J. Cent. South Univ. Technol. (2008) 15(s1): 167-171 

DOI: 10.1007/s11771-008-339-6 

Effects of thermal-oxidative aging on rheological properties of

montmorillonite modified bitumen

FENG Peng-cheng(冯鹏程), YU Jian-ying(余剑英), WANG Xiao(王 骁), TAO Yuan-yuan(陶园园)

(School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China)

Abstract: Organic montmorillonite (OMMT) modified bitumen nanocomposites was prepared by melt blending. The effects of thin-film oven test(TFOT) and pressure ageing vessel(PAV) on rheological properties of pristine bitumen and OMMT modified bitumen were investigated by dynamic shear rheometer (DSR). The results show that complex modulus (G*) increases, phase angle (δ) decreases and rutting factor (G*/sin δ) is enhanced for the pristine bitumen after TFOT, whereas G*, δ and G*/sin δ of OMMT modified bitumen have a little change before and after TFOT. Besides, the pristine bitumen exhibits a large increase of G* and a great decrease of δ after PAV aging. However, the changes in G* and δ of OMMT modified bitumen are small before and after PAV. Compared with the pristine bitumen, OMMT modified bitumen presents a lower fatigue factor (G*sin δ) after PAV. As a consequence, resistance to thermal-oxidative aging of bitumen is remarkably improved due to the introduction of OMMT.

Key words: montmorillonite; bitumen; aging; rheological property

1 Introduction

As other polymeric materials evolve with time, bitumen is also prone to become hardening and embrittlement by oxidating decomposing or polymerizing due to exposure to the heat source, UV radiation and atmospheric oxygen during the mixing process, laying on the road and the service life in the pavement. Bitumen ageing is one of the principal factors causing negative change of physical structures and chemical compositions, and results in the deterioration of its physical behaviors^[1-3]. Therefore, the durability of pavement bitumen is reduced dramatically, which severely shortens the lifespan of bitumen pavement.

Montmorillonite can impart polymers with some unique properties, such as thermal, mechanical and barrier properties in virtue of its silicate sheets dispersed in the polymers at nanoscale^[4-5]. The pristine sodium   montmorillonite (Na-MMT) and organic montmorillonite (OMMT) as modification have been adopted to improve the properties of bitumen and SBS modified bitumen in our previous researches. The findings were obtained that addition of Na-MMT and OMMT to bitumen can obviously improve the physical properties of bitumen. At the same time, the aging properties of bitumen are also can be enhanced to some extent^[6], which can be contributed to the formation of intercalated and exfoliated structures for Na-MMT and OMMT respectively in bitumen matrix^[7-8]. Bitumen is a typical viscoelastic material, numerous studies address the rheological properties of the pristine and modified bitumens^[9-12], and the changes in viscoelastic parameters, such as complex modulus (G*) and phase angle (δ) of bitumens during the aging process, have a very large effect on the utility of bitumen in the pavement^[13-14]. Consequently, it is of great importance to improve the negative effects of aging on the rheological properties of bitumen.

In the present work, OMMT modified bitumen was prepared by melt blending. The thin film oven test (TFOT) was used to simulate the age hardening occurring of bitumen during plant mixing and laydown, and the pressure ageing test (PAV) was employed to simulate long-term thermal-oxidative ageing during service of bitumen. The effects of the thermal-oxidative aging on the rheological properties of pristine bitumen and OMMT modified bitumen were evaluated by means of dynamic rheological techniques.

2 Experimental

2.1 Materials

Bitumen and SK-70 bitumen were supported by SK Corp., Korea. The used OMMT in this study was made by the Na-MMT exchanged with hexadecyl dimethyl benzyl ammonium, was supplied by Fenghong Clay Chemical Factory, Zhejiang, China.

2.2 Preparation of OMMT modified bitumen

The modified bitumen was prepared using a high shear mixer (Weiyu Machine Co., Ltd, China). Bitumen was first heated until it became a well fluid at around  150 ℃ in an iron container. Then 3% OMMT was added into bitumen, and the mixtures were blended at        5 000 r/min for 60 min to ensure uniform dispersion of OMMT in the matrix. The pristine bitumen was also processed under the same conditions in order to compare with the modified bitumen.

