Role of the rubber particle and polybutadiene cis content on the toughness of high impact polystyrene

High impact polystyrene (HIPS) is a classical reactor polymer blend produced by in situ polymerization of styrene in solution with polybutadiene rubber. The importance of the particle size and rubber crosslink density on the particle cavitation capability and the controlling of toughening mechanisms in the styrenic matrix is well established in current literature. In the present work, the role of the rubber particle on the HIPS toughness has been investigated for two commercial grades with low and high cis polybutadiene. Transmission electron microscopy (TEM) was employed for observation of particle size distribution and digital imaging applied for quantitative analysis of the micrographs. Measurements of apparent volume fraction and average particle size were determined in TEM images for both grades, while the gel content and swelling index were employed to evaluate the effect of the polybutadiene cis isomer on the rubber crosslink density. Grade morphology and crosslink effects on mechanical properties were assessed by slow three-point bending and uniaxial tensile testing. The results illustrate that polybutadiene cis content in HIPS grades has strong influence on the mechanical properties, particularly affecting yielding and energy to failure. Accordingly, it was observed that HIPS grades with equivalent average particle size and apparent volume fractions present a much higher energy to failure and a lower yield stress with high cis content polybutadiene when compared to their lower cis polybutadiene counterparts.     

Nylon-6-rubber blends

The macroscopic cavitation and yield behaviour of nylon-6/rubber blends was studied. The type of rubber (poly(butadiene), ethylene propylene copolymer (EPDM) or polyethylene (LDPE), the rubber concentration and the rubber particle size was varied. The onset of cavitation was determined by measuring the intensity of the transmitted light from an incident laser beam. Both the yield stress and the cavitation stress appeared to increase with increasing strain rate and rubber modulus. No linear relation between the shear modulus and the cavitation stress was found. The data indicate that blends with a very small particle size have a relatively high cavitation stress. In all cases, a high cavitation stress of the elastomer resulted in a high yield stress of the blend.     

Modes of deformation in rubber-modified thermoplastics during tensile impact

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.     

Rubber toughening of plastics

Measurements of particle size distribution, particle volume fraction?/, Young's modulus, tensile and compressive yield stress and Charpy impact strength were made on a series of 14 high-impact polystyrene (HIPS) polymers of widely varying structure. In materials throughout the series containing 8.5 wt % polybutadiene, it was found that?/ varied between 0.17 and 0.44 as the mean particle size increased from 0.2 to 1.8 μm. Modulus and yield stresses depended principally upon particle volume fraction but the ratio of polybutadiene to polystyrene within the particles also appeared to have some influence upon properties. By contrast, variations in ? provided only a partial explanation for the observed differences in Charpy impact strength. It is concluded that impact strength is affected by rubber particle size to a much greater extent than properties measured at low strain rates.     

HIPS性能与橡胶粒子结构的关联及落锤冲击断裂的增韧机理

研究了橡胶粒子的结构对高抗冲聚苯乙烯(HIPS)性能及落锤冲击断裂的影响。结果表明,屈服强度随着橡胶体积分数的增加而线性递减;材料的落锤冲击韧性主要由分散相粒子的形态及粒子-界面间的粘接强度决定。透射电子显微镜(TEM)和扫描电镜(SEM)对落锤冲击断裂下的微观形变机理研究显示:(1)塑性形变或多重银纹是落锤冲击断裂过程中主要的能量耗散形式;(2)落锤冲击增韧机理可以通过橡胶粒子的形态设计及组成进行调控。另外,采用热重分析法(TGA)研究了HIPS中基质和凝胶的热稳定性。     

高抗冲聚苯乙烯的断裂韧性

综述了高抗冲聚苯乙烯断裂韧性方面工作的进展。评述了高聚物的脆性断裂－韧性断裂转变、分散相粒子大小及其分布、基材与分散相间的界面粘结等因素对高抗冲聚苯乙烯冲击韧性的影响。     

