Bioinspired Catecholic Flame Retardant Nanocoating for Flexible

Article of Materials is BioinspiredChemistry Catecholic published by the American Chemical Society. 1155 Flame Retardant Sixteenth Street N.W., Washington, DC 20036 Published by American SubscriberNanocoating access provided by CMU Libraries Society. -Chemical http://library.cmich.edu Copyright © American Chemical Society. However, no copyright

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R. Jones, Sergei Nazarenko, and Christopher J Ellison Chemistry of Materials is

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DIRECT EXPOSURE TO TORCH

Page 1 of 24Chemistry of Materials

1 2 Coated Foam Control Foam 3 4 AFTER 5 ACS Paragon Plus Environment REMOVAL Melt 6 Dripping OF TORCH 7

Chemistry of Materials

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Bioinspired

Catecholic

Flame

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Retardant

Nanocoating for Flexible Polyurethane Foams Joon Hee Cho,§ Vivek Vasagar,‡ Kadhiravan Shanmuganathan,& Amanda R. Jones,§ Sergei Nazarenko,‡ and Christopher J. Ellison*,§,† §

McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, TX

78712 †

Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712

‡

School of Polymers and High Performance Materials, University of Southern Mississippi,

Hattiesburg, Mississippi 39402 &

Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune,

Maharashtra 411008, India

ABSTRACT An efficient, environmentally friendly, and water-applied flame retardant surface nanocoating based on polydopamine (PDA) was developed for foamed materials such as polyurethane (PU). The PDA nanocoating, deposited by simple dip-coating in an aqueous dopamine solution, consists of a planar sublayer and a secondary granular layer structure that evolve together, eventually turning into a dense, uniform and conformal layer on all foam surfaces. In contrast to flexible PU foams that are known to be highly flammable without flame

ACS Paragon Plus Environment

1

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retardant additives, micro combustion calorimetry (MCC) and thermogravimetric analysis (TGA) confirm that the neat PDA is relatively inflammable with a strong tendency to form carbonaceous, porous char that is highly advantageous for flame retardancy. By depositing nanocoatings of PDA onto flexible PU foams, the flammability of the PU foam was significantly reduced with increasing coating thickness. For the thickest coating (3 days of PDA deposition), the foam quickly self-extinguished and its original shape was completely preserved after exposure to a flame in a torch burn test. In addition to the char forming ability of PDA, it is hypothesized that its catechol units likely scavenge nearby radicals that typically evolve additional fuel for the fire as they attack surrounding materials. This multiple flame retardancy action of PDA (i.e., char formation + radical scavenging) enables flame retardant foams with a peak heat release rate (P-HRR) that is significantly reduced (up to 67%) relative to control foams, representing much better performance than many conventional additives reported in the literature at comparable or higher loadings. INTRODUCTION

The National Fire Protection Association (NFPA) reported that in 2013 the United States had 1.24 million fires, resulting in 3,240 civilian deaths, 15,925 injuries, and $11.5 billion of direct property loss.1 The issues and concerns associated with such statistics highlight the need to develop flame retardant materials that additionally comply with strict safety regulations.2 Furthermore, environmental concerns continue to grow regarding the most common flame retardant chemistries like halogenated small molecule compounds.3-4 Toxic halogenated flame retardants can bioaccumulate even in isolated locations, raising environmental concerns. Hence, the need is undeniable to develop flame retardant materials that combine efficiency with environmentally friendly aspects.


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PU foam is one example of a highly flammable material that is used widely in consumer products (e.g. furniture, bedding, mattresses, and automotive seating).5-6 Recently, layer-by-layer (LbL) assembly has been used to reduce flammability of flexible PU foams.2, 7-8 LbL assembly efficiently constructs a fire-resistant barrier coating on the foam surface. All flame retardants are placed on the foam’s surface, precisely where they are needed without compromising the mechanical properties; in contrast, the simple mixing of small molecule flame-retardant additives into foams often results in adverse changes to the foam mechanical properties given that they are used internally within the foam wall support structure at loadings of 10 wt% or more. The present study focuses on polydopamine (PDA) coatings as nontoxic and environmentally friendly flame retardants. PDA, a catechol containing molecule, is an excellent mimic of adhesive proteins present in the feet of common mussels (Mytilus edulis) in significant quantities. In fact, this accounts for mussels’ ability to strongly tether themselves to a variety of surfaces. In turn, PDA has been extensively exploited as a universal surface modification agent for a wide range of applications in biomedical engineering,9-12 electrochemistry,13-14 membrane technology,15-16 nanotechnology,17-18 and more. Researchers have studied the interaction of other catechol containing molecules, such as synthetic melanin synthesized from L-dopa or related monomers, with specifically generated free radicals and reported that they are powerful scavengers of carbon-centered radicals.19-20 Recently, Ju et al. synthesized PDA nanoparticles and demonstrated that the catechols in nanoparticle form also had strong radical-scavenging activity.21 The radical scavenging activity of these catechol containing molecules has been exploited in a number of ways. For example, melanins have been reported to display a significant thermal and thermooxidative stabilization effect on commercial polymers22 and PDA containing copolymers with PMMA23 substantially delayed the peak decomposition of PMMA.


