In the last few decades, the rubber industry has faced a major problem in finding a suitable way for dealing with the huge amount of rubber products, especially tires, at the end of their life. Millions of tons of used tires and similar products at the end of their limited service life are dumped in natural environments. The environmental problems created by waste rubber and discarded tires have become a serious issue. Many attempts have been made to recycle waste rubber for both environmental and economic reasons [1, 2]. Every year, approximately 3.3 million tons of used tires (at the end of their useful life) are generated in Europe (etrma.org) and approximately 180,000 tons in Turkey (lasder.org.tr). The recycling of discarded tires has some very important outcomes including environmental protection, energy savings and provision of raw materials. Therefore, the development of an efficient way to utilize rubber waste is an emergent economic and environmental task faced by the rubber industry worldwide [2, 3].
Rubber recycling is a difficult task because of the vulcanization reaction, which takes place in three-dimensional structures. After undergoing reactions, vulcanized rubber turns into an infusible and insoluble form and cannot be converted into other forms [4, 5]. The crosslinks in the main skeleton of the polymer, which form during the vulcanization process, turn the thermoplastic structures into thermosets that are impossible to reshape by heating. Therefore, physical and chemical treatments for waste tires are necessary to destroy the three-dimensional structure [6, 7].
A lot of study has revealed that ground tire rubber (GTR) recycling should target the production of thermoplastic elastomers, and its reuse in the rubber industry . Rubber recycling currently is receiving special attention, because other disposal methods, such as burning for energy generation, are dangerous for the environment . Some new physical and chemical devulcanization techniques for recycling are being developed to recycle used tires. However, all of these devulcanization methods are unable to produce a product that is similar to virgin rubber. Rubber recycling technologies such as mechanical [9, 10], mechanochemical [11–15], ultrasonic , chemical [17–19], microwave [3, 20–22] and biological methods  have been developed with the targets of higher product quality and percent yield. Devulcanization provided the advantage of rendering the rubber suitable for reformulating and recycling into usable products .
The microwave devulcanization technique is based on the application of a controlled amount of microwave energy to the material at a certain energy level that is sufficient to cleave carbon-sulfur (302 kJ/mol) and sulfur-sulfur bonds (273 kJ/mol), but insufficient to cleave carbon-carbon bonds, which have a higher bond energy (349 kJ/mol). In this method, tire waste can be reclaimed without depolymerization as a material that is capable of being recompounded and revulcanized and has physical properties essentially equivalent to the original vulcanizate [6, 25]. The use of microwaves for heating has the advantage of volumetric heating, which is faster and supports a more homogeneous heating than other conventional methods based on different heat transfer mechanisms, such as convection and conduction. However, one critical requirement of the microwave process is the presence of polar groups or components in the polymer. The presence of carbon black in nonpolar rubbers makes the rubbers receptive to microwave energy. This fact makes it possible to use the microwave heating to devulcanize waste tire rubber [7, 26].
The goals of this research were to obtain devulcanized rubber (DV-R) from waste tires and then to combine it with a styrene butadiene rubber (SBR) matrix to produce a new composite material. The curing characteristics, mechanical properties and morphology of these materials were investigated and compared with the characteristics of untreated GTR/SBR composites.
2 Materials and methods
2.1 Materials and characterization
The GTR used in this study was supplied by Un-sal Rubber Co. Ltd (Adana, Turkey). The GTR, which was prepared by grinding under ambient conditions, was an unclassified ground rubber from the treads and sidewalls of car, truck and bus tires. The original rubber, SBR 1502, was obtained from Petkim (Izmir, Turkey). Toluene was purchased from Merck (Germany). Sulfur, cyclohexyl benzothiazyl sulfenamide (CBS), stearic acid and zinc oxide (ZnO) for vulcanization studies were procured from local sources.
The particle size distribution of the GTR was characterized using 35, 60, 120 and 230 mesh sieves. The thermal characterization was performed by thermogravimetric analysis (TGA) to determine the composition and thermal degradation properties of the samples. The analysis was performed with a Perkin Elmer thermogravimetric analyzer under nitrogen from ambient temperature to 800°C with a heating step of 10°C/min.
