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Superior Flame-Resistant Cellulose Nanofibril Aerogels Modified with Hybrid Layer-by-Layer Coatings Oruç Köklükaya, Federico Carosio, and Lars Wågberg ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08018 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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ACS Applied Materials & Interfaces

Superior Flame-Resistant Cellulose Nanofibril Aerogels Modified with Hybrid Layer-by-Layer Coatings Oruç Köklükaya*, †, Federico Carosio‡ and Lars Wågberg*, †, ᵮ †

Fiber and Polymer Technology, KTH Royal Institute of Technology,

Teknikringen 56, SE-100 44 Stockholm, Sweden ‡

Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Alessandria Site

Viale Teresa Michel 5, 15121, Alessandria, Italy ᵮ

Wallenberg Wood Science Center, KTH Royal Institute of Technology,

SE-100 44 Stockholm, Sweden KEYWORDS layer-by-layer assembly, flame-retardant, thermal stability, cellulose nanofibril, aerogel

ACS Paragon Plus Environment

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ABSTRACT

Nanometer thin films consisting of cationic chitosan (Ch), anionic poly(vinylphosphonic acid) (PVPA) and anionic montmorillonite clay (MMT) are deposited on highly porous, wet stabilized cellulose nanofibril (CNF) aerogels using the Layer-by-Layer (LbL) technique. Model experiments with silicon oxide surfaces are used to study the details of the LbL formation and the multilayer structure. The formation of the LbLs on the aerogels is also investigated as a function of solution concentration using polyelectrolyte titration. Thermogravimetric analysis indicates that the LbL-coating significantly improves the thermal stability of the CNF aerogel. Horizontal flame test shows that aerogels coated with 5 quadlayers of Ch/PVPA/Ch/MMT, using solutions/dispersion of high concentration, are able to self-extinguish immediately after the removal of the flame and LbL-coated aerogels do not ignite under heat flux (35 Kw/m2) in cone calorimetry. The LbL-coated aerogel can prevent flame penetration from a torch focused on the surface achieving temperature drops up to 650 °C across the 10 mm thick specimen for several minutes. LbL treatment is hence a rapid and highly effective way to specifically tailor the surface properties of CNF aerogels in order to confer unprecedented flame-retardant characteristics.

1. INTRODUCTION The desire for alternatives to oil-based materials has become a strong driving force for the use of natural and renewable resources. In this scenario, cellulose -the most abundant natural polymer available- has attracted extensive attention.1 Indeed, the chemical and physical properties of cellulose offer numerous possibilities for its use in a multitude of application areas. In particular, nanocellulose prepared from wood fibers is currently being used to study novel applications as gels, nanopapers, batteries, supercapacitors and in fibril-reinforced composites due to its


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nanoscale dimensions, easy to tune surface chemistry and excellent mechanical properties.2–4 Turbak et al. and Herrick et al. described the preparation of nanocellulose from wood employing a homogenization process to disintegrate the wood pulp and release the cellulose nanofibrils (CNF) constituting the wood fiber wall.5,6 In contrast to macroscopic pulp fibers, CNFs have nano-dimension, a very high aspect ratio and excellent mechanical properties due to the preferential orientation of the crystalline regions of the glucan chains along the fibril axis. The CNF entanglements can favor the formation of dense networks with excellent macroscopic mechanical integrity. In an effort to reduce the high cost of the process and to manufacture a more homogeneous CNF, different pretreatments have been investigated such as enzymatic and mechanical treatments,7 carboxymethylation,8 and TEMPO oxidation.9 All these different types of CNF have been successfully employed as building blocks for the production of nanoengineered materials such as highly ordered nanopaper or complex aerogels.2–4 Aerogels are solid state materials manufactured from a gel state by replacing the liquid component with gas using supercritical drying or freeze drying. These processes yield three-dimensional scaffolds with nano- and micro-sized pores that make possible further inclusion of functional materials resulting in aerogels with advanced uses in the medical, cosmetic and pharmaceuticals fields. Due to their unique properties such as low density, high specific area, high porosity and low thermal conductivity aerogels have been studied extensively.10,11 The most common aerogels are prepared using inorganic metal oxides,12 silica nanoparticles,13 organic and organic-inorganic (hybrid) compounds.14 The use of natural materials in the manufacturing of aerogels has provided access to highly engineered and bio-based materials, thus broadening the application areas and in recent years, cellulose and cellulose derivatives have been successfully exploited to manufacture aerogels.15–20 Due to the facile tunability of the process, CNF-based aerogels have


