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Fabrication of highly efficient thermal energy storage composite from waste polystyrenes
Journal Pre-proofs Fabrication of highly efficient thermal energy storage composite from waste polystyrenes Changhui Liu, Xiaotian Ma, Peixing Du, Zhonghao Rao PII: DOI: Reference:
S0009-2509(20)30009-9 https://doi.org/10.1016/j.ces.2020.115477 CES 115477
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
21 October 2019 16 December 2019 3 January 2020
Please cite this article as: C. Liu, X. Ma, P. Du, Z. Rao, Fabrication of highly efficient thermal energy storage composite from waste polystyrenes, Chemical Engineering Science (2020), doi: https://doi.org/10.1016/j.ces. 2020.115477
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Fabrication of highly efficient thermal energy storage composite from waste polystyrenes Changhui Liu,a,b Xiaotian Ma,a,b Peixing Du,a,b Zhonghao Raoa,b a
Jiangsu Province Engineering Laboratory of High Efficient Energy Storage Technology and
Equipments, China University of Mining and Technology, Xuzhou 221116, China. [email protected]. b
Laboratory of Energy Storage and Heat Transfer, School of Electrical and Power Engineering,
China University of Mining and Technology, Xuzhou, 221116, China. Abstract: To high value-added utilization of waste polystyrene (PS) foam, an in-situ phase change material (PCM) encapsulation protocol was realized by using waste PS foam as a holding material for thermal energy storage with a shape-stabilization methodology. Mechanistic study suggested that the robust Lewis acid catalysed Friedel-Crafts reaction involved was the key to the success of this method owing to a simultaneous process of porous structure formation and PCM impregnation, by which the PCM encapsulation rate can attain to 68.7% without leakage. Lewis acid catalysts used in the process were able to be converted to their corresponding metal oxides via a simple alkali treatment, which not only diminished the tedious metal species isolation step but also made an external thermal conductivity enhancement that an utmost 61.0% thermal conductivity enhancement can be achieved compared with that of pristine paraffin wax. Additionally, the PCM composite preparation process can be facilely scaled up which strongly demonstrated its real application feasibility. Moreover, besides waste PS foam, other waste PS based materials were proven to be feasible towards PCM holding materials with the same approach. Introduction In China, plastic production has achieved to 9.8 million tons in 2017 with a 10.5% increasing rate,
and 60% of them were disposed as waste.(Hahladakis and Iacovidou, 2019) Polystyrene (PS) is of great importance in plastic family which was estimated to be produced 2.95 million tons annually (Merchant Research & Consulting, Expandable Polystyrene (EPS): 2017 World Market Outlook and Forecast up to 2027, UK, (2017))(Fonseca et al., 2015). Due to its low cost, low density, insulation, water repellent and chemical inertness, it has been intensively produced and applied in many areas, such as food packing material, building material and electronic appliances. Landfill and incineration are the two main ways to dispose the waste plastic.(Alassali et al., 2019; Yang et al., 2018b) While, landfill will bring a secondary pollution to the land as the waste plastic should take a long time to degrade in the absence of ultraviolet and microbes. Incineration process is a toxic gas emitting process which will also cause several air pollution and green house problems.(Chen and Huang, 2007; Mangalara and Varughese, 2016) Recycle or reuse of this material are problematic and little business interest owing to the relatively low economic return, which will inevitably cause world-wide pollution along with the increasing PS demand.(Kumar et al., 2018) Therefore, a sustainable processing technique which facilitates the recycle or reuse waste PS is highly desirable. As the advancement of chemistry and material science, many routes for high value-added reuse waste PS have been reported.(Chaukura et al., 2016; Rajaeifar et al., 2017; Ramanan et al., 2018; Zhao et al., 2019) On the one hand, it can be adopted as a raw material to produce adsorbent for some pollutants removal. For example, very recently, Liu et al. developed a chemically hypercrosslinked PS adsorbent derived from the waste PS. This material exhibited a good selectivity towards Cd over Na under a column adsorption condition.(Jia et al., 2019) The same group also found that the waste PS was able to be converted to an absorbent for carbon dioxide capture and separation with the aid of dichloromethane and AlCl3 as crosslinking reagent and catalyst
respectively.(Fu et al., 2017) Sun et al. applied waste PS as an absorbent for phenol removal from coking plant effluent, in which a concentrated sulfuric acid induced sulfonation was assumed to responsible for the exchange properties due to the grafting of sulfonate group on the skeleton of PS.(Sun et al., 2010) On the other hand, it can be converted into high value-added activated carbon through a high temperature pyrolysis. For example, Santamaría et al. prepared an activated carbon bearing an ultrahigh BET up to 2700 m2g-1, which thus has a good potential to be applied as a supercapacitor electrode.(de Paula et al., 2018) Chen et al. reported a pyrolysis synthetic method for converting waste PS to its corresponding porous carbon material for supercapacitor.(Zhang et al., 2018) Tian et al. investigated the combination of sewage sludge and waste PS with the aim of fabricating a hollow spherical sludge carbon. Further study revealed that this hollow spherical sphere can be easily recycled and efficiently regenerated by a simple thermal treatment.(Wu et al., 2015) In addition, waste PS has been proven to be feasible for the production of liquid fuel,(Shadangi and Mohanty, 2015) PS-derived membranes,(Zhuang et al., 2016) cement materials additive,(Eskander and Tawfik, 2011) microencapsulated phase change material (PCM),(Shao et al., 2018) graphite/expanded foam board,(Yang et al., 2018a) oxygen reduction reaction (ORR) catalyst supporting material,(You et al., 2014) photocatalyst for the degradation of rhodamine B,(de Assis et al., 2018) and so on. Despite various methodologies have been reported for reuse of waste PS, either high investment or less merits of the final product still makes the demand of a highly efficient and more economic strategy. Energy storage technique is of great significance for the alleviation of energy shortage risks.(Ager and Lapkin, 2018; Olabi, 2017) Thermal energy conservation has a crucial role in energy storage because it can solve the problems of time and spatial mismatch between thermal energy
supply and demand.(Yan and Yang, 2019) PCM, alternatively recognized as “latent heat-storage materials” have been intensively studied in various implements owing to its high energy storage density and chemical stability.(Liu and Rao, 2017; Lv et al., 2019) However, the leakage of liquid state during the phase change process, which will cause the loss of PCM, corrosion of container and volume change, heavily hampers its advancement.(Liu et al., 2015; Liu et al., 2017) Shapestabilization of PCM emerges as a feasible technology with the aim at addressing the leakage problem that has drawn vast attention and has been adopted in real applications.(Fang et al., 2014) In spite of many research regarding the preparation of shape-stabilization of PCM composite, the inevitable latent heat capacity dropping down derived from the blending of supporting material, which was unable to contribute the enthalpy, still hinders its rapid advancing pace. Among the numerous shape-stabilization strategies, physically blending a PCM with a porous structural supporting material has a key role owing to its convenient fabrication procedure.(Kenisarin and Kenisarina, 2012; Liu and Rao, 2017) However, the poor encapsulation owing to the surface tension between two individuals has a negative impact on its impregnation.(Qi et al., 2015) Out of this consideration, we suspected that a simultaneous impregnation and generation of porous structure in which the starting state of both PCM and precursors of supporting materials is homogeneous might result in a promising encapsulation efficiency because the mutual contact of the already formed heterogeneous interface can be avoided. Hyper-crosslinking reaction of polymers have an important role in the preparation of porous materials due to their simple making procedure and stable final material possessing micro or macro porous structure and high surface area.(Tan and Tan, 2017) They have been widely applied in gas storage,(Yang et al., 2016) pollutant isolation,(Vilela et al., 2012) catalysis(Li et al., 2012) etc., but the research on their using as holding material for shape-
