Kamis, 12 April 2012
Materials Sciences and Applications, 2011, 2, 486-496
doi:10.4236/msa.2011.25066 Published Online May 2011 (http://www.SciRP.org/journal/msa)
Copyright © 2011 SciRes. MSA
Processing and Material Characteristics of a
Reclaimed Ground Rubber Tire Reinforced
Styrene Butadiene Rubber
Debapriya De1*, Debasish De2
1Chemistry Department, MCKV Institute of Engineering, Liluah, India; 2Chemistry Department, Meghnad Saha Institute of Technology,
Received January 21st, 2011; revised May 6th, 2011; accepted May 12th, 2011.
Mechanochemically partially devulcanized ground rubber tire (GRT) was revulcanized in composition with virgin styrene butadiene rubber (SBR). Reclaiming of GRT was carried out by tetra methyl thiuram disulfide (TMTD) in presence of spindle oil. The cure characteristics and tensile properties of SBR compounds were investigated. Results indicate that the minimum torque and Mooney viscosity of the SBR compounds increase with increasing reclaim rubber (RR) loading whereas the scorch time remain unaltered but optimum cure time exhibit a decreasing trend. Increasing RR loading also gives SBR compounds better resistance towards swelling but the 100% modulus, 200% modulus tensile strength, and the elongation at break increases. Thermogravimetric analysis of SBR/RR vulcanizates was carried out in order to get thermal stability of the vulcanizates. Scanning electron microscopy (SEM) studies further indicate the coherency and homogeneity in the SBR/RR vulcanizates.
Dwindling source and rising price of crude oil have become a serious concern for availability of feed stock to the synthetic rubber industry. Huge quantities of rubbers are used in tire industry and the world’s rubber markets are dominated by two rubbers – one being natural rubber (NR) and the other being styrene butadiene rubber (SBR). Among different rubbers, tire industry consumed largest quantity of SBR which is mainly used in car tire tread. But after a long run when these tires are not serviceable only a few grams or kilograms of rubber are abraded out from the tire. The entire amount of rubber from the worn-out tires is discarded, which again require very long time for environmental degradation due to cross- link structure of rubbers. Various approaches such as land filling , incineration , pyrolysis [3,4], civil engineering [5,6] applications etc. have been adopted for reuse of waste discarded rubber products. But in almost all the applications no value is added from the product side. Tire recycling or reclaiming are one of the preferable routes, according to the so-called waste management hierarchy, under environmental aspect. The recycling of tires not only saves our valuable resource of petroleum from which the synthetic rubbers are originated, but also protects our precious environment. Thus reclaiming of tires in alternative applications is actually the use of base polymer in new formulations with simultaneous cost saving in raw material and preserving both natural resource and environment. Incorporation and dispersion of reclaim rubber in fresh rubber play important roles towards the product quality, production economy and market competition. Compatibility of reclaim rubber with virgin rubber is an essential requirement for optimum mechanical properties of the vulcanizates. Therefore it is necessary to evaluate the performances of such fresh rubber/reclaim rubber blends. In a review paper the author  have discussed the various reclaiming processes of vulcanized rubber in presence of different chemicals. The mechanical reclaiming of GRT by TMTD as reclaiming agent was studied by the author . Here the extent of reclaiming was monitored by measurement of sol content, inherent viscosity of sol rubber, crosslink density and Mooney visProcessing and Material Characteristics of a Reclaimed Ground Rubber Tire Reinforced Styrene Butadiene Rubber cosity of reclaim rubber as function of milling time and concentration of reclaiming agent. The optimization of the reclaiming agent concentration and time of reclaiming was also reported. The performance evaluations of such type of reclaim rubber/virgin natural rubber blend was also studied  by the author. In this paper mechanochemically partially devulcanized GRT was blended with fresh NR in 20% - 60% level. The reclaim rubber, prepared in this investigation, when blended with fresh NR has been found to reduce the tensile strength by about 7% for 20% reclaim containing vulcanizate and 46% for 60% reclaim containing vulcanizate. It is observed that the aging performances of the reclaim rubber containing vulcanizates are better than the