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fatty acid composites as form stable phase change materials
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Energy Conversion and Management 49 (2008) 373–380 www.elsevier.com/locate/enconman
Preparation, characterization and thermal properties of styrene maleic anhydride copolymer (SMA)/fatty acid composites as form stable phase change materials ¨ nal Ahmet Sarı *, Cemil Alkan *, Ali Karaipekli, Adem O Department of Chemistry, Gaziosmanpasßa University, 60240, Tokat, Turkey Received 20 March 2006; received in revised form 17 October 2006; accepted 4 June 2007 Available online 20 July 2007
Abstract Fatty acids such as stearic acid (SA), palmitic acid (PA), myristic acid (MA) and lauric acid (LA) are promising phase change materials (PCMs) for latent heat thermal energy storage (LHTES) applications, but high cost is the major drawback of them, limiting their utility area in thermal energy storage. The use of fatty acids as form stable PCMs will increase their feasibilities in practical applications due to the reduced cost of the LHTES system. In this regard, a series of styrene maleic anhydride copolymer (SMA)/fatty acid composites, SMA/SA, SMA/PA, SMA/MA, and SMA/LA, were prepared as form stable PCMs by encapsulation of fatty acids into the SMA, which acts as a supporting material. The encapsulation ratio of fatty acids was as much as 85 wt.% and no leakage of fatty acid was observed even when the temperature of the form stable PCM was over the melting point of the fatty acid in the composite. The prepared form stable composite PCMs were characterized using optic microscopy (OM), viscosimetry and Fourier transform infrared (FT-IR) spectroscopy methods, and the results showed that the SMA was physically and chemically compatible with the fatty acids. In addition, the thermal characteristics such as melting and freezing temperatures and latent heats of the form stable composite PCMs were measured by using the differential scanning calorimetry (DSC) technique, which indicated they had good thermal properties. On the basis of all the results, it was concluded that form stable SMA/fatty acid composite PCMs had important potential for practical LHTES applications such as under floor space heating of buildings and passive solar space heating of buildings by using wallboard, plasterboard or floors impregnated with a form stable PCM due to their satisfying thermal properties, easy preparation in desired dimensions, direct usability without needing additional encapsulation thereby eliminating the thermal resistance caused by the shell and, thus, reducing the cost of the LHTES system. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Form stable phase change material (PCM); Styrene maleic anhydride copolymer (SMA); Fatty acid; Latent heat thermal energy storage (LHTES)
1. Introduction Thermal energy storage (TES) is a useful tool to increase energy efficiency and energy savings. Latent heat thermal energy storage (LHTES) using a PCM is one of the most attractive methods of TES due to allowing storage and release of energy in large quantities per unit weight of *
Corresponding authors. Tel.: +90 3562521616; fax: +90 3562521585. E-mail addresses: [email protected] (A. Sarı), [email protected] (C. Alkan). 0196-8904/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.06.006
PCM at nearly constant temperature during the phase change [1–3]. As conventional PCMs, fatty acids have been characterized by their proper solid–liquid phase change temperature, high latent heat storage capacity, good thermal properties and thermal reliability [4–8], but, their cost is high compared to other types of PCMs especially salt hydrates and paraffins. PCMs require special LHTES devices in different shapes or elements such as shell and tube PCM heat exchanger or a lot of cans to encapsulate them since they change from solid to liquid during the energy storage period. Although the use
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of such type materials solved the solid–liquid phase change problem, it increases not only the heat resistance but also the cost of the LHTES system. However, these problems can be overcome using a form stable PCM, which can be prepared by encapsulation of the PCM into a polymeric network. Moreover, the main advantages of such a kind of PCM are the following [4–9]: (1) not allowing leakage of melted PCM during phase change process; (2) not needing an additional storage container for encapsulation, thereby reducing the cost of the LHTES system; (3) eliminating the thermal resistance caused by the capsule shell; (4) reducing the reactivity of PCM with the environment and controlling the volume change of the PCM during the solid–liquid phase change; and (5) easy preparation in the desired dimensions, and therefore, they become feasible for some heating applications in buildings such as underfloor space heating and reducing electric peak load in heating in winter by using wallboard and plasterboard prepared by absorption of a PCM. These beneficial properties directed the researchers to develop new kinds of form stable PCMs and use them in practical LHTES applications. Hong and Xin-Shi [10] prepared a form stable PCM that consists of paraffin as the dispersed PCM and high density polyethylene (HDPE) with different melting index as supporting material, and they reported that the total stored energy by the form stable PCM is comparable with that of traditional PCMs since the mass fraction of paraffin can be as much as 80%. Inaba and Tu [11] studied the thermal performance of a form stable PCM composed of paraffin and HDPE. Sarı [12] investigated the latent heat storage properties and thermal conductivity of a paraffin/HDPE composite as form stable PCM and suggested the composite with mass fraction of 3% expanded graphite as a promising form stable PCM for LHTES applications. Xiao et al. [13] selected a styrene–butadiene–styrene copolymer as supporting material and paraffin as PCM to prepare a form stable PCM. This material, with 80 wt.