Enhanced Performance of LDPE/SBS Blend Modified Asphalt Through Dynamic Vulcanization

Transcription

1 Enhanced Performance of LDPE/SBS Blend Modified Asphalt Through Dynamic Vulcanization Enhanced Performance of LDPE/SBS Blend Modified Asphalt Through Dynamic Vulcanization Guangtao Gao, Yong Zhang and Yinxi Zhang Research Institute of Polymer Materials, Shanghai Jiao Tong University, Shanghai , China Kang Sun State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai , China Received: 8 January 2002 Accepted: 22 May 2002 SUMMARY A blend of a Low-Density Polyethylene (LDPE) and a Styrene-Butadiene-Styrene triblock copolymer (SBS) was mixed with an asphalt, and a storage-stable modified asphalt was prepared by dynamic vulcanization. The vulcanization characteristics of this LDPE/SBS blend-modified asphalt were studied using a strain-controlled rheometer. The vulcanization of the SBS in the blend-modified asphalt resulted in a marked increase in the torque. In addition, the physical properties of the LDPE/SBS blend-modified asphalt containing even a small quantity of sulfur were improved compared to the modified asphalt without sulfur. The storage stability of the LDPE/SBS blend-modified asphalt was also improved by the addition of sulfur. The rheological properties of the LDPE/SBS blend-modified asphalt before and after adding sulfur were characterized using a dynamic shear rheometer and a rotational viscometer. INTRODUCTION It is well known that polymer modified asphalt (PMA) has been used in road technology with the intention of reducing road deterioration such as rutting, cracking and stripping 1-6. Currently, a large variety of polymers, such as Styrene-Butadiene- Styrene triblock copolymer (SBS), Styrene-Butadiene Rubber (SBR) and Polyethylene (PE) are being used for PMA. However, the major obstacle to its widespread employment in paving practice has been the tendency of PMA towards gross phase separation under quiescent conditions at elevated temperatures, because of the poor compatibility between polymer and asphalt. 7 It has been reported that the stability of PMA can be improved by the addition of sulfur, frequently in the elemental form It is commonly believed that sulfur chemically crosslinks the polymer molecules and chemically couples polymer and asphalt through sulfide or polysulfide bonds. Most of the studies described here concentrate on the preparation processes, composition and performance of PMA after the addition of sulfur. In our opinion, the preparation of storage-stable PMA with sulfur is readily achieved by dynamic vulcanization, and this concept can be applied in practice on a large scale. Dynamic vulcanization, which is widely used in rubber/plastic blends, is a route to new thermoplastic elastomers which have properties (such as ultimate mechanical properties, stability of phase morphology in the melt, and so on) as good as or even, in some cases, better than those of elastomeric block copolymers 12,13. Dynamic vulcanization can be described as follows: after sufficient melt mixing of rubber and plastic, e.g., in an internal mixer or a compounding extruder (to form a well-mixed blend), vulcanizing agents are added. Vulcanization then occurs while mixing continues. Fine vulcanized rubber particles, which are formed by shearing and shattering, are dispersed homogeneously in a continuous plastic phase 13. As for the processing procedure, the preparation of PMA with sulfur is similar to the dynamic vulcanization of rubber/plastic blends. The objective of the present work is to confirm the occurrence of vulcanization in the LDPE/SBS blendmodified asphalt, and to investigate the physical Polymers & Polymer Composites, Vol. 10, No. 6,

