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Superwetting polypropylene aerogel supported form-stable phase change materials with extremely high organics loading and enhanced thermal conductivity
Solar Energy Materials and Solar Cells 174 (2018) 307–313
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Superwetting polypropylene aerogel supported form-stable phase change materials with extremely high organics loading and enhanced thermal conductivity
MARK
Haizhi Hong, Yu Pan, Hanxue Sun, Zhaoqi Zhu, Chonghua Ma, Bing Wang, Weidong Liang, ⁎ Baoping Yang, An Li College of Petrochemical Technology, Lanzhou University of Technology, Langongping Road 287, Lanzhou 730050, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polypropylene Aerogel Form-stable Phase change material Thermal conductivity
The development of high efficient materials or devices for storage and utilization of sustainable thermal energy should be of great importance in alleviating energy crisis. Herein, we reported the fabrication of superwetting polypropylene (PP) aerogel as the support materials for construction of form-stable phase change materials (PCMs) composites. Due to its abundant porosity, light weight of aerogel in nature, inherent superhydrophobic and superoleophilic properties, lipophilic organic PCMs can be loaded into PP aerogel (PP/PCM) with a high PCMs loading of up to 1060 wt%, which is nearly two orders of magnitudes higher than those mostly reported form-stable PCMs systems. The PP/PCM composites show high latent heat in the range of 141.1 kJ kg−1 to 159.5 kJ kg−1 and excellent thermal stability and recyclability where their latent heat nearly remains unchanged even after 50 times of melting/freezing cycles. Results obtained from the X-ray diffraction (XRD) show that the incorporation of organic PCMs into PP aerogel network decreases the crystal size of PCMs in the composites. More interestingly, the PP/PCM composites show an enhanced thermal conductivity, in the case of PP/Paraffin composite, which is over two times of that of paraffin. Having the advantages of low cost and abundant resource of PP, simple preparatory process, high PCMs loading, good stability, recyclability and thermal conductivity, this the PP/PCM composites may have great potential for renewable energy saving applications.
1. Introduction Every year, huge amounts of energy were consumed all over around the world. In response to the energy crisis of fossil fuels as well as emerging ecological concerns and global warming, the exploitation of clean, renewable, and sustainable energy resource and techniques has become increasingly important than ever. So far, a number of new energy resources have been exploited, for example, solar energy, wind energy and nuclear energy, etc. However, there are still a lot of issues, including energy intermittence, geographical restrictions, etc., for utilization of those mentioned energy remain to be solved. Therefore, the development of high efficient materials or devices for capture and storage of that energy should be of great significance. Phase change materials (PCMs) are a kind of materials with a high heat of fusion which, melting and solidifying at a certain temperature, are capable of storing and releasing large amounts of thermal energy when their phase changes from solid state to liquid state or vice versa.
⁎
Also, this enables thermal energy being stored from one process or period of time and used at a later point in time or transferred to a different location, which shows great potential for various applications such as solar energy saving [1–3], industrial waste heat recovery [4,5], building energy saving [6,7], electronic temperature control equipment [8] and functionally thermal fluid [9], etc. To date, a number of materials such as inorganic compounds, organic compounds, and their mixtures can be used as PCMs, including hydrated salts [10], paraffin [11,12], fatty acids [13], and polyethylene glycol (PEG) [14,15]. Compared with inorganic PCMs, organic PCMs have the characteristic of low corrosion and melting/freezing cycles without phase segregation, which have been widely used in thermal energy saving systems. In the most cases, however, the direct use of these organic PCMs has the limitations for the leakage of the liquid phase above their melting point. To address this issue, microencapsulated phase change materials with organic PCMs as core and microcapsules as shell have been proven to be an efficient strategy and
Corresponding author. E-mail address: [email protected] (A. Li).
http://dx.doi.org/10.1016/j.solmat.2017.09.026 Received 18 June 2017; Received in revised form 27 August 2017; Accepted 12 September 2017 0927-0248/ © 2017 Published by Elsevier B.V.
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2.2. Preparation of PP aerogel and film support materials
thus have been well investigated. However, the drawbacks for the microcapsule method relied in the need of multistep procedures, complicated techniques and relative high cost, thus hindering their practical applications. In contrast, direct storing the PCMs into porous support materials to form form-stable PCM composites should be one of promising remedies to this issue. To date, several kinds of porous materials including gypsum [6,16], diatomite [17], graphene aerogel [18–20], SiO2 [21,22], conjugated microporous polymers [23] and ceramic composites [24] have been used to fabricate form-stable PCM composites by means of natural immersion approach. Along this line, we have recently developed the development of several kinds of porous materials with superhydrophobic and superoleophilic wettability, including superwetting graphene-nickel foam (G-NF) [25], conjugated microporous polymers (CMPs) [23] and spongy attapulgite (s-ATP) [26], for construction of form-stable PCM composites with enhanced significantly enhanced affinity of lipophilic organic PCMs (e.g. Paraffin) to support materials In a continuation of our previous works on PCMs [23,26,27], herein, we report the first example of polypropylene (PP) based aerogel with superwetting property as porous medium for fabrication of form-stable PCM composites (PP/PCM). Our primary design for construction of such superwetting PP aerogel was based on the following three main aspects: (1) The superhydrophobic and superoleophilic properties of the PP based aerogel originated from its inherent lipophilic chemistry in combination with porous morphology, which is easy to be fabricated and thus quite different from those artificial superwetting support materials that usually need complicated and multistep to prepare. (2) Compared with those reported porous materials for preparation of form-stable PCM composites which are usually capable of loading the PCMs in the range from ten percent to tens of percent [28,29], and the results of PP/PCM composites are at least one order of magnitude higher owing to its light weight in nature of aerogel. (3) The thermal conductivity of the PP/PCM composites was notably elevated over one fold after the addition of PP as compared with that of the pure PCMs. We suggest that such superwetting PP aerogel may find diverse applications for thermal energy saving systems. On the other hand, the findings of this study may also provide useful guidance for future design and creation of high performance form-stable PCMs composites.
