The synergistic effect of organic montmorillonite and thermoplastic polyurethane on properties of asphalt binder

Construction and Building Materials 229 (2019) 116867

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

The synergistic effect of organic montmorillonite and thermoplastic polyurethane on properties of asphalt binder Meng Jia a, Zengping Zhang a,⇑, Haiting Liu b, Biao Peng a, Hongliang Zhang a, Wenjiang Lv c, Qiang Zhang d, Zhiyong Mao e a

Key Laboratory for Special Area Highway Engineering of Ministry of Education, Chang’an University, Xi’an, Shaanxi 710064, PR China Jiangsu Sinoroad Engineering Technology Research Institute Co., Ltd., Nanjing, Jiangsu 211800, PR China Shaanxi Province Transportation Construction Group Co., Ltd., Xi’an, Shaanxi 710075, PR China d Chinese Sanan Construction Co. Ltd, Xi’an, Shaanxi 710043, PR China e Ningbo Xinming Construction Engineering Test Co., Ltd., Ningbo, Zhejiang 315000, PR China b c

h i g h l i g h t s
The synergistic effect of OMMT and TPU on properties of asphalt is studied.
OMMT can improve the storage stability of TPU modified binder.
OMMT and TPU present a good synergy in asphalt matrix.

a r t i c l e

i n f o

Article history: Received 25 April 2019 Received in revised form 7 August 2019 Accepted 1 September 2019 Available online 7 September 2019 Keywords: Asphalt binder TPU OMMT Content Synergistic effect Property

a b s t r a c t This paper aims at investigating the synergistic effect of organic montmorillonite (OMMT) and thermoplastic polyurethane (TPU) on physical and rheological properties of asphalt binder. Herein, the structure and content of OMMT and TPU in asphalt binder were first determined. OMMT/TPU modified asphalt binders were then evaluated by XRD (X-ray diffraction), FTIR (Fourier transform infrared), penetration, softening point, ductility, viscosity, storage stability, DSR (dynamic shear rheometer), BBR (bending beam rheometer) and LOI (limiting oxygen index) tests. Results show that, the increase of OMMT content facilitates storage stability of OMMT/TPU modified binder. Existence of OMMT and TPU with appropriate contents is able to enhance high and low temperature properties, elastic behavior and flame retardancy of asphalt binder. TPU content is the dominant factor of low temperature property, while OMMT content plays a key role in flame retardancy of OMMT/TPU modified binder. Additionally, it is recommended to use 2% OMMT and 9% TPU in asphalt binder. All in all, OMMT and TPU present a good synergy in asphalt matrix. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Polyurethane (PU), one of polymer materials, has been extensively utilized in many fields (e.g. construction, petroleum industry, textile industry), mainly in forms of foam plastics, elastomer, adhesive [1–3]. In general, the synthesis of PU is divided into two steps: PU prepolymer is first synthesized from a certain mass ratio of isocyanate (e.g. TDI, MDI) and polyol (e.g. polyester polyol, polyether polyol); the PU prepolymer then reacts with chain extender to prepare the PU product [4–6]. As for the molecular structure, PU is a block polymer comprising of alternating hard segments ⇑ Corresponding author. E-mail address: [email protected] (Z. Zhang). https://doi.org/10.1016/j.conbuildmat.2019.116867 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

and soft segments (Fig. 1), where hard segments are supplied by the reaction products of isocyanate and chain extender, and polyol makes up soft segments [7–10]. In the majority of cases, the more soft segments lead to the larger flexibility of PU, whereas the PU with more hard segments has greater stiffness and mechanical property [9,11,12]. As a consequence, the ratio of soft segment to hard segment has a great influence on PU properties, which can be controlled by several parameters (i.e. isocyanate/polyol ratio, terminal –NCO content of PU prepolymer and chain extension coefficient) [7,13,14]. With this regard, PU modified binders with desirable properties can be designed by adjusting these parameters. A growing number of studies in recent years have been conducted regarding PU modified binder [5,6,15–18]. By summarizing

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Hard segment

3. Materials and methods

Soft segment 3.1. The selection of materials The materials used to prepare modified binders include base binder, OMMT, TPU prepolymer, chain extender, the details of which are as follows. 3.1.1. Base binder The 90# base binder was provided by SK Co., Ltd., Korea, and its properties are listed in Table 1. Fig. 1. The molecular structure of PU.

literatures, we notice that, the PU structure does greatly influence asphalt properties. For example, as reported by Xu et al [5] and Xia et al [15], they adopted a PU prepolymer (terminal –NCO content: 3 wt%–6 wt%) as asphalt modifier and then found that high and low temperature properties of base binder were improved. To further enhance the properties, Sun et al [6] and Ban [19] prepared PU modified binders by adding a certain amount of chain extender to prepolymer/asphalt binder composites, and the result indicated that the prepared PU modified binders had an excellent low temperature property as well as a good high temperature property. However, Bazmara et al. [17] added a synthetic PU to asphalt binder. It was found that PU had no significant impact on the low temperature property of base binder. Although these advantages, PU modified binders usually have a poor storage stability [6,20]. To solve this problem, some chemical additives like compatibilizers can be introduced [19,20]. However, these chemical additives are generally volatile; also, they have disadvantages like high cost and single function. So, some cost-effective and environmentalfriendly alternatives should be attempted to improve storage stability of PU modified binder. There are some reports on using organic montmorillonite (OMMT) as a stabilizer to improve the storage stability of polymer modified binders, reasons of which are summarized as two points: first, OMMT is one of layered silicates at nanoscale, which determines that some polymers (e.g. SBS, SBR) and asphalt molecules can intercalate into OMMT galleries to form a stable ternary composite system [21–23]; besides, incorporation of OMMT can equalize the density difference between asphalt binder and polymers [21,24,25]. Therefore, OMMT, as an environmental-friendly alternative to chemical additives, is expected to improve the poor storage stability of PU modified binder.

