Thermal comfort properties of Kevlar and Kevlar/wool fabrics

Abstract

Recent research on ballistic vests has focused on comfort performance by enhancing thermal comfort and moisture management. Kevlar/wool fabric has been developed as a potential material for ballistic vests. This study investigates the thermal comfort properties of woven Kevlar/wool and woven Kevlar ballistic fabrics. In this context, the thermal resistance, water-vapor resistance, moisture management performance, air permeability and optical porosity of 100% Kevlar and Kevlar/wool ballistic fabrics were compared. The effects of fabric physical properties on laboratory-measured thermal comfort were analyzed. This study also presents the fabric bursting strength and tear strength for comparison. Experimental results showed a clear difference in thermal comfort properties of the two fabrics. It was found that Kevlar/wool possesses better moisture management properties and improved mechanical properties than Kevlar fabric.

Keywords

ballistic fabric Kevlar/wool blend thermal comfort moisture management fabric porosity tear strength

High-performance ballistic fabrics made of Kevlar have the necessary properties for protection against high-velocity projectiles. A multi-layered ballistic vest can be very heavy and uncomfortable to wear, particularly in hot climatic conditions. Advanced body armor technologies aim to reduce the vest weight in order to enhance the comfort level.^1–3 It was reported that users were reluctant to wear an uncomfortable protective vest.⁴ Therefore, the interaction between the protective vest and the body is an important factor that needs to be considered while designing a body armor. One of the critical factors is selecting the ballistic fabrics, especially when both protection and comfort are considered.⁵

Thermal comfort can be considered as the wearer's subjective satisfaction with the thermal environment.⁶ It can also be defined as the ability to maintain constant body temperature through the thermal balance of heat generated by the body and transferring it to the environment.^7,8 In fact, the human body requires elimination of excess heat generated within the body.⁹ This occurs through dry heat losses and perspiration from the body to the environment. The dry heat losses rely on the clothes and the air gap between the skin and the garment layers. The evaporative losses on the other hand rely on sweat evaporation from the skin to and through the garment layers.^7–9 Thus, thermal comfort is a complex phenomenon. It can be influenced by the human body, the properties of garments worn and the surrounding atmosphere.⁸ In order to be thermally comfortable when the body is heating up and sweating, the garment should be able to transfer heat and moisture away from the skin to the atmosphere.⁷ Since there may be occasions when a single-layer ballistic fabric is used, the effective moisture transfer and thermal conductivity characteristics of a ballistic fabric can be closely related to user comfort.^4,10 In addition, because the thermal comfort behavior of a multi-layer protective panel is governed by its constituent layers, it is necessary to understand thermal comfort properties of individual fabrics. Furthermore, the knowledge of individual fabrics could be used to design a multi-layer fabric assembly and predict its comfort performance in end use.

Earlier research ascertained the utility of wool in improving the effectiveness of high-velocity ballistic protective fabric.¹¹ The function of wool in a Kevlar/wool (KW) fabric was to increase friction between yarns to restrict lateral yarn separations during impact. The enhanced fabric ballistic performance allows a reduction in the number of layers needed in the body armor panel. Hence, the increased weight of the KW fabric would not affect the performance of a ballistic panel.¹¹ However, the moisture management and thermal comfort properties of the fabric were not evaluated at that time. Since wool can pick up significant amounts of moisture, the KW fabric should have improved moisture management capability to absorb and transfer moisture away compared to the pure Kevlar fabric (KA), which contains the same amount of Kevlar yarn as KW. This study investigates and compares the thermal comfort properties of a woven ballistic KW fabric and a KA fabric, which have been proven to have equivalent ballistic performance per unit fabric mass in the wet condition. This evaluation was quantified in terms of moisture transport behavior and comfort as determined by selected laboratory-based experiments. The fabric optical porosity and air permeability were also investigated.

