Indian Journal of Fibre & Textile Research Vol. 25, December 2000, pp. 24 1 -245

Flame retardancy of Nomex/cotton ring-spun blended yarn fabrics

R S Rengasamy', V K Kothari & R L Gotipamul Department of Textile Technology, Indian Institute of Technology, New Delhi 1 1 0 0 1 6, India

Received 2 November 1999; revised received and accepted 25 January 2000

Fabrics knitted from Nomex, cotton and Nomex I cotton blended yams have been treated with a FR finish 'Flovan CGN' of phosphoric acid salt containing nitrogen and tested for char length, l imiting oxygen index (LOI), mechanical properties and wash fastness of FR finish. It is observed that LOI increases and the char length decreases with an increase in Nomex content in blends with cotton. The wash fastness of FR finish after three washes is better on cotton fabrics than on Nomex/cotton blended yam fabrics. Fabrics made from Nomex yams have lower shrinkage and high bursting strength than those made from cotton and Nomex/cotton yams. The FR finish improves the LOI values and reduces the char length and times of afterglow and afterflame for all the fabrics. The bursting strength of FR-treated fabrics shows no significant effect of FR finish.

Keywords: Char length, Cotton, Flame-retardant finish, Limiting oxygen index, Nomex

1 Introduction Industrial workers are prone to accidents and

protective clothing is essential for workers to prevent such accidents. One of the most demanding fields of protective clothing is fire protection. Protective clothing should have the following characteristics, viz. slow flame spread, low heat generation 1 , high degree of insulation to the wearer, higher dimensional stability, strength and abrasion resistance, and high strength retention . They should not melt or form chars that may disintegrate under flame2• They are also expected to give good breathability, excellent handling and wearing qualities to the wearer3. The techniques and raw materials used to make flameretardant garments are often expected to meet the demanding industrial specifications.

Nomex fibre is inherently flame-retardant and has a moisture regain of 4.5%. The fabrics made from Nomex fibre can be laundered or dry-cleaned without losing its flame retardancyl . Nomex fibre loses 50% of its strength when exposed to direct sunlight for 50 h. It retains half of its strength at 2850 C. Fabrics made from Nomex fibres are uncomfortable due to the hot feel provided by them to the wearer. This may be corrected by blending with flame-retardant (FR) viscose, polynosic and some FR-treated natural fibres at the cost of abrasion resistance. Kotresh et af. 4 have developed FR-outer garments for anti G-suit from

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friction spun hybrid yarns using Kevlar and FR viscose in core sheath form.

Cheaper work-wears are made from 'Karvin', which contains 30% Nomex, 5% Kevlar and 65% FRviscose. Many flame-retardant fibres, viz. BasofiI, TPI I , Teij in-Conex, Kermel, Lenzing' s P84, Kynol and Philene, have been developed to meet the evergrowing demand for fire protection clothing5,6. Fabrics using such expensive fibres may not be acceptable for certain applications including personal protection . In addition, the protective clothing needs to provide some degree of comfort to the wearer. In this respect, the demand for hybrid yarns made from fibre components is constantly growing.

The recent trend is to use blended yarns for fire retardancy and other functional applications . Mixing cheaper fibres to the costly flame-retardant fibres in making the yarns can minimize the cost of flameretardant fabrics. In addition, the fabrics can be made from these yarns with a range of mechanical, comfort and functional properties, required for various applications. Fabrics made from yarns spun from single fibre type can ' t have a wide range of properties.

Anand and Garvey? have studied the effect of blend variables, type of spun yarns and fabric structural parameters on the flammabil ity characteristics of weft knitted fabrics made from Visil , modacrylic, wool and their blends. For the same blend ratios, the fabrics made from rotor-spun yarns exhibit higher limiting oxygen index (LOI) values and lower char lengths

242 INDIAN 1. FIBRE TEXT. RES. , DECEMBER 2000

than those for made from ring-spun yams. It was also observed that flammability characteristics could be substantially improved by combining two different . yam types in the same fabric rather than using blended fibre yams.

Cotton is known for its comfort and aesthetic properties. Transport of perspiration in liquid form is an important contributor to the comfort of the garment to the wearer8• I O• In this respect, the presence of cotton or FR-viscose in blends with synthetics improves the comfort of fabrics? Nomex fibre has an excellent resistance to combustion, dimensional stability up to 450° C, very good abrasion resistance and average tensile strength. Nomex fabrics start decomposing at about 370 °C and undergo shrinkage and puckering at about 370 - 375°C. FR-treated fabrics made from cotton blended yams in blends with Nomex fibres are the potential candidates for work-wear, where, reasonable comfort to the wearer is one of the primary requirements. Moreover, such blend combination can be made cheaper, yet meeting the required flammabil ity characteristics. This 'is possible by proper selection of blend ratio and engineering the blended yam structures.

