MWCNTs Elastomer Nanocomposite, Part 2: The Addition of MWCNTs ...

Fullerenes, Nanotubes and Carbon Nanostructures, 17: 55–66, 2009 Copyright # Taylor & Francis Group, LLC ISSN 1536-383X print/1536-4046 online DOI: 10.1080/15363830802515923

MWCNTs Elastomer Nanocomposite, Part 2: The Addition of MWCNTs to an Oil-extended SBR-based Carbon Black-filled Rubber Compound Franco Cataldo,1 Ornella Ursini,2 and Giancarlo Angelini2 1

Trelleborg Wheel Systems spa, Tivoli, Rome, Italy Institute of Chemical Methodologies, Rome, Italy

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Abstract. Multi-walled carbon nanotubes (MWCNTs) were tested in a styrenebutadiene copolymer (E-SBR)-based rubber compound as partial replacement of N375 carbon black at 5 and 10 phr level. The composite studied was characterized by a high content of plasticizing oil. It is shown by the increase of the apparent shape factor of the MWCNTs that the oil permits a better dispersion and alignment of the MWCNTs in the rubber matrix. The reinforcing effect of MWCNTs has been studied both on sulphur-cured and radiation-cured nanocomposites. In both cases, a strong reinforcing effect has been measured at low elongations, but the extra-reinforcement tends to disappear at higher elongations due to the poor interaction between the MWCNTs surface and the rubber matrix. The presence of high levels of plasticizer permits to partially reduce the strong heat build up and hence the strong mechanical hysteresis normally observed in nanocomposites prepared with MWCNTs. Keywords: Multi-walled carbon nanotubes, MWCNTs, Nanocomposite, Styrenebutadiene copolymer, E-SBR, Reinforcement, Rubber compound, Mechanical hysteresis, Sulphur curing, Radiation curing

Address correspondence to Franco Cataldo, Trelleborg Wheel Systems spa, Via Tiburtina 143, I-00010 Villa Adriana, Tivoli, Rome, Italy. E-mail: [email protected] 55

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INTRODUCTION

In the previous work we explored the effect of the addition of increasing levels of multi-walled carbon nanotubes (MWCNTs) to a natural rubber compound in presence of a small amount of plasticizer (1). The melt mixing procedure which consists in the addition of the filler directly into the polymer matrix in a liquid-like state suffers of limited dispersion especially if the filler is a problematic filler such as MWCNTs. Furthermore, other works already appearing in literature on carbon nanotubes nanocomposites have evidenced the relatively poor dispersion and poor interfacial bonding which limit the carbon nanotubes from expressing their full potential as reinforcing fillers even at low loading levels (2). The purpose of the present work is to explore if the addition of a high amount of a plasticizer to a nanocomposite may help in a more uniform dispersion of the MWCNTs. It is well known that natural rubber cannot be compounded with large levels of oils (3). Therefore, we have decided to evaluate a compound based on a styrene-butadiene copolymer (SBR), a rubber that is normally compounded with large amount of plasticizer (3). The formulation we have chosen is characterized by an oil-extended SBR which already contains 37.5 phr of oil plus additional 23 phr of free oil. The meaning of ‘‘phr’’ is one part every 100 parts of rubber, which is the traditional way to express the quantitative amount of the various princeprincecomponents of a rubber formulation. By using the Einstein-Guth equation (1,4,5), we will evaluate the shape factor fMWCNT in the present formulation. Furthermore, the nanocomposites were both sulphur-cured and radiation cured in order to assess that the reinforcement exerted by the MWCNTs is completely independent from the crosslinking system adopted.

2. 2.1.

EXPERIMENTAL Materials and Equipment

MWCNTs were from Nanocyl as reported in Part 1 of this work (1). All the chemicals and compounding ingredients are the same described in Part 1 of this paper with the exception of the oil-extended styrenebutadiene copolymer (S1712) and of the curing accelerator MBTS-80, which is mercaptobenzothiazole 80% active predispersed in a rubber matrix. The equipment used in the present work was the same as in Part 1 (1). Radiation curing was made with gamma cell from Atomic Energy of Canada at a dose rate of 2.0 kGy/h to a total dose of 500 kGy using a 60 Co radiation source.

MWCNTs Elastomer Nanocomposite, Part 2

2.2.

