Improvement of PBO fiber surface and PBO/PPESK composite interface properties with air DBD plasma treatment

The interface of fibrous composites is a key factor to the whole properties of the composites. In this study, the effects of air dielectric barrier discharge (DBD) plasma discharge power density on surface properties of poly(p‐phenylene benzobisoxazole) (PBO) fiber and the interfacial adhesion of PBO fiber reinforced poly(phthalazinone ether sulfone ketone) (PPESK) composite were investigated by several characterization methods, including XPS, SEM, signal fiber tensile strength, interlaminar shear strength, and water absorption. After the air DBD plasma treatment at a power density of 41.4 W/cm³, XPS analysis showed that some polar functional groups were introduced on the PBO fiber surface, especially the emergence of a new oxygen‐containing group (?O–C = O group). SEM observations revealed that the air DBD plasma treatment had a great influence on surface morphologies of the PBO fiber, while the signal fiber tensile strength results showed only a small decline of 5.9% for the plasma‐treated fiber. Meanwhile, interlaminar shear strength value of PBO/PPESK composite was increased to 44.71 MPa by 34.5% and water absorption of the composite decreased from 0.46% for the untreated specimen to 0.27%. The results showed that the air DBD plasma treatment can effectively improve the properties of the PBO fiber surface and the PBO/PPESK composite interface. Results obtained from the above analyses also showed that both the fiber surface and the composite interface performance would be reduced when an undue plasma discharge power density was applied. Copyright © 2011 John Wiley & Sons, Ltd.     

The reinforcing effect of clay on the mechanical properties of nitric acid–treated fluorosilicone rubber filled PI composites

The mechanical behaviour of fluorosilicone rubber–filled PI composites with and without clay was investigated. The clay filled fluorosilicone rubber composite had the highest interlaminar shear strength value of all the combinations because its higher bond strength may have hindered a large fibre/matrix debonding. The maximum tensile strength was observed for 20 vol% fluorosilicone rubber/PI/5vol%clay composite. The interlaminar shear strength of clay filled fluorosilicone rubber/PI composite was greater than that of fluorosilicone rubber/PI composite, which shows that the adhesion factor of the combination of the PI and fluorosilicone rubber was greater.     

Effect of surface modification by bromination and metalation on Kevlar fibre-epoxy adhesion

This research uses metalation, bromination, and grafting to modify the surface of Kevlar fibre. The resulting fibre was characterized with tensile measurement, infrared spectroscopy, intrinsic viscosity, and scanning electron microscopy (SEM). From the SEM micrographs, the surface morphology of the bromine etched Kevlar fibre has a rougher surface than the untreated fibre. Furthermore, the tensile strength of Kevlar fibre decreases with the increase of bromine treating time. The interlaminar shear strength (ILSS) of the fibre-reinforced epoxy composite was measured. The results show that the ILSS of the bromoacetic acid-grafted Kevlar sample can be increased by 12%, while that of epichlorohydrin-grafted Kevlar can be increased by 8%.     

The mechanical properties of plasma‐treated carbon fiber reinforced PA6 composites with CNT

The aim of this work is the evaluation of the effects of plasma treatment and the addition of CNT on the mechanical properties of carbon fibre/PA6 composite. A powder impregnation process with integrated inline continuous plasma of carbon fibers was used to produce CF/PA6 composite. CF/PA6 composite was processed into test laminates by compression moulding, and interface dominated composite properties were studied. The tensile and impact strength of composites containing CNT and plasma‐treated carbon fibres improved obviously. The tensile strength of nanocomposite largely increases with the increasing of the CNT content and then decreases when the CNT content is over 2%. The hydroxyl groups of the fibers surface are in favor of the wettability of carbon fibers by the polar matrix resin, which is resulting in a further interaction of the fiber surface with the curing system of the matrix resin.     

