INTRODUCTION:

Cotton is the most abundant used natural textile material and used in industrial and domestic applications. However, it carries with it increased fire hazards, since most of polymers including cotton are flammable. This problem becomes more serious today because of the present living phenomenon of great number of people in confined space at heights. In developing as well as in developed countries, deaths in hundreds and injuries in thousands result from fire each year¹. Therefore, the development of flame retarded materials has become crucial for safety. Many different approaches have been reported for improving the flame retardancy like intrinsically thermally stable polymers,fire retardant additives², intumescent fire retardant systems³ and chemical treatments including our study⁴ of polymeric materials. Further, there are doubts about the common use of halogen containing compounds as flame retardants (FRs) because of release of highly toxic and corrosive fumes during combustion⁵. Therefore, environmental concerns and legislations are driving the search for new flame retardants that reduces flammability with minimum change to the physico-mechanical properties of material.

In the present work, eco-friendly intumescent formulation (acid source, blowing agent and char forming agent (3:1:1) containing metal ions (Cr, Fe, Cu and Zn) was used for coating on cotton fabric (CF). Thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses in nitrogen atmosphere were carried out to study the thermal behaviour of coated fabric. Limiting oxygen index (LOI) study was also carried out to evaluate the flammability behaviour of the cotton fabric.

MATERIALS AND METHODS:

Cotton fabric (CF) of area density (230.4 g/m²) was used for back coating. The chemicals used to improve flame retardant property of cotton fabric were ammonium polyphosphate (APP) as an acid source, melamine as blowing agent, pentaerythritol (PER) as char forming agent (Clariant Co., Germany), and acrylic based resin (Zytrol-7800) (Zydex Industries, India) as binder.

A formulation has been prepared containing intumescent components (ammonium polyphosphate, melamine and pentaerythritol) in ratio 3:1:1 % w/w of pure cotton fabric in the form of paste using water as solvent. This formulation in paste form was coated on plane woven cotton fabric. The resin binder (30 % w/w of cotton fabric) used for coating the formulation was acrylic latex (Zytrol-7800). The fabric was coated with simple knife-blade technique, giving rise to approximate 30 % add on weight on the cotton fabric substrate. The 10 % intumescent coated cotton fabric sample in this study is abbreviated as CCF-Int. Similarly other set of formulations with 20 %, 30 %, 40 % Intumescent w/w of cotton fabric and with 10% intumescent plus 3% metal salts (Cr, Fe, Cu and Zn) w/w of CF were also prepared. Each coated fabric was placed in an oven to dry and cured at 120-125°C for 4 minutes.

Thermal Analysis:

TG of cotton and its coated cotton samples were obtained using DuPont 951 thermal analyzer from ambient temperature to 600°C at the heating rate of 10°C/min and are shown in Fig 1. The DSC thermograms of cotton and its coated samples were recorded using DSC Q10 V9.0 build 275 instrument from ambient temperature to 550°C at the heating rate of 10°C/min and are shown in Fig 2. The nitrogen gas was used as purge gas with flow rate of 100 mL/min. Alumina pan was used as a sample container.

Fig 1: TG curves in nitrogen at heating rate of 10°C/min of CF, CCF–Int, CCF–Int–Cr/Fe/Cu/Zn.

Limiting oxygen index:

Limiting oxygen index (LOI) values of the fabrics were measured using a Stanford Redcroft FTA flammability unit BS-2782 instrument. Fabric samples of size 150×50 mm were tested according to standard method ASTM D2863, ISO-4589.

Kinetics study:

Where, n is the reaction order, A is the pre-exponential factor and E represents the activation energy of the reaction. The alpha (α, the extent of reaction) is equal to (m₀-m/m₀-m_f), where m₀ represents the initial sample mass in an experiment, m denotes sample mass at any time t and m_f represents the final sample mass. The activation energy (E) and pre-exponential factor (A) were determined by applying the equation (1) on the TG data obtained from experiments performed at heating rate of 10°C/min.

RESULTS AND DISCUSSION:

TG curve of cotton fabric (Fig 1) shows only one stage of thermal degradation in temperature range 312–425°C with weight loss 78 % in nitrogen atmosphere. On heating cotton fabric, the weight loss occurs due to dehydration followed by pyrolytic thermal degradation releasing volatilesand flammable compounds⁷in nitrogen. The weight loss occurs due to pyrolysis and random chain scission^7,8. This stage is supported by DTG peak at 398°C in nitrogen atmosphere.

Cotton coated with intumescent:

TG curve of cotton fabric coated with intumescent (CCF–Int) shows three stages (Fig 1) of thermal degradation in nitrogen. The TG data of the samples in nitrogen atmosphere are given in Table 1.

