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Preparation and performance of a novel thermoplastics polyurethane solid–solid phase change materials for energy storage
Solar Energy Materials & Solar Cells 102 (2012) 36–43
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Preparation and performance of a novel thermoplastics polyurethane solid–solid phase change materials for energy storage Peng Xi a,b,n, Lei Xia a, Pengfei Fei a, Dong Zhang a, Bowen Cheng a a b
Tianjin Municipal Key Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, 399 Bin Shui West Road, 300387 Tianjin, China State Key laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, 100080 Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2011 Received in revised form 15 March 2012 Accepted 29 March 2012 Available online 13 April 2012
Phase change materials (PCMs) are a series of functional materials with storing and releasing energy properties. PCMs are able to adjust and control the environment around them through storing and releasing energy during phase change process. Based on the phase change theory, bis(1,3-dihydroxypropan-2-yl)4,40 -methylenebis(4,1-phenylene)dicarbamate, a novel tetrahydroxy compound, was designed and synthesized. Depending on the spatial structure of the tetrahydroxy compound, the form-stable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was prepared. In the molecular structure of TPUPCM, polyethylene glycol was employed as the soft segments, and the hard segments were made up of 4,40 -diphenylmethane diisocyanate and tetrahydroxy compound made up multi-benzene ring structure. The composition and structure, crystalline morphology, phase change behaviors, thermal performances and mechanical properties for tetrahydroxy compound and TPUPCM were investigated by using fourier transform infrared spectrometer (FT-IR), 1H nuclear magnetic resonance spectrometer (1H NMR), wide-angle X-ray diffraction (WAXD), polarizing optical microscopy (POM), differential scanning calorimentry (DSC), thermogravimetry analysis system (TGA) and Instron 5566 tensile tester. The TPUPCM’s the weight average molecular weight, number average molecular weight, dissolving and mechanical abilities were also tested. The results show that the solid–solid phase-change material possesses excellent phase-change properties and an applicable temperature range. The heating cycle phase-change enthalpy is 137.4 J/g, and the cooling cycle phase-change enthalpy is 127.6 J/g. The decomposition of TPUPCM starts and reaches a maximum at 323.5 1C and 396.2 1C, respectively. Furthermore, the solid–solid phase-change material is dissolvable, meltable and can be processed directly, and has potential applications in thermal energy storage. & 2012 Elsevier B.V. All rights reserved.
Keywords: Phase change Polyethylene glycol Heat storage material Crystalline Energy storage
1. Introduction The energy crisis and the increase of greenhouse gas emissions have made the utilization of renewable energy sources to be a crucial topic [1]. Solar energy, acts as clean energy, is attracting more and more attention in modern energy technology. However, the extensive use of solar energy is limited due to its timedependence. So the energy storage becomes imperative. Among a series of energy storage methods, thermal energy storage is extremely important. It can be applied not only in solar energy storage [2] but also widely in waste heat recovery [3], smart housing [4], temperature control greenhouses and textiles [5], heat regulation of electronics [6], telecommunications and microprocessor equipment[7], and so on. n Correspond author at: Tianjin Municipal Key Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, 399 Bin Shui West Road, 300387 Tianjin, China. Tel.: þ 86 22 27637689; fax: þ 86 22 83955164. E-mail address: [email protected] (P. Xi).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.03.034
Phase change materials (PCMs) are effective thermal storage media. According to the phase change states, PCMs are divided into three categories: solid–solid PCMs, solid–liquid PCMs, and liquid–gas PCMs. The liquid–gas PCMs have no practical application value because of the larger volume change during phase change [8]. The solid–liquid PCMs, such as paraffin wax and fatty acid, can neither be directly used. They must be sealed in container or made into microcapsule. So their applications are hampered by high cost of encapsulation [9]. The major problems of the inorganic solid–solid PCMs are their chemical and thermal instability, super cooling and corrosive behavior [10]. In comparison with these phase change materials, the organic solid–solid phase change materials are considered as ideal candidates because they have been found to exhibit many desirable characteristics, for example, no liquid or gas generation, small volume change, no need to seal them in receptacle, non-corrosiveness and non-toxicity [11]. There are two approaches to prepare form-stable organic solid–solid PCMs, which are physical approach and chemical
P. Xi et al. / Solar Energy Materials & Solar Cells 102 (2012) 36–43
approach. The physical approach is a simple and cheap one, through which two kinds of form-stable organic solid–solid PCMs have been obtained by dispersing PCMs into inorganic porous materials (halloysite, nanotube [12], active carbon [13], carbon nanotube [14], expanded graphite [15], diatomite [16]) or higher melting point polymeric materials(cellulose, agarose [17], PAM [18], PMMA [19]). In the former, because support materials are inorganic material and cannot be directly processed, the applications of these PCMs are limited. In the latter, as long as the temperature is below the melting point of the supporting materials, the whole PCMs can keep their solid shape even when phase change unit changes from solid to liquid. However, the materials prepared via the physical blending tend to not undergo phase change after several heating–cooling cycles due to the loss of PEG. For the chemical approach, chemical grafting, blocking copolymerization are used to make polymer-based solid–solid PCMs as the energy storage. In these PCMs, the phase change unit is mainly PEG because it has higher reaction active end group, high latent heat storage capacity, suitable melting temperature, low vapor pressure, high thermal and chemical stability, and non-corrosiveness. Zhou [20] prepared a solid–solid PCM via a copolymerization reaction of high molecular weight poly (ethylene glycol) (PEG4000) with poly (vinyl alcohol) (PVA) and 4,40 -diphenylmethane diisocyanate (MDI). However, the maximum phase change enthalpy is 72.8 J/g. The value is still very low in comparison with that of pure PEG. Jiang et al. [21] prepared a network solid–solid PCM with rigid polymer cellulose diacetate (CDA) serving as a skeleton and the PEG as a branch chain. However, because of the covalent network of the PEG grafted CDA, the material is not fit for melt processing. Qi et al. [22] prepared a hyperbranched polyurethane solid–solid PCM using hyperbranched polyester (BoltornsH20) as chain extender via a two-step reaction of PEG, MDI and H20. The phase transition enthalpy of the PCM is more than 100 J/g. However, the polymer aggregation is difficultly to restraint, and the cross-link formed easily. Furthermore, the value of H20 is too expensive to be suitable to extensive application. Su [23] synthesized a polymeric solid–solid PCM (PUPCM) with polyurethane block copolymer structure composed of PEG10000 as soft segment, 4,40 -diphenylmethane diissyanate and 1,4-butanediol as a chain extender. The PUPCM is an excellent linear segment block copolymer and beneficial to fabrication. Li [24] synthesized a cross-linking copolymer PCM (PEG/MDI/PE) via the two-step condensation reaction of PEG10000 with pentaerythritol (PE) and 4,40 -diphenylmethane diissyanate. Its phase change enthalpy reaches 152.97 kJ/kg. Although the cross-linking copolymer is not directly processed, the research results provide an important way for the synthesis of PCM with high phase change enthalpy. In recent research work, we designed and synthesized a novel tetrahydroxy compound, bis(1,3-dihydroxypropan-2-yl)4,40 -methylenebis(1,4-phenylene)dicarbamate (named as THCD here for the convenience) (shown in Scheme 1). Relying on the spatial structure of THCD, a form-stable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was prepared via employing PEG as soft segments, while 4,40 -diphenylmethane diisocyanate and
Scheme 1. The chemical structure of THCD.
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THCD comprising of multi-benzene ring structure as hard segments. The material’s dissolving and melting processing ability, phase change behaviors, thermal performance, and crystalline morphology were investigated by various techniques. The results show that the solid–solid PCM owns excellent phase change properties, can be processed directly and is likely to be produced in large scale.
