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natural rubber compounds cured using conventional and efficient vulcanization systems
Accepted Manuscript Mechanical and rheometric properties of gilsonite/carbon black/natural rubber compounds cured using conventional and efficient vulcanization systems Juan Sebastián Vélez Herrera, Sandra Velásquez Restrepo, Diego Giraldo Vasquez PII:
S0142-9418(16)30438-X
DOI:
10.1016/j.polymertesting.2016.09.005
Reference:
POTE 4756
To appear in:
Polymer Testing
Received Date: 12 May 2016 Revised Date:
16 August 2016
Accepted Date: 4 September 2016
Please cite this article as: J.S. Vélez Herrera, S.V. Restrepo, D. Giraldo Vasquez, Mechanical and rheometric properties of gilsonite/carbon black/natural rubber compounds cured using conventional and efficient vulcanization systems, Polymer Testing (2016), doi: 10.1016/j.polymertesting.2016.09.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Mechanical and rheometric properties of gilsonite/carbon black/natural rubber compounds cured using conventional and efficient vulcanization systems
Juan Sebastián Vélez Herrera1*, Sandra Velásquez Restrepo1, Diego Giraldo Vasquez2 1
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Grupo BIOMATIC - Biomecánica, Materiales, TIC, Diseño y Calidad para el Sector cuero, plástico, caucho y sus cadenas productivas, Centro de Diseño y Manufactura del Cuero del SENA, Itagüí, Antioquia. 2 Grupo de Materiales Poliméricos, Departamento de Ingeniería Metalúrgica y de Materiales, Facultad de Ingeniería, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia
Abstract
The effect of adding gilsonite micrometric particles on the properties of N330 carbon black
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(CB) reinforced natural rubber (NR) compounds was investigated. Formulations with 30/0, 22.5/7.5, 15/15, 7.5/22.5 and 0/30 parts per hundred of rubber (phr) of CB/gilsonite were used, comparing the effect of conventional and efficient vulcanization systems. Gilsonite was characterized by X-ray fluorescence (XRF), thermogravimetric analysis (TGA), elemental analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Tension,
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uniaxial compression, compression-set, abrasion resistance and dielectric strength tests were carried out on specimens that were moulded using the optimal curing time measured by oscillating disc rheometry (ODR). Abrasion wear resistance, and mechanical and rheometric properties varied with the CB/gilsonite content and on the vulcanization system. It was found
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that gilsonite facilitated the incorporation of carbon black during mixing, diminished the reversion during rheometric tests of compounds with efficient vulcanization cure systems and Some gilsonite/CB/NR compounds showed similar
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increased the dielectric strength.
rheometric properties, compressive modulus and wear resistance to CB/NR compounds, which evidenced the use of gilsonite as an available filler for NR-based materials. Keywords: vulcanization systems, gilsonite, natural rubber, rubber-filler interaction
Introduction Natural rubber (NR) reaches 40% of rubbers worldwide consumption. NR is a widely used elastomer obtained from the latex of some plant species, among which the Heveas brasiliensis tree is the most commonly used in the industry [1]. An NR article manufacturing begins with
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ACCEPTED MANUSCRIPT the mixing of NR with organic and mineral additives; then the mixture is cured by the vulcanization process at temperatures usually between 140 and 180 °C. As a result, the compound acquires high elasticity that distinguishes elastomeric materials from any other type of polymer. In the NR vulcanization process, a cross-linked structure is obtained with improved environment and solvent resistance, tensile strength, elongation at break, tear
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strength, abrasion resistance, compressive stiffness and other mechanical properties [1] [2]. NR-based compounds employ vulcanization systems that are formed by: activators, curing agents and reaction accelerators or retardants [3] [4]. It was found that from a greater accelerator/sulphur ratio, a faster reaction is obtained, and promotes the formation of
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monosulphide and disulphide cross-links over polysulfide cross-links. Quantity and type of crosslinks affects the rheological, mechanical and thermal properties of cured rubbers [3][4][5]. Hence, properties of vulcanized rubber compounds depend on the vulcanization
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process, the type and structure of the elastomer, and on the type and concentration of the ingredients of the vulcanization system [6]. Based on the sulphur content and accelerator/sulphur ratio, vulcanization is classified into efficient (EV), semi-efficient (SEV) and conventional (CONV) vulcanization systems; each exhibits a different performance with regard to thermal, mechanical and chemical behaviour [7].
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Rubber industry uses fillers to improve properties and/or reduce costs [5] [8]. Some fillers result in a reinforcing effect, therefore an improving of mechanical properties is achieved, but other fillers do not alter the mechanical properties or can even reduce it [9] [10] [11]. The most used reinforcement filler in the rubber industry is carbon black [5] [12], whose
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morphology, size and affinity with the elastomeric matrix have not been surpassed by any other reinforcing filler [13] [14]. However, one of the main disadvantages of carbon black is its volatility that difficult its incorporation to the compound during mixing. In the case of
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rubber articles where electrical insulation is needed, reinforcement with carbon black could be a problem due to its low dielectric strength when compared with other mineral fillers [15]. For this reason the rubber industry is constantly in search of alternatives to improve rubber processing and properties, and/or reduce costs. Gilsonite could be an interesting additive for rubber compounds owing to its bituminous composition, availability and relative low price. It is a bitumen-impregnated rock known for its high content of asphaltene, commonly added to asphalts to improve the performance of asphalt binder at high temperatures [16] [17]. Savage and Hitchcock have studied gilsonite in rubber formulations for electrical wiring [18], but no other studies have been reported on mechanical, thermal or processing properties of gilsonite/rubber compounds. As stated by Roland [19], the interest in the electrical properties
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ACCEPTED MANUSCRIPT of rubber compounds is primarily due to the strong effect of fillers, hence the relevance of assessment non-conventional fillers for rubber industry. This study is aimed at the characterization of gilsonite available in Colombia, and to evaluate its processability, mechanical properties and abrasion resistance in NR compounds using different gilsonite and carbon black contents and comparing conventional and efficient
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vulcanization system behaviour. The effect of the addition of gilsonite on dielectric strength is also evaluated, since it is a subject of interest for some rubber articles as shoe soles and
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vibration isolators.
Experimental
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Materials
NR grade TSR-10 from Guatemala, N330-grade CB from Cabot Corporation, zinc oxide (ZnO), stearic acid, N-butylbenzothiazole-2-sulfenamide (TBBS) and sulphur (S) from Bayer Co. were purchased from Industrias del Caucho, in Medellín, Colombia. Grounded and pulverized gilsonite was provided by Gilsontech, in Cúcuta, Colombia. Gilsonite preparation and characterization
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Grounded and pulverized gilsonite was sieved by using a 200 mesh sieve. The particle-size distribution (PSD) was measured at a Malvern equipment reference Mastersizer 2000. Elemental analysis was carried out to determine carbon, sulphur, nitrogen, hydrogen and
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oxygen content in gilsonite and CB, by using a LECO elemental analyser. Carbon, hydrogen and nitrogen contents were determined at 1050 °C, while oxygen content was measured at 1250 °C in helium atmosphere. Sulphur content was measured at 1350 °C with oxygen flow
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and an IR detector. The main elemental composition was analyzed by X-ray fluorescence (XRF) in a spectrometer model Optim’x Thermo. Thermogravimetric analysis (TGA) was carried out on a TA Instruments Q500 equipment between 25 and 900°C under nitrogen atmosphere with a heating rate of 20 °C/min. A following TGA analysis was carried out using a temperature interval of 25 to 550°C with a heating rate of 20 °C/min under a nitrogen atmosphere. Once a temperature of 550 °C was reached, the atmosphere was changed to oxygen. At this state an isotherm for 5 minutes was applied. Finally the temperature was increased up until 900°C at 20 °C/min.
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ACCEPTED MANUSCRIPT X-ray diffraction (XRD) analysis was carried out by using a Rigaku Miniflex diffractometer. The samples were scanned from 10º to 90º diffraction angle (2θ) in scanning steps of 2º. Particle morphology was analysed in a scanning electron microscope (SEM) JSM-590LV
Compounding, rheometric tests and specimens fabrication
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(JEOL).
