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.     

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.     

Effect of modifier concentration on the fracture behaviour of rubber-modified PMMA

Izod fracture surfaces of blends of PMMA with various amounts of a rubber modifier were studied using scanning electron microscopy. Attention was focused on modes of crack initiation and propagation and on the role of the modifier in the fracture process. It was found that the impact strength of this class of materials increased monotonically with an increase in modifier concentration, at least up to 40 wt% modifier. Unmodified PMMA was studied to provide a basis for understanding the morphological features on the fracture surfaces of the rubber-modified blends. It was confirmed that PMMA fractures through the formation and rupture of crazes. This phenomenon was also found to occur in blends containing 10 wt% modifier. However, blends with 20 wt% modifier crazed only in the later stages of the fracture process, when the crack speed had exceeded some critical value. No evidence of crazing was found in blends with 30 and 40 wt% modifier loadings, although extensive plastic deformation was observed on the fracture surfaces.     

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.     

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.     

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.     

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.     

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.     

Effect of notch severity on fatigue fracture in a rubber-modified glassy polymer

The research concerned the effect of notch severity on the fatigue behaviour of series of rubber-modified glassy polymers, consisting of a styrene-acrylonitrile copolymer with different amounts of an olefin rubber. Tests were conducted under displacement control and two different loading conditions. Both stages of fatigue lifetimes, that is, fracture initiation and crack propagation, were examined. It was observed that the initial notch severity determines the duration of the crack-initiation stage, while crack propagation does not depend on it. The crack velocity appears to be controlled by the maximum applied stress intensity factor, and the correlation does not depend on the rubber content. The results obtained have been interpreted by considering three different zones in the specimen during the fracture process: a far-field viscoelastic continuum, a process zone and a failure zone.     

Effect of fillers on the degradation of automotive epoxy adhesives in aqueous solutions

Absorption of water in epoxy adhesives containing different types of fillers was studied after immersion in distilled water and in NaCl solutions during several periods of time. The amount of water uptake in the adhesives was found to increase with the concentration of water-soluble fillers incorporated in the adhesive matrix. The water absorption behaviour of the adhesives investigated was found to be non-Fickian. Owing to reverse osmosis, the amount of water absorbed in the adhesives decreases with the concentration of the bulk NaCl solution.     

Effect of moisture on the static and viscoelastic shear properties of epoxy adhesives

The effects of moisture on the structural properties of two commercially available adhesives, FM73M and FM300M, are investigated. The experimental study consists of static and viscoelastic shear measurements made from bonded joint specimens soon after they were cured and after lengthy exposure in 63% and 95% relative humidity environments. Static shear modulus and shear creep compliance data for each adhesive at each moisture level throughout a wide range of temperatures are illustrated. Also shown are the effects of temperature and moisture on the ultimate shear strength behaviour, and the temperature and moisture viscoelastic shift functions. It is concluded that the effect of moisture as an external plasticizer on the shear properties of these adhesives is equivalent to raising the environmental temperature.     

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.     

Effect of different types of silver and epoxy systems on the properties of silver/epoxy conductive adhesives

In this study, the effect of amine adduct powder (AAP) on the electrical conductivity, thermal expansion, and flexural modulus of one-part system conductive adhesives are investigated. One-part system conductive adhesives are prepared using two types of silver (silver A and silver B) and different percentage of filler loadings, 10 vol.% ?40 vol.%. Silver A adhesive systems exhibit higher electrical conductivity with lower percolation thresholds, high flexural moduli, and low coefficients of thermal expansion (CTE) compared silver B adhesive systems. A comparison of the electrical conductivity and thermal expansion properties of the one part and two-part of silver A adhesives system is undertaken. The one-part silver A adhesives system shows high electrical conductivity and low CTE values compared to the two-part system. This is due to the higher cross linking density of the one-part system compared to that of the two-part system.     

Study on fracture behavior of surface treated montmorillonite/epoxy nanocomposites

It is known that the mechanical properties of clay-reinforced nanocomposites are significantly affected by the dispersion of clay particles in the matrix. In this study, the effect of surface-treatment of Montmorillonite (MMT) on the fracture behavior of MMT/epoxy nanocomposite was investigated. For this purpose, fracture tests were performed using samples with three different clay concentration level. After fracture tests, SEM analysis was made on the fracture surfaces to examine the fracture mechanism. It was found that the MMT treatment using 3-aminopropyltriethoxysilane enhanced the fracture toughness increased of the MMT/epoxy nanocomposite. This is due to the improved intercalation effect and interfacial strength between MMT and epoxy matrix.     

