Devulcanization Process Research Articles

Transforming waste into valuable resources: Recycled nitrile-butadiene rubber scraps filled with electric arc furnace slag

This study investigates the valorization of two industrial waste streams - nitrile butadiene rubber (NBR) scraps and electric arc furnace (EAF) slag – in the development of recycled NBR compounds filled with EAF slag as filler. A new recycling method for NBR scraps is employed via calendering at room temperature without the need for curatives, chemical agents, or pre-grinding. The resulting recycled NBR is then used as a matrix for EAF slag particles to produce sustainable rubber compounds that are entirely recycled. Characterization reveals that incorporating EAF slag enhances the devulcanization process of recycled NBR during recycling. The filler grain size affects composite properties like hardness, crosslink density, and tensile modulus, with finer slag particles (<50 μm) exhibiting improved reinforcement due to increased interaction surface with the rubber matrix. Dynamic mechanical analysis indicates that recycled NBR filled with EAF slag exhibits a more significant Payne effect compared to unfilled recycled NBR, due to filler-matrix interactions. Interestingly, EAF slag facilitates the rapid fractional recovery of the low-strain storage modulus after experiencing high-amplitude strain, ascribed to the formation of a rigid rubber layer around the slag particles.Overall, the findings highlight the potential to valorize these two waste materials effectively by producing functional recycled NBR/EAF slag composites with desirable properties through a simple, industrially viable recycling method without capital-intensive equipment. This represents both environmental and economic benefits through waste valorization and industrial symbiosis.

Natural Rubber Latex Wastes from Balloon Production as Valuable Source of Raw Material: Processing, Physico-Mechanical Properties, and Structure

This study explores the potential for recycling natural rubber (NR) latex waste from balloon production through the devulcanization and revulcanization processes. The mechanical devulcanization of colored latex balloon waste was conducted, followed by revulcanization using a sulfur-based system. The reclaimed rubber’s properties, including crosslink density, tensile strength, and abrasion resistance, were compared with those of virgin NR. The results demonstrate that the reclaimed rubber maintains a crosslink density close to that of virgin NR. Hardness and abrasion resistance were comparable, indicating successful material recovery. Structural analyses, including FTIR and SEM microscopy, revealed that the devulcanization process effectively allowed for successful revulcanization. This study concludes that NR latex waste can be effectively recycled and reused in rubber composite formulations, offering a sustainable approach to waste management in the rubber industry and contributing to developing eco-friendly materials. In the context of this research, integrating advanced chemical and physical methods, such as solubility parameter calculations and enhanced devulcanization techniques, could further optimize the devulcanization process. These methods quantitatively enhance the efficiency of material recovery, offering a path to more sustainable recycling practices. The findings suggest that combining such advanced methodologies could significantly improve recycled NR latex’s overall performance and applicability in industrial applications.

Thermomechanical-chemical devulcanization of ground tire rubber

Abstract We developed a thermomechanical devulcanization process of ground tire rubber (GTR) by incorporating chemicals to achieve better devulcanization. The samples were devulcanized in a twin-screw extruder with the help of three types of chemicals that aid devulcanization: N-(cyclohexylthio) phthalimide (PVI), dibenzamid diphenyl disulfide (DBD) and dialkyl-pentasulfide (DPS, commercialized as Aktiplast 79). We characterized the resulting devulcanizates by Soxhlet-extraction, and produced vulcanized sheets, which we characterized mechanically and examined the vulcanization process. We found that all chemicals facilitated better devulcanization and the devulcanizate had a more rubber-like behavior during mixing. Vulcanization was also slightly improved and reversion was reduced. When revulcanized, the rubber had improved strain at break and tear strength.

Rubber devulcanization in the planetary extruder – processing properties and structure

Thermo-mechanical devulcanization of ground tire rubber (GTR) in a planetary extruder was investigated. The processing parameters of recycled rubber were studied based on standardized protocols using a Mooney viscometer and a rubber processing analyzer (RPA). Fourier transform infrared spectroscopy and scanning electron microscopy were used to determine rubber structure. Devulcanization process upscaling limits processing properties of recycled rubber, which for partially devulcanized rubber can be determined only by RPA rheometer.

