Citric acid based durable and sustainable flame retardant treatment for lyocell fabric

Carbohydrate Polymers 153 (2016) 78–88

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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Citric acid based durable and sustainable flame retardant treatment for lyocell fabric Naveed Mengal a,b , Uzma Syed b , Samander Ali Malik b , Iftikhar Ali Sahito a,b , Sung Hoon Jeong a,∗ a b

Department of Organic and Nano Engineering, Hanyang University, Seoul 133-791, Republic of Korea Department of Textile Engineering, Mehran University of Engineering & Technology, Jamshoro, 76062, Pakistan

a r t i c l e

i n f o

Article history: Received 28 March 2016 Received in revised form 15 July 2016 Accepted 18 July 2016 Available online 19 July 2016 Keywords: Flame retardant Sustainable Lyocell Easy care finishes Cross-linker Citric acid

a b s t r a c t Pyrovatex CP New, is a commonly used organophosphorus based flame retardant (FR) reagent for cellulosic materials. However, it has a drawback of high formaldehyde release when used with methylated melamine (MM) based cross-linker, a known carcinogenous compound. In the present approach, a durable and sustainable flame retarding recipe formulation for lyocell fabrics is developed using citric acid (CA) as a cross-linker. The FR finish was applied by pad-dry-cure process. The treated fabrics were characterized for surface morphology, elemental analysis, TG analysis, char study and FT-IR spectroscopy. Furthermore, flame retardancy, washing durability, formaldehyde release and breaking strength were also assessed, and compared with the conventional MM based FR recipe. The fabric samples treated with 400 g L−1 of FR with either 40 or 80 g L−1 of CA demonstrate flame retardancy even after 10 washing cycles. Furthermore, a 75% reduction in formaldehyde release is achieved. Higher char yield and lower decomposition temperature are found compared to untreated and FR+ MM treated lyocell. Such an improved sustainable recipe formulation can be used for lyocell fabric without any health risk in apparel wear. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Lyocell fiber has a unique semi-micro fibrillar structure, which gives it excellent moisture transportation and comfort properties (Bharathi Yazhini & Gurumallesh Prabu, 2015; Kwon et al., 2014; Zhang, Okubayashi, & Bechtold, 2005). Because of environmental sustainability, and excellent aesthetic properties, there has been a gradual increase in its apparel and home textile market share. However, like other cellulosic fibers, lyocell is also prone to catch fire and burns into ash, causing serious damage to any nearby material. Nevertheless, such accidents can be avoided by treating apparel with flame retardant (FR) chemical finishes. However, most finishing agents have been found to have adverse effects on the environment and human skin because of carcinogenous chemical groups in their structure (Horrocks, 2011; Malucelli et al., 2014). In order to achieve durable flame retarding performance on cellulosic fibers, chemicals such as tetrakis (hydroxymethyl) phosphonium salt (THPX) and reactive organophosphorus based N-methylol dimethylphosphonopropionamide (N-MDMPA) have

∗ Corresponding author. E-mail address: [email protected] (S.H. Jeong). http://dx.doi.org/10.1016/j.carbpol.2016.07.074 0144-8617/© 2016 Elsevier Ltd. All rights reserved.

been widely used (Yang, Wu & Xu, 2005). However, TPHX is very expensive and involves the release of ammonia fumes and also requires an ammoniation chamber for a controlled chemical process. On the other hand, N-MDMPA involves the use of a MM based cross-linking agent which can lead to high levels of formaldehyde release during the finishing process and in the use as well ´ 2012). Increased amount of formalde(Bischof-Vukuˇsic´ & Katovic, hyde content, pose a serious threat and can potentially lead to skin cancer and other respiratory diseases. Many countries have banned FR finished apparel that does not comply with local and international standards with respect to formaldehyde content. One such international standard is OEKO-TEX® standard 100, which categorizes the finished textile article according to the amount of formaldehyde present. According to this standard, the allowable limit of formaldehyde content for babies wear fabric is less than 16 ppm; direct skin contact apparel is 75 ppm or mg/kg of fabric, and for fabrics, not in direct contact, such as curtains and upholstery, the level is 300 ppm (Anonymous, 2016b). This stringent criterion stimulated the development of FR recipe formulations with zero associated safety and health implications (Horrocks, 2011). In the present study, the flame retardant properties of lyocell fabric using a polycarboxylic acid based cross-linker in the FR

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Table 1 FR finishing recipes for lyocell fabric using Pyrovatex CP New, MM and CA. Chemical Used

Pyrovatex CP New MM PA Add-on%

Recipes using MM as cross-linker M1 [g L−1 ]

M2 [g L−1 ]

M3 [g L−1 ]

M4 [g L−1 ]

