Chitosan Nanobrick Wall on Flexible

Subscriber access provided by University of Sunderland

Applications of Polymer, Composite, and Coating Materials

Extreme Heat Shielding of Clay/Chitosan Nanobrick Wall on Flexible Foam Simone Lazar, Federico Carosio, Anne-Lise Davesne, Maude Jimenez, Serge Bourbigot, and Jaime C. Grunlan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10227 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

To be submitted to ACS AMI

Extreme Heat Shielding of Clay/Chitosan Nanobrick Wall on Flexible Foam Simone Lazar1ǂ, Federico Carosio2*ǂ, Anne-Lise Davesne5ǂ, Maude Jimenez5, Serge Bourbigot5*, Jaime Grunlan1-3* 1

Department of Chemistry, 2Department of Materials Science and Engineering, 3Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA 4 Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Alessandria campus, Viale Teresa Michel 5, 15121 Alessandria, Italy 5 University of Lille – ENSCL, Unité Matériaux et Transformations (UMET) - CNRS UMR 8207 Av. Mendeleiev - CS 90108, 59652 Villeneuve d’Ascq Cedex – France

Keywords: layer by layer, chitosan, vermiculite, polyurethane foams, fire protection, insulation

Abstract Flexible polyurethane foam is widely used in bedding, transportation, and furniture, despite being highly flammable. In an effort to decrease the flammability of the polymer, an environmentally-friendly flame retardant coating was deposited on polyurethane foam (PUF) via layer-by-layer assembly. Treated foam was subjected to three different fire scenarios – 10 s torch test, cone calorimetry, and a 900 s burn-through test – to evaluate the thermal shielding behavior of an eight bilayer chitosan/vermiculite clay nanocoating. In each fire scenario, the nanocoating acts as a thermal shield from the flames by successfully protecting the backside of the PUF, while the side directly exposed to the flame results in a hollowed nanocoating that maintains the complex three-dimensional porous structure of the foam. Cone calorimetry reveals that the coating reduces the peak heat release rate and total smoke release by 53% and 63%, respectively, while a temperature gradient greater than 200°C is observed across a 2.5 cm thick coated foam sample during the rigorous burn-through fire test. The thermal shielding behavior of this

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26


polymer/clay nanocoating makes this system very attractive in improving the fire safety of polyurethane foam used for insulating applications.

1. Introduction The heat and flame protection of polymeric materials continues to be a major area of concern due to their widespread use and contribution to our unprecedented quality of life. Catastrophic events, such as the fire that recently devoured the Grenfell Tower and took numerous lives in West London,1 highlight the importance of flame retardants (FR) in building and construction materials. Flexible polyurethane foam (PUF), for example, is commonly used in mattresses or seat cushions in furnishings found in homes, work environments, and various methods of transportation such as automobiles, trains, and aircraft.2 Polyurethane foam is very flammable and melt drips, leading to rapid fire propagation.3-4 Improving the heat and fire resistance of PUF is crucial in preventing or delaying the spread of fire and protecting nearby items from extreme heat, which can ultimately save lives by providing longer escape time.

Halogenated flame retardants, such as bromine, have traditionally been used as additives to delay and limit the occurrence of fires. These types of molecules are usually small in size and tend to migrate out of the host polymer over time, leading to bioaccumulation.5 With increasing environmental concerns and health regulations, halogenated flame retardants are being scrutinized due to their inherent toxicity when released into the atmosphere.6-9 The application of non-halogenated flame retardant coatings on the surface of polymers is thus an attractive alternative that can reduce the environmental impact of flame retarded PUF and potentially


Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



reduce the likelihood of altering mechanical properties, which is often observed with flame retardant additives.10-11

Layer-by-layer (LbL) assembly has been heavily studied as a means to apply flame retardant nanocoatings to a variety of substrates.12-13 This technique is based on electrostatic interactions occurring between two oppositely charged components in aqueous solution,14-16 but can also employ hydrogen-bonds,17-19 van der Waals forces,20 or covalent bond formation between layers.21 Layer-by-layer assembly is a valuable tool for applying conformal flame retardant coatings, especially to the complex three-dimensional open-cellular structure of polyurethane foam. Hybrid multilayer nanocoatings, comprising organic polyelectrolytes and inorganic platelets, are promising thermal shielding systems for flexible polyurethane foam.22 The nanobrick wall structure of these coatings acts as a barrier that shields the core of the foam from the flame,23 while minimizing heat and mass transfer through the material. Recently, LbL deposition of halloysite clay has been used to render flexible polyurethane foam selfextinguishing.24 In another study, a layered double hydroxide (LDH) multilayer coating was deposited on PUF.25 The use of LDH substantially increased the thickness of the coating relative to montmorillonite clay, and reduced the peak heat release rate. Despite a growing body of work focused on the fire behavior of these hybrid nanocoatings,23-27 very little work has been done to evaluate and understand their extraordinary thermal shielding behavior.

In the present study, a completely renewable LbL system, consisting of chitosan (CH) and vermiculite clay (VMT), has been deposited on flexible polyurethane foam to impart significant thermal shielding behavior. This nanocoating contains relatively few bilayers (i.e. few processing



Page 4 of 26


steps). Chitosan was chosen because of its charring ability,28 while vermiculite clay was chosen because of its high aspect ratio.4 The chitosan acts as an organic mortar that holds together the high aspect ratio vermiculite nanobricks (i.e. nanoplatelets). An 8 BL CH/VMT coated foam withstands a novel bench-scale fire test that is comparable to large-scale fire-resistant tests found in industry.29 When exposed to an 1100°C flame, the temperature measured by a thermocouple 5 mm from the backside of a 2.5 cm thick foam is maintained at only 328°C. Cone calorimetry reveals that the coated foam strongly decreases heat release rates and maintains intact its original shape after combustion, while the novel burn-through test reveals that the coating can also reduce conductive and convective heat transfer. This scalable, nanobrick wall coating could allow commodity polyurethane foam to be used in a variety of new insulating applications to protect various types of dwellings or vehicles from heat and fire.

