Disintegration of periodate–chlorite oxidized hardwood pulp fibres to cellulose microfibrils: kinetics and charge threshold

Periodate–chlorite oxidized bleached hardwood kraft pulp fibre samples with six levels of charge densities ranging from 0.5 to 1.8 mmol/g were gradually disintegrated to microfibrils using a high-shear homogenizer. The disintegration kinetics and mechanisms were studied by a flow fractionation method, and the properties of the resulting particles were determined using low shear viscosity and transmittance measurements. The particles formed during the disintegration were visualized with a charge-coupled device camera and by field-emission scanning electron microscopy. The result showed that cellulose fibres with a low charge density disintegrated at a low rate and produced ragged fibres and bunches of microfibrils via bursting of the fibre walls, whereas those with a higher charge density broke down at a high rate and microfibrils were formed through swelling and the creation of balloon structures. A carboxyl content of 1.2 mmol/g was found to be the threshold value for the efficient formation of high aspect ratio microfibrils and also for the change in the disintegration mechanism in the high-shear homogenizer.     

Effects of wet-pressing-induced fiber hornification on enzymatic saccharification of lignocelluloses

This article reports the effect of wet-pressing-induced fiber hornification on enzymatic saccharification of lignocelluloses. A wet cellulosic substrate of bleached kraft eucalyptus pulp and two wet sulfite-pretreated lignocellulosic substrates of aspen and lodgepole pine were pressed to various moisture (solids) contents by variation of pressing pressure and pressing duration. Wet pressing reduced substrate moisture content and produced irreversible reduction in fiber pore volume—fiber hornification—as reflected in reduced water retention values (WRVs), an easily measurable parameter, of the pressed substrates. Wet pressing resulted in a reduction in substrate enzymatic digestibility (SED) by approximately 20% for the two sulfite-pretreated substrates when moisture content was reduced from approximately 75% to 35%. The reduction in SED for the cellulosic substrate was less than 10% when its moisture content was reduced from approximately 65% to 35%. The results indicated that reduction in SED is negligible when samples were pressed to solids content of 40% but observable when pressed to solids content of 50%. It was also found that WRV can correlate to SED of hornified substrates resulting from the same never-dried or pressed sample independent of the hornification process (e.g., pressing or drying). This correlation can be fitted using a Boltzmann function. Cellulase adsorption measurements indicated that wet-pressing-induced fiber hornification reduced cellulose accessibility to cellulase. The results obtained in this study provide guidelines to high-solids enzymatic saccharification of pretreated biomass.     

Influence of fibre–fibre joint properties on the dimensional stability of paper

Measurements have been performed to clarify the connection between fibre–fibre joint properties and dimensional stability using laboratory sheets prepared from never-dried fibres, from heavily hornified fibres having a low molecular contact area between the fibres, and from both hornified and never-dried fibres treated with a polyelectrolyte multilayer (PEM) technique to increase the molecular contact area in the fibre–fibre joint. The influence of the drying mode, i.e. whether the sheets are dried freely or under restraint, was also evaluated. The results showed that neither paper strength nor fibre–fibre joint contact area had any significant influence on the dimensional stability of sheets dried under restraint. On the other hand, when the sheets were dried freely, the PEM-treated sheets expanded to the same extent as, or to an even greater extent than the non-PEM-treated sheets, even though they adsorbed less water for a given change in relative humidity. There was also a correlation between drying shrinkage and dimensional stability, where greater shrinkage was associated with a greater hygroexpansion in the freely dried sheets.     

Key role of the hemicellulose content and the cell morphology on the nanofibrillation effectiveness of cellulose pulps

The effect of the hemicellulose content and that of the fibre morphology on the nanofibrillation behaviour of delignified cellulose pulps were studied. For this purpose, pulps from two non-woody plants, alfa (Stipa tenacissima) and sunflower (Helianthus annuus), were delignified using NaClO₂/acetic acid and the NaOH pulping processes to obtain fibres with different hemicellulose contents. The ensuing fibres were characterized by chemical analysis, SEM, FTIRS and X-ray diffraction. The fibres were then disintegrated into nanofibrillated cellulose (NFC) using either a high pressure homogenizer or a domestic blender. The degree of fibrillation and the morphology of the nanofibrillated fractions were evaluated by centrifugation and Field-emission scanning electron microscopy. Pulps containing the highest hemicellulose content showed higher yields of the nanofibrillated fraction and a better aptitude for the individualization of the microfibrils. Furthermore, it was shown that fibres from sunflowers exhibiting a thinner cell wall were easier to fibrillate and could be disintegrated into NFC by just using a simple domestic-blender once deliginification process was carried out using the NaClO₂/acetic acid method. Eucalyptus fibres were also used to further confirm the key role of hemicelluloses in the nanofibrillation process of woody plants.     