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been utilized to develop functional materials by embedding magnetic particles,21 carbon nanotubes22 or clay particles23 within the aerogel structure. Another key feature of CNF-based aerogels is the very large surface area available for further modification. Cervin et al. and Aulin et al. modified aerogels with the aid of chemical vapor deposition of fluorinated silane in order to produce respectively highly hydrophobic and superoleophobic aerogels.24,25 Hamedi et al. exploited the high ionic charge of the wet-stabilized aerogels to demonstrate a rapid method for the layer-by-layer (LbL) assembly of conducting polymers, biomolecules and carbon nanotubes in order to develop high charge-storage capacity of supercapacitors.26 In wider application areas, their inherent high flammability strongly limits the possible utilization of bio-based aerogels. Thus, finding a solution to this problem would lead to a new class of highly functional, strong and lightweight materials. As far as the flame-retardant bio-based aerogels are concerned, different strategies have been proposed.27-29 Wu et al. investigated the fire-resistant properties of carbon nanofiber aerogels prepared from pyrolysis treatment of bacterial cellulose. However, there are limited numbers of strategies focusing on flame-retardant properties of CNF-based aerogels. In the present study, carboxymethylated cellulose nanofibrils were used as building blocks for preparing light, mechanically strong, and wet-stable aerogels. The layer-by-layer technique was then exploited in an effort to coat every accessible surface of the cross-linked aerogel in order to impart flame-retardant properties to the substrate without the need for increasing the inorganic component to high ratios. The LbL assembly technique was presented as a robust and versatile method to modify the surface of a substrate using polyelectrolytes or nanoparticles.30,31 During the LbL treatment, selected reagents are consecutively adsorbed at the solid-liquid interface in an alternating fashion. The most widely studied LbL processes use the electrostatic attraction, i.e.


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entropic driving forces, as main interaction although other possibilities exist; such as donor/acceptor,32 hydrogen bonds,33 and covalent bonds34 where either enthalpic or entropic driving forces form the base for the interactions. Using the LbL technique, a broad range of functionalities has been imparted to various substrates (e.g. gas barrier,35 antibacterial,36 antireflection37 and electrical conductivity38). In recent years, the LbL assembly technique has been exploited as a novel method to impart flame-retardant characteristics to cotton fabrics,39 polyester fabrics,40 plastic thin films41 and complex 3D polyurethane foams.42,43 The LbL approach has been shown to be particularly effective for substrates characterized by a high surface-to-bulk ratio. For example, the high specific surface area and porous structure of opencell polyurethane foams paved the way for a flame-retardant LbL assembly through a dipping and squeezing procedure.43,44 Like PU foams, CNF aerogels are a perfect substrate to be modified by the LbL approach, given the large surface area available and their tunable surface charge; despite this, to the best of our knowledge, the LbL approach has never been attempted to confer flame-protection properties to such a substrate from a totally renewable raw material. In the present contribution, we describe a considerably fast LbL deposition of a hybrid coating where the required constituents of an intumescent flame-retardant coating (i.e. carbon source, acid source and blowing agent) are assembled in combination with inorganic lamellar nanoparticles.45 Figure 1 is a schematic representation of the process used to deposit a quadlayer (QL) system (Ch/PVPA/Ch/MMT) onto the CNF-based aerogels.


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Figure 1 Schematic representation of the LbL assembly of Ch, PVPA and MMT onto CNF aerogels, with rapid washing and filtration between each adsorption step (one sequence of deposition represents one quadlayer). The process was repeated 5 times in order to deposit 5 QLs. LbL model experiments were also performed using silicon oxide surfaces with the quartz crystal microbalance (QCM) technique in an effort to investigate the effect of solution concentration on the film growth and the mechanisms behind the flame-retardant performance. The morphology of the thin films deposited onto the aerogels was monitored by scanning electron microscopy (FE-SEM). The flame-retardant properties have been assessed by thermogravimetric analysis, flammability tests (direct methane flame application), cone calorimetry test (heat flux application) and the flame penetration test (butane torch). The deposition of 5QL of LbL system adds 19±1 wt % to the aerogel weight. LbL-treated aerogels showed self-fire-extinguishing properties during the horizontal flame test, immediately after the removal of the flame source, no


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ignitability under cone calorimetry heat flux and no flame-penetration when exposed to butane flame torch. This hybrid nano-coating provides a very effective flame-retardant coating for the highly porous cellulose nanofibril-based aerogel with very few LbL coating layers. 2. EXPERIMENTAL SECTION 2.1. Chemicals and Substrates Ch (molecular weight, Mw = 60 000, 95% deacetylation, G.T.C Union Corp., Qingdao, China), PVPA (Mw = 24 000, Polysciences Inc., Eppelheim, Germany), MMT (Cloisite Na+, BYK Additives & Instr., Wesel, Germany) were all used as received. Sodium chloride (NaCl), hydrochloric acid (HCl) and sodium hydroxide (NaOH) were all of analytical grade (Merck). Silicon wafers (Addison Engineering, Inc., San Jose, USA) and AT-cut quartz crystal sensors (Q-Sense AB, Västra Frölunda, Sweden) were used for film characterization. Cellulose nanofibrils (CNF) (Innventia AB, Stockholm, Sweden) were received as 2 wt% gel. All solutions were prepared using 18.2 MΩ cm milli-Q grade water. 2.2. Aerogel Preparation The wet-stable CNF aerogels were prepared according to a procedure described previously.26 The 1,2,3,4-butanetetracarboxylic acid (BTCA) cross-linker and sodium hypophosphite (SHP) catalyst were mixed with CNF gel (2wt %) and the gel was then placed in aluminum molds and frozen using dry ice followed by freeze-drying to obtain an aerogel. The dried aerogels were heated to 170 °C for 5 minutes to ensure a covalent cross-linking reaction with the BTCA/SHP combination. The aerogels were then washed with deionized water and dewatered until the conductivity of the filtrate was below 5 µS/cm, in order to remove unreacted SHP and BTCA from the bulk.


