Cellulose nanofibrils as filler for adhesives: effect on specific fracture energy of solid wood-adhesive bonds

Cellulose nanofibrils were prepared by mechanical fibrillation of never-dried beech pulp and bacterial cellulose. To facilitate the separation of individual fibrils, one part of the wood pulp was surface-carboxylated by a catalytic oxidation using (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as a catalyst. After fibrillation by a high pressure homogenizer, the obtained aqueous fibril dispersions were directly mixed with different urea–formaldehyde-(UF)-adhesives. To investigate the effect of added cellulose filler on the fracture mechanical properties of wood adhesive bonds, double cantilever beam specimens were prepared from spruce wood. While the highest fracture energy values were observed for UF-bonds filled with untreated nanofibrils prepared from wood pulp, bonds filled with TEMPO-oxidized fibrils showed less satisfying performance. It is proposed that UF-adhesive bonds can be significantly toughened by the addition of only small amounts of cellulose nanofibrils. Thereby, the optimum filler content is largely depending on the adhesive and type of cellulose filler used.     

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.     

Conversion of cellulose materials into nanostructured ceramics by biomineralization

Synthesis of hierarchically ordered silica materials having ordered wood cellular structures has been demonstrated through in-situ mineralization of wood by means of surfactant-directed mineralization in solutions of different pH. At low pH, silicic acid penetrates the buried interfaces of the wood cellular structure without clogging the pores to subsequently “molecularly paint” the interfaces thereby forming a positive replica following calcinations. At high pH, the hydrolyzed silica rapidly condenses to fill the open cells and pits within the structure resulting in a negative replica of the structure. Surfactant-templated mineralization in acid solutions leads to the formation of micelles that hexagonally pack at the wood interfaces preserving structural integrity while integrating hexagonally ordered nanoporosity into the structure of the cell walls following thermal treatment in air. The carbothermal reduction of mineralized wood with silica at high temperature produces biomorphic silicon carbide (SiC) materials, which are typical aggregations of β-SiC nanoparticles. To understand the roles of each component (lignin, crystalline cellulose, amorphous cellulose) comprising the natural biotemplates in the transformation to SiC rods, three different cellulose precursors including unbleached and bleached pulp, and cellulose nanocrystals have been utilized. Lignin in unbleached pulp blocked homogeneous penetration of silica into the pores between cellulose fibers resulting in non-uniform SiC fibers containing thick silica layers. Bleached pulp produced uniform SiC rods with camelback structures (80 nm in diameter; ∼50 μm in length), indicating that more silica infiltrates into the amorphous constituent of cellulose to form chunky rather than straight rod structures. The cellulose nanocrystal (CNXL) material produced clean and uniform SiC nanowires (70 nm in diameter; >100 μm in length) without the camelback structure.     

Cellulose nanofibrils—adsorption with poly(amideamine) epichlorohydrin studied by QCM-D and application as a paper strength additive

In this paper cellulose nanofibrils were used together with a cationic polylelectrolyte, poly(amideamine) epichlorohydrin (PAE), to enhance the wet and the dry strength of paper. The adsorption of nanofibrils and PAE on cellulose model surfaces was studied using quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM). The differences in fibril and polyelectrolyte adding strategies onto cellulose fibres were studied by comparing layer-structures and nano-aggregates formed by the nanofibrils and PAE. The results showed that when PAE was first adsorbed on the model fibre surface a uniform and viscous layer of nanofibrils could be adsorbed. When PAE and nanofibrils were adsorbed as cationic aggregates a non-uniform and more rigid layer was adsorbed. Paper sheets were prepared using both the bi-layer and nano-aggregate adding strategy of the nanofibrils and PAE. When PAE and nanofibrils were adsorbed on pulp fibres as a bi-layer system significant increase in both wet and dry tensile strength of paper could be achieved even at low added amounts of PAE. When the substances were added as nano-aggregates the improvements in paper strength properties were not as significant. Bulk and surface nitrogen content analyses of the paper samples showed that the adding strategy does not affect the total adsorbed amount of PAE but it has a strong effect on distribution of substances in the paper matrix which has a crucial effect on paper wet and dry strength development.     

