Elsevier

Construction and Building Materials

Volume 71, 30 November 2014, Pages 53-62
Construction and Building Materials

Preparation and analysis of composite phase change material used in asphalt mixture by sol–gel method

https://doi.org/10.1016/j.conbuildmat.2014.07.100Get rights and content

Highlights

Abstract

Based on the principle of sol–gel method and the mechanism of silane coupling agent can be used to adjust the gel structure, four different kinds of composite phase change materials (CPCMs) were prepared by using pure phase change material (PCM), silica powder adsorbed PCM, floating bead adsorbed PCM and activated carbon adsorbed PCM as raw materials via sol–gel process, the four CPCMs were named as PCPCM, SCPCM, FCPCM and ACPCM, respectively. Their coating effectiveness and the latter two CPCMs’ latent heat storage capacities were studied by scanning electron microscope (SEM) and differential scanning calorimetry (DSC). The results show: silane coupling agent can improve particle distribution of CPCMs positively, its proper mixing amount is 10% of the mass of tetraethyl orthosilicate (TEOS); compared to PCPCM and SCPCM, FCPCM and ACPCM have better coating effectiveness; compared to FCPCM, ACPCM has larger latent heat storage capacity and proper phase change temperature, it also has better high temperature stability and can meet the demand of being used in asphalt mixture.

Introduction

Asphalt pavement is widely used all over the world for its good service performance. However, the emerging issue of global warming and urban heat island in recent years made the dark asphalt paving absorbed large quantities of heat, which leads to higher temperature in asphalt pavement during summer. The rising temperatures at the paving surface will affect the properties of the surface layer. According to Yoder and Witzak, high surface temperatures influence the performance of asphalt pavement such as rutting, aging and fatigue. This problem has been especially pronounced in permafrost regions because the settlement of paving surfaces has been a serious problem as ice-rich layers thaw due to recent climate change [1]. In addition, asphalt pavement may crack due to extreme low temperature or rapid temperature change during winter. These distresses will have an inverse influence on the service performance and the age of asphalt pavement. The literatures also show that temperature is one of the key factors impact the asphalt pavement performance [2], [3]. Therefore, solving the impact of temperature on asphalt pavement is one of the hottest issues faced with road engineers.

Latent heat storage (LHS) is a form of thermal energy storage (TES) which has been used in buildings in the past three decades. It is based on the heat absorption or release when phase change material (PCM) undergoes a phase change from solid to liquid or liquid to gas or vice versa, and it has the ability to provide high-energy storage density and to store heat at constant temperature corresponding to the phase-transition temperature of PCM [4]. In recent years, some researches carried out a lot of experiments to study the effect of mixing PCMs into asphalt mixture on asphalt pavement temperature. Asphalt pavement constructed with PCMs is expected to have more proper service temperature for the reasons that PCMs can prevent the pavement temperature from getting too high or too low, which will lead to less high temperature or low temperature distresses. Zhang et al. [5] tested and analyzed the energy storage capacities of various PCMs and the effect of these PCMs’ properties on asphalt pavement performance, the results show that polyethylene glycol 4000 can be used to mix into asphalt mixture. Cao et al. [6] analyzed the influences of polyethylene glycol on asphalt binder and asphalt mixture, the results illustrate that polyethylene glycol and asphalt binder are physically mixed, asphalt binder mixed with polyethylene glycol has good adhesion with calcareous aggregates; asphalt mixture prepared with polyethylene glcol has good energy storage capacity. Ma and Li [7] have carried out a lot of studies on mixing organic solid–liquid PCM into asphalt mixture, the results show that organic solid–liquid PCM has positive influence on asphalt pavement temperature to some extent, but it also affects pavement service performance. According to their further studies, the organic PCM may soften asphalt binder because it will dissolve when being mixed into asphalt binder. Although silicon dioxide (SiO2) was used as supporting material to prevent PCM from dissolving, but due to the high temperature during asphalt mixture mixing process (higher than 140 °C), solid PCM absorbed too much heat and became liquid, and then leaked out from the supporting material mostly. This phenomenon has large influence on the energy storage capacity of asphalt mixture and affects asphalt pavement service performance.