stabilization of PCM has rarely been seen or fully studied.(Liu et al., 2019) Owing to the homogenous state of the starting materials and the final porous structure, we suspected that it could meet the requirements for the above mentioned in-situ PCM holding and thus satisfy a promising impregnation efficiency. In this work, we firstly adopted waste PS as supporting materials with the purpose of PCM shape-stabilization. Additionally, the impact of the dose of cross-linking reagent and catalyst was firstly considered in current work. Notably, besides waste PS foam, other types of PS based waste plastic materials were also investigated for understanding their thermal energy storage performance. Experimental section Paraffin (the melting temperature is 60 oC) was purchased from Joule wax Co. Ltd. Shanghai, China. Formaldehyde dimethyl acetal (FDA) was purchased from Aladdin chemical reagent Corp. (Shanghai, China) and used without further purification. 1,2-Dichloroethane (DCE), methanol, and anhydrous ferric chloride were purchased from National Medicines Corporation Ltd. China used as received. Elemental and density analysis with respect to various PS based plastic was listed in table S1. The other daily PS based materials used in this manuscript were obtained in cafeterias of China University of Mining and Technology. FT-IR spectra were recorded using a Bruker VERTEX 70 FT-IR as KBr discs. Thermal gravimetric analysis (TGA) was carried out on a TA SDT Q600 instrument under a nitrogen atmosphere by heating from room temperature to 800 °C at a rate of 10 °C min−1. Differential scanning calorimetry (DSC) was performed in a TA Q200 at a heating (or cooling) rate of 2 oC min1
under a nitrogen atmosphere. All the samples were heated to 80 oC, then cooled to 20 oC and
equilibrated for 10 min. X-ray diffraction (XRD) patterns were recorded on a diffractometer
(Smartlab, Rigaku) with Ni-filtered CuK α radiation (k = 0.154 nm) at a tube current of 30 mA and a generator voltage of 40 kV. Scanning was performed at a speed of 8 °C min−1, from 0 to 80° of 2θ. Thermal conductivity of the samples was measured by using a thermal conductivity meter (Hot Disk 2500-OT, Sweden), based on the transient plane source method, the testing temperature was adjusted and controlled by a water bath and an insulated chamber. X-ray photoelectron spectra (XPS) were recorded on a SHIMADZU-Kratos AXIS-ULTRA DLD-600W X-ray photoelectron spectrometer at a base pressure of 2 × 10−9 Pa in the analysis chamber using Al Kα radiation. Scanning electron microscopy (SEM) images were recorded using a FEI Sirion 200 field-emission scanning electron microscope operating at 10 kV. Transmission electron microscopy (TEM) were recorded using a FEI Tecnai G2 F20 equipped with an X-ray energy dispersive spectroscopy (EDS).The surface areas were calculated from nitrogen adsorption data by Brunauer-Emmett-Teller (BET) or Langmuir analysis. Pore size distributions were calculated by DFT methods from the adsorption branch. Magnetic analysis was conducted by using a commercially available magnet. Leakage proof performance was determined by recording the weigh difference between the original samples and being heated at 80 oC for 2 h, digital camera was also used to monitor the leakage phenomenon in this work. Uncertainty analysis is taken into account. Due to the uncertainty of the data acquisition device and thermocouples, the temperature measurement error range for each measuring point is 0.5 oC. The thermal conductivity analyzer has an accuracy of 3% at a working temperature from -200 to 150 oC. DSC has an accuracy of 0.01 oC and 0.1% with respect to temperature and enthalpy respectively at a working temperature from -180 to 725 oC. A typical preparation step for waste PS encapsulated PCM
The reaction was carried out in a 100 mL round bottom flask. Paraffin (10 g) was added into the flask and allowed to heat to 80 oC. DCE (25 mL) was added when paraffin was completely melting. Waste PS foam (1.5 g) was slowly added into the flask under the condition of magnetic stirring. Then FDA (3.2 mL) and anhydrous FeCl3 (1.16 g) were added sequentially after PS was completely dissolved. After stirring at 80 oC for 20 h, DCE was removed through an evaporation. Ethanol (25 mL) was added to disperse the resulted HCPs/paraffin composite and ammonia (3 mL) was then added at room temperature. The mixture was then allowed to stir at room temperature for 24 h. After the reaction, ethanol was removed by evaporation and the final PCM composite (12 g) was obtained as a dark brown powder after washing with water and drying in a vacuum oven at 80oC for 12 h. Results and discussion Initially, waste PS encapsulated PCM was fabricated referring a literature reported elsewhere.(Fu et al., 2017; Li et al., 2012) The preparation process was mainly constituted by dissolving in an organic solvent, hyper-crosslinking reaction, evaporative removal of organic solvent and ammonia treatment (Fig 1a).(Yang et al., 2016) A diagrammatic illustration about the encapsulation process was depicted in Fig. S1. Mechanism behind the reaction was illustrated in Fig 1b, briefly, the phenyl ring of PS skeleton reacted with FDA with the aid of FeCl3 to form an intermediate I which then reacted with the other PS unit to complete the hyper-crosslinking process. It should be mentioned that the hyper-crosslinking reaction, which attributes to the robustness and rapid kinetic of FriedelCrafts reaction, renders a large specific surface area and porous nanostructure with good stability. Meanwhile, to further improve the atomic economy, FeCl3, which used as a catalyst was in-situ converted to its corresponding iron oxide (Fig 1c). This operation not only omits the isolation process of Fe species but also makes an external thermal conductivity enhancement of the as-
obtained PCM composite thanks to the existence of iron oxide. In order to clarify the mechanism behind, two waste hyper-crosslinked polystyrene (WHCP) materials, WHCP1, fabricated in the absence of paraffin, and WHCP2, which was prepared in the presence of paraffin and washed completely with petroleum ether to remove paraffin, were submitted to BET and pore size analysis, as it is shown in table 1, there is no significant difference between these two kinds of WHCP materials, which demonstrates the fact that the presence of inertness paraffin does not have effect towards the Friedel-Crafts reaction between PS and FDA behind. On the other hand, the existence of iron oxide was confirmed by means of FT-IR (Fig. S2), XPS (Fig. S3) and magnetic analysis (Fig. S4). All of the evidences obtained strongly proved the feasibility of our catalyst in-situ reusing strategy. Table 1 Porous information of WHCP.
aWHCPs1:
Samplesa
BET surface area/(m2/g)
Pore volume /(cm3/g)
Average pore diameter /(nm)
WHCP1 WHCP2
328 331
0.25 0.25
2.89 2.91
prepared in the absence of paraffin; HCPs2: prepared in the presence of paraffin.