400 75 20 23

400 37.5 20 20.7

200 75 20 17.2

200 37.5 20 13.6

C1 [g L−1 ]

C2 [g L−1 ]

C3 [g L−1 ]

C4 [g L−1 ]

400 80 64 20 19.3

400 40 32 20 19

200 80 64 20 18

200 40 32 20 15

Recipes using CA as cross-linker

Pyrovatex CP New CA SHP PA Add-on%

recipe were assessed. Different polycarboxylic acids have been reported as non-formaldehyde cross linking agents with Sodium hypophosphite (SHP, a phosphorus based salt), as the most effective catalyst (Huang, Xing, Yu, Shang, & Dai, 2011; Yang, He & Voncina, 2011). Blanchard et al. achieved flame retardancy in cotton carpet and cotton/polyester blended fleece by using polycarboxylic acids (Blanchard & Graves, 2002, 2005). Mohsin et al. reported durable flame retardancy in cotton fabric by using carboxylic based crosslinker (Mohsin, Ahmad, Khatri, & Zahid, 2013). However, these studies did not quantify the level of formaldehyde in the FR finished fabric, which is very demanding criterion from a consumer point of view. Recently, a few novel approaches for sustainable flame retardancy of cellulosic substrate have been reported using layer by layer assembly (Carosio & Alongi, 2015), the use of spinach herbal extract (Basak, Samanta, & Chattopadhyay, 2015), green coconut shell extract (Basak, Patil, Shaikh, & Samanta, 2016) and banana pseudostem sap (Basak, Saxena, Chattopadhyay, Narkar, & Mahangade, 2015). Similarly, the efficacy of low cost, eco-friendly CA (a polycarboxylic acid) as a cross-linker has also been proven for durable press (DP) finishing (Yao, Wang, Ye, & Yang, 2013) and ´ 2009). However, to the FR treatment of paper (Grgac, Lozo, & Banic, best of our knowledge critic acid as cross-linker has not been used for the flame retardancy of lyocell woven fabric. Herein, the ecological, physical and thermal properties of lyocell fabric treated with flame retardant formulations comprising of CA and MM have been investigated and compared in order to have better understanding of these cross-linkers and their effect on the flame retardancy, thermal stability, and formaldehyde content of treated fabric. 2. Experimental 2.1. Materials Desized lyocell (STANDARD TENCEL® ) plain woven fabric with construction 30 × 30 (Ne)/93 × 75 (per inch) weighing 140 g m−2 supplied by Lenzing AG, Austria was used as received. Pyrovatex CP New, an organophosphorus based flame retardant (FR) and Knittex® CHN, a MM based cross-linker were supplied by SwissSpecialty Chemicals, Pakistan. Whereas, Sigma-Aldrich reagent grade CA (99% purity), SHP (99% purity) and phosphoric acid (PA, conc. 85%) were purchased from Al-Beruni chemicals, Hyderabad, Pakistan. 2.2. FR treatment on lyocell fabric The lyocell fabric samples were treated with different recipe compositions of FR agent, cross-linking agents, catalysts as shown

in Table 1. The recipes containing MM are represented as M1, M2, M3 and M4, whereas C1, C2, C3 and C4 denote recipe formulations containing CA. The purpose of varying the concentrations of recipe ingredients was to optimise the actual amount of ingredients. In all CA containing recipe formulations the ratio between CA and SHP was maintained to 1:0.8 as suggested by (Mohsin et al., 2013). In addition, an equal quantity of PA has been used in all recipe formulations to initiate the reaction and to enhance the performance of FR agent. Single bath conventional pad-dry-cure method was used for the treatment of recipe on to fabric sample. Each sample was dipped and padded with the recipe formulation (FR + cross linking agent + catalysts) using Rapid® horizontal padders until a wet pick-up of 80% was achieved. After the chemical padding, the treated samples were dried in Rapid® mini dryer at 110 ◦ C for 90 s followed by a curing step in the same machine. The curing was performed at 150 ◦ C for 300 s for (FR + MM) samples and for (FR + CA) samples at 180 ◦ C for 90 s respectively. Afterward, the samples were rinsed with deionized water (DI) at room temperature and dried at 70 ◦ C for 5 min. The real uptake of recipe chemicals (add-on%) on fabric after the aforementioned treatment was calculated using Eq. (1) and the results are presented in Table 1. Add-on% = [WF − WI /WI ] × 100

(1)