Exploring the Impact of Reinforcing Filler Systems on Devulcanizate Composites.

Composites revolutionize material performance, fostering innovation and efficiency in diverse sectors. Elastomer-based polymeric composites are crucial for applications requiring superior mechanical strength and durability. Widely applied in automotives, aerospace, construction, and consumer goods, they excel under extreme conditions. Composites based on recycled rubber, fortified with reinforcing fillers, represent a sustainable material innovation by repurposing discarded rubber. The integration of reinforcing agents enhances the strength and resilience of this composite, and the recycled polymeric matrix offers an eco-friendly alternative to virgin elastomers, reducing their environmental impact. Devulcanized rubber, with inherently lower mechanical properties than virgin rubber, requires enhancement of its quality for reuse in a circular economy: considerable amounts of recycled tire rubber can only be applied in new tires if the property profile comes close to the one of the virgin rubber. To achieve this, model passenger car tire and whole tire rubber granulates were transformed into elastomeric composites through optimized devulcanization and blending with additional fillers like carbon black and silica-silane. These fillers were chosen as they are commonly used in tire compounding, but they lose their reactivity during their service life and the devulcanization process. Incorporation of 20% (w/w) additional filler enhanced the strength of the devulcanizate composites by up to 15%. Additionally, increased silane concentration significantly further improved the tensile strength, Payne effect, and dispersion by enhancing the polymer-filler interaction through improved silanization. Higher silane concentrations reduced elongation at break and increased crosslink density, as it leads to a stable filler-polymer network. The optimal concentration of a silica-silane filler system for a devulcanizate was found to be 20% silica with 3% silane, showing the best property profile.

Optimization of scrap tire devulcanization process using Taguchi and DEAR method in a supercritical fluid extraction pilot scale setup

Recycling of scrap tires is gaining more significance to ensure sustainable consumption and production on a commercial scale. The supercritical CO2 extraction technique has been classified as green technology under United Nation's sustainable development guidelines. Literature shows that research on scrap tire devulcanization using supercritical CO2 was performed on a laboratory-scale setup only. However, significant pilot-scale research needs to be conducted before commercial use and technology transfer. This study utilized a pilot-scale supercritical fluid extraction equipment to devulcanize waste tire-derived crumb rubber. Using Taguchi's design of experiment (DOE) model along with the Data Envelopment Analysis Based Ranking (DEAR) approach, the effect of the input process variables on response factors was examined. The input functions, devulcanizing agent's concentration, and pump pressure were discovered to be the most influential parameters in the devulcanization process. Further optimization was carried out via the DEAR-Taguchi technique. The optimal process parameters were determined to be a pump pressure of 350 bar, an extractor temperature of 80 °C, a holding time of 30 min in the extractor, a CO2 circulation of 120 min, and a devulcanizing agent concentration of 15 vol.%. Finally, a validation experiment was conducted at the optimum conditions to verify the acceptability of the model. It was noticed that the divergence for the confirmation experiment was within 10 %, indicating that the approach used for the devulcanization of crumb rubber is acceptable.

Research of Insertion Loss of Multilayered Construction with Devulcanized Waste Rubber

Abstract In this study, the insertion loss of devulcanized waste rubber baffles were evaluated. Acoustic baffles are suitable to reduce noise from the devices or machines by interfering with their emitting sound waves. Knowledge of the acoustic properties of the material used is of significant importance in ensuring the effectiveness of the acoustic properties of the baffle. Basic properties include airborne sound insulation, which is usually determined during laboratory tests. Baffles consists of sound absorbing and sound insulating materials. In this study, plasterboards were used as sound insulating material and devulcanized waste rubber as sound absorbing material. During the devulcanization process, porous granules are obtained, which can be used as an acoustic material. In this study, two types of rubber granules were devulcanized by grinding method and one other type was chemically devulcanized. Three types of rubber granules were mixed together in increasing 25 % proportion steps and glued with patented polyurethane glue. A total of 15 different composition devulcanized waste rubber granule boards were made. Rubber boards were attached together with the plasterboards. Insertion loss of the different composite baffles was measured in semi-anechoic chamber in a purposefully designed stand in 1/3rd octave bands. The results showed that the insertion loss of the baffles was mostly dependent on the rubber granule board density. When the density of the rubber board increased, the insertion loss also increased. The 5–6 dB insertion loss difference was measured between the most and least dense rubber granule board baffles.

