- Email: [email protected]
Cure characteristics and vulcanizate properties of a natural rubber compound extended with convoluted rubber powder
Polymer Testing 19 (2000) 507–521
Material Properties
Cure characteristics and vulcanizate properties of a natural rubber compound extended with convoluted rubber powder U.S. Ishiaku*, C.S. Chong, H. Ismail School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia Received 15 January 1999; accepted 19 March 1999
Abstract Fine convoluted rubber powder (RP) obtained from the sanding process of polishing rubber balls was used as an extender for a natural rubber compound. The cure characteristics of the extended compound were studied with the Monsanto MDR 2000 rheometer while the mechanical properties of the vulcanizates were measured in accordance with ASTM standards. The results indicated that the incorporation of rubber powder does not significantly increase the viscosity of the compounds but it reduces both scorch time and cure time. The tendency towards reversion increases with increasing rubber powder concentration. The compound can be extended with up to 30% RP without compromising tensile properties although the modulus increases continuously, albeit not so significantly. The resilience of the compounds remains almost unchanged even at higher loading, thus indicating that fine rubber powder is a true extender for rubber compounds. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In the burgeoning discourse on what to do with the piles of rubber and elastomeric materials, the size of rubber particles is emerging as the major determining factor in rubber reuse. Tyres account for over 60% of all elastomeric products and the fate of scrap tyres apparently dictates the trend of rubber recycling. The era when the landfill was the ultimate destination of scrap rubber is gradually being swept into the annals of history as rubber recycling is fast becoming a profitable venture. * Corresponding author. Tel.: + 604-657-7888; fax: + 604-657-3678; e-mail: [email protected] 0142-9418/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 2 1 - 5
508
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
As a fuel, scrap tyres have found their way into the energy systems of pulp and paper mills, cement kilns, industrial boilers and power plants while other possibilities are still being explored. Tyres are increasingly shredded and used to make rubber modified asphalt [1]. Very fine ground rubber is treated with gases which modify the outer few molecular layers of rubber particles to enable them bond with materials like polyurethane, latex and other polymers. Untreated crumb rubber can be mixed with resin and glue for some applications but the bonding is physical rather than chemical and the end products tend to have lower performance specifications. Treated crumb rubber is increasingly being used in shoe soles where it has been found that the treated rubber actually improves fraction and wear [2]. While a ‘whole tyre’ has a negative economic value as it costs money to transport and burn even for fuel, shredding adds value to the scrap tyre [3]. Roughly shredded tyres (10.16–15.24 cm long pieces) fetch between $8 and $15/t and 5.08 cm chips are worth $8–20/t. However, the value goes up tremendously upon grinding as 14 inch minus grind generally sells for $280–440/t while 80 mesh or less sells for $600–900/t. Consequently, the terminologies used in rubber recycling although not so well defined, denote the relative sizes of rubber particles. Tyre derived fuel chips (TDF) are 5.08 or 7.62 cm square. Crumb rubber includes materials measuring 0.635 cm inches (3 mesh) i.e. rubber particles through a screen with 3 holes/linear inch) down to 0.0419 cm (40 mesh) and smaller. While ground rubber ranges in size from 30 mesh (0.5944 cm) down to 100 mesh (0.015 cm) [2]. Three main techniques (or a combination) are used to produce the different grades of rubber for recycling. Cracker mills make use of grinding forces between two rollers, granulation utilizes shearing and chopping while impaction relies on a two stage process of super cooling of tyres or chips with liquid nitrogen and then shattering them in a hammer mill. The latter method yields powder morphology in which particles are smooth, angular and pitted, with the surface showing striated lines where fracture has occurred [4,5]. The ambient-type products from the first two methods on the other hand are convoluted and sponge-like in appearance [5]. While the economics and technology favour fine regrinds, it is necessary that the science keeps pace. In this study the potential of powdered rubber, obtained as a by-product in the sanding process of polishing rubber balls, is investigated as extender for a virgin rubber compound, with attention being paid to cure characteristics and vulcanizate properties. A number of studies have considered the cure characteristics [4,6–8] as well as the mechanical properties of ground rubber [4–10]. The use of cured rubber as an extender for rubber compounds has been hindered by unsatisfactory wear and failure properties, particularly in highly demanding applications such as tyres. However, it has been demonstrated that the extent of quality deterioration associated with ground scrap extension can be minimised and almost eliminated as particle size decreases [9]. In their investigation involving ultra fine recycled rubber in which more than 90% or more of the particles fall below 20 µm, Swor et al. demonstrated that up to 50 phr of recycled rubber (RR) can be added to a rubber compound without compromising tensile strength. 100 and 150 phr rubber powder contents showed 87.5 and 80% retention of tensile strength respectively. Fatigue life improved with the addition of 100 phr of RR while a tread wear rating of 100 was recorded for the same composition. Other workers also reported little or no changes upon the addition of limited quantities of ground rubber to rubber compounds. Up to 30% of rubber powder with 20 µm size or less can be incorporated according to Burford and Pittolo [5]. No significant change in vulcanizate physical properties were found when loadings of cryo-crumb
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
509
with diameters less than 500 µm were kept at 10% or less. A major breakthrough has been made by Michelin Tire Corp. USA with the report that it has been able to incorporate in excess of 10% rubber from scrap tyres into new tyres while maintaining quality [3].
