Effect of knitting structure and yarn composition on thermal comfort properties of bi-layer knitted fabrics

Abstract

In this work, nine bi-layer knitted samples with varied knitting structures and made up of different yarn compositions were fabricated, and their thermal comfort properties were investigated. The thermal comfort properties were evaluated by breathability, water transfer properties, thermo-physiology properties and dynamic cooling properties, and their relationship with fabric knitting structure and yarn composition were investigated statistically. It was observed that bi-layer knitted fabrics with meshes at one side had better air permeability, moisture management properties, drying performance, thermo-physiological properties and dynamic cooling function, but lower wicking height than bi-layer knitted fabrics with trim and symmetrical structure (without meshes). The composition of nylon and polyester filaments with varied wettability as outer and inners layer of bi-layer knitted fabrics, respectively, improved the water one-way transport capacity significantly. In particular, bi-layer fabrics with asymmetric structure and made up of yarns with varied hydrophilicity as each layer have excellent moisture management capacity. Moreover, fabrics made up of yarns with finer fibers exhibited better thermal comfort properties.

Keywords

knitting structure yarn composition thermal comfort nylon polyester

The thermal comfort property is one of most important features of fabrics for consumers in terms of casual clothing. It is expected that clothing maintains the microenvironment of the human body and clothing comfortable in different atmospheric conditions.¹ In particular, fabrics used to manufacture summer or sports clothing should have excellent thermal and moisture transfer properties to keep the microclimate of the body and clothing cool, dry and comfortable. It is generally known that there are three forms of dry heat exchange, convection, conduction and radiation, between the human body and the environment. In fact, this dry heat exchange is not effective enough to release heat from the human body after sweating excessively, and excessive heat has a negative effect on thermal comfort and work efficiency,² whereas the sweat evaporation increases the heat loss according to the principle of energy conservation, which is crucial for the cooling function of clothing. Moreover, it has been verified that poor thermal or moisture transfer properties of fabrics diminish their overall comfort³ and even threaten human health⁴ due to bacteria breeding in the excessive sweat.

The evaluation of thermal comfort capacity plays a critical role in the development of fabrics, and it is mainly conducted in the three aspects of breathability and heat and moisture transfer properties. Breathability is related to the size and number of pores within the fabrics. With the increase of the number and size of pores, air and moisture permeability increases. In a warm and humid environment, better breathability leads to a pleasant state of psychological and physical harmony of the human being with the clothing–body microclimate.¹ Thermal and moisture transfer properties are crucial for the thermal comfort of fabrics. Massive thermal conduction is conducive to heat produced by the human body transferring to the outside environment through clothing,⁵ which is expected for summer and sports clothing. Conversely, fabrics with limited thermal conduction are often used to produce winter clothing due to good heat insulation. Excellent moisture absorption and transfer capacity of fabric can help to keep the microclimate near skin dry and comfortable after sweating excessively. Meanwhile, a large amount of heat also could be absorbed and diffused during the moisture transfer process.^6,7 The evaluation of thermal and moisture transfer properties has focused on the wicking effect,⁸ moisture management properties,^8–11 drying performance,^8,10 thermo-physiological properties^8,10,11 and cooling performance.¹² The thermo-physiological properties were defined as wear comfort in terms of thermal and moisture effects,¹³ and they are estimated through thermal and water vapor resistance.¹⁴ Generally, the wicking height indicates the liquid moisture absorption and transfer property; moisture management properties explain the liquid water absorption, spreading and one-way transfer properties; the drying performance is a common method to evaluate fast-dry fabrics; thermo-physiological comfort is assessed by thermal resistance and water vapor resistance; and cooling performance shows the overall thermal properties of dry and wet fabrics.

