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Materials and Design xxx (2009) xxx–xxx

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Contents lists available at ScienceDirect

Materials and Design

journal homepage: www.elsevier .com/locate /matdes

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Technical Report

Effect of zinc oxide type on ageing properties of Styrene Butadiene Rubbercompounds

Nurhan Vatansever a,*, S�eyda Polat b

a Kocaeli University, Köseköy Vocational School, 41135 Kartepe-Kocaeli, Turkeyb Kocaeli University, Metallurgical and Materials Engineering Department, Umuttepe Campus, 41380 Kocaeli, Turkey

17181920

a r t i c l e i n f o

Article history:Received 19 June 2009Accepted 7 September 2009Available online xxxx

2122

0261-3069/$ - see front matter � 2009 Published bydoi:10.1016/j.matdes.2009.09.015

* Corresponding author. Address: Kocaeli Üniversikokulu, 41135 Kocaeli, Türkiye. Tel.: +90 262 373 515

E-mail addresses: vatansever@kocaeli.edu.tr (N.edu.tr (S�. Polat).

Please cite this article in press as: Vatansever NDesign (2009), doi:10.1016/j.matdes.2009.09.01

PRa b s t r a c t

The presence of zinc oxide in rubber compounds is tried to be minimized due to environmental concerns.In this study; the effect of different zinc oxide types on ageing properties of SBR compounds was inves-tigated. Active zinc oxide and zinc oxide coat on CaCO3 core were used in SBR compounds and their age-ing characteristics were compared. The changes in tensile properties, hardness and rebound resiliencewere followed and evaluated.

� 2009 Published by Elsevier Ltd.

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1. Introduction

Styrene Butadiene Rubber (SBR), a copolymer of styrene andbutadiene, is a synthetic rubber which is widely used all over theworld. Different ratios of styrene and butadiene are being usedfor different applications, but the ratio of 23.5% styrene and76.5% butadiene is the most commonly used one for general pur-pose rubbers. The major areas of consumption of SBR are in tireindustry, automotive industry and in the production of technicalparts, cable insulation material, shoe sole, mat, hose, etc. [1].

SBR compound recipes are similar to those of natural rubberand contain generally fillers, sulfur, accelerators, zinc oxide, stearicacid, softeners or extenders. Zinc oxide (ZnO) is added with stearicacid for cure activation and it is also reported that it improves flexfatigue and ageing resistance [1].

Although zinc is classified as a heavy metal, it is essential forhuman, animals and plants. Its deficiency has adverse effects onhuman growth, immunity, reproduction and cognity. But excessamounts of zinc should also be avoided as it causes pollution. Ex-cess zinc oxide may be released as a result of abrasion of rubbergoods and during their production and leaching into the rain water.Zinc has toxic effect on specially algae rather than other microor-ganisms. Therefore release of zinc to the environment should becontrolled and thus zinc oxide content in tires and rubber goodsshould be kept as low as possible for environmental and economicreasons [2–5]. Besides these concerns, the formation of excess zincsulfide during vulcanization which is deposited inside the surfaceof the mold as a residue is to be removed and this affects produc-tivity adversely [6].

Elsevier Ltd.

tesi, Köseköy Meslek Yükse-8; fax: +90 262 373 2719.Vatansever), seyda@kocaeli.

, Polat S�. Effect of zinc oxide ty5

ESurface area of ZnO is a parameter which affects the activity ofZnO. Zinc oxides having surface areas between 30 and 70 m2/g arecalled active ZnO whereas the conventional ZnO has the surfacearea of about 6 m2/g. Active ZnO has better dispersion and higheractivity resulting in less consumption of ZnO. The latest develop-ment in this field is the use of nano-sized ZnO having even lowerparticle sizes and providing rubber compounds with improvedabrasion resistance and tear strength. The impact of nano materialson health and environment were studied by several researchersand their potential toxicity were evaluated [7,8].

It is reported that zinc loaded clay can replace ZnO which is anovel way to reduce ZnO levels in rubber compound and minimizeits environmental impact significantly [4,5]. In a US Patent, theproducts containing ZnO coated particles are reported to have sim-ilar or better properties compared with conventional filler/ZnO andmay result in cost savings for the corresponding product formula-tions [9].

In this work; two different types of zinc oxides were used; oneis being active ZnO, the other one is ZnO coat on CaCO3 core. Thesecond material contains 40–45% less ZnO and is more environ-mental friendly. SBR compounds using these materials were pre-pared and their physical properties and ageing characteristicswere investigated to contribute to the present literature [10–13].

