Unraveling structural evolution of crumb rubber derived from end-of-life tires in supercritical fluid environments

Unraveling structural evolution of crumb rubber derived from end-of-life tires in supercritical fluid environments | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Unraveling structural evolution of crumb rubber derived from end-of-life tires in supercritical fluid environments Jin Li, Jiayu Wang, Mohsen Alae, Feipeng Xiao This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6994293/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 29 Sep, 2025 Read the published version in Journal of Polymer Research → Version 1 posted 5 You are reading this latest preprint version Abstract The crumb rubber (CR) derived from end-of-life tires (ELTs) are widely used in paving asphalt modification, while the pretreatments are generally necessary to improve the CR-asphalt compatibility. This study explores the structural changes in CR under various pretreatment scenarios, with an emphasis on mechanisms in supercritical carbon dioxide (ScCO 2 ) reaction environments. Two supercritical pretreatments were designed, including supercritical de-crosslinking (SCD) and supercritical swelling (SCS). Two other pretreatments were also considered for comparison: high-pressure de-crosslinking (HPD) and unpretreated (UPT). The structural evolutions of CR underwent these pretreatments are systematically analyzed through different physicochemical approaches. The results show that SCS causes slight random scission of rubber crosslinking structure, while SCD achieves uniform and thorough de-crosslinking of CR. However, high temperatures also unavoidably cause some structural damage during both supercritical pretreatments. Thermal analysis reveals that SCS induces minor “sol” content increase, while HPD and SCD greatly increase “sol,” with SCD further improving thermal stability of CR. Microstructural observations show distinct morphology changes, ranging from increased porosity with SCS to complete structural disruption under SCD. The supercritical pretreatment processes involve ScCO 2 -induced swelling, enabling efficient and uniform de-crosslinking, accompanied by filler release under high temperatures. These findings provide insight into the mechanisms underlying CR pretreatment in ScCO 2 environments. Solid waste valorization End-of-life tires Crumb rubber Supercritical carbon dioxide Pretreatment Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1 Introduction Based on the concept of circular economy, the recycling and reuse of end-of-life tires (ELTs) maximize resource utilization throughout the tire lifecycle, reducing environmental pollution and waste (Antoniou and Zorpas, 2019; Li et al., 2024). Currently, crumb rubber (CR) modified asphalts, prepared using the tire-derived CR as a modifier, have been widely applied in road pavement construction (F. Li et al., 2022; Picado-Santos et al., 2020). However, CR modified asphalt produced using traditional wet processes often exhibits poor storage stability and workability in practical applications (Liang et al., 2015; Yu et al., 2021). The primary underlying issue is the incompatibility between CR and asphalt, mainly caused by significant differences in their chemical properties (e.g., molecular size and polarity) and the resultant physical property disparities (e.g., solubility) (J. Li et al., 2022b; Li et al., 2021a; Polacco et al., 2015). This incompatibility can be summarized into two key reasons: 1) CR has an inert surface with weak chemical affinity to asphalt components, preventing effective interfacial bonding. 2) CR possesses an internal three-dimensional crosslinked network structure, which inhibits asphalt components from penetrating and interacting with its interior (J. Li et al., 2022a; Li et al., 2021b). An effective solution to this problem is to pretreat CR to intentionally modify its chemical and/or physical properties, thereby enhancing its chemical reactivity with the asphalt matrix. To date, mainstream CR pretreatment techniques can be classified into two categories: surface activation and internal de-crosslinking (devulcanization or desulfurization in some contexts) (Guo et al., 2020; Liang et al., 2022; Zheng et al., 2021). Among them, surface activation enhances or restores the surface activity of CR through methods such as pre-reaction, oxidation, grafting, polymer coating, solution immersion, plasma treatment, or gamma-ray irradiation (Phiri et al., 2022). Internal de-crosslinking selectively breaks crosslinking bonds within CR using physical radiation, mechanical, biological, or chemical methods, without affecting the polymer backbone (Yin et al., 2021). However, each of these existing methods has limitations. Surface activation processes are often overly complex and have low treatment efficiency, while internal de-crosslinking tends to result in either incomplete or excessive de-crosslinking, making efficient pretreatment of CR challenging. Consequently, the use of pretreated CR as an asphalt modifier still faces various drawbacks, necessitating the exploration of novel pretreatment approaches. In this context, the authors introduced supercritical fluid (SCF) technology for CR pretreatment and further applying SCF-pretreated CR in the field of pavement materials. SCF is a fluid above its critical temperature and pressure, possessing both gas-like diffusivity and liquid-like solvating ability (Boyère et al., 2014; Chen et al., 1995; Gao et al., 2019; Li and Xu, 2019). Compared with conventional fluids, SCFs exhibit larger free space and greater compressibility. By simply adjusting pressure, temperature, or both, SCFs can transition between “gas-like” and “liquid-like” states (Asaro et al., 2020). In this study, supercritical carbon dioxide (ScCO 2 ) is employed as a reaction medium for CR pretreatment, serving as both a swelling agent and a carrier agent for the pre-swelling and de-crosslinking of CR. On the one hand, ScCO 2 is expected to penetrate the micropores of CR, promoting the swelling of its internal crosslinked network and creating free space. On the other hand, ScCO 2 has significant potential to dissolve and transport the de-crosslinking agent molecules into the interior of CR. To the authors’ best knowledge, the structural evolution of particulate CR in the SCF environment remains unclear and lacks quantitative characterization. Therefore, the primary objective of this study is to address this research gap by elucidating the structural evolution behavior of CR in different ScCO 2 environments through a range of physicochemical characterization methods. 2 Methodology 2.1 Material preparation 2.1.1 Raw materials This study utilized 30-mesh CR derived from ELTs through an ambient grinding process, based on its widespread availability and common particle size for asphalt modification. Dry ice, namely solid carbon dioxide, was employed in this study to create the ScCO 2 reaction environment. The use of dry ice offers practical advantages due to its high purity, accessibility, and capacity to achieve the critical temperature and pressure required for ScCO 2 formation. The diphenyl disulfide (DD) was selected as the de-crosslinking agent in this study. This compound is known for its high reactivity with sulfur crosslinks in vulcanized rubber, facilitating the selective cleavage of crosslinked bonds while preserving the polymer backbone structure. 2.2.2 Material processing This study designed four CR pretreatment scenarios to compare and analyze the physicochemical properties of CR under different conditions, aiming to clarify the reaction mechanisms involved. The four pretreatment scenarios are described as follows: 1) Supercritical de-crosslinking (SCD) The CR was subjected to a ScCO 2 environment under preset pressure and temperature conditions, with a specified dosage of the de-crosslinking agent (i.e., DD) added. This process aimed to induce both swelling and de-crosslinking of the CR. 2) Supercritical swelling (SCS) All conditions were identical to the SCD process, except that no de-crosslinking agent was loaded. Under this scenario, the CR underwent primarily physical swelling. This setup was designed to isolate the effect of the de-crosslinking agent. 3) High-pressure de-crosslinking (HPD) The process parameters of HPD were also consistent with the SCD scenario, except that conventional high-pressure carbon dioxide was used instead of the ScCO 2 . This scenario aimed to evaluate the influence of the SCF reaction environment. 4) Unpretreated (UPT) The CR was kept in its original state, without undergoing any pretreatment, served as the control group for this study. The key parameters of the four CR pretreatment scenarios are summarized in Table 1 . Note that all other condition parameters not listed in the table were kept constant across the scenarios. Table 1 Key parameters of four CR pretreatment scenarios CR pretreatment scenario ScCO 2 temperature (°C) ScCO 2 pressure (MPa) Pretreatment duration (h) DD agent dosage (%) UPT 0 0 0 0 HPD 205 2 2 15 SCS 205 10 2 0 SCD 205 10 2 15 2.2 Material testing 2.2.1 Sol fraction test The rubber component in CR is composed of sol and gel fractions. The sol fraction consists primarily of linear rubber molecular chains formed by the breaking of crosslink bonds during de-crosslinking and is soluble in toluene. The gel fraction consists of the residual rubber in a crosslinked structure, which is insoluble in toluene. To determine the sol content, a precise amount of CR sample was weighed using an analytical balance. The sample was wrapped in slow quantitative filter paper and subjected to Soxhlet extraction using toluene as the solvent. After extraction, the residue retained in the filter paper was dried to a constant weight in a vacuum oven at 50°C and weighed. The sol fraction was calculated using the following formula: $$\:\begin{array}{c}{f}_{s}=\frac{{m}_{0}-{m}_{1}}{a{m}_{0}}\#\left(1\right)\end{array}$$ Where: \(\:{f}_{s}\) is the sol content of the CR (%); \(\:{m}_{0}\) is the initial mass of the CR before extraction (g); \(\:{m}_{1}\) is the final mass of the CR after vacuum drying (g); \(\:a\) is the rubber content in the CR (%). 