It is quite predictable that the amount of cross-linking reagent and catalyst will give a direct effect on the crosslinking rate and kinetics, which will then in turn determine the encapsulation rate and structure of PCM composite. Based on this consideration, different quantities of FDA and FeCl3 were screened to know the impact of dose of cross-linking reagent and catalyst on the performance of PCM composite. Leakage-proof ability is one of the most important parameters to evaluate the performance of PCM composite, hence, to facilitate discussion, we chose the leakage rate as the main parameter to know the performance of the PCM. At beginning, the model reaction parameters were set as 1.5 g waste PS foam, 10 g paraffin, 3.2 mL FDA (2.5 equiv) and 1.16 g anhydrous FeCl3 (0.5 equiv) in the solvent of 1,2-dichloromethane reacted at 80 oC for 24 h. As evidenced in table 2,
the leakage rate is 1.6% under the model reaction conditions (entry 1) and gradually raised with the decrease of FDA amount. The leakage rate is over 10% when 0.75 equivalent of FDA was employed. This phenomenon clearly reveals that the encapsulation rate has a great dependence on the amount of hyper-crosslinking reagent. The tendency is roughly followed a rule that more FDA used, the better. Further, our effort for the encapsulation of paraffin fell into failure when declined the amount of FDA to 0.5 equivalent. To further prove our hypothesis, we prepared PCM composite with a FDA amount more than the model one. These results demonstrated that the leakage rate can be further decreased to 0.8% and 0.2% when 2.75 and 3.0 equivalent of FDA was used as cross-linking reagent respectively (entries 7 and 8). Therefore, an assumption was achieved that the amount of FDA has a great impact on the cross-linking rate due to its effect on the Friedel-crafts induced hypercrosslinking reaction which thus determined the encapsulation capacity of the hyper-crosslinking PS. It should be noted that hyper-crosslinking reaction in this process plays a significant role in the preparation of waste PS encapsulated PCM since the pristine waste PS is unable to offer any positive effect on the supporting of melting paraffin as demonstrated in Fig. S5. After knowing the effect of cross-linker amount, the dose of FeCl3 was then considered. As shown in table 2, the leakage rate became much lower than the model condition that only 0.1% leakage of paraffin was observed after decreasing the amount of FeCl3 to 0.45 equivalent (entry 9). But, to our surprised, the PCM composite cannot be formed when a further lower amount of FeCl3 was employed due to the failure of the hyper-crosslinking reaction (entry 10). Based on this result, a rising amount of FeCl3 was examined. It was found that the leakage rate increased obviously in the cases of 0.55 and 0.6 equivalent of catalyst. We suspected the reason responsible for this phenomenon was divided into twofold. At the first, a larger amount of FeCl3 might lead to a faster
kinetic for cross-linking reaction that the cross-linking process occurred too rapidly to hold the paraffin in the reaction system. On the other hand, the existence of iron oxide which derived from FeCl3 will inevitably bring the clogging of the pore of the resulted hyper-crosslinked PS, which thus led to an inferior encapsulation rate. In addition, the leakage phenomenon was recorded by a digital camera, the wet part in the filter paper stands for the leakage of paraffin during the heating process. As it is displayed in table 3, the results obtained in a digital camera show a good agreement with the data calculated with a high accuracy balance.
a
b
c Figure 1. (a) Schematic illustration of preparation procedure, (b) proposed mechanism behind the PCM encapsulation process, (c) the formation mechanism for Fe3O4.
Table 2. Study on the different reaction parameters.a Sample
FDA (mL)
FDA
FeCl3 (g)
FeCl3 (equiv)
(equiv)
Leakage rate (%)
S1
3.2
2.5
1.16
0.5
1.6
S2
2.9
2.25
1.16
0.5
1.9
S3
2.6
2.0
1.16
0.5
2.3
S4
1.9
1.5
1.16
0.5
2.9
S5
1.0
0.75
1.16
0.5
10.2
S6
0.3
0.25
1.16
0.5
NO
S7
3.5
2.75
1.16
0.5
0.8
S8
3.8
3.0
1.16
0.5
0.2
S9
3.2
2.5
1.05
0.45
0.1
S10
3.2
2.5
0.93
0.4
NO
S11
3.2
2.5
1.28
0.55
5.8
S12
3.2
2.5
1.40
0.6
6.9
Paraffin:10 g, waste PS: 1.5 g, NH3·H2O: 5.0 equiv. to FeCl3, reaction temperature: 80 oC, NO:
a
not obtained. The structure of the as-prepared PCM composite was then subjected to analysis. As shown in Fig. 2, SEM images of the samples were displayed. WHCP exhibited a porous and amorphous structure (Fig. 2a) in the absence of paraffin, which was in a good accordance with the literature results.(Fu et al., 2017) The PCM composite features a sphere-like structure under the model reaction condition which can be ascribed to the impregnation or coating of paraffin in the pore or surface of WHCP.
The sphere phenomenon gradually fades away when a less amount of FDA was used (Fig. 2c to 2e). Almost no such kind of spherical structure can be found after decreasing the amount of FDA to 0.75 equivalent. It was indirectly indicated that the specific surface area of WHCP was declined owing to a lower crosslinking rate caused by a less dose of FDA. This phenomenon was also in line with the poor leakage-proof capacity of PCM composite in the case of S5. The importance of the quantity of FDA was further proven that the failure of PCM shape stabilization caused by an employment of further less dose of FDA (S6). Thus it can be concluded that this spherical shape demonstrated a fine encapsulation or coating of paraffin on the skeleton of hyper-crosslinking PS. The size of the spherical nanoparticle became smaller for S7, S8 and S9, probably caused by the lower hypercrosslinking kinetic derived from a lower catalyst concentration, which thus resulted in a superior encapsulation over the other samples. As shown in Fig. 2j to 2k, a rising the dose of FeCl3 would afforded some big stacking cubes, which had no contribution to the impregnation of PCM.
a
1 m
b
c
2 m
2 m
d
e
2 m
2 m 4
g
f
2 m
2 m
i
h
2 m
2 m
k
j
2 m
2 m
Figure 2. SEM images of waste PCM encapsulated PCM. (a) WHCP, (b) S1, (c) S2, (d) S3, (e) S4, (f) S5, (g) S7, (h) S8, (i) S9, (j) S11, (k) S12. Besides SEM image analysis, some other characterizations were subjected to test. As it is shown in Fig. 3a and 3b, FT-IR shows the main response of the paraffin in the composite materials, where the peaks at around 2900 cm−1 and 2850 cm−1 are ascribed to the –CH2 asymmetrical stretching vibration and –CH2 symmetrical stretching vibration, respectively. The peaks at 1465 cm−1 and 720 cm−1 stand for the rocking vibration of –CH2 group and the peak at 1377 cm−1 comes from the deformation vibration of –CH3.(Wu et al., 2019) It is found that peaks of samples weakene to some extend, proves that WHCP is able to adsorb infrared light, which thus results in a weaker response of paraffin component. In addition, no extra peaks were observed except for the response of HCP and paraffin in FT-IR and XRD patterns, which indicated that no further chemical reaction occurred in the system between the supporting materials and paraffin, which makes the feasibility of this insitu one-pot hyper-crosslinking and paraffin encapsulation strategy thanks to the inertness of
paraffin towards the acid aided Friedel-Crafts reaction. It is worth noting that the lattice structure of paraffin has no obvious change that further demonstrates the good encapsulation efficiency of our in-situ PCM shape-stabilization strategy. TGA analysis was also subjected to consideration for the above samples (Fig. 3c), it was obvious that the main weight loss of the sample was constituted by two aspects, the holding material and PCM material, which proved the generation of HCP material even in the presence of paraffin and also excluded the occurrence of the other side-reactions during this process in the same time. On the other hand, it is obvious that the weight loss temperature corresponds to paraffin is delayed for the shape stabilized PCM composite due to the protection effect of organic WHCP materials, which is able to endow a good affinity to organic PCM. Table 3. Leakage proof performance of the as-prepared PCM composites. Sample
Paraffin
S1
S2
S3
S4
0 min
5 min
10 min
20 min
40 min
S5
S7
S8
S9
S11
S12
a
b
10
c
20
WHCP S3 S8
30
40
Paraffin S4 S9
50
2 (degrees)
60
S1 S5 S11
70
S2 S7 S12
80
Paraffin S4 S9
0.4
S1 S5 S11
S2 S7 S12
b
S3 S8
2
Heat flow (W/g)
a
Thermal conductivity (W/m·K)
Figure 3. Characterization of the as-prepared PCM composites, (a) FT-IR, (b) XRD, (c) TGA.
