Investigation of Swelling and Dissolution Process of Natural Rubber in Aromatic Oil

(Department of Petroleum Processing, East China University of Science & Technology, Shanghai 200237)

Investigation of Swelling and Dissolution Process of Natural Rubber in Aromatic Oil

Wang Feng; Kuang Minming; Li Guanlong; Zhou Xiaolong; Li Chenglie

(Department of Petroleum Processing, East China University of Science & Technology, Shanghai 200237)

Aromatic oil has been used to promote the properties of crumb rubber modified asphalt which is an ideal method to deal with the resource utilization of waste rubber tires and by-product of refinery. Furfural extract oil (FEO) was separated into the light fraction and the heavy fraction. Swelling and dissolution process of natural rubber sheet in these three oil samples was investigated to shed light on the interaction mechanism. Crumb rubber also interacted on FEO and asphalt respectively. Energy dispersive spectrometer (EDS), thermo-gravimetric analysis (TGA) and scanning electron microscope (SEM) were used to characterize the chemical and structural properties of processed rubber. The chemical composition of processed oils and asphalt was investigated by using the hydrocarbon group analysis (SARA) and gel permeation chromatography. The results revealed that the swelling rate and mass loss of rubber in oils were much higher than those in asphalt and rose with an increasing processing temperature. The heavy fraction of FEO had more diffusion and dissolving capability than the light fraction, whilst compatibility was observed between the heavy fraction and the light fraction. Selective absorption was not observed in the study and detachment of dissolved rubber was disseminated from the outside to the inside. The cross-linking degree of the residue rubber was unchanged with the processing time, and sulfur predominantly remained in the undissolved rubber. Dissolution of crumbed rubber in oils was attributed to devulcanization, while that in the asphalt was mainly attributed to depolymerization.

rubber; aromatic oil; asphalt; swelling; dissolution

1 Introduction

The development of automotive industry may produce approximately 1000 million tires worldwide every year[1]. This number is still rapidly increasing in this decade; hence, the disposal of scrap tires has become a serious issue in solid waste management. The only ideal market with a high potential for expansion that can solve the scrap tire pollution problem is considered to be the development of crumb-rubber-modified asphalt binder (CRMA), because asphalt pavements are also extensively produced every year[2]. Compared to neat asphalt, CRMA has many benefits, including the road resistance to rutting, fatigue cracking, and thermal cracking, which are required to meet the challenge of increasing traffic loads in varying climate environments. CRMA has also been proved in practice to be a very effective and economical alternative to polymer-modified asphalt.

Two methods have been used to add crumb rubber into asphalt: the dry process and the wet process[1-4]. In the dry process, crumb rubber is used as a kind of solid filler in the hot mix to replace some of the solid fraction. In the wet process, crumb rubber is mixed with asphalt at elevated temperature (170—220 ℃) for a period of between half an hour to two hours. It has been proved that the binder prepared by the wet process has a better performance than that of the dry process.

Swelling and dissolution are two stages of interaction of rubber and asphalt in the wet process. Rubber firstly swells in asphalt by absorbing the light components[2,5], especially n-alkanes and n-alkylbenzenes[3], while changing the chemical composition and rheological properties of the residual asphalt, resulting in improvement of rutting, fatigue and thermal cracking resistance[1,6-8]. Dissolution of rubber, which usually consists of devulcanization and depolymerization of rubber, takes place during swelling under extreme interaction conditions like high tem-perature or high shearing[3,9], releasing polymeric compounds into asphalt. Particular dissolution contributes a tangled construction to the rubber-asphalt binder to soften the binder, prevent the thermal cracking at a low temperature and reduce the phase separation[6,10-12].

The factors influencing rubber swelling in asphalt have been investigated[3,13-15], illustrating that swelling is controlled by nature of rubber and asphalt, i. e., the rubber type and the asphalt penetration grade, and is also significantly affected by the interaction temperature, and the mix rate. However, investigation of the dissolution process is rarely presented in the literature. As an excessive devulcaization and depolymerization can bring about the deterioration of binder properties, it is necessary to clarify how CR is dissolved in the asphalt under varying conditions, especially the composition of asphalt, which can help us in the preparation of better CRMA binders.

