Failure mechanics in ternary composites of polypropylene with inorganic fillers and elastomer inclusions

The effects of phase morphology, interfacial adhesion and filler particle shape and volume fraction on the fracture toughness of polypropylene (PP) filled with CaCO₃ or Mg(OH)₂ and ethylene-propylene elastomer (EPR) were investigated. Separation of the inorganic filler and elastomer particles was achieved using maleic-anhydride-grafted PP (MPP) to enhance the inorganic filler-matrix adhesion. Encapsulation of the rigid filler by the elastomer was achieved by using maleic-anhydride-grafted EPR (MEPR) to increase the inorganic filler-elastomer adhesion. The two limiting morphologies differed significantly in fracture toughness under impact loading at the same material composition. A model for a mixed mode of failure, accounting for the plane strain and plane stress contributions to the strain energy release rate,G _c, was used to predict the upper and lower limits forG _c for the two limiting morphologies over an interval of elastomer volume fractions,v _e, from 0–0.2 at a constant filler volume fraction,V _f = 0.3, and over the filler volume fraction from 0–0.4 at constant EPR content. The role of material yield strength in controlling fracture toughness has been described successfully using Irwin's analysis of plastic zone size. The presence of elastomer enhances both the critical strain energy release rate for crack initiation,G _c, and the resistance to crack propagation as expressed by Charpy notched impact strength for the two limiting morphologies. Satisfactory agreement was found between the experimental data and predictions of upper and lowerG _c limits.     

Control of the structure and properties of barium sulphate-filled blends of polypropylene and ethylene propylene copolymers

The structure and properties of polypropylene (PP) and ethylene propylene copolymer (EPR) blends filled with BaSO₄ have been investigated. The aspect of structure control concerned was the separate dispersion of filler and rubber in the PP matrix or encapsulation of the filler in the rubber phase. The former structure prevails in the PP/EPR/BaSO₄ systems, and addition of maleic anhydride-grafted polypropylene (MAPP) enhances the adhesion between the PP matrix and the filler. Encapsulation of the filler particles into the elastomer takes place when maleated EPR-rubber (EPMA) is used, and the encapsulated structure prevails even under the severe shearing conditions of injection molding. The improved matrix/filler adhesion resulted in increased yield stress and tensile strength, but decreased impact resistance. The particle size of the filler proved to be a crucial factor; below a certain particle size aggregation becomes a dominating factor. Extensive aggregation leads to the deterioration of all mechanical properties, especially to decreased impact strength.     

The mechanical properties of ternary composites of polypropylene with inorganic fillers and elastomer inclusions

The effect of elastomer volume fraction and phase morphology on the elastic modulus of ternary composites polypropylene (PP)/ethylene-propylene rubber (EPR)/inorganic filler containing 30 vol % of either spherical or lamellar filler has been investigated. Phase morphology was controlled using maleated polypropylene (MPP) and/or maleated ethylene-propylene elastomer (MEPR). As revealed by SEM observations, composites of MPP/EPR/filler exhibit separation of the filler and elastomer and good adhesion between MPP and the filler, whereas composites of PP/MEPR/filler exhibit encapsulation of the filler by MEPR. Composite models were utilized to estimate upper and lower bounds for the elastic modulus of these materials, which is strongly dependent on the morphology of the ternary composite. A model based on the Kerner equation for perfect separation of the soft inclusions and rigid fillers gives a good prediction of the upper limit for relative elastic modulus as a function of filler and elastomer volume fractions. The lower limit, achieved in the case of perfect encapsulation, depends significantly on the particle shape. Good agreement was found between experimental data and lower limits predicted using the Halpin-Tsai equation for lamellar filler and the Kerner-Nielsen equation for spherical filler. In order to calculate reinforcing efficiency of the core-shell inclusions, the finite element method (ANSYS 4.4A, GT STRUDL) has been used.     