Effects of cellulose nanofibrils on the structure and properties on PVA nanocomposites

A green method—joint mechanical grinding and high pressure homogenization—was used to defibrillate paper pulp into nanofibrils. The prepared cellulose nanofibrils (CNF) were then blended with PVA in an aqueous system to prepare transparent composite film. The size and morphology of the nanofibrils and their composites were observed, and the structure and properties were characterized. The results showed that CNFs are beneficial to improve the crystallinity, mechanical strength, Young’s modulus, T _g and thermal stability of the PVA matrix because of their high aspect ratio, crystallinity and good compatibility. Therefore, nano cellulosic fibrils were proven to be an effective reinforcing filler for the hydrophilic polymer matrix. Moreover, the green fabrication approaches will be helpful to build up biodegradable nanocomposites with wide applications in functional environmentally friendly materials.     

Structure of cellulose and microcrystalline cellulose from various wood species, cotton and flax studied by X-ray scattering

The structure of microcrystalline cellulose (MCC) made by mild acid hydrolysis from cotton linter, flax fibres and sulphite or kraft cooked wood pulp was studied and compared with the structure of the starting materials. Crystallinities and the length and the width of the cellulose crystallites were determined by wide-angle X-ray scattering and the packing and the cross-sectional shape of the microfibrils were determined by small-angle X-ray scattering. The morphological differences were studied by scanning electron microscopy. A model for the changes in microfibrillar structure between native materials, pulp and MCC samples was proposed. The results indicated that from softwood or hardwood pulp, flax cellulose and cotton linter MCC with very similar nanostructures were obtained with small changes in reaction conditions. The crystallinity of MCC samples was 54–65%. The width and the length of the cellulose crystallites increased when MCC was made. For example, between cotton and cotton MCC the width increased from 7.1 nm to 8.8 nm and the length increased from 17.7 nm to 30.4 nm. However, the longest crystallites were found in native spruce wood (35–36 nm).     

DMA analysis and wood bonding of PVAc latex reinforced with cellulose nanofibrils

Suspensions of commercial refined beech pulp (RBP) were further processed through mechanical disintegration (MD-RBP), chemical modification (CM-RBP) and through chemical modification followed by mechanical disintegration (CM-MD-RBP). Nanocomposites were prepared by compounding a poly(vinyl acetate) (PVAc) latex adhesive with increasing contents of the different types of nanofibrils, and the resulting nanocomposites were analyzed by dynamic mechanical analysis (DMA). Also, the suitability of using the CM-RBP fibrils to formulate PVAc adhesives for wood bonded assemblies with improved heat resistance was studied. The presence of cellulose nanofibrils had a strong influence on the viscoelastic properties of PVAc latex films. For all nanocomposites, increasing amounts of cellulose nanofibrils (treated or untreated) led to increasing reinforcing effects in the glassy state, but especially in the PVAc and PVOH glass transitions. This reinforcement primarily resulted from interactions between the cellulose fibrils network and the hydrophilic PVOH matrix that led to the complete disappearance of the PVOH glass transition (tan δ peak) for some fibril types and contents. At any given concentration in the PVOH transition, the CM-MD-RBP nanofibrils provided the highest reinforcement, followed by the MD-RBP, CM-RBP and the untreated RBP. Finally, the use of the CM-RBP fibrils to prepare PVAc reinforced adhesives for wood bonding was promising since, even though they generally performed worse in dry and wet conditions, the boards showed superior heat resistance (EN 14257) and passed the test for durability class D1.     

Determination of nanocellulose fibril length by shear viscosity measurement

The lengths of ten types of cellulose nanofibrils were evaluated by shear viscosity measurement of their dilute dispersions. Aqueous dispersions of surface-carboxylated cellulose nanofibrils with a uniform width of ~3 nm were prepared from wood cellulose by 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidation and successive mechanical treatment. Cellulose nanofibril samples with different average lengths were prepared by controlling the conditions of the oxidation or mechanical treatment. The viscosity-average lengths, L _visc, of the nanofibrils were calculated by applying the shear viscosities of the dilute dispersions to an equation for the dilute region flow behavior of rod-like polymer molecules. The obtained L _visc values ranged from 1,100 to 2,500 nm and showed a linear relationship to the length-weighted average length, L _w, measured by microscopic observation; the relation was described as L _visc = 1.764 × L _w + 764. The influences of the electric double-layer of the nanofibrils and surface-carboxylate content on the value of L _visc were also investigated.     