In addition, thermal comfort properties are the function of fabric materials, yarns and knitting structures.¹⁵ To fabricate knitted fabrics with good thermal comfort properties, researchers have developed blended fabrics with different yarns or fibers, such as cotton-rich/polylactic acid fiber,¹⁶ modal/polypropylene, modal/polyester,¹⁷ Viloft/cotton and Viloft/polyester.¹⁸ Moreover, the hydrophobicity or hydrophilicity of the fiber greatly influences the wicking behavior.¹⁹ Meanwhile, fabricating fabrics with a wettability gradient between the inside and outside surfaces was demonstrated to be an effective way to enhance the moisture management properties. This gradient wettability could be achieved by using different materials with varied hydrophilicity as the inner and outer layers of bi-layer fabrics.²⁰ For example, bi-layer knitted fabrics made up of micro-fiber polyester (inner layer) and modal (outer layer) bi-layer knitted fabric have a better moisture management property.²¹ To improve the comfort properties of sports clothing, bi-layer knitted fabrics made up of hydrophobic polypropylene as the inner layer and hydrophilic modal as the outer layer were developed to produce volleyball sportswear²² and bi-layer knitted fabrics made up of Tencel yarn as the outer layer and acrylic/micro-fiber polyester yarn as the inner layer were developed to make tennis sportswear.²³ In addition, these knitted fabrics made up of hydrophobic fibers (polypropylene, polyester) as the inner layer were good candidates for next-to-skin applications.¹⁷ What is more, this type of fabric with a wettability gradient could be fabricated by designing the fabric structure with biomimetic networks, and larger loops were formed at the inner side and smaller loops at the outer side of fabrics.^24–26 It was reported that the yarn count, yarn twist and combing process affected different thermal comfort properties of cotton knitted fabrics.¹ Moreover, the fiber arrangement of yarns influenced the fabric porosity and thermal transfer behavior.²⁷ The loop length and yarn linear density were also important parameters to be considered in the development of knitted fabrics with an excellent thermal transfer property.²⁸ Furthermore, the thickness and porosity of fabric²⁹ played important roles in the thermal comfort properties of fabrics, thermal insulation improved with the increase of fabric thickness and increasing porosity promoted breathability.

Polyester and nylon have been widely used in the fabrication of textiles due to their excellent physical and chemical characters.^30,31 The hydrophobicity (official regain, 0.4%) and poor thermal transfer capacity of common polyester limit its wide application for fabrics used for summer and sports clothing.³² To solve this problem, researchers fabricated irregular cross-section polyester fibers to improve their water transfer properties^33,34 and cool-touch polyester yarns containing nanoparticles with excellent thermal conductivity to enhance their thermal transfer ability.^35,36 Typically, jade nanoparticles^37,38 have been used widely due to their low cost and high thermal conductivity. In contrast, nylon fibers have moisture sensitive performance (official regain, 4.5%) and a better thermal transfer property.³¹ Many researchers have tried to further improve the thermal transfer properties through conducting metal sputtering,³⁹ adding nanoparticles to a nylon matrix^40–42 and modifying the fiber cross-section.^43–45 Considering the wide application of cool-touch polyester and nylon in textiles, developing bi-layer knitted fabrics made up of cool-touch polyester as the inner layer and nylon as the outer layer could be an effective way to improve the thermal comfort of clothing due to their different reactions to moisture. However, the thermal comfort properties of bi-layer knitted fabrics made up of polyester fibers as the inner layer and nylon fibers as the outer layer have not been investigated. Furthermore, the fineness of fibers and knitting structures have been demonstrated to affect the thermal-wet comfort properties of fabrics significantly.^13,46 Thus, this work focuses on the effect of the knitting structure and yarn compositions on the thermal comfort properties of polyester and nylon bi-layer knitted fabrics by investigating the breathability, wicking effect, moisture management properties, drying performance, thermo-physiological comfort characteristics and dynamic cooling function. This work further enriches the research and development of knitted fabrics with good thermal comfort properties.

Materials

Considering the enhancement of the water transport property of irregular fibers and different surface structures formed by changes of the tuck stitch knitting structure, three types of filaments, 75D(denier)/72F(filaments) cool-touch polyester (CPET) with a tri-groove cross-section, 70D/68F cool-touch nylon with a crisscross-section (CPA 68) and 70D/34F cool-touch nylon with a crisscross-section (CPA 34), were selected, and three types of tuck stitch knitting structure with similar inner surfaces but different outer surfaces were designed to investigate the effects of the knitting structure and composition on the thermal-wet properties of bi-layer knitted fabrics. Then nine knitted fabrics were fabricated using a circular knitting machine (XL-XST, Shishi Xinlong Machinery, Co., Ltd, China, 34G/25.4 mm). To ensure that the differences of weight and thickness of the knitted fabrics are mainly triggered by the knitting structures, similar knitting parameters were designed to keep a similar density in the production process of all samples. The fiber appearance of the filament samples is shown in Figure 1, and the fiber cross-section information of cool-touch nylon filaments is shown in Table 1. From Figure 1 and Table 1, it is found that fibers of yarn CPA 34 are obviously coarser than those of CPA 68, but the grooves on the fiber of CPA 34 are larger than those of CPET and CPA 68. The knitting structure appearance and knitting pattern are shown in Figure 2. It shows different surface textures with varied sized meshes. In this work, tuckings 1–3 were denoted as a “plane” structure, “small mesh” structure and “large mesh” structure, respectively. For samples with mesh structure, the mesh surface is made up of cool-touch nylon filaments and the other surface is made up of cool-touch polyester filaments. Knitted fabric specifications are shown in Table 2, which indicates that samples with the same structure have similar weight and thickness.

