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Deformation processes in high impact polystyrene as revealed by analysis of arrested cracks
Abstract
Fracture of rubber-toughened polystyrene (HIPS) has been studied via post-mortem analysis of specimens with arrested cracks formed under impact loading conditions. This methodology involves the study of the deformed region around an arrested crack tip by electron microscopy. It was applied to two commercial HIPS products and two experimental products with sub-micron rubber particles. For the HIPS materials, the arrested crack tip has a blunted shape with a width from 50 to 100μm depending of the material and its impact resistance. As expected, massive crazing was found in all these materials around the crack. In a narrow zone just ahead of the crack tip, where the crack would have continued to grow if not arrested, considerable elongation and deformation of the rubber particles was observed. This zone appears to be the precursor of the crack. In this region, “dilatation” and “shear” bands of more than 50μm long were observed. Higher magnification TEM observations show highly deformed rubber particles (“S” or “tear-drop” shape) along the shear bands of 1–6μm width. Three different kind of cavitation was observed just ahead of the crack tip: (1) small voids inside the membranes of the rubber particles; (2) large voids in the interface between rubber particles and the matrix and (3) interfacial cavities within the particles between the rubber membrane and the polystyrene occlusions. Within the narrow shear-bands there is evidence of shear yielding in the fracture of HIPS, similar to that observed in more ductile polymers.
... Conforme Figura 3b e Tabela 1, o PS apresenta o valor mais alto de módulo de elasticidade sob flexão. Isto pode ser atribuído à estrutura química do PS que não possui butadieno em sua cadeia, o qual confere flexibilidade ao polímero [17]. Observa-se no geral que a introdução de uma fase elastomérica na matriz rígida de PS, promove uma diminuição no valor do módulo de elasticidade independente se a blenda é compatibilizada ou não. ...
... The morphology of the disperse phase is of crucial importance, particularly for the impact resistance. In the case of HIPS, whose structure is similar to that of the materials considered in this paper, research has been carried out on the effect of many variables on their properties 4,5,6 . Other studies have concentrated on nylons 7 and unsaturated polyester (UP) resins 8 . ...
A thermoplastic polystyrene-based material with styrene-butadiene-styrene dispersed particles and a thermoset polyester toughened with butadiene-acrylonitrile particles were studied. Photomicrographs of materials that had different proportions of elastomer were processed and the images analysed. The changes in the morphological characteristics were correlated with the fracture toughness and modulus of elasticity.
... Since HIPS is in its nature composed of multicomponent and multi-phase polymeric materials with glassy and rubbery phases, end-use properties are dependent on many variables, such as the molecular weight and molecular weight distribution of PS polymerized and rubber employed, the composition and con-centration of rubber, the particle size and size distribution of rubber-dispersed phase, rubber-phase volume, the degree of graft and cross-linking and so on (Fischer and Hellman, 1996;Rios-Guerrero et al., 2000). It has been noticed that one of the main factors affecting the impact strength and toughness of HIPS is the rubberphase particle size and size distribution. ...
High impact polystyrene has been a typical thermoplastic to improve the impact strength and toughness of polystyrene. One of the main factors affecting these properties is the particle size and size distribution of the rubber phase. The rubber-phase particle size is related to the prepolymerization time that is determined by the phase inversion between polystyrene and rubber phases. However, the phase inversion in highly viscous oil-in-oil emulsions proceeds with a transitional fashion, not a catastrophic inversion. It has been tried to elucidate this inversion point by using viscosity, conversion and conductivity measurement techniques. They are used to find the point of phase inversion, indicative of the end of phase inversion, but they cannot produce its progression consecutively. In this study, a laser light scattering technique is utilized to characterize the evolution of the rubber particle size distribution depending on reaction time as well as to find the point of phase inversion. The progression of phase inversion during polymerization is monitored by this technique, which in turn is used to establish the proper prepolymerization time. Comparisons, limitations, and applications are dealt with.
... High-impact polystyrene (HIPS) is widely used in the audio, video, automotive, and aircraft industries, and in electric appliances, among others. Small rubber particles embedded in the polystyrene matrix provide high-impact resistance [1,2]. However, this polymer is highly flammable when exposed to fire, due to its chemical structure, where aliphatic groups bound to aromatic moieties contribute to the combustion reaction without forming any char layer that could prevent fire propagation [3,4]. ...