2. Experimental Natural Rubber (SMR L) was purchased from Kumpulan Guthrie Sdn Bhd., Seremban, Malaysia. The rubber powder, which is a waste product from the sanding process (polishing) of rubber balls and artificial eggs, was obtained from Watas Holdings (M) Sdn Bhd., Penang, Malaysia. The control compound, which was also made from SMR L, was formulated with conventional sulphur vulcanization system as shown in Table 1. The formulation used is a variation of the standard ASTM D 3184-89 formulation. 2.1. Particle size distribution 350 g of rubber powder was placed in a Retsch sieve and shaken for 10 min, after which the weight of rubber powder on each filter was obtained and the percentage was calculated. 2.2. Resilience Wallace Dunlop Tripsometer was initially set at an angle of 45°. The sample was placed in the sample holder and the tripsometer was released. The ‘indentor’ rebounds after impacting the sample and the maximum angle of rebound was noted. Rebound resilience is calculated according to the equation: Percentage resilience =
1 ⫺ cosθ2 × 100% 1 ⫺ cosθ1
(1)
where θ1 = initial angle = 45°θ2 = maximum rebound angle
Table 1 Rubber compound formulation with different rubber powder content Ingredients
Weight (g)
SMR L Zinc oxide Stearic acid MBT Sulphur Rubber pwder
100 6 0.5 1.0 3.5 Variable
510
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
2.3. Hardness A Wallace Dead Load Tester was used to measure hardness in accordance with ASTM D 1415-88 used for rubber with hardness ranging between 30 and 85 IRHD (International Rubber Hardness Degree). 2.4. Mixing procedure The rubber powder was incorporated into the control compound for 8 min on a two roll mill. The concentration of rubber powder added to the control compound was varied from 0 to 70% at intervals of 10%. The 0% concentration was subjected to the same mixing procedure to ensure similar mechano–chemical histories as the extended compounds. 2.5. Cure characteristics Cure characteristics were studied with the Monsanto Moving Die Rheometer (MDR 2000) according to ASTM D 2084-93. About 4 g samples of the respective compounds were tested at 150°C. 2.6. Vulcanization process 2 mm thick sheets were compression moulded at 150°C with a force of 10 MPa using a hot press according to the respective cure time, t90 determined with the MDR 2000. 2.7. Tensile tests Dumbbell shaped samples were cut from the moulded sheets according to ASTM D412. Tensile tests were performed at a cross-head speed of 500 mm/min after conditioning at 25°C. Tensile tests were carried out with a Monsanto Tensometer T10. 2.8. Tear test Tear test was carried out according to ASTM D624-81. The ‘Die B’ crescent shaped test pieces were mounted on the Monsanto Tensometer T10 and a test speed of 500 mm/min was used at 25 ± 3°C. 3. Results and discussion 3.1. Particle size distribution The rubber powder obtained from the sanding process exhibits the morphology normally associated with ambient type grinding of vulcanized rubber as the rubber powder is convoluted and sponge-like [5]. This appearance is in contrast to cryo-ground rubber particles which are normally
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
511
angular and smooth [4,5]. The particle size distribution is shown in Fig. 1. As 91.5% of the rubber particles range in sizes which are less than 250 µm, the rubber powder can be classified as a fine grind [5] although it falls short of the description of ultra fine grind [9]. 4. Cure characteristics 4.1. Reversion Fig. 2 shows the cure curves for the compound containing varying concentrations of RP. It can be seen that reversion sets in with the addition of 40% although it is minimal. Above this concentration, the tendency towards reversion becomes more prominent. This trend is similar to that recorded by other workers [5,7]. 4.2. Torque minimum and maximum Torque minimum (ML)) initially decreases slightly with increase in concentration up to 20% after which an increase is noted (Fig. 3). The increase in viscosity may be explained with the help of Einstein’s equation [11] which accounts for the viscosity of a suspension containing spherical particles in a medium. The medium is assigned a viscosity ηo and the viscosity of the suspension is expressed as: η = ηo(1 + 2.5c)
(2)
Fig. 1.
Size distribution of rubber powder particles.
512
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
Fig. 2.
Cure curves of SMRL and the extended compounds at varying rubber powder concentration.
Fig. 3. Torque maximum, torque minimum and the torque difference for SMRL and the extended compounds at varying rubber powder concentration.
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
513
where c is the volume fraction of the filler. From Eq. (1), viscosity is expected to increase with increase in filler loading. The increase in viscosity could also be due to agglomeration of the rubber particles in the compound at low viscosities. Besides, the matrix content decreases with increase in PR content. As the rubber particles are already crosslinked, they will not flow so easily as the matrix so an increase in the concentration of these particles will reduce flow. A similar observation was noted by Baharin et al. [12] in their investigation of the use of prevulcanized latex powder as a filler in natural rubber compound. It is worth noting, however, that the increase in ML is not so significant, thus indicating that the addition of RP has no significant adverse effect on processibility. The torque maximum (MH) increases continuously with increase in the concentration of rubber powder (Fig. 3). This is because the rubber particles which are vulcanized and have relatively higher modulus compared to the matrix (rubber compound), so this will lead to an increase in torque maximum. A similar trend is shown for torque difference (Fig. 3) which is an indication of the extent of crosslinking. This is a result of the combined effects of the new crosslinks being formed and those originally present in the rubber particles. This trend is similar to that obtained by Phadke et al. [4,7,8]. 4.3. Scorch time (t2) and cure time (t90) It is obvious from Fig. 4 that scorch time decreases with increase in rubber powder content. This trend is similar to that shown by cure time t90 on the same figure. Similar trends were observed by Baharin et al. [12] and Mathew et al. [13].
