Flame-Retardant Polyurethane Foams: One-Pot, Bioinspired Silica

Article Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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Flame-Retardant Polyurethane Foams: One-Pot, Bioinspired Silica Nanoparticle Coating Daniel J. Brannum, Erik J. Price, Daniel Villamil, Susan Kozawa, Michelle Brannum, Cindy Berry, Robert Semco, and Gary E. Wnek* Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States

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ABSTRACT: The replacement of halogen-free flame retardants, driven by health concerns, has sparked a large demand for new “green” flame-retardant alternatives. Inspired by the natural flame-retardant properties of Cladophora sp. algae coated with silica diatoms, a silica sol−gel method has been employed to improve the fire resistance of common, open-cell polyurethane (PU) foams. The Stöber process with components 2-propanol, water, tetraethyl orthosilicate (TEOS), and ammonium hydroxide was employed for silica nanoparticle synthesis on the inside walls and struts of PU foam. Upon ignition, the treated foams briefly burn, followed by formation of a propagating char front that leads to self-extinguishment. Most importantly, the coating of silica nanoparticles prevents dripping of flaming residues seen in common untreated PU foams. Microcomputerized tomography of silica-treated foam after burning reveals that char formation is confined to the outer edges of the bulk foam. Via cone calorimetry, the peak heat release rate of a 0.5 M TEOS foam was reported as dropping from 560 to 262 kW/m2, relative to untreated foam. These results, coupled with the ease of application of the silica coatings, suggest a viable and scalable approach to the mitigation of burning of common open-cell PU foams. KEYWORDS: flame-retardant materials, sol−gel coatings, biomimicry, bioinspired, polyurethane foam, scalable

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inherent fire resistance.4 A previously unpublished transmission electron microscopy (TEM) image on such a material revealed a plethora of silica diatoms (Figure 1). This tempted us to postulate a synergistic effect of algal cellulose and silica diatoms: (1) silica can act as both a thermal barrier to underlying cellulose and a diffusion barrier to oxygen needed for combustion as well as combustible volatiles to the flame, and (2) the silica serves to drive cellulose toward dehydration to carbonaceous products. This dehydration occurs when pyrolysis of cellulose occurs in oxygen-free or oxygen-limited conditions.5 Moreover, the carbonaceous layer can further serve as a thermal and diffusion barrier. Therefore, a synthetic analogue of the silica diatom-coated algae was envisioned to mitigate the flammability of common open-cell polyurethane (PU) foam. A crude replication without the exquisite detail of diatoms was proposed through

INTRODUCTION Flammability of commodity polymeric materials and its mitigation have been a topic of interest since their inception in commercial use many decades ago.1 The use of these materials in housing, transportation, and packaging is steadily increasing. However, this increase in demand comes with an increasing environmental and legislative pressure to limit the choices of flame retardants commonly employed to mitigate flammability.2 A particular example is halogenated flame retardants. Upon arrival to the market, these materials dominated due to their highly effective radical scavenging flame-retardant mechanism. However, upon investigation, they became the subject of scrutiny due to concerns about toxicity and environmental impact.3 While new polymers with broad applications and inherent fire resistance may yet be discovered, there is an urgent need to develop new approaches to mitigate flammability of existing, commodity materials. Inspiration for the present work derives from the interesting observation that a species of fresh-water algae, specifically Cladophora sp. prevalent along the shores of Lake Erie, exhibits © XXXX American Chemical Society

Received: March 27, 2019 Accepted: July 9, 2019 Published: July 9, 2019 A

DOI: 10.1021/acsapm.9b00283 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

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ACS Applied Polymer Materials

In this report, we aim to employ a system which is attractive not only due to its ability to offer superior surface area coverage (and ultimately superior flame mitigation) of PU foam struts but also because of its ease of scaling and reusability of reaction solvents. While reactions shown herein are performed for 3 h, literature has shown that such reactions can be performed in less than an hour.10 Additionally, as no catalysts or carcinogenic ingredients are used, reaction components can be easily separated and recycled for further use. Particularly, residual silica can be separated out of the reaction medium via sedimentation or centrifugation for a plethora of applications in the biomedical field.11−17 Herein, we report the development of a facile, scalable sol−gel silica coating process for open-cell PU foams, which not only selfextinguish but also fully prevent dripping of flaming residue.

