Study of the Effect of Systems of Vulcanization on the Properties of Natural Rubber (RSS).


Abstract:
Dynamic vulcanization of natural rubber (RSS) was studied. The effect of methods of vulcanization, i.e. sulphur, sulphurless and peroxide on mechanical, rheological properties, thermal ageing and weathering tests were investigated. Swelling behavior of when using sulphur, sulphurless and peroxide were investigated. Accelerators of mercapto class gave medium scorch time they gave faster onset of cure than peroxide but slower than thiurams which gave the fastest onset of cure of accelerators as a whole, Viscosity of the compounds tended to increase from peroxide, sulphurless, to sulphur system. Vulcanizates with thiuram and a little sulphur showed higher tensile strength, elongation at break and lower modulus than sulphur system. The peroxide system was lower in tensile strength than both sulphur and sulphur-less system but had higher elasticity value more than other systems Thiuram cured accelerators with low sulphur gave better resistance to ageing than sulphur system. Peroxide vulcanizates gave excellent ageing characteristics. In toluene the resistance was weak for all systems however it showed the smallest ratio in peroxide system. Sulphur system showed the best resistance for processing oil (37) and peroxide system showed more resistivety than sulphur-less system

Key words: rubber, mechanical properties, viscosity, swelling, ATR-FTIR, ageing

1. Introduction
Vulcanization is an important stage for manufacturing rubber products, which deals with the formation of three dimensional networks. By this means, the overall elasticity and strength of rubber products could be achieved. In general, there are three main types of rubber vulcanization, namely, sulphur, sulphur-less and peroxide vulcanization. Sulphur vulcanization is the most popular system for general purpose diene rubbers (NR, IR, SBR, and BR). Owing to its low cost, easy availability, good processing and physical properties. By using accelerators, the efficiency of the rubber-sulphur reaction can be improved the activation energy of vulcanization decreases from 270 kJ/mol to 80-125 kJ/mol the number of sulphur atoms required to form each crosslink reduces from 40-50 to below 10 (1 and 2).Articles cured on the low-sulphur vulcanization have good resistance to ageing . It should be notted that that this property depends on the amount of free sulphur at the vulcanizate. The smaller this amount, the better the resistance to ageing. Very small proportion of sulphur don’t appreciably reduce the ageing resistance but they raise the degree of cross linkage very much and it is therefore customary to use some sulphur in conjunction with thiuram accelerator (3 and 4).Despite giving relatively low mechanical properties, the peroxide vulcanization is still used in the rubber products requiring high thermal resistance. Peroxides typically react with the rubber molecules via the hydrogen abstraction, leading to highly active sites on rubber molecules known as free radicals. The main key responsible for properties of the peroxide cured vulcanizates is, in general, the state of cure normally depends mainly on cure temperature (5).

2. Materials and methods

Table (1):Details of materials:

Material. Ingredients.
Elastomer. Ribbed Smoked Sheets (RSS).
Filler. High Abrasion Furnace
Carbon black (HAF).
Accelerators Tetra methyl thiuram disulphide
(TMTD)
Mercaptobenz thiazole (MBT).
Dibenz thiazyl disulphide (MBTS).
Dicumyl peroxide (DCP)
Di-o-tolylguanidine(DOTG)

Activators. Zinc oxide
Stearic acid
Softeners. Processing oil 37
Anti oxidant. Antioxidant irganox 1010.

2.1. Mixing and vulcanization procedures
This was accomplished on a laboratory calender with two horizontal cylinders (200mm. diameter and working length of 400 mm) the revolving speed of the front slower cylinder was 16 r.p.m. and hind roll speed 20 r.p.m . The hollow cylinders were cooled by means of flushing water in order to regulate temperature not to exceed 60 ?C during different stages of mixing. The mix kept at room temperature for 24 h before testing.

Table (2): Compounding ingredients incorporated in RSS during vulcanization according to different systems of vulcanization.

