INTRODUCTION:

Cotton textile materials are used extensively to make life pleasant, comfortable and colorful. The use of textile materials in industry and domestic purposes increases with the progress of society. But unfortunately, cellulosic materials are combustible and cause fire hazards. The risk due to high flammability of cotton textile remains always with us as a result of the personal quality of most textiles, mainly cloths and immediate homely environment. Therefore, the development of flame retarded materials has become essential for safety. The public demand for increased safety has led to greater interest in flame retardant (FR) materials. Many different approaches have been reported^1-4 for improving the flame retardancy, intumescent coating is one of the methods used to impart flame retardancy to textiles. The thermal degradation process of cellulosic material has been the focus of wide research in the analysis of flame retardancy, release of harmful substances during waste ignition, recovering of chemical raw materials and optimizing the ignition processes to produce energy from fossil fuels.

Many flame retardant systems have been developed and commercialized productively but without taking into account the toxic effect of flame retardant especially of antimony-bromine system^5,6. The role of flame retardant intumescent coating is to protect the substrate from atmospheric oxygen and prevent flow of heat inward from flame as well as to shield the escape of low molecular weight volatile compounds into burning vapour phase.

In view of the above, environmental friendly phosphorus based intumescent system as coating formulations containing the nanoclay has been selected as a flame retardant for cotton fabric. An intumescent system consist an acid source, a swelling agent and a char forming agent^7-11. In this paper ammonium polyphosphate (APP) as an acid source, melamine as a swelling agent and pentaerythritol (PER) as a carbon source were used intumescent system. The nanoclay (sodium bentonite) as a thermal barrier is also introduced in intumescent system.

EXPERIMENTAL:

Materials and Methods

The chemicals used to improve flame retardant property of cotton fabric were ammonium polyphosphate (APP), melamine, pentaerythritol (PER) (Clariant Co., Germany), sodium bentonite nanoclay (Sud-Chemi. and acrylic based resin (Zytrol- 7800) as binder (Zydex Industries, India). Cotton fabric (CF) of area density (230.4 g/m²) was used for coating.

A formulation has been prepared containing intumescent components (ammonium polyphosphate, melamine and pentaerythritol) in ratio 3:1:1 i.e. (6:2:2 % w/w) weight by weight of pure cotton fabric with nanoclay (1-7 %NC). The formulation was coated on plane woven cotton fabric. Table 1 gives the description of the samples. The method of preparation of samples and procedure of coating has been discussed in detail in our earlier publication¹².

Thermal Analysis: TG and DTA thermograms of cotton and its coated cotton samples were obtained using Perkin Elmer (Pyris Diamond) instrument in nitrogen atmosphere with flow rate of 100 mL/min. Alumina pan was used as a sample container. DTA and TG thermograms obtained from ambient temperature to 600°C at a heating rate of 10°C/min are shown in Figures 1 and 2, respectively.

Limiting Oxygen Index (LOI): LOI values that measure performance of flame retardancy were measured using a Stanford Redcroft FTA flammability unit BS-2782 instrument. Fabric samples (150 mm×50 mm) were tested according to standard method ASTM D2863, ISO-4589.

Figure 1 DTA curves in nitrogen at heating rate of 10 ⁰C/min of (a) CF; (b) CCF–Int–(2–7%) NC.

RESULTS AND DISCUSSIONS:

DTA Study: DTA curves of all the samples are shown in Figure1. Table 2 shows the DTA peaks of all the samples in nitrogen atmosphere. DTA curve of cotton fabric shows single sharp endothermic peak with maximum at 357°C in nitrogen atmosphere. This endothermic peak may be due to pyrolysis and random chain scission of cotton cellulose molecules^13-15. Coated cotton fabric samples (CCF–Int–(2–7%NC) shows only one endothermic peak with maxima in temperature range 299–342°C. This peak for every sample corresponds to first and second stage of thermal degradation in TG. This DTA peak corresponds to the decomposition of APP and release of phosphoric acid. DTA peak supports second stage of TG due to decomposition of melamine releasing NH₃ continuously which further swells intumescent complex layer and increases its thickness. Therefore, this DTA peak may also be attributed to melamine reaction with APP and PER forming a complex. The process of the decomposition of polyphosphate/PER/melamine complex, cross–linking and aromatization of char are not resolved by DTA and as a result process is not seen in DTA separately.

