Fiber Suspensions Research Articles

The shear and compressive yield stress of fibrillated acacia pulp fiber suspensions

Abstract The degree of interactions between fibers and the tendency of fibers to form flocs play an important role in effective unit operation in pulp and paper industry. Mechanical treatments may damage the structure of the fiber cell wall and geometrical properties, and ultimately change the fiber-fiber interactions. In this study, the gel crowding number, compressive and shear yield stress of fibrillated acacia pulps were investigated, and the results showed that the gel crowding number of the refined pulp samples ranged from 8.7 to 10.7, which were much lower than that of un-refined pulps. As the concentration increased, both the compressive yield stress P y {P_{y}} and shear yield stress τ y {\tau _{y}} of all suspensions increased accordingly, and the yield stress was found to depend on a power law of the crowding number. Moreover, the values of τ y / P y {\tau _{y}}/{P_{y}} were also examined and the variation of τ y / P y {\tau _{y}}/{P_{y}} became largely dependent on the fiber morphology and mass concentration.

Predicting scattering properties of fiber suspensions using Mie theory and probabilistic cross-sectional diameter of fibers

Scattering of visible light by micrometer-scale natural wood fibers is usually treated by assuming fibers to be perfect long cylindrical scatterers. In industrial processes, however, fibers experience deformations and are far from ideal cylinders. Variation in fiber morphology affects their scattering properties and it poses a challenge for reliable process measurements. In this paper, we have studied experimentally scattering of both deformed natural and ideal artificial non-absorbing fibers in aqueous suspension and their response to mass concentration of fibers. Experimental results are compared with the predictions of the Mie theory which is combined with cross-sectional diameter probability distribution of fibers. It is shown that the diameter distribution of the fibers together with Mie theory provides results that agree with experiments in case of both natural and ideal fibers.

Inertial settling of flexible fiber suspensions

Inertial settling of rigid and flexible fibers in suspension is investigated using numerical simulations. Clumping of fibers in the semidilute regime leads to faster settling; flexible fibers exhibit more clumping and higher settling velocities. At higher fiber concentrations, mobility becomes limited, thus fiber orientation and deformation become insensitive to concentration.

The effects of process conditions on rheological properties of functional citrus fibre suspensions

A deeper knowledge of rheological effects of dietary fibre in aqueous suspensions and their dependence on operating conditions adopted in their preparation seems necessary to design properly systems with expected rheological properties. It is known that changes in “mixing intensity” can modify the observed rheological properties of fibre suspensions; nevertheless only a small number of previously published data is available, and, usually, they are related to technological parameters such as speed of mixing (rpm), homogenisation pressure, residence time, etc. In the present work, the energy and the power of mixing were used, in a systematic investigation, and it was observed that by increasing both of them a stronger three-dimensional network is obtained, as the result of the interactions among the dispersed particles. Nevertheless, when higher levels of energy or power are used, no further increase in network strength can be obtained. The proposed approach and the obtained results can be relevant for industrial applications in order to scale-up the production of fibres’ suspensions.

On the development of a continuous methodology to fractionate microfibriallated cellulose

Abstract The focus of this study is the development of a methodology to mechanically separate or fractionate micro-fibrillated fibre suspensions (MFC) into different size classes. We extend the principle outlined by Madani et al. (2010) and create a continuous separation in an annular gap undergoing spiral Poiseuille flow (solid body rotation superimposed on pressure driven flow). Achieving hydrodynamic stability of this flow was the main scientific challenge for scale-up. This work is presented in two different studies. In the first study, we perform a series of batch-wise centrifugation tests to develop the criteria for motion of the individual classes of particles which compose a Eucalyptus MFC suspension. Here, we suspend the MFC in a weak gel and demonstrate a linear reduction in average particle size with increasing centrifugal force; motion is initiated in heavier particles before the lighter ones. In the second study, we use this batch-wise data to design a continuous prototype and we successfully demonstrate a continuous separation with performance similar to that achieved in the batch-wise tests.

