Radiochemical ageing of an amine cured epoxy network. Part I: change of physical properties

An aromatic rich, amine cured epoxy network (initial glass transition temperature 250degreesC), was irradiated in air (pressure 0.22 MPa), at 30 and 120degreesC, by gamma rays with two dose rates 2 and 20 kGy/h, for doses upto 70 MGy. The following characteristics were recorded, thickness of oxidised layer (TOL) from IR microspectrophotometry, flexural strength sigma(R), toughness K-1C and glass transition temperature T-g. sigma(R) decreases from 120 MPa to about 40 MPa in the most degraded samples. This decrease is sharply linked to TOL showing the key role of the oxidised layer in crack initiation. K-1C decreases from 0.7 to 0.55 MPa m(1/2). Data are too much scattered to allow a kinetic study but it appears that, in the early period of exposure, K-1C decreases more rapidly at 120degreesC than at 30degreesC. T-g decreases from 250 to 140degreesC in the most degraded samples, and the decrease is faster at 30degreesC than at 120degreesC. The decrease of T-g is attributed to a predominant chain scission process. The decrease of K-1C can be attributed to a combination of chain scission and physical ageing or chain scission and crosslinking. A relationship between T-g and the number of chain scissions, derived from the Di Marzio's theory, is proposed. (C) 2004 Elsevier Ltd. All rights reserved.