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Dicarboxylic acids may compete withstandard vulcanisation processes forcrosslinking epoxidised natural rubberMyriam Pirea, Sophie Norveza, Ilias Iliopoulosa, Benoît LeRossignolb & Ludwik Leiblera

a Matière Molle et Chimie, ESPCI ParisTech-CNRS, 10 rueVauquelin, UMR-7167, 75005, Paris, France.b Hutchinson SA Centre de Recherche, Chalette sur Loing, France.Published online: 06 Sep 2013.

To cite this article: Myriam Pire, Sophie Norvez, Ilias Iliopoulos, Benoît Le Rossignol& Ludwik Leibler (2014) Dicarboxylic acids may compete with standard vulcanisationprocesses for crosslinking epoxidised natural rubber, Composite Interfaces, 21:1, 45-50, DOI:10.1080/15685543.2013.830527

To link to this article: http://dx.doi.org/10.1080/15685543.2013.830527

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Dicarboxylic acids may compete with standard vulcanisationprocesses for crosslinking epoxidised natural rubber

Myriam Pirea, Sophie Norveza*, Ilias Iliopoulosa, Benoît Le Rossignolb and LudwikLeiblera

aMatière Molle et Chimie, ESPCI ParisTech-CNRS, 10 rue Vauquelin, UMR-7167, 75005 Paris,France; bHutchinson SA Centre de Recherche, Chalette sur Loing, France

(Received 11 December 2012; accepted 5 April 2013)

The work focuses on crosslinking epoxidised natural rubber by dicarboxylic acidswhich react with the epoxy sites along the chain. The crosslinking reaction at 180 °Cis followed in a rheometer, and the cured materials are characterised by stress–strainexperiments. The cure process is tremendously accelerated (from 3 h to 20min) inthe presence of an equimolar amount of 1,2-dimethylimidazole. Thermal stabilityand fatigue life experiments were carried out on samples charged with carbon blackand antioxidants. The results are found in between the ones obtained with standardsulphur or peroxide crosslinked samples.

Keywords: crosslink; epoxidised natural rubber; ENR; dicarboxylic acids; DMI;fatigue life; ageing resistance

Introduction

A long-lasting challenge in rubber crosslinking consists in finding a way to improve thethermal stability while maintaining the dynamic performance characteristics of theresulting materials.[1–4] Indeed, it is well known that sulphur vulcanisation givesmaterials having longer fatigue life than materials crosslinked by peroxides, due to theability of the S–S crosslinks to break before the carbon-carbon bonds of the primaryelastomer network and subsequently to reform. However, this is at the expense of heatresistance, yet because of the breakability of the bonds. Many studies have documentedboth the advantages (increased ageing resistance), and the disadvantages (impairedfatigue resistance) of efficient and conventional vulcanisation systems.[5,6] Conversely,the crosslinks provided by peroxide cure are carbon–carbon bonds very similar instrength to other bonds in the polymer backbone, giving rise to a good ageingresistance, but to poor dynamic performances over time. In this work, we tackled thisproblem by introducing long crosslinking bridges based on α,ω-dicarboxylic acidsreacting with the oxirane groups of epoxidised natural rubber (ENR).

ENR is a unique elastomeric material retaining most of the properties of naturalrubber (NR) from which it derives.[7,8] Owing to the presence of the epoxide groupsalong the chain, ENR demonstrates oil resistance, low gas permeability, high wet grip,as well as enhanced compatibility with polymers bearing polar groups.[8–11] ReactiveENR, which presents a dual functionality (double bonds and epoxy sites) available for

*Corresponding author. Email: sophie.norvez@espci.fr

Composite Interfaces, 2014Vol. 21, No. 1, 45–50, http://dx.doi.org/10.1080/15685543.2013.830527

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crosslinking, can be vulcanised via a radical way using sulphur or peroxides.[12,13]Our group showed recently that ENR can also be efficiently crosslinked by dicarboxylicacids (DAs) reacting with the epoxy groups, producing β-hydroxy-esters along the chainas proved by solid-state NMR spectroscopy [14–16] (Figure 1). The present workshows that this system presents a potential to meet the unavoidable compromisebetween fatigue life and ageing resistance.

