Polypropylene-natural fibre composites. Analysis of fibre structure modification during compounding and its influence on the final properties

The final properties of composite materials are highly dependent on the residual geometrical parameters (length, diameter, aspect ratio), orientation and distribution of the fibres in the matrix, which in turn are related to the processing conditions. This study analysed the fibre structure variation during the processing of a polypropylene matrix reinforced with cellulose flax pulp for different reinforcement concentrations. The fibre's geometrical parameters, length, diameter and aspect ratio have been measured and their statistical distributions have been assessed for each concentration. Furthermore, the effect of the microstructure variation on the final mechanical properties was analysed. In particular, changes in the interfacial area were evaluated based on the hypothesis that the fibres were cylindrical in shape and considering the average values of the diameters and the lengths calculated using a statistical distribution approach. The fibre interfacial area after the process decreases as the fibre concentration increases and this evaluation explains how the adhesion methods that are used for fibre surface modification fail because of the decrement in the modifier interfacial density. The Halpin–Tsai approach was used to model the experimental data obtained from tensile tests for different composites, so as to confirm the effect of fibre parameters, such as aspect ratio and interfacial area values, in the PP/cellulose blends final properties.     

Nanocellulose enhanced interfaces in truly green unidirectional fibre reinforced composites

One of the main problems in fabricating natural fibre reinforced polymers is the poor adhesion between intrinsically polar plant fibres and non-polar polymer matrices. We have developed a truly green technique of modifying natural fibre (hemp and sisal) surfaces to improve the interaction between the fibres and polymers by attaching nano-scale bacterial cellulose to the fibre surfaces. These modified natural fibres were then incorporated into the renewable polymers cellulose acetate butyrate (CAB) and poly-L-lactic acid (PLLA). Unidirectional natural fibre reinforced composites were manufactured to investigate the impact of the surface modification on the fibre and interface dominated composite properties. Both the tensile strength parallel as well as perpendicular to the fibres of the composites reinforced by bacterial cellulose modified natural fibres were found to increase significantly, especially in the case of a PLLA matrix. In case of modified sisal reinforced PLLA the parallel strength increases by 44% and the off-axis composite strength by 68%. Scanning electron microscopy observations of the composite fracture surfaces confirm the improved interaction between the fibre and the polymer matrix.     

Studies on interfacial adhesion in unidirectional isora fibre reinforced polyester composites

Unidirectional isora fibre reinforced polyester composites were prepared by compression moulding. Isora is a natural bast fibre separated from the Helicteres isora plant by a retting process. The effect of alkali treatment on the thermal properties of the fibre was studied using TGA, DTA and DSC in oxygen and nitrogen atmosphere. Mechanical properties like tensile strength, Young's modulus, flexural strength, flexural modulus and impact strength of the composites containing untreated and alkali-treated fibres have been studied as a function of fibre loading. The optimum loading for tensile properties of the composite containing untreated fibre was found to be 45% by volume and on alkalization of the fibre, the optimum loading increased to 66%. For flexural properties the loading was optimized at about 56% and 66%, for the composites containing untreated and alkali treated fibres, respectively. From DMA studies it was observed that the alkali-treated fibre composites have higher E′ and E″ values compared to untreated fibre composites. From swelling studies in styrene it was observed that the mole percent uptake of the solvent by the treated fibre composites is less than by the untreated fibre composites. From these results it can be concluded that in composites containing alkalized fibres there is enhanced interfacial adhesion between the fibre and the matrix leading to better properties, compared to untreated fibre composites.     

Application of nano-indentation, nano-scratch and single fibre tests in investigation of interphases in composite materials

