Surface chemical modification of microfibrillated cellulose: improvement of barrier properties for packaging applications

Heterogeneous acetylation of microfibrillated cellulose (MFC) was carried out to modify its physical properties and at the same time to preserve the morphology of cellulose fibrils. The overall reaction success was assessed by FTIR together with the degree of substitution (DS) defined by titration and the degree of surface substitution (DSS) evaluated by means of XPS. Dynamic contact angle measurements confirmed the hydrophobicity improvement relative to non-modified samples. The increase of contact angle upon reaching a certain reaction time and some decrease following the further acetylation was confirmed. Mechanical properties of MFC films made from chemically modified material were evaluated using tensile strength tests which showed no significant reduction of tensile strength. According to SEM images, dimension analysis and tensile strength data, the acetylation seemed not to affect the morphology of cellulose fibrils.     

Topochemical acetylation of cellulose nanopaper structures for biocomposites: mechanisms for reduced water vapour sorption

Moisture sorption decreases dimensional stability and mechanical properties of polymer matrix biocomposites based on plant fibers. Cellulose nanofiber reinforcement may offer advantages in this respect. Here, wood-based nanofibrillated cellulose (NFC) and bacterial cellulose (BC) nanopaper structures, with different specific surface area (SSA), ranging from 0.03 to 173.3 m²/g, were topochemically acetylated and characterized by ATR-FTIR, XRD, solid-state CP/MAS ¹³C-NMR and moisture sorption studies. Polymer matrix nanocomposites based on NFC were also prepared as demonstrators. The surface degree of substitution (surface-DS) of the acetylated cellulose nanofibers is a key parameter, which increased with increasing SSA. Successful topochemical acetylation was confirmed and significantly reduced the moisture sorption in nanopaper structures, especially at RH = 53 %. BC nanopaper sorbed less moisture than the NFC counterpart, and mechanisms are discussed. Topochemical NFC nanopaper acetylation can be used to prepare moisture-stable nanocellulose biocomposites.     

Morphological investigation of polylactide/microfibrillated cellulose composites

Optical microscopy and transmission electron microscopy have been used to investigate the morphology of polylactide (PLA)/microfibrillated cellulose (MFC) composites prepared by: compression molding of wet-comingled MFC and PLA latex or powder, twin-screw extrusion of the wet-comingled compounds, and solvent mixing of PLA with MFC or acetylated MFC. Compression molding of wet-comingled MFC and PLA latex or powder compounds resulted in a cellular MFC network, whereas solvent-cast films showed a more uniform dispersion of MFC fibers. Somewhat lower aggregate diameters observed in the acetylated MFC were assumed to be due to decreased MFC hydrophilicity and improved chemical affinity with the PLA matrix. The MFC networks in the commingled compounds were severely disrupted after twin-screw extrusion. This confirmed the limited deformability of the networks inferred from the extensive syneresis during the initial compression molding step, and accounted for substantial losses in stiffness reinforcement by the MFC after extrusion.     

Role of hydrogen bonding in cellulose deformation: the leverage effect analyzed by molecular modeling

The axial modulus of the cellulose Iβ crystal is as high as 120–160 GPa. The importance of hydrogen bonds is often emphasized in this context, although intrinsic stiffness of the hydrogen bonds is relatively low. Here, hydrogen bond–covalent bond synergies are investigated quantitatively using molecular mechanics and molecular dynamics simulations for the so-called leverage effect, a model introduced recently in which strains for intra-molecular hydrogen bonds are higher than for the cellulose chain as a whole, thereby amplifying their contribution to the total stiffness. The present work also includes simulation of the hydrogen bonding band shifts in vibrational spectra during cellulose deformation, which are compared with FT-IR data. The leverage effect hypothesis was supported by the results, although the total contribution to cellulose stiffness is only 12 %. Hydrogen bonding is still critically important and would lower the modulus much more than 12 %, if “artificially” removed in the model. The reason is that intra-molecular hydrogen bonding preserves the crystal structure and directs axial deformation mechanisms towards higher energy deformation and high stiffness.     

