Accepted Manuscript Title: Water-based chitosan/melamine polyphosphate multilayer nanocoating that extinguishes fire on polyester-cotton fabric Author: Marcus Leistner Anas A. Abu-Odeh Sarah C. Rohmer Jaime C. Grunlan PII: DOI: Reference:
S0144-8617(15)00403-8 http://dx.doi.org/doi:10.1016/j.carbpol.2015.05.005 CARP 9908
To appear in: Received date: Revised date: Accepted date:
27-2-2015 15-4-2015 8-5-2015
Please cite this article as: Leistner, M.,Water-based chitosan/melamine polyphosphate multilayer nanocoating that extinguishes fire on polyester-cotton fabric, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights Water-based chitosan / melamine polyphosphate nanocoating for fabric.
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Polyester-cotton blend extinguishes immediately with only 12 wt% coating deposited.
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Nanocoating is free of organic solvents and toxic chemicals.
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Chitosan:melamine ratio used to tailor flame retardant behavior and growth of coating.
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Layer-by-layer assembly produces melamine polyphosphate in-situ from water.
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Water-based chitosan / melamine polyphosphate multilayer nanocoating that extinguishes fire on polyester-cotton fabric
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Marcus Leistner, Anas A. Abu-Odeh, Sarah C. Rohmer, Jaime C. Grunlan*
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Anas A. Abu-Odeh E-mail: [email protected]
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Contact information: Marcus Leistner E-mail: [email protected]
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Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA
Sarah C. Rohmer E-mail: [email protected]
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Jaime C. Grunlan* Tel.: +1 9798453027 Fax: +1 9798453081 E-mail: [email protected]
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Full Paper
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Abstract
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Polyester-cotton (PECO) blends are widely used in the textile industry because they combine the
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softness of cotton and the strength and durability of polyester. Unfortunately, both fiber types
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share the disadvantage of being flammable. The layer-by-layer coating technique was used to
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deposit a highly effective flame retardant (melamine polyphosphate) from water onto polyester-
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cotton fabric. Soluble melamine and sodium hexametaphosphate form this water-insoluble flame
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retardant during the coating procedure. This unique nanocoating imparts self-extinguishing
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properties to PECO with only 12 % relative coating weight. Vertical flame testing, pyrolysis
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combustion flow calorimetry (PCFC), thermogravimetric analysis (TGA) and scanning electron
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microscopy were used to evaluate the quality of the coating as well as its flame retardant
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performance. A combination of both condensed and gas phase activity appears to be the reason
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for this effective flame retardancy. Degradation pathways of both cotton and polyester are
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affected by the applied coating, as shown by PCFC and TGA. Use of environmetally benign and
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non-toxic chemicals, and the ease of layer-by-layer deposition, make this coating an industrially
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feasible alternative to render polyester-cotton fabric self-extinguishing.
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Keywords
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Layer-by-layer assembly, chitosan, flame retardant nanocoating, polyester-cotton, melamine
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polyphosphate
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Compounds studied in the article
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Chitsoan (CID: 21896651), melamine (CID: 7955), melamine polyphosphate (CID: 92612),
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poly(ethylene terephthalate) (CID: 16212789), sodium hexametaphosphate (CID: 24968), cotton
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1. Introduction
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Synthetic fibers, such as polyesters, represent the largest component of the textile industry, with
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an increasing demand over the past ten years (Shui & Plastina, 2013). Blends of cotton and
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synthetic fibers are widely used for apparel, particularly workwear. Polyester-cotton (PECO)
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blends are used to combine the comfort and breathability of cotton with the strength and
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durability of the polyester (Day, Suprunchuk & Wiles, 1986). As with most organic polymers, the
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flammability of these textiles can be problematic. Some common ways to reduce flammability of
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PECO fabric are the use of an inherently flame retardant polyester, halogenated flame retardants
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used as additives in the polyester or back-coatings containing either halogenated flame retardants
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or phosphorus compounds (Drevelle et al., 2005; Horrocks & Kandola, 2004; Horrocks, Wang,
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Hall, Sunmonu & Pearson, 2000; Weil & Levchik, 2008). Aliphatic polyesters form volatiles
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rather than char, which leads to melt-dripping during decomposition (Horrocks, 2011). This melt
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flow is an important part of extinguishing pure synthetic textiles, so the introduction of any
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substance into (or onto) the polymer that reduces the melt flow can actually increase
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flammability. Even some print pigments and dyes are known to have this adverse effect (Ozcan,
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Dayioglu & Candan, 2004; Weil & Levchik, 2008).
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Back-coating is often used to create an effective flame retardant coating on textiles and carpets.
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These coatings consist of a curable binder (e.g., acrylic resins) and flame retardant additives such
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as brominated compounds, ammonium polyphosphate (APP) or other phosphorus-based additives
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(Drevelle et al., 2005; Herrlich, Steib & Lang, 2014; Horrocks, Wang, Hall, Sunmonu & Pearson,
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2000). Layer-by-layer (LbL) assembly does not need a matrix, so it offers an opportunity for an
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improved flame retardant treatment. Oppositely charged polyelectrolytes that incorporate the
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desired functionality are deposited onto the substrate. Other interactions, such as hydrogen
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bonding or even covalent bonding can also be used to deposit functional layers onto a substrate
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(Alongi, Carosio & Malucelli, 2014; Borges & Mano, 2014; Broderick & Lynn, 2013). This
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water-based coating technique can be used to modify various properties of substrates such as gas
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permeability, flammability or antistatic and antimicrobial properties (Dvoracek, Sukhonosova,
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Benedik & Grunlan, 2009; Holder, Spears, Huff, Priolo, Harth & Grunlan, 2014; Laufer,
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Kirkland, Morgan & Grunlan, 2013; Park, Ham & Grunlan, 2010). Active flame retardant
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ingredients in LbL coatings can be polyelectrolytes or even particles that are embedded in the
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coating (Laufer, Carosio, Martinez, Camino & Grunlan, 2011; Pan, Wang, Pan, Song, Hu &
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Liew, 2015). Chitosan (CH) in combination with polyphosphates like sodium hexametaphosphate
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(PSP) or APP are well-known layer-by-layer systems that reduce flammability of cotton, but have
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no significant influence on the burning behavior of polyester-cotton blends (Carosio, Alongi &
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Malucelli, 2012; Mateos, Cain & Grunlan, 2014). The use of insoluble and non-melting additives
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for thermoplastics alters the physical properties of the polymer, so the material is less suitable for
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fiber production or other flexible applications.
