Rheological Properties of Polymer-Modified
Bitumen
Jiqing Zhu
Swedish National Road and Transport Research Institute (VTI), Olaus Magnus väg 35, SE-581 95 Linköping, Sweden. Email: jiqing.zhu@vti.se
Xiaohu Lu
Nynas AB, SE-149 82 Nynäshamn, Sweden. Email: xiaohu.lu@nynas.com
Abstract This paper employs isothermal annealing at different temperatures to
obtain different microstructures of the same modified bitumen with styrene-butadiene-styrene (SBS) copolymer. The effect of polymer-modified bitumen (PMB) morphology on high-temperature rheological properties is investigated based on the same material. The characteristic wavelength ξ of PMB microstruc-ture was calculated from the fluorescence microscopy images. Dynamic shear rheometer (DSR) measurements and multiple stress creep and recovery (MSCR) tests were performed. The PMB morphology-rheology correlation was discussed within the studied temperature and frequency range. Comparing with a binary droplet-in-matrix microstructure, a homogenous PMB morphology tends to store more energy during shear cycles and reach higher recovery after the loading.
Keywords Polymer-modified bitumen, Morphology, Rheology, Creep.
1
Introduction
The morphology of polymer-modified bitumen (PMB) has important influences on its rheological properties (Lu et al. 2010). Some previous studies (Liang et al. 2017, Wang et al. 2017) used additives to obtain different PMB microstructures
and investigated their effects on PMB rheology. However, the use of additives may bring unknown impacts into PMB. It is still preferred to study various PMB microstructures based on the same material composition.
Soenen et al. (2006) reported that thermal history can largely affect the PMB morphology and thus the rheology, especially at low frequencies. This might have provided a feasible and practical way to control PMB morphology, i.e. by control-ling its thermal history. This paper employs isothermal anneacontrol-ling at different tem-peratures to obtain different microstructures of the same modified bitumen with styrene-butadiene-styrene (SBS) copolymer. The effect of PMB morphology on high-temperature rheological properties is investigated.
2
Materials and method
2.1 Materials
A base bitumen of penetration grade 70/100 was used to prepare PMB. Its SARA fractions were 8% of saturates, 55% of aromatics, 22% of resins and 15% of as-phaltenes according to thin-layer chromatography with flame ionisation detection (TLC-FID). A linear triblock SBS copolymer was mixed with the base bitumen at 180 °C. The polymer content was 5% by weight of the blend. The related proper-ties were tested and are presented in Table 1.
Table 1 Properties of the base bitumen and polymer-modified bitumen
Property Base bitumen Polymer-modified bitumen
Penetration, 25 °C (0.1 mm) 86 56
Softening point, ring & ball (°C) 43.4 77.8
Penetration index -1.8 4.3
2.2 Method
To obtain different microstructures, the prepared PMB samples were conditioned by isothermal annealing for 1 h at 160 °C and 120 °C respectively before each test. Fluorescence microscopy was employed to capture the PMB morphology and the two-dimensional fast Fourier transform (2D-FFT) method was applied to ana-lyse the captured images (Zhu et al. 2018). As for rheology, dynamic shear rhe-ometer (DSR) was used to measure the complex modulus and phase angle at dif-ferent test temperatures (64 °C, 70 °C, 76 °C and 82 °C) and frequencies (10 rad/s, 1 rad/s and 0.1 rad/s) according to ASTM D7175. In addition, multiple stress
creep and recovery (MSCR) tests were performed at different temperatures (76 °C, 70 °C and 64 °C) according to ASTM D7405. The relation between PMB morphology and rheological properties is discussed.
3
Results and discussion
3.1 Morphology
The captured PMB morphology images are shown in Figure 1. It can be observed that the PMB presents a homogenous microstructure after isothermal annealing for 1 h at 160 °C, but a two-phase pattern with SBS-rich droplets in the bitumen-rich matrix after conditioning at 120 °C. For further morphological analysis, please re-fer to Lu et al. (2010). In this paper, values of the characteristic wavelength ξ of the observed PMB microstructures are calculated, as defined by:
ξ = 2π/km (1)
where km is the characteristic spatial frequency corresponding to a peak in the
2D-FFT power spectrum (Figure 1). For the sample after isothermal annealing for 1 h at 160 °C, the curve does not present a peak. Its km is read as infinite. Thus, the
calculated ξ value is 0 mm. But the calculated ξ value is 2.356 mm for the sample after conditioning at 120 °C. These results indicate that the isothermal annealing at different temperatures has resulted in significantly different PMB microstructures.
Fig. 1 Microscopy images and the analysis by two-dimensional fast Fourier transform (2D-FFT)
3.2 Complex modulus and phase angle
The DSR measurement results of complex modulus G*, phase angle δ and other derivative parameters (G*/sinδ and elastic modulus G’) at different test
tempera-tures and frequencies are presented in Figure 2. It is indicated that the two samples after isothermal annealing at different temperatures show only very limited differ-ence between each other at 10 rad/s, both fulfilling the 1000 Pa criterion of G*/sinδ at 76 °C (ASTM D6373).