Fig. 4. Cure time (t90) and scorchtime (t2) for SMRL and the extended compounds at varying rubber powder concentration.
514
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
Investigations involving the cure characteristics of rubber compounds containing vulcanized rubber powder revealed that sulphur migrates from the matrix into the rubber powder [13–15]. On the other hand, the unreacted accelerator complex migrates from the rubber powder into the matrix [14]. The existence of unreacted accelerator in the rubber powder was proven by Mathew et al. [13]. Consequently, with increase in rubber powder concentration, the accelerator concentration in the compound increases and this accounts for the decrease in scorch time and t90.
5. Stress–strain curves Fig. 5(a) and (b) show the stress–strain curves of the rubber compound containing varying amounts of rubber powder. Generally, the vulcanizates become weak and brittle as the concentration of rubber powder increases although the modulus slightly increases. 5.1. Tensile strength The addition of rubber powder gradually raises tensile strength (TS) until a maximum is attained at 20% as shown in Fig. 6. Further increase leads to a gradual decrease until 40% after which a rapid decline in strength occurs. At 20% loading, the tensile strength is higher than that of the original compound and thus synergism is indicated. The 30% shows strength which is the same value as the original compound while the 40% RP content does not show a significant decrease in strength. This signifies that up to 30% of RP can be added to the original rubber compound without compromising its gum strength. This observation is in agreement with earlier investigations on ground rubber [5,9]. Similar observations involving other properties are discussed later. The initial increase in TS at low RP concentration could be due to improved interfacial bonding between the crosslinked rubber particles and the matrix. RP particles have higher modulus and therefore act as a reinforcing filler. although the reinforcing capability is limited and falls far short of the high modulus reinforcing fillers such as carbon black (CB). Beyond 20% RP content, agglomeration and hence particle–particle interaction of the rubber powder account for the observed decrease in tensile strength. At much higher RP content (60% and above) the matrix content is small and not enough to wet all the rubber particles. This, coupled with agglomeration, accounts for the rapid decline in tensile strength. The vulcanizates also show good tensile strength retention. This is indicated, for example, by a retention of 85% with 50% rubber powder content. At higher concentrations, the decrease in retention is significant. 5.2. Elongation at break Elongation at break (EB) decrease with increase in RP content (Fig. 7) although the decrease is gradual so the percentage retention of EB is good. For example, the compound containing 40% of RP retains almost 90% EB. A similar observation was reported by Baharin et al. [12]. Thus indicating that the addition of RP does not adversely affect this property.
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
515
Fig. 5. Stress–strain curves for SMRL and the extended compounds at varying rubber powder concentration (a) 0– 30%; (b) 40–70%.
5.3. Modulus at 100 and 300% elongation (M 100 and M 300) Modulus is a direct function of rubber powder content as shown in Fig. 8, for example, the addition of 70% RP raised M 100 and M 300 by as much as 104.1 and 74.4% respectively. This
516
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
Fig. 6. Tensile strength and its retention for the original and the extended compounds at varying rubber powder concentration.
Fig. 7. Elongation at break and its retention for the original and the extended compounds at varying rubber powder concentration.
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
517
Fig. 8. The modulus of the original and the extended compounds at varying rubber powder concentration.
trend is similar to that shown by torque maximum and hardness, thus indicating that the rigidity of the vulcanizates increase with increase in RP content. This means that the RP behaves like rigid particulate fillers since it has a higher modulus than the matrix. However, the increase in rigidity is much lower compared to the high modulus fillers such as CB, silica or CaCO3. Rigid particulate fillers are not easily deformed and their addition to a relatively soft matrix results the development local stretching in the matrix which exceed the overall strain of the composite. Thus, the rubber will respond with a higher stress when deformed. This phenomenon known as ‘strain amplification’ results in a significant increase in modulus [16]. 5.4. Tear strength The tear strength of the vulcanizates decreases with increase in RP content as shown in Fig. 9. This trend is similar to that shown by EB (Fig. 7) to which tear strength bears a relationship. However, the retention of EB and other mechanical properties are superior to tear strength retention. For example the addition of 30% RP only retains 80.4% of the tear strength of the gum vulcanizate. Even then, this level of property retention is good enough for some applications requiring mechanical integrity. 5.5. Resilience The resilience remains more or less constant up to 30% concentration of rubber powder after which a gradual decrease which is not so significant is observed (Fig. 10). The decrease is due to the rubber particles acting as fillers which introduces a mechanism whereby the strain energy fades [16,17]. This is caused by the decreased segment mobility of the matrix molecules due to
518
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
Fig. 9. Tear strength and its retention for the original and the extended compounds at varying rubber powder concentration.
