Efficiency Of Devulcanization Research Articles

Waste tire rubber with low and high devulcanization level prepared in the planetary extruder

Waste tires management is serious and global environmental problem. Therefore, searching for new and industrially applicable solutions to convert waste tire rubber into high-value added products is gaining more and more attention. Rubber devulcanization is step forward for further developing rubber recycling and upcycling technologies. Thermo-mechanical treatment of ground tire rubber (GTR) performed in twin screw extruders is currently the most popular solution, while literature information about alternative continuous methods are very limited. In this field of research, using of planetary extruders as multi-screw reactors seems to be interesting solution, which is due to their high mixing efficiency, good heat exchange and devolatilization capacity.In this work, thermo-mechanical treatment of GTR was performed in a lab-scale planetary extruder. Devulcanization effectiveness has been investigated by extruder torque monitoring, Mooney viscosity, swelling measurements, Horikx theory, Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetry, curing characteristics and tensile tests. Volatile organic compounds emitted from untreated waste tire rubber and reclaimed rubbers were determined using a gas chromatography combined with flame ionization detector or mass spectrometry. The results showed that GTR devulcanization efficiency and emission levels of volatile organic compounds increased with higher temperature during planetary extrusion. Mooney viscosity and tensile properties of GTR treated using a planetary extruder were comparable to the commercially available reclaimed rubbers. This work confirms that waste tire rubber devulcanization in the planetary extruder is a promising approach for further studies towards sustainable development of rubber recycling technologies and circular economy.

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.

Influence of filler network on thermo-chemical de-vulcanization efficiency of carbon black filled natural rubber

Carbon black is often used as the reinforcing filler in tires since it plays an important role in improvement of tires mechanical properties such as abrasion, stiffness, modulus and fatigue life. In this study, natural rubber (NR) filled with various loadings of carbon black was prepared. Then, the NR vulcanizates were de-vulcanized via thermo-chemical method using diphenyl disulfide as de-vulcanization aid. The de-vulcanization efficiency was analyzed by relationship between sol fraction and crosslink density of the de-vulcanizates. It is found that the de-vulcanization efficiency is influenced by filler loading. This is attributed to the degree of filler network formation in a rubber matrix which is depended on the filler loadings. In the unfilled de-vulcanizates the results showed that almost 100% of sol fraction and the crosslink density reduced to almost zero are observed. Adding carbon black results in a decrease of sol fraction and increase of crosslink densities. This is due to during de-vulcanization some occurred reactive radicals reacted with active site of carbon black surfaces in filler network to form gels of complex compound. Hence, the de-vulcanization efficiency is lower with increasing carbon black loadings.

Effective devulcanization of ground tire rubber using choline chloride-based deep eutectic solvents

The effect of deep eutectic solvents (DES) in devulcanization of ground tire rubber (GTR) was explored for the first time in this work. Three different DESs, namely ChCl:Urea, ChCl:ZnCl2 and ZnCl2:Urea were prepared and their physicochemical properties were examined. GTR was mixed with various DESs which were further sonicated and heated at various time intervals. The treated GTR was analyzed by energy dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscope (FESEM) and thermogravimetric analysis (TGA). Additionally, the extent of devulcanization was assessed using soluble fraction, cross-link density and related by Horikx theory. The relation between DESs’ viscosities towards devulcanization efficiency was also evaluated.

Recycled Rubber from Waste of Natural Rubber Gloves Blending with Polypropylene for Preparation of Thermoplastic Vulcanizates Compatibilized by Maleic Anhydride

Recycled rubber from waste of natural rubber gloves was prepared via thermo-mechanical de-vulcanization using diphenyl disulfide as a de-vulcanizing aid. After de-vulcanization, the recycled rubber has a higher soluble fraction and a lower cross-link density than that of untreated rubber waste. The de-vulcanization efficiency was then analyzed by Horikx’s method and it was found that random scission or non-selective breakdown of the rubber network, due to both main chain and cross-link scission had occurred during de-vulcanization. Thermoplastic vulcanizates based on recycled rubber and polypropylene blends were prepared using the dynamic vulcanization technique in an internal mixer. Maleic anhydride was used as a chemical modifier in the thermoplastic vulcanizates. The effect of the maleic anhydride loading level was also studied. It was found that the mechanical and rheological properties of the thermoplastic vulcanizates improved with an increasing maleic anhydride content. Furthermore, it was confirmed by SEM morphological investigation that an increasing maleic anhydride content caused a decrease in the size of the spherical dispersed vulcanized rubber domains in the continuous polypropylene matrix. A maleic anhydride content of 80 meq was found to be the optimum content for the improvement of the phase compatibility of the thermoplastic vulcanizates.