3.1.2. OMMT OMMT was prepared from MMT by treatment with octadecyl dimethyl benzyl ammonium chloride, the relevant parameters of which are detailed in the Ref. [26]. Fig. 2 provides the SEM image that demonstrates the intercalated structure of OMMT. It is known from the literature [23,26–28] that 2–3 wt% of layered silicates are frequently used to modify base binder. Considering the adverse effect of layered silicates on low temperature property of binder, this work attempted to select four low OMMT contents, as 0.5 wt %, 1.0 wt%, 1.5 wt% and 2.0 wt% (by asphalt weight). 3.1.3. TPU prepolymer As described in the introduction, by varying the type and number of functional groups, PU with different properties can be gained. To compensate for the disadvantageous effect of OMMT on asphalt low temperature property, this study employed a thermoplastic polyurethane (TPU) synthesized from 2,4-TDI and a polyether prepolymer, which was supplied by Huatianpu Co., Ltd., in Zibo city, China. Its molecular structure and basic information are shown in Fig. 3 and Table 2, respectively. Terminal –NCO groups (in Fig. 3) serve as active points that are able to react with –NH2 groups in chain extender, thus extending the molecular chain of prepolymer. Nevertheless, as a rule, the more –NCO groups lead

2. Objectives The main objectives of this study are listed in order: (1) To design the structure of thermoplastic polyurethane (TPU). (2) To investigate the effect of OMMT on storage stability of the TPU modified binder. (3) To evaluate the synergistic effect of OMMT and TPU on physical and rheological properties of asphalt binder and then recommend an appropriate OMMT and TPU content.

Fig. 2. The SEM image of OMMT.

Table 1 Properties of base binder. Test item

Unit

Test result

Technical requirement

Standard test method

Penetration (25 °C, 100 g, 5s) Ductility (5 cm/min, 15 °C) Softening point Density RTFOT (163 °C, 85 min)

0.1 mm cm °C g/cm3 % % cm

93 ˃100 46.1 1.002 0.070 63.3 21.5

80–100 100 44–54 – ±0.8 55.0 8

ASTM D5 ASTM D113 ASTM D36 ASTM D1505 AASHTO T240-06 ASTM D5 ASTM D113

Weight loss Penetration ratio Ductility (15 °C)

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Fig. 3. The molecular structure of TPU prepolymer.

Fig. 4. The molecular structure of MOCA.

to the larger proportion of hard segments which is detrimental to the TPU flexibility and low temperature cracking resistance of TPU modified binder. So, it is necessary to control the amount of terminal –NCO groups to a low range [5,15,19]. Through the preliminary experimental research, we find that, TPU prepolymer within an appropriate content range (7–11 wt%) significantly improves low temperature property of base binder. In view of this, five TPU contents (5 wt%, 7 wt%, 9 wt% and 11 wt%) are adopted in this work. 3.1.4. Chain extender On the whole, introducing an appropriate amount of chain extender allows the extension of molecular chains of TPU prepolymer, thereby obtaining the TPU characterized by better comprehensive properties than TPU prepolymer [4,6,11]. The 3,30 -dichloro-4,40 -diaminodiphenylmethane (MOCA), one kind of aromatic diamine chain extenders, is commonly used in the synthesis of TPU based on TDI [29,30]. Fig. 4 and Table 3 present the molecular structure and basic information of MOCA, respectively. According to some literatures [11,31,32], the chain extender coefficient (r), representing the molar ratio of –NH2 in chain extender to –NCO in prepolymer, has a remarkable impact on TPU flexibility and subsequently may affect low temperature cracking resistance of TPU modified binder. More precisely, the r value smaller than 1 means the formation of some TPU crosslinked structures in asphalt matrix, which is bad for low temperature property; on the contrary, excessive MOCA, acting as a plasticizer, adversely affects comprehensive properties of TPU modified binder. In addition, considering that there exists a small amount of –OH groups in OMMT that can react with –NCO groups in prepolymer, the r value is initially determined to be 0.93 (which will be validated by FTIR result). Accordingly, the MOCA content is calculated by the following equation.

mMOCA ¼ mprepolymer 

M MOCA  NCO%  r 2  M NCO

ð1Þ

where, mMOCA is the MOCA mass (g); mprepolymer is the mass of TPU prepolymer (g); MMOCA and MNCO are the relative molecular mass of MOCA and –NCO group (367.16 and 42), respectively; NCO% is the –NCO content in TPU prepolymer; r is the chain extender coefficient. 3.2. Preparation The preparation procedure of OMMT/TPU modified asphalt binder is presented in Fig. 5. It should be noted that after stirred,

asphalt samples were placed in an oven for adequate reaction development at 135 °C for 1 h [4]. 3.3. Experimental methods 3.3.1. Characterization X-ray diffraction (XRD) measurements were made in Cu-Ka radiation mode, with a wavelength (k) of 0.154 nm at a 2°/min scan rate. Previous studies have shown that although an frequently used technique to characterize the microstructure of nanocomposites, transmission electron microscope (TEM) was generally not suitable for testing asphalt binder, since some asphalt molecules was easy to evaporate under the vacuum environment [25,33]. For the sake of accurately determining the microstructure of OMMT modified binder, the XRD test was conducted on the samples after the dissolution-filtration procedure. Detailed procedures were as follows: the OMMT/TPU modified binder was dissolved in trichloroethylene; the solution was then filtered to separate OMMT; the microstructure of separated OMMT particles was characterized by the XRD test [25]. Based on XRD patterns, the basal spacing (d001) can be obtained using the Bragg equation (Eq. (2)).