A comparative study of the effect of different rigid fillers on the fracture and failure behavior of polypropylene based composites

Polypropylene (PP) composites with 5 wt% of different rigid particles (Al₂O₃ nanoparticles, SiO₂ nanoparticles, Clay (Cloisite 20A) nanoparticles or CaCO₃ microparticles) were obtained by melt mixing. Composites with different CaCO₃ content were also prepared. The effect of fillers, filler content and addition of maleic anhydride grafted PP (MAPP) on the composites fracture and failure behavior was investigated. For PP/CaCO₃ composites, an increasing trend of stiffness with filler loading was found while a decreasing trend of strength, ductility and fracture toughness was observed. The addition of MAPP was beneficial and detrimental to strength and ductility, respectively mainly as a result of improved interfacial adhesion. For the composites with 5 wt% of CaCO₃ or Al₂O₃, no significant changes in tensile properties were found due to the presence of agglomerated particles. However, the PP/CaCO₃ composite exhibited the best tensile behavior: the highest ductility while keeping the strength and stiffness of neat PP. In general, the composites with SiO₂ or Clay, on the other hand, displayed worse tensile strength and ductility. These behaviors could be probably related to the filler ability as nucleating agent. In addition, although the incorporation of MAPP led to improved filler dispersion, it was damaging to the material fracture behavior for the composites with CaCO₃, Al₂O₃ or Clay, as a result of a higher interfacial adhesion, the retardant effect of MAPP on PP nucleation and the lower molecular weight of the PP/MAPP blend. The PP/MAPP/SiO₂ composite, on the other hand, showed slightly increased toughness respect to the composite without MAPP due to the beneficial concomitant effects of the presence of some amount of the β crystalline phase of PP and the better filler dispersion promoted by the coupling agent which favor multiple crazing. From modeling of strength, the effect of MAPP on filler dispersion and interfacial adhesion in the PP/CaCO₃ composites was confirmed.     

Effect of interfacial interaction on morphology and mechanical properties of PP/POE/BaSO4 ternary composites

Ternary composites of Polypropylene (PP)/ethylene-octene copolymer (POE)/Barium Sulfate (BaSO₄)(PP/POE/BaSO₄) were prepared through a two-step process: BaSO₄ master-batches were first prepared through blending of BaSO₄ and POE, then blending with PP. Two families of phase structure were confirmed through SEM and DSC, depending on their interfacial interaction. Separation of POE and BaSO₄ filler was found when untreated or titanate coupling agent treated BaSO₄ filler were used. Encapsulation of BaSO₄ particles by POE elastomer was achieved by using BaSO₄ master-batch prepared through reactive blending of BaSO₄ with POE in the presence of maleic anhydride (MAH) and dicumyl peroxide (DCP). The mechanical properties of the composites greatly rely on the morphology. The yield strength and the impact toughness of a composite with core-shell morphology are higher than those of composites with separated morphologies, but the former has lower flexural modulus and elongation at break than the latter. The interfacial interaction was evaluated by semi-empirical equations developed previously. The deformation and toughening mechanisms of the composites were also investigated.     

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.     

Abrasive wear in filled elastomers

The abrasive wear of rubbers is strongly affected by the filler particles dispersed in the elastomer matrix. The fillers are incorporated usually for the purposes of mechanical reinforcement and improving the conductivity of the neat resins. It is found that the wear rates of the filled silicone rubbers increase slowly with filler concentration until a critical volume fraction,v _c, is reached, at which point they increase very rapidly with increasing filler concentration. This behaviour appeared to be universal in all the filled silicones we studied, regardless of the type of filler and silicone rubber used. However the magnitude of the critical filler fraction,v _c, can be changed significantly with the filler shape, resin cross-linking density and filler surface treatments. No reasonable relationship could be found between this wear behaviour and the mechanical properties measured in a macroscopic manner. Experimental evidence suggests that the incipient cracks that lead to wear losses may start within the thin layers of highly stressed material, the damage zones, surrounding the rigid particles. A simple model taking into account the stress concentration induced by the rigid fillers shows excellent correlation between the wear rate and the damage zones volume. With this new model, the observed wear behaviours can be explained satisfactorily.     

