Investigating the effect of some fabric parameters on the thermal comfort properties of flat knitted acrylic fabrics for winter wear

Abstract

In this paper, the thermal comfort properties of flat knitted acrylic fabrics differing in terms of knit structure, tightness, thickness and porosity were investigated within the perspective of its usage in winter wear products. Measured and calculated using the data from Permetest and Alambeta devices, the thermal comfort properties were handled in three aspects, namely thermoregulation characteristics, breathability and thermo-physiological characteristics, and their relationship with fabric structural parameters were investigated statistically. The results indicated that rib 2 × 2 structures provide the optimum condition in terms of thermoregulation, breathability and thermo-physiological comfort, whose thickness and porosity values should be adjusted accordingly, since the thickness improves thermal insulation and porosity improves breathability.

Keywords

thermal comfort breathability thermal resistance thermal conductivity thermal absorptivity water vapor permeability acrylic

Comfort is one of the most important features of textiles for consumers in terms of casual wear product. Due to its simple manufacturing technique, low production cost and several advantages they provide for consumers, such as high elasticity, freedom of movement, good handle, ease of care, etc.,¹ flat knitted fabrics are much more preferred in the production of casual wear products. The usage of yarns made of acrylic fiber and its blends are also very common in the market, especially for winter wear² because of their wool-like touch, low density and easy care.

Clothing comfort is related to the thermoregulation and moisture transport characteristics of the fabrics, which explain the transfer of heat and mass between the clothed body and the environment. Thermoregulation characteristics are measured basically with thermal resistance, thermal conductivity and thermal absorptivity. Being directly proportional to thickness and inversely proportional to thermal conductivity, thermal resistance is relevant to thermal insulation.³ As a new concept, thermal absorptivity is about the feeling of warm or cold. Although it is not common to sweat because of weather conditions in winter time, air permeability and water vapor permeability of the fabrics also have an impact on the thermoregulation characteristics of the fabrics. Known as breathability,⁴ which is the ability of clothing to allow the transmission of moisture vapor by diffusion and to facilitate evaporative cooling,⁵ water vapor permeability depends on air permeability.⁶ On the other hand, from the perspective of thermo-physiological comfort, relative water vapor permeability determines the transportation of moisture through the fabric, since water is a good conductor of heat and thermal resistance is influenced by the amount of moisture that is present in the fabric.⁷ Being in indirect relation with the relative water vapor permeability, the lower values of water vapor resistance are desirable for better moisture transport⁸ and better breathability. Together with thermal resistance, water vapor resistance can be used for calculating the moisture permeability index, which is an overall indication of thermo-physiological comfort.⁴ According to Woodcock,⁹ the moisture permeability index emphasizes the fact that clothing should maintain a level of thermal equilibrium.

There are many studies within the literature that aimed to investigate and analyze thermal comfort properties that basically covered the parameters related to heat and mass transfer, including different combinations of thermoregulation, breathability and even moisture transmission parameters. Whereas some studies considered the final uses of the products, most of them tried to explain the relationship between those parameters and the fiber, yarn and fabric structural characteristics on a general basis.

Different type of fibers and yarns were selected by researchers to examine the thermal comfort properties. Oglakcioglu and Marmarali¹ studied polyester and cotton fibers. Oglakcioglu et al.¹⁰ investigated the characteristics of single jersey knitted structures that were produced using channeled and hollow polyester fibers. Ozcelik et al.⁸ compared the thermo-physiological properties of interlock knitted fabrics produced with air-jet textured, false-twist textured and non-textured filament polyethylene terephthalate (PET) yarns. Studying again the micro-denier polyester filament, filament polyester and spun polyester, Sampath et al.³ also investigated knitted fabrics that are blends of polyester and cotton and 100% cotton. Gericke and Pol⁴ studied about the selected comfort properties of cotton, regenerated bamboo, and viscose rayon. Gun et al.³ investigated the thermal comfort properties of plain knitted fabrics made from modal viscose yarns having microfibers and conventional fibers. Lizák et al.¹¹ concentrated on the thermal transport characteristics of polypropylene fiber-based knitted fabrics. Having investigated many fibers, Amber et al.¹² studied the relative effects of fine wool, mid-micron wool and acrylic. Thus, it was observed that the studies mostly concentrated on the most common fibers, such as polyester or cotton.
Figure 1.
Sketches and photos for the fabric specimens.