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The radical scavenging capability of PDA along with its universal adhesive nature strongly suggests that PDA could be useful for a flame retardant surface coating system, which is exceptionally suitable for highly flammable foamed materials with low densities, complicated geometries, and large surface areas. Moreover, given its biological origin (in living animals), PDA is intrinsically nontoxic and biocompatible.24 This biomimetic coating could serve as an effective and environmentally friendly flame retardant system for flexible PU foams. EXPERIMENTAL Materials. Dopamine hydrochloride was purchased from Sigma-Aldrich. Tris (tris(hydroxymethyl)aminomethane) base was obtained from Fisher Scientific. Ultrapure water (18.2 MΩ-cm) was obtained from a Thermo Scientific Barnstead E-pure water purification system. Polyether-based PU foam (type 1850, Future Foams, High Point, NC) without flameretardant additives (density of 28 kg/m3) was used as received. PDA coating. PU foam was immersed into dopamine solution (2 mg dopamine hydrochloride per mL of 10 mM Tris-HCl buffer solution, pH 8.5). During the initial stage of dopamine polymerization, the foam was compressed several times in the solution to draw the solution into the foams. The solution was then aggressively stirred, continuously, for 1-3 days. The solution color turned black by the alkaline pH-induced oxidative polymerization of dopamine. To facilitate the homogeneous circulation of dopamine and growing PDA in the solution, the foam was shaken in the solution every 2 h during the first 10 h until the solution color darkened and became consistent. PDA-coated foams were rinsed with ultrapure water and then immersed in deionized water under mild stirring for 3 days to remove unbound PDA while the deionized water was continuously changed during the purification. Subsequently, these


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foams were dried with a mild air stream for 2 days and then under vacuum for one day until a constant weight was reached. Finally, foam samples were dried at 70 °C under vacuum for 2 h before testing. The foam weight was measured before and after the coating to determine the % mass PDA of each sample. In addition, the weight of a control foam decreased upon immersion in the buffer solution because some unattached molecules (e.g., residual unreacted monomers or uncrosslinked polymers) leached out of the foam. The weight loss leveled out after 1 day of immersion in the buffer, reaching a steady state. Since the weight loss from a control foam upon immersion in buffer was about 4.8 wt%, this weight loss was taken into account when % mass PDA added was calculated. Select samples were subjected to compression force deflection testing according to ASTM D3574-11 (discussed in Supporting Information, Section S1) to evaluate the impact of the PDA coatings on the foam mechanical properties. Scanning Electron Microscopy (SEM). Images of the foam surfaces, cross-sections of the foams, and the char morphology were acquired by SEM (Zeiss Supra 40 VP). For SEM images of foam surface and char morphology, the samples were sputter coated with an Au-Pd target prior to imaging to prevent charging. In addition, to obtain SEM cross-sectional images of the PDA deposition on the foam, the sliced foam samples were cryo-fractured and then coated with osmium tetroxide for 20 h to prevent charging, followed by grounding the samples with strips of copper tape. SEM cross-sectional images were analyzed by using ImageJ software and the average thickness of PDA coating on PU surface was determined. 10 locations per sample were analyzed to measure the average thickness of the PDA coating. Thermogravimetric analysis (TGA). The thermal stability of the samples was investigated with a thermogravimetric analyzer (DSC/TGA 1, Mettler Toledo). Samples were heated from 30 °C to 800 °C at 10 °C/min in nitrogen gas.


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Torch burn test. Foam flammability was qualitatively evaluated by exposure to direct flame from a butane torch for 10 s.2, 8 A blue flame from a butane torch implies that the flame temperature was approximately 1300 °C. The burning behaviors of samples (e.g., flame spread, dripping, charring, and receding away from flame) were evaluated visually and videos were recorded. To ascertain the flammability of the material, observations from the torch burn test were analyzed in correlation with combustion tests. Combustion tests. To evaluate flame retardancy, samples were tested in both a micro combustion calorimeter and a cone calorimeter. Microscale combustibility was assessed using a Govmark MCC-2 according to ASTM D7309 method A. The sample size was 4 mg and samples were tested at a heating rate of 1 °C/s under nitrogen, from 80 to 900 °C. Cone calorimeter measurements were performed with the Govmark cone instrument at a radiant flux of 35 kW/m2 with an exhaust flow of 24 L/s based on ASTM E 1354 “Standard Test Method for Heat and Visible Smoke Release rates for Materials and Products using an Oxygen Consumption Calorimeter.” The sample dimensions used were 100 × 100 mm2 with a thickness of 25 mm. The samples were wrapped in aluminum foil without frame or grid, and for each sample three repetitions were performed. Results and Discussion PDA Coating Growth and Microstructure. Since Lee et al. developed the first mussel-inspired surface coating protocol,25 PDA coatings have been extensively exploited in a variety of fields, while recently much effort has been devoted to fundamental study of the film growth mechanism.26 Attachment of this bioinspired coating is understood to be universal, but its thickness, adhesion force, deposition


