Effect of elastomer modification on the adhesive characteristics of maleimide-functional phenolic resins

The effect of addition of elastomeric modifiers on the adhesive properties like lap shear strength and T-peel strength of an addition curable, maleimi de functional novolac phenolic resin (PMF), self-cured and cocured with a n ovolac epoxy resin, was studied using aluminium adherends. The modifiers us ed were (I) two grades of carboxyl terminated butadiene acrylonitrile copol ymer (CTBN) of different molecular weights, (2) a low molecular weight, epo xidized hydroxyl-terminated polybutadiene, and (3) a high molecular weight acrylate terpolymer containing pendant epoxy functionality. The adhesive pr operties, when examined as a function of the varying concentrations of the additives, ranging from 10 to 30 parts per hundred parts (phr) of the resin , were found to depend on the nature of the matrix being modified as well a s on the nature and concentration of the elastomer. The adhesive properties at ambient temperature of the self-cured, highly brittle PMF resin were dr amatically improved by the inclusion of all the elastomers, the increase be ing substantial in the case of high molecular weight CTBN. For the more rig id, less ductile, epoxy-cured PMF system, the adhesive properties were marg inally improved by the high molecular weight CTBN, whereas the other elasto mers were practically ineffective. For both self-cured and epoxy-cured PMF systems, the inclusion of these elastomers generally decreased the high-tem perature adhesive properties, implying impairment of thermal characteristic s, evidenced also from their dynamic mechanical spectra. The presence of ph ase-separated elastomer particles in the modified systems has been evidence d from scanning electron micrographs. (C) 1999 John Wiley & Sons, Inc.