Mechanism of Peroxide Crosslinking of EPDM Rubber R

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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 mechanism of peroxide cure of EPDM. Firstly, a study on peroxide crosslinking of low-molecular-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 peroxide-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 decomposition 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 crosslinks. 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 interpretation 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 decrease of the alkane conversion. The conver- sions increase in the series ENBH _ DCPDH < VNBH (_1:1:2). This agrees fairly well with experimental data on peroxide curing efﬁ- ciencies determined by rheometry of peroxide cure of EPDMs with various diene monomers [3, 12] and demonstrates, again, the higher reactivity of the VNB(H) terminal unsaturation. It is also shown that the total amount of combination products in clusters I, II and III is constant, i.e. independent of 4 Concentration (in arbitrary units) of reaction products of 10 wt% ENBH, DCPDH and the alkane and alkene structures and con- VNBH in n-octane with 5wt% DCP (30minutes at 433 K) (red and blue shades indicate centrations, suggesting that it is simply de- various combination and addition crosslink structures, respectively; dashed line termined by the peroxide content. However, indicates total concentration of combination crosslinks) the total amount of addition products from clusters II and III does depend on the diene structure, following again the sequence 5 5 Effect of selective ENBH _ DCPDH < VNBH, and the concentra- 13C-labeling of ENB tion of the alkene. It is ﬁnally calculated that unsaturation on MAS solid-state in a 10 wt% alkene solution H-abstraction 13C NMR spectra on a molar basis from the alkane and alkene of (crosslinked) is approximately 1:1 and subsequent for- ENB-EPDM mation of dimers via combination follows simple statistics, viz. alkyl/alkyl : alkyl/allyl : allyl/allyl _ 1:2:1.

13C NMR study of peroxide-cured, 13C-labeled ENB-EPDM [16] The development of high-temperature Mag- ic-Angle-Spinning (MAS) solid-state NMR spectroscopy has enabled a breakthrough in the chemical characterisation of crosslinked rubbers [19]. Unfortunately, the diene monomer content of EPDM is rather low, the conversion of the diene upon crosslinking is low and, finally, a large number of 6 6 Assignment 13 new crosslink structures are formed. As a of solid state C 13 NMR spectrum result, MAS C NMR of crosslinked EPDM of step-wise does not provide any insight in the cross- peroxide-cured, linking mechanism. Actually, Figure 5 (top) 13C-labeled shows that the diene unsaturation (ENB: C2 ENB-EPDM and C8) is hardly visible for crosslinked com- mercial EPDM. Therefore, EPDM with 13C labels at the olefinic C2 and C8 positions has been prepared [20]. Subsequently, peroxide has been added via a solution route and step-wise crosslinking at 433K as a function of time has been performed. Figures 5 (centre) and (bottom) show the very intense olefinic signals, both for the starting and the peroxide-cured, 13C-labeled EPDM. Figures 5 sum of M of two ENBH molecules, and formed via combination of two ENBH allyl (centre) and (bottom) clearly show that the 244Da, i.e. equal to the M sum minus 2 Da. radicals, in analogy to the structures from intensities of the olefinic C2 and C8 signals The ENBH dimers with M 242 Da are formed cluster II. decrease upon peroxide curing, as a result via addition of an ENBH allyl radical to the GC-MS has not only enabled the identifica- of the addition reactions. ENBH unsaturation, followed by H-transfer, tion of the various species, but also allows Figure6 shows a magnification of the solid- and the ENBH dimers with M 244 Da are the quantification of their respective con- state 13C NMR spectrum of peroxide-cured

460 KGK · September 2009 13C-labeled EPDM. New aliphatic signals be- 7 7 Evolution of EPR tween 45 and 60ppm are observed, which spectra upon are assigned to saturated C2 and C8 atoms heating of 1.2wt% formed upon addition reactions. Next to the DCPD-EPDM with non-reacted olefinic C2 and C8 atoms at 147 5 wt% DCP at 440K and 110ppm, respectively, a large number and assignment of new olefinic signals at 111 – 118 ppm, of structure 143 – 146 and 148 – 157 ppm are observed, of the sharp line component which are assigned to crosslink structures with intact unsaturation, resulting from combination reactions of allylic ENB-EPDM macro-radicals. Combination at the C3 and C9 allylic positions is the most probable. Fi- 8 nally, new peaks are observed between 60 and 100ppm and between 160 and 220ppm. These are assigned to a variety of oxidation products with C-O (ether, alcohol and/or [hydro]peroxide) and C=O (acid, ketone and aldehyde) units, respectively. The intensity of these oxidation-related signals is rather high relatively to the new aliphatic and olefinic crosslink-related signals. This is probably due to the fact that the peroxide cure experiment has been carried out step- wise to study crosslinking kinetics [16] with a small amount of labeled EPDM, resulting in repeated exposure to air. Indeed, by per- forming a second peroxide curing experiment with labeled EPDM in one step, the NMR signals of the oxidised species were strongly decreased in intensity.

