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paraffin wax blends as phase change materials
Renewable Energy 68 (2014) 140e145
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermal and mechanical characterization of injection moulded high density polyethylene/paraffin wax blends as phase change materials M.E. Sotomayor a, *, I. Krupa b,1, A. Várez a, 2, B. Levenfeld a, 3 a b
Materials Science and Engineering Department, Carlos III University of Madrid, Avda. Universidad 30, 28911 Leganés, Spain Center for Advanced Materials, Office of Research, Qatar University, P.O. Box: 2713, Doha, Qatar
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2013 Accepted 26 January 2014 Available online
Thermal and mechanical properties of blends based on high density polyethylene and paraffin wax were investigated. The blends were prepared from 5 to 50 vol. % of paraffin wax employing a twin-screw extruder. Thermal behaviour of samples was determined by differential scanning calorimetry, thermogravimetric and dynamic mechanical analyses. A displacement of melting temperature of polyethylene was detected as a consequence of the plasticization effect of wax. These results revealed that melting temperatures and latent heats of samples are suitable for their application as phase change materials. Blends were processed by injection moulding which is an advantageous method to obtain pieces of this kind of materials. The evolution of loss tangent versus temperature of injected samples showed the lack of miscibility between the components of the blend. Tensile tests were carried out to characterize the mechanical strength of blends. Elongation at break decreased as paraffin wax content increased, and Young’s modulus decreased with wax content but in the case of blends with a 30 vol. % of wax and more, brittle rupture occurred and no yield point was observed. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Polyethylene/wax blends Phase change material Injection moulding Thermal properties Mechanical properties
1. Introduction Phase change materials (PCMs) are substances with high heats of fusion that are able, through melting and solidifying at certain temperatures, to store and release large amounts of energy [1]. These materials store energy in the process of changing their aggregate state from solid to liquid. When PCMs reach the temperature at which the phase change takes place, large amounts of heat are absorbed without they get hotter. When the ambient temperature in the space around the PCM drops, the PCM solidifies, releasing its stored latent heat. PCMs absorb and emit heat while maintaining a nearly constant temperature [2]. These materials must have a large latent heat and high thermal conductivity. They should have a melting temperature lying in the practical range of operation, melt congruently with minimum subcooling and be chemically stable, low in cost, non-toxic and non-corrosive [3]. Materials that have been studied are hydrated salts, paraffin waxes, fatty acids and
* Corresponding author. Tel.: þ34 916248863; fax: þ34 916249430. E-mail addresses: [email protected] (M.E. Sotomayor), [email protected]. qa (I. Krupa), [email protected] (A. Várez), [email protected] (B. Levenfeld). 1 Tel.: þ974 44035671. 2 Tel.: þ34 916249484; fax: þ34 916249430. 3 Tel.: þ34 916249915; fax: þ34 916249430. http://dx.doi.org/10.1016/j.renene.2014.01.036 0960-1481/Ó 2014 Elsevier Ltd. All rights reserved.
eutectics of organics and non-organic compounds. PCMs provide solutions in very specific areas, for example related to the time delay and available power between production or availability of energy and its consumption in receiving systems (solar energy, cogeneration, etc) and in applications of thermal protection [4]. Commercial paraffin waxes are cheap with moderate thermal storage densities (w200 J/g), they undergo negligible subcooling, and they are chemically inert and stable with no phase segregation. However, they have low thermal conductivity (w0.2 W/m C) which limits their applications as passive systems in PCM [3]. The encapsulation for preventing leakage of melted PCM would decrease the thermal conductivity, so another possibility to keep waxes in a stable shape during their application is to blend them with convenient polymers. Blends of high density polyethylene (HDPE) and several kind of paraffin wax (PW) become suitable materials for applications as phase change materials [5e10]. Inaba et al. [5] obtained a shape-stabilized paraffin (pentacosane) as a solideliquid phase change material employing HDPE as a supporting material. The test shape-stabilized paraffin was composed of the paraffin 74 mass% and HDPE 26 mass%. The whole compound can keep the same shape in a solid state without leakage of the melted paraffin even if the paraffin melts during a heat storage process. The work carried out by Hong et al. [7] deals with the preparation of HDPE and paraffin wax blends and the analysis of the structure by using
M.E. Sotomayor et al. / Renewable Energy 68 (2014) 140e145
scanning electron microscopy. On the other hand the thermal properties were studied. Sari [8] focused his work in the preparation of samples and the determination of their thermal properties. In order to improve the thermal conductivity, expanded and exfoliated graphite was added to the melted composite. Molefi et al. [9] presented the preparation of PCM based on paraffin wax and three different matrices as LDPE, LLDPE and HDPE. The samples were melt pressed and thermal properties were determined. Paraffin wax is the phase change material and HDPE serves as the supporting material, which provides structural strength and prevents the leakage of the melted paraffin. In this thermal energy storage system, while dispersed paraffin wax changes in state from solid to liquid, the HDPE keeps the material in a compact shape. As long as the operating temperature is below the melting point of the supporting material, the compound material can keep its shape even when the PCM melts during a heat storage process. The net structure of the PCM prevents leakage of liquid to occur when paraffin melts. Lee et al. [6] studied the durability of injected high density polyethylene/paraffin blends as a thermal energy storage material by investigation of the seepage behaviour of paraffin. They also determined that the crystalline morphological characteristics of HDPE are responsible of the excellent sealant property of the blend. Mngomezulu et al. [10] characterized melt pressed blends of HDPE, a Ficher-Tropsch paraffin wax and alkali-treated wood flour, and concluded that wax influences the viscoelastic behaviour of the HDPE matrix in the blends. Plastic injection moulding, which is a very cost effective method to produce very large quantities of parts, involves heating the material to a temperature at which flow is possible, forcing the plastic to pass through a nozzle into a shaped cavity, and cooling it. HDPE/PW blends are presented as phase change materials suitable for applications where it is necessary to maintain stable the temperature. The melting temperature of wax establishes the field of application, and dissipation of heat in several devices is a critical point where these materials would be able to play an important role. So, they can be employed as thermal protection systems of electronic devices. In this investigation, injection moulding is proposed as a profitable method to process phase change materials, and provides advantages not only related to this kind of processing route but final mechanical properties of parts as well. The evaluation of thermal and mechanical properties of injected HDPE/PW samples was realized under the requirements of PCMs by means of DSC, TGA, DMA and tensile tests.
