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The main objective of this study was to examine the impact of cellulose nanocrystals (CNCs) in advanced waterborne wood coatings such as polycarbonate urethane (PCU) and hybrid alkyd varnish (HAV) in terms of coating performance, mechanical properties, optical properties, and water permeation and uptake properties. The influence of CNCs on the overall quality of the various waterborne wood coatings was investigated by incorporating different percentages of CNCs. Varying CNC content in coating formulations showed that CNCs are effective for waterborne wood coatings; CNCs offer both higher scratch and impact resistance as compared to neat coatings and have a significant reduction in water vapor permeation through a film with little increase in water vapor uptake at high concentrations. It was observed that the CNC darkened and reduced gloss in the coatings and viscosified the dispersion. These research findings suggest that CNCs are well-dispersed at lower concentrations, but at high concentrations, agglomeration occurred. Thus, while CNCs can give better mechanical and permeation performances at contents of up to 5 wt %, at 1 wt % CNCs can still provide modest scratch and chip resistance improvement without loss of optical properties (gloss and color) while retaining a similar water uptake. Overall, it can be concluded that CNCs have the potential to be used as a reinforcement filler in high-performance waterborne wood coatings.

In nature, cellulose nanofibers form hierarchical structures across multiple length scales to achieve high-performance properties and different functionalities. Cellulose nanofibers, which are separated from plants or synthesized biologically, are being extensively investigated and processed into different materials owing to their good properties. The alignment of cellulose nanofibers is reported to significantly influence the performance of cellulose nanofiber-based materials. The alignment of cellulose nanofibers can bridge the nanoscale and macroscale, bringing enhanced nanoscale properties to high-performance macroscale materials. However, compared with extensive reviews on the alignment of cellulose nanocrystals, reviews focusing on cellulose nanofibers are seldom reported, possibly because of the challenge of aligning cellulose nanofibers. In this review, the alignment of cellulose nanofibers, including cellulose nanofibrils and bacterial cellulose, is extensively discussed from different aspects of the driving force, evaluation, strategies, properties, and applications. Future perspectives on challenges and opportunities in cellulose nanofiber alignment are also briefly highlighted.

Cellulose nanocrystal (CNC) based composites have been explored as protective organic coatings for metal surfaces, ceramics, and wood. However, the inherent hygroscopic nature of CNCs hinders the t...

Mechanically fibrillated cellulose nanofibril (CNF) sheets of varying thicknesses were fabricated by using a wet stacking lamination technique without the use of solvents other than water or binders. The use of this technique allowed for the creation of multilayer structures with a working area of 117 mm by 117 mm and thickness of up to 0.547 ± 0.03 mm in under 2 h, which represents the shortest total processing time reported for such thickness of CNF sheets. To highlight the capabilities of utilizing wet stacking, the thickest reported 100% pure multilayer CNF sheet with a thickness of 1.65 ± 0.02 mm was produced. To gauge the effect of processing parameters on the mechanical performance of the produced sheets, thickness (85–547 μm thick), pressing time (35 min, 1 h, and 2 h), pressing pressure (0–5.17 MPa), and loading rate (4 min, 2 min, and 20 s) were varied. Tensile testing results show that the ultimate strength increased as the thickness increased and subsequently reached a plateau at a value of 20...

Nanocellulose has potential as a reinforcing agent to improve stiffness and strength in polymer fiber; however, the inherent difference in hydrophilicity makes it challenging to incorporate it into...

Cellulose nanomaterials are promising materials for the polymer industry due to their abundance and renewability. In packaging applications, these materials may impart enhanced gas barrier performance due to their high crystallinity and polarity. In this work, low barrier to superior gas barrier pristine nanocellulose films were produced using a shear-coating technique to obtain a range of anisotropic films. Induction of anisotropy in a nanocellulose film can control the overall free volume of the system which effectively controls the gas diffusion path; hence, controlled anisotropy results in tunable barrier properties of the nanocellulose films. The highest anisotropy materials showed a maximum of 900-fold oxygen barrier improvement compared to the isotropic arrangement of nanocellulose film. The Bharadwaj model of nanocomposite permeability was modified for pure nanoparticles, and the CNC data were fitted with good agreement. Overall, the oxygen barrier performance of anisotropic nanocellulose films was 97 and 27 times better than traditional barrier materials such as biaxially oriented poly(ethylene terephthalate) (BoPET) and ethylene vinyl alcohol copolymer (EVOH), respectively, and thus could be utilized for oxygen-sensitive packaging applications.

Cellulose nanocrystal (CNC) composite coatings may impart many benefits in packaging, electronic, optical, etc. applications, however, large-scale cellulose coating production is a major engineering challenge. A versatile roll-to-roll reverse gravure process for the manufacture of cellulose nanocrystal-poly(vinyl alcohol) (CNC–PVA) coatings on a flexible polymer substrate was investigated in the present work. CNC content was varied from 0 to 100% to determine the effect of CNC concentration on coating anisotropy. Coatings were characterized by polarized light microscopy, UV–Vis spectrophotometry, cross-hatch adhesion testing and optical profilometry. This method produced uniform, highly transparent coatings with surface roughness less than 100 nm for all CNC–PVA weight ratios examined. The isotropic-anisotropic coating transformation was observed above 50% CNC, with a maximum anisotropy at 70% CNC along the shear direction. Anisotropic CNC–PVA coatings exhibited increased water vapor barrier performance due to the increased CNC packing and density.

