Impact properties of glass fibre/impact modifier/polypropylene hybrid composites

The impact properties of glass-fibre/impact-modifier/polypropylene (GF/IM/PP) hybrid composites were characterized using a number of impact test methods. For the IM/PP blends, the impact fracture toughness can be measured using linear elastic fracture mechanics (LEFM) approach. For the GF/IM/PP hybrids, due to their non-compliance with LEFM, the essential work of fracture approach was employed. The impact properties of the IM/PP blends increased with IM concentration, while that of the GF/IM/PP hybrids did not change very much with IM content. It was concluded that cavitation of the PP matrix around the IM particles was the major toughening mechanism in the IM/PP blends. However, in the GF/IM/PP hybrids, the toughening effect due to cavitation was suppressed due to the introduction of short glass fibres (15 vol%). It is believed that the local stress in the matrix was relieved by fibre/matrix debonding of the relatively weak fibre/matrix interface. Thus, the presence of the IM particles was rendered insignificant in the GF/IM/PP hybrids.     

Fracture resistance of polyblends and polyblend matrix composites: Part II Role of the rubber phase in Nylon 6,6/ABS alloys

A comprehensive study was undertaken on the specific role of rubber on toughening when other rigid polymer or non-polymer phases were present. Nylon 6,6/SAN blends of various SAN concentrations ranging from pure SAN to pure nylon 6,6 were investigated with and without fibre reinforcements. These results could be compared with the toughness values of unreinforced and fibre-reinforced nylon 6,6/ABS alloys from a previous study in order to elucidate the role of rubber. Fracture behaviour was investigated rigorously by characterizing the fracture initiation toughness, JIC, and the steady-state fracture toughness, Jss. These were then related to the microstructure and failure modes determined by microscopy and fractography methods. It was found that rubber increased both fracture initiation and propagation toughness in the presence of the rigid phase, while the rigid phase toughened the alloy only when the rigid phase/matrix interface was strong enough. The role played by glass fibres was found to be critically related to the fibre/matrix interfacial strength. Toughening was generally observed, both in the presence and absence of rubber, when the interface was strong. In all cases toughening could be related to the enhancement of plasticity in the crack tip by the presence of the rubber phase or the reinforcing glass phase. This revised version was published online in November 2006 with corrections to the Cover Date.     

Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites

The local microstructure can be altered significantly by various fibre surface modifications, causing property differences between the interphase region and the bulk matrix. By using tapping mode phase imaging and nanoindentation tests based on the atomic force microscope (AFM), a comparative study of the sized fibre surface topography and modulus as well as the local mechanical property variation in the interphase of E-glass fibre reinforced epoxy resin and E-glass fibre reinforced modified polypropylene (PPm) matrix composites was conducted. The phase imaging AFM was found a highly useful tool for probing the interphase with much detailed information. Nanoindentation experiments indicated the chemical interaction during processing caused by a gradient profile in the modulus across the interphase region of γ-aminopropyltriethoxy silane (γ-APS) and polyurethane (PU)-sized glass fibre reinforced epoxy composite. The interphase with γ-APS/PU sizing is much softer than the PPm matrix, while the interphase with the γ-APS/PP sizing is apparently harder than the matrix, in which the modulus was constant and independent of distance away from the fibre surface. The interphase thickness varied between less than 100 and ≈300 nm depending on the type of sizing and matrix materials. Based on a careful analysis of ‘boundary effect’, nanoindentation with sufficient small indentation force was found to enable measuring of actual interphase properties within 100 nm region close to the fibre surface. Special emphasis is placed on the effects of interphase modulus on mechanical properties and fracture behaviour. The interphase with higher modulus and transcrystalline microstructure provided simultaneous increase in the tensile strength and the impact toughness of the composites.     

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.     

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.     

Tensile and fracture behaviour of PP/wood flour composites

Polypropylene/wood flour composites with different fibre content were prepared. The effect of composition and the incorporation of maleinated polypropylene on the materials tensile and fracture and failure behaviour was investigated. Reliable fracture toughness data that will be useful for structural applications were obtained. In unmodified composites an increase in Young´s modulus was found with the addition of wood flour to PP, whereas tensile strength, strain at break and fracture toughness were observed to decrease as fibre content increased. The presence of MAPP was beneficial to tensile strength and ductility and had no significant effect on fracture toughness, as a result of enhanced fibre dispersion within the matrix and improved interfacial adhesion. Although reduced ductility and toughness were observed for the composites respect to the matrix, in the case of modified composites, environmentally friendly stiffer materials were obtained with cost saving without sacrificing strength.     