Fig. 10. Rebound resilience and its retention for the original and the extended compounds at varying rubber powder concentration.
their interaction with filler particles. This will result in increased hysteresis. Thus hysteresis increases at high RP content and of course the opposite trend is exhibited in terms of resilience. The decrease in resilience with increasing RP content is relatively small and thus the addition of 70% RP only reduces the resilience by 16.6%. In other words, 83.4% of the resilience of the
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
519
gum vulcanizate is retained. This is not surprising since the filler in question is of the same material as the matrix but for the cross links. So the rubber particles do not behave as typical rigid particulate fillers and the rubber particles being themselves elastic will only hinder matrix molecular movement to a limited extent. Thus, the RP behaves as an extender to the rubber compound. 5.6. Hardness Hardness increases with increase in RP content as shown in Fig. 11. This trend is similar to those observed for torque maximum (Fig. 3) and M 100 and M 300 (Fig. 8). The hardness increased because the vulcanized rubber powder particles have relatively higher modulus than the rubber matrix. Furthermore, during the vulcanization stage vulcanization agents diffuse into the rubber powder resulting in the formation of more crosslinks [15]. This further increases the modulus of the RP filler. However, the increase in hardness due to the addition of the rubber powder is not as much as that resulting from the more rigid fillers. For example, 70% RP content raises hardness by a mere 36.8% (35 to 44 IRHD). As explained earlier, this is due to the fact that both the filler and the matrix comprise of the same material and the rubber powder particles are only semi-rigid. 6. Conclusions
1. The torque–time curves from the rheometer indicate that torque minimum which is synony-
Fig. 11. Hardness and its retention for the original and the extended compounds at varying rubber powder concentration.
520
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
mous to viscosity remains relatively unchanged up to 40% loading and only rises slightly at higher loadings. This is an indication that the incorporation of rubber powder does not inhibit processibility. On the other hand the addition of the powder reduces processing safety (scorch time) and cure time. 2. Up to 30% of the fine grind can be added to a rubber compound without compromising tensile strength while the addition of 20% shows tensile strength which is higher than that of the original compound, thus imparting a synergistic effect. EB reduces with increasing concentration of rubber powder but the percentage retention of EB is very good even at high loadings. The modulus increases with increasing rubber powder concentration but the overall increase is not so significant. 3. Tear strength decreases continuously but the retention is very good at lower loadings as 80% of the compound’s tear strength is retained at 30% RP concentration. 4. The percentage resilience of the compound remains almost unchanged even at high loadings of the rubber powder. This means that the elasticity of the compound is not significantly altered thus indicating that the ground rubber is behaves as an extender and not a filler.
Acknowledgements The authors gratefully acknowledge the support given by Mr M. Murakannoo and Mr Wong On of Watas Holdings Sdn. Bhd., Penang.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Loesel A. Elastomers ’92—fuel or filler. Chemical Marketing Reporter 1992;242:SR5–7. Riggle D. A finer grind for rubber recycles. BioCycle: Emmaus 1995;36:42–7. Steuteville R. Puncturing the scrap tire problem. BioCycle: Emmaus 1995;36:51–2. Phadke AA, Chakraborty SK, De SK. Cryoground rubber–natural rubber blends. Rubb Chem Technol 1983;57:19–33. Burford RP, Pittolo M. Characterization and performance of powdered rubber. Rubb Chem Technol 1982;55:1234–49. Acetta A, Vergnaud JM. Upgrading of scrap rubber powder by vulcanization without new rubber. Rubb Chem Technol 1981;54:302–10. Phadke AA, Bhattacharya AK, Chakraborty SK, De SK. Studies of vulcanization of reclaimed rubber. Rubb Chem Technol 1983;56:726–36. Phadke AA, Bhowmick AK, De SK. Effect of cryoground rubber on properties of NR. J Appl Polym Sci 1986;32:4063–74. Swor RA, Jensen LW, Budzol M. Ultrafine recycled rubber. Rubb Chem Technol 1980;53:1215–25. Acceta A, Vergnaud JM. Rubber recycling—upgrading of scrap rubber powder by vulcanization II. Rubb Chem Technol 1982;55:961–6. Mullins L. Effect of fillers in rubber. In: Bateman L, editor. The chemistry and physics of rubber-like substances. London: Maclaren and Sons Ltd, 1963. p. 301–28. Baharin A, Nasir M, dan Rohaidah Abd. Rahim. Rubber powder from waste natural rubber latex and its usage. International Rubber Conference 1997, K.L., Malaysia, 6–9 October 1997. p. 277–83.
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
521
[13] Mathew G, Singh RP, Lakshminarayanan R, Thomas S. Use of natural rubber prophylactics waste as a potential filler in styrene–butadiene rubber compounds. J Appl Polym Sci 1996;61:2035–50. [14] Rigbi Z. Powdered and granulated rubber. In: Narkis M, dan Rosenzweig N, editors. Polymer powder technology. Chichester: John Wiley and Sons, 1995. p. 483–90. [15] Hilyard NC, Tong SG, Harrison K. Influence of the cure system on the properties of vulcanisates incorporating whole tyre scrap rubber crumb. Plas Rubb Process Applic 1983;3:315–22. [16] Kraus G. Reinforcement of elastomer by particulate fillers. In: Eirich FR, editor. Science and technology of rubber. New York: Academic Press, 1978. p. 339–65. [17] Rodok JRM, Tai CL. A theory of inclusions of viscoelastic materials. J Appl Polym Sci 1962;6:518–28.
