Physicochemical properties and mechanical characters of methanol ...

Colloid Polym Sci (2011) 289:111–119 DOI 10.1007/s00396-010-2328-1

ORIGINAL CONTRIBUTION

Physicochemical properties and mechanical characters of methanol-modified melamine-formaldehyde (MMF) shell microPCMs containing paraffin Jun-Feng Su & Sheng-Bao Wang & Yun-Yi Zhang & Zhen Huang

Received: 17 September 2010 / Revised: 20 October 2010 / Accepted: 28 October 2010 / Published online: 10 November 2010 # Springer-Verlag 2010

Abstract Microcapsules containing phase change materials (microPCMs) with melamine-formaldehyde (MF) shells have been applied in many thermo-regulation or thermosaving fields. However, it is still essential to decrease the residual formaldehyde and enhance the mechanical properties of MF shells. The objective of this work was to fabricate a series of microPCMs containing paraffin by an in situ polymerization method using methanol-modified melamine-formaldehyde (MMF) prepolymer as shell material and investigate the physicochemical properties and mechanical characters of these microPCMs. FT-IR analysis indicates that the methanol-modified method can reduce the free formaldehyde in shell material through increasing the cross-linking structure. Optical microphotographs and SEM morphologies show that the microPCMs have regular globe shape with smooth surface. With the increasing of emulsion stirring rates from 1,000 to 5,000 rpm, the average diameters decreased sharply from 27 to 2.5 μm. The phase change temperature (Tm) of microPCMs samples with the core/shell ratios of 3/1, 2/1, 1/1, and 2/1 are 22.6, 23.0, 23.4, and 23.9 °C, which are nearly equaled to the Tm of

J.-F. Su Tianjin Key Laboratory of Refrigeration Technology, Tianjin University of Commerce, Tianjin 300134, People’s Republic of China J.-F. Su (*) : Y.-Y. Zhang : Z. Huang Institute of Materials Science and Chemical Engineering, Tianjin University of Commerce, Tianjin 300134, People’s Republic of China e-mail: [email protected] S.-B. Wang Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China

pure paraffin (22.5 °C). Mechanical properties test data show that the MMF shells have larger yield point value than that of MF shell for microPCMs with the same core/ shell ratio, which means that the methanol-modified method shell can greatly enhanced the resistance of deformation for MF shells. Moreover, MMF shells can resist the interface extrusion force in epoxy resin owing to their higher yield point of enhanced MMF shell. Keywords Microcapsules . Phase change materials . Melamine-formaldehyde . MicroPCMs . Methanol . Mechanical properties

Introduction Polymeric solid–liquid phase change materials (PCMs) have been applied in energy storage fields because of their great ability of absorbing and releasing large latent heat in a phase change process [1]. To date, N-alkanes, their mixtures and paraffin waxes have been widely proposed as PCMs [2]. Although very high latent heat can be obtained, the bulk PCMs are not easy to handle in practical application. To overcome their inherent shortcomings of these solid– liquid PCMs, such as liquid migration, supercooling, and volume expansion, the technology of microencapsulation of PCMs (microPCMs) have been paid more attention in many PCMs functional products including thermalregulation building materials, clothing fabric, PCMs heat transmittance emulsion, and infrared camouflage materials [3–5]. MicroPCMs can handle PCMs as core material with many advantages: decreasing PCMs tolerate volume change, preventing PCMs contaminate with the outside environment, and increasing the PCMs heat transfer area, etc. [6–8].