The falling weight impact properties of maleic anhydride compatibilized polypropylene–polyamide blends

A series of polypropylene–polyamide 6 (PP–PA) blends of composition 80: 20, 50:50 and 20:80 have been prepared in a twin screw extruder followed by injection moulding. Maleic anhydride grafted polypropylene was used as a compatibilizer for these blends. Static mechanical and falling weight impact tests were performed on these blends. The fracture surfaces of impact specimens were subsequently examined by scanning electron microscopy (SEM). The mechanical properties of the blends were found to be strongly dependent on the PP–PA blend ratios. The Young’s modulus, tensile strength and impact energy were observed to increase with increasing PA content. The impact strength was better in blends when the PA content approached 80 wt %. SEM observations revealed that the addition of compatibilizer resulted in good adhesion between the PP dispersed domains and PA matrix in the PP–PA 20:80 blend. Furthermore, the SEM fractographs also indicated that the cold drawn of PA matrix and debonding of PP domains were responsible for the high impact strength of this blend. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fracture resistance of a glass-fibre reinforced rubber-modified thermoplastic hybrid composite

Toughening mechanisms in a hybrid amorphous thermoplastic composite containing both distributed rubber particles and rigid glass fibres have been investigated. Tensile properties were measured for a range of materials with varying rubber particle and glass-fibre contents, and different rubber particle sizes. Fracture toughness was characterized by separating the overall fracture into its initiation and propagation components. Deformation and fracture modes at crack tips were optically characterizedin situ during loading. The results indicate that both initiation and propagation toughness are enhanced by rubber particle additions to the glass-fibre reinforced composite. Synergistic effects between glass fibres and rubber particles are identified: for example, glass fibres inhibit crazing at rubber particles, and rubber particles tend to promote crazing at fibre/matrix interfaces and also void initiation at fibre ends. Toughening mechanisms are discussed in the light of available models.     

A study of the mechanical behaviour on fibre reinforced hollow microspheres hybrid composites

This paper presents the results of an investigation into the effects of hollow glass microsphere fillers and of the addition of short fibre reinforcements on the mechanical behaviour of epoxy binding matrix composites. Properties like flexural stiffness, compressive strength, fracture toughness and absorbed impact energy, were studied. The specimens were cut from plates produced by vacuum resin transfer moulding having a microsphere contents of up to 50% and with fibre reinforcement up to 1.2% by volume. The tests performed with unreinforced composites show that flexural and compressive stiffness, maximum compressive stresses, fracture toughness and impact absorbed energy decrease significantly with increasing filler content. However, in terms of specific values, both flexural and compressive stiffness and impact absorbed energy increase with microsphere content. The addition of glass fibre produces only a slight improvement in the flexure stiffness and fracture toughness, while increasing significantly the absorbed impact energy. In contrast, the addition of a small percentage of carbon fibres produces an important improvement in both fracture toughness and flexure stiffness, when hybrid composites with 0.9% carbon fibre are compared to unreinforced foam, but did not improved absorbed impact energy.     

The toughness of epoxy polymers and fibre composites modified with rubber microparticles and silica nanoparticles

The present paper investigates the effect of adding silica nanoparticles to an anhydride-cured epoxy polymer in bulk and when used as the matrix of carbon- and glass-fibre reinforced composites. The formation of ‘hybrid’ epoxy polymers, containing both silica nanoparticles and carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber microparticles, is also discussed. The structure/property relationships are considered, with an emphasis on the toughness and the toughening mechanisms. The fracture energy of the bulk epoxy polymer was increased from 77 to 212 J/m² by the presence of 20 wt% of silica nanoparticles. The observed toughening mechanisms that were operative were (a) plastic shear-yield bands, and (b) debonding of the matrix from the silica nanoparticles, followed by plastic void-growth of the epoxy. The largest increases in toughness observed were for the ‘hybrid’ materials. Here a maximum fracture energy of 965 J/m² was measured for a ‘hybrid’ epoxy polymer containing 9 wt% and 15 wt% of the rubber microparticles and silica nanoparticles, respectively. Most noteworthy was the observation that these increases in the toughness of the bulk polymers were found to be transferred to the fibre composites. Indeed, the interlaminar fracture energies for the fibre-composite materials were increased even further by a fibre-bridging toughening mechanism. The present work also extends an existing model to predict the toughening effect of the nanoparticles in a thermoset polymer. There was excellent agreement between the predictions and the experimental data for the epoxy containing the silica nanoparticles, and for epoxy polymers containing micrometre-sized glass particles. The latter, relatively large, glass particles were investigated to establish whether a ‘nano-effect’, with respect to increasing the toughness of the epoxy bulk polymers, did indeed exist.     

