Study of a newly structuralized meta-aramid/cotton blended yarn for fabrics with enhanced flame-resistance

Abstract

This paper presents a study on a newly structuralized meta-aramid/cotton blended yarn for fabrics with enhanced flame-resistance. In this study, a new type of “marl yarn” resembling structure for cotton/aramid yarns was proposed with an aim to lower the flammability of cotton fiber strands within the yarn and thus enhance the flame resistance of the blended yarns and the resultant fabrics. To facilitate the formation of marl yarn structure, a modified device was developed that can be attached to the ring spinning machine for yarn production. Yarn structure was examined and the effects of the blending ratio of aramid/cotton fibers and yarn structure on the yarn flammability and physical properties were investigated. The results showed that a marl-like yarn structure was formed wherein a small amount of meta-aramid fibers were concentrated to form fiber strands, which served as effective fire barriers, hindering the afterflame/afterglow of cotton fibers. The experimental results demonstrated that the marl structured yarn exhibited lower yarn flammability in terms of afterflame, afterglow, damage length and limiting oxygen index (LOI) as well as possessing similar physical properties compared with conventional evenly blended yarn. By using the marl structured yarns developed, meta-aramid/cotton blended woven fabrics were produced and their flammability and physical properties were evaluated. The results showed that the fabrics using the marl structured yarns had a higher minimal flame application time for ignition and LOI as well as a lower flame spread speed than fabrics using evenly blended yarns.

Keywords

yarn structure cotton meta-aramid fiber blended yarn flammability

Consumers nowadays are not only seeking fashion and garment comfort but also demanding functionalities related to health and safety. As a natural and cellulosic fiber composing of anhydroglucose units joined by β-1,4-glucosidic bonds, cotton possesses good strength and abrasion resistance, high moisture absorbency, good heat conductivity, and is soft to the touch. Therefore, cotton has been widely used for producing textiles and garments for various applications due to such characteristics. Fabrics commonly used for professional uniforms for example for chefs and security personnel, and for furnishings such as in hotels and for use in home bedding are made of pure cotton, manmade fibers or a blend of the two.^1,2 However, cotton fiber ignites easily and burns rapidly.³ Therefore, the question of how to lower the flammability of cotton fabrics is a key issue for such kinds of applications. There are two approaches to lowering the flammability of cotton fabric and thus enhance its flame resistance. One approach is to apply flame-retardant chemicals onto the fabric via fabric finishing. The advantages of this approach are its simplicity and low product cost, however the chemicals commonly used are toxic and, as a result, the physical properties and comfort of the fabric could degrade due to the finishing process.^4-7 Moreover, the treated fabric is not very durable and therefore has a relatively short service life. Another method is to blend the cotton with fibers that are inherently flame-resistant or have been treated for flame resistance.^8–9 Such fabric is comfortable to wear, has durable flame-resistance, good physical properties but a relatively higher material cost.^10–12 Among the flame retardant fibers, the meta-aramid fiber (poly(m-phenylene isophthalamide)) is composed of aromatic rings linked by amide bonds in the form of meta-linkages that are inherently flame-resistant. It also has high thermal stability and insulation, non-melt drop characteristics, good anti-radiation performance and moisture absorption as well as being soft to handle.^13–14 However, the meta-aramid fiber does have certain disadvantages such as poor dyeability and low weatherability. For such reasons, its application is currently limited to areas requiring unusually high thermal and flame resistance such as for use in firefighter clothing and high temperature filtration. Furthermore, the static charge generated by the meta-aramid fiber in the yarn production process can introduce difficulties in relation to the yarn’s spinnability and have negative effects on the yarn’s properties, for example, causing hairiness. Therefore, it is highly beneficial to lower the flammability of cotton fabrics by blending the meta-aramid fibers and cotton fibers. This blending can extend the application of meta-aramid fibers to common functional garments and home furnishing that only require a low to medium level of flame resistance. This approach also improves the spinnability of the meta-aramid fiber and general yarn properties, as well as reducing the cost of the material.

Various blended fabrics with flame retardant properties have been developed wherein the flame resistance of fabric depends on the fiber components as well as the yarn and fabric structure. Huang investigated the thermal property of fabric made of Nomex and Viscose flame retardant fibers.¹⁵ An optimum blending ratio of 80/20 (Nomex/Viscose FR) was identified as producing a fabric with excellent flame-resistance at an economical cost. Rengasamy et al. developed flame retardant knitted fabrics by blending Nomex and cotton fibers in various blending ratios.¹⁶ They found that the limiting oxygen index (LOI) of the fabric increased, and its char length and times of afterflame and afterglow decreased with an increase in the proportion of Nomex. The mechanical properties of meta-aramid/cotton blended fabrics were investigated by Liu et al.¹⁷ The experimental results showed that with an increased blending ratio of meta-aramid fibers, the yarn strength decreased and then increased, and the tensile and tearing strength of fabric increased. Ndlovu et al. studied cotton/polysulfonamide fiber blended and core-spun yarns for flame retardancy and mechanical performance.¹⁸ The test results indicated that the core-spun yarn exhibited higher flame retardancy performance while the blended yarn showed higher tensile strength. The effect of structure on a fabric’s flame resistance, studied by Wang et al., demonstrated that the weave, thickness, and tightness had a significant influence.¹⁹ Further studies focusing on fabric flammability indicated that differences in the structure significantly influenced fabric burning behavior, including properties such as fiber arrangement, fiber-to-fiber contact within the yarn, and yarn bulk density.^20–22 However, the effects of yarn geometry and structure on the flammability of blended fabrics have not been explored in depth.