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kinetics, and surface morphology are highly varied depending on many parameters like coating time, pH and dopamine concentration of the solution, surface chemical composition of substrates, and kinds of oxidants used.26-31 Several studies on silicon substrates noted that granular PDA films grow continually without any slowing in growth kinetics by consecutive immersion into freshly prepared dopamine solutions29 or increased dopamine concentration higher than 2 mg/mL.26 However, the investigation into PDA growth kinetics has been focused on bare or chemically modified silicon surfaces with a planar geometry. Less effort has been devoted to investigating its growth kinetics on polymer surfaces as well as surfaces with complicated geometries. It is also worth noting that the exact molecular structure of PDA is still under debate in the literature and several reviews have highlighted this issue.32-34 Therefore, we first undertook a thorough study of the evolution of conformal PDA coatings on open-cell PU flexible foam. PDA-coated PU foams were prepared using an established procedure with simple modifications as described in the Experimental Section. This simple dip-coating procedure resulted in a conformal nanolayer on the complicated geometry of open cell foam. The foam color darkened and the % mass PDA of the coated foams showed linear growth as a function of PDA deposition time (Figure 1a, b). The % mass PDA was 5.3%, 10.5%, and 15.9% for PU flexible foams PDA-coated for 1 day (PDA1D), for 2 days (PDA2D) and for 3 days (PDA3D), respectively. Cross-sections of PDA-coated foams also showed uniform color without any gradation, demonstrating uniform PDA growth on the foam surface regardless of location (Figure 1a). A cross-sectional SEM image of a strut of PDA3D is shown in Figure 1c and the PDA coating on the PU surface is further magnified in Figure 1d. These images demonstrate uniform and conformal coverage of the PDA along with its globular surface topography after PDA coating for 3 days.


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PDA pH=8.5 10 mM Tris Buffer

3

Figure 1. (a) Pictures of 25.4×25.4×25.4 mm control and PDA-coated foam cubes (top row) and their respective cross-sections (bottom row). (b) % mass PDA of coated foams as a function of coating time with inset describing the polymerization of PDA. SEM images of (c) a crosssection of a strut of PDA3D coated foam and (d) a higher magnification image that shows the globular surface topography of the PDA3D. Figure 2 displays the evolution of surface topography of the PDA coating on PU foam with increasing coating time. In contrast to the smooth surface of the control foam (Figure 2a), a PDA film grown for 1 day on PU foam was composed of small islets on top of a thin film (Figure 2b) and displayed a uniform texture that confirmed the conformal nature of PDA deposition. Furthermore, in Figure S1 a representative cross section of PU foam coated for 1 day is provided, which shows a conformal and coherent PDA film without any significant pores or cracks. Interestingly, the PDA coating consists of a planar and smooth sublayer, along with a


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distinct heterogeneous secondary-layer of granules; the significant morphological discordance between a planar sublayer and the attached protruding large grains implies that the bulk solution and surface have different PDA growth mechanisms.31,

35

With additional PDA deposition,

surface grains became larger as well as greater in number while the texture of the sublayer was also growing (Figure 2c). The large grains (i.e., PDA nanoparticles) are grown in solution and attach themselves to the smooth PDA film on the foam surface, by virtue of the mussel-inspired strong wet adhesion of PDA; eventually, they grow together with the PDA coating on the surface of the foam. These growing surface structures eventually coalesce into a grainy, yet integrated, film with additional deposition of PDA (Figure 2d).

Figure 2. SEM surface images of (a) control, (b) PDA1D, (c) PDA2D and (d) PDA3D PU foams. Cross-sections of the PDA layers on PU were explored with deposition time (Figure 3). Findings strongly supported the aforementioned PDA coating growth mechanism. Consequently,


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the PDA thin film initially composed of a smooth surface and a few particles turned into a coarsely textured and consistently thick film. Dip-coating PU foam in dopamine solution for 24 h and 48 h resulted in ~55 and ~90 nm thick planar PDA sublayers, respectively, suggesting that the PDA sublayer gradually grew and, even after 24 h, had not yet attained a saturation thickness. The thicker PDA deposition and prolonged PDA growth kinetics on this open-cell PU foam compared to those on more extensively studied Si surfaces might be due to PU’s more hydrophobic surface.31, 36 However, further investigation of substrate geometry effects on PDA growth is needed. The secondary layer granular structure and the underlying planar sublayer gradually grew together and were eventually integrated into a consolidated globular thick layer (~240 nm) after 3 days of deposition, as shown in Figure 3c. The PDA deposition on PU appeared to be quite robust and was not delaminated, in contrast to that on Si.27 In addition, the PDA layers look continuous and compact in Figure 3; others have reported such layers to be dense enough to be impermeable to electroactive ions.29 Lastly, a few specimens were subjected to compression force deflection (CFD) testing to evaluate the effect of the coating on mechanical properties. The protocol used is discussed in the Supporting Information section S1 and the results are tabulated in Table S1. A 15% increase (maximum) in CFD was observed in PDA3D compared to the control foam, which could be attributed to thickness increase of each foam strut after PDA coating.