Table 1 displays the compound formulation of gilsonite/CB/NR compounds with conventional (CONV) and efficient (EV) vulcanization systems. As showed in Table 1, five compounds for each vulcanization system were evaluated varying the gilsonite and carbon
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black content. In all the compounds the total content of CB and/or gilsonite was 30 phr. Table 1. Formulations studied in this work.
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CONV system
Material Natural rubber (NR) Zinc oxide (ZnO) Stearic acid Sulphur (S) Accelerant/sulphur ratio
phr*
phr*
100
100
5
5
2
2
0.8
3.5
2.75
0.6
0.3
5.8
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TBBS
EV system
30/0, 7.5/22.5, 15/15, 22.5/7.5, 0/30
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Carbon black N330 / 30/0, 7.5/22.5, 15/15, gilsonite 22.5/7.5, 0/30 * Parts per hundred parts of rubber
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The compounding process was carried out on a Bolling two-roll mill with 6” diameter x 12” length rolls. Rheometric properties of three samples randomly cut from each compound were measured at 160ºC by using a Monsanto Oscillating Disc Rheometer (ODR). Specimens for mechanical tests were vulcanized into a hot press moulding machine at 160ºC, using the t100 time determined by rheometric tests. Tensile specimens were obtained from a 2 mm thickness sheet moulded for each compound, and three dumbbell-shaped specimens were punched out from each sheet. For uniaxial compression and compression-set tests, three cylinders according to ASTM D575 standard were moulded for each compound. Cylinders of 6 mm in diameter x 12 mm of length were moulded to obtain samples for wear tests.
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ACCEPTED MANUSCRIPT Tensile and compressive properties Tensile modulus values at 100 % (M100) and 300 % (M300) of elongation were evaluated from the stress-strain curves of dumb bell samples according the ASTM D412 standard. The grip separation speed was set to 500 mm/min and initial gauge length to 25 mm. Tensile forces were applied by using a Digimess universal testing machine and a maximum
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elongation of 550% was applied during tests. Compressive modulus values were measured from uniaxial compression tests that were carried out in a Shimadzu Instron universal testing machine at a crosshead speed of 20 mm/min, compressing the specimens up to 25% of strain. Compression-set tests were carried
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out according to method B specified in ASTM D395 standard, at 70 °C during 22 hours, using the device recommended by the standard. Hardness of dumbbell-shaped specimens was
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measured using a Shore A-type durometer according the ASTM D2240 standard. Abrasion wear tests
Abrasion wear resistance of cylindrical specimens was carried out by using a Maqtest apparatus according to ASTM D5963, applying 10 N as normal load.
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Dielectric strength
Dielectric strength of 7 cm diameter x 2 mm thickness specimens was measured in a Baur equipment reference DPA. Cylindrical electrodes were used, and 1 KV/s was applied during the tests. The maximum applied voltage was 75 KV. Tests were carried out at room
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temperature.
Results and discussions
Gilsonite characterization Figures 1 and 2 show the particle-size distribution and SEM micrographs respectively of gilsonite added to rubber compounds. It can be observed that gilsonite particles presented sizes between 0.4 and 40 micrometres, with approximately 80% with sizes between 2 and 15 micrometres. Particles appeared to be polyhedral with smooth and flat surfaces, which do not favour the mechanical interlocking with the rubber matrix, since it is well known that irregular shapes and high surface area are preferred to increase the reinforcing effect of fillers.
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Figure 1. Particle-size distribution of gilsonite
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Figure 2. SEM micrographs of gilsonite particles added to the rubber compounds. Table 2 indicates the results of gilsonite and CB elemental analysis, and table 3 shows the
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gilsonite composition detected by XRF, regarding the latter is a semiquantitative technique. The elemental composition of gilsonite showed lower percentages of carbon and hydrogen when compared to CB, but higher percentages of oxygen, sulphur and other elements. Sulphur content in gilsonite could alter the vulcanization kinetics, but it must be evaluated at rheometric tests. Zinc and iron could also influence the vulcanization, but they are present in low contents.
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ACCEPTED MANUSCRIPT Table 2. Gilsonite and CB composition detected by elemental analysis Element (%w/w) Filler H
N
O
S
Others
Gilsonite
75.2
7.0
1.5
8.4
1.5
6.4
Carbon black
86.3
12.8
0.1
0
0.8
0
Ni
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C
K
Pb
Ca
Zn
1.5
1.1
0.9
0.6
0.5
S
Fe
V
Si
Ti
Al
% w/w
72.1
7.5
5.4
4.6
3.6
1.6
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Element
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Table 3. Gilsonite composition detected by X-ray fluorescence (XRF) spectrometer
Figure 3 shows the thermogram obtained by TGA for gilsonite. Under nitrogen atmosphere, one single weight loss of 49.73 % was observed; this decomposition started at 380 °C and ended at 470 °C, and it is attributed to the hydrocarbon decomposition. The residue mass at the end of the test was 43.69% w/w. Under nitrogen-air atmosphere, the weight loss occurred
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in two-step losses. In the first step, the weight loss was of 42.75%, at the same temperature intervals observed in the nitrogen atmosphere test. In the second step, 53.86% weight loss is attributed to the hydrocarbon decomposition when gilsonite was exposed to oxygen in at 550 °C when the isothermal process was applied. At 900 °C 3.2 % of the specimen remained as
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residue.
Figure 4 shows the diffractogram obtained from DRX analysis. According to the Inorganic Crystal Structure Database for quartz (ICSD 01-083-0539) and for aluminium oxide (ICSD
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00-031-0026), the peaks at diffraction angles 2θ of 20.827°, 26.593°, 50.046°, 59.857° and 68.193° correspond to the hexagonal SiO2, whereas the peaks at 20.935°, 24.921°, 25.652°, 40.472° and 50.167° were identified as aluminium oxide (Al2O3).
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ACCEPTED MANUSCRIPT 100
N itr o g e n N itr o g e n - a ir
1.5
60
1.0
0.5
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Derivat. weigth change (%/°C)
Weigth (%)
80
40
0.0
0
200
400
600
800 1000
Temperature (°C) 20
A ir
0 0
100
200
300
400
500
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N itro g e n
600
700
800
900
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T e m p e ra tu re (°C )
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Figure 3. TGA and DTGA thermograms of gilsonite in nitrogen and air atmospheres.
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Figure 4. Diffractogram of gilsonite.
Rheometric characteristics
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Figure 5 shows the rheometric cure curves at 160 °C using conventional and efficient vulcanization systems. Rheometric properties of the compounds studied in this work are presented in Table 4. The rheometric properties considered in this work were the minimum torque ML, maximum torque MH, difference between MH and ML, the scorch time ts2, the time for reaching the 100% of the curing t100 and the curing rate index CRI.