Sodium dodecyl sulphate (SDS) residue analysis of foam-formed cellulose-based products

AbstractSurfactant residues in foam forming are related to the circulation of surfactant in the foaming process, but they may also affect the properties of the foam-formed product. In this work, residues of an anionic surfactant, sodium dodecyl sulphate (SDS), in foam-formed products were studied. Two different methods were applied for the determination of SDS: ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy) and solvent extraction spectrophotometry. The dry matter content of the sample before the drying step has a significant effect on the SDS residue. Another factor that has a significant effect on the SDS residues is the amount of precipitated dodecyl sulphate (DS). It was found that SDS does not degrade in the drying phase and will remain in the salt form in the final product. SDS residue decreased when the temperature of the fibre suspension was increased. Theoretical values for SDS residues were estimated from the original surfactant concentration and the amount of water remaining in the product before the drying phase. When all the surfactant was in soluble form, measured SDS residues were close to the theoretical values, which indicates that SDS was not permanently adsorbed onto softwood cellulose fibres.

Towards improved understanding of the viscoelastic properties of functionalized lemon peel fibers in suspension based on microstructure, hydration value and swelling volume

Lemon peel fibers were obtained using different processing conditions (pH, addition of salts, mechanical processing and storage), they showed distinctly different viscoelastic properties when dispersed in an aqueous solution. In order to gain insight in the mechanism underlying their texturizing properties, the volume occupation in dispersion was determined through small molecule diffusion experiments and the swelling volume was measured by a conventional method. Based on the diffusion coefficient of glucose as a function of fiber concentration, the hydration value and volume occupancy of differently functionalized fibers were calculated. The diffusion based volume occupancy and the swelling volume were related with the fiber suspension microstructure and its rheological properties. High pressure homogenization (HPH) improved the viscoelastic properties, mainly due to an increase of the swelling volume. Salt addition and pH influenced the hydration value, the swelling volume and viscoelastic properties. These results show that the texturizing properties of lemon peel fibers induced by HPH, are mainly determined by the extent of swelling and network formation rather than an increased fiber hydration value.

Impact of Drying on Meso- and Nanoscale Structures of Citrus Fiber: A Study by SFG, ATR-IR, XRD, and DLS

Citrus fibers are a sidestream of the pectin extraction process from citrus peel. This sidestream can be converted into a functional ingredient through a shear-induced homogenization process. One technical challenge with this material is that dehydration and subsequent rehydration result in reduction of viscosity compared to the original product. In this study, various drying methods were compared with never-dried fibers to investigate the structural changes underlying the viscosity loss. Infrared and X-ray diffraction analyses confirmed no changes in chemical composition and crystalline structure of citrus fibers. The dynamic light scattering and sum frequency generation analyses of citrus fiber suspension showed that the rehydration process could not fully disperse aggregated fibers, which appears to be the main cause for the viscosity loss.

Impact of microfluidization on the microstructure and functional properties of pea hull fibre

The goal of the present work was to evaluate the impact of the particle size (D90) of microfluidized pea hull fibre suspensions on the physical microstructure, composition and functional properties (i.e. water retention capacity and rheological behaviour). To that end, fibre suspensions with D90 of 120, 100, 80 and 60 μm were produced. Among them, the suspensions of 100 and 80 μm were produced employing two different processing conditions to evaluate the impact of processing on the microstructure and functionality. Suspensions with smaller particles (D90 ≤ 80 μm) presented a thermal stable, pseudoplastic, thixotropic and viscoelastic behaviour varying their elastic modulus in the linear viscoelastic region from 19 to 89 Pa for suspensions with D90 of 80 and 60 μm. By decreasing the D90, it was observed a continuous defibrillation of aggregates of cellulosic macrofibrils, into macrofibrils and microfibrils. Consequently, the number of particles and their interactions by electrostatic and friction forces increased. Alcohol insoluble substances, released from the hemicellulosic and pectic networks, were only detected for the suspensions with D90 = 60 μm. In this case, particles presented inter-fibrillar voids that may increase their flexibility and functionality. For a given particle size distribution, the processing conditions had negligible effect on the functionality. In summary, functionality of microfluidized fibre suspensions may be tuned by controlling the release of alcohol insoluble substances and the defibrillation of the cellulosic network, process conditions then can follow energetic or economic criteria.