2. Experimental 2.1 Materials Branched polyethylenimine (PEI, 100 000 g mol-1) and poly(acrylic acid) (PAA, 25 000 g mol-1, 35 wt% in water) were purchased from Sigma-Aldrich (Milwaukee, WI). Chitosan (CH, 60 000 g mol-1, deacetylation 95%) was purchased from G.T.C. Union Group Ltd. (Qingdao, China) and Microlite 963++ vermiculite clay (VMT, 7.8 wt% in water) was purchased from Specialty Vermiculite Corp (Cambridge, MA). All solutions were prepared with 18 MΩ deionized (DI) water and the pH of solutions were adjusted using diluted solutions that were prepared from 37% hydrochloric acid, 97% sodium hydroxide pellets, or 70% nitric acid purchased from SigmaAldrich. Open cell, polyether-based polyurethane foam (type 1850, 1.75 lbs ft-3 density) was purchased from Future Foam (High Point, NC).


Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



2.2 Layer-by-Layer Deposition Silicon wafers were thoroughly rinsed with DI water and methanol and then dried prior to a 5 min plasma treatment to impart negative charge on the surface. A lab-built robot constructed for LbL assembly was used to deposit the nanocoating on the wafers.30 The wafers were first primed with 0.1 wt% PEI for 5 min, followed by a 5 min dip into 1.0 wt% VMT. All consecutive bilayers consisted of 1 min dips into aqueous solutions of CH and VMT, with rinsing and drying in between each step. Polyurethane foam (20 x 20 x 2.5 cm3) was rinsed with DI water and dried in an oven at 70°C overnight prior to deposition. The foam was first exposed to a 1.0 wt% PAA solution (adjusted to pH 2 using 2M nitric acid) for 1 min to induce a negative surface charge. The foam was then exposed to a 0.1 wt% PEI solution (unaltered pH of 10.3) for 5 min, followed by exposure to a 1.0 wt% VMT solution (unaltered pH of 7.8) for 5 min to complete the initial bilayer. All consecutive bilayers consisted of CH/VMT with 1 min deposition times until the desired number of bilayers was achieved. 0.1 wt% CH solution was prepared by adjusting the pH to 1.6 using 5M hydrochloric acid to fully dissolve the chitosan. The chitosan solution was left to stir overnight and then adjusted to pH 6 using 5M sodium hydroxide prior to deposition. Each deposition involved compressing the foam three times in a given solution. The foam was then thoroughly rinsed with DI water and wrung out using mechanical rollers before submerging the foam in the next solution. Coated foam samples were dried overnight in an oven at 70°C before testing. The overall layer-by-layer process for coating polyurethane foam is illustrated in Figure 1.



Page 6 of 26


Figure 1. Schematic of layer-by-layer deposition of CH/VMT nanobrick wall coating on flexible polyurethane foam.

2.3 Characterization Film thickness was measured on polished silicon wafers (University Wafer, Boston, MA) using an alpha-SE Ellipsometer (J.A. Woollman Co., Inc., Lincoln, NE), equipped with a 632.8 nm laser. The surface morphology of the uncoated and coated polyurethane samples was observed by sputter coating the samples with 5 nm of platinum/palladium alloy prior to imaging using a field-emission scanning electron microscope (SEM) (Model JSM-7500, JEOL; Tokyo, Japan). Electron probe micro-analysis aluminum x-ray (EPMA Al X-ray) mapping was carried out on cross-sections of the samples at 15 kV and 4 nA using a CAMECA SX100. Samples were embedded in epoxy resin and polished up to 0.25 µm with carbon using a Bal-Tec SCD005 sputter coater. The nature of the nanocoating on PUF was also observed using a FEI Tecnai G220 twin transmission electron microscope (TEM).

The initial fire screening of the control and coated polyurethane foam (5 x 5 x 2.5 cm3) were carried out using a small-scale torch test. Samples were placed on a wired grid and directly subjected to the flame (~1400°C) of a butane hand torch (Triggertorch MT-76 K, Master Appliance Corps.; Racine, WI) for 10 s. In order to compare the flame resistance of the coated 6 ACS Paragon Plus Environment

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



foam to the control foam, samples were weighed before and after the torch test to calculate mass loss. The morphology of the residue from the torch test was analyzed using the SEM procedure previously described to characterize the morphology of the PUF prior to testing.

A cone

calorimeter (Fire Testing Technology (FTT), East Grinstead, UK) was employed to investigate the combustion behavior of untreated and LbL-treated foam (10 x 10 x 2.5 cm3) under 35 kW/m2 heat flux in a horizontal configuration, following the ISO 5660 standard. Time to ignition (TTI, [s]), peak heat release rate (pkHRR, [kW/m2]), total heat release (THR, [MJ/m2]), smoke production rate (SPR, [m2/s]), total smoke release (TSR, [m2/m2]) and final residue were measured. The morphology of the cone combustion residue was imaged using a LEO-1450VP SEM (LEO Electron Microscopy, Thornwood, New York). Fourier transform infrared spectroscopy in attenuated total reflectance (FTIR ATR) spectra of the combustion residue were collected in the range 4000-700 cm-1 (16 scans and 4 cm-1 resolution) at room temperature using a FT-IR/FIR spectrophotometer (Perkin Elmer mod. Frontier, Waltham, MA, USA) equipped with a diamond crystal. A burn-through fire test designed by the R2Fire team in Lille was performed on the neat and treated foam samples (20 x 20 x 2.5 cm3).29 The PUF sample was fixed vertically in a sample holder made of two plates of refractory material with a 10 x 10 cm2 window. A flame was applied using a propane burner under 116 ± 10 kW/m² heat flux for 15 min or until the flame penetrated the sample. Mass loss was measured by weighing the sample before and after testing. Temperature changes through the thickness of the sample were measured using thermocouples that were placed at 5, 10, 15, and 20 mm from the front side. The front and back side of the coated PUF residue obtained after the burn-through fire-test was imaged using a Hitachi S4700 SEM, as well as EPMA Al X-Ray mapping and TEM that were previously described.



