Environ. Sci. Technol. 2009, 43, 503–508
Asphalt Modified with Nonmetals Separated from Pulverized Waste Printed Circuit Boards JIUYONG GUO,† JIE GUO,† S H I F E N G W A N G , ‡ A N D Z H E N M I N G X U * ,† School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China, and Research Institute of Polymer Materials, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China
Received August 18, 2008. Revised manuscript received October 22, 2008. Accepted November 12, 2008.
Nonmetals separated from pulverized waste printed circuit boards (PCBs) were reused as a new modifier to improve the performance of asphalt. The classical and rheological properties of unmodified asphalt and non-metal-modified asphalt (NMA) were determined. Specifically, the influence of nonmetals content and particle size on these properties has been studied. When the nonmetals content was 25 wt% and the particle size group was 0.07-0.09 mm, the NMA had a viscosity of 1225 cP at 135 °C, a penetration of 53.7 dmm at 15 °C, a ring and ball softening point of 54 °C, a ductility of 43.5 cm at 15 °C, a G*/sin δ of 3995.27 Pa at 60 °C, and an upper limit temperature (G*/sin δ ) 1 kPa) of 69.4 °C, all of which showed that the high temperature performance of asphalt was improved significantly. Therefore, this study gives a fundamental understanding of NMA and represents a novel attempt to deal with the fast increasing quantities of nonmetals from waste PCBs, which is significant from an environmental and economic standpoint.
Introduction Recycling of waste printed circuit boards (PCBs) is an important subject not only from a standpoint of the protection of the environment but also of the recovery of valuable materials (1). If waste PCBs are not recycled properly, it can cause considerable damage to the local environment (2, 3). Nowadays, various recycling processes have been applied to waste PCBs, of which mechanical-physical processes have attracted more attention than hydrometallurly and pyrometallurgy (4, 5). However, one of the most difficult problems faced by all of the recycling processes is how to deal with the nonmetals reclaimed from waste PCBs in an environmentally friendly and economical manner. The nonmetals consist of thermosetting polymers, glass fibers, brominated flame retardants, and other additives. Thermosetting polymers cannot be remelted or reformed because of their network structure. Most recycling approaches practiced could only recover metal contents of PCB scraps to an extent of 28 wt% of the total weight. More than 70 wt% of PCB scraps which are nonmetals are not efficiently recycled or recovered and * Corresponding author tel: +86 21 54747495; fax: +86 21 54747495; e-mail:
[email protected]. † School of Environmental Science and Engineering. ‡ School of Chemistry and Chemical Engineering. 10.1021/es8023012 CCC: $40.75
Published on Web 12/12/2008
2009 American Chemical Society
have to be incinerated or land-filled (6). However, incineration of the nonmetals is not economical or suitable because the large percent (as high as 50 wt%) of inorganic components contained in the nonmetals leads to a low combustion value of the nonmetals and the common incineration process has to be upgraded to inhibit the formation of highly toxic polybrominated dibenzodioxins and dibenzofurans (PBDD/ Fs) with proper tail gas treatment. As an option to treat the nonmetals, land-filling will take up a mass of land, raise the cost of recycling of waste PCBs, and waste valuable resources. Therefore, it is urgent to develop economical and environmentally sound recycling methods for the nonmetals from waste PCBs. To resolve the environmental problem caused by waste PCBs, researchers such as Masatoshi Iji in relevant fields have carried out many studies (7) on the recycling of epoxy resin compounds for molding electronic components, and the nonmetals were reused as reinforcing fillers in phenolic molding compound (8) and unsaturated polyester molding compound (9) in our previous research. Asphalts are widely used in road paving. But unmodified asphalts are highly susceptible to changes in temperature because of their rheological properties. To accommodate ever increasing traffic loadings in different climatic environments and to resist failures