Flame-Retardant and Smoke-Suppressed Silicone Foams with

Article pubs.acs.org/IECR

Flame-Retardant and Smoke-Suppressed Silicone Foams with Chitosan-Based Nanocoatings Shi-Bi Deng, Wang Liao,* Jun-Chi Yang, Zhi-Jie Cao, and Yu-Zhong Wang* 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 ABSTRACT: Flexible silicone foams (SiFs) are high-performance but flammable materials that emit dense smoke in a fire. Two type of low-cost, eco-friendly nanocoatings, i.e., chitosan (CH)/ ammonium polyphosphate (APP) and CH/montmorillonite (MMT), were fabricated on SiFs through a layer-by-layer (LbL) assembly. With seven bilayers (BL) of CH/APP coatings, the limiting oxygen index (LOI) increases from 20.2% to 23.8%, the peak heat release rate (pHRR) decreases by 27.6%, and the total smoke production (TSP) decreases 42%. Further deposition of CH/APP, however, partly damaged the fire resistance. In contrast, fire hazard and smoke release of CH/MMT coated SiFs were monotonously reduced. Every 7 BL of coating leads to ca. 12% reduction of TSP. Moreover, thermogravimetric analysis (TGA) was used to follow the pyrolysis of the coated foams, and scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX) were applied to analyze the char residues systematically.

1. INTRODUCTION Flexible polymeric foams are high-performance materials with the advantages of lightweight, insulation for noise, heat and electricity, and absorption of impact energy.1,2 However, based on their high porosity and organic, polymeric nature, flammability and the release of massive amounts of smoke limit their application. Polysiloxane foams (SiFs) are a series of elemento-organic polymers with −Si−O−Si− comprising their main chains3 (Scheme 1). They release no poisonous gases or

avoided in practical application. To improve the mechanical performance and flame retardancy of SiFs, white carbon black was added during the foaming process. Exposed under an open flame for 60 s, SiFs extinguished in 2 s with only 1.27 cm burnt out and no toxic gas release.7,8 The addition of 2.5% glass fiber and 5% glass microsphere could extend the burning time to 5 times that of a 12-cm-thick SiF9. Generally, inorganic fillers (asbestos, for instance) and artificial fibers formed by glass, carbon, alumina, silicates are widely used during the foaming process at present.7−11 Further design of flame retardant (FR) SiFs, to the best of our knowledge, is rarely reported. The layer-by-layer (LbL) assembly has now become a worldwide standard for the rapid construction of versatile coatings.12−17 Grunlan et al.18 deposited intumescent polymeric multilayers of poly(sodium phosphate) and poly(allylamine) on fabrics. The coatings, although extremely thin, effectively extinguished fire on fabrics and reduced the heat hazards. Later, clay-based nanocoatings were proved to be effectively fire-resistant on polyurethane foams (PUFs).19,20 Laachachi et al. prepared flame-retarded polyamide and polyamide fabrics using LbL assemblies.21−23 Yang et al.24 developed a nanocoating by the LbL method on melamine foams, the limiting oxygen index (LOI) of which increased from 34.5% to 47.0% with only two bilayers. In addition, the efficacy of FR LbL coating has also been proved in cotton,25−28 polyester

Scheme 1. Structure of Silicone Derivatives

droplets during burning. Because of their unique chain structure and properties, SiFs show excellent performance in ultrahigh-/low-temperature resistance, chemical stability, antiaging, ozone resistance, and physiological inertness;4 therefore, these can be used as filling materials for propellant, wings, and rudder, and also protective thermal insulation materials for aircrafts and missiles.3,5,6 However, SiFs still have a tendency to smolder, combust, and release choking smoke in fire conditions, which should be © XXXX American Chemical Society

Received: February 11, 2016 Revised: June 9, 2016 Accepted: June 16, 2016

A

DOI: 10.1021/acs.iecr.6b00532 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Scheme of layer-by-layer (LbL) assembly with (1) chitosan (CH) and montmorillonite clay (MMT), or (2) CH and ammonium polyphosphate (APP) on polysiloxane foams (SiFs).