We know that the light hydrocarbon oil, especially the aromatic oil, is used to improve the properties and stability of the rubber modified asphalt[11,16]. In this subject, furfural extract oil (FEO), which is a kind of by-product produced in lubricating oil solvent refining process at refineries and an ideal material for rubber filling oil, was utilized to evaluate the effect of oil composition on both swelling and dissolution process of natural rubber sheet. Chemical and structural properties of rubber and oil samples were determined before and after interaction.

2 Experimental

2.1 Main materials

2.1.1 Natural rubber sheet

Crumb rubber used for asphalt modification is smaller than 1 mm in diameter and is therefore difficult to be separated from the oil sample after mixing. In addition, CR from truck tires contains a high percentage of hydrocarbons, and exhibits a stronger interaction activity with asphalt than synthetic rubber[11]. In our study, natural rubber sheets, measuring 10 mm in length, 10 mm in width, and 2 mm in thickness, were used to simulate the swelling and dissolution process of CR in oil.

2.1.2 Crumb rubber

CR from truck tires produced at ambient temperature was chosen because the ambient-temperature CR presents more surface area than cryogenic CR and is therefore found to be more effective for producing CRMA[7,17-18]. It is known that CR particles with a size ranging from 30 mesh to 80 mesh are most suitable for CRMA preparation[1,11,15,17-19]. Thus, CR particles with a size range of between 40 mesh (425 μm) and 60 mesh (250 μm) were used in this work.

2.1.3 Asphalt and oil

The asphalt used in this study had a penetration grade of 60/70. Furfural extract oil (FEO) was obtained from lubricating oil solvent refining process at refineries. FEO was produced through extraction by N-methyl-2-pyrrolidone (NMP), which could selectively dissolve aromatics, and during this extraction process the extract fraction (FEOE) and the raffinate fraction (FEOR) were obtained. Consequently, three hydrocarbon oils and one asphalt sample were used to simulate the swelling and dissolution process.

2.2 Swelling and dissolution experiments

Swelling: Eight NR sheets were weighed respectively and put into the glass bottle apiece. The bottle was filled with 50 g of oil (FEO, FEOE, FEOR and asphalt, respectively), and then placed into an oven, the temperature of which was set at 100 ℃, 120 ℃, 150 ℃, and 190 ℃, respectively, during each experiment. After swelling for different time duration (2 h, 4 h, 6 h, 8 h, 12 h, 24 h, 36 h, and 48 h), respectively, the NR sheet was taken out of the bottle one by one. The surface of each NR sheet was wiped with dry cloth and then weighed.

The swelling index (SI), which reflects the process of rubber absorbing asphalt, is defined as follows:

where m0is the mass of original rubber sheet, and m1is the mass of swollen rubber sheet.

The swelling index is related to the swelling time and the thickness of rubber sheet, according to Frantzis’ research[20]:

where D is the diffusion coefficient of solvent (oils in our study), t is the swelling time, and d is the thickness of rubber sheet which is considered to be unchanged during the process. Dissolution: It is known that the un-vulcanized or devulcanized rubber is soluble in toluene, while the vulcanized rubber which contains a reticulated chemical structure is insoluble[21-22]. Toluene was used to extract the oil, and the devulcanized or depolymerized fraction from the swollenrubber sheets along with the insoluble fraction were dried and weighed.

The mass loss (ML) is the percentage of dissolved rubber after interaction within the original rubber, which is defined as follows:

where m2is the mass of residual swollen rubber sheet extracted by toluene.

2.3 Characterization of rubber and oil samples after interaction

Five NR sheets were put into a bottle filled with 20 g of oil, and were then subject to swelling for 8 h, 24 h, and 48 h, respectively, at a temperature of 100 ℃, 120 ℃, and 150 ℃, respectively. After that procedure, the swollen NR sheets were extracted by toluene, and were then dried to give pure rubber for further tests. The rubber sheets processed in FEOR at 120 ℃ after extraction were labeled as NR8, NR24, and NR48, respectively. The residual liquid phase after interaction under different conditions was also tested respectively.

