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.     

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.     

Cytotoxicity tests of cellulose nanofibril-based structures

Cellulose nanofibrils based on wood pulp fibres are most promising for biomedical applications. Bacterial cellulose has been suggested for some medical applications and is presently used as wound dressing. However, cost-efficient processes for mass production of bacterial cellulose are lacking. Hence, fibrillation of cellulose wood fibres is most interesting, as the cellulose nanofibrils can efficiently be produced in large quantities. However, the utilization of cellulose nanofibrils from wood requires a thorough verification of its biocompatibility, especially with fibroblast cells which are important in regenerative tissue and particularly in wound healing. The cellulose nanofibril structures used in this study were based on Eucalyptus and Pinus radiata pulp fibres. The nanofibrillated materials were manufactured using a homogenizer without pre-treatment and with 2,2,6,6-tetramethylpiperidine-1-oxy radical as pre-treatment, thus yielding nanofibrils low and high level of anionic charge, respectively. From these materials, two types of nanofibril-based structures were formed; (1) thin and dense structures and (2) open and porous structures. Cytotoxicity tests were applied on the samples, which demonstrated that the nanofibrils do not exert acute toxic phenomena on the tested fibroblast cells (3T3 cells). The cell membrane, cell mitochondrial activity and the DNA proliferation remained unchanged during the tests, which involved direct and indirect contact between the nano-structured materials and the 3T3 cells. Some samples were modified using the crosslinking agent polyethyleneimine (PEI) or the surfactant cetyl trimethylammonium bromide (CTAB). The sample modified with CTAB showed a clear toxic behaviour, having negative effects on cell survival, viability and proliferation. CTAB is an antimicrobial component, and thus this result was as expected. The sample crosslinked with PEI also had a significant reduction in cell viability indicating a reduction in DNA proliferation. We conclude that the neat cellulose nanostructured materials tested in this study are not toxic against fibroblasts cells. This is most important as nano-structured materials based on nanofibrils from wood pulp fibres are promising as substrate for regenerative medicine and wound healing.     

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.     

Multi-layer nanopaper based composites

Native cellulose nanofibrils (CNF) were prepared from bleached birch pulp without any chemical or enzymatic pretreatment. These CNF were modified by adsorption of a small amount of water-soluble polysaccharides and used to prepare nanopapers, which were processed into composites by lamination with an epoxy resin and subsequently cured. The results were compared to the properties of composites prepared using bacterial cellulose nanopapers, since bacterial cellulose constitutes highly pure and crystalline cellulose. It was found that both types of nanopapers significantly improved both the thermal stability and mechanical properties of the epoxy resin. As anticipated, addition of only 2 wt% of water-soluble polysaccharides efficiently hindered crack-propagation within the nanopaper and significantly improved the tensile strength and work of fracture compared to composites containing a conventional nanopaper reinforcement. The mechanical properties of the composites thus reflected the improvement of the nanopaper properties by the polysaccharides. Moreover, it was possible to predict the properties of the final composite from the mechanical performance of the nanopapers.     

Reduction of water wettability of nanofibrillated cellulose by adsorption of cationic surfactants

Adsorption isotherms of single and double chain cationic surfactants with different chain length (cetyltrimethyl-, didodecyl- and dihexadecyl ammonium bromide) onto cellulose nanofibrils were determined. Nanofibrillated cellulose, also known as microfibrillated cellulose (MFC), with varying contents of carboxyl groups (different surface charge) was prepared by TEMPO-mediated oxidation followed by mechanical fibrillation. The fibril charge was characterized by potentiometric and conductometric titration. Surfactant adsorption was verified by Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS). Wetting and adhesion of water onto fibril films was determined by contact angle measurements. Small aggregates (admicelles) of surfactant were shown to form on the nanofibril surfaces, well below critical micelle concentrations. The results demonstrate the possibility of using cationic surfactants to systematically control the degree of water wettability of cellulose nanofibrils.     