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A survey of literatures shows that, in all the microPCMs fabrication methods (physical, chemical, and physiochemical), the in situ polymerization is a well-known approach to obtain microcapsules with core–shell structure [9, 10]. In addition, formaldehyde resins, like urea-formaldehyde (UF) and melamine-formaldehyde (MF), are usually as typical shell materials applied to encapsulate PCMs because of their low price, easy controlling, high compatibility, and good thermal stability [7, 10–13]. However, some of the residues of these formaldehyde resin in shells can cause environmental and health problems [11, 14]. To solve this puzzle, Zhang [15] has synthesized low toxic ureaformaldehyde resin through the method of adding formaldehyde once while putting in urea for three times. Wang [16] has fabricated microPCMs using resorcinol-modified melamine-formaldehyde shell material. Although many recent works have focused on the permeability, morphology, thickness, and compactability of MF shells for microPCMs, there is still little knowledge available on reducing the remnant formaldehyde content of microPCMs. On the other hand, mechanical properties of the microPCMs is another crucial character for manufacturing thermo-regulated microPCMs/polymer composites. Because of the volume expansion of encapsulated PCMs and the different of expansion coefficients for microPCMs and matrix polymer, the shell may be torn or broken by the internal stress in microPCMs/polymer composites. Moreover, the tiny microPCMs embedded in polymeric matrix are easy be damaged by micro-cracks coming from the outside acting force. This phenomenon has been investigated by single-capsule measurements [17]. Mechanical properties of microPCMs are obviously important for stability issues. Theoretically, the mechanical strength of microcapsules is determined by their chemical composition, structure, size, and shell thickness. It has been found that MF shells are stronger and rupture at larger deformations than UF shells for a given size and shell thickness [18]. In our previous study [19], we have fabricated high compact MF shell microPCMs through a special two-step method. Though MF shells have high compressive strength, these shells also must have optimum toughness to cope with the repeated rigorous absorbing–releasing thermal transmittance process and stabbing of micro-cracks. Instead, microcapsules must be tailored in their complex deformation characteristics rather to efficiently perform their tasks. Based on these considerations, the objective of this work was to fabricate microPCMs containing paraffin by an in situ polymerization method using methanol-modified melamine-formaldehyde (MMF) as shell material. Paraffin is usually a PCM desirable for usage in thermal storage and releasing for its availability in a reasonable phase change temperature range and its large amount latent heat [11]. Styrene–maleic anhydride (SMA) copolymer solid was

Colloid Polym Sci (2011) 289:111–119

used as a nonionic dispersant. Through the methanolmodified process, the remnant formaldehyde in shells can be reduced. The chemical structure, surface morphologies, average diameter, encapsulation efficiencies, and thermal stability properties of these MMF shell microPCMs were investigated systemically. In addition, the enhanced rigidity and toughness of MMF shells were measured comparing it with the MF shells.

Experimental Materials Paraffin (Tianjin Kemel Chemical Reagent Development Center, Tianjin, China) was used as the PCM material (core material). The shell material was prepolymer of melamineformaldehyde modified by methanol (solid content was 78.0%, Aonisite Chemical Trade Co., Ltd., Tianjin). SMA copolymer (Scripset® 520, Hercules, USA) was applied as dispersant. Organic diluent (butyl glycidyl ether), bisphenol-A epoxy resin (E-51) and curing agent (amine) were supplied by Tianjin Synthetic Material Research Institute (Tianjin, China). Fabrication of MMF shell microPCMs The encapsulation was carried out in a 500 ml three-neck round-bottomed flask. 3.0 g SMA and 0.8 g sodium hydroxide (NaOH) were dissolved in 100 ml of water (50 °C). The pH value was adjusted to 4–5 by acetic acid solution; 10.0 g paraffin was added to the aqueous SMA solution, and the mixture was emulsified mechanically under a vigorous stirring rate for 10 min using QSL highspeed disperse-machine (Shanghai Hongtai Ltd., Shanghai, China). The emulsion was dropped in a bottle dipping in steady temperature flume with a stirring speed of 1,500 rpm; 12.8 g prepolymer was dropped in this bottle with a rate of 0.25 g/min. The shell formed after 2.5 h by heating slowly to a temperature of 60 °C. Then the temperature elevated to 75 °C. After polymerization for 1.5 h, temperature was dropped slowly at 2 °C/min to room temperature. The resultant microcapsules were filtered and washed with deionized water and dried in a vacuum oven. Fourier transform infrared spectra analysis The chemical structures of microPCMs were analyzed by a Nicolet Magna 750 (Germany) Fourier transform infrared spectra (FT-IR) spectrometer. FT-IR spectra in transmittance mode were recorded among the range of 400–4,000 cm−1.


Surface morphologies of microPCMs After the microPCMs fabrication, 1 ml of the colloidal solution was extracted and spread on a clean glass slide (1 cm×3 cm). Photographs of the microcapsules retained in emulsion were taken by an optical microscope (MiVnt Image Analyze System, China). The morphologies of dried microPCMs (in a vacuum oven at 40 °C for 24 h) were examined by means of a XL30 PHILIPS scanning electron microscopy (SEM). SEM experiments were performed at 10 kV after carefully coating with gold-palladium without cracking the shells.