We analyze the effect of montmorillonite clay nanoparticles on the burning rate (BR), mechanical and rheological properties of high-impact polystyrene (HIPS) and blends of polyethylene terephthalate (PET) and HIPS produced by a twin-screw extrusion process. It is found that the BR of the HIPS-PET system is considerably reduced at relatively high clay concentrations. On the other hand, the increase in clay concentration in the HIPS matrix none leads to an unexpected rise in the BR. The degradation temperature is determined by measurements of complex viscosity as a function of temperature. It is found that degradation temperature decreases with clay concentration, particularly in the HIPS-PET-clay nanocomposite, diminishing by 28°C with respect to that of HIPS at high clay contents. This low degradation temperature means that a char layer is formed at lower temperatures, so the BR decreases as well. Mechanical properties are also affected, such as the fracture strain.
... 5 Several studies have been conducted to explore an effective/sensitive method for monitoring rubber particle cavitation in different polymeric blends. In addition to electron microscopy techniques, the limitations of which have been mentioned previously, 6 some other procedures that have been examined include measuring the intensity of the transmitted light from an incident laser beam (light scattering method), 7 detecting incipient rubber particle cavitation with dynamic mechanical tests, 8 and monitoring dimensional changes through the application of controlled thermal contraction/expansion cycles to specimens to provide a definite degrees of thermally induced cavitation. 4,8 Moreover, some attempts have been made to elucidate the effects of cavitation on the final mechanical properties. ...
Rubber particle cavitation has been the focus of many investigations because it dramatically affects the mechanical properties of polymeric blends. In this work, the effect of rubber particle cavitation on the mechanical behavior of high-impact polystyrene was studied. The extent of cavitation in rubber particles was varied via different thermal contraction/expansion cycles in the range of −100 to 23°C. Tensile, creep, and Charpy impact tests were conducted to evaluate the effects of the degree of cavitation on the general mechanical properties. The notch-tip damage zone and deformation micromechanisms were also investigated by a transmitted optical microscopy technique to reveal the effects of cavitation on toughness. The results of this investigation illustrate a close relationship between the degree of rubber particle cavitation and the mechanical performance of high-impact polystyrene. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1110–1117, 2007
... Because HIPS is composed of multicomponent and multiphase polymeric materials, with glassy and rubbery phases, end-use properties are dependent on many variables, such as the molecular weight (MW) and molecular weight distribution (MWD) of the polymerized PS and rubber used, the composition and concentration of rubber, the particle size and particle size distribution of the rubber-dispersed phase, the rubber-phase volume, the degree of grafting, and crosslinking, for example. [1][2][3][4] It has been noticed that one of the main factors affecting the impact strength and toughness of HIPS is the rubber-phase particle size and its size distribution. The rubber particle size distribution is interrelated with operating conditions, such as the geometry of the reactor and impeller, the type and amount of rubber and initiator, polymerization temperature, residence time distribution, prepolymerization time, agitation speed, and the concentration of chain transfer agent. ...
High-impact polystyrene (HIPS) was prepared by the bulk or low-solvent polymerization of styrene in the presence of dissolved rubber and characterized to measure the dispersed particle size of the rubber phase. Before preparation, the prepolymerization time was established by measuring the evolution of particle size distribution of the dispersed phase as a function of reaction time. The measurement technique by laser light scattering was found to be efficient enough not only to lead to the right prepolymerization time but also to predict rubber-phase particle size distribution. Polymerization experiments were then conducted to investigate the effect of solvent contents on the particle size distribution of the rubber phase, in which ethylbenzene was introduced as a solvent at levels of 0, 3, 10, and 15%. As the solvent content increased, the size of rubber-phase particles initially increased, reaching a maximum, and then decreased. It is speculated that a decrease in the molecular weight of the matrix, a decrease in the viscosity ratio between polybutadiene to polystyrene phases, and a change in rubber morphology all contributed to the change in the rubber particle size of HIPS. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3672–3679, 2003
... Because HIPS is composed of multicomponent and multiphase polymeric materials, with glassy and rubbery phases, end-use properties are dependent on many variables, such as the composition and concentration, particle size, and particle size distribution of the rubber. It has been noticed that one of the main factors affecting the impact strength and toughness of HIPS is the rubber-phase particle size and its size distribution [7][8][9][10][11]. ...
This study analyses the impact properties of high impact polystyrene (HIPS). HIPS is one of the well-known toughened polymers.