control formulation,which does not contain any reclaim rubber. TGA shows that the thermal stability of the vulcanizate increases with increasing reclaim rubber content. Noordermeer et al.  reclaimed waste latex rubber by diphenyl disulfide, 2-aminopheny disulfide and 2, 2'-dibenzamidodiphenyl disulfide as a function of concentration of reclaiming agent, time and temperature. The comparative study of the reclaiming efficiency of the three reclaiming agents was carried out. From the study it was evident that 2, 2'-dibenzamidodiphenyl disulfide is able to break the crosslink bonds at around 20°C. Sombatsompop and Kumnuantrip [11,12] incorporated reclaim rubber from tire tread into two grades of NR and investigated
various properties of the blend. They found that Mooney number, shear viscosity and cure rate increased with RR content but optimum cure time is independent of it. Sreeja and Kutty  studied the cure characteristics and mechanical properties of NR/RR blends, using the EV system. They reported that scorch time and tensileproperties of the blend reduced with RR loading. The curing characteristics and mechanical properties of SBR/RR blend system was reported by the author . Here reclaiming of waste rubber was carried by a simple process with an eco-friendly renewable resource material [15,16]. The main constituent of this material is diallyl disulfide. Other constituents are disulfides, mono sulfides, poly sulfides and thiol compounds. Nevatia et al.  mixed RR with recycled poly ethylene and evaluated physical properties, dynamic mechanical properties and rheological behaviors of the blend. They reported that 50/50, RR/PE vulcanizate showed optimum processibility, ultimate elongation and set properties. Here sulfur- accelerator cured system gave superior product properties
than peroxide cured system. This paper describes a new elastomer products based on virgin SBR and reclaimed GRT. Initial step involves the mechanochemical reclaiming of GRT by TMTD and subsequent incorporation of reclaimed GRT into virgin
SBR in different proportion. The (re)vulcanization of different SBR/RR blends was found to give a new low cost product with adequate properties. The term (re)vulcanization was used because there are two simultaneous processes such as vulcanization of the virgin rubber and (re)vulcanization of partially devulcanized GRT and even the co-vulcanization of them. Curing characteristics and mechanical properties before and after aging of SBR/RR blend system have been studied. Thermal behavior of RR, SBR and different SBR/RR vulcanizates wasalso studied. Finally the dispersion of reclaim rubber into SBR was evaluated by scanning electron microscopy
GRT, purchased from local market was used in this investigation. The GRT was an unclassified ground rubber from the tread and side walls of passenger and truck tires.The particles of GRT were of various sizes ranging from a few millimeters to 100 microns. Styrene butadiene rubber (SBR 1502, Synthetics & Chemicals Ltd. India), tetramethylthiuram disulfide (TMTD) (Alpha Chemika, Maharashtra, India), zinc oxide (S. D. Fine Chem. India),stearic acid (Loba Chemi. India), sulfur (S. D. Fine Chem. India), spindle oil (MCI, India), carbon black (N330,Philips Carbon Ltd. India) and toluene (S. D. Fine Chem. India) were used as received.
2.2. Experimental Procedure
The optimization of reaction conditions and the concentration of TMTD used for reclaiming of GRT were reported in the author’s previous work . Hundred grams of ground rubber was thoroughly mixed with 2.75 g TMTD and 10 mL spindle oil. The mixture was then reclaimed mechanically in an open two-roll mixing mill at a friction ratio of 1.2 for 40 minutes near ambient temperature. It has been found that with progress of milling the materials become soft, sticky and band formation occurs on the roll. The extent of reclaiming was monitored by measurement of sol content (30.3%), inherent viscosity of sol rubber (0.3944), crosslink density (6.587 × 10−4 mol/cm3), molecular weight between crosslink bonds (33.093 × 103), swelling ratio (4.099) and Mooneyviscosity [ML (1 + 4) 100°C] (70.6) of reclaim rubber
2.3. Preparation of SBR/RR Vulcanizates
Mixing of fresh SBR, various proportions of reclaim rubber and compounding ingredients was carried out for 15 minutes at room temperature on an open two-roll mixing mill. Compound formulations are presented in Table 1. The amount of add itives such as ZnO, stearic Processing and Material Characteristics of a Reclaimed Ground Rubber Tire Reinforced Styrene Butadiene Rubber
Table 1. Mix formulation and curing characteristics of SBR/RR Compounds.
