Study on rheological properties and phase-change temperature control of asphalt modified by polyurethane solid–solid phase change material

Solar Energy 194 (2019) 893–902

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Study on rheological properties and phase-change temperature control of asphalt modified by polyurethane solid–solid phase change material ⁎

T

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Kun Weia, , Xiaoqing Wanga, Biao Maa, , Wenshuo Shia, Shiyu Duanb, Fangshu Liua a b

Key Laboratory of Ministry of Transportation Road Structure and Materials, Chang’ an University, Xi’an 710064, China Mechanical Engineering College, Xi’an Shiyou University, Xi’an 710065, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Polyurethane solid–solid PCM Rheology properties Temperature control Specific heat capacity Coefficient of heat conductivity

Polyurethane solid–solid phase change material (PCM) with low phase change temperature is synthesized via prepolymer method. The synthetic polyurethane solid–solid PCM had lower crystallinity than PTMEG2000. The polyurethane solid–solid PCM was still in solid state during the heating process, and no liquid leakage was observed. After many phase change cycles, the polyurethane solid–solid PCM exhibited good stability of phase change cycles. Hence, the asphalt modified by polyurethane solid–solid PCM was prepared by high-speed shearing method. With increasing polyurethane solid–solid PCM proportion, the high temperature deformation resistance of modified asphalt gradually increased. At low temperature (< −18 °C), the addition of the polyurethane solid–solid PCM improved the low temperature performance of asphalt. With increasing polyurethane solid–solid PCM proportion, the specific heat capacity of the modified asphalt gradually increased. When the polyurethane solid–solid PCM proportion was > 3 wt%, the specific heat capacity of the modified asphalt had an evident peak ranging from 13 °C to 25 °C. In the range of the proportion, the coefficient of the heat conductivity of the modified asphalt gradually increased with increasing PCM content. During the cooling process, the modified asphalt cooled relatively slow before the phase transition of the polyurethane solid–solid PCM. With continued decrease in the temperature, the phase transition of the polyurethane solid–solid PCM occurred, and the cooling rate of the modified asphalt further decreased. With increasing polyurethane solid–solid PCM content, the temperature control ability of modified asphalt and the efficiency of temperature control gradually increased.

1. Introduction Asphalt pavement with good driving comfort and excellent performance in service is easy to construct and maintain. Asphalt pavement is the main pavement structure in high-grade highway. The asphalt pavement is affected by the repeated action of vehicle load and by the climatic and environmental factors, such as temperature change (Luo et al., 2018; Li et al., 2017; Yang et al., 2017), moisture (Elshaer et al., 2019), wind speed (Chen et al., 2019), and solar radiation (Anting et al., 2018), in its course of service. Among these environmental factors, temperature is one of the most important factors leading to asphalt pavement problems. In many cold areas locally and internationally, temperature shrinkage cracks will occur on asphalt pavement because of the sudden temperature drop. These thermal cracks on the asphalt pavement are extremely common in cold areas, which is a worldwide problem. Thermal cracks destroy the integrity, continuity, and beauty of asphalt pavement. More importantly, the existence of cracks also

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provides favorable conditions for water to enter the road structure, which weakens the strength of the base or even the roadbed, lowers the bearing capacity of the pavement, and accelerates the damage to the pavement. The thermal cracks of the asphalt pavement are directly correlated with the low temperature crack resistance of the asphalt and asphalt mixture. The improvement of the low temperature performances of the asphalt and asphalt mixture is significant to reduce and prevent thermal cracks on the asphalt pavement. PCMs are functional materials. Their physical state (phase state) changes with varying temperatures, accompanied by the absorption or release of energy (commonly known as latent heat of phase change) in the process of phase change. Phase change depends only on temperature. In the process, the temperature of the PCMs themselves can be essentially kept constant. At present, PCMs are widely applied in the fields of solar energy utilization (Qiu et al., 2019; Kahwaji et al., 2018), construction (Wijesuriya et al., 2018; Lee et al., 2018), textile (Kim et al., 2016) and electronic devices (Emam et al., 2019). In the field of

Corresponding authors. E-mail addresses: [email protected] (K. Wei), [email protected] (B. Ma).

https://doi.org/10.1016/j.solener.2019.11.007 Received 29 August 2019; Accepted 3 November 2019 Available online 15 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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asphalts with different polyurethane solid–solid PCM proportions were prepared. High/low temperature rheological properties, thermophysical parameters of polyurethane solid–solid PCM-modified asphalt, and temperature control in the cooling process were systematically analyzed and evaluated. Results of this study promote the popularization and application of polyurethane solid–solid PCM in asphalt pavement in low temperature areas.