Effect of cellulase-assisted refining on the properties of dried and never-dried eucalyptus pulp

The effect of two different cellulases on the hornification phenomenon,in which drainability (Schopper–Riegler method) and mechanical propertiesdiminish when pulps are dried, was studied. The enzyme applications testedincluded a commercial enzyme named ComC (Pergalase A40 from CIBA) and alaboratory enzyme from Paenibacillus sp. strain BP-23namedCelB. Industrial never-dried Eucalyptus globulus bleachedkraft pulp was split in two halves and one of them was dried at ambientcontrolled conditions. We compared enzyme effects on both pulps (wet pulp anddried pulp) before and after PFI mill refining. Enzyme applications increaseddrainability (Schopper–Riegler method) and water retention value (WRV) ofnever-dried bleached pulp, although this did not imply an enhancement of themechanical properties of paper. Cellulase treatment of dried pulps, bycontrast,gave rise to increased drainability and WRV and also to improved mechanicalproperties. The changes caused by drying became less significant after enzymeapplication. Handsheets from CelB-treated dried pulps showed an improvement oftensile and burst indexes while tear decreased. The effect produced by CelB canbe considered a biorefining step. In fact, by means of enzyme treatment withCelB the properties of paper manufactured from dried pulp equalled theproperties attained from wet fibres, with the exception of tear index. Changeswere also found in surface fibre morphology, such as flakes and peeling due tocellulase treatment. The surface modification of fibres with cellulases givesrise to better bonding properties and a closer structure of paper. The finalconclusion is that treatment with cellulases could compensate the hornificationeffect and lead to an important saving of refining energy. The novel enzyme,CelB, was the most effective in improving paper properties and counterbalancingthe hornification effect caused by drying.     

Adsorption of a Trichoderma reesei endoglucanase and cellobiohydrolase onto bleached Kraft fibres

Cellulases can be used to modify pulp fibres. For the development of biotechnical applications, a better understanding of the adsorption of cellulases onto commercial wood fibres is needed. In this work, the adsorption behaviour of purified CBH I and EG II on bleached Kraft fibres was investigated. Three variables were studied with respect to their effect on adsorption: fibre type (hardwood or softwood), fibre history (never-dried or once-dried), and ionic strength. The results showed that fibre history had the largest influence on the extent of adsorption of each enzyme. The effect of ionic strength was shown to be dependent on the enzyme and fibre type. At high ionic strength, CBH I exhibited a higher affinity for both once-dried and never-dried fibres at low enzyme concentrations; however, salt was shown to decrease the extent of adsorption at higher enzyme dosages. In contrast, salt increased the maximum adsorption of EG II, most notably on the once-dried hardwood fibres. Fibre type was also shown to affect adsorption behaviour. CBH I had a higher affinity for softwood fibres than for hardwood fibres at low enzyme concentrations. The maximum adsorption of EG II onto once-dried softwood fibres increased by 80% compared to the once-dried hardwood fibres. Interestingly, this did not correlate to in creased fibre hydrolysis. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reinforcing effect of carboxymethylated nanofibrillated cellulose powder on hydroxypropyl cellulose

Bionanocomposites of hydroxypropyl cellulose (HPC) and nanofibrillated cellulose (NFC) were prepared by solution casting. The various NFC were in form of powders and were prepared from refined, bleached beech pulp (RBP) by mechanical disintegration, optionally combined with a pre- or post mechanical carboxymethylation. Dynamic mechanical analysis (DMA) and tensile tests were performed to compare the reinforcing effects of the NFC powders to those of their never-dried analogues. For unmodified NFC powders an inferior reinforcing potential in HPC was observed that was ascribed to severe hornification and reagglomeration of NFC. In contrast, the composites with carboxymethylated NFC showed similar behaviors, regardless of the NFC suspensions being dried or not prior to composite preparation. SEM characterization confirmed a homogeneous dispersion of dried, carboxymethylated NFC within the HPC matrix. These results clearly demonstrate that drying of carboxymethylated NFC to a powder does not decrease its reinforcing potential in (bio)nanocomposites.     

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.     

How does the never-dried state influence the swelling and dissolution of cellulose fibres in aqueous solvent?