Isotactic polypropylene solution (20 mg mL−1) was prepared by adding granular PP and 10 mL of p-xylene in a round-bottomed flask under silicone oil bath at 130 °C [30]. When granular PP was fully dissolved, a homogeneous solution was formed. Then, different operation methods were adopted to fabricate the aerogel and film. As for aerogel, the isopropyl alcohol was added into the solution, and then the mixtures were precipitated and lyophilized as aerogel for latter process. The film was obtained through salivation method. 2.3. Preparation of phase change materials composites A certain amount of PP aerogel was immersed into a small beaker filled with paraffin in water bath or dry oven at 90 °C. The aerogel was permeated for just a short moment. Afterwards, the treated PP/Paraffin composites were taken out and dried in the oven at 90 °C to remove the excessive paraffin until the weight loss keeping constant. The procedures are same for preparation of PP/MA and PP/SA. The following equation was used to calculate the loading rate for PP/PCM composites:
ϕ=
m1 − m 0 m0
where m0 is the weight before loading of PCMs, m1 is the weight after loading of PCMs and Φ is the loading rate, respectively. 2.4. Characterization Surface morphology was observed on a JSM-6701F cold field emission scanning electron microscope (SEM). All samples were gold coated before analysis. Energy Dispersive Spectrometer (EDS) analysis was performed on a cold field emission scanning electron microscope (Oxford X-Max). The X-ray diffraction (XRD) was performed on a Rigaku D/Max-2400 diffractometer with a Cu tube source and scans were taken from 2θ at 2° to 80°. The Brunauer-Emmett-Teller (BET) surface area and the porous volume of the PP aerogel were detected with a nitrogen adsorption/desorption isotherm measured at 77 K using a physisorption analyzer (ASAP 2020, Micromeritics). The water contact angle (CA) was measured on a contact anglemeter (DSA100, Kruss). Differential scanning calorimeter (DSC, 200F3) was utilized to learn about their thermodynamic properties. The samples were detected from 20 °C to 120 °C with a heating-cooling rate of 5 °C min−1 in nitrogen atmosphere at a flow rate of 10 mL min−1. The thermal conductivity of PP/PCM composites were investigated on flash method thermal analyzer (LFA 447, Netzsch).
2. Experimental 2.1. Materials Isotactic polypropylene (PP, Mw = ~ 250,000) was obtained from Sigma-Aldrich Co. LLC. P-xylene was purchased from Shanghai Zhongqin Chemical Reagent Co., Ltd. Isopropyl alcohol was supplied by Tianjin Fuyu Fine Chemical Co., Ltd. Paraffin was used as phase change energy storage material and ordered from Shanghai Huashen Rehabilitation Equipment Co., Ltd. Myristic acid (MA) and stearic acid (SA) were purchased from BASF chemical Co. Let. All chemicals were used as received without purification.
3. Results and discussion In this work, isotactic polypropylene or waste polypropylene-based materials, which is commercially available and widely used in industry, was used as raw material for preparation of porous support material to fabricate the PCM composites. To this end, isotactic polypropylene (or Fig. 1. The preparation of polypropylene aerogel and film.
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large pore size would facilitate the impenetration and loading the organic PCM inside to the PP aerogel network. The microscopic morphologies of PP aerogel and film were evaluated by SEM. As shown in Fig. 3a, the PP aerogel is composed of agglomerated micrometer-sized microgel particles, showing a loosely porous network morphology originated by p-xylene template effect under freeze-drying. For PP thin film (Fig. 3b), it also exhibits a porous surface microstructure in the presence of large pores formed by interconnected PP chains. To further investigate their surface wettability, static water and contact angle (CA) measurements were performed. As expected, the PP aerogel shows superhydrophobic surface wettability with a water CA of 153° (Fig. 3c). Such superhydrophobicity should be attributed to the combination of microporous morphological structures and its hydrophobic chemical compositions, which are the two key factors for the surface superhydrophobicity. In the case of PP thin film, a water CA of 143° was obtained (Fig. 3d), also indicating a strong hydrophobicity. The PP aerogel also exhibits excellent surface superoleophilicity. When a droplet of diesel oil was introduced on the surface of PP aerogel, it can be adsorbed quickly inside of aerogel, implying a strong affinity to oils. Based on such surface superhydrophobicity and superoleophilicity of the PP aerogel, the lipophilic paraffin and n-carboxylic acids such as MA and SA can be easily incorporated into the PP aerogel to form-stable PCM composites. Though using superwetting porous materials to prepare form-stable PCMs composites has been reported previously [23,25,33], quite different from those artificial superwetting surfaces constructed by controlling the surface morphological features where surface wettability is greatly influenced by experimental conditions or techniques adopted [34,35], our strategy takes advantage of inherent chemistry of PP which allows both strong hydrophobic chemical compositions and porous characteristics are easily obtained at the same time to create superwetting property of PP aerogel only through a facile one-step freeze-drying process without using any complicated techniques, thus it is of greatly technological significance. As shown in Fig. 3e–g, the pores of PP aerogel are homogeneously filled by paraffin, SA and MA, illustrating that higher alkanes and ncarboxylic acids were completely confined into aerogel porous network due to the effects of superoleophilicity, pore capillary effect and the surface adsorption of the PP aerogel, which is similar to the observations reported in our previous studies [23]. As a result, nearly no leakage of the phase change substances loaded in the PP aerogel was observed even when the temperature is higher than their melt points. Furthermore, in order to investigate the dispersion of the adsorbed substance in PP aerogel network, the EDS measurement (oxygen was used as tagged element) for PP/PCM composites were performed. It can be seen from the EDS images (Fig. 3f inset and g inset) that the organic phase change substances