oxidation of natural rubber-based magnetorheological elastomers

5
Oxidation of natural rubber-based magnetorheological elastomers Mattias Lokander a , Torbjo¨rn Reitberger b , Bengt Stenberg a, * a Department of Fibre and Polymer Technology, KTH, Teknikringen 56-58, SE - 100 44 Stockholm, Sweden b Department of Chemistry, Nuclear Chemistry, KTH, Teknikringen 56, SE - 100 44 Stockholm, Sweden Received 1 April 2004; received in revised form 17 May 2004; accepted 31 May 2004 Abstract The rheological properties of magnetorheological (MR) materials can be changed continuously, rapidly and reversibly by an applied magnetic field. Solid MR materials consist of magnetically polarisable particles, generally iron, in an elastomer matrix. The high iron concentrations required (about 30% by volume) in order to get a substantial magnetorheological effect should influence the long-term stability of the materials. In this paper, the oxidative stability of natural rubber-based magnetorheological elastomers has been studied by chemiluminescence and oven ageing. The results show that the oxidative stability of natural rubber decreases dramatically when large amounts of iron particles are incorporated in the matrix. This is probably due to the large amounts of oxygen on the surface of the particles. Conventional antioxidants can be used to prolong the lifetime of magnetorheological elastomers, but in order to get acceptable lifetime of the materials a careful selection of the antioxidant system has to be made. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Magnetorheology; Natural rubber; Iron; Oxidation 1. Introduction The rheological properties of magnetorheological (MR) materials can be changed continuously, rapidly and reversibly by an applied magnetic field. Magneto- rheological fluids, where magnetically polarisable par- ticles are dispersed in a carrier oil, were introduced by Rabinow in 1948 [1]. Interest in solid analogues of magnetorheological fluids, where the carrier oil is replaced by a rubber or a gel, has increased recently. The MR effect in MR fluids is seen as a field-responsive yield stress, whereas in MR solids the shear modulus is field-responsive [2,3]. In most MR solids, the polarisable particles, generally pure iron, have been aligned by an applied magnetic field prior to the curing of the matrix [4e7]. However, although the particles have been aligned, the iron concentration needed in order to get a good MR effect is about 30% by volume [5,7,8]. It is possible to achieve a good MR effect without aligning the particles if the particles used have a relatively low critical concentration (the concentration where the particles are in touch with each other). In such cases, the actual iron concentration in the material has to be close to the critical concentration of the particles [9e11]. The largest magnetorheological effect in MR solids reported so far is about 60% [12]. The high iron concentrations required in order to get a substantial magnetorheological effect may influence the long-term stability of the materials. The surface of ‘‘pure’’ iron particles is covered with a thin layer of iron oxides. This results in large amounts of oxygen incorporated into the material. Furthermore, iron ions are known to enhance the oxidation of rubber materials [13e17]. * Corresponding author. Tel.: C46 8 790 8269; fax: C46 8 10 07 75. E-mail address: [email protected] (B. Stenberg). 0141-3910/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2004.05.019 Polymer Degradation and Stability 86 (2004) 467e471 www.elsevier.com/locate/polydegstab

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Polymer Degradation and Stability 86 (2004) 467e471

www.elsevier.com/locate/polydegstab

Oxidation of natural rubber-based magnetorheologicalelastomers

Mattias Lokandera, Torbjorn Reitbergerb, Bengt Stenberga,*

aDepartment of Fibre and Polymer Technology, KTH, Teknikringen 56-58, SE - 100 44 Stockholm, SwedenbDepartment of Chemistry, Nuclear Chemistry, KTH, Teknikringen 56, SE - 100 44 Stockholm, Sweden

Received 1 April 2004; received in revised form 17 May 2004; accepted 31 May 2004