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ACCEPTED MANUSCRIPT Table 4. Rheometric properties of the compounds studied in this work Gilsonite/CB content (phr)
Parameter
0/30
EV
15/15
22.5/7.5
30/0
1.22 ± 0.05 2.07 ± 0.52 1.16 ± 0.09 0.97 ± 0.37 1.64 ± 0.92
ML (dN.m)
27.81 ± 0.54
29.15 ± 2.02
27.33 ± 1.51
25.73 ± 1.07
21.33 ± 1.50
MH - ML (dN.m)
26.59 ± 0.49
27.08 ± 1.70
26.17 ± 1.42
24.76 ± 0.70
19.69 ± 0.68
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MH (dN.m)
ts2 (min)
3.54 ± 0.22 3.57 ± 0.21 3.54 ± 0.13 3.15 ± 0.13 3.79 ± 0.36
t100 (min)
9.25 ± 1.35 8.67 ± 1.14 9.54 ± 0.69 7.11 ± 0.91
CRI (min-1)
0.18 ± 0.04 0.20 ± 0.03 0.17 ± 0.02 0.25 ± 0.03 0.12 ± 0.05
ML (dN.m)
1.25 ± 0.00 1.27 ± 0.21 0.88 ± 0.11 0.74 ± 0.05 1.17 ± 0.10
MH (dN.m)
26.00 ± 0.72
MH - ML (dN.m)
24.75 ± 0.72
ts2 (min)
24.88 ± 1.25
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CONV
7.5/22.5
23.61 ± 0.94
12.06 ± 2.09
21.97 ± 0.31
19.33 ± 0.27
24.10 ± 0.54
27.09 ± 0.20
18.59 ± 0.22
22.93 ± 0.44
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System
5.02 ± 0.10 4.61 ± 0.42 4.50 ± 0.17 4.58 ± 0.10 3.34 ± 0.18
t100 (min)
13.65 ± 0.61
13.73 ± 0.69
14.24 ± 8.58 ± 0.46 0.90
0.12 ± 0.02 0.15 ± 0.06 0.11 ± 0.02 0.10 ± 0.03 0.19 ± 0.01
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CRI (min-1)
11.42 ± 4.78
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It can be observed that curves for CB/NR and 7.5 gilsonite/22.5 CB/NR compounds are very similar, for both CONV and EV systems. Increase of gilsonite up to 15 and 22.5 phr at the same time that CB content was decreased at 15 and 7.5 phr, respectively, diminished the
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stiffness of the cured compounds, which indicates a lower reinforcing effect of fillers at those levels; this behaviour was observed for both vulcanization systems. 15 gilsonite/15 CB/NR and 22.5 gilsonite/7.5 CB/NR compounds presented lower vulcanization times than CB/NR and 7.5 gilsonite/22.5 CB/NR compounds, because of the reactivity of hydrocarbon chains and sulphur at gilsonite.
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ACCEPTED MANUSCRIPT 30 30 CB 7.5 Gils - 22.5 CB 15 Gils - 15 CB 22.5 Gils - 7.5 CB 30 Gils
20 15 10
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Torque (dN.m)
25
5 0 0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
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Time (min)
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a. Conventional vulcanization system 30
20 15
30 CB 7.5 Gils - 22.5 CB 15 Gils - 15 CB 22.5 Gils - 7.5 CB 30 Gils
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Torque (dN.m)
25
10
0 0
1
2
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5
3
4
5
6
7
8
9
10 11 12 13 14 15
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Time (min)
b. Efficient vulcanization system
Figure 5. Rheometric curves at 160 °C using (a) conventional and (b) efficient vulcanization systems.
As expected, CB reinforced NR cured with a conventional vulcanization system showed reversion i.e. decreasing of torque with increasing testing time after the end of the curing. A plateau curing curves after the end of the vulcanization was observed for efficient vulcanization system. For compounds exhibiting reversion behaviour, t100 was taken at the time where maximum torque was presented; exhibiting a plateau after ending the curing
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ACCEPTED MANUSCRIPT reaction, t100 was taken at the time where torque values reached a steady behaviour. It is remarkable that addition of gilsonite to CB/NR compounds did not affect the reversion of compounds cured using conventional vulcanization systems or the plateau of compounds cured using efficient vulcanization systems. However, compounds only with gilsonite showed rheometric curves with different behaviour to compounds with some content of CB.
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Gilsonite/NR compound cured with conventional vulcanization system showed the highest reaction time and did not exhibit reversion; the compound only with gilsonite and an efficient vulcanization system showed the lowest reaction time and presented reversion. This indicates the synergic effect of CB and gilsonite, and influence of gilsonite on the vulcanization
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kinetics. It is well known that reversion in NR sulphur-based vulcanization is related to accelerant/sulphur ratio [3] [4] [6] [7], therefore, differences in the behaviour after the end of
the vulcanization systems.
Tensile and compressive properties
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the vulcanization process could be associated to gilsonite interaction with the ingredients of
Tensile curves until 550% of elongation are shown in Figure 6. One representative curve is showed for each material seeking to facilitate the visualization of the curves. 100% and 300%
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modulus, hardness and compressive modulus are shown in figures 7, 8 and 9, respectively. It is important to note that lines between data are presented in figures 7, 8, 9, 11 y 12 only to facilitate the visualization, but it was observed that CB/NR compounds exhibited a behaviour with some remarkable differences when compared with CB/gilsonite/NR compounds or
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gilsonite/NR compounds as will be discussed in the next sections. It can be observed that CB presented a reinforcing effect as expected; however, stiffness decreased with increasing the gilsonite content and decreasing the CB content.
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It is observed that all the compounds using conventional vulcanization system showed higher stiffness than those with efficient vulcanization (EV) system. This behaviour is attributed to the formation of polysulfide crosslinks which enhances stiffness more than monosulfide or disulfide crosslinks, regarding that using conventional vulcanization systems promotes more polysulfide crosslinks than EV systems in sulphur-based vulcanization of NR [6] [7]. The setback in hardness, tensile and compressive modulus is attributed to the combined effects of the decreasing content of CB and a poor rubber–gilsonite interaction. Urrego [6] studied the same vulcanization systems used in this work, in NR compounds without the addition of any filler. He found the same tensile modulus and hardness obtained in this work for gilsonite/NR cured with the conventional vulcanization system. Cured gilsonite/NR
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ACCEPTED MANUSCRIPT compounds using the efficient system show increased hardness, tensile and compressive modulus compared to the same formulation cured with conventional system, an opposite behaviour than materials with some CB content. This could suggest that gilsonite affect the structure of the cross-linked network as described in the rheometric properties presented
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previously in this work.
20
30 CB
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7.5 Gils - 22.5 CB
10
15 Gils - 15 CB
5
0 0.0
0.5
1.0
1.5
2.0
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Stress (MPa)
15
2.5
3.0
3.5
4.0
4.5
5.0
22.5 Gils - 7..5 CB 30 Gils
5.5
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Strain (mm/mm)
a. Conventional vulcanization system
15
7.5 Gils - 22.5 CB
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Stress (MPa)
30 CB
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20
10
15 Gils - 15 CB 22.5 Gils - 7..5 CB
5
30 Gils
0 0
1
2
3
4
Strain (mm/mm)
b. Efficient vulcanization system
5
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ACCEPTED MANUSCRIPT Figure 6. Stress-strain curves of CB, gilsonite and CB/gilsonite NR compounds with (a) conventional and (b) efficient vulcanization systems. Carbon black content (phr) 30,0
22,5
15,0
7,5
0,0
3.5
9
M100 CONV M100 EV M300 CONV M300 EV
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8
5
2.0
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4
1.5
1.0
0.5 0.0
7.5
15.0
22.5
3 2
300% Modulus (MPa)
7 6
2.5
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100% Modulus (MPa)
3.0
1 0 30.0
Gilsonite content (phr)
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Figure 7. Tensile modulus M100 and M300 of CB, gilsonite and CB/gilsonite NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization.
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Figure 8. Hardness of CB, gilsonite and CB/gilsonite NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization.
Figure 9. Compressive modulus of CB, gilsonite and CB/gilsonite NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization.
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ACCEPTED MANUSCRIPT SEM micrographs of the failed fracture during tensile tests of some compounds at various curing systems are shown in Figure 10. Decrease of CB content and increase of gilsonite content result in less tearing lines and surface roughness of the rubber matrix. This indicates a reduction of rubber–filler interaction when gilsonite content is increased, which altered the crack paths in rubber decreasing resistance to crack propagation. Some isolated particles of
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gilsonite appeared in the gilsonite/NR and CB/gilsonite/NR compounds, which is another evidence of low rubber–gilsonite interaction when mechanical stresses are applied.
