Alkanolamide as an Accelerator, Filler-dispersant and a Plasticizer in Silica-filled Natural Rubber Compounds

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--Material properties

Alkanolamide as an accelerator,

_ﬁ

ller-dispersant and a

plasticizer in silica- lled natural rubber compounds

_ﬁ

Indra Surya

a

, H. Ismail

b,*

_{, A.R. Azura}

b

a_{On study leave from Department of Chemical Engineering, Engineering Faculty, University of Sumatera Utara, Medan 20155, Sumatera}

Utara, Indonesia

b_{School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong Tebal 14300, Penang,}

Malaysia

a

i

r

n

t

f

i

o

c

l

e

Article history: Received 21 May 2013 Accepted 25 July 2013

Keywords:

A feasibility study was carried out on the utilization of Alkanolamide (ALK) on silica reinforcement of natural rubber (NR) by using a semi-ef cient cure system. The ALK wasﬁ incorporated into the NR compound at 1.0, 3.0, 5.0, 7.0 and 9.0 phr. An investigation was carried out to examine the effect of ALK on the cure characteristics and properties of NR

[17]

compounds.It was found that ALKgaveshorter scorch and cure timesfor silica- lled NRﬁ

[17]

compounds. ALK also exhibited higher torque differences, tensile modulus, tensile strength, ha rdness and crosslink densityof up to 5.0[17]phr of ALK loading, and then decreased with further increasesof ALK loading.The resilience increased with increased

[17]

ALK loading.Scanning electron microscopy (SEM) micrographs proved that 5.[37]0 phr of ALK in thesilica- lledﬁ NR compound exhibited the greatest matrix tearing line and surface roughness due to higher reinforcement level of the silica, as well as better dispersion and cure enhancement.

1. Introduction

Carbon blacks and silicas of varying forms and particle-sizes are very popular and have been widely utilized as reinforcingﬁllers in the rubber industry. In general, the properties of silica-reinforced rubber vulcanisates are usually inferior to those of carbon blacks, even when they are of comparable size[1]. This is attributed to the apparent dissimilarity in the surface chemistry of the two materials. Carbon black will react with sulphur during vulcanization

[30]

and form sulphur bonds that link the rubber chains, and also tie the carbon black to the rubber.This is known as

ﬁller-rubber crosslinking, another type of crosslink to the

rubber system, and is de ned as coupling bondsﬁ [2,3].[In 3 0 ]

marked contrast to the hydrocarbon functionality of carbon

black, silica does not react with sulphur, and coupling bonds will not be formed due to its hydrophilic silanol groups that are relatively incompatible with hydrocarbon rubbers, such as natural rubber[2].

For many applications, silicas are not satisfactory alter-natives to carbon blacks because of their relatively lower reinforcement levels. The fundamental problem is their surface chemistry, which is more polar and hydrated than carbon blacks, and which cause them to be dif cult to wet,ﬁ

disperse and interact with hydrocarbon rubbers. Numerous methods have been carried out to improve the reactivity of

[39]

silicas with the rubber phases. One such method is the utilization of a silane-coupling agent to modify the surface

[39]

of the silica. The modi edﬁ silica provide chemically active surfaces that can participate in vulcanization, providing coupling bonds between the silane and both the silica and the rubber phases[4,5]. Those products show signi cantﬁ

improvement in performance compared to their base materials.

*Corresponding author.

E-mail address:hana @eng.usm.myﬁ (H. Ismail).

Contents lists available at ScienceDirect

Polymer Testing

j o uh ro nm awe l pw awv g.ic eeeo :lrms. /e l o c a t e / p o l y t e s t

(5)

Another alternative method to overcome the de ciencyﬁ

of silica is the utilization of Alkanolamide. The ingredient is synthesized from Re ned Bleached Deodorized Palmﬁ

Stearin (RBDPS), a product of crude palm oil fractionation. This article describes the preparation and utilization of the new rubber ingredient as an additive in order to improve the reinforcement level of silica to natural rubber.

