The effect of interfacial microstructure on the interfacial strength of glass fiber/polypropylene resin composites

In this study, a new method to form resin droplets on fibers has been developed, and samples for the single fiber pull-out test were prepared using this method. The effects of microstructure of polypropylene (PP) resin and the microstructure of interface between the glass fiber and PP resin on the interfacial strength have been investigated. In addition, the influence of the microstructure of the interface on the interfacial strength of glass fiberreinforced PP composites have been discussed. It has been found that in the pull-out test, the transcrystallinity formed at the interface between the glass fiber and PP resin improved the interfacial strength when no spherulites developed in the PP matrix. On the other hand, it has been found that when the spherulites were well developed in the PP matrix, the transcrystallinity formed at the interface reduced the interfacial strength. Finally, rapid cooling has been shown to improve the interfacial strength between the fiber and resin in the crystalline polymer matrix composites. © 1994 John Wiley & Sons, Inc.     

Influence of Residual Stresses, Interface Roughness, and Fiber Coatings on Interfacial Properties in Ceramic Composites

Fiber pushout tests are performed on zircon-matrix composites especially fabricated with a variety of silicon carbide reinforcing fibers and fiber coatings in order to create samples with different interfacial properties, surface roughness, and possibly in different states of residual stress to demonstrate their role on the interfacial and mechanical properties of fiber-reinforced composites. The data obtained from fiber pushout tests are analyzed using linear, shear-lag, and progressive debonding models to extract important interfacial properties, residual stresses, and surface roughness. The nature and magnitude of residual stresses in composites are independently characterized by measuring the coefficient of thermal expansion of the fiber, the matrix, and the composite for comparison with similar values measured using the fiber pushout tests. These results are then compared for self-consistency among different ways of analyzing data and with independently measured and calculated values. The results have shown that independent and complementary methods of data acquisition and analysis are required to fully understand interfacial properties in ceramic composites. In particular, independent measures of the coefficient of thermal expansion, residual stresses, and surface roughness are required to confidently interpret interfacial properties obtained by different analytical approaches and then relate them to the overall mechanical response of composites. It is also shown that composites with optimum mechanical response can be created by suitably engineering the interface using multiple fiber coatings.     

Effect of interfacial interaction on the crystallization and mechanical properties of PP/nano‐CaCO3 composites modified by compatibilizers

To investigate the effect of interfacial interaction on the crystallization and mechanical properties of polypropylene (PP)/nano‐CaCO₃ composites, three kinds of compatibilizers [PP grafted with maleic anhydride (PP‐g‐MA), ethylene–octene copolymer grafted with MA (POE‐g‐MA), and ethylene–vinyl acetate copolymer grafted with MA (EVA‐g‐MA)] with the same polar groups (MA) but different backbones were used as compatibilizers to obtain various interfacial interactions among nano‐CaCO₃, compatibilizer, and PP. The results indicated that compatibilizers encapsulated nano‐CaCO₃ particles, forming a core–shell structure, and two interfaces were obtained in the compatibilized composites: interface between PP and compatibilizer and interface between compatibilizer and nano‐CaCO₃ particles. The crystallization and mechanical properties of PP/nano‐CaCO₃ composites were dependent on the interfacial interactions of these two interfaces, especially the interfacial interaction between PP and compatibilizer. The good compatibility between PP chain in PP‐g‐MA and PP matrix improved the dispersion of nano‐CaCO₃ particles, favored the nucleation effect of nano‐CaCO₃, increased the tensile strength and modulus, but reduced the ductility and impact strength of composites. The partial compatibility between POE in POE‐g‐MA and PP matrix had little effect on crystallization and mechanical properties of PP/nano‐CaCO₃ composites. The poor compatibility between EVA in EVA‐g‐MA and PP matrix retarded the nucleation effect of nano‐CaCO₃, and reduced the tensile strength, modulus, and impact strength. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Interfacial adhesion between ionic copolymers and high-modulus surface-treated carbon fibers