2. Materials and methods 2.1. Materials Polyethylene glycol (PEG, analytical grade, Mn¼2000, 4000, 6000, 8000, from Shanghai Medical Chemical Reagent Co. Inc., China) is dried at 80 1C under high vacuum (20 Pa) for overnight prior to use. Polyethylene glycol monomethylether (MPEG, analytical grade, Mn¼4000, from China Medical Group, China) is degassed and dried under high vacuum (20 Pa) at 100–110 1C for 3–4 h. Benzaldehyde glycerin acetal (BGA, analytical grade, from Shanghai Ruiteng Chemical Reagent Co. Inc., China), Tetrahydrofuran (THF, analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China), Ethanol (EtOH, analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China), petroleum ether (analytical grade, Shanghai Medical Chemical Reagent Co. Inc., China) and N, N-dimethylformamide (DMF, analytical grade, from Tianjin third Chemical Reagent Co. Inc., China) were dried by 5 A˚ molecular sieve for 24 h followed by distillation before use. 4,40 diphenylmethane diisocyanate (MDI, analytical grade, from Tianjin Kemiou Chemical Reagent Co. Inc., China) was used as received. 2.2. Synthesis of THCD The synthetic route of THCD is shown in Scheme 2. THCD was prepared in a three necks round bottomed flask fitted with an overhead stirrer, nitrogen inlet, and an addition funnel. First, 0.01 mol MDI were introduced into the flask and dissolved in 20 ml THF. And then, 0.02 mol BGA was dropped into the reaction flask. The mixture was stirred under nitrogen at 60 1C. Completion of the reaction was indicated by the disappearance of strong absorbed peaks of –OH and –NCO using FT-IR spectroscopy. Then THF was evaporated and recovered. The white precipitate and 40 ml mixture of THF and EtOH (VTHF: VEtOH ¼ 1:2) were mixed and yielded clear solution. The solution was stirred under H2 (1 atm) at 25 1C for 24 h, filtered and evaporated, and crude product was attained. The product was washed via 5 ml petroleum ether and for twice. The sample was kept in a vacuum oven at 80 1C until further characterization (white powder, yield is 95.13%). 2.3. Synthesis of TPUPCM The synthesis was conducted in a two step polymerization process under an inert atmosphere of nitrogen in three necks round bottomed flask fitted with an overhead stirrer. The synthesis route is shown in Scheme 3. First, 0.002 mol MDI and 20 ml freshly distilled DMF were mixed and yielded clear solution. Predetermined amount of dried PEG was dropped into the solution under nitrogen at 0 1C and stirred for 5 min. And then, the reaction continued under nitrogen at 80 1C. Completion of the reaction was indicated by the disappearance of strong absorbed peaks of –OH in PEG molecule using FT-IR spectroscopy. The NCO-terminated modified MPEG (MMPEG) was synthesized using similar procedure as mentioned earlier. In the second step reaction, predetermined amount of THCD and 10 ml freshly distilled DMF were mixed and added into the three necks round bottomed flask by dropping. The mixture was stirred at 80 1C for 6 h, and MMPEG was added. The reaction continued at 80 1C for 2 h. At last, the
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Scheme 2. Synthesis route to THCD.
Scheme 3. Synthesis route to TPUPCM.
reaction mixture was cast in a special glass pan and set in vacuum oven for further reaction at 80 1C for 48 h, so that the reactions could proceed to completely, and the DMF was volatilized. The samples were kept in vacuum at room temperature for two weeks prior to test. In the whole reaction process, the total amount of – NCO groups is kept to equal to that of –OH groups. 2.4. Characterization Fourier transform infrared (FT-IR) spectra of the samples were obtained on a Nicollet NEXUS-670 FT-IR spectrometer. Transition
of attenuated total reflectance (ATR) spectra of samples between 500 cm 1 and 4000 cm 1 were collected by 32 scans for each spectrum and the resolution for the ATR spectra was 2 cm 1. 1 H Nuclear magnetic resonance (1H NMR) spectra of the samples were collected by using a JEOL EX-400 NMR spectrometer at room temperature with DMSO-d6 as the solvent. The weight average molecular weight and number average molecular weight of the PCM samples were determined with a multiangle laser light scattering apparatus (DAWN-DSP, Wyatt Technology Co., Santa Barbara, CA) combined with a P100 pump (Thermo Separation Products) equipped with a TSKGEL G5000 HHR column (7.8–300 mm). The solvent used was high-performance liquid chromatograph grade dimethylformamide (DMF). The Analysis of the PEG and PCM samples by wide-angle X-ray diffraction (WAXD) was carried out with a Philip PW 1710 at 30 kV and 20 mA. Bragg’s angle 2y is set from 101 to 501 with a rate of 31/min. An observation of the samples by polarizing optical microscopy (POM) was performed on Olympus BX51 Polarizing Microscope. The sample was placed between a microscope glass and a cover slip, and then heated with hot stage. Differential scanning calorimentry (DSC) analysis of the samples was carried out with a Perkin Elmer DSC-7 to monitor the variations in melting temperature (Tm), crystallizing temperature (Tc), latent heat of fusion (DHf) and latent heat of crystallization (DHc) of the samples. A typical thermal cycle consisted of a consecutive heating and cooling process (in the temperature interval 0–100 1C). The heating and cooling process times were maintained at approximately 15 and 25 min, respectively. Thermal cycles test continued until 1000 complete heating–cooling processes of TPUPCM. Thermogravimetry and derivative of thermogravimetry curves of the TPUPCM were obtained by using a Netzsch STA409PC thermal analysis system. About 10 mg dried sample was set into an alumina crucible and weighed. The curve was recorded at a heating rate of 5 1C/min in air atmosphere. The melting point of TPUPCM was tested through XP-201 digital melting point meter at a heating rate of 10 1C/min. The mechanical behaviors of TPUPCM were conducted by an Instron 5566 tensile tester. The test samples were cut out from the hot pressed film. Their size and the gage length are
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40 mm 5 mm 0.5 mm and 25 mm, respectively. Rate of extension is 15 mm/min.
3. Results and discussion 3.1. Structural properties of TPUPCM The results of IR and 1H NMR spectra for THCD (Figs. 1 and 2) verify that the composition and structure of THCD agree with Scheme 1. In the IR spectrum of THCD (shown in Fig. 1(a)), the characteristic peak of –NCO for MDI at 2279 cm 1 and the stretching vibration peaks of –OH of BGA at 3437 cm 1 disappear. The stretching vibration peaks of –NH, –C¼O and –C–O– emerge at 3313 cm 1, 1723 cm 1, and 1259–1001 cm 1, respectively. These results prove that the chemical bond of –NHCOO– has formed. The strong absorption peak at 3415 cm 1 is assigned to the stretching vibration peaks of terminal hydroxyl group of THCD. The peaks locating at 3005, 1598, 1506 and 1445 cm 1 are the characteristic peaks of benzene ring [25]. In order to further verify the FT-IR analysis results, the 1H NMR analysis was performed (shown in Fig. 2). Through the analysis Fig. 3. FT-IR spectrum of TPUPCM. (a) TPUPCM; (b) PEG6000; (c) THCD.
results of 1H NMR spectrum of THCD, it can be found that the structure of THCD in the Scheme 1 corresponds to the 1H NMR spectrum. Both the IR spectrum and 1H NMR spectrum of THCD indicate that the novel tetrahydroxy compound has been successfully prepared. To reveal composition and structure of TPUPCM, FT-IR and 1H NMR spectra test were performed as shown in Figs. 3 and 4. For a comparison, FT-IR spectra of THCD and PEG6000 were also shown in Fig. 3. From the Fig. 3, it can be found that the hydroxyl groups of THCD at 3415 cm 1 tend to polymerize. Moreover, the strong peaks emerge at 2890 and 1106 cm 1, which belong to the stretching vibration peaks of –CH2– and –C–O– for soft segment. The characteristic peaks of –NH, –C¼O and benzene ring locate at 3313 cm 1, 1725 cm 1, 1596 cm 1, 1546 cm 1, 1512 cm 1 and 1465 cm 1. These results prove that TPUPCM has been synthesized. In 1H NMR spectrum of TPUPCM, the proton peaks correspond to the structure of TPUPCM. The analysis results of 1H NMR spectrum of TPUPCM verify further those of FT-IR spectra. Fig. 1. FT-IR spectrum of THCD. (a) THCD: (b) BGA; (c) MDI.