Since no differences between tensile modulus were observed for gilsonite/NR compounds cured using conventional and efficient vulcanization systems, gilsonite appeared to be
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responsible for setback in rigidity instead of the type of crosslinking related to the vulcanization system. Additionally, the sulfur at gilsonite reacted with the rubber chains and the vulcanization system for producing additional crosslinks. The formation of new links
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when a load was applied to the rubber
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between the rubber chains and the sulfur of the gilsonite reduced chain mobility and slippage
b) 22.5 phr CB/7.5 phr gilsonite-NR. Efficient vulcanization system
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a) 30 phr CB-NR. Conventional vulcanization system
c) 15 phr CB/15 phr gilsonite-NR. Conventional vulcanization system
d) 30 phr gilsonite-NR. Efficient vulcanization system
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f) 30 phr gilsonite-NR. Conventional vulcanization system at higher magnification that e)
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e) 30 phr gilsonite-NR. Conventional vulcanization system
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Figure 10. SEM micrographs of the fracture surfaces during tensile tests of CB, CB/gilsonite
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and gilsonite filled NR compounds using conventional and efficient vulcanization systems.
Compression-set results are shown in figure 11. It was found that compounds only with CB or only with gilsonite showed no dependance of the vulcanization system, but CB/gilsonite/NR compounds cured with conventional systems presented higher values of compression-set than those cured with efficient systems. Compounds cured with efficient vulcanization system
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showed a continuous increase in compression-set with the increase in gilsonite content. This behavour is explained by the decrease in the reinforcing effect of CB when gilsonite was added, which allows the relaxation of NR chains. Same behavior is observed in compounds
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cured with conventional vulcanization system, with the noticeable exception of gilsonite/NR specimens. Low compression-set of gilsonite/NR compound vulcanized with conventional system is due to interaction between gilsonite particles and NR matrix, which was not
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affected by the effect of CB.
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ACCEPTED MANUSCRIPT Carbon black content (phr) 30,0
22,5
15,0
7,5
0,0
CONV EV
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25
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20
15
10 0.0
7.5
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Compression-set (%)
30
15.0
22.5
30.0
Gilsonite content (phr)
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Figure 11. Variation of compression-set values of the CB, CB/gilsonite and gilsonite NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization.
Results of abrasion tests are shown in figure 12. It is observed that CB/gilsonite/NR compounds with efficient vulcanization systems showed lower abrasion resistance than the
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same compounds cured with conventional vulcanization system. This difference is attribuited to the prevalence of polysulphide cross-links in latter, which are more resistant to abrasion
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wear than mono or disulphide cross-links that are prevalent on compounds cured with efficient system. CB/NR and gilsonite/NR compounds showed different behaviour to CB/gilsonite/NR compounds, indicating a synergic effect on the abrasion resistance when CB and gilsonite were added.
No diference was observed between the wear resistance of 30 phr CB/NR and 22.5 phr CB/7.5 phr gilsonite/NR compounds cured with a conventional vulcanization system. A slightly increase in volumetric loss was found with the conventional vulcanization system for 15 phr CB/15 gilsonite/NR and 7.5 phr CB/22.5 gilsonite/NR compounds. A remarkable increase in volumetric loss was observed for gilsonite/NR compound using a conventional vulcanization system, which is related to the poor interaction between gilsonite particles and NR matrix when CB is not present as reinforcing filler.
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ACCEPTED MANUSCRIPT CB reinforced compounds cured with efficient vulcanization systems exhibit increased volumetric loss with increasing gilsonite content. However, gilsonite/NR without CB compound show an improved abrasion resistance than 7.5 phr CB/22.5 phr gilsonite/NR. This is attributed to the synergic effect of CB and gilsonite on the volumetric loss setback.
22.5
15.0
7.5
CONV EV
0.0
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3
Volumetric loss (mm )
600
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Carbon black content (phr) 30.0
200
0.0
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400
7.5
15.0
22.5
30.0
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Gilsonite content (phr)
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Figure 12. Volumetric loss in abrasion tests of CB, CB/gilsonite and gilsonite filled NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization. As presented in figure 13, addition of gilsonite improved the dielectric strength of the
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compounds, because of the well-known dielectric behaviour of asphaltites, as the gilsonite used in this study. Since CB has low dielectric strength when compared to gilsonite, no synergic CB/gilsonite effect was observed. Regarding NR is a non-polar polymer, gilsonite appeared to be responsible for the improvement on dielectric strength of the compound.
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ACCEPTED MANUSCRIPT Carbon black content (phr) 30.0
22.5
15.0
0.0
7.5
15.0
7.5
0.0
35
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30 25 20 15 10
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Dielectric strength (KV/mm)
40
22.5
30.0
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Gilsonite content (phr)
Figure 13. Dielectric strength of CB, CB/gilsonite and gilsonite filled NR compounds. Lines between data are presented only to facilitate the visualization.
Conclusions
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Micrometric gilsonite acts as filler in NR and CB/NR compounds, and low mechanical interlocking with the rubber matrix is observed. Rheometric tests and mechanical properties indicate a synergic effect of gilsonite and CB in some composites. This behaviour could be
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related to sulphur and zinc found in gilsonite which modifies the kinetics of the vulcanization reaction and the structure of the cross-linked network, because of the well-known effect of
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sulphur and zinc in natural rubber curing process. The effect of gilsonite on mechanical properties depended of the vulcanization system, especially the modulus under compressive loads and abrasion resistance. Gilsonite increases the dielectric strength of carbon black reinforced natural rubber and facilitates the incorporation of carbon black during mixing. These results suggest that gilsonite could be used as a filler in natural rubber compounds regarding the low cost and availability of gilsonite. Acknowledgments
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ACCEPTED MANUSCRIPT The authors would like to thank the SENNOVA at SENA (the Spanish acronym for the National Training Service of Colombia), by its support through the SENNOVA Leaders program for making this research possible.
References
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[1] J. Clavijo, “El caucho natural, alternativa viable para tierras marginales cafeteras y cultivo promisorio para la sustitución manual de cultivos ilícitos,” Universidad Nacional de Colombia, 2004. [2] A. Torres, “Vulcanización de elastómeros con peróxidos orgánicos,” Universidad Complutense de Madrid, 2010.
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[3] R. Datta, “Rubber Curing Systems,” Rapra Review Reports, vol. 12, no. 12, p. 150, 2002. [4] M. Akiba and A. Hashim, “Vulcanization and crosslinking in elastomers,” Progress in Polymer Science, vol. 22, no. 96, 1997. pp. 475–521.
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[5] J. Peña, “Evaluación de formulaciones de caucho natural con cargas orgánicas e inorgánicas,” Universidad central de Venezuela, 2007. [6] W. Urrego, “Efecto del sistema de vulcanización en la cinética de reacción y en las propiedades físico-químicas de un caucho natural Colombiano”, Universidad de EAFIT, 2014.
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[7] I. Suryaa, H. Ismail, “The effect of the addition of alkanolamide on properties of carbon black-filled natural rubber (SMR-L) compounds cured using various curing systems”, Polymer Testing, vol. 50, 2016, pp 276–282 [8] M. Vargas, “Evaluación de formulaciones de EPDM con cargas orgánicas e inorgánicas,” Universidad Central de Venezuela, 2004
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[9] R. E. Yanez Bolivar, “Estudio de la influencia de cargas nanométricas en las propiedades de formulaciones de caucho nitrilo,” Universidad Simon Bolivar, 2007.
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[10] A. Kreher, G. Kuhner, L. Rothbuhr, G. Turk, and S. Wolff, “Process for the production of mixed granulate from carbon black and light filler,” U.S Patent 4 430 280. 1984. [11] T. S. Mroczkowski, “Tire tread rubber composition,” U.S Patent 5 162 409. 1992. [12] D. W. Carlson and W. D. Breach, “Carbon black system and improved rubber stock,” U.S Patent 5294253. 1994. [13] Z. H. Li, J. Zhang, and S. J. Chen, “Effects of carbon blacks with various structures on vulcanization and reinforcement of filled ethylene-propylene-diene rubber,” eXPRESS Polymer Letters, vol. 2, no. 10, 2008. pp. 695–704. [14] S. Chuayjuljit, A. Imvittaya, N. Na-ranong, and P. Potiyaraj, “Effects of Particle Size and Amount of Carbon Black and Calcium Carbonate on Curing Characteristics and Dynamic Mechanical Properties of Natural Rubber”, Journal of Metals, Materials and Minerals, vol. 12, no. 1, 2002. pp. 51–57.
