Flame retardant and leaking preventable phase change materials for thermal energy storage and thermal regulation

Highlights

• An innovative and facile chelation-deposition strategy to prepare FRPCMs with a bio-based flame retardant shell.

• The FRPCMs have large phase change enthalpy ranging from 117 to 158 J/g.

• The FRPCMs possess good leaking prevention performance at 90 ℃.

• The LOI value of the EP/FRPCM composites increases from 18.5 to 21.6 and the THR is reduced by 39.5%.

Abstract

Phase change materials (PCMs) possess great potential in thermal energy storage and thermoregulation scopes. However, liquid leaking and high flammability restricts their practical applications seriously. In this work, an innovative and facile chelation-deposition strategy is proposed to prepare flame retardant PCMs (FRPCMs) with 1-octadecane as the phase change substance and bio-based magnesium phytate (PAMg) as the flame retardant shell. In this way, the liquid leaking and flammability issues are addressed simultaneously. We have undertaken a comprehensive study on the structure and properties of the FRPCMs via various tests. It is confirmed that the 1-octadecanol is successfully microencapsulated by the PAMg shell and its crystal structure is not affected. The thermal storage capacity is evaluated with differential scanning calorimetry (DSC) and the latent heat is ranged from 117 to 158 J/g. Limited oxygen index (LOI) and cone calorimeter are performed to assess the flame retardancy of FRPCMs. The LOI value of the epoxy resin/FRPCM composites is increased from 18.5 to 21.6 and the total heat release (THR) is reduced by 39.5% with the addition of FRPCMs particles. The FRPCMs are proved to possess good leaking prevention performance at 90 °C (higher that the melting point of 1-octadecanol). Furthermore, the long term thermal reliability, thermal regulation, leaking prevention and corrosive characteristics of FRPCMs are characterized.

Introduction

In recent years, phase change materials (PCMs) have gained major attention due to the increasing worldwide concern on energy crisis and the growing environmental pollution problems [1], [2], [3], [4]. PCMs are attractive materials that can absorb, storage and release large amounts of heat energy during the phase transition process at a constant temperature [5], [6], [7]. Besides, PCMs possess many desirable characteristics, including high energy storage density, narrow phase change temperature range, low corrosivity, good reusability, low cost and easy processing [8], [9], [10]. Therefore, numerous PCMs, including n-alkanes [11], paraffin wax [12,13], fatty acids [14,15], and polyethylene glycols [16], have been extensively researched for various applications, such as energy-saving buildings [17], thermoregulated textiles [18], solar energy harvesting and storage [19], and waste heat recovery [20]. However, the practical applications of PCMs are severely restrained by the drawback of liquid leakage during the solid-liquid phase transition process [21], [22], [23]. Besides, most of PCMs are highly flammable and easily cause fire disaster, making it difficult to meet the demand of some fireproof required fields [24]. Therefore, it is of great importance to address the liquid leaking problem and enhance the flame retardancy of PCMs [25].

Previous researches have demonstrated that the liquid leaking of PCMs can be alleviated by microencapsulation technology and the flammability can be reduced through the addition of flame retardants, i.e., the aims of leaking prevention and flame retardant are realized individually [26,27]. For instance, Fang et al. [28] prepared a SiO₂ shell microencapsulated paraffin composite and demonstrated an improved thermal stability due to the inorganic shell, but the flame retardant test was lacked. Li et al. [29] developed a flame retardant and form-stable PCMs where polypropylene acted as the supporting material and ammonium polyphosphate/triazine char forming agent functioned as the flame retardant. Kazanci et al. [30] synthesized a composite microcapsule with polystyrene as the shell material, paraffin as the phase change material and halogen-free ortho-phosphoric acid/pentaerythritol as the flame retardants. Unfortunately, the incorporation of leakage-proof materials and flame retardants will inevitably lead to unwanted reduction of latent heat, thereby resulting in a major decrease in energy storage capacity. Therefore, we can get the inspiration that an ideal candidate is one that integrates the functions of leakage-proof material and flame retardant, with the double benefit of addressing the liquid leaking problem and decreasing flammability. Recently, Du et al. [31] prepared a phosphorous-containing PMMA shell to microencapsulated PCMs and ameliorated the flame retardant property. However, the fabrication process is complex and uses some organic solvents, which is not in line with the concept of green chemistry.

Phytic acid (PA) is a biomass-derived organophosphorus compound that mainly exists in seeds, beans, roots and stems of plants [32]. Owing to the high content of phosphorus, PA exhibits great potential value in preparing bio-based flame retardants [33]. On the other hand, the six phosphate groups show strong ability to chelate metal cations and form complexes. Therefore, a variety of metallic phytates (PA-M, where M is Al³⁺, Cu²⁺, Zn²⁺, Mg²⁺, or Sn⁴⁺) have been synthesized and applied in flame retardant polymers, including PVC [34], EVA [35], PLA [36] and PP [37]. Among these metallic phytates, the magnesium phytate (PAMg) shows obvious advantages over other candidates, such as heavy metal-free and easily controlled synthesis process. Furthermore, the PAMg can be precipitated in a controlled way when the pH of the aqueous solution is increased gradually. Derived from the renewable source, PAMg as a bio-based flame retardant holds great promise as a shell material for PCMs. However, as far as we know, there is no research on encapsulating PCMs with a bio-based flame retardant shell which helps to improve the leaking prevention and flame retardancy simultaneously.

In this work, 1-octadecanol is selected as the phase change material because of its low cost, nontoxicity, large enthalpy of phase transition (about 220 J/g), and appropriate phase transition temperature (approximately 45 °C-65 °C) [38]. The flame retardant PCMs (FRPCMs) with a 1-octadecanol core and PAMg shell are prepared via a facile chelation-deposition route. The 1-octadecane plays the role as phase change substance and the bio-based PAMg acts as both the shell material and flame retardant. The results show that the leakage problem and flame retardancy of PCMs are well addressed. Moreover, the prepared FRPCMs possess large thermal storage capacity and good thermoregulation performance, thus greatly contributing to enlarge the application scopes of PCMs.