The swelling and dissolution capacity of dried and never-dried hardwood and softwood pulps and cotton linters was compared in two aqueous solvents, N-methylmorpholine-N-oxide (NMMO)-water at 90 °C with water contents ranging from 16 to 22% and NaOH—water at −6 °C with NaOH contents ranging from 5 to 8%. Swelling and dissolution mechanisms were observed by optical microscopy and dissolution efficiency was evaluated by recovering insoluble fractions. The results show a contrasted picture towards the effect of the never-dried state on the swelling and the dissolution capacity depending on the origin of the fibres and the type of aqueous solvent. In the case of NMMO—water, the presence of water within and around the fibre does not seem to favour dissolution initiation but after 2 h of mixing the dissolution yield appears to be similar for either dried or never-dried state. The limiting factor for dissolution in NMMO—water is not the penetration of the solvent inside the cellulose fibres, but only the local concentration of NMMO molecules around the fibre. For NaOH—water, both optical microscopy observations on individual fibres and dissolution yield measurements show that the never-dried state is more reactive for softwood pulps and cotton linters and has no significant effect on hardwood pulps. In this case, the local decrease of solvent strength is counteracted by the opening of the structure in the never-dried state which should enable the Na⁺ hydrated ions to penetrate easier.     

Carbon-13 NMR evidence for cocrystallization of cellulose as a mechanism for hornification of bleached kraft pulp

Solid-state ¹³C NMR spectroscopy was used to characterize a bleached softwood kraft pulp in the never-dried state and after cycles of drying and remoistening. Changes in NMR signal strengths indicated that growth of crystalline domains involved cocrystallization rather than accretion of cellulose from noncrystalline domains. A cluster of C-4 signals at 89.4 ppm, assigned to the interiors of crystalline domains, grew at the expense of C-4 signals at 84.0 and 84.9 ppm, assigned to the well-ordered surfaces of crystalline domains. Irreversible changes were not detected until the moisture content dropped below 18%. They were enhanced by a second drying/remoistening cycle, but showed little further change on subsequent cycles. The necessary conditions resembled those reported for hornification, suggesting that cocrystallization might provide a mechanism for hornification.     

Susceptibility of never-dried and freeze-dried bacterial cellulose towards esterification with organic acid

The susceptibility of (1) never-dried and (2) freeze-dried bacterial cellulose (BC) towards organic acid esterification is reported in this work. When never-dried BC (BC which was solvent exchanged from water through methanol into pyridine) was modified with hexanoic acid, it was found that the degree of substitution (DS) was significantly lower than that of hexanoic acid modified freeze-dried BC. The crystallinity of freeze-dried BC hexanoate was found to be significantly lower compared to neat BC and never-dried BC hexanoate. This result, along with the high DS indicates that significant bulk modification occurred during the esterification of freeze-dried BC. Such results were not observed for never-dried BC hexanoate. All these evidence point towards to fact that freeze-dried BC was more susceptible to organic acid esterification compared to never-dried BC. A few hypotheses were explored to explain the observed behaviour and further investigated to elucidate our observation; the effect of residual water in cellulose, the accessibility of hydroxyl groups and the crystal structure of never-dried and freeze-dried BC on the susceptibility of cellulose fibrils to esterification, respectively. However, the investigation of these hypotheses raised more questions and we are still left with the main question; why do BC nanofibres behave differently when modifying freeze-dried BC or never-dried BC?     

Qualitative evaluation of microfibrillated cellulose using the crill method and some aspects of microscopy

It has been a challenge to develop rapid online characterisation techniques for nanocellulose given the fibrillar structure of the nanoparticles. The crill optical analyser uses optical response signals in the infrared (IR) and ultraviolet (UV) wavelength ranges to evaluate the particle size properties of micro/nanofibrillar cellulosic materials. In this work, the crill analyser was used to measure the projected areas of UV and IR light sources by measuring the light blocked by nanocellulosic particles. This work uses the crill methodology as a new, simplified technique to characterise the particle size distribution of nanocellulosic material based on chemi-thermomechanical pulp (CTMP), thermomechanical pulp (TMP), and sulphite pulp (SP). In the first part, hydrogen peroxide pretreatment of CTMP and TMP in a wing mill refiner followed by high-pressure homogenisation to produce microfibrillated cellulose (MFC) was evaluated using the crill method. In the second part, TEMPO oxidation of CTMP and SP combined with high-shear homogenisation to produce MFC was studied using the crill method. With 4 % hydrogen peroxide pretreatment, the crill values of the unhomogenised samples were 218 and 214 for the TMP and CTMP, respectively, improving to 234 and 229 after 18 homogenisation passes. The results of the TEMPO method indicated that, for the 5 mmol NaClO SP-MFC, the crill value was 108 units at 0 min and 355 units after 90 min of treatment, a 228 % improvement. The CTMP and TMP fibres and the MFC were freeze dried and fibrillar structure of the fibres and microfibrils was visualised using scanning electron and transmission electron microscopy.     