EPR study of peroxide decomposition in EPDM [17] A variety of EPDMs with different diene structures and contents was mixed with 5 wt% of DCP via a solution route. EPR spectra were recorded at 440 K as a function of time. In the following discussion, the results for a 1.2wt% DCPD-EPDM (Fig.7) will be ex- 8 Mechanism of peroxide crosslinking of DCPD-EPDM rubber plained in detail as an illustration for the whole series of EPDMs studied. The EPR spectra consist of a superposition of two In most spectra, the double integral inten- peroxide-derived radicals or radicals formed sub-spectra, both at the same field position. sity of the broad line component is signifi- via H-abstraction from the EPM backbone The first sub-spectrum shows a clear hyper- cantly larger than that of the sharp line are observed. This is most probably due to fine coupling with sharp spectral features, component. The intensity of the sharp line the high reactivity of these alkoxy and alkyl whereas the second sub-spectrum is just a component increases steeply at short times radicals, resulting in concentrations which broad (_15G wide) Gaussian line contribu- and then decreases slowly to zero at longer are too low to be detected by EPR. The latter tion. The first spectrum fully agrees with times. The intensity of the broad line com- results indicate that it is not possible to the simulated spectrum of the allyl radical ponent usually increases more slowly and study the sequence of free-radical reactions of DCPD-EPDM (compare first and second also decays more slowly to zero at even occurring during peroxide cure of EPDM us- spectra in Figure 7; hyperfine coupling con- longer reaction times. It is surprising to note ing EPR. Actually, the observation of just the stants are also given). It is noted that radi- that rheometry indicates that peroxide cur- two allyl radicals of un-reacted DCPD-EPDM cals are not formed at the allylic DCPD ing of EPDM is more or less completed after and of DCPD-EPDM crosslinks, shows that bridgehead position as this would violate 10 to 15 minutes at 440 K, which agrees EPR in this case is “merely” a technique to Bredt’s rule. The broad line feature is tenta- with 6 to 9 times the half life time of DCP probe the network structure as a function tively assigned to a second allyl radical, (440 K: _100sec.), i.e. >98 % conversion of of time. The observation of the broad line which has the same structure as the allyl DCP. However, strong EPR signals of both feature and its assignment to the allyl radi- radical of DCPD-EPDM, but is positioned in sharp and broad line features are still ob- cal of a DCPD unit in a crosslink is fully in a crosslink. The low mobility of the cross- served after one hour or even longer. In ad- accordance with the results of the model link explains the lack of sharp EPR features. dition, it is noted that no signals related to and 13C NMR studies, discussed above, and

