This study revealed that Kevlar and Kevlar/wool fabric thermal properties. The effect of fabric physical properties on thermal comfort was analyzed. The tests conducted on moisture management techniques were: sweating guarded hot plate (SGHP), moisture management tester, and surface tester. A conclusion for the paper is given which recommends the use of Kevlar and Kevlar/wool in the textile industry. The report examines the different thermal properties of Kevlar A363 and Kevlar/wool in an effort to determine which fabric is best suited for different weather conditions. In addition, the tests carried out shows clearly the difference in thermal properties of the two materials depending on their structure based on yarn count, fabric thickness, and mass. From the research, it was found out that Kevlar/Wool possess the most properties and is best suited in all seasons. Its structure has good porosity in terms of air and water transfer between the fabric and the body. The results agreed with the theoretical values tested in the past implying that the tests were perfect.
Keywords
Thermal comfort, porosity, permeability and heat transfer
Introduction
Researchers have developed body armour of high quality, and capable of protecting the wearer from harmful radiations. Advanced body armour technologies aim at enhancing the thermal comfort management on users, and reducing the number of layers in order to reduce the garment weight (Textile Technology 2012). The interaction between the garment and the skin is an important factor that manufacturers should put into consideration while designing textiles. According to Ashrae (1992), thermal comfort is the mind condition that shows satisfaction with the thermal environment. The present day textiles used by individuals have led into durability, lightness, and flexibility as people look for protection and comfort. The most important features for these clothes are protection and comfort. High performance fibers have been developed with the necessary properties. This body armour is capable of withstanding high operating temperatures, flame retardancy, resistant to chemicals, and high tensile strengths (Flambard, Ferreira, Vermeulen and Bourbigot 2003; Rupp 2001).
Human body requires eliminating the excess heat generated from inside the body. This occurs through water evaporation from the body to the environment. Bearing in mind that almost 80 percent of the body is covered, a lot of aeration is required in removing heat. In addition, the type of garment used should be able to release the heat to the environment (David 2000). Wool is a comfortable fabric because it enhances moisture absorption of a fabric and has a high thermal evaporation rate. The effective moisture transfer and thermal conductivity characteristics of body armour relate to user comfort (Huang 2006).
Fabric porosity
Porosity of a material refers to the ration of the total amount of void space to the bulk volume occupied. Fabric porosity assists in assessment of clothing comfort and determining technical properties of textiles (Elnashar 2005). The porosity of a material depends on the fabric and yarns construction. In addition, porosity forms one of the most important features that represent a textile structure. The efficient of fabric porosity determines the thermal comfort and the protection of the wearer. In addition, the multi-layer structure, the warp and weft densities, and the type of weave of a woven fabric allow air transmission, thus making it a porous material. The warp-weft density of a material has a great impact on the porosity of the material (Szosland 1999). Woven materials have different warp-weft densities whose effects contribute to the comfort of the material. Moreover, porosity of a fabric determines heat energy retention and liquid perspiration (Elnashar 2005).
The study investigates the thermal comfort of woven ballistic Kevlar/wool fabric. The woven ballistic/wool fabric is chosen because it can withstand varying thermal characteristics and is the most common used protective garment in health care centers. The tested material was compared to the current woven Kevlar A363 ballistic fabric in terms of thermal comfort using moisture management test methods. In particular, the study investigated the moisture transfer properties and transfer behavior of fabrics. Moreover, fabric surface properties, porosity and air permeability were investigated.
Fabrics physical properties
Table 1 shows the thermal properties of Kevlar A363 and Kevlar/wool (73/27).
Table 1: Fabric physical properties
Property
Test method
Yarn count (text)
AS/NZS 2001.1.2:1998
Fabric thickness
AS 2001.2.15-1989
Picks/end per inch
AS 2001.2.5-1991
Mass per unit area (g/m2)
AS2001.2.13-1987
Kevlar A363
95 text
0.31 mm
26 yarn in picks
26 yarn in end
Kevlar/wool
[1]
95 text Kevlar
two-fold ~35 text wool
0.51 mm
26 wool
26 Kevlar
= 52 yarn in picks
26 wool
26 Kevlar
= 52 yarn in end
Thermal comfort tests methods
The thermal comfort of Kevlar/wool fabrics was tested and evaluated against 100 percent woven Kevlar (A363). This evaluation tested how wool can enhance fabrics with moisture transport and comfort. In addition, the test investigates the ability of wool fabric to transfer heat from the outer skin layer to the environment.