such as permanent deformation, cracking, and water damage, major emphasis has been placed on improving the performance of asphalt (10). This approach has led to a fundamental variation in the design of long lasting asphalt pavements. To improve asphalt characteristics, specific performance enhancers have been investigated. These include additive modification, polymer modification, and chemical reaction modification (11). The best known form of modification is by means of polymer modification, traditionally used to improve the temperature susceptibility of asphalt by increasing binder stiffness at high service temperatures and reducing stiffness at low service temperatures (12). In order to reduce the cost of modified asphalt, recycled materials such as ground tire rubber (13) and soft wood bark charcoal (14) are used as asphalt modifiers. The use of waste polymers as asphalt modifiers is considered a rather new and interesting way of modification because it involves two important aspects which are waste material utilization and asphalt property enhancement. As mentioned in the first paragraph, in our previous research the nonmetals were reused as reinforcing fillers in phenolic molding compound (8) and unsaturated polyester molding compound (9), proving that the nonmetals can be a good filler for different polymer matrixes. The glass fibers and resins powder contained in the nonmetals can be used to strengthen the asphalt. In addition, the leaching test result in another paper (15) has suggested that the concentrations of leached heavy metals and the reactive flame retardant (mainly tetrabrombisphenyl-A (TBBA)) are very low and in line with the standards. Therefore, the nonmetals may be suitable and promising modifiers for asphalt. Recycling of the nonmetals from waste PCBs is the primary motive, and the high temperature performance of non-metalmodified asphalt (NMA) and the large quantity of added nonmetals are two main concerns in this exploratory study. In order to develop fundamental understanding of the NMA, the influence of addition content and particle size of nonmetals on classical and the rheological properties of the NMA was investigated in the study.
Experimental Procedures Materials and Preparation of the NMA. The nonmetals separated from pulverized waste PCBs were produced VOL. 43, NO. 2, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
503
FIGURE 1. Flowchart of nonmetals preparation for the NMA.
FIGURE 2. Viscosity (a), penetration (b), R&B softening point (c), and ductility (d) versus nonmetals content for the NMA. through a process of two-step crushing combined with corona electrostatic separation, which is illustrated in Figure 1. The waste PCBs used in this study were without mounted components or with all the hazardous mounted components (capacitor, etc.) being removed. The PCBs were first pulverized in a process consisting of a coarse crushing step and a fine-pulverizing step, using a shearing machine and a hammer grinder. Then the nonmetals were separated from metals by a corona electrostatic separator. After that, the nonmetals were sieved into four groups based on particle size, 0.07-0.09, 0.09-0.15, 0.15-0.30, and 0.30-0.45 mm, 504
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 2, 2009
by a vibrating screen. At last, the nonmetals were dried at 120 °C for 2 h. AH-70 paving asphalt was obtained from Baoli Asphalt Co. Ltd. in Jiangsu Province, China, with a penetration of 72.2 dmm at 25 °C according to ASDM D 5, a ductility of greater than 100 cm at 15 °C according to ASTM D 113-86, a softening point of 48 °C according to ASTM D 36, and a viscosity of 450.0 cP at 135 °C according to ASDM D 4402. The original asphalt was heated in a bucket at 160 °C until it was a fluid and subdivided. A calculated amount of nonmetals was mixed into the asphalt and was stirred for 30
TABLE 1. Effects of Nonmetals Content on Penetration Index (PI), Equivalent Softening Point (T800), and Equivalent Brittle Point (T1.2) of the NMA nonmetals content
unmodified asphalt
10 wt%
15 wt%
20 wt%
25 wt%
30 wt%
PI PIrequired, minimum T800 T800, required, minimum T1.2 T1.2, required, maximum
1.0092 -1.0 55.99 55.23 -25.86 -26.58
1.5385 -1.0 59.59 60.02 -28.66 -28.23
0.8696 -1.0 57.34 57.17 -22.88 -23.06
