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A Green Approach to Improving the Strength and Flame Retardancy of Poly(vinyl alcohol)/Clay Aerogels: Incorporating bio-based Gelatin Yu-Tao Wang, Hai-Bo Zhao, Kimberly C Degracia, Lin-xuan Han, Hua Sun, Mingze Sun, Yu-Zhong Wang, and David A. Schiraldi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14958 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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ACS Applied Materials & Interfaces

A Green Approach to Improving the Strength and Flame Retardancy of Poly(vinyl alcohol)/Clay Aerogels: Incorporating bio-based Gelatin Yu-Tao Wang1,2, Hai-Bo Zhao1, Kimberly Degracia2, Lin-Xuan Han1, Hua Sun2, Mingze Sun2, Yu-Zhong Wang1*, David A. Schiraldi2*

1

Center for Degradable and Flame-Retardant Polymeric Materials, College of

Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China 2

Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7202, USA

∗

Corresponding authors: E-mail address: [email protected] (Y.-Z. Wang); [email protected] (David A. Schiraldi)

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ACS Paragon Plus Environment


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Abstract: Bio-based gelatins were used to improve the compressive and flammability properties poly(vinyl alcohol)/montmorillonite (PVA/MMT) aerogels, fabricated using a simple and environmentally-friendly freeze-drying method. Due to the excellent compatibility and strong interfacial adhesion between PVA and gelatin, the compressive moduli of aerogels were enhanced dramatically with the incorporation of gelatin. PVA/MMT/porcine gelatin aerogels exhibit compressive modulus values as much as 12.4 MPa, nearly 300% that of the control PVA/MMT aerogel. The microstructure of the PVA/MMT/gelatin aerogels show a three-dimensional co-continuous network. Combustion testing demonstrated that with the addition of gelatin, the self-extinguishing time of aerogel was cut by half and the limiting oxygen index values increased to 28.5%. The peak heat release rate, obtained from cone calorimetry, also decreased with the incorporating of gelatin. Thermogravimetric analysis demonstrated that the gelatins slowed the sharp decomposition of PVA matrix polymer and increased the thermal stability of aerogels at the major decomposition stage of the composite aerogels. These results indicate that as a green bio-based material, gelatin could improve the mechanical properties as well as the properties of flame-retardancy simultaneously. Keywords: aerogel; gelatin; flammability; mechanical properties; poly(vinyl alcohol); clay

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Introduction Aerogels, known as one of the lowest density families of materials in the world (typically ranging from 0.005 to 0.1 g/cm3), are becoming increasingly important in the global economy.1 First introduced by Kistler in the early 1930s using silicon alkoxides as matrixes, followed by sol-gel polymerization, aging and drying,2 their novel methods of preparation, and the unique sets of properties (such us low densities, low thermal conductivities, high porosities and high specific surface areas) have attracted attention for over a half century.3-8 These porous materials have been considered as candidates to replace tradition foam materials in packaging and insulation applications.9-12 Inorganic/organic composite aerogels are a family that has been developed and actively studied more recently.9, 13-17 Since inorganic aerogels, such as those based on silica, are brittle and exhibit poor mechanical properties, organic polymers were incorporated to improve the material toughness. Poly(vinyl alcohol) (PVA) is a good candidate for fabricating the polymer/clay aerogels and is frequently used in aerogel research.18-21 As a water soluble polymer with abundant hydroxyls, PVA can interact with montmorillonite clay (MMT) strongly via exfoliation and hydrogen bonding. A facile and environmentally-friendly freeze drying method can be used to prepare this kind of hybrid aerogel, potentially at low cost if processing parameters are optimized. It has been reported that the PVA/MMT aerogels are promising substitutes for foams in the area of fire safety because of their low flammabilities in many cases.22

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Although the PVA/MMT aerogel composites exhibit better mechanical and flammability properties than the corresponding polymer-only materials, two areas of potential improvement are: (1) the compressive strengths of PVA/MMT aerogels, which are still relatively low for specific applications; (2) The flammability of these aerogels improved only slightly compared to pure polymer systems (for example, the LOI values of PVA/clay aerogel increased from 21% to 24% with incorporating of 50 wt% clay in PVA-based aerogels).22