Changes in cellulose structure during dissolution in LiCl:N,N-dimethylacetamide and in the alkaline iron tartrate system EWNN

The influence of different solvents on the morphology of cellulose during the dissolution process was studied. Spruce sulfite pulp, cotton linters and hydrolysed cotton linters were treated for a short time with lithium chloride: N,N-dimethylacetamide (LiCl:DMAc) and an alkaline solution of iron sodium tartrate (EWNN), respectively. The changes occurring at the fibre surfaces and within the cell walls were observed by scanning as well as by transmission electron microscopy. The cellulose fibres show significant differences in the dissolution behaviour when comparing the reaction of the two solvents. Using LiCl:DMAc, the cotton linters fibres become lamellar separated and within the spruce sulfite pulp fibres solvent channels appear in the first step with the fibrils becoming separated. In contrast, EWNN has a swelling effect on the surface of the cellulose fibres. Both solvent systems predominantly affect the ends of the fibres and places where the wall structure has been damaged.     

Changes in lignin and cellulose by irradiation

The effect of high-energy radiation on wood and cellulose was investigated. By irradiation of beech wood, changes in lignin, in carbohydrates and in wood structure take place. Furthermore, new lignin carbohydrate complexes are formed. A way is shown to prevent undesirable reactions. Irradiated pulp possesses a lower degree of polymerization and a higher accessibility for chemical reactions. Processing irradiated pulp to viscose fibres will be more efficient.     

Characterisation of the undissolved residuals ID CMC-solutions

The undissolved fibre and gel residuals that had not completely reacted to form fully dissolved carboxymethyl cellulose (CMC) ID the production of CMC were studied to clarify the reactivity of wood components ID the pulp. The undissolved residuals, the pulp and the CMC were therefore analysed on the fibre level, the cell-wall level and the chemical composition level. The results may be interpreted as indicating that the presence of undissolved residuals ID the CMC was not due to any chemical difference. The undissolved residuals were shown to consist mainly of swollen cell wall parts and some whole wood cells, mainly thick-walled compression wood and summerwood cells. They react more slowly ID the mercerisation and etherification, probably because of a greater diffusion resistance due to their larger dimensions or to a more dense structure. These cells are assumed to be less accessible for chemical penetration, but they may also contain supramolecular structures that slow down the CMC reaction.     

A comparative CP/MAS 13C-NMR study of cellulose structure in spruce wood and kraft pulp

CP/MAS ¹³C-NMR spectroscopy in combination with spectral fitting was used to study the supermolecular structure of the cellulose fibril in spruce wood and spruce kraft pulp. During pulping, structures contributing to inaccessible surfaces in the wood cellulose are converted to the cellulose I allomorph, that is, the degree of order is increased. This increase is also accompanied by a conversion of cellulose I to cellulose I. Cellulose from wood composed of different cell types, that is, compression wood, juvenile wood, earlywood, latewood and normal wood exhibited a similar supermolecular structure. Assignments were made for signals from hemicellulose which contribute significantly to the spectral C-4 region (80–86 ppm) in kraft pulp spectra but substantially less to the corresponding region in wood spectra.     