First stage of thermal degradation: In the first stage of thermal degradation of CCF–Int, 50% weight loss takes place in the temperature range 290–360°C which may be mainly due to the dehydration of cotton and partial degradation of ammonium polyphosphate (APP) releasing phosphoric acid⁹. The released phosphoric acid starts catalyzing the dehydration of cellulose as well as of pentaerythritol (PER) above 290°C. Fusion of PER also takes place at around 300°C., APP starts releasing phosphoric acids at about 275°C, which in turn phosphorylates cellulose as well as PER. Ammonia and water are released on further heating of APP, and cross–linked phosphate is obtained which further reacts with PER at about 300°C. Phosphorylated cellulose as well as polyolphosphate decomposes to phosphoric acid, NH₃ and H₂O resulting in char formation⁹. There is also possibility of formation of polyphosphoric acid¹⁰ from phosphoric acid with release of water vapour.

Melamine, the third constituent of intumescent formulation as a spumific agent, sublimes at 250°C and starts releasing NH₃ at 270°C and releases ammonia up to 400°C. In this process a layer of swelled material starts forming on the polymer substrate as a thermal barrier which is expected to protect the substrate from atmospheric oxygen. The diffusion of flammable volatiles from the burning substrate to the combustion phase is also prevented. The weight loss in this stage was found maximum in comparison to other two stages. This stage is supported by DTG first maximum at 323°C of CCF–Int in N₂ atmosphere.

Second stage of thermal degradation: The second stage of thermal degradation in the temperature range 360–445°C shows 15 % weight loss of CCF–Int in nitrogen atmosphere. The weight loss in this stage is considered due to release of NH₃ from melamine apart from pyrolytic decomposition of cellulose and of released volatile compounds.

Table 1: TG data and LOI value of cotton fabric and its coated samples in nitrogen

Name of sample	Stage	Temp. range (⁰C)	Wt. loss (%)	DTG peak (⁰C)	LOI (%)	Char at 600 ⁰C (%)
Cotton (CF)	1^st	312–425	78.1	398	18.0	12.8
CCF–Int	1^st 2^nd 3^rd	290–360 360–445 445–600	50.5 14.9 4.5	323 430 –	21.8	25.4
CCF–Int–Cr	1^st 2^nd 3^rd	270–370 370–460 460–600	59.1 17.5 3.1	335 435 –	21.9	15.8
CCF–Int–Fe	1^st 2^nd 3^rd	280–370 370–460 460–600	58.1 12.9 3.7	330 440 –	21.9	21.4
CCF–Int–Cu	1^st 2^nd 3^rd	280–380 380–465 465–600	62.1 10.9 2.9	338 432 –	22.0	18.9
CCF–Int–Zn	1^st 2^nd 3^rd	285–365 365–455 455–600	56.6 13.8 4.3	335 438 –	21.9	20.2

Table 2: DSC data of cotton fabric and its coated samples in nitrogen

Name of sample

DSC temperature (⁰C)

Heat flow (J/g)

Nature of peaks

Initiation

Maximum

310

370

116

340

380

120.7

50.4

–

Endo

Endo, large

Exo, small

CCF–Int

180

220

320

360

190

250, 300

350

400

85.4

4.7

63.5

–

Endo

Endo very small

Endo, small

Exo, Broad

Endo, large

CCF–Int–Cr

180

230

300

360

190

260

330

400

63.3

13.7

21.7

–

Endo, large

Endo, very small

Endo, broad

Exo, Broad

Endo, large

CCF–Int–Fe

190

230

330

370

200

250, 290

370

400

40.5

11.4

–

Endo, large

Endo, very small

Endo, small

Exo, broad

Endo, large

CCF–Int–Cu

170

280

360

250

320, 360

400

81.1

5.5

50.4

–

Endo, large

Exo, large, broad

Endo, large

CCF–Int–Zn

180

290

370

100

250

320, 360

400

124.5

28.3

32.8

–

Endo,large, sharp

Endo, sharp

Exo, sharp, broad

Endo, large

Swelling of mass also occurs in this stage. Melamine can react with polyolphosphate to form a complex structure of polyphosphate/PER/melamine. The deoxygenation and dehydrogenation as the additional reactions also occur in this decomposition stage. Second DTG maximum at temperature 430°C corresponds to the weight loss in this stage in nitrogen atmosphere.

Third stage of thermal degradation: In this stage, the 5 % weight loss occurs in the temperature range 445–600°C. In this stage, decomposition of polyolphosphate and polyphosphoric acid takes place releasing phosphoric acid again. Further, the decomposition of melamine complex is also occurred apart from the pyrolysis and interaction of char⁹. In case of nitrogen atmosphere only solidification through cross–linking reaction of residual char occurs and no oxidative degradation takes place which is also supported by less weight loss in this stage. No DTG peak is observed in this stage because of gradual weight loss.