STABLE FREE RADICAL–ASSISTED MECHANICAL DEVULCANIZATION OF CARBON BLACK–FILLED NATURAL RUBBER VULCANIZATES

ABSTRACT The issue of recombining of the radicals formed from cross-link scission and the consequent reformation of once-cleaved cross-links during mechanical devulcanization of carbon black–filled NR vulcanizates was investigated in a two-roll mill by the addition of the stable free radical 4-hydroxy-TEMPO (4HT), formerly 4-hydroxy-2, 2, 6, 6-tetramethylpiperidine-N-oxyl free radical. The effect of concentration of 4HT (0–6 pounds per hundred of rubber [phr]) and the effect of cure system, that is, conventional, semi-efficient (EV), and EV, used for vulcanization of the original sample on the efficiency of devulcanization were determined from the residual cross-link density of the gel part of the devulcanized rubber and the percent devulcanization, and further from revulcanizate properties. Chemical probe analysis of the devulcanized sample disclosed that (i) mechanical shearing predominantly breaks the majority cross-link type that decides the associated percent devulcanization; (ii) shear-induced chain-shortening reactions of polysulfidic cross-links occur at ambient conditions, increasing the absolute value of monosulfidic links irrespective of the significant reduction in total cross-link density due to cross-link scission; (iii) re–cross-linking of once cleaved bonds is being arrested in the presence of 4HT, thereby increasing the percent devulcanization. Both the bond energy of the cross-link type to be cleaved and the recombining capacity of the broken cross-links seem to play a decisive role in percent devulcanization, and the probable mechanism involved is suggested.

EFFECT OF DEVULCANIZATION ON CROSSLINK DENSITY AND CROSSLINK DISTRIBUTION OF CARBON BLACK FILLED NATURAL RUBBER VULCANIZATES

ABSTRACTCarbon black filled natural rubber (NR) vulcanizates were devulcanized at ambient temperature in a two roll mill. The effect of cure system, that is, conventional vulcanization (CV), semiefficient vulcanization (semi EV), and efficient vulcanization (EV) systems, used for vulcanization of the original sample, on the efficiency of devulcanization was studied. The efficiency of devulcanization expressed as percentage devulcanization of the samples calculated from residual crosslink density measurements was correlated with the sol fraction of the devulcanized samples based on Horikx analysis. Using chemical probe analysis, we determined (i) the crosslink distribution pattern of the original sample, (ii) the extent to which the different types of crosslinks—that is, polysulfidic, disulfidic, and monosulfidic crosslinks—have been debonded or broken during the shearing process in the two roll mill, and (iii) the pattern of bond formation during revulcanization. Mechanical shearing predominantly breaks the majority crosslink type (polysulfidic crosslinks in CV and semi EV cure systems and disulfidic crosslinks in EV samples). Irrespective of the significant reduction in total crosslink density in all three sets of samples, chain shortening reactions similar to the post-crosslinking chemical reactions at curing temperatures also occur during mechanical shear at ambient conditions, which increased the absolute value of monosulfidic links in CV and semi EV systems. However, in the devulcanized EV system, the absolute value of polysulfidic crosslinks increased, which might be due to the re-crosslinking of the cleaved bonds. All the devulcanized samples were revulcanized, and the mechanical and morphological properties were analyzed. The percentage retention of the vulcanizate properties after revulcanization of the devulcanized samples correlated very well with efficiency of devulcanization.

Upscaling of a Batch De-Vulcanization Process for Ground Car Tire Rubber to a Continuous Process in a Twin Screw Extruder.

As a means to decrease the amount of waste tires and to re-use tire rubber for new tires, devulcanization of ground passenger car tires is a promising process. Being an established process for NR and EPDM, earlier work has shown that for ground passenger car tire rubber with a relatively high amount of SBR, a devulcanization process can be formulated, as well. This was proven for a laboratory-scale batch process in an internal mixer, using diphenyl disulfide as the devulcanization aid and powder-sized material. In this paper, the devulcanization process for passenger car tire rubber is upscaled from 15 g per batch and transformed into a continuous process in a co-rotating twin screw extruder with a capacity of 2 kg/h. As SBR is rather sensitive to devulcanization process conditions, such as thermal and mechanical energy input, the screw design was based on a low shear concept. A granulate with particle sizes from 1–3.5 mm was chosen for purity, as well as economic reasons. The devulcanization process conditions were fine-tuned in terms of: devulcanization conditions (time/temperature profile, concentration of devulcanization aid), extruder parameters (screw configuration, screw speed, fill factor) and ancillary equipment (pre-treatment, extrudate handling). The influence of these parameters on the devulcanization efficiency and the quality of the final product will be discussed. The ratio of random to crosslink scission as determined by a Horikx plot was taken for the evaluation of the process and material. A best practice for continuous devulcanization will be given.