% paraffin, had a melting temperature of 56–58 °C and a heat of fusion of 165.21 kJ/kg. Peng et al. [15] encapsulated low temperature melting materials like paraffin in bisphenol-A epoxy and styrene–ethylene–butylene–styrene (SEBS) polymers and showed that the interaction between paraffin and SEBS resulted in a material with adequate thermal and mechanical performance. Liu et al. [16] analyzed the thermal properties and hydrophilic–lipophilic properties of a form stable HDPE/paraffin PCM encapsulated in silica gel, and they proposed this PCM for use in the building field because of its good thermal properties and better hydrophilicity and better fire proofing properties. Alkan et al. [17] studied polyethylene glycol/acrylic polymer blends as form stable PCMs for LHTES systems and recommended these blends as suitable materials for day time solar energy storage for space heating. Moreover, the polymer-based form stable composite PCMs could be shaped into plates or added into concrete to be directly applied as floor or wallboard [18]. In recent years, some applications of such material in energy efficient buildings (e.g. in floor space heating system and
wallboard or floor to absorb solar energy and narrow the temperature swing of a day in winter) was studied by Xu et al. [19] and Lin et al. [20]. On the other hand, polymer encapsulants provide an opportunity to utilize fatty acids as PCMs within a unique composite structure. Ozonur et al. [21] prepared microcapsules of a natural coco fatty acid mixture for TES systems using urea–formaldehyde resin, melamin–formaldehyde resin, b-naphthol–formaldehyde resin and gelatin–gum Arabic and suggested that the coco acid mixture microencapsulated in gelatin-gum Arabic can be a candidate for PCM applications in heating and cooling of building with further development. Feldman et al. [14] developed a matrix type phase change thermal storage tile module by using blends of fatty acids with polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate and vinyl acetate–vinyl chloride copolymer, and they reported that the new type tile module could be used as latent heat storage material with no outer container. In addition, Pielichowski and Flejtuch [22–24] studied binary blends of polyethylene glycol, polyethers and polyethylene oxide with selected fatty acids and characterized the blends using calorimetric, spectroscopic and optic methods. They indicated that the polymer-based binary blends as PCMs were feasible for TES systems working at large temperature ranges. The copolymerization of styrene with maleic anhydride creates a copolymer (SMA) that has a higher glass transition temperature than polystyrene and is chemically reactive with certain functional groups. SMA is often used in blends or composites where interaction or reaction of the maleic anhydride provides desirable interfacial effects [25]. Moreover, SMA has two functional groups, carbonyl oxygen (AC@O) and ether oxygen (AOA) that can interact with the hydroxyl (AOH) functional group of a fatty acid. Therefore, SMA can be used to encapsulate fatty acids as PCMs at room temperature. It can be recognized from the literature that the majority of the investigations deal with the encapsulation of paraffin into a polymer structure, and the studies regarding polymer encapsulation of fatty acids are limited. Therefore, this paper reports the preparation, characterization and thermal properties of SMA/fatty acid composites (SMA/SA, SMA/PA, SMA/MA and SMA/LA) as form stable PCMs for LHTES applications. The compatibility of SMA with the fatty acids was characterized using morphology (OM), viscosimetry (dilute solution viscosimetry) and spectroscopy (FT-IR) methods. In addition, the thermal properties such as melting and freezing temperatures and latent heats of the prepared form stable composite PCMs were determined using the DSC analysis technique. 2. Experimental 2.1. Materials Styrene maleic anhydride copolymer (SMA, Mw: 224,000 g mol1) was obtained from the Merck company
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and used without purification. The chemical formula of SMA is given in Fig. 1. Stearic acid (SA), palmitic acid (PA), myristic acid (MA) and lauric acid (LA) were also supplied from the Merck Company. The thermal properties of the fatty acids (>96% purity) measured by DSC analysis are given in Table 1. Chloroform with Merck grade was used to dissolve the SMA and fatty acids. 2.2. Preparation of form stable composite PCMs The SMA/fatty acid composites were prepared by the solution casting method. Solutions of SMA and fatty acid were dissolved in chloroform prepared separately and the fatty acid solution was added to the SMA solution drop wise. Then, chloroform was cast at room temperature for 15 days. The composites were prepared at different mass fractions of fatty acid (50%, 60%, 70%, 80%, 85% and 90%) to find the maximum encapsulation ratio without leakage of the fatty acid from the composite surface when the temperature was between the melting points of the SMA and fatty acids. 2.3. Characterization of form stable composite PCMs Firstly, the composites with different mass fraction of fatty acid were subjected to heat treatment over the melting point of the PCM (fatty acid) to determine the highest encapsulation fraction without leakage of the fatty acid from the composite. The composites with this property were defined as form stable composite PCMs. In order to prove the physical and chemical compatibility between the components of the composite, the form stable composite PCMs were characterized by morphology and spectroscopy methods. The morphology of the PCMs was investigated using an optical microscope (OM, Laica model). The spectroscopic analysis was performed on a KBr disk by using a Jasco 430 model FT-IR.