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ACCEPTED MANUSCRIPT [15] N. González, M. D. À. Custal, S. Lalaouna, J. R. Riba & E. Armelin, “Improvement of dielectric properties of natural rubber by adding perovskite nanoparticles”, European Polymer Journal, 75, 210–222, 2016. [16] H. Rondon, A. Hernández, C. Urazan, “Behavior of gilsonite-modified hot mix asphalt by wet and dry processes”, Journal of Materials in Civil Engineering, vol. 7, pp. 1–10, 2015.
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[17] M. Ameri, A. Mansourian, A. Hossein, “Investigating effects of ethylene vinyl acetate and gilsonite modifiers upon performance of base bitumen using Superpave tests methodology”, Construction and Building Materials, vol. 36, pp. 1001–1007, 2012. [18] H. Hitchcock & M. Savage, “Electrical Cable”. U.S Patent 2 235 536. March 18, 1941.
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[19] C. M. Roland, “Electrical and dielectric properties of rubber”, Rubber Chemistry and Technology, vol. 89, No. 1, pp. 32-53, 2016
S0142-9418(16)30438-X
DOI:
10.1016/j.polymertesting.2016.09.005
Reference:
POTE 4756
To appear in:
Polymer Testing
Received Date: 12 May 2016 Revised Date:
16 August 2016
Accepted Date: 4 September 2016
Please cite this article as: J.S. Vélez Herrera, S.V. Restrepo, D. Giraldo Vasquez, Mechanical and rheometric properties of gilsonite/carbon black/natural rubber compounds cured using conventional and efficient vulcanization systems, Polymer Testing (2016), doi: 10.1016/j.polymertesting.2016.09.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Mechanical and rheometric properties of gilsonite/carbon black/natural rubber compounds cured using conventional and efficient vulcanization systems
Juan Sebastián Vélez Herrera1*, Sandra Velásquez Restrepo1, Diego Giraldo Vasquez2 1
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Grupo BIOMATIC - Biomecánica, Materiales, TIC, Diseño y Calidad para el Sector cuero, plástico, caucho y sus cadenas productivas, Centro de Diseño y Manufactura del Cuero del SENA, Itagüí, Antioquia. 2 Grupo de Materiales Poliméricos, Departamento de Ingeniería Metalúrgica y de Materiales, Facultad de Ingeniería, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia
Abstract
The effect of adding gilsonite micrometric particles on the properties of N330 carbon black
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(CB) reinforced natural rubber (NR) compounds was investigated. Formulations with 30/0, 22.5/7.5, 15/15, 7.5/22.5 and 0/30 parts per hundred of rubber (phr) of CB/gilsonite were used, comparing the effect of conventional and efficient vulcanization systems. Gilsonite was characterized by X-ray fluorescence (XRF), thermogravimetric analysis (TGA), elemental analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Tension,
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uniaxial compression, compression-set, abrasion resistance and dielectric strength tests were carried out on specimens that were moulded using the optimal curing time measured by oscillating disc rheometry (ODR). Abrasion wear resistance, and mechanical and rheometric properties varied with the CB/gilsonite content and on the vulcanization system. It was found
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that gilsonite facilitated the incorporation of carbon black during mixing, diminished the reversion during rheometric tests of compounds with efficient vulcanization cure systems and Some gilsonite/CB/NR compounds showed similar
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increased the dielectric strength.
rheometric properties, compressive modulus and wear resistance to CB/NR compounds, which evidenced the use of gilsonite as an available filler for NR-based materials. Keywords: vulcanization systems, gilsonite, natural rubber, rubber-filler interaction
Introduction Natural rubber (NR) reaches 40% of rubbers worldwide consumption. NR is a widely used elastomer obtained from the latex of some plant species, among which the Heveas brasiliensis tree is the most commonly used in the industry [1]. An NR article manufacturing begins with
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ACCEPTED MANUSCRIPT the mixing of NR with organic and mineral additives; then the mixture is cured by the vulcanization process at temperatures usually between 140 and 180 °C. As a result, the compound acquires high elasticity that distinguishes elastomeric materials from any other type of polymer. In the NR vulcanization process, a cross-linked structure is obtained with improved environment and solvent resistance, tensile strength, elongation at break, tear
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strength, abrasion resistance, compressive stiffness and other mechanical properties [1] [2]. NR-based compounds employ vulcanization systems that are formed by: activators, curing agents and reaction accelerators or retardants [3] [4]. It was found that from a greater accelerator/sulphur ratio, a faster reaction is obtained, and promotes the formation of
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monosulphide and disulphide cross-links over polysulfide cross-links. Quantity and type of crosslinks affects the rheological, mechanical and thermal properties of cured rubbers [3][4][5]. Hence, properties of vulcanized rubber compounds depend on the vulcanization
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process, the type and structure of the elastomer, and on the type and concentration of the ingredients of the vulcanization system [6]. Based on the sulphur content and accelerator/sulphur ratio, vulcanization is classified into efficient (EV), semi-efficient (SEV) and conventional (CONV) vulcanization systems; each exhibits a different performance with regard to thermal, mechanical and chemical behaviour [7].
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Rubber industry uses fillers to improve properties and/or reduce costs [5] [8]. Some fillers result in a reinforcing effect, therefore an improving of mechanical properties is achieved, but other fillers do not alter the mechanical properties or can even reduce it [9] [10] [11]. The most used reinforcement filler in the rubber industry is carbon black [5] [12], whose
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morphology, size and affinity with the elastomeric matrix have not been surpassed by any other reinforcing filler [13] [14]. However, one of the main disadvantages of carbon black is its volatility that difficult its incorporation to the compound during mixing. In the case of
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rubber articles where electrical insulation is needed, reinforcement with carbon black could be a problem due to its low dielectric strength when compared with other mineral fillers [15]. For this reason the rubber industry is constantly in search of alternatives to improve rubber processing and properties, and/or reduce costs. Gilsonite could be an interesting additive for rubber compounds owing to its bituminous composition, availability and relative low price. It is a bitumen-impregnated rock known for its high content of asphaltene, commonly added to asphalts to improve the performance of asphalt binder at high temperatures [16] [17]. Savage and Hitchcock have studied gilsonite in rubber formulations for electrical wiring [18], but no other studies have been reported on mechanical, thermal or processing properties of gilsonite/rubber compounds. As stated by Roland [19], the interest in the electrical properties
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ACCEPTED MANUSCRIPT of rubber compounds is primarily due to the strong effect of fillers, hence the relevance of assessment non-conventional fillers for rubber industry. This study is aimed at the characterization of gilsonite available in Colombia, and to evaluate its processability, mechanical properties and abrasion resistance in NR compounds using different gilsonite and carbon black contents and comparing conventional and efficient
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vulcanization system behaviour. The effect of the addition of gilsonite on dielectric strength is also evaluated, since it is a subject of interest for some rubber articles as shoe soles and
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vibration isolators.
Experimental
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Materials
NR grade TSR-10 from Guatemala, N330-grade CB from Cabot Corporation, zinc oxide (ZnO), stearic acid, N-butylbenzothiazole-2-sulfenamide (TBBS) and sulphur (S) from Bayer Co. were purchased from Industrias del Caucho, in Medellín, Colombia. Grounded and pulverized gilsonite was provided by Gilsontech, in Cúcuta, Colombia. Gilsonite preparation and characterization
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Grounded and pulverized gilsonite was sieved by using a 200 mesh sieve. The particle-size distribution (PSD) was measured at a Malvern equipment reference Mastersizer 2000. Elemental analysis was carried out to determine carbon, sulphur, nitrogen, hydrogen and
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oxygen content in gilsonite and CB, by using a LECO elemental analyser. Carbon, hydrogen and nitrogen contents were determined at 1050 °C, while oxygen content was measured at 1250 °C in helium atmosphere. Sulphur content was measured at 1350 °C with oxygen flow
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and an IR detector. The main elemental composition was analyzed by X-ray fluorescence (XRF) in a spectrometer model Optim’x Thermo. Thermogravimetric analysis (TGA) was carried out on a TA Instruments Q500 equipment between 25 and 900°C under nitrogen atmosphere with a heating rate of 20 °C/min. A following TGA analysis was carried out using a temperature interval of 25 to 550°C with a heating rate of 20 °C/min under a nitrogen atmosphere. Once a temperature of 550 °C was reached, the atmosphere was changed to oxygen. At this state an isotherm for 5 minutes was applied. Finally the temperature was increased up until 900°C at 20 °C/min.