highway construction, PCMs are added into asphalt or asphalt mixture. Considering the existence of PCMs, the heating/cooling rate of the asphalt and asphalt mixture can be actively regulated to delay and shorten the occurrence time and duration of extreme temperature. This method can solve the temperature-related diseases of the asphalt pavement. At present, the PCMs used in studies in asphalt and asphalt mixture are solid–liquid PCMs. Zhang et al. (2019) prepared stereotyped composite PCM by using porous expanded graphite to absorb solid–liquid PCM polyethylene glycol (average molecular weight of 2000) and also prepared modified asphalt via phase change by adding asphalt into the stereotyped composite PCM. The stereotyped composite PCM had good compatibility with asphalt. Near the phase change temperature of the stereotyped composite PCM (endothermic temperature: 40–50 °C, exothermic temperature: 35–25 °C), the heating and cooling rates of the modified asphalt by phase change are lower than those of the unmodified asphalt. Wei et al. (2019) synthesized melamine resin by in situ polymerization as a cystic wall. The capsule core was a microcapsule PCM of tetradecane, which was a solid–liquid PCM. Then, the microcapsule PCM was added into asphalt. The impact of the microcapsule PCM on the performance of asphalt was analyzed. The melamine resin cystic wall protected tetradecane and effectively prevented PCM leakage. The addition of the microcapsule in asphalt (< 5 wt%) did not have a significant adverse effect on the conventional technical performance of asphalt. In the range of phase change temperature (endothermic temperature: 4–9 °C), the temperature change rate of the modified asphalt was lower than that of unmodified asphalt. Chen et al. (2019) first used SiO2 to adsorb polyethylene glycol (average molecular weight of 4000) and mixed the stereotyped PCM with cement paste. Then, cement was used to cover and encapsulate the stereotyped PCM. The composite PCM with excellent thermal stability was obtained. With the composite PCM replacing fine aggregate with corresponding particle size, the researchers studied the impact of the composite PCM on the performance of OGFC-13. With increasing composite PCM proportion, the temperature control ability of the composite PCM gradually increased. The addition of PCM reduced the adhesion strength of ice to the mixture and delay ice formation. With regard to the temperature control ability and road performance of the mixture, the recommended proportion of the composite PCM was in the range of 0.8–1.1 wt%. Jin et al. (2018) first used ceramsite to adsorb polyethylene glycol (average molecular weight of 6000), and epoxy resin to encapsulate the ceramsite pores. Then, the encapsulated composite PCM was used to replace rough aggregate with corresponding particle size. The phase change energy storage asphalt mixture was prepared. After epoxy resin encapsulation, the composite PCM had excellent exudation stability and heat storage/exothermic performance. In the heating process, the maximum temperature difference between phase change energy storage asphalt mixture and ordinary asphalt reaches up to 9.1 °C. Compared with solid–liquid PCMs, polymer solid–solid PCMs had many advantages, such as no container required for encapsulation, long service life, excellent durability, and high thermal temperature. In our previous study (Wei et al., 2019), we successfully synthesized polyurethane solid–solid PCM with high phase change temperature (endothermic temperature: 38.6–55.5 °C, exothermic temperature: 16.5–2.0 °C). The synthesized polyurethane solid–solid PCM had excellent thermal stability, with the thermal degradation temperature of up to 280 °C. In the heating process, the temperature changing rate of modified asphalt was lower than that of base asphalt. If the content added was high, then the temperature change rate of the modified asphalt was low. The maximum temperature difference between modified asphalt with the addition proportion of 5 wt% and base asphalt was 8 °C. To solve the low temperature issues of asphalt pavement, we used 4,4′-diphenyl-methane-diisocyanate, polytetrahydrofuran glycol (PTMEG) 2000, and di-o-chlorotoluene diphenylmethane as the synthetic materials. Polyurethane solid–solid PCM with low phase change temperature was synthesized via prepolymer method. Hence, modified

2. Materials and methods 2.1. Test materials 4,4′-Diphenyl-methane-diisocyanate (MDI, 98% pure) was obtained ¯ n = 2000, analyfrom Aladdin Agent (Shanghai) Co., Ltd. PTMEG (M tically pure) and di-o-chlorotoluene diphenylmethane (MOCA, analytically pure) were acquired from J&K Scientific. N,NDimethylformamide (DMF, analytically pure) was obtained from Sinopharm Chemical Reagent Co., Ltd. Karamay 70# base asphalt was adopted as the asphalt in this study. All technical indexes of the asphalt met the requirements of Technical Specification for Construction of Highway Asphalt Pavement (JTG F402004). The test results are shown in Table 1. 2.2. Polyurethane solid–solid PCM synthesis In this paper, polyurethane solid–solid PCM was prepared using prepolymer method. The specific preparation process is as follows. A certain amount of PTMEG2000 was added into a four-mouth flask equipped with agitator, reflux condenser, thermometer, and N-venting device to dehydrate at 110 °C in vacuum for 2 h. After the temperature dropped to 75 °C, a certain amount of anhydrous DMF was added and stirred until PTMEG2000 was dissolved completely. The temperature was controlled at 75 °C, and then MDI was added. After reacting for 2 h under the protection of N2, the prepolymer containing the active group –NCO was obtained. The –NCO content was determined by N-butadiene diamine method. Then, the weighed chain extender MOCA was added, and the mixture was stirred and mixed quickly. After vacuum defoaming, the sample was poured into the mold, which was preheated to 110 °C and coated with demolding agent for 24 h. With the temperature drop to room temperature, polyurethane solid–solid PCM was obtained. Its composition is shown as Table 2. 2.3. Preparation of polyurethane solid–solid PCM-modified asphalt The high-speed shearing method was adopted to prepare polyurethane solid–solid PCM-modified asphalt. The specific process was as follows. First, the synthesized polyurethane solid–solid PCM was crushed into particles by a shear mill. The base asphalt was preheated to 160 °C. After adding the polyurethane solid–solid PCM particles, the mixture was stirred for 10 min. Then, polyurethane solid–solid PCMmodified asphalt was prepared by continuous shearing at a speed of Table 1 Properties of the base bitumen.

