Dynamic Devulcanization and Dynamic Vulcanization for Recycling of Crosslinked Rubber 1)

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1 Crosslinked rubber Recycle Automotive rubber waste Devulcanization Reclaimed rubber Thermoplastic elastomer A new recycling technology for crosslinked rubber was developed using continuous reactive processing methods. In the process of producing reclaimed rubber, a breakage of crosslinking points in the rubber matrix occurs selectively under optimized controls of shear stress, reaction temperature and internal pressure in a twin screw extruder corresponding to the dynamic devulcanization process. The reclaimed rubbers show excellent mechanical properties applicable to new rubber compounds. Furthermore, an enhanced rubber recycling process including both the dynamic devulcanization and dynamic vulcanization for producing thermoplastic elastomer (TPE) based on rubber waste has been established. The obtained TPE exhibits a highly recoverable rubber elasticity and mechanical properties comparable to commercial TPE. Dynamische Devulkanisation und dynamische Vulkanisation zur Wiederverwertung von vernetzten Kautschuken Vernetzter Kautschuk Wiederverwertung Automobilgummiabfälle Devulkanisation regenerierter Kautschuk thermoplastische Elastomere Eine neue Recyclingtechnologie für vernetzten Kautschuk auf der Basis eines kontinuierlichen reaktiven Verfahrens im Zweischneckenextruder wurde entwickelt. In dem Prozess wird Regenerat durch die selektive Aufspaltung von Vernetzungsstellen in der Kautschukmatrix in einem Scherfeld und unter Einwirkung von Druck und Temperatur hergestellt. Das Regenerat zeigt ausgezeichnete mechanische Eigenschaften im Vergleich zu neuen Compounds. Weiterhin wurde die dynamische Devulkanisation von Altgummi zur Herstellung von TPV in einem dynamischen Vulkanisationsprozess eingesetzt. Die erhaltenen TPV zeigen eine hohe Elastizität und mechanische Eigenschaften die den kommerziell erhältlichen TPV vergleichbar sind. Dynamic Devulcanization and Dynamic Vulcanization for Recycling of Crosslinked Rubber 1) Recently recycling of waste materials is growing importance for all the industries in the world. The automotive and transportation industries are the biggest consumers of raw rubber. Rubber waste is usually generated from manufacturing products during the process and by the post-consumer (retired) products, mainly including scrap tires. In Japan about one million tons of scrap tires are generated annually, to example. From the point of energy balances a material recycling of rubber waste is preferable to any other recycling techniques [1]. However, the material recycling in form of crumb rubber and reclaimed rubber results in only about 11.5 % of the whole scrap tires in 2004 [2]. The oldest and simplest reclaiming method in the rubber recycling industry is called pan method. The reclaimed rubber obtained by this method is much inferior in physical properties to the virgin (new) rubber. Technological developments for the tires such as steel belted and radial ones cause some limitation to the amount of recycled rubber in higher quality rubber compounds for new tires [1]. Hence, new material recycling technologies such as microwave method [3] and ultrasonic method [4] have been developed for the aim of shorter reaction times. However, the reclaimed rubbers obtained by these methods are not quite excellent in quality as to be widely applicable in practical rubber products. In this study a new continuous rubber recycling technology has been developed for crosslinked rubber for the purpose of obtaining recycled materials in high quality as reclaimed raw rubber and thermoplastic elastomer (TPE) based on rubber waste. Rubber recycling technology Automotive rubber waste For motor vehicles many crosslinked rubber products are used as weather strips, hoses, vibration insulators and miscellaneous parts except tires as shown in Figure 1 [5]. The sum of their weights is corresponding to about 3 % of the total weight of the vehicle. Among rubbery polymers for these automotive rubber products, the use of ethylene-propylene-diene rubber (EPDM) amounts to be about one half of them by weight as shown in Figure 1. In this study a continuous rubber recycling technology has been developed mainly for crosslinked EPDM waste. Recycling process of producing reclaimed rubber Principle of producing reclaimed rubber Material recycling of crosslinked polymers including rubber is generally thought of as being a difficult by using simple heating procedure, because of the three-dimensional network structure restricting the material from melting. In the rubber recycling process using the conventional the pan method, finely ground rubber powder mixed with oils and reagents is heated with steam in the pressure vessel at the temperature of 200 C for more than 5 hours. Moreover, this process usually has to be followed by several procedures (refining and straining) for obtaining the final re- Autoren K. Fukumori, M. Matsushita, M. Mouri, H. Okamoto, N. Sato, K. Takeuchi and Y. Suzuki, Aichi (Japan) Corresponding author: Dr. Kenzo Fukumori Toyota Central R & D Labs. Inc Nagakute Aichi, , Japan Tel. + 81/ Fax: + 81/ fukumori-k@mosk.tytlabs.co.jp 1 This paper was partly presented at IRC 2005 YOKOHAMA, Oct , KGK Juli/August

chain; (b) deformation of the network chain (particularly, S-S- bonds) by shearing &4 &4 Dynamic devulcanization process for the product of reclaimed rubber In the newly developed continuous