In order to address the above-mentioned problems, in this study we aimed to develop a series of meta-aramid/cotton blended yarns and fabrics with lowered flammability and thus enhanced flame-resistance. The key issues included how to select an appropriate fiber blending ratio and design the yarn structure that utilized the aramid fiber to obtain enhanced flame-resistance while maintaining the cotton fabric features. In this study, a new type of ‘marl yarn’ resembling structure for aramid/cotton yarns was proposed. Based on this specially designed yarn architecture, a newly structuralized meta-aramid/cotton blended yarn was developed and produced in a modified ring spinning system. The flammability and physical properties of the new blended yarn at different blending ratios were measured. The structural effects of the blended yarns in terms of internal structure and surface appearance were examined to better understand the spinning mechanism of the yarn. Twill woven fabrics using this blended yarn were produced and their flame-retardant and mechanical properties were evaluated, including minimal flame application time for ignition, flame spread speed, LOI, tensile and tearing strength, abrasion resistance, and transfer properties. Furthermore, the effect of different proportions of blended meta-aramid and cotton fibers on fabric flammability and other physical properties was analyzed.

Design of the marl structured yarn and the modified device attached to the ring spinning machine

The properties and functionality of textiles depend on both the content and arrangement of the blending fibers within the yarn. For cotton yarns blended with a small to medium amount of meta-aramid fibers to lower the flammability of the cotton or to enhance the flame resistance of the yarn and fabric, how to utilize meta-aramid fibers within the blended yarn for the desired functionality becomes a key issue that depends on the fiber distribution within the yarn. Figure 1 illustrates the architecture of the marl yarn, which consists of two blending fiber strands twisted together through spinning. With this specially designed yarn construction, the small amount of meta-aramid fibers could be concentrated to form fiber strands with the cotton fiber strands (as opposed to an even blending of fibers); such a construction could provide an effective fire barrier by reducing the ease of ignition and flame propagation rates or even by hindering the afterflame/afterglow of the cotton fibers within the yarn and fabric. Consequently, the flammability of the cotton/aramid yarn and fabric can be lowered and thus their flame resistance (char length, afterflame/afterglow time) may be increased. Furthermore, by avoiding constructing continuous cotton fiber strands along the yarn axis it is anticipated that flame spread along the flammable cotton strand would be hindered, which could further improve the flame resistance of yarns and fabrics. In relation to fiber and manufacturing cost, functional fibers such as the meta-aramid used in this study are generally expensive compared with cotton. Therefore, the goal for the proposed marl yarn resembling structure was to achieve a higher flame resistance than that of common evenly blended fiber yarn and fabric; in other words, for the marl structured yarn and fabric to obtain the same level of flame resistance by using a lower proportion of aramid fibers for blending. Furthermore, compared to the evenly blended yarn spinning method, the developed marl yarn spinning technology shortens the spinning process by eliminating the fiber blending steps, and thus reduces the manufacturing cost.

Figure 1.

The newly structuralized blended yarn.

In order to facilitate the construction of the cotton and meta-aramid fiber yarn structure, a modified device was developed that can be attached on the existing ring spinning frame. The device has, at a minimum, the following essential functions: 1) to arrange two input rovings during the drafting process; 2) to control the width of the drafted fiber ribbon exiting from the front roller nip; and 3) to exchange the relative position of two input rovings. The modified device developed consists of two beams with roving feeding eyes, namely the top and bottom reciprocating beam, the transmission unit, and the motor with its controller. As shown in Figure 2, the top and bottom reciprocating beam was installed at the rear of the back roller wherein the top beam was placed on the bottom beam with their respective roving feeding eyes. During the marl structured yarn spinning, the two individual cotton and meta-aramid rovings in typical packaging were fed into the top and bottom reciprocating beams through their feeding eyes, respectively, and then passed through the nip of the back roller according to the rotation of the roller. Consequently, these two rovings were fed into the conventional ring spinning system and were individually drafted. After passing through the front roller, the two drafted roving strands converged to form a final marl yarn resembling structuralized cotton/aramid blended yarn. Both top and bottom reciprocating beams moved in a reverse direction at a certain distance and speed, which caused the relative positions of the two rovings in the drafting zone to regularly switch to avoid the formation of continuous cotton fiber strands along the yarn axis. The moving distance and speed of both the top and bottom reciprocating beams were the same and could be adjusted by the transmission unit and the motor via its controller, respectively, according to requirements.

Figure 2.

Modified ring spinning system for the production of the newly structuralized aramid/cotton yarns.

Experimental details

Fiber materials

In this experiment, the meta-aramid and cotton fibers were supplied by fiber manufacturers for the yarn production with the short staple yarn spinning system. The meta-aramid fiber was 38 mm in length with a fineness of 1.84 dtex. The tensile strength and elongation of the meta-aramid fiber were 39.2 cN/tex and 34.5%, respectively, which were tested by using an Instron Tester following the ASTM D3822 standard. The geometry and properties of the cotton fibers used for blending with the meta-aramid fibers were examined on the Spinlab 900 (Uster, Switzerland) and Instron Tester (Instron 4411, US). The results showed that the cotton fiber possessed the length, micronaire, tenacity, and elongation of 25.4 mm, 4.6, 31.1 cN/tex, and 7.1%, respectively.

