Application of Benzyl Ester of Modified Vegetable Oils as Rubber Processing Oils

Hasleena Boontawee1,a, Charoen Nakason1, Azizon Kaesaman1,

Anoma Thitithammawong1 and Sopa Chewchanwuttiwong2

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

Keywords: Benzyl ester, Vegetable oil, Processing oil, Esterification

Abstract. Benzyl esters of fatty acids based on three types of vegetable oils (i.e., coconut, palm,

and soybean oils) were in-house prepared. They were used as alternative rubber processing oil to

replace conventional aromatic oil which has been banned by European community since December

2009. Fatty acids were first prepared by hydrolysis of vegetable oils and thereafter esterified with

benzyl alcohol in the presence of sulfuric acid as a catalyst. The reaction based on molar ratio of

fatty acid:benzyl alcohol:sulfuric acid was set at 1.5:1.0:0.05 gave yield of benzyl esters higher than

80%. Rubber compounds containing different types of benzyl ester were prepared according to the

standard formulation of ASTM 3184. It was found that the processing oil in the form of benzyl

esters is possible to use instead of aromatic oil in rubber formulation. Various parameters and

properties include mixing energy, Mooney viscosity, curing, mechanical and dynamic mechanical

properties of rubber compounds and vulcanizates have been investigated.

Introduction

Distillate aromatic extracts (DAE) with high content of polycyclic aromatic hydrocarbons (PAH)

are widely used as processing oils for rubber manufacturing especially tires production. However,

the European legislation classified the DAE as ‘carcinogenic’ [1]. Non-carcinogenic alternatives

have been developed to replace DAE in rubber and tire formulations. These new products, also

known as mild extraction solvate (MES) and treated distillate aromatic extract (TDAE) process oils

[2–3]. Moreover, there are some reports on natural oils in natural rubber-based truck tire tread cap

compound. These oils were found to be suitable on the basis of low PAH content [1]. In this

research work, extensive studies have been carried out to prepare the processing oil with low level

of PAH based on modified vegetable oils (i.e., their fatty acids) in the form of benzyl ester for

applying to rubber compounds. Fatty acids based on three types of vegetable oils (i.e., coconut,

palm and soybean oils) were used in the present investigation. The coconut, palm and soybean oil

have low, medium and high double bonds in molecular structure, respectively. Furthermore, they

also have the molecular weight in the same trend.

Experimental

2.1 Preparation of fatty acid benzyl ester

Saponification value (according to AOAC-920.160), acid value (according to AOAC-940.28)

and iodine value (according to AOAC-993.20) were tested for vegetable oils before the preparation

of fatty acid. Fatty acid was prepared by hydrolysis with NaOH at 70°C for 9 h with a molar ratio of

oil:NaOH = 1:6. The reaction mixture was separated by adding saturated salt solution. Hydrochloric

acid (4M) was then incorporated into the upper layer of the mixture to perform acidification. The

solution was neutralized with water. After that, the fatty acid product was preheated to 100ºC to

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evaporate the trace water content before esterification. A fixed amount of aromatic alcohol (i.e.,

benzyl alcohol) and H2SO4 catalyst were added into the fatty acid and then refluxed for 3 h. The

molar ratio of fatty acid:benzyl alcohol:sulfuric acid was fixed at 1.5:1:0.04. The residual fatty acid

in the mixture was eliminated by extracting with 10% w/v Na2CO3 solution. As a result, benzyl

ester with the yield of 80% was achieved.

Compound mixing

There are two steps for the preparation of rubber compounds. The formulation (according to

ASTM D3184) was adjusted with process oil 10 phr and high abrasion furnace black (N330) 50 phr.

In the first step, non-productive was done by using an internal mixer with a capacity of 0.5 L. Their

compounding was employed at 60 ± 3ºC, 60 rpm and Fill factor at 0.7, and the mixing energy data

was also recorded. Second step, the productive compound was mixed with sulfur and also sheeted

out through the two-roll mill at room temperature. Cure properties, mooney viscosity (ASTM D

1646-94), Cross-link density (Flory–Rehner equation), mechanical properties (ASTM D 412-06)

and dynamic mechanical properties (by DMA technique) were investigated.

