Indian Journal of Fibre & Textile Research Vol. 30, December 2005. pp. 396-40 1

Flammability evaluation of cotton fabrics in vertical and horizontal burning modes

T M Kotresh", R Indu Shekar, M S Subbu Lakshmi & S N Vijay Lakshmi

Defence B io Engineering & Electro Medical laboratory, C V Raman Nagar, Bangalore 560 093, India

Received 30 August 2004; revised received alld accepted 9 February 2005

The burning behaviour of cotton fabrics has been evaluated using the horizontal and vertical modes of burning. The performance of fabrics has been studied in terms of rate of burning and temperature of burning fabric by placing the thermocouples at a distance of 0, 0.5 and 1 .0 cm from the burning fabric. The results show that the weight of fabric affects the rate of burning and the temperature of burning fabric. The rate of burning is found to decrease, whereas flame temperature increases with the increase in mass of fabric. The rate of burning is found to be independent of spacing both in horizontal and vertical modes of burning.

Keywords: Cloth cover, Cotton, Fabric tightness, Flame retardancy IPC Code: Int. CI.? D06H3/00, GOI N33/36

1 Introduction Clothing for protection against heat, flame, molten

metal splashes, radiation sources and chemicals has become essential for both civilian and defence personnel I . The increased pace of research and development activity on problems of textile flammability has been reflected in a number of technological developments. The choice of a fibre has a fundamental effect on the flammabi lity of work wear, since it affects the ease of ignition, rate of flame spread and the resultant combustion products . The selection of fibre/fabric has to be made after a careful consideration of parameters, viz. ease of ignition, rate of flame spread, production of char/molten drip, production of smoke, toxic gases and their density, after glow, extent of damage to adjacent skin , and heat of combustion of the materiae.

There is a growing interest among the scientific community to render mil itary and personnel protective clothing with flame retardant properties because of the increased level of threats encountered by the service personnel. The defence personnel are involved in various operations and exposed to various environments, i nvolving the threat of fire. Hence, there is an urgent need to develop effective flame retardant structures to protect the lives of soldiers, precious equipment and systems.

Although a significant amount of work has been carried out by various research organizations in the

"To whom al l the correspondence should be addressed. Phone : 25058392; Fax +9 1 -80-2528201 1 ; E-mail: tmkotresh @yahoo.co.in

past, a scientific and meaningful evaluation of flame retardancy has not been carried out so as to understand and achieve high level of flame retardancy. This stems from the fact that the subject is restricted to the defence related stores and does not provoke much interest in the civi l sector because of the techno-economic reasons.

Further, significant technological improvements have been made in the field of polymer science, flame retardants and evaluation techniques which necessitate the need to take up the work and exploit the technological inputs. The flammability of an organic polymer is determined by its chemical composition3. Although modification of this flammability is possible through the use of flame retardants, it is important to understand the chemical nature of the base material and its constructional particulars to select and optimize the flame retardant . If a polymer is exposed to an ignition source which does not provide enough heat to cause cleavage of the molecular chains into smaller fragments, the polymer will not support combustion. The molecular composition of the material to be flame retarded governs the thermal stability of the polymer as well as the selection of flame retardant chemicals to be used3.4.

The flammabi lity behaviour of textiles is influenced by a number of factors, such as mass/unit area, cloth cover, porosity, surface properties and finishing effects, like napped or lofty construction5.6. The conventional evaluation technique, such as char length evaluation used to ascertain the extent of flame

KOTRESH el al.: FLAMMABILITY EVALUATION OF COTTON FABRICS 397

retardancy, is merely based on pass/fail criteria and the data generated is of little use to enhance the extent of flame retardancy. Therefore, the conventional technique needs to be modified to generate relevant data. Alvares and Blackshear7 described an apparatus to measure the burning rate and heat transfer of vertically hanging cotton cloth panels. Carroll­Porczynski8 has reported the modified limiting oxygen index equipment to measure the temperature generated and rate of burning for the downward burning of a fabric.

In the present work, an attempt has been made to assess the hazard potential of 100 % cotton fabrics using the horizontal and vertical modes of burning. The present work also emphasizes on the parameters of base fabric which are expected to influence the flame retardant properties. The effect of these parameters on the rate of burning (RB) and flame temperature (Ff) during burning has also been studied. The microclimate conditions that exist between the fabric and the skin have been simulated and studied by placing the thermocouple at a distance of 0.5 cm and 1 .0 cm from the burning fabric. This is based on the fact that the position of thermocouple with respect to the burning fabric affects the sensing of the flame temperature. However, the work is restricted to untreated fabric and the data so generated may aid in optimizing the process parameters that are involved in the flame retardant treatment.

