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POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. (2008)

Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/pat.1163E-mail: suig2004@yahoo.com.cnessential modulus and strength of neat rubber are low, an

additional reinforcing phase is necessary for the practical

uses of rubbermaterials.4 Rubber is generally reinforcedwith

fibers, carbon blacks (CBs), and silicates. The full reinforcing

effects from these fillers are diminished due to their large size

and agglomeration, and application of the well-dispersed

incorporating CNTs into rubber1719 because a large loading

is needed for reinforcement due to material characteristics of

rubber, i.e. the framework of the reinforcement phase must

be formed in rubber for enhancing the low modulus and

strength of a neat rubber. At present, the batch production of

CNTs is available because of the progress in developing

synthetic methods.20,21 So further study and development of

CNT-reinforced rubber nanocomposites are now feasible.

Some research on the pretreatment of CNTs and interfacial

modification techniques have been reported, which is helpful

to the preparation of CNT-filled rubber composites.2224 But

*Correspondence to: G. Sui, The Key Laboratory of Beijing City onPreparation and Processing of Novel Polymer Materials, BeijingUniversity of Chemical Technology, Beijing 100029, China.with the addition of the treated CNTs. Copyright # 2008 John Wiley & Sons, Ltd.

DS: carbon nanotubes; polymer; nanocomposites; mechanical properties; pretreatment

DUCTION

er strengthen the properties of current polymer

, the addition of many kinds of inorganic fillers has

been researched and applied extensively. The size

ersion characteristics of inorganic particles have a

ect on the comprehensive properties of polymer

tes. The application of nanometer fillers to polymer

is a promising channel for property modification.

no-fillers have improved polymer performance

ly because of their high specific surface area

d to conventional fibers or particles.13

important type of polymer material, rubber is widely

nano-fillers into rubber to obtain beneficial mechan

physical properties is becoming crucial.5,6

In recent years, a great deal of attention has been

carbon nanotubes (CNTs), which consist of ro

graphite sheets built from sp2 carbon units, becaus

possess outstanding electrical, thermal, and me

characteristics.710 As ideal reinforcing fillers in p

composites, CNTs are expected to provide a

reinforcement effect compared to other nano-fillers

their inherent superior properties. As a consequenc

have been widely exploited in different kinds of po

and considerable enhancements of electrical cond

and mechanical properties have been achieved istorage modulus of the CNT/NR nanocomposites greatly exceeds that of neat NR and CB/NR

composites under all temperature regions. The thermal stability of NR was also obviously improvedG. Sui1,2*, W. H. Zhong2, X. P. Yang1, Y. H.1The Key Laboratory of Beijing City on Preparation and Processing o

Beijing 100029, China2School of Mechanical and Materials Engineering, Washington State

Received 18 September 2007; Revised 15 December 2007; Accepted 1

In order to achieve dramatic improvements in th

of carbon nanotube (CNT)-reinforced rubber co

(NR) nanocomposite was prepared through sol

Thermal properties, vulcanization characteristi

CNT/NR nanocomposites were characterized in

Through the addition of the CNTs treated usi

(hydrated silica, resorcinol, and hexamethylen

melting peak in differential scanning calorime

rate of NR slightly decreased.Meanwhile, the ov

alleviated. The dispersion of the treated CNTs in

themwere rather good. The mechanical properti

increase compared to the neat NR and traditionanatural rubber composites

bon nanotubes

1 and S. H. Zhao1

vel Polymer Materials, Beijing University of Chemical Technology,

ersity, Pullman, WA 99164, USA

rch 2008

formance of rubber materials, the development

osites was attempted. The CNT/natural rubber

mixing on the basis of pretreatment of CNTs.

nd physical and mechanical properties of the

trast to the carbon black (CB)/NR composite.

cid bath followed by ball milling with HRH

tramine) bonding systems, the crystallization

DSC) curves of NR weakened and the curing

ring reversion of CNT/NR nanocomposites was

rubber matrix and interfacial bonding between

the CNT-reinforced NR showed a considerable

/NR composite. At the same time, the CNT/NRCopyright # 2008 John Wiley & Sons, Ltd.

the dispersion of CNTs into rubber materials is still

problematic due to possible entanglement of the high

aspect ratio CNTs and the high viscosity of rubbers.

