Fracture toughness of multiphase polypropylene composites containing rubbery and particulate inclusions

The fracture toughness of binary and ternary phase polypropylene (PP) composites containing ethylene–propylene rubber (EPR) and glass beads, has been studied using the J-integral method at 23 and − 20 °C. For determining J _c, analysis of the stress-whitening zone was found to be more meaningful than the commonly used blunting line approach. Functionalized EPR was found to be more effective toughening additive for PP than EPR, in both binary and ternary phase compositions. Crack growth mechanisms were examined by scanning electron microscopy. In rubber-modified blends, cavitation and shear yielding were found to be the primary toughening mechanisms, while in ternary phase composites particle–matrix debonding played a major role. This revised version was published online in November 2006 with corrections to the Cover Date.     

The role of poly(ethylene terephthalate-co-isophthalate) as interfacial agent in polypropylene–matrix composites

The effect of modifying the particle/matrix interfacial region on the morphology and tensile behaviour of glass bead-filled polypropylene (PP) composites was studied. The interface modification was promoted by blending PP with a small concentration (5% by weight) of poly(ethylene terephthalate-co-isophthalate) (co-PET). Ten different PP/co-PET/glass beads ternary composites were prepared, characterized and compared with the homologous PP/glass beads binary ones. Maleic anhydride-grafted PP was added as a compatibilizing agent for PP and co-PET in some of the studied formulations, and its effect studied. Furthermore, four different silane-treated glass beads were used to prepare the composites (50 wt.%). Results showed that three different interfaces, corresponding to three different levels (low, middle and high) of particle/matrix adhesion, could be obtained in these composites by varying the matrix composition and the silane coupling agent on the glass bead surface, which resulted in a wide range of tensile properties, from ductile composites with low tensile strength and high elongation to brittle ones with high tensile strength. It was found that co-PET embeds glass bead surface independently of the silane coupling agent employed. Finally, the adhesion degree differences between the different composite phases seemed to be the main cause to explain the differences found in the sensitivity of the composite tensile characteristics to the strain rate.     

Fracture behavior of nylon hybrid composites

Hybrid composite systems consisting of liquid crystalline polymer (LCP), short glass fibers and toughened nylon in varied ratios were studied. Dynamic mechanical results indicated that, elastomeric phase in toughened nylon 6,6 promoted a better compatibilization between nylon 6,6 and LCP in a hybrid system containing short glass fibers in comparison with one without glass fibers. Improved compatibility facilitated fibrillation of LCP phase in the skin region of the hybrid composite, thereby providing superior tensile strength. Without the presence of LCP, glass fiber reinforced toughened nylon 6,6 exhibited the least tensile strength. J-integral analysis and essential work of fracture (EWF) method were used to compare the fracture behavior of composites. Results showed that specific essential work of fracture were consistent with the critical J-integral. Matrices reinforced by LCP alone showed the best crack initiation and propagation toughnesses, followed by glass fiber reinforced and hybrid composites. The better compatibility between nylon 6,6 and LCP appeared to inhibit the interfacial debonding process, resulting in brittle fracture.     

Continuous-glass-fibre-reinforced polypropylene composites: I. Influence of maleic-anhydride-modified polypropylene on mechanical properties

This study investigates the influence of maleicanhydride-modified polypropylene (m-PP) on monotonic mechanical properties of continuous-glass-fibre-reinforced polypropylene (PP) composites. Maleicanhydride-modified polypropylene was added to the PP homopolymer to improve the adhesion between the matrix and the glass fibre. Three-point bending tests were performed on 0° and 90° unidirectional glass-fibre/PP laminates with various weight fractions of m-PP in the PP matrix. These tests showed an increase in both longitudinal and transverse flexural strength up to 10 wt% m-PP, whereas at higher weight fractions of m-PP a decrease in flexural strength was observed. No significant influence of m-PP on composite stiffness was observed. Additional mechanical tests on unidirectional glass/PP composites with 0 wt% and 10 wt% m-PP showed only a small increase in fibre-dominated properties such as longitudinal tensile strength and strain, whereas composite properties that are governed by the interphase, such as transverse, shear and compressive strength, showed significant increases as a result of matrix modification and an enhanced interaction between the glass fibres and the PP matrix.     