Different approaches have been introduced to enhance the mechanical behaviors of PVA–based aerogels; these include: heat treatment,23-24 multiple freeze-thaw processing,25 radiation19-20, 26-27 and incorporating of cross-linking agent for PVA such as aldehydes,28 divinylsulfone29 or fibers cellulose nanofibrils.30 Few of the aforementioned methods improve the flame-retardance of the composites while improving their strengths. To suppress the flammability of PVA-based aerogel, efficient flame retardants (FRs), such as ammonium polyphosphate (APP) and silica gel have been evaluated in PVA-based aerogels.31 The addition of FRs, however, decrease the compressive properties of PVA aerogels while reducing their flammability. Borax is one material that not only improved the mechanical properties but also suppressed the flammability of PVA-based aerogels.32 With the addition of borax, however, the aerogel precursor of aerogel became gelatinous, making flow and processing difficult. The development of a novel approach to improve the strengths and flame-retardancy of PVA/MMT aerogels, simultaneously, therefore is still an opportunity of interest. 4


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As a relatively low cost, bio-based material that is easily obtained from skin, connective tissue and bones in nature, gelatin has been wildly used in the area of pharmaceutical, food, cosmetic and photographic industries because of its biocompatibility and biodegradability.33-38 With abundant hydroxyl and amino groups on the chains, gelatin dissolves in water easily, and may be compatible with PVA (interacting via hydrogen bonding).39-40 Meanwhile, the high nitrogen content from the amino acids in gelatin may raise the possibility of these being low flammability materials.41-42 Compared with traditional flame retardants, based on halogens or phosphorus, gelatins can be considered to be “green” and nontoxic. This family of polypeptides could therefore contribute to improved mechanical properties and increased flame-retardance of PVA/clay aerogel simultaneously. In present work, gelatin was incorporated into the PVA/clay aerogel system, and the resultant material densities, compression properties, microstructures, thermal stabilities and combustion behaviors were studied.

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Experimental section Materials. Poly(vinyl alcohol) (PVA; Mn=31000-50000 D), fish gelatin (FG) and porcine gelatin (PG); Sigma-Aldrich, St louis, MO, USA) sodium montmorillonite (Na-MMT; PGW grade, density 2.6 g/cm3, cation exchange capacity (CEC) 145 mequiv/100 g; Nanocor Inc., Arlington Heights IL USA), were used without further purification. Deionized (DI) water was obtained using a Barnstead RoPure reverse-osmosis, low-pressure system.

Preparation of aerogels. The compositions of aerogels are shown in Table 1. All of the hybrid aerogels were fabricated from aqueous precursors that consisted of 5 wt% PVA, 5 wt% MMT and 1 or 2 wt% of the gelatins. For example, to prepare PVA/MMT aerogel with fish gelatin, 5 g of PVA was added into 50 g of deionized water at 90°C, stirring for 6 h. Simultaneously, 5 g of Na-MMT, 1 g of fish gelatin and 50 g of deionized water were well mixed at the high speed setting of a Waring model MC2 mini laboratory blender for 10 min. Then the aforementioned PVA solution (50 g, 10 %) was slowly added into the clay/fish gelatin mixture and stirred for another 1 h to create a uniform suspension. The resulting precursors were poured into a mold and frozen in a solid carbon dioxide/ethanol bath (-70°C) immediately. The frozen samples were transferred to a VirTis AdVantange freeze dryer (Warminster, PA USA) for ice sublimation. This final product was named as P5/M5/FG1, where letter P referred to PVA, letter M denoted Na-MMT and the subscripts mean their content in every 100 g water. Other samples were identified as P5/M5/FG2, P5/M5/PG1 and 6


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P5/M5/PG2 were prepared by a similar method. P5/M5 aerogels were fabricated as the general control. P6/M5 and the gelatin aerogels were also prepared as control for specific test. All the samples were placed in the desiccator after freeze-drying for further characterizations and tests. Table 1. The compositions of PVA/MMT aerogel and PVA/MMT aerogels with gelatins Sample