Although some studies on CNT-reinforced rubbers have

been performed, presently the overall performance of these

rubber nanocomposites has not reached the expected

potential.1719,2527

In our previous work, the powder CNT-reinforced rubber

composites were prepared using rubber suspension by

means of the spray drying process,28,29 and the overall

performance of these powder rubber nanocomposites was

increased. For further exploring the preparation and proper-

ties of solid rubber nanocomposites in bulk, the solvent

mixing assisted with a two-roll mill was adopted in this

study. After a great deal of exploratory experiments, some

good properties of the resulting rubber composites and

valuable results were obtained. The effects of the treated

CNTs on thermal properties, vulcanization characteristics,

and physical andmechanical properties of the natural rubber

(NR) composites were characterized in contrast to the

resorcinol, and hexamethylene tetramine with a weight ratio

milling was undertaken in a purified argon protect

atmosphere on a planetary ball mill. Steel balls with

diameters of 4 and 6mm were used as a grinding medium.

Ball milling was conducted with the disk revolution speed of

180 rpm. The scanning electron microscope (SEM) image of

the ball-milled CNTs is shown in Fig. 2. It can be found that

after the ball-milling treatment, the CNTs were loose in bulk,

which was beneficial to the dispersion and penetration of

CNTs inside the rubber macromolecular chains. Further-

more, there was no drastic decrease in the aspect ratio of the

CNTs due to the short milling time (0.5 hr), which can insure

stress transfer ability in the composites. The chemical

properties of the ball-milled CNTs and the interaction

between the CNTs and NR are reported in another of our

papers.34 CB (N326) was supplied by Tianjin Carbon Black

Factory, China. The NR used in this study was a Standard

Malaysian Rubber 5, and provided by Beijing Academy of

Rubber Industry, China. The ball-milled CNTs and CB were

mixed with NR in experiment, respectively.

Polym. Adv. Technol. (2008)

G. Sui et al.15:10:6) bonding systems. The dry tri-component system

Figure 1. TEM micrographs of raw CNTs.

Copyright # 2008 John Wiley & Sons, Ltd.traditional CB/NR composites.

EXPERIMENTAL

MaterialsThe CNTs used in the experiments were prepared by

chemical vapor deposition (CVD) of propylene gas at 7008Cusing Ni as a catalyst. The diameters of raw nanotubes vary

from 20 to 50 nm, and lengths vary from 100 to 1mm. The

transmission electron microscope (TEM) image of raw CNTs

is shown in Fig. 1. The raw CNTs were purified in

hydrofluoric acid (HF) for 24 hr, and then cleaned them

continuously with de-ionized water until chemically neutral.

The purified CNTs were obtained after drying for 24 hr in a

vacuum oven to remove moisture. To create some functional

groups on the surface of the CNTs and improve the bonding

between the CNTs and NR matrix, the purified CNTs were

further treated with blended acid and HRH (hydrated silica,comprising hydrated silica, resorcinol, and hexamethylene

tetramine, popularly known as HRH, was used as the

bonding system to achieve improved bonding between the

fillers and the rubber matrix.30,31 This effective and eco-

nomical silica-containing bonding system was introduced in

CNT/NR nanocomposites in this study. The dry HRH

bonding system was prepared in the experiment. The

purified CNTs were dipped in a blended acid solution with

a volume ratio between nitric acid and sulfuric acid of 1:3.

The loading of CNTs was 1 g for 10ml of blended acid

solution. The mixing solution was boiled and refluxed for

0.5 hr, and then the CNTs were carefully washed and

filtrated with de-ionized water until chemically neutral.