k ¼ 2d001 sinh

ð2Þ

where, k is the wavelength of X-ray; 2h is the diffraction angle. Fourier transform infrared (FTIR) spectra were tested with a scan range of 400–4000 cm1 and scan frequency of 32 times/min. 3.3.2. Storage stability The storage stability test allows the evaluation of storage stability or phase integrity of modified binders at construction temperature [34]. About 50 g specimen was poured into an aluminum tube with standard dimensions. The tube was then sealed and maintained in vertical position at 163 °C for 48 h. Following that, the tube was cut into three equal sections after cooled at 5 °C for 4 h. The softening point differences between the top and bottom sections should be lower than 2.5 °C to guarantee the good stability and phase integrity of modified binders during high temperature storage [16]. 3.3.3. Physical properties The 25 °C penetration, softening point, 5 °C ductility and dynamic viscosity were tested in terms of ASTM D5, ASTM D36, ASTM D113 and ASTM D4402, respectively. Three replicates were used in these tests.

Table 2 The basic information of TPU prepolymer. NCO content (%)

Viscosity(85 °C) mPas

Gel time min

Hardness HA

Tensile strength MPa

Density (25 °C) g/cm3

3.3 ± 0.2

450

7–9

75 ± 2

18

1.08

4

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Table 3 The basic information of MOCA.

MOCA

Relative molecular mass

Melting point (°C)

Hardness Shore A/D

Density (g/cm3)

Chlorine content (%)

Amine value (mmol/g)

367.16

100–109

A75

1.44

26

7.4–7.6

Fig. 5. The preparation process of modified binder.

3.3.4. Rheological behaviors The high and low temperature rheological properties were measured via a dynamic shear rheometer (DSR) and bending beam rheometer (BBR), in accordance with ASTM D7175 and ASTM D6648, respectively. 3.3.5. Flame retardancy Limiting oxygen index (LOI) tests proceeded as follows: the specimen with standard dimensions (120 mm  7 mm  3 mm) was vertically installed on the fixture before the combustion cylinder was covered. The flows of oxygen and nitrogen were then regulated by valves. Following this, the top surface of specimen was fully touched by the gas flame which was stopped once the specimen was ignited. Through repeated tests, the minimum volume flow of oxygen to maintain the sample burning was recorded, along with the volume flow of nitrogen. The oxygen index (LOI) can be expressed as follows:

½O  100% LOI ¼ ½O þ ½N

ð3Þ

Fig. 6. The XRD patterns of (a) pristine OMMT, (b) separated OMMT, (c) 2%OMMT/ 9%TPU modified binder.

where, ½O is the volume flow of oxygen at critical oxygen concentration; ½N  is the volume flow of nitrogen at critical oxygen concentration.

of asphalt or TPU molecules that enlarges the distance between layers of pristine OMMT, which has been reported in literatures [22,35–39].

4. Results and discussion

4.1.2. FTIR The typical functional groups of materials for this study are analyzed by FTIR spectra (Fig. 7). In the FTIR spectra of MOCA (Fig. 7a), the characteristic peaks appear at 3441 cm1 and 3358 cm1, which is associated with the stretching vibrations of –NH groups. For TPU prepolymer (Fig. 7b), the absorption peak at 2269 cm1 is assigned to the asymmetric stretching vibrations of terminal –NCO groups. In Fig. 7c, the weak band of –OH stretching vibrations at 3618 cm1 results from adsorption water molecules in OMMT galleries [40]. However, after preparation of modified binder is completed, the characteristic peaks mentioned above all disappear (as seen in Fig. 7e), while a new characteristic peak at 1728 cm1, caused by TPU, is observed in the FTIR spectra of modified binder. Therefore, the disappearance of the band at 2269 cm1 can be attributed to the chemical reaction of –NCO groups. According to relevant reports [4,5,11], it is obtained that –NCO groups are able to react with composites containing active hydrogen (e.g. –OH, –NH), the schematic diagrams of which are displayed as follows. Since there exists a small amount of –OH groups in OMMT structure (as seen in the weak peak at 3618 cm1), most of the –NCO groups (from TPU prepolymer) seem to react with –NH groups (from MOCA) to form TPU in asphalt matrix. Furthermore, the chain extension coefficient adopted in this study (0.93) ensures the complete reaction of TPU prepolymer.