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Figure 1. SEM image of silica diatoms on the backbone of cladophora fibrous algae.

EXPERIMENTAL SECTION

Materials. Open-cell PU foam was obtained from JoAnn Fabrics. Each piece was 2 in. × 22 in. × 22 in. and sold as replacement cushion foam with no flame-retardant additives. Tetraethyl orthosilicate (TEOS), 2-propanol, and ammonium hydroxide were purchased from Sigma-Aldrich and used as received. Water used was purified through reverse osmosis. Preparation of Flame-Retardant PU Foam. Silica sol−gel deposition was performed by using the method of Wang et al.18 In a typical experiment, a cube piece of foam with dimensions of 1.5 cm × 1.5 cm × 1.5 cm was immersed in a solution containing 1.67 mL of TEOS, 15 mL of 2-propanol, 0.85 mL of H2O, and 0.115 mL of 14.8 M NH4OH. The 2-propanol, water, NH4OH, and foam were all added to a 20 mL vial and mixed for 5 min. TEOS was then added and mixed typically for 3 h at room temperature. At the end of the 3 h the solution is fully reacted, and the excess was poured off and washed four times with ethanol and four times with water. The resulting foam was left in air to dry. Water and ammonium hydroxide concentration were based on the concentration of TEOS used in the system. The concentration of TEOS used in this paper ranged from 0.25 to 5 M. Ammonium hydroxide concentration was calculated to be equal to the concentration of TEOS times 0.23, whereas the concentration of

deposition of silica particles via a sol−gel chemistry based on the Stöber process (Scheme 1A−C) in which tetraethyl orthosilicate (TEOS) hydrolyzes followed by condensation to form silica nanoparticles embedded on (and perhaps into) the surface of the foam struts. Open-cell PU foams were chosen due to their ubiquitous presence in many commodity items (e.g., seat cushions, bedding, etc.). If untreated, these foams burn extensively and readily drip flaming material, which is an additional source of ignition for underlying materials.6 Significant investigation into flame mitigation of PU foams has been reported,7 but research concerning sol−gel depositions for flame retardation of PU foams has received much less attention. Verdoltti et al. reported on a polyurethane−silica hybrid in which polysiloxane domains were incorporated into the urethane prior to polymerization.8 In addition, Bellayer et al. employed silicate-based intumescent coatings of PU foams.9

Scheme 1. Schematic of the Stöber Process: (A) Hydrolysis of the TEOS Occurs, Creating Orthosilithic Acid; (B) Condensation Occurs, Dimerizing the Orthosilithic Acid; and (C) Continued Condensation until Silica Nanospheres Are Formed; (D) General Polyurethane Structure and Its Subsequent Silica Treatment via the Stöber Process

B


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Figure 2. (A) Compression testing for ten cycles. (B) First Compression Cycle. (C) The last nine cycles.

Figure 3. SEM images at various magnifications: (A−C) untreated standard; (D−F) 0.25 M TEOS; (G−I) 0.50 M TEOS; (J−L) 1.0 M TEOS. water was 6.25 times the concentration of TEOS. Coated PU foams for UL-94 (V and HBF), LOI, cone calorimetry, and compression testing all utilized larger foams to best accommodate ideal conditions for each instrument. For these sizes, sample volumes were linearly scaled based on the increased volume. The previously mentioned procedure was followed with the exception that the larger foam was immersed in a large beaker equipped with a stir bar rather than a small vial. Characterization. Electron microscopy was performed by using a Helios 650 Nanolab field emission scanning electron microscope

