Copper nanoparticles interlocked phase-change microcapsules for thermal buffering in packaging application

Abstract

A novel kind of functionalized copper nanoparticles (CuNPs) interlocked polydivinylbenzene (PDVB) microcapsules containing hexadecane as phase change material (PCM) was synthesized targeting thermal buffering application in food packaging. Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM) characterizations confirmed the successful functionalization of CuNPs and their existence in the PDVB shell. Scanning electron microscopy (SEM) results indicated that produced microcapsules exhibited well-defined core–shell microstructure with spherical morphology, and their compositions and chemical structures were confirmed by a series of spectroscopic characterizations. Atomic force microscopy (AFM) results showed that average and mean square roughness is higher in CuNPs/microPCMs than bare microcapsules. Differential scanning calorimetry (DSC) analysis confirmed that microPCM with 1.0% CuNPs achieved a maximum melting enthalpy of 132 J/g with an encapsulation ratio of 60.5% and could maintain it even after 100 melting-freezing cycles. The thermal conductivity was significantly increased by 319.5% as compared with that of bare microcapsules. Most importantly, optimized microcapsules provided an excellent thermal buffering effect of more than 6.5 h for the 240 g of chocolate to raise its temperature from 5 °C to 35 °C, confirming great potential in food packaging application.

Introduction

Maintenance of cold chain during storage and transportation of food products (chocolates, fruits, vegetables, etc.) is essential to ensure the quality of food. Usually, temperatures between 8 °C to 20 °C can maintain structural integrity and retard microbial/physiological/chemical changes occurring with food products due to microbial/ enzymatic activity [1]. During the cold chain, temperature fluctuations can cause detrimental effects on the food due acceleration of chemical reactions or microorganism growth, which may not only lead to deterioration of quality but also shorten the shelf-life of the food products. Thus, there remains intense demand to develop new strategies to buffer the temperature fluctuations along the cold chain. In this sense, the packaging can be designed to play an active role in maintaining the food temperature within desired limits and, thus, to ensure the quality, safety and increase the shelf-life of the products. Generally, traditional commercial packages such as low-density polyethylene and polystyrene do not provide thermal buffering during cold chains due to their low thermal conductivity [2]. However, there are different strategies that could contribute to improving the thermal buffering capacity of packaging. One of them is to use of thermal energy storage materials comprised of a unifying system of phase change material (PCM) and packaging material [3].

PCMs have attracted tremendous attention due to their capability of absorbing, storing, and releasing large amounts of latent heat over a well-defined temperature range. With the advancement in PCM research, numerous organic, inorganic, and eutectic PCMs have been investigated. Among all, n-alkanes (organic PCMs) with low phase change temperature, high latent heat, inertness, non-corrosiveness, and low cost have been extensively investigated. PCMs can be used either in bulk form (macro-encapsulated) or it may be incorporated inside porous matrix to have a composite. For instance, Johnston et al. [4] used calcium silicate-alkane based PCM composite for thermal buffering in paperboard packages. Unal et al. [5] employed halloysite /polyethylene /polyethylene glycol nanocomposite for thermal buffering in packaging films for cold storage. However, some serious shortcomings such as low thermal conductivity, flammability, rapid volume expansion, and leakage issue restrict the practical applicability of bulk PCM [6]. While PCM composites suffer from particle size limitation, fix composite geometry, low encapsulation efficiency, leakage, and low surface area [7], [8]. Therefore, encapsulation of PCMs becomes crucial for its safe application in packaging material.