Ingredients (phr) 1 2 3 4 5 6 7 8 9 10
Styrene Butadiene rubber (SBR) 100 80 70 60 50 40 100 80 80 80
Reclaim rubber (RR) - 20 30 40 50 60 - 20 20 20
Zinc oxide 5 5 5 5 5 5 5 5 5 5
Stearic acid 2 2 2 2 2 2 2 2 2 2
TMTD 2.16 1.61 1.335 1.06 0.785 0.51 2.16 1.61 1.61 1.61
Sulfur 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
carbon Black (N330) - - - - - - 40 20 30 40
Spindle oil - - - - - - 4 2 3 4
Optimum cure time (t90, min) 7.5 5.5 5.25 4.75 4.5 4.25 10 5.25 6.0 4.5
Scorch time (ts2, min) 1.5 0.5 1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.5
Extent of cure, (dNm) 49 54.2 55.5 55.6 56 58 68.5 58 63 68
Cure rate index, (min−1) 16.7 20 23.5 26.7 28. 6 30. 8 10. 5 21 18.2 25
100 % Modulus, MPa 1.255 1.59 1.62 2.10 2.13 2.28 3.5 2.146 3.267 4.196
200 % Modulus, MPa 1.698 2.25 2.31 3.08 3.19 3.32 5.477 3.264 4.985 6.465
Tensile Strength, MPa 2.335 2.781 2.974 3.835 4.573 5.017 12.014 7.515 9.946 12.577
% Elongation at break 432 377 382 390 427 445 537 587 521 509
Hardness, (Shore A) 50 60 65 64 62 60 65 60 63 65
Crosslinking value, (1/Q) 0.238 0.292 0.324 0.358 0.376 0.406 0.300 0.300 0.325 0.337
Mooney Viscosity [ML (1 + 4) 100°C] 40.7 51.0 65.7 70.6 71.8 75.0 - - - -
acid and sulfur were used based on 100 g of rubber irrespective of the amount of reclaim rubber, because it was reported that the additives in reclaim rubber originated from parent compound are inactive . Theamount of TMTD was maintained at 9 m mol in all the vulcanizates based on the amount of TMTD used during reclaiming of GRT. This is due to the fact that in sulfur, TMTD vulcanization system the optimum concentration of TMTD is chosen as 9 m mol i.e. 2.16 g per hundred gm of fresh rubber (i.e. RR and SBR). Formulation 1 contains no reclaim rubber and formulation 2 - 6 contains different proportion of reclaim rubber from 20 - 60 wt%.In order to study the effect of carbon black, various proportion of carbon black was added in SBR/RR (80/20) blend system. Formulation 7 contains only SBR with 40 phr carbon black and formulation 8 - 10 contain different Processing and Material Characteristics of a Reclaimed Ground Rubber Tire Reinforced Styrene Butadiene Rubber
proportion of carbon black (20, 30 and 40 phr) in SBR/RR blend. It has been observed that with increase in the proportion of carbon black its incorporation and dispersion become gradually difficult. With higher proportion of carbon black loading the compounds become stiff and the temperature rises due to high shearing action required for better dispersion. The cure characteristics of SBR/RR compounds were determined with the help of a Monsanto Oscillating Disc Rheometer, R-100 at 160°C. It has been found that all cure curves were level off in the region of 60 minutes, where torque-time gradient of each sample was constant or did not change significantly. The compounded rubber stock were then cured in a compression molding machine at 160°C and at applied pressure of 34.5 MPa for the respective optimum cure time (t = t90) obtained from rheographs. After curing, the vulcanized sheet was taken out of the mold and immediately cooled under tap to restrict from further curing.
2.4. Measurement of Mechanical Properties
The mechanical properties such as modulus, tensile strength and elongation at break was measured by a Hounsfield, model H10 KS tensile testing machine as per ASTM D 412-51T at room temperature (25 ± 2°C) at a uniform speed of separation 500 mm/min. Hardness (Shore A) of the vulcanizates were measured by a Hirosima Hardness tester as per ASTM D 1415-56T. The values reported were based on the average of five measurement of each sample. The aging characteristics of the vulcanizates was evaluated by accelerated aging test in an air aging oven at 100 ± 2°C after 24, 48 and 72 h aging. Mooney viscosities of rubber compounds were determined by a Monsanto Mooney viscometer 2000 at ML (1 + 4) 100°C as per ASTM D 1646
2.5. Swelling Value of the Vulcanizates
The swelling value (Q) was determined with about 0.5 g of cured samples (accurately weighed). The sample was immersed in 250 mL toluene for 72 h to attain equilibrium swelling. After equilibrium swelling the sample was taken out and the solvent was blotted from the surface of the sample and weighed immediately. It was then dried under vacuum at 100°C upto constant weight. The crosslinking value of the vulacanizates was calculated from the following equation . where S , D , O W W W and F W are swollen weight, dried weight, weight of the original sample and formula weight respectively. Formula weight ( F W ) is the total weight of rubber plus compounding ingredients based on 100 parts of rubber.
2.6. Determination of Crosslink Density
The crosslink densities of SBR/RR vulcanizates were determined by the Flory-Rehner equation  by using swelling value measurement.where r V is the volume fraction of rubber in the swollen gel, S V is the molar volume of the toluene (106.2 cm3·mol−1 in this study), is the rubber-solvent interaction parameter(0.378 in this study), swell is cross-link density ofthe rubber (mol·cm−3) and f is functionality of the crosslinks (being 4 for sulphur curing system).The volume fraction of a rubber network in the swollen phase is calculated from equilibrium swelling data as: where 1 W is the weight fraction of solvent, 1 d is the density of the solvent, 2 W is the weight fraction of the polymer in the swollen specimen and 2 d is the density of the polymer.
2.7. Thermo Gravimetric Analysis (TGA)
The thermo gravimetric analysis (TGA) of SBR/RR vulcanizate was carried out by using a TGA 50, Shimadzu, Japan, thermo-gravimetric analyzer in nitrogen (flow rate 50 mL/min) within the temperature range of 20 to 800°C.All these analysis were carried out at heating rate of 10°C/min.
2.8. Scanning Electron Microscopy
The tensile fracture surface of the samples were studied in scanning electron microscope (SEM) (JEOL, JSM5800) at 0° tilt angle after coating the surface with sputtered gold.
