Use of phase change materials to develop electrospun coatings of interest in food packaging applications

1 Use of Phase Change Materials to develop Electrospun Coatings of interest in Food 1 Packaging Applications 2 3 Wilson Chalco-Sandoval, María José Fabra, Amparo López-Rubio and Jose M. 4 Lagaron* 5 Novel Materials and Nanotechnology Group, IATA-CSIC, Avda. Agustin Escardino 7, 6 46980 Paterna (Valencia), Spain. 7 8 * Corresponding author. Prof.; Tel.: (+34) 96 390 00 22 ext: 2512; Fax: (+34) 96 363 63 9 01; E-mail address: [email protected]s 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 *Manuscript Click here to view linked References 2 ABSTRACT 30 In the present study, a heat management PS tray containing an ultrathin fiber-structured 31 PS/PCM coating was prepared by using high throughput electrohydrodynamic 32 processing. To this end, polystyrene (PS) was used as the encapsulating matrix of a 33 commercial phase change material (PCM) called RT5 (a blend of paraffins with a 34 transition temperature at 5ºC), by using the electrospinning technique. With the aim of 35 imparting heat management capacity to the trays, the PS tray was coated by the 36 PS/PCM ultrathin fiber mats and a soft heat treatment was applied to improve the 37 adhesion between the layers. Results showed that RT5 could be properly encapsulated 38 inside the PS matrix, with a good encapsulation efficiency (ca. 78%) and the developed 39 PS fibers had a heat storage capacity equivalent to ~34 wt.% of the neat PCM. The 40 effect of storage time and temperature was evaluated on the heat storage capacity of the 41 developed PS-trays with the ultrathin fiber-structured PS/PCM layer. The heat storage 42 capacity was affected not only by the storage time, but also by the temperature. This 43 work adds a new insight on the development of heat management polymeric materials 44 of interest in food packaging applications, in order to preserve the quality of refrigerated 45 packaged food products. Although the electrohydrodynamic processing seems to be a 46 promising alternative to develop heat management materials, further works will be 47 focused on the improvement of heat storage capacity and efficiency of the developed 48 packaging materials along storage time. 49 50 Keywords Phase change material · Electrohydrodynamic processing · Heat 51 Management materials · Food packaging · Encapsulation 52 53 1. INTRODUCTION 54 Maintaining the cold chain during the commercialization of certain food products is one 55 of the key aspects to ensure food safety and food quality. Refrigeration temperatures are 56 used for preventing or slowing down microbial, physiological and chemical changes in 57 food produced by microbial, chemical and/or enzymatic activity. Along the cold chain 58 there can be temperature variations which will consequently have negative effects on 59 food due to crystal ice growth, acceleration of chemical reactions and/or microorganism 60 growth, which could result in a reduction of quality and may shorten the shelf-life of the 61 food products. Therefore, there is a great interest on finding new strategies to reduce 62 temperature fluctuations along the cold chain. In this sense, the packaging can be 63 designed to play an active role to maintain the food temperature within desired limits 64 and, thus, to ensure the quality, safety and increase the shelf-life of the products (James 65 et al., 2006). However, traditional commercial packages do not provide any protection 66 for maintaining the cold chain. An strategy already proposed to impart thermal 67 buffering capacity to standard packaging materials is based on the development of 68 thermal energy storage structures through the addition of, for example, phase change 69 materials (PCMs) (Chalco-Sandoval et al., 2014, Gin and Farid 2010 Oró et al. 2012) 70 within the polymeric structures (Oró et al. 2013). This strategy has been used by several 71 researchers such as Yannick (2006) who patented a method to manufacture an insulated 72 container used to transport and store ice cream, and Laguerre et al. (2008) who 73 developed and validated a mathematical model to predict the product temperature at 74 certain locations within an insulated container equipped with PCM. However, little 75 information exists in the literature about the incorporation of encapsulated PCM 76 structures into polymeric matrices for food packaging purposes, either in the form of 77 multilayer or in nanocomposites. Chalco-Sandoval et al., 2014 developed PS multilayer-78 based heat storage structures based on PS films coated with PCL/PCM electrospun 79 layers. An additional PCL electrospun layer (without PCM) was also electrospun in 80 some cases to retain PCM during film storage. 