Surface selective removal of xylan from refined never-dried birch kraft pulp

In this study, the effect of enzyme treatment on refined, never-dried bleached birch kraft pulp was investigated, using an endo-1,4-β-xylanase, that is substantially free from cellulase activity. The xylanase treatment of refined never-dried pulp revealed a rapid initial hydrolysis rate with a time-dependent saturation level in the amount of hydrolyzed pulp carbohydrates. Surprisingly short xylanase treatment times were found to have an impact on the fiber surface structure and on the physicochemical properties of kraft pulp fibers. Xylanase treatment led to mild microscopic differences in the ultrastructure of a never-dried fiber, whereas local topographical differences were distinguishable with atomic force microscopy. Results from the analysis of dissolved carbohydrates and the interfacial properties of the xylanase-treated never-dried fibers thus confirm a selective removal of xylan from the fiber surfaces. The zeta-potential charge and dewatering properties of the pulp slurry, fiber morphology, and strength properties of the paper were affected, which is a concomitant of xylanase treatment. However, the papermaking properties of the fibers were mainly preserved with simultaneous improvement in the dewatering rate of the pulp. Thus, optimized xylanase treatment of refined bleached kraft pulp provides a fiber for papermaking or fiber modification purposes with a selectively modified chemical composition of the fiber surface layer.     

Flocculated flow of microfibrillated cellulose water suspensions: an imaging approach for characterisation of rheological behaviour

Our aim was to characterise the suspension rheology of microfibrillated cellulose (MFC) in relation to flocculation of the cellulose fibrils. Measurements were carried out using a rotational rheometer and a transparent cylindrical measuring system that allows combining visual information to rheological parameters. The photographs were analyzed for their floc size distribution. Conclusions were drawn by comparing the photographs and data obtained from measurements. Variables selected for examination of MFC suspensions were degree of disintegration of fibres into microfibrils, the gap between the cylinders, sodium chloride concentration, and the effects of changing shear rate during the measurement. We studied changes in floc size under different conditions and during network structure decomposition. At rest, the suspension consisted of flocs sintered together into a network. With shearing, the network separated first into chain-like floc formations and, upon further shear rate increase, into individual spherical flocs. The size of these spherical flocs was inversely proportional to the shear rate. Investigations also confirmed that floc size depends on the geometry gap, and it affects the measured shear stress. Furthermore, suspension photographs revealed an increasing tendency to aggregation and wall depletion with sodium chloride concentration of 10⁻³ M and higher.     

Agriculture crop residues as a source for the production of nanofibrillated cellulose with low energy demand

Nanofibrillated cellulose (NFC) from three agricultural crop (rice straw, corn and rapeseed stalk) residues was isolated with high-yield production using either high pressure homogenisation or a high speed blender. The fibres were extracted from the neat biomass via an NaClO₂/acetic acid and alkali pulping process. TEMPO-mediated oxidation pretreatment at pH 7 and 10 was accomplished to facilitate the release of the cellulose microfibrils. The fibrillation yield, transparency degree and morphological characteristics of the ensuing NFC were analysed using the centrifugation method, transmittance measurement and SEM observation. The energy consumption during the disintegration process was also accessed. It was shown that the mode of lignin removal and the fibre pretreatment notably affected the nanofibrillation efficiency and energy demand. A successful production of NFC with yield exceeding 90 %, using a simple Waring blender, was achieved when the NaClO₂/acetic acid delignification followed by a TEMPO-NaBr–NaClO oxidation at pH 10 was adopted.     

Indirect evidence of supramolecular changes within cellulose microfibrils of chemical pulp fibers upon drying

Dried and never-dried chemical pulps were subjected to strong sulfuric acid hydrolysis and the dimensions of the resulting cellulose nanocrystals (CNCs) were characterized by AFM image analysis. Although the average length of CNCs was fairly similar in all samples (55–65 nm), the length distribution histograms revealed that a higher number of longer crystals and a lower number of shorter crystals were present in the CNC suspensions prepared from never-dried pulps. The distinction was hypothetically ascribed to tensions building in individual cellulose microfibrils upon drying, resulting in irreversible supramolecular changes in the amorphous regions. The amorphous regions shaped by tensions were deemed as more susceptible to acid hydrolysis.     