Production of yarn and fabric samples

The 18.5 tex marl structured yarns in different blending ratios of cotton and meta-aramid fibers were produced by using a Zinser 351 ring spinning machine (Zinser 351, Germany) with the above-mentioned modified device. For a comparison, conventional evenly blended meta-aramid/cotton fiber yarns in the same blending ratios, together with the 100% cotton and 100% meta-aramid fiber yarns, were also spun. Table 1 presents the detailed specifications of the yarn samples. In order to produce these yarns, cotton, meta-aramid and their blended rovings in the designed roving counts and blending ratios were used in this study. During production of the rovings for conventional evenly blended meta-aramid/cotton fiber yarns, the meta-aramid and cotton slivers were mixed in the drawing frame three times and then the roving process was conducted to form aramid/cotton blended rovings.

Table 1.

Specifications of yarn samples

Yarn sample	Yarn structure	Composition and blending ratio	Yarn linear density (tex)	Twist (tpm)
YC	Conventional	100% cotton	18.5	935
YM1	Marl structured	24% meta-aramid and 76% cotton	18.5	935
YM2	Marl structured	48% meta-aramid and 52% cotton	18.5	935
YM3	Marl structured	65% meta-aramid and 35% cotton	18.5	935
YE1	Evenly blended	24% meta-aramid and 76% cotton	18.5	935
YE2	Evenly blended	48% meta-aramid and 52% cotton	18.5	935
YE3	Evenly blended	65% meta-aramid and 35% cotton	18.5	935
YA	Conventional	100% meta-aramid	18.5	935

Using the above-produced marl structured and evenly blended aramid/cotton, pure cotton, and aramid fiber yarns, eight twill woven fabric samples were produced using a JWG 1728 rapier weaving machine (JWG 1728, China). Table 2 shows the specifications of fabrics produced. For fabric samples FM1, FM2, and FM3, the marl structured aramid/cotton (48/52) yarns (YM2) were selected as warp yarns and the marl structured aramid/cotton yarns in different blending ratios (YM1, YM2, and YM3) were used as weft yarns. The evenly blended aramid/cotton (48/52) yarns (YE2) were employed as warps, while evenly blended aramid/cotton yarns in various blending ratios (YE1, YE2, and YE3) were adopted as wefts for fabricating the samples FE1, FE2, and FE3.

Table 2.

Specifications of fabric samples

Fabric sample	Fabric content		Yarn linear density (tex)		Fabric density (threads/cm)		Weave
Fabric sample	Warp	Weft	Warp	Weft	Warp	Weft	Weave
FC	YM2	YC	18.5 × 2	18.5 × 2	28	20	1/3 twill
FM1	YM2	YM1	18.5 × 2	18.5 × 2	28	20	1/3 twill
FM2	YM2	YM2	18.5 × 2	18.5 × 2	28	20	1/3 twill
FM3	YM2	YM3	18.5 × 2	18.5 × 2	28	20	1/3 twill
FE1	YE2	YE1	18.5 × 2	18.5 × 2	28	20	1/3 twill
FE2	YE2	YE2	18.5 × 2	18.5 × 2	28	20	1/3 twill
FE3	YE2	YE3	18.5 × 2	18.5 × 2	28	20	1/3 twill
FA	YM2	YA	18.5 × 2	18.5 × 2	28	20	1/3 twill

Measurements

All the specimens were conditioned for at least 24 h under 20℃ ± 2℃ and 65% ± 4% relative humidity (RH) following ISO 139 in order to follow the standards and acquire the most accurate results from further testing. The internal structure and surface appearance of the yarns were examined using a digital microscope to identify the yarn blending structure according to the distribution of meta-aramid and cotton fibers within the yarn.

Fabric flammability or flame-resistance depends on not only the fabric structure but also the yarn’s flammability. Referring to ASTM D1230 and past studies on the flammability of yarns and fabrics, a testing protocol for aramid/cotton blended yarns was established based on a 45° flammability tester (Govmark, Model: TC-45, USA), as shown in Figure 3.^23–24 In this experiment, the yarns were arranged along the length direction on the center hollow area of the yarn preparation holder, at a density of 57 yarns per centimeter and with an effective testing size of 38 mm × 145 mm, as shown in Figure 3(a). Six specimens for each cop yarn were tested and the mean value was obtained. Following the ASTM D1230 testing procedure, each specimen after preparation and conditioning was inserted in the specimen holder and mounted on the 45° flammability tester as shown in Figure 3(b). A flame 16 mm in length was applied to the surface of the yarn specimen for 3 s. The afterflame time (i.e., the time the flame on the yarn continues after the removal of the ignition source), the afterglow time (i.e., the time the afterglow on the yarn lasts when the ignition source is removed and the yarn flaming ceases), and damage length were recorded to indicate the level of flammability of the yarn. The char length of the yarn sample after the test was recoded as the damage length. The LOI of the yarn was measured by a commercial testing company according to ASTM D2863. The yarns were arranged along the horizational direction on the specimen holder at a density of 57 yarns per centimeter and with an effective testing size of 38 mm × 72 mm. Fifteen specimens for each cop yarn were tested and mean value was obtained. A multiple comparison least significant difference (LSD) test was carried out to assess the significance of samples at a 0.05 level using SPSS statistical software.

