Controlled Interfaces in Low‐temperature‐cured Phenolic Composites

Abstract

The surface chemistry and interfacial adhesion of silanetreated E-glass fibers and low-power, oxygen or water plasma-treated ultrahigh-modulus polyethylene (UHMPE) fibers with acid-catalyzed, low-temperature-cured phenolic resins have been measured by X-ray photoelectron spectroscopy, (XPS) and static secondary ion mass spectrometry (SIMS), fibers bundle pullout and flexural and interlaminar shear strength properties of unidirectional composites. The most effective treatment for E-glass involved an epoxy silane rather than an amino silane owing to the protonation by the acid catalyst of the amine as well as the loss of reactive sites by the formation of carbon dioxide adducts stable at the conditions of cure. Scanning electron microscope examination of the cured phenolic resin showed phase-separated water domains; the diameter of these domains decreased towards the fiber surface in a choppedstrand mat composite owing to the lowering of the surface tension produced by the dissolution of the binder by the phenolic resin. Both water and oxygen-plasma treatments of UHMPE increased the fiber bundle pullout force by the same amount and this was attributed to direct chemical bonding to hydroxyl (and possibly epoxy) groups detected by XPS, SSIMS and derivatization. The best composite properties were obtained when oxygen plasma-treated UHMPE was used and this was attributed to the slower restructuring and higher total oxygen content enabling efficient wet-out of the fibers by the resin.