894

Test

Specification

Results

Specification limits

Penetration (at 25 °C; 1/10 mm) Softening point (°C) Ductility (at 25 °C; 5 cm/min) Viscosity (at 135 °C; Pa s) Specific gravity (at 25 °C; g/cm3) Flash point (°C) After RTFO (at 163 °C; 85 min) Mass loss (%) Residual penetration ratio (at 25 °C; %) Residual ductility (cm)

T0604-2011 T0606-2011 T0605-2011 T0619-2011 T0603-2011 T0611-2011

70.5 49.4 51.2 0.72 1.039 270

60–80 ≥46 > 20 ≤3 Measured records ≥230

T0610-2011 T0604-2011

0.65 67

≤2 ≥61

T0605-2011

12.6

≥8

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Table 2 Polyurethane solid–solid PCM composition. Sample

Hard segment

Soft segment

Soft segment content (wt%)

Polyurethane solid–solid PCM

MDI/MOCA

PTMEG2000

80

5000 r/min for 40 min at 180 °C by using a high-speed shearing emulsifier. The base and modified asphalt samples with the polyurethane solid–solid PCM mass fractions of 0, 3, 5, and 7 wt% were obtained using the process above. 2.4. Performance testing and characterization 2.4.1. X-Ray diffraction testing TTRIII X-ray diffractometer from Japan Rigaku Corporation was adopted to analyze the crystal structure of the polyurethane solid–solid PCM and its PTMEG2000. The conditions included a Cu target, a graphite monochromator, pipe voltage of 40 kV, 2θ scanning range of 5–50°, testing temperature of −20 °C, and scanning rate of 3°/min.

Fig. 1. Photograph of a bitumen specimen with temperature sensor.

length was prepared. Then, the mass of asphalt was approximately 240 g. The calibrated Pt100 thermal resistance temperature sensor was buried in the asphalt sample. The temperature sensor was located at the center of asphalt sample, as shown in Fig. 1. The asphalt sample embedded with temperature sensor was placed in the high and low temperature test boxes. After 5 h at 40 °C, the temperature dropped to −35 °C at the cooling rate of 2 °C/min. The test was over when the sample temperature became −35 °C. The change in the asphalt sample temperature in the cooling process was measured and recorded using an embedded temperature sensor.

2.4.2. Differential scanning calorimetry DSC200F3 differential scanning calorimeter from German NETZSCH was used to determine the enthalpy change in polyurethane solid–solid PCM and its PTMEG2000 in dynamic heating/cooling process. The test sample weight was 5 mg. Under the conditions of N2 atmosphere and flow rate of 30 mL/min, the temperature decreased from 55 °C to −20 °C and then increased from −20 °C to 55 °C. The heating and cooling rates were both 10 °C/min. 2.4.3. Dynamic shear rheological test In this paper, DHR-1 dynamic shear rheometer from TA Company was used in the DSR test. According to ASTM D 7175-15, DSR test was conducted at 64 °C, 70 °C, 76 °C, 82 °C, and 88 °C to collect the complex shear modulus (G*) and phase angle (δ) of the prepared asphalt sample. The strain control model was adopted in the test. The strain value was 12%, and the testing frequency was 10 rad/s. The diameter of parallel plate was 25 mm, and the thickness of asphalt sample was 1 mm.

3. Results and discussion 3.1. Analysis on crystallization characteristics of polyurethane solid–solid PCM In the molecular structure of the polyurethane solid–solid PCM, taking organic solid–liquid PCM (PTMEG2000) with low melting point as the soft segment, the absorption and release of energy are achieved through the transition between crystalline and amorphous form. Fig. 2 shows XRD test results of the synthesized polyurethane solid–solid PCM and its PTMEG2000. The PTMEG2000 had two distinct crystal diffraction peaks (Fig. 2), which suggested that the PTMEG2000 had good crystallization ability. Among these peaks, the 2θ angles of the crystal diffraction peaks were located at 20.0° and 24.7°. With almost the same 2θ angle, the synthesized polyurethane solid–solid PCM also had crystalline diffraction peaks. This result indicated that the crystallization of polyurethane solid–solid PCM was formed by the soft segment PTMEG2000 in the molecular structure. According to the enlarged image in Fig. 2, compared with PTMEG2000, the strength of crystalline diffraction peak of the polyurethane solid–solid PCM was small. In the XRD diagram, the strength of the crystalline diffraction peak was proportional to the crystallization ability of the material. Fig. 2 shows that the crystallization ability of polyurethane solid–solid PCM was lower than that of PTMEG2000, because polyurethane solid–solid PCM consisted of two parts, namely, soft and hard segments, which restrained the crystallization of soft segment. To analyze the change of crystallization ability of polyurethane solid–solid PCM intuitively further, we calculated the crystallinity of polyurethane solid–solid PCM and PTMEG2000 by Jade 6.5 software according to the obtained X-ray diffraction patterns (Gu et al., 2015; Zheng et al., 2017). The calculation formula is as follows:

2.4.4. Bending beam rheological test According to the test method in ASTM D6648-01, TE-BBR bending rheometer from US Cannon Company was adopted to make bending beam rheological test to analyze the low temperature rheological performance of the asphalt sample. The test temperatures were −12 °C, −18 °C, and −24 °C. The creep stiffness (S) and creep rate (m) of the modified asphalt sample at 60 s were used as the testing results. 2.4.5. Measurement of specific heat capacity of asphalt The MDSC test mode of DSC Q2000 differential scanning calorimeter from TA Company was used to measure the specific heat capacity of the asphalt sample under the conditions of the N2 flow rate of 50 mL/ min, modulation period of 80 s, temperature amplitude of ± 0.75 °C, heating rate 2 of °C/min, and testing temperature ranging from −20 °C to 30 °C. 2.4.6. Thermal conductivity measurement XIATECH TC 3000E thermal conductivity meter from Xi’an Xiatech Electronics Co., Ltd. was used to measure the thermal conductivity of the asphalt sample at room temperature (25 °C). Each asphalt sample was tested thrice as a parallel test, and the average value was taken as the test result. 2.4.7. Temperature control test First, the asphalt sample that was 80 mm in diameter and 50 mm in 895

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Fig. 2. X-ray diffraction patterns of polyurethane solid–solid PCM and PTMEG2000.

