Comparative Thermal Behaviour Study of Thermal Layers Made Out of Woven and Nonwoven Fabric Using FR Viscose and p-Aramid Fibres

Para-aramid from Dupont and Lenzing’s fire-retardant viscose were the fibres used in this study. The physical attributes of each fibre were examined. Tenacity moisture regain, length, elongation, and density are among the qualities that were examined, shown in Table 1.

The table shows that the lengths of FR viscose and p-aramid are 51 and 53 mm, respectively. The tenacity for p-aramid was determined to be 112.41 g/tex and 24.21 g/tex for FR viscose. P-aramid and FR viscose showed 26.48% and 13.78% elongation, respectively. Increased elongation is beneficial while spinning yarn. In spinning, lower elongation increased the amount of yarn breakage [25]. Spinning at high speeds is highly challenging with poor elongation. The tenacity of fibres was assessed using an instrument called Vibrodyn, which was used in conjunction with a single fibre fineness tester Vibroscop., The moisture regain was found to be 7.11% for FR viscose and 5%. for p-aramid. Viscose with higher moisture regain will result in more comfortable fabric. The density was found to be 1.98 g/cc for FR viscose and 1.67 g/cc for p-aramid. Because of the use of chemicals to give fire retardancy, FR viscose has a higher density.

Para-aramid and FR viscose were utilized to create 1Ne roving with a 2.5 TPI. This high tpi roving is created for the direct creation of thermal layers using roving rather than turning it into yarn. To complete the investigation, yarn made of 10Ne, FR viscose and p-aramid was also prepared.

15 different fabric combinations were produced, shown in Table 2. Out of these, 9 fabric samples are woven structures and 6 samples are non-woven. The combinations of woven fabrics are: yarn/yarn, yarn/roving, and roving/roving. A CCI sample loom was used to create woven samples. First of all, the warp yarn was sized using a CCI sizing machine. The sizing was done using polyvinyl alcohol (PVA) solution, followed by drying at 100 °C for five minutes. The warping of the sized yarn was done on a CCI warping machine SUU 550. Finally, 1/1 plain weave structure fabric was manufactured on a CCI rapier loom. For weaving purposes the ends/inch and pick/inch were kept at 52/20, 33/13, and 1/16 for fabric samples made out of yarn Combinition-1, Combination-2, and Combination-3, respectively.

Non-woven samples of 330 g/m² and 665 g/m² were manufactured using a Trytex make needle punching set-up.

The desized fabric samples developed were examined for areal density, and tensile strength in accordance with ISO 3801 and ISO 5081, respectively. According to ISO 15025, ISO 6942, and ISO 9151 test methodologies, these fabric samples were evaluated for limited flame spread, radiant heat resistance, and convective heat resistance, respectively [17].

The flame spread test was performed using a limited flame spread tester in accordance with ISO 15025 Method A (surface ignition). According to this standard, the surface or bottom edge of the vertically oriented sample is exposed to the appropriate flame for 10 seconds. Data pertaining to the flame spread, hole development, debris formation, after-flame time, and afterglow time is documented.

In order to test the radiant heat transfer index (RHTI24), the ISO 6942 B test technique was used. In accordance with the EN 469 standard for clothing that protects against heat and flame, the textiles were exposed to a radiant heated source that produced heat fluxes of 40 kW/m². The amount of time (s) it took for the calorimeter’s temperature to increase by (24 ± 0.2) °C was noted. RHTI24 was examined ten times for each sample. Using the following equation, the transmitted heat flux density (Qc), expressed in kW/m², was calculated: 1 Qc=M.Cp.12A.(t24−t12) where M is the copper plate’s mass in kilograms; Cp is copper’s specific heat of 0.385 kJ/kg at 0 °C; 12/(t24 - t12) is the calorimeter’s average rate of temperature rise in decimal degrees per second in the range of 12 to 24 °C rise, and A is the copper plate’s area in square meters.

According to the ISO 9151 test technique, the convective heat transmission (HTI24) across a material was calculated. The horizontally oriented test specimen was subjected to an incident heat flux of 80 kW/m² from the flame of a gas burner that was positioned beneath it and was partially restricted from moving. A tiny copper calorimeter placed on top of and in contact with the specimen was used to quantify the amount of heat that was going through it. The amount of time (s) it took for the calorimeter’s temperature to increase to (24 ± 0.2) °C was noted. For HTI24, each sample was examined ten times.

HTI24 is the time to achieve a temperature rise of 24 °C in the calorimeter at a specified incident heat flux density, which is roughly equivalent to the time it takes to experience a second-degree burn. The HTI12 for radiant and convective heat is the time to achieve a temperature rise of 12 °C in the calorimeter at a specified incident heat flux density. The difference between the two (HTI24-HTI12) is the amount of time it takes for a second-degree burn to develop. A reliable indicator of the skin discomfort warning time was provided by the time disparities between HTI24 and HTI12.

Before testing, the specimens were conditioned for at least 24 h at a temperature of (20±2) °C and relative humidity of (65±2) %.

