Recycling of tire‐curing bladder by ultrasonic devulcanization

The recycling of butyl rubber‐based tire‐curing bladder was carried out by means of a grooved barrel ultrasonic extruder. Die pressure and ultrasonic power consumption were measured as a function of flow rate and ultrasonic amplitude. Gel fraction and crosslink density of the ultrasonically devulcanized rubber were substantially reduced. In turn, this led to some reduction in gel fraction and crosslink density in the revulcanized rubber. These findings were correlated with dynamic properties and the cure behavior of the devulcanized rubber. The mechanical properties of the revulcanized rubber, dependent on processing conditions during devulcanization, were compared with that of the virgin vulcanizate. Good mechanical properties of revulcanized rubber was achieved with 86 and 71% retention of the tensile strength and the elongation at break respectively, and with modulus increased by 44%. The devulcanized rubber was found to contain tiny gel particles of a wide size distribution with a predominant size of <4 μm. POLYM. ENG. SCI., 46:8–18, 2006. © 2005 Society of Plastics Engineers     

Continuous ultrasonic devulcanization of unfilled butadiene rubber

The devulcanization of sulfur‐cured unfilled butadiene rubber (BR) with a grooved‐barrel ultrasonic reactor under various processing conditions was carried out. The experiments indicated that BR had a narrow devulcanization window. Outside this window, significant degradation or no devulcanization occurred. Gel permeation chromatography (GPC) measurements were carried out with the sol part of virgin and devulcanized samples to study the breakdown of the polymeric chains. The GPC data showed a significant molecular weight reduction and a broadening of the molecular weight distribution upon devulcanization, indicating that the devulcanization and degradation of BR occurred simultaneously. The rheological properties showed that devulcanized BR was more elastic than the virgin gum. The vulcanizates of the blends of virgin and devulcanized BR showed a considerable enhancement of the mechanical properties. The thermal behaviors of the virgin and devulcanized BR were found to be different. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1166–1174, 2004     

Continuous ultrasonic devulcanization of unfilled butyl rubber

The devulcanization of resin‐cured unfilled butyl rubber with a grooved‐barrel ultrasonic reactor under various processing conditions was carried out. The experiments indicated that, because of the lower unsaturation and good thermal stability of butyl rubber, its devulcanization could be successfully accomplished only under severe ultrasonic‐treatment conditions. Gel permeation chromatography measurements were carried out for the virgin gum and sol part of devulcanized samples to study the changes in the rubber network during the devulcanization process. The obtained data showed a significant molecular weight reduction and a broadening of the molecular weight distribution upon devulcanization, which indicated that the devulcanization and degradation of butyl rubber occurred simultaneously. The rheological properties showed that devulcanized butyl rubber was more elastic than the virgin gum. The vulcanizates of the devulcanized butyl rubber showed mechanical properties comparable to those of the virgin vulcanizate. The thermal behaviors of the virgin and devulcanized butyl rubber were different and were correlated to the double‐bond content. The structural characteristics of the devulcanized butyl rubber were simulated with the Dobson–Gordon theory of rubber network statistics. A fairly good agreement between the experimental data and theoretical prediction was achieved. The simulation of devulcanized butyl rubber indicated that the rate of crosslink rupture was much higher than that of the main chain. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1316–1325, 2004     

Continuous ultrasonic devulcanization of unfilled NR vulcanizates

Sulfur-cured unfilled natural rubber (NR) is successfully devulcanized in a continuous extrusion process under the application of high-power ultrasonic energy. The die characteristics and ultrasonic power consumption are measured. A unique correlation is found between the crosslink density and gel fraction of the devulcanized NR. This correlation is independent of the processing parameters, such as barrel temperature, die gap, flow rate, and amplitude of ultrasound. However, these parameters do influence the degree of devulcanization. In most cases, the degree of devulcanization is found to pass through a maximum at an intermediate level of ultrasonic energy. It is hypothesized that simultaneous breakup and reformation of crosslinks occur during the devulcanization of NR, with the relative contribution of each being determined by the process parameters. The cure curves and mechanical properties of the revulcanized NR are studied. The mechanical properties are found to depend on the revulcanization recipe. On optimizing it, tensile strength as high as 14.2 MPa is achieved, which is about 70% of that of the virgin NR vulcanizate. Ultimate elongation as high as 670% is obtained, which is the same as that of the virgin NR vulcanizate. Such stress–strain behavior is an indication that the devulcanized NR maintains the strain-induced crystallization characteristics inherent to the virgin NR vulcanizates. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2007–2019, 1998     

