CROSS-LINKING AND MODIFICATION OF SATURATED ELASTOMERS USING FUNCTIONALIZED AZIDES The studies described in this thesis are part of the Research Programme of the Dutch Polymer Institute, Eindhoven, the Netherlands, project #580. Graduation committee Chairman prof. dr. F. Eising University of Twente Promotor prof. dr. ir. J.W.M. Noordermeer University of Twente Assistant promotor dr. A.G. Talma University of Twente/ Akzo Nobel, Polymer Chemicals B.V. Members prof. dr. ir. M.M.C.G. Warmoeskerken University of Twente prof. dr. P.J. Dijkstra University of Twente prof. dr. ir. D.M. Bieliński Technical University of Łódź dr. ir. M. van Duin DSM Elastomers Global R&D Cross-linking and modification of saturated elastomers using functionalized azides By Agata Joanna Zielińska PhD Thesis, University of Twente, Enschede, the Netherlands With summary in English, Dutch and Polish Copyright © 2011 A.J. Zielińska, Enschede, the Netherlands All rights reserved Cover design by A.J. Zielińska, picture by Piotr Długołęcki ISBN: 978-90-365-3208-2 CROSS-LINKING AND MODIFICATION OF SATURATED ELASTOMERS USING FUNCTIONALIZED AZIDES DISSERTATION to obtain the degree of doctor at the University of Twente, on the authority of the rector magnificus, prof. dr. H. Brinksma, on account of the decision of the graduation committee, to be publicly defended on Friday, 1st of July 2011 at 16.45 by Agata Joanna Zielińska born on 4th of November 1981 in Tomaszów Mazowiecki, Poland This dissertation has been approved by: prof. dr. ir. J.W.M. Noordermeer Promotor dr. A.G. Talma Assistant Promotor 1 If I speak in the tongues of men and of angels, but do not have love, I am only a resounding gong or a clanging cymbal. 2 If I have the gift of prophecy and understand all mysteries and all knowledge, and if I have a faith that can move mountains, but do not have love, I am nothing. 3 If I give all I possess to the poor and give over my body to be burned, but do not have love, I gain nothing. 4 Love is patient, love is kind. It does not envy, it does not boast, it is not proud. 5 It does not dishonor others, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. 6 Love does not delight in evil but rejoices with the truth. 7 It always protects, always trusts, always hopes, always perseveres. 1 Corinthians 13:1-7 1 Korinthiërs 13:1-7 1 Kor 13:1-7 To my son and husband 3 Table of contents Chapter 1 Introduction: Cross-linking/modification of saturated 1 hydrocarbon based elastomers with organic azides Chapter 2 Literature Review: Organic peroxides and azides in cross-linking 7 and modification of polymers Chapter 3 Comparison of different azides with respect to their reactivity 23 towards EPM rubber Chapter 4 Cross-linking of EP(D)M-rubbers with di-azides: Mechanical 41 properties of vulcanizates Chapter 5 Coagents as potential vulcanization aids in di-azides cross- 61 linking Chapter 6 Modification of EPM-rubber using mono-azides 79 Chapter 7 Mechanistic study on the reaction between SA/AF functionalities 103 and saturated hydrocarbons Chapter 8 Di-azides cross-linked, iPP/EPDM-based thermoplastic 123 vulcanizates Appendix A Recycling of di-azides cross-linked EPM 139 Summary 145 Samenvatting 149 Podsumowanie 153 Symbols and abbreviations 157 Bibliography 161 Acknowledgements 163 Chapter 1 3 4 Introduction: Cross-linking/modification of saturated hydrocarbon based elastomers with organic azides 5 Chapter 1 EP(D)M-rubber While the commodity polymers: poly(ethylene) (PE) and poly(propylene) (PP) are hard, semi-crystalline materials, the random copolymer of ethylene and propylene (EPM) is a soft, easily flowing, amorphous polymer with interesting elastic properties. The letters (E) and (P) stand for ethylene and propylene respectively, while (M) indicates the class of elastomers with fully saturated main chain, as defined in ISO 1629. The EPM- elastomers have typical ethylene-contents of 45-70 wt %. At high ethylene incorporation a low-level of crystallinity can develop which usually melts at 30-90 °C. The higher ethylene content allows for the copolymer to be extended with larger amounts of fillers and shows enhanced strength. Main limitation however, is inferior elastic recovery especially at low temperatures. The structures of highly unsaturated natural rubber (NR) and saturated EPM-rubber are given in Scheme 1.1. The absence of double bonds in the backbone causes that the EPM has very good ozone resistance as well as better thermal stability compared to unsaturated rubbers like NR. Atmospheric factors, particularly ozone, greatly accelerate weathering of elastomers with double bonds located in the main chain.1,2 Another advantage of ethylene-propylene rubber is that it can be highly extended with fillers and mineral oil what allows for economical compounding. CH3 (A) CH CH (B) CH CH CH CH 2 2 n 2 2 n 2 m CC CH H 3 Scheme 1.1: Structures of: (A) NR