4.1. Structure analysis 4.1.1. XRD Fig. 6 provides the XRD patterns of pristine OMMT, separated OMMT and OMMT/TPU modified binder. The d001-reflection peak of pristine OMMT appears at 2h = 3.15°, indicating that pristine OMMT forms an intercalated structure with d001 value of 2.81 nm. By contrast, no peak is observed for OMMT/TPU modified binder, which does not prove complete exfoliation because a small amount of OMMT in binder may be not enough for diffraction peaks to be seen. In order to exactly determine the type of layered structure, OMMT is separated from OMMT/TPU modified binder and then tested by diffractometer. The result (Fig. 6b) shows that for separated OMMT, there is a strong peak at 2h = 1.65° corresponding to the d001 value of 5.35 nm. It seems that not all OMMT sheets are exfoliated; in other words, some OMMT particles with intercalated structure still exist in asphalt matrix. As a result, OMMT/TPU modified binder is likely to form a part-exfoliated nanostructure. Additionally, comparing Fig. 6a and b demonstrates that the d001-reflection peak of pristine OMMT is shifted towards a smaller angle, and the d001 value correspondingly increases from 2.81 nm to 5.35 nm, the reason of which seems to the intercalation

M. Jia et al. / Construction and Building Materials 229 (2019) 116867

5

TPU (within the test range) generally corresponds to the better storage stability of OMMT/TPU modified binder, where the optimal mass ratio is required to be further studied. It has been reported that asphaltenes in asphalt binder could intercalate into OMMT galleries, achieving a good stability of OMMT modified binder [44,45]. For OMMT/polymer modified binder, OMMT particles with a large specific surface area, can effectively hinder the moving path of all ingredients including asphaltenes, polymers and maltenes, and as a rule, a larger OMMT content has a stronger effect. Therefore, these ingredients are hard to move; instead, they are welldistributed [44,45]. 4.3. Physical properties

Fig. 7. The FTIR spectra of (a) MOCA, (b) TPU prepolymer, (c) OMMT, (d) base binder and (e) OMMT/TPU modified binder.

slow

fast

R  NCO þ R  OH ! R  NHCOOH ! R  NH2 þ CO2 fast

R  NCO þ R  NH2 ! R  NHCONH  R 4.2. Storage stability analysis With regard to the improvement effect of OMMT on the storage stability of TPU modified binder, Fig. 8 presents the storage stability test results for modified binders involved various OMMT/TPU contents. It can be observed that, regardless of the OMMT content, softening point difference between top and bottom sections (DS) increases along with incorporation of TPU, indicating that an increased TPU content is bad for the storage stability of OMMT/ TPU modified binder. This phenomenon chiefly originates in two aspects: first, there are some differences between TPU and asphalt binder in parameters like relative molecular mass, solubility, density, leading to relatively poor compatibility between them [41,42]; on the other hand, TPU and asphalt binder, in most cases, are physical combined and have no occurrence of chemical reaction (no characteristic peaks reacted with binder are also observed from Fig. 7), the blend of which is thermodynamic incompatible [4,43]. It can also be seen that, in Fig. 8, existence of OMMT dramatically decreases the DS value, which is an indication of OMMT to improve the storage stability. The greater mass ratio of OMMT to

Various physical parameters of binders with different OMMT/ TPU contents are displayed in Fig. 9. As expected, with the increase of OMMT and TPU contents, penetration decreases while softening point correspondingly increases. This manifests that both OMMT and TPU can enhance high temperature stability of base binder. Fig. 9a and b also indicate that, in most cases, the increase of TPU content from 9% to 11% makes little contribution to the high temperature stability enhancement. It is known from the literature [17,46] that TPU is likely to adsorb lightweight components in asphalt matrix and then swell to form a stable three-dimensional network structure through physical crosslinking effect; however, the swelling TPU can reach saturation so that the further increase of TPU content has a relatively slight effect on high temperature stability. By contrast, OMMT acts as filler instead of being soluble into asphalt matrix, which determines that OMMT particles dispersed in asphalt matrix can restrict the molecular motion and then steadily improve the high temperature stability with content [47]. From Fig. 9c, modified binders has a ductility larger than that of base binder, suggesting that incorporation of OMMT and TPU, within the test range, helps to low temperature cracking resistance of base binder. When the TPU content remains constant, ductility gradually decreases as the OMMT content rises, which may be due to the restriction of rigid OMMT particles on the movement of asphalt molecules [35]. On the contrary, irrespective of the OMMT content, ductility goes up with the TPU content increasing. This confirms that the TPU structure designed in this study endows binder with good low temperature property; more specifically, we adopted a thermoplastic PU prepolymer and controlled the chain extender coefficient (r = 3.3 ± 0.2) to guarantee a large proportion of soft segments in TPU structure, where these soft segments with excellent flexibility are primarily responsible for the improvement in low temperature property of binder. Similar results have been reported in another research [15]. Interestingly, ductility usually surges as the TPU content increases from 7% to 9%. It may be that when asphalt binders involved more than 7% TPU, TPU with a great tensile property may play a dominant role in low temperature deformation resistance of OMMT/TPU modified binder. Overall, the 9% TPU may be preferred from the aspect of high and low temperature properties. 4.4. Viscosity analysis

Fig. 8. The storage stability test results for binders containing various OMMT/TPU contents.

Viscosity of asphalt binder evaluates its flowability [48], of which the results are shown in Fig. 10. As observed in this figure, the temperature dependency of viscosity is noticeable that viscosity decreases along with the temperature rising, irrespective of the OMMT/TPU content. Compared with base binder, modified binders have a remarkably larger viscosity. It appears that adding OMMT and TPU leads to a drop of asphalt flowability, thereby improving the thermal stability of asphalt binder, which is line with the penetration and softening point results.