3.2. Physical properties of TPUPCM
Fig. 2. 1H NMR spectrum of THCD.
The physical properties of the novel polyurethane solid–solid phase change material synthesized are illustrated in Table 1. From Table 1, it can be found that the sample as-obtained shows excellent dissolving and melting-processing properties. It is dissolvable in tetrahydrofuran (THF), N, N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), respectively. The TPUPCM obtained has an of 2.56 105 and an of 1.68 105. The melting point and the first degradation temperature of the polyurethane phase change material is 186 1C and 323 1C (TGA curve), respectively. It can be manufactured to form any given shape (film, strip, particle and filament) when the temperature is at 186–323 1C. These results verify that the novel polyurethane phase change material as-synthesized is thermoplastic polymer, which benefits from the steric structure of THCD. In the synthesis of TPUPCM, the multi-benzene rings structure in the main chain is inflexible due to the hyperconjugation effect among multi-benzene rings and acyl groups. The space steric effect inhibits the second hydroxyl to participate reaction and leads to the linear polymer main chain to be formed.
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Fig. 4. 1H NMR spectrum of TPUPCM.
Table 1 Physical properties of TPUPCM6000. Dissolved properties
Melt processed properties
The average molecular weight
THF
DMF DMSO Melting point
The first degradation temperature
Mw
Slightly soluble
Easily soluble
323 1C
2.56 105 1.68 105
186 1C
Mn
3.3. Crystallization properties of TPUPCM To reveal the crystalline morphology of the TPUPCM, WAXD technique was employed and the XRD patterns of pristine PEG6000 and TPUPCMs with different molecular weight PEG are shown in Fig. 5. It shows that both pure PEG and TPUPCM have similar diffraction patterns, and so are their diffraction angles and crystal plane distances. In the diffraction pattern of pure PEG and TPUPCMs, two strong diffraction peaks appear at 18.81 and 23.21. The result indicates that PEG and TPUPCMs possess similar crystal structure and crystal cell type [24]. The differences between them are that the diffraction peak height of TPUPCM is lower and the half width is broader than that of pure PEG, which means the crystallites become smaller and the degree of crystallinity decreases. Furthermore, the diffraction peak intensity gradually increases in the diffraction patterns of TPUPCM with the increase of molecular weight of PEG, when the molecular weight of PEG is 6000, the diffraction peak intensity reaches maximum, and then decreases. These results show that TPUPCM with PEG6000 (TPUPCM6000) has the best crystallization properties. To further verify the crystallization properties of TPUPCM, the POM was used for recording the micro-morphologies of pure PEG and TPUPCM. Fig. 6(a) and (b) show the POM photos of PEG6000 and TPUPCM6000 at room temperature. It is observed that both micrographs show obvious cross-extinction patterns, which suggests that both of them are crystalline and their crystalline morphologies are spherulites. However, the soft segments were limited by the hard segments in TPUPCM, the crystallization process of PEG becomes heterogeneous nucleation and confined crystallization, and the crystallites are smaller [24]. When the temperatures approaches transition point of melting, the spherulites fade away and eventually disappear, and the soft segments experience the transformation from crystalline state to amorphous one. These results are in good agreement with WXRD results.
Fig. 5. XRD patterns of the TPUPCM.
3.4. Phase change properties of TPUPCM To study the phase change properties of TPUPCM, the Differential scanning calorimetry (DSC) was employed. The DSC curves of pure PEG and TPUPCMs with different molecular weight PEG are shown in Fig. 7, and the data of their transition temperature and enthalpy are summarized in Table 2. By contrast with the analysis results of DSC, it can be found that PEG and TPUPCMs undergo all phase transition, but their phase transition states are quite different. The phase transition of pure PEG is a process from solid to liquid. When the temperature is raised to about 65.9 1C, the pure PEG undergoes a change from a white crystal solid to a transparent liquid [26]. While, TPUPCM remains solid in the whole heating process, even if the temperature is raised to 100 1C or higher, indicating that TPUPCM is solid–solid phase material. Fig. 7 shows that the DSC curves of TPUPCMs have strong melting and releasing heat peaks during the heating and cooling procedure. Moreover, the phase change enthalpy values of TPUPCM are gradually enhanced with the
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Fig. 6. POM photos of PEG and TPUPCM: (a) PEG6000; (b) TPUPCM6000.
Fig. 8. Heating and cooling DSC cures of TPUPCM after 1000 thermal cycles.
Fig. 7. DSC curves of the TPUPCM. Table 2 Phase change temperature and enthalpy of PEG6000 and TPUPCM. Sample
PEG6000 TPUPCM2000 TPUPCM4000 TPUPCM6000 TPUPCM8000
Phase transition
Solid–liquid Solid–solid Solid–solid Solid–solid Solid–solid
WH(J/g)
Tp ( 1C)
Heating cycle
Cooling cycle
Heating cycle
Cooling cycle
189.6 97.5 126.4 137.4 127.1
167.2 88.9 112.4 127.6 115.2
65.9 56.3 55.8 57.1 57.5
37.6 31.8 33.4 31.2 30.1
Tp ¼Peak transition temperature of samples; samples.
WH ¼phase transition enthalpy of
increase of molecular weight of PEG. This demonstrates that the phase change enthalpy of TPUPCM corresponds to the melting enthalpy of the soft segments in TPUPCM. It’s worth noting that the phase change enthalpy of TPUPCM with PEG6000 is 137.4 J/g (heating cycle) and 127.6 J/g (cooling cycle) and the transition temperature is suitable. These results prove that prepared TPUPCMs are reversible phase transition latent storage material, which possess excellent phase change properties and have a wide application in the energy storage.
Fig. 9. The FT-IR spectrum of synthesized TPUPCM after 1000 thermal cycles. (a) First thermal cycle; (b) 1000th thermal cycle.
Compared with PEG/MDI/PE cross-linking copolymer [24], the phase change enthalpy of TPUPCM is 137.4 J/g, lower than that of PEG/MDI/PE cross-linking copolymer, the phase change enthalpy
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of PEG/MDI/PE cross-linking copolymer is 152.97 J/g. This is because the phase change enthalpy of the PCM is intimately related to the weight percentage of phase change unit. In the molecular structure of TPUPCM, the phase change units include PEG (in the main chain) and MPEG (in the branch chain). When the main chain structure remains unchanged, the weight percentage of phase change unit of TPUPCM gradually increases with the increase of molecular weight of MPEG. Li chose PEG10000 as phase change unit in the synthesis of PEG/MDI/PE cross-linking copolymer PCM. In view of the mechanical properties of TPUPCM,
Fig. 10. TGA curve of TPUPCM6000.
we chose MPEG4000 as branch chain of TPUPCM. This affects the phase change enthalpy of TPUPCM to an extent. Furthermore, through the analysis of FT-IR spectrum of TPUPCM, it can be found that there are a few of hydroxy groups which have not reacted. This also makes the phase change enthalpy of TPUPCM reduced. 3.5. Thermal reliability and stability of TPUPCM The DSC curves of synthesized TPUPCM after 1500 and 1000 thermal cycles (melting/freezing) are shown in Fig. 8. From Fig. 8, it can be found that the phase change temperatures and latent heat values of the TPUPCM slightly change after 1000 cycles. Based on these results, it is note worthy that the as-synthesized PCMs have good thermal reliability in terms of thermal properties after thermal cycles. The FT-IR spectrum of synthesized TPUPCM after 1000 thermal cycles (melting/freezing) is shown in Fig. 9. By comparison of the analysis results, it can be found that any degradation does not occur in the chemical structure of the TPUPCM after 1000 thermal cycles. This means that the synthesized TPUPCMs have good thermal stability during a long utility period. The thermogravimetric and derivative thermogravimetric analysis curves of TPUPCM6000 are shown in Fig. 10. The mechanism of thermal decomposition involves two steps, as can be seen in TGA curves. When the temperature reaches 323 1C, slight thermal decomposition appears. The decomposition of TPUPCM reaches to a maximum at 396.2 1C. These results indicate that TPUPCMs have a wider processing temperature range. This is very important for fabrication and application of TPUPCM. 3.6. Fabrication and mechanical properties of TPUPCM
Fig. 11. The sheets of TPUPCM.
