Analysis of thermal comfort properties of tri-layer knitted fabrics

ABSTRACT The tri-layer knitted fabrics created for the aim of active sportswear have been enhanced with the help of microdenier filament polyester yarn, spun polyester yarn, polypropylene, and cotton in this study. These developed tri-layer knitted fabrics are then examined for the thermal comfort properties. The results evidently showed that Microdenier Polyester/Microdenier Polyester/Cotton tri-layer knitted fabrics combination shows exceptionally appreciable thermal comfort properties due to their structural factors such as filamentous nature, lesser thickness, low areal density, and lesser bulkiness. The effect of fiber chosen also plays a crucial part with respect to the thermal comfort properties of tri-layer fabrics developed. Samples such as Microdenier Polyester/Polypropylene/Cotton, Polypropylene/Microdenier Polyester/Cotton also performed better next to that of the Microdenier Polyester/Microdenier Polyester/Cotton combination because polypropylene also possesses a good wicking characteristic. A poor thermal behavior was found in the Microdenier Polyester/Cotton/Polypropylene sample because of the reasons such as protruding fibers of cotton, increased thickness, high areal density, etc. Also on comparing between the filament and the spun yarn, the filament yarn is highly recommended due to its appreciable behavior. Results evidently show that Microdenier Polyester/Microdenier Polyester/Cotton combination possesses an exceptionally appreciable thermal comfort property.


Introduction
The human body temperature is in general maintained by the process of thermo regulation. A normal human body temperature is 37°C. Whenever a human performs heavy physical activity such as active sports a higher amount of sweat has been released from the human body. The human body begins to heat up during these moments, and the body goes through a self-thermoregulation process to bring the make various trilayer composite fabric combinations' thermal and permeability properties better. The test findings demonstrated that the loop length and yarn linear density of the wool knitted materials had a significant impact on the thermo-physiological comfort of the trilayer composite fabric. The bulk density of the polyester fabric also had an effect on the thermo-physiological comfort of the tri-layer composite fabric. Tesinova and Atalie (2022) investigated the effects of layer maintenance and structure parameters on the thermal properties of sports fabrics. Clothing made from a blend of 100% Elastane and 100% PES had a lower thermal conductivity than athletic wear made of polyamide.
A tri-layer structure made from the Bamboo, lycoell and micro-fiber polyester combinations were constructed and analyzed for their comfort properties (Suganthi and Senthilkumar 2018). Analyzing the moisture management characteristic of a double-layered fabric made of two layers with cotton on the side that touches the body and polypropylene on the side that is exposed to the environment revealed that the fabric has excellent Overall Moisture Management Capacity properties due to the quicker and faster occurrence of moisture transmission and evaporation (Karthikeyan et al. 2017). Compared to materials made entirely of wool or bamboo, viscose fabrics made of wool/polyester and wool/bamboo offer significantly better thermal qualities. The total moisture-management effectiveness of cotton/bamboo knitted fabrics declines as the percentage of bamboo exceeds 50% (Guruprasad et al. 2015;Manshahia and Das 2014). Micro polyester knitted fabric structures exhibit good moisture management comfort qualities like absorption, wicking, and drying rate (Prakash, Ramakrishnan, and Koushik 2013;Srinivasan et al. 2007). All thermal comfort properties, such as wicking, drying rate, air, and water vapor permeability, were shown to be high in multi-layered knitted materials made of micropolyester yarn with a few tuck points (Divya et al. 2022). The thermal characteristics of sports fabrics were studied by Atalie et al. (2021), along with how structure aspects impacted them and how different layers maintained them. Their findings show that the geometry and properties of the material have an effect on the water vapor permeability of bi-layered sportswear. According to Kandhavadivu, Rathinamoorthy, and Surjit (2015) bamboo charcoal, lyocell, bamboo, and micropolyester yarns have reportedly been used to enhance the functional fabrics' capacity to regulate moisture and heat. The findings show that, tri-layer weft knitted materials have exceptionally good functional characteristics including air permeability, water vapor permeability, transverse wicking, and drying rate when compared to tri-layer woven fabrics.
This study is to examine how the tri-layer knitted structure made of microdenier polyester, polypropylene, and cotton affects its ability to provide thermal comfort. The developed tri-layer knit materials exhibit high thermal conductivity and wickability, increasing the user-required comfort of the clothing. Strong functional characteristics of these materials include moisture absorption, for instance, where the moisture is absorbed via the base layer and transferred through the different other layers, enhancing the fabric's comfort value.