According to the analyzing results of previous infrared

spectroscopy,32,33 some functional groups such as hydroxyl,

carboxyl, and carbonyl were loaded on the CNT surface after

the blended acid treatment. The dry acid-treated CNTs were

blended with the HRH bonding systems with a weight ratio

of 25:3. To untie the entanglement of CNTs and facilitate the

co-mixing of CNTs and HRH bonding systems, the mixture

was milled for 0.5 hr in a ball-milling machine. Dry ball

Figure 2. SEM micrographs of ball-milled CNTs.DOI: 10.1002/pat

tested using a TGA 2050 thermogravimetric analyzer from

room temperature to 5008C at a heating rate of 108C/minunder nitrogen purge.

RESULTS AND DISCUSSION

Table 1. The composition of NR composites

eat NR CNT/NR CB/NR

100phr5.0phr3.0phr2.8phr1.3phr0.1phr

25phr 25phr

Preparation and properties of natural rubber compositesPreparation of the CNT/NR nanocompositesThe formulation of NR composites used in experiments is

shown in Table 1. The incorporation of treated CNTs into NR

was carried out by a solvent mixingmethod. Firstly, a certain

quantity of CNTs and NR was added into the toluene

solution, respectively. The solution of CNTs was sonicated

using a Brasonic1 Ultrasonic cleaner 1210 (Branson Ultra-

sonics Corporation) for 2 hr. Meanwhile, the solution of NR

was stirred and kept for certain duration until the rubber was

uniformly dissolved in the toluene. The toluene solution of

CNTs was subsequently dispersed into the solution of NR

with stirrer and ultrasonication simultaneously for 2 hr. Then

the dry CNT/NR mixture was obtained by evaporating the

solvent off at 808C under vacuum. Subsequently, thepreparation of CNT/NR nanocomposites was accomplished

by adding other ingredients including vulcanizing agent in

the formulation of composites in an open two-roll mill

(160 320mm2) at room temperature with the nip gap ofabout 1mm. At the same time, the neat NR and CB/NR

composites were prepared in an open two-roll mill

(160 320mm2) at room temperature by the traditionalmechanical mixing method.

Characterizations of the materialsDifferential scanning calorimetry (DSC) analysis was per-

formed using TA Instruments DSC 2910 in a nitrogen

atmosphere. The measurement was performed at tempera-

ture varying from100 to 2008Cwith a heating rate of 108C/min. The vulcanization behavior of neat NR and NR

composites were determined at the processing temperature

of 1508C using non-rotor rheometer with a model ofMM4130C2 produced by Beijing Huanfeng Mechanical

Factory, China. The microstructures of raw CNTs were

observed using JEM-200CX TEM produced by NEC

Company, Japan. The microstructures of treated CNTs and

the fracture surface morphologies of the cured NR compo-

sites were observed using S-3500N model SEM produced by

Constituent N

NRZinc oxideStearic acidSulfurN-Cyclohexyl-2-benzothiazole-sulfenamide2-MercaptobenzothiazoleBall-milled CNTsCBHitachi Company, Japan. The neat NR and NR composites

were pressed using a hot press, and then cut into standard

specimens. The tests of all mechanical properties of NR

vulcanizates were carried out according to the ASTM

standards. Five specimens were measured for every case

and the average values were taken. The dynamic mechanical

performances of neat NR and NR nanocomposites were

studied using a Perkin-Elmer 7 dynamic mechanical

analyzer in the pressing mode at a frequency of 11Hz.