Highly ductile fibres and sheets by core-shell structuring of the cellulose nanofibrils

A greater ductility of cellulosic materials is important if they are to be used in increasingly advanced applications. This study explores the potential for using chemical core-shell structuring on the nanofibril level to alter the mechanical properties of cellulose fibres and sheets made thereof. The structuring was achieved by a selective oxidation of the cellulose C2–C3 bonds with sodium periodate, followed by a reduction of the aldehydes formed with sodium borohydride, i.e. locally transforming cellulose to dialcohol cellulose. The resulting fibres were morphologically characterised and the sheets made of these modified fibres were mechanically tested. These analyses showed a minor decrease in the degree of polymerisation, a significantly reduced cellulose crystal width and a greater ductility. At 27 % conversion of the available C2–C3 bonds, sheets could be strained 11 %, having a stress at break of about 90 MPa, and consequently a remarkable tensile energy absorption at rupture of about 9 kJ/kg, i.e. 3–4 times higher than a strong conventional paper. Zero-span tensile measurements indicated that the treatment increased the ductility not only of sheets but also of individual fibres. This suggests that the amorphous and molecularly more mobile dialcohol cellulose is located as a shell surrounding the crystalline core of the cellulose fibrils, and that, at deformations beyond the yield point, this facilitates plastic deformation both within and between individual fibres.     

Development of continuous process enabling nanofibrillation of pulp and melt compounding

Microfibrillated cellulose (MFC), a mechanically fibrillated pulp mostly consisting of nanofibrils, is a very attractive material because of its high elastic modulus and strength. Although much research has been done on composites of MFC and polypropylene (PP), it has been difficult to produce such composites at an industrial level because of the difficulties in using MFC in such composites are not only connected to the polarity (that can be improved with compatibilizers), but also with the challenge to make a homogeneous blend of the components, and also the low temperature stability of cellulose that could cause problems during processing. We developed a new processing method which enables continuous microfibrillation of pulp and its melt compounding with PP. Never-dried kraft pulp and powdered PP were used as raw materials to obtain MFC by kneading via a twin-screw extruder. Scanning electron microscopy showed nano to submicron wide fibers entangled in the powdered PP. MFC did not aggregate during the melt compounding process, during which the water content was evaporated. Maleic anhydride polypropylene (MAPP) was used as a compatibilizer to reinforce interfacial adhesion between the polar hydroxyl groups of MFC and non-polar PP. We investigated the effect of MAPP content on the mechanical properties of the composite, which were drastically improved by MAPP addition. Needle-leaf unbleached kraft pulp (NUKP)-derived MFC composites had better mechanical properties than needle-leaf bleached kraft pulp (NBKP)-derived MFC composites. Injection molded NUKP-derived MFC composites had good mechanical and thermal properties. The tensile modulus of 50 wt% MFC composite was two times, and the tensile strength 1.5 times higher than that of neat PP. The heat distortion temperature of 50 wt% MFC content composite under 1.82 MPa flexural load was increased by 53 °C, from 69 to 122 °C. This newly developed continuous process using powder resin has the potential for application at an industrial level.     

Effect of preparation process of microfibrillated cellulose-reinforced polypropylene upon dispersion and mechanical properties

Microfibrillated cellulose (MFC)-reinforced polypropylene (PP) was prepared via two engineering approaches: disintegration of the pulp by a bead mill followed by a melt-compounding process with PP (B-MFC-reinforced PP); and disintegration of the pulp mixed with PP by a twin screw extruder followed by a melt-compounding process (T-MFC-reinforced PP). The effects that the engineering process and the microfibrillation of the pulp had upon the dispersion and mechanical properties were investigated through tensile tests, rheological analysis and X-ray computed tomography. The bead-milling method enabled a uniform microfibrillation of the pulp to under 100 nm, which corresponded to a surface area of 133–146 m²/g for the pulp, found by the Brunauer–Emmett–Teller (BET) analysis. The T-MFC-reinforced PP with 30 wt% MFC content exhibited a tensile modulus of 5.3 GPa and a strength of 85 MPa, whereas the B-MFC-reinforced PP composites with the same content of MFC exhibited values of 4.1 GPa and 59.6 MPa, respectively. Rheological analysis revealed that the complex viscosity and storage modulus at 170 °C of T-MFC-reinforced PP with 30 wt% MFC content are 5–7 and 5–8 times higher than that of B-MFC-reinforced PP, respectively. This indicated that T-MFC was more dispersed in the PP than B-MFC. Therefore, T-MFC produced a more rigid interconnected network in the matrix during the melting state than B-MFC.     