As TPUPCMs are thermoplastic polymer, they can be easily processed into different shapes such as bar, rod tube and so on through extrusion and molding techniques. Fig. 11 shows the sheet products of TPUPCM, which were processed by molding techniques. The appearance of sheets of TPUPCM is white. The surface of sheets is smooth. The formation of sheets is easy. To investigate the mechanical properties of TPUPCM, the Instron 5566 tensile tester was used. Fig. 12 presents the change of sample in tension process. By contrast with these photos, it can be seen that the sample shows obvious necking in tension process. In Table 3, the results prove that the maximum elongation at break and the breaking strength of the sample achieve 354% and 61 N, respectively. These results indicate that TPUPCM owns excellent mechanical properties.
Fig. 12. The change of the sample in tension process.
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Table 3 The mechanical properties of TPUPCM. Sample Breaking strength (N)
Extension at break (mm)
Elongation at break (%)
Fracture work (Nm)
1 2 3
47.6 88.5 56.3
190.4 354.0 225.0
2302.8 4628.2 2861.9
56.0 61.0 61.0
4. Conclusion Depending on design and synthesis of the novel tetrahydroxy compound, form-stable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was prepared. According to the results analysis, TPUPCM has the same crystal structure and crystal cell type as PEG, and shows outstanding phase change properties. Furthermore, TPUPCM owns good thermal stability during a long utility period. It is note worthy that TPUPCM can be processed directly, and has a wider processing temperature range, good mechanical properties. These results indicate that TPUPCM has a great potential for thermal energy storage applications.
Acknowledgement The authors thank for the financial supports from Application Fundamental and Advanced Technology Research Proposal Project of Tianjin, China (No. 10 JCYBJC03100) and Key project of Chinese Ministry of Education (No. 208005). References [1] Aydin Ahmet Alper, Okutan Hasancan, High-chain fatty acid esters of myristyl alcohol with odd carbon number: novel organic phase change materials for thermal energy storage-2, Solar Energy Materials and Solar Cells 95 (2011) 2417–2423. [2] Yanlai Zhang, Shuangfeng Wang, Zhonghao Rao, Experiment on heat storage characteristic of microencapsulated phase change material slurry, Solar Energy Materials and Solar Cells 95 (2011) 2726–2733. [3] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al, A review on phase change energy storage: materials and applications, Energy Conversion and Management 45 (2004) 1597–1615. [4] H. Li, G.Y. Fang, Experimental investigation on the characteristics of polyethylene glycol/cement composites thermal energy storage materials, Chemical Engineering and Technology 33 (2010) 1650–1654. [5] C.Z. Chen, L.G. Wang, Y. Huang, Crosslinking of the electrospun polyethylene glycol/cellulose acetate composite fibers as shape-stabilized phase change materials, Materials Letters 63 (2009) 569–571. [6] T. Kousksou, A. Jamil, T. El Rhafiki, Paraffin wax mixtures as phase change materials, Solar Energy Materials and Solar Cells 94 (2010) 2158–2165.
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[7] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renewable & Sustainable Energy Reviews 13 (2009) 318–345. [8] X.H. Gu, P. Xi, B.W. Cheng, Synthesis and characterization of a novel solid– solid phase change luminescence material, Polymer International 59 (2010) 772–777. [9] Deng Zhang Xiaoxing, Feng Pengfei, Rongxiu, novel gelatinous shape-stabilized phase change materials with high heat storage density, Solar Energy Materials and Solar Cells 95 (2011) 1213–1218. [10] Xiaoxi Weilong Wanga, Yutang Yang, Jing Fang, Ding, Preparation and performance of form-stable polyethylene glycol/silicon dioxide composites as solid–liquid phase change materials, Applied Energy 86 (2009) 170–174. [11] Aydin Ahmet Alper, Okutan Hasancan, High-chain fatty acid esters of myristyl alcohol with even carbon number: novel organic phase change materials for thermal energy storage-1, Solar Energy Materials and Solar Cells 95 (2011) 2752–2762. [12] Mei Dandan, Zhang Bing, Liu Ruichao, Preparation of capric acid/halloysite nanotube composite as form-stable phase change material for thermal energy storage, Solar Energy Materials and Solar Cells 95 (2011) 2772–2777. [13] Feng Lili, Zheng Jie, Yang Huazhe, Preparation and characterization of polyethylene glycol/active carbon composites as shape-stabilized phase change materials, Solar Energy Materials and Solar Cells 95 (2011) 644–650. [14] Yanbin Cui, Caihong Liu, Shan Hu, The experimental exploration of carbon nanofiber and carbon nanotube additives on thermal behavior of phase change materials, Solar Energy Materials and Solar Cells 95 (2011) 1208–1212. [15] L. Xia, P. Zhang, Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage, Solar Energy Materials and Solar Cells 95 (2011) 2246–2254. [16] Sedat Karaman, Ali Karaipekli, Ahmet Sari, Polyethylene glycol (PEG)/diatomite composite as a novel form-stable phase change material for thermal energy storage, Solar Energy Materials and Solar Cells 95 (2011) 1647–1653. [17] S.B. Senturk, D. Kahraman, C. Alkan, Biodegradable PEG/cellulose, PEG/ agarose and PEG/chitosan blends as shape stabilized phase change materials for latent heat energy storage, Carbohydrate Polymers 84 (2011) 141–144. [18] Xiao-Ming Zhou, Study on phase change characteristics of PEG/PAM coupling blend, Journal of Applied Polymer Science 116 (2010) 1591–1595. [19] A. Sari, C. Alkan, A. Karaipekli, Poly(ethylene glycol)/poly(methyl methacrylate) blends as novel form-stable phase-change materials for thermal energy storage, Journal of Applied Polymer Science 116 (2010) 929–933. [20] Xiao-Ming Zhou, Preparation and characterization of PEG/MDI/PVA copolymer as solid–solid phase change heat storage material, Journal of Applied Polymer Science 113 (2009) 2041–2045. [21] Y. Jiang, E. Ding, G. Li, Study on transition characteristics of PEG/CDA solid– solid phase change materials, Polymerica . 43 (2002) 117. [22] C. Qi, L. Liao, H.L. Xu, Study on the influence of thermal characteristics of hyperbranched polyurethane phase change materials for energy storage, Journal of Applied Polymer Science 115 (2010) 2228–2235. [23] Su Jing-Cang, Liu Peng-Sheng, A novel solid–solid phase change heat storage material with polyurethane block copolymer structure, Energy Conversion and Management 47 (2006) 3185–3191. [24] Li Wei-Dong, Ding En-Yong, Preparation and characterization of cross-linking PEG/MDI/PE copolymer as solid–solid phase change heat storage material, Solar Energy Materials and Solar Cells 91 (2007) 764–768. [25] Xi Peng, Gu XiaoHua, Huang XiangAn, Synthesis, characterization and luminescent properties of new highly luminescent organic ligand and complexes of trivalent rare earth, Spectrochimica Acta Part A 66 (2007) 667–671. [26] C. Alkan, A. Sari, O. Uzun, Poly (ethylene glycol)/acrylic polymer blends for latent heat thermal energy storage, AIChE Journal 52 (2006) 3310–3314.