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Figure 3. SEM cross-sections of PDA coating, deposited on PU foam struts, for (a) PDA1D, (b) PDA2D, and (c) PDA3D PU foams. Thermal Analysis and Micro Combustion Calorimetry The thermal properties and flammability of pure PDA (not coated on foam) and neat PU control foam were first measured using TGA (Figure 4a) and MCC (Figure 4b), followed by exposure to a torch test and cone calorimetry. TGA was used to characterize thermal degradation as measured by volatilization of degradation products, while MCC was used to measure the heat released upon complete oxidation of all volatiles. One of the key advancements of MCC is the ability to use substantially less material than other approaches.37 However, for a complete evaluation of flame retardancy, MCC should be combined with a larger scale flammability evaluation protocol such as cone calorimetry.38 Upon being heated to 600 °C in TGA under N2, the PU flexible control foam quickly reduced in weight and left no residue. It has been classified as one of the most flammable polymeric materials, as represented by the large peaks in a MCC plot (Figure 4b). In the MCC test, a double peak behavior was observed37, 39 where the peak heat release rate (P-HRR) and total heat release (HR) of the control foam were 498.3 W/g and 22.7 kJ/g, respectively. In contrast, pure PDA had 63.6 wt% residue left after heating to 600 °C (by TGA), showed an undetectable P-HRR, and displayed an HR of only 2.4 kJ/g in a MCC test, demonstrating PDA is a char forming and nonflammable material. When the MCC test was applied to PDA coated foams such as PDA3D, the original shape and cellular structure of the small MCC sample (~ 4 mg) was largely retained after the MCC test, although the weight was slightly reduced. The shape retention could be the result of the formation of a consolidated carbonaceous char on the surface of each foam strut when exposed to high heat or fire that can serve to isolate the fuel/molten polymer and prevent the foam from collapsing into a liquid pool.


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In addition, given the radical scavenging effect of catecholic compounds of melanin and PDA,21, 23

a PDA coating could inhibit radicals, lowering the heat release values such as the average and

P-HRR during MCC testing.

(a) 100

(b) 600 500

80

HRR (W/g)

Remaining weight (%)

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60 40 20

400 300 200 100

0

0 0

200 400 Temperature (°°C)

600

100

200

300 400 500 Temperature (°°C)

600

Figure 4. (a) Thermogravimetric analysis in nitrogen and (b) micro-combustion calorimetry (MCC) of uncoated PU foam control (red solid line) and preformed PDA (blue dashed line). Flame Retardant Behavior PU foams (50 × 50 × 25 mm3) were subjected to a direct flame from a butane torch for 10 s and the foam flammability was qualitatively investigated. Upon exposure to direct flame, the neat control foam immediately ignited, formed a PU melt pool, and was completely consumed while the fire was transferred to underlying materials by melt dripping (see video in Supporting Information). For PDA coated foams, no melt dripping was exhibited regardless of thickness, but an improvement in overall performance was observed as the PDA deposition thickness/time was increased. PDA3D foams retained the original shape of the foam upon exposure to the torch and the flame self-extinguished quickly, leaving a part of the foam unburned, while PDA1D foams collapsed and were severely damaged (Figure 5a, b). To


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evaluate the extent of damage, a cross-section of the post-burn PDA3D (Figure 5c) was evaluated using SEM. The cellular structure was well-preserved throughout the foam (Figure 5e), although the surface that was directly scorched by the flame displayed a contracted foam char structure (Figure 5d). The surface char morphology is shown in Figure 5f, revealing a consolidated and grainy surface which closely resembles the pristine PDA coating, indicating the PDA coating structure was retained during exposure to the torch flame. Furthermore, flame spread was substantially delayed upon exposure of PDA3D to the torch flame, which is likely due to the radical scavenging capability of catechols (see video in Supporting Information). In contrast, after immediate ignition of the PDA1D foam specimen, the flame quickly spread over the entire surface with substantial heat generation, which caused significant thermal shrinkage (Figure 5b, video in Supporting Information). However, the conformal PDA thin film on the PU surface left a highly consolidated char residue on the foam surface struts after the fire, which resulted in preservation of the cellular structure (Figure S2). The surface char of PDA1D revealed leather-like features with a few grains, indicating the existence of PDA residue on the surface. Also, PDA2D foam specimens did not collapse, upon exposure to flame, and burned much less aggressively than PDA1D foams (see Supporting Information). Finally, to further confirm the effect of the coating, specimens were also weighed after the torch test. As the coating weight increased, the mass lost due to the torch test was reduced (up to 40 wt%) after the torch burn test (Figure S3).