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The compatibility of the components in the solutions was also characterized by the viscometry technique. Viscosity measurements were obtained with an Ubbelohde viscometer in a constant temperature water bath at 25 °C using chloroform as a solvent. A solution concentration of 0.5 g/dl was not exceeded in the viscosity measurements. The obtained viscosity data were evaluated by the following equation expressing the specific viscosity (gsp) of the polymer as a function of the concentration (C) 2
gsp =C ¼ ½g þ K½g C
ð1Þ
where gsp/C is the reduced viscosity and [g] is the intrinsic viscosity that corresponds to zero concentration of the solution. K and K[g]2 are the Huggins coefficient and the interaction term, respectively. The thermal properties of the form stable SMA/fatty acid composites such as melting and freezing points and latent heats of melting and freezing were measured by a SETARAM DSC 131 model instrument. Indium was used as a reference for temperature calibration. The analyses were performed at temperatures of 20–100 °C and 5 °C/ min heating rate under a constant stream of argon at a flow rate of 60 ml/min. The heat flow repeatability was 0.2 lW. A 5–10 mg of sample was sealed in an aluminum pan. The melting and freezing point were taken as the onset temperature obtained by drawing a line at the point of maximum slope of the leading edge of the DSC peak and extrapolating the base line on the same side as the leading edge of the peak. The latent heat capacity was determined by numerical integration of the area of the DSC peak. Reproducibility was tested by conducting three measurements, and the mean deviation was ±0.14 °C in phase change temperature and ± 1.82 J/g in latent heat capacity. 3. Results and discussion 3.1. Physical and chemical characterization of form stable composite PCMs
Fig. 1. Chemical structures of styrene–maleic anhydride copolymer (SMA).
Table 1 Thermal properties of fatty acids Fatty acid
Melting point (°C)
Latent heat of Freezing melting (J/g) point (°C)
Latent heat of freezing (J/g)
Stearic acid (SA) Palmitic acid (PA) Myristic acid (MA) Lauric acid (LA)
66.87 60.45 52.44 42.14
242.15 221.42 210.70 190.12
246.74 226.56 212.65 194.23
66.36 59.88 52.49 42.20
Each of the fatty acids was encapsulated successfully into the polymer matrix of SMA as much as 85 wt.%. The SMA/fatty acid (15/85 wt.%) composites were characterized as form stable PCMs since they did not allow leakage of the melted fatty acid from the surface even when subjected to heat treatment over the melting points of the fatty acids. Fig. 2 shows the optical micrographs of form stable SMA/SA, SMA/PA, SMA/MA and SMA/LA (15/ 85 wt.%) composite PCMs. A single phase behavior was observed from the optic images, indicating that the fatty acids were encapsulated by the SMA. This case verified strongly the association of fatty acid with SMA, and the surface was stable against leakage of fatty acid from the composite surface. If there were no interactions between the components of the composites, the solid phase of SMA and the liquid phase of fatty acid would be detected
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Fig. 2. The OM micrographs of form stable PCMs. (a) SMA/SA (15/85 wt.%), (b) SMA/PA (15/85 wt.%), (c) SMA/MA (15/85 wt.%) and (d) SMA/LA (15/85 wt.%).
separately due to seepage of fatty acids at higher temperatures than the melting points of fatty acids. In addition, the morphology investigation revealed that SMA was compatible physically with the fatty acids. Because of its simplicity, viscosimetry has become an attractive method for studying the compatibility of polymers in solution. Therefore, the compatibility of SMA with the fatty acids was investigated using the dilute solution viscosimetry method. The solution viscosity of the composite determined to be higher than that of the pure components, according to this method, indicates strong attractive interaction between the components and good compatibility. Fig. 3 shows the variation of the reduced viscosity with the concentration of the dilute solution of form stable composite PCMs (15/85 wt.%). The slope and intercept of the plots correspond to the K[g]2 and intrinsic viscosity, [g] of the composite with respect to Eq. (1), respectively. It is clearly seen from the figure that the [g] and K[g]2 values of all the composites were higher than that of their pure components due to the presence of strong attractive interactions between the SMA and the fatty acids. This result is also evidence of good chemical compatibility between the components of the form stable composite PCMs. Furthermore, FT-IR spectroscopy is a powerful technique to investigate the specific interactions between the pure components of a composite material. Fig. 4 shows the FT-IR spectra of the pure components (LA and SMA) and their corresponding composite (SMA/LA, 15/ 85 wt.%), given as an example. The following remarks were derived based on the FT-IR spectroscopy analysis:
1. The attractive interaction between the hydroxyl group (AOH) in the LA molecules and the ether oxygen in SMA was proved by the shift of the hydroxyl absorption band in the 3410–3550 cm1 and 920–940 cm1 ranges. 2. An additional analysis of the FT-IR spectrum of the LA indicated that carboxylic acid formed a dimer structure (AC@O stretching band at 1698 cm1), and there was no esterification reaction between the SMA and the acid, as evidenced by SMA/LA acid spectrum. 3. The shape (strong single peak) and location (1698 cm1) of the carbonyl groups absorption band in the FT-IR spectrum of LA (proving that LA forms dimers) were very similar as in the form stable SMA/LA (15/ 85 wt.%) composite PCM (single peak, 1699 cm1). This was evidence indicating that the carbonyl group of LA did not take part in the interaction, and it occurred between the hydroxyl group of LA and ether oxygen of the SMA. The FT-IR analysis results obtained for form stable SMA/SA, SMA/PA and SMA/MA composite PCMs (15/85 wt.%) were almost the same. The results also confirmed the existence of good chemical compatibility between the SMA and the fatty acids. Similar spectral findings were reported for different kinds of polymer/fatty acid composite PCMs by Feldman et al. [14] and Pielichowski and Flejtuch [23,24].
