A study on the toughening mechanism of rubber-modified polyfumctional epoxy resins

The toughening mechanism of CTBN rubber-toughened tri functional epoxy resins was studied using a scanning electron microscope. The SEM micrographs show that the CTBN rubber-modified tri functional epoxy resins are composite materials consisting of a continuous phase of epoxy resins and a CTBN rubbery disperse phase. The toughening mechanism is discussed mainly in terms of the initiation and propagation of secondary microcracks during or before propagation of the primary cracks, and the several important models of the secondary microcrack formation are discussed in more detail. Our task was to control the morphology of the composite, the structures of separate phases and the interfacial adhesion in such a way as to obtain the best balance of properties.     

Modelling the properties of rubber-modified epoxy polymers

A finite-element model for rubber particles in a polymeric matrix has recently been proposed which is based upon a collection of spheres, each consisting of a sphere of rubber surrounded by an annulus of matrix. We have used this model to investigate in detail the stress distributions in and around a rubber particle, or a void, in a matrix of epoxy polymer. We have deduced the bulk modulus of the rubber-toughened epoxy and considered the implications of the stress distributions on the observed toughening micromechanisms. Of particular concern has been the effects of the volume fraction and the properties of the rubber phase.     

Craze-like damage in a core-shell rubber-modified epoxy system

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.     

Development of a process zone in rubber-modified epoxy polymers

The effects of a process zone on toughness and on R-curve behavior were investigated for a model, rubber-modified epoxy polymer. The system studied was one in which the bridging mechanism of toughening does not operate. The characteristic features of R-curve behavior, a rise in toughness with crack extension until an approximate steady-state is reached, were observed using double-cantilever-beam tests. The evolution of the process zone was studied using transmission-optical microscopy. As the crack grew, the process zone appeared to fan out until it reached a steady-state thickness; it then remained a uniform size upon further crack advance. The features of the experimental R-curves were shown to be directly correlated to the evolution of the process zone. Furthermore, the effect of the portion of the process zone in the crack wake was examined by a series of experiments in which the wake was partially removed, and the R-curve re-established by subsequent loading. These experiments demonstrated that removal of the crack wake caused the crack-growth resistance to drop. The toughness then built back up to the steady-state value as the crack wake re-developed. This unambiguously demonstrated a contribution to toughening from the crack wake despite the absence of any observable bridging mechanism. These results support the accepted notion that an extrinsic toughening mechanism is responsible for the increased toughness observed upon adding rubber particles to an epoxy matrix This revised version was published online in August 2006 with corrections to the Cover Date.     

Plastic deformation in highly cross-linked rubber-modified epoxy resins

Plastic yielding behaviour of three different cross-link density rubber-modified epoxy resins, at different rubber levels and temperature, were investigated. All the systems studied show decrease in Young's modulus,E, and yield stress, _Y, with increasing temperature and rubber content. The deformation process was analysed using both Bowden and Argon theories. Molecular parameters from each. theory were then compared with chemical structures of the epoxy systems.     

Modelling of the toughening mechanisms in rubber-modified epoxy polymers

A mathematical model has been developed to quantify the relationships between the microstructure and fracture properties of multiphase rubber-toughened epoxy polymers. Good agreement between predictions from the model and experimental results have been found. The model also reveals that localized plastic shear banding in the epoxy matrix, running between the rubbery particles, is the dominating mechanism under all testing conditions. Plastic void growth in the epoxy matrix is the other main toughening mechanism. This latter mechanism is initiated by internal cavitation of the rubbery particle, or by debonding at the particle-matrix interface, and is particularly significant at higher test temperatures.     

Effects of pre-cracking methods on fracture behaviour of an Araldite-F epoxy and its rubber-modified systems

The fracture behaviour of an Araldite-F epoxy and its rubber-modified systems was evaluated using compact tension specimens pre-cracked by three methods, namely, razor blade pressing, razor blade tapping and fatigue pre-cracking. The results show that the razor blade tapping method produces a lowest critical stress intensity factor, K _lc, while the razor blade pressing produces an abnormally high K _lc, being about five times higher than the former for the pure epoxy. Transmission polarized optical microscopy reveals that the crack tip produced by razor blade pressing in the pure epoxy specimen was completely surrounded by a plastic deformation zone with compressive residual stress, but the crack tip produced by razor blade tapping was free of residual stress and plastic deformation. It was found that the sensitivity of the fracture toughness value to the pre-cracking methods decreases after the pure epoxy was modified by 10% core-shell rubber or 10% liquid rubber.     

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.     

