MODELLING AND CORRELATING THE PERMEABILITY OF PULP AND PAPER

ABSTRACT

A sheet of paper is modelled as a network of cellulose fibres, either cylindrical or band-shaped. The equations for creeping flow through such structures are solved, and The calculated permeabilities are compared with measured values. Flow through some paper structures such as pulp sheets and handsheets of unbeaten sulphate pulp is adequately described by the structural model involving band-shaped fibres when a fibre aspect ratio of 3.5 is used. For newsprint sheets the measured permeability is lower than that predicted by the models when physically realistic values of the aspect ratio are taken. A total of 19 different paper grades have been characterised by measurement of the total specific surface area and The fibre orientation ratio in addition to the measurements of effective diffusivity, permeability and porosity. Permeability and effective diffusivity correlate with each other and permeability correlates with fibre orientation, so that at constant porosity, permeability decreases with increasing fibre orientation.     

Fibre orientation structures and their effect on crack resistance of injection moulded transverse ribbed plate

Abstract

An extensive study of the fibre orientation structures developed in a transverse ribbed plate during injection moulding, and the use of these structures to investigate the effect of local fibre orientation state on crack initiation resistance, is reported. The fibre orientation results for the ribbed plate, measured using large area image analysis system developed at Leeds University, showed that after an initial settling down period, the central core region, where the fibres are aligned perpendicular to the flow direction, decreased in size monotonically, with an associated monotonic increase in the outer shell regions, where the fibres are aligned preferentially along the injection direction. Interestingly, the level of orientation in the two regions remained almost constant: only the proportions of the two regions were found to change with flow length. Across the plate, close to the gate, the central core region was found to have a lens-like shape, while at the other end of the plate the core was thinner and also consistent in thickness across the sample width. The transverse rib was found to cause little disturbance to the fibre orientation of the base plate. The different proportions of the shell and core regions at different locations over the ribbed plate provided an ideal case to test the proposition of Friedrich that the crack resistance of a short fibre reinforced material depends on the number of fibres that are perpendicular to the crack tip. The impact test results gathered in this way confirmed this hypothesis of Friedrich.     

Controlled orientation of short fibre reinforcement for anisotropic performance of rubber compounds

Abstract

The potential advantages of introducing anisotropic properties into a rubber compound by replacing some particulate filler with short fibre have been investigated. The control compound (containing no fibre) was based on NR and contained 60 pphr (N330) carbon black. In the fibre filled compound, 15 pphr of the carbon black was replaced with an equivalent volume of Santoweb fibre (10 pphr). Specimens with strong fibre orientation were produced and tensile, tear, compression, and shear properties measured at different angles to the orientation. Stretching parallel to fibre orientation resulted in a stiffness at low elongations × 3·5 higher than in the control compound. However, the tensile and tear strengths were lower than the control compound, probably owing to the high stresses at the fibre surfaces leading to fibre pullout. Maximum tensile and tear strengths were obtained when fibre orientation was at angles of 20° and 45°, respectively, to the stretching direction. Stiffness at a small (5%) compression was greater when fibres were oriented parallel to the compression direction. At greater compressions, fibre buckling reduced the stiffening effect. When fibres were oriented at 45° to the plane of shearing, shear stiffness in one direction was twice the shear stiffness in the opposite direction and twice the value for the control compound. Together with greater stiffness, this orientation led to low heat loss during dynamic shearing at a particular stress amplitude (40% of the value for the control compound).     

Smart composite materials for self-sensing and self-healing

Abstract

The various methods of self-sensing and self-healing developed within the Composite Systems Innovation Centre, University of Sheffield, are reviewed. Damage sensing using electrical resistance in carbon fibre reinforced composite or using the fibres as optical sensing elements in glass fibre reinforced composite is demonstrated. Amelioration of low level damage is demonstrated in both monolithic composite materials and sandwich structures using direct chemical reactions within the matrix without the use of encapsulants. These reactions can be activated by resistive heating of the material itself. The use of a combination of these techniques could create a truly smart structure able to both sense and repair damage and degradation.     

Computer Simulation of the Flow Field and Particle Deposition by Diffusion in a 3-D Human Airway Bifurcation

ABSTRACT

Air flow and ultrafine particle diffusion patterns within an airway bifurcation are investigated by numerically solving the corresponding full Navier-Stokes equations in a three-dimensional curvilinear coordinate system using PHOENICS flow-simulation software. Weibel's human lung morphology, airway generation I = O, was applied in the computer simulation. Computational parameters such as grid sizes, boundary condition treatments, iteration numbers, and convergence criteria have been verified by experimental data measured within the same (mathematically specified) bifurcation geometry. Velocity patterns generated by computer simulation show excellent agreement with the experimental results. Detailed velocity and particle concentration distributions are discussed. Results show that both the inlet and outlet flow boundary conditions have significant effects on the flow and concentration distribution patterns within the bifurcation. The scales of secondary flow within the daughter branches are shown be particularly sensitive to the inlet boundary condition.     