Results and discussion

3.1 Physico-chemical characterization of oils

The physical characteristics of raw vegetable oils and benzyl esters are as shown in Table 1 and

2. Coconut oil shows much higher acid value than palm and soybean oils. Acid value indicates the

content of free fatty acid group in oil to react with benzyl alcohol in esterification. Saponification

values of coconut oil are higher than palm and soybean oils, respectively. These high saponification

values indicate the presence of ester groups, especially long-chain alkyl ester and low molecular

weight [1]. Iodine value indicates the presence of unsaturation in the material, with higher iodine

value representing higher unsaturation. Among the oils, soybean oil shows higher iodine value.

Comparative results of physico-chemical characteristics of benzyl esters with aromatic oil are

shown in Table 2. All types of benzyl esters are liquid with pale yellow color, whereas the aromatic

oil is a viscous fluid and dark brown. The benzyl esters gave lower viscosity, aromatic content and

glass transition temperature than those of the aromatic oil. This implies that the benzyl esters are

easier for processing and less toxic. It also shows that the coconut benzyl ester has higher aromatic

content than the other benzyl esters. This is due to its high carboxylic content with low molecular

weight. The esterification is occurred more in the coconut based product compared to others. The

crucial factor of Tg is the structure of processing oil. The C-O bonds in the chain of the ester group

make it more flexible than in C-C bonds in benzyl group. The aromatic oil is much less flexible

than benzyl esters, consequently the aromatic oil shows a high Tg. Moreover, it is found that

coconut oil shows a lower Tg value than other benzyl esters, this indicates that coconut oil has the

highest C-O bond relative to the acid value [4].

3.2 Processability properties of the rubber compound

Table 3 shows the processability properties of the rubber compounds with various types of

processing oil. It can be seen that the mixing energy of rubber compound using benzyl esters

decreased and they show similar mixing energy as aromatic oil. Rubber compounds with aromatic

oil show lower Mooney viscosity and rubber compounds with benzyl esters show higher values.

The higher value attributes to the lower degree of unsaturated structure of benzyl ester. The

processing oil with high degree of unsaturated structure is easier to insert into natural rubber matrix

(i.e. unsaturated isoprene unit) [5]. The torque differences (MH-ML) of rubber compounds with

benzyl esters are higher than rubber compounds with aromatic oil. High torque difference indicates

high degree of crosslinking rubber molecules. It is reported that the fatty acid ester can react as an

activator in vulcanization process and benzyl esters promote increasing of the degree of

crosslinking [6]. The cross-link density values are correlated well with the values of torque

differences.

Advanced Materials Research Vols. 415-417 1165

Table 1. Saponification value/ molecular weight/ acid value /iodine value of raw vegetable oils.

Name of oils

Acid value

[mg KOH/g oil]

Saponification value

[mg KOH/ g oil]

Molecular weight

[g/mol]

Iodine value

[mg l2/ g oil]

Coconut oil 8.75 249 674 9

Palm oil 0.76 198 847 51

Soybean oil 0.68 187 897 132

Table 2. Physico-chemical characteristics of the processing oil

Process oil

Aromatic oil Coconut

benzyl ester

Palm

benzyl ester

Soybean

benzyl ester

Appearance Viscous fluid,

dark brown

Liquid,

pale yellow

Liquid,

pale yellow

Liquid,

pale yellow

Specific gravity at 25ºC 0.9780 0.9965 0.9551 0.9793

Refractive index at 20ºC 1.570 1.489 1.477 1.484

Viscosity at 25ºC [cp] 60.6 3.3 7.5 5.2

Aromatics contents [%wt] 73 26.6 20.7 20.8

Glass transition point, Tg [ºC] -43 -112 -107 -110

Table 3. Processing properties of rubber compound with varies types of processing oil

Type of oil

in rubber compound

Total

energy [J]

Mooney

viscosity

[ML(1+4),

100°C]

Curing properties Cross-link

density

(×10-4)

[mol/cm3]

Scorch time

[min]

Cure time

[min]

MH-ML

[dNm]

Without oil 57,496 77 1.07 10.16 26.64 1.36

Aromatic oil 49,626 56 1.26 10.99 20.47 0.96

Coconut benzyl ester 50,807 60 1.28 10.59 21.04 1.12

Palm benzyl ester 50,247 62 0.97 10.56 23.36 1.21

Soybean benzyl ester 49,598 58 1.12 11.03 21.52 0.96

3.3 Mechanical and dynamic mechanical properties of the rubber compounds

The tensile strength and elongation at break of the rubber vulcanizates using various types of

benzyl ester are given in Table 4. It is found that the rubber compound with benzyl esters shows the

higher tensile strength than rubber compound with aromatic, as a result of hydrocarbon chain length

of benzyl esters which act as a good activator in vulcanizing system of rubber compound.