Test procedure
The porosity was investigated by analyzing image segments through a microscope, camera and computer system. The digital images from light transmission were acquired by the multi-media software Motic Images Plus 2.0 ML. This software is designed to analyze the dark color segments on image having resolution of 752x524 pixels.
Test results
(a) (b)
Figure 1: Images for (a) Kevlar A 363 and (b) Kevlar Wool
Air permeability
The tests method is according to International Organisation for Standardisation (ISO) EN ISO 9237.1995[2]
The fabric sample size (0.31mm) was bigger than the inner diameter of measuring chamber (0.25mm), avoiding the selvedge area of the fabric and five measurements were taken. The calculation method for the air permeability expressed as: volume flow of air per unit water pressure per unit area of fabric.
The air permeability, R expressed in millimetres per second was calculated using equation 1.
R = 1
Where
q͞v is the arithmetic mean flow-rate of air.
A is the area of fabric under test in square centimetres. A= 4.908 cm2.
167 is the conversion factor from the cubic decimetres.
Test results for Air permeability:
Descriptive Statistics: Kevlar A 363, Kevlar/Wool
Table 2
Variance
Mean
(mm)
Standard Dev
Coefficient of Var
Minimum
Maximum
Kevlar A363
95% Confidence Intervals
Kevlar A363 fabric calculation C1
Kevlar/wool fabric calculation C2
Air permeability results
Figures 1 a, and b show two images percentage illumination observed from the Motic Images processor for two fabrics. From the observation, Kevlar/Wool has a low air permeability compared to Kevlar A363. From table 1, Kevlar/wool has a thickness of 0.51 mm while Kevlar A363 has a thickness of 0.31. This indicates that more light would pass through Kevlar A363 creating a bigger percentage illumination. Bigger pore dimensions allow more illumination through that small pore dimensions. The pore dimensions were seen to be highly variable between the two fabrics. On the other hand, Kevlar A363 allowed less light through resulting into dull image as shown in figure 1 (a). Kevlar/wool specimen was found to be less porous than Kevlar A363 because its yarns are tightly knitted and allows less light to pass through. Kevlar A363 is a light fabric allowing more light to pass through; hence air and water can easily diffuse through the fabric. On the other hand, Kevlar/wool has tightly knitted yarns that allow less light to pass through resulting into darker shades from the Motic Image method.
Discussion
The relationship between air permeability and porosity of a fabric is a complicated matter. Kevlar A363 and Kevlar/wool showed different characteristics on air permeability in table 2. The air permeability of Kevlar A363 was 6.5 mL/cm3 while, that for Kevlar/wool was 30.6 mL/cm3. The experiment shows that fabric porosity is affected by the fabric thickness, which determines illumination percentage. The effect of fabric texture on colour yielded by the shadow is very significance on this test. The low penetration of light from tight fabrics indicates low porosity due to the warp and weft density. In addition, the warp and weft density determines the permeability of a fabric (Lane 1996). Loose fabrics have less number of bonding points of the yarn per unit area, are more deformable and have poor thermal properties in terms of thermal insulation (Hsieh 1995).
Moisture Management Experimental Methods
Sweating Guarded Hotplate
Sweating Guarded Hotplate (SGHP) was used to simulate the heat and moisture transfer processes that occur between the skin and the fabric according to International Organisation for Standardisation(ISO) NO 11092(E).[4] The thermal resistance test measured the energy required to maintain a constant temperature of 35°C of the surface of the measuring plate. The result was used to calculate the thermal resistance of the fabric sample. The wet vapour test measured the power requires to keep the constant vapour pressure between the top and the bottom layer of the fabric.
Test
The test specimens were cut in square 35×35cm.Three samples were tested for each fabric. The fabrics have been conditioned for thermal resistance under the testing atmosphere of relative humidity (RH) of 65% and air temperature (Ta) of 20°Cfor 24 hours.The vapour resistance specimens have been conditioned under testing atmosphere of relative humidity (RH) of 40% and air temperature (Ta) of 35°Cfor 24 hours.
The thermal measurement unit temperature (Tm) and the thermal guards’ temperature (Ts) were 35.0oCo. Air temperature (Ta) was 20. 0oCo and the relative humidity (RH) was 65%. The water vapour test measurement unit (Tm), thermal guards’ temperature (Ts) and air temperature (Ta) were 35.0oCo with the relative humidity of 40%. The air speed was 1m/sec for both tests.