Other researchers concentrated on the influence of fabric structures on thermal properties, which were limited in either different fabric structures or the aspects of thermal properties. Ozdil et al.¹³ evaluated the effect of yarn count, twist coefficient, combing process and the tightness on the thermal properties of 1 × 1 cotton rib fabrics. The effect of single jersey, 1 × 1 rib and interlock structures on thermal properties was investigated by Oglakcioglu and Marmarali.¹ Amber et al.¹² investigated the thermal and moisture transfer properties of single jersey, half terry and terry sock fabrics that differed in fiber type and yarn structure. Ucar and Yilmaz¹⁴ analyzed the natural and forced convective heat transfer characteristics of 1 × 1, 2 × 2 and 3 × 3 rib knitted fabrics produced from acrylic yarns.

Nonetheless, the number of the studies regarding the thermal comfort characteristics of flat knitted fabrics made up of acrylic fibers, which are especially used for winter clothing, is still low and thus further investigation is needed. The present studies about flat knitted acrylic fabrics do not cover all the parameters related with thermoregulation, breathability and thermo-physiological comfort. Moreover, the studies within the literature usually try to explain the influence of the fabric structural characteristics independently. Considering the points above, this study aimed to extend the studies about the flat knitted fabrics for winter wear made of acrylic yarn by investigating the thermoregulation, breathability and thermo-physiological comfort characteristics of different type of fabrics and their relation with the fabric structural properties, such as knit structure, tightness, thickness and porosity, with a further aim of analyzing the combined effect of these parameters.

Materials and method

Single jersey, 1 × 1 interlock, 1 × 1 rib and 2 × 2 rib structures were knitted using 100% acrylic yarn in the count of 28/2 Nm in two different tightness levels, that is, slack and tight. The structures were knitted by using a 7 fine, 672 needle Shima Seiki SES 124S V bed flat knitting machine. The properties of the knitted fabric structures are given in Table 1 and the sketches and photos of the specimen were given in Figure 1.
Table 1.
Properties of knitted fabric structures

Sample code Courses per cm Wales per cm Loop density (loops/cm²) Weight (g/m²) Thickness (mm) Loop length l (cm) Air permeability (mm/s)

SJ-Slack 5.00 4.30 21.50 294.4 1.58 0.87 753.33

SJ-Tight 6.00 4.30 28.50 349.1 1.64 0.81 735.33

Rib 1 × 1-Slack 8.00 10.70 82.40 326.6 1.67 0.72 810.67

Rib 1 × 1-Tight 10.70 10.30 112.89 383.8 1.71 0.46 509.33

Rib 2 × 2-Slack 7.30 11.70 78.11 422.2 2.03 0.62 841.00

Rib 2 × 2-Tight 9.00 12.70 105.30 453.5 2.12 0.53 644.33

Int 1 × 1-Slack 8.00 11.70 47.20 431.4 2.25 0.68 431.33

Int 1 × 1-Tight 9.70 12.00 57.72 514.1 2.25 0.60 266.33

The number of courses and wales per cm, loop length, weight and thickness of the fabric samples was measured according to the relevant standards.^15–18 The air permeability of the samples was measured by using Prowhite test apparatus according to standard EN ISO 9237:1995¹⁹ by applying 100 Pa constant air pressure to each sample attached to a 20 cm² circular holder. Within this study air permeability was investigated in two aspects. It was taken as an individual parameter in the discussion of breathability and thermo-physiological characteristics, whereas it was considered as an indicator for the porosity of the fabric in the discussion of thermoregulation characteristics. Actually, it is known that showing the openness of the fabric, the porosity of the fabric is defined as the ratio of the void area to total area of the fabric. However, air permeability depends heavily on the porosity of the fabric^20–22 and, therefore, it can be considered as a proxy for porosity.