were well dispersed in PP aerogel matrix. The XRD patterns of PP aerogel, paraffin and PP/Paraffin composites are showed in Fig. 4. There are no obvious diffraction peaks can be observed for the PP aerogel, which means that PP aerogel is amorphous in nature. For paraffin, the main two strong characteristic diffraction peaks at 21.6° and 24.04° are observed. In the case of the PP/Paraffin composite, the characteristic peaks of paraffin also exist in the product, indicating that paraffin has been incorporated successfully into PP aerogel [22]. Notably, the peak intensities are obvious higher for pure paraffin than that of PP/Paraffin composite. By applying with Scherrer equation, the crystal size was calculated to be 18.11 nm for pure paraffin and 15.58 nm for PP/Paraffin, which shows that the crystal size decreased after the paraffin was incorporated into the porous network of the PP aerogel. Such decrease in crystal size of paraffin should be attributed to the attractive interaction of PP aerogel to paraffin as well as the pore size effect which would hinder the movement of paraffin in the composite during the melting and crystallization process, which can be found in other PCM composite systems [22,36]. The prepared PP/PCM composites exhibit excellent thermal properties which were evaluated by DSC measurements. Fig. 5a shows the DSC thermograms of paraffin, PP/Paraffin composite and PP/Paraffin
commercial available polypropylene pipes, as shown in Fig. 1) was dissolved completely in p-xylene under vigorous stirring at 130 °C, followed by precipitation in isopropyl alcohol solvent and then filtering. The filter residues then can be used to prepare PP aerogel by freeze-drying method or thin film by salivation method. The preparation procedures for PP aerogel and film are illustrated in Fig. 1. Interestingly, the PP aerogel shows superhydrophobic wettability. As shown in Fig. 1, a water droplet keeps spherical shape, just like on the surface of lotus. Since our primary design was to prepare PCM composites, we focused on the PP aerogel due to its abundant porosity and, more importantly, inherent superhydrophobic wettability which would enhance its affinity to organic PCM and thus facilitate to prepare form-stable PCM composites [23,25,31]. On the other hand, PP aerogel prepared under our conditions is light weight and which can be placed on the dandelion. Its density was estimated to be 0.0271 g cm−3. Taking advantages of its low cost, easy availability, abundant porosity, inherent superhydrophobicity and light weight in nature, the as-prepared PP aerogel would be promising candidate for preparation form-stable PCM composites. The surface areas and pore structures of as-prepared PP aerogel were evaluated by nitrogen adsorption and desorption measurements at 77 K. As shown in Fig. 2a, the N2 adsorption and desorption isotherms of the aerogel present IV style with H3 type hysteresis loops [32]. The BET surface area of PP aerogel was measured to be 38.66 m2 g−1 and the (Barrett-Joyner-Halenda) BJH adsorption cumulative surface area of pores (radius range of 0.85–150 nm) is evaluated separately 44.08 m2 g−1. Fig. 2b shows the pore-size distributions of samples. As can be seen, a relative large pore-size distribution (PSD) was observed. Its pore diameter and the pore volume were calculated to be 22.54 nm and 0.17 cm3 g−1 (P/P0 = 0.97), respectively, suggesting that the PP aerogel is consisted of mesopores (pore size larger than 2 nm). Such
Fig. 2. (a) N2 adsorption-desorption isotherms of PP aerogel measured at 77.3 K. (b) Pore size distribution curves of PP aerogel.
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Fig. 3. The SEM images of (a) PP aerogel, (b) PP film, (e) PP/Paraffin, (f) PP/SA, (g) PP/MA. (c) The contour profile of a water drop on a tablet of PP aerogel with the CA of 153°, (d) and the CA of 143° on PP film. The insets of (f) and (g) are the corresponding EDS images.
the samples were operated for 50 times of melting/freezing cycles and then evaluated by DSC measurements. As shown in Fig. 5a, the thermograms of PP/PCM composites before and after 50 times of thermal cycles (named as PP/Paraffin50) are nearly overlapping and no obvious change was seen. The ΔHm and ΔHs were calculated to be 141.4 J g−1 and 147.8 J g−1 after 50 times of thermal cycles, respectively. Clearly, the quantitative value difference in enthalpy of the PP/PCM composites is nearly negligible before and after thermal cycles test, suggesting an excellent thermal stability. In general, most of organic PCMs suffer the drawback of poor thermal conductivity as well as high degree of supercooling due to their chemistry nature, especially for those organic PCMs were usually used and packaged into polymeric microcapsules to form a core/shell structure in case of leakage of PCMs during the melting/freezing cycles [20,41], of which the polymeric shell with low thermal conductivity also hinders the heat transfer from environment to PCMs core. Such poor thermal conductivity may result in hysteresis of thermal response and thus may decreases their thermal energy storage performance [15]. In this study, interestingly, the PP/PCM composites show better thermal conductivity than that of pure organic PCMs. In this case, we used the paraffin and PP/Paraffin composite as models to evaluate their thermal conductivities. As shown in Fig. 6, the PP/Paraffin composite has a thermal conductivity of 0.534 W m−1 K−1 at 25 °C, which is two times of that of paraffin (0.207 W m−1 K−1). We also measured their thermal conductivities at different temperature and the values were evaluated as 0.674 W m−1 K−1 at 35 °C and 0.423 W m−1 K−1 at 45 °C, respectively. Table 2 lists the thermal conductivity data of some PCM composites reported previously. In fact, with these values, the PP/paraffin composite even exhibits better thermal conductivity than a variety of PCM composites such as that listed in Table 2, which is greatly advantageous for their practical applications. More importantly, owing to the abundant porosity and light weight in nature of PP aerogel, it shows extremely high loading of organic PCMs. In this case, we also selected paraffin as target organic PCM to evaluate the loading capacity of PP aerogel. As shown in Fig. 7, the PP aerogel possesses an initial paraffin loading of 1060 wt% of its weight.