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The present study involves an effective layer-by-layer coating for polyester-cotton fabric, which
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is completely free of toxic additives and organic solvents. The active ingredients form the coating
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by themselves and the ingredients are environmentally-benign and water-soluble. The addition of
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melamine (Mel) to a CH / PSP multilayer, by including it in the chitosan solution, forms
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insoluble melamine polyphosphate (MPP) during deposition. MPP is known as a flame retardant
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additive for synthetic polymers and it improves the flame retardancy of this LbL nanocoating
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(Burke & Mogul, 2010; Jahromi, Gabriëlse & Braam, 2003; Sullalti, Colonna, Berti, Fiorini &
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Karanam, 2012). With only 12 wt% of this coating deposited on PECO fabric, it self-extinguishes
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immediately in a vertical flame test. Conformal coating of individual fibers, good hand of the
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coated fabric, and ease of the coating procedure make this technology promising for commercial
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use.
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2. Experimental
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2.1 Materials
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Sodium hexametaphosphate (crystalline, 96%), melamine (99%), sodium hydroxide (98%), and
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hydrochloric acid (37%) were purchased from Sigma-Aldrich (Milwaukee, WI). Chitosan
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(60,000 g mol-1, 95 % degree of deacetylation) was purchased from G.T.C. Bio Corporation
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(Qingdao, China). All chemicals were used as received. Polyester-cotton fabric (65%
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polyethylene terephthalate, 4.5 oz yd-2) was obtained from Testfabrics, Inc. (West Pittston, PA).
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The fabric was washed thoroughly in deionized water to remove impurities and dried for 60 min
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at 70°C before use. Deionized water (18 MΩ) was used to prepare all solutions. After complete
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dissolution of the ingredients, the pH of each solution was adjusted to pH 4 using 5 M
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hydrochloric acid or 1 M sodium hydroxide solution. Mixtures of chitosan and melamine were
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prepared by adding melamine to an acidic solution of chitosan (pH < 4) and immediately
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adjusting to pH 4 with 5 M hydrochloric acid.
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2.2 Coating Procedure
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A manual dip coating procedure was used to deposit the flame retardant nanocoating onto the
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fabric. The polyester-cotton fabric was first soaked for 5 min in the solution containing the CH
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polycation, referred to as “cationic solution”, to allow complete wetting and good adhesion of the
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coating to the substrate. All subsequent dips in the PSP solution and the cationic solution
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(producing one bilayer) were 1 min. After each deposition step, the fabric was briefly rinsed in
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deionized water to remove weakly adhered material. After each soak and rinse, the fabric was
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squeezed by hand in order to minimize contamination of the next coating solution or rinse water.
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Each solution and the rinse water were renewed after every 5 bilayers. Once the desired number
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of bilayers were deposited, the fabric was rinsed in deionized water and dried for 60 min at 70°C.
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The weight add-on deposited on the fabric is controlled by the number of bilayers deposited. In
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an effort to achieve the same weight gain with different concentrations, the number of bilayers of
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each recipe was adjusted. Figure 1 shows the general coating procedure and the chemical
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structures of the primary ingredients. All solutions were used at room temperature. Both the
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cationic solution and the PSP solution were adjusted to pH 4, as mentioned above.
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Figure 1. Schematic of the layer-by-layer coating procedure and chemical structures of the flame retardant components.
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2.3 Analytical methods
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Burning behavior of the coated fabric was evaluated with a vertical flame test (VFT), following
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ASTM D6413-08 using a VC-2 vertical flame cabinet (Govmark, Farmingdale, NY). Pyrolysis
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combustion flow calorimetry (PCFC) was performed, according to method A of ASTM D7309, at
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the University of Dayton Research Institute (Dayton, OH), with a heating rate of 1 K s-1 and a
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sample weight of 5 mg. This test revealed the influence of the coating on the fabric’s heat release.
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All flammability tests were performed in triplicate. Thermogravimetric analysis (TGA) was
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performed in nitrogen with a constant heating rate of 20 K min-1 and a sample weight of 15 mg,
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using a TA Q50 TGA (TA Instruments, New Castle, DE). A JEOL (Tokyo, Japan) JSM-7500F
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field emission scanning electron microscope (FE-SEM) was used to image selected samples.
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Fabric samples were sputter-coated with 4 nm of platinum prior to SEM imaging.
164 3. Results and discussion
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3.1 Film growth
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The influence of varying the chitosan:melamine ratio within the cationic solution was evaluated.
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Figure 2a shows the weight gain of 15 bilayers (BL) of CH:Mel / PSP deposited on polyester-
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cotton fabric, as a function of varying the CH concentration in the aqueous solution. The total
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concentration of cationic ingredients (chitosan and melamine) was held constant at 1.4 wt% in
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water and the concentration of sodium hexametaphosphate was 2 wt% in the anionic solution.
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Weight gain on the fabric increases as the ratio of CH:Mel increases (i.e., higher concentration of
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CH in the cationic solution). Melamine and chitosan have competing interactions with PSP.
