INTRODUCTION:

Cotton has taken a very important place as textile since ages particularly in developing countries due to numerous advantages. But when cotton comes in contact with fire it gets ignited easily and starts burning vigorously with flame and produces smoke and gases during combustion and cause fire hazards. This is a major disadvantage of cotton textiles being organic in nature and limits its applications. Therefore, flame retardancy of cotton textiles has attracted much attention from the view point of fire hazard prevention and led to greater interest in flame retardant (FR) treatments^1,2. The flame retardant additives are of growing importance as they are durable and environment friendly flame retardants in textiles industries with the increasing health and safety concerns. A major challenge to increase the safety in applications of cotton fabric is to enhance the fire retardant properties of polymers. Flame retardant treatments alter the decomposition path of cotton and prevent the formation of volatiles products³ mainly laevoglucosan^4,5.The function of intumescent coating as a flame retardant is to increase char^6,7, reduce the amount of flammable volatiles and make a protective char layer which protect the burning material from the fire^8,9.

In this study, cotton fabric has been coated with intumescent formulations containing inorganic additives such as aluminium trihydrate, zinc borate and zeolite with kaolin nanoclay. Thermal degradation and burning behaviour of coated fabric samples were investigated using thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques, and auto flammability test, respectively. Mechanical properties of coated fabric samples were also studied.

EXPERIMENTAL:

Materials:

Cotton fabric (area density = 78.57 g/m²) was donated by Raymond Zambaiti Ltd. Kolhapur, which was already desized, mercerized, bleached, washed with water and dried. The components taken for intumescent flame retardant system were ammonium polyphosphate (APP) as Exolit AP 422 donated by Clariant Inc., USA, melamine (MEL) and pentaerythritol (PER) purchased from CDH Chemicals Co., India. An acrylic resin (Zytrol 25) was purchased from Zydex Industries, India and was used as binder. Aluminium trihydrate (ATH) and kaolin (KLN) nanoclay were purchased from Sigma Aldrich Co., India. Zinc borate (ZB) and zeolite (ZL) chemicals were purchased from Himedia Chemicals Co. India. All these materials were used as received.

Table 1. Intumescent components required for formation of slurry containing additive.

Components	Parts	Solid content (%)	ODrequired (g)	Slurry(g)
APP	60	100	9.60	9.60
MEL	20	100	3.20	3.20
PER	20	100	3.20	3.20
Binder	15	26	2.40	9.23
ATH/ZB/ZL	10	86/100/89	1.60/1.60/1.60	1.86/1.60/1.79
Total	125	-	20.00	27.09/26.83/27.02
Water	-	-	-	22.90/23.16/22.97
Target solid	-	-	-	40.00
Total amount	-	-	-	50.00

^*OD (oven dry) required = (parts/total) × (target solid × amount of slurry)/100

^#Slurry = (OD required/solid content) × 100

Preparation of intumescent formulations and coating method:

The various samples of cotton fabric were prepared by coating with intumescent formulations containing kaolin nanoclay and other inorganic additives. The basic intumescent formulation (INT) for coating was prepared by taking 60 parts of APP, 20 parts of MEL and 20 parts of PER and 15 parts of binder in water. The intumescent formulation slurry of 50 g was prepared in each case for coating on fabric and firstly, the solid content of each component was determined by heating at 100^oC for 1 hr in an oven. The amount of different components required for preparation of intumescent formulations containing different additives (10 parts) with kaolin nanoclay (6 parts) were calculated accordingly. The detail calculation of amount of components required for intumescent formulation containing different additives is shown in Tables 1.

The fabric coating with intumescent formulation was carried out using an automatic K-control coater (RK-Print Coat Instrument Ltd. UK, model K 101) as shown in Figure 1. The coating paste was placed on one end of cotton fabric of 20 cm × 30 cm size and then spread with a rod coater. The coated fabric sample was placed in an oven immediately to dry for 2 min at 105 ^oC to eliminate the residual water and to crosslink the coating layer.

Figure 1. K-control coater used for coating.

Flammability study:

The ATLAS 45^o Automatic Flammability Tester (Model M233G AFC 45^o flammability chamber) was used to evaluate flammability of coated fabric samples according to ASTM D1230. In the 45° Automatic Flammability Test, the fabric sample was mounted at an angle of 45° with test specimen of size 15 cm x 6 cm of the fabric. The specimen was then exposed to a standard butane flame for 12 seconds to cause ignition, and then burning time and burning characteristics were recorded.