In Eq. (1), WF is the final oven dried weight of treated sample, and WI is the initial oven dried weight of control (untreated) sample. The chemicals add-on% on the sample was maximum (23%) using recipe M1, indicating excellent bonding of FR with cellulose structure in the presence of an MM cross-linker, the add-on% decreased as the quantity of FR and MM was reduced. A similar trend was observed with FR + CA samples where sample using recipe C1 has a maximum add-on% (19.3%) among all FR + CA samples. Chemical add-on% measurement was beneficial in forecasting the flammability behaviour of different formulations. 3. Characterization 3.1. Flammability test The flammability of fabric samples were assessed using Govmark 45◦ flammability tester in accordance to ASTM D 1230-94, re-approved in 2001. It was observed that untreated lyocell fabric was not burned at 1 s standard impingement time, consequently, it was adjusted to 5 s, and all treated samples were tested using this adjusted impingement time (5 s). Subsequently with the adjustment in impingement time, amendment in classification of sample’s flammability characteristics was done, similar to other researchers (Lam, Kan, & Yuen, 2011; Poon & Kan, 2015; Siriviriyanun, Edgar, & Yanumet, 2008). Detail of sample flamma-

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bility classification is presented in section 1.1 of Supplementary information. The wash durability of FR treated fabric was assessed at zero wash (before washing), after 5 and 10 wash cycles, the specimens were laundered in accordance to standard ISO 6330- 4A in Electrolux® wash cater, a horizontal rotating drum type machine. The standard deviation (SD) for char length is presented in Table 2.

3.5. Formaldehyde content measurement To meet the international standard requirement for formaldehyde content, FR finished specimens were assessed for free formaldehyde content by the guidelines given in ISO 14184-1:2011 standard at zero wash, after 05 and 10 washing cycles. 3.6. Breaking strength measurement

3.2. Thermal stability analysis Thermogravimetric (TG) analysis was performed to measure the pyrolysis and thermal decomposition phenomenon of fabric samples using SDT Q600 Thermogravimetric analyser, TA instruments (USA) from ambient temperature to 800 ◦ C. The heating rate was set at a heating rate of 20 ◦ C min−1 under nitrogen and air atmosphere in 100 mL min−1 flow rate. The test data collected included: Tonset10% (temperature at 10% of weight loss) and char residue ◦ (%) at 700 C. Derivative thermogravimetric (dTG) data was also acquired to calculate the Tmax (temperature at maximum weight loss) and the residue at Tmax . All the collected data is presented in Tables 3 and 4. 3.3. Surface morphology and elemental analysis

The universal strength tester (Titan1 ) by James H. Heal & Co. was utilized to evaluate the breaking strength of FR treated lyocell fabric according to standard ASTM D 5035-95 – ravelled strip method (reapproved in 2003). The result and discussion of which is given in section 1.2 of Supplementary information. 3.7. Fabric whiteness Data colour spectrometer was used to measure the whiteness of treated samples using light reflectance method. Whiteness was measured at zero, 5 and 10 wash cycles and the analysis were made by comparing the CIE whiteness index of treated samples with the CIE whiteness index of the control sample (untreated lyocell). 4. Results and discussion

Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) was carried out for the surface morphology, elemental analysis, P and N mapping of untreated and treated samples using Nova NanoSEM 450 equipped with TEAMTM EDS analysis system by In-TEC Co. USA. The EDS of the samples was conducted at a beam energy of 15 KeV. SEM was also used for char analysis of samples after flammability test. 3.4. Chemical structure analysis The FT-IR assessment of untreated and treated lyocell fabric samples for chemical structure was done by using NicoletTM iSTM 10 FT-IR spectrometer from Thermo Fisher Scientific Inc., USA with attenuated total reflectance (ATR) mode.

4.1. Flame retardancy Fabric treated with Pyrovatex CP New with either of the crosslinkers, such as MM or CA, exhibited considerable flame retardancy even after 10 wash cycles (refer Table 2) when compared to control sample (C) that burned completely in 12 s. In Table 2 for simplicity, the samples treated with different recipe formulations are designated with sample codes by including the letter ‘L’ as the prefix to the recipe codes already given in Table 1. The results reported in Table 2 reveal that the samples LC2 and LC1 treated with 400 g L−1 of Pyrovatex CP New with either 40 or 80 g L−1 of CA completely retarded the flame. In addition, no spread of flame has been found from impingement point; even after 10 wash cycles the specimen has only 3 cm spot of solid carbonaceous char on the fabric surface

Table 2 Flammability characteristics of untreated and treated lyocell fabric by 45◦ flammability test. Sample code

Sample description

No. of samples

Burning time [s]

Char length [mm] ± SD

Classification

C LC1

Control sample At zero wash After 5 wash cycles After 10 wash cycles At zero wash After 5 wash cycles After 10 wash cycles At zero wash After 5 wash cycles After 10 wash cycles At zero wash After 5 wash cycles After 10 wash cycles At zero wash After 5 wash cycles After 10 wash cycles At zero wash After 5 wash cycles After 10 wash cycles At zero wash After 5 wash cycles After 10 wash cycles At zero wash After 5 wash cycles After 10 wash cycles

