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Fabrication and Characterization of Flame-retardant Nanoencapsulated noctadecane with Melamine–formaldehyde Shell for Thermal Energy Storage Xiaosheng Du, Yuanlai Fang, Xu Cheng, Zongliang Du, Mi Zhou, and Haibo Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03980 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

1

Fabrication and Characterization of Flame-retardant Nanoencapsulated

2

n-octadecane with Melamine–formaldehyde Shell for Thermal Energy

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Storage

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Xiaosheng Du, Yuanlai Fang, Xu Cheng, Zongliang Du, Mi Zhou, and Haibo Wang*

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Textile Institute, College of Light Industry, Textile and Food Engineering, Sichuan University, No.24

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South Section 1, Yihuan Road, Chengdu, 610065, China.

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Corresponding Author

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* E-mail: [email protected] (H.B. Wang). Tel: 86-28-85401296. Fax: 86-28-85401296.

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ABSTRACT: An innovative reactive phosphorus–nitrogen containing diamine, PNDA, was obtained by

10

dehydration reaction between 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and

11

4,4′-diaminobenzophenone (DABP). Then, flame-retardant nanoencapsulated n-octadecane (NanoC18)

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with PNDA-modified melamine–formaldehyde (MF) as shell was fabricated via in situ polymerization.

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N-octadecane was successfully encapsulated in the PNDA-modified MF polymer shell and the diameter

14

of NanoC18 ranged within 80–140 nm, as evidenced by scanning electronic microscopy (SEM) and

15

Fourier transform infrared (FTIR) spectroscopy. The thermal property and flame retardancy of NanoC18

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were researched by thermogravimetric (TGA), differential scanning calorimetry (DSC), cone

17

calorimetry measurement, and limiting oxygen index (LOI) test. DSC analysis indicated that NanoC18

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exhibited a relatively high phase change enthalpy within the range of 110.8–141.3 J/g. The results of the

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combustion test indicated that the introduction of the phosphorus–nitrogen containing PNDA into 1

ACS Paragon Plus Environment

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NanoC18 considerably increased the LOI and residual weight of EP/ NanoC18 composites, as well as

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suppressed the release of heat and smoke. Moreover, the thermal properties, thermal stability, and

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durability of NanoC18 were barely changed upon PNDA introduction into NanoC18. In conclusion, this

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nanoencapsulated n-octadecane with an excellent phase change properties and flame-retardant properties

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exhibit considerable potential for energy saving construction, thermoregulated textile and other thermal

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energy storage applications.

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KEYWORDS: nanoencapsulation, n-octadecane, thermal energy storage, melamine–formaldehyde

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resin, flame-retardant

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INTRODUCTION

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In recent years, phase change materials (PCMs) based thermal energy storage (TES) systems have

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attracted extensive attention because of increasing energy consumption and serious environmental

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concern.1 PCMs are attractive materials that can absorb and release prodigious amounts of energy during

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phase transition. Various PCMs, including n-alkanes, polyethylene glycols, fatty acid esters, fatty acids,

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salt hydrates, salts, and metals, have been studied for TES applications.2-4 Among the numberous PCMs,

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n-alkanes are considered to be the most suitable PCM for their desirable properties, including their

35

chemical inertness and stability, outstanding energy storage density, appropriate phase transition

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temperature range, abundance, environmental friendliness, low cost, and recyclability.5,6 However, the

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drawbacks of leakage during phase transition and poor thermal conductivity of n-alkanes seriously

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restrict their TES applications. 7-9 2


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Encapsulated phase change material (EPCM) are micro-sized core–shell particles that include PCM as

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core and a protective layer as shell. Encapsulation of n-alkanes with a solid shell can not only prevent

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the leakage and minimize the reactivity of n-alkanes but also expand heat exchange area.10,11

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Encapsulated n-alkanes have been extensively studied for TES applications, such as solar energy storage

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utilization, recovery of waste heat, thermal regulated textiles, and energy saving construction.12-14 The

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shell materials of the EPCMs are primarily classified into inorganic materials and organic materials.