0.7334 -1.0 60.63 59.95 -18.03 -18.71
0.8108 -1.0 58.31 58.04 -21.23 -21.50
2.0482 -1.0 66.04 65.97 -28.72 -28.79
TABLE 2. Effects of Nonmetals Particle Size on Penetration Index (PI), Equivalent Softening Point (T800), and Equivalent Brittle Point (T1.2) of the NMA nonmetals particle size
unmodified asphalt
0.07-0.09 (mm)
0.09-0.06 (mm)
0.06-0.03 (mm)
0.03-0.15 (mm)
PI PI required, minimum T800 T800, required, minimum T1.2 T1.2, required, maximum
1.0092 -1.0 55.99 55.23 -25.86 -26.58
0.7334 -1.0 60.63 59.95 -18.03 -18.71
0.1351 -1.0 56.11 54.05 -15.93 -17.98
-0.1961 -1.0 53.79 52.16 -14.75 -16.38
-0.09901 -1.0 53.81 52.29 -15.75 -17.26
min. A series of the NMA with nonmetals concentrations ranging from 10 to 30 wt% and particle sizes ranging from 0.07 to 0.30 mm were prepared. The NMA was cooled at room temperature and stored in a refrigerator at -4 °C. It should be emphasized that the parameter of the nonmetals particle size group was fixed at 0.07-0.09 mm when the effects of nonmetals addition content on all properties of the NMA were investigated, and the parameter of nonmetals addition content was fixed at 20 wt% when the effects of nonmetals particle size on all properties of the NMA were investigated. Classical Properties Testing. High temperature viscosity of the NMA was measured by Brookfield viscometer (Model DV-II +, Brookfield Engineering, Inc., Middleboro, MA) according to ASDM D 4402. The NMA was heated in an iron container in an oven at 135 °C for 30 min. Approximate 10 g of heated NMA was poured into the sample chamber. The sample chamber containing the NMA sample was then placed in the thermocontainer. The testing temperature was set at 135 °C, and the spindle speed was 20 rpm. The viscosity value was measured when the temperature had been stabilized for 30 min. Penetration of the NMA was tested at 15 °C, 25 °C, and 30 °C according to ASDM D 5 specifications. Penetration is expressed in units of 1/10 mm, which is a value of how deep a standard needle under a 100 g load penetrates into the
asphalt binder sample after 5 s loading time at a certain temperature. The measurement at 25 °C defines the penetration grade of the tested asphalt. The penetration index (PI), equivalent softening point (T800), and equivalent brittle point (T1.2) can be derived from the following formulas according to Chinese Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (16). If the value of penetration is converted to logarithm, then the relationship between penetration (log P) and temperature (T) is a linear equation, and it can be expressed as log P ) K + Alog pen × T
(1)
where P is penetration, T is testing temperature, the temperature index Alog pen is the slope of the simulation line which is obtained through the linear regression, and K is the intercept of the simulation line. The penetration index (PI) is defined by eq 2 PI )
20 - 500Alog pen 1 + 50Alog pen
(2)
The equivalent softening point (T800) is the temperature when the penetration is 800 and it is defined by eq 3 T800 )
log 800 - K 2.9031 - K ) Alog pen Alog pen
(3)
The equivalent brittle point (T1.2) is the temperature when the penetration is 1.2, which is to investigate the low temperature performance of asphalt binder, and is defined by eq 4 T1.2 )
FIGURE 3. Amplitude sweeps as a function of strain at 35 °C and 1 Hz for unmodified asphalt and the NMA.
log 1.2 - K 0.0792 - K ) Alog pen Alog pen
(4)
The softening point is defined as the temperature at which an asphalt sample can no longer support the weight of a 3.5 g steel ball. The softening point was tested according to ASTM D 36. A steel ball weighing 3.5 g was placed on a disk of asphalt sample contained in a horizontal, vertically supported, 20 cm diameter metal ring. The assembly was heated in a water bath at 5 °C/min. The temperature at which the sample became soft enough to allow the ball, enveloped in the sample material, to fall a distance of 25.4 mm was recorded. The ductility test measures asphalt ductility by stretching a standard-sized briquette (defined by ASTM D 113: Ductility VOL. 43, NO. 2, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
505
FIGURE 4. Temperature sweeps for the NMA with different nonmetals contents in terms of the complex modulus (a) and the phase angle (b).