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2.3. Layer-by-layer (LbL) Deposition 0.1 wt% and 0.5 wt% Ch solutions were prepared using 1 v/v % acetic acid (Sigma Aldrich), 1 wt% MMT dispersion was prepared by first magnetic stirring of the suspension overnight in order to achieve full dispersion, then stirred with an Ultra Turrax mixer (IKA, T 25 Basic, Germany) at 15 000 rpm for 15 min, followed by sonication using a Vibra-Cell ultrasonic processor (Sonics and Materials, Inc., USA) for 10 min at ambient temperature. The dispersion was then placed in a centrifuge at 4 500 rpm for 1 hour. The supernatant MMT suspension was used to prepare 0.1 wt% and 0.5 wt% dispersions. PVPA was diluted to 0.1 wt%. The solutions were magnetically stirred for 24 hours to ensure complete dissolution. The final pH of the solutions was adjusted to 5 with sodium hydroxide (NaOH, 1M) and hydrochloric acid (HCl, 1M) prior to the deposition. The silicon wafers were cleaned consecutively with milli-Q water, ethanol and milli-Q water and were then blown dry with nitrogen. In order to clean and activate the surface, the silicon wafers were then placed in air plasma cleaner (PDC 002, Harrick Scientific Corporation, USA) for 3 min. The cleaned silicon wafers were then sequentially dipped into the solutions in the order: Ch, PVPA, Ch and MMT, and the wafers were rinsed with milli-Q water and dried with nitrogen after each deposition. One cycle of adsorption leads to the formation of one quadlayer (QL). All the solutions had the same concentration (1g/L and 5g/L), electrolyte concentration (10mM NaCl) and adjusted pH (pH 5) values. The rinsing milli-Q water had the same electrolyte concentration and pH. During the first quadlayer formation, the deposition times were 5 min both for each polymer solution and for each rinsing step. For the following layers, adsorption and rinsing times of 1 min were used. The wet-stable CNF-aerogels were alternately placed in polyelectrolyte solutions then transferred to Büchner funnel. By applying a vacuum pressure on the bottom surface of the aerogels polyelectrolyte solutions were


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forced through the aerogels. Excess and unadsorbed polyelectrolyte solutions were thoroughly rinsed with rinsing and vacuum filtration (Figure 1). The amount of polymer and clay adsorbed onto an aerogel in each layer of deposition was determined by polyelectrolyte titration of the residual polymer or MMT in solution after deposition using a polyelectrolyte titrator system consisting of a Metrohm 716 DMS titrino (Switzerland) and a Stabino Particle Metrix Gmbh (Germany). Polyvinylsulfate potassium salt (KPVS) was used to titrate Ch and polydiallyldimethyl ammonium chloride (PDADMAC) was used to titrate MMT clay and PVPA. 2.4. Quartz Crystal Microbalance with Dissipation Model experiments of the LbL assembly of the multilayer film were also investigated using a quartz crystal microbalance equipment with dissipation (QCM-D) supplied by Q-sense AB (Västra Frölunda, Sweden). The AT-cut quartz crystals (QSX303) cleaned by following the procedure described above. The polymer solutions and clay dispersion were alternately deposited on a quartz crystal with a continuous flow of 150 µL/min and this adsorption was monitored in real time in situ. 2.5. Atomic Force Microscopy (AFM) The surface morphology of LbL thin films assembled on silicon wafers was imaged using an atomic force microscope (AFM), Nanoscope IIIa (Bruker AXS, Santa Barbara, CA). The images were acquired in air using E- and J-type piezoelectric scanners and Scanassyst cantilevers that have a nominal resonance frequency of 70 kHz and a spring constant of 0.4 N/m. The surface roughness, defined as the root mean square (Rq) value, was calculated by averaging three different 4 and 100 µm2 areas. The AFM software, (Nanoscope version 6.13), section function,


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was used to calculate the thickness of the thin film on the silicon wafer surface by imaging the scalpel-scratched area. 2.6. Thermal Stability The thermal and thermo-oxidative degradation of CNF aerogel were investigated before and after the LbL coating by thermogravimetric analysis (TGA) using a TA Q500 (TA Instruments, South Carolina, USA) in nitrogen and air, respectively. The samples were weighed ( ca.10 mg) and heated from 50 to 800 °C at a heating rate of 10 °C/min under a constant gas flow rate of 60 ml/min. 2.7. Flammability and Cone Calorimetry The flammability of aerogels was evaluated in a horizontal configuration following the UL-94 standard for what concerns sample positioning and employed flame characteristics. The sample (20x80x20 mm3) was placed on a wire mesh and exposed to a 20 mm methane flame on the short side of the sample for 5 seconds. The horizontal flame test (HFT) was conducted three times to enable the repeatability to be assessed. Both the burning behavior and the residue were investigated during the test. The combustion of CNF aerogels and LbL-coated aerogels was investigated under an irradiative heat flux (35 kW/m2) using cone calorimetry (Fire Testing Technology, FTT, UK) in accordance with the ISO 5660 Standard. The samples (50x50x10 mm3) were placed in a horizontal configuration, and the time to ignition (TTI, s), heat release rate (HRR, kW/m2), peak heat release rate (pkHRR, kW/m2), and total heat release (THR, kW/m2) were measured. The experiments were conducted four times for each sample to assess the reproducibility and significant data. The uncertainty was evaluated as standard deviation.