05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05 05

11.66, DNI DNI DNI DNI DNI DNI IBE, 37.33 34.00 31.32 IBE, 36.30 34.00 31.00 DNI DNI DNI DNI DNI DNI DNI DNI DNI DNI IBE, 35.00 IBE, 14.22

Completely burned 22 ± 1 26 ± 3 35 ± 4 22 ± 2 27 ± 6 36 ± 7 82 ± 6 130 ± 3 135 ± 0 85 ± 5 130 ± 4 135 ± 0 20 ± 3 26 ± 2 26 ± 4 20 ± 3 29 ± 4 31± 3 30 ± 4 32 ± 4 36 ± 5 34 ± 3 85 ± 5 105 ± 6

Class 3 Class 1 Class 1 Class 1 Class 1 Class 1 Class 1 Class 1 Class 3 Class 3 Class 1 Class 3 Class 3 Class 1 Class 1 Class 1 Class 1 Class 1 Class 1 Class 1 Class 1 Class 1 Class 1 Class 1 Class 1

LC2

LC3

LC4

LM1

LM2

LM3

LM4

FR measurements: DNI = did not ignite, IBE = Ignite but extinguished, Class 1 = Pass/Normal flammability, Class 3 = Fail/Dangerously flammable, SD = Standard deviation.

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Scheme 1. Cross linking mechanism of CA with Pyrovatex CP New and lyocell.

as presented in Supplementary information Fig. S1. This appreciable washing durability of sample may be attributed to effective ester crosslinking by CA between FR and lyocell fabric, also illustrated by a reaction mechanism given in Scheme 1, which is also reported elsewhere (Mohsin et al., 2013). In addition, it is an acknowledged fact that the organophosphorus based FRs, when applied to cellulose slow down the burning process and produce carbonaceous char and water (Horrocks, 1983). This residual carbonaceous char forms an insulating layer around the fiber and protect it from heat and discourages the burning as the igniting source is moved away from test specimen (Siriviriyanun et al., 2008). This is also obvious from the SEM micrograph of the char residue for LC1 sample in Fig. 4. Furthermore, the samples LC3 and LC4 kindled initially, extinguished giving a scorched length of approx. 85 mm in 37 s, achieving a Class 1 classification. However, LC3 and LC4 burned progressively to the specified length of 130–135 mm after 5 and 10 wash cycles. Thus, by following the explanation given in section 1.1 of Supplementary information, samples LC3 and LC4 are marked as Class 3 (i.e., fail to pass the FR test). Samples LC3 and LC4 have inferior flame retardancy due to lesser concentration of FR agent in their recipes while the lesser amount of CA in LC4 also resulted in reduced number of ester linkages between the lyocell substrate and the FR agent. Samples treated with MM based cross-linker demonstrate effective flame retardancy in inhibiting the flame efficiently as stated in Table 2. All samples (LM1-LM4) have resistance to catch ignition at zero wash cycle. The samples LM1-LM3 prevented ignition even after ten wash cycles; however, sample M4 initially caught fire and was then extinguished giving a burnt length of 85 & 105 mm after 5 and 10 wash cycles, respectively. This performance of sample LM4 may be attributed to the lesser quantities of FR and cross-linker in the recipe that results in lesser covalent bonds between the fiber OH groups and the FR agent. The overall appreciable washing durability in the sample LM1-LM4 is due to the presence of N-methylol reactive groups in the structure of Pyrovatex CP New that form covalent bonds with the hydroxyl groups of cellulose as presented in Fig. S2 of Supplementary information, also previously reported by (Poon & Kan, 2015). The covalent bonds have high resistance to hydrolysis during laundering. Therefore, samples LM1-LM3 exhibit admirable

flame retarding durability even after 10 wash cycles. Furthermore, the use MM based cross-linker in the FR recipe also increases the number of covalent bonds with cellulose substrate in the presence of PA. Presence of nitrogen in MM also improves flame retardancy of substrate through Phosphorus-Nitrogen synergism (Wu & Yang, 2004; Wu & Yang, 2006). 4.2. Thermal stability analysis The pyrolysis and thermal degradation phenomenon of treated lyocell fabric samples was investigated using TG analysis at zero wash and 10 wash cycles. It was compared with that of control sample as shown in Fig. 1. The obtained TG and dTG data are presented in Table 3 and Table 4, respectively. Under the nitrogen atmosphere, cellulose was progressively degraded through dehydration and depolymerization pathways as depicted in the TG curves in Fig. 1a. The TG curve pattern of the control sample (C) in Fig. 1a displays that the sample experienced a 10% gradual weight loss up to 313 ◦ C (given as Tonset10% in Table 3), corresponding to the loss of water molecules, usually held by the fabric at room temperature (moisture content). Later, as the pyrolysis temperature was increased, the weight loss accelerated, giving a weight loss of 60% (Fig. 1b, Table 3) at 365 ◦ C (Tmax ) mainly due to the decomposition of the glycosyl unit and formation of flammable volatiles, generally levoglucosan. These formed flammable compounds further accelerated the pyrolysis behavior until the sample was completely converted in carbonaceous char with a residue of 5.8% at 700 ◦ C (Fig. 1a, Table 3). In contrast, the presence of coating on treated samples significantly changed the degradation profile (Fig. 1a). The TG curve of the treated lyocell fabric either with CA or MM formulations showed two stages of degradation. The first stage of degradation occurred due to catalyzed dehydration and the presence of FR coating, which lowered the sample’s decomposition temperature as compared to the control sample (refer to Tonset10% in Table 3 and dTG curves in Fig. 1b). This low decomposition temperature of the treated samples may be attributed to the fact that the flame retardant coating decomposes prior to the substrate decomposition to interfere with the burning process (Nehra, Hanumansetty, Edgar, & Dahiya, 2014).