Flexural properties and micromorphologies of wood flour/carbon nanofiber/maleated polypropylene/polypropylene composites

The effects of carbon nanofibers (CNF) on the performance of three- or more phase composites are complicated. CNFs formulated into wood flour (WF)/maleated polypropylene (MAPP)/polypropylene (PP) composites by high shear blending alone improved flexural properties. Addition of an extrusion step after high shear blending enhanced CNF dispersion and improved the composites’ flexural moduli, but in these systems CNF did not contribute to flexural property enhancement. The addition of 1.0 wt% CNF to WF/MAPP/PP by high shear blending followed by extrusion did not affect either the WF dispersion or WF/PP adhesion. The adhesion between WF and PP/MAPP matrix was good (SEM), but CNF adhesion to the PP/MAPP matrix was poor.     

The J-integral fracture toughness of PP/CaCO3 composites

The J-integral method was introduced to investigate the fracture process of PP/CaCO₃ composites. The results showed that the resistance of PP/CaCO₃ composites to crack initiation and propagation was greatly improved with the addition of CaCO₃ filler. Large scale plasticity was caused in PP/CaCO₃ composites, from which a large amount of energy was absorbed by the PP matrix. The reason for the increase in the fracture toughness of PP/CaCO₃ composites was attributed to the partial micro-drawing ahead of the crack tip in the PP matrix, which was formed by the stress concentration caused by the filler particles in the PP matrix and/or by the interfacial debonding between filler particles and the PP matrix. It was indicated that the presence of CaCO₃ filler could augment the ductility of composites locally, resulting in higher fracture energy in the crack initiation and propagation of the PP/CaCO₃ composites in a certain CaCO₃ content range.     

Micromechanical deformation processes in PP/wood composites: Particle characteristics,adhesion, mechanisms

Polypropylene (PP) was reinforced with four natural fillers having different particle characteristics. Interfacial adhesion was changed by the introduction of maleated polypropylene (MAPP). The properties of the studied PP/wood composites depended strongly on interfacial adhesion and on the particle characteristics of the wood. Coupling with functionalized polymer is necessary for the preparation of composites with acceptable properties if the size of the particles is large and their aspect ratio is small. The effect of adhesion is smaller for particles with large aspect ratio. Several micromechanical deformation processes may occur in PP/wood composites including matrix yielding, debonding, fiber pull-out and fiber fracture both parallel and perpendicular to the fiber axis. The processes are competitive and may take place simultaneously and/or consecutively. The inherent properties of the reinforcement may limit the improvement of composite strength. Micromechanical deformation processes determine composite properties irrespectively of their mechanism.     

Studies of surface-modified wood flour/polypropylene composites

Wood flour (WF)/polypropylene (PP) composites have been made by extrusion and hot press compression molding. The composite water uptake and flexural properties were investigated. The composite fracture surfaces were studied by SEM. WF esterified with octanoyl chloride was used in WF/PP composites to improve the composites’ water resistance. Maleated polypropylene (MAPP) was also studied and compared with esterification by acid chlorides. Esterification by octanoyl chloride reduced the composite water uptake. However, the C₈ chain is still not long enough to form effective entanglements with the PP matrix. So, despite enhancements in hydrophobic interactions, flexural strengths and flexural moduli decreased. MAPP (MW = 47000) polymer chains can entangle with the matrix polypropylene molecules. Therefore, when MAPP’s maleic anhydride functions esterify WF surface hydroxyls, improved water resistance and composite flexural properties were achieved. The modifier chain length is of critical importance and more important than the surface density of hydrophobic groups for improving WF–PP interfacial adhesion and composite mechanical performance.     