Length-controlled cellulose nanofibrils produced using enzyme pretreatment and grinding

The length of cellulose nanofibrils (CNFs) is a significant parameter for various applications. The goal of this research was to employ a fabrication method to produce length-controlled CNFs; the chosen technique was enzy-grinding (enzyme pretreatment followed by mechanical grinding). Here, we presented the results of the optimization of the diameter and length, the characterization of the properties of CNFs and nanofilms prepared using these fibrils. The cellulose morphology, crystallinity index (CrI), chemical structure, and thermal stability were investigated as functions of the enzyme loading and hydrolysis time. The results showed that enzy-grinding could effectively reduce the diameter and length of cellulose fibrils. The average diameter was about 8.6 ± 3.6 nm, and the length could be controlled over the range from 0.76 ± 0.38 μm to ≥ 4 μm (i.e. aspect ratios from 43 to ≥ 328). After the grinding process, the CNFs maintained high thermal stability and no change in the chemical structure compared to the original pulp. The transmittance and mechanical properties of the CNF films were strongly dependent on the fibril length. The fabrication of length-controlled CNFs using the enzy-grinding process is meaningful and significant research which could be relevant to the optimization of such materials for various applications.     

Mechanical,thermal and barrier properties of pectin/cellulose nanocrystal nanocomposite films and their effect on the storability of strawberries (Fragaria ananassa)

In this work, two formulations of pectin/cellulose nanocrystals/glycerol nanocomposites were employed as packaging to extend storage life of strawberries. The effects of incorporating cellulose nanocrystals extracted from bleached Kraft wood pulp on the mechanical, thermal, and barrier properties of pectin‐based nanocomposites were evaluated. Nanocomposite films with different filler levels of cellulose nanocrystals (1, 2, 4 and 8% w/w) were prepared by casting. Compared with the neat film of pectin, improvements in the mechanical properties of the nanocomposites were observed, but these films became fragile. To improve the film flexibility, glycerol was added as a plasticizer and then new variations in the mechanical, thermal, and barrier properties of these nanocomposites were evaluated. The effects of nanocomposite films on storability of strawberries were compared with Poly vinyl chloride packaging films. The Poly vinyl chloride film and the nanocomposites showed similar behavior regarding weight loss by the strawberries, especially in the initial days of storage. The results show that pectin/cellulose nanocrystals/glycerol nanocomposites could be considered as a viable packaging alternative for replaced the Poly vinyl cloride film. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Switchable ionic liquids enable efficient nanofibrillation of wood pulp

Use of switchable ionic liquid (SIL) pulp offers an efficient and greener technology to produce nanofibers via ultrafine grinding. In this study, we demonstrate that SIL pulp opens up a mechanically efficient route to the nanofibrillation of wood pulp, thus providing both a low cost and chemically benign route to the production of cellulose nanofibers. The degree of fibrillation during the process was evaluated by viscosity and optical microscopy of SIL treated, bleached SIL treated and a reference pulp. Furthermore, films were prepared from the fibrillated material for characterization and tensile testing. It was observed that substantially improved mechanical properties were attained as a result of the grinding process, thus signifying nanofibrillation. Both SIL treated and bleached SIL treated pulps were fibrillated into nanofibers with fiber diameters below 15 nm thus forming networks of hydrophilic nature with an intact crystalline structure. Notably, it was found that the SIL pulp could be fibrillated more efficiently than traditional pulp since nanofibers could be produced with more than 30% less energy when compared to the reference pulp. Additionally, bleaching reduced the energy demand by further 16%. The study demonstrated that this switchable ionic liquid treatment has considerable potential in the commercial production of nanofibers due to the increased efficiency in fibrillation.     

Preparation of tough cellulose II nanofibers with high thermal stability from wood

Well-dispersed cellulose II nanofibers with high purity of 92 % and uniform width of 15–40 nm were isolated from wood and compared to cellulose I nanofibers. First, ground wood powder was purified by series of chemical treatments. The resulting purified pulp was treated with 17.5 wt% sodium hydroxide (NaOH) solution to mercerize the cellulose. The mercerized pulp was further mechanically nanofibrillated to isolate the nanofibers. X-ray diffraction patterns revealed that the purified pulp had been transformed into the cellulose II crystal structure after treatment with 17.5 wt% NaOH, and the cellulose II polymorph was retained after nanofibrillation. The cellulose II nanofiber sheet exhibited a decrease in Young’s modulus (8.6 GPa) and an increase in fracture strain (13.6 %) compared to the values for a cellulose I nanofiber sheet (11.8 GPa and 7.5 %, respectively), which translated into improved toughness. The cellulose II nanofiber sheet also showed a very low thermal expansion coefficient of 15.9 ppm/K in the range of 20–150 °C. Thermogravimetric analysis indicated that the cellulose II nanofiber sheet had better thermal stability than the cellulose I nanofiber sheet, which was likely due to the stronger hydrogen bonds in cellulose II crystal structure, as well as the higher purity of the cellulose II nanofibers.     