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2. Experimental

2.1. Materials

Materials used in compound preparation are given in Table 1;SBR 1502 produced by TOGLIATTI – Russia, having ML 1 + 4

(100 �C) value of 52, Corax N330 carbon black produced by DEGUS-SA, Ravenna-Italy, Nyflex 222B, severely hydro treated process oil,with aniline point of 103 �C produced by NYNAS, Stearic acid

pe on ageing properties of Styrene Butadiene Rubber compounds. J Mater

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Table 1Materials used in compound preparation.

Material Source

SBR 1502 TogliattiN330 DegussaProcess oil-Nyflex222B NynasStearic acid, MMFA 1810 MedanSulfur 80% MixlandTBBS MixlandZnO-RAC 75% BrüggemannZnO-NC105 Global chemical

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MMFA 1810 produced by MEDAN, Indonesia, Sulfur S80 GA F500produced by MIXLAND, France, TBBS 75 GA F200 (Nitrosaminefree), produced by MIXLAND, France, ZnO RAC (active) with surfacearea of 70 m2/g, having 93.5% Zn as ZnO produced by BRÜGGE-MANN CHEMICAL, Heilbronn and ZnO NC105 with active ZnO (sur-face area 20–30 m2/g) coating on an inert calcium carbonate coreproduced by GLOBAL CHEMICAL, Thailand. Active ZnO coat is about55–60% of the total weight and 40–45% is the inert CaCO3 core.Coat material contains 93–98% ZnO. Properties of the differenttypes of ZnO are given in Table 2.

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2.2. Compound preparation

SBR master batches were prepared in a Carter Bros Laboratoryscale mixer with 1.5 l capacity. The mixer has intermeshing rotorsand is equipped by an external heating unit. The recipes used inthis work are given in Table 3.

Master batch was prepared first by mixing ‘‘SBR + ZnO + stearicacid” for 30 s with a rotor speed of 47 rpm, then the door was openedand ‘‘processing oil + carbon black” were added and mixed for 80 s.Finally the compound was mixed 40 more seconds and dumped.Dump temperature of the master batch was about 105–110 �C.

For the final batch; first the master batch was mixed alone for75 s with a rotor speed of 20 rpm, then ‘‘sulfur + TBBS” were addedand mixed for 150 more seconds and dumped. Dump temperaturefor the final batch was about 85–90 �C.

Tensile test specimens were cured in vulcanization press at185 �C for 10 min. Discs, for rebound resilience tests, were curedat 185 �C for 15 min.

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Table 2Properties of different types of ZnO investigated.

Type Specific surfacearea (m2/g)

Core material ZnO%

Type % (by weight)

ZnO-RAC (active) Minimum 70 – – Minimum 92ZnO-NC105 20–30a CaCO3 40–45 93–98b

a Total area both coat and core material.b Zinc oxide content of the coat material.

Table 3SBR recipesa.

Material SBR-1 SBR-2

SBR 1502 100 100N330 50 50Process oil 10 10ZnO RAC 3 0ZnO NC105 0 3Stearic acid 1 1Sulfur 1.75 1.75TBBS 1 1

Total 166.75 166.75

a Values are given in phr (per hundred rubber).

Please cite this article in press as: Vatansever N, Polat S�. Effect of zinc oxide tyDesign (2009), doi:10.1016/j.matdes.2009.09.015

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2.3. Cure properties

Cure properties were tested using MDR (Moving Die Rheome-ter) – Alpha Technologies MDR 2000 according to ASTM D5289.Test conditions of 180 �C and 15 min were used. Mooney viscositymeasurements of the compounds were carried out using MooneyViscometer – Alpha Technologies Mooney MV 2000; test condi-tions were 100 �C and 145 �C for 4 and 20 min, respectively, with1 min pre-heating according to ASTM D1646.

2.4. Ageing studies

2.4.1. Thermal ageingTensile test specimens were prepared and tested for original

properties and then accelerated thermal ageing studies were car-ried out at in a laboratory type oven (Electro-mag with sensitivityof ±1 �C), kept at 125 �C in accordance with ASTM D573. Followingthe exposure to high temperature for different periods of time upto 240 h, samples were taken out of the oven and tested for tensileproperties and hardness.