The high toughness is given by the rubbery phase. The impact fatigue behavior of HIPS was studied with a Ceast pendulum type
tester (Resil 25). The fracture mechanisms were examined with a scanning electron microscopy. The nature of crack initiation
and propagation was investigated for small impact angles and three different spans. The impact angles of charpy hammer were
chosen as 5°, 10°, 15°, 20°, and 25°. The fracture characteristics varied with the impact angle, the number of impacts, and
the distance between supports. The rate of crack propagation was high at higher impact angles with lower endurance, and low
at lower impact angles with higher endurance.
... An improvement in the toughness of glassy polymers can be achieved by dispersion of rubber particles into the polymer matrix123. According to the literature, rubber particles with diameters between 1 and 2 μm give the best results in the preparation of high-impact polystyrene (HIPS) [4, 5]. ...
Batch polymerization of isoprene was carried out at 25∘C in a normal microemulsion stabilized with sodium dodecyl sulfate and initiated with the redox couple tert-butyl hydroperoxide/tetraethylene-pentamine. Characterization by transmission electronic microscopy showed that polyisoprene nanoparticles with number-average diameter close to 20 nm were obtained. The low molecular weights obtained, as determined by gel permeation chromatography, were probably due to chain scission as inferred from the oxidative ambient at which polymerization was carried out. Microstructure calculated from infrared spectroscopy data indicates that the obtained polyisoprene contains around 80% total 1,4 units, which is in accordance with its glass transition temperature (-60.8∘C) determined by differential scanning calorimetry.
The structure property correlation of different commercial grades of high-impact polystyrene (HIPS) with varying polybutadiene rubber (PBR) content has been investigated. The average molecular weights of polystyrene, rubber content, gel content, swelling index, degree of grafting, morphology of rubber, and rubber particle size distribution were characterized and correlated with the mechanical properties of HIPS. The dynamic light scattering and transmission electron microscopy techniques were employed to characterize the loss of submicron tiny PBR gel particles. The obtained results reveal the fact that submicron tiny PBR gel particles that are lost during gel estimation by ultracentrifugation process lower the gel content and rubber efficiency, which significantly contribute to the overall impact performance evaluation of the final HIPS product.
Polymer blending has been identified as the most versatile and economical method to produce new multiphase polymeric materials that are able to satisfy the complex demands for performance. Over the past few decades the number of polymer blends has grown tremendously. In fact the design and development of these multiphase polymer blend materials are strongly dependent on two major parameters: the control of the interface and the control of the morphology. In general, the term morphology refers to the shape and organization on a scale above the atomic level (e.g., the arrangement of polymer molecules into amorphous or crystalline regions) and the manner in which they are organized into more complex units. On the other hand, morphology of a polymer blend indicates the size, shape, and spatial distribution of the component phases with respect to each other. Since it is well established that most of the properties — mechanical, optical, rheological, dielectrical, and barrier properties — of polymer blends are strongly influenced by the type and fineness of the phase structure, the study of the control of the morphology of polymer blends has emerged as an area of continuous interest to polymer material scientists in the last few decades (1–5).
The mechanical properties of different high impact polystyrene (HIPS) were determined by means of impact and tensile tests. Impact strength (IS), yield stress (σy) and Young’s modulus (E) were evaluated as a function of the rubber phase morphological features such as type and particle size (Dp), volume fraction (Φ) and interparticle distance (IPD). In order to evaluate the changes produced in the rubber phase, the initiator, chain transfer agent (CTA), and type and concentration of SB copolymer were varied during the synthesis of HIPS. Transmission electron microscopy was used to analyze the materials’ morphology and the data obtained reveal that the Dp and Φ increases mainly with an increase in the PB content and/or with the use of CTA and decreases with an increase in initiator concentration. In addition, E and yield stress diminishes when increasing the dimensions of the rubber phase (Dp and Φ), contrary to the IS. The IPD/Dp ratio also indicates the dependency of the mechanical properties with respect to the rubber phase features. An increase in IPD/Dp provokes an increases in E and σy, whereas IS diminishes.
A single particle finite element model has been used to analyse the effects of thermal stresses on the stress distributions near the interface between the rubber particle and the matrix in rubber modified polymers. The thermal stresses are due to the mismatch of thermal contraction between the rubber particle and the matrix during cooling. The study is to determine whether the thermal stresses are significant enough to affect the distribution of normal stress and von Mises stress at the particle/matrix interface. Results from the single particle model show that a temperature decrease of 60 K, i.e. from 100 to 40°C, can generate a circumferential compressive stress at the particle/matrix interface, which is of the same magnitude as the tensile stress required to cause failure in most of the glassy polymers. Although the effect on the circumferential normal stress is significant, its effect on the von Mises stress is very small. The results also show that when the cavitation occurs in the rubber particle, the thermal stress effect is drastically reduced. This study provides encouraging evidence for the importance of thermal stresses in determining the stress distributions in rubber modified polymers and suggests that thermal stresses should be considered in the deformation analysis of these materials.
