Enhanced mechanical properties of epoxy composites embedded with MF/TiO2 hybrid shell microcapsules containing n-octadecane
Graphical abstract
Introduction
Materials that can autonomously store and release thermal energy provide great opportunities for efficient and compact thermal energy management. As carriers of latent thermal energy, phase change materials (PCMs) implement storage and release of thermal energy through phase transition [1], [2]. PCMs can greatly improve energy efficiency compared with sensible heat storage and have attracted widespread attention in the energy field. Microencapsulated phase change materials (MPCMs) utilize a film-forming material to coat PCMs for forming microspheres with core–shell structure to prevent the leakage of melting PCMs and increase the heat transfer area [3], [4], [5]. MPCMs have broad application prospects in the fields of construction [6], [7], [8], aerospace coatings [9], [10], [11], and textiles [12], [13]. MPCMs are mixed with matrix materials such as epoxy, gypsum, and foam to form the phase change composites for energy storage and intelligent temperature regulation [14], [15]. In practical application, suffering from the external force or the thermal stress generated during the phase transition, MPCMs are easily debonded from the matrix or even ruptured [16], which significantly shortens the service life of phase change composites. Therefore, increasing the mechanical strength of MPCMs and enhancing the bonding strength between MPCMs and matrix are the keys to improving the mechanical performance of composites and their lifetime.
The mechanical strength of MPCMs is mainly related to the cross-linking degree of the microcapsule shell and the ratio of shell thickness to microcapsule diameter [17]. The interface bonding strength is affected by the bonding area and the degree of molecular entanglement between MPCMs and the matrix [14]. Thus, appropriate shell material selection and the method for synthesizing microcapsule shells are crucial. In recent years, it has been favored by researchers to synthesize organic–inorganic hybrid microcapsules by doping inorganic nanoparticles into an organic shell to combine the advantages of organic and inorganic materials [18], [19], [20], [21]. Due to the superiorities of excellent thermal and mechanical properties, inorganic nanoparticles often play a functional role in the hybrid shell. For example, Zhang et al. [22] synthesized hybrid microcapsules with MF/hydrophobic-silicon carbide (H-SiC) as the shell by in situ polymerization. The thermal conductivity of the hybrid microcapsules with 2% H-SiC nanoparticles was increased by 55.82% compared with microcapsules without H-SiC nanoparticles.
Melamine formaldehyde (MF) resin, one of the most commonly used organic materials, has the advantages of low cost, easy preparation, and good mechanical strength [23], [24]. MF resin can play a structural role in the hybrid shell. For MF shell, in situ polymerization is applied as the synthesis method. In situ polymerization forms a shell on the surface of PCMs droplets through the deposition and polymerization of MF prepolymer [25]. Titanium dioxide (TiO2) nanoparticle, which has nontoxicity, high thermal conductivity, and outstanding mechanical property, is an excellent candidate for nanoparticle filler [26], [27]. The inorganic nanoparticle fillers can be added to the emulsion system or the MF prepolymer system to synthesize organic–inorganic hybrid microcapsules [28]. For the approach of adding nanoparticles to the emulsion system, the presence of nanoparticles promotes emulsification due to the emulsifying effect of nano-scale particles [19], [29]. This leads the prepared emulsion droplets to have smaller diameters. Since the forming of the microcapsule is the polymerization of MF prepolymer on the surface of core droplets, the diameter of microcapsules strongly depends on the size of the core droplets. Therefore, the prepared microcapsules have a smaller particle size at the same cumulative distribution. For the approach of adding nanoparticles to the MF prepolymer system, it has little effect on the particle size of microcapsules. However, due to the surface effect of nanoparticles, nano-TiO2 has enormous specific surface area and surface energy, which results in a strong tendency of agglomeration of TiO2 nanoparticles [30], [31]. It is difficult for TiO2 nanoparticles to migrate uniformly into the MF shell, which degrades the mechanical properties of microcapsules. To improve the dispersion stability of nanoparticles, it is feasible to modify nanoparticles by using silane coupling agents to lower surface energy and produce chemical bonds on the surface of nanoparticles [32], [33], [34]. In terms of matrix materials, epoxy resin is an excellent candidate due to its strong cohesiveness, good mechanical properties, and outstanding corrosion resistance. In the aerospace field, many aerospace composites are based on the blends of epoxy resin [35].
In the present work, TiO2 nanoparticles were modified by a silane coupling agent to reduce agglomeration. Based on in situ polymerization, the modified TiO2 nanoparticles were doped into the microcapsule shell via MF prepolymer system, forming the n-octadecane@MF/TiO2 hybrid shell microcapsules (HS-MPCMs). The effects of nanoparticle doping on the microcapsules were analyzed from the aspects of microstructures, thermal performance, and mechanical properties.
Section snippets
Materials
Titanium dioxide (TiO2, rutile, 99.8%) with an average size of 40 nm, as an inorganic nanofiller, was used to synthesize hybrid shells. 3-Glycidoxypropyltrimethoxysilane (KH560, ≥98.0%, Sinopharm Chemical Reagent Co., Ltd) was used to modify TiO2 nanoparticles. n-Octadecane (99.0%) was employed as the core material. Melamine (99.0%) and formaldehyde solution (37 wt.%) were monomers for synthesizing the organic shell. Polyvinyl alcohol (PVA, 97.5–99.0 mol%) and sodium dodecyl sulfate (SDS,
Chemical characterization of nanoparticles and microcapsules
The FTIR spectra of TiO2, modified TiO2, n-octadecane, MS-MPCMs, and HS-MPCMs were shown in Fig. 2. In the spectra of TiO2 and modified TiO2 nanoparticles, the absorption peaks around 530–850 cm−1 belong to Ti-O stretching vibration and Ti-O-Ti bridging stretching vibration [20]. The spectrum of TiO2 displays a broad peak in the range of 3200–3600 cm−1, which is attributed to the stretching vibration of the hydroxyl group on the surface of TiO2 nanoparticles [37]. However, in the spectrum of
Conclusion
Applying in situ polymerization, n-octadecane@MF/TiO2 hybrid shell microcapsules with better mechanical properties were successfully synthesized by doping KH560 modified TiO2 nanoparticles in the MF shell. The rupture strength of HS-MPCMs is 30.4% higher than that of pure MF shell microcapsules. The doping of modified TiO2 nanoparticles to the MF shell also enhances the interface bonding strength between microcapsules and epoxy matrix by improving chemical bonding. The curing agent, as a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to gratefully acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 11772302, 12172332, 11727803 and 11972037), the Fundamental Research Funds for the Provincial Universities of Zhejiang (RF-A2020013), and the Johns Hopkins University start-up fund for Sung Hoon Kang (United States).
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