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Preparation and characterization of microencapsulated phase change material with low remnant formaldehyde content
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Materials Chemistry and Physics 106 (2007) 437–442
Preparation and characterization of microencapsulated phase change material with low remnant formaldehyde content Li Wei, Zhang Xing-Xiang ∗ , Wang Xue-Chen, Niu Jian-Jin Tianjin Municipal Key Lab of Fiber Modification and Functional Fibers, Institute of Functional Fibers, Tianjin Polytechnic University, Tianjin 300160, China Received 21 July 2006; received in revised form 20 June 2007; accepted 20 June 2007
Abstract Microencapsulated phase change materials (MicroPCMs) were synthesized by in situ polymerization using melamine–formaldehyde (MF) resin as shell and n-octadecane as core. The employed MF prepolymer was prepared by incorporating formaldehyde once and melamine for three times. The effects of dropping rate of MF prepolymer on the surface morphology, dispersibility and thermal stability of the microcapsules were systematically investigated using scanning electronic microscopy (SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and thermogravimetry analysis (TG). The results show that, with the dropping rate of the MF prepolymer decreasing, the flocculation phenomenon of microcapsules decreases and the globular surface becomes smoother; and the thermal stability increases regularly. The average diameter of the microcapsules is about 2.2 m and the diameter distribution is narrow. The enthalpy of the microcapsules containing 59 wt% n-octadecane is 144 J g−1 . In addition, the remnant formaldehyde content of the microcapsules is 68.6 mg kg−1 , which is highly attractive for the application of MicroPCMs. © 2007 Elsevier B.V. All rights reserved. Keywords: Phase change material; Microcapsule; Dropping rate; Thermal stability; Remnant formaldehyde content
1. Introduction Microcapsules are tiny particles that contain active agent or core material surrounded by a coating or shell [1], protecting specific functional from, or releasing them into an outer phase for a long period [2]. The diameters of the microcapsules are usually in the range of 1–1000 m. The core materials of the microcapsules can be drugs [2], cells [3], fragrant oils [4], dyes, etc. The core material can also be micro-particles as used in electronic ink [5]. MicroPCMs have attracted more and more attention since 1990s [6–8]. MicroPCMs have been widely studied as active or pumped coolants [9–12], solar and nuclear heat storage systems [13] and as a heat exchanger in a packed bed. They have also been used in the manufacture of thermoregulated fibers, fabrics, and foams [14,15]. MicroPCMs have been synthesized with melamine–formaldehyde (MF) [10,16], urea–formaldehyde (UF) [17,18], gelatin–formaldehyde [14], polyurethane (PU) [19], etc. as shell materials. MicroPCMs containing n-octadecane with MF shell has attracted many
researchers since its unique inherent properties such as good seal tightness, endurance, water resistance, alkaline resistance and fire resistance [20–22]. However, the MF, urea–formaldehyde and gelatin–formaldehyde shell microcapsules usually release poisonous formaldehyde in the application, which limits their application. Zhang et al. [23] have successfully synthesized low toxic urea–formaldehyde resin through the method of adding formaldehyde once while putting in urea for three times, the content of free formaldehyde in the products meets GB/T 14732 completely in China. There is still little information available on reducing the remnant formaldehyde content of microcapsules, however. Low remnant formaldehyde content of MF shell MicroPCMs was synthesized successfully by in situ polymerization with n-octadecane as core in this study. The effects of MF prepolymer dropping rate on the surface morphology, dispersibility and thermal stability of the microcapsules were systematically studied. 2. Experimental 2.1. Materials
∗
Corresponding author. Tel.: +86 22 24528144; fax: +86 22 24348894. E-mail address: [email protected] (X.-X. Zhang).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.030
Melamine (98 wt%, Tianjin Resins Material Factory) and formaldehyde (37 wt% aqueous, A.R., Tianjin Chemical Reagent Factory) were used as
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W. Li et al. / Materials Chemistry and Physics 106 (2007) 437–442
Table 1 Dropping rates and components Sample number
(mL min−1 )
Dropping rates n-Octadecane (g) Formaldehyde (37 wt% solution) (mL) Melamine (g) Distilled water (mL) TA (19 wt% solution) (g)
A0
A1
A2
A3
A4
A5
Max 0
Max 25.0
2.08 25.0
0.82 25.0 13.0
0.51 25.0
0.27 25.0
7.0 14.0 13.0
Max: rate of pouring in once. monomers; n-octadecane (purity 99 wt%, Union Lab. Supplies Limited, Hong Kong) was used as core material. TA (sodium styrene–maleic anhydride copolymer, 19 wt% aqueous solution, Shanghai Leather Chemical Works) was employed as an emulsifier. Triethanolamine (95 wt%), acetic acid (36 wt%) and sodium hydroxide (A.R.) all purchased from Tianjin Chemical Regents, Inc. were used as pH regulators. Urea (A.R.) was also obtained from Tianjin Chemical Regents, Inc.
