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cellulose nanofibers hybrid carbon aerogel by freeze drying with ultra-low phosphorus
Journal Pre-proof Flame-retardant polyvinyl alcohol/cellulose nanofibers hybrid carbon aerogel by freeze drying with ultra-low phosphorus
Yajun Huang, Ting Zhou, Song He, Huan Xiao, Huaming Dai, Bihe Yuan, Xianfeng Chen, Xiaobing Yang PII:
S0169-4332(19)32591-7
DOI:
https://doi.org/10.1016/j.apsusc.2019.143775
Reference:
APSUSC 143775
To appear in:
Applied Surface Science
Received date:
9 July 2019
Revised date:
19 August 2019
Accepted date:
23 August 2019
Please cite this article as: Y. Huang, T. Zhou, S. He, et al., Flame-retardant polyvinyl alcohol/cellulose nanofibers hybrid carbon aerogel by freeze drying with ultra-low phosphorus, Applied Surface Science(2018), https://doi.org/10.1016/j.apsusc.2019.143775
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© 2018 Published by Elsevier.
Journal Pre-proof Flame-ratardant polyvinyl alcohol/cellulose nanofibers hybrid carbon aerogel by freeze drying with ultra-low phosphorus Yajun Huanga, b, c, Ting Zhoub, Song Hea, 1, Huan Xiaoa, Huaming Daia , Bihe Yuana, Xianfeng Chena, Xiaobing Yangd,
2
a. School of Satety Science and Emergency Management, Wuhan University of Technology, Luoshi Road 122, Wuhan, 430070, PR China b. State Key Laboratory of Fire Science, University of Science and Technology of China, Jinzhai Road 96, Hefei, 230027, PR China c. Division of Fire Safety Engineering, Lund University, Box 118, Lund, SE-22100,
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Sweden
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d. State Key Laboratory of NBC Protection for Civilian, Beijing 100191, PR China Abstract
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Polyvinyl alcohol/cellulose nanofibers hybrid aerogel was prepared under freeze drying method. To improve the aerogels’ anti-combustion performance, 0.8wt%
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microencapsulated ammonium polyphosphate (MCAPP) was loaded as the flame retardant. Aerogels with extremely low density (~0.06 g/cm3) and excellent
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mechanical performance (Young’s modulus: 1.045 MPa) can be obtained. The resulted aerogel also exhibit considerable thermal insulation ability (thermal
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conductivity: ~0.04 W/m·K). Experimental results indicate that the value of limiting oxygen index increases from 19.5% to 37.5% when loading 0.8 wt% MCAPP.
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Accordingly, the aerogels’ peak heat release rate decreased significantly from 222.44 to 107.84 kW/m2. The char residue rises when introducing MCAPP and the char’s integrity improves a lot after combustion. The fire performance index and fire growth index increases and falls respectively, indicating improved anti-combustion performance. X-ray photoelectron spectroscopy results show C=O bonds would be increased for the esterification of phosphoric acid from MCAPP. In addition, the production of carbonate can be prohibited while combustion when loading MCAPP. Keywords: polyvinyl alcohol; cellulose nanofibers; flame retardant; aerogel 1. Introduction Aerogels are porous material comprised of porous cells. With several outstanding properties, large specific surface area, high porosity, excellent thermal 1 2
Corresponding author. Tel: 861 807 154 7487 E-mail: [email protected] (Song He) Corresponding author. Tel: 861 381 087 1208 E-mail: [email protected] (Xiaobing Yang) 1
Journal Pre-proof insulation property [1, 2], etc., for example, considerable amount of attention has been caught by silica aerogel for their great market prospect to be used in lots of fields [3-7]. Aerogels are prepared by wiping the in-pore solvent out of the wet gel while making sure the integrity of pore structure. Supercritical fluid drying (SFD) method [8-10] can wipe out the pore liquid within the wet gel with the slightest damage to the pore structure. However, the supercritical drying condition is harsh (5–10MPa) to reach [8], which need expensive facilities and much energy should be cost. Ambient pressure drying (APD) method cannot get rid of capillary pressure, which may damage the pore structure [11, 12]. Freeze drying (FD) method can provide capillary
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pressure free environment while displacing the solvent out [13]. And the freeze drying
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condition is not so hard to reach.
Silica aerogel is the most used aerogel product which has been commercialized.
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However, the poor mechanical performance of the pure silica aerogel has largely limited its application. Several reinforcement strategies have been proposed, such as
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chemical strengthening [14] and physical reinforcement [15]. However, important parameters like density, thermal conductivity may be degraded after physical or
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chemical reinforcement [15, 16]. In addition, the force between aerogel particles and reinforcement substance may not so strong for application [17].
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Organic aerogels originated from biocompatible and renewable raw material, like cellulose, have drawn people’s attention a lot for their excellent mechanical properties.
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The cellulose is kind of biocompatible and renewable material which is abundant in terms of sources [18]. Hayase et. al, [19] prepared biocomposite aerogels with high thermal insulation and bendability under SFD utilizing polymethylsilsesquioxane and cellulose as the raw materials. High tensile strength (97 MPa tensile strength, 161 MPa·m3·kg−1 specific strength) cellulose nanofibril aerogel membranes has been made using APD method [20]. Pragya et. al, [21] have prepared low density and high strength nano-fibrillated cellulose aerogel under FD. The cellulose based aerogel can be widely used in many fields including thermal insulation [22], adsorption [23-25], food packing [26] and catalyst [27]. However, the low thermal stability and high flammability of cellulose based aerogel restrict its application [28]. TG-DSC analysis indicated that nanocellulose based aerogel can be thermal decomposed in ~200℃ [29]. Improving the fire-resistant performance and thermal stability have become a hot 2
Journal Pre-proof topic. And several strategies have been provided, including loading retardant additives. According
to
the
previous
studies,
flame
retardant
additives
like
sodium-montmorillonite (MMT) [30], graphene oxide [31], magnesium hydroxide [32] et. al, have been introduced to prepare cellulose based flame retardant aerogel. Among the flame retardants, ammonium polyphosphates (APP) attracts lots of attention [33]. It has the priority over the others due to its high flame retardant efficiency [34]. Moreover, APP does not produce any other pollutants [35]. In this study, microencapsulated APP (MCAPP) which can form a more stable charred layer than a conventional APP [36] was introduced in the cellulose based
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aerogel as the flame retardant additive. Polyvinyl alcohol (PVA), acting as chemical
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reinforcement, was used to improve the aerogel’s mechanical performance. 2. Experimental Section
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2.1 Preparation of Cellulose Nanofibers (CNFs)
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The CNF used in this research were prepared in accordance with our previous study [18].