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ACCEPTED MANUSCRIPT X-ray diffraction (XRD) analysis was carried out by using a Rigaku Miniflex diffractometer. The samples were scanned from 10º to 90º diffraction angle (2θ) in scanning steps of 2º. Particle morphology was analysed in a scanning electron microscope (SEM) JSM-590LV
Compounding, rheometric tests and specimens fabrication
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(JEOL).
Table 1 displays the compound formulation of gilsonite/CB/NR compounds with conventional (CONV) and efficient (EV) vulcanization systems. As showed in Table 1, five compounds for each vulcanization system were evaluated varying the gilsonite and carbon
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black content. In all the compounds the total content of CB and/or gilsonite was 30 phr. Table 1. Formulations studied in this work.
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CONV system
Material Natural rubber (NR) Zinc oxide (ZnO) Stearic acid Sulphur (S) Accelerant/sulphur ratio
phr*
phr*
100
100
5
5
2
2
0.8
3.5
2.75
0.6
0.3
5.8
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TBBS
EV system
30/0, 7.5/22.5, 15/15, 22.5/7.5, 0/30
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Carbon black N330 / 30/0, 7.5/22.5, 15/15, gilsonite 22.5/7.5, 0/30 * Parts per hundred parts of rubber
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The compounding process was carried out on a Bolling two-roll mill with 6” diameter x 12” length rolls. Rheometric properties of three samples randomly cut from each compound were measured at 160ºC by using a Monsanto Oscillating Disc Rheometer (ODR). Specimens for mechanical tests were vulcanized into a hot press moulding machine at 160ºC, using the t100 time determined by rheometric tests. Tensile specimens were obtained from a 2 mm thickness sheet moulded for each compound, and three dumbbell-shaped specimens were punched out from each sheet. For uniaxial compression and compression-set tests, three cylinders according to ASTM D575 standard were moulded for each compound. Cylinders of 6 mm in diameter x 12 mm of length were moulded to obtain samples for wear tests.
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ACCEPTED MANUSCRIPT Tensile and compressive properties Tensile modulus values at 100 % (M100) and 300 % (M300) of elongation were evaluated from the stress-strain curves of dumb bell samples according the ASTM D412 standard. The grip separation speed was set to 500 mm/min and initial gauge length to 25 mm. Tensile forces were applied by using a Digimess universal testing machine and a maximum
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elongation of 550% was applied during tests. Compressive modulus values were measured from uniaxial compression tests that were carried out in a Shimadzu Instron universal testing machine at a crosshead speed of 20 mm/min, compressing the specimens up to 25% of strain. Compression-set tests were carried
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out according to method B specified in ASTM D395 standard, at 70 °C during 22 hours, using the device recommended by the standard. Hardness of dumbbell-shaped specimens was
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measured using a Shore A-type durometer according the ASTM D2240 standard. Abrasion wear tests
Abrasion wear resistance of cylindrical specimens was carried out by using a Maqtest apparatus according to ASTM D5963, applying 10 N as normal load.
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Dielectric strength
Dielectric strength of 7 cm diameter x 2 mm thickness specimens was measured in a Baur equipment reference DPA. Cylindrical electrodes were used, and 1 KV/s was applied during the tests. The maximum applied voltage was 75 KV. Tests were carried out at room
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temperature.
Results and discussions
Gilsonite characterization Figures 1 and 2 show the particle-size distribution and SEM micrographs respectively of gilsonite added to rubber compounds. It can be observed that gilsonite particles presented sizes between 0.4 and 40 micrometres, with approximately 80% with sizes between 2 and 15 micrometres. Particles appeared to be polyhedral with smooth and flat surfaces, which do not favour the mechanical interlocking with the rubber matrix, since it is well known that irregular shapes and high surface area are preferred to increase the reinforcing effect of fillers.
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Figure 1. Particle-size distribution of gilsonite
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Figure 2. SEM micrographs of gilsonite particles added to the rubber compounds. Table 2 indicates the results of gilsonite and CB elemental analysis, and table 3 shows the
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gilsonite composition detected by XRF, regarding the latter is a semiquantitative technique. The elemental composition of gilsonite showed lower percentages of carbon and hydrogen when compared to CB, but higher percentages of oxygen, sulphur and other elements. Sulphur content in gilsonite could alter the vulcanization kinetics, but it must be evaluated at rheometric tests. Zinc and iron could also influence the vulcanization, but they are present in low contents.
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ACCEPTED MANUSCRIPT Table 2. Gilsonite and CB composition detected by elemental analysis Element (%w/w) Filler H
N
O
S
Others
Gilsonite
75.2
7.0
1.5
8.4
1.5
6.4
Carbon black
86.3
12.8
0.1
0
0.8
0
Ni
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C
K
Pb
Ca
Zn
1.5
1.1
0.9
0.6
0.5
S
Fe
V
Si
Ti
Al
% w/w
72.1
7.5
5.4
4.6
3.6
1.6
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Element
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Table 3. Gilsonite composition detected by X-ray fluorescence (XRF) spectrometer
Figure 3 shows the thermogram obtained by TGA for gilsonite. Under nitrogen atmosphere, one single weight loss of 49.73 % was observed; this decomposition started at 380 °C and ended at 470 °C, and it is attributed to the hydrocarbon decomposition. The residue mass at the end of the test was 43.69% w/w. Under nitrogen-air atmosphere, the weight loss occurred
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in two-step losses. In the first step, the weight loss was of 42.75%, at the same temperature intervals observed in the nitrogen atmosphere test. In the second step, 53.86% weight loss is attributed to the hydrocarbon decomposition when gilsonite was exposed to oxygen in at 550 °C when the isothermal process was applied. At 900 °C 3.2 % of the specimen remained as
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residue.
Figure 4 shows the diffractogram obtained from DRX analysis. According to the Inorganic Crystal Structure Database for quartz (ICSD 01-083-0539) and for aluminium oxide (ICSD
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00-031-0026), the peaks at diffraction angles 2θ of 20.827°, 26.593°, 50.046°, 59.857° and 68.193° correspond to the hexagonal SiO2, whereas the peaks at 20.935°, 24.921°, 25.652°, 40.472° and 50.167° were identified as aluminium oxide (Al2O3).
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ACCEPTED MANUSCRIPT 100
N itr o g e n N itr o g e n - a ir
1.5
60
1.0
0.5
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Derivat. weigth change (%/°C)
Weigth (%)
80
40
0.0
0
200
400
600
800 1000
Temperature (°C) 20
A ir
0 0
100
200
300
400
500
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N itro g e n
600
700
800
900
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T e m p e ra tu re (°C )
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Figure 3. TGA and DTGA thermograms of gilsonite in nitrogen and air atmospheres.
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Figure 4. Diffractogram of gilsonite.
Rheometric characteristics
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Figure 5 shows the rheometric cure curves at 160 °C using conventional and efficient vulcanization systems. Rheometric properties of the compounds studied in this work are presented in Table 4. The rheometric properties considered in this work were the minimum torque ML, maximum torque MH, difference between MH and ML, the scorch time ts2, the time for reaching the 100% of the curing t100 and the curing rate index CRI.