2.2.2 Crosslinking density test The crosslink density of CR was measured using the equilibrium swelling method. First, a precise mass of CR was weighed and then immersed in toluene for 72 h to reach equilibrium swelling. After removal, the sample was blotted dry to remove surface solvent and weighed. The sample was then dried to a constant weight in a vacuum oven at 60°C and weighed again. The crosslink density was calculated based on the Flory-Rehner equation (Flory and Rehner, 1943): $$\:\begin{array}{c}{\nu\:}_{x}=-\frac{\text{l}\text{n}\left(1-{\varphi\:}_{r}\right)+{\varphi\:}_{r}+\chi\:{{\varphi\:}_{r}}^{2}}{{v}_{s}\left({{\varphi\:}_{r}}^{\frac{1}{3}}-\frac{1}{2}{\varphi\:}_{r}\right)}\#\left(2\right)\end{array}$$ Where: \(\:{\nu\:}_{x}\) is the crosslink density (mol/cm 3 ); \(\:\chi\:\) is the interaction parameter between rubber and solvent, set to 0.4; \(\:{v}_{s}\) is the molar volume of the solvent (mol/ml), set to 106.2 mol/ml; \(\:{\varphi\:}_{r}\) is the volume fraction of rubber in the swollen CR system, calculated as follows: $$\:\begin{array}{c}{\varphi\:}_{r}=\frac{\left({m}_{3}-{m}_{0}\varphi\:\right)/{\rho\:}_{r}}{\left({m}_{3}-{m}_{0}\varphi\:\right)/{\rho\:}_{r}+\left({m}_{2}-{m}_{3}\right){\rho\:}_{s}}\#\left(3\right)\end{array}$$ Where: \(\:{\rho\:}_{r}\) is the density of rubber without carbon black (g/cm 3 ), set to 0.95 g/cm 3 ; \(\:{\rho\:}_{s}\) is the density of toluene (g/ml), set to 0.865 g/ml; \(\:\varphi\:\) is the volume fraction of carbon black in the CR, approximately 0.35; \(\:{m}_{0}\) is the initial mass of the CR (g); \(\:{m}_{2}\) is the mass of the swollen CR after surface drying (g); \(\:{m}_{3}\) is the final mass of the swollen CR after vacuum drying (g). 2.2.3 FTIR test The molecular structure and functional groups of CR were analyzed using a Fourier transform infrared spectrometer equipped with an Attenuated Total Reflectance (ATR) accessory. The sample was placed on the ATR crystal and fixed with a pressure head. The spectral range was set to 4000 − 600 cm − 1 , with 32 scans per measurement at a resolution of 4 cm − 1 . After each test, the ATR accessory was cleaned with tetrahydrofuran-soaked lint-free cotton, and background single-channel spectra were remeasured. Each CR sample was tested at least three times to ensure accuracy. 2.2.4 TGA test The thermal stability and composition of CR were analyzed using a thermogravimetric analyzer (TGA). Approximately 15 mg of CR was placed in a crucible, and the test began after the mass stabilized. The initial temperature was set to 50°C, maintained for 1 min, and then increased to 600°C at a heating rate of 10°C/min under a nitrogen atmosphere. 2.2.5 SEM observation The microstructure of CR particles was observed using scanning electron microscopy (SEM). As CR is non-conductive, samples were coated with a gold-palladium alloy using an Oxford Quorum SC7620 sputter coater (10 mA for 45 seconds). Images were captured with a ZEISS Gemini 300 SEM at an accelerating voltage of 3 kV using an SE2 secondary electron detector. SEM magnifications of 200×, 500×, 1000×, and 2000× were used based on particle size. 3 Results and discussion 3.1 Crosslinking structure The sol content and crosslink density of CR pretreated using four different processes were plotted on the Horikx model diagram to analyze the changes in the crosslinking structure after pretreatments, as shown in Fig. 1 (Horikx, 1956; Seghar et al., 2019). It should be noted that in vulcanized rubber crosslinking networks, the relationships between sol content and the crosslink density ratio, under scenarios where only main-chain scission or only crosslinking bond cleavage occurs, are represented by the red and blue curves, respectively. The experimental data point for unpretreated CR (i.e., UPT) is located at the starting position of the curves, representing its initial state. The experimental data points for CR pretreated using HPD are situated near and slightly above the red curve. This indicates that, under high-pressure conditions, a certain degree of de-crosslinking occurs. However, due to the inferior solubility and permeability of compressed high-temperature CO 2 gas compared to ScCO 2 , the devulcanizing agent cannot fully contact the crosslinked network. Consequently, de-crosslinking reactions are limited to the CR surface, leading to poor uniformity. Additionally, under these high-temperature conditions, the scission of rubber main chains contribute to a degree comparable to the cleavage of crosslink bonds. For CR pretreated using SCS, the experimental data points are closer to the starting position of the curves. This suggests that, in the absence of a de-crosslinking agent, the CR undergoes random scission of main chains and crosslink bonds driven by the penetration of ScCO 2 . As a result, the crosslink density decreases slightly, accompanied by the generation of a small amount of sol content. In contrast, the experimental data points for CR pretreated using SCD lie between the red and blue curves. This indicates that, under these conditions, the CR undergoes a more thorough de-crosslinking reaction dominated by the combined effects of ScCO 2 and the chemical attack of DD. Crucially, DD achieves sufficient contact with the crosslinking network, allowing de-crosslinking to occur uniformly in both the surface and interior of the CR. Nonetheless, it is evident that some degree of unavoidable main chains scission occurs under high temperatures, leading to an additional increase in sol content after pretreatment. 3.2 Functional groups The FTIR spectra of CR subjected to four different pretreatment processes and the characteristic peaks of interests are shown in Fig. 2 . The peak at 700 cm − 1 corresponds to the phenyl structure. As the degree of de-crosslinking increases, more de-crosslinking agents interact with the crosslinking network within the CR and undergo reactions. The de-crosslinking agent used in this study, namely DD, contains a phenyl structure, and its reaction and incorporation into the CR enhance this characteristic peak. The peaks at 570 cm − 1 and 475 cm − 1 correspond to the C-S and S-S bonds, respectively. As the de-crosslinking (including both SCD and HPD) pretreatment progresses, the crosslinked bonds (C-S and S-S) in the vulcanized rubber are gradually cleaved, leading to a decrease in the intensity of these two peaks. The peak at 2725 cm − 1 represents the S-H bond, and its intensity diminishes progressively with the occurrence of the de-crosslinking reaction. Additionally, the peak at 1310 cm − 1 is associated with the C-C bonds in the main chain of rubber macromolecules, while the peaks at 1670 cm − 1 and 840 cm − 1 correspond to the RCH = RCH structure, and the peak at 3000 cm − 1 is attributed to the = CH structure. During the reaction, the intensity of these peaks weakens as the reaction progresses, indicating that the main chain structure of rubber macromolecules is also damaged to some extent under high-temperature conditions. Furthermore, the peak at 970 cm − 1 corresponds to the C 4 H 6 structure of butadiene. As the de-crosslinking (including both SCD and HPD) reaction proceeds, more vinyl side groups are cleaved due to crosslinking, forming 1,4-double bonds, which enhance the intensity of this peak. The peaks at 1725 cm − 1 and 1080 cm − 1 represent the C = O and S-O structures, respectively. After pretreatment, the intensities of these two peaks decrease, suggesting that these structures are also disrupted during the reaction process. In summary, during the SCD/HPD pretreatment, the radicals generated by the DD selectively couple with the radicals from the vulcanized rubber, breaking the crosslinking bonds and introducing the phenyl structure of the DD into the CR. However, due to indiscriminate high-temperature effects, the rubber main chain and other unstable structures in the CR are also inevitably damaged. 3.3 Chemical composition To evaluate the thermal stability and analyze the composition of CR subjected to four different pretreatment processes, TGA was conducted to obtain weight loss curves. Additionally, the weight loss curves were differentiated with respect to temperature to derive derivative thermogravimetric (DTG) curves. Both the TGA and DTG curves for CR under different pretreatment processes are shown in Fig. 3 . In analyzing TGA and DTG curves, the rubber type is typically determined by the temperature corresponding to the maximum decomposition rate. Specifically, for the main components of tire rubber, natural rubber (NR) exhibits a maximum decomposition temperature around 375°C, while synthetic rubbers, such as styrene-butadiene rubber (SBR) and butadiene rubber (BR), generally decompose at a maximum rate near 450°C. The inorganic fillers, which do not decompose within the temperature range studied (50–600°C), are considered the residual components. As shown in Fig. 3 , the DTG curves of unpretreated CR (i.e., UPT) display a single peak at approximately 450°C, indicating that the primary rubber component in samples is synthetic rubber. However, the samples also show a weight loss of about 10% near 375°C, suggesting the presence of natural rubber in the unpretreated CR. In contrast, the DTG curves of the three pretreated CRs reveal an additional peak at approximately 280°C, though its intensity varies. This peak is probably due to the thermal decomposition of sol and partially degraded gel produced during the pretreatment process. Among these, the peak is relatively less pronounced for CR subjected to the SCS pretreatment. However, for the HPD or SCD processes, where a higher proportion of sol is generated, the compositional changes result in a more prominent peak at this temperature. The temperature ranges corresponding to the two peaks can be used to broadly classify the components of CR based on their pyrolysis characteristics into three categories: “sol,” rubber hydrocarbons, and inert fillers. Note that the term “sol” used here differs from the sol discussed in previous sections. In this context, “sol” refers to rubber components within the pretreated CR that become more prone to pyrolysis. Based on this classification, the compositional proportions of the three components under different pretreatment processes are summarized in Table 2 . It can be observed that the proportion of “sol” increases after pretreatments, while the proportions of rubber hydrocarbons and inert fillers decrease. The changes are relatively small for CR subjected to SCS pretreatment but are more pronounced for those treated with HPD and SCD processes. The increase in “sol” proportion results from the synergistic effects of targeted cleavage of crosslinking bonds by DD, enhanced penetration of ScCO 2 , and random thermal degradation under high-temperature conditions. These combined factors loosen the crosslinking network structure within the CR, making some “sol” components more susceptible to pyrolysis. Consequently, the proportion of rubber hydrocarbons that retain their original crosslinking network structure decreases. The reduction in filler content after pretreatments may be attributed to the breakdown of the crosslinking network, which releases fillers such as carbon black that were previously constrained by the rubber molecular network. However, it is worth noting that after SCD pretreatment, the CR loses its original granular form, and the small sample size for TGA analysis introduces some variability despite repeated experiments. Additionally, the peak temperature on the DTG curve represents the temperature at which the maximum weight loss rate occurs during