High-modulus carbon-fiber-reinforced thermoplastic composites typically fail at the interface due to poor adhesion between fiber and matrix. To increase interfacial strength, the research described herein focuses on modifying the fiber surface (via high-temperature acid treatment or zinc electrolysis) to facilitate chemical functional groups on the fiber that might increase fiber-matrix inter-actions. The thermoplastic matrix materials used in this study were random copolymers of ethylene and methacrylic acid in which the carboxyl groups in the methacrylic acid segments were neutralized with either sodium or zinc counterions. Mechanical tests were performed to determine the macroscopic effects of fiber pretreatment on the ultimate mechanical properties of the composites. Fabrication was designed such that fiber-matrix separation provides the dominant contribution to mechanical gracture. Composites containing fibers treated with nitric acid, or a mixture of nitric and sulfuric acids exhibit a 20 to 25 percent increase in transverse (tensile) fracture stress relative to composites fabricated with as-received fibers. Scanning electron microscopy of the fiber-matrix interface at fracture allows one to “zoom-in” and obtain qualitative details related to adhesion. Fracture surface micrographs of the above-mentioned acid-treated fiber-reinforced composites reveal an increase in the amount of matrix material that adhered to the fiber surface relative to the appearance of the fracture surface of composites fabricated with as-received fibers. The presence of acid functionality in the matrix, rather than the divalent nature of the zinc counterions, produces the largest relative enhancement of transverse (tensile) fracture stress in the above-mentioned composites containing surface-treated carbon fibers.     

Interfacial Characteristics of Sisal Fiber and Polymeric Matrices

Sisal-fiber-reinforced composites, as a class of eco-composites, have attracted much attention from materials scientists and engineers in recent years. In this article, the effects of fiber surface treatment on fiber tensile strength and fiber-matrix interface characteristics were determined by using tensile and single fiber pullout tests, respectively. The short beam shear test was also employed to evaluate the interlaminar shear strength of the composite laminates. Vinyl ester, epoxy, and high-density polyethylene (HDPE) were chosen as matrix materials. To enhance the interfacial strength, two kinds of fiber surface-treatment methods, namely, chemical bonding and oxidisation, were used. The results obtained showed that different fiber surface-treatment methods produced different effects on the tensile strength of the sisal fiber and fiber-matrix interfacial bonding characteristics. Hence, valuable information on the interface design of sisal fiber–polymer matrix composites can be obtained from this study.     

Effect of Interfacial Shear Strength on Crack-Fiber Interaction Behavior in Ceramic Matrix Composites

Zircon matrix composites uniaxially reinforced with SiC fibers were fabricated with different interfacial properties by changing the fiber coatings. The phenomenon of crack interaction with fibers and/or fiber coatings and its dependence on the interfacial properties were studied using a microindentation technique. The influence of the fiber orientation relative to the crack extension direction on the crack-fiber interaction was also investigated. Crack deflection was observed at the fiber-matrix interface in composites having low interfacial shear strength, and the crack deflection was mostly single-sided, but double-sided deflection was also observed. Crack penetration into the fiber occurred in composites with high values of the interfacial shear strength. These observations were in general agreement with the theoretical predictions of the crack deflection behavior based on the bimaterial interfaces in ceramic composites, but additional observations were made on crack deflection at multiple fiber-matrix interfaces.     

Interfacial Characteristics of Sisal Fiber and Polymeric Matrices

Sisal-fiber-reinforced composites, as a class of eco-composites, have attracted much attention from materials scientists and engineers in recent years. In this article, the effects of fiber surface treatment on fiber tensile strength and fiber-matrix interface characteristics were determined by using tensile and single fiber pullout tests, respectively. The short beam shear test was also employed to evaluate the interlaminar shear strength of the composite laminates. Vinyl ester, epoxy, and high-density polyethylene (HDPE) were chosen as matrix materials. To enhance the interfacial strength, two kinds of fiber surface-treatment methods, namely, chemical bonding and oxidisation, were used. The results obtained showed that different fiber surface-treatment methods produced different effects on the tensile strength of the sisal fiber and fiber-matrix interfacial bonding characteristics. Hence, valuable information on the interface design of sisal fiber-polymer matrix composites can be obtained from this study.     

Mechanical,flammability, and crystallization behavior of polypropylene composites reinforced by aramid fibers

In this article, we report the mechanical and thermal properties, together with the crystallization and flammability behaviors, of pure polypropylene (PP) and PP/aramid fiber (AF) composites with AF loadings of 5, 10, 20, 30, and 40 wt %. The mechanical properties of the samples were evaluated by tensile and izod notched impact tests, and the results show that the tensile strength of the composites could reach up to 67.8 MPa and the izod notched impact strength could rise to 40.1 kJ/m². The structure and morphology were observed by scanning electron microscopy and polarized optical microscopy, respectively. This demonstrated that a solid interface adhesion between the matrix and fibers was formed. The thermal and crystalline behaviors of the PP/AF composites were also investigated by thermogravimetric analysis and differential scanning calorimetry analysis, and the results show that the char residue of the PP/AF composites improved greatly with increasing AF loading, and the highest value could reach up to 23.7% in the presence of 40 wt % AF. The supercooling degree, initial crystallization temperature, and crystallization percentage were used to characterize the crystallization behavior of the PP/AF composites, and the results indicate that the AFs had positive effects on the promotion of PP nucleation, which can usually improve the mechanical properties of composites. Moreover, the flammability analysis of the PP/AF composites demonstrated that the presence of AFs could significantly decrease the peak heat release rate and the total heat release and reduce the melt-dripping of the PP/AF composites. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