Figure 3.

Setup of flame-resistance test: (a) yarn sample for testing, (b) tester (Govmark, Model: TC-45, USA).

The physical properties of the yarns were also evaluated, including tenacity and elongation, hairiness and evenness. Three cops for each type of yarn were prepared and measured. The yarn strength and elongation were tested on the Uster Tensorapid tester (Switzerland). Fifty readings were obtained and averaged, with a testing speed of 5000 mm/min and testing length of 500 mm. Yarn hairiness was tested on the Zweigle hairiness tester (G566, Switzerland). Three readings were used and averaged for each sample, at a testing speed of 100 m/min. The Uster III tester (Switzerland) was used for the measurement of yarn evenness. Each yarn sample was tested at a speed of 400 m/min for 1 min.

The resultant fabric flammability was measured following ASTM D1230. Firstly, the ease of ignition of the fabric was evaluated to indicate the initiation of combustion. For each fabric sample, 10 specimens 50 mm × 150 mm in size were tested. The standard length of flame applied to the fabric samples was 16 mm. The minimal flame application time for ignition of fabric was recorded and the mean value was generated for each fabric sample. A short minimal flame application time for ignition indicates the ease of ignition of a fabric. Secondly, eight specimens with a testing size of 50 mm × 150 mm were prepared and a stop cord was placed above at a position of 150 mm along the length direction of the specimen. The standard length and time of flame applied to the fabric samples were 16 mm and 12 seconds, respectively. The flame spread time was recorded during the test when the stop cord broke. Based on this, the flame spread speed of the fabric specimen could be calculated and the mean value obtained. Finally, the LOI of the fabrics was measured by a commercial testing company according to ASTM D2863, with fifteen testing specimens for each type of fabric. The minimum oxygen concentration required to make the fabric burn was determined.

The common fabric properties examined included fabric tensile strength, tearing strength, air permeability, water vapor permeability, thermal conductivity, and the water absorption rate. The tensile and tearing strength were measured on the Intron tester (Instron 4411, USA) in accordance with ASTM D5034 and ASTM D2261, respectively. The air permeability was tested using SDL ATLAS Air Permeability Tester (M021A, US) following ASTM D737. For each type of fabric, five readings were obtained and averaged. The abrasion resistance of fabrics was evaluated on the Martindale Abrasion Tester (SDL ATLAS M235, US) according to ASTM D4966. The water vapor permeability and water absorption rate of the blended fabrics were measured according to ASTM E96 Option B and GB/T 21655.1, respectively. The thermal conductivity of fabrics was evaluated on the KESF Thermo Labo-II (Japan).

Results and discussion

Yarn surface structure and internal structure

To check whether the above-proposed marl yarn resembling structure was formed during the yarn spinning as expected, a 18.5 tex meta-aramid/cotton yarn with black-dyed aramid fibers and cotton fibers in a blending ratio of 48% and 52% respectively was produced, using the modified device attached to the ring spinning machine. Then, a length of yarn was produced and wrapped on a blackboard, as shown in Figure 4(a). Based on the yarn surface structure exhibited in Figure 4(a), it can be seen that there are two types of surface structures in the marl yarn: Type 1 and Type 2. Type 1 is a plied yarn-like structure, which the majority of yarn structure types are. Figure 4(b) and (c) illustrate enlarged images of the surface structure and cross-section of the yarn. As expected, the meta-aramid fibers were concentrated to form a fiber strand within the yarn due to the two rovings feeding, drafting, and twisting during the ring spinning process. As shown in Figure 4(a), (d), and (e), Type 2 is an aramid/cotton fiber melange structure with aramid fibers that were concentrated on the surface layer. Such a yarn structure was formed when the two rovings of aramid and cotton fibers swapped their feeding positions by using the above-mentioned modified device during yarn spinning. For a comparison, a 18.5 tex evenly blended yarn in the same blending ratio of 48% and 52% for meta-aramid and cotton respectively was also produced using the same ring spinning machine. The yarn surface structure and cross-section are presented in Figure 4(f) and (g) wherein the meta-aramid and cotton fibers are arranged almost evenly, showing a significant difference of fiber distribution compared to that of the marl structured yarn.

Figure 4.

Structure of marl structured and evenly blended yarns: (a) a small amount of marl structured yarn wrapped on a blackboard, (b) surface structure of a plied structure (Type 1) of marl structured yarn, (c) cross-section of a plied structure (Type 1) of marl structured yarn, (d) surface structure of a melange structure (Type 2) of marl structured yarn, (e) cross-section of a melange structure (Type 2) of marl structured yarn, (f) surface structure of evenly blended yarn, (g) cross-section of evenly blended yarn.

When a flame is applied to the yarn and its resultant fabric, compared to the evenly blended yarn structure, the marl structured yarns have two advantages in terms of surface appearance and internal structure. The first advantage comes from the plied structure, in which the black meta-aramid and white cotton strands were plied together. This facilitated the properties of the meta-aramid fiber to act as a barrier against the flame. The second advantage derives from the melange structure, which increases the potential to block the flame spreading of the cotton fiber strands as well as increasing the surface flame-resistance. From the above discussions it appears that both structural characteristics in the marl structured yarn probably lower the flammability and thus improve flame retardance of the resultant aramid/cotton blended yarn and fabric.