Thermal studies:

The thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses were carried out using a NETZSCH STA 449F1 TG instrument at heating rate 10 K/min in air atmosphere with flow rate 100 mL/min from ambient temperature to 700 ^oC. Sample weight of about 10 mg was taken each time for thermal analysis.

Mechanical studies:

The thickness measurement of fabric samples was done by using the Prolific Thickness Tester instrument (BS 2544:154).

The stiffness measurement of fabric samples was done by using Paramount Stiffness Tester (BS 3356:1961) with test specimens of 2.5 cm x 12 cm size. The tests were repeated for all the samples and an average was calculated.

RESULTS AND DISCUSSION:

Flammability behaviour:

The flammability behaviour of pure cotton fabric and coated fabric samples were evaluated by auto flammability test and compared at the ignition time of 12 sec (Figure 2). For pure cotton fabric (CF), the fabric sample burned entire length within 13 sec after removing the ignition source and thus failed this test. The coated cotton fabric samples did not ignite in this test. The char spot formation of char length in a range of 1.8-2.2 cm without any flame were observed, hence coated cotton fabric samples passed the flammability test (Table 2). Table 2 reveals that addition of nanoclay along with ATH, ZB and ZL in intumescent slurry shows no significant difference in burning behaviour. The coated cotton fabric samples exhibit good flame retardant properties due to formation of protective barrier layer of char on the surface of burning material⁸ to prevent it from the fire and made the degradation of the fabric difficult. The coated cotton fabric samples showed reduction in flammable volatiles as indicated by increase in char formation.

Figure 2. Burning behaviour of (1) CF, (2) CF-INT-ATH, (3) CF-INT-KLN-ATH, (4) CF-INT-ZB, (5) CF-INT-KLN-ZB, (6) CF-INT-ZL and (7) CF-INT-KLN-ZL samples.

Table 2. Flammability parameters of pure cotton fabric and coated cotton fabric samples.

Sample	Auto flammability test
	Flame spread time (sec)	Char length (cm)	Burning speed (m/hr)	Pass/Fail
CF	13	BEL^#	41.53	fail
CF-INT-ATH	DNI	2.0	--	pass
CF-INT-KLN-ATH	DNI	1.8	--	pass
CF-INT-ZB	DNI	1.8	--	pass
CF-INT-KLN-ZB	DNI	2.2	--	pass
CF-INT-ZL	DNI	2.2	--	pass
CF-INT-KLN-ZL	DNI	1.9	--	pass

^*DNI-Did Not Ignite, ^#BEL-Burn Entire Length

Thermal behaviour:

TG analysis:

TG curves of pure cotton and coated fabric samples are shown in Figures 3 and 4. The various parameters such as T_10wt%, (temperature at 10 % mass loss), T_50wt% (temperature at 50 % mass loss) and char yield at 600 ^oC are observed for comparing thermal stability and are given in Table 3. Pure cotton fabric (CF) reported in our earlier study¹⁰ shows two stages of thermal degradation which degraded completely upto 500 ^oC. The major weight loss during first stage degradation of pure cotton (CF) is due to dehydration, decomposition process and formation volatile products mainly laevoglucosan^11,12. The weight loss in second stage degradation of pure cotton fabric (CF) thermal is due to the oxidation of residue left after degradation in previous stage.

After coating with intumescent slurry containing INT and ATH, TG curve for this CF-INT-ATH sample shows three stages of thermal degradation. The weight loss for first two stages (100-310 and 310-600^oC) is almost same (40 %) with DTG peak at 293^oC, which is due to many processes of coated cotton fabric such as dehydration, dephosphorylation, and oxidations of volatile products and aromatic charred residues¹³ as well as dehydration and decomposition of ATH. Third stage of thermal degradation (600-700^oC) of CF-INT-ATH shows 8.1 % weight loss, which may be due to the oxidation of carbonaceous residue formed in previous stage. The onset temperature of degradation of CF-INT-ATH sample is found 279 ^oC which is decreased by 34 ^oC as compared to that of CF. The temperature at mid-point of decomposition (T_50wt%) is 371 ^oC, which is increased by 40 ^oC as compared to that of CF. When kaolin nanoclay is added in formulation for sample (CF-INT-KLN-ATH), no significant difference in weight loss pattern as well as in onset temperature is observed except decrease in the T_50wt% by 16^oC as compared to CF-INT-ATH.