Xc =

Ac Ac + KAa

segment, thereby resulting in the corresponding reduction of the phase change temperature and enthalpy value of polyurethane solid–solid PCM (Alkan et al., 2012; Chen et al., 2015; Du et al., 2017). The synthesized polyurethane can control the temperature field of asphalt pavement in a low temperature environment and improve the low temperature performances of the asphalt and asphalt mixture because of the low phase change temperature of the polyurethane solid–solid PCM. Fig. 3b shows that PTMEG2000 is a typical solid–liquid phase change material. PTMEG2000 exhibited a white waxy solid state at 5 °C, whereas PTMEG2000 melted into a colorless transparent liquid at 50 °C. However, the synthesized polyurethane solid–solid PCM was crystalline solid with opaque appearance at 5 °C. When the temperature increased to 50 °C, the synthesized polyurethane solid–solid PCM changed from crystalline to amorphous, and its appearance became transparent solid. However, the synthesized polyurethane solid–solid PCM remained in a solid state, and no liquid leakage was observed, because the hard segment consisting of MDI and MOCA played a skeleton role in polyurethane solid–solid PCM, which restricted the flow of soft segment material above the phase change temperature for the material to remain at a solid state above the phase change temperature. With the unique structure and performance, polyurethane solid–solid PCM fully met the requirement of asphalt pavement for PCMs. Thermal stability is also an important performance evaluation index for PCMs. Fig. 4 shows the DSC patterns of the polyurethane solid–solid PCM obtained by the test after different times (5, 10, 20, and 40) of the heating/cooling cycles. The figure shows that under different phase change cycles, the DSC patterns of the polyurethane solid–solid PCM obtained by the test almost coincided with each other, whereas the phase change temperature and enthalpy have barely changed. The synthesized polyurethane solid–solid PCM had good phase change cycle stability.

(1)

where Xc is the crystallinity of material, Ac is the area of diffraction peak in the crystalline zone below the diffraction curve, Aa is the area of diffraction peak in the amorphous zone below the diffraction curve, and K is correction factor that is set to 1. The results are shown in Table 3. The table shows that the crystallinity of PTMEG2000 was 70.78%, whereas that of polyurethane solid–solid PCM was only 43.34%. This result indicated that compared with solid–liquid PCM PTMEG2000, the crystallinity of synthesized polyurethane solid–solid PCM was significantly reduced, and the crystallization ability was remarkably weakened. 3.2. Thermal performance analysis on polyurethane solid–solid PCM Fig. 3 shows DSC patterns of PTMEG2000 and synthesized polyurethane solid–solid PCM and macrostate changed during heating. Fig. 3a shows that the starting point of the phase change in PTMEG2000 during cooling was 10 °C, the ending point of phase change was −1 °C, the peak temperature of the phase change was 3.9 °C, and the exothermic enthalpy value of the phase change was 104.3 J/g. Compared with PTMEG2000, the phase change temperature and enthalpy value of the synthesized polyurethane solid–solid PCM decreased slightly. The starting point of the phase change during cooling decreased to −3.5 °C, the ending point of the phase change was −13.6 °C, the peak temperature of the phase change was −8.5 °C, and the exothermic enthalpy value was 49.49 J/g. This result is observed, because in the molecular structure of the polyurethane solid–solid PCM, the hard segment consisting of MDI and MOCA inhibited the crystallization of soft segment, which made the entry of the soft segment difficult at the interfaces of soft and hard segments in the microzone in the soft segment to participate into crystallization, reduced the crystalline ability of soft

3.3. High temperature rheological properties of polyurethane solid–solid PCM-modified asphalt

Table 3 Crystallinity results. Sample

Crystallinity (%)

PTMEG2000 Polyurethane solid–solid PCM

70.78 43.34

Superpave standard specifies that rutting factor (G*/sinδ) is adopted to evaluate the high temperature rutting resistance of asphalt materials. If G*/sinδ is large, then the rutting resistance of the asphalt materials is good. According to the G* and δ data of the asphalt sample at different temperatures obtained by test, the obtained G*/sinδ values are shown 896

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Fig. 3. (a) DSC curves of A and B; (b) Photos of A and B at 5 °C and 50 °C.