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

The 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     

Improved properties of blends of ultrasonically treated unfilled polyurethane rubber

The aim of this investigation is to improve the properties of the polyurethane rubber that has been devulcanized using the coaxial ultrasonic reactor. After devulcanization of the cured rubber under varying gap sizes and amplitudes, blends of the devulcanized and virgin rubber at different ratios are prepared. Rheological and mechanical properties, hardness as well as gel fraction and crosslink density of the vulcanized blends have been investigated. Results show that compared to the vulcanizates of the devulcanized samples, the vulcanizates of the blends show a remarkable improvement in properties, which are sometimes comparable to or even better than the virgin vulcanizate. Blends have also prepared using different proportions of ground samples and virgin rubber, and a comparison of properties between these blends and the blends of the devulcanized rubber has been carried out. The results show that compared to the ground samples, the blends of the devulcanized samples are easier to mix and exhibit enhanced and more uniform properties. Polym. Eng. Sci. 44:794–804, 2004. © 2004 Society of Plastics Engineers.     

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     

Continuous ultrasonic devulcanization of NR/SBR blends

The ultrasonic devulcanization of sulfur‐cured natural rubber (NR)/styrene–butadiene rubber (SBR) blends was studied with the goal of understanding the devulcanization of rubber vulcanizates in which two networks of different natures were present. Also, similarities and differences in the devulcanization behaviors of NR, SBR, and their blends were found. During the devulcanization of cured NR/SBR blends, we observed that, as for NR, the ultrasonic power consumption for 75/25 and 50/50 (w/w) NR/SBR blends passed through a maximum at 7.5 μm. For SBR and 25/75 (w/w) NR/SBR blends, the power consumption increased with increasing ultrasonic amplitude. The higher power consumption led to a higher degree of devulcanization. The crosslink densities of the devulcanized 25/75, 50/50, and 75/25 (w/w) NR/SBR blends were lower than those of the devulcanized NR and SBR, possibly because of the reduced degree of unsaturation. The tensile properties of the revulcanized blends were lower than those of the virgin vulcanized blends. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 160–168, 2002     

Properties of natural rubber vulcanizates containing mechanochemically devulcanized ground tire rubber

The devulcanization of ground tire rubber (GTR) was carried out with a self-designed pan-mill type mechanochemical reactor. Gel fraction and crosslink density measurements confirmed the occurrence of stress induced mechanochemical devulcanization of GTR. The partially devulcanized GTR (dGTR) was blended with virgin natural rubber (NR) at different ratios. The curing characteristics and mechanical properties of these composites were investigated and compared with those composites of raw ground tire rubber (rGTR) and NR. The results showed that the tensile properties of the dGTR/NR vulcanizates were much better than those of the rGTR/NR vulcanizates, which are comparable to or even better than the virgin vulcanizate, indicating the significant benefit of mechanochemical devulcanization. At the GTR content of 10%, the tensile strength of the dGTR/NR blends increased to 23.2 MPa from 13.7 MPa of the rGTR/NR blends, enhanced by 69% through partial devulcanization of GTR, and the elongation at break increased by 47%.     