2 &1 &2 claimed rubber. The reclaimed rubber obtained by this method is much inferior in physical properties to a new rubber, due to the occurrence of the breakages of both the crosslinking points and main chain (C-C) bonds unselectively. &1 Rubber products for motor vehicles &2 Schematic illustration of the reactor for the dynamic devulcanization &3 &3 Breakages of crosslinking points in high shear flow: (a) model for the network chain; (b) deformation of the network chain (particularly, S-S- bonds) by shearing &4 &4 Dynamic devulcanization process for the product of reclaimed rubber In the newly developed continuous recycling process [6], various chemical reactions corresponding to the selective breakage, of crosslinking points (so-called, devulcanization) can be efficiently controlled by optimizing the parameters in the reactor such as the shear stress, the temperature and the internal pressure. This dynamic devulcanization process for crosslinked rubber waste is performed on a modular screw type reactor (i.e., twin screw extruder) as schematically shown in Figure 2. For this equipment, the screw configuration with the modular screw elements (right-handed screw, kneading disk, etc.) is suitably designed to be applicable to the dynamic devulcanization process. The rubber waste was roughly crashed into small pieces in the size of about 5 mm for the following recycling process. In the first pulverization zone of this process roughly crashed rubber material becomes into fine particles by high shearing, and heated to the devulcanization reaction temperature quickly. The residence time is assured to be enough to complete the devulcanization reaction under a shear flow in the next devulcanization zone. In this reaction zone, fine particles of crosslinked rubber become highly elongated by filling and shearing with the kneading disk elements, as well as being plasticized eventually. A basic understanding for the cleavages of crosslinking bonds under high shear stress is suggested as following. As shown in Figure 3 there appears to be a small difference in the bonding energy between C-C bonds and C-S or S-S bonds. Hence by simple heating in the pressure vessel, the cleavages of both the C-C and C-S or S-S bonds may occur unselectively. This causes the lowering of the physical properties for reclaimed rubber by the conventional method. On the other hand, with regard to the elastic constant k for these bonds (approximately estimated on the basis of the values for crystals), the k- value for the S-S bonds can be estimated to be about 1/30th of the one for the C-C bonds as shown in Figure 3a. Generally, it is understood that mechanical behavior of crosslinked rubber may be mainly controlled by the entropic term in the strain energy. In the contrast with this entropic deformation behavior at extremely higher shear stress induced by filling and kneading in the reactor, most of the rubbery molecules may become to be fully elongated to their limited extensibility. Under these conditions, the spring having a lower elastic constant (the S-S bonds) may become more extended as compared with the spring having a higher elastic constant (the C-C bonds) in an elastic manner as shown in the Figure 3b. The elastic energy induced by high shearing may be particularly concentrated at the S-S bonds causing the breakages of crosslinking points selectively. 406 KGK Juli/August 2006

&5 &5 Appearances and Mooney viscosity of rubbers sampled from the positions A E &6 &7 Change in the network structure during the dynamic devulcanization process The temporal changes in the rubber

sampled from positions A E under a typical test condition were studied by pulling out the screw quickly from the barrel of the reactor on the way and sampling the materials along the axial length of

$As for the materials sampled from various positions, the Mooney viscosity, the fractions of the sol and gel components, the molecular weight distribution for the sol component and the effective$

