Improved anti-oxidation properties of electrospun polyurethane nanofibers achieved by oxyfluorinated multi-walled carbon nanotubes and aluminum hydroxide

Materials Chemistry and Physics 126 (2011) 685–692

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Improved anti-oxidation properties of electrospun polyurethane nanofibers achieved by oxyfluorinated multi-walled carbon nanotubes and aluminum hydroxide Ji Sun Im a , Byong Chol Bai a , Tae-Sung Bae b , Se Jin In c , Young-Seak Lee a,∗ a

Department of Fine Chemical Engineering and Applied Chemistry, BK21-E2 M, Chungnam National University, Daejeon 305-764, Republic of Korea Korea Basic Science Institute (KBSI), Jeonju 561-765, Republic of Korea c Department of Fire and Disaster Protection Engineering, Woosong University, Daejeon 300-718, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 19 August 2010 Received in revised form 21 October 2010 Accepted 16 December 2010 Keywords: Thermal properties Thermogravimetric analysis (TGA) X-ray photo-emission spectroscopy (XPS) Inorganic compounds

a b s t r a c t Polyurethane fibers were fabricated using an electrospinning method with aluminum hydroxide and multi-walled carbon nanotubes (MWCNTs) as flame-retardant additives to improve the thermal oxidation stability of the polyurethane fibers. The MWCNTs were incorporated into the polyurethane fibers after oxyfluorination treatment to improve the dispersivity and compatability. The thermal properties and anti-oxidation stabilities of these polyurethane fibers were investigated under nitrogen and oxygen flows from room temperature to 600 ◦ C to determine the effects of the MWCNTs and aluminum hydroxide additives. The aluminum hydroxide acted as an energy storage tank by releasing water, resulting in an endothermic reaction. The MWCNTs promoted the formation of a charred layer that acted as a protective film to prevent the decomposition of the polyurethane by oxygen radicals. The flame-retardant properties were also improved by an enhanced gel-type structural network generated by the MWCNTs. The integral procedure decomposition temperature and activation energy increased significantly, indicating that the flame-retardant properties of the polyurethane fibers improved. These results are attributed to the aluminum hydroxide, MWCNT additives, and the oxyfluorination treatment. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Recently, polyurethanes have been extensively used in numerous commercial applications due to their wide range of possible compositions. They have been used as shape memory foams, coatings, adhesives, sealants, synthetic leathers, membranes, elastomers, and biomedical applications [1–3]. For this reason, the fabrication of polyurethane has been widely studied. Polyurethane fibers are of special interest because the fibers have outstanding mechanical properties and high surface area when compared with bulk polymers. These advantages of fabrication can be enhanced by manufacturing at the nanoscale. Electrospinning methods can be used to produce nanoscaled fibers. Electrospinning is a fiber spinning technique that uses electric fields in the kilovolt range, with magnitudes as high as 40 kV. Under an applied electrostatic force, the polymer is ejected from a nozzle. The ejected polymer’s diameter is significantly reduced as it is transported to a template. The template also serves as the ground for the electrical charge. Such thin fibers provide unusually high surface area to

∗ Corresponding author. Tel.: +82 42 821 7007; fax: +82 42 822 6637. E-mail address: [email protected] (Y.-S. Lee). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.12.061

volume ratios and are of interest for many applications ranging from textiles to composite reinforcement, sensors, biomaterials, and membrane technology [4,5]. Unfortunately, these fibers have not yet had wide-spread application due to their poor thermal and anti-oxidation properties [1–3]. To improve the thermal and anti-oxidation properties of electrospun polyurethane fibers, traditional halogen fire retardants, such as P, Br, and Cl, have been used to suppress the radicals that arise during combustion. However, the possibility of generating hazardous gases and compounds, such as dioxin, has become a serious environmental concern [6,7]. To solve this problem, the introduction of inorganic flame retardants to organic polymer materials has been widely investigated [8–11]. The addition of inorganic hydroxides increases flame resistance due to a decrease in the proportion of combustible polymer present. In addition, due to the endothermic nature of inorganic hydroxide dehydration, the ignition temperature increases with the decomposition temperature of the polymer [10,12–14]. However, polyurethane exhibits weakened mechanical properties when used with inorganic flame retardants such as aluminum hydroxide. Carbon nanotubes (CNTs) have also been used as a flameretardant filler because the stability of the polymer can be improved by the free radical scavenger effect of CNTs and because the CNTs