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ACCEPTED MANUSCRIPT Table 4. Rheometric properties of the compounds studied in this work Gilsonite/CB content (phr)
Parameter
0/30
EV
15/15
22.5/7.5
30/0
1.22 ± 0.05 2.07 ± 0.52 1.16 ± 0.09 0.97 ± 0.37 1.64 ± 0.92
ML (dN.m)
27.81 ± 0.54
29.15 ± 2.02
27.33 ± 1.51
25.73 ± 1.07
21.33 ± 1.50
MH - ML (dN.m)
26.59 ± 0.49
27.08 ± 1.70
26.17 ± 1.42
24.76 ± 0.70
19.69 ± 0.68
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MH (dN.m)
ts2 (min)
3.54 ± 0.22 3.57 ± 0.21 3.54 ± 0.13 3.15 ± 0.13 3.79 ± 0.36
t100 (min)
9.25 ± 1.35 8.67 ± 1.14 9.54 ± 0.69 7.11 ± 0.91
CRI (min-1)
0.18 ± 0.04 0.20 ± 0.03 0.17 ± 0.02 0.25 ± 0.03 0.12 ± 0.05
ML (dN.m)
1.25 ± 0.00 1.27 ± 0.21 0.88 ± 0.11 0.74 ± 0.05 1.17 ± 0.10
MH (dN.m)
26.00 ± 0.72
MH - ML (dN.m)
24.75 ± 0.72
ts2 (min)
24.88 ± 1.25
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CONV
7.5/22.5
23.61 ± 0.94
12.06 ± 2.09
21.97 ± 0.31
19.33 ± 0.27
24.10 ± 0.54
27.09 ± 0.20
18.59 ± 0.22
22.93 ± 0.44
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System
5.02 ± 0.10 4.61 ± 0.42 4.50 ± 0.17 4.58 ± 0.10 3.34 ± 0.18
t100 (min)
13.65 ± 0.61
13.73 ± 0.69
14.24 ± 8.58 ± 0.46 0.90
0.12 ± 0.02 0.15 ± 0.06 0.11 ± 0.02 0.10 ± 0.03 0.19 ± 0.01
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CRI (min-1)
11.42 ± 4.78
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It can be observed that curves for CB/NR and 7.5 gilsonite/22.5 CB/NR compounds are very similar, for both CONV and EV systems. Increase of gilsonite up to 15 and 22.5 phr at the same time that CB content was decreased at 15 and 7.5 phr, respectively, diminished the
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stiffness of the cured compounds, which indicates a lower reinforcing effect of fillers at those levels; this behaviour was observed for both vulcanization systems. 15 gilsonite/15 CB/NR and 22.5 gilsonite/7.5 CB/NR compounds presented lower vulcanization times than CB/NR and 7.5 gilsonite/22.5 CB/NR compounds, because of the reactivity of hydrocarbon chains and sulphur at gilsonite.
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ACCEPTED MANUSCRIPT 30 30 CB 7.5 Gils - 22.5 CB 15 Gils - 15 CB 22.5 Gils - 7.5 CB 30 Gils
20 15 10
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Torque (dN.m)
25
5 0 0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
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Time (min)
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a. Conventional vulcanization system 30
20 15
30 CB 7.5 Gils - 22.5 CB 15 Gils - 15 CB 22.5 Gils - 7.5 CB 30 Gils
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Torque (dN.m)
25
10
0 0
1
2
EP
5
3
4
5
6
7
8
9
10 11 12 13 14 15
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Time (min)
b. Efficient vulcanization system
Figure 5. Rheometric curves at 160 °C using (a) conventional and (b) efficient vulcanization systems.
As expected, CB reinforced NR cured with a conventional vulcanization system showed reversion i.e. decreasing of torque with increasing testing time after the end of the curing. A plateau curing curves after the end of the vulcanization was observed for efficient vulcanization system. For compounds exhibiting reversion behaviour, t100 was taken at the time where maximum torque was presented; exhibiting a plateau after ending the curing
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ACCEPTED MANUSCRIPT reaction, t100 was taken at the time where torque values reached a steady behaviour. It is remarkable that addition of gilsonite to CB/NR compounds did not affect the reversion of compounds cured using conventional vulcanization systems or the plateau of compounds cured using efficient vulcanization systems. However, compounds only with gilsonite showed rheometric curves with different behaviour to compounds with some content of CB.
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Gilsonite/NR compound cured with conventional vulcanization system showed the highest reaction time and did not exhibit reversion; the compound only with gilsonite and an efficient vulcanization system showed the lowest reaction time and presented reversion. This indicates the synergic effect of CB and gilsonite, and influence of gilsonite on the vulcanization
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kinetics. It is well known that reversion in NR sulphur-based vulcanization is related to accelerant/sulphur ratio [3] [4] [6] [7], therefore, differences in the behaviour after the end of
the vulcanization systems.
Tensile and compressive properties
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the vulcanization process could be associated to gilsonite interaction with the ingredients of
Tensile curves until 550% of elongation are shown in Figure 6. One representative curve is showed for each material seeking to facilitate the visualization of the curves. 100% and 300%
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modulus, hardness and compressive modulus are shown in figures 7, 8 and 9, respectively. It is important to note that lines between data are presented in figures 7, 8, 9, 11 y 12 only to facilitate the visualization, but it was observed that CB/NR compounds exhibited a behaviour with some remarkable differences when compared with CB/gilsonite/NR compounds or
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gilsonite/NR compounds as will be discussed in the next sections. It can be observed that CB presented a reinforcing effect as expected; however, stiffness decreased with increasing the gilsonite content and decreasing the CB content.
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It is observed that all the compounds using conventional vulcanization system showed higher stiffness than those with efficient vulcanization (EV) system. This behaviour is attributed to the formation of polysulfide crosslinks which enhances stiffness more than monosulfide or disulfide crosslinks, regarding that using conventional vulcanization systems promotes more polysulfide crosslinks than EV systems in sulphur-based vulcanization of NR [6] [7]. The setback in hardness, tensile and compressive modulus is attributed to the combined effects of the decreasing content of CB and a poor rubber–gilsonite interaction. Urrego [6] studied the same vulcanization systems used in this work, in NR compounds without the addition of any filler. He found the same tensile modulus and hardness obtained in this work for gilsonite/NR cured with the conventional vulcanization system. Cured gilsonite/NR
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ACCEPTED MANUSCRIPT compounds using the efficient system show increased hardness, tensile and compressive modulus compared to the same formulation cured with conventional system, an opposite behaviour than materials with some CB content. This could suggest that gilsonite affect the structure of the cross-linked network as described in the rheometric properties presented
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previously in this work.
20
30 CB
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7.5 Gils - 22.5 CB
10
15 Gils - 15 CB
5
0 0.0
0.5
1.0
1.5
2.0
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Stress (MPa)
15
2.5
3.0
3.5
4.0
4.5
5.0
22.5 Gils - 7..5 CB 30 Gils
5.5
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Strain (mm/mm)
a. Conventional vulcanization system
15
7.5 Gils - 22.5 CB
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Stress (MPa)
30 CB
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20
10
15 Gils - 15 CB 22.5 Gils - 7..5 CB
5
30 Gils
0 0
1
2
3
4
Strain (mm/mm)
b. Efficient vulcanization system
5
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22,5
15,0
7,5
0,0
3.5
9
M100 CONV M100 EV M300 CONV M300 EV
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8
5
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1.5
1.0
0.5 0.0
7.5
15.0
22.5
3 2
300% Modulus (MPa)
7 6
2.5
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100% Modulus (MPa)
3.0
1 0 30.0
Gilsonite content (phr)
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Figure 7. Tensile modulus M100 and M300 of CB, gilsonite and CB/gilsonite NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization.
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Figure 8. Hardness of CB, gilsonite and CB/gilsonite NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization.
Figure 9. Compressive modulus of CB, gilsonite and CB/gilsonite NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization.
