The flammability of fabrics is determined by a variety of factors, such as fiber type, fabric structure, fabric weight, and fabric finish. Generally, natural fibers like cotton, wool, and silk are more likely to ignite than synthetic fibers like polyester, nylon, and acrylic. However, they are also likely to burn slower and self-extinguish faster. Also, fabrics with a loose or open structure like knits, lace, and fleece are more likely to ignite than those with a tight or dense structure such as woven or nonwoven fabrics. In addition, fabrics with a low weight or thickness like chiffon, voile, and organza are more likely to ignite than those with a high weight or thickness such as denim, canvas, and leather. Moreover, fabrics with a flammable or combustible finish such as starch, wax, or resin are more likely to ignite than those with a flame-retardant or fire-resistant finish such as borax, alum, or bromine compounds. External conditions like temperature, humidity, oxygen level, and wind speed can also affect the flammability of fabrics; higher temperature and lower humidity can increase the flammability while lower temperature and higher humidity can decrease it.
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Knitting parameters of all samples developed were kept at a constant level, such as stitch length 0.35 cm, tightness factor at 14 and input tension 25 N. As the samples were developed using different fibers or blends, there is variation in GSM of samples before and after wet processing. In some cases, the GSM increases while in some others the GSM decreases or remains the same after wet processing. It was observed that sample S20 containing 70% FR-Polyester and 30% Protex has highest areal density 176 g/m 2 and sample S11 containing 50% FR-viscose and 50% Nomex has the lowest GSM value of 90. Most of the samples have GSM around 120. Thickness was measured for all the samples. Sample containing 100% carbon fibers has highest thickness because of bulkiness of yarn and the air gaps. Air trapped in the yarn and fabric, increases the thickness of fabric. The carbon fiber is stiffer as compared to other fibers used. The fiber stiffness is responsible for having a bulkier fiber bundle/yarn and a thicker fabric. This is well known in the textile field and available literature [ [17] , [18] , [19] , [20] ].
It was observed that in the blend containing 50% Protex fiber and 50% Nomex fiber, the Nomex tends to enhance the LOI value of the blended sample. In case of 50/50 blended carbon and Protex fibers sample S14, the overall LOI of the sample decreased as compared to the individual LOI of carbon fibers. Thus, it can be inferred that Protex fibers tend to decrease the LOI. The highest LOI among the blended samples was achieved in the blend comprising 50% Polyester fiber and 50% carbon fibers (S15). It means that combination of Nomex fibers helps in enhancing the overall LOI value of Protex, FR-viscose rayon and Polyester.
Nomex fiber shows the highest LOI value as meta-aramids are inherently flame resistant. The higher the LOI value, the higher the non-flammability. The higher the hydrogen to carbon ratio in the polymer, the greater is the tendency to burning (other factors being constant). Presence of benzene rings and hydrogen bonded molecules in polymer chains tends to increase the flame resistance and LOI. FR-Viscose is a cellulose-based fiber which was found to be minimum flame resistant. During synthesis of fire-resistant cellulose, the polymer solution is mixed with fire retardant agent to impart flame resistance. In general, high flammability is the inherent behavior of all cellulosic materials. When FR-viscose fibers were blended with other high flame-resistant fibers like Nomex, carbon and FR-polyester it tends to decrease the overall LOI values.
Generally, textiles having LOI values greater than oxygen level (21%) tend to be considered flame retardant fibers. Higher LOI value give the indication how harder for the material to catch fire . LOI tests were performed only for the samples comprising 100% or 50/50 ratio of component fibers in the blend. The results are shown in Table 3 . The purpose was to test the individual limiting oxygen and flammability behavior of fibers while in 50/50 blends the aim was to investigate the combined effect of both fibers to increase/decrease flame retardancy.
Highest after flame time (40s) was observed in sample S14 containing 50% Protex and 50% Carbon. Samples S2, S3, S5 and S9 showed 0s while S7 showed highest afterglow time 9s. According to standard ASTM F , maximum char length is 15 cm while NFPA demands maximum 10 cm char length is acceptable. 15 cm char length is universally accepted.