PVA

MMT

Gelatin

DI Water

P5/M5

5g

5g

-

100 g

P6/M5

6g

5g

-

100 g

P5/M5/FG1

5g

5g

Fish Gelatin

1g

100 g

P5/M5/FG2

5g

5g

Fish Gelatin

2g

100 g

P5/M5/PG1

5g

5g

Porcine Gelatin

1g

100 g

P5/M5/PG2

5g

5g

Porcine Gelatin

2g

100 g

Characterization. Fourier transform infrared spectroscopy (FTIR) was conducted on Agilent Cary 680 FTIR spectrometer (Agilent Technologies, United States). Rheological tests were performed by using the ARES G2 advanced dynamic rheometric expansion system (TA Instruments, United States) at room temperature with a fixed frequency of 1 rad s−1.41 The apparent densities of the dried aerogels were calculated by measuring the weight and dimension data using analytical balances and digital calipers; three aerogel specimens were calculated for each composition and their values averaged. 7



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Compression testing was studied on an Instron model 5500 universal testing machine (Instron, United States), fitted with a 1 kN load cell and tested at a constant rate of 10 mm/min. Three cylindrical samples (∼2 cm in diameter and height) were tested for each composition and their values averaged. A JEOL JSM 6510LV scanning electron microscopy (SEM) (JEOL, Japan) was used to characterize the morphological microstructure of the aerogels. The specimens were fractured in liquid nitrogen bath (-196°C) and then platinum was coated on the cross-section of samples before testing. A TGA Q500 (TA Instruments, United States) thermogravimetric analyzer was used to study the thermal stabilities of aerogel samples. The specimens were placed in a platinum pan, equilibrated at 100°C for 2 min and then heated up to 700°C at 10°C/min under the nitrogen flow of 40 mL/min. The testing method of vertical burning test was similar to that of UL-94 test, but using cylindrical specimens (∼2 cm in diameter and 3 cm in height). The samples were burned in a horizontal-vertical flame chamber (Skyline Instruments Co., China) with a 2 cm blue flame. The limit oxygen index (LOI) values were obtained by an HC-2C oxygen index meter (Jiangning, China) according to ASTM D 2863-2009. The size of the specimens tested was 120 mm×10 mm×10 mm. The combustion behaviors of aerogel samples were performed by a cone calorimeter device (Fire Testing Technology, UK) in accordance with the ASTM E 1354 standard. The specimens (100×100 mm) with an average thickness of 10 mm, wrapped in 8


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aluminum foil, were exposed at a heat flux of 50 kW/m2.

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Results and discussion

Scheme 1. Preparption procedures of PVA/MMT aerogels and PVA/MMT/Gelatins.

Table 2. Compression properties and densities of PVA/MMT aerogels and PVA/MMT aerogels with gelatins Sample

Density(g/cm3)

Modulus(MPa)

Specific Modulus （MPa cm3/g）

P5/M5

4.0±0.5

0.095±0.003

42.1±4.0

P5/M5/FG1

7.1±1.2

0.105±0.008

67.6±6.2

P5/M5/FG2

8.5±0.4

0.118±0.012

72.0±5.9

P5M5/PG1

11.8±2.2

0.100±0.005

118±14

P5/M5/PG2

12.4±2.2

0.108±0.006

114±13

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Figure 1. Compressive stress–strain curves of PVA/MMT aerogels and PVA/MMT/Gelatins. Structure Characterization Scheme

1

illustrates

the

preparation

process

of

PVA/MMT

and

PVA/MMT/Gelatin aerogels. With the incorporation of gelatins which can bond strongly with PVA, rigid aerogel samples could be obtained.43-44 The bonding between these two material is supported by FTIR results, which are reported in Figure S1 of the Supporting Information; the effectiveness of porcine gelatin (PG) blending to PVA is supported by the FTIR peak at ~1645 cm-1, which is ascribed the stretching of C=O bond (amide I) in PG.45-46 PVA only exhibits a small shoulder at 1645 cm-1 while a strong peak appears in PVA/PG aerogel composites.47 Other characteristic bands of stretching vibration of –CH2- (at ~2920 cm−1) of PVA, the amide II (at ~1545 cm−1) and amide III (at ~1238 cm−1) of gelatin are also witnessed 11