TABLE 3. Effects of Nonmetals Modification on High Temperature Performance Enhancement for Asphalt nonmetals content 0 wt% 10 wt% 15 wt% 20 wt% 25 wt% 30 wt%
G*/sin δ at 60 °C (Pa)
upper limit temperature, G*/sin δ ) 1 kPa (°C)
2083.46 2411.85 3001.05 3406.33 3995.27 4482.26
65.4 66.2 67.9 68.4 69.4 70.8
of Bituminous Materials) of asphalt binder to its breaking point. The stretched distance in centimeters at breaking is then reported as ductility. In this paper, ductility was characterized at 15 °C with an extensional speed of 5 cm/ min according to ASTM D 113-86. Rheological Properties Characterization. The rheological properties of the asphalt were determined by means of dynamic mechanical methods consisting of temperature and frequency sweeps in an oscillatory-type testing mode performed within the region of linear viscoelastic (LVE) response. The oscillatory tests were conducted on a Bohlin Gemini 200 Advanced Rheometer using two parallel plate testing geometries consisting of 25 mm diameter plates with a plate gap adjusted to 2 mm. The rheological properties of asphalt were measured in terms of their complex shear modulus (G*) and phase angle (δ). The amplitude sweeps tests, at the frequency of 1 Hz, were previously carried out on each sample to determine the linear viscoelasticity region. The temperature sweeps in the linear viscoelasticity range were undertaken at a constant frequency of 1 Hz over a temperature range from 35 to 90 °C. The frequency sweep tests were performed under controlled strain loading conditions using frequencies between 0.01 to 10 Hz at 10 °C temperature intervals between 45 and 85 °C. Morphological Analysis. The morphologies of the nonmetals and the NMA samples were observed using a polarizing optical microscope, made by Leica Microsystems GmbH, Germany. A small portion of the nonmetals or drop of the NMA was placed between two heated microscope glass slides and pressed to form a thin film which was viewed under the microscope.
Results and Discussion Classical Properties. Figure 2a illustrates the viscosity values at 135 °C for the NMA. The trend is that the viscosity of the 506
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 2, 2009
FIGURE 5. Micrograph of the NMA. NMA increased with the increase of addition content of nonmetals. The increase in viscosity is thought to be caused by glass fibers and fine powders contained in the nonmetals. When the addition content exceeded 25 wt%, the increase was significant, and this may lead to a difficulty in the pumping of the NMA. According to the Technical Specifications for Construction of Highway Asphalt Pavements (17), the viscosity at 135 °C should not exceed 3000 cP. The effects of nonmetals content on penetration at 25 °C are shown in Figure 2b. It is found that the addition of nonmetals reduced the penetration of the NMA considerably. The reduction indicates that the addition of nonmetals increased the stiffness and rutting resistance of the NMA. According to (17), as for AH-70 paving asphalt used in this paper, there is a maximum value of 80 and a minimum value of 40. From Figure 2b it can be seen that the penetration of the NMA was in line with the standard. The effects of nonmetals content on penetration index (PI), equivalent softening point (T800), and equivalent brittle point (T1.2) of the NMA are shown in Table 1. PI, T800, and T1.2 are a series of parameters to investigate the temperature sensibility of asphalt. T800, required and T1.2, required are derived from formula 5 and formula 6, separately. T800,required )
50 × (2.9031 - log P25) × (PI + 10) + 25 (20 - PI)
(5)
50 × (log P25 - 0.0792) × (PI + 10) 20 - PI
(6)
T1.2,required ) 25 -
FIGURE 6. Black diagrams of rheological data for the NMA compared to unmodified asphalt: (a) temperature sweep tests from 35 to 90 °C at 1 Hz; (b) frequency sweep tests from 0.01 to 10 Hz. where P25 is the penetration at 25 °C and PI is the penetration index defined in eq 2. The larger PI indicates a lower temperature sensibility, which is demanded when the asphalt is used in high temperature areas. The results show that PI and T800 of the NMA were higher than the required values, except for the T800 when the content was 10 wt%. The T1.2 of the NMA was almost the same as the T1.2, required, and the gap between them can be ignored, considering the anticipated use of the NMA in high temperature areas. The effects of nonmetals particle size on PI, T800, and T1.2 of the NMA are shown in Table 2. The results show that particle size had a significant influence on these three parameters. The NMA with the smallest particle size of nonmetals had the best performance in PI, T800, and T1.2. The large size nonmetals reduced the low temperature performance of asphalt, which was indicated by the increase of T1.2. The reason for this effect can be explained by particle-particle interactions: a smaller particle size has more surface area per unit volume and therefore will lead to greater asphalt/particle interactions. The R&B softening point of the NMA is illustrated by Figure 2c. The results show that the softening point increased with