2 Guangtao Gao, Yong Zhang, Yinxi Zhang and Kang Sun properties and rheological behaviour of the LDPE/SBS blend-modified asphalts before and after adding sulfur. EXPERIMENTAL Materials Asphalt, AH-90 paving asphalt, was obtained from the Zhongyou Xingneng Asphalt Factory of Jiangyin, Jiangsu, China. The physical properties of this asphalt were as follows: penetration: 90 dmm (deci-millimetre, 25 C, ASTM D5); softening point: 47.5 C (ASTM D36); viscosity: 0.35 Pa.s (135 C, ASTM D4402). SBS, Grade 4303, was produced by the Yanshan Petrochemical Co., Ltd., China. SBS 4303 was a starlike polymer, containing 30 wt% styrene, M w =350,000. LDPE, Grade N220, with a melt flow index of 2.0 g/ 10 min (190 C/21.168N), was provided by the Shanghai Petrochemical Co., Ltd., China. Sulfur was an industrial grade product, made by the Shanghai Jinghai Chemical Co., China. LDPE/SBS Blends Preparation LDPE and SBS were mixed to form LDPE/SBS blends in the mixing chamber of a HAAKER rheometer RC90 at the rotor speed of 80 rpm and at a defined temperature. The blend compositions in this paper were chosen only to illustrate the effect of the blends on the asphalt modification. Vulcanization Characteristics of the Polymer-Modified Asphalt ( PMA) Asphalt was heated to 160 C in a small container until it flowed fully. A pre-weighed amount of polymer blend was mixed into the asphalt under high-speed stirring for 1h and the blend became essentially homogeneous. When the temperature of the blend was reduced below 110 C, a given level of sulfur was added and the stirring continued for 15 min at a low stirring speed (200 r.p.m.) so as to obtain unreacted homogeneous blends. Below 110 C, neither the reaction between asphalt and sulfur nor the vulcanization of SBS/sulfur blend would take place, according to the literature 14,15. A strain-controlled rheometer (Advanced Rheology Expanded System (ARES) of the Rheometric Scientific Co., USA) with cup and plate geometry was used to determine the vulcanization characteristics of the blends. A sample (2.0 g) was put in the lower shadow cup (30 mm in diameter). After the sample was heated to the flow point, the upper parallel plate (25 mm in diameter) was lowered for a tight fit with the sample. All the samples were held at a constant temperature of 80 C for 10 min, then heated rapidly to a high temperature (always 180 C in this paper) and kept there for 1 h. The applied frequency was 50 rad/s. The maximum torque (M H ), the minimum torque (M L ), t 0, t 50, t 90, and curing rate (1/t 50 ), were obtained from the curing curves. t 0 is the time when the torque is equal to M L. t 50 and t 90 are the times when the torques are equal to ((M H - M L ) x 50%) + M L and ((M H - M L ) x 90%) + M L, respectively. These were measured after the minimum on the curves of torque versus time. Polymer Modified Asphalt Preparation The polymer modified asphalt was prepared using a high-shear mixer at 180 C at a speed of 4000 rpm. First, 600 g asphalt was heated to become a fluid in the mixer. When the temperature reached about 170 C, a given amount of polymer (1.5 pph, 3 pph, 4.5 pph, or 6 pph, based on 100 parts of asphalt) was added to the asphalt in the mixer. The temperature increased to 180 C and the mixing was continued at this temperature for 1 h. After that a given amount of sulfur (5% based on the weight of SBS) was added, and the mixing continued for another 1 h. Measurement of Physical Properties and Storage Stability The softening points of the asphalt were measured in accordance with ASTM standard D36. In the test, a metal ring with an asphalt sample, on which a metal ball was placed, was put into a water or glycerine bath. The temperature was increased at 5.0 C/min. The softening point of an asphalt sample was the temperature when the ball fell through the ring. The penetration of the asphalts was measured according to ASTM standard D5. Generally the penetration was the depth that a needle, with the defined weight, penetrated vertically into the sample at 25 C over a time of 5 s. The storage stability of the modified asphalt was measured as follows: some of the prepared polymer modified asphalt was transferred into an aluminum 434 Polymers & Polymer Composites, Vol. 10, No. 6, 2002

