Bastfaserverstärkte,biologisch abbaubare Polyester – Einfluß der Faserverfeinerung durch Dampfdruckaufschluß

A biodegradable copolyester was compounded in a twin-screw extruder with 25 wt.-% hemp fibres and then injection moulded into tensile test bars. Two different types of fibres were used: An exclusively mechanically treated fibre (MEC) and a fibre refined by steam explosion (STE). The mechanical properties of STE fibres are 30% lower than those of MEC fibres, nevertheless the same reinforcing effect was obtained. This is mainly due to the higher aspect ratio of STE and the better fibre distribution in the STE composites. A maximal reinforcement was not achieved, because of a too weak fibre matrix adhesion in combination with an insufficient fibre length caused by fibre damage in the extrusion and injection moulding process.     

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.     

Verstärkte Thermoplaste – Theorie und Praxis

Reinforced thermoplastics are composites consisting of a viscoelastic plastics matrix of low strength and a material (mostly inorganic) which has a high strength and a high modulus of elasticity, showing purely elastic strain. Globular and fibrous particles can be used for reinforcing. In the first case, the reinforcing action is largely due to an isotropic reduction of the strainability of the thermoplastic. In the second, more important case, the fibres can also accumulate stress from the matrix. In both cases, the elastic and viscous components of strain are pushed closer together. In practice, irregularities in the orientation of the fibres and continually changing patterns of stress cannot be avoided. These give rise to difficulties in checking the theoretical treatment of thermoplastics reinforced with short fibres. The basic element itself, i.e., the individual fibre embedded in the matrix has several variants, i.e., the length of the fibre, the ratio of the fibre length to the diameter, the critical fibre length determined from the interfacial shear strength and fibre strength, and the tensile strength in the transverse direction.     

Graphitization of carbon fibre/ glassy carbon composites

Carbon fibre/glassy carbon composites were prepared by aligning unidirectionally in furfuryl alcohol condensate the PAN-based carbon fibres treated at different temperatures and with different degrees of stretching. The graphitization of the composites was found to start at the boundary between the fibres and glassy carbon matrix, and to proceed into the matrix. This is considered to be due to the stress accumulation at the boundary caused by a large shrinkage of the matrix. The carbon fibres remain nongraphitized even after a heat-treatment at 2800°C. The composites heat-treated at high temperature (2800°C) are found to show a high overall degree of graphitization, unexpected on basis of the known graphitization behavior of carbon fibres and of glassy carbon, and a high degree of uniaxial preferred orientation of crystallites.     

Mechanical Properties of Thermoplastic Butadiene–Styrene (SBS) and Oxidized Short Carbon Fibre Composites

This work reports on some results of research conducted on composite materials consisting of a butadiene–styrene (SBS) thermoplastic elastomer matrix filled with short carbon fibres previously subjected to oxidative treatment to increase the surface functionality. Scanning electron microscopy confirms the existence of interactions between the matrix and the fibre, which are not observed for commercial fibre fillers and which translate into mechanical strength increments, in terms of the Young’s modulus, tensile and tear strengths for the oxidized fibre composites. The stress–strain curves of the composites show yield point phenomena as strain is applied longitudinally to the main fibre orientation. In oxidized fibre composites the stress and strain coordinates are a function of the degree of oxidation (greater strain for more strongly oxidized fibre) and fibre strength (lower stress for longer treatment times). © 1997 SCI.     

Reorientation of short steel fibres during the flow of self-compacting concrete mix and determination of the fibre orientation factor

A simple method has been developed to assess the orientation and distribution of short steel fibres in self-compacting concrete mixes during flow. The flow of self-compacting fibre reinforced concrete has been simulated using three-dimensional Lagrangian smooth particle hydrodynamics (SPH) which is simpler and more appropriate to use to simulate the flow and to monitor the distribution of fibres and their orientation during the flow. A probability density function (PDF) has been introduced to represent the fibre orientation variables in three dimensions. Moreover, the orientation variables of each individual fibre in an arbitrary two dimensional cross-section have been calculated using the geometrical data obtained from the three dimensional simulations. From these a new definition of the fibre orientation factor has been introduced and a method proposed for its determination from the fibre orientations monitored during the simulations. It is shown that this new definition gives results that are consistent with the expected reorientation of fibres towards the principal direction of flow. A method has also been proposed for its determination from image analysis on cut sections.     