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While CH forms a complex with PSP [x CHn+ + y PSP6– → (CH)x(PSP)y] that leads to film
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growth, the addition of Mel to this system forms mixed compounds [x CHn+ + y PSP6– + z Mel+
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→ (CH)x(Mel)z(PSP)y] and melamine polyphosphate crystals embedded in the coating [6 Mel+ +
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PSP6– → (Mel)6(PSP)]. While the formation of MPP [(Mel)6(PSP)] is essential to impart flame
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retardant properties, Mel and PSP alone do not grow layer-by-layer under these conditions. When
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depositing Mel/PSP in the absence of CH, there is no weight gain on the fabric, suggesting no
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film growth. When adjusting the CH:Mel ratio in the cationic solution, there is a tradeoff between
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film growth and flame retardancy. Film growth as a function of the number of deposited layers is
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shown in Figure 2b, using 0.5 wt% CH and 0.9 wt% Mel in the cationic solution. This particular
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CH:Mel ratio is of interest because it imparts the most effective flame retardancy to the fabric, as
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discussed below. Linear film growth was also observed in a recent study depositing CH and PSP
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on cotton fabric and is believed to be due to an interdiffusing polyelectrolyte system (Guin,
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Krecker, Milhorn & Grunlan, 2014). The addition of Mel does not seem to change this growth
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behavior.
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Figure 2. Weight gain as a function of (a) CH concentration (in a cationic solution with Mel) and (b) number of bilayers deposited with PSP.
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3.2 Flame retardant behavior
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Flame retardant behavior of coated PECO fabric was evaluated with vertical flame testing and
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pyrolysis combustion flow calorimetry. VFT reveals the fabric’s response to an open flame and
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gives some indication of the flame spread after ignition. PCFC reveals the combustion behavior,
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by exposing the sample to a constant heating rate, from an energetic point of view. The fabric
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becomes much more flame retardant as the CH:Mel ratio is decreased in the cationic deposition
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solution. With only a 12 wt% coating, the fabric is self-extinguishing at a CH:Mel ratio of 1:1
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and lower. Figure 3 shows the reduction in char length of coated samples with increasing Mel
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concentration in the cationic solution. More melamine leads to more formation of melamine
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polyphosphate during the coating procedure, which is the most important flame retardant
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component. While CH is mainly used as a binder in this system, to secure the MPP inside the
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film, it also acts as a charring agent. The charring of the 8 BL CH/PSP coating is very clear in
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Figure 3, although it does not self-extinguish in the absence of melamine. The uncoated sample
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on the far left burnt off and left just a melted residue (i.e., no char formation).
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Figure 3. Fabric after vertical flame testing. Each coated sample had 12.5 ± 0.4 wt% deposited. Concentration of melamine in the cationic deposition solution is increasing from left to right. Concentration of PSP in the anionic solution was held constant at 2 wt%.
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The SEM images in Figure 4 show the surface structure of the coated PECO fabric before and
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after VFT. These micrographs reveal that the LbL coating exhibits less complexation and
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bridging on the fabric’s surface when melamine is present in the coating (Figure 4a). The less
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uniform CH/PSP coating (Figure 4c) impairs the hand of these textiles, making them relatively
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stiff (Guin, Krecker, Milhorn & Grunlan, 2014). With melamine present, the coating contains
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MPP and forms a much denser char (Figure 4b), which decreases the transport of flammable
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volatiles from the substrate to the flame. This dense char can additionally act as a heat shield to
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prevent further degradation of the underlying fabric. In addition to helping char formation,
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melamine can be released at the start of a fire and dilute the gaseous phase by itself and with its
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inflammable decomposition products, such as ammonia and carbon dioxide (Ledeti, Vlase, Vlase,
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Doca, Bercean & Fulias, 2014). This dilution results in a decreased oxygen concentration near the
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fabric surface that reduces the combustion heat and slows the flame spread. Melamine can also
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condense when heated to form intermediates like melam and melem, which polymerize further to
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increase the density of the char by simultaneously releasing ammonia (Wirnhier, Mesch, Senker
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& Schnick, 2013). Melamine and its decomposition products, as well as melamine salts, can also
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alter the decomposition process of polyesters, such as poly(butylene terephthalate) that could lead
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to increased char formation (Balabanovich, 2004; Hoffendahl, Duquesne, Fontaine & Bourbigot,
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2014). As terephthalate polyesters with a linear component of less than 7 CH2 units show similar
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thermal decomposition, comparable effects are expected to occur with the PET used here (Ohtani,
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Kimura & Tsuge, 1986).
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Figure 4. SEM micrographs of polyester-cotton before (top) and after VFT (bottom). The samples were coated with (a,b) 15 BL of CH/PSP and (c,d) 8 BL of CH:Mel/PSP, each with 12.5 ± 0.4 wt% added to the fabric.
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The flame retardant efficiency of these CH:Mel/PSP nanocoatings (also called CH/MPP) is also
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higher Mel concentration, as shown in Figure 5a. Peak heat release rate (PHRR) and total heat
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release (THR) of the coating using the lowest and highest CH:Mel ratio, along with the uncoated
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control and fabric coated with only CH/PSP, are summarized in Table 1 (along with key
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parameters from vertical flame testing). Both test methods show evidence of flame retardant
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behavior, even with samples that burned up during the VFT. The longer burning time measured
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in the vertical flame test is the result of slower flame spread caused by insufficient, but
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measurable flame suppression of the coating. The uncoated control sample shows a relatively
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high residue weight after the VFT, without formation of an actual char. This residue is caused by
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the typical melting behavior of the polyester, which leads to self-extinguishment of pure
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polyester fabric. PECO blends do not self-extinguish, because the cotton partially suppresses this
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so-called “runaway effect”. Addition of char forming substances, such as CH and MPP, further
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deteriorates the burning behavior, which leads to the observed decrease in residue. With
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sufficient MPP on the fabric, the runaway effect of polyester can become a char forming flame
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retardant mechanism, which can be observed using a CH:Mel ratio of 1:1 or less.