2. Experimental

2.1. Laboratory preparation of Alkanolamide

The reaction was carried out at atmospheric pressure in a 1000 ml reaction vesselfitted with a stirrer. Typically, 1.0 mol of RBDPS and 3.0 mol each of diethanolamine, so-dium methoxide and methanol (as catalysts) were placed in the reactionflask. The speci cations of the RBDPS arefi

[59]

given in Table 1.Themixture was stirred and heated;the reaction temperature was kept constant at 70 _{C for about} ﬁve hours. The resultant mixture was extracted with diethyl ether, and washed with saturated sodium chloride solution. The crude Alkanolamide was puri ed withﬁ

anhydrous sodium sulphate, and then concentrated by a rotary evaporator. The ﬁnal product, a cream-coloured, waxy, solid material, namely Alkanolamide, was used as an additive in silica- lled natural rubber compounds. Theﬁ

reaction procedure was as shown in Fig. 1.

2.2. Materials

Natural rubber grade SMR L obtained from Guthrie (M) Sdn. Bhd., Seremban, Malaysia was used. Other com-pounding ingredients such as sulphur, zinc oxide, stearic acid, N-isopropyl-N0_{-phenyl-p-phenylenediamine (IPPD),}

benzothiazolil disul de (MBTS), and precipitated silicaﬁ

(grade Vulcasil–S) were supplied by Bayer Co. (M) Sdn. Bhd., Petaling Jaya, Selangor, Malaysia.

2.3. Compounding

A semi-ef cient vulcanization system was used for theﬁ

compounding. The recipes for the preparation of the nat-ural rubber compounds are given in Table 2. The com-pounding was done on a two-roll mill (Model XK-160).

Table 3 shows the designation and composition of the

[94]

NR-based recipes used in this study.

[62]

2.4. Cure characteristics

The cure characteristics of the silica- lled NR com-ﬁ

pounds were obtained using a Monsanto Moving Die

Rheometer (MDR 2000), whichwas used to determine the

scorch time (ts2) cure time (t90), and torque difference

(MHdML) according to ISO 3417. Samples[62] of the respective

compounds were tested at 150 _C_._{The compounds were} subsequently compression moulded using a stainless steel mould at 150 _{C with a pressure of 10 MPa using a}

labo-ratory hot-press based on the respective curing times.

2.5. Tensile, hardness and resilience properties

Dumbbell-shaped samples were cut from the moulded sheets. Tensile tests were performed at a cross-head speed of 500 mm/min using an Instron 3366 universal tensile

[45]

machine according to ISO 37.The tensile strength andstress

at 100% elongation (M100), 300% elongation (M300),and

[22]

elongation at break wereinvestigated. The hardness

mea-surementsof the samples were performed according to ISO

[21]

7691-I using a Shore A type manual durometer. The

resil-ience was studied using a Wallace Dunlop Tripsometer

[21]

according toBS 903 Part A8. The reboundresilience was calculated according to the following equation:

% Resilience¼ ð1cos ½ [21]

q

2Þ=ð 1 cos

q

1Þ 100 (1)

where,

q

1is the initial angleof displacement (45), and

q

2is

the maximum rebound angle.

2.6. Scanning electron microscopy (SEM)

The tensile fractured surfaces of the compounds were examined by using a Zeiss Supra-35VP scanning electron microscope (SEM) to obtain information regarding the llerﬁ

dispersion and to detect the possible presence of

micro-[70]

defects.The fractured pieces were coatedwith a layer of gold to eliminate electrostatic charge build-up during examination.

2.7. Fourier transform infra-red (FTIR) spectroscopy

The FTIR spectra were obtained using a Perkin-Elmer 2000 series instrument. The spectrum resolution was 4 m1and the scanning range was from 550 to 4000 cmc 1.

2.8. Measurement of crosslink density

Swelling tests on the rubber vulcanisates were

per-[21]

formed in toluene in accordance with ISO 1817.The cured

test pieces (30 mm5 m2 mm) were weighed usingm

an electrical balance andswollen in toluene until equilib-[21]

rium, which took 72 h at room temperature.The samples

weretaken out of the liquid, thetoluene was removed from

thesample surfaces and the weight was determined.The

samples were then dried in the oven at 60 _{C until constant}

weight was obtained. The swelling results were used to

Table 1

Speci cation of RBDPS.ﬁ

Acid value 0.30 mg KOH

Free fatty acid 0.14%

Iodine value 35.3

Moisture 0.01%

Colour 2.8 red/20 yellow

Saturated

C12:0 (lauric acid) 0.1%

C14:0 (myristic acid) 1.2%

C16:0 (palmitic acid) 59.1%

C18:0 (stearic acid) 4.6%

C20:0 (arachidic acid) 0.4%

Unsaturated

C18:1 (oleic acid) 28.2%

C18:2 (linoleic acid) 6.3%

(6)

