S. Basu, S. Malik, G. Joshi et al.
Carbohydrate Polymer Technologies and Applications 2 (2021) 100050
Fig. 3. Structure of chitosan and its cyanoethyl derivative.
ture found in the exoskeletons of crustaceans, molluscs, insects and also fungi. The main commercial sources of chitin are shrimp and crab shells. Chitosan can be obtained by enzymatic deacetylation of chitin or treatment of the later with NaOH at high temperature. The chemical backbone of chitin unlike cellulose consists of linear chains of complex glycan 𝛽 -(1–4) − 2-acetamido-2-deoxy- d -glucopyranose, from which chi- tosan has been derived to possess amine instead of acetamide group. Chitosan unlike its parent molecule has greater solubility due to this substitution ( Cissé, Montet, Tapia, Loiseau & Ducamp-Collin, 2012 ; Puvvada, Vankayalapati & Sukhavasi, 2012 ; Rinaudo, 2006 ). Commercially used chitosans are low (50–150 kDa) and medium (700–1000 kDa) molecular weight varieties. Chitosan has been suc- cessfully explored to have influenced mechanical, hydrophilic, reten- tion and barrier properties when used at low concentrations (1–2%) ( Fernandes et al., 2010 ). When used in surface engineering, ‘low molec- ular weight-low concentration’ (LMW-LC) chitosan has been found to reduce water adsorption by ∼ 20% (increase water barrier properties) in paper than ‘medium molecular weight-higher concentrations’ (MMW- HC). This fact can be positively correlated with identical studies result- ing in reduction of Cobb-60 Index (water absorption capacity) by 80– 90% and reduction in water vapour transmission rate (WVTR). The same has also been found to lower the hardness and roughness by ∼ 50% (with and without fillers) of papers, hence proving to be an excellent surface sizing agent. LMW-LC enhances the dry strength of paper (dry strength is inversely proportional to the concentration of LMW chitosan) which is established by a considerable increase in tensile index (reported to be ∼ 10%) and burst index (reported to be ∼ 10%) ( Habibie, Hamzah, Anggaravidya & Kalembang, 2016 ). Electron-micrographs and other chemical analytics in earlier studies have confirmed the equilibrium sto- ichiometry between LMW-LC chitosan and cellulose present in paper. This property may be due the fact that LMW-LC presents with greater area (homogeneity) and hence scope for inter fibre bond formation than MMW-HC. When it comes to wet end addition, chitosan (concentra- tion ∼ 1–3%) has also been reported to improve the burst strength by 20–30%, breaking length by 20–30% and wax pick number (or sur- face strength index) by ∼ 2 units in paper boards from reclaimed pulp ( Bhardwaj, Bhardwaj & Negi, 2016 ). Wax pick number is the average highest numerical designation of the wax that does not disturb the sur- face of the paper. It is also called the Critical Wax Strength Number and it corresponds to the surface strength of paper. Generally, a pick occurs when the surface of the paper specimen blisters, breaks, or lifts and/or coating substance adheres to the surface of the wax. Chitosan, chitosan derivatives like cyanoethyl chitosan ( Fig. 3 ) and other biopoly- mers (e.g. carboxymethyl cellulose etc.) have also been found to have synergistic effects on paper strength properties ( Bhardwaj et al., 2016 ; Nada, El-Sakhawy, Kamel, Eid & Adel, 2006 ). The dielectric properties and thermal stabilities also improve in paper by the addition of chitosan derivatives. Chitosan in solution form (in acetic acid) has been found to
confer antimicrobial properties against both gram positive and gram- negative bacteria in paper. Evidently, the diverse array of functional attributes makes chitosan a promising bio-polymeric additive for paper industry ( Bhardwaj et al., 2016 ; Fatehi et al., 2010 ; Fernandes et al., 2010 ; Habibie et al., 2016 ; Miranda et al., 2013 ; Nada et al., 2006 ; Nassar, El-Sakhawy, Madkour, El-ziaty & Mohamed, 2014 ; Rastogi & Samyn, 2015 ; Vainio & Paulapuro, 2005 ; Zakaria et al., 2015 ).
5. Proteins
5.1. Soy-based
Soy flour is an abundantly available bio-product of soybean oil in- dustry that has been reported to be comprised of 51% protein, contain- ing 18 polar and non-polar amino acids. The polar amino acids take part in inter-fibre bonding through chemical cross-links thereby confer- ring enhanced mechanical, thermal and hydrophilic properties. Cross- linked soy protein has been proved to have increased the mechanical strength of bio-degradable polymers and paper ( Chabba, Matthews & Netravali, 2005 ; Dastidar & Netravali, 2013 ; Kellor, 1974 ). Chemically cross-linked [by using diethylenetriaminepentaacetic acid (DTPA) and sodium hypophosphite] and conjugated soy protein (with chitosan) have been explored for strength enhancing effects on virgin as well as re- cycled (hornified) pulp fibres. The same has been found to have in- creased tensile index of different types of pulp. In a detailed study of soy flour ( Salam et al., 2015 ), the authors established the capability of soy flour glycoprotein as a mechanical property enhancer. In the mentioned study, when added at a concentration of 1.5% on reclaimed pulp, soy flour glycoprotein has increased the tensile and burst indices of handsheets by ∼ 5%. Soy flour protein also under similar conditions enhanced the tensile and burst indices by ∼ 5%. Soy flour protein con- jugated with chitosan increased the tensile and burst indices by ∼ 20% whereas soy flour glycoprotein enhanced the same by ∼ 10% and 20% respectively. Soy flour protein (as well as glycoprotein) conjugated with DTPA and chitosan increased the tensile and burst indices by ∼ 50%. Soy flower glycoprotein-DTPA-chitosan conjugate (1%) improved the ten- sile index of reclaimed and kraft pulp handsheets by 52.6% and 57.8% respectively; burst index by 39.2% and 42.5% respectively and com- pression index by 39.9% and 48.6% respectively. The superlative rise in mechanical properties are conferred by intense inter-fibre bond for- mation (H-bond and esterification) by -OH, -COOH and -NH 2 functional groups in glycoprotein conjugates. In addition to the soy protein, cotton seed protein was also used as a wet and dry strength additive in paper ( Cheng, Villalpando, Easson & Dowd, 2017 ).
5.2. Cottonseed protein
The global production of cottonseed accounts to 39–44 million met- ric tons (MMT) per annum during the year 2014–17 which imply 8.3
8
Made with FlippingBook - Online magazine maker