Thermogravimetric analysis (TGA) was performed in a Perkin Elmer TGA1 thermogravimetric analyzer from 30 to 600 C at a heating rate of 10 C/min in nitrogen atmosphere. Rheological experiments were carried out in a capillary rheometer Haake Rheocap S20. The dimensions of the die were 1 mm of diameter and a length of 30 mm in order to keep a L/D ratio of 30. The shear rate was chosen in a range from 100 to 10,000 s1 and a melting time of 5 min was employed for each test. The parts were conformed in an Arburg 220S 250-60 injection moulding machine. The injection parameters were optimized: the temperature profile of the barrel was set to 145/150/155/160 C from the feeding zone to the die the mould temperature was 40 C. The holding pressure profile was established in three steps from injection pressure till 25 bar. Dynamic mechanical analyses (DMA) were performed in single cantilever using a TA Instruments DMAQ800 instrument. A frequency of 1 Hz and an amplitude of 20 mm were employed and samples were heated from 135 to 100 C at a heating rate of 4 C/ min. The width and thickness of the injected parts employed in DMA were close to 5.90 and 3.00 mm. The maxima on tan d-temperature plot were determined to identify the relaxations associated to glass transitions. Finally, mechanical properties of injected samples were determined through tensile tests in a Schimadzu AG-I testing machine at room temperature. The mechanical tests were carried out on the basis of standard EN ISO 527-1:1996, and the dimensions of the samples correspond to the specimen numbered as 5B. The deformation speed was 50 mm/min and results were obtained testing five parts of each blend. 3. Results and discussion 3.1. Determination of density Pycnometric density of blends against wax content is plotted in Fig. 1. The dashed line represents density values calculated from rule of mixing [11]. When increasing wax content density of blends decreases. Experimental results agree with theoretical ones which indicates that there was no paraffin wax loss during mixing. 3.2. Differential scanning calorimetry Fig. 2 shows DSC heating scans of pure components and blends. In the case of pure HDPE there is only one peak at 130.7 C and, in the case of pure PW, two endothermic peaks at 32.3 and 53.1 C are
2. Materials and methods The materials employed were a high density polyethylene supplied by Dow Plastics with a MFI of 25 g/10 min (190 C and 2.16 kg), and a soft paraffin wax which is refined from petroleum, supplied by Panreac (carbon distribution C18eC50). Pure components were firstly mechanically mixed in a Turbula for 15 min, and then they were blended in a twin-screw extruder Haake Rheomex CTW100p at 160 C and 40 r.p.m. in order to obtain larger amounts of sample. Blends with different volume percentages of paraffin (5, 10, 20, 30, 40 and 50) were prepared with no leakage of this component. Differential scanning calorimetry (DSC) experiments were carried out in a Perkin Elmer Diamond calorimeter with nitrogen as purge gas. Samples with a mass of w10 mg were sealed in a 50 ml aluminium pan, and an empty pan was used as reference. They were heated from 20 to 160 C at a heating rate of 10 C/min and then cooled at the same rate. Subsequently, they were heated again at the same conditions. Peak temperatures and enthalpies of melting were determined from the second scan.
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Fig. 1. Pycnometric density of HDPE/PW blends.
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M.E. Sotomayor et al. / Renewable Energy 68 (2014) 140e145
Fig. 2. DSC heating thermograms of pure components and HDPE/PW blends.
lower, at 122.4 C and this fact reveals plasticization of HDPE by paraffin wax [15]. On the other hand, the melting peak of PW displaces from 53.1 C to 48.2 C (in the blend with a 10 vol. % of PW) when wax content decreases in blend. DSC cooling thermograms of pure components and blends are shown in Fig. 3. Crystallization temperatures of polyethylene and paraffin wax are 111.9 C and 43.8 C, respectively. Parameters obtained from DSC measurements are summarized in Table 1. The degree of crystallinity of HDPE related to a 100% crystalline polyethylene with a specific enthalpy of 293 J/g [16] was calculated. As it can be seen, the degree of crystallinity of polymer remains almost constant and close to 60% in all the samples. The crystallization temperature of wax does not vary but, the crystallization temperature of HDPE in the blends decreases with an increase of wax content. The results in Table 1 show that the latent heat of paraffin decreases slightly till the blend with a 20 vol. % of wax when it is compared with pure component. This occurs because the threedimensional net structure formed by HDPE restricts the thermal molecular movements of paraffin during the phase change. It is believed that higher HDPE amount causes the formation of more compact three-dimensional net structure, thus thermal molecular movements of paraffin are confined more excessively [17]. Moreover, in the sample with a 10 vol. % of wax there is a strong decrease of latent heat of melting of wax and in the case of 5 vol. % this peak disappears. This fact suggests that wax could crystallize with HDPE and/or belong to the amorphous phase of polymer in this low concentration region [9]. The maximum working temperature recommended for this PCM should be established taking into account the melting temperature of wax and HDPE in the blend. Initially, the working temperature should be higher than Tm of wax and lower than Tm of HDPE. Moreover, it is critical to realize a deep study of degradation of the material after several thermal cycles at working temperature. 3.3. Thermogravimetric analysis
Fig. 3. DSC cooling thermograms of pure components and HDPE/PW blends.