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Fig. 1. TGA and dTG curves of the untreated and treated samples (a, b) under N2 atmosphere, and (c, d) under air atmosphere.

Table 3 TG data of the untreated and treated lyocell fabric at zero and after 10 wash cycles in nitrogen atmosphere. Sample C LC1 LC2 LM1 LM2 LC1 LC2 LM1 LM2 a

Washing stage

At zero Wash

After 10 wash cycles

Tonset10% [◦ C]

Tmax a [◦ C]

Weight loss [%] at Tmax

Char residue at 700 ◦ C [%]

313 251 265 264 270 273 234 263 260

365 302 303 301 303 320 325 309 306

60 29.5 32 24 26 31 53 37 35

5.8 43.8 33.5 42.2 40.2 40.9 24.2 38.8 37.7

From the derivative curves.

Table 4 TG data of untreated and treated lyocell fabric at zero and after 10 wash cycles in air atmosphere. Sample C LC1 LC2 LM1 LM2 LC1 LC2 LM1 LM2 a

Washing stage

At zero Wash

After 10 wash cycles

From derivative curves.

T onset10% [◦ C]

Tmax1 a [◦ C]

Weight loss [%] at Tmax

Tmax2 a [◦ C]

Char at 700 ◦ C [%]

305 266 261 268 269 281 283 261 259

336 301 302 299 302 321 318 305 308

47.6 26.1 30.1 26.6 28.7 32.2 33.6 28.3 31.7

474 530 529 576 564 518 523 536 530

4.6 35.4 19.1 21.6 17.6 25.2 11.8 14.9 10.6

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Fig. 2. SEM micrograph of: (a) untreated lyocell, (b) LC1 at zero wash, (c) LM1 at zero wash, (d) LC1 after 10 wash cycles and (e) LM1 after 10 wash cycles.

In the second stage, the phosphorus present on the sample surface underwent further phosphorylation and dephosphorylation (condensed phase flame retardant activity), causing a reduction in the formation of flammable volatiles and enhancing the generation of thermally stable char (Gaan & Sun, 2007). Amongst all the samples, LC1 remarkably resisted the combustion process and rendered in 43.8% char residue at 700 ◦ C, which is 750% more than the control sample and slightly higher than that of the LM1 residue (42.2%). The treated samples with the same formulations were also assessed for TG analysis after 10 wash cycles. All the samples showed appreciable thermal stability with slightly less char residue (at 700 ◦ C) when compared to the zero wash samples (see Table 3). The better flame retardancy to LC1 may be attributed to the higher amount of phosphorus content (due to the addition of SHP) and its effective crosslinking to the cellulose by CA. Furthermore, As far as thermo-oxidative stability (under air environment) is concerned, the TG and dTG curves of sample C demonstrated that the thermal degradation of cellulose occurred in a two main steps; one is the breakdown of glyosidic linkages and the second involves dehydration, decarboxylation and decarbonilation (Scheirs, Camino, & Tumiatti, 2001). The TG curve of sample C illustrates that it degraded completely with no significant char residue. In oxidative atmosphere, the dehydration and depolymerization came about simultaneously but at comparatively lower decomposition temperature (i.e. 336 ◦ C vs 365 ◦ C in nitrogen), which is also