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.     

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.     

Chemically modified cellulose micro- and nanofibrils as paper-strength additives

Chemically modified cellulose micro- and nanofibrils were successfully used as paper strength additives. Three different kinds of cellulose nanofibrils (CNFs) were studied: carboxymethylated CNFs, periodate-oxidised carboxymethylated CNFs and dopamine-grafted carboxymethylated CNFs, all prepared from bleached chemical fibres of dissolving grade, and one microfibrillated cellulose from unbleached kraft fibres. In addition to mechanical characterization of the final paper sheets the fibril retention, sheet density and sheet morphology were also studied as a function of addition of the four different cellulose fibrils. In general, the cellulose fibrils, when used as additives, significantly increased the tensile strength, Young’s modulus and strain-at-break of the paper sheets. The effects of the different fibrils on these properties were compared and evaluated and used to analyse the underlying mechanisms behind the strengthening effect. The strength-enhancing effect was most pronounced for the periodate-oxidised CNFs when they were added together with polyvinyl amine (PVAm) or poly(dimethyldiallylammonium chloride) (pDADMAC). The addition of periodate-oxidised CNFs, with pDADMAC as retention aid, resulted in a 37% increase in tensile strength at a 2 wt% addition and an 89% increase at a 15 wt% addition (from 67 to 92 and 125 kNm/kg, respectively) compared to a reference with only pDADMAC. Wet-strong sheets with a wet tensile index of 30 kNm/kg were also obtained when periodate-oxidised CNFs and PVAm were combined. This significant increase in wet strength is suggested to be the result of a formation of cross-links between the aldehyde groups, introduced by the periodate oxidation, and hydroxyl groups on the lignocellulosic fibres and the primary amines of PVAm. Even though less significant, there was also an increase in wet tensile strength when pDADMAC was used together with periodate-oxidised fibrils which shows that the aldehyde groups are able to increase the wet strength without the presence of the primary amines of the PVAm. As an alternative method to strengthen the fibre network, carboxymethylated CNFs grafted with dopamine, by an ethyl dimethylaminopropyl carbodiimide coupling, were used as a strength additive. When used as an additive, these CNFs showed a strong propensity to form films on and around the fibres and significantly increased the mechanical properties of the sheets. Their addition resulted in an increase in the Young´s modulus by 41%, from 5.1 to 7.2 GPa, and an increase in the tensile strength index of 98% (from 53 to 105 kNm/kg) with 5 wt% retained dopamine-grafted CNFs.     

Electrically conductive composites of polyethylene filled with polyamide particles coated with silver

New types of electrically conductive polymeric composites were prepared on a base of high-density polyethylene (HDPE) matrix filled with silver-coated polyamide (PA) particles. The electrical, mechanical and adhesive properties of those composites are reported in this paper. The percolation concentration of the filler within a matrix was found to be 4 vol.%. Composites filled with high filler content were highly electrically conductive; their electrical conductivity reached the value of 6.8 × 10² S cm⁻¹. Mechanical properties and rheology of these composites were discussed. The adhesive properties of the composites to metal sharply increased with an increase in the filler content.     

Effect of filler shape on mechanical properties of rigid polyurethane composites containing plant particles

The effect of types of fillers on mechanical properties of rigid polyurethane composite samples was investigated. Polyurethane (PU) composites were prepared using a molasses polyol (MP, a mixture of molasses and polyethylene glycol, Mw=200) diphenylmethane diisocyanate (MDI) and fillers. The following plant particles, bamboo powder, roast bamboo powder, wood meal, coffee grounds, ground coffee bean parchment and cellulose powder, were used as fillers. The mixture of MP and fillers was reacted with MDI by adding an adequate amount of acetone as a solvent. The content of fillers was defined as the ratio of filler weight to total weight of polyol and fillers. The filler content was varied from 10 to 90 wt%. Polyurethane (PU) composites were prepared using fillers with MP. Lengths of major axis and minor axis for each particle regarded as an ellipse were measured using an optical microscope. Averages of diameter and aspect ratio were derived for each plant particle. The relationships between these average values and the mechanical properties, such as strength and elastic modulus, determined by the compression tests were investigated. The effect of filler content was estimated using the apparent volume ratio which is determined as the ratio of the apparent volume of fillers to the reciprocal values of the apparent density of samples. The master curves of the relationships between the specific values of mechanical properties and the apparent volume ratio were obtained. It was found that the compression strength and the elastic modulus for composite samples with different fillers showed maximum values at average aspect ratio around 3. It was also found that the apparent volume ratio, where the mechanical properties showed maximums, decreases with increasing aspect ratio. Using master curves, it is possible to evaluate the mechanical properties of plant particle filled polyurethane composites are described.     

