Nanostructured wood hybrids for fire retardancy prepared by clay

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Nanostructured wood hybrids for fire retardancy prepared by clay impregnation into the cell wall Qiliang Fu, Lilian Medina, Yuanyuan Li, Federico Carosio, Alireza Hajian, and Lars A. Berglund ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10008 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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ACS Applied Materials & Interfaces

Nanostructured wood hybrids for fire retardancy prepared by clay impregnation into the cell wall Qiliang Fu†, Lilian Medina†, Yuanyuan Li†, Federico Carosio‡,*, Alireza Hajian†, Lars A. Berglund†,* † Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden ‡ Politecnico di Torino, Alessandria Site, Viale Teresa Michel 5, 15121 Alessandria, Italy Abstract: Eco-friendly materials need “green” fire retardancy treatments, which offers opportunity for new wood nanotechnologies. Balsa wood (Ochroma pyramidale) was delignified to form a hierarchically structured and nanoporous scaffold mainly composed of cellulose nanofibrils. This nanocellulosic wood scaffold was impregnated with colloidal montmorillonite clay to form a nanostructured wood hybrid of high flame retardancy. The nanoporous scaffold was characterized by scanning electron microscopy, and gas adsorption. Flame retardancy was evaluated by cone calorimetry, whereas thermal and thermo-oxidative stability were assessed by thermogravimetry. The location of well-distributed clay nanoplatelets inside the cell walls was confirmed by energy dispersive X-ray analysis. This unique nanostructure dramatically increased thermal stability due to thermal insulation, oxygen depletion and catalytic charring effects. A coherent organic/inorganic charred residue was formed during combustion, leading to strongly reduced heat release rate peak and reduced smoke generation. Keywords: biocomposite; nanocomposite; layered silicate; nanocellulose; nanostructured, inorganic hybrid, wood nanotechnology 1 ACS Paragon Plus Environment


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Introduction Wood is essentially a nanocellulosic material, and has low density, relatively high mechanical properties and low cost,1,2 and is therefore used in load-bearing applications.3–7 However, in order to use wood in a safe manner, the flammability may need to be addressed.8,9 Fire protection can be obtained by deposition of fire retardant additives as coatings or by impregnation. Coatings often need renewed application with time. Another issue is leakage of hazardous and/or unhealthy fire retardants.10,11 Possible replacements of halogens are often nitrogen or phosphorus compounds.12–14 Since those compounds may also have problems, there is a need for more environmentally friendly and efficient solutions to the fire retardancy problem. Recently, wood mineralization concepts have been used for multi-functional hybrid organicinorganic materials.15–20 Such hybrids can exhibit extraordinary performance in terms of mechanical properties, thermal resistance, fire retardancy, barrier effects and ultraviolet resistance.21–24 For instance, Merk et al. prepared fire retardant hybrid wood using a bioinspired mineralization process based on the synthesis of CaCO3 inside the cell walls16 or in the wood lumen space.17 It is of interest to further explore the possibility to bring nanoscale mineral particles into the cell wall, although the low specific surface area (SSA) of native wood is a potential problem. Montmorillonite clay (MTM), consists of about 1 nm thick stacked layers of silicate.25 Typical diameters of MTM platelets are in the 100-400 nm range. MTM has been used in polymer nanocomposites to provide gas barrier, mechanical strength and fire retardant properties.26–28 Preparation of nanocomposites include melt blending, water-based colloidal processes such as layer by layer assembly or paper making in combination with cellulose nanofibrils.29–31 These materials showed impressive mechanical properties and thermal/fire

