Transforming silica-silane reinforced rubber into a high quality devulcanizate
Rounak Ghosh is a PhD student in the department Elastomer Technology and Engineering. Co)Promotors are dr. W.K. Dierkes, prof.dr. A. Blume and dr. A.G. Talma from the faculty of Engineering Technology.
This general summary provides a comprehensive overview of the thesis, commencing with the research motivation. The summary highlights the key findings derived from the results of this study and their discussion, and concludes it with the investigative limitations and future research prospects.
Research motivation
Recycling of vulcanized elastomers presents a substantial challenge due to their thermoset nature. In contrast to thermoplastics, when heated, rubber crosslinks preventing reshaping. This renders thermosets more complex to recycle. The commonly used process known as "reclaiming" involves breaking crosslinks but at the same time also scission of polymer chains. A promising solution is "devulcanization," which reverts rubber to a reusable state by selective breakdown of crosslinks. Devulcanization preserves polymer chains, thereby maintaining mechanical properties, and makes it a sustainable rubber recycling solution.
This research focuses on devulcanization of end-of-life tires (ELTs), particularly natural rubber (NR), styrene-butadiene rubber (SBR), and butadiene rubber (BR) based. NR is rather easy to recycle due to its softening effect at high temperatures, whereas SBR-BR rubber becomes brittle, hindering devulcanization. The filler system significantly impacts the process, with carbon black-filled rubber being easier to devulcanize than silica-filled rubbers, in which chemical filler-polymer bonds pose challenges. In the current technology for tire treads, SBR-BR as polymer and silica-silane as filler system are increasingly used due to improved fuel efficiency.
Rubber devulcanization holds increasing importance for sustainable tire production by 2030 and beyond. Incorporating high-quality devulcanized rubber into new tires can enhance the circular tire economy, reduce the carbon footprint and promote a greener future. This study addresses the lack of a dedicated devulcanization process for SBR-BR based silica-silane filled tires, proposing an efficient solution to fill this gap.
This research aims to develop an environmentally sustainable devulcanization process for passenger car tire rubber, resulting in a high-quality devulcanizate which can be reused in considerable quantities in new tires. Earlier work has shown, that silanes are efficient devulcanization aids, besides being commonly used in passenger car tire tread compounding and therefore a non-critical additive to tire rubber. It involves evaluating silane-based devulcanization aids (DA), optimizing the process with model as well as ground tire rubber, investigating the reactivity of the DA’s, elucidating the reaction mechanisms, enhancing the devulcanizate quality with additives, exploring applications in tire compounding, and finally upscaling of the developed process in an extruder.
Results summary
In Chapter 3, various silanes were chosen based on existing knowledge about functionalities boosting the efficiency of devulcanization. They were evaluated concerning their effectiveness in devulcanizing a SBR-BR based silica-silane filled model passenger car tire tread rubber, focusing on mechanical properties, network breakdown, and miscibility. The SBR-silica network presented a formidable challenge for devulcanization, with vinyl silane emerging as a promising DA. The presence of a peroxide activator further improved the efficiency of this silane, making it the most promising DA in this study. Vinyl silane (VTEO) and its activated variant (VP) yielded the best mechanical and network breakdown results, with polysulphidic and amino silane also showing favorable outcomes. These silanes demonstrated a higher crosslink-to-random-scission ratio, enhancing devulcanizate quality compared to the traditional ones. An increased degree of devulcanization led to higher miscibility and a low number of immiscible particles. Tensile strength recovery reached approximately 50–55%, with a network breakdown percentage of 55–60%, compared to the feed material. The resulting mechanical and network breakdown properties were 22–25% superior to those of the benchmark devulcanizate.
Based on the screening described in Chapter 3, Chapter 4 is dedicated to the optimization of the devulcanization process parameters. The first part was performed with the model passenger car tire tread material, and the second part with whole passenger car tire (WT) granulates. For both the materials, devulcanization was performed using vinyl silane with peroxide (VP) as DA. The resulting model devulcanizate showed a tensile strength of 9.4 MPa, an elongation at break of 112%, and a network breakdown of 60%. The optimized process parameters are:
- Temperature: 155⁰C
- Residence time: 6 minutes
- TDAE oil (process oil): 5%
- DA concentration: 5%
- Rotor speed: 150 RPM
- Fill factor: 80%
The WT devulcanizate exhibited a tensile strength of 8.6 MPa, an elongation at break of 162%, and a network breakdown of 75%. The devulcanization process was fine-tuned to a temperature of 180⁰C, and the other parameters were similar to the ones of the model devulcanizate.
