Solid-State ¹³C NMR Study of Accelerated-Sulfur-Vulcanized ¹³C-Labeled ENB−EPDM

Abstract

Accelerated-sulfur-vulcanized 13 C-labeled EPDM with and without carbon black and extender oil was analyzed using 13 C solid-state NMR to determine the chemical structure of the network. Highresolution solid-state 13 C NMR reveals that sulfur cross-linking takes place at the allylic positions of the ENB independent of the presence of carbon black and oil. From the integrated intensities of the 13 C signals, the conversion of ENB into a cross-link can be quantitatively determined during the vulcanization process. The ENB conversion for gumstock EPDM is ∼10% after 10 min of vulcanization at 150 °C, which is a typical optimum vulcanization time in commercial applications. In the presence of carbon black the ENB conversion is marginally faster and reaches ∼12% in 10 min, while a maximum conversion of ∼20% was obtained. The efficiency of the ENB conversion was ∼20% less at 150 °C and ∼30% less at 180 °C in the compound with carbon black and oil compared to the compound without carbon black and oil. The substitution at the 9-position of ENB is always preferred over each of the two 3-positions. In turn, the substitution on the 3-exo position is always preferred over the 3-endo position, which is different from earlier model studies. When the material is heated for extended periods (10-20 min), oxidation and reversion of the cross-links starts to occur. Reversion is enhanced upon a temperature increase to 180 °C and yields a 4,5,6,7-tetrahydro-4,7-methanobenzo[b]thiophene compound. The length of the sulfur bridge in compound A and B is rather short, i.e., 1 or 2.

Sulfuration and reversion reactions of brominated poly(isobutylene‐co‐isoprene)

“…2(d)]. Analogous thiophene oxidation products were reported by Hahn et al11 for 2,3‐dimethyl‐2‐butene vulcanizates and by Winters et al13 for sulfur‐based cures of ethylene–propylene–diene monomer (EPDM). The possibility that thiophene could originate from the reaction of a conjugated diene ( 2 ) with sulfur14 was eliminated when these reagents were heated to 140 °C for 20 min without 5 or its cyclic precursors being produced.…”

“…The prolonged heating of 2,3‐dimethyl‐2‐butene‐based disulfides to 150 °C is reported to yield thiophene in addition to 2,3‐dimethyl‐2‐butene and H 2 S 11. Similarly, Winters et al13 characterized a thiophene reversion product of ethylidenenorbornene‐derived allylic sulfides found in EPDM systems. Although the mechanism of allylic sulfide oxidation is not known, the dehydrocyclization of polysulfides at elevated temperatures has been well established 32.…”

Influence of the diene monomer on devulcanization of EPDM rubber

“…Solid‐state 13 C NMR of 13 C‐labeled ENB‐EPDM, which was sulfur vulcanized for prolonged times (120 min at 150°C or for 30 min. at 180°C) has shown that decrosslinking occurs and a cyclic thiophene species (Scheme ) is formed 37. This suggests that the scission of the CS bond of one alkenylsulfide unit of the sulfur crosslink occurs and that a new CS bond is formed at the other allylic position of the second alkenylsulfide unit.…”

Simple kinetic numerical model based on rheometer data for ethylene–propylene–diene monomer accelerated sulfur crosslinking

“…In particular, because of the double bond outside the macromolecular chain of the backbone, a very high stability, both at high temperatures and in the presence of oxidizing agents, is expected. The formation of carbonyls at C‐5 and/or C‐8 of ENB–EPDM due to oxidation was shown to be linked to the accelerated sulfur vulcanization of EPDM 22. To summarize, with all of the vulcanization issues previously pointed out taken into consideration, the basic reaction scheme represented in Figure 1 can be adopted.…”