Material

The ballistic KW woven fabric developed by Sinnappoo et al.¹¹ was used in this study. It has been engineered with the same number of picks and ends per centimeter as KA fabric by integrating the wool and Kevlar yarns together as an equivalent yarn into the warp and weft of a simple square-sett plain weave. The wool yarn, spun from non-shrinkproofed wool of about 20 µm, was 35 tex low-twist two-fold. The Kevlar yarn for KW was T964C Kevlar 129.

Testing methods

Fabric physical properties

The fabric physical properties were tested according to relevant Australian standard methods, including yarn count (AS/NZS 2001.1.2:1998),¹² fabric thickness (AS 2001.2.15-1989),¹³ picks/ends per centimeter (AS 2001.2.5-1991)¹⁴ and mass per unit area (AS2001.2.13-1987).¹⁵ The fabric specimens were conditioned under standard conditions of 65 ± 3% relative humidity (RH) and temperature of 20 ± 2℃ for 24 hours according to AS 2001.1-1995.¹⁶ Both KW and KA fabrics were evaluated in this study based on a single layer instead of a multi-layered protective panel.

Moisture regain

The moisture regain of the KA and KW was measured according to ISO 6741.¹⁷ Moisture regain is the mass of water absorbed by a known mass of completely dry material exposed to the standard atmosphere mentioned above for at least 24 hours.¹⁸ Moisture regain was calculated using Equation (1):

Regain = \frac{W \times 100}{D} %

(1)

where W is the mass of absorbed water and D is the oven dry mass of the fabric.

Fabric optical porosity

The fabric optical porosity determines the openness of the fabric. The optical porosity was investigated by analyzing fabric surface-image segments obtained with a microscope. Digital images from light transmission were acquired by the multi-media software Motic Images Plus 2.0 ML. This software analyzes the dark shadow segments on an image of 752 × 524 pixels. The optical porosity result was calculated based on the percentage illumination of the air spaces that the microscope image captured.¹⁹

Air permeability

Air permeability tester M021S, manufactured by SDL Atlas, was used to measure the air permeability of the fabrics according to EN ISO 9237.1995.²⁰ The fabric sample size was 80 mm × 80 mm and five measurements were taken. The air permeability R was calculated using Equation (2):

R = \frac{\bar{qv}}{A} \times 167

(2)

where is the arithmetic mean flow-rate of air, A is the area of fabric under test (A = 4.908 cm² in this paper) and 167 is the conversion factor from cubic decimeters.

Sweating Guarded Hotplate

The Sweating Guarded Hotplate (SGHP), manufactured by SDL Atlas, was used to simulate the heat and moisture transfer processes that occurs between the skin and the fabric according to ISO 11092:1993(E).²¹ The value of the arithmetic mean of three readings from each specimen of the fabric and the standard deviation was calculated according to the standard. Three specimens, each measuring 300 mm × 300 mm were tested for each fabric.

The thermal resistance test measured the energy required to maintain a constant temperature of 35℃ on the surface of the measuring plate. The energy value as well as the temperature difference between the plate surface and the surrounding ambient air were used to calculate the thermal resistance of the fabric sample using Equation (3). The fabrics were conditioned under the standard conditions¹⁶ before thermal resistance measurement. The thermal resistance measurement unit temperature (T_m) was 35℃, air temperature (T_a) was 20℃ and the RH was 65%. The air speed was 1 m/s.

R_{ct} = \frac{S_{a} (T_{m} - T_{a})}{Q} - R_{ct 0}

(3)

where R_ct is the thermal resistance [m²K/W], R_ct0 is the constant bare plate measurement of thermal resistance [m²K/W], S_a is the surface area of the measuring unit (S_a = 0.04 m²), T_m is the temperature [℃] of the plate surface, T_a is the temperature [℃] of the ambient air and Q is the power [watt] required to maintain a constant plate surface temperature.

The water-vapor test measured the power required to keep a constant vapor pressure between the top and bottom layer of the fabric. The test recorded the average power required to keep the measuring unit at its selected temperature based on a 15-minute integration.²¹ The vapor resistance test specimens were conditioned under an atmosphere of RH of 40% and temperature of 35℃ for 24 hours as specified in ISO 11092:1993(E).²¹ In this case, the T_m and T_a in Equation (3) were 35℃ and the RH was 40% for the testing atmosphere. The air speed was 1 m/s. The water-vapor resistance of fabric is calculated by the vapor pressure difference between the plate surface and the ambient air using Equation (4).

