The effect of the addition of alkanolamide on properties of carbon black- filled natural rubber (SMR-L) compounds cured using various curing systems
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Polymer Testing
j o u r n a l h o m e p a g e :
Material properties The effect of the addition of alkanolamide on properties of carbon black-filled natural rubber (SMR-L) compounds cured using various curing systems
- a Indra Surya , H. Ismail b
Department of Chemical Engineering, Engineering Faculty, University of Sumatera Utara, Medan, 20155, Sumatera Utara, Indonesia School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong Tebal, 14300, Penang, Malaysia a r t i c l e i n f o a b s t r a c t Article history:
The properties of carbon black (CB)-filled natural rubber (SMR-L) compounds, with and without the
Received 25 November 2015
addition of Alkanolamide (ALK), based on various curing systems such as efficient, semi-efficient and
Accepted 12 January 2016
conventional vulcanisation systems were investigated. The ALK loading was fixed at 5.0 phr. It was found
Available online 21 January 2016
that ALK gave improvements to the cure rate, torque difference, crosslink density, filler dispersion, rubber efiller interaction and reinforcing efficiency of CB. ALK also enhanced the tensile modulus, hardness,
Keywords:
resilience, tensile strength and elongation at break of CB-filled SMR-L vulcanisates for each curing sys-
Alkanolamide
tem. The degree of improvement of cure characteristics and mechanical properties depended on the level
Curing systems
of sulphur and the ratio of accelerator to sulphur in each system. Scanning electron microscopy (SEM)
Carbon black
proved that the CB-filled SMR-L vulcanisates with ALK for each curing system displayed a greater matrix
Filler dispersion tearing line and surface roughness due to greater rubberefiller interaction. Rubber-filler interaction
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction combination form (as hybrid filler) for the purpose of achieving their synergistic effect in order to produce better overall mechan-
Through vulcanisation, weak and plastic raw rubbers are hard- ical properties . However, at a relatively higher loading of CB or ened or cured by sulphur and converted into strong elastic rubber silica, the filler particles tend to form agglomerates and will reduce the properties of the rubber vulcanisates. Practically, to overcome vulcanisates. When vulcanisation was first discovered, the sulphur reaction with sulphur alone took several hours to be completed. the filler dispersion problem, special additives such as processing Utilisation of accelerators (organic sulphur donor ingredients), with aids, dispersant aids, etc. are utilised. the combination of other ingredients such as zinc oxide and fatty In our previous work the preparation and application of acids, allowed the sulphur reaction to be accomplished in a shorter Alkanolamide (ALK) in silica-filled SMR-L compounds was re- time and was recognised as accelerated sulphur vulcanisation ported. The ALK enhanced the tensile properties and hardness of
Based on the level of sulphur and the ratio of accelerator to
the silica-filled SMR-L vulcanisates. The enhancement of these properties was attributed to the improvement of silica dispersion sulphur, accelerated sulphur vulcanisation can be classified into and the excelling crosslink density that stemmed from the incor- three categories: efficient (EV), semi-efficient (semi-EV) and con- ventional (CV) vulcanisation or curing systems. poration of ALK. The results also indicated that ALK could function
The strength and elasticity of a rubber vulcanisate can be further as an accelerator and internal plasticiser.
The comparison of ALK and aminopropyltriethoxy Silane enhanced by the addition of reinforcing filler. Carbon black (CB) and silica are the most popular reinforcing fillers for rubbers, and have (APTES)-silane coupling agent on the properties of silica-filled been widely employed in the rubber industry. CB is commonly SMR-L compounds was also reported Due to its combined utilised for producing black rubber products, while silica is used in and unique function as an accelerator and internal plasticiser, ALK coloured products. Sometimes, they are also utilised in produced a higher reinforcing efficiency than APTES at a similar loading.
A further study regarding the effect of ALK loading on properties Corresponding author. of CB-filled SMR-L, epoxidised natural rubber (ENR) and styrene
- E-mail addresses: (H. Ismail).
0142-9418/© 2016 Elsevier Ltd. All rights reserved.
