Properties of cellulose films prepared from NaOH/urea/zincate aqueous solution at low temperature

Cellulose films were successfully prepared from NaOH/urea/zincate aqueous solution pre-cooled to −13 °C by coagulating with 5% H₂SO₄. The cellulose solution and regenerated cellulose films were characterized with dynamic rheology, ultraviolet–visible spectroscope, scanning electron microscopy, wide angle X-ray diffraction, Fourier transform infrared (FT-IR) spectrometer, thermogravimetry and tensile testing. The results indicated that at higher temperature (above 65 °C) or lower temperature (below −10 °C) or for longer storage time, gels could form in the cellulose dope. However, the cellulose solution remained a liquid state for a long time at 0–10 °C. Moreover, there was an irreversible gelation in the cellulose solution system. The films with cellulose II exhibited better optical transmittance, high thermal stability and tensile strength than that prepared by NaOH/urea aqueous solution without zincate. Therefore, the addition of zincate in the NaOH/urea aqueous system could enhance the cellulose solubility and improve the structure and properties of the regenerated cellulose films.     

The study of regenerated cellulose films toughened with thermoplastic polyurethane elastomers

Regenerated cellulose blend film with thermoplastic polyurethane (TPU) was successfully prepared by coagulating cellulose/TPU solution with water in the presence of a thermoplastic polyurethane elastomer (TPU). Compared with pristine regenerated cellulose film, the toughness and thermal stability of the blend film was significantly improved. For example, the elongation at break was increased from 11% of pristine cellulose film to 51% of blend film with 20 wt. % TPU. The 50% weight loss temperature of this blend film was increased by 33 °C compared to neat cellulose. The relaxation transition temperature of cellulose was decreased with the addition of TPU through dynamic mechanical thermal analysis. The oxygen permeability was decreased from 2.3 × 10⁻¹⁰ cm³ cm/cm² s Pa of pristine cellulose film to 0.08 × 10⁻¹⁰ cm³ cm/cm² s Pa of the blend film with 20 wt.%. TPU The X-ray diffraction spectra showed that the crystallinity of cellulose decreased with incorporation of TPU. The images of scanning electron microscope discovered that there was good compatibility between cellulose and TPU. TPU was nano-dispersed in cellulose matrix. The blend film still maintained quite good transparency.     

The dissolution of cellulose in NaOH-based aqueous system by two-step process

A new dissolution method, a two-step process, for cellulose in NaOH/urea aqueous system was investigated with ¹³C NMR, wide X-ray diffraction (WXRD), and solubility test. The two steps were as follows: (1) formation and swelling of a cellulose–NaOH complex and (2) dissolution of the cellulose–NaOH complex in aqueous urea solution. The dissolution mechanism could be described as strong interaction between cellulose and NaOH occurring in the aqueous system to disrupt the chain packing of original cellulose through the formation of new hydrogen bonds between cellulose and NaOH hydrates, and surrounding the cellulose–NaOH complex with urea hydrates to reduce the aggregation of the cellulose molecules. This leads to the improvement in solubility of the polymer and stability of the cellulose solutions. By using this two-step process, cellulose can be dissolved at 0–5 °C in contrast to the known process that requires −12 °C. Regenerated cellulose (RC) films with good mechanical properties and excellent optical transmittance were prepared successfully from the cellulose solution.     

Effects of temperature and molecular weight on dissolution of cellulose in NaOH/urea aqueous solution

Dissolution of cellulose having different viscosity-average molecular weight (M _η ) in 7 wt%NaOH/12 wt%urea aqueous solution at temperature from 60 to −12.6°C was investigated with optical microscope, viscosity measurements and wide X-ray diffraction (WXRD). The solubility (S_a) of cellulose in NaOH/urea aqueous solution strongly depended on the temperature, and molecular weight. Their S_a values increased with a decrease in temperature, and cellulose having M _η below 10.0 × 10⁴ could be dissolved completely in NaOH/urea aqueous solution pre-cooled to −12.6°C. The activation energy of dissolution (E_a,s) of the cellulose dissolution was a negative value, suggesting that the cellulose solution state had lower enthalpy than the solid cellulose. The cellulose concentration in this system increased with a decrease of M _η to achieve about 8 wt% for M _η of 3.1 × 10⁴. Moreover, cellulose having 12.7 × 10⁴ could be dissolved completely in the solvent pre-cooled to −12.6°C as its crystallinity (χ _c) decreased from 0.62 to 0.53. We could improve the solubility of cellulose in NaOH/urea aqueous system by changing M _η , χ _c and temperature. In addition, the zero-shear viscosity (η ₀ ) at 0°C for the 4 wt% cellulose solution increased rapidly with an increase of M _η , as a result of the enhancement of the aggregation and entanglement for the relatively long chains.     