As the frequency decreases, however, their difference starts to enlarge, espe-cially in phase angle δ and elastic modulus G’. This is attributed to the different microstructures of the two samples. Comparing with a binary droplet-in-matrix microstructure, the homogenous PMB morphology after isothermal annealing for 1 h at 160 °C can assist in storing more energy during the shear cycles at 1 rad/s and 0.1 rad/s. This means that, assuming a slow traffic, PMB with homogenous morphology would provide higher resistance to the loading than the one with bina-ry droplet-in-matrix microstructure.
Fig. 2 Complex modulus, phase angle and other derivative parameters
3.3 Multiple stress creep and recovery (MSCR)
Figure 3 shows the MSCR test results at different temperatures. The results indi-cate that, at all the tested temperatures, the sample after isothermal annealing for 1 h at 120 °C has a higher accumulated strain level than that at 160 °C. For both two samples, the recovery is very limited at 76 °C, resulting in a very slight difference between them. As the temperature decreases, however, the recovery starts to in-crease. Their difference also starts to enlarge, especially at the higher stress level of 3200 Pa. The related parameters, including the average total strain ε1, percent
recovery R, non-recoverable creep compliance Jnr at both two stress levels and the
percent difference between stress levels, are calculated according to ASTM D7405. Calculation results are listed in Table 2.
Fig. 3 Multiple stress creep and recovery (MSCR) test results at different temperatures Table 2 Calculation of multiple stress creep and recovery (MSCR) test results
Tests @ 76 °C @ 70 °C @ 64 °C 160 °C 120 °C 160 °C 120 °C 160 °C 120 °C ε1100 0.413 0.454 0.250 0.244 0.136 0.139 ε13200 15.8 16.8 9.32 9.32 4.69 4.99 R100 10.7% 8.4% 46.9% 37.5% 55.5% 45.4% R3200 4.6% 2.2% 28.8% 13.9% 47.1% 24.3% Rdiff 56.7% 74.2% 38.7% 62.9% 15.2% 46.6%
Jnr100 3.69 kPa-1 4.16 kPa-1 1.33 kPa-1 1.53 kPa-1 0.605 kPa-1 0.757 kPa-1
Jnr3200 4.72 kPa-1 5.15 kPa-1 2.08 kPa-1 2.51 kPa-1 0.775 kPa-1 1.18 kPa-1
Jnr-diff 27.9% 23.8% 56.8% 64.3% 28.0% 56.2%
The calculation shows that the two samples mostly reach similar levels of total strain ε1. The higher conditioning temperature (160 °C) has leaded to higher
per-cent recovery. This confirms that the difference between samples is not caused by different levels of aging at high temperatures but does result from the different microstructures. Comparing with a binary droplet-in-matrix microstructure, the homogenous PMB morphology after isothermal annealing for 1 h at 160 °C can provide higher resistance to permanent deformation in the tested temperature range. The percent difference between stress levels is mostly lower for the ho-mogenous PMB morphology, indicating a lower stress sensitivity. These results mean that, to some extent, PMB with homogenous morphology would be resistant to heavier traffic loading than one with binary droplet-in-matrix microstructure.
4
Summary: Towards PMB morphology-rheology relation
In the previous sections, it has been discussed that PMB morphology has influ-ences on the rheological properties, although not necessarily with the same signif-icance in the full temperature and frequency range. In certain range, however, it is possible to build the quantitative correlation between the characteristic wavelength ξ of the PMB microstructure and rheological parameters, such as the elastic modu-lus G’ and MSCR percent recovery at 3200 Pa (Table 3). Comparing with a binary droplet-in-matrix microstructure, a homogenous PMB morphology tends to store more energy during shear cycles and reach higher recovery after the loading.Table 3 Correlation between morphological and rheological parameters of PMB
Sample ξ G’ @76°C & 1 rad/s R3200 @70°C R3200 @64°C
160 °C 0 mm 134 Pa 28.8% 47.1%
120 °C 2.356 mm 36.2 Pa 13.9% 24.3%
Due to the limited number of samples and tests presented in this paper, the PMB morphology-rheology correlation is only preliminarily discussed within the studied temperature and frequency range. Towards a complete relation, however, this paper provides a quantitative morphological parameter for future studies, i.e. the characteristic wavelength ξ of the PMB microstructure by 2D-FFT method. In addition, it is demonstrated in this paper that controlling thermal history is a feasi-ble and practical way to control PMB morphology based on the same material. As a recommendation, it would be interesting to further investigate the effect of PMB morphology on the linear viscoelastic region and zero-shear viscosity.
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