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Preparation and performance of a novel thermoplastics polyurethane solid–solid phase change materials for energy storage Peng Xi a,b,n, Lei Xia a, Pengfei Fei a, Dong Zhang a, Bowen Cheng a a b
Tianjin Municipal Key Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, 399 Bin Shui West Road, 300387 Tianjin, China State Key laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, 100080 Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 December 2011 Received in revised form 15 March 2012 Accepted 29 March 2012 Available online 13 April 2012
Phase change materials (PCMs) are a series of functional materials with storing and releasing energy properties. PCMs are able to adjust and control the environment around them through storing and releasing energy during phase change process. Based on the phase change theory, bis(1,3-dihydroxypropan-2-yl)4,40 -methylenebis(4,1-phenylene)dicarbamate, a novel tetrahydroxy compound, was designed and synthesized. Depending on the spatial structure of the tetrahydroxy compound, the form-stable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was prepared. In the molecular structure of TPUPCM, polyethylene glycol was employed as the soft segments, and the hard segments were made up of 4,40 -diphenylmethane diisocyanate and tetrahydroxy compound made up multi-benzene ring structure. The composition and structure, crystalline morphology, phase change behaviors, thermal performances and mechanical properties for tetrahydroxy compound and TPUPCM were investigated by using fourier transform infrared spectrometer (FT-IR), 1H nuclear magnetic resonance spectrometer (1H NMR), wide-angle X-ray diffraction (WAXD), polarizing optical microscopy (POM), differential scanning calorimentry (DSC), thermogravimetry analysis system (TGA) and Instron 5566 tensile tester. The TPUPCM’s the weight average molecular weight, number average molecular weight, dissolving and mechanical abilities were also tested. The results show that the solid–solid phase-change material possesses excellent phase-change properties and an applicable temperature range. The heating cycle phase-change enthalpy is 137.4 J/g, and the cooling cycle phase-change enthalpy is 127.6 J/g. The decomposition of TPUPCM starts and reaches a maximum at 323.5 1C and 396.2 1C, respectively. Furthermore, the solid–solid phase-change material is dissolvable, meltable and can be processed directly, and has potential applications in thermal energy storage. & 2012 Elsevier B.V. All rights reserved.
Keywords: Phase change Polyethylene glycol Heat storage material Crystalline Energy storage
1. Introduction The energy crisis and the increase of greenhouse gas emissions have made the utilization of renewable energy sources to be a crucial topic [1]. Solar energy, acts as clean energy, is attracting more and more attention in modern energy technology. However, the extensive use of solar energy is limited due to its timedependence. So the energy storage becomes imperative. Among a series of energy storage methods, thermal energy storage is extremely important. It can be applied not only in solar energy storage [2] but also widely in waste heat recovery [3], smart housing [4], temperature control greenhouses and textiles [5], heat regulation of electronics [6], telecommunications and microprocessor equipment[7], and so on. n Correspond author at: Tianjin Municipal Key Lab of Fiber Modification and Functional Fiber, Tianjin Polytechnic University, 399 Bin Shui West Road, 300387 Tianjin, China. Tel.: þ 86 22 27637689; fax: þ 86 22 83955164. E-mail address: [email protected] (P. Xi).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.03.034
Phase change materials (PCMs) are effective thermal storage media. According to the phase change states, PCMs are divided into three categories: solid–solid PCMs, solid–liquid PCMs, and liquid–gas PCMs. The liquid–gas PCMs have no practical application value because of the larger volume change during phase change [8]. The solid–liquid PCMs, such as paraffin wax and fatty acid, can neither be directly used. They must be sealed in container or made into microcapsule. So their applications are hampered by high cost of encapsulation [9]. The major problems of the inorganic solid–solid PCMs are their chemical and thermal instability, super cooling and corrosive behavior [10]. In comparison with these phase change materials, the organic solid–solid phase change materials are considered as ideal candidates because they have been found to exhibit many desirable characteristics, for example, no liquid or gas generation, small volume change, no need to seal them in receptacle, non-corrosiveness and non-toxicity [11]. There are two approaches to prepare form-stable organic solid–solid PCMs, which are physical approach and chemical
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approach. The physical approach is a simple and cheap one, through which two kinds of form-stable organic solid–solid PCMs have been obtained by dispersing PCMs into inorganic porous materials (halloysite, nanotube [12], active carbon [13], carbon nanotube [14], expanded graphite [15], diatomite [16]) or higher melting point polymeric materials(cellulose, agarose [17], PAM [18], PMMA [19]). In the former, because support materials are inorganic material and cannot be directly processed, the applications of these PCMs are limited. In the latter, as long as the temperature is below the melting point of the supporting materials, the whole PCMs can keep their solid shape even when phase change unit changes from solid to liquid. However, the materials prepared via the physical blending tend to not undergo phase change after several heating–cooling cycles due to the loss of PEG. For the chemical approach, chemical grafting, blocking copolymerization are used to make polymer-based solid–solid PCMs as the energy storage. In these PCMs, the phase change unit is mainly PEG because it has higher reaction active end group, high latent heat storage capacity, suitable melting temperature, low vapor pressure, high thermal and chemical stability, and non-corrosiveness. Zhou [20] prepared a solid–solid PCM via a copolymerization reaction of high molecular weight poly (ethylene glycol) (PEG4000) with poly (vinyl alcohol) (PVA) and 4,40 -diphenylmethane diisocyanate (MDI). However, the maximum phase change enthalpy is 72.8 J/g. The value is still very low in comparison with that of pure PEG. Jiang et al. [21] prepared a network solid–solid PCM with rigid polymer cellulose diacetate (CDA) serving as a skeleton and the PEG as a branch chain. However, because of the covalent network of the PEG grafted CDA, the material is not fit for melt processing. Qi et al. [22] prepared a hyperbranched polyurethane solid–solid PCM using hyperbranched polyester (BoltornsH20) as chain extender via a two-step reaction of PEG, MDI and H20. The phase transition enthalpy of the PCM is more than 100 J/g. However, the polymer aggregation is difficultly to restraint, and the cross-link formed easily. Furthermore, the value of H20 is too expensive to be suitable to extensive application. Su [23] synthesized a polymeric solid–solid PCM (PUPCM) with polyurethane block copolymer structure composed of PEG10000 as soft segment, 4,40 -diphenylmethane diissyanate and 1,4-butanediol as a chain extender. The PUPCM is an excellent linear segment block copolymer and beneficial to fabrication. Li [24] synthesized a cross-linking copolymer PCM (PEG/MDI/PE) via the two-step condensation reaction of PEG10000 with pentaerythritol (PE) and 4,40 -diphenylmethane diissyanate. Its phase change enthalpy reaches 152.97 kJ/kg. Although the cross-linking copolymer is not directly processed, the research results provide an important way for the synthesis of PCM with high phase change enthalpy. In recent research work, we designed and synthesized a novel tetrahydroxy compound, bis(1,3-dihydroxypropan-2-yl)4,40 -methylenebis(1,4-phenylene)dicarbamate (named as THCD here for the convenience) (shown in Scheme 1). Relying on the spatial structure of THCD, a form-stable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was prepared via employing PEG as soft segments, while 4,40 -diphenylmethane diisocyanate and
Scheme 1. The chemical structure of THCD.
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THCD comprising of multi-benzene ring structure as hard segments. The material’s dissolving and melting processing ability, phase change behaviors, thermal performance, and crystalline morphology were investigated by various techniques. The results show that the solid–solid PCM owns excellent phase change properties, can be processed directly and is likely to be produced in large scale.