3. Results and Discussion
3.1. Curing Characteristics
Curing characteristics of different SBR/RR blend system and SBR/RR (80/20) blend system with various proportions of carbon black loading are given in Table 1. The optimum cure time decreases but scorch time remain unaltered with increase in reclaim rubber content in all Processing and Material Characteristics of a Reclaimed Ground Rubber Tire Reinforced Styrene Butadiene Rubber the cases. From Table 1 it has been found that with increase in the proportion of reclaim rubber extent of cure increases due to the presence of crosslinked gel in the reclaim rubber. It is also evident that with increase in carbon black loading the optimum cure time and scorch time remain unaffected but extent of cure increases with increasing carbon black loading.
3.2. Tensile Properties of Rubber Compound
The stress-strain behavior of SBR/RR vulcanizates with various proportion of reclaim rubber are shown in Figure1. The moduli at 100% and 200% elongation increases with increasing reclaim rubber content in all the SBR/RR vulcanizates. The stress strain was decreased with increasing reclaim rubber content. From the Figure it is shown that the stress-strain of SBR/RR vulcanizates was decreased after 50% elongation compare to that of the fresh SBR vulcanizate. Tensile properties, Mooney viscosity and crosslinking value of the SBR/RR vulcanizates are presented in Table 1. From the values in Table 1 it is seen that moduli at 100% and 200% elongation, tensile strength, elongation at break and hardness increases with increasing reclaim rubber content. The reason for higher 100% and 200% moduli may be due to higher crosslink density (Figure 2) of the vulcanizates, arising out of the gel present in reclaim rubber or may be due to the presence of active functional sites in reclaim rubber which may participate in crosslinking during the process of vulcanization. As crosslink density of the vulcanizates increases with increasing reclaim rubber content, chain mobility decreases and more load is required for 100% and 200% elongation. The increasing value of tensile strength with reclaim rubber content is probably due to the presence of carbon black left in the reclaimrubber . It is known [7,21,22] that the partial devulcanization of ground rubber tire (GRT) facilitates the interface adhesion between the surface chains of reclaim rubber (RR) particles and surrounding rubber matrix due to their co-crosslinking in the interphase layer. The increase in the value of elongation at break with reclaim rubber content can indicate a better compatibility in interphase layer of the rubber matrix and reclaimed rubberparticles [23,24]. The growth of the values of hardness observed at increasing RR content in SBR/RR (re)vulcanizates obviously evidences of more intensive post vulcanization process in the blends due to presence of additional sulfur released at the devulcanization of GRT .Effect of carbon black loading was studied in SBR/RR (80/20) blend system. It is seen that with increase in carbon
black loading 100% and 200% moduli, tensile strength increases. This can be explained by corresponding increase in crosslinking value data. The higher crosslinking value of the vulcanizates is conclusive evidence
Figure 1. Stress – Strain behavior of SBR/RR blend system.
Figure 2. Crosslink density of SBR/RR blend system as a
function of reclaim rubber content and carbon black loading.
for the presence of a network with a greater number of filler polymer interactions, constraining molecular mobility in the polymer. The tensile strength of 40 phr carbon black loaded SBR/RR (80/20) vulcanizate is higher compared to that of the control formulation 7 i.e. vulcanizate containing no reclaim rubber. Elongation at break decreases with increasing carbon black loading. Hardness increases because with increasing carbon black loading vulcanizates become stiff and hard. The increase in the value of Mooney viscosity with carbon black loading further indicates that the processing of the rubber compoundbecome difficult if higher amount of carbon black is incorporated into the rubber matrix.
3.3. Effect of Thermal Aging
The aging characteristics of SBR/RR vulcanizates should Processing and Material Characteristics of a Reclaimed Ground Rubber Tire Reinforced Styrene Butadiene Rubber be given proper attention because reclaim rubber itself is a degraded mass. Thus accelerated aging test were performed for SBR/RR vulcanizates. Percent retention of 100% and 200% modulus are shown in Figures 3 and 4.In all the cases it was observed that percent retention of 100% and 200% modulus increases with increasing reclaim rubber content and with progress of aging. This may be due to residual crosslinking during progress of aging which is further enhanced due to the presence of reclaim rubber, containing active functional sites. Percent retention of tensile strength is shown in Figure 5. From the figure it is evident that percent retention of tensile strength decreases with progress of aging but increases with increasing reclaim rubber content at a particular time of aging. This may be due to increasing crosslink density of the vulcanizates with progress of aging. The percent retention of elongation at break for the various SBR/RR systems at different RR content after aging is shown in Figure 6. But percent retention of elongation at break values at higher reclaim content such as 50 and 60 wt% reduces considerably; this is a result of both thermooxidative degradation and post vulcanization of SBR/RR (re)vulcanizates studied. Percent retention of hardness is shown in Figure 7. With progress of aging as the vulcanizates become stiff therefore hardness increases.Effect of carbon black loading on % retention of properties after 72 h aged SBR/RR (80/20) blend system was shown in Figure 8. The percent retention value of both 100% and 200% modulus continuously increases with increasing carbon black loading. But the percent retention values of tensile strength and elongation at break shows a different behavior. Both the properties shows maximum percent retention values for 20 phr carbon black loading compared to that of the control formulation.