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2.2 Preparation of Cross-Linked PVA/CNF/APP Aerogels MCAPP, microencapsulated by melamine-formaldehyde resin, was purchased
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from KeYan (Anhui, China) where the weight ratio of melamine formaldehyde : APP is 1:10. Poly(vinyl alcohol) (PVA, 99% hydrolyzed) whose molecular weight ranges
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31000－50000 came from Sigma-Aldrich. MCAPP with various weights were mixed in the CNF solution (30 g, 10 wt %) under condition of 10000 rpm stirring within 30 min. The resulted solution was denoted as A. At the same time, 10 wt % PVA/deionized water solution was prepared under 90 ℃ and the solution was agitated for 8 h. And then solution B can be obtained. In the next step, solution B was slowly added to the A solution and they were stirred for 2h under ultrasonic bath during which hydroxyls within CNF chains and PVA chains [18] crosslinked together. As a result, the gel can be obtained. The composition of the samples are displayed in Table 1. The resulting gels were dried under freeze & vacuum condition [18]. Table 1 Overview of cross-linked aerogels with MCAPP prepared by freeze drying
Sample ID
PVA g
CNF g
MCAPP g
Densidy g/cm3
Thermal conductivity W/m·K
PVA/CNF
20
30
0
0.047±0.001
0.0380±0.0003
3
Journal Pre-proof PVA/CNF/APP1
20
30
0.16
0.050±0.001
0.0390±0.0003
PVA/CNF/APP2
20
30
0.24
0.052±0.002
0.0398±0.0005
PVA/CNF/APP3
20
30
0.32
0.055±0.002
0.0405±0.0005
PVA/CNF/APP4
20
30
0.40
0.060±0.002
0.0413±0.0007
2.3 Characterization The mechanical performance was tested by electronic dynamic & static fatigue testing machine (E3000K8953, Instron). The samples’ microstructure was investigated by a field emission scanning electron microscope (FESEM) from FEI
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(FE-SEM, SIRION200). The chemical structure of the samples were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet 8700, Thermo Fisher
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Scientific) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, Thermo-VG Scientific). The thermal stability was studied by thermogravimetric
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analyzer (SDT Q600, TA). The transient hot wire method was applied to investigate
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the samples’ thermal insulation property (λ) (TC3000E, Xiaxi technology). 3. Results and Discussion
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3.1 Apparent Density and Thermal Conductivity Table 1 shows the density and thermal conductivity of cross-linked aerogels. The
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aerogel samples exhibit relatively low density ranging from 0.047 ~ 0.061 g/cm3. The thermal conductivity and density share the same variation tendency. Both the thermal
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conductivity and density increase monotonously with the rise of MCAPP. According to the previous study [18], PVA/CNF aerogel is porous material containing lots of pores. MCAPP particles can take up some of the pores, leading to the reduction of porosity. As a result, the density would increase. On the other hand, the reduction of porosity would intensify the heat transfer through solid phase, giving rise to the increasing of apparent thermal conductivity. 3.2 Thermal Stability
4
Journal Pre-proof 100
weight (%)
80 70
Deriv. Weight (a.u.)
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
90
60 50 40 30
PVA/CNF PVF/CNF/APP1 PVF/CNF/APP2 PVF/CNF/APP3 PVF/CNF/APP4
20 10
a
100
200
300
400
500
600
700
800
temperature C) 250
100
b
200
300
400
500
600
700
800
temperature C) PVA/CNF PVA/CNF/APP1
200
PVA/CNF/APP2
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HRR (W/g)
PVA/CNF/APP3 PVA/CNF/APP4
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150 100
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50
100
200
c
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0 300
400
500
600
Temperature (C)
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Figure 1 TG (a) and DTG (b) and heat release rate by micro combustion calorimeter (c) of the as-prepared samples
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Table 2 Related thermal data of PVA/CNF and its composite aerogels
278.4
PVA/CNF/APP1
138.1
218.6
253.0
273.1
309.2
385.2
257.2
449.9
22.9
PVA/CNF/APP2
143.4
212.8
242.1
258.3
289.7
397.9
257.2
449.9
27.3
PVA/CNF/APP3
184.7
218.3
243.6
261.3
308.1
420.6
257.2
449.9
34.7
PVA/CNF/APP4
171.3
215.3
243.1
264.3
304.5
418.2
257.2
449.9
33.3
Tini% (℃)
PVA/CNF
200.4
T10% (℃)
T20% (℃)
T30% (℃)
T40% (℃)
T50% (℃)
Tpeak1 (℃)
Tpeak2 (℃)
Char residue （800 ℃）(%)
245.3
267.9
285.5
291.8
291.1
/
14.6
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Samples
The samples’ thermal stability were investigated through thermal gravity analysis in N2 atmosphere. And the results are shown in Figure 1 and Table 2. The temperature at 5, 10, 20, 30, 40 and 50 wt% weight loss are denoted as Tini, T10%, T20%, T30%, T40%, and T50%, respectively. It’s obvious that all of the samples exhibited similar mass loss patterns with three main stages. In the first stage (<100 °C), the slight weight loss can be mainly attributed to the evaporation of moisture [37]. The next stage occurred at ~250℃-~400℃. For the sample PVA/CNF, the weight loss can be related to the decomposition
of
PVA/CNF
chains
[18]. 5
However,
the
weight
loss
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Journal Pre-proof PVA/CNF/APP1, PVA/CNF/APP2, PVA/CNF/APP3 and PVA/CNF/APP4 can be the synergistic effect of APP [38] and PVA/CNF decomposition. At this stage, APP could be thermally decomposed to NH3 and H2O for the decomposition of the terminal chain [-OP(O)(ONH4)2] and midchain [-OP(O)(ONH4)-] [38]. Owing to the carbonization reaction between CNF/PVA chains induced by APP in CNF/PVA aerogel, the carbonaceous char with strengthened structure formed [36]. And the strengthened char could further act as protective layer by covering the surface of aerogel matrix and thus limit the permeability of volatile gases [39]. As a result, the weight loss of samples with MCAPP turned to be less heavily than that of PVA/CNF
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aerogel. (Figure 1a). Meanwhile, the heat release of PVA/CNF aerogel is much
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smaller than that of samples with MCAPP. The last stage happens at ~ 450℃, on account of degradation of the formed char [40, 41], combustible volatile gases
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escaped and oxidized. Thus, remarkable exothermic peaks can be observed at MCC curves of PVA/CNF/APP1, PVA/CNF/APP2, PVA/CNF/APP3 and PVA/CNF/APP4
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(Figure 1c).