Effects of yarn structure and fiber blending ratio on yarn flammability

Based on the above protocol setup for the testing of yarn flammability, the afterflame time, afterglow time, and damage length of yarn were used to evaluate yarn flammability. The LOI of the yarn was also measured to identify its level of flame resistance. Table 3 and Figure 5 show the effects of yarn structure and blending ratio on the yarn flammability and flame resistance. It was found that by introducing the flame-resistant meta-aramid fibers into the cotton fibers, the afterglow time and damage length of the yarn were greatly decreased and the LOI value increased significantly compared with the 100% cotton yarn. This result indicates that the cotton yarn flammability is greatly lowered after blending with the meta-aramid fibers and thus the flame resistance of the yarn is enhanced. Figure 5 also shows that the afterflame time, afterglow time, and damage length of the yarn decreases with an increase in the proportion of aramid fibers in the aramid/cotton blended yarn. For the marl structured yarn, the afterflame time, afterglow time, and damage length decreased from 7.2 s, 222 s, and 68 mm at the aramid blending ratio of 24% to 2.5 s, 0 s, and 34 mm at the aramid blending ratio of 48%, but the LOI of yarn increased from 23.5% to 26.5%.

Figure 5.

Effects of yarn structure and fiber blending ratio on yarn flammability: (a) afterflame time, (b) afterglow time, (c) damage length, and (d) LOI value.

Table 3.

Results of yarn flammability

Yarn sample	Afterflame time (s) (CV%)	Afterglow time (s) (CV%)	Damage length (mm) (CV%)	LOI (%) (CV%)
YC	5.28 (27.19)	296.40 (7.32)	145 (0)	15.45 (0.77)
YM1	7.19 (24.95)	221.82 (12.39)	68.30 (16.22)	23.45 (0.60)
YM2	2.50 (26.31)	0 (0)	34.20 (17.37)	26.49 (0.56)
YM3	2.58 (28.56)	0 (0)	34.40 (17.51)	26.48 (0.80)
YE1	10.08 (25.52)	256.60 (10.02)	91.10 (19.74)	23.46 (0.68)
YE2	8.93 (28.78)	13.63 (22.50)	52.90 (25.63)	23.87 (0.68)
YE3	4.25 (28.59)	0 (0)	33.60 (16.43)	25.88 (0.64)
YA	0.31 (49.11)	0 (0)	24.34 (12.48)	34.45 (0.46)

Note: LOI = limiting oxygen index.

As shown in Figure 5, it is clear that the marl structured yarns show better flame-retardant performance than evenly blended yarns. At the blending ratios of aramid/cotton (24/76) and aramid/cotton (48/52), the marl structured yarns have significantly shorter afterflame time, afterglow time, and damage length compared with the evenly blended yarns using the same blending ratios. In the case of aramid/cotton at a ratio of 48/52, the marl structured yarn has an LOI increase of around 11% from 23.9% to 26.5% when compared to evenly blended yarn. This demonstrates that the marl structured yarns has lower flammability and better flame resistance compared to evenly blended yarns with a low to medium meta-aramid blending ratio in the yarns. One possible reason is that, as already mentioned, the flame retardance of aramid fibers is enhanced when blended with the cotton fibers within the marl structured aramid/cotton blended yarn. A multiple comparisons LSD test of the yarn samples was conducted to statistically assess the effects of the fiber blending ratios and yarn structures on flame resistance. As shown in Table 4, with the exception of the pair of samples FM2 and FM3, it is clear that there is a significant difference in yarn flammability between yarn samples using different blending ratios of aramid fibers as well as between samples using marl structured yarn and evenly blended yarn when a small to medium amount of aramid fibers are blended with the cotton fibers for aramid/cotton blended yarns,.

Table 4.

Results of multiple comparisons least significant difference (LSD) test of yarn flame-resistance

Yarn sample		Afterflame time		Afterglow time		Damage length		LOI
Yarn sample		Mean difference	Sig.	Mean difference	Sig.	Mean difference	Sig.	Mean difference	Sig.
FC	FM1	−1.909*	0.010	74.580*	0.000	76.700*	0.000	−8.000*	0.000
	FM2	2.777*	0.000	296.400*	0.000	110.800*	0.000	−11.040*	0.000
	FM3	2.701*	0.000	296.400*	0.000	110.600*	0.000	−11.027*	0.000
	FA	4.973*	0.000	296.400*	0.000	120.660*	0.000	−19.000*	0.000
FM1	FM2	4.686*	0.000	221.820*	0.000	34.100*	0.000	−3.040*	0.000
FM1	FM3	4.610*	0.000	221.820*	0.000	33.900*	0.000	−3.027*	0.000
FM2	FM3	−0.076	0.917	0.000	1.000	−0.200	0.963	0.013	0.820
FM1	FE1	−2.886*	0.000	−34.780*	0.000	−22.800*	0.000	−0.007	0.910
FM2	FE2	−6.428*	0.000	−13.626	0.052	−18.700*	0.000	2.627*	0.000
FM3	FE3	−1.671*	0.024	0.000	1.000	0.800	0.853	0.600*	0.000

Note: LSD = xxx; LOI = limiting oxygen index. The mean difference is significant at the 0.05 level.

