Setting relationships between structure and devulcanization of ground tire rubber and their effect on self-healing elastomers

  Citation: Alonso Pastor, L.E.; Núñez Carrero, K.C.; Araujo-Morera, J.; Hernández Santana, M.; Pastor, J.M. Setting Relationships between Structure and Devulcanization of Ground Tire Rubber and Their Effect on Self-Healing Elastomers. Polymers 2022,14, 11. https://doi.org/ 10.3390/polym14010011 Academic Editors: Elnaz Esmizadeh and Ali Vahidifar Received: 19 November 2021 Accepted: 17 December 2021 Published: 21 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Setting Relationships between Structure and Devulcanization of Ground Tire Rubber and Their Effect on Self-Healing Elastomers Luis E. Alonso Pastor 1, Karina C. Núñez Carrero 2,* , Javier Araujo-Morera 3, Marianella Hernández Santana 3,* and JoséMaría Pastor 1,2 1 Department of Condensed Matter Physics, University of Valladolid, Paseo del Cauce, 47010 Valladolid, Spain; [email protected] (L.E.A.P.); [email protected] (J.M.P.) 2Foundation for Research and Development in Transport and Energy (CIDAUT), Parque Tecnológico de Boecillo, Plaza Vicente Aleixandre Campos 2, 47051 Valladolid, Spain 3Institute of Polymer Science and Technology (ICTP-CSIC), Juan de la Cierva 3, 28006 Madrid, Spain; [email protected] *Correspondence: [email protected] (K.C.N.C.); [email protected] (M.H.S.) Abstract: The use of devulcanized tire powder as an effective reinforcement in self-healing styrenebutadiene rubber (SBR) compounds has been investigated for the first time in this work. For this purpose, the evolution of the microstructure of the rubber from end-of-life tires (ELTs) was studied during granulation, grinding and devulcanization through an exhaustive characterization work in order to relate the final microstructure with the mechanical response of the repaired systems. Different morphologies (particle size distribution and specific surface area) obtained by cryogenic and water jet grinding processes, as well as different devulcanization techniques (thermo-mechanical, microwave, and thermo-chemical), were analyzed. The results demonstrated the key influence of the morphology of the ground tire rubber (GTR) on the obtained devulcanized products (dGTR). The predictions of the Horikx curves regarding the selectivity of the applied devulcanization processes were validated, thereby; a model of the microstructure of these materials was defined. This model made it possible to relate the morphology of GTR and dGTR with their activity as reinforcement in self-healing formulations. In this sense, higher specific surface area and percentage of free surface polymeric chains resulted in better mechanical performance and more effective healing. Such a strategy enabled an overall healing efficiency of more than 80% in terms of a real mechanical recovery (tensile strength and elongation at break), when adding 30 phr of dGTR. These results open a great opportunity to find the desired balance between the mechanical properties before and after self-repair, thus providing a high technological valorization to waste tires. Keywords: end-of-life tires (ELTs); ground tire rubber (GTR); grinding process; devulcanization; self-healing 1. Introduction Tires are mandatory products for the mobility of people and goods. However, these vital elements are not capable of being recycled due to their complex structures. Nowadays, the amount of waste tires discarded worldwide is approximately 800 million units, 10 million tons per year. If one considers that the amount of natural and synthetic rubber in tires is about 60%, 6 million tons of rubber from tires are disposed each year [ 1 , 2 ]. In this sense, giving a second life and valorization to end-of-life tires (ELTs) has become a global priority. Some of the previously considered solutions for the disposal of ELTs (e.g., landfilling and incineration) have become less and less viable because of the associated environmental problems and high recovery costs. Instead, the reuse and valorization of the constituent materials have become the preferential solution. Methods and technologies that transform Polymers 2022,14, 11. https://doi.org/10.3390/polym14010011 https://www.mdpi.com/journal/polymers Polymers 2022,14, 11 2 of 19 ELTs into raw materials primarily depend on the use of the tire (passenger cars, trucks, airplanes, etc.) since each application involves different rubber compounds. These technologies are grouped into three levels [ 3 – 5 ], following the circular economy model [ 6 ]. Level 1 includes the direct use of the tire, as well as mechanical treatments that destroy its structure (bead, sidewall or tread removal, cutting, compression, baling) [ 3 – 5 ]. Level 2 is made up of technologies that further reduce ELTs’ rubber size [ 3 – 5 ]. The resulting product, commonly called ground tire rubber (GTR), comes in different particle sizes in the form of chips (10–50 mm), granules (1–10 mm), and powder (<1 mm). There are three main technologies included in level 2: mechanical grinding, cryogenic grinding, and water jetting. Regarding GTR characteristics, there are significant differences between technologies. The GTR obtained with mechanical milling has a higher degree of oxidation and degradation caused by the large amount of heat generated during grinding; as the size is reduced, more damage is exerted to the rubber. Cryogenic GTR has a relatively smooth surface and a wide particle size distribution. GTR obtained with a water jet is porous and has a larger specific surface area compared to its cryoground peers [ 1 ]. In general, distinctions in the field of application are based on the particle size but not on the technology. The main uses of higher-size particles (chips and granules) include road and sport foundations, soil treatments, noise barriers, playground and sport surfaces, and footwear [ 4 , 7 ]. Meanwhile, powder is principally used as filler in rubberized asphalt [ 7 – 10 ], in building and construction and in concrete [ 7 , 9 , 11 , 12 ], as porous bitumen binders [ 4 ], in playground surfaces and trails for athletics [ 7 , 13 ] in automotive floor mats [ 7 ], and as filler for tires [ 4 ]. In addition, the outputs of level 2 are often used as feedstock for next-level processes. Level 3 mainly includes the pyrolysis and devulcanization [ 14 ] of ELT rubber. Pyrolysis refers to a thermal degradation in the absence of oxygen at a temperature range of 400–800 ◦ C. The valuable products generated are pyrolysis gas, oil, and char (which consists of carbon black (CB), ashes, and inorganic particulates) [ 5 ]. Meanwhile, devulcanization is, by definition, the rupture of the sulfur–sulfur (S–S) and/or carbon–sulfur (C–S) bonds that constitute the three-dimensional structure formed during the vulcanization of the rubber [ 1 , 13 , 15 ]. However, this definition is somehow misleading, since it is commonly used to describe any process that aims to obtain a rubber compound that can be processed and vulcanized similarly to pristine rubber, regardless of whether the crosslinks or the main chain are broken. Currently, devulcanization is being approached by thermal, mechanical [ 16 ], chemical [ 17 ], biological, microwave [ 18 ], and ultrasonic techniques, as well as their combinations (e.g., thermo-chemical, thermo-mechanical, and mechano-chemical) [ 5 , 19 – 21 ]. Devulcanized products are very diverse and mainly distinguished by having different degrees of network breakage with the subsequent crosslink rupture and main chain scissions. Devulcanized GTR (dGTR) is primarily used to obtain thermoplastic elastomers (TPEs) [ 22 ], to obtain automotive parts, and as a secondary ingredient for tires [ 4 ]. However, the balance between network breakage and backbone chain scissions has an important effect on the final performance of the rubber product [1]. The main current challenge is related to improving the efficiency of the abovementioned technologies and to transforming ELTs into new raw materials and products with high added value. In line with this objective, research in which, for example, recycled fibers from car tire waste were used to successfully develop a rubber aerogel, has been carried out. Aerogels are ideal for many high-value applications such as drug-delivery pharmaceuticals, filters for pollutants, and building insulation materials [ 23 – 25 ]. It has also been discovered that tire pyrolysis produces value-added products such as tire pyrolysis oil (TPO) and char. TPO can be used as precursor in the synthesis of carbon nanotubes, and char can be converted to porous carbon structures that can be used as adsorption and energy-storage materials such as supercapacitors and batteries [ 26 ]. One previous work of the authors also contributed to the aforementioned objective, where GTR was used as effective reinforcement in self-healing materials [ 27 ]. In contrast to “classical” materials, self-healing materials are those capable of partially/completely restoring their initial properties and/or functionalities without significant human intervention [ 10 ]. There Polymers 2022,14, 11 3 of 19 are basically two ways to develop self-healing polymers [ 28 – 31 ]. The first is based on the integration of discrete containers (capsules, fibers, or vascular networks) loaded with active components into the matrix material. When damage occurs, the containers break and release the healing agent to repair the damage. This is the so-called extrinsic concept. The second approach concerns the development of so-called intrinsically self-healing materials, that is, materials containing dynamic bonds that can restore their chemical or physical connections after damage under the influence of a non-disruptive external stimulus. A number of dynamic bonds have been shown to enable healing: Diels–Alder (DA) and retro-Diels–Alder (retro-DA) based bonds; hydrogen bonds in supramolecular networks; coordination complexes; disulfide-based chemistries; among others. However, these materials face a trade-off between mechanical performance and repairability—the higher the repairability, the lower the mechanical performance. One possible solution to overcome this trade-off is by reinforcing them. Hence, in this study, styrene-butadiene rubber (SBR) compounds that combine self-healing properties and good mechanical performance were developed thanks to the use of GTR as reinforcement. Contrary to analogue CB compounds, the mechanical performance of the self-healing rubber was enhanced (up to 80%) without adversely affecting the healing efficiency, resolving the problem of achieving both good reparability and good mechanical performance [27]. The work presented here is part of an ongoing research line that aims to develop elastomeric systems that can combine self-healing properties with the use of ELT rubber by acting on the tire rubber compound and using devulcanized rubber as reinforcement in self-healing matrices. We report the systematic and comparative microstructural analyses of diverse ground tire rubber (GTR) and devulcanized ground tire rubber (dGTR) originating from end-of-life truck tires (mainly composed of natural rubber (NR)) and obtained by different methods and technologies, thus enabling the definition of a structural model related to the selectivity of the applied devulcanization process. This is intended to demonstrates the possibilities of both the secondary raw material and the available recycling levels (i.e., level 2: grinding and level 3: devulcanization) used to valorize rubber from ELTs. Accordingly, self-healing SBR compounds filled with GTR and dGTR were prepared and characterized. These results were used to establish a relationship between the microstructures of GTR and dGTR and the self-healing efficiency and mechanical performance of SBR compounds. Finally, it is expected that this work will contribute to the perception of the recycling of ELTs as not only a benefit for the environment and the economy but also an opportunity for scientific-technological progress. 