S. Basu, S. Malik, G. Joshi et al.
Carbohydrate Polymer Technologies and Applications 2 (2021) 100050
Lima et al. (2003) have emphatically portrayed the property en- hancement of paper-sheets made from Eucalyptus pulp by the action of hemicellulose from different legumes. The article experimentally demonstrated that after addition of hemicellulose, tear and burst indices significantly improved upto a maximum of 30% and 20% respectively. Furthermore, retention of fines enhanced by 3% [Increase in Apparent Specific Weight (ASW in kg/m 3 )], tensile index by 11%, TEA by 19% and specific elastic modulus by 3% as compared to control (without added hemicellulose) samples. An interesting fact has been established by Mobarak and Fahmy (1973) in their research on hemicellulose addi- tives. They have shown that the retention of added hemicellulose which was previously extracted from hardwood has lowered (by ∼ 4%) in hard- wood pulp itself with respect to the retention of hemicellulose obtained from straw. Besides, straw pulp has 5% enhanced capacity to retain hemicelluloses than hardwood. Owing to this fact, sheet prepared from straw pulp was more compact than that of hardwood pulp after hemi- cellulose addition.
sulfonium, phosphonium etc. forms. However, alkaline charging fol- lowed by a reaction with 3 ‑chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) is the most common method for producing cationic starch ( Fig. 2 ). Gelatinized (gelatinization of starch increases the reac- tion rate) starch in presence of alkali and CHPTAC forms quaternary ammonium cationic starch ether at temperatures around 40 °C with a starch concentration of 25–35% (w/v) ( Ghasemian, Ghaffari & Ashori, 2012 ). Sodium sulphate is often used to maintain the integrity of the unswollen starch granule during the entire reaction period. A reaction efficiency of around 85% and degree of substitution around 0.5 is con- sidered legible for the application of the modified product. Interest- ingly, amylose and amylopectin have different adsorption potential in pulp fibres and amylose is more potent in getting adsorbed. Although cationic starch shows low Langmuir type isotherms (rate of adsorption as a function of applied pressure), its affinity to fibre-fibre bonding is comparatively higher than other modified starch forms. The adsorption of cationic starch onto fibres depends upon the electrolyte concentration in the system ( Ulbrich, Radosta, Kießler & Vorwerg, 2012 ). The properties conferred by cationic starch can be grouped in four main sections: (i) Enhancement of mechanical strength, (ii) Retention aids to fines and fillers, (iii) Improved drainage, and (iv) Generating low toxic paper-mill effluent. The reactive cationic groups provides strong adsorption in- terface between fibres, fines and fillers through electrostatic at- traction ( Carr & Bagby, 1981 ; Ghasemian et al., 2012 ; Lee et al., 2002 ; Pettersson, Höglund & Wågberg, 2007 ; Solarek, 1986 ; Ulbrich et al., 2012 ; Xie, Yu, Liu & Chen, 2006 ; Yang, Qiu, Qian & Shen, 2013 ; Zakraj š ek, 2008 ; Zakraj š ek & Golob, 2009 ). Ghasemian et al. (2012) have repored that cationic starch can sig- nificantly increase the tear, tensile and burst indices of virgin and mixed pulps. A maximum tensile index of 29.2 Nm/g, tear index of 18.7 mKm 2 /g and burst index of 2.74 kPam 2 /g was observed in handsheets made from mixed pulps with cationic starch additive. The strength properties have increased by 2–3 folds as compared to untreated pulp. Ulbrich et al. (2012) have statistically established the correlation between cationic starch adsorption onto cellulose fibres and its effect on strength enhancement.
