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Polymer Testing 28 (2009) 812Contents lists avaPolymer Testing

journal homepage: www.elsevier .com/locate/polytestMaterial Properties

Comparison of the mechanical properties at similar hardness level ofnatural rubber filled with various reinforcing-fillers

N. Rattanasoma,b,*, S. Prasertsri c, T. Ruangritnumchai c

a Institute of Science and Technology for Research and Development, Mahidol University, Salaya, Nakhon Pathom 73170, ThailandbCentre for Rubber Research and Technology, Mahidol University, Salaya, Nakhon Pathom 73170, ThailandcDepartment of Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailanda r t i c l e i n f o

Article history:Received 6 June 2008Accepted 13 August 2008

Keywords:Natural rubberSilicaCarbon blackClayMechanical propertiesFiller dispersion* Corresponding author. Institute of Science and TTel.: 66 81 4313039; fax: 66 2 4410511.

E-mail address: stnrt@mahidol.ac.th (N. Rattana

0142-9418/$ see front matter 2008 Elsevier Ltddoi:10.1016/j.polymertesting.2008.08.004a b s t r a c t

Commercially, the alteration of a rubber formulation is usually made in such a way as tokeep the hardness of the rubber product constant. This is because a specific hardness of therubber product sets the limit to its practical applications. Therefore, in this paper, naturalrubber (NR) vulcanizates containing various fillers were prepared to have the samehardness level, and their mechanical properties were compared and related to the degreeof filler dispersion. The results show that higher amounts of carbon black (CB) and silicaare needed for CB- and silica-filled natural rubber vulcanizates to achieve the samehardness value as a NR vulcanizate containing 6 phr of montmorillonite clay. At equalloading of fillers, clay-filled vulcanizate exhibits higher modulus, hardness, tensile strengthand compression set, but lower heat build-up resistance and crack growth resistance thanthose of the vulcanizates containing conventional fillers. For the vulcanizate having thesame hardness value, CB-filled vulcanizate gives the better overall mechanical propertiesfollowed by the clay-filled and silica-filled vulcanizates, respectively. The explanation isgiven as the better dispersion of carbon black, as can be seen in the SEM micrograph.

2008 Elsevier Ltd. All rights reserved.1. Introduction

Natural rubber (NR) exhibits outstanding propertiessuch as green strength and tensile strength because it cancrystallize spontaneously when it is strained. However,some properties of natural rubber such as modulus, hard-ness and abrasion resistance need to be improved for somespecific applications. Carbon black and silica are theconventional reinforcing-fillers used to enhance themechanical properties of various rubbers. In general,a carbon black-reinforced rubber exhibits higher modulusthan a silica-reinforced one. However, silica providesa unique combination of tear strength, aging resistance andadhesion properties [1].echnology for Research and D

som).

. All rights reserved.In recent years, nanoclays have attracted much atten-tion because of their ability to enhance the mechanicalproperties of rubber vulcanizates [29]. The improvementin properties can be achieved at remarkably low claycontent (less than 10 phr) if the clay silicate layers are ableto disperse into the polymer matrix at the nanoscale level[7,8]. It has been reported that the incorporation of a smallamount of clay gives rise to a more rigid material, which isreflected in a marked increase of hardness and modulus.The nanocomposites of natural rubber have been found togive optimum tensile strength and tear strength whencontaining 58 phr of organomodified montmorillonite [9].

Although, the effect of types of filler on the properties ofrubber vulcanizates has been extensively investigated atevelopment, Mahidol University, Salaya, Nakhon Pathom 73170, Thailand.

mailto:stnrt@mahidol.ac.thwww.sciencedirect.com/science/journal/01429418www.elsevier.com/locate/polytest

N. Rattanasom et al. / Polymer Testing 28 (2009) 812 9equal loading [911], it is also interesting to elucidate theeffect of types of filler on the mechanical properties of thevulcanizates at the same hardness level. This is becausea specific hardness is required for numerous industrialproducts. In this study, vulcanizates having the samehardness were prepared by adjusting the amount of eachfiller. Then, the mechanical properties of the vulcanizateswere determined. Also, the mechanical properties ofnatural rubber vulcanizates filled with equal amounts ofcarbon black, silica and montmorillonite clay (6 phr) werecompared. The degree of filler dispersion of all vulcanizateswas also examined and related to their properties.