TGA analysis revealed variations in filler type and concentration within the WT granulates. The inhomogeneity of this feed material in terms of type of rubber and filler type as well as content, and aging during service life contributed to a lower tensile strength observed in WT material compared to the model compound. On the other hand, the presence of natural rubber led to higher elongation at break and lower viscosity in contrast to the model compound.
Chapter 5 is dedicated to three primary aspects:
i. Evaluating combinations of DAs in terms of additional or synergistic effect.
ii. Investigating the impact of individual DA constituents.
iii. Assessing devulcanization using peroxides, which are part of the VP DA, as sole agents.
Samples devulcanized with VP demonstrated superior network breakdown properties and mechanical strength after revulcanization compared to all tested DA combinations: no synergistic effect was measured. Samples devulcanized with the combination of VTEO and BPO (benzoyl peroxide) exhibited better tensile strength, miscibility, and viscosity than those devulcanized with VTEO and DCP: BPO in conjunction with vinyl silane turned out to be a promising DA, surpassing DCP for this purpose. BHT did not significantly impact the devulcanization reaction; however, plays a vital role in maintaining the chemical stability of the peroxide. Peroxides alone were inefficient as DAs for these unsaturated polymer systems based on NR, BR and SBR.
In Chapter 6, the devulcanization reaction mechanisms were investigated for two different devulcanization aids, VTEO (vinyl silane) and VP (vinyl silane with peroxide). The primary devulcanization reaction involved radical addition reactions: recombining broken crosslinks and polymer chains with active DA radicals to achieve stabilization. FTIR characterization including side product analysis was performed on educts and products of the devulcanization reaction using liquid model compounds (LMCs), confirming the proposed devulcanization mechanism. GC analysis provided insights into the efficacy of devulcanization, particularly with VP. Comparable retention times in GC spectra for devulcanized LMC and uncured LMC supported the effectiveness of the decrosslinking process. GC and NMR analyses of VP and decomposed VP confirmed the presence of dimerized and oligomerized structures within decomposed VP.
Chapter 7 investigated the influence of the type of processing aids. This series of experiments was performed with WT rubber granulate and using the most efficient DA. It aimed to achieve two primary objectives: firstly, to evaluate the impact of different process aids on the devulcanization procedure, and secondly, to identify an optimal process aid and concentration for enhancing the devulcanization process and devulcanizate properties. An optimum additive quantity of 5% process aid (PA) was determined, as exceeding this amount resulted in nip slippage in the mixer and comparatively poorer devulcanizate quality. Silane-terminated liquid polybutadiene rubber (SLBR) and Polyoctenamer (PO) exhibited superior properties due to the contribution of the long polymer chains and unsaturation in the backbone compared to other PA's. Unsaturated PA’s contributed to the additional crosslink formation during revulcanization resulting in superior revulcanizate quality. Samples pre-swollen for 6 hours to let the DA and PA migrate into the granules exhibited similar properties to those pre-swollen for one day. Additionally, additives compounded after devulcanization displayed a plasticization effect rather than an influence on material quality.
Chapter 8 delves into an examination of the influence of a silanization step to activate the silica in the WT rubber, and of separately added silica and silane to the devulcanizates. Additionally, it encompasses the process of blending devulcanizates or ground rubber with a virgin compound for a comparative analysis of the influence of the recycled rubber on the properties of a blend. Compounding with around 20% of additional filler (silica + silane) can improve tensile strength of the revulcanizates by up to 15%. In addition to the filler cost, a sacrifice in elongation at break, dispersion, and abrasion resistance was noticed. With an additional 20% silica and increasing silane concentration, tensile strength, Payne effect, and dispersion improved due to better polymer-filler interaction from enhanced silanization. However, network breakdown of the devulcanizate (after compounding and before revulcanization) got decreased due to an increase in crosslink density caused by the formation of short, stable filler-polymer bonds during the silanization reaction. The optimum concentration for improvement of the properties of the devulcanizate was elaborated to be 20% silica and 3% silane.
