Fracture mechanism of toughened epoxy resin with bimodal rubber-particle size distribution

A bimodal rubber-particle distributed epoxy resin was made by simultaneous addition of two kinds of liquid rubbers, CTBN1300X9 and CTBN1300X13. These rubbers were added at a constant total rubber content but with varying weight ratios. The microstructure and fracture behaviour of these rubber-modified epoxy resins have been studied. A strong increase in the fracture resistance was found for the bimodal rubber-particle distributed epoxy resin. The role of the small particle is thought to toughen the shear bands between large particles. The role of large particle is thought to induce a large-scale shear deformation in the crack front. The synergistic effect of these particles gives rise to a strong increase in the toughness of these bimodal rubber-particle distributed epoxy systems.     

Shear ductility and toughenability study of highly cross-linked epoxy/polyethersulphone

The objective of the present study was to determine whether the ductility and toughenability of a highly cross-linked epoxy resin, which has a high glass transition temperature, T _g, can be enhanced by the incorporation of a ductile thermoplastic resin. Diglycidyl ether of bisphenol-A (DGEBA) cured by diamino diphenyl sulphone (DDS) was used as the base resin. Polyethersulphone (PES) was used as the thermoplastic modifier. Fracture toughness and shear ductility tests were performed to characterize the materials. The fracture toughness of the DDS-cured epoxy was not enhanced by simply adding PES. However, in the presence of rubber particles as a third component, the toughness of the PES–rubber-modified epoxy was found to improve with increasing PES content. The toughening mechanisms were determined to be rubber cavitation, followed by plastic deformation of the matrix resin. It was also determined, through uniaxial compression tests, that the shear ductility of the DDS-cured epoxy was enhanced by the incorporation of PES. These results imply that the intrinsic ductility, which had been enhanced by the PES addition, was only activated under the stress state change due to the cavitation of the rubber particles. The availability of increasing matrix ductility seems to be responsible for the increase in toughness. This revised version was published online in November 2006 with corrections to the Cover Date.     

Toughening mechanisms of modified unsaturated polyester with novel liquid Polyurethane rubber

Unsaturated polyester (UPE) has been toughened by incorporating novel liquid polyurethane (PU) rubber. PU rubber was synthesized using toluene di-isocyanate and polyols such as poly (propylene glycol) and poly (tetramethylene ether) glycoi, whose molecular weights vary from 650 to 4000. Particle size was varied from 0.1 to 3 m by changing the polyol and the molecular weight of PU rubber, and the effects of particle size on the fracture toughness of PU rubber-modified UPE were investigated. Hydroxyl terminated PU rubber (HTPU) and isocyanate terminated PU rubber (ITPU) were used to study the effects of rubber-matrix adhesion. The toughening mechanisms observed by scanning electron microscope are debonding between rubber and matrix in HTPU-modified UPE, and cavitation in the rubber particle in ITPU-modified UPE. However, shear bands were not observed as UPE is a highly cross-linked thermoset with very short chain length between the cross-links. A 1.9-times increase in fracture toughness of UPE was achieved with the formation of cavitated particles. In order to measure the process zone size at the crack tip, the thin sections of tested double-notched four-point bending specimens were examined by optical microscope.     

自乳化水性环氧树脂的制备

采用聚醚醇二缩水甘油醚(PEGGE)、乙醇胺(MEA)及冰乙酸对双酚A环氧树脂(DGEBA)化学改性,在不需Lewis酸催化剂条件下制备低污染、高性能水性环氧树脂。首先,在物料摩尔比(MEA/PEGGE)2∶1,反应温度55℃,反应时间4h下,用PEGGE对MEA扩链合成MEA-PEGGE加成物;然后,在物料摩尔比(DGEBA/MEA-PEGGE)2∶1,反应温度65℃,反应时间5h下,用MEA-PEGGE加成物对DGEBA扩链,合成DGEBA-MEA-PEGGE加成物;再采用冰乙酸与DGE-BA-MEA-PEGGE加成物成盐,制备出具有良好水溶解分散性能的自乳化水性环氧树脂。该树脂涂膜性能优良,具有良好柔韧性和耐冲击性,改善了普通环氧树脂性能较脆的缺陷。     

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.     