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Inorganic materials, such as silicon dioxide,15 calcium carbonate,16 titanium dioxide,17 zirconium

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oxide,18 and zinc oxide19, possess high thermal stability and thermal conductivity; however, their

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encapsulation efficiency and tenacity are low. As alternatives, organic materials, such as melamine–

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formaldehyde resin (MF),20 urea–formaldehyde resin (UF),21 polyacrylate,22 polyuria–urethane resin

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(PU),23 polystyrene,24 and gelatin–arabic gum,25 have been intensively studied as shell materials for

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EPCMs. Among the various shell materials, MF resin is considered to be a promising candidate for

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PCMs encapsulation because of its effective encapsulation, high tensile/tear strength, low cost, and

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easily controlled synthesis.26-28

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Numerous studies in the literature have extensively studied the fabrication route, microstructure

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control, thermal conductivity, latent heat storage, and mechanical properties of encapsulated n-alkanes

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with polymer shells. However, rare attention has been focused on improving the flame retardancy of

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encapsulated n-alkanes with polymer shells. Silica based phase change composites have been

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synthesized to improve flame retardancy of the organic PCM.29 However, no relevant research has

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reported the fabrication of the encapsulated PCMs with organic flame-retardant shell. Similar to other 3


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organic materials, both n-alkanes and the polymer shells are highly flammable, thereby restricting the

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application of encapsulated n-alkanes in building construction, thermoregulated textile, and electric and

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electronic industries.30-32 Hence, the improvement of the flame retardancy for the encapsulated n-alkanes

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is imperative.

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Recently, phosphorus–nitrogen possessing intumescent flame retardants have become increasingly

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popular on account of their advantageous features which included halogen free, low smoke, and high

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flame retardancy.33,34 Phosphorus–nitrogen containing flame retardants usually experience intense

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expansion and form a protective char layer, which prevents thermal transmission and protect the

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underlying material.35 Furthermore, phosphorus–nitrogen containing flame retardants displayed

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synergistic effect between nitrogen and phosphorus during combustion.36

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In this study, an innovative reactive phosphorus–nitrogen containing diamine, PNDA, was

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synthesized through the dehydration reaction between 4,4′-diaminobenzophenone (DABP) and

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9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide

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nanoencapsulated n-octadecane (NanoC18) with PNDA-modified MF resin as shell was obtained via in

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situ polymerization. The flammability properties of NanoC18 were studied by cone calorimeter test and

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limiting oxygen index (LOI) measurement. Moreover, the morphologies, chemical structures, phase

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change properties, thermal reliabilities, and thermal stabilities of NanoC18 were systemically analyzed.

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EXPERIMENTAL SECTION

77

Materials

(DOPO).

4


Then,

flame-retardant

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N-octadecane (98 wt% purity), DOPO (97 wt% purity), and 4,4′-diamino-diphenylmethane (DDM,

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98.5 wt% purity) were obtained from Chengdu Best Reagent (China). DABP, melamine (99 wt% purity),

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formaldehyde (37.0 wt% aqueous solution) were obtained from Aladdin Reagent Co., Inc. Styrene–

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maleic anhydride copolymer (SMA, Mw=60,000-70,000) was provided by Nanjing Yinxin Factory

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(China) and used as an emulsifier. Epoxy resin (EP, epoxy value was 0.44) was obtained from Hefei

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Jiangfeng Factory (China). Triethanolamine (99 wt% purity) and citric acid (97 wt% purity) were used

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as received without further treatment.

