intumescent multilayer nanocoating, made with renewable polyelectrolytes, for flame-retardant cotton

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Intumescent Multilayer Nanocoating, Made with Renewable Polyelectrolytes, for Flame-Retardant Cotton Galina Laufer, Christopher Kirkland, Alexander B. Morgan, and Jaime C. Grunlan* ,Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States Multiscale Composites and Polymers Division, University of Dayton Research Institute, Dayton, Ohio 45469, United States * S Supporting Information ABSTRACT: Thin lms of fully renewable and environ- mentally benign electrolytes, cationic chitosan (CH) and anionic phytic acid (PA), were deposited on cotton fabric via layer-by-layer (LbL) assembly in an eort to reduce ammability. Altering the pH of aqueous deposition solutions modies the composition of the nal nanocoating. CHPA lms created at pH 6 were thicker and had 48 wt % PA in the coating, while the thinnest lms (with a PA content of 66 wt %) were created at pH 4. Each coating was evaluated at both 30 bilayers (BL) and at the same coating weight added to the fabric. In a vertical ame test, fabrics coated with high PA content multilayers completely extinguished the ame, while uncoated cotton was completely consumed. Microcombustion calorimetry conrmed that all coated fabric reduces peak heat release rate (pkHRR) by at least 50% relative to the uncoated control. Fabric coated with pH 4 solutions shows the greatest reduction in pkHRR and total heat release of 60% and 76%, respectively. This superior performance is believed to be due to high phosphorus content that enhances the intumescent behavior of these nanocoatings. These results demonstrate the rst completely renewable intumescent LbL assembly, which conformally coats every ber in cotton fabric and provides an eective alternative to current ame retardant treatments. INTRODUCTION Most household textiles are highly ammable, and this has led to new standards and legislation directed at reducing the risk of res, particularly with respect to upholstery and mattresses. 1 Numerous studies have focused on imparting ame-retardant properties to common textiles such as cotton. 25 One of the more recent methods to render a given substrate (e.g., cotton fabric or polyurethane foam) ame retardant is through applying a multilayered nanocoating using layer-by-layer (LbL) assembly. 69 LbL assembly involves alternate adsorption of oppositely charged electrolytes to form thin multilayered lms, as shown schematically in Figure 1. 10,11 Each positivenegative pair deposited is referred to as a bilayer (BL). This technique allows for tailoring of a coatings structure and composition at the nanoscale. 1214 In addition to ame- retardant behavior, 7,15,16 varying composition has led to the development of LbL lms with electrochromic, 1719 oxygen barrier, 2022 water repellant, 23,24 controlled drug release, 25,26 and antimicrobial properties. 2729 The combination of nano thickness and deposition from dilute aqueous solutions provides the ability to use LbL deposition to conformally coat complex substrates such as polyester or cotton fabric 6,7,30 and open-celled foam. 8,9 Intumescent coatings are a subset of ame retardant technology commonly used to protect building materials. 31 The coating does not modify the intrinsic properties (e.g., strength) of the substrate and can be easily applied to a variety of surfaces. 3234 An intumescent system typically requires three components bound together with a binder: a source of carbon, an acid source, and a blowing agent. 35 These components react upon heating to generate a swollen multicellular insulating layer that protects the underlying material from heat and ame. 36 Intumescent nanocoatings were recently applied to cotton fabric using LbL assembly. 7 Layers of polyallylamine (PAAm) and poly(sodium phosphate) (PSP) were conformally deposited on individual bers, which eliminated the need for a binder. In this case, cotton as well as the PAAm served as the carbon source, creating a complete intumescent system. Twenty bilayers of PAAmPSP prevented the ignition of Received: June 6, 2012 Revised: August 15, 2012 Published: August 17, 2012 Figure 1. Schematic of LbL assembly using CH and PA. Steps 14 are repeated until the desired number of bilayers are deposited. Article pubs.acs.org/Biomac © 2012 American Chemical Society 2843 dx.doi.org/10.1021/bm300873b | Biomacromolecules 2012, 13, 28432848

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Page 1: Intumescent Multilayer Nanocoating, Made with Renewable Polyelectrolytes, for Flame-Retardant Cotton

Intumescent Multilayer Nanocoating, Made with RenewablePolyelectrolytes, for Flame-Retardant CottonGalina Laufer,† Christopher Kirkland,† Alexander B. Morgan,‡ and Jaime C. Grunlan*,†

†Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States‡Multiscale Composites and Polymers Division, University of Dayton Research Institute, Dayton, Ohio 45469, United States