Materials
Eight tri-layer knitted fabrics were developed in the yarn combinations (inner, outer, and middle layers) listed in Table 1. For tri-layer knitted fabrics, four different types of yarn were chosen, including polyester staple fiber yarn (PSF) − 2/72 s double yarn, cut staple length of 38 mm, 0.8 denier, polypropylene (PP): 150 denier-108 filaments, microdenier polyester filament (MDP)-150 denier, 108 filaments and cotton (CC) (36s): Unevenness percent − 9.33, Thick/km (+50%) − 12, Thin/km (−50%) − 2, Hairiness longer than 3 mm/km − 23, Neps/km − 18. Polyester yarn wicks moisture away from the body well. Excellent moisture and thermal transport qualities are found in polypropylene and cotton, respectively. A circular knitting machine, Arizio tri layer, model 2000, diameter 34 inch, 24 gauge, loop length 0.30 mm, and 15 rev/min was used to create eight tri-faced garments. The samples were kept in a typical environment for 24 h to allow for conditioning and relaxation during the knitting process, which was accomplished using continuous machine settings (Figures 1 and 2).

Methods
According to the process outlined by the "Starfish" recommendations, knitted samples were given the following relaxing treatments. Cut samples were kept in the dry relaxation for 48 h on a level surface in a tension-free state conditioning cabinet. In the cabinet, a standard environment of 21°C and 65% RH humidity was maintained. Following this wet relaxation, knitted samples were placed in a stainless steel water bath with 0.05 g/l standard wetting agent and water temperature maintained at approximately 38°C. Samples were then left to dry for 24 h with little agitation. Following a one-minute hydroextraction, samples were dried for 48 h on a level surface. Samples were placed to the usual environment of 21°C, 65% relative humidity and no pressure for 48 h. Following the full relaxation stage, wet relaxed samples were properly cleaned, quickly extracted with water for 1 min, then tumble dried for 60 min at a temperature of 70°C. Following that, samples were placed on a flat surface in a conditioning cabinet set to 21°C, 65% RH, and no tension for 48 h. Every relaxing technique was applied in accordance with ASTM D 1284-76.
The tri-layer knitted materials' loop length, stitch density, thickness, and areal density (mass per unit area) were all measured. Areal density was measured using the standard ASTM D3776-09. Wales and course lengths per unit were assessed using the standard ASTM D3887-96(2008). Courses

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per centimeter and wales per centimeter of the fabric samples were measured at five places of each sample using counting glass. Five specimens were tested for each sample 100 wales in a single course were counted to determine the stitch length. The stitch length is the result of dividing the measured cut length by 100. Using a Shirley thickness gauge, the textiles' thickness was measured in accordance with ASTM D1777-96(2019). The overall volume of a fabric and the amount of fiber in the sample were calculated in order to assess porosity. When expressed as a percentage of the entire volume, the difference between these two values is regarded as air space and provides the porosity. The following equation was used to calculate the porosity: Where P is the porosity in %, A is the sample's area in cm 2 , W is its weight in g, T is its thickness in cm, and D is the fiber density in g/cm 3 .
According to British Standard BS 5636 1990, the air permeability of the tri-layer knitted fabrics was evaluated using the KES-F8 AP1 Air Permeability Tester. The thermal conductivity of the three-layer knitted construction was measured in accordance with ASTM D7340-07(2018)e1 using a Lee's disc device. The formula used to get the thermal resistance is R = h/(m 2 K/W), where h is the fabric's thickness in meters and m is its thermal conductivity in Watts per meter Kelvin. According to BS 7209:1990, the tri-layer knitted fabrics were tested for water vapor permeability using an evaporative dish. Each of the knitted textiles had ten readings done, and the averages were then computed. The SAS System was used to examine the quantitative results of the objective measures (version 8 for Windows). To determine the importance of variations in yarn mix and thermal comfort characteristics, a two-way ANOVA was performed. If the P value was 0.05 or lower, any changes were considered significant.