2.3 Bitumen aging experiment

TFOT was executed in Bitumen Film Oven (Model 82, Huanan Testing Machines Co., Ltd, China) with a plate and axis, and the rotation of plate was carried out around the axis. The bitumen which was put on the plates was heated for 5 h at 163 ℃ according to ASTM D1754. The PAV apparatus (Model 9300, Prentex Inc., USA) consisted of the pressure aging vessel and temperature chamber. A cylinder of dry and clean compressed air was provided by a pressure regulator with air pressure. The standard aging procedure of 100 ℃, 2.1 MPa and 20 h for the PAV was utilized, the residue from TFOT aging was used according to ASTM D 6521.

2.4 Dynamic rheological characterization

Dynamic rheological measurements for all the samples (pristine and modified bitumen) were performed in parallel plate mode in the Dynamic Shear Rheometer (Model AR2000, TA Co., USA). A temperature sweep (from 5 ℃ to 70 ℃) with 2 ℃ increments was applied at a fixed frequency of 10 rad/s and at variable strain. In this study, parallel plates of 2.5 mm and 8 mm diameters were used in different temperature ranges. The rheological parameters were measured for calculating viscoelastic parameters, such as G*, δ, G*/sin δ and G*sin δ.

3 Results and discussion

3.1 Effect of montmorillonite on dynamic rheological properties of bitumen

Dynamic shear rheological tests involve the application of a periodically oscillatory strain or stress. This loading model can just reflect the rheological behaviors of bitumen under the traffic loading^[15]. Fig.1 shows the curves of G* versus temperature for the pristine and the OMMT modified bitumen. According to Fig.1, significant increase in G* exhibits a more viscoelastic behavior of the modified bitumen than that of the unmodified, which may be caused by the exfoliation of OMMT layers in bitumens^[7]. The effect of OMMT on δ of the pristine and modified bitumen is also shown in Fig.1. It can easily be seen that δ of modified bitumen apparently decreases due to the adding of OMMT, which means that the elastic behavior of bitumen is enhanced effectively in the test temperature range. As the same as the effect of OMMT on G*, the reason can similarly ascribed to the dispersion structure of OMMT in the bitumen matrix.

Fig.1 Effects of OMMT on rheological properties of pristine and modified bitumen

3.2 Effect of TFOT on rheological properties

3.2.1 Effects on G*

Fig.2 shows the curves of G* vs temperature for the pristine and OMMT modified bitumen after TFOT. Bitumen thermal-oxidative aging dramatically decreases the content of oily composition in the bitumen matrix, in company with the accumulation of asphaltene composition, which made the bitumen become hardening and embrittlement^[16]. Accordingly, as shown in Fig.2, G* is increased during the aging process. Furthermore, according to Fig.2, it can also be seen that the modified bitumen with OMMT exhibits lower magnitude of change in G* in comparison with the pristine, which suggests that the effect of TFOT aging on G* of modified

Fig.2 Curves of G* vs temperature for pristine and OMMT modified bitumen after TFOT

bitumen is lower. The reason can be attributed to the dispersed situation of OMMT silicate sheets in the bitumen. The nanoscale silicate layers can efficiently reduce the permeability to oxygen in the bitumen, presenting reduction of oxidative degree. Therefore, the trends of hardening of bitumen are decreased, resulting in lessening the change in G* of modified bitumen.

3.2.2 Effects on δ

The effect of TFOT aging on δ of the pristine and the OMMT modified bitumen is shown in Fig.3. It should be noted that δ of both the pristine and modified bitumen is reduced. The reduction of δ suggests the decrease of viscous part and the increase of elastic part, indicating hardening process of the bitumen during the thermal-oxidative aging. In addition, Fig.3 also shows that there is a smaller magnitude of variation in δ for the modified bitumen during the TFOT aging. Analogous to the reason of influence of OMMT on G* during the TFOT aging, the OMMT distinctly improves the aging resistance of bitumen, thereby, lessens the effect of TFOT aging on δ evidently.

Fig.3 Curves of δ vs temperature for pristine and OMMT modified bitumen after TFOT

3.2.3 Effects on G*/sin δ

In accordance with the strategic highway research program(SHRP) specification, G*/sin δ at 10 rad/s as the rutting factor has been selected for measuring the contribution of a binder to the permanent deformation^[17]. Fig.4 shows the effect of TFOT aging on G*/sin δ for the pristine and OMMT modified bitumen. OMMT layered silicates increase the complex modulus and improve the elasticity of bitumen (increase G* and decrease δ), thereby, increasing G*/sin δ values of the bitumen as indicated in Fig.4.