In this work, the Al-MCM-41 catalytic pyrolysis of styrene–butadiene copolymers in a thermobalance has been studied. The behaviour of such copolymers and the corresponding to high impact polystyrene (HIPS), physical blend of PS and PB with a given grafting degree, has been compared, and the importance of the degree of contact between the catalyst and the different polymer domains has been pointed out. Different particle size copolymer particles have been mixed with the catalyst, and in addition, samples have been prepared by solving the copolymer and mixing it with the catalyst, thus assuring an intimate contact. Different decomposition steps which can be related to the degradation of the different domains of the copolymer (polystyrene (PS) and polybutadiene (PB)) have been observed, despite the decomposition processes of the PB and PS domains are not completely independent, showing certain interaction. The importance of to carefully controlling, defining and characterizing the experimental conditions of catalytic pyrolysis of PS–PB experiments in order to generalize or to extend the results obtained in such experiments is clearly demonstrated, and pseudokinetic models capable of reproducing the amount of material evolving trough each decomposition step have been suggested. The possibility of combining the two criteria: (1) the assignment of each decomposition step and (2) the application of a pseudokinetic model is suggested as a potential tool for the characterization of the composition of commercial copolymers or mixtures of PB and PS, once the adequate calibration runs have been performed.
In this work, the particular behaviour of a commercial high-impact polystyrene (HIPS) during the catalytic pyrolysis over Al-MCM-41 has been studied. The results obtained in a thermobalance showed differences in the number and/or the relative importance of the reaction steps involved in the pyrolysis, depending on the polymer particle size, which can be related to the differences in the nature of the polymeric phase being decomposed in each stage. Moreover, the relative importance of each step is very dependent on the particle size, revealing differences in the distribution of the different copolymer domains (i.e., styrene and butadiene domains) when the different particle size samples are mixed with the catalyst. The type of contact of the pure PS and PB polymers has also been studied revealing that, contrary to other results in literature, the catalyst may have an important effect both on the PS and PB pyrolysis. The results obtained showed that catalytic pyrolysis of these polymers could be a powerful tool for providing a fast and simple method for the characterization of copolymers of styrene and butadiene units.
While it is recognized that the heterogeneous particles in HIPS play the dual role of providing multiple sites for craze initiation in the polystyrene (PS) matrix and allow the stabilization of the crazing process through cavitation/fibrillation in the PB phase within the particle, the precise role of particle morphology is not well understood or quantified. This work probes the micromechanics of uniaxial tensile deformation and failure in rubber-toughened PS through axi-symmetric finite element representative volume element (RVE) models that can guide the development of blends of optimal toughness. The RVE models reveal the effect on craze morphology and toughness by various factors such as particle compliance, particle morphology, particle fibrillation and particle volume fraction. The principal result of our study is that fibrillation/cavitation of PB domains within the heterogeneous particle provides the basic key ingredient to account for the micro- and macro-mechanics of HIPS.
A model for dilatational yielding in rubber-toughened polymers, which was proposed in an earlier paper, is developed further. According to the model, deformation begins with cavitation of the rubber particles, and progresses through the growth of dilatational bands, which are cavitated planar yield zones combining inplane shear with extension normal to the band. The model is used to predict band angles and other characteristics of yielding in toughened plastics, including the conditions under which polymers yield without cavitating. Electron micrograph evidence for the formation of dilatation bands is presented, and the influence of these bands on deformation and fracture behaviour is discussed, using a novel method of presentation based on cavitation diagrams.
A theory is advanced to explain the effects of rubber particle cavitation upon the deformation and fracture of rubber-modified plastics. The criteria for cavitation in triaxially-stressed particles are first analysed using an energy-balance approach. It is shown that the volume strain in a rubber particle, its diameter and the shear modulus of the rubber are all important in determining whether void formation occurs. The effects of rubber particle cavitation on shear yielding are then discussed in the light of earlier theories of dilatational band formation in metals. A model proposed by Berg, and later developed by Gurson, is adapted to include the effects of mean stress on yielding and applied to toughened plastics. The model predicts the formation of cavitated shear bands (dilatational bands) at angles to the tensile axis that are determined by the current effective void content of the material. Band angles are calculated on the assumption that all of the rubber particles in a band undergo cavitation and the effective void content is equal to the particle volume fraction. The results are in satisfactory agreement with observations recorded in the literature on toughened plastics. The theory accounts for observed changes in the kinetics of tensile deformation in toughened nylon following cavitation and explains the effects of particle size and rubber modulus on the brittle-tough transition temperature.