calculate the molecular weight between two crosslinks the volume fraction of the polymer in the swollen

spec-imen,Qmis the weight increase of the blends in toluene and

c

is the interaction parameter of the rubber

network-solvent (

c

[21]

of NR¼ 0.393).The degree of the crosslink density is given by;

Vc ¼ 1

2Mc

a (4)

3. Results and discussion

3.1. Characterization of Alkanolamide

Fig. 2 shows the typical infrared spectrum of the Alka-nolamide (ALK) obtained, andTable 4shows the wave-number of the functional groups in the ALK molecule. The strong broadband at 3370 cm1is an O H stretch of the–

ethanol. The C H stretches above and below 3000 cm– 1

indicate saturated and unsaturated portions exist in the molecule[7]. The umbrella mode at 1364 cm 1_{con rms}_ﬁ

the presence of a methyl group (CH3) which is attached to

the carbon atom. The (CH2) rocking band at 719 cm1

means that there are more than 4 methylene atoms in a row in the molecule. The C]O stretch is at 1615 cm 1, and

the amide C N stretch is at 1248 cm– 1. The spectrum

clearly shows the presence of wavenumbers of the func-tional groups of ALK.

3.2. Cure characteristics and crosslink density

The effects of ALK on the cure characteristics of the silica- lled NR compound can be seen inﬁ Fig. 3andTable 5. Fig. 3 shows that the scorch and the cure times decreased with increased ALK loading. It was observed that there was an enhancement in the cure characteristics, indicating that ALK could act as a co-curing agent or secondary accelerator in the curing process of the silica- lled NR compounds. Theﬁ

cure enhancement can be attributed to the amine content

[18]

of the ALK.Amine is an alkaline substance which will

in-crease the pH ofthe rubber compoundand, in most

in-[18]

stances,enhance the cure rate.Any material thatmakesthe

rubber compound more basicwillenhance the cure rate since acidic materials tend to retard the effect of the

[84]

accelerator[8].Increasing the ALK loadinghas the same effect as increasing the amount ofamine in the silica- lledﬁ

[17]

NR compound. Hence, it can be saidthat the higher the ALK

loading, thegreater the amount of amine andthe shorter

thecure rate.

According to Bateman[9], and Ismail & Ng[10][18], a certain organic substance containing nitrogen atoms promotes the

curing process of ole nic rubber, comprising sulphur andﬁ

primary accelerators, throughthe formation of complexes

whichare responsible for theﬁssion of the sulphur

mole-cules to form crosslinksbetween the linear rubber chains.It

is expected that the amine-content of the ALK would accelerate the cure and is responsible for enhancing the cure rate and the cure state of the silica- lled NRﬁ

compound.

As presented in Table 5, the incorporation of up to

5.[18]0 phr of ALK into the silica- lled rubber compoundﬁ

increased the torque difference (MHdML), i.e. the

maximum torque minus the minimum torque. Further

Fig. 1.Chemical reaction of the preparation of Alkanolamide.

Table 2

[17]

Compositionof the rubber compounds.

Ingredients Content (phr)a

Natural rubber (SMR L) 100.0

Alkanolamide 0.0; 1.0; 3.0; 5.0; 7.0; 9.0 Precipitated silica (Vulcasil - S) 30.0

Sulphur 1.5

Zinc oxide 5.0

IPPD 2.0[72]

Stearic acid 2.0

MBTS 1.5

a _{parts per hundred parts of rubber.}

Table 3

Designation and Composition of the NR-based recipes.

