Rubber compounds contain several additives besides the elastomer matrix, such as fillers, curing agents, activators, antioxidants, among others. However, some of these additives causes a negative impact on the environment, e.g., zinc oxide commonly used as activator in the vulcanization process. Thus, in this study, ecofriendly synthesized magnesium oxide was evaluated as substitute to ZnO in a standard nitrile rubber composition. Additionally, commercial magnesium oxide was also included in study to be used as a reference. All rubber compounds were evaluated in terms of rheometric (minimum and maximum torques, curing times), rheological (dynamic moduli), and mechanical (tear resistance, tensile resistance, hardness, and stress relaxation) properties and crosslink density. The marching-modulus was more noticeable when MgO and green-MgO was employed as activator. The vulcanization of NBR with MgO and green-MgO resulted in rubber materials with higher crosslink density, higher elastic moduli, lower phase angle δ, and lower stress relaxation than ZnO composites. Besides that, no significant difference was observed for the mechanical performance regarding tear strength, tensile strength, and hardness.
This study reports the reinforcement degree investigation of two types of rockwool fibers (F1 and F2), in nitrile rubber composites. The micro-computed tomography (micro-CT) 3D images showed that both fibers were well-dispersed in the NBR matrix, without a preferential orientation. The micro-CT analysis also allowed quantifying volume fraction, inter-fiber distance, and aspect ratio. Those morphometric parameters were used for supporting the composites rheological behavior assessment. Changes in the elastic modulus and phase angle followed the same trend of the inter-fiber distance values, regardless the type of fiber. Both volume fraction and aspect ratio data from the micro-CT analysis were used to predict theoretical values of elastic modulus using the Guth-Gold and modified Guth-Gold equations, and the results obtained were compared to the rheological experimental data. This analysis was helpful to better understand the rockwool fibers reinforcement degree differences in the production of the nitrile rubber composites.
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