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Table 1. VFT and PCFC results of selected fabric samples, each with 12.5 ± 0.4 wt% coating added. Vertical Flame Test
CH : Mel
conc. (wt%)
BL
burning time (s)
char (in)
residue (%)
n/a
n/a
12 ± 2
no char
49 ± 1
0
8
26 ± 2
entire sample
28 ± 2
0.9 : 0.5
10
33 ± 2
entire sample
31 ± 1
0.5 : 0.9
15
0
4.5 ± 0.5
93 ± 1
1.4 :
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PCFC* -1
PHRR (W g ) 127 at 387°C 242 at 463°C 55 at 311°C 199 at 457°C 27 at 318°C 147 at 428°C 25 at 322°C 171 at 418°C
-1
THR (kJ g ) 15.2 ± 0.2 10.9 ± 0.2 9.4 ± 0.1 9.6 ± 0.1
* PHRR = peak heat release rate, THR = total heat release
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Table 1 shows that the total heat release of all coated fabric is significantly lower than the heat
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release of the uncoated control. A shift to lower decomposition temperature can also be observed
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(from PCFC) for the cotton portion of the coated fabrics. Increased burning time, lower heat
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release and altered decomposition temperatures are evidence of flame retardant activity. PCFC
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and TGA reveal complementary information about the decomposition behavior, as shown in
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Figure 5. The CH/PSP coating leads to a strong shift of the cotton decomposition to lower
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temperature, while the polyester is unaffected. This is due to PSP reacting with cotton, but neither
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of the coating’s ingredients interact with polyester. The fabric sample coated with CH/MPP shifts
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both heat release peaks to lower temperatures, as shown in Figure 5a. This behavior was
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confirmed by TGA of this sample (Figure 5b). The polyester portion decomposes at a lower
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temperature as a result of the altered decomposition of polyester in the presence of Mel
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(Balabanovich, 2004). Samples coated with a CH:Mel ratio of either 5:9 or 9:5 in the cationic
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solution have a similar behavior in PCFC measurements, but act differently in the vertical flame
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test. In this flame retardant system, melamine acts primarily in the gas phase through dilution,
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and only a small portion interacts with polyester in the condensed phase. Gas phase effects like
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this are not very well accounted for using PCFC, as this method uses oxygen consumption
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calorimetry to determine the heat of combustion of pyrolysis products. Thermal decomposition of
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MPP is an endothermic process, which creates a heat sink during the burning process.
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Endothermic reactions during the pyrolysis are also not detected with PCFC (Lyon & Walters,
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2004).
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ip t cr Figure 5. (a) Heat release rate as a function of temperature, measured by PCFC, and (b) TGA of uncoated PECO fabric [and samples coated with 12.5 wt% of CH/MPP (CH:Mel ratio was 5:9) or CH/PSP].
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PECO fabric degrades starting at 350°C, with peak rates at 384°C (cotton) and 451°C (polyester).
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If coated with CH/PSP, the degradation of cotton starts at a significantly lower temperature
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(250°C), while the degradation temperature of polyester is unaffected. This is caused by the
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phosphate catalyzed dehydration reaction of cellulosic material like cotton. Pure cotton forms
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laevoglucosan above 300°C, which decomposes further to volatile products. Below 300°C,
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dehydration reactions are preferred, which lead to increased char formation. These dehydration
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reactions are catalyzed by phosphoric acid and cause the lower pyrolysis temperature of the
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cotton part within the blended fabric (Kandola & Horrocks, 1996). Accordingly, an increased
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char formation is observed in TGA of the coated fabric. The entire cotton content of the uncoated
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control (35 wt%) is volatilized, while only a 21 % mass loss is observed with the CH/PSP coated
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fabric. Only 16 wt% volatiles were formed by the pyrolysis of the cotton portion coated with
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CH/MPP. All samples, regardless of coating, exhibit a mass loss due to the polyester portion of
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51 ± 1 %, indicating no significant influence of the coating on the pyrolysis of polyester.
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Differences in the residual weight at 500°C (Figure 5b) confirm this observation, as they are
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equivalent to the differences in weight loss caused by pyrolysis of the cotton portion.
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295 4. Conclusions
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An environmentally benign, water-based nanocoating was deposited layer-by-layer onto
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polyester-cotton fabric. The insoluble flame retardant MPP was formed at ambient conditions
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during the coating procedure by a reaction between melamine in one solution and sodium
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hexametaphosphate in the other. Chitosan serves as a counterpart to PSP in the layering process,
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as well as an additional carbon source for increased char formation. PECO fabric, coated with 15
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bilayers of CH/MPP, was shown to be self-extinguishing during vertical flame testing. A relative
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coating weight of at least 12 % is sufficient to protect PECO in both the condensed and gas
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phases. Char formation by the altered pyrolysis of cotton, caused by the polyphosphate, and
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dilution of the gas phase by released melamine result in an effective flame retardant combination
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to extinguish the ignited fabric. With its ease of deposition and use of relatively non-toxic
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components, this unique coating offers an interesting alternative for fire protection of polyester-
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cotton.
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References
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Alongi, J., Carosio, F., & Malucelli, G. (2014). Current emerging techniques to impart flame
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retardancy to fabrics: An overview. Polymer Degradation and Stability, 106(0), 138-149. Balabanovich, A. I. (2004). The effect of melamine on the combustion and thermal
314
decomposition behaviour of poly(butylene terephthalate). Polymer Degradation and
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Stability, 84(3), 451-458.
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Borges, J., & Mano, J. F. (2014). Molecular Interactions Driving the Layer-by-Layer Assembly of Multilayers. Chemical Reviews, 114(18), 8883-8942.
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Broderick, A. H., & Lynn, D. M. (2013). Covalent Layer-by-Layer Assembly Using Reactive Polymers. Functional Polymers by Post-Polymerization Modification, 371-406. Burke, J., & Mogul, F. (2010). First flame retardant PET. Specialty Chemicals Magazine, 30(9), 37-38.
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resistance of cotton fabric through ultrasonication rinsing of multilayer nanocoating.
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Cellulose, 21(4), 3023-3030.
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Herrlich, T., Steib, C., & Lang, A. (2014). flame-resistant coating for the rear side of a carpet. In WIPO (Ed.) (Vol. WO2014026741 A1, pp. 1-32). Switzerland: Clariant Int Ltd.
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Hoffendahl, C., Duquesne, S., Fontaine, G., & Bourbigot, S. (2014). Decomposition mechanism
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Stretchable Gas Barrier Achieved with Partially Hydrogen-Bonded Multilayer Nanocoating.
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Macromolecular Rapid Communications, 35(10), 960-964.
