Hye Jung Youn

Preparation of cross-linked cellulose nanofibril aerogel with water absorbency and shape recovery

Abstract Cellulose nanofibril (CNF) aerogels are promising materials for various applications because of their highly porous and ultralight characteristics. The fiber network of CNF aerogels held together by hydrogen bonding and mechanical entanglement of adjacent fibers is easily destroyed when it is exposed to water. In this study, cross-linked CNF aerogels were prepared using maleic acid and sodium hypophosphite as cross-linking agents. In the first step of treatment, CNF dispersed in water was reacted with maleic acid to form an ester linkage. Sodium hypophosphite was then added to the maleic acid-functionalized CNF suspension and the suspension was rapidly frozen using liquid nitrogen. CNF aerogel was obtained after freeze drying. The cross-linking of the cellulose was formed by the reaction between the carbon–carbon double bond of maleic acid-functionalized CNF and hypophosphite. The cross-linked CNF aerogel exhibited good network stability in water and springiness after compression.

An Experimental Investigation of the Effect of Pulp Types, Mechanical Treatments and Crill Contents on Fibre Network Strength

To obtain good formation and even basis weight profile, uniform dispersion of pulp suspension is of great importance. Since the fibre network or floc is the prime cause of uneven mass distribution in fibre suspension, it is critically important to reduce the floc size either by preventing them to form or breaking them down. In this study the network strengths of pulp fibre suspensions prepared from two recycled pulps including OCC and ONP and two virgin pulps were evaluated to examine the possibility of increasing stock consistency in the headbox. Also the influence of mechanical treatments and crill contents on the network strength of pulp fibres was evaluated. Results show fibre network strength increased with an increase of stock concentration. Crill addition of pulp suspension decreased the fibre network strength dramatically, especially when the consistency of pulp suspension was low. DIP and KOCC pulp suspensions formed a weaker fibre network than virgin pulps since they contained significant amount of fines and their fibre quality was deteriorated substantially, which suggested that papermaking with recycled pulps would require less shear or turbulence to disrupt flocs. This indicates that these recycled pulps are more suitable for high consistency forming than virgin pulps.

Structural characteristics of nanofibrillated cellulose mats: Effect of preparation conditions

Nanofibrillated cellulose (NFC) mats, which have porous or dense structures, were prepared in this study. The effects of the number of grinding passes, suspension solids content, and drying conditions on the structural changes of the NFC mats were investigated. Water removal in the NFC suspensions for forming the mats was carried out using pressurized dewatering equipment. The field-emission scanning electron microscope observation showed that the nanofibrils were preserved by solvent exchange drying, whereas the partial aggregation of nanofibrils occurred by freeze drying. Properties such as shrinkage, density, porosity, and specific surface areas of the NFC mats changed depending on the preparation conditions of the NFC mats. The NFC mats, which have low density and high porosity, could be prepared by solvent exchange drying and freeze drying. Porosity of the NFC mats varied from 76 % to 96 %. The specific surface area of the NFC mat increased to 175 m2/g with an increase in the number of grinding passes.

Flocculation behavior of cellulose nanofibrils under different salt conditions and its impact on network strength and dewatering ability

Flocculation of cellulose nanofibrils (CNF) was induced by charge neutralization with different types and concentrations of salt, and its effects on the network strength and dewatering ability of the CNF suspension were investigated. Aggregation of the CNF suspension was evaluated by measurement of light transmission using Turbiscan equipment. This procedure enabled us to characterize the aggregation and sedimentation behavior of nanofibrils under different salinity conditions. Aggregation and sedimentation of CNF occurred on salt addition because the degree of compression of the electrical double layer was changed by adsorption of cations onto the CNF, depending on the type and concentration of salt. The changes in network strength of the CNF suspension due to flocculation were evaluated using a conventional oscillatory rheometer. The viscosity, storage modulus, and yield stress of the CNF suspension increased with an increase in the ionic strength of the suspension. Microrheology measurements using the dynamic light scattering technique were also adopted to characterize the viscoelastic properties of the CNF suspension, revealing that CNF suspension containing a high concentration of salt showed more solid-like behavior. In addition, the aggregation degree of the CNF affected the dewatering ability of the CNF suspension. Bivalent cations were more effective for increasing the network strength and dewatering for a small amount of added salt compared with monovalent cations.

