Jun-Feng Su, Xiao-Yan Yuan, Zhen Huang, and Yun-Hui Zhao. Interfacial debonding behaviors affect the mechanical properties of microencapsulated phase ...
10.1002/spepro.003865
Behavior of microencapsulated phase change material/epoxy composites Jun-Feng Su, Xiao-Yan Yuan, Zhen Huang, and Yun-Hui Zhao
Interfacial debonding behaviors affect the mechanical properties of microencapsulated phase change material/epoxy composites. Microcapsules containing phase change materials (microPCMs) make a functional material with a ‘core-shell’ structure. MicroPCMs have attracted much attention in the fields of energy storage and smart materials, where they are widely applied as temperature-regulating fibers, construction materials, fabric and clothing, and as anti-ice coatings.1–4 For example, microPCMs have been used in a ski jacket, where the encapsulated PCM initially absorbs the skier’s body heat and stores it until the body temperature drops from the outside environment. MicroPCMs can also be used in conjunction with solar heating and radiant heat flooring as a latent heat-storage device. PCMs can provide a larger heat transfer area per unit volume and thus a higher heat transfer rate when in microPCM form.5 The addition of particulate fillers to a polymeric resin is known to have a significant influence on its mechanical properties. Particulate fillers can increase fracture toughness, for instance, by crack pinning or bridging, micro-cracking, and crack deflection.6–9 These phenomena will greatly decrease the mechanical properties of composites. Interfacial interactions and interphases play a key role in all multicomponent materials, irrespective of the number and type of their components or their actual structure (see Figure 1). In a repeated, vigorous thermal absorbing-releasing process, microcracks and separation will occur at the composite interface. The microcapsules and polymeric matrix have different thermal expansion coefficients. Consequently, the different parts of the composite will expand and shrink to different extents during a repeated thermal phase change process. If the PCM inside the microPCM changes in volume during temperature changes, it can cause micro-cracks or fractures in the matrix. This spoils the thin microcapsule shell, and the PCM loses the shell’s protection (see Figure 1). In this way, the mechanical properties of these composites may decrease with internal cracking or
Figure 1. Illustration of interfacial failure caused by the residual stresses due to expansion and shrinkage in the thermal transmission process and mismatch of molecule movement among the components.
microcapsule rupture. Interfacial changes influence the properties and shorten the service life of microPCM/matrix composites. We evaluated the effect of particle size, particle-matrix interfacial adhesion, particle loading, and repeated cycles of rapid heating and cooling on the mechanical properties of microPCM/epoxy composites. A rigorous quenching control was enforced for composites in this study: composites were treated at 50ı C (increase rate of 5ı Cmin 1 / and then immediately immersed in cold water (15ı C). We subjected each composite to between one and three cycles of this heating and cooling. We found that the Young’s modulus of the epoxy composite with same-average-diameter filled microPCMs was reduced by more, the more times we repeated the thermal treatment: see Figure 2(A). The energetic thermal transmission destroys the internal structure of these
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Figure 2. Young’s modulus (A) and tensile strength (B) of various average-diameter microencapsulated phase change material (microPCM)-filled epoxy composites after experiencing (a) one, (b) two, and (c) three cycles of a thermal absorbing-releasing process. The microPCMs’ weight contents are 5%.
samples. We also found that, for composites with the same weight ratio of microPCMs, Young’s modulus decreased as the average diameter of the filled microPCMs increased. A single thermal cycle reduced the tensile strength values for composite samples filled with microPCMs, and subsequent cycles caused the tensile strength to drop further: see Figure 2(B). We used scanning electron microscopy to elucidate the interfacial behavior of the composites after thermal treatment cycles. It is clear that some cracks appeared in the interphase, which became flexible after a single cycle of thermal treatment (see Figure 3). The original close interface could not resist this cracking, because of the mismatch in
Figure 3. Scanning electron microscope images of the interfacial morphologies of microPCM/epoxy composites after three cycles of a thermal absorbing-releasing process.
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expansion coefficients between the shell and epoxy matrix.10 After two and three cycles of heat transmission, the phase separation in the interphase region, and the shell and matrix was still linked by an interface molecule. In summary, we examined the robustness to rapid cycles of heating and cooling of microcapsules containing phase change materials, which, among other things, are used to absorb, store, and radiate thermal energy in building materials or clothing. We found that cycles of rapid heating and cooling reduce the tensile strength and Young’s modulus of the microPCM/epoxy composites. This can be readily explained by considering how differences in the thermal expansion and contraction of the PCM affect structural boundaries. We will next undertake large-scale fabrication of microPCM/epoxy composites for temperature sensing. We will also develop a model for the relationship between the interfacial debonding and mechanical properties, to predict adhesion between the filler microPCMs and the matrix, and the stability and usefulness of the temperature sensor.
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Jun-Feng Su and Zhen Huang Tianjin University of Commerce Tianjin, China Jun-Feng Su is an associate professor. He received his doctorate from Tianjin University (2009). His main research interests include polymer functional materials, polymer composites, biodegradable polymers, and nanoscale materials. Xiao-Yan Yuan and Yun-Hui Zhao Tianjin University Tianjin, China
c 2011 Society of Plastics Engineers (SPE)