3.2. Thermal properties of form stable composite PCMs Thermal properties such as phase change temperature and phase change enthalpy of a PCM are important parameters for an effective LHTES application [1–4]. In
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Fig. 3. The reduced viscosity versus concentration of form stable composite PCMs in chloroform at 25 °C.
Fig. 4. FT-IR spectra of LA, SMA and form stable SMA/LA (15/85 wt.%) composite PCM.
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form stable PCMs, the magnitude of these properties depends on the mass fraction of PCM encapsulated by the supporting material. The phase change enthalpy or latent heat capacity of a form stable composite PCM increases with increasing mass percentage of the PCM in the composite. The thermal properties of the prepared form stable composite PCMs were measured using the DSC technique, and the results were given in Figs. 5–8 and Table 2. When comparing the phase change temperatures of the form stable PCMs with those of the pure fatty acids (Table 1), it can be seen that the change in melting point was 0.48, 0.41, 0.69 and 0.66 °C and the change in freezing point was 0.32, 0.61, 0.42 and 0.42 °C for the form stable SMA/SA, SMA/PA, SMA/MA and SMA/ LA (15/85 wt.%) composite PCMs, respectively. These results can be considered as an indicator of the interaction that occurred between the functional groups of the SMA and the fatty acids. It can also be noted that the form stable composite PCMs have proper phase change temperatures for some practical LHTES applications such as under-floor space heating and absorbing solar energy in wallboard, plaster board or floor impregnated with the form stable PCM. Moreover, the form stable SMA/SA, SMA/PA, SMA/MA and SMA/LA (15/85 wt.%) composite PCMs have a latent heat of melting of 202.23, 184.54, 176.49 and 160.83 J/g and a latent heat of freezing of 204.82, 185.18, 177.78 and 163.43 J/g, respectively. By comparing these values with those of some polymer/fatty acid blends in the literature [14,22–24], it can be remarkably noted that the form stable SMA/fatty acid composites have high latent heat storage capacity, and therefore, they are promising PCMs for practical LHTES applications performed at temperature range of 40–70 °C. On the other hand, the latent heat values of the form stable PCMs were calculated using the equation DH SMA=FA ¼ W FA DH FA
Fig. 6. DSC curve of SMA/PA (15/85 wt.%) composite PCM.
Fig. 7. DSC curve of SMA/MA (15/85 wt.%) composite PCM.
ð2Þ
where DHSMA/FA, WFA and DHFA are the calculated latent heat of form stable SMA/fatty acid composite PCM, mass
Fig. 8. DSC curve of SMA/LA (15/85 wt.%) composite PCM.
Fig. 5. DSC curve of form stable SMA/SA (15/85 wt.%) composite PCM.
percentage of fatty acid and the measured latent heat of the pure fatty acid (Table 1), respectively. The calculated latent heat values are given in Table 3. Comparing the measured latent heat values of the form stable PCMs (Table 2) with the calculated values using Eq.
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Table 2 Measured thermal properties of form stable SMA/fatty acid composite PCMs (15/85 wt.%) using DSC
needing an extra encapsulation, thereby reducing the cost of the LHTES system.
Form stable composite PCM
Melting point (°C)
Latent heat of melting (J/g)
Freezing point (°C)
Latent heat of freezing (J/g)
Acknowledgement
SMA/SA SMA/PA SMA/MA SMA/LA
67.35 60.04 51.75 41.48
202.23 184.54 176.49 160.83
66.68 59.27 52.07 41.78
204.82 185.18 177.78 163.43
The authors would like to acknowledge the financial support (Project No. 2003K120510) by the Turkish State Planning Organization. References
Table 3 Calculated latent heat values of form stable SMA/fatty acid composite PCMs (15/85 wt.%) using Eq. (2) Form stable composite PCM
Latent heat of melting (J/g)
Latent heat of freezing (J/g)
SMA/SA SMA/PA SMA/MA SMA/LA
205.83 188.21 180.00 161.67
209.73 192.58 180.75 166.00
(2) (Table 3), it was clearly seen that the measured values were slightly less than the calculated values. This was due to the interaction between the components of the form stable composite PCMs. 4. Conclusions The SMA/fatty acid composites were prepared as new kinds of form stable PCMs for LHTES systems by encapsulation of the fatty acids (SA, PA, MA and LA) as PCMs into the polymer matrix of SMA as supporting material. The fatty acids were encapsulated successfully as much as 85 wt.%, and no leakage of fatty acid was observed even when the temperature of the form stable PCM was over the melting point of the fatty acid. The prepared form stable SMA/SA, SMA/PA, SMA/MA and SMA/LA (15/ 85 wt.%) composite PCMs were characterized using optic microscopy, viscosimetry and FT-IR spectroscopy methods. The results proved the availability of the attractive interaction between the hydroxyl group of the fatty acids and the ether oxygen of the SMA, which resulted in good physical and chemical compatibility of the components of the composites. Moreover, the thermal properties of the form stable composite PCMs were measured using the DSC technique. The DSC results indicated that the form stable composite PCMs had proper phase change temperatures and satisfying latent heat storage capacities for practical LHTES applications. In this work, it was concluded that the form stable SMA/fatty acid composites can be considered as candidate PCMs for LHTES applications such as under-floor space heating of buildings and solar energy storage using wallboard and plasterboard impregnated with form stable PCM due to having good thermal properties, simple preparation in desired dimensions, elimination of the heat resistance caused by the capsule shell, direct usability without
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Energy Conversion and Management 49 (2008) 373–380 www.elsevier.com/locate/enconman