Moreover, according to Fig.4, it is easily observed that there is a great increase in G*/sin δ of the pristine bitumen after TFOT. The reason may be due to the increase in G* and the decrease in δ during the TFOT

Fig.4 Curves of G*/sin δ vs temperature for pristine and OMMT modified bitumen after TFOT

Aging, leading to an obvious increase in G*/sin δ. Whereas the increased magnitude of modified bitumen is seemingly slight due to the adding of OMMT.

3.3 Effect of PAV on rheological properties

3.3.1 Effects on G*

Fig.5 shows the curves of G* vs temperature for the pristine and OMMT modified bitumen after PAV. It can be seen that there is a bigger G* value after the PAV aging than TFOT. The phenomena can be interpreted that the loss of volatile components and the increase of asphaltene in view of the oxidation of bitumen becomes stronger in the accessional pressure than that of TFOT aging, thereby presenting a bigger G* value. Moreover, compared with the pristine bitumen, the modified bitumen exhibits lower increasing extent of G*, even presenting a smaller G* value than the pristine after PAV. The results can also be contributed to the dispersed structure of OMMT modified bitumen.

Fig.5 Curves of G* vs temperature for pristine and OMMT modified bitumen after PAV

3.3.2 Effects on δ

Fig.6 shows the curves of G* vs temperature for the pristine and OMMT modified bitumen after PAV. Compared to TFOT, it should be noted a continued decrease of δ value during the PAV aging, which means a further reduction in viscous part of bitumen. In addition, since the introduction of OMMT can obviously improve the aging property of bitumen during PAV aging process, the influence of the aging on δ of the modified bitumen is reduced obviously. As a consequence, it exhibits lower increasing extent of δ than that of the pristine as shown in Fig.6.

Fig.6 Curves of δ vs temperature for pristine and OMMT modified bitumen after PAV

3.3.3 Effects on G*sin δ

The influences of OMMT on the fatigue factor (G*sin δ) of pristine and the modified bitumens after PAV aging were shown in Fig.7. G*sin δ at 10 rad/s as the fatigue factor is used to measure the contribution of a binder to the fatigue ability resistance^[17]. The high the fatigue factor value, the more quick the shearing energy

Fig.7 Curves of G*sin δ vs temperature for pristine and OMMT modified bitumen after PAV

loss under the relative loads, namely, the worse the fatigue ability resistance of bitumen. As shown in Fig.7, G*sin δ of modified bitumen is lower than pristine bitumen. In accordance with SHRP method^[18], when G*sin δ= 5 000 kPa, the temperature was 24.5 ℃ for the pristine bitumen. However, it is reduced to 21.5 ℃ for the modified bitumen, which suggests that it is well to improve the fatigue ability resistance of bitumen as a result of the introduction of OMMT.

4 Conclusions

1) OMMT modified bitumen was prepared by melt blending, and the effects of OMMT on the dynamic rheological behaviors of the modified bitumen were investigated. The results show that the modified bitumen exhibits higher G*, lower δ, higher G*/sin δ in comparison with the pristine bitumen, which suggests that the rheological properties of modified bitumen is significantly improved.

2) G* increases, δ decreases and G*/sin δ is remarkably enhanced for the pristine bitumen after TFOT, whereas G*, δ and G*/sin δ of OMMT modified bitumen have a little change before and after TFOT, which suggests that the effect of TFOT thermal-oxidative aging on the rheological properties of modified bitumen is weaker than the pristine. The reason can be contributed to the formation of an exfoliated structure in the OMMT modified bitumen.

3) Compared with the pristine bitumen, OMMT modified bitumen exhibits smaller changes in both G* and δ after PAV, meanwhile G*sin δ of modified bitumen transferred to the low temperature region. As a consequence, the resistance to thermal-oxidative aging of bitumen is significantly enhanced due to the introduction of OMMT. Thereby the negative effect of thermal-oxidative aging on the rheological properties of OMMT modified bitumen is improved remarkably.

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(Edited by CHEN Ai-hua)

Foundation item: Project(50773061) supported by the National Natural Science Foundation of China

Received date: 2008-06-25; Accepted date: 2008-08-05

Corresponding author: YU Jian-ying, PhD, Professor; Tel: +86-27-59735080; E-mail: jyyu@whut.edu.cn