3 Enhanced Performance of LDPE/SBS Blend Modified Asphalt Through Dynamic Vulcanization toothpaste tube (32 mm in diameter and 160 mm in height). The tube was sealed and stored vertically in an oven at 163 C for 48 h, then taken out, cooled to room temperature, and cut horizontally into three equal sections. The samples taken from the top and bottom sections were used to evaluate the storage stability of the polymer modified asphalts by measuring their softening points and viscosities at 135 C. If the difference in softening points between the top and the bottom sections was less than 2.5 C and the viscosities of asphalts from the top and the bottom sections varied within 10%, the sample was considered to have good storage stability. If the softening points differed by more than 2.5 C or the difference of the two viscosities was above or below 10%, the polymer modified asphalts were classified as less stable on storage 16. Rheological Characterization A strain-controlled rheometer, ARES, with parallel plate geometry (25 mm in diameter) was used to determine the rheological characteristics of PMA. A temperature sweep was applied over the range 30~100 C at a fixed frequency of 10 rad/s. About 1.0 g sample was put onto the low plate. After the sample was heated to become molten, the upper parallel plate was lowered to contact tightly with the sample and trimmed. The final gap was adjusted to 1.2 mm. All the samples were held at 30 C for 10 min, and then the temperature was raised in 2 C increments. Various viscoelastic parameters, such as G*, G, G, and tan δ were collected automatically by the RSI Orchestrator software. of the sulfur structures, the evolution of H 2 S was also evident in a side reaction. The reaction between asphalt and sulfur is stated by Filippis et al. 14,17 to begin at about 130 C. In our experimental study of the vulcanization characteristics of LDPE/SBS blend-modified asphalts in the presence of sulfur, all the asphalts were kept at 180 C and the variations in torque with time were compared, as shown in Figures 1-3. The initial fall in torque over about the first 5 minutes came from a delay in achieving the correct temperature of 180 C. It can be seen in Figure 1, the torque of LDPEmodified asphalt, in the absence or presence of sulfur, was little changed with time after 10 minutes at the high temperature. But for SBS-modified asphalt, the torque increased with time in the presence of sulfur over the period of 5 to 30 minutes, as shown in Figure 2. The torque was also slightly dependent on time for the asphalt/sulfur blend, which indicates that the reaction between asphalt and sulfur has a small effect on the torque of the SBS-modified asphalt with a small amount of sulfur. Obviously, the measurable increase in the torque can be only realized through the reaction between sulfur and SBS, as reported by Wen. 11 Moreover, the torque of LDPE/SBS blend-modified asphalt kept unchanged with increasing time (as shown in Figure 3), which implies that the chemical reaction between asphalt and LDPE/SBS blend could Viscosity Measurement The flow properties of the asphalt samples were determined by a rotational viscometer (Model DV- II+, Brookfield Engineering Inc., USA) at fixed temperatures of 110, 120, 135, 160, or 180 C and a rotating speed of 60 rpm. Figure 1 Vulcanization of LDPE modified asphalt at 180 C RESULTS AND DISCUSSION Vulcanization Characteristics It has been reported that sulfur (up to 50wt% in asphalt) can improve the physical and mechanical properties of an asphalt. 17 The improvement was related to a reaction between asphalt and sulfur, which occurs within the aromatic centers (Ar) in the asphaltene molecules, giving rise to higher asphaltene/ resin ratios by the formation of sulfur structures such as Ar-SS-Ar and Ar-(S) x -Ar 18. Besides the formation Polymers & Polymer Composites, Vol. 10, No. 6,

4 Guangtao Gao, Yong Zhang, Yinxi Zhang and Kang Sun Figure 2 Vulcanization of SBS-modified asphalt at 180 C Table 1 Vulcanization parameters of polymer modified asphalts in the presence of sulfur F ormulation Asphalt/3% SBS/0.3% Sulfur Asphalt/4.5%LD- PE/SBS (34:66) Blend/0.3%Sulfur 5 M H x10, N. m M L x10 5, N. m ( M -M ) x10 5, H L N.m , min t 0 t 50 min t 90 min, , Curing rate 1/t, 100 mi -1 ( ) n Figure 3 Vulcanization of LDPE/SBS blend-modified asphalt at 180 C not be detected by means of the torque measurement. The torque of the LDPE/SBS blend-modified asphalt, in the presence of sulfur, increased significantly with increasing time, which reflects the process of gradual formation of high molecular weight polymers. 11 Some important vulcanization parameters, such as M H, M L, t 0, t 50 and t 90, in the torque versus time curves are given in Table 1. It can be seen in Table 1 that the cure rates (1/t 50 ) of asphalt/sbs/sulfur binder and asphalt/ldpe/sbs blend/sulfur binder are about the same. Physical Properties and Storage Stability The physical properties and storage stability of the LDPE/SBS blend-modified asphalts were improved to a great extent by the addition of sulfur, as shown in Figure 4 and Table 2, respectively. Before adding sulfur, the softening points of the modified asphalt increased and the penetrations decreased with increasing LDPE/SBS blend content. After the addition of sulfur, the softening points of all the modified asphalts were higher than those of the binders without sulfur. The penetration decreased slightly, especially when the LDPE/SBS blend content was above 3%. Compared to the big differences in softening points and viscosities between the top and bottom sections of the modified binders without sulfur, the differences between the modified binders containing sulfur became smaller, as shown in Table 2, which indicates that the storage stability of the LDPE/SBS blend-modified asphalt has been effectively improved with the addition of sulfur. Rheological Properties For rutting resistance at high temperatures, a high complex modulus (G*) and elastic modulus (G ) and low tan δ values are favourable because they reflect a more elastic behaviour. Figure 5 shows the dynamic properties of the LDPE/SBS blend-modified asphalts as functions of the LDPE/SBS blend content. In the absence of sulfur, the G* of the modified binders increased with increasing LDPE/SBS blend content at high temperature, as shown in Figure 5(a). Obviously, in the medium temperature range (30-50 C), G* increased slightly and the tan δ became 436 Polymers & Polymer Composites, Vol. 10, No. 6, 2002