in Fig. 5. The figure shows that under the same proportion of polyurethane solid–solid PCM, the G*/sinδ of the base and modified asphalts decreased with increasing temperature. The rutting resistance of asphalt decreased with increasing temperature, and high temperature performance became poor. At the same testing temperature, with increasing proportion of polyurethane solid–solid PCM, the G*/sinδ of the asphalt sample gradually increased. The addition of polyurethane solid–solid PCM improved the high temperature performance of asphalt and enhanced its high temperature deformation resistance. According to previous research findings (Wei et al., 2019), polyurethane solid–solid PCM is physically blended into the asphalt without chemical reaction. However, as a kind of polymer material, polyurethane solid–solid PCM may adsorb some light components in asphalt and change the colloidal structure of asphalt, which relatively increases the asphaltene and gum contents in asphalt and the viscosity of asphalt, to improve the high temperature performance of asphalt. At the same time, after polyurethane solid–solid PCM compound with asphalt, due to the restriction of polyurethane solid–solid PCM particles, the flow deformation of asphalt molecular chain was hindered. Therefore, the ability of resisting deformation was enhanced under the action of load, and the ability to resist deformation at high temperature was gradually increased. Fig. 5 shows that when the temperature is > 82 °C, with increasing proportion of the polyurethane solid–solid PCM, the improvement of polyurethane solid–solid PCM on the high temperature performance of asphalt gradually weakened. For example, at 64 °C, 70 °C, 76 °C, 82 °C, and 88 °C, the differences in G*/sinδ between modified asphalt with the proportion of 7 wt% and base asphalt were 0.11, 0.15, 0.11, 0.07, and 0.05 kPa, respectively. This result was observed, because with increasing temperature, the strength of polyurethane solid–solid PCM and the constraint capacity of asphalt molecular chains gradually weakened. Therefore, at high testing temperature, polyurethane solid–solid PCM had poor improvement effect on the high temperature performance of asphalt.

temperature crack resistance was strong. If the m value was large, then the stress relaxation performance of asphalt and its crack resistance were good. At the same time, to avoid the contradiction between S and m values in the evaluation of the low temperature performance of asphalt, some researchers adopted the S/m ratio, that is, the ratio of S and m at 60 s of testing time, as a new index in evaluating the low temperature performance of asphalt (Li et al., 2016; Wang et al., 2012; Liu et al., 2010). If the value was large, then the low temperature performance of asphalt was poor and vice versa. Fig. 6 shows the m and S values of the base and modified asphalts obtained at 60 s of testing time, as well as the temperature-dependent curve of the S/m ratio. Fig. 6a shows that the S values of the base and polyurethane solid–solid PCM-modified asphalts had the same changing trend, that is, it gradually increased with decreasing temperature. When the test temperature was −12 °C, the S value of base asphalt was extremely close to those of modified asphalt with different polyurethane solid–solid PCM proportions. With decreasing temperature, the S values of the base and polyurethane solid–solid PCM-modified asphalts gradually showed a difference. If the test temperature was low, then the difference was remarkable. When the test temperature was −24 °C, with increasing polyurethane solid–solid PCM proportion, the S value of the asphalt gradually decreased. With regard to S, at the low temperature of < −18 °C, the addition of polyurethane solid-solid PCM in asphalt is beneficial to improve its flexibility and low temperature crack resistance. Fig. 6b shows that with the decrease in temperature, the m values of the base and polyurethane solid–solid PCM-modified asphalts gradually decreased. Fig. 6b also shows that at the same test temperature, with increasing polyurethane solid–solid PCM proportion, the m value of the asphalt decreased gradually. With regard to m, the addition of polyurethane solid–solid PCM in asphalt was not beneficial to the stress relaxation and low temperature crack resistance of asphalt. As shown in Fig. 6a and b, when the test temperature was < −18 °C, either the S or m value of the asphalt showed limitation in the correct evaluation of the impact of polyurethane solid–solid PCM on its low temperature performance. Hence, we further analyzed the low temperature performance of the polyurethane solid–solid PCM-modified asphalt by using the S/m ratio. Fig. 6c shows that when the test temperatures were −12 °C and −18 °C, the evaluation index S/m ratio for the low temperature performance of the base asphalt was extremely close to that of the modified asphalt with different polyurethane solid–solid PCM proportions. This result indicated that the addition of

3.4. Low temperature rheological properties of polyurethane solid–solid PCM-modified asphalt Superpave standard indicated that the stiffness modulus and relaxation performance of asphalt are used as the core indices to evaluate the low temperature performance of asphalt. If the S value was small, then the low temperature flexibility of asphalt was good, and the low 897

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Fig. 4. DSC curves of polyurethane solid–solid PCM after performing different numbers of cold-hot cycle excitations. (a): 5 cycles; (b): 10 cycles; (c): 20 cycles; (d): 40 cycles.

changing curves of the specific heat capacities of the base and polyurethane solid–solid PCM-modified asphalts with increasing temperature. Fig. 7 shows that the specific heat capacity of base asphalt increased with increasing temperature. When the temperature was low (< 0 °C), the specific heat capacity of base asphalt increased rapidly. When the temperature is high, the specific heat capacity of base asphalt grew slowly. The specific heat capacity of asphalt increased with increasing temperature, because the increase in temperature caused the acceleration of the molecular movement and the increase of the required energy. Asphalt is a viscoelastic material. At low temperature, asphalt is in the glass state, and the molecular chain of the asphalt is in the freezing state. Asphalt needs high heat to intensify the molecular movement. Therefore, the growth rate of the specific heat capacity with temperature is high. At high temperature, asphalt is in a high elastic state, and the movement ability of the asphalt molecular chain is enhanced. Low heat can intensify the molecular movement. Therefore, the growth rate of specific heat capacity with temperature is low.

polyurethane solid–solid PCM had insignificant effect on the improvement or damage to the low temperature performance of asphalt. When the test temperature was −24 °C, with increasing polyurethane solid–solid PCM proportion, the evaluation index for the low temperature performance and S/m ratio gradually decreased. For example, the index S/m ratio for the low temperature performance of base asphalt was 6071.4, whereas that for the low temperature performance of modified asphalt with the polyurethane solid–solid PCM proportion of 7 wt% was 5548.3. At low temperature (< −18 °C), the addition of the polyurethane solid–solid PCM improved the low temperature performance of asphalt. 3.5. Analysis on thermophysical parameters of polyurethane solid–solid PCM-modified asphalt In terms of the phase change-modified asphalt, thermophysical parameters are significant for its temperature control. Fig. 7 shows the 898

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Fig. 5. Rutting parameter (G*/sinδ) for base and polyurethane solid–solid PCMmodified asphalt binder.