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2.8. Flame Penetration Test The capability of CNF aerogel to resist flame penetration was evaluated by a flame penetration test, which simulates the severe conditions observed during an ignition event. The sample (50x50x10 mm3) was placed in a ceramic frame in a vertical configuration. The center of the specimen was exposed to a butane flame from a distance of 100 mm so that the temperature on the exposed surface was in the range of 750 °C. The flame was applied continuously until complete penetration of the sample or up to 3 min. The temperatures on the front and back sides of the specimen were measured throughout the test using a thermocouple (stainless steel sheathed, k-type, 1.5 mm diameter, Tersid S.r.l, Milano, Italy). The tests were monitored with a camera in real time. The tests were repeated four times to assess reproducibility. 2.9. Scanning Electron Microscopy (SEM) The surface morphologies of CNF aerogels and LbL-coated aerogels were investigated before and after the flammability test using a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) to obtain secondary electron images. Aerogel samples were cut with a razor blade, fixed with conductive carbon tape and coated with a platinum/palladium layer for 50 s using a Cressington 208 HR high resolution sputter coater.


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3. RESULTS AND DISCUSSION 3.1. Influence of Solution Concentration of Polyelectrolytes and Nanoparticles on the Formation of LbL Thin Films QCM-D was used to study the formation of the multilayer film on model silicon oxide surfaces, this provides the amount of adsorbed polymer and clay together with immobilized water using Sauerbrey model.46 Figure 2a and 2b show the normalized change in frequency and dissipation for the third overtone and the calculated total adsorbed mass after consecutive adsorption of each layer, during the build-up of 5QL (Ch/PVPA/Ch/MMT). Each adsorption step took 10 min and was followed by 10 min of rinsing with 10 mM NaCl solution at pH 5.


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Figure 2 LbL build-up of (Ch/PVPA/Ch/MMT)5 on model silicon oxide surfaces from QCM-D measurements. (a) change in normalized frequency and energy dissipation for the third overtone, (b) total adsorbed mass of multilayer film calculated using the Sauerbrey model. (c) AFM tapping mode height images of (Ch/PVPA/Ch/MMT) quadlayers (1QL, 3QL and 5QL) deposited on a silicon wafer substrate using 1 g/L and 5 g/L solutions. The images are 2 x 2 µm2 and the zrange is indicated in the scale bar to the right of the images.


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As shown in Figure 2a, the frequency decreased after the deposition of each layer constituent. Two different solution concentrations (0.1 g/L and 0.5 g/L) were used at 10 mM NaCl for adsorption, i.e. the concentrations for the QCM-D measurements were one tenth of those used for multilayer formation on aerogel samples. This reduction is necessary because the high viscosity at high concentrations limits the use of the QCM-D instrument. The build-up of the multilayer shows a similar trend for both the solution concentrations, but the frequency shift at 0.5 g/L concentration is much higher than that at 0.1 g/L concentration, indicating a higher adsorbed amount. The addition of MMT leads to a larger frequency decrease than Ch and PVPA. The difference between the normalized frequency curves of different overtones (Figure S1) indicates the build-up of a rather inhomogeneous LbL structure for the lower concentration since there is a larger difference between the different overtones. This suggests that the adsorption is higher and the organization is better in the adsorbed layers when using the higher concentration. The change in the dissipation is related to the viscoelastic properties of the thin film47 and a low dissipation value indicates a rather thin film with rigid properties whereas a high value shows a more swollen and mobile film. Figure 2a shows that during the build-up of the initial quadlayer, the energy dissipation increases when Ch is adsorbed, following the MMT layer, while there is a decrease when Ch is adsorbed following the PVPA layer. A possible explanation is that at pH 5 chitosan is moderately charged (pKa 6) due to the primary amine groups in its structure and is obviously able to deposit a flat and thin film, while the PVPA with pKa values of 3.3 and 9.2 in the presence of NaCl,48 is partially dissociated and probably exists in a wormlike chain.49 Ch and PVPA hence form a thin homogeneous film in agreement with earlier published results.50 Additional information on the build-up of the quadlayers is given by the relationship between energy dissipation and the absolute change in frequency (Figure S2). The increase in dissipation