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Synthesis of phosphorus–nitrogen containing diamine (PNDA)

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PNDA was synthesised through the dehydration reaction between DOPO and DABP. In brief, DOPO

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(43.20 g, 0.20 mol) and DABP (6.37, 0.03 mol) were added into a flask at 180 °C with mechanical

88

stirring for 4 h. Subsequently, 150 mL toluene was added to the mixture at 100 °C. Then, the precipitant

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was filtered and washed with toluene. The raw product was purified by recrystallization in

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tetrahydrofuran. The white powder PNDA was obtained by drying at 60 °C for 8 h with a yield of 78%

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(14.65 g). The illustration of preparation process of PNDA is represented in Scheme 1.

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Scheme 1. Synthetic route of PNDA

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Fabrication of flame-retardant NanoC18 As demonstrated in Scheme 2, the flame-retardant NanoC18 were obtained by an in situ polymerization method, including the synthesis of PNDA-modified MF prepolymer, the fabrication of n-octadecane emulsion, as well as the formation of NanoC18. The recipes for NanoC18 are listed in Table 1. Scheme 2. Schematic fabrication of NanoC18

Synthesis of PNDA-modified MF prepolymer: a predetermined amount of melamine, 10 mL distilled water, and 37.0 wt% formaldehyde solution were mixed in a round-bottomed flask. Triethanolamine was added to adjust the pH of the mixture to 8.5. Then a transparent PNDA-modified MF prepolymer solution was obtained with mechanical stirring at 70 °C for 100 min. Preparation of n-octadecane emulsion: 1.0 g SMA, 0.17 g sodium hydroxide, and 60 mL distilled water were added in a flask and stirred at 85 °C for 2 h. The pH of the mixture was adjusted to 4.0 with a saturated solution of citric acid. Then, 10.0 g n-octadecane was added and the system was emulsified at 6


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40 °C with a stirring speed of 3,000 rpm for 60 min. Fabrication of NanoC18: PNDA-modified MF prepolymer was added into the previously prepared n-octadecane emulsion dropwise with a stirring speed of 500 rpm. Then, the mixture was continuously stirred at 70 °C for 2 h. Then, the suspension was filtered and washed using a 50 wt% aqueous ethanol to eliminate impurity and unencapsulated n-octadecane. The flame-retardant NanoC18 were dried under vacuum for 24h. Table 1. Fabrication recipes of flame-retardant NanoC18 n-octadecane

Melamine

Formaldehyde

PNDA

n-octadecane

PNDA content

(g)

(g)

solution (g)

(g)

content (wt%)

(wt%)

NanoC18-1

10.00

4.17

6.76

-

60

-

NanoC18-2

10.00

3.83

6.32

0.50

60

3

NanoC18-3

10.00

3.49

5.88

1.00

60

6

NanoC18-4

10.00

3.16

5.44

1.50

60

9

NanoC18-5

10.00

2.82

5.00

2.00

60

12

NanoC18-6

10.00

5.44

9.08

1.20

50

6

NanoC18-7

10.00

4.38

7.33

1.09

55

6

NanoC18-8

10.00

2.74

4.65

0.92

65

6

NanoC18-9

10.00

2.10

3.59

0.86

70

6

Sample

Characterization 1H

and

31P

nuclear magnetic resonance (1H NMR and

31P

NMR) spectra of PNDA were

recorded at 400 MHz by Bruker AV-Ⅱ spectrometer with dimethyl sulfoxide-d6 as the solvent. The chemical structures of PNDA, n-octadecane, PNDA-modified MF shell material, and the synthesized NanoC18 were analysed by Nicolet NEXUS-670 Fourier transform infrared spectrometer via KBr sampling method. The morphology of the NanoC18 was observed by SEM (JOEL JSM-7500F, Japan). 7