Sub critical crack advance vs. impact fracture on high impact polystyrene with different second phase volume contents and structures

Also for polymers, many fractures in service occurs after a period in which an existing crack has propagated in a sub-critical manner, while the laboratory tests are mainly concentrated on impact fractures. Aim of this paper is then to investigate the sub critical fracture in some high impact polystyrene (HIPS) materials with different second phase volume fraction and particle size and to compare it with the outcomes of impact Fracture Mechanics experiments. Large differences in the results of the two mechanical test procedures are evidenced: the materials behaviour is then examined from the structural point of view and an interesting case of interfacial failure, which disappears at high strain rate, is attested on some HIPSs by means of different techniques, i.e. electron microscopy, nuclear magnetic resonance spectroscopy and dynamic mechanical spectroscopy, indicating that the slow crack fracture behaviour can be influenced by parameters that do not affect ordinary mechanical tests.     

Computer modelling of rubber-toughened plastics

Coarse models of high-impact polystyrene (HIPS) have been created by computer simulation of the rubber particle spacing in the resin. Interparticle surface-surface distance parameters can be calculated from the models to help explain properties of real materials and predict the properties of hypothetical impact-polystyrene resins. Calculations of the geometric spacing of rubber particles in a group of hypothetical HIPS resins show that a narrow rubber-particle size distribution gives smaller interparticle distance and more reinforcing particles compared to broad distributions for a given average particle size.     

弹性体改性高抗冲聚苯乙烯的力学行为：Ⅰ.拉伸和动态力学性能

研究了高抗冲聚苯乙烯（ＨＩＰＳ）同苯乙烯－丁二烯－苯乙烯热塑性弹性体（ＳＢＳ）、丁苯橡胶（ＳＢＲ）、高顺式聚丁二烯橡胶（ＰＢＲ）共混物的拉伸性能及动态力学性质。ＳＢＳ改性的ＨＩＰＳ试样具有最大的断裂能，最大的断裂伸长率，较高的拉伸杨氏模量、屈服强度，屈服点过后有明显的应变硬化现象。ＳＢＲ改性的ＨＩＰＳ也呈现韧性断裂，拉伸过程中有明显的应力发白现象。ＲＢＲ改改ＨＩＰＳ试样表现为脆性断裂，力学性能变劣     

Dilatational bands in rubber-toughened polymers

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.     

Computer modelling of rubber-toughened plastics

A computer model has been developed which shows that high-impact polystyrene (HIPS) surface roughness (gloss) depends heavily on rubber phase volume and rubber particle size distribution parameters. The model has been developed in a series of steps. First, several tools have been created for isolation and display of rubber particles near the surface of computer-generated resins. Next, a technique for choosing surface points has been devised, using an algorithm which allows the surface to be disturbed by any particle near the surface. In step three, a non-linear fit of the surface points produces an abstract surface in the form of a grid. The variability in the array of grid points is a measure of surface roughness. The measured surface roughness of conventional high-impact polystyrene resins correlates to the variables identified by the model. A high percentage of the surface roughness variability has been explained in a correlation using average rubber particle size and rubber phase volume, showing the linear regression approach to be good for prediction of the surface roughness of conventional HIPS resins.     

Toughening mechanism for a rubber-toughened epoxy resin with rubber/matrix interfacial modification

