Durability of rubber products Publisher: Twente University Press, P.O. Box 217, 7500 AE Enschede, The Netherlands, www.tup.utwente.nl Print: Océ Facility Services, Enschede, The Netherlands Cover design: by Jos Peters © N.M. Huntink, Zutphen, 2003. No part of this work may be reproduced by print, photocopy, or other means without the permission in writing from the publisher. ISBN 90 365 1946 2 DURABILITY OF RUBBER PRODUCTS DEVELOPMENT OF NEW ANTIDEGRADANTS FOR LONG-TERM PROTECTION PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Universiteit Twente, op gezag van de rector magnificus, prof. dr. F.A. van Vught, volgens besluit van het College voor Promoties in het openbaar te verdedigen op vrijdag 7 november 2003 om 15:00 uur door Nicolaas Maria Huntink geboren op 8 mei 1963 te Doetinchem Dit proefschrift is goedgekeurd door: Promotor: prof. dr. ir. J.W.M. Noordermeer Assistent-promotor: dr. R.N. Datta Voorwoord Tijdens de laatste jaren van mijn loopbaan bij Flexsys B.V. in Deventer heb ik gewerkt aan een promotieonderzoek, uitgevoerd binnen de ‘New Products Group’, in samenwerking met de onderzoeksgroep “Rubbertechnologie” van de Universiteit Twente. Dit onderzoek is nu ten einde gekomen. Veel personen hebben door hun aanwezigheid en hulp bijgedragen aan het tot stand komen van dit proefschrift. Ik wil graag gebruik maken van de mogelijkheid om deze personen te bedanken. Als eerste wil ik mijn promotor, professor Noordermeer, bedanken voor de gelegenheid die hij mij gegeven heeft om in zijn groep te werken aan mijn promotieonderzoek. Door de goede begeleiding en kritische beoordeling van mijn werk heb ik veel geleerd. Veel dank ben ik verschuldigd aan mijn assistent-promotor, Rabin Datta, die mij heeft geadviseerd en gemotiveerd dit promotieonderzoek uit te voeren. Zijn enthousiaste en doortastende manier van begeleiden zal ik niet gauw vergeten, net zoals zijn vriendschap en de vele anekdotes uit India. De dagen die we samen op kantoor of tijdens een dienstreis hebben doorgebracht waren altijd gezellig en vaak zeer leerzaam. Auke Talma wil ik ook graag bedanken voor al zijn ideeën en hulp bij de ontwikkeling en synthese van de verschillende antidegradanten die beschreven staan in dit proefschrift. Auke was mede verantwoordelijk voor de goede sfeer binnen de ‘New Products Group’. Bertus Oldehanter wil ik graag bedanken voor de vele syntheses die hij voor mij heeft uitgevoerd. Zijn kennis van computers is ook regelmatig van pas gekomen. Sumana Datta wil ik graag bedanken voor de analytische ondersteuning en hulp bij de verschillende modelonderzoeken. De vele discussies die we hebben gevoerd hebben een grote bijdrage geleverd aan dit proefschrift. Wasil Maslow wil ik graag bedanken voor het DOSY 1H-NMR werk. Zijn expertise was zeer waardevol voor het ophelderen van het werkingsmechanisme van PPD-C18. I would like to thank the students Olivier Courier and Geraline van ‘t Slot for their help with the ozonolysis experiments described in Chapter 6. Johan Baaij, Gerard Hogeboom, Martin Hondeveld, Andre Roolvink, Remco Meijer en Rene Willemsen wil ik graag bedanken voor hun hulp in het rubberlab en voor de plezierige tijd die ik daar heb doorgebracht. Johan was altijd aanwezig en bereid om de meest uiteenlopende problemen op te lossen. Gerard, bedankt dat je, nadat Flexsys besloten had om de ‘New Products Group’ op te heffen, toch nog bereid was om de vele rubbermengsels voor mij te testen. De samenwerking met het rubberlab was altijd goed en plezierig. I would like to thank Leona Baclowski and Horn-Jau Lin for their help with the DSC-measurements described in Chapter 8. Henk Schreurs, Arie de Hoog, Hans Hofstraat, Enno Klop, Rob van Puijenbroek, Brenda Rossenaar en Minie Janssen-Mulders wil ik graag bedanken voor hun hulp en inzet bij het onderzoek naar het effect van antidegradanten op de thermische stabiliteit van onoplosbaar zwavel, beschreven in hoofdstuk 9. De projectbesprekingen eens in de zes tot acht weken waren altijd zeer interessant en leerzaam. I would like to thank the colleagues of the RBT group at the University Twente for their hospitality and the fun we had, especially during the study trips to London and Limburg. Richard en Joost, bedankt dat jullie mijn paranimfen willen zijn. Gerda, bedankt voor je hulp bij allerlei administratieve zaken. Mijn familie wil ik graag bedanken voor alles wat ze voor mij hebben gedaan en de interesse voor mijn werk. Marion, bedankt dat je de laatste maanden wat vaker alleen hebt klaar gestaan voor Luc en Mart, zodat ik ongestoord aan mijn proefschrift kon werken. Luc en Mart, bedankt dat jullie de meeste nachten lekker hebben doorgeslapen. Contents Chapter 1 General introduction 1 Chapter 2 Durability of rubber compounds 12 Chapter 3 Synthesis and characterization of potential long-lasting 53 antidegradants Chapter 4 Development of test protocols for screening potential 77 slow-migrating antidegradants Chapter 5 Evaluation of slow release antidegradants in typical passenger 93 and truck tire sidewall compounds Chapter 6 Ozonolysis of model olefins - Efficiency of antiozonants - 115 Chapter 7 Quinonediimine as bound antioxidant in silica compounds with 137 the possibility to reduce the level of silane coupling agent Chapter 8 Ranking of several antidegradants for their effectiveness to 159 protect rubber against oxidation using differential scanning calorimetry and by accelerated aging of steelcord skim compounds Chapter 9 The interaction of antidegradants with sulfur vulcanization 175 agents Main symbols and abbreviations 195 Summary 199 Samenvatting 203 Chapter 1 General introduction 1.1 Introduction The first people using rubber were the natives of Haiti. They played a ball game, the balls being made from the sap, a white milky fluid, of a tree.1 Natural rubber (NR) or caoutchouc is the product condensed from this