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temperature. The microPCMs/epoxy composite samples were treated at 50 °C for 10 min, and immediately immersed in cold water (15 °C). This thermal treatment process of each sample was repeated for ten times in a temperature-controlling machine (Jiangsu Haian Shentai Elec Co., Ltd, China). The interface morphologies of the composites were observed by SEM and the deformation of shells was estimated.

Results and discussion Chemical structure analysis of MMF shell

Average diameter of microPCMs The average diameters of microPCMs were measured using a partial diameter distribution machine (LA950 V2, Japan). Thermal analysis The thermal analyses of microPCMs were carried out on a DSC analyzer (Perkin-Elmer, DSC7). The encapsulation efficiency of the core material content was measured from the heat of fusion (ΔHf) of PCM [20]. Ten micrograms of the dried microPCMs sample was tested with a heating rate of 5 °C/min and nitrogen (N2) as the purging and protective gas. Each sample was analyzed at least twice and the average value was recorded. The thermal stability characterization was performed on a Dupont SDT-2960 Thermogravimetric analysis (TGA) at a scanning rate of 5 °C/min in a flow of 40 ml/min N2. Measurements of mechanical properties of MMF shells The rigidity of the MMF shells was tested according to our reported method [21]. Microcapsules were placed between two pieces of glass (4×2 cm) and compressed. A pressure sensor under the bottom glass measured the intensity and data were directly obtained. When the microcapsules were compressed before a certain point, there was no change on the surface. In addition, a “yield point” was obtained when the deformation showed a plastic behavior. The deformation morphologies of the shell were observed by means of a XL30 PHILIPS SEM. Deformation behavior of MMF shells in epoxy matrix The toughness of the MMF shells in matrix was evaluated through a self-designed thermal transmission process. About 10 g MMF shell microPCMs was embedded in 50 g epoxy resin. The samples of microPCMs/epoxy composite were cured completely at 50 °C for 24 h and the temperature was dropped slowly at 2 °C/min to room

Figure 1 illustrates the microPCMs fabrication process through the in situ polymerization method using SMA as a nonionic dispersant. SMA molecules can hydrolyze in water by NaOH and form carboxyl (−COOH) groups. These hydrophilic polar groups, alternatively arranging along the SMA backbone chains, thus associate with water molecules and trimly cover the oil droplets surface with hydrophobic groups oriented into oil droplets and hydrophilic groups out of oil droplets [20]. This orientation of molecular groups results in a relatively strong electron negative field on the surface of oil particles [20]. Anionic polyelectrolyte hydrolyzed SMA has anionic carboxyl groups that can interact with positively charged below the ζ potential, and then, the MMF prepolymer will be adsorbed by static on oil particles and occurred polymerization. Figure 2 shows the chemical reaction structures of methanol-modified MF prepolymer with four steps. Step 1 is the reaction between the formaldehyde and melamine. With the help of acid catalysis, the prepolymer melamineformaldehyde can be modified by methyl alcohol forming cross-linking structure, as shown in steps 2–3. Further chemical cross-linking reaction will occur between the methanol-modified MF prepolymer molecular chains (step 4). From the MMF cross-linking structure, we can conclude that the free formaldehyde will be greatly decreased by methyl alcohol. Moreover, free formaldehyde in emulsion do not be cross-linked will be consumed by methyl alcohol. Because of the complexity MMF structure, we applied a sold MMF prepolymer product with stability to fabricate a series of microPCMs with the same shell chemical structures. Figure 3 shows the FT-IR spectra of (a) paraffin, (b) SMA, (c) prepolymer MMF, (d) cured MMF, and (e) microPCMs, respectively. As the core material, paraffin has the characteristic peaks of 2,920, 1,461, and 1,376 cm−1 (spectrum a). In spectrum b for SAM, the peaks at approximately 1,494 cm−1 is assigned to the C=C stretching vibrations of benzene ring and the strong peak at approximately 1,775 cm−1 is the C=O stretching vibrations

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Fig. 1 Illustration of microPCMs fabrication process through an in situ polymerization method using SMA as a nonionic dispersant

of anhydride. The chemical structures of pure MMF prepolymer and cured MMF prepolymer were contrastively analyzed by FT-IR as shown in spectra of c and d. The wide absorption peak at approximately 3,340 cm−1 is attributed to the superposition of N–H stretching vibration. According Fig. 2 Chemical reaction structures of methanol-modified MF prepolymer

to the previous work [20], the peaks at 1,556 and 815 cm−1 are assigned to the vibrations of triazine ring; and the corresponding peaks of cured MMF lie at 1,559 and 815 cm−1. Comparing to spectrum a, the absorbance of characteristic peaks at approximately 2,920, 1,461, and