Fractionation of lignin macromolecules by sequential organic solvents systems and their characterization for further valuable applications

Lignin solvent fractionation is one of the promising methods for homogenizing and utilizing lignin commercially. In this work, fractionation characteristics of two lignin fractions were compared to investigate the potential of utilization of fractionated lignin. Two lignins [milled wood lignin(MWL) and organosolv lignin(OL) from yellow poplar] were sequentially fractionated with ethyl acetate(F1), 2-butanone(F2), methanol(F3), acetone(F4), and dioxane/water(F5). Yields of five MWL fractions F1 to F5 were 11.7%, 11.7%, 15.3%, 11.8%, and 49.6%, respectively, and yields of OL fractions were 26.2%, 26.1%, 18.7%, 3.7% and 25.4%. Average molecular weight of F1 (lowest molecular weight fraction) ranged from 1000 to 2400Da, whereas that of F5 (highest molecular weight fraction) was above 10000Da. According to functional group analysis, contents of phenolic hydroxyl groups and methoxyl groups decreased gradually with increasing molecular weight. DFRC analysis was performed to investigate the frequency of β-O-4 linkages and it revealed that the higher molecular weight fractions yielded larger amounts of DFRC monomers, indicating that those fractions more frequently contain aryl ether linkages. TG/DTG showed that the low molecular weight fractions generally have lower initial thermal stability. Tg of the fractions ranged from 126°C to 156°C, increasing as the molecular weight of the lignin fraction increased.

Subnanomolar Sensitivity of Filter Paper-Based SERS Sensor for Pesticide Detection by Hydrophobicity Change of Paper Surface

As a cost-effective approach for detecting trace amounts of pesticides, filter paper-based SERS sensors have been the subject of intensive research. One of the hurdles to overcome is the difficulty of retaining nanoparticles on the surface of the paper because of the hydrophilic nature of the cellulose fibers in paper. This reduces the sensitivity and reproducibility of paper-based SERS sensors due to the low density of nanoparticles and short retention time of analytes on the paper surface. In this study, filter paper was treated with alkyl ketene dimer (AKD) to modify its property from hydrophilic to hydrophobic. AKD treatment increased the contact angle of the aqueous silver nanoparticle (AgNP) dispersion, which consequently increased the density of AgNPs. The retention time of the analyte was also increased by preventing its rapid absorption into the filter paper. The SERS signal was strongly enhanced by the increased number of SERS hot spots owing to the increased density of AgNPs on a small contact area of the filter surface. The reproducibility and sensitivity of the SERS signal were optimized by controlling the distribution of AgNPs on the surface of the filter paper by adjusting the concentration of the AgNP solution. Using this SERS sensor with a hydrophobicity-modified filter paper, the spot-to-spot variation of the SERS intensity of 25 spots of 4-aminothiophenol was 6.19%, and the limits of detection of thiram and ferbam as test pesticides were measured to be 0.46 nM and 0.49 nM, respectively. These proof-of-concept results indicate that this paper-based SERS sensor can serve for highly sensitive pesticide detection with low cost and easy fabrication.