Influence of surface-modified TiO2 nanoparticles on fracture behavior of injection molded polypropylene

We prepared surface-modified TiO₂ nanoparticle (21 nm)/polypropylene nanocomposites using a twinscrew extruder and an injection molding machine. The TEM (transmission electron microscopy) and SEM (scanning electron microscopy) images showed homogeneous dispersion of nano-TiO₂ at 1 vol.% filler content and weak nanoparticle matrix interfacial adhesion. It was found that the essential work of fracture (EWF) approach, usually characterizing fracture toughness of ductile materials, was no longer applicable to the nanocomposite samples because of the extreme crack blunting and tearing processes observed in the EWF tests. As an alternative approach, the specific essential work-related yield was used for assessment of the plane-strain toughness, as suggested in the literature. The results indicated that the addition of 1 vol.% nano-TiO₂ did not toughen the polypropylene (PP) matrix at all. On the other hand, it was observed from the EWF tensile curves that the nanoparticles enhanced the ductility of the PP matrix greatly, the reason of which was probably ascribed to the high level of molecular orientation of the injection molded samples, as revealed by the polarized optical microscopy (POM). Because of the highly ductile behavior induced by the nanoparticles, the fracture energy achieved two-to three-fold increase, depending on the ligament lengths of the samples. The difference between the toughness and ductility of nanocomposites was discussed.     

Influence of the filler particle shape on the elastic moduli of PP/CaCO3 and PP/Mg(OH)2 composites

The effects of enhancing the interfacial adhesion between fillers and PP matrix on the Youngs and shear moduli were observed within wide concentration intervals of both filler and adhesive agent. Calcium carbonate and magnesium hydroxide were used as-received and/or with the surface hydrophobized by the long-chain fatty acids. A copolymer of polypropylene and maleic anhydride containing about 1 wt/wt% of grafted maleic anhydride was used for the interface modification. The analysis, based on the classical models, showed the necessity of their simple modification considering the increase in the amount of immobilized matrix with enhancing interfacial adhesion. Maleated polypropylene influenced both the matrix-filler interface and matrix bulk due to the interactions of carboxyl groups with basic centres on the filler surface or on the impurities in the matrix bulk. The greater extent of the immobilized matrix on the filler surface caused the creation of the particles hyperstructure at a lower Mg(OH)₂ content than in systems with zero adhesion.     

Investigation of thermal conductivity and dielectric properties of LDPE-matrix composites filled with hybrid filler of hollow glass microspheres and nitride particles

The hybrid filler of hollow glass microspheres (HGM) and nitride particles was filled into low-density polyethylene (LDPE) matrix via powder mixing and then hot pressing technology to obtain the composites with higher thermal conductivity as well as lower dielectric constant (D_k) and loss (D_f). The effects of surface modification of nitride particles and HGMs as well as volume ratio between them on the thermal conductivity and dielectric properties at 1 MHz of the composites were first investigated. The results indicate that the surface modification of the filler has a beneficial effect on thermal conductivity and dielectric properties of the composites due to the good interfacial adhesion between the filler and matrix. An optimal volume ratio of nitride particles to HGMs of 1:1 is determined on the basis of overall performance of the composites. The thermal conductivity as well as dielectric properties at 1 MHz and microwave frequency of the composites made from surface-modified fillers with the optimal nitride to HGM volume ratio were investigated as a function of the total volume fraction of hybrid filler. It is found that the thermal conductivity increases with filler volume fraction, and it is mainly related to the type of nitride particle other than HGM. The D_k values at 1 MHz and microwave frequency show an increasing trend with filler volume fraction and depend largely on the types of both nitride particles and HGMs. The D_f values at 1 MHz or quality factor (Q × f) at microwave frequency show an increasing or decreasing trend with filler volume fraction and also depend on the types of both nitride particle and HGM. Finally, optimal type of HGM and nitride particles as well as corresponding thermal conductivity and dielectric properties is obtained. SEM observations show that the hybrid filler particles are agglomerated around the LDPE matrix particles, and within the agglomerates the smaller-sized nitride particles in the hybrid filler can easily form thermally conductive networks to make the composites with high thermal conductivity. At the same time, the increase of the value D_k of the composites is restricted due to the presence of HGMs.     

