Tensile properties of TiO2-filled poly(vinyl acetate) in the transition region

The stress–strain properties of TiO₂-filled poly(vinyl acetate) have been studied at filler percentages of 0, 10, 20, 30, and 40% TiO₂ over a strain-rate range of 100–5000%/ min at 24°C. Tensile strength, Young's modulus, and offset yield strengths all were found to increase with higher strain rates and higher TiO₂ contents. Ultimate elongations decreased with greater TiO₂ content and higher strain rates. Shift factors for volume fraction of filler were estimated for tensile properties as function of test rate. Stress relaxation studies have shown a reduction in relaxation times with increasing TiO₂ content. Calculations of the out-of-phase Young's modulus were made as a function of filler content employing a box-type of distribution of relaxation times. A possible explanation for the stress–strain behavior observed is that introduction of TiO₂ changes the internal viscosity of the system, similar to the effect of temperature. This would also mean that the ultimate properties would be dependent on filler content and strain rate because viscous resistance to chain deformation would be altered. The effect of filler on stress relaxation could be thought of being due to an increase in short-range chain motion.     

Poly(phenylene oxide) composites containing crosslinked polystyrene microspheres. I. Stress–strain properties

The stress–strain properties of poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene composites containing crosslinked polystyrene microspheres have been measured at strain rates of 0.167, 1.67, and 16.7 min^?1. It is found that Young's modulus almost has no increase with the filler content. The elongation at break and tensile strength decrease with the volume fraction of the filler, but both tend to flatten out at the volume fraction ν_f > 0.25 at the strain rate of 1.67 min^?1. The two ultimate tensile properties also have maximum values in the relationship with strain rate at the same filler concentration and strain rate conditions. Considering that elongation can be brought about by both matrix and filler, the well-known equation of elongation at break becomes     

Mechanical properties and volume dilatation of HDPE/CaCO3 blends with and without impact modifier

Different blends of high‐density polyethylene (HDPE) with calcium carbonate (CaCO₃) were mechanically tested under uniaxial tension with or without poly(ethylene‐co‐octene) elastomer grafted with maleic anhydride (POEg), as an impact modifier. In some materials, the surface of the CaCO₃ was treated with an amino acid and in others the mineral particles were left untreated. The stress–strain behavior were determined at constant true strain rate by using the VidéoTraction^© system. Also, the volume changes upon stretching was assessed by means of the video extensometer and correlated with X‐ray densitometry measurements. The dependence of modulus, yield stress, and cavitation is shown to depend on the relative percentage of the three constituents. In particular, the cavitation rate increases markedly with the CaCO₃ content and decreases with the POEg content. By contrast, the surface pretreatment of the CaCO₃ particles appear to be of much lesser importance. POLYM. ENG. SCI., 46:1512–1522, 2006. © 2006 Society of Plastics Engineers     

Rheological properties of PDMS filled with CaCo3: The effect of filler particle size and concentration

The effects of filler particle size and concentration on the rheological properties of hydroxyl terminated polydimethylsiloxane (HO‐PDMS) filled with calcium carbonate (CaCO₃) were investigated by an advanced rheometric expansion system (ARES). The Casson model was used to describe the relationship between shear stress and shear rate for steady‐state measurement. Micron‐CaCO₃ could not afford the CaCO₃/HO‐PDMS suspensions obvious shear thinning behavior and a yield stress high enough, whereas nano‐CaCO₃ could provide the suspensions with remarkable shear thinning behavior and high yield stress. Incorporation of nano‐CaCO₃ into HO‐PDMS resulted in the transformation of HO‐PDMS from a mainly viscous material to a mainly elastic material. With increasing nano‐CaCO₃ content, shear thinning behavior of nano‐CaCO₃/HO‐PDMS suspensions became more obvious. Remarkable yield stress was observed in nano‐CaCO₃/HO‐PDMS suspensions with high filler content, and increased with increasing nano‐CaCO₃ content. The degree of thixotropy was quantitatively determined using a thixotropic loop method. It was found that nano‐CaCO₃ favored more the buildup of filler network structure in the suspensions than micron‐CaCO₃ at the same weight fraction. Furthermore, increasing nano‐CaCO₃ content accelerated the establishment of filler network structure in the nano‐CaCO₃/HO‐PDMS suspensions. An overshoot phenomenon was observed in the nano‐CaCO₃/HO‐PDMS suspensions at high shear rates. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3395–3401, 2006     