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Thermal properties of NanoC18 were performed on a NETZSCH DSC 214 differential scanning calorimeter at a heating/cooling rate of 10 °C/min under nitrogen atmosphere. An accelerated thermal cycling test was performed to study the reversible stability of NanoC18. The thermal cycling procedure consisted of 100 heating/cooling thermal cycles within the range of −10 °C–80 °C. DSC analysis was performed to measure the changes of the thermal properties for NanoC18. The reproducibility was checked by performing five measurements. The thermal stabilities of n-octadecane and NanoC18 were studied using a thermogravimetric analyser (TGA, Netzsch STA409PC, Germany) under air atmosphere. The samples were heated from 50 °C to 800 °C at 10 °C/min. To investigate the flammability properties of NanoC18, EP/NanoC18 composites were prepared by mixing NanoC18, EP and DDM at a mass ratio of 1:2.75:0.6. The LOI of the EP/NanoC18 composites was measured by HC-2 oxygen index instrument (Jiangning Instrument, China) according to ASTM D 2863. A FTT cone calorimeter was also used to research the flammability properties of EP/NanoC18 composites (10 cm × 10 cm × 0.5 cm) according to ISO 5660-1. The morphology of residual char of EP/NanoC18 composites after cone calorimeter test was observed by FEI Quanta 250 SEM. The X-ray photoelectron spectroscopy (XPS) of the residual char was measured using XSAM80 spectrometer.

RESULTS AND DISCUSSION 8


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Structure analysis of PNDA

Figure 1. 1H NMR and 31P NMR spectra of PNDA. PNDA was synthesized from DABP and DOPO through dehydration reactions between the DABP carbonyl group and DOPO P–H group. The synthesized PNDA was investigated using 1H NMR,

31P

NMR, and FTIR to confirm its chemical structure. Figure 1 displays 1H NMR and 31P NMR spectra of PNDA. 1H NMR spectrum of PNDA displayed several characteristic peaks: the peaks at 5.88 and 6.82 ppm corresponded to benzene protons near –NH2; the peaks at 6.9–8.7 ppm corresponded to the benzene protons near to P=O; and the peak at 4.94 ppm corresponded to –NH2. Furthermore, the peaks of

31P

NMR spectrum at around 29.51 and 31.12 ppm demonstrated that the synthesized PNDA retained the cyclic DOPO groups.

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Figure 2. FTIR spectrum of PNDA. Figure 2 represents the FTIR spectrum of PNDA. As shown in Figure 2, the strong and broad absorption peaks from 3500–3000 cm−1 were associated with the stretching vibration of N–H, meanwhile the N–H bending vibration appeared at 1678 cm−1. The peaks located at 1197 and 910 cm−1 were associated with the stretching vibration of P–O–Ph. The peak located at 1242 cm−1 was indexed to the stretching vibration of P=O bond, while the peak at 1510 cm−1 corresponded to the stretching vibration of P–Ph. All the analysis results verified the successful preparation of PNDA.

Chemical characterization of NanoC18 In this study, flame-retardant nanocapsules were fabricated using PNDA-modified MF as shell. Figure 3 presents the FTIR spectra of n-octadecane, PNDA-modified MF resin, and flame-retardant NanoC18-3 with PNDA-modified MF resin shell. Figure S1 presents the FTIR spectra of NanoC18 with different PNDA contents and n-octadecane contents. In the spectrum of n-octadecane, strong peaks located at 2849 and 2916 cm−1 were attributed to C–H stretching vibrations of –CH3 and –CH2–, while 10


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the absorption peaks at 1377 and 1472 cm−1 were attributed to bending vibration of –CH3 and –CH2–. The peak at 717 cm−1 was typical absorption peak ascribed to in-plane rocking vibration of the –(CH2)n–. In the spectra of PNDA-modified MF resin and NanoC18-3, the wide and strong absorption peak at approximately 3400 cm-1 was associated with the superposition of N–H and O–H stretching vibrations, while the peaks at 1345 and 1563 cm-1 were associated with the stretching vibration of C=N and C–N of triazine ring. The sharp peak at 815 cm−1 in the spectra of PNDA-modified MF resin and NanoC18-3 was assigned to the bending vibration of triazine ring, which was the typical characteristic peak of the MF resin. In addition, the peaks at 1510, 1242, 1197, and 910 cm−1 corresponding to P–Ph, P=O, and P– O–Ph could be observed in the spectra of PNDA-modified MF resin and NanoC18-3. Furthermore, in the FTIR spectrum of NanoC18-3, the characteristic peaks of n-octadecane overlapped with the peaks of PNDA-modified MF resin shell. The results showed that n-octadecane was successfully encapsulated into the PNDA-modified MF polymer shell.