Mechanical and thermal expansion properties of a participate filled polymer

The use of particulate filled polymer formulations over wide temperature ranges has resulted in a need to understand their mechanical behaviour better. In this investigation, a crosslinked epoxy-urethane polymer filled with Al₂O₃ particles was studied. Mechanical and thermal expansion properties were determined at ambient and liquid nitrogen temperatures and compared to theoretical predictions. Other parameters under consideration were volume fraction and adhesion between filler and matrix. The theoretical equations employed for predicting mechanical properties appear fairly reliable at ambient temperature but unreliable at liquid nitrogen temperatures. The degree of bonding between filler and matrix influences mechanical properties at ambient temperature but at liquid nitrogen temperature no difference in properties owing to matrix filler bonding was evident. This result is attributed to compressive stresses on the filler particles resulting from the lower thermal expansion of the filler.     

Compatibilizing effect of mesoporous fillers on the mechanical properties and morphology of polypropylene and polystyrene blend

Polypropylene(PP)/Polystyrene(PS) (PP/PS = 80/20) blend with different types of fillers were prepared by using melt method. Four different types of fillers, namely mesoporous MCM-41 (without template), nano-SiO₂, Polymethylmethacrylate (PMMA)/MCM-41 and PMMA/SiO₂ were considered. For PMMA/MCM-41 filler, the synthesis of the filler consisting of entrapped strand of PMMA within the pores of mesoporous MCM-41 (without template) was described. The mechanical properties of the blend determined as the nano-fillers contents and the different types of blend were found to vary with the different interface between fillers and the matrix. SEM revealed a good interaction between the matrix phases and PMMA/MCM-41 or MCM-41 (without template). The decreased T_g of PS implied that the good adhesion between PP and PS blend was obtained by adding PMMA/MCM-41 nano-filler.     

Soundproofing effect of polypropylene/inorganic particle composites

Two inorganic particle-filled polypropylene (PP) composites including PP/hollow glass bead (HGB) composite and PP/nanometer calcium carbonate (nano-CaCO₃) composite were prepared by means of a twin-screw extruder in the present paper. The transmission loss was measured, to identify the effects of sound frequency and the filler content on the sound insulation properties of these filled systems. The results showed that the sound insulation effect of the PP/nano-CaCO₃ obeyed roughly the law of mass, the transmission loss of the two composites increased nonlinearly with an increase of the filler volume fraction, and the value of the transmission loss for the PP/HGB system was higher than that of the PP/nano-CaCO₃ system under the same level of sound frequency. The transmission loss increased roughly with an increase of sound frequency for the two composites except to individual sound frequency. The mechanisms of the sound insulation of these composites were discussed.     

Percolation based enhancement in effective thermal conductivity of HDPE/LBSMO composites

Thermal conductivity of composites with electrically conducting La_0·7Ba_0·15Sr_0·15MnO₃ (LBSMO) filler of nanometric grain size in HDPE matrix is investigated. Volume fraction of LBSMO fillers was varied between 0 and 0·30. SEM photographs of the composites show the presence of clusters and percolative paths, particularly for composites prepared with higher filler volume fractions. The effective thermal conductivity of the composites displays significant enhancement with increasing filler content in HDPE. A maximum enhancement of ~65% compared to that for pure HDPE has been observed for composite with 0·30 volume fraction of LBSMO filler. Most of the models those are generally used to predict the properties of two phase mixtures, has been found either to under/overestimate the measured effective thermal conductivity of the composites. We confirm that the observed rapid increase in the effective thermal conductivity of HDPE/LBSMO composite over the studied range of filler volume fraction (viz. 0?C0·30), is predicted very well, considering the effect of percolation as proposed by Zhang et al (2009).     