Fig. 4. XRD patterns of PP aerogel, paraffin and PP/Paraffin composites.
composite after 50 times of thermal cycles. Obviously, there are two endothermic and exothermic peaks. The sharp or main peak corresponds to the solid-liquid phase change of the paraffin used as PCMs, while the minor peaks appeared at the left of the main peak are in concert with the solid-solid phase transition of paraffin [37,38]. The phase change latent heat of the paraffin and PP/Paraffin composite were calculated by the area under the exothermic peak in the DSC curves and presented in Table 1, which show the peak melting/solidification temperatures (Tm/TS) and melting/solidification enthalpies (ΔHm/ΔHs) between the stages of solid-liquid phase transition. Compared with the melting and solidification enthalpies data of paraffin and PP/Paraffin composite, a decrease in both melting and solidification enthalpies was clearly seen, which was similar to other PCM composites systems after incorporation of organic PCMs into porous support materials [18,39]. The ΔHm and ΔHs were respectively calculated to be 141.9 J g−1 and 149.9 J g−1, respectively. With these value, the phase change latent heat of the PP/Paraffin composite is higher than that of PEG2000/Ag [14], palmitic acid/SiO2 [36], paraffin/graphene oxide [40], and can compete with PEG/cellulose/GNP composites [18]. In order to investigate the thermal stability of PP/PCM composites,
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Fig. 6. The thermal conductivity of PP/Paraffin composite in different temperature.
Table 2 Thermal conductivity properties of some PCM composites in literature.
Fig. 5. The DSC curves for melting and freezing of (a) paraffin, PP/Paraffin composite and PP/Paraffin composite after 50 times of thermal cycles, (b) MA and PP/MA composite and (c) SA and PP/SA composite.
Paraffin PP/Paraffin PP/Paraffin50 MA PP/MA SA PP/SA
Melting process Tm (°C)
ΔHm (J g
58.5 60.6 58.7 59.5 56.1 59.0 56.9
168.4 141.9 141.4 200.2 159.9 185.6 143.3
)
Ts (°C)
ΔHs (J g
48.4 47.5 47.9 45.4 45.5 47.7 45.5
172 149.9 147.8 202.7 156.7 188.1 145.3
Ref.
MH-GA-P Paraffin/carbon fiber SA-ODE SA-ODE + 5%HBN 40% Al2O3/ESBR
0.248 0.5 0.298 0.317 0.26
[20] [42] [43] [43] [44]
Table 3 The PCMs content of various form-stable PCM composites.
Freezing process −1
Thermal conductivity (W m−1 K−1)
Fig. 7. The loading rate of PP/Paraffin composite after different times of melting/freezing cycles under operation.
Table 1 The DSC data of paraffin, PP/Paraffin composite, PP/Paraffin50, MA, SA, PP/SA and PP/ MA composites. Samples
Sample
−1
)
311
Composites
PCM content
Ref.
Erythritol/EG Paraffin/EG Gypsum/C18-C24 C-SiO2-PA C-SiO2-OD Paraffin/HDPE PEG/SiO2 MA-PA-SA/EG PP/Paraffin
15–20 vol% 10 wt% 18 wt% 61 wt% 73 wt% 77 wt% 80 wt% 92.86 wt% 900–1060 wt%
[45] [38] [6] [28] [28] [37] [22] [29] This work
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Compared with those reported porous materials for preparation of form-stable PCM composites which are usually capable of loading the PCMs in the range from ten percent to tens of percent (Table 3), our PP/ Paraffin composites are nearly two orders of magnitudes as high, which is of technological significance for their industrial applications. Furthermore, due to its superoleophilic wettability, the PP aerogel shows strong affinity to the loaded organic PCMs. It also can be seen that the loaded paraffin is very stable, only a slight loading loss of 9.7% was observed after 10 times of melting/freezing cycles. In the range of 10 times to 20 times, a smaller loading loss of 5.2% was measured. It should be noted that nearly no obvious leakage of organic PCMs was observed during the whole melting/freezing cycles even operated for many times. The main weight loss in this case is mostly caused by the contact loss during transferring procedure. These results also confirmed the strong affinity of PP aerogel to organic PCMs which should be attributed to its inherent superwetting properties [25]. In addition, PP aerogel is very stable in acid or base environments, so it can withstand harsh conditions during operation.
[5]
[6]
[7]
[8] [9]
[10]
[11]
[12] [13]
4. Conclusions In summary, we have demonstrated the fabrication of superwetting PP aerogel, which was originated from bulk chemical materials or industrial PP based wastes, as the support materials for construction of form-stable PCMs composites. Due to its abundant porosity, light weight of aerogel in nature, inherent superhydrophobic and superoleophilic properties, lipophilic organic PCMs can be loaded into PP aerogel with an extremely high PCMs loading of up to 1060 wt%, which is nearly two orders of magnitude higher than those mostly reported form-stable PCMs systems. The PP/PCM composites show high latent heat in the range from 141.1 kJ kg−1 to 159.5 kJ kg−1 and excellent thermal stability and recyclability where their latent heat nearly remains unchanged even after 50 times of melting/freezing cycles. More interestingly, the PP/PCM composites show an enhanced thermal conductivity, in the case of PP/Paraffin composite, which is over two times of that of paraffin. Having the advantages of low cost and abundant resource of PP, simple fabrication process, high PCMs loading, good stability, recyclability and thermal conductivity, the PP/PCM composites may have great potential for renewable energy saving applications. Also, we suggest that such three-in-one strategy for preparation of aerogel materials with superwetting properties may open a new possibility for future design and creation of high performance form-stable PCMs composites, which should be of great importance in alleviating energy crisis.
[14]
[15]
[16] [17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
Acknowledgements
[25]
The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 51403092, 51462021 and 51663012 and 41361070), the Natural Science Foundation of Gansu Province, China (Grant No. 1610RJYA001), Support Program for Hongliu Young Teachers (Q201411), Hongliu Elitist Scholars of LUT (J201401), Support Program for Longyuan Youth and Fundamental Research Funds for the Universities of Gansu Province, and Program for Young Talent of State Ethnic Affairs Commission ([2016]57).
