Mechanism of peroxide cure of EPDM rubber

Mechanism of peroxide cure of EPDM rubber
Martin van Duin¹, Ramona Orza², Ron Peters³ and Victor Chechik⁴

1. DSM Elastomers Global R&D,
   PO Box 1130
   , 6160BC Geleen, The Netherlands
   
2. Technical University of Eindhoven, The Netherlands
   
3. DSM Research, PO Box 18, 6160 MD Geleen, The Netherlands
   
4. University of York, United Kingdom
   

Ter-polymerisation of ethylene, propylene and a diene monomer yields EPDM rubber with a saturated polymer backbone and residual unsaturation in the side group. As a result, EPDM has a superior resistance against oxygen, ozone, heat and irradiation over polydiene rubbers, such as NR, BR and SBR. EPDM is mainly cross-linked with sulfur or peroxide, yielding rubber networks with thermo-labile S-S or thermo-stable C-C bonds, respectively. Peroxide cure has the obvious advantage of allowing full exploitation of the excellent heat resistance of EPDM.
Recently, several studies have been performed to obtain further insight in the chemical mechanism of peroxide cure of EPDM. First, a study on peroxide cross-linking of low-molecular-weight models for EPDM has been performed. GC-MS allowed the identification and quantification of a wide variety of “cross-linked” products. The precise structure of the various products could be obtained by careful interpretation of the MS data, distinguishing products formed via combination and addition reaction and enabling identification of the original free-radical species responsible for the cross-linked species. Secondly, a solid state ¹³C NMR study has been performed on peroxide-cured ¹³C-labeled EPDM. ¹³C labeling results in a dramatic increase in sensitivity when studying the new structures formed during peroxide cure, such as the actual cross-link structures again formed via combination and addition reactions, but also side products such as formed via oxidation. Finally, an EPR study has been performed on the decomposition of peroxide in EPDMs with various diene monomers. Free-radical species were only identified at the later stages of cross-linking and even when according to rheology cross-linking was completed. Two free-radical species were identified, both with an allylic structure. A mobile species is formed via H-abstraction from un-reacted diene along the polymer chain, whereas an immobile species is formed from unsaturation in cross-links.
Based on these studies an expanded chemical mechanism for the cross-linking of EPDM is proposed. In agreement with the commonly accepted scheme for peroxide cure, a distinction has to be made between cross-links formed via combination of two free radicals and via addition of a free radical to an EPDM unsaturation. However, there is now conclusive evidence that the free radicals participating in these reactions do not only originate from the polymeric backbone, but also from the residual diene unsaturation. Finally, this mechanism explains in a straightforward fashion the high reactivity of EPDM with 5-vinyl-2-norbornene (VNB) towards peroxide cure against EPDMs with either 5-ethylidene-2-norbornene or dicyclopentadiene as diene monomers. Steric hindrance is the decisive factor, favouring VNB-EPDM with its terminal unsaturation.