Composition (phr) S S-less Peroxide
RSS 100 100 100
Zinc oxide 5 5 5
Stearic acid 1 1 1
Antioxidantirganox1010 1 1 1
Carbon black (HAF) 45 45 45
Processing oil (37) 5 5 5
Sulphur 2.5 0.5 -
MBTS 1 - -
MBT 1.5 - -
TMTD - 2.5 -
DOTG - 1 -
DCP - - 3

2.2. Cure Characteristics
Cure characteristics were studied using a Monsanto Moving Die Rheometer (Zwick 4308) according to ASTM D 2240-93. Samples (4 g) of the respective compounds were tested at the vulcanization temperature (150 ?C). The use of this curemeter and standardized values read from the curve is specified in ASTM D 2084. Some of these recommended values that are important to know for this study are
ML: Minimum torque in N.m or lbf.in.
MH: Maximum torque where curve plateaus are in N.m or lbf.in.
tx: Minutes to x% of torque increase, tx = minutes for torque value equivalent to In rubber terminology, t90 is defined as “optimum cure time” ML + x(MH ? ML)/100.
2.3. Mooney Viscosity.
Mooney viscometer is probably the most widely used method for measuring the quality of natural rubber (6). This viscometer was invented by Melvin Mooney, US Rubber Company, in the 1930s and is now used to measure the viscosity of both natural and synthetic rubber worldwide. This method consists of rotating a special serrated rotor while embedded in a rubber sample within a sealed, pressurized, serrated, temperature controlled cavity. The rotor turns at a constant rate of 2.0 revolutions min?1 (0.21 rad s?1) and the resulting torque is measured. This test imparts a shear rate of only 1 s?1 (7). The Mooney viscosity results are reported in arbitrary Mooney Units (MU) which is based on torque as defined by ISO 289 and ASTM D1646. The Mooney viscosity was determined by using a Monsanto automatic Mooney viscometer (MV 2000) at 120 ?C. The testing procedure was conducted according to the method described in ASTM D 1646-94. :
2.4. Vulcanization Process
Rubber sheets (3 mm thick) were compression moulded at 150 ?C with force of 10 MPa using a hot press according to respective cure times, t90, determined with the (Zwick 4308).
2.5. Tensile Properties
Dumb-bell-shaped samples were cut from the moulded sheets according to ASTM D 412. Tensile test were performed at a cross-head speed of 500 mm/min. Tensile testing was carried out with (universal testing material Zwick 1425)
2.6 Hardness Properties
Samples of at least 12 mm thickness with flat surface were cut for hardness test. The measurement was according to ASTM D2240 using Durometer of Model 306L Type A. The unit of hardness is expressed in (A Shore).
2.7. Rebound Resilience
Rebound resilience is a very basic form of dynamic test in which the test Piece is subjected to one half-cycle of deformation only. The strain is applied by impacting the test piece with an indentor which is free to rebound after the impact. Rebound resilience is defined as the ratio of the energy of the indentor after impact to its energy before impact expressed as a percentage and, hence, in the case where the indentor falls under gravity, is equal to the ratio of rebound height to the drop height, which is the measure square of velocities before and after impact and timing gates have been added to apparatus to enable automation of the data reading. The test is performed by (digi test Ruckprall 567 BJ.06).
2.8. Swelling Study
Swelling was studied in toluene, benzene and processing oil 37; according to ASTM D 471-79. Cured test pieces of the compounds of dimension 30×5×2 mm were weighed using an electrical balance and this was taken to be the initial weight, M1(8). Calculation of the change in mass was as follows:
Swelling percentage= [(M2?M1)/M1] ×100
Where M1 is the initial mass of specimen (g) and M2 is the mass of specimen (g) after immersion. When a cross linked polymer is brought into contact with a solvent, the network absorbs a certain amount of liquid which depends strongly on the molecular weight of this liquid and the degree of cross linking of the polymer (9and 10).The mass and dimensions of the polymer will be changed due to the penetration of the solvent into the swollen specimen. Therefore, the swelling process may lead to deformation or destruction of the sample microstructure. May result in the absorption of the liquid, extraction of soluble constituents and chemical reaction. The volume change is a good general measure of the resistance of a rubber to a given liquid. A high degree of swelling indicates that the rubber is not suitable for use in that environment (11).
2.9. ATR-FTIR Measurements
Were run with a JASCO instrument (FT/IR-6100typeA in the following conditions: wave number range: 600-4000 cm-1; aperture setting: 3.5mm mm; scanner velocity: 2.2 kHz; background scan time: 32 sec; sample scan time: 32 sec; resolution6 cm–1; beam splitter: KBr; angle of incident radiation: 45o. After recording, the ATR-FTIR spectra were converted into transmission FTIR spectra. The plate samples (6 x 6 mm) were simply posed on the sampling stage, in intimate contact with the optical element, a hemi cylindrical prism of SeZn (called Internal Reflection Element (IRE)). The incident radiation arrives onto the sample with a certain angle usually, between 30 and 60o) to the normal of the sample plane. Then, the reflected (beam is collected by a mirror, which focuses the reflected radiation onto the detector.
2.10. Aging of Rubber
The effects of aging on rubber were studied in many researches to determine the time-dependent effects on NR compounds. The bulk of the research into the aging of rubber has concentrated on the oxidative effects. Oxidative effects occur when oxygen attacks the unsaturated bond along the backbone of the poly isoprene (12). Braden and Gent (13). have defined the characteristics of static crack growth due to ozone. They concluded the following: critical tearing energy is necessary for cracks to grow, crack length increases linearly with time, and the rate of crack growth is proportional to the ozone concentration. Lake and Lindley (14). expanded the work of Braden and Gent by examining the role of ozone in the cracking and fatigue of rubber. Lake (15). demonstrated that there is a threshold value for tearing energy below which all crack growth is attributable to ozone. LaCounta et al. (16). Studied tire rubber subject to aging due to multiple factors. They developed an outdoor accelerated aging simulator, using a number of aging factors. The aging factors included heat, ozone, UV light, dynamic stretching, and aqueous solutions.
2.10. 1. Thermal Ageing
The air ageing was conducted in an air oven (modelFC712, Blue M Electrical Co.) at various times at 90 ?C the aged samples were allowed to rest at room temperature for 16 h and the physical properties were then measured.
2.10. 2. Weathering Ageing
Experiments were carried out in a modified Q-Panel QUV® weathering device equipped with UVA-340 lamps. A partition of polycarbonate panels was placed down the centerline of the instrument and sealed with silicone RTV to isolate the two sides. The water chamber was sealed on one side to eliminate humidity. Copper coils cooled with flowing tap water were placed near the center wall of both sides. Two 4? muffin fans were mounted inside each half to circulate air. The fans were mounted in unused sample positions to bring in outside air and more cooling.) The wiring was modified to allow heating of the water chamber independently from the light cycle timer. By passing dry compressed air into the “dry” side, the relative humidity could be maintained at nearly 0%. By bubbling air though the water bath (maintained at 45 °C), the relative humidity could be maintained at about 45%.Samples with formulations shown in Table1were cut to 0.5? × 1.25? (1.3 × 3.2 cm) and adhered to the center portion of a 6? × 12? aluminum panel using silicone RTV in a single 3 × 9 array. In this series, the samples were exposed to U.V for 200 hours. The samples were subjected to rain, wind and humidity conditions in an alternative manner every 3 minutes for a half minute.
3. Results and Discussion
3.1. Rheometer properties