2.3.1 Characterization of processed rubber

The thermogravimetric analyses (TG/DTG) were carried out using a Q600SDT type instrument. About 8 mg of the sample (NR8, NR24, and NR48) were weighed and heated from room temperature up to 700 ℃ under N2atmosphere at a heating rate of 10 ℃/min.

An S-3400N type scanning electron microscope (SEM) was utilized to observe the microstructure of rubber sheet samples. A Falion 60S energy dispersive spectrometer (EDS) was used to analyze the elemental composition of the sample.

2.3.2 Components analysis of oils

Furfural extract oil and asphalt are both complex mixtures of organic molecules, produced from crude oil with its composition varying according to its origin, and thus it is difficult to determine the precise hydrocarbon groups. However, it is possible to separate oil or asphalt into four main hydrocarbon groups, viz.: saturates, aromatics, resins and asphaltenes (SARA), which is widely used in analysis of heavy oil and asphalt components.

The SARA analyses of residual oil after swelling and analysis of pure oil that was aged under the same condition were conducted according to the industry standard SH/T 0509—1992 of China.

2.4 Interaction between crumb rubber particles and oil and asphalt

It is known that CR particles size can influence the properties of CRMA binder because of dimensional and interaction effects[1,11,15,17-19]. Smaller dimension of CR particles can accelerate the absorption of light components of asphalt on CR particles, leading to digestion of the rubber into asphalt. However, too small CR particles cannot function as filler in the binder and are easily broken down. Temperature and time also play significant roles in the interaction, because higher temperature and longer time can accelerate both the swelling and dissolution process[8,11,17].

In this study, CR particles and FEO or asphalt with a mass of four times that of CR particles were mixed at a temperature of 190 ℃ for 20 min at a stirring rate of 600 r/min. The resulting mixture was extracted by toluene, and the soluble fraction after solvent removal was tested by a PLGPC50 type gel permeation chromatograph, using tetrahydrofuran as the dissolvent.

3 Results and Discussion

3.1 Swelling and dissolution of NR sheet in oils

3.1.1 Variation of swelling index with oil at different temperatures

Variation of swelling index versus swelling time for NR sheet at 100 ℃, 120 ℃, 150 ℃, and 190 ℃ (a, b, c and d respectively) in FEOR, FEOE, FEO, and asphalt (only at 190 ℃), respectively, are presented in Figure 1. In general, the swelling index increases versus swelling time along the logarithmic trend lines, which are similar to those suited for rubber cured in asphalt[5,13]. However, the difference was that a swelling equilibrium condition was not reached after having been subject to swelling for 48 h in FEOR, FEOE, and FEO, demonstrating a better swelling capability of light oil as compared to the asphalt. At the same swelling duration, the higher the temperature is, the higher the swelling index would be. The reason is that when the viscosity of oils decreases, the molecular chains of rubber will move faster with an increasing temperature. In addition, most NR sheets were dissolved in oils after 24 h at 150 ℃ and 190 ℃, respectively.

Figure 1 Swelling curves of NR sheet in different oils at different temperatures

At 100 ℃ and 120 ℃, the swelling index decreased in the following order: FEOR>FEO>FEOE, but it turned into the following decreasing order: FEO>FEOR>FEOE at 150 ℃, and then changed to the following decreasing order: FEO>FEOE> FEOR at 190 ℃, respectively. The swelling of rubber is dependent on the viscosity of solvent and the nature of the solvent[5]. FEOR containing lighter components had lower viscosity than the other two oil samples at low temperature and could diffuse more easily into rubber; the difference in viscosity between these three oils was reduced with an increasing temperature, thus the swelling progress was controlled by the nature of oil samples. It can be concluded that oil sample containing heavy components, especially aromatics, possessed better swelling capability than the alkanes-rich light oil samples.