J.S. Im et al. / Materials Chemistry and Physics 126 (2011) 685–692

can promote structural network formation for improved mechanical properties of the polymer matrix [15–17]. However, strong van der Waals forces or interactions between CNTs result in the formation of aggregates. This makes it difficult to obtain a fine, homogeneous dispersion in a polyurethane matrix [18–20]. The agglomeration of CNTs also results in poor interfacial interactions with the matrix. Therefore, achieving a homogeneous dispersion of CNTs in a polyurethane matrix is a key requirement in obtaining the desired flame-retardant properties of such composites. In our research, we use a fluorination method to improve the dispersivity and compatibility of CNTs with a polymer matrix [21–23]. This is a simple and efficient method for the modification of polymer surfaces. In this study, the synergistic effects of aluminum hydroxide and CNT additives on the anti-oxidation properties of electrospun polyurethane fibers were investigated. The hydrophobic surface of the CNTs was modified by an oxyfluorination treatment, which can be carried out within the span of a few minutes with high modification efficiency. This induces hydrophilic functional groups for improved dispersivity and compatibility of CNTs in hydrophilic polyurethane fibers. The effects of the flame-retardant additives and the oxyfluorination treatment were investigated based on the decomposition of the electrospun polyurethane fibers. 2. Experimental 2.1. Preparation of the polyurethane solution A mixture of polycaprolactone diol (Mn: 530, 19.5 g) and polycaprolactone triol (Mn: 900, 3.75 g) was heated at 45 ◦ C for 30 min. It was then heated again to 60 ◦ C with 30 min of holding time before being cooled to 40 ◦ C. Isophorone diisocyanate (purity: 98%, 14.5 g) was added to the mixture and then stirred for 24 h [24,25]. 2.2. Surface modification of the MWCNTs by oxyfluorination The surfaces of the MWCNTs were modified by an oxyfluorination treatment. As a pretreatment step, the MWCNTs were heated to 120 ◦ C for 2 h to remove any impurities. The oxyfluorination was carried out at a pressure of 1 bar for 3 min with various mixing volume ratios of oxygen and fluorine (3:7, 5:5, and 7:3). The details of this oxyfluorination treatment have been described in previous reports [21–24,26,27]. The oxyfluorinated MWCNTs were designated as CNT, CNT(O3/F7), CNT(O5/F5), and CNT(O7/F3), according to their respective oxyfluorination conditions. 2.3. Preparation of the electrospinning solutions Five polymer solutions were prepared to investigate the effects of aluminum hydroxide and MWCNT additives and oxyfluorination treatment; 1: no additives, 2: MWCNT additive, 3: aluminum hydroxide (Al2 O3 content: 50–57%) additive, 4: MWCNT (outer diameter: 110–170 nm, length: 5–9 ␮m) and aluminum hydroxide additives, and 5: oxyfluorinated MWCNT and aluminum hydroxide additives. In the case of the oxyfluorinated MWCNTs, CNT(O5/F5) was used for further analysis under a dispersion test in the polyurethane solution (as further discussed in Section 3.1). 2.4. Preparation of the electrospun nanofibers The five prepared polymer solutions were ejected from a syringe tip onto an aluminum-foil-covered collector using an electrospinning apparatus. The electrospinning was performed under the following conditions: feeding rate of the polymer solution, 1 mL/h; supplied voltage, 18 kV; tip-to-collector distance, 10 cm; and collector rpm, 100. The five samples were termed as PU, CN-PU, AH-PU, CA-PU and OCA-PU, based on their respective electrospun polymer solutions: 1, 2, 3, 4 or 5 (as described in Section 2.3). 2.5. Characterization An ultraviolet (UV) visible spectrometer (Optizen 2120 UV visible, Mecasys, Korea) was used to investigate the dispersion of the MWCNTs in the polyurethane solution. A diluted polyurethane solution was prepared by adding 100 mL of water (for easy measurement) to decrease the viscosity of the polyurethane solution. The measurement was performed following the general method presented in the literature for the use of UV–vis spectrometry [28,29]. To determine the measurement wavelength, blends of diluted polyurethane solutions with MWCNTs were prepared with three known MWCNT concentrations. These blends were scanned across a broad wavelength range from 325 to 1400 nm. The wavelength showing the highest