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Rubber Formulation and Testing

The rubber compound formulation was based in a copolymer butadienestyrene with 23.5% styrene content (E-SBR, S1712) which is oil extended with 37.5 phr of aromatic oil extract. E-SBR: 137.5 phr, carbon black N375: variable (see Table 1), ZnO: 3.5 phr, stearic acid 1.5 phr, 6PPD: 1 phr, aromatic: oil 23 phr, paraffin wax: 1 phr, accelerator MBTS-80: 0.75 phr, accelerator TBBS: 0.6 phr, soluble sulphur: 2.2 phr. As reported in Part 1 (1), a special sheeting treatment was reserved to the composites after mixing and before curing: the repeated passage through the rolls of the open mill always in the same direction to ensure the alignment of the nanotubes. Sulphur curing was achieved with curing steam presses operating at 151uC for 30 minutes. For radiation curing use was made of the same procedure detailed in a previous work (6) where the green uncured rubber compound slabs were put inside the gamma cell for irradiation at room temperature. The samples were tightly wrapped in polypropylene sheets during irradiation (6).

3. RESULTS AND DISCUSSION 3.1.

General Observation

As reported in Table 1, the reference compound was loaded with 80 phr of N375 carbon black. The preparation of the two nanocomposites consisted in the partial replacement of carbon black with MWCNTs, respectively, at 5 and 10 phr but maintaining the total filler content (carbon black + MWCNTs) at the same amount of the reference compound, that is, 80 phr.

3.2.

Cure Kinetics

In Table 1 are reported the data concerning the rheometrics of the reference compound and the two nanocomposites prepared with the addition of MWCNTs. Surprisingly the cure speed parameters at the level of T50 and T90 (respectively the time to reach 50% and 90% of optimum cure) of the two nanocomposites with 5 and 10 phr MWCNTs do not show any significant differences in comparison to the reference compound. Only at the level of T10, it is possible to observe that the cure speed is significantly higher but only for the nanocomposite with 10 phr of MWCNTs. Similarly, also the scorch time in these compounds is not affected by the addition of MWCNTs as in the case of the natural

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Table 1.

Summary of test results on nanocomposite based on styrene-butadiene copolymer E-SBR TYPE S1712 CARBON BLACK N375 CARBON NANOTUBES PLASTICIZER DAE OIL

137,5 phr 80 phr none 23 phr

137,5 phr 75 phr 5 phr 23 phr


MH [dNm] ML [dNm] MH-ML [dNm] t 10 [min] t 50 [min] t90 [min]

13,37 2,326 11,04 4,28 9,49 19,73

14,61 3,536 11,07 4,25 9,59 19,76

17,6 4,586 13,01 1,54 9,08 20,09

Mooney ML (1+4) Scorch [min] Hardness IRHD scale ˆ 3] Density [g/cm

at 100uC at 127uC IRHD scale ˆ 3] [g/cm

59,8 21,35 68,3 1,162

77,1 19,65 73,2 1,155

98,3 20,76 76,8 1,156

Unaged modulus Sulphur cured at 151uC 6 309

Tensile (MPa) Elongation at break (%) M50% (MPa) M100%(MPa) M200%(MPa) M300%(MPa)

20,60 448 1,77 3,10 8,15 13,86

16,70 366 2,32 4,00 8,92 13,97

16,30 344 2,92 5,00 10,16 14,85

Unaged Modulus Radiation cured at

Tensile (MPa) Elongation at break (%)

16,80 547

14,40 559

15,00 508

F. Cataldo et al.

Rheometer test MDR 151uC 6 309 Arc +/2 0,5 100 cpm

Continued E-SBR TYPE S1712 CARBON BLACK N375 CARBON NANOTUBES PLASTICIZER DAE OIL

137,5 phr 80 phr none 23 phr



500 kGy

M50% (MPa) M100%(MPa) M200%(MPa) M300%(MPa)

1,22 2,17 5,87 10,06

1,60 2,71 5,65 8,81

2,18 3,65 7,04 10,27

GOODRICH HBU 151uC 6 30 min

TEMPERAT. INCREASE (uC) DEFLECTION %

69,1 28,8

77,5 212,8

86,8 214,0

METRAVIB CURE 151uC 6 30 MIN

TAN DELTA @ 30uC TAN DELTA @ 60uC

0,361 0,308

0,335 0,292

0,349 0,304


Table 1.

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rubber-based compound studied in Part 1 (1). Although it is well known that natural rubber is much more sensitive than synthetic rubber to basic substances, which accelerate dramatically the cure speed, the differences observed in the present work in comparison to the previous part can be rationalized in terms of volume fraction of MWCNTs filler. In fact, in Part 1, the MWCNTs reached a volume fraction of 0.039 and 0.058, respectively, at 10 and 15 phr and only at those loading levels was there a dramatic increase in the cure speed. In the present paper, also because the oil dilution, the volume fraction of MWCNTs is only 0.013 and 0.025, respectively, at 5 and 10 phr, significantly less than the levels reached in the previous work.