We have conducted a study of toughening mechanisms in rubber modified polycarbonate systems in order to evaluate the sequence of deformation events which improve fracture toughness. We conclude that cavitation of the rubber particles occurs first, followed by massive shear yielding of the matrix material. The size and shape of the deformation zone created in front of the crack is governed by the mechanical properties of the rubber particles and the stress state at the crack tip. The importance of using a variety of analytical techniques to characterize deformation mechanism is also illustrated. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/28299/1/0000053.pdf
To explore the particle-size effect on craze plasticity and on the level of toughening of high-impact polystyrene (HIPS), a commercial grade HIPS (Mobile PS 4600) with a composite particle fraction of 0.217 and average particle size of 2.51 μm was used. The matrix fractions of the material were dissolved in toluene and the unaffected particles were harvested as a gel fraction which was levitated in fresh toluene as a dilute suspension. This dilute particle suspension was centrifuged to separate the particles into two non-overlapping small and large particle populations of average size 1.03 and 3.97 μm, respectively. With these separated particles, two new blends of HIPS-type material with narrow particle distributions were reconstituted using commercial grade Lustrex HH-104 PS, together with a reconstituted blend of HIPS made up of the original broad particle-size distribution to be used as a standard for comparison. All three blends had the same volume fraction as the original HIPS. Stress-strain experiments performed on the reconstituted blends showed that the mechanical properties and toughness levels of the reconstituted HIPS were nearly identical to the properties of the original HIPS. While the toughness of the reconstituted material with larger particles was roughly halved at the same flow stress level, the flow stress of the reconstituted blend with small particles had a craze flow stress 5% higher than that of the other two reconstituted blends, and a very severely reduced level of toughness. The behaviour of these blends was analysed with the aid of a theoretical model developed by Piorkowska et al. and furnished additional support for the correctness of the particle-size effect based on the principle of ‘stress-induced displacement misfit’ proposed by Argon et al. previously.
Multiple craze formation is well-known to be an essential part of the toughening mechanism induced by appropriate incorporation of rubber particles into glassy matrices. Details of the craze structure of stress-whitened high impact polystyrene (HIPS) have been examined using transmission and scanning electron microscopy. In addition to the usual OsO4 straining procedure for TEM, examination by SEM of surfaces created by fracturing at liquid nitrogen temperatures of previously crazed specimens proved useful. The results are discussed in the context of previous observations on craze structure and the relationship to formation of useful materials.
Blends of polystyrene containing concentric spherical shell particles produced from KRO-1 resin by means of a morphological transformation were disassembled to harvest the particles. These particles were in turn separated into two nearly nonoverlapping populations of average sizes of 0.32 and 2.3 μm and were subsequently reassembled by solvent blending into homo-PS to create two separate sets of heterogeneous polymers of large and small particles at particle weight fractions of 0.05 and 0.15, respectively. It was found that the craze flow stress increases systematically with decreasing particle volume fraction at constant particle size and that the blends with the small particles that are less potent craze initiators have substantially higher flow stresses than blends with the large particles.
The deformation process in rubber-modified polystyrene was studied by static SAXS on deformed tensile samples under loaded conditions. The typical scattering pattern of these systems was interpreted as originating from two components: crazing in the polystyrene matrix and a dominant cavitational component associated with the rubbery phase. We have used and checked a new method to separate and quantify the crazing component and to give an estimation of the noncrazing component. The contribution of matrix crazing to the plastic deformation is of minor importance. With an increasing content of rubber, the amount of crazing decreases strongly whereas the (impact) toughness increases. In both PS/SEBS blends and HIPS the main source of plastic energy absorption might be cavitation-induced microscopic shear yielding.
Cavitation mechanism related with crazing is investigated in high impact polystyrene (PS) with rubber particles of a rubbery core/glassy polymer shell. By transmission electron microscopic work using the annealing effect on craze, we verified that the rubber particles were cavitated after the crazing of PS matrix and that rubber components from rubbery core were sorbed into the crazes. And finite elemental analysis supported that the sorption to craze fibrils would plasticize the craze.