Microfiltration performance of regenerated cellulose membrane prepared at low temperature for wastewater treatment

A series of regenerated cellulose membranes with pore diameters ranging from 21 to 52 nm have been prepared by dissolving cellulose in 5 wt% LiOH/12 wt% urea aqueous solution re-cooled to −12 °C. The influences of cellulose concentration on the structure, pore size, and the mechanical properties of the membrane were studied by using Wide angle X-ray diffraction, scanning electron micrography and tensile testing. Their pore size, water permeability, equilibrium-swelling ratio and fouling behaviors of the cellulose membranes were characterized. The water-soluble synthetic and natural polymers as organic matter were used to evaluate the microfiltration performance of the regenerated cellulose membrane for wastewater treatment in aqueous system. The results revealed that the organic matter with molecular weight more than 20 kDa effected significantly on the membrane pore density, and reducing factor a _2, whereas that having molecular weight less than 20 kDa exhibited a little influence on the membrane pore size reducing factor a ₁. Furthermore, a simple model to illustrate of microfiltration process of the RC membrane for wastewater treatment was proposed.     

Influence of coagulation temperature on pore size and properties of cellulose membranes prepared from NaOH–urea aqueous solution

The morphology and structure of the regenerated cellulose membranes prepared from its NaOH–urea aqueous solution by coagulating with 5 wt% H₂SO₄–10 wt% Na₂SO₄ aqueous solution with different temperatures and times were investigated. The pore size, water permeability and physical properties of the membranes were measured with scanning electron micrograph (SEM), wide X-ray diffraction (WXRD), Fourier transfer infrared spectroscopy (FTIR), flow rate method, and tensile testing. The SEM observation revealed that the structure and pore size of the membranes changed drastically as a function of the coagulation temperature. The membranes coagulated at lower temperatures tended to form the relatively small pore size than those at higher temperatures. On the contrary, the membranes coagulated at different times exhibited similar pore size. Interestingly, the mean pore size and water permeability of the membranes increased from 110 nm with standard deviation (SD) of 25 nm and 12 ml h⁻¹ m⁻² mmHg⁻¹ respectively to 1,230 nm with SD of 180 nm and 43 ml h⁻¹ m⁻² mmHg⁻¹ with an increase in coagulation temperature from 10 to 60°C. However, the membranes regenerated below 20°C exhibited the dense structure as well as good tensile strength and elongation at break. The result from FTIR and ultraviolet-visible (UV-vis) spectroscopy indicated that the relatively strong intermolecular hydrogen bonds exist in the cellulose membranes prepared at lower coagulation temperatures. This work provided a promising way to prepare cellulose materials with different pore sizes and physical properties by controlling the coagulation temperature.     

Nonalkali swelling solutions for regenerated cellulose

Swelling of regenerated cellulose in nonalkali aqueous solutions containing lithium chloride and urea (LiCl/urea/water) was examined. The effect of solution concentration on fiber properties was studied using microscopy, weight gain (swelling), and mechanical strength tests. The regenerated cellulose samples included lyocell fibers, viscose fibers, and fibers spun from alkali. The change in the mechanical properties of treated fibers was smaller than that of fibers treated with alkali to the same level of swelling. The degree of swelling in these solutions was related to the propensity for the formation of Li–cellulose coordination complexes, and these were enhanced by reductions in both urea and water content.     

Concentration enrichment of urea at cellulose surfaces: results from molecular dynamics simulations and NMR spectroscopy

A combined solid-state NMR and Molecular Dynamics simulation study of cellulose in urea aqueous solution and in pure water was conducted. It was found that the local concentration of urea is significantly enhanced at the cellulose/solution interface. There, urea molecules interact directly with the cellulose through both hydrogen bonds and favorable dispersion interactions, which seem to be the driving force behind the aggregation. The CP/MAS ¹³C spectra was affected by the presence of urea at high concentrations, most notably the signal at 83.4 ppm, which has previously been assigned to C4 atoms in cellulose chains located at surfaces parallel to the (110) crystallographic plane of the cellulose Iβ crystal. Also dynamic properties of the cellulose surfaces, probed by spin-lattice relaxation time ¹³CT ₁ measurements of C4 atoms, are affected by the addition of urea. Molecular Dynamics simulations reproduce the trends of the T ₁ measurements and lends new support to the assignment of signals from individual surfaces. That urea in solution is interacting directly with cellulose may have implications on our understanding of the mechanisms behind cellulose dissolution in alkali/urea aqueous solutions.     

