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 morphology of woven oriented polypropylene tapes and fibres has been studied both before and after processing by hot compaction. In this technique bundles of oriented tapes or fibres are subjected to suitable conditions of temperature and pressure so that just sufficient of each tape or fibre is selectively melted; on cooling, this material recrystallizes to bind the whole structure together. Three different polymers were studied, woven into various individual weave styles, in relation to optimum processing conditions. The weave style, the adhesion between neighbouring interfaces after compaction and the direction of crack propagation along the neighbouring interfaces on peeling were examined leading to clear correlations between the observed morphology and mechanical peel strength data. 相似文献
Abstract The mechanical properties of as-spun poly(ethylene 2,6-naphthalene-dicarboxylate) (PEN) fibres were studied in order to characterize this relatively new material near its glass-rubber transition. Tensile tests were carried out on amorphous (low-speed spun) PEN filaments. The temperature range of 90°C up to 160°C was covered using increments of 10°C. A transition from necking and cold drawing to rubber-like behavior was observed in the stress-strain relationship. Dynamic mechanical experiments were carried out on PEN yarns spun at speeds from 500 to 4000 m min?1. Both temperature and frequency were varied. The maxima in loss modulus depend on spinning speed. Tensile behavior and dynamic mechanical behavior of PEN fibres demonstrate that the glass-rubber transition temperature of PEN is approximately 125°C. 相似文献
Summary: The melting temperature difference between poly(propylene) (PP) fibre and random poly(propylene‐co‐ethylene) (PPE) was exploited to establish processing conditions for all‐PP composite. Under these conditions, the matrix must be liquid to ensure good wetting and impregnation of fibres, though temperatures must be low enough to avoid melting of fibres. The high chemical compatibility of the two components allowed creation of strong physico‐chemical interactions, favouring strong interfacial adhesion. Static and dynamic mechanical properties and morphology of all‐PP composites were investigated according to method of preparation and compared with the behaviour of hot compacted composites, prepared under different moulding conditions. The composites were compacted with varying pressure and time, and mechanical and thermal properties of the resulting sheets were measured. With increased moulding time, more fibres melted or their original properties deteriorated. Fast cooling or quenching caused imperfect morphology. Moulding pressure played an important role. Morphology of the optimum hot compacted composite was investigated using scanning electron microscopy before and after tensile testing. Tensile fracture surfaces showed a melted phase epitaxially crystallised onto the remaining orientated phase. Compacted composites showed fibre shapes under a thin layer of PPE with all of the gaps between fibres filled by melted PPE matrix.
SEM of compacted all‐PP composite without quenching. 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 production of solid section products from highly oriented fibers by a novel compaction procedure is described for melt-spun and gel-spun polyethylene fibers, poly(ethylene terephthalate) and polypropylene fibers and Vectran liquid crystalline copolyester fibers. Differential scanning calorimetry and electron microscopy have been used to study the structure of the compacted polymers. For the most successful compaction, selective surface melting of a small fraction of each fiber enables the formation of a fiber composite of high integrity, where the matrix phase is formed by epitaxial crystallization of the melted fraction on the initial fibers, retaining a high proportion of their initial strength and stiffness. A wide range of potential applications is envisaged for the composites produced by hot compaction. In many cases these composites will be produced by thermoforming. In addition to the obvious advantages of high stiffness and strength, in several instances the unrestricted exploitation of unique properties of the fibers such as transparency to microwave radiation or low thermal expansion coefficients offer additional incentives for the use of these hot compacted materials rather than conventional fiber/resin composites. 相似文献
AbstractThis paper describes an investigation into the thermoformability of a new class of oriented polymeric material recently developed, namely hot compacted polypropylene sheet. Exploitation of any new material requires an intimate understanding of a whole range of factors, amongst which thermoformability is pre-eminent. This is particularly true for oriented polymeric materials, for while the preferred molecular alignment gives enhanced properties such as stiffness, strength, and resistance to impact, the downside is that the stretched molecular chains tend to limit further flow under stress, making thermoforming difficult. The aim of the present study was to establish the critical parameters for successful thermoforming of hot compacted polypropylene sheet.Elevated temperature tensile tests were used to investigate the stress–strain behaviour of the compacted materials. The crucial parameters were found to be the post-yield modulus, which gives a measure of the resistance of the material to large scale deformation, and the strain to failure, which gives the upper limit on deformation. The post-yield modulus was found to be significantly affected by the test temperature and the high strain hardening behaviour of the material confirmed that significant force is required to thermoform the compacted polypropylene sheets. A hemispherical mould, with built-in gripping plate, was used to carry out a study of the thermoforming behaviour of the compacted sheets, and the results were found broadly to confirm the conclusions of the tensile tests. A linear relationship was found between the tensile force and the postforming force, reinforcing the synergy between the two tests. In addition the forming tests showed that the best temperatures to use were either side of the melting point of the melted and recrystallised phase, depending on the amount of postforming deformation required. Different gripping arrangements were investigated both in which the sheet was fully gripped and in which the sheet was allowed to flow into the mould during forming. The different schemes were found to control whether a successful component could be produced under different conditions and at different ultimate strains. Finally, the tests with the hemispherical mould showed that thermoforming this shape requires significant interlaminar shear deformation, and above 15% strain this resulted in destruction of the interlayer bond. For strains greater than this, successful thermoforming could only be achieved by allowing the material to flow into the mould. 相似文献