Recycling of unfilled polyurethane rubber using high‐power ultrasound

This investigation deals with the recycling of polyurethane rubber by the application of high‐power ultrasound in a continuous ultrasonic coaxial reactor. The cured rubber has been devulcanized at various feed rates and various gap sizes and then revulcanized again with certain adjustments in the curing recipe. The die pressure and the total power consumption have been recorded as a function of the processing conditions. The rheological and mechanical properties, hardness, gel fraction, and crosslink density of the original, devulcanized, and revulcanized samples have been measured and compared in an attempt to determine the optimum condition for devulcanization. Gel permeation chromatography (GPC) has been carried out with the sol part of the devulcanized samples to study the devulcanization and degradation. The results show that at low flow rates and narrow gaps, the material is degraded very quickly and, therefore, exhibits very poor mechanical properties. However, increasing the feed rate results in an improvement of the mechanical properties. Measured values of the crosslink densities and gel fractions indicate the processing conditions under which greater devulcanization and degradation of the samples take place. The lower molecular weights of the sol, extracted from the devulcanized samples, obtained in the GPC experiments in comparison with polyurethane gum indicate a breakdown of the polymeric chains as a result of devulcanization. The devulcanized samples show a higher activation energy of viscous flow, possibly because of the formation of branched structures. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 980–989, 2003     

Devulcanization of Natural Rubber Vulcanizates by Mechanochemical Process

Using a suitable disulfide-based devulcanizing agent, which cleaved the sulfur cross-links in vulcanized rubber at high temperature, devulcanization of gum natural rubber was carried out. High sulfur and medium sulfur, as well as low sulfur–containing rubber vulcanizates were used to study the cleavage of sulfidic bonds. The cure characteristics and mechanical properties of vulcanized natural rubber and revulcanized natural rubber were studied. Thermal properties of the rubber were analyzed by thermogravimetric analysis (TGA), which indicates that the onset degradation temperature further increased on revulcanization with higher amount of disulfide. The properties of the revulcanized natural rubber increased with increasing disulfide concentration, also the mechanical properties of the devulcanized natural rubber were increased by decreasing the sulfur content in the original rubber vulcanizate. From the rheometric study increases in optimum cure time were observed when ground rubber vulcanizates were treated with higher amounts of disulfide. The scanning electronic microscopy (SEM) study suggested the change in failure mechanism as influenced by the type of cross-linking present and the devulcanizing agent used. From infrared (IR) spectroscopy it was observed that the oxidation of the main polymeric chain did not occur at the time of high temperature milling.     

EPDM rubber reclaim from devulcanized EPDM

Two types of ethylene–propylene–diene monomer (EPDM) rubbers, namely an efficient vulcanized (EV) and a semiefficient vulcanized (SEV), have been used to produce devulcanizates in a continuous setup. The devulcanizates are re‐cured using the same recipes as for the virgin rubber. The influence of mixing it with virgin rubber compound, the addition of extra sulfur, the operating devulcanization conditions, and the excess of devulcanizing agent on the mechanical properties (hardness, tensile strength, and compression set) of the reclaim rubbers are studied. Most of the reclaims produced show slightly inferior mechanical properties compared to the virgin rubber. Surface imperfection was observed on the devulcanizate with high devulcanizing agent content. Excellent mechanical properties (all above the standards) of the reclaim were found when the devulcanized profile material was used (EV‐EPDM) to replace the virgin one for application as a roofing sheet material (SEV‐EPDM). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5948–5957, 2006     

Effects of shear stress and subcritical water on devulcanization of styrene–butadiene rubber based ground tire rubber in a twin‐screw extruder

The devulcanization reaction of styrene–butadiene rubber (SBR) based ground tire rubber (GTR) in GTR/ethylene–propylene–diene monomer rubber (EPDM) blend was investigated through a compound‐induced reaction by increasing screw rotation speed and being in the presence of subcritical water. The effects of temperature, pressure, screw rotation speed, or promoting agents on the gel content, Mooney viscosity, and Fourier transform infrared spectra of the sol of the devulcanized blends (devulcanized ground tire rubber (DGTR)/EPDM) were measured, and the mechanical properties and microstructures of the revulcanized blend ((DGTR/EPDM)/SBR) were characterized. The results show that subcritical water as a swelling agent and reaction medium promotes the devulcanization reaction, increases the selectivity of the crosslink breakage, keeps the extrusion material from oxidative degradation, reduces the gel particle size of the devulcanized blends, and significantly improves the mechanical properties of the revulcanized SBR/(DGTR/EPDM) blends. In subcritical water, the suitable promoting agents (alkylphenol polysulfide 450, hydrogen peroxide H₂O₂, or 450/H₂O₂) accelerate the devulcanization reaction, keep the double bond content, and lead to further decrease of the gel content and Mooney viscosity of the devulcanized blends and further increase of the mechanical properties of the revulcanized SBR/(DGTR/EPDM) blends. Especially the compound promoting agent (450/H₂O₂) improves the selectivity of the crosslink breakage in devulcanization of SBR‐based GTR. When 450/H₂O₂ is added as a compound promoting agent at the best reaction condition in subcritical water (200°C, 1.6 MPa and 1000 rpm), the tensile strength and elongation at break of the revulcanized SBR/(DGTR/EPDM) blends reach to 85.4% and 201% of vulcanized SBR (24.0 MPa, 356%), respectively. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1845–1854, 2013     