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Referring to Table 2, the decomposition temperatures of PVA/CNF at 5%, 10%, 20% and 30% mass losses are observed higher than those of the samples loading with MCAPP. However, the decomposition temperatures of PVA/CNF at 40% and 50%
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mass losses are obviously lower than those of the samples loading with MCAPP. The reduction in decomposition temperatures of Tini, T10%, T20% and T30% when loading
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MCAPP is mainly attributed to the phosphorylation and dehydration effect of APP particles in aerogel. And the phosphorylation and dehydration effect could effectively promote the formation of carbonaceous char during heating process, leading to the rising of T40% and T50%. And for the same reason, the char residue at 800℃ increases with the rising of MCAPP loading. 3.3 Combustion Behavior To study the samples’ combustion behavior, cone calorimeter tests were conducted. The heat flux was set 35 kW/m2. And the results are presented in Figure 2 and Table 3. It’s obvious the heat release rate (HRR) and total heat released (THR) decrease with the rise of MCAPP (Figure 2a and b). Specifically, the THR falls from 7.48 MJ/m2 to 2.76 MJ/m2 and the peak heat release rate (pHRR) decreases to 107.84 kW/m2 from 222.44 kW/m2 monotonously (Table 3). Similarly, both the CO and CO2 production show decreasing tendency as the increase of MCAPP (Figure 2c and d). 6
Journal Pre-proof This is due to the existence of APP in aerogel matrix of PVA/CNF which promotes the carbonization between the PVA and CNF chains and strengthens the structure of carbonaceous char [34]. Considerable amount of C is fixed by carbonization, result in low releasing rate of C series volatile like CO and CO2 as well as the relevant heat release (ie. pHRR and THR). In order to evaluate the samples’ fire hazard, the fire performance index (FPI) and fire growth index (FGI) are introduced. The FPI is ratio of TTI (time to ignition) to pHRR, indicating the propensity to flashover in a full-scale fire. And a smaller FPI means a higher possibility for flashover [42]. Shown in Table 3, the FPI rises from
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0.05 s·m2/kW to 0.13 s·m2/kW, indicating less probability of flashover would happen
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to the sample if more MCAPP is introduced. The fire growth index (FGI) refers to the ratio of the peak heat release rate (PHRR) to the peak time, and a greater FGI
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indicates that the fire would grow quickly once lit. The FGI falls to 4.94 kW/ m2·s from 3.00 kW/ m2·s when increasing the MCAPP content (Table 3). Both the FPI and
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FGI indicate lower fire hazard when MCAPP is introduced. 8
250
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
0 0
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50
50
100
150
5
250
300
2
0
b
50
100
150
200
250
300
Time (s) 2.5 PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
2.0
CO2 (vol %)
0.05
3
0
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
0.06
4
1
Time (s)
0.07
CO (mg/m3)
200
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
6
THR (MJ/m2)
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100
a
7
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150
2
HRR (kW/m )
200
0.04 0.03 0.02
1.5 1.0 0.5
0.01 0.0
0.00
c
0
50
100
150
200
250
300
time (s)
d
0
50
100
150
200
250
300
time (s)
Figure 2 Heat release rate (a), total heat released (b), CO (c) and CO2 (d) production of the samples in cone calorimeter tests Table 3 Relevant parameters in the cone calorimeter tests under 35 kW/m2 Samples
THR (MJ/m2)
pHRR (kW/m2)
TTI (s) 7
FPI (s·m2/kW)
FGI (kW/ m2·s)
Char residue (%)
Journal Pre-proof PVA/CNF
7.48
222.44
12.10
0.05
4.94
22.22
PVA/CNF/APP1
4.34
176.08
14.70
0.08
4.19
29.73
PVA/CNF/APP2
3.15
163.05
17.20
0.11
4.18
33.33
PVA/CNF/APP3
3.67
134.88
16.20
0.12
3.21
31.58
PVA/CNF/APP4
2.76
107.84
14.00
0.13
3.00
39.39
The sample pictures before and after cone caloriemeter test is presented in Figure 3a, and the char residue is listed in Table 3. It’s clear that the integrity of char turns to be better with the rise of MCAPP (Figure 3). And more char residue can be obtained
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after exposed under 35kW/m2 when more MCAPP is introduced.
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Figure 3 Sample pictures before and after cone calorimeter tests (35kW/m2)
Figure 4 Raman spectra of the samples after cone calorimeter tests (35kW/m2)
Raman spectra is used to investigate the char residue of the samples after combustion which is shown in Figure 4. It’s clear that 2 strong peaks at ~1357 cm−1 (D band) and 1605 cm−1 (G band) are displayed in the spectra. The intensity proportion of D and G bands (denoted as ID/IG) is often used to assess the 8
Journal Pre-proof graphitization degree of char residue. In detail, low value of ID/IG indicates high graphitization degree of char, where the char residue can better shield the underlying structure from been destroyed under high temperature/heat flux. It’s obvious that the ID/IG values of samples with MCAPP series are almost the same (Figure 4). While lower ID/IG values are realized after introduction of MCAPP, demonstrating the enhancing function of MCAPP on the graphitization degree of char. The width of D band is another indicator of disordered canbon. The introduction of MCAPP also causes the narrowing of D band (Figure 4), indicating the improvement of graphitization degree of char [43, 44]. Thus, the combustion of underlying polymer
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can be prohibited effectively.
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Figure 5 SEM iamges of the samples before (a, b, c, d, e) and after (f, g, h, i, j) cone calorimeter tests (a, f: PVA/CNF b, g: PVA/CNF/APP1; c, h: PVA/CNF/APP2; d, i: PVA/CNF/APP3; e, j: PVA/CNF/APP4)
Figure 6 EDS mapping of C (a, d), O (b, e), P (c, f) in PVA/CNF/APP1 (a, b, c) and PVA/CNF/APP4 (d, e, f)
The microstructure of the samples are presented in Figure 5a-j. It’s clear that no prominent difference can be found among the samples before cone calorimeter tests (Figure 5a, b, c, d and e). To figure out the element distribution, the EDS mapping of the samples PVA/CNF/APP1 and PVA/CNF/APP4 has been coducted (Figure 6). It’s clear that the phosphorus disperses uniformly within the samples. However, for the existence of MCAPP, various appearance can be seen on the samples after cone 9
Journal Pre-proof calorimeter tests. Compared with PVA/CNF (Figure 5f), lots of intumescence can be found on the surface of PVA/CNF/APP1 (Figure 5g) and PVA/CNF/APP2 (Figure 5h). The intumescence on PVA/CNF/APP2 (Figure 5h) distributed more densely than that of PVA/CNF/APP1 (Figure 5g). The intumescence can be caused by MCAPP. MCAPP decomposed itself and dehydrated PVA and CNF when heated under 35kW exposure. Then the char formed. Under the shock of gases generated from decomposition, the intumescence shape. As to PVA/CNF/APP3 and PVA/CNF/APP4, excess MCAPP was loaded, the intumescence connect with each other to form integral one, as a result, no prominent intumescence can be observed on the surface
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(Figure 5i and j).
Figure 7 LOI and UL-94 test results of the samples
The samples’ flammability were explored by LOI (Limiting Oxygen Index) and UL-94 (Vertical Burning Test), and the results are shown in Figure 7. LOI is an important parameter and defined as the minimum volume of oxygen for maintaining the combustion of the material. In the experiment, the PVA/CNF aerogel was ignited immediately with vigorous flame, and finally no material remained. It is indicated that neat PVA/CNF aerogel is of high flammability whose LOI value is as low as 19.5%. With the addition of APP, the flame-retardant performance of the samples improved significantly. As the percentage of APP increased from 10% to 15%, the LOI increased from 27% to 37.5%, all samples self-extinguished and passed the V-0 rating in the UL-94 teats. For PVA/CNF/APP1, The sample had droplets during the burning process and the black residue was the product of carbonized PVA and cellulose. Above all, both the acid source and gases source can be originated from the MCAPP, 10
Journal Pre-proof
of
making it intumescent flame retardant.