and (B) EPM The common disadvantage of the ethylene/propylene based polymers whether it is PE, PP or EPM-rubber is lack of chemical functionalities and inadequate compatibility with other polymers, what limits their applications. This disadvantage is difficult to overcome due to low reactivity of the fully saturated hydrocarbon chains. The problem is especially significant for EPM-rubber, as in order to obtain the elastic properties, the raw polymer needs to be cross-linked. A general way to overcome the lack of reactivity of saturated polymers is by pursuing free-radical reactions, usually initiated by thermal decomposition of peroxides. Peroxide cross-linking of EPM is commercially applied. Also grafting of unsaturated monomers, for example maleic anhydride (MA) initiated by peroxides is performed on an industrial scale.3 One major disadvantage of peroxide- induced processes however is the lack of selectivity which leads to a number of side reactions. The peroxide chemistry is described more extensively in Chapter 2. Compared to the most common sulfur cross-linking, peroxide vulcanization leads to inferior dynamic mechanical properties. Moreover, the EPM compounds can not be 2 Introduction extended with aromatic oils, which are highly reactive towards free radicals, and even when more expensive paraffinic plasticizers are applied the efficiency of the peroxide- curing is still low and requires addition of coagents.4,5 To allow for sulfur-curing, ethylene-propylene-terpolymers were developed which contain a certain amount of unsaturation. In the history of EPDM (ethylene-propylene-diene rubber) many dienes have been tested as third monomer, and 5-ethylidene-2-norbornene (ENB) is currently the most commonly used.6 Other commercial dienes are dicyclopentadiene (DCPD) and vinyl-norbornene (VNB). Out of the two double bonds of these dienes, Scheme 1.2, one in the strained ring is most reactive and thus consumed during polymerization, while the second double bond allows for sulfur vulcanization. The lower reactivity of the second double bond towards polymerization is to minimize branching reactions. It is due to branching that the amount of diene which can be incorporated into the polymer is significantly limited. (A) (B) (C) CH3 (D) CH CH CH CH CH CH 2 2 n p 2 m CH CH3 Scheme 1.2: Structures of commercially applied dienes (A) ENB; (B) DCPD and (C) VNB, and most common EPDM-rubber: (D) ENB-EPDM (ethylene-propylene-5-ethylidene-2-norbornene) The double bonds present after polymerization are not located in the polymer backbone and therefore EPDM is still considered as a saturated M-class rubber, Scheme 1.2(D). EPDM shows practically the same high ozone-resistance as EPM. Although incorporation of the diene into the ethylene-propylene chain allows for sulfur vulcanization, the efficiency of the curing-reaction is much lower compared to butadiene- or isoprene-based R-rubbers containing C=C unsaturation on every fourth carbon atom along the polymer chain. The amount of diene which can be incorporated into EPDM is typically below 10 wt %. Still most of the EPDM products are sulfur-cured and also when peroxides are applied the efficiency of the reaction is significantly improved in comparison to EPM. Consequently, the amount of coagents required for peroxide vulcanization of ENB- and DCPD-EPDMs is greatly reduced relative to EPMs. In case of VNB-EPDM, due to its highly reactive towards free radical terminal unsaturation, even lower amounts of peroxides are needed than for ENB- and DCPD- EPDMs still resulting in similar mechanical properties.7 3 Chapter 1 Aim of the thesis Organic azides are known to be reactive not only toward alkenes but more remarkably also towards alkanes. Thus it is interesting to investigate their reaction especially with saturated types of elastomers, like EP(D)M, which are most difficult to cross-link. The aim of the project is to design and synthesize new azide compounds which will be applied for cross-linking purposes and modification. The general concept is that the substances carrying one azide functionality can be grafted on the side of the polymer chain while the di-functional compounds are tested as cross-linking agents. The mono-azides used for modification may, besides an azide group which will react with the elastomeric chain, also contain a second functional group designed to provide desired properties. It should be understood however that in the present study the main focus was on understanding and quantifying of the azide/polymer reaction rather than on tailoring the elastomer’s properties for any specific purpose. The di-azides are desired to react with high efficiency to ensure a sufficient cross-link density. Generally, it is expected that di-azide vulcanization can result in superior dynamic/mechanical properties over peroxide-curing while maintaining good thermal stability.