Fig. 3 FT-IR spectra of a paraffin, b SMA, c prepolymer MMF, d cured MMF, e microPCMs

1,376 cm−1 in spectrum e are weakened indicating that paraffin has been almost microcapsulated by MMF resin. Surface morphologies and average diameters of microPCMs Optical microphotographs of microPCMs were taken to illuminate the encapsulation details. Figure 4a, b show morphologies of core material dispersed by hydrolyzed SMA after 10 min at room temperature and the formed microPCMs with MMF shells. It can be seen that the Fig. 4 Surface morphologies and average diameters of microPCMs fabricated by in situ polymerization: a, b optical photographs of MMF shell microPCMs in emulsion, c SEM surface morphologies of dried MMF shell microPCMs, and d average diameters of microPCMs

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organic core material is dispersed into particles in water with the help of hydrolyzed SMA molecules. However, these particles have not been separated from each other directly due to the molecule linkage of the hydrolyzed SMA. Being encapsulated by shell material, the core particles are ultimately separated through the regulation of hydrolyzed SMA molecules. These microPCMs are regular globe shape with smooth surfaces. Figure 4c shows the SEM surface morphology of microPCMs with core/shell ratio of 2/1 fabricated by 3,000 rpm emulsion stirring rate. The reason of selecting this sample is that we have drawn a conclusion that microPCMs with MF shells fabricated under stirring speed of 3,000 rpm usually has the perfect shell structure [21]. Figure 4d shows the average diameters of microPCMs (core/shell ratio of 2/1) under various emulsion stirring rates in range of 1,000–5,000 rpm. With the decreasing of stirring rates, the average diameters decreased sharply from 27 to 2.5 μm. This result accords with our previous study and indicates that the average diameter is mainly determined by emulsion stirring rates [7]. The MMF shells applied in this study will not influence this rule. In addition, it means that the MMF cross-linked with a compact structure forming thin shells. Thermal properties of microPCMs A series of microPCMs were fabricated with different core/ shell weight ratios (3/1, 2/1, 1/1, and 1/2) to investigate the effect of shell on thermal properties. Table 1 lists the phase

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Table 1 Phase change behaviors of pure paraffin and various microPCMs with different core/shell ratios MicroPCMs samples (w/w, core/shell) Pure paraffin 3:1 2:1 1:1 1:2

Tm (°C)

22.5 22.6 23.0 23.4 23.9

ΔHf,

microPCMs

182.0 130.4 117.5 87.0 55.9

(J/g)

Ef (%) – 85.4 87.4 89.9 92.3

change Tm and encapsulation efficiency (Ef) data of these microPCMs samples evaluated by DSC. The Ef was measured from the heat of fusion (ΔHf) of PCM by Eq. 1 [20], Ef ¼

ΔHmicroPCMs 100% ΔHf ; PCM þ mPCM

ð1Þ

Where ΔHmicroPCMs is the calculated heat fusion of microPCMs by DSC, ΔHf, PCM is the latent heat of fusion of PCM, and mPCM is the total weight of the PCM in emulsion. ΔHmicroPCMs values of microPCMs were calculated through the integral of the curves area by software Origin 8.0 Professional. Pure paraffin applied in this study has the Tm and ΔHmicroPCMs data of 22.5 °C and 182.0 J/g. The Tm of microPCMs samples with the core/shell ratios of 3/1, 2/1, 1/ 1, and 2/1 are 22.6, 23.0, 23.4, and 23.9 °C, which are nearly equaled to the pure paraffin. It indicates that the polymer shell of microcapsules do not influence the proprieties of the phase change behavior of paraffin, and the MMF polymer can be successfully utilized to encapsulate PCM to absorb and release heat. Moreover, the Ef data of these microPCMs are more than 85.4%. As the microcapsules are consisted of core and shell material with different weight ratios, the microPCMs have various absorbed ΔHmicroPCMs values. The sample with core and shell ratio of 1/2 has the maximum E f of 92.3%. MicroPCMs samples with the core/shell ratios of 3/1, 2/1, 1/1, and 2/1 have the ΔHmicroPCMs data of 130.4, 117.5, 87.0, and 55.9 J/g. TGA has been widely applied to investigate the encapsulation effect and shell compactness of microcapsules [2, 21]. Figure 5 shows the TGA curves of microPCMs containing paraffin synthesized with different core/shell weight ratios of (a) 3/1, (b) 2/1, (c) 1/1, and (d) 1/ 2. Pure paraffin decreased its weight in the temperature range of 130–207 °C sharply. Contrastively, all microPCMs samples containing paraffin lost their weight at the temperature of nearly 200 °C. About 10% weight was lost before 250 °C owing to some water and other little