Preparation, characterization and thermal properties of styrene maleic anhydride copolymer (SMA)/fatty acid composites as form stable phase change materials ¨ nal Ahmet Sarı *, Cemil Alkan *, Ali Karaipekli, Adem O Department of Chemistry, Gaziosmanpasßa University, 60240, Tokat, Turkey Received 20 March 2006; received in revised form 17 October 2006; accepted 4 June 2007 Available online 20 July 2007
Abstract Fatty acids such as stearic acid (SA), palmitic acid (PA), myristic acid (MA) and lauric acid (LA) are promising phase change materials (PCMs) for latent heat thermal energy storage (LHTES) applications, but high cost is the major drawback of them, limiting their utility area in thermal energy storage. The use of fatty acids as form stable PCMs will increase their feasibilities in practical applications due to the reduced cost of the LHTES system. In this regard, a series of styrene maleic anhydride copolymer (SMA)/fatty acid composites, SMA/SA, SMA/PA, SMA/MA, and SMA/LA, were prepared as form stable PCMs by encapsulation of fatty acids into the SMA, which acts as a supporting material. The encapsulation ratio of fatty acids was as much as 85 wt.% and no leakage of fatty acid was observed even when the temperature of the form stable PCM was over the melting point of the fatty acid in the composite. The prepared form stable composite PCMs were characterized using optic microscopy (OM), viscosimetry and Fourier transform infrared (FT-IR) spectroscopy methods, and the results showed that the SMA was physically and chemically compatible with the fatty acids. In addition, the thermal characteristics such as melting and freezing temperatures and latent heats of the form stable composite PCMs were measured by using the differential scanning calorimetry (DSC) technique, which indicated they had good thermal properties. On the basis of all the results, it was concluded that form stable SMA/fatty acid composite PCMs had important potential for practical LHTES applications such as under floor space heating of buildings and passive solar space heating of buildings by using wallboard, plasterboard or floors impregnated with a form stable PCM due to their satisfying thermal properties, easy preparation in desired dimensions, direct usability without needing additional encapsulation thereby eliminating the thermal resistance caused by the shell and, thus, reducing the cost of the LHTES system. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Form stable phase change material (PCM); Styrene maleic anhydride copolymer (SMA); Fatty acid; Latent heat thermal energy storage (LHTES)
1. Introduction Thermal energy storage (TES) is a useful tool to increase energy efficiency and energy savings. Latent heat thermal energy storage (LHTES) using a PCM is one of the most attractive methods of TES due to allowing storage and release of energy in large quantities per unit weight of *
Corresponding authors. Tel.: +90 3562521616; fax: +90 3562521585. E-mail addresses: [email protected] (A. Sarı), [email protected] (C. Alkan). 0196-8904/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.06.006
PCM at nearly constant temperature during the phase change [1–3]. As conventional PCMs, fatty acids have been characterized by their proper solid–liquid phase change temperature, high latent heat storage capacity, good thermal properties and thermal reliability [4–8], but, their cost is high compared to other types of PCMs especially salt hydrates and paraffins. PCMs require special LHTES devices in different shapes or elements such as shell and tube PCM heat exchanger or a lot of cans to encapsulate them since they change from solid to liquid during the energy storage period. Although the use
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of such type materials solved the solid–liquid phase change problem, it increases not only the heat resistance but also the cost of the LHTES system. However, these problems can be overcome using a form stable PCM, which can be prepared by encapsulation of the PCM into a polymeric network. Moreover, the main advantages of such a kind of PCM are the following [4–9]: (1) not allowing leakage of melted PCM during phase change process; (2) not needing an additional storage container for encapsulation, thereby reducing the cost of the LHTES system; (3) eliminating the thermal resistance caused by the capsule shell; (4) reducing the reactivity of PCM with the environment and controlling the volume change of the PCM during the solid–liquid phase change; and (5) easy preparation in the desired dimensions, and therefore, they become feasible for some heating applications in buildings such as underfloor space heating and reducing electric peak load in heating in winter by using wallboard and plasterboard prepared by absorption of a PCM. These beneficial properties directed the researchers to develop new kinds of form stable PCMs and use them in practical LHTES applications. Hong and Xin-Shi [10] prepared a form stable PCM that consists of paraffin as the dispersed PCM and high density polyethylene (HDPE) with different melting index as supporting material, and they reported that the total stored energy by the form stable PCM is comparable with that of traditional PCMs since the mass fraction of paraffin can be as much as 80%. Inaba and Tu [11] studied the thermal performance of a form stable PCM composed of paraffin and HDPE. Sarı [12] investigated the latent heat storage properties and thermal conductivity of a paraffin/HDPE composite as form stable PCM and suggested the composite with mass fraction of 3% expanded graphite as a promising form stable PCM for LHTES applications. Xiao et al. [13] selected a styrene–butadiene–styrene copolymer as supporting material and paraffin as PCM to prepare a form stable PCM. This material, with 80 wt.