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Horrocks, A. R. (2011). Flame retardant challenges for textiles and fibres: New chemistry versus innovatory solutions. Polymer Degradation and Stability, 96(3), 377-392. Horrocks, A. R., & Kandola, B. K. (2004). Flame Retardant Textiles. In J. Troitzsch (Ed.).
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Plastics Flammability Handbook: Principles, Regulations, Testing, and Approval (pp. 182-
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retardants in textile back-coating formulations. Polymer International, 49(10), 1079-1091.
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Kandola, B. K., & Horrocks, A. R. (1996). Complex char formation in flame-retarded fibre-
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Laufer, G., Carosio, F., Martinez, R., Camino, G., & Grunlan, J. C. (2011). Growth and fire
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resistance of colloidal silica-polyelectrolyte thin film assemblies. Journal of Colloid and
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Laufer, G., Kirkland, C., Morgan, A. B., & Grunlan, J. C. (2013). Exceptionally Flame Retardant
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Polyelectrolyte Solutions. ACS Macro Letters, 2(5), 361-365. Ledeti, I., Vlase, G., Vlase, T., Doca, N., Bercean, V., & Fulias, A. (2014). Thermal
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decomposition, kinetic study and evolved gas analysis of 1,3,5-triazine-2,4,6-triamine.
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Fabric. Industrial & Engineering Chemistry Research, 53(15), 6409-6416.
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Park, Y. T., Ham, A. Y., & Grunlan, J. C. (2010). High Electrical Conductivity and Transparency
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Sullalti, S., Colonna, M., Berti, C., Fiorini, M., & Karanam, S. (2012). Effect of phosphorus
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based flame retardants on UL94 and Comparative Tracking Index properties of
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Wirnhier, E., Mesch, M. B., Senker, J., & Schnick, W. (2013). Formation and Characterization of
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S0144-8617(15)00403-8 http://dx.doi.org/doi:10.1016/j.carbpol.2015.05.005 CARP 9908
To appear in: Received date: Revised date: Accepted date:
27-2-2015 15-4-2015 8-5-2015
Please cite this article as: Leistner, M.,Water-based chitosan/melamine polyphosphate multilayer nanocoating that extinguishes fire on polyester-cotton fabric, Carbohydrate Polymers (2015), http://dx.doi.org/10.1016/j.carbpol.2015.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1 2 3 4
Highlights Water-based chitosan / melamine polyphosphate nanocoating for fabric.
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Polyester-cotton blend extinguishes immediately with only 12 wt% coating deposited.
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Nanocoating is free of organic solvents and toxic chemicals.
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Chitosan:melamine ratio used to tailor flame retardant behavior and growth of coating.
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Layer-by-layer assembly produces melamine polyphosphate in-situ from water.
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Water-based chitosan / melamine polyphosphate multilayer nanocoating that extinguishes fire on polyester-cotton fabric
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Marcus Leistner, Anas A. Abu-Odeh, Sarah C. Rohmer, Jaime C. Grunlan*
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Anas A. Abu-Odeh E-mail: [email protected]
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Contact information: Marcus Leistner E-mail: [email protected]
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Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA
Sarah C. Rohmer E-mail: [email protected]
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Jaime C. Grunlan* Tel.: +1 9798453027 Fax: +1 9798453081 E-mail: [email protected]
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Full Paper
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Abstract
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Polyester-cotton (PECO) blends are widely used in the textile industry because they combine the
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softness of cotton and the strength and durability of polyester. Unfortunately, both fiber types
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share the disadvantage of being flammable. The layer-by-layer coating technique was used to
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deposit a highly effective flame retardant (melamine polyphosphate) from water onto polyester-
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cotton fabric. Soluble melamine and sodium hexametaphosphate form this water-insoluble flame
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retardant during the coating procedure. This unique nanocoating imparts self-extinguishing
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properties to PECO with only 12 % relative coating weight. Vertical flame testing, pyrolysis
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combustion flow calorimetry (PCFC), thermogravimetric analysis (TGA) and scanning electron
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microscopy were used to evaluate the quality of the coating as well as its flame retardant
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performance. A combination of both condensed and gas phase activity appears to be the reason
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for this effective flame retardancy. Degradation pathways of both cotton and polyester are
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affected by the applied coating, as shown by PCFC and TGA. Use of environmetally benign and
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non-toxic chemicals, and the ease of layer-by-layer deposition, make this coating an industrially
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feasible alternative to render polyester-cotton fabric self-extinguishing.
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Keywords
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Layer-by-layer assembly, chitosan, flame retardant nanocoating, polyester-cotton, melamine
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polyphosphate
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Compounds studied in the article
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Chitsoan (CID: 21896651), melamine (CID: 7955), melamine polyphosphate (CID: 92612),
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poly(ethylene terephthalate) (CID: 16212789), sodium hexametaphosphate (CID: 24968), cotton
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1. Introduction
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Synthetic fibers, such as polyesters, represent the largest component of the textile industry, with
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an increasing demand over the past ten years (Shui & Plastina, 2013). Blends of cotton and
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synthetic fibers are widely used for apparel, particularly workwear. Polyester-cotton (PECO)
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blends are used to combine the comfort and breathability of cotton with the strength and
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durability of the polyester (Day, Suprunchuk & Wiles, 1986). As with most organic polymers, the
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flammability of these textiles can be problematic. Some common ways to reduce flammability of
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PECO fabric are the use of an inherently flame retardant polyester, halogenated flame retardants
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used as additives in the polyester or back-coatings containing either halogenated flame retardants
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or phosphorus compounds (Drevelle et al., 2005; Horrocks & Kandola, 2004; Horrocks, Wang,
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Hall, Sunmonu & Pearson, 2000; Weil & Levchik, 2008). Aliphatic polyesters form volatiles
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rather than char, which leads to melt-dripping during decomposition (Horrocks, 2011). This melt
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flow is an important part of extinguishing pure synthetic textiles, so the introduction of any
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substance into (or onto) the polymer that reduces the melt flow can actually increase
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flammability. Even some print pigments and dyes are known to have this adverse effect (Ozcan,
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Dayioglu & Candan, 2004; Weil & Levchik, 2008).
