Observation of transcrystalline growth of PLA crystals on sisal fibre bundles and the effect of crystal structure on interfacial shear strength

Hot-stage microscopy was used to characterise crystal growth at the interface between sisal fibre bundles and a polylactic acid (PLA) matrix in order to better understand the mechanical properties of sisal fibre–PLA composites. Cooling rates and crystallisation temperatures and times were varied to influence crystalline morphology at the interface. Single sisal fibre bundles were evaluated in their as received state or treated with 6 wt.% caustic soda solution for 48?h at room temperature. A microbond shear test was used to characterise the shear strength of the interface as a function of fibre surface treatment. These tests were performed on sisal fibre bundles carefully embedded in flat films of PLA supported on card mounts. Fibre bundles in a PLA matrix were cooled from 180?°C at rates from 2 to 9?°C/min and then crystallised isothermally. For as received fibre bundles uneven growth of PLA spherulites occurred at all cooling rates and crystallisation temperatures. For caustic soda treated fibres, uneven spherulitic growth was observed at crystallisation temperatures at and above 125?°C. In contrast, transcrystalline growth was observed for samples cooled to 120?°C at cooling rates from 2 to 6?°C/min and then allowed to crystallise. The microbond shear strengths of untreated and caustic soda treated fibre bundles were evaluated using Weibull statistics and the caustic soda treated fibres exhibited higher interfacial shear strengths in comparison to untreated fibres, reflecting the development of a transcrystalline layer at the fibre to matrix interface.     

Study of the influence of thermal history on the load transfer efficiency and fibre failure in carbon/polypropylene microcomposites using Raman spectroscopy

The influence of thermal history on the interfacial load transfer efficiency and fibre failure in carbon/polypropylene microcomposites has been studied using Micro Raman spectroscopy. Microcomposites were manufactured by cooling from the melt at different constant cooling rates or isothermally crystallized. Thermal residual strains were measured during and after manufacture of the microcomposites. The residual strains resulted in compressive fibre failure. Based on the experimental data, interfacial load transfer efficiency was determined quantitatively for the different cooling procedures. Results indicate that thermal history has a very large influence on the interfacial load transfer efficiency of the microcomposites. This was shown to be due to the influence of thermal history on transcrystallinity and interfacial residual stresses. A transcrystalline interphase provides a more effective load transfer compared to the non-transcrystalline interphase. Furthermore, decreasing cooling rates leads to an increase in load transfer efficiency due to increased transcrystallinity and higher crystallization temperature resulting in higher interfacial stresses.     

Weibull analysis of microbond shear strength at sisal fibre–polyester resin interfaces

An analysis has been made of the tensile strength of sisal fibres and the interfacial adhesion between fibres and polyester resin droplets. Density and microscopy methods were used to determine the cross-sectional area of the sisal fibres. The average tensile strength of treated sisal fibres decreased by a modest amount following treatment with 0.06 M NaOH. However, this treatment resulted in a substantial increase in the interfacial shear strength at the sisal fibre to polyester resin interface. Weibull analysis has been used successfully to analyse variability in tensile strengths and interfacial shear strength using probability of failure plots. Scanning electron microscopy has revealed the shape of resin droplets on the surface of treated and untreated sisal fibres and contact angles are much lower for droplets on treated fibres. Damage to the surface of fibres has been examined following shear testing. Weibull analysis is an effective tool for characterising highly variable fibre properties and evaluating the level of adhesion between polymer resin and the fibre surface.     

Improving the interfacial strength of PMMA resin composites by chemically grafting graphene oxide on UHMWPE fiber

Controlling interfacial microstructure and interactions between (ultra high molecular weight polyethylene) UHMWPE fiber and matrix is of crucial importance for the fabrication of advanced polymer composites. In this paper, (UHMWPE fiber-g-graphene oxide [GO]) was prepared. GO nanoparticles distributed onto the ?ber surface uniformly, which could increase surface polarity and roughness. Increases of interlaminar shear strength (ILSS) and interfacial shear strength (IFSS) of UHMWPE fiber-g-GO composites were achieved. These enhancements can be attributed to the existent of GO interface with providing chemical bonding and strong mechanical interlocking between the ?ber and matrix. Moreover, impact resistance of UHMWPE fiber-g-GO composites was enhanced.     