Effects of yarn structure and fiber blending ratio on yarn physical properties

The overall properties of the blended yarns are greatly affected by the blend ratio, properties of the fiber component and their interactions.^15, ¹⁶ The effects of blending ratio and yarn structure on the physical properties of yarns were investigated, including yarn tenacity and elongation, hairiness, and evenness; the results are presented in Figure 6. As shown in Figure 6(a) and (b), after blending the aramid fibers with cotton fibers, the tenacity of the resultant aramid/cotton blended yarn slightly decreases compared with the 100% cotton yarn because of the big difference in elongation between cotton and aramid fibers. However, no significant difference was found in relation to the blending ratio on the tenacity of either marl structured or evenly blended meta-aramid/cotton yarns. The tenacity and elongation of yarn samples in the marl structured yarn was similar to that of the evenly blended using the same blending ratio. The pure meta-aramid yarn exhibited the highest tenacity and elongation among the eight pure and blended aramid/cotton yarns, which can be attributed to the higher strength and elongation of meta-aramid fiber.

Figure 6.

Effects of yarn structure and fiber blending ratio on yarn properties: (a) tenacity, (b) elongation, (c) hairiness, and (d) evenness.

As shown in Figure 6(c), when blending with aramid fibers, the hairiness of both the marl structured and evenly blended yarns at the blending ratio of aramid/cotton (24/76) and aramid/cotton (48/52) is reduced compared with pure meta-aramid and cotton yarn. When the blending ratio of aramid fiber further increases, yarn hairiness also increases. Compared with the evenly blended yarn using the blending ratios of 24% and 48%, the marl structure yarn exhibited a lower hairiness, attributable to its two roving feeds for the yarn spinning. The pure meta-aramid yarns exhibited the highest hairiness value among the yarn samples due to the accumulation of the static charge generated from the production process. Figure 6(d) illustrates that the yarn structure and blending ratio do not have a significant influence on yarn evenness. At the three blending ratios, the evenness of the marl structured yarn is slightly higher than the evenly blended yarn.

Effects of yarn structure and fiber blending ratio on flammability of the resultant woven fabrics

The flammability of fabrics made of both marl structured and evenly blended yarns in varied blending ratios of aramid fiber and cotton were evaluated in terms of minimal flame application time for ignition, flame spread speed, and LOI. For a comparison, the flammability of fabrics produced using 100% cotton yarns and 100% aramid fibers yarns as wefts in the same fabric specification was also measured. Table 5 lists the results. Figure 7 shows photographs of the fabric samples before and after the flame resistance testing. In Table 5, it can be seen that through blending the cotton fibers with meta-aramid fibers, fabric flammability is greatly lowered. When compared with the fabric sample FC by using 100% cotton yarns as wefts, the fabric sample FM1 made of marl structured yarns with a small amount of aramid fibers (blending ratio of 24%) has an increase of minimal flame application time for ignition of around 33.3%, from 6 to 8 s, and an LOI of about 4.2%, from 21.81% to 22.73%, respectively. The reduction of flame spread speed reaches around 22.9% from 4.15 mm/s to 3.20 mm/s. The multiple comparisons LSD test found the difference to be significant (see Table 6).

Figure 7.

Photographs of fabric samples before and after flame resistance testing: (a) to (d) samples FM1, FM2, FE1, and FE2 before the testing, respectively; (e) to (h) samples FM1, FM2, FE1, and FE2 after the flame was applied for 3 s, respectively; (i) and (j) samples FM1 and FE1 after the flame was applied for 5 s, respectively; and (k) and (l) samples FM2 and FE2 after the flame was applied for 6 s, respectively.

Table 5.

Flame retardant properties of woven fabrics

Fabric sample	Minimal flame application time for ignition (s) (CV%)	Flame spread speed (mm/s) (CV%)	LOI value (%) (CV%)
FC	6 (12.82)	4.15 (7.55)	21.81 (2.03)
FM1	8 (7.21)	3.20 (9.14)	22.73 (1.33)
FM2	9 (7.58)	2.17 (4.57)	24.61 (2.16)
FM3	10 (4.50)	1.94 (2.24)	24.39 (1.79)
FE1	5 (12.02)	3.41 (7.39)	21.73 (1.40)
FE2	6 (7.94)	2.52 (7.60)	22.80 (1.03)
FE3	7 (4.69)	2.18 (5.10)	24.65 (0.99)
FA	13 (6.06)	No ignition	26.51 (1.67)

Note: LOI = limiting oxygen index.

Table 6.

Results of multiple comparisons LSD test of fabric flame-resistance

Fabric sample		Minimal flame application time for ignition		Flame spread speed		LOI
Fabric sample		Mean difference	Sig.	Mean difference	Sig.	Mean difference	Sig.
FC	FM1	−2.000*	0.000	0.951*	0.000	−0.920*	0.000
	FM2	−2.700*	0.000	1.975*	0.000	−2.800*	0.000
	FM3	−3.700*	0.000	2.209*	0.000	−2.573*	0.000
	FA	−6.800*	0.000	1.970*	0.000	−4.693*	0.000
FM1	FM2	−0.700*	0.010	1.025*	0.000	−1.880*	0.000
FM1	FM3	−1.700*	0.000	1.258*	0.000	−1.653*	0.000
FM2	FM3	−1.000*	0.000	0.234*	0.043	0.227*	0.022
FM1	FE1	3.100*	0.000	−0.951*	0.000	1.000*	0.000
FM2	FE2	1.800*	0.000	−1.237*	0.000	1.813*	0.000
FM3	FE3	2.700*	0.000	−0.584*	0.000	−0.260*	0.009

Note: LOI = limiting oxygen index. The mean difference is significant at the 0.05 level.