It was observed that sample S2 consisting of 100% FR-Polyester and sample S20 consisting of 70% FR-polyester and 30% Nomex showed dripping behavior. This is because of thermoplastic nature of FR-polyester which causes the dripping and melting during burning [ [21] , [22] , [23] , [24] ].
It can be observed that sample S10 containing 50% FR-polyester and 50% Protex shows char length of 18 cm. Sample S1 containing 100% Nomex produces lowest char length of 6 cm due to the fire retardance nature of Nomex which provides necessary protection and resistance to flammability as reported in previous research [ 19 ]. It was noted earlier that char length of Nomex increases with increase in weight change. A minor weight change was observed with respect to the other samples. Sample S9 consisting of 50% Carbon and 50% Nomex produced 7 cm char length. These two samples showed minimum char length due to presence of Nomex fiber in the blend. It was also previously noted that Nomex produces minimal char length [ 20 ]. Maximum weight loss was observed in sample S4 containing 100% FR-viscose fibers. This can also be attributed to the lowest limiting oxygen index of FR-viscose fibers. Char forming indicates excellent barrier properties if non-thermoplastic fibers are used.
TGA was performed to analyze the thermal stability of 15 samples which contain 100% and 50/50 blends of fibers so that the effect of the fiber composition on thermal stability could be analyzed. In thermogravimetric analysis, mass of material was measured and change in mass was recorded as the function of temperature. All the samples were subjected to high temperature up to 600 °C. The samples behaved differently, and variable mass degradation was observed. The TGA curves of samples with 100% and 50/50% composition are shown in Fig. 2(a). DTGA curves of samples with 100% and 50/50% composition are shown in Fig. 2(b).
Thermogravimetric analysis, (a) TGA curves of samples with 100% and 50/50% composition, (b) DTGA curves of samples with 100% and 50/50% composition.
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The results suggest that sample S2 containing 100% FR-polyester fibers showed very good thermal stability as minimum mass degradation and highest residual mass (84.12%) was obtained at temperature of 600 °C. Sample S5 consisting of 100% carbon fibers showed residual mass of 64.52%. Sample S4 containing 100% FR-viscose showed minimum thermal stability as it was observed that maximum mass was decomposed and only 19.79% was residual mass at 600 °C. TGA results also showed that very small amount of residual mass (20.79%) was observed in sample S10 containing 50% FR-polyester and 50% Protex fibers.
For the sample S1 containing 100% Nomex, only 2% of mass change occurred for first 100 °C which was due to evaporation of moisture absorbed in the fibers. The moisture regain of meta-aramid is only 0.5%. In the 2nd stage up to 300 °C, only 3% mass decomposed because the pyrolysis starts at this temperature range. This small mass loss can be due to release of water caused by the breaking of hydrogen bonds. Small amount of heat also causes change in crystalline nature of fibers which is reported in previous research [[25], [26], [27], [28]]. In the 3rd stage, with an increase of 300 °C up to temperature of 600 °C, maximum mass change (41%) occurs. The combustion temperature of meta-aramids is around 500 °C when the degradation starts. Total mass decomposition of Nomex samples is estimated as 46%. Charring starts at around 400 °C and then with the increasing temperature, Nomex burns into the residual mass, it doesn't melt or drip. In the last stage the decrease in slope suggests the condensation reactions which cause the yield of polyaromatic compounds. In this phase, the C Created by potrace 1.16, written by Peter Selinger - N bonds break frequently and then followed by C Created by potrace 1.16, written by Peter Selinger - O. By increasing the temperature further, gases like CH4 are formed and this increases the pyrolysis energy [[29], [30], [31], [32]].