81 Phase change materials (PCMs) are substances that undergo a phase transition at a 82 specific temperature and, as a result, they are able to absorb and release latent heat with 83 a very small variation in temperature (Jin et al. 2010). PCMs could be used during 84 transport, storage and distribution stages to maintain the cold chain of solid food, 85 beverages, pharmaceutical products, textile industry, blood derivatives, electronic 86 circuits, cooked food, biomedical products and many others (Oró et al. 2012). The most 87 commonly used phase change materials are paraffin waxes, fatty acids, eutectics and 88 hydrated salts (Farid et al. 2004). Paraffin compounds fulfill most of the requirements 89 for being used as PCMs, as they are reliable, predictable, non-toxic, chemically inert 90 and stable below 500ºC. They also show little volume changes on melting and have low 91 vapor pressure in the melt form (Sharma et al. 2009). Direct incorporation of PCMs into 92 packaging structures is difficult because of their low thermal stability, low thermal 93 conductivity and some of them are liquid at ambient temperature (Fang et al. 2009). 94 Microencapsulation of the PCMs is a plausible solution because it allows protecting 95 them against the influences of the outside environment, increasing the heat-transfer 96 area, and permitting the core material to withstand changes in volume of the PCM 97 which take place as the phase change occurs, thus, allowing the development of small 98 and portable thermal energy storage systems (Alkan et al. 2011). 99 Electrohydrodynamic processing is one technique increasingly being used for the 100 microencapsulation of substances. This technique has proven to be a suitable method for 101 encapsulation of several components, including biomedical compounds, functional food 102 ingredients, PCMs and others substances within polymer matrices (Goldberg et al., 103 2007; Lopez-Rubio et al., 2012; Pérez-Masiá et al., 2013). The electrohydrodynamic 104 processing, commonly termed as electrospinning, is a technique whereby long non-105 woven ultrafine structures, typically fibers with diameters of several tens to several 106 hundreds of nanometers, may be formed by applying a high-voltage electric field to a 107 solution containing polymers (Teo and Ramakrishna, 2006). As a result of the applied 108 electric field, a polymer jet is ejected from the tip of a capillary through which a 109 polymer solution is pumped, accelerated toward a grounded target and deposited 110 thereon (Arecchi et al., 2010). 111 The aim of this work was to develop heat management materials of interest in food 112 packaging for refrigeration applications by means of developing a electrospun coating 113 incorporating a PCM which melts at 5ºC (RT5), to be used onto polystyrene (PS) trays. 114 The effects of storage temperature and ageing on the performance of the trays were also 115 evaluated. 116 117 2. MATERIALS AND METHODS 118 2.1 Materials 119 Rubitherm RT5, a technical grade paraffin wax, was chosen as the PCM for refrigerated 120 storage. It is based on a cut resulting from refinery production and it consists entirely of 121 normal paraffin waxes (C14-C18). RT5 was purchased from Rubitherm Technologies 122 GmbH (Berlin, Germany). Polystyrene trays were purchased from Poliestirenos 123 Asturianos S.L (Asturias, Spain). Polystyrene (PS) commercial grade foam was 124 supplied by Traxpo (Barcelona, Spain). N, N-dimethylformamide (DMF) with 99% 125 purity and trichloromethane (99 %) were purchased from Panreac Quimica S.A. 126 (Castellar del Vallés, Spain). All products were used as received without further 127 purification. 