Page 8 of 26


The thermal degradation behavior of the residues from the burn-through fire test was measured, using a Discovery thermogravimetric analyzer (TGA) from TA Instruments, and compared with unburnt coated and uncoated foam. The samples weighed approximately 6 mg and tests were conducted in an air atmosphere from 40°C to 1000°C, at a heating rate of 10°C/min. The purge flow and the sample purge flow were set respectively at 15 mL/min and at 50 mL/min. TGA was coupled with a Nicolet iS10 for the FTIR analysis of the evolving gases. The gases were brought to the spectrometer by a transfer line heated at 225°C to avoid condensation. The background was acquired before each analysis and the number of scans were set at 8 with a resolution of 4 cm-1. The gases that evolved during the burn-through fire test were analyzed with a Fourier transform infrared spectrometer provided by an AntarisTM Industrial Gas System (Thermo Fisher, Waltham, MA), equipped with a gas cell (2 m long optical pathway, chamber filled with dry air) with two KBr windows. The transfer line was heated up to 200°C to avoid condensation of hot evolved gases. The temperature of the FTIR gas cell was set at 186°C and its pressure was set at 653 Torr. The spectra were analyzed using OMNIC software. This set up allowed for analysis of gases such as carbon dioxide, sulfur dioxide, nitric oxide, nitrogen dioxide, water, hydrogen bromide, hydrogen chloride, hydrogen cyanide, ammonia, ethene, acetylene, carbon monoxide, and propane.

Chemical structure was explored using solid state magic angle spinning nuclear magnetic resonance (MAS NMR), performed on a Bruker Advance II 400 MHz spectrophotometer.

13

C

Solid State MAS NMR analysis with proton cross-polarization was performed at a rotating speed of 12,500 Hz to obtain a good signal to noise ratio. Single-pulse analysis was also performed using a rotating speed of 10,000 Hz and 512 scans. Both analyses were carried out in a low-field


Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



4 mm probe using glycerine as the standard. For 29Si Solid State MAS NMR, the rotating speed was set at 5,000 Hz and experiments were carried out with 64 scans using a 7 mm probe. Proton cross-polarization was performed when possible, with 2048 scans. The probe and the rotating speed were the same as single pulse experiments and the standard used was tetramethysilane. 27

Al Solid State MAS NMR was also carried out using a rotating speed of 20,000 Hz and a 3.2

mm probe. The standard used was Al(H2O)63+.

3. Results & Discussion 3.1 Nanocoating Structure Vermiculite clay is comprised of magnesium, aluminum, iron, silicon, hydrogen, and oxygen that are arranged into two tetrahedral sheets that stack above and below an octahedral sheet.31 The anionic vermiculite clay platelets are held together by cationic chitosan that, when deposited layer-by-layer on a silicon wafer, results in a linearly growing system (Figure 2a). In order to deposit the same system on polyurethane foam, the sample was first primed with a PAA solution to induce a negative surface charge via hydrogen bonding.4 Bilayers comprised of CH and VMT were then deposited on the PUF, with PEI substituting for chitosan in the first bilayer to improve adhesion.32 After deposition, the polyurethane foam’s white color is a visually homogeneous beige (Figure 2b), adding 17.5 ± 0.5 wt% to the PUF with 8 bilayers (BL). Low magnification SEM of the uncoated PUF shows the typical three-dimensional open cell porosity that is characterized by a smooth, even morphology (Figure 2c). This morphology is evidently changed after LbL deposition, in which the walls of the foam are conformally coated and possess some texture due to the presence of the clay-containing nanocoating (Figure 2d). Electron probe microscopy, using aluminum x-ray mapping, provides a quantitative elemental analysis of



Page 10 of 26


aluminum that is characteristic of vermiculite found within the nanocoating (Figure 2e). The aluminum x-ray mapping is found to be mainly in the blue/green region of the quantifiable scale, which confirms the homogeneous nature of the coating along a PUF strut compared to the uncoated PUF that lacks the presence of aluminum. A cross-sectional TEM image reveals the multilayered structure of the deposited coating (Figure 2f). The darker lines of the micrograph represent the VMT nanoplatelets that are preferentially oriented parallel to the foam surface and are embedded within a CH matrix that appears lighter in color due to the difference in electron density. Although the thickness observed by TEM is not very accurate as a result of the sample preparation, it is representative of how the nanocoating deposits on PUF (i.e. three-dimensional porous substrate), which differs from how the system deposits on a silicon wafer. The coating of foam is not a “clean” process due to the rigorous rinsing and drying process that involves compressing the foam in water and thoroughly wringing out the foam after each deposited layer. Nonetheless, the micrograph clearly shows how the nanobrick wall coating follows the PUF’s complex three-dimensional structure by bending on the cell wall edges and conformally coating every surface available.