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Superwetting polypropylene aerogel supported form-stable phase change materials with extremely high organics loading and enhanced thermal conductivity
MARK
Haizhi Hong, Yu Pan, Hanxue Sun, Zhaoqi Zhu, Chonghua Ma, Bing Wang, Weidong Liang, ⁎ Baoping Yang, An Li College of Petrochemical Technology, Lanzhou University of Technology, Langongping Road 287, Lanzhou 730050, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polypropylene Aerogel Form-stable Phase change material Thermal conductivity
The development of high efficient materials or devices for storage and utilization of sustainable thermal energy should be of great importance in alleviating energy crisis. Herein, we reported the fabrication of superwetting polypropylene (PP) aerogel as the support materials for construction of form-stable phase change materials (PCMs) composites. Due to its abundant porosity, light weight of aerogel in nature, inherent superhydrophobic and superoleophilic properties, lipophilic organic PCMs can be loaded into PP aerogel (PP/PCM) with a high PCMs loading of up to 1060 wt%, which is nearly two orders of magnitudes higher than those mostly reported form-stable PCMs systems. The PP/PCM composites show high latent heat in the range of 141.1 kJ kg−1 to 159.5 kJ kg−1 and excellent thermal stability and recyclability where their latent heat nearly remains unchanged even after 50 times of melting/freezing cycles. Results obtained from the X-ray diffraction (XRD) show that the incorporation of organic PCMs into PP aerogel network decreases the crystal size of PCMs in the composites. More interestingly, the PP/PCM composites show an enhanced thermal conductivity, in the case of PP/Paraffin composite, which is over two times of that of paraffin. Having the advantages of low cost and abundant resource of PP, simple preparatory process, high PCMs loading, good stability, recyclability and thermal conductivity, this the PP/PCM composites may have great potential for renewable energy saving applications.
1. Introduction Every year, huge amounts of energy were consumed all over around the world. In response to the energy crisis of fossil fuels as well as emerging ecological concerns and global warming, the exploitation of clean, renewable, and sustainable energy resource and techniques has become increasingly important than ever. So far, a number of new energy resources have been exploited, for example, solar energy, wind energy and nuclear energy, etc. However, there are still a lot of issues, including energy intermittence, geographical restrictions, etc., for utilization of those mentioned energy remain to be solved. Therefore, the development of high efficient materials or devices for capture and storage of that energy should be of great significance. Phase change materials (PCMs) are a kind of materials with a high heat of fusion which, melting and solidifying at a certain temperature, are capable of storing and releasing large amounts of thermal energy when their phase changes from solid state to liquid state or vice versa.
⁎
Also, this enables thermal energy being stored from one process or period of time and used at a later point in time or transferred to a different location, which shows great potential for various applications such as solar energy saving [1–3], industrial waste heat recovery [4,5], building energy saving [6,7], electronic temperature control equipment [8] and functionally thermal fluid [9], etc. To date, a number of materials such as inorganic compounds, organic compounds, and their mixtures can be used as PCMs, including hydrated salts [10], paraffin [11,12], fatty acids [13], and polyethylene glycol (PEG) [14,15]. Compared with inorganic PCMs, organic PCMs have the characteristic of low corrosion and melting/freezing cycles without phase segregation, which have been widely used in thermal energy saving systems. In the most cases, however, the direct use of these organic PCMs has the limitations for the leakage of the liquid phase above their melting point. To address this issue, microencapsulated phase change materials with organic PCMs as core and microcapsules as shell have been proven to be an efficient strategy and
Corresponding author. E-mail address: [email protected] (A. Li).
http://dx.doi.org/10.1016/j.solmat.2017.09.026 Received 18 June 2017; Received in revised form 27 August 2017; Accepted 12 September 2017 0927-0248/ © 2017 Published by Elsevier B.V.
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2.2. Preparation of PP aerogel and film support materials
thus have been well investigated. However, the drawbacks for the microcapsule method relied in the need of multistep procedures, complicated techniques and relative high cost, thus hindering their practical applications. In contrast, direct storing the PCMs into porous support materials to form form-stable PCM composites should be one of promising remedies to this issue. To date, several kinds of porous materials including gypsum [6,16], diatomite [17], graphene aerogel [18–20], SiO2 [21,22], conjugated microporous polymers [23] and ceramic composites [24] have been used to fabricate form-stable PCM composites by means of natural immersion approach. Along this line, we have recently developed the development of several kinds of porous materials with superhydrophobic and superoleophilic wettability, including superwetting graphene-nickel foam (G-NF) [25], conjugated microporous polymers (CMPs) [23] and spongy attapulgite (s-ATP) [26], for construction of form-stable PCM composites with enhanced significantly enhanced affinity of lipophilic organic PCMs (e.g. Paraffin) to support materials In a continuation of our previous works on PCMs [23,26,27], herein, we report the first example of polypropylene (PP) based aerogel with superwetting property as porous medium for fabrication of form-stable PCM composites (PP/PCM). Our primary design for construction of such superwetting PP aerogel was based on the following three main aspects: (1) The superhydrophobic and superoleophilic properties of the PP based aerogel originated from its inherent lipophilic chemistry in combination with porous morphology, which is easy to be fabricated and thus quite different from those artificial superwetting support materials that usually need complicated and multistep to prepare. (2) Compared with those reported porous materials for preparation of form-stable PCM composites which are usually capable of loading the PCMs in the range from ten percent to tens of percent [28,29], and the results of PP/PCM composites are at least one order of magnitude higher owing to its light weight in nature of aerogel. (3) The thermal conductivity of the PP/PCM composites was notably elevated over one fold after the addition of PP as compared with that of the pure PCMs. We suggest that such superwetting PP aerogel may find diverse applications for thermal energy saving systems. On the other hand, the findings of this study may also provide useful guidance for future design and creation of high performance form-stable PCMs composites.