sensitivity was selected as the measurement wavelength. For this study, measurements of MWCNTs were acquired at 635 nm [28,29]. These measurements were conducted after centrifugal separation of the polyurethane/MWCNT blend solution for 10 min at 1500 rpm. X-ray photoelectron spectroscopy (XPS) spectra of the MWCNTs were obtained with a MultiLab 2000 spectrometer (Thermo Electron Co., England) to evaluate changes in the chemical species on the surface before and after oxyfluorination. The XPS measurements were performed using Al K␣ (1485.6 eV) X-rays with a 14.9-keV anode voltage, a 4.6-A filament current, and a 20-mA emission current. All samples were pretreated at 10−9 mbar to remove impurities. The XPS survey spectra were obtained with a 50-eV pass energy and a 0.5-eV step size. Core-level spectra were obtained at a 20-eV pass energy with a 0.05-eV step size. The morphology of the samples was investigated with a field emission scanning electron microscope (FE-SEM, Hitachi, S-5500) to determine the effects of oxyfluorination. The size of the average diameter was calculated by a software program (Hitachi, S-5500) installed on the FE-SEM apparatus. Thermogravimetric analysis (TGA) was performed using a Shimadzu TGA-50H thermoanalyzer at scan rates of 5, 10, 20, and 40 ◦ C/min under air or nitrogen with flow rates of 2 × 10−5 m3 /min. The TGA provided information regarding the thermal oxidation stability and integral procedure decomposition temperature (IPDT). Differential scanning calorimetry (DSC) measurements were taken using a SHIMADZU DSC-50H instrument with an empty cell as the reference. The samples were placed in a liquid cell of aluminum and were heated from room temperature to 600 ◦ C under an air flow of 2 × 10−5 m3 /min, at a rate of 10 ◦ C/min with an accuracy of 0.1 ◦ C.

3. Results 3.1. Effects of oxyfluorination based on improved dispersion of MWCNTs The effects of oxyfluorination based on improved dispersion of MWCNTs were demonstrated by UV–vis spectra, as shown in Fig. 1. All samples showed a trend of increased transmission intensity with time. The non-treated MWCNTs showed the highest transmission intensity, indicating the poorest dispersion. The intensity of the transmitted light decreased as a result of the oxyfluorination treatment. When a higher oxygen content was used, the transmission intensity decreased significantly. An inflection point (in the transmission intensity versus time plot) was observed for the non-treated MWCNTs; this point separates regions of high and low slopes. The steep slope may have been caused by the aggregation of MWCNTs in the polymer solution, and the lower slope may have been caused by the force of gravity. The aggregation region was attenuated due to the oxyfluorination surface modification. Thus, the degree of dispersion of the MWCNTs in the polymer solution was effectively improved by inducing functional groups via oxyfluorination treatment. This is further supported by the XPS results explained in Section 3.2. The CNT(O5/F5) samples exhibited

CNT(O7/F3) CNT(O5/F5) CNT(O3/F7) CNT

Transmitted intensity

686

0

200

400

600

800

1000

Time (min) Fig. 1. Effects of oxyfluorination based on the dispersivity of MWCNTs in a polyurethane solution.