2.4.2. Ageing with nitric acid (HNO3) vaporExtra pure nitric acid (65%) purchased from Merck was used for

ageing studies. Nitric acid was put in a beaker which was placed atthe bottom of a laboratory desiccator. Tensile test specimens wereplaced over the beaker in a way that they were exposed to HNO3

vapor and the cover of desiccator was closed. Tests were carriedout at room temperature for different periods up to 720 h. Follow-ing the exposure of SBR compounds to nitric acid vapor, measure-ment of tensile properties and hardness was carried out and resultswere compared with the original values.

2.4.3. Photo ageingPhilips HPR 125 W Hg lamp k > 300 nm) was used for UV ageing

studies. Samples were placed at a distance of 20 cm from thesource and were irradiated at 40 ± 2 �C for 72 h. Following the UVexposure, tensile properties and hardness values were measured.

2.4.4. Weatherometer ageingHygros 250 ACS model weatherometer (Angelantoni Industrie)

was used for ageing studies. In the first step; samples were keptat �30 �C for 7.5 h then temperature was raised to 50 �C and keptat this temperature for 7.5 h and finally the samples were kept at80 �C for 7.5 more hours. This cycle was repeated for three times.Following the completion of three cycles, tensile properties andhardness values were measured and compared with the originalones.

2.4.5. Oil ageingFor oil ageing studies, AKSOIL SAE 30 was used, having SAE vis-

cosity of 30, density 0.891 g/ml, flash point 248 �C, and viscosity at100 �C, in cSt, of 14–15.5.Tensile test specimens were immersed inoil at room temperature for 2, 5, 9 days, then they were taken outand immersed in acetone, blotted with filter paper. Initial weightand the weight after completing the immersion period in oil weremeasured. Amount of oil intake and tensile properties after oiltreatment were determined according to ASTM D471.

3. Results and discussion

3.1. Evaluation of cure properties

MDR results taken at 180 �C for 15 min are given in Table 4.Cure properties of both compounds are similar to each other. Opti-mum cure times (t90) are 4–5% shorter for SBR-2 compound indi-

pe on ageing properties of Styrene Butadiene Rubber compounds. J Mater

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Table 4MDR results (180 �C, 15 min).

SBR-1 SBR-2

S0 ML (dNm) 1.09 1.12S0 0 ML (dNm) 0.86 0.88t10 (min.) 1.76 1.75t50 (min.) 4.28 4.15t90 (min.) 8.28 7.90S0 MH (dNm) 7.50 7.56S0 0 MH (dNm) 1.04 1.09

Table 5Mooney viscosity results.

SBR-1 SBR-2

ML (1 + 4) 100 �C 41.6 43.1ML (1 + 20) 145 �C 27 28

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OriginalUV aged

Fig. 2. Hardness measurements after UV ageing.

Original

Weatherometer aged

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cating this compound is curing slightly faster. Mooney viscositiesof both compounds at 100 �C and 145 �C are given in Table 5. Moo-ney viscosity values are similar having only 3–4% difference be-tween the compounds. Scorch time ‘‘t5” and ‘‘t35” values couldnot be measured at this temperature.

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Fig. 3. Hardness measurements after weatherometer ageing.

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3.2. Evaluation of hardness tests

The hardness measurements were carried out with a Shore Atype durometer (Frank) in accordance with ASTM D2240. Resultsare given in Fig. 1. Hardness values of the original compounds weremeasured as 50 Shore A.

Upon ageing at 125 �C for 240 h hardness levels of both com-pounds reached to 90 Shore A. At this point samples were extre-mely brittle. At lower ageing times, the increase in hardnesslevels of the SBR-2 compound which contains ZnO NC105 isslightly higher compared to that of SBR-1 compound which con-tains active ZnO (RAC ZnO).

Hardness values before and after UV ageing were measured andplotted in Fig. 2. It is seen that hardness levels have increased by0.58% in SBR-1 and 1.35% in SBR-2. The increase in SBR-2 is moresignificant.

Hardness values before and after weatherometer ageing weremeasured and plotted in Fig. 3. It is seen that hardness levels haveincreased by 1.93% in SBR-1 and 1.92% in SBR-2. The increase ofhardness values in both compounds are about the same.

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Fig. 1. Hardness measurements after ageing at 125 �C.