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Figure 5. Images of foams after exposure to a torch flame. (a) PDA3D, (b) PDA1D, and (c) a cross-section of PDA3D foam. SEM images of the cross section of the PDA3D specimen at the surface (d) and in the center of the specimen (e), showing a retention of the cellular structure. (f) SEM image of char surface of post-burn PDA3D foam. For a more detailed investigation of flame retardancy, samples were tested in a cone calorimeter according to a standard protocol (ASTM E-1354/ISO 5660). This test is a method for assessing the flammability of materials; it is widely employed to examine the performance of fire retardant materials, but requires significantly larger sample sizes than TGA or MCC. The cone calorimeter test provides the heat release rate (HRR) of a burning sample by measuring the oxygen consumption. P-HRR is one of the most important parameters to evaluate fire safety and


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represents the point in a fire where the heat generation is most likely to propagate the flame or ignite adjacent material. Therefore, significant reduction in P-HRR by the PDA coating indicates inhibition of the combustion process and factors associated with rapid flame spread.40 The HRR curves of the coated foams and uncoated foam control are shown in Figure 6. Table 1 displays the summary of the cone calorimeter tests. All foams ignited rapidly, within a few seconds after being exposed to the radiant heat flux of 35 kW/m2. The untreated PU foam control exhibited the least fire retardant capacity. The HRR trace for that foam was short lasting and intense with a large P-HRR. The trace consisted of two overlapping processes similar to prior research.37, 41 We attribute the first process to the initial pyrolysis and combustion of the gaseous products released by the solid foam followed by its thermal shrinkage, rapid collapse of the cellular structure and ultimately conversion into a pool of liquid polymer and thermal decomposition products. The second, more intense heat release process, was ascribed to a rapid volatilization of that liquid pool leading to a more intense combustion. After the liquid fuel was consumed, the flame and the heat release ceased. No solid char was found at the end of the cone calorimeter test for control PU foam specimens, which is in agreement with the outlined combustion mechanism that involves solid-liquid transformation followed by rapid volatilization of the combustible products from the liquid phase (Figure S4).

Similar to the torch test

results, thicker PDA coatings led to improved fire retarding performance. The HRR trace of PDA1D foam specimens looked quite similar to the control, although the PDA coating caused a marginal reduction in P-HRR. However, PDA2D and PDA3D specimens showed a significant reduction in key flammability parameters such as P-HRR and average heat release rate (A-HRR), implying the PDA coating substantially altered the inherent burning behavior of the PU foam. Eventually, the PDA coating totally eliminated the second peak and induced a prolonged burn


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time due to the delayed pyrolysis of the PU decomposition products. As a result, PDA3D revealed a sharp reduction in P-HRR (67%) and A-HRR (69%) along with a dramatic decrease in average mass loss rate (72%) compared to control foam, as listed in Table 1. The PDA2D coating also substantially reduced the flammability of the foam, represented by a reduction in PHRR (50%) and A-HRR (45%) compared to the control foam. Meanwhile, the HRR curve of PDA2D showed a significantly delayed second pyrolysis peak relative to the control foam. Like in the case of torch burn test, PDA3D retained its original cellular structure upon exposure to flame in the cone calorimetry test and PDA2D exhibited considerable thermal shrinkage (Figure S4). The PDA nanocoating acted to interrupt the self-sustained combustion cycle and to inhibit and suppress the combustion process. This unique protective nanobarrier is envisioned to perform at the physical and chemical level, in both the condensed phase and gas phase. First, the combustion process was physically retarded by forming a consolidated protective carbon residue on the PU surface, which prevented the foam from collapsing during fire and consequently led to the absence of a second peak in the HRR curve of PDA3D. Second, catechol functional groups of the PDA coating scavenged free radicals during the fire and the fuel supply was considerably suppressed. Lastly, the PDA coating also drastically reduced carbon monoxide and carbon dioxide emission (Table 1).


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800 700

HRR (kW/m2)

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600 500 400 300 200 100 0 -50

0

50

100

150

200

250

Time (s)

Figure 6. Heat release rate (HRR) of control (black solid line), PDA1D (green short-dashed line), PDA2D (blue long-dashed line), and PDA3D (red dotted line), as a function of time during cone calorimeter testing. The literature has reported cone calorimetry analysis on flexible PU foams modified with a variety of commercial flame retardant additives.42 The results are summarized in Table 2. When comparing PDA coated foams with conventional flame retardant systems for % reduction of P-HRR, PDA2D and PDA3D showed much more dramatic reductions in the P-HRR than the best-performing conventional flame retardant systems despite the low loading of PDA.

Table 1. Cone calorimeter results for control foam, PDA1D, PDA2D, and PDA3D coated foams. The samples were tested in triplicate and the values of measured parameters were averaged.