Crystallographic texturing in single poly(p-phenylene benzobisoxazole) fibres investigated using synchrotron radiation

A micro-focus synchrotron beam has been used to investigate crystallographic texturing in poly(p-phenylene benzobisoxazole) fibres. By generating diffraction patterns across single fibres, it is possible to produce profiles showing scattering intensity as a function of beam position across the fibre. A straightforward model has been developed to allow the degree of texturing to be quantified for direct comparison between fibre types. The experimental results are found to fit a radial fibrillar-texturing model, which incorporates a distribution in radial orientation about the fibre axis. Previous studies reporting the a-axis of the PBO unit cell to be aligned radially within fibrils about the fibre axis are found to be correct.The degree of radial fibrillar texturing is in the same fibre order as tensile modulus and crystalline domain orientation for PBO fibres with different processing histories. It is proposed that the degree of radial fibrillar texturing is therefore related to fibre homogeneity. An extrapolation of tensile modulus to that of a perfectly homogeneous fibre results in a value in good agreement with the PBO crystal modulus. This further supports the proposal that the degree of radial fibrillar texturing is related to fibre homogeneity.     

Effects of heat treatment temperatures on microstructures and mechanical properties of the chopped carbon fibres SiC composites

ABSTRACT

A simple wet papermaking technique was used to fabricate chopped carbon fibre paper (C_sf-paper) with random fibre orientation and self-supporting network structures in this work. The C_sf-papers were laminated layer by layer and further infiltrated with PyC interface and SiC matrix via chemical vapour infiltration (CVI) to obtain C_sf/SiC composites. The effects of heat treatment temperatures on microstructures, phase composition, and mechanical properties were investigated. Results showed that the C_sf-paper has played a good self-supporting role and its fibres form a completely random fibre orientation in 2D plane. The fibres almost remained undamaged and unbroken during the CVI. Proper heat treatment could improve the mechanical properties. At 1200°C, the maximum values of flexural strength and Young's modulus reached about 306 MPa and 196.5 GPa, respectively. Meanwhile, compared with tape casting, reaction sintering and spark plasma sintering, composites fabricated by wet papermaking and CVI can improve the mechanical properties.     

Modelling method based on idealised fibre bundles

Abstract

In the fibrous structures such as textiles and composites there are fibre assemblies exhibiting statistical bundle like behaviour. This paper presents a modelling method and software FibreSpace, based on a system of structuralised statistical fibre bundles, so called fibre bundle cells. These fibre bundle cells introduced before represent different idealised and typified fibre properties such as fibre shape, state of deformation, gripping as a connection with the vicinity, and the characteristic of force-transmitting and damage. With the help of the weighted parallel connection of the fibre bundle cells the mechanical behaviour and the damage process of real fibrous systems can be modelled as well as some structural properties or the strength data of single fibres can be determined by a fibre bundle cells model identified on the basis of measurements. The applicability of the fibre bundle cells method and modelling program developed is demonstrated by modelling the load and damage process of real textile structures and unidirectional composites during tensile or flexural test.     

Molecular Mechanisms of Disperse Dyeing of Polyester and Nylon Fibres

The kinetics of the dyeing of polyester fibres with disperse dyes have previously been discussed in terms of a model in which the only forces of interaction between dye and polymer are those that give rise to mutual solubility. The rate and extent of dyeing depend on the size and shape of the dye molecules and the detailed structure of the fibre. The concentration profiles of dye within single filaments of polyesters and nylons have now been determined by a new method termed ‘optical sectioning’, in which an image of a filament is scanned by a narrow slit. The dye distributions are found by comparing experimental transmission values with those calculated by a computer for a model system in which various parameters can be adjusted. For the polyester-dye systems the rate constant of transfer of dye from dyebath to fibre (k₁) was the same as the rate constant of diffusion away from the interface (k₂). Some nylon-dye systems, on the other hand, behaved as if k₂ = k₁, whereas others behaved initially as if k₁ = k₂ but later as if k₂ > k₁. Apparent diffusion coefficients were also derived from sorption measurements and were found to fall as the dye concentration in the fibre increased. Measurement of the orientation of the dye molecules by optical dichroism showed that the dye molecules entering the filaments late in the dyeing process were more highly orientated than the earlier ones. The decrease in diffusion coefficient has been interpreted in terms of the observed higher orientation, as resulting from a greater entropy of activation in the diffusion process.     