Strength and toughness of carbon fibers reinforced rigid polyurethane composites by adsorbing tannic acid and refining Ni grains on carbon fibers surface

In this article, short carbon fibers (CFs) reinforced rigid polyurethane (RPU) composites were prepared with the aim of improving both strength and toughness. A tannic acid (TA)‐nickel (Ni) composite coating was spontaneously co‐deposited onto CFs surface by a one‐step electrodeposition method to strengthen the interface bonding of the composites. The satisfactory mechanical properties of the composites were mainly attributed to the superior interfacial adhesion. On the one hand, TA could play a role in refining Ni grain during electrodeposition. On the other hand, the hydroxyl groups attached to composite coating, which were introduced by TA, could react with the RPU matrix to form chemical bonds. When the composites were under stress, the chemical bonds could effectively transfer the stress from matrix to the interface, while the refined Ni crystals could greatly increase the stress transfer path, and thus improve the strength and toughness of the material. Compared with pure RPU, the tensile strength, bending strength,interlaminar shear strength, and impact strength of TA‐Ni‐coated CFs/RPU composites were improved by 14.8%, 83.1%, 28.7%, and 121.4%, respectively.     

Study of the carbon fiber–Poly(Ether–Ether–Ketone) (PEEK) interfaces, 4: influence of fiber–matrix adhesion on the mechanical properties of unidirectional composites

The aim of the last part of this general study is to analyze the influence of the interfacial properties and, more precisely, the adhesion energy, between carbon fibers and PEEK on the final performance of unidirectional composites. A set of mechanical properties, i.e. interlaminar shear strength, longitudinal tensile and compressive and transverse tensile properties, of different unidirectional laminates with the same content (60% by volume) of carbon fibers is determined. It is first shown that the interlaminar shear strength is constant, whatever the type of materials. Therefore, this test is not appropriate to characterize the strength of the fiber–matrix interface in PEEK-based composites. On the contrary, in agreement with previous work on other systems, it appears that the ultimate properties (longitudinal tensile and compressive as well as transverse tensile strengths and strains) of the laminates increase with the interfacial adhesion energy, whereas the stiffness of these composites remains unaffected in all cases.     

Flame retardancy of fibre-reinforced epoxy resin composites for aerospace applications

The applicability of phosphorus-containing reactive amine, which can be used in epoxy resins both as crosslinking agent and as flame retardant, was compared in an aliphatic and an aromatic epoxy resin system. In order to fulfil the strong requirements on mechanical properties of the aircraft and aerospace applications, where they are mostly supposed to be applied, carbon fibre-reinforced composites were prepared. The flame retardant performance was characterized by relevant tests and mass loss type cone calorimeter. Besides the flame retardancy, the tensile and bending characteristics and interlaminar shear strength were evaluated. The intumescence-hindering effect of the fibre reinforcement was overcome by forming a multilayer composite, consisting of reference composite core and intumescent epoxy resin coating layer, which proved to provide simultaneous amelioration of flame retardancy and mechanical properties of epoxy resins.     

Cellulose man-made fibre reinforced polypropylene—correlations between fibre and composite properties

A series of viscose fibres from the tyre cord type varying in mechanical parameters and titre were compounded with polypropylene to produce fibre reinforced composites. Single fibre strength is analysed in detail and conclusions are drawn with respect to effective strength values in composite applications. Composites were analysed in terms of tensile and impact properties. Correlations between single fibre and composite properties are studied. High fibre elongation leads to favourable composite impact properties via high composite elongations at break. Using water as a plastisizing agent increasing fibre elongation, notched Charpy impact strength can be improved by more than 50%. Using a modified rule of mixtures and a shear lag model for the composite modulus it was shown how a titre reduction improves the composite stiffness by an increased interfacial area. A direct fibre-composite strength correlation was not found.     

Energy absorbing polymer-based composites

The ability of polymer-based fibre reinforced composite materials to absorb energy when subjected to high stress depends on the micromechanics of deformation and fracture of the individual laminae and the interaction between laminae. The micromechanismsdepend on the properties of the matrix, fibres and fibre-matrix interface. The capacity of material to absorb energy is important in many applications and particular reference is made to the response of structures to impact.     