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Sample

P-HRR (kW/m2)

A-HRR (kW/m2)

Average Mass Loss Rate (g/m2s)

CO Yield (kg/kg)

CO2 Yield (kg/kg)

Total HR (MJ/m2)

Mass Loss (wt%)

Control

734

732

17.5

0.053

2.792

23

99.4

PDA1D

709

697

17.1

0.069

1.766

22

99.2

PDA2D

368

403

6.7

0.045

0.041

21

95.9

PDA3D

239

230

4.9

0.013

0.028

21

79.6

Table 2. Cone calorimeter results reported in literature for flexible PU foams with commercial flame-retardants.42

Additive Type

Flame Retardants in Flexible PU Foam (wt%)

Commercial Product / Supplier

P-HRR Reduction (%)

Zn stearate (10)

Synpro® Zinc stearate / Ferro

17

DE-60F / Great Lakes Chemical

37

Fyrol® FR2 / Akzo Nobel Chemicals

21

Non-Halogen Additive Pentabromodiphenyl oxide Halogen Additive blend (20) Halogen-Phosphorus Additive

Tris(1.3-dichloro isopropyl) phosphate (20) Firemaster® HP-36 / Great Lakes

Halogen-Phosphorus Halogenated phosphate + Non-Halogen

Chemical + Antimony Oxide / Laurel

43

ester (28) + Sb2O3 (7) Additives

Industries

CONCLUSION


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An environmentally-friendly, bioinspired flame-retardant nanocoating was deposited, via a simple dip-coating process, on the surface of flexible PU foam. As a function of coating time, % mass PDA linearly increased up to 15.9 %, corresponding to a 240 nm thick layer, derived from simultaneous evolution of a planar sublayer and a secondary grainy structure. Though coating time was lengthy in this study, future work could optimize the deposition time by adjusting the pH, oxidant, or dopamine concentration. The HRR of neat PDA was negligible in MCC tests and large char yields of 63.6 wt% were observed in TGA under N2, demonstrating advantageous thermal and fire-related characteristics imparted by PDA. In contrast, the control foam exhibited a high P-HRR of 498.3 W/g in MCC and was totally consumed during a TGA run in N2. In the torch burn test, the PDA coating largely remained on the surface with a high level of integrity, prevented melt-dripping of PU, and retarded the pyrolysis process by preventing the foam from collapsing into a liquid pool. Cone calorimetry showed that the PDA coating led to a dramatic reduction (by up to 67%) in the P-HRR of flexible PU foam when compared to uncoated foam, better than many commercial foams with incorporated flame retardants. Overall, the synergistic combination of physical and chemical actions resulted in a reduction in the flammability of PDA-coated foams, which is directly aligned with % mass PDA coating applied.

ASSOCIATED CONTENT Supporting Information Description of compression force deflection testing, SEM cross-sectional image of PDA1D coating, photograph and SEM images of post-burn PDA1D, remaining weight of foams after


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torch burn test, photographs of residue of foams after cone calorimetry, and supporting videos. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel: 512-471-6300, Fax: 512-471-7060, Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS CJE thanks the Welch Foundation (grant# F-1709), DuPont Young Professor Award and the Norman Hackerman Advanced Research Program (grant# ARP- 003658-0037-2011) for partial financial support. We also gratefully acknowledge partial financial support from the National Science Foundation through the CLiPs NSF-STC, DMR-0423914. JHC acknowledges partial financial support from LG Chem Graduate Research Fellowship and Graduate Dean’s Prestigious Fellowship Supplement. REFERENCES 1. Karter, M.J., Fire Loss in the United States during 2013. National Fire Protection Association: Quincy, Massachusetts, 2014. 2. Laufer, G.; Kirkland, C.; Morgan, B. A.; Grunlan, J.C. Exceptionally Flame Retardant Sulfur-Based Multilayer Nanocoating for Polyurethane Prepared from Aqueous Polyelectrolyte Solutions. ACS Macro Lett. 2013, 2, 361-365. 3. Leu, T. S.; Wang, C. S. Synergistic Effect of a Phosphorus-Nitrogen Flame Retardant on Engineering Plastics. J. Appl. Polym. Sci. 2004, 92, 410-417. 4. Watanabe, I.; Sakai, S. Environmental Release and Behavior of Brominated Flame Retardants. Environ. Int. 2003, 29, 665-682. 5. Flambard, X.; Bourbigot, S.; Kozlowski, R.; Muzyczek, M.; Mieleniak, B.; Ferreira, M.; Vermeulen, B.; Poutch, F. Progress in Safety, Flame Retardant Textiles and Flexible Fire Barriers for Seats in Transportation. Polym. Degrad. Stabil. 2005, 88, 98-105. 6. Kozlowski, R.; Mieleniak, B.; Muzyczek, M. Fire Resistant Composites for Upholstery. Polym. Degrad. Stabil. 1999, 64, 511-515.