Combined effect of fiber content and microstructure on the fracture toughness of SGF and SCF reinforced polypropylene composites

Composites of polypropylene (PP) reinforced with short glass fibers (SGF) and short carbon fibers (SCF) were prepared with extrusion compounding and injection moulding techniques. The fracture behavior of the two types of composites was studied. The fracture toughness (K _c of the composites was measured in the T-direction [main crack transverse to mould flow direction (MFD)] and in the L-direction (main crack parallel to the MFD) using compact tension (CT) specimens made from the plaques manufactured. The study was focused on the combined effect of fiber volume fraction and microstructure (fiber length and alignment) on the fracture toughness of short fiber composites. It was observed that the addition of fibers effectively enhanced the fracture toughness for both SGF/PP and SCF/PP systems in the T-direction but only improved the composite toughness in the L-direction for the case of a low fiber volume fraction (8%). The composite fracture toughness kept almost unchanged in the T-direction and decreased in the L-direction with increasing fiber volume fraction. These were explained using the combined effect of fiber volume fraction and microstructure.     

Biocomposites from Musa textilis and polypropylene: Evaluation of flexural properties and impact strength

Abaca (Musa textilis)-reinforced polypropylene composites have been prepared and their flexural mechanical properties studied. Due to their characteristic properties, M. textilis has a great economic importance and its fibers are used for specialty papers. Due to its high price and despite possessing very distinctive mechanical properties, to date abaca fibers had not been tested in fiber-reinforced composites. Analysis of materials prepared showed that, in spite of reduced interface adhesion, flexural properties of the PP composites increased linearly with fiber content up to 50 wt.%. Addition of a maleated polypropylene coupling agent still enhanced the stress transfer from the matrix to the reinforcement fiber. As a result, composites with improved flexural properties were obtained. The mechanical properties of matrix and reinforcing fiber were evaluated and used for modelling both the flexural strength and modulus of its composites. In addition, the impact strength of materials was evaluated. Comparison with mechanical properties of composites reinforced with fiberglass points out the potentiality of abaca-reinforced polypropylene composites as suitable substitutes in applications with low impact strength demands.     

The effect of fibre content on the mechanical properties of glass fibre mat/polypropylene composites

Glass fibre mat was prepared by the fibre mat-manufacturing machine developed in our laboratory. Glass fibre mat reinforced polypropylene (PP) composites were fabricated with the variation of glass fibre content. Tensile, flexural and high rate impact test was conducted to investigate the effect of glass fibre content on the mechanical properties of the glass fibre mat/PP composite. Deformation and fracture behaviour of the glass fibre mat/PP composites was investigated to study the relationship with the mechanical property data. The tensile and flexural modulus increased with the increment of glass fibre content. However, the tensile and flexural strengths exhibited maximum values and showed a decrease at the higher glass fibre content than this point. The impact absorption energy also exhibited a similar result with the tensile and flexural property data.     

NaBF4 as a hygrothermal durability enhancer for glass fibre reinforced polypropylene composites

The long term performance of composite materials is highly desired for their expanding application range. Tuning the interphase properties has been proven to be a practical way to enhance the performance of composites. In this study, short glass fibre (GF) reinforced polypropylenes (PPs) with improved hygrothermal durability were obtained by incorporating NaBF₄ into the sizing and thus the interphases of GF/PP composites. Detailed investigations were performed on the surface properties of sized GFs and the mechanical properties of virgin and aged composites. It was found that the retention in both ultimate tensile strength and Charpy impact toughness of aged composites monotonically increased with increasing NaBF₄ content. The improvement in hygrothermal durability was related to the enhanced fibre/matrix adhesion strength induced by the presence of NaBF₄ as indentified by fracture surface analysis using field-emission scanning electron microscopy and single fibre pull-out test.     

Effect of coupling agents on reinforcing potential of recycled carbon fibre for polypropylene composite

Polypropylene (PP) composites reinforced with recycled carbon fibre have been prepared through extrusion compounding and injection moulding. The reinforcing potential of the recycled fibre was increased by improving the interfacial adhesion between the fibre and PP matrix and this was done by the addition of maleic anhydride grafted polypropylene (MAPP) coupling agents. Three MAPP couplers with different molecular weights and maleic anhydride contents were considered. The effects on the mechanical properties of the composite were studied, and scanning electron microscopy (SEM) was used to study the fracture morphology of the tensile specimens. It was observed that with the addition of MAPP the interfacial adhesion was improved as fewer fibres were pulled-out and less debonding was seen. A microbond test was performed and a significant improvement in interfacial shear strength was measured. This resulted in composites with higher tensile and flexural strengths. The maximum strength was achieved from MAPP with the highest molecular weight. Increased modulus was also achieved with certain grades of MAPP. It was also found that the composite impact strength was improved significantly by MAPP, due to a higher compatibility between the fibre and matrix, which reduced crack initiation and propagation.     