distinguished. The first peak is related to the solidesolid transition between different crystalline wax structures [12,13], and the second peak is associated to the melting transition. Considering the blend with a 5 vol. % of PW, only the melting peak corresponding to HDPE can be detected, and this fact could indicate a certain miscibility degree between the components in this concentration region [14]. However when the quantity of PW is 10 vol. % and higher, the melting peak of this second component appears. Moreover, by means of addition of higher quantities of PW a displacement of the melting peak of HDPE to lower temperatures can be observed. For example, in the case of the sample with a 50 vol. % of PW, the melting temperature of HDPE appears 8 C
Fig. 4 shows TGA curves of pure components and blends. Decomposition of pure components takes place in one step, however in the case of blends this process occurs in two steps: the first one is related to wax decomposition and the second one to HDPE. This degradation behaviour is typical of immiscible blends whose components have very different decomposition temperatures. The starting and ending degradation temperatures of HDPE, PW and their blends can be examined in Table 2. Pure HDPE has the highest thermal stability and its decomposition begins at 410 C. Thermal stability of blends is lower than HDPE but their decomposition range is enlarged if it is compared with pure components. A similar behaviour is found in the case of blends of polypropylene and paraffin wax [15,18,19]. This characteristic is beneficial for blends used in powder injection moulding technology because it favours a gradual decomposition of the binder [20,21]. On the other
Table 1 Parameters obtained from DSC experiments of HDPE/PW blends.a HDPE/PW (vol. %)
HDPE ( C) Tm
HDPE (J/g HDPE) DHm
Crystallinity (%)
TcHDPE ( C)
PW ( C) Tm
PW (J/g PW) DHm
TcPW ( C)
100/0 95/5 90/10 80/20 70/30 60/40 50/50 0/100
130.8 129.4 128.4 125.8 124.3 123.3 122.4 e
178.6 179.8 180.9 185.0 184.1 205.5 210.8 e
61 62 62 64 63 71 72 e
111.9 114.9 113.6 111.4 109.0 108.6 107.3 e
e e 48.2 49.6 50.5 50.4 51.4 53.1
e e 43.9 105.2 139.6 135.6 127.8 183.5
e e 42.7 44.1 43.3 43.5 42.6 43.8
a
T: temperature, DH: enthalpy, m: melting, c: crystallization.
M.E. Sotomayor et al. / Renewable Energy 68 (2014) 140e145
Fig. 4. TGA curves of pure components and HDPE/PW blends.
143
Fig. 6. Log viscosity at 1000 s1 plotted against wax content of HDPE/PW blends.
Table 2 Starting and ending degradation temperatures of pure components and their blends.a HDPE/PW
Ts ( C)
Te ( C)
TeeTs ( C)
100/0 95/5 90/10 80/20 70/30 60/40 50/50 0/100
410 234 225 222 227 243 236 200
491 510 495 490 500 515 496 309
81 276 270 268 273 272 260 109
a
T: temperature, s: starting, e: ending.
hand, the mass loss percentages can be clearly correlated to the initial amount of components of the blends which is another evidence of no wax loss during processing.
Fig. 7. Injection pressure and viscosity at 1000 s1 and 160 C of HDPE/PW blends.
3.4. Rheological behaviour According to injection moulding requirements, a previous rheological study of blends in the melting state was performed. The viscosity of samples as a function of shear rate is presented in Fig. 5. This shear rate range was selected because similar shear rate values are reached during injection process. In all the samples, the viscosity decreases as shear rate increases according to a pseudoplastic behaviour which is the most suitable for the process. And on
Fig. 5. Viscosity plotted against shear rate of HDPE/PW blends at 160 C.
the other hand, if wax content increases the viscosity of the blend decreases. Paraffin wax presents a Newtonian behaviour but its viscosity at 160 C is too much low to be measured through this method, that is why this value was obtained by means of a logarithmic additivity rule according to Eq. (1) [22]:
Fig. 8. Loss tangent plotted against temperature of injected HDPE/PW blends.
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3.5. Dynamic mechanical analysis
Fig. 9. Storage modulus plotted against temperature of injected HDPE/PW blends.
DMA was performed in order to evaluate the possible changes in glass transition temperature of blend components as a function of wax content. Loss tangent of pure HDPE and some representative blends is plotted against temperature in Fig. 8. Unfortunately, it was not possible to analyse pure wax by DMA because it was too brittle to sustain the dynamic forces applied during the test. In the case of HDPE, an unique peak appears at 110 C which corresponds to arelaxation (Tg) and, in the case of blends two peaks can be seen. The first one corresponds to Tg of polyethylene and, the peak centred in 65 C corresponds to Tg of PW. This fact reveals that there is not miscibility between both components of the blend and this behaviour emerges as a consequence of the big difference in mechanical properties of the pure components. The storage modulus of pure HDPE and HDPE/PW blends is shown as a function of temperature in Fig. 9. It is possible to identify two kinds of behaviour around 40 C which could be considered as the melting temperature of wax. Paraffin wax in its solid state, below 40 C, reinforces the polyethylene matrix. The wax immobilizes the polymer chains leading to a higher modulus of the polymeric matrix. However, after wax melting the decrease of the modulus is more pronounced in the blends containing higher wax content [15]. 3.6. Mechanical properties
Fig. 10. Stressestrain curves of pure HDPE, 80/20 and 70/30 blends.
log h ¼
X
wi log hi ¼ wPW log hPW þ wHDPE log hHDPE
(1)
where h is the viscosity of the blend, hi is the component viscosity, wi is the component weight fraction, and n is the number of components. From the slope of the graph (see Fig. 6), an estimation of viscosity of paraffin wax could be done. In the case of a shear rate of 1000 s1 it was 0.002 Pa s. Once the rheological behaviour of blends was evaluated, they were injection moulded. The injection pressure reached by blends with increasing paraffin wax contents is presented in Fig. 7. As it can be observed, there is a correlation between injection pressure and viscosity of blends. The lower viscosity of the blend the lower is the injection pressure needed to fill the mould.