Anomalous reinforcing effects in cellulose gel-based polymeric nanocomposites

Cellulose-synthetic polymer nanocomposite films were prepared by immersion of cellulose gel in polymer solutions followed by dry casting. The cellulose hydrogel was prepared from aqueous alkali-urea solution. As the synthetic polymer, polystyrene (PS) and poly(methyl methacrylate) (PMMA) were used. The polymer content could be changed between 10 and 80% by changing polymer concentration of immersing solution. While the mechanical properties of the cellulose-PMMA composite films showed a nearly linear dependence on PMMA content, those of cellulose-PS composites showed an anomalous behavior; both tensile strength and Young’s modulus showed prominent maxima at 15–30 wt% PS contents. This anomaly may have resulted from the specific interaction between the aromatic ring of PS and the hydrophobic plane of the glucopyranoside. Both PMMA and PS composite films showed significant improvements in dimensional thermal stability; up to 25 wt% synthetic polymer content, the coefficient of thermal expansion (CTE) was as low as ca. 30 ppm/K, about 1/3 of the pure polymers. This indicates that the regenerated cellulose network is effective in suppressing thermal expansion of the synthetic polymers.     

Effects of polymer concentration and coagulation temperature on the properties of regenerated cellulose films prepared from LiOH/urea solution

Aqueous 5 wt% LiOH/12 wt% urea solution pre-cooled to −12 °C has a more powerful ability to dissolve cellulose compared to that of NaOH/urea and NaOH/thiourea solution system. The influences of the cellulose concentration and coagulation temperature on the structure, pore size and mechanical properties of the cellulose films prepared from LiOH/urea system were investigated. The cellulose films exhibited good mechanical properties either at wet or dry state and their pore size and water permeability at wet state can be controlled by changing the cellulose concentration or coagulation temperature. With a decrease of the coagulation temperature, the mechanical properties and optical transmittance of the cellulose films enhanced, as a result of the formation of relative smaller pore size and denser structures. This work provided a promising way to prepare cellulose films with different pore sizes at wet state and good physical properties at dry state.     

Sorption of dyes on cellulose II: effect of alkali treatment of fibre and dye structure

Cellulose is a linear 1,4-β-glucan polymer where the units are able to form highly ordered structures, as a result of extensive interaction through intra- and intermolecular hydrogen bonding of the three hydroxyl groups in each cellulose unit. Alkali has a substantial influence on morphological, molecular and supramolecular properties of cellulose II polymer fibres causing changes in crystallinity. Lyocell fibres pre-treated with 0.0, 2.0, and 4.0 mol dm⁻³ aqueous NaOH solution were dyed with hydrolyzed reactive dyes that had different molecular shapes and sizes. Overall exhaustion (q _e), value of K, and −ΔG increased for lyocell samples pre-treated with aqueous NaOH solution in the following order: 2.0 > 4.0 > 0.0 mol dm⁻³ NaOH. The same trends were observed for colour strength (K/S) values of the dyeings. Pre-treatment of lyocell with 2.0 mol dm⁻³ NaOH creates the substrate that achieves the most thermodynamically favourable system for sorption of hydrolyzed reactive dyes, as at this concentration crystallinity decreases (with respect to 0.0 mol dm⁻³ NaOH treated lyocell) to afford higher sorption; however, at higher alkali concentrations the macro-sorbent forms a compacted unit that limits diffusion within the sorbent interior. Molecular size of the sorbate dye has a significant effect on the sorption process: for the largest dye structure the sorption isotherm is most closely correlated to a Langmuir isotherm; as the size of the dye decreases correlation to a Langmuir isotherm is observed, but with good correlation to the Freundlich isotherm; as the size of the dye is decreased further sorption is more typical of a Freundlich isotherm.     

Dissolving of cellulose in PEG/NaOH aqueous solution

Here, a new solvent system for cellulose is reported. The solvent is a mixed aqueous solution of 1.0 wt.% poly(ethylene glycol) (PEG) and 9.0 wt.% of NaOH. Cellulose powder was added into the mixture at room temperature at first, and freezing it at −15 °C for 12 h following a thaw of the mixture at room temperature under strong stirring. There formed a clean solution of cellulose, and the optical microscopy was used to record the dissolving process. ¹³C-NMR, FT-IR, XRD, and intrinsic viscosity measurements revealed that there forms a homogeneous solution of cellulose in the new solvent system. The maximum solubility of cellulose with average molecular weight of 1.32 × 10⁵ g mol⁻¹ in the solvent system is 13 wt.%. The cellulose solution in the new solvent system is stable, even for 30 days storage at room temperature.     