Determination of interfacial shear strength of white rot fungi treated hemp fibre reinforced polypropylene

Short untreated and white rot fungi treated hemp fibre, polypropylene (PP) and maleated polypropylene (MAPP) coupling agent were extruded and injection moulded into composite tensile test specimens. The tensile properties of untreated and treated fibre and their composites were measured. The fibre length distributions in the composite were obtained by dissolving the PP/MAPP matrix in boiling xylene to extract the fibre. Both the Single Fibre Pull-Out test and the Bowyer and Bader model were used to determine the interfacial shear strength (IFSS) of these composites. IFSS was found to be lower for the Single Fibre Pull-Out test, which was considered to be largely due to axial loading of fibre and the resulting Poisson’s contraction occurring during this technique. This suggests that the Bowyer and Bader model provides a more relevant value of IFSS for composites. The results obtained from both methods showed that IFSS of the treated fibre composites was higher than that for untreated fibre composites. This supports that the hemp fibre interfacial bonding with PP was improved by white rot fungi treatment.     

Estimation of thermal conductivity for polypropylene/hollow glass bead composites

The thermal conductivity of hollow glass bead (HGB)-filled polypropylene (PP) composites was estimated using the thermal conductivity equation of inorganic hollow microsphere-filled polymer composites published in the previous paper. The estimations were compared with the measured data of the PP composites filled with two kinds of HGB with different size (the mean diameter was respectively 35 μm and 70 μm). The results showed that the predictions of the thermal conductivity were in good agreement with the measured data except to individual data points. Furthermore, both the estimated and measured thermal conductivity decreased roughly linearly with increasing the HGB volume fraction when the HGB volume fraction was less than 20%; the influence of the particle diameter on the thermal conductivity was insignificant.     

Full exploitation of Cannabis sativa as reinforcement/filler of thermoplastic composite materials

Hemp strands and cane straw of hemp have been used as reinforcement and filler of polypropylene composites obtained by injection moulding. The aim of the work was to improve the tensile properties of hemp composites and make them more similar to those obtained with glass fibre composites. For this reason, maleated polypropylene (MAPP) was used as compatibility agent in hemp strand and hemp straw composites. MAPP decreases the hydrophobic nature of PP matrix and enhances the dispersion and the adhesion at interface between both constituents. The hydrophilic property of material’s surface was determined by colloidal titration. The length and diameter of the fibre reinforcement during processing were analysed and their aspect ratio calculated. The tensile results obtained showed that the mechanical properties of hemp strand composites can amount to 80% of the mechanical properties of glass fibre composites.     

FRACTURE BEHAVIOUR OF UNTREATED AND SILANE-TREATED TALC-FILLED POLYPROPYLENE COMPOSITES

Abstract— Fracture behaviour of injection-moulded polypropylene filled with silane-treated talc was studied as a function of filler volume fraction (0–20%) and compared to that of polypropylene filled with untreated talc. High-rate tests (0.57 m/s) on SENB specimens were carried out using an instrumented Charpy impact pendulum, and linear elastic fracture mechanics (LEFM) was applied to calculate the fracture parameters, K _C and G _C. It was found that moderate fractions of talc which were added to the polypropylene matrix increased the fracture toughness of the composite independent of the talc surface treatment. This general improvement seems to be due to the peculiar orientation of the talc platelets in the injection-moulded specimens. The fracture behaviour of the composites was also studied at low strain rate (1 mm/min) by tests on J -integral type specimens with the same SENB geometry. In this case, the composites with silane-treated talc presented poor J -integral values compared to those of the samples with untreated talc. This was attributed to a reduction of the plastic zone at the crack tip, since the improved coupling between the talc platelets and matrix increased the yield strength of the composite. All the results are explained on a basis of morphological and microstructural details.     