Enzymatic Hydrolysis of Recovered Office Printing Paper with Low Enzyme Dosages to Produce Fermentable Sugars

The use of recovered paper and paper manufacturing wastes are a potentially large, concentrated, and convenient raw material for ethanol production via enzyme hydrolysis and fermentation. However, many previous studies in the area have investigated impractically high enzyme charges. In this research, low dosages of enzymes on copy paper (CP) were investigated for the conditions of 5% consistency (w/v) and 50 °C for 48 h. The removal of inorganic filler (mainly calcium carbonate) by washing prior to hydrolysis led to higher sugar yields than the unwashed CP as well as CP acidified to remove the ash. Enzyme adsorption measurement showed that both acid-soluble ash and acid-insoluble ash adsorb enzymes with a greater affinity than fibers. Drying of the fibers (termed hornification) decreased the efficiency of enzyme hydrolysis, confirming previous results. The mechanical refining at 10% consistency in a laboratory refining mill of previously dried fibers improved the sugar recovery to similar or higher levels as never-dried fibers. By plotting water retention value (WRV) versus corresponding sugar recovery, it was shown that WRV is more useful at low enzyme charges reflecting the use of refining and the reversal of hornification. For de-ashed and refined copy paper, the sugar recovery was determined to be 82% and 97% with enzyme dosages of 4 and 8 Filter Paper Unit (FPU)/g oven-dried (OD) substrate.     

Characterization of magnetic membranes based on bacterial and man-made cellulose

Ferrites were synthetized in situ in two different neutral cellulose gels: a never-dried bacterial cellulose membrane and a never-dried cast film using N-methylmorpholine-N-oxide as solvent. X-ray diffraction, transmission electron microscope (TEM), vibrating sample magnetometry (VSM) and Mossbauer spectroscopy were used to characterize the resulting magnetic nanocomposites. TEM micrographs showed the presence of ferrites in two different shapes, acicular and equiaxial, respectively hydrated ferric oxides (FeOOH) and the spinel oxides: maghemite (gamma-Fe2O3) or magnetite (Fe3O4). Thin sections of bacterial cellulose showed these particles to be located along the cellulose microfibrils, which are assumed to provide a site for their nucleation. Room temperature magnetization curves showed all samples to be superparamagnetic     

The porous structure of pulp fibres with different yields and its influence on paper strength

The porous structure of the interior of papermaking fibres is a well-known important property of the fibres. Changes of this structure will influence tensile and burst strength of paper formed from the fibres and a change in pore size of the pores within the fibre wall is also important for the ability of molecules to diffuse in and out of the fibre wall. Relevant examples of this latter effect are the removal of lignin during cooking and the addition of performance chemicals during papermaking. In this paper, pore sizes and the pore size distribution of unbleached softwood fibres have been studied. A well-characterised fibre material consisting of laboratory cooked spruce and pine pulp of various lignin contents was used. Pore size and pore size distribution were measured by studies of the relaxation behaviour of ²H in fibres saturated with ²H₂O. Beside this the total and surface charge of the fibres were also measured together with strength properties of papers from unbeaten fibres. For both pulps, there is a maximum in pore radius at a yield around 46%. Calculations of fibre wall volume from water retention values and yield levels show that there is a discontinuity in pore radius as a function of the fibre wall volume around a yield of 51%. It is suggested that this discontinuity is caused by the breakdown of the hemicellulose/lignin matrix within the fibre wall at this yield level. The strength of the papers formed from the fibres shows a correlation with the surface charge of the fibres. Based on the change in surface charge with yield and the change in total charge with yield, this correlation is suggested to be due to an opening up of the external part of the fibre wall. This stresses the importance of the chemical composition and physical structure of the outer layer of the fibre wall.     

Swelling of carboxymethylated cellulose fibres

Swelling of cotton cellulose fibres having different proportions of carboxyl groups in the H-form was studied. The carboxyl groups were introduced by carboxymethylation under different reaction conditions. By studying the swelling of modified cellulose samples (water retention value of non-dried fibre) it was shown that the concentration of sodium hydroxide was the dominant factor among the investigated reaction parameters. The number of acidic groups was found to play a significant but not determinative role in the level of improvement in swelling caused by carboxymethylation. A linear correlation was observed between swelling and iodine sorption capacity. The degree of collapse of the highly accessible structure of cellulose during drying (hornification) was larger in the case of more accessible carboxymethylated fibres than for the alkali treated sample. The degree of hornification increased with growing swellability and with growing number of carboxyl groups in the investigated interval (40–120 mmol carboxyl/mol cellulose). This type of modified cellulosic fibre could be used for enhanced entrapping and release of chemicals. This revised version was published online in August 2006 with corrections to the Cover Date.