0.3
0.2
0.1
1 100 200
1
0
-1
300 500
-2 0.0 25
30
35
40
45 o
Temperature ( C)
c
50
55
60
20
30
40
50
o
60
Temperature ( C)
70
80
Figure 4. (a) Thermal conductivity of the as-prepared PCM composites, (b) the measured latent heat of S9 during 500 melting-freezing cycles, (c) bearing the compression by a 500 grams force at 70 °C. After determination the structure, thermo-physical properties with respect to the as-prepared materials were studied. Thermal conductivity is one of the most important parameters to evaluate the performance of PCM composite as it heavily determines the heat transfer efficiency. As shown in Fig. 4a, thermal conductivity of the samples as a function of temperature was summarized in a diagram. Firstly, it can be verified that there is a little dependence of thermal conductivity upon the testing temperature that most of samples lay on a range from 0.2 W/m·K to 0.3 W/m·K regardless of testing temperature. Another clear trend can be concluded that S11, S12 bear a relatively superior thermal conductivity over the other samples presumably owing to a higher iron oxide concentration. Moreover, the samples exhibit a higher thermal conductivity at a vicinal phase change temperature, it can be ascribed to the thermal conductive resistance decrease owing to the air discharging effect led by the volume swelling of phase change process. It is worth noting that thermal conductivity of samples features an obvious enhancement compared with that of pure paraffin thanks to the existence of the iron oxide. Phase change temperature and latent heat enthalpy of the as-prepared PCM composites were listed in table 4. It was found that incorporation of paraffin into the network of the hyper-crosslinking PS led to a slight phase change variation, in which the melting peak temperature and the freezing peak temperature shifted from 63.28 oC and 60.69 oC to 56.70 oC and 48.98 oC for S9, respectively. This phenomenon is presumably attributed to the strong affinity of hyper-crosslinking PS to the paraffin molecules, which thus impacts the vicinal organic molecules. Moreover, as it is evidenced in Fig. 4b, energy storage and release stability of this waste PS
encapsulated PCM with respect to both phase change temperature and latent heat is much promising that both of them kept almost unchanged after 500 melting and freezing because of the good affinity between the paraffin wax and this kind of organic based supporting material. Compared with already reported results on stability of shape-stabilized PCM composite,(Maleki et al., 2019; Tang et al., 2017; Yang et al., 2017) our material shows superiority that the variation regarding phase change temperature is less than 0.2 oC, the difference of latent heat is also less than 2.0% during the 500 heating/cooling phase transitions, which further demonstrates the advantageous of our shapestabilization strategy. Additionally, compared with pure paraffin, the mechanic performance exhibits a significant enhancement owing to the stability of the hyper-crosslinking network. As it is shown in Fig. 4c, this WHCP encapsulated PCM composite is well-preserved after a 500 g grams force compression at a temperature higher than the melting point of paraffin, while, the shape of paraffin collapsed completely under the same condition. Table 4. Phase change temperature and latent heat of the as-prepared PCM composites. Sample
Melting process
Freezing process
Onset (oC)
Peak (oC)
Hm (J/g)
Onset (oC)
Peak (oC)
Hf (J/g)
Paraffin
48.18
63.28
175.88
63.31
60.69
175.53
S1
50.90
56.52
106.58
54.05
50.93
106.38
S2
51.45
58.76
132.23
53.73
47.72
128.33
S3
51.45
56.74
110.51
53.85
50.55
110.56
S4
51.34
57.97
131.48
53.80
49.09
126.28
S5
51.47
60.48
102.64
53.16
46.26
107.83
S7
51.24
56.45
106.19
53.95
50.15
106.52
S8
49.70
55.55
111.30
54.20
51.98
111.96
S9
50.86
56.70
120.71
53.83
48.98
120.53
S11
51.10
58.70
140.01
53.34
47.09
140.19
S12
51.17
58.44
152.32
53.62
47.52
152.21
Figure 5. Large scale synthesis of PCM composite from waste PS foam. In order to demonstrate the real application of our waste PS encapsulation method and verify the practical use of this wastes PS reusing protocol, a large lab scale synthesis experiment was conducted. As it is displayed in Fig. 5, the main preparation steps, both in-situ encapsulation, evaporation and ammonia treatment can be easily dealt in a 250 g lab scale thanks to the simplicity of the operation procedure and robustness of the organic reaction behind. Importantly, no emission or release were involved in the whole process because the solvent 1,2-dichloromethane was facilely
recycled through a laboratory set vacuum evaporator and the catalyst FeCl3 was converted into iron oxide for thermal conductivity enhancement. Moreover, the final obtained PCM composite exhibits good thermo-physical properties in terms of energy storage capacity and efficiency. Thus, this waste PS recycling protocol really implies a promising sustainability, a good atomic economy and paves an avenue for highly efficient reuse of PS based plastic. Finally, to further extend the reuse scope of our waste PS strategy, some other PS made waste plastics were subjected to examination under the optimized preparation condition. As it is shown in table 5, five different kinds of waste PS materials, namely, disposable dish container, disposable cup, disposable milk tea cap, disposable tableware and plastic tea pot can be used as holding materials with the aid of catalytic amount of FeCl3 without significant paraffin leakage. Among these waste PS, plastic tea pot afforded the outstanding performance in terms of encapsulation rate. The reason behind might ascribe to the higher purity than the other materials. The milk tea cap exhibited the poorest encapsulation rate due to the existence of some unreacted impurities (55.8%). It is noteworthy that the successful preparation of different kinds of PS waste materials encapsulated PCM further evidences the robustness and feasibility of our protocol. Therefore, this recycle methodology indeed provides a new approach for highly efficient and practical reusing waste PS materials. Table 5. Preparation of form-stable PCM from waste PS plastic. Disposable
Disposable
Disposable
Disposable
Plastic tea
dish
cup
milk tea cap
tableware
pot
container
Real application Raw material in this work Photograph of sample Melting
5 cm
5 cm
5 cm
5 cm
5 cm
108.7
128.6
98.1
113.2
133.7
0.235
0.228
0.229
0.237
0.259
61.8
73.1
55.8
64.4
76.0
0.9
1.1
1.0
1.3
1.5
latent heat (J/g) Thermal conductivity (W/m·K) Encapsulatio n rate (%) Leakage rate (%)
Conclusion In conclusion, we developed a facile approach to reuse waste PS foam as a holding material for PCM shape-stabilization with a 68.7% encapsulation rate without observation of leakage by using an FeCl3 catalyzed hyper-crosslinking process. FeCl3 used in this process was elaborately converted to its corresponding Fe3O4 for an extra thermal conductivity enhancement, which not only omitted
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Highlights 1.
Hypercrosslinking reaction enabled phase change material encapsulation protocol.
2.
Waste polystyrenes were used as starting materials.
3.
Catalysts were converted to metal oxide for thermal conductivity enhancement.
4.
The preparation process can be facilely scaled up.
Author contributions Changhui Liu conceived the project and designed the experiment. Xiaotian Ma and Peixing Du conducted the experiment. Changhui Liu and Zhonghao Rao analyzed the data and discussed the results. Changhui Liu wrote the manuscript.
Conflicts of interest There are no conflicts to declare.