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shows an overall proportionality with the decrease in frequency for the first three quadlayers for both solution concentrations but for the fourth and fifth quadlayers, the increase in dissipation, due to MMT adsorption, is about the same for each adsorption step but the frequency change is greater for the higher solution concentration, and is probably because the MMT particles adsorbed in the outer layers form a more expanded and flexible structure than in the initial layers for the lower solution concentration. Since the adsorption will be faster at a higher concentration it can be suggested that a locking of a non-equilibrium structure in the adsorbed layers is the most probable mechanism for the higher adsorbed amounts and a more even packing in the adsorbed layer (Figure 2b). Figure 2c shows the AFM height images of thin films adsorbed using two different solution concentrations and different number of quadlayers (1, 3 and 5QL) where the structure was investigated after consolidation of the swollen film. The surface roughness of the adsorbed films is rather similar for the films formed using high and low concentrations and shows a small increase as the number of deposited quadlayer increases (Figure S3). Since the roughness is in the order of 3-5 nm, the dry adsorbed layers must be considered to be rather smooth and it is interesting to note that even the higher concentrations result in a smooth dry layer. It should be stressed that the wet layers, as studied with the QCM, contains a large amount of water and during drying, there is a considerable consolidation of the layers most probably creating a collapse of the adsorbed layers. The high anisotropy of the clay platelets will naturally lead to a higher orientation in the dried state. To investigate this effect further, the thickness of deposited dry films on silicon oxide surfaces was investigated by scratch height method and results showed similar trends to the roughness measurements but with a much larger difference between the solution concentrations. The higher solution concentration resulted in thicker films (74±6 nm) compared to that of low solution concentration (33±2 nm ) which would be expected


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from the adsorbed amount assuming a similar density of the dry films. The increase in thickness with an increasing solution concentration (Figure S4) was also expected from the QCM measurements but the increase is higher than the increase in the adsorbed amount. This also indicates that the consolidation of the viscoelastic multilayers leads to similarly smooth dry films but with a significantly different thickness. The LbL-coatings were then successfully deposited on CNF aerogels using the same procedure as in the model studies. After the adsorption of 5QL using 1 g/L and 5 g/L solutions, the total weight gain was calculated by weighing the samples before and after the LbL assembly to be 12% and 19%, respectively (Table S1). The analysis of the deposited layers on the surface of the aerogel, using polyelectrolyte titration (Figure S5), showed a trend similar to that obtained in QCM-D. Figure 3 shows SEM images of untreated and LbL treated aerogels.

Figure 3 SEM micrographs of untreated aerogel (a) and (d), (Ch/PVPA/Ch/MMT) 5QL treated with 1 g/L solutions (b) and (e), 5QL treated with 5 g/L solutions (c) and (f) at different magnifications.


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The SEM images of the porous structure of the aerogel show a rather smooth surface (Figure 3d). When the aerogels are coated with 5QL, the surface morphology of the pores changes and becomes a bit rougher (Figure 3e). This effect is more pronounced for the solutions with a higher concentration (Figure 3f). Considering the similar roughness values as detected with the model films it can be suggested that this roughening might be due to shrinking, and wrinkling of the surface layers of the aerogel during drying. Moreover, the macroscopic pores retained their shape and macroscopic dimensions even after 5QL deposition which demonstrates the wet mechanical properties of the aerogels (Figure 3a-c). The results obtained from QCM-D, AFM and polyelectrolyte titration clearly show that the solution concentration influences the adsorbed amount, which in turn influences the dry properties of the formed layers. 3.2. Thermal Stability and Flame-Retardancy of CNF Aerogel The thermal and thermo-oxidative stabilities of untreated and LbL-coated CNF aerogels were investigated by thermogravimetric analysis in both nitrogen and air. Table S2 summarizes the thermal analysis and Figure 4 shows the weight loss (TG) and the derivative (dTG) as a function of temperature in nitrogen and air.


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Figure 4 Weight loss and the derivative of the weight loss for untreated and LbL-treated CNF aerogel in (a), (c) nitrogen and (b), (d) air. Cellulose thermal degradation in nitrogen proceeds in one-step through two competitive pathways; depolymerization that produces flammable volatiles and dehydration which favors char formation (Figure 4a). On the other hand, in air cellulose usually undergoes a two-step thermo-oxidation. In the first step (300-400 °C), the aliphatic char and flammable volatiles are formed. In the second step (400-600 °C), this aliphatic char is further oxidized to aromatic char with a consecutive release of CO and CO2 (Figure 4b).51,52 LbL-assembled coating did not change the thermal and thermo-oxidative degradation of the aerogel that still occurred in one and two steps, respectively. The presence of the coating led to a slight reduction in the cellulose decomposition temperature in both nitrogen and air, as shown in Figure 4a,b and by the Tonset 10% values in Table S2. Nevertheless, the degradation of the LbL-treated aerogels gave a remarkably higher residue in both nitrogen and air. In air, the high residue produced in the first step was oxidized in the 400-600 °C range; this process was quick and pronounced for 1 g/L samples but it was slower for 5 g/L samples as shown by the derivative curves in Figure 4c,d. Even in air, the residue from quadlayer coated aerogels increased significantly (11 and 13% for 1