Influence of carbon fibre orientation on reaction‐to‐fire properties of polymer matrix composites

The influence of the orientation of carbon fibres on the reaction‐to‐fire characteristics of a layered composite has been investigated in detail. 8552/IM7 prepregs were laid up to give unidirectional and quasi‐isotropic laminates. Specimen thickness (0.25 to 8.0 mm) and heat flux (15 to 80 kW/m²) were varied for irradiation. Fundamental reaction‐to‐fire properties of this composite are interpreted on the basis of the matrix components: epoxy resin and polyethersulfone. Cone calorimetry and temperature distributions through the laminate showed that the velocity and degree of combustion are dominated by fibre orientation for a given resin. In general, a quasi‐isotropic fibre orientation leads to faster ignition, because of preferred delaminations, but retards combustion processes more effectively than a unidirectional lay‐up. Migration velocities of the pyrolysis zone were measured. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluating unidirectional composite thermal conductivities through engineered interphase

ABSTRACT

Effect of fibre/matrix interphase parameters, including thickness and material properties on the equivalent thermal conductivities of unidirectional fibre-reinforced polymer composites. A unit cell-based micromechanical method is proposed to evaluate the thermal conductivities of unidirectional multi-phase composites. The longitudinal thermal conductivity of unidirectional fibre-reinforced polymer matrix composites is seen to be independent of interphase region. When the thermal conductivity of interphase is higher than that of matrix, the increase of interphase thickness leads to an improvement in transverse thermal conductivity of fibre-reinforced polymer composites. The influences of fibre volume fraction, orientation angle and shape of cross-section as well as temperature on the thermal conducting behaviour are widely examined. The model predictions are in good agreement with the experimental data reported in the literature.     

Piecewise Polynomial Solutions of Aerosol Dynamic Equation

This paper presents a discretization technique for particle dynamics equation based on piecewise continuous approximations of the solution. The growth terms are re-solved using a Discontinuous Galerkin approach, and the coagulation by a collocation approach. The method fits in the general framework recently proposed by the author. The discontinuous approximation allows better representations of distributions with sharp peaks. Numerical tests reveal that accurate solutions can be obtained with a very small number of size bins. An efficient software package AeroSolve which implements the proposed algorithms was developed.     

CMC material for train brake systems

Abstract

A continuous carbon fibre/silicon nitride matrix composite material has been produced by an inexpensive method. According to this method, the space between 2D carbon fibre preforms is filled with a S₃ N₄ powder by a pressure infiltration method. High particle packing densities are achieved within the fibre preforms in this way. The compact body is heat treated to form a porous framework without shrinkage, and is then strengthened with an inorganic matrix synthesised from a liquid pre-ceramic polymer. The densification degree, microstructure, and thermal and mechanical properties of the composite material are characterised. The C/Si₃ N₄ composite material pyrolysed at 1300°C is considered to be a very promising material for low temperature applications such as brake discs for rapid train systems.     

Hemp fibre as alternative to glass fibre in sheet moulding compound Part 1 – influence of fibre content and surface treatment on mechanical properties

Abstract

Hemp fibre mat reinforced unsaturated polyester composites were fabricated using a conventional sheet moulding compound process. The influence of fibre and CaCO₃ filler content on strength and stiffness of these hemp fibre reinforced sheet moulding compounds is reported and compared with data for chopped glass fibre reinforced sheet moulding compounds. In addition the influence of alkaline and silane treatments of the hemp fibres is evaluated. The experimental data are compared to modified versions of the Cox–Krenchel and Kelly–Tyson models, supplemented with parameters of composite porosity to improve the prediction of composite tensile properties. A good agreement was found between the modified models and experimental data for strength and stiffness. The results indicate that hemp fibre reinforced sheet moulding compounds are of interest for low cost engineering applications that require high stiffness to weight ratios.     

Determination of the Orientation of Crystallites in PET Fibres

Abstract

A method for the determination of the average angle of orientation of crystallites φ from the azimuthal record of the meridional reflection of the 1 05 diatropic plane for PET fibres is widely used. The 1 05 reflection is the closest to the meridian and the normal to this plane making an angle of approximately 10[ddot] with the c axis. The intensity distribution measured by this method reflects the distribution of the 1 05 plane normals and therefore it comprises: the distribution of the 1 05 plane normals relative to the c crystallographic axis, the distribution of the crystallites relative to the reference fibre axis and the tilted orientation; that is the molecular chain axis inclined by some degrees with respect to the fibre axis. By the studying of the diffraction intensity distribution from the 1 05 diatropic plane this particularly should be taken into account. The sub-meridional (1 05) reflection was scanned in two different geometries: omega and chi scan.

Supercritical CO₂ fluid as an alternative dyeing medium changes the fibre structure to a certain extent in dependence on the treatment temperature and pressure used. Therefore the changes of crystalline orientation in poly(etbylene terephthalate) (PET) fibres as brought about under the influence of supercritical CO₂ fluid were investigated.     