For a rubber-toughened piperidine-DGEBA epoxy resin, the interface between the rubber particle and the epoxy resin matrix was modified by an epoxide end-capped carboxyl terminated butadiene and acrylonitrile random copolymer (CTBN). The end-capping epoxides used were a rigid diglycidyl ether of bisphenol-A (Epon 828), a short-chain flexible diglycidyl ether of propylene glycol (DER 736), and a long-chain flexible diglycidyl ether of propylene glycol (DER 732). The microstructures and the fracture behaviour of these rubber-modified epoxy resins were studied by transmission electron microscopy and scanning electron microscopy. Their thermal and mechanical properties were also investigated. In the rubber-modified epoxy resins, if the added CTBNs were end-capped by a flexible diglycidyl ether of propylene glycol (DER 732 or DER 736) before curing, the interfacial zone of the undeformed rubber particle, the degree of cavitation of the cavitated rubber particle on the fracture surface and the fracture energy of the toughened epoxy resin were all significantly increased. The toughening mechanism based on cavitation and localized shear yielding was considered and a mechanism for the interaction between cavitation and localized shear yielding that accounts for all the observed characteristics is proposed.     

Numerical and experimental studies on the fracture behavior of rubber-toughened epoxy in bulk specimen and laminated composites

To study the toughening mechanisms of liquid rubber (LR) and core-shell rubber (CSR) in bulk epoxy and composite laminate, experimental and numerical investigations were carried out on compact tension (CT) and double-cantilever-beam (DCB) specimens under mode-I loading. The matrix materials were pure epoxy (DGEBA), 15% LR (CTBN) and 15% CSR modified epoxies. Experimental results and numerical analyses showed that both liquid rubber (LR) and core-shell rubber (CSR) could improve significantly the fracture toughness of pure epoxy (DGEBA). However, the high toughness of these toughened epoxies could not be completely transferred to the interlaminar fracture toughness of the unidirectional carbon fibre reinforced laminate. The main toughening mechanism of CSR in bulk epoxy was the extensive particle cavitation, which greatly released the crack-tip triaxiality and promoted matrix shear plasticity. The poor toughness behavior of CSR in the carbon fibre laminate was thought to be caused by the high constraint imposed by the stiff fibre layers. No particle cavitation had been observed in LR modified epoxy and the main toughening mechanism was merely the large plastic deformation near the crack-tip due to the rubber domains in the matrix which results in a lower yield strength but a higher elongation-to-break.     

高压水射流破碎胎面胶的脆化效应与胶粉形成

采用霍布金森压杆试验模拟子午线轮胎胎面胶的破碎回收过程,分析高压水射流冲击下胎面胶材料受力及响应状态,试验表明材料存在韧脆转变现象,进而发生脆性断裂。橡胶断口与胶粉微观形貌表明,裂纹扩展区呈现典型的放射状脆性断面形貌,并形成大量与胶粉尺寸匹配的平整光滑区域,直接验证了脆性断裂的存在并阐述其发生过程。然后利用应力波传播判据和脆断力学分析解释了胎面胶材料出现脆化效应的原因。对材料韧脆转变的影响因素进行分析后可知,高压水射流冲击过程中,材料质点变形速度远大于韧脆转变临界速度,在力学性能上表现为断裂应力小于屈服应力,致使材料发生脆性断裂并形成精细胶粉。     

Fracture toughness and fracture mechanisms of PBT/PC/IM blend

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques were employed in the morphology and fracture mechanisms studies on a commercial polybutylene terephthalate/polycarbonate/impact modifier (PBT/PC/IM) blend. The fracture mechanisms involved at different temperatures under both impact and static loading were revealed. It was found that massive plastic deformation of the matrix material occurred after rubber particle cavitation; and it was this plastic deformation that was responsible for the drastic enhancement in fracture toughness although the widespread cavitation did absorb a considerable amount of energy as well. The major source of toughness was the same for both impact and static fracture tests, but the toughening processes became effective at a much lower temperature under static than impact conditions. The sequence of toughening events was also observed using TEM.     

A predictive model for particulate filled composite materials

Our predictive model for particulate filled composite materials is applied to epoxy resin toughened by rubber spheres. Good agreement is found between predicted values of stiffness and experimental measurements. The variability of yield stress with volume fraction is explained. The observed fracture processes, including rubber cavitation, are explored; the importance of thermal stresses is highlighted. The fracture behaviour of these materials is discussed in the light of these predictions.     