Serving as supporting, HDPE plays an important role in PCM. It must offer a high structural strength with a relatively small percentage of HDPE. The paraffin wax content strongly influences the mechanical properties of HDPE. Two different mechanical behaviours are exhibited by the different tested blends and they are represented schematically in Fig. 10. In the case of pure HDPE, a typical curve stressestrain for this kind of polymer can be observed. As expected, the yield stress of the samples decreases with wax content and the material also presents a lower drawability (the blend with a 20 vol. % of PW e.g. is shown) breaking at lower deformation. And, when wax content increases up to 30 vol. %, the behaviour changes drastically because of the yield point disappears, the material losses completely its drawability and becomes brittle. From mechanical testing of HDPE and HDPE/PW blends, elongation at yield, yield stress, elongation at break, stress at break and Young’s modulus values were calculated. These results are summarized in Table 3. In the case of blends with an amount of equal or higher than 30 vol. % of PW, there are no values for elongation at yield and yield stress because of the absence of yield point before rupture. On the other hand, stress at break is always lower than yield stress. Stress and elongation at break decrease with an increase in wax content. Molefi et al. [9] also observed the same behaviour blending HDPE with a Ficher-Tropsch paraffin wax and they showed that wax
Table 3 Mechanical properties of HDPE/PW blends.a
sy Ssy (MPa)
S3 y
HDPE/PW
3y
100/0 95/5 90/10 80/20 70/30 60/40 50/50
0.123 0.16 0.17 0.161 e e e
0.004 0.01 0.01 0.004
23.6 22.6 22.1 18.7 e e e
0.3 0.3 0.4 0.5
3B
sB SsB (MPa)
S3 B
6.0 5.5 5.59 2.9 0.3 0.06 0.020
0.4 0.5 0.07 0.6 0.1 0.01 0.006
13.3 13.1 11 9.5 15 12.7 6
0.2 0.3 2 0.2 1 0.7 1
Et SEt (MPa) 710 590 609 513 464 441 361
14 45 19 15 24 14 16
a 3 y, sy, 3 B, sB and Et are elongation at yield, yield stress, elongation at break, stress at break and Young’s modulus of elasticity, respectively. S3 y, Ssy, S3 B, SsB and SEt are their standard deviations.
M.E. Sotomayor et al. / Renewable Energy 68 (2014) 140e145
crystals in the amorphous phase of the polymer act as defect points for the initiation and propagation of stress cracking. A high Young’s modulus of HDPE can be observed as a consequence of injection moulding processing route. Young’s modulus of the blends decreases with an increase in PW content as a consequence of modulus of paraffin wax is much lower than the modulus of HDPE. It is known that modulus depends on crystallinity degree and paraffin wax has a lower crystallinity than HDPE. 4. Conclusions Phase change materials based on high density polyethylene and paraffin wax were thermal and mechanically characterized in this work. The blends containing from 5 to 50 vol. % of wax were extruded, and in spite of different melting temperatures of both components no wax loss was detected after processing. This fact was evaluated by means of thermogravimetric analyses. All the prepared blends presented a pseudoplastic behaviour, being all of them suitable to be injection moulded. DSC results showed a displacement of melting temperature of polyethylene as a consequence of the plasticization effect of wax. Glass transition temperature of both components of blend was detected by dynamic mechanical analysis revealing that there is not miscibility between them. A decrease of Young’s modulus with wax content indicates that the modulus of paraffin wax is lower than the modulus of HDPE. As a consequence of this study, injection moulded blends of HDPE and PW are highly suitable in phase change materials applications owing to their lack of miscibility and their profitable thermal and mechanical properties. The material is cheap, easy to prepare, and posses a suitable latent heat and mechanical strength. Acknowledgements Authors thank financial support received from MICINN (MAT2010-19837-CO6 project) and Madrid regional government (MATERYENER S2009 PPQ-1626 program). References [1] Abhat A. Low temperature latent heat thermal energy storage heat storage materials. Solar Energy 1983;30(4):313e32. [2] Demirbas MF. Thermal energy storage and phase change materials: an overview. Energy Sources Part B 2006;1(1):85e95.
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[3] Farid MM, Khudhair AM, Razack SAK, Al-Hallaj S. A review on phase change energy storage: materials and applications. Energy Convers Manage 2004;45(9e10):1597e615. [4] Zalba B, Marín JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Thermal Eng 2003;23(3):251e83. [5] Inaba H, Tu P. Evaluation of thermophysical characteristics on shapestabilized paraffin as a solideliquid phase change material. Heat Mass Transfer 1997;32:307e12. [6] Lee CH, Choi HK. Crystalline morphology in high-density polyethylene/paraffin blend for thermal energy storage. Polymer Compos 1998;19(6):704e8. [7] Hong Y, Ge XS. Preparation of polyethylene- paraffin compound as a formstable solideliquid phase change material. Solar Energy Mater Solar Cells 2000;64(1):37e44. [8] Sari A. Form-stable paraffin/high density polyethylene composites as solide liquid phase change material for thermal energy storage: preparation and thermal properties. Energy Convers Manage 2004;45(13e14):2033e42. [9] Molefi JA, Luyt AS, Krupa I. Comparison of LDPE, LLDPE and HDPE as matrices for phase change materials based on a soft Fischer-Tropsch paraffin wax. Thermochim Acta 2010;500(1e2):88e92. [10] Mngomezulu ME, Luyt AS, Krupa I. Structure and properties