Preparation and application of composite phase change materials stabilized by cellulose nanofibril-based foams for thermal energy storage.

The leakage issue and inferior heat conduction of organic phase change materials (PCMs) limit their actual applications. In the present study, cellulose nanofibril (CNF)-based foams were prepared as the porous scaffolds for polyethylene glycol (PEG) and paraffin wax (Pw) to prevent their leakage, and multiwalled carbon nanotubes (CNTs) were incorporated to improve the heat transfer performance. The prepared foams had low density (<67.3 kg/m3) and high porosity (>94.5 %). Selective chemical modifications of nanocellulose foams enhanced their shape-stability and compatibility with PCMs. The highly porous foam structure and favorable compatibility resulted in high PCM loading levels (93.63 % for PEG and 91.77 % for Pw) and negligible PCM leakage (<2 %). CNTs improved the heat transfer performance of PCMs, as evidenced by the improved thermal conductivities and boosted temperature rises during solar heating. Meanwhile, the composite PCMs exhibited improved thermal stability over the control. PEG-based composite PCM exhibited a phase change enthalpy of 143 kJ/kg with a melting temperature of 25.2 °C; Pw-based composite PCM exhibited a phase change enthalpy of 184 kJ/kg with a melting temperature of 53.4 °C. Novel PCM sandwich structures based on these composite PCMs and a thermoelectric generator were designed and displayed promising potential for solar energy harvesting and utilization.

A waterproof cellulose nanofibril sheet prepared by the deposition of an alkyl ketene dimer on a controlled porous structure.

This study prepared a waterproof cellulose nanofibril (CNF) sheet via the deposition of an alkyl ketene dimer (AKD) on the sheet's controlled porous structure. The porosity of the CNF sheet was controlled by drying under different conditions, which included hot-press drying (HD) and solvent-exchange drying (SD), and the effect on the hydrophobization and water-related barrier performance of the sheet were investigated. When the SD sheet was immersed in an AKD wax solution, the sheet exhibited super-hydrophobicity and a lower water vapor transmission rate, compared with the HD sheet. This indicated that the porous structure of the SD sheet enabled AKD to be adsorbed on both the surface and the inner surface and it filled in the pores of the sheet, thereby giving rise to excellent waterproofing properties. The performance of a hydrophobized SD sheet as a water barrier material was comparable to a linear low-density polyethylene film. This study confirms the possibility for AKD wax to be immersed in a porous CNF sheet and used as a potential barrier material in hydrogel packaging.

Cellulose nanofibril/carbon nanotube composite foam-stabilized paraffin phase change material for thermal energy storage and conversion.

The leakage and low thermal conductivity of paraffin phase change material (PCM) must be addressed to achieve a more efficient energy storage process. In this study, cellulose nanofibril (CNF) foams were prepared as the porous support of paraffin to prevent its leakage, and multiwalled carbon nanotubes (CNTs) were incorporated in the foams to improve heat transfer performance. Treatment of CNF with methyltrimethoxysilane improved compatibility between the foams and paraffin. The prepared highly porous (porosity >96%) foams had paraffin absorption capacities exceeding 90%. The form-stable PCM composites displayed negligible paraffin leakage and had a compact structure. The prepared PCM composites had enhanced heat transfer performance, reasonable phase change properties and thermal stabilities. The enthalpy of the SCNF/CNT50-Pw PCM composite decreased by 6% after 100 melting/freezing cycles. Compared with pristine paraffin, the PCM composites exhibited superior form-stabilities and improved thermal properties, which suggested application in a solar-thermal-electricity energy harvesting and conversion system.

Improving the Barrier Properties of Packaging Paper by Polyvinyl Alcohol Based Polymer Coating-Effect of the Base Paper and Nanoclay.

The poor barrier properties and hygroscopic nature of cellulosic paper impede the wide application of cellulosic paper as a packaging material. Herein, a polyvinyl alcohol (PVA)-based polymer coating was used to improve the barrier performance of paper through its good ability to form a film. Alkyl ketene dimer (AKD) was used to enhance the water resistance. The effect of the absorptive characteristics of the base paper on the barrier properties was explored, and it was shown that surface-sized base paper provides a better barrier performance than unsized base paper. Nanoclay (Cloisite Na+) was used in the coating formulation to further enhance the barrier performance. The results show that the coating of PVA/AKD/nanoclay dispersion noticeably improved the barrier performance of the paper. The water vapor transmission rate of the base paper was 533 g/m2·day, and it decreased sharply to 1.3 g/m2·day after the application of a double coating because of the complete coverage of the base paper by the PVA-based polymer coating. The coated paper had excellent water resistance owing to its high water contact angle of around 100°. The grease resistance and mechanical properties of the base paper also improved after coating. This work may provide inspiration for improving the barrier properties of packaging paper through the selection of a suitable base paper and coating formulation.