Results and discussions
Eight developed tri-layer samples' physical characteristics were examined, and they are listed in Table 2. After knitting, the developed tri-layer sample textiles were allowed to relax under specific standard climatic conditions of 65% relative humidity and 27°C. The geometrical characteristics of the generated tri-layer fabrics with regard to the CPI and WPI were obviously impacted significantly when the yarn type changed. According to the yarn parameters, the areal density of the fabrics is also altered.

Air permeability
Any fabric structure's comfort properties are mostly determined by the air circulation between the inter-yarn voids. Because of the channels and voids that the fabric is made of, the yarns chosen and the way it was built have a significant impact on how permeable the fabric is to air. The fabric's air permeability is generally influenced by a variety of elements. These include the type of yarn used, the fabric's design, its GSM, its porosity, its areal density, etc.
The MDP/MDP/CC tri-layer air permeability of the knitted structure is excellent among all the other tri-layer knitted samples followed by the MDP/PPR/CC, PPR/MDP/CC MDP/CC/PPR, and the various other knitted structures made out of the spun polyester yarn. The thickness and mass per unit area of this MDP/MDP/CC structure are the key reasons for its outstanding air permeability. The thickness and mass per unit area of the MDP/MDP/CC structure are found to be minimal when compared to all other knitted constructions. As the thickness and mass per unit area is less the air gap thickness is low which results in less amount of air getting entrapped between the layers resulting in an excellent air permeability nature. The presence of the microdenier filament polyester in the skin touching and middle layer results in an excellent wicking characteristic and moisture transfer making it very suitable with good air permeability. The polypropylene yarn present in the middle layer with the microdenier polyester yarn as the skin touching layer shows a good air permeability at a second level next to the MDP/MDP/CC tri-layer knitted structure. From the results, it was evidently proved that either the microdenier filament polyester yarn or the polypropylene yarn in the middle layer shows some amount of moisture transmission compared to that of the cotton yarn present as the middle layer. The main reason for the cotton yarn to possess less air permeability nature is the presence of the protruding fibers in the surface of the yarn. The water and moisture molecules that have to be transferred are highly hindered by the protruding fibers in the surface of the cotton, which in turn results in the poor air permeability. Also, the thickness of the MDP/CC/PPR sample is comparatively higher than the other entire tri-layer structure sample. This may be also one of the reasons for the air permeability to get decreased. The yarn spaces and the structure of the fabric mainly influence the air flow through the inter yarn pores. When compared to cotton yarn, microdenier polyester yarn has a more porous nature and a less torturous continuous path, making it more ideal for the thermal comfort nature. This is due to the microdenier polyester structure's lack of hairiness. Figure 3 shows that the air permeability value of MDP/MDP/CC tri-layer knitted fabrics is appreciable than the other fabrics due to more porous nature when compared to all the other trilayer knitted fabrics. Knitted fabrics made of MDP/MDP/CC have high air permeability and offer enough ventilation to maintain a suitable microclimate for the wearer. As per the principle of air flow, the thickness and air flow is inversely proportional. If the thickness of the fabric is more, the air flow will be less and vice-versa. The less bulkiness of the MDP/MDP/CC tri-layer knitted structure is one more important factor to improve air permeability. Also, the results evidently show that the filament yarns only exhibit good thermal comfort properties compared to that of the spun yarns. This is mainly because of the uninterrupted path exhibited by the filament yarns and a more uniform and a smooth structure. Another important factor to increase air permeability is to make the MDP/MDP/CC trilayer knitted structure less bulky. This is because as yarn gets thinner and the pores between loops get larger, the air permeability will definitely increase. It is undeniably established that an increase in yarn thinness affects porosity by expanding pore space. Similar results were represented by Benltoufa et al. (2007). According to most studies, a fabric's air permeability depends on its air porosity, which affects how open it is (Yoon and Buckley 1984).