128 129 2.2. Preparation of polystyrene-based tray structures 130 131 2.2.1 Preparation of heat management PS-trays. 132 PS trays were coated with PS/PCM mats produced by means of the high throughput 133 electrohydrodynamic processing. The full process of the PCM encapsulation through a 134 high voltage spinning methodology has been previously developed (patent application 135 number: P201131063). The electrospun PS/PCM fibers were prepared according to 136 Perez-Masia et al. (2013), by dissolving the required amount of PS, under magnetic 137 stirring, in a solvent prepared with a mixture of trichloromethane:N,N-138 dimethylformamide (70:30 w/w) in order to reach a 10% in weight (wt.-%) of PS. 139 PS/PCM fiber mats were directly electrospun onto a metal collector over 5 hours by 140 means of a Fluidnatek® electrospinning pilot plant equipment from Bioinicia S.L. 141 (Valencia, Spain) equipped with a variable high-voltage 0-60 kV power supply. 142 PS/PCM solutions were electrospun under a steady flow-rate using a motorized high 143 throughput multinozzle injector, scanning vertically towards a metallic grid used as 144 collector. The distance between the needle and the collector was 28 cm and experiments 145 were carried out at ambient temperature. The voltage of the collector and injector were 146 set at 52 kV and 44 kV, respectively. 147 The electrospun PS/PCM coatings presented a whitish appearance and, with the aim of 148 obtaining a continuous pellicle, the PS/PCM coating (~ 50g) was deposited onto the PS 149 trays and was annealed at 145 ºC for 1.5 min using a hot-plate hydraulic press (Carver, 150 Inc., Wabash, USA) which also favoured the adhesion between materials. 151 152 2.2.2 Samples conditioning and storage 153 Samples were equilibrated in desiccators at 0% RH by using silica gel and at two 154 different temperatures 4 and 25ºC for three months. PS-trays containing the PS/PCM 155 coating were taken from the desiccators at different time intervals (0, 7, 15, 30, 45, 60 156 and 90 days) and DSC and FTIR analysis were carried out. 157 158 2.3. Characterization of PS trays with the ultrathin fiber-structured PS/PCM 159 coating. 160 161 2.3.1. Scanning Electron Microscopy (SEM). 162 SEM was conducted on a Hitachi microscope (Hitachi S-4100) at an accelerating 163 voltage of 10 kV. Samples were cryo-fractured after immersion in liquid nitrogen and 164 subsequently sputtered with a gold–palladium mixture under vacuum before their 165 morphology was examined using SEM. The thickness of the coating layer was 166 measured by means of the Adobe Photoshop CS3 extended software from the SEM 167 micrographs in their original magnification. 168 169 2.3.2. Differential Scanning Calorimetry (DSC) 170 Thermal analyses of the samples were carried out on a DSC-7 calorimeter (Perkin 171 Elmer Inc., Norwalk, USA) from -20 to 20ºC under a nitrogen atmosphere using a 172 refrigerating cooling accessory (Intracooler 2) (Perkin Elmer Inc., Norwalk, USA). The 173 scanning rate was 2ºC/min in order to minimize the influence of this parameter in the 174 thermal properties. The amount of material used for the DSC experiments was adjusted 175 so as to have a theoretical PCM content of 1-2 mg approximately. The enthalpy results 176 obtained were, thus, corrected according to this PCM content. All tests were carried out 177 in triplicate. 178 179 2.3.3. Attenuated Total Reflectance Infrared Spectroscopy (ATR-FTIR). 180 ATR-FTIR spectra of polystyrene (PS) polymer, PS tray, pure RT5 (PCM), PS/PCM 181 fibers and PS tray structures were collected at 25ºC in a FTIR Tensor 37 equipment 182 (Bruker, Rheinstetten, Germany). The spectra were collected in the different materials 183 by averaging 20 scans at 4 cm-1 resolution. The experiments were repeated twice to 184 verify that the spectra were consistent between individual samples. 185 186 2.3.4. Temperature profiles. 187 The temperature profiles of the PS trays with and without the PS/PCM coating were 188 compared. To this end, all samples were frozen at -18ºC for 1 day. Then, the surface 189 temperature evolution was registered at room temperature (20ºC) by using an infrared 190 thermometer MS Plus (PCE Instruments, Tobarra, Spain). 