Dynamic observation of fracture in particulate-filled epoxy resins reinforced with rubber

Crack propagation in epoxy resins filled with alumina trihydrate has been observed by dynamic in situ scanning electron microscopy. Double torsion specimens were fractured inside a scanning electron microscope (SEM) connected to a video recorder. Characteristic features of the crack propagation process were observed. The fracture mode was mainly intergranular at low crack velocities, (ca. 10 m/s) with evidence of filler particle cracking (transgranular fracture) at higher velocities or after acid-washing the particles. Extensive shear yielding of the epoxy matrix occurred between closely spaced filler particles within the crack tip damage zone. Post-mortem static observations of the fracture surfaces were also carried out. The addition of rubber toughening agents modified the crack propagation process. In some cases the rubber was present as fine, evenly distributed particles while in others there were coarser precipitates and/or what appeared to be rubber-rich epoxy phases. Ligamentary bridges across crack faces in the crack wake necked to fracture at low crack velocities but failed by cavitation under rapid loading. Energy dissipation by local shear yielding of the matrix was still prominent. Vinyl terminated rubber addition induced especially widespread yielding. Out-of-SEM determinations of tensile and fracture parameters were consistent with dynamic SEM observations.     

不同柔性链丙烯酸松香环氧树脂性能比较

以丙烯酸松香二缩水甘油酯(ARE)、丙烯酸松香乙二醇二缩水甘油醚(AR-EDGE)和丙烯酸松香丁二醇二缩水甘油醚(AR-BDGE)为研究对象,通过测定固化反应的凝胶时间、固化产物的耐热性、力学性能及耐溶剂性,研究了不同柔性链对松香基环氧树脂性能的影响。结果表明,松香基的稠环结构有着优异的耐热性,然而也会带来较大的脆性,需引入合适长度的柔性链。三种体系中,AR-EDGE综合性能最好,虽耐热性不及ARE,但其力学性能强于另两种体系;柔性链稍短的ARE因脆性大力学性能不好;柔性链稍长的AR-BDGE耐热性及耐丙酮性不佳,力学性能一般。     

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.     

Influence of particle size and particle size distribution on toughening mechanisms in rubber-modified epoxies

The principal toughening mechanism of a substantially toughened, rubber-modified epoxy has again been shown to involve internal cavitation of the rubber particles and the subsequent formation of shear bands. Additional evidence supporting this sequence of events which provides a significant amount of toughness enhancement, is presented. However, in addition to this well-known mechanism, more subtle toughening mechanisms have been found in this work. Evidence for such mechanisms as crack deflection and particle bridging is shown under certain circumstances in rubber-modified epoxies. The occurrence of these toughening mechanisms appears to have a particle size dependence. Relatively large particles provide only a modest increase in fracture toughness by a particle bridging/crack deflection mechanism. In contrast, smaller particles provide a significant increase in toughness by cavitation-induced shear banding. A critical, minimum diameter for particles which act as bridging particles exists and this critical diameter appears to scale with the properties of the neat epoxy. Bimodal mixtures of epoxies containing small and large particles are also examined and no synergistic effects are observed.     

橡胶增韧环氧树脂的增韧力学模型综述

对橡胶增韧环氧树脂的增韧机理模型进行了回顾,其主要增韧机理为局部部切屈服,孔洞或空穴的塑料性体积膨胀和橡胶颗粒桥联,在细观结构上采用典型轴对称单元的有限元分析来预测增韧环氧树脂的力学行为,断裂力学的J积分模型可用于估算增韧程序,基于断裂韧性GIC的增韧模型定量分析已取得一定进展。     

Epoxy resin/liquid natural rubber system: secondary phase separation and its impact on mechanical properties