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stability due to the production of structure in which clay nanoparticles are aligned and ordered within a continuous matrix (e.g. catatonic polyelectrolyte or cellulose nanofibrils) in a “brick and mortar” or “nacre-mimetic” structure.32–37 The key challenge in terms of nanostructural control is to prepare the wood-clay hybrids so that MTM clay platelets become located inside the wood cell wall. Several researchers have prepared mixed wood/clay materials. Wang et al. investigated Cathay poplar (Populous cathayana Rehd) impregnated with organoclay to improve thermal stability.18 In another study, clay nanoparticles were mixed with melamine-urea-formaldehyde, and impregnated into the wood and cured resulting in improved mechanical properties, dimensional stability and water repellency. In none of the reported studies was clay nanoplatelets located within the cell wall. The purpose is to prepare truly nanostructured wood/clay hybrids with strongly improved fire retardant properties. In the present study, balsa is the selected wood species. It serves as model material since the high porosity facilitates modification effects. It is also, however, used in load-bearing applications as a sandwich core material. The porosity of wood can be utilized to impart new functionalities.38–42 In a previous study, wood architectures with increased nanoporosity were prepared by delignification.43 Transparent wood for engineering purposes were later prepared by infiltration of the delignified structure with poly(methyl methacrylate) or epoxy.43–45 Here, delignified wood scaffolds are impregnated by an MTM hydrocolloid to prepare organic/inorganic wood hybrids. Although inorganics have been combined with wood in commercial material applications, such procedures do not attempt to control the nanoscale structure. In the present study, the aim is, for the first time, to bring montmorillonite (MTM) clay nanoplatelets inside the cell wall of a delignified wood scaffold using a hydrocolloidal suspension. This may have implications for the use of other types of prefabricated nanoparticles for wood modification purposes. 3 ACS Paragon Plus Environment


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Results and discussion The multistep process used to prepare the nanocomposite is schematically illustrated in Figure 1. Delignification was the first step, where lignin was removed from the cell wall, in particular from cell wall corners and the middle lamella where the local lignin content is high. This is expected to facilitate diffusion of nanoclay into the cell wall in the next stage, in order to tailor the nanostructure. The delignified scaffold shows porosity at several length scales while preserving mechanical robustness, and may be functionalized in many different ways to extend property range and functionalities in wood-based composites.

Figure 1. Schematic illustration of the preparation process for the nanostructured fire-retardant wood hybrid. A nanoporous wood-based scaffold was obtained by removing lignin from native wood using peracetic acid delignification. The wood hybrid was formed by impregnation of a nanoclay hydrocolloidal suspension.

Morphology and porosity 4 ACS Paragon Plus Environment

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The SEM microscope graphs show that delignification results in nano-scale pore formation in the cell wall and middle lamellae (Figure 2). The delignified scaffold becomes white with respect to original “Balsa-Native” (Figure 2a, the right side). The specimen dimensions and honeycomb structure are well preserved (Figure 2a and Figure S1). The native cell wall structure is apparent in Figures 2b and 2d. In Figure 2b, the lignin-rich middle lamella is the central layer between cells, and it dominates at the center of the cell wall corner. In the high resolution image Figure 2d, cellulose nanofibrils are apparent as white “dots” sticking out of the surface. They are reinforcing elements embedded in a molecular polymer matrix mixture of lignin and hemicelluloses. After delignification, nano- and micro-scale pores in the cell wall and cell wall corners are apparent (see Figures 2c and 2e). The lignin content decreased from 24.9% for Balsa-Native to 5.5% for Balsa-Delignified as evaluated with Klason lignin analyses (Table 1). The hemicellulose content was also reduced. Since the cellulose mass in the sample was preserved, the relative cellulose content was increased from 50.7 to 74.2% as lignin and some of the hemicelluloses were removed. Table 1 gives component weight fractions. If data are expressed as reduction in lignin and hemicellulose mass, delignification removed 85% of the lignin and more than 40% of the hemicellulose. Some remaining lignin and hemicellulose are important to provide the necessary mechanical robustness to the scaffold.



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Figure 2. Photograph and SEM micrographs of native wood and delignified scaffold. a) The digital photograph of untreated Balsa-Native (yellow) and Balsa-Delignified (white). Cell wall cross-section morphologies of b) Balsa-Native wood and c) Balsa-Delignified scaffold at low magnification. High resolution SEM images of cell wall S2 layer in the d) Balsa-Native wood and e) Balsa-Delignified wood.