2 Materials and Methods

2.1 Materials

The constructional particulars of the various commercial cotton fabrics studied are shown in Table 1 . Iron-constantan thermocouples of the range 200° -750°C were used in the present study. The above range of temperature was found to be adequate as the temperature generated during the initial trials was in the range 300°-600°C.

2.2 Methods

Selected 17 fabrics were subjected to vertical mode of burning and evaluated for RB and Ff using the set­up and the method as discussed earlier9• For the horizontal mode of burning, the test set-up used for the vertical mode of burning was suitably modified to mount the thermocouples on the top surface of the fabric at regular intervals. For ease of igniting the fabric in the horizontal mode, 1 2.5 mm of fabric was allowed to drop vertically into the bunsen burner flame.

Ff and RB were evaluated for both the horizontal and vertical modes of burning as reported in previous stud/ . In the present study, Ff and RB were evaluated by positioning the thermocouples at three positions, viz. (a) in contact with the burning fabric (0 cm space), and (b) 0.5 cm and (c) 1 .0 cm away from the burning fabric. At least four samples of each

Tablel- Constructional particulars of fabric

Fabric code

2 3 4 5 6 7 8 9 10 1 1 1 2 1 3 1 4 1 5 1 6 1 7

Mass gsm

54 76 78 1 08 1 1 8 1 20 1 30 1 35 1 36 1 40 1 40 1 50 1 54 220 265 270 426

Thickness Weave mm

0. 1 8 Plain

0.24 Plain

0.27 Plain

0.44 Plain

0.32 Plain

0.39 Plain

0.39 Plain

0.34 Plain

0.33 Plain

0.34 Plain

0.3 1 Plain

0.35 Plain

0.32 Plain

0.42 311 twill

0.6 1 3/1 twill

0.68 Inter lock

1 .46 3/1 twill

Air porosity Cloth cover Air permeability % Kc cc/cm2/s

30 14.00 40.32 32 1 8.70 36.38 29 1 5 .40 26.50 25 1 6.80 3 1 .20 37 1 6. l l 27.05 3 1 1 7.20 32.78 33 1 8.00 22. 1 5 40 1 9.60 41 .60 4 1 1 2.80 22.60 4 1 1 8.20 1 9.58 45 19 .70 1 8.23 43 2 1 .00 2 1 .42 48 1 8.00 23.20 52 25.82 9.36 43 1 6. 1 0 >60.00 40 1 9.40 1 7 .00 29 24.64 5.26

398 INDIAN J. FIBRE TEXT. RES., DECEMBER 2005

fabric were evaluated and the average reading was noted. If the difference between the four readings exceeded ±5% of the mean value, testing was repeated until four consecutive readings within the above limits were obtained.

3 Results and Discussion

3.1 Flame Temperature

The burning behaviour data of cotton fabrics in terms of flame temperature and rate of burning are shown in Tables 2 and 3 , and graphically represented in Figs 1 and 2. For ease of presentation, the FT in horizontal mode is represented as FTH while FTV represents FT in vertical mode. The generated data for all the three different spacings show that the FTH and FTV increase with the increase in mass and thickness of the samples. This finding is in the expected lines as the heavy fabrics provide more amount of fuel for burning as also reported earlier9• However, the magnitude of FTV at all three spacings is higher vis­a-vis that of FTH values and is attributed to the heat loss to the surroundings in the horizontal mode of burning, while in the vertical mode a part of the heat generated is used in pre-heating the unburnt fabric. FTH values show a significant decrease as the spacing increases, whereas the FTV values also decrease but not as markedly as FTH values; this is attributed to the cumulative sensing of the temperature by the thermocouples mounted in the vertical orientation. As can be seen from Table 2, the fabric<: in the similar weight range (fabric codes 8- 1 1 ) show almost similar flame temperature values recorded at any one of the three spacings. The correlation coefficient values (ro, rO.5 and rl .o) between FT and mass are 0.92, 0.92 and 0.96 for horizontal mode of burning and 0.94, 0.96 and 0.97 for vertical mode of burning when the thermocouples are \n contact, at 0.5 cm spacing and 1 .0 cm spacing respectively (Table 4). The fabric parameters, like air permeability and thickness, exhibit fairly high correlation while the cloth cover exhibits poor correlation.

