Scientific Research China Petroleum Processing and Petrochemical Technology 2012,Vol. 14, No. 3, pp 39-43 September 30, 2012 The Mechanism of Asphalt Modification by Crumb Rubber Zhang Xiaoying 1 ; Xu Chuanjie 2 (1. State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying 257061; 2. Shtar Science & Technology Group, China University of Petroleum) Abstract: To investigate the mechanism of asphalt modification by crumb rubber, the interactions between FCC slurry and rubber particles were evaluated at different and time. The rules of the change in mass of SARA composition in FCC slurry were obtained before and after its interaction with rubber particles, which showed that crumb rubber not only absorbed saturates and aromatics but also resins. Asphaltenes promoted the desulfurization or degradation of crumb rubber during the interaction between asphalt and crumb rubber. Key words: asphalt; crumb rubber; mechanism; modification; SARA composition 1 Introduction With an increasing number of scrap tires [1-2], treating and making use of scrap tires become more important for environmental protection, because it will take more than one hundred years for scrap tires to degrade naturally [3]. Scrap tires can be made into crumb rubber, which can be used to modify the performance of asphalt. There are two kinds of processes to produce the crumb rubber-modified asphalt mixture, i.e.: the dry process and the wet process [4]. In the wet process, the crumb rubber is dispersed in bitumen to produce the crumb rubber-modified asphalt, which is then mixed with the aggregate to form a mixture. In the dry process, the recycled rubber from tire waste is mixed with the aggregate before introducing the binder into the mixture, since some large size recycled rubber acts as a part of the aggregate. The wet process can make better use of the highly elastic nature of crumb rubber [5], which is the predominant way to improve the pavement performance by using crumb rubber. Most studies are focused on the use of crumb rubber in asphalt by the wet process. There are many advantages resulted from using the crumb rubber-modified asphalt, including improving the performance of pavement at low, reducing noise, reducing pavement smoothness on icy and snowy days, enhancing anti-aging properties of asphalt, improving resistance to rutting (permanent deformation), extending the life of the road and improving the driving comfort [1, 6-7]. At present, the mechanism of modification of asphalt by crumb rubber supposes that the crumb rubber can absorb aromatic oils and light fractions from the bitumen and swells at low and then is subjected to desulfurization or degradation at high. This mechanism was inferred from the property change of modified asphalt [4, 8-13]. For example, the viscosity of the crumb rubber-modified asphalt increases with an increase of the temperature or the time under specific condition, which means that the interaction between asphalt and crumb rubber in swelling process, the broken network of cross-linked bonds in crumb rubber and the desulfurization or degradation of crumb rubber can lead to rapid loss of the viscosity [5], elastic modulus [12], elasticity and increase the phase angle. The 40 120 mesh crumb rubber is usually used in modified asphalt studies. This crumb rubber-modified asphalt cannot be directly used to explore the modification mechanism because there is no approach to separate crumb rubber from asphalt effectively, the components exchange between asphalt and crumb rubber cannot be determined. In order to achieve the goal, the FCC slurry is used as a simulated asphalt, and large-size rubber particles are used to substitute for the crumb rubber, and the changes in the SARA composition of FCC slurry before and after its interaction with rubber particles are determined to calculate Corresponding Author: Ms. Zhang Xiaoying, Telephone: +86-532-86984325; E-mail: zzxxyy1725@126.com. 39
China Petroleum Processing and Petrochemical Technology 2012,14(3):39-43 whether the rubber particles absorb components from or release components into the FCC slurry. These results can serve as a direct evidence to illustrate the mechanism of asphalt modification by crumb rubber. 