Microchip-based strategy for enrichment of acetylated proteins

We have developed a simple microchip-based method for the separation and enrichment of acetylated proteins and peptides using a microchip technique. Poly (dimethylsiloxane) (PDMS) microfluidic channels were modified by passing an acidic solution of hydrogen peroxide through them. This resulted in hydrophilic silanol-covered surfaces onto which poly (diallyldimethylammonium chloride) (PDDA) can be coated. Protein A/G beads were then captured by the PDDA layer and antibodies can then be immobilized via the protein A/G. This technique enables efficient capture of antigens due to the optimal spacing and orientation of surface molecules. Two solutions, one containing 72.5 fmol?μL^?1 of acetylated bovine serum albumin (BSA-Ac), the other 72.5 fmol?μL^?1 of tryptic BSA-Ac digest were then enriched. High selectivities were obtained, and a 82.4 % recovery of the acetylated proteins was attained. This on-chip platform was then coupled to MALDI-MS to provide information on the acetylation sites of proteins and peptides. Additional peaks were observed in the mass spectra after enrichment and were assigned to acetylated peptides. This is significant with respect to understanding the mechanism and function of acetylation. In our opinion, this microchip-based technique has a large potential for detecting acetylated proteins and peptides in complex biological mixtures, and in acetylomics in general.

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.     

Preparation and Properties of Alginate/Water‐Soluble Chitin Blend Fibers

Water‐soluble chitin (half‐deacetylated chitin) was prepared from chitosan by N‐acetylation with acetic anhydride. Alginate/water‐soluble chitin blend fibers were prepared by spinning their mixture solution through a viscose‐type spinneret into a coagulating bath containing aqueous CaCl₂ and ethanol. The structure and properties of the blend fibers were studied with the aids of infrared spectra (IR), X‐ray diffraction (XRD) and scanning electron microscopy (SEM). structure analysis indicated good miscibility existed between alginate and water‐soluble chitin, due to the strong interaction from the intermolecular hydrogen bonds and electrostatic interactions. Best values for the dry tensile strength and breaking elongation were obtained when the water‐soluble chitin content was 30 wt%. The wet tensile strength and breaking elongation decreased with the increase of water‐soluble chitin content. The introduction of water‐soluble chitin in the blend fiber can improve the water‐retention properties of the blend fiber compared to pure alginate fiber. The fibers treated with aqueous solution of silver nitrate have good antibacterial activity to Staphylococcus aureus.     

Microfibrillated cellulose (MFC): pullulan bionanocomposite films

The aim of this work was to develop and characterize microfibrillated cellulose (MFC)/pullulan bionanocomposites. Fourier transform infrared spectroscopy suggested that the affinity between the two polymers resulted in new hydrogen bonding of the nanocomposite materials compared to pristine pullulan. At the same time, an increase in crystallinity was observed proportional to the amount of MFC used, as shown by the X-ray analyses. Accordingly, final films showed improved mechanical properties proportionally to the filler loading, with impressive elastic modulus and tensile strength of ~4.50 GPa and ~60 MPa, respectively, for the sample containing 10 % MFC. However, as demonstrated by the moisture sorption isotherms, the addition of MFC did not help reduce the amount of water adsorbed by the samples. In addition, the oxygen and water vapor permeability data clearly showed that final films still suffered high relative humidity values, whereas their barrier performance toward oxygen was excellent under dry conditions, with O₂ permeability coefficients (P′O ₂) comparable with those of common high barrier films/coatings. Finally, while the nanocomposites in the form of films had high haze values (from 23 to 40 %), the same nanocomposites in the form of coatings were decidedly more transparent, which suggests that their use as thin layers could be more suitable when the “see-through” capability must be preserved, for example in food packaging applications.     

Superior non-woven sheet forming characteristics of low-density cationic polymer-cellulose nanofibre colloids

This work examines the addition of cationic polymers, cationic polyacrylamide (CPAM) and polyamide–amine–epichlorohydrin (PAE), to cellulose nanofibres to produce superior forming characteristics. The addition of 2 mg of high MW CPAM/g of nanofibres halved the drainage time to under 1 min at 0.1 wt% solids content due to increasing the floc size and the fibre forming a bulky and porous filter medium during drainage. The more open structure created in the wet state was partially preserved during the drying process, reducing the sheet density from 760 to 680 kg/m³, at the highest level of polymer addition. The addition of CPAM resulted in significant additional bridging between nanofibres, which then substantially increased the non-uniformity of the filter medium. PAE addition at 10 mg/g of micro fibrillated cellulose (MFC), also reduced drainage time, while increasing retention, but without changing the sheet uniformity. Wet strength increased continuously with PAE addition level, reaching 31.6 kN m/kg at the highest level of 20 mg of PAE/g of MFC.     