Yielding and impact behaviour of pp/sgf/epr ternary composites with controlled morphology

Dependencies of the yield strength (_yc), yield strain (_yc) and Charpy notched impact strength (CNIS) of polypropylene (PP) reinforced with 30 wt% short glass fibres (SGF) and ethylene — propylene random copolymer (EPR) inclusions on EPR volume fraction (v _e) were investigated within the interval of v _e varying from 0–0.2. Only one limiting phase morphology has been attained reproducibly using a procedure based on chemical modification of PP. Adhesion enhancement between SGF and PP and complete separation between SGF and EPR was achieved by grafting PP with 2wt % maleic anhydride (MAH). Two regions existed on _yc versus v _e curves in the case of complete separation of the reinforcement and elastomer. The observed increase of _yc with increasing v _e within the interval 0<v _e < 0.05 was attributed to the change in the mode of fracture from brittle to quasi-ductile. Such an explanation has been supported by a several fold increase in _yc. Above v _e=0.05, a monotonic decrease of _yc with increasing v _e was observed corresponding well with an explanation based on a reduction of matrix effective cross-section. In this interval of v _e the concentration dependence of _yc was described quantitatively using existing composite models and satisfactory agreement between predictions and experimental data was obtained. The CNIS increased monotonically up to v _e=0.1 for both homo- and copolymer based composites. Above v _e=0.1, CNIS, measured at -20°C using 6 × 4 × 50 mm bars, notched accordingly ASTM D256 standard, increased for copolymer based composites while it remained constant for homopolymer based materials. Physical meaning of these data is, however, obscured by the inability to separate effects of v _e from those of specimen geometry using only a single standard impact strength data.     

Influence of filler particle shape on elastic moduli of PP/CaCO3 and PP/Mg(OH)2 composites

The effects of filler particle shape on the Young's and shear moduli of PP/CaCO₃ and PP/Mg(OH)₂ composites were studied in the concentration interval up to 50 vol/vol % filler. Calcium carbonate had irregular, approximately spherical particles and magnesium hydroxide had particles either in the form of hexagonal plates or micro-needles. The analysis based on the classical models together with structural observations enabled explanation of the composition dependences of elastic moduli of the blends studied. It was found that immobilization of PP matrix on the filler surface prominently influenced the values ofG′ andE′ moduli of PP/CaCO₃ and PP/Mg(OH)₂ composites. The presence of the strongly immobilized PP with increasing geometrical anisotropy of the filler particles enabled a hyperstructure creation in the composites PP/Mg(OH)₂.     

Skin-Inspired Thermosensitive Tactile Sensor Based on Thermally Conductive and Viscous Interface Composites for Rocks

Achieving high thermal conductivity and exceptional interfacial adhesion simultaneously in thermosensitive tactile recognition sensors poses a significant challenge. A copolymer, poly([[(butylamino)carbonyl]oxy]ethyl-ester)-co-polydimethylsiloxane (referred to as PP), is synthesized and subsequently complexed with alumina particles coated with liquid metal (LMAl₂O₃) to prepare a composite material called PP/LMAl₂O₃ with high thermal conductivity and strong interfacial adhesion to address this challenge. The best thermal conductivity (4.43 W m⁻¹ K⁻¹), electrical insulation (10⁻⁶–10⁻⁷ S m⁻¹), and adhesion properties derived from hydrogen bonding (1316 N m⁻²) are obtained by adjusting the volume fraction of PP and LMAl₂O₃ in PP/LMAl₂O₃. PP/LMAl₂O₃ with high thermal conductivity and high interface adhesion can efficiently transfer heat between thermal flux sensors and the objects being sensed, reliably detecting small thermal flux variations and ensuring accurate thermal flux measurements. In this study, PP/LMAl₂O₃ is used to make up thermosensitive tactile sensor. Surprisingly, PP/LMAl₂O₃ demonstrates high thermal signal sensitivity for tactile recognition applications, allowing the smart thermosensitive tactile sensor system to distinguish unknown rock materials even in the dark. Overall, PP/LMAl₂O₃ may function as a fundamental material in thermosensitive tactile sensors for lithology identification.     