Figure 1.

Fibers appearance of filament samples. CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Figure 2.

Appearance and knitting structure of knitted samples.

Table 1.

Profile of the cool-touch nylon fiber cross-section

Fiber specification	Groove depth (µm)	Groove width (µm)	Groove area (µm²)	Fiber diameter (µm)
CPA 68	1.89	6.37	6.02	14.91
CPA 34	2.53	8.69	10.99	22.86

CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Table 2.

The specification of nine bi-layer knitted fabric samples

No.	Knitting structure	Composition (weaving number)	Weight (g/m²)	Thickness (mm)
F1	Tucking 1 (plane)	1# CPET (3, 6) / 2# CPA 68 (1, 2, 4, 5)	168.85 (3.25)	0.91 (0.015)
F2	Tucking 1 (plane)	1# CPET (3, 6) / 3# CPA 34 (1, 2, 4, 5)	175.25 (4.47)	0.94 (0.014)
F3	Tucking 1 (plane)	3# CPA 34 (1–6)	170.30 (4.56)	0.95 (0.030)
F4	Tucking 2 (small mesh)	1# CPET (2, 4, 6, 8, 10, 12) /2# CPA 68 (1, 3, 5, 7, 9, 11)	127.45 (4.12)	0.63 (0.008)
F5	Tucking 2 (small mesh)	1# CPET (2, 4, 6, 8, 10, 12) /3# CPA 34(1, 3, 5, 7, 9, 11)	124.15 (5.73)	0.63 (0.022)
F6	Tucking 2 (small mesh)	3# CPA 34 (1–12)	128.95 (4.23)	0.68 (0.014)
F7	Tucking 3 (large mesh)	1# CPET (2, 4, 6, 8, 10, 12) /2# CPA 68 (1, 3, 5, 7, 9, 11)	106.80. (3.18)	0.57 (0.013)
F8	Tucking 3 (large mesh)	1# CPET (2, 4, 6, 8, 10, 12) /3# CPA 34 (1, 3, 5, 7, 9, 11)	108.90 (4.26)	0.59 (0.032)
F9	Tucking 3 (large mesh)	3# CPA 34 (1–12)	106.30 (4.62)	0.60 (0.056)

Note: values in brackets of weight and thickness are standard deviations.

CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Methods

This work was focused on the evaluation of bi-layer knitted fabric thermal comfort properties, including permeability, wicking height, moisture management properties, drying performance, thermal-physiological properties and the dynamic cooling property.

Air permeability plays a key role in the breathability of fabrics. It was measured by an automatic gas permeability tester (YG461E-III, Ningbo Textile Instrument Factory, China). In the testing process, the test area was set as 20 cm² and pressure was set as 100 Pa according to the standard ISO 9237-1995 (R2017).

Wicking describes the liquid transport ability of fabric though its complex fibrous structure.¹⁷ The vertical wicking test method was used to investigate the wale and course wicking height of the fabric samples. It is categorized as in-plane wicking performance. The test area was 2.5 cm × 30 cm in this work. The lower ends of the prepared fabric strips were suspended vertically, and immersed in a reservoir with stilled water. The wicking height every 5 minutes was measured by a scale adjacent to the strips for 30 minutes.

Moisture management decides the comfort level of fabrics, and it is one of the key performance criteria in today's apparel industry.⁴⁷ This property was evaluated using a Moisture Management Tester (M290, SDL Atlas, USA), according to the ASTM D1776-2008 standard. There are two sensor groups in this tester, upper sensors and the lower group. The tested sample (8.0 × 8.0 cm²) is placed between these two groups, and liquid moisture transport behaviors are recorded in real time. The testing solution is synthetic sweat, which is prepared by mixing 1 l distilled water and approximately 9 g sodium chloride (NaCl). All samples should be washed and ironed to remove excessive water and wrinkled before conducting the test. Through the internal procedure of this instrument, the accumulative one-way transport capability (OWT, %) from the fabric inner side to outer side and the overall moisture management capacity (OMMC) can be calculated according to the test indexes. The OMMC describes the overall capability of the fabric, where its value was kept between 0 and 1. Fabric samples had better water management property if this value was closer to 1.⁴⁸ In this work, for samples made up of two types of yarn, the surface made up of polyester filaments is the test surface and the surface without meshes is the test surface for samples with mesh structures.

Drying performance was determined by the drying time of fabric with set water; with the decrease of drying time, the drying performance is better. The drying time was tested by a RF4008HP Drying Rate Tester (heated-plate method, REFOND Equipment Co., China) according to standard AATCC MT 201, and the test area is 15 × 15 cm². The temperature is detected by an infrared temperature sensor to determine whether the test fabric is dry or not based on the principle that moisture evaporation will take away heat and reduce the surface temperature.