J.S. Im et al. / Materials Chemistry and Physics 126 (2011) 685–692

687

a

a

294

292

290

C1s C(1) C(2) C(3) C(4) C(5) C(6)

Intensity (cps)

Intensity (cps)

C1s C(1) C(2) C(3)

288

286

284

282

296

280

294

292

290

288

286

284

282

280

Binding energy (eV)

Binding energy (eV)

b

b

540

538

O1s O(1) O(2)

Intensity (cps)

Intensity (cps)

O1s O(1) O(2)

536

534

532

530

540

538

Binding energy (eV)

3.2. Chemical analysis of oxyfluorinated MWCNTs by XPS To investigate the functional groups induced on the MWCNTs by oxyfluorination, C1s peaks were deconvoluted to several pseudo-Vogit functions (sum of the Gaussian–Lorentzian function). The deconvolution was performed with a peak analysis program obtained from Unipress Co., U.S.A., and the results are shown in Figs. 2 and 3. The pseudo-Vogit function is given by [30]: 2

532

530

528

526

F1s F(1) F(2) F(3)



S 1 + (E − E0 /FWHM)

c

Intensity (cps)

a curve similar to that of the CNT(O7/O3) samples. For this reason, CNT(O5/F5) was used for further analysis in this study to avoid the higher fluorine usage.



534

Binding energy (eV)

Fig. 2. C1s and O1s peak deconvolution of pristine MWCNTs: (a) C1s and (b) O1s.

F(E) = H (1 − S) exp(− ln(2)(E − E0 /FWHM) ) +

536

2

,

where F(E) is the intensity at energy E, H is the peak height, E0 is the peak center, FWHM is the full-width at half-maximum, and S is the shape function related to the symmetry and Gaussian–Lorentzian mixing ratio. The surface compositions and assignments of the C1s components are listed in Tables 1 and 2, respectively. The deconvoluted C1s peaks of pristine MWCNTs and oxyfluorinated MWCNTs are presented in Figs. 2(a) and 3(a), respectively. The C(1) peak corresponds to non-functionalized sp2 carbon atoms, which originate from the aromatic carbons of MWCNTs. The concentration of this component was 85.72% for the pristine MWCNTs

696

694

692

690

688

686

684

682

680

Biding energy (eV) Fig. 3. C1s, O1s and F1s peak deconvolution of oxyfluorinated MWCNTs: (a) C1s, (b) O1s and (c) F1s.

[31]. The concentration of C(1) decreased to 69.21% due to the oxyfluorination process. Fluorine gas seemed to react with the carbon more actively than oxygen, resulting in a breakage of the sp2 carbon aromatic structure. The C(2) and C(3) components were assigned to C–O and C O, respectively. As a result of the

688

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Fig. 4. FE-SEM images: (a) PU, (b) and (c) CA-PU and (d) and (e) OCA-PU.

oxyfluorination treatment, the total content of carbon–oxygen single and double bonds increased by approximately 25%. These carbon–oxygen bonds contributed to the improved hydrophilicity of the MWCNTs [32]. Fluorinated carbon bonds were observed in C(4), C(5), and C(6), indicating semi-ionic, covalent, and perfluorinated C–F bonds, respectively [30,33,34]. The O1s and F1s peaks were also investigated to provide supplementary data for the interpretation of the C1s peaks. The O1s peaks were deconvoluted to investigate changes in the oxygen content of the MWCNTs caused by oxyfluorination. The O1s deconvolution peaks are presented in Figs. 2(b) and 3(b), and their surface compositions and assignments are presented in Tables 1 and 2, Table 1 Assignments of the different components of the C1s, O1s and F1s spectra. Component

Assignment

C(1) C(2) C(3) C(4) C(5) C(6) F(1) F(2) F(3) O(1) O(2)

Aliphatic non-functionalized sp2 or sp3 C Carbon oxygen double bond (C O) Carbon oxygen single bond (C–O) Semi-ionic C–F bond Covalent C–F bond Perfluorinated C–F bond Semi-ionically bound fluorine (C–CF) Covalent CF Perfluorinated CF bonding Carbon oxygen double bond (C O) Carbon oxygen single bond (C–O)