Table (3): Data obtained by an oscillating disc rheometer for the vulcanization process using different accelerators and different system of vulcanization of RSS.

Rheometer Properties S S-less Peroxide
Scorch time(minute) 2.3 1.746 1.976
Optimum cure time 10.19 4.28 30
(minute)
Rate cure index 12.67 39.46 3.56
(minute-1)
t1( initiation time) 2.2 1.7 1.9
M max (Nm) 3.3 3.6 3.05
(maximum torque).
M min (Nm) 0.4 0.9 0.885
(minimum torque).
The lowest value of optimum cures of the three vulcanization systems (highest rate cure index) in the sulphur-less system, low sulphur vulcanization with thiuram accelerator gave a very good vulcanization plateau, the sulphur system of vulcanization using mercapto accelerators (semi ultra accelerator) gave a faster onset of cure than peroxide system which gave longer cure time and shorter induction time .The minimum torque, a measure of the stock viscosity, showed a slight increase with sulphur-less system this indicated that the processing ability of the compounds became a little more difficult, rubber was already cross-linked, and didn’t easily flow in the matrix, so would reduce the flow and consequently increased the torque
3.2 Viscosity Properties

Table (4): Viscosity results according to different systems of vulcanization

Viscosity Properties S S-less Peroxide
MV (Mooney Viscosity) 36 31.5 29.9
LM (Lowest Mooney) 35.7 31.4 29.8
MAX.Mooney 74.2 58.5 54.9
(Maximum Mooney)
MrLX Mooney 2.1 1.3 1
(Stress Relaxation)
Table (4) represents the effect of accelerator type on Mooney viscosity Even though only a small amount of accelerator it noticeably influenced the Mooney viscosity of the compound. Obviously, the results revealed that the compound viscosity depends on the accelerator type, i.e., viscosity of the compound tended to increase from DCP, TMTD, MBTS to MBT. It could be observed that the effect of accelerator type on compound viscosity corresponds well with the melting point of the accelerators (the melting points of DCP, TMTD, MBTS and MBT are 38, 137,175 and 179.1 °C, respectively). The higher the melting point of the accelerator, the greater the Mooney viscosity of the compound In addition to the compound viscosity the torque increases for a few seconds to a maximum value (Vmax), then decreases very rapidly to reach a minimum value LM) in many cases, and lastly rises again at varying speeds to a plateau (VR or ML (1+4)120) reached after 2–4 min depending on the sample.
3.3. Physico-mechanical Properties
Vulcanizates with thiuram and a little sulphur has a synergism effect giving a good cross linking. Thus having higher tensile strength, elongation at break and lower modulus than sulphur system. The peroxide system was lower in tensile strength than both sulphur and sulphur-less system but had higher elasticity value more than other systems

Table (5): Physico-mechanical properties of RSS according to different system of vulcanization.