The thermal-physiological property of fabric depends on the thermal resistance and water vapor resistance under the steady-state condition.¹⁰ Thermal and water vapor resistances were measured through a Sweat Guarded Hotplate apparatus (YG606, Ningbo Textile Instrument Factory, China), according to standard ASTM D1776/D1776M-16-2008. Thermal resistance (10⁻³ m²ċK/W) indicates the ability of thermal insulation, and it is determined by the heat flow conducted through the given area of fabric at the set difference of temperature between the test board and the environment. With the increase of thermal resistance, the thermal transfer ability decreases. Water vapor resistance [g/(m²ċhċPa)] simulated the moisture transport through fabrics, and it is an indirect method for measuring the water vapor transfer capacity of a fabric.¹⁰ Thermal resistance and water vapor resistance of the fabric were calculated using the following equations

R_{c t} = R_{c} - R_{c 0} = \frac{T_{m} - T_{a}}{H_{c}} - R_{0}

(1)

where

R_{c 0} (1 0^{- 3} m^{2} \cdot K W)

is thermal resistance without the sample;

H_{c}

(W/m²) is the energy supplied to the test board to be kept at 35℃;

T_{m}

and

T_{a}

are the test board and test environment temperature (K), respectively

R_{e t} = R_{e} - R_{e 0} = \frac{P_{m} - P_{a}}{H_{e}} - R_{e 0}

(2)

where

R_{e 0} (m^{2} \cdot Pa W)

is water vapor resistance without the sample;

H_{e}

(W/m²) is the energy supplied to the test board to be kept at 35℃;

P_{m}

and

P_{a}

are the test board and test environment water vapor pressure (Pa), respectively.

The cooling property of fabric describes the thermal and moisture transfer capacity. It was performed by refitted Sweat Guarded Hotplate apparatus (YG606 II, Ningbo Textile Instrument Factory, China) with an added program to control heating power (22.0 W). This test was conducted by measuring the thermal property of the dry sample and the wetted sample with set water content ,respectively, and the temperature of the fabric in the heating process with time was recorded to evaluate the cooling function of the fabrics.⁴⁹ The cooling property of dry fabric indicates the heat transfer capacity of the fabric, whereas the cooling property of wetted fabric demonstrates the combined effect of the coupled heat–moisture property of the wet fabric and cooling function of moisture evaporation and diffusion.⁴⁹ Before conducting this test, samples should be preprocessed. The dry fabrics were well conditioned for 48 hours in a standard environment (temperature 20 ± 1℃ and relative humidity 65 ± 2%). The wet fabric was treated with 20 g liquid water uniformly.

Before conducting tests, all samples are conditioned for 48 hours in a standard environment of temperature 20 ± 1^oC and relative humidity 65 ± 2%. An analysis of variance (ANOVA) was conducted at the 95% confidence interval.

Results and discussion

Air permeability

From Figure 3, there is a highly significant difference in the air permeability between samples with varied knitting structures. Samples with a large mesh structure have the highest air permeability, followed by samples with a small mesh structure, and samples with a plane structure exhibited the lowest air permeability. This is because air permeability is mainly determined by pores in the fabrics, and fabrics with a large mesh structure have more pores, which promoted air permeation. Meanwhile, higher thickness (Table 2) of samples with a plane structure prevented air from permeating through them. A significant difference in air permeability can be found between samples made up of two types of yarns (CPET and CPA (68/34)) and samples made up of CPA 34. Samples made up of CPA 34 have higher air permeability than samples made up of CPET and CPA (68/34). This is caused by the fineness gap of yarns CPET and CPA. The coarser yarn CPET results in tighter fabric when using the same knitting process parameters, and fewer pores are formed in its fabrics. Samples made up of CPET and CPA (68/34) have no obvious gap of air permeability due to their similar composition of yarns, whereas the air permeability of samples made up of CPET and CPA 34 is slightly higher than that of samples made up of CPET and CPA 68.

Figure 3.

Air permeability of knitted samples with different knitting structures (**significant difference at the 99% confidence interval; *significant difference at the 95% confidence interval). CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Wicking height

As shown in Figure 4, samples F1, F2 and F3 with a plane structure have the highest wicking height. It is known that the amount, size and distribution of pores within fabrics are important to determine the wicking performance.⁵⁰ In particular, smaller pores are filled first and influence the liquid front movement.⁵¹ Yarns of knitted fabrics with a plane structure are arranged more tightly than others (see Figure 2). This promotes the formation of smaller pores within the fabric, which enhances the wicking effect. However, there is no obvious difference in the wale and course wicking height between samples (F4, F5 and F6) with a small mesh structure and samples (F7, F8 and F9) with a large mesh structure. This is mainly caused by their similar structure and yarn composition; more specifically, one side of the samples is tight polyester yarn and the other side is arranged loosely by nylon yarns. Moreover, the larger pore size of the loose structure is not an effective capillary for the wicking effect.