2.2 Microwave treatment of GTR and characterization of DV-R
The GTR was treated in an adapted microwave apparatus consisting of a domestic-type microwave oven (Samsung MW71E) and a stirring apparatus. The power of the magnetron was set to 800 W, and 100 g of the GTR was put in a 1000 ml beaker and stirred at a speed of 6 rpm. The processing variable was the exposure time, which was varied from 1 to 5 min. The samples were identified as DV-R followed by a number corresponding to the exposure time in min. The temperature of the samples was measured after each treatment.
The sol content of the GTR and DV-R material was measured with a Soxhlet apparatus using toluene as solvent. The extraction was completed in 24 h, and it was made with 2 g of both GTR and DV-R. After the extraction, the sample was dried for 24 h at 70°C, and its weight was recorded. The percent sol content was calculated from the following equation :
where Wi is the weight of the sample before extraction and W is the weight of the dry rubber sample after extraction.
The chemical groups of GTR, before and after the sample was devulcanized, were analyzed with a Perkin Elmer Spektrum 400 [Fourier transform infrared spectroscopy (FTIR)] from 4000 to 450 cm-1 with a 4 cm-1 resolution and then the efficiency of devulcanization was determined.
2.3 Preparation of DVR and SBR composites
Virgin SBR, other additives and various proportions of the DV-R samples were mixed for 15 min on an open two-roll mixing mill at room temperature. The formulations of the composites are presented in Table 1. Formulation 1 does not contain DV-R and is a control sample. Formulations 2, 3 and 4 were prepared at 10, 30 and 50 phr DV-R. For comparison, untreated GTR was mixed with SBR in the same series. In the formulation of GTR/SBR composites, GTR was used as a filler. As DV-R was considered to be an elastomeric component, it was used as a component of raw rubber and not a filler. Because DV-R contains rubber and carbon black, the amounts of SBR and carbon black were adjusted in the formulations of the composites. The amounts of the other additives (ZnO, stearic acid and sulfur) in the formulations were based on 100 g of rubber without taking the DV-R into account, because it was assumed that the additives in the DV-R originated from the parent compound and were inactive .
The maximum sol content and suitable temperature (Table 2) were obtained in the samples that were exposed for 4 and 5 min but not the samples that were exposed for 1, 2 and 3 min. Therefore, DV-R/SBR composites were prepared by exposing the samples in the microwave for 4 and 5 min.
2.4 Characterization of DV-R/SBR composites
2.4.1 Cure characteristics
The cure characteristics of the rubber composites were investigated by using a rheometer, Beijing RADE MR-C3, at 170°C. From the rheographs, the minimum torque (ML), scorch time (ts2), maximum torque (MH) and optimum curing time (t90) were obtained. The ML is a measure of the rubber’s resistance to flow during processing, and the MH relates to the compound’s final stiffness. The number of min to reach 90% of the MH or t90 briefly expresses the cure time required to almost complete the vulcanization process. These samples were identified as R followed by two numbers corresponding to the exposure time in min and the DV-R content (1/10).
2.4.2 Measurement of mechanical properties
The mechanical properties of the DV-R/SBR and GTR/SBR composites were measured with a Shimadzu AG-IC tensile testing machine according to ASTM D 412 under laboratory conditions (23±2°C). The testing speed was 500 mm/min. Dumbbell-shaped specimens were punched from the compression-molded sheets, and the hardness (Shore A) of the new composites was measured with a Bareiss Shore meter according to ASTM D 2240. At least three samples for each composition were tested, and the average value is reported.
2.4.3 Determination of crosslink density
The chemical characteristics of the DV-R/SBR and GTR/SBR composites were investigated by determining their crosslink densities. The test specimens, which weighed approximately 0.2 g each, were immersed in a solvent containing 100 ml of toluene at room temperature (23°C) for 72 h. The excess solvent on the surfaces of the specimens was removed by drying with filter paper. The weights of the swollen specimens were measured, and then the specimens were dried to a constant weight. The crosslink densities of the composites were determined by the Flory-Rehner equation with the Kraus correction [28, 29]:
where ρc is the crosslink density of composite (mol/m3), Vs is the molar volume of the solvent (1.069×10-4 m3/mol), vr is the volume fractions of filled rubber in the swollen specimen and X is the rubber-solvent interaction parameter (0.3795 in this study ).