Invented by Hes et al.,^23,24 the Alambeta instrument was used to measure the thermal conductivity, thermal resistance and thermal absorptivity of the samples.²⁵ Thermal conductivity λ (W/mK) is evaluated as the quantity of heat that passes in unit time through the unit area of a slab of infinite extent and unit thickness when a unit difference of temperature exists between its faces. Thermal resistance R (m²K/W) is evaluated as the ratio of the temperature difference between two faces of the fabric to the rate of flow of heat per unit area normal to faces. R = h/λ; where R is thermal resistance, h is thickness and λ is thermal conductivity.^1,26 Thermal absorptivity b is evaluated as the heat flow that passes between the human skin and the contacting textile fabric. Thermal absorptivity b is given by the equation below, where ρ is the density and c is the specific heat capacity of the sample
$b (Ws 1 / 2 / m 2 K) = (λ ρ \cdot c) 1 / 2$
(1)

Invented by Hes and Araugo,²⁷ the Permetest instrument was used to measure thermal resistance R_ct (m²K/W), water vapor resistance R_t (Pa m²/W) and relative water vapor permeability p_wv of the fabric samples.²⁸ Actually, water vapor permeability was calculated according to the equation below
$P_{wv} (%) = 100 (u_{s} / u_{o})$
(2)
where u_s is heat loss from the measuring head with the fabric sample and u_o expresses heat loss from the measuring head without fabric.²⁹ Besides these values that are directly measured by the Permetest instrument, the parameter water vapor permeability index i_m was calculated from equation (3)³⁰
$i_{m} = 60.6 (R_{ct} / R_{t})$
(3)

All tests were carried out three times under standard atmospheric conditions, that is, 20 ± 2℃ temperature and 65 ± 2% relative humidity. The samples were conditioned for a minimum of 24 hours before tests.

Analysis of variance (ANOVA) and post hoc tests were used to analyze the test results for significance in differences of the mean values of the measured properties, since the fabric structural parameters of tightness and knit structures have a categorical type of data.³¹ Correlation analyses were used to define the relationships between the measured properties for the fabric structural parameters of fabric thickness and air permeability, which were used as an indicator for the fabric porosity. In order to show the relationship visually, for whom the correlation values were found to be significant for 24 measurements, the diagrams were drawn from the average values of the characteristics of the specimen in different knitted structures using a trendline. All the statistical tests were established at the 0.05 significance interval using SPSS 21³² statistical software.

Results and discussion

Table 2 displays the thermoregulation properties of the fabric structures knitted from acrylic yarns.
Table 2.
Thermal properties of knitted structures measured using Alambeta with standard deviations

Sample code Thermal conductivity (W/mK) Thermal resistance (m² mK/W) Thermal absorptivity (Ws^1/2/ m² K)

SJ-Slack 0.048 (0.0005) 0.040 (0.001) 119.00 (6.24)

SJ-Tight 0.049 (0.0002) 0.039 (0.002) 110.53 (13.06)

Rib 1 × 1-Slack 0.050 (0.0005) 0.043 (0.001) 111.67 (3.05)

Rib 1 × 1-Tight 0.053 (0.0004) 0.038 (0.001) 123.67 (4.73)

Rib 2 × 2-Slack 0.049 (0.0006) 0.056 (0.001) 97.63 (7.59)

Rib 2 × 2-Tight 0.051 (0.0006) 0.054 (0.001) 107.80 (8.30)

Int 1 × 1-Slack 0.052 (0.0007) 0.051 (0.001) 112.00 (13.45)

Int 1 × 1-Tight 0.054 (0.0003) 0.051 (0.001) 132.00 (10.58)

The influences of knit structure and tightness on these characteristics were evaluated with two-way ANOVA statistical analysis. According to the significance values in Table 3, the knit structure was found to affect all three characteristics. Tightness was also found to be influential on thermal conductivity and thermal absorptivity, whereas the combination of tightness and the knit structure was found to be influential only on thermal conductivity. These relationships are better seen in Figures 2 –4.
Figure 2.
Thermal conductivity values of knitted structures.

Figure 3.
Thermal resistance values of knitted structures.

Figure 4.
Thermal absorptivity values of knitted structures.