thermal decomposition, as presented Table 2 . The peak temperatures of CR pretreated with HPD and SCS processes remain relatively unchanged. In contrast, the peak temperature of CR subjected to SCD increases by approximately 50°C. This is because the degree of de-crosslinking is limited in the first two processes due to the absence of a ScCO 2 reaction environment or a de-crosslinking agent. However, in the latter case, the combined action of ScCO 2 and the de-crosslinking agent DD enables thorough de-crosslinking. The de-crosslinking agent used in this study contains phenyl groups that react with the crosslinking bonds within the CR and form side chains attached to the rubber macromolecular backbone. This increases the thermal stability of the rubber hydrocarbons in the CR after this pretreatment process. Table 2 Peak temperature in DTG curves of CR with different pretreatments CR pretreatment Peak temperature (°C) “Sol” fraction Rubber hydrocarbon Filler UPT 429.93 7.59 50.44 41.97 HPD 432.18 28.73 30.95 40.32 SCS 428.74 15.85 51.88 32.28 SCD 482.04 23.03 42.62 34.35 3.4 Surface morphology To investigate the changes in the microstructure of CR under different pretreatment processes, SEM was used to capture images of CR samples at various magnifications, as shown in Fig. 4 . Unpretreated CR (i.e., UPT) exhibits a rough surface with irregular particle shapes and pore structures of varying sizes. In contrast, the microstructure of CR underwent significant changes after the three pretreatment processes, with a common feature being pronounced swelling, where the particle volume expanded considerably. However, each process also displayed distinct characteristics. Under the SCS pretreatment process, the CR underwent physical swelling while largely retaining its original particle morphology, rough surface texture, and pore structure. Its surface became more porous and fluffier. This indicates that this process primarily involves physical treatment rather than chemical reactions. In the HPD pretreatment process, the CR also maintained its particle morphology, but its surface became smoother, and the pore structures gradually disappeared. This is primarily due to the action of the de-crosslinking agent, which caused crosslink breakage on the surface of the CR, forming a layer of sol that covered the original surface. Finally, the CR subjected to the SCD pretreatment process completely lost its original particle morphology and other surface features, presenting an entirely different microstructure. This is because the extensive de-crosslinking reaction significantly disrupted the internal crosslinked network of the CR. At the microscopic scale, the morphology appeared nearly smooth and fluid-like, which can essentially be interpreted as an external manifestation of the high sol content in terms of microstructural characteristics. 3.5 Reaction mechanisms Based on the characterizations of CR after pretreatments detailed above, the main reaction mechanism of the supercritical pretreatment of CR in ScCO 2 can be summarized as follows: 1) Swelling effect of ScCO 2 : Due to the high permeability of ScCO 2 , CR can be penetrated by ScCO 2 molecules and undergo significant physical swelling, causing an obvious volume expansion. Although the internal crosslinked network of the rubber is not completely broken, it becomes more loosened, providing a foundation for the subsequent de-crosslinking agent molecules to enter and participate in the reaction, as shown in the second step in Fig. 5 . 2) Targeted attack on the crosslinking network by the de-crosslinking agent under the assistance of ScCO 2 : After the addition of the de-crosslinking agent, the high solubility and permeability of ScCO 2 , combined with the targeted attack by the de-crosslinking agents on the crosslinking bonds, as well as the activation of the de-crosslinking agent’s reactivity under high-temperature conditions, enable a more thorough, uniform, and efficient de-crosslinking compared to conventional high-temperature and high-pressure conditions. Most of the crosslinked structures are broken down, as shown in the third step of Fig. 5 . However, it should be noted that due to the high-temperature conditions, some breaking of the main chain is unavoidable during this process. 3) Release and migration of fillers: As the crosslinked network inside the CR disintegrates, fillers that were originally confined within the rubber, such as carbon black and silica, are gradually released. They then migrate with the stirring of the reactants and the flow of ScCO 2 , as shown in the last step of Fig. 5 . The specific reaction mechanisms of CR during the SCD pretreatment process are shown in Fig. 6 . Initially, the de-crosslinking agent (i.e., DD) and CR undergo reactions (a) and (b) under high-temperature conditions. The de-crosslinking agent decomposes to generate free radicals, while the rubber inside the CR also undergoes thermal decomposition, with some crosslink bonds and macromolecular rubber chains randomly breaking. Subsequently, the free radicals of the de-crosslinking agent further react with the vulcanized rubber inside the CR, as expressed in reaction (c). This reaction pathway includes two forms: 1) The free radicals of the de-crosslinking agent break the crosslinking bonds (S-S bonds) inside the CR; 2) The free radicals break the C = C double bonds on the rubber main chain. Throughout the pretreatment process, the thermal breaking of the rubber macromolecular chains in reaction (b) is continuous and inevitable, but overall, it does not dominate the process. 4 Conclusion This study investigates the structural changes of CR under different pretreatment scenarios by different physicochemical means, with a focus on the pretreatment mechanisms in ScCO 2 environments. The key findings are summarized as follows: The pretreatment processes significantly change the crosslinking structure of CR. Compared to the initial state (UPT), HPD induces partial surface de-crosslinking, SCS causes slight random scission, and SCD achieves uniform and thorough de-crosslinking throughout the material in ScCO 2 , though some main-chain scission is unavoidable at high temperatures. FTIR analysis shows that SCD and HPD cleave crosslinking bonds and incorporate the phenyl structure of de-crosslinking agent into the CR network, while high-temperature conditions also cause damage to rubber main chains and unstable structures. Thermal analysis reveals that SCS induces minor “sol” content increase compared to UPT, and HPD and SCD significantly increase “sol,” with SCD further improving thermal stability by incorporating phenyl side chains. Microstructural observations highlight distinct effects of pretreatment. UPT retains its rough, irregular structure; SCS increases porosity without changing particle morphology; HPD forms a smoother sol layer; and SCD destroyed the structure completely, yielding a smooth, fluid-like appearance. The supercritical pretreatment of CR involves several key mechanisms: physical swelling by ScCO 2 loosens the crosslinked network, facilitating efficient and uniform de-crosslinking by the de-crosslinking agent. This process also leads to the release and migration of fillers, while some unavoidable main-chain scission occurs under high-temperature conditions. Declarations Funding Declaration This study was supported by the Research Fund for Central Universities of China under grant numbers of 22120240341, National Natural Science Foundation of China at the number of 52308441 and National Natural Science Foundation of Sichuan Province at the number of 2024NSFSC0930. Data Availability Data available on request from the authors Author Contribution Declaration Jin Li: Conceptualization, Data Collection, Writing - Original Draft; Jiayu Wang: Data Collection, Validation; Mohsen Alae: Formal analysis, Data Curation; Feipeng Xiao: Conceptualization, Writing - Review & Editing, Funding acquisition Competing Interest Declaration The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Antoniou, N.A., Zorpas, A.A., 2019. Quality protocol and procedure development to define end-of-waste criteria for tire pyrolysis oil in the framework of circular economy strategy. Waste Management 95, 161–170. https://doi.org/10.1016/j.wasman.2019.05.035 Asaro, L., Gratton, M., Poirot, N., Seghar, S., Aït Hocine, N., 2020. Devulcanization of natural rubber industry waste in supercritical carbon dioxide combined with diphenyl disulfide. Waste Management 118, 647–654. https://doi.org/10.1016/j.wasman.2020.09.026 Boyère, C., Jérôme, C., Debuigne, A., 2014. Input of supercritical carbon dioxide to polymer synthesis: An overview. European Polymer Journal 19. Chen, D.T., Perman, C.A., Riechert, M.E., Hoven, J., 1995. Depolymerization of tire and natural rubber using supercritical fluids. Journal of Hazardous Materials 44, 53–60. https://doi.org/10.1016/0304-3894(95)00047-X Flory, P.J., Rehner, J., 1943. Statistical Mechanics of Cross‐Linked Polymer Networks I. Rubberlike Elasticity. The Journal of Chemical Physics 11, 512–520. https://doi.org/10.1063/1.1723791 Gao, H., Liu, H., Song, C., Hu, G., 2019. Infusion of graphene in natural rubber matrix to prepare conductive rubber by ultrasound-assisted supercritical CO2 method. Chemical Engineering Journal 368, 1013–1021. https://doi.org/10.1016/j.cej.2019.03.026 Guo, F., Zhang, J., Pei, J., Ma, W., Hu, Z., Guan, Y., 2020. Evaluation of the compatibility between rubber and asphalt based on molecular dynamics simulation. Front. Struct. Civ. Eng. 14, 435–445. https://doi.org/10.1007/s11709-019-0603-x Horikx, M.M., 1956. Chain Scissions in a polymer network. Journal of Polymer Science 19, 445–454. Li, D., Leng, Z., Yao, L., Cao, R., Zou, F., Li, G., Wang, Hainian, Wang, Haopeng, 2024. Mechanical, economic, and environmental assessment of recycling reclaimed asphalt rubber pavement using different rejuvenation schemes. Resources, Conservation and Recycling 204, 107534. https://doi.org/10.1016/j.resconrec.2024.107534 Li, F., Zhang, X., Wang, L., Zhai, R., 2022. The preparation process, service performances and interaction mechanisms of crumb rubber modified asphalt (CRMA) by wet process: A comprehensive review. Construction and Building Materials 354, 129168. https://doi.org/10.1016/j.conbuildmat.2022.129168 Li, J., Chen., Z., Xiao, F., Amirkhanian, S.N., 2022a. Solution Soaking Pretreatments of Crumb Rubber for Improving Compatibility of Rubberized Asphalt. J. Mater. Civ. Eng. 34, 04021377. https://doi.org/10.1061/(ASCE)MT.1943-5533.0004015 Li, J., Chen, Z., Xiao, F., Amirkhanian, S.N., 2021a. Surface activation of scrap tire crumb rubber to improve compatibility of rubberized asphalt. Resources, Conservation and Recycling 169, 105518. https://doi.org/10.1016/j.resconrec.2021.105518 Li, J., Wang, J., Xiao, F., Amirkhanian, S.N., 2021b. Characterizing Compatibility of Crumb Rubber Modified Asphalt by Customized Drainage Method. J. Test. Eval. 49, 20190856. https://doi.org/10.1520/JTE20190856 Li, J., Xiao, X., Chen, Z., Xiao, F., Amirkhanian, S.N., 2022b. Internal de-crosslinking of scrap tire crumb rubber to improve compatibility of rubberized asphalt. Sustainable Materials and Technologies 32, e00417. https://doi.org/10.1016/j.susmat.2022.e00417 Li, K., Xu, Z., 2019. A review of current progress of supercritical fluid technologies for e-waste treatment. Journal of Cleaner Production 227, 794–809. https://doi.org/10.1016/j.jclepro.2019.04.104 Liang, M., Qiu, Z., Luan, X., Qi, C., Guo, N., Liu, Z., Su, L., Yao, Z., Zhang, J., 2022. The Effects of Activation Treatments for Crumb Rubber on the Compatibility and Mechanical Performance of Modified Asphalt Binder and Mixture by the Dry Method. Front. Mater. 