Preparation and properties studies of halogen-free flame retardant form-stable phase change materials based on paraffin/high density polyethylene composites
Introduction
Energy needs for a wide variety of applications mainly depend on time and energy resources available. Therefore, energy storage is often required to meet various demands for energy. The latent energy storage has become one of the most attractive techniques among the different methods of thermal energy storage. Much attention has been paid to the form-stable phase change materials (PCM) in recent years, which represent a rational alternative to traditional PCM. Among the various kinds of the form-stable PCM of interest, paraffin has been found to exhibit many desirable characteristics, such as high heat of fusion, varied phase change temperature, negligible super-cooling, lower vapor pressure in the melt, chemically inert and stable, self-nucleating, no phase segregation and commercial availability at reasonable cost [1], [2], [3], [4].
There are many literatures which have reported the preparation, physical–chemical, thermal energy storage and thermal conductivity properties of the form-stable PCM based on the paraffin [5], [6], [7], [8], [9], [10], [11]. It has been realized that the thermal conductivity is one of the important properties for the form-stable PCM in the practical applications. Various studies have been conducted to enhance the thermal conductivity property of the form-stable PCM. For example, Sari [7] has found that the thermal conductivity of the form-stable P1/HDPE and P2/HDPE composites was, respectively, increased about 14% and 24% by the addition of expanded graphite as little as 3 wt%. Xiao [8], [9] has prepared a shape-stabilized PCM by blending paraffin with a thermoplastic–elastomer poly (styrene-butadiene-styrene, SBS) and established its thermal performances during the melting and solidification processes. They concluded that the shape-stabilized PCM exhibited the same phase transition characteristics of paraffin and could be up to 80% of the latent heat of paraffin. And the thermal conductivity of shape-stabilized PCM has been increased significantly by introducing expanded graphite. Moreover, Zhang [10] has improved the thermal conductivity of the shape-stabilized PCM by adding some solid powders with high thermal conductivity, such as diatomite, wollastonite, organic bentonite, CaCO3 and graphite. The theoretical models (a cubic additive model and a spherical model) have well predicted the effective thermal conductivity property. Zhang [11] has prepared the paraffin/expanded graphite composite phase change thermal storage materials with a large thermal storage capacity, improved thermal conductivity and no occurrence of liquid leakage during solid–liquid phase change.
Although the preparation, thermal conductivity and thermal storage properties have been extensively studied, the reports on the thermal stability and flammability properties of the form-stable PCM are relatively rare. The lower thermal stability and inflammability properties have severely restricted the wide applications, especially in the building fields. Therefore, the studies of improved thermal and flammability properties of the form-stable PCM are very important. It is well known that the expandable graphite not only can improve thermal conductivity of the form-stable PCM composites, but also can be acted as flame retardant. In the present work, the intumescent flame retardant system based on expandable graphite (EG) with different synergistic additives, such as ammonium polyphosphate (APP) and zinc borate (ZB) was chosen. The preparation and properties of the halogen-free flame retardant form-stable PCM composites were investigated. The TGA and Cone calorimeter results revealed that there was a synergistic effect between the EG and APP, contributing to the improved thermal stability and flammability properties.
Section snippets
Materials
The high density polyethylene (HDPE) was supplied as pellets by Daqing Petrochemical Company, China Petroleum. The paraffin used was a commercial product with melting temperature (Tm = 54–56 °C) and latent heat of 163.58 kJ/kg. The ammonium polyphosphate (APP, white powders, average particle size: 92% < 10 μm), zinc borate (ZB, 2ZnO · 3B2O3 · 3.5H2O) and expandable graphite (EG) with average size of 300 μm were used.
Preparation of halogen-free flame retardant form-sable PCM composites
The twin-screw extruder (TE-35, China) was used to prepare the form-stable PCM composites.
Morphology characterizations
The morphology of the expandable graphite (EG) characterized by SEM is presented in Fig. 1. The SEM images with low and high imaginations clearly reveal the round-like lamellar structures with an average size of about 300 μm. Fig. 2 shows the SEM photographs of the paraffin/HDPE composites and the halogen-free flame retardant form-stable PCM composites. The paraffin looks dispersed well in the three-dimensional net structure formed by the HDPE, as indicated in Fig. 2a. It is observed from Fig. 2
Conclusions
Halogen-free flame retardant form-stable phase change materials (PCM) based on paraffin/HDPE composites were prepared through the twin-screw extruder technique. The SEM images showed that the EG was a kind of round-like lamellar structure. The HDPE acted as the supporting material and form the three-dimensional net structure. The paraffin acted as a phase change material and dispersed in the network structure. The flame retardant dispersed well in the matrix and did not show notable
Acknowledgements
The work was financially supported by the Program for New Century Excellent Talents in University (No. NCET-06-0485) and the specialized Research Fund for the Doctoral Program of higher education (No. 20060295005) and Program of Jiangnan University (No. 206000-21050737).
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