the increase of nonmetals content. This is due to the reinforcing effect by glass fibers in the nonmetals. The results indicate that the addition of nonmetals decreased the temperature susceptibility of the asphalt. When the addition content was 30 wt%, the softening point had been increased by more than 8 °C. Ductility is another parameter besides T1.2 to investigate the low temperature performance of asphalt, and higher ductility means better low temperature performance. Figure 2d illustrates the effects of nonmetals content on ductility of the NMA at 15 °C. The trend is that the addition of nonmetals decreased the asphalt ductility significantly. The reduction may be caused by the stress concentration in the interfaces between nonmetals particle and asphalt. According to ref 17, the minimal ductility at 15 °C of AH-70 paving asphalt should not be less than 40 cm. Rheological Properties. The amplitude sweeps undertaken on the asphalt were used to determine the linear viscoelasticity limits for the NMA as well as to quantify the strain (or stress) dependency of the NMA. Strain dependency, a term that defines viscoelasticity materials, represents material parameters including modulus, viscosity, and phase angle, which depend on the strain. The amplitude sweeps for the NMA at 35 °C are presented in Figure 3. It can be found that the linear viscoelasticity region was 0 to 0.1. The
strain corresponding to the curve turning point of modified asphalt was smaller than that of unmodified asphalt. The temperature sensitivity of the NMA with different nonmetals contents has been assessed by means of isochronal plots of the complex modulus (G*) and phase angle (δ) versus temperature at 1 Hz as shown in Figure 4. The complex modulus (G*) consists of the storage (or elastic) modulus (G′) and the loss (or viscous) modulus (G′′). In order to resist rutting, asphalt should be stiff and elastic. Therefore, the complex shear modulus elastic portion (G′) should be large. When rutting is of greatest concern, superpave specifies a minimum value for the elastic component of the complex shear modulus. In SHRP specifications, permanent deformation is controlled by limiting G*/sin δ (rutting resistance parameter) to at least 1.0 kPa before aging in an RTFO (rolling thin-film oven). From Figure 4 it can be seen that the modification increased G* for asphalt and when the temperature was above 60 °C it slightly increased the phase angle for asphalt as well. The rutting resistance parameter G*/sin δ at 60 °C and upper limit temperature for asphalt are shown in Table 3. The results in Table 3 show that the rutting resistance of the NMA was obviously greater than that of unmodified asphalt, and this phenomenon is desired. The phenomenon can be explained by the stiffening effect, and it is due to the reinforcement caused by the glass fibers and resin powders in the nonmetals, which can be seen in Figure 5. In order to achieve better understanding of the modification effect, the rheology of the NMA was compared with that of unmodified asphalt by means of using black diagrams as shown in Figure 6. Black diagrams are considered as rheological “fingerprints” to assess the rheological data and the rheological behavior of asphalt (18-20). From Figure 6a it can be seen that the NMA has its black curve shifted toward a stiffer and more elastic response, which is highly desirable for the high temperature performance improvement of asphalt by the nonmetals modification. From Figure 6b it can be seen that the NMA shows a relatively smooth curve in the black space and is less brittle than unmodified asphalt as indicated by the higher complex modulus of the NMA at a given elastic response. Overall, the black diagrams show potential benefit offered by the nonmetals modification, as indicated by the improved rheological properties. On the basis of the study, a fundamental understanding of the NMA has been obtained (1): the nonmetals modification has obviously reduced the temperature susceptibility of asphalt binder by increasing binder stiffness at high service temperatures (2); the low temperature performance of the VOL. 43, NO. 2, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
507
NMA expressed in the deduced equivalent brittle point (T1.2) is near or better than that of unmodified asphalt (3); the NMA with the smallest particle size of the nonmetals has the best overall performance (4); the weight content of the nonmetals can even reach as high as 25% with a good overall performance of the NMA. On one hand, the glass fibers and resins powder contained in the nonmetals have strengthened the asphalt and the nonmetals modification has improved the performance of asphalt; on the other hand, adding the nonmetals to asphalt will reduce the cost of asphalt, a material of large usage. Furthermore, it is important to find applications for nonmetals of waste PCBs. Therefore, asphalt modification with nonmetals is a promising choice and beneficial from both environmental and economic viewpoints. The low temperature rheology and other relevant aspects of the NMA will be studied in the future.