Fig. 7. Changes in the curves of the specific heat capacity of asphalt specimens with temperature.

Fig. 7 also shows that at the same temperature, the specific heat capacity of polyurethane solid–solid PCM-modified asphalt was higher than that of base asphalt. If the polyurethane solid–solid PCM proportion was high, then the specific heat capacity of the modified asphalt was large. When the polyurethane solid–solid PCM proportion was >

3 wt%, the specific heat capacity of the modified asphalt had an evident peak ranging from 13 °C to 25 °C. The peak temperature was 21 °C. This result was consistent with the DSC changing curve of the polyurethane solid–solid PCM during the heating process. In this paper, the

Fig. 6. BBR test results of base and polyurethane solid–solid PCM-modified asphalt binder. 899

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relatively slow. At the same time, the temperature of modified asphalt was high. Stage 2: The temperature ranged from −3 °C to approximately −14 °C (phase change of polyurethane solid–solid PCM). At this stage, the temperature difference between polyurethane solid–solid PCM-modified and base asphalts increased significantly. With increasing polyurethane solid–solid PCM proportion, the temperature difference between modified and base asphalts gradually increased. For example, at 5300 s, the temperature difference between modified asphalt with the polyurethane solid–solid PCM proportions of 3, 5, and 7 wt% and base asphalt were 1.9 °C, 3.0 °C, and 3.8 °C, respectively. This result was observed because at this stage, polyurethane solid–solid PCM begins to change from crystalline to amorphous. In the phase change process, polyurethane solid–solid PCM released the stored heat in the form of latent heat, thereby reducing the cooling rate of modified asphalt. If the polyurethane solid–solid PCM proportion was high, then the latent heat released was high, and the cooling rate of modified asphalt was slow. Therefore, the temperature difference between modified asphalt and base asphalt was significant. Stage 3: The temperature ranged from −14 °C to the end of cooling (after phase change of polyurethane solid–solid PCM). At this stage, the temperature difference between polyurethane solid–solid PCM-modified and base asphalts decreased gradually and finally became consistent with the simulated ambient temperature. To further analyze the temperature control effect of polyurethane solid–solid PCM modified asphalt during cooling process, we adopted latent heat accumulated temperature value (LHATV) and latent heat thermoregulation index (LHTI) to analyze the temperature regulation performance of polyurethane solid–solid PCM-modified asphalt(Wei et al., 2019; Ma et al., 2019). Among them, LHATV was used to reflect phase change temperature regulation effect of polyurethane solid–solid PCM-modified asphalt (see Eq. (2) for the specific calculation). LHTI was used to characterize the efficiency of polyurethane solid–solid PCM-modified asphalt to play the role of phase change temperature regulation (see Eq. (3) for the specific calculation).

Table 4 The thermal conductivity coefficient of polyurethane solid–solid PCM, base and modified asphalt binder at room temperature. Sample

Contents of polyurethane solid–solid PCM (wt %)

Thermal conductivity coefficient (W/(m K))

Polyurethane solid–solid PCM Base asphalt binder Modified asphalt binder Modified asphalt binder Modified asphalt binder

100

0.2212

0 3 5 7

0.1781 0.1556 0.1596 0.1666

starting point of the phase change in synthesized polyurethane solid–solid PCM during the heating process was 13.6 °C, the ending point of phase change was 24.5 °C, and the phase change peak temperature was 20.9 °C. When the temperature was > 13.6 °C, the phase change occurred in polyurethane solid–solid PCM inside the modified asphalt from crystalline state to amorphous state. In this process, a large amount of latent heat of phase change was absorbed, and the specific heat capacity of modified asphalt gradually increased. When the phase change ended, the latent heat absorbed decreased gradually, and the specific heat capacity of the modified asphalt decreased gradually. Therefore, in the entire phase change temperature range, the specific heat capacity of the modified asphalt increased first and then decreased gradually, with a certain peak value. The coefficients of the heat conductivity of the base asphalt, polyurethane solid–solid PCM, and polyurethane solid–solid PCM-modified asphalt that were obtained at room temperature are shown in Table 4. The table shows that the coefficient of heat conductivity of the synthesized polyurethane solid–solid PCM was higher than that of the base asphalt. The polyurethane solid–solid PCM was added into the base asphalt. The coefficients of the heat conductivity of modified asphalt with three polyurethane solid–solid PCM proportions were lower than that of the base asphalt in this paper. However, with increasing polyurethane solid–solid PCM proportion, the coefficient of the heat conductivity of the modified asphalt gradually increased, because that when the proportion was low, and the addition of polyurethane solid–solid PCM destroyed the homogeneity of asphalt itself and worsened the heat conductivity of modified asphalt. However, with the gradual increase in the polyurethane solid–solid PCM proportion, due to the good heat conductivity of polyurethane solid–solid PCM, their high heat conductivity was gradually dominant in modified asphalt, and the heat conductivity of the polyurethane solid–solid PCM-modified asphalt system was gradually improved. Generally, when the proportion was within 7 wt%, the coefficient of the heat conductivity of polyurethane solid–solid PCM-modified asphalt was lower than that of base asphalt.

