Investigation in the state of cure of elastomers through nanoindentation

A novel nanoindentation procedure was developed for measuring the mechanical properties of elastomers as they exhibit surface tackiness. A small dynamic load was superimposed to a larger static deformation after a dwell time for allowing stress relaxation. The outcomes of measurements on a fully crosslinked filled acrylonitrile butadiene rubber (NBR) were analyzed to verify the reliability of this procedure. This method was then employed to investigate the mechanical properties of the same elastomer cured at different temperatures and times. As expected, a change from viscoplastic to viscoelastic behavior due to the formation of chemical crosslinks over curing time was found, and mechanical properties plateaus were reached more rapidly at higher manufacturing temperatures. The results were then compared with compression set data: similar trends were detected for the loss factor and the compression set, both decreasing down to a plateau level. Overall, the proposed method demonstrated the ability to recognize undercured regions in elastomer samples - particularly through the determination of the loss factor (tanδ) - and may represent a new tool for improving quality control for the rubber industry.