Abstract

The rheological properties of magnetorheological (MR) materials can be changed continuously, rapidly and reversibly by anapplied magnetic field. Solid MR materials consist of magnetically polarisable particles, generally iron, in an elastomer matrix. Thehigh iron concentrations required (about 30% by volume) in order to get a substantial magnetorheological effect should influence

the long-term stability of the materials. In this paper, the oxidative stability of natural rubber-based magnetorheological elastomershas been studied by chemiluminescence and oven ageing. The results show that the oxidative stability of natural rubber decreasesdramatically when large amounts of iron particles are incorporated in the matrix. This is probably due to the large amounts ofoxygen on the surface of the particles. Conventional antioxidants can be used to prolong the lifetime of magnetorheological

elastomers, but in order to get acceptable lifetime of the materials a careful selection of the antioxidant system has to be made.� 2004 Elsevier Ltd. All rights reserved.

Keywords: Magnetorheology; Natural rubber; Iron; Oxidation

1. Introduction

The rheological properties of magnetorheological(MR) materials can be changed continuously, rapidlyand reversibly by an applied magnetic field. Magneto-rheological fluids, where magnetically polarisable par-ticles are dispersed in a carrier oil, were introduced byRabinow in 1948 [1]. Interest in solid analogues ofmagnetorheological fluids, where the carrier oil isreplaced by a rubber or a gel, has increased recently.The MR effect in MR fluids is seen as a field-responsiveyield stress, whereas in MR solids the shear modulus isfield-responsive [2,3]. In most MR solids, the polarisableparticles, generally pure iron, have been aligned by anapplied magnetic field prior to the curing of the matrix

* Corresponding author. Tel.:C46 8 790 8269; fax:C46 8 10 07 75.

E-mail address: [email protected] (B. Stenberg).

0141-3910/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2004.05.019

[4e7]. However, although the particles have beenaligned, the iron concentration needed in order to geta good MR effect is about 30% by volume [5,7,8]. It ispossible to achieve a good MR effect without aligningthe particles if the particles used have a relatively lowcritical concentration (the concentration where theparticles are in touch with each other). In such cases,the actual iron concentration in the material has to beclose to the critical concentration of the particles [9e11].The largest magnetorheological effect in MR solidsreported so far is about 60% [12].

The high iron concentrations required in order to geta substantial magnetorheological effect may influence thelong-term stability of thematerials. The surface of ‘‘pure’’iron particles is covered with a thin layer of iron oxides.This results in large amounts of oxygen incorporated intothe material. Furthermore, iron ions are known toenhance the oxidation of rubber materials [13e17].

468 M. Lokander et al. / Polymer Degradation and Stability 86 (2004) 467e471

In this paper, the oxidative stability of naturalrubber-based magnetorheological elastomers has beenstudied by chemiluminescence and oven ageing. Naturalrubber was chosen as the matrix because it has beensuccessfully used in magnetorheological rubbers [4,11],and because it is known to give a good chemilumines-cence signal when oxidised [18].

2. Experimental

2.1. Materials

The iron particles used were irregularly shaped pureiron particles from Hoganas AB, Sweden, ASC300(particle size !60 mm), or spherical carbonyl iron CMfrom BASF (particle size �10 mm). The matrix materialwas conventionally cured natural rubber, SMR GP. Therecipe was 100 phr rubber, 6 phr zinc oxide (ZnO),0.5 phr stearine, 3.5 phr sulphur, and 0.5 phr MBT(phrZ grams per hundred grams rubber). Some materi-als were stabilised by addition of 2 phr Irganox 1076.The iron particles were mixed into the rubber togetherwith the vulcanisation system in a Brabender internalmixer. Samples were vulcanised at 140 �C for 40 minunder a pressure of approximately 12 MPa.

2.2. Chemiluminescence

Chemiluminescence (CL) measurements were per-formed on a CLD100 CL-Detector from TohokuElectronic Industrial Co. The instrument is sensitive towavelengths between 280 and 650 nm, with peaksensitivity in the region 400e450 nm. The measurementswere performed at constant temperature, 120 �C or90 �C, with an airflow of 50 ml/min through the testchamber. The sample weight was chosen so the amountof matrix material was about 3 mg.