LHATV =

∫t

t1

0

|f (t ) - f (t0) − (y (t ) − y (t0 ))| dt ≈

∑ Δti × ΔTi,

(2)

where f(t) refers to the function of temperature–time relationship curve for base asphalt samples, y(t) refers to the function of temperature–time relationship curve for polyurethane solid–solid PCM-modified asphalt samples, t0 is the starting time of phase change (s), t1 is the time of the end of phase change (s), t refers to the testing time (s), and T refers to the asphalt sample temperature (°C).

LHTI = LHATV /(Δt × ΔT ),

(3)

where Δt refers to the duration of phase change (s), and ΔT is the maximum temperature difference between polyurethane solid–solid PCM-modified and base asphalt samples (°C). The calculation results of LHATV and LHTI for modified asphalt are shown in Table 5. With increasing polyurethane solid–solid PCM proportion, the LHATV of the modified asphalt increased gradually. With increasing polyurethane solid–solid PCM content, additional latent heat of the phase change was released during the phase change process, and the phase change temperature regulation ability of the modified asphalt was gradually increased. Therefore, LHATV increased with increasing the polyurethane solid–solid PCM proportion. Table 5 also shows that with increasing polyurethane solid–solid PCM proportion, the LHTI of the modified asphalt also increased gradually, which means that within the polyurethane solid–solid PCM proportion that was determined in this paper, its improvement helped polyurethane solid–solid PCMmodified asphalt improve the efficiency of phase change temperature regulation.

3.6. Analysis on temperature control effect of polyurethane solid–solid PCM-modified asphalt during cooling process During the cooling process, the temperature variation curves of the base and polyurethane solid–solid PCM-modified asphalts and the temperature difference curves between polyurethane solid–solid PCMmodified and base asphalts are shown in Fig. 8. Fig. 8a and b show that the cooling process of polyurethane solid–solid PCM-modified asphalt can be divided into three stages, as follows. Stage 1: Asphalt started cooling down to approximately −3 °C (before the phase change of polyurethane solid–solid PCM). During the cooling stage, the temperature of the polyurethane solid–solid PCM-modified asphalt was gradually higher than that of the base asphalt, because the coefficient of the heat conductivity of the former was lower than that of the latter, and the specific heat capacity of the former was higher than that of the latter. Therefore, under the same environmental conditions, the cooling process of polyurethane solid–solid PCM-modified asphalt was

4. Conclusions A kind of polyurethane solid–solid PCM with low phase change 900

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Fig. 8. (a) The cooling curves of asphalt specimens; (b) The temperature difference between base and modified asphalt binder in the cooling process.

improvement effect of polyurethane solid–solid PCM on the high temperature performance of asphalt weakened gradually. When the test temperatures were −12 °C and −18 °C, the evaluation index S/m ratio of the low temperature performance of the base asphalt is extremely close to that of modified asphalt with different polyurethane solid–solid PCM contents. When the test temperature is −24 °C, with increasing polyurethane solid–solid PCM proportion, the S/m ratio of the low temperature performance decreased gradually. Improving the polyurethane solid–solid PCM proportions helped enhance the low temperature performance of the modified asphalt at low test temperature (< −18 °C). At the same temperature, the specific heat capacity of the polyurethane solid–solid PCM-modified asphalt was higher than that of base asphalt. The polyurethane solid–solid PCM proportion is high, and the specific heat capacity of the modified asphalt is high. When the polyurethane solid–solid PCM proportion was > 3 wt%, the specific heat capacity of modified asphalt has an evident peak at the temperature ranging from 13 °C to 25 °C. When the polyurethane solid–solid PCM proportion was within 7 wt%, the coefficient of the heat conductivity of polyurethane solid–solid PCM-modified asphalt was lower than that of the base asphalt. However, with increasing polyurethane solid–solid PCM proportion, the coefficient of the heat conductivity of the modified asphalt increased gradually in this range. The cooling process of the polyurethane solid–solid PCM-modified asphalt can be divided into three stages on the basis of the phase change in polyurethane solid–solid PCM. Stage 1: The sample starts cooling down to approximately −3 °C. At this stage, the differences in the temperature between polyurethane solid–solid PCM-modified and base asphalts were great. During the cooling stage, the temperature of polyurethane solid–solid PCM-modified asphalt was gradually higher than that of base asphalt. Stage 2: The temperature ranged from −3 °C to approximately −14 °C. At this stage, the difference in temperature between polyurethane solid–solid PCM-modified and base asphalts increased significantly. With increasing polyurethane solid–solid PCM proportion, the temperature difference between modified and base asphalts gradually increased. Stage 3: The temperature ranged from −14 °C to the end of cooling. At this stage, the temperature difference between polyurethane solid–solid PCM-modified and base asphalts decreased gradually and finally became consistent with the simulated ambient temperature. With increasing polyurethane solid–solid PCM proportion, the LHATV and LHTI of the modified asphalt increased gradually. Within the range of polyurethane solid–solid PCM proportion that is determined in this paper, increasing the polyurethane solid–solid PCM proportion in modified asphalt can enhance the phase change

