Hydrophobic modification of nanocellulose and all-cellulose composite films using deep eutectic solvent as a reaction medium

Cellulose - In this work, deep eutectic solvent (DES) based on imidazole and triethylmethylammonium chloride was used as a reaction medium for the esterification of cellulose nanofiber (CNF) and...     

Disintegration of periodate–chlorite oxidized hardwood pulp fibres to cellulose microfibrils: kinetics and charge threshold

Periodate–chlorite oxidized bleached hardwood kraft pulp fibre samples with six levels of charge densities ranging from 0.5 to 1.8 mmol/g were gradually disintegrated to microfibrils using a high-shear homogenizer. The disintegration kinetics and mechanisms were studied by a flow fractionation method, and the properties of the resulting particles were determined using low shear viscosity and transmittance measurements. The particles formed during the disintegration were visualized with a charge-coupled device camera and by field-emission scanning electron microscopy. The result showed that cellulose fibres with a low charge density disintegrated at a low rate and produced ragged fibres and bunches of microfibrils via bursting of the fibre walls, whereas those with a higher charge density broke down at a high rate and microfibrils were formed through swelling and the creation of balloon structures. A carboxyl content of 1.2 mmol/g was found to be the threshold value for the efficient formation of high aspect ratio microfibrils and also for the change in the disintegration mechanism in the high-shear homogenizer.     

Fragment analysis of different size-reduced lignocellulosic pulps by hydrodynamic fractionation

Micro- and nanocelluloses are typically produced using intensive mechanical treatments such as grinding, milling or refining followed by high-pressure homogenization to liberate individual nano- and microcellulose fragments. Even though chemical and enzymatic pretreatments can be used to promote fiber disintegration, the required mechanical treatments are still highly energy consuming and very costly. Therefore, it is important to understand the kinetics and factors affecting the disintegration tendency of cellulose. In this study, the disintegration tendency of three different wood cellulose pulps with varying chemical composition processed in a PFI mill was examined by analyzing the fractional composition of the microparticles formed. The fractional compositions of the microfibrils and microparticles formed were measured with novel analyzers, which fractionated particles using a continuous water flow in a long tube. The hydrodynamic fractionators used in this study gave valuable information about different size of particles. Results showed that the amount of lignin and hemicelluloses clearly affected the kinetics and the mechanics of cellulose degradation. The P and S1 layers were peeled off from the Kraft fibers, causing the S2 layer to be cropped out. The thermomechanical pulp (TMP) fibers were first degraded by comminution and delamination from the middle lamella and the primary wall. As the refining process progressed, the fibers and fiber fragments began to unravel. Surprisingly, the semi-chemical pulp (SCP) fibers degraded more like Kraft fibers than TMP fibers despite their high lignin and extractive content.     

Sulfonated cellulose nanofibrils obtained from wood pulp through regioselective oxidative bisulfite pre-treatment

The consecutive pre-treatment of cellulose with periodate and bisulfite was used as a new potential method to promote nanofibrillation of hardwood pulp and to obtain nanofibrils with sulfonated functionality. Nanofibrils having typical widths of 10–60 nm were obtained from sulfonated celluloses having low anionic charge densities (0.18–0.51 mmol/g) by direct high-pressure homogenization without the use of any mechanical pre-treatments. The aqueous nanofibrils existed as highly viscous and transparent gels and possessed cellulose I crystalline structures with crystallinity indexes of approximately 40 %. A transparent film was obtained from sulfonated nanofibrils having tensile strength of 164 ± 4 MPa and Young’s modulus of 13.5 ± 0.4 MPa. Oxidative sulfonation was shown to be a potential green method to promote nanofibrillation of cellulose, as it avoids the production of halogenated wastes, because the periodate used can be efficiently regenerated and recycled as shown in the preliminary experiments.     

The role of hornification in the disintegration behaviour of TEMPO-oxidized bleached hardwood fibres in a high-shear homogenizer

The effect of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation on the structure of hornified fibres and their disintegration behaviour was studied by a method combining gradual disintegration of the fibre structure in an in-line homogenizer with a chromatographic separation technique. It was seen that hornification prior to TEMPO-mediated oxidation had a notable effect on the disintegration behaviour of bleached cellulose fibres in a high-shear homogenizer and on the properties of the resulting particles. Field-emission scanning electron microscopy imaging of the suspensions and viscosity and transmittance measurements revealed that never-dried oxidized fibres disintegrated into bunches of microfibrils and at higher charge densities into thinner and more individual microfibrils. These microfibrils were obtained from fibres through swelling and ballooning. The hornified fibres were mainly cut into shorter ones as the charge density increased. After reversing the hornification and allowing the fibres to swell further, however, microfibrils were also obtained from this source. The charge threshold for efficient microfibril production from never-dried fibres in the high-shear homogenizer used here was 0.7 mmol/g.     