Thermal conductivity
A material's capacity to move heat is referred to as thermal conductivity. Sweat is mainly created on the surface of the human skin due to the heat dissipation. This, in turn results in the accumulation of the moisture on the human body. The performance of the fabric's thermal conduction is significantly influenced by the fabric's thickness, material choice, and structural design, among other factors. Figure 4 clearly shows that MDP/MDP/CC fabric combination exhibits higher thermal conductivity in comparison with all the other tri-layer knitted structure because it is more porous than the other developed fabrics. The most important elements affecting a fabric's capacity to transfer heat are its warm nature, stiffness, and weight, all of which are largely determined by its thickness (Benltoufa et al. 2007). The thermal conductivity increases with the stiffness of the fabric. This is because of when the fabric is stiff, it will feels more flat and allows more space to conduct and permit the heat to the other These combinations also indicate that, in addition to fabric thickness, the fabric's structure and the type of yarn used also have an impact on the fabric's ability to conduct heat. The wearer is more comfortable since MDP/MDP/CC knitted materials have high air permeability and adequately ventilate the microclimate. The microdenier polyester filament yarn's channeled structure creates a compatible conduit for heat movement across the fabric's numerous layers. The inherent properties of the cotton fibers result in a poor thermal conduction resulting in a high thermal resistivity due to its bulkiness and thickness. So MDP/CC/PPR tri-layer structure shows a poor  thermal conductivity. A poor thermal conductivity results from a diminished heat transmission characteristic when more air becomes trapped inside the fabric structure. The lower heat conductivity of the trilayer fabrics is probably due to the increased porosity value of the trilayer fabrics made from the finer yarns.

Thermal resistance
The thickness of the fabric affects how much heat is transferred through any fabric surface. The thermal properties are significantly influenced by areal density, fabric structure, and thickness. Figure 5 shows the thermal resistance of the developed tri-layer fabrics. The thermal insulation property of a cloth is mostly determined by the air volume fraction. The type of the yarn chosen, the spinning system and count of the yarn also influence the thermal resistivity of the fabric. Figure 5 it is shows that MDP/CC/PPR has a high thermal resistance due to the protruding hairs in the cotton yarn. The moisture absorbed by the microdenier filament polyester yarn is easily wicked to the middle layer, but the middle layer, which is a cotton yarn, interrupts the moisture and heat transmission to a great level resulting in a higher thermal resistance. This is because of the moisture occupied on the surfaces of the fabric will reduce the insulation behavior. Further, this moisture on the surface will lead to conduction hence increases the thermal conductivity and decreases the thermal resistance (Zhu 2020). More air is being trapped inside the layers of the fabric due to its increased thickness resulting in poor conduction and high thermal insulation. Cotton fiber's specific heat is relatively high when compared to the other fibers. A very high energy is required for any temperature to get raise and the heat to get transferred to the other layers of the fabric. In direct proportion to the increase in fabric-specific heat, the thermal resistance rises. The fabrics increased thickness, air gaps, and the volume of trapped air all enhance heat resistance (Atalie et al. 2021).

