Influence of benzyl ester oil on processability of silica filled NR compound

Hasleena Boontawee1,a, Charoen Nakason1,b, Azizon Kaesaman1,c, Anoma Thitithammawong1,d and Sopa Chewchanwuttiwong2,e

1Center of Excellence in Natural Rubber Technology (CoE-NR), Department of Rubber Technology and Polymer Science, Faculty of Science and Technology, Prince of Songkla University, Pattani

94000, THAILAND. 2Department of Science, Faculty of Science and Technology, Prince of Songkla University, Pattani

94000, THAILAND. ah.boontawee@gmail.com, bncharoen@bunga.pn.psu.ac.th, ckazizon@bunga.pn.psu.ac.th,

danoma-t@bunga.pn.psu.ac.th, ehsopa@bunga.pn.psu.ac.th

Keywords: benzyl ester, rubber, silica

Abstract. Modified benzyl esters from coconut, palm, and soybean oil were used as alternative

rubber processing oil to replace distillate aromatic extract oil (DAE). The effect of benzyl esters on

mixing, curing properties, filler–filler interaction and Mooney viscosity was investigated in silica

filled natural rubber compound. Benzyl ester oil was found to be effective processing oil with

decreasing of mixing energy and Mooney viscosity. Furthermore, it was found that coconut and

palm benzyl esters showed higher torque difference than the aromatic oil filled compound. Also,

benzyl ester oil gave similar level of cure time and dynamic properties compared with the rubber

compound with aromatic oils.

Introduction

Aromatic oil has been widely used in tire production. However, this type of oil contains high

concentration of polycyclic aromatic hydrocarbons (PAHs) which are identified as carcinogens.

Tires containing plasticizers with high concentrations of polyaromatic hydrocarbons have been

banned from the European market since January 2010. Therefore, there have been many research

work try to study the replacement of aromatic oil by others type of oil. Dasgupta et al. (2010)

studied chemical and physical properties for ten types of natural oils and six types of petroleum-

based oils. They found that natural oils are the best alternative processing aids for rubber. Some of

the natural oils, such as rubber seed, neem, dolma, and castor oils showed improvement of

processing properties, filler–filler interaction and dispersion properties in NR compound [1].

Adding small amount of vegetable oil into rubber compound caused significantly decreasing of

rheometric torque, abrasion and modulus because of high plasticization [2]. Therefore, vegetable oil

and modified vegetable oil might be important alternative processing oils that could be replaced

aromatic oil used in the rubber formulation. In this research work, extensive studies have been

carried out to utilize benzyl ester oil in silica filled natural rubber compound. Benzyl esters were

prepared from esterification of fatty acid of vegetable oil (i.e., coconut, palm, and soybean oil) and

benzyl alcohol in the presence of acid catalyst under appropriate temperature and reaction time [3].

Experimental

Benzyl ester oils were prepared by modification of fatty acid from three types of vegetable oils

(i.e., coconut, palm, and soybean oils). The preparation and characterization procedures of the

benzyl ester oils were carried out using a method described elsewhere in our previous work [3].

The rubber compound was prepared using a formulation according to ASTM 3184, as shown in

Table 1. The compound was mixed by using double mixing steps. The first step was done using an

internal mixer with a mixing chamber of 500 ml. The mixer was operated using a rotor speed of 60

rpm, a fill factor of 0.75 and mixing temperature of 60±3ºC. The mixing energy was recorded

during mixing operation. Zinc oxide, stearic acid, silica, silane coupling agent, and processing oil

Advanced Materials Research Vol. 844 (2014) pp 221-224Online available since 2013/Nov/15 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.844.221

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were added to natural compounding in first step. In the second step, the curatives were mixed with

the rubber mix on a two roll mill at room temperature. Then, cure properties (ASTM D2084 - 11),

Mooney viscosity (ASTM D 1646-94), and filler–filler interaction were investigated using moving

die processing tester at 110˚C, 0.2 Hz for 4 min.

Table 1. Compounding formulation

Ingredients phr

Natural rubber 100

Zinc oxide 5

Stearic acid 2

Processing oil* 10

Silica (Ultrasil VN3) 35

TESPT, 8 wt% relative to silica 2.8

TBBS 0.7

Sulfur 2

*Aromatic oil, Coconut benzyl ester, Palm benzyl ester and Soybean benzyl ester

Results and discussion

The mixing energy of silica filled NR with various type of processing oil is shown in Fig. 1. It

can be seen that incorporation for all types of vegetable oil and benzyl ester resulted in reduction of

mixing energy compared with that of the aromatic oil filled compound. It is also seen that the

soybean benzyl ester filled compound shows the lowest mixing energy. This might be attributed to

long molecular chain of soybean benzyl ester. Also, it is noted that the specific aromatic rings in

soybean benzyl ester caused lower friction in the rubber compound [4].