On incorporating zinc borate additive in place of ATH for CF-INT-ZB sample, no significant change in TG curve is observed except slightly decrease in DTG peak. The onset temperature of degradation of CF-INT-ZB sample is observed at 265 ^oC, which is 14^oC lower than that of CF-INT-ATH and 48 ^oC lower than that of pure cotton fabric. The T_50wt% for CF-INT-ZB is 397 ^oC, which is 26 ^oC higher than that of CF-INT-ATH and 66 ^oC higher than that of CF. When kaolin nanoclay is added in formulation for sample (CF-INT-KLN-ZB), the T_50wt%is decreased by 73^oC. A great decrease in char yield (8.4 %) is also observed as compared to CF-INT-ZB (28.4 %). The addition of zeolite in CF-INT slurry in place of ATH and ZB did not give encouraging results (Table 2) for improvement in flame retardancy of cotton fabric.

In this study, addition of ZB increases the stability with greater contribution in increase of char yield. It is reported earlier¹⁴ that zinc borates dehydrate endothermically and released water which dilutes gaseous flammable products. It is also stated in literature¹⁴ that zinc borate can change the oxidative decomposition pathway of polymers but the reason for which is not known clearly whether this is due to an inhibition effect of boron oxide or the oxidation of graphite structure in the char¹⁵, or is due purely to the formation of a protective sintered layer on fabric substrate.

Figure 3. TG curve of (1) CF, (2) CF-INT-ATH and (3) CF-INT-KLN-ATH samples in air atmosphere.

Table 3. TG data of pure and coated cotton fabric samples in air atmosphere.

Sample	Stages	Temp. range (^oC)	Weight loss (%)	DTG (^oC)	T_10wt% (^oC)	T_50wt% (^oC)	Char yield at 600 ^oC (%)
CF	1^st 2^nd	100-360 360-510	78.0 20.5	330 470	313	331	0.35
CF-INT-ATH	1^st 2^nd 3^rd	100-310 310-600 600-700	40.0 38.4 8.1	293	279	371	21.1
CF-INT-KLN-ATH	1^st 2^nd 3^rd	100-310 310-600 600-700	41.5 40.9 7.2	293	274	355	16.6
CF-INT-ZB	1^st 2^nd 3^rd	100-310 310-600 600-700	38.3 32.6 9.4	286	265	397	28.4
CF-INT-KLN-ZB	1^st 2^nd 3^rd	100-310 310-600 600-700	46.5 44.5 6.7	288	267	324	8.40
CF-INT-ZL	1^st 2^nd 3^rd	100-310 310-600 600-700	41.7 37.7 10.8	288	272	353	19.6
CF-INT-KLN-ZL	1^st 2^nd 3^rd	100-310 310-600 600-700	42.7 40.6 8.15	287	270	349	15.8

Figure 4. TG curve of (1) CF-INT-ZB, (2) CF-INT-KLN-ZB, (3) CF-INT-ZL and (4) CF-INT-KLN-ZL samples in air atmosphere.

DSC analysis:

DSC thermograms of fabric samples were obtained from 40 to 700 ^oC in air atmosphere and are shown in Figures 5 and 6. The initiation and maximum temperatures alongwith the nature of various DSC peaks in air atmosphere were measured and are given in Table 4. DSC thermograms of pure cotton fabric sample (CF) shows two major exothermic peaks with maxima at 350 ^oC due to dehydration and oxidation process of the products with the formation of laevoglucosan and at 472 ^oC may be due to the oxidation process of char formed¹².

DSC curve of CF–INT-ATH shows the exotherms with maxima at 290 and 322 ^oC, which are lowered in comparison to pure cotton fabric (CF) due to catalyzed dehydration, phosphorylation and cross-linking process. The third DSC exotherm of CF–INT-ATH is observed at 451^oC, which may be due to aromatization reactions of the char residue¹⁶ formed in air atmosphere. On addition of kaolin nanoclay in this sample (CF-INT-KLN-ATH), no difference is observed in DSC curve in comparison to CF-INT-ATH.

The intumesecnt coated samples containing ZB additive show significant changes in DSC curves in which the intensity of second or last exotherm peak is reduced as well as shifted to higher temperatures (Table 4), which may be due to dehydration as well as formation of a protective sintered layer. DSC curve of CF–INT-ZB shows two exotherms at 281 and 498 ^oC, which were observed at lower temperature in comparison to pure cotton fabric (CF). On addition of kaolin nanoclay along with ZB for the above sample (CF-INT-KLN-ZB), a significant different thermal behaviour is observed by DSC analysis as compared to other combinations (Table 4). In case of CF-INT-KLN-ZB sample, the exotherm is seen at higher temperature (493 0C) in comparison to CF (472 0C), which indicates synergy of ZB and kaolin, and increase in char formation at the fabric surface by insulating cotton fabric from flame and atmospheric oxygen. The samples containing zeolite show almost similar DSC behavior to samples containing ATH.