2. Materials and methods 2.1. Materials Polyethylene glycol (PEG, analytical grade, Mn¼2000, 4000, 6000, 8000, from Shanghai Medical Chemical Reagent Co. Inc., China) is dried at 80 1C under high vacuum (20 Pa) for overnight prior to use. Polyethylene glycol monomethylether (MPEG, analytical grade, Mn¼4000, from China Medical Group, China) is degassed and dried under high vacuum (20 Pa) at 100–110 1C for 3–4 h. Benzaldehyde glycerin acetal (BGA, analytical grade, from Shanghai Ruiteng Chemical Reagent Co. Inc., China), Tetrahydrofuran (THF, analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China), Ethanol (EtOH, analytical grade, from Tianjin Keetong Chemical Reagent Co. Inc., China), petroleum ether (analytical grade, Shanghai Medical Chemical Reagent Co. Inc., China) and N, N-dimethylformamide (DMF, analytical grade, from Tianjin third Chemical Reagent Co. Inc., China) were dried by 5 A˚ molecular sieve for 24 h followed by distillation before use. 4,40 diphenylmethane diisocyanate (MDI, analytical grade, from Tianjin Kemiou Chemical Reagent Co. Inc., China) was used as received. 2.2. Synthesis of THCD The synthetic route of THCD is shown in Scheme 2. THCD was prepared in a three necks round bottomed flask fitted with an overhead stirrer, nitrogen inlet, and an addition funnel. First, 0.01 mol MDI were introduced into the flask and dissolved in 20 ml THF. And then, 0.02 mol BGA was dropped into the reaction flask. The mixture was stirred under nitrogen at 60 1C. Completion of the reaction was indicated by the disappearance of strong absorbed peaks of –OH and –NCO using FT-IR spectroscopy. Then THF was evaporated and recovered. The white precipitate and 40 ml mixture of THF and EtOH (VTHF: VEtOH ¼ 1:2) were mixed and yielded clear solution. The solution was stirred under H2 (1 atm) at 25 1C for 24 h, filtered and evaporated, and crude product was attained. The product was washed via 5 ml petroleum ether and for twice. The sample was kept in a vacuum oven at 80 1C until further characterization (white powder, yield is 95.13%). 2.3. Synthesis of TPUPCM The synthesis was conducted in a two step polymerization process under an inert atmosphere of nitrogen in three necks round bottomed flask fitted with an overhead stirrer. The synthesis route is shown in Scheme 3. First, 0.002 mol MDI and 20 ml freshly distilled DMF were mixed and yielded clear solution. Predetermined amount of dried PEG was dropped into the solution under nitrogen at 0 1C and stirred for 5 min. And then, the reaction continued under nitrogen at 80 1C. Completion of the reaction was indicated by the disappearance of strong absorbed peaks of –OH in PEG molecule using FT-IR spectroscopy. The NCO-terminated modified MPEG (MMPEG) was synthesized using similar procedure as mentioned earlier. In the second step reaction, predetermined amount of THCD and 10 ml freshly distilled DMF were mixed and added into the three necks round bottomed flask by dropping. The mixture was stirred at 80 1C for 6 h, and MMPEG was added. The reaction continued at 80 1C for 2 h. At last, the
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Scheme 2. Synthesis route to THCD.
Scheme 3. Synthesis route to TPUPCM.
reaction mixture was cast in a special glass pan and set in vacuum oven for further reaction at 80 1C for 48 h, so that the reactions could proceed to completely, and the DMF was volatilized. The samples were kept in vacuum at room temperature for two weeks prior to test. In the whole reaction process, the total amount of – NCO groups is kept to equal to that of –OH groups. 2.4. Characterization Fourier transform infrared (FT-IR) spectra of the samples were obtained on a Nicollet NEXUS-670 FT-IR spectrometer. Transition
of attenuated total reflectance (ATR) spectra of samples between 500 cm 1 and 4000 cm 1 were collected by 32 scans for each spectrum and the resolution for the ATR spectra was 2 cm 1. 1 H Nuclear magnetic resonance (1H NMR) spectra of the samples were collected by using a JEOL EX-400 NMR spectrometer at room temperature with DMSO-d6 as the solvent. The weight average molecular weight and number average molecular weight of the PCM samples were determined with a multiangle laser light scattering apparatus (DAWN-DSP, Wyatt Technology Co., Santa Barbara, CA) combined with a P100 pump (Thermo Separation Products) equipped with a TSKGEL G5000 HHR column (7.8–300 mm). The solvent used was high-performance liquid chromatograph grade dimethylformamide (DMF). The Analysis of the PEG and PCM samples by wide-angle X-ray diffraction (WAXD) was carried out with a Philip PW 1710 at 30 kV and 20 mA. Bragg’s angle 2y is set from 101 to 501 with a rate of 31/min. An observation of the samples by polarizing optical microscopy (POM) was performed on Olympus BX51 Polarizing Microscope. The sample was placed between a microscope glass and a cover slip, and then heated with hot stage. Differential scanning calorimentry (DSC) analysis of the samples was carried out with a Perkin Elmer DSC-7 to monitor the variations in melting temperature (Tm), crystallizing temperature (Tc), latent heat of fusion (DHf) and latent heat of crystallization (DHc) of the samples. A typical thermal cycle consisted of a consecutive heating and cooling process (in the temperature interval 0–100 1C). The heating and cooling process times were maintained at approximately 15 and 25 min, respectively. Thermal cycles test continued until 1000 complete heating–cooling processes of TPUPCM. Thermogravimetry and derivative of thermogravimetry curves of the TPUPCM were obtained by using a Netzsch STA409PC thermal analysis system. About 10 mg dried sample was set into an alumina crucible and weighed. The curve was recorded at a heating rate of 5 1C/min in air atmosphere. The melting point of TPUPCM was tested through XP-201 digital melting point meter at a heating rate of 10 1C/min. The mechanical behaviors of TPUPCM were conducted by an Instron 5566 tensile tester. The test samples were cut out from the hot pressed film. Their size and the gage length are
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40 mm 5 mm 0.5 mm and 25 mm, respectively. Rate of extension is 15 mm/min.
3. Results and discussion 3.1. Structural properties of TPUPCM The results of IR and 1H NMR spectra for THCD (Figs. 1 and 2) verify that the composition and structure of THCD agree with Scheme 1. In the IR spectrum of THCD (shown in Fig. 1(a)), the characteristic peak of –NCO for MDI at 2279 cm 1 and the stretching vibration peaks of –OH of BGA at 3437 cm 1 disappear. The stretching vibration peaks of –NH, –C¼O and –C–O– emerge at 3313 cm 1, 1723 cm 1, and 1259–1001 cm 1, respectively. These results prove that the chemical bond of –NHCOO– has formed. The strong absorption peak at 3415 cm 1 is assigned to the stretching vibration peaks of terminal hydroxyl group of THCD. The peaks locating at 3005, 1598, 1506 and 1445 cm 1 are the characteristic peaks of benzene ring [25]. In order to further verify the FT-IR analysis results, the 1H NMR analysis was performed (shown in Fig. 2). Through the analysis Fig. 3. FT-IR spectrum of TPUPCM. (a) TPUPCM; (b) PEG6000; (c) THCD.
results of 1H NMR spectrum of THCD, it can be found that the structure of THCD in the Scheme 1 corresponds to the 1H NMR spectrum. Both the IR spectrum and 1H NMR spectrum of THCD indicate that the novel tetrahydroxy compound has been successfully prepared. To reveal composition and structure of TPUPCM, FT-IR and 1H NMR spectra test were performed as shown in Figs. 3 and 4. For a comparison, FT-IR spectra of THCD and PEG6000 were also shown in Fig. 3. From the Fig. 3, it can be found that the hydroxyl groups of THCD at 3415 cm 1 tend to polymerize. Moreover, the strong peaks emerge at 2890 and 1106 cm 1, which belong to the stretching vibration peaks of –CH2– and –C–O– for soft segment. The characteristic peaks of –NH, –C¼O and benzene ring locate at 3313 cm 1, 1725 cm 1, 1596 cm 1, 1546 cm 1, 1512 cm 1 and 1465 cm 1. These results prove that TPUPCM has been synthesized. In 1H NMR spectrum of TPUPCM, the proton peaks correspond to the structure of TPUPCM. The analysis results of 1H NMR spectrum of TPUPCM verify further those of FT-IR spectra. Fig. 1. FT-IR spectrum of THCD. (a) THCD: (b) BGA; (c) MDI.