Figure 4.

Wale and course wicking height of knitted samples within 30 minutes.

There is an interesting phenomenon that occurred in samples with different yarn composition. For samples with a plane structure, sample (F3) fabricated by yarn CPA 34 had the lowest wicking height, and the sample made up of yarns CPET and CPA 34 had the highest wicking height. However, samples with a mesh structure show a converse change of wicking height. It is well known that polyester is hydrophobic³⁰ and nylon is moisture sensitive.³¹ Generally, pores and moisture absorption have a positive effect on the wicking height. However, the presence of adsorbing groups on the fiber surface has high affinity toward water molecules in capillaries, inhibiting the water transfer along the capillary channels formed by the fibers.⁵² Thus, for samples with a plane structure, F3 made up of nylon has a lower wicking height than that of F1 and F2, made up of a combination of nylon and polyester. For samples with mesh structures, F6 and F9 fabricated by nylon yarns promote the wicking effect of their fabric due to hydrophilic performance. Samples F4 and F7 have a higher wale and course wicking height than samples F5 and F8, respectively, as shown in Figure 4. This illustrates that finer fiber is more conducive to the wicking effect than coarser fiber due to the smaller pores formed within the fabrics.

Moisture management properties

The water content changes of samples F3, F6 and F9 made up of cool-touch nylon CPA 34 are shown in Figure 5. This test was conducted on two sides of samples F3, F6 and F9. From this figure, it is shown that there is no obvious difference between the two sides of sample F3, but an obvious difference was observed between the two sides of samples F6 and F9. This illustrates that the plane structure of bi-layer fabric is nearly symmetric, and the minor gap of structures between the two sides of F3 have an ignorable effect on the water management properties, whereas the asymmetric structures of samples F6 and F9 have an obvious effect on the water transfer properties. Thus, the difference in the water management properties of samples F1, F2 and F3 is mainly triggered by the varied yarn compositions of these samples, and the differences in the water management properties of samples F4–F6 and samples F7–F9 are determined not only by yarn compositions but also knitting structures.

Figure 5.

Water content of the inner and outer sides with time of samples F3, F6 and F9 (both sides of the fabric were tested).

The water one-way transport index shows the capacity for water transport from one side of the fabric to the other. Fabric has a good one-way transport property when the value of its one-way transport index is higher than 200, and an excellent one-way transport property when this index is higher than 400.⁴⁸ From Figure 6, samples made up of CPET and CPA with a mesh structure all have excellent one-way transport capacity, and samples with a plane structure have a good one-way transport capacity. This is caused by the difference of hydrophilicity between nylon and polyester. The one-way transport property of samples made up of CPA 34 yarns is very poor due to their minor wettability gradient triggered by the difference in structures between the inner and outer layers. Furthermore, there is significant difference in the one-way transport index between samples made up of CPET and CPA 68 and samples made up of CPET and CPA 34. Samples made up of CPET and coarser nylon yarn CPA 34 exhibited higher values. This is because the larger gap of groove size on the fiber surface between CPA 34 and CPET causes a better differential capillary effect and water directional transport capacity. Samples with mesh structures have an obviously higher one-way transport index than samples with a plane structure because the mesh structure provided more surface area for water transfer.²⁴ It also indicates that the asymmetric structure of the knitted fabric between the inner and outer sides is conducive to the water one-way transport property.

Figure 6.

One-way transport index of samples. CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

According to the differential capillary effect, using polyester and nylon yarns with a wettability gap as the inner and outer layers of bi-layer knitted fabric is an effective way to improve the OMMC. As shown in Figure 7, samples made up of two types of yarns with different wettability, CPET and CPA, have an obviously better OMMC than samples just made up of a single type of yarn, CPA 34.

Figure 7.

Overall moisture management capacity (OMMC) of samples. CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Figure 8 shows the wetted radius of knitted samples with different knitting structures and yarn compositions. The wetted radius displays the spreading capacity along the fabric surface. It is determined by the moisture absorption and moisture transfer capacity of the fabric. As show in Figure 8, samples with a plane structure have a larger wetted radius than samples with a mesh structure. This is because yarns are arranged closely in samples with a plane structure, and more pores formed by tight yarn assemblies enhance the wicking effect of fabric. Moreover, samples made up of nylon CPA 34 yarns also have a larger wetted radius than samples made up of polyester and nylon yarns. This is mainly because a lower amount of liquid stayed in the fabric to spread, due to the good water one-way transport capacity of samples made up of polyester and nylon yarns.