Mechanical study:

Stiffness measurement:

The stiffness in warp wise direction of pure cotton fabric (CF) was 3.2 cm but for coated cotton fabric samples (CF-INT-ATH, CF-INT-ZB, CF-INT-ZL) the stiffness was observed in range of 5.7-6.2 cm and for samples containing kaolin (CF-INT-KLN-ATH, CF-INT-KLN-ZB, CF-INT-KLN-ZL) the stiffness was observed as 4.3-4.4 cm. In case of weft wise direction, stiffness was observed 2.6 cm for pure cotton fabric (CF) and increased to 5.6-6.3 cm for samples (CF-INT-KLN-ATH, CF-INT-KLN-ZB, CF-INT-KLN-ZL) and 4.3-4.4 cm for samples (CF-INT-ATH, CF-INT-ZB, CF-INT-ZL) as given in Table 5. The addition kaolin showed reverse effects on stiffness of fabric warp wise and weft wise.

Figure 5. DSC curve of (1) CF, (2) CF-INT-ATH and (3) CF-INT-KLN-ATH samples in air atmosphere.

Figure 6. DSC curve of (1) CF-INT-ZB, (2) CF-INT-KLN-ZB, (3) CF-INT-ZL and (4) CF-INT-KLN-ZL samples in air atmosphere.

Thickness measurement:

The thickness of pure cotton fabric (CF) was observed 2.2 mm and thickness of cotton fabric coated with intumescent containing additives with or without nanoclay for samples (CF-INT-ATH, CF-INT-KLN-ATH, CF-INT-ZB, CF-INT-KLN-ZB, CF-INT-ZL, CF-INT-KLN-ZL) varies from 2.9 to 3.3 mm as given in Table 5. Thickness and stiffness of coated cotton fabric samples are not too high than pure cotton fabric which indicates that the properties of cotton fabric would have not been affected significantly.

Table 4. DSC data of pure cotton fabric and coated cotton fabric samples in air atmosphere.

Sample

DSC temperature (^oC)

Nature of peak

Initiation temp. (^oC)

Maximum temp. (^oC)

337

445

350

472

Exo (large and sharp)

CF-INT-ATH

280

305

414

290

322

451

Exo (small and sharp)

Exo (small and broad)

CF-INT-KLN-ATH

280

308

411

295

331

461

Exo (small and sharp)

Exo (small and broad)

CF-INT-ZB

259

403

281

498

Exo (small and sharp)

Exo (small and broad)

CF-INT-KLN-ZB

286

477

303

493

Exo (medium and sharp)

Exo (small and broad)

CF-INT-ZL

280

317

440

291

333

467

Exo (small and sharp)

Exo (small and broad)

CF-INT-KLN-ZL

269

310

407

285

320

469

Exo (small and sharp)

Exo (small and broad)

Table 5. Stiffness and thickness values of pure cotton fabric and coated fabric samples.

Sample	Stiffness		Thickness(mm)
Sample	Warpwise (cm)	Weftwise (cm)	Thickness(mm)
CF	3.2	2.6	2.2
CF-INT-ATH	6.0	4.2	3.0
CF-INT-KLN-ATH	4.4	5.6	2.9
CF-INT-ZB	5.7	4.9	2.9
CF-INT-KLN-ZB	4.3	6.3	2.9
CF-INT-ZL	6.2	4.8	3.3
CF-INT-KLN-ZL	4.3	5.6	3.0

CONCLUSION:

Intumescent coating containing supportive additives as well as nanoclay successfully imparts flame retardant properties to cotton fabric. The thermal analysis of coated cotton fabric showed decrease in the onset temperature of degradation but increase in mid-point temperature of degradation as well as char yield. The auto flammability test results showed that all the coated fabric samples did not ignite and achieved better flame retardant properties. Mechanical study indicated that no significant change in mechanical properties of cotton fabric is observed while making the cotton fabric flame retardant.

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Received on 04.11.2013 Modified on 09.11.2013

Asian J. Research Chem. 6(12): December 2013; Page 1140-1145