2.3. Oven ageing and tensile testing

Ageing was carried out in an oven at 90 �C or 70 �C.‘‘Dog bones’’ with a cross-section of approximately2! 3 mm, were made from three different materials:without iron particles, and filled with about 9 and 37%by volume. The different samples were hanging incotton threads without contact with each other or anymetal parts in the oven, and they were randomlydistributed in the oven. The stressestrain properties ofthe materials were evaluated using an Instron 5566tensile testing machine. Five samples of each materialwere evaluated at each ageing time. The crossheadspeed was 500 mm/min.

2.4. Density

In order to confirm the iron particle content of thesamples the density of each sample was measured beforethe measurements. The density was calculated applyingthe Archimedes principle according to:

Density¼ Mass in air

Buoyancy in ethanol!Density of ethanol

Density of ethanol= 0.79 g cm�3.A Mettler Toledo AE100 balance and a Mettler

density determination kit 33360 were used for themeasurements. The volume percentage of iron particleswas calculated according to:

ciron ¼rtot � rrubber

riron � rrubber

where rtot is the density of the whole material, rrubber isthe density of the matrix material, and riron is the densityof iron= 7.9 g cm�3.

3. Results and discussion

Chemiluminescence curves at 120 �C for materialswith different content of ASC300 particles are shown inFig. 1. As can be seen, the time to reach the peak of theCL curve is shorter for the materials with iron. Thenumber of counts per minute is a measure of the reactionrate, but the iron particles shields some of the lightemitted, which causes the signal to be weaker when theiron content increases although the reaction is actuallyfaster. The maximum in the CL curve corresponds to theformation of a visible oxidised skin [18]. The time for theCL curves to reach this maximum is plotted in Fig. 2.The inverse of this time could be seen as a measure of therate of the skin formation. The dependence of this rateon the iron concentration is shown in Fig. 3. As can be

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Fig. 1. Chemiluminescence curves at 120 �C for materials with

different content of ASC300 particles.

469M. Lokander et al. / Polymer Degradation and Stability 86 (2004) 467e471

seen the rate increases linearly with increasing ironconcentration.

The oven ageing tests confirm that the peak of the CLcurve corresponds to the formation of the oxidised skin.The mechanical strain at break decreases with increasingageing time, whereas the stress at break and tensilemodulus initially decreases due to chain scission withinthe matrix, before it increases rapidly due to theformation of the crosslinked oxidised skin. The mini-mum of the stress at break and modulus correspond wellto the maximum in the CL curve (Fig. 4). If theminimum value of the modulus, related to the originalvalue, is plotted versus the ageing time to reach thisvalue, some interesting aspects can be seen (Fig. 5). Theincorporation of iron particles into the rubber not onlyresults in faster formation of the oxidised skin, but alsoin faster degradation within the sample. This is seen asa larger relative decrease in the modulus before it startsto increase for the iron filled materials. The antioxidantin these cases decreases both the degradation within thematerial and the rate of the skin formation. For theunfilled material, the antioxidant prolongs the time toform the oxidised skin, which leads to a lower minimummodulus of the material due to which more oxygen hastime to diffuse into the material [19].

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Fig. 2. Time to CL max versus iron content.

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Fig. 3. Rate of skin formation (1/time to CL max) as a function of iron

content.

There are some possible explanations for the in-crease in oxidation rate due to incorporation of largeamounts of iron particles into the rubber. The ironparticles will increase the heat transfer through thematerial, which could lead to faster degradation of thematrix. However, it has been reported that incorpora-tion of large amounts of aluminium particles, whichalso increases the heat transfer through the material,does not decrease the oxidative stability [20]. Anotherexplanation could be that the increased heat transferwill influence the vulcanisation reactions. This is thecase for aluminium particles [21], and should be thesame for iron particles. The conventional sulphurvulcanisation system used is sensitive to reversion;thus, the crosslink density in the iron filled materialscould be somewhat lower than that in the unfilledmaterials, and the fraction of monosulfidic crosslinks ishigher. This will influence the oxidative behaviour ofthe materials [22].