fabrics,29−31 polyester-cotton blends fabrics,32−34 polyamide,21,22 polycarbonate,35 and ramie fabrics.36,37 In this work, two different nanocoatingsi.e., chitosan (CH)/ammonium polyphosphate (APP) and CH/montmorillonite (MMT)are fabricated on/in SiFs via LbL assembly. CH is an amino polysaccharide derived from the second-mostabundant natural biopolymer, i.e., chitin. Ammonium polyphosphate (APP) is a very popular commercial flame retardant.38−41 The layered clay, MMT, has been fully proven to form a physical barrier in a fire, protecting substrates from burning out.42−44 Both systems have been proven to be effective FR coatings.20,33,45 Therefore, the CH/APP coatings represent a typical intumescent FR system, in which CH acts as carbon and gas sources, and APP plays the role of acid source, which generates phosphoric acids for charring. For the other system (i.e., CH/MMT coatings), MMT not only forms a physical barrier in burning, but also may suppress smoke release. Therefore, the current study aims to improve fire resistance and also the smoke suppression of SiFs without compromising the foaming formula or the mechanical properties, or destroying the open-celled structure of the foam. Meanwhile, the two types of nanocoatings built with CH/APP or CH/MMT using the LbL method are investigated in detail.

unless otherwise stated. All materials were used as received, without further purification. 2.2. Layer-by-Layer (LbL) Deposition. The cationic deposition solution was prepared by adjusting the pH of DI water to 4 with acetic acid (HAc) and then adding 1 wt % CH. This aqueous solution was stirred magnetically for 12 h until the CH powder completely dissolved and it was used without adjusting pH value. APP solution, one of the anionic solutions was prepared by adding APP to DI water and the concentration was 1 wt %. Simultaneously, the other anionic solution was prepared by adding 1 wt % of MMT, and both of them were stirred magnetically for 12 h before being used. Figure 1 shows the schematic representation of preparing process of LbL assemblies of CH/APP and CH/MMT nanocoatings. First, the untreated SiF was presoaked in a 2 wt % PAA solution (pH 2) for 5 min to create a primer layer with negative charge to improve adhesion. After that, SiFs were immersed in the positively charged CH solution for 5 min, rinsed in a stream of DI water, and dried in an air-circulating oven at 80 °C for 40 min. This procedure was followed by an identical immersion; SiFs was rinsed in the MMT suspension or APP solution and then was dried in an oven. After the initial bilayer (BL) finished, the same procedure was followed with CH (immersion time = 1 min) and MMT or APP (immersion time = 1 min) for each subsequent bilayer until the desired number of layers were deposited. Each deposition was followed by the substrate being immersed and rinsed in DI water twice each for 2 min to remove excess particles and then dried in an air-circulating oven at 80 °C for 40 min. It was noticed that the coating time may have an influence in LbL results. However, in this paper, we did not optimize the coating time, which needed to be studied in the future work. Finally, the coated foams were dried under vacuum at 70 °C for 12 h prior to testing. Besides, quartz wafers, which were used as the coated substrate under the monitor of ultraviolet−visible light (UV-vis) spectrometer, were washed in piranha solution (volume ratio of 25% H2O2 (30% strength), 75% H2SO4 (98% strength)) at 90 °C for 1 h prior to LbL treatment, and rinsed with DI water several times. The wafers were treated using the previously described

2. EXPERIMENTAL SECTION 2.1. Materials. The commercial silicone foam without any flame-retardant additives (SiF; density = 0.18 g/cm3) was provided by BlueStar Chengrand Research & Design Institute of Chemical Industry. Chitosan (CH; MW 150 kDa, 85%−95% deacetylated) was purchased from Jinan Haidebei Marine Bioengineering Co., Ltd. Ammonium polyphosphate was purchased from Shifang Taifeng New Flame Retardant Co., Ltd. Polyacrylic acid (PAA; MW 100 kDa) was supplied by Sigma−Aldrich. Ammonium polyphosphate was supplied by Shifang Taifeng New-Type Flame Retardants, Co., Ltd. China. The clay Closite Na (Na+-MMT; PGW grade, cation-exchange capacity (CEC) equal to 145 mequiv/100 g, density = 2.6 g/ cm3) was supplied by Nanocor, Inc. Acetic acid (HAc) was purchased from Tianjin Bodi Chemical Co., Ltd. Deionized (DI) water (18.2 MΩ, pH 6) was used for all experiments, B


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Figure 2. UV-vis absorbance spectra of the CH/APP (a) and the CH/MMT (b) nanocoatings deposited on the quartz wafers. The insets show the absorption intensity at 269 nm as a function of the bilayer numbers.