Table 5 shows the effect of the blending ratio of cotton and aramid fibers on fabric flammability. The minimal flame application time for ignition and LOI increase and flame spread speed decreases with an increase in the blending ratio of aramid fibers for both marl structured and evenly blended yarn fabrics (with the exception of the LOI of fabric sample FM3). Table 6 presents the results of the multiple comparisons LSD test conducted on the fabric’s flame resistance. The blending ratio of the aramid fibers on fabric flammability was significant.

In this study, a marl-like structure was designed for producing the cotton and aramid blended yarn. The test results show that such marl structured yarn has lower yarn flammability and thus better flame resistance compared with conventional evenly blended yarn. The comparison of flammability between fabrics using marl structured yarns and evenly blended yarns is presented in Table 5. From the comparisons, it can be seen that the fabrics using the marl structured yarns possess higher minimal flame application time for ignition and LOI as well as lower flame spread speed compared with the fabrics using evenly blended yarns. In this regard, the trend is similar to the yarn flammability. One possible reason is that the small amount of aramid fibers is concentrated when forming several fiber strands within the yarn via building the marl yarn structure, which probably hinders the ignition of flames as well as the flame spread of flammable cotton strands and thus lowers the flammability of the cotton/aramid blended fabrics. For example, when compared with fabric sample FE2 using the evenly blended yarn, fabric sample FM2 using the marl structured yarn has an increase in minimal flame application time for ignition of around 50% from 6 to 9 s, and an increase in LOI of about 7.9% (percentage increase) from 22.8% to 24.61% as well as a decrease in flame spread speed of around 13.9% from 2.52 mm/s to 2.17 mm/s. The multiple comparisons LSD test on fabric flammability was performed to assess whether there was a significant difference between the fabric samples using marl structured yarns and evenly blended yarns. The results shown in Table 5 demonstrate that there is a significant difference between these two types of aramid/cotton blended fabrics.

Effects of yarn structure and fiber blending ratio on physical properties of resultant woven fabric

The blending ratio of fibers and yarn structure affect not only the resultant fabric flammability but also the physical properties and comfort of the fabric. Table 7 shows the durability of cotton/aramid blended fabrics made with the marl structured and evenly blended yarns, in terms of tensile and tearing strength as well as abrasion resistance. The majority of cotton/aramid blended fabrics exhibit higher tensile and tearing strength in both warp and weft directions than the 100% cotton yarns as weft yarns. The fabric samples FM1 and FM2 using the marl structured yarns as wefts possess a higher abrasion resistance compared to the fabric sample FC using cotton yarns as wefts, while the abrasion resistance of fabric samples FE1, FE2, and FE3 is lower than that of fabric sample FC. A possible reason for this is that the marl structured yarn has a plied structure, which probably benefits the yarn and thus the fabric abrasion resistance. As the yarns in different fiber blending ratios and yarn structures were used as weft yarns for the fabrics, the analysis on fabric tensile and tearing strength will focus on the weft direction of fabrics. Considering the effect of the blending ratio of aramid and cotton fibers on the fabric physical properties, the results in Table 7 reveal that the tensile and tearing strength of fabrics in the weft direction increase when the aramid blending ratio increases from 24% to 48%; however, a further increase of blending ratio to 65% leads to a decrease in fabric strength. With an increase of the aramid blending ratio, both fabrics using marl structured and evenly blended yarns exhibited a decreased fabric abrasion resistance. Compared with fabric samples using the evenly blended yarns in the same blending ratio of the aramid fiber, the fabrics using the marl structured yarn showed a higher tensile and tearing strength in the weft direction of fabric when the aramid blending ratios of 24% and 48% were adopted. The abrasion resistance of fabrics using the marl structured yarns was higher than that of the fabrics using the evenly blended yarns, while the fabric sample FM1 made of marl structured yarns had the highest abrasion resistance among the eight fabric samples in this experiment.

Table 7.

Tensile and tearing strength and abrasion resistance of fabrics

Fabric sample	Tensile strength (N) (CV%)		Tearing strength (N) (CV%)		Abrasion resistance (weight reduction) (%) (CV%)
Fabric sample	Warp	Weft	Warp	Weft	Abrasion resistance (weight reduction) (%) (CV%)
FC	601 (2.25)	415 (4.14)	50 (5.72)	42 (4.13)	0.89 (0.38)
FM1	669 (1.34)	399 (2.48)	56 (5.91)	44 (11.07)	0.27 (0.40)
FM2	685 (2.68)	517 (3.13)	53 (8.29)	50 (9.42)	0.68 (0.86)
FM3	610 (3.41)	443 (1.74)	54 (12.62)	44 (13.02)	0.95 (0.47)
FE1	597 (4.48)	359 (7.55)	51 (9.40)	43 (10.05)	1.54 (1.50)
FE2	646 (2.03)	439 (2.53)	49 (7.29)	50 (9.01)	1.91 (1.69)
FE3	617 (1.95)	457 (5.28)	56 (6.27)	46 (10.44)	2.51 (0.25)
FA	643 (3.09)	495 (3.73)	53 (5.42)	51 (8.53)	0.40 (0.40)