The role of damage-softened material behavior in the fracture of composites and adhesives

Failure mechanisms of materials under very high strain experienced at and ahead of the crack tip such as the formation, growth and interaction of microvoids in ductile materials, microcracks in brittle solids or crazes in polymers and adhesives are represented by one-dimensional, nonlinear stress-strain relations possessing different post-yield softening (strain-softening) behavior. These reflect different ways by which the material loses capacity to carry load up to fracture or total separation. A DCB type specimen is considered in this study. ^* The nonlinear material is confined to a thin strip between the two elastic beams loaded by a wedge. The problem is first modelled as a beam on a nonlinear foundation. The pertinent equation is solved numerically as a two-point boundary value problem for both the stationary and the quasi-statically propagating crack. A finite element model is then used to model the problem in more detail to assess the adequacy of the beam model for the use of experimental data to determine in-situ properties of the thin interlayer.It is found that the energy release rate derived by assuming built-in conditions at the crack tip can be used to calculate the fracture (surface) energy more accurately and conveniently than Berry's scheme [2] even in cases where the built-in assumption is apparently invalid. The analyses suggest that it is possible to infer from detailed macroscopic measurements of the deformations of the beam prior to and during crack growth the approximate characteristics of the complete (uniaxial) material stress-strain behavior of the cohesive interlayer including loading and strain-softening.

---

Résumé On représente les mécanismes de rupture de matériaux sous de trés fortes déformations appliquées à l'extrémité ou en amont de fissures tels que la formation, la croissance et l'interaction de micro-lacunes dans les matériaux ductiles, les micro-fissures dans les matériaux fragiles ou les craquelures dans les polymères et les adhésifs, par des relations contraintes-déformations non linéaires, unidimensionnelles et présentant des comportements différents d'adoucissement à l'écrouissage. Ces mécanismes reflètent diverses voies selon lesquelles un matériau perd sa capacité de supporter une charge avant rupture ou séparation totale. Dans cette étude, on considère une éprouvette de type double Cantilever, un matériau non élastique étant ramené à une bande mince comprise entre deux poutres élastique étant ramené à une bande mince comprise entre deux poutres élastique étant ramené à une bande mince comprise entre deux poutres élastiques sollicitées par un coin. On commence par représenter le problème comme une poutre reposant sur un support non linéaire. Par voie numérique, on résoud l'équation correspondante dans les cas d'une fissure stationnaire et d'une fissure en propagation quasi-statique. On utilise ensuite un modèle par éléments finis pour représenter avec plus de détails le problème en vue de vérifier l'adéquation du type d'éprouvette pour l'utilisation des données expérimentales à la détermination des propriétés in situ de l'intercouche mince.On trouver que la vitesse de relaxation de l'énergie obtenue en supposant des conditions de consolidation à l'extrémité de la fissure peut être utilisée pour la calcul de l'énergie de rupture (en surface) de mainère plus précise et plus commode que l'approche de Berry, même dans les cas où une hypothèse de consolidation n'est apparemment pas valable. L'analyse suggère qu'il est possible de déduire les caractéristiques approchées du comportement complet du matériau du point de vue contraintes-dilatations (uniaxiales) relatives à l'intercouche de liaison, tant pour la mise en charge que pour l'adoucissement, à partir de mesures macroscopiques détaillées des déformations de la poutre avant et durant l'expansion de la fissure.

---

---

This paper is a shortened version of part of [1].

---

An experimental program is being conducted in our laboratories to study the strain-strain characteristics of a number of polymeric solids using the DCB model discussed in this paper.

---

An experimental program is being conducted in our laboratories to study the strain-strain characteristics of a number of polymeric solids using the DCB model discussed in this paper.     

Influence of interface modification and phase separation on damping properties of epoxy concrete

A preliminary study has been conducted to investigate the influences of interface modification and rubber phase separation on damping behaviour of epoxy concrete (EPC). The main parameters studied here are loss factor (η), i.e. damping, and dynamical modulus of elasticity (E_d). It is found that the tenacity of EPC can be improved by controlling rubber phase separation, and that the mechanical vibration can be absorbed while keeping a high enough E_d value by forming an elastic interlayer using a coupling agent and acrylonitrilebutadiene rubber with end carboxyl group. The values of η and E_d obtained in this paper are in the range of 0.0436–0.0534 and 20.97–26.72 GPa, respectively. The interface microstructure of EPC has been characterized by means of XPS and FTIR here. The results suggest that a weak Lewis acid-base reaction has taken place and chemically bonded joints have formed in the interlayer of EPC, and the rubber phase separation has been characterized by the technique of microimage analysis.