3.3.

Mooney Viscosity and Hardness

Figure 1 shows the trend in Mooney viscosity measured at 100uC on the green compound and also the trend in the hardness measured on the cured compound. In both cases the contribution of the MWCNTs is manifested as an extra reinforcing effect. Coming back to the EinsteinGuth equation (1,4) and using the same calculation approach used in

Figure 1. Mooney viscosity at 100uC and hardness of the cured compounds at different levels of MWCNTs. In both cases it is evident a linear increment.


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Part 1 (1) of this work, we can get a shape factor fMWCNTs 5 62.8 from the compound filled with 5 phr of MWCNTs and fMWCNTs 5 49.4 for the other compound loaded at 10 phr. Therefore, in the present formulation the dependence of the shape factor of MWCNTs from their volume fraction can be described from the following equation: f MWCNT ~13:844 WMWCNT {0:3463

ð1Þ

The shape factors found in the present compound rich in plasticizer is significantly higher than we have calculated previously in the case of the natural rubber-based compound where fMWCNTs was found in the range of 52.5–34.0 (1). From these considerations it appears evident that the plasticizer exerts a beneficial effect on the dispersion of the MWCNTs so that the shape factor becomes slightly closer the theoretical value of 157.9. The validity of our approach is supported by the fact that similar shape factor values and similar dependence of the shape factor from MWCNTs have been found in two completely different formulations.

3.4.

Mechanical Properties

The mechanical properties of the cured compounds are reported in Table 1. Only for the mechanical properties two different curing systems were adopted. One is the traditional sulphur curing which was used also to vulcanize the specimens for all the other tests (if cured) reported in Table 1. The other method is radiation curing that involves the use of high energy radiation from a c rays source. It is evident from the data in Table 1 that the extra reinforcing effect imparted by MWCNTs is maximum when measured on moduli at low elongation and vanishes almost completely at higher elongations. This can be visualized more easily in Figures 2 and 3 where the data of Table 1 have been reported as index values making 100 the data derived from the reference compound filled exclusively with 80 phr of N375 carbon black. The data show that the maximum reinforcement achieved at short elongations is completely independent from the vulcanization system adopted, whether sulphurbased or radiation-based. This implies that the reinforcement is exclusively dominated by the filler-polymer and filler-filler interaction and that the network formed by different type of crosslinks and crosslinking density display only a minor effect. On the other hand, the tensile strength of the nanocomposites, that is, the modulus at the maximum elongation before breaking, appears lower than that of the reference compound (see Table 1). This trend is not surprising at all since one can find an explanation in terms of filler network destruction at higher elongation as well as on the so-called filler de-wetting mechanism (5) and filler flocculation. This behavior has already been observed in

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Figure 2. Index values of the sulphur-cured moduli calculated from Table 1 making 100 the values of the reference compound without MWCNTs. It can be observed that the maximum reinforcement is obtained at lower elongations and becomes less pronounced at higher elongations.

other studies on nanocomposites such as the effect of nanoclay on the rubber reinforcement (7, 8). A similar trend, although less pronounced, has been observed in Part 1 of the present work (1) where MWCNTs were tested in a natural rubber based compound with a very small amount of plasticizer. In Figure 4 it is possible to follow the moduli trend in such natural rubber-based compound from data taken from Part 1 of the present work.

3.5.

Dynamic Properties

One of the main drawbacks derived from the addition of MWCNTs to rubber compound regards the strong mechanical hysteresis measurable under dynamic conditions, that is, under cyclic deformation. In a compound without plasticizer, the mechanical hysteresis due to the presence of MWCNTs is manifested both under compression and under extension (1). In the present case with a SBR matrix and a large amount of plasticizer, the increase of mechanical hysteresis derived by the


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Figure 3. Index values of the radiation-cured moduli calculated from Table 1 making 100 the values of the reference compound without MWCNTs. It can be observed that the maximum reinforcement is again obtained at lower elongations and becomes less pronounced at higher elongations demonstrating that this behaviour is independent from the curing system (sulphur-based or radiationbased) employed.