Three novel experimental techniques were employed in this work in order to investigate the influence of the interphase region in polymer–glass composites on the bulk material properties: (i) the microdroplet test is a single fibre test designed to characterize the fibre–matrix bond (interface region) and to determine the interfacial shear stress in composite material; (ii) the nano-indentation test, a novel nano-hardness technique with ability to produce an indent as low as a few nanometres was employed in order to measure nano-hardness of the fibre–matrix interphase region; and (iii) the nano-scratch test, used in conjunction with the nano-indentation test for measurement of the interphase region width. The microdroplet test (MDT) has been used to characterize the interfacial bond in fibrous composite materials. The specimen consists of a fibre with a drop of cured resin pulled while the drop is being supported by a platinum disc with a hole. A properly tested specimen fails at the droplet’s tip–fibre interface, revealing the ultimate interfacial shear strength. In this study, finite element analysis (FEA) of the MDT has been focused toward simulation of the fibre–matrix interphase region. The influence of several functional variations of the material properties across the interphase layer on the stress distribution at the droplet’s tip was analysed. The results showed that the variation of the interphase properties significantly affects the stress distribution at the fibre–droplet interface, and, therefore, the stress redistribution to composite material. These results led to further experimental investigation of the interphase region, in order to obtain the material properties essential for the interfacial stress analysis. The interphase region in dry and water aged polymer–glass composite materials was investigated by means of the nano-indentation and the nano-scratch techniques. The nano-indentation test involved indentation as small as 30 nm in depth, produced along a 14 μm path between the fibre and the matrix. The distinct properties of the interphase region were revealed by 2–3 indents in dry materials and up to 15 indents in water aged, degraded materials. These results indicated interdiffusion in water aged interphase regions. The nano-scratch test involves moving a sample while being in contact with a diamond tip. The nano-scratch test, used in conjunction with the nano-indentation test, accurately measured the width of the interphase region. The results showed that the harder interphase region dissolved into the softer interphase region (both regions being harder/stronger than the matrix) expanding its width after aging in water.     

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.     

Dielectric properties of short sisal/coir hybrid fibre reinforced natural rubber composites

The dielectric properties, such as dielectric constant, volume resistivity and dielectric loss factor, of sisal/coir hybrid fibre reinforced natural rubber composites have been studied as a function of fibre loading, fibre ratio, frequency, chemical modification of fibres and the presence of a bonding agent. The dielectric constant values have been found to be higher for fibre filled systems than pure natural rubber. This has been attributed to the polarization exerted by the incorporation of fibres into the matrix. Dielectric constant values were observed to be decreased with increase in frequency due to the decreased interfacial and orientation polarization at higher frequencies. Whereas dielectric constant increases with fibre loading because of the increment in number of polar groups after the addition of hydrophilic lignocellulosic fibres. The volume resistivity of the composites was found to be decreased with fibre loading and a percolation threshold has been obtained at 15.6% volume of fibres. Fibre treatment, such as alkali, acetylation, benzoylation, peroxide and permanganate, were carried out to improve the adhesion between fibres and matrix. The dielectric constant values were lower for systems consisting of fibres subjected to chemical treatments due to the increased hydrophobicity of fibres. The addition of a two-component dry bonding agent consisting of hexamethylene tetramine and resorcinol, used for the improvement of interfacial adhesion between the matrix and fibres, reduced the dielectric constant of the composites. When the weight percentage of sisal fibre was increased in the total fibre content of the hybrid composites, the dielectric constant was found to increase. The added fibres and different chemical treatments for them increased the dielectric dissipation factor. A dielectric relaxation has been observed at a frequency of 5 MHz.     

Silane-modified polymers as interphases in glass-fibre-reinforced composites: 2. Grafting and crosslinking of interphase materials

_—This paper presents an interphase engineering technique suitable for grafting silane-modified polymers onto glass fibres to be used in composites with enhanced impact tolerance. The silane-modified polymers include ethylene polymers grafted with γ-methacryloxypropyltrimethoxysilane (MPS) and a copolymer of butyl acrylate (BuA) and MPS. The grafting of functionalized interphase materials onto glass fibres is performed in solution. By changing the concentrations of the solutions, different amounts of polymer can be deposited on the fibres. Water crosslinking of the polymer gives the possibility of producing stabilised interfacial polymer coatings over a range of thicknesses. It is concluded that acidic conditions (1) promote the grafting of silane-modified polymers on glass fibres and (2) for a given reaction time, increase the amount of crosslinked polymer in the interphase, i.e. yield more stable interphases. It is also likely that preserving acidic conditions at the fibre/polymer interface is important for maintaining bonding across the interface. It is shown that polystyrene/glass-fibre composites having SEBS at the interface are promising candidates for high-impact-tolerance composites.     

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.     

