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6: Low-molecular-weight model study of peroxide cross-linking of EPDM rubber using gas chromatography–mass spectrometry : addition and combination
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Characterisation of polymeric network structures
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Low-molecular-weight model study of
peroxide cross-linking of EPDM rubber
using gas chromatography and mass
Addition and combination
The dicumyl-peroxide-initiated addition and combination reactions of mixtures of alkanes (n-octane, n-decane) and alkenes (5,6-dihydrodicyclopentadiene (DCPDH), 5-ethylidene-2-norbornane (ENBH) and 5-vinylidene-2-norbornane (VNBH)) were studied to mimic the peroxide cross-linking reactions of terpolymerised ethylene, propylene and a diene monomer (EPDM). The reaction products of the mixtures were separated by both gas chromatography (GC) and comprehensive two-dimensional gas chromatography (GC×GC). The separated compounds were identified from their mass spectra and their GC and GC×GC elution pattern. Quantification of the various alkyl/alkyl, alkyl/allyl and allyl/allyl combination products show that allylic-radicals comprise approximately 60% of the substrate radicals formed. The total concentrations of the products formed by combination are found to be independent of the concentration and the type of alkene. In addition, the total concentration of the formed addition products depends strongly on the type of the alkene used, viz. VNBH > ENBH ≈ DCPDH, which is a consequence of differences in steric hindrance of the unsaturation. The peroxide curing efficiency, defined as the number of moles of cross-linked products formed per mol of peroxide, is 173% using 9% (w/w) VNBH. This indicates that the addition reaction is recurrent. In addition, the present results provide more-detailed structural information. The described approach to use low-molecular-weight model compounds has proven to be a very powerful tool to study the cross-linking of EPDM.
R. Peters, M. van Duin, D. Tonoli, G. Kwakkenbos, Y. Mengerink, R. van Benthem, C.G. de Koster, P. Schoenmakers, Sj. van der Wal, Journal of Chromatography A, 1201 (2008) 151160.
EPDM rubber is an elastomer, which is terpolymerised from ethylene, propylene and a diene monomer. Due to its excellent resistance to heat, ozone, oxygen and water, it can be used for a wide range of outdoor and demanding applications . It dominates the market for non-tyre rubber applications (e.g. window or door sealing). EPM (a copolymer of ethylene and propylene) can be cross-linked with peroxides, but the introduction of a diene ter-monomer strongly improves the cross-linking efficiency. The two dienes commonly used in commercial EPDM are dicyclopentadiene (DCPD) and 5-ethylidene-2-norbornene (ENB) (Fig. 6.1) . Recently a new advanced catalyst technology was developed, which enables the incorporation of up to 3% (w/w) of 5-vinylidene-2-norbornene (VNB) (Fig. 6.1) into EPDM, without the occurrence of polymer-reactor fouling or excessive polymer branching . The concentration and the type of the diene significantly influence the peroxide curing efficiency. It has been shown that the curing efficiency is governed by steric effects . VNB is the most...
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