Effects of Crosslinking Degree on the Coating Properties

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PEER REVIEWED ARTICLE bioresources com, Bio polymers have drawn increasing attention in recent years because of its non. toxicity low cost abundance and sustainability Vieira et al 2011 Xiang and Runge. 2016 The food application of bio polymers has been investigated for over a century and. applications of bio polymers as films and coatings in paper have recently received. increased interest Aulin and Lindstr m 2011 Barhoum et al 2014 Anthony et al 2015. The bio polymers that are widely used for films and coatings can be divided into three. major categories polysaccharides proteins and lipids Aulin and Lindstr m 2011. Polysaccharides are widely used in films and coatings because of their natural abundance. and sustainability Aulin and Lindstr m 2011 Vieira et al 2011 Major polysaccharides. include cellulose hemicellulose starch and chitosan Vieira et al 2011 Cellulose starch. chitosan and their derivatives have been widely studied and used in the film and coating. industries Mikkonen et al 2009 Xiang and Runge 2016 However unlike cellulose and. starch hemicellulose which is widely distributed in plant biomass is still utilized at a low. level A large amount of waste hemicellulose produced from pulping or other bio refinery. industries is used as low value fuels or animal feed Xing et al 2011 Xiang et al 2014. Xiang and Runge 2016 Achieving a high value conversion of hemicellulose has gained. interest in recent years, Hemicelluloses are complex branched hetero polysaccharides that consist of. different sugar units and uronic acids Mikkonen et al 2009 Eg s et al 2013 Their. structures depend on the plant species and extraction conditions Arabinoxylan AX is the. dominant type of hemicellulose found in annual plants and agro industrial residues Eg s. et al 2013 The structure of AX is based on a linear backbone that consists of 1 4 linked. D xylopyranose and branches that contain acetyl arabinosyl and glucuronosyl residues. Mikkonen et al 2009 Arabinoxylan consists of many hydroxyl groups which can form. strong hydrogen bonds that lead to a good film formation ability Eg s et al 2013 Zhong. et al 2013 However films made from AX demonstrate limitations in material. applications because of its brittleness low mechanical properties and hygroscopic. properties Goksu et al 2007 Escalante et al 2012 Therefore chemical modification is. necessary to enhance the material application range of AX Kayserilio lu et al 2003. Escalante et al 2012 The esterification of hemicellulose can be achieved in homogeneous. or heterogeneous solutions Sun et al 1999 2002 2003 Carboxymethyl hemicellulose. can be synthesized by reaction with sodium chloroacetate in a slurry media Petzold et al. 2006 Etherification of hemicellulose can be achieved in an alkaline solution by reaction. with propylene oxide Jain et al 2000 Laine et al 2013 However these modifications. are still not able to tackle all of the disadvantages that arise when utilizing hemicelluloses. Gr ndahl et al 2003 Hansen and Plackett 2008 Consequently hemicelluloses can be. used as coating or coating binder materials which could minimize its material. disadvantages such as brittleness and low strength Xiang et al 2014 Anthony et al. Crosslinking may be a modification method that can help better utilize. hemicellulose as a paper coating binder material because a network structure can bind the. coating surface and inorganic pigments Furthermore the molecular mass of hemicellulose. is lower so some improvements might be needed for the formation of a strong film. Crosslinking is an effective method that can effectively increase the molecular mass of. hemicellulose Glutaraldehyde GA is a dialdehyde compound that is widely used as a. crosslinking agent in the modification of starch El Tahlawy et al 2007 protein Matsuda. et al 1999 guar gum Sandolo et al 2009 and chitosan Monteiro Jr and Airoldi 1999. The previous study by the authors showed that crosslinking hemicelluloses with GA. Hu et al 2019 Coating properties of arabinoxylan BioResources 14 1 70 86 71. PEER REVIEWED ARTICLE bioresources com, produced a paper coating binder that had a comparable performance to that of PVOH. Xiang et al 2016 In this previous work the effects of the amount of GA on the film. properties and the effects of the degree of substitution DS on the coating properties were. investigated Xiang et al 2016 The AX that had been crosslinked at a low DS with GA. showed better coating properties compared with that of GA crosslinked with AX at a high. DS However the viscosity of the low DS AX solution suspension was higher than that of. the high DS AX solution suspension Xiang et al 2016 High viscous coatings are difficult. to disperse evenly and the addition of more solvent may result in wasting solvent It also. requires more time and energy to dry the coatings Therefore it is essential and useful to. reduce