FTIR spectroscopic and thermogravimetric characterization of ground tyre rubber devulcanized by microwave treatment

In this work, the phenomena involved in the microwave devulcanization of ground tyre rubber (GTR) were investigated. Three types of GTR with different content of organic compounds (elastomers, plasticizers, etc.), carbon black and ash have been studied. The chemical structure of GTR before and after a microwave devulcanization process was analyzed by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Furthermore, the efficiency of the microwave devulcanization conducted at different treatment time was evaluated based on the values of crosslinking density and sol fraction content. FTIR spectroscopy results showed that devulcanization of GTR causes a decrease in carbon black with generation of CO2 due to its thermo oxidation, a decrease in the structural groups related to elastomeric components (mainly methylene and methine) and a breaking of CS groups and SS bridges. The presented results indicate the strong correlation between the content of SiO2 in the GTR and the achieved degree of devulcanization. It was observed that GTRs with a high content of SiO2 are devulcanized more easily than the samples with low content of SiO2, which suggest that the presence of silica fillers improve microwave devulcanization efficiency.

Characterization of styrene butadiene rubber and microwave devulcanized ground tire rubber composites

Abstract Ground tire rubber (GTR) was devulcanized by microwaves at the same heating rate (constant power) and different times of exposure. The devulcanized rubber (DV-R) and untreated GTR were characterized physically and thermally. Composite materials were prepared from different proportions of the GTR, which was used as a filler, and the DV-R, which was used as part of the styrene butadiene rubber (SBR) matrix, and by varying the exposure time of the microwave power. These composites were compared with a control sample that was prepared from virgin SBR. The sol content (soluble part) and Fourier transform infrared spectroscopy (FTIR) analyses of the devulcanized samples were examined to define the efficiency of devulcanization. The cure characteristics and tensile properties of the SBR composites were researched. In this study, it was found that using DV-R as part of the rubber matrix produced much better properties than using GTR as a filler, thereby showing the significant benefits of microwave devulcanization. At the DV-R content of 50 phr, the elongation at break of the DV-R 5 min/SBR composites increased to 445.06% from 217.25% for the GTR/SBR composites, i.e., the elongation at break was enhanced by 105% by the devulcanization of GTR. Scanning electron microscopy (SEM) photographs displayed a better interface coherence between the DV-R 5 min and SBR matrix than the GTR/SBR composites.

Degradation kinetics of ethylene propylene diene terpolymer residues devulcanized by microwaves

The reclaiming of elastomer residues is one of the main problems of recycling materials. In this work, the efficiency of devulcanization of recycled ethylene propylene diene terpolymer (EPDM) was determined by subjecting the elastomer samples to 2, 3 and 4 min of microwaves exposure. Scanning electron microscopy and thermogravimetric analysis using different heating rates to determine the kinetic degradation parameters according to the Flynn–Wall–Ozawa and Criado methods were studied. The results obtained showed that up to 4 min devulcanization exposure period, the degradation temperature of EPDM did not exceed. Microwaves exposure caused modifications in the morphology and reduction in activation energy values upon increased samples exposure period. Degradation mechanisms were altered, mainly for the 4-min exposure period, where the nucleation mechanism shifts to diffusion, suggesting that the microwave method was able to promote the breakage of the elastomeric three-dimensional network.