2. Materials and Methods 2.1. Materials 2.1.1. Ground Tire Rubber (GTR) Rubber granules from end-of-life truck tires were used. Two grinding technologies were further employed: cryogenic grinding and water jetting. Lehigh Technologies supplied cryogenic GTR (GTR-Cryo) and Rubber Jet supplied the GTR obtained with water jet technology (GTR-WJ), both within the same particle size range. Details on the particle size and composition of the as-received departure material (granules) and the resulting ground material (powder) are reported in Supplementary Material Table S1. 2.1.2. Devulcanized Ground Tire Rubber (dGTR) Three robust devulcanization techniques, based on different principles, were chosen to devulcanize the GTR powder samples. The procedures are described below. Thermo-Mechanical (TM) Devulcanization TM devulcanization was carried out in an internal mixer (Thermo Electron Corporation, Karlsruhe, Germany) with a filling volume of 70%, using Banbury-type rotors at room temperature and a rotor speed of 30 rpm for 10 min. Although the equipment was Polymers 2022,14, 11 4 of 19 initially at room temperature, friction heated the material. As a result, the average process temperature was 70 ◦C. Microwave (MW) Devulcanization Microwave devulcanization was carried out in a microwave prototype [ 32 , 33 ]. Samples of 30 g were used. Devulcanization was performed at 700 W and 80 rpm/min, with an exposure time of 6 min. Mechano-Chemical (CH) Devulcanization Bis(3-triethoxysilyl propyl) tetrasulfide (TESPT) was used as devulcanizing agent following previously reported procedures [ 17 , 34 , 35 ]. GTR was extensively mixed with TESPT at a ratio of 6 mL of TESPT/100 g of GTR and subsequently soaked for 24 h prior to devulcanization. Then, the mixture was transferred to an internal mixer (Thermo Electron Corporation, Karlsruhe, Germany)) using Banbury-type rotors and a filling volume of 70%. The device was at room temperature at the beginning, the process time was 40 min, and the average temperature of the material was 67 ◦C. 2.1.3. Self-Healing SBR Compounds Styrene-butadiene rubber (E-SBR Europrene 1502) and commercial-grade vulcanizing additives supplied by Sigma-Aldrich (Burlington, MA, USA) were used as-received. Table 1 compiles all the prepared formulations. Table 1. SBR compounds recipes in phr (parts per hundred parts of rubber). Ingredient (phr) Compound SBR SBR/GTR- Cryo SBR/GTRWJ SBR/dGTR- Cryo SBR/dGTR- WJ SBR 100 100 100 100 100 ZnO 5 5 5 5 5 SA11111 CBS 1 1 1 1 1 S11111 GTR 30 30 dGTR 30 30 Mixing was performed in an open two-roll mill (Comerio Ercole, Busto Arsizio, Italy) at room temperature using a rotor speed ratio of 1:1.5. First, rubber was passed through the rolls until a band was formed. The activating complex (zinc oxide (ZnO) and stearic acid (SA)) and filler (GTR or dGTR powder) were then progressively added to the rubber; finally, the curatives (N-cyclohexylbenzothiazole-2-sulphenamide (CBS) and sulfur (S)) were added. The crosslinking process was followed using a Rubber Process Analyzer (Alpha Technologies, Bellingham, WA, USA) at curing temperature T c = 160 ◦ C, frequency of 0.833 Hz , and 2.79% strain for 60 min. The composites were then vulcanized in an electrically heated hydraulic press (Gumix, Fort Lee, NJ, USA) at 160 ◦ C and 200 MPa according to their t 90 , as derived from the corresponding curing curves (see Supplementary Material Table S2 ). Samples were cut out from press-cured sheets to perform all the characterization and testing. 2.2. Characterization 2.2.1. GTR and dGTR Characterization Sol Fraction About 5 g of GTR or dGTR were extracted (Soxhlet extraction) in acetone for 24 h and subsequently extracted in toluene for 72 h. After extraction, each sample was dried at Polymers 2022,14, 11 5 of 19 45 ◦C until a constant weight was reached. The sol fraction for each of the extractants was calculated with Equation (1). Sol (%)= 1−Wgel Wsample !·100 (1) where Wgel is the weight of the extracted sample and Wsample is the weight of the sample before extraction. The sol fraction was defined as the sum of the soluble fractions in acetone and toluene. Crosslink Density The crosslink density ( ν ) in the mass-basis form, which is the number of moles of sulfur crosslinks per unit mass of rubber, was determined through equilibrium swelling experiments. About 0.2 g of the acetone-extracted powder sample were placed in toluene at room temperature and in darkness—to avoid molecular changes—for 72 h. Analyzing samples in powder form without any additional processing avoids possible alterations of the crosslink density [ 33 ]. The toluene was refreshed every 24 h in order to ensure equilibrium swelling. The swollen samples were taken out from the solvent, carefully removing any solvent excess, and then weighed again. After that, samples were dried at 45 ◦C until a constant weight (~48 h). Crosslink density was calculated using the Flory–Rehner equation [ 36 ] considering tetra-functional crosslinks; Equation (2). For further details, see Supplementary Material S3. v=−1 2·ρr·Vs·ln(1−Vr)+Vr+χ·Vr2 Vr1/3 −Vr/2(2) Horikx Plots M.M. Horikx derived a theoretical relationship between the soluble fraction generated after the degradation of a polymer network and the relative decrease in crosslink density as a result of either main-chain scission or crosslink breakage [ 37 ]. The application of the Horikx approach to evaluate devulcanization was experimentally verified by Verbruggen [ 38 ] and Seghar [ 39 ] in different ways. According to Horikx, when only main chain scission takes place, the relative decrease in crosslink density is given by Equation (3): 1−vf vi =1−1−√sf2 1−√si2(3) where vi is the crosslink density of the untreated vulcanizate, vf is the crosslink density of the vulcanizate after treatment, si is the soluble fraction of the untreated vulcanizate, and sfis the soluble fraction after treating the vulcanizate. On the other hand, when only crosslink breakage takes place, the soluble fraction is related to the relative decrease in crosslink density by Equation (4): 1−vf vi =1− γf1−√sf2 γi1−√si2(4) where the new parameters γi and γf are the