Compatibilizing effect of maleated polypropylene on the mechanical properties of injection molded polypropylene/polyamide 6/functionalized-TiO2 nanocomposites

TiO₂ nanoparticles were pretreated with excessive toluene-2,4-diisocyanate to synthesize TDI-functionalized TiO₂ (TiO₂-NCO), and then the polypropylene/polyamide 6/(PP/PA6, 70/30 wt%) blends containing 3 phr of the TDI-functionalized TiO₂ were prepared using twin-screw extruder followed by injection molding. Maleated polypropylene (PP-g-MAH) was used to compatibilize the blends. The mechanical properties of PP/PA6 blends based nanocomposites were studied through tensile and flexural tests. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to assess the fracture surface morphology and the dispersion of the TDI-functionalized TiO₂, respectively. The dynamic mechanical properties of PP/PA6 based nanocomposites were analyzed by using dynamic mechanical thermal analyzer (DMTA). The strength and stiffness of the PP/PA6 compounds were improved significantly in the presence of PP-g-MAH. This has been attributed to the synergistic effect of TDI-functionalized TiO₂ and PP-g-MAH. The PP-g-MAH compatibilized PP/PA6 compounds showed a homogeneous morphology supporting the compatibility improvement between PP, PA6 and TDI-functionalized TiO₂. TEM results revealed that the TDI-functionalized TiO₂ nanoparticles were exfoliated and uniformly dispersed in blends matrix. Possible chemical interactions between PP, PA6, TDI-functionalized TiO₂ and PP-g-MAH were proposed based on the experimental work.     

On the post-mortem fracture surface morphology of short fiber reinforced thermoplastics

The fracture surface morphology of short fiber reinforced thermoplastics (SFRTs) has often been used to assess qualitatively the degree of fiber–matrix interfacial adhesion. Mechanical properties such as tensile strength, fracture toughness and failure strain, etc. are then correlated with the morphology. Fracture surfaces showing fibers surrounded by a large amount of matrix material is commonly regarded as indication of strong fiber–matrix interfacial adhesion while smooth fibers are characteristic of weak interfacial adhesion. Many experimental results of SFRTs have been so interpreted. However, it is shown in this paper that strictly speaking, such interpretations are generally incorrect. Moreover, the amount of matrix material does not provide a quantitative measure of the adhesion. Correct implication of the morphology of fracture surfaces is clarified. Short glass fiber reinforced polyamide 6,6/polypropylene (PA 6,6/PP) blends toughened by rubber are employed as examples for SFRTs since the PA 6,6/PP blend system by changing PA 6,6 concentration in the matrix blend represents a wide range of matrix materials. It is demonstrated that the fracture surface morphology of such composites is dependent on both fiber–matrix interfacial adhesion strength and matrix shear yield strength. Consequently, tensile failure strain is well correlated with the post-mortem fracture surface morphology of these SFRTs.     

Effect of fibre content on the interlaminar fracture toughness of unidirectional glass-fibre/polyamide composite

In this paper, the effects of fibre content on the interlaminar fracture in continuous glass-fibre/polyamide 12 composite have been investigated under model I (DCB) loading condition. The specimens were fabricated with different fibre volume contents (21%, 26%, 34% and 39%) by using a powder impregnation method. It was observed that the values of G_IC(NL) and G_IC(PROP) of this material have a dropping tendency with increasing fibre volume content in the range of 21%–39%, while no general trends in G_IC(5%) and G_IC(VIS). Results show that the glass-fibre/polyamide 12 composites possess high mode I fracture toughness, which is mainly attributed to the high ductility of the polyamide 12 matrix, and the increased fibre bridging caused by the increasing of the fibre volume content can not change the decrease tendency of G_IC(PROP). The fracture surfaces of the specimens were observed by scanning electron microscopy, and the fracture mechanism was analysed.     