Table 3.
The results of the two-way analysis of variance for thermal comfort characteristics

Parameters and variables Df F Sig

Thermal conductivity

Knit structure 3 92.735 0.000

Tightness 1 122.567 0.000

Knit structure * tightness 3 5.465 0.009

Thermal resistance

Knit structure 3 13.965 0.000

Tightness 1 4.091 0.060

Knit structure * tightness 3 2.458 0.100

Thermal absorptivity

Knit structure 3 410.030 0.013

Tightness 1 425.884 0.037

Knit structure * tightness 3 217.562 0.085

The thermal conductivity values, which show the ability of the fabric to conduct heat, are given in Figure 2. According to the figure, interlock structures have the highest thermal conductivity and single jersey structures have the lowest thermal conductivity values in both tight and slack forms, as in parallel with the previous results.^1,26 This can be explained with the characteristics of interlock structures. The interlock structure is the tightest and has the lowest porosity among these three knitted structures and, thus, it has maximum thermal conductivity values.²⁶

Figure 2 also shows the influence of tightness on thermal conductivity. Although the thermal conductivity values of rib 1 × 1 and rib 2 × 2 structures take place between the thermal conductivities of the single jersey and interlock structures, their sequence differs according to tightness. Tight structures have higher thermal conductivities when compared with the slack structure of the same knit structure, as in the case of previous findings,³³ since it is known that tightness increases the amount of fiber in a specific area of the fabric and the thermal conductivity of fiber is higher than the thermal conductivity of the entrapped air.¹ Nonetheless, the change in tightness is more influential on the rib structures than on single jersey and interlock structures, as pointed out statistically. This may be related with the firmer and more inflexible structure of the single jersey and interlock structures. Tightness might not have much influence on the inflexible structures.

Thermal resistance values of the samples, which show the ability of the fabric to prevent heat from flowing through it, are given in Figure 3. According to the figure, thermal resistance values increase depending on the knitted structure, namely single jersey, 1 × 1 rib, interlock and 2 × 2 rib. This result was in parallel with the findings of Oglakcioglu and Marmaralı,¹ who found that the thermal resistance values of single jersey structures were lower than both the 1 × 1 rib and interlock structures. Moreover, Majumdar et al.²⁶ found that interlock fabrics had maximum thermal resistance, followed by rib and plain knitted fabrics.

Within the study, it was also observed that the thermal resistance values were close to each other for two fabric structures: single jersey and 1 × 1 rib structures and interlock structure and rib 2 × 2. Considering the fact that thermal resistance expresses the insulation capability of the fabric structures,^3,34 this revealed that these pairs of fabric structures show more or less the same insulation characteristics. Thus, it may be better to use rib 2 × 2 or interlock structures for winter wear. Nonetheless, the tightness is observed to have no important impact on the thermal resistance values, although it was stated by Ucar and Yılmaz¹⁴ that heat loss decreased as fabrics became tight. This result may be attributed to the fact that the tightness can be more influential in different yarn counts and densities.

Thermal absorptivity, which shows the warm cool feeling of the fabric structures, is given in Figure 4. According to the figure, the highest thermal absorptivity values were obtained for the single jersey and interlock structures. Single jersey fabric had the highest thermal absorptivity value in the slack form, whereas interlock fabric had the highest value in the tight form. This point was actually confirmed with the results of a two-way ANOVA stating that tightness was another influential factor for thermal absorptivities. Nonetheless, the thermal absorptivity values of the rib structures seemed to be lower than the thermal absorptivity values of the single jersey and interlock structures. Even rib 2 × 2 was seen to be the structure that had the lowest thermal absorptivity value. Having lower thermal absorptivity values, rib structures can be said to have warmer feelings³⁵ when compared with the interlock and single jersey structures that give a cooler feeling at the beginning of skin contact. The reason why single jersey and interlock fabrics have higher thermal absorptivity and rib structures have lower absorptivity is that thermal absorptivity is very much related to the surface characteristics. Actually, a smoother surface increases the contact area between the skin and the fabric and the heat flows easily, creating a cooler feeling.³⁶ Rib structures have ribs that create peak and valley patterns on the fabric surface and these decrease the contact area and so the flow of the heat. In fact, rib 1 × 1 structures, which provide a smoother surface than rib 2 × 2 structures, have higher thermal absorptivity values. Tightness, on the other hand, is found to be influential on thermal absorptivity because fabric structure and tightness change the surface structure.³⁷ Considering the thermal absorptivity values, it can be stated that rib structures can be more preferred for winter wear because of giving a warmer feeling.

