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Mechanism of Peroxide Crosslinking of EPDM Rubber R

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Mechanism of Peroxide Crosslinking of EPDM Rubber R PRÜFEN UND MESSEN TESTING AND MEASURING EPDM · Peroxide · Crosslinking · Structure · Mechanism Mechanism of Peroxide The results of three experimental 1 studies, i.e. GC-MS of peroxide-cross- Crosslinking of EPDM Rubberr linked alkene/alkane mixtures, 13C NMR of peroxide-cured, 13C-labeled ENB- EPDM and EPR of peroxide curing of EPDM, are combined, yielding a more accurate description of the mechanism of peroxide cure of EPDM. Both alkyl and allyl macro-radicals are formed via Ter-polymerisation of ethylene, propylene tion of the peroxide, yielding primary alkoxy H-abstraction from the EPM main chain and a non-conjugated diene yields EPDM (ROx) or secondary alkyl radicals (Rx). Subse- and the residual diene unsaturation, rubber with a saturated polymer backbone quent abstraction of H-atoms from the respectively. Combination of these and residual unsaturation in the side groups EPDM polymer results in the formation of macro-radicals yields alkyl/alkyl, alkyl/ (Fig.1 top). As a result, EPDM has a superior EPDM macro-radicals (EPDMx). Calculations allyl and allyl/allyl crosslinks and resistance against oxygen, ozone, heat and based on kinetic data for H-abstraction in- addition of these macro-radicals to the residual diene unsaturation yields alkyl/ irradiation over polydiene rubbers, such as dicate that H-abstraction mainly occurs alkene and allyl/alkene crosslinks. NR, BR and SBR [1, 2]. This makes EP(D)M the along the saturated EPM polymer backbone elastomer of choice for outdoor and elevat- [11], whereas several electron paramagnet- ed-temperature applications, such as auto- ic resonance (EPR) studies have shown the Mechanismus der motive sealing systems, window gaskets, selective formation of allyl radicals derived Peroxidvernetzung von EPDM roof sheeting, waste water seals, electrical from the diene monomer. The actual cross- cable & wires and V belts. Nowadays, EPDM linking proceeds via two pathways, which EPDM · Peroxid · Vernetzung · Struktur · is the largest non-tire rubber with a world- have been shown to be additive [12]. Two Mechanismus wide consumption of approximately EPDM macro-radicals either combine or, al- Die Ergebnisse von drei experimentel- 1100kton/year. As other elastomers, EPDM ternatively, a macro-radical adds to an len Untersuchungen basierend auf GC- has to be crosslinked to achieve its opti- EPDM unsaturation. Optical spectroscopy MS Analysen von peroxidvernetzten mum performance in terms of elasticity, has confirmed the conversion of the EPDM Alken/Alkan Mischungen, der 13C NMR tensile and tear strength and solvent resist- unsaturation upon peroxide cure [13]. The Spektroskopie von peroxidvernetzten ance. Introduction of unsaturation via in- high reactivity of VNB-EPDM towards per- 13 und C markierten ENB-EPDM und EPM corporation of the diene monomer enables oxide cure is explained by the low steric führen zusammen zu einer genaueren sulfur vulcanisation (_80% of EPDM appli- hindrance of the terminal VNB unsatura- Beschreibung des Mechanismus der cations) and enhances the peroxide curing tion [4]. In practical EPDM/peroxide com- peroxidischen Vernetzung von EPDM. Sowohl Alkyl- als auch Allylmakroradi- efficiency (_15%). The obvious advantage pounds usually co-agents, such as triallyl kale und entsprechende ungesättigte of peroxide cure over sulfur vulcanisation is (iso)cyanurate, trimethylolpropane trimeth- Gruppen werden durch H-Abstraktion the formation of thermo-stable C-C bonds acrylate or m-phenylenebis(maleimide), are an der Hauptkette gebildet. Die Kombi- in stead of thermo-labile S-S links. Peroxide included to increase the peroxide curing eff- nation dieser Makroradikale führt zu cure thus allows the full exploitation of the ficiency [14], which obviously affects the Alkyl/Alkyl, Alkyl/Allyl und Allyl/Allyl excellent heat resistance of EPDM. mechanism of peroxide cure. Netzknoten und Addition der Makrora- The peroxide curing efficiency of EPDM is More details on the mechanism of peroxide dikale an die verbliebenen Diengruppen determined by the structure of the diene [3, cure of EPDM and the structures formed are führt zu Alkyl/Alken und Allyl/Alken 4]. 