Figure 2. a) Layer-by-layer growth of the CH/VMT nanocoating deposited on a silicon wafer, b) digital images of uncoated and coated PUF, SEM image of c) uncoated and d) coated PUF, e) EPMA Al X-Ray mapping of coated PUF, and f) TEM cross-sectional micrograph of 8BL CH/VMT deposited on PUF. 10 ACS Paragon Plus Environment

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



3.2 Thermal Behavior of Nanocoated Foam The initial fire screening of the coated polyurethane foam was carried out using a 10 s butane torch test to evaluate the flame retardancy of the coated and uncoated PUF. When the torch test is carried out on uncoated foam, the flame entirely consumes the sample and melt dripping is observed. When the 8BL CH/VMT coated PUF is exposed to the same torch test for 10s, the nanocoating shields the flame from the inner core of the PUF allowing it to maintain its original shape and structure, losing only 25.2 ± 0.4 wt% of its initial mass (Figure 3a). As seen in the cross-sectional image of the treated PUF, only the outer surface of the sample is charred, while the inner core is preserved (Figure 3b). Figure 3c shows that the 3D porous structure observed from the charred area of the treated PUF has been hollowed (circled in yellow), suggesting that the polyurethane substrate was consumed by the flame, leaving behind nothing but the nanocoating. The hollowed structure of the charred surface of the foam is further confirmed when discussing the mechanism for thermal shielding (see Section 3.3).

Figure 3. a) Digital images of (left) 8BL CH/VMT PUF and (right) cross-section of PUF after 10 s torch test (the foam sample was cut through the middle to reveal the undamaged material inside) and SEM images of b) core and c) char of 8BL CH/VMT PUF after torch test.



Page 12 of 26


Cone calorimetry was employed to evaluate the reaction of modified PUF to a radiant heat flux normally found in developing fires (35 kW/m2).33 The sample exposed to the heat flux degrades and releases combustible volatiles. A spark is generated periodically, so that when the released volatile gases reach the proper concentration the material ignites and is subjected to flaming combustion. Figure 6a shows heat release and smoke production data, as well as the post combustion residue analysis performed with SEM. Table S1 summarizes the cone data. Upon exposure to the cone heat flux, the unmodified PUF quickly ignites and its structure starts collapsing, while the heat release rate sharply increases. The foam then completely melts, generating a pool of a low viscosity liquid that vigorously burns, achieving the maximum HRR (427 kW/m2). The flames almost completely consume the sample, leaving a residue that is just 3% of the initial mass.

Figure 4. Heat release rate as a function of time, as measured by cone calorimetry, and post combustion residue morphology: a) HRR and THR, b) SPR and TSR, c) digital images of post combustion residues (left is neat PUF and right is 8BL coated PUF), and d) SEM micrographs of 8 BL coated PUF.


Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



The presence of the CH/VMT coating completely changes the burning behavior of the foam. After ignition, no collapsing occurs and the foam shows substantially reduced HRR values that peak at only 201 kW/m2 (Figure 4a). This behavior is ascribed to the CH/VMT coating that can mechanically sustain the foam and produce a protective barrier. Indeed, upon heating, chitosan undergoes a char forming reaction that leads to the production of thermally stable aromatic char.34 This carbon-based matrix holds together the VMT nanoplatelets and results in a protective coating that is capable of limiting mass and heat transfer from/to the flame, thereby reducing the heat release rate. Smoke generated is also dramatically reduced, as evidenced by a 63% reduction in total smoke released. This is an important result because reduced smoke production in the early stages of a fire will allow people to escape more easily (and without suffering from long term health problems due to smoke inhalation).35 The final residue of the 8BL coated foam is increased to 22% indicating that the presence of the coating also improves the char forming ability of PUF. This additional reduction of the combustible volatile released is reflected in the THR values (see Table S1). This char was further confirmed by ATR spectroscopy (Figure S1), where the presence of a peak located at 1600 cm-1 can be ascribed to C=C stretching in conjugated aromatic carbonaceous structures.36 The residue collected at the end of the test has been imaged by SEM microscopy (Figure 4d), revealing that the CH/VMT coating efficiently preserves the original PUF three-dimensional structure, similar to what was already observed in flame torch tests (compare Figure 3b, c with Figure 4d).

Uncoated and coated foam were subjected to a burn-through fire test at high heat flux (116 kW/m2) to assess the behavior of the samples in a fire scenario similar to those commonly used as an evaluation tool in industrial practice.29 Thermocouples (T1-T4) were embedded in the foam



Page 14 of 26


at various distances to measure the temperature gradient across the sample (Figure 5a,b). Upon exposure, the uncoated polyurethane foam melts immediately and completely degrades under the flame in a matter of seconds. The 8BL CH/VMT coated PUF, however, withstands the high heat flux for more than ten minutes. Figure S2 shows photographs taken at regular intervals during the burn-through fire test. No melt dripping is observed, but high amounts of smoke is released during the first minute of the test. Moreover, as soon as the flame from the burner impinges the sample, it turns from blue to orange, suggesting the release of decomposition products from the foam. This reaction occurs for the first minute of the test, before the smoking nearly stops and the flame turns blue again, suggesting that all flammable material has either reacted or been consumed.

Figure 5. a) Side view schematic of the 10 x 10 cm2 portion of the foam sample exposed to the flame. b) Back side schematic of the sample showing the relative positions of the thermocouples. c) Temperature change within the 8BL-coated foam during the burn-through fire test, as measured by thermocouples.

The front side of the foam begins to glow red only after a few seconds of being exposed to the flame. This phenomenon is similar to that observed when ceramic material is submitted to high temperatures, suggesting that the front side of the sample turned into a ceramic residue. The side exposed to the flame becomes brittle, light and powdery, with an ashy feel, and did not sustain


Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



even weak probing. In contrast, the backside of the sample turns into a black and rigid crust that grows during the test and is surrounded by yellowish particles. Most importantly, it is possible to see that the open-cell structure of the foam is kept intact, and the foam does not collapse during the test. Weight measurements before and after testing show that the sample loses approximately 25 wt% of its mass in the case of the treated foam. Table S2 summarizes the results of the various fire tests performed on the PUF samples.

The temperature registered by the thermocouples (Figure 5c) shows that a steady state regime is reached 3 minutes after exposure of the sample to the flame. The graph shows that the temperature difference between the thermocouples gradually increases from the front side of the foam to the back side (T1-T2 = 37°C, T2-T3 = 87°C, and T3-T4 = 121°C). Moreover, a significant temperature variation of 245°C between thermocouple 1 (T1 = 573°C) and thermocouple 4 (T4 = 328°C) is observed. This means that the 8BL CH/VMT nanocoating exhibits thermal shielding behavior that may be of great interest for insulation.