Isotactic polypropylene solution (20 mg mL−1) was prepared by adding granular PP and 10 mL of p-xylene in a round-bottomed flask under silicone oil bath at 130 °C [30]. When granular PP was fully dissolved, a homogeneous solution was formed. Then, different operation methods were adopted to fabricate the aerogel and film. As for aerogel, the isopropyl alcohol was added into the solution, and then the mixtures were precipitated and lyophilized as aerogel for latter process. The film was obtained through salivation method. 2.3. Preparation of phase change materials composites A certain amount of PP aerogel was immersed into a small beaker filled with paraffin in water bath or dry oven at 90 °C. The aerogel was permeated for just a short moment. Afterwards, the treated PP/Paraffin composites were taken out and dried in the oven at 90 °C to remove the excessive paraffin until the weight loss keeping constant. The procedures are same for preparation of PP/MA and PP/SA. The following equation was used to calculate the loading rate for PP/PCM composites:
ϕ=
m1 − m 0 m0
where m0 is the weight before loading of PCMs, m1 is the weight after loading of PCMs and Φ is the loading rate, respectively. 2.4. Characterization Surface morphology was observed on a JSM-6701F cold field emission scanning electron microscope (SEM). All samples were gold coated before analysis. Energy Dispersive Spectrometer (EDS) analysis was performed on a cold field emission scanning electron microscope (Oxford X-Max). The X-ray diffraction (XRD) was performed on a Rigaku D/Max-2400 diffractometer with a Cu tube source and scans were taken from 2θ at 2° to 80°. The Brunauer-Emmett-Teller (BET) surface area and the porous volume of the PP aerogel were detected with a nitrogen adsorption/desorption isotherm measured at 77 K using a physisorption analyzer (ASAP 2020, Micromeritics). The water contact angle (CA) was measured on a contact anglemeter (DSA100, Kruss). Differential scanning calorimeter (DSC, 200F3) was utilized to learn about their thermodynamic properties. The samples were detected from 20 °C to 120 °C with a heating-cooling rate of 5 °C min−1 in nitrogen atmosphere at a flow rate of 10 mL min−1. The thermal conductivity of PP/PCM composites were investigated on flash method thermal analyzer (LFA 447, Netzsch).
2. Experimental 2.1. Materials Isotactic polypropylene (PP, Mw = ~ 250,000) was obtained from Sigma-Aldrich Co. LLC. P-xylene was purchased from Shanghai Zhongqin Chemical Reagent Co., Ltd. Isopropyl alcohol was supplied by Tianjin Fuyu Fine Chemical Co., Ltd. Paraffin was used as phase change energy storage material and ordered from Shanghai Huashen Rehabilitation Equipment Co., Ltd. Myristic acid (MA) and stearic acid (SA) were purchased from BASF chemical Co. Let. All chemicals were used as received without purification.
3. Results and discussion In this work, isotactic polypropylene or waste polypropylene-based materials, which is commercially available and widely used in industry, was used as raw material for preparation of porous support material to fabricate the PCM composites. To this end, isotactic polypropylene (or Fig. 1. The preparation of polypropylene aerogel and film.
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large pore size would facilitate the impenetration and loading the organic PCM inside to the PP aerogel network. The microscopic morphologies of PP aerogel and film were evaluated by SEM. As shown in Fig. 3a, the PP aerogel is composed of agglomerated micrometer-sized microgel particles, showing a loosely porous network morphology originated by p-xylene template effect under freeze-drying. For PP thin film (Fig. 3b), it also exhibits a porous surface microstructure in the presence of large pores formed by interconnected PP chains. To further investigate their surface wettability, static water and contact angle (CA) measurements were performed. As expected, the PP aerogel shows superhydrophobic surface wettability with a water CA of 153° (Fig. 3c). Such superhydrophobicity should be attributed to the combination of microporous morphological structures and its hydrophobic chemical compositions, which are the two key factors for the surface superhydrophobicity. In the case of PP thin film, a water CA of 143° was obtained (Fig. 3d), also indicating a strong hydrophobicity. The PP aerogel also exhibits excellent surface superoleophilicity. When a droplet of diesel oil was introduced on the surface of PP aerogel, it can be adsorbed quickly inside of aerogel, implying a strong affinity to oils. Based on such surface superhydrophobicity and superoleophilicity of the PP aerogel, the lipophilic paraffin and n-carboxylic acids such as MA and SA can be easily incorporated into the PP aerogel to form-stable PCM composites. Though using superwetting porous materials to prepare form-stable PCMs composites has been reported previously [23,25,33], quite different from those artificial superwetting surfaces constructed by controlling the surface morphological features where surface wettability is greatly influenced by experimental conditions or techniques adopted [34,35], our strategy takes advantage of inherent chemistry of PP which allows both strong hydrophobic chemical compositions and porous characteristics are easily obtained at the same time to create superwetting property of PP aerogel only through a facile one-step freeze-drying process without using any complicated techniques, thus it is of greatly technological significance. As shown in Fig. 3e–g, the pores of PP aerogel are homogeneously filled by paraffin, SA and MA, illustrating that higher alkanes and ncarboxylic acids were completely confined into aerogel porous network due to the effects of superoleophilicity, pore capillary effect and the surface adsorption of the PP aerogel, which is similar to the observations reported in our previous studies [23]. As a result, nearly no leakage of the phase change substances loaded in the PP aerogel was observed even when the temperature is higher than their melt points. Furthermore, in order to investigate the dispersion of the adsorbed substance in PP aerogel network, the EDS measurement (oxygen was used as tagged element) for PP/PCM composites were performed. It can be seen from the EDS images (Fig. 3f inset and g inset) that the organic phase change substances