2 Materials and Methods

2.1 Materials J-34 cotton (26 mm) and Nomex staple fibre (2.2

dtex, 5 1 mm) were used. The fibre characteristics are given in Table I . 2.2 Yarn and Fabric Production

Nomex and cotton were manually opened and processed separately through pilot and normal cards respectively with appropriate settings . The carded slivers of 0. 1 8s Ne were given one draw frame passage and then blended at second draw frame, further processed through two passages of draw frame. The set blend ratios for Nomex/cotton yams were 67 :33, 50:50 and 33:67 . The material was processed on the speed frame and ring fram� to

Cotton

Table I -Properties of fibres used in the study

Nomex

2.5% Span length, 26.44 mm 50% Span length, 1 2. 8 1 mm Uniformity ratio, 48.4 % Bundle strength, 22.8 eN/tex Mieronaire value, 3 .8 Maturity, 0.693 (eaustieaire method)

Length, 5 1 mm. Fineness, 2.2 dtex Tenacity, 35 eN/tex Elongation at break, 40% Modulus, 963 eN/tex

produce yams of 1 2s Ne (49 tex). Optimum twist multiplier (cotton count system) selected for 100% Nomex, 100% cotton and blended yams of 67 :33, 50:50 and 33 :67 blend ratios were 2.4, 4.5, 2.54, 3 . 1 1 and 3 . 14 respectively. Single yams were cleared through the Autoconer, two plied and twisted with a folding twist of 7 .8 twists/in. (307tpm). All the blended yams were waxed before knitting on a flat knitting machine. The machine gauge was kept at 10 needles/in . . The courses per inch were kept at 16 . Scouring was carried out on the fabric samples before applying the FR-finish. The scouring conditions were as follows: Lissapol-N 0.2%; NaOH 1 .5% and Na2CO] 1 .5% (owf). The temperature of the bath was kept at 1 25 °C for 2 h.

2.3 Flame-retardant Finishing A commercial flame-retardant finish 'Flovan CGN'

of phosphoric acid salt containing nitrogen was used with cold water. The pH of the finish is 4.5-6.0 at 1 00gllitre at 20°C. The percentage of nitrogen and phosphorus in the finish is not known. The finish was applied on fabrics made from cotton and Nomex/cotton blended yams. The add-on percentage was set at 1 8 (w/w) based on the cotton content in the fabrics. To see the effect of FR finish on Nomex, the FR finish was also applied on 1 00% Nomex fabrics with 1 8% add-on.

The treated fabrics were passed through rollers of a laboratory model padding machine, where the pressure on the top roller can be varied up to 7 kg/cm2. The pressure on the top roller pressure was varied to get a wet pick of 82 % for cotton fabrics. The % add-on was adjusted for other samples by varying the pressure and concentration of FR finish. The pressure was varied from 1 kg/cm2 to 1 .5 kg/cm2

and the concentration of the solution from 1 50 gpl to 300 gpl depending on the sample. The padded samples were dried for 5 min at 100°C and then cured at a stenter at 140°C for 5 min. The samples were weighed before and after FR finishing and the add-on percentage was calculated from these weights.

2.4 Fabric Testing The fabrics (both treated and untreated) were tested

for LOI, char length, and times of afterflame and afterglow, after conditioning in a standard atmosphere for 8 h. For the LOI test, ASTM standard D2863 was followed. The fabric was placed vertically in the centre of glass chimney of a commercial instrument. Regulating the flow of nitrogen and oxygen separately can control the atmosphere in the chimney. The fabric

RENGASAMY et al.: FLAME RETARDANCY OF NOMEX/COTTON RING-SPUN BLENDED YARN FABRICS 243

specimen was ignited at the upper end and the nitrogen-oxygen ratio was adjusted until combustion was just supported. The minimum percentage of oxygen to support combustion is the LOI value. Ten specimens, five each in wale and course direction, were tested for LO!. Frum these values, the average LOI value for each fabric was calculated.

For testing the char length, and times of afterflame and afterglow, the ASTM standard D4723 (identification No. 36) was followed. The fabric specimen was vertically suspended over a propane flame for 1 2 s. The time taken by the flame on the fabric to extinguish itself after removal of the fabric from the flame of the burner is the time of afterflame. The time of glow on the fabric after extinguishing of the flame is called the time of afterglow.