The morphology of the GTR surface and the fracture surfaces of the DV-R/SBR and GTR/SBR composites were evaluated with a ZEISS EVO LS 10 scanning electron microscope (SEM). The sample surfaces were sputter-coated with gold powder using a Sputter Coater Auto 108.
3 Results and discussion
3.1 Physical and thermal characterization of GTR
Figure 1 shows the particle size distribution of the GTR used in these experiments. Particle size analysis showed that the majority of the ground rubber particles were in the 35–120 mesh (0.5–0.125 mm) range. The particles in the size range higher than 120 mesh (smaller than 0.125 mm) were found in small quantities. It is well known that particles of GTR that are suitable for utilization in new formulations and for devulcanization processes should be smaller than 0.6 mm and have high surface areas [31–34].
An SEM photograph of representative GTR is shown in Figure 2. It was observed that the GTR particles had irregular shapes and their surfaces were rough.
The partial composition of the GTR was determined by TGA and results are given in Figure 3. The thermogram in Figure 3 shows that the first weight loss (13%) below 350°C corresponded to a loss of water and highly volatile materials, such as low molecular weight plasticizers, oils, waxes and antioxidants that were used for tire manufacture. The second weight loss, at 350–500°C, was a large weight change (52%), and it was assigned to the degradation of the polymers and moderately volatile materials related to the elastomers and curing agent. Finally, the last degradation phase, above 500°C when approximately 35% of the original weight of the GTR remained, corresponded to the decomposition of carbon black.
3.2 Characterization of DV-R
The temperatures of the samples after microwave exposure are shown in Table 2. The Soxhlet extraction results, including sol contents of the samples, are given in the same table. It can be observed that, as expected, the sample temperatures increased immediately after the microwave exposure and were almost linear with the time of exposure.
Soxhlet extraction evaluates only the fraction of soluble matter present in the sample, which is a good parameter to use to evaluate the efficiency of the devulcanization process [3, 7]. The initial sol content of the GTR was 2.30% and it rose to 25.36% and 34.77% after microwave devulcanization for 4 and 5 min, respectively. The higher sol content showed that the devulcanization process was more efficient. It can be observed that the time of treatment in the microwave has a significant influence on the sol content. This was expected because when the starting material was treated for longer periods of time, it reached higher temperatures and generated materials with higher sol contents. The sol content of the devulcanized GTR in this study was compared with the sol content obtained with different devulcanization methods in other references in Table 3.
To obtain information about the structure and functional groups after microwave devulcanization, FTIR spectra are presented in Figure 4, where the FTIR spectra of the DV-R (after microwave exposure times of 4 and 5 min) and GTR were compared. During the microwave devulcanization of GTR, the amount of energy released is utilized for breaking S-S bonds instead of C-C bonds, because the S-S bond energy is lower than the energies of various organic bonds. However, a few C-C and C-S bonds may break along with the S-S bonds at higher temperatures and after longer durations of heating. The devulcanization was carried out for a maximum of 5 min at 370±10°C (maximum temperature) and the rubber crosslinks can break easily without main chain scission. The peaks of the C-H stretching vibration (3000–2800 cm-1) and the -CH2 deformation (1447 cm-1 and 1372 cm-1) did not show significant differences between GTR and DV-R [23, 36]. The peak at 1538 cm-1 was assigned to the C-S bond, which represents untreated GTR. After the devulcanization process, the peak value of the C-S bond decreased (especially for the DV-R5 sample), which showed the devulcanization of the GTR during the microwave devulcanization.
3.3 Characterization of DV-R/SBR composites
3.3.1 Cure characteristics
The cure characteristics of the DV-R/SBR and GTR/SBR composites are given in Table 4. The DV-R content and microwave exposure time affected the curing behavior.
