Functionalized nanocelluloses and their use in barrier and membrane thin films

Abstract Nanocellulose is envisioned as one of the key product innovations of future biorefineries, since it can potentially function in numerous high-end applications and replace many current petroleum-based products due to its superior properties, abundance and renewable nature. The main difficulty hindering the industrial upscaling of nanocellulose is the lack of feasible techniques for processing cellulose fibres on a nanoscale. At the same time, ongoing research efforts have concentrated on charting the suitability of nanocellulose for various novel applications. The chemical functionalization of cellulose is currently regarded as a significant step for both enhancing nanocellulose fabrication and increasing its value as a product by virtue of its adjustable surface properties. This thesis reports on the surface functionalization of cellulosic fibres by means of two new chemical pre-treatments based on periodate oxidation and sequential chlorite oxidation or reductive amination for use in the fabrication of nanocelluloses. The properties of the resulting nanocelluloses were characterized and their applicability to novel film structures was investigated. Both nanoporous thin films for composite membranes and self-standing barrier films were manufactured and studied for their suitability in water purification and packaging applications, respectively. The oxidation of cellulose to 2,3-dicarboxylic acid cellulose (DCC) significantly enhanced the nanofibril production as only 1-4 passes through the homogenizer were required for disintegration of the fibres down to nano-scale. The fabricated DCC-nanofibrils had both high optical transmittance and viscosity comparable to that of TEMPO-oxidized cellulose nanofibrils. DCC-nanofibrils with a carboxyl content of 1.75 mmol/g showed a potential for functioning as a nanoporous thin-film membrane layer in ultrafiltration tests. The second pre-treatment introduced an acid-free fabrication of amphiphilic cellulose nanocrystals (CNCs) with uniform width and length into nanocellulose production for the first time. Reaction conditions of periodate oxidation were presumed to be one of the key factors to impact the formation of either CNCs or cellulose nanofibrils. The butylamino-functionalized CNCs were used to fabricate barrier films that showed good mechanical strength and high resistance to permeation by oxygen even at elevated relative humidity.Tiivistelmä Yksi metsäteollisuuden viimeisimmistä tuoteinnovaatiosta on nanoselluloosa, jolle on esitetty lukuisia uusia sovellusmahdollisuuksia sekä potentiaalia toimia korvaavana raaka-aineena öljypohjaisille tuotteille sen erinomaisten materiaaliominaisuuksien sekä globaalin saatavuuden ja uusiutuvuuden takia. Nanoselluloosan teollista hyödyntämistä on kuitenkin hidastanut kustannustehokkaiden valmistusmenetelmien puuttuminen. Samanaikaisesti on tehty laaja-alaista tutkimustyötä nanoselluloosan soveltuvuudesta uusiin käyttökohteisiin. Selluloosan kemiallista funktionalisointia pidetään tällä hetkellä yhtenä lupaavimpana menetelmänä tehostamaan sekä nanoselluloosan valmistusta että tuomaan lisäarvoa nanokuiduille, joiden pintaominaisuuksia voidaan muokata. Tässä työssä tutkittiin selluloosakuitujen funktionalisointia perjodaattihapetukseen sekä kloriittihapetukseen tai pelkistävään aminointiin perustuen ja nanoselluloosan valmistusta esikäsitellystä selluloosasta. Työssä tutkittiin erityisesti valmistettujen nanoselluloosien ominaisuuksia ja selvitettiin niiden soveltuvuutta uudentyyppisiin filmirakenteisiin. Filmirakenteita muokkaamalla tehtiin nanohuokoisia komposiittimembraaneita vedenpuhdistukseen sekä barrier-filmejä pakkausmateriaaleihin. Selluloosan hapetus 2,3-dikarboksyylihapposelluloosaksi tehosti nanoselluloosan valmistusta huomattavasti ja kuidut saatiin hajotettua 1-4 läpäisyllä homogenisaattorissa. Valmistetut DCC-nanofibrillit olivat optisesti läpinäkyviä sekä niiden viskositeetti oli yhtä korkea kuin aiemmin raportoiduilla TEMPO-hapetettuilla nanofibrilleillä. Ultrasuodatuskokeissa DCC-nanofibrilleistä pystyttiin muodostamaan nanohuokoinen kerros membraaninpinnalle, jota on mahdollista käyttää vedenpuhdistuksessa. Pelkistävällä aminointiesikäsittelyllä selluloosakuiduista onnistuttiin ensimmäistä kertaa valmistamaan kooltaan yhdenmukaisia amfifiilisiä selluloosananokiteitä ilman yleisesti käytettyä happohydrolyysiä. Siten työssä nanoselluloosien valmistukseen käytetyn perjodaattihapetuksen havaittiin soveltuvan sekä selluloosananokiteiden että selluloosananofibrillien valmistukseen. Butyyliamino-funktionalisoiduista selluloosananokiteistä valmistetut barrier-filmit olivat mekaanisesti vahvoja ja ne ehkäisivät hapenläpäisyä jopa korkeassa ilmankosteudessa