Water vapor permeability
The transport of perspiration by any textile material and its capability is termed as the water vapor permeability. The high level of heat generated in a human body is balanced only if the perspiration takes place well. For the perspiration to occur well, the water vapor permeability nature should be good. If the fabric possesses poor water vapor permeability, then the perspiration process will not occur effectively which, in turn, will result in the sweat accumulation to the wearers' garment making the wearer feel clinged to moisture resulting in a higher level of discomfort. The developed tri-layer knitted fabrics water vapor permeability was examined and the findings are displayed in Figure 6. Figure 6 shows that tri-layer knitted MDP/MDP/CC fabrics have higher water vapor permeability than all other materials. The MDP/MDP/CC fabric is far thinner than all previous developed tri-layer fabrics. This is an important characteristic, as all other fabrics have low water vapor permeability due to the increase in thickness. This is because of the water vapor moves rapidly in the thinner fabrics (Ndlovu et al. 2015). The fabric's water vapor permeability quality is significantly influenced by both the geometry and material composition of the fabric. When compared to all the other samples, MDP/MDP/CC shows more water transport because of its less thickness and areal density resulting in the less air entrap between the layers. Thickness is a main parameter in determining the water vapor permeability because it has to take care of the distance of the liquid moisture transfer through the various layers of the fabric. The channel structure of the microdenier polyester yarn makes it more suitable to quickly transfer the moisture through its various layers. This channel structure of the microdenier polyester yarn shows an excellent pulling nature of the moisture through all the layers of the fabric. Also, in comparison to that of the spun yarn, the filament yarn possesses good water vapor permeability nature because of the more uninterrupted, filament structure. Due to its greater thickness and higher areal density, the MDP/CC/PPR sample has the lowest water vapor permeability. The fabric water vapor permeability reduces with increasing fabric thickness.

Data analysis: variance statistics
The SAS System (version 8 for Windows) was used to analyze the experimental data for the tri-layer knitted fabrics, and analysis of variance (ANOVA) was used to determine the significance of the yarn combination and thermal comfort qualities at a 95% confidence level. To determine if the parameters are significant or not, p values were examined. Table 3 presents the Two-way ANOVA's findings.
According to the table's statistical analysis, the yarn combination and loop length are significantly influenced by thermal comfort with p > .05, respectively. These results unequivocally demonstrate the validity of our experimental strategy.

Conclusion
The tri-layer knitted fabrics of the MDP/MDP/CC combination exhibit exceptionally notable thermal comfort properties because of their structural characteristics, such as their filamentous nature, less thickness, low areal density, and less bulkiness, which were developed and analyzed for the eight trilayer knitted fabrics' thermal comfort characteristics. The development of tri-layer fabrics' thermal comfort characteristics is greatly influenced by the choice of fiber. Samples such as MDP/PPR/CC, PPR/MDP/CC also performed better next to that of the MDP/MDP/CC combination because polypropylene also possesses a good wicking characteristic. A poor thermal conductivity behavior was found in the PSF/CC/PPR sample because of the reasons such as protruding fibers of cotton, increased thickness, high areal density, etc. The protruding fibers in the cotton yarns interrupt the free flow of air and moisture through the surface of the fabric. Also, more void spaces entrap a high level of air resulting in the thickness of the fabric which, in turn, raises the real density of the fabric and reduces the thermal behaviors of the fabric. Also, comparing to the spun yarn, the filament yarn is highly recommended due to its appreciable behavior. Results evidently show that MDP/MDP/CC combination possesses an exceptionally appreciable thermal comfort property. According to the findings, microdenier filament polyester yarn for the skincontacting layer, cotton for the outermost fabrics exposed to the environment, and microdenier filament polyester yarn for the middle layer of the fabric all exhibit excellent thermal comfort properties with high levels of comfort, making them ideal for summer and active sports. The fabric exhibits exceptionally strong functional qualities, such as moisture absorption, wherein the moisture is absorbed through the bodycontacting layer of microdenier polyester and it excellently penetrated the water through the middle layer of microdenier polyester, and it evaporates the absorbed moisture to the environment well through the outer layer of cotton, making the sample more suitable for sporting activities.

Highlights
• In this research, cotton, microdenier filament polyesteryarn, spun polyester yarn, and polypropylene have been chosen to enhance thethermal comfort properties of the tri-layer knitted fabrics developed for thepurpose of active sportswear. • The results evidently showed that the tri-layer knitted fabrics of MDP/MDP/CC combination show exceptionally appreciable thermal comfort properties due to their structural factors such as filamentous nature, lesser thickness, low areal density, and lesser bulkiness. • The effect of fiber chosen also plays a major role with respect to the thermal comfort properties of tri-layer fabrics developed.

Disclosure statement
No potential conflict of interest was reported by the authors.