The effects of the DV-R content on the ML and the MH are shown in Figure 5A and B. The ML increased slightly with the addition of DV-R in all cases (4 and 5 min), but it increased remarkably with the addition of untreated GTR. This indicates firstly that there was some interaction between the DV-R and the virgin rubber and secondly that the processing of composites containing GTR was more difficult than for composites containing DV-R. The reason for the increase in the ML could be the agglomeration of waste rubber particles in the SBR matrix .
The MH, a measure of crosslink density, decreased slightly with the addition of DV-R and the microwave exposure time, but it increased with the addition of untreated GTR. The MH increased remarkably with the addition of 30 phr of GTR, and further additions of GTR resulted in a decrease in the torque. This relationship indicated that the elastic modulus was lower in the composites containing DV-R. The decrease in the value of the MH may be due to the presence of short rubber chains and crosslink precursors in the DV-R .
Figure 6A and B show the ts2 and optimum cure time (t90) as functions of DV-R content. The ts2 of the composites decreased slightly with an increase in both the GTR and DV-R concentrations. Furthermore, the ts2 of the DV-R/SBR composites are lower than those of the GTR/SBR composites. It is clear that there are more unsaturated rubber bonds in the DV-R than in the GTR. The shorter ts2 for the composites indicate that the crosslinking reactions start earlier in the vulcanization process. Ishiaku et al.  showed that a reaction between the SBR matrix and the DV-R can catalyze crosslinking. Diffusion of the accelerator from the DV-R into the virgin rubber can reduce the ts2 [27, 38]. De et al.  reported that the revulcanized rubber has active crosslinking sites, which lower the ts2.
The t90 is the vulcanization time required to obtain a product with optimum physical characteristics. The t90 of the composite decreased with an increase in the concentration of both the GTR and DV-R in the composite skeleton. Both ts2 and t90 decreased considerably with the addition of DV-R5 to the matrix, which demonstrates the presence of active functional sites in these composites.
3.3.2 Mechanical properties of composites
The DV-R content and the exposure time were found to have a strong, distinct effect on the mechanical properties of the DV-R/SBR and GTR/SBR composites. The mechanical properties of the SBR composites with different concentrations of GTR and DV-R are shown in Table 5. These composites tend to become weak and brittle with increasing concentrations of GTR and DV-R. The results in Table 5 show that the mechanical properties of the DV-R/SBR were far superior to the properties of SBR filled with GTR at the same loading.
The effects of the GTR and DV-R contents on the tensile strength and the elongation at break are shown in Figure 7A and B. With the addition of 10 phr of DV-R and GTR to the composite, the values of the tensile strength and elongation at break increased. These increases may be because the smaller DV-R and GTR particles act as reinforcing agents in the materials. This result was supported by prior work by Rooj et al. . However, the values of the tensile strength and elongation at break decreased for 30 and 50 phr loadings of both DVR and GTR. The deterioration of the mechanical properties of the GTR/SBR composites is related to the weak adhesion between the GTR particles and the SBR matrix. Because the GTR particles are not well-dispersed in the matrix of SBR, they are weak sites for stress-transmission and result in lower mechanical properties for the composite. The values of the properties for composites that contained DV-R were much better than for the composites with GTR. Because DV-R takes part in the crosslinking reaction, there is strong interfacial bonding between the SBR matrix and DV-R that leads to good tensile properties.
The tensile strength and elongation at break increased with the time of exposure to microwave power. The higher tensile strength and elongation at break may be due to the decrease in the number of crosslinked sites in the DV-R5/SBR and the strong interfacial bonds between the SBR matrix and DV-R, which transmit stress very well in the composite structure.