Figure 3. FTIR spectra of n-octadecane, PNDA-modified MF resin and flame-retardant NanoC18-3.

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Morphology of NanoC18 The morphologies of NanoC18 with different PNDA contents were observed by SEM, and the micrographs of NanoC18 are shown in Figure 4. As shown, flame-retardant NanoC18 exhibits a regularly spherical shape with a smooth surface and no destruction was observed in the nanocapsules, thereby manifesting that n-octadecane was successfully encapsulated by PNDA-modified MF resin. The diameters of the NanoC18 fabricated in this research were mainly ranged from 80 nm to 140 nm. The particle size of NanoC18 was not uniform because of different the shear stresses of fluid at different locations during emulsification process. The particle size distribution curves of NanoC18 are presented in Figure S2. The average diameter of NanoC18 was measured to be 109.6 nm for NanoC18-1 (0% content of PNDA), 109.3 nm for NanoC18-2 (3% content of PNDA), 108.8 nm for NanoC18-3 (6% content of PNDA), 110.2 nm for NanoC18-4 (9% content of PNDA), and 111.7 nm for NanoC18-5 (12% content of PNDA). The result showed that the introduction of PNDA into the encapsulation processes did not influence the geometric profile of NanoC18.

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Figure 4. SEM micrographs of NanoC18 with different PNDA contents.

Thermal properties of NanoC18 The phase change characteristics of NanoC18 and n-octadecane were characterized by DSC, and the DSC curves are shown in Figure 5. The thermal parameters of NanoC18 and n-octadecane are summarized in Table 2. The phase change behaviour of NanoC18 were similar to those of n-octadecane, because n-octadecane was encapsulated by PNDA-modified MF resin without chemical reaction during the preparation of NanoC18. As shown in Table 2, the measured melting point (Tm) and crystallization point (Tc) of n-octadecane were 34.3 and 17.2 °C, respectively. When encapsulated, the Tm of the NanoC18 were lower than the Tm of bulk n-octadecane by 1.4–2.3 °C, while the Tc of the NanoC18 was higher than the Tc of bulk n-octadecane by 1.5–2.5 °C. This result occurred because 13



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the extremely small size of NanoC18 led to a high ratio of surface area to volume, which enhanced the heat transportation of NanoC18.

Figure 5. DSC curves of NanoC18 and n-octadecane: (a) melting process and (b) freezing process. Table 2. Thermal properties of NanoC18 and n-octadecane Tm

ΔHm

Tc

ΔHc

(°C)

(J/g)

(°C)

(J/g)

n-octadecane

34.3

241.1

17.2

238.8

-

NanoC18-1

32.6

126.1

18.9

124.3

87.0

NanoC18-2

32.5

125.4

19.4

123.5

86.4

NanoC18-3

32.0

126.5

19.7

123.7

86.9

NanoC18-4

32.4

124.5

19.5

123.4

86.1

NanoC18-5

32.1

125.9

19.7

124.1

86.8

NanoC18-6

32.3

110.8

19.6

108.4

91.4

NanoC18-7

32.6

118.4

19.3

116.7

89.1

NanoC18-8

32.9

134.0

18.7

131.8

85.2

NanoC18-9

32.7

141.3

19.0

139.6

83.6

Sample

E (%)

The enthalpies of melting and crystallization (ΔHm and ΔHc) for n-octadecane were 241.1 and 238.8 J/g, respectively (Table 2). The encapsulation efficiency (E) of NanoC18 can be estimated by the following equation: 14


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E=

 H

H m,NanoC18  H c,NanoC18 m,n -octadecane

 H c,n -octadecane   wPCM

(1)