Figure 5. Effect of reclaim rubber content on % retention of
However, for 30 and 40 phr carbon black loading the percent retention values considerably decreases due toaging. Although the percent retention of tensile strength for 30 and 40 phr carbon black loading is not less than that of the control formulation. It is seen from Figure 8 that percent retention of hardness was almost constant with increasing carbon black loading. These results show the effectiveness of 20 phr carbon black in the SBR/RR (80/20) blend system. Thus the aging performances of formulation containing reclaim rubber are superior than that of the control formulation which does not contain any reclaim rubber. This phenomenon indicates the antiaging characteristics of reclaim rubber.
3.4. Thermogravimetric Analysis
Thermal degradation of different SBR/RR vulcanizates Processing and Material Characteristics of a Reclaimed Ground Rubber Tire Reinforced Styrene Butadiene Rubber
Figure 7. Effect of reclaim rubber content on % retention of
in inert atmosphere was analyzed and corresponding resultsare given in Figure 9. The thermogravimetric analysis were performed in nitrogen atmosphere, which shows only thermal degradation behavior compare to the combine effect of thermal as well as termooxidative degradation in presence of oxygen or air. The tempera- ture interval of degradation stages evaluated from DTG curves, temperature of the stages maximum rate of degradation, sample weight loss at the temperatures and char residue values are listed in Table 2. All SBR/RR blend system shows two characteristic degradation peak in DTG curve between 320°C - 530°C (Figure 9). But for all the vulcanizates initial weight loss occurs between 50 - 300°C. Pure SBR shows 3.6% wt loss up to 300°C whereas with increase in reclaim rubber content it goes up to 10% for SBR/RR blend system. This initial weight loss under N2
Figure 8. Effect of carbon black loading on % Retention of
properties after 72 h aged SBR/RR (80/20) blend system.
atmosphere between 50°C - 300°C is due to the volatilization of processing oil or any other low boiling point component present in pure and blend system. As reclaim rubber contain large amount of processing additives compare to virgin rubber, so with increasing reclaim rubber content initial weight loss increases from 3.6% to 10%. As RR contain some NR for that reason SBR/RR blend shows two distinct peak in DTG curve which was also observed for NR/SBR blend system . Pure SBR shows 10.57% wt loss in 1st degradation step. With increase in reclaim rubber content % wt loss in 1st degradation step gradually increases and maximum 24.13% was observed at 40phr SBR containing SBR/RR blend system. 1st degradation was faster with increase in RR content in SBR/RR blend system. Low temperature degradation was much pronounced with increase in RR content in SBR/RR blend system. If we compare our TGA results with pure RR system, it was observed that reclaim rubber shows three steps degradation. Out of three steps, 2nd degradation step occurs in between 334.4°C - 507.5°C. Actually 2nd degradation step of pure reclaim rubber partially merge with the 1st degradation step of SBR/RR blend system. As a result of which % wt loss increases in 1st degradation step of SBR/RR blend system with increase in reclaim rubber content. But in case of second degradation the complete opposite train was observed. Pure SBR shows around 80% weight loss in second degradation step. This is the characteristic degradation peak for pure SBR in thermo-gravimetric analysis. The % wt. loss of blend vulcanizates is decreased in 2nd degradation step with increase in reclaim rubber content. This may be due to the low SBR content in SBR/RR blend system. The same reason was also apProcessing and Material Characteristics of a Reclaimed Ground Rubber Tire Reinforced Styrene Butadiene Rubber
3.5. SEM Study
Scanning electron micrograph of the tensile fractured surface of fresh SBR and different SBR/RR vulcanizates are displayed in Figure 10. The micrograph of fresh SBR vulcanizate showed homogeneous surface morphology. Whereas in the reclaim rubber containing vulcanizates several number of crack paths in different directions was observed which is making the vulcanizate susceptible under mechanical stress. Here in case of reclaim rubber containing vulcanizate, the fracture mode showed the failure mode that was less rubbery in nature, due to highercross link density of the vulcanizate.
Mechanochemical reclaiming of GRT was carried out by multifunctional reclaiming agent, TMTD. The reclaimed rubber prepared in this investigation, when mixed with fresh SBR has been found to increase the tensile strength by about 19% for 20% reclaim containing vulcanizateand 115% for 60% reclaim containing vulcanizates. It isobserved that the aging characteristics of the reclaim rubber containing vulcanizates are superior compared to that of the control formulation, which does not contain any reclaim rubber. TGA shows that the thermal stability of the vulcanizate increases with increasing reclaim rubber proportion. SEM studies indicate that reclaim rubber containing vulcanizates are vulnerable under mechanical stress. Another advantage of this reclaiming agent is the reduced smell during the reclamation process and of the final reclaims, one of the most important short comings of other disulfides used for this purpose.5. Acknowledgements
The authors, Debapriya De thankfully acknowledge the financial support by the Department of Science and Technology (DST), New Delhi, India for carrying out the present research work.