An investigation was carried out to explore the morphology and mechanical properties of diglycidyl ether of bisphenol A epoxy resin (DGEBA) with liquid natural rubber possessing hydroxyl functionality (HLNR). Though modification of epoxies by synthetic rubber has been extensively studied not much attention has been paid to liquid natural rubber. Photo depolymerisation of natural rubber enables us to synthesise low molecular weight oligomers by varying the experimental parameters. Epoxy resin was cured using nadic methyl anhydride as hardener in presence of N,N-dimethyl benzyl amine accelerator. Hydroxylated natural rubber of different concentrations is used as modifier for epoxy resin. The addition of such chemically modified liquid rubber to an anhydride hardener–epoxy resin mixture has given rise to the formation of a two-phase microstructure in the cured systems, consisting of spherical particles of liquid natural rubber strongly bonded to the surrounding matrix, there by providing the required mechanism for toughness enhancement. Subinclusions of epoxy resin were present in the elastomer domains as secondary particles (particle in particle morphology) as evidenced from the SEM (scanning electron micrograph) photomicrographs. The origin of the so-called secondary phase separation is due to the combined effect of hydrodynamics, viscoelastic effects of rubber phase, diffusion, surface tension, polymerisation reaction and phase separation. In a dynamic asymmetric system, the diffusion of the fast dynamic phase is prevented by the slow dynamic phase, and hence the growth of fast dynamic phase gets retarded due to the slow dynamic phase. In the case of low viscosity blends the growth of fast dynamic phase turns fast and hence diffusion of fast dynamic phase cannot follow geometrical growth and cannot establish local concentration equilibrium and hence double phase separation takes place. The double phase separation is responsible for the enhanced impact and toughness behaviour of the blends. The mechanical behaviour of the liquid rubber-modified epoxy resin was evaluated in terms of tensile and flexural properties.     

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.     

Phase separation mechanism of rubber-modified epoxy

The phase separation mechanism during the cure reaction of a liquid rubber-modified epoxy resin was investigated by light scattering, light microscopy, torsional braid analysis, electron microscopy, and differential scanning calorimetry. The binary mixture of epoxy oligomer (diglycidyl ether of bisphenol A) and carboxyl-terminated butadiene-acrylonitrile copolymer (liquid rubber) exhibited the upper critical solution temperature-type phase behaviour. The mixture loaded with curing agent was a single-phase system in the early stage of curing. When the cure reaction proceeded, phase separation took place via the spinodal decomposition induced by the increase in the molecular weight of epoxy. This was supported by the characteristic change of light scattering profile with curing time. Electron microscopy revealed that, in cured resin, the spherical rubber domains are dispersed somewhat regularly in an epoxy matrix. The regular domain arrangement seems to result from a specific situation; the competitive progress of the spinodal decomposition and polymerization; i.e. the coarsening process to irregular domain structure seems to be suppressed by network formation in the epoxy phase. It was also shown that curing at higher temperatures resulted in the suppression at an earlier stage of spinodal decomposition, and hence, shorter interdomain spacing.     

Damage zone around crack tip and fracture toughness of rubber-modified epoxy resin under mixed-mode conditions

Damage zones that form around crack tips before the onset of fracture provide significant data for evaluating the fracture behavior of polymeric materials. The size of the damage zone correlates closely with the fracture toughness of the resin. In this study, we investigate the relationship between the fracture toughness and damage zone size around crack tips of a rubber-modified epoxy resin under mixed-mode conditions. The fracture toughness, G_C, based on the energy release rate, is measured using an end-notched circle type (ENC) specimen. The deformation of rubber particles in the damage zones is also observed using an optical microscope. The results show that the fracture toughness, G_C, of the rubber-modified epoxy resin is closely related to the area of the damage zone. In the specimen with a loading angle of 30°, the rubber particles were deformed ellipsoidally due to the difference between the first and second principal stresses.     