% paraffin, had a melting temperature of 56–58 °C and a heat of fusion of 165.21 kJ/kg. Peng et al. [15] encapsulated low temperature melting materials like paraffin in bisphenol-A epoxy and styrene–ethylene–butylene–styrene (SEBS) polymers and showed that the interaction between paraffin and SEBS resulted in a material with adequate thermal and mechanical performance. Liu et al. [16] analyzed the thermal properties and hydrophilic–lipophilic properties of a form stable HDPE/paraffin PCM encapsulated in silica gel, and they proposed this PCM for use in the building field because of its good thermal properties and better hydrophilicity and better fire proofing properties. Alkan et al. [17] studied polyethylene glycol/acrylic polymer blends as form stable PCMs for LHTES systems and recommended these blends as suitable materials for day time solar energy storage for space heating. Moreover, the polymer-based form stable composite PCMs could be shaped into plates or added into concrete to be directly applied as floor or wallboard [18]. In recent years, some applications of such material in energy efficient buildings (e.g. in floor space heating system and
wallboard or floor to absorb solar energy and narrow the temperature swing of a day in winter) was studied by Xu et al. [19] and Lin et al. [20]. On the other hand, polymer encapsulants provide an opportunity to utilize fatty acids as PCMs within a unique composite structure. Ozonur et al. [21] prepared microcapsules of a natural coco fatty acid mixture for TES systems using urea–formaldehyde resin, melamin–formaldehyde resin, b-naphthol–formaldehyde resin and gelatin–gum Arabic and suggested that the coco acid mixture microencapsulated in gelatin-gum Arabic can be a candidate for PCM applications in heating and cooling of building with further development. Feldman et al. [14] developed a matrix type phase change thermal storage tile module by using blends of fatty acids with polyvinyl chloride, polyvinyl alcohol, polyvinyl acetate and vinyl acetate–vinyl chloride copolymer, and they reported that the new type tile module could be used as latent heat storage material with no outer container. In addition, Pielichowski and Flejtuch [22–24] studied binary blends of polyethylene glycol, polyethers and polyethylene oxide with selected fatty acids and characterized the blends using calorimetric, spectroscopic and optic methods. They indicated that the polymer-based binary blends as PCMs were feasible for TES systems working at large temperature ranges. The copolymerization of styrene with maleic anhydride creates a copolymer (SMA) that has a higher glass transition temperature than polystyrene and is chemically reactive with certain functional groups. SMA is often used in blends or composites where interaction or reaction of the maleic anhydride provides desirable interfacial effects [25]. Moreover, SMA has two functional groups, carbonyl oxygen (AC@O) and ether oxygen (AOA) that can interact with the hydroxyl (AOH) functional group of a fatty acid. Therefore, SMA can be used to encapsulate fatty acids as PCMs at room temperature. It can be recognized from the literature that the majority of the investigations deal with the encapsulation of paraffin into a polymer structure, and the studies regarding polymer encapsulation of fatty acids are limited. Therefore, this paper reports the preparation, characterization and thermal properties of SMA/fatty acid composites (SMA/SA, SMA/PA, SMA/MA and SMA/LA) as form stable PCMs for LHTES applications. The compatibility of SMA with the fatty acids was characterized using morphology (OM), viscosimetry (dilute solution viscosimetry) and spectroscopy (FT-IR) methods. In addition, the thermal properties such as melting and freezing temperatures and latent heats of the prepared form stable composite PCMs were determined using the DSC analysis technique. 2. Experimental 2.1. Materials Styrene maleic anhydride copolymer (SMA, Mw: 224,000 g mol1) was obtained from the Merck company
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and used without purification. The chemical formula of SMA is given in Fig. 1. Stearic acid (SA), palmitic acid (PA), myristic acid (MA) and lauric acid (LA) were also supplied from the Merck Company. The thermal properties of the fatty acids (>96% purity) measured by DSC analysis are given in Table 1. Chloroform with Merck grade was used to dissolve the SMA and fatty acids. 2.2. Preparation of form stable composite PCMs The SMA/fatty acid composites were prepared by the solution casting method. Solutions of SMA and fatty acid were dissolved in chloroform prepared separately and the fatty acid solution was added to the SMA solution drop wise. Then, chloroform was cast at room temperature for 15 days. The composites were prepared at different mass fractions of fatty acid (50%, 60%, 70%, 80%, 85% and 90%) to find the maximum encapsulation ratio without leakage of the fatty acid from the composite surface when the temperature was between the melting points of the SMA and fatty acids. 2.3. Characterization of form stable composite PCMs Firstly, the composites with different mass fraction of fatty acid were subjected to heat treatment over the melting point of the PCM (fatty acid) to determine the highest encapsulation fraction without leakage of the fatty acid from the composite. The composites with this property were defined as form stable composite PCMs. In order to prove the physical and chemical compatibility between the components of the composite, the form stable composite PCMs were characterized by morphology and spectroscopy methods. The morphology of the PCMs was investigated using an optical microscope (OM, Laica model). The spectroscopic analysis was performed on a KBr disk by using a Jasco 430 model FT-IR.