I. Surya, H. Ismail / Polymer Testing 50 (2016) 276e282
1 Þ 100 (4)
The tensile fractured surfaces of the CB-filled SMR-L with and without ALK at various curing systems were examined by using a Zeiss Supra-35VP scanning electron microscope (SEM) to obtain information regarding the filler dispersion, rubberefiller interac-
2.6. Scanning electron microscopy (SEM) analysis
is the maximum rebound angle.
2
is the initial angle of displacement (45 ) and q
1
where q
2 Þ=ð1 cos q
Table 1 The SMR-L compound formulation.
% Resilience ¼ ½ð1 cos q
37. The tensile strength (TS), stress at 100% elongation (M100), stress at 300% elongation (M300) and elongation at break (EB) were determined. The hardness measurements of the samples were performed according to ISO 7691-I, using a Shore A type manual durometer. The resilience was studied by using a Wallace Dunlop Tripsometer, according to BS 903 Part A8. The rebound resilience was calculated according to Equation .
Tensile tests were performed at a cross-head speed of 500 mm/min using an Instron 3366 universal tensile machine, according to ISO
2.5. Tensile, hardness and resilience properties Dumbbell-shaped samples were cut from the moulded sheets.
(3)
2M c
1
¼
Fig. 1. Molecular structure of Alkanolamide.
Ingredients Curing system EV Semi-EV CV SMR-L 100.0 100.0 100.0 ZnO
L ¼ 0.393). The crosslink density is given by;
1.5
30.0 ALK 0.0; 5.0 0.0; 0.0; 5.0 a parts per hundred parts of rubber. b 5.0 phr was the optimum loading of ALK for CB-filled SMR-L compound with a semi-EV recipe
30.0
30.0
2.5 CB N330
1.5
0.8
0.8 S
3.0
5.0
2.0 MBTS
2.0
2.0
2.0 IPPD
2.0
2.0
5.0 Stearic acid
5.0
V c
butadiene rubber (SBR) compounds revealed that ALK gave cure enhancement, better filler dispersion and greater rubberefiller interaction to three different types of rubbers ALK enhanced the mechanical properties, especially up to 5.0 phr of ALK in SMR-L and SBR compounds, and at 1.0 phr of ALK in ENR-25 compound.
It is important to further investigate the applicability of ALK as a new rubber additive in rubber vulcanisation. Hence, through the examination of the properties of CB-filled SMR-L compounds in the presence of ALK, the applicability of ALK in vulcanisation of CB- filled SMR-L compounds with various curing systems was stud- ied. This study focused on the cure characteristics and mechanical properties of CB-filled SMR-L compounds with and without ALK, which were cured by EV, semi-EV and CV systems.
), cure time (t
Swelling tests on the CB-filled SMR-L vulcanisates were per- formed in toluene in accordance with ISO 1817. The cured test pieces (30 mm 5 mm 2 mm) were weighed using an electric balance and swollen in toluene until equilibrium, which took 72 h at room temperature. The samples were taken out from the liquid, the toluene was removed from the sample surfaces and the weight was determined. The samples were then dried in an oven at 60 C until constant weight was obtained. The swelling results were used to calculate the molecular weight between two crosslinks (M
2.4. Measurement of crosslink density
C, with a pressure of 10 MPa, and applying a laboratory hot-press based on respective curing times.
) according to ISO 3417. Samples of the respective compounds were tested at 150 C. The CB-filled SMR-L compounds were subsequently compression-moulded using a stainless steel mould at 150
H dM L
) and torque difference (M
90
2
) by applying the FloryeRehner equation
The cure characteristics of the CB-filled SMR-L compounds with and without ALK at various curing systems were obtained using a Monsanto Moving Die Rheometer (MDR 2000), which was employed to determine the scorch time (ts
2.3. Cure characteristics
The EV, semi-EV and CV vulcanisation recipes were applied in rubber compounding. The compounding procedure was performed on a two-roll mill (Model XK-160). displays the compound formulation of CB-filled SMR-L compounds with ALK and without ALK at various curing systems.