of phase change materials based on HDPE, soft Fischer-Tropsch paraffin wax, and wood flour. J Appl Polymer Sci 2010;118(3):1541e51. [11] German RM. Powder injection molding. Princeton, NJ, USA: Metal Powder Industries Federation; 1990. [12] Tiwan GB, Srivastava SP, Pandey SP, Purohit RC, Saxena AK, Goyal SK, et al. Thermally induced phase-transitions in petroleum waxes from some typical Indian crude oils. Petrol Sci Technol 1997;15(3):335e46. [13] Genovese A, Amarasinghe G, Glewis M, Mainwaring D, Shanks RA. Crystallisation, melting, recrystallisation and polymorphism of n-eicosane for application as a phase change material. Thermochim Acta 443(2); 2006:235e44. [14] Krupa I, Luyt AS. Physical properties of blends of LLDPE and an oxidized paraffin wax. Polymer 2001;42(17):7285e9. [15] Krupa I, Miková G, Luyt AS. Polypropylene as a potential matrix for the creation of shape stabilized phase change materials. Eur Polymer J 2007;43(3): 895e907. [16] Wilfong DL, Knight GW. Crystallization mechanisms for LLDPE and its fractions. J Polymer Sci Part B: Polymer Phys 1990;28(6):861e70. [17] Cai Y, Wei Q, Huang F, Lin S, Chen F, Gao W. Thermal stability, latent heat and flame retardant properties of the thermal energy storage phase change materials based on paraffin/high density polyethylene composites. Renew Energy 2009;34:2117e23. [18] Rodríguez-Senín E. Desarrollo del proceso de moldeo por inyección de polvos para la obtención de ferritas blandas utilizando sistemas ligantes termoplásticos y termoestables. Doctoral Thesis. Carlos III University of Madrid; 2007. [19] Aggarwal G, Smid I, Jin Park S, German RM. Development of niobium powder injection molding: Part I. Feedstock and injection molding. Int J Refract Metals Hard Mater 2006;24(3):253e62. [20] Thomas-Vielma P, Cervera A, Levenfeld B, Várez A. Production of alumina parts by powder injection molding with a binder based on high density polyethylene. J Eur Ceramic Soc 2008;28(4):763e71. [21] Herranz G, Levenfeld B, Várez A, Torralba JM. Development of new feedstock formulation based on high density polyethylene for MIM of M2 high speed steels. Powder Metal 2005;48(2):134e8. [22] German RM, Bose A. Injection molding of metals and ceramics. Princeton, New Jersey: Metal Powder Industries Federation; 1997.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermal and mechanical characterization of injection moulded high density polyethylene/paraffin wax blends as phase change materials M.E. Sotomayor a, *, I. Krupa b,1, A. Várez a, 2, B. Levenfeld a, 3 a b
Materials Science and Engineering Department, Carlos III University of Madrid, Avda. Universidad 30, 28911 Leganés, Spain Center for Advanced Materials, Office of Research, Qatar University, P.O. Box: 2713, Doha, Qatar
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2013 Accepted 26 January 2014 Available online
Thermal and mechanical properties of blends based on high density polyethylene and paraffin wax were investigated. The blends were prepared from 5 to 50 vol. % of paraffin wax employing a twin-screw extruder. Thermal behaviour of samples was determined by differential scanning calorimetry, thermogravimetric and dynamic mechanical analyses. A displacement of melting temperature of polyethylene was detected as a consequence of the plasticization effect of wax. These results revealed that melting temperatures and latent heats of samples are suitable for their application as phase change materials. Blends were processed by injection moulding which is an advantageous method to obtain pieces of this kind of materials. The evolution of loss tangent versus temperature of injected samples showed the lack of miscibility between the components of the blend. Tensile tests were carried out to characterize the mechanical strength of blends. Elongation at break decreased as paraffin wax content increased, and Young’s modulus decreased with wax content but in the case of blends with a 30 vol. % of wax and more, brittle rupture occurred and no yield point was observed. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Polyethylene/wax blends Phase change material Injection moulding Thermal properties Mechanical properties
1. Introduction Phase change materials (PCMs) are substances with high heats of fusion that are able, through melting and solidifying at certain temperatures, to store and release large amounts of energy [1]. These materials store energy in the process of changing their aggregate state from solid to liquid. When PCMs reach the temperature at which the phase change takes place, large amounts of heat are absorbed without they get hotter. When the ambient temperature in the space around the PCM drops, the PCM solidifies, releasing its stored latent heat. PCMs absorb and emit heat while maintaining a nearly constant temperature [2]. These materials must have a large latent heat and high thermal conductivity. They should have a melting temperature lying in the practical range of operation, melt congruently with minimum subcooling and be chemically stable, low in cost, non-toxic and non-corrosive [3]. Materials that have been studied are hydrated salts, paraffin waxes, fatty acids and
* Corresponding author. Tel.: þ34 916248863; fax: þ34 916249430. E-mail addresses: [email protected] (M.E. Sotomayor), [email protected]. qa (I. Krupa), [email protected] (A. Várez), [email protected] (B. Levenfeld). 1 Tel.: þ974 44035671. 2 Tel.: þ34 916249484; fax: þ34 916249430. 3 Tel.: þ34 916249915; fax: þ34 916249430. http://dx.doi.org/10.1016/j.renene.2014.01.036 0960-1481/Ó 2014 Elsevier Ltd. All rights reserved.