respectively. The variations in the O1s peaks correspond well to the C1s peak variations due to oxyfluorination. More single C–O bonds were produced by the oxyfluorination process. The F1s deconvolution peaks are presented in Fig. 3(c), and their surface composition and assignments are presented in Tables 1 and 2, respectively. The variation in the F1s peaks also corresponds well with the C1s peak variation due to oxyfluorination. Based on the above results, it can be confidently stated that the hydrophobic surface of the MWCNTs was modified by hydrophilic functional groups (C–O and C O bonds). Additionally, the dispersion of the MWCNTs in hydrophilic polyurethane solution was improved by hydrophilic functional groups induced on the MWCNTs. 3.3. Surface morphology investigation using FE-SEM images Fig. 4 shows images of the surface morphology of the PU, CAPU, and OCA-PU samples, based on the effects of oxyfluorination treatment of the MWCNTs. The PU samples showed an average electrospun fiber diameter of 240 ± 50 nm. Oval-shaped beads were observed after the addition of MWCNTs and aluminum hydroxide, as shown in Fig. 4(b) and (c). These oval-shaped beads may form as a result of aluminum hydroxide clustering. The CN-PU sample did not show any oval-shaped beads (data not shown). MWCNTs were observed on the surface of the aluminum hydroxide beads, as displayed in Fig. 4(d) and (e), indicating that the interfacial inter-

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689

Table 2 Peak parameters of the different components of the C1s, O1s and F1s spectra. Component

Peak position (eV)

C(1) C(2) C(3) C(4) C(5) C(6) O(1) O(2) F(1) F(2) F(3)

284.52 285.91 286.72 287.73 289.32 292.85 531.23 532.54 686.73 687.74 689.52

Pristine MWCNTs

Oxyfluorinated MWCNTs

FWHM (eV)

Concentration (%)

FWHM (eV)

Concentration (%)

1.87 1.84 1.38 – – – 1.78 2.12 – – –

85.72 8.45 5.83 – – – 37.64 62.36 – – –

1.68 1.48 0.87 1.32 1.71 1.42 2.12 2.11 2.18 3.19 1.68

69.21 13.13 4.75 2.98 8.48 1.45 27.81 72.19 23.27 61.24 15.49

actions between the aluminum hydroxide and the MWCNTs were promoted by oxyfluorination of the MWCNTs.

(IDT) increased due to the flame-retardant additives, as shown in Fig. 5(a). The PU sample had an IDT around 130 ◦ C, whereas the IDT of the CA-PU and OCA-PU samples was observed around 250 ◦ C. This demonstrates the effect of additives in delaying the initial degradation of polyurethane. For temperatures higher than the IDT, all samples showed a significant weight loss in the temperature range of 300–450 ◦ C. This weight loss was caused by the degradation of

3.4. Anti-oxidation behavior characterized by TGA The anti-oxidation behavior was investigated by TGA, with the results shown in Fig. 5. The initial degradation temperature

a

b

100 80 60

PU CN-PU

40

CA-PU OCA-PU

20

Weight (%)

Weight (%)

AH-PU

5-PU

80

40-PU

60 40

0 0

100

200

300

400

500

600

0

100

80

10-CN-PU

20-CN-PU 40-CN-PU

60 40

Weight (%)

d

100 5-CN-PU

200

300

400

500

600

o Temperature ( C)

o Temperature ( C)

c

20

100 5-AH-PU

80

20-AH-PU

10-AH-PU

40-AH-PU

60 40 20

0

0 0

100

200

300

400

500

600

0

100

o Temperature ( C)

200

300

400

500

600

o Temperature ( C)

e

f 100 80

10-CA-PU

20-CA-PU 40-CA-PU

60 40 20

Weight (%)

100 5-CA-PU

Weight (%)

20-PU

10-PU

20

0

Weight (%)

100

5-OCA-PU 10-OCA-PU

80

20-OCA-PU 40-OCA-PU

60 40 20

0

0 0

100

200

300

400

Temperature ( oC)

500

600

0

100

200

300

400

500

600

Temperature ( oC)

Fig. 5. TGA curves for heating rates of 5, 10, 20, and 40 ◦ C/min; the heating rate is indicated as a number in front of the sample name.