Surface Modification of Ultrahigh-molecular-weight Polyethylene Fibers with Coupling Agent during Extraction Process

Ultrahigh molecular weight polyethylene (UHMWPE) fibers were treated with a coupling agent following the extraction of gel fibers, resulting in modified fibers after subsequent ultra-drawing. The structure and morphology of the modified UHMWPE fibers were characterized and their surface wetting, interfacial adhesion, and mechanical properties were investigated. It was found that the coupling agent was absorbed into the UHMWPE fiber and trapped on the fiber surface. Compared with unmodified UHMWPE fibers, the modified fibers had smaller contact angle, higher crystallinity, and smaller crystal size. The interfacial adhesion and mechanical properties of UHMWPE fibers were significantly improved with increasing coupling agent concentration and gradually reached a plateau value. After treatment with 1.5 wt% solution of a silane coupling agent (γ -aminopropyl triethoxysilane, SCA-KH-550), the interfacial shear strength of the UHMWPE-fiber/epoxy composites was increased by 108% and the tensile strength and modulus of modified UHMWPE fibers were increased by 11% and 37% respectively.     

Interfacial strength in glass fibre-polypropylene composites: influence of chemical bonding and physical entanglement

For glass fibre–polypropylene (PP) composites, the non-polar nature of polypropylene presents a problem. The present investigation shows that it is necessary to introduce a functionalised PP, for example PP-g-MAH, in order to enhance the bond strength between the PP matrix and aminosilane treated glass fibre. To achieve a better bonding between the substances, three different systems (1–3) in addition to a reference system (0), have been investigated in this study. The two first are based on PP-g-MAH coupling agents, with different concentrations of acid anhydride groups, and the third is a directly reacting system. In the first system, the silane treated glass fibre is exposed to molten mixture of 95 wt% PP homopolymer and 5 wt% PP-g-MAH. In the second system, the silane treated glass fibre is covered by a thin layer of PP-g-MAH and thereafter exposed to the molten PP. The interfacial shear strength is highest for the systems with the pre-compounded graft-copolymer. The resulting influence of the selected coupling systems on the interfacial bond strength of single fibre composite is studied by fragmentation testing. The intermolecular shear strength between fibre and matrix increases with the intermolecular entanglement length of the PP-g-MAH and not by the degree of functionalisation. The PP-g-MAH mixed into the PP gave better results than the route of first covering the glass fibre with a thin layer of PP-g-MAH. This is explained in terms of the probability of generating entanglements and in terms of a weak boundary layer at the glass surface. This conclusion is also supported by the results from using the third principle, i.e. direct reaction between the PP matrix and azidosilane treated glass fibres.     

Polymer–Polymer and Single Polymer Composites Involving Nanofibrillar Poly(vinylidene Fluoride): Manufacturing and Mechanical Properties

Nanofibrillar polymer–polymer composites (NFCs) and single polymer composites (SPCs) were produced using linear low density polyethylene (LLDPE) and poly(vinylidene fluoride) (PVDF). The NFCs were fabricated by means of a microfibrillar composite concept comprising melt blending, cold drawing, and compression molding retaining the highly oriented PVDF reinforcing nanofibrils (diameter of approximately 250 nm) dispersed without any agglomeration in the isotropic LLDPE matrix. The SPC films were prepared by partial surface premelting of neat PVDF nanofibrils (diameter of about 130 nm) using hot compaction at 148°C (about 20°C below the complete melting of PVDF), thus preserving the PVDF nanofibrillar identity. Tensile testing of NFCs based on LLDPE and PVDF showed an increase in the tensile modulus by 135% and in the tensile strength at break by 211%, as compared to those of an isotropic LLDPE film. Furthermore, the PVDF SPCs showed an enhancement of tensile modulus of 30% and strength at break of 305% when compared to those of an isotropic PVDF film.     