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ACCEPTED MANUSCRIPT SEM micrographs of the failed fracture during tensile tests of some compounds at various curing systems are shown in Figure 10. Decrease of CB content and increase of gilsonite content result in less tearing lines and surface roughness of the rubber matrix. This indicates a reduction of rubber–filler interaction when gilsonite content is increased, which altered the crack paths in rubber decreasing resistance to crack propagation. Some isolated particles of
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gilsonite appeared in the gilsonite/NR and CB/gilsonite/NR compounds, which is another evidence of low rubber–gilsonite interaction when mechanical stresses are applied.
Since no differences between tensile modulus were observed for gilsonite/NR compounds cured using conventional and efficient vulcanization systems, gilsonite appeared to be
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responsible for setback in rigidity instead of the type of crosslinking related to the vulcanization system. Additionally, the sulfur at gilsonite reacted with the rubber chains and the vulcanization system for producing additional crosslinks. The formation of new links
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when a load was applied to the rubber
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between the rubber chains and the sulfur of the gilsonite reduced chain mobility and slippage
b) 22.5 phr CB/7.5 phr gilsonite-NR. Efficient vulcanization system
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a) 30 phr CB-NR. Conventional vulcanization system
c) 15 phr CB/15 phr gilsonite-NR. Conventional vulcanization system
d) 30 phr gilsonite-NR. Efficient vulcanization system
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f) 30 phr gilsonite-NR. Conventional vulcanization system at higher magnification that e)
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e) 30 phr gilsonite-NR. Conventional vulcanization system
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Figure 10. SEM micrographs of the fracture surfaces during tensile tests of CB, CB/gilsonite
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and gilsonite filled NR compounds using conventional and efficient vulcanization systems.
Compression-set results are shown in figure 11. It was found that compounds only with CB or only with gilsonite showed no dependance of the vulcanization system, but CB/gilsonite/NR compounds cured with conventional systems presented higher values of compression-set than those cured with efficient systems. Compounds cured with efficient vulcanization system
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showed a continuous increase in compression-set with the increase in gilsonite content. This behavour is explained by the decrease in the reinforcing effect of CB when gilsonite was added, which allows the relaxation of NR chains. Same behavior is observed in compounds
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cured with conventional vulcanization system, with the noticeable exception of gilsonite/NR specimens. Low compression-set of gilsonite/NR compound vulcanized with conventional system is due to interaction between gilsonite particles and NR matrix, which was not
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affected by the effect of CB.
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ACCEPTED MANUSCRIPT Carbon black content (phr) 30,0
22,5
15,0
7,5
0,0
CONV EV
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10 0.0
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Compression-set (%)
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15.0
22.5
30.0
Gilsonite content (phr)
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Figure 11. Variation of compression-set values of the CB, CB/gilsonite and gilsonite NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization.
Results of abrasion tests are shown in figure 12. It is observed that CB/gilsonite/NR compounds with efficient vulcanization systems showed lower abrasion resistance than the
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same compounds cured with conventional vulcanization system. This difference is attribuited to the prevalence of polysulphide cross-links in latter, which are more resistant to abrasion
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wear than mono or disulphide cross-links that are prevalent on compounds cured with efficient system. CB/NR and gilsonite/NR compounds showed different behaviour to CB/gilsonite/NR compounds, indicating a synergic effect on the abrasion resistance when CB and gilsonite were added.
No diference was observed between the wear resistance of 30 phr CB/NR and 22.5 phr CB/7.5 phr gilsonite/NR compounds cured with a conventional vulcanization system. A slightly increase in volumetric loss was found with the conventional vulcanization system for 15 phr CB/15 gilsonite/NR and 7.5 phr CB/22.5 gilsonite/NR compounds. A remarkable increase in volumetric loss was observed for gilsonite/NR compound using a conventional vulcanization system, which is related to the poor interaction between gilsonite particles and NR matrix when CB is not present as reinforcing filler.
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ACCEPTED MANUSCRIPT CB reinforced compounds cured with efficient vulcanization systems exhibit increased volumetric loss with increasing gilsonite content. However, gilsonite/NR without CB compound show an improved abrasion resistance than 7.5 phr CB/22.5 phr gilsonite/NR. This is attributed to the synergic effect of CB and gilsonite on the volumetric loss setback.
22.5
15.0
7.5
CONV EV
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Volumetric loss (mm )
600
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Carbon black content (phr) 30.0
200
0.0
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22.5
30.0
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Gilsonite content (phr)
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Figure 12. Volumetric loss in abrasion tests of CB, CB/gilsonite and gilsonite filled NR compounds using conventional and efficient vulcanization systems. Lines between data are presented only to facilitate the visualization. As presented in figure 13, addition of gilsonite improved the dielectric strength of the
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compounds, because of the well-known dielectric behaviour of asphaltites, as the gilsonite used in this study. Since CB has low dielectric strength when compared to gilsonite, no synergic CB/gilsonite effect was observed. Regarding NR is a non-polar polymer, gilsonite appeared to be responsible for the improvement on dielectric strength of the compound.
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ACCEPTED MANUSCRIPT Carbon black content (phr) 30.0
22.5
15.0
0.0
7.5
15.0
7.5
0.0
35
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30 25 20 15 10
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40
22.5
30.0
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Figure 13. Dielectric strength of CB, CB/gilsonite and gilsonite filled NR compounds. Lines between data are presented only to facilitate the visualization.
Conclusions
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Micrometric gilsonite acts as filler in NR and CB/NR compounds, and low mechanical interlocking with the rubber matrix is observed. Rheometric tests and mechanical properties indicate a synergic effect of gilsonite and CB in some composites. This behaviour could be
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related to sulphur and zinc found in gilsonite which modifies the kinetics of the vulcanization reaction and the structure of the cross-linked network, because of the well-known effect of
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sulphur and zinc in natural rubber curing process. The effect of gilsonite on mechanical properties depended of the vulcanization system, especially the modulus under compressive loads and abrasion resistance. Gilsonite increases the dielectric strength of carbon black reinforced natural rubber and facilitates the incorporation of carbon black during mixing. These results suggest that gilsonite could be used as a filler in natural rubber compounds regarding the low cost and availability of gilsonite. Acknowledgments
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ACCEPTED MANUSCRIPT The authors would like to thank the SENNOVA at SENA (the Spanish acronym for the National Training Service of Colombia), by its support through the SENNOVA Leaders program for making this research possible.
References
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[9] R. E. Yanez Bolivar, “Estudio de la influencia de cargas nanométricas en las propiedades de formulaciones de caucho nitrilo,” Universidad Simon Bolivar, 2007.
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[10] A. Kreher, G. Kuhner, L. Rothbuhr, G. Turk, and S. Wolff, “Process for the production of mixed granulate from carbon black and light filler,” U.S Patent 4 430 280. 1984. [11] T. S. Mroczkowski, “Tire tread rubber composition,” U.S Patent 5 162 409. 1992. [12] D. W. Carlson and W. D. Breach, “Carbon black system and improved rubber stock,” U.S Patent 5294253. 1994. [13] Z. H. Li, J. Zhang, and S. J. Chen, “Effects of carbon blacks with various structures on vulcanization and reinforcement of filled ethylene-propylene-diene rubber,” eXPRESS Polymer Letters, vol. 2, no. 10, 2008. pp. 695–704. [14] S. Chuayjuljit, A. Imvittaya, N. Na-ranong, and P. Potiyaraj, “Effects of Particle Size and Amount of Carbon Black and Calcium Carbonate on Curing Characteristics and Dynamic Mechanical Properties of Natural Rubber”, Journal of Metals, Materials and Minerals, vol. 12, no. 1, 2002. pp. 51–57.
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ACCEPTED MANUSCRIPT [15] N. González, M. D. À. Custal, S. Lalaouna, J. R. Riba & E. Armelin, “Improvement of dielectric properties of natural rubber by adding perovskite nanoparticles”, European Polymer Journal, 75, 210–222, 2016. [16] H. Rondon, A. Hernández, C. Urazan, “Behavior of gilsonite-modified hot mix asphalt by wet and dry processes”, Journal of Materials in Civil Engineering, vol. 7, pp. 1–10, 2015.
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