191 192 2.3.4. Optical properties. 193 Internal transmittance of the PS/PCM coating and PS-trays was determined through the 194 surface reflectance spectra with a spectrocolorimeter CM-3600 (Minolta Co, Tokyo, 195 Japan) with a 10 mm illuminated sample area. Measurements were taken from three 196 replicates by using both a white and black background and Kubelka-Munk theory for 197 multiple scattering was applied to the sample reflection spectra. Internal transmittance 198 (Ti) was calculated from the reflectance of the sample layer backed by a known 199 reflectance and the reflectance of the film on an ideal black background (Hutchings 200 1999). Moreover, CIE-L∗a∗b∗ coordinates (CIE, 1986) were obtained by the infinite 201 reflection spectra of the samples, using D65 illuminant/10° observer in order to 202 calculate the whiteness index (WI) of the samples (Eq. (1)). 203 Equation (1) 204 205 2.4. Statistical Analysis. 206 Statgraphics Plus for Windows 5.1 (Manugistics Corp., Rockville, USA) was used for 207 carrying out statistical analyses of data through analysis of variance (ANOVA). Fisher’s 208 least significant difference (LSD) was used at the 95% confidence level. 209 210 3. RESULTS 211 3.1. Morphology and optical properties 212 The main objective of this work was to develop PS trays containing PS/PCM coatings 213 to maintain the chilling temperature of fresh food products along the cold-chain. The 214 PS/PCM coating was previously observed by SEM (cf. Figure 1a). The surface images, 215 showed a dense but opened structure with many beaded areas (10.3 ± 4.2 µm) within the 216 fibrous (1.6 ± 0.6 µm) mat, and the cross-section images of the PS-trays (cf. Figure 1b) 217 gave an idea of the coating´s thickness (~ 78µm) and compactness. 218 Optical properties of the PS trays containing, or not, the PS/PCM coating were 219 evaluated and compared by means of the internal transmittance (Ti) where an increase 220 in the internal distribution of transmittance is ascribed to an increase in transparency. 221 Spectral distribution curves of internal transmittance are plotted in Figure 2. Lower Ti 222 values (Ti ≤ 15%) were obtained in all the samples as compared to those previously 223 obtained for multilayer structures prepared with PS (Ti ≥ 50%) which can be ascribed to 224 the different nature of the PS used (Chalco-Sandoval et al. 2014). Considering these 225 results, PS-trays and the corresponding PS-trays containing the ultrathin fiber-structured 226 coating can be considered to have low transparency. The highest internal transmittance 227 values were found for the PS tray whereas Ti values decreased with the addition of the 228 electrospun PS/PCM coating. 229 Laguerre, O., Ben Aissa, M. F., & Flick, D. (2008). Methodology of temperature 379 prediction in an insulated container equipped with Phase Change Materials. (Vol. 802, 380 pp. 83-90). 381 Lopez-Rubio, A., Sanchez, E., Wilkanowicz, S., Sanz, Y., & Lagaron, J.M. (2012). 382 Electrospinning as a useful technique for the encapsulation of living bifidobacteria in 383 food hydrocolloids. Food Hydrocolloids, 28, 159-167. 384 Oró, E., de Gracia, A., & Cabeza, L. F. (2013). Active phase change material package 385 for thermal protection of ice cream containers. International Journal of Refrigeration, 386 36(1), 102-109. 387 Oró, E., de Gracia, A., Castell, A., Farid, M. M., & Cabeza, L. F. (2012). Review on 388 phase change materials (PCMs) for cold thermal energy storage applications. Applied 389 Energy, 99, 513-533. 390 Perez-Masia, R., Lopez-Rubio, A., Fabra, M. J., & Lagaron, J. M. (2013). 391 Biodegradable polyester-based heat management materials of interest in refrigeration 392 and smart packaging coatings. Journal of Applied Polymer Science, 130(5), 3251-3262. 393 Pérez-Masiá, R., López-Rubio, A., & Lagarón, J. M. (2013). Development of zein-394 based heat-management structures for smart food packaging. Food Hydrocolloids, 395 30(1), 182-191. 396 Sharma, A., Tyagi, V. V., Chen, C. R., & Buddhi, D. (2009). Review on thermal energy 397 storage with phase change materials and applications. Renewable and Sustainable 398 Energy Reviews, 13(2), 318-345. 