Figure 8.

The wetted radius of knitted samples in the Moisture Management Tester test. CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

For samples made up of polyester and nylon yarns, samples with a plane structure have a significantly larger wetted radius than samples with a mesh structure, and there is no obvious gap in the wetted radius between samples with a small mesh structure and samples with a large mesh structure. The excellent water one-way transport capacity of these samples promotes water transfer from the inner side to outer side quickly, and a small amount of water that remained in the inner side limited the water wetted radius. Meanwhile, the mesh structure in outer side results in the discontinuous capillary channels, which prevent water from spreading around. Moreover, the inner wetted radius of samples (mesh structure) made up of CPET and CPA 68 is larger than their outer wetted radius. Conversely, the inner wetted radius of samples (mesh structure) made up of CPET and CPA 34 is lower than their outer wetted radius. This is mainly caused by the better one-way transport capacity of samples made up of CPET and CPA 34 than samples made up of CPET and CPA 68, as shown in Figure 6. For samples made up of nylon CPA 34 yarns, there is minor difference in the wetted radius between samples with different knitting structures. Although water could spread under the combined action of the wicking effect and water absorption of nylon due to the moisture sensitive property of nylon in both sides of these fabrics, the discontinuous capillary channels of samples with a mesh structure results in a decrease of the water wetted radius.

From Figures 9 and 10, for bi-layer knitted samples made up of polyester and nylon as the inner and outer sides, respectively, the water spreading speed and water absorption rate of the outer layers are higher than those of the inner layers due to the better hydrophilicity of nylon. Meanwhile, a higher spreading speed and absorption rate are demonstrated by samples made from CPET and CPA 68. This is caused by the finer fiber, which enhances the wicking effect and increases the specific surface area for water interaction. There is no obvious gap in the spreading speed and absorption rate between the inner and outer layers of samples made up of CPA 34 because both layers of these samples are made up of same materials. Samples with a plane structure have a higher spreading speed and absorption rate than samples with mesh structures because of the more closely arranged yarns of the plane structure.

Figure 9.

Spreading speed of samples. CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Figure 10.

Absorption rate of samples. CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Drying performance

The drying time shows the time that is required for a fabric containing a settled amount of water to dry. Fabric has a better drying performance with decreasing drying time. Good drying performance of fabrics enhances the sweat evaporation and keeps the clothing dry and comfortable after sweating excessively. As shown in Figure 11, the knitting structure and yarn composition both have significant effects on the drying performance of knitted samples. This is consistent with previous results.⁵³ The drying time decreases from plane to large mesh structures. This illustrates that fabrics with a large mesh structure have the best drying property, followed by samples with a small mesh structure, and the worst drying performance is demonstrated by samples with a plane structure. This is because the large mesh structure increases the contact area between moisture and air, which benefits moisture evaporation. Meanwhile, the decreasing thickness of these samples with different structures is conducive to their drying performance. Moreover, samples made up of CPA 34 have the highest drying time in every group of samples with the same knitting structure, since it is known that nylon has better hydrophilicity than polyester, which prevents moisture from evaporating. Samples made up of CPET and CPA 34 have a lower drying time than samples made up CPET and CPA 68. This shows that coarser hydrophilic fibers have a more positive effect on drying performance than finer fibers. This is because finer fibers increase the contact area between fibers and moisture, and the moisture sensitive property of nylon inhibits moisture evaporation.

Thermo-physiological property

Thermo-physiological properties are determined by thermal and water vapor resistance. They show the fabric heat transfer capacity and moisture permeable capacity due to the temperature gradient in the vertical direction of the fabric. Table 3 shows the test results of thermal resistance and water vapor resistance of bi-layer knitted samples. Thermo-physiological properties are the function of many factors, such as fabric structure, thickness and composition of yarns. There is a highly significant difference in thermal resistance and water vapor resistance between samples with different structures. This indicates that fabric structures affect the fabric heat dissipation and water vapor permeability. As shown in Figures 12 and 13, samples with a plane structure have the highest thermal resistance and water vapor resistance, followed by samples with a small mesh structure, and samples with a large mesh structure showed the lowest values in every group of yarn composition. This is mainly triggered by the varied thickness (Table 2) and meshes on the surface of these samples with different structures. Lower thickness of fabric and meshes on the fabric surface benefit the thermal transfer and water vapor permeation.

Figure 11.

The drying time of samples with different knitting structures (**significant difference at the 99% confidence interval). CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Figure 12.

Thermal resistance of samples (**significant difference at the 99% confidence interval). CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Figure 13.

Water vapor resistance of samples (**significant difference at the 99% confidence interval). CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

Table 3.