Figure 3. SEM images of pure (a) and nanocoated SiF samples with CH/APP (b-1, b-2, and b-3) and CH/MMT deposition (c-1, c-2, and c-3). The bilayer numbers (0 BL, 7 BL, 14 BL, or 21 BL) of the coating are listed above the figures.

procedure, and, especially during each cycle, wafers were dried in the air. 2.3. Characterization. UV-vis absorption spectra were obtained using a Varian Cary 50 spectrophotometer. Morphological microstructure of the uncoated and coated foams were investigated using a Model JSM 5900LV scanning electron microscopy (SEM) system (JEOL, Japan) at an accelerating voltage of 20 kV. The elemental analysis in the surface scanning model was carried out by the energy-dispersive X-ray spectrometer (EDX). Thermogravimetric analysis (TG) was performed using a TG system (Model TG 209F1, Netzsch, Germany) to study the thermal stabilities of the foams, and the specimens were heated from 40 °C to 700 °C at a heating rate of 10 °C/min under a nitrogen flow of 50 mL/min. For the TGA tests, the SiFs were cut and the inside parts were picked out and ground to a powder. At least three samples were tested from each foam. Furthermore, coating contents were calculated

by measuring the weights before and after LbL assembly treatment. The LOI tests were performed using an HC-2C oxygen index meter (Jiangning, China) in accordance with ASTM Standard D 2863-2009. The size of the specimens tested was 120 mm × 10 mm × 10 mm. The combustion behaviors of the foams were carried out by a cone calorimeter device (Fire Testing Technology, UK) according to ISO Method 5660-1. The specimens (100 mm × 100 mm) with an average thickness of 25 mm were exposed to a beaming cone at a heat flux of 50 kW/m2.

3. RESULTS AND DISCUSSION 3.1. Characterization of LbL Assemblies on SiFs. The flame-retardant coatings were deposited on the skeleton of foams by LbL method in aqueous solutions. In order to figure out whether the nanocoatings were successfully assembled on the substrates and the corresponding amount, CH/APP and C


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Figure 4. TG and DTG curves of pure SiF and SiFs coated with 7, 14, and 21 BL of CH/APP (a-1 and a-2) and CH/MMT (b-1 and b-2) at a heating rate of 10 °C/min in N2.

CH/MMT bilayers were first deposited on quartz wafers in the same procedure. UV-vis measurements were then taken to monitor the growth of the LbL assembly process.24,37 Figures 2a and 2b present the UV-vis absorption spectra of the 4, 8, 12, 16, and 20 bilayers of CH/APP and CH/MMT nanocoatings on quartz wafers, respectively. In Figure 2a, a weak absorption peak at 269 nm, which is characteristic peak of APP, can be observed clearly in each curve. This absorbance increases along with the number of bilayers, indicating an incremental trend of the film thicknesses.24 For CH/MMT coatings in Figure 2b, a uniform increase of the spectra also indicates a buildup of the coating, which leads to complete absorption and the scattering of incident light. In order to make a better comparison between the CH/APP and CH/MMT coatings, two insert plots showing the absorbance at 269 nm were added in Figure 2 to indicate the bilayer numbers. The linear trends imply that both depositions were relatively continuous and uniform during each assembly cycle. To clearly observe the nanocoatings on the SiFs, the morphologies of these layers were scanned by SEM (Figure 3). Cells of the pure SiF shows smooth surfaces (Figure 3a), while the coated foams show relatively rough and uneven surfaces with aggregated platelets (see Figures 3b and 3c). Figure 3b shows that CH/APP coatings on SiFs are randomly distributed in a form of “sea−islands” morphology.46 In contrast, the CH/MMT coatings cover more cell surfaces and are more homogeneous (Figure 3c). The reason could be attributed to the nanoscale laminated layered structure of MMT, which does not exist in polymeric APP. The platelets of MMT evenly spread out on a CH prelayer, and as the LbL cycle continues, inter-infiltration between different layers may enhance their bonds. These behaviors may also contribute to a higher weight gain of CH/MMT pairs (see Table 2, presented later in this work). Generally, nanostructure layers have been