Comparison of failure mechanisms between rubber-modified and unmodified epoxy adhesives under mode II loading condition

The effect of rubber modification on fracture toughness of adhesive joints under mode II loading condition was investigated in comparison with that under mode I loading, wherein the two adhesives rubber-modified and unmodified were used. To evaluate the fracture toughness on the basis of R-curve characteristics under mode II loading condition, four-point bend tests had been conducted for the adhesively bonded end-notched flexure (ENF) specimens. Thus obtained R-curves revealed the following trend: its behavior did not appear for the unmodified adhesive, whereas the rubber-modified adhesive exhibited a typical behavior. In the initial stage of crack propagation, G _IIC of the rubber-modified adhesive is lower than that of the unmodified adhesive, but becomes greater in the range of Δa > 25 mm. Nevertheless, the significant improvement of the fracture toughness with the rubber modification under mode I loading condition was not observed under mode II loading. Moreover, FEM analysis was made to elucidate the relation between the above fracture behavior and stress distributions near the crack tip. The results gave the reasonable relationship between evolution of plastic zone and the area with high void-fraction as well as the R-curves behavior. In addition, macroscopic and SEM observations for the fracture surfaces were also conducted.     

用单轴拉伸实验研究HIPS的银纹增韧现象

高抗冲聚苯乙烯 (HIPS)是通过颗粒填充进行改性以提高其力学性能的工程塑料 ,通过单轴拉伸实验对其银纹损伤及增韧现象做初步探讨 ,发现银纹增韧机理是橡胶增韧中主导因素 ,分析得到受拉HIPS的弹塑性损伤本构方程的一般形式 .     

增韧环氧树脂相结构

用SEM分别对不同配比的热塑/热固(TP/TS)原位共混增韧体系和"离位"增韧体系的浇注体固化后的相形貌进行了观察。结果显示: 对于共混增韧体系, 当热塑性树脂含量为16.7％~20.0%时体系发生相反转。通过对发生相反转体系的热固性颗粒尺寸的统计分析可知, 随着TP含量的增加, 热固性颗粒的粒子直径越来越小, 且粒子直径的分散性也越来越小; 在统计分析结果的基础上由数学分析给出了粒子直径与TP含量的经验关系式, 由该经验式可知, 热固性粒子直径的上下限分别是1.838μm和0.925μm, 对应的质量分数分别约为16.0％和39.0％。对"离位"增韧的相形貌进行了分析, "离位"增韧体系形成了增韧层-过渡层-本体层的复合结构。TP含量在增韧层保持较高水平, 在过渡层中迅速降低, 在本体层中几乎不存在; 同时为达到较好的增韧效果, 过渡层厚度不应超过增韧层厚度。      

Ductile-to-brittle transition of rubber-modified polypropylene

The irreversible deformation mechanisms of polypropylene (PP) blended with an ethylenepropylene rubber (EPR) were investigated in the region of the ductile-to-brittle (D-B) transition. The nature of the D-B transition over the composition range of 0 to 25% EPR was studied as a function of temperature and strain rate. Optical microscopy and scanning electron microscopy were used to examine the irreversible microdeformation processes in the fractured specimens. At –40° C, the controlling irreversible deformation process in PP was crazing. In the blends, two kinds of damage zones were observed: a diffuse zone due to voiding at rubber particles and an intense damage zone due to craze-like damage and deformation bands. In general, the size and density of the damage zones increase in a gradual manner through the D-B transition whether examined as a function of temperature, strain rate or blend composition.     

Crystalline order in rubber-modified thermoplastics

The crystalline order in blends of thermoplastics such as polyethylene (PE) and isotactic polypropylene (iPP) with trans octenylene rubber (TOR) has been investigated by small-angle X-ray scattering. Interface distribution functions have been used to evaluate the SAXS. It was found, that the crystal morphology depends strongly on sample composition. At 10% TOR content both thermoplastics change their supermolecular structure in a characteristic way: while in the PE/TOR blends the crystal morphology of polyethylene becomes bimodal, the lamellar order is increased in the iPP/TOR blends. This behaviour is a consequence of finer dispersion of TOR in the matrix which causes the interface area to increase.     

Effective separation and alignment of long entangled carbon nanotubes in epoxy

This paper investigated the microstructural change of long entangled carbon nanotubes in an epoxy matrix under continuous shear. It was found that a critical composite viscosity is required to effectively separate the entangled nanotubes and align them along the shear direction. The major nanotube separation-alignment in composite processing is reflected by the plateaus on the viscosity variation curve. An improvement of the nanotube dispersion and composite flexural modulus was obtained in aligned MWNTs-epoxy. It was also found that the addition of MWNTs decreases the crosslinking rate of the epoxy.