Cellulose nanocrystals (CNCs) can impart desirable barrier properties in film packaging applications; however, commercial production of these materials is inhibited by the absence of a large-scale manufacturing process for producing CNC coatings. To fill this knowledge gap, a potential large-scale manufacturing process, roll-to-roll gravure printing, for cellulose nanocrystal coating on a flexible polyethylene terephthalate substrate has been described in this work. Processing parameters which control the coating structure and properties were examined. For a given gravure roll, gravure speed, substrate speed, and ink viscosity were determined to be the most important parameters that control the liquid transfer from the ink bath to the substrate, which determined the coating thickness (2–6 µm). After successful fabrication, CNC coating adhesion was investigated with a crosshatch adhesion test. The adhesive strength of the CNC coating was correlated with coating thickness, and the maximum coating strength was observed for the lowest coating thickness. Coatings were characterized using atomic force microscopy and UV–Vis spectroscopy. Finally, the crystalline domain arrangement of coatings was determined for coatings made from three different CNC concentrations, and the effect of viscosity on CNC alignment was explained by variation of shear rate, which was controlled by the micro-gravure rotation.

Cellulose nanomaterials (CNMs) are a class of materials that have recently garnered attention in fields as varied as structural materials, biomaterials, rheology modifiers, construction, paper enhancement, and others. As the principal structural reinforcement of biomass giving wood its mechanical properties, CNM is strong and stiff, but also nontoxic, biodegradable, and sustainable with a very large (Gton yr-1 ) source. Unfortunately, due to the relatively young nature of the field and inherent incompatibility of CNM with most man-made materials in use today, research has tended to be more basic-science oriented rather than commercially applicable, so there are few CNM-enabled products on the market today. Herein, efforts are presented for preparing and forming cellulose nanomaterial nanocomposites. The focus is on recent efforts attempting to mitigate common impediments to practical commercialization but is also placed in context with traditional efforts. The work is presented in terms of the progress made, and still to be made, on solving the most pressing challenges-getting properties that are competitive with currently used materials, removing organic solvent, solving the inherent incompatibility between CNM and polymers of interest, and incorporation into commonly used industrial processing techniques.

When diethanolamine (DEA) is used as a curative for a DGEBA epoxy, a rapid “adduct-forming” reaction of epoxide with the secondary amine of DEA is followed by a slow “gelation” reaction of epoxide with hydroxyl and with other epoxide. Through an extensive review of previous investigations of simpler, but chemically similar, reactions, it is deduced that at low temperature the DGEBA/DEA gelation reaction is “activated” (shows a pronounced induction time, similar to autocatalytic behavior) by the tertiary amine in the adduct. At high temperature, the activated nature of the reaction disappears. The impact of this mechanism change on the kinetics of the gelation reaction, as resolved with differential scanning calorimetry, infrared spectroscopy, and isothermal microcalorimetry, is presented. It is shown that the kinetic characteristics of the gelation-reaction of the DGEBA/DEA system are similar to other tertiary-amine activated epoxy reactions and consistent with the anionic polymerization model previously proposed for this class of materials. Principle results are the time-temperature-transformation diagram, the effective activation energy, and the upper stability temperature of the zwitterion initiator of the activated gelation reaction. It is established that the rate of epoxide consumption cannot be generically represented as a function only of temperature and degree of epoxy conversion. The complex chemistry active in the material requires specific consideration of the dilute intermediates in the reaction sequence in order to define a model of the reaction kinetics applicable to all time-temperature cure histories.

A description of enthalpy recovery for an amine-cured, digylcidyl ether of bisphenol A (DGEBA) epoxy (Epon 828/Jeffamine T403) was acquired through a series of experiments targeting the effects of the individual parameters: cooling rate, heating rate, aging temperature, and aging time. From these experiments, the peak temperature, T p , and fictive temperature, T f , were tracked as measures of the extent of structural relaxation. The KAHR formalism provides a standard for comparison to similar epoxies and was parametrized using the peak shift method. The resulting parameterization was then used to assess the relation between volumetric and shear relaxation. In particular, the KAHR model was used to predict volumetric relaxation time-temperature shift factors that are directly compared to measured time-temperature shift factors in shear. This unique methodology to assess equivalence between volumetric and shear relaxation shift factors further establishes the basis for this key assumption of continuum constitutive models based on rheological simplicity.

In the present study, very small concentrations of CNCs and CNFs were melt-compounded into PLA using a solvent-free process where poly(ethylene glycol) (PEG) was used to disperse the nanoparticles in PLA. As a comparison, the commercial nucleant, talc, was processed similarly into PLA. CNC and CNF were shown to be efficient nucleants for PLA, similar to talc, and conventional Avrami and Lauritzen-Hoffman analysis of the crystallization behavior was performed for isothermal temperatures from 90 °C to 130 °C across all compositions. From the Avrami analysis, the crystallization rate, half-time, and Avrami exponent were calculated and suggested a synergistic effect of nanocellulose and PEG, even at very small concentrations. The crystallization half-time was lower than talc at higher temperatures indicating faster crystallization for samples containing nanocellulose under certain conditions. Analysis of secondary nucleation revealed a decrease in the surface energy for CNC containing samples, further suggesting that, given the enhanced mobility of plasticized PLA, very small concentrations of CNC are effective nucleation agents. Lastly, scanning electron microscopy was used to correlate the crystal morphology of chemically etched samples with the thermal analysis. Coarsening of the microstructure was observed initially with the addition of PEG, and further coarsening was observed upon the addition of nanocellulose.