2.2. Compounding
The reaction procedures and molecular characterisations of the ALK were given in our previous report The molecular structure of ALK is presented in
Bhd., Seremban, Malaysia, and N330-grade CB was supplied by the Cabot Corporation. Other compounding ingredients such as sulphur (S), zinc oxide (ZnO), stearic acid, N-isopropyl-N'-phenyl- p-phenylenediamine (IPPD), benzothiazyl disulfide (MBTS) were supplied by Bayer Co. (M) Sdn. Bhd., Petaling Jaya, Selangor, Malaysia. The ALK was synthesised in the laboratory using Refined Bleached Deodorized Palm Stearin (RBDPS) and diethanolamine.
2.1. Materials NR grade SMR-L was used and obtained from Guthrie (M) Sdn.
2. Experimental
c
.
is the weight increase of the vulcanisate in toluene and c is the interac- tion parameter of the rubber networkesolvent (
), V
m
is the volume fraction of the rubber in the swollen specimen, Q
r
/mol), V
3
s ¼ 106.4 cm
is the molar volume of the toluene (V
s
3
M c ¼ r p
where r is the polymer/rubber density ( r of SMR-L ¼ 0.92 g/cm
1 1 þ Q m (2)
V r ¼
(1)
2 r
V r Þ þ V r þ cV
V 1=3 r lnð1
V s
c of SMR- fractured pieces were coated with a layer of gold to eliminate electrostatic charge build-up during analysis.
2.7. Measurement of rubberefiller interaction
5.0
7.40
5.0
8.95
0.22
8.73 Semi e EV
0.0
8.43
0.33
8.10
9.93
7.69
0.30
9.63 CV
0.0
10.55
0.20
10.35
5.0
12.64
0.19
0.29
0.0
The rubberefiller interactions were determined by swelling the cured CB-filled SMR-L compounds in toluene, according to ISO 1817. Test pieces with dimensions of (30 mm 5 mm 2 mm) were prepared from the moulded sheets. The initial weights were recorded prior to testing. The test pieces were then immersed in toluene and conditioned at room temperature in a dark environ- ment for 72 h. After the conditioning period, the weights of the swollen test pieces were recorded. The swollen test pieces were then dried in the oven at 70 C for 15 min and were allowed to cool at room temperature for another 15 min before the final weights were recorded. The Lorenz and Park's equation
The cure characteristics of CB-filled SMR-L compounds, with and without the presence of ALK at various curing systems, are shown in The addition of 5.0 phr of ALK into the CB-filled SMR-L compound for each curing system caused a decrease in scorch and cure times and an increase of torque dif- ference. Since amine is an ingredient of accelerators and also an accelerator activator , the amine part of ALK, together with ZnO and fatty acid, activated the MBTS-accelerator more pronouncedly and, consequently, improved the rate of sulphur reaction of CB- filled SMR-L compounds. The increase of torque difference value was attributed to the additional function of ALK, as an internal plasticiser agent, which plasticised and softened the filled com- pounds. This resulted in reduced viscosity and improved process- ability due to the CB dispersion and SMR-LeCB interaction. The SMR-LeCB interaction may be defined as additional physical crosslinks and, together with sulphide crosslinks, contrib- uted to total crosslink density of the CB-filled SMR-L com- pound. Degree of crosslink density of a rubber vulcanisate was indicated by its own torque difference value
was applied in this study. The swelling index was calculated according to Equation
Qf=Qg ¼ ae z
þ b (5)
where the subscripts f and g referred to filled and gum vulcanisates, respectively; z was the ratio by weight of filler to hydrocarbon rubber in the vulcanisate; while a and b were constants. The lower the Qf/Qg value, the greater the rubberefiller interaction becomes.
In this study, the weight of the toluene uptake per gram of hy- drocarbon rubber (Q) was calculated based on Equation .
Q ¼ ½Swollen Dried weight=½Initial weight 100=Formula weight (6)
3. Results and discussion
3.1. The cure characteristics and crosslink density
Curing systems Loading of ALK (phr) Torque difference properties M H , dN.m M L , dN.m M H dM L , dN.m EV
. The higher the torque difference value, the higher the degree of crosslink density.