*S Supporting Information

ABSTRACT: Thin films of fully renewable and environ-mentally benign electrolytes, cationic chitosan (CH) andanionic phytic acid (PA), were deposited on cotton fabric vialayer-by-layer (LbL) assembly in an effort to reduceflammability. Altering the pH of aqueous deposition solutionsmodifies the composition of the final nanocoating. CH−PAfilms created at pH 6 were thicker and had 48 wt % PA in thecoating, while the thinnest films (with a PA content of 66 wt %) were created at pH 4. Each coating was evaluated at both 30bilayers (BL) and at the same coating weight added to the fabric. In a vertical flame test, fabrics coated with high PA contentmultilayers completely extinguished the flame, while uncoated cotton was completely consumed. Microcombustion calorimetryconfirmed that all coated fabric reduces peak heat release rate (pkHRR) by at least 50% relative to the uncoated control. Fabriccoated with pH 4 solutions shows the greatest reduction in pkHRR and total heat release of 60% and 76%, respectively. Thissuperior performance is believed to be due to high phosphorus content that enhances the intumescent behavior of thesenanocoatings. These results demonstrate the first completely renewable intumescent LbL assembly, which conformally coatsevery fiber in cotton fabric and provides an effective alternative to current flame retardant treatments.

■ INTRODUCTIONMost household textiles are highly flammable, and this has ledto new standards and legislation directed at reducing the risk offires, particularly with respect to upholstery and mattresses.1

Numerous studies have focused on imparting flame-retardantproperties to common textiles such as cotton.2−5 One of themore recent methods to render a given substrate (e.g., cottonfabric or polyurethane foam) flame retardant is throughapplying a multilayered nanocoating using layer-by-layer(LbL) assembly.6−9 LbL assembly involves alternate adsorptionof oppositely charged electrolytes to form thin multilayeredfilms, as shown schematically in Figure 1.10,11 Each positive−negative pair deposited is referred to as a bilayer (BL). Thistechnique allows for tailoring of a coating’s structure and

composition at the nanoscale.12−14 In addition to flame-retardant behavior,7,15,16 varying composition has led to thedevelopment of LbL films with electrochromic,17−19 oxygenbarrier,20−22 water repellant,23,24 controlled drug release,25,26

and antimicrobial properties.27−29 The combination of nanothickness and deposition from dilute aqueous solutionsprovides the ability to use LbL deposition to conformallycoat complex substrates such as polyester or cotton fabric6,7,30

and open-celled foam.8,9

Intumescent coatings are a subset of flame retardanttechnology commonly used to protect building materials.31

The coating does not modify the intrinsic properties (e.g.,strength) of the substrate and can be easily applied to a varietyof surfaces.32−34 An intumescent system typically requires threecomponents bound together with a binder: a source of carbon,an acid source, and a blowing agent.35 These components reactupon heating to generate a swollen multicellular insulating layerthat protects the underlying material from heat and flame.36

Intumescent nanocoatings were recently applied to cottonfabric using LbL assembly.7 Layers of polyallylamine (PAAm)and poly(sodium phosphate) (PSP) were conformallydeposited on individual fibers, which eliminated the need fora binder. In this case, cotton as well as the PAAm served as thecarbon source, creating a complete intumescent system.Twenty bilayers of PAAm−PSP prevented the ignition of

Received: June 6, 2012Revised: August 15, 2012Published: August 17, 2012

Figure 1. Schematic of LbL assembly using CH and PA. Steps 1−4 arerepeated until the desired number of bilayers are deposited.

Article

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© 2012 American Chemical Society 2843 dx.doi.org/10.1021/bm300873b | Biomacromolecules 2012, 13, 2843−2848

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cotton fabric, demonstrating the first LbL-based intumescentnanocoating. This effective antiflammable nanocoating, pro-duced with manmade (i.e., synthetic) molecules, can also beprepared with phosphorus and nitrogen-rich renewableingredients.Interest in green chemistry and concern over toxicity and

environmental issues associated with various flame-retardanttreatments on textiles that come in close contact with skin, havecreated a desire to use “green” materials. Phytic acid (PA),which is the major storage form of phosphorus in cereal grains,beans, and oil seeds, is one such molecule.37 Its structureconsists of six phosphate groups (Figure 1), which is similar toPSP mentioned above. PA has already been reported as anantioxidant,38,39 anticancer agent,40,41 and a means of loweringblood glucose level for diabetics.42,43 Furthermore, it isenvironmentally friendly, biocompatible, and nontoxic. PAand its salts are able to interact with positively chargedmolecules (pKa 1.9−9.5),44 which makes it a good candidate forLbL assembly.45 From a flame-retardant perspective, moleculeswith a higher phosphorus content can deliver more active flameretardant atoms per molecule,46 and PA has 28 wt % P basedupon its molecular weight. Chitosan (CH) is a nitrogencontaining molecule that can be paired with PA to provide acompletely renewable intumescent system. CH is an aminopolysaccharide obtained from the shells of crustaceans (e.g.,lobsters and shrimp) (Figure 1),47 that becomes positivelycharged at low pH (pKa 6−6.5).48,49 Much like PA, CH isbiodegradable, biocompatible, and environmentally benign.These properties have led to significant study of CH’s use inbiomedical applications,50−52 agriculture,53 and the foodindustry.54 CH is expected to be an effective intumescentadditive, because it will act as a char-forming agent, it is carbonrich, and can be a blowing agent, releasing nitrogen gas as itdegrades.55−57 Additionally, there is the potential for favorablephosphorus−nitrogen (P−N) condensed phase char forma-tion/flame retardant reactions between CH and PA.58,59