3 &5 &5 Appearances and Mooney viscosity of rubbers sampled from the positions A E &6 &7 Change in the network structure during the dynamic devulcanization process The temporal changes in the rubber matrix during the dynamic devulcanization process &6 Spatial heterogeneity in structure of reclaimed rubber by devulcanization &7 Changes in the average molecular weight of sol component in rubbers sampled from positions A E under a typical test condition were studied by pulling out the screw quickly from the barrel of the reactor on the way and sampling the materials along the axial length of the screw. As for the materials sampled from various positions, the Mooney viscosity, the fractions of the sol and gel components, the molecular weight distribution for the sol component and the effective network chain density for the gel component were evaluated. Furthermore in this study, the change in the crosslink structure for these materials were analyzed with the chemicals such as thiol/amine reagents, which are used to cleave sulfur bonds. A mixture of propane-2-thiol and piperidine in heptane is used to cleave polysulfide bonds, and with hexane-1-thiol in piperidine, polysulfide and disulfide bonds can be cleaved [6]). According to this procedure a full characterization of partially decrosslinked network structure during the dynamic devulcanization processing can be investigated. Under suitable process conditions (i.e., screw configuration, reaction temperature, screw rotation speed, etc.) for EPDM, the reclaimed EPDM in good surface appearance can be continuously obtained from the head of the reactor as shown in Figure [4]. This behavior was closely connected with the appropriate values of the Mooney viscosity for the reclaimed EPDM according to the rubbery polymer. Figure 5 shows the changes in the appearance and Mooney viscosity of the EPDM sampled from the various positions along the axial length of the screw in the reactor. In the first pulverization zone, roughly crashed rubber material is turned into fine particles by high shearing. In the next devulcanization zone, the fine particles of the crosslinked EPDM become to be plasticized. The Mooney viscosity of the EPDM samples decreases along the axial length of the screw, corresponding to the progress of the devulcanization. The reclaimed EPDM obtained by devulcanization is composed of the sol (the fraction soluble in toluene) and gel (the insoluble fraction) components. As for the reclaimed EPDM under typical devulcanization conditions, the fractions of the sol and gel components were evaluated to be about 44 % and 56 %, respectively. Furthermore, the effective network chain density for the gel component was evaluated to be 1/20th of the originally vulcanized rubber. Figure 6 shows schematically the spatial heterogeneity in the reclaimed rubber, for which there appears to be mixtures of uncrosslinked polymer chains assigned to the sol component and loosely crosslinked ones assigned to the gel component. Figure 7 shows the changes in the average molecular weight of the sol component during the process. The average molecular weight of KGK Juli/August

&8 &9 &10 &8 Changes in network structure during the dynamic devulcanization process: the numbers of mono-, di- and polysulfide bonds &9 Deodorization during the dynamic devulcanization: (a)

4 &8 &9 &10 &8 Changes in network structure during the dynamic devulcanization process: the numbers of mono-, di- and polysulfide bonds &9 Deodorization during the dynamic devulcanization: (a) deodorization mechanism (b) signals from odorous components in rubbers &10 Stress-strain curves of crosslinked rubbers stage of the process, the numbers of the di- and polysulfidic bonds decrease, while the number of the monosulfidic bonds increases. At the late stage of the process the number of the monosulfide bonds decreases, and finaly becomes to be negligibly small. On the other hand, only by heating in a pressure vessel with flowing nitrogen gas, the number of the poly- and disulfidic bonds decrease, but the number of the monosulfidic bonds increases. During the dynamic devulcanization process in the reactor, the di- and polysulfide bonds are firstly converted into the form of the monosulfidic bonds, and then the monosulfidic bonds finally may come to be cleaved under high shear stress and internal pressure. Deodorization of reclaimed rubber The characteristic odor from reclaimed rubber may cause the limits to its practical applications. Hence, a deodorization procedure for reclaimed rubber is also included in the dynamic devulcanization process by a newly developed method [7], in which high-pressured water is injected into a barrel. The deodorization mechanism during the dynamic devulcanization process is schematically shown in Figure 9a: odorous components in the rubber compound come to be trapped into the high-pressured water vapor and removed efficiently through vents with vacuum pumps. The barrel temperature and screw elements around the positions for the water injection are suitably adjusted to avoid an inadequate progress of the devulcanization caused by the decreased temperature through the diffusion of water molecules. Figure 9b shows the GC-peaks of the odorous components in reclaimed rubbers (both untreated and treated) and new rubber compound. For the reclaimed rubber, the odorous components in the treated rubber are hardly detected similar to those in the new rubber compound while there are many odorous components detected in the untreated rubber. the sol component in the reclaimed EPDM is nearly constant, comparable to the one in new EPDM, from the position B to the position D. This result supports the occurrence of selective breakages of crosslinking points in the EPDM during the dynamic devulcanization process. Figure 8 shows the changes in the the amounts of the mono-, di- and polysulfidic bonds in the network structure during the process. Initially, for the crosslinked rubber, the fraction of mono-, di- and polysulfide bonds were evaluated to be about 20 %, 20 % and 60 %, respectively. In the early Properties of reclaimed rubber and its practical uses for automotive rubber products The reclaimed EPDM can be compounded and crosslinked with accelerated sulfur vulcanization system according to the conventional recipe. Figure 10 shows typical stressstrain curves of crosslinked rubbers for the reclaimed EPDM by the developed method and new rubber. The reclaimed EPDM exhibits excellent tensile stress-strain property almost comparable to the new one. 408 KGK Juli/August 2006