-p
25000 20000 15000
lP
10000 5000
25000
Intensity (a.u.)
20000 15000 10000 5000
c
20000
288 286 Binding Energy (eV)
original curve 284.8 286.3 287.95 289.28 fitted curve
284
282
15000 10000 5000 0 292
b
290
25000
288 286 Binding Energy (eV)
284
282
original curve 284.8 286.3 287.95 289.28 290.3 fitted curve
20000 Intensity (a.u.)
30000
290
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0 292
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a
original curve 284.8 286.24 288.79 290.3 fitted curve
25000
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Intensity (a.u.)
30000
30000
original curve 284.8 286.24 288.79 fitted curve
Intensity (a.u.)
35000
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Figure 8 FTIR spectra of the samples
15000 10000 5000
0 292 291 290 289 288 287 286 285 284 283 282 Binding Energy (eV)
d
0 292
290
288 286 Binding Energy (eV)
284
282
Figure 9 C 1s spectrum of PVA/CNF (a, b (35 kW/m2)) & PVA/CNF/APP4 (c, d (35 kW/m2)) Table 4 XPS results of the samples before and after combustion
Sample
C-C
C-O
C=O
O=C-O
CO32-
0 2.4%
0 0
Before cone calorimeter test PVA/CNF PVA/CNF/APP4
52.5% 45.0%
42.8% 51.0%
4.7% 1.6%
After cone calorimeter test (35 kW/m2) PVA/CNF PVA/CNF/APP4
62.7% 58.3%
21.0% 28.8% 11
2.0% 6.5%
0 3.5%
14.3% 2.9%
Journal Pre-proof The FTIR spectra of PVA/CNF aerogel and PVA/CNF/APP4 aerogel are shown in Figure 8. The peak at 3408 cm-1 can be ascribed to –OH stretching vibration in CNF [37]. For these two samples, the characteristic peaks of PVA are observed at 2918 and 1088 cm-1 which are caused by the asymmetric stretching vibrations of – CH2 and the stretching vibrations of C-O, respectively [45]. The typical absorption peak at 1256 cm-1 is due to the stretching vibration of P=O bonds in MCAPP [46]. The peaks at 881 cm-1 represent the asymmetric stretching vibration of P-O group [47]. The 2 typical characteristic peaks indicate the effective introduction of APP in the resulted sample.
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To figure out the chemical composition of the samples before and after
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combustion, XPS analysis [48, 49] was conducted and the results are listed in Figure 9 and Table 4. According to the study of Tsuguyuki et al. [50], bonds like -COONa and
-p
-CHO do exist in CNF. The C 1s core-level spectrum of PVA/CNF and PVA/CNF/APP4 can be curve-fitted with several peak components (Figure 9), whose
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binding energies locate at 284.8 eV for C-C bonds, 286.3 eV for C-O species [51], 287.95 and 288.79 eV for C=O species [52, 53] and 289.28 eV for O=C-O bonds [52].
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The C-C bonds increase signicantly after combustion for both PVA/CNF and PVA/CNF/APP4 for the carbonization effect of phosphoric acid. In addition, the C=O
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or O=C-O bonds rise clearly for PVA/CNF/APP4 when treated under 35kW heat flux radiation which can be attributed to the esterification of phosphoric acid to PVA and
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CNF. Nevertheless, both C-O and C=O bonds in PVA/CNF dramatically decrease after treated under 35kW heat flux radiation which can be turned to CO32- after combustion (Table 4).
3.4 Mechanical performance
Shown in Figure 10, the as-prepared aerogels exhibit excellent mechanical property. According to Figure 10b, the samples go through typical strain hardening behavior with the rise of MCAPP content. The Young’s modulus (Figure 10d) which was obtained within the deformation range of 25% - 35% increases from 0.367 MPa to 1.045 MPa with the rise of density. The specific modulus (Figure 10d), calculated based on the modulus to density ratio, rises from 7.81 m2/s2 to 17.42 m2/s2 when the density increases to 0.060 g/cm3 from 0.047 g/cm3. As a result, the introduction of MCAPP can significantly improve the samples’ compression resistance. As shown in Figure 5a, b, c, d and e, APP is well dispersed in the PVA/CNF matrix with no evident 12
Journal Pre-proof orientation (Figure 6). In addition, APP contributes to the overall filler loading by 0.32wt% (PVA/CNF/APP1), 0.48wt% (PVA/CNF/APP2), 0.64wt% (PVA/CNF/APP3) and 0.8wt% (PVA/CNF/APP4). The additional filler could increase the stress transfer from the PVA/CNF matrix to the filler [54] resulting in the increase of Young’s modulus. And the Young’s modulus increases with the rising of the filler (MCAPP)
5.0
4.5
4.5
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
2.5
2.5 2.0
1.5
1.5
0.5 0.0 0
10
20
30
40
strain (%)
50
60
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b
3.0
2.0
1.0
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
0.75
Stress (MPa)
3.0
1.00
3.5
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stress (MPa)
3.5
4.0
lP
4.0
1.00
re
5.0
-p
ro
of
loading level (Figure 10).
70
0.75
0.50
0.50
0.25
0.25
0.00
0.00
1.0 0.5 0.0
80
-0.25
c
20
25
30
35
-0.25 40
Strain (%)
Figure 10 Uniaxial compression test (PVA/CNF/APP4, a), stress-strain curve (b, c) and modulus (d) of the samples
4. Conclusions In this study, PVA/CNF hybrid aerogel was prepared under freeze drying method while introducing MCAPP as the intumescent flame retardant. Aerogels with 13
Journal Pre-proof extremely low density (~0.06 g/cm3) and remarkable mechanical performance (Young’s modulus increases to 1.045 MPa when loading 0.8 wt% MCAPP ) can be obtained. The aerogel’s thermal conductivity can be controlled within 0.04 W/m·K. Experimental results indicate that LOI value increases from 19.5% to 37.5% when loading 0.8 wt% MCAPP. Accordingly, THR decreases significantly from 7.48 to 2.76 MJ/m2. The char residue rises and the char’s integrity improves a lot after combustion when loading MCAPP. The fire performance index (FPI) and fire growth index (FGI) increases and falls respectively, indicating improved anti-combustion performance. XPS results show C=O bonds would be increased for the esterification of phosphoric
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acid from MCAPP. In addition, the production of carbonate can be prohibited in case
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of combustion when loading MCAPP. Conflicts of interest
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None. Acknowledgement
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This research was financially supported by the National Key Research and Development Program of China (2017YFC0804900 & 2017YFC0804907), the
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Fundamental Research Funds for the Central Universities (WUT: 2018IVB058) and the Natural Science Foundation of China (No. 51706165). Y.J.Huang thanks for the
References
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support from China Scholarship Council (CSC).
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Highlights 1. Monolithic PVA/CNF/APP hybrid aerogels are obtained by freeze drying. 2. Aerogel flammability decreases significantly while loading 0.8 wt% APP.