3.2. Physical properties of TPUPCM
Fig. 2. 1H NMR spectrum of THCD.
The physical properties of the novel polyurethane solid–solid phase change material synthesized are illustrated in Table 1. From Table 1, it can be found that the sample as-obtained shows excellent dissolving and melting-processing properties. It is dissolvable in tetrahydrofuran (THF), N, N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), respectively. The TPUPCM obtained has an of 2.56 105 and an of 1.68 105. The melting point and the first degradation temperature of the polyurethane phase change material is 186 1C and 323 1C (TGA curve), respectively. It can be manufactured to form any given shape (film, strip, particle and filament) when the temperature is at 186–323 1C. These results verify that the novel polyurethane phase change material as-synthesized is thermoplastic polymer, which benefits from the steric structure of THCD. In the synthesis of TPUPCM, the multi-benzene rings structure in the main chain is inflexible due to the hyperconjugation effect among multi-benzene rings and acyl groups. The space steric effect inhibits the second hydroxyl to participate reaction and leads to the linear polymer main chain to be formed.
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Fig. 4. 1H NMR spectrum of TPUPCM.
Table 1 Physical properties of TPUPCM6000. Dissolved properties
Melt processed properties
The average molecular weight
THF
DMF DMSO Melting point
The first degradation temperature
Mw
Slightly soluble
Easily soluble
323 1C
2.56 105 1.68 105
186 1C
Mn
3.3. Crystallization properties of TPUPCM To reveal the crystalline morphology of the TPUPCM, WAXD technique was employed and the XRD patterns of pristine PEG6000 and TPUPCMs with different molecular weight PEG are shown in Fig. 5. It shows that both pure PEG and TPUPCM have similar diffraction patterns, and so are their diffraction angles and crystal plane distances. In the diffraction pattern of pure PEG and TPUPCMs, two strong diffraction peaks appear at 18.81 and 23.21. The result indicates that PEG and TPUPCMs possess similar crystal structure and crystal cell type [24]. The differences between them are that the diffraction peak height of TPUPCM is lower and the half width is broader than that of pure PEG, which means the crystallites become smaller and the degree of crystallinity decreases. Furthermore, the diffraction peak intensity gradually increases in the diffraction patterns of TPUPCM with the increase of molecular weight of PEG, when the molecular weight of PEG is 6000, the diffraction peak intensity reaches maximum, and then decreases. These results show that TPUPCM with PEG6000 (TPUPCM6000) has the best crystallization properties. To further verify the crystallization properties of TPUPCM, the POM was used for recording the micro-morphologies of pure PEG and TPUPCM. Fig. 6(a) and (b) show the POM photos of PEG6000 and TPUPCM6000 at room temperature. It is observed that both micrographs show obvious cross-extinction patterns, which suggests that both of them are crystalline and their crystalline morphologies are spherulites. However, the soft segments were limited by the hard segments in TPUPCM, the crystallization process of PEG becomes heterogeneous nucleation and confined crystallization, and the crystallites are smaller [24]. When the temperatures approaches transition point of melting, the spherulites fade away and eventually disappear, and the soft segments experience the transformation from crystalline state to amorphous one. These results are in good agreement with WXRD results.
Fig. 5. XRD patterns of the TPUPCM.
3.4. Phase change properties of TPUPCM To study the phase change properties of TPUPCM, the Differential scanning calorimetry (DSC) was employed. The DSC curves of pure PEG and TPUPCMs with different molecular weight PEG are shown in Fig. 7, and the data of their transition temperature and enthalpy are summarized in Table 2. By contrast with the analysis results of DSC, it can be found that PEG and TPUPCMs undergo all phase transition, but their phase transition states are quite different. The phase transition of pure PEG is a process from solid to liquid. When the temperature is raised to about 65.9 1C, the pure PEG undergoes a change from a white crystal solid to a transparent liquid [26]. While, TPUPCM remains solid in the whole heating process, even if the temperature is raised to 100 1C or higher, indicating that TPUPCM is solid–solid phase material. Fig. 7 shows that the DSC curves of TPUPCMs have strong melting and releasing heat peaks during the heating and cooling procedure. Moreover, the phase change enthalpy values of TPUPCM are gradually enhanced with the
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Fig. 6. POM photos of PEG and TPUPCM: (a) PEG6000; (b) TPUPCM6000.
Fig. 8. Heating and cooling DSC cures of TPUPCM after 1000 thermal cycles.
Fig. 7. DSC curves of the TPUPCM. Table 2 Phase change temperature and enthalpy of PEG6000 and TPUPCM. Sample
PEG6000 TPUPCM2000 TPUPCM4000 TPUPCM6000 TPUPCM8000
Phase transition
Solid–liquid Solid–solid Solid–solid Solid–solid Solid–solid
WH(J/g)
Tp ( 1C)
Heating cycle
Cooling cycle
Heating cycle
Cooling cycle
189.6 97.5 126.4 137.4 127.1
167.2 88.9 112.4 127.6 115.2
65.9 56.3 55.8 57.1 57.5
37.6 31.8 33.4 31.2 30.1
Tp ¼Peak transition temperature of samples; samples.
WH ¼phase transition enthalpy of
increase of molecular weight of PEG. This demonstrates that the phase change enthalpy of TPUPCM corresponds to the melting enthalpy of the soft segments in TPUPCM. It’s worth noting that the phase change enthalpy of TPUPCM with PEG6000 is 137.4 J/g (heating cycle) and 127.6 J/g (cooling cycle) and the transition temperature is suitable. These results prove that prepared TPUPCMs are reversible phase transition latent storage material, which possess excellent phase change properties and have a wide application in the energy storage.
Fig. 9. The FT-IR spectrum of synthesized TPUPCM after 1000 thermal cycles. (a) First thermal cycle; (b) 1000th thermal cycle.
Compared with PEG/MDI/PE cross-linking copolymer [24], the phase change enthalpy of TPUPCM is 137.4 J/g, lower than that of PEG/MDI/PE cross-linking copolymer, the phase change enthalpy
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of PEG/MDI/PE cross-linking copolymer is 152.97 J/g. This is because the phase change enthalpy of the PCM is intimately related to the weight percentage of phase change unit. In the molecular structure of TPUPCM, the phase change units include PEG (in the main chain) and MPEG (in the branch chain). When the main chain structure remains unchanged, the weight percentage of phase change unit of TPUPCM gradually increases with the increase of molecular weight of MPEG. Li chose PEG10000 as phase change unit in the synthesis of PEG/MDI/PE cross-linking copolymer PCM. In view of the mechanical properties of TPUPCM,
Fig. 10. TGA curve of TPUPCM6000.
we chose MPEG4000 as branch chain of TPUPCM. This affects the phase change enthalpy of TPUPCM to an extent. Furthermore, through the analysis of FT-IR spectrum of TPUPCM, it can be found that there are a few of hydroxy groups which have not reacted. This also makes the phase change enthalpy of TPUPCM reduced. 3.5. Thermal reliability and stability of TPUPCM The DSC curves of synthesized TPUPCM after 1500 and 1000 thermal cycles (melting/freezing) are shown in Fig. 8. From Fig. 8, it can be found that the phase change temperatures and latent heat values of the TPUPCM slightly change after 1000 cycles. Based on these results, it is note worthy that the as-synthesized PCMs have good thermal reliability in terms of thermal properties after thermal cycles. The FT-IR spectrum of synthesized TPUPCM after 1000 thermal cycles (melting/freezing) is shown in Fig. 9. By comparison of the analysis results, it can be found that any degradation does not occur in the chemical structure of the TPUPCM after 1000 thermal cycles. This means that the synthesized TPUPCMs have good thermal stability during a long utility period. The thermogravimetric and derivative thermogravimetric analysis curves of TPUPCM6000 are shown in Fig. 10. The mechanism of thermal decomposition involves two steps, as can be seen in TGA curves. When the temperature reaches 323 1C, slight thermal decomposition appears. The decomposition of TPUPCM reaches to a maximum at 396.2 1C. These results indicate that TPUPCMs have a wider processing temperature range. This is very important for fabrication and application of TPUPCM. 3.6. Fabrication and mechanical properties of TPUPCM
Fig. 11. The sheets of TPUPCM.