Utilization of Rubber Waste for Manufacture of Rubber Conical Bonded

Rubber Conical Bonded (RCB) is a component that connects the wheel with the train bogie, which functions to dampen vibration and noise. This RCB is made of rubber. In this research, RCB vulcanizate will be made with a mixture of natural rubber (RSS 1) and rubber waste. Rubber waste is the residue from the production process of technical rubber goods or rubber technical goods that are no longer used. This rubber waste goes through a devulcanization process to be reused as a raw material. In this study, rubber waste derived from the residue of the production process was varied with the phr ratio as follows: RSS 1 / Rubber waste / Devulcanized rubber waste (RCB0: 100/0/0; RCB10: 90/10/0; RCB30: 70/30/0; RCB50: 50/50/0; RCB10X: 90/0/10; RCB30X: 70/0/30). The vulcanizate obtained was then subjected to physical characterization (before and after aging 24 hrs; 70oC) and compared with the technical specifications of the RCB. Physical characterization includes Tensile Strength (min 14 N/mm2), Tear Strength (min 26 N/mm2), Hardness Shore A (50±5), Elongation at Break (min 400%), and Compression Set (max 30%). The results show that all variations meet the specifications for tensile strength, tear strength, and hardness.

Thermomechanical devulcanization of butyl rubber using twin-screw extruder: Process parameters, viscoelastic and compatibility properties

Thermomechanical devulcanization of butyl rubber (BR1), bromobutyl rubber (BR2) vulcanizates, both prepared in laboratory, and a waste commercial butyl rubber (BR3) using a twin-screw extruder is reported. The commercial butyl rubber vulcanizate (BR3) was used as a tire curing bladder and the waste rubber received from a tire manufacturing company. The devulcanization process was carried out at various barrel temperatures and screw speeds (40, 80 and 120 r/min). The lowest sol fraction was obtained for devulcanized waste commercial butyl rubber (D-BR3). According to Horikx theory, devulcanized butyl rubber (D-BR1) presented mainly crosslink breakup while devulcanized bromobutyl rubber (D-BR2) and devulcanized waste commercial butyl rubber (D-BR3) showed a mixture of main chain and crosslink scission. As cure rheographs at 190°C indicated, the addition of devulcanized rubbers to the corresponding virgin ones influenced both scorch and optimal cure times. The tensile properties of devulcanized/virgin rubber blends changed depending on the blend type and virgin rubber. The stress relaxation experiment revealed that the addition of devulcanized rubber to virgin one mostly influenced short relaxation times and also decreased G0. BR1/D-BR3 blends exhibited higher elastic component than BR1/D-BR1 and BR2/D-BR2 samples. The Han plots revealed higher compatibility for BR2/D-BR2 blends.

Real-time monitoring of ground-tire rubber microwave devulcanization with thermal and electrochemical sensors

Devulcanizing ground-tire rubber (GTR) properly requires the removal of the sulphur linkages that crosslink the polymer. The volatile sulphur compounds (VSCs) released during the process must be extracted from the reactor to avoid any chemical recombination, and the sulphur gas concentrations conveniently sensed during the extraction, along with a thermal sensoring of the payload, can be used to monitor the whole devulcanization process. In this contribution, a modified conventional microwave oven was used to devulcanize the GTR. The microwave-processed GTR was evaluated by determining the values of its mass loss (ML), soluble fraction (sol fraction), and the variation of its electric permittivity. Results show a direct relationship between the energy delivered, sol fraction, the VSCs concentrations, the ML, and the permittivity values. Thus, this paper demonstrates that monitoring the VSCs can provide a reliable indication of ML and, consequently, devulcanization evolution even at non-uniform temperature conditions.

Optimization of Crumb Rubber Modified Binder Formulation through Compatibility Analysis.