2.2. Preparation of microcapsules Prepolymer solution synthesis: 3.0 g melamine and 13.0 mL formaldehyde were mixed with 14.0 mL of distilled water. The mixture was stirred at 70 ◦ C and adjusted to pH 8.5 with triethanolamine. When it became clearly transparent, another 2.0 g melamine was added and stirred till it was dissolved completely. Similarly, the last 2.0 g melamine was added and stirred till it became transparent absolutely. The prepolymer solution was prepared. Emulsion preparation: 25.0 g n-octadecane, 13.0 g TA and 200 mL of distilled water were emulsified mechanically at 70 ◦ C with a stirring rate of 8000 rpm for 45 min. The emulsion was adjusted to pH 4.5 with 10.0 wt% acetic acid solution. Microcapsule fabrication: the prepolymer solution was dropped into the emulsion at different rates while the emulsion was stirred at a rate of 600 rpm (Table 1). After all of the prepolymer was added, it was continuously stirred for 90 min. The pH of the emulsion was adjusted to 9 with 10 wt% triethanolamine solution, which terminated the reaction. Then 1.0 g of urea was added to remove the remnant formaldehyde. The resultant microcapsules were filtered and washed with 30 wt% ethanol–water solution at approximately 50 ◦ C until pH 7 was reached. The wet cake was dried in a vacuum oven. The polymer shell powder was prepared by adjusting the prepolymer solution to pH 9 with 50 wt% triethanolamine solution. Then the mixture of polymer and water was filtered and the wet cake was dried in an oven at 100 ◦ C for 8 h to remove the water.
2.3. Characterization of the microcapsules FTIR spectra of n-octadecane, microcapsules and shell material were obtained using a spectrophotometer (BRUKER UECIOR22, wavelength 400–4000 cm−1 ) at room temperature. The morphologies of the microcapsules were obtained by using a scanning electronic microscope (SEM, Philips XL30). A drop of the microcapsulesethanol dispersion was dripped on a stainless steel SEM stub and allowed air-dry overnight. The samples were silver-coated. The diameters of microcapsules were measured on the SEM photos. More than 200 microcapsules were counted. The diameter distribution was processed with Origin 7.5 Professional. The diffraction patterns of microencapsulated n-octadecane and the polymer shell were obtained using a wide-angle X-ray diffraction (WAXD, Bruker Aux D8 Advance, 40 kV, 40 mA, Cu K␣1 ) at 20 ◦ C and 35 ◦ C, respectively. The scanning range was 10–45◦ (2θ). The thermal properties of the microcapsules and n-octadecane were measured using a differential scanning calorimetry (DSC, Perkin-Elmer, DSC7) in the range of 0–80 ◦ C at a heating or cooling rate of 10 ◦ C min−1 in a nitrogen atmosphere.
The thermal stabilities of dried microcapsules were obtained by using a thermogravimetric analyzer (NETZSCH STA 409 PC/PG TG-DT) at a scanning rate of 10 ◦ C min−1 in the temperature range of 25–600 ◦ C in a nitrogen atmosphere. The absorbance of extraction solution of microcapsules was estimated by a spectral photometer (Beijing Rayleigh Analytical Instrument Corp. VIS-7220) at room temperature; then the remnant formaldehyde content of the microcapsules can be obtained.
2.4. Measurement of remnant formaldehyde content in the microcapsules Determination of formaldehyde concentration in the microcapsules with phenolphthalein reagent (C6 H4 SN(CH3 )CNNH2 ·HCl, MBTH) [24] is as follows: the remnant formaldehyde extracted from the microcapsules can react with phenolphthalein reagent to form azine. It can be oxidized by ferrate ion in an acidic solution; then a blue-green compound can be obtained. The intensity of the color is proportional to the quantity of formaldehyde present. Briefly, approximately 150 mg of dried microcapsules was wrapped with a filter paper, and immersed in 100 mL water for 1 h. Then the wrapped microcapsules were removed from the solution. An extraction solution was obtained. Five milliliters of 0.05 g L−1 phenolphthalein solution and 0.4 mL 1.0 wt% ferrate ion solution were added in 1.0 mL extraction solution for coloration. The absorbance at wavelength of 630 nm of this solution was measured using a spectral photometer at room temperature; then the concentration of the formaldehyde solution was obtained from the formaldehyde calibration curve. Finally, the remnant formaldehyde content of MicroPCMs was calculated according to the volume and concentration of the formaldehyde solution and mass of MicroPCMs.
2.5. Calculation of n-octadecane content in the microcapsules The specific heat of n-octadecane was a constant in the measured temperature range. The content of n-octadecane in the microcapsule can be estimated according to the measured enthalpy: n-octadecane content (%) =
|Hm | × 100 |Hmo |
(1)
where |Hm | is the enthalpy of microcapsules (J g−1 ) and |Hmo | is the enthalpy of n-octadecane, here it is 244 J g−1 (DSC melting enthalpy).