The specimens were heated from 80 to 1008C at a heatingrate of 58C/min. The thermal stability of specimens was

Copyright # 2008 John Wiley & Sons, Ltd.Thermal properties of uncured rubberTo study the effect of the different fillers on the material

characteristics of NR, the uncured neat NR and NR

composites were subjected to DSC analyses. The DSC curves

are shown in Fig. 3. From these DSC curves, the thermal

properties of specimens were analyzed. The glass transition

region of NR occurred below608C, and the exotherm peaksbetween 160 and 2008C can be attributed to the vulcanizationreaction. The glass transition temperatures (Tg), the onset

temperature of curing (the temperature at the intersection of

the baseline and the tangent of the low temperature side

of the exotherm peak resulting from curing, Tc), and the peak

temperature of exothermal peak (Tp) were obtained and

listed in Table 2. Because functional groups introduced on

the surface of CNTs through the treatment with acid and ball

mill could result in physical adsorption and chemical

interaction between CNTs and rubber molecules, the

molecular mobilization of the rubber matrix in composites

was restricted. A slight increase in the Tg of NR with the

addition of fillers is indicated. At the same time, the

crystalline structure of NR was also affected by the activity

of the macromolecular chains. As NR is a type of semi-

crystalline material, the heat absorption peak appeared

around 308C in the curves due to the melting of the crystals.Figure 3. DSC curves of neat NR and NR composites.

Polym. Adv. Technol. (2008)

DOI: 10.1002/pat

The fillers can obstruct the mobilization of the rubber

macromolecular chains, and prevent macromolecular seg-

ments from obtaining ordered alignment of the crystal

lattices. Moreover, the percentage of the rubber matrix in the

reduced due to the addition of CNTs, when compared to

the neat NR and CB/NR composites. It would be an

advantage to eliminate the difference of crosslinking degree

between the surface and inside the thick rubber product

arising from the low thermal conductivity of the rubber

materials.

Physical and mechanical properties of NRvulcanizatesSeveral important physical and mechanical properties of the

neat NR and NR composites are tested and listed in Table 3.

Compared to the neat NR and CB/NR composites, the

addition of the CNTs brought about remarkable increase in

hardness, tensile modulus, and tensile strength to the rubber

material. The rebound resilience and dynamic compression

properties of the CNT/NR nanocomposites are better than

that of CB-filled NR composites, which is beneficial for the

actual application such as tire, etc., under a dynamic

condition. The fracture morphology of the cured CNT/NR

nanocomposites is shown in Fig. 5. It is noted that no obvious

Table 2. Thermal property parameters of NR and NR composites

Tg (8C) Tc (8C) Tp (8C) TpTc (8C)

NR 64.22 168.85 177.25 8.40CNT/NR 62.55 164.68 179.48 14.80CB/NR 62.78 163.66 174.96 11.30

N

G. Sui et al.composites decreased because of the addition of the fillers.

Therefore, the degree of crystallinity of the NR composite

specimens decreased accordingly and the heat absorption

peaks in DSC curves weakened after the addition of the

fillers. After adding fillers, the value of TpTc of NRcomposites showed an increasing tendency which indicated

that the vulcanizing rate of the NR matrix was decreasing.

Some studies indicated that the addition of CNTs could

absorb the basic accelerator species and delay vulcaniza-

tion.17,35 Therefore, the vulcanizing rate of the NR matrix

decreased owing to the addition of CNTs in this study.

Vulcanization characteristicsVulcanization curves of the neat NR and NR composites are

shown in Fig. 4. It can be seen that the scorch time of NR,

which is the measurement of premature vulcanization of

rubber, showed a slight reduction after the addition of the

fillers. Although some studies indicated that the addition of

CNTs could delay the onset of vulcanization,17,27 the

addition of the CNTs could improve the thermal conduc-

tivity of the NR material because of the superior thermal

conductivity of the CNTs, and could promote occurrence of

the vulcanization. Therefore, the effect of CNTs on the scorch

timemight depend on the composition andmicrostructure of

rubber composites. During the curing reaction period, the

torque of rubber specimens gradually increased. The time for

reaching the maximum torque of the nanocomposite

containing treated CNTs was a little longer than the other

two specimens, which reflected that the vulcanizing rate of

CNT/NR nanocomposites is lower. This is consistent with

the results of DSC analyses. Meanwhile, the maximum

torque of the CNT/NR nanocomposite specimen was the

highest among the materials. This is because the CNTs with

ultra-high modulus effectively restricted the changes in

polymer molecular configuration, and then enhanced the

modulus of the rubber composites. In the last region of the

vulcanization process, the over-curing reversion of NR

Table 3. The mechanical properties of NR composites

SamplesHardness (Shore A) 41Modulus at 300% (MPa) 1.8Tensile strength (MPa) 7.2Elongation at break (%) 680Rebound degree (%)Goodrich compression permanent set (%)Goodrich compression heat accumulation (8C)