(FEI, Hillsboro, OR) with 10 keV beam. Samples of silica-coated urethane form, and Cladophora sp. algae were coated with a 7 nm layer of Pd to mitigate charging in the electron beam. The sample of algae imaged in Figure 1 was first treated with household bleach overnight in an attempt to destroy living matter followed by copious washing with water and then was dried for at least 1 week under ambient conditions. Compression testing was done on a Zwick/Roell tensile/ compression tester (Zwick Roell Group, Ulm, Germany) using a 100 N load cell at room temperature. Tests were ran at 1 cm/min for C


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Figure 4. MicroCT images of (A) untreated and (B) treated (0.5 M TEOS) PU foam. then filtered to remove spurious noise, and visualized by using Avizo’s volume-rendering module.

10 cycles with a 1 min hold cycle in between each full cycle. Square samples were made at a 2:1 ratio being 5 cm × 5 cm × 2.5 cm. Thermal gravimetric analysis (TGA) was performed using a TA Instruments Q500. Approximately 10 mg samples were ramped at 10 °C/min from room temperature to 700 °C. The weight of the residue remaining after a TGA run with an air flow of 60 mL/min was used to calculate the amount of silica in the sample. Limiting oxygen index (LOI) testing was performed at room temperature according to ASTM D2863 using a LOI apparatus constructed by the Combustion Fire Research Laboratory in the Department of Mechanical and Aerospace Engineering at Case Western Reserve University. Each sample was repeated twice to examine the accuracy of the experimental setup and absolve the introduction of operator error. Samples were precisely cut to 12.5 cm × 1.25 cm × 1.25 cm dimensions to comply with standardized testing procedures. Flammability was tested by using the UL-94 V specification by applying a 5 cm propane flame for 5 s to samples cut to 12.5 cm by 1.25 cm by 1.25 cm dimensions. Each molar concentration was characterized via testing with three repetitions to ensure statistical significance. For UL-94 HBF (ASTM D4986-18), sample foams were cut to 50 mm × 150 mm × 13 mm. Samples were oriented horizontally, 13 mm above the wing top burner. The burner was turned on and left on for 60 s, and each sample was performed five times for statistical significance. Cone calorimeter testing was performed using samples prepared at the size 100 mm × 100 mm with a thickness between 23 and 27 mm using the FTT cone calorimeter (Fire Testing Technologies Ltd., East Grinstead, UK). Samples were ran by using a heat flow of 50 kW/m2, with data presented including time to ignition (TTI), heat release rate (HRR), peak of heat release rate (PHRR), time to peak of heat release (TTPHR), total heat release (THR), specific extinction area (SEA), total smoke release (TSR), carbon monoxide (CO) production, and carbon dioxide (CO2) production. Optical microscopy was performed on a Nikon Eclipse TS100 optical microscope with an attached Micropublisher 3.3 RTV imaging from QImaging. Samples were characterized by using 10×, 20×, and 40× magnifiers on 10× optical lenses, allowing scale bars of 25 μm on the associated QCapture software. Microcomputerized tomography (MicroCT) experiments were performed at Procter & Gamble Co. and utilized ∼8 mm diameter sections of foams placed in a Scanco mCT50 X-ray scanner (Scanco Medical, Zurich, Switzerland). Scanning was performed with an energy of 45 keV, with 3000 projections and an integration time of 9.6 s per projection. The resulting data set was 5694 × 5694 × 1506 voxels with attenuation values represented as 16-bit integers. Each voxel had a diameter of 1.8 μm. To visualize the data, the sample was then read into a visualization platform, Avizo 9.2.0 (FEI, Hillsboro, OR). The data were first converted into an appropriate 8-bit window,