The bursting strength of untreated and treated fabric samples was tested using diaphragm bursting strength tester. Fabric shrinkage was measured after soaking the fabrics in soap solution for 30 min, washing and then air drying at 100°e. Fabric thickness was measured under a pressure of 1 9.6 cN/cm2 in fabric thickness gauge.

2.S Wash Fastness of FR Finish To study the wash fastness of FR finish, the fabric

samples were subjected to three washing cycles and dried. With each washing cycle, the following conditions were maintained: soap (Lissapol N), 5gpl; soda ash, 2gpl; M:L ratio, I :50; temperature, 60°C; and time 30 min. LOI values were also measured for dried fabric samples after third washing. The percentage decrease in LOI values after washing in relation to the LOI values of fabrics before washing was calculated.

3 Results and Discussion

3.1 Processability of Fibres in Yarn Manufacturing and Yarn Properties

The processability of fibres during yarn manufacturing was found to be better with cotton and Nomex/cotton blends than for 100% Nomex. However, this needs to be quantified. Table 2 shows the tensile properties of ring-spun yarns made from Nomex, cotton and their blends.

As the proportion of Nomex in the blend increases, the tenacity, strain at break and modulus increase due to better tensile properties of Nomex. The yarns made from cotton fibres suffer from low tensile strength. However, the addition of Nomex can overcome this deficiency.

Table 2-Tensile properties of ring-spun yarns made from Nomex, cotton and their blends

Property 1 00% 67/33 50/50 33/67 1 00% Nomex N / C N/C N/C Cotton

Tenacity at break, cN/tex 24.0 1 8 .2 1 2.3 1 1 .4 1 0.2

Strain at break ,% 24.7 23.0 1 4.0 9.6 6.7

Initial modulus, cN/tex 340 288 294 275 220

3.2 Flame Retardant Properties of Fabrics The fabric particulars and flame retardant

properties for fabrics made from Nomex, cotton and their blends are shown in Table 3 . It is observed that the LOI values are insensitive to course and wale directions.

3.2.1 Effect of Blend Ratio For both the treated and untreated fabrics, the LOI

is high for Nomex, followed by Nomex-rich blends and then cotton. An increase in LOI with an increase in th� proportion of Nomex for blended yarn fabrics may be ascribed to the increase in the pyrolysis temperature. Aramides do not easily breakdown into combustible molecular fragments even in the absence of FR chemical elements2•

For cotton fabrics, the increase in LOI after finish is only 2 1 %, whereas for N omex fabrics, the increase is 85%. The % increase in LOI after FR treatment for blended fabrics is 92, 75 and 8 1 % for Nomex/cotton of 67:33, 50:50 and 33 :67 blend ratios respectively. FR-treated Nomex/cotton blended yarn fabrics have higher LOI values than the untreated Nomex fabrics. In addition, the other flammability characteristics of blended fabrics improve largely after FR finishing. This is due to the additional effect of reduction in the amount of flammable gases in the presence of FR chemical. This suggest that FR-treated fabrics from Nomex/cotton yarns with blend ratio of 50:50 or 33 :67 can be suitably employed for flame-retardant applications. Moreover, the cost of these blended fabrics is favourable compared to 1 00% Nomex fabrics.

Among the unfinished fabrics, the Nomex fabric has very low char length followed by Nomex-rich blends and then cotton. Times of afterflame ' and afterglow do not exist for 1 00% Nomex fabrics. This is due to the inherent flame-retardant properties of the Nomex fibers. An increase in char length and times of afterflame and afterglow is observed with an increase

244 INDIAN J. FIBRE TEXT. RES., DECEMBER 2000

Table 3-Fabric construction particulars, flame retardancy and other properties of Nomexlcotton ring-spun blended yam fabrics

Particular 1 00% Nomex

Yam count, tex 92

Thickness, mm

Before finish 2.42 After finish 2.54

% Add-on of FR-finish 1 7.9 (w/w)

LOI before finish 30.5 Wale-wise 30.5 Course-wise 30.4

LO! after finish 56.3 Wale-wise 56.5 Course-wise 56.0

Char length, cm Before finish 2 After finish

Time of afterflame, s Before finish Nil After finish

Time of afterglow, s Before finish Nil After finish

Bursting strength. N/cm2

Before finish 1 74.4 After finish 1 73.5

Strength retention, % 99

Fabric shrinkage, % 1 . 1 8

Wash fastness LOI after 3 washes Reduction in LOI after 3 washes, %

N - Nomex; and C-cotton

in cotton proportion for both the treated and untreated fabrics. Cotton fabric has more char length and longer times of afterflame and afterglow. Char length decreases considerably on treatment with FR finish.