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Fig. 9. The results of physical properties: (a) Penetration at 25 °C; (b) Softening point; (c) Ductility at 5 °C.

log ðBVÞ ¼ k  T þ t

ð4Þ

in which, T is the test temperature in Fig. 10; k and t are regression parameters. The regression parameters, mixing and compaction temperatures are listed in Table 4. Correlation coefficients (R2) are found to be higher than 0.96, demonstrating a good linear correlation between viscosity and temperature. Besides, Table 4 also shows that mixing and compaction temperatures of modified binders all increase by at least 10 °C as compared to them of base binder. It is important that these mixing and compaction temperatures provide a guide for asphalt pavement construction. 4.5. Rheological analysis

Fig. 10. The viscosity results for binders containing various OMMT/TPU contents.

The construction temperature of asphalt pavement includes mixing temperature and compaction temperature which correspond to the regressed viscosities of 170 ± 20 mPas and 280 ± 30 mPas, respectively. The regressed viscosity is determined by linear regression to the data in Fig. 10 [49], where the regression model can be expressed as follows:

4.5.1. DSR test With the intention of studying the influence of OMMT and TPU on high temperature viscoelastic behaviors of asphalt binder, the complex modulus (G*) and phase angle (d) results within the range of 60 °C–80 °C are provided in Fig. 11 [50]. From this figure, modified binders have an obviously larger G* value with respect to base binder, which is contrary to the d results. This means that existence of OMMT and TPU has an improvement in stiffness and elastic property of base binder, which can account for the results of physical properties. No matter it is OMMT or TPU, the increased content causes an increase in G* value and yet a decrease in d value; thus,

7

M. Jia et al. / Construction and Building Materials 229 (2019) 116867 Table 4 The regression parameters, construction temperatures of asphalt binders. Asphalt sample

k

t

R2

Base 0.5%OMMT + 9%TPU 1.0%OMMT + 9%TPU 1.5%OMMT + 9%TPU 2.0%OMMT + 9%TPU 1.0%OMMT + 5%TPU 1.0%OMMT + 7%TPU 1.0%OMMT + 11%TPU

0.0169 0.01823 0.01871 0.01860 0.01863 0.01814 0.01821 0.01828

4.8979 5.3265 5.4703 5.5081 5.5703 5.2509 5.3207 5.4450

0.9730 0.9800 0.9749 0.9769 0.9819 0.9791 0.9805 0.9663

Tmixing (°C)

Tcompaction (°C)

Min.

Max.

Ave.

Min.

Max.

Ave.

154.7 167.0 170.3 173.4 176.3 163.5 166.8 172.8

160.2 172.2 175.6 178.4 181.3 169.0 172.1 178.1

157.5 169.6 173.0 175.9 178.8 166.3 169.5 175.5

146.8 155.7 159.3 162.2 165.4 152.1 155.4 161.5

149.3 160.4 164.0 167.0 170.0 157.0 160.3 166.4

148.1 158.1 161.7 164.6 167.7 154.6 157.9 164.0

Fig. 11. The complex modulus and phase angle results for binders with various OMMT/TPU contents: (a) and (b) Complex modulus; (c) and (d) Phase angle.

both OMMT and TPU are conductive to the enhancement of stiffness and elastic property. In addition, by comparing Fig. 11a and b (also Fig. 11c and d), it can be concluded that OMMT seems to be stronger in influencing viscoelastic behaviors of binder when compared with TPU. Fig. 12 presents the rutting resistance factor (G*/sind) results for binders containing various OMMT/TPU contents. After OMMT and TPU are added to base binder, G*/sind value dramatically increases, regardless of the temperature. This result implies that incorporation of OMMT and TPU reduces rutting susceptibility of binder at high temperature, which coherent with the results of physical properties. Moreover, the comparison between OMMT and TPU demonstrates that the influence of OMMT content on asphalt rutting susceptibility is generally more significant.

Based on the G*/sind results, failure temperatures of these binders are obtained (Fig. 13) [50]. As illustrated in Fig. 13, failure temperatures of the binders are higher than 70 °C except for base binder (around 66 °C). It is evident that the composite modification by OMMT and TPU enhances high temperature performance grade of base binder at least one grade. By comparing the effect of OMMT and TPU on failure temperature, it is obtained that OMMT appears to be more noticeable in elevating the failure temperature. Furthermore, it can also be noted that, among these binders, only the binder involved 2% OMMT and 9% TPU has a failure temperature higher than 76 °C, therefore being classified as PG 76; the further increase of OMMT content (from 2% to 3%) probably has no ability to enhance high temperature performance grade but weakens low temperature property of binder. As a result, combined with

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M. Jia et al. / Construction and Building Materials 229 (2019) 116867

Fig. 12. The rutting resistance factor results for binders containing various OMMT/ TPU contents.

the results of physical properties, it is recommended to use 2% OMMT and 9% TPU in asphalt binder. 4.5.2. BBR test For BBR test, creep stiffness (S) and creep rate (m-value) are two important outputs (Fig. 14). Generally speaking, the larger S value means the more tensile stress, and the smaller m-value corre-

Fig. 13. The failure temperature results for binders containing various OMMT/TPU contents.

sponds to the less stress relaxation [51]. So, the small S value and large m-value are desirable for asphalt binder. It can be seen from Fig. 14 that modified binders exhibit a smaller S value and lar-

Fig. 14. The BBR test results: (a) and (b) Creep stiffness; (c) and (d) Creep rate.

