NMR‐Based Cross‐Link Densities in EPDM and EPDM/ULDPE Blend Materials and Correlation with Mechanical Properties

Abstract

The role of cross‐linking in dictating the microstructural and mechanical properties in ethylene‐propylene‐diene‐monomer rubber (EPDM) and EPDM/ULDPE blends cross‐linked by different sulfur amounts is investigated by solid‐state 1H time‐domain NMR spectroscopy and tensile‐tests. Analyses of spin‐spin relaxation time (T2), by combining free‐induction decay (FID), magic‐sandwich echo‐FID, and Hahn‐echo experiments demonstrate a reduction in crystal‐amorphous interface regions of pure ultralow‐density polyethylene (ULDPE) upon curative addition. The blends demonstrate a complete loss of these fractions due to curative‐induced plasticization and solvation by polyethylene segments of EPDM. Cross‐link densities, quantified by the magnitude of residual dipolar coupling constant (Dres), arising from topological restrictions to segmental motions, are measured by multiple‐quantum experiments. The entanglement‐dominated EPDMs demonstrate a significant reduction in ultimate tensile properties with increasing Dres. The analogous blends yield similar Dres values up to 0.36 phr of free sulfur. Thereafter, a deviation from the cross‐linking trend of the EPDMs is observed with the blends approaching a cross‐linking limit, thus emphasizing the migration of additives to the amorphous phase of the ULDPE. From the additional contributions of solvation and complex entanglement scenarios in the blends, restoration and even significant enhancement in ultimate tensile strength are achieved. Limitations in applying the popular Mooney–Rivlin analysis are also briefly discussed.

Mechanical Properties of Polyisoprene-Based Elastomer Composites

“…This may thus lead to a high D res tail of an inhomogeneous D res distribution. To accommodate for a small contribution from an immobilized surface layer, a bimodal fitting function is used, wherein the constrained fraction is force-fitted to a coupling value of 10 kHz and corresponding distribution width of 0.5 . These two values are adequately chosen based on the quality of the fit and account for a mobility gradient.…”

“…To accommodate for a small contribution from an immobilized surface layer, a bimodal fitting function is used, wherein the constrained fraction is force-fitted to a coupling value of 10 kHz and corresponding distribution width of 0.5. 69 These two values are adequately chosen based on the quality of the fit and account for a mobility gradient. Since this first component was very small and varied nonsystematically, we will not discuss it further.…”

Flame retardant, mechanical, and hot air aging properties of ethylene‐propylene‐diene monomer/ammonium polyphosphate/nano‐silic composite rubber

“…12 PFRs increase the stability of coke by introducing Patoms, which usually produces more aromatic residual coke and increases the amount of residual carbon with a glassy surface. 13 In addition, PFRs can cause flame suppression by releasing phosphorus-containing molecules into the combustion matrix, which form free radicals acting as radical scavengers in the gas phase. 14 However, PFRs' primary disadvantage is that they also require high loading, which surely leads to deterioration in mechanical properties of the modified rubbers.…”

“…Phosphorus‐based flame retardants (PFRs) are commonly used, highly efficient, and non‐toxic, which have a variety of flame retardant methods 12 . PFRs increase the stability of coke by introducing Patoms, which usually produces more aromatic residual coke and increases the amount of residual carbon with a glassy surface 13 . In addition, PFRs can cause flame suppression by releasing phosphorus‐containing molecules into the combustion matrix, which form free radicals acting as radical scavengers in the gas phase 14 .…”

Effects of artificial weathering in NR/SBR elastomer blends

“…Though quantitative, a complete distinction of phase-specific cross-link densities could not be established for these systems as the components were made of similar dipolar couplings. This method has also been extended to elastomer/plastic blends of EPDM and ultralow-density polyethylene (ULDPE) blends, 30 where a spin-spin relaxation time (T2) filter was used to suppress the signals arising from crystalline and crystalline-amorphous interface fractions of the ULDPE phase and to obtain cross-link density of the EPDM phase.…”