Structure of a High Efficiency Glass Fiber Filter Medium

ABSTRACT

The analysis of the real internal structure of a high efficiency glass fiber filter medium was carried out to provide detailed structure data, e.g., for more realistic calculations of the filter performance. The procedure of the structure analysis is described in detail and the most important results are presented and discussed. The fiber diameter distribution was found to be constant throughout the HEPA medium studied. The fibers tended to be oriented perpendicular to the direction of air flow, but there was no preferred direction of orientation within the plane of the filter medium. The interfiber distance distributions in parallel as well as in perpendicular orientation to air flow direction were found to be of the expected exponential shape for distances > 1 μm. On the other hand, the frequencies of the distances < 1 μm increase and the exponential shape of the curves is no longer observed.     

Characterisation of the microstructure and deformation of high modulus cellulose fibres

This paper reports on the microstructure and deformation of one type of high modulus cellulose fibre characterised using the techniques of Raman spectroscopy and synchrotron X-ray diffraction and it compares this fibre to a lower modulus conventional viscose fibre. The crystallinity of the fibres has been determined using X-ray diffraction. The orientation parameter has been determined by measuring the width of the (200) equatorial reflections for each fibre using microfocus synchrotron radiation and it has also been shown that the crystal orientation parameter varies from the skin to core of the fibres and is different for each type. Mechanical properties of the fibres are reported and it is shown that the high modulus cellulose fibres have very different stress-strain behaviour to the viscose fibres. Finally, it is shown for the high modulus fibre that the 1414, 1260, 1095 and 895 cm⁻¹ Raman bands shift under the application of tensile deformation towards a lower wavenumber with the 1095 cm⁻¹ band giving information about the backbone chain stretching of the cellulose. The viscose fibres show less significant shifts in this peak. The crystal modulus of the high modulus cellulose fibre has also been determined by calculating the change in the c-spacing upon the application of tensile deformation to individual cellulose monofilaments. This change in the c-spacing is determined from the change in position of the (002) meridional reflection giving a crystal modulus of 77 GPa. This value is a little low compared to other published data, and reasons for this are discussed. The shear modulus between crystallites is also calculated and compared to previously published data.     

Integrated compounding and injection moulding of short fibre reinforced composites

Abstract

Composites of high density polyethylene (HDPE) and carbon fibre (C fibre) were compounded and moulded into tensile test bars in compounding injection moulding (CIM) equipment that combines a twin-screw extruder and an injection moulding unit. Two HDPE grades exhibiting different rheological behaviours were used as matrices. The mechanical properties of the moulded parts were assessed by both tensile and impact tests. The respective morphologies were characterised by scanning electron microscopy (SEM) and the semicrystalline structures of the matrices investigated by X-ray diffraction. The final fibre length distribution and fibre orientation profiles along the part thickness were also quantified. The composites with lower viscosity exhibit higher stiffness, higher strength and superior impact performance. Both composites exhibit a three layer laminated morphology, featuring two shell zones and a core region. Interfacial interaction is favoured by a lower melt viscosity that enhances the wetting of the fibre surfaces and promotes mechanical interlocking. The composites display a bimodal fibre length distribution that accounts for significant fibre length degradation upon processing. The dimensions of the transversely orientated core differ for the two composites, which is attributed to the dissimilar pseudoplastic behaviour of the two HDPE grades and the different thermal levels of the compounds during injection moulding. Further improvements in mechanical performance are expected through the optimisation of the processing conditions, tailoring of the rheological behaviour of the compound and the use of more adequate mould designs.     

Investigations into lightweight solutions for epoxy composite fire property improvement

Abstract

The aim is to study methods to improve the fire properties of carbon fibre reinforced epoxy composites with interlayers without compromising structural performance, i.e. using a carbon fibre veil with expandable graphite, an intrinsically flame retardant thermoplastic film or a carbon fibre reinforced thermoplastic (CFTP) layer. The CFTP layer seems the most promising, with a significant improvement in fire properties as determined with cone calorimetry, while the mechanical performance of the modified composite appears unaffected.     

A comparative investigation of interfacial adhesion behaviour of polyamide based self-reinforced polymer composites by single fibre and multiple fibre pull-out tests

Abstract

Interfacial adhesion of composite materials plays an important role in their mechanical properties and performance. In the present investigation, analysis of the interfacial properties of self-reinforced polyamide composites by using microbond multiple fibre pull-out test is emphasised. Microbond specimens prepared through thermal processing are tested for their interfacial properties by multiple fibre bundle pull-out tests and compared with that of traditional single fibre pull-out test specimens. Multiple fibre pull-out addresses the volume fraction as well as eliminates the possibility of fibre breakage before matrix shear. Higher scatter in the data in the samples is addressed in the present studies. FTIR and Fractographic studies are carried out for deep understanding of the post pull-out interfacial adhesion.