Physical properties of Kevlar and Nomex plied and covered yarns

Abstract

This article experimentally investigates various physical properties of Kevlar and Nomex plied yarns. The tensile properties of a yarn decide the performance of post spinning operations – warping, weaving and knitting – hence its accurate technical evaluation carries much importance in industrial applications. For this purpose, Nm 50 Kevlar and Nomex yarns were twisted in various combinations with a DirecTwist machine. Multi-yarn twisting and cross-yarn covering methods were used to produce plied yarns. Surface roughness (both pin friction coefficient and yarn-to-yarn friction coefficient), lint generation, hairiness, breaking force and breaking elongation properties of the yarns were tested. Furthermore, the effect of twisting technique, twist amount and yarn type on these physical properties of yarns were statistically investigated.

Keywords

Kevlar Nomex plied yarns surface roughness breaking force lint generation

Many flame-resistant clothing systems and ballistic protection clothes need to be made from Nomex® and/or Kevlar® fabrics to meet the protection requirements.¹ Kevlar and Nomex are registered trademarks of DuPont for a group of synthetic organic fibers in the aromatic polyamide family.^2,3 Kevlar fibers have many desired engineering properties, such as high modulus, high strength, light weight, good chemical resistance and thermal stability. Thus, Kevlar fibers are widely used as the functional material in flexible body armors.³ Nomex brand fiber, a member of the aramid family of fibers, offers excellent flame resistance, good textile properties, dimensional stability and resistance to degradation by a wide range of chemicals and industrial solvents. Nomex has a flexible polymer chain that gives it more textile-like qualities, while retaining high-temperature properties similar to Kevlar.⁴

Depending on the application, the choice between Kevlar and Nomex is one of comfort and protection requirements. Kevlar is much more abrasive resistant then Nomex and is therefore used in a higher concentration percentage in professional fire-fighters’ turnout gear. Nomex is a softer feeling fiber and is used to a higher degree in daily use garments because of its greater comfort provided to the wearer. Kevlar is, however, added to Nomex to provide a higher degree of strength to the apparel.⁵ Kevlar is also added to the Nomex to help establish a stronger break open point for the garments.

Aramid and other high-strength fibers and fabrics have been studied extensively due to their application in a wide range of products, such as bullet-proof vests, confinement chambers for jet engines, cut-resistant gloves, etc. Furthermore, Kevlar and Nomex fibers have been widely investigated in reporting the ballistic and flame resistance performances of a fabric or composite material in the previous researches.^6,7 DuPont reported only the tensile properties of Nomex filament yarns and effect of yarn twist on these tensile properties.⁴ It was concluded that with increasing twist, elongation increases, initial modulus decreases sharply, tenacity increases as twist increases for 200 denier yarn and tenacity peaks at approximately 4 TPI, then decreases with further twist for 1200 denier yarn. Zhu et al.⁸ tested Kevlar single yarns in tension to investigate the strain rate effect on the dynamic material properties in terms of Young’s modulus, tensile strength, maximum strain and toughness. The yarns were also in filament form. Thus, in this study, surface roughness (both pin friction coefficient and yarn-to-yarn friction coefficient), lint generation, hairiness, breaking force and breaking elongation properties of the yarns were experimentally investigated using two types of Kevlar and Nomex (staple) plied yarns with varying twist factors.

Materials and method

In order to investigate the effects of yarn type and yarn twist on yarn properties, 12 different yarns were produced for the experiment by using a DirecTwist machine. Nm 50 Kevlar and Nomex staple yarns were used for this purpose. The first group of yarns contained three yarns (three-ply yarns) that were produced in two different twist levels, whereas the second group of yarns contained a core yarn in the middle and two surrounding yarns in the structure (cross-covered yarns). Three kilograms of each yarn type was manufactured for the experiments in the same twisting unit in order to prevent spindle-related deviations.

In this study, the twist levels, 150 and 300 T/m for three-ply yarns and 200 and T/m for covered yarns, were chosen to investigate the effect of degree of twist on yarn physical properties. These levels of twist, which gave considerably different yarn compactness to produced yarn samples, were chosen according to advice given by DirecTwist company representatives. The structural properties of the produced yarns are given in Tables 1 and 2.

Table 1.

The structural properties of the three-ply yarns

Yarn structure	Yarn type	Twist (T/m)
Three-ply yarns	1 Kevlar + 2 Nomex	150 S
	1 Kevlar + 2 Nomex	300 S
	2 Kevlar + 1 Nomex	150 S
	2 Kevlar + 1 Nomex	300 S
	3 Kevlar	150 S
	3 Kevlar	300 S
	3 Nomex	150 S
	3 Nomex	300 S

Table 2.