Designation Composition NR/Silica/Alkanolamide (phr)

(7)

increases in the ALK loading decreased the torque

differ-[53]

ence. It is also evident that the minimum torque (ML)

decreased withincreases in the ALK loading.It has been reported that ML corresponds to the ﬁller- ller interﬁ

[67]

agglomeration [11]andis a measure of thestock viscosity [12]. The incorporation of ALK into the silica- lled NRﬁ

compound reduced the viscosity of the compound and facilitated a relatively strong interaction between the ALK and the silica ller, while the silica particles had a weakerﬁ

tendency to interact with each other and form a ller- llerﬁ ﬁ [18]

agglomeration.Consequently, the processability of the sil-ica dispersion in the rubber phase became easier and increased the degree of silica dispersion.

Theoretically, the torque difference (M

[18]

HdML)

repre-sents the shear dynamic modulus, which is indirectly

related to the total crosslink density of a rubbercompound [18]

[10,13–51 ].The total crosslink density is contributed by the

sulphide crosslinks and physical crosslinks [16,17]. The addition of up to 5.0 phr of ALK into the silica- lled NRﬁ

compounds increased the torque difference (MHdML) f o [51]

the silica- lled NR vulcanisates.ﬁ This is clearly attributed to the action of theALK, not only as a secondary accelerator,

but also as ﬁ ller dispersant. The ALK accelerated the

sulphur reaction and enhanced the state of the sulphide

[51]

crosslink.The ALKincreased the degree of silica dispersion and formed the additional physicalcrosslinks by silica NR–

attachments (which will be seen in FTIR result).

The reduction of the torque difference (MHdML) fter a

the 5.0 phr loading, i.e. the E/7.0 NR compound, is most probably due to the softening or lubricating effect of the

[46]

excessive ALK, which caused a lower crosslink density.This

can be attributed to thephenomenon of dissolving a part of the elemental sulphur and silica particles into the excessive ALK and, as a consequence, less sulphur and silica were

attached to the NR chains.The ALK is a unique molecule

which is structured by a non-polar hydrocarbon chain and polar terminal groups. The non-polar hydrocarbon chain dissolved in the non-polar NR, forming a monolayer and lubricating the NR; whereas the polar terminal groups together with other curatives and silica formed interme-diate complexes that remained insoluble. The excessive amount of ALK caused a more pronounced lubricating ef-fect, and produced boundary layers which coated and absorbed not only the curatives but also the silicaﬁller inside the layers. Through this mechanism, the ability of sulphur to form a sulphide crosslink and silica to form a

[46]

physical crosslink would be less.This explanation could be

supported by amorphological study of the tensile fractured

surface of the E/7.0 NR compound. The fractured surface

seemed smoother than that of the D/5.0 NR compound due to excessive ALK loading. ALK was synthesized from the

Fig. 2.The infrared spectrum of Alkanolamide.

Table 4

The Wavenumbers of Functional Groups of Alkanolamide Molecule.

Vibration Wavenumber (cm1

)

O H stretch– 3370

Unsaturated (C]C) 3006

Saturated (CH3) 2852

C]O stretch 1615

CH3Umbrella mode 1364

C N stretch– 1248

CH2rocking 719

[17]

Fig. 3.Scorch time ( )t2and cure time (t90) of thesilica- lled NR compoundsﬁ

at various ALK loadings. I. Surya et al. / Polymer Testing 32 (2013) 1313–3 211

(8)

RBDPS, a palm oil fraction. Palm oil has the effect of plas-ticizing or lubricating rubbers [18].

The crosslink density of the silica- lled NR vulcanisatesﬁ

was determined by the FlorydRehner approach [Eq.(2)]. Fig. 4 shows the crosslink density of the vulcanisates at room temperature. The addition of up to 5.[17]0phr of ALKinto

the silica- lled rubber compound increased the crosslinkﬁ

density, andfurther increases in the ALK loadingdecreased

[18]

the crosslink density. This observation is in line with that of

the torque difference (MHdML), which is an indirect

measure of thecrosslink density ofthe rubber vulcanisates.

3.3. Silica dispersion

The degree of silica dispersion in the NR phase can be determined quantitatively by equation [19,20],

L¼

h

rmr

in which:

h

r¼[MLf/MLg], mr¼[MHf/MHg], where MLfand

MHf are the minimum and the maximum torques of the ﬁlled vulcanisates; MLgand MHgare the minimum and the

maximum torques of the gum NR vulcanisates. Based on the previous study [21] (for NR gum compound vulcanized with the same semi-ef cient formulation), Mﬁ Lg ¼ 0.04;

MHg ¼4.81.[18]The lower the value of L at a particular silica

loading, the better is the silica dispersion in the rubber

[18]

phase. The value of L forthe silica dispersion in theNR phase is presented in Table 6.