The Effect of a Polymer-Stabilized Latex Cobinder on the Optical and Strength Properties of Pigment Coating Layers.

Coated paper with a porous coating layer may have enhanced light-scattering ability and thus favorable optical properties. However, the increased porosity of such a coating layer is likely to decrease the strength of the coated paper, thereby adversely affecting the quality of the paper in the printing and converting processes. In this research, polymer-stabilized (PS) latex was prepared and used as a cobinder for the pigment coating of the paper. The PS latex particles were colloidally stabilized by a 3:1 mixture of starch and polyvinyl alcohol. The influence of the PS latex cobinder on the viscosity, sedimentation, and consolidation of coating colors was investigated. In addition, the effect of the cobinder on the properties of coating layers, namely, their porosity and surface, optical, and tensile properties, was examined. The results revealed that the PS latex cobinder formed microstructures in the coating colors and affected their viscosity. The addition of PS latex also led to enhanced interactions between coating color components, which affected the consolidation of the coating color, resulting in the formation of dried coating layers with greater porosity and improved optical properties (i.e., higher brightness and opacity) relative to coatings without the PS latex cobinder. Furthermore, the addition of PS latex improved the tensile strength of the coating layers, which was attributable to the small size and the polymeric protective shell of the cobinder particles. Thus, these results show that this PS latex cobinder has the potential to be used for the production of high-quality coated paper products.

Use of cellulose nanofibril (CNF)/silver nanoparticles (AgNPs) composite in salt hydrate phase change material for efficient thermal energy storage.

Salt hydrate phase change materials (PCMs) possess the challenge of supercooling, which must be addressed to allow more efficient energy storage and utilisation. In this work, cellulose nanofibril (CNF), a versatile biopolymer was used to support and disperse silver nanoparticles (AgNPs), and the synthesised CNF/AgNPs composite was used to improve the performance of sodium acetate trihydrate (SAT). Results showed that CNF dispersed the AgNPs uniformly and prevented their aggregation. Through the synergistic effect of 1% CNF/AgNPs and 2% sodium phosphate dibasic dodecahydrate, a low supercooling degree of 1.2 °C was achieved. Moreover, AgNPs were uniformly distributed in the prepared PCM composite. Differential scanning calorimetry results indicated that the prepared PCM@CNF/AgNPs 0.02 composite showed a similar melting point (57.4 °C) and enthalpy (269 kJ/kg), compared to those of pure SAT. Thermogravimetric analysis showed that the PCM composite did not lose all moisture until a heating temperature of 160 °C, showing improved thermal stability. The thermal conductivity of PCM@CNF/AgNPs 0.02 composite was 31.6% higher than that of SAT. The enthalpy of this composite decreased only around 2% after 100 melting/freezing cycles, showing satisfying thermal reliability. This composite can therefore be used to fabricate high-performance TES systems with negligible supercooling and improved thermal properties.

Comparison of Effects of Sodium Chloride and Potassium Chloride on Spray Drying and Redispersion of Cellulose Nanofibrils Suspension.

Cellulose nanofibrils (CNFs) were exposed to the same levels of potassium chloride (KCl) and sodium chloride (NaCl) before being subjected to spray drying. The effect of NaCl and KCl on the size of atomized droplets and the hydrogen bond retardation between CNFs was investigated by characterizing product morphology, particle size distribution, dispersion stability in aqueous system, and surface chemistry. The results showed that the CNF suspensions treated with KCl could be atomized into smaller droplets during spray drying, and then CNF powder with smaller sizes could be obtained. As the agglomeration was less, and the CNF with KCl addition had good dispersion stability after redispersion compared with CNF treated by NaCl. Therefore, KCl treatment was an effective method to reduce the agglomeration of CNF during spray drying.

Incorporation of CNF with Different Charge Property into PVP Hydrogel and Its Characteristics.

Cellulose nanofibril (CNF)-added polyvinylpyrrolidone (PVP) hydrogels were prepared using different types of CNFs and their properties were investigated. CNFs with different morphology and surface charge properties were prepared through quaternization and carboxymethylation pretreatments. The quaternized CNF exhibited the narrow and uniform width, and higher viscoelastic property compared to untreated and carboxymethylated CNF. When CNF was incorporated to PVP hydrogel, gel contents of all hydrogels were similar, irrespective of CNF addition quantity or CNF type. However, the absorptivity of the hydrogels in a swelling medium increased by adding CNF. In particular, the quaternized CNF-added PVP hydrogel exhibited the highest swelling ability. Unlike that of hydrogels with untreated and carboxymethylated CNFs, the storage modulus of PVP hydrogels after swelling significantly increased with an increase in the content of the quaternized CNF. These indicate that a PVP hydrogel with a high absorptivity and storage modulus can be prepared by incorporating the proper type of CNF.