3.2.2 Effect of FR Finish FR treatment increases the LOI values and

decreases the char length of fabrics considerably. Afterflame and afterglow were not observed for FRtreated fabrics, except in the case of cotton fabrics. The FR-treated fabrics made from Nomex and blends do not catch fire in 1 2s. This indicates the excellent

67/33 50/50 33/67 1 00 % N/C N/C N/C Cotton

98 95 9 1 88

2.8 1 2.65 2.63 2.33 2.96 2.82 2.84 2.49

5 .5 8 .8 1 2.2 1 8.2

27. 1 26.4 23.4 25. 1 27.2 26.2 23.4 25.2 26.9 26.5 23.3 25.0

52. 1 46. 1 42.3 30.3 52.2 46. 1 42.4 30.2 52.0 46.0 42. 1 30.4

4 \ 0 1 0.8 20.3 2.8 3.2 4.7 1 3.5

3.5 4.8 5.5 1 0.8 Nil Nil Nil 9.8

2.8 3.5 5 .8 7 .5 Nil Nil Nil 6.8

1 25.4 1 22.5 1 1 1 .7 1 1 0.7 1 25.4 1 20.5 1 1 0.7 1 08.8 1 00

1 .30

48.4

7. 1

98 99 98

1 .25 1 .50 4.20

43.8 4 1 .8 30.0

6.6 1 .2 1 .0

flame-retardant properties of Nomex and its blends with cotton. In the case of FR-tr�ated cotton fabrics, the treated sample catches the fire in 1 2s and starts to propagate, so more char length is observed for these fabrics.

The wash fastness of FR finish is found to be better on 1 00% cotton fabrics than on Nomex/cotton blended yarn fabrics. The percentage decrease in LOI values of FR-treated blended yarn fabrics after three washes shows that the decrease in LOI is higher for fabrics of high Nomex contents. The durability of FR finish containing phosphorus and nitrogen elements

RENGASAMY et al. : FLAME RETARDANCY OF NOMEX/COTTON RING-SPUN BLENDED YARN FABRICS 245

used in this study is better on cotton than on Nomex due to the better reactivity of the FR elements with cotton than with Nomex.

3.3 Mechanical Properties of NomexlCotton Blended Yarn Fabrics

Fabric made from Nomex yarn has higher bursting strength and better dimensional stability than the fabrics made from cotton or blended yarns. The bursting strength increases linearly with an increase in Nomex proportion. It is to be noted that the fabric constructions are the same for all the fabrics. The yarns used in constructing the fabrics were produced at optimum twist level (to get maximum tenacity). It is clear that the high bursting strength of Nomex fabric is due to the rigid backbone chain of metaaramid fibre. The bursting strength of fabrics is not affected by the presence of FR finish.

Cotton fabric shows higher shrinkage as compared to the fabrics made from Nomex and Nomex/cotton blends. As the cotton content in the blend increases, the fabric shrinkage also increases.

4 Conclusions As the proportion of Nomex in the Nomex/cotton

ring-spun yarns increases, the tenacity and strain at break increase. Nomex tends to absorb FR finish based on phosphorus origin. The increase in LOI values of FR-treated fabrics from that for unfinished fabrics is more for Nomex fabrics than for cotton fabrics. Fabrics made from al l Nomex yarns have higher LOI than the fabrics made from 67:33 and 50:50 Nomex/cotton blended yarns. The other flameretardant properties, viz char length and times for afterflame and afterglow, are superior for Nomex fabrics as compared to those for the fabrics made from cotton and Nomex/cotton blends.

Fabrics made from Nomex yarns have higher bursting strength and lower fabric shrinkage than those made from cotton and Nomex/cotton blended yarns. The FR finish based on phosphorus used in the study does not reduce the bursting strength of treated fabrics. Cotton fabric shows the higher shrinkage than the fabrics made from Nomex and Nomex/cotton blends. Addition of Nomex to cotton improves the dimensional stability of the fabrics. The wash fastness of FR finish based on phosphorus is better on cotton than on Nomex.

The flammability characteristics of FR-finished Nomex/cotton blended yarn fabrics with blending ratios of 50:50 and 33:67 are comparable to those of unfinished fabrics made from 1 00% Nomex fibre yarns. From economical and functional points of view, the Nomex/cotton blended yarn fabric with 67% cotton has potential for flame- retardant applications.

Acknowledgement The authors are thankful to Mis Dupont for

providing Nomex fibres. They are also thankful to Northern India Textile Research Association, Ghaziabad, for supplying cotton fibres and allowing to use their testing facilities in carrying out this work.

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