单螺杆挤出机连续脱硫制备再生胶的结构与性能

利用本课题组研制的单螺杆挤出机,对废旧轮胎胶(GTR)粉进行连续脱硫制备再生胶(DGTR),并对DGTR进行再硫化制成DGTR硫化试样.结果显示,随螺杆转速增加,DGTR凝胶含量和DGTR硫化试样交联密度逐渐降低,DGTR硫化试样的拉伸强度和拉断伸长率有所提高;螺杆转速设定为40 r·min^-1时,在205℃脱硫温度下取得了最好的脱硫效果,DGTR硫化试样的拉伸强度和拉断伸长率分别约为11 MPa和386%.从红外光谱和光电子能谱的分析结果推测,在单螺杆挤出机的剪切和热等的共同作用下,GTR粉中部分S-S键和C-S键发生了断裂.     

Devulcanization of waste tires using a twin‐screw extruder: The effects of processing conditions

During recent decades, for both economic and environmental reasons, recycling of waste tires that are based on SBR/NR has been widely considered. In this study the devulcanization process of the tread section of waste tires was carried out by using a twin‐screw extruder. The effects of barrel temperature and screw speed were investigated. Percent of devulcanization, sol fraction, and curing behavior of devulcanized samples were studied. After the addition of curing agents into the devulcanizates, the general behavior of the rheometry test for rubber compounds was observed. Percent of devulcanization and sol fraction depended on the screw speed and barrel temperature, respectively. The devulcanized samples were formulated with virgin rubber (15/85 wt% ratio) and re‐cured successfully. Tensile strength, elongation at break, compression set, hardness, and resilience were evaluated. It was found that the mechanical properties of the compound containing devulcanizates were slightly inferior to those of the virgin compound. J. VINYL ADDIT. TECHNOL., 2011. © 2011 Society of Plastics Engineers     

Mechanochemical preparation of devulcanized ground fluoroelastomers for the enhancement of the thermal stability of nitrile–butadiene rubber vulcanizates

Mechanochemically devulcanized ground fluoroelastomer (FKM) was used as a low‐cost functional additive for the enhancement of the thermal stability of nitrile–butadiene rubber (NBR) vulcanizates. Without the use of any chemicals, the stress‐induced mechanochemical devulcanization of ground FKM was achieved through solid‐state mechanochemical milling at ambient temperature. The sol fraction of the ground FKM was increased from its original 1.4 to 19.8% after milling; this confirmed the realization of the mechanochemical devulcanization of FKM. Moreover, the oxygen‐containing polar groups on the surface of the mechanochemically milled FKM benefitted its interfacial adhesion with the polar NBR matrix. The curing characteristics and mechanical properties of the devulcanized, FKM‐filled NBR vulcanizates were investigated and compared with those of the untreated FKM‐filled NBR vulcanizates. The results show that the mechanical properties of the devulcanized FKM‐filled NBR vulcanizates were much better than those of the untreated FKM‐filled NBR vulcanizates. The presence of the reclaimed FKM significantly increased the onset degradation temperature of the NBR vulcanizates as a result of the improved polymer–filler interaction, uniform dispersion, and high thermal stability of the reclaimed FKM. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of processing parameters on the properties of microwave‐devulcanized ground tire rubber/polyethylene dynamically revulcanized blends

This work aimed to evaluate the effects of processing parameters on properties of dynamically revulcanized blends based on 60 wt % of devulcanized ground tire rubber (GTR) and 40 wt % of high density polyethylene (HDPE). Devulcanization of the GTR was carried out in a system comprised of a conventional microwave oven adapted with a motorized stirring system with speed control. The resulting devulcanized GTR contained 26 wt % soluble in toluene. It was processed with HDPE in a twin screw extruder to produce a dynamically revulcanized blend. Processing parameters such as screw speed and feeding mode were varied. Their effects were evaluated based on tensile, dynamic mechanical, thermal and rheological properties, as well as on morphology. The results show the importance of the processing parameters involved in the production of blends with dynamically revulcanized rubber phase in the extruder, and good match between the residence time of the rubber and its revulcanization kinetics. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43503.     