Table 1. Composition of native wood (Balsa-Native) and delignified scaffold (BalsaDelignified). Sample

Lignin [%]

Hemicellulose [%]

Cellulose [%]

Balsa-Native

24.9

24.4

50.7

Balsa-Delignified

5.5

20.3

74.2


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The change in specific surface area (SSA) due to delignification was evaluated by BET analysis of gas adsorption (Figure 3). Various drying procedures (see methods section) were used to preserve the structure and evaluate SSA. The reason is that drying of nanocellulosic structures tends to change the original structure and reduce SSA. Figure 3 reports the acquired SSA values and nitrogen adsorption-desorption isotherms of native wood and delignified scaffolds.

Figure 3. a) BET specific surface area (SSA) data for different drying methods. b) Nitrogen adsorption-desorption isotherms of materials subjected to different processes. Balsa-Delignified is dried from water. Balsa-Delignified/Solvent exchange was prepared by solvent exchange (ter-butanol) followed freeze drying. Balsa-Delignified/Super critical dried was achieved by organic solvent exchange (pure ethanol) followed by super critical drying (CO2).



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The BET specific surface areas of the Balsa-Delignified wood significantly increased compared to Balsa-Native wood (1.4 m2/g). SSA values range from 9 m2/g to 41m2/g depending on dehydration and drying methods (Figure 3a). From the nitrogen absorption results, the highest SSA value is obtained by supercritical drying and organic solvent exchange. This can be ascribed to lower surface tension of both CO2 gas and organic solvents, so that the pore structure is better preserved.46 In contrast, drying from water (BalsaDelignified) leads to agglomeration of cellulose nanofibers during the drying process and reduced SSA values. Effects from different drying methods are observed also in nitrogen adsorption-desorption isotherm data (Figure 3b) and in the differences in pore size distribution data (Figure S2). Note the small pore size for super-critically dried samples as estimated from the BET data in Figure S2. Clay location in cell wall of wood hybrid nanocomposite Unmodified wood and delignified balsa scaffolds were impregnated with clay suspension in order to prepare organic/inorganic wood hybrids. In Balsa-Native-Clay, the weight percent gain (WPG) was 4.1%, whereas it was high as 17.3% in Balsa-Delignified-Clay (Table S1). In Figure 4a, Balsa-Native-Clay samples show similar morphology to Balsa-Native (Figure 2b); a compact and dense cell wall structure (compare Figure 4a and 2b). Only a few pores in the cell wall are detected even at high magnification (Figure 4b). The lumen space inside the cells were empty for both materials, indicating that if any, clay nanoplatelet content in lumen is low (see also lower magnification insets of Figure 4a and 4c). It is important to note that cell walls of Balsa-Delignified samples completely change morphology after impregnation with the clay suspension. Pores are no longer visible and the structure appears denser (compare Figure 4d and 2e). This is ascribed to diffusion and absorption of nanoclay particles inside the cell wall, see high magnification micrographs (Figure 4c and 4d), as well as images of a single cell (Figure S3). The filling of microscale pore space in cell wall corners is apparent from these 8 ACS Paragon Plus Environment

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data, but nanoscale clay presence in the secondary cell wall and clay distribution are of critical importance. In order to assess this, EDX analyses were performed on cell wall cross sections.

Figure 4. FE-SEM micrographs of Balsa-Native-Clay (a,b) and Balsa-Delignified-Clay (c,d) samples. The insert SEM images in a) and c) are the wood honeycomb-like cell walls of Balsa-Native-Clay and Balsa-Delignified-Clay. The rectangles in a) and c) are positions of the high resolution images in b) and d), respectively. The arrows in c) and d) indicate location of nanoclay particles.

The spectrum in Figure 5a shows relative intensities measured along the cell wall (yellow line). The signals of silicon (Figure 5b, orange) and aluminum (Figure 5b, yellow) indicate a high concentration of clay platelets homogeneously distributed across the cell wall thickness. This is highly significant, since it means it is possible to put nanoscale particles in colloidal suspension inside the cell wall of delignified wood scaffolds. One explanation may be the