9, 845718. https://doi.org/10.3389/fmats.2022.845718 Liang, M., Xin, X., Fan, W., Luo, H., Wang, X., Xing, B., 2015. Investigation of the rheological properties and storage stability of CR/SBS modified asphalt. Construction and Building Materials 74, 235–240. https://doi.org/10.1016/j.conbuildmat.2014.10.022 Phiri, M.M., Phiri, M.J., Formela, K., Wang, S., Hlangothi, S.P., 2022. Grafting and reactive extrusion technologies for compatibilization of ground tyre rubber composites: Compounding, properties, and applications. Journal of Cleaner Production 369, 133084. https://doi.org/10.1016/j.jclepro.2022.133084 Picado-Santos, L.G., Capitão, S.D., Neves, J.M.C., 2020. Crumb rubber asphalt mixtures: A literature review. Construction and Building Materials 247, 118577. https://doi.org/10.1016/j.conbuildmat.2020.118577 Polacco, G., Filippi, S., Merusi, F., Stastna, G., 2015. A review of the fundamentals of polymer-modified asphalts: Asphalt/polymer interactions and principles of compatibility. Advances in Colloid and Interface Science 224, 72–112. https://doi.org/10.1016/j.cis.2015.07.010 Seghar, S., Asaro, L., Aït Hocine, N., 2019. Experimental Validation of the Horikx Theory to be Used in the Rubber Devulcanization Analysis. J Polym Environ 27, 2318–2323. https://doi.org/10.1007/s10924-019-01513-z Song, P., Wan, C., Xie, Y., Formela, K., Wang, S., 2018. Vegetable derived-oil facilitating carbon black migration from waste tire rubbers and its reinforcement effect. Waste Management 78, 238–248. https://doi.org/10.1016/j.wasman.2018.05.054 Yin, L., Yang, X., Shen, A., Wu, H., Lyu, Z., Li, B., 2021. Mechanical properties and reaction mechanism of microwave-activated crumb rubber-modified asphalt before and after thermal aging. Construction and Building Materials 267, 120773. https://doi.org/10.1016/j.conbuildmat.2020.120773 Yu, H., Deng, G., Zhang, Z., Zhu, M., Gong, M., Oeser, M., 2021. Workability of rubberized asphalt from a perspective of particle effect. Transportation Research Part D: Transport and Environment 91, 102712. https://doi.org/10.1016/j.trd.2021.102712 Zheng, W., Wang, H., Chen, Y., Ji, J., You, Z., Zhang, Y., 2021. A review on compatibility between crumb rubber and asphalt binder. Construction and Building Materials 297, 123820. https://doi.org/10.1016/j.conbuildmat.2021.123820 Cite Share Download PDF Status: Published Journal Publication published 29 Sep, 2025 Read the published version in Journal of Polymer Research → Version 1 posted Reviewers agreed at journal 10 Aug, 2025 Reviewers invited by journal 10 Aug, 2025 Editor invited by journal 07 Aug, 2025 Editor assigned by journal 16 Jul, 2025 First submitted to journal 15 Jul, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6994293","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":498220863,"identity":"2886e5ed-bfef-4a46-aec9-9272ebaec74b","order_by":0,"name":"Jin Li","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Jin","middleName":"","lastName":"Li","suffix":""},{"id":498220864,"identity":"d922f965-01a4-406a-b057-edf8d12e5b5d","order_by":1,"name":"Jiayu Wang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Jiayu","middleName":"","lastName":"Wang","suffix":""},{"id":498220865,"identity":"9e608bb7-8ab5-45ba-afce-43b9c5fed0f6","order_by":2,"name":"Mohsen Alae","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Mohsen","middleName":"","lastName":"Alae","suffix":""},{"id":498220866,"identity":"b9d91889-09f7-4eba-bf48-e446a9eabdb0","order_by":3,"name":"Feipeng Xiao","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5klEQVRIiWNgGAWjYBAC9gYGhgMMDDZgjkECAwNjAyEtPAfAWtIkSNMCBIclYAJEaJHuMTzwo+J8Hb90+4WCBww2shsOMD97gFeLzLGEgz1nbktIzjlTAHRYmvGGA2zmBvi02EskHzjA23ZbwuBGTgJQy+HEDQd42CTwaeGRSGw4+LftHEzLf2K0JB84zNt2AKgl/QBQywEitAD9cljmTLLkzBk5wEA2SDaeeZjNDL8W6R7jj28q7Pj5JdKfGf6osJPtO978DK8WBoQsj5kBAyiomPGqR9HC/vgBIcWjYBSMglEwMgEAFYVL0Pb1Fq4AAAAASUVORK5CYII=","orcid":"","institution":"Tongji University","correspondingAuthor":true,"prefix":"","firstName":"Feipeng","middleName":"","lastName":"Xiao","suffix":""}],"badges":[],"createdAt":"2025-06-27 20:29:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6994293/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6994293/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10965-025-04595-7","type":"published","date":"2025-09-29T15:57:18+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":89096487,"identity":"4def28dd-2381-41e6-93f1-139302ae95a2","added_by":"auto","created_at":"2025-08-14 15:33:56","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":562580,"visible":true,"origin":"","legend":"\u003cp\u003eCrosslinking structure of CR with different pretreatments\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6994293/v1/2d1e6802be4433d9505bec8a.png"},{"id":89098116,"identity":"643bcaad-29b6-4c99-b8a6-d866e0e0bbf5","added_by":"auto","created_at":"2025-08-14 15:49:56","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":329009,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR analysis of CR with different pretreatments, (a) FTIR spectra, (b) Peak intensities\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6994293/v1/ecf2fbe5b068ea6d9f030fe5.png"},{"id":89095971,"identity":"4cdfc843-14b5-4d7f-994b-e85854c4202f","added_by":"auto","created_at":"2025-08-14 15:25:56","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":163395,"visible":true,"origin":"","legend":"\u003cp\u003eTGA analysis of CR with different pretreatments, (a) TGA curves, (b) DTG curves\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6994293/v1/ebbf19f175939b90c5d7e5ff.png"},{"id":89095969,"identity":"738d5ec5-f654-47a4-9ec1-3fcd5aebe77c","added_by":"auto","created_at":"2025-08-14 15:25:55","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":2012513,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of CR with different pretreatments\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6994293/v1/298bed0f373258b4c00a20cd.png"},{"id":89095974,"identity":"bf5baa5b-ddce-4da8-b908-0d21be8e2ee1","added_by":"auto","created_at":"2025-08-14 15:25:56","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":270439,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of reaction mechanisms of supercritical pretreatments\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-6994293/v1/10047a7aed3651fd8e23fbc0.png"},{"id":89096489,"identity":"d1831345-d174-4a36-8985-af19a30c114e","added_by":"auto","created_at":"2025-08-14 15:33:56","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":112315,"visible":true,"origin":"","legend":"\u003cp\u003eSupercritical de-crosslinking reaction equation of CR, (a) Self-decomposition reaction of DD; (b) Decomposition reaction of CR under high-temperature conditions; (c) De-crosslinking reaction between DD and CR (Song et al., 2018)\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-6994293/v1/f066e0124c0fa7308c4128a3.png"},{"id":92883931,"identity":"e58d5417-7fd0-4766-9393-57799d146050","added_by":"auto","created_at":"2025-10-06 16:11:10","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4146503,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6994293/v1/4734b12d-dc1d-4366-a938-dce561f76a7a.pdf"}],"financialInterests":"","formattedTitle":"Unraveling structural evolution of crumb rubber derived from end-of-life tires in supercritical fluid environments","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eBased on the concept of circular economy, the recycling and reuse of end-of-life tires (ELTs) maximize resource utilization throughout the tire lifecycle, reducing environmental pollution and waste (Antoniou and Zorpas, 2019; Li et al., 2024). Currently, crumb rubber (CR) modified asphalts, prepared using the tire-derived CR as a modifier, have been widely applied in road pavement construction (F. Li et al., 2022; Picado-Santos et al., 2020). However, CR modified asphalt produced using traditional wet processes often exhibits poor storage stability and workability in practical applications (Liang et al., 2015; Yu et al., 2021).\u003c/p\u003e\u003cp\u003eThe primary underlying issue is the incompatibility between CR and asphalt, mainly caused by significant differences in their chemical properties (e.g., molecular size and polarity) and the resultant physical property disparities (e.g., solubility) (J. Li et al., 2022b; Li et al., 2021a; Polacco et al., 2015). This incompatibility can be summarized into two key reasons: 1) CR has an inert surface with weak chemical affinity to asphalt components, preventing effective interfacial bonding. 2) CR possesses an internal three-dimensional crosslinked network structure, which inhibits asphalt components from penetrating and interacting with its interior (J. Li et al., 2022a; Li et al., 2021b).\u003c/p\u003e\u003cp\u003eAn effective solution to this problem is to pretreat CR to intentionally modify its chemical and/or physical properties, thereby enhancing its chemical reactivity with the asphalt matrix. To date, mainstream CR pretreatment techniques can be classified into two categories: surface activation and internal de-crosslinking (devulcanization or desulfurization in some contexts) (Guo et al., 2020; Liang et al., 2022; Zheng et al., 2021). Among them, surface activation enhances or restores the surface activity of CR through methods such as pre-reaction, oxidation, grafting, polymer coating, solution immersion, plasma treatment, or gamma-ray irradiation (Phiri et al., 2022). Internal de-crosslinking selectively breaks crosslinking bonds within CR using physical radiation, mechanical, biological, or chemical methods, without affecting the polymer backbone (Yin et al., 2021).\u003c/p\u003e\u003cp\u003eHowever, each of these existing methods has limitations. Surface activation processes are often overly complex and have low treatment efficiency, while internal de-crosslinking tends to result in either incomplete or excessive de-crosslinking, making efficient pretreatment of CR challenging. Consequently, the use of pretreated CR as an asphalt modifier still faces various drawbacks, necessitating the exploration of novel pretreatment approaches.