Please cite this article in press as: Vatansever N, Polat S�. Effect of zinc oxide tyDesign (2009), doi:10.1016/j.matdes.2009.09.015

Hardness values up to 720 h of exposure to nitric acid vaporwere measured and plotted in Fig. 4. First an increase in hardnesslevels up to 360 h of exposure and a decrease in hardness at 720 hof ageing were observed. Surface of the specimen was more ef-fected from acid vapors and surface hardness increased more rap-idly compared to the inside. As a result of exposure to acid vapors;surface of the samples lost their smooth appearance to rough andirregular appearance (Fig. 5). The decrease of hardness levels at theend of 720 h acid vapor ageing may be explained by this irregularsurface, since the tip of the indentor may penetrate into the innerparts of the sample where the hardness levels are lower.

3.3. Evaluation of resilience properties

For resilience tests; ‘‘Santam Digital Rubber Resilience TestingMachine SRT-5” and ASTM D1054 test method were used. In thistest, a pendulum is released with a certain height (90� angle) andafter strike the rebound height was measured by an incrementalencoder. The ratio of rebounded height to the initial height is dis-played digitally on the machine panel in percentage (%). Six testswere carried out for each sample and the average of last threewas reported. Resilience tests were carried out for only thermallyaged specimens.

For both SBR-1 and SBR-2 compounds similar decreasing trendupon ageing at 125 �C was observed. The ratio of resilience values

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Ageing time (hours)

original4 hours9,5hours17hours24 hours45hours168 hours360 hours720 hours

Fig. 4. Hardness measurements after nitric acid ageing.

Fig. 5. Surface photographs of (a) SBR-1 and (b) SBR-2 samples after 720 h of nitric acid vapor ageing.

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of SBR-1/SBR-2 before and after thermal ageing is about the same(Fig. 6). SBR-2 compound has originally 2,9% lower resilience valuecompared with SBR-1 compound, after 74 h of ageing at 125 �C, theratio is 3%.This means that SBR-2 absorbs more energy, and is lesselastic in character.

3.4. Evaluation of tensile properties

For measurements of tensile properties ‘‘Instron Tensile TesterModel 3345” was used and tests were carried out in accordancewith ASTM D412. Test speed of 100 mm/min was utilized. Change

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original24 hours48 hours74 hours

Fig. 6. Resilience measurements after ageing at 125 �C.

Please cite this article in press as: Vatansever N, Polat S�. Effect of zinc oxide tyDesign (2009), doi:10.1016/j.matdes.2009.09.015

of tensile stress at break upon ageing at 125 �C and elongation aregiven in Figs. 7 and 8. The ageing behavior of SBR-1 and SBR-2compounds looks similar but in SBR-1 compound, retention of ten-sile stress and elongation upon ageing is mostly higher comparedwith SBR-2. This fact may be explained with the different typesof zinc oxides used in these compounds. In SBR-2, ZnO coat onCaCO3 core was used and the ZnO (which is reported to improveageing resistance) content is 40% lower compared with SBR-1 com-pound. The change of 100% modulus values upon ageing at 125 �Cis given in Fig. 9. The increase in 100% modulus values is higher inSBR-2 compound which appears to be more affected from ageing.

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Table 6Retention of tensile stress, elongation and 100% modulus upon ageing at 125 �C.

Ageing period (h) Retention of

Tensile stress (%) Elongation (%) 100% Modulus (%)

SBR-1 SBR-2 SBR-1 SBR-2 SBR-1 SBR-2

0 100 100 100 100 100 10026.5 75 60 59 48 132 14350.5 47 48 17 16 360 39775.5 53 50 14 7 491 –99.5 51 55 10 5 – –117.5 48 53 7 4 – –123.5 51 46 3 4 – –146 51 47 4 2 – –164 51 53 2 1 – –

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Fig. 12. Change of 100% modulus upon ageing in nitric acid vapor.

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UComparison of retention of tensile stress, elongation and 100%modulus values for SBR-1 and SBR-2 compounds are given inTable 6. Retention values are calculated with the formula (agedvalue/original value) � 100.

The change of tensile stress, elongation and 100% modulus uponageing with nitric acid vapor are given in Figs. 10–12. First a sharpdecrease in tensile strength and elongation is observed and thenthe rate of decay decreases. The influence of nitric acid vapor aremore pronounced at the exposed surface of the sample rather thaninside. Therefore although the appearance seemed effected fromnitric acid vapor heavily (Fig. 5), the overall properties are not af-

Please cite this article in press as: Vatansever N, Polat S�. Effect of zinc oxide tyDesign (2009), doi:10.1016/j.matdes.2009.09.015

fected to the same extent. Increase in 100% modulus values andafter about 300 h of ageing a stabilization followed by a slight de-crease was observed. Retention of tensile strength, elongation and100% modulus are given in Table 7. SBR-2 seems to be slightly af-fected from nitric acid vapor especially at higher exposure times.