 100%

where wPCM is the theoretical mass fraction of n-octadecane in NanoC18. The encapsulation efficiencies of n-octadecane in NanoC18 were calculated to be in the range of 83.6-91.4%. Notably, the phase change enthalpies and E of NanoC18 almost unchanged by increasing the PNDA content in NanoC18. Thus, the addition of PNDA did not influence phase change enthalpy and encapsulation efficiency of NanoC18. In addition, DSC results showed with the addition of n-octadecane into the encapsulation system from 50 wt% to 70 wt%, the phase change enthalpy of NanoC18 increased from 110.8 J/g to 141.3 J/g, and it’s encapsulation efficiency decreased from 91.4% to 83.6%. This phenomenon can be explained as follows: n-octadecane was not being completely encapsulated owing to the shortage of shell material, thereby resulting in the diffusion of n-octadecane from nanocapsules.

Reversible stability of NanoC18 The phase transition reversibility of NanoC18 was evaluated though an accelerated thermal cycling test in the range of −10 °C to 80 °C. The phase change enthalpies of NanoC18 after thermal cycling test were tested by DSC measurement. The DSC curves of NanoC18 before and after 100 heating/cooling thermal cycles are presented in Figure S3. The loss ratio of phase change enthalpies for NanoC18 can be estimated according to the following equation:

Loss ratio =

H m,0  H m,t H m,0

 100%

(2)

where ΔHm,0 and ΔHm,t are the melting enthalpies of NanoC18 before and after 100 heating/cooling 15


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thermal cycles, respectively. The phase change enthalpy (ΔHm) and loss ratio after thermal cycling test of NanoC18 with different PNDA contents and n-octadecane contents are presented in Figure 6. The phase change enthalpies of NanoC18 slightly decreased after 100 heating/cooling thermal cycles, and the loss ratios of ΔHm for NanoC18 ranged from 1.62% to 2.41%, indicating that NanoC18 exhibited good thermal reversibility during phase transition. In addition, the thermal reversibility of NanoC18 was almost unchanged by the addition of PNDA to the nanocapsule. Moreover, the thermal reversibility of NanoC18 deteriorated with the increasing n-octadecane content in the nanocapsule; this finding was because of the diffusion of n-octadecane from nanocapsules because of the shortage of the shell material.

Figure 6. Phase change enthalpy (ΔHm) and loss ratio after thermal cycling test of NanoC18 with different (a) PNDA contents and (b) n-octadecane contents.

Thermal stability of NanoC18 TGA curves of NanoC18 and n-octadecane with different PNDA contents are presented in Figure 7. Pure n-octadecane presented only one thermal decomposition stage from 159 °C to 232 °C because of 16


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volatilization of linear alkane molecules. When encapsulated, two thermal decomposition stages were founded in the TGA curves of NanoC18. The first decomposition stage of NanoC18 from 163 °C to 235 °C was due to the decomposition of n-octadecane, while the second decomposition stage from 335 °C to 600 °C was associated to the decomposition of the polymer shell. NanoC18-1, NanoC18-2, NanoC18-3, NanoC18-4, and NanoC18-5 exhibited T10% (the

temperature at which 10% weight loss

occur) values of 179 °C, 177 °C, 177 °C, 180 °C, and 181 °C, respectively. The T10% value of NanoC18 was higher than the T10% of n-octadecane because of the protection provided by the polymer shell. In addition, the char residues of NanoC18-1, NanoC18-2, NanoC18-3, NanoC18-4, and NanoC18-5 at 900 °C were calculated as 3.22%, 5.86%, 7.52%, 8.77%, and 9.87%, respectively. Notably, the char residue of NanoC18 considerably increased with the increasing PNDA content in the nanocapsule. This result is due to the following mechanism: as an intumescent flame retardant, the phosphorus–nitrogen containing PNDA produced a swollen char layer, which acted as a physical barrier against heat transmission and protected NanoC18 from further decomposition.