2. Experimental

2.1. Preparation of rubber/clay composite

Concentrated natural rubber latex (60% dry rubbercontent, high ammonia) and the clay (Na-montmoril-lonite), with a cationic exchange capacity of 119 meq/100 gwere used to prepare the rubber/clay composite. Concen-trated NR latex was diluted with water to 30% dry rubbercontent. The amount of clay used was calculated such thatthe dried natural rubber would contain 6 phr of clay. Theclay suspension was prepared before mixing with NR latex.The clay was added into the excess water (2 g per 100 ml ofwater) andwas stirred vigorously for 4 h in order to achievegood dispersion of the silicate layers in the medium. Then,the mixture of clay suspension and NR latex was stirredgently for 30 min. Thereafter, the mixture was cast ina stainless steel tray and left at room temperature for a day.Finally, it was further dried in an oven at 50 C. The rubber/clay obtained was further mixed with other ingredients toprepare the rubber compound.2.2. Preparation of NR compounds and NR vulcanizates

The compound formulations are given in Table 1. Theconventional fillers used in this experiment were carbonblack (N330) andprecipitated silica (Hisil-233). The sampleswere designated as gum, C6, CB6, CB14, S6, S35. The lettersC, CB and S refer to clay, carbon black and silica, respec-tively. Thenumber followed the letters indicates theamountof filler in phr. The gum samplewas prepared for comparingTable 1NR compound formulations

Ingredients Gum C6 CB6 CB14 S6 S35

NR 100 100 100 100 100 100Clay 6 Carbon black (N330) 6 14 Silica (Hisil-233) 6 35Zinc oxide 5 5 5 5 5 5Stearic acid 2 2 2 2 2 2CBSa 1 1 1 1 1 1TMTDb 0.1 0.1 0.1 0.1 0.1 0.1Sulfur 2 2 2 2 2 2PEGc 2 2

a N-Cyclohexyl-2-benzothiazolesulfenamide.b Tetramethylthiuram disulfide.c Polyethylene glycol.properties with those of the filled samples. All ingredients,except the curatives, were mixed with rubber (or rubber/clay) in a laboratory-size internalmixer at a set temperatureof 50 C with a rotor speed of 50 rpm and a fill factor of 0.7.The total mixing time in the internal mixer was 5.5 min.After discharging, the compound was mixed on a two roll-mill for 1 min. Then, the curatives were added and furthermixed for 4 min. Finally, 10 end-roll passes were madebefore sheeting off. The compounds were finally compres-sion molded at 150 C. The cure time used for preparing theNRvulcanizateswas the timeatwhich the rheometer torqueincreased to90%of the total torque changeon the cure curve.The mechanical properties, hardness, tensile strength, tearstrength, heat build-up resistance and crack growth resis-tance, were measured on vulcanizates, a) containing equalamounts of conventional fillers and montmorillonite clay(6 phr), and b) containing different amounts of each fillersuch as to give the same hardness level.2.3. Mechanical property measurement

The hardness was measured using a Wallace Shore Adurometer, according to ISO 7619-1. Compression moldedsheets having a thickness of about 1.2 mm were used fortear and tensile testing. Tear and tensile properties of thespecimens were measured following ISO 34-1 and ISO 37,respectively. Crescent test pieces were used for deter-mining the tear strength. The measurements were carriedout using an Instron Universal Tester (Model 4301) witha crosshead speed of 500 mm/min and initial clamp sepa-ration of 65 mm. The values of tear and tensile propertieswere the average of 45 specimens.

The heat build-up and crack growth resistance of thevulcanizates were measured, in accordance with ISO 4666and ISO 132 using Goodrich flexometer and De Mattia typemachines, respectively. Dynamic compression set wasevaluated using the same specimens as for heat build-uptesting. The original height of the specimen was measuredprior to heat build-up testing and the specimen was left at25 C for 1 h after the heat build-up test. Thereafter, its finalheight was measured and the compression set was calcu-lated using the following equation.

Dynamic compression set% Ho Hf

Ho 100

where Ho original height (mm).Hf final height (mm).The fracture surfaces of the vulcanizates were examined

using a scanning electron microscope (SEM, JEOL JSM-6301) in order to view the degree of filler dispersion. Thesamples were sputtered with gold before examination toprevent charging on the surface.