3. Results and discussion 3.1. FTIR of microcapsules The FTIR spectra of n-octadecane, polymer shell and microencapsulated n-octadecane are presented in Fig. 1. The strong absorption peaks at approximately 2960 and 717 cm−1 in the spectra of n-octadecane and the microcapsules are associated with the aliphatic C–H stretching vibration and the in-plane rocking vibration of the CH2 group, respectively. None of these specific peaks are observed in the spectrum of the shell polymer, however. These characterized peaks in the spectra of the microcapsules prove the formation of microencapsulated noctadecane. 3.2. Effects of dropping rate on morphology SEM micrographs of microcapsules prepared by various dropping rates are shown in Fig. 2. With the decrease of dropping rate, the flocculation phenomenon of the microcapsules decreases and the globular surface becomes smoother. There is enough time for the prepolymer to concentrate on the surface of emulsion droplets as the dropping rate decreases and then
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Fig. 3. Diameter distribution of microcapsules. Fig. 1. FTIR spectra of (a) n-octadecane, (b) shell polymer and (c) microencapsulated n-octadecane.
condensation reacts. There was little difference among samples A3 , A4 and A5 . The reason may be that the dropping rates of these samples are slow enough for the prepolymer to evenly surround on the droplets. In addition, the average diameter of A3 microcapsules is 2.2 m; and the diameter distribution is shown in Fig. 3.
locate at 19◦ , 23◦ and 25◦ (2θ), respectively, appear in the WAXD pattern of the microcapsules measured at 20 ◦ C; by contrary, none of the crystallization diffraction peaks can be observed in the X-ray pattern of the microcapsules at 35 ◦ C except for the amorphous diffraction peak at approximately 19◦ (2θ). There is only an amorphous diffraction peak in the pattern of the shell polymer, which locates at 24◦ (2θ). 3.4. Thermal properties of microcapsules
3.3. Crystallography of microcapsules WAXD patterns of microencapsulated n-octadecane and polymer shell are shown in Fig. 4. Obvious diffraction peaks
The melting and crystallizing properties of n-octadecane and microcapsules at a heating and cooling rate of 10.0 ◦ C min−1 are presented in Table 2. The DSC cooling curves of n-octadecane
Fig. 2. SEM micrographs of surface of microcapsules synthesized at various dropping rates.
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Fig. 4. WAXD patterns of (a) microencapsulated n-octadecane at 20 ◦ C, (b) microencapsulated n-octadecane at 35 ◦ C and (c) MF shell.
Fig. 6. TG diagrams of microcapsules synthesized at various dropping rates.
lower than that of bulk n-octadecane, which is so-called supercooling. The multiple peaks on the DSC cooling curve of the microcapsules are not caused by the differences of the cooling rates in the measured range, but caused by the difference in the average diameters [25]. The DSC cooling curve of the microcapsules with an average diameter above 6.4 m is almost the same as that of n-octadecane bulk [25]. The exothermic peak α on the DSC cooling curve is likely the heterogeneously nucleated liquid-rotator transition, the peak β is attributed to rotator-crystal transition, and peak γ, the largest peak, is attributed to homogeneously nucleated liquid-crystal transition [26]. Rotator phase is a weakly ordered crystallization phase occurring in equilibrium at temperature between the liquid phase and the melt. Besides, to some extent, the super cooling may also be affected by the low thermal conductivity of polymer shell. The enthalpy of Hm is almost equal to that of Hc , which indicates that super cooling has little effect on enthalpy.
Fig. 5. DSC curves of n-octadecane and microencapsulated n-octadecane.
and microcapsules are shown in Fig. 5. The enthalpy of the dried microcapsules is 144 J g−1 , which indicates the microcapsules containing 59 wt% of n-octadecane. As shown in Table 2, the melting peak temperature of the microcapsules is 1.7 ◦ C higher than that of bulk n-octadecane; and the peak becomes wider than that of bulk n-octadecane, which may be attributed to the low thermal conductivity of the polymer shell. There are three peaks, α, β, and γ respectively, visible from the DSC cooling curve of the microcapsules, while only one peak is visible from that of noctadecane. In addition, the temperature of peak γ is almost 6 ◦ C
3.5. Thermal stability of microcapsules Fig. 6 presents TG diagrams of the microcapsules synthesized with various dropping rates. The mass of the material decreases with the temperature increasing. Polymer shell loses weight at 370 ◦ C rapidly; and this is caused by the decomposition of the MF shell, which is presented among samples A1 –A5 at nearly 370 ◦ C as shown in Table 3. The thermal stability of the microcapsules rises with the decrease of the dropping rate. As the dropping rate decreases, there is enough time for prepolymer to
Table 2 Phase change properties of n-octadecane and microencapsulated n-octadecane PCM/microPCMs
Tm (◦ C) α
n-Octadecane Microcapsule
Hm (J g−1 ) β
38.9 40.6
244 144
Tc (◦ C) α
β
γ
22.7 28.7
– 25.5
– 16.6
Hc (J g−1 )
n-Octadecane content (wt%)
243 145
100 59
Tm : peak temperature on DSC heating curve; Hm : enthalpy on DSC heating curve; Tc : peak temperature on DSC cooling curve; Hc : enthalpy on DSC cooling curve (sum of α, β, γ).