In the present study, thin films of CH and PA were depositedonto cotton fabric in an effort to create the first biobased (i.e.,renewable) intumescent nanocoating. Film thickness, as well asphosphorus-to-nitrogen ratio, was tailored by changing the pHof the polyelectrolyte solutions. Three different CH−PAformulations were applied to cotton fabric, and their flame-retardant properties were studied using vertical flame testingand microcombustion calorimetry (MCC). The thinnestcoating (30 BL is ∼10 nm thick) was deposited using pH 4solutions, and this multilayer film has the highest PA content(66 wt %) relative to films deposited from pH 5 and 6solutions. With regard to antiflammability, this pH 4 nano-coating completely stopped flame propagation on cotton fabric,leaving more than 90% residue following the vertical burn test.Calorimetry revealed that all fabrics coated with 30 CH−PAbilayers, exhibited peak heat release rates (pkHRRs) that werereduced by at least 50% compared to an uncoated control.When CH−PA nanocoatings are normalized by weightdeposited on cotton fabric, it appears that greater phosphoruscontent produces the most effective flame retardant. This workdemonstrates the first fully biorenewable intumescent nano-coating for cotton fabric, whose deposition from aqueoussolutions makes this a promising alternative to currentantiflammable treatments for fabric.

■ EXPERIMENTAL SECTIONMaterials. Cationic deposition solutions were prepared by

adjusting the pH of deionized (DI) water (18.2 MΩ, pH ∼ 5.5) to2 with 1 M hydrochloric acid (HCl) and then adding 0.5 wt % CH(MW 50−190 kDa, 75−85% deacetylated) purchased from Aldrich(Milwaukee, WI). This solution was magnetically stirred for 24 h untilthe CH was completely dissolved. Anionic solutions were prepared byadding 2.0 wt % of PA sodium salt hydrate (Aldrich, Milwakee, WI) toDI water and stirred for 24 h. The pH of these solutions were adjustedto 4, 5, and 6 with 1 M NaOH or 1 M HCl just prior to deposition.Branched polyethylenimine (1.0 wt % in water) (MW 25 000 g/mol,Aldrich) was used as a primer layer to improve adhesion to cotton.Single-side-polished (1 0 0) silicon wafers (University Wafer, SouthBoston, MA) were used as a substrate for film thickness character-ization. Cotton fabric, having a balanced weave (with approximately 80threads per inch in both the warp and fill direction) and a weight of119 g/m2, was supplied by the USDA Southern Regional ResearchCenter (New Orleans, LA).

LbL Deposition. Prior to deposition, silicon wafers were rinsedwith acetone and DI water, and then dried with filtered air. All filmswere deposited on a given substrate using the procedure shownschematically in Figure 1. Substrates were alternately dipped intopositive and negative solutions. Initial dips were 5 min each andsubsequent dips were 1 min. Each dip was followed by rinsing with DIwater and, in the case of silicon wafer, drying with air. Fabrics werewringed out to expel liquid as an alternative to the traditional dryingstep. After the desired number of bilayers was deposited, fabrics weredried at 70 °C in an oven for 2 h before testing.

Characterization of Film Growth, Structure and Flamma-bility Properties. Film thickness was measured with an alpha-SEEllipsometer (J. A. Woollam Co., Inc., Lincoln, NE). Weight perdeposited layer was measured with a Maxtek Research Quartz CrystalMicrobalance (RQCM) (Infinicon, East Syracuse, NY), with afrequency range of 3.8−6 MHz, in conjunction with 5 MHz quartzcrystals. Surface morphology of thin films deposited on silicon waferswas imaged with a Nanosurf EasyScan 2 Atomic Force Microscope(AFM) (Nanoscience Instruments, Inc., Phoenix, AZ). Surface imagesof virgin and coated cotton fabric were acquired with a JEOL JSM-7500F FESEM (JEOL Ltd., Tokyo, Japan). A platinum coating of 6nm was deposited on all samples prior to scanning electronmicroscope (SEM) imaging to prevent charging. Vertical flame testswere conducted on coated and uncoated fabrics, according to ASTMD6413-08, using an Automatic Vertical Flammability Cabinet, modelVC-2 (Govmark, Farmingdale, NY). Microcombustion calorimeter,model MCC-1 (Govmark), testing was performed with a 1 °C/secheating rate under nitrogen, from 200 to 700 °C, using method A ofASTM D7309-07. There was no additional conditioning prior to MCCtesting.