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3. Tripling aerogel’s Young’s modulus has been achieved by just loading
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0.8 wt% APP.
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Yajun Huang, Ting Zhou, Song He, Huan Xiao, Huaming Dai, Bihe Yuan, Xianfeng Chen, Xiaobing Yang PII:
S0169-4332(19)32591-7
DOI:
https://doi.org/10.1016/j.apsusc.2019.143775
Reference:
APSUSC 143775
To appear in:
Applied Surface Science
Received date:
9 July 2019
Revised date:
19 August 2019
Accepted date:
23 August 2019
Please cite this article as: Y. Huang, T. Zhou, S. He, et al., Flame-retardant polyvinyl alcohol/cellulose nanofibers hybrid carbon aerogel by freeze drying with ultra-low phosphorus, Applied Surface Science(2018), https://doi.org/10.1016/j.apsusc.2019.143775
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© 2018 Published by Elsevier.
Journal Pre-proof Flame-ratardant polyvinyl alcohol/cellulose nanofibers hybrid carbon aerogel by freeze drying with ultra-low phosphorus Yajun Huanga, b, c, Ting Zhoub, Song Hea, 1, Huan Xiaoa, Huaming Daia , Bihe Yuana, Xianfeng Chena, Xiaobing Yangd,
2
a. School of Satety Science and Emergency Management, Wuhan University of Technology, Luoshi Road 122, Wuhan, 430070, PR China b. State Key Laboratory of Fire Science, University of Science and Technology of China, Jinzhai Road 96, Hefei, 230027, PR China c. Division of Fire Safety Engineering, Lund University, Box 118, Lund, SE-22100,
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Sweden
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d. State Key Laboratory of NBC Protection for Civilian, Beijing 100191, PR China Abstract
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Polyvinyl alcohol/cellulose nanofibers hybrid aerogel was prepared under freeze drying method. To improve the aerogels’ anti-combustion performance, 0.8wt%
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microencapsulated ammonium polyphosphate (MCAPP) was loaded as the flame retardant. Aerogels with extremely low density (~0.06 g/cm3) and excellent
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mechanical performance (Young’s modulus: 1.045 MPa) can be obtained. The resulted aerogel also exhibit considerable thermal insulation ability (thermal
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conductivity: ~0.04 W/m·K). Experimental results indicate that the value of limiting oxygen index increases from 19.5% to 37.5% when loading 0.8 wt% MCAPP.
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Accordingly, the aerogels’ peak heat release rate decreased significantly from 222.44 to 107.84 kW/m2. The char residue rises when introducing MCAPP and the char’s integrity improves a lot after combustion. The fire performance index and fire growth index increases and falls respectively, indicating improved anti-combustion performance. X-ray photoelectron spectroscopy results show C=O bonds would be increased for the esterification of phosphoric acid from MCAPP. In addition, the production of carbonate can be prohibited while combustion when loading MCAPP. Keywords: polyvinyl alcohol; cellulose nanofibers; flame retardant; aerogel 1. Introduction Aerogels are porous material comprised of porous cells. With several outstanding properties, large specific surface area, high porosity, excellent thermal 1 2
Corresponding author. Tel: 861 807 154 7487 E-mail: [email protected] (Song He) Corresponding author. Tel: 861 381 087 1208 E-mail: [email protected] (Xiaobing Yang) 1
Journal Pre-proof insulation property [1, 2], etc., for example, considerable amount of attention has been caught by silica aerogel for their great market prospect to be used in lots of fields [3-7]. Aerogels are prepared by wiping the in-pore solvent out of the wet gel while making sure the integrity of pore structure. Supercritical fluid drying (SFD) method [8-10] can wipe out the pore liquid within the wet gel with the slightest damage to the pore structure. However, the supercritical drying condition is harsh (5–10MPa) to reach [8], which need expensive facilities and much energy should be cost. Ambient pressure drying (APD) method cannot get rid of capillary pressure, which may damage the pore structure [11, 12]. Freeze drying (FD) method can provide capillary
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pressure free environment while displacing the solvent out [13]. And the freeze drying
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condition is not so hard to reach.
Silica aerogel is the most used aerogel product which has been commercialized.
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However, the poor mechanical performance of the pure silica aerogel has largely limited its application. Several reinforcement strategies have been proposed, such as
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chemical strengthening [14] and physical reinforcement [15]. However, important parameters like density, thermal conductivity may be degraded after physical or
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chemical reinforcement [15, 16]. In addition, the force between aerogel particles and reinforcement substance may not so strong for application [17].
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Organic aerogels originated from biocompatible and renewable raw material, like cellulose, have drawn people’s attention a lot for their excellent mechanical properties.
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The cellulose is kind of biocompatible and renewable material which is abundant in terms of sources [18]. Hayase et. al, [19] prepared biocomposite aerogels with high thermal insulation and bendability under SFD utilizing polymethylsilsesquioxane and cellulose as the raw materials. High tensile strength (97 MPa tensile strength, 161 MPa·m3·kg−1 specific strength) cellulose nanofibril aerogel membranes has been made using APD method [20]. Pragya et. al, [21] have prepared low density and high strength nano-fibrillated cellulose aerogel under FD. The cellulose based aerogel can be widely used in many fields including thermal insulation [22], adsorption [23-25], food packing [26] and catalyst [27]. However, the low thermal stability and high flammability of cellulose based aerogel restrict its application [28]. TG-DSC analysis indicated that nanocellulose based aerogel can be thermal decomposed in ~200℃ [29]. Improving the fire-resistant performance and thermal stability have become a hot 2
Journal Pre-proof topic. And several strategies have been provided, including loading retardant additives. According
to
the
previous
studies,
flame
retardant
additives
like
sodium-montmorillonite (MMT) [30], graphene oxide [31], magnesium hydroxide [32] et. al, have been introduced to prepare cellulose based flame retardant aerogel. Among the flame retardants, ammonium polyphosphates (APP) attracts lots of attention [33]. It has the priority over the others due to its high flame retardant efficiency [34]. Moreover, APP does not produce any other pollutants [35]. In this study, microencapsulated APP (MCAPP) which can form a more stable charred layer than a conventional APP [36] was introduced in the cellulose based
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aerogel as the flame retardant additive. Polyvinyl alcohol (PVA), acting as chemical
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reinforcement, was used to improve the aerogel’s mechanical performance. 2. Experimental Section
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2.1 Preparation of Cellulose Nanofibers (CNFs)
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The CNF used in this research were prepared in accordance with our previous study [18].