Copyright # 2008 John Wiley & Sons, Ltd.aggregates of treated CNTs in rubber matrix were found. A

few outcrops of the CNTs existed on the fracture surface,

which indicated that the strong interaction between the

CNTs and the rubber macromolecules in the NR composites

Figure 4. Vulcanizing curves of neat NR andNR composites.

R CNT/NR CB/NR2 63 2 54 20.2 12.5 0.3 9.5 0.30.6 24.8 2.0 22.5 1.820 495 15 480 20

73 2 54 312.5 0.3 13.5 0.53.2 0.3 4.4 0.3

Polym. Adv. Technol. (2008)

DOI: 10.1002/pat

was formed. Therefore, the CNTs could carry stress

throughout the rubber matrix, and play an effective

recoverable strain energy in a deformed specimen, and the

loss factor is related to the energy damped due to energy

dissipation as heat.

The storage modulus of the neat NR and NR composites

versus temperature curves is shown in Fig. 6. It is found from

Fig. 6(a) that the storage modulus of the CNT/NR

nanocomposites is higher than that of the neat NR and

CB/NR composites from low temperature to high tempera-

ture regions. The high modulus and specific surface areas of

CNTs enhance the stiffness of the NR, which results in an

increment of storage modulus of the CNT/NR nano-

composite. At low temperature, the modulus of the neat

NR exhibits a high value due to the semi-crystalline

characteristic of NR, and a small variation of modulus of

NR appeared after the addition of fillers. With the increase in

temperature, although the E0 of all specimens manifesteddeclining trends, it is evident that stiffness of NR increases as

a result of addition of fillers. A shift of rapid decreasing

region of E0 towards higher temperature can be seen for NRafter adding the CNTs and the reinforcing effect is more

Figure 5. SEM micrograph of CNT/NR nanocomposites.

Preparation and properties of natural rubber compositesreinforcement role in the resulting nanocomposites. Pre-

dictably, the NR composites containing the CNTs possessed

the best mechanical properties among these specimens. This

CNT/NR nanocomposite exhibited great enhancements in

Shore A hardness, tensile modulus, and tensile strength by

16, 32, and 10%, respectively, compared to NR composites

reinforced with the same loading level of CB. The theoretical

modeling studies on the mechanical properties of CNT/NR

composites will be the subject of a future work.

Dynamic mechanical properties of NRvulcanizatesThe effect of the fillers on the dynamic mechanical property

of NR material was analyzed by DMA in this work. The

elastic modulus (E0) and the loss factor (tan d) of the neat NRand NR composites were characterized as functions of

temperature. Under an oscillating force, the resultant strain

in specimen depends upon both elastic and viscous behavior

of materials. The storage modulus reflects the elastic

modulus of the rubber materials which measures theFigure 6. The dynamic storage modulus as a function of

temperature for neat NR and NR composites.

Copyright # 2008 John Wiley & Sons, Ltd.obvious above the Tg of NR. The storage modulus of CNT/

NR nanocomposite in the rubbery region is the highest

among all specimens and keeps the steady value within the

testing temperature.