The deformation mechanisms during fracture of a Nylon 6/ABS blend compatibilized with an imidized acrylic polymer were compared to those of an uncompatibilized blend. A postmortem examination of deformed zones in samples loaded to failure in a double-notch four-point-bend geometry was made using transmission electron microscopy (TEM). For the compatibilized blend, cavitation of the rubber particles followed by massive shear yielding of the polyamide matrix was concluded to be the sequence of events leading to toughness; while, for the brittle uncompatibilized blend, the evidence indicated that a lack of adhesion at the Nylon-ABS interface prevented the rubber particles from cavitating and the subsequent plastic deformation of the polyamide matrix. © 1994 John Wiley & Sons, Inc.
The double-notch four-point-bend (DN-4PB) technique is known to produce fruitful information revealing the toughening mechanisms around the sub-critically propagated crack tip. In this study, correlations among the single-edge-notch three-point-bend (SEN-3PB), the single-edge-notch four-point-bend (SEN-4PB), and the DN-4PB toughness measurement techniques have been conducted using various modified-epoxy systems. The toughnesses of both the neat and rubber-toughened epoxy systems are found to be independent of the testing techniques used. Specifically, when the peak load is plotted against B. W1/2/Y (B: thickness, W : width, and Y: correction factor), the data obtained from SEN-3PB, SEN-4PB, and DN-4PB all fall on the same line. The slope of this line is defined as the stress intensity factor. These results imply that with a single DN-4PB test, it is possible to gain the information needed to describe both the toughening mechanisms and the fracture toughness value of relatively brittle (KIC ≤ 1.2 MPa. M1/2) polymeric materials.
Quantitative transmission electron microscopy and optical microscopy is used to study craze initiation and growth in thin films of high-impact polystyrene (HIPS). Dilution of the HIPS with unmodified polystyrene reduces the craze–craze interactions, permitting equilibrium growth and craze micromechanics to be studied. It is found that the equilibrium craze length depends on the size of the nucleating rubber particle, but not the internal structure; no short crazes less than a particle diameter are observed. The long crazes can be adequately modelled by the Dugdale model for crazes grown from crack tips. The effects of particle size and particle internal occlusion structure on craze nucleation have been separated. Craze nucleation is only slightly enhanced at highly occluded particles relative to craze nucleation at solid rubber particles of the same size. There is a strong size effect, however, which is independent of particle internal structure. Crazes are rarely nucleated from particles smaller than ∼1 μm in diameter, even though these make up about half the total number. These craze nucleation and growth effects may be understood in terms of two hypotheses for craze nucleation: (1) the initial elastic stress enhancement at the rubber particle must exceed the stress concentration at a static craze tip and (2) the region of this enhanced stress must extend at least three fibril spacings from the particle into the glassy matrix. Since the spatial extent of the stress enhancement scales with the particle diameter, the second hypothesis accounts in a natural way for the inability of small rubber particles to nucleate crazes.
Toughening mechanisms of a core-shell rubber-modified epoxy were investigated using various microscopic techniques. It was found that the crack tip damage zone of the rubber-modified epoxy appeared to consist of multiple craze-like damage and massive shear banding using optical microscopy. The craze-like damage was further analysed using transmission electron microscopy (TEM) and actually found to be a collection of line arrays of highly cavitated rubber particles. The matrix material around the cavitated particles appeared to have plastically deformed, while the material outside of the array was undeformed. The structure and physical nature of this highly localized dilatational process are substantially different from those of the commonly known craze. The sequence of events leading to the formation of these craze-like line arrays is discussed.
Large strains in rubber toughened polymers cause void formation and growth in the rubber particles and yielding in the matrix. Void formation usually precedes plasticity in the matrix around the particle and previous papers have proposed models for the relationship between rubber surface energy, volume strain energy and void growth. In this paper, it is shown that another volume criterion must also be satisfied arising from the fact that in all these models, no decohesion is allowed at the particle-matrix interface. A fracture mechanics approach, where linear and nonlinear elasticity are assumed for the matrix and the rubber particle, respectively, is used to define a void formation criterion depending on the rubber fracture surface energy. After formation, the stability of the void is examined, taking into account the volume conservation between matrix and particle and the stress due to surface tension when the void size is very small. A size effect is observed, indicating that voids cannot grow in small particles. The required value of fracture energy in a particle on a microscopic scale is discussed.
An energy-balance criterion for cavitation of rubber particles, which was proposed in an earlier paper [A. Lazzeri and C. B. Bucknall, J. Mater. Sci.