S0009-2509(20)30009-9 https://doi.org/10.1016/j.ces.2020.115477 CES 115477
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Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
21 October 2019 16 December 2019 3 January 2020
Please cite this article as: C. Liu, X. Ma, P. Du, Z. Rao, Fabrication of highly efficient thermal energy storage composite from waste polystyrenes, Chemical Engineering Science (2020), doi: https://doi.org/10.1016/j.ces. 2020.115477
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Fabrication of highly efficient thermal energy storage composite from waste polystyrenes Changhui Liu,a,b Xiaotian Ma,a,b Peixing Du,a,b Zhonghao Raoa,b a
Jiangsu Province Engineering Laboratory of High Efficient Energy Storage Technology and
Equipments, China University of Mining and Technology, Xuzhou 221116, China. [email protected]. b
Laboratory of Energy Storage and Heat Transfer, School of Electrical and Power Engineering,
China University of Mining and Technology, Xuzhou, 221116, China. Abstract: To high value-added utilization of waste polystyrene (PS) foam, an in-situ phase change material (PCM) encapsulation protocol was realized by using waste PS foam as a holding material for thermal energy storage with a shape-stabilization methodology. Mechanistic study suggested that the robust Lewis acid catalysed Friedel-Crafts reaction involved was the key to the success of this method owing to a simultaneous process of porous structure formation and PCM impregnation, by which the PCM encapsulation rate can attain to 68.7% without leakage. Lewis acid catalysts used in the process were able to be converted to their corresponding metal oxides via a simple alkali treatment, which not only diminished the tedious metal species isolation step but also made an external thermal conductivity enhancement that an utmost 61.0% thermal conductivity enhancement can be achieved compared with that of pristine paraffin wax. Additionally, the PCM composite preparation process can be facilely scaled up which strongly demonstrated its real application feasibility. Moreover, besides waste PS foam, other waste PS based materials were proven to be feasible towards PCM holding materials with the same approach. Introduction In China, plastic production has achieved to 9.8 million tons in 2017 with a 10.5% increasing rate,
and 60% of them were disposed as waste.(Hahladakis and Iacovidou, 2019) Polystyrene (PS) is of great importance in plastic family which was estimated to be produced 2.95 million tons annually (Merchant Research & Consulting, Expandable Polystyrene (EPS): 2017 World Market Outlook and Forecast up to 2027, UK, (2017))(Fonseca et al., 2015). Due to its low cost, low density, insulation, water repellent and chemical inertness, it has been intensively produced and applied in many areas, such as food packing material, building material and electronic appliances. Landfill and incineration are the two main ways to dispose the waste plastic.(Alassali et al., 2019; Yang et al., 2018b) While, landfill will bring a secondary pollution to the land as the waste plastic should take a long time to degrade in the absence of ultraviolet and microbes. Incineration process is a toxic gas emitting process which will also cause several air pollution and green house problems.(Chen and Huang, 2007; Mangalara and Varughese, 2016) Recycle or reuse of this material are problematic and little business interest owing to the relatively low economic return, which will inevitably cause world-wide pollution along with the increasing PS demand.(Kumar et al., 2018) Therefore, a sustainable processing technique which facilitates the recycle or reuse waste PS is highly desirable. As the advancement of chemistry and material science, many routes for high value-added reuse waste PS have been reported.(Chaukura et al., 2016; Rajaeifar et al., 2017; Ramanan et al., 2018; Zhao et al., 2019) On the one hand, it can be adopted as a raw material to produce adsorbent for some pollutants removal. For example, very recently, Liu et al. developed a chemically hypercrosslinked PS adsorbent derived from the waste PS. This material exhibited a good selectivity towards Cd over Na under a column adsorption condition.(Jia et al., 2019) The same group also found that the waste PS was able to be converted to an absorbent for carbon dioxide capture and separation with the aid of dichloromethane and AlCl3 as crosslinking reagent and catalyst
respectively.(Fu et al., 2017) Sun et al. applied waste PS as an absorbent for phenol removal from coking plant effluent, in which a concentrated sulfuric acid induced sulfonation was assumed to responsible for the exchange properties due to the grafting of sulfonate group on the skeleton of PS.(Sun et al., 2010) On the other hand, it can be converted into high value-added activated carbon through a high temperature pyrolysis. For example, Santamaría et al. prepared an activated carbon bearing an ultrahigh BET up to 2700 m2g-1, which thus has a good potential to be applied as a supercapacitor electrode.(de Paula et al., 2018) Chen et al. reported a pyrolysis synthetic method for converting waste PS to its corresponding porous carbon material for supercapacitor.(Zhang et al., 2018) Tian et al. investigated the combination of sewage sludge and waste PS with the aim of fabricating a hollow spherical sludge carbon. Further study revealed that this hollow spherical sphere can be easily recycled and efficiently regenerated by a simple thermal treatment.(Wu et al., 2015) In addition, waste PS has been proven to be feasible for the production of liquid fuel,(Shadangi and Mohanty, 2015) PS-derived membranes,(Zhuang et al., 2016) cement materials additive,(Eskander and Tawfik, 2011) microencapsulated phase change material (PCM),(Shao et al., 2018) graphite/expanded foam board,(Yang et al., 2018a) oxygen reduction reaction (ORR) catalyst supporting material,(You et al., 2014) photocatalyst for the degradation of rhodamine B,(de Assis et al., 2018) and so on. Despite various methodologies have been reported for reuse of waste PS, either high investment or less merits of the final product still makes the demand of a highly efficient and more economic strategy. Energy storage technique is of great significance for the alleviation of energy shortage risks.(Ager and Lapkin, 2018; Olabi, 2017) Thermal energy conservation has a crucial role in energy storage because it can solve the problems of time and spatial mismatch between thermal energy
supply and demand.(Yan and Yang, 2019) PCM, alternatively recognized as “latent heat-storage materials” have been intensively studied in various implements owing to its high energy storage density and chemical stability.(Liu and Rao, 2017; Lv et al., 2019) However, the leakage of liquid state during the phase change process, which will cause the loss of PCM, corrosion of container and volume change, heavily hampers its advancement.(Liu et al., 2015; Liu et al., 2017) Shapestabilization of PCM emerges as a feasible technology with the aim at addressing the leakage problem that has drawn vast attention and has been adopted in real applications.(Fang et al., 2014) In spite of many research regarding the preparation of shape-stabilization of PCM composite, the inevitable latent heat capacity dropping down derived from the blending of supporting material, which was unable to contribute the enthalpy, still hinders its rapid advancing pace. Among the numerous shape-stabilization strategies, physically blending a PCM with a porous structural supporting material has a key role owing to its convenient fabrication procedure.(Kenisarin and Kenisarina, 2012; Liu and Rao, 2017) However, the poor encapsulation owing to the surface tension between two individuals has a negative impact on its impregnation.(Qi et al., 2015) Out of this consideration, we suspected that a simultaneous impregnation and generation of porous structure in which the starting state of both PCM and precursors of supporting materials is homogeneous might result in a promising encapsulation efficiency because the mutual contact of the already formed heterogeneous interface can be avoided. Hyper-crosslinking reaction of polymers have an important role in the preparation of porous materials due to their simple making procedure and stable final material possessing micro or macro porous structure and high surface area.(Tan and Tan, 2017) They have been widely applied in gas storage,(Yang et al., 2016) pollutant isolation,(Vilela et al., 2012) catalysis(Li et al., 2012) etc., but the research on their using as holding material for shape-
stabilization of PCM has rarely been seen or fully studied.(Liu et al., 2019) Owing to the homogenous state of the starting materials and the final porous structure, we suspected that it could meet the requirements for the above mentioned in-situ PCM holding and thus satisfy a promising impregnation efficiency. In this work, we firstly adopted waste PS as supporting materials with the purpose of PCM shape-stabilization. Additionally, the impact of the dose of cross-linking reagent and catalyst was firstly considered in current work. Notably, besides waste PS foam, other types of PS based waste plastic materials were also investigated for understanding their thermal energy storage performance. Experimental section Paraffin (the melting temperature is 60 oC) was purchased from Joule wax Co. Ltd. Shanghai, China. Formaldehyde dimethyl acetal (FDA) was purchased from Aladdin chemical reagent Corp. (Shanghai, China) and used without further purification. 1,2-Dichloroethane (DCE), methanol, and anhydrous ferric chloride were purchased from National Medicines Corporation Ltd. China used as received. Elemental and density analysis with respect to various PS based plastic was listed in table S1. The other daily PS based materials used in this manuscript were obtained in cafeterias of China University of Mining and Technology. FT-IR spectra were recorded using a Bruker VERTEX 70 FT-IR as KBr discs. Thermal gravimetric analysis (TGA) was carried out on a TA SDT Q600 instrument under a nitrogen atmosphere by heating from room temperature to 800 °C at a rate of 10 °C min−1. Differential scanning calorimetry (DSC) was performed in a TA Q200 at a heating (or cooling) rate of 2 oC min1