Impact of the Fiber Length Distribution on Porous Sponges Originating from Short Electrospun Fibers Made from Polymer Yarn

AbstractUltralight highly porous sponges made of short electrospun polymer fibers have gained significant attention for a variety of applications. According to the established procedures, short electrospun fibers are obtained by cutting or homogenization of electrospun fibers in suspension, which yield fibers with inhomogeneous fiber length. The role of the fiber length distribution and the fiber length in the mechanical compressibility of the sponges is unknown. Therefore, as a model study, sponges made from suspensions of short electrospun poly(acrylonitrile) (PAN) fibers with controlled fiber length distribution are investigated, and the role of the fiber length distribution in the compressibility of the sponges is analyzed quantitatively. These sponges are also compared to the ones prepared by established procedure as a benchmark. It is found that the compression stress and modulus of ultralight sponges with monodisperse short fibers are respectively 32% and 45% higher than that made with polydisperse short fibers. The study also shows that sponges made from longer fibers have higher modulus in comparison to the sponges made from shorter fibers.

Microrheology imaging of fiber suspensions - a case study for lyophilized collagen I in HCl solutions.

In fiber suspensions with low optical contrast, the in situ characterization of structural properties with conventional microscopy methods fails. However, overlaying subsequent images of multiple particle tracking (MPT) videos including short trajectories usually discarded in MPT analysis allowed for direct visualization of individual fibers and the network structure of lyophilized collagen I (Coll) distributed in hydrochloric acid solutions. MPT yielded a broad distribution of mean square displacements (MSDs). Freely diffusing tracer particles yielded viscosities indicating that, irrespective of concentration, a constant amount of Coll is dissolved in the aqueous phase. Particles found elastically trapped within fibrous Coll structures exhibited a broad range of time-independent MSDs and we propose a structure comprising multiple fiber bundles with dense regions inaccessible to tracers and elastic regions of different stiffness in between. Bulky aggregates inaccessible to the 0.2 μm tracers exist even at low Coll concentrations, a network of slender fibers evolves above the sol-gel transition and these fibers densify with increasing Coll concentration. This novel MPT-based imaging technique possesses great potential to characterize the fiber distribution in and structural properties of a broad range of biological and technical suspensions showing low contrast when imaged with conventional techniques. Thus, MPT imaging and microrheology will help to better understand the effect of fiber distribution and network structure on the viscoelastic properties of complex suspensions.

A Correlation of fiber orientation on dynamic viscoelasticity in fiber suspension

A Correlation of fiber orientation on dynamic viscoelasticity in fiber suspension

Flow behavior of Cellulose Nano Fiber Suspension in Planar Channels with an Abrupt Contraction

Flow behavior of Cellulose Nano Fiber Suspension in Planar Channels with an Abrupt Contraction

Application of constitutive equation considering fiber orientation in shear flow of fiber suspension

Application of constitutive equation considering fiber orientation in shear flow of fiber suspension

Dispersion of 24-mm staple fibers with foam

In addition to carding and airlaid processes, nonwovens are produced from staple fibers by a wet-laid process. A drawback of this process is the necessity to use very dilute fiber suspensions to avoid fiber entanglement and consequent poor fiber web uniformity. As a result, flow volumes are very high and process speeds are rather low compared to water forming used in the paper and board industry. A promising option for making nonwovens is foam laying. The bubbles in foam keep the fibers apart until the foam is removed, and much higher fiber weight consistencies can be used compared to traditional wet laying. A key challenge in foam forming of nonwovens is to obtain uniform dispersion of the fibers in the foam. In this work, we studied this with 24-mm staple fibers, and analyzed the homogeneity of the obtained foams by making fibrous sheets from them in a laboratory sheet mold. We found that dispersion was highly dependent on the mixing conditions, such as mixing time, foam air content, and fiber weight consistency. Remarkably, excellent fiber disintegration and uniform sheets were obtained without mechanical pre-treatment of staple fibers, with fiber consistencies as high as 0.3%. By comparison, conventional wet-laid processes typically operate with fiber consistencies lower than 0.05%. Thus, at an industrial scale, foam forming promises production of uniform webs from stable fibers with clearly lower water volumes and possibly also higher machine speeds compared to conventional wet-laid forming.