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is evidence for the participation of allyl combination crosslinks is constant and in- [4] M.van Duin and H.G.Dikland, Rubber Chem. EPDM radicals in crosslink formation. dependent of the EP(D)M composition. The Technol. 76 (2003) 132. [5] M. van Duin, M.Dees and H.G.Dikland, Kautsch. total amount of addition crosslinks increas- Gummi Kunstst 61 (2008) 233. Discussion es with increasing diene content and in the [6] M.Dees and M. van Duin, Rubber World, (Aug./ Based on the three experimental studies, series ENB _ DCPD < VNB. Typical diene con- Sept. 2008). discussed above in some detail, a more ac- versions are _25 % for ENB and DCPD and [7] M. van Duin, Kautsch. Gummi Kunstst. 55 (2002) curate chemical mechanism for the perox- _50% for VNB. 150. ide crosslinking of EPDM is proposed (Fig.8). [8] H.G.Dikland and M. van Duin, “Crosslinking of Thermal decomposition of the peroxide Conclusions EPDM and Polydiene Rubbers Studied by Optical Spectroscopy” in “Spectroscopy of Rubbers and yields primary alkoxy radicals, which may Based on these studies a more accurate Rubbery Materials”, Rapra Technology Ltd., convert to alkyl radicals via C-scission. These chemical mechanism for the peroxide cross- V. M. Litvinov and P. P. De (Eds.), Shawbury, radicals abstract H-atoms from the EPDM linking of EPDM is proposed. In agreement Shrewsbury, Shropshire (2002), p. 207. polymer, both at the CH2 and CH units in the with the commonly accepted scheme for [9] W. C.Endstra, C. T. J. Wreesman, in “Elastomer EPM main chain and at the allylic positions peroxide cure [7-10], a distinction is made Technology Handbook”, N.P.Cheremisinoff (Ed.), of the diene unit, yielding EPDM alkyl and between crosslinks formed via combina- CRC Press, Boca Raton (1993), p.495. allyl macro-radicals, respectively, in a _1:1 tion of two EPDM macro-radicals and via [10] P. Dluzneski, Rubber Chem. Technol. 74 (2001) 451. ratio. The allylic H-atoms of the diene mon- addition of a macro-radical to an EPDM un- [11] M.van Duin and B.Coussens: “(Re)evaluation of omer have a higher intrinsic reactivity for saturation. Conclusive evidence is now pro- the Importance of Hydrogen Abstraction during H-abstraction than the aliphatic H-atoms of vided showing that the free radicals partici- Radical Grafting of Polyoleﬁns“, 11th Polymer the EPM chain. However, H-abstraction also pating in these reactions originate both Processing Society meeting, Stuttgart (1995). occurs from the polymer backbone as a re- from the EPM polymer backbone and the [12] H.G.Dikland, Kautsch. Gummi Kunstst. 49 sult of the large molar excess of aliphatic residual diene unsaturation, yielding a com- (1996) 413. versus allylic H-atoms in EPDM. Crosslinking plicated mixture of crosslink structures [13] K.Fujimoto and S.Nakade, J.A ppl. Polym. Sci. 13 (1969) 1509. is achieved via two routes. Firstly, termina- with varying number of diene units and un- [14] S.K.Henning and R. Costin, Rubber World 233 tion of two EPDM macro-radicals via combi- saturations in the actual crosslinks. (2006) 28. nation yields alkyl/alkyl, alkyl/allyl and allyl/ [15] R.Peters, M. van Duin, D. Tonoli, G.Kwakkenbos, allyl crosslinks with 0, 1 and 2 diene units, Acknowledgements Y. Mengerink, R. van Benthem, Ch. de Koster, respectively, and with 0, 1 and 2 unsatura- Susana Camara, Bruce Gilbert, Adrian Whit- P. J.Schoenmakers and S. van der Wal, J.Chro- tions, respectively, in the actual crosslink wood and Mouna Zachary (University of mat. A 1201 (2008) 151. between the two rubber chains. Secondly, York), Marcel Aussems, Victor Litvinov, Rob [16] R.Orza, P. Magusin, V. Litvinov and M. van Duin, “Solid state 13C NMR study of peroxide cross- an EPDM macro-radical adds to a EPDM un- Meier, Rudy Parton and David Tonoli (DSM linking of 13C labeled ENB-EPDM”, to be submit- saturation. For steric reasons the macro- Research) and Pieter Magusin (Technical ted to Macromolecules. radical thus formed will probably not prop- University of Eindhoven) are acknowledged [17] M.Zachary, S.Camara, A. C.Whitwood, B. C. Gil- agate with a second diene unsaturation. It for their various contributions. Herman Dik- bert, M. van Duin, R. Meier and V. Chechik, Eur. will more likely terminate via H-transfer, land (DSM Elastomers) is thanked for the Polym. J. 44 (2008) 1099. yielding allyl/alkene and alkyl/alkene never-ending discussions on peroxide cure [18] R.Peters, D.Tonoli, M. vanDuin, G.Kwakkenbos, crosslinks with 2 and 1 diene units, respec- of EPDM. Y. Mengerink, R. van Benthem, Ch. de Koster, P. J.Schoenmakers and S. van der Wal, J. Chro- tively, and with 1 and 0 unsaturation, re- mat. A 1201 (2008) 141. spectively, in the actual crosslink. Note that References [19] “Spectroscopy of Rubbers and Rubbery Materi- in the case of peroxide cure of EPM without [1] W. Hofmann, “Rubber Technology Handbook”, als”, Rapra Technology Ltd., V.M. Litvinov and diene only saturated alkyl/alkyl combina- Hanser Publishers, Munich (1989). P. P. De (Eds.), Shawbury, Shrewsbury, Shropshire tion crosslinks are formed. Next, the macro- [2] J.A.Riedel and R.Vander Laan, “Ethylene Propyl- (2002). radical addition path does not terminate ene Rubbers”, in “The Vanderbilt Rubber Hand- [20] W. Heinen, L. N.Ballyns, W. J. A. Wittenburg, th the radical species. Therefore, the contribu- book”, 13 Ed. (1973), p.123. R.Winters, J. Lugtenburg and M. van Duin, Poly- mer 40 (1999) 4353. tions of macro-radical combination and ad- [3] F. P. Baldwin, P. B.Orzel, C. A.Cohen, H. S.Ma- kowski and J. F. van de Castle, Rubber Chem. dition are additive. The total amount of Technol. 43 (1970) 522.

462 KGK · September 2009