Microencapsulation is a process of forming a core–shell structure with the core as a PCM and shell as a polymeric or inorganic material. Microencapsulated PCMs (microPCMs) have some decent benefits over the pristine PCMs, including leakage prevention, increased heat transfer area, volume change control, and increased applicability [9]. The development of microPCMs in terms of thermal energy storage capacity, synthetic strategies, morphology control, and mechanical properties were extensively investigated [6]. Polymeric microcapsules can be easily tailored to meet these aspects, but their shells suffer from low thermal conductivity [10]. Low thermal conductivity of polymeric shells pose an extra time to store and release the latent heat energy of PCM. This signifies that the process of thermal energy storage unit will delay the phase change process, which will lead to more energy consumption and thereby cost increase. Since preservation of many food items at chilling conditions requires a quick response of thermal energy to avoid/postpone microbial, physical, and chemical changes, the application of microencapsulated PCM in thermoregulating food packaging gets baffled due to poor thermal conductivity of polymeric shell. Therefore, some more attempts are needed to improve the thermal conductivity and stability of polymeric shell to make them application suitable. So far, two approaches are commonly practiced: (i) introducing thermally conductive shell materials and (ii) incorporating nanoparticles (NPs) in the polymeric shell (i.e., composite shell). Adopting former approach microPCMs with various thermally conductive shells were reported e.g. calcium carbonate (CaCO₃): 0.61 W/m.K [11], zirconia: 0.89 W/m.K [12], AlOOH: 0.70 W/m.K [13], titanium dioxide (TiO₂): 0.72 W/m.K [14], etc. In the above examples, though the thermal conductivities increased significantly, their encapsulation efficiencies were extremely low, i.e., ~ <25%. Moreover, since most of microPCMs are applied in circulatory systems (heat pumps, heat exchangers, etc.), the inorganic shells are too fragile to be applied. Therefore, the second approach of incorporating thermally conductive NPs in the polymeric shell became more popular as it has the potential of imparting multiple functionalities to the shell besides enhancement of thermal conductivity [15]. The most recent literature survey indicates that very few NPs such as boron nitride [16], silver [17], silicon carbide [18], aluminium oxide [19], graphene oxide [20], carbon nanotubes [7], etc. were reported as reinforcers to fabricate microPCMs with a composite shell. However, most of the NPs cited above were used with MF resin, UF resin, MUF resin, and PU and were not surface modified. Since NPs used were not sufficiently hydrophobic (amphiphilic character), they were not compatible with hydrophobic polymer shells. Therefore, unmodified NPs got easily desorbed while processing and post-application stage which limit their employability [21]. Hence, an appropriate grafting methodology or surface modification of NPs is needed to make these NPs loaded and enhance their compatibility with the polymeric shell.

In this article, we fabricated novel microcapsules using hexadecane as core PCM surrounded by functionalized CuNPs interlocked polydivinylbenzene (PDVB) as composite shell. PDVB was chosen as a shell to achieve good mechanical stability and improved barrier (leakage of PCM) through its highly cross-linked network. Moreover, the toxicity and processability of PDVB precursors are considerably lower and simpler than precursors of MF, UF, and PU. Since many food products (chocolates, vegetables, fruits, etc.) need to be store between the chilling temperature of range of 8–20 °C, PCM melting temperature within a chilling range was selected. Targeting application of microPCM in thermoregulated food packaging within the chilling range, hexadecane was chosen as PCM due to its relatively large enthalpy of 218 ± 2 J/g and stable melting temperature of 18 ± 1 °C. The selection of CuNPs was made as a reinforcing material due to their anti-bacterial property, high thermal conductivity, low cost, and versatile chemistry. It is postulated that CuNP embedded microPCM will not only improve thermal conductivity but also enhances the rate of energy transport (storage/discharge) during phase transition [11]. Previously, PCM microcapsules were extensively reported only for thermal energy storage performance, synthetic strategies, morphology control, and mechanical properties with the variable parameters such as in core: shell ratio, reaction temperature, stirring speed, amount of surfactant, variation in a single component in the reactants, etc. At present, there are hardly any article which reports direct effect of variation of CuNPs concentration (especially in acrylic-based microcapsules) for composite shell microcapsules. Therefore, variation in CuNPs for systematic quantification of CuNPs to study the microstructure, the size and shape, thermal conductivity, latent heat density, thermal stability, heat response performance was selected. To enhance compatibility within the polymer matrix and promote interfacial interactions, novel silane chemistry was employed for CuNPs functionalization. During suspension polymerization, these functionalized CuNPs were subsequently incorporated into the polymer backbone chain of PDVB to synthesize formaldehyde-free microcapsules. Finally, the suitability of produced microcapsules for the food packaging application was investigated. The layer of microcapsules was applied in the temperature-controlled packaging (TCP), and its thermal buffering ability was studied.