Hak Lae Lee

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.

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.

Microwave Synthesis and Application of Cationic Bagasse Hemicelluloses

Novel microwave synthesis cationic hemicelluloses from sugarcane bagasse were obtained by quaternization with CHMAC using sodium hydroxide as a catalyst in microwave synthesis condition. The parameters including the dosage of hemicellulose/CHMAC, the microwave time and the microwave power were optimized and the extent of quaternization was measured by zeta potential. As paper additive, the effect of cationic hemicellulose on bleached bagasse was researched. Then pulp handsheet properties were studied. The results indicated that adding the cationic hemicellulose could increase paper mechanical strengths, and its effect was better than original bleached bagasse pulp. At the dosage of 0.8% on oven dried pulp, paper tensile index, tensile strength, breaking length, burst index and folding endurance can be increased by 43.79%, 36.95%, 33.11%, 17.35%, 33.33% respectively, much better than the paper without the cationic hemicellulose.

Cellulose nanofibrils coated paper substrate to detect trace molecules using surface-enhanced Raman scattering

Acquisition of reproducible surface-enhanced Raman scattering (SERS) signals using paper-based SERS sensors is difficult because of the non-uniformity of the paper surface, which possesses a large number of pores and high surface roughness. To overcome the poor reproducibility of paper-based SERS sensors, they were coated with cellulose nanofibrils (CNF) obtained from pulp fibers, which fill the pores and decrease the local height differences on the paper’s surface. The combination of hydrophobic modification and the improved uniformity and smoothness of the surface after CNF coating increased the coverage of silver nanoparticles (AgNPs) in the SERS spots from 87 to 95% and gave a more uniform distribution of the AgNPs on the SERS substrate. When a high magnification lens was used for sensitive molecular detection, the hydrophobic CNF-coated paper-based SERS sensor reduced the relative standard deviation of the SERS intensity and improved the limit of detection for rhodamine 6G. The CNF coating on paper was found to afford a more effective and reproducible paper-based SERS sensor than conventional filter paper.Graphical Abstract

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.