It was also observed that the scorch times of CB-filled com- pounds, with and without ALK, decreased when the curing system was changed from EV to Semi-EV and CV. All curing systems used MBTS as the accelerator, and it was functionally classified as a primary accelerator which usually provides scorch delay to a rub- ber compound . The lower the amount of MBTS, the lower was the scorch safety. This explained why the scorch times tended to slightly decrease when the curing system was changed through the above sequence.
When the curing system was changed from EV to Semi-EV and CV, the cure time and torque difference tended to increase. A possible explanation may be due to the effect of sulphur content of each curing system. The EV system possesses the least amount of sulphur and the CV system possesses the greatest. The higher amount of sulphur requires longer time to complete the sulphur- isation, or crosslinking reaction, hence it produces higher crosslink density. This explains why the cure time and torque difference of the CB-filled SMR-L compounds, with and without ALK, increased in sequence: EV, Semi-EV and CV.
displays the crosslink density of CB-filled SMR-L com- pounds, with and without the presence of ALK, for various curing systems. The crosslink density was determined by the Flor- yeRehner approach [Eq.
The addition of 5.0 phr of ALK into the
CB-filled SMR-L compounds increased the crosslink density. This observation was in line with the data in . Torque difference
Fig. 2. Scorch times (ts Fig. 3. Cure times (t 90 ) of the CB-filled SMR-L compounds at various curing systems.
Table 2 Torque difference properties of the CB-filled and unfilled SMR-L compounds at various curing systems.
12.45 I. Surya, H. Ismail / Polymer Testing 50 (2016) 276e282
I. Surya, H. Ismail / Polymer Testing 50 (2016) 276e282
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. This is rubber-filler crosslinking which is consid- ered as another type of crosslink to the rubber system, and defined as coupling bonds The higher the sulphur content, the higher the solubility of CB in the sulphur phase, and hence the higher the degree of CB dispersion.
3.3. The rubberefiller interaction
Improved filler dispersion means greater rubberefiller in- teractions. Based on Lorenz and Park's equation (Equation ), the rubberefiller interaction of CB-filled SMR-L compounds at various curing systems is presented in . It can be seen that the Qf/Qg values decreased with the addition of ALK for all various curing systems. The decreased Qf/Qg indicated that the rubberefiller interaction in CB-filled SMR-L systems became greater, which was attributed to the capability of ALK to plasticise and soften the CB- filled SMR-L compounds and, therefore, improving the CB dispersion.
The rubber-filler interaction of the filled SMR-L compounds, with and without ALK, was enhanced from EV, Semi-EV and CV. Again, this was attributed to the sulphur content of the curing system. The higher the sulphur content, the more pronounced the sulphureCB interaction, and hence the better the CB dispersion and the greater the SMR-L-CB interaction.
3.4. The reinforcing efficiency (RE)
The degree of reinforcement provided by the filler can be calculated through its reinforcing efficiency (RE), which in its simplest form, was given by Equation .
RE ¼ ðM H M L
Þ f
ðM H M L
Þ g
=ðM H M L
Þ g
in which: (M
The crosslink density of the filled compounds with and without ALK increased when the curing system was changed from EV to Semi-EV and CV. This was simply due to the sulphur content of each curing system. A curing system with higher sulphur content would produce a higher crosslink density .
H
M
L
)
f
¼ difference in torque value of filled compound
(M
H
M
L
)
g ¼ difference in torque value of un filled/gum
compound A higher RE value meant greater rubber-filler interaction, which Fig. 4. Crosslink density of the CB-filled SMR-L compounds at various curing systems.
The L values of CB-filled compounds, with and without ALK, decreased from EV to Semi-EV and CV. This meant that the degrees of CB dispersion were the lowest in EV and the highest in CV. This phenomenon was also attributed to the sulphur content, since CB reacts with sulphur during the curing process and forms CB- sulphur bonds that link the rubber chains and tie the filler onto the rubber
values of CB-filled compounds with ALK were higher than those of CB-filled compounds without ALK. This indicated that the crosslink density of CB-filled compounds with ALK was higher than that of CB-filled compounds without ALK.