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2.2 Preparation of Cross-Linked PVA/CNF/APP Aerogels MCAPP, microencapsulated by melamine-formaldehyde resin, was purchased
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from KeYan (Anhui, China) where the weight ratio of melamine formaldehyde : APP is 1:10. Poly(vinyl alcohol) (PVA, 99% hydrolyzed) whose molecular weight ranges
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31000－50000 came from Sigma-Aldrich. MCAPP with various weights were mixed in the CNF solution (30 g, 10 wt %) under condition of 10000 rpm stirring within 30 min. The resulted solution was denoted as A. At the same time, 10 wt % PVA/deionized water solution was prepared under 90 ℃ and the solution was agitated for 8 h. And then solution B can be obtained. In the next step, solution B was slowly added to the A solution and they were stirred for 2h under ultrasonic bath during which hydroxyls within CNF chains and PVA chains [18] crosslinked together. As a result, the gel can be obtained. The composition of the samples are displayed in Table 1. The resulting gels were dried under freeze & vacuum condition [18]. Table 1 Overview of cross-linked aerogels with MCAPP prepared by freeze drying
Sample ID
PVA g
CNF g
MCAPP g
Densidy g/cm3
Thermal conductivity W/m·K
PVA/CNF
20
30
0
0.047±0.001
0.0380±0.0003
3
Journal Pre-proof PVA/CNF/APP1
20
30
0.16
0.050±0.001
0.0390±0.0003
PVA/CNF/APP2
20
30
0.24
0.052±0.002
0.0398±0.0005
PVA/CNF/APP3
20
30
0.32
0.055±0.002
0.0405±0.0005
PVA/CNF/APP4
20
30
0.40
0.060±0.002
0.0413±0.0007
2.3 Characterization The mechanical performance was tested by electronic dynamic & static fatigue testing machine (E3000K8953, Instron). The samples’ microstructure was investigated by a field emission scanning electron microscope (FESEM) from FEI
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(FE-SEM, SIRION200). The chemical structure of the samples were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet 8700, Thermo Fisher
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Scientific) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, Thermo-VG Scientific). The thermal stability was studied by thermogravimetric
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analyzer (SDT Q600, TA). The transient hot wire method was applied to investigate
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the samples’ thermal insulation property (λ) (TC3000E, Xiaxi technology). 3. Results and Discussion
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3.1 Apparent Density and Thermal Conductivity Table 1 shows the density and thermal conductivity of cross-linked aerogels. The
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aerogel samples exhibit relatively low density ranging from 0.047 ~ 0.061 g/cm3. The thermal conductivity and density share the same variation tendency. Both the thermal
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conductivity and density increase monotonously with the rise of MCAPP. According to the previous study [18], PVA/CNF aerogel is porous material containing lots of pores. MCAPP particles can take up some of the pores, leading to the reduction of porosity. As a result, the density would increase. On the other hand, the reduction of porosity would intensify the heat transfer through solid phase, giving rise to the increasing of apparent thermal conductivity. 3.2 Thermal Stability
4
Journal Pre-proof 100
weight (%)
80 70
Deriv. Weight (a.u.)
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
90
60 50 40 30
PVA/CNF PVF/CNF/APP1 PVF/CNF/APP2 PVF/CNF/APP3 PVF/CNF/APP4
20 10
a
100
200
300
400
500
600
700
800
temperature C) 250
100
b
200
300
400
500
600
700
800
temperature C) PVA/CNF PVA/CNF/APP1
200
PVA/CNF/APP2
of
HRR (W/g)
PVA/CNF/APP3 PVA/CNF/APP4
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150 100
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50
100
200
c
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0 300
400
500
600
Temperature (C)
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Figure 1 TG (a) and DTG (b) and heat release rate by micro combustion calorimeter (c) of the as-prepared samples
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Table 2 Related thermal data of PVA/CNF and its composite aerogels
278.4
PVA/CNF/APP1
138.1
218.6
253.0
273.1
309.2
385.2
257.2
449.9
22.9
PVA/CNF/APP2
143.4
212.8
242.1
258.3
289.7
397.9
257.2
449.9
27.3
PVA/CNF/APP3
184.7
218.3
243.6
261.3
308.1
420.6
257.2
449.9
34.7
PVA/CNF/APP4
171.3
215.3
243.1
264.3
304.5
418.2
257.2
449.9
33.3
Tini% (℃)
PVA/CNF
200.4
T10% (℃)
T20% (℃)
T30% (℃)
T40% (℃)
T50% (℃)
Tpeak1 (℃)
Tpeak2 (℃)
Char residue （800 ℃）(%)
245.3
267.9
285.5
291.8
291.1
/
14.6
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Samples
The samples’ thermal stability were investigated through thermal gravity analysis in N2 atmosphere. And the results are shown in Figure 1 and Table 2. The temperature at 5, 10, 20, 30, 40 and 50 wt% weight loss are denoted as Tini, T10%, T20%, T30%, T40%, and T50%, respectively. It’s obvious that all of the samples exhibited similar mass loss patterns with three main stages. In the first stage (<100 °C), the slight weight loss can be mainly attributed to the evaporation of moisture [37]. The next stage occurred at ~250℃-~400℃. For the sample PVA/CNF, the weight loss can be related to the decomposition
of
PVA/CNF
chains
[18]. 5
However,
the
weight
loss
of
Journal Pre-proof PVA/CNF/APP1, PVA/CNF/APP2, PVA/CNF/APP3 and PVA/CNF/APP4 can be the synergistic effect of APP [38] and PVA/CNF decomposition. At this stage, APP could be thermally decomposed to NH3 and H2O for the decomposition of the terminal chain [-OP(O)(ONH4)2] and midchain [-OP(O)(ONH4)-] [38]. Owing to the carbonization reaction between CNF/PVA chains induced by APP in CNF/PVA aerogel, the carbonaceous char with strengthened structure formed [36]. And the strengthened char could further act as protective layer by covering the surface of aerogel matrix and thus limit the permeability of volatile gases [39]. As a result, the weight loss of samples with MCAPP turned to be less heavily than that of PVA/CNF
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aerogel. (Figure 1a). Meanwhile, the heat release of PVA/CNF aerogel is much
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smaller than that of samples with MCAPP. The last stage happens at ~ 450℃, on account of degradation of the formed char [40, 41], combustible volatile gases
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escaped and oxidized. Thus, remarkable exothermic peaks can be observed at MCC curves of PVA/CNF/APP1, PVA/CNF/APP2, PVA/CNF/APP3 and PVA/CNF/APP4
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(Figure 1c).