Figure 7 shows the temperature dependence of tan d of

neat NR andNR composites. The position of tan d peak in tan

dtemperature curve can also be used to identify the Tg of the

rubber materials. It can be seen that the peak of tan d of the NR

composite slightly shifts to a higher temperature compared to

that of the neat NR. It denotes that the mobilization of rubber

macromolecules is restricted due to the addition of fillers. The

enhancement of tan d peak position of NR composites

corresponds to the DSC experiment results. The tan d value of

NR composites is related to both NR and fillers material

characteristics. The adding of CNTs reduces the percentage

of the NR in composites, which lowers hysteresis loss of

the rubber under an oscillating force. Therefore, the height of

the tan d peak of NR decreases after the adding of fillers. In

general, the higher tan d value of a rubber from 20 to 108Ccan be used to predict the preferable anti-skid properties of

rubber material under wet conditions, and the small tan d

Figure 7. The loss factor as a function of temperature for

neat NR and NR composites.

Polym. Adv. Technol. (2008)DOI: 10.1002/pat

process of polymer materials affects the thermal stability

bulk scenarios through a facile techniquewhich improves the

G. Sui et al.value of a rubber in the region of 50608C implies the lowrolling resistance of rubber material.36,37 Therefore, it can be

deduced from Fig. 6(b) that compared to CB, the CNTs can

offer rubber materials significantly better anti-skid proper-

ties and low rolling resistance, which is of great value for

ideal tire materials.

Thermal stability of NR vulcanizatesFigure 8 shows TGA curves of the neat NR and NR

composites. By comparing the weight loss as a function of

temperature, the effect of CNTs on the thermal stability of

NR can be analyzed. Although there is no obvious variation

at the onset temperature of thermal degradation for different

specimens, a shift of rapid degradation region towards

higher temperature can be seen for NR after adding the

fillers, especially adding the CNTs. Meanwhile, the weight

loss of CNT/NR nanocomposites at the same temperature is

the smallest among all studied specimens. The reason is that

CNTs can impose the restriction on the mobilization of

rubber macromolecules and conduct heat homogeneously

and avoid the heat concentration. Therefore, the thermal

stability of NR was further improved by adding CNTs

compared to CB/NR composites.

However, inconsistent results had been reported by Puglia

et al.,38 where they found the thermal degradation rate

increased and the onset temperature of thermal degradation

decreased for epoxy resin after the addition of CNTs. It was

attributed to the high thermal conductivity and specific

Figure 8. TGA curves of neat NR and NR composites.surface areas of CNTs which can accelerate the transfer of

heat. This interpretation is not completely substantiated. The

improved thermal stability of PMMA was also proved in

thermal degradation experiment of CNT/PMMA.39 The

study of Troitskii et al. indicated that C60 can combine with

free radicals produced in the thermal degradation process of

PMMA, and form relative steady macro-free radicals.40

Therefore, the CNTs can also combine with free radicals

produced in the thermal degradation process of NR, forming

relative steady-free radicals, which then delay the pro-

gression of thermal degradation. Because the thermal

degradation of epoxy usually proceeds according to a cation

mechanism and the thermal degradation of PMMA and NR

proceeds according to a free radical reaction, the different

chemical mechanism during the thermal degradation

11. Stephan C, Nguyen TP, Chapell ML, Lefrant S, Journet C,Bernier P. Characterization of singlewalled carbon nanotu-

Copyright # 2008 John Wiley & Sons, Ltd.bes-PMMA composites. Synthetic Met. 2000; 108: 139149.

Polym. Adv. Technol. (2008)industrial processing, a requirement for high quality

industrial-scale production. In summary the results indicate

(1) improved performance for the eventual NR products, and

(2) improved processing for production (less complicated

procedures, higher quality, new manufacturing efficiencies).

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CONCLUSIONS

The CNT/NR nanocomposites were fabricated through

solvent mixing assisted with a two-roll mill. With the

addition of the treated CNTs, Tg of the resulting composites

increased slightly. At the same time, the area of crystal-

lization melting peak of NR in DSC curves and curing rate of

NR decreased. Compared to the neat NR and traditional CB/

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improved by adding treated CNTs. In conclusion, the NR

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excellent overall performance improvements due to the

reinforcement effect of the high aspect ratio functionalized

CNTs. Furthermore, advanced rubber nanocomposites with

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