1. Introduction Carbon fiber-reinforced composites that are widely used in aerospace, automotive, and other industries are growing with an exponential rate [1–3]. Carbon fiber has important advantages because of its high strength, specific fatigue behavior, and low density [4, 5]. The interlaminar properties are key factors for the high performance of carbon fiber composites [6, 7]. The interaction between the fiber and neat epoxy is always a problem for many composite systems, including other fibers. The fiber/matrix interaction can be improved by using physical and chemical treat- ments to achieve enhanced interfacial and interlam- inar properties. One of these treatments is to modify the matrix material used in the manufacturing of the composite [8]. Nanoparticles are one of the most promising solutions to modify the matrix material. Carbon nanomaterials have been used to improve the mechanical and thermal properties of composites. In this context, Garcia et al. [9] used carbon nanotubes (CNT) to improve the fracture toughness of carbon fiber epoxy composite. The bridge effect helped to connect the laminates better and stronger. Further- more, it was stated that the study could be improved with surface modification of carbon nanotubes. Sim- ilarly, the effect of CNT on the carbon fiber epoxy composites for mechanical improvement was stud- ied by Yokozeki et al. [10] It was observed that the mechanical properties were improved to a certain ex- tent. Therefore, CNT materials were successfully 1106 Graphene oxide modified carbon fiber prepregs: A mechanical comparison of the effects of oxidation methods V. Acar 1 , S. Erden 2 , M. Sarıkanat 2 , Y. Seki 3 , H. Akbulut 1 , M. Ö. Seydibeyoğlu 4* 1 Ataturk University, Department of Mechanical Engineering, Erzurum, Turkey 2 Ege University, Department of Mechanical Engineering, Izmir, Turkey 3 Dokuz Eylul University, Department of Chemistry, Izmir, Turkey 4 Izmir Katip Celebi University, Department of Materials Science and Engineering, Izmir, Turkey Received 25 March 2020; accepted in revised form 15 May 2020 Abstract. The objective of this study is to understand the significance of graphene oxide type for the improvement of me- chanical properties of carbon fiber reinforced epoxy composites. Modified epoxy systems were prepared by adding two dif- ferent graphene oxide nanoparticles prepared by the modified Hummers method (GOH) and Laachachi method (GOL). In the second stage, carbon fiber reinforced epoxy composites were produced using prepreg manufacturing technique. X-ray Photoelectron Spectroscopy characterizations of the pure graphene and modified graphene nanoparticles (GOH and GOL) were performed to confirm the oxidation of the nanoparticles. The morphology of the nanocomposite epoxy resin was in- vestigated with scanning electron microscopy. Mechanical tests were conducted with composites to determine the properties of the final materials. It was observed that the addition of the GOH improved the longitudinal tensile and flexural strength values by 41 and 33%, respectively. Interlaminar shear strength (ILSS) values of epoxy/carbon fiber significantly increased up to 58% with GOH addition. Moreover, the results show that nano-enhanced resins could be a key component for many composites, and it can be a suitable material for long term usage and resistance of sudden failure of composites. Keywords: nanocomposites, graphene oxide, hummers method, prepreg, mechanical properties eXPRESS Polymer Letters Vol.14, No.12 (2020) 1106–1115 Available online at www.expresspolymlett.com https://doi.org/10.3144/expresspolymlett.2020.90 * Corresponding author, e-mail: seydibey@gmail.com © BME-PT