The structural properties of the cross-covered yarns

Yarn structure	Yarn type	Twist of the 1st yarn (T/m)	Twist of the 2nd yarn (T/m)
Cross-covered yarns	Nomex core (Nomex core – 2 Kevlar yarns cross-covered)	200 S	200 Z
	Nomex core (Nomex core – 2 Kevlar yarns cross-covered)	400 S	400 Z
	Kevlar core (Kevlar core – 2 Nomex yarns cross-covered)	200 S	200 Z
	Kevlar core (Kevlar core – 2 Nomex yarns cross-covered)	400 S	400 Z

Surface roughness (both pin friction coefficient and yarn-to-yarn friction coefficient), lint generation, hairiness, breaking force and breaking elongation properties of the yarns were tested and all measurements were performed after conditioning of the yarns for 24 hours under the standard atmosphere conditions (20 ± 2℃ temperature, 65% ± 2 relative humidity).

The coefficient of friction values of the yarns and lint generation properties were measured using a Lawson-Hemphill CTT (Constant Tension Transport) tester (Figure 1(a)). The CTT friction test modules allow measuring the coefficients of yarn-to-yarn friction and yarn-to-pin friction. The machine provides constant input tension on the yarn as it is running during the test. This is the principle of dynamic yarn testing and it is maintained by the specially designed tension arms and the yarn drive mechanism (which maintains the output rolls at constant speed while changing the input roll speed on demand to keep the input tension constant), which is illustrated in Figure 1(b).

Figure 1.

Constant Tension Transport (CTT) instrument (a) and its elements (b).⁹

The yarn-to-solid material tests were performed according to ASTM D 3108 − 01, while the yarn-to-yarn friction tests were carried out according to ASTM D 3412. In the yarn-to-solid material (pin friction) test, a relative speed of 100 m/min was used. The test method covers the measurement of the coefficient of kinetic friction between the yarn and solid surface or surfaces of constant radius in the contact area. In the yarn-to-yarn friction test, the test yarn is wrapped around itself. As a result of this contact, the output tension on the yarn changes. The software calculates the coefficient of yarn friction using the input and output tension values, as well as the number of wraps and apex angle.¹⁰ The experiments were carried out on each yarn type with 100 randomly selected replications for both pin friction and yarn-to-yarn friction tests.

The lint generation test is helpful to analyze the amount of yarn shedding. Excessive lint creates problems for fabric production machines and the work environment, and appears as defects on the fabric.⁹ The lint generation test measures the amount of the lint that is generated during a 1 km long test while the yarn is running under constant tension at test speeds up to 360 m/min. Tensile strength and breaking elongation of yarns were tested using a Lloyd LRX Plus instrument according to the ASTM D 2256 standard. The crosshead speed was 250 mm/min and the gauge length was 250 mm for tensile tests.

Yarn hairiness was measured by the CTT instrument using its charge-coupled device (CCD) camera, which measures the yarn diameter values with 3.25 micron precision as the yarn is moving under constant tension at test speeds up to 360 m/min. By using its software modules, the number of hairs in different lengths can be determined.

Results and discussion

In order to determine the statistical significance of yarn twist level and yarn type on yarn properties, independent samples t-tests and analysis of variance (ANOVA) tests were applied to the achieved test results after the control of normal distribution status and homogeneity of variables. To deduce whether the parameters were significant or not, p-values were examined for multi-yarn twisted and cross-yarn covered plied yarns, respectively, in Tables 3 and 4. The smaller the p value, the more strongly the test rejects the null hypothesis, which represents that there is no difference between two measured data sets. A p-value of 0.05 or less rejects the null hypothesis at the 5% significance level.

Table 3.

Variance analysis (p-values) of multi-yarn twisted plied yarns

(I) Yarn type	(J) Yarn type	Sig. (p)
(I) Yarn type	(J) Yarn type	Coefficient of pin friction	Coefficient of yarn-to-yarn friction	Lint generation	Breaking force	Elongation
1 Kevlar + 2 Nomex	2 Kevlar + 1 Nomex	0.000	0.000	0.001	0.000	0.001
	3 Kevlar	0.000	0.041	0.000	0.000	0.000
	3 Nomex	0.000	0.000	0.020	0.000	0.000
2 Kevlar + 1 Nomex	1 Kevlar + 2 Nomex	0.000	0.000	0.001	0.000	0.001
	3 Kevlar	0.000	0.000	0.000	0.000	0.970
	3 Nomex	0.000	0.000	0.000	0.000	0.000
3 Kevlar	1 Kevlar + 2 Nomex	0.000	0.041	0.000	0.000	0.000
	2 Kevlar + 1 Nomex	0.000	0.000	0.000	0.000	0.970
	3 Nomex	0.000	0.000	0.000	0.000	0.000
3 Nomex	1 Kevlar + 2 Nomex	0.000	0.000	0.020	0.000	0.000
	2 Kevlar + 1 Nomex	0.000	0.000	0.000	0.000	0.000
	3 Kevlar	0.000	0.000	0.000	0.000	0.000

Table 4.