■ RESULTS AND DISCUSSION

Film Growth and Microstructure. Growth of three CH-PA films, assembled from solutions at pH 4, 5, or 6, wasmonitored using ellipsometry. Figure 2a shows that all recipesgrow linearly, but thickness increases significantly with pH.There are likely two factors causing films to be extremely thinat pH 4. First, CH is highly charged at this low pH (pKa 6−6.5)due to the presence of primary amine groups in its structure(Figure 1) and exists in an extended form that deposits very flat(or thin). Thin deposition associated with high charge densitypolymers has also been observed with polyethylenimine andpoly(acrylic acid).21 Second, there is considerable stiffness inpolysaccharides that can hinder the adsorption process andresult in island growth.60 Aggregates observed on the surfacesof 10 BL films are believed to be remnants of these islands. AtpH 4 (Figure 2b), these raised features are fewer and furtherbetween than at pH 6 (Figure 2c). Thinner deposition andfewer islands produce the anemic layer thickness growth

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observed at pH 4. It is also possible that rinsing with unalteredDI water (pH ∼5.5) reduces the degree of CH ionization andleads to some desorption.As the pH of the CH solution increases, CH molecules take

on a more compact, coiled conformation and deposit in a moreglobular conformation. This more weakly charged state is alsoaccompanied by reduced self-repulsion that allows moremolecules to deposit in a given layer. As a result, the thicknessof thin film assemblies increases with increasing CH pH. Amore uniform surface can also be observed in the AFM surfacescan at pH 6 (Figure 2c) relative to pH 4 (Figure 2b). Thesechanges in deposition are also reflected in film composition, asshown in Figure 3. CH content decreases with pH due to its

thinner deposition. This situation is exacerbated by the fact thathigher charge density CH (at pH 4) requires fewer chains toovercompensate the charge of the previously adsorbed PAlayer. As the concentration of PA decreases, from 66 wt % atpH 4 to less than 50 wt % at pH 6, the flammabilityperformance is expected to change due to reduced phosphoruscontent.61

Flame Retardant Behavior of Cotton Fabric. Cottonfabric was coated with 30 BL of CH−PA at pH 4, 5, and 6 andsubjected to vertical flame testing (ASTM D6413-08). Figure 4shows these fabric samples following a 12 s exposure to directflame. All coated fabric exhibited a less vigorous flame than thecontrol fabric. Afterflame time was also significantly reduced,

and afterglow was eliminated. The uncoated control fabric wascompletely consumed during the test, while fabric coated with30 BL of pH 5 coating completely stopped the flame frompropagating almost immediately after ignition and more than90 wt % of the fabric was preserved (Figure 4). Fabric coatedwith pH 4 solutions also showed significant flame-retardantcapacity, leaving about 80 wt % of the fabric, while adding only16 wt % to the fabric weight before burning. Fabric coated withpH 6 solutions was burned much more extensively, but still hadsome unburned white spots and a significant amount of residueleft (42 wt %). Differences in these burn results could beattributed to the difference in the coating weight, as the bestflame-retardant coating had the highest weight gain added tocotton (∼18 wt %). This result matches the efficacy of apreviously reported intumescent assembly prepared withnonrenewable ingredients.7

In an effort to separate the influence of coating weight andcomposition, fabric coated with 18 wt % at each pH wasevaluated. This normalized comparison required 32 and 37 BLcoated at pH 4 and 6, respectively. It is interesting to note thatthere appears to be a substrate influence because pH 6 growsthe thickest on the Si wafer (Figure 2a), which should providethe highest weight gain at a given number of bilayers. It ispossible that the combination of film rigidity and low chargedensity at pH 6 contribute to some flaking or peeling of thecoating, which would explain this unexpectedly low weight gainon cotton fabric. When comparing the same coating weight, it isclear that higher PA content yields a more effective flameretardant. There was no noticeable change for pH 6 with extrabilayers, but there was a significant improvement for pH 4 withjust two additional bilayers. This pH 4 coating has the highestconcentration of PA among the three recipes tested. In regardsto flammability, with phosphorus/phosphate being the activechemistry that induces charring (which in turn lowers heatrelease and flame propagation), increasing levels of phosphorusshould lower flammability in a cellulosic material such ascotton.62−64 With PA containing 28 wt % P, higher levels of PAin the pH 4 system yield higher levels of phosphorus/

Figure 2. Growth of CH−PA assemblies as a function of depositionsolution pH, as measured by ellipsometry (a). AFM height images of10BL assemblies deposited from pH 4 (b) and pH 6 (c) solutions.

Figure 3. Film composition as a function of deposition solution pH forCH−PA assemblies (see also Supporting Information).

Figure 4. Images of uncoated control and fabrics coated with 30 BL ofCH−PA (top) and 18 wt % CH−PA (bottom) deposited at varyingpH level.

