Low Curing Temperature CTBN-Toughened Epoxy Adhesive
A carboxyl-terminated butadiene-acrylonitrile copolymer (CTBN)-toughened epoxy adhesive cured with 2-ethyl-4-methyl-imidazole (EMI) is described. In the presence of EMI the reaction of carboxyl end-group of the liquid rubber with epoxy group is very rapid, it is practically completed before the appearance of phase separation or gelation. At low CTBN concentrations thie in situ formed domain are discrete globules, which toughen the resin matrix. Aggregation of the dispersed phase begins at the CTBN concentration 30 parts per hundred parts resin (phr); at much higher CTBN concentrations the rubber phase becomes continuous. The EMI concentration is a critical factor for the toughening of the adhesive. A room-temperature lap shear strength maximum of adhesive joint was found at CTBN concentration in the range of 30 to 40 phr. The epoxy adhesive KH-225, toughened with CTBN, has superior fatigue strength to those modified with a nonfunctional liquid butadiene-acrylonitrile copolymer or a liquid statistical copolymer of butadiene-acrylonitrile-acrylic acid.
KeywordsFatigue Acetone Titration Epoxy Rubber
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