Physico-mechanical S S-less Peroxide
Properties
Tensile strength 155 214 122
(Kg/cm2)
Elongation at break % 718 816 540
Modulus at 200% 33 31 35
Hardness(shore A) 64.7 63.3 59.1
Elasticity% 35 37 39
Hardness showed a slight increase via vulcanization with sulphur system rather than vulcanization with thiuram system and the modulus value was higher in sulphur system than sulphur-less system which was higher than peroxide system.
3.4. Results of Equilibrium Swelling in Toluene, Benzene and processing oil.
The obtained value of swelling in processing oil 37 showed that sulphur system gave the best resistance for oil and peroxide system showed more resistivety than sulphur-less system, in toluene and benzene peroxide system showed the best resistance

Figure 1: Equilibrium swelling of RSS in toluene, benzene and processing oil 37

3.5. ATR-FTIR Measurement of RSS According to Different Systems ofVulcanization

By changing the system ofvulcanization
,incorporation of different accelerators characteristic peaks appeared elucidating each system and difference between bonds formed via each system
3.5.1.ATR-FTIR Measurement of Sulphur System of RSS

Figure 2: IR spectra of sulphur system of RSS

Very weak signals for the samples range4000-3200cm-1 for the samples the vibrations responsible for bands in this region are O-H stretching organic acids and phenols it was obvious at peak 3703Cm-1. we assume that the activator (stearic acid) and the antioxidant ignorax1010 were completely incorporated in the polymer blends the same assumption should be considered in the case of the other polymer additives , the peak at 2926Cm-1 corresponded to C-H stretch, the C=O group of saturated aliphatic carboxylic acids was absorbed at 1687 Cm-1, the peak at 1606 Cm-1 due to symmetric ring stretch corresponded to aromatic accelerators, antioxidant and processing oil incorporated , peak at 1528 Cm-1 due to carboxylate ester., CH2 scissoring bend is represented by peak at 1459 Cm-1,C-N tertiary amine due to thiazole accelerators represented by peak at 1283 Cm-1, peak at 889 Cm-1 due to vinyl C-H bend, peak at 724 Cm-1 represented CH2S,Peak at 574Cm-1due to S-S stretching
3.5.2. ATR-FTIR Measurement of Sulphur- less System of RSS

Figure 3: IR spectra of sulphur-less system of RSS

Peak at 3297Cm-1 and Very weak signals for the samples range 4000-3300 Cm-1 vibrations responsible for bands in this region are O-H stretching organic acids, phenols and N-H stretch aromatic secondary amine due to incorporation of DOTG accelerator , the peak at 2926Cm-1 corresponds to C-H stretch the C=O group of saturated aliphatic carboxylic acids absorbed at 1748 Cm-1 , the peak at 1600 Cm-1 due to the symmetric ring stretch , peak at 1550 Cm-1 due to carboxylate ester., CH2 scissoring bend appeared at 1428 Cm-1,C-N tertiary amine due to incorporation of TMTD represented by peak at 1375 ,1283 Cm-1 , Peak at 1247 Cm-1 indicated DOTG aromatic secondary amine stretch , Peak at 1043Cm-1due to C-O ether linkage, peak at 878 Cm-1 due to vinyl C-H bend , Peak at 746 Cm-1 due to CH2 rocking, peak at 665 Cm-1 due to thio ether stretch, peak at 724 Cm-1 represented CH2S, Peak at 574 Cm-1 indicated S-S stretching.