The research is devoted to developing the production of crumb rubber-modified bitumen with improved stability. It has been established that the most suitable semi-empirical coefficient for determining the compatible plasticizer to crumb rubber is based on the ratio of paraffin-naphthenic compounds to resinous-asphaltene compounds. With the help of differential scanning calorimetry, temperature regimes of crumb rubber destruction and preparation of rubber-containing components (210 °C) were studied and determined. It was established that determining the dynamic viscosity of hydrocarbon concentrates with crumb rubber on a rotary viscometer is not applicable due to elastic components, making it difficult to measure and obtain reliable data. The most suitable method is the shear viscosity method. Using fluorescent microscopy, it was established that the formation of a branched structure of crumb rubber is achieved in the waste industrial oil, indicating devulcanization processes. It was found that hydrocarbon plasticizer with high naphthenic oil content is the most compatible with crumb rubber. Synthetic wax was found to be of greater interest as a devulcanizing/stabilizing agent, and its application in an amount of 3% allows the formation of a stable CRMB structure and stabilizes the devulcanization process.

New Route of Tire Rubber Devulcanization Using Silanes.

The disposal of tires at the end of their lifespan results in societal and environmental issues. To tackle this, recycling and reuse are effective solutions. Among various recycling methods, devulcanization is considered to be a very sustainable option, as it involves the controlled breakdown of crosslinks while maintaining the polymer backbones. The objective of this study is to develop a sustainable devulcanization process for passenger car tire rubber using silanes. In this study, a thermo-mechanical-chemical devulcanization process was conducted to screen six potential devulcanization aids (DAs). Silanes were chosen as they are widely used in tire rubber as coupling agents for silica. The efficiency of the devulcanization was studied by the degree of network breakdown, miscibility of the devulcanized material, and mechanical properties of the de- and revulcanized material. Compared to the parent compound, a 55-60% network breakdown was achieved for the devulcanizate along with 50-55% of tensile strength recovery. In addition to superior devulcanization efficiency, this DA offers a sustainable alternative to the conventional ones, such as di-phenyl-di-sulphide, due to its compliance with safety regulations. The devulcanizate can be utilized in high-performance applications, such as tires and seals, while 100% devulcanizate can be employed in low-strength technical rubber products.

Adsorption of aqueous Pb(II) using non-devulcanized and devulcanized tyre rubber powder: a comparative study.

This study aimed to compare the adsorption of Pb(II) ions from an aqueous solution using non-devulcanized (NTR) and devulcanized tyre rubber (DTR) powder. Both types of rubber particles were prepared from used truck tyres, with DTR processed through mechano-chemical devulcanization. The adsorption experiments were conducted using 100-200µm particles, with adsorbent doses ranging from 5 to 15g/L. Effects of adsorbent dose, initial metal concentration and contact time were investigated. Characterization of both adsorbents was done using SEM-EDS, FTIR, and XRD analysis. Different adsorption isotherm and kinetic models were used to analyse the adsorption mechanisms. The results of the study showed that DTR was significantly more efficient at adsorbing Pb(II) compared to NTR. The maximum adsorption capacities estimated from the Langmuir equation were 75.1mg/g and 6.61mg/g for DTR and NTR, respectively. Among the kinetic models tested, pseudo 2nd order kinetic model was found to be the most suitable for tyre rubber adsorbents. The optimal dose and contact time were found to be 5g/L and 120min, respectively, for both adsorbents. The superior performance of DTR in Pb(II) adsorption was attributed to the change in the surface morphology of the rubber during the devulcanization process, resulting in increased surface roughness. The adsorption of Pb(II) was accompanied by the leaching of Zn from both types of rubber, suggesting that an ion exchange mechanism might be involved in the adsorption process. In conclusion, devulcanization appears to be a viable method for improving the adsorption properties of tyre rubber.