界面对ABS/PA6共混体系脆韧转变及断裂行为的影响

合成了反应性核壳结构增韧剂ABS-g-MA和ABS-g-AA增韧PA6, 2种增韧剂的唯一差别在于壳层接枝的反应性单体不同, 从而在与PA6共混过程中相界面存在差异, 研究在其他参数都相同的条件下, 界面性质对ABS/PA6共混体系脆韧转变及断裂行为的影响。Molau试验与扭矩试验证实ABS-g-MA/PA6共混物具有更佳的界面强度。TEM结果表明2种增韧剂在PA6中均匀分散, 然而力学测试结果表明ABS-g-MA/PA6共混物在分散相质量分数为25%~30%时发生脆韧转变,冲击强度可以达到900 J/m以上, ABS-g-AA/PA6共混物在分散相质量分数为30%~35%时发生脆韧转变。SEM结果表明ABS-g-MA/PA6共混物断面发生显著的塑性形变。TEM表明ABS-g-MA/PA6共混物的形变机制为橡胶粒子的空洞化与塑料基体的剪切屈服, 而ABS-g-AA/PA6体系没有空洞化现象, 基体剪切屈服不明显。Vu---Khanh方法测试表明, 由于ABS-g-MA/PA6共混物更高的界面强度, 共混物具有更高的G_i值, 因此冲击韧性极佳。      

Interlaminar fracture toughness and associated fracture behaviour of bead-filled epoxy/glass fibre hybrid composites

To investigate enhancement of matrix-dominated properties (such as interlaminar fracture toughness) of a composite laminate, two different bead-filled epoxies were used as matrices for the bead-filled epoxy/glass fibre hybrid composites. The plane strain fracture toughness of two different bead-filled epoxies have been measured using compact tension specimens. Significant increases in toughness were observed. Based on these results the interlaminar fracture toughness and fracture behaviour of hybrid composites, fabricated using bead-filled epoxy matrices, have been investigated using double cantilever beam and end notch flexure specimens for Mode I and Mode II tests, respectively. The hybrid composites based on carbon bead-filled matrix shows an increase in both G _IC initiation and G _IIC values as compared to a glass fibre reinforced plastic laminate with unmodified epoxy matrix. The optimum bead volume fraction for the hybrid composite is between 15% and 20%. However, the unmodified epoxy glass-fibre composite shows a higher G _IC propagation value than that of hybrid composites, due to fibre bridging, which is less pronounced in the hybrids as the presence of the beads results in a matrix-rich interply region.     

Kinetic and structural interpretations of post-yield crack resistance behavior of polyamide-6/polyolefin-g-maleic anhydride blends

Binary blends of polyamide-6 (PA-6)/polypropylene-grafted-maleic anhydride (PP-g-MA) and PA-6/low density polyethylene-grafted-maleic anhydride (LDPE-g-MA) were prepared with varied concentration (0–30 wt%) of maleic anhydride-grafted polyolefinic (PP) moiety as the impact modifier. The microstructural attributes and thermal properties of the blends were characterized by WAXD, FTIR, SEM, DSC, and TGA. The WAXD/DSC studies have revealed that the crystallinity of the blends decreased with the increase in the PP modifier whereas the onset of degradation temperature remained nearly unaffected. Comparative assessment of the crack toughness behavior of the blends has been carried out following the essential work of fracture (EWF) approach based on post-yield fracture mechanics (PYFM) concept. The kinetics of crack propagation of the blends has been discussed in the realms of structural and compositional parameters. An enhancement in the toughness (resistance to stable crack propagation) as indicated by a maximum in the non-EWF (βw _p) values have been observed at 10 and 20 wt% followed by a sharp and consistent drop in the composition regime of 10–20 and 20–30 wt% of PP-g-MA and LDPE-g-MA, respectively; conceptually implying possible ductile-to-semiductile transitions in the blend systems. The equivalence of PYFM–EPFM fracture parameters have been discussed following inequality criterion. Fractured surface morphology investigations revealed that the failure mode of the blends undergo a systematic transition from matrix-controlled homogeneous flow/deformation in the PP/polyamide phase to blend composition-dependent changes in the modes and extent of fibrillation via cavitation and shear-banding mechanisms.     