◦

obvious from the first sharp peak in dTG curve at 336 C in Fig. 1d. The carbonaceous char formed at this step is further oxidized to form CO and CO2 at high temperatures (468 ◦ C in Fig. 1c) giving a second sharp but relatively small dTG peak in Fig. 1d. In contrast, the presence of coating on the treated sample strongly sensitized the cellulose decomposition and favored the char formation, apparent from significant reduction in Tonset10% and Tmax values (Table 4). This char formation greatly slowed down the combustion process and shifted it toward a higher temperature, as evident from small bumpy curves in the range of 510–580 ◦ C (Tmax2 ) as shown in Fig. 1d and Table 4. Here, the sample LC1 once again performed exceptionally well by giving a char residue of 35.4% at 700 ◦ C as compared to the control sample (4.6%) and LM1 (21.6%). Similarly, in air too, the thermal stability of the treated samples after 10 wash cycles was commendable owing to successful binding of the char enrichment species to the cellulose fabric. This acted to protect the fiber against the thermal activity and resulted in 25.2% char residue. 4.3. Surface morphology and elemental analysis The morphology of untreated, treated samples at zero wash and after 10 wash cycles were examined using SEM (Fig. 2). The smooth rod like appearance without any striations in Fig. 2a reveals the fibrous structure to be untreated lyocell fiber. Fig. 2b displays a uniform surface coating (Pyrovatex CP New + CA) on lyocell fiber in

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Fig. 3. EDS P, N mapping and elemental spectral profile of (a) Control sample (b) LC1 at zero wash (c) LC1 after 10 wash cycles (d) LC1 at zero wash cycles and (e) LM1 after 10 wash cycles.

sample LC1. However, the surface is rougher and shows a bumpy appearance compared to the untreated sample. This surface roughness along the fiber longitudinal axis may be ascribed to damage cause by the use of PA during the finishing treatment. Similarly, In Fig. 2c the presence of coating (Pyrovatex CP New + MM) is obvious from change in fiber’s surface texture in sample LM1. Correspondingly, Fig. 2(d, e) depict the surface characteristics of FR treated lyocell for CA and MM after 10 wash cycles respectively, confirming the presence of coating on the fiber surface, though the quantity of coating seems diminished because of successive washing cycles. From the SEM micrographs assessment (Fig. 2), it is clear that the FR formulations were deposited evenly on the fiber surface and remained attached to the surface even after 10 wash cycles. In order to further support this finding, phosphorus (P) and nitrogen (N) mapping was carried out using energy dispersive spectroscopy (EDS), as presented in Fig. 3. It can be noticeably seen from P and N mapping in Fig. 3a that the coating covers the fiber surface uniformly and the P and N distribution was homogenous. Fig. 3(b, c) represent the coating consisting of CA at zero and after 10 wash cycles, whereas Fig. 3(d, e) illustrates the coating comprising of the MM cross-linker at zero and after 10 wash cycles, respectively. From the Phosphorus mapping analysis presented in Fig. 3, the uniform adherence of the phosphorus on fiber is confirmed with both

recipe formulations even after 10 wash cycles (verified by the presence of P). Fig. 3 also consists of the elemental spectral profile for untreated and treated samples as shown in the far right (Fig. 3a–e). As expected, the untreated sample had large presence of carbon and oxygen atoms (Fig. 3a). However, in the treated samples, the atomic peaks corresponding to nitrogen and phosphorus are easily visible in Fig. 3(b–e). In addition, it was noticed that the phosphorus peak height was reduced after 10 wash cycles (Fig. 3c & e) when compared to the zero washed treated sample (Fig. 3b & d) due to removal of the unreacted surface finish. Moreover, elemental analysis was also carried using SEM-EDS. The elemental analysis technique provides an insight to the surface composition of materials. Table 5 lists the amount of these elements (atomic%) for untreated lyocell and selected samples at zero wash and after 10 wash cycles. In control sample, negligible traces of phosphorus are found, as it does not contain any phosphorus contents. Only carbon and oxygen were found as its main constituents. The amount of phosphorus observed in LC1 was 2.93% and after 10 wash cycles it remained 2.07%. Similarly for LM1 phosphorus, the content was 2.61% and after 10 wash cycles it became 1.96%. The decrease in phosphorus content in treated lyocell fabric after 10 wash cycle is due to removal of unreacted monomer or oligomers from the fabric surface by intense washing. In addi-

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Fig. 4. Photographic images and SEM micrographs of char residue of (a) LC1 at zero wash (b) LC1after 10 wash cycles (c) LM1 at zero wash and (d) LM1after 10 wash cycles.

Table 5 EDS elemental analysis for control and treated samples at different wash cycles. Sample

Control LC1 LM1 LC1 LM1

Washing stage

Zero wash After 10 washing cycles

Elements Carbon

Oxygen

Nitrogen

Phosphorus

Atomic (%)

Atomic (%)

Atomic (%)

Atomic (%)

44.12 51.13 53.71 54.27 55.14

42.32 31.43 26.22 28.99 28.72

13.26 14.51 17.46 14.68 14.18

0.3 2.93 2.61 2.07 1.96

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Fig. 5. FTIR-ATR of untreated and the treated lyocell with FR + CA at zero wash and after 10 wash cycles.

tion, the high amount of phosphorus content in LC1 is as result of SHP in the recipe formulation. This increased amount of phosphorus enhanced the flame retardant characteristics of treated lyocell, also witnessed during flammability and Thermogravimetric analysis. Moreover, in LM1 sample, the amount of nitrogen was higher than LC1 because of the greater nitrogen presence in MM. From the SEM micrographs and EDS analysis, the formation of the thin layer of phosphorus is confirmed on lyocell fabric.