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Back-coating is often used to create an effective flame retardant coating on textiles and carpets.
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These coatings consist of a curable binder (e.g., acrylic resins) and flame retardant additives such
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as brominated compounds, ammonium polyphosphate (APP) or other phosphorus-based additives
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(Drevelle et al., 2005; Herrlich, Steib & Lang, 2014; Horrocks, Wang, Hall, Sunmonu & Pearson,
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2000). Layer-by-layer (LbL) assembly does not need a matrix, so it offers an opportunity for an
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improved flame retardant treatment. Oppositely charged polyelectrolytes that incorporate the
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desired functionality are deposited onto the substrate. Other interactions, such as hydrogen
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bonding or even covalent bonding can also be used to deposit functional layers onto a substrate
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(Alongi, Carosio & Malucelli, 2014; Borges & Mano, 2014; Broderick & Lynn, 2013). This
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water-based coating technique can be used to modify various properties of substrates such as gas
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permeability, flammability or antistatic and antimicrobial properties (Dvoracek, Sukhonosova,
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Benedik & Grunlan, 2009; Holder, Spears, Huff, Priolo, Harth & Grunlan, 2014; Laufer,
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Kirkland, Morgan & Grunlan, 2013; Park, Ham & Grunlan, 2010). Active flame retardant
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ingredients in LbL coatings can be polyelectrolytes or even particles that are embedded in the
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coating (Laufer, Carosio, Martinez, Camino & Grunlan, 2011; Pan, Wang, Pan, Song, Hu &
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Liew, 2015). Chitosan (CH) in combination with polyphosphates like sodium hexametaphosphate
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(PSP) or APP are well-known layer-by-layer systems that reduce flammability of cotton, but have
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no significant influence on the burning behavior of polyester-cotton blends (Carosio, Alongi &
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Malucelli, 2012; Mateos, Cain & Grunlan, 2014). The use of insoluble and non-melting additives
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for thermoplastics alters the physical properties of the polymer, so the material is less suitable for
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fiber production or other flexible applications.
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The present study involves an effective layer-by-layer coating for polyester-cotton fabric, which
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is completely free of toxic additives and organic solvents. The active ingredients form the coating
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by themselves and the ingredients are environmentally-benign and water-soluble. The addition of
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melamine (Mel) to a CH / PSP multilayer, by including it in the chitosan solution, forms
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insoluble melamine polyphosphate (MPP) during deposition. MPP is known as a flame retardant
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additive for synthetic polymers and it improves the flame retardancy of this LbL nanocoating
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(Burke & Mogul, 2010; Jahromi, Gabriëlse & Braam, 2003; Sullalti, Colonna, Berti, Fiorini &
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Karanam, 2012). With only 12 wt% of this coating deposited on PECO fabric, it self-extinguishes
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immediately in a vertical flame test. Conformal coating of individual fibers, good hand of the
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coated fabric, and ease of the coating procedure make this technology promising for commercial
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use.
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2. Experimental
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2.1 Materials
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Sodium hexametaphosphate (crystalline, 96%), melamine (99%), sodium hydroxide (98%), and
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hydrochloric acid (37%) were purchased from Sigma-Aldrich (Milwaukee, WI). Chitosan
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(60,000 g mol-1, 95 % degree of deacetylation) was purchased from G.T.C. Bio Corporation
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(Qingdao, China). All chemicals were used as received. Polyester-cotton fabric (65%
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polyethylene terephthalate, 4.5 oz yd-2) was obtained from Testfabrics, Inc. (West Pittston, PA).
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The fabric was washed thoroughly in deionized water to remove impurities and dried for 60 min
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at 70°C before use. Deionized water (18 MΩ) was used to prepare all solutions. After complete
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dissolution of the ingredients, the pH of each solution was adjusted to pH 4 using 5 M
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hydrochloric acid or 1 M sodium hydroxide solution. Mixtures of chitosan and melamine were
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prepared by adding melamine to an acidic solution of chitosan (pH < 4) and immediately
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adjusting to pH 4 with 5 M hydrochloric acid.
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2.2 Coating Procedure
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A manual dip coating procedure was used to deposit the flame retardant nanocoating onto the
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fabric. The polyester-cotton fabric was first soaked for 5 min in the solution containing the CH
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polycation, referred to as “cationic solution”, to allow complete wetting and good adhesion of the
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coating to the substrate. All subsequent dips in the PSP solution and the cationic solution
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(producing one bilayer) were 1 min. After each deposition step, the fabric was briefly rinsed in
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deionized water to remove weakly adhered material. After each soak and rinse, the fabric was
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squeezed by hand in order to minimize contamination of the next coating solution or rinse water.
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Each solution and the rinse water were renewed after every 5 bilayers. Once the desired number
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of bilayers were deposited, the fabric was rinsed in deionized water and dried for 60 min at 70°C.
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The weight add-on deposited on the fabric is controlled by the number of bilayers deposited. In
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an effort to achieve the same weight gain with different concentrations, the number of bilayers of
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each recipe was adjusted. Figure 1 shows the general coating procedure and the chemical
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structures of the primary ingredients. All solutions were used at room temperature. Both the
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cationic solution and the PSP solution were adjusted to pH 4, as mentioned above.
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Figure 1. Schematic of the layer-by-layer coating procedure and chemical structures of the flame retardant components.
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2.3 Analytical methods
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Burning behavior of the coated fabric was evaluated with a vertical flame test (VFT), following
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ASTM D6413-08 using a VC-2 vertical flame cabinet (Govmark, Farmingdale, NY). Pyrolysis
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combustion flow calorimetry (PCFC) was performed, according to method A of ASTM D7309, at
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the University of Dayton Research Institute (Dayton, OH), with a heating rate of 1 K s-1 and a
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sample weight of 5 mg. This test revealed the influence of the coating on the fabric’s heat release.
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All flammability tests were performed in triplicate. Thermogravimetric analysis (TGA) was
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performed in nitrogen with a constant heating rate of 20 K min-1 and a sample weight of 15 mg,
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using a TA Q50 TGA (TA Instruments, New Castle, DE). A JEOL (Tokyo, Japan) JSM-7500F
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field emission scanning electron microscope (FE-SEM) was used to image selected samples.