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Referring to Table 2, the decomposition temperatures of PVA/CNF at 5%, 10%, 20% and 30% mass losses are observed higher than those of the samples loading with MCAPP. However, the decomposition temperatures of PVA/CNF at 40% and 50%
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mass losses are obviously lower than those of the samples loading with MCAPP. The reduction in decomposition temperatures of Tini, T10%, T20% and T30% when loading
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MCAPP is mainly attributed to the phosphorylation and dehydration effect of APP particles in aerogel. And the phosphorylation and dehydration effect could effectively promote the formation of carbonaceous char during heating process, leading to the rising of T40% and T50%. And for the same reason, the char residue at 800℃ increases with the rising of MCAPP loading. 3.3 Combustion Behavior To study the samples’ combustion behavior, cone calorimeter tests were conducted. The heat flux was set 35 kW/m2. And the results are presented in Figure 2 and Table 3. It’s obvious the heat release rate (HRR) and total heat released (THR) decrease with the rise of MCAPP (Figure 2a and b). Specifically, the THR falls from 7.48 MJ/m2 to 2.76 MJ/m2 and the peak heat release rate (pHRR) decreases to 107.84 kW/m2 from 222.44 kW/m2 monotonously (Table 3). Similarly, both the CO and CO2 production show decreasing tendency as the increase of MCAPP (Figure 2c and d). 6
Journal Pre-proof This is due to the existence of APP in aerogel matrix of PVA/CNF which promotes the carbonization between the PVA and CNF chains and strengthens the structure of carbonaceous char [34]. Considerable amount of C is fixed by carbonization, result in low releasing rate of C series volatile like CO and CO2 as well as the relevant heat release (ie. pHRR and THR). In order to evaluate the samples’ fire hazard, the fire performance index (FPI) and fire growth index (FGI) are introduced. The FPI is ratio of TTI (time to ignition) to pHRR, indicating the propensity to flashover in a full-scale fire. And a smaller FPI means a higher possibility for flashover [42]. Shown in Table 3, the FPI rises from
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0.05 s·m2/kW to 0.13 s·m2/kW, indicating less probability of flashover would happen
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to the sample if more MCAPP is introduced. The fire growth index (FGI) refers to the ratio of the peak heat release rate (PHRR) to the peak time, and a greater FGI
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indicates that the fire would grow quickly once lit. The FGI falls to 4.94 kW/ m2·s from 3.00 kW/ m2·s when increasing the MCAPP content (Table 3). Both the FPI and
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FGI indicate lower fire hazard when MCAPP is introduced. 8
250
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
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Time (s) 2.5 PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
2.0
CO2 (vol %)
0.05
3
0
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
0.06
4
1
Time (s)
0.07
CO (mg/m3)
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PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
6
THR (MJ/m2)
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100
a
7
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150
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HRR (kW/m )
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0.04 0.03 0.02
1.5 1.0 0.5
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c
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time (s)
d
0
50
100
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200
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300
time (s)
Figure 2 Heat release rate (a), total heat released (b), CO (c) and CO2 (d) production of the samples in cone calorimeter tests Table 3 Relevant parameters in the cone calorimeter tests under 35 kW/m2 Samples
THR (MJ/m2)
pHRR (kW/m2)
TTI (s) 7
FPI (s·m2/kW)
FGI (kW/ m2·s)
Char residue (%)
Journal Pre-proof PVA/CNF
7.48
222.44
12.10
0.05
4.94
22.22
PVA/CNF/APP1
4.34
176.08
14.70
0.08
4.19
29.73
PVA/CNF/APP2
3.15
163.05
17.20
0.11
4.18
33.33
PVA/CNF/APP3
3.67
134.88
16.20
0.12
3.21
31.58
PVA/CNF/APP4
2.76
107.84
14.00
0.13
3.00
39.39
The sample pictures before and after cone caloriemeter test is presented in Figure 3a, and the char residue is listed in Table 3. It’s clear that the integrity of char turns to be better with the rise of MCAPP (Figure 3). And more char residue can be obtained
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after exposed under 35kW/m2 when more MCAPP is introduced.
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Figure 3 Sample pictures before and after cone calorimeter tests (35kW/m2)
Figure 4 Raman spectra of the samples after cone calorimeter tests (35kW/m2)
Raman spectra is used to investigate the char residue of the samples after combustion which is shown in Figure 4. It’s clear that 2 strong peaks at ~1357 cm−1 (D band) and 1605 cm−1 (G band) are displayed in the spectra. The intensity proportion of D and G bands (denoted as ID/IG) is often used to assess the 8
Journal Pre-proof graphitization degree of char residue. In detail, low value of ID/IG indicates high graphitization degree of char, where the char residue can better shield the underlying structure from been destroyed under high temperature/heat flux. It’s obvious that the ID/IG values of samples with MCAPP series are almost the same (Figure 4). While lower ID/IG values are realized after introduction of MCAPP, demonstrating the enhancing function of MCAPP on the graphitization degree of char. The width of D band is another indicator of disordered canbon. The introduction of MCAPP also causes the narrowing of D band (Figure 4), indicating the improvement of graphitization degree of char [43, 44]. Thus, the combustion of underlying polymer
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can be prohibited effectively.
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Figure 5 SEM iamges of the samples before (a, b, c, d, e) and after (f, g, h, i, j) cone calorimeter tests (a, f: PVA/CNF b, g: PVA/CNF/APP1; c, h: PVA/CNF/APP2; d, i: PVA/CNF/APP3; e, j: PVA/CNF/APP4)
Figure 6 EDS mapping of C (a, d), O (b, e), P (c, f) in PVA/CNF/APP1 (a, b, c) and PVA/CNF/APP4 (d, e, f)
The microstructure of the samples are presented in Figure 5a-j. It’s clear that no prominent difference can be found among the samples before cone calorimeter tests (Figure 5a, b, c, d and e). To figure out the element distribution, the EDS mapping of the samples PVA/CNF/APP1 and PVA/CNF/APP4 has been coducted (Figure 6). It’s clear that the phosphorus disperses uniformly within the samples. However, for the existence of MCAPP, various appearance can be seen on the samples after cone 9
Journal Pre-proof calorimeter tests. Compared with PVA/CNF (Figure 5f), lots of intumescence can be found on the surface of PVA/CNF/APP1 (Figure 5g) and PVA/CNF/APP2 (Figure 5h). The intumescence on PVA/CNF/APP2 (Figure 5h) distributed more densely than that of PVA/CNF/APP1 (Figure 5g). The intumescence can be caused by MCAPP. MCAPP decomposed itself and dehydrated PVA and CNF when heated under 35kW exposure. Then the char formed. Under the shock of gases generated from decomposition, the intumescence shape. As to PVA/CNF/APP3 and PVA/CNF/APP4, excess MCAPP was loaded, the intumescence connect with each other to form integral one, as a result, no prominent intumescence can be observed on the surface
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(Figure 5i and j).
Figure 7 LOI and UL-94 test results of the samples
The samples’ flammability were explored by LOI (Limiting Oxygen Index) and UL-94 (Vertical Burning Test), and the results are shown in Figure 7. LOI is an important parameter and defined as the minimum volume of oxygen for maintaining the combustion of the material. In the experiment, the PVA/CNF aerogel was ignited immediately with vigorous flame, and finally no material remained. It is indicated that neat PVA/CNF aerogel is of high flammability whose LOI value is as low as 19.5%. With the addition of APP, the flame-retardant performance of the samples improved significantly. As the percentage of APP increased from 10% to 15%, the LOI increased from 27% to 37.5%, all samples self-extinguished and passed the V-0 rating in the UL-94 teats. For PVA/CNF/APP1, The sample had droplets during the burning process and the black residue was the product of carbonized PVA and cellulose. Above all, both the acid source and gases source can be originated from the MCAPP, 10
Journal Pre-proof
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making it intumescent flame retardant.
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25000 20000 15000
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10000 5000
25000
Intensity (a.u.)