Figure 7. TG curves of NanoC18 and n-octadecane. 17


ACS Sustainable Chemistry & Engineering 1 2 3 4 260 5 6 7 261 8 9 10262 11 12263 13 14 15264 16 17 18265 19 20 266 21 22 23 24 25 26 27 28 29 30 31 32267 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49268 50 51269 52 53 54270 55 56 57 58 59 60

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Flame-retardant properties of NanoC18 To research the flame-retardant properties of NanoC18, the combustion behaviours of EP/NanoC18 composites were measured. Various characteristic combustion parameters obtained from cone calorimetry and LOI tests, including LOI value, total heat release (THR), the peak of heat release rate (pHRR), residual weight, and total smoke release (TSR) are presented in Table 3. Figure 8 presented the curves of heat release rate (HRR) and THR versus time for EP/NanoC18 composites. Table 3. The pHRR, THR, residual weight, TSR, and LOI value of EP/NanoC18 composites Sample

THR (MJ/m2)

pHRR (kW/m2)

Residual weight (wt%)

TSR (m2/m2)

LOI (%)

EP/NanoC18-1

269.0

817.5

2.9

8548.0

20.4

EP/NanoC18-2

241.4

725.8

5.1

8017.4

22.0

EP/NanoC18-3

218.9

635.8

6.9

7673.1

22.6

EP/NanoC18-4

198.5

595.2

8.6

7302.6

24.3

EP/NanoC18-5

187.6

549.7

9.8

6956.0

25.1

Figure 8. The curves of HRR (a) and THR (b) versus time for EP/NanoC18 composites. As shown in Figure 8, after ignition, EP/NanoC18-1 (without PNDA added) burned very rapidly, and

18


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EP/NanoC18-1 presented the highest pHRR, THR, and TSR values of 817.5 kW/m2, 269.0 MJ/m2, and 8548.0 m2/m2, respectively, in all composites. With the addition of PNDA into NanoC18, the THR, pHRR, and TSR of EP/NanoC18 composites decreased distinctly, considerably increasing people’s escape probability during fire hazard. Compared with EP/NanoC18-1, the addition of 12 wt% PNDA into NanoC18-5 brought a 32.8% decrease in pHRR, a 30.3% decrease in THR, and a 18.6% decrease in TSR of EP/NanoC18-5. Moreover, with 12 wt% addition of PNDA into NanoC18, the residual weight of EP/NanoC18 composites increased from 2.9 wt% to 9.8 wt%, while the LOI of EP/NanoC18 composites increased from 10.4 % to 25.1 %. Phosphorus and nitrogen in PNDA markedly promoted the generation of protective char layers cover the surface of EP/NanoC18 composites, thereby preventing the interior material from the flame and heat flux during burning. All results indicated that the introduction of phosphorus–nitrogen containing PNDA into NanoC18 significantly improved its flame-retardant properties.

Morphology and structure of residual char To further elucidate flame-retardant mechanism, SEM and digital camera were applied to observe the morphology and structure of residual char for EP/NanoC18 composites after combustion. The SEM images and digital photographs of residual char for EP/NanoC18-1 and EP/NanoC18-4 are showed in Figure 9. As shown in Figure 9 (A1), the residue of EP/NanoC18-1 showed only a small amount of severely broken char, thereby indicating that EP/NanoC18-1 cannot block flame and heat during combustion. As seen in Figure 9 (A2) and Figure 9 (A3), the exterior surface of neat EP/NanoC18-1 19



Page 20 of 30

exhibited a multihole cracked char layer structure, whereas the inner surface was in accordance with exterior layer. By contrast, a relatively compact, smooth, and continuous layer with a considerable amount of char was observed in residual char for EP/NanoC18-4 (Figure 9 (B1) and Figure 9 (B2)), thereby effectively protecting the internal structure and preventing heat transmission during combustion. Meanwhile, intumescent and porous structure was observed in the interior char layer of EP/NanoC18-4 in Figure 9 (B3) because of the P–N bond crack, which produced nonflammable NH3 and PO·.

