Surface Coating for Flame-Retardant Behavior of Cotton Fabric

Article pubs.acs.org/IECR

Surface Coating for Flame-Retardant Behavior of Cotton Fabric Using a Continuous Layer-by-Layer Process SeChin Chang,† Ryan P. Slopek,† Brian Condon,*,† and Jaime C. Grunlan§ †

Southern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture,, New Orleans, Louisiana 70124, United States § Department of Mechanical Engineering, Materials Science and Engineering Program, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: Cotton’s exceptional softness, breathability, and absorbency have made it America’s best-selling textile fiber; however, cotton textiles are generally more combustible than their synthetic counterparts. In this study, a continuous layer-bylayer self-assembly technique was used to deposit polymer−clay nanocoatings on cotton fabrics to enhance their flame retardancy. Alternating layers of positively charged branched polyethylenimine (BPEI) with urea and diammonium phosphate and negatively charged clay nanoparticles were continuously applied to the fabrics in a single process without rinsing. The morphology and flame-retardant properties of the coated fabrics were characterized using scanning electron microscopy (SEM) and a variety of flammability tests. The treated fabrics exhibited improved thermal stability, as evidenced by increased ignition times and lower heat release rates. The results of this study show that flame-retardant nanocoatings can be readily applied to textile fabrics using a continuous process that is ideal for commercial and industrial applications.

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INTRODUCTION According to Cotton Incorporated, cotton is America’s bestselling textile fiber, outselling all synthetic fibers combined. Cotton fiber is widely used to produce apparel, home furnishings, and various industrial products, such as medical supplies, industrial thread, and tarpaulins. Although cotton is prized for its softness, breathability, and ability to absorb moisture, cotton is generally more combustible than most synthetic fibers.1 To meet fire safety regulations and expand the use of cotton in textile applications that require flame resistance, a significant number of flame-retardant treatments for textiles were developed in the second half of the last century. The majority of these flame-retardant treatments can be classified into four distinct groups: inorganic, halogenated organic, organophosphorus, and nitrogen based.2−6 Recent concerns over the toxicological and environmental consequences of using several of these chemical treatments on textiles,4,7−13 which typically have high surface areas and are in close contact with skin, have fueled the search for environmentally friendly alternatives. Recently, the layer-by-layer (LBL) assembly of nanocoatings onto fabrics has attracted a lot of attention as a simple and costeffective method of imparting specific chemical finishes to natural and synthetic fabrics. The LBL process was modernized by G. Decher14−18 and is based on the alternating deposition of charged anionic and cationic polyelectrolytes to create nanometer thin multilayer coatings on the surface of a material via electrostatic interactions. A wide variety of functional molecules can be incorporated within the coatings, including nanoparticles, dyes, and proteins, to modify the physical and chemical properties of the material surface. Although the LBL process is dependent on such parameters as the chemistry of the polyelectrolyte, temperature, and pH,19−27 the process is © 2014 American Chemical Society

not limited by the size, shape, and topography of the material surface, which makes the technique well-suited for tailoring the surface properties of nonplanar of textile fabrics.21,26 In recent years, researchers have used the LBL process to modify the surface of textile fabrics to impart or improve upon numerous surface properties including mechanical integrity,28−30 UV protection,28−32 chemical resistance/reactivity,33 hydrophobicity/hydrophilicity,31,34−37 dyability,38,39 antimicrobial activity,30,40−43 and flame retardancy.28,44−49 Grunlan and co-workers have extensively studied the LBL deposition of clay49 and silica46 nanoparticles with branched polyethylenimine (BPEI) on cotton fabric to produce flame-retardant multilayer coatings. By varying the number of bilayers, the pH of the aqueous BPEI solutions, nanoparticle size, and concentration, the researchers discovered that thicker films with a high loading of nanoparticles were produced when the pH and nanoparticle concentrations were increased. When directly compared to untreated cotton fabrics, most of the coated fabrics showed a flame resistance observation by vertical flame test ASTM standard method D6413-0850 from direct flame. In addition to examining inorganic-based flame-retardant nanocoatings, Grunlan and co-workers used LBL deposition of poly(sodium phosphate) and poly(allylamine) bilayers to produce an environmentally friendly, all-polymer nanocoating capable of extinguishing flame on cotton fabrics.47 Other research groups have employed the LBL technique to create multilayer coatings of α-zirconium phosphate44 and aluminum Received: Revised: Accepted: Published: 3805