W. Li et al. / Materials Chemistry and Physics 106 (2007) 437–442
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Table 3 Thermal stability of microcapsules synthesized at various dropping rates Sample number
(◦ C)
T0.05 Mass loss starting point (◦ C)
A0
A1
A2
A3
A4
A5
102 371
148 376
164 353
180 378
174 384
180 369
T0.05 : the temperature at 5% mass loss. Table 4 Absorbance values of different formaldehyde concentration Experimental
F A
(mg kg−1 )
1
2
3
4
5
6
7
8
9
0 0.056
0.02 0.068
0.04 0.088
0.08 0.111
012 0.132
0.16 0.149
0.20 0.172
0.30 0.219
0.40 0.281
F: formaldehyde concentration; A: absorbance values. Table 5 Remnant formaldehyde content of the microcapsules Sample number
Absorbance values
Formaldehyde content (g)
Weight of microcapsules (g)
Formaldehyde concentration (mg kg−1 )
S1 S2 S3
0.076 0.080 0.077
10.723 11.233 10.839
0.1521 0.1673 0.1594
70.5 67.1 68.0
concentrate on the surface of emulsion droplet and the prepolymer reacts sufficiently. Then the microcapsules are formed with higher cross-linking degree shell. There is little change of T0.05 among sample A3 , A4 and A5 ; this may be explained as the dropping rate of A3 providing enough time for shell condensation reaction. 3.6. Free formaldehyde concentration of microcapsules We can get formaldehyde criteria curve and equation using the data showed in Table 4. The remnant formaldehyde content of the microcapsules is presented in Table 5. The remnant formaldehyde content of the microcapsules synthesized by in situ polymerization is only 67.1 mg kg−1 . The employed prepolymer was prepared by putting in formaldehyde once and melamine for three times. In the field of textile, remnant formaldehyde content is restricted. The polyacrylonitrile–vinylidene chloride fibers containing up to 30 wt% MicroPCMs were wet-spun in our laboratory [27]. The remnant formaldehyde content of this fiber is 158.4 mg kg−1 . This fiber can be used for the fabric that does not contact skin. According to the MicroPCMs content in the fibers above, if the microcapsules in this paper were added into the fiber and the remnant formaldehyde concentration would be quite low. It is estimated that the microcapsules with low remnant formaldehyde content have a wider application area and promising prospect. 4. Conclusion MicroPCMs were synthesized by in situ polymerization using melamine–formaldehyde as shell and n-octadecane as core and
the employed melamine–formaldehyde prepolymer was prepared by incorporating formaldehyde once and melamine for three times. With the dropping rate of the MF prepolymer decreasing, the flocculation phenomenon of the microcapsules decreases and the globular surface becomes smoother. The thermal stability increases with the decrease of the dropping rate. Microcapsules with average diameter 2.2 m have smooth globular surface. The enthalpy of the microcapsules containing 59 wt% n-octadecane is 144 J g−1 . The remnant formaldehyde content of microcapsules is only 67.1 mg kg−1 , which indicates their wider application area and promising prospect. Acknowledgements The authors are thankful to the National Natural Science Foundation of China (No. 50573058) and Specialized Research Foundation for the Doctoral Program of Higher Education (No. 20050058004) for the financial supports. References [1] S. Benita, Microencapsulation: Methods and Industrial Applications, Marcel Dekker, Inc., New York, 1996, pp. 1–3. [2] S.J. Park, S.H. Kim, J. Colloid Interf. Sci. 271 (2004) 336. [3] F.T. Meng, G.H. Ma, Y.D. Liu, et al., Colloids Surf. B: Biointerf. 33 (2004) 177. [4] K.J. Hong, S.M. Park, Polymer 41 (2000) 4567. [5] B. Comiskey, J.D. Albert, H. Yoshizawa, J. Jacobson, Nature 394 (1998) 253. [6] R. Yang, H. Xu, Y.P. Zhang, Solar Energy Mater. Solar Cells 80 (2003) 405. [7] B. Boh, E. Knez, M. Staresinic, J. Microencapsul. 22 (2005) 715. ¨ ¨ Paksoy, H. Evliya, Int. J. Energy Res. 30 [8] Y. Ozonur, M. Mazman, H.O. (2006) 741.
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¨ [9] N. Sarier, E. Onder, Thermochim. Acta 452 (2006) 65. [10] J.F. Su, L.X. Wang, L. Ren, Z. Huang, J. Appl. Polym. Sci. 103 (2007) 1295. [11] M.N.A. Hawlader, M.S. Uddin, M.M. Khin, Appl. Energy 74 (2003) 195. [12] A. Sarı, Energy Convers. Manag. 45 (2004) 2033. [13] P.B.L. Chaurasia, Energy Convers. Manag. 41 (2000) 1569. [14] Y. Shin, D. Yoo, K. Son, J. Appl. Polym. Sci. 97 (2005) 910. [15] D.P. Colvin, Proceedings of the 2nd International Conference on Safety and Protective Fabrics, USA, 2000, pp. 28–30. [16] X.X. Zhang, Y.F. Fan, X.M. Tao, K.L. Yick, Mater. Chem. Phys. 88 (2004) 300. [17] P. Charunyakorn, S. Sengupta, S.K. Roy, Int. J. Heat Mass Transf. 34 (1991) 819.