Mechanical,morphological and thermal properties of short carbon and aramid fibres-filled bromo-isobutylene-isoprene rubber vulcanised with 4, 4’ bis(maleimido)diphenylmethane

ABSTRACT

This paper describes the use of a combination of 4, 4’ bis(maleimido)diphenylmethane and ZnO as a high-temperature processable vulcanising agent for the short aramid and carbon fibre-filled bromo-isobutylene-isoprene rubber. The fibre breakage analysis, cure characteristics, mechanical, thermal and morphological properties of the composites were evaluated with different fibre loading. The fibre breakage analyses revealed that the aramid fibres have good length retention property compared to carbon fibres. The morphological analysis of the extracted aramid fibres showed severe surface roughness primarily due to fibrillation after shear mixing. The fibrillated aramid fibres lead to aggregation and poor dispersion of the fibres in the rubber matrix. However, fibrillation imparted surface roughness and increased surface area on the aramid fibres which improved the fibre–matrix interaction via mechanical anchoring. On the other hand, the carbon fibre-filled composite showed poor fibre–matrix interaction and inferior strength and modulus.     

Silane coupling agents in basalt-reinforced polyester composites

A new reinforcing mineral fibre, comparable in strength (3 MPa) and modulus (90 MPa) with E-glass fibres, has recently been produced from naturally-occurring basalt rock. Following earlier studies of basalt fibre/polymer matrix interactions by single fibre pull-out tests, the interfacial interaction in basalt/polyester composite systems has now been investigated. The effect of silane coupling agents applied under various conditions is reported here. The controlling effects of silane hydrolysis, condensation, orientation on the basalt surface and chemical bonding on the surface are revealed in corresponding variations in flexural strengths.     

Fibre orientation distribution in turbulent fibre suspensions flowing through an axisymmetric contraction

The Reynolds averaged Navier–Stokes equation was solved numerically with the Reynolds stress model to get the mean fluid velocity and the turbulent kinetic energy in turbulent fibre suspensions flowing through an axisymmetric contraction. The fluctuating fluid velocity was represented as a Fourier series with random coefficients. Then the slender‐body theory was used to predict the fibre orientation distribution and orientation tensor. Some numerical results are compared with the experimental ones in the turbulent fibre suspensions flowing through a contraction with a rectangular cross‐section. The results show that the fibres with high aspect ratio tend to align its principal axis with the flow direction much easier. High contraction ratio makes the fibre alignment with the flow direction much easier. The contraction ratio has a strong effect on the fibre orientation distribution. Only a small part of the fibre is aligned with the flow direction in the inlet region, while most fibres are aligned with the flow direction when they approach to exit. The fibres are aligned with the flow direction rapidly in the inlet region, after that the fibre orientations change little in the most of the downstream region. The fibres with high aspect ratio are aligned with the flow direction faster when they enter the contraction. The randomising effect of the turbulence becomes significant in the downstream region because of the high turbulent intensity.     

Experimental and numerical investigation into fatigue damage mechanisms in multifibre microcomposites

Abstract

Polarised light microscopy has been used to investigate the influence of stress level, interfibre spacing, and fibre–matrix adhesion on the fatigue micromechanisms in carbon–epoxy model composites consisting of a planar array of five intermediate modulus carbon fibres. It was found that an increase in fatigue stress results in an increase in the number of fibre breaks, a more coordinated fibre fracture pattern as a result of fibre–fibre interaction, and extensive interfacial damage. In addition, it was shown that a smaller interfibre spacing results in a higher level of fibre–fibre interaction. Finally, in the case of surface treated carbon fibres (good fibre–matrix adhesion), a more coordinated fibre failure pattern was observed owing to stronger fibre–fibre interaction, whereas in the case of untreated carbon fibres (poor fibre–matrix adhesion), extensive debonding was observed which resulted in a more random fibre failure pattern. Finally, the experimental results were validated by means of a three-dimensional finite element analysis.     