28 (1993) 6799], is developed by including a term for the energy stored in the matrix and released during expansion of the voids. The model relates the critical volume strain at cavitation to the radius of the rubber particle, and to the shear modulus, surface energy and failure strain of the rubber. The effects of temperature, strain rate and type of stress field upon cavitation behaviour and the resulting toughness of the two-phase polymer are discussed in terms of the model.
Mechanical damage was investigated in polymethylmethacrylate toughened with core-shell (hard core) rubber particles. During a tensile experiment, volume changes, light absorption, light scattering and a small strain elastic modulus were recorded. Light scattering was quantitatively related to the number of damaged particles and a fast partial unloading technique allowed determination of the non-elastic part of these changes in material properties. Experiments performed between 10–5 and 10–1s–1 and between 20 and 70 C showed time-temperature transitions. These appeared to be different for each property, and measurement of the activation energy for each parameter enabled microscopic damage mechanisms to be inferred. Three types of microstructural damage were observed: pure matrix plasticity at very low strain rates or high temperatures, rubber cavitation at correlated locations at medium strain rates and temperatures, and disordered cavitation, rubber tearing and matrix plasticity at high strain rates or low temperatures. The experimental mean stress triggering rubber cavitation was compared with the predicted value.
In situ scanning electron microscope crack propagation experiments have been performed on a number of polystyrene and high impact polystyrene blends so that dynamic observations can be made of the mechanisms of failure. Brittle fracture is observed in low rubber phase volume systems, whereas high rubber phase volume systems exhibit a ductile tearing mode of fracture. As the rubber phase volume is increased there is an increased density of crazes, which leads to a reduction in width of material between them. The subsequent failure of the crazes leaves bridging ligaments. Under increasing load these fail in a manner dependent on their thickness such that there is a brittle-ductile transition at a ligament thickness around 3m. We argue that this alteration in mechanism could be caused by either the loss of the triaxial stress state or the reduced probability of extrinsic flaws being found in the smaller ligaments, resulting in inhibition of crazing. The stress required for failure at the crack tip consequently increases from that for craze formation to the yield stress of the dense polymer. This in turn allows a larger crazed deformation zone (already increased due to the stress relief effects of crazing) to form, hence a further toughness increase.
Real-time small-angle X-ray scattering (RTSAXS) studies were performed on a series of rubber-modified thermoplastics. Scattering patterns were measured at successive time intervals as short as 1.8 ms and were analysed to determine the plastic strain due to crazing. Simultaneous measurements of the absorption of the primary beam by the sample allowed the total plastic strain to be computed. The plastic strain due to other deformation mechanisms, e.g. particle cavitation and macroscopic shear deformation was determined by the difference. Samples of commercial thicknesses can be studied at high rates of deformation without the inherent limitations of microscopy and its requirement of thin samples (i.e., plane strain constraint is maintained on sample morphology).
Contrary to the conclusions drawn from many previous dilatation-based studies, it has been demonstrated that the strain due to non-crazing mechanisms, such as rubber particle cavitation, and deformation of the glassy ligaments between rubber particles, occurs before that due to crazing mechanisms. Crazing accounts for at most only half of the total plastic strain in HIPS (high impact polystyrene) and ABS (rubber-modified styrene-acrylonitrile copolymer) materials. The proportion of strain attributable to crazing can be much less than half the total in thermoplastic systems with considerable shear yield during plastic deformation.
The predominant deformation mechanism in polycarbonate-ABS blends is shear in the PC (polycarbonate) with associated rubber gel particle cavitation in the ABS. This cavitation means that there appears to be a direct relationship between gel particle rubber content in the ABS and toughness of the blend. The mechanism is the same whether the tensile stress is in the direction parallel or perpendicular to the injection-moulded orientation, with simply less total strain being reached before fracture in the weaker perpendicular direction. Crazing, although the precursor to final fracture, occurs after the predominant mechanism and contributes only a few per cent to the total plastic deformation.
The fracture properties of blends of poly(butylene terephthalate), PBT, with acrylonitrile-butadiene-styrene materials, ABS, compatibilized by a methyl methacrylate–glycidyl methacrylate–ethyl acrylate terpolymer, MGE, have been characterized by Izod impact and single-edge notch, three-point bend (SEN3PB) type tests. The impact properties have been shown to be very sensitive to specimen thickness and mildly sensitive to notch sharpness. Blends containing 30 wt% ABS and molded into 3.18 mm samples are super tough in the absence of a compatibilizer; however, 6.35 mm specimens require higher ABS contents and compatibilization to achieve significant toughness. Low quantities of MGE (1 wt%) are required to produce super tough blends of 6.35 mm thickness; whereas, higher quantities of MGE result in a decrease in the impact strength. A dual mode of deformation during Izod impact testing has been observed for uncompatibilized blends molded into 6.35 mm samples where brittle failure occurs in the region of fracture initiation and ductile failure occurs in the region of crack termination. Similarly, a more brittle mode of failure occurs for SEN3PB samples with long ligament lengths and ductile failure for samples with short ligament lengths. The distance a crack can propagate and the size of the stress whitened zone created during Izod impact testing have been shown to be related to the impact properties determined by Izod and SEN3PB tests.