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RESULTS AND DISCUSSION Physical, Morphological, and Thermal Characterization. Foams were treated via the modified sol−gel process using TEOS concentrations ranging from 0.25 to 5.0 M, depositing variable weight percent amounts of silica in to the sample (Table S1). Concentration selection was dictated by the desire to balance the attainment of flame retardancy without noticeable impact on the foam’s mechanical properties, a crucial variable in application of these foams. Toward that end, cyclic compression testing (Figure 2) was performed. From the results, negligible change in mechanical properties are observed until 1.0 M TEOS. Shown in Figure 2B, the 1.0 M sample showed a nearly 30% increase in engineering stress during the first cycle that persisted through the remaining cycles (Figure 2C). Discussed in detail later, foams treated with 0.5 M TEOS afford arguably the best combination of fire retardancy with no apparent compromise of compression properties. Scanning electron microscopy was performed on foam samples to visualize the silica coatings. Shown in Figure 3A−C, the pristine foam reveals a pore size ranging from approximately 200 to 500 μm with an average of about 340 μm. Silica sol−gel deposition at three different TEOS molarities (Figure 3D−L) indicates the presence of silica particles and eventual formation of plate-like structures at 1.0 M TEOS. As the molarity of TEOS used increases (or as the concentration of silica embedded in the PU foam increases), the pore size did not vary (Table S2). To further ensure silicon presence on the treated foams, SEM-EDX was performed. A representative SEM image and elemental profiles for a 1.0 M TEOS foam are shown in Figure S3A−D. Of the particular strut shown, EDX analysis on the foam strut’s cross section shows a statistically significant amount silica (Figure S4). To complement SEM, microCT is an especially useful technique to nondestructively probe the interiors of foams. Interestingly, resultant images of a 0.5 M TEOS treated foam (Figure 4B) reveals what appear to be occasional thin membranes which are believed to be the result of incomplete rupture of some cells during foaming. Because such thin membranes cannot as readily be seen in the untreated foam, this suggests the interesting application of sol−gel silica as a microCT contrast agent for polymer foam structures. D


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particular untreated foams studied. Thus, for the case of 0.5 M TEOS-treated foam, the silica content is ∼12%. A serendipitous discovery was made doing TGA on higher molarity samples (1.0 M TEOS). Upon exiting the TGA, the foam architecture persisted, even with the clear removal of the organic PU. Shown in Figure 6 are SEM images of a section of aforementioned sample, and further imaging via microCT is shown in Figure S5. Discussed later in detail during the mechanistic consideration section, this is indicative of a thick, continuous coating. Flammability of Untreated and Treated Foams. Qualitative visual experiments with 1.5 cm cubes of untreated foams were performed first. Briefly, samples were suspended from a ring stand in a fume hood and ignited with a match to probe general flammability. In the case of the untreated foams, samples showed rapid combustion with both significant dripping of molten products and little residue after complete combustion (Figure S6). In contrast, foams treated with 0.5 M TEOS shown in Figure 7 display vastly different combustion profiles. Under the same conditions as the untreated foams, initial burning of the treated foams briefly occurs but is followed by the rapid formation of char and propagation of a “char front”. Interestingly, 0.5 M TEOS and the above-treated samples exhibited no dripping whatsoever, and the charred foam retained its original dimensions. Cutting of the burned foam revealed essentially pristine material toward the center of the specimenan observation supported by a microCT scan (Figure S7). Further analysis via optical microscopy on 1.0 M TEOS PU foams burnt via TGA heating to 700 °C (Figure S8) shows thin layers of silica over what were apparently thin, intact membranes that did not rupture during foaming, as noticed in the microCT of sol−gel-treated but unburned foam in Figure 5b. These observations prompted characterization via the UL-94 testing standardboth the vertical and horizontal burning flame (HBF) test. Testing methods were designed to evaluate the flammability of a rectangular material exposed to an intermittent flame source; the UL-94 method is an staple test standard for plastic flammability.20,21 It must be stated that for the vertical test, per the stipulation that no flame and/or combustion be observed at the clamp−sample interface, no treated PU foam samples genuinely obtained the classification of “V” given to successful samples. This also holds for the HBF test, in which no damage can surpass the 60 mm line on the sample if seeking the HF 1 and HF 2 classifications. As observed in Figure 7, the lack of these classifications is not representative of the treated foams’ flame-retardant capabilities.