R_{et} = \frac{S_{a} (P_{m} - P_{a})}{Q} - R_{et 0}

(4)

where R_et is the water-vapor resistance [m²Paw⁻¹], R_et0 is the constant bare plate measurement of water-vapor resistance [m²Pa/W], S_a is the surface area of the measuring unit (S_a = 0.04 m² in this paper), P_m is the saturation water-vapor partial pressure [Pa] at the surface of the measuring unit, P_a is the water-vapor partial pressure [Pa] of the air in the test enclosure and Q is the power [watt] required to maintain a constant plate surface temperature.

Moisture Management Tester

The Moisture Management Tester (MMT) measures, evaluates and classifies the liquid moisture management properties of textiles. Five specimens each of the KA and KW fabrics measuring 80 mm × 80 mm were conditioned at the standard conditions.¹⁶ The mass of each specimen was measured. Normal saline solution (9 grams of sodium chloride per liter) was dripped freely onto the top surface at the center of the fabric. The dropped solution then might spread outward on the top surface and through the fabric to the bottom surface of the fabric and spread outward on the bottom surface. As the solution moved, MMT measured the liquid moisture transport behavior in different directions of the sample.²² The results were rated according to the following grading table as suggested in the test method:²²

wetting time [s] in top and bottom: (1) ≥120S no wetting; (2) 20-119S slow; (3) 5-19S medium; (4) 3-5S fast; (5) <3S very fast;

absorption rate in [%/s] top and bottom: (1) 0–10%/s very slow; (2) 10–30%/s slow; (3) 30–50%/s medium; (4) 50–100%/s fast; (5) >100%/s very fast;

max wetted radius [mm] in top and bottom: (1) 0–7 mm no wetting; (2) 7–12 mm small; (3) 12–17 mm medium; (4) 17–20 mm fast; (5) >22 mm very fast;

spreading speed [mm/s] in top and bottom: (1) 0–1 mm/s very slow; (2) 1–2 mm/s slow; (3) 2–3 mm/s medium; (4) 3–4 mm/s fast; (5) >4 mm/s very fast;

one-way transport capacity (OWTC): (1) <−50 very poor; (2) −50–100 poor; (3) 100–200 good; (4) 200–400 very good; (5) >400 excellent;

overall moisture management capacity (OMMC): (1) 0–0.2 very poor; (2) 0.2–0.4 poor; (3) 0.4–0.6 good; (4) 0.6–0.8 very good; (5) >0.8 excellent.

Bursting strength

The ball burst method was used to determine the bursting force of compression fabrics using modified Australian Standard AS 2001.2.19 (determination of bursting force of textile fabrics-ball burst method). The tests were performed on a Lloyd instrument under the dry conditions (65 ± 3% RH and 20 ± 2℃). The compression rate was set at 1000 mm per minute, which is the maximum speed for the instrument. As a simulant bullet, the polished spherical steel ball was 10 mm in diameter. After clamping, the center of a fabric area of 45 mm in diameter was compressed by the ball and the compression force was recorded. An average of three bursting strength results was reported for both KA and KW fabrics in this paper.

Results and discussion

Fabric weight and regain

The physical properties of the fabrics are shown in Table 1. From Table 1, it can be seen that the KW has the same number of picks and ends per centimeter to KA but the total yarn count for KW was different due to the wool component (27% of the total). Therefore, both KW and KA contain the same amount of Kevlar yarn per unit fabric area, but the mass and thickness of KW has increased due to the wool yarn. The moisture regain measured for the KA fabric was 3.2%, while for the KW fabric it was 5.9%. The KW fabric has higher moisture regain due to its wool component, which can absorb more moisture than Kevlar.

Table 1.