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g/L and 5 g/L, respectively) if compared with the control aerogel (4%) at 800 °C. This increase in amount of residue can be ascribed to enhanced barrier properties of the coating due to presence of MMT clay leading to more residue in both nitrogen and air, as shown in Figure 4a,b. This increased thermal stability and higher cellulose char formation are due to the combined effect between phosphorus-containing polymer and the barrier properties of the inorganic clay platelets.53 PVPA generates phosphoric acid during decomposition which dehydrates the cellulose and chitosan and promotes the char formation, while the aligned nanoplatelets of clay generate an inorganic barrier that insulates the substrate and protects it from oxygen. The effect is more pronounced as the adsorbed amount increases. This is particularly evident in air where, the 1 g/L treated aerogels show a rapid oxidation indicating that the coating did not completely prevent oxygen diffusion and probably crack at high temperatures. The 5 g/L solution has a higher chitosan concentration and, upon degradation, this could most probably provide a continuous and stable charred structure embedding oriented clay nanoplatelets. Flammability tests were performed in a horizontal configuration to investigate the reaction to a direct methane flame of untreated and LbL-treated CNF aerogels. On the other hand, the cone calorimetry test simulates the burning behavior of a material when exposed to the heat fluxes typically found in developing fires. Figure 5 shows flammability and cone calorimetry data.


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Figure 5 Burning time measured during the horizontal flame test and the residue amount at the end of the test (a), (b) Photographs taken during the horizontal flame test of the different aerogels, 10 s after removal of methane flame and at the end of the test, (c) Heat release rate curves for the untreated and LbL-treated aerogels, and (d) Photographs of the reference and of the LbL-treated CNF aerogel 20 and 60 s after the application of heat flux (35 kW/m2). The flammability data is summarized in Table S3. The untreated aerogel ignited immediately when exposed to the flame and the flame propagated mainly on the surface then selfextinguished after an average of 46 s (Figure 5a,b). This can be attributed to the inherent charforming ability of cellulose and to the fact that the highly porous structure of the aerogel collapsed upon flame exposure to form a dense char layer which reduced the amount of volatile gases released from the aerogel. The aerogels coated with 5QL of Ch/PVPA/Ch/MMT using a 1 g/L solutions showed self-extinguishing behavior immediately after the removal of the methane flame; however the flameless combustion (thermal oxidation in the solid state) normally referred


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to “afterglow” slowly consumed the sample leaving a brittle charred residue (10% of the original weight). This detrimental behavior appears to be related to an insufficient amount of coating. The coating almost immediately extinguishes the flame, and its presence on the surface of the aerogel also prevents collapse. This sustains the flameless combustion of the material as air can easily flow through the uncollapsed aerogel structure. The same behavior was observed in the TG analysis in air, where the aerogel coated with 5QL-1 g/L showed a faster degradation at 450 °C than the reference and 5QL-5 g/L coated aerogels. Conversely, the aerogels coated with 5QL using 5 g/L solution was able to form a more efficient protective layer limiting the flammability of the underlying aerogel and eliminating the afterglow phenomenon. The final residue left was as high as 98% thus demonstrating that increasing the solution concentration improves the barrier properties of the film, which in turn results in better flame-retardant properties. Figure 5c shows results from the cone calorimetry measurements, summarized as the heat release rate (HRR) curves and peak heat release rate (pkHRR) for both the reference and the LbL-treated samples. Photographs of aerogels during cone calorimetry measurements are also shown in Figure 5d. Under 35 kW/m2 heat flux, the untreated CNF aerogel quickly ignites and burns eventually showing a glowing phenomenon after the flame is extinguished. The deposition of a thin multilayer film dramatically altered the combustion behavior of the aerogels. Indeed, LbL-treated aerogels did not ignite and flaming combustion was not observed. Instead of flaming combustion, oxidation took place in the solid state and the samples glowed (Figure 5d). This process consumes oxygen and is measured by cone calorimetry and converted to a very low HRR signal. The heat release rate peak (pkHRR) of the reference was 149 kW/m2 and those of the 5QL aerogels based on 1 g/L and 5 g/L concentrations were 40 and 31 kW/m2 respectively. It is important to stress that this behavior is unprecedented for cellulose-based materials, and is in


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contrast to the behavior of CNF nanocomposite foams containing graphene oxide, sepiolite and boric acid which, despite impressive thermal stability and flammability properties, still showed ignition (50% of the samples) when tested in the cone calorimeter.54 In the present study, the LbL-treated aerogels showed only a glowing phenomenon resulting in a very low HRR. As shown in Figure S6, aerogels coated with low concentrations solutions left a shrunken char residue whereas the aerogels treated with higher concentration retained their original shape. Since 5QL-5 g/L-treated aerogels shown better dimensional stability and flame retardant properties than the 5QL-1 g/L, this sample was selected, along with the unmodified aerogel, for further studying by using a flame penetration test. This test is schematically described in Figure 6 where the photographs of untreated and LbL-treated aerogels taken during the flame penetration test are shown together with the average temperature plots from measurements on the front and rear of the samples.