不同冲击断裂方式下橡胶粒子形态对HIPS脆韧转变的影响

核-壳型/蜂窝型高抗冲聚苯乙烯(HIPS)树脂在Izod冲击和落锤冲击断裂过程中的脆-韧转变行为不同。透射电镜(TEM)对亚断裂形变区的观察表明,蜂窝型HIPS对落锤冲击作用下的脆韧转变敏感,应力集中能有效引发基体的塑性形变,呈韧性断裂,但在v-notched Izod冲击断裂过程中,却呈脆性;而核-壳型HIPS的冲击断裂性能与之相反。将LDO假说引入冲击破坏研究并结合损伤竞争准数Da和Vincent图探讨了分散相特性及冲击方式对脆韧转变行为的影响。     

Fracture behaviour of unmodified and rubber-modified epoxies under hydrostatic pressure

The fracture toughness and uniaxial tensile yield strengths of unmodified and CTBN-rubber-modified epoxies were measured under hydrostatic pressure. The purpose of these experiments was to learn how suppressing cavitation in rubber particles affects the deformation mechanisms and the fracture toughness of rubber-modified epoxy. It was found that the cavitation of CTBN-rubber could be suppressed at a relatively low pressure (between 30 and 38 M Pa). With cavitation suppressed, the rubber particles are unable to induce massive shearyielding in the epoxy matrix, and the fracture toughness of the rubber-modified epoxy is no higher than that of the unmodified epoxy in the pressure range studied. Unmodified epoxy shows a brittle-to-ductile transition in fracture toughness test. The reason for this transition is the postponement of the cracking process by applied pressure.Work performed while on a sabbatical leave at the University of Michigan.     

The effect of dispersed paint particles on the mechanical properties of rubber toughened polypropylene composites

The influence of dispersed paint particles on the mechanical properties of rubber toughened PP was investigated. The matrix was basically a hybrid of PP, rubber and talc. Model systems with spherical glass bead filled matrix were also studied to examine the effect of filler shape and size. Properties like tensile strength, strain at break, impact strength, and fracture toughness were influenced by the dispersed inclusions. Tensile strength at yield decreased linearly according to Piggott and Leinder's equation. Strain at break decreased more drastically with paint particles than glass beads, revealing that irregularly shaped particles offered greater stress concentrations. The tensile strength and strain at break were less influenced by the size of paint particles whereas a slight decrease in the modulus values was observed with decreasing particle size. Impact strength and fracture toughness also decreased with increasing filler fraction. Lack of stress transfer between filler and matrix aided in reduction of impact strength. Decrease in fracture toughness was influenced by volume replacement and constraints posed by fillers. The size of paint particles had little effect on the impact strength and fracture properties at the filler concentration levels used in this investigation.     

Toughening of polystyrene by natural rubber-based composite particles: Part III Fracture mechanisms

Impact testing has allowed the toughness of PS blends to be correlated with the morphology of the dispersed rubber phase, which was a natural rubber (NR) in particle form, coated with a shell of polystyrene (PS) or polymethylmethacrylate (PMMA). PS subinclusions were also introduced into the NR core. The impact resistance of the prepared PS blends began to rise steeply at a particle content of about 18 wt %. Transmission electron microscopy (TEM) in combination with osmium tetroxide staining techniques, allowed direct analysis of the crazing and cavitation processes in the composite natural rubber particle-toughened PS blends. Bulk samples were studied at high and slow deformation speeds. Different deformation mechanisms were effective, depending on the location of the observed stress-whitened zone relative to the notch tip. The apparent fracture mechanisms in rubber-toughened PS blends were also studied by scanning electron microscopy. PS blends containing polydisperse natural rubber-based particles or monodisperse poly(n-butylacrylate)-based particles, and commercial high-impact polystyrene, were compared. This revised version was published online in November 2006 with corrections to the Cover Date.