Preparation and characterization of water-redispersible nanofibrillated cellulose in powder form

Water-redispersible, nanofibrillated cellulose (NFC) in powder form was prepared from refined, bleached beech pulp (RBP) by carboxymethylation (c) and mechanical disintegration (m). Two routes were examined by altering the sequence of the chemical and mechanical treatment, leading to four different products: RBP-m and RBP-mc (route 1), and RBP-c and RBP-cm (route 2). The occurrence of the carboxymethylation reaction was confirmed by FT-IR spectrometry and ¹³C solid state NMR (¹³C CP-MAS) spectroscopy with the appearance of characteristic signals for the carboxylate group at 1,595 cm⁻¹ and 180 ppm, respectively. The chemical modification reduced the crystallinity of the products, especially for those of route 2, as shown by XRD experiments. Also, TGA showed a decrease in the thermal stability of the carboxymethylated products. However, sedimentation tests revealed that carboxymethylation was critical to obtain water-redispersible powders: the products of route 2 were easier to redisperse in water and their aqueous suspensions were more stable and transparent than those from route 1. SEM images of freeze-dried suspensions from redispersed RBP powders confirmed that carboxymethylation prevented irreversible agglomeration of cellulose fibrils during drying. These results suggest that carboxymethylated and mechanically disintegrated RBP in dry form is a very attractive alternative to conventional NFC aqueous suspensions as starting material for derivatization and compounding with (bio)polymers.     

Cellulose II nanoelements prepared from fully mercerized, partially mercerized and regenerated celluloses by 4-acetamido-TEMPO/NaClO/NaClO2 oxidation

A softwood bleached kraft pulp (SBKP) and cotton lint cellulose were fully or partially mercerized, and these along with celluloses and commercially available regenerated cellulose fiber and beads were oxidized by 4-acetamido-TEMPO/NaClO/NaClO₂ at 60 °C and pH 4.8. Weight recovery ratios and carboxylate contents of the oxidized celluloses were 65–80% and 1.8–2.2 mmol g⁻¹, respectively. Transparent and viscous dispersions were obtained by mechanical disintegration of the TEMPO-oxidized celluloses in water. These aqueous dispersions showed birefringence between cross-polarizers, indicating that mostly individualized cellulose nanoelements dispersed in water were obtained by these procedures. Transmission electron microscopy observation showed that the cellulose nanoelements prepared from mercerized SBKP, repeatedly mercerized SBKP, mercerized cotton lint cellulose, regenerated cellulose beads and 18% NaOH-treated SBKP, i.e. partially mercerized SBKP, had similar morphologies and sizes, 4–12 nm in width and 100–200 nm in length. The 18% NaOH-treated SBKP was converted to cellulose nanoelements consisting of both celluloses I and II.     

New promising composite materials useful in the adsorption of Cu(II) in ethanol based on cellulose and cellulose acetate

This work describes for the first time the synthesis and characterization of new promising materials based on cellulose (Cel) and cellulose acetate (Celac), previously modified with aluminum oxide (CelAl and CelacAl) and post functionalized with 1,4-diazabicyclo [2.2.2] octane-n-propyltrimethoxysilane chloride (SiDbCl₂), resulting the chemically modified hybrid materials CelAl/SiDbCl₂ and CelacAl/SiDbCl₂. The materials have shown to be useful in the adsorption of CuCl₂ from ethanol, presenting high effective adsorption capacity. In the adsorption process, the copper ions diffuse into the solid solution interface and are retained as anionic complexes CuCl₃⁻ or CuCl₄²⁻. An expressive effective adsorption capacity t_Q, as well as the stability constants β₁ and β₂, were found for both adsorbents: (a) CelAl/DbCl₂: t_Q = 0.33 × 10⁻³ mol g⁻¹, log β₁ = 4.23 (±0.04) and log β₂ = 6.99 (±0.03); (b) CelacAl/DbCl₂: t_Q = 0.48 × 10⁻³ mol g⁻¹, log β₁ = 5.1 (±0.1) and log β₂ = 8.3 (±0.1). Both adsorbent materials are potentially useful in the pre-concentration and further analysis of Cu(II) present in trace amounts in ethanol, extensively used as an automotive fuel in Brazil. Regeneration of the adsorbents requires a very simple procedure consisting in their immersion in aqueous solution which causes the immediate release of the Cu(OH₂)_n²⁺ species to the solution phase.     