When ground rubber was added to the virgin compound, a significant loss in properties was observed, much more compared to the addition of a devulcanizate. Mechanically ground samples yielded better blend properties than cryogenically ground rubber due to their higher surface area and improved interlocking from the rough particle surface. VP devulcanizates could be blended with a virgin tread compound up to 20% without a significant reduction in tensile strength; a 50% blend resulted in around 25% and 30% sacrifice in tensile strength and elongation at break respectively. Additionally filled devulcanizates exhibited relatively higher mechanical properties compared to the no additionally filled ones. A considerably higher amount of devulcanizate can be used compared to the reclaim or powder.
Based on the promising results of the devulcanizate-virgin rubber blends, in Chapter 9 a more elaborated application study was performed, including also properties relevant for tire applications. An improvement in aging resistance due to the addition of devulcanizate was clearly observed. Compared to the model compound, for the 20% devulcanizate blend, tear strength and wet grip were improved. The devulcanizates exhibited good processability during blending and processing. Overall, a 20% devulcanizate blend can be utilized for high-performance applications without a significant sacrifice in the required properties, while a 50% devulcanizate blend or 100% devulcanizate is suitable for low-performance applications.
Chapter 10 entails a comprehensive comparison between the developed devulcanizates and their commercial counterparts. The truck tire and aircraft tire tread based samples exhibited similar properties, characterized by a tensile strength of approximately 14 MPa and an elongation at break of around 200%. In contrast, the model devulcanizate displayed a comparatively lower tensile strength (approximately 9.4 MPa) and elongation at break (about 110%). Two other devulcanizates based on truck tire treads and a blend of whole passenger car and truck tire rubber samples demonstrated even lower tensile strength (around 8 MPa) and elongation at break (about 150%). These variations can be attributed to factors such as differences in the devulcanization process and variation in the feed material, especially the presence of natural rubber and carbon black, and variations in the devulcanization process. Higher temperature and shear forces within the extruder facilitated a more extensive network breakdown, contributing to improved tensile strength. However, there is a limit at which polymer scission becomes dominant and properties decrease again. Moreover, a correlation was established between tensile strength of the devulcanizate and miscibility with a virgin compound: VP-devulcanized samples exhibited superior mechanical strength and miscibility, followed by VTEO. TESPT displayed an intermediate performance between vinyl silanes and the reference DPDS, and their comparative efficiency can be summarized as VP > VTEO > TESPT > DPDS.
Chapter 11 consolidates all the findings and extends them to an upscaled extruder process, with primary focus on the transition from batch to continuous processing to enhance productivity and enable industrialization. This upscaling endeavor entailed adjustments in screw configuration and process optimization. The optimal parameters for the extruder process and the properties were determined as follows:
- Temperature profile: 210⁰C for all zones
- Screw configuration: high shear screw
- Screw speed: 75 RPM
- DA concentration and type: 5% VP
- PA concentration and type: 5% SLBR
- Tensile strength: 9.6 MPa
- Elongation at break: 189%
- Network breakdown: 74%
- Mooney viscosity: 80 MU
These optimized parameters can serve as an initial reference for further upscaling of the process in an industrial environment. The extruder process has demonstrated its capability to yield superior quality devulcanizates compared to the internal mixer. VP emerges as the most promising devulcanization aid within this study as well, and SLBR proves to be a viable alternative for process aids.
Outlook
The future outlook for rubber recycling through devulcanization appears promising, poised to address environmental concerns and foster sustainable practices in the rubber industry. Devulcanization, with its capability to selectively break down crosslinks in vulcanized rubber, presents a key solution for recycling rubber products, particularly tires. Advancements in devulcanization methods, including innovative chemical agents and processing techniques, hold the potential to enhance efficiency and product quality. As research explores alternative devulcanization aids adhering to stringent environmental regulations, the landscape of rubber recycling is expected to evolve further. Integration of more sustainable practices, such as the use of supercritical CO2 in devulcanization processes, aligns with global efforts toward eco-friendly solutions.
Moreover, the development of continuous devulcanization processes could streamline operations, making rubber recycling more scalable and economically viable. Collaboration between academia, industry, and policymakers is crucial to drive research, implement regulations, and create a supportive framework for the widespread adoption of devulcanization in rubber recycling. As circular economy initiatives gain traction, the role of devulcanization in closing the loop for rubber materials is likely to expand. The future of rubber recycling through devulcanization holds promise for reducing waste, conserving resources, and establishing a sustainable paradigm in the rubber industry.