Microstructure and mechanical properties of continuous Al2O3 fibre reinforced Ni45Al45Cr7.5Ta2.5 alloy (IP75) matrix composites

Single crystalline Al₂O₃ fibres (sapphire), coated with the NiAl alloy IP75 by physical vapour deposition (PVD), were assembled to fabricate composites by means of diffusion bonding. The microstructure and chemistry of both as-coated fibre and as-diffusion bonded composites were investigated by electron microscopy and microanalysis. The interface shear stress for complete debonding was measured by fibre push-out tests at room temperature, and the composite tensile strength was measured at 900°C and 1100°C. An amorphous layer with a thickness of about 400?nm formed between the fibre and the matrix during the PVD process and was maintained during diffusion bonding. A Laves phase precipitated along NiAl grain boundaries in the IP75 matrix. This caused a lower tensile strength of the IP75/Al₂O₃ composite at high temperatures compared to as-cast monolithic IP75 and rendered the composite useless for structural applications.     

Effect of transcrystallization in carbon fiber reinforced poly(p-phenylene sulfide) composites on the interfacial shear strength investigated with the single fiber pull-out test

The effect of transcrystallinity in carbon fiber reinforced poly(p-phenylene sulfide (PPS) composites on the apparent shear strength was investigated with the single fiber pull-out test. Transcrystalline zones around the reinforcing fibers do not seem to improve the adhesion level significantly. Neighbor fibers hinder the formation of the transcrystalline zone and a ductile fracture behavior can be observed. However, the apparent strength level is slightly higher for composites containing such reinforcing neighbor fibers compared with single fiber composite samples. During annealing a brittle interface can be formed in the multifiber composite yielding a higher level of the apparent shear strength.     

Improving the adhesion between a aqueous PU and a polyamide fibre with silane

Glycidoxypropyltrimethoxysilane (GPS) and γ-aminopropyltrimethoxysilane (APS) were used to modify the surface chemistry of polyamide fibre. The surface chemistry was characterised using X-ray photoelectron spectroscopy. The silanol functional group was designed to be introduced on the surface of polyamide fibre to increase its chemical activity by N-alkylation of GPS and hydrolysis of APS, and to improve the poor interfacial adhesion between a polyamide 66 fibre and an aqueous polyurethane polymer adhesive. The microbond test was used to measure the interfacial shear strength between the waterborne PU adhesive and the polyamide fibre. It has been found that APS hydrolysis and GPS-alkylated fibre surface can be used to improve the interfacial adhesion of polyamide fibre to PU. The IFSS can be improved by N-alkylation of GPS from 5.0 to 8.4?MPa. After water immersion at 50?°C for 48?h, then drying, the IFSS increased to 8.8?MPa due to the plasticisation of PU in water. Better interfacial adhesion was also observed by the hydrolysis of APS, but not significantly improved by this method due to the relatively weak hydrogen bond at the interface between APS and polyamide fibre.     

Author Index to Volume 8

Pull-out experiments have been carried out with Kevlar fibres embedded in epoxy resin. Friction accompanied debonding, and had to be allowed for in the analysis. The debonding stress was about equal to the matrix strength for 80°C cured epoxies. However, debonding appears to be a brittle fracture process, and the works of fracture corresponding to the apparent interface strengths are very low, ranging from ca. 20-40 Jm^-2 depending on the surface treatment and degree of cure of the resin. Water immersion for 2300 h at room temperature reduced the apparent strengths and works of fracture with some of the surface treated fibres, but not with the untreated fibres. Interface pressures during debonding were 10-15 MPa for the 20°C cured specimens and 20-30 MPa for the 80°C cure. Water soaking markedly reduced the friction coefficients. Post-debonding friction was high, but estimates of the parameters was probably unreliable due to the fibre having a somewhat thick end due to fibrillation when being cut.     