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strong capillary forces exerted by the nano- and micro-scale pores on the aqueous suspension with exfoliated nanoplatelets. The “wet” SSA of the delignified wood is higher than that of dried scaffold, because the wet scaffold preserved the real pore structure. All pore space generated by delignification may be available and the “wet” SSA of the samples is a minimum of 41 m2/g (Figure 3a). Consequently, particle size distribution is of crucial importance for the fraction of clay particles able to penetrate inside the cell wall. To this aim, a detailed analysis of the clay particle dimensions was conducted (Figure S4). AFM height measurements show an average thickness of 1.2 nm for individual platelet, in accordance with the literature.33 The particle size distribution (platelet diameter) was estimated from SEM images (Figure S4). The average size was 131 nm, with 37% below 50 nm and 48% between 50 nm and 150 nm (Figure S4). The scaffold pore size and clay particle size distributions were plotted in Figure 5c. The two curves partially overlap in the 45 to 90 nm region, confirming that a considerable fraction of the nanoclay is small enough to enter the pore structure in the Balsa-Delignified scaffold. The large size clay particles (larger than 100 nm) are probably not able to diffuse into the cell wall, but are deposited on the surface of the cell wall. With respect to mechanisms, clay platelets diffuse into the liquid filled cell wall in order to simply equilibrate clay concentration gradients. In the real wet structure, pore size is a complex parameter, but as a starting point for interpretation of mechanisms, the concept is helpful.


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Figure 5. Clay particle distribution in the cell wall of Balsa-Delignified-Clay. a) SEM in the line scanning mode and b) element distribution across the cell wall; c) size distribution of clay particles (blue) and pore volume distribution in the cell wall (black) after delignification Densities, porosities and WGP weight gain are presented in Table S1. The average weight gain was 17.3% for Balsa-Delignified-Clay samples. The reason for this comparably high value is due to the delignification (43 wt % weight loss) and the small dimensions of clay platelets, affinity with the cell wall and the high SSA of Balsa-Delignified samples. In contrast, the native wood reference material, Balsa-Native-Clay, showed low weight gain (4.1%, see Table S1). The reason is lower SSA, lower porosity and possibly smaller pore size. Although water stability of the present material is unknown, molecular scale clay-cellulose interaction may be tailored to avoid leaching problem. Thermal stability The thermal and thermo-oxidative stability of native and modified wood was evaluated by thermogravimetric (TG) analysis in nitrogen and air, respectively (Figure 6). Flaming combustion behavior is simulated by nitrogen TG under essentially anaerobic atmosphere. This is relevant to combustion below the surface of the burning material. Air TG simulates decomposition of the polymeric substances until the volatilization part of polymer combustion starts. Air TG also helps interpretation of oxidation of the solid combustion residue, after flame extinction. The TG and dTG curves are reported in Figure 6 while Table 2 collects temperature and weight fraction data.



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Figure 6. Thermogravimetric (TG) data and the differential (dTG) of TG curves for native wood and wood/clay nanocomposite hybrids in nitrogen (a and b) or air atmosphere (c and d). The values in a) and c) are the final residue weights at 800°C. Note that Balsa-Native-Clay has a clay content of 4.1wt%, and Balsa-Delignified-Clay has a clay content of 17.3wt%. In inert atmosphere, after an initial weight loss due to adsorbed water evaporation, wood components undergo pyrolysis. This process is the result of two competitive pathways that involve the release of volatile products and the formation of a thermally stable carbonaceous structure. The process occurs in different temperature ranges: 200-260°C for hemicellulose, 240-350 °C for cellulose and 280-500°C for lignin.12,47–49 Hemicellulose is the first component to degrade, see the slight shoulder in all the dTG curves. The main peak in the dTG is ascribed to cellulose and appears at a temperature corresponding to the maximum decomposition rate.50 Lignin decomposition takes place over a broader temperature range and results in higher char yields.48 The processes overlap in the TG curve of Balsa-Native to yield 12 ACS Paragon Plus Environment

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a final residue composed of a thermally stable char (Figure 6a, 18% of the initial mass). For Balsa-Delignified tested in nitrogen, the removal of lignin increases the Tmax value as depicted in Figure 6b. This is due to increased crystalline cellulose content.51 Despite this, the very low lignin content strongly reduces the final residue (9% as compared with 18% for Balsa-Native).