\u003c/p\u003e\u003cp\u003eIn this context, the authors introduced supercritical fluid (SCF) technology for CR pretreatment and further applying SCF-pretreated CR in the field of pavement materials. SCF is a fluid above its critical temperature and pressure, possessing both gas-like diffusivity and liquid-like solvating ability (Boy\u0026egrave;re et al., 2014; Chen et al., 1995; Gao et al., 2019; Li and Xu, 2019). Compared with conventional fluids, SCFs exhibit larger free space and greater compressibility. By simply adjusting pressure, temperature, or both, SCFs can transition between \u0026ldquo;gas-like\u0026rdquo; and \u0026ldquo;liquid-like\u0026rdquo; states (Asaro et al., 2020). In this study, supercritical carbon dioxide (ScCO\u003csub\u003e2\u003c/sub\u003e) is employed as a reaction medium for CR pretreatment, serving as both a swelling agent and a carrier agent for the pre-swelling and de-crosslinking of CR. On the one hand, ScCO\u003csub\u003e2\u003c/sub\u003e is expected to penetrate the micropores of CR, promoting the swelling of its internal crosslinked network and creating free space. On the other hand, ScCO\u003csub\u003e2\u003c/sub\u003e has significant potential to dissolve and transport the de-crosslinking agent molecules into the interior of CR.\u003c/p\u003e\u003cp\u003eTo the authors\u0026rsquo; best knowledge, the structural evolution of particulate CR in the SCF environment remains unclear and lacks quantitative characterization. Therefore, the primary objective of this study is to address this research gap by elucidating the structural evolution behavior of CR in different ScCO\u003csub\u003e2\u003c/sub\u003e environments through a range of physicochemical characterization methods.\u003c/p\u003e"},{"header":"2 Methodology","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Material preparation\u003c/h2\u003e\u003cdiv id=\"Sec4\" class=\"Section3\"\u003e\u003ch2\u003e2.1.1 Raw materials\u003c/h2\u003e\u003cp\u003eThis study utilized 30-mesh CR derived from ELTs through an ambient grinding process, based on its widespread availability and common particle size for asphalt modification.\u003c/p\u003e\u003cp\u003eDry ice, namely solid carbon dioxide, was employed in this study to create the ScCO\u003csub\u003e2\u003c/sub\u003e reaction environment. The use of dry ice offers practical advantages due to its high purity, accessibility, and capacity to achieve the critical temperature and pressure required for ScCO\u003csub\u003e2\u003c/sub\u003e formation.\u003c/p\u003e\u003cp\u003eThe diphenyl disulfide (DD) was selected as the de-crosslinking agent in this study. This compound is known for its high reactivity with sulfur crosslinks in vulcanized rubber, facilitating the selective cleavage of crosslinked bonds while preserving the polymer backbone structure.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\u003ch2\u003e2.2.2 Material processing\u003c/h2\u003e\u003cp\u003eThis study designed four CR pretreatment scenarios to compare and analyze the physicochemical properties of CR under different conditions, aiming to clarify the reaction mechanisms involved. The four pretreatment scenarios are described as follows:\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\n\u003ch3\u003e1) Supercritical de-crosslinking (SCD)\u003c/h3\u003e\n\u003cp\u003eThe CR was subjected to a ScCO\u003csub\u003e2\u003c/sub\u003e environment under preset pressure and temperature conditions, with a specified dosage of the de-crosslinking agent (i.e., DD) added. This process aimed to induce both swelling and de-crosslinking of the CR.\u003c/p\u003e\n\u003ch3\u003e2) Supercritical swelling (SCS)\u003c/h3\u003e\n\u003cp\u003eAll conditions were identical to the SCD process, except that no de-crosslinking agent was loaded. Under this scenario, the CR underwent primarily physical swelling. This setup was designed to isolate the effect of the de-crosslinking agent.\u003c/p\u003e\n\u003ch3\u003e3) High-pressure de-crosslinking (HPD)\u003c/h3\u003e\n\u003cp\u003eThe process parameters of HPD were also consistent with the SCD scenario, except that conventional high-pressure carbon dioxide was used instead of the ScCO\u003csub\u003e2\u003c/sub\u003e. This scenario aimed to evaluate the influence of the SCF reaction environment.\u003c/p\u003e\n\u003ch3\u003e4) Unpretreated (UPT)\u003c/h3\u003e\n\u003cp\u003eThe CR was kept in its original state, without undergoing any pretreatment, served as the control group for this study.\u003c/p\u003e\u003cp\u003eThe key parameters of the four CR pretreatment scenarios are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Note that all other condition parameters not listed in the table were kept constant across the scenarios.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eKey parameters of four CR pretreatment scenarios\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCR pretreatment scenario\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eScCO\u003csub\u003e2\u003c/sub\u003e temperature (\u0026deg;C)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eScCO\u003csub\u003e2\u003c/sub\u003e pressure (MPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003ePretreatment duration (h)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eDD agent dosage (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eUPT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHPD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e205\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSCS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e205\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSCD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e205\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Material testing\u003c/h2\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e2.2.1 Sol fraction test\u003c/h2\u003e\u003cp\u003eThe rubber component in CR is composed of sol and gel fractions. The sol fraction consists primarily of linear rubber molecular chains formed by the breaking of crosslink bonds during de-crosslinking and is soluble in toluene. The gel fraction consists of the residual rubber in a crosslinked structure, which is insoluble in toluene. To determine the sol content, a precise amount of CR sample was weighed using an analytical balance. The sample was wrapped in slow quantitative filter paper and subjected to Soxhlet extraction using toluene as the solvent. After extraction, the residue retained in the filter paper was dried to a constant weight in a vacuum oven at 50\u0026deg;C and weighed. The sol fraction was calculated using the following formula:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}{f}_{s}=\\frac{{m}_{0}-{m}_{1}}{a{m}_{0}}\\#\\left(1\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eWhere: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{f}_{s}\\)\u003c/span\u003e\u003c/span\u003e is the sol content of the CR (%); \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{m}_{0}\\)\u003c/span\u003e\u003c/span\u003e is the initial mass of the CR before extraction (g); \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{m}_{1}\\)\u003c/span\u003e\u003c/span\u003e is the final mass of the CR after vacuum drying (g); \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:a\\)\u003c/span\u003e\u003c/span\u003e is the rubber content in the CR (%).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003e2.2.2 Crosslinking density test\u003c/h2\u003e\u003cp\u003eThe crosslink density of CR was measured using the equilibrium swelling method. First, a precise mass of CR was weighed and then immersed in toluene for 72 h to reach equilibrium swelling. After removal, the sample was blotted dry to remove surface solvent and weighed. The sample was then dried to a constant weight in a vacuum oven at 60\u0026deg;C and weighed again. The crosslink density was calculated based on the Flory-Rehner equation (Flory and Rehner, 1943):\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}{\\nu\\:}_{x}=-\\frac{\\text{l}\\text{n}\\left(1-{\\varphi\\:}_{r}\\right)+{\\varphi\\:}_{r}+\\chi\\:{{\\varphi\\:}_{r}}^{2}}{{v}_{s}\\left({{\\varphi\\:}_{r}}^{\\frac{1}{3}}-\\frac{1}{2}{\\varphi\\:}_{r}\\right)}\\#\\left(2\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eWhere: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\nu\\:}_{x}\\)\u003c/span\u003e\u003c/span\u003e is the crosslink density (mol/cm\u003csup\u003e3\u003c/sup\u003e); \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\chi\\:\\)\u003c/span\u003e\u003c/span\u003e is the interaction parameter between rubber and solvent, set to 0.4; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{v}_{s}\\)\u003c/span\u003e\u003c/span\u003e is the molar volume of the solvent (mol/ml), set to 106.2 mol/ml; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\varphi\\:}_{r}\\)\u003c/span\u003e\u003c/span\u003e is the volume fraction of rubber in the swollen CR system, calculated as follows:\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}{\\varphi\\:}_{r}=\\frac{\\left({m}_{3}-{m}_{0}\\varphi\\:\\right)/{\\rho\\:}_{r}}{\\left({m}_{3}-{m}_{0}\\varphi\\:\\right)/{\\rho\\:}_{r}+\\left({m}_{2}-{m}_{3}\\right){\\rho\\:}_{s}}\\#\\left(3\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eWhere: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{r}\\)\u003c/span\u003e\u003c/span\u003e is the density of rubber without carbon black (g/cm\u003csup\u003e3\u003c/sup\u003e), set to 0.95 g/cm\u003csup\u003e3\u003c/sup\u003e; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{s}\\)\u003c/span\u003e\u003c/span\u003e is the density of toluene (g/ml), set to 0.865 g/ml; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varphi\\:\\)\u003c/span\u003e\u003c/span\u003e is the volume fraction of carbon black in the CR, approximately 0.35; \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{m}_{0}\\)\u003c/span\u003e\u003c/span\u003e is the initial mass of the CR (g); \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{m}_{2}\\)\u003c/span\u003e\u003c/span\u003e is the mass of the swollen CR after surface drying (g); \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{m}_{3}\\)\u003c/span\u003e\u003c/span\u003e is the final mass of the swollen CR after vacuum drying (g).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\u003ch2\u003e2.2.3 FTIR test\u003c/h2\u003e\u003cp\u003eThe molecular structure and functional groups of CR were analyzed using a Fourier transform infrared spectrometer equipped with an Attenuated Total Reflectance (ATR) accessory. The sample was placed on the ATR crystal and fixed with a pressure head. The spectral range was set to 4000\u0026thinsp;\u0026minus;\u0026thinsp;600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, with 32 scans per measurement at a resolution of 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. After each test, the ATR accessory was cleaned with tetrahydrofuran-soaked lint-free cotton, and background single-channel spectra were remeasured. Each CR sample was tested at least three times to ensure accuracy.