successfully and evenly built on the skeleton of SiFs and the open cells maintain their integration. 3.2. Thermal Decomposition. TG is used to study the thermal stability and the degradation process of the pure and coated SiFs. The obtained TG curves and derived differential thermogravimetry (DTG) are presented in Figure 4. The key data of decomposition temperatures at 5% mass loss (T5%), maximum decomposition rate (Tmax), weight loss rate at Tmax, and the residue amount at 700 °C are summarized in Table 1. Table 1. TG Data in N2 of Pure and Coated SiFs with Different Bilayers of CH/APP or CH/MMTa sample pure (CH/ APP)7 (CH/ APP)14 (CH/ APP)21 (CH/ MMT)7 (CH/ MMT)14 (CH/ MMT)21 a

T5% (°C)

Tmax (°C)

weight loss rate at Tmax (%/min)

residue at 700 °C (%)

277 290

486 617

5.2 2.4

37.1 45.4

277

604

2.3

47.0

262

600

2.1

49.3

292

544

1.5

59.8

288

546

1.3

61.3

286

545

1.2

63.0

Superscripts are as denoted.

In accord with reported results,1 pure SiF degrades gradually after 277 °C. Later, the C−Si bonds break down when the temperature is >300 °C, as does the side organic groups. The maximum mass loss rate of pure SiF is observed at 486 °C. In temperature ranges higher than that, main chains decompose and form a ceramic-like material that is composed of Si, C, and D


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Industrial & Engineering Chemistry Research O. Finally, 37% char residue still remains at 700 °C. In contrast, coated SiFs exhibit different thermal degradation behaviors, in which protecting effects are enhanced as the bilayer number increases. In Figure 4, the CH/APP coatings shift the Tmax to a high value of over 600 °C than that of CH/MMT coatings (∼545 °C), and CH/APP layers slow the weight loss rate (WLR) more, illustrating that the addition of APP, which is a widely used acid source and gas source in an intumescent FR system, improves the thermal stability of SiFs more efficiently. In addition, compared with the WLR at the Tmax value of pure SiF (5.2%/min), the values of coated SiFs with CH/APP and CH/MMT show an apparent decline (21 BL CH/APP decreases it to 2.1%/min, 60% reduction; 21 BL CH/MMT decreases it to 1.2%/min, 77% reduction). The CH/MMT coatings leave more residue; even 7 bilayers of CH/MMT increased the residue of SiF from a value of 37.1% to 59.8% (an increase of 61.2%). By comparison, the highest residue of CH/ APP-coated SiF with 21 BL is only 49.3%. The reason may partly be attributed to higher add-on mass of the polysaccharide−clay pair. Generally, both nanocoatings effectively improved the initial decomposition temperature, significantly slowed the WLR, shifted the thermogravimetric peaks to higher temperatures, and left more residue. Therefore, for the high-temperature resistance application of SiFs, both nanocoatings successfully endowed the foam with a wider applicable range of temperature by improving its thermostability. 3.3. Flame Retardant and Smoke Suppression Behaviors. The limiting oxygen index (LOI) tests, which indicate the minimum oxygen concentration (vol %) supporting the combustion of materials, were performed. The LOI values and the mass fraction derived from the nanocoatings on SiF are listed in Table 2. The LOI of pure SiF was 20.2%. When