average numbers of crosslinked units per chain before and after treatment, respectively. Horikx plots are a representation of sf as a function of the relative decrease in the crosslink density, defined as: Relative decrease in crosslink density =1−vf vi (5) Polymers 2022,14, 11 6 of 19 Horikx theoretical curves were drawn using Equations (3) and (4). For this, the initial soluble fraction si was determined by the swelling test according to Equation (1), sf varied between si and 1, and the crosslink indexes γi and γf were calculated from sol fraction by Equation (6), as described by Verbruggen et al. [38]. s=(2+γ)−γ2+4γ1/2 2γ(6) The values of the treated vulcanizates were then plotted on the graph, and their positions with respect to the theoretical lines was evaluated. The quantitative values of the network rupture and the selectivity of the devulcanization process could be estimated from the Horikx diagrams via the calculation of the percentage of devulcanization according to the methodology proposed by Edwards et al. [ 40 ], as detailed in Supplementary Material, S4. Thermogravimetric Analysis (TGA) Thermogravimetric curves were obtained using a thermal analyzer (Mettler Toledo, Columbus, OH, USA). Samples of ~10 mg were heated from 25 to 600 ◦ C under a nitrogen atmosphere (inert medium) and in air (oxidant medium) from 600 to 1000 ◦ C at a heating rate of 10 ◦C/min. Scanning Electron Microscopy (SEM) The morphological analysis of the GTR and dGTR powder and SBR compounds was achieved with scanning electron microscopy (Hitachi, Chiyoda, Tokyo, Japan). Samples were sputter-coated with gold–palladium prior to observation. Particle Size Distribution GTR and dGTR powder (~0.05 g) were previously dispersed in 20 mL of a water/ethanol 70/30 solution with 0.2 mL of the surfactant Triton X-100. The suspension was sonicated in an ultrasound bath (Elmasonic, Singen, Germany) for about 2 h. The particle size distribution was obtained by means of a laser scattering particle size distribution analyzer (Coulter, Barcelona, Spain). A volume-standard cumulative distribution was measured under stabilized conditions. Each sample was subjected to a 60 s optical measurement. BET Surface Area The BET surface areas of GTR powder were determined by nitrogen volume adsorption at − 196 ◦ C using a surface area and porosity analyzer (Micromeritics, Norcross, Georgia, USA). GTR was previously vacuum dried at 80 ◦C. X-ray Photoelectron Spectroscopy X-ray photoelectron measurements were performed on the surfaces of the GTR samples using a spectrometer (Fison Instruments, Ipswich, United Kingdom) equipped with a hemispherical electron analyzer (CLAM 2) and an Mg K α X-ray source (1253.6 eV) operated at 300 W. Binding energies were corrected to the carbon 1s peak located at 285 eV. Fourier Infrared Spectroscopy–Attenuated Total Reflectance (FTIR–ATR) The FTIR–ATR spectra of dGTR and the products extracted with acetone and toluene were obtained using a Tensor 27 model Bunker spectrometer. The gel fractions were analyzed. Spectra were normalized to the SiO 2 signal [ 33 ], and the relevant signals were analyzed [33,41] (see Supplementary Material Figure S5). Polymers 2022,14, 11 7 of 19 2.2.2. Self-Healing Rubber Compounds Characterization Tensile Testing Dog-bone shape specimens (Type 2, UNE-ISO 37) were used for uniaxial tensile testing. Tests were done on a universal mechanical testing machine (Instron, Norwood, MA, USA) equipped with a 1 kN load cell. Samples were stretched until failure at a constant crosshead speed of 200 mm/min at room temperature. Stress at break (ultimate stress) and strain at break (ultimate strain) were determined in order to mechanically characterize the SBR compounds. Healing Protocol Dog-bone specimens were manually cut in the center with the aid of a razor blade, thus creating a proper joining area. In order to heal the specimens, the two separated parts were carefully repositioned together and fastened with clamps. Then, they were placed in an oven at 130 ◦ C for 1 h. These conditions were selected as optimal after evaluating different healing protocols and using 70 ◦ C and 7 h as departing conditions [ 42 ]. The thermally treated specimens were subjected to a tensile test with the abovementioned testing conditions. Healing efficiency (η) was calculated by Equation (7): η(%)=PHealed PPristine ·100 (7) where PHealed and PPristine are the property of interest (tensile strength or elongation at break) of the healed and pristine specimen, respectively, determined under the same test conditions. 3. Results and Discussion The results derived from this research are divided in three sections. In the first section, we discuss the effect of the grinding technology (cryogrinding and water jet) on the microstructure of GTR. The second section is devoted to evaluating systematically various devulcanization techniques on different GTR samples and discussing their effectiveness towards selective devulcanization. In the last section, the incorporation of GTR and dGTR into a self-healing SBR compound is discussed, establishing proper relationships between the healing capability and the optimal and selective devulcanization technology. 