Cotton coated with intumescent and metal (CCF–Int–Cr/Fe/Cu/Zn):

TG curves (Fig 1) of CCF–Int–Cr/Fe/Cu/Zn samples show three stages of degradation similar to that of CCF–Int sample supported by three DTG peaks (Table 1) in nitrogen atmosphere. Weight loss in nitrogen atmosphere of CCF–Int–Cr/Fe/Cu/Zn samples is found 56–62% in first stage of thermal degradation (270–365°C); 11–17% in second stage of thermal degradation (355–400°C); and 3–4% in third stage of degradation (450–600°C). On inclusion of transition metals (Cr, Fe, Cu and Zn) in intumescent formulation, the increase in char yield is not observed.

DSC study of cotton fabric and its coated samples:

DSC thermograms of samples were obtained upto 550°C in nitrogen atmosphere and are shown in Fig 2. The initiation and maximum temperatures of various DSC peaks in nitrogen atmosphere were measured and are given in Table 2.

Fig 2: DSC curves in nitrogen at heating rate of 10°C/min of CF, CCF–Int, CCF–Int–Cr/Fe/Cu/Zn.

The first endotherm maximum in each sample is found below 116°C. This is due to the sorbed moisture and dehydration reaction. In nitrogen, cotton fabric shows large second endotherm with maximum at 340°C, which may be due to dehydration, depolymerization and pyrolysis reactions with the formation of major volatile component i.e. laevoglucosan and its evaporation¹¹. The cotton cellulose shows small exotherm with maximum at 380°C, which may be due to pyrolytic decomposition, randon chain scission, cross–linking and formation of char¹¹. The DSC reveals that 50.4 J/g heat was absorbed during the pyrolytic decomposition of cotton fabric in nitrogen atmosphere.

DSC thermograms of CCF–Int, CCF–Int–Cr and CCF–Int–Fe in nitrogen atmosphere show the second endotherm (very small) peaks at 190, 190 and 200°C, respectively may be due to many reactions such as dehydration of cotton cellulose, fusion of pentaerythritol and partial degradation of APP–I to more stable form APP–II. CCF–Int–Cu and CCF–Int–Zn samples do not show this endotherm may be because of concurrence of endothermic reactions with subsequent exothermic reactions due to dehydration of cotton cellulose and decomposition of phosphate.

The third DSC endotherm each of CCF–Int, CCF–Int–Cr and CCF–Int–Fe, CCF–Int–Cu and CCF–Int–Zn with maxima at (250, 300 double peak), 260, (250, 290 double peak), 250 and 250°C, respectively represents the decomposition of APP releasing phosphoric acid, catalyzed dehydration of cellulose, cross–linking of ammonium polyphosphates and phosphorylation of PER as well as cotton cellulose⁹. During same temperature range the partial sublimation and decomposition of melamine also takes place resulting the evolution of ammonia. The heat in the range 4-28 J/g is absorbed during dephosphorylation and catalyzed reactions for all the coated samples in nitrogen atmosphere (Table 2).

The fourth peak in DSC curve of each of CCF–Int, CCF–Int–Cr, CCF–Int–Fe, CCF–Int–Cu and CCF–Int–Zn is exothermic in nature with maxima at 350, 330, 370°C, (320, 360°C double peak) and (320, 360°C double peak), respectively. This peak represents decomposition of cotton cellulose, decomposition of polyolphosphate and melamine structure complex. This exothermic peak is considered more than compensated of endothermic reactions such as crosslinking of phosphates, releasing of ammonia etc. This exotherm corresponds to the interface of first and second stage of thermal degradation of TG of coated samples. Last endotherms of the samples peaking at about 400°C are considered due to cross-linking, deoxygenation and aromatization reactions of the char formed in nitrogen atmosphere.

Thermal degradation kinetics:

Activation energies of thermal degradation of cotton fabric and its coated samples were determined using first order Coats-Redfern method⁶ on data obtained from TG and are given in Table 3. The values of activation energy and pre–exponential factor were obtained for CF, CCF–Int and CCF–Int–Cr/Fe/Cu/Zn samples, in the range of degree of conversion α = 0.2–0.85, 0.15–0.7, 0.1–0.75, 0.1–0.7, 0.1–0.75, 0.1–0.75 respectively, which falls in first stage of thermal degradation of maximum mass loss. The activation energy of CCF-Int, 161.2 kJ mol^–1, was found lower than that of CF (207.1 kJ mol^–1) which is further decreased in case of CCF-Int-NC-Cr/Fe/Cu/Zn samples (97-138 kJ mol^–1). The decrease in activation energy is due to catalyzing effect of phosphoric acid and metals and show that the dehydration path during thermal degradation of cotton is chosen in which more char is formation at the expense of tar¹¹.