[18] R.C. Brown, J.D. Rasberry, S.P. Overmann, Powder Technol. 98 (1998) 217. [19] J.S. Cho, A. Kwon, C.G. Cho, Colloid Polym. Sci. 280 (2002) 260. [20] Y.H. Lee, C.A. Kim, W.H. Jang, et al., Polymer 42 (2001) 8277. [21] M. Zanetti, A. Pizzi, J. Appl. Polym. Sci. 88 (2003) 287. [22] Y.F. Fan, X.X. Zhang, S.Z. Wu, et al., Thermochim. Acta 429 (2005) 25. [23] C.J. Zhang, B.Z. Lei, Y.Y. Bi, J. China Adhes. 14 (2005) 22. [24] M.G. Yang, T.R. Wang, Y.F. Jin, Chin. J. Curr. Pract. Med. 14 (2005) 80. [25] X.X. Zhang, Y.F. Fan, X.M. Tao, K.L. Yick, J. Colloid Interf. Sci. 281 (2005) 299. [26] H. Kraack, E.B. Sirota, M. Deutsch, J. Chem. Phys. 112 (2000) 6873. [27] X.X. Zhang, X.M. Tao, K.L. Yick, X.C. Wang, Text. Res. J. 76 (2006) 351.
Materials Chemistry and Physics 106 (2007) 437–442
Preparation and characterization of microencapsulated phase change material with low remnant formaldehyde content Li Wei, Zhang Xing-Xiang ∗ , Wang Xue-Chen, Niu Jian-Jin Tianjin Municipal Key Lab of Fiber Modification and Functional Fibers, Institute of Functional Fibers, Tianjin Polytechnic University, Tianjin 300160, China Received 21 July 2006; received in revised form 20 June 2007; accepted 20 June 2007
Abstract Microencapsulated phase change materials (MicroPCMs) were synthesized by in situ polymerization using melamine–formaldehyde (MF) resin as shell and n-octadecane as core. The employed MF prepolymer was prepared by incorporating formaldehyde once and melamine for three times. The effects of dropping rate of MF prepolymer on the surface morphology, dispersibility and thermal stability of the microcapsules were systematically investigated using scanning electronic microscopy (SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and thermogravimetry analysis (TG). The results show that, with the dropping rate of the MF prepolymer decreasing, the flocculation phenomenon of microcapsules decreases and the globular surface becomes smoother; and the thermal stability increases regularly. The average diameter of the microcapsules is about 2.2 m and the diameter distribution is narrow. The enthalpy of the microcapsules containing 59 wt% n-octadecane is 144 J g−1 . In addition, the remnant formaldehyde content of the microcapsules is 68.6 mg kg−1 , which is highly attractive for the application of MicroPCMs. © 2007 Elsevier B.V. All rights reserved. Keywords: Phase change material; Microcapsule; Dropping rate; Thermal stability; Remnant formaldehyde content
1. Introduction Microcapsules are tiny particles that contain active agent or core material surrounded by a coating or shell [1], protecting specific functional from, or releasing them into an outer phase for a long period [2]. The diameters of the microcapsules are usually in the range of 1–1000 m. The core materials of the microcapsules can be drugs [2], cells [3], fragrant oils [4], dyes, etc. The core material can also be micro-particles as used in electronic ink [5]. MicroPCMs have attracted more and more attention since 1990s [6–8]. MicroPCMs have been widely studied as active or pumped coolants [9–12], solar and nuclear heat storage systems [13] and as a heat exchanger in a packed bed. They have also been used in the manufacture of thermoregulated fibers, fabrics, and foams [14,15]. MicroPCMs have been synthesized with melamine–formaldehyde (MF) [10,16], urea–formaldehyde (UF) [17,18], gelatin–formaldehyde [14], polyurethane (PU) [19], etc. as shell materials. MicroPCMs containing n-octadecane with MF shell has attracted many
researchers since its unique inherent properties such as good seal tightness, endurance, water resistance, alkaline resistance and fire resistance [20–22]. However, the MF, urea–formaldehyde and gelatin–formaldehyde shell microcapsules usually release poisonous formaldehyde in the application, which limits their application. Zhang et al. [23] have successfully synthesized low toxic urea–formaldehyde resin through the method of adding formaldehyde once while putting in urea for three times, the content of free formaldehyde in the products meets GB/T 14732 completely in China. There is still little information available on reducing the remnant formaldehyde content of microcapsules, however. Low remnant formaldehyde content of MF shell MicroPCMs was synthesized successfully by in situ polymerization with n-octadecane as core in this study. The effects of MF prepolymer dropping rate on the surface morphology, dispersibility and thermal stability of the microcapsules were systematically studied. 2. Experimental 2.1. Materials
∗
Corresponding author. Tel.: +86 22 24528144; fax: +86 22 24348894. E-mail address: [email protected] (X.-X. Zhang).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.030
Melamine (98 wt%, Tianjin Resins Material Factory) and formaldehyde (37 wt% aqueous, A.R., Tianjin Chemical Reagent Factory) were used as
438
W. Li et al. / Materials Chemistry and Physics 106 (2007) 437–442
Table 1 Dropping rates and components Sample number
(mL min−1 )
Dropping rates n-Octadecane (g) Formaldehyde (37 wt% solution) (mL) Melamine (g) Distilled water (mL) TA (19 wt% solution) (g)
A0
A1
A2
A3
A4
A5
Max 0
Max 25.0
2.08 25.0
0.82 25.0 13.0
0.51 25.0
0.27 25.0
7.0 14.0 13.0
Max: rate of pouring in once. monomers; n-octadecane (purity 99 wt%, Union Lab. Supplies Limited, Hong Kong) was used as core material. TA (sodium styrene–maleic anhydride copolymer, 19 wt% aqueous solution, Shanghai Leather Chemical Works) was employed as an emulsifier. Triethanolamine (95 wt%), acetic acid (36 wt%) and sodium hydroxide (A.R.) all purchased from Tianjin Chemical Regents, Inc. were used as pH regulators. Urea (A.R.) was also obtained from Tianjin Chemical Regents, Inc.