5-Vinyl-2-norbornene (VNB) provides a lacking, mainly due to the complexity of the Verntzungsstellen. substantially higher efficiency for peroxide system (large number of structures formed cure than 5-ethylidene-2-norbornene (ENB) at low concentrations) combined with the and dicyclopentadiene (DCPD), which ex- relatively difficult, analytical accessibility of plains the growing interest in recently de- crosslinked polymer networks. Recently, veloped VNB-EPDM polymers [5,6]. The mechanism of peroxide crosslinking of EPDM rubber has been reviewed in refer- Authors ences 7 and 8 and the subsequent practical M. van Duin, R.Peters, Geleen consequences in references 9 and 10. The (The Netherlands), R. Orza, basic steps in the generally accepted mech- Eindhoven (The Netherlands), anism of peroxide cure of EPDM are pre- V. Chechik, York (United Kingdom) sented in Figure2. The chain of free-radical reactions is initiated by thermal decomposi- Corresponding author: Martin van Duin DSM Elastomers 1 This study was presented during the Polymer P. O. Box 1130 Network Group meeting (Larnaca; June 2008) and 6160 BC Geleen the Fall Rubber Colloquium (Hannover; November The Netherlands 2008). It will also be published in Macromolecular E-mail: Symposia. [email protected] 458 KGK · September 2009 several studies have been performed to ob- 1 tain further insight in the chemical mecha- nism of peroxide cure of EPDM. Firstly, a study on peroxide crosslinking of low-mo- lecular-weight model compounds for EPDM has been performed [15]. The use of gas chromatography (GC) and especially GCxGC allowed the separation of a wide variety of “crosslinked” model products. The precise structure of the various products could be obtained by careful interpretation of the mass spectrometry (MS) data. Secondly, a solid state 13C nuclear magnetic resonance (NMR) study has been performed on perox- ide-cured, 13C-labeled EPDM [16]. 13C labe- 1 ling results in a dramatic increase in sensi- Conversion of 5-ethylidene-2-norbornene (ENB) via ter-polymerisation with ethylene (E) and propylene (P) into ENB-EPDM rubber (top) and hydrogenation of ENB to tivity when studying the new structures 2-ethylidene norbornane (ENBH) model compound (bottom) formed upon peroxide cure. Finally, an EPR study has been performed on the decompo- sition of peroxide in EPDMs with various 2 2 Generally accepted diene monomers [17]. Although this third mechanism for study was initiated to investigate the free- peroxide cure of radical species formed in the course of per- EPDM [7-10] oxide cure, it eventually provided informa- tion on the structure of some of the cross- links. In this review the results of these three studies will be combined, providing a 3 3 GCxGC-MS more accurate mechanism for peroxide cure chromatogram of of EPDM. reaction product of n-octane with Experimental ENBH (10wt%) and For full experimental details on the GC-MS, DCP (5wt%) 13C NMR and EPR studies on peroxide cross- (30minutes at 433 K) linking of EPDM (models) see the original papers [15-17]. Results and discussion GC-MS study of model compound crosslinking [15] Mixtures of an alkene (5 – 20 wt%), viz. hy- drogenated ENB (Fig. 1), DCPD and VNB (ENBH, DCPDH and VNBH, respectively), with an alkane, viz. n-octane or n-decane, have been “crosslinked” with 5wt% dicumyl dimers with a molecular weight (M) of showed species with M of 234 Da, i.e. equal peroxide (DCP) for 30minutes at 433 K. The 226Da, i.e. equal to the sum of M of two n- to the sum of M of n-octane and ENBH, and products have been analysed with GC-MS octane molecules minus 2 Da, but varying 236 Da, i.e. equal to the M sum minus 2Da. and GCxGC-MS. In the following discussion, in branching structure. These dimers are The first series of products are formed via the results for an ENBH/n-octane/DCP mix- formed as a result of combination of two addition of an octyl radical to the ENBH un- ture will be explained in detail as an illustra- octyl radicals, formed in their turn via H-ab- saturation, followed by H-transfer. The sec- tion for the whole series of alkene/alkane straction from n-octane by the peroxide ond series of products are formed via com- model experiments performed. In addition radicals. In a more in-depth study a series of bination of an octyl and an ENBH allyl radi- to the expected DCP decomposition prod- linear and branched alkanes has been cross- cal, both formed in their turn by H-abstrac- ucts (cumylalcohol and acetophenone), sev- linked with DCP [18]. Detailed interpreta- tion from n-octane and ENBH, respectively. eral clusters of other reaction products tion of the MS spectra has allowed the iden- A variety of coupling products with differ- have been observed (Fig. 3). Products result- tification of the precise structure of the ent branching structures are observed, as ing from the addition of peroxide-derived various dimers, which thus provides infor- for cluster I. Detailed interpretation of the radicals to the alkenes and new unsaturat- mation on the selectivity for H-abstraction MS spectra showed that H-abstraction from ed species due to C-scission or dispropor- from the various C positions: CH > CH2 > CH3. ENBH preferable occurs at the allylic C3 and tionation of alkyl radicals have not been Cluster II consists of coupling products of C9 positions. Cluster III consists of ENBH identified. Cluster I consists of n-octane one n-octane and one ENBH molecule. MS dimers with M of 242Da, i.e. equal to the KGK · September 2009 459 PRÜFEN UND MESSEN TESTING AND MEASURING centrations (Fig. 4). It was shown that in- 4 creasing the alkene content in the alkene/ alkane mixture results in a linear increase of the alkene conversion and in a linear de- crease of the alkane conversion.
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