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section3\"\u003e\u003ch2\u003e2.2.4 TGA test\u003c/h2\u003e\u003cp\u003eThe thermal stability and composition of CR were analyzed using a thermogravimetric analyzer (TGA). Approximately 15 mg of CR was placed in a crucible, and the test began after the mass stabilized. The initial temperature was set to 50\u0026deg;C, maintained for 1 min, and then increased to 600\u0026deg;C at a heating rate of 10\u0026deg;C/min under a nitrogen atmosphere.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\u003ch2\u003e2.2.5 SEM observation\u003c/h2\u003e\u003cp\u003eThe microstructure of CR particles was observed using scanning electron microscopy (SEM). As CR is non-conductive, samples were coated with a gold-palladium alloy using an Oxford Quorum SC7620 sputter coater (10 mA for 45 seconds). Images were captured with a ZEISS Gemini 300 SEM at an accelerating voltage of 3 kV using an SE2 secondary electron detector. SEM magnifications of 200\u0026times;, 500\u0026times;, 1000\u0026times;, and 2000\u0026times; were used based on particle size.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Crosslinking structure\u003c/h2\u003e\u003cp\u003eThe sol content and crosslink density of CR pretreated using four different processes were plotted on the Horikx model diagram to analyze the changes in the crosslinking structure after pretreatments, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e (Horikx, 1956; Seghar et al., 2019). It should be noted that in vulcanized rubber crosslinking networks, the relationships between sol content and the crosslink density ratio, under scenarios where only main-chain scission or only crosslinking bond cleavage occurs, are represented by the red and blue curves, respectively.\u003c/p\u003e\u003cp\u003eThe experimental data point for unpretreated CR (i.e., UPT) is located at the starting position of the curves, representing its initial state. The experimental data points for CR pretreated using HPD are situated near and slightly above the red curve. This indicates that, under high-pressure conditions, a certain degree of de-crosslinking occurs. However, due to the inferior solubility and permeability of compressed high-temperature CO\u003csub\u003e2\u003c/sub\u003e gas compared to ScCO\u003csub\u003e2\u003c/sub\u003e, the devulcanizing agent cannot fully contact the crosslinked network. Consequently, de-crosslinking reactions are limited to the CR surface, leading to poor uniformity. Additionally, under these high-temperature conditions, the scission of rubber main chains contribute to a degree comparable to the cleavage of crosslink bonds.\u003c/p\u003e\u003cp\u003eFor CR pretreated using SCS, the experimental data points are closer to the starting position of the curves. This suggests that, in the absence of a de-crosslinking agent, the CR undergoes random scission of main chains and crosslink bonds driven by the penetration of ScCO\u003csub\u003e2\u003c/sub\u003e. As a result, the crosslink density decreases slightly, accompanied by the generation of a small amount of sol content. In contrast, the experimental data points for CR pretreated using SCD lie between the red and blue curves. This indicates that, under these conditions, the CR undergoes a more thorough de-crosslinking reaction dominated by the combined effects of ScCO\u003csub\u003e2\u003c/sub\u003e and the chemical attack of DD. Crucially, DD achieves sufficient contact with the crosslinking network, allowing de-crosslinking to occur uniformly in both the surface and interior of the CR. Nonetheless, it is evident that some degree of unavoidable main chains scission occurs under high temperatures, leading to an additional increase in sol content after pretreatment.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Functional groups\u003c/h2\u003e\u003cp\u003eThe FTIR spectra of CR subjected to four different pretreatment processes and the characteristic peaks of interests are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eThe peak at 700 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the phenyl structure. As the degree of de-crosslinking increases, more de-crosslinking agents interact with the crosslinking network within the CR and undergo reactions. The de-crosslinking agent used in this study, namely DD, contains a phenyl structure, and its reaction and incorporation into the CR enhance this characteristic peak.\u003c/p\u003e\u003cp\u003eThe peaks at 570 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 475 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e correspond to the C-S and S-S bonds, respectively. As the de-crosslinking (including both SCD and HPD) pretreatment progresses, the crosslinked bonds (C-S and S-S) in the vulcanized rubber are gradually cleaved, leading to a decrease in the intensity of these two peaks. The peak at 2725 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e represents the S-H bond, and its intensity diminishes progressively with the occurrence of the de-crosslinking reaction.\u003c/p\u003e\u003cp\u003eAdditionally, the peak at 1310 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is associated with the C-C bonds in the main chain of rubber macromolecules, while the peaks at 1670 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 840 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e correspond to the RCH\u0026thinsp;=\u0026thinsp;RCH structure, and the peak at 3000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to the =\u0026thinsp;CH structure. During the reaction, the intensity of these peaks weakens as the reaction progresses, indicating that the main chain structure of rubber macromolecules is also damaged to some extent under high-temperature conditions.\u003c/p\u003e\u003cp\u003eFurthermore, the peak at 970 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the C\u003csub\u003e4\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e structure of butadiene. As the de-crosslinking (including both SCD and HPD) reaction proceeds, more vinyl side groups are cleaved due to crosslinking, forming 1,4-double bonds, which enhance the intensity of this peak. The peaks at 1725 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1080 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e represent the C\u0026thinsp;=\u0026thinsp;O and S-O structures, respectively. After pretreatment, the intensities of these two peaks decrease, suggesting that these structures are also disrupted during the reaction process.\u003c/p\u003e\u003cp\u003eIn summary, during the SCD/HPD pretreatment, the radicals generated by the DD selectively couple with the radicals from the vulcanized rubber, breaking the crosslinking bonds and introducing the phenyl structure of the DD into the CR. However, due to indiscriminate high-temperature effects, the rubber main chain and other unstable structures in the CR are also inevitably damaged.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec19\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Chemical composition\u003c/h2\u003e\u003cp\u003eTo evaluate the thermal stability and analyze the composition of CR subjected to four different pretreatment processes, TGA was conducted to obtain weight loss curves. Additionally, the weight loss curves were differentiated with respect to temperature to derive derivative thermogravimetric (DTG) curves. Both the TGA and DTG curves for CR under different pretreatment processes are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eIn analyzing TGA and DTG curves, the rubber type is typically determined by the temperature corresponding to the maximum decomposition rate. Specifically, for the main components of tire rubber, natural rubber (NR) exhibits a maximum decomposition temperature around 375\u0026deg;C, while synthetic rubbers, such as styrene-butadiene rubber (SBR) and butadiene rubber (BR), generally decompose at a maximum rate near 450\u0026deg;C. The inorganic fillers, which do not decompose within the temperature range studied (50\u0026ndash;600\u0026deg;C), are considered the residual components.\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the DTG curves of unpretreated CR (i.e., UPT) display a single peak at approximately 450\u0026deg;C, indicating that the primary rubber component in samples is synthetic rubber. However, the samples also show a weight loss of about 10% near 375\u0026deg;C, suggesting the presence of natural rubber in the unpretreated CR.\u003c/p\u003e\u003cp\u003eIn contrast, the DTG curves of the three pretreated CRs reveal an additional peak at approximately 280\u0026deg;C, though its intensity varies. This peak is probably due to the thermal decomposition of sol and partially degraded gel produced during the pretreatment process. Among these, the peak is relatively less pronounced for CR subjected to the SCS pretreatment. However, for the HPD or SCD processes, where a higher proportion of sol is generated, the compositional changes result in a more prominent peak at this temperature.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe temperature ranges corresponding to the two peaks can be used to broadly classify the components of CR based on their pyrolysis characteristics into three categories: \u0026ldquo;sol,\u0026rdquo; rubber hydrocarbons, and inert fillers. Note that the term \u0026ldquo;sol\u0026rdquo; used here differs from the sol discussed in previous sections. In this context, \u0026ldquo;sol\u0026rdquo; refers to rubber components within the pretreated CR that become more prone to pyrolysis. Based on this classification, the compositional proportions of the three components under different pretreatment processes are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003eIt can be observed that the proportion of \u0026ldquo;sol\u0026rdquo; increases after pretreatments, while the proportions of rubber hydrocarbons and inert fillers decrease. The changes are relatively small for CR subjected to SCS pretreatment but are more pronounced for those treated with HPD and SCD processes. The increase in \u0026ldquo;sol\u0026rdquo; proportion results from the synergistic effects of targeted cleavage of crosslinking bonds by DD, enhanced penetration of ScCO\u003csub\u003e2\u003c/sub\u003e, and random thermal degradation under high-temperature conditions. These combined factors loosen the crosslinking network structure within the CR, making some \u0026ldquo;sol\u0026rdquo; components more susceptible to pyrolysis. Consequently, the proportion of rubber hydrocarbons that retain their original crosslinking network structure decreases.