under a nitrogen atmosphere. All the samples were heated to 80 oC, then cooled to 20 oC and
equilibrated for 10 min. X-ray diffraction (XRD) patterns were recorded on a diffractometer
(Smartlab, Rigaku) with Ni-filtered CuK α radiation (k = 0.154 nm) at a tube current of 30 mA and a generator voltage of 40 kV. Scanning was performed at a speed of 8 °C min−1, from 0 to 80° of 2θ. Thermal conductivity of the samples was measured by using a thermal conductivity meter (Hot Disk 2500-OT, Sweden), based on the transient plane source method, the testing temperature was adjusted and controlled by a water bath and an insulated chamber. X-ray photoelectron spectra (XPS) were recorded on a SHIMADZU-Kratos AXIS-ULTRA DLD-600W X-ray photoelectron spectrometer at a base pressure of 2 × 10−9 Pa in the analysis chamber using Al Kα radiation. Scanning electron microscopy (SEM) images were recorded using a FEI Sirion 200 field-emission scanning electron microscope operating at 10 kV. Transmission electron microscopy (TEM) were recorded using a FEI Tecnai G2 F20 equipped with an X-ray energy dispersive spectroscopy (EDS).The surface areas were calculated from nitrogen adsorption data by Brunauer-Emmett-Teller (BET) or Langmuir analysis. Pore size distributions were calculated by DFT methods from the adsorption branch. Magnetic analysis was conducted by using a commercially available magnet. Leakage proof performance was determined by recording the weigh difference between the original samples and being heated at 80 oC for 2 h, digital camera was also used to monitor the leakage phenomenon in this work. Uncertainty analysis is taken into account. Due to the uncertainty of the data acquisition device and thermocouples, the temperature measurement error range for each measuring point is 0.5 oC. The thermal conductivity analyzer has an accuracy of 3% at a working temperature from -200 to 150 oC. DSC has an accuracy of 0.01 oC and 0.1% with respect to temperature and enthalpy respectively at a working temperature from -180 to 725 oC. A typical preparation step for waste PS encapsulated PCM
The reaction was carried out in a 100 mL round bottom flask. Paraffin (10 g) was added into the flask and allowed to heat to 80 oC. DCE (25 mL) was added when paraffin was completely melting. Waste PS foam (1.5 g) was slowly added into the flask under the condition of magnetic stirring. Then FDA (3.2 mL) and anhydrous FeCl3 (1.16 g) were added sequentially after PS was completely dissolved. After stirring at 80 oC for 20 h, DCE was removed through an evaporation. Ethanol (25 mL) was added to disperse the resulted HCPs/paraffin composite and ammonia (3 mL) was then added at room temperature. The mixture was then allowed to stir at room temperature for 24 h. After the reaction, ethanol was removed by evaporation and the final PCM composite (12 g) was obtained as a dark brown powder after washing with water and drying in a vacuum oven at 80oC for 12 h. Results and discussion Initially, waste PS encapsulated PCM was fabricated referring a literature reported elsewhere.(Fu et al., 2017; Li et al., 2012) The preparation process was mainly constituted by dissolving in an organic solvent, hyper-crosslinking reaction, evaporative removal of organic solvent and ammonia treatment (Fig 1a).(Yang et al., 2016) A diagrammatic illustration about the encapsulation process was depicted in Fig. S1. Mechanism behind the reaction was illustrated in Fig 1b, briefly, the phenyl ring of PS skeleton reacted with FDA with the aid of FeCl3 to form an intermediate I which then reacted with the other PS unit to complete the hyper-crosslinking process. It should be mentioned that the hyper-crosslinking reaction, which attributes to the robustness and rapid kinetic of FriedelCrafts reaction, renders a large specific surface area and porous nanostructure with good stability. Meanwhile, to further improve the atomic economy, FeCl3, which used as a catalyst was in-situ converted to its corresponding iron oxide (Fig 1c). This operation not only omits the isolation process of Fe species but also makes an external thermal conductivity enhancement of the as-
obtained PCM composite thanks to the existence of iron oxide. In order to clarify the mechanism behind, two waste hyper-crosslinked polystyrene (WHCP) materials, WHCP1, fabricated in the absence of paraffin, and WHCP2, which was prepared in the presence of paraffin and washed completely with petroleum ether to remove paraffin, were submitted to BET and pore size analysis, as it is shown in table 1, there is no significant difference between these two kinds of WHCP materials, which demonstrates the fact that the presence of inertness paraffin does not have effect towards the Friedel-Crafts reaction between PS and FDA behind. On the other hand, the existence of iron oxide was confirmed by means of FT-IR (Fig. S2), XPS (Fig. S3) and magnetic analysis (Fig. S4). All of the evidences obtained strongly proved the feasibility of our catalyst in-situ reusing strategy. Table 1 Porous information of WHCP.
aWHCPs1:
Samplesa
BET surface area/(m2/g)
Pore volume /(cm3/g)
Average pore diameter /(nm)
WHCP1 WHCP2
328 331
0.25 0.25
2.89 2.91
prepared in the absence of paraffin; HCPs2: prepared in the presence of paraffin.
It is quite predictable that the amount of cross-linking reagent and catalyst will give a direct effect on the crosslinking rate and kinetics, which will then in turn determine the encapsulation rate and structure of PCM composite. Based on this consideration, different quantities of FDA and FeCl3 were screened to know the impact of dose of cross-linking reagent and catalyst on the performance of PCM composite. Leakage-proof ability is one of the most important parameters to evaluate the performance of PCM composite, hence, to facilitate discussion, we chose the leakage rate as the main parameter to know the performance of the PCM. At beginning, the model reaction parameters were set as 1.5 g waste PS foam, 10 g paraffin, 3.2 mL FDA (2.5 equiv) and 1.16 g anhydrous FeCl3 (0.5 equiv) in the solvent of 1,2-dichloromethane reacted at 80 oC for 24 h. As evidenced in table 2,
the leakage rate is 1.6% under the model reaction conditions (entry 1) and gradually raised with the decrease of FDA amount. The leakage rate is over 10% when 0.75 equivalent of FDA was employed. This phenomenon clearly reveals that the encapsulation rate has a great dependence on the amount of hyper-crosslinking reagent. The tendency is roughly followed a rule that more FDA used, the better. Further, our effort for the encapsulation of paraffin fell into failure when declined the amount of FDA to 0.5 equivalent. To further prove our hypothesis, we prepared PCM composite with a FDA amount more than the model one. These results demonstrated that the leakage rate can be further decreased to 0.8% and 0.2% when 2.75 and 3.0 equivalent of FDA was used as cross-linking reagent respectively (entries 7 and 8). Therefore, an assumption was achieved that the amount of FDA has a great impact on the cross-linking rate due to its effect on the Friedel-crafts induced hypercrosslinking reaction which thus determined the encapsulation capacity of the hyper-crosslinking PS. It should be noted that hyper-crosslinking reaction in this process plays a significant role in the preparation of waste PS encapsulated PCM since the pristine waste PS is unable to offer any positive effect on the supporting of melting paraffin as demonstrated in Fig. S5. After knowing the effect of cross-linker amount, the dose of FeCl3 was then considered. As shown in table 2, the leakage rate became much lower than the model condition that only 0.1% leakage of paraffin was observed after decreasing the amount of FeCl3 to 0.45 equivalent (entry 9). But, to our surprised, the PCM composite cannot be formed when a further lower amount of FeCl3 was employed due to the failure of the hyper-crosslinking reaction (entry 10). Based on this result, a rising amount of FeCl3 was examined. It was found that the leakage rate increased obviously in the cases of 0.55 and 0.6 equivalent of catalyst. We suspected the reason responsible for this phenomenon was divided into twofold. At the first, a larger amount of FeCl3 might lead to a faster
kinetic for cross-linking reaction that the cross-linking process occurred too rapidly to hold the paraffin in the reaction system. On the other hand, the existence of iron oxide which derived from FeCl3 will inevitably bring the clogging of the pore of the resulted hyper-crosslinked PS, which thus led to an inferior encapsulation rate. In addition, the leakage phenomenon was recorded by a digital camera, the wet part in the filter paper stands for the leakage of paraffin during the heating process. As it is displayed in table 3, the results obtained in a digital camera show a good agreement with the data calculated with a high accuracy balance.
a
b
c Figure 1. (a) Schematic illustration of preparation procedure, (b) proposed mechanism behind the PCM encapsulation process, (c) the formation mechanism for Fe3O4.