Acknowledgments This project was supported by the National High Technology Research and Development Program of China (863 program 2006AA06Z364).
Literature Cited (1) Li, J.; Lu, H.; Guo, J.; Xu, Z.; Zhou, Y. Recycle technology for recovering resources and products from waste printed circuit boards. Environ. Sci. Technol. 2007, 41, 1995–2000. (2) Hui, R.; Li, P.; Guo, Y.; Jia, M.; Ping, A. Severe PCDD/F and PBDD/F pollution in air around an electronic waste dismantling area in China. Environ. Sci. Technol. 2007, 41, 5641–5646. (3) Leung, A. O. W.; William, J. L.; Wong, A. S.; Wong, M. H. Spatial distribution of polybrominated diphenyl ethers and polychlorinated dibenzo-p-dioxins and dibenzofurans in soil and combusted residue at Guiyu, an electronic waste recycling site in southeast China. Environ. Sci. Technol. 2007, 41, 2730–2737. (4) Cui, J.; Forssberg, E. Mechanical recycling of waste electric and electronic equipment: a review. J. Hazard. Mater. 2003, 99, 243– 263. (5) Veit, H. M.; Bernardes, A. M.; Ferreira, J. Z.; Tenoirio, J. A. S.; Malfatti, C. D. F. Recovery of copper from printed circuit boards scraps by mechanical processing and electrometallurgy. J. Hazard. Mater. 2006, 137, 1704–1709.
508
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 2, 2009
(6) Li, J.; Shrivastava, P.; Gao, Z.; Zhang, H. Printed circuit board recycling: a state-of-the-art survey. IEEE Trans. Electron. Packag. Manuf. 2004, 27, 33–42. (7) Iji, M. Recycling of epoxy resin compounds for moulding electronic components. J. Mater. Sci. 1998, 33, 45–53. (8) Guo, J.; Li, J.; Rao, Q.; Xu, Z. Phenolic molding compound filled with nonmetals of waste PCBs. Environ. Sci. Technol. 2008, 42, 624–628. (9) Guo, J.; Cao, B.; Guo, J. Y.; Xu, Z. A plate produced by nonmetallic materials of pulverized waste printed circuit boards. Environ. Sci. Technol. 2008, 42, 5267–5271. (10) Sengoz, B.; Isikyakar, G. Evaluation of the properties and microstructure of SBS and EVA polymer modified bitumen. Constr. Build. Mater. 2008, 22, 1897–1905. (11) Isacsson, U.; Lu, X. Characterization of bitumens modified with SEBS, EVA and EBA polymers. J. Mater. Sci. 1999, 34, 737–45. (12) Airey, G. D. Rheological evaluation of ethylene vinyl acetate polymer modified bitumens. Constr. Build. Mater. 2002, 16, 473–487. (13) Navarro, F. J.; Partal, P.; Martı´nez-Boza, F.; Valencia, C.; Gallegos, C. Rheological characteristics of ground tire rubber-modified bitumens. Chem. Eng. J. 2002, 89, 53–61. (14) Chebila, S.; Chaalaa, A.; Roya, C. Use of softwood bark charcoal as a modifier for road bitumen. Fuel 2000, 79, 671–683. (15) Zheng, Y.; Shen, Z.; Cai, C.; Ma, S.; Xing, Y. The reuse of nonmetals recycled from waste printed circuit boards as reinforcing fillers in the polypropylene composites. J. Hazard. Mater. doi: 10.1016/ j.jhazmat.2008.07.008. (16) Chinese industry standard. Standard test methods of bitumen and bituminous mixtures for highway engineering. JTJ 0522000, 32-38. (17) Chinese industry standard. Technical specifications for construction of highway asphalt pavements. JTG F40-2004, 8-15. (18) Airey, G. D. Use of black digrams to identify inconsistencies in rheological data. Road Mater. Pavement Des. 2002, 3 (4), 403– 24. (19) Airey, G D.; Mohammed, M. H.; Fichter, C. Rheological characteristics of synthetic road binders. Fuel 2008, 87, 1763– 1775. (20) Widyatmoko, I.; Elliott, R. Characteristics of elastomeric and plastomeric binders in contact with natural asphalts. Constr. Build. Mater. 2008, 22, 239–249.
ES8023012