2 Experimental 2.1 Materials The rubber particles used in this study were produced from scrap tires and processed by grinding with coarse surfaces to obtain irregular shapes and relatively large surface areas. The average diameter of each rubber particle was about 10 mm. The rubber particles processed by grinding can promote their interaction with the asphalt. FCC slurry was used to replace the asphalt in the research, because the components and the molecular structure of FCC slurry are similar to those of asphalt, and also some of its components are likely present in asphalt, while its molecular weight is lower than that of petroleum asphalt. The data of SARA composition of FCC slurry are listed in Table 1. Table 1 SARA composition of FCC slurry Item Content, m% Saturates 46.30 Aromatics 26.04 Resins 24.78 Asphaltenes 2.88 It can be seen from Table 1 that the relative content of SARA components in FCC slurry is similar to that of some asphalt grades in China. 2.2 Experimental procedure A definite amount of rubber particles and FCC slurry was put into a glass tube, which was covered with an aluminum foil, followed by heating for some time. After the heating procedure, the mixture was poured out from the tube and the FCC slurry was removed from the surface of rubber particles with n-heptane. Both the rubber particles and the FCC slurry residue were weighed to determine the mass change of the components absorbed by or released from the rubber particles. The composition of remaining FCC slurry was measured by the SARA composition analysis. The test were set at 130, 160, 190, 210, and 230, respectively, and the interaction time between FCC slurry and rubber particles was specified at 1 h, 3 h, 5 h, and 7 h, respectively. The mass ratio of rubber particles to FCC slurry before addition to the glass tube was kept constant. 2.3 Changes in SARA composition and calculation process The test method divided petroleum asphalt into four components based on their polarity and their diffusion ability in different solvents. The SARA composition analysis was carried out with a thin layer chromatograph scanner (IATROSCAN MK-5), which was made by the Intermass Fischer-Asia Pte Ltd, equipped with a chromatographic column of silica and IATROCORDER TC-II recorder, using n-heptane, toluene, and tetrahydrofuran as developing solvents to separate saturates, aromatics and resins, respectively. The mass of the remaining FCC slurry after interacting with rubber particles was determined by subtracting the mass of substance absorbed by rubber particles from the mass of original FCC slurry. The changes in the mass of SARA components in FCC slurry before and after interacting with rubber particles were calculated as follows: Change in the mass of certain component in rubber particles = (the mass of original FCC slurry percentage of the component in original FCC slurry - the mass of remaining FCC slurry percentage of the component in residual FCC slurry)/the mass of rubber particles 100%. Whether the components were absorbed by or released from the rubber crumbs was determined through the changes in SARA composition of the FCC slurry before and after interacting with rubber particles. 3 Results and Discussion Logically, if the rubber particles absorbed any component from the FCC slurry during the interaction of FCC slurry with rubber particles, the mass of that component in the FCC slurry would decrease, and then the change in mass of the component per gram of rubber particles would be positive. If the rubber particles released any component into the FCC slurry, then the mass of the component in the FCC slurry would increase, and the change in the mass 40
Zhang Xiaoying, et al. The Mechanism of Asphalt Modification by Crumb Rubber of that component per gram of rubber particles would be negative. So the change of the component in the FCC slurry would vary against that of the rubber particles. 3.1 Change in the mass of saturates The changes in mass of saturates in rubber particles with the interaction time and temperature are listed in Figure 1. It can be seen from Figures 1 that the rubber particles would absorb saturates during all interactions and the absorption could cause swelling of the crumb rubber. The absorption or release of saturates by rubber particles depended on the degree of the cross-linked networks of vulcanized rubber during swelling of rubber particles and the molecular structures of components contained in the FCC slurry. In this study, rubber particles could interact with FCC slurry in different ways under different conditions, which could lead to different trends on the change in the mass of saturates. Figure 1 The change of saturates with time at different During processing of rubber tire, the rubber extender oil was added which contained most aromatics and some saturates. The material exchange between the FCC slurry and rubber particles was carried out mainly via penetrative diffusion. The concentration of saturates in FCC slurry was higher than that in rubber particles and the concentration