P-values of independent samples t-test for cross-yarn covered plied yarns

Yarn type (I)	Yarn type (J)	Sig. (p)
Yarn type (I)	Yarn type (J)	Coefficient of pin friction	Coefficient of yarn-to-yarn friction	Lint generation	Breaking force	Elongation
Nomex core	Kevlar core	0.000	0.000	0.000	0.000	0.000

As seen in Tables 3 and 4, the material types and their combinations were found to be statistically significant for all the investigated psychical yarn properties of plied and covered yarns. Table 5 shows the p-values of independent samples t-test found to analyze the effect of twist on the properties of yarns.

Table 5.

P-values of independent samples t-test (indicating the effect of twist on the properties of yarns)

Yarn types	Yarn groups	Twist per meter (I)/(J)	Sig. (p)
Yarn types	Yarn groups	Twist per meter (I)/(J)	Coefficient of pin friction	Coefficient of yarn-to-yarn friction	Lint generation	Breaking force	Elongation
Three-ply yarns	1 Kevlar + 2 Nomex	150/300	0.000	0.000	0.029	0.036	0.005
	2 Kevlar + 1 Nomex	150/300	0.000	0.000	0.035	0.010	0.001
	3 Kevlar	150/300	0.185	0.317	0.038	0.031	0.890
	3 Nomex	150/300	0.000	0.000	0.048	0.000	0.001
Cross-yarn covered yarns	Nomex core	200/400	0.114	0.000	0.010	0.036	0.000
Cross-yarn covered yarns	Kevlar core	200/400	0.000	0.000	0.017	0.020	0.000

The twist level was also found to be statistically effective on most of the yarn properties analyzed in this study (Table 5). It was also observed that twist had no significant effect on surface roughness and breaking elongation properties of three-ply Kevlar yarns.

Results of yarn-to-pin friction test

The result of the yarn-to-solid material test (pin friction test) gives the coefficient of friction value, which represents the surface characteristics and smoothness of yarn. The value can give the frictional property of yarn during a continuous contact with solid materials.¹¹ An important aspect of dynamic tensile testing of spun yarns is the possibility of predicting the performance of yarn in subsequent processes. Dynamic tensile properties measured by continuous testing simulate actual manufacturing conditions more closely than the static tensile testing.⁹

The results for coefficient of pin friction values are given in Figure 2. It can be seen that yarn roughness is higher for the yarns including three-ply Nomex yarns, whereas it is lower for the three-ply Kevlar yarns. Plied yarns, which contain Kevlar in the structure, have a relatively lower coefficient of pin friction, which means a smoother surface and lower friction. A yarn may break while knitting because of the strength of the yarn and/or if the yarn has high friction. Thus, it can be predicted that yarns including Nomex will generate more yarn breakage than 100% Kevlar yarns because of the high friction. Yarn breakages cause productivity loss, thus increasing the manufacturing costs. An improvement in material quality and production efficiency is possible by testing possible yarn breakage before the yarn is used in production.

Figure 2.

Coefficient of pin friction values of yarns.

In the case of core yarns, higher coefficient of friction values were obtained from the yarns with Kevlar as the core and Nomex as the sheath. This is again caused by the surface characteristics of the Nomex fibers, which generate a rougher yarn surface.

The coefficient of pin friction value generally increases with the increasing yarn twist. Increased twist level reduces the surface hairiness of the spun yarns.¹² As the twist increases, more torque is applied at the yarn-forming zone and thus the probability of a fiber being embedded into the main yarn body increases, which results in less hairiness.¹³ It was also found in the study carried out by Atlaş and Kadoğlu¹⁴ that the coefficient of pin friction increases as the hairiness decreases. This could be related with the protruding yarn hairs preventing full contact between the yarn and the metal surface.¹⁵

The hairiness values of plied yarns were determined to confirm the effect of hairiness on coefficient of pin friction values (Figure 3). The highest hairiness values were obtained for 150 T/m three-ply Kevlar yarns where the twist factor was low. This yarn was also the one with the lowest coefficient of pin friction value.