Fig. 2 shows Mooney viscosity of silica filled NR compounds with various types of processing

oil. It can be seen that the natural rubber compounds with all types of benzyl ester oil showed lower

Mooney viscosity than that of the rubber compound with aromatic oil. Furthermore, the coconut and

palm benzyl ester shows the lowest Mooney viscosity. This may be due to aromatic ring and short

molecular hydrocarbon chains in molecular structures of coconut and palm benzyl ester. They are

easily distributed into natural rubber matrix.

Fig. 3 and Table 2 show cure characteristics and curing properties of the natural rubber

compounds with various types of processing oil. It is clear that the aromatic oil filled rubber showed

longer scorch time than those of the compound with benzyl ester oils. This may be attributed to the

benzyl ester oil could be possibly act as an activator in vulcanization reaction of the natural rubber

molecules. In Table 2, it is seen that cure time (TC90) of all rubber compounds with different types

of oil are more or less very similar. It is also seen that higher torque difference (MH-ML) of the

rubber compounds with coconut and soybean benzyl ester oils compared with the rubber compound

with aromatic oil was observed. Basically, higher torque difference indicates higher degree of cross-

linking of the natural rubber molecules [5]. Therefore, it is concluded that the coconut and soybean

benzyl ester could act as a cure activator in the vulcanization process. This may cause higher extent

of cross-linking density as indicated by the torque difference. Cure rate index is a measure for rate

of vulcanization based on the difference between optimum vulcanization and incipient scorch time.

Table 2 shows the cure rate index of the silica filled NR compound at various type of the processing

oil. Only a slight difference in the rate of vulcanization was observed upon comparing the type of

processing oils in the compound. Therefore, adding 10 phr of processing oils to silica filled

compound have no effect in rate of vulcanization.

222 Advances in Rubber

Strain sweep experiments at a fixed oscillating frequency and temperature were performed on

the NR filled compounds with various types of processing oils. Results in terms of relationship

between elastic modulus (G’) versus strain magnitude are shown in Fig.4. These results reveal a

typical nonlinear viscoelastic behavior, i.e. the well-known Payne effect. Payne effect was occurred

when G’ decreases, as strain amplitude increases [6]. Therefore, it is seen that the all silica filled

compounds show the Payne effect. Furthermore, the silica filled compound with processing oils

show more linear viscoelastic behavior than the one without oil. This result is confirmed that adding

processing oil into the NR filled compound will caused decreasing of Payne effect with increasing

of filler-filler interaction [6]. However, there is no different of Payne effect among different types

of oils used in the silica filled NR compounds.

Fig. 3. Curing curevs of silica filled NR

compounds with various type of

processing oil.

Fig. 4. Relationship between storage

modulus and strain amplitude of silica

filled NR compounds with various type of

processing oil.

Fig. 2 Mooney viscosity of silica filled

NR compound with various type of

processing oil.

Fig. 1 Mixing energy of silica filled NR

compound with various type of processing

oil.

Soybean benzyl ester

Coconut benzyl ester Without oil

Coconut oil

Aromatic

oil

Palm benzyl ester

Soybean oil

Palm oil

Advanced Materials Research Vol. 844 223

Table 2 Curing properties of silica filled natural rubber compound with various types of processing

oil

Oils type ML MH MH-ML TS1 TC90 CRI

(dN.m) (dN.m) (dN.m) (min) (min) (min-1

)

Without oil 1.14 6.80 5.67 7.62 22.09 6.91

Aromatic oil 0.79 4.71 3.92 9.56 22.72 7.60

Coconut oil 0.66 5.14 4.48 8.58 22.89 6.99

Palm oil 0.72 4.02 3.30 10.18 23.87 7.30

Soybean oil 0.7 4.06 3.36 9.81 23.64 7.23

Coconut benzyl ester 0.69 5.78 5.08 7.36 20.40 7.67

Palm benzyl ester 0.42 3.78 3.36 8.79 22.94 7.06

Soybean benzyl ester 0.62 5.41 4.79 8.50 21.61 7.63

Summary

The benzyl ester oils could be used to reduce mixing energy, Mooney viscosity of the silica filled

natural rubber compound. Furthermore, the compound with benzyl ester oil showed higher torque

difference than that of the silica filled natural rubber compound with the aromatic oil. Furthermore,

cure time and dynamic properties in terms of filler–filler interaction of the rubber compounds with

benzyl ester oil gave similar level to that of the natural rubber compound with aromatic oils.

Acknowledgements

This work was financially supported by the PSU research fund, the Center of Excellence in Natural

Rubber Technology (CoE-NR) and the Graduate School, Prince of Songkla University, Pattani,

Thailand.

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224 Advances in Rubber

Advances in Rubber 10.4028/www.scientific.net/AMR.844 Influence of Benzyl Ester Oil on Processability of Silica Filled NR Compound 10.4028/www.scientific.net/AMR.844.221

DOI References

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properties and hysteresis loss of CB-filled vulcanized natural rubber, Int. J. Mec. Sci. 52 (2010) 168-174.

http://dx.doi.org/10.1016/j.ijmecsci.2009.09.001