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Fabric samples were sputter-coated with 4 nm of platinum prior to SEM imaging.
164 3. Results and discussion
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3.1 Film growth
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The influence of varying the chitosan:melamine ratio within the cationic solution was evaluated.
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Figure 2a shows the weight gain of 15 bilayers (BL) of CH:Mel / PSP deposited on polyester-
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cotton fabric, as a function of varying the CH concentration in the aqueous solution. The total
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concentration of cationic ingredients (chitosan and melamine) was held constant at 1.4 wt% in
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water and the concentration of sodium hexametaphosphate was 2 wt% in the anionic solution.
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Weight gain on the fabric increases as the ratio of CH:Mel increases (i.e., higher concentration of
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CH in the cationic solution). Melamine and chitosan have competing interactions with PSP.
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While CH forms a complex with PSP [x CHn+ + y PSP6– → (CH)x(PSP)y] that leads to film
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growth, the addition of Mel to this system forms mixed compounds [x CHn+ + y PSP6– + z Mel+
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→ (CH)x(Mel)z(PSP)y] and melamine polyphosphate crystals embedded in the coating [6 Mel+ +
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PSP6– → (Mel)6(PSP)]. While the formation of MPP [(Mel)6(PSP)] is essential to impart flame
178
retardant properties, Mel and PSP alone do not grow layer-by-layer under these conditions. When
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depositing Mel/PSP in the absence of CH, there is no weight gain on the fabric, suggesting no
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film growth. When adjusting the CH:Mel ratio in the cationic solution, there is a tradeoff between
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film growth and flame retardancy. Film growth as a function of the number of deposited layers is
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shown in Figure 2b, using 0.5 wt% CH and 0.9 wt% Mel in the cationic solution. This particular
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CH:Mel ratio is of interest because it imparts the most effective flame retardancy to the fabric, as
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discussed below. Linear film growth was also observed in a recent study depositing CH and PSP
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on cotton fabric and is believed to be due to an interdiffusing polyelectrolyte system (Guin,
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Krecker, Milhorn & Grunlan, 2014). The addition of Mel does not seem to change this growth
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behavior.
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Figure 2. Weight gain as a function of (a) CH concentration (in a cationic solution with Mel) and (b) number of bilayers deposited with PSP.
192
3.2 Flame retardant behavior
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Flame retardant behavior of coated PECO fabric was evaluated with vertical flame testing and
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pyrolysis combustion flow calorimetry. VFT reveals the fabric’s response to an open flame and
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gives some indication of the flame spread after ignition. PCFC reveals the combustion behavior,
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by exposing the sample to a constant heating rate, from an energetic point of view. The fabric
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becomes much more flame retardant as the CH:Mel ratio is decreased in the cationic deposition
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solution. With only a 12 wt% coating, the fabric is self-extinguishing at a CH:Mel ratio of 1:1
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and lower. Figure 3 shows the reduction in char length of coated samples with increasing Mel
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concentration in the cationic solution. More melamine leads to more formation of melamine
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polyphosphate during the coating procedure, which is the most important flame retardant
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component. While CH is mainly used as a binder in this system, to secure the MPP inside the
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film, it also acts as a charring agent. The charring of the 8 BL CH/PSP coating is very clear in
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Figure 3, although it does not self-extinguish in the absence of melamine. The uncoated sample
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on the far left burnt off and left just a melted residue (i.e., no char formation).
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Figure 3. Fabric after vertical flame testing. Each coated sample had 12.5 ± 0.4 wt% deposited. Concentration of melamine in the cationic deposition solution is increasing from left to right. Concentration of PSP in the anionic solution was held constant at 2 wt%.
211
The SEM images in Figure 4 show the surface structure of the coated PECO fabric before and
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after VFT. These micrographs reveal that the LbL coating exhibits less complexation and
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bridging on the fabric’s surface when melamine is present in the coating (Figure 4a). The less
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uniform CH/PSP coating (Figure 4c) impairs the hand of these textiles, making them relatively
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stiff (Guin, Krecker, Milhorn & Grunlan, 2014). With melamine present, the coating contains
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MPP and forms a much denser char (Figure 4b), which decreases the transport of flammable
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volatiles from the substrate to the flame. This dense char can additionally act as a heat shield to
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prevent further degradation of the underlying fabric. In addition to helping char formation,
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melamine can be released at the start of a fire and dilute the gaseous phase by itself and with its
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inflammable decomposition products, such as ammonia and carbon dioxide (Ledeti, Vlase, Vlase,
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Doca, Bercean & Fulias, 2014). This dilution results in a decreased oxygen concentration near the
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fabric surface that reduces the combustion heat and slows the flame spread. Melamine can also
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condense when heated to form intermediates like melam and melem, which polymerize further to
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increase the density of the char by simultaneously releasing ammonia (Wirnhier, Mesch, Senker
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& Schnick, 2013). Melamine and its decomposition products, as well as melamine salts, can also
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alter the decomposition process of polyesters, such as poly(butylene terephthalate) that could lead
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to increased char formation (Balabanovich, 2004; Hoffendahl, Duquesne, Fontaine & Bourbigot,
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2014). As terephthalate polyesters with a linear component of less than 7 CH2 units show similar
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thermal decomposition, comparable effects are expected to occur with the PET used here (Ohtani,
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Kimura & Tsuge, 1986).
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Figure 4. SEM micrographs of polyester-cotton before (top) and after VFT (bottom). The samples were coated with (a,b) 15 BL of CH/PSP and (c,d) 8 BL of CH:Mel/PSP, each with 12.5 ± 0.4 wt% added to the fabric.