2.2. Preparation of microcapsules Prepolymer solution synthesis: 3.0 g melamine and 13.0 mL formaldehyde were mixed with 14.0 mL of distilled water. The mixture was stirred at 70 ◦ C and adjusted to pH 8.5 with triethanolamine. When it became clearly transparent, another 2.0 g melamine was added and stirred till it was dissolved completely. Similarly, the last 2.0 g melamine was added and stirred till it became transparent absolutely. The prepolymer solution was prepared. Emulsion preparation: 25.0 g n-octadecane, 13.0 g TA and 200 mL of distilled water were emulsified mechanically at 70 ◦ C with a stirring rate of 8000 rpm for 45 min. The emulsion was adjusted to pH 4.5 with 10.0 wt% acetic acid solution. Microcapsule fabrication: the prepolymer solution was dropped into the emulsion at different rates while the emulsion was stirred at a rate of 600 rpm (Table 1). After all of the prepolymer was added, it was continuously stirred for 90 min. The pH of the emulsion was adjusted to 9 with 10 wt% triethanolamine solution, which terminated the reaction. Then 1.0 g of urea was added to remove the remnant formaldehyde. The resultant microcapsules were filtered and washed with 30 wt% ethanol–water solution at approximately 50 ◦ C until pH 7 was reached. The wet cake was dried in a vacuum oven. The polymer shell powder was prepared by adjusting the prepolymer solution to pH 9 with 50 wt% triethanolamine solution. Then the mixture of polymer and water was filtered and the wet cake was dried in an oven at 100 ◦ C for 8 h to remove the water.
2.3. Characterization of the microcapsules FTIR spectra of n-octadecane, microcapsules and shell material were obtained using a spectrophotometer (BRUKER UECIOR22, wavelength 400–4000 cm−1 ) at room temperature. The morphologies of the microcapsules were obtained by using a scanning electronic microscope (SEM, Philips XL30). A drop of the microcapsulesethanol dispersion was dripped on a stainless steel SEM stub and allowed air-dry overnight. The samples were silver-coated. The diameters of microcapsules were measured on the SEM photos. More than 200 microcapsules were counted. The diameter distribution was processed with Origin 7.5 Professional. The diffraction patterns of microencapsulated n-octadecane and the polymer shell were obtained using a wide-angle X-ray diffraction (WAXD, Bruker Aux D8 Advance, 40 kV, 40 mA, Cu K␣1 ) at 20 ◦ C and 35 ◦ C, respectively. The scanning range was 10–45◦ (2θ). The thermal properties of the microcapsules and n-octadecane were measured using a differential scanning calorimetry (DSC, Perkin-Elmer, DSC7) in the range of 0–80 ◦ C at a heating or cooling rate of 10 ◦ C min−1 in a nitrogen atmosphere.
The thermal stabilities of dried microcapsules were obtained by using a thermogravimetric analyzer (NETZSCH STA 409 PC/PG TG-DT) at a scanning rate of 10 ◦ C min−1 in the temperature range of 25–600 ◦ C in a nitrogen atmosphere. The absorbance of extraction solution of microcapsules was estimated by a spectral photometer (Beijing Rayleigh Analytical Instrument Corp. VIS-7220) at room temperature; then the remnant formaldehyde content of the microcapsules can be obtained.
2.4. Measurement of remnant formaldehyde content in the microcapsules Determination of formaldehyde concentration in the microcapsules with phenolphthalein reagent (C6 H4 SN(CH3 )CNNH2 ·HCl, MBTH) [24] is as follows: the remnant formaldehyde extracted from the microcapsules can react with phenolphthalein reagent to form azine. It can be oxidized by ferrate ion in an acidic solution; then a blue-green compound can be obtained. The intensity of the color is proportional to the quantity of formaldehyde present. Briefly, approximately 150 mg of dried microcapsules was wrapped with a filter paper, and immersed in 100 mL water for 1 h. Then the wrapped microcapsules were removed from the solution. An extraction solution was obtained. Five milliliters of 0.05 g L−1 phenolphthalein solution and 0.4 mL 1.0 wt% ferrate ion solution were added in 1.0 mL extraction solution for coloration. The absorbance at wavelength of 630 nm of this solution was measured using a spectral photometer at room temperature; then the concentration of the formaldehyde solution was obtained from the formaldehyde calibration curve. Finally, the remnant formaldehyde content of MicroPCMs was calculated according to the volume and concentration of the formaldehyde solution and mass of MicroPCMs.