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phosphate available to cross-link the cellulosic fiber and CH toform an intumescent char.Higher magnification SEM images of burned fabric, as shown

in Figure 5, provide further insight into the observedantiflammable behavior of these CH−PA nanocoatings. Thecoating does not obscure the weave structure of the fabric, and,at higher magnification (middle row of images in Figure 5), theconformal coating of individual cotton fibers can be clearlyseen. All coated fibers look similar to the uncoated control inshape and structure, but some of the coated fibers are linkedtogether. Fabric coated with pH 6 solutions appears to have theroughest surface with numerous bridges formed betweenindividual fibers. This roughness suggests that this thicklydepositing system probably starts to form those bridges early inthe deposition process and as more layers get deposited, andthe fabric gets twisted during processing steps, the coatingconnecting the fibers peels off and contributes to a low finalweight gain. These gaps in the coating serves as sites where heatwill penetrate through the protective coating and thermallydamage the underlying cotton, which can lead to pyrolysis offlammable gases and subsequent localized flame propagation.Thin deposition at lower pH delays this interfiber linking,which preserves the coating and therefore eliminates the gaps inthe protective coating.Images of the postburn samples (in the bottom row of Figure

5) show that cotton fibers retain their shape and integrity afterburning, and there is also evidence of intumescent behavior.There are bubbles on top and in the gap between fibers due toexpansion of the intumescent carbon layer in the coating as gasevolves during burning. These images clearly demonstrate theprotective nature of this coating by forming a swollen, cellularlayer. Although this intumescent coating does not expand to 10times its original size, like most conventional intumescentcoatings, it remains able to slow, and in some cases stop, theflame propagation. The pH 6 coating exhibits much lessbubbling (i.e., weaker intumescent effect), which was expectedbased upon its diminished flame-retardant behavior (Figure 4).This coating has the lowest phosphorus content (Figure 3),which is a contributing factor, and the weak points/gaps in thiscoating likely result in uneven fire protection on the surface ofthe fibers.

Microcombustion calorimetry (MCC) was used to furtherinvestigate the flame-retardant properties of coated anduncoated cotton. Figure 6 shows heat release rate (HRR)

curves generated with fabric containing 18 wt % of CH−PAdeposited at pH 4, 5, and 6. It appears that the presence of theintumescent coating decreases the onset decompositiontemperature for coated fabrics, and this becomes morepronounced as the PA concentration in the coating increases.This temperature decrease, which has been observed with otherphosphorus-containing flame retardants,7,65 is due to thecatalyzed dehydration of cellulose, which aids in char formationby the decomposition of phosphorus compounds. The bestperforming pH 4 fabric exhibits the greatest reduction inpkHRR and total heat release (HR) of 60% and 76%respectively, compared to the uncoated control. Althoughsomewhat counterintuitive, it is important for the componentsof the coating to degrade before the onset of cottondegradation in order to preemptively form the protectivelayer. This lower temperature of reaction is good because char

Figure 5. SEM images of coated and uncoated cotton fabric before (top two rows) and after (bottom row) vertical burn testing. The uncoated fabricwas completely consumed during burning, so no postburn image can be shown.

Figure 6. Heat release rate as a function of temperature for uncoatedcontrol and CH−PA coated cotton fabric. All coated fabrics contained18 wt % CH−PA.

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formation is initiated early in the fire, thus preventing furtherpyrolysis of the cotton that in turn retards flame spread.In an effective intumescent system, the acid source must be

the first component to decompose to ensure dehydration of thecarbon source.66 PA begins to decompose around 200 °C,which is below the decomposition temperature of cotton (320°C) or CH (250 °C).57,67 Next, the carbon source (CH) isconverted into char by a dehydration reaction with PA. Thischar is then expanded by gases produced by decomposition ofthe blowing agent. In this case, CH serves as a carbon sourceand a blowing agent, so there is a continuous release of foaminggases over the entire char forming area. Eventually, the coatingsolidifies into a multicellular material that slows the heat flowfrom the fire to the substrate.66 To put the HRR reduction intoperspective, lowering peak HRR and total HR of a burningmaterial means it is easier for the material to self-extinguishonce the flame source is removed. Lower heat release meansthat the heat from the burning material is less, so it is harder topropagate flame and for heat to damage unburned portions ofthe material adjacent to the site of flaming combustion.64,68