20


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

Page 22 of 24

7. Pan, H.; Pan, Y.; Wang, W.; Song, L.; Hu, Y.; Liew, K. M. Synergistic Effect of Layerby-Layer Assembled Thin Films Based on Clay and Carbon Nanotubes to Reduce the Flammability of Flexible Polyurethane Foam. Ind. Eng. Chem. Res. 2014, 53, 14315-14321. 8. Laufer, G.; Kirkland, C.; Cain, A. A.; Grunlan, J. C. Clay-Chitosan Nanobrick Walls: Completely Renewable Gas Barrier and Flame-Retardant Nanocoatings. ACS Appl. Matr. Interfaces 2012, 4, 1643-1649. 9. Shultz, M. D.; Reveles, J. U.; Khanna, S. N.; Carpenter, E. E. Reactive Nature of Dopamine as a Surface Functionalization Agent in Iron Oxide Nanoparticles. J. Am. Chem. Soc. 2007, 129, 2482-2487. 10. Lee, H.; Rho, J.; Messersmith, P. B. Facile Conjugation of Biomolecules onto Surfaces via Mussel Adhesive Protein Inspired Coatings. Adv. Mater. 2009, 21, 431-434. 11. Chien, H. W.; Kuo, W. H.; Wang, M. J.; Tsai, S. W.; Tsai, W. B. Tunable Micropatterned Substrates Based on Poly(dopamine) Deposition via Microcontact Printing. Langmuir 2012, 28, 5775-5782. 12. Ku, S. H.; Ryu, J.; Hong, S. K.; Lee, H.; Park, C. B. General Functionalization Route for Cell Adhesion on Non-Wetting Surfaces. Biomaterials 2010, 31, 2535-2541. 13. Luczak, T. Preparation and Characterization of the Dopamine Film Electrochemically Deposited on a Gold Template and its Applications for Dopamine Sensing in Aqueous Solution. Electrochim. Acta 2008, 53, 5725-5731. 14. Zhang, Y.; Wang, H.; Nie, J.; Zhou, H.; Shen, G.; Yu, R. Mussel-Inspired Fabrication of Encoded Polymer Films for Electrochemical Identification. Electrochem. Commun. 2009, 11, 1936-1939. 15. McCloskey, B. D.; Park, H. B.; Ju, H.; Rowe, B. W.; Miller, D. J.; Freeman, B. D. A Bioinspired Fouling-Resistant Surface Modification for Water Purification Membranes. J. Membr. Sci. 2012, 413, 82-90. 16. McCloskey, B. D.; Park, H. B.; Ju, H.; Rowe, B. W.; Miller, D. J.; Chun, B. J.; Kin, K.; Freeman, B. D. Influence of Polydopamine Deposition Conditions on Pure Water Flux and Foulant Adhesion Resistance of Reverse Osmosis, Ultrafiltration, and Microfiltration Membranes. Polymer 2010, 51, 3472-3485. 17. Kim, B. H.; Lee, D. H.; Kim, J. Y.; Shin, D. O.; Jeong, H. Y.; Hong, S.; Yun, J. M.; Koo, C. M.; Lee, H.; Kim, S. O. Mussel-Inspired Block Copolymer Lithography for Low Surface Energy Materials of Teflon, Graphene, and Gold. Adv. Mater. 2011, 23, 5618-5622. 18. Fei, B.; Qian, B. T.; Yang, Z. Y.; Wang, R. H.; Liu, W. C.; Mak, C. L.; Xin, J. H. Coating Carbon Nanotubes by Spontaneous Oxidative Polymerization of Dopamine. Carbon 2008, 46, 1795-1797. 19. Dunford, R.; Land, E. J.; Rozanowska, M.; Sarna, T.; Truscott, T. G. Interaction of Melanin with Carbon-Centered and Oxygen-Centered Radicals from Methanol and Ethanol. Free Radical Biol. Med. 1995, 19, 735-740. 20. Rozanowska, M.; Sarna, T.; Land, E. J.; Truscott, T. G. Free Radical Scavenging Properties of Melanin Interaction of Eu- and Pheo-Melanin Models with Reducing and Oxidising Radicals. Free Radical Biol. Med. 1999, 26, 518-525. 21. Ju, K. Y.; Lee, Y.; Lee, S.; Park, S. B.; Lee, J. K. Bioinspired Polymerization of Dopamine to Generate Melanin-Like Nanoparticles Having an Excellent Free-RadicalScavenging Property. Biomacromolecules 2011, 12, 625-632.