TGA was used to compare the degradation of pristine and silica-coated foams in air as a function of temperature (Figure 5). Standard TGA curves (weight loss vs temperature) and

Figure 5. (A) TGA weight loss and (B) DTG curves of neat PU foam and TEOS-treated foams.

differential curves (DTG) were recorded. The decomposition of PU foam in air occurs in two phases as shown in the DTG curves. The first peak at 250 °C is representative of the degradation of hard segments, and the second at 375 °C corresponds to the soft segments in the foam.19 The residue is the percent by weight of silica in the foams minus ∼1.4%. Note that since PU char yield varies with variables like isocyanate and polyol structure, heating rate, and so on, this designation of 1.4% was empirically defined as the char yield for the

Figure 6. SEM of a 1.0 M TEOS sample foam fully degraded in TGA at various magnifications: (A) 100 μm, (B) 20 μm, and (C) 10 μm. E


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Figure 7. Time-lapse photos of burning of silica-treated (0.5 M TEOS) PU foam. The total elapsed time is ∼15 s.

Table 1. Burning Parameters of PU Coated and Uncoated Foam sample

mass (g)

LOI (%)

TTI (s)

PHRR (kW/m2)

TTPHRR (s)

THR (MJ/m2)

FIGRA (W/s)

TSR (m2/m2)

mean SEA (m2/kg)

residue (%)

standard 0.25 M treated 0.50 M treated 1.00 M treated

6.28 6.72 6.57 8.41

16.2 18 19 19.5

1 1 1 1

560 533 282 254

30 25 10 5

19 19 17 15

19 21 28 51

154 175 168 55

211 222 277 77

5.9 0.3 16.5 24.9

Figure 8. Data from cone calorimetry on coated PU foams and controls as a function of time. Plots include (A) HHR, (B) THR, (C) mass loss, (D) TSR, (E) CO2 production, and (F) CO production.

While the propagating char front mechanism of flame retardation is fundamentally not ideal for evaluation via these tests, insightful information can be gleaned nonetheless. Resultant observations were divided into two categories: (1) samples made at TEOS of 0.25 M and below (low concentration), demonstrating some of the characteristic behaviors of PU foam, and (2) those at 0.5 M and above (high concentration), revealing notably different activity. While low TEOS concentration samples consistently displayed nonflaming drip and complete combustion, the heat shrinking

mechanism and rate of combustion decreased noticeably by 10 s or more relative to unmodified foams. In contrast, the high TEOS concentration samples performed remarkably with reproducible prevention of any dripping and heat shrinkage. Sample combustion times were reduced, with 1.0 M TEOS samples creating faster char fronts and extinguishing slightly faster than 0.5 M TEOS samples. Additionally, a dimensionally conserved char was formed, encapsulating unburnt PU foam. Barring the “disqualification” from the flame spread, both 0.5 and 1.0 M TEOS foams could qualify for V-2 rating. F