Table 1 summarizes the results of MCC testing. All coatedfabric exhibits reduced total HR by at least 70%. Additionally,the amount of char increased by a factor of 7 and is significantlygreater than the coating weight, confirming that cotton fabricitself was preserved during the burning. It is also clear, fromTable 1 and Figure 4, that burn behavior improves with higherPA concentration. It has been shown previously that flame-retardant properties of cellulostic fabrics improve with increaseof phosphorus content due to more effective dehydration ofcellulose, prevention of the formation of volatile material, andaccelerating the formation of char.65,69 Even though theseintumescent nanocoatings demonstrate lower flammability ofcotton fabric in MCC testing, these results do not directlycorrelate with postburn residues. The pH 4 coating exhibits thelowest heat release rate even at 30 BL (i.e., lower weight)relative to pH 5 (at the same number of BL), while the latterappears to perform better in the vertical flame test (Figure 4).These seemingly conflicting results could be attributed touneven surface coverage on the fabric with pH 4 coating, whichexhibits the thinnest growth (Figure 2). With just two more BL,the pH 4 coating performs the best in every category andcompletely prevents flame propagation on cotton. Even withthe worst performing pH 6 coating, the heat release wassignificantly reduced, which is one of the desired characteristicsfor a flame-retardant coating.

■ CONCLUSIONSIntumescent nanocoatings were deposited on cotton fabricusing environmentally benign components (CH and PA) toimpart flame retardant behavior. By varying the pH of aqueousdepositing solutions, coating thickness and composition weremodified. Using solutions at pH 6 resulted in relatively thick

coatings with the lowest PA content, while the opposite wasachieved at pH 4 (i.e., thin growth and high PA content). Theflame-retardant properties of 30 BL-coated fabric were testedusing the vertical flame test, where the pH 5 coating (with theheaviest weight gain) was able to extinguish the flame. Toeliminate the variable of coating weight, fabrics with equalweight gain (∼18 wt %) were subjected to the vertical flametest. There was a clear trend of improvement in burn behaviorwith higher PA content. Fabric coated with pH 4 (66 wt % PA)left as much as 95% preserved fabric (completely unburnedmaterial with a small amount of char) after burning. Smallbubbles formed on top of and in the space between the fiberswere observed by SEM, which is evidence of the intumescenteffect. Additionally, microcalorimeter testing revealed a lowerpkHRR for all coated fabrics. Coatings made with pH 4solutions resulted in better flame-retardant properties relativeto those made at higher pH due to greater phosphorus content(in the form of PA). Higher phosphorus concentration isknown prevent formation of volatile material and accelerate theformation of char more effectively. This study demonstrates thefirst completely renewable intumescent LbL nanocoating forcotton fabric, which provides an effective and environmentallyfriendly alternative to current flame-retardant treatments.

■ ASSOCIATED CONTENT*S Supporting InformationThe weight of each CH and PA layer deposited, as measured byQCM, allows CH-PA assembly composition to be determined.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel: +1 979 845 3027; fax: +1 979 862 3989. E-mail address:[email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe Materials Flammability Group of the Fire ResearchDivision at NIST is gratefully acknowledged for financialsupport of this work (Cooperative Agreement No.70NANB11H171). We also thank Dr. Jodie Lutkenhaus forhelpful discussions regarding QCM. A.B.M. also wishes tothank Mary Galaska and Kathleen Beljan for their assistance incollecting the PCFC measurements.

■ REFERENCES(1) Kamath, M. G.; Bhat, G. S.; Parikh, D. V.; Condon, B. D. J. Ind.Text. 2009, 38, 251−262.(2) Liodakis, S.; Fetsis, I. K.; Agiovlasitis, P. J. Therm. Anal. Calorim.2009, 98, 289−291.

Table 1. MCC Results for CH−PA Coated Cotton Fabric and an Uncoated Control

# BL (% weight gain) char yield (wt%) pkHRR (W/g)a pkHRR (°C)a Total HR (kJ/g)b

Control 5.6 ± 0.1 259 ± 6.7 382 ± 2.1 12.0 ± 0.1pH 4 30 BL (∼16%) 41.7 ± 1.9 99 ± 3.5 311 ± 0.9 2.8 ± 0.1

32 BL (∼18%) 42.4 ± 0.3 100 ± 1.8 313 ± 0.8 2.8 ± 0.1pH 5 30 BL (∼18%) 41.8 ± 0.5 116 ± 3.3 318 ± 0.3 3.2 ± 0.1pH 6 30 BL (∼13%) 38.7 ± 0.4 134 ± 1.4 318 ± 5.0 3.8 ± 0.1

