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

MARı´A L. ARIAS; PATRICIA M. FRONTINI; ROBERTO J.J. WILLIAMS

POLYMER

ELSEVIER SCI LTD

Lugar: Amsterdam44 (2003) 1537?1546; Año: 2003 p. 1537 - 1546

0032-3861

One inevitable effect of adding soft rubber particles to a rigid polymer is that its yield stress, sY; is lowered by the stress concentrationeffect produced by dispersed particles. This effect may be enhanced if a fraction of the added rubber remains dissolved in the matrix, loweringits glass transition temperature, thus producing an extra decrease in sY: Both effects lead to an increase in the critical stress intensity factor,KIC, originated by crack tip blunting. The toughenability of an epoxy matrix may be defined by its ability to promote deformationmechanisms that can increase KIC beyond the value attained by crack tip blunting. The aim of this manuscript was to analyze thetoughenability of two different epoxy systems exhibiting similar relative increases in KIC by rubber addition: (a) diglycidylether of bisphenolA (DGEBA) cured with piperidine; (b) DGEBA cured with a stoichiometric amount of 4,40-diamine-3,30-dimethyldicyclohexylmethane(3DCM). Both epoxies were modified with 15 wt% CTBN (carboxyl-terminated butadiene-acrylonitrile copolymer), that phase-separated inthe 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. Forthe DGEBA-piperidine system, dilatation bands (croids) and massive shear yielding in the region close to the crack tip, were observed. For theDGEBA-3DCM system, a region of cavitated particles was observed close to the crack tip without any evidence of shear deformation in thematrix. The higher toughenability of the former system compared to the latter, was associated to the different mechanical behavior observedin uniaxial compression tests of the pure epoxy matrices.