For sample S2 containing 100% FR-polyester fibers, there was only 4.5% mass change in first stage of 100 °C. In 2nd stage up to 300 °C, again 4.5% mass degradation occurred. In the 3rd stage up to maximum range of temperature from 300 °C to 600 °C, there was 12% mass change. As melting point of polyester is about 250 °C, the fibers start melting, and the fabric shrinks. The FR-polyester (Recron) sample shows the smallest mass change due to highly crystalline nature of the FR-polyester fiber. Pyrolysis temperature of FR-polyester is 420 °C which is higher than others. That is why FR-polyester fibers burnt at higher temperature and a smaller mass change occurred [33,34]. Thermal degradation of FR-polyester is usually done by the random chain breakages which cause significant mass loss [35].
In the sample S3 consisting of 100% modacrylic (Protex), there was only 1% mass change in first stage of 100 °C. This is due to very low moisture regain of modacrylic fibers which is 0.3&#;0.4% [34]. Also up to 200 °C only 1% mass degradation was observed. In the 2nd stage, with temperature increase of up to 300 °C, there was suddenly 50% mass degradation. This is confirmed by the previous research in which modacrylic fibers showed similar decomposition behavior [36].
In case of sample S4 containing 100% FR-viscose fibers, there was 10% mass change in the first stage of temperature rise to 100 °C. This is because of higher moisture regain of FR-viscose fibers (10&#;11%). In the 2nd stage of temperature rise by further 100 °C, there was only 1% mass change. There are numerous reactions happening successively, various volatile compounds are released from the samples. The third stage denotes an increase of 400 °C, from 200 °C to 600 °C. During this phase the maximum mass change of 70% occurred due to degradation of cellulosic polymer.
FR-Viscose fiber showed highest mass reduction as it decomposed completely. In the earlier works, it was reported that only 14% mass was left as residual upon a temperature of 600 °C [[37], [38], [39], [40], [41], [42]]. The excessive mass loss in FR-viscose occurs due to dehydration and decarboxylation reactions which causes the emission of combustible gases like aldehydes, ketones, ethers, etc. Weight loss of fire-retardant FR-viscose fiber corresponds to the dehydration of polysilicic acid. The dehydration of polysilicic acid provides a flame and facilitates the emission of combustible gases generated from FR-viscose fiber [[41], [42], [43]].
Sample S5 which consists of 100% carbon fibers, showed a mass loss of 8% in the first stage when temperature rises to 100 °C. In the 2nd stage, when temperature increased from 100 °C to 300 °C, there was even smaller mass change of 6%. In the 3rd stage up to 600 °C, 17% mass degradation occurred. In PAN based carbon fibers, the mass loss happens due to dehydrogenation and chain rupture. The dehydrogenation is due to oxidation and evolves H2O. As heating rate increases, there is more and more chain rupture. Heating at the rate 10 °C per minute causes lower oxygen uptake to polymers chains. Mass loss in nitrogen is significant because of no mass gain due to oxygen introduction [44]. Mass reduction above 250 °C is due to thermal shrinkage caused by the cyclization in PAN based carbon fibers. At higher temperatures, significant amount of mass loss is observed. The previous findings reported the decomposition of carbon fibers results in emission of various products including H2O, CO, CO2 and CH4 [45].
The 50:50 blended samples of the constituent fibers exhibited mass loss based on rule of mixture and the behavior was intermediate based on the fiber composition. The charred samples were analyzed with SEM. The microscopic images of fibers which have not lost their different inherent structure are presented in Fig. 3. Fig. 3(a) shows FR-Polyester fibers, (b) shows Protex fibers, (c) shows Nomex fibers, (d) shows FR-Viscose fibers, and (e) shows Carbon fibers, which are unchanged even after burning of the sample.
Charred surface morphologies of FR-fibers which have not lost their different inherent structure (a) FR-Polyester fibers, (b) Protex fibers, (c) Nomex fibers, (d) FR-Viscose fibers, and (e) Carbon fibers.
The SEM images show that the samples are not completely decomposed or charred at 600 °C. This is indication of thermal resistance performance in the fabrics developed from the inherently flame-resistant fibers.
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