Figure 9. Digital photographs, SEM images of exterior surface, and SEM images of inner surface of residual chars for EP/NanoC18-1 (A1, A2, and A3) and EP/NanoC18-4 (B1, B2, and B3).

XPS analysis of char residues Figure 10 present the XPS spectra and C1s spectra of residual char after combustion for the obtained EP/NanoC18 composites. In Figure 10 (a), the relative contents of C1s (284.6 eV), O1s (531.9 eV), and 20


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N1s (399.8 eV) for EP/NanoC18-1 residual char were 72.1%, 20.6%, and 7.3%, respectively. In addition to C, O, and N, 2.7 % P (134.3 eV) was appeared in residual char of EP/N5 (Figure 10 (b)). As shown in Figure 10 (c) and Figure 10 (d), C1s spectra of residual char for EP/NanoC18 composites can be split into three characteristic bands at 284.5, 285.7, and 288.1 eV, which are indexed to C–C/C–H in aromatic and aliphatic species, C–O/C–OH, and C=O, respectively. The detailed C1s XPS data of residual char after combustion for EP/NanoC18-1 and EP/NanoC18-4 are presented in Table 4. To evaluate the thermal oxidative resistance of residual char, Cox/Caα ratio was calculated. As tabulated in Table 4, the Cox/Caα values of the residual char for EP/NanoC18-1 and EP/NanoC18-4 were 1.25 and 0.87, respectively. This result indicated that the incorporation of phosphorus-nitrogen containing PNDA into NanoC18 considerably decreased the oxidation degree of the residual char. According to previous studies,37,38 this graphitized residual char can significantly prevent the diffusion of flammable gaseous and restrain combustion, thereby enhancing flame retardancy. Table 4. C1s XPS data of residual char for EP/NanoC18-1 and EP/NanoC18-4 C–C/C–H

C–O/C–OH

C=O

Cox/

area (%)

area (%)

area (%)

Caα

EP/NanoC18-1

44.5

46.2

9.3

EP/NanoC18-4

53.5

27.7

18.8

Sample

αC ox

1.2 5 0.8 7

denotes oxidized carbons (C–O/C–OH, C=O); Ca denotes aromatic and aliphatic carbons.

21



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Figure 10. XPS and C1s spectra of residual char for EP/NanoC18-1 (a, c) and EP/NanoC18-4 (b, d).

CONCLUSION In conclusion, novel flame-retardant NanoC18 containing n-octadecane with PNDA-modified MF as shell was prepared via in situ polymerization. SEM and FTIR indicated that n-octadecane was successfully encapsulated in the PNDA-modified MF resin and the diameter of NanoC18 ranged within 80–140 nm. NanoC18 possessed a relatively high phase change enthalpy within the range of 110.8– 141.3 J/g. The results of combustion test indicated that the introduction of phosphorus–nitrogen 22


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containing PNDA into NanoC18 considerably increased the LOI value and residual weight of EP/NanoC18 composites, as well as suppressed the release of heat and smoke. Moreover, the thermal properties, durability, and thermal stability of NanoC18 were barely changed upon PNDA introduction into NanoC18. Hence, the flame-retardant NanoC18 exhibit enormous potential for TES applications, especially in energy saving construction and thermoregulated textile fields. 39

ASSOCIATED CONTENT Supporting Information. FTIR spectra of NanoC18 (Figure S1), particle size distribution curves of NanoC18 (Figure S2), DSC curves of NanoC18 before and after 100 heating/cooling thermal cycles (Figure S3).

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (NO. 51773129, 51503130); and Postdoctoral research foundation of Sichuan University (2018SCU12049).

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Table of Contents

Novel flame-retardant nanoencapsulated n-octadecane with melamine–formaldehyde resin shell was successfully fabricated via in situ polymerization.

30


Fabrication and Characterization of Flame ... - ACS Publications

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