The role of devulcanizing agent for mechanochemical devulcanization of styrene butadiene rubber vulcanizate

More or less universally, bis(3‐triethoxysilyl propyl) tetrasulfide (TESPT) has been used as a coupling agent during dispersion of silica filler in a virgin nonpolar rubber compound. It is for the first time that TESPT has been used as a devulcanizing agent and as‐grown devulcanized rubber facilitates the silica dispersion in nonpolar rubber compound without any coupling agent. Dual functionalities of TESPT have been modeled and validated in this work. Various factors like the role of sol‐gel content, inherent viscosity of sol rubber, crosslink density, and degree of devulcanization were investigated as a function of devulcanization time and amount of TESPT to optimize devulcanization time and TESPT amount. To study the silica reinforcement, revulcanization of devulcanized SBR was carried out with silica filler and the curing characteristics of the material were evaluated. From the mechanical properties and thermogravimetry analysis the optimum time for devulcanization is determined. Further, scanning electron microscopy (SEM) studies were undertaken to check the coherency and homogeneity of the material. POLYM. ENG. SCI., 58:74–85, 2018. © 2017 Society of Plastics Engineers     

Reclaiming of ground rubber tire by a novel reclaiming agent. I. virgin natural rubber/reclaimed GRT vulcanizates

Mechanochemically devulcanized ground rubber tire (GRT) was (re)vulcanized in compositions with virgin natural rubber (NR). The NR/GRT (re)vulcanizates with GRT content from 20 to 60 wt% have been prepared and studied. Reclaiming of GRT was successfully carried out by tetra methyl thiuram disulfide (TMTD) in presence of spindle oil at around ambient temperature. The cure characteristics and mechanical properties of the virgin NR/reclaim GRT blend were studied. The optimum cure time decreases but scorch time remain unaltered with increasing reclaim rubber content in the blend. Effect of carbon black was studied in NR/RR (80/20) blend. Ageing characteristics of different NR/RR blends were evaluated. Scanning electron microscopy (SEM) studies further indicate the coherency and homogeneity in the NR/RR vulcanizate. The dynamic mechanical properties and swelling behavior of NR/RR blend vulcanizates were examined. The elastic and storage modulus of the NR/RR vulcanizates become improve with increasing reclaim rubber content. Electrical properties of NR/RR vulcanizates were also studied. POLYM. ENG. SCI., 47:1091–1100, 2007. © 2007 Society of Plastics Engineers     

A novel devulcanization technology for vulcanized natural rubber

Devulcanization of waste rubber poses a challenging environmental, economical, and disposal problem in the world because of their cross‐linked three‐dimensional network structure. Devulcanization of gum natural rubber vulcanizate containing three different sulfur/accelerator ratios was carried out in 90°C for 10 min with the help of open two‐roll cracker cum mixing mill in presence and absence of thiol acid as a devulcanizing agent. The vulcanizate properties markedly depended on devulcanizing agent and also devulcanization techniques. Revulcanized rubber obtained by devulcanizing with thiol acid offered better mechanical properties. Decrease in scorch time and increase in rheometric torque were observed for revulcanized rubber containing devulcanizing agent. The onset degradation temperature largely depended on presence of thiol acid. IR spectroscopic results revealed that the main polymeric chain did not oxidize at the time of milling above the room temperature. Increase in storage modulus and decrease in loss modulus were observed for revulcanized rubber from DMA study. The SEM was considered to study the failure mechanism and homogeneity of the vulcanizate. By adopting this devulcanization technique, more than 85% mechanical property of vulcanized natural rubber was retained. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 2831–2840, 2006