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The flame retardant efficiency of these CH:Mel/PSP nanocoatings (also called CH/MPP) is also
236
apparent in terms of energy generation. PCFC shows a significant decrease in heat release with - 11 -
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higher Mel concentration, as shown in Figure 5a. Peak heat release rate (PHRR) and total heat
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release (THR) of the coating using the lowest and highest CH:Mel ratio, along with the uncoated
239
control and fabric coated with only CH/PSP, are summarized in Table 1 (along with key
240
parameters from vertical flame testing). Both test methods show evidence of flame retardant
241
behavior, even with samples that burned up during the VFT. The longer burning time measured
242
in the vertical flame test is the result of slower flame spread caused by insufficient, but
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measurable flame suppression of the coating. The uncoated control sample shows a relatively
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high residue weight after the VFT, without formation of an actual char. This residue is caused by
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the typical melting behavior of the polyester, which leads to self-extinguishment of pure
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polyester fabric. PECO blends do not self-extinguish, because the cotton partially suppresses this
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so-called “runaway effect”. Addition of char forming substances, such as CH and MPP, further
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deteriorates the burning behavior, which leads to the observed decrease in residue. With
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sufficient MPP on the fabric, the runaway effect of polyester can become a char forming flame
250
retardant mechanism, which can be observed using a CH:Mel ratio of 1:1 or less.
251 252
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Table 1. VFT and PCFC results of selected fabric samples, each with 12.5 ± 0.4 wt% coating added. Vertical Flame Test
CH : Mel
conc. (wt%)
BL
burning time (s)
char (in)
residue (%)
n/a
n/a
12 ± 2
no char
49 ± 1
0
8
26 ± 2
entire sample
28 ± 2
0.9 : 0.5
10
33 ± 2
entire sample
31 ± 1
0.5 : 0.9
15
0
4.5 ± 0.5
93 ± 1
1.4 :
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PCFC* -1
PHRR (W g ) 127 at 387°C 242 at 463°C 55 at 311°C 199 at 457°C 27 at 318°C 147 at 428°C 25 at 322°C 171 at 418°C
-1
THR (kJ g ) 15.2 ± 0.2 10.9 ± 0.2 9.4 ± 0.1 9.6 ± 0.1
* PHRR = peak heat release rate, THR = total heat release
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Table 1 shows that the total heat release of all coated fabric is significantly lower than the heat
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release of the uncoated control. A shift to lower decomposition temperature can also be observed
257
(from PCFC) for the cotton portion of the coated fabrics. Increased burning time, lower heat
258
release and altered decomposition temperatures are evidence of flame retardant activity. PCFC
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and TGA reveal complementary information about the decomposition behavior, as shown in
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Figure 5. The CH/PSP coating leads to a strong shift of the cotton decomposition to lower
261
temperature, while the polyester is unaffected. This is due to PSP reacting with cotton, but neither
262
of the coating’s ingredients interact with polyester. The fabric sample coated with CH/MPP shifts
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both heat release peaks to lower temperatures, as shown in Figure 5a. This behavior was
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confirmed by TGA of this sample (Figure 5b). The polyester portion decomposes at a lower
265
temperature as a result of the altered decomposition of polyester in the presence of Mel
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(Balabanovich, 2004). Samples coated with a CH:Mel ratio of either 5:9 or 9:5 in the cationic
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solution have a similar behavior in PCFC measurements, but act differently in the vertical flame
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test. In this flame retardant system, melamine acts primarily in the gas phase through dilution,
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and only a small portion interacts with polyester in the condensed phase. Gas phase effects like
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this are not very well accounted for using PCFC, as this method uses oxygen consumption
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calorimetry to determine the heat of combustion of pyrolysis products. Thermal decomposition of
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MPP is an endothermic process, which creates a heat sink during the burning process.
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Endothermic reactions during the pyrolysis are also not detected with PCFC (Lyon & Walters,
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2004).
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ip t cr Figure 5. (a) Heat release rate as a function of temperature, measured by PCFC, and (b) TGA of uncoated PECO fabric [and samples coated with 12.5 wt% of CH/MPP (CH:Mel ratio was 5:9) or CH/PSP].
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PECO fabric degrades starting at 350°C, with peak rates at 384°C (cotton) and 451°C (polyester).
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If coated with CH/PSP, the degradation of cotton starts at a significantly lower temperature
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(250°C), while the degradation temperature of polyester is unaffected. This is caused by the
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phosphate catalyzed dehydration reaction of cellulosic material like cotton. Pure cotton forms
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laevoglucosan above 300°C, which decomposes further to volatile products. Below 300°C,
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dehydration reactions are preferred, which lead to increased char formation. These dehydration
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reactions are catalyzed by phosphoric acid and cause the lower pyrolysis temperature of the
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cotton part within the blended fabric (Kandola & Horrocks, 1996). Accordingly, an increased
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char formation is observed in TGA of the coated fabric. The entire cotton content of the uncoated
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control (35 wt%) is volatilized, while only a 21 % mass loss is observed with the CH/PSP coated
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fabric. Only 16 wt% volatiles were formed by the pyrolysis of the cotton portion coated with
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CH/MPP. All samples, regardless of coating, exhibit a mass loss due to the polyester portion of
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51 ± 1 %, indicating no significant influence of the coating on the pyrolysis of polyester.
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Differences in the residual weight at 500°C (Figure 5b) confirm this observation, as they are
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equivalent to the differences in weight loss caused by pyrolysis of the cotton portion.
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295 4. Conclusions
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An environmentally benign, water-based nanocoating was deposited layer-by-layer onto
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polyester-cotton fabric. The insoluble flame retardant MPP was formed at ambient conditions
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during the coating procedure by a reaction between melamine in one solution and sodium
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hexametaphosphate in the other. Chitosan serves as a counterpart to PSP in the layering process,
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as well as an additional carbon source for increased char formation. PECO fabric, coated with 15
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bilayers of CH/MPP, was shown to be self-extinguishing during vertical flame testing. A relative
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coating weight of at least 12 % is sufficient to protect PECO in both the condensed and gas
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phases. Char formation by the altered pyrolysis of cotton, caused by the polyphosphate, and
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dilution of the gas phase by released melamine result in an effective flame retardant combination
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to extinguish the ignited fabric. With its ease of deposition and use of relatively non-toxic
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components, this unique coating offers an interesting alternative for fire protection of polyester-
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cotton.
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