Numerical study of the single fibre push-out test: Part III. Singularity of interface stresses

The singular behaviour at the free edges of the fibre-matrix interface is analysed for the fibre push-out test geometry based on the boundary element method. The fibre push-out test has been extensively used to measure the fibre-matrix interfacial properties in polymer, ceramic and metal matrix composites. There are two free edges in the fibre push-out specimen: one is at the loaded fibre end and the other at the supported fibre end. The singular stresses can be expressed as a function of singular exponent and singular stress intensity. It is shown that the singular exponents obtained at both fibre ends are characteristic of composite constituent properties, such as Young's moduli of fibre and matrix, and does not vary with specimen dimensions. The singular exponents are real and identical for the shear and radial stress components at fibre ends where the wedge angles are the same. The singular stress intensities are also implicit in material properties, and vary with specimen dimensions, such as fibre to matrix radius ratio, fibre aspect ratio and support hole size. An interfacial failure criterion is proposed here based on the average stress concept to determine the critical singular stress intensities in mode I and mode II loads.     

Ultrasonic assembly of anisotropic short fibre reinforced composites

We report the successful manufacture of short fibre reinforced polymer composites via the process of ultrasonic assembly. An ultrasonic device is developed allowing the manufacture of thin layers of anisotropic composite material. Strands of unidirectional reinforcement are, in response to the acoustic radiation force, shown to form inside various matrix media. The technique proves suitable for both photo-initiator and temperature controlled polymerisation mechanisms. A series of glass fibre reinforced composite samples constructed in this way are subjected to tensile loading and the stress–strain response is characterised. Structural anisotropy is clearly demonstrated, together with a 43% difference in failure stress between principal directions. The average stiffnesses of samples strained along the direction of fibre reinforcement and transversely across it were 17.66 ± 0.63 MPa and 16.36 ± 0.48 MPa, respectively.     

Interfacial characterisation and mechanical properties of heat treated non-woven kenaf fibre and its reinforced composites

This paper reports on the comprehensive characterisation of heat treated kenaf fibre (KF) and its composites. The kenaf fibres were modified by heating for 2.5–12.5 h inside a drying oven. Heat treatment produces an increase in the crystallinity index and fibre strength of KF. The highest value of KF strength was recorded by applying heat treatment of 10 h on KF. The heat treatment results in the partial removal of impurities/extractives on the KF surface which is detected by scanning electron microscopy and X-ray photoelectron spectroscopy. Atomic force microscopy results signify the decrease of roughness, the increase in peak area density and the increase of the adhesion force on the surface area of heat treated KF. The effect of the heat treatment in enhancing the interface bonding characteristics between the KF and unsaturated polyester matrix can be reflected by the interlaminar shear strength (ILSS) and dynamic mechanical analysis value of the composites. The flexural properties of the composites showed a similar trend to ILSS. However, the fracture toughness revealed contrasting results. Water absorption induced a drastic loss of the mechanical properties of the composites albeit better retention of properties was observed in the case of heat-treated KF composites.     

Stress transfer efficiency in model composites under dynamic loading

The micromechanics of tension–tension fatigue loading in model single-fibre composite geometries is investigated in this paper. In an attempt to emulate the conditions encountered in full carbon fibre composites, the fibres were prestrained prior to the curing process to ensure that they were free of high residual compressive stresses as a result of resin shrinkage. The resulting specimens were grouped into two categories depending on the level of the initial fibre prestrain (case A low, case B high). The cyclic load is designed to be well below the endurance fatigue limit of the polymer matrix (∼0.6%), and to have a frequency low enough to avoid unwanted thermal post curing. Throughout the preparation procedure, as well as during fatigue loading, the fibre stress (strain) was constantly monitored by means of laser Raman spectroscopy. The fibre axial stress distributions at each fatigue step were converted to interfacial shear stress (ISS) distributions, from which important parameters such as the maximum ISS the system can accommodate, the transfer length for efficient stress built-up and the length required for the attainment of maximum ISS were obtained. The results showed that, up to 2×10⁶ loading cycles, the main parameters which affected the stress transfer efficiency at the interface were the fibre fracture process itself and the viscoelastic behaviour of the matrix material. Received: 7 November 2001 / Accepted: 22 March 2002 / Published online: 5 July 2002     

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.     