Table 2. Thermogravimetric data for native wood and wood/clay hybrids in nitrogen and air. Sample

Nitrogen

Air

Tonset10%

T*max

Residue

Tonset10%

T*max1

T*max2

Residue

[°C]

[°C]

at 800°C [%]

[°C]

[°C]

[°C]

Balsa-Native

277

345

18

263

304

393

3

Balsa-Native-Clay

280

342

23

261

301

405

7

Balsa-Delignified

272

357

9

267

277

356

1

Balsa-Delignified-Clay

275

330

42

262

285

386

33

at 800°C [%]

The presence of clay has a strong effect on final residues and effects of delignification. For Balsa-Native-Clay, the increase in final residue is due to the 4.1% clay WPG weight gain (Table S1). Balsa-Delignified-Clay shows as much as 42% residue at 800°C. This cannot be explained by simply adding the clay content (17.3% clay WPG as reported in Table S1) to the measured residue from Balsa-Delignified (9%). Clay nanoparticles increases the char production of cellulose partly by acting as a thermal insulator and provide Na+ catalytic sites enhancing degradation paths towards char formation.52,53 The cellulose-clay nanostructure in the cell wall improves stable char formation from 9 to 25%, comparing Balsa-Delignified and Balsa-Delignified-Clay (Figure 6a and Table 2), respectively. Similar results are observed in oxidative environment (air). TG curves (Figure 6c) show two degradation steps, the first, occurs between 250-350°C and it’s related to the production of



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char from hemicellulose, cellulose and lignin. This char is then oxidized to mainly CO and CO2 during the second degradation step.54 Both Balsa-Native and Balsa-Delignified samples yielded final residues in the range of 1-3% (Table 2). The presence of clay in wood-clay reduces degradation kinetics as observable from dTG curves (Figure 6d) and Tmax2 values in Table 2. The final residue is comprised of the sum of clay and wood residue. When clay is nanostructurally distributed within the cell wall the final residue was as high as 33% (Table 2). The second degradation step, related to char oxidation, is significantly reduced. The presence of clay probably slows down oxygen diffusion within the charred residue. Similar results have been reported for nanocellulose/clay brick and mortar structures and have been related to both the preferential orientation of clay within the structure and intimate contact between clay and cellulose nanofibrils.35 The final residue evaluated at 800°C still contains 16% of organic char at 800°C, showing strong contribution from clay to formation of thermally stable structures. Cone calorimetry Cone calorimetry was investigated to obtain information about the burning behavior. This technique investigates forced combustion of a material exposed to a heat flux typical of developing fires (the 35 kW/m2 adopted here represents the initial stage of a fire).55 During the tests, the samples were quickly heated by a conical heater. This triggers thermal degradation decomposition reactions with subsequent release of volatile and combustible species that are ignited by a spark igniter to initiate forced combustion of the sample. Heat release rate HRR and total heat release THR plots are reported in Figure 7, while Table 3 collects main parameters evaluated during the tests.


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Figure 7. Cone calorimetry data of unmodified and modified wood and wood scaffolds: a) Average Heat release rate (HRR) plots, b) average Total Heat Release (THR) plots and c) snapshots of residues collected at the end of the tests.

Table 3. Cone calorimetry data of unmodified and modified samples. TTI time to ignition, pKHRR peak heat release rate, THR total heat release. Sample

TTI [s]

pKHRR [kW m-2]

THR [MJ m-2]

SPR [10-4 m2 s-1]

TSP [m2]

Balsa-Native

8±1

245±9

9.3±1.6

7.5±1.3

0.12

Balsa-Native-Clay

8±1

229±12

8.6±0.8

6.8±1.5

0.1

Balsa-Delignified

8±1

296±11

10.8±1.3

5.5±1

*

Balsa-Delignified-

8±1

157±9

7.4±1.2

3±1

*

Clay Note: “*” the value is too small to be detected. Balsa-Native samples quickly ignite after 8 seconds (TTI) with vigorous burning (Table 3). During the early stages, a protective char, mainly from charred lignin, is produced on the 15 ACS Paragon Plus Environment


