Analysis of the damage zone around the crack tip for two rubber-modified epoxy matrices exhibiting different toughenability

doi:10.1016/S0032-3861(02)00829-7

Polymer

Volume 44, Issue 5, March 2003, Pages 1537-1546

https://doi.org/10.1016/S0032-3861(02)00829-7 Get rights and content

Abstract

One inevitable effect of adding soft rubber particles to a rigid polymer is that its yield stress, σ_Y, is lowered by the stress concentration effect produced by dispersed particles. This effect may be enhanced if a fraction of the added rubber remains dissolved in the matrix, lowering its glass transition temperature, thus producing an extra decrease in σ_Y. Both effects lead to an increase in the critical stress intensity factor, K_IC, originated by crack tip blunting. The toughenability of an epoxy matrix may be defined by its ability to promote deformation mechanisms that can increase K_IC beyond the value attained by crack tip blunting. The aim of this manuscript was to analyze the toughenability of two different epoxy systems exhibiting similar relative increases in K_IC by rubber addition: (a) diglycidylether of bisphenol A (DGEBA) cured with piperidine; (b) DGEBA cured with a stoichiometric amount of 4,4′-diamine-3,3′-dimethyldicyclohexylmethane (3DCM). Both epoxies were modified with 15 wt% CTBN (carboxyl-terminated butadiene-acrylonitrile copolymer), that phase-separated in the course of polymerization. The damage zone around the surviving crack tip produced by the double-notch four-point-bend (DN-4PB) technique, was analyzed for both rubber-modified epoxies by transmission optical microscopy and transmission electron microscopy. For the DGEBA-piperidine system, dilatation bands (croids) and massive shear yielding in the region close to the crack tip, were observed. For the DGEBA-3DCM system, a region of cavitated particles was observed close to the crack tip without any evidence of shear deformation in the matrix. The higher toughenability of the former system compared to the latter, was associated to the different mechanical behavior observed in uniaxial compression tests of the pure epoxy matrices.

Introduction

Thermosets exhibit high values of stiffness and strength together with good heat and solvent resistance, due to their cross-linked nature. One major drawback is their poor resistance to impact and crack initiation. Improvements in their fracture resistance result from the blend with different types of modifiers that generally form a second dispersed phase. The most frequently used modifiers are liquid rubbers that phase-separate in the course of polymerization leading to a dispersion of rubber particles in the thermoset matrix [1].

The main toughening mechanism present in rubber-modified epoxies is the cavitation of rubber particles followed by void growth and induced shear yielding of the matrix [2]. Shear bands connecting cavitated rubber particles, called ‘dilatation bands’ or ‘croids’, have been observed [3], [4], [5], [6]. In these bands voids are not interconnected and their free surface is formed within the rubber phase. They propagate by generating peripheral zones of high elastic strain where the matrix becomes strain softened and yields more easily [6]. The eventual strain hardening prevents the occurrence of fracture at relatively early stages of the deformation.

From the description given above, the toughenability of an epoxy matrix might be related to its capacity to exhibit strain softening and large strains at relatively low stress values. The aim of this manuscript is to analyze this relationship for two epoxy systems exhibiting different toughenability: (a) diglycidylether of bisphenol A (DGEBA) cured with piperidine (a classic system for the study of toughening mechanisms), and (b) DGEBA cured with a stoichiometric amount of 4,4′-diamine-3,3′-dimethyldicyclohexylmethane (3DCM) (an epoxy formulation with poor toughenability). Both epoxies were modified with 15 wt% CTBN (carboxyl-terminated butadiene-acrylonitrile copolymer), that phase-separated in the course of polymerization.

Toughening mechanisms were analyzed by observing the damage zone around the surviving crack tip produced by the double-notch four-point-bend (DN-4PB) technique [4], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Both transmission optical microscopy (TOM) and transmission electron microscopy (TEM) were used. The mechanical behavior of both unmodified epoxy matrices was analyzed in uniaxial compression. This is a useful test for studying yielding and strain softening since, because the stress is compressive, tensile fracture is suppressed and plastic yielding can be obtained for materials that under other conditions exhibit a brittle behavior.

Section snippets

Materials

Two different epoxy matrices based on diglycidylether of bisphenol A (DGEBA, MY790, Ciba) were investigated. The first one was cured with piperidine (5 g per 100 g of DGEBA), during 16 h at 120 °C. The second one was cured with a stoichiometric amount of 4,4′-diamino-3,3′-dimethyldicyclohexylmethane (3DCM, Laromin C260, BASF), using the following thermal cycle: 1 h at 50 °C, 2 h at 100 °C and 2 h at 150 °C. Fig. 1 shows the chemical structures of the corresponding epoxy networks. The DGEBA-piperidine

Generated morphologies

For both rubber-modified epoxies, phase separation of a rubbery phase was produced in the course of polymerization. Fig. 3(a) and (b) shows TEM micrographs of the generated morphologies. The average size of rubber particles present in the DGEBA-3DCM system (0.17 μm) is lower than the corresponding average size for the DGEBA-piperidine system (0.77 μm). However, in the latter case a bimodal distribution seems to be present, with a large fraction of very small particles.

Thermal and mechanical characterization

Thermal and mechanical

Conclusions

The mere increase of K_IC by rubber-modification should not be used, as is frequently found in the literature, as a criterion for assessing its efficiency on the toughening of an epoxy formulation. The stress concentration effect produced by dispersed particles and the decrease in the glass transition temperature produced by the fraction of rubber remaining dissolved in the matrix, lead to a decrease in σ_Y and a corresponding increase in the critical stress intensity factor, K_IC, originated by

Acknowledgements

The authors are grateful to Drs Carina Cano and Marı́a J. Yañez for carrying out the electronic microscopy experiments.

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Analysis of the damage zone around the crack tip for two rubber-modified epoxy matrices exhibiting different toughenability

Abstract

Introduction

Section snippets

Materials

Generated morphologies

Thermal and mechanical characterization

Conclusions

Acknowledgements

Polymer

Polymer

Polymer

Thermosetting polymers

J Mater Sci

J Mater Sci

J Mater Sci

J Mater Sci

Polymer

J Mater Sci