Raymond A. Pearson

Rubber-Toughened Epoxies: A Critical Review

Epoxy resins have been used as structural materials since the late 1940s. Despite their desirable properties such as high strength, excellent creep resistance, and good adhesion, they suffer from low fracture energy. Rubber modification as a major toughening approach to overcome the inherent brittleness of epoxy polymers was introduced during the early 1970s. Since then, a large number of investigations have been conducted to elucidate different aspects of rubber-toughened epoxies. The present work is a critical review of the field focusing on the important parameters affecting rubber-toughening. The studies reviewed are classified in five categories including roles of matrix ductility, rubber concentration, blend morphology, particle cavitation, and particle/matrix interface. It has been tried to provide an in-depth view of the state-of-the-art knowledge in the field and to direct future studies towards exploring new approaches for toughening of epoxy polymers.

Role of particle cavitation in rubber-toughened epoxies : 1. Microvoid toughening

The significance of particle cavitation in rubber-toughened polymers remains controversial. While some researchers believe void formation promotes shear yielding in the polymer matrix, others consider it as a secondary energy consuming process. The research described here was undertaken to further elucidate the role of particle cavitation in toughening through comparative examination of epoxies modified by conventional rubber modifiers and hollow plastic particles. The results of this study illustrate that rubber particles with different cavitation resistance and pre-existing microvoids toughen the present epoxy matrix in the same manner. Therefore, we conclude that the cavitation resistance of the rubbery phase does not directly contribute to toughness, but instead simply allows the matrix to deform by shear. An additional mechanism of microcracking was observed when 40-μm hollow plastic particles were employed. Despite the similar behaviour in fracture toughness testing, rubber particles and microvoids differ considerably in how they affect the compressive yield strength of the blend. The results of this study suggest the possible importance of inter-particle distance in toughening of epoxies. This concept will be examined in part 2 of this study.

Role of particle cavitation in rubber-toughened epoxies: II. Inter-particle distance

Abstract Two types of conventional rubber modifiers and a series of hollow plastic micro-spheres were employed as toughening agents in a diglycidyl ether of bisphenol A (DGEBA) epoxy in Part I (Bagheri R, Pearson RA. Polymer 1996;37:4529) of this study. It was found that the rubber modifiers with different cavitation resistance and hollow plastic micro-spheres which act as pre-existing microvoids toughen epoxies in the same manner. The current study is composed to further examine the previous results in terms of the role inter-particle distance in rubber/microvoid toughened epoxies. It is shown that the fracture toughness in toughened blends goes through a ductile-to-brittle transition with inter-particle distance. The source of this transition is found to be a stress state change provided by voided particles. The ligament between neighboring particles, thus, experiences a transition from plane strain to plane stress state by decreasing the inter-particle distance. Interestingly, it is shown that the transition in toughened blends does not occur at a specific inter-particle distance as frequently proposed in literature, but varies with the size of the modifier. Therefore, there is an influence of particle size on yielding of the toughened epoxies that is responsible for the shift in transition.

Role of blend morphology in rubber-toughened polymers

The influence of blend morphology on mechanical behaviour of rubber-toughened polymers was investigated. Diglycidyl ether of bisphenol A epoxies toughnened by core-shell rubber particles were employed as the model systems. The blend morphology was varied by changing the composition of the shell of particles, the curing agent, and the extent of agitation prior to casting. It is shown that the most uniform dispersion of particles is obtained when the shell of the modifiers contains reactive groups. In the absence of the reactive groups and when a slow curing agent is employed, however, a highly connected microstructure is obtained. It was found that a blend with a connected microstructure provides significantly higher fracture toughness compared to a similar blend containing uniformly dispersed particles. The reason for this observation is that the connected morphology enables the shear bands to grow further from the crack tip and thus consume more energy before fracture occurs. Also, the yield strength in uniaxial tensile testing is significantly lower in the blend with the connected morphology. Therefore, it should contribute to a larger plastic zone size.

Multicomponent latex IPN materials: 2. Damping and mechanical behavior

The integrals of the linear loss shear modulus vs. temperature (loss area, LA) and linear tan δ vs. temperature (tan δ area, TA) were characterized for various core/shell latex particles with synthetic rubber, poly(butadiene-stat-styrene) [P (Bd/S), 90/10], and interpenetrating polymer networks (IPN) as the cores. The IPN cores were composed of P(Bd/S) (Tg ≃ − 70°C) and an acrylate based copolymer (Tg around 10°C) for potential impact and damping improvement in thermoplastics. Poly(styrene-stat-acrylonitrile) (SAN, 72/28) was the shell polymer for all these polymers. Under the same loading, for both toughening and damping controls, among the IPN core/shell, blend of separate core/shell, and multilayered core/shell polymers, the IPN core/shell polymers were the best dampers. However, the other core/shell polymers also showed higher LA values than P(Bd/S)/SAN core/shell polymer. A comparison of LA values via a group contribution analysis method was made, the effect of particle morphology and phase continuity on damping being studied. Inverted core/shell latex particles (glassy polymer SAN was synthesized first) showed much higher LA and TA values than normal core/shell ones (rubbery polymer was synthesized first). Models for maximum LA and TA behavior are proposed. The damping property was essentially controlled by the phase miscibility and morphology of the core/shell latex particles. The LA values for each peak in these multiphase materials provided some indication of the several fractional phase volumes.