addition of MWCNTs is measured with the Goodrich flexometer at 35 Hz under compression. Figure 5 shows an almost linear increase in the heat buildup as function of the content of MWCNTs. Surprisingly, at low frequency (5Hz) and in tension mode, the increase in hysteresis has not been detected (see Figure 5). Evidently, under light stress conditions the increase of the viscous modulus is compensated by a simultaneous increase of the elastic modulus so that the tand parameter remains almost constant even after the addition of 10 phr of MWCNTs. Under high stress conditions, the conditions obtained by working with the Goodrich flexometer, the mechanical hysteresis caused by the poor surface interaction between the MWCNTs surface and the rubber matrix (1) becomes largely evident under the form of a strong temperature increase of the test specimen. Again the difference in mechanical hysteresis measured at low strain in comparison to the hysteresis generated in compression under more drastic conditions can find an explanation in the

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Figure 4. Index values of the moduli taken from Part 1 of this work (1). Also in the case of a natural rubber based compound with very low level of plasticizer the maximum reinforcement exerted by MWCNTs can be observed at low elongation. However, at higher elongation there is still a substantial increment.

reinforcing mechanism and polymer-filler as well as filler-filler interaction proposed by Kraus (5). The other undesired phenomenon, the permanent set after compression (see Table 1), attributable to the poor surface interaction with the polymer matrix of MWCNTs has been observed also in the present case of a SBR-based rubber compound and large level of plasticizing oil.

CONCLUSIONS MWCNTs tested in a SBR-based rubber compound as partial replacement of N375 carbon black and with a high content of a plasticizer show the same mechanical features observed in the case of natural rubberbased compound with low levels of plasticizer studied in Part 1. In particular, the increment in Mooney viscosity imparted by the addition of MWCNTs has permitted us to calculate the shape factor of MWCNTs. Such shape factor resulted higher than that measured in the previous Part 1 of this work suggesting a better dispersion and alignment of the MWCNTs achieved by the presence of high levels of plasticizer. The


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Figure 5. Dynamic properties measured at the Goodrich flexometer as heat built up (HBU) and at the mechanical spectrometer as tand, the ratio between viscous and elastic modulus.

reinforcing effect of MWCNTs addition on both sulphur-cured and radiation-cured compounds appears almost identical showing strong reinforcement at low elongations but negligible effect at high elongations and at the ultimate properties where phenomena like filler dewetting and filler flocculation become dominant. The high hysteresis of MWCNTs nanocomposites and their excessive permanent set remains a drawback of the compounds studied which is only partially mitigated by the presence of large excess of a plasticizer. The present study and the previous one (1) show that MWCNTs have a great potential to be used as reinforcing fillers, especially as partial replacement of carbon black in rubber compounds. The advantages in terms of extra reinforcement are undisputable and may find application in rubber tread compounds of the future provided that the cost of this material will become much more popular than today. The drawback of the poor interaction of the MWCNTs surface with rubber matrix can be overcome with tailor-made chemical functionalization of the nanotubes surface.

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ACKNOWLEDGMENTS Our thanks all the laboratory staff of Trelleborg Wheel Systems Central Laboratory, Tivoli, Italy, where large part of the present work has been made.

REFERENCES 1. Cataldo, F., Ursini, O., and Angelini, G. (2009) MWCNTs elastomer nanocomposite: Part 1. The addition of MWCNTs to a natural rubber-based carbon black filled rubber compound. Fullerenes Nanotub. Carbon Nanostruct, 17: 38–54. 2. Bokobza, L. (2007) Multi-walled carbon nanotube elastomeric composites: A review. Polymer, 48: 4907–4920. 3. Franta I. (ed.). (1989) Elastomers and Rubber Compounding Materials, Elsevier: Amsterdam. 4. Boonstra, B.B. (1987) Reinforcing fillers. Chapter 7. In Rubber Technology and Manufacture; Blow, C.M. and Hepburn, C. (eds.), Butterworths: London. 5. Kraus, G. (1978) Reinforcement of elastomer by particulate fillers. Chapter 8. In Science and Technology of Rubber; Eirich, P.F. (ed.), Academic Press: New York. 6. Cataldo, F., Ursini, O., and Angelini, G. (2007) Radiation-cured nanocomposites based on diene rubber and nanoclay. Progr. Rubber Plast. Recycl. Technol., 23: 209–222. 7. Cataldo, F. (2007) Preparation and properties of nanostructured rubber composites with montmorillonite. Macromol. Symp., 247: 67–77. 8. Cataldo, F. (2007) Effekte von nanoclays in gummi nanoverbundstrukturen. Gummi Fasern Kunststoffe, 60: 651–657.