As TPUPCMs are thermoplastic polymer, they can be easily processed into different shapes such as bar, rod tube and so on through extrusion and molding techniques. Fig. 11 shows the sheet products of TPUPCM, which were processed by molding techniques. The appearance of sheets of TPUPCM is white. The surface of sheets is smooth. The formation of sheets is easy. To investigate the mechanical properties of TPUPCM, the Instron 5566 tensile tester was used. Fig. 12 presents the change of sample in tension process. By contrast with these photos, it can be seen that the sample shows obvious necking in tension process. In Table 3, the results prove that the maximum elongation at break and the breaking strength of the sample achieve 354% and 61 N, respectively. These results indicate that TPUPCM owns excellent mechanical properties.
Fig. 12. The change of the sample in tension process.
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Table 3 The mechanical properties of TPUPCM. Sample Breaking strength (N)
Extension at break (mm)
Elongation at break (%)
Fracture work (Nm)
1 2 3
47.6 88.5 56.3
190.4 354.0 225.0
2302.8 4628.2 2861.9
56.0 61.0 61.0
4. Conclusion Depending on design and synthesis of the novel tetrahydroxy compound, form-stable thermoplastic polyurethane solid–solid phase change material (TPUPCM) was prepared. According to the results analysis, TPUPCM has the same crystal structure and crystal cell type as PEG, and shows outstanding phase change properties. Furthermore, TPUPCM owns good thermal stability during a long utility period. It is note worthy that TPUPCM can be processed directly, and has a wider processing temperature range, good mechanical properties. These results indicate that TPUPCM has a great potential for thermal energy storage applications.
Acknowledgement The authors thank for the financial supports from Application Fundamental and Advanced Technology Research Proposal Project of Tianjin, China (No. 10 JCYBJC03100) and Key project of Chinese Ministry of Education (No. 208005). References [1] Aydin Ahmet Alper, Okutan Hasancan, High-chain fatty acid esters of myristyl alcohol with odd carbon number: novel organic phase change materials for thermal energy storage-2, Solar Energy Materials and Solar Cells 95 (2011) 2417–2423. [2] Yanlai Zhang, Shuangfeng Wang, Zhonghao Rao, Experiment on heat storage characteristic of microencapsulated phase change material slurry, Solar Energy Materials and Solar Cells 95 (2011) 2726–2733. [3] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al, A review on phase change energy storage: materials and applications, Energy Conversion and Management 45 (2004) 1597–1615. [4] H. Li, G.Y. Fang, Experimental investigation on the characteristics of polyethylene glycol/cement composites thermal energy storage materials, Chemical Engineering and Technology 33 (2010) 1650–1654. [5] C.Z. Chen, L.G. Wang, Y. Huang, Crosslinking of the electrospun polyethylene glycol/cellulose acetate composite fibers as shape-stabilized phase change materials, Materials Letters 63 (2009) 569–571. [6] T. Kousksou, A. Jamil, T. El Rhafiki, Paraffin wax mixtures as phase change materials, Solar Energy Materials and Solar Cells 94 (2010) 2158–2165.
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[7] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renewable & Sustainable Energy Reviews 13 (2009) 318–345. [8] X.H. Gu, P. Xi, B.W. Cheng, Synthesis and characterization of a novel solid– solid phase change luminescence material, Polymer International 59 (2010) 772–777. [9] Deng Zhang Xiaoxing, Feng Pengfei, Rongxiu, novel gelatinous shape-stabilized phase change materials with high heat storage density, Solar Energy Materials and Solar Cells 95 (2011) 1213–1218. [10] Xiaoxi Weilong Wanga, Yutang Yang, Jing Fang, Ding, Preparation and performance of form-stable polyethylene glycol/silicon dioxide composites as solid–liquid phase change materials, Applied Energy 86 (2009) 170–174. [11] Aydin Ahmet Alper, Okutan Hasancan, High-chain fatty acid esters of myristyl alcohol with even carbon number: novel organic phase change materials for thermal energy storage-1, Solar Energy Materials and Solar Cells 95 (2011) 2752–2762. [12] Mei Dandan, Zhang Bing, Liu Ruichao, Preparation of capric acid/halloysite nanotube composite as form-stable phase change material for thermal energy storage, Solar Energy Materials and Solar Cells 95 (2011) 2772–2777. [13] Feng Lili, Zheng Jie, Yang Huazhe, Preparation and characterization of polyethylene glycol/active carbon composites as shape-stabilized phase change materials, Solar Energy Materials and Solar Cells 95 (2011) 644–650. [14] Yanbin Cui, Caihong Liu, Shan Hu, The experimental exploration of carbon nanofiber and carbon nanotube additives on thermal behavior of phase change materials, Solar Energy Materials and Solar Cells 95 (2011) 1208–1212. [15] L. Xia, P. Zhang, Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage, Solar Energy Materials and Solar Cells 95 (2011) 2246–2254. [16] Sedat Karaman, Ali Karaipekli, Ahmet Sari, Polyethylene glycol (PEG)/diatomite composite as a novel form-stable phase change material for thermal energy storage, Solar Energy Materials and Solar Cells 95 (2011) 1647–1653. [17] S.B. Senturk, D. Kahraman, C. Alkan, Biodegradable PEG/cellulose, PEG/ agarose and PEG/chitosan blends as shape stabilized phase change materials for latent heat energy storage, Carbohydrate Polymers 84 (2011) 141–144. [18] Xiao-Ming Zhou, Study on phase change characteristics of PEG/PAM coupling blend, Journal of Applied Polymer Science 116 (2010) 1591–1595. [19] A. Sari, C. Alkan, A. Karaipekli, Poly(ethylene glycol)/poly(methyl methacrylate) blends as novel form-stable phase-change materials for thermal energy storage, Journal of Applied Polymer Science 116 (2010) 929–933. [20] Xiao-Ming Zhou, Preparation and characterization of PEG/MDI/PVA copolymer as solid–solid phase change heat storage material, Journal of Applied Polymer Science 113 (2009) 2041–2045. [21] Y. Jiang, E. Ding, G. Li, Study on transition characteristics of PEG/CDA solid– solid phase change materials, Polymerica . 43 (2002) 117. [22] C. Qi, L. Liao, H.L. Xu, Study on the influence of thermal characteristics of hyperbranched polyurethane phase change materials for energy storage, Journal of Applied Polymer Science 115 (2010) 2228–2235. [23] Su Jing-Cang, Liu Peng-Sheng, A novel solid–solid phase change heat storage material with polyurethane block copolymer structure, Energy Conversion and Management 47 (2006) 3185–3191. [24] Li Wei-Dong, Ding En-Yong, Preparation and characterization of cross-linking PEG/MDI/PE copolymer as solid–solid phase change heat storage material, Solar Energy Materials and Solar Cells 91 (2007) 764–768. [25] Xi Peng, Gu XiaoHua, Huang XiangAn, Synthesis, characterization and luminescent properties of new highly luminescent organic ligand and complexes of trivalent rare earth, Spectrochimica Acta Part A 66 (2007) 667–671. [26] C. Alkan, A. Sari, O. Uzun, Poly (ethylene glycol)/acrylic polymer blends for latent heat thermal energy storage, AIChE Journal 52 (2006) 3310–3314.