Thermal resistance and water vapor resistance of samples with standard deviations

Structure Yarn composition	Thermal resistance (10⁻³ m² ċ K/W)			Water vapor resistance (m² ċ Pa/W)
Structure Yarn composition	Plane	Small mesh	Large mesh	Plane	Small mesh	Large mesh
CPET/CPA 68	20.64 (0.14)	16.74 (0.05)	14.29 (0.21)	2.71 (0.05)	1.42 (0.04)	1.24 (0.04)
CPET/CPA 34	24.35 (0.16)	19.81 (0.10)	19.09 (0.08)	3.24 (0.08)	1.76 (0.04)	1.33 (0.03)
CPA 34	21.55 (0.15)	18.72 (0.03)	18.36 (0.08)	2.79 (0.08)	2.66 (0.05)	2.04 (0.05)

Note: values in brackets are standard deviations.

CPET: 75D(denier)/72F(filaments) cool-touch polyester; CPA 68: 70D/68F cool-touch nylon with a crisscross-section; CPA 34: 70D/34F cool-touch nylon with a crisscross-section.

In addition, the yarn composition of fabrics has a significant effect on their thermo-physiological properties. The thermal resistance and water vapor resistance of samples made up of CPET and CPA 68 are lower than those of samples made up of CPET and CPA 34. This illustrates that fabric made up of yarns with finer fibers is conducive to enhancing thermal and moisture transport properties. This is because the decreasing fineness increases the contact area between fibers, which promotes heat conduct through the fibers. Meanwhile, with the finer fiber of fabrics, more pores are formed for water vapor permeation. Moreover, samples made up of CPET and CPA 34 exhibited higher thermal resistances in the groups of PET/CPA34 and CPA 34. This is because nylon has a better thermal transfer performance than polyester. However, for samples with a mesh structure, samples made up of CPA have a higher water vapor resistance than samples made up of CPET and CPA. This is mainly caused by the better hydrophilicity of nylon than polyester, which benefits moisture absorption but prevents moisture from permeating and evaporating.

Dynamic cooling performance

The thermal and moisture transfer is a dynamic process in the microclimate of the body–clothing system. The cooling property is the function of heat conduction and moisture diffusion. The dynamic cooling property is better with the decrease of sample temperature in the settled heating time.⁴⁹ Figure 14 shows the temperature variation of dry samples in the settled heating time. As shown in Figure 14(a), the temperature of all samples is higher than that of the test board due to the thermal resistance of fabric. Figure 14(b) shows the difference of temperature between the test board and samples in the settled heating time. It is known that the cooling function is better with the increasing temperature gap. From this figure, samples with a plane structure have the lowest temperature difference due to their high thickness. There is no obvious gap of temperature between samples with mesh structures, whereas samples F4 and F7 made up of CPET and CPA 68 have a higher temperature gap than samples F5 and F8 made up of CPET and CPA 34. Moreover, samples F6 and F9 made up of CPA 34 also have a higher temperature gap than samples F5 and F8. This is consistent with the results of thermo-physiological performance. It also can be explained by finer fibers enhancing the thermal transfer capacity, and hence better thermal conductivity of nylon.

Figure 14.

The heating properties of dry knitted samples.

Figure 15 shows the heating properties of the wetted samples. From Figure 15(a), it is found that the temperature of all wetted samples is considerably lower than that of the test board with heating time, which is the opposite with dry knitted samples. This indicates that water contained in the fabric benefits the cooling function, because the cooling effect is determined by the combined thermal conductivity of moisture and fabric, and moisture evaporation. In other words, sweating is not only a method of heat dissipation for the human body, but also an advantage for cooling the microclimate of the clothing–body system. It is also shown in the two stages in the heating process, which is similar to previous results.⁴⁹ The temperature increases slowly in the first stage, and increases fleetingly in the second stage. At the beginning, the moisture evaporation absorbs much heat, and much water is filled in pores, which increases the thermal conductivity of fabric and further improves the heat loss of the fabric. However, the evaporation rate and heat conductivity decrease with the decrease of water content, resulting in a rapid increase of temperature. Figure 15(b) indicates the temperature gap between the test board and samples with the heating time. It can be seen that samples with a large mesh structure have the highest temperature gap with the heating time, and the lowest one is shown by the sample with a plane structure. This is because samples with a mesh structure have better moisture management, drying performance and thermo-physiological properties than samples with a plane structure. Similarly, samples F7, F4 and F1 made up of finer fibers have a higher temperature gap than samples F8, F5 and F2, respectively, due to finer fibers having a positive effect on the improvement of thermal and moisture transfer properties.

Figure 15.

The heating properties of wetted knitted samples.