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The compatibility of the components in the solutions was also characterized by the viscometry technique. Viscosity measurements were obtained with an Ubbelohde viscometer in a constant temperature water bath at 25 °C using chloroform as a solvent. A solution concentration of 0.5 g/dl was not exceeded in the viscosity measurements. The obtained viscosity data were evaluated by the following equation expressing the specific viscosity (gsp) of the polymer as a function of the concentration (C) 2
gsp =C ¼ ½g þ K½g C
ð1Þ
where gsp/C is the reduced viscosity and [g] is the intrinsic viscosity that corresponds to zero concentration of the solution. K and K[g]2 are the Huggins coefficient and the interaction term, respectively. The thermal properties of the form stable SMA/fatty acid composites such as melting and freezing points and latent heats of melting and freezing were measured by a SETARAM DSC 131 model instrument. Indium was used as a reference for temperature calibration. The analyses were performed at temperatures of 20–100 °C and 5 °C/ min heating rate under a constant stream of argon at a flow rate of 60 ml/min. The heat flow repeatability was 0.2 lW. A 5–10 mg of sample was sealed in an aluminum pan. The melting and freezing point were taken as the onset temperature obtained by drawing a line at the point of maximum slope of the leading edge of the DSC peak and extrapolating the base line on the same side as the leading edge of the peak. The latent heat capacity was determined by numerical integration of the area of the DSC peak. Reproducibility was tested by conducting three measurements, and the mean deviation was ±0.14 °C in phase change temperature and ± 1.82 J/g in latent heat capacity. 3. Results and discussion 3.1. Physical and chemical characterization of form stable composite PCMs
Fig. 1. Chemical structures of styrene–maleic anhydride copolymer (SMA).
Table 1 Thermal properties of fatty acids Fatty acid
Melting point (°C)
Latent heat of Freezing melting (J/g) point (°C)
Latent heat of freezing (J/g)
Stearic acid (SA) Palmitic acid (PA) Myristic acid (MA) Lauric acid (LA)
66.87 60.45 52.44 42.14
242.15 221.42 210.70 190.12
246.74 226.56 212.65 194.23
66.36 59.88 52.49 42.20
Each of the fatty acids was encapsulated successfully into the polymer matrix of SMA as much as 85 wt.%. The SMA/fatty acid (15/85 wt.%) composites were characterized as form stable PCMs since they did not allow leakage of the melted fatty acid from the surface even when subjected to heat treatment over the melting points of the fatty acids. Fig. 2 shows the optical micrographs of form stable SMA/SA, SMA/PA, SMA/MA and SMA/LA (15/ 85 wt.%) composite PCMs. A single phase behavior was observed from the optic images, indicating that the fatty acids were encapsulated by the SMA. This case verified strongly the association of fatty acid with SMA, and the surface was stable against leakage of fatty acid from the composite surface. If there were no interactions between the components of the composites, the solid phase of SMA and the liquid phase of fatty acid would be detected
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Fig. 2. The OM micrographs of form stable PCMs. (a) SMA/SA (15/85 wt.%), (b) SMA/PA (15/85 wt.%), (c) SMA/MA (15/85 wt.%) and (d) SMA/LA (15/85 wt.%).
separately due to seepage of fatty acids at higher temperatures than the melting points of fatty acids. In addition, the morphology investigation revealed that SMA was compatible physically with the fatty acids. Because of its simplicity, viscosimetry has become an attractive method for studying the compatibility of polymers in solution. Therefore, the compatibility of SMA with the fatty acids was investigated using the dilute solution viscosimetry method. The solution viscosity of the composite determined to be higher than that of the pure components, according to this method, indicates strong attractive interaction between the components and good compatibility. Fig. 3 shows the variation of the reduced viscosity with the concentration of the dilute solution of form stable composite PCMs (15/85 wt.%). The slope and intercept of the plots correspond to the K[g]2 and intrinsic viscosity, [g] of the composite with respect to Eq. (1), respectively. It is clearly seen from the figure that the [g] and K[g]2 values of all the composites were higher than that of their pure components due to the presence of strong attractive interactions between the SMA and the fatty acids. This result is also evidence of good chemical compatibility between the components of the form stable composite PCMs. Furthermore, FT-IR spectroscopy is a powerful technique to investigate the specific interactions between the pure components of a composite material. Fig. 4 shows the FT-IR spectra of the pure components (LA and SMA) and their corresponding composite (SMA/LA, 15/ 85 wt.%), given as an example. The following remarks were derived based on the FT-IR spectroscopy analysis:
1. The attractive interaction between the hydroxyl group (AOH) in the LA molecules and the ether oxygen in SMA was proved by the shift of the hydroxyl absorption band in the 3410–3550 cm1 and 920–940 cm1 ranges. 2. An additional analysis of the FT-IR spectrum of the LA indicated that carboxylic acid formed a dimer structure (AC@O stretching band at 1698 cm1), and there was no esterification reaction between the SMA and the acid, as evidenced by SMA/LA acid spectrum. 3. The shape (strong single peak) and location (1698 cm1) of the carbonyl groups absorption band in the FT-IR spectrum of LA (proving that LA forms dimers) were very similar as in the form stable SMA/LA (15/ 85 wt.%) composite PCM (single peak, 1699 cm1). This was evidence indicating that the carbonyl group of LA did not take part in the interaction, and it occurred between the hydroxyl group of LA and ether oxygen of the SMA. The FT-IR analysis results obtained for form stable SMA/SA, SMA/PA and SMA/MA composite PCMs (15/85 wt.%) were almost the same. The results also confirmed the existence of good chemical compatibility between the SMA and the fatty acids. Similar spectral findings were reported for different kinds of polymer/fatty acid composite PCMs by Feldman et al. [14] and Pielichowski and Flejtuch [23,24].