were well dispersed in PP aerogel matrix. The XRD patterns of PP aerogel, paraffin and PP/Paraffin composites are showed in Fig. 4. There are no obvious diffraction peaks can be observed for the PP aerogel, which means that PP aerogel is amorphous in nature. For paraffin, the main two strong characteristic diffraction peaks at 21.6° and 24.04° are observed. In the case of the PP/Paraffin composite, the characteristic peaks of paraffin also exist in the product, indicating that paraffin has been incorporated successfully into PP aerogel [22]. Notably, the peak intensities are obvious higher for pure paraffin than that of PP/Paraffin composite. By applying with Scherrer equation, the crystal size was calculated to be 18.11 nm for pure paraffin and 15.58 nm for PP/Paraffin, which shows that the crystal size decreased after the paraffin was incorporated into the porous network of the PP aerogel. Such decrease in crystal size of paraffin should be attributed to the attractive interaction of PP aerogel to paraffin as well as the pore size effect which would hinder the movement of paraffin in the composite during the melting and crystallization process, which can be found in other PCM composite systems [22,36]. The prepared PP/PCM composites exhibit excellent thermal properties which were evaluated by DSC measurements. Fig. 5a shows the DSC thermograms of paraffin, PP/Paraffin composite and PP/Paraffin
commercial available polypropylene pipes, as shown in Fig. 1) was dissolved completely in p-xylene under vigorous stirring at 130 °C, followed by precipitation in isopropyl alcohol solvent and then filtering. The filter residues then can be used to prepare PP aerogel by freeze-drying method or thin film by salivation method. The preparation procedures for PP aerogel and film are illustrated in Fig. 1. Interestingly, the PP aerogel shows superhydrophobic wettability. As shown in Fig. 1, a water droplet keeps spherical shape, just like on the surface of lotus. Since our primary design was to prepare PCM composites, we focused on the PP aerogel due to its abundant porosity and, more importantly, inherent superhydrophobic wettability which would enhance its affinity to organic PCM and thus facilitate to prepare form-stable PCM composites [23,25,31]. On the other hand, PP aerogel prepared under our conditions is light weight and which can be placed on the dandelion. Its density was estimated to be 0.0271 g cm−3. Taking advantages of its low cost, easy availability, abundant porosity, inherent superhydrophobicity and light weight in nature, the as-prepared PP aerogel would be promising candidate for preparation form-stable PCM composites. The surface areas and pore structures of as-prepared PP aerogel were evaluated by nitrogen adsorption and desorption measurements at 77 K. As shown in Fig. 2a, the N2 adsorption and desorption isotherms of the aerogel present IV style with H3 type hysteresis loops [32]. The BET surface area of PP aerogel was measured to be 38.66 m2 g−1 and the (Barrett-Joyner-Halenda) BJH adsorption cumulative surface area of pores (radius range of 0.85–150 nm) is evaluated separately 44.08 m2 g−1. Fig. 2b shows the pore-size distributions of samples. As can be seen, a relative large pore-size distribution (PSD) was observed. Its pore diameter and the pore volume were calculated to be 22.54 nm and 0.17 cm3 g−1 (P/P0 = 0.97), respectively, suggesting that the PP aerogel is consisted of mesopores (pore size larger than 2 nm). Such
Fig. 2. (a) N2 adsorption-desorption isotherms of PP aerogel measured at 77.3 K. (b) Pore size distribution curves of PP aerogel.
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Fig. 3. The SEM images of (a) PP aerogel, (b) PP film, (e) PP/Paraffin, (f) PP/SA, (g) PP/MA. (c) The contour profile of a water drop on a tablet of PP aerogel with the CA of 153°, (d) and the CA of 143° on PP film. The insets of (f) and (g) are the corresponding EDS images.
the samples were operated for 50 times of melting/freezing cycles and then evaluated by DSC measurements. As shown in Fig. 5a, the thermograms of PP/PCM composites before and after 50 times of thermal cycles (named as PP/Paraffin50) are nearly overlapping and no obvious change was seen. The ΔHm and ΔHs were calculated to be 141.4 J g−1 and 147.8 J g−1 after 50 times of thermal cycles, respectively. Clearly, the quantitative value difference in enthalpy of the PP/PCM composites is nearly negligible before and after thermal cycles test, suggesting an excellent thermal stability. In general, most of organic PCMs suffer the drawback of poor thermal conductivity as well as high degree of supercooling due to their chemistry nature, especially for those organic PCMs were usually used and packaged into polymeric microcapsules to form a core/shell structure in case of leakage of PCMs during the melting/freezing cycles [20,41], of which the polymeric shell with low thermal conductivity also hinders the heat transfer from environment to PCMs core. Such poor thermal conductivity may result in hysteresis of thermal response and thus may decreases their thermal energy storage performance [15]. In this study, interestingly, the PP/PCM composites show better thermal conductivity than that of pure organic PCMs. In this case, we used the paraffin and PP/Paraffin composite as models to evaluate their thermal conductivities. As shown in Fig. 6, the PP/Paraffin composite has a thermal conductivity of 0.534 W m−1 K−1 at 25 °C, which is two times of that of paraffin (0.207 W m−1 K−1). We also measured their thermal conductivities at different temperature and the values were evaluated as 0.674 W m−1 K−1 at 35 °C and 0.423 W m−1 K−1 at 45 °C, respectively. Table 2 lists the thermal conductivity data of some PCM composites reported previously. In fact, with these values, the PP/paraffin composite even exhibits better thermal conductivity than a variety of PCM composites such as that listed in Table 2, which is greatly advantageous for their practical applications. More importantly, owing to the abundant porosity and light weight in nature of PP aerogel, it shows extremely high loading of organic PCMs. In this case, we also selected paraffin as target organic PCM to evaluate the loading capacity of PP aerogel. As shown in Fig. 7, the PP aerogel possesses an initial paraffin loading of 1060 wt% of its weight.