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the polyurethane. The degradation rate was lowered due to the effects of the flame-retardant additives. These results suggest that MWCNTs are useful for generating a charred layer that can act as a protectant from oxygen radicals (see Section 4 for a more detailed explanation). In the case of the added aluminum hydroxide, the weight loss was apparently caused by the endothermic degradation of the aluminum hydroxide, as shown in the following [35]: 2Al(OH)3 → 2AlO·OH + 2H2 O

(1)

2Al(OH)3 → Al2 O3 + 3H2 O

(2)

2AlO·OH → Al2 O3 + H2 O,

(3)

where reactions (1)–(3) occurred around 245 ◦ C, 320 ◦ C, and 550 ◦ C,

3.5. Integral procedure decomposition temperature (IPDT) of the samples The IPDT was calculated by the following equations [36]: IPDT = A∗ K∗ (T f − T i ) + T i

(4)

∗

(5)

∗

(6)

A = (S 1 + S 2 )/(S 1 + S 2 + S 3 ) K = (S 1 + S 2 )/S 1 ,

where Ti is the initial experimental temperature and Tf is the final experimental temperature. The areas of S1 , S2 , and S3 are depicted by Doyle’s proposition in Fig. 6(a) [37], and the calculated IPDT is presented in Fig. 6(b). For all samples, the IPDT increased with increasing heating rate, and the IPDT increased due to the effects of the retardant additives and oxyfluorination treatment. This indicates that the flame-retardant properties were improved. 3.6. Activation energy The activation energy was evaluated to investigate the antioxidation behavior of the samples. The activation energy (Ed ) was calculated using the following equation, which was derived from the equations published by Flynm, Wall, and Ozawa in [37]: Ed = −

  R  log ˚

C

(1/Tr )

,

(7)

where Tr is the weight loss temperature and ˚ is the heating rate (◦ C/min). C is a constant equal to 0.4521. The calculated activation energy is presented in Fig. 7. The activation energy increases in the following order: PU, CN-PU, AH-PU, CA-PU, and OCA-PU.

Fig. 6. IPDT of samples at various heating rates: (a) Doyle’s proposition and (b) IPDT.

The activation energy increased from 110 to 314 kJ/mol, which is almost a threefold increase between the PU and OCA-PU samples. This increased activation energy, meaning that a greater energy is required to activate the sample, is attributed to the retardant additives and the oxyfluorination treatment of the MWCNTs. 3.7. Endothermic effects of the flame-retardant additives To demonstrate the thermal effects of aluminum hydroxide and MWCNTs, DSC curves are presented in Fig. 8. The original DSC curve

Activation energy (KJ/mol)

respectively [35]. By generating water, this endothermic reaction would be carried out in the polyurethane fibers. The synergistic effects of MWCNTs and aluminum hydroxide were observed in the CA-PU sample, as shown by the higher IDT and residual weight. These synergistic effects may be attributed to the retardant volatilization process of water in the polyurethane fibers caused by the absorption of the MWCNTs (MWCNTs have a high absorption ability). The endothermic reaction might then be sustained by avoiding rapid water consumption. Oxyfluorination effects can be observed over 450 ◦ C by comparing the CA-PU and OCA-PU samples. In this comparison, OCA-PU showed a higher residual weight than CA-PU. This result is attributed to the improved interfacial affinity between the polyurethane and MWCNTs, caused by the oxyfluorination treatment. TGA investigations of each sample were also performed at various heating rates to predict the anti-oxidation behavior during combustion and to calculate the activation energy. The TGA results evaluated at 5, 10, 20, and 40 ◦ C/min are depicted in Fig. 5. In every case, the curve is shifted to a higher temperature, as shown in Fig. 5. The degree of shifting decreased with the additives, indicating that the additives reduced the effects of the heating rate on the anti-oxidation properties of the sample.

350 300 250 200 150 100 50 0 PU

CN-PU AH-PU CA-PUOCA-PU

Fig. 7. Activation energy of samples.