4.4. Char analysis In order to deeply investigate the mechanism of flame inhibition in the treated lyocell sample, the char morphologies were studied using SEM at 200 x and 1500 x magnifications. The SEM analysis was carried out for both the recipe formulations at zero wash and after 10 wash cycles. In Fig. 4a, the SEM micrographs of sample LC1 char reveals that the coating transformed the cellulose (lyocell) into phosphorylated cellulose that resisted fiber depolymerization into volatile species, but underwent catalytic dehydration at fiber surface finally resulting in formation of carbonaceous char. This formed char served as a protecting layer and delayed the fiber degradation during burning, also evident from the swollen fibrous structure in Fig. 4a. However, in Fig. 4b, the char residue length of LC1 was slightly higher than that in Fig. 4a, suggesting the removal of unattached oligomers due to 10 washing cycles and causing more damage to the fibrous structure (evident from the SEM micrograph, Fig. 4b). A similar phenomenon was observed with char study of LM1 sample where phosphorus in FR and nitrogen in MM worked synergistically to inhibit the flame by promoting carbonaceous char formation even after 10 wash cycles (Fig. 4d). However after 10 wash cycles the SEM micrographs of the LM1 char demonstrates rupture in fibrous network and a few dehydrated bubbles on the fiber surface. Hence, from the char analysis, it is proposed that the treated samples prevented the generation of flammable volatiles (gas and hydrocarbons) and encouraged the char formation even after 10 wash cycles.

4.5. FT-IR spectroscopy FT-IR technique helps to measure the chemical structure of samples. It was carried out to further validate the attachment of FR on to lyocell fabric using CA. The measured characteristics bands of untreated lyocell and FR treated lyocell are shown in Fig. 5. In order to investigate the attachment of FR on to the lyocell fabric, the FTIR spectra for untreated, FR + CA treated lyocell fabric was assessed in the range from 4000 to 750 cm−1 wavenumber. All the major bands for lyocell fabric can be seen in the top most spectra in Fig. 5. The peaks observed are the characteristics peaks of cellulose including the OH stretching at 3331 cm−1, C H stretching at 2892 cm−1 ,O H bending at 1638 cm−1 , CH2 bending at 1419 cm−1 and the CH bending at 1313 cm−1 (Sahito, Sun, Arbab, Qadir, & Jeong, 2015; Su & Li, 2010). The broad range between 900 and 1200 cm−1 in the spectra also correspond to cellulosic fiber, which in this case is lyocell fiber. However different visible peaks appeared in the spectra of FR+ C-A treated lyocell such as a strong peak, at 1655 cm−1 which denotes C O stretching of amide that is present in the FR, another strong band near 1546 cm−1 involves the CNH bending and C N stretching. Furthermore, the peak at 829 cm−1 is contributed by phosphonates structures that have P-CH − R groups. However, at 1320–1140 cm−1 the vibration of phosphorus bonding (P = O) is difficult to identify due to overlapping with the primary and secondary OH deformation of lyocell (Poon & Kan, 2015). The peak visible at 1726 cm−1 corresponds to carbonyl band of esters (Wu, Yang, & He, 2010) thus confirming the presence of CA at zero wash and after 10 wash cycles. From the FT-IR analysis it is evident that the FR + CA formulation is successfully coated and is durable to multiple wash cycles. 4.6. Formaldehyde content In this research, along with durable flame retardancy and thermal stability, the environmental and health impacts were also studied. Therefore, formaldehyde content is measured in treated samples and reported in Fig. 6, depicting that sample LC2 has maximum formaldehyde content of 58 ppm and LC4 has minimum

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Fig. 6. Formaldehyde content of FR treated lyocell fabric: using (a) CA and (b) MM based cross-linker.

Fig. 7. CIE Whiteness Index of FR treated lyocell fabric using: (a) CA and (b) MM based cross linkers.