eutectics of organics and non-organic compounds. PCMs provide solutions in very specific areas, for example related to the time delay and available power between production or availability of energy and its consumption in receiving systems (solar energy, cogeneration, etc) and in applications of thermal protection [4]. Commercial paraffin waxes are cheap with moderate thermal storage densities (w200 J/g), they undergo negligible subcooling, and they are chemically inert and stable with no phase segregation. However, they have low thermal conductivity (w0.2 W/m C) which limits their applications as passive systems in PCM [3]. The encapsulation for preventing leakage of melted PCM would decrease the thermal conductivity, so another possibility to keep waxes in a stable shape during their application is to blend them with convenient polymers. Blends of high density polyethylene (HDPE) and several kind of paraffin wax (PW) become suitable materials for applications as phase change materials [5e10]. Inaba et al. [5] obtained a shape-stabilized paraffin (pentacosane) as a solideliquid phase change material employing HDPE as a supporting material. The test shape-stabilized paraffin was composed of the paraffin 74 mass% and HDPE 26 mass%. The whole compound can keep the same shape in a solid state without leakage of the melted paraffin even if the paraffin melts during a heat storage process. The work carried out by Hong et al. [7] deals with the preparation of HDPE and paraffin wax blends and the analysis of the structure by using
M.E. Sotomayor et al. / Renewable Energy 68 (2014) 140e145
scanning electron microscopy. On the other hand the thermal properties were studied. Sari [8] focused his work in the preparation of samples and the determination of their thermal properties. In order to improve the thermal conductivity, expanded and exfoliated graphite was added to the melted composite. Molefi et al. [9] presented the preparation of PCM based on paraffin wax and three different matrices as LDPE, LLDPE and HDPE. The samples were melt pressed and thermal properties were determined. Paraffin wax is the phase change material and HDPE serves as the supporting material, which provides structural strength and prevents the leakage of the melted paraffin. In this thermal energy storage system, while dispersed paraffin wax changes in state from solid to liquid, the HDPE keeps the material in a compact shape. As long as the operating temperature is below the melting point of the supporting material, the compound material can keep its shape even when the PCM melts during a heat storage process. The net structure of the PCM prevents leakage of liquid to occur when paraffin melts. Lee et al. [6] studied the durability of injected high density polyethylene/paraffin blends as a thermal energy storage material by investigation of the seepage behaviour of paraffin. They also determined that the crystalline morphological characteristics of HDPE are responsible of the excellent sealant property of the blend. Mngomezulu et al. [10] characterized melt pressed blends of HDPE, a Ficher-Tropsch paraffin wax and alkali-treated wood flour, and concluded that wax influences the viscoelastic behaviour of the HDPE matrix in the blends. Plastic injection moulding, which is a very cost effective method to produce very large quantities of parts, involves heating the material to a temperature at which flow is possible, forcing the plastic to pass through a nozzle into a shaped cavity, and cooling it. HDPE/PW blends are presented as phase change materials suitable for applications where it is necessary to maintain stable the temperature. The melting temperature of wax establishes the field of application, and dissipation of heat in several devices is a critical point where these materials would be able to play an important role. So, they can be employed as thermal protection systems of electronic devices. In this investigation, injection moulding is proposed as a profitable method to process phase change materials, and provides advantages not only related to this kind of processing route but final mechanical properties of parts as well. The evaluation of thermal and mechanical properties of injected HDPE/PW samples was realized under the requirements of PCMs by means of DSC, TGA, DMA and tensile tests.
Thermogravimetric analysis (TGA) was performed in a Perkin Elmer TGA1 thermogravimetric analyzer from 30 to 600 C at a heating rate of 10 C/min in nitrogen atmosphere. Rheological experiments were carried out in a capillary rheometer Haake Rheocap S20. The dimensions of the die were 1 mm of diameter and a length of 30 mm in order to keep a L/D ratio of 30. The shear rate was chosen in a range from 100 to 10,000 s1 and a melting time of 5 min was employed for each test. The parts were conformed in an Arburg 220S 250-60 injection moulding machine. The injection parameters were optimized: the temperature profile of the barrel was set to 145/150/155/160 C from the feeding zone to the die the mould temperature was 40 C. The holding pressure profile was established in three steps from injection pressure till 25 bar. Dynamic mechanical analyses (DMA) were performed in single cantilever using a TA Instruments DMAQ800 instrument. A frequency of 1 Hz and an amplitude of 20 mm were employed and samples were heated from 135 to 100 C at a heating rate of 4 C/ min. The width and thickness of the injected parts employed in DMA were close to 5.90 and 3.00 mm. The maxima on tan d-temperature plot were determined to identify the relaxations associated to glass transitions. Finally, mechanical properties of injected samples were determined through tensile tests in a Schimadzu AG-I testing machine at room temperature. The mechanical tests were carried out on the basis of standard EN ISO 527-1:1996, and the dimensions of the samples correspond to the specimen numbered as 5B. The deformation speed was 50 mm/min and results were obtained testing five parts of each blend. 3. Results and discussion 3.1. Determination of density Pycnometric density of blends against wax content is plotted in Fig. 1. The dashed line represents density values calculated from rule of mixing [11]. When increasing wax content density of blends decreases. Experimental results agree with theoretical ones which indicates that there was no paraffin wax loss during mixing. 3.2. Differential scanning calorimetry Fig. 2 shows DSC heating scans of pure components and blends. In the case of pure HDPE there is only one peak at 130.7 C and, in the case of pure PW, two endothermic peaks at 32.3 and 53.1 C are
2. Materials and methods The materials employed were a high density polyethylene supplied by Dow Plastics with a MFI of 25 g/10 min (190 C and 2.16 kg), and a soft paraffin wax which is refined from petroleum, supplied by Panreac (carbon distribution C18eC50). Pure components were firstly mechanically mixed in a Turbula for 15 min, and then they were blended in a twin-screw extruder Haake Rheomex CTW100p at 160 C and 40 r.p.m. in order to obtain larger amounts of sample. Blends with different volume percentages of paraffin (5, 10, 20, 30, 40 and 50) were prepared with no leakage of this component. Differential scanning calorimetry (DSC) experiments were carried out in a Perkin Elmer Diamond calorimeter with nitrogen as purge gas. Samples with a mass of w10 mg were sealed in a 50 ml aluminium pan, and an empty pan was used as reference. They were heated from 20 to 160 C at a heating rate of 10 C/min and then cooled at the same rate. Subsequently, they were heated again at the same conditions. Peak temperatures and enthalpies of melting were determined from the second scan.
141
Fig. 1. Pycnometric density of HDPE/PW blends.
