ROOM-TEMPERATURE CURING OF CTBN-TOUGHENED EPOXY ADHESIVE WITH ELEVATED-TEMPERATURE SERVICE CAPABILITY

The adhesive joint strengths at room temperature (25°C) and at 120°C for an epoxy resin cured at room temperature with tris (N,N‐dimethylaminomethyl) phenol and containing varying amounts of a carboxyl‐terminated polybutadiene‐acrylonitrile copolymer (CTBN) were investigated and reported. It was observed that as CTBN content increased, lap shear strength and T‐peel strength increased significantly and passed through a maximum at about 10 parts CTBN content per 100 parts epoxy resin and then decreased. To explain the observed results, the effect of CTBN content on the bulk tensile strength, impact energy, glass transition temperature (ETg), and morphology were also evaluated. SEM observations showed microphase separation between CTBN and epoxy matrix for the composition containing 10 parts CTBN per 100 parts epoxy resin, and this composition exhibited maximum bulk tensile strength and impact energy. The enhancement of bulk mechanical properties, hence the adhesive joint strength, was attributed to the higher toughness produced by the dispersed rubber particles. At higher CTBN content, the rubber phase became continuous, and the system exhibited a fall in mechanical properties, hence in adhesive joint strength, which was attributed to the flexibilization effect. The glass transition temperature of the cured epoxy resin was little influenced by CTBN incorporation up to 10 parts per 100 epoxy resin. Higher levels of CTBN content due to the flexibilization effect, decreased the ETg, hence the elevated temperature capability. The results of this study demonstrate that epoxy resins containing low levels of CTBN, cured at room temperature with tris (N,N‐dimethylaminomethyl) phenol can be used to formulate structural adhesives for use in elevated temperature environments. Copyright © 1990 John Wiley & Sons, Inc.