 F. G. Smith, “Rubber ACS Division,” Meeting, IRC, Orlando, 26-28 October 1993.
 H. F. Mark, N. M. Bikales, C. G. Overberger and G.Menges, “Encyclopedia of Polymer Science and Engineering,”Vol. 14, Wiley, New York, 1988, pp. 787-804.
 A. A. Merchant and M. A. Petrich, “Pyrolysis of ScrapTires and Conversion of Chars to Activated Carbon,”AIChE Journal, Vol. 39, No. 8, 1993, pp. 1370-1376.
 J. P. Lin, C. Y. Chang and C. H. Wu, “PyrolyticTreatment of Rubber Waste: Pyrolysis Kinetics of Styrene-Butadiene Rubber,” Journal of Chemical Technology & Biotechnology, Vol. 66, No. 1, 1996, pp. 7-14.doi:10.1002/(SICI)1097-4660(199605)66:1<7::AID-JCT
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Senin, 09 April 2012
Mixing of Powder Rubber
Ali Amash*, Martin Bogun and Robert-H. Schuster
Deutsches Institut für Kautschuktechnologie e. V.,
Eupener Straße 33, 30519 Hannover, Germany
Udo Görl and Martin Schmitt
Pulver Kautschuk Union GmbH (PKU), 45772 Marl, Germany
Paper presented at the
International Rubber Conference, IRC 2001
Co-ordinated by the Rubber Division of the Institute of Materials.
Organised by IOM Communications Ltd.
12 - 14 June 2001
ICC, Birmingham, UK
A markedly high filler dispersion degree of >95% was determined for all compounds produced by
the varying process parameters. Consistently, rheological and mechanical measurements revealed only negligible to weak differences between the sample properties. For comparison of results, compounds based on NR in bale and powder form were prepared by the discontinuous mixing process.
One of the most desired developments is still a reliable continuous compounding process of conventional rubber compounds. It gains an increasing interest, due to the related attractive advantages including economic efficiency, as compared to the nowadays mostly widespread multi-stage rubber compounding technology. In recent years, a real progress towards the continuous mixing process, which primarily requires a continuous feed state of all compound ingredients, occurred by manufacturing of new powder rubber types based on E-SBR and NR [1-3] as well as on gas-phase EPDM [4-6]. These free-flowing rubbers are suggested to replace the conventional rubber supplied in bale form.
The distinctive characteristic of the granular batches produced from polymer emulsions (SBR, NR) and non-pelletized carbon black (fluffy) is the markedly constant rubber-filler ratio, and that their both components are homogeneously distributed in an intimate mix with a high degree of filler dispersion . Compounds on the basis of carbon black filled E-SBR types, prepared by the discontinuous kneader mixing process, revealed an unique combination of favourable mixing
behaviour, high dispersion and good in-rubber properties.
In previous works, remarkable efforts are made on the development of an appropriate continuous mixing process for powder rubber based on SBR by using the twin-screw extruder (TSE) and considering several aspects such as the influence of mutual process parameters [7-9]. Twin-screw extruders cover a large variety of configurations including (i) co-rotating, (ii) counter-rotating and (iii) partially or fully intermeshing screws as well as (iv) with constant or variable helix angle . In order to improve mixing performance, TSE design may include special mixing sections and conventional or modified kneading blocks for dispersiv and distributive mixing. It is noteworthy that the various types of twin-screw extruder as well as of mixing elements are primarily designed for processing plastics. The real challenge lie in the transfer of the overall extrusion technology to powder rubber processing. Due to recent progress in gravimetric and volumetric proportioning technology, powder materials and fillers can nowadays also be fed with high accuracy.
This paper is a further contribution of advanced concepts to the development of an appropriate continuous rubber mixing process for powder rubber compounds on the basis of NR by using the twin-screw extruder. The main target of the work is the optimization of mixing parameters, such as feed rate, screw speed, temperatures and energy input. In addition to these a screw configuration should be optimized, in order to improve the extrudate output with a still high dispersion degree and homogenous distribution. An important aim is also the study of the physical behaviour of the materials. A comparison of powder to bale rubber compounds produced by discontinuous process is done.
Material and Feeding
Powder rubber (PR) on the basis of 100 phr natural rubber (NR) and 47 phr carbon black (CB) was supplied from PKU GmbH (Marl / Germany) for performance of investigations. The average particle size of the granules lie in the range of 0.5-3 mm. For producing compounds and vulcanizates, different ingredients including antioxidants and curing system have to be incorporated. Table 1 shows the compound recipe used. It should be noticed that the chemicals were premixed and separately added to the powder rubber during mixing process.