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Fig. 2. DSC heating thermograms of pure components and HDPE/PW blends.
lower, at 122.4 C and this fact reveals plasticization of HDPE by paraffin wax [15]. On the other hand, the melting peak of PW displaces from 53.1 C to 48.2 C (in the blend with a 10 vol. % of PW) when wax content decreases in blend. DSC cooling thermograms of pure components and blends are shown in Fig. 3. Crystallization temperatures of polyethylene and paraffin wax are 111.9 C and 43.8 C, respectively. Parameters obtained from DSC measurements are summarized in Table 1. The degree of crystallinity of HDPE related to a 100% crystalline polyethylene with a specific enthalpy of 293 J/g [16] was calculated. As it can be seen, the degree of crystallinity of polymer remains almost constant and close to 60% in all the samples. The crystallization temperature of wax does not vary but, the crystallization temperature of HDPE in the blends decreases with an increase of wax content. The results in Table 1 show that the latent heat of paraffin decreases slightly till the blend with a 20 vol. % of wax when it is compared with pure component. This occurs because the threedimensional net structure formed by HDPE restricts the thermal molecular movements of paraffin during the phase change. It is believed that higher HDPE amount causes the formation of more compact three-dimensional net structure, thus thermal molecular movements of paraffin are confined more excessively [17]. Moreover, in the sample with a 10 vol. % of wax there is a strong decrease of latent heat of melting of wax and in the case of 5 vol. % this peak disappears. This fact suggests that wax could crystallize with HDPE and/or belong to the amorphous phase of polymer in this low concentration region [9]. The maximum working temperature recommended for this PCM should be established taking into account the melting temperature of wax and HDPE in the blend. Initially, the working temperature should be higher than Tm of wax and lower than Tm of HDPE. Moreover, it is critical to realize a deep study of degradation of the material after several thermal cycles at working temperature. 3.3. Thermogravimetric analysis
Fig. 3. DSC cooling thermograms of pure components and HDPE/PW blends.
distinguished. The first peak is related to the solidesolid transition between different crystalline wax structures [12,13], and the second peak is associated to the melting transition. Considering the blend with a 5 vol. % of PW, only the melting peak corresponding to HDPE can be detected, and this fact could indicate a certain miscibility degree between the components in this concentration region [14]. However when the quantity of PW is 10 vol. % and higher, the melting peak of this second component appears. Moreover, by means of addition of higher quantities of PW a displacement of the melting peak of HDPE to lower temperatures can be observed. For example, in the case of the sample with a 50 vol. % of PW, the melting temperature of HDPE appears 8 C
Fig. 4 shows TGA curves of pure components and blends. Decomposition of pure components takes place in one step, however in the case of blends this process occurs in two steps: the first one is related to wax decomposition and the second one to HDPE. This degradation behaviour is typical of immiscible blends whose components have very different decomposition temperatures. The starting and ending degradation temperatures of HDPE, PW and their blends can be examined in Table 2. Pure HDPE has the highest thermal stability and its decomposition begins at 410 C. Thermal stability of blends is lower than HDPE but their decomposition range is enlarged if it is compared with pure components. A similar behaviour is found in the case of blends of polypropylene and paraffin wax [15,18,19]. This characteristic is beneficial for blends used in powder injection moulding technology because it favours a gradual decomposition of the binder [20,21]. On the other
Table 1 Parameters obtained from DSC experiments of HDPE/PW blends.a HDPE/PW (vol. %)
HDPE ( C) Tm
HDPE (J/g HDPE) DHm
Crystallinity (%)
TcHDPE ( C)
PW ( C) Tm
PW (J/g PW) DHm
TcPW ( C)
100/0 95/5 90/10 80/20 70/30 60/40 50/50 0/100
130.8 129.4 128.4 125.8 124.3 123.3 122.4 e
178.6 179.8 180.9 185.0 184.1 205.5 210.8 e
61 62 62 64 63 71 72 e
111.9 114.9 113.6 111.4 109.0 108.6 107.3 e
e e 48.2 49.6 50.5 50.4 51.4 53.1
e e 43.9 105.2 139.6 135.6 127.8 183.5
e e 42.7 44.1 43.3 43.5 42.6 43.8
a
T: temperature, DH: enthalpy, m: melting, c: crystallization.
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Fig. 4. TGA curves of pure components and HDPE/PW blends.
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Fig. 6. Log viscosity at 1000 s1 plotted against wax content of HDPE/PW blends.
Table 2 Starting and ending degradation temperatures of pure components and their blends.a HDPE/PW
Ts ( C)
Te ( C)
TeeTs ( C)
100/0 95/5 90/10 80/20 70/30 60/40 50/50 0/100
410 234 225 222 227 243 236 200
491 510 495 490 500 515 496 309
81 276 270 268 273 272 260 109
a
T: temperature, s: starting, e: ending.
hand, the mass loss percentages can be clearly correlated to the initial amount of components of the blends which is another evidence of no wax loss during processing.
Fig. 7. Injection pressure and viscosity at 1000 s1 and 160 C of HDPE/PW blends.
3.4. Rheological behaviour According to injection moulding requirements, a previous rheological study of blends in the melting state was performed. The viscosity of samples as a function of shear rate is presented in Fig. 5. This shear rate range was selected because similar shear rate values are reached during injection process. In all the samples, the viscosity decreases as shear rate increases according to a pseudoplastic behaviour which is the most suitable for the process. And on
Fig. 5. Viscosity plotted against shear rate of HDPE/PW blends at 160 C.
the other hand, if wax content increases the viscosity of the blend decreases. Paraffin wax presents a Newtonian behaviour but its viscosity at 160 C is too much low to be measured through this method, that is why this value was obtained by means of a logarithmic additivity rule according to Eq. (1) [22]:
Fig. 8. Loss tangent plotted against temperature of injected HDPE/PW blends.
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3.5. Dynamic mechanical analysis
Fig. 9. Storage modulus plotted against temperature of injected HDPE/PW blends.