Experimental

Materials

ENR25, containing 25mol% epoxide groups, was supplied by the Tun Abdul RazakResearch Centre (Malaysia). Dodecanedioic acid (C12DA, 99%) and polyethylene glycoldicarboxylic acid (PEG600DA, Mw=600 gmol�1) were obtained from Acros and Fluka,respectively. The Sigma–Aldrich Co. provided 1,2-dimethylimidazole (DMI, 98%).

Blend preparation

Mixtures of ENR, DA and DMI were prepared in a Haake Polydrive mixer equippedwith cam rotors at 40 °C for about 12min. The ratio DA/epoxide was 1 diacid moleculefor 25 epoxy sites, i.e., 1 for 100 monomers. The ratio DMI/DA was 200 or 400%, i.e.,1 or 2 DMI molecules per acid group. The mechanical properties of the gums weretested in the presence of different carbon blacks leading to the choice of a mixture ofSRF N772 (10 phr) and HAF N330 (34 phr). An aromatic oil (Plaxolene 25) wasadded (2 phr) to facilitate the mixing. For ageing experiments, antioxidants were alsoadded, IPPD (N-isopropyl-N′-phenyl-p-phenylenediamine, 1 phr) and Vulcanox BKF(2,2′-methylene-bis-(4-methyl-6-tert-butylphenol, 1 phr). After blending, samples werecured in a CARVER press under 8 tons pressure in appropriate conditions of time andtemperature.

Measurements

Curing

The crosslinking process was followed using a TA Instruments ARES rheometer, inplate–plate geometry (25-mm diameter) (1Hz frequency, 0.1% strain). The temperature

Figure 1. Crosslinking of ENR25 by C12DA or PEG600DA. The ratio DA/epoxide is 1 DA for25 epoxy sites, i.e., for 100 total monomer units of the ENR25 polymer backbone.

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was raised from 25 to 180 °C at 5 °C/min, and then the sample was left underoscillatory shear for curing at 180 °C.

Mechanical properties

Dumbbells of 25mm effective length and 4mm width were cut in a 1.5-mm thick curedrubber sheet. Strain–stress behaviour was studied using an Instron machine equippedwith a video extensometer at a crosshead speed of 500mm/min, until break.

Fatigue tests

Dumbbell test pieces cut from a vulcanised rubber sheet of 1.5mm thickness weresubmitted to a strain of 100% at the frequency of 1.8 Hz. For each formulation, sixspecimens were tested and the number of cycles needed for failure (fatigue life) wasrecorded using a home-made Fatigue to Failure Tester.

Ageing experiments

Ageing tests were performed on three blends of ENR25 crosslinked either byPEG600DA (in presence of 400% DMI) or by sulphur (conventional vulcanisationrecipe, CV) or dicumylperoxide. All blends were reinforced by carbon black (44 phr)and protected by antioxidants (2 phr). The DA-containing blend was cured at 180 °C,the others at 160 °C. The evolution of mechanical properties of cured materials aged for24, 72 and 168 h at 115 °C was followed by stress–strain experiments. Sample size: H2,2-mm thickness according to ISO37 standard. Five specimens were tested for eachexperiment.