Endo

J.S. Im et al. / Materials Chemistry and Physics 126 (2011) 685–692

aluminum hydroxide additive (AH-PU, CA-PU, and OCA-PU) exhibited a sharp peak around 420 ◦ C. This peak can be attributed to the release of water from aluminum hydroxide, as explained in Section 3.4. The peak intensity increased with the addition of MWCNTs and with oxyfluorination of the MWCNTs. The release of water was apparently occurred slowly by the absorption properties of the MWCNTs. This optimizes the retardant effects of aluminum hydroxide by hindering the initial burst of released water. This effect was maximized by the improved interface between the water-absorbant MWCNTs and the polyurethane.

PU CN-PU AH-PU CA-PU OCA-PU 100

200

691

4. Discussion

300

400

500

600

4.1. Suggested mechanism of the aluminum hydroxide flame retardant

o

Temperature ( C) Fig. 8. DSC curves of samples.

from the PU sample shows the endothermic peak in the range of 370–420 ◦ C. This is caused by the decomposition of the synthesized polyurethane. The CN-PU sample also presented similar curves without any significant changes, but the samples with the

The suggested retardant role of aluminum hydroxide is presented in Fig. 9(a). Oval-shaped aluminum hydroxide beads were incorporated into the polyurethane fibers, as explained in Section 3.3. These aluminum hydroxide beads played an important role, acting as energy storage tanks by releasing water upon decomposition of the aluminum hydroxide (as explained in Section 3.4). This allows for the attenuation of external heat energy, resulting in improved anti-oxidation properties.

Fig. 9. Suggested mechanism of flame-retardant additives and oxyfluorination: (a) effects of aluminum hydroxide, (b) and (c) MWCNTs and (d) oxyfluorination.

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4.2. Suggested mechanism of the MWCNT flame retardant In general, the decomposition of a polymer is primarily caused by oxygen radicals [38]. The reaction of oxygen radicals can be blocked by the formation of a thin protective film, as depicted in Fig. 9(b). The MWCNTs seem to assist in promoting a thin protective film due to their high thermal conductivity, allowing them to generate a charred layer on the polyurethane fibers. One can assume that the free radical scavenger effect of the MWCNTs is the main reason for the improvement in the anti-oxidation property of the fibers. Thus, the main role of the MWCNTs on the surface of the polyurethane fibers is to form compact charred layers that act as a heat barrier and as thermal insulation. MWCNTs inside the polyurethane fibers also play an important role in the anti-oxidation properties. Recently, some researchers have reported that carbon nanotubes can act as effective flameretardant additives if they form a jammed network structure in the polymer matrix, such that the material behaves rheologically like a gel [39]. This result might be attributed to the enhanced structural network inside the polyurethane fibers [40]. Although the strength of polyurethane fibers would be weakened by the addition of aluminum hydroxide, MWCNTs can compensate for this disadvantage. Flame retardation by MWCNTs may also hinder the spread of fire due to the formation of a gel-type polymer, even during combustion. 4.3. Oxyfluorination effects of MWCNTs for improved flame-retardant properties To display the improved anti-oxidation properties of the polyurethane fibers, the oxyfluorination effect on the MWCNTs is presented in Fig. 9(c) and (d). The oxyfluorination treatment of the MWCNTs caused increased affinity between the MWCNTs and the aluminum hydroxide, as depicted in Fig. 9(c) (see also the SEM images and explanation in Fig. 4). Without oxyfluorination treatment, the MWCNTs were aggregated and showed poor dispersion. However, the dispersion of MWCNTs was improved by the oxyfluorination treatment. This improved dispersion of MWCNTs can promote both the generation of a protective thin film and the formation of a structural network inside and outside the polyurethane fibers. 5. Conclusions Both aluminum hydroxide and MWCNTs were incorporated into polyurethane fibers as flame-retardant additives. These additives were incorporated using the electrospinning method. The hydrophobic surfaces of the MWCNTs were modified with hydrophilic functional groups by using oxyfluorination to improve the dispersivity of the MWCNTs in the polyurethane fibers. Aluminum hydroxide acted as an energy storage tank by releasing water, resulting in an endothermic reaction. The MWCNTs promoted the formation of a charred layer as a protective film for preventing the decomposition of the polyurethane by oxygen

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