Optimization of carboxymethylation reaction as a pretreatment for production of cellulose nanofibrils

We investigated the optimal reaction conditions for carboxymethylation as a pretreatment method for the production of cellulose nanofibrils (CNF). The influence of the reaction sequence, solvent composition, and presence of water in the reaction medium on the carboxymethylation of pulp was studied. We also investigated the effects of water in the reaction medium on CNF properties. The most effective carboxymethylation of pulp was achieved with non-solvent exchanged pulp and isopropanol. An increase in pulp consistency increased the carboxyl group content. The optimum reaction condition used only one-third the amounts of monochloroacetic acid and sodium hydroxide for the same level of carboxymethylation. The number of passes required for mechanical fibrillation of the pulp, the morphology and dispersion instability of CNF were all strongly influenced by the carboxyl content introduced during the carboxymethylation reaction. The number of mechanical treatment steps required to produce CNF decreased as the carboxyl content increased. Pulp with a high carboxyl content resulted in a more stable suspension due to the increased electrostatic repulsion between the fibrils.

Strengthening effect of polyelectrolyte multilayers on highly filled paper

Abstract Polyelectrolyte multilayering (PEM) treatment is regarded as one of the surface modification techniques to increase paper strength. In this study, the strengthening effect of PEM treatment of highly filled paper was investigated. PEM treatment was performed on both fiber and filler with cationic starch and anionic polyacrylamide systems. Both approaches provided an improvement in the strength of filled paper. However, the strengthening effect of PEM treatment on fibers became weaker as filler content increased, whilst the PEM treatment on fillers was more effective in improving the strength of the highly filled paper. This was because PEM treatment on fillers improved bonding strength between fiber and filler as well as bonding strength between fillers. This was confirmed by the evaluation of specific debonding factor between fiber and filler, and the measurement of the breaking strength of dense film composed of fillers.

Effect of salt concentration in polyelectrolyte multilayering on properties of modified GCC and filled paper - OPEN ACCESS

This study investigated the effects of salt concentration in polyelectrolyte multilayering (PEM) treatment on the properties of ground calcium carbonate (GCC) and the mechanical strength of the filled paper with the PEM treated GCC. Polydiallyldimethyl-ammonium chloride (PDADMAC) and poly(sodium 4-styrene sulfonate) (PSS) were used as cationic and anionic polyelectrolyte, respectively, for the PEM treatment. The PEM treatment was performed in 0 0.1 M NaCl solutions. Polyelectrolyte multilayers could be formed without flocculation of the PEM treated GCC particles, which was confirmed by the zeta potential and particle size of GCC. The amount of polymer adsorbed onto GCC showed different behavior depending on salt concentration. At relatively higher salt concentration (0.1 M NaCl) in this study, more adsorption of polyelectrolytes occurred, which resulted in a thick multilayer on GCC. In addition, it provided strength improvement of the filled paper when the PEM treated GCC was used as filler. As a results, salt concentration during PEM treatment affected both of properties of the PEM treated GCC and mechanical properties of filled paper. ADDRESSES OF THE AUTHORS: Jegon Lee (ljg234@snu.ac.kr), Kyujeong Sim (sim0828@gmail.com), and Hye Jung Youn (page94@snu.ac.kr), Department of Forest Sciences, College of Agriculture and Life Sciences, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826 Corresponding author: Hye Jung Youn

Effects of Fractionation and Mechanical Treatments of Korean OCC on Paper Properties

Linerboards and corrugating mediums are being produced using recycled old corrugated containers (OCC) as major raw materials. However, it causes many quality and process problems including strength reduction, deterioration of machine runnability, etc. To find the most appropriate recycling process for the highly recycled papers like Korean OCC, several approaches of fractionation and mechanical treatment of the fractionated stocks were examined and compared. The stock obtained after refining of the whole stock containing both long and short fiber fractions gave better strength but lower in freeness. Refining of the long fiber fraction and remixing it with unrefined short fiber fraction also improved strength, but the improvement was less than the case of refining whole stock. Mechanical treatment of fines fraction did not show any changes in strength. Results indicated that not only long fiber fraction but also short fiber fraction should be mechanically treated to improve strength properties of highly recycled fibers. Removal of the contaminants in the long fiber fraction was crucial for strength improvement.