\u003c/p\u003e\u003cp\u003eThe reduction in filler content after pretreatments may be attributed to the breakdown of the crosslinking network, which releases fillers such as carbon black that were previously constrained by the rubber molecular network. However, it is worth noting that after SCD pretreatment, the CR loses its original granular form, and the small sample size for TGA analysis introduces some variability despite repeated experiments.\u003c/p\u003e\u003cp\u003eAdditionally, the peak temperature on the DTG curve represents the temperature at which the maximum weight loss rate occurs during thermal decomposition, as presented Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The peak temperatures of CR pretreated with HPD and SCS processes remain relatively unchanged. In contrast, the peak temperature of CR subjected to SCD increases by approximately 50\u0026deg;C. This is because the degree of de-crosslinking is limited in the first two processes due to the absence of a ScCO\u003csub\u003e2\u003c/sub\u003e reaction environment or a de-crosslinking agent. However, in the latter case, the combined action of ScCO\u003csub\u003e2\u003c/sub\u003e and the de-crosslinking agent DD enables thorough de-crosslinking. The de-crosslinking agent used in this study contains phenyl groups that react with the crosslinking bonds within the CR and form side chains attached to the rubber macromolecular backbone. This increases the thermal stability of the rubber hydrocarbons in the CR after this pretreatment process.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003ePeak temperature in DTG curves of CR with different pretreatments\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCR pretreatment\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePeak temperature (\u0026deg;C)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u0026ldquo;Sol\u0026rdquo; fraction\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRubber hydrocarbon\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eFiller\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eUPT\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e429.93\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e7.59\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e50.44\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e41.97\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHPD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e432.18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e28.73\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e30.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e40.32\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSCS\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e428.74\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e15.85\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e51.88\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e32.28\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSCD\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e482.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e23.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e42.62\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e34.35\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Surface morphology\u003c/h2\u003e\u003cp\u003eTo investigate the changes in the microstructure of CR under different pretreatment processes, SEM was used to capture images of CR samples at various magnifications, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Unpretreated CR (i.e., UPT) exhibits a rough surface with irregular particle shapes and pore structures of varying sizes. In contrast, the microstructure of CR underwent significant changes after the three pretreatment processes, with a common feature being pronounced swelling, where the particle volume expanded considerably. However, each process also displayed distinct characteristics.\u003c/p\u003e\u003cp\u003eUnder the SCS pretreatment process, the CR underwent physical swelling while largely retaining its original particle morphology, rough surface texture, and pore structure. Its surface became more porous and fluffier. This indicates that this process primarily involves physical treatment rather than chemical reactions.\u003c/p\u003e\u003cp\u003eIn the HPD pretreatment process, the CR also maintained its particle morphology, but its surface became smoother, and the pore structures gradually disappeared. This is primarily due to the action of the de-crosslinking agent, which caused crosslink breakage on the surface of the CR, forming a layer of sol that covered the original surface.\u003c/p\u003e\u003cp\u003eFinally, the CR subjected to the SCD pretreatment process completely lost its original particle morphology and other surface features, presenting an entirely different microstructure. This is because the extensive de-crosslinking reaction significantly disrupted the internal crosslinked network of the CR. At the microscopic scale, the morphology appeared nearly smooth and fluid-like, which can essentially be interpreted as an external manifestation of the high sol content in terms of microstructural characteristics.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec21\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Reaction mechanisms\u003c/h2\u003e\u003cp\u003eBased on the characterizations of CR after pretreatments detailed above, the main reaction mechanism of the supercritical pretreatment of CR in ScCO\u003csub\u003e2\u003c/sub\u003e can be summarized as follows:\u003c/p\u003e\u003cp\u003e1) Swelling effect of ScCO\u003csub\u003e2\u003c/sub\u003e: Due to the high permeability of ScCO\u003csub\u003e2\u003c/sub\u003e, CR can be penetrated by ScCO\u003csub\u003e2\u003c/sub\u003e molecules and undergo significant physical swelling, causing an obvious volume expansion. Although the internal crosslinked network of the rubber is not completely broken, it becomes more loosened, providing a foundation for the subsequent de-crosslinking agent molecules to enter and participate in the reaction, as shown in the second step in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e2) Targeted attack on the crosslinking network by the de-crosslinking agent under the assistance of ScCO\u003csub\u003e2\u003c/sub\u003e: After the addition of the de-crosslinking agent, the high solubility and permeability of ScCO\u003csub\u003e2\u003c/sub\u003e, combined with the targeted attack by the de-crosslinking agents on the crosslinking bonds, as well as the activation of the de-crosslinking agent\u0026rsquo;s reactivity under high-temperature conditions, enable a more thorough, uniform, and efficient de-crosslinking compared to conventional high-temperature and high-pressure conditions. Most of the crosslinked structures are broken down, as shown in the third step of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. However, it should be noted that due to the high-temperature conditions, some breaking of the main chain is unavoidable during this process.\u003c/p\u003e\u003cp\u003e3) Release and migration of fillers: As the crosslinked network inside the CR disintegrates, fillers that were originally confined within the rubber, such as carbon black and silica, are gradually released. They then migrate with the stirring of the reactants and the flow of ScCO\u003csub\u003e2\u003c/sub\u003e, as shown in the last step of Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe specific reaction mechanisms of CR during the SCD pretreatment process are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Initially, the de-crosslinking agent (i.e., DD) and CR undergo reactions (a) and (b) under high-temperature conditions. The de-crosslinking agent decomposes to generate free radicals, while the rubber inside the CR also undergoes thermal decomposition, with some crosslink bonds and macromolecular rubber chains randomly breaking. Subsequently, the free radicals of the de-crosslinking agent further react with the vulcanized rubber inside the CR, as expressed in reaction (c). This reaction pathway includes two forms: 1) The free radicals of the de-crosslinking agent break the crosslinking bonds (S-S bonds) inside the CR; 2) The free radicals break the C\u0026thinsp;=\u0026thinsp;C double bonds on the rubber main chain. Throughout the pretreatment process, the thermal breaking of the rubber macromolecular chains in reaction (b) is continuous and inevitable, but overall, it does not dominate the process.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eThis study investigates the structural changes of CR under different pretreatment scenarios by different physicochemical means, with a focus on the pretreatment mechanisms in ScCO\u003csub\u003e2\u003c/sub\u003e environments. The key findings are summarized as follows:\u003c/p\u003e\u003cp\u003eThe pretreatment processes significantly change the crosslinking structure of CR. Compared to the initial state (UPT), HPD induces partial surface de-crosslinking, SCS causes slight random scission, and SCD achieves uniform and thorough de-crosslinking throughout the material in ScCO\u003csub\u003e2\u003c/sub\u003e, though some main-chain scission is unavoidable at high temperatures.\u003c/p\u003e\u003cp\u003eFTIR analysis shows that SCD and HPD cleave crosslinking bonds and incorporate the phenyl structure of de-crosslinking agent into the CR network, while high-temperature conditions also cause damage to rubber main chains and unstable structures.\u003c/p\u003e\u003cp\u003eThermal analysis reveals that SCS induces minor \u0026ldquo;sol\u0026rdquo; content increase compared to UPT, and HPD and SCD significantly increase \u0026ldquo;sol,\u0026rdquo; with SCD further improving thermal stability by incorporating phenyl side chains.\u003c/p\u003e\u003cp\u003eMicrostructural observations highlight distinct effects of pretreatment. UPT retains its rough, irregular structure; SCS increases porosity without changing particle morphology; HPD forms a smoother sol layer; and SCD destroyed the structure completely, yielding a smooth, fluid-like appearance.\u003c/p\u003e\u003cp\u003eThe supercritical pretreatment of CR involves several key mechanisms: physical swelling by ScCO\u003csub\u003e2\u003c/sub\u003e loosens the crosslinked network, facilitating efficient and uniform de-crosslinking by the de-crosslinking agent. This process also leads to the release and migration of fillers, while some unavoidable main-chain scission occurs under high-temperature conditions.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding Declaration\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported by the Research Fund for Central Universities of China under grant numbers of 22120240341, National Natural Science Foundation of China at the number of 52308441 and National Natural Science Foundation of Sichuan Province at the number of 2024NSFSC0930.