非异氰酸酯聚氨酯的制备与性能研究

采用双酚F型环氧树脂NPEF-170与聚乙二醇400二缩水甘油醚为原料制备了双官能度的环碳酸酯，再与聚醚胺T403反应合成了非异氰酸酯聚氨酯（NIPU），并且利用FT-IR时产物的结构进行了表征。同时探讨了反应条件对环氧树脂转化率的影响。研究了不同质量比的原料对非异氰酸酯聚氨酯力学性能的影响，研究表明，当原料的质量比为3：7时所制备的NIPU材料的力学性能最好。最大拉伸强度为7．58MPa，扯断伸长率为347％。     

Fracture toughness of multiphase polypropylene composites containing rubbery and particulate inclusions

The fracture toughness of binary and ternary phase polypropylene (PP) composites containing ethylene–propylene rubber (EPR) and glass beads, has been studied using the J-integral method at 23 and − 20 °C. For determining J _c, analysis of the stress-whitening zone was found to be more meaningful than the commonly used blunting line approach. Functionalized EPR was found to be more effective toughening additive for PP than EPR, in both binary and ternary phase compositions. Crack growth mechanisms were examined by scanning electron microscopy. In rubber-modified blends, cavitation and shear yielding were found to be the primary toughening mechanisms, while in ternary phase composites particle–matrix debonding played a major role. This revised version was published online in November 2006 with corrections to the Cover Date.     

Three-dimensional elastoplastic finite element modelling of deformation and fracture behaviour of rubber-modified polycarbonates at different triaxiality

A periodic face-centred cuboidal cell model is provided to account for inter-particle interaction, and a particle-crack tip interaction model is developed to study the interaction between a blunting model I crack tip and the closest array of initially spherical rubber particles in an effective medium. Three-dimensional elastoplastic finite element analysis has been preformed to study the deformation and fracture behaviour of rubber-modified polycarbonates. The effective elastoplastic constitutive relation is derived by the method of homogenisation and local stress and strain distributions are obtained to explore the role of rubber cavitation in the toughening process at different stress triaxiality. 3D elastoplastic finite element results are compatible with experimental observations, that is, rubber particles can act as stress concentrators to initiate crazing or shear yielding in the matrix but they behave differently from voids at high triaxiality. Rubber cavitation plays an important role in the toughening process under high tensile triaxial stresses.     

形状记忆氢化双酚A型环氧树脂的制备与性能

为研究聚丙二醇二缩水甘油醚(JH-230)对热固性形状记忆环氧树脂基本性能的影响,将异佛尔酮二胺(IPDA)与具有不同分子量比的氢化双酚A型环氧树脂(AL-3040)、聚丙二醇二缩水甘油醚(JH-230)共混,经完全固化制备出一种新型的形状记忆氢化双酚A型环氧树脂。借助傅里叶红外光谱仪(FT-IR)、差示扫描量热仪(DSC)、动态热机械分析仪(DMA)和拉伸-回复形状记忆测试分析了热固性形状记忆环氧树脂的分子结构以及JH-230对固化体系玻璃化转变温度、储能模量和形状记忆性能的影响。研究表明,JH-230可以增加固化体系链段的柔韧性;固化体系的玻璃化转变温度与动态模量随JH-230含量的增加而降低;该形状记忆氢化双酚A型环氧树脂体系具有良好的形状记忆性能,且形变完全回复时间随JH-230含量的增加而延长。     

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.     

The fracture of an epoxy polymer containing elastomeric modifiers

The fracture energies of elastomer-modified epoxy polymers have been determined over a range of strain rates from 10⁻² to 10³ sec⁻¹. The modifiers included a liquid carboxyterminated butadiene acrylonitrile and a solid rubber. They were used alone and also in combination. In all cases, the modifiers increased the toughness of the base resin by orders of magnitude and one combination of liquid and solid rubber increased toughness by 60 times. There was a general decrease in fracture energy with increasing strain rate but even during impact testing the modified epoxys were 10 to 20 times tougher than the base polymer. Scanning electron microscopy revealed that, when combined with the liquid rubber, the solid rubber induced a localized shear yielding.