The use of Nuclear Magnetic Resonance to characterize and evaluate rubber degradation

Abstract

Nuclear Magnetic Resonance (NMR) is a characterization technique in which the (non-zero spin) nuclei of the constituent elements in the material can be excited to resonance in an applied magnetic field around a frequency specific to each NMR active nucleus. The power of this technique stems from its ability to distinguish between like atoms in magnetically/chemically unique environments. For 1H NMR in natural and synthetic rubbers designed for industrial use, the transverse (or spin-spin) relaxation time, T2, of the resonance can be used to determine the degree of sulfur (or other) cross-linking between the polymer chains. In this work, NMR is used to monitor the degradation of natural rubber due to oxidation and/or ozonolysis. This degradation causes bond cleavage and increases the cross-link density within the material. These processes lead to a rubber becoming brittle, to the point where it can no longer withstand the operating conditions. These structural/chemical changes in a degraded rubber liner used in service are characterized by 13C NMR spectroscopy. Although the work here is laboratory based and has been performed on relatively sophisticated NMR equipment, another less accurate, but potentially more useful NMR method is also being investigated for in-service, non-destructive testing. For this work low powered NMR equipment was used to follow the changes in 1H T2 as a function of degradation time in rubber samples placed aged at 100 °C. This latter technique measures the average (over all similar nuclei) relaxation times, and hence loses its specificity. However our work has shown that even this simpler method is sensitive to structural changes in the rubber. As rubber degrades it becomes stiffer and this overall bond relaxation time decreases. This type of equipment is simple enough to be made portable to be used in the field. This monitoring of degradation could then lead to using NMR to determine the performance of a part in service. The residual life could be estimated such that replacement could be scheduled before failure is likely.