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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
"To whom all the correspondence should be addressed. Phone: 659
14 18; Fax: 009 1 -0 1 1 -6857757; E mail: [email protected]
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
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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
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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
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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
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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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