3.1.2 Variation of diffusion coefficient with oil type and temperature

The diffusion data of oil samples into rubber at 100 ℃, 120 ℃, 150 ℃ and 190 ℃ has been depicted in Figure 2 (a), (b), (c) and (d), respectively. The slope of the fitting line is 4D1/2π1/2, and the diffusion coefficient of oil samples at different temperatures is calculated and listed in Table 1. It can be found that diffusion coefficient of these three oils dramatically increased and the difference between them was reduced as temperature rose, attesting to the same pattern as the swelling curve. The diffusion coefficient of FEO was higher than that of FEOR andFEOE at 190 ℃, demonstrating the compatibility between light components and heavy components. In addition, the diffusion coefficient of three oil samples was almost 10 times higher than that of asphalt, indicating that the light hydrocarbon oil like FEO could be used to swell rubber more efficiently. However, fitting correlation coefficient (R2) of these curves reduced with an increasing temperature, demonstrating that the diffusion theory is more suitable at low temperature.

Table 1 Diffusion coefficient of oil samples and asphalt at different temperatures

Figure 2 Effect of oil type and temperature on diffusion coefficient

3.1.3 Variation of dissolution of NR sheet by oil at different temperatures

Figure 3 presents the dissolution process of NR sheet in three oil samples at 100 ℃, 120 ℃, 150 ℃ and 190 ℃, respectively. The results showed that the NR sheet was slightly dissolved in oil samples at 100 ℃, but could be readily dissolved in FEOR at 120 ℃. The NR sheets were almost completely dissolved after 24 h of immersion at 150 ℃and 190 ℃, respectively, whilst FEOE and FEO showed stronger dissolving capability than FEOR, attesting to the same trend as the swelling index. It can also be seen that light oils were more efficient in dissolving rubber than the asphalt.

3.2 Properties of processed NR sheets and oils

3.2.1 Energy dispersive spectrometric analysis of processed NR sheets

The chemical elements of NR8, NR24 and NR48 were analyzed by energy dispersive spectrometry. The networks of rubber materials mainly consist of polymer chains and sulfur bridges, therefore the content of carbon and sulfur is denotative of showing the structure changes in the remaining composition of rubber after dissolution. It can be noticed from Table 2 that the carbon content in rubber decreased with the processing time, while its sulfur content increased. It can be suggested that most sulfurcompounds remained in the undissolved rubber and the dissolved rubber molecules contained more carbon.

Table 2 Changes in carbon and sulfur contents depending upon interaction time and temperature during processing of NR sheets

Figure 3 Mass loss of NR sheet in different oil samples at different temperatures

3.2.2 Thermogravimetric analysis of processed NR sheets

Thermal analysis is a useful tool for the characterization of polymeric materials. Thermogravimetry is a powerful technique that shows the composition identification, cross-linking, and thermal behavior of rubber materials. Figure 4(a) and Figure 4(b) show the mass loss profiles, recorded by TG and DTG measurements. It can be seen from Figure 4(a) that the mass loss of rubber started at about 300 ℃ and ended at 500 ℃, which was associated with the thermal cracking of rubber hydrocarbons. The remainder consisted of carbon black and inorganic filler. It can be found out that the content of rubber hydrocarbons of NR8, NR24, NR48 was reduced successively, which was also verified by the dissolution process mentioned above. It can be seen from Figure 4b that thermal crack-ing started from about 300 ℃ and reached a maximum rate at about 370 ℃. There existed a shoulder peak at about 430 ℃, which was the characteristic peak specific for the natural rubber. It has been proved that the peak temperature of thermal cracking indicated the cross-linking of sulfur compounds in rubber materials[23]. The profiles of these three specimens were almost the same, indicating that no difference was identified in their cross-linked sulfur compounds.

