Avaliação do uso de borracha NBR pré-reticulada em composições de O-rings: processabilidade e desempenho mecânico

Abstract

This work aims to study the use of NBR N8 (pre-crosslinked) in formulations for NBR O-rings with a Shore A hardness of 60-70, already used in the industry, seeking improvements in processability and mechanical performance. The experimental design consisted of a single factor of study, which is the amount of pre-crosslinked rubber to be added to the formulation, replacing NBR N608. The experimental coding used was XX/YY, where XX and YY represent the amounts in phr of the non-crosslinked and pre-crosslinked rubbers, respectively. The raw NBR rubbers N608, N8, and N615 were characterized using Thermogravimetric Analysis (TGA), Fourier Transform Infrared Spectroscopy in Attenuated Total Reflection mode (FTIR-ATR), and Mooney Viscosity. No significant differences were observed in the TGA and FTIR-ATR results of the rubbers studied. NBR N608 and NBR N8 showed higher initial viscosity and Mooney viscosity values than NBR N615, with NBR N8 exhibiting a more elastic behavior due to the presence of crosslinking. The elastomeric compositions were prepared in an open two-roll mixer as described in ASTM D-3187. An increase in the Mooney viscosity of the NBR/carbon black compositions in the non-vulcanized state was observed due to the contribution of the carbon black, which causes a filler network formation, characterized as the Payne Effect. The formulations with NBR N8 showed no surface imperfections on the extrudate and had a lower die swell value. In the study of crosslink density (CLD), no changes were observed with the increase in NBR N8 content. For the mechanical properties, a slight trend of reduction in tensile strength and elongation at break was observed with the replacement of NBR N608 by NBR N8. In the Shore A hardness test, results were very similar across all formulations, as expected due to the specific formulation used. In the stress relaxation test, the compositions projected a normalized retained stress (σ∞/σ0) of 77% of the original clamp value, regardless of the type of rubber present in the formulation. The compression set results showed an excellent value; however, the formulation results were quite similar, indicating no contribution from the NBR N8 content. Although the mechanical properties were similar, the rheological and rheometric properties were significantly influenced by the NBR N8 content.