Compression and tensile properties of self-reinforced poly(ethylene terephthalate)-composites

Tensile and compression properties of self-reinforced poly(ethylene terephthalate) (SrPET) composites has been investigated. SrPET composites or all-polymer composites have improved mechanical properties compared to the bulk polymer but with maintained recyclability. In contrast to traditional carbon/glass fibre reinforced composites, SrPET composites are very ductile, resulting in high failure strains without softening or catastrophic failure. In tension, the SrPET composites behave linear elastically until the fibre-matrix interface fails, at which point the stiffness starts decreasing. As the material is further strained, strain hardening occurs and the specimen finally fails at a global strain above 10%. In compression, the composite initially fails through fibre yielding, and at higher strains through fibre bending. The stress-strain response is reminiscent of an elastic-perfectly plastic material with a high strain to failure (typically over 10%). This indicates that SrPET composites are not only candidates as semi-structural composites but also as highly efficient energy absorbing materials.     

IFSS, TG, FT-IR spectra of impregnated sugar palm (Arenga pinnata) fibres and mechanical properties of their composites

This study aimed to investigate the effect of resin impregnation on the interfacial shear strength (IFSS), thermogravimetric (TG) and fourier transform infrared (FT-IR) of sugar palm (Arenga pinnata) fibres. In addition, the effect of resin impregnation on the mechanical properties of sugar palm fibre reinforced unsaturated polyester (UP) composites was also studied. The fibres were impregnated with UP via vacuum resin impregnation process at a pressure of 600 mmHg for 5 min. Composites of 10, 20, 30, 40 and 50 % fibre loadings were fabricated and tested for tensile and flexural properties. It was observed that the impregnation process caused the fibres to be enclosed by UP resin and this gave a strong influence to the increase of its interfacial bonding by the increase of its IFSS from single fibre pull-out test. It was also observed with TG and FT-IR spectra that the impregnated fibre had lower moisture uptake than the control and there was no significant increase in thermal stability of the impregnated fibre. The sequence of fibre decomposition started from the evaporation of moisture, hemicelluloses, cellulose, lignin and finally ash content and the presence of these components were proven by FT-IR spectra. For the composite specimens, due to the high interfacial bonding of the impregnated fibre and the matrix, the impregnated composites showed consistently higher tensile strength, tensile modulus, elongation at break, flexural strength, flexural modulus and toughness than the control samples. It was also observed that 30 % fibre loading gave optimum properties.     

Accelerated weathering of unbleached and bleached Kraft wood fibre reinforced polypropylene composites

In this paper, combined moisture/ultraviolet (UV) weathering performance of unbleached and bleached Kraft wood fibre reinforced polypropylene (PP) composites was studied. Composites containing 40 wt% fibre with 3 wt% of a maleated polypropylene (MAPP) coupling agent were fabricated using extrusion followed by injection moulding. Composite mechanical properties were evaluated, before and after accelerated weathering for 1000 h, by tensile and impact testing. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) were also carried out to assess the changes occurring during accelerated weathering. Bleached fibre composites initially showed higher tensile and impact strengths, as well as higher thermal stability and greater crystallinity. During accelerated weathering, both unbleached and bleached fibre composites reduced tensile strength (TS) and Young's modulus (YM), with the extent of the reduction found to be similar for both unbleached and bleached fibre composites. Evidence supported that the reduction of TS and YM was due to PP chain scission, degradation of lignin and reduced fibre-matrix interfacial bonding.     

Determination of tensile strength of UHMWPE fiber-reinforced polymer composites

Quasi-static tensile test of UHMWPE fiber-reinforced composite laminate is challenging to perform due to low interlaminar shear strength and low coefficient of friction. Tensile tests proposed in the literature were conducted and limitations associated with each method led to the evolution of a new method. Tensile test of single-ply was realized as the best representative of tensile strength of a composite than tensile test of UHMWPE laminate. A fixture was developed for single-ply tests which increased friction and provided the mechanical constraint to slipping. The fixture is easy to fabricate and has provided repeatable results for eight grades of UHMWPE fiber-based (0/90) fabrics. Reported tensile strengths are in quite high range of 900–1500 MPa.     