The morphology of the foam was analyzed after exposing the 8BL coated foam sample to the burn-through fire test. Digital images of the front and backside of the coated foam provide a visual observation of the sample as a result of exposing it to the 900s burn-through test (Figure 6a,b). Cross sectional X-Ray mapping of aluminum in the coated foam residues (front side and backside) were obtained by EPMA (Figure 6c,d). This confirms that the vermiculite within the coating is still present after burning, and that it maintains the open-cell structure. This is also confirmed by SEM micrographs of the front side of the residue (Figure 6c), which was directly exposed to the flame. Similar to the charred surface of the PUF after torch testing, the polyurethane is completely consumed, leaving only a hollow porous skeleton behind. SEM of the 15 ACS Paragon Plus Environment


Page 16 of 26


sample closer to the backside (Figure 6f) also shows the preserved foam structure, which is somewhat damaged (cracks and coarse surface) because of the fire test. The insets in Figure 6 show some holes and round protuberances at the surface of the cell walls. This is likely the result of the release (or trapping) of combustion gases during the burn-through fire test.

Figure 6. Digital images of coated PUF during the burn-through test: a) front view and b) back view. Aluminum cross section EPMA X-ray mapping of coated foam residues after the burn-through fire test: c) front side and d) backside of PUF. SEM images of residue from the burn-through fire test: e) front side and f) back side of PUF. Insets reveal the formation of cracks and bubbles.

3.3 Mechanism of Thermal Shielding To examine any potential gas phase action of the nanocoating, in-situ gas phase analysis was performed using a gas-picking system coupled with FTIR. It is observed that the gases emitted during the test are mainly carbon dioxide (CO2), carbon monoxide (CO), nitric oxide (NO), water (H2O), and propane (C3H8). No significant differences were observed between the neat PUF and 8BL coated PUF (Figure 7). CO2 and H2O were emitted continuously throughout the test, along with CO and NO emissions. With regard to C3H8, quantities emitted seem higher for the coated PUF than for the neat PUF. These disparities can be explained by the different test durations, as the neat PUF test was stopped only seconds after it began to avoid damaging


Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



equipment from the significant flaming of the sample. When comparing the two samples on the same time scale, the level of C3H8 emitted by the 8BL-coated foam during the first few seconds of the test corresponds to the level emitted by the neat foam. This result suggests that the nanocoating does not have gas phase action.

Figure 7. In-situ FTIR analysis of the emitted gases from a) uncoated PUF and b) 8BL CH/VMT coated PUF during the burn-through fire test.

Temperature measurements from the burn-through fire test, as well as the evolution in the appearance of the residue through the thickness of the sample, suggests a gradient in the composition of the residue depending on its distance from the flame. Therefore, the char was divided into four parts, A to D, based on the position of the thermocouples in the burn-through fire test (Figure 8a). Thermogravimetric analysis was performed on these four portions of the residue to gain insight about its chemical composition and help elucidate the thermal protection mechanism. TGA of the four parts of the coated foam residue are compared to the uncoated and coated foam prior to the fire test (Figure 8b). Most residues exhibit degradation steps attributed to the degradation of residual organic matter. In other words, a charred residue was formed during the fire test, which further degrades during thermogravimetric analysis. Interestingly, this 17 ACS Paragon Plus Environment


Page 18 of 26


char proportion increases through the thickness of the sample, with position A (directly exposed to the flame) degrading only very slightly, meaning that no organic part remains at the end of the fire test, whereas position D (backside of the foam) loses more than 70 wt%. Weight loss during the main degradation steps in TGA is attributed to the decomposition of the organic matter, suggesting an evolution in the organic/inorganic proportions of the residue as analysis is carried out further from the side of direct exposure to the flame.

Figure 8. a) Cross-sectional image of the coated foam residue from the burn-through fire test, with schematic showing the position of the four parts (Char A to D) collected for analysis. b) Thermogravimetric analysis of the residues from the burn-through fire test under air atmosphere, along with uncoated and 8BL CH/VMT coated PUF prior to fire testing. Residual organic matter and residues are also indicated. The residual weight of Char A to C is attributed to degradation of the organic part and dehydration, while the organic content was determined by the weight percentage degraded during the main degradation step. The residual weight and organic content of Char D, coated, and uncoated PUF were determined by the weight percentage degraded during the two main decomposition steps of the TGA curve. The inset in b) shows the thermal stability of VMT clay. 18 ACS Paragon Plus Environment

Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



A more detailed analysis reveals that two degradation behaviors are observed in TGA. Initially, Char A to C exhibit only one main degradation step between 500 and 600°C, attributed to the degradation of a stable char formed by the exposure of polyurethane to high temperature during the fire test. The proportion of this char increases across the sample (4, 13, and 36% for char A, B, and C, respectively). FTIR analysis of the gases released were recorded during thermogravimetric testing to support the hypothesis of the degradation of residual organic matter. Figure S3 shows that the degradation consists mostly of carbon and nitrogen (emission of CO2, CO, HCN, and H2O), while Table S3 provides the complete list of peak assignments. Finally, the mass loss not accounted for in the TGA graph (5, 3, and 2% for Char A, B, and C, respectively) is attributed to dehydration of the mineral part (i.e. vermiculite).37-38 Char D, furthest from the flame, exhibits a different behavior, where two degradation steps are observed. The first one occurs between 200 and 300°C, while the second one is between 450 and 500°C. Both correspond to the degradation steps observed during the thermal degradation of unburnt polyurethane foam. FTIR analysis of the gases show that they are identical to the gases produced by thermal degradation of the unburnt coated foam (HCN, alkanes, ester and ether compounds, CO2 and CO) (Figure S4). It seems reasonable to assume that neat polyurethane was preserved during the fire test despite the high heat flux.