Composites of high density polyethylene and different grades of calcium carbonate: Mechanical,rheological, thermal,and morphological properties

Calcium carbonate (CaCO₃)/high density polyethylene (HDPE) composites were prepared in a HAAKE twin screw extruder, using the experimental conditions defined by the factorial experimental design presented in a prior study. In this study, the effect of different grades (Ca₁ and Ca₂) and CaCO₃ content (varying from 0 to 15 wt %) on the mechanical, rheological, thermal, and morphological properties was evaluated. The results showed that the addition of the filler provoked a decrease on the impact strength, stress at break, and yield stress properties in relation to the pure HDPE. A consequent increase on the modulus of elasticity, indicating an increase on the rigidity of the composite, was observed. It was also verified a tendency to increase the toughness and the viscosity of the composites as CaCO₃ was added. Scanning electron micrographs showed that as the filler was incorporated to HDPE matrix, CaCO₃ particles tended to agglomerate, especially that grade constituted of particles of smaller size. The thermal analysis showed that the addition of mineral filler caused a decrease on the crystallinity degree. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2559–2564, 2006     

Rate‐Dependent function in the correlation between hardness and yield stress of polyethylene composites

A study of the validity of a correlation between compressive yield stress and Vickers hardness of CaCO₃/high density polyethylene composites was carried out. Composites with calcium carbonate in the range of 0–0.40 were produced by twin screw extrusion followed by compression molding. Hardness was implemented by indenting the samples with Vickers indenter and compression test were carried out at various crosshead speeds in the range of 0.2–50 mm min⁻¹. It was found that increasing calcium carbonate content increased both hardness and yield stress. In addition, compressive yield stress increased with increasing crosshead speed. The correlation between hardness and yield stress was observed to be independent on filler content and valid when the deformation rate employed in both tests were within the similar ranges which was in the order of 10⁻¹ mm min⁻¹. Tabor's equation was modified to take into account for the rate‐dependent behavior of composites and was found to be applicable and yield relatively accurate results.     

The yield strength of particulate reinforced thermoplastic composites

The effects of the filler volume fraction and strength of adhesion on the mode of tensile failure of a particulate reinforced polypropylene (PP) are investigated using finite element simulation (FES). When there is perfect adhesion between constituents, an upper bound for tensile yield strength is found to be 1.33 times the matrix yield strength above a critical volume of particulate concentration. Utilizing Sjoerdsma's model for interacting stress concentration fields, one can determine the concentration dependence of the yield strength below the critical filler volume fraction. When there is no adhesion between constituents, a modified version of an equation by Nicolais and Narkis adequately describes a lower bound for the tensile yield strength. The particulate concentration and the matrix ductility are the prime factors in controlling the brittle failure of the composite. Upper and lower bounds for brittle failure strength are characterized using a strength-of-materials approach and stress concentration factors for both “perfect” and “zero” adhesion. The properties of calcium carbonate filled PP homopolymer were measured over a wide range of filler volume fractions. CaCO₃ was either treated with stearic acid to prevent adhesion between constituents or used as received. Maleic anhydride grafted PP (MPP) was used to promote adhesion. For filler volume fractions below 0.2, yielding of the composite by combined microcavitation and shear deformation was the principal failure mechanism. At v_f above 0.35, a brittle failure mechanism dominated. In the range between 0.2 and 0.35, both failure modes were observed in the populations tested. Good agreement was found between the experimental results and the proposed model.     

Influence of Filler Size on Impact Properties of PP/Elastomer/Filler Ternary Composites

Influence of filler size on impact properties for polypropylene (PP)/elastomer/filler ternary composites was investigated. Calcium carbonate (CaCO₃) particles with a diameter in the range from 120 to 1200 nm were used as a filler and polystyrene-block-poly(ethylene-butene)-block-polystyrene triblock copolymer (SEBS) was used as an elastomer. In the PP/SEBS/CaCO₃ ternary composite, CaCO₃ particles and SEBS particles were dispersed in the PP matrix separately. In the case that SEBS elastomer volume fraction was below 0.12, the impact strength improved gradually with a decrease of CaCO₃ mean diameter from 1200 to 160 nm. In the case that SEBS volume fraction was above 0.17, the impact strength improved significantly by the incorporation of CaCO₃ particles with a mean diameter in the range from 120 to 900 nm. However, the impact strength hardly improved by the incorporation of CaCO₃ particles with a mean diameter of 1200 nm.     