23 ppm, whereas, LC1 and LC3 has 42 and 36 ppm respectively when measured prior washing (at zero wash). From these findings it is evident that all Pyrovatex CP New + CA samples i.e. LC1-LC4 have acceptable formaldehyde content i.e. <75 ppm and they fulfills the criterion of OEKO-TEX® standard 100 for apparel fabrics. This amount further diminished below 10 ppm and up to 5 ppm after 5 and 10 laundering cycles respectively due to hydrolysis of unreacted finish. Likely, the fabric samples treated with MM based cross-linker have higher value of formaldehyde as shown in Fig. 6b, due to the presence of N-methylol group in FR and in MM which converts to formaldehyde during curing step. The reported value of formaldehyde content when measured prior to washing were 226, 160, 103 and 94 ppm in samples LM3, LM1, LM2 and LM4 respectively, which verifies that these recipe formulations cannot be used for apparel or direct skin contact fabrics (>75 ppm). From the findings stated above it is inferred that CA has the potential to be used as a cross-linker with N-methylol dimethylphosphonopropionamide (N-MDMPA), commercially marketed as Pyrovatex CP New. It is also proposed that if the recipes for samples LC1 and LC2 are further optimized, they might meet the OEKO-TEX® standard 100 for children wear (Formaldehyde content < 16 ppm).

4.7. CIE whiteness index To investigate the effect of FR and cross-linkers on the color of treated lyocell fabric, the CIE whiteness index was measured and compared with control value (C) of 69.06. The results reported in Fig. 7a reflect that CIE whiteness index (WI) of samples LC1-LC4 lies between 43.79 and 59 at zero wash stage. Thereafter, it improved with subsequent washing cycles. A sharp decline in whiteness was observed in samples LC3 and LC1, due to higher concentrations of CA (Yao et al., 2013). This decrease in whiteness may also be attributed to the acidic pH of the finishing bath as well as to elevated curing temperature (180 ◦ C). Furthermore, the WI of fabric treated with FR + MM also reduced as shown in Fig. 7b, though the effect is not as high as compared to FR + CA, and thereafter it also improved gradually with successive washing cycles.

5. Conclusions A durable eco-friendly FR finishing system for lyocell fabric was developed using CA and compared with a conventional MM containing FR system for flame retardancy, thermal stability, washing

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durability, free formaldehyde content and physical properties. At higher FR concentrations (400 g L−1 ), both finishing recipe formulations gave excellent and durable flame retardancy even after 10 washing cycles. TGA and dTG results reveal that the decomposition temperature of the treated fabrics shifted to a lower temperature giving a high amount of char residue. Particularly in sample LC1, the char residue was 43.5% under nitrogen gas and 35.4% under air atmosphere. Char studies and the FTIR analysis also proved effective crosslinking by CA even after 10 wash cycles. It is also observed that both the cross-linkers have an adverse effect on tensile strength and CIE whiteness index of the treated fabric. In addition, the FR + CA formulation also has the advantage of low formaldehyde release, thus meeting the requirement of OEKO-TEX® standard 100 for apparel fabric. From these findings, it is inferred that CA can be used as a cross-linker with Pyrovatex CP New on commercial scale for the production of durable and sustainable FR apparel fabrics. Acknowledgments The authors acknowledge Mr. Jim Taylor, Lenzing AG, Austria for his guidance and supply of lyocell fabric for the research and Mr. Shoaib Siddiqui, Assistant Manager Technical, Gul Ahmed Textile Mills Karachi for his valuable suggestions and help throughout this project. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbpol.2016.07. 074. References Anonymous (2016). OEKO-TEX® Association, Limit values and fastness (Vol. 2016). Basak, S., Patil, P. G., Shaikh, A. J., & Samanta, K. K. (2016). Green coconut shell extract and boric acid: New formulation for making thermally stable cellulosic paper. Journal of Chemical Technology and Biotechnology, Basak, S., Samanta, K. K., & Chattopadhyay, S. (2015). Fire retardant property of cotton fabric treated with herbal extract. The Journal of The Textile Institute, 106(12), 1338–1347. Basak, S., Saxena, S., Chattopadhyay, S., Narkar, R., & Mahangade, R. (2015). Banana pseudostem sap: A waste plant resource for making thermally stable cellulosic substrate. Journal of Industrial Textiles, http://dx.doi.org/10.1177/ 1528083715591580 Bharathi Yazhini, K., & Gurumallesh Prabu, H. (2015). Study on flame-retardant and UV-protection properties of cotton fabric functionalized with ppy-ZnO-CNT nanocomposite. RSC Advances, 5(61), 49062–49069. Blanchard, E. J., & Graves, E. E. (2002). Polycarboxylic acids for flame resistant cotton/polyester carpeting. Textile Research Journal, 72(1), 39–43. Blanchard, E. J., & Graves, E. E. (2005). Improved flame resistance of cotton/polyester fleece with phosphorus based polycarboxylic acids. AATCC Review, 5(2), 26–30. ´ S., & Katovic, ´ A. (2012). Formaldehyde free binding system for Bischof-Vukuˇsic, flame retardant finishing of cotton fabrics. FIBRES & TEXTILES in Eastern Europe, 20(1), 90.

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