Powder rubber NR 100
Premix Stearic acid2
Twin Screw Extruder
Compounding Experiments were performed by using the twin-screw extruder (TSE; Farrel FTX80) equipped with co-rotating screws . Key data about the machine are given in Table 2. The extruder barrel consists of nine replaceable parts of 4 L/D Length, respectively. The first metering zone barrel should be permanently cooled by water to prevent material caking. The following eight processing segments are mainly used for mixing and can be heated up to temperatures of 370°C, where open cylinders are suitable for feeding additional ingredients and for degassing purpose.
Max. process length 36 L/D
Extruder hole diameter 37 mm
Screw outer diameter 36.5 mm
Screw inner diameter 23.5 mm
Diameter ratio 1.55
Channel depth 6.25 mm
Alignment 31 mm
Max. screw speed 500 min-1
Max. Power 18.5 kW
Max. torque per screw 176 Nm
Compounds on the basis of the new powder rubber as well as of a conventional NR type obtained in bale form were produced by the multi-stage discontinuous mixing process carried out on a 1.5 l laboratory internal mixer for mixing time of 6 min. The test formulation is given in Table 1, where the bale rubber type SMR10 (100 phr) and CB N234 (50 phr) would replace the powder rubber/filler-batch used also for extrusion. In the case of bale rubber compound, all ingredients were mixed at 140°C after the necessary mastication of the matrix, while the curatives were added in a separated mixing stage at 115°C. Powder rubber compounds were prepared in a similar manner, however, no separated mastication is ever needed, the curing system and antioxidants are added in the second stage, the mixing time was varied and the loading degree of PR slightly increased (10%).
s a slightly lower cross-linking density, which may be attrSample Preparation and Measurements
All materials produced by extrusion and kneader were shortly threaded over a roll mill of a large nap (5 mm) at 70°C, in order to get rolling hides.
The Mooney viscosity of the rubber compounds was detected for an evaluation of the processing behaviour. This important property depends on filling degree, dispersion and the decrease of the molecular weight (i.e. mixing time) during the mixing cycle. Stress-strain measurements were performed on Dumbbell specimens of vulcanizates at room temperature. Tensile strength, elongation at break, modulus and hardness were determined.
Results and Discussion
Discontinuous Rubber Mixing
Table 3 gives some results for the compounds of the internal mixer. In general, only weak to moderate differences can be established between their properties. The powder rubber compound mixed for 6 min exhibit the lowest Mooney viscosity and the highest dispersion degree of 98.5%, while its mechanical behaviour is comparable to that of the bale rubber vulcanizate. This mixture yields also a good dispersion degree of ca. 95.5% and a clearly higher viscosity in comparison to the two-stage PR mix. Reducing mixing time of powder rubber to 4.5 min, only marginal changes can be observed in the compound properties. The one-stage PR-compound mixed for 3 min exhibits values of dispersion and Mooney viscosity comparable to
Mixing time mastication 6 min 4.5 min 3 min
+ 6 min one stage
Dispersion degree [ %] 95.6 98.5 97.6 95.3
Vulcam. torque S`max-S`min [dNm] 17 16 16 16.3
Elongation at break, εb [%] 532 537 533 540
200% modulus [MPa] 7.8 7.5 7.7 7.3
Hardness (shore A) 67.5 68.8 69.5 70
Screw Elements and Configuration
It is well known, that the geometrics for co-rotating screw were basically developed by Erdmenger, with the objective of screw's touching at any screw angle for selfcleaning purpose . In this work, various types of screw elements were available for varying and optimizing screw configuration. Figure 1 illustrates these “loose”screw parts: (Fig. 1a) single-flighted undercut elements of long pitch for particulate feed handling; (1b) forward pumping conveying elements of Erdmenger-profile and various pitches; (1c) kneading blocks for dispersiv mixing, made up of five disks turned by 45° and taking up a total length of 1 L/D (overall offset of 180°; outer diameter 36 mm); (1d) Farrel Asymmetric Modular Mixing Element (FAMME or CME) for dispersiv mixing, which is a variation of the Erdmenger profile exhibit a very steep pitch; FAMME and kneading blocks can be designed as forward-pumping or reverse-pumping elements; (1e) distributive turbine mixing elements for the homogenization and dispersion of low-viscosity additives at low shear rates; (1f) non-intermeshing elements of polygon profile, primarily, for distributive mixing.