Effect of adhesion on the fracture toughness of calcium carbonate–filled polypropylene

The effect of adhesion on the strain energy release rate (G_c) and Charpy notched impact strength (NIS) of calcium carbonate (CaCO₃)-filled polypropylene (PP) at room temperature is investigated over a wide interval of particulate filler volume fractions. The concentration dependence of G_c and NIS are discussed in terms of competition between the effects of increasing stiffness, decreasing effective matrix cross section, and the transition from a plane strain to a plane stress mode of failure. In all cases the plane stress and plane strain limits of the critical strain energy release rate for initiation of cracks were not affected by the presence of the filler and are the same as those for neat matrix. In the case of no adhesion between components, the size of the crack tip plastic zone increases with increasing filler volume fraction (v_f) because of the reduction of the material yield strength. In the region 0 < v_f < 0.12, there is a mixed mode of failure, and the measured value of G_c for crack initiation increases steadily as the sample cross section approaches a fully plane stress state. The reduction in yield strength also results in the increase in G_c for crack propagation as reflected by an increase in NIS. Above v_f= 0.12, the specimen cross section is in a fully plane stress state, and further increase in filler volume fraction (decrease in matrix effective cross section) has the net effect of reducing both G_c and NIS. In the case of “perfect” adhesion, the yield strength increases only slightly with v_f. In the region 0 < yr < 0.05 there is also a mixed mode of failure, but the increase in G_c is much less than that for the no-adhesion case since the size of the plastic zone in front of the crack is much smaller. Above v_f= 0.05, the combined effects of increasing stiffness, reduction of the size of the plastic zone, and decreasing matrix cross section dominate the behavior, causing a steady reduction in both G_c and NIS. Good agreement was found between experimental data and calculations based on fracture mechanics principles.     

Binary and ternary particulated composites: UHMWPE/CACO3/HDPE

A study of the influence of employing ultrahigh molecular weight polyethylene (UHMWPE) on the toughness of CaCO₃/high‐density polyethylene (HDPE) composites was carried out. Binary and ternary HDPE‐based composites with calcium carbonate in the range of 0–40% and UHMWPE in the range of 0–50% were produced by twin‐screw extrusion followed by compression molding. From tensile and impact tests, it was found that increasing calcium carbonate content increased tensile modulus, but decreased tensile strength, strain at break, and impact resistance. The addition of UHMWPE helped to increase the strain at break and impact resistance of composites moderately without decreasing modulus or strength. The degree of toughening was found to increase with increasing UHMWPE content, but to decrease as the filler volume fraction was increased. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1503–1513, 2000     

Crystallization of i-PP/CaCO3 Composites and its Correlation with Tensile Properties

Abstract

Investigation of crystallization behaviour of isoatactic polypropylene (i-PP) in i-PP/CaCO₃ composites is carried out through Differential Scanning Calorimetry (DSC) and wide angle X-ray diffraction measurements. The effect of CaCO₃ and its surface treatment with titanate coupling agent on nucleation and growth rate of crystallization, crystallite size distribution and crystallinity, is determined from exothermic crystallization peaks of the composites. The filler concentration dependence of crystallinity determined by both the techniques shows good qualitative agreement. Tensile properties viz. tensile modulus, yield stress and elongation were also measured as functions of filler concentration for both untreated and treated CaCO₃ filled composites. Crystallinity, tensile strength and elongation decreased with increasing filler content in both the cases whereas tensile modulus increased. Crystallization parameters have been correlated with the tensile properties of i-PP/CaCO₃ composites.     