Fig. 4. XRD patterns of PP aerogel, paraffin and PP/Paraffin composites.
composite after 50 times of thermal cycles. Obviously, there are two endothermic and exothermic peaks. The sharp or main peak corresponds to the solid-liquid phase change of the paraffin used as PCMs, while the minor peaks appeared at the left of the main peak are in concert with the solid-solid phase transition of paraffin [37,38]. The phase change latent heat of the paraffin and PP/Paraffin composite were calculated by the area under the exothermic peak in the DSC curves and presented in Table 1, which show the peak melting/solidification temperatures (Tm/TS) and melting/solidification enthalpies (ΔHm/ΔHs) between the stages of solid-liquid phase transition. Compared with the melting and solidification enthalpies data of paraffin and PP/Paraffin composite, a decrease in both melting and solidification enthalpies was clearly seen, which was similar to other PCM composites systems after incorporation of organic PCMs into porous support materials [18,39]. The ΔHm and ΔHs were respectively calculated to be 141.9 J g−1 and 149.9 J g−1, respectively. With these value, the phase change latent heat of the PP/Paraffin composite is higher than that of PEG2000/Ag [14], palmitic acid/SiO2 [36], paraffin/graphene oxide [40], and can compete with PEG/cellulose/GNP composites [18]. In order to investigate the thermal stability of PP/PCM composites,
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Fig. 6. The thermal conductivity of PP/Paraffin composite in different temperature.
Table 2 Thermal conductivity properties of some PCM composites in literature.
Fig. 5. The DSC curves for melting and freezing of (a) paraffin, PP/Paraffin composite and PP/Paraffin composite after 50 times of thermal cycles, (b) MA and PP/MA composite and (c) SA and PP/SA composite.
Paraffin PP/Paraffin PP/Paraffin50 MA PP/MA SA PP/SA
Melting process Tm (°C)
ΔHm (J g
58.5 60.6 58.7 59.5 56.1 59.0 56.9
168.4 141.9 141.4 200.2 159.9 185.6 143.3
)
Ts (°C)
ΔHs (J g
48.4 47.5 47.9 45.4 45.5 47.7 45.5
172 149.9 147.8 202.7 156.7 188.1 145.3
Ref.
MH-GA-P Paraffin/carbon fiber SA-ODE SA-ODE + 5%HBN 40% Al2O3/ESBR
0.248 0.5 0.298 0.317 0.26
[20] [42] [43] [43] [44]
Table 3 The PCMs content of various form-stable PCM composites.
Freezing process −1
Thermal conductivity (W m−1 K−1)
Fig. 7. The loading rate of PP/Paraffin composite after different times of melting/freezing cycles under operation.
Table 1 The DSC data of paraffin, PP/Paraffin composite, PP/Paraffin50, MA, SA, PP/SA and PP/ MA composites. Samples
Sample
−1
)
311
Composites
PCM content
Ref.
Erythritol/EG Paraffin/EG Gypsum/C18-C24 C-SiO2-PA C-SiO2-OD Paraffin/HDPE PEG/SiO2 MA-PA-SA/EG PP/Paraffin
15–20 vol% 10 wt% 18 wt% 61 wt% 73 wt% 77 wt% 80 wt% 92.86 wt% 900–1060 wt%
[45] [38] [6] [28] [28] [37] [22] [29] This work
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Compared with those reported porous materials for preparation of form-stable PCM composites which are usually capable of loading the PCMs in the range from ten percent to tens of percent (Table 3), our PP/ Paraffin composites are nearly two orders of magnitudes as high, which is of technological significance for their industrial applications. Furthermore, due to its superoleophilic wettability, the PP aerogel shows strong affinity to the loaded organic PCMs. It also can be seen that the loaded paraffin is very stable, only a slight loading loss of 9.7% was observed after 10 times of melting/freezing cycles. In the range of 10 times to 20 times, a smaller loading loss of 5.2% was measured. It should be noted that nearly no obvious leakage of organic PCMs was observed during the whole melting/freezing cycles even operated for many times. The main weight loss in this case is mostly caused by the contact loss during transferring procedure. These results also confirmed the strong affinity of PP aerogel to organic PCMs which should be attributed to its inherent superwetting properties [25]. In addition, PP aerogel is very stable in acid or base environments, so it can withstand harsh conditions during operation.
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4. Conclusions In summary, we have demonstrated the fabrication of superwetting PP aerogel, which was originated from bulk chemical materials or industrial PP based wastes, as the support materials for construction of form-stable PCMs composites. Due to its abundant porosity, light weight of aerogel in nature, inherent superhydrophobic and superoleophilic properties, lipophilic organic PCMs can be loaded into PP aerogel with an extremely high PCMs loading of up to 1060 wt%, which is nearly two orders of magnitude higher than those mostly reported form-stable PCMs systems. The PP/PCM composites show high latent heat in the range from 141.1 kJ kg−1 to 159.5 kJ kg−1 and excellent thermal stability and recyclability where their latent heat nearly remains unchanged even after 50 times of melting/freezing cycles. More interestingly, the PP/PCM composites show an enhanced thermal conductivity, in the case of PP/Paraffin composite, which is over two times of that of paraffin. Having the advantages of low cost and abundant resource of PP, simple fabrication process, high PCMs loading, good stability, recyclability and thermal conductivity, the PP/PCM composites may have great potential for renewable energy saving applications. Also, we suggest that such three-in-one strategy for preparation of aerogel materials with superwetting properties may open a new possibility for future design and creation of high performance form-stable PCMs composites, which should be of great importance in alleviating energy crisis.
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The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 51403092, 51462021 and 51663012 and 41361070), the Natural Science Foundation of Gansu Province, China (Grant No. 1610RJYA001), Support Program for Hongliu Young Teachers (Q201411), Hongliu Elitist Scholars of LUT (J201401), Support Program for Longyuan Youth and Fundamental Research Funds for the Universities of Gansu Province, and Program for Young Talent of State Ethnic Affairs Commission ([2016]57).
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