Material Properties
Cure characteristics and vulcanizate properties of a natural rubber compound extended with convoluted rubber powder U.S. Ishiaku*, C.S. Chong, H. Ismail School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia Received 15 January 1999; accepted 19 March 1999
Abstract Fine convoluted rubber powder (RP) obtained from the sanding process of polishing rubber balls was used as an extender for a natural rubber compound. The cure characteristics of the extended compound were studied with the Monsanto MDR 2000 rheometer while the mechanical properties of the vulcanizates were measured in accordance with ASTM standards. The results indicated that the incorporation of rubber powder does not significantly increase the viscosity of the compounds but it reduces both scorch time and cure time. The tendency towards reversion increases with increasing rubber powder concentration. The compound can be extended with up to 30% RP without compromising tensile properties although the modulus increases continuously, albeit not so significantly. The resilience of the compounds remains almost unchanged even at higher loading, thus indicating that fine rubber powder is a true extender for rubber compounds. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In the burgeoning discourse on what to do with the piles of rubber and elastomeric materials, the size of rubber particles is emerging as the major determining factor in rubber reuse. Tyres account for over 60% of all elastomeric products and the fate of scrap tyres apparently dictates the trend of rubber recycling. The era when the landfill was the ultimate destination of scrap rubber is gradually being swept into the annals of history as rubber recycling is fast becoming a profitable venture. * Corresponding author. Tel.: + 604-657-7888; fax: + 604-657-3678; e-mail: [email protected] 0142-9418/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 2 1 - 5
508
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
As a fuel, scrap tyres have found their way into the energy systems of pulp and paper mills, cement kilns, industrial boilers and power plants while other possibilities are still being explored. Tyres are increasingly shredded and used to make rubber modified asphalt [1]. Very fine ground rubber is treated with gases which modify the outer few molecular layers of rubber particles to enable them bond with materials like polyurethane, latex and other polymers. Untreated crumb rubber can be mixed with resin and glue for some applications but the bonding is physical rather than chemical and the end products tend to have lower performance specifications. Treated crumb rubber is increasingly being used in shoe soles where it has been found that the treated rubber actually improves fraction and wear [2]. While a ‘whole tyre’ has a negative economic value as it costs money to transport and burn even for fuel, shredding adds value to the scrap tyre [3]. Roughly shredded tyres (10.16–15.24 cm long pieces) fetch between $8 and $15/t and 5.08 cm chips are worth $8–20/t. However, the value goes up tremendously upon grinding as 14 inch minus grind generally sells for $280–440/t while 80 mesh or less sells for $600–900/t. Consequently, the terminologies used in rubber recycling although not so well defined, denote the relative sizes of rubber particles. Tyre derived fuel chips (TDF) are 5.08 or 7.62 cm square. Crumb rubber includes materials measuring 0.635 cm inches (3 mesh) i.e. rubber particles through a screen with 3 holes/linear inch) down to 0.0419 cm (40 mesh) and smaller. While ground rubber ranges in size from 30 mesh (0.5944 cm) down to 100 mesh (0.015 cm) [2]. Three main techniques (or a combination) are used to produce the different grades of rubber for recycling. Cracker mills make use of grinding forces between two rollers, granulation utilizes shearing and chopping while impaction relies on a two stage process of super cooling of tyres or chips with liquid nitrogen and then shattering them in a hammer mill. The latter method yields powder morphology in which particles are smooth, angular and pitted, with the surface showing striated lines where fracture has occurred [4,5]. The ambient-type products from the first two methods on the other hand are convoluted and sponge-like in appearance [5]. While the economics and technology favour fine regrinds, it is necessary that the science keeps pace. In this study the potential of powdered rubber, obtained as a by-product in the sanding process of polishing rubber balls, is investigated as extender for a virgin rubber compound, with attention being paid to cure characteristics and vulcanizate properties. A number of studies have considered the cure characteristics [4,6–8] as well as the mechanical properties of ground rubber [4–10]. The use of cured rubber as an extender for rubber compounds has been hindered by unsatisfactory wear and failure properties, particularly in highly demanding applications such as tyres. However, it has been demonstrated that the extent of quality deterioration associated with ground scrap extension can be minimised and almost eliminated as particle size decreases [9]. In their investigation involving ultra fine recycled rubber in which more than 90% or more of the particles fall below 20 µm, Swor et al. demonstrated that up to 50 phr of recycled rubber (RR) can be added to a rubber compound without compromising tensile strength. 100 and 150 phr rubber powder contents showed 87.5 and 80% retention of tensile strength respectively. Fatigue life improved with the addition of 100 phr of RR while a tread wear rating of 100 was recorded for the same composition. Other workers also reported little or no changes upon the addition of limited quantities of ground rubber to rubber compounds. Up to 30% of rubber powder with 20 µm size or less can be incorporated according to Burford and Pittolo [5]. No significant change in vulcanizate physical properties were found when loadings of cryo-crumb
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
509
with diameters less than 500 µm were kept at 10% or less. A major breakthrough has been made by Michelin Tire Corp. USA with the report that it has been able to incorporate in excess of 10% rubber from scrap tyres into new tyres while maintaining quality [3].