Matrix properties and composite failure

The influence of matrix extensibility on the properties of a composite was studied using two glassy polymers of almost identical chemical structure but differing crosslink densities. The lower crosslink density gave a 73 % increase in tensile elongation at break and a 56% increase in specific fracture energy. Unidirectional laminates of glass, carbon, and Kevlar^® fibres were prepared with these two polymers and tested for shear strength, transverse tension, and dynamic fatigue.The shear strengths of the polymers were found to be almost independent of crosslink densities (about 100 MPa). The interlaminar shear strengths of the carbon fibre laminates corresponded to those of the matrix polymers (Kevlar^® fibre laminates failed at 60 %). In accordance with Griffith's equation the more extensible polymer and its laminates performed better in tensile tests transverse to the fibres due to improved fracture energy. Failure criteria based on strain magnification were useful in the case of glass fibre laminates, but proved inadequate for laminates based on anisotropic fibres such as carbon and Kevlar^®.The dynamic fatigue strengths of the two matrix polymers were unaffected by the difference in crosslink densities. Almost the same fatigue strengths were obtained for the matrix polymers as for the laminates (carbon, glass) transverse to the fibres. A lack of processability of the polymer with high functionality was identified as a source of deteriorating effects.     

Interface characteristic of aramid fiber reinforced poly(phthalazinone ether sulfone ketone) composite

Interface is an important microstructure for advanced polymer‐matrix composite. The composite interface is the bridge and the link for stress transferring between the fiber and the matrix resin. In this work, oxygen plasma treatment was used for modification of aramid fiber surface. The effects of plasma treatment power on interlaminar shear strength of composite were evaluated by short‐beam shear test. The morphologies of both the aramid fiber surface and its composite interface fracture were observed by SEM. The chemical structure and surface chemical composition of the plasma‐treated and separated fibers were analyzed by Fourier transform infrared (FTIR) and XPS, respectively. The results showed that the interlaminar shear strength of composite was enhanced by 33% with plasma treatment power of 200 W. The FTIR and XPS results indicated that the active functional groups were introduced onto the aramid fiber surface by plasma treatment forming chemical bonds with the poly(phthalazinone ether sulfone ketone) resin. The SEM results proved that the aramid fiber surface was roughened by plasma treatment enhancing the mechanical bond with the poly(phthalazinone ether sulfone ketone) resin. The composite rupture occurred from the composite interface to the fiber or the matrix resin. Copyright © 2017 John Wiley & Sons, Ltd.     

Effect of phase change microcapsules on the thermo-mechanical,fracture and heat storage properties of unidirectional carbon/epoxy laminates

This work aims at producing and characterizing unidirectional carbon/epoxy composites containing different fractions of paraffin microcapsules (MC) for thermal management applications. The viscosity of the epoxy/MC mixtures increases with the MC content, thereby increasing the final matrix weight and volume fraction and reducing that of the fibers. This is at the basis of the decrease in mechanical properties of the laminates with high MC concentration (the elastic modulus decreases up to 53% and the flexural strength up to 67%), but the application of theoretical models shows that this decrease is only due to the lower fiber volume fraction, and not to a change in the properties of the constituents or the fiber/matrix interaction. The MC phase is preferentially distributed in the interlaminar zone, which leads to a thickening of this region and a decrease in matrix-related properties, such as the interlaminar shear strength, which decreases of up to 70%. However, a modest MC fraction causes an increase in the mode I interlaminar fracture toughness of 48%, due to the introduction of new toughening mechanisms. On the other hand, an excessive MC content lets the crack propagating through the matrix and not at the fiber/matrix interface, thereby reducing the toughening mechanism provided by fiber bridging. For the thermal properties, the phase change enthalpy increases with the MC fraction up to 48.7 J/g, and this is reflected in better thermal management performance, as proven by thermal imaging tests. These results are promising for the development of multifunctional polymer composites with thermal energy storage and thermal management properties, and future works will be focused on a deeper study of the micromechanical properties of PCM microcapsules and on the improvement of the capsule/matrix adhesion.     