3.1. Effect of Grinding Technology on the Microstructure of GTR TGA was conducted to study the effect of the grinding technology on the thermal stability of GTR and its composition (rubbers, fillers, ash, etc.). In Figure 1a, one can see three main losses that correspond to natural rubber (NR) (1st loss); a mixture of styrenebutadiene rubber (SBR), butadiene rubber (BR) (2nd loss), and carbon black (CB) (3rd loss) , as previously reported by the authors [ 27 ]. It should be noted that GTR with a high NR content was purposely selected because this type of rubber more easily devulcanizes than SBR. When SBR is the major component of GTR, the network is more stable and less prone to devulcanize [ 1 ]. By looking at the first two losses, one can also notice that both industrial technologies (cryogrinding and water jet) did not seem to affect the degradation and relative content ratio of the resulting GTR powder. On the other hand, differences were observed in the third loss. A further analysis was carried out with XPS, which is discussed in Section 3.2. Polymers 2022,14, 11 8 of 19 Polymers 2022, 13, x FOR PEER REVIEW 8 of 21 Regarding the morphology of both GTR, SEM micrographs (Figure 1b) show important differences between the two technologies, as expected. The cryogenic technology seemed to produce bigger particles with a smooth surface; meanwhile, water jet powder seemed to be composed of more irregular particles with a broader size distribution. BET measurements and laser scattering measurements confirmed this observation, as seen in Figure 1c and Table 2. Other authors have found equivalent results [1,13,43]. Figure 1. (a) DTG curves, (b) SEM micrographs, and (c) particle size distribution of the GTR obtained with cryogenic and water jet technologies. Figure 1. ( a ) DTG curves, ( b ) SEM micrographs, and ( c ) particle size distribution of the GTR obtained with cryogenic and water jet technologies. Regarding the morphology of both GTR, SEM micrographs (Figure 1b) show important differences between the two technologies, as expected. The cryogenic technology seemed to produce bigger particles with a smooth surface; meanwhile, water jet powder seemed to be composed of more irregular particles with a broader size distribution. BET measurements Polymers 2022,14, 11 9 of 19 and laser scattering measurements confirmed this observation, as seen in Figure 1c and Table 2. Other authors have found equivalent results [1,13,43]. Table 2. Average particle size, BET surface, and surface relative element content of the GTR obtained with cryogenic and water jet technologies. Powder Sample GTR-Cryo GTR-WJ Average particle size (µm) 317 (6) 193 (33) BET surface area (m2/g) 0.0209 0.1696 Element content (%) C 82.37 88 O 13.59 10.67 Si 4.04 1.33 O/C 0.16 0.12 The chemical composition of the GTR surface was also investigated by means of XPS. The C 1s and O 1s core spectra of GTR-Cryo and GTR-WJ are shown in Figure 2. The deconvolution of the C 1s shows the characteristic peaks at binding energies of 284.5 eV (C=C), 285 eV (C–H), and 286.5 eV (C–OH) [ 6 ], with no noticeable differences between the two grinding methods. Meanwhile, the O 1s can be deconvoluted into two peaks related to double (O=C) and single (O–C) bonds at 530 and at 532.4 eV, respectively [ 44 , 45 ]. The intensity of both contributions seemed higher for GTR-Cryo, assuming that more oxygenated groups were present in this ground powder. The high compressive shear stress during the cryogrinding process could have generated active chains that could have subsequently been converted into oxidation products. Data in Table 2show the relative element content (carbon (C), oxygen (O), and silicon (Si)) in each GTR. The higher O content in GTR-Cryo and the higher O/C ratio are evidence of the slight oxidation process occurring during the cryogrinding. As is discussed in the next sections, the morphology and structure of both GTR samples and their dispersion in the rubber matrix play decisive roles for achieving good mechanical and healing performance. Polymers 2022, 13, x FOR PEER REVIEW 10 of 21 Figure 2. XPS C and O core spectra of: (a,b) GTR-Cryo and (c,d) GTR-WJ. 3.2. Comparison of Different Devulcanization Processes of GTR and Their Effect on Crosslink Breakage Selectivity Three devulcanization technologies based on different principles—thermo-mechan- ical (TM), microwave (MW), and mechano-chemical (CH)—were applied to the GTR studied in the previous section. Based on the trigger (temperature, shear forces, and radiation), it is possible to excite the atoms to enable the vibration of bonds and their rupture depending on the bond energy (S–S, 268 kJ/mol; C–S, 285 kJ/mol; C–C, 346 kJ/mol) [46]. Figure 3 shows a comparison of the properties of the devulcanized powder (dGTR) after applying the abovementioned techniques. Special attention has been paid to the soluble fractions extracted in both acetone and toluene, associated with free surface short and long chains, respectively (Standard ASTM D297-93), in order to correlate them with the resulting microstructures. Figure 2. XPS C and O core spectra of: (a,b) GTR-Cryo and (c,d) GTR-WJ. Polymers 2022,14, 11 16 of 19 Polymers 2022, 13, x FOR PEER REVIEW 18 of 21 Figure 7. (a) Healing efficiency of SBR compounds with 30 phr of GTR/dGTR. (b) Overall healing performance of SBR compounds. The symbol size is properly scaled according to the recovery of the maximum strain. 4. Conclusions This study explored the effect of free polymeric chains on the surface of partially devulcanized recycled tire powder (from end-of-life truck tires) in self-healing SBR compounds. The importance of understanding the evolution of the microstructure during the different levels of the recycling of ELTs (i.e., grinding (GTR) and devulcanization (dGTR)) has also been demonstrated via the establishment of relationships between particle shape, composition, and specific surface area with the efficiency of various devulcanization processes. First, we found that the microstructure of the departing GTR influenced the obtainment of devulcanized products (dGTR), affecting the decrease in the network density and selectivity. Secondly, the thermo-mechanical devulcanization (TM) provided the highest amount of free long chains on the surface of the ground powder, regardless of the type of departing grinding method (cryogenic or WJ). This devulcanization technique also proved to be the most selective crosslink scission method when dealing with end-of-life truck tires. A selected content (30 phr) of GTR and TM-dGTR particles was further added to a self-healing SBR compound. The morphological and dynamic features of the grains suggested that the key factors leading to the recovery of the mechanical properties are the high percentage of free surface polymeric chains and high devulcanization selectivity, achieving a healing efficiency of more than 80% based on the recovery of both stress and strain. In conclusion, the research discussed here gathered information at different levels (molecular and micro/macro), and it presents a good phenomenological approach for a better understanding of the underlying healing mechanism taking place in elastomeric materials. Future work will deal with the optimization of devulcanized particle content, as well as the incorporation of GTR/dGTR in filled SBR compounds, in the search of better mechanical properties to be scaled up to real-life applications. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1. Table S1: Composition and particle size of as-received rubber granules and GTR powder, reported by the supplier; Table S2: Data derived from the curing curves and crosslink density of SBR compounds; S3: Crosslink density, S4: Quantification of selectivity parameter from Horikx plots, S5. (a) FTIRATR spectra of dGTR_WJ-TM and the extracted products (gel fractions). Zoom on the regions of Figure 7. ( a ) Healing efficiency of SBR compounds with 30 phr of GTR/dGTR. ( b ) Overall healing performance of SBR compounds. The symbol size is properly scaled according to the recovery of the maximum strain. The second aspect worth analyzing is the effect of the devulcanization on the healing capability of SBR compounds. From Figure 7a, one can confirm that such a process favored healing in this study. During the TM devulcanization, the selective homolytic scission of S–S took place, thus enabling viscous flow and enhancing the mobility of free short and long polymer chains. Consequently, the interdiffusion of rubber chains and the rearrangement of broken, reversible S–S bonds at the healed interface were favored. If we correlate this behavior with the structural model, one can state that the more selectively devulcanized the material (dGTR-Cryo), the higher mechanical recovery one can achieve in terms of tensile strength. Meanwhile, if we analyze the recovery in terms of elongation at break, the dGTR- WJ showed the highest value. Tensile testing is a standard technique for the determination of self-healing efficiency [ 55 ]. The most commonly used parameter is the maximum load at failure of the specimen (tensile strength). However, healing in terms of maximum deformation is of paramount importance in the field of elastomeric materials. The recovery beyond low strains indicates that significant load transfer from the damaged/repaired zone to the bulk occurred, leading to higher deformation before failure. In this sense, the SBR/dGTR-WJ compound seems to be the principal choice showing the best overall healing performance in terms of a real mechanical recovery (stress and strain). Figure 7b illustrates a very good representation of these results. 4. Conclusions This study explored the effect of free polymeric chains on the surface of partially devulcanized recycled tire powder (from end-of-life truck tires) in self-healing SBR compounds. The importance of understanding the evolution of the microstructure during the different levels of the recycling of ELTs (i.e., grinding (GTR) and devulcanization (dGTR)) has also been demonstrated via the establishment of relationships between particle shape, composition, and specific surface area with the efficiency of various devulcanization processes. First, we found that the microstructure of the departing GTR influenced the obtainment of devulcanized products (dGTR), affecting the decrease in the network density and selectivity. Secondly, the thermo-mechanical devulcanization (TM) provided the highest amount of free long chains on the surface of the ground powder, regardless of the type of departing Polymers 2022,14, 11 17 of 19 grinding method (cryogenic or WJ). This devulcanization technique also proved to be the most selective crosslink scission method when dealing with end-of-life truck tires. A selected content (30 phr) of GTR and TM-dGTR particles was further added to a self-healing SBR compound. The morphological and dynamic features of the grains suggested that the key factors leading to the recovery of the mechanical properties are the high percentage of free surface polymeric chains and high devulcanization selectivity, achieving a healing efficiency of more than 80% based on the recovery of both stress and strain. In conclusion, the research discussed here gathered information at different levels (molecular and micro/macro), and it presents a good phenomenological approach for a better understanding of the underlying healing mechanism taking place in elastomeric materials. Future work will deal with the optimization of devulcanized particle content, as well as the incorporation of GTR/dGTR in filled SBR compounds, in the search of better mechanical properties to be scaled up to real-life applications. Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/polym14010011/s1. Table S1: Composition and particle size of as-received rubber granules and GTR powder, reported by the supplier; Table S2: Data derived from the curing curves and crosslink density of SBR compounds; S3: Crosslink density, S4: Quantification of selectivity parameter from Horikx plots, Figure S5. (a) FTIR-ATR spectra of dGTR_WJ-TM and the extracted products (gel fractions). Zoom on the regions of interest: (b) C–H and (c) C=C signals, Figure S6. Particle size distribution of GTR and dGTR from cryogrinding and water jet technologies. Author Contributions: L.E.A.P.: conceptualization, methodology, formal analysis, investigation, data curation, writing—original draft, and writing—review and editing; K.C.N.C.: conceptualization, formal analysis, resources, data curation, writing—original draft, writing—review and editing, visualization, and supervision; J.A.-M.: investigation and data curation; M.H.S.: conceptualization, formal analysis, resources, data curation, writing—original draft, writing—review and editing, visualization, supervision, project administration, and funding acquisition; J.M.P.: resources, supervision, project administration, and funding acquisition. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the State Research Agency of Spain (AEI) through grants RYC2017-22837 (M.H.S.) and PRE2018-084732 (L.E.A.P.). Furthermore, this work is framed within the OSIRIS Network, which is funded by the Ministerio de Ciencia e Innovación and the CDTI through the 2020 call for grants to Technology Centres of Excellence “Cervera” (CER-2021 1009). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available on request from the corresponding author, M.H.S. Acknowledgments: All authors acknowledge Lehigh Technologies and Rubber Jet for kindly providing GTR samples. The authors also acknowledge Xavier Colom Fajula from Universitat Politècnica de Catalunya (Barcelona, Spain) for providing MW devulcanized samples. Conflicts of Interest: The authors declare no conflict of interest. References 1. Asaro, L.; Gratton, M.; Seghar, S.; Aït Hocine, N. Recycling of rubber wastes by devulcanization. Resour. Conserv. Recycl. 2018 , 133, 250–262. [CrossRef] 2. Zhang, X.; Lu, C.; Liang, M. Properties of natural rubber vulcanizates containing mechanochemically devulcanized ground tire rubber. J. Polym. Res. 2009,16, 411–419. [CrossRef] 3. Shulman, V.L. Tire Recycling. In Waste; Shulman, V.L., Ed.; Elsevier: Amsterdam, The Netherlands, 2019; pp. 489–515. ISBN 9780123814753. 4. Shulman, V.L. Introduction to Tyre Recycling: Twenty-Five Years of Tyre Recycling in the EU; Shulman, V.L., Ed.; European Tyre Recycling Association (ETRA): Brussels, Belgium, 2018. 5. Saputra, R.; Walvekar, R.; Khalid, M.; Mubarak, N.M.; Sillanpää, M. Current progress in waste tire rubber devulcanization. Chemosphere 2021,265, 129033. [CrossRef] Polymers 2022,14, 11 18 of 19 6. Araujo-Morera, J.; Verdejo, R.; López-Manchado, M.A.; Hernández Santana, M. Sustainable mobility: The route of tires through the circular economy model. Waste Manag. 2021,126, 309–322. [CrossRef] [PubMed] 7. Nuzaimah, M.; Sapuan, S.M.; Nadlene, R.; Jawaid, M. Recycling of waste rubber as fillers: A review. IOP Conf. Ser. Mater. Sci. Eng. 2018,368, 012016. [CrossRef] 8. Lo Presti, D. Recycled Tyre Rubber Modified Bitumens for road asphalt mixtures: A literature review. Constr. Build. Mater. 2013 , 49, 863–881. [CrossRef] 9. Shu, X.; Huang, B. Recycling of waste tire rubber in asphalt and portland cement concrete: An overview. Constr. Build. Mater. 2014,67, 217–224. [CrossRef] 10. Utrera-Barrios, S.; Verdejo, R.; López-Manchado, M.A.; Hernández Santana, M. Evolution of self-healing elastomers, from extrinsic to combined intrinsic mechanisms: A review. Mater. Horizons 2020,7, 2882–2902. [CrossRef] 11. Moustafa, A.; ElGawady, M.A. Mechanical properties of high strength concrete with scrap tire rubber. Constr. Build. Mater. 2015 , 93, 249–256. [CrossRef] 12. Sousa, S.P.B.; Ribeiro, M.C.S.; Cruz, E.M.; Barrera, G.M.; Ferreira, A.J.M. Mechanical behaviour analysis of polyester polymer mortars reinforced with tire rubber fibres. Ciência Tecnol. dos Mater. 2017,29, e162–e166. [CrossRef] 13. Ramarad, S.; Khalid, M.; Ratnam, C.T.; Chuah, A.L.; Rashmi, W. Waste tire rubber in polymer blends: A review on the evolution, properties and future. Prog. Mater. Sci. 2015,72, 100–140. [CrossRef] 14. Noordermeer, B.J.; Dierkes, W.; Blume, A.; Hoek, H.; Van Reuvekamp, L. Lifetime Recycling Loops for Elastomer Products Dependable Seals Start with DeWAL. Rubber & Plastics News, 30 April 2018, pp. 14–17. 15. Joseph, A.M.; Madhusoodhanan, K.N.; Alex, R.; George, B. Stable free radical–assisted mechanical devulcanization of carbon black–filled natural rubber vulcanizates. Rubber Chem. Technol. 2018,91, 469–491. [CrossRef] 16. Formela, K.; Cysewska, M.; Haponiuk, J.T. Thermomechanical reclaiming of ground tire rubber via extrusion at low temperature: Efficiency and limits. J. Vinyl Addit. Technol. 2016,22, 213–221. [CrossRef] 17. Ghosh, J.; Ghorai, S.; Bhunia, S.; Roy, M.; De, D. The role of devulcanizing agent for mechanochemical devulcanization of styrene butadiene rubber vulcanizate. Polym. Eng. Sci. 2018,58, 74–85. [CrossRef] 18. Simon, D.Á.; Pirityi, D.; Tamás-Bényei, P.; Bárány, T. Microwave devulcanization of ground tire rubber and applicability in SBR compounds. J. Appl. Polym. Sci. 2020,137, 48351. [CrossRef] 19. Araujo-Morera, J.; Verdugo-Manzanares, R.; González, S.; Verdejo, R.; Lopez-Manchado, M.A.; Hernández, M. On the Use of Mechano-Chemically Modified Ground Tire Rubber (GTR) as Recycled and Sustainable Filler in Styrene-Butadiene Rubber (SBR) Composites. J. Compos. Sci. 2021,5, 68. [CrossRef] 20. Buitrago-Suescún, O.; Britto, R. Devulcanization of ground tire rubber: Thermo-oxidation followed by microwave exposure in the presence of devulcanizing agent. Iran. Polym. J. 2020,29, 553–567. [CrossRef] 21. Si, H.; Chen, T.; Zhang, Y. Effects of high shear stress on the devulcanization of ground tire rubber in a twin-screw extruder. J. Appl. Polym. Sci. 2013,128, 2307–2318. [CrossRef] 22. Sripornsawat, B.; Saiwari, S.; Pichaiyut, S.; Nakason, C. Influence of ground tire rubber devulcanization conditions on properties of its thermoplastic vulcanizate blends with copolyester. Eur. Polym. J. 2016,85, 279–297. [CrossRef] 23. Thai, Q.B.; Le, D.K.; Do, N.H.N.; Le, P.K.; Phan-Thien, N.; Wee, C.Y.; Duong, H.M. Advanced aerogels from waste tire fibers for oil spill-cleaning applications. J. Environ. Chem. Eng. 2020,8, 104016. [CrossRef] 24. Thai, Q.B.; Ee Siang, T.; Khac Le, D.; Shah, W.A.; Phan-Thien, N.; Duong, H.M. Advanced fabrication and multi-properties of rubber aerogels from car tire waste. Colloids Surf. A Physicochem. Eng. Asp. 2019,577, 702–708. [CrossRef] 25. Thai, Q.B.; Chong, R.O.; Nguyen, P.T.T.; Le, D.K.; Le, P.K.; Phan-Thien, N.; Duong, H.M. Recycling of waste tire fibers into advanced aerogels for thermal insulation and sound absorption applications. J. Environ. Chem. Eng. 2020,8, 104279. [CrossRef] 26. Sathiskumar, C.; Karthikeyan, S. Recycling of waste tires and its energy storage application of by-products—A review. Sustain. Mater. Technol. 