Fig. 5 TGA curves of pure paraffin and microPCMs containing paraffin synthesized with different core/shell (w/w) ratios: a 3/1, b 2/1, c 1/1, and d 1/2

molecule ingredients. With the temperature increasing, the weights of microPCMs decreased with different speeds. The sample with core/shell ratio of 3/1 lost its 80% weight at 310 °C. However, the sample with core/shell ratio of 1/2 lost its 80% weight at 450 °C. It proves that core material has been protected by shell and more shell will provide higher compact condition for core material. But because of the cracks in the shell, the temperature increases the weight loss more rapidly at 200 °C for microPCMs Mechanical properties of MMF shells Research on microcapsules is a highly interdisciplinary field that benefits from contributions from various branches of natural sciences and engineering. Besides designing microcapsules for specific tasks requires understanding and controlling their physicochemical properties, the key characteristics are adhesion properties, permeability of the capsule membranes and mechanical properties [17]. Mechanical properties of microPCMs are obviously important for stability issues. In applications of both storing and embedding in composites, microPCMs have to hold enough robustness to avoid shells rupturing due to wear and tear. The shell rupture can in contrast serve as a pathway for fast and efficient release and thus might be desired under certain conditions. Stability is not ensured by simply increasing the elastic modulus of the shell material. Instead, microcapsules must be tailored in their complex deformation characteristics rather than to efficiently perform their tasks. However, there is limited number of tools to analyze mechanical properties of micron-sized capsules on the single-capsule level. Sun [22] had investigated the strength of microcapsules made of three different shells by a micromanipulation technique. Single microcapsules were compressed to


large deformation or rupture, and the force being imposed on them were measured simultaneously. This method was acquired through a special apparatus and the difficulty to see the surface shape changes in straightway. For microPCMs applied in piled state, the strength of a single microcapsule could not reflect the actuality strength, as the microcapsules were piled together [21]. Based on our previous study method, we have identified that microcapsules will show a plastic behavior when a press on shells is beyond their yield point. To understand the methanol-modified effect on mechanical properties of MF shells, we compared the mechanical properties of yield points for MF shell and MMF shell microPCMs. Figure 6 yield point values for MF shell with core/shell ratios of 3/1, 2/1, 1/1, and 1/2 are 0.65, 0.79, 0.85, and 0.90×105 Pa, and MMF shell with core/shell ratios of 3/1, 2/1, 1/1, and 1/2 are 0.70, 0.97, 1.12, and 1.30×105 Pa. The yield point of MF shells are nearly equal to our previous results [21]. With the increasing of shell ratios in microcapsules, both yield points for MF and MMF shells have increased owing to the enhancement of shell thickness and compactness. It is obvious that for the microPCMs with the same core/shell ratio, the MMF shell have larger yield point value than that of MF shell. For example, the yield point of MMF shell with core/shell ratio of 2/1 is about 140% of yield point of MF shell. It indicates that the methanol-modified method can enhanced the resistance of deformation for MF shells. Figure 7 shows the SEM morphologies of microPCMs after a press test to directly understand their yield point with a plastic behavior. The deformation of MF shell microPCMs were investigated firstly after a 0.8×105 Pa pressure to investigate the effect of core/shell ratios (3/1, 2/1, 1/1, and 1/2) on mechanical properties of shells. In Fig. 7a, it can be seen that the MF shell microPCMs (1/1) sample nearly all had broken under the pressure. It means

Fig. 6 Yield point values of microPCMs with different core/shell ratios after a press