OMMT/TPU content (%)

M. Jia et al. / Construction and Building Materials 229 (2019) 116867

TPU plays a dominant role in low temperature relaxation capacity of OMMT/TPU modified binders, while OMMT has an adverse effect.

/11 1.0 7 / 1.0 /5 1.0 /9 2.0 /9 1.5 /9 1.0 /9 0.5 l o ntr Co

20.0

9

4.6. Flame retardancy

20.5

21.0

21.5

22.0

22.5

Limiting oxygen index (%)

The limiting oxygen index (LOI) results for binders containing various OMMT/TPU contents are shown in Fig. 15. The LOI value is observed to increase after the composite modification, indicating that existence of OMMT and TPU is able to improve fire resistance of base binder. Fig. 15 also demonstrates that a small amount of OMMT can improve flame retardancy of asphalt binder, which has been corroborated by some researchers [52,53]. In fact, OMMT at nanoscale has a large specific surface area in asphalt matrix and meanwhile, OMMT has expansibility in a wide temperature range, which can determine that OMMT particles in asphalt binder are capable of blocking the oxygen penetration and release of flammable volatiles effectively (Fig. 16) [54–56].

Fig. 15. The limiting oxygen index test results for binders containing various OMMT/TPU contents.

5. Conclusions ger m-value than base binder, irrespective of the temperature, suggesting that introduction of OMMT and TPU endows base binder with better low temperature property. Besides, with respect to the influence of the modifier type, an increase of OMMT content increases S value whereas decreases m-value, but the opposite trend can be found for TPU. Therefore, it can be concluded that,

In this work, the synergistic effect of OMMT and TPU on properties of asphalt binder was mainly investigated. The appropriate OMMT/TPU content in asphalt matrix was obtained according to the physical and rheological properties by a series of Superpave tests. Based on the results and discussion, conclusions can be drawn:

Fig. 16. The flame retardancy mechanism of OMMT.

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M. Jia et al. / Construction and Building Materials 229 (2019) 116867

(1) Compared with pristine OMMT, OMMT separated from composite modified binder has a larger interlayer spacing, which may be due to the intercalation of asphalt and TPU molecules. (2) An increase of OMMT content helps to the storage stability of OMMT/TPU modified binders. (3) Adding OMMT and TPU with appropriate contents can improve high and low temperature properties, elastic behavior and flame retardancy of asphalt binder, where OMMT has potential to improve flame retardancy of polymer modified binder. (4) Both OMMT and TPU are conductive to high temperature property of binder; TPU content is the dominant factor of low temperature property, whereas OMMT has an adverse effect; OMMT content plays a key role in flame retardancy of OMMT/TPU modified binder. (5) It is recommended to use 2% OMMT and 9% TPU in asphalt binder.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the Shaanxi Provincial Communication Construction Group (No. 17-06K), the Fundamental Research Funds for the Central Universities of Chang’an University (No.300102218523) and the National Natural Science Foundation of China (NSFC) (Grant No. 51208043). References [1] T. Tantisattayakul, P. Kanchanapiya, P. Methacanon, Comparative waste management options for rigid polyurethane foam waste in Thailand, J. Clean. Prod. 196 (2018) 1576–1586. [2] M. Gonzalez, M. Nelly, S. Turri, Development of polyester binders for the production of sustainable polyurethane coatings: technological characterization and life cycle assessment, J. Clean. Prod. 164 (2017) 171–178. [3] D. Dobslaw, C. Woiski, F. Winkler, K. Engesser, C. Dobslaw, Prevention of clogging in a polyurethane foam packed biotrickling filter treating emissions of 2-butoxyethanol, J. Clean. Prod. 200 (2018) 609–621. [4] X. Sheng, M. Wang, T. Xu, J. Chen, Preparation, properties and modification mechanism of polyurethane modified emulsified asphalt, Constr. Build. Mater. 189 (2018) 375–383. [5] C. Xu, Z. Zhang, F. Zhao, F. Liu, J. Wang, Improving the performance of RET modified asphalt with the addition of polyurethane prepolymer (PUP), Constr. Build. Mater. 206 (2019) 560–575. [6] M. Sun, M. Zheng, G. Qu, K. Yuan, Y. Bi, J. Wang, Performance of polyurethane modified asphalt and its mixtures, Constr. Build. Mater. 191 (2018) 386–397. [7] S. Cao, M. Li, S. Li, J. Xia, K. Huang, Influence of soft and hard segment ratio on the properties of self-healing polyurethanes, Thermosetting Resin 33 (3) (2018) 5–9. [8] M. Fuensanta, J. Jofre-Reche, F. Rodríguez-Llansola, V. Costa, J. MartínMartínez, Structure and adhesion properties before and after hydrolytic ageing of polyurethane urea adhesives made with mixtures of waterborne polyurethane dispersions, Int. J. Adhes. Adhes. 85 (2018) 165–176. [9] J. Qu, L. Tian, X. Wang, Effect of soft and hard segment on structure and properties of polyurethane elastomers, J. Funct. Polym. 23 (2) (2010) 160–165. [10] Q. Yin, X. Wang, J. Chen, X. Wu, J. Zheng, Synthesis of blocked waterborne polyurethane with 3,5-dimethylpyrazole for industrial application in digital inkjet printing, J. Appl. Polym. Sci. 136 (29) (2019) 8. [11] Z. Cao, Q. Zhou, S. Jie, B. Li, High cis-1,4 hydroxyl-terminated polybutadienebased polyurethanes with extremely low glass transition temperature and excellent mechanical properties, Ind. Eng. Chem. Res. 55 (6) (2016) 1582– 1589. [12] S. Taheri, G. Sadeghi, Microstructure–property relationships of organomontmorillonite/polyurethane nanocomposites: Influence of hard segment content, Appl. Clay Sci. 114 (2015) 430–439. [13] C. Fang, S. Pan, Z. Wang, X. Zhou, W. Lei, Y. Cheng, Synthesis of waterborne polyurethane using snow as dispersant: structures and properties controlled by polyols utilization, J. Mater. Sci. Technol. 35 (7) (2019) 1491–1498.