Surface topography and wear mechanisms in polyamide 66 and its composites

A study has been made of the tribological behaviour of polyamide 66 (PA66) running against itself, in unlubricated, non-conformal and rolling-sliding contact. Tests were conducted over a wide range of loads and slip ratios using a twin-disc test rig. The wear and friction behaviour of unreinforced PA66 is dominated mainly by three major features: a critical slip ratio under a fixed load and running speed, macro-transverse cracks and a layer of film on the contact surface. Both the wear and friction properties of unreinforced PA66 can be improved considerably by filling with 20wt% PTFE, and the tribological mechanisms are changed significantly. This reinforcement prevents both the initiation and propagation of transverse cracks on the contact surfaces which occurred in the unreinforced material. It also decreases both the wear rate and the friction coefficient substantially. The 30wt% short glass-fibre reinforced PA66 also suppresses the transverse cracks from initiation on the surfaces. A thin film on the contact surfaces plays a dominant role in reducing wear and friction of the composite and in suppressing the transverse cracks. These results offer the prospect of enhanced applicability of PA66 in engineering components.     

Structure property relation of hybrid biocomposites based on jute,viscose and polypropylene: The effect of the fibre content and the length on the fracture toughness and the fatigue properties

In the present study, the extent of jute and viscose fibre breakage during the extrusion process on the fracture toughness and the fatigue properties was investigated. The composite materials were manufactured using direct long fibre thermoplastic (D-LFT) extrusion, followed by compression moulding. The fracture toughness (K_IC) and the fracture energy (G_IC) of the PP–J30 composites were significantly improved (133% and 514%, respectively) with the addition of 10 wt% viscose fibres, indicating hindered crack propagation. The addition of viscose fibres resulted in three times higher fatigue life compared with that of the unmodified jute composites. Further, with the addition of (2 wt%) MAPP, the PP–J30–V10 resulted in a higher average viscose fibre length of 8.1 mm, and the fracture toughness and fracture energy increased from 9.1 to 10.0 MPa m^1/2 and 28.9 to 31.2 kJ/m², respectively. Similarly, the fatigue life increased 51% compared with the PP–J30–V10, thus demonstrating the increased work energy due to hindrance of the propagation of cracks.     

Fracture toughness investigation of the dynamically vulcanized EPDM/PP/ionomer ternary blends using the J-integral via the locus method

The fracture mechanics investigation of the dynamically vulcanized EPDM and PP/ionomer ternary blends was performed in terms of the J-integral by measuring fracture energy via the locus method. The ternary blends consisting of EPDM, PP and ionomer were prepared in a laboratory internal mixer by blending and vulcanizing simultaneously. Vulcanization was performed with dicumyl peroxide (DCP) and the composition of EPDM and PP was fixed at 50/50 by weight. Two kinds of poly(ethylene-co-methacrylic acid) (EMA) ionomers were used. The J-integral values at crack initiation, J _c, of the dynamically vulcanized EPDM and PP/EMA ionomer ternary blends were affected by the cation types (Na⁺ or Zn²⁺) and contents (5–20 wt%) of the added EMA ionomers. The ternary blend containing 20 wt% zinc-neutralized EMA ionomer and 1.0 p.h.r. DCP showed the highest J _c values of the blends. The results have been discussed with regard to the fracture topology observed by scanning electron microscopy (SEM).     

The effect of post-cure duration on the mode I interlaminar fracture toughness of glass-fibre reinforced vinylester

The effect of different post-cure conditions on the mode I fracture toughness of a vinylester resin and its glass-fibre reinforced composite counterpart has been studied. Two sets of parameters were investigated. The first was the post-cure duration at constant temperature; 90°C for 1, 4 and 24 h. The second, for resin toughness only, was a combination of post-cure temperature and duration; 90°C/4 h, 80°C/8 h and 70°C/16 h. The results show that the post-cure increases toughness. This trend is consistent between the pure resin and the fibre composite for all treatments, except at 90°C/24 h. It is believed that the prolonged post-cure duration of 24 h has weakened the bond strength at the fibre–matrix interface, thus reducing the effectiveness of toughness transfer from the matrix to the composite. The study concludes that the post-cure enhances the toughness of the glass-fibre/vinylester composite, mainly due to the increase of resin toughness.