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surface. This barrier reduces the amount of combustible volatiles and momentarily reduces the HRR as demonstrated by a shoulder in the HRR plot (see Figure 7a). This protection eventually breaks down and there is a sudden increase in HRR which reaches its maximum value (pKHRR=245 kW/m2). The final residue consists of fragile and light black ashes (Figure 7c). The removal of lignin changes the burning behavior; see difference in HRR curves between Balsa-Native and Balsa-Delignified. No protective surface char is generated for BalsaDelignified samples and the HRR plot steeply reaches its maximum value which is higher than for Balsa-Native (296 vs 245 kW/m2 as reported in Table 3). Due to the absence of lignin almost no residue is collected at the end of the test (Balsa-Native as reported in Figure 7c). The presence of clay does not change the HRR plot of unmodified wood as observable from Figure 7. On the other hand, Balsa-Delignified-Clay samples show a peculiar burning behavior. Upon ignition a charred protective layer is produced on the surface. In contrast to Balsa-Native-Clay, the protection does not break and therefore the combustion kinetics is reduced. The pkHRR value is the lowest registered among all different samples yielding a 47 and 36% reduction with respect to Balsa-Delignified and Balsa-Native, respectively. THR values are reduced as well. The residue appears more compact and less fragile (BalsaDeliginifed-Clay in Figure 7c) The results can be ascribed to the unique nanostructure of clay nanoplatelets embedded within the cell wall. This hybrid nanocomposite is efficient in reducing oxygen permeability and promoting char formation of the organic cellulose scaffold. The efficiency of the present approach has not been compared with “classical” wood flame retardancy treatments.56 Instead, clay addition is interesting as an eco-friendly methodology, which operates by new mechanisms. Clay nanoplatelets are embedded and held together by a thermally stable char.


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This char forms on the surface and is then extended through the thickness. The clay improves thermal insulation and mechanical properties of the charred layer. The surface protection layer does not break and combustion kinetics is reduced (Figure 7c). Smoke parameters are reported in Figure S5. Balsa-Delignified-Clay showed the lowest CO/CO2 release, smoke production rate (SPR) and total smoke release (TSR) (Table 3). This further proves the fire retardant effects exerted by clay nanoplatelets when distributed at nanoscale and embedded in the secondary cell wall structure (Figure S5). Analysis of residues after combustion The char residue was fragmented and discontinuous (mainly charred on the surface) for BalsaNative-Clay while more compact and coherent for Balsa-Delignified-Clay. Figure 8 reports micrographs along with elemental analyses performed on Balsa-Delignified-Clay residues.



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Figure 8. SEM cross-section images of Balsa-Native-Clay residue (a,b,c), Balsa-DelignifiedClay residue (d,e,f), Energy-Dispersive X-ray spectroscopy (EDX) of Balsa-Delignified-Clay residue based on mapping model investigation (g,h,i). In Figure 8a, the charred portion of Balsa-Native-Clay maintained the original wood structure. The combustion of most hemicellulose and cellulose decreased the thickness of cell walls, nanoscale texture is lost and it appears smoother with respect to the original samples (compare Figure 8b and 4a). This change in morphology is from the pyrolysis processes generating a mixture of aliphatic/aromatic charred structures. The structures are not found within the bulk of the sample but are characteristic of the first 100-200 µm below the surface. Balsa-Delignified-Clay residue also preserved the original structure with reduced cell wall thickness (Figure 8d). In high magnification micrographs, the presence of clay particles are observed as embedded within the produced charred residue (Figure 8e and 8f). Elemental analyses further confirm the distribution of clay platelets inside the cell wall (Figure 8g, 8h and 8i). Signals from other elements based on EDX results provide further support for clay platelet location in the secondary cell wall (Figure S6 and Table S2). Conclusions The possibility to bring clay into the cell wall of delignified wood scaffolds is promising for new wood nanotechnologies and an opportunity for green fire retardancy treatment. A method is presented for colloidal clay nanoplatelet diffusion into the swollen secondary cell wall of nanoporous cellulosic wood scaffolds, so that true nanocomposites are formed. The presence of fairly homogeneously distributed clay platelets inside the cell wall is indeed confirmed by two sets of energy-dispersive x-ray diffraction data (wood-clay hybrid before and after cone calorimetry). The nanoporous cellulosic wood scaffold is a critical element in the process and shows a specific surface area as high as 41 m2/g. 18 ACS Paragon Plus Environment