[14] H. Zhou, H. Chen, D. Liu, X. Zhang, X. Chen, Y. Li, Study on properties of polyester polyurethane elastomer based on IPDI, China Plast. Ind. 40 (12) (2012) 84–87. [15] L. Xia, D. Cao, H. Zhang, Y. Guo, Study on the classical and rheological properties of castor oil-polyurethane pre polymer (C-PU) modified asphalt, Constr. Build. Mater. 112 (2016) 949–955. [16] F. Khairuddin, M. Alamawi, N. Yusoff, K. Badri, H. Ceylan, S. Tawil, Physicochemical and thermal analyses of polyurethane modified bitumen incorporated with Cecabase and Rediset: optimization using response surface methodology, Fuel 254 (2019) 115662. [17] B. Bazmara, M. Tahersima, A. Behravan, Influence of thermoplastic polyurethane and synthesized polyurethane additive in performance of asphalt pavements, Constr. Build. Mater. 166 (2018) 1–11. [18] R. Yu, X. Zhu, X. Zhou, Y. Kou, M. Zhang, C. Fang, Rheological properties and storage stability of asphalt modified with nanoscale polyurethane emulsion, Petrol. Sci. Technol. 36 (1) (2017) 85–90. [19] X. Ban, Study on Preparation and Properties of Polyurethane (PU) Modified Asphalt, China Master’s Thesis, Chang’an University, 2017. [20] H. Zhang, G. Zhang, F. Han, Z. Zhang, W. Lv, A lab study to develop a bridge deck pavement using bisphenol A unsaturated polyester resin modified asphalt mixture, Constr. Build. Mater. 159 (2018) 83–98. [21] B. Zhang, M. Xi, D. Zhang, H. Zhang, B. Zhang, The effect of styrene–butadiene– rubber/montmorillonite modification on the characteristics and properties of asphalt, Constr. Build. Mater. 23 (10) (2009) 3112–3117. [22] M. Sureshkumar, S. Filippi, G. Polacco, I. Kazatchkov, J. Stastna, L. Zanzotto, Internal structure and linear viscoelastic properties of EVA/asphalt nanocomposites, Eur. Polym. J. 46 (4) (2010) 621–633. [23] S. Liu, S.B. Zhou, Y. Xu, Evaluation of cracking properties of SBS-modified binders containing organic montmorillonite, Constr. Build. Mater. 175 (2018) 196–205. [24] S. Galooyak, B. Dabir, A. Nazarbeygi, A. Moeini, Rheological properties and storage stability of bitumen/SBS/montmorillonite composites, Constr. Build. Mater. 24 (3) (2010) 300–307. [25] H. Zhang, C. Zhu, B. Tan, C. Shi, Effect of organic layered silicate on microstructures and aging properties of styrene–butadiene–styrene copolymer modified bitumen, Constr. Build. Mater. 68 (2014) 31–38. [26] M. Jia, Z. Zhang, L. Wei, X. Wu, X. Cui, H. Zhang, W. Lv, Q. Zhang, Study on properties and mechanism of organic montmorillonite modified bitumens: view from the selection of organic reagents, Constr. Build. Mater. 217 (2019) 331–342. [27] C. Zhu, H. Zhang, G. Xu, C. Shi, Aging rheological characteristics of SBR modified asphalt with multi-dimensional nanomaterials, Constr. Build. Mater. 151 (2017) 388–393. [28] H. Zhang, C. Zhu, K. Yan, J. Yu, Effect of rectorite and its organic modification on properties of bitumen, J. Mater. Civ. Eng. 27 (8) (2015). [29] X. Wei, T. Zhang, Z. Sui, J. Zhang, Z. Jiang, Effect of different chain extenders on cast polyurethane elastomer, Chem. Propell. Polym. Mater. 15 (6) (2017) 54– 57. [30] Y. Feng, H. Zhou, X. Zhang, Y. Li, Y. Li, Preparation and characterization of polyurethane damping materials derived from mixed-base prepolymers containing numerous side methyls, e-Polym 15 (5) (2015) 323–327. [31] X. Liu, X. Hua, J. Liu, Factors influencing the glass transition temperature of polyether polyurethane elastomer, Thermosetting Resin 33 (3) (2018) 22–25. [32] W. Peng, G. Wang, S. Ye, z. Xue, X. Mo, Synthesis and properties of polyurethane elastomers for seals, Polyurethane Ind. 31 (6) (2016) 37–39. [33] H. Zhang, H. Xu, X. Wang, J. Yu, Microstructures and thermal aging mechanism of expanded vermiculite modified bitumen, Constr. Build. Mater. 47 (2013) 919–926. [34] R. Padhan, A. Sreeram, Enhancement of storage stability and rheological properties of polyethylene (PE) modified asphalt using cross linking and reactive polymer based additives, Constr. Build. Mater. 188 (2018) 772–780. [35] J. Yu, X. Zeng, S. Wu, L. Wang, G. Liu, Preparation and properties of montmorillonite modified asphalts, Mater. Sci. Eng.: A 447 (1–2) (2007) 233–238. [36] A. Rehab, N. Salahuddin, Nanocomposite materials based on polyurethane intercalated into montmorillonite clay, Mater. Sci. Eng.: A 399 (1–2) (2005) 368–376. [37] F. Ortega, C. Roman, F. Navarro, M. García-Morales, T. McNally, Physicochemistry control of the linear viscoelastic behaviour of bitumen/montmorillonite/MDI ternary composites: effect of the modification sequence, Fuel Process. Technol. 143 (2016) 195–203. [38] S. Sinha Ray, M. Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Prog. Polym. Sci. 28 (11) (2003) 1539–1641. [39] S. Bee, M. Abdullah, S. Bee, L. Sin, A. Rahmat, Polymer nanocomposites based on silylated-montmorillonite: a review, Prog. Polym. Sci. 85 (2018) 57–82. [40] J. Jin, Y. Tan, R. Liu, F. Lin, Y. Wu, G. Qian, H. Wei, J. Zheng, Structure characteristics of organic bentonite and the effects on rheological and aging properties of asphalt, Powder Technol. 329 (2018) 107–114. [41] G. Polacco, A. Muscente, D. Biondi, S. Santini, Effect of composition on the properties of SEBS modified asphalts, Eur. Polym. J. 42 (5) (2006) 1113–1121. [42] Z. Zhang, M. Jia, W. Jiao, B. Qi, H. Liu, Physical properties and microstructures of organic rectorites and their modified asphalts, Constr. Build. Mater. 171 (2018) 33–43. [43] V. Carrera, A. Cuadri, M. García-Morales, P. Partal, Influence of the prepolymer molecular weight and free isocyanate content on the rheology of polyurethane modified bitumens, Eur. Polym. J. 57 (2014) 151–159.