Figure 3.

Hairiness of yarns.

Results of yarn-to-yarn friction test

The result of the yarn-to-yarn friction test represents the performance of yarn during weaving and other production steps as it is in a friction with the other yarns.¹¹

Results of coefficient of yarn-to-yarn friction were found to be similar to the results of coefficient of pin friction by means of twist (Figure 4). As the number of twists increases, the coefficient of yarn-to-yarn friction generally increases, except for three-ply Kevlar yarns, which have higher hairs in the structure. Furthermore, the effect of material type on the coefficient of yarn-to-yarn friction was found to be statistically significant, according to the results given in Tables 3 and 4. It can be also stated from Figure 4 that three-ply Nomex yarns have the highest coefficient value, which produce the highest friction force during the contact of yarn to itself. Thus, under the same weaving conditions, more problems will occur with three-ply Nomex yarns than other plied yarns, due to the yarn-to-yarn fiction forces.

Figure 4.

Coefficient of yarn-to-yarn friction values of yarns.

The coefficient of yarn-to-yarn friction results of the yarns, which have Kevlar as the core, are higher than the ones that have Nomex as the core due to the higher roughness of Nomex yarns.

Results of lint generation test

At textile mills, fiber lint (fly) generation is a great concern at almost every stage of the production, including knitting and weaving operations.¹⁶ The amount of lint produced by the yarns is given in Figure 5.

Figure 5.

Amount of lint generation.

According to Figure 5, three-ply Kevlar yarns generate lower levels of lint, while three-ply Nomex yarns generate higher amounts. As the number of Kevlar yarns in the plied yarn structure increases, the amount of lint decreases. In addition, twist has a significant effect on the lint generation property, since twist insertion make the yarn structure more compact and more durable for fluffing. In the case of covered yarns, a significant difference was found between the lint generation properties of the yarns (Table 5). Yarns that include Kevlar as core material generate higher lint due to the higher fluffing of Nomex, which is used as a cross-covering.

Results of tensile strength test

Tensile strength results of the yarns are shown in Figure 6. It can be denoted from Figure 6 that the breaking force values of three-ply Kevlar yarns are the highest, whereas the values of Nomex yarns are the lowest, due to the huge fiber tenacity of Kevlar fibers¹⁷ used in the yarn construction. Twist was found to be significant for the breaking force value, as expected.

Figure 6.

Breaking force values of yarns.

If the results are evaluated in terms of core yarns, it can be stated that the yarns which have two Kevlar yarns as a sheath are stronger than the one which has one Kevlar yarn as a core component. Thus, according to these results, it can be predicted that in applications where the strength of the fabric is more important than the comfort of fabric, 2 Kevlar +1 Nomex yarns can be used instead of Nomex core yarns, which have higher strength and elongation values.

The results of the breaking force analyses show that there seems to be an opposite relationship between the lint generation and breaking force, as expected. When the breaking force of yarns increases, the amount of generated lint also increases.

Results of breaking elongation test

Breaking elongation results of the yarns are shown in Figure 7. The results show that breaking elongation values of three-ply Nomex yarns are the highest, whereas elongation values of Kevlar yarns are the lowest. However, as the number of Nomex yarns in plied and covered yarns increases, the elongation values increase slightly, since the breakage of the Kevlar yarns occurs before breakage of Nomex yarns.

Figure 7.

Breaking elongation values of yarns.

When cross-covered yarns are investigated, it can be seen that the elongation values are higher for Kevlar core yarns than Nomex core yarns. This is due to the fact that cross-covered Kevlar yarns around the Nomex core prevent the elongation of Nomex yarn. In addition to this, the twist level also affects the elongation values of both plied and cross-covered yarns. As the twist level increases, the elongation values accelerate.

Conclusion

In order to investigate the effects of yarn type and yarn twist on yarn properties, Ne 50 Kevlar and Nomex yarns were twisted in various combinations. Surface roughness (both pin friction coefficient and yarn-to-yarn friction coefficient), lint generation, hairiness, breaking force and breaking elongation properties of the yarns were tested. According to the analyses, the effect of yarn type and twist level on yarn properties was found to be statistically significant. Yarn roughness was determined to be higher for the yarns, including three-ply Nomex yarns, whereas it was lower for the three-ply Kevlar yarns. It was also observed that yarn hairiness had a detractive influence on the coefficient of pin friction. Furthermore, as the number of twists increased, the coefficient of yarn-to-yarn friction and yarn-to-pin friction generally increased, whereas the amount of lint generation was decreased.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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