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Since typical “diameters” of clay platelets are around 130 nm, diffusion into the swollen cell wall is made possible by pore channels larger than the clay platelets. The most likely diffusion mechanism is elimination of the clay concentration gradient. The clay suspension is impregnated through the tube-like lumen channels, which have diameters in the range of tens of micrometers. The clay nanoparticles diffuse from the lumen into the water-swollen cell wall, with a diffusion distance of only 1-10 µm. During cone calorimetry, the peak heat release rate for nanostructured wood hybrids is reduced by 36% compared with native wood and 32% compared with clay-treated native wood. Conversion of wood-clay tissue into thermally stable char-clay residue is the most important mechanism. Enhanced charring of wood tissue is induced by the clay in the cell wall, as confirmed by TG. The clay reinforced char protects underlying tissue and reduces combustion kinetics and smoke production rates. The reason is that clay with controlled nanoscale distribution reduces thermal diffusivity, oxygen diffusion and catalyzes favorable charring reactions in cellulose. The present wood hybrid has inorganic nanoparticles in the secondary cell wall, a uniquely structured nanocomposite created by nanoparticle diffusion from a hydrocolloidal suspension. The favorable performance of this wood hybrid is in strong contrast with clay-modified native wood. Simple impregnation of wood by an identical clay suspension did not significantly improve thermal and flame retardant characteristics. This illustrates the importance of the delignified and nanoporous scaffold. The results may inspire new wood nanotechnology procedures for preparation of wood hybrids containing other nanoparticles.

Experimental section



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Delignification: Samples of Balsa wood (Ochroma pyramidale, purchased from Wentzels Co. Ltd., Sweden) were prepared with different sizes: 20×20 ×1 mm3 (small size, L×R×T), 20×5×5 mm3 (medium size, L×R×T) and 50×50×5 mm3 (large size, L×R×T), and dried in oven at 105±3 °C for 24 h. The schematic description of delignification and clay impregnation has been reported in Figure 1. The wood samples were treated by 4 wt% peracetic acid (C2H4O3, Sigma Aldrich) with deionized water for 80 °C at different time depending on the size of the sample. The samples were chemically extracted 3 h, 6 h and 12 h for small, medium and large size, respectively. Every 6 h extracted procedure changed fresh PAA solution for large size sample. The extracted wood samples were completely washed using deionized water. Part of delignified wood samples was exchanged water by using ethanol, acetone and tur-butanol followed by doing freeze drying. Part of extracted samples was dehydration by 96 % ethanol and pure ethanol over-night and then dried by supercritical drying in carbon dioxide.46 There samples are followed by doing freeze drying. The pore analyses of the delignified samples were carried out on nitrogen absorption. A 2.6 wt % Montmorillonite (MTM) (Cloisite Na+, density of 2.86 g cm-3, Southern Clay Products, Inc.) suspension was prepared by strong stirring using Ultra Turrax blender (IKA, DI25 Basic) at 25000 rpm for 20 min followed by sonication using Vibra-Cell (Sonics and Materials, Inc.) for 8 min. Then, the clay aggregates were removed by centrifugation at 4500 rpm for 20 min.57 This procedure was repeated three times, yielding a stable 2 wt % MTM suspension. Clay impregnation procedure The delignified wood samples were exchanged water three times by using ethanol and acetone solution. The unmodified and delignified wood samples were dipped into the MTM 20 ACS Paragon Plus Environment