The four portions of the residual foam were also analyzed by NMR, as shown in Figure 9. Spectra for the uncoated and coated foam show peaks at 130 and 136 ppm, corresponding to aromatic carbons. The peak at 156 ppm is linked to the C=O bond from urethane and the peaks at 40 and 30 ppm correspond to aliphatic carbons from the diisocyanate monomer. The peaks at 75,



Page 20 of 26


73, 71 and 69 ppm are linked to C-O bonds from the diol monomer, whereas the peak at 18 ppm is from the - (CH2)n - aliphatic region.39 Regarding the residues (Char A to C), no signal was recorded in 13C solid-state MAS NMR with 1H cross-polarization. This finding suggests a lack of protons, due to the significant degradation. As a result, single pulse experiments (direct observation) were carried out to observe their structure. A peak at 132 ppm is observed in the spectra of Char B and C, suggesting that carbon is still present in aromatic form. It should be noted that the peak is asymmetric, which indicates oxidation of the aromatic carbon.40 Regarding the spectrum of Char A, no signal was detected. This result confirms that the part of the sample directly exposed to the flame is almost completely inorganic, due to the total degradation of the polymer.

Figure 9. a) 13C Solid State MAS NMR with 1H cross-polarization of the residues (A, B and C) from the burn-through test. b) 13C Solid State NMR with 1H cross-polarization on Char D from the burn-through test and on the unburnt coated and uncoated foam, with peak attributions.39 c) 13C Solid State NMR (direct observation) on the residues (A, B, and C) from the burn-through fire test.

On the contrary, the NMR spectrum of Char D shows a broadening of the aromatic peak near 130 ppm, and a general loss of resolution, although the characteristic peaks of polyurethane remain. Two peaks appear at -0.30 and -3.97 ppm, which may correspond to the formation of a 20 ACS Paragon Plus Environment

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



methyl-silica bond,41-43 resulting from a reaction between vermiculite and the organic part of the sample during burning. Figure S5 proposes the possible structures of these methyl-silica bonds,41 where the D-Site and T-Site refer to the number of oxygens surrounding the Si atom. 27

Al MAS solid state NMR was also carried out on the residual coated foam to provide more

information on the eventual transformation of vermiculite during the burn-through fire test. Spectra of 8BL CH/VMT coated PUF, Char D and Char C show two peaks at 57.4 and at -0.5 ppm, corresponding to the respective presence of tetrahedral (AlO4) and octahedral sites (AlO6) found in vermiculite clay (Figure S6). Spectra of Char A and Char B show only one peak at 57.4 ppm. These two pieces of information suggest that vermiculite is preserved during the fire test.

Figure 10 shows TEM micrographs used to compare the structure of the coating before and after the burn-though fire test. As previously shown in Figure 2f, the nanocoating possesses a well organized structure, with layers oriented parallel to the surface of the polyurethane foam. After the burn-through test, it can be seen that the highly organized nanostructure of the coating is preserved even after the application of a high heat flux. Clay nanoparticles are proposed to be the sole material staying intact during the test and remain organized on the walls of the foam, thus preserving its structure. Vermiculite sheets are clearly visible, even more than before the test, due to the degradation of the organic material. Moreover, closer to the backside, the coating retains the same morphology as observed prior to the burn-through fire test. Finally, the areas of the images circled in yellow, suggest that the charred residue concentrates in and around the coating, as observed by the analysis after cone calorimetry. The presence of vermiculite within a carbonaceous matrix prevents the sample from crumbling into ashes due to the fragility of the other parts of the residue.



Page 22 of 26


Figure 10. TEM image of the residue a, b) close to front side after the burn-through fire test, and c, d) close to backside. Grey and white areas correspond respectively to epoxy resin (used to prepare the sample for TEM analysis) and voids.

4. Conclusions An environmentally-benign nanocoating, consisting of chitosan and vermiculite clay, was deposited on flexible polyurethane foam via layer-by-layer assembly. A nanobrick wall thin film was conformally deposited throughout the complex three-dimensional porous structure of the foam. This nanocoating was found to act only in the condensed phase by providing thermal shielding. It provides an inorganic exoskeleton that maintains the structure of the foam, through nano-organization of the vermiculite platelets. At the surface of the foam, only inorganic matter is observed following fire testing, which serves to reduce the temperature felt deeper within the foam. The resulting temperature gradient promotes char formation and a stable residue. Chemical analyses reveal that the char is primarily aromatic. This mechanism takes place across the sample from the front side to the backside and demonstrates the protection of polyurethane, which is


Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



shown to degrade only partially at the backside of sample after being exposed to an 1100°C flame. This tremendous heat shielding behavior, from such a thin and conformal coating, is promising for improving the fire safety of polyurethane foam, especially in insulating applications.

ASSOCIATED CONTENT Supporting Information Supporting information is available free of charge via the Internet at http://pubs.acs.org. FTIR analysis, NMR spectra, and summary of fire test results.

AUTHOR INFORMATION Corresponding Authors *Jaime Grunlan (Email: [email protected], Phone: 979-845-3027) *Serge Bourbigot (Email: [email protected], Phone: +33 (0)3 20 43 48 88) *Federico Carosio (Email: [email protected], Phone: +39 0131 229303)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ǂ These authors contributed equally to this work.

ACKNOWLEDGEMENTS The Polymer NanoComposites Laboratory would like to acknowledge funding from the United States Air Force Research Laboratory (Award No. FA9300-15-C0004), while the R2Fire



Page 24 of 26


research team would like to acknowledge the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement n°670647/FireBarConcept/2014-2020) for funding. This study is also based upon research supported by the Chateaubriand Fellowship of the Office for Science & Technology of the Embassy of France in the United States. The authors would also like to acknowledge the Texas A&M Materials Characterization Facility (MCF), for infrastructural support, as well as, Pierre Bachelet and Johann Sarazin for fire testing assistance, Bertrand Revel for NMR assistance, and Séverine Bellayer for imaging assistance at the University of Lille.