Mechanochemical surface activation of ground tire rubber by solid‐state devulcanization and grafting

AbstractCured rubber powder was successfully devulcanized under the application of high‐energy ball milling. An attempt was made to improve the efficiency of devulcanization with a phenolic antioxidant (2,6‐di‐tert‐butyl‐4‐methyl‐phenol). In addition, the grafting of virgin natural rubber to the surface of rubber powder was promoted by milling under an inert atmosphere. The extent of mechanochemical devulcanization was studied by estimation of the sol–gel fractions and crosslink densities. Our findings suggest that the process led to a preferential breakage of sulfur crosslinks. The curing behavior of the compounds and the mechanical properties of the vulcanizates containing treated rubber powder and fresh rubber were investigated. The chemical and mechanical properties depended strongly on the experimental milling conditions, which could consequently be controlled to improve the quality of the reclaimed rubber. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1631–1638, 2003

Comparative analysis of ultrasonically devulcanized unfilled SBR, NR, and EPDM rubbers

AbstractThe comparative study of the continuous ultrasonic devulcanization of various unfilled rubbers [natural rubber, styrene–butadiene rubber (SBR), ethylene–propylene–diene rubber (EPDM)] is carried out by means of a ultrasonic reactor. The power consumption, gel fraction, crosslink density, cure behavior, and physical properties of devulcanized rubbers were measured. The glass transition temperatures of virgin, vulcanized, and devulcanized rubbers were determined in order to characterize the difference in the mobility of rubber molecules for each rubber before and after devulcanization. Thermogravimetric analysis was also used to determine thermal stability of the various rubbers. A unique correlation between gel fraction and crosslink density indicated significant differences in the efficiency of devulcanization of various rubbers. Under certain devulcanization conditions, the mechanical properties of revulcanized SBR and EPDM rubbers were found to improve compared to those of the original rubbers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 434–441, 2003

Ultrasonic Devulcanization of Unfilled SBR under Static and Continuous Conditions

Abstract A static ultrasonic treatment device was used to investigate the effect of ultrasound on devulcanization of unfilled SBR disks in the absence of shearing effect. Some visible bubbles were observed at the center of the disk sample at low pressure, as well as at the edges at high pressure, as the treatment time increases. Evidently, these bubbles are a proof of the existence of cavitation during the ultrasonic devulcanization. Stronger reduction of crosslink density observed at a higher pressure in the treatment is a direct indication of the significant role of pressure in improving the efficiency of ultrasonic devulcanization. Treatment of the ground sample, having higher concentration of voids, indicated lower crosslink density and gel fraction than those of the sheet sample not having visible voids. The unique correlation between gel fraction and crosslink density of devulcanized rubbers observed earlier in continuous process, was also obtained in the present case. However, this curve is shifted towards higher values of crosslink density. It is concluded that shearing effect has an influence on improving the efficiency of devulcanization.

Ultrasonic Devulcanization Reactors for Recycling of GRT: Comparative Study

Abstract The present paper describes the comparative study of a continuous ultrasonic devulcanization of ground tire rubber (GRT) using two reactors. The devulcanization zone in the first reactor (coaxial reactor) is located at the exit from an extruder without imposition of additional shearing. In the second reactor (barrel reactor), the devulcanization zone is located in barrel where additional shearing takes place due to screw rotation. Gel fraction, crosslink density, cure behavior and physical properties of GRT obtained in these two reactors were measured. Also, the distribution of gel fraction and crosslink density at various locations in the devulcanization zone of the barrel reactor were determined. Gel fraction and crosslink density show that GRT in both reactors is partially devulcanized. A unique correlation between gel fraction and crosslink density obtained in both reactors indicated that additional shearing had a positive influence on improving the efficiency of devulcanization. Under the optimal devulcanization condition, the physical properties of revulcanized GRT and the output are higher in the barrel reactor. Power consumption density in the barrel reactor is found to be significantly higher.

Continuous ultrasonic devulcanization of carbon black-filled NR vulcanizates

The continuous ultrasonic devulcanization of natural rubber (NR) filled with various concentrations of carbon black (CB) indicated a minimum of crosslink density and gel fraction at an intermediate amplitude, which is independent of CB content. An attempt was made to improve the efficiency of devulcanization by use of various chemicals (1,3 Diphenylguanidine, 2-Mercaptobenzothiazole, Thianaphthene). However, these experiments did not indicate any improvement in comparison with devulcanization without chemicals. An idea of adding fresh CB into devulcanized compound, which has been shown to improve mechanical properties in the case of styrene–butadiene rubber (SBR), was tested in the present study for CB filled NR compound. The obtained result indicated that an addition of fresh CB to devulcanized CB-filled NR did not lead to an improvement in mechanical properties upon revulcanization. The revulcanization recipe was optimized to improve the mechanical properties of revulcanized CB-filled NR vulcanizates. It was found that CB-filled NR upon revulcanization retained its strain-induced cystallizability with the tensile strength and elongation at break at about 50 and 70% level of the virgin vulcanizates. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2340–2348, 2001