sap (latex). The name caoutchouc was derived from the Indian word “caa-o-chu”, meaning “weeping tree”. In 1770 the Englishman, Joseph Priestley, recommended the material for use as an eraser, or rubber, the latter term being adopted by the English-speaking world as a generic term for materials of high reversible elasticity.2 In those days, Europeans were rubbing out pencil marks with small cubes of rubber. The rubber however was not an easy substance to work with because it deteriorated very easily. There are hundreds of latex-producing plants, belonging to different botanical families, and they are predominantly found in tropical climates. Not all caoutchouc- producing plants are harvested for industrial purposes, because the yield is either too small, the caoutchouc content in the latex too low, or the caoutchouc contains too many resinous impurities. Early plantation economies used Ficus elastica, Funtumia, de Castilloa, and Manihot plants, but they were soon replaced by the Hevea brasiliences, because the latter gives a much higher yield of a superior caoutchouc.3 The invention of useful rubber is commonly attributed to Charles Goodyear.4 Natural or India rubber, as it was then known, had little uses. Rubber products melted in hot weather, froze and cracked in cold, and adhered to everything they touched. In 1830 Goodyear began experimenting with raw rubber to turn it into a useable product. In 1839 he managed to harden it by mixing the rubber with sulfur, white lead and oil of turpentine and drying it near a hot stove. The pieces, which had come into direct contact with the stove, had changed into an elastic, non-sticky product. The process known as vulcanization had been born. The term vulcanization was named after Vulcan, the Roman god of fire. In 1906, Bayer started a research program to produce synthetic rubber because Germany was resentful of the dominance that the UK and Holland held over supplies of natural rubber.5 The first synthetic rubber suitable for industrial scale production, methyl rubber (polydimethylbutadiene), was developed in 1910. It was very susceptible to oxidative break-down.6 Early users of vulcanized natural rubber soon became aware of the sensitivity of the material to deterioration under a variety of conditions and in a number of ways. These included the change to a sticky mass on general aging, the formation of deep 1 Chapter 1 cracks in a direction perpendicular to the application of a stress (now associated with ozone attack), deterioration in contact with copper wire (a serious problem when natural rubber was widely used as an electrical insulator) and the surface hardening that could be observed after exposure to light. At the beginning of the 1920’s passenger car tires rarely survived 5.000 km of use for reasons other than oxygen attack.7 However, car manufactures and the public started demanding longer life of tires and other rubber products. Application of reinforcing carbon blacks resulted in a significantly increased tread life. At the same time the degrading effects of oxygen came to the foreground and research was started to find means that would prevent oxygen and related compounds from destroying rubber products, especially tires. Rubber history credits three chemists with being the first people to develop commercially successful antioxidants: Herbert Winkelman and Harold Gray at B.F. Goodrich and, independently, Sidney Cadwell at US Rubber. All three served later as American Chemical Society Rubber Division chairmen, and Cadwell (1956) and Winkelman (1961) became Charles Goodyear Medal winners, at least in part for their pioneering work on these substances.7 Their two non- accelerating antioxidants differed in chemical detail, but both were condensation products of an aromatic amine and an aliphatic aldehyde. Commercial exploitation of antidegradants started after World War I. The American army, still suffering from the chemical-warfare horrors of World War I, wanted to make and store for long term hundreds of thousands of gas masks. Aware that the kinds of rubber products, with which they were then familiar, deteriorated rather rapidly in air, the first plan was to store the masks in some inert atmosphere like nitrogen, a plan entailing some difficulties. Cadwell was able to convince the military organization that there was a simpler measure and that he could guarantee it would work. He proved that masks compounded from a rubber containing his antioxidant remained serviceable for a long time, even if simply stored in air, given a reasonable degree of shelter from sunlight and heat. The American army ordered a huge number of masks made of rubber containing Cadwell’s antioxidant. In the decades since, hundreds of new antioxidants have been developed, falling in the class of staining or non-staining products. The staining antioxidants are members of the huge family of amino compounds. Because they discolor rubber, these antioxidants are used primarily in black rubbers. The non- staining antioxidants are commonly designated as phenolics and phosphites. The history of protection against ozone attack is somewhat shorter. Until the mid-1950s, wax was the only available antiozonant. The waxes with their low solubility in rubber gradually migrate to the air-exposed product surfaces and form a layer of bloom, through which ozone cannot penetrate. These waxes gave only protection against ozone attack in static applications but not in dynamic applications. In 1954 three chemists working at the Rock Island Arsenal: R.F. Shaw, Z.T. Ossefort and W.J. Touhey, discovered that the addition of dialkyl-phenylene diamines and/or alkyl-aryl-phenylene diamines to the waxes provided also protection against ozone in dynamic applications.7,8 The combination of wax and diamine is still the most widely used package of antidegradants today. 2