3.2 Rate of Burning Table 3 shows that the RB decreases as the fabric

mass increases in both the modes of burning at all the three spacings. This finding is in line with those reported by other researchers9• 1 O• Rate of burning in the horizontal mode (RBH) is significantly lower than that in vertical mode (RBV) and is attributed to the heat loss to the surroundings in the horizontal mode of

Table 2-Buming behaviour of fabric in terms of flame temperature

Fabnc ____ �

------�F�la=m�e�t�em

�p.�, o�C�--��

-----code At 0 cm At 0.5 cm At 1 .0 cm

2

3

4

5

6

7

8

9

10

I I

1 2

1 3

14

15

16

17

spacing spacing spacing H V H V H V

207

282

276

300

309

320

330

345

357

362

362

389

395

4 1 5 482

505

521

3 1 3

37 1

380

456

440

453

463

475

454

493

487

489

494

597

622

628

685

92

124

1 27

1 84 1 95

200

204

2 1 1

220

2 1 5

220

235

247

274

300

307

359

240

266

280

30 1

3 14

3 1 8

340

352

345

365

377

385

394

502

535

554

625

49

52

52

58

59

60

62

65

65

66

65

73

73

80

88

92

102

173

195

203

242

256

256

296

280

279

299

262

3 1 5

3 1 1

380

405

4 1 1

5 12 H-Horizontal mode; and V-Vertical mode

Table 3-Buming behaviour of fabric in terms of rate of burning

Fabric Rate of burning, cmls code At 0 cm At 0.5 cm At 1 .0 cm

2

3

4 5 6

7

8

9

10 1 1 12

13 1 4 15

16

17

spacing spacing spacing H V H V H V

0.60 1 1 .40 0.60

0.49 8.58 0.50

0.49 8.4 0.50

0.34 4.85 0.34 0.32 4. 14 0.30

0.3 1 4. 15 0.3 1

0.29 3.96 0.30

0.28 3.96 0.29 0.28 3 .81 0.29 0.26 0.26

0.24

0.24

0. 1 9

0. 17

0. 1 7

0. 1 1

3.8 1 3.8 1

3.68

3.26 2.34

2.0

1 .90

0.84

0.25

0.26 0.24 0.25

0. 1 8

0. 17

0. 1 7

0. 1 2

1 1 .30 0.60 1 1 .43

8.60 0.48 8.90 8.4 0.47 8.9

4.90 0.34 5.00

4.30 0.30 4.50

4.30 0.30 4.40

3.80 0.29 3.30 3.70 0.29 3.40 3.80 0.29 3.26

3.80 3.80 3.70

3.20

2.30

2.0

1 .85

0.88

0.25

0.26 0.24

0.25

0. 1 8

0. 17

0. 17

0. 1 2

3.26

3.30 3.04

3.00

2.35

1 .90

2.00

0.90

KOTRESH et ai. : FLAMMABILITY EVALUATION OF COTTON FABRICS 399

700 700 Y 0= 0.8524x + 225.8 (a) Yo = 2 1 6 . 1 4x + 268.22 (b)

600 · 600 Y 0.5 = 1 80.87 x + 1 39.84 �u Y o , = 0.6965 x + 1 07.04 oU Y . o = 41 .674x + 50 . 1 78

500 · 500 ci. ci. � 400 E 400 § .2! OJ 300 OJ 300 E E '" 200 '" 200 u.. u..

1 00 • - 1 00

0 0

0 50 1 00 1 50 200 250 300 350 400 450 0 0.2 0.4 0.6 0.8 1 .2 1 .4 1 .6

Mass. gsm Thickness. mm

7 ;;- 7 ;;- Yo = -0.01 08x + 4.6734 (c ) Yo = -2.73 1 6x + 4.1 521 (d) � 6 Y 0 5 = -0.01 1 2x + 4.7735 � 6 Y 0.5 = -2.6839x + 4.1 491 </l Y ' .0 = -2.6 1 2 x + 4.0766 </l Y ' .0 = -0.01 08x + 4.6734 E E 5 5 u u C; 4 V1 4 c c :: ;: 3 3 ::J :J .0 -0 2 '0 2 '0 OJ .2! iO r;; CI: CI:

0 0

0 " 5 1 50 225 300 375 450 0 0 .3 0.6 0.9 1 .2 1 .5 1 .8

Mass. gsm Thickness, m m

Fig. 1 - Influence of (a) mass on flame temperature, (b) thickness on flame temperature, (c) mass on rate of burning, and (d) thickness on rate of burning during horizontal mode of burning [(0) TC in contact, (0) TC at 0.5 cm spacing, and (. ) TC at 1 .0 cm spacing]

800 800 · (a) (b)

700 700 ·

600 600 � � c: 500 ci. 500 ·

E � 400 E

400 · � .. .. E 300 E 300 Yo = 262.17x + 374.27 !! Yo = 1 .0068x + 327.1 5 !! u.. ... Yo., = 303.77x + 249.89 200 Yo., = 1 .1614x + 1 96.1 1 200 · Y, o = 253.92x + 1 88.1 5 1 00 Y, o = 0.9344x + 149.02 1 00

0 0

0 50 100 1 50 200 250 300 350 400 450 0 0.25 0.5 0.75 1 .25 1 .5 1 .75

Mass, gsm Thickness , mm 1 3 1 3 (c) (d) Yo = -0.0224x + 7.986

1 1 Y o = -5.4804x + 6.7876 1 1 Y O.s = -0.0223x + 7.9604 !E. If) Y, o = -0.0225x + 7.8785 E Yo.s = -5.4314x + 6.751 1 E u 9 Y • . o = -5.3295x + 6.60 1 5 u

9 o:ii o:ii .E c: c: 7 'E 7 :; :::I ,Q aI --5 '0 5 0 .. .. ... iO 0:: 3 0:: 3

1

0 0.2 0.4 0.6 0.8 1 .2 1 .4 1 .6 0 50 1 00 1 50 200 250 300 350 400 450

Thick ness , m m M a s s , gsm Fig . 2- Influence of (a) mass on flame temperature, (b) thickness on flame temperature, (c) thickness on rate of burning, and (d) mass on

rate of burning during vertical mode of burning [(0) TC i n contact, (0) TC at 0.5 cm spacing, and (.) TC at 1 .0 cm spacing]

400 l NDIAN J. FIBRE TEXT. RES . . DECEMBER 2005

7 1 2 0- 6 (a) Y = -0.0014x + 0.7964 ( b ) Y = -0.0246x + 1 6.41 7 ..- 1 0 � � 5 E 8 u Oi 4 c: 6 E 3 :J .c 4 - 2 0 2 2 <tI Ck::

0 0 0 1 00 200 300 400 500 600 0 1 00 200 300 400 500 600 700 800

Flame tem perature , °C Fig. 3-lnfluence of flame temperature on rate of burning [(a) horizonwl mode. and (b) vertical mode]

Table 4-Relationship between constructional particulars of the colton fabric anel flame temperature (IT) or rate of burning (RB)

Relationship 1"0 ro.) H v H v H v

Mass V.I' IT 0.92 0.94 0.92 0.96 0.96 0.97

Thickness V.I' IT 0.75 0.78 0.77 0.8 1 0.82 0.85

Mass V.I R B -0.80 -0.76 -0.79 -0.76 -0.80 -0.72

Thickness vs RB -0.62 -0.59 -0.60 -0.59 -0.6 1 -0.55

Cloth cover vs IT 0.56 0.63 0.59 0.62 0.60 0.62

Cloth cover vs RB -0.57 -0.52 -0.57 -0.52 -0.58 -0.50

AP vs IT -0.82 -0.84 -0.82 -0.84 -0.8 1 -0.83

AP vs R B

RB vs IT 0.77 0.73 0.77 0.72 0.78 0.7 1

-0.9 1 -0.89 -0.96 -0.80 -0.87 -0.84

I'll> 1"0.5 and I"I-Correlation coefficients at 0, 0.5 and 1 .0 cm spacings. AP-Air permeabi l ity

burning wherein the flame is perpendicular to the surface of burning fabric. This results in the loss of heat energy to the surroundings which otherwise would have been utilized partly to heat the unburnt portion of the fabric as in the case of vertical mode of burning. The heat loss, though not quantified in the present study, is presumed to be higher and is reflected in the lower values of RBH. Also, no reduction in either the RBH or the RBV values is observed at the three levels of spacing.