3.2. Thermal properties of form stable composite PCMs Thermal properties such as phase change temperature and phase change enthalpy of a PCM are important parameters for an effective LHTES application [1–4]. In
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Fig. 3. The reduced viscosity versus concentration of form stable composite PCMs in chloroform at 25 °C.
Fig. 4. FT-IR spectra of LA, SMA and form stable SMA/LA (15/85 wt.%) composite PCM.
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form stable PCMs, the magnitude of these properties depends on the mass fraction of PCM encapsulated by the supporting material. The phase change enthalpy or latent heat capacity of a form stable composite PCM increases with increasing mass percentage of the PCM in the composite. The thermal properties of the prepared form stable composite PCMs were measured using the DSC technique, and the results were given in Figs. 5–8 and Table 2. When comparing the phase change temperatures of the form stable PCMs with those of the pure fatty acids (Table 1), it can be seen that the change in melting point was 0.48, 0.41, 0.69 and 0.66 °C and the change in freezing point was 0.32, 0.61, 0.42 and 0.42 °C for the form stable SMA/SA, SMA/PA, SMA/MA and SMA/ LA (15/85 wt.%) composite PCMs, respectively. These results can be considered as an indicator of the interaction that occurred between the functional groups of the SMA and the fatty acids. It can also be noted that the form stable composite PCMs have proper phase change temperatures for some practical LHTES applications such as under-floor space heating and absorbing solar energy in wallboard, plaster board or floor impregnated with the form stable PCM. Moreover, the form stable SMA/SA, SMA/PA, SMA/MA and SMA/LA (15/85 wt.%) composite PCMs have a latent heat of melting of 202.23, 184.54, 176.49 and 160.83 J/g and a latent heat of freezing of 204.82, 185.18, 177.78 and 163.43 J/g, respectively. By comparing these values with those of some polymer/fatty acid blends in the literature [14,22–24], it can be remarkably noted that the form stable SMA/fatty acid composites have high latent heat storage capacity, and therefore, they are promising PCMs for practical LHTES applications performed at temperature range of 40–70 °C. On the other hand, the latent heat values of the form stable PCMs were calculated using the equation DH SMA=FA ¼ W FA DH FA
Fig. 6. DSC curve of SMA/PA (15/85 wt.%) composite PCM.
Fig. 7. DSC curve of SMA/MA (15/85 wt.%) composite PCM.
ð2Þ
where DHSMA/FA, WFA and DHFA are the calculated latent heat of form stable SMA/fatty acid composite PCM, mass
Fig. 8. DSC curve of SMA/LA (15/85 wt.%) composite PCM.
Fig. 5. DSC curve of form stable SMA/SA (15/85 wt.%) composite PCM.
percentage of fatty acid and the measured latent heat of the pure fatty acid (Table 1), respectively. The calculated latent heat values are given in Table 3. Comparing the measured latent heat values of the form stable PCMs (Table 2) with the calculated values using Eq.
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Table 2 Measured thermal properties of form stable SMA/fatty acid composite PCMs (15/85 wt.%) using DSC
needing an extra encapsulation, thereby reducing the cost of the LHTES system.
Form stable composite PCM
Melting point (°C)
Latent heat of melting (J/g)
Freezing point (°C)
Latent heat of freezing (J/g)
Acknowledgement
SMA/SA SMA/PA SMA/MA SMA/LA
67.35 60.04 51.75 41.48
202.23 184.54 176.49 160.83
66.68 59.27 52.07 41.78
204.82 185.18 177.78 163.43
The authors would like to acknowledge the financial support (Project No. 2003K120510) by the Turkish State Planning Organization. References
Table 3 Calculated latent heat values of form stable SMA/fatty acid composite PCMs (15/85 wt.%) using Eq. (2) Form stable composite PCM
Latent heat of melting (J/g)
Latent heat of freezing (J/g)
SMA/SA SMA/PA SMA/MA SMA/LA
205.83 188.21 180.00 161.67
209.73 192.58 180.75 166.00
(2) (Table 3), it was clearly seen that the measured values were slightly less than the calculated values. This was due to the interaction between the components of the form stable composite PCMs. 4. Conclusions The SMA/fatty acid composites were prepared as new kinds of form stable PCMs for LHTES systems by encapsulation of the fatty acids (SA, PA, MA and LA) as PCMs into the polymer matrix of SMA as supporting material. The fatty acids were encapsulated successfully as much as 85 wt.%, and no leakage of fatty acid was observed even when the temperature of the form stable PCM was over the melting point of the fatty acid. The prepared form stable SMA/SA, SMA/PA, SMA/MA and SMA/LA (15/ 85 wt.%) composite PCMs were characterized using optic microscopy, viscosimetry and FT-IR spectroscopy methods. The results proved the availability of the attractive interaction between the hydroxyl group of the fatty acids and the ether oxygen of the SMA, which resulted in good physical and chemical compatibility of the components of the composites. Moreover, the thermal properties of the form stable composite PCMs were measured using the DSC technique. The DSC results indicated that the form stable composite PCMs had proper phase change temperatures and satisfying latent heat storage capacities for practical LHTES applications. In this work, it was concluded that the form stable SMA/fatty acid composites can be considered as candidate PCMs for LHTES applications such as under-floor space heating of buildings and solar energy storage using wallboard and plasterboard impregnated with form stable PCM due to having good thermal properties, simple preparation in desired dimensions, elimination of the heat resistance caused by the capsule shell, direct usability without
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