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in the spectrum of PVA2.5/PG2.5.In the FTIR spectrum of PVA, the peak at 3305 cm-1 corresponds to the symmetrical hydroxyl group stretch.48 The peak at 3313 cm-1 in the porcine gelatin aerogel spectrum, can be assigned to the stretching of C-N and N-H bonds.49 In the spectrum of PVA/PG aerogels, this board bands, influenced by the stretching O-H and N-H bonds, shifts to the lower position (3301 cm-1). The peak of C-O stretching vibration shifts from 1092 cm-1 to 1086 cm-1. The shifts of these bands were probably attributed to the intermolecular interactions between PVA and gelatin.39

The complex viscosities of the precursors of PVA (PVA2.5 and PVA5), porcine gelatin (PG2.5) and PVA/gelatin (PVA2.5/PG2.5) are illustrated in Figure S2 by rheological tests. PVA precursor solutions exhibited extremely low values (＜1 Pa s), as is typical of such systems, however, with the incorporating of PG, the complex viscosity of PVA/PG increased dramatically, higher than those of the individual PVA and PG solutions, indicating the presence of the strong interaction between PVA and PG.41

Apparent density and compressive behavior The densities of PVA-based aerogels are listed in Table 2. All of the aerogels produced with gelatin kept their shapes in the mold and exhibited no apparent volume shrinkage during the process of freeze drying. PVA/MMT samples exhibited the lowest densities since they contained the lowest solid contents. With the addition of gelatin, the densities of aerogel samples increased accordingly. 12


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The compressive properties of PVA/MMT/gelatins and the control sample are shown in Table 2 and Figure 1. The specific moduli of aerogel samples in Table 2 were calculated by the ratio of initial compressive modulus to the density. The initial compressive modulus of PVA/MMT aerogels averaged 4.0±0.5 MPa, less than those observed with the PVA/MMT/gelatin aerogels. Compared with traditional fire retardants (FR) which were reported to decrease the mechanical properties of aerogel composites and to limit the interaction between polymer matrixes and clay particles,31 the potential bio-based flame retardant, gelatin, increased the compressive modulus of PVA/MMT. The initial compressive modulus of P5/M5/PG1 was 11.8±2.2 MPa, nearly triple that of the control; this result may be attributed to the good compatibility and strong interfacial adhesion between PVA and gelatin. The high content of amino in gelatin could provide numerous physical crosslinking points for PVA via hydrogen bonding.

As shown in Figure 1, the P5/M5/FG2 aerogels exhibited the highest stress in the high strain area while the compressive modulus of P5/M5/PG2 aerogels were higher than those of other samples. Aerogels with different kinds of gelatins had different compressive behaviors because of the different sequences and contents of functional groups in different sources of gelatin.

The gelatin aerogels (with or without clay) were prepared as the controls and the data of compression moduli are listed in the Table S1 of the Supporting Information. Fish gelatin aerogels are too soft to survive (keep the original shape)

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after cutting, and the compression tests for FG aerogels cannot be finished. For the porcine gelatin aerogels, the compression modulus of the PG5 aerogel is 8.5±1.2 MPa, lower than those of PVA/MMT aerogels with porcine gelatin listed in Table 2. However, with incorporating of 5 wt% clay, the compression moduli of aerogels decreased to 0.38±0.04 MPa probably because the clay particles could not disperse well in the gelatin matrix due to the extremely high precursor viscosity of Gelatin5/MMT5. The similar phenomenon can be observed for the FG5/MMT5 (the compression modulus was only 1.7±0.30 MPa). These results revealed that the improvement of compression properties of PVA/MMT/Gelatins was attributed to the interaction of gelatin and the PVA/Clay systems.

Microstructures of Aerogels.

Figure 2. SEM images of PVA/MMT aerogel and PVA/MMT aerogels with gelatins

SEM was used to investigate the morphologies and study the relationship between structure and properties of PVA-based aerogels. The microstructures of the P5/M5 14