REFERENCES (1) Rawlinson, K.; Sherwood, H.; Dodd, V., Grenfull Tower Final Death Toll: Police Say 71 Lives Lost as Result of Fire. The Guardian 2017. (2) Szycher, M. Flexible and Semiflexible Foams. In Szycher's Handbook of Polyurethanes; CRC Press: 2012; pp 181-186. (3) Alongi, J.; Carosio, F. Flame Retardancy of Flexible Polyurethane Foams: Traditional Approaches versus Layer-by-Layer Assemblies. In Novel Fire Retardant Polymers and Composite Materials; Woodhead Publishing: 2017; pp 171-200. (4) Cain, A. A.; Plummer, M. G. B.; Murray, S. E.; Bolling, L.; Regev, O.; Grunlan, J. C. Iron-Containing, High Aspect Ratio Clay as Nanoarmor that Imparts Substantial Thermal/Flame Protection to Polyurethane with a Single Electrostatically-Deposited Bilayer. Journal of Materials Chemistry A 2014, 2 (41), 1760917617. (5) Segev, O.; Kushmaro, A.; Brenner, A. Environmental Impact of Flame Retardants (Persistence and Biodegradability). International Journal of Environmental Research and Public Health 2009, 6 (2), 478491. (6) Koch, C.; Sures, B. Environmental Concentrations and Toxicology of 2,4,6-Tribromophenol (TBP). Environmental Pollution 2018, 233, 706-713. (7) Jarosiewicz, M.; Duchnowicz, P.; Włuka, A.; Bukowska, B. Evaluation of the Effect of Brominated Flame Retardants on Hemoglobin Oxidation and Hemolysis in Human Erythrocytes. Food and Chemical Toxicology 2017, 109, 264-271. (8) Linares, V.; Bellés, M.; Domingo, J. L. Human Exposure to PBDE and Critical Evaluation of Health Hazards. Archives of Toxicology 2015, 89 (3), 335-356, DOI: 10.1007/s00204-015-1457-1. (9) Watanabe, I.; Sakai, S.-i. Environmental Release and Behavior of Brominated Flame Retardants. Environment International 2003, 29 (6), 665-682. (10) Chatterjee, S.; Shanmuganathan, K.; Kumaraswamy, G. Fire-Retardant, Self-Extinguishing Inorganic/Polymer Composite Memory Foams. ACS App. Mater. Interfaces 2017, 9 (51), 44864-44872. (11) Bourbigot, S.; Duquesne, S. Fire Retardant Polymers: Recent Developments and Opportunities. Journal of Materials Chemistry 2007, 17 (22), 2283-2300.


Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To be submitted to ACS AMI (12) Qiu, X.; Li, Z.; Li, X.; Zhang, Z. Flame Retardant Coatings Prepared using Layer by Layer Assembly: A Review. Chemical Engineering Journal 2018, 334, 108-122. (13) Holder, K. M.; Smith, R. J.; Grunlan, J. C. A Review of Flame Retardant Nanocoatings Prepared using Layer-by-Layer Assembly of Polyelectrolytes. Journal of Materials Science 2017, 52 (22), 12923-12959. (14) Borges, J.; Mano, J. F. Molecular Interactions Driving the Layer-by-Layer Assembly of Multilayers. Chemical Reviews 2014, 114 (18), 8883-8942. (15) Dubas, S. T.; Schlenoff, J. B. Factors Controlling the Growth of Polyelectrolyte Multilayers. Macromolecules 1999, 32 (24), 8153-8160. (16) Decher, G.; Hong, J. D.; Schmitt, J. Buildup of Ultrathin Multilayer Films by a Self-Assembly Process: III. Consecutively Alternating Adsorption of Anionic and Cationic Polyelectrolytes on Charged Surfaces. Thin Solid Films 1992, 210-211, 831-835. (17) Xiang, F.; Parviz, D.; Givens, T. M.; Tzeng, P.; Davis, E. M.; Stafford, C. M.; Green, M. J.; Grunlan, J. C. Stiff and Transparent Multilayer Thin Films Prepared Through Hydrogen-Bonding Layer-by-Layer Assembly of Graphene and Polymer. Advanced Functional Materials 2016, 26 (13), 2143-2149. (18) Kharlampieva, E.; Kozlovskaya, V.; Sukhishvili, S. A. Layer-by-Layer Hydrogen-Bonded Polymer Films: From Fundamentals to Applications. Advanced Materials 2009, 21 (30), 3053-3065. (19) Quinn, J. F.; Johnston, A. P. R.; Such, G. K.; Zelikin, A. N.; Caruso, F. Next Generation, Sequentially Assembled Ultrathin Films: Beyond Electrostatics. Chemical Society Reviews 2007, 36 (5), 707-718. (20) Richardson, J. J.; Cui, J.; Björnmalm, M.; Braunger, J. A.; Ejima, H.; Caruso, F. Innovation in Layer-byLayer Assembly. Chemical Reviews 2016, 116 (23), 14828-14867. (21) Bergbreiter, D. E.; Liao, K.-S. Covalent Layer-by-Layer Assembly- An Effective, Forgiving Way to Construct Functional Robust Ultrathin Films and Nanocomposites. Soft Matter 2009, 5 (1), 23-28. (22) Ding, F.; Liu, J.; Zeng, S.; Xia, Y.; Wells, K. M.; Nieh, M.-P.; Sun, L. Biomimetic Nanocoatings with Exceptional Mechanical, Barrier, and Flame-Retardant Properties from Large-Scale One-Step Coassembly. Science Advances 2017, 3 (7), 1-9. (23) Holder, K. M.; Huff, M. E.; Cosio, M. N.; Grunlan, J. C. Intumescing Multilayer Thin Film Deposited on Clay-Based Nanobrick Wall to Produce Self-Extinguishing Flame Retardant Polyurethane. Journal of Materials Science 2015, 50 (6), 2451-2458. (24) Smith, R. J.; Holder, K. M.; Ruiz, S.; Hahn, W.; Song, Y.; Lvov, Y. M.; Grunlan, J. C. Environmentally Benign Halloysite Nanotube Multilayer Assembly Significantly Reduces Polyurethane Flammability. Advanced Functional Materials 2017, 1703289. (25) Yang, Y.-H.; Li, Y.-C.; Shields, J.; Davis, R. D. Layer Double Hydroxide and Sodium Montmorillonite Multilayer Coatings for the Flammability Reduction of Flexible Polyurethane Foams. Journal of Applied Polymer Science 2015, 132 (14), 41767. (26) Kim, Y. S.; Harris, R.; Davis, R. Innovative Approach to Rapid Growth of Highly Clay-Filled Coatings on Porous Polyurethane Foam. ACS Macro Letters 2012, 1 (7), 820-824. (27) Kim, Y. S.; Li, Y.-C.; Pitts, W. M.; Werrel, M.; Davis, R. D. Rapid Growing Clay Coatings to Reduce the Fire Threat of Furniture. ACS App. Mater. Interfaces 2014, 6 (3), 2146-2152. (28) Uddin, K. M. A.; Ago, M.; Rojas, O. J. Hybrid films of chitosan, cellulose nanofibrils and boric acid: Flame retardancy, optical and thermo-mechanical properties. Carbohydrate Polymers 2017, 177, 13-21. (29) Tranchard, P.; Samyn, F.; Duquesne, S.; Thomas, M.; Estèbe, B.; Montès, J.-L.; Bourbigot, S. Fire Behaviour of Carbon Fibre Epoxy Composite for Aircraft: Novel Test Bench and Experimental Study. Journal of Fire Sciences 2015, 33 (3), 247-266. (30) Gamboa, D.; Priolo, M. A.; Ham, A.; Grunlan, J. C. Influence of Rinsing and Drying Routines on Growth of Multilayer Thin Films using Automated Deposition System. Review of Scientific Instruments 2010, 81 (3), 036103. (31) Ismadji, S.; Soetaredjo, F. E.; Ayucitra, A. Natural Clay Minerals as Environmental Cleaning Agents. In Clay Materials for Environmental Remediation; Springer International Publishing: Cham, 2015; pp 5-37. 25 ACS Paragon Plus Environment