Table 2. Study on the different reaction parameters.a Sample
FDA (mL)
FDA
FeCl3 (g)
FeCl3 (equiv)
(equiv)
Leakage rate (%)
S1
3.2
2.5
1.16
0.5
1.6
S2
2.9
2.25
1.16
0.5
1.9
S3
2.6
2.0
1.16
0.5
2.3
S4
1.9
1.5
1.16
0.5
2.9
S5
1.0
0.75
1.16
0.5
10.2
S6
0.3
0.25
1.16
0.5
NO
S7
3.5
2.75
1.16
0.5
0.8
S8
3.8
3.0
1.16
0.5
0.2
S9
3.2
2.5
1.05
0.45
0.1
S10
3.2
2.5
0.93
0.4
NO
S11
3.2
2.5
1.28
0.55
5.8
S12
3.2
2.5
1.40
0.6
6.9
Paraffin:10 g, waste PS: 1.5 g, NH3·H2O: 5.0 equiv. to FeCl3, reaction temperature: 80 oC, NO:
a
not obtained. The structure of the as-prepared PCM composite was then subjected to analysis. As shown in Fig. 2, SEM images of the samples were displayed. WHCP exhibited a porous and amorphous structure (Fig. 2a) in the absence of paraffin, which was in a good accordance with the literature results.(Fu et al., 2017) The PCM composite features a sphere-like structure under the model reaction condition which can be ascribed to the impregnation or coating of paraffin in the pore or surface of WHCP.
The sphere phenomenon gradually fades away when a less amount of FDA was used (Fig. 2c to 2e). Almost no such kind of spherical structure can be found after decreasing the amount of FDA to 0.75 equivalent. It was indirectly indicated that the specific surface area of WHCP was declined owing to a lower crosslinking rate caused by a less dose of FDA. This phenomenon was also in line with the poor leakage-proof capacity of PCM composite in the case of S5. The importance of the quantity of FDA was further proven that the failure of PCM shape stabilization caused by an employment of further less dose of FDA (S6). Thus it can be concluded that this spherical shape demonstrated a fine encapsulation or coating of paraffin on the skeleton of hyper-crosslinking PS. The size of the spherical nanoparticle became smaller for S7, S8 and S9, probably caused by the lower hypercrosslinking kinetic derived from a lower catalyst concentration, which thus resulted in a superior encapsulation over the other samples. As shown in Fig. 2j to 2k, a rising the dose of FeCl3 would afforded some big stacking cubes, which had no contribution to the impregnation of PCM.
a
1 m
b
c
2 m
2 m
d
e
2 m
2 m 4
g
f
2 m
2 m
i
h
2 m
2 m
k
j
2 m
2 m
Figure 2. SEM images of waste PCM encapsulated PCM. (a) WHCP, (b) S1, (c) S2, (d) S3, (e) S4, (f) S5, (g) S7, (h) S8, (i) S9, (j) S11, (k) S12. Besides SEM image analysis, some other characterizations were subjected to test. As it is shown in Fig. 3a and 3b, FT-IR shows the main response of the paraffin in the composite materials, where the peaks at around 2900 cm−1 and 2850 cm−1 are ascribed to the –CH2 asymmetrical stretching vibration and –CH2 symmetrical stretching vibration, respectively. The peaks at 1465 cm−1 and 720 cm−1 stand for the rocking vibration of –CH2 group and the peak at 1377 cm−1 comes from the deformation vibration of –CH3.(Wu et al., 2019) It is found that peaks of samples weakene to some extend, proves that WHCP is able to adsorb infrared light, which thus results in a weaker response of paraffin component. In addition, no extra peaks were observed except for the response of HCP and paraffin in FT-IR and XRD patterns, which indicated that no further chemical reaction occurred in the system between the supporting materials and paraffin, which makes the feasibility of this insitu one-pot hyper-crosslinking and paraffin encapsulation strategy thanks to the inertness of
paraffin towards the acid aided Friedel-Crafts reaction. It is worth noting that the lattice structure of paraffin has no obvious change that further demonstrates the good encapsulation efficiency of our in-situ PCM shape-stabilization strategy. TGA analysis was also subjected to consideration for the above samples (Fig. 3c), it was obvious that the main weight loss of the sample was constituted by two aspects, the holding material and PCM material, which proved the generation of HCP material even in the presence of paraffin and also excluded the occurrence of the other side-reactions during this process in the same time. On the other hand, it is obvious that the weight loss temperature corresponds to paraffin is delayed for the shape stabilized PCM composite due to the protection effect of organic WHCP materials, which is able to endow a good affinity to organic PCM. Table 3. Leakage proof performance of the as-prepared PCM composites. Sample
Paraffin
S1
S2
S3
S4
0 min
5 min
10 min
20 min
40 min
S5
S7
S8
S9
S11
S12
a
b
10
c
20
WHCP S3 S8
30
40
Paraffin S4 S9
50
2 (degrees)
60
S1 S5 S11
70
S2 S7 S12
80
Paraffin S4 S9
0.4
S1 S5 S11
S2 S7 S12
b
S3 S8
2
Heat flow (W/g)
a
Thermal conductivity (W/m·K)
Figure 3. Characterization of the as-prepared PCM composites, (a) FT-IR, (b) XRD, (c) TGA.