Atactic polystyrene was subjected to an elevated pressure–temperature cycle with the resulting densification, mechanical properties, and thermal scanning behavior observed. Most densifications were carried out with the PST as a viscous liquid. In this manner, ambient residual compactions greater than 2% were produced. Pressures up to 90,000 psi and temperatures to 320°C were employed. The technique used for vitrification from the high pressure–temperature region was found to drastically affect the mechanical behavior. If the polystyrene was vitrified from the treatment region by lowering the temperature, the material exhibited enhanced yield strength, by up to 40%. If the polystyrene was quenched by raising the pressure, the samples exhibited much lower mechanical strength. While the mechanical behavior of temperature-vitrified samples is enhanced compared to the pressure-vitrified materials, their densities are comparable. The compaction achieved is primarily determined by the pressure applied as the polymer vitrifies. Thermal scanning behavior of the pressure-vitrified materials show endothermic and exothermic responses below Tg, while the temperature-vitrified materials do not. Annealing the compacted polystyrene at room temperature caused little change in density. However, at temperatures above 60°C, the density relaxed rapidly. Samples which had been temperature vitrified and annealed such that the compaction completely relaxed, still maintained the enhanced mechanical properties of the densified materials. 相似文献
Poly(arylene ether nitrile) (PEN) is a class of high-performance engineering plastics of poly(arylene ether) with cyano groups as side groups, which can get improved thermal, mechanical, and electrical properties through simple molecular structure design. In this work, a series of PEN (BPA/PP based PEN) copolymers were synthesized with varying amounts of phenolphthalein and bisphenol A. The influence of the copolymer molecular structure variations on the thermal, mechanical, and dielectric properties of PEN copolymer films was investigated. The results demonstrated that the BPA/PP based PEN copolymer films have great mechanical properties and low dielectric constant, as well as enhanced thermal properties. The highest 5% weight loss temperature of 494.9°C was obtained by PEN-B7P3, while the highest glass transition temperature of 238.6°C was obtained by PEN-B3P7. Porous BPA/PP based PEN films prepared by non-solvent induced phase separation (NIPS) exhibited satisfactory mechanical properties and the highest tensile strength of 9.4 MPa was achieved. Moreover, the introduction of the phenolphthalein structure into the PEN molecular chain can improve the heat resistance of the PEN copolymers without deteriorating the dielectric properties, which gives the copolymers great potential as candidates for applications in flexible electronics and wireless communication. 相似文献
The influence of untreated and benzoylated oil palm empty fruit bunch (OPEFB) short fiber loading on the mechanical properties of the poly(vinyl chloride) (PVC) composite was studied. Benzoylated OPEFB was produced by mixing OPEFB with NaOH solution and agitating vigorously with benzoyl chloride. The PVC resin, various additives, and OPEFB were first dry blended using a laboratory mixer before being milled into sheets on a two-roll mill at 165°C and then hot pressed into composite samples at 180°C. The tensile and impact strength of untreated and benzoylated OPEFB composites decreased whereas the tensile modulus increased with increasing fiber loading from 0 to 40 phr. However, the benzoylated OPEFB was able to improve the tensile properties and impact strength of composites when compared to the untreated fiber. The enhancement of mechanical properties showed that the treatment improved the OPEFB fiber-PVC matrix interfacial adhesion. The improvement of adhesion was clarified by SEM micrographs, the increase of water resistance, and the reduction of glass transition temperature of the composites. 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
Polymer dielectrics, are commonly used as insulating materials for electronic products. Light weight, good mechanical properties and high thermal conductivity are important properties. However, electrical and thermal parameters are interrelated, and it is challenging to have a dielectric polymer that is also resistant to high temperatures and high thermal conductivity. Hence, high-performance composite films were prepared by the method of post-solid phase chemical reaction using polyarylene ether nitrile (PEN) and divinyl siloxane-bisbenzocyclobutene (BCB) as raw materials. First, parameters of the curing reaction were determined by rheological and activation energy calculations. Then, through adjusting the content of BCB resin and treatment temperature, the performance of PEN/BCB composites could be tuned. Thermal properties have been studied by differential scanning calorimetry, dynamic mechanical analysis, thermal gravimetric analysis, and hot-disk method. Here, the PEN/BCB composite electric insulating materials with outstanding thermal performance (Tg: 208–400°C, T5%: 469–544°C, thermal conductivity: 1.270–2.215 W/m K). Besides, its mechanical and dielectric properties were investigated in detail. It is noteworthy that the tensile strength of composite film can exceed a maximum of 130 MPa, which is 23.19% higher compared to the untreated one. Also, PEN/BCB composites own low dielectric constant (2.27–4.08 at 1 KHz), and the relationship between frequency or a wide temperature range and dielectric constant/loss is stable. Thus, it has a greater potential for applications in electronics in high-temperature environments. 相似文献