Table 5 Calculation results of LHATV and LHTI for modified asphalt. Polyurethane solid–solid PCM proportion (wt %)

Time zone (s)

LHATV (°C s)

LHTI

3 5 7

4470–5601 4535–5720 4535–5798

285.7 614.4 1383.7

0.021 0.043 0.091

exothermic temperature range was synthesized, and the crystallization characteristics and thermal performance of the synthesized polyurethane solid–solid PCM were tested and analyzed. Hence, the synthesized polyurethane solid–solid PCM was added into asphalt. The high/low temperature rheological properties of modified asphalt and the thermophysical parameters and temperature control effect of modified asphalt during cooling process were analyzed. According to the experimental results and analysis, the following conclusions can be drawn: In the molecular structure of the polyurethane solid–solid PCM, the hard segment composed of MDI and MOCA binds the crystallization of soft segment PTMEG2000, thereby decreasing the crystallinity of the material. The crystallinity of PTMEG2000 was 70.78%, whereas that of synthesized polyurethane solid–solid PCM was only 43.34%. Considering the inhibition of hard segment on soft segment crystallization, compared with PTMEG2000, the phase change temperature and enthalpy of the polyurethane solid–solid PCM decreased correspondingly. During the cooling process, the starting point of the phase change in polyurethane solid–solid PCM decreased to −3.5 °C, the ending point of phase change was −13.6 °C, the peak temperature of the phase change was −8.5 °C, and the exothermic enthalpy of phase change was 49.49 J/g. The structure and properties of the polyurethane solid-solid PCM allowed the control of the temperature field of the asphalt pavement at low temperature and improved the low temperature performance of asphalt and asphalt mixture. Considering the unique structural characteristics of polyurethane solid–solid PCM, polyurethane solid–solid PCM was still in solid state during the heating process, and no liquid leakage is observed. After many times of phase change cycles, the phase change temperature and enthalpy of polyurethane solid–solid PCM have barely changed. With increasing proportion of polyurethane solid–solid PCM, the G*/sinδ of the asphalt sample increased gradually. The addition of polyurethane solid–solid PCM in asphalt improved the high temperature performance of asphalt. When the test temperature was > 82 °C, with increasing polyurethane solid–solid PCM proportions, the

901

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thermoregulation ability of modified asphalt and improve the efficiency of polyurethane solid–solid PCM-modified asphalt in phase change thermoregulation.

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Acknowledgements This research is supported by the National Natural Science Foundation of China (51608044), China Postdoctoral Science Foundation (2015M572513), Young Talent fund of University Association for Science and Technology in Shaanxi, China (20170507), Transportation Industry High-Level Technical Personnel Training project (2018-019), Science and Technology Planning Project of Tibet Autonomous Region of China (XZ201801-GD-04), and Fundamental Research Funds for the Central Universities of Chang'an University (300102218523). The authors declare that they have no conflict of interest to this work. References Alkan, C., Günther, E., Hiebler, S., Ensari, Ö.F., Kahraman, D., 2012. Polyurethanes as solid-solid phase change materials for thermal energy storage[J]. Sol. Energy 86, 1761–1769. Anting, N., Din, M.F.M., Iwao, K., Ponraj, M., Siang, A.J.L.M., Yong, L.Y., Prasetijo, J., 2018. Optimizing of near infrared region reflectance of mix-waste tile aggregate as coating material for cool pavement with surface temperature measurement[J]. Energ. Build. 158, 172–180. Chen, C.Z., Liu, W.M., Wang, Z.Q., Peng, K.L., Pan, W.L., Xie, Q., 2015. Novel form stable phase change materials based on the composites of polyethylene glycol/polymeric solid-solid phase change material[J]. Sol. Energ. Mat. Sol. C. 134, 80–88. Chen, J., Li, J.H., Wang, H., Huang, W., Sun, W., Xu, T., 2019a. Preparation and effectiveness of composite phase change material for performance improvement of Open Graded Friction Course[J]. J. Clean. Prod. 214, 259–269. Chen, J.Q., Wang, H., Xie, P.Y., 2019. Pavement temperature prediction: theoretical models and critical affecting factors[J]. Appl. Therm. Eng. 158, UNSP 113755. Du, X.S., Wang, H.B., Wu, Y., Du, Z.L., Cheng, X., 2017. Solid-solid phase-change materials based on hyperbranched polyurethane for thermal energy storage[J]. J. Appl. Polym. Sci. 134, 45014. Elshaer, M., Ghayoomi, M., Daniel, J.S., 2019. Impact of subsurface water on structural performance of inundated flexible pavements[J]. Int. J. Pavement Eng. 20, 947–957. Emam, M., Ookawara, S., Ahmed, M., 2019. Thermal management of electronic devices and concentrator photovoltaic systems using phase change material heat sinks: experimental investigations[J]. Renew. Energ. 141, 322–339. Gu, Y., Zhang, X.J., Lu, S.G., Jiang, D.P., Wu, A., 2015. High rate performance of LiF modified LiFePO4/C cathode material[J]. Solid State Ionics 269, 30–36. Jin, J., Xiao, T., Zheng, J.L., Liu, R.H., Qian, G.P., Xie, J., Wei, H., Zhang, J.H., Liu, H.F., 2018. Preparation and thermal properties of encapsulated ceramsite-supported phase

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