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suspension, and low vacuum (0.3 bars for 2 h) was performed at the same time. Subsequently, the wet clay impregnated samples were frozen in the fridge over night at -20 °C and then freeze dried for 24-48 h. FE-SEM: The cross section of unmodified and modified wood samples were imaged with a Field-Emission Scanning Electron Microscope (FE-SEM, Hitachi S-4800, Japan) using an accelerating voltage of 1 kV at a 3-8 mm working distance. The cross sections were achieved by fragile fracture surface in liquid nitrogen. The so prepared samples were coated with platinum-palladium prior to FE-SEM observation. EDX: Elemental analyses of Balsa-Native-Clay and Balsa-Delignified-Clay cross-section’s surface were carried out using an Energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments, X-MAX N 80, UK) with accelerating voltage of 15 kV. Klason lignin and sugar analysis: The lignin (Klason lignin) content of wood samples was measured according to the TAPPI method (TAPPI T 222 om-02).58 200 mg wood sample (40 meshes) were dissolved in a 3 ml 72 wt % sulfuric acid solution (H2SO4, Sigma-Aldrich) at low vacuum for 1 h followed the addition of 84 ml of Mill Q water. The solution was hydrolyzed in autoclave at 120±5 °C for 1 h. Then the precipitate and hydrolyzed solutions were separated by filtration with a glass microfibers filer. Finally, the hydrolyzed solution was diluted to 3 % v/v H2SO4 solution, and analyzed for sugar content on a Dionex ICS-3000 high performance anion chromatography instrument. The precipitate (Klason lignin) was dried in the oven at 105±3 °C for 24 h. The lignin content was determined by following Eq. (1). Lignin %= Ml/Ms × 100% (1)



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Where Ml (mg) is the mass of precipitate (lignin) and Ms (mg) is the originally dried mass of the wood sample. BET: Nitrogen physisorption was performed on an ASAP 2020 (Micromeritics) at -196 °C. Wood samples (0.2-0.3 g) were degassed at 70°C for 5 h before the Brunauer−Emmett−Teller (BET) specific surface area measurement. The results were collected at a relative pressure between 0.1 and 0.3.59 TG (N2/Air): Thermogravimetric analysis (TG) was conducted on a Toledo TGA/SDTA851 instrument. The wood samples (10±1 mg) were placed in the alumina pans and heated from 25 °C to 800 °C with a heating rate of 10 °C/min, in an N2 or air flow (50 ml/min). The TG measurement in air atmosphere was carried out in order to understand the thermo-oxidation performance of the samples, while TG performed in the N2 atmosphere was discussed the thermal degradation behavior of cellulose in wood/clay composite. Cone calorimetry: Oxygen consumption cone calorimetry (Fire testing technology, FTT) testing was used to investigate the combustion of wood samples (50×50×5 mm3) under a heat flux of 35 kW/m2. The test was repeated three times for each sample. The following parameters were registered: time to ignition (TTI, [s]), peak of heat release rate (pKHRR, [kW m-2]) and total heat release (THR, [MJ m-2]) were evaluated. AFM: AFM images were captured using a multimode Nanoscope IIIa (Bruker Corp., USA). Scanning topography was operated in tapping mode and RTESP-150 cantilevers, having a nominal tip radius of 8 nm and a nominal spring constant of 5 N/m (Bruker Corp., USA). To measure the exact thickness of the clay particles, surfaces were prepared by dipping layer-bylayer assembly (one bi-layer) of Polyethylenimine (0.1 g/L) and clay (0.3 g/L). This technique provides a more accurate height measurement of the particles from the dispersions deposited onto the surface without the effect of drying as occurs upon dry-casting.60 22 ACS Paragon Plus Environment

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Associated content Supporting information SEM images of wood honeycomb cells structure and single cell structure. BET result shows the distribution of pore size in the cell wall. Nanoclays size statistics based on SEM and AFM images. Elements statistics characterized via SEM/EDX. Gases and smoke production data were collected from cone calorimetry tests. Author information Corresponding author *Lars A. Berglund, Email: [email protected] *Federico Carosio, Email: [email protected] Notes The authors declare no competing financial interest. Acknowledgements We acknowledge funding from Knut and Alice Wallenberg foundation via Wallenberg Wood Science Center (WWSC). In addition, the authors would like to acknowledge the SSF FireFoam project (RMA11-0065). We thank Dr. Yao for help with nanoclay size measurement. Q. Fu is grateful to China Scholarship Council (CSC) for funding his PhD program.



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Table of Content Wood/Clay Hybrids

Balsa wood was delignified to form a hierarchical, cellulose-based scaffold. This scaffold with its nanoporous cell wall, was impregnated with an aqueous suspension of montmorillonite clay to form a nanostructured wood hybrid of high thermal stability and flame retardance.


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Nanostructured wood hybrids for fire retardancy prepared by clay

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