3.2. The filler dispersion
presents the values of L for CB dispersion in the SMR-L
¼ [M
The degree of CB dispersion in SMR-L compounds using various curing systems, due to the addition of ALK, can be quantitatively determined by Equation
.
L ¼ h r m r
(7)
where: h
r
¼ [M
Lf
/M
Lg
], and m
r
Hf
HG ), respectively.
/M
Hg
]; where M
Lf
and M
Hf
were the minimum and maximum torques of the filled compounds, and M
Lg
and M
Hg
were the minimum and the maximum torques of the unfilled/gum rubber compound. A lower value of L, at a particular CB loading, meant a better degree of CB dispersion. The cure characteristics of gum compounds of SMR-L using different curing systems (i.e. EV, Semi-EV and CV; M LG and M HG ) were 0.05, 0.07 and 0.05 (M
LG
); and 4.85, 4.88 and 5.91 (M
phase, with and without ALK, using various curing systems. It can be seen that the L values of CB-filled compounds with ALK were lower than those of CB-filled compounds without ALK. The reduced values of L indicated that ALK had improved the CB dispersion through its plasticisation effect in CB-filled SMR-L compounds.
I. Surya, H. Ismail / Polymer Testing 50 (2016) 276e282
As presented in ALK increased the RE of CB on SMR-L compounds. This was attributed to the combined effects of better filler dispersion and greater rubberefiller interaction.
5.0 1.36 ± 0.23 5.15 ± 0.22 29.2 ± 1.2 966.3 ± 23.0 54 ± 0.6 65.5 ± 0.8 CV 0.0 1.39 ± 0.18 5.56 ± 0.17 26.4 ± 0.9 758.3 ± 21.2 51 ± 0.3 61.7 ± 0.6 5.0 1.58 ± 0.22 6.02 ± 0.20 29.5 ± 1,1 799.3 ± 20.2 57 ± 0.9 68.8 ± 0.7
Curing systems Loading of ALK (phr) Mechanical properties M100 (MPa) M300 (MPa) TS (MPa) EB (%) Hardness (Shore A) Resilience (%) EV 0.0 1.12 ± 0.19 4.28 ± 0.16 23.8 ± 0.7 880.4 ± 19.5 48 ± 0.4 55.1 ± 0.5 5.0 1.22 ± 0.10 4.88 ± 0.18 25.6 ± 0.9 978.3 ± 19.2 51 ± 0.7 60.9 ± 0.5 Semi e EV 0.0 1.28 ± 0.20 4.92 ± 0.18 25.3 ± 0.9 835.8 ± 20.2 50 ± 0.4 58.4 ± 0.6
Table 3 The mechanical properties of CB-filled SMR-L compounds at various curing systems.
Fig. 7. Reinforcing efficiency of the CB-filled SMR-L compounds at various curing systems.
3. Degree of improvement of the cure characteristics and me- chanical properties of carbon black-filled natural rubber (SMR- L) compounds with Alkanolamide depended on the curing
2. Alkanolamide also improved the tensile modulus, hardness, resilience, tensile strength and elongation at break of the effi- cient, semi-efficient and conventional curing systems of carbon black-filled natural rubber (SMR-L) compounds.
1. Alkanolamide increased the cure rate, torque difference value, crosslink density, degree of filler dispersion, rubberefiller interaction and reinforcing efficiency of efficient, semi-efficient and conventional curing systems of carbon black-filled natural rubber (SMR-L) compounds.
4. Conclusions From this study, the following conclusions were drawn:
filled SMR-L compounds with ALK. An enhancement in rupture energy, due to a greater rubberefiller interaction, was responsible for the roughness and the matrix tearing line of the fractured sur- face. The micrographs of the tensile fractured surfaces were in good agreement with the results obtained by other researchers who reported that an increase in rupture energy was responsible for the roughness and the matrix tearing line of the fractured surfaces.