2.5. Calculation of n-octadecane content in the microcapsules The specific heat of n-octadecane was a constant in the measured temperature range. The content of n-octadecane in the microcapsule can be estimated according to the measured enthalpy: n-octadecane content (%) =
|Hm | × 100 |Hmo |
(1)
where |Hm | is the enthalpy of microcapsules (J g−1 ) and |Hmo | is the enthalpy of n-octadecane, here it is 244 J g−1 (DSC melting enthalpy).
3. Results and discussion 3.1. FTIR of microcapsules The FTIR spectra of n-octadecane, polymer shell and microencapsulated n-octadecane are presented in Fig. 1. The strong absorption peaks at approximately 2960 and 717 cm−1 in the spectra of n-octadecane and the microcapsules are associated with the aliphatic C–H stretching vibration and the in-plane rocking vibration of the CH2 group, respectively. None of these specific peaks are observed in the spectrum of the shell polymer, however. These characterized peaks in the spectra of the microcapsules prove the formation of microencapsulated noctadecane. 3.2. Effects of dropping rate on morphology SEM micrographs of microcapsules prepared by various dropping rates are shown in Fig. 2. With the decrease of dropping rate, the flocculation phenomenon of the microcapsules decreases and the globular surface becomes smoother. There is enough time for the prepolymer to concentrate on the surface of emulsion droplets as the dropping rate decreases and then
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Fig. 3. Diameter distribution of microcapsules. Fig. 1. FTIR spectra of (a) n-octadecane, (b) shell polymer and (c) microencapsulated n-octadecane.
condensation reacts. There was little difference among samples A3 , A4 and A5 . The reason may be that the dropping rates of these samples are slow enough for the prepolymer to evenly surround on the droplets. In addition, the average diameter of A3 microcapsules is 2.2 m; and the diameter distribution is shown in Fig. 3.
locate at 19◦ , 23◦ and 25◦ (2θ), respectively, appear in the WAXD pattern of the microcapsules measured at 20 ◦ C; by contrary, none of the crystallization diffraction peaks can be observed in the X-ray pattern of the microcapsules at 35 ◦ C except for the amorphous diffraction peak at approximately 19◦ (2θ). There is only an amorphous diffraction peak in the pattern of the shell polymer, which locates at 24◦ (2θ). 3.4. Thermal properties of microcapsules
3.3. Crystallography of microcapsules WAXD patterns of microencapsulated n-octadecane and polymer shell are shown in Fig. 4. Obvious diffraction peaks
The melting and crystallizing properties of n-octadecane and microcapsules at a heating and cooling rate of 10.0 ◦ C min−1 are presented in Table 2. The DSC cooling curves of n-octadecane
Fig. 2. SEM micrographs of surface of microcapsules synthesized at various dropping rates.
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Fig. 4. WAXD patterns of (a) microencapsulated n-octadecane at 20 ◦ C, (b) microencapsulated n-octadecane at 35 ◦ C and (c) MF shell.
Fig. 6. TG diagrams of microcapsules synthesized at various dropping rates.
lower than that of bulk n-octadecane, which is so-called supercooling. The multiple peaks on the DSC cooling curve of the microcapsules are not caused by the differences of the cooling rates in the measured range, but caused by the difference in the average diameters [25]. The DSC cooling curve of the microcapsules with an average diameter above 6.4 m is almost the same as that of n-octadecane bulk [25]. The exothermic peak α on the DSC cooling curve is likely the heterogeneously nucleated liquid-rotator transition, the peak β is attributed to rotator-crystal transition, and peak γ, the largest peak, is attributed to homogeneously nucleated liquid-crystal transition [26]. Rotator phase is a weakly ordered crystallization phase occurring in equilibrium at temperature between the liquid phase and the melt. Besides, to some extent, the super cooling may also be affected by the low thermal conductivity of polymer shell. The enthalpy of Hm is almost equal to that of Hc , which indicates that super cooling has little effect on enthalpy.
Fig. 5. DSC curves of n-octadecane and microencapsulated n-octadecane.