of saturates in rubber particles was limited by the degree of cross-linked networks while the diffusion of saturates in FCC slurry was not constrained. So the diffusion rate of saturates from rubber particles to FCC slurry was slower than that from FCC slurry to rubber particles. The mass of saturates absorbed by rubber particles was more than that released from rubber particles under the conditions of low temperature and short contact time, so the change in the mass of saturates was positive. The main reason leading to the decreased mass change of saturates at 130 was the release of saturates from rubber particles to FCC slurry. As the temperature increased and the time elapsed, the rubber particles swelled sufficiently, and the diffusion rate of saturates became higher and higher, resulting in an increased absorption of saturates by rubber particles. The S S bonds and C C bonds in the cross-linked network of rubber became unstable and then broke down when the temperature was above 210 after 5 hours of reaction, and small molecules produced by rubber particles were released into FCC slurry, which caused a decreasing mass change of saturates in rubber. The broken cross-linked bonds and the release of small molecules caused a decreased volume of rubber particles. 3.2 Change in the mass of aromatics The changes in the mass of aromatics absorbed by rubber particles are listed in Figure 2. It can be seen from Figure 2 that the mass of absorbed aromatics increased with time when the temperature was below 210, and it decreased after 5 hours of reaction when the exceeded 210. The absorption of aromatics by rubber particles peaked at 210 over a reaction time of five hours. Similar to saturates, changes in the mass of aromatics were positive under all the conditions evaluated in the present study. Figure 2 Change of aromatics with time at different These data directly verified that rubber particles absorbing light fractions contained in asphalt caused swelling of 41
China Petroleum Processing and Petrochemical Technology 2012,14(3):39-43 crumb rubber. The mass of component absorbed into the rubber particles depended on the extent of the swelling of the cross-linked network and on the compatibility between the components in FCC slurry and those in the rubber particles. According to the principles of similarity and compatibility, aromatics in FCC slurry made crumb rubber swell more easily than saturates did. Since aromatics have condensed ring structures and higher molecular weight than saturates, the mass of aromatics absorbed by rubber particles was less than that of saturates. 3.3 Change in the mass of resins As shown in Figure 3, the change in the mass of resins in rubber particles increased at first and then decreased with the reaction time. After 5 hours of reaction, this change reached its maximum value at a temperature of about 190, while these changes reached their maximum values at other after a reaction time of 5 hours. Resins have aromatic structures and some polar functional groups which differ from those of aromatics. The molecular weight of resins is higher than that of aromatics and saturates and is lower than that of asphaltenes. Molecules in rubber particles are polymers of styrene and butadiene containing aromatic structures and polar functional groups with sulfur atoms. According to the compatibility and similarity principle, when the cross-linked network of rubber swells enough, resins can enter the cross-linked network too. So resins could be absorbed by rubber particles too. As shown in Figure 3, most of the changes in the mass of the resins absorbed by rubber particles were posi- Figure 3 Change of resins with time at different tive. There was no any impact of resins contained in the asphalt on the mechanism of modification, which was described in previous literature. In the current study, resins were found to be absorbed by crumb rubber to make crumb rubber swell. 3.4 Change in the mass of asphaltenes It can be seen from Figure 4 that changes in the mass of asphaltenes were negative and the absolute value of the change decreased with time at a constant temperature for reaction durations up to 6 hours. The change became positive after 6 hours of reaction, regardless of the reaction temperature. There were more asphaltenes in the FCC slurry after interaction with rubber particles. The amount of additional asphaltenes was related to reaction temperature and time. At a constant reaction time which was less than 5 hours, the higher the temperature was, the more asphaltenes released by rubber particles would be. At a reaction time of 7 hours, the higher the temperature was, the more asphaltenes absorbed by rubber particles would be. Figure 4 Change of asphaltenes with time at different Rubber particles tended to release asphaltenes into the FCC slurry within 6 hours and would absorb or consume asphaltenes from the slurry after 6 hours at all. The transition in the mass of asphaltenes from negative to positive showed that some chemical interactions had occurred. When the temperature was low or the reaction time was short, the structure of cross-linked network of rubber particles did not swell enough. So the rubber particles released molecular fragments into the FCC slurry at first. The molecular weight of those molecular fragments 42