Effect of emulsifier content of sizing agent on the surface of carbon fibres and interface of its composites

In this work, carbon fibres were sized with different emulsifier content sizing agent in order to improve the performances of carbon fibres and the interface of carbon fibres composites. The surface characteristic changing after modification was investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM). Wetting and surface energy along with contact angles were determined by the dynamic contact angle analysis test (DCAT). At the same time, the single fibre strengths and weibull distributions were also studied in order to understand the effect of the emulsifier content of sizing agent on the carbon fibres. The interfacial shear strength and hygrothermal ageing of the composites were measured which showed a different enhancement, respectively. The results revealed that sizing agent E-3 showed better interface adhesion between fibres and matrix and sizing agent E-2 sized carbon fibre has better ageing resistant properties.     

Enhancements in crystallinity,thermal stability,tensile modulus and strength of sisal fibres and their PP composites induced by the synergistic effects of alkali and high intensity ultrasound (HIU) treatments

In this investigation, sisal fibres were treated with the combination of alkali and high intensity ultrasound (HIU) and their effects on the morphology, thermal properties of fibres and mechanical properties of their reinforced PP composites were studied. FTIR and FE-SEM results confirmed the removal of amorphous materials such as hemicellulose, lignin and other waxy materials after the combined treatments of alkali and ultrasound. X-ray diffraction analysis revealed an increase in the crystallinity of sisal fibres with an increase in the concentration of alkali. Thermogravimetric results revealed that the thermal stability of sisal fibres obtained with the combination of both alkali and ultrasound treatment was increased by 38.5 °C as compared to the untreated fibres. Morphology of sisal fibre reinforced composites showed good interfacial interaction between fibres and matrix after the combined treatment. Tensile properties were increased for the combined treated sisal fibres reinforced PP composites as compared to the untreated and pure PP. Tensile modulus and strength increased by more than 50% and 10% respectively as compared to the untreated sisal fibre reinforced composite. It has been found that the combined treatment of alkali and ultrasound is effective and useful to remove the amorphous materials and hence to improve the mechanical and thermal properties.     

Interphase characterization in epoxy resin/carbon fibre composites

In this paper, interphase properties of carbon fibre/epoxy resin single-fibre model and unidirectional (UD) composites are reported. To study the contribution of the carbon fibre surface chemistry and morphology and of the resin itself to the overall properties of the composites, untreated, oxidized and sized fibres are used with bi- and tetrafunctional, diglycidylether of Bisphenol A, DGEBA and tetraglycidyl 4,4'-diaminodiphenylmethane, TGDDM-based resins, cured with amine and anhydride hardeners. Adsorption measurements and single fibre contact angle experiments, as well as the pull-out test were applied to characterize the surface of carbon fibre and the interfacial shear strength with different matrices. It was shown that the presence of the size on the surface can drastically affect the wettability as well as the starting rate of the cure reaction of epoxide in the vicinity of the fibre surface, as revealed by FTIR microscopy. Different elastic-plastic behavior of model composites before debonding is found for untreated, oxidized and sized fibres, due to the various interphase structures formed. Both micro-and macromechanical properties of the composites are found to be significantly affected by the matrix properties. The role of the surface treatment of fibers becomes especially important in high performance resin systems.     

Surface modification of ultra high molecular weight polyethylene fibers via the sequential photoinduced graft polymerization

In this study, a sequential photoinduced graft polymerization process was proposed to improve the poor interfacial bonding property of ultra high molecular weight polyethylene (UHMWPE) fibers. The polymerization was initiated by dormant semipinacol (SP) groups and carried out in a thin liquid layer. Methacrylic acid (MAA) and acryl amide (AM) were grafted stepwise onto the surface of UHMWPE fibers. Attenuated total reflectance infrared spectroscopy (ATR-IR) and thermo gravimetric analysis (TGA) confirmed the grafting. The analysis result of pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) indicated the structure of grafted chains. Scanning electron microscopy (SEM) images and atomic force microscopy (AFM) images revealed the apparent morphology changing, and the grafted layers were observed. Interfacial shear stress (IFSS) test of the modified fibers showed an extensively improved interfacial bonding property. The active groups grafted onto the fibers would supply enough anchor points for the chemical bonding with various resins or further reactions.     