Results and discussion

Acceleration of the crosslinking avoids a degradation of mechanical properties

Figure 1 shows the reaction scheme leading to the crosslinking of ENR25 by DAs. Thesimple binary blend ENR+DA does not react readily, since after a 2-h heat treatmentat 180 °C the crosslinking reaction of the binary blend is not complete. This long curingtime was detrimental to the tensile properties of the cured rubber.[14] To overcome this,we searched for catalysts and found that the esterification reaction was tremendouslyaccelerated using 1,2-dimethylimidazole (DMI).[15] Figure 2(A) compares the kineticsof crosslinking reaction without or with DMI introduced in stoichiometric amount(200mol% vs. DA, i.e., 1 DMI molecule per acid group). During the 31-min heatingramp of temperature from 25 to 180 °C, the elastic modulus G’ first decreases becausethe rubber softens. Then, the temperature is maintained at 180 °C. The sudden increasein G’ shows the beginning of crosslinking reaction. In the presence of DMI, the cross-linking reaction is complete in only 20min. This is clearly beneficial for the mechanicalproperties (Figure 2(B)). The mechanism of the acceleration is not catalytic, since anequimolar amount of DMI is required to make the reaction efficient. This was associ-ated with the formation of an imidazolium dicarboxylate which would solubilise theDA in the elastomer matrix, as suggested by differential scanning calorimetry (DSC)experiments.[15] The occurrence of this intermediate species enabling the activation ofthe crosslinking agent in the matrix was further attested by solid-state NMR experi-ments.[16]

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Towards a compromise between fatigue life and ageing resistance

Mixtures with optimal composition were formulated by adding carbon black and, forageing experiments, antioxidants. We made a selection of the most efficient antioxidantsfor our system, using thermal analysis in DSC. The technique consisted in applying toa polymer sample an isothermal treatment under an oxidative atmosphere, until anexotherm appears. At this point, the efficiency of added antioxidants may be evaluated.Two phr of different protective agents were introduced in charged samples of ENR25(to be crosslinked by PEG600 diacid in the presence of 400% of DMI). Some antioxi-dants delayed the crosslinking reaction, for instance those containing zinc: the diacidcan form a stable complex with the metal and thus be inhibited. The selected antioxi-dants were finally IPPD and Vulcanox BKF.

The charged samples were submitted to fatigue tests and ageing experiments. Theresults were compared with the ones obtained with charged ENR25 crosslinked withsulphur or peroxide, and also with natural rubber crosslinked with sulphur.

Figure 2. (A) The crosslinking reaction of ENR25 by C12DA, without DMI or in the presenceof 200% DMI is followed by rheology. (B) Tensile tests on the corresponding samples,crosslinked at 180 °C.

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Fatigue tests

The number of cycles before failure were 102,000, 150,000, 104,000 and 91,000, forENR25 +C12DA, ENR25 + PEG600DA, ENR25 + sulphur and NR+ sulphur, respec-tively. Therefore the blends crosslinked by C12DA or PEG600DA present a fatigue lifecomparable with the one of rubbers vulcanised by sulphur, deemed to allow a betterfatigue resistance than peroxides.

Ageing experiments

Figure 3 shows the evolution of the strain at break with the ageing time at 115 °C. Asexpected, the S-vulcanised sample demonstrates a poor ageing resistance, while samplecrosslinked with peroxides presents a very stable behaviour over time, due to thepresence of strong C–C bridges. The sample crosslinked by DAs presents an intermedi-ate behaviour, with an initial decrease similar to sulphur blends, but a better ageingresistance.

Our system using DAs thus opens the gate to new developments towards a compro-mise between the two classical ways of rubber crosslinking, using sulphur or peroxides.

Conclusion

The in bulk-crosslinking of ENR with DAs is efficient using 1,2-dimethylimidazole,despite the fact that the epoxy groups are sterically hindered. The mechanism ofacceleration involves the quantitative formation of an imidazolium dicarboxylate thatenables the dispersion and the activation of the crosslinking agent. The formulation ofthe blends was optimised by adding carbon black and antioxidants. Thermal stabilityand fatigue life experiments on the charged materials give results corresponding to aninteresting compromise between the ones obtained with standard sulphur- or peroxide-crosslinked samples.

AcknowledgementThis study was funded by a Cifre grant Hutchinson/ANRT (National Association of Research andTechnology, France).

Figure 3. Evolution with ageing time at 115 °C of the strain at break of ENR25 crosslinked by(squares) sulphur; (diamonds) DA; (circles) peroxide. A standard deviation of 5–10% may beapplied to each measurement.

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