2. Experimental Natural Rubber (SMR L) was purchased from Kumpulan Guthrie Sdn Bhd., Seremban, Malaysia. The rubber powder, which is a waste product from the sanding process (polishing) of rubber balls and artificial eggs, was obtained from Watas Holdings (M) Sdn Bhd., Penang, Malaysia. The control compound, which was also made from SMR L, was formulated with conventional sulphur vulcanization system as shown in Table 1. The formulation used is a variation of the standard ASTM D 3184-89 formulation. 2.1. Particle size distribution 350 g of rubber powder was placed in a Retsch sieve and shaken for 10 min, after which the weight of rubber powder on each filter was obtained and the percentage was calculated. 2.2. Resilience Wallace Dunlop Tripsometer was initially set at an angle of 45°. The sample was placed in the sample holder and the tripsometer was released. The ‘indentor’ rebounds after impacting the sample and the maximum angle of rebound was noted. Rebound resilience is calculated according to the equation: Percentage resilience =
1 ⫺ cosθ2 × 100% 1 ⫺ cosθ1
(1)
where θ1 = initial angle = 45°θ2 = maximum rebound angle
Table 1 Rubber compound formulation with different rubber powder content Ingredients
Weight (g)
SMR L Zinc oxide Stearic acid MBT Sulphur Rubber pwder
100 6 0.5 1.0 3.5 Variable
510
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
2.3. Hardness A Wallace Dead Load Tester was used to measure hardness in accordance with ASTM D 1415-88 used for rubber with hardness ranging between 30 and 85 IRHD (International Rubber Hardness Degree). 2.4. Mixing procedure The rubber powder was incorporated into the control compound for 8 min on a two roll mill. The concentration of rubber powder added to the control compound was varied from 0 to 70% at intervals of 10%. The 0% concentration was subjected to the same mixing procedure to ensure similar mechano–chemical histories as the extended compounds. 2.5. Cure characteristics Cure characteristics were studied with the Monsanto Moving Die Rheometer (MDR 2000) according to ASTM D 2084-93. About 4 g samples of the respective compounds were tested at 150°C. 2.6. Vulcanization process 2 mm thick sheets were compression moulded at 150°C with a force of 10 MPa using a hot press according to the respective cure time, t90 determined with the MDR 2000. 2.7. Tensile tests Dumbbell shaped samples were cut from the moulded sheets according to ASTM D412. Tensile tests were performed at a cross-head speed of 500 mm/min after conditioning at 25°C. Tensile tests were carried out with a Monsanto Tensometer T10. 2.8. Tear test Tear test was carried out according to ASTM D624-81. The ‘Die B’ crescent shaped test pieces were mounted on the Monsanto Tensometer T10 and a test speed of 500 mm/min was used at 25 ± 3°C. 3. Results and discussion 3.1. Particle size distribution The rubber powder obtained from the sanding process exhibits the morphology normally associated with ambient type grinding of vulcanized rubber as the rubber powder is convoluted and sponge-like [5]. This appearance is in contrast to cryo-ground rubber particles which are normally
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
511
angular and smooth [4,5]. The particle size distribution is shown in Fig. 1. As 91.5% of the rubber particles range in sizes which are less than 250 µm, the rubber powder can be classified as a fine grind [5] although it falls short of the description of ultra fine grind [9]. 4. Cure characteristics 4.1. Reversion Fig. 2 shows the cure curves for the compound containing varying concentrations of RP. It can be seen that reversion sets in with the addition of 40% although it is minimal. Above this concentration, the tendency towards reversion becomes more prominent. This trend is similar to that recorded by other workers [5,7]. 4.2. Torque minimum and maximum Torque minimum (ML)) initially decreases slightly with increase in concentration up to 20% after which an increase is noted (Fig. 3). The increase in viscosity may be explained with the help of Einstein’s equation [11] which accounts for the viscosity of a suspension containing spherical particles in a medium. The medium is assigned a viscosity ηo and the viscosity of the suspension is expressed as: η = ηo(1 + 2.5c)
(2)
Fig. 1.
Size distribution of rubber powder particles.
512
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
Fig. 2.
Cure curves of SMRL and the extended compounds at varying rubber powder concentration.
Fig. 3. Torque maximum, torque minimum and the torque difference for SMRL and the extended compounds at varying rubber powder concentration.
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
513
where c is the volume fraction of the filler. From Eq. (1), viscosity is expected to increase with increase in filler loading. The increase in viscosity could also be due to agglomeration of the rubber particles in the compound at low viscosities. Besides, the matrix content decreases with increase in PR content. As the rubber particles are already crosslinked, they will not flow so easily as the matrix so an increase in the concentration of these particles will reduce flow. A similar observation was noted by Baharin et al. [12] in their investigation of the use of prevulcanized latex powder as a filler in natural rubber compound. It is worth noting, however, that the increase in ML is not so significant, thus indicating that the addition of RP has no significant adverse effect on processibility. The torque maximum (MH) increases continuously with increase in the concentration of rubber powder (Fig. 3). This is because the rubber particles which are vulcanized and have relatively higher modulus compared to the matrix (rubber compound), so this will lead to an increase in torque maximum. A similar trend is shown for torque difference (Fig. 3) which is an indication of the extent of crosslinking. This is a result of the combined effects of the new crosslinks being formed and those originally present in the rubber particles. This trend is similar to that obtained by Phadke et al. [4,7,8]. 4.3. Scorch time (t2) and cure time (t90) It is obvious from Fig. 4 that scorch time decreases with increase in rubber powder content. This trend is similar to that shown by cure time t90 on the same figure. Similar trends were observed by Baharin et al. [12] and Mathew et al. [13].