November 25, 2013 February 10, 2014 February 20, 2014 February 20, 2014 dx.doi.org/10.1021/ie403992x | Ind. Eng. Chem. Res. 2014, 53, 3805−3812

Industrial & Engineering Chemistry Research

Article

oxide28 nanoplatelets to successfully improve the flame retardancy of both cotton and synthetic fabrics. The nanocoating of textile materials to impart flame retardancy is a relatively new field, and although the use of LBL technology to modify the surfaces of textile fabrics and fibers has been widely studied in recent years, the overall process is still not well understood. Previous LBL flameretardant research has primarily focused on the development and characterization of simple clay/polyelectrolyte thin films with no additional small molecule flame retardants added to the formulation. In addition, current laboratory-based LBL techniques that use multiple immersion/rinse cycles to develop multilayer coatings can be labor-intensive and time-consuming, which often limits the use of the coating technique in commercial applications. In the present study, an attempt was made to improve the flame retardancy of three cotton fabrics, print cloth, mercerized print cloth, and twill, by the deposition of multilayer films containing BPEI, diammonium phosphate, urea, and clay nanoparticles using a newly developed continuous LBL process. Scanning electron microscopy (SEM) measurements were performed to verify the presence of the deposited nanolayers and study the morphology of the various nanocoatings. The imparted flame retardancy of the cotton fabrics due to the character of the deposited clay nanolayers was tested using thermogravimetric analysis (TGA), microscale combustion calorimetry, limiting oxygen index, and vertical flammability testing.

Figure 1. Schematic of the continuous LBL deposition process used to apply the 50 BL clay−BPEI−DAP−urea coatings. Arrows indicate the directional flow of the fabric as it is processed.

Table 1. Fabric Treatment and TGA Data for Treated and Untreated Print Cloth, Mercerized (M) Print Cloth, and Twill Cotton Fabrics

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EXPERIMENTAL PROCEDURES Materials. Print cloth (102 g/m2), mercerized print cloth (110 g/m2), and twill (258 g/m2) cotton fabrics were obtained from Test Fabrics Inc. and used as received. Commercial grade diammonium phosphate (dibasic) and urea were obtained from Magnolia Chemical Co. (New Orleans, LA, USA). Kaolin powder, sodium hydroxide, and BPEI with a molecular weight of 1200 were purchased from Aldrich and used as received. Continuous Layer-by-Layer Coating Process. A modified laboratory pad-steam unit (Mathis, model PSA-HTF) was used to apply the 50 BL clay−BPEI−diammonium phosphate− urea coatings to the print cloth, mercerized print cloth, and twill cotton fabrics in a continuous layer-by-layer process. The steam range of the pad-steam unit was bypassed, and the fabrics were directly loaded onto the rinsing section of the unit, which consisted of two separate rinsing chambers. The fabrics were cut to a width of 16 in. (405 mm) for processing and hand fed through the individual rollers of the rinsing chambers before the ends were sewn together to create a continuous piece of fabric. A schematic of the continuous layer-by-layer process is shown in Figure 1. The first rinsing chamber was filled with 10 L of an aqueous-based 1.5 wt % BPEI solution (cationic) containing 10−20 wt % urea and diammonium phosphate (DAP). The amount of urea and DAP added to the 1.5 wt % BPEI solution was varied to determine the effects of adding nonionic small molecule flame retardants to the nanocoating formulation. The second chamber was filled with 10 L of an aqueous solution of 1.5% Kaolin (anionic) that was brought to a pH of 11 using NaOH. The clay nanoparticles were dispersed in deionized water using a shear mixer (Silverson, model L5MA) set to 3000 rpm, and a stable colloidal solution was obtained after 3 h of mixing. Three different chemical formulations were used in this study, and the detailed contents of each treatment are outlined in Table 1. The multilayer continuous deposition process consisted of two steps, the immersion of the cotton

treatment control

clay and BPEI (1.5 wt % Kaolin, 1.5 wt % BPEI)

formulation 1 (1.5 wt % Kaolin, 1.5 wt % BPEI, 10 wt % urea, and 10 wt % DAP)



fabric sample

add-ons (wt %)

onset of degradation (°C)