4. Starch and derivatives
4.1. Starch
Starch is the widely used additive in paper industry showing di- verse array of functional attributes owing to its chemical compatibil- ity, abundance and inexpensiveness. Commercial starches are extracted from grain crops, tubers and legume plants. It is a biopolymer formed by the amalgamation of polymeric glucans viz. amylose and amylopectin linked by glycosidic bonds. Amylose is a linear chain of repetitive glucose units linked by 𝛼 − 1,4-glucosidic bond, whereas amylopectin is a highly branched macromolecule, which consists of short chains of (1 → 4)-linked 𝛼 - d -glucose with (1 → 6)- 𝛼 -linked branches. The crys- tallinity of starch depends upon the relative content as well the struc- tural orientation of amylose and amylopectin. The high viscosity (be- cause of extensive hydrogen bonding of amylopectin) of native or un- modified starch limits its overall accessibility when used as additives in paper making. This drawback is counteracted by plasticization (using solvents like glycerol, sorbitol etc.) or depolymerisation and/or modi- fication (chemical or enzymatic) or blending with other compounds or combination of ionic starch forms ( Tomasik & Zaranyika, 1995 ). Chem- ical modification is the most frequently used method of commercial starch derivation into functional products like cationic starch, oxidized starch, hydroxyethyl starch etc. For chemical modification, the substi- tutable hydroxyl groups of starch are to be taken into account. Hydroxyl group at C6 in starch is primary alcohol whereas C2 & C3 are secondary ones; in the presence of glucose ring portion –CHOH –CHOH- instead of C2 & C3 carbons, starch is a triol, whereas glycosidic bonds at this juncture makes it a hemiacetal. The three hydroxyl groups in glucose subunit make starch susceptible to a number of possible modifications. The basic chemical modification of starch is usually done via oxida- tion, esterification and etherification. Unmodified starch as an additive has been proved to have conferred mechanical strength (to some ex- tent), surface smoothness and optical property to paper ( Bajpai, 1999 ; Hermansson & Svegmark, 1996 ; Jane, 1995 ; Larotonda, Matsui, So- bral & Laurindo, 2005 ; Matsui et al., 2004 ; Perry & Donald, 2000 ; Santayanon & Wootthikanokkhan, 2003 ; Tomasik & Zaranyika, 1995 ; Vásconez, Flores, Campos, Alvarado & Gerschenson, 2009 ). Prepara- tion route and structure of some important starch derivatives are pre- sented in Fig. 2 ( Colussi et al., 2015 ; Fu, Zhang, Ren & BeMiller, 2019 ; Nasir, Abdulmalek & Zainuddin, 2020 ; Yi, Zhang & Ju, 2014 ).
4.3. Oxidized starch
Oxidative modification of starch is another popular modification pro- cedure employed to make highly viscous native starch to be industrially compatible. The primary and secondary alcohol groups are oxidized un- der controlled set of conditions to substitute hydroxyl groups with re- active aldehyde and/or carboxyl groups. The modification agents are chosen on the basis of origin and final utilization of the product. Com- mercially produced oxidized starch are modified at 25–30 °C with low oxidant concentrations ( < 3%) in controlled batch processes (in presence of oxygen and/or transition metal ion catalysts). Although the most com- mon oxidant used is sodium hypochlorite ( Fig. 2 ), other chemicals such as permanganates, hydrogen peroxide, periodates, persulphates, dichro- mates etc. were also practiced in recent past ( Jonhed, 2006 ). The car- bonyl or carboxylic groups present in anionic (oxidized) starch prevents retrogradation of starch and hence generates a stable modified product that can exert its function under high temperature (cooking) systems be- sides lowering gelatinizing temperature of starch itself ( Vanier, El Ha- lal, Dias & da Rosa Zavareze, 2017 ). Ascribed to this property, oxidized starch is popularly used as a coating material in paper industry since it prevents clustering of pigments in coating formulations ( Jonhed, 2006 ; Lee et al., 2002 ; Lewicka, Siemion & Kurcok, 2015 ; Shen et al., 2014 ; Vanier et al., 2017 ; Xie et al., 2006 ).
4.2. Cationic starch
4.4. Other starch-derived additives
Cationic starch is the most popular starch derivative in paper in- dustry imparting useful properties to various kinds of pulp for qual- ity enhancement. It is commercially produced in amino, ammonium,
Acetylated starch is another popular starch derivative of commercial value in paper and food industries. It is produced commercially by alkali
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