Fig. 4. Cure time (t90) and scorchtime (t2) for SMRL and the extended compounds at varying rubber powder concentration.
514
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
Investigations involving the cure characteristics of rubber compounds containing vulcanized rubber powder revealed that sulphur migrates from the matrix into the rubber powder [13–15]. On the other hand, the unreacted accelerator complex migrates from the rubber powder into the matrix [14]. The existence of unreacted accelerator in the rubber powder was proven by Mathew et al. [13]. Consequently, with increase in rubber powder concentration, the accelerator concentration in the compound increases and this accounts for the decrease in scorch time and t90.
5. Stress–strain curves Fig. 5(a) and (b) show the stress–strain curves of the rubber compound containing varying amounts of rubber powder. Generally, the vulcanizates become weak and brittle as the concentration of rubber powder increases although the modulus slightly increases. 5.1. Tensile strength The addition of rubber powder gradually raises tensile strength (TS) until a maximum is attained at 20% as shown in Fig. 6. Further increase leads to a gradual decrease until 40% after which a rapid decline in strength occurs. At 20% loading, the tensile strength is higher than that of the original compound and thus synergism is indicated. The 30% shows strength which is the same value as the original compound while the 40% RP content does not show a significant decrease in strength. This signifies that up to 30% of RP can be added to the original rubber compound without compromising its gum strength. This observation is in agreement with earlier investigations on ground rubber [5,9]. Similar observations involving other properties are discussed later. The initial increase in TS at low RP concentration could be due to improved interfacial bonding between the crosslinked rubber particles and the matrix. RP particles have higher modulus and therefore act as a reinforcing filler. although the reinforcing capability is limited and falls far short of the high modulus reinforcing fillers such as carbon black (CB). Beyond 20% RP content, agglomeration and hence particle–particle interaction of the rubber powder account for the observed decrease in tensile strength. At much higher RP content (60% and above) the matrix content is small and not enough to wet all the rubber particles. This, coupled with agglomeration, accounts for the rapid decline in tensile strength. The vulcanizates also show good tensile strength retention. This is indicated, for example, by a retention of 85% with 50% rubber powder content. At higher concentrations, the decrease in retention is significant. 5.2. Elongation at break Elongation at break (EB) decrease with increase in RP content (Fig. 7) although the decrease is gradual so the percentage retention of EB is good. For example, the compound containing 40% of RP retains almost 90% EB. A similar observation was reported by Baharin et al. [12]. Thus indicating that the addition of RP does not adversely affect this property.
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
515
Fig. 5. Stress–strain curves for SMRL and the extended compounds at varying rubber powder concentration (a) 0– 30%; (b) 40–70%.
5.3. Modulus at 100 and 300% elongation (M 100 and M 300) Modulus is a direct function of rubber powder content as shown in Fig. 8, for example, the addition of 70% RP raised M 100 and M 300 by as much as 104.1 and 74.4% respectively. This
516
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
Fig. 6. Tensile strength and its retention for the original and the extended compounds at varying rubber powder concentration.
Fig. 7. Elongation at break and its retention for the original and the extended compounds at varying rubber powder concentration.
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
517
Fig. 8. The modulus of the original and the extended compounds at varying rubber powder concentration.
trend is similar to that shown by torque maximum and hardness, thus indicating that the rigidity of the vulcanizates increase with increase in RP content. This means that the RP behaves like rigid particulate fillers since it has a higher modulus than the matrix. However, the increase in rigidity is much lower compared to the high modulus fillers such as CB, silica or CaCO3. Rigid particulate fillers are not easily deformed and their addition to a relatively soft matrix results the development local stretching in the matrix which exceed the overall strain of the composite. Thus, the rubber will respond with a higher stress when deformed. This phenomenon known as ‘strain amplification’ results in a significant increase in modulus [16]. 5.4. Tear strength The tear strength of the vulcanizates decreases with increase in RP content as shown in Fig. 9. This trend is similar to that shown by EB (Fig. 7) to which tear strength bears a relationship. However, the retention of EB and other mechanical properties are superior to tear strength retention. For example the addition of 30% RP only retains 80.4% of the tear strength of the gum vulcanizate. Even then, this level of property retention is good enough for some applications requiring mechanical integrity. 5.5. Resilience The resilience remains more or less constant up to 30% concentration of rubber powder after which a gradual decrease which is not so significant is observed (Fig. 10). The decrease is due to the rubber particles acting as fillers which introduces a mechanism whereby the strain energy fades [16,17]. This is caused by the decreased segment mobility of the matrix molecules due to
518
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
Fig. 9. Tear strength and its retention for the original and the extended compounds at varying rubber powder concentration.