which showed the lower L and Qf/Qg values of CB-
was influenced by the degree of filler dispersion. The improved filler dispersion provided a greater surface area for rubberefiller interactions. RE of CB on SMR-L compounds, with and without ALK, at various curing systems is shown in
vulcanisates of CB-filled SMR-L, with and without ALK, for various curing systems, taken at 300 magnification. It can be clearly observed that the CB-filled SMR-L vulcanisates with 5.0 phr of ALK for each curing system (micrographs of (b), (d) and (f)) exhibited greater matrix tearing lines and surface roughness compared to those of CB-filled SMR-L vulcanisates without ALK (
3.6. Scanning electron microscopy (SEM) study displays the SEM micrographs of fractured surfaces of the
The mechanical properties of CB-filled SMR-L vulcanisates, with and without ALK, of the CV system were the greatest due to the highest degree of filler dispersion, greatest rubber-filler interaction, and hence highest RE. The mechanical properties of CB-filled SMR-L vulcanisates, with and without ALK, of the EV system were the lowest due to the lowest degree of filler dispersion, weakest rubber-filler interaction, and hence lowest RE.
The elongations at break of CB-filled SMR-L compounds with ALK were higher than those of CB-filled compounds without ALK. Again, this was attributed to the function of ALK as an internal plasticiser agent which modified the flexibility of CB-filled SMR-L vulcanisates. The ALK provided a free volume which allowed more flexibility for the SMR-L chains to move.
and the SEM micrographs later in The micrographs of CB-filled SMR-L vulcanisates with ALK exhibited greater matrix tearing lines and surface roughness. This indicated greater rubberefiller interaction which altered the crack paths, leading to increased resistance to crack propagation, thus causing an increase in tensile strength.
The enhancement in tensile strength was attributed to a higher RE, or the combined effects of better filler dispersion and greater rubberefiller interaction. This explanation was in line with the results in
Tensile modulus and hardness of a rubber vulcanisate are mainly dependent on the degree of crosslinking . Resilience is enhanced, to some extent, as the crosslink density rises Hence, the enhancements of M100, M300, hardness and resilience were attributed to the enhancement of crosslink density, as dis- played in .
showed the mechanical properties of CB-filled SMR-L, with and without the addition of ALK, for various curing systems. Obviously, the tensile modulus (M100 and M300), hardness, resil- ience, tensile strength and elongation at break were significantly increased using various curing systems with the addition of ALK.
The RE values of CB, with and without ALK, were the lowest in EV and were the highest in CV. This was due to the lowest degree of CB dispersion and the weakest rubber-filler interaction in EV, and the highest degree of CB dispersion and the greatest rubber-filler interaction in CV.
(c) and (e)). This indicated better filler dispersion and greater rubberefiller interaction, and the micrographs of the tensile fractured surfaces were in good agreement with the graphs in
3.5. The mechanical properties
I. Surya, H. Ismail / Polymer Testing 50 (2016) 276e282
Fig. 8. SEM micrographs of the failed fracture of CB-filled SMR-L vulcanisate at a magnification of 300 ; (a) 0.0 phr ALKd(EV), (b) 5.0 phr ALKd(EV), (c) 0.0 phr ALKd(semi-EV), (d)
5.0 phr ALKd(semi-EV), (e) 0.0 phr ALKd(CV), (f) 5.0 phr ALKd(CV).[8]
system, especially the level of sulphur and ratio of accelerator to . sulphur of each curing system.
[9]
4. Morphological studies of the tensile fractured surfaces of carbon
[10]
black-filled natural rubber (SMR-L) vulcanisates of each curing
system with Alkanolamide exhibited a greater matrix tearing
line and surface roughness due to greater rubberefiller
[11]
interaction.
[12]
Acknowledgements .
[13]
The authors would like to thank Universiti Sains Malaysia for .
[14]
providing the research facilities for carrying out the experiment .
and for making this research work possible. One of the authors
[15]
(Indra Surya) is grateful to the Directorate General of Higher Edu-
[16]
cation (DIKTI) Tahun Anggaran 2011, Ministry of Education and .
Culture (Kemdikbud) of the Republic of Indonesia, for the award of
[17]
a scholarship under the fifth batch of the Overseas Postgraduate .
Scholarship Program. [18]
.
[19]
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I. Surya, H. Ismail / Polymer Testing 50 (2016) 276e282
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