21

Page 23 of 24

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


22. Shanmuganathan, K.; Cho, J. H.; Iyer, P.; Baranowitz, S.; Ellison, C. J. Thermooxidative Stabilization of Polymers Using Natural and Synthetic Melanins. Macromolecules 2011, 44, 9499-9507. 23. Cho, J. H.; Shanmuganathan, K.; Ellison, C. J. Bioinspired Catecholic Copolymers for Antifouling Surface Coatings. ACS Appl. Mater. Interfaces 2013, 5, 3794-3802. 24. Postma, A.; Yan, Y.; Wang, Y. J.; Zelikin, A. N.; Tjipto, E.; Caruso, F. SelfPolymerization of Dopamine as a Versatile and Robust Technique to Prepare Polymer Capsules. Chem. Mater. 2009, 21, 3042-3044. 25. Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-Inspired Surface Chemistry for Multifunctional Coatings. Science 2007, 318, 426-430. 26. Ball, V.; Del Frari, D.; Toniazzo, V.; Ruch, D. Kinetics of Polydopamine Film Deposition as a Function of pH and Dopamine Concentration: Insights in the Polydopamine Deposition Mechanism. J. Colloid Interface Sci. 2012, 386, 366-372. 27. Bernsmann, F.; Ball, V.; Addiego, F.; Ponche, A.; Michel, M.; Gracio, J. J. D.; Toniazzo, V.; Ruch, D. Dopamine-Melanin Film Deposition Depends on the Used Oxidant and Buffer Solution. Langmuir 2011, 27, 2819-2825. 28. Bernsmann, F.; Frisch, B.; Ringwald, C.; Ball, V. Protein Adsorption on DopamineMelanin Films: Role of Electrostatic Interactions Inferred from Zeta-Potential Measurements versus Chemisorption. J Colloid Interface Sci. 2010, 344, 54-60. 29. Bernsmann, F.; Ponche, A.; Ringwald, C.; Hemmerle, J.; Raya, J.; Bechinger, B.; Voegel, J. C.; Schaaf, P.; Ball, V. Characterization of Dopamine-Melanin Growth on Silicon Oxide. J. Phys. Chem. C 2009, 113, 8234-8242. 30. Bernsrnann, F.; Ersen, O.; Voegel, J. C.; Jan, E.; Kotov, N. A.; Ball, V. MelaninContaining Films: Growth from Dopamine Solutions versus Layer-by-Layer Deposition. ChemPhysChem 2010, 11, 3299-3305. 31. Klosterman, L.; Riley, J. K.; Bettinger, C. J. Control of Heterogeneous Nucleation and Growth Kinetics of Dopamine-Melanin by Altering Substrate Chemistry. Langmuir 2015, 31, 3451–3458. 32. Dreyer, D. R.; Miller, D. J.; Freeman, B. D.; Paul, D. R.; Bielawski, C. W. Elucidating the Structure of Poly(dopamine). Langmuir 2012, 28, 6428-6435. 33. Liebscher, J.; Mrowczynski, R.; Scheidt, H. A.; Filip, C.; Hadade, N. D.; Turcu, R.; Bende, A.; Beck, S. Structure of Polydopamine: A Never-Ending Story? Langmuir 2013, 29, 10539-10548. 34. Liu, Y.; Ai, K.; Lu, L. Polydopamine and its Derivative materials: Synthesis and Promising Applications in Energy, Environmental, and Biomedical Fields. Chem. Rev. 2014, 114, 5057-115. 35. Wei, Q.; Zhang, F. L.; Li, J.; Li, B. J.; Zhao, C. S. Oxidant-Induced Dopamine Polymerization for Multifunctional Coatings. Polym. Chem. 2010, 1, 1430-1433. 36. Jiang, J. H.; Zhu, L. P.; Zhu, L. J.; Zhu, B. K.; Xu, Y. Y. Surface Characteristics of a Self-Polymerized Dopamine Coating Deposited on Hydrophobic Polymer Films. Langmuir 2011, 27, 14180-14187. 37. Krämer, R. H.; Zammarano, M.; Linteris, G. T.; Gedde, U. W.; Gilman, J. W. Heat Release and Structural Collapse of Flexible Polyurethane Foam. Polym. Degrad. Stabil. 2010, 95, 1115-1122. 38. Schartel, B.; Pawlowski, K. H.; Lyon, R. E. Pyrolysis Combustion Flow Calorimeter: A Tool to Assess Flame Retarded PC/ABS Materials? Thermochim. Acta 2007, 462, 1-14.


22


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

Page 24 of 24

39. Chen, X.; Huo, L.; Jiao, C.; Li, S. TG–FTIR Characterization of Volatile Compounds from Flame Retardant Polyurethane Foams Materials. J. Anal. Appl. Pyrolysis 2013, 100, 186191. 40. Schartel, B.; Hull, T. R. Development of Fire-Retarded Materials - Interpretation of Cone Calorimeter Data. Fire Mater. 2007, 31, 327-354. 41. Lefebvre, J.; Bastin, B.; Le Bras, M.; Duquesne, S.; Ritter, C.; Paleja, R.; Poutch, F. Flame Spread of Flexible Polyurethane Foam: Comprehensive Study. Polym. Test. 2004, 23, 281-290. 42. Fire and Polymers: Materials and Solutions for Hazard Prevention. Gordon, L. N.; Wilkie, C. A. (Eds.); ACS Symp. Ser. 797; American Chemical Society: Washington, D.C., 2001.


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Bioinspired Catecholic Flame Retardant Nanocoating for Flexible

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