Characterization and pyrolysis behaviour of different paper mill waste materials

Paper industry generates a considerable amount of wastes. Their composition mainly depends on the type of paper produced and the origin of cellulose fibres. Nowadays, in Spain, 40% of solid wastes generated by the paper and pulp industry are deposited directly in landfill, 25% are used in the agriculture, 13% in the ceramic industry and 7% in the concrete production. In the last years, thermal treatment methods like combustion, pyrolysis and gasification have been widely study as alternative techniques for the valorization of different organic waste materials. The main objective of the present work is to study the pyrolysis behaviour of different paper mill waste materials. For this reason, a wide characterization of eight paper mill waste materials from different origins was performed using SEM, FTIR, DRX and thermogravimetric techniques. Paper mill sludges from recycled paper, mainly wastes obtained from deinking process, showed high CaCO₃ and clays contents. Compared with the elevated total organic matter content (TOM) of paper mill waste materials their low organic carbon content determined by Cr₂O₇²⁻ oxidation reveals the elevated chemical stability of organic matter, due to high content on cellulose fibres. Analysis of samples by SEM indicates that successive recycled processes of paper leads to paper mill waste materials with more degraded fibres. XRD analyses show as crystalline cellulose was present in reject and primary sludge from paper mills that produced paper from virgin wood. However, crystalline cellulose content significantly decreased in waste materials from recycled paper. Finally, thermogravimetric analysis indicates that presence or mineral matter and degradation of cellulose significantly influences their pyrolysis behaviour. In general, weight loss of paper mill waste materials started at lower temperatures than pure cellulose. In waste materials from recycled paper weight loss continues at temperatures highest than 500 °C due to kaolinite dehydration and carbonates decomposition.     

Water molecule-induced hydrogen bonding between cellulose nanofibers toward highly strong and tough materials from wood aerogel

Lightweight, highly strong and bio-based structural materials remain a long-lasting challenge. Here, inspired by nacre, a lightweight and high mechanical performance cellulosic material was fabricated via a facile and effective top-down approach and the resulting material has a high tensile strength of 149.21 MPa and toughness of 1.91 MJ/m³. More specifically, the natural balsawood (NW) was subjected to a simple chemical treatment, removing most lignin and partial hemicellulose, follow by freeze-drying, forming wood aerogel (WA). The delignification process produced many pores and exposed numerous aligned cellulose nanofibers. Afterwards, the WA absorbed a quantity of moisture and was directly densified to form above high-performance cellulosic material. Such treatment imitates highly ordered “brick-and-mortar” arrangement of nacre, in which water molecules plays the role of mortar and cellulose nanofibrils make the brick part. The lightweight and good mechanical properties make this material promising for new energy car, aerospace, etc. This paper also explains the strengthening mechanism for making biomimetic materials by water molecules-induced hydrogen bonding and will open a new path for designing high-performance bio-based structural materials.     

The potential of TEMPO-oxidized nanofibrillar cellulose beads for cell delivery applications

The advanced development of cell carriers for regenerative medicine and cell therapy demands materials able to sustain cell viability prior to their delivery to the target tissue, an ability that can be controlled by the shape, size and degradability of the matrix. TEMPO-oxidized nanofibrillar cellulose (ToNFC) macromolecules are negatively charged and therefore can be easily formulated by ionotropic gelation into beads of varying sizes that can release their payload through an erosion-controlled process. We report here for the first time on the preparation of ToNFC beads via ionic gelation using CaCl₂ and on their loading with OSTEO-1 rat bone cells, with a view to examine their capacity of sustaining the cell viability and of releasing the bone cells in a controlled manner. The initial results obtained demonstrate that ToNFC is able to protect the OSTEO-1 cells and to maintain their viability for at least 2 weeks. Following gradual disintegration of the beads, a significant cell release and subsequent proliferation was observed after 7 days. These results indicate the considerable potential of nanofibrillar cellulose (ToNFC) for applications in cell therapy and regenerative medicine.     

Comparison study of TEMPO-analogous compounds on oxidation efficiency of wood cellulose for preparation of cellulose nanofibrils

The effect of chemical structures of TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy radical) derivatives and its analogous compounds on oxidation efficiency of C6 primary hydroxyls of wood cellulose was investigated using the NaClO/NaBr system at pH 10. Because the oxidation takes place selectively on the surfaces of cellulose microfibrils, individualized and surface-oxidized cellulose nanofibrils can be obtained by simple mechanical treatment in water, when sufficient amounts of carboxylate groups are formed homogeneously in cellulose microfibrils. 4-acetamide-TEMPO and 4-methoxy-TEMPO showed efficient catalytic behavior with short reaction times (<4 h) and high carboxylate contents (>1.1 mmol/g) in oxidation of wood cellulose, comparable to TEMPO. Correspondingly, these TEMPO derivatives as well as TEMPO gave high nanofibril yields >56%. On the other hand, the use of 4-hydroxy-TEMPO and 4-oxo-TEMPO resulted in the lowest efficiency in oxidation: oxidation times >24 h, carboxylate contents <0.3 mmol/g, and individualized and surface-oxidized nanofibril yields <2%.     