Fabric physical properties (mean ± standard deviation)

Fabric	Structure and source	Yarn count (tex)	Picks /cm	Ends /cm	Moisture regain (%)	Fabric thickness (mm)	Mass per unit area (g/m²)	Optical porosity (%)	Air permeability mm/s
KA	Plain weave (Commercial)	95	11	11	3.2	0.32 ± 0.01	210 ± 1	2.5 ± 0.7	6.5 ± 1.4
KW	Plain weave (RMIT)	Kevlar 95 Wool 35	11	11	5.9	0.51 ± 0.01	299 ± 2	6.1 ± 0.8	30.6 ± 4.2

Fabric air permeability and optical porosity

The results in Table 1 also show that the air permeability of KW (30.6 mm/s) is higher than the air permeability of KA (6.5 mm/s); meanwhile, the optical porosity of KW (6.1%) is also higher than the optical porosity of KA (2.5%). Figures 1(a) and 2(a) show the optical microscope images in the form of the light transmission through the fabric for KA and KW fabric structures, respectively. Figures 1(b) and 2(b) highlight the porous areas observed from the Motic image processor for KA and KW, respectively. The results indicate that even though KW contains the same amount of Kevlar yarn as KA, the addition of wool yarn made the fabric thicker and created voids between yarns to allow air to easily pass through. This observation agrees with the report that air permeability depends on the physical properties of fabric such as construction, mass, thickness and yarn count.²³

Figure 1.

Kevlar fabric optical porosity: (a) original; (b) processed.

Figure 2.

Kevlar/wool optical porosity: (a) original; (b) processed.

Fabric air permeability and optical porosity are related to each other. A fabric that has a high percentage of optical porosity is more likely to have high air permeability.²⁴ The experimental results in Table 1 agreed well with the relationship between optical porosity and air permeability. The optical porosity is affected by the fabric structure.²⁵ It is an important feature to gauge textile permeability.²⁶ The low optical porosity of KA fabric is due to the small pores between the warp and the weft threads (Figure 1). In contrast, the pore size in KW fabric (Figure 2) is larger than that in KA fabric. As a result, the KW fabric shows higher optical porosity and air permeability than the KA fabric.

Thermal and water-vapor resistance

The thermal resistance and water-vapor resistance results of the two fabrics are shown in Figures 3 and 4, respectively. The thermal resistance of KW fabric (0.011 m²K/W) was higher than that of KA fabric (0.008 m²K/W). The difference is due to the wool component, which resulted in different physical properties between the two fabrics. Fabric mass and thickness determine the amount of heat transfer between the body and the surrounding air.²⁷ The KA fabric shown in Figure 5(a) is made from fine Kevlar continuous filaments with low crimp and no twist; hence the fabric is thin. As the KA fabric mass is lower than KW (Table 1), a single layer of KA fabric allows easier transfer of heat from the body to the surrounding air. However, when comparing the thermal resistance for the same weight fabric, that is, the thermal resistance is normalized by its fabric mass per unit area, the normalized thermal resistance for KW is slightly lower than that for KA. This suggests that KW could have similar equivalent thermal resistance as KA when fabric thickness and weight are considered. More detailed technical information can be revealed when the KW and KA fabrics are designed to have the same weight.

Figure 3.

Fabric thermal resistance.

Figure 4.

Fabric water-vapor resistance.

Figure 5.

Scanning electron microscope images: (a) KA; (b) KW.

The standard scale of R_ct value for the medium weight woven fabric (170–240 g/m²) should be 0.01–0.018 m²K/W. Since the KA fabric weight is 209.6 g/m² and its thermal resistance is 0.008 m²K/W, less than 0.018 m²K/W, KA can be considered as comfortable in standard condition.^27,28 In contrast, the KW fabric (Figure 5(b)) has two types of yarns: the Kevlar filament yarn as used in KA and spun wool yarn. Hence, less heat would pass through the thick KW fabric due to the additional thermal insulation from wool. However, the R_ct value for KW does not approach the standard scale of 0.02–0.025 m²K/W for a heavy weight woven fabric (240–375 g/m²). Therefore, the KW has reasonable thermal resistance for industrial fabric that has a heavy weight of 299.2 g/m² and it can be considered as comfortable in standard conditions as well.