PP-g-MAH对PP/纳米SiO_2复合材料性能的影响

采用熔融共混法制备了聚丙烯(PP)/纳米SiO2复合材料,研究了相容剂马来酸酐接枝聚丙烯(PP-g-MAH)对复合材料力学性能、结晶性能及界面作用的影响。结果表明:PP-g-MAH能有效地增强纳米SiO2与PP基体间的界面作用,提高复合材料的力学性能;同时,PP-g-MAH增强了纳米SiO2的成核活性,使PP的结晶温度升高,球晶细化。     

The interfacial bond stress in steel fiber cement composites

In fiber cement composites most fibers are in a state of partial bond due to internal stresses arising from moisture migration during fabrication and subsequent volume changes in the matrix. A wide variation in the computed interfacial bond strength therefore occurs depending upon the type of test or when derived from phenomena such as crack spacing. In practice debonding of the fibers occurs in flexural tension in the presence of a strain gradient. This paper presents further data on steel fiber mortar and concrete to confirm the validity of the composite mechanics approach to predict the composite flexural strength. It is shown that the composition of the matrix and its strength properties influence the fiber-matrix interfacial bond stress and the relative contributions of the matrix and the fibers to the composite flexural strength.     

Enhancement of mechanical properties and interfacial adhesion of PP/EPDM/flax fiber composites using maleic anhydride as a compatibilizer

In order to improve the compatibility between natural fibers and polypropylene (PP) and polypropylene‐ethylene propylene diene terpolymer (PP‐EPDM) blends, the functionalization of both matrices with maleic anhydride (MA) is investigated in this study. The morphological observations carried out by scanning electron microscopy show that the incorporation of small amounts of functionalized polymer considerably improves the adhesion at the fiber‐matrix interface. In these cases, the fibers are perfectly embedded in the matrix in relation to the composites prepared with the pure homopolymers, and a significant increase in the composite strength is also observed, particularly, after the incorporation of both modified polymers (MAPP and MAEPDM). Thus, it is possible to correlate better interfacial adhesion with the improvement of mechanical properties. It is assumed that the functionalization of the matrix reduces interfacial stress concentrations and may prevent fiber‐fiber interactions, which are responsible for premature composite failure. The crystallization kinetics of PP were also analyzed by differential scanning calorimetry (DSC). It was observed that both flax fiber and rubber behave as effective nucleant agents, accelerating PP crystallization. Moreover, these results are particularly relevant when the grafted matrices are added to the composite. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2170–2178, 2003     

Microstructure and fracture behavior of polypropylene/barium sulfate composites

The microstructure, mechanical properties, and fracture behavior of polypropylene (PP)/barium sulfate (BaSO₄) composites were studied. Four composite samples with different PP‐BaSO₄ interface were prepared by treating the filler with different modifiers. The fracture behavior of the composites under different strain rates was studied by means of Charpy impact tests and essential work of fracture (EWF) tests. It is shown that a moderate interfacial adhesion is favorable for toughening, which ensures that the particles transfer the stress and stabilizes the cracks at the primary stage of the deformation, and satisfies the stress conditions of plastic deformation for matrix ligaments subsequently via debonding. Very strong interfacial adhesion is not favorable for toughness, especially under high strain rate, because the debonding‐cavitation process may be delayed and the plastic deformation of matrix may be restrained. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1207–1213, 2006     

Measurement of residual stress effects by means of single-particle composite tests

The effects of thermal residual stresses on adhesion have been quantitatively examined by means of a single-particle composite technique. Residual stresses were varied systematically by controlling the molding time-temperature profile for single, surface-modified glass spheres embedded in a thermoplastic polymeric matrix. Interfacial failure was induced by tensile straining of single-particle composites until debonding at the geometric poles was observed between the matrix and spherical particle. The interfacial strength was calculated and is reported as a function of residual hoop stresses, the magnitude of the latter determined by the processing conditions, i.e. different residual stress states within the matrix. A preliminary study was conducted using two interfaces: an octylsilane/poly(vinyl butyral) interface and a di-aminosilane/poly(vinyl butyral) interface. The former is a weak bond with only Lifshitz-van der Waals forces operating across the interface, and the latter a strong bond featuring a molecular penetration mechanism. Both interfaces were deleteriously affected by increases in residual stresses, with the former having the highest level of residual stress with complete interfacial strength loss. Experimental requirements for a more quantitative study are described.     

