A NMR Investigation of Filler Effects in EPDM

The Hahn Echo (HE) Nuclear Magnetic Relaxation (NMR) relaxation method is regularly used to evaluate the total crosslinking level or state of cure in thermoset rubbers.  Nonetheless, it is known that the relaxation signal is influenced by other ingredients in the compound recipe, namely plasticizer and reinforcing fillers.  In this investigation, a series of compounds based on peroxide cured EPDM will be mixed with varying amounts of plasticizer and carbon black.  In both cases, the dynamic properties (storage modulus and tan δ) will be measured for the unvulcanized compounds according to ASTM D6204.  Vulcanized compounds will be characterized by stress-strain and hardness measurements.  All compounds were subjected to analysis by HE NMR.  It was found the degree of plasticization has a direct correlation with the average molar mass between chain entanglements (M_e) calculated from the short decay constant T₂₁. M_e correlates very well with the dynamic properties in the unvulcanized state and plasticizer addition did not affect the level of chemical crosslinking.  In the carbon black study, an increase in its concentration brought about a decrease in M_e.  The lowering of the M_e value is likely due to increased interactions between the filler and the polymer chain.  A new parameter M_chain-filler is introduced to estimate the average molar mass between polymer chains and fillers.  Compared to the chain entanglement density, the overall magnitude of this interaction appears to be weak in the mobile zone of the compound matrix.   Upon vulcanization, the filler’s surface chemistry adversely affected the peroxide vulcanization.  As in the case of plasticization, a relatively good correlation is obtained between M_e and dynamical properties in the unvulcanized state.  Compression set resistance is shown to directly follow both M_c and M_c after aging.  The carbon black study results can be understood within the context of a morphological model containing different zones of chain mobility – a thin shell of immobilized chains, an intermediate zone of limited mobility and a dominant mobile phase consisting mainly of entangled and crosslinked polymer chains.