Upscaling of a Mechanochemical Devulcanization Process for EPDM Rubber Waste from a Batch to a Continuous System

The present work is a comparative study of the effects of mechanical shear, temperature, and concentration of a chemical agent on the devulcanization process of post-industrial ethylene propylene diene (EPDM) rubber waste. Devulcanization was carried out in a heating press (no shear), an internal mixer (low shear), and a co-rotating twin screw extruder (high shear) at temperatures ranging from 100 to 200 °C. The efficiency of pure dibenzamido diphenyl disulfide (DBD) and a commercial devulcanizing agent, Struktol A89®, containing DBD were studied. Based on the results, the devulcanization process was upscaled from 40 g per batch to a continuous process with a capacity of 270 g/h. The parameters were fine-tuned regarding flow rate, screw speed, and temperature. Blends of virgin rubber (VR) and 25, 50, and 75 wt% recyclates were compared with blends of VR and 25, 50, and 75 wt% of untreated RWP. The quality of the recyclate was determined by rheometer tests, SEM images, TGA, and mechanical properties. The best results were obtained with 2 wt% DBD in the extruder with a temperature profile of 120 to 80 °C, 50 rpm, and 4.5 g per minute (gpm). The tensile strength and strain at break of the recyclate already met the requirements of DIN EN 681-1:2006 for the production of sealing systems. The compression set and Shore A hardness were restored by mixing recyclate with 25 wt% VR.

Features of structure formation in crumb rubber modified bitumen

The research is devoted to developing the science-based technological solution for obtaining crumb rubber modified bitumen (CRMB) with improved stability. The technological solution provides a preliminary process of devulcanization of crumb rubber in hydrocarbon plasticizer in the presence of stabilizing agent and subsequent preparation of CRMB. Using fluorescent microscopy, it was established that the formation of a branched structure of crumb rubber is achieved in the waste industrial oil, indicating devulcanization processes. It was found that the most compatible with crumb rubber is a hydrocarbon plasticizer with a high content of naphthenic oil. Synthetic wax was found to be of greater interest as a stabilizing agent, and its application in an amount of 3% allows the formation of a stable CRMB structure and stabilizes the devulcanization process. The test results of obtained CRMB showed that their viscosity corresponds to the requirements for PG bitumen

Effect of different devulcanization agents on the mechano-chemical devulcanization process of waste tires

Herein, the effects of two devulcanization agents, i.e., disulfide oil (DSO) and tetramethyl thiuram disulfide (TMTD), byproducts of gas refineries, on the mechano-chemical devulcanization process of waste tires were investigated. The response surface methodology (RSM) involving Box-Behnken Design (BBD) was utilized for the optimization of different factors, i.e., reaction temperature, reaction time, and the content of devulcanization aids. Samples were prepared in a Brabender mixer at a temperature of 100°C, and a rotor speed of 80 RPM for 15 min, and the devulcanization percentage of the samples was determined using swelling test, Mooney viscosity, and Horikx’s theory. The results demonstrated that when 5 phr devulcanization agents were used, the devulcanization percentages for samples containing DSO and TMTD were 73 and 80%, respectively. In addition, the effect of reaction temperature, reaction time, and the content of devulcanization agents on the crosslink scission was also investigated. It was found that by increasing these three parameters, the crosslink density decreased while the devulcanization percentage increased. All in all, the results indicated that the performance of TMTD as devulcanization agent was better than that of DSO. We believe that our presented research study will open new avenues for effective recycling of waste rubbers.

Recycling of EPDM via Continuous Thermo-Mechanical Devulcanization with Co-Rotating Twin-Screw Extruder.

Devulcanization represents the recycling of choice for a homogenous rubber waste stream because it allows revulcanization of samples previously devulcanized, making the life of the rubber virtually endless, according to the principles of circular economy. Among the many devulcanization processes, the thermo-mechanical one is the most appealing because it is a continuous process, easy to be industrialized. In this paper a comprehensive set of analyses (FTIR, TGA, DSC, elemental analyses, Py-GC/MS, swelling tests) were carried out on a post-industrial ethylene propylene diene monomer (EPDM), thermo-mechanical devulcanized in a co-rotating twin-screw extruder with different process parameters (thermal and screw profile, rpm). Results of the swelling test according to the Flory-Rehner theory and Horikx analyses show that the higher the thermal profile and the higher the rpm, the higher is the percentage of devulcanization. The quality of the devulcanized sample in terms of sol fraction and percentage of random scissions depends on the process conditions. The screw profile concurs to the efficiency of the devulcanization: the different number of kneading elements and more in general the screw profile composition affects the percentage of devulcanization, making the results in some tests more dependent on the screw speed.