Yet another explanation could be that the particlesare covered with a thin layer of iron oxide. In such cases,smaller particles, with larger surface area, should

Modulus

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Fig. 4. Modulus after different ageing times, and chemiluminescence

curve at 90 �C for natural rubber filled with about 9% of ASC300.

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NRNR+9%ASC300NR+37%ASC300NR+AONR+AO+9%ASC300NR+AO+37%ASC300

Fig. 5. Minimum modulus related to original modulus versus the

ageing time to reach the minimum value.

470 M. Lokander et al. / Polymer Degradation and Stability 86 (2004) 467e471

decrease the stability even more than large particles. CLcurves of materials with 27% of ASC300 and carbonyliron CM at 120 �C are shown in Fig. 6. The shielding ofthe light is very efficient for the smaller particles, whichleads to a very weak signal. If the curves are normalisedto their maximum and minimum (Fig. 7), it is clear thatthe peak of the signal for the small carbonyl ironparticles comes earlier than for the larger ASC300particles. This indicates that the decrease in oxidativestability is due to the surface of the particles.

The last, and maybe most probable, explanation isthat iron ions catalyse the decomposition of hydro-peroxides and thereby accelerate the oxidation [14,15].The iron ions may be able to diffuse from the particlesurfaces into the rubber. It has been shown that copperions are able to diffuse into a polymer matrix froma copper surface [23]. Such diffusion of metal ions intothe matrix would provide an efficient catalyst of theoxidation. If this is the main reason for the accelerationof the oxidation, complex forming metal deactivatorsmay be used for decreasing the oxidation rate of MRsolids [14,24e26].

For NR and other general-purpose elastomers,ageing conditions of 7e14 days at 70 �C have become

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Fig. 6. Chemiluminescence curves at 120 �C of materials with 27% of

ASC300 and carbonyl iron CM.

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Fig. 7. Normalised chemiluminescence curves at 120 �C of materials

with 27% of ASC300 and carbonyl iron CM.

commonplace in product specifications and in antioxi-dant evaluations [27]. Thus, such a test was performedon stabilised natural rubber and iron filled naturalrubber. The mechanical properties, as related to thevalues of unaged samples are shown in Fig. 8. As can beseen, the antioxidant Irganox 1076 is a reasonablechoice for natural rubber. About 75% of the strain, and85% of the stress at break remains after 14 days.However, for the iron filled samples a more efficientantioxidant system has to be chosen. The stress andstrain at break for this material has decreased to about35% of the original value after only 7 days at 70 �C. Asmentioned, metal deactivators, which form complexeswith the metal ions and thereby inhibit the catalysis ofthe oxidation, is one group of stabilisers that should beinvestigated. Another group of stabilisers that could beinteresting to use in these materials is the recentlydeveloped organotellurium antioxidants [28].

4. Conclusions

The oxidative stability of natural rubber decreasesdramatically when large amounts of iron particles areincorporated in the matrix. This is probably due to thecatalytic effect of iron ions on the decomposition ofhydroperoxides, and the large amounts of oxygen on thesurface of the particles. Conventional antioxidants canbe used to prolong the lifetime of magnetorheologicalelastomers, but in order to get acceptable lifetime of thematerials a careful selection of the antioxidant system,which should include some metal deactivator, has to bemade.

Acknowledgements

The European Commission is gratefully acknowl-edged for financial support (Contract no: G5RD-CT-1999-00125).

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Fig. 8. Mechanical properties after ageing at 70 �C, related to the

properties of unaged material.

471M. Lokander et al. / Polymer Degradation and Stability 86 (2004) 467e471

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