Moreover, it can be seen that the rubber compound with soybean benzyl ester shows the lower

tensile strength than other benzyl esters, because the double bond in hydrocarbon chain may

complexes the interaction with curing agent. It affects by decreasing the performance of curing

agent for vulcanizing system [6]. However, the addition aromatic oil provides the rubber with better

elasticity due to its compatibility with natural rubber molecules by increasing the free

volumebetween rubber molecular chains [7]. Glass transition temperature (Tg) and loss tangent (tan

δ) as a function of testing temperatures are summarized in Table 4. It can be seen that the addition

of oil into rubber compound reduces the Tg value by the formation of more flexible rubber

molecular chains. Moreover, benzyl esters provided Tg values of the rubber compounds slightly

lower than that of the compound with aromatic oil. For this reason, the processing oil containing

high content of benzene rings yields higher Tg value of the rubber than processing oil containing

1166 Advanced Materials, ICAMMP 2011

low content of aromatic structure. This agrees with original Tg values of the oil and benzyl ester

(Table 2). In the dynamic mechanical properties study, it can also consider the capability of using

for industrial applications from the tested data. The compounds with aromatic oil and palm benzyl

ester showed lower tan δ at 70 and 100 °C than those of the compounds with coconut and soybean

benzyl esters, which indicates better rolling resistance and lower heat buildup. Whiles the

compound with coconut benzyl ester showed the highest tan δ at 0 °C. This means that the coconut

benzyl ester offers good wet grip property to the rubber compound [1].

Table 4. Mechanical and dynamic properties of rubber compound with various types of processing

oil

Type of oil

in rubber compound

Mechanical properties Dynamic mechanical properties

Tensile

strength [MPa]

Elongation at

break [%]

Tg

[°C]

Tan δ

at 0 °C

Tan δ

at 70 °C

Tan δ

at 100 °C

Without oil 27.28 519.4 -38.1 0.14 0.13 0.16

Aromatic oil 23.51 626.2 -39.3 0.12 0.09 0.11

Coconut benzyl ester 25.82 577.5 -43.8 0.15 0.14 0.17

Palm benzyl ester 27.05 572.8 -41.3 0.11 0.09 0.14

Soybean benzyl ester 24.64 572.4 -43.3 0.13 0.14 0.20

Summary

Benzyl ester based on coconut, palm and soybean oils can be used as alternative processing oil in

the rubber compounding. The properties of rubber compounds prepared from benzyl esters were

comparable to those of rubber compounded with aromatic oil. However, palm benzyl ester provided

effective properties compared to other benzyl esters. Mixing energy and dynamic mechanical

property of the rubber compound with palm benzyl ester showed similar results as the compound

with aromatic oil. Moreover, palm benzyl ester gives better mechanical properties to the rubber

compound than the one with aromatic oil.

Acknowledgements

This work was financially supported by National Research Council of Thailand. Graduate

School, Prince of Songkla University, Pattani, Thailand and the Center of Excellence in Natural

Rubber Technology (CoE-NR)

References

[1] S.L. Dasgupta, S. Agrawal, R. Bandyopadhyay, Eco-friendly Processing Oils: A new tool to

achieve the improved mileage in tyre tread, J. Polym. Test., 28 (2009) 251-263.

[2] J.E. Pocklington, A safer alternative to aromatic process oils, Tire Technol. Int., Hamburg,

Germany, 1998.

[3] V. Null, Safe Process Oils for Tires with Low Environmental Impact, Tire Technol. Int.,

Hamburg, Germany, 1999.

[4] P. Opaprakasit, M. Opaprakasit, Thermal Properties and Crystallization Behaviors of

Polylactide and Its Enantiomeric Blends, Macromolecular Symposia (Special Issue: Advances in

Petrochemicals and Polymers),264 (2008) 113-120.

[5] A. Whelan and K.S. Lee, Developments in Rubber Technology-1, Essex, England, 1979.

[6] F.W. Barlow, Rubber Compounding, Dekker, New York, 1988.

[7] W. Pechurai, Influence of oil and fillers on properties of thermoplastic elastomer based on

natural rubber and polyethylene blends, Prince of songkla university, Pattani, Thailand, 2009.

Advanced Materials Research Vols. 415-417 1167

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