According to EN ISO 15025 (Procedure A), a flame spread test was performed on all 15 samples. Every sample complies with EN 469 requirements. All samples yielded identical findings, listed in Table 5. The findings of the flame spread tests are in accordance with expectations because all of the fabric samples are made out of inherent flame-retardant material [26].

According to EN 469, the mean radiant heat resistance index (RHTI₂₄) for performance level 2 should be ≥18 sec, and RHTI₂₄ - RHTI₁₂ should be ≥ 4 sec. In contrast, the mean heat transmission index (HTI₂₄) for convective heat should be ≥13 sec, and HTI₂₄ - HTI₁₂ should be ≥4 sec. Although these values of RHTI₂₄ and HTI₂₄ as per EN 469 are for a component assembly of all layers of firefighter suits (outer layer, moisture barrier, and thermal liner), only the thermal liner is taken in this study to achieve these values so that a firefighter suit with higher resistance to radiant and convective heat can be developed.

From Table 6, the RHTI₂₄ value for yarn/yarn (Combination-1) is found to be 11.4 sec for 100 % p-aramid (TW 1 Y/Y), 9.3 sec for 100 % FR viscose (TW 3 Y/Y), and 10.7 seconds for a blend of 50 % p-aramid and 50 % FR viscose (TW 5 Y/Y The convective heat resistance (HTI₂₄) value of the yarn/yarn combination is found at 9.6 seconds for TW 1 Y/Y, 8.5 seconds for TW 3 Y/Y, and 8.7 seconds for TW 5 Y/Y. In the case of yarn/roving (Combination-2), RHTI₂₄ values of the TW 1 Y/R sample are found to be 14.1 seconds and 12.3 seconds for TW 3 Y/R and 12.4 seconds for TW 5 Y/R. The convective heat resistance (HTI₂₄), for the TW 1 Y/R sample is found to be 14.4 seconds, 12.9 seconds for the TW 3 Y/R sample, and 13 seconds for the TW 5 Y/R sample. Table 6 also reveals that in roving/roving (Combination-3), samples TW 1 R/R, TW 3 R/R, and TW 5 R/R showed 19.9 seconds, 18 seconds, and 18.2 seconds of RHTI₂₄ values, respectively. The HTI₂₄ values of these samples (TW 1 R/R, TW 3 R/R, and TW 5 R/R) were 17.8 seconds, 17.7 seconds, and 15.3 seconds respectively. From this study, it is found that the RHTI₂₄ and RHT₂₄ values of Combination-3 are higher than for Combination-2 and Combination-1. Higher values were shown because of the increase in thickness due to roving insertion in both directions, increasing heat resistance values. Combination-1 shows the lowest values of RHTI₂₄ and RHT₂₄ as these samples are made out of yarn only. The RHTI₂₄ and RHT₂₄ values of Combination-2 are greater than for Combination-1 as this combination has yarn in the warp-wise direction and roving in the weft-wise direction.

Due to the more open structure of roving in nonwoven fabric, as compared to yarn, it entrapped air in the pores and thus provided more thermal resistance [24].

Besides, Combination-3 samples’ thickness is higher than for Combination-2 and Combination-3. As per the earlier studies, it was found that the thermal insulation of nonwoven batting materials increases with the batting thickness [27-29].

From Table 6, RHTI₂₄ values for the 330 gsm group non-woven composition are found to be 13.7 seconds for NW3301, 15.3 seconds for NW3302, and 14.6 seconds for NW3303. While convective heat resistance (HTI₂₄) values for the 330 gsm group, are 14.4 seconds for NW3301, 11.7 seconds for NW3302, and 12.5 seconds for NW3303.

In the same Table, RHTI₂₄ and HTI₂₄ values of the 665 gsm non-woven composition group are also given. The RHTI₂₄ values of NW6654, NW6655, and NW6656 are found to be 23.7 seconds, 30.0 seconds, and 27.2 seconds, respectively. While HTI₂₄ values of NW6654, NW6655, and NW6656 are found to be 25.9 seconds, 26.3 seconds, and 26.0 seconds.

It is clear from the table that the 665 gsm group has higher thermal resistance values (RHTI₂₄ and HTI₂₄) compared to the 330 gsm group. The reason for the higher thermal resistance values is that the thickness and areal density of the 665 gsm group are higher than for the 330 gsm group. Due to this, the thermal resistance values of the 665 gsm group are higher than for the 330 gsm group [27-29].

The study also revealed that the RHTI₂₄ and HTI₂₄ values of the 665 gsm non-woven composition group are higher than all the woven and 330 gsm groups of nonwoven samples. The reason is clear: due to the higher thickness and areal density, the 665 gsm non-woven composition group showed higher RHTI₂₄ and HTI₂₄ values.

It is also explicit from Table 6 that the RHTI₂₄ and HTI₂₄ values of Combination-3 are higher than for the 330 gsm group of samples. The thickness of samples of the Combination-3 and 330 gsm groups, as shown in Table 3 and Table 4, are almost similar. The reason behind these higher values of Combination-3 is that it has a higher areal density compared to the 330 gsm group of samples.