5 Enhanced Performance of LDPE/SBS Blend Modified Asphalt Through Dynamic Vulcanization Table 2 Storage stabilities analysis of asphalt modified with LDPE/SBS (34:66) blend Material Formulations LDPE/SBS (34:66) blend, phr Sulfur, phr Storage stability analysis V iscosity at 135 C, Pa. s Top* # Bottom* Top/bottom ratio R ing & ball softening point, C Top* # Bottom* Top-Bottom Difference *The samples were taken from the top and bottom of a sample tube. #The polymer blend in the top section sample was agglomerated, so the test sample of the viscosity and softening point could not be made Figure 4 Effect of LDPE/SBS blend content on physical properties of LDPE/SBS blend-modified asphalts before and 1 hour after adding sulfur. Formula: asphalt 100, LDPE/SBS blend variable, sulfur 5.0 phr (by the weight of SBS). (a) Softening point; (b) Penetration a b smaller with increasing content of LDPE/SBS in the blend. But at high temperatures ( C), G* increased significantly with increasing LDPE/SBS blend content, and tan δ became smaller. That is, the temperature susceptibility of the asphalt was reduced to some extent. The addition of sulfur resulted in smaller changes in tan δ with increasing temperatures than that of the modified binders without sulfur. For the modified asphalt containing 3wt% LDPE/SBS blend, the change in tanδ was less than 10 at temperatures ranging from 30 to 100 C, as shown in Figure 5(b). As the LDPE/ Polymers & Polymer Composites, Vol. 10, No. 6,

6 Guangtao Gao, Yong Zhang, Yinxi Zhang and Kang Sun Figure 5 Effect of polymer content on rheological properties of the LDPE/SBS blend-modified asphalts (Frequency: 10 rad/s). Formula: asphalt 100, LDPE/SBS blend variable, sulfur 5.0 phr (by the weight of SBS). (a) before adding sulfur; (b)1 hour after adding sulfur a b SBS blend content increased, the tan δ of the modified asphalt changed only slightly, and G* of the modified binders increased significantly within the test temperature range. The temperature of the modified binders, when G*/ sin δ is equal to 1kPa, was calculated to indicate the high-temperature performance. The results are shown in Figure 6. Obviously, the high temperature performance of the binders has been improved effectively with the addition of sulfur. Viscosity Analysis The effect of polymer content on the viscosity of the LDPE/SBS blend-modified asphalts is shown in Figure 7. In the absence of sulfur, the increase in the viscosity of the asphalt was not directly proportional to the polymer content. And a somewhat marked change was observed when the LDPE/SBS blend content exceeded 3 wt%. After adding sulfur, the asphalt viscosities were a little higher than those of the blend without sulfur. The vulcanization of SBS in modified asphalts and other reactions between sulfur Figure 6 Effect of LDPE/SBS blend content on the temperature of the modified asphalts when G*/sinδ=1 kpa. Formula: asphalt 100, LDPE/SBS blend variable, sulfur 5.0 phr (by the weight of SBS) Figure 7 Effect of the amount of LDPE/SBS blend on asphalt viscosity at 135 C. Formula: asphalt 100, LDPE/ SBS blend variable, sulfur 5.0 phr (by the weight of SBS) 438 Polymers & Polymer Composites, Vol. 10, No. 6, 2002