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aerogel, P5/M5/FG1 aerogel and P5/M5/PG1 aerogel are exhibited in Figure 2. It had been reported that the PVA/MMT aerogels possessed a lamellar architecture which followed the direction of ice growth.22 As shown in Figure 2, PVA/MMT aerogel exhibited a dendrite growth layered structure. The polymer phase is linking between the different clay layers as the “branch”; the presence of significant interspace between the clay layers implies that the interaction between polymer matrix and the clay is not strong. The microstructure of aerogel converted to a co-continuous 3D network structure with the incorporating of fish gelatin. Instead of the lamellar architecture, a uniform porous structure was created by the strong interaction between PVA and gelatin, presumably via hydrogen bonding. Compared with PVA/MMT aerogels, PVA/MMT/PG aerogels exhibit a more stereoscopic lamellar morphology, with thick layers and denser organic “bridge” linking different layers, as was observed from SEM images of PVA/MMT/PG aerogels. Porcine gelatin was found to embed between the layers as “bridge”, likely stabilized by the good compatibility and the interaction via multi-hydrogen bonding, which could be responsible for the highest compressive modulus of PVA/MMT/PG among all the samples.

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Figure 3. TGA curves of PVA/MMT aerogels and PVA/MMT/Gelatins at a heating rate of 10°C/min under nitrogen

Figure 4. TGA curves of PVA/MMT aerogels and PVA/MMT/Gelatins at a heating rate of 10°C/min under nitrogen

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Table 3. TGA data of PVA/MMT and PVA/MMT/Gelatin aerogels. Td10%(°C)

Td20%(°C)

TdMAX(°C)

dW/dT(%/°C)

Weight at 100°C (%)

Residue (%)

P5/M5

236

251

251

0.80

96.1

47.8

P6/M5

239

255

261

1.0

94.7

40.7

P5/M5/FG1

250

268

265

0.58

96.4

45.0

P5/M5/FG2

255

279

275

0.45

95.6

40.8

P5/M5/PG1

245

268

269

0.58

93.6

44.3

P5/M5/PG2

249

277

280

0.44

93.6

40.3

Samples

Thermal Stability. The thermal properties of PVA/MMT aerogels and PVA/MMT/Gelatins were studied by thermogravimetric analysis (TGA; Figure 3) and differential thermogravimetry (DTG; Figure 4). The related TGA data, including the decomposition temperatures at 10% weight loss (Td10%), 20% weight loss (Td20%), and at maximum decomposition rate (Tdmax) are listed in Table 3. The values at maximum mass decomposition rate (dW/dT) and the residue amount are also summarized in Table 3. To avoid the influence of moisture of the hydrophilic aerogel samples, the thermogravimetric analyzer was rapidly heated to 100°C from room temperature, equilibrated at 100°C for 2 min and then ramped to 700°C at a heating rate of 10°C/min under nitrogen. As hydrophilic materials rich in surface hydroxyl and amino groups, aerogels with PVA and gelatin have measurable levels of physisorbed water which were difficult to remove via regular drying methods. The initial weight loss before 100°C 17



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be may attributed to the loss of this water. The main step of mass loss of PVA/MMT aerogels, occurring between 200 to 300ºC, was caused by the decompositions of molecular chains of PVA. The corresponding sharp weight loss peaks for the main decomposition of control samples were exhibited in Figure 4 with the maximum mass decomposition rate (dW/dT) from 0.8 to 1.0. Compared with P5/M5 aerogel, Fish gelatin and porcine gelatin increased the Td10%, Td20% and Tmax of aerogels although the samples with gelatins had a higher weight loss rate before 100°C. The maximum decomposition rates decreased from 0.8 to 0.44, implying that the gelatins slowed down the sharp decomposition of PVA and increased the thermal stability of aerogels at the major decomposition stage between 200°C to 300°C. P6/M5, P5/M5/FG1 and P5/M5/PG1 samples all contained 6 wt% of organic materials but showed different thermal stabilities. These three aforementioned compositions could be used to determine the function of gelatin during the thermal decomposition of hybrid aerogel composites and compare the charring abilities of PVA and gelatin. The thermal stabilities of aerogels produced with gelatin were superior to that of the PVA/MMT aerogels, as shown by their higher Td10%, residue amounts and their lower maximum mass decomposition rates. It is likely that gelatin can produce more high carbon-content structures via dehydration and the release of other small molecular substances during the decomposition than PVA; this may result in a charred layer which could hinder heat transfer and limit further decomposition of PVA matrix. 18