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Figure 6 Schematic description of the flame penetration test (a), photographs of the front and rear side of the reference aerogel (b) and 5QL-5 g/L (c) 20 s after the butane flame application and at the end of the test. The temperature measured during the flame penetration test, on the flame-exposed and un-exposed side of the reference aerogel (d) and 5QL-5 g/L-treated aerogel (e). The results in Figure 6b shows that the flame of the torch easily penetrated through the untreated aerogel which was completely destroyed after 40 seconds. In contrast, 5QL-5 g/L-treated aerogels preserved their structure, successfully insulating the unexposed side of the sample (Figure 6c). The coating helped to maintain the aerogel structure most probably due to the high clay concentration in the coating on every available surface (Figure 3c,f). The temperature on the


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rear gradually increased up to 250 °C and then slowly decreased to a plateau of 100 °C. This means that there was an initial temperature drop of 500 °C through the 10 mm thickness and the temperature difference increased to 650 °C when the temperature stabilized (Figure 6d,e). 3.3 Post-Combustion Residue Analysis and Flame-Retardant Mechanism In order to investigate the fire protection provided by the coatings, 5QL-5 g/L coated silicon wafer surfaces were exposed to a butane flame for 3 s and then studied by AFM and SEM, as shown in Figure 7.

Figure 7 AFM scanassyst mode: (a) Scratched surfaces of LbL treated silicon wafers showing the thickness of a (Ch/PVPA/Ch/MMT)5 quadlayer deposited on a silicon wafer substrate with 5 g/L concentration and (b) after exposure to a butane flame. The images are 10 x 10 µm2 and the z-range is indicated in the scale bar to the right of the images. (c) SEM micrograph of crosssections of the (Ch/PVPA/Ch/MMT)5 quadlayer based on 5 g/L solution and (d) cross section of the same film after 3 s exposure to a butane flame. The surface roughness increased after the flame exposure to a final value of 57±13 nm, and the thickness also increased (Figure 7a,b). Since AFM analysis of the burned thin film was difficult


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due to the soft fragile structure of the film, the cross-section of 5QL-5 g/L film on a silicon oxide surface was also imaged by SEM before and after the flame application and Figure 7c,d show that the thickness of the film increased by a factor of 20 from the original 80±7 nm to 1.55±0.1 µm (in accordance with AFM measurement shown in Figure 7a,b). SEM was also used to investigate the structure change of the aerogel surface after the flame test. Figure 8 shows collected micrographs as well as the schematic representation of the flame protection mechanism for layers formed from the 1 g/L and 5 g/L solutions.


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Figure 8 SEM micrographs of aerogels after a horizontal flame test (a) and (d) reference sample, (b) and (e) (Ch/PVPA/Ch/MMT) 5QL based on 1 g/L dispersions, (c) and (f) 5QL based on 5 g/L dispersions, (g) schematic illustration of the flame-retardant mechanism differences between the aerogels treated with 1 g/L solutions and 5 g/L solutions.


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The untreated CNF aerogel shrinks and that the porous structure collapses on exposure to a flame (Figure 8a). In contrast the structure of the aerogel coated with 5QL-1 g/L appears less collapsed maintaining a portion of the original porosity (Figure 8b) and the aerogel coated with 5QL-5 g/L completely maintains its integrity (Figure 8c). The high magnification micrographs reveal the morphology of the aerogel cell walls. The uncoated aerogel shows a smooth morphology and a damaged structure (Figure 8d). A swollen coating is clearly seen on the LbL-coated aerogels, i.e. bubbles. The bubbles are damaged and cracked on the 5QL-1 g/L coated aerogel (Figure 8e), whereas they appear intact and undamaged on the 5QL-5 g/L coatings (Figure 8f). On the basis of AFM and SEM observations, the flame retardant mechanism is proposed as shown in Figure 8g. Upon exposure to flame or heat, the coating components strongly interact, providing an expanded hybrid organic/inorganic coating. This protection is brittle and fragile when deposited from 1g/L concentration and its protection efficiency is limited but, at higher solution/dispersion concentrations, the resulting expanded structure is stronger and does not crack upon exposure to a flame or heat flux. For this reason 5 g/L coated aerogels resulted in the better flame retardant properties granting self-extinguishing behavior during flammability test, no ignition during cone calorimetry tests and impressively stopping the flame penetration of a 750 °C flame torch. 4. CONCLUSION Freeze-dried and crosslinked CNF aerogels were prepared and used as substrates for films consisting of quadlayers of Ch, PVPA and MMT clay deposited on the aerogels by a rapid layerby-layer technique. The effects of two different solution concentrations (i.e. 1 g/L and 5 g/L) on the achieved flame retardant properties have been thoroughly evaluated. The total adsorbed amount increased with increasing concentrations of the quadlayer component solutions. The same trend was found for the roughness and the thickness of LbL assemblies but the films still