Cotton is one kind of natural fiber that makes the fabric both comfortable and soft to the touch; however, it is easily ignited and burns rapidly. Conversely, the meta-aramid fiber not only possesses excellent flame resistance but also high thermal insulation, good moisture absorption, but is also soft handle. In this regard, the transfer property and water absorbency of aramid/cotton fiber blended fabrics were evaluated, which are related to fabric comfort. Table 8 shows the results of air and water vapor permeability, thermal conductivity, and the water absorption rate of fabrics. Compared with sample FC using cotton yarns, the air permeability of fabric samples FM1, FM2, and FM3 using the marl structured yarns decreased and the water vapor permeability of the fabric slightly increased after the aramid fibers were introduced into the yarn. On the contrary, fabric FM1 possesses a lower water absorption rate, while the absorption rate of fabrics FM2 and FM3 was higher compared with sample FC. Fabric sample FM1 showed a lower thermal conductivity, while samples FM2 and FM3 exhibited higher thermal conductivity than sample FC. With the increase of the blending ratio of aramid fibers, the air and water vapor permeability as well as the thermal conductivity of the fabric varies, but there seemed to be a significant increase in the water absorption rate of fabrics using marl structured yarns. Table 8 also shows that there was no significant difference in relation to air and water vapor permeability or thermal conductivity between the fabric samples using the marl structured and evenly blended yarns. More interestingly, the fabrics using the marl structured yarn exhibited a much higher rate of water absorption when compared to fabrics using evenly blended yarns, probably due to the plied yarn structure of the marl structured yarn.

Table 8.

Air and water vapor permeability, thermal conductivity, and water absorption rate of fabrics

Fabric sample	Air permeability (CV%)	Water vapor permeability (g/(m².24 h)) (CV%)	Thermal conductivity (ksi/(W/m.K)) (CV%)	Water absorption rate (%) (CV%)
FC	30.0 (8.36)	28.59 (6.23)	0.0465 (5.36)	85.53 (8.65)
FM1	20.4 (9.05)	29.37 (10.27)	0.0480 (3.99)	72.68 (14.33)
FM2	26.2 (5.81)	32.04 (11.34)	0.0459 (3.22)	126.63 (5.77)
FM3	26.4 (5.88)	31.89 (8.17)	0.0444 (2.51)	130.00 (12.02)
FE1	20.0 (8.36)	29.85 (6.89)	0.0474 (7.33)	60.85 (9.99)
FE2	27.0 (9.44)	30.34 (4.97)	0.0454 (5.17)	56.86 (3.47)
FE3	27.3 (6.72)	31.11 (4.41)	0.0441 (5.86)	63.60 (4.24)
FA	31.9 (5.88)	30.78 (4.23)	0.0453 (2.69)	140.06 (2.84)

Conclusion

In this study, a newly structuralized meta-aramid/cotton blended yarn was developed and produced in a ring spinning system with an attachment. The attachment was developed to facilitate the construction of a marl yarn resembling structure for cotton/aramid yarns wherein the small amount of meta-aramid fibers could be concentrated to form fiber strands with cotton fiber strands within the yarn compared with conventional evenly blended fibers. The aramid fiber strands can serve as effective fire barriers to hinder the afterflame/afterglow of cotton fibers and thus to lower the flammability of yarn and the resultant fabric. Examination of the yarn surface structure and cross-section found that the designed marl yarn resembling structure was formed during the yarn spinning, which consists of two types of yarn structure: plied and melange. The results of the investigation of the effects of yarn structure and fiber blending ratio on yarn flammability and its physical properties demonstrated that the afterglow time and damage length of the yarn were greatly decreased and the LOI value increased significantly compared with the 100% cotton yarn by introducing the meta-aramid fibers into the cotton fibers to form the blended yarns. The afterflame time, afterglow time, and damage length of the yarn decreased with an increase in the proportion of aramid fibers in the aramid/cotton blended yarn. The experimental results also found that the marl structured yarns possessed better flame-retardant properties, comparable tenacity, lower hairiness, and slightly higher unevenness than evenly blended yarns. The measured properties of aramid/cotton blended fabrics using the developed marl structured yarns revealed that different levels of flame resistance can be achieved on the aramid/cotton blended fabrics. Moreover, fabrics made of the marl structured yarns showed a significantly enhanced flame-resistance in terms of minimal flame application time for ignition, flame spread time, and LOI value compared with fabrics using conventional evenly blended yarns. The fabrics using the marl structured yarns also showed a higher tensile and tearing strength in the weft direction of the fabric when the aramid blending ratios of 24% and 48% were adopted.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors wish to thank the Innovation and Technology Commission of Hong Kong SAR Government, The Hong Kong Research Institute of Textiles and Apparel, X-Fiper (Hong Kong) Limited and Hang Hui Holdings (H.K.) Limited for funding support the project titled “Manufacturing technology of high value added flame resistant textiles” (Grant No. ITP/054/16TP).

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