&11 &12 &11 Applications of reclaimed rubber for automotive products &12 Changes in tread depth for standard and test truck tires during actual road tests Application of continuous recycling

about 70 % of the total weight of raw rubber materials annually consumed in Japan. This results in one million tons of scrap tires generated.

5 &11 &12 &11 Applications of reclaimed rubber for automotive products &12 Changes in tread depth for standard and test truck tires during actual road tests Application of continuous recycling technology to tire rubber waste The consumed amount of tire rubbers such as natural rubber (NR), styrene-butadiene rubber (SBR) and butyl rubber (IIR) for the production of new tires correspond to about 70 % of the total weight of raw rubber materials annually consumed in Japan. This results in one million tons of scrap tires generated. Hence, the continuous rubber recycling technology in this study was also applied to tire rubber waste, generated from both manufacturing products and scrap tires [7]. From the standpoint of the engineering practice, test truck tires were prepared by mixing the NR-based reclaimed rubber (10wt%) from manufacturing products, with the NR-based new rubber (90wt%) for the tread rubber compound. Standard truck tires with only the NR-based new rubber were also prepared for the comparison. Actual road tests were carried out in mileage up to km. During the road tests the changes in the tread depth for both the test and standard tires were examined against mileage, respectively. The changes in the tread depth for the test and standard truck tires are shown in Figure 12, including equipped positions of those tires on the vehicle. The tread depths were measured against mileage for the tires on the rear positions. The tread wear behavior of the test truck tire was estimated to be almost similar to that of the standard one in mileage up to km. From these results it was confirmed that the reclaimed tire rubber obtained by the continuous recycling technology can be applied to new tire rubber compounds with its appropriate content in the engineering practice. &13 &13 Dynamic devulcanization and dynamic vulcanization processes for the product of TPE based on EPDM waste In 1997 this technology was practically applied to the material recycling of EPDM waste generated in manufacturing processes of automotive weather strips in a plant having the production capacity of 500 tons/year for reclaimed rubber [5]. The reclaimed EPDM has been currently utilized for producing various automotive rubber products with its appropriate content as shown in Figure 11. Recycling process of producing TPE Principle of producing TPE TPE provides highly recoverable properties similar to conventional crosslinked rubber, and can be processed with the moldability and efficiency of thermoplastics. In addition to the simpler processing, the principal advantages of TPE compared to crosslinked rubber include an easier recycling of waste, design flexibility, product quality and dimensional consistency. Hence, owing to these excellent characteristics, TPE has come to be widely used in a variety of applications including automotive products as substitute materials for rubbers and soft plastics. In this study, as the next step for the development of rubber recycling technology, a more enhanced recycling method of producing a dynamically vulcanized(crosslinked) TPE based on EPDM waste has been established [8]. The enhanced recycling process of producing TPE is composed of pulverization, devulcanization, blend and vulcanization. Figure 13 shows a schematic illustration of the recycling of producing the TPE based on EPDM waste. In the first zone, the crashed EPDM comes to be pulverized by shearing. KGK Juli/August

&14 &15 &16 &14 Temporal changes in the phase structure of the EPDM/PP blends during the dynamic devulcanization and dynamic vulcanization &15 Elongation-retraction property of TPE Besed on EPDM

The aiming morphology of TPE corresponds to the formation of the phase structure with chemically crosslinked EPDM phase (major component) dispersed in PP matrix (minor component).