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ACS Applied Polymer Materials To obtain more quantitative flammability data, LOI testing was performed on untreated and treated foams. LOI is another flammability test that burns samples in a controlled atmosphere to determine the minimum concentration of oxygen required for sustained combustion. All tests were performed by the same operator on the same day to eliminate sources of human error and variance due to humidity and pressure changes. Experimental results are outlined in the first section of Table 1, which illustrates the averaged LOI results (reported as O2 concentration) for each sample concentration group. As expected, neat PU foam presented an LOI of 16.2 during testing. Relative to the untreated foam, all samples showed reduced flame propagation and slowed the combustion, with LOI values ranging from 18% to 19.5%. A general trend can be established that as an increased TEOS concentration is used to treat the foams, the LOI subsequently increases. This increasing LOI trend is likely due to the surface silica acting as a thermal and oxygen barrier, retarding the combustion process. While these changes are minor and still fall below the 20% oxygen concentration that air is commonly composed of the flame mitigation mechanism observed these coatings utilize does not eliminate combustion but instead rapidly self-extinguish the combusting material. To further provide quantitative flammability data, cone calorimetry was performed. A versatile machine and one of the most popular benchmarks of material flammability, cone calorimetry captures the combustion behavior of the materials tested via a number of conditions, including time to ignition (TTI), heat release rate (HRR), peak of heat release rate (PHRR), time to peak of heat release (TTPHR), total heat release (THR), specific extinction area (SEA), total smoke release (TSR), carbon monoxide production (CO), and carbon dioxide production (CO2). Data for standards and the treated foams are outlined in Table 1 and Figure 8. The differences between the untreated standard and 0.25 M TEOS sample are negligible in all measured properties except the peak release of CO2 and LOI. These similarities and differences, namely the reduction in peak release of CO2, have been attributed to a change in pyrolysis conditions. As more silica is introduced in to the combustion system via these coatings, incomplete combustion becomes more prevalent. For the 0.5 M TEOS foam the PHRR declines by half from 560 to 282 kW/m2 relative to the untreated sample. Also, in the CO and CO2 graphs the barrier properties of the coating become apparent. The second peak in the CO graph correlates to the lack of oxygen, indicative of incomplete combustion. Though it prevents the oxygen incorporation, the flammable nature of CO contributes to the longer burn time. The 1.0 M TEOS sample shows the best flammability characteristics, with reduced PHRR and TSR alongside a residual char yield of 25% at the end of test. The barrier properties of the coating are represented in the CO and CO2 graphs. As seen, the rate of CO2 production is very small, and a larger formation rate of CO is shown as the test proceeds, relative to the other samples. These reduced PHRR directly correspond to an increased fire growth rate (FIGRA), which is defined as PHRR divided by time to peak heat release rate (TTPHRR) (e.g., 1.0 M = 254/5 = 51 W/s). This increase is simply because the PHRR (albeit lower) occurs at an earlier time relative to samples with lower TEOS concentration. Mechanism of Fire Retardation. The mechanism of flame mitigation regarding the silica coating on polymers is a well-documented procedure.22 Briefly, nonflammable nano-

particles are paired with polymers that possess low char yield. Upon combustion, the polymeric layer burns away until a silicate-rich layer is formed on the surface, affording an effective oxygen and heat barrier. In coatings discussed in this paper, different concentrations of TEOS result in different flame-retardant mechanisms. We observe that in concentrations of 0.25 M TEOS and below there is not enough silica formation to create an effective barrier to complete combustion. However, it is observed that even without this effective barrier that systems convey minor flame mitigation properties in that drips are no longer flaming and the rate of combustion is significantly decreased relative to untreated foams. We attribute this to the reduction of combustible material concentration in the immediate combustion zone. With higher concentration coatings (0.5 M TEOS), samples contain a large enough concentration of silica to create an insulative coating. As such, upon combustion, these coatings follow the mechanism outlined in the previous paragraph. At higher concentrations still (1.0 M TEOS), coatings are hypothesized to deposit enough silica to create a thick, selfsupporting continuous layer of silica at the surface. This is supported by the SEMs in Figure 6 and microCT in Figure S5. Like 0.5 M, these coatings also following a nanoparticle percolation model. With both 0.5 and 1.0 M TEOS, it is hypothesized that more than just heat and oxygen are being blocked via the silica coatings. It is hypothesized that the coating additionally acts to radiate heat away from the foam and to restrict release of the highly reactive isocyanate degradation product that standard or lower TEOS concentration samples commonly show. As a result, the normally large PHRR shown in Figure 7A is truncated at an earlier time, explaining the increased FIGRA values as TEOS concentration increases.