Confocal Raman Spectroscopy – Applications on Wood,Pulp, and Cellulose Fibres

The manifold possibilities using dispersive Raman spectroscopy along the production chain from wood to cellulose fibre are presented here: The distribution of the lignin, cellulose, and added resins in the wood cell; additives inside regenerated cellulose fibres; their accessibility and reactivity. On-line measurements during the phase transition of magnesium sulphite -hexahydrate into -trihydrate are performed in a stirring vessel.     

Using a fully recyclable dicarboxylic acid for producing dispersible and thermally stable cellulose nanomaterials from different cellulosic sources

We fabricated cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) from different cellulose materials (bleached eucalyptus pulp (BEP), spruce dissolving pulp (SDP) and cotton based qualitative filter paper (QFP) using concentrated oxalic acid hydrolysis and subsequent mechanical fibrillation (for CNFs). The process was green as acid can be easily recovered, and the prepared cellulose nanomaterials were carboxylated and thermally stable. In detail, the CNC yield from the different materials was similar. After hydrolysis, the DP of the cellulose materials decreased substantially, whereas the mechanical fibrillation of the cellulosic solid residues (CSRs) did not dramatically reduce the DP of cellulose. CNCs with different aspect ratios were produced from different starting materials by oxalic acid hydrolysis. The CNCs and CNFs obtained from BEP and QFP possessed more uniform dimensions than those from SDP. On the other hand, CNFs derived from SDP presented the best suspension stability. FTIR analyses verified esterification of cellulose by oxalic acid hydrolysis. The results from both XRD and Raman spectroscopy indicated that whereas XRD crystallinity of CNCs from BEP and QFP did not change significantly, there was some change in Raman crystallinity of these samples. Raman spectra of SDP CNCs indicated that the acid hydrolysis preferably removed cellulose I portion of the samples and therefore the CNCs became cellulose II enriched. TGA revealed that the CNCs obtained from QFP exhibited higher thermal stability compared to those from BEP and SDP, and all the CNCs possessed better thermal stability than that of CNCs from sulfuric acid hydrolysis. The excellent properties of prepared cellulose nanomaterials will be conducive to their application in different fields.     

In situ wound sprayable double-network hydrogel: Preparation and characterization

In clinical settings the wound-dressing was required easy to use and can match the wound area immediately, at the same time they need to have the properties of hemostats, anti-inflammation and promoting wound healing. To get an ideal wound dressing, we developed a type of gel-like wound adhesive patch from spraying double-network hydrogel, which own the properties of self-antibacterial and can promote wound healing. By spraying, the gel-like wound adhesive patch can match the wound area immediat...     

Microstructural cumulative material degradation and fatigue-failure micromechanisms in wood-pulp fibres

This paper establishes the fundamental micro-mechanisms associated with the conversion of single wood pulp fibres into fibres suitable for the production of paper. It deals with an examination of the morphological and structural changes taking place in pulp fibres being subjected to cyclic mechanical actions that are representative of those experienced by fibres in mechanical refiners. Implementing the experimental procedure previously described (Hamad, 1994), qualitative answers are provided to such questions as what material property changes are associated with the various identifiable micro-mechanisms and how is the extent of damage accumulation related to wood species, pulping type, refining energy, and the number of cycles? A collation of the underlying themes responsible for material degradation indicates that a recognition of the regions of high-localized deformation and the manner in which cracks grow as well as the general weakening of the material due to structural damage and mechanical degradation of the fibre cell wall material, provide an insight into the way in which single fibres are rendered suitable for papermaking by mechanical refining.