6 &14 &15 &16 &14 Temporal changes in the phase structure of the EPDM/PP blends during the dynamic devulcanization and dynamic vulcanization &15 Elongation-retraction property of TPE Besed on EPDM waste tion conditions have to be suitably adjusted to form the desirable domain structure during the continuous process. The aiming morphology of TPE corresponds to the formation of the phase structure with chemically crosslinked EPDM phase (major component) dispersed in PP matrix (minor component). Figure 14 shows the morphology development of the EPDM/PP blend during the continuous process. At the early stage of the process the EPDM component forms the continuous phase, depending on its volume amount in the blend. At the following intermediate and late stages of the process under suitable reaction conditions, the phase inversion occurs and thus the desirable domain structure for TPE, in which the vulcanized(re-crosslinked) EPDM phase of less than 1 lm is finely dispersed in the continuous PP phase, finally comes be formed. Properties of TPE based on EPDM waste and its applications for automotive products The obtained TPE pellets can be molded by extrusion molding or injection molding like conventional thermoplastics. The injection molded TPE specimen shows good tensile properties and recovers its original length after elongation in the elongation-retraction experiment as shown in Figure 15. In the engineering practice the TPE based on EPDM waste is comparable to commercial TPE in various properties (tensile property, processability, surface appearance, etc.). The mass production of the TPE based on foamed EPDM waste, which is generated in the manufacturing process of automotive weather strips, has been started in July 2002 with a new plant having the product capacity of 350 tons/year for TPE. The development of automotive products made with the TPE as shown in Figure 16 is presently working on to achieve its practical uses [9]. &16 Applications of TPE for automotive products In the following zones, the dynamic devulcanization (selective breakages of crosslinking points in the EPDM), blend of the reclaimed EPDM (ca.80wt%) with polypropylene (PP, ca.20wt%), and dynamic vulcanization of the EPDM component in the blend proceed continuously and finally result in the formation of some desirable domain structure. Changes in phase morphology In order to obtain TPE showing preferable physical properties, the screw geometry and configuration for the reactor and reac- Conclusion In this study, a new recycling technology for automotive rubber waste has been developed with twin screw extruder as a modular screw type reactor. With optimized screw geometry and configuration in the reactor, the breakages of crosslinking points may predominantly occur under suitable conditions of temperature, shear stress and internal pressure. Reclaimed rubber from EPDM waste generated in the manufacturing process of weather strips exhibits excellent good properties, which are almost equal to those of new rubber. The reclaimed rubber has been currently mass-produced and utilized for manufacturing automotive rubber products since It is also confirmed that this technology can be applied to the recycling of tire rubber waste including scrap tires. Furthermore, as a next phase of the development an enhanced rubber recycling method of producing TPE based on rubber waste has been established. The TPE shows physical properties comparable to commercial TPE. The mass production of the TPE based EPDM waste has been started in July KGK Juli/August 2006

7 It is expected that the newly developed rubber recycling technology in this study would contribute to both protecting the environment and saving resources with regard to rubber waste generated in the world. Acknowledgement This work was partly carried out by joint research and development with International Center for Environmental Technology Transfer in , commissioned by the Ministry of Economy Trade and Industry. References [1] Recycling of Rubber"", RAPRA Report 99 (1997) 9. [2] TIRE INDUSTRY OF JAPAN 2005, JATMA, [3] Goodyear Tire & Rubber Co. Ltd., Plast. Rubb. News, Sept. 9 (1979). [4] A.I. Isayev, J. Chen and A. Tukachinsky, Rubber Chem. Technol. 68 (1995) 267. [5] S. Otsuka, M. Owaki, Y. Suzuki, H. Honda, K. Nakashima, M. Mouri and N. Sato, SAE Paper (1998) [6] K. Fukumori, M. Mouri, N. Sato, H. Okamoto, M. Matsushita, H. Honda, K. Nakashima, Y. Suzuki and M. Owaki, Gummi FASERN Kunststoe 54 (2001) 48. [7] K. Fukumori, M. Matsushita, H. Okamoto, N. Sato, Y. Suzuki and K. Takeuchi, JSAE Review 23 (2002) 259. [8] H. Okamoto, K. Fukumori, M. Matsushita, N. Sato, Y. Tanaka, T. Okita, K. Takeuchi, N. Suzuki and Y. Suzuki, Prepr. of IUPAC-PC2002, Dec.2-5, 2002, 638. [9] Y. Tanaka, T. Watanabe, T. Okita, M. Matsushita, H. Okamoto, K. Fukumori, N. Suzuki and Y. Suzuki, SAE Paper (2003). KGK Juli/August

Continuous recycling of vulcanisates

Gummi Fasern Kunststoffe, No., 2, p. 48 Continuous recycling of vulcanisates K. Fukumori, M. Mouri, N. Sato, H. Okamoto and M. Matsushita * Translation submitted by C. Hinchliffe Selected from International