char % yield at 600 °C

print cloth M print cloth twill

0

346

1.2

0

345

2.2

0

354

8.0


2.9

340

11.1

2.6

337

11.0

2.2

345

12.5


10.0

277

27.3

9.4

286

21.6

5.8

286

17.1

print cloth M print cloth Ttwill

17.8

275

26.4

16.1

275

26.6

12.7

275

21.6

20.2

273

28.9

18.2

273

26.3

19.5

274

21.5


fabrics in one of the three 1.5 wt % BPEI solutions, immediately followed by immersion of the fabrics in the 1.5% Kaolin solution. The two steps were continuously repeated for each formulation, without rinsing, for a total of 50 cycles using a roller speed of 2 m/min and a pad pressure of 3 bar (300 kPa). Upon completion of the deposition process, the rinsing chambers of the pad-steam unit were drained and the fabric 3806

dx.doi.org/10.1021/ie403992x | Ind. Eng. Chem. Res. 2014, 53, 3805−3812


Article

was cut for removal. The 50 BL coated cotton fabrics were dried at 110 °C and cured at 140 °C via a continuous dryer (Mathis, model KTF-S) set to a speed of 2 m/min. Thermal, Flammability, and Combustion Analysis. The vertical flame test was conducted using a flame control module (Govmark Organization Inc.) according to ASTM standard method D6413-08.50 Photographs of the untreated and ureatreated samples during the vertical flame test were taken with a digital camera (NV3, Samsung). LOI values were measured according to ASTM standard method D2863-0951 using a limiting oxygen index chamber (Dynisco Polymer Test). TGA was measured using a TGA Q500 thermal gravimetric analyzer (TA Instruments). A sample of 5−6 mg was heated from room temperature to 600 °C at a heating rate of 10 °C/min under nitrogen at a flow rate of 60 mL/min. Three runs were performed to obtain average thermal decomposition parameters. Microscale combustion calorimeter (MCC) measurement was performed using a FAA microcalorimeter produced by Fire Testing Technology Limited, United Kingdom, according to the ASTM D7309-07a method. Specimens of fabric weighing approximately 5 mg were heated from 80 to 700 °C at a heating rate 1 °C/s in a stream of nitrogen flowing at 80 mL/min. The combustor temperature was set at 900 °C, and the oxygen/ nitrogen flow rate was set at 20/80. The reported data are averages of three replicate measurements. Surface Analysis of Nanocoatings. SEM was used to observe the microstructure and the surface morphology of the coated and uncoated cotton samples. The instrument was a Phillips XL 30 ESEM with the acceleration voltage set at 12 kV and a beam current of 0.5 nA. The samples were coated with a gold−palladium alloy to provide a 200 Å gold−palladium layer of thickness using a vacuum sputter coater. Both the burned and unburned cotton samples were examined at magnifications ranging from 100× to 1500×.

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RESULTS AND DISCUSSION Fabric Treatment and TGA. Table 1 shows percent addon values following wet pickup, dry, cure, and reconditioning of 50 bilayer coated print cloth, mercerized print cloth, and twill fabric samples. Following application, the treated fabrics appeared colorless as the original fabrics. Formulations 1, 2, and 3 showed 10.0, 17.8, and 20.2 wt % add-ons for print cloth, 9.4, 16.1, and 18.2 wt % add-ons for mercerized print cloth, and 5.8, 12.7, and 19.5 wt % add-ons for twill fabrics, respectively. When treated with clay and BPEI alone, the percent add-ons were significantly reduced, yielding 2.9, 2.6, and 2.2 wt % addons for the print cloth, mercerized print cloth, and twill fabrics, respectively. The majority of add-ons appear to be the result of adding DAP and urea to the BPEI solution. In fact, when the fabrics were treated with an aqueous solution of 20 wt % DAP and urea, the percent add-ons ranged from 15.5 to 17.3% depending on the fabric. At this stage no attempt was made to test the durability of the flame-resistant formulations with multiple laundering tests. In addition to the values of add-ons for the three different formulations, Table 1 shows the TGA values for degradation onset temperature (°C) and char yield (% at 600 °C), obtained in nitrogen atmospheres, for untreated and treated cotton fabrics. All untreated cotton fabrics degraded between 345 and 354 °C by TGA and showed char residues between 1.2 and 8.0% of original weight after 600 °C. As in Figure 2, treating the cotton with clay and BPEI alone increased the char yield, but did not significantly alter the overall degradation curves of the