The effects of the GTR and DV-R content on the modulus at 100%, 200% and 300% elongations are presented in Figure 8A–C. The values of the modulus increased with increases in the GTR content, especially at 50 phr, indicating that the GTR loading in the rubber matrix enhanced the rigidity of the composite. The important increase in the modulus is attributed to the higher modulus of the GTR than the composite matrix; therefore, GTR acts like a rigid particulate filler that cannot be deformed easily. The addition of GTR to a soft composite matrix leads to the development of a local stretching in the composite matrix that exceeds the overall strain of the composite. The 300% modulus was not obtained for the GTR/SBR composites with 30 and 50 phr loadings. Hence, DV-R is comprised of a gel fraction, which has a higher modulus, and a sol fraction, which could be co-crosslinked with the SBR matrix, and the values of the modulus of the GTR/SBR composites were lower than the values that were obtained by loading GTR with DV-R5.
The hardness of the composites showed an increase with longer microwave exposure time and higher DV-R loading (Figure 9A). The increase in the hardness of the composites is most likely due to the increase in the crosslink density of the composites.
Figure 9B shows the variations of the crosslink densities of the composites as functions of DV-R content. Because the crosslink density of the DV-R was reduced by devulcanization, the crosslink density of the DV-R/SBR composites was a little lower than that of the GTR-filled SBR composites. The moderate increase in crosslink density of the composites with the addition of DV-R was due to the presence of active crosslinking sites in the DV-R, which may take part in crosslinking during the vulcanization process. It is proven that the crosslink density of the DV-R/SBR composites is lower than that of the GTR/SBR composites, which further confirmed that the devulcanization was accomplished by the microwave treatment (Figure 9B).
The changes in the mechanical properties of the virgin rubber and devulcanized GTR for the different devulcanization methods are compared in Table 6. In this table, we bring attention to the references that are related to the devulcanization of waste tire rubber.
SEM micrographs of tensile fractured surfaces of GTR/SBR and DV-R/SBR composites with 30 phr and various exposure times are shown in Figure 10A–C. The surfaces of SBR composites filled with the GTR exhibited a morphology of brittle fracture, which is evidence of inferior properties, especially elongation at break. There were some cracks, pores and many cavities on the fracture surfaces of the composites (Figure 10A). This morphology showed that the adhesion of GTR to the SBR matrix was poor. The weak adhesion results in damage or alteration of the mechanical properties, which is in agreement with the other conclusions from our experimental results. The exposure time also affected the fracture surface morphology. The fracture surface of the DV-R4/SBR composite was similar to the GTR filler/SBR composite (Figure 10B) because the adhesion was poor between the DV-R4/SBR composites. The tensile fractured surface of composites with 30 phr DV-R5 is shown in Figure 10C. Desirable adhesion exists between the DV-R filler and the SBR matrix and a crosslinking reaction occurs between the DV-R and the SBR matrix.
DV-R was produced by a microwave technique and then DV-R and untreated GTR were blended with virgin SBR in different proportions. A considerable increase in the sol fraction occurred during the devulcanization process. An FTIR study confirmed the microwave devulcanization of GTR. The sol content of the untreated GTR was 2.3%, and the sol content reached its maximum of 34.77% for DV-R5. The curing characteristics and mechanical properties of the DV-R/SBR composites were compared with the properties of the GTR/SBR composites. The ML increased and the MH decreased to a small extent with an increase in the DV-R, but both characteristics showed a remarkable increase with the addition of untreated GTR. It was a fact that the processability of the composites did not change meaningfully with the DV-R loadings. At 10 phr DV-R and GTR loadings of SBR composites, the tensile strength and elongation at break were improved in comparison to the virgin SBR. At higher concentrations of DV-R and GTR, these properties deteriorated. As a result of microwave devulcanization, the DV-R/SBR composites had better mechanical properties than the GTR-filled SBR composites at the same loadings. However, the elongation at break of the DV-R5/SBR composites decreased only 15.17% versus the control sample at 50 phr loading, while the elongation at break was 30.18% and 58.59% for DV-R4/SBR and GTR/SBR composites, respectively. This research has shown that the microwave treatment can be used as a convenient technique for rubber recycling studies.
The authors acknowledge the support of the Coordination Committee of Scientific Research Projects of Selcuk University, Turkey (Project no: 10201043).
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About the article
Published Online: 2014-05-01
Published in Print: 2014-08-01