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Both the sample and bare plate moisture heat transfer were calculated according to the test standard. The test result intended on average power required to keep the measuring unit at its selected temperature based on 15 minute integration. The value of the arithmetic mean of three reading results from each specimen of the fabric and the standard deviation has been calculated according to the standard and evaluated regards to 50⨯10-3 m²·K/W in thermal resistance and for vapor resistance up to 10 m²·Pa/w
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Figure 3: The water vapor resistance
Results and discussions for SGHP
Sweating guarded hot plate apparatus approximates the heat and moisture transfer from a body surface through a clothing system. The test measures the thermal resistance and evaporative resistance of body armour. The results of the two fabrics are shown in figures 2 and 3.
The heat flow through Kevlar/wool is less compared to Kevlar A363 as seen from figure 2. The difference in heat flow between the two fabrics comes from the structure of their fibers. The thermal conductivity in the direction of thickness for both Kevlar A363 and wool determines the amount of heat transfer between the body and the surrounding. Wool has a high resistance to heat (0.123 m2-K/WS) and therefore allows a small amount of heat to pass through. On the other hand, Kevlar A363 has smaller yarn thickness that allows easier transfer of heat from the surrounding to the body.
An average standard deviation of 0.91 was discovered in the test for vapor resistance, which falls under the theoretical value of 0.86 to 0.92. Vapor resistance determines the ability of a fabric to withstand high vapor density. Kevlar A363 has a higher vapor resistance than wool. This indicates that Kevlar A363 is more applicable in low temperatures when the humidity is low because it can withstand high vapor densities. Moreover, its thickness is small allowing faster heat loss and easier transfer of air in and out of the body. On the other hand, Kevlar/wool has low vapor resistance because it has dense fibers with a thickness of 0.51mm, as shown in table 1, which absorb a lot of vapor, and take long to release it. In addition, fabrics with low vapor resistance are readily soaked during wet weather making the users uncomfortable.
Kevlar A363 is made of thin yarns that allow more heat to penetrate hence, it has low thermal resistance. Such clothes would be uncomfortable during cold seasons because they lose heat at a high rate. On the other hand, wool can be used in all seasons because its thermal qualities align with the natural environment.
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Test principle
MMT measures, evaluates, and classifies liquid moisture management properties of textiles. The properties are tested by placing the test specimen between two horizontal electrical sensors. A test solution for measuring the electricity conductivity changes is placed at the centre of the specimen. The solution moves freely in three directions spreading on the top surface. As the solution moves, the electrical resistance of the test specimen is measured and recorded. These readings were used in calculating fabric liquid moisture content changes. The changes determine the liquid moisture transport behavior in different directions of the garment (AATC Test Committee 2009).
Test procedure
1. The Kevlar A363 and Kevlar/wool fabrics were cut each into 5 specimens, each measuring 8cm radius.
2. The mass of each fabric was measured.
3. The specimens were conditioned under the standard atmosphere of 65 % relative humidity and 20 degrees centigrade air temperature for 24 hours.
4. The specimens were placed on the test material and the amount of electricity resistance measured and recorded.
Test
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MMT Test result
Figure 4: (a) Water absorption time for Kevlar A363
(b) Water absorption time for Kevlar/wool
Test
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Figure 4 shows the test results for the Moisture Management Tester using the two fabrics. Kevlar A363 has low liquid absorption rate from the low wetted radii, indicating low spread rates. The fabric has a bottom radius of 0.88mm indicating an absorption rate of 0.044mm/s. The above liquid absorption rate indicates that sweat from the body cannot easily diffuse to the body armour, and evaporate to the atmosphere. Kevlar A363 has low water absorption rate because its rate of heat transfer is higher than that of Kevlar/wool. On the other hand, Kevlar/wool has a higher liquid absorption rate because it shows a large wetted area. This indicates that Kevlar/wool has a high capacity of heat transfer from the outer skin to the outer surface. The fabric has a high spreading rate with 8.0cm radius of bottom wetted area and 7.8 cm radius top wetted area. Kevlar/wool has a liquid absorption rate of 0.4mm/s. This shows that liquid can easily spread on the bottom and dry quickly.
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The surface tester measures surface properties including surface smoothness, crispness, and roughness in fabrics. The test determines the relationship between friction and a tactile property of a fabric. The coefficients of friction of the fabrics are measured using the textile Friction Analyzer, or Kawabata Evaluation System.
Test procedure
1. The specimen were cut into 20cm by 20cm, and conditioned to Australia standards ISO 2001.1-1995.
2. The Kevlar A363 and wool fabric were placed under the test sensor and measured the friction at three different positions on each specimen. Three samples of each of the two specimens were tested.