and microcapsules are shown in Fig. 5. The enthalpy of the dried microcapsules is 144 J g−1 , which indicates the microcapsules containing 59 wt% of n-octadecane. As shown in Table 2, the melting peak temperature of the microcapsules is 1.7 ◦ C higher than that of bulk n-octadecane; and the peak becomes wider than that of bulk n-octadecane, which may be attributed to the low thermal conductivity of the polymer shell. There are three peaks, α, β, and γ respectively, visible from the DSC cooling curve of the microcapsules, while only one peak is visible from that of noctadecane. In addition, the temperature of peak γ is almost 6 ◦ C
3.5. Thermal stability of microcapsules Fig. 6 presents TG diagrams of the microcapsules synthesized with various dropping rates. The mass of the material decreases with the temperature increasing. Polymer shell loses weight at 370 ◦ C rapidly; and this is caused by the decomposition of the MF shell, which is presented among samples A1 –A5 at nearly 370 ◦ C as shown in Table 3. The thermal stability of the microcapsules rises with the decrease of the dropping rate. As the dropping rate decreases, there is enough time for prepolymer to
Table 2 Phase change properties of n-octadecane and microencapsulated n-octadecane PCM/microPCMs
Tm (◦ C) α
n-Octadecane Microcapsule
Hm (J g−1 ) β
38.9 40.6
244 144
Tc (◦ C) α
β
γ
22.7 28.7
– 25.5
– 16.6
Hc (J g−1 )
n-Octadecane content (wt%)
243 145
100 59
Tm : peak temperature on DSC heating curve; Hm : enthalpy on DSC heating curve; Tc : peak temperature on DSC cooling curve; Hc : enthalpy on DSC cooling curve (sum of α, β, γ).
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Table 3 Thermal stability of microcapsules synthesized at various dropping rates Sample number
(◦ C)
T0.05 Mass loss starting point (◦ C)
A0
A1
A2
A3
A4
A5
102 371
148 376
164 353
180 378
174 384
180 369
T0.05 : the temperature at 5% mass loss. Table 4 Absorbance values of different formaldehyde concentration Experimental
F A
(mg kg−1 )
1
2
3
4
5
6
7
8
9
0 0.056
0.02 0.068
0.04 0.088
0.08 0.111
012 0.132
0.16 0.149
0.20 0.172
0.30 0.219
0.40 0.281
F: formaldehyde concentration; A: absorbance values. Table 5 Remnant formaldehyde content of the microcapsules Sample number
Absorbance values
Formaldehyde content (g)
Weight of microcapsules (g)
Formaldehyde concentration (mg kg−1 )
S1 S2 S3
0.076 0.080 0.077
10.723 11.233 10.839
0.1521 0.1673 0.1594
70.5 67.1 68.0
concentrate on the surface of emulsion droplet and the prepolymer reacts sufficiently. Then the microcapsules are formed with higher cross-linking degree shell. There is little change of T0.05 among sample A3 , A4 and A5 ; this may be explained as the dropping rate of A3 providing enough time for shell condensation reaction. 3.6. Free formaldehyde concentration of microcapsules We can get formaldehyde criteria curve and equation using the data showed in Table 4. The remnant formaldehyde content of the microcapsules is presented in Table 5. The remnant formaldehyde content of the microcapsules synthesized by in situ polymerization is only 67.1 mg kg−1 . The employed prepolymer was prepared by putting in formaldehyde once and melamine for three times. In the field of textile, remnant formaldehyde content is restricted. The polyacrylonitrile–vinylidene chloride fibers containing up to 30 wt% MicroPCMs were wet-spun in our laboratory [27]. The remnant formaldehyde content of this fiber is 158.4 mg kg−1 . This fiber can be used for the fabric that does not contact skin. According to the MicroPCMs content in the fibers above, if the microcapsules in this paper were added into the fiber and the remnant formaldehyde concentration would be quite low. It is estimated that the microcapsules with low remnant formaldehyde content have a wider application area and promising prospect. 4. Conclusion MicroPCMs were synthesized by in situ polymerization using melamine–formaldehyde as shell and n-octadecane as core and
the employed melamine–formaldehyde prepolymer was prepared by incorporating formaldehyde once and melamine for three times. With the dropping rate of the MF prepolymer decreasing, the flocculation phenomenon of the microcapsules decreases and the globular surface becomes smoother. The thermal stability increases with the decrease of the dropping rate. Microcapsules with average diameter 2.2 m have smooth globular surface. The enthalpy of the microcapsules containing 59 wt% n-octadecane is 144 J g−1 . The remnant formaldehyde content of microcapsules is only 67.1 mg kg−1 , which indicates their wider application area and promising prospect. Acknowledgements The authors are thankful to the National Natural Science Foundation of China (No. 50573058) and Specialized Research Foundation for the Doctoral Program of Higher Education (No. 20050058004) for the financial supports. References [1] S. Benita, Microencapsulation: Methods and Industrial Applications, Marcel Dekker, Inc., New York, 1996, pp. 1–3. [2] S.J. Park, S.H. Kim, J. Colloid Interf. Sci. 271 (2004) 336. [3] F.T. Meng, G.H. Ma, Y.D. Liu, et al., Colloids Surf. B: Biointerf. 33 (2004) 177. [4] K.J. Hong, S.M. Park, Polymer 41 (2000) 4567. [5] B. Comiskey, J.D. Albert, H. Yoshizawa, J. Jacobson, Nature 394 (1998) 253. [6] R. Yang, H. Xu, Y.P. Zhang, Solar Energy Mater. Solar Cells 80 (2003) 405. [7] B. Boh, E. Knez, M. Staresinic, J. Microencapsul. 22 (2005) 715. ¨ ¨ Paksoy, H. Evliya, Int. J. Energy Res. 30 [8] Y. Ozonur, M. Mazman, H.O. (2006) 741.
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