Effect of orientation stretch on structure of a composite fibre based on high-density polyethylene and polybutylene terephthalate

Conclusions Due to the effect of the HDPE matrix in composite fibres based on HDPE-PBTP it is possible to draw the polybutylene terephthalate component to degrees of orientation which exceed the degree of orientation attained for pure PBTP.The introduction of chemical cross-links between the HDPE and the PBTP under radiative treatment permits one to expand the temperature service range of polyethylene fibres containing a small amount of the polyester.The possibility of transition from orthorhombic chain packing to a mesomorpohic state in a HDPE composite fibre on heating above the equilibrium melting point under isometric conditions has been demonstrated.Translated from Khimicheskie Volokna, No. 5, pp. 33–35, September–October, 1989.     

Processing and properties of mineral‐interfaced cellulose fibre composites

Polypropylene (PP) or, in some cases, poly (lactic acid) (PLA) were compounded with cellulosic fibres. The amount of fibres used was in the range 10–30 vol % and, in case of PP, a series of compounds was prepared with a minor amount of maleated PP as a compatibiliser. This matrix was denoted MAPP. Before compounding the polymers and the fibres, undelaminated bentonite (industrial scale) or delaminated clay (nanoclay) was deposited on the fibres in different amounts to improve the dispersion of the fibres in the polymer matrix, i. e., to avoid detrimental fibre bundles. The PP‐based compounds were either extruded or injection moulded, whereas the PLA‐compounds were only injection moulded. The mechanical properties were primarily evaluated for the injection moulded specimens. In general, the fibres had a strong effect on the mechanical behaviour of the materials, especially in the case of PLA and MAPP. Treating the fibres with undelaminated clay or nanoclay improved to some extent the dispersion of the fibres and the mechanical performance of the composites, but further optimization of the function of the mineral in this respect is probably required. The combination of the mineral treatment with a debonding agent appeared to be an interesting route here. With such a combination, a visually very good dispersion of the fibres in the PP‐based matrix could be obtained, partly at the expense of the mechanical performance. The compounding of the cellulosic fibres with PP led in this case to a marked decrease in the fibre length. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Compressive behaviour of rigid rod polymer fibres and their adhesion to composite matrixes

Abstract

The deformation behaviour of the new high performance polymer fibres, poly(p-phenylene benzobisoxazole) (PBO) and polypyridobisimidazole (PIPD) and their adhesion to an epoxy composite matrix have been investigated. Both fibres give well defined Raman spectra, and the deformation micromechanics of PBO and PIPD single fibres and composites were studied from stress induced Raman band shifts. Single fibre stress-strain curves were determined in both tension and compression, thus providing an estimate of the compressive strength of these fibres. It was found that the PIPD fibre has a higher compressive strength (~1 GPa) than PBO (~0·3 GPa) and other high performance polymer fibres, because hydrogen bond formation is possible between PIPD molecules. It has been shown that when PBO and PIPD fibres are incorporated into an epoxy resin matrix, the resulting composites show very different interfacial failure mechanisms. The fibre strain distribution in the PBO-epoxy composites follows that predicted by the full bonding, shear lag model at low matrix strains, but deviations occur at higher matrix strains due to debonding at the fibre/matrix interface. For PIPD-epoxy composites, however, no debonding was observed before fibre fragmentation, indicating better adhesion than for PBO as a result of reactive groups on the PIPD fibre surface.     

Characterization of the adhesion of single-walled carbon nanotubes in poly(p-phenylene terephthalamide) composite fibres

Poly(p-phenylene terephthalamide)/single-walled carbon (PPTA/SWNT) composite fibres with different draw ratios have been spun using a dry-jet wet spinning process and their structure and deformation behaviour analysed using Raman spectroscopy. The dispersion of nanotube has been examined by Raman scattering intensity mapping along the fibre. The nanotubes improved the polymer orientation in composite fibre with a draw ratio of 2 but degraded the orientation at higher draw ratios. The mechanical reinforcing effect by nanotubes is related to the change of polymer orientation, suggesting a dominant role of polymer orientation in mechanical performance of the composite fibre. High efficiency of stress transfer within the strain range of 0-0.35% and breakdown of the interface at higher strains has been found in the composite fibres through an in situ Raman spectroscopic study during fibre deformation. Cyclic loading applied on the fibre has indicated reversible deformation behaviour at low strain and gradual damage of the interface at high strains.     