Anionic wood nanofibers produced from unbleached mechanical pulp by highly efficient chemical modification

Abstract Chemical modification of lignocellulosic materials, especially as a pre-treatment in nanocellulose production, has mainly been conducted with lignin-free bleached cellulose pulps. However, non-bleached pulp exhibits several advantages over bleached pulp, namely excluding of the use of hazardous bleaching chemicals, higher yield, and lower cost. In this study, the chemical modification of lignocellulose (groundwood pulp, GWP) with a high lignin content (27.4 wt%) was investigated using a deep eutectic solvent (DES) as a reaction medium. A low-melting DES was easily obtained within one hour by mixing triethylmethylammonium chloride (TEMACl) and imidazole at room temperature. Carboxylated GWP was obtained by adding succinic anhydride to the DES. In mild reaction conditions (2 h at 70 °C), carboxylic acid contents of 1.88–3.34 mmol/g were obtained depending on the anhydride dosage used (5–20 mmol/1 g of pulp) with excellent yield (over 90%). The GWP was more reactive in the pre-treatment step, measuring carboxylic acid contents higher than those of bleached cellulose pulps treated in identical reaction conditions (containing less than 0.5 wt% lignin). After deprotonation of the carboxylic acid groups, highly anionic wood nanofibers (AWNFs) were produced using a microfluidizer. Vacuum filtration was applied in the preparation of self-standing films, which had good mechanical properties and were transparent. The fabricated AWNFs have many potential uses—for instance, in sustainable water purification because of their adjustable surface charge

Rapid preparation of all-cellulose composites by solvent welding based on the use of aqueous solvent

Abstract In the present study, the dissolution of softwood fibers and fabrication of all-cellulose composites was investigated using aqueous tetraethylammonium hydroxide solution (TEAOH, 35 wt%) at room temperature. The bulk of the cellulose fibers were instantly dissolved when mixed with the solvent, and suspensions up to 3 wt% were fabricated with ease. During the dissolution, a few remaining larger fiber flocks were fully dissolved within 3 h of mixing at room temperature. TEAOH was further studied in the production of all-cellulose composites by solvent welding (partial dissolution) of cellulose fiber sheets. Tensile strength of the original cellulose sheet increased from 3.3 to 55.0 MPa only after 30 s of room-temperature solvent welding, and an increase of over 23-fold (up to 76.7 MPa) was obtained by high-pressure-aided hot-pressing of the partially dissolved sheets before drying. The demonstrated fabrication of all-cellulose composites yielded many benefits in comparison to the current dissolution systems, such as short treatment time, ambient operation conditions, and readily applicable processing solutions, which makes it a potential methodology applicable even at an industrial scale

Lignin-rich sulfated wood nanofibers as high-performing adsorbents for the removal of lead and copper from water

Abstract Lignin-rich wood nanofibers (WNFs) were investigated as adsorbents for heavy metals. Lignin-free cellulose nanofibers (CNFs) produced from bleached cellulose fibers were used as a reference. Two raw materials were used to produce WNFs: groundwood pulp as industrially produced wood fibers and sawdust as an abundantly available low-value industrial side stream. WNFs and reference CNFs were produced using a reactive deep eutectic solvent to obtain nanofibers with abundant sulfate groups on their surfaces. With a similar amount of sulfate groups, WNFs had a higher adsorbent performance compared to CNFs and, at low metal concentrations (0.24 mmol/l), the removal of both metals was almost quantitate with WNFs. However, it was noted that, at pHs 4 and 5, the sodium present in the buffer solution interfered with the adsorption, leading to lower adsorption capacities compared to the capacity at pH 3. In addition, in the case of lead, the adsorption capacity dramatically decreased at a high metal concertation, indicating that a high lead concentration results in the saturation of adsorption sites of sulfated nanofibers, leading to a decreased adsorption capacity. Nevertheless, it was observed that WNFs had a higher tolerance to high metal concentrations than CNFs

Highly transparent nanocomposites based on poly(vinyl alcohol) and sulfated UV-absorbing wood nanofibers