37 BL (∼18%) 39.1 ± 0.8 161 ± 12.8 322 ± 1.2 3.8 ± 0.1apkHRR = peak heat release rate. bHR = heat release.

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(3) Wu, X.; Yang, C. Q. J. Fire Sci. 2008, 26, 351−368.(4) Price, D.; Horrocks, A. R.; Akalin, M.; Faroq, A. A. J. Anal. Appl.Pyrolysis 1997, 40−41, 511−524.(5) Tsafack, M. J.; Levalois-Grutzmacher, J. Surf. Coat. Technol. 2006,201, 2599−2610.(6) Li, Y. C.; Schulz, J.; Mannen, S.; Delhom, C.; Condon, B.; Chang,S.; Zammarano, M.; Grunlan, J. C. ACS Nano 2010, 4, 3325−3337.(7) Li, Y.-C.; Mannen, S.; Morgan, A. B.; Chang, S.; Yang, Y.-H.;Condon, B.; Grunlan, J. C. Adv. Mater. 2011, 23, 3926−3931.(8) Kim, Y. S.; Davis, R.; Cain, A. A.; Grunlan, J. C. Polymer 2011, 52,2847−2855.(9) Laufer, G.; Kirkland, C.; Cain, A. A.; Grunlan, J. C. ACS Appl.Mater. Interfaces 2012, 4, 1643−1649.(10) De Villiers, M. M.; Otto, D. P.; Strydom, S. J.; Lvov, Y. M. Adv.Drug Delivery Rev. 2011, 63, 701−715.(11) Decher, G.; Schlenoff, J. B. Multilayer Thin Films − SequentialAssembly of Nanocomposite Materials; Wiley-VCH: Weinheim,Germany, 2003.(12) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213−4219.(13) Sui, Z. J.; Salloum, D.; Schlenoff, J. B. Langmuir 2003, 19,2491−2495.(14) Tan, H. L.; McMurdo, M. J.; Pan, G. Q.; Van Patten, P. G.Langmuir 2003, 19, 9311−9314.(15) Laufer, G.; Carosio, F.; Martinez, R.; Camino, J.; Grunlan, J. C.J. Colloid Interface Sci. 2011, 356, 69−77.(16) Laachachi, A.; Ball, V.; Apaydin, K.; Toniazzo, V.; Ruch, D.Langmuir 2011, 27, 13879−13887.(17) Park, Y. T.; Grunlan, J. C. Electrochim. Acta 2010, 55, 3257−3267.(18) DeLongchamp, D. M.; Kastantin, M.; Hammond, P. T. Chem.Mater. 2003, 15, 1575−1586.(19) Liu, S.; Kurth, D. G.; Mohwald, H.; Volkmer, D. Adv. Mater.2002, 14, 225−228.(20) Priolo, M.; Gamboa, D.; Holder, K.; Grunlan, J. C. Nano Lett.2010, 10, 4970−4974.(21) Yang, Y.-H.; Haile, M.; Park, Y. T.; Malek, F. A.; Grunlan, J. C.Macromolecules 2011, 44, 1450−1459.(22) Svagan, A. J.; Akesson, A.; Cardenas, M.; Bulut, S.; Knudsen, J.C.; Risbo, J.; Plackett, D. Biomacromolecules 2012, 13, 397−405.(23) Bravo, J.; Zhai, L.; Wu, Z.; Cohen, R. E.; Rubner, M. F.Langmuir 2007, 23, 7293−7298.(24) Jisr, R.; Rmaile, H.; Schlenoff, J. B. Angew. Chem., Int. Ed. 2004,44, 782−785.(25) Wood, K. C.; Boedicker, J. Q.; Lynn, D. M.; Hammond, P. T.Langmuir 2005, 21, 1603−1609.(26) Yan, Y.; Such, G. K.; Johnston, A. P. R.; Lomas, H.; Caruso, F.ACS Nano 2011, 5, 4252−4257.(27) Dvoracek, C. M.; Sukhonosova, G.; Benedik, M. J.; Grunlan, J.C. Langmuir 2009, 25, 10322−10328.(28) Podsiadlo, P.; Paternel, S.; Rouillard, J. M.; Zhang, Z. F.; Lee, J.;Lee, J. W.; Gulari, L.; Kotov, N. A. Langmuir 2005, 21, 11915−11921.(29) Lichter, J. A.; Van Vliet, K. J.; Rubner, M. F. Macromolecules2009, 42, 8573−8586.(30) Carosio, F.; Laufer, G.; Alongi, J.; Camino, G.; Grunlan, J. C.Polym. Degrad. Stab. 2011, 96.(31) Weil, E. D. J. Fire Sci. 2011, 29, 259−296.(32) Duquesne, S.; Magnet, S.; Jama, C.; Delobel, R. Surf. Coat.Technol. 2003, 180−181, 302−307.(33) Horrocks, A. R.; Wang, M. Y.; Hall, M. E.; Sunmonu, F.;Pearson, J. S. Polym. Int. 2000, 49, 1079−1091.(34) Chen-Yang, Y. W.; Chuang, J. R.; Yang, Y. C.; Li, C. Y.; Chiu, Y.S. J. Appl. Polym. Sci. 1998, 69, 115−122.(35) Duquesne, S.; Magnet, S.; Jama, C.; Delobel, R. Polym. Degrad.Stab. 2005, 88, 63−69.(36) Leca, M.; Cioroianu, L.; Cioroianu, G.; Damian, G.; Costea, C.;Matei, A. M. Rev. Roum. Chim. 2007, 52, 745−752.(37) Duskova, D.; Marounek, M.; Brezina, P. J. Sci. Food Agric. 2000,81, 36−41.