Time‐resolved small‐angle X‐ray scattering study of void fraction evolution in high‐density polyethylene during stress unloading and strain recovery

By means of time‐resolved small‐angle X‐ray scattering, we developed an analysis methodology to assess the void volume fraction ?_v in high‐density polyethylene (HDPE) during tensile testing. The specimens were first drawn up to different imposed strains, and subsequently were subjected to stress unloading and strain recovery stages. During the loading stage, ?_v progressively increased with the strain level, starting from a well‐defined onset strain prior to the yield point. In particular, ?_v reached a maximum of 8.75 vol% for a strain of 12.5% in the case of a HDPE grade with a molecular weight of 105 000 g mol^?1. Stress unloading and strain recovery caused a decrease in ?_v attained at the end of the loading stage. For a HDPE grade with a molecular weight of 55 000 g mol^?1, ?_v was more important during the loading stage and the decrease in ?_v was less marked during the stress unloading stage when compared to the HDPE with molecular weight of 105 000 g mol^?1. The residual and reversible components of void volume fraction were revealed. © 2015 Society of Chemical Industry     

The fracture toughness of natural fibre- and glass fibre-reinforced SMC

The fracture toughness properties, in terms of stress intensity factor K_Ic and strain energy release rate G_Ic, of hemp fibre mat-reinforced sheet moulding compound (H-SMC) are measured using the compact tension (CT) method and compared with those of glass fibre-reinforced SMC (G-SMC). Three material parameters were considered for composite optimisation: fibre volume fraction, CaCO₃ filler content and hemp fibre surface treatments using either alkaline, silane or a combination of these two treatments. The highest fracture toughness for H-SMC composites was obtained at a fibre loading of around 30?vol.-%, while it was also shown that the fracture toughness properties of H-SMC are sensitive to mineral filler content. Surface treatment of the hemp fibres using a combined alkaline-silane treatment resulted in a significant improvement in fracture toughness of H-SMC composites. Optimised H-SMC composites exhibited fracture toughness properties similar to those of G-SMC at fibre contents of 20?vol.-%, with K_Ic values of around 6?MPa.m^?1/2.     

Comparison of mechanical properties of PP/SEBS blends at intermediate and high strain rates with SiO2 nanoparticles vs. CaCO3 fillers

The present article focuses on the effect of two types of inorganic fillers (SiO₂ and CaCO₃) on the mechanical properties of PP/SEBS blend. The nominal particle diameters of SiO₂ and CaCO₃ are 7 nm and 1 μm, respectively. The studied blend ratios were PP/SEBS/SiO₂ (CaCO₃) = 75/22/3 and 73/21/6 vol %. The morphology of polymer blends was observed and the distributions of the SEBS, SiO₂, and CaCO₃ particles were analyzed by transmission electron microscopy (TEM). Tensile tests were conducted at nominal strain rates from 3 × 10^?1 to 10² s^?1. The apparent elastic modulus has the local strain‐rate dependency caused by SiO₂ nanoparticles around SEBS particles in the blend of PP/SEBS/SiO₂. The yield stress has weak dependency of morphology. The absorbed strain energy has strong dependency of the location of SiO₂ nanoparticle or CaCO₃ fillers and SEBS particle in the morphology. It is considered that such morphology, in which inorganic nanoparticles are located around SEBS particles, can prevent the brittle fracture while the increased local strain rate can enhance the apparent elastic modulus of the blend at the high strain rate. On the basis of the results of this study, the location and size of inorganic nanoparticles are the most important parameters to increase the elastic modulus without decreasing the material ductility of the blend at both low and high strain rates. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Ductility of filled polymers

The ductility of a calcium carbonate-filled amorphous copolyester PETG in a uniaxial tensite test was examined as a fiction other filler volume fraction. A ductile-to-quasibrittle transition occurred as the volume fraction of filler increased. This transition was from propragation of a stable neck through the entire gauge length of the specimen to fracture in the neck without propagation. The draw stress (lower yield stress) did not depend on the filler content and was equal to the draw stress of the unfilled polymer. It was therefore possible to use a simply model to predict the dependence of the fracture strain on the filler volume fraction. It was proposed that when the fracture strain decreases to the draw strain of the polymer the fracture mechanism changes and the fracture strain drops sharply. The critical filler content at which the fracture mode changes is determined primarily by the degree of strain-hardening of the polymer. © 1994 John Wiley & Sons, Inc.     