From the data shown in Table 6, it can be seen that ALK

[17]

caused a lowering in the value of L.The higher the ALK loading, thelower isthe value ofL.The lower value of L indicates better silica dispersion, which provides a greater available surface area for the silica-NR interaction. The

greater the available surface area for the interaction, the higher is the total crosslink density [16,17]. However, as presented inFig. 4, the total crosslink densities of the E/7.0 and F/9.0 silica- lled NR vulcanisates were lower thanﬁ

those of the B/1.0, C/3.0 and D/5.0 silica- lled NR vulcani-ﬁ

sates, and even the former vulcanisates had lower values of L. This phenomenon can be attributed to the dominant plasticizing effect of ALK. As discussed before, the excessive amount of ALK trapped the silica inside its layers, thus causing less formation of physical crosslinks.

3.4. Mechanical properties

Figs. 5 8– show the effect of ALK on the tensile, hardness

and resilience of the silica- lled NR vulcanisates. It can beﬁ Table 5

The Effect of Alkanolamide Loading on Torque Difference of Silica- lled NRﬁ Compounds.

Torques NR-based compound

A/0.0 B/1.0 C/3.0 D/5.0 E/7.0 F/9.0 Maximum

torque, (MH), dN.m

10.34 10.29 11.40 11.60 9.40 9.16 Minimum

torque, (ML), dN.m

1.23 0.78 0.65 0.51 0.24 0.23 Tork diff.,

(MHdML), dN.m

9.11 9.51 10.75 11.09 9.16 8.93

Fig. 4.Crosslink density of the silica- lled NR vulcanizates at various ALKﬁ loadings.

Table 6

The Value of L for Silica Dispersion in NR Phase. Torque Property NR-based compound

A/0.0 B/1.0 C/3.0 D/5.0 E/7.0 F/9.0 Minimum torque,

(ML), dN.m

1.23 0.78 0.65 0.51 0.24 0.23

hr¼[MLf/MLg] 17.50 11.14 9.29 7.29 3.43 3.29

Maximum torque, (MH), dN.m

10.34 10.29 11.40 11.60 9.40 9.16 mr¼[MHf/MHg] 2.12 2.11 2.34 2.38 1.93 1.88

L¼hrdmr 15.45 9.03 6.95 4.91 1.50 1.41

Fig. 5.Modulus at 100% and 300% elongations of the silica- lled NR vul-ﬁ canizates at various ALK loadings.

Fig. 6.Tensile strength of the silica- lled NR vulcanizates at various ALKﬁ loadings.

(9)

seen that M100 and M300 increased up to the maximum level (5.[17]0 phr)and then decreased with further increases in

[17]

the ALK loading.The results of the tensile strength and

hardness also exhibited a similar trend.According to Hertz

[22], and Ismail & Chia [23], the modulus or stiffness/ hardness and tensile properties are dependent only on the degree of crosslinking. According to Ignatz-Hoover & To [24], as the crosslink density of a rubber vulcanisates in-creases, the elastic properties, such as tensile modulus, hardness and tensile strength increase, whereas viscous loss properties, such as hysteresis, decrease. Further in-creases in the crosslink density will produce a vulcanisate that tends towards brittle behaviour. Thus, at a higher crosslink density, such elastic properties as mentioned earlier begin to decrease.

The enhancement in tensile modulus, tensile strength and hardness with up to 5.0 phr ALK loading is attributed to a higher reinforcement level of silica to natural rubber due to a better degree of silica dispersion or wetting in the NR phase, as well as the cure rate and cure state enhancements phenomenon of the silica- lled NR compound. Thisﬁ

explanation is in line with the data in Table 6, and the SEM micrographs of the silica- lled NR vulcanisates. Theﬁ

micrograph of D/5.[22]0 exhibits a homogeneous micro dispersion ofsilicaas a result of the incorporation of ALK.

According to Hamed[25], in order for a ller to give sig-ﬁ

ni cant reinforcement, the size of its particles must beﬁ

small ( 1

m

m) and well dispersed in the rubber matrix. The deterioration of those properties after 5.0 phr can be attributed to the lower degree of crosslink density and the more pronounced softening effect of the excessive ALK.