High impact polystyrene (HIPS) particles with sizes well below 1 μm have been demonstrated to interact with growing crazes giving rise to some local, non-catastrophic fibril texture damage, which increases with increasing particle size and volume fraction of the second phase. The influence of such damage on the sub-critical fracture behaviour of some core-shell HIPSs, which have been well characterized from the morphological and structural points of view, is investigated. A statistical craze fibril fracture model is adopted that considers a two-parameter Weibull distribution and a Paris law for the sub-critical crack advance is derived. The meaning and dependence of the Weibull parameters on the structural and morphological characteristics of the considered materials are discussed. The proposed approach is not limited to the statistical idea and can, in principle, be used to derive the Paris behaviour in a larger class of materials.
The criteria for internal cavitation of rubber particles have been evaluated. It is shown that internal rubber cavitation can be considered as an energy balance between the strain energy relieved by cavitation and the surface energy associated with the generation of a new surface. The model predicts that there exists a critical particle size for cavitation. Very small particles (100–200 nm) are not able to cavitate. This critical-particle-size concept explains the decrease in toughening efficiency in different rubber-modified systems involving very small particles.
A relationship between the macroscopic toughness, the intrinsic network (entanglement and/or crosslink) density and the relative thickness of polymeric systems, is presented. Toughness of amorphous, glassy polymers is mainly determined by the strain to break, since the yield stress generally only varies between 50 and 80 MPa. It was found that the strain to break strongly depends on the absolute thickness of the specimen or, equivalently, the local thickness within the (micro) structure of the material. Only below a certain critical thickness can the intrinsic strain at break of a polymer be reached. The absolute value of this critical thickness and the intrinsic strain at break of a polymer are both determined by the network density. In this paper polystyrene (PS), a polymer that is generally considered to be very brittle, was investigated with respect to the influence of absolute thickness on its strain to break. For thin isotropic tapes of PS it was demonstrated that this critical thickness is below 1 μm. Based on experiments with macroscopically ‘thick’ PS samples (3 mm), which are made locally thin by the introduction of small, non-adhering rubbery particles (‘holes’), we could identify that the critical thickness is 0.05 μm for PS.
This paper presents a new model for the impact fracture characterization of ductile polymers. The model takes into account the energy dissipated in plastic deformation during the crack propagation period. Two material parameters are used in the model: the fracture energy at initiation and an equivalent tearing modulus which represents the variation of the fracture energy as the crack grows. The method was used to measure the impact fracture toughness of a toughened nylon 66 and of a polycarbonate/polyethylene blend. Measurements were made in impact as well as at a low loading rate. The material fracture toughness was also measured by the J-integral method in nonlinear fracture mechanics at a low loading rate. The fracture energy at initiation determined from the proposed model is in good agreement with that of the J-integral method. Inconsistency and the negative intercept which result from the conventional method of analysis are also discussed in terms of the two material parameters.
High speed fracture behaviour of nylon 6/SEBS-g-MA blends with rather small rubber particles near the lower limit for rubber toughening is characterized by the standard Izod impact test and the Vu-Khanh methodology. This characterization expands on previous reports that have examined the standard Izod impact strength of nylon 6 blends with various maleic anhydride grafted styrene-(ethylene-co-butylene)-styrene (SEBS-g-MA) materials including the ductile-brittle transition behaviour that occurs when the rubber particle size and the test temperature are varied. Load-deflection curves and impact strength of blends with different rubber particle sizes, using both thick and thin specimens, are also reported. Load-deflection curves of tough blends do not show significant differences after normalization by specimen thickness. Morphological features near crack tips formed at high speed were examined by microscopy to gain insight about the sequence of events that occur during crack propagation. All blends examined here show four different regions, i.e. an extensive shear yield zone, a shear yield zone, a cavitation zone and an apparent non-deformed zone within a visible whitened zone. Vu-Khanh parameters, fracture energy at initiation and tearing modulus, show a strong relation to the average rubber particle size and the deformed zone size.