Versatile Yolk-Shell Encapsulation: Catalytic, Photothermal, and Sensing Demonstration.

Here, a novel, versatile synthetic strategy to fabricate a yolk-shell structured material that can encapsulate virtually any functional noble metal or metal oxide nanocatalysts of any morphology in a free suspension fashion is reported. This strategy also enables encapsulation of more than one type of nanoparticle inside a single shell, including paramagnetic iron oxide used for magnetic separation. The mesoporous organosilica shell provides efficient mass transfer of small target molecules, while serving as a size exclusion barrier for larger interfering molecules. Major structural and functional advantages of this material design are demonstrated by performing three proof-of-concept applications. First, effective encapsulation of plasmonic gold nanospheres for localized photothermal heating and heat-driven reaction inside the shell is shown. Second, hydrogenation catalysis is demonstrated under spatial confinement driven by palladium nanocubes. Finally, the surface-enhanced Raman spectroscopic detection of model pollutant by gold nanorods is presented for highly sensitive environmental sensing with size exclusion.

Analysis of Spotty Deposits on Fine Paper and Investigation of Key Factors Affecting Alkyl Ketene Dimer Spot Formation.

Defect spots degrade paper quality and often cause sheet breaks and other problems in the subsequent coating process. We encountered sporadic surges of defect spots on paper from a paper machine producing printing and writing grades of paper. We found that white or light-colored filler spots and waxy, translucent, elliptical spots accounted for 27.3 and 34.4% of these defects, respectively, and that both types of defect contained unhydrolyzed AKD as a major component. Paper machine's wet-end deposits, however, did not contain unhydrolyzed AKD as a deposit component. We hypothesized that the colloidal stability of AKD was the main factor causing the increase in spot defects, and we verified this by deposit testing experiments. Among the papermaking additives tested, bentonite caused the most significant increase in AKD deposition during testing, which was attributed to its causing the electrostatic coagulation of cationically charged AKD emulsion particles. In contrast, anionically charged AKD emulsions showed improved stability and less deposit formation in the presence of bentonite.

Development and Application of Nanosized Polymer-Stabilized Cobinders and Their Effect on the Viscoelastic Properties and Foaming Tendencies of Coating Colors.

Polymer latexes have long been used as coating binders by various branches of industry due to their capacity to adhere coating components and increase the strength of the dried final coatings. In addition, these latexes have been known to affect the rheology of coating dispersions. Currently, emulsion polymerization is the most widely used method of producing polymer latexes. While the stability of these latexes is primarily provided by electrostatic repulsion between surfactants, this property also causes foaming problems during coating processes. In this research, these problems were addressed by preparing polymer-stabilized (PS) latexes that contained different concentrations of acrylic acid. Steric protection of the latexes was provided by a protective shell consisting of starch and poly(vinyl alcohol) (PVA). The viscosity, particle size, ζ-potential, and viscoelastic behavior of the prepared latexes were investigated as a function of pH, and their surface tension and foaming tendencies were evaluated. The latexes were applied as coating cobinders in calcium carbonate and clay coating dispersions, and the viscoelastic properties, surface tensions, and foaming tendencies of these mixtures were studied. The presence of acrylic acid monomers was found to be an important factor affecting the viscosity, particle size, and ζ-potential of the PS latexes prepared in this work, which were further found to generate less foam than comparable emulsion-polymerized latexes. Finally, coating color viscoelastic properties were modified via the partial substitution of styrene-butadiene (S/B) latexes with PS latexes.

UV/Vis Spectrometry-Based Analysis of Alkyl Ketene Dimer (AKD) Retention to Solve the Waxy Spot Problem in the Papermaking Process.

A sudden surge in the number of translucent and oval-shaped waxy spots caused a serious production loss of the papermaking process. The investigation of the spots revealed that the alkyl ketene dimer (AKD) sizing agent caused the waxy spot problem. A ultraviolet/visible (UV/vis) spectrometry method for the quantitative analysis of AKD was developed and used to reduce the waxy spot problem in paper products. The results showed that the method could be used to quantify AKD in both papermaking stock and white water. The major factors in the papermaking wet end that were associated with the waxy spot problem were evaluated, and practical approaches to solving the AKD retention problem and the waxy spot problem were proposed and implemented. The dosage of a retention aid was found to be the principal factor controlling AKD retention. However, varying the retention aid dosage resulted in the deterioration of the paper formation; therefore, this was not a suitable solution to the waxy spot problem. The type of fixing agent and AKD used was found to be the secondary factor affecting the AKD retention and papermaking system cleanliness. Mill trials were conducted on a paper machine to examine the effects of different fixing agents and AKD types on AKD retention and the waxy spot count at the reel. This approach identified a combination of fixing agent and AKD type that substantially improved AKD retention and reduced the formation of translucent waxy spots in the resulting paper products.