DMA was performed in order to evaluate the possible changes in glass transition temperature of blend components as a function of wax content. Loss tangent of pure HDPE and some representative blends is plotted against temperature in Fig. 8. Unfortunately, it was not possible to analyse pure wax by DMA because it was too brittle to sustain the dynamic forces applied during the test. In the case of HDPE, an unique peak appears at 110 C which corresponds to arelaxation (Tg) and, in the case of blends two peaks can be seen. The first one corresponds to Tg of polyethylene and, the peak centred in 65 C corresponds to Tg of PW. This fact reveals that there is not miscibility between both components of the blend and this behaviour emerges as a consequence of the big difference in mechanical properties of the pure components. The storage modulus of pure HDPE and HDPE/PW blends is shown as a function of temperature in Fig. 9. It is possible to identify two kinds of behaviour around 40 C which could be considered as the melting temperature of wax. Paraffin wax in its solid state, below 40 C, reinforces the polyethylene matrix. The wax immobilizes the polymer chains leading to a higher modulus of the polymeric matrix. However, after wax melting the decrease of the modulus is more pronounced in the blends containing higher wax content [15]. 3.6. Mechanical properties
Fig. 10. Stressestrain curves of pure HDPE, 80/20 and 70/30 blends.
log h ¼
X
wi log hi ¼ wPW log hPW þ wHDPE log hHDPE
(1)
where h is the viscosity of the blend, hi is the component viscosity, wi is the component weight fraction, and n is the number of components. From the slope of the graph (see Fig. 6), an estimation of viscosity of paraffin wax could be done. In the case of a shear rate of 1000 s1 it was 0.002 Pa s. Once the rheological behaviour of blends was evaluated, they were injection moulded. The injection pressure reached by blends with increasing paraffin wax contents is presented in Fig. 7. As it can be observed, there is a correlation between injection pressure and viscosity of blends. The lower viscosity of the blend the lower is the injection pressure needed to fill the mould.
Serving as supporting, HDPE plays an important role in PCM. It must offer a high structural strength with a relatively small percentage of HDPE. The paraffin wax content strongly influences the mechanical properties of HDPE. Two different mechanical behaviours are exhibited by the different tested blends and they are represented schematically in Fig. 10. In the case of pure HDPE, a typical curve stressestrain for this kind of polymer can be observed. As expected, the yield stress of the samples decreases with wax content and the material also presents a lower drawability (the blend with a 20 vol. % of PW e.g. is shown) breaking at lower deformation. And, when wax content increases up to 30 vol. %, the behaviour changes drastically because of the yield point disappears, the material losses completely its drawability and becomes brittle. From mechanical testing of HDPE and HDPE/PW blends, elongation at yield, yield stress, elongation at break, stress at break and Young’s modulus values were calculated. These results are summarized in Table 3. In the case of blends with an amount of equal or higher than 30 vol. % of PW, there are no values for elongation at yield and yield stress because of the absence of yield point before rupture. On the other hand, stress at break is always lower than yield stress. Stress and elongation at break decrease with an increase in wax content. Molefi et al. [9] also observed the same behaviour blending HDPE with a Ficher-Tropsch paraffin wax and they showed that wax
Table 3 Mechanical properties of HDPE/PW blends.a
sy Ssy (MPa)
S3 y
HDPE/PW
3y
100/0 95/5 90/10 80/20 70/30 60/40 50/50
0.123 0.16 0.17 0.161 e e e
0.004 0.01 0.01 0.004
23.6 22.6 22.1 18.7 e e e
0.3 0.3 0.4 0.5
3B
sB SsB (MPa)
S3 B
6.0 5.5 5.59 2.9 0.3 0.06 0.020
0.4 0.5 0.07 0.6 0.1 0.01 0.006
13.3 13.1 11 9.5 15 12.7 6
0.2 0.3 2 0.2 1 0.7 1
Et SEt (MPa) 710 590 609 513 464 441 361
14 45 19 15 24 14 16
a 3 y, sy, 3 B, sB and Et are elongation at yield, yield stress, elongation at break, stress at break and Young’s modulus of elasticity, respectively. S3 y, Ssy, S3 B, SsB and SEt are their standard deviations.
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crystals in the amorphous phase of the polymer act as defect points for the initiation and propagation of stress cracking. A high Young’s modulus of HDPE can be observed as a consequence of injection moulding processing route. Young’s modulus of the blends decreases with an increase in PW content as a consequence of modulus of paraffin wax is much lower than the modulus of HDPE. It is known that modulus depends on crystallinity degree and paraffin wax has a lower crystallinity than HDPE. 4. Conclusions Phase change materials based on high density polyethylene and paraffin wax were thermal and mechanically characterized in this work. The blends containing from 5 to 50 vol. % of wax were extruded, and in spite of different melting temperatures of both components no wax loss was detected after processing. This fact was evaluated by means of thermogravimetric analyses. All the prepared blends presented a pseudoplastic behaviour, being all of them suitable to be injection moulded. DSC results showed a displacement of melting temperature of polyethylene as a consequence of the plasticization effect of wax. Glass transition temperature of both components of blend was detected by dynamic mechanical analysis revealing that there is not miscibility between them. A decrease of Young’s modulus with wax content indicates that the modulus of paraffin wax is lower than the modulus of HDPE. As a consequence of this study, injection moulded blends of HDPE and PW are highly suitable in phase change materials applications owing to their lack of miscibility and their profitable thermal and mechanical properties. The material is cheap, easy to prepare, and posses a suitable latent heat and mechanical strength. Acknowledgements Authors thank financial support received from MICINN (MAT2010-19837-CO6 project) and Madrid regional government (MATERYENER S2009 PPQ-1626 program). References [1] Abhat A. Low temperature latent heat thermal energy storage heat storage materials. Solar Energy 1983;30(4):313e32. [2] Demirbas MF. Thermal energy storage and phase change materials: an overview. Energy Sources Part B 2006;1(1):85e95.
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