Fatigue of rubber-modified epoxies: effect of particle size and volume fraction

A change in crack-tip plastic zone/rubber particle interactions induces a transition in the fatigue crack propagation (FCP) behaviour of rubber-modified epoxy polymers. The transition occurs at a specific K level, KT, which corresponds to the condition where the size of the plastic zone is of the order of the size of the rubber particles. At ΔK>ΔKT, rubber-modified epoxies exhibit improved FCP resistance compared to the unmodified epoxy. This is because the size of the plastic zone becomes large compared to the size of the rubber particles and, consequently, rubber cavitation/shear banding and plastic void growth mechanisms become active. At ΔK>ΔKT, both neat and rubber-modified epoxies exhibit similar FCP resistance because the plastic zone size is smaller than the size of the rubber particles and hence, the rubber cavitation/shear banding and plastic void growth mechanisms are not operating. As a result of these interactions, the use of smaller 0.2 μm rubber particles in place of 1.5 μm rubber particles results in about one order of magnitude improvement in FCP resistance of the rubber-modified system, particularly near the threshold regime. Such mechanistic understanding of FCP behaviour was employed to model the FCP behaviour of rubber-modified epoxies. It is shown that the near threshold FCP behaviour is affected by the rubber particle size and blend morphology but not by the volume fraction of the modifiers. On the other hand, the slope of the Paris-Erdogan power law depends on the volume fraction of the modifiers and not on the particle size or blend morphology.

The role of dispersed phase morphology on toughening of epoxies

The use of structural core/shell latex particles as toughening agents provides a model system which allows independent control of several key factors that influence the fracture toughness of modified plastics. This paper focuses on varying the shell composition of poly(butadiene-co-styrene) [P(B-S)] core/poly(methyl methacrylate) (PPMA) shell particles by incorporating acrylonitrile (AN) comonomer into the PMMA shell at various AN/MMA ratios and by crosslinking of the shell at various AN/MMA ratios. It was found that the degree of particle dispersability in the epoxy matrix can be precisely controlled by the AN content in the PMMA shell and by crosslinking the PMMA in the shell. It was also found that the degree of particle dispersability plays a crucial role on the fracture toughness of the modified epoxies. A microclustered morphology provides a much higher toughness than a uniform particle distribution.

The use of microvoids to toughen polymers

The concept of microvoid toughening of polymers has been proposed by several investigators. This investigation illustrates the similarities between the function of rubber particles in rubber-toughened plastics with that of microvoids. While previous researchers created microvoids within polymeric materials by means of non-adhering particles, our novel approach is based on using hollow plastic micro-spheres to generate holes. An epoxy polymer is used as a model polymer matrix. The results of this investigation show that hollow plastic particles toughen the epoxy resin in the same manner as rubber particles. Surprisingly, mechanical characterization of modified blends revealed that the use of hollow plastic micro-spheres provides greater yield strength than that of similar blends with equivalent rubber content.

The effect of particle - matrix adhesion on the mechanical behavior of glass filled epoxies: Part 1. A study on yield behavior and cohesive strength

Basic properties of glass filled epoxy such as yield behavior and cohesive strength, which are important in understanding toughening mechanism, are studied using compression tests and double notch 4 point bending (DN-4PB) tests. Three different glass reinforcements, large glass spheres, small glass spheres, and glass fibers, were treated by different methods and used at different volume fractions ranging from 10 to 30 vol%. The effect of different surface treatments and moisture exposure on the yield behavior is studied. It was found that in all types of formulation the yield stress decreased after moisture exposure and that the yield stress was dependent on the surface treatment both before and after moisture exposure. No treatment and treatment of glass reinforcements with aminopropyltrimethoxysilane coupling agent resulted in relatively higher yield stress. Close observation of microstructure by transmission OM showed that the degree of debonding in the specimens for compression tests was quite dependent on the surface treatment after moisture exposure. The decrease in cohesive strength of the neat specimens were observed after moisture exposure.

Toughening of Epoxy Nanocomposites: Nano and Hybrid Effects

In this paper, we review recent progress made in the field of epoxy-based binary and ternary nanocomposites containing three-, two-, and one-dimensional (i.e., 3D-, 2D-, and 1D) nano-size fillers with a special focus on their fracture behaviors. Despite investigations conducted so far to evaluate the crack-resistance of epoxy nanocomposites and attempts made to clarify the controlling toughening mechanisms of these materials, some questions remain unsolved. It is shown that silica nanoparticles can be as effective as rubber particles in improving the fracture toughness/energy; but incorporation of carbon nanotubes (CNTs) or clay platelets in epoxy matrices delays crack growth only modestly. The “nano” effects of silica (<25 vol.%) and rubber (>10 wt.%) nanoparticles in toughening epoxy resin are confirmed by comparison with silica and rubber micro-particles of the same loading. There is clear evidence of both synergistic and additive toughening effects in the silica/rubber/epoxy ternary nanocomposites. In addition, positive hybrid toughening effect has been observed in the nano-rubber/CNT/epoxy composites; however, a negative hybrid effect is predominant in nano-clay/nano-rubber/epoxy ternary nano-composites. Future research directions for epoxy-based nanocomposites towards multi-functional applications are discussed.