3. Results and discussion

Hardness and 300% modulus of all vulcanizates areillustrated in Figs. 1 and 2, respectively. As expected, thegum gives the lowest hardness and modulus while hard-ness and modulus increase noticeably when 6 phr of clay isadded to the NR. At equal amounts of filler, the clay-filledvulcanizate exhibits the highest stiffness, followed by

40

45

50

55

60

Gum C6 CB6 CB14 S6 S35

Hard

ness (S

ho

re A

)

Fig. 1. Hardness of NR vulcanizates filled with various fillers at variouscontents.

0

5

10

15

20

25

30

35

Gum C6 CB6 CB14 S6 S35

Ten

sile stren

gth

(M

Pa)

Fig. 3. Tensile strength of NR vulcanizates filled with various fillers atvarious contents.

N. Rattanasom et al. / Polymer Testing 28 (2009) 81210CB-filled and silica-filled, respectively. In addition, theresults show that 14 phr of CB or 35 phr of silica is requiredfor the vulcanizates to reach the same hardness level as thevulcanizate containing 6 phr of clay. Compared to CB,a higher amount of silica is needed in order to achieve thesame hardness level as the clay-filled vulcanizate. This isthought to be due to the decrease in crosslink density whenhigh silica loading is used. In a previous study, crosslinkdensity of NR vulcanizates gradually decreases when silicaloading is more than 20 phr [12]. The explanation is givenas the adsorption of zinc complex on the silica surface, thuslowering the sulfur vulcanization efficiency.

Tensile strength of all vulcanizates is shown in Fig. 3. Ascan be seen, clay-filled vulcanizate exhibits the highesttensile strength while tensile strength of the other vulcani-zates is notmuch different. Likewise, tensile strength of gumandfilledNRvulcanizatescontaining50 phrofCB is showntobe similar [13]. The strain induced crystallization is known tobe responsible for the high strength of gumNR vulcanizates.

Fig. 4 displays the tear strength of various NR vulcani-zates. The gum exhibits the lowest tear strength while thevulcanizates having equal amounts of fillers give similartear strength. Although, it is established that sphericalparticles can blunt the crack tip more effectively than theplate-shaped filler having high aspect ratio particles [14],such a small amount of CB and silica (6 phr) used in thisexperiment may not be sufficient to effectively blunt thetear tip. However, the tear strength of CB- and silica-filledNR vulcanizates markedly increases when they wereprepared to have similar hardness to that of clay-filled0

2

4

6

8

300%

M

od

ulu

s (M

Pa)

Gum C6 CB6 CB14 S6 S35

Fig. 2. 300% Modulus of NR vulcanizates filled with various fillers at variouscontents.vulcanizate. At similar hardness level, CB-filled sampleexhibits similar tear strength to that of silica-filledvulcanizate, but much higher than that of the clay-filledvulcanizate. The higher amount of fillers in CB- andsilica-filled vulcanizates may obstruct the tear path moreeffectively than that for the clay-filled vulcanizate. It hasalso been reported that if the clay platelets are aligned inthe perpendicular direction to the applied force, the stressmay be concentrated at the sharp edges of particles andpromote earlier failure compared to spherical fillers [14].

Crack growth resistance, expressed as the length ofcrack growth (lower length of crack growth indicateshigher crack growth resistance), is shown in Fig. 5. In thisexperiment, the gum exhibits the highest crack growthresistance. The explanation is given as its lower modulusresulting in the lower stress concentration at its crack tip[13,15]. For the vulcanizates having equal amounts of filler,clay-filled vulcanizate shows the lowest crack growthresistance followed by CB- and silica-filled vulcanizates,respectively. It is apparent that these results correspondwell with their moduli. The highest modulus of C6 leads tothe highest stress concentration at its crack tip compared toCB6 and S6. On the other hand, a decrease in crack growthresistance is observed for S35 although its modulus issimilar to that of C6 and CB14. It is thought that the lowercrack growth resistance of S35 results from its poor silicadispersion or lower filler-rubber interaction, whichoverrides the effect of the modulus.

The fracture surfaces of NR vulcanizates filled withvarious fillers are illustrated in Fig. 6. As can be seen, the0

30

60

90

120

150

180

Gum C6 CB6 CB14 S6 S35

Tear s

tren

gth

(N

/m

m)

Fig. 4. Tear strength of NR vulcanizates filled with various fillers at variouscontents.