Structural Changes of the Coating Layer by Styrene/Acrylate Latex with Hydroxyethyl Methacrylate.

The coating structure is directly related to quality of coated paper, especially printability. Its structure mainly consists of pigment, binder, and additives. Binder has a significant influence on the coating structure because it is involved in the structure forming process and takes a key role in maintaining the structure. In this study, styrene acrylate (S/A) latex with hydroxyethyl methacrylate (HEMA) as a comonomer was used as a binder, and the influence of HEMA contents on latex swelling and coating structure was investigated. The S/A latex particle swelled more with an increasing HEMA content, which significantly influenced coating color and coating layer characteristics. The viscosity of coating color became thick, and the dewatering of coating color was suppressed. The HEMA monomer that swelled latex particle allowed the coating layer to consolidate rapidly, which made the coating layer more porous and more uniform.

Upconverting Oil-Laden Hollow Mesoporous Silica Microcapsules for Anti-Stokes-Based Biophotonic Applications.

A recyclable, aqueous phase functioning and biocompatible photon upconverting system is developed. Hollow mesoporous silica microcapsules (HMSMs) with ordered radial mesochannels were employed, for the first time, as vehicles for the post-encapsulation of oil phase triplet-triplet annihilation upconversion (TTA-UC), with the capability of homogeneous suspension in water. In-depth characterization of such upconverting oil-laden HMSMs (UC-HMSMs) showed that the mesoporous silica shells reversibly stabilized the encapsulated UC oil in water to allow efficient upconverted emission, even under aerated conditions. In addition, the UC-HMSMs were found to actively bind to the surface of human mesenchymal stem cells without significant cytotoxicity and displayed upconverted bright blue emission under 640 nm excitation, indicating a potential of our new TTA-UC system in biophotonic applications. These findings reveal the great promise of UC-HMSMs to serve as ideal vehicles not only for ultralow-power in vivo imaging but also for stem cell labeling, to facilitate the tracking of tumor cells in animal models.

Recycling of isopropanol for cost-effective, environmentally friendly production of carboxymethylated cellulose nanofibrils.

An approach to recycling isopropanol used in the carboxymethylation of pulp fiber was investigated as a cost-effective and environmentally friendly method of producing cellulose nanofibrils (CNF). Carboxymethylation of pulp fiber was carried out using isopropanol (IPA) as the sole solvent. IPA was recovered after carboxymethylation reaction and recycled in the next carboxymethylation reaction. Simple recycling of IPA decreased the reaction efficiency of carboxymethylation due to the increase of water content in the IPA. To dehydrate the recovered IPA, a 4 Å molecular sieve was used as a drying material. It was shown that dehydration restored carboxymethylation efficiency to the same level as when fresh IPA was used. The characteristics of the carboxymethylated CNFs produced using the recycled IPA were evaluated, including fibrillation tendency, average width, and width distribution, and it was shown that the use of recycled IPA after dehydration treatment did not cause any changes in carboxymethylated CNF properties. Recycling IPA after simple dehydration using a molecular sieve is thus a cost-effective and environmentally friendly method of producing carboxymethylated CNF.

Flexible and Micropatternable Triplet-Triplet Annihilation Upconversion Thin Films for Photonic Device Integration and Anticounterfeiting Applications.

Triplet-triplet annihilation upconversion (TTA-UC) has recently drawn widespread interest for its capacity to harvest low-energy photons and to broaden the absorption spectra of photonic devices, such as solar cells. Although conceptually promising, effective integration of TTA-UC materials into practical devices has been difficult due to the diffusive and anoxic conditions required in TTA-UC host media. Of the solid-state host materials investigated, rubbery polymers facilitate the highest TTA-UC efficiency. To date, however, their need for long-term oxygen protection has limited rubbery polymers to rigid film architectures that forfeit their intrinsic flexibility. This study introduces a new multilayer thin-film architecture, in which scalable solution processing techniques are employed to fabricate flexible, photostable, and efficient TTA-UC thin films containing layers of oxygen barrier and host polymers. This breakthrough material design marks a crucial advance toward TTA-UC integration within rigid and flexible devices alike. Moreover, it introduces new opportunities in unexplored applications such as anticounterfeiting. Soft lithography is incorporated into the film fabrication process to pattern TTA-UC host layers with a broad range of high-resolution microscale designs, and superimposing host layers with customized absorption, emission, and patterning ultimately produces proof-of-concept anticounterfeiting labels with advanced excitation-dependent photoluminescent security features.

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.