The aim of this work was to establish the important parameters that control the hot compaction behaviour of woven oriented polypropylene. Five commercial woven cloths, based on four different polypropylene polymers, were selected so that the perceived important variables could be studied. These include the mechanical properties of the original oriented tapes or fibres, the geometry of the oriented reinforcement (fibres or tapes), the mechanical properties of the base polymer (which are crucially dependant on the molecular weight and morphology), and the weave style. The five cloths were chosen so as to explore the boundaries of these various parameters, i.e. low and high molecular weight: circular or rectangular reinforcement (fibres or tapes): low or high tape initial orientation: coarse or fine weave.A vital aspect of this study was the realisation that hot compacted polypropylene could be envisaged as a composite, comprising an oriented ‘reinforcement’ bound together by a matrix phase, formed by melting and recrystallisation of the original oriented material. We have established the crucial importance of the properties of the melted and recrystallised matrix phase, especially the level of ductility, in controlling the properties of the hot compacted composite. 相似文献
The development of morphology in tapes and woven cloths of oriented melt-spun Tensylon polyethylene has been studied both before and after hot compaction over a range of temperatures below and above the optimum. For both the unidirectional fibres and the woven cloths, the optimum temperature was found to be where approximately 30% of the original structure was lost which, for Tensylon tapes, was ∼2 K below the point of major crystalline melting, giving a processing window roughly twice as wide as for other previously studied polyethylene materials. Transverse sections show a two-component morphology after etching of cratered ribbons emerging from a flat, relatively featureless landscape. This morphology disappears at the highest temperature studied when the longitudinal morphology consists of oriented walls from which transcrystalline units have grown during cooling. Morphological comparison with other polyethylenes and their compactions places Tensylon behaviour alongside Dyneema, Spectra and Tekmilon rather than the melt-spun Certran. 相似文献
It is shown that the incorporation of interleaved films has major advantages for the production of polyethylene single polymer composites by the process of selective melting (termed hot compaction). The key issue is to choose a compaction temperature which melts the minimum amount of the original oriented elements whilst achieving acceptable bonding within the compacted structure. Utilising an interleaved film, excellent interlayer peel strengths can be achieved at lower compaction temperatures giving greater retention of the oriented fraction of the original fibres or tapes and a wider processing window. For example, using a very high modulus, ultra-high molecular weight, polyethylene tape, together with an interleaved film, resulted in an in-plane modulus of 25 GPa, an in-plane strength of 500 MPa, and an interlayer strength of >10 N/10 mm. These are amongst the highest values reported for a single polymer composite. Other important factors have been investigated including fabric weave style and whether it is better to use fibres or tapes as the oriented reinforcement. 相似文献
The current work is a major extension of two very different studies carried out previously to investigate factors that affect the peel strength of single-polymer composites produced by the Leeds hot compaction process. First, it was found that the peel strength was significantly increased by introducing interleaved films, of the same polymer, between the layers of woven oriented tapes that make up the composite. Secondly, it was shown that incorporation of carbon nanofibres (CNF) into the oriented tapes prior to hot compaction could also increase peel strength.In the present study we have investigated the amalgamation of these two approaches, in particular to see if there are synergistic advantages in the combination. Samples were produced with and without interleaved films, and with and without carbon nanofibres, located either in the oriented polypropylene tapes, in the interleaved film or in both. Maximum peel strength was achieved with the combination of the interleaved film and the incorporated nanofibres, but importantly this could be achieved with the CNF located only in the film. This has significant processing and performance advantages as the incorporation of CNF into the oriented tapes tends to limit the drawability of the polypropylene due to internal voiding around the particles.Scanning electron micrographs of the hot compacted composites show a strong correlation between the observed damage on the peel surfaces and the measured peel loads. It is shown that the peel load is dependent on the fraction of melted matrix at the interface and hence the interleaved films give additional matrix material at this point. It is also shown that the incorporation of CNF promotes fibrillation, and so increases the amount of energy absorbed during peeling. 相似文献