3.5.3. ATR-FTIR Measurement of Peroxide System of RSS

Figure 4: IR spectra of peroxide system of RSS
Peak at 3254 Cm-1 and Very weak signals for the samples range 4000-3200 cm-1 responsible for bands in this region are O-H stretching organic acids and phenols the C=O group of saturated aliphatic carboxylic acids absorbed at 1750 Cm-1 the peak at 2926Cm-1 corresponded to C-H stretch, the peak at 1600 Cm-1 due to the symmetric ring stretch , peak at 1533 cm–1 due to carboxylate ester,CH2 scissoring bend is represented by peak at 1422 Cm-1, peak at 1262 Cm-1 due to aromatic ether , peak at 1031 Cm-1 due to C-O ether linkage, peak at 877 Cm-1 due to vinyl C-H bend , peak at 745 Cm-1 due to CH2 rocking , peak at 685 Cm-1 due to Cis C-H bend , here it was observed the lack of S-bonding characteristic peaks

3.6. Ageing Properties
3.6.1. Change in Physico-mechanical Properties of Aged Samples at 90 ?C (Geer Ageing) for Different Periods According to Different Systems of Vulcanization of RSS.
Peroxide vulcanizates gave excellent ageing characteristics, thiuram cured accelerators with low sulphur had a better resistance to ageing than sulphur system.
3.6.1.1.Change in Tensile Strength (Kg/Cm2)
Upon thermal ageing at 90 ?C, all the systems of vulcanization showed a further increase in tensile strength. And then decreased .This was due to the formation of additional crosslinks during thermal ageing Rubber samples are usually cured in industry only to 90%. The allowance of 10% is generally kept to accommodate the introduction of crosslinks in the matrix during service. When were subjected to thermal ageing at 90 ?C, the formation of additional crosslinks got accelerated. However, the tensile strength of all samples decreased due to the degradation of crosslinks.

Figure 5: Effect of thermal ageing on tensile strength of RSS according to different systems of vulcanization

3.6.1.2. Change in Elongation at Break%
The effects of different cross linking systems on elongation at break of un aged and aged samples are represented below. It had been found that the elongation at break of the vulcanisates decreased due to thermal ageing. The decrease in elongation at break could be attributed to the weakening of the matrix after thermal ageing. However, in peroxide system the increase in number of cross links may also contribute it.

Figure 6: Effect of thermal ageing on elongation at break % of RSS according to different systems of vulcanization

3.6.1.3. Change in Modulus at 200%
The modulus increased after ageing at 90 ?C. This was probably due to the formation of additional crosslinks. The modulus was found to increase with the periods of subjecting the samples to accelerated ageing till 250 hours.

Figure 7: Effect of thermal ageing on modulus at 200 % of RSS according to different systems of vulcanization
3.6.1.4. Change in Hardness (Shore)
The hardness was found to increase with the periods of subjecting the samples to accelerated ageing till 250 hours.

Figure 8: Effect of thermal ageing on hardness of RSS according to different systems of vulcanization

3.6.1.5. Change in Elasticity %
The elasticity was deteriorated for all the samples and the deterioration increased with increasing the period of subjection to accelerated ageing.

Figure 9: Effect of thermal ageing on elasticity % of RSS according to different systems of vulcanization

3.6.2. Deterioration in Physico-mechanicalProperties of RSS After 200 Hours Using Xenon Apparatus (Weathering Test Results)

It was found that peroxide system gave the best resistance of all systems used to U.V, radiation, artificial rain and hot air produced by xenon apparatus. The effects of different crosslinking systems on elongation at break of un aged and aged systems are represented in Table 6. It had been found that the tensile strength and elongation at break of the vulcanisates decreased due to weathering ageing. The decrease in elongation at break could be attributed to the weakening of the matrix. The modulus and hardness were found to increase with the periods of subjecting the samples to accelerated weathering test. The elasticity was deteriorated for all the samples

Table (6): Deterioration effect due to subjecting RSS mixtures to weathering ageing

Deterioration% S S-less peroxide
Tensile strength
(kg/cm2) 10.32 8.87 5.7
Elongation at break% 13.64 8.08 6.8
Modulus at 200% -14.2 -12.1 -9.6
Hardness(shore) -4.48 -4.2 -3.38
Elasticity% 8.5 5.1 3.5
(deterioration %) = (value before ageing – value after ageing) / value before ageing × 100

Conclusion
Since the crosslinking are carbon-carbon bonds peroxide vulcanizates gave excellent ageing characteristics and high resilience, but their strength tear, and mechanical properties were inferior to sulphur and sulphurless vulcanizates the cure rate was slow and the induction periods was short. Thiuram cured accelerators with low sulphur have better resistance to ageing than sulphur system and good physico-mechanical properties that is characteristic of sulphurless crosslinking reactions. In toluene the resistance was very weak for all systems however it showed the smallest ratio in peroxide system. Sulphur system showed the best resistance for processing oil (37) and peroxide system showed more resistively than sulphurless system. The weathering test showed that deterioration is at least in peroxide system.

Acknowledgements: The authors are thankful to all staff members of National Research Center, A.R.E for helpful suggestions.
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