Correlation analysis of thermal comfort properties

To analyze the correlation between thermal comfort properties, a Pearson's correlation analysis was conducted. The cooling property was subject to quantitative evaluation by calculating the ratio of the maximum temperature gap (Figure 15(b)) to the relevant heating time. As shown in Table 4, there are significant correlations between some thermal comfort parameters. The yarn composition is significantly correlated with the water one-way transport property, and the knitting structure is significantly correlated with most of the thermal comfort properties. This is consistent with the purpose of the experimental design. That is to say, the experimental design in this work is reasonable and scientific. There is a negative correlation between air permeability and drying time. The drying time decreases with the increase of air permeability. This is mainly because more pores of fabrics with higher air permeability benefits moisture evaporation. Meanwhile, the correlations between the wicking height and water vapor resistance and the drying time are positive. This is caused by better moisture absorption and worse water vapor permeation of fabric higher wicking height and water vapor resistance. Water vapor resistance and thermal resistance show a highly positive correlation due to their similar test conditions. It also can be found that there are significantly negative correlations between the cooling property and the thermal and water vapor resistance. This indicates that good thermal and moisture transport properties promote the cooling property of the fabric. As a result, the cooling property could be defined as an index for the evaluation of knitted fabrics used for summer and sports clothing. More specifically, with the increase of the index (cooling property), the thermal comfort of fabric increases.

Table 4.

Results of Pearson's correlation analysis between thermal comfort properties

Properties	Yarn composition	Knitting structure	Air permeability	Wicking height	One-way transport	Drying time	Thermal resistance	Water vapor resistance	Cooling property
Yarn composition	1.000	0.000	0.302	0.187	−0.735*	0.291	0.351	0.414	−0.533
Knitting structure	0.000	1.000	0.939**	−0.765*	0.274	−0932**	−0.746*	−0.807**	0.555
Air permeability	0.302	0.939**	1.000	−0.628	−0.049	−0.776*	−0.652	−0.605	0.435
Wicking height	0.187	−0.765*	−0.628	1.000	−0.608	0.840**	0.709*	0.895**	−0.548
One-way transport	−0.735*	0.274	−0.049	−0.608	1.000	−0.580	−0.278	−0.643	0.307
Drying time	0.291	−0.932**	−0.776*	0.840**	−0.580	1.000	0.663	0.864**	−0.531
Thermal resistance	0.351	−0.746*	−0.652	0.709*	−0.278	0.663	1.000	0.822**	−0.909**
Water vapor resistance	0.414	−0.807**	−0.605	0.895**	−0.643	0.864**	0.822**	1.000	−0.674*
Cooling	−0.533	0.555	0.435	−0.548	0.307	−0.531	−0.909**	−0.674*	1.000

**Significant difference at the 99% confidence interval; *significant difference at the 95% confidence interval.

Conclusion

In order to investigate the effect of knitting structure and yarn composition of both sides of bi-layer knitted fabrics on thermal-wet comfort properties, three knitting structures with different appearance of the two layers and three types of yarn composition with varied hydrophilicity and fabricated nine bi-layer knitted samples were prepared and investigated via breathability, water transfer properties, thermo-physiology properties and dynamic cooling properties. Results showed that knitting structures and yarn compositions affected the thermal comfort properties obviously. An asymmetric structure with meshes of bi-layer knitted fabrics benefit improvement of their thermal comfort properties. Meshes at one side enhanced air permeability, moisture management properties, drying performance, thermo-physiological properties and the dynamic cooling function of bi-layer knitted fabrics. However, large meshes had a negative effect on the wicking height. Using hydrophilic nylon filaments as the outer layer and hydrophobic polyester filaments as the inner layer is an effective way to fabricate bi-layer knitted fabrics with good water one-way transport capacity. In particular, bi-layer fabrics with an asymmetric structure and made up of yarns with varied hydrophilicity as each layer have an excellent moisture management capacity. Moreover, finer fibers of yarns were beneficial for the development of knitted fabrics with good thermal comfort properties. Pearson's correlation analysis demonstrated that the ratio of maximum temperature gap to the relevant heating time could be used as an index for the quantitative evaluation of knitted fabric used for summer and sports clothing. More specifically, the thermal comfort increases with the increase of this index. This study contributes to improve the knowledge of developing knitted fabrics made up of polyester and nylon yarns, as well as other types of yarns, for summer and sports clothing.

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: This work was supported by the National Key R&D Program of China (Grant No. 2017YFB0309100), the Fundamental Research Funds for the Central Universities (Grant No. CUSF-DH-D-2018024), the College of Textiles, Donghua University, Shanghai, China, and the China Scholarship Council (Grant No. 201806630035).

ORCID iD

Peihua Zhang

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