Figure 2. Heat release rate versus temperature curves of the untreated and treated cotton (a) print cloth, (b) mercerized print cloth, and (c) twill fabrics coated with 50 bilayers of polyelectrolyte−clay nanoparticles at different concentrations.

fabrics. According to Figure 2a, however, when the print cloth was treated with the various formulations and degraded, it showed onsets of degradation between 273 and 277 °C and provided char yields between 26.4 and 28.9%. Mercerized print cloth (Figure 2b) degraded between 273 and 286 °C and showed char yields between 21.6 and 26.6%. Treated twill fabric (Figure 2c) degraded between 274 and 286 °C and provided char residues between 17.1 and 21.5% of original weight. Decomposition temperatures of treated fabrics were 3807



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Table 2. Vertical Flame (ASTM D6413-08) and Limiting Oxygen Index (ASTM D2863-09) Tests of Untreated and Treated Fabricsa

a

Add-ons data are single-observation values.

5 cm line during the LOI test for all samples are provided. LOI values of untreated fabrics are shown between 18 ± 5 and 19 ± 5% oxygen in nitrogen. LOI data for treated fabrics are shown between 23 ± 5 and 40 ± 5%. Fabrics treated with clay and BPEI in the absence of added flame retardants yielded a slight increase in LOI values compared to the control samples. Print cloth treated with the three formulations containing DAP and urea provided LOI values of 28 ± 5, 38 ± 5, and 39 ± 5% when add-on values were 10.0, 17.8, and 20.2 wt %. For treated mercerized print cloth, LOI values are very similar to those of treated print cloth, 29 ± 5, 38 ± 5, and 40 ± 5%, when add-on values were 9.4, 16.1, and 18.2 wt %. Additionally, treated twill fabric has also a high LOI values such as 27 ± 5, 36 ± 5, and 38 ± 5% when add-on values are 5.8, 12.7, and 19.5 wt %. For each type of fabric construction, the values for LOI increase in Table 2 before reaching a limiting value with respect to add-on, which strengthens the case for concentration of formulations that afford both higher phosphorus and nitrogen content and crosslinking of polymer for improved flame resistance. Convinced that formulations with our new monomers will afford flame resistance to fabrics of different constructions, we tested the treated print cloth, mercerized print cloth, and twill fabrics by the vertical flame test (ASTM D6413-08) and observed gratifying results (Table 2). Following a 12 s exposure to flame, most of the treated fabric samples showed no observable after-flame or after-glow times and char lengths were less than half the sample length. Most add-on samples except

lower than those of untreated fabrics. This may be due to the phosphonic acid derivatives that might accelerate fabric degradation. From the data in Table 1 it is evident that the char yields generated by high add-ons (wt %) are 10−20% greater than those produced by low add-ons (wt %) due to the higher phosphorus and nitrogen contents. Chars protect against heat and flame propagation because they generate thermally stable cohesive phases having decomposition temperatures that exceed the temperatures of the oxidizing zones of flames. Furthermore, chars are intumescing; they foam and release gases that suppress flammability. Therefore, it is very important to design flame retardants that support intumescing and char formation.50−52 Limiting Oxygen Indices and Vertical Flame Tests of Fabrics. In the LOI (ASTM D2863-09) test, LOI values indicate the minimum amount of oxygen needed to sustain a candle-like flame when a sample is burned in an atmosphere of oxygen and nitrogen. Textiles are considered to be flammable when LOI values are below 21% oxygen in nitrogen and are considered to be flame-retardant when LOI values fall in the range of 26−28%. At these LOI values flame-retarded test fabric samples are expected to pass open flame tests in either the horizontal or vertical direction.50−52 Passing an open flame test means that the ignited test sample self-extinguished following a very short after-flame time; the sample did not glow after the flame extinguished by itself. Table 2 shows LOI and vertical flame test data. The average LOI values and time to burn to the 3808



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mercerized print cloth 9.4% add-on sample passed the vertical flame test when add-on values were 10.0−20.2 wt % add-ons for print cloth, 16.1−18.2 wt % add-ons for mercerized print cloth, and 5.8−19.5 wt % add-ons for twill fabrics. In these cases the char lengths of treated fabrics that passed the vertical flame test were

Surface Coating for Flame-Retardant Behavior of Cotton Fabric

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