To improve the form-stability of PCM, researchers mostly used encapsulation method to synthesize composite phase change material (CPCM) and some good results were obtained [8], [9], [10]. CPCM is constituted by PCM and encapsulation medium, the PCM will not leak out when undergoes solid–liquid or liquid–solid phase change process because of the existence of encapsulation medium. In CPCM, PCM serves as latent heat storage material and encapsulation medium acts as supporting material. Therefore, the key to synthesize CPCM is choosing appropriate PCM, encapsulation medium and preparation method. At present, several preparation methods of CPCM are: direct immersion method, melt blending method, graft copolymerization, intercalation method, microencapsulation method, sintering method and sol–gel method.

Direct immersion method is based on the capillary effect, this method immerges liquid PCM directly into porous supporting material to synthesize CPCM. Lee et al. [11] prepared phase change concrete by utilizing this method, and compared energy storage capacity between common concrete and phase change concrete. Hadjieva et al. [12] used this method to prepare porous concrete absorbed with sodium thiosulfate pentahydrate, it improves heat storage capacity and structural stability of the composite PCM concrete system. Direct immersion method is easy to carry out, but the PCM’s thermophysical properties will decline and PCM may leak out after multiple thermal cycling.

Melt blending method is carried out by melting PCM and supporting material first, then blending them together to synthesize CPCM. Ahmet [13] prepared two kinds of paraffin/high density polyethylene composites by using this method. Melt blending method is simple and easy to operate, but phase separation may occur easily and PCM may leak out from the CPCM.

In principle, graft copolymerization is a process in which side chain grafts are covalently attached to a main chain of a polymer backbone to form branched copolymer [14]. Ahmet et al. [15] synthesized a series of polystyrene graft palmitic acid (PA) copolymers as novel polymeric solid–solid phase change materials (PCMs) via this method. In these solid–solid PCMs, polystyrene is the skeleton and PA is a functional side chain that stores and releases heat during its phase transition process. The drawback of graft copolymerization is that composites prepared by it have low latent heat.

For intercalation method, layered inorganic substance serves as main part, and organic monomer or polymer is intercalated into the main part to form composite. Fang et al. [16] used melting intercalation method to prepare RT20/MMT composite PCM by blending RT20 with an organically modified montmorillonite (MMT). This kind of composite PCM is a good candidate for building applications due to its large latent heat, suitable phase change temperature and good performance stability. Chen et al. [17] prepared a kind of novel shape-stabilized phase change material (SSPCM) by using a melting intercalation technique. In this kind of SSPCM, lauric acid (LA) acts as phase change material and organophilic montmorillonite (OMMT) acts as support material. Intercalation method can form nano-layered composite, but application of this method is narrow.

For microencapsulation method, core material is microencapsulated with polymer which acts as shell material. Sarier et al. [18] prepared four different kinds of microencapsulation by taking urea and formaldehyde as shell materials, and by taking n-octadecane, n-octadecane/PEG600, n-eicosane/n-hexadecan, PEG1000/Na2CO3·10H2O/n-hexadecane as core materials, respectively. Ma et al. [19] prepared composite shape-stabilized phase change material by using a membrane layer to encapsulate the PCM to ensure stability. Liu et al. [20] took paraffin and high density polyethylene (HDPE) as core materials, and sodium silicate monohydrate (Na2SiO3.9H2O) and hydrochloric acid (HCl) were used as coating materials to prepare microencapsulation. The shell material can protect the core material from influence of oxygen, water or light, but the composite prepared by this method may have poor stability which will shorten its service lifetime.

Sintering method can increase density and strength of raw material, and improve its other physical properties through a series of physical and chemical changes. Zhang et al. [21] prepared two kinds of salt/ceramic composites via this method, the two composites are testified that can be used to save energy and decrease facility volume. The composite prepared by sintering method has higher hardness, strength, and better abrasion resistance, but application of this method is narrow.