(38) Graf, E.; Eaton, J. W. Free Radical Biol. Med. 1990, 8, 61−69.(39) Stodolak, B.; Starzynska, A.; Czyszczon, M.; Zyla, K. Food Chem.2007, 101, 1041−1045.(40) Shamsuddin, A. M.; Vucenik, I.; Cole, K. E. Life Sci. 1997, 61,343−354.(41) Fox, C. H.; Eberl, M. Complementary Ther. Med. 2002, 10, 229−234.(42) Kunyanga, C. N.; Imungi, J. K.; Okoth, M. W.; Biesalski, H. K.;V., V. Ecol. Food Nutr. 2011, 50, 452−471.(43) Lee, S.-H.; Park, H.-J.; Chun, H.-K.; Cho, S.-Y.; Cho, S.-M.;Lillehoj, H. S. Nutr. Res. (N.Y.) 2006, 26, 474−479.(44) Evans, W. J.; McCourtney, E. J.; Shrager, R. I. J. Am. Oil Chem.Soc. 1982, 59, 189−191.(45) Tang, F.; Wang, X.; Xu, X.; Li, L. Colloids Surf. Physicochem. Eng.Aspects 2010, 369, 101−105.(46) Levchik, S. V.; Weil, E. D. J. Fire Sci. 2006, 24, 345−364.(47) El-Tahlawy, K.; Hudson, S. M. J. Appl. Polym. Sci. 2006, 100,1162−1168.(48) Lee, D. W.; Lim, H.; Chong, H. N.; Shim, W. S. Open Biomed. J.2009, 1, 10−20.(49) Richert, L.; Lavalle, P.; Payan, E.; Shu, X. Z.; Prestwich, G. D.;Stoltz, J. F.; Schaaf, P.; Voegel, J. C.; Picart, C. Langmuir 2004, 20,448−458.(50) Mao, J.; Zhao, L.; Yao, K.; Shang, Q.; Yang, G.; Cao, Y. J.Biomed. Mater. Res. A 2003, 64A, 301−308.(51) Huang, D.; Zuo, Y.; Zou, Q.; Wang, Y.; Gao, S.; Wang, X.; Liu,H.; Li, Y. J. Biomed. Mater. Res. B Appl. Biomater. 2011, 100B, 51−57.(52) Prabaharan, M. J. Biomater. Appl. 2008, 23, 5−36.(53) Hadrami, A. E.; Adam, L. R.; Hadrami, I. E.; Daayf, F. Mar.Drugs 2010, 8, 968−987.(54) No, H. K.; Meyers, S. P.; Prinyawiwatkul, W.; Xu, Z. J. Food Sci.2007, 72, R87−R100.(55) Carosio, F.; Alongi, J.; Malucelli, G. Carbohydr. Polym. 2012, 88.(56) Alongi, J.; Carosio, F.; Malucelli, G. Cellulose 2012, 19, 1041−1050.(57) Zeng, L.; Qin, C.; Wang, L.; Li, W. Carbohydr. Polym. 2011, 83,1553−1557.(58) Leu, T.-S.; Wang, C.-S. J. Appl. Polym. Sci. 2004, 92, 410−417.(59) Toldy, A.; Toth, N.; Anna, P.; Marosi, G. Polym. Degrad. Stab.2006, 91.(60) Skovstrup, S.; Hansen, S. G.; Skrydstrup, T.; Schiott, B.Biomacromolecules 2010, 11, 3196−3207.(61) Fontaine, G.; Bourbigot, S. J. Appl. Polym. Sci. 2009, 113, 3860−3865.(62) Weil, E. D.; Levchik, S. V. Flame Retardants for Plastics andTextiles: Practical Applications; Hanser Publishers: Cincinnati, OH,2009.(63) Lecoeur, E.; Vroman, I.; Bourbigot, S.; Lam, T. M.; Delobel, R.Polym. Degrad. Stab. 2001, 74, 487−492.(64) Yang, C. Q.; He, Q.; Lyon, R. E.; Hu, Y. Polym. Degrad. Stab.2010, 95, 108−115.(65) Gaan, S.; Sun, G. Polym. Degrad. Stab. 2007, 92, 968−974.(66) Mouritz, A.; Gibson, A. Flame Retardant Composites. In FireProperties of Polymer Composite Materials, Gladwell, G. M. L., Ed.;Springer: Dordrecht, The Netherlands, 2006; pp 237−286.(67) Reda, S. Y. Ciene. Tecnol. Aliment. 2011, 31, 475−190.(68) Lyon, R. E.; Crowley, S.; Walters, R. N. Fire Mater. 2008, 32,199−212.(69) Gaan, S.; Sun, G. J. Anal. Appl. Pyrolysis 2007, 78, 371−377.

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