Self-reinforced polypropylene composites (SR-PP) possess an exceptional property spectrum and are predestined for use in a multitude of structural or semi-structural applications. However, the underlying macromolecular orientation can only be transferred into the layered composites consolidated out of highly stretched fibres and tapes of semi-finished textile products. Specific preservation of the self-reinforcement throughout processing and beyond is necessary and creates special challenges for processing technology. Depending on the processing temperatures and pressures selected, the highly oriented fibres and tapes are influenced by their own degree of self-reinforcement, which, in turn, affects the microstructure and remaining composite properties. In this publication, three different semi-finished textile products are introduced as basic materials. Exemplary selected test samples, which display a low degree of compaction on the one hand and a high degree of compaction on the other, were subject to wet chemical etching to enable confocal laser scanning microscopic images to be created. These images were then used to compare the microstructures of the semi-finished textile products that were used. 相似文献
AbstractThe discovery of methods to make highly oriented polymers has given tremendous stimulus to both basic polymer science and industrial developments in the period 1970 to the present. High modulus, high strength fibres for aramids and polyethylene, are based on very different methodologies but the ultimate result is similar in producing fully extended polymer chains. For polymers in solid sections, the enhancement of properties is less dramatic but still very worthwhile. In this case, three methods are described: hydrostatic extrusion, die-drawing and hot compaction of oriented fibres and tapes. Hot compaction is a new technique with many possibilities for which a wide range of applications has already been identified. 相似文献
Summary: The difference between the melting temperatures of poly(propylene) (PP) fibre and random poly(propylene‐co‐ethylene) (PPE) was exploited in order to establish processing conditions for an all PP composite. Under these conditions the matrix must be a liquid in order to ensure good wetting and impregnation at the fibres, though the temperature must not be too high to avoid melting the fibres. The high chemical compatibility of the two components allowed creation of strong physico‐chemical interactions, which favour strong interfacial adhesion. The static and dynamic mechanical properties and morphology of poly(propylene) woven fabric reinforced random PPE composites have been investigated with reference to the woven geometry that influenced the properties of the woven composites. Among the various cloth architectures that were used in the PP‐PPE composites, the satin weave imparted overall excellent mechanical properties due to the weave parameters, such as high float length and fibre count, low interlace point and crimp angle, etc. Morphology of the composite has been investigated by macro photography and scanning electron microscopy. Images from scanning electron microscopy provided confirmation of the above results by displaying the consolidation and good fibre‐matrix wetting of the composites.
Loss modulus of poly(propylene) woven‐matrix composites with different types of woven geometry. 相似文献
This paper describes the incorporation of carbon nanofibres (CNF) into polypropylene (PP) single polymer composites, materials where both the reinforcing phase and the matrix phase are PP. The CNF/PP composites were produced from an assembly of highly oriented tapes. The process of making the composites involves heating the tapes to a critical temperature such that a small fraction of the surface of each tape is melted; on cooling this recrystallises to form the matrix of the composite. The production of the composites required optimisation of three stages; incorporation of CNF into PP tapes, orientation of CNF/PP tapes by tensile drawing and hot compaction of the tapes. Results are presented to describe the research and findings in each of these key stages.Preliminary studies showed that the introduction of small amounts of carbon nanofibres (CNF) significantly improved the properties of isotropic PP. For example, 5% volume addition of CNF gave a 60% increase in the room temperature Young's modulus and a reduction of 35% in the thermal expansion coefficient. Moreover, the percentage enhancement of properties was greater at high temperatures where the stiffness of the PP is much reduced. These results can be very well understood in terms of conventional composite modelling.In unidirectional CNF/PP hot compacted composites the major improvements in mechanical behaviour are in the direction transverse to the orientation direction, where the CNF can make a proportionately greater contribution to the properties, and as shown by dynamic mechanical behaviour, this is most marked at high temperatures. Composite modelling based on uniform strain with appropriate allowance for the CNF aspect ratio predicts the behaviour extremely well. A very interesting result is that the peel strength of composites produced by hot compaction of woven CNF/PP shows a four-fold increase over woven PP composites and this is increased by another factor of two by the addition of a maleic anhydride compatibiliser. A further interesting result, of some practical significance, is that although the incorporation of CNF into PP causes voiding and some loss of molecular orientation during drawing, the hot compaction process closes and seals the voids, so that the original PP density is recovered. 相似文献