3. The arithmetic mean of the three specimens’ results was calculated and recorded.
Test
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Table 4: frictional coefficient and the roughness measurements for Kevlar A363 and Kevlar/wool fabrics
Fabrics
Mean Warp Direction
Mean Weft Direction
Kevlar A363
Textile manufacturing industries are mostly concerned with clothing comfort based on the human sensory response to clothing materials. The surface test method determines the relationship between the fabric and the body in terms of friction. The coefficient of friction of a fabric depends on the structural properties of that fabric. Kevlar A363 has a lower coefficient of friction than wool. On the other hand, the differences between the coefficients of friction between the two fabrics are very small. In addition, Kevlar A363 has loose yarns, lightly knitted creating a smooth texture that offers less friction to the outer body layer.
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The latest textile technology follows the analyses of wool and Kevlar fabrics. The ideology behind the process involves using wool and Kevlar together hence making a blend of two materials such that the final product formulated can be one which consists of higher thermal comfort for the user. Kevlar/wool has the ability of withstanding varying weather conditions because the tests carried out indicated so. On the other hand, Kevlar A363 is more applicable in low temperature seasons because it has low ability of loosing heat, and cannot allow cool temperatures from the environment to the outer body layer. Moreover, the tests carried out play a significant role in the textile industry by determining the best amour to produce at different weather conditions.
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AATCC Technical Manual
ASHRAE., 1992. ASHRAE Standards, Thermal Environmental Conditions for
Human Occupancy Atlanta. 55
AATCC, Liquid Moisture Management Propertied of Textile Fabrics, 2009, American
Association of Textile Chemists and Colourists, pp. 361-365.
Congalton, D., 1999. Heat and Moisture Transport Through Textiles and Clothing Ensembles
Utilizing the Hohenstein Skin Model, J. Coat. Fabr., 28, 183–196
David, H. 2000, Performance characteristics of waterproof breathable
fabrics, J. Ind. Text. 29 (4), p. 306.
Elnashar, E. A. 2005. Volume porosity and permeability in double-layer woven fabrics, AUTEX
Research Journal, 5(4); 216-217
Flambard, X., Ferreira, M., Vermeulen, B., and Bourbigot, S., 2003. “Mechanical and thermal
behavior of first choice, second choice, and recycled P-Aramid fibers”, Journal of textile and apparel, technology and management. 3(2); 1
Hsieh, Y.L., 1995. Liquid Transport in Fabric Structures, Textile Res. Journal., vol. 65 (5), pp.\
299-307,
Huang, J. 2006. Sweating guarded hot plate test method. China; School of Textile and Materials.
Lane K. M., 1996. “Analysis of Knitted Fabric Models Using Image Processing”, Masters
Thesis, Georgia Institute of Technology.
Mair K. (2012). What are the properties of Kevlar? 24 October 2012. Retrieved from
http://www.ehow.co.uk/info_8084577_properties-kevlar.html
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http://www.textile-technology.com/
Rupp J, et al. 2001, Textiles for protection against harmful ultraviolet radiation. International
Textiles Bulletin;47 (6): 8 – 20.
Szosland, J. 1999. ‘Air Permeability of Woven Multilayer Composite Textiles’, Fibres &
Textiles in Eastern Europe, 7(24), pp.34-37
Appendix I: Kevlar Grading table for all indices
Wetting Time
Top(sec)
Wetting Time
Bottom(sec)
Top
Absorption
Rate(%/sec)
Bottom
Absorption
Rate(%/sec)
Top Max
Wetted Radius
(mm)
Bottom Max
Wetted Radius
(mm)
Top
Spreading Speed
(mm/sec)
Bottom
Spreading Speed
(mm/sec)
Accumulative
one-way transport
Index (%)
OMMC
Kevlar 1
Kevlar 2
Kevlar 3
Kevlar 4
Kevlar 5
Appendix II: Kevlar/wool grading table
Wetting Time
Top(sec)
Wetting Time
Bottom(sec)
Top
Absorption
Rate(%/sec)
Bottom
Absorption
Rate(%/sec)
Top Max
Wetted Radius
(mm)
Bottom Max
Wetted Radius
(mm)
Top
Spreading Speed
(mm/sec)
Bottom
Spreading Speed
(mm/sec)
Accumulative
one-way transport
Index (%)
OMMC
Kevlar/wool 1
Kevlar/wool 2
Kevlar/wool 3
Kevlar/wool 4
Kevlar/wool 5
0.7461