21 BL of CH/APP were coated, the LOI reached 24.8% (increase by 23%). In contrast, when 21 BL of CH/MMT were deposited, the LOI value increased to 25.7% (increased by 27%). During the burning tests of LOI, all SiFs released some white intumescent powder, spreading on their surface, which has been reported to be inorganic silica residue;1 however, the inner part of the pure and coated SiF exhibits different colors. The pure SiF itself only contains a small amount of carbon and leaves a small char after its ignition in a small-scale flame in LOI test. As we know, dense char layer formed during burning is relatively important for the self-extinguishing of a material. The compact char residue could impede the transfer of oxygen, heat, and combustible volatiles, and efficiently suppress burning. It is believed that the formation of compact char could lead to a high LOI value.47 In this work, because little protective residue formed under these mild combustion conditions, only a low LOI value could be obtained. However, there were protective coatings attached on the skeleton of the foams, which contain a certain amount of carbon and other elements, such as N, P, and Al. These coatings enhanced the charring capability of SiFs, and protective char formed during burning can effectively improve the LOI values. Cone calorimetry (CC) is widely used to investigate the combustion performance of materials under a simulation of real fire scenario. This test offers data of heat and smoke evolution versus time. From the curves, critical parameters to describe the fire safety of the materials can be obtained, including the time to ignition (TTI), peak of heat release rate (pHRR), time to pHRR (TTpHRR), heat release rate (HRR), total heat release (THR), smoke produce rate (SPR), and total smoke production (TSP). The HRR and THR, as a function of time, are shown in Figures 5 and 6, respectively. Smoke hazards are the most fatal in real fires. Smoke release behaviors of SPR and TSP curves are shown in Figures 7 and 8, respectively. Corresponding key parameters are summarized in Table 3. All the samples were ignited within 6 s under a heat flux of 50 kW/m2. For those CH/APP nanocoated SiFs, an apparent reduction in the pHRR was first observed. When the coating is 7 bilayers, the pHRR value decrease from 208.6 kW/m2 to 151.0 kW/m2 (a reduction of 28%). However, as Figure 5 shows, the reduction trend did not increase as the bilayer number increased. On the contrary, the pHRR becomes higher when the numbers of the bilayers increase from 7 to 14 or 21. The THR of the materials exhibited similar results. We thus inferred that 7 BL of CH/APP nanocoating was “saturated” to play a sufficiently effective role in improving the flame retardancy of SiFs, which means that 7 BL of CH/APP

Table 2. Mass Percentage of Add-On and LOI Results of Pure and Coated SiFs with CH/APP or CH/MMT Nanocoatings sample CH/APP add-on (wt %) LOI (vol %) CH/MMT add-on (wt %) LOI (vol %)

pure

7 BL

14 BL

21 BL

0 20.2

2.4 23.8

3.6 24.3

7.5 24.8

0 20.2

4.1 23.4

6.8 24.7

13.2 25.7

Figure 5. HRR curves of pure and coated SiFs with 7, 14, and 21 bilayers of (a) CH/APP or (b) CH/MMT. E


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Figure 6. THR curves of pure and coated SiFs with 7, 14, and 21 bilayers of (a) CH/APP or (b) CH/MMT.

Figure 7. SPR curves of pure and coated SiFs with 7, 14, and 21 bilayers of (a) CH/APP or (b) CH/MMT.

Figure 8. TSP curves of pure and coated SiFs with 7, 14, and 21 bilayers of (a) CH/APP or (b) CH/MMT.

Table 3. Cone Calorimeter Data of Pure and Coated SiFs with Different Bilayers of CH/APP or CH/MMTa

a

sample

TTI (s)

pHRR (kW m−2)

TTpHRR (s)

THR (MJ m−2)

TSP (m2)

FIGRA (kW m−2 s−1)

residue (%)

pure SiF (CH/APP)7 (CH/APP)14 (CH/APP)21 (CH/MMT)7 (CH/MMT)14 (CH/MMT)21

4 4 3 5 4 2 6

208.6 151.0 152.9 194.9 169.8 150.6 149.8

25 25 25 30 35 20 25

78.2 66.6 76.1 87.6 82.1 76.5 77.5

14.2 8.2 14.5 20.2 11.6 10.0 8.3

8.3 6.0 6.1 6.5 4.9 7.5 6.0

59.8 67.2 59.6 62.9 67.9 71.6 72.9

Superscripts are as denoted.

gas sauce to get the good flame-retardant effects. With an increased deposition, the ratio of the coating components was changed, which may not meet the desired flame retardancy of SiFs and might lead to higher pHRR and THR. By comparison,

nanocoating for SiFs was effective enough and more bilayers would not contribute more. This phenomenon may be explained as follows: the intumescent flame retardants always need a suitable ratio of components among acid, carbon, and F


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Figure 9. Digital photos of the residues of pure SiF (a) and the ones coated with (CH/APP)7 (b) or (CH/MMT)7 (c) after cone calorimetry tests. For clarity, numbered pilot lamps are used to indicate different sublayers in the residues.