Figure 4 Thermogravimetric analysis of processed NR sheets

3.2.3 Microstructure characterization

The original rubber sheet was a plane but ravine-like surface in Figure 5(a), which was favorable for diffusion of oil inside the rubber. The ravines had disappeared on the surface of the swollen rubber because of expanding of polymer chains, as shown in Figure 5(b). Figure 5(c) shows a rugged surface of processed rubber sheet, which should be ascribed to the occurrence of devulcanization and depolymerzation of rubber fraction. However, no deep holes appeared inside the rubber sheet, demonstrating that the dissolution spread out from the surface to the inner side.

Figure 5 SEM micrographs at 300 times of magnification: (a) original NR sheet; (b) swollen NR24; (c) processed NR24

3.2.4 Composition of oil interacting with NR sheet at different temperatures

It has been verified that absorption of light fraction into rubber modified the chemical composition of the liquid phase during the interaction of rubber and asphalt. Since the viscosity of FEOR, FEOE and FEO was much lower than that of asphalt, the chemical composition of these oils changed quite little after interaction, as shown in Figure 6. The samples in these tests were processed for 24 h,at 100 ℃ and 150 ℃, respectively. It can be seen that the SARA composition of oil samples was almost the same before and after use, attesting to the non-selective absorption of oil into rubber during this study.

Figure 6 Changes of SARA analyses of processed oils with temperature and time

3.3 Interaction of crumb rubber with FEO and asphalt

3.3.1 Dissolution of crumb rubber in FEO and asphalt

Figure 7 depicts the dissolution of crumb rubber in FEO and asphalt. The interaction rate was accelerated as the rubber particles were much smaller than the rubber sheets. 49 percent of crumb rubber were dissolved in FEO, which was tantamount to the result of NR sheets processed at 190 ℃ for 48 h. Dissolution of crumb rubber in asphalt showed the same trend as the result of NR sheet and its dissolution rate was much lower than that of FEO.

Figure 7 Dissolution of crumb rubber in FEO and asphalt

3.3.2 GPC of FEO and asphalt

The gel permeation chromatography study on the interaction of toluene extract with crumb rubber–FEO and rubber–asphalt mixtures, respectively, is shown in Figure 8(a) and 8(b). The aged FEO and the processed FEO showed almost the same molecular weight distribution in Figure 8(a), denoting that rubber molecules were not detected, which was contrary to the fact that nearly half of crumb rubber had been dissolved in FEO. It has been noticed that polymer chains of unvulcanized natural rubber were not dissolved in THF, suggesting that the dissolved rubber molecules in FEO were too large to be dissolved in THF during the GPC study.

Figure 8 Molecular weight distribution curve: (a) FEO; (b) asphalt

The molecular weight of the aged asphalt and the processed asphalt was distributed in two regions, namely: 150—350 and 350—100 000, respectively, as shown in Figure 8(b). The MW distribution curve of the aged asphalt ended at 30 000, while that of processed asphalt ended at about 100 000, demonstrating that more large molecules existed in the processed asphalt. The average molecular weight, calculated from the GPC, was 2 300 and 3 890 for the aged asphalt and the processed asphalt, respectively. By comparing the data obtained thereby, it can be concluded that a part of crumb rubber broke down into smaller molecules which could be dissolved in THF and might diffuse into the liquid phase.

3.4 Interaction of rubber on oil or asphalt

The possible interaction scheme is presented in Figure 9: the sulfur bridges were broken down, while rubber is soaked in light oils like FEOR, FEOE, and FEO at high temperature. Thereby, the cyclic poly-sulfide compounds were probably produced[24-25]because the cross-linking degree of residue rubber was unchanged according to the result of TG analyses. Sulfur preferred to be attached to the undissolved rubber as the sulfur content in residue rubber increased with an increasing processing time. Long polymeric chains were released into liquid phase as a result of devulcanization, while C—C bonds remained unchanged, and the detachment happened from the outside to the inside. The difference between light oil and asphalt is that a part of C—C bonds were broken up in the asphalt and small molecules were then produced.

Figure 9 Scheme for interaction of rubber on oil samples or asphalt

4 Conclusions

This study simulated the interaction of oil samples on crumb rubber and asphalt by using NR sheet and aromatic oils. The effect of oil type and temperature on the swelling index, diffusion coefficient, and mass loss of NR sheet was investigated.