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that this pressure have beyond its yield point, which accords to the results in Fig. 6. Figure 7b–d shows the morphologies of microPCMs with core/shell ratios of 2/1, 1/1, and 1/2 under the same pressure, respectively. These samples keep the global shape without cracks and the shells without being broken. Figure 7e shows the morphology of MF shell microPCMs with core/shell ratios of 1/1 after a 1.0×105 Pa pressure. A typical microcapsule has broken with left hollow shell. These results indicate that more shell material in microPCMs may increase the mechanical properties of shells by the way of increasing the shell thickness and compactness. Figure 7f–h show the MMF shell microPCMs with core/shell ratios of 2/1, 1/1, and 1/2 after a 1.2×105 Pa pressure. Obviously, three microcapsule samples exhibit the plastic deformation behavior without shell broken under this pressure. The microPCMs with core/shell ratios of 2/1 has the largest scale of deformations. Interestingly, the microPCMs with core/shell ratios of 1/2 nearly has not deformation of shells. This result is similar to the properties of MF shell that more shell material in microcapsule can enhance the yield point of shells. At the same time, it indicates that the MMF shell has the higher mechanical properties than MF shells. Stability of microPCMs in epoxy matrix MicroPCMs are usually embedded in polymer matrix as functional composite material applied in thermal storing– releasing field. The mechanical properties of the shells can be enhanced by modifying its chemical structure and controlling all the synthesis conditions [23]. However, we still have little knowledge about the shell stability in polymer matrix during a repeated thermal transmittance process. To investigate the shell stability of microPCMs in matrix, we design a drastic thermal transmission process to stimulate the practical application environment. Figure 8a, b show the interface morphologies of MF shell and MMF shell microPCMs after the designed thermal process. Both microPCMs samples have the same core/shell ratio (2/1). Each composite has 10 g MMF shell microPCMs and 50 g epoxy resin, and the composite samples were treated at 50 °C for 10 min and immediately immersed in cold water (15 °C), repeated ten times in a temperature-controlling machine. As shown in Fig. 8a, MF shells can have an obvious deformation after this thermal process. The main reason for these phenomena is that the MF shell has lower yield point. During a thermal absorbing process, the volume PCM in shell will expend but be resisted by shell. When the latent heat of PCM is releasing, its volume is decreasing faster than the shell and epoxy resin. The difference in the expended coefficients between the PCM and the matrix polymer would occur as an interface extrusion on the shells. While this extrusion force

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Fig. 7 SEM morphologies of various microPCMs with different core/ shell ratios after a press: a–d MF shell microPCMs with core/shell ratios of 3/1, 2/1, 1/1, and 1/2 after a 0.8×105 Pa pressure, e MF shell

microPCMs with core/shell ratios of 1/1 after a 1.0×105 Pa pressure, f–h MMF shell microPCMs with core/shell ratios of 2/1, 1/1, and 1/2 after a 1.2×105 Pa pressure

is beyond the yield point of the shell, a deformation of shell will be formed without cracks. Figure 8b shows the MMF shell state in epoxy matrix after the same thermal process. The shell keeps globe shape without shrunken phenomenon. It means that the MMF shell can resist the interface extrusion force owing to its higher yield point. These results accord to the analysis of the mechanical properties of MF shell and MMF shell.

various methods. The following conclusions can be drawn from this study:

Conclusions In this study, a series of MMF shell microPCMs containing paraffin were successfully synthesized by in situ polymerization. The physicochemical properties and mechanical characters of these microPCMs were investigated by Fig. 8 SEM interface morphologies of microPCMs imbedded in epoxy resin after a thermal treatment: a MF shell microPCMs and b MMF shell microPCMs

1. Methanol-modified method can reduce the free formaldehyde in shell material through increasing the crosslinking structure. MMF resin can be successfully used to fabricate microcapsules with globe shapes with smooth surface. 2. The average diameter can be controlled though modifying the emulsion speed. With the decreasing of stirring rates between 1,000 and 5,000 rpm, the average diameters decrease sharply from 27 to 2.5 μm. 3. MMF shells have high encapsulation ability of PCM (Ef >85.4%) with good compact condition for core materials. MMF shells do not greatly affect the Tm of paraffin.


4. MMF shell have larger yield point value than that of MF shell, which means that the methanol-modified method for MF shell can greatly enhanced the resistance of deformation for shells. MMF shell can also resist the interface extrusion force owing to the higher yield point of shell in epoxy resin.

Acknowledgments The authors are grateful to the financial support of the National Natural Science Foundation of China (No. 50803045).

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