PSI - Issue 61

J.M. Parente et al. / Procedia Structural Integrity 61 (2024) 285–290 J.M. Parente et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The use of nanocomposite materials has been increasing significantly in the most diverse industrial applications due to their ability to offer improved mechanical performance without increasing the weight of the structure. Carbon based nanoparticles are the ones that have received the most attention among all those used to improve the mechanical properties of composite materials. In this context, carbon nanofibers (CNFs) seem to be the most promising due to their unique properties (Al-Saleh et al. (2011), Feng et al. (2014)). Although their composition is similar to many others, carbon atoms are arranged in stacked conical (bamboo-type) or cylindrical (herringbone) configurations that extend over several microns. Therefore, these structures provide the ability to improve the mechanical properties of the nanocomposite to which they are added. For example, literature reports significant benefits in terms of bending properties, microhardness, and viscoelastic response when CNFs are added to the epoxy resin (Bal (2010), Santos et al. (2023)). Furthermore, Poveda et al. (2012) noted that using 10 wt.% of CNFs the thermal expansion coefficient can decrease by about 12%. Shokrieh et al. (2013) studied the effect of CNFs content and found that 0.25 wt.% promoted the highest tensile and flexural strengths, with values 23% and 10% higher than those obtained for neat epoxy resin, respectively. On the other hand, in a similar study, Sun et al. (2011) found improvements in tensile strength and Young's modulus of about 8% and 17%, respectively, for 1.0 wt.% CNFs. These different contents can be explained by different polarities or by the effect of CNFs on the curing processes (curing reaction conditions, degree of cure, or crosslinking network) (Santos et al. (2021)). However, more than evaluating the benefits achieved with nanoparticles in terms of mechanical performance, it is necessary to understand how the interaction between them and the resin occurs during the manufacturing process and, consequently, how the mechanical properties are affected (Parente et al. (2020), Schmidt et al. (2003)). So, the objective of this study is to study the properties of different epoxy resins nanoreinforced with carbon nanofibers with the aim of improving their mechanical properties, with a focus on the effect of suspension viscosity on the mechanical properties. This subject is very important because epoxy resin-based materials are widely used in the most diverse industrial fields, i.e., railway, automotive, aeronautics, and process industries. 2. Materials and methods AH 150 resin and an IP430 from Ebalta hardener as well as an SR 8100 epoxy resin and an SD 8824 hardener from Sicomin, were used for this work. the carbon nanofibers were supplied by Merck. Viscosity tests were performed using a Haake RS150 rheometer with a conical plate device (C35/2Ti). The resin exhibited Newtonian behavior at shear rates between 0 and 100 s -1 , and the reported values corresponded to the stabilized viscosity and the tests were limited to the resin to avoid polymerization affecting the measurements. To determine the contact angle between carbon nanofibers and resins, the carbon nanofibers were compressed to create wafers with 1 cm diameter. For this test the sessile method was used, with 5 µl of epoxy resin droplet dripped to the surface of the carbon nanofiber wafer, and the variation of the contact angle was recorded for 5 seconds using the dataphysics OCAH 200 system. Shrinkage values were obtained using cylindrical tubes with 37 mm in height and 25 mm of diameter. Finally, the epoxy/CNF mixture was transferred into the tubular molds until filled, and after the cure process, the height of the central part of the sample was measured and compared with the height of the mold. For this test only Sicomin resin was used because the behavior is similar independently of the resin used The cure temperature, 0.75 wt.% of carbon nanofibers were mixed with the epoxy resin for three hours using a at 1000 rpm with a mechanical mixer assisted by an ultrasound bath. The carbon nanofiber content is the ideal value for this type of resin according the study developed by Santos et al. (2023). Subsequently, the hardener was added to the carbon nanofiber/resin mixture and mixed at 300 rpm. Finally, the samples were put in a vacuum chamber to remove air bubbles and put into molds with a dimension of 120×80×3 mm³. The cure was carried out at different temperatures: 5, 20, and 40ºC for 48 hours for the Ebalta resin, and 5, 20, and 40ºC for 24 hours for the Sicomin resin. Post-cure was performed at 40ºC for 24 hours for Sicomin and 80ºC for 5 hours for the Ebalta resin following the supplier datasheets specifications. The plates produced (120×80×3 mm³) were cut with an Accutom Struers cutting machine into 100×10×3 mm³ dimension samples for three-point bending (3PB) tests, which were carried out according to the EN ISO 178:2003 standard using a Shimadzu model Autograph AG-X universal testing machine, with a 10 kN load cell. For all tested temperatures, 5 specimens were tested at a rate of 2 mm/min and at room temperature. To calculate