Vapor resistance determines the ability of a fabric to resist and/or transfer water-vapor from the fabric to the atmosphere. Figure 4 shows that the water-vapor resistance for KA fabric is 9.2 m²Pa/W, which is higher than that for KW, 7.3 m²Pa/W. This indicates that KA is more applicable in low-temperature environments because of its high vapor resistance.²⁹ According to Horrocks and Anand,³⁰ protective clothing with water-vapor resistance that is less than 20 m²Pa/W can perform best breathability and comfortable to wear when the humidity is low. In other words, both KA and KW have lower water-vapor resistance to moisture transfer and therefore higher breathability.²¹ In particular, with wool fibers, KW can absorb a lot of vapor and release it away from the body to the atmosphere.³¹ Since the KW fabric vapor resistance (7.3 m²Pa/W) is lower than KA, the KW fabric has better breathability and improved comfort properties compared to KA when the humidity inside the garment is high.

Moisture management

The moisture absorption radius shown in Figure 6 and the test results in Table 2 reveal that the KW fabric is able to absorb more moisture than KA, which is expected. Despite this, the KW fabric has very poor OMMC of 0.012 and very poor OWTC of –444%. However, the KW fabric has a moderate liquid moisture spreading capability on both sides as can be seen from the water location image in Figure 6(b). According to the test results in Table 2, the KW fabric has a slow water spreading speed of 1.0 mm/s on the bottom surface and 0.9 mm/s on the top surface. The small wetted radii of the bottom surface (11 mm) and top surface (10 mm) indicate that the liquid can spread through the top surface, be transferred from the next-to-skin surface to the opposite surface, and spread out on the fabric bottom surface. Therefore, the KW has the capacity for moisture to transfer from the inner surface to the outer surface, where it evaporates moisture easily into the environment. The KA, on the other hand, has very poor liquid moisture management properties without any wet-out radius and 0 liquid moisture spreading rate on the bottom surface of the fabric. The OMMC was 0 and OWTC was −737.3% (Table 2). This indicates that the liquid could not diffuse from the inner surface into the fabric and it was accumulated on the top surface of the fabric, as illustrated in Figure 6(a). Consequently, the KA fabric cannot effectively evaporate water into the environment as it would keep the sweat between skin and the next-to-skin surface of the fabric. Overall, both KA and KW fabrics have poor moisture management capacity; however, the KW fabric has improved liquid moisture management properties compared to the KA fabric.

Figure 6.

Water absorption radius at 120 seconds after saline solution was dripped: (a) KA; (b) KW.

Table 2.

Moisture management – all indexes (mean ± standard deviation)

Fabric	Wetting Time (s)		Absorption Rate (%/s)		Max Wetted radius (mm)		Spreading Speed (mm/s)		Accumulative one-way transport (%)	OMMC
Fabric	Top	Bottom	Top	Bottom	Top	Bottom	Top	Bottom	Accumulative one-way transport (%)	OMMC
KA	8.2 ± 1.7	120 ± 0	322 ± 32	0 ± 0	5 ± 0	0 ± 0	0.6 ± 0.1	0 ± 0	−737 ± 42	0 ± 0
KW	11.1 ± 2.5	12.5 ± 3.1	119 ± 35	13.2 ± 1.9	10 ± 0	11.0 ± 2.2	0.9 ± 0.1	1.0 ± 0.1	−444 ± 54	0.012 ± 0.009

OMMC: overall moisture management capacity.

The single-layer KW may find applications that require body armor with low-level protection and improved thermal comfort performance. In a recent survey³² in Jeddah Prison in Saudi Arabia, it was found that the prison officers were attacked by prisoners, but the prison officers did not wear any body armor vests on duty because of discomfort of the protective equipment. A uniform from the KW fabric could provide a low-level protection for the prison officers against the stabbing threat while maintaining a certain level of thermal comfort and mobility. For engineering multi-layer body armor, understanding the performance of a panel containing either KW or KA fabric is important for material selection and body armor design. This may be further investigated and reported in the future.