20000 15000 10000 5000
c
20000
288 286 Binding Energy (eV)
original curve 284.8 286.3 287.95 289.28 fitted curve
284
282
15000 10000 5000 0 292
b
290
25000
288 286 Binding Energy (eV)
284
282
original curve 284.8 286.3 287.95 289.28 290.3 fitted curve
20000 Intensity (a.u.)
30000
290
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0 292
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a
original curve 284.8 286.24 288.79 290.3 fitted curve
25000
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Intensity (a.u.)
30000
30000
original curve 284.8 286.24 288.79 fitted curve
Intensity (a.u.)
35000
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Figure 8 FTIR spectra of the samples
15000 10000 5000
0 292 291 290 289 288 287 286 285 284 283 282 Binding Energy (eV)
d
0 292
290
288 286 Binding Energy (eV)
284
282
Figure 9 C 1s spectrum of PVA/CNF (a, b (35 kW/m2)) & PVA/CNF/APP4 (c, d (35 kW/m2)) Table 4 XPS results of the samples before and after combustion
Sample
C-C
C-O
C=O
O=C-O
CO32-
0 2.4%
0 0
Before cone calorimeter test PVA/CNF PVA/CNF/APP4
52.5% 45.0%
42.8% 51.0%
4.7% 1.6%
After cone calorimeter test (35 kW/m2) PVA/CNF PVA/CNF/APP4
62.7% 58.3%
21.0% 28.8% 11
2.0% 6.5%
0 3.5%
14.3% 2.9%
Journal Pre-proof The FTIR spectra of PVA/CNF aerogel and PVA/CNF/APP4 aerogel are shown in Figure 8. The peak at 3408 cm-1 can be ascribed to –OH stretching vibration in CNF [37]. For these two samples, the characteristic peaks of PVA are observed at 2918 and 1088 cm-1 which are caused by the asymmetric stretching vibrations of – CH2 and the stretching vibrations of C-O, respectively [45]. The typical absorption peak at 1256 cm-1 is due to the stretching vibration of P=O bonds in MCAPP [46]. The peaks at 881 cm-1 represent the asymmetric stretching vibration of P-O group [47]. The 2 typical characteristic peaks indicate the effective introduction of APP in the resulted sample.
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To figure out the chemical composition of the samples before and after
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combustion, XPS analysis [48, 49] was conducted and the results are listed in Figure 9 and Table 4. According to the study of Tsuguyuki et al. [50], bonds like -COONa and
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-CHO do exist in CNF. The C 1s core-level spectrum of PVA/CNF and PVA/CNF/APP4 can be curve-fitted with several peak components (Figure 9), whose
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binding energies locate at 284.8 eV for C-C bonds, 286.3 eV for C-O species [51], 287.95 and 288.79 eV for C=O species [52, 53] and 289.28 eV for O=C-O bonds [52].
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The C-C bonds increase signicantly after combustion for both PVA/CNF and PVA/CNF/APP4 for the carbonization effect of phosphoric acid. In addition, the C=O
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or O=C-O bonds rise clearly for PVA/CNF/APP4 when treated under 35kW heat flux radiation which can be attributed to the esterification of phosphoric acid to PVA and
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CNF. Nevertheless, both C-O and C=O bonds in PVA/CNF dramatically decrease after treated under 35kW heat flux radiation which can be turned to CO32- after combustion (Table 4).
3.4 Mechanical performance
Shown in Figure 10, the as-prepared aerogels exhibit excellent mechanical property. According to Figure 10b, the samples go through typical strain hardening behavior with the rise of MCAPP content. The Young’s modulus (Figure 10d) which was obtained within the deformation range of 25% - 35% increases from 0.367 MPa to 1.045 MPa with the rise of density. The specific modulus (Figure 10d), calculated based on the modulus to density ratio, rises from 7.81 m2/s2 to 17.42 m2/s2 when the density increases to 0.060 g/cm3 from 0.047 g/cm3. As a result, the introduction of MCAPP can significantly improve the samples’ compression resistance. As shown in Figure 5a, b, c, d and e, APP is well dispersed in the PVA/CNF matrix with no evident 12
Journal Pre-proof orientation (Figure 6). In addition, APP contributes to the overall filler loading by 0.32wt% (PVA/CNF/APP1), 0.48wt% (PVA/CNF/APP2), 0.64wt% (PVA/CNF/APP3) and 0.8wt% (PVA/CNF/APP4). The additional filler could increase the stress transfer from the PVA/CNF matrix to the filler [54] resulting in the increase of Young’s modulus. And the Young’s modulus increases with the rising of the filler (MCAPP)
5.0
4.5
4.5
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
2.5
2.5 2.0
1.5
1.5
0.5 0.0 0
10
20
30
40
strain (%)
50
60
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b
3.0
2.0
1.0
PVA/CNF PVA/CNF/APP1 PVA/CNF/APP2 PVA/CNF/APP3 PVA/CNF/APP4
0.75
Stress (MPa)
3.0
1.00
3.5
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stress (MPa)
3.5
4.0
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4.0
1.00
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5.0
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loading level (Figure 10).
70
0.75
0.50
0.50
0.25
0.25
0.00
0.00
1.0 0.5 0.0
80
-0.25
c
20
25
30
35
-0.25 40
Strain (%)
Figure 10 Uniaxial compression test (PVA/CNF/APP4, a), stress-strain curve (b, c) and modulus (d) of the samples
4. Conclusions In this study, PVA/CNF hybrid aerogel was prepared under freeze drying method while introducing MCAPP as the intumescent flame retardant. Aerogels with 13
Journal Pre-proof extremely low density (~0.06 g/cm3) and remarkable mechanical performance (Young’s modulus increases to 1.045 MPa when loading 0.8 wt% MCAPP ) can be obtained. The aerogel’s thermal conductivity can be controlled within 0.04 W/m·K. Experimental results indicate that LOI value increases from 19.5% to 37.5% when loading 0.8 wt% MCAPP. Accordingly, THR decreases significantly from 7.48 to 2.76 MJ/m2. The char residue rises and the char’s integrity improves a lot after combustion when loading MCAPP. The fire performance index (FPI) and fire growth index (FGI) increases and falls respectively, indicating improved anti-combustion performance. XPS results show C=O bonds would be increased for the esterification of phosphoric
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acid from MCAPP. In addition, the production of carbonate can be prohibited in case
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of combustion when loading MCAPP. Conflicts of interest
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None. Acknowledgement
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This research was financially supported by the National Key Research and Development Program of China (2017YFC0804900 & 2017YFC0804907), the
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Fundamental Research Funds for the Central Universities (WUT: 2018IVB058) and the Natural Science Foundation of China (No. 51706165). Y.J.Huang thanks for the
References
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support from China Scholarship Council (CSC).
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Journal Pre-proof
Highlights 1. Monolithic PVA/CNF/APP hybrid aerogels are obtained by freeze drying. 2. Aerogel flammability decreases significantly while loading 0.8 wt% APP.
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3. Tripling aerogel’s Young’s modulus has been achieved by just loading
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0.8 wt% APP.
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