Segregated MWCNT Structure Formation in Conductive Rubber Nanocomposites by Circular Recycling of Rubber Waste

The reclamation and devulcanization processes of rubber waste change its cross-link density and sol–gel proportions, which are essential in the reprocessing of reclaimed rubber (RR), especially in the dispersion of nanofillers throughout the RR matrix. In this work, conductive elastomers based on RR/carbon nanotubes (CNTs) were prepared. Proper control of the quality of RR is needed to ensure the effective formation of conductive pathways within the matrix. Therefore, this work aimed to control a segregated CNT network by manipulating how the RR is prepared. Three alternative devulcanization conditions were tested: (a) physical reclaiming, (b) thermomechanical reclaiming, and (c) thermochemical reclaiming. These techniques resulted in three types of RRs. Nanocomposite based on the physically RR presented the highest tensile strength, reaching approximately 15 MPa with addition of 2 phr of CNT. This was due to a comparatively lesser destruction of the rubber network and the formation of a segregated structure of CNT filler in the nanocomposite. Therefore, the conductive performance was significantly enhanced from 4.5 × 10–10 S/cm for unfilled to 1.25 × 10–6 S/cm with CNT dose as low as 1 vol %. The segregated CNT network, whose formation was assisted by gel fraction and excluded volume in RR, also resulted in a very low electrical percolation threshold φc, which was significantly reduced from 0.78 vol % for low cross-linked RR to 0.14 vol % for high cross-linked RR. This could reduce CNT filler use by 80% whereas 100% of pristine rubber would be replaced, and mechanical and electrical properties would improve. The approach described in this work will be helpful in manufacturing conductive rubber products efficiently yet sustainably with recycling of rubber waste.

Dynamic Permittivity Measurement of Ground-Tire Rubber (GTR) during Microwave-Assisted Devulcanization.

Many efforts are being made to find innovative ways of recycling rubber from end-of-life tires (ELTs), also called ground tire Rubber (GTR). Recycling through devulcanization allows the reintroduction of rubber back into the manufacturing industry. Such a process requires providing enough energy to break the sulfur links, while preventing damage to the polymeric chain. Microwave heating is controllable, efficient, and it does not rely on conventional heating mechanisms (conduction, convection) which may involve high heating losses, but rather on direct dielectric heating. However, to adequately control the microwave-assisted devulcanization performance, a thorough knowledge of the GTR permittivity versus temperature is required. In this work, GTR permittivity was monitored during its devulcanization. A resonant technique based on a dual-mode cylindrical cavity was used to simultaneously heat rubber and measure its permittivity at around 2 GHz. The results show sharp changes in the GTR permittivity at 160 and 190 °C. After the GTR cooled down, a shifted permittivity evidences a change in the GTR structure caused by the devulcanization process. Microwave-assisted devulcanization effectiveness is proven through time-domain nuclear magnetic resonance (NMR) measurements, by verifying the decrease in the cross-link density of processed GTR samples compared to the original sample.

Environmental assessment of rubber recycling through an innovative thermo-mechanical devulcanization process using a co-rotating twin-screw extruder

Under the umbrella of the transition toward a circular economy, the evaluation of the environmental sustainability of processes aiming to the recycling of materials is essential. Rubbers, like ethylene-propylene diene monomer (EPDM), represent a class of materials that have become an environmental, social and economic issue, so the evaluation of the end-of-life management of these materials is of primary relevance. This study investigates the environmental burdens associated with a thermo-mechanical devulcanization process for recycling of EPDM wastes by co-rotating twin-screw extruders. The Life Cycle Assessment methodology was used to carry out the analysis and eight environmental impact categories have been evaluated: Climate change, Ozone depletion, Photochemical ozone formation, Acidification, Eutrophication (freshwater), Ecotoxicity (freshwater), Resource use (fossils) and Resource use (minerals and metals). In the first step, the analysis focused on the identification and quantification of main environmental hotspots of the process. Then, the study was extended by including two comparative analyses, to better understand the magnitude of the environmental burdens generated by the devulcanization process. The results of the hotspots analysis showed that the main contribution to the environmental impacts of the devulcanization process is due to energy consumption. However, the comparison underlines how the devulcanization process for recycling of EPDM waste allows to reduce the environmental burden associated with the life cycle of rubber products.