Creep of multidirectionally fibre-reinforced composites

A model for the creep of metal matrix composites multidirectionally reinforced by short fibres is proposed. The reinforcement is described by the effective stiffness tensor of a multidirectional arrangement of continuous fibres and the internal damage of the composite during creep due to fibre fragmentation is introduced by assigning a heuristic nonlinear stress–strain relationship to the fibres. Based on the model, the load partitioning between matrix and fibres is computed. The macroscopic creep behaviour is simulated for composites exhibiting different fibre orientation distributions and different heuristic nonlinear stress–strain functions. The computational results rationalize the creep behaviour of multidirectional fibre-reinforced composites. For a two-dimensional random orientation distribution, a good qualitative match between simulation and experimental results is obtained for compressive loading and for in-plane tensile loading. For loading normal to the reinforcement plane, the model overestimates the creep resistance. In this case, the formation and growth of cavities seems to govern the creep deformation of the composite.     

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.     

Effects of fibre surface treatment on fracture-mechanical properties of sisal-fibre composites

Sisal fibre reinforced composites, one class of a broad range of eco-composite materials, were studied in connection with the effects of fibre surface treatment on their fracture-mechanical properties. Previous investigations on sisal fibre and its composites have been fully reviewed [1], which provided an impetus for this research. Two fibre surface treatment methods, chemical coupling based on silane and oxidization based on permanganate and dicumyl peroxide, together with untreated sisal fiber composites were used to set up different levels of interface bonding strength. The interface effects on the mechanical properties and fracture toughness of sisal fibre reinforced vinyl-ester composites were completely assessed based on the test results obtained and theoretical analyses. Many aspects of studies reported in this paper are original, such as single fiber pull-out tests and toughness evaluation of sisal composites aided by scanning electron microscopy. The results showed that fibre surface treatment could improve interfacial bonding properties between sisal fibre and vinylester resin. These in turn influenced the fracture-mechanical characteristics of this class of ecocomposites.     

Multi-response analysis in the material characterisation of electrospun poly (lactic acid)/halloysite nanotube composite fibres based on Taguchi design of experiments: fibre diameter, non-intercalation and nucleation effects

Poly (lactic acid) (PLA)/halloysite nanotube (HNT) composite fibres were prepared by using a simple and versatile electrospinning technique. The systematic approach via Taguchi design of experiments (DoE) was implemented to investigate factorial effects of applied voltage, feed rate of solution, collector distance and HNT concentration on the fibre diameter, HNT non-intercalation and nucleation effects. The HNT intercalation level, composite fibre morphology, their associated fibre diameter and thermal properties were evaluated by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), imaging analysis and differential scanning calorimetry (DSC), respectively. HNT non-intercalation phenomenon appears to be manifested as reflected by the minimal shift of XRD peaks for all electrospun PLA/HNT composite fibres. The smaller-fibre-diameter characteristic was found to be sequentially associated with the feed rate of solution, collector distance and applied voltage. The glass transition temperature (T _g) and melting temperature (T _m) are not highly affected by varying the material and electrospinning parameters. However, as the indicator of the nucleation effect, the crystallisation temperature (T _c) of PLA/HNT composite fibres is predominantly impacted by HNT concentration and applied voltage. It is evident that HNT’s nucleating agent role is confirmed when embedded with HNTs to accelerate the cold crystallisation of composite fibres. Taguchi DoE method has been found to be an effective approach to statistically optimise critical parameters used in electrospinning in order to effectively tailor the resulting physical features and thermal properties of PLA/HNT composite fibres.     

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.     

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.     

Room vs. temperature studies of model composites: modes of failure of carbon fibre/epoxy interfaces

Laser Raman spectroscopy (LRS), transmission polarised optical microscopy (TPOM) and scanning electron microscopy (SEM) have been used to characterise the interface of model single-fibre composites. The composites consisted of single-carbon fibres embedded in epoxy resin. Local stress measurements as a function of applied strain were performed using LRS at both room temperature (RT) and 60?°C. Consecutively, the coupons were strained to failure and field emission SEM was used to study the fracture surfaces. In a parallel study, identical systems were subjected to incremental tension and fracture events were recorded as a function of applied strain. At RT, TPOM was used to provide additional insight in the local stress transfer. The stress transfer was found to depend on the combined effect of interfacial chemistry and thermal stresses. Thus, in the case of sized fibres, there is a distinct change in the interfacial failure mode at high temperature, whereas in the case of unsized fibres, the stress transfer is dominated by thermal stresses: at high temperature it is weak, due to the relief of the thermal stress field.