Fabric mechanical properties

The ball bursting force results in Figure 7(a) show that, although there is no statistically significant difference, the average bursting strength of KW fabric was marginally higher than that of KA fabric. This could be due to the addition of the wool component, which provided frictional force to restrict movement of Kevlar yarn. Hence the steel ball broke slightly more Kevlar filaments in KW than in KA. Since the fabric being tested was mounted in such a way that outside of the area of 45 mm in diameter was tightened by clamps, it is unlikely that the yarns being compressed by the steel ball would be pulled out of the fabric structure between the clamps. This was confirmed from observation after testing. In addition, the bursting strength of lightweight 100% wool fabric is normally below 200 N, which is far less than that of KA fabric. Therefore, the contribution of wool to the overall bursting strength is very marginal.

Figure 7.

Ball bursting strength (a), and ball penetration force versus extension curves (b).

Figure 7(b) shows typical ball penetration force versus extension curves. Both KA and KW have a similar extension at the maximum bursting strength. However, the KA curve is not as smooth as the KW fabric curve. This was again due to fabric tightness and friction between yarns. The KA fabric was thin and the steel ball could push yarns sideways, resulting in a serrated compression force profile when the yarns slid on the ball surface. On the other hand, because the wool in KW restricted Kevlar yarn movement, the compression force profile of KW is much smoother. Therefore, the advantage of adding wool may not be significant for achieving a high bursting strength. Furthermore, the burst strength testing speed was only 1 m/min, which is extremely slow compared to the ballistic impact. Earlier research has concluded that the ballistic properties of KW fabric at least matched, if not surpassed, the ballistic properties of KA fabric tested in the range of 431–440 m/s.¹¹ It appears that, for comparing high-velocity protective fabrics, the burst strength tested at 1 m/min could be a good indication of differences in ballistic performance.

The tear strength results in Figure 8 were from a previous study.¹¹ They show that the KW fabric is significantly stronger to tearing than the KA in both weft and warp directions. The wool contributed approximately 38.7% improvement in fabric mean tear strength. Furthermore, wool makes positive contributions to the energy absorption mechanism of pulling yarns out of the weave due to the increased longitudinal frictional force along the yarns, especially near free edges of tested samples.¹¹ As a result, the KW fabric can at least match the dry or wet ballistic performance of an equivalent pure KA when tested under National Institute of Justice (NIJ) Ballistic Standard Level III A.¹¹

Figure 8.

Mean tear strength (dry tests) for KA and KW fabrics.

Conclusion

The properties relating to thermal comfort of KA and KW fabric were investigated, including thermal and water-vapor resistance, moisture management and fabric permeability. The test results of water-vapor resistance reveal that the KW fabric can transfer water vapor to the atmosphere more easily than KA. Test results of thermal resistance indicate that the thermal resistance of KW fabric is higher than that of KA. However, when the fabric weight and thickness are considered, the thermal resistance of the KW fabric could be lower than or equivalent to that of the KA. Compared to KA, KW fabric has the capability to manage moisture transfer because water can wet the KW fabric and spread through the fabric, whereas KA has very poor moisture management capability. The KW fabric has higher air permeability and optical porosity than KA fabric, which indicates that the KW is a breathable fabric as well. Furthermore, the wool component contributed marginally higher bursting strength of KW fabric than that of KA, and improved the mean tear strength of KW fabric approximately 38.7% compared to KA. Overall, wool enhanced the thermal comfort properties of the KW woven fabric without compromising the fabric mechanical properties.

Footnotes

Funding

This work was supported by the Saudi Cultural attaché in Australia on behalf of the King Abdulla Scholarship Program, the MINISTRY of Higher Education of Saudi Arabia.

Acknowledgments

The authors would like to thank all astute reviewers for their suggestions to improve this paper. Also the authors acknowledge the Brunswick sustainable laboratory team at RMIT University for their support during fabric testing.

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