0.3
0.2
0.1
1 100 200
1
0
-1
300 500
-2 0.0 25
30
35
40
45 o
Temperature ( C)
c
50
55
60
20
30
40
50
o
60
Temperature ( C)
70
80
Figure 4. (a) Thermal conductivity of the as-prepared PCM composites, (b) the measured latent heat of S9 during 500 melting-freezing cycles, (c) bearing the compression by a 500 grams force at 70 °C. After determination the structure, thermo-physical properties with respect to the as-prepared materials were studied. Thermal conductivity is one of the most important parameters to evaluate the performance of PCM composite as it heavily determines the heat transfer efficiency. As shown in Fig. 4a, thermal conductivity of the samples as a function of temperature was summarized in a diagram. Firstly, it can be verified that there is a little dependence of thermal conductivity upon the testing temperature that most of samples lay on a range from 0.2 W/m·K to 0.3 W/m·K regardless of testing temperature. Another clear trend can be concluded that S11, S12 bear a relatively superior thermal conductivity over the other samples presumably owing to a higher iron oxide concentration. Moreover, the samples exhibit a higher thermal conductivity at a vicinal phase change temperature, it can be ascribed to the thermal conductive resistance decrease owing to the air discharging effect led by the volume swelling of phase change process. It is worth noting that thermal conductivity of samples features an obvious enhancement compared with that of pure paraffin thanks to the existence of the iron oxide. Phase change temperature and latent heat enthalpy of the as-prepared PCM composites were listed in table 4. It was found that incorporation of paraffin into the network of the hyper-crosslinking PS led to a slight phase change variation, in which the melting peak temperature and the freezing peak temperature shifted from 63.28 oC and 60.69 oC to 56.70 oC and 48.98 oC for S9, respectively. This phenomenon is presumably attributed to the strong affinity of hyper-crosslinking PS to the paraffin molecules, which thus impacts the vicinal organic molecules. Moreover, as it is evidenced in Fig. 4b, energy storage and release stability of this waste PS
encapsulated PCM with respect to both phase change temperature and latent heat is much promising that both of them kept almost unchanged after 500 melting and freezing because of the good affinity between the paraffin wax and this kind of organic based supporting material. Compared with already reported results on stability of shape-stabilized PCM composite,(Maleki et al., 2019; Tang et al., 2017; Yang et al., 2017) our material shows superiority that the variation regarding phase change temperature is less than 0.2 oC, the difference of latent heat is also less than 2.0% during the 500 heating/cooling phase transitions, which further demonstrates the advantageous of our shapestabilization strategy. Additionally, compared with pure paraffin, the mechanic performance exhibits a significant enhancement owing to the stability of the hyper-crosslinking network. As it is shown in Fig. 4c, this WHCP encapsulated PCM composite is well-preserved after a 500 g grams force compression at a temperature higher than the melting point of paraffin, while, the shape of paraffin collapsed completely under the same condition. Table 4. Phase change temperature and latent heat of the as-prepared PCM composites. Sample
Melting process
Freezing process
Onset (oC)
Peak (oC)
Hm (J/g)
Onset (oC)
Peak (oC)
Hf (J/g)
Paraffin
48.18
63.28
175.88
63.31
60.69
175.53
S1
50.90
56.52
106.58
54.05
50.93
106.38
S2
51.45
58.76
132.23
53.73
47.72
128.33
S3
51.45
56.74
110.51
53.85
50.55
110.56
S4
51.34
57.97
131.48
53.80
49.09
126.28
S5
51.47
60.48
102.64
53.16
46.26
107.83
S7
51.24
56.45
106.19
53.95
50.15
106.52
S8
49.70
55.55
111.30
54.20
51.98
111.96
S9
50.86
56.70
120.71
53.83
48.98
120.53
S11
51.10
58.70
140.01
53.34
47.09
140.19
S12
51.17
58.44
152.32
53.62
47.52
152.21
Figure 5. Large scale synthesis of PCM composite from waste PS foam. In order to demonstrate the real application of our waste PS encapsulation method and verify the practical use of this wastes PS reusing protocol, a large lab scale synthesis experiment was conducted. As it is displayed in Fig. 5, the main preparation steps, both in-situ encapsulation, evaporation and ammonia treatment can be easily dealt in a 250 g lab scale thanks to the simplicity of the operation procedure and robustness of the organic reaction behind. Importantly, no emission or release were involved in the whole process because the solvent 1,2-dichloromethane was facilely
recycled through a laboratory set vacuum evaporator and the catalyst FeCl3 was converted into iron oxide for thermal conductivity enhancement. Moreover, the final obtained PCM composite exhibits good thermo-physical properties in terms of energy storage capacity and efficiency. Thus, this waste PS recycling protocol really implies a promising sustainability, a good atomic economy and paves an avenue for highly efficient reuse of PS based plastic. Finally, to further extend the reuse scope of our waste PS strategy, some other PS made waste plastics were subjected to examination under the optimized preparation condition. As it is shown in table 5, five different kinds of waste PS materials, namely, disposable dish container, disposable cup, disposable milk tea cap, disposable tableware and plastic tea pot can be used as holding materials with the aid of catalytic amount of FeCl3 without significant paraffin leakage. Among these waste PS, plastic tea pot afforded the outstanding performance in terms of encapsulation rate. The reason behind might ascribe to the higher purity than the other materials. The milk tea cap exhibited the poorest encapsulation rate due to the existence of some unreacted impurities (55.8%). It is noteworthy that the successful preparation of different kinds of PS waste materials encapsulated PCM further evidences the robustness and feasibility of our protocol. Therefore, this recycle methodology indeed provides a new approach for highly efficient and practical reusing waste PS materials. Table 5. Preparation of form-stable PCM from waste PS plastic. Disposable
Disposable
Disposable
Disposable
Plastic tea
dish
cup
milk tea cap
tableware
pot
container
Real application Raw material in this work Photograph of sample Melting
5 cm
5 cm
5 cm
5 cm
5 cm
108.7
128.6
98.1
113.2
133.7
0.235
0.228
0.229
0.237
0.259
61.8
73.1
55.8
64.4
76.0
0.9
1.1
1.0
1.3
1.5
latent heat (J/g) Thermal conductivity (W/m·K) Encapsulatio n rate (%) Leakage rate (%)
Conclusion In conclusion, we developed a facile approach to reuse waste PS foam as a holding material for PCM shape-stabilization with a 68.7% encapsulation rate without observation of leakage by using an FeCl3 catalyzed hyper-crosslinking process. FeCl3 used in this process was elaborately converted to its corresponding Fe3O4 for an extra thermal conductivity enhancement, which not only omitted
the tedious isolation step but also made the whole process more atom economic. An outstanding advantage of this strategy is the homogenized state of the starting materials, for which an even mixing could be reasonably achieved. Besides that, owing to the encapsulation occurring alongside the formation of the supporting material, a highly efficient encapsulation rate was able to be expected. Furthermore, this process can be easily scaled-up and extended to the other PS based waste plastic besides PS foam, sufficiently demonstrated the feasibility and sustainability of our waste PS reuse strategy. Acknowledgements The authors thanks for the Fundamental Research Funds for the Central Universities (No. 2019XKQYMS69). Reference Ager, J.W., Lapkin, A.A., 2018. ENERGY STORAGE Chemical storage of renewable energy. Science 360, 707-708. Alassali, A., Abis, M., Fiore, S., Kuchta, K., 2019. Classification of plastic waste originated from waste electric and electronic equipment based on the concentration of antimony. Journal of Hazardous materials 380, 120874-120874. Chaukura, N., Gwenzi, W., Bunhu, T., Ruziwa, D.T., Pumure, I., 2016. Potential uses and valueadded products derived from waste polystyrene in developing countries: A review. Resources, Conservation and Recycling 107, 157-165. Chen, J.-C., Huang, J.-S., 2007. Theoretical and experimental study on the emission characteristics of waste plastics incineration by modified O-2/RFG combustion technology. Fuel 86, 2824-2832. de Assis, G.C., Skovroinski, E., Leite, V.D., Rodrigues, M.O., Galembeck, A., Alves, M.C.F., Eastoe, J., de Oliveira, R.J., 2018. Conversion of "Waste Plastic" into Photocatalytic Nanofoams for Environmental Remediation. ACS Appl Mater Interfaces 10, 8077-8085. de Paula, F.G.F., de Castro, M.C.M., Ortega, P.F.R., Blanco, C., Lavall, R.L., Santamaría, R., 2018. High value activated carbons from waste polystyrene foams. Microporous and Mesoporous
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Highlights 1.
Hypercrosslinking reaction enabled phase change material encapsulation protocol.
2.
Waste polystyrenes were used as starting materials.
3.
Catalysts were converted to metal oxide for thermal conductivity enhancement.
4.
The preparation process can be facilely scaled up.
Author contributions Changhui Liu conceived the project and designed the experiment. Xiaotian Ma and Peixing Du conducted the experiment. Changhui Liu and Zhonghao Rao analyzed the data and discussed the results. Changhui Liu wrote the manuscript.
Conflicts of interest There are no conflicts to declare.