7 Enhanced Performance of LDPE/SBS Blend Modified Asphalt Through Dynamic Vulcanization and asphalt components are indicated by the significantly increased viscosity in this study. The viscosity of the modified asphalt at the LDPE/SBS blend content of 6.0% was about nine times higher than that of original asphalt, and about twice as high as that of the LDPE/SBS blend-modified asphalts with no sulfur. The effect of temperature on the asphalt viscosity can be described by the Arrhenius function, η = η 0 exp(e η / RT). By plotting log η versus 1/T (as shown in Figure 8, where η is the viscosity and η 0 is a constant), the activation energy of the asphalts was calculated from the slope of the plots, and listed in Table 3. The activation energy (E η ) of the asphalts was affected slightly by the addition of LDPE/SBS blends before and after adding sulfur, but η 0 was much more sensitive. CONCLUSIONS A storage stable LDPE/SBS blend-modified asphalt has been successfully prepared by dynamic vulcanization. The vulcanization characteristics of the LDPE/SBS blend-modified asphalt, in the presence of sulfur, can be determined by a strain-controlled rheometer. The physical properties of the modified asphalt and the storage stability were improved by the addition of sulfur. The high temperature performance of the asphalt was improved, and the temperature susceptibility was also reduced considerably by the presence of sulfur at a 0.1 to 0.2% level. The LDPE/SBS blend content in the asphalt has a great effect on the rheological properties of the asphalt. As determined by the rotational viscometer, the asphalt viscosity was directly proportional to the polymer content, and was further enhanced by the addition of sulfur. ACKNOWLEDGEMENT Figure 8 logη~1/t relationship of asphalt and LDPE/ SBS blend-modified asphalts before and 1 hour after adding sulfur The authors acknowledge the financial support provided by the National Natural Science Foundation of China. REFERENCES 1. Lewandowski, L. H., Rubber Chemistry and Technology, 67, (1994), Lu, X., and Isacsson, U., Fuel, 76, (1997), Jew, P., Shimizu, J.A., Svazic, M., and Woodhams, R.T., Journal of Applied Polymer Science, 31, (1986), Lee, Y. J., France, M. L., and Hawley, M. C., Rubber Chemistry and Technology, 70, (1997), Yousefi, A. A., Ait-Kadi, A., and Roy, C., Journal of Materials in Civil Engineering, 12, (2000), 113 Table 3 Effect of polymer content on the activation energy of the LDPE/SBS (34:66) blend -modified asphalts LDPE/SBS (34:66) blend content, pph Before E η adding sulfur 8, kj/mol η 0 x 10, Pa. s After adding sulfur Before adding sulfur After adding sulfur Polymers & Polymer Composites, Vol. 10, No. 6,

8 Guangtao Gao, Yong Zhang, Yinxi Zhang and Kang Sun 6. Shuler, T.S., Collins, T. H., and Kirkpatrick, J. P., Asphalt: Relationship to Mixture. ASTM STP 941, O. E. Briscoe, Ed., American Society for Testing and Materials, Philadelphia, (1987), Hesp, S. A. M., and Woodhams, R. T., Colloid Polymer and Science, 269, (1991), Maldonado, P., Mas, J., and Phung, T.K., US Patent (1979) 9. Planche, J. P., PCT WO (1990) 10. Bellomy, R. C., and McGinnis, E.L., US Patent (1994) 11. Wen, G. A., Zhang, Y., Zhang, Y. X., Sun, K., and Chen, Z. Y., Journal of Applied Polymer Science, 82,(2001), Coran, A. Y., Das, B, and Patel, R.P., US Patent (1978) 13. Coran, A. Y., Rubber Chemistry and Technology, 68, (1995), Filippis, P. D., Giavarini, C., and Santarelli, M. I., Petroleum Science and Technology, 15, (1997), Morrison N.J., Porter M., Rubber Chemistry and Technology, 57, (1984), Moran, L.E., U.S. Patent, US (1991) 17. Filippis, P. D., Giavarini, C., Santarelli, M. I., Fuel, 77, (1998), Al-Dobouni, A., Shahab, Y. A., and Tawfiq, K.S., Fuel Science and Technology International, 12, (1994), Polymers & Polymer Composites, Vol. 10, No. 6, 2002