For UV ageing; only 72 h of ageing was carried out at 40 �C andretention of tensile stress, elongation and 100% modulus values aregiven in Table 8. SBR-2 compound retained its tensile properties100% whereas SBR-1 compound has lost 3–5% of original values.

For weatherometer ageing; the ageing conditions are given inSection 2, 67.5 h of ageing was applied in total. Retention of tensileproperties is given in Table 9. Results reveal that SBR-2 is againbetter or equal in terms of retaining the original properties.

The change of weight upon oil ageing for different periods oftime is given in Fig. 13. Both compounds exhibit similar behavioras far as the weight gain is considered. The change of tensilestress at break, elongation at break and 100% modulus are given

pe on ageing properties of Styrene Butadiene Rubber compounds. J Mater

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Table 7Tention of tensile stress, elongation and 100% modulus upon ageing in nitric acidvapors.

Ageing period (h) Retention of

Tensile stress (%) Elongation (%) 100% Modulus (%)

SBR-1 SBR-2 SBR-1 SBR-2 SBR-1 SBR-2

0 100 100 100 100 100 1004 87 93 93 94 102 1009.5 76 76 78 89 118 10717 77 85 78 84 111 10624 76 77 78 80 125 12645 71 71 80 78 141 148168 67 63 69 66 185 166360 58 53 62 56 193 217720 45 48 64 61 135 130

Table 8Retention of tensile stress, elongation and 100% modulus upon UV ageing.

Ageing period (h) Retention of

Tensile stress (%) Elongation (%) 100% Modulus (%)

SBR-1 SBR-2 SBR-1 SBR-2 SBR-1 SBR-2

0 100 100 100 100 100 10072 94 101 97 101 96 100

Table 9Retention of tensile stress, elongation and 100% modulus upon weatherometerageing.

Ageing period (h) Retention of

Tensile stress (%) Elongation (%) 100% Modulus (%)

SBR-1 SBR-2 SBR-1 SBR-2 SBR-1 SBR-2

0 100 100 100 100 100 10067.5 89 105 89 96 108 110

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Fig. 15. Change of elongation at break upon ageing in oil.

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ulus SBR-1

SBR-2

Fig. 16. Change of 100% modulus upon ageing in oil.

Table 10Retention of tensile stress, elongation and 100% modulus upon ageing in oil.

Ageing period (h) Retention of

Tensile stress (%) Elongation (%) 100% Modulus (%)

SBR-1 SBR-2 SBR-1 SBR-2 SBR-1 SBR-2

0 100 100 100 100 100 1002 82 90 83 89 92 915 81 81 80 88 92 919 79 87 79 88 87 87

14 76 74 76 77 84 85

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in Figs. 14–16. In Fig. 14., tensile stress initially drops upon oilageing and then remains almost same. Similar trend in elongationvalues were also observed (Fig. 15). In Fig. 16 100% modulus val-ues displayed a gradual decay upon completion of 14 h oil ageingwhich may be due to the plasticizing effect of the oil which is ta-ken in the rubber compound. Retention of tensile strength, elon-gation and 100% modulus were given in Table 10. SBR-2 seemsretained its original tensile strength and elongation at break val-ues slightly better.

Please cite this article in press as: Vatansever N, Polat S�. Effect of zinc oxide tyDesign (2009), doi:10.1016/j.matdes.2009.09.015

4. Conclusions

As the result of investigating the effect of different zinc oxidetypes (active zinc oxide and zinc oxide coat on CaCO3 core) on age-

pe on ageing properties of Styrene Butadiene Rubber compounds. J Mater

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ing properties of SBR compounds; it is concluded that similar cure,mechanical and ageing properties may be obtained by using lessZnO. Consumption of lower amounts of ZnO is preferred for envi-ronmental as well as economic reasons and studies for this purposeshould be continued.

Acknowledgements

The authors acknowledge the financial support provided fromthe Unit of Scientific Research Projects at Kocaeli University (Pro-ject No. 2006-19). The authors also acknowledge the cooperationfrom Standard Profil company for compound preparation and cur-ing tests. The authors would like to thank Mr. Yusuf Güner, Com-pound Technologies Project Leader at Standard Profil Co., for hisgenerous help and suggestions throughout the work.

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