first, fully burnt parts (i.e., heat and gases); second, partially burnt parts (or smoke); and third, the burnt residue. In this study, it was observed that pure SiF turned into a residue with a white, powdered surface layer and a wholly black inner residue (Figure 9a), while coated SiFs exhibited a hierarchical char residue layers with a white, powdered surface and a heterogeneous inner char piece, which consists of multiple morphologies from surface to lower layers (Figures 9b and 9c), suggesting the incomplete combustion of substrates. We divided the residue into two layersa white surface and a dark inner pieceand the latter was finely divided into three sublayers, labeled as “1”, “2”, or “3”, from top to bottom (the colors of the sublayers were approximately marked along the side in Figure 9). Furthermore, to quantitatively study the microstructure of the residues, the element distribution of the defined layers were calculated by EDX mapping and the data are listed in Table 4. As the table shows, element contents of

laminar clays act as “bricks” and the polysaccharide plays a role of “concrete”, which forms a “brick-and-mortar”-like protection layer on the surface of the substrate.20 The clay particles, such as MMT, lose little weight, even when the temperatures reaches 800 °C or above. The physical barrier of CH/MMT layers reduces the pHRR value, a major indicator for heat hazards, in a more regular trend than that of CH/APP coatings. The claybased char blocks the diffusion of flammable vapors, oxygen, and produced heat, inhibiting the burning process. This function not only protects the SiF from degrading fast, but also reduces the pHRR and THR values of foams. The chief parameters of smoke damage in fire were also recorded during the cone calorimeter tests. Lower TSP and peak smoke produce rate (pSPR) imply a slower spread of smoke and, thus, more time for people to be rescued. Figure 8a shows the curves of smoke release for CH/APP coated SiFs, as a function of time. The TSP values of (CH/APP)7 is only 8.2 m2, decreasing to 58% of the original value, while that of (CH/ APP)21, in contrast, increased 42%. The reason may be same with that of the higher THR for (CH/APP)21, which is that the ratio of the CH/APP coating components cannot meet the desired flame retardancye of SiFs at a high number of bilayers. Compared with CH/APP coatings, the TSP and pSPR values of CH/MMT systems monotonously decrease as the coating number increases. Every 7 bilayers of coating leads to an ∼12% reduction of TSP. The reasons for less smoke could also be attributed to an intumescent char or a barrier effect. During a combustion process, a char layer with a small amount of bubbles, which were derived from APP, was formed on the surface of SiF with (CH/APP)7, indicating that a suitable compounding ratio of the layer components between CH and APP would result in an acceptable compact char layer. For those clay−polysaccharide bilayers, they could form thicker and more-compact char layers when more bilayers were deposited on the skeleton of the foams. Their barrier effects to smoke release are based on the sandwich structure of MMT, which hinders the transmission of substances and also forms a heat center for charring reactions. Wilkie et al. prepared a PS/clay nanocomposite with a clay component as low as 0.1% but resulted in a 40% reduction on pHRR.48 Therefore, they proposed a flame retardant mechanism, in which paramagnetic iron within the clay trapped the radicals. In this study, the accumulation of MMT and CH at high bilayers in a sandwich structure has a tendency to maintain better mechanical integrity of a protective layer playing a role of thermal insulation layer, and also prevents the substrates from further degrading and releasing products to the gas phase. Generally, the intumescent flame retardant system of 7 BL of CH/APP nanocoating could produce enough swollen char to protect the substrate from combustion, while 21 BL of CH/MMT nanocoating can form effective protection layers. 3.4. Residue Analysis. In fact, during a combustion process, a material will change into three different forms:

Table 4. Relative Elemental Composition from EDX Mapping Analysis of the Chars of the Pure and Coated SiFs with (CH/APP)7 or (CH/MMT)7 after Cone Calorimeter Testsa SiF Material residue pure white surface dark inner (CH/APP)7 white surface dark inner dark inner dark inner (CH/MMT)7 white surface dark inner dark inner dark inner