the viscosity of crosslinked AX and still maintain its enhanced coating properties. Two different types of crosslinking can take place during polymer crosslinking. Gebben et al 1985 One type is intermolecular crosslinking which leads to an increase. in the viscosity because of the increase in the radius of gyration Cherian et al 2007 Foster. et al 2009 The other one is crosslinking within a single molecule called intramolecular. crosslinking which leads to a decrease in the viscosity because of the volume contraction. of the polymer coils Fordyce and Ferry 1951 At high monomer concentration 0 1. intermolecular crosslinking is dominant Fordyce and Ferry 1951 Tuteja et al 2010 To. induce intramolecular crosslinking the use of ultra dilute solutions with concentrations as. low as 0 1 has been proposed Fordyce and Ferry 1951 Therefore it is feasible to reduce. the viscosities of a crosslinked AX solution suspension by reacting it in a dilute solution. However even in a diluted solution intermolecular crosslinking is still evident Wang et. al 2011 Furthermore if the reaction is completed in a dilute solution the production. efficiency is low In order to overcome these difficulties a continuous dropwise addition. strategy was proposed Harth et al 2002 Consequently crosslinking with a continuous. method has been developed Harth et al 2002, In this work the possibility of obtaining low viscosity GA modified AX GAX. that still maintains its enhanced coating properties was investigated The AX concentration. and amount of GA were the two variables used to control the degree of crosslinking During. crosslinking under acidic condition or high concentration GA may polymerize with itself. through forming hemihydrate multimers or an aldol condensation reaction forming. unsaturated aldehydes Walt and Agayn 1994 Matsuda et al 1999 Monteiro Jr and. Airoldi 1999 Therefore it is not explicit enough to express the degree of crosslinking. using the DS As such the crosslink index CI represented the degree of crosslinking for. the GAX The viscosity of the GAX was controlled or reduced by varying the degree of. intermolecular crosslinking or even possibly inducing intramolecular crosslinking The AX. solution was added dropwise to the dilute GA solution to control the degree of crosslinking. The effects of crosslinking degree on the film and coating properties of the GAX were also. investigated,EXPERIMENTAL, Sugarcane bagasse was obtained from Guangxi Laibin Donta Paper Co Ltd.
Nanning China Sodium hydroxide ethanol and hydrogen peroxide were purchased. from Sinopharm Chemical Reagent Co Ltd Shanghai China Ethylenediamine. tetraacetic acid disodium salt dihydrate EDTA GA 50 in water acetic acid calcium. carbonate sodium hydroxide hydroxylamine hydrochloride and polyvinyl alcohol. Hu et al 2019 Coating properties of arabinoxylan BioResources 14 1 70 86 72. PEER REVIEWED ARTICLE bioresources com, PVOH Mw 27000 were purchased from Shanghai Macklin Biochemical Co Ltd. Shanghai China Hydrochloric acid was purchased from Guangzhou Chemical Reagent. Factory Guangzhou China, Arabinoxylan was extracted from sugarcane bagasse using an alkaline peroxide. solution Briefly sugarcane bagasse was pre extracted with 95 ethanol for 16 h in a. Soxhlet extractor to remove any waxy compounds Prior to extraction the sugarcane. bagasse was washed with 0 2 w v EDTA for 1 h at 90 C to remove metal cations as. these ions promote decomposition of hydrogen peroxide and reduce its delignification. performance The sugarcane bagasse was then reacted in a 2 w v NaOH solution with. 4 w v H2O2 at 50 C and a 1 10 solid to liquid ratio w v for 3 h Then the pH of the. alkali soluble part was adjusted to 5 5 and slowly poured into three volumes of 95 ethanol. before being stirred The precipitated solid was then washed with 70 ethanol several. times before being freeze dried, The extracted AX was in a brown powder form and consisted of 85 1 xylose. 7 0 arabinose 4 6 glucose 0 32 galactose 1 14 glucuronic acid and 0 72. galacturonic acid relative dry weight of hemicelluloses w w. Crosslinking of arabinoxylan, Arabinoxylan was dissolved in deionized water to make a 10 w v solution. Different amounts of GA GA AXU mole ratios 0 1 0 3 0 5 and 1 0 were then. dissolved in deionized water The AX mono sugar unit has a molar mass of 132 g mol and. was referred to as AXU Afterward the pH of the GA solution was adjusted to 3 0 by acetic. acid Then the GA solution was diluted to a certain concentration to reach final AX. concentrations of 0 1 0 25 0 5 and 1 0 The AX solution was added to the GA. solution dropwise 1 mL 3 min for 150 min at 25 C After the reaction the pH was. adjusted to 6 0 with a 1 NaOH solution The modified AX was precipitated by pouring. the mixture into three volumes of 95 v v ethanol and washed three times with 70 v v. ethanol Three samples were made for each modification. Film preparation, All of the films were prepared by a casting method Glutaraldehyde modified AX.