On the nanostructure of micrometer-sized cellulose beads

The analysis of the porosity of materials is an important and challenging field in analytical chemistry. The gas adsorption and mercury intrusion methods are the most established techniques for quantification of specific surface areas, but unfortunately, dry materials are mandatory for their applicability. All porous materials that contain water and other solvents in their functional state must be dried before analysis. In this process, care has to be taken since the removal of solvent bears the risk of an incalculable alteration of the pore structure, especially for soft materials. In the present paper, we report on the use of small-angle X-ray scattering (SAXS) as an alternative analysis method for the investigation of the micro and mesopores within cellulose beads in their native, i.e., water-swollen state; in this context, they represent a typical soft material. We show that even gentle removal of the bound water reduces the specific surface area dramatically from 161 to 109 m² g⁻¹ in cellulose bead sample type MT50 and from 417 to 220 m² g⁻¹ in MT100. Simulation of the SAXS curves with a bimodal pore size distribution model reveals that the smallest pores with radii up to 10 nm are greatly affected by drying, whereas pores with sizes in the range of 10 to 70 nm are barely affected. The SAXS results were compared with Brunauer–Emmett–Teller results from nitrogen sorption measurements and with mercury intrusion experiments.     

Approximate solution of the autocatalytic hydrolysis of cellulose

Depolymerization of cellulose in homogeneous acidic medium is analyzed on the basis of autocatalytic model of hydrolysis with a positive feedback of acid production from the degraded biopolymer. The normalized number of scissions per cellulose chain, S(t)/n° = 1 − C(t)/C₀, follows a sigmoid behavior with reaction time t, and the cellulose concentration C(t) decreases exponentially with a linear and cubic time dependence, C(t) = C₀exp[−at − bt ³], where a and b are model parameters easier determined from data analysis.     

Effect of chitosan and ions on actuation behavior of cellulose–chitosan laminated films as electro-active paper actuators

Cellulose-chitosan laminated films were made for Electro-Active Paper (EAPap) actuators and the effect of chitosan and different types of free ions namely, Cl⁻, NO₃⁻ and CF₃COO⁻ were investigated on the actuation behavior. The fabrication process of the films was explained and the actuation performance was tested in terms of bending displacement of the actuators. It was observed that, chitosan content and type of free ions influence the tip displacement of the actuators. Cl⁻ was found the best free ion among others, and detail observations are explained. By laminating chitosan layer on the cellulose films, the humidity sensitiveness of cellulose EAPap actuators was reduced.     

Wood cellulose nanofibrils prepared by TEMPO electro-mediated oxidation

A softwood bleached kraft pulp (SBKP) was subjected to electro-mediated oxidation in water with TEMPO or 4-acetamido-TEMPO without any chlorine-containing oxidant. Solid recovery ratios of water-insoluble fractions of the oxidized SBKPs were more than 80%, and C6-carboxylate contents increased up to approximately 1 mmol g⁻¹ after oxidation for 48 h. Significant amounts of C6-aldehyde groups (0.17–0.38 mmol g⁻¹) were also formed in the oxidized SBKPs. The degree of polymerization decreased from 2,200 to 520 and 1,400 by the oxidation for 48 h with TEMPO at pH 10 and 4-acetamido-TEMPO at pH 6.8, respectively. The original cellulose I crystal structure and crystallinity of SBKP were maintained after the oxidation, indicating that all C6-oxidized groups were selectively formed on crystalline cellulose microfibril surfaces. The oxidized SBKPs with carboxylate contents of more than 0.9 mmol g⁻¹ were convertible to individual cellulose nanofibrils in yields of more than 80% by disintegration in water.     

Strength and barrier properties of MFC films

The preparation of microfibrillar cellulose (MFC) films by filtration on a polyamide filter cloth, in a dynamic sheet former and as a surface layer on base paper is described. Experimental evidence of the high tensile strength, density and elongation of films formed by MFC is given. Typically, a MFC film with basis weight 35 g/m² had tensile index 146 ± 18 Nm/g and elongation 8.6 ± 1.6%. The E modulus (17.5 ± 1.0 GPa) of a film composed of randomly oriented fibrils was comparable to values for cellulose fibres with a fibril angle of 50°. The strength of the films formed in the dynamic sheet former was comparable to the strength of the MFC films prepared by filtration. The use of MFC as surface layer (0–8% of total basis weight) on base paper increased the strength of the paper sheets significantly and reduced their air permeability dramatically. FEG-SEM images indicated that the MFC layer reduced sheet porosity, i.e. the dense structure formed by the fibrils resulted in superior barrier properties. Oxygen transmission rates (OTR) as low as 17 ml m⁻² day⁻¹ were obtained for films prepared from pure MFC. This result fulfils the requirements for oxygen transmission rate in modified atmosphere packaging.