The swelling index and mass loss of NR sheet and diffusion coefficient of oils all increased with a rising temperature, and the reason was that the viscosity of oil samples decreased correspondingly and the molecular chains of rubber could move faster. These three indicators of FEOR, which contained more alkanes, were higher than those of FEOE and FEO containing more aromatics at lower temperature (100 ℃ and 120 ℃in this study), but the situation was reversed at higher temperature (150 ℃and 190 ℃). It can be concluded that aromatic components could penetrate more readily into the internal side of the NR polymer, showing more dissolving capability, in compliance with the conclusion mentioned in the literature. It is noticed that the value of the three indicators of FEO was higher than those of FEOR and FEOE at 190 ℃, demonstrating the compatibility between light components and heavy components.

The NR sheets processed at different time durations showed almost the same thermal properties, indicating that the cross-linking degree of the residual rubber was unchanged. The sulfur content of processed NR rubber increased with an increasing processing time, suggesting thatsulfur preferred to remain in the residual rubber. The micrograph of processed NR sheet showed that the detachment of dissolved rubber spread from the outside to the inside.

The SARA composition of different processed oils was almost the same, indicating that a non-selective absorption took place during interaction of light oil on rubber in this study. Based on the results of GPC analyses, it can be concluded that the dissolution of crumb rubber in FEO was attributed to devulcanization, and that phenomenon in asphalt was mainly ascribed to depolymerization.

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The Project “Commercial Application Test of High-Performance Catalyst for Gas-Phase Hydrogenation of Octenal”Passed Review and Appraisal

The project “Commercial application of high-performance catalyst for gas-phase hydrogenation of octenal”undertaken by the Research Institute of Nanjing Chemical Company under SINOPEC has passed the research achievement appraisal organized by the SINOPEC’s Science and Technology Division.

It is learned that the NCH6-2 type catalyst for gas-phase hydrogenation of octenal developed by the research team has made great strides in the pore structure and distribution of active phase, resulting in improvements in catalytic activity, selectivity and stability. The process technology for preparation of the catalyst is reliable, and the product quality is stable, while the production process is safe with HSE meeting the relevant regulations. The formulation and preparation technique of this catalyst has its innovative nature, and the research institute has applied for two Chinese invention patents. The said catalyst developed by this research institute has been running for two years during the commercial application test in the 85 kt/a unit for octenal hydrogenation at the Qilu Petrochemical Company. The outcome of operation over two years has revealed that the unit runs smoothly to completely convert octenal (without octenal being detected at the reactor outlet), and the octanol selectivity can be more than 99.5% to achieve apparent energy conservation effect and good economic and social benefits.

Successful Startup of First in China Integrated Propylene/Isobutylene Production Unit

The propylene/isobutylene integrated unit using the C3/C4Oleflex dehydrogenation process licensed by UOP constructed at the Shandong Jingbo Petrochemical Co., Ltd. has been successfully put on stream. It is learned that this is the first in China and the second in the world integrated C3/C4dehydrogenation unit.

This new unit located in Binzhou city, Shandong province, adopts the UOP’s Oleflex technology for catalytic dehydrogenation of C3/C4hydrocarbons, which can be converted to propylene and isobutylene, respectively, with a propylene output of 116 kt/a and an isobutylene output of 104 kt/a. This technology applies the scheme of dehydrogenation of a mixture of C3/C4hydrocarbons in an integrated unit. Compared with other similar technologies, this technology is proved to be one with the lowest production cost coupled with the highest return of investment. Furthermore, the Dongming Petrochemical Company which will have a propylene/isobutene production capacity of 265 kt/a, and the Dongying Liyuan Petrochemical Company which will have a propylene/isobutene production capacity of 220 kt/a all intend to put on stream their new integrated propylene/isobutylene units in 2017.

date: 2015-03-04; Accepted date: 2015-05-05.

Zhou Xiaolong, Telephone: +86-021-64252041; E-mail: xiaolong@ecust.edu.cn.