0 48 g and PVA 0 12 g were dissolved in deionized water and stirred at 300 rpm for 40. min at 80 C to make a 7 5 w v suspension Then the solution was poured onto a. polytetrafluoroethylene dish 60 mm 45 mm and allowed to dry at 23 C and a relative. humidity of 50 The film thickness was measured by a micrometer precision 1 m. Measurements were taken at 10 different locations on each film and the mean values were. used in the calculations to determine the mechanical test measurement The thickness of. the films was approximately 0 17 mm,Paper coated by modified arabinoxylan. For paper coated with only AX or GAX 20 w w coating suspensions were. prepared with a pH of approximately 3 When GAX or AX were used as paper coating. adhesives 40 w w coating suspensions were prepared which contained 10 w w. GAX or AX and 90 w w calcium carbonate The same amount of PVA was also added. to calcium carbonate as a control, Prior to coating printing paper 70 mm 210 mm was dried in a drying oven at. 105 C for 8 h and kept in a desiccator for 24 h The paper was coated with a manual. Hu et al 2019 Coating properties of arabinoxylan BioResources 14 1 70 86 73. PEER REVIEWED ARTICLE bioresources com, coating machine to control the coating weight which was approximately 6 g m2 The. coated paper was dried at 170 C for 5 min and further dried at 105 C for 60 min The. coated paper was kept in a desiccator for 24 h After that the weight of the paper samples. was measured to determine the weight of the coating Two sheets of coated paper were. prepared for each modification,Characterization of the crosslinked arabinoxylan. The DS of the modified AX was the moles of the GA reacted per mole of AXU. The free aldehyde content of the modified AX samples was determined by following the. method using hydroxylamine hydrochloride Yu et al 2010 with some modifications 0 15. g sample was added in 25 mL hydroxylamine hydrochloride solution 0 25 M The pH. was adjusted to 5 0 by using NaOH solution 0 1 M The conversion of aldehydes into. oximes continues at 50 C for 2 h The mixture was then titrated back to pH 5 0 using 0 1. M NaOH The aldehyde content was calculated with Eq 1. CHO 0 1 V sample V blank m 1, where Vsample and Vblank represent the consumption of NaOH by the sample and the blank.
0 1 is the molar concentration of NaOH solution and m is the weight of the sample. The aldehyde content in the GAX filtrate was also measured to determine the. unreacted GA content The reacted GA was calculated by subtracting the amount of. unreacted GA from the amount of GA added and then the DS values of the GAX samples. were determined The samples were labelled as GAXN where N is the CI. The Fourier transform infrared FT IR spectroscopy analysis was performed in an. absorbance range of 4000 cm 1 to 400 cm 1 using a Bruker TENSOR27 FT IR spectrometer. Karlsruhe Germany, The suspension viscosities of the AX and GAX were measured with a rotary. viscometer DV II HE Brookfield USA The viscosity of the 20 w w AX and GAX. suspension was measured at different rotational speeds. Characterization of the crosslinked arabinoxylan films and coatings. Films from each modification were cut into five rectangular specimens with a 40. mm length and 10 mm width The tensile strength of the specimens was measured using a. tensile strength machine Universal Testing Machine 5565 Instron Canton USA at 23. C and a 50 relative humidity The initial length of the grips was 25 mm and the testing. speed was 5 mm min with a preload of 0 5 N, The contact angle of the films was measured with an OCA40 contact angle system. Effects of Crosslinking Degree on the Coating Properties of Arabinoxylan 2019 Coating properties of arabinoxylan BioResources 14 1 70 86 71 Bio polymers have drawn increasing attention in recent years because of its non toxicity low cost abundance and sustainability Vieira et al 2011 Xiang and Runge 2016 The food application of bio polymers has been investigated for

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