The role of maleic anhydride functionalized graphene oxide in improving the interfacial properties of carbon fibre/bismaleimide composites

This work aims to assess the effect of maleic anhydride functionalized graphene oxide (MAH‐f‐GO) on the interfacial properties of carbon fibre/bismaleimide (BMI) composites by experimental and finite element (FE) methods. Transverse fibre bundle (TFB) specimens with different contents of MAH‐f‐GO nanoparticles were manufactured to investigate the interfacial strength of the carbon fibre/BMI composites. The fracture surface of the TFB specimens was examined by scanning electron microscopy to observe the morphologies of the fibre ? matrix interface. The coefficient of thermal expansion, cure shrinkage and elastic modulus were measured and included in the FE simulation. An FE analysis model was established to simulate the thermal residual stress distribution around the carbon fibre and to estimate the interfacial bonding strength of the TFB specimens. The combination of experimental and FE analysis results indicated that the addition of MAH‐f‐GO nanoparticles noticeably reduced the concentration of residual stress at the fibre ? matrix interface and enhanced the interfacial properties of the carbon fibre/BMI composites.© 2017 Society of Chemical Industry     

Effect-of Processing Varcables on Interfacial Properties of an SiG-Fiber-Reinforced Reaction-Bonded Si3N4 Matrix Composite

Fiber/matrix interfacial debonding and frictional sliding stresses were evaluated by single-fiber pushout tests on unidirectional continuous silicon-carbide-fiber-reinforced, reaction-bonded silicon nitride matrix composites. The debonding and maximum pushout loads required to overcome interfacial friction were obtained from load–displacement plots of pushout tests. Interfacial debonding and frictional sliding stresses were evaluated for composites with various fiber contents and fiber surface conditions (coated and uncoated), and after matrix densification by hot isostatic pressing (HIPing). For as-fabricated composites, both debonding and frictional sliding stresses decreased with increasing fiber content. The HIPed composites, however, exhibited higher interfacial debonding and frictional sliding stresses than those of the as-fabricated composites. These results were related to variations in axial and transverse residual stresses on fibers in the composites. A shear-lag model developed for a partially debonded composite, including full residual stress field, was employed to analyze the nonlinear dependence of maximum pushout load on embedded fiber length for as-fabricated and HIPed composites. Interfacial friction coefficients of 0.1–0.16 fitted the experimental data well. The extremely high debonding stress observed in uncoated fibers is believed to be due to strong chemical bonding between fiber and matrix.     

The effect of transcrystallinity on the transverse mechanical properties of single-polymer polyethylene composites

The microstructure of polyethylene (PE)/PE composites, consisting of the high-density PE (HDPE) matrix and ultrahigh molecular-weight PE (UHMWPE) fibers, was investigated. Single-fiber composites were prepared and analyzed in a hot-stage crystallization unit attached to a polarizing microscope, aiming to find out how the conditions of crystallization affected the transcrystallinity (tc) growth at the fiber-matrix interface. Thermal treatments leading to two extreme microstructures, of either maximum or minimum thickness of the transcrystalline zone, were sought. It was found that a uniform transcrystalline layer was developed on the UHMWPE fiber from the HDPE melt under isothermal conditions, whereas rapid cooling from the melt prevented the generation of tc. The mechanical properties of unidirectional composite laminae either with or without the transcrystalline zone were measured. A comparison of the transverse strength predicted by theoretical models with the experimental values revealed good interfacial adhesion in the PE/PE system. It was shown that the tc growth had a negligible effect on the composite mechanical properties in the longitudinal direction, whereas it resulted in a 50% decrease of the transverse tensile strength and strain to failure. Scanning electron microscopy attributed that observation to premature brittle failure at tc/tc contact regions. © 1995 John Wiley & Sons, Inc.     