Preparation and properties of biocomposites composed of bio‐based epoxy resin,tannic acid,and microfibrillated cellulose

As new bio‐based epoxy resin systems, glycerol polyglycidyl ether (GPE) and sorbitol polyglycidyl ether (SPE) were cured with tannic acid (TA) at various conditions. When the curing conditions were optimized for the improvement of thermal and mechanical properties, the most balanced properties were obtained for the GPE/TA and SPE/TA cured at 160 °C for 2–3 h at the epoxy/hydroxyl ratio of 1/1. The cured SPE/TA had a higher glass transition temperature (T_g) and tensile strength than the cured GPE/TA. Next, biocomposites of GPE/TA and SPE/TA with microfibrillated cellulose (MFC) were prepared by mixing aqueous solution of the epoxy/curing reagent with MFC, and subsequent drying and curing at the optimized condition. For both the GPE/TA/MFC and SPE/TA/MFC biocomposites, T_g and the storage modulus at rubbery plateau region increased with increasing MFC content over the studied range of 3–15 wt %. The tensile strength at 25 °C for GPE/TA/MFC biocomposite with MFC content 10 wt % was 76% higher than that of control GPE/TA, while the tensile modulus was little improved. On the other hand, the tensile strength and modulus of SPE/TA/MFC biocomposite with MFC content 10 wt % were 30 and 55% higher than those of control SPE/TA, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 425–433, 2010     

Dual-functional chitosan–methylisothiazolinone/microfibrillated cellulose biocomposites for enhancing antibacterial and mechanical properties of agar films

In order to enhance the antibacterial and mechanical properties of agar films, the chitosan-methylisothiazolinone (C–MIT) complex was first prepared by the ionic gelation method, and the characterization of the C–MIT complex was carried out by Fourier transform infrared spectroscopy, transmission electron microscopy, and Thermo gravimetry. Chitosan was successfully crosslinked with tripolyphosphate for the nanoencapsulation of methylisothiazolinone, and the C–MIT complex was spherical in shape with a diameter of about 10 nm. The C–MIT/MFC biocomposites obtained through the adsorption of the C–MIT complex on the microfibrillated cellulose (MFC) was then incorporated into the agar films. In comparison with the pure agar films, the tensile strength of the agar composite films was increased by about 19 % at the loading of 10 wt% of C–MIT/MFC biocomposites, and antibacterial tests demonstrated that the agar composite films exhibited remarkable antibacterial activities against both Escherichia coli and Staphylococcus aureus. This work provides a new approach to utilizing multifunctional agar films in the medical field.     

Arabinoxylan structure affects the reinforcement of films by microfibrillated cellulose

The chemical structure of rye arabinoxylan (rAX) was systematically modified, exploiting selective enzymes to mimic different naturally occurring xylans, i.e., its degree of substitution (DS) was decreased using α-l-arabinofuranosidase, and a controlled decrease in the degree of polymerization (DP) was performed using endo-1,4-β-d-xylanase. The arabinose to xylose ratio was decreased from 0.45 to 0.27 and the weight-average molar mass was decreased from 184,000 to 49,000 g/mol. The resulting samples were used to prepare films, as such, and with 15% (wt. − %) softwood-derived microfibrillated cellulose (MFC) to obtain novel plant-derived biocomposite materials. The enzymatic tailoring of rAX increased the crystallinity of films, evidenced by X-ray diffraction studies, and the addition of MFC to the debranched, low DS rAX induced the formation of ordered structures visible with polarizing optical microscopy. MFC decreased the moisture uptake of films and increased the relative humidity of softening of the films, detected with moisture scanning dynamic mechanical analysis. For the first time, the chemical structure of xylan was proven to significantly affect the reinforcement potential of nano-sized cellulose, as the tensile strength of films from high DP rAXs, but not that of low DP rAXs, clearly increased with the addition of MFC. At the same time, MFC only increased the Young’s modulus of films from rAX with high arabinose content, regardless of DP.     

On the mechanism of the unwanted acetylation of polysaccharides by 1,3-dialkylimidazolium acetate ionic liquids: part 2—the impact of lignin on the kinetics of cellulose acetylation

Cellulose acetylation has been reported as a side reaction of cellulose treatment with the ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) (Karatzos et al. in Cellulose 19:307–312, 2012) and other 1,3-dialkylimidazolium acetate ionic liquids. 1-Acetylimidazole (AcIm), an [EMIm][OAc] impurity, has been found to be the actual acetylating agent (Zweckmair et al. in Cellulose 22:3583–3596, 2015), and the degree of acetylation was relatively low, below a DS of approx. 0.1%. Higher degrees of cellulose acetylation (DS > 10%) have been observed when the entire wood was mixed with [EMIm][OAc] instead of cellulosic pulp only (Abushammala et al. in Carbohydr Polym 134:609–616, 2015). In this paper, we explore the impact of wood constituents, mainly lignin, on cellulose acetylation using AcIm. The results demonstrate that lignin itself can be readily acetylated upon mixing with AcIm, and—noteworthy—that lignin presence significantly accelerates cellulose acetylation. The initial rate of cellulose acetylation by AcIm increased from 1.8 to 4.7%/h when only 1% of lignin, based on cellulose mass, was added. A mechanistic study employing cellulose and lignin model compounds showed lignin to be more susceptible to acetylation than cellulose and to act as an intermediate acetyl group source for further cellulose acetylation in a catalytic scenario.     