0

4

8

12

16

Gum C6 CB6 CB14 S6 S35

Len

gh

t o

f crack g

ro

wth

(m

m)

Fig. 5. Crack growth resistance at 20 kilocycles of NR vulcanizates filled withvarious fillers at various contents.

Fig. 6. SEM micrographs of NR vulcanizates filled with various fillers a

N. Rattanasom et al. / Polymer Testing 28 (2009) 812 11dispersion of various fillers in NR is not much differentwhen using 6 phr of filler. However, at the same hard-ness level, S35 has the poorest filler dispersion becausethe aggregate of silica can be readily observed inFig. 6(e). It has been also shown in the previous studythat filled rubber vulcanizate having poor silica disper-sion exhibits lower crack growth resistance despite itslower modulus compared to other vulcanizates [12].

Dynamic compression set results for various NR vulca-nizates are shown in Fig. 7. It is evident that compressionset is low for the gum. When a low loading of CB or silica(6 phr) is added, the compression set is not significantlychanged. Nevertheless, clay-filled vulcanizate gives some-what higher compression set than that of CB- and silica-filled vulcanizates when equal amounts of fillers are used.At the same hardness level, silica-filled vulcanizate (S35)t various contents; (a) C6, (b) CB6, (c) S6, (d) CB14 and (e) S35.

0

1

2

3

4

5

6

Gum C6 CB6 CB14 S6 S35

Co

mp

ressio

n set (%

)

Fig. 7. Dynamic compression set of NR vulcanizates filled with various fillersat various contents.

0

2

4

6

8

10

12

Gum C6 CB6 CB14 S6 S35

Heat b

uild

up

(C

)

Fig. 8. Heat build-up of NR vulcanizates filled with various fillers at variouscontents.

N. Rattanasom et al. / Polymer Testing 28 (2009) 81212shows the highest compression set when compared to thatof C6 and CB14. This is attributed to the poor filler-rubberinteraction or poor filler dispersion when the silica loadingis high. Another possible explanation is the decrease incrosslink density when silica loading is high. This resultalso agrees well with the previous study showing that thedynamic compression set tends to increase when the silicaloading is more than 30 phr [12].

It can be seen in Fig. 8 that heat build-up of the vulca-nizates containing a small amount of filler is not muchdifferent from that of the gum. However, it should be notedthat the clay-filled sample shows slightly higher heat build-up than CB-filled and silica-filled vulcanizates when usingequal amounts of filler.When higher amounts of CB or silicais added to prepare vulcanizates with similar hardness tothat of C6, CB-filled sample (CB14) exhibits the same heatbuild-up value as that of C6, while heat build-up of silica-filled sample (S35) increases considerably. The possibleexplanation is also given as the combined effects of thepoor silica dispersion and lower crosslink density of silica-filled sample when a high amount of silica is loaded. Heatbuild-up of a vulcanizate is reported to increase withdecreasing crosslink density [13].

4. Conclusions

The results show that modulus, hardness and tearstrength of NR filled with conventional fillers and clay aresuperior to those of the gum. Hardness and modulusincrease noticeably when 6 phr of clay is added. At equalloading of fillers, clay-filled NR vulcanizate exhibits highermodulus, hardness, tensile strength compression set butlower heat build-up resistance and crack growth resistancethan those of NR vulcanizates containing conventionalfillers. However, their tear strength is not much different.Compared to CB, a higher amount of silica is needed toachieve the same hardness as that of the clay-filled sample.At the same hardness value, CB-filled vulcanizate gives thebetter overall mechanical properties followed by the clay-filled and silica-filled vulcanizates, respectively. The poorsilica dispersion and lower crosslink density of silica-filledvulcanizate (S35) are thought to be the causes of its poorerproperties compared to the other vulcanizates.

Acknowledgements

The authors gratefully acknowledge Mr. WoothichaiThaicharoen for providing the clay/rubber composite usedin this experiment. Sincere appreciation is extended to staffof National Metal and Materials Technology Center forcarrying out the SEM measurement.

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Comparison of the mechanical properties at similar hardness level of natural rubber filled with various reinforcing-fillersIntroductionExperimentalPreparation of rubber/clay compositePreparation of NR compounds and NR vulcanizatesMechanical property measurement

Results and discussionConclusionsAcknowledgementsReferences