additive in paper making (a) Sodium alginate; (b) guar galactomannan; (c) 𝜅 -carrageenan; (d) 𝜆 -carrageenan; (e) 𝜄 - carrageenan; (f) xanthan.
derivatives) and cationic guar (derivatized with glycidyltrimethylam- monium chloride) in synergism or alone enhances the tensile index of paper without compromising with the softness, when added to a bleached pulp furnish. Cationic guar alone (at a concentration of 1.0%) has been reported to increase tensile strength by ∼ 15%; whereas an- ionic guar (at a concentration of 1.0%) enhances the tensile index by ∼ 17% ( Dasgupta, 1999 ). The blends of cationic and anionic guar (at
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