Abstract Unbleached lignocellulose fibers were studied for the fabrication of wood-based UV-absorbing nanofibers and were used to produce transparent nanocomposites. Groundwood pulp (GWP) and sawdust were selected as raw materials thanks to their low processing degree of fibers and abundant availability as a low-value industrial side stream. Both materials were first sulfated using a reactive deep eutectic solvent. The sulfated wood and sawdust nanofibers (SWNFs and SSDNFs, respectively) were fabricated using a mild mechanical disintegration approach. As a reference material, sulfated cellulose nanofibers (SCNFs) were obtained from bleached cellulose pulp. Our results showed that both GWP and sawdust exhibited similar reactivity compared with bleached cellulose pulp, whereas the yields of sulfated lignin-containing pulps were notably higher. The diameters of both SWNFs and SSDNFs were approximately 3 nm, which was similar to those of the SCNFs. When 10 wt % of lignin-containing nanofibers were mixed together with poly(vinyl alcohol), the fabrication of nanocomposites with only a minimal decrease in transparency in the visible light spectrum was achieved. Transmission in the UV region, on the other hand, was significantly reduced by SWNFs and SSDNFs, whereas SCNFs had only a minor UV-absorbing property. Although the reinforcing effect of lignin-containing nanofibers was lower compared with that of SCNFs, it was comparable with those of other UV-absorbing additives reported in the literature. Overall, the wood-based UV-absorbing nanofibers could have a valuable use in optical applications such as lenses and optoelectronics

Synthesis of imidazolium-crosslinked chitosan aerogel and its prospect as a dye removing adsorbent

Abstract The potential utility of Debus–Radziszewski imidazole synthesis in the fabrication of crosslinked chitosan was studied. Three-component crosslinkingwas achieved by using glyoxal and propionaldehyde to connect amine groups of chitosan via imidazolium crosslinking. A water-insoluble (at pH range of 2–10) chitosan was obtained at room temperature with a degree of substitution of 0.45 and aerogel was obtained after freezedrying. The ability of the imidazolium-crosslinked chitosan (ICC) aerogel to absorb an anionic dye, Direct Yellow 27, from a model water was then studied. Based on the Langmuir isotherm, at a pH of 4, an adsorption maximum of 2340 mg g⁻¹ (3.5 mmol g⁻¹) was obtained. In addition, due to the permanent cationic charge of imidazolium group, ICC exhibited excellent adsorption capacity, even under alkaline conditions. Methylglyoxal and benzaldehyde were also used to obtain other types of ICC, demonstrating the versatility of Debus–Radziszewski imidazole synthesis for fabrication of modified chitosan

A fast method to prepare mechanically strong and water resistant lignocellulosic nanopapers

Abstract This study covers a green method to prepare hybrid lignocellulosic nanopapers by combining wood nanofibres (WNFs) and cellulose nanofibres (CNFs). The WNFs and CNFs behave synergistically to compensate for the drawbacks of each other resulting in enhanced hybrid nanopapers. The draining time of hybrid nanopapers was improved by up to 75% over CNF nanopaper, and the mechanical properties, modulus, strength and elongation, were respectively improved up to 35%, 90% and 180% over WNF nanopaper. Additionally, the water resistance of hybrid nanopapers was considerably improved with a water contact angle of 95°; the neat CNF nanopaper had a contact angle of 52°. The morphology of nanopapers, studied by electron microscopy, indicated that lignin acts as a matrix, which binds the nanofibres together and makes them impervious to external environmental factors, such as high humidity. The reported hybrid nanopapers are 100% bio-based, prepared by a simple and environmentally friendly processing route. Reported hybrid nanopapers can be used in novel applications such as gas barrier membranes and printable electronics

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

Abstract 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

Effect of plasticizers on the mechanical and thermomechanical properties of cellulose-based biocomposite films

Abstract Biocomposites based on natural cellulose fibers (CF) and hydroxyethyl cellulose (HEC), were produced in the form of green packaging films. The effect of the different single-component plasticizers (glycerol, propylene carbonate and ethylene carbonate) on the mechanical and dynamic thermomechanical properties of the films were studied. Moreover, the softening effect of the two-component plasticizer based on deep eutectic solvents (DESs) was addressed. Of the single-component plasticizers, glycerol was found to be the most efficient by increasing the elongation at break of the composite by 53%. A similar, or even better, increase in elongation at break (up to 81%) was obtained with DESs based on choline chloride and glycerol, glucose or urea. Based on the dynamic mechanical analysis at varying humidity, the performance of plasticizers was strongly attributed to the humidity. The DES based on tetrabutylammonium bromide and propylene carbonate was most efficient at providing thermoformability to the composite by lowering the thermal softening temperature. Based on the obtained results, DESs are a highly promising plasticizers for the cellulose-based biocomposites with similar or even better plasticizing effect compared to conventional plasticizer. In addition, DESs can be used to improve the thermoformability of biocomposites, by lowering the thermal softening temperature