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Data Availability\u003c/p\u003e\n\u003cp\u003eData available on request from the authors\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Author Contribution Declaration\u003c/p\u003e\n\u003cp\u003eJin Li: Conceptualization, Data Collection, Writing - Original Draft; Jiayu Wang: Data Collection, Validation; Mohsen Alae: Formal analysis, Data Curation; Feipeng Xiao: Conceptualization, Writing - Review \u0026amp; Editing, Funding acquisition\u003c/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eCompeting Interest Declaration\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAntoniou, N.A., Zorpas, A.A., 2019. Quality protocol and procedure development to define end-of-waste criteria for tire pyrolysis oil in the framework of circular economy strategy. Waste Management 95, 161\u0026ndash;170. https://doi.org/10.1016/j.wasman.2019.05.035\u003c/li\u003e\n\u003cli\u003eAsaro, L., Gratton, M., Poirot, N., Seghar, S., A\u0026iuml;t Hocine, N., 2020. Devulcanization of natural rubber industry waste in supercritical carbon dioxide combined with diphenyl disulfide. Waste Management 118, 647\u0026ndash;654. https://doi.org/10.1016/j.wasman.2020.09.026\u003c/li\u003e\n\u003cli\u003eBoy\u0026egrave;re, C., J\u0026eacute;r\u0026ocirc;me, C., Debuigne, A., 2014. Input of supercritical carbon dioxide to polymer synthesis: An overview. European Polymer Journal 19.\u003c/li\u003e\n\u003cli\u003eChen, D.T., Perman, C.A., Riechert, M.E., Hoven, J., 1995. Depolymerization of tire and natural rubber using supercritical fluids. Journal of Hazardous Materials 44, 53\u0026ndash;60. https://doi.org/10.1016/0304-3894(95)00047-X\u003c/li\u003e\n\u003cli\u003eFlory, P.J., Rehner, J., 1943. Statistical Mechanics of Cross‐Linked Polymer Networks I. Rubberlike Elasticity. The Journal of Chemical Physics 11, 512\u0026ndash;520. https://doi.org/10.1063/1.1723791\u003c/li\u003e\n\u003cli\u003eGao, H., Liu, H., Song, C., Hu, G., 2019. Infusion of graphene in natural rubber matrix to prepare conductive rubber by ultrasound-assisted supercritical CO2 method. Chemical Engineering Journal 368, 1013\u0026ndash;1021. https://doi.org/10.1016/j.cej.2019.03.026\u003c/li\u003e\n\u003cli\u003eGuo, F., Zhang, J., Pei, J., Ma, W., Hu, Z., Guan, Y., 2020. Evaluation of the compatibility between rubber and asphalt based on molecular dynamics simulation. Front. Struct. Civ. Eng. 14, 435\u0026ndash;445. https://doi.org/10.1007/s11709-019-0603-x\u003c/li\u003e\n\u003cli\u003eHorikx, M.M., 1956. Chain Scissions in a polymer network. Journal of Polymer Science 19, 445\u0026ndash;454.\u003c/li\u003e\n\u003cli\u003eLi, D., Leng, Z., Yao, L., Cao, R., Zou, F., Li, G., Wang, Hainian, Wang, Haopeng, 2024. Mechanical, economic, and environmental assessment of recycling reclaimed asphalt rubber pavement using different rejuvenation schemes. Resources, Conservation and Recycling 204, 107534. https://doi.org/10.1016/j.resconrec.2024.107534\u003c/li\u003e\n\u003cli\u003eLi, F., Zhang, X., Wang, L., Zhai, R., 2022. The preparation process, service performances and interaction mechanisms of crumb rubber modified asphalt (CRMA) by wet process: A comprehensive review. Construction and Building Materials 354, 129168. https://doi.org/10.1016/j.conbuildmat.2022.129168\u003c/li\u003e\n\u003cli\u003eLi, J., Chen., Z., Xiao, F., Amirkhanian, S.N., 2022a. Solution Soaking Pretreatments of Crumb Rubber for Improving Compatibility of Rubberized Asphalt. J. Mater. Civ. Eng. 34, 04021377. https://doi.org/10.1061/(ASCE)MT.1943-5533.0004015\u003c/li\u003e\n\u003cli\u003eLi, J., Chen, Z., Xiao, F., Amirkhanian, S.N., 2021a. Surface activation of scrap tire crumb rubber to improve compatibility of rubberized asphalt. Resources, Conservation and Recycling 169, 105518. https://doi.org/10.1016/j.resconrec.2021.105518\u003c/li\u003e\n\u003cli\u003eLi, J., Wang, J., Xiao, F., Amirkhanian, S.N., 2021b. Characterizing Compatibility of Crumb Rubber Modified Asphalt by Customized Drainage Method. J. Test. Eval. 49, 20190856. https://doi.org/10.1520/JTE20190856\u003c/li\u003e\n\u003cli\u003eLi, J., Xiao, X., Chen, Z., Xiao, F., Amirkhanian, S.N., 2022b. Internal de-crosslinking of scrap tire crumb rubber to improve compatibility of rubberized asphalt. Sustainable Materials and Technologies 32, e00417. https://doi.org/10.1016/j.susmat.2022.e00417\u003c/li\u003e\n\u003cli\u003eLi, K., Xu, Z., 2019. A review of current progress of supercritical fluid technologies for e-waste treatment. Journal of Cleaner Production 227, 794\u0026ndash;809. https://doi.org/10.1016/j.jclepro.2019.04.104\u003c/li\u003e\n\u003cli\u003eLiang, M., Qiu, Z., Luan, X., Qi, C., Guo, N., Liu, Z., Su, L., Yao, Z., Zhang, J., 2022. The Effects of Activation Treatments for Crumb Rubber on the Compatibility and Mechanical Performance of Modified Asphalt Binder and Mixture by the Dry Method. Front. Mater. 9, 845718. https://doi.org/10.3389/fmats.2022.845718\u003c/li\u003e\n\u003cli\u003eLiang, M., Xin, X., Fan, W., Luo, H., Wang, X., Xing, B., 2015. Investigation of the rheological properties and storage stability of CR/SBS modified asphalt. Construction and Building Materials 74, 235\u0026ndash;240. https://doi.org/10.1016/j.conbuildmat.2014.10.022\u003c/li\u003e\n\u003cli\u003ePhiri, M.M., Phiri, M.J., Formela, K., Wang, S., Hlangothi, S.P., 2022. Grafting and reactive extrusion technologies for compatibilization of ground tyre rubber composites: Compounding, properties, and applications. Journal of Cleaner Production 369, 133084. https://doi.org/10.1016/j.jclepro.2022.133084\u003c/li\u003e\n\u003cli\u003ePicado-Santos, L.G., Capit\u0026atilde;o, S.D., Neves, J.M.C., 2020. Crumb rubber asphalt mixtures: A literature review. Construction and Building Materials 247, 118577. https://doi.org/10.1016/j.conbuildmat.2020.118577\u003c/li\u003e\n\u003cli\u003ePolacco, G., Filippi, S., Merusi, F., Stastna, G., 2015. A review of the fundamentals of polymer-modified asphalts: Asphalt/polymer interactions and principles of compatibility. Advances in Colloid and Interface Science 224, 72\u0026ndash;112. https://doi.org/10.1016/j.cis.2015.07.010\u003c/li\u003e\n\u003cli\u003eSeghar, S., Asaro, L., A\u0026iuml;t Hocine, N., 2019. Experimental Validation of the Horikx Theory to be Used in the Rubber Devulcanization Analysis. J Polym Environ 27, 2318\u0026ndash;2323. https://doi.org/10.1007/s10924-019-01513-z\u003c/li\u003e\n\u003cli\u003eSong, P., Wan, C., Xie, Y., Formela, K., Wang, S., 2018. Vegetable derived-oil facilitating carbon black migration from waste tire rubbers and its reinforcement effect. Waste Management 78, 238\u0026ndash;248. https://doi.org/10.1016/j.wasman.2018.05.054\u003c/li\u003e\n\u003cli\u003eYin, L., Yang, X., Shen, A., Wu, H., Lyu, Z., Li, B., 2021. Mechanical properties and reaction mechanism of microwave-activated crumb rubber-modified asphalt before and after thermal aging. Construction and Building Materials 267, 120773. https://doi.org/10.1016/j.conbuildmat.2020.120773\u003c/li\u003e\n\u003cli\u003eYu, H., Deng, G., Zhang, Z., Zhu, M., Gong, M., Oeser, M., 2021. Workability of rubberized asphalt from a perspective of particle effect. Transportation Research Part D: Transport and Environment 91, 102712. https://doi.org/10.1016/j.trd.2021.102712\u003c/li\u003e\n\u003cli\u003eZheng, W., Wang, H., Chen, Y., Ji, J., You, Z., Zhang, Y., 2021. A review on compatibility between crumb rubber and asphalt binder. Construction and Building Materials 297, 123820. https://doi.org/10.1016/j.conbuildmat.2021.123820\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Solid waste valorization, End-of-life tires, Crumb rubber, Supercritical carbon dioxide, Pretreatment","lastPublishedDoi":"10.21203/rs.3.rs-6994293/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6994293/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe crumb rubber (CR) derived from end-of-life tires (ELTs) are widely used in paving asphalt modification, while the pretreatments are generally necessary to improve the CR-asphalt compatibility. This study explores the structural changes in CR under various pretreatment scenarios, with an emphasis on mechanisms in supercritical carbon dioxide (ScCO\u003csub\u003e2\u003c/sub\u003e) reaction environments. Two supercritical pretreatments were designed, including supercritical de-crosslinking (SCD) and supercritical swelling (SCS). Two other pretreatments were also considered for comparison: high-pressure de-crosslinking (HPD) and unpretreated (UPT). The structural evolutions of CR underwent these pretreatments are systematically analyzed through different physicochemical approaches. The results show that SCS causes slight random scission of rubber crosslinking structure, while SCD achieves uniform and thorough de-crosslinking of CR. However, high temperatures also unavoidably cause some structural damage during both supercritical pretreatments. Thermal analysis reveals that SCS induces minor \u0026ldquo;sol\u0026rdquo; content increase, while HPD and SCD greatly increase \u0026ldquo;sol,\u0026rdquo; with SCD further improving thermal stability of CR. Microstructural observations show distinct morphology changes, ranging from increased porosity with SCS to complete structural disruption under SCD. The supercritical pretreatment processes involve ScCO\u003csub\u003e2\u003c/sub\u003e-induced swelling, enabling efficient and uniform de-crosslinking, accompanied by filler release under high temperatures. These findings provide insight into the mechanisms underlying CR pretreatment in ScCO\u003csub\u003e2\u003c/sub\u003e environments.\u003c/p\u003e","manuscriptTitle":"Unraveling structural evolution of crumb rubber derived from end-of-life tires in supercritical fluid environments","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-14 15:25:51","doi":"10.21203/rs.3.rs-6994293/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-08-10T08:07:34+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-08-10T06:33:15+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Journal of Polymer Research","date":"2025-08-07T19:53:47+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-16T11:49:33+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Polymer Research","date":"2025-07-15T09:35:18+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"03a943b3-e491-49f6-9e55-2e88183af4c7","owner":[],"postedDate":"August 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-10-06T16:05:36+00:00","versionOfRecord":{"articleIdentity":"rs-6994293","link":"https://doi.org/10.1007/s10965-025-04595-7","journal":{"identity":"journal-of-polymer-research","isVorOnly":false,"title":"Journal of Polymer Research"},"publishedOn":"2025-09-29 15:57:18","publishedOnDateReadable":"September 29th, 2025"},"versionCreatedAt":"2025-08-14 15:25:51","video":"","vorDoi":"10.1007/s10965-025-04595-7","vorDoiUrl":"https://doi.org/10.1007/s10965-025-04595-7","workflowStages":[]},"version":"v1","identity":"rs-6994293","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6994293","identity":"rs-6994293","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}