Aggregation of CaCO3 particles in PP composites: Effect of surface coating

The occurrence and effect of aggregation in PP composites containing seven different precipitated CaCO₃ fillers coated with stearic acid are described in this study. The particle size and specific surface area of the filler varied in a relatively wide range, the latter changed between 2 and 12 m²/g. The fillers were characterized by various methods including surface area, particle size and bulk density. PP composites were prepared in an internal mixer in the composition range of 0–0.3 volume fraction filler content and their structure was studied by optical microscopy. The tensile and rheological properties of the composites were related to their structure. The results prove that the unambiguous detection of the presence of aggregation is difficult in particulate filled composites. The coating of CaCO₃ fillers with a surfactant changes the behavior of the particles considerably, but does not eliminate aggregation completely. The association of filler particles depends on the relative magnitude of adhesion and separating forces. Although coating decreases the surface free energy of the filler significantly, gravitational forces are much smaller than adhesion between either uncoated or coated fillers thus powder particles always aggregate. Different forces act in suspensions used for the determination of the particle size distribution of the filler; the shape of the distribution may indicate the presence of aggregation. Coated fillers form much looser aggregates with more diffuse interphases, than uncoated particles. Composite stiffness is completely insensitive to changes in structure or interaction, but the direct evaluation of other tensile properties may also lead to erroneous conclusions. Model calculations, oscillatory viscometry, as well as the proper representation of the results allow the unambiguous detection of aggregation.     

Effect of ultra fine particles on the elastic properties of oriented polypropylene composites

Fine spherical particles with various diameters (70, 160 and 40 nm, and 35, 65 and 125m) were mixed with isotactic polypropylene (PP). For the oriented composites having hexagonal symmetry produced by drawing, the elastic properties were determined by five compliances,S, or stiffness constants,C. Four of these, namely,S ₃₃,S ₁₁,S ₁₃ andS ₄₄ (orC ₃₃,C ₁₁,C ₁₃ andC ₄₄) were determined for the oriented composites filled with particles whose average diameters were 7 nm and 65m. For the composites filled with the smaller particles (7 nm), all the stiffness constants (C ₃₃,C ₁₁,C ₁₃ andC ₄₄) increased with the filler content, whereas for those with larger particles (65m), this relation was reversed. The Young's moduli of the oriented composites filled with relatively small particles (7, 16 and 40 nm) in each re-stretching direction increased with increasing filler content and with decreasing filler size, whereas those of the composites filled with larger filler (35, 65 and 125m) decreased with increasing filler content and size. It was concluded that the modulus of the oriented composite is determined by three factors, namely: (1) molecular orientation of matrix polymer; (2) the volume-fraction and size of filler; and (3) the fraction of void introduced by drawing. The moduli observed for the oriented composites are well explained by an equation derived on an assumption of the independence of the three effects. It was also concluded that extremely small fillers with particle sizes comparable to that of the crystalline region in PP matrix have a prominent reinforcing effect in the oriented polymer matrix.     

Predicting the effects of microstructural features on strain localization of a two-phase titanium alloy

Strain localization influenced by microstructural features has an important effect on mechanical properties of α + β titanium alloy. To address the effect, a microstructure-based finite element model is established. In this model, regions for primary α (α_p) and transformed β matrix (β_t) are generated from real microstructures of a two-phase titanium alloy (TA15 alloy); the plastic flow behaviors of these two features are determined directly rather than from single phase alloy. A constitutive equation of α_p is developed with consideration of dislocation–obstacle interaction, whereas the constitutive equation of β_t is determined by nanoindentation tests. Finally, the calculated stress–strain responses of the alloy are verified by experiments. The simulated results show that strain localization bands (SLBs) have two morphologies: short and long-continuous SLBs. Lots of short SLBs appear mainly in β_t and α_p when the volume fraction of α_p is small and moderate, respectively. Long-continuous SLBs appear mainly in α_p when the volume fraction of α_p is large. With the increase of α_p in SLBs, the strength of the alloy decreases while the ductility increases. By decreasing the disparity of strength between α_p and β_t, the strain gradient in SLBs reduces and the ductility of the alloy increases.