Structural characterization and mechanical properties of wet-processed fibreboard based on chemo-thermomechanical pulp, furanic resin and cellulose nanocrystals

Fibreboards are made of lignocellulosic fibres and synthetic adhesive which connect them. These synthetic adhesives, while relatively low-cost, are usually non-biodegradable and may cause health and environmental issues. Therefore, in recent years, there has been an increased demand for replacing these adhesives with bio-derived adhesives. The present study aims to develop fibreboards from chemo-thermomechanical pulp (CTMP) and a furanic resin based on prepolymers of furfuryl alcohol via wet-processing. To improve the bonding properties, maleic acid, aluminium sulphate, and cellulose nanocrystals (CNCs) were added. The resulting fibreboards were evaluated for their structural features and mechanical properties. The bending strength was improved when CNCs were added into the fibre's suspension, and the morphology indicated a more compact structure. The combination of the CTMP with CNC and Biorez resulted in the same mechanical behaviours as those noted for CTMP alone, the best performance being observed for the boards in which Al2(SO4)3 was added. Infrared spectroscopy and X-ray diffraction also proved the presence of cellulose nanocrystals and resin in the boards by increased specific bands intensity and crystallinity index, respectively.

Learning the Macroscopic Flow Model of Short Fiber Suspensions from Fine-Scale Simulated Data.

Fiber–fiber interaction plays an important role in the evolution of fiber orientation in semi-concentrated suspensions. Flow induced orientation in short-fiber reinforced composites determines the anisotropic properties of manufactured parts and consequently their performances. In the case of dilute suspensions, the orientation evolution can be accurately described by using the Jeffery model; however, as soon as the fiber concentration increases, fiber–fiber interactions cannot be ignored anymore and the final orientation state strongly depends on the modeling of those interactions. First modeling frameworks described these interactions from a diffusion mechanism; however, it was necessary to consider richer descriptions (anisotropic diffusion, etc.) to address experimental observations. Even if different proposals were considered, none of them seem general and accurate enough. In this paper we do not address a new proposal of a fiber interaction model, but a data-driven methodology able to enrich existing models from data, that in our case comes from a direct numerical simulation of well resolved microscopic physics.

Dewatering saturated, networked suspensions with a screw press

A model is presented for the dewatering of a saturated two-phase medium in a screw press. The model accounts for the detailed two-phase rheological behaviour of the pressed material and splits the press into two zones, an initial well-mixed constant-pressure region followed by an axial transport region in which the total pressure steadily increases. In this latter region, a slowly varying helical coordinate transformation is introduced to help reduce the dynamics to an annular bi-axial compression of the two-phase medium. Unlike previous modelling, the transition point between the two zones is determined self-consistently, rather than set a priori, and the pressure along the length of the press is deduced from the rheology of the two-phase flow rather than averaging the two-phase dynamics over a cross section of the press. The model is compared to experimental observations of the dewatering of a paper-making fibre suspension and of a clay slurry, and is shown to reproduce operational data.

Morphologies and characteristic of glass fiber suspensions basing on various beating speeds

Glass fibers commonly flocculate in suspensions and slurries, which can be largely prevented by a beating process. In this paper, morphologies and characteristics of glass fiber suspensions resulting from various beating speeds are explored. By increasing the speed (ranging from 1500 revolutions to 12000 revolutions), glass fibers can be translated, rotated, bended and broken, which leads the drainage resistances of glass fiber suspensions increase dramatically from 19.5 °SR to 23.5 °SR, then fluctuate and settle close to 22.5 °SR. Decreasing the fiber length leads to reduction in fiber–fiber contact and improves the uniformity of fiber suspensions. The separation and random distribution of glass fibers decreases in the viscosity of the fiber suspension.

Oriented suspension mechanics with application to improving flow linear dichroism spectroscopy.

Flow linear dichroism is a biophysical spectroscopic technique that exploits the shear-induced alignment of elongated particles in suspension. Motivated by the broad aim of optimizing the sensitivity of this technique, and more specifically by a hand-held synthetic biotechnology prototype for waterborne-pathogen detection, a model of steady and oscillating pressure-driven channel flow and orientation dynamics of a suspension of slender microscopic fibres is developed. The model couples the Fokker–Planck equation for Brownian suspensions with the narrow channel flow equations, the latter modified to incorporate mechanical anisotropy induced by the particles. The linear dichroism signal is estimated through integrating the perpendicular components of the distribution function via an appropriate formula which takes the biaxial nature of the orientation into account. For the specific application of pathogen detection via binding of M13 bacteriophage, it is found that increases in the channel depth are more significant in improving the linear dichroism signal than increases in the channel width. Increasing the channel depth to 2 mm and pressure gradient to 5 × 104 Pa m−1 essentially maximizes the alignment. Oscillating flow can produce nearly equal alignment to steady flow at appropriate frequencies, which has significant potential practical value in the analysis of small sample volumes.