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CONCLUSION In this work, sol−gel-based silica coatings of common, opencell PU foams were shown to effectively and reliably attenuate combustion. For the sake of application, these coatings were first shown to have a negligible impact on mechanical properties of foams until significantly large concentrations of TEOS was used. SEM-EDX was utilized to visualize and confirm the presence of silica on the internal foam structures. The concentration of silica relative to the original foam was determined via TGA and mass difference, with concentrations ranging from 1.8 to 20.8 wt % silica on the coatings. Flammability testing was performed, beginning with qualitative burn tests alongside Micro-CT and optical microscopy to demonstrate proof of concept. To supply more quantitative data, UL-94, LOI, and cone calorimetry testing were performed, showing effective mitigation of flammability via significant reductions in data like PHRR and THR. Utilizing inexpensive and benign ingredients under mild reaction conditions, this scalable process is an attractive project for further development.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00283. Figures S1−S8 (PDF) G


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(13) Bitar, A.; Ahmad, N.; Fessi, H.; Elaissari, A. Silica-based nanoparticles for biomedical applications. Drug Discovery Today 2012, 17 (19−20), 1147−1154. (14) de Oliveira, L. F.; de Almeida Goncalves, K.; Boreli, F. H.; Kobarg, J.; Cardoso, M. B. Mechanism of interaction between colloids and bacteria as evidenced by tailored silica-lysozyme composites. J. Mater. Chem. 2012, 22, 22851. (15) de Oliveira, L. F.; Bouchmella, K.; Goncalves, K. d. A.; Bettini, J.; Kobarg, J.; Cardoso, M. B. Functionalized Silica Nanoparticles As an Alternative Platform for Targeted Drug-Delivery of Water Insoluble Drugs. Langmuir 2016, 32 (13), 3217−3225. (16) Wibowo, N.; Chuan, Y. P.; Seth, A.; Cordoba, Y.; Lua, L. H. L.; Middelberg, A. P. J. Co-administration of non-carrier nanoparticles boosts antigen immune response without requiring protein conjugation. Vaccine 2014, 32 (29), 3664−3669. (17) Kim, S.; Na, H.; Won, C.; Min, D. In-depth study on the gene silencing capability of silica nanoparticles with different pore sizes: degree and duration of RNA interference. RSC Adv. 2016, 6, 27143− 27150. (18) Wang, X.; Shen, Z.; Sang, T.; Cheng, X.; Li, M.; Chen, L.; Wang, Z. Preparation of spherical silica particles by Stöber process with high concentration of tetra-ethyl-orthosilicate. J. Colloid Interface Sci. 2010, 341 (1), 23−29. (19) Gaboriaud, F.; Vantelon, J. P. Mechanism of Thermal Degradation of Polyurethane Based on MDI and Propoxylated Trimethylol Propane. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 2063− 2071. (20) UL 94. Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, Underwriter Laboratories, 2013. (21) ASTM D4986-18. Standard Test Method for Horizontal Burning Characteristics of Cellular Polymeric Materials, 2018. (22) Morgan, A. B. Flame retarded polymer layered silicate nanocomposites: a review of commercial and open literature systems. Polym. Adv. Technol. 2006, 17, 206−217.

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Erik J. Price: 0000-0002-5733-8550 Gary E. Wnek: 0000-0001-7358-8878 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Wesley Mahan for introducing us to fireresistant algae and for providing samples of Cladophora from Lake Erie for electron microscopy and to Underwriters Laboratories for support of the CWRU Combustion Fire Research Laboratory. This work was supported in part by an NSF Partnership for Innovation grant to G.E.W. and a GAANN Fellowship to D.J.B. We are grateful to Dr. Thomas E. Dufresne and Paula Chmielewski of Procter & Gamble Co. for carrying out the micro-CT experiments and image analyses. In addition, we thank Wayne Jennings and Kevin Abassi of the Swagelok Center for Surface Analysis of Materials for assistance in obtaining the SEM images. Many thanks to Prof. James Ti’en for useful discussions. We also thank the manuscript’s referees for insightful comments.

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REFERENCES

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Flame-Retardant Polyurethane Foams: One-Pot, Bioinspired Silica

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