Relative Atomic Composition (at. %)

position

C

O

Si

0.4 0.7

68.0 79.7

29.5 18.0

1 2 3

0.9 1.4 6.8 4.9

70.8 83.3 71.3 75.6

28.4 12.9 17.0 12.4

1 2 3

8.6 12.5 35.1 23.4

64.2 68.1 59.2 66.3

27.2 19.4 5.6 10.2

P

N

0.1 0.2 0.3

2.1

Al

6.5

0.03 0.10 0.11

a

The numbers (1, 2, and 3) indicate different layers in the dark inner piece, as marked in Figure 9.

white surface varies between the pure SiF, (CH/APP)7, and (CH/MMT)7 coated SiFs. The oxygen contents are all at an exceptionally high level, which may be caused by the instrumental error and probable absorption of water for the porous char residues derived from the SiF. Nevertheless, the elemental carbon contents are different over a variety of times, which could be an indicator of the charring behaviors. There was very little carbon remaining in the white surface char of pure SiF (∼0.4%), while that of (CH/APP)7 and (CH/MMT)7 were 0.9% and 8.6%, respectively (both of which are greater than that observed for the pure SiFs). For the CH/APP coated SiF, a cohesive residue layer that consisted of thermally stable G


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Figure 10. SEM images of the dark inner parts of the char residues of pure (a) and 7, 14, and 21 bilayers coated SiFs with CH/APP (b-1, b-2, and b3) or CH/MMT (c-1, c-2, and c-3) formed in a muffle furnace at 600 °C.

silicon phosphate could be formed to reduce the flammability and the smoke production of the material.49 In contrast, the sample coated with CH/MMT left much more carbon than the pure SiF and the CH/APP-coated SiFs. The reasons may be attributed to the following: (1) the add-on mass of CH/MMT was more than that of CH/APP, and, hence, the carbon content of CH/MMT was originally higher; and (2) the barrier effects produced by MMT platelets inhibited the combustion of SiF substrate and allowed the carbon to be retained in the residue. Moreover, the elemental distribution also show that coated SiFs contain more carbon and less silicon in inner layers, compared with that of pure SiF, especially for the CH/MMT-coated foams. Comparing the carbon contents of the inner residues, that of the (CH/MMT)7-coated SiF was determined to be was much higher than that of the (CH/APP)7-coated SiF. The reason may not be simply attributed to the protective CH/ MMT layer, which preserves the organic substrates; the reason may also be attributed to the flammable compounds permeating into the spaces between MMT platelets in nanoscale, which consequently enhanced the stability and the homogeneity of the intercalated char layer in combustion.50−52 The carbon contents in the char residues of coated SiFs are higher than that of pure SiF at any part in the char. This result indicates that the coated foams, especially for those with two or three sublayers, did not fully combust under the protection of the nanocoatings. Therefore, they released less total heat and left more residue. Furthermore, dark inner char residues were characterized by SEM (Figure 10). Tiny bubbles can be observed with diameters ca. 3 μm (Figure 10b), formed by the fragments of SiF and CH in combustion and were blown up by the gas released from

APP in combustion. For the 21 BL CH/APP-coated SiF, dense bubbles covered the foam skeleton overall. Besides, during combustion of the coated SiF, the “mortar” of CH converted to char, which filled the gap between clays and formed continuous and compact layers wrapping the foam skeleton (Figure 10c).

4. CONCLUSION In this work, we demonstrated successful fabrication of sustainable and eco-friendly layer-by-layer (LbL) assembles of chitosan (CH)/ammonium polyphosphate (APP) and chitosan (CH)/montmorillonite (MMT) nanocoatings on polysiloxane foam. The coatings exhibited their effectiveness in flame retardancy and smoke suppression without compromising the processing and properties or destroying the open-celled structure of the foams. Compared with CH/APP nanocoatings on flexible silicon foams (SiFs), the CH/MMT layers provide a more effective barrier on smoke release and form denser chars during combustion. Systematical char analysis again supports the better protective function of CH/MMT nanocoatings. Both coatings offer low cost and eco-friendly fire safety to SiFs, which widens their range of application.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W. Liao). *Tel./Fax: +86-28 85410755. E-mail: [email protected] (Y.Z. Wang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51320105011 and H


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51421061) and Program for Changjiang Scholars and Innovative Research Team in University (No. IRT 1026).

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