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Combustion Behavior. Limiting oxygen index (LOI) measurements were carried out, investigating combustion performances of the samples. Table 4 gives the LOI values of the PVA/MMT aerogel and the aerogels produced with gelatins. The LOI values of aerogels increased monotonically with the addition of gelatin. The aerogels produced with the incorporation of 16.7 wt% gelatin (P5/M5/FG2 and P5/M5/PG2) exhibited LOI values greater than 28% and could be the flame-retardant materials that were generally self-extinguishing.50 The Vertical burning tests were another kind of small fire combustion test which could estimate samples’ flammability. The fire out (extinguishing) time of the aerogels are shown in Table 5. From the Table 5, the t1 denotes fire out (extinguishing) time after the 1st ten-seconds ignition, the t2 denotes fire out (extinguishing) time after the 2nd ten-seconds ignition and the time of t1+t2 means the totally fire out (extinguishing) time in the vertical burning tests. Compared with P5/M5 aerogels, the aerogels containing fish or porcine gelatin were easier to extinguish. With increasing content of gelatin in hybrid aerogel systems, the self-extinguish time of aerogel decreased gradually. A sizzling sound was noted when the samples with gelatins ignited; it is likely that the water vapor released while the burning process of PVA/MMT/Gelatins. The resulting water vapor could help to dilute the concentration of flammable gas and the stable char layer could be produced through dehydration effect of gelatins. The results of vertical burning tests and LOI tests indicated that 19



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gelatin was a kind of effective bio-FR in small fire combustion test.

Table 4. The LOI values of PVA-based aerogels Sample

P5/M5

P5/M5/FG1

P5/M5/FG2

P5/M5/PG1

P5/M5/PG2

LOI (%)

24.0

26.8

28.5

26.5

28.0

Table 5. The vertical burning test results of PVA-based aerogels Sample

t1

t2

t1+t2

P5/M5

15.6±2.3s

1.6±0.5s

17.2±2.5s

P5/M5/FG1

12.1±2.3s

1.5±0.5s

13.7±2.5s

P5/M5/FG2

9.5±0.5s

1.0±1.2s

10.5±1.1s

P5M5/PG1

5.7s±0.5s

11.0±0.8s

16.7±0.5s

P5/M5/PG2

5s±0.7s

3.75±1.7

8.8±1.6s

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Figure 5. HRR plots of PVA/MMT and PVA/MMT/Gelatin aerogels.

Figure 6. THR curves of PVA/MMT and PVA/MMT/Gelatin aerogels.

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Table 6. Cone Calorimeter data of PVA/MMT and PVA/MMT/Gelatin aerogels. Sample

Weight (g)

TTI (s)

pHRR 2

(kW/m )

TTPHRR (s)

FIGRA (KW/s.

THR 2

(MJ/m )

2

m)

THR/mass 2

(MJ/m *g)

P5/M5

11.0

3

140.2

10

14.0

12.0

1.09

P5/M5/FG1

10.3

4

139.5

10

14.0

11.9

1.16

P5/M5/FG2

17.3

3

126.8

10

12.7

19.7

1.14

P5/M5/PG1

14.1

3

124.3

10

12.4

15.0

1.06

P5/M5/PG2

13.3

3

122.0

10

12.2

12.7

0.95

Cone calorimetry tests were conducted to evaluate the flame-retardant performances of aerogel samples and the relevant burning data including the time to ignition (TTI), peak of heat release (pHRR), time to peak of heat release (TTPHRR) and total heat release (THR) were given in Table 6. All the aerogels were easily ignited under the heat flux of 50 kW/m2 and had a relatively short TTI values. Figure 5 illustrates the heat release rate (HRR) curves of PVA/MMT/gelatin aerogels and the control sample. PVA/MMT aerogel exhibited a relatively sharp doublet peaks in HRR curve, with a pHRR of 140.2 kW/m2. In a contrast, aerogels with gelatins generally had broad doublet peaks in HRR plots and the pHRRs of PVA/MMT/Gelatin aerogels decreased to around 125 kW/m2 except with P5/M5/FG1. With increasing gelatin concentration in aerogel system, the pHRRs of PVA/MMT/Gelatin aerogels decreased continuously. In accordance with the results of vertical burning tests, P5/M5/PG2 exhibited the lowest pHRR among all the samples, 22