Cellulose-precipitated calcium carbonate composites and their effect on paper properties

A cellulose-PCC composite was synthesised using the CaCl₂ reaction with dimethyl carbonate (DMC) under alkaline conditions and in the presence of cellulose fibrils made from highly refined bleached softwood kraft pulp. The results showed that the ash content in the synthesised cellulose-PCC composite increased by increasing the reaction temperature from 25°C to 70°C, the reaction time from 3.5 min to 7.5 min and the cellulose consistency from 0.05 % to 0.1 %. The ratio of calcium carbonate generated to the calcium chloride used initially was increased by increasing the reaction temperature and time. The XRD pattern of the cellulose-PCC composite indicated no modification micro-crystal habit of the deposited CaCO₃. The SEM images showed that the cellulose-PCC composite filler had a rhombohedral shape as opposed to the scalenohedral shape of common PCC. The paper filled with the cellulose-PCC composite had much higher bursting and tensile strengths, at a tearing strength similar to common PCC.     

Mechanical properties of cellulose nanomaterials studied by contact resonance atomic force microscopy

Quantification of the mechanical properties of cellulose nanomaterials is key to the development of new cellulose nanomaterial based products. Using contact resonance atomic force microscopy we measured and mapped the transverse elastic modulus of three types of cellulosic nanoparticles: tunicate cellulose nanocrystals, wood cellulose nanocrystals, and wood cellulose nanofibrils. These modulus values were calculated with different contact mechanics models exploring the effects of cellulose geometry and thickness on the interpretation of the data. While intra-particle variations in modulus are detected, we did not observe a measureable difference in modulus between the three types of cellulose particles. Improved practices and experimental complications for the characterization of cellulosic nanomaterials with atomic force microscopy are discussed.     

木质纤维素纳米纤丝制备及形态特征分析

以阔叶树材杨木木粉为原料,始终保持纤维处在水润涨的状态下,利用亚氯酸钠在酸性条件下脱除木质素,氢氧化钾脱除半纤维素,然后借助高强度超声波的空化作用,依次制备了综纤维素、纯化纤维素及木质纤维素纳米纤丝(WCNF).通过傅里叶变换红外吸收光谱(FTIR)、X射线衍射(XRD)、扫描电镜(SEM)对WCNF制备过程中的化学组分、晶型结构、结晶度及形态特征变化进行了表征,并进一步利用图像分析系统对WCNF的直径分布进行了测量统计.结果表明,WCNF的主要成分为纤维素,其晶型结构仍为纤维素Ⅰ型,结晶度为65.68%,较之原料木粉提高了12.33%.所得纤丝的直径集中分布在5～32nm之间,长度大于10μm,长径比高于300,纤丝间相互交织成网状缠结结构.WCNF的高结晶度、高长径比、纳米尺度、网状缠结结构,显示其为一种十分理想的增强增韧材料.     

TEMPO氧化和高频超声下埃米级厚度纤维素纳米纤丝的制备

通过不同TEMPO氧化体系对商品竹浆进行氧化处理,经高频超声纳米纤丝化后,可以制得长度在数百纳米,宽度小于5.0 nm,厚度仅为几个埃的纤维素纳米纤丝(TEMPO-oxided cellulose nanofibrils,TOCNs).这种纳米带状(nanostrip)的TOCNs是由纤维素片层构成的.本文通过场发射扫描电子显微镜(FE-SEM)探究了原料和2组TOCNs样品的形貌变化,利用透射电子显微镜(TEM)和原子力显微镜(AFM)对2组TOCNs样品的三维尺寸(长、宽、厚)进行测量统计.通过不同氧化体系产物的TOCNs三维尺寸差异,并结合傅里叶红外吸收光谱(FTIR)、X射线衍射(XRD)及交叉极化和魔角旋转13C固态核磁共振光谱(CP/MAS 13C-NMR)揭示了不同氧化体系对纤维素Iβ层内氢键及长轴方向的作用机理.