TG Study: TG curve of cotton and all coated samples are shown in Figure 2. The TG data such as weight loss, degradation stage, DTG peak and char yield of the samples in nitrogen atmosphere are given in Table 3. The thermal degradation of cotton (CF) and cotton coated with intumescent (CCF-Int) has been discussed in detail in our earlier publication¹⁶.

Figure 2 TG of cotton fabric and its coated samples with intumescent–nanoclay in nitrogen atmosphere at heating rate of 10°C/min.

In case of CCF-Int, major weight loss occurs in first stage of thermal degradation upto 360°C temperature in nitrogen atmosphere. Decomposition of APP gives phosphoric acid which catalyzes the dehydration of cellulose as well as of pentaerythritol (PER) above 200°C. Phosphorylated cellulose as well as polyolphosphate decomposes to phosphoric acid, NH₃ and H₂O resulting in char formation. Melamine sublimes at about 250°C releasing NH₃ up to 400°C. A thermal barrier is formed on the cotton substrate which is expected to protect the cotton from atmospheric oxygen. About 15 % weight loss occurs in second stage of thermal degradation in the temperature range 360–445°C which is due to continuous release of NH₃ from melamine resulting in swelling of mass. A complex structure of polyphosphate/PER/melamine is formed at about 400°C temperature. On further heating upto 600°C the decomposition of melamine complex is also occurred

Table 1 Description of intumescent formulations for cotton fabric

Samples	Description of the samples
Cotton (CF)	100% cotton woven fabric
Coated cotton fabric (CCF)
CCF–Int	Cotton fabric coated with intumescent components (APP, PER and melamine) in ratio of 3:1:1 (6%: 2%: 2%) w/w of pure cotton fabric (CF)
CCF–Int–1% NC	Cotton fabric coated with intumescent components (APP, PER and melamine) in ratio of 3:1:1 (6%: 2%: 2%) + nanoclay (NC) 1% w/w of pure cotton fabric (CF)
CCF–Int–2% NC	CF coated with intumescent components (APP, PER and melamine) in ratio of 3:1:1 (6%: 2%: 2%) + nanoclay (NC) 2% w/w of CF.
CCF–Int–4% NC	CF coated with intumescent components (APP, PER and melamine) in ratio of 3:1:1 (6%: 2%: 2%) + nanoclay (NC) 4% w/w of CF.
CCF–Int–5% NC	CF coated with intumescent components (APP, PER and melamine) in ratio of 3:1:1 (6%: 2%: 2%) nanoclay (NC) 5% w/w of CF.
CCF–Int–6% NC	CF coated with intumescent components (APP, PER and melamine) in ratio of 3:1:1 (6%: 2%: 2%) + nanoclay (NC) 6% w/w of CF.
CCF–Int–7% NC	CF coated with intumescent components (APP, PER and melamine) in ratio of 3:1:1 (6%: 2%: 2%) + nanoclay (NC) 7% w/w of CF.