Effect of PAN-based and pitch-based carbon fibres on microstructure and properties of continuous Cf/ZrB2-SiC UHTCMCs

In this paper the microstructure and mechanical properties of two different C_f/ZrB₂-SiC composites reinforced with continuous PyC coated PAN-derived fibres or uncoated pitch-derived fibres were compared.Pitch-derived carbon fibres showed a lower degree of reaction with the matrix phase during sintering compared to PyC/PAN-derived fibres. The reason lies in the different microstructure of the carbon. The presence of a coating for PAN-derived fibres was found to be essential to limit the reaction at the fibre/matrix interface during SPS. However, coated bundles were more difficult to infiltrate, resulting in a less homogeneous microstructure.As far as the mechanical properties are concerned, specimens reinforced with coated PAN-derived fibres provided higher strengths and damage tolerance than uncoated pitch-derived fibres, due to the higher degree of fibre pull-out. On the other hand, the weaker fibre/matrix interface resulted in lower interlaminar shear, off-axis strength and ablation resistance.     

Effect of fibre length and chemical modifications on the tensile properties of intimately mixed short sisal/glass hybrid fibre reinforced low density polyethylene composites

Hybrid composites prepared by the incorporation of two or more different types of fibres into a single polymer matrix deserve much attention. This method of hybridisation of composites offers a profitable procedure for the fabrication of products while the resulting materials are noted for their high specific strength, modulus and thermal stability. The influence of the relative composition of short sisal/glass fibres, their length and distribution on the tensile properties of short sisal/glass intimately mixed polyethylene composites (SGRP) was examined. Different compositions of sisal and glass such as 70/30, 50/50 and 30/70 have been prepared with varying fibre lengths in the range of 1–10 mm. Emphasis has also been given to the variation of fibre–matrix adhesion with several fibre chemical modifications. Chemical surface modifications such as alkali, acetic anhydride, stearic acid, permanganate, maleic anhydride, silane and peroxides given to the fibres and matrix were found to be successful in improving the interfacial adhesion and compatibility between the fibre and matrix. The nature and extent of chemical modifications were analysed by infrared spectroscopy while improvement in fibre–matrix adhesion was checked by studying the fractography of composite samples using a scanning electron microscope. Assessment of water retention values has been found to be a successful tool to characterize the surface of the stearic acid modified fibres. It was found that the extent of improvement in tensile properties of SGRP varied with respect to the nature of chemical modifications between fibre and matrix. Improved mechanical anchoring and physical and chemical bonding between fibre and polyethylene matrix are supposed to be the reasons for superior tensile strength and Young's modulus in treated composites. Several secondary reasons such as high degree of fibre dispersion and reduced hydrophilicity in chemically modified fibres also are believed to play a role. Among the various chemical modifications, the best tensile strength and modulus was exhibited by the SGRP with benzoyl peroxide treated fibres. This is attributed to the peroxide‐initiated grafting of polyethylene on to the fibres. Copyright © 2004 Society of Chemical Industry     

Natural fibre-reinforced biopolymers as construction materials – new discoveries

Originally coming from aerospace technology, fibre reinforced plastics (FRP) are successfully used for various applications because of their excellent specific properties, e. g. high strength and stiffness, low weight and the potential of optimisation by orientating (esp. continuous) fibres along the load paths. In order to successfully meet the environmental problems of these classic composites, the DLR Institute of Structural Mechanics developed an innovative idea in 1989: By embedding natural and near natural reinforcing fibres, e. g. flax, hemp, ramie, cellulose etc., into a biopolymeric matrix from cellulose, starch or lactic acid derivatives, etc. (thermoplastics as well as thermosets), new fibre reinforced materials, called biocomposites, were created and are still being developed. In terms of mechanical properties being comparable to glass fibre reinforced plastics (GFRP), latest developments on new fibre/matrix combinations and environmentally compatible flame retardants enable biocomposites to replace GFRP in most cases. Biocomposites are designed to meet the processing requirements for commonly used manufacturing techniques, e. g. pressing, injection moulding, filament winding, BMC, SMC etc. Apart from anisotropic and specially tailored lightweight structural parts with continuous fibre reinforcements, biocomposites are very well suited for panelling elements in cars, railways and aeroplanes, etc., using different kinds of nonwovens from single fibres (needlefelt nonwovens, fleeces, etc.) to be easily adapted to the usually curved shapes of panellings, fairings, etc.     

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 AND CORRELATING THE PERMEABILITY OF PULP AND PAPER

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.     

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.