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indicating that as a kind of bio-based material, gelatin could decrease the fire risk of PVA-based aerogel in both small and big fire tests. THR was used to evaluate the total heat release of material during burning process, which was determined by the mass of the sample and the decomposition products. Figure 6 demonstrated the THR curves of control sample and the aerogels with gelatin. The relative data were also listed in Table 6. Compared with the control sample, THRs of PVA/MMT/Gelatin aerogels increased because the mass of aerogel with gelatin and the concentration of flammable material in aerogel composites were higher than the PVA/MMT aerogel. THR/mass of the aerogels was computed by the ratio of THR to the mass, and could better illustrate the heat release character of aerogels by removing mass differences from the comparison. The THR/mass values of aerogels with gelatins were similar to those of the control. P5/M5/PG2 aerogel exhibited the lowest THR/mass value, which was 87% of that of the control, probably because the PG help to produce a dense char layer to limit the decomposition of PVA matrix. The fire growth rates (FIGRA) of samples with gelatins, displayed in Table 6, also decreased, indicating an improving survival chance under room fire conditions.

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Figure 7. SEM images of surface char residue of PVA-based aerogel. (A and B) PVA/MMT aerogel, (C and D) PVA/MMT/Porcine Gealtin2 aerogel. To get a better understanding of the flame retardant behavior of gelatin in PVA/MMT aerogel system, the residue microstructures of P5/M5/PG2 and P5/M5 were examined, and are presented in Figure 7. Aerogel produced with PG exhibited fewer gaps on the surface of the char residue, compared with the control. A denser surface char structure was produced by the help of gelatin during the combustion of aerogel. It was reported that the dehydrated gelatin has a relatively low melting point and the melting gelatin could help to produce the denser char on the surface of the material during the combustion.51 On the basis of thermal stability test and combustion tests, the flame-retardant mechanism of the PVA/MMT/gelatin aerogels can be postulated. The low flammability of gelatin and the physical crosslinking structure between gelatin and PVA/clay system make aerogel composites difficult to ignite, especially in the small fire burning test. The char structure enabled by the interaction between gelatin and PVA/MMT system could prevent the transfer of heat and combustible volatiles, limiting the further decomposition of the polymer matrix and keeping the relative low heat release rate of aerogel for cone calorimeter tests.52 24


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4. Conclusions The preparation of PVA/clay aerogels with fish or porcine gelatin by a simple and environmentally friendly freeze-drying method was demonstrated. The structure characterization,

apparent

densities,

compressive

properties,

morphologies,

combustion behaviors and thermal stabilities of aerogels were determined. Likely because of hydrogen bonding between PVA and gelatin, the compressive moduli of PVA/MMT aerogel with gelatins enhanced dramatically. The P5/M5/PG2 exhibited the highest compressive modulus, nearly triple that of the P5/M5 aerogel. The co-continuous microstructure of aerogel with gelatin implied an increasing interaction between different phases in PVA/MMT/gelatin aerogels, compared with the control. Combustion testing showed decreased self-extinguishing time and increased LOI values with the addition of gelatin. The results of vertical burning tests and LOI tests indicated that gelatin was a kind of effective bio-FR in small fire combustion test. Compared with the control, the pHRR of PVA/MTT/gelatins also decrease slightly although the THR increased because the gelatin is itself an organic flammable material. The values of THR/mass were still similar to those of the controls, indicating that the heat suppression of PVA/MMT aerogels could be maintained with the addition of gelatin. The TGA tests of hybrid aerogels, compared with that of the P5/M5 aerogel, demonstrate that both fish and porcine gelatins increase the Td10%, Td20%, Tmax and decreased the dW/dT of the aerogels. The comparison between P5/M5/FG1, P5/M5/PG1 and P6/M5 indicates that gelatin can improve the charring ability of the hybrid aerogel system. As a green bio-based material, gelatin had an 25



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excellent compatibility with PVA matrix via hydrogen bonding and could enhance the properties of compression and flame-retardancy.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

FTIR spectra, complex viscosity curves and the table of compression moduli of gelatin aerogels.

Acknowledgment The authors of this paper thank the China Scholarship Council and this work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51320105011 and 51121001) and Program for Changjiang Scholars and Innovative Research Team in University (IRT. 1026).

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TOC Graphic

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338x190mm (96 x 96 DPI)


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Green Approach to Improving the Strength and ... - ACS Publications

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