The Study of Reactive Functional Groups in Adhesive Bonding at the Aramid-Epoxy Interface

In polymeric composites, reactive functional groups on the fiber surface are assumed to enhance the mechanical strength of the fiber-matrix interface greatly by forming covalent bonds with the matrix. To test this assumption, we sought to promote covalent bonding at the aramid fiber-epoxy matrix interface by attaching flexible reactive pendent groups to the fiber surface. Other factors that could affect interfacial adhesion were kept constant, i.e., surface energy and surface topography. Quantitative analysis showed a pendent group attachment level of 1.5 to 4.5 groups per 100 Ų of fiber surface, a level that agrees well with the theoretical amount. Surprisingly, in adhesive performance tests, the presence of these reactive pendent groups did not improve the fiber-matrix interface strength. Specific chemical tests for covalent bond formation between the terminal amine of the pendent group and the epoxy molecule showed that covalent bonding did not occur, thus explaining the unexpected lack of improvement in adhesive performance.     

A new method for study of the fiber-matrix interface in composites: single fiber pull-out from a microcomposite

—A new method, single fiber pull-out from a microcomposite (SFPOM), was developed to study the fiber/matrix interface in composites. By pulling a fiber out of a seven-fiber microcomposite, the SFPOM test provides the real feeling of a fiber pulled out of an environment similar to that in a real composite. Interfacial shear strength decreased as the fiber volume fraction increased in the fiber-matrix system tested in the experiment. Three factors were suggested to be responsible for the phenomenon: (1) poor bonding between fibers when close to each other; (2) shear stress concentration in the matrix between neighboring fibers; and (3) possible change in matrix properties, thus altering the failure mechanism from interfacial debonding to a mixture of interfacial debonding and matrix fracture.     

Effects of the interfacial stress-induced crystallization on the matrix crystalline morphology and the mechanical properties of glass fiber-reinforced high density polyethylene composites

In the melt-mixing process of high density polyethylene (HDPE) and glass fibers (GF), four types of composites with various interfacial bond strength were obtained by adding maleic low molecular weight polyethylene (MPEW) or maleic anhydride (MAH) and initiator, etc. The mechanical properties of these composites and their dependence on the matrix crystalline morphology were investigated by scanning electron microscopy, small-angle light scattering, differential scanning calorimetry, wide-angle X-ray diffraction, and a material universal mechanical testing machine. The occurrence of the interfacial transition regions made of extended-chain crystals around glass fibers was found to be the result of crystallization effects induced by the interfacial stress. The interfacial stress was mainly produced from the matrix shrinkage in the specimen molding process. Under high interfacial bond strength conditions, the forming process of the extended-chain crystals was found to both relax the interfacial stress and at the same time, enhance the interfacial phase modulus and improve the mechanical properties of the composites. Under a 30% glass fiber content condition, the extended-chain crystals formed along the normal direction of glass fiber surfaces connected with each other, fully filled the matrix, and led to a significant increase in the Charpy impact strength of the composites. By contrast, under weak interfacial adhesion, the interfacial stress was released by a dewetting process of the interfacial phase and by the formation of interfacial cracks. Consequently, the interfacial stress did not influence the growth of the spherulites in the matrix and at the same time, the Charpy impact strength of the composite was lower.     

Effect of coupling agents on the thermal and mechanical properties of polypropylene–jute fabric composites

Composites with different jute fabric contents and polypropylene (PP) were prepared by compression molding. The composite tensile modulus increased as the fiber content increased, although the strain at break decreased due to the restriction imposed on the deformation of the matrix by the rigid fibers. Moreover, and despite the chemical incompatibility between the polar fiber and the PP matrix, the tensile strength increased with jute content because of the use of long woven fibers. The interfacial adhesion between jute and PP was improved by the addition of different commercial maleated polypropylenes to the neat PP matrix. The effect of these coupling agents on the interface properties was inferred from the resulting composite mechanical properties. Out‐of‐plane instrumented falling weight impact tests showed that compatibilized composites had lower propagation energy than uncompatibilized ones, which was a clear indication that the adhesion between matrix and fibers was better in the former case since fewer mechanisms of energy propagation were activated. These results are in agreement with those found in tensile tests, inasmuch as the compatibilized composites exhibit the highest tensile strength. Scanning electron microscopy also revealed that the compatibilized composites exhibited less fiber pullout and smoother fiber surface than uncompatibilized ones. The thermal behavior of PP–compatibilizer blends was also analyzed using differential scanning calorimetry, to confirm that the improvements in the mechanical properties were the result of the improved adhesion between both faces and not due to changes in the crystallinity of the matrix. Copyright © 2006 Society of Chemical Industry