399 Teo, W. E., & Ramakrishna, S. (2006). A review on electrospinning design and 400 nanofibre assemblies. Nanotechnology, 17(14), R89-R106. 401 Zhang, S., Wu, J.-Y., Tse, C.-T., & Niu, J. (2012). Effective dispersion of multi-wall 402 carbon nano-tubes in hexadecane through physiochemical modification and decrease of 403 supercooling. Solar Energy Materials and Solar Cells, 96, 124-130. 404 Zhang, X.-x., Tao, X.-m., Yick, K.-l., & Wang, X.-c. (2004). Structure and thermal 405 stability of microencapsulated phase-change materials. Colloid & Polymer Science, 406 282(4), 330-336. 407 Yannick, A., 2006. European Patent No FR 2930739. 408 409 Table 1. Colour coordinates (L*, h*and C*) and whiteness index (WI) of PS-trays 410 containing or not PS/PCM pad. 411 Samples L* h* C* WI PS-tray 90.5 (0.4)a 102 (0.9)a 0.10 (0.06)a 90.5 (1.1)a PS-tray with PS/PCM pad 93.3 (0.6)b 98 (1.0)b 0.22 (0.05)a 93.3 (0.5)b a-b: Different superscripts within the same column indicate significant differences between samples. 412 Table 2. Ratio of PS/PCM of non-stored and stored samples at 4 and 25ºC. Mean value (standard deviation). 413 Material Non-stored Stored 3 months at 4°C Stored 3 months at 25°C PS-trays with the coating 1.29 (0.2) 1.1 (0.3) 0.84 (0.2) 414 Table 3. Thermal properties of the Rubitherm 5 (RT5) and the PS trays structures. 415 Mean value (standard deviation). 416 Time (days) Tm (ºC) ΔHm (J/g PCM) Tc1(ºC) Tc2 (ºC) ΔHc (J/g PCM) Supercooling (ºC) 4°C 25°C 4°C 25°C 4°C 25°C 4°C 25°C 4°C 25°C 4°C 25°C Pure RT5 7.2 (0.1) 142 (3) 5.3 (0.2) 142 (3) 1.9 (0.2) 0 7.2(0.1)a 1 7.2(0.1)a 1 107(1) a1 107(1) a1 5.7(0.2)a 1 5.7(0.1)a 2 3.2(0.1)a1 3.2(0.1)a 2 109(1) a1 107(1) a1 1.5(0.2) a1 1.5(0.1) a1 7 7.3(0.1)a 1 7.3(0.1)a 1 104(1) b1 98(1)b2 5.7(0.1)a 1 5.7(0.1)a 2 3.3(0.1)ab 1 3.3(0.1)a b2 103(1) b1 98(1)b2 1.6(0.1) a1 1.6(0.1) a1 15 7.4(0.1)a b1 7.4(0.1)a b1 95(1)c1 87(1)c2 5.8(0.1)a b1 5.8(0.2)a b1 3.4(0.1)bc 1 3.4(0.1)b 2 93(3)c1 87(1)c2 1.6(0.1) a1 1.6(0.3) a1 30 7.5(0.2)b 1 7.7(0.1)c 1 92(3)c1 70(1)d2 5.9(0.1)b 1 6.1(0.1)c 1 3.5(0.2)bc d1 3.6(0.1)c 2 92(5)c1 70(1)d2 1.6(0.1) a1 1.6(0.3) a1 45 7.7(0.2)b c1 7.8(0.1)c 1 85(1)d1 55(2)e2 6.1(0.1)c 1 6.2(0.1)c 2 3.6(0.1)cd e1 3.9(0.1)d 2 82(2)d1 55(2)e1 1.6(0.1) a1 1.6(0.1) a1 60 7.7(0.2)b c1 8.0(0.1)c d1 80(1)e1 49(3)e2 6.2(0.1)c d1 6.4(0.2)c d2 3.7(0.2)de 1 3.9(0.1)d 2 80(1)d1 49(1)f2 1.6(0.1) a1 1.6(0.2) a1 75 8.0(0.1)c 1 8.3(0.1)d 1 79(1)e1 47(4)f2 6.3(0.1)d 1 6.6(0.1)d 1 3.9(0.2)ef 1 4.1(0.2)d e2 79(1)d1 47(4)fg 2 1.7(0.1) a1 1.7(0.1) a1 90 8.1(0.1)d 1 8.5(0.1)e 2 75(2)f1 44(2)f2 6.5(0.2)d e1 6.8(0.1)e 1 4.1(0.1)f1 4.4(0.1)e 2 73(3)e1 44(2)g2 1.6(0.1) a1 1.7(0.1) a1 417 a-f: Different superscripts within the same column indicate significant differences due to storage time (p < 0.05). 418 1-2: Different superscripts within the same line indicate significant differences due to the temperature used (p < 0.05). 419 420 421 422 423 424 425 426 Figure captions 427 Figure 1. Surface (a) and cross-section (b) SEM images of the PS tray with the ultrathin 428 fiber-structured PS/PCM coating. Scale markers correspond to 20 and 200 µm for the 429 surface and cross-section, respectively. 430 Figure 2. Spectral distribution of internal transmittance (Ti) of PS trays with and 431 without the ultrathin fiber-structured PS/PCM coating. 432 Figure 3. ATR-FTIR spectra of the neat PS polymer, PS tray, pure RT5 and non-433 stored/stored PS tray with the PS/PCM coating measured at 4 and 25ºC. (A) Non-stored 434 PS tray containing the PS/PCM coating; (B) and (C) PS tray containing the PS/PCM 435 coating stored for 3 months at 4 and 25ºC, respectively. 436 Figure 4. Encapsulation efficiency and the calculated amount of the RT5 (%) 437 encapsulated in the PS-tray systems. (a) Efficiency (%) at 4°C and 25°C; (b) % RT5 at 438 4°C and 25°C. 439 Figure 5. Surface temperature as a function of time for PS tray with and without the 440 ultrathin fiber-structured PS/PCM coating. 441 442 Figure 1 Click here to download high resolution image Figure 2 Click here to download high resolution image Figure 3 Click here to download high resolution image Figure 4 Click here to download high resolution image