The sol–gel process involves first the formation of a sol followed by that of a gel, and then through heat treatment to form oxide or other compounds. Li et al. [22] obtained a form-stable paraffin/silicon dioxide (SiO2)/expanded graphite (EG) composite phase change material (PCM) by using sol–gel method. Silica gel acts as the supporting material and EG is used to increase the thermal conductivity. Zhang et al. [23] synthesized a novel microencapsulated phase-change material based on an n-octadecane core and an inorganic silica shell via sol–gel process. The silica shell material was successfully fabricated onto the surface of the n-octadecane core. The sol–gel method has a temperate process and is easy to carry out, the composite prepared by this method has uniform structure, but there are many micro pores exist in the gel.

In summary, direct immersion method and melt blending method cannot prevent PCM from leaking out to some extent; CPCM prepared via graft copolymerization has low latent heat that cannot meet the demand of regulating asphalt pavement temperature; intercalation method and sintering method have narrow applications; CPCM prepared via microencapsulation method may have poor stability and short service lifetime; sol–gel method is simple and has temperate reacting process, CPCM prepared via the sol–gel method has good properties. Therefore, in view of the high temperature during asphalt mixture mixing process and the demand for PCM durability during pavement service period, this paper aims at preparing CPCM that can be used in asphalt mixture via sol–gel process, the results will provide a reference for using PCM in road engineering.

Section snippets

Materials

The materials for preparing CPCMs in this study are shown in Table 1.

Sol–gel method

Sol–gel method is an effective way to form organic/inorganic composite. This method implies starting the process with a sol, then gelation of the sol is accomplished through low-temperature hydrolysis and condensation reactions [23]. Sol is defined as a dispersion of colloidal particles or polymeric species in a solvent. Gel is a continuous solid network surrounding and supporting a continuous liquid medium. The sol to gel

Experimental conditions

  • (1) Dosage of ethanol (EtOH)

In this paper, tetraethyl orthosilicate (TEOS) was used as precursor and ethanol (EtOH) was used as solvent to produce sol. The more content of EtOH will lead to smaller gel density and larger void ratio, this may reduce the strength of gel structure; meanwhile, more EtOH means longer distance between reactants which will decrease the reaction rate and prolong gelation time. Therefore, the molar ratio of TEOS and EtOH was chosen as 1:4 in this study.

  • (2) Dosage of

SEM results of PCPCM (CPCMs prepared by pure PCM)

This paper prepared PCPCMs through sol–gel process, their microstructures were observed by using SEM (Scanning Electron Microscope, VEGAII-XMU, TESCAN company, Czech Republic). Fig. 1, Fig. 2 show two kinds of PCPCMs, the difference between them is coating times (once coated or twice coated). Fig. 1 shows once coated PCPCM. Fig. 2 shows twice coated PCPCM, it was formed by using once coated PCPCM as raw material via sol–gel process, and it is used to testify whether coating times has

DSC results and analysis

From the above study, it is clear that few PCMs were left in PCPCM and SCPCM, so these two kinds of CPCMs must have poor latent heat storage capacity. However, the voids exist on the particles’ surfaces of floating bead and activated carbon can provide adsorption spaces for PCMs, so FCPCM and ACPCM may have better latent heat storage capacity. In this paper, DSC (Differential Scanning Calorimetry, DSC204F1Phoenix, NETZSCH Company, Germany) was used to compare latent heat storage capacity

Conclusion

In this paper, four different kinds of composite phase change materials were prepared by using pure PCM, silica powder adsorbed PCM, floating bead adsorbed PCM and activated carbon adsorbed PCM as raw materials through sol–gel method, the four CPCMs were named as PCPCM, SCPCM, FCPCM and ACPCM, respectively. Their coating effectiveness and the latter two CPCMs’ latent heat storage capacities were studied by SEM and DSC, respectively. The SEM results show: silane coupling agent has positive

Acknowledgements

The writers wish to acknowledge the financial support of this research by the “Twelfth five-year” National Science and Technology Support Plan (No. 2014BAG05B04), and the National Natural Science Foundation of China (No. 51310105029) and the Construction Science and Technology Project of Ministry of Transport of China (No. 2013 318 490 010).

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