Page 26 of 26

To be submitted to ACS AMI (32) Yang, Y.-H.; Haile, M.; Park, Y. T.; Malek, F. A.; Grunlan, J. C. Super Gas Barrier of All-Polymer Multilayer Thin Films. Macromolecules 2011, 44 (6), 1450-1459. (33) Schartel, B.; Hull, T. R. Development of Fire-Retarded Materials—Interpretation of Cone Calorimeter Data. Fire and Materials 2007, 31 (5), 327-354. (34) Corazzari, I.; Nisticò, R.; Turci, F.; Faga, M. G.; Franzoso, F.; Tabasso, S.; Magnacca, G. Advanced Physico-Chemical Characterization of Chitosan by means of TGA Coupled On-line with FTIR and GCMS: Thermal Degradation and Water Adsorption Capacity. Polymer Degradation and Stability 2015, 112, 1-9. (35) Gann, R. G. Estimating Data for Incapacitation of People by Fire Smoke. Fire Technology 2004, 40 (2), 201-207. (36) Socrates, G. Infrared and Raman Characteristic Group Frequencies. Tables and Charts. , 3rd ed.; 2004; Vol. 35, p 905. (37) Balek, V.; Pérez-Rodríguez, J.; Pérez-Maqueda, L.; Subrt, J.; Poyato, J. Thermal Behaviour of Ground Vermiculite, 2007; Vol. 88, p 819-823. (38) Valásková, M.; Simha Martynková, G. Vermiculite: Structural Properties and Examples of the Use, 2012. (39) Sophie, D.; Michel, L. B.; Serge, B.; René, D.; Giovanni, C.; Berend, E.; Chris, L.; Toon, R.; Hervé, V. Mechanism of Fire Retardancy of Polyurethanes using Ammonium Polyphosphate. Journal of Applied Polymer Science 2001, 82 (13), 3262-3274. (40) Bourbigot, S.; Le Bras, M.; Delobel, R.; Tremillon, J.-M. Synergistic Effect of Zeolite in an Intumescence Process. Study of the Interactions between the Polymer and the Additives. Journal of the Chemical Society, Faraday Transactions 1996, 92 (18), 3435-3444. (41) Brindle, R.; Albert, K.; Harris, S. J.; Tröltzsch, C.; Horne, E.; Glennon, J. D. Silica-Bonded Calixarenes in Chromatography: I. Synthesis and Characterization by Solid-State NMR Spectroscopy. Journal of Chromatography A 1996, 731 (1), 41-46. (42) Fyfe, C. A.; Zhang, Y.; Aroca, P. An Alternative Preparation of Organofunctionalized Silica Gels and their Characterization by Two-Dimensional High-Resolution Solid-State Heteronuclear NMR Correlation Spectroscopy. Journal of the American Chemical Society 1992, 114 (9), 3252-3255. (43) Smitrovich, J. H.; Woerpel, K. A. Oxidation of Sterically Hindered Alkoxysilanes and Phenylsilanes under Basic Conditions. The Journal of Organic Chemistry 1996, 61 (17), 6044-6046.

TOC Graphic:


Chitosan Nanobrick Wall on Flexible

Recommend Documents