Effect of coupling agents on the crystallinity and viscoelastic properties of composites of rice hull ash-filled polypropylene

This paper focuses on the application of silica rich, rice hull ash (RHA) from the rice industry as filler in polypropylene (PP). The dynamic rheological behaviour and the crystallinity of its composites with semi-amorphous polypropylene were used as performance indicators. The effect of coupling agents on RHA/PP compatibility was also investigated. Addition of RHA increased storage modulus (G′), and the onset (T _co) and peak (T _c) crystallinity temperatures, and decreased the degree of crystallinity of the system. Two coupling agents, maleated polypropylene (MAPP) and an amino functional silane, were used to improve the interfacial adhesion of RHA and PP. Addition of MAPP increased G′, T _c and T _co and the crystalline phase of the system. Addition of silane had mixed effects. It increased the rheological properties at higher concentrations, it increased T _c and T _co at all the concentrations and increased crystallinity at lower concentrations and decreased it at higher concentrations. Results from a thermogravimetric analyser were useful in revealing the mechanism by which silane bond to RHA and its effect on the rheology and crystallisation behaviour of the composites.     

Spatially resolved characterization of cellulose nanocrystal-polypropylene composite by confocal Raman microscopy

Raman spectroscopy was used to analyze cellulose nanocrystal (CNC) -polypropylene (PP) composites and to investigate the spatial distribution of CNCs in extruded composite filaments. Three composites were made from two forms of nanocellulose (CNCs from wood pulp and the nano-scale fraction of microcrystalline cellulose) and two of the three composites investigated used maleated PP as a coupling agent. Raman maps, based on cellulose and PP bands at 1098 and 1460 cm(-1), respectively, obtained at 1 μm spatial resolution showed that the CNCs were aggregated to various degrees in the PP matrix. Of the three composites analyzed, two showed clear existence of phase-separated regions: Raman images with strong PP and absent/weak cellulose or vice versa. For the third composite, the situation was slightly improved but a clear transition interface between the PP-abundant and CNC-abundant regions was observed, indicating that the CNC remained poorly dispersed. The spectroscopic approach to investigating spatial distribution of the composite components was helpful in evaluating CNC dispersion in the composite at the microscopic level, which helped explain the relatively modest reinforcement of PP by the CNCs.     

Ductile-<Emphasis Type="Italic">to</Emphasis>-brittle transition in cenosphere-filled polypropylene composites

Cenosphere-filled polypropylene (PP) composites were fabricated and characterized for their structural/morphological and fracture mechanical behaviour. The fracture properties were studied following the essential work of fracture (EWF) approach based on post-yield fracture mechanics (PYFM) concept. The structural attributes and its consequent effects on the dynamic mechanical properties were characterized by wide angle X-ray diffraction (WAXD), hot-stage polarized light optical microscopy (PLOM) and dynamic mechanical analysis (DMA). The WAXD studies have revealed a decrease in crystallinity of the composites with increase in cenosphere content. PLOM studies reveals a threefold reduction in the diameter of the spherulite in case of composite with 10 wt% of cenosphere compared to that of PP followed by an increase of ~50% in the composite with 20 wt% of cenosphere compared to that of the composite with 10 wt% cenosphere. DMA revealed an enhancement in the energy dissipation ability of the composite with 10 wt% of cenosphere and an increase in the storage modulus up to ~30% in the composites relative to the soft PP phase. The non-essential work of fracture (NEWF: βw _p) as the resistance to stable crack propagation has shown a maximum at 10 wt% of cenosphere followed by a sharp drop at higher cenosphere content indicating a cenosphere-induced ductile-to-brittle transition (DBT). Fractured surface morphology investigations revealed that the failure mode of the composites undergo a systematic transition from matrix-controlled shear deformation to filler-controlled quasi-brittle modes above a cenosphere loading of 10 wt% in the composites reiterating the possibility of filler-induced semiductile-to-DBT transition.