LOI and Char Yield:

Char yield at 600°C and Limiting Oxygen Index (LOI) values of cotton fabric and its coated samples were obtained and are given in Table 1. The higher the value of LOI and char yield the better is the flame resistance of the material. The LOI value for pure cotton fabric was found 18% which is increased to 21.8% for intumescent coated fabric (CCF-Int). When add–on percentage of intumescent formulation was taken 10, 20, 30 and 40% the LOI values were found increased in the order of 21.8, 22.2, 25.6 and 28%. From this study, it was observed that the effect of intumescent alone is sufficient in increasing the LOI to make the cotton fabric flame retarded. LOI values for coated samples containing metal ions were found almost the same to that of CCF–Int. No significant effect of metals was observed on LOI values in present formulations and composition.

Cotton fabric produced 12.8% char yield which was increased to 25.4% for coated cotton fabric (CCF–Int) at 600°C. The char yield was found decreased to the values in the range 15−21% when metals (Cr, Fe, Cu and Zn) were introduced in coating formulations but still it is higher than that of pure cotton fabric. This decrease in char yield from 25.4 of CCF–Int to (15–21%) on introducing metals may be due to more catalyzing effect of metals on the degradation of cotton fabric in nitrogen atmosphere^12,13 On adding metals in intumescent formulation, LOI remains same but char yield decreases. It shows that non–flammable gases like NH₃, CO₂ etc are formed in large quantity.

CONCLUSIONS:

TG curves of coated cotton fabric with intumescent (CCF-Int) show three stages of thermal degradation mainly due to dehydration, decomposition and cross-linking of char, respectively. The action of intumescent is found concurrent with the process of decomposition of cotton fabric on heating. The activation energy of coated fabric (CCF-Int) was found lower than that of cotton fabric which was further decreased in case of CCF-Int-Cr/Fe/Cu/Zn samples. The decrease in activation energy supports the fact that the dehydration path during thermal degradation of cotton is preferred in which more char is formed at the expense of volatile compounds. The LOI value for pure cotton fabric was found 18% which was increased to 21.8% for intumescent coated fabric (CCF-Int). When add–on percentage of intumescent formulation was increased from 10 to 40% the LOI values were found increased further from 21.8 to 28%. From this study, it was observed that the effect of intumescent alone was sufficient in increasing the LOI to make the cotton fabric flame retarded. No significant effect of metals was observed on LOI values in the present formulations and composition.

REFERENCES:

1. Horrocks AR and Price D, Fire Retardant Materials, Woodhead Publ., Ltd, 2001.

2. Davis PJ, Horrocks AR and Alderson A, Polym Degrad Stab. 2005; 88:114-122.

3. Lecoeur E, Vroman I, Bourbigot S et al., Polym Degrad Stab. 2006; 91:1909-1914.

4. Dahiya JB and Kumar K, J Sci Ind Res. 2009; 68: 548-554.

5. Camino G, Martinasso G and Costa L, Polym Degrad Stab. 1990; 27: 285.

6. Coats AW and Red fern JP, Kinetic parameters from thermodynamic data, Nature, 1964; 201: 68-69.

7. Ming G and Qiu-ju D, The Chine J Proc Engg. 2006; 6(2): 242-246.

8. Hirata T and Nishimoto T, Thermochim Acta, 1991; 193: 99-106.

9. Kandola BK and Horrocks AR, Polym Degrad Stab. I 996; 54: 289-303.

10. Camino G. Fire Retardant Polymeric Materials, Edited by Nelson G. ACS, USA, 1995; p p. 461-492.

11. Soares S, Camino G and Levchik S, Polym Degrad Stab. 1995; 49: 275-283.

12. Khelfa1 A, Finqueneisel1 G, Auber M and Weber JV, J. Therm Anal Calori. 2008; 92: 795–799.

13. Wang JS, Liu Y, Zhao HB et al., Polym Degrad Stab. 2009; 94: 625–631.

Received on 28.04.2010 Modified on 21.05.2010

Asian J. Research Chem. 3(4): Oct. - Dec. 2010; Page 911-915

Name of sample	1-α	Temp. range ⁰C	E kJmol^-1	ln A min^-1
CF	0.80–0.15	373–410	207.1	35.6
CCF–Int	0.85–0.30	310–347	161.2	30.2
CCF–Int–Cr	0.90–0.25	297–369	105.0	18.2
CCF–Int–Fe	0.90–0.30	299–346	138.3	25.4
CCF–Int–Cu	0.90–0.25	297–361	115.1	20.4
CCF–Int–Zn	0.90–0.25	278–356	97.5	17.2