Adhesion-Promoting Properties of Plasma Polymer Layers in Epoxy Resin for Glass Fiber-Reinforced Plastics

Glass fiber-reinforced plastics (GFRPs) rely on strong fiber–matrix adhesion to achieve optimal mechanical performance. However, conventional sizing systems provide only partial surface coverage and may limit interfacial bonding. The current study investigates atmospheric-pressure plasma-enhanced chemical vapor deposition (APECVD) as an alternative approach for depositing adhesion-promoting plasma coatings on glass fibers intended for epoxy-based composites. Organosilane precursors were deposited using an atmospheric plasma jet under different operating conditions, and the chemical composition and the morphology of the resulting plasma coatings were extensively characterized. Adhesion performance was evaluated by pull-off tests on glass sheets and pull-out tests on glass fibers embedded in epoxy resin. Results show that the plasma coatings provide complete surface coverage and present a tunable chemistry containing several adhesion-promoting functional groups, including silanol and amine-containing species. Pull-off tests on glass sheets demonstrate adhesion improvements of at least 68% compared with untreated glass. In pull-out tests, plasma-coated glass fibers achieved interfacial shear strengths up to 102.9% higher than untreated fibers and 24.4% higher than an industrial reference material. These results demonstrate how atmospheric plasma polymerization can produce effective adhesion-promoting coatings on glass surfaces, thus offering a promising and industrially scalable alternative to conventional adhesion promoters for epoxy-based GFRPs.