2019,22, e00125. [CrossRef] 27. Hernández Santana, M.; Huete, M.; Lameda, P.; Araujo, J.; Verdejo, R.; López-Manchado, M.A. Design of a new generation of sustainable SBR compounds with good trade-off between mechanical properties and self-healing ability. Eur. Polym. J. 2018 ,106, 273–283. [CrossRef] 28. Blaiszik, B.J.; Kramer, S.L.B.; Olugebefola, S.C.; Moore, J.S.; Sottos, N.R.; White, S.R. Self-Healing Polymers and Composites. Annu. Rev. Mater. Res. 2010,40, 179–211. [CrossRef] 29. van der Zwaag, S. (Ed.) Self Healing Materials; Springer Series in Materials Science; Springer Netherlands: Dordrecht, The Netherlands, 2007; Volume 100, ISBN 978-1-4020-6249-0. 30. Wu, D.Y.; Meure, S.; Solomon, D. Self-healing polymeric materials: A review of recent developments. Prog. Polym. Sci. 2008 ,33, 479–522. [CrossRef] 31. Duenas, T.; Enke, A.; Chai, K.; Castellucci, M.; Sundaresan, V.B.; Wudl, F.; Murphy, E.B.; Mal, A.; Alexandar, J.R.; Corder, A.; et al. Smart Self-Healing Material Systems Using Inductive and Resistive Heating. In Smart Coatings III; American Chemical Society: Washington, DC, USA, 2010; pp. 45–60. 32. Colom, X.; Marín-Genescà, M.; Mujal, R.; Formela, K.; Cañavate, J. Structural and physico-mechanical properties of natural rubber/GTR composites devulcanized by microwaves: Influence of GTR source and irradiation time. J. Compos. Mater. 2018 ,52, 3099–3108. [CrossRef] Polymers 2022,14, 11 19 of 19 33. Colom, X.; Faliq, A.; Formela, K.; Cañavate, J. FTIR spectroscopic and thermogravimetric characterization of ground tyre rubber devulcanized by microwave treatment. Polym. Test. 2016,52, 200–208. [CrossRef] 34. Das, A.; De, D.; Ghorai, S.; Mondal, D.; Hait, S.; Ghosh, A.K.; Wiessner, S.; Das, A.; De, D. Devulcanization of Waste Rubber and Generation of Active Sites for Silica Reinforcement. ACS Omega 2019,4, 17623–17633. [CrossRef] 35. Ghorai, S.; Bhunia, S.; Roy, M.; De, D. Mechanochemical devulcanization of natural rubber vulcanizate by dual function disulfide chemicals. Polym. Degrad. Stab. 2016,129, 34–46. [CrossRef] 36. Flory, P.J.; Rehner, J. Statistical mechanics of cross-linked polymer networks I. Rubberlike elasticity. J. Chem. Phys. 1943 ,11, 512–520. [CrossRef] 37. Horikx, M.M. Chain Scissions in a Polymer Network. Rubber Chem. Technol. 1956,29, 1166–1173. [CrossRef] 38. Verbruggen, M.A.L.; van der Does, L.; Dierkes, W.K.; Noordermeer, J.W.M. Experimental validation of the Charlesby and Horikx models applied to de-vulcanization of sulfur and peroxide vulcanizates of NR and EPDM. Rubber Chem. Technol. 2016 ,89, 671–688. [CrossRef] 39. Seghar, S.; Asaro, L.; Aït Hocine, N. Experimental Validation of the Horikx Theory to be Used in the Rubber Devulcanization Analysis. J. Polym. Environ. 2019,27, 2318–2323. [CrossRef] 40. Edwards, D.W.; Danon, B.; van der Gryp, P.; Görgens, J.F. Quantifying and comparing the selectivity for crosslink scission in mechanical and mechanochemical devulcanization processes. J. Appl. Polym. Sci. 2016,133, 1–10. [CrossRef] 41. Aoudia, K.; Azem, S.; Aït Hocine, N.; Gratton, M.; Pettarin, V.; Seghar, S. Recycling of waste tire rubber: Microwave devulcanization and incorporation in a thermoset resin. Waste Manag. 2017,60, 471–481. [CrossRef] 42. Hernández, M.; Grande, A.M.; Dierkes, W.; Bijleveld, J.; van der Zwaag, S.; García, S.J. Turning Vulcanized Natural Rubber into a Self-Healing Polymer: Effect of the Disulfide/Polysulfide Ratio. ACS Sustain. Chem. Eng. 2016,4, 5776–5784. [CrossRef] 43. Zefeng, W.; Yong, K.; Zhao, W.; Yi, C. Recycling waste tire rubber by water jet pulverization: Powder characteristics and reinforcing performance in natural rubber composites. J. Polym. Eng. 2018,38, 51–62. [CrossRef] 44. Stobinski, L.; Lesiak, B.; Malolepszy, A.; Mazurkiewicz, M.; Mierzwa, B.; Zemek, J.; Jiricek, P.; Bieloshapka, I. Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods. J. Electron Spectros. Relat. Phenomena 2014 , 195, 145–154. [CrossRef] 45. Drewniak, S.; Muzyka, R.; Stolarczyk, A.; Pustelny, T.; Kotyczka-Mora´nska, M.; Setkiewicz, M. Studies of Reduced Graphene Oxide and Graphite Oxide in the Aspect of Their Possible Application in Gas Sensors. Sensors 2016,16, 103. [CrossRef] 46. Diao, B.; Isayev, A.I.; Levin, V.Y. Basic Study of Continuous Ultrasonic Devulcanization of Unfilled Silicone Rubber. Rubber Chem. Technol. 1999,72, 152–164. [CrossRef] 47. Saiwari, S.; Dierkes, W.K.; Noordermeer, J.W.M. Devulcanization of whole passenger car tire material. KGK Kautsch. Gummi Kunstst. 2013,66, 20–25. 48. Bilgili, E.; Arastoopour, H.; Bernstein, B. Pulverization of rubber granulates using the solid state shear extrusion process. Part II. Powder characterization. Powder Technol. 2001,115, 277–289. [CrossRef] 49. Formela, K.; Hejna, A.; Zedler, L.; Colom, X.; Canavate, J. Microwave treatment in waste rubber recycling—recent advances and limitations. Express Polym. Lett. 2019,13, 565–588. [CrossRef] 50. Material, A.S. Standard Test Methods for Rubber Products-Chemical Analysis. Astm D297 2011,9.01, 1–37. [CrossRef] 51. Hernández, M.; den Brabander, M.; García, S.; van der Zwaag, S.; Hernández Santana, M.; den Brabander, M.; García, S.; van der Zwaag, S.; Hernández, M.; den Brabander, M.; et al. Routes to Make Natural Rubber Heal: A Review. Polym. Rev. 2018 ,58, 585–609. [CrossRef] 52. Zhang, Y.; Zhang, Z.; Wemyss, A.M.; Wan, C.; Liu, Y.; Song, P.; Wang, S. Effective Thermal-Oxidative Reclamation of Waste Tire Rubbers for Producing High-Performance Rubber Composites. ACS Sustain. Chem. Eng. 2020,8, 9079–9087. [CrossRef] 53. Song, P.; Wan, C.; Xie, Y.; Formela, K.; Wang, S. Vegetable derived-oil facilitating carbon black migration from waste tire rubbers and its reinforcement effect. Waste Manag. 2018,78, 238–248. [CrossRef] 54. Mangaraj, D. Rubber Recycling by Blending with Plastics. In Rubber Recycling; De, S.K., Isayev, A., Khait, K., Eds.; CRC Press: Boca Raton, FL, USA, 2005; ISBN 9780203499337. 55. Bode, S.; Enke, M.; Hernandez, M.; Bose, R.K.; Grande, A.M.; van der Zwaag, S.; Schubert, U.S.; Garcia, S.J.; Hager, M.D. Characterization of Self-Healing Polymers: From Macroscopic Healing Tests to the Molecular Mechanism. Adv. Polym. Sci. 2015 , 273, 113–142. [CrossRef]