The behaviour of the rubber compounding process on extruder is essentially determined and influenced by the chosen screw configuration . The available screw elements allow modification of screw geometry and great variety of mixing sections longitudinally from the feeding zone towards the screw tip. However, the optimization of the screw assembly and the overall continuous mixing process has to be based on principles applied by discontinuous compounding in an internal mixer or on the roll mill. With other words, suitable extrusion concepts require that the powder rubber goes through regions of high shear forces and long duration, alternating withsections of low shear exposure and short residence time. The former can be realized by dispersiv and distributive mixing elements at low temperatures for fully rubber mastication and high filler dispersion. In this work, several screw configurations composed of different screw elements and varying arrangement were basically tested. Preliminary extruder experiments were performed on the free-flowing powder NR-rubber/filler batch (PR) by using screw configurations of 24 or 36 L/D process length with mostly conveying elements and up to four mixing elements designed for forward- and reverse-pumping. The results of these initial stages were relatively insufficient regarding material properties, but, important test points and improvement options of the process could be defined as well as useful effects were observed during compounding. Therefore, the next step lie in the development of a screw configuration based on the overall establishment. Further investigations revealed that a certain arrangement and a minimum number of all types of the available screw elements with a process length of 36 L/D, would result in a considerably efficient use of the individual distinctive mixing and other advantages of the different elements. In Figure 2, an optimized screw configuration is illustrated for the continuous mixing process of the powder rubber compounds based on NR. It can be seen, that a kneading block placed in the plasticizing zone at 80°C and one FAMME in the following section are needed for a highly level of mastication and filler dispersion.
Both block types consist of forward-pumping and reverse-pumping elements, respectively, in order to expose the granular rubber to high shear stress for long residence time. After material conveyance, turbine elements are useful for the homogeneous incorporation of the chemical premix at clearly low shear and temperatures. The compound goes through conveying elements, alternating with a FAMME and a polygon element, which are suitable for further dispersion and
effective distribution, respectively. In the last screw segment, conveying elements of small pitch and Erdmenger profile are placed.
Process Parameters and Properties
Table 4: Properties of NR compounds produced by the continuous mixing process.
Feed rate [kg/h] 10 20 30 40 50
Screw speed [rpm] 150 220 310 390 460
Specific energy input [kWh/kg] 0.505 0.374 0.348 0.332 0.313
Temperature [°C] 76 88 107 120 132
Mooney Viscosity @100°C [MU] 55.5 57 59 61.5 63
Dispersion degree [%] 98.5 98.0 97.2 96.0 95.3
Torque S`max-S`min [dNm] 15.5 15.4 15.0 14.4 13.7
Scorching time, TS1 [min] 3.3 3.3 3.1 3.0 2.8
Tensile strength, σmax [MPa] 31.4 31.2 30.7 30.3 30.0
200% modulus [MPa] 8.4 8.2 7.6 7.6 7.4
Hardness (shore A) 67.5 67 67.5 68 68
vulcameter measurements revealed an obvious scorching safety for all samples of the tested throughput rates, where a reasonable scorching time higher than ca. 2.8 min was recorded. It is assumed, that the short residence time of the compound in great shear regions, due to high screw speed and conveying capability, would compensate the eventual scorching effects of increasing temperature. A further effect observed on the vulcameter curves is a gradually small reduction of the corresponding maximum torque with increasing output and screw speed. This can be primarily attributed to the slightly decreasing cross-linking density, as the mixing time becomes shorter and, consequently, the quality of diffusion and homogeneity of the chemicals in the entire rubber matrix is also marginal lowered. Certainly, metering some chemicals (i.e. zinc oxide) in an earlier stage for better dispersion would improve the cross-linking density, since the later absorption and homogeneity of the curatives can be considerably facilitated. An important advantage of raising feed rate and screw speed is doubtless the overall reduction in specific energy input needed for the continuous mixing process. Increasing feed rate, screw speed and temperature results in a certain enhancement of the Mooney viscosity, which is determined by the screw configuration and mixing quality. Short duration of the powder rubber at the mixing elements imposing high shear forces, lowers the level of mastication and, consequently, the Mooney viscosity. However, this important process property still lie in a clearly good value range, as compared to that of internal mixer compounds. An analogous interpretation is given for the slight decrease observed in the filler dispersion of the extrudates by increasing the mentioned parameters. In dependence of the screw speed, the residence time of the material in the high shear regions becomes shorter as well as the duration of dispersion and distribution. But also in this case, the values of the dispersion degree remain in a narrow range of 95-99%. This is evidentially referred to a great suitability and capability of the optimized screw configuration (Fig. 2) to meet important requirements of the continuous rubber mixing process, i.e., good dispersion at maximized output.
The development of an appropriate continuous mixing process for NR- powder rubber compounds on the twin-screw extruder requires a simultaneous pursuit of several mutual aims, including the achievement of a good quality of morphology (filler dispersion and distribution) and related final properties as well as a maximization of the output at low to moderate temperatures for scorching safety. However, the most important basis of the mixing process is an optimum extruder screw configuration containing various types of screw elements having different functions. A significant raise of the throughput rate is achieved by increasing screw speed without a negative influence on the process and final material properties. These are comparable to the properties of compounds produced by discontinuous mixing. The defined limits of the extruder, i.e., screw speed, torque and cooling are crucial for the optimization of continuous mixing concept.
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