Table 2 DTA peaks of cotton fabric and its coated samples with intumescent–nanoclay in nitrogen atmosphere

Name of sample	DTA temperature (⁰C)		Nature of peak
Name of sample	Initiation	Maximum	Nature of peak
CF	316	357	Endo
CCF–Int–2%NC	295	342	Endo
CCF–Int–4%NC	283	314	Endo
CCF–Int–5%NC	278	299	Endo
CCF–Int–6%NC	288	307	Endo
CCF–Int–7%NC	280	306	Endo

Table 3 TG data and LOI of cotton fabric and its coated samples with intumescent–nanoclay in nitrogen atmosphere

Name of Sample

Stage

Temp. range (⁰C)

Wt. loss (%)

DTG peak (⁰C)

Char at 600 ⁰C (%)

LOI (%)

1^st

312–425

78.1

398

12.8

18.0

CCF–Int

1^st

2^nd

3^rd

290–360

360–445

445–600

50.5

14.9

4.5

323

425

–

25.4

21.8

CCF–Int–1% NC

1^st

2^nd

3^rd

270–395

395–555

555–650

65.5

14.4

4.4

357

502

–

12.8

21.9

CCF–Int–2% NC

1^st

2^nd

3^rd

260–340

340–410

410–650

54.9

12.7

5.7

304

382

–

19.7

22.0

CCF–Int–4% NC

1^st

2^nd

3^rd

250–350

350–440

440–650

53.4

13.0

4.8

307

384

–

23.2

22.1

CCF–Int–5% NC

1^st

2^nd

3^rd

265–320

320–415

415–650

47.2

12.5

6.5

295

377

–

25.2

21.9

CCF–Int–6% NC

1^st

2^nd

3^rd

265–315

315–360

360–650

30.6

19.2

18.3

297

349

384

27.4

22.1

CCF–Int–7% NC

1^st

2^nd

3^rd

250–345

345–430

430–650

47.2

13.5

6.1

301

385

–

27.4

22.3

Cotton coated with intumescent and nanoclay (CCF–Int–(1-7%) NC): TG curves (Figure 2) of CCF–Int–(1–7%) NC samples show three stages of degradation similar to that of CCF–Int sample supported by three DTG peaks (Table 3) in nitrogen atmosphere. Weight loss in nitrogen atmosphere of CCF–Int–(1–7%) NC samples is found 31–65 % in first stage of thermal degradation (250–395°C); 13–19 % in second stage of thermal degradation (315–555°C); and 4–18 % in third stage of degradation (360–650°C).

Char yields: Char yields of base cotton fabric and coated cotton fabric were calculated from TG curves at 600°C in nitrogen atmosphere (Table 3). Cotton fabric gives 12.8 % char yield which is increased to 25.4 % for coated cotton fabric (CCF–Int). The char yield is found in the range 15–27 % when nanoclay was added in coating intumescent formulations. Figure 3 shows that with the addition of 2–3 % nanoclay with intumescent, char yield is found decreased but LOI value remains the same to that of CCF–Int. This shows that NC has some interactions and as result of this non–flammable products are evolved. But with further increase of nanoclay addition (>4 %), the char yield increased without change in value of LOI. This indicates that with about 4–5 % nanoclay addition the reinforcement of char takes place. We can conclude that the addition of nanoclay (4–5 %) reduces the diffusion of volatile/low molecular weight products from the substrate to the flame region and thus nanoclay provides the thermal stability due to intercalation and cross linking phenomenon.

Figure 3 LOI and Char yield (%) of CF and CCF–Int–(1–7%)NC samples in nitrogen atmosphere.

Limiting oxygen index (LOI): We know that the higher the value of LOI (Table 3) and char yield better is the flame resistance of the material. LOI values for coated samples (CCF–Int–(1–7%)NC) containing nanoclay are found in the range 21.9–22.3 % which are almost the same to that of CCF–Int (21.8 %). No significant effect of nanoclay was observed on LOI values (Figure 3) in present intumescent formulations.

CONCLUSIONS:

From this study, it is observed that the effect of intumescent alone is sufficient in increasing the LOI to make the cotton fabric flame retarded. The addition of nanoclay (4–5 %) reduces the diffusion of volatile/low molecular weight products from the substrate to the flame region and thus nanoclay provides the thermal stability due to intercalation and cross linking phenomenon.

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Received on 22.06.2010 Modified on 02.07.2010

Asian J. Research Chem. 3(4): Oct. - Dec. 2010; Page 1007-1010