The fabrics in the l ight weight category (up to 1 00 gsm) exhibit high RB, whereas fabrics in the weight range beyond 1 00 gsm show low RB, suggesting that this class of fabrics is ideal for personnel protective clothing to achieve a minimum level of flame retardancy. Further, it has been observed by the practical experience that it is difficult to achieve a

satisfactory level of flame retardancy with low add-on of flame retardants in l ight weight fabrics . Higher add-on results in poor comfort properties and strength reduction associated with tt-.ldering. ThL decrease in the rate of burning with the increase in weight and thickness is attributed to (i) the increased amount of fuel in unit volume, ( i i) the fabric tightness, and (iii) less amount of air entrapped between the yarns. It is generally found that with the increase in weight of the fabrics the cloth cover increases with a corresponding decrease in the air volume and the avai l able air spaces.

Table 4 shows that the functional parameters FT and RB have an inverse correlation and are attributed to the mass of the fabrics. It is observed that as the mass of the fabric increases, the rate of burning decreases. This is because in heavier fabrics more amount of material is available for combustion, which. in turn, results in higher amounts of energy being released with corresponding increase in flame temperature. The relationship between IT and RB in horizontal and vertical modes of burning IS graphically represented in Fig. 3 .

4 Conclusions The rate of burning decreases while flame

temperature increases as the mass of fabric increases. A significant reduction in flame temperature is observed with the increase in spacing of thermocouples in the horizontal mode of burning . The rate of burning is found to be independent of spacing either in the horizontal mode or in the vertical mode of burning, though the magnitude of rate of burning in vertical mode is much higher than in the horizontal mode.

KOTRESH eT al. : FLAMMABILITY EVALUATION OF COTION FABRICS 40 1

The study encompasses a spectrum of fabrics in the light, medium and heavy mass range and reveals that it is the mass of the fabric that has over riding influence on the rate of burning and flame temperature. This finding will immensely help in selecting the base fabrics for flame retardant treatment . The flame temperature values recorded in the present study (49°-52 1 DC for horizontal mode and 173°-685DC for vertical mode) are of much higher magnitude as the human skin suffers irreversible skin damage at a temperature of nDc (ref. 1 1 ) . The study also suggests the necessity of flame retarding the apparel fabrics to restrict, if not eliminate, the potential danger for the human beings involved in any of the unforeseen fire scenario.

Acknowledgement The authors are extremely thankful to Shri G P

Agrawal , Director, DEBEL, for his keen interest and encouragement during the study. They are also

thankful to Ms Anita Bettaiah and A R Tejeswini for their assistance in carrying out the experimental work .

References I Bajaj P. Chakrapani S & Jha N K, TeXT Res 1. 54 ( 1 984) 6 1 9. 2 John Ford, TexT Horiz.ion. May ( 1 989) 40. 3 Horrocks A R. Tunc M & Price D. The burning behaviour of

textiles and its assessment by oxygen i ndex methods. TexT

Prog. 1 8( 1 12/3) ( 1 989). 4 Thomas Mitchell. 1 Cowed Fabric. 23 ( 1 994) 298. 5 Tesoro G C & Meiser C H (Jr), TeXT Res 1. 40 ( 1 970)430. 6 Tesoro G C, Sel la S B & Wil lard J J, TexT Res 1. 39 ( 1 969)

1 80. 7 Alvares N J & Blackshear P L, FLammability of Fabrics­

Fire and Flammability Series, Vol 9, edited by C J Hi lado (Technomic, Westport, USA), 1 974. 1 1 5 .

8 Carrol l-Porczynski C Z. The FlammabiLiTY o{ ComposiTe Fabrics (Astex, Gui ldford, UK), 1 976.

9 Kotresh T M. llldian 1 Fibre TexT Res, 2 1 ( \ 996) 1 57- 1 60. 1 0 Backer S . Tesoro G C . Tong T Y & Moussa N A. TexTile

Fabric FlalllmabiliTy (MIT Press, Cambridge Mass. USA). 1 976.

1 1 Stoll A M, i n Advances in HeaT Trallsfer. Vol 4. edited by H P Harnett & T F I rv ine (Academic Press, New York, USA). 1 967, 1 1 5 .