In situ preparation,characterization and performance of magnesium carbonate whiskers/cellulose fibers hybrid paper

Magnesium carbonate whiskers/cellulose fibers hybrid paper was successfully prepared via in situ synthesis process, and the flame retardancy and tensile strength properties were investigated. The effects of magnesium salt type, initial magnesium salt concentration, temperature, time and pH value were also detected. The hybrid paper was characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and thermogravimetry-derivative thermogravimetry techniques, respectively. The results showed that the same whiskers/fibers hybrid paper could be prepared from different magnesium salts. If the pH was above 9.4, hydrated basic magnesium carbonate would be created, which would lead to the decrease of oxygen index (OI). Reaction time had no effect on the OI and tensile strength of the hybrid paper. Under the optimized conditions, the OI of the hybrid paper reached 28 %. The in situ synthesis process was much better than the direct addition process.     

The effect of Fenton chemistry on the properties of microfibrillated cellulose

A fully bleached birch kraft pulp was treated with acidic hydrogen peroxide in the presence of ferrous ions (Fenton’s reagent) and thereafter treated mechanically in a colloid mill to produce a product containing microfibrillated cellulose (MFC). The produced MFC products were chemically and morphologically characterized and compared with MFC products produced without pretreatment as well as with enzymatic hydrolysis. Fenton treatment resulted in an increase in total charge and number of carbonyl groups while the intrinsic viscosity decreased. The Fenton treated pulps were easier to process mechanically i.e. they reached a higher specific surface area at a given mechanical treatment time and the MFC produced had a stable water-fibre suspension for at least 8 weeks compared to enzymatic pretreated pulps and pulps not subjected to any pretreatment.     

Fiber properties of eucalyptus kraft pulp with different carboxyl group contents

Fiber properties (fiber swelling ability, crystal structure of cellulose, fiber surface morphology, and etc.) of eucalyptus kraft pulp with different contents of carboxyl group in Na-form were studied. There was a direct proportional relationship between water retention value and carboxyl content of pulp. When the carboxyl content increased from 35.6 to 315.7 mmol/kg, tensile index and burst index increased by 56.1 and 117.8 %, respectively, and crystallinity of cellulose decreased by 11.8 %. Environmental scanning electron microscope showed that more fibrillation was observed on the carboxymethylated fiber surface, compared with the control sample. The results from Fourier transform infrared spectra analysis suggested that the relative intensity of the band at 1,633/cm was increased after carboxymethylation treatment, which showed that the carboxyl content increased. The increase in the carboxyl content not only could increase the fiber strength properties, but also could increase the recycling times of the fiber.     

A Well-defined Hierarchical Hydrogen Bonding Strategy to Polyureas with Simultaneously Improved Strength and Toughness

A well-defined quadruple hydrogen bonding strategy involving dimerization of 2-ureido-4[1H]-pyrimidone(UPy) units is innovatively designed to prepare polyureas with high overall mechanical properties. Three polyureas containing different amounts of UPy units were synthesized by replacing a portion of isophorone diisocyanate(IPDI) with a UPy-derived diisocyanate. The formation of quadruple hydrogen bonds in hard segments via UPy dimers was confirmed by nuclear magnetic resonance(NMR) and Fourier transform infrared spectroscopy(FTIR). The mechanical properties of the polyureas were evaluated by uniaxial tensile testing. Compared to the polyurea without UPy units, remarkable improvements in Young's modulus, tensile strength, and toughness were simultaneously achieved when UPy units were incorporated. The mechanism behind the strong strengthening effect rooted in the stronger intermolecular forces among hard segments brought by the quadruple hydrogen bonds, which were stronger than the inherent bidentate and monodentate hydrogen bonds among urea groups, and the slower soft segmental dynamics reaveled by both increased Tg and relaxation time of the soft segments. The mechanism behind the strong toughening effect was ascribed to more effective energy dissipation brought by the quadruple hydrogen bonds that served as stronger sacrificial bonds upon deformation. This work may offer new insight into the design of polyurea elastomers with comprehensively improved mechanical properties.