Influence of fibre extraction method,alkali and silane treatment on the interface of Agave americana waste HDPE composites as possible roof ceilings in Lesotho

Surface treatment is often necessary for strong composites. But the challenge for developing countries is to find chemicals and treatment procedures that are cheap and simple but maintain good composite properties. Mercerization followed by silane treatment of natural fibres is among the simplest and cheapest methods used to improve composite interfaces. This study investigates the effectiveness of this method to improve the bond between Agave americana fibres and post consumer HDPE. The influence of fibre extraction method, mercerization and mercerization followed by silane treatment on interfacial shear strength (ISS) and fibre properties is determined. The results indicate that ISS values are generally low but mercerization doubles the ISS values between Agave americana fibres extracted by traditional boiling of leaves and post consumer HDPE. Mercerization also improves fibre tensile and thermal properties. While triethoxyvinylsilane treatment of fibres after mercerization does not improve the ISS, it does not reduce it either, nor does it reduce tensile and thermal strengths of mercerized fibres. Fibres from non-boiled leaves resulted in poor fibre tensile strengths but improved ISS. There is a potential to use mercerization as cheap, simple technique to make Agave americana HDPE composites to provide cheap roof ceilings in Lesotho.     

Surface characterisation of carbon fibre recycled using fluidised bed

X-ray photoelectron spectroscopy (XPS) was used to investigate the surface of carbon fibres recycled using a high-temperature fluidised bed. The interfacial shear strength of the recycled carbon fibres with epoxy resin was examined using a micro-droplet test. The corresponding as received carbon fibres were used as control samples. It was shown that the recycling process converted some of the surface hydroxyl groups into carbonyl and carboxylic groups due to the effect of heat in atmosphere of air. The overall O/C ratio was not changed significantly. The interfacial shear strength with epoxy resin was not affected by the change of surface oxygen composition. It was also shown that surface texture may play a dominant role in interfacial bonding performance.     

Relationship between fibre-matrix adhesion and the interfacial shear strength in polymer-based composites

A model is proposed to correlate the interfacial shear strength at the fibre-matrix interface, measured by means of a fragmentation test on single fibre composites, to the reversible work of adhesion between both solids, this quantity being defined as the sum of the dispersive and the acid-base interactions (physical interactions) between the fibre and the matrix. Whatever the nature of the fibres and the matrices, a linear relationship, passing through the origin, is established between the interfacial shear strength and the reversible work of adhesion. However, the slope of this straight line depends on the elastic properties and, more precisely, on the elastic moduli of both the fibre and the matrix. This leads us to express the reversible work of adhesion as the product of a mean intermolecular distance at the interface and an adhesive pressure related to the interfacial shear strength. The limits of the theoretical and experimental approaches leading to the establishment of such a model, as well as its domain of validity, are discussed.     

Modelling of the transverse strength of fibre reinforced epoxy composite at low and high temperature

Process-induced thermal residual stresses and matrix failure of unidirectional carbon fibre reinforced composites (CFRP) have been investigated by finite element analysis (FEA). We used a partial discrete FEA model based on a unidirectional composite consisting of a microscopic area of fibres and matrix surrounded by a homogenised composite area. The FEA provided information about the stress state in the matrix and the fibre–matrix interface. The transverse strength of the composite was calculated regarding matrix failure and fibre matrix debonding. The influence of the temperature on the Young's modulus, the non-linear stress–strain behaviour and the strength of the matrix were investigated in detail. Following this approach it was possible to incorporate the resulting microresidual stresses on the transverse strength of the composite. Tensile tests of the neat resin and of the composite were performed in the temperature range of ?40°C to 60°C. The results of the FEA modelling are in good agreement with the experimental results of the transverse tests.     

Transcrystallinity phenomena in PET and PAN fibres/polyhydroxybutyrate matrix systems

The interface in polyhydroxybutyrate/ PAN- and polyhydroxybutyrate/ PET fibres model composites has been investigated by polarizing optical and scanning electron microscopy. Under certain melting/cooling conditions in PAN fibre system a transcrystalline interphase is clearly observed, whereas in PET/PHB model composites the transcrystallization phenomenon occurs only when fibres with significant surface roughness are used.