PSI - Issue 37

J.M. Parente et al. / Procedia Structural Integrity 37 (2022) 820–825 J.M. Parente/ Structural Integrity Procedia 00 (2019) 000 – 000

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graphene appears as one of the most promising due to its excellent mechanical and electrical properties (Dimiev and Eigler (2013)). Graphene basically is a single layer of carbon atoms with sp 2 hybridization in a honeycomb structure in a lattice configuration (Parente et al. (2019)). This configuration enables the graphene to be functionalized. It´s a material created from the exfoliation of graphite, existing in natural form as a powder which can be mixed with the matrix of the desired material or deposited in its surface. Graphene can be divided into three groups graphene nanosheets, graphene oxide and graphene nanoplatelets. Graphene nanosheets, for example, is the basic unit of graphene consisting of single layer of carbon forming a sheet like surface (Kuilla et al. (2010)). Similar to graphene nanosheets, by adding oxide groups, a better graphene/matrix interaction is obtained, acting, in this context, as anchor points in the creation of functionalized graphene (Yu et al. (2016)). Finally, graphene nanoplatelets consist of several layers of graphene connected by each other through Van de Waals forces possessing the larger overall size of the three types of graphene (Anwar et al. (2016)). The development of epoxy resin-based materials with added graphene for use in the most diverse industrial fields (especially in railways, aircraft or process industries) requires an understanding of how their interaction is made during the manufacturing process as well as their mechanical properties are affected by the presence of graphene. Therefore, the main goal of this work is to analyze the properties of an epoxy resin enhanced with graphene, in order to improve its mechanical properties. Special focus will be given to the effect of suspension viscosity on the mechanical properties. 2. Material and experimental procedure A SR 8100 epoxy resin and a SD 8100 hardener, both supplied by Sicomin, were used in conjunction with graphene nanoplatelets (supplied by Graphenest) to produce nanocomposites. Viscosity tests were performed in a Haake RS150 rheometer with a conical plate device (C35/2Ti) and the tests only contemplated the resin in order to the polymerization would not affect the measurement. The resin exhibits Newtonian behaviour at the shear rate between 0 and 100 s -1 , and the values reported correspond to the stabilized viscosity. To measure the contact angle of graphene with the resin, the nano-reinforcements were previously compressed in order to form wafers with a diameter of 1 cm. Using the sessile methodology, 5 µl of epoxy resin were dropped onto the surface of the graphene wafer and the contact angle variation was recorded for 8 sec, using the DataPhysics OCAH 200 system. Finally, to study the effect of the mixing conditions between resin and hardener on the polymerization rate, the system temperature was monitored using a temperature sensor inserted directly into the reaction medium. The shrinkage values were obtained using cylindrical specimens with a height of 37 mm and diameter of 25 mm. The system was poured into the moulds until complete filling (up to the top) and, after the curing process, the height of the central part of the sample was measured and compared with the height of the mould. To assess the effect of hardener mixing time, resin and hardener were mixed for a certain time (0.5, 2.5 and 5 min) in a mechanical mixer at 300 rpm. After mixing, the system was placed in a vacuum chamber to remove air bubbles. Subsequently, the mixture was poured into a mould with 120×80×3 mm 3 and, after the curing process, specimens of these plates were obtained to be tested with dimensions of 100×10×3 mm 3 . Regarding the study of the effect of pre cure temperature (7, 25, 30 and 45 ºC) and graphene content (0, 0.25, 0.5 and 1.0 wt.%), the nanoparticles were mixed to the epoxy resin for three hours using a mechanical mixer at 1000 rpm and ultrasound bath. After this process, the hardener was added to the graphene/resin mixture using the same mechanical mixer but now at 300 rpm. Afterwards, the system was placed in a vacuum chamber to remove any air bubbles and, finally, poured into moulds similar to those described above (120×80×3 mm 3 ). Pre-cure was performed at different temperatures (7, 25, 30 and 45 ºC) for 24 hours and the final cure was carried out at 40ºC for 24h. One more time, specimens of these plates were obtained to be tested with dimensions of 100×10×3 mm 3 . Three-point bending (3PB) static tests were performed according to the ASTM 768 standard. A Shimadzu universal testing machine, model Autograph AG-X, equipped with a load cell of 10 kN was used and, for each condition, 5 specimens were tested at room temperature and at a rate of 2 mm/min. The bending strength was calculated as the nominal stress at middle span section obtained using maximum value of the load, while the stiffness modulus was obtained by linear regression of the load-displacement curves considering the interval in the linear segment with a correlation factor greater than 95%.