As presented in Fig. 7, the addition of 1.0 phr ALK into the silica- lled NR compound resulted in the vulcanisatedﬁ

B/1.0 NR compound having a higher elongation at break. Increasing the ALK loading caused a further increase in the percentage of elongation at break. This can be attributed to the function of the ALK as an internal plasticizer to the silica- lled NR system. According to Moneypenny et al.ﬁ

[24], a plasticizer is a compounding material that is used not only to improve the rubber compound processing but also to modify the physical properties such as hardness,

ﬂexibility or distensibility of the rubber vulcanisates, and one of the sources from which it is made is natural oil/fat [18]. However, ALK is a waxy, solid material, and is derived from RBDPS-Crude Palm Oil, a type of natural oil. Although ALK has a smaller molecular size compared to NR, that additive could provide a monolayer in the rubber com-pound which provides free volume, thus allowing more mobility/ exibility for the rubber chains. Increasing theﬂ

ALK loading has the same effect as an increase in free vol-ume that will enhance the exibility or the extensibility ofﬂ

the silica- lled NR vulcanisates.ﬁ

Fig. 9 shows the effect of ALK on the resilience of the

[47]

silica- lled NR vulcanisates.ﬁ Resilienceis the ratio of energy

released by therecovery from deformation tothatrequired

to produce the deformation[26]. According to Hofmann [27], and Ignatz-Hoover & To[24][90], the rebound resilience is enhanced to some extent as the crosslink density rises, and

the resilience isrelated to theﬂexibility of the molecular

chains;the moreﬂexible the molecular chains, the better the resilience. The incorporation of ALK into the silica- lledﬁ

NR compound enhanced the resilience of the silica- lled NRﬁ

vulcanisates. Again, this can be attributed to the functions of the ALK as an accelerator andfiller dispersant which en-hances the total crosslink density, and as a plasticizer which improves the exibility of the lled rubber chains.fl fi

[17]

3.5. Scanning electron microscopy (SEM) study

Scanning electron microscopy (SEM) was used to

[46]

determine thesilica dispersion in the NR matrix.Fig.10

provides the SEM micrographs of the tensile fractured

surfaces of the silica- lled NR vulcanisates with andﬁ

withoutthe addition ofALK:(A) A/0.0; (B) C/3.0; (C) D/5.0;

and (D) E/7.0 at 300magni cation.ﬁ

From the micrographs, it can clearlybe observed that

[55]

the presence of ALK improved the silica dispersion.The

Fig. 7.Hardness of the silica- lled NR vulcanizates at various ALK loadings.ﬁ

Fig. 8.Elongation at break of the silica- lled NR vulcanizates at various ALKﬁ

loadings. Fig. 9.Resilience of the silica- lled NR vulcanizates at various ALK loadings.ﬁ I. Surya et al. / Polymer Testing 32 (2013) 1313–3 211

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dispersion of the ller is the least homogeneous in A/0.0,ﬁ

where large silica agglomerates can be observed. The SEM micrograph of A/0.0 also seemed relatively smooth compared to those of the others, which indicates that A/0.0 is less ductile than the others. However, the SEM micro-graph for D/5.[17]0 exhibitsthe greatest matrix tearing line and

[18]

surface roughness. Better silica dispersion in the D/5.0

altered the crack path, which led to increased resistance to crack propagation, and thus caused an increase in elastic properties, such as tensile modulus, tensile strength, and

[73]

hardness.The micrographs of the tensile fractured surfaces

werein good agreement with the results obtained by Ismail

[22]

and Mathialagan[28]as well as Nabil et al.[29]which

re-portedthat an increase in energy was responsible for the

Fig.10.SEM micrographs of the failure fracture surface of silica- lled compound at a magni cation of 300 , (a) A/0.0, (b) C/3.0, (c) D/5.0, and (d) E/7.0.ﬁ ﬁ 

Fig. 11.FTIR spectrums of (a) A/0.0; (b) D/5.0.

[22]

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roughness and the matrix tearing line of the fractured

[22]

surface. However, the matrix tearing line and surface roughnessof E/7.0 was smoother than those of D/5.0, which indicated the more pronounced plasticizing or lubricating effect of the excessive ALK.

