Effect of Grit-Blasting on the Surface Energy of Graphite/Epoxy Composites

Abstract

ABSTRACT Contact-angle measurements were used to determine the surface energies of graphite/epoxy composites before and after grit-blasting with 80- and 220-grit garnet particles. Two different composite systems cured at 350°F were considered but they behaved similarly. Contact angles made by a series of liquids, including water, ethylene glycol, glycerol, formamide, and methylene iodide on as-tooled and grit-blasted composite panels were measured using a contact-angle goniometer. The contact angles were used to determine the dispersive and polar components of the surface energy . However, instead of using the contact angles made by two liquids to determine the two components of the surface energy, we used the contact angles made by several liquids and a graphical technique to determine and to improve the accuracy of the measurements. It was found that the surface energies of as-tooled composites were approximately 35 mJ/m2 and were mostly dispersive; the polar components were small. After grit-blasting with 80- or 220-grit garnet, the dispersive component of the surface energy decreased somewhat whereas the polar component increased significantly; the total surface energy after grit-blasting with 80- or 220-grit garnet was approximately 50 mJ/m2. Etching composites in oxygen plasmas had a similar effect on the dispersive and polar components of the surface energy and on the total surface energy. Grit-blasting with 220-grit alumina resulted in higher dispersive components of surface energy and lower polar components than grit-blasting with 220-grit garnet even though the total surface energy was similar after grit-blasting with 220-grit garnet and alumina. Correcting the measured surface energies for the effects of roughness resulted in small decreases in the total surface energy of the grit-blasted composites. However, even after correcting for roughness, the surface energies of the grit-blasted composites were still significantly greater than those of the as-tooled composites, indicating that grit-blasting resulted in changes in surface chemistry as well as changes in surface morphology of the composites. These changes in surface chemistry certainly included the removal of mold release agents from the as-tooled composites but probably also involved the creation of free radicals and their reaction with atmospheric oxygen to introduce oxygen-containing functional groups on the surfaces of the composites.