Mechanical fabrication of high-strength and redispersible wood nanofibers from unbleached groundwood pulp

In the past, the direct production of lignin-containing nanofibers from wood materials has been very limited, and nanoscale fibers (nanocelluloses) have been mainly isolated from chemically delignified, bleached cellulose pulp. In this study, we have introduced a newly adapted, heat-intensified disc nanogrinding process for the enhanced nanofibrillation of wood nanofibers (WNF) with a high lignin content (27.4 wt%). The WNF produced this way have many unique and intriguing properties in their naturally occurring form, for example, being able to be dispersed in ethanol and having ethanol solution viscosities higher than water solution viscosities. When WNF nanopapers were formed with ethanol, the properties of the nanofibers were recoverable without a notable decrease in the viscosity or mechanical strength after redispersing them in water. The preservation of lignin in the WNF was noticed as an increase in the water contact angles (89°), the rapid removal of water in the fabrication of the nanopapers, and the enhanced strength of the nanopapers when subjected to high pressure and heat. The nanopapers fabricated from the WNF were mechanically stable, having an elastic modulus of 6.2 GPa, a maximum stress of 103.4 MPa, and a maximum strain of 3.5%. Throughout the study, characteristics of the WNF were compared to those of the delignified and bleached reference cellulose nanofibers. We envision that the exciting characteristics of the WNF and their lower cost of production compared to that of bleached cellulose nanofibers may offer new opportunities for nanocellulose and biocomposite research.     

Cationic wood cellulose films with high strength and bacterial anti-adhesive properties

In this work, periodate oxidized birch wood pulp and microfibrillated cellulose (MFC) were cationized using Girard’s reagent T or aminoguanidine. Cationic celluloses were used to obtain films via solvent-casting method, and the effects of the cationization route and the cellulose fiber source on the properties of the films were studied. Thermal and optical properties of the films were measured using differential scanning calorimetry and UV–Vis spectrometry, and the morphology of the films was examined using an optical microscope and a field emission scanning electron microscope. Bacterial anti-adhesive properties of the films were also studied using a modified leaf print method and against Staphylococcus aureus and Escherichia coli. Both cationizing agents exhibited similar reactivity with periodate oxidized celluloses, however, MFC had significantly higher reactivity compared to birch pulp. The films with high tensile strength (39.1–45.3 MPa) and modulus (3.5–7.3 GPa) were obtained from cationized birch pulp, aminoguanidine modification producing a film with slightly better mechanical properties. Modulus of the films was significantly increased (up to 14.0 GPa) when MFC was used as a cellulose fiber source. Compared to the unmodified MFC films, the cationic MFC films were less porous and significantly more transparent; however, they had slightly lower tensile strength values. It was found that aminoguanidine modified celluloses had no culturable bacteria on its surface and also exhibited resistance to microbial degradation, whereas there were culturable bacteria on the surface of Girard’s reagent modified films and they were partially degraded by the bacteria.     

Effect of tempo and periodate-chlorite oxidized nanofibrils on ground calcium carbonate flocculation and retention in sheet forming and on the physical properties of sheets

Nanofibrils (NFC) or microfibrils (MFC) are potential candidates for high filler-loaded papers and board as they are able to compensate for strength loss caused by the filler itself. However, the interaction of nanofibrils and the filler during sheet forming is not yet well understood. The aim here was to examine 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and periodate-chlorite oxidized (DCC) anionic nanofibrils during sheet forming in order to determine their effects on flocculation, filler retention and the strength and optical properties of the handsheets. The experiments were carried out by manufacturing filler-loaded sheets from refined kraft fibres and ground calcium carbonate (GCC) with various added levels of TEMPO and DCC nanofibrils. The results showed that both types of nanofibril caused pronounced agglomeration of the GCC filler, which increased its retention in the paper web. Given the same filler content, the strength properties were the same or slightly better than in a sheet formed without any chemical agent, while light scattering was slightly inferior. Poorer formation seemed to be the explanation for why the increased bonding induced by NFCs was not reflected in obviously better sheet strengths. The physical properties of sheets containing NFC were superior to those of sheets formed with cationic polyacrylamide as a retention aid with the same filler content and level of formation. Thus NFCs seem to be potential retention aids for use in fine paper production instead of traditional polymers.     

Castor oil-based biopolyurethane reinforced with wood microfibers derived from mechanical pulp

Wood fibers with high lignin content show promise to function in numerous applications with advantageous properties if the fiber features are appropriately exploited. The present study introduces a new approach to disintegrate and disperse wood fibers from groundwood pulp (GWP) directly to polyol without additional solvent exchanges or chemical modifications. In comparison bleached chemical pulp with low lignin content was ground in the polyol, but only low consistency (1 wt%) operation was possible, whereas up to 5 wt% consistency with GWP was carried out with ease. The micron sized fibers in polyol were reacted with polymeric diphenylmethane diisocyanate to produce fiber reinforced biopolyurethane (bioPU) composites. The mechanical properties of the composites improved compared to reference bioPU showing 14.6% increase in Young’s modulus, 54.5% in tensile strength and 26.1% in strain at break. The tan δ peaks shifted to higher temperature from 5.5 to 10.4 °C when fibers up to 5.1 wt% were incorporated to bioPU. Overall, the bulk microfibers from GWP with low degree of processing were cost-effective reinforcements for bioPUs, which improved the qualities of the fabricated composites and showed good compatibility with polyurethane.