Mechanical properties of i-PP/CaCO3 composites

Tensile and impact behavior of CaCO₃-filled polypropylene was studied in the composition range 0–60 wt % filler. Tensile modulus increased while tensile strength and breaking elongation decreased with increase in CaCO₃ content. The modulus increase and elongation decrease were attributed to increased filler–polymer interaction resulting in reduction in molecular mobility, while increased amorphization and obstruction to stress transfer accounted for the tensile strength decrease. Analysis of tensile strength data showed introduction of stress concentration in the composites. Izod impact strength at first increased up to a critical CaCO₃ content, beyond which the value decreased. Surface treatment of CaCO₃ with a titanate coupling agent LICA 12 enhances the adhesion of the filler and polymer, which further modifies the strength properties. Scanning electron microscopic studies indicated better dispersion of CaCO₃ particles upon surface treatment, which effected the changes in the strength properties of the composites.     

Influence of excessive filler coating on the tensile properties of LDPE-calcium carbonate composites

Calcium carbonate fillers are usually coated with stearic acid to reduce their surface energy and improve their dispersion in polymers. Commercial products are often over-coated and contain an excess of surfactant. It was found that stearic acid linearly increases the modulus and yield stress of LDPE but reduces its tensile strength, yield strain, and ultimate elongation. The influence of surfactant excess on the tensile properties of low-density polyethylene (LDPE)-CaCO₃ composites was investigated. Compounds of LDPE and optimally coated filler or with excess surfactant were prepared and their properties compared. CaCO₃ increased the stiffness and yield stress of the polymer but reduced all its other tensile properties. Over-coating the filler did not lead to linear accumulation of the effects of filler and stearic acid on the polymer matrix. In fact, surfactant excess amplifies the reinforcing effect on the stiffness but reduces all other mechanical properties of the composite. Calcium stearate, which is sometimes used as acid scavenger, lubricant or processing aid, has the same effect on the polymer properties as stearic acid, but to a smaller extent. It is concluded that it is most advantageous to coat the filler with the optimal amount of surfactant necessary to cover its surface with an organic monolayer unless the influence of excessive coating is required for a certain application. Care must also be taken in interpreting some of the published results, where the quality of the filler coating was not investigated.     

Mechanical behavior of highly filled natural CaCO3 composites: Effect of particle size distribution and interface interactions

The mechanical behavior of poly(di‐methyl siloxane) (PDMS) composites containing high volume fractions of natural CaCO₃ particles of various particle size distributions was studied under tensile and oscillatory bending stresses, emphasizing the unique behavior of high filler loaded compositions. Composites containing the maximal possible solid loading of raw CaCO₃ were investigated for the effect of fatty acids surface treatment. The elastic modulus increased with increasing filler loading, following Chantler's model for dental composites when correlated with the absolute filler volume fraction. Good fit to “traditional” models, e.g., Frankle‐Acrivos and Halpin‐Tsai, was obtained by correlating the modulus values with the volume fraction relative to the maximal possible filler loading. A master curve of different particle size distributions and filler levels composites was obtained by using the relative volume fraction values, illustrating the effect of particle packing characteristics on small deformation mechanical behavior. A minor increase in T_g was found in parallel to the appearance of a T_m relaxation peak at approximately −40°C. A peak temperature shift at T_m and a pronounced increase in this peak with increasing filler fraction was found as well. The changes in the melting transition are attributed to the constraints of the filler particles acting on the crosslinked melting polymer. Surface treatment with fatty acids significantly degraded the tensile properties. Interestingly, an increase of 4 vol% filler was enabled owing to the surface treatment, while restoring reasonable tensile properties. No significant effect was observed for excess of fatty acids resulting from physically adsorbed acids. Tan δ curves reveal low PDMS‐CaCO₃ particles interactions, and mobility of the PDMS chains in the increased filler fraction as in the treated 64 vol% composite, both higher than those in the raw composite. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Tensile yield in phenolphthalein polyether ketone

Uniaxial tension tests to the yield point were performed on phenolphthalein polyether ketone (PEK-C) from room temperature to near the glass transition temperature (T_g) at a constant rate of 0.02 min^?1. At room temperature, some measurements were also made at strain rates from 0.002 min^?1 to 2 min^?1. Yield stress was a linear function of temperature and log strain rate. The temperature and the strain rate dependence of yield stress could be modeled using Eyring theory. Yield energy was found to be a linear function of temperature. Young's modulus, yield strain, elastic strain, and plastic strain all decreased with temperature. © 1994 John Wiley & Sons, Inc.