M. Jia et al. / Construction and Building Materials 229 (2019) 116867 [44] H. Lu, F. Ye, J. Yuan, W. Yin, Properties comparison and mechanism analysis of naphthenic oil/SBS and nano-MMT/SBS modified asphalt, Constr. Build. Mater. 187 (2018) 1147–1157. [45] H. Zhang, J. Yu, S. Wu, Effect of montmorillonite organic modification on ultraviolet aging properties of SBS modified bitumen, Constr. Build. Mater. 27 (1) (2012) 553–559. [46] S. Zapien-Castillo, J. Rivera-Armenta, M. Chavez-Cinco, B. Salazar-Cruz, A. Mendoza-Martinez, Physical and rheological properties of asphalt modified with SEBS/montmorillonite nanocomposite, Constr. Build. Mater. 106 (2016) 349–356. [47] H. Zhang, Z. Chen, G. Xu, C. Shi, Physical, rheological and chemical characterization of aging behaviors of thermochromic asphalt binder, Fuel 211 (2018) 850–858. [48] Z. Chen, J. Pei, T. Wang, S. Amirkhanian, High temperature rheological characteristics of activated crumb rubber modified asphalts, Constr. Build. Mater. 194 (2019) 122–131. [49] S. Liu, A. Peng, J. Wu, S.B. Zhou, Waste engine oil influences on chemical and rheological properties of different asphalt binders, Constr. Build. Mater. 191 (2018) 1210–1220. [50] D. Sun, G. Sun, Y. Du, X. Zhu, T. Lu, Q. Pang, S. Shi, Z. Dai, Evaluation of optimized bio-asphalt containing high content waste cooking oil residues, Fuel 202 (2017) 529–540.

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[51] H. Zhang, Y. Gao, G. Guo, B. Zhao, J. Yu, Effects of ZnO particle size on properties of asphalt and asphalt mixture, Constr. Build. Mater. 159 (2018) 578–586. [52] K. Zhu, K. Wu, B. Wu, Z. Huang, Investigations of the montmorillonite and aluminium trihydrate addition effects on the ignitability and thermal stability of asphalt, J. Chem. (2014) 8. [53] X. Liu, J. Guo, W. Tang, H. Li, X. Gu, J. Sun, S. Zhang, Enhancing the flame retardancy of thermoplastic polyurethane by introducing montmorillonite nanosheets modified with phosphorylated chitosan, Compos. Part A-Appl. S 119 (2019) 291–298. [54] H. Zhang, C. Shi, J. Han, J. Yu, Effect of organic layered silicates on flame retardancy and aging properties of bitumen, Constr. Build. Mater. 40 (2013) 1151–1155. [55] K. Liu, K. Zhang, J. Wu, B. Muhunthan, X. Shi, Evaluation of mechanical performance and modification mechanism of asphalt modified with graphene oxide and warm mix additives, J. Clean. Prod. 193 (2018) 87–96. [56] Q. Yu, X. Tan, M. Bi, Q. Gao, Y. Ke, Research on application of modified montmorillonite, Chem. World 56 (6) (2015) 374.