Zhang Xiaoying, et al. The Mechanism of Asphalt Modification by Crumb Rubber was smaller than that of rubber molecules and higher than that of asphaltenes in the original FCC slurry. Those molecular fragments belonged to the asphaltenes which caused the decrease of asphaltenes concentration in the rubber particles and the increase of asphaltenes concentration in FCC slurry. As the temperature and the reaction time increased, these fragments in the FCC slurry could break down into smaller molecules including saturates, aromatics, and resins at the expense of asphaltenes. Thus the change in the mass of asphaltenes became positive. 4 Conclusions This paper described the investigation of the changes in the SARA composition of FCC slurry before and after the interactions between FCC slurry and rubber particles. Based on the results of this investigation, the following conclusions were drawn up: The changes in the total mass of the SARA components could directly evidence that crumb rubber swelled at low and that the reaction involving desulfurization or degradation of crumb rubber took place at higher. The asphalt components absorbed by crumb rubber not only included saturates and aromatics but also resins, and the interactions between these components and crumb rubber caused swelling of crumb rubber. The substances released from crumb rubber before the desulfurization or degradation reactions were mainly molecular fragments of crumb rubber, which were actually composed of large molecular asphaltenes. The reaction of desulfurization or degradation consumed asphaltenes and produced smaller molecules of saturates, aromatics, and resins and entered into asphalt during the interaction between these components and crumb rubber. References [1] Cheng Yuan. Prospect for application of waste rubber powder [J]. China Synthetic Rubber Industry, 2001, 24(2): 65-66 (in Chinese) [2] Qiu Qinghua, Jia Demin, Wang Feidi. Research progress on utilization of waste rubber powder [J]. Rubber Industry, 1997, 44(11): 691-695 (in Chinese) [3] Takallou H B, Hicks R G. Development of improved mix and construction guidelines for rubber-modified asphalt pavements[j]. Transportation Research Record, 1998, 1171: 113-120 [4] Abdurrahman M A. Engineering characterization of the interaction of asphalt with crumb rubber modifier (CRM)[J]. Science & Engineering, 1997, 57(8): 5197-5208 [5] Takallou H B, Sainton A. Advances in Technology of Asphalt Paving Materials Containing Used Tire Rubber[R]. Transportation Research Record, 1992, 1339: 23-29 [6] Terrel R L, Walter J L. Modified Asphalt Materials - The European Experience[J], Proceedings of AAPT. 1986, 55: 482-518 [7] Rameshchandra S V. Rheology of Crumb Rubber Modified Asphalt Binder and Mixes[D]. Texas A&M University, 1997. [8] Green E, Tolonen W. The chemical and physical properties of asphalt-rubber mixture: Part-I. Basic material behavior[r]. FHWA-AI-HPR14-162, Arizona Department of Transportation, 1977-03 [9] Bahia H, Davis, R, Effect of Crumb Rubber Modifier (CRM) on Performance-Related Properties of Asphalt Binders[C]// The 1994 Annual Meeting of the Association of Asphalt Paving Technologists, St Louis, Missouri, 1994: 53-56 [10] Zhang Xiaoying, Xu Chuanjie, Zhang Yuzhen. Effect of the solubility of crumb rubber on the properties of modified asphalt[j]. Petroleum Processing and Petrochemicals, 2006, 37(3): 53-56 (in Chinese) [11] Billiter T C, Chun J S, Pavision R R, et al. Investigation of the curing variables of asphalt rubber binder[j]. ACS Division of Fuel Chemistry Preprints, 1996, 41(4): 1221-1226 [12] Oliver J W H. Crumb rubber asphalt fatigue study phase 1: Binder testing[r]. Contract report CR IC6496C. ARRB Transport Research Ltd., Australia, 1997-11 [13] Zhang Xiaoying, Xu Chuanjie, Zhang Yuzhen. Effect of shearing temperature on the thermal depolymerization of crumb rubber asphalt system[j]. Petroleum Processing and Petrochemicals, 2010, 41(1): 55-58 (in Chinese) 43