The dynamic elastic properties are important characteristics of composite materials. They control the vibrational behaviour of composite structures and are also an ideal tool for monitoring of the development of CFRCs’ mechanical properties during their processing (heat treatment, densification). The present studies have been performed to explore relations between the dynamic tensile and shear moduli and some structural features (viz., fibre fraction, fibre type, porosity, weave pattern of woven reinforcement) of various unidirectional or bi-directional fibre reinforced carbon/carbon composites, made out of PAN- or pitch-based fibres as reinforcements and phenolic resin or coal tar pitch as matrix precursors. The dynamic tensile and in-plane shear moduli were determined from resonant frequencies of a beam with free ends. The longitudinal dynamic Young’s modulus of unidirectional CFRC composites – besides its dependence on the original fibre modulus and fibre volume contents – also reflects changes induced in matrix and fibres by heat treatment. The in-plane shear modulus does not depend on the fibre type but there exists its distinct tendency to increase with increasing fibre fraction. For bi-directionally reinforced composites, the longitudinal tensile modulus is more sensitive to the fabric weave pattern than to the fibre type. Tensile modulus of diagonally cut specimens and in-plane shear modulus of longitudinally cut ones are mutually correlated and, therefore, simultaneously controlled by densification steps and graphitisation heat treatment. 相似文献
This work demonstrated a novel and potentially important application of two-dimensional small-angle X-ray scattering (2D-SAXS) to investigate powder compaction. SAXS from powder compacts of three materials commonly used for pharmaceutical tabletting exhibited azimuthal variations, with stronger intensity in the direction of the applied compaction force, relative to the transverse direction. This implied that compaction of a (macroscopic) powder could also produce changes on the molecular (nanometre) scale, which can be probed by 2D-SAXS. Two possible explanations for this effect were suggested. A combination of anisometric (i.e. elongated or flattened) granules with anisotropic morphologies could result in azimuthal variation in X-ray scattering due to granule orientation. It is expected that this mechanism would require relatively low packing density, so may operate during die filling. Granule re-orientation appeared less likely at higher packing densities and compaction pressures, however. Under these conditions, the changes in the 2D-SAXS patterns would be consistent with the powder granules becoming relatively flattened in the compression direction, with corresponding changes in their nano-scale morphology. The magnitude of this effect was found to vary between the materials used and increased with compaction pressure. This suggested that 2D-SAXS studies could provide useful information on force-transmission within a compressed powder. Further analysis of the data also suggested differences in the compaction mechanisms (i.e. granule re-orientation, deformation or fragmentation) between the materials studied. 相似文献
Abstract Works on woven composite both thermoset and thermoplastic are numerous, however in most instances they involve the use of preimpregnated fabric. It is apparent that woven thermoplastic system has significant potential due to the combined properties such as better damage tolerence, recyclability, easy processing, storage, etc. Here work on woven thermoplastic composites based on Continuous Fiber Impregnated towpreg (COFIT) tape rather than conventional approach is reported. The influences of different woven geometry and ply effect were investigated. Correlations between different woven geometry and weave characteristics were also noted. In general, the woven composite properties are influenced by many process variables such as type of towpreg, woven geometry, number of plies etc. 相似文献
The aim of this study is to characterise the mass and volume of wet unsaturated granular media for an equilibrium state obtained after a compaction or kneading process. For two mixing processes for powder and water, under constant operating conditions, the dry density is correlated to water content by a characteristic relation. As observed in the dynamic compaction of soils, this phenomenon exists also for the kneading and static compaction of different raw materials. An analytical expression is suggested for modelling the relation between these two parameters. 相似文献
Previous analysis of published experimental results on compaction and relaxation of textile reinforcements allowed the effects of some processing parameters on the mechanical behavior of the reinforcements to be identified. However, a limited number of relaxation results are available; also, the effect of some parameters on compaction received limited attention, and the behavior observed with fluid-saturated reinforcements has not been investigated. In this paper, the results of a structured experimental program of compaction and relaxation performed on three woven reinforcements are reported. In half the trials, a relaxation period was imposed on the samples. In the other half, samples saturated with distilled H2O were compacted. The selection of the processing parameters was found to be as important as the selection of the reinforcement itself for the definition of a manufacturing operation. The processing parameters governing the compaction and relaxation were seen not to be the same, and the fiber reorganization that occurs during the compaction phase was found to have a different effect on successive compaction cycles than the reorganization occurring during the relaxation. 相似文献