3.6. Infrared spectroscopic study

Fig. 11 shows the results of the FTIR spectroscopic study on the vulcanisates of A/0.0 and D/5.0, the control and the control with 5.0 phr. of ALK. All the gures show that the Cﬁ –

H stretches above and below 3000 cm1_{, indicating that the}

vulcanisates has saturated and unsaturated portions [7]. The strong bands occurred around 2954-2956 cm1_,

2916 cm1 _{and 2848 cm}1_{, which are attributed to the}

symmetric and asymmetric stretching vibration of the C H–

bonds based on CH2and CH3[96].Also, the bands at 1538 cm1

and 1456 cm1are relatedto the bending vibration of the

methylene(CH2) and methyl (CH3) groups, respectively. The spectra clearly show the characteristic wavenumbers of the NR vulcanisates. These characteristic wavenumbers of the NR were also reported by Wang et al.[30]for starch/NR composites, and Ooi et al. [31] for oil palm ashﬁlled NR vulcanisates.

[52]

Fig.11(a) shows thatthe presence of theO H group–

peak at 3380 cm1_{is related}_{to the presence of}_{silanol Si}

–

OH.In addition, two peaks are observed, i.e. at 890 cm 1

and 1096 cm1, which can be related to the Si O Si sym-– –

metric stretch, and Si O Si asymmetric stretch, respec-– –

tively. The spectrum clearly shows the presence of silica in the NR vulcanisates.

The IR spectrum of D/5.0 (Fig. 11b) showed similar

[102]

characteristic bands to that of A/0.0.In addition, new bands are observed, i.e. at 3830 cm1_{(O H stretch), 1398 cm}

– 1

(NO2 symmetric stretch), 1279 cm1 (C N stretch), and–

666 cm1_(NO

2bend), the occurrence of which are due to

the effect of ALK on the silica lled-NR vulcanisates. It isﬁ

also observed that the Si O Si asymmetric stretching peak– –

has shifted towards a lower frequency (1095 cm1_{). This}

spectral feature indicates that there are strong interactions, not only between ALK and silica, but also between ALK and NR.

According to Limper[32], due to its low boiling point, alcohol, which is attached to the TESPT coupling agent, can be classi ed as a Volatile Organic Compound (VOC). Duringﬁ

curing, the ethanol in the ALK is released, and the amine group becomes reactive and forms coupling bonds with the silanol Si O group of silicas, instead of the non-polar hy-–

drocarbon of ALK interacting physically through moment dipole interaction with the long chains of NR. By this mechanism, the silica-ALK-NR bonding/interaction is formed, and the probable mechanism for such an interac-tion is presented inFig. 12.

4. Conclusions

Alkanolamide was synthesized from RBDPS-palm oil and diethanolamine. Alkanolamide can be utilized as a new ad-ditive in silica- lled natural rubber compounds. The incor-ﬁ

poration of Alkanolamide enhanced the cure rate, and reduced the scorch time of silica- lled natural rubber com-ﬁ

pounds. The results also indicated that Alkanolamide can be utilized as a plasticizer to increase the elongation at break and resilience. The incorporation of Alkanolamide also improved the reinforcement level of silica to natural rubber compounds. The tensile modulus, hardness, tensile strength and crosslink density of the silica-ﬁlled natural rubber vul-canisates were enhanced, especially up to 5.0 phr loading.

Acknowledgements

The authors would like to thank Universiti Sains Malaysia for providing the research facilities for carrying out the experiment and for making this research work

Fig. 12.The probable mechanism of coupling bond between NR and silica in the presence of Alkanolamide. I. Surya et al. / Polymer Testing 32 (2013) 1313–3 211

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possible, and the Division of Oleochemicals, PT. Bakri Sumatera Plantation Tbk., Medan, 20362, Sumatera Utara,

[85]

Indonesia, for supplying the RBDPS. One of the authors

(Indra Surya) is grateful to the Directorate General of

Higher Education (DIKTI), Ministry of Education and

Cul-ture (Kemdikbud) of the Republic of Indonesia, for the

award of a scholarship under the fth batch of the Overseasﬁ

Postgraduate Scholarship Program.

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