Fig. 10. Rebound resilience and its retention for the original and the extended compounds at varying rubber powder concentration.
their interaction with filler particles. This will result in increased hysteresis. Thus hysteresis increases at high RP content and of course the opposite trend is exhibited in terms of resilience. The decrease in resilience with increasing RP content is relatively small and thus the addition of 70% RP only reduces the resilience by 16.6%. In other words, 83.4% of the resilience of the
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
519
gum vulcanizate is retained. This is not surprising since the filler in question is of the same material as the matrix but for the cross links. So the rubber particles do not behave as typical rigid particulate fillers and the rubber particles being themselves elastic will only hinder matrix molecular movement to a limited extent. Thus, the RP behaves as an extender to the rubber compound. 5.6. Hardness Hardness increases with increase in RP content as shown in Fig. 11. This trend is similar to those observed for torque maximum (Fig. 3) and M 100 and M 300 (Fig. 8). The hardness increased because the vulcanized rubber powder particles have relatively higher modulus than the rubber matrix. Furthermore, during the vulcanization stage vulcanization agents diffuse into the rubber powder resulting in the formation of more crosslinks [15]. This further increases the modulus of the RP filler. However, the increase in hardness due to the addition of the rubber powder is not as much as that resulting from the more rigid fillers. For example, 70% RP content raises hardness by a mere 36.8% (35 to 44 IRHD). As explained earlier, this is due to the fact that both the filler and the matrix comprise of the same material and the rubber powder particles are only semi-rigid. 6. Conclusions
1. The torque–time curves from the rheometer indicate that torque minimum which is synony-
Fig. 11. Hardness and its retention for the original and the extended compounds at varying rubber powder concentration.
520
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
mous to viscosity remains relatively unchanged up to 40% loading and only rises slightly at higher loadings. This is an indication that the incorporation of rubber powder does not inhibit processibility. On the other hand the addition of the powder reduces processing safety (scorch time) and cure time. 2. Up to 30% of the fine grind can be added to a rubber compound without compromising tensile strength while the addition of 20% shows tensile strength which is higher than that of the original compound, thus imparting a synergistic effect. EB reduces with increasing concentration of rubber powder but the percentage retention of EB is very good even at high loadings. The modulus increases with increasing rubber powder concentration but the overall increase is not so significant. 3. Tear strength decreases continuously but the retention is very good at lower loadings as 80% of the compound’s tear strength is retained at 30% RP concentration. 4. The percentage resilience of the compound remains almost unchanged even at high loadings of the rubber powder. This means that the elasticity of the compound is not significantly altered thus indicating that the ground rubber is behaves as an extender and not a filler.
Acknowledgements The authors gratefully acknowledge the support given by Mr M. Murakannoo and Mr Wong On of Watas Holdings Sdn. Bhd., Penang.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Loesel A. Elastomers ’92—fuel or filler. Chemical Marketing Reporter 1992;242:SR5–7. Riggle D. A finer grind for rubber recycles. BioCycle: Emmaus 1995;36:42–7. Steuteville R. Puncturing the scrap tire problem. BioCycle: Emmaus 1995;36:51–2. Phadke AA, Chakraborty SK, De SK. Cryoground rubber–natural rubber blends. Rubb Chem Technol 1983;57:19–33. Burford RP, Pittolo M. Characterization and performance of powdered rubber. Rubb Chem Technol 1982;55:1234–49. Acetta A, Vergnaud JM. Upgrading of scrap rubber powder by vulcanization without new rubber. Rubb Chem Technol 1981;54:302–10. Phadke AA, Bhattacharya AK, Chakraborty SK, De SK. Studies of vulcanization of reclaimed rubber. Rubb Chem Technol 1983;56:726–36. Phadke AA, Bhowmick AK, De SK. Effect of cryoground rubber on properties of NR. J Appl Polym Sci 1986;32:4063–74. Swor RA, Jensen LW, Budzol M. Ultrafine recycled rubber. Rubb Chem Technol 1980;53:1215–25. Acceta A, Vergnaud JM. Rubber recycling—upgrading of scrap rubber powder by vulcanization II. Rubb Chem Technol 1982;55:961–6. Mullins L. Effect of fillers in rubber. In: Bateman L, editor. The chemistry and physics of rubber-like substances. London: Maclaren and Sons Ltd, 1963. p. 301–28. Baharin A, Nasir M, dan Rohaidah Abd. Rahim. Rubber powder from waste natural rubber latex and its usage. International Rubber Conference 1997, K.L., Malaysia, 6–9 October 1997. p. 277–83.
U.S. Ishiaku et al. / Polymer Testing 19 (2000) 507–521
521
[13] Mathew G, Singh RP, Lakshminarayanan R, Thomas S. Use of natural rubber prophylactics waste as a potential filler in styrene–butadiene rubber compounds. J Appl Polym Sci 1996;61:2035–50. [14] Rigbi Z. Powdered and granulated rubber. In: Narkis M, dan Rosenzweig N, editors. Polymer powder technology. Chichester: John Wiley and Sons, 1995. p. 483–90. [15] Hilyard NC, Tong SG, Harrison K. Influence of the cure system on the properties of vulcanisates incorporating whole tyre scrap rubber crumb. Plas Rubb Process Applic 1983;3:315–22. [16] Kraus G. Reinforcement of elastomer by particulate fillers. In: Eirich FR, editor. Science and technology of rubber. New York: Academic Press, 1978. p. 339–65. [17] Rodok JRM, Tai CL. A theory of inclusions of viscoelastic materials. J Appl Polym Sci 1962;6:518–28.