Rigidity analysis of polypropylene/vegetal fibre composites after recycling

Polypropylene/hemp or sisal fibre composites exhibit interesting recyclability [Bourmaud A, Baley C. Investigations on the recycling of hemp and sisal fibre-reinforced polypropylene composites. Polymer Degradation and Stability 2007;92(6):1034-45]. The obtained results prove that the tensile modulus of these polypropylene/vegetal fibre composites is well conserved with the number of reprocessing cycles. In this work, we investigated the relationship between the mechanical properties of the fibres and those of the composites by taking the influence of the recycling into account.In the first part of this study we carried out nanoindentation and tensile tests on vegetal fibres to obtain transversal and longitudinal Young's moduli. The experimental values were then introduced into micromechanical models, taking the aspect ratio changes into account, to estimate the stiffness of the PP/vegetal fibre injected composites before and after recycling. The first results show an interesting correlation between experimental results and model predictions; however a general underestimation of tensile stiffness of composites can be noticed.     

Seawater ageing of flax/poly(lactic acid) biocomposites

Natural fibre reinforced biopolymer composites, or biocomposites, are an alternative to the glass fibre reinforced thermoset composites widely used today in marine applications. Biocomposites offer good mechanical properties and total biodegradability, but if they are to be adopted for marine structures their durability in a seawater environment must be demonstrated. In the present study unreinforced PLLA (Poly(l-Lactic acid)), injected and film stacked flax composites with the same PLLA matrix have been examined. All the samples were aged in natural seawater at different temperatures in order to accelerate hygrothermal ageing. Changes to physico-chemical and mechanical behaviour have been followed by weight measurements, thermal and gel permeation chromatography (GPC) analyses, and tensile testing, completed by acoustic emission recording and scanning electron microscopy (SEM) examination. The matrix tensile stiffness is hardly affected by seawater at temperatures to 40 °C but the composite loses stiffness and strength. Fibre/matrix interface weakening is the main damage mechanism induced by wet ageing, but both matrix and fibre cracks also appear at longer periods.     

Tensile behavior of quasi-unidirectional glass fiber/polypropylene composites at room and elevated temperatures

In the present study, the tensile behavior of quasi-unidirectional glass fiber/polypropylene composites at room and elevated temperatures were investigated by both micro- and macromechanical test methods. In the micromechanical studies, a single fiber fragmentation test was employed for measuring the interfacial shear strength at fiber-polypropylene interface in the temperature range from 23 °C to 90 °C. The results show that interfacial shear strength decreases with increasing testing temperature. In the macromechanical studies, experimental results show that the elastic modulus of polypropylene and transverse elastic modulus of composites are sensitive to the testing temperature. The weakened fiber-polypropylene interface due to elevated temperatures led to the vanishing of “knee” in transverse tensile stress-strain curves. A function was proposed to evaluate the dependence of the elastic modulus of quasi-unidirectional glass fiber/polypropylene composites on the testing temperatures and tested against experimental data. Tensile failure mechanisms of composites were demonstrated to evolve with the testing temperature.     

Effect of strain rate on the dynamic tensile behaviour of UHMWPE fibre laminates

Ultra-high molecular weight polyethylene (UHMWPE) fibre has great potential for strengthening structures against impact or blast loads. A quantitative characterization of the mechanical properties of UHMWPE fibres at varying strain rates is necessary to achieve reliable structural design. Quasi-static and high-speed tensile tests were performed to investigate the unidirectional tensile properties of UHMWPE fibre laminates over a wide range of strain rates from 0.0013 to 163.78 s⁻¹. Quasi-static tensile tests of UHMWPE fibre laminates were conducted at thicknesses ranging from 1.76 mm to 5.19 mm. Weibull analysis was conducted to investigate the scatter of the test data. The failure mechanism and modes of the UHMWPE fibre laminates observed during the test are discussed. The test results indicate that the mechanical properties of the UHMWPE fibre laminate are not sensitive to thickness, whereas the strength and the modulus of elasticity increase with strain rate. It is concluded that the distinct failure modes at low and high strain rates partially contribute to the tensile strength of the UHMWPE fibre laminates. A series of empirical formulae for the dynamic increase factor (DIF) of the material strength and modulus of elasticity are also derived for better representation of the effect of strain rate on the mechanical properties of UHMWPE fibre laminates.