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remained smooth with a roughness of the order of 3-5 nm. The LbL coating was easily deposited onto the aerogel substrate homogeneously covering all the available surfaces without altering the pore structure. During the horizontal flammability tests, the aerogel LbL-coated with the higher concentration (5 g/L) solutions extinguished the flame immediately after the removal of the flame due to the formation of a compact carbonaceous layer. All the modified samples showed an unprecedented non-ignition behavior when exposed to a heat flux, whereas the unmodified sample ignited and was completely destroyed, and the aerogels treated with quadlayers of the higher concentration were able to withstand the penetration of a butane flame torch, successfully insulating the unexposed side of the sample with a temperature drop of up to 650 °C with respect to the exposed side. The use of a highly exfoliated colloidally stable clay dispersion enhanced the mechanical, barrier and thermal properties of the coated material. The impressive flame retardancy performances achieved can be attributed to a combined effect of MMT, PVPA and Ch, which favors char formation and reduces the release of flammable volatiles. The rapid adsorption of a multilayer thin film on a complex, highly porous light-weight aerogel can impart unprecedented flame-retardant characteristics able to extend the fields of application of promising class of material. The layer by layer technique makes it possible to design efficient and green fireproof materials in a manner which can easily be exploited and extended to other substrates. ASSOCIATED CONTENT Supporting Information The following files are available free of charge. QCM-D data for different overtones; QCM-D data of change in frequency as a function of change in dissipation; AFM roughness profile and thickness profile images of thin films


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deposited on silicon wafer substrate; total adsorbed amount on aerogel calculated by polyelectrolyte titration; photographs of CNF aerogels before and after cone calorimetry; weight gain calculation data; TGA results and flammability data of aerogel. AUTHOR INFORMATION Corresponding Author * Email: [email protected] * Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Lars Wågberg, Oruç Köklükaya and Federico Carosio acknowledge financial support from SSF (The Swedish Foundation for Strategic Research) and Lars Wågberg also acknowledges The Wallenberg Wood Science Centre for financial support. REFERENCES 1.

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54. Wicklein, B.; Kocjan, A.; Salazar-Alvarez, G.; Carosio, F.; Camino, G.; Antonietti, M.; Bergström, L. Thermally Insulating and Fire-Retardant Lightweight Anisotropic Foams Based on Nanocellulose and Graphene Oxide Nat. Nanotechnol. 2015, 10, 277-283. Table of contents graphic


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Schematic representation of the LbL assembly of Ch, PVPA and MMT onto CNF aerogels, with rapid washing and filtration between each adsorption step (one sequence of deposition represents one quadlayer). The process was repeated 5 times in order to deposit 5 QLs. 97x53mm (300 x 300 DPI)


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LbL build-up of (Ch/PVPA/Ch/MMT)5 on model silicon oxide surfaces from QCM-D measurements. (a) change in normalized frequency and energy dissipation for the third overtone, (b) total adsorbed mass of multilayer film calculated using the Sauerbrey model. (c) AFM tapping mode height images of (Ch/PVPA/Ch/MMT) quadlayers (1QL, 3QL and 5QL) deposited on a silicon wafer substrate using 1 g/L and 5 g/L solutions. The images are 2 x 2 µm2 and the z-range is indicated in the scale bar to the right of the images. 156x138mm (300 x 300 DPI)



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SEM micrographs of untreated aerogel (a) and (d), (Ch/PVPA/Ch/MMT) 5QL treated with 1 g/L solutions (b) and (e), 5QL treated with 5 g/L solutions (c) and (f) at different magnifications. 84x39mm (300 x 300 DPI)


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Weight loss and the derivative of the weight loss for untreated and LbL-treated CNF aerogel in (a), (c) nitrogen and (b), (d) air. 60x43mm (300 x 300 DPI)



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Burning time measured during the horizontal flame test and the residue amount at the end of the test (a), (b) Photographs taken during the horizontal flame test of the different aerogels, 10 s after removal of methane flame and at the end of the test, (c) Heat release rate curves for the untreated and LbL-treated aerogels, and (d) Photographs of the reference and of the LbL-treated CNF aerogel 20 and 60 s after the application of heat flux (35 kW/m2). 94x50mm (300 x 300 DPI)


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Schematic description of the flame penetration test (a), photographs of the front and rear side of the reference aerogel (b) and 5QL-5 g/L (c) 20 s after the butane flame application and at the end of the test. The temperature measured during the flame penetration test, on the flame-exposed and un-exposed side of the reference aerogel (d) and 5QL-5 g/L-treated aerogel (e). 127x90mm (300 x 300 DPI)



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AFM scanassyst mode: (a) Scratched surfaces of LbL treated silicon wafers showing the thickness of a (Ch/PVPA/Ch/MMT)5 quadlayer deposited on a silicon wafer substrate with 5 g/L concentration and (b) after exposure to a butane flame. The images are 10 x 10 µm2 and the z-range is indicated in the scale bar to the right of the images. (c) SEM micrograph of cross-sections of the (Ch/PVPA/Ch/MMT)5 quadlayer based on 5 g/L solution and (d) cross section of the same film after 3 s exposure to a butane flame. 60x42mm (300 x 300 DPI)


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SEM micrographs of aerogels after a horizontal flame test (a) and (d) reference sample, (b) and (e) (Ch/PVPA/Ch/MMT) 5QL based on 1 g/L dispersions, (c) and (f) 5QL based on 5 g/L dispersions, (g) schematic illustration of the flame-retardant mechanism differences between the aerogels treated with 1 g/L solutions and 5 g/L solutions. 198x222mm (300 x 300 DPI)


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