The relation between thermal properties and the structural characteristics of the fabrics, which are thickness and air permeability, were also analyzed using correlation analysis. Despite the fact that it is not possible to build a linear relationship between these two parameters, as there are other parameters influencing them, the approximation of these relations were given with a chart that shows the average values for the fabrics in different structures, as shown in Figure 5.
Figure 5.
The relation between thermal properties and the fabric parameters of different knitted structures: (a) the relationship between thermal resistance and thickness; (b) the relationship between thermal conductivity and air permeability; (c) the relationship between thermal absorptivity and air permeability.

The statistical analyses showed that there is a strong negative relation between thermal conductivity and air permeability, with the Pearson correlation coefficient of –0.903 (p = 0.002, n = 24). The fabrics with higher porosity values have lower thermal conductivities,²⁶ since thermal conductivity is the combination of the conductivity of air and fiber. Tight constructions containing more fibers and less air in a structure have higher thermal conductivity values.⁷On the other hand, the relation between thickness and thermal resistance of the fabrics was found to be directly proportional to the Pearson correlation coefficient of 0.86 (p = 0.006, n = 24). This meant that thicker fabrics offer more resistance to heat flow across the fabric. This result is exactly parallel with the previous findings^34,38 and it was confirmed that in expressing the thermal insulation of the fabrics, thermal resistance mainly depends on the fabric thickness.^12,30,38,39 This is because actually increasing the thickness of the fabric increases the amount of air within the fabric and the insulation characteristics of the air are better than the fiber. The relation between air permeability and thermal absorptivity was also found to be statistically significant with the Pearson correlation coefficient of –0.738 (p = 0.037, n = 24). This meant that as the porosity of the fabric increases, the thermal absorptivity decreases, giving a warmer feeling to the user. This finding may be attributed to the fact that thermal absorptivity depends on the thermal capacity and thermal conductivity of the fabric.⁸ Within the porous structures, the fibers are not closely placed to each other and this makes the thermal conductivity to become lower. Since the thermal absorptivity is dependent on the thermal conductivity, as the porosity of the fabric increases, the thermal absorptivity decreases.

Regarding the thermoregulation characteristics, it can be stated that interlock and rib 2 × 2 structures provide better characteristics in terms of thermal insulation and warmer feeling, respectively, for the usage of fabrics for winter wear products. In addition, the larger the porosity and thickness of the fabrics, the more the fabric improves the thermal comfort in terms of thermoregulation.

The parameters regarding the breathability and thermo-physiological comfort are summarized in Table 4.
Table 4.
The breathability and thermo-physiological comfort properties of knitted structures with standard deviations

Sample code Air permeability (mm/s) Thermal resistance, R_ct (m²mK/W) Water vapor resistance, R_t (Pa m²/W) Relative water vapor permeability (%) Water vapor permeability index i_m

SJ-Slack 753.33 (11.504) 0.051 (0.005) 7.50 (0.35) 43.53 (1.10) 0.42 (0.042)

SJ-Tight 735.33 (6.807) 0.055 (0.003) 7.77 (0.59) 42.40 (1.97) 0.43 (0.053)

Rib 1 × 1-Slack 810.67 (11.547) 0.058 (0.004) 8.30 (0.44) 40.83 (1.27) 0.42 (0.015)

Rib 1 × 1-Tight 509.33 (5.132) 0.046 (0.004) 7.33 (0.70) 43.60 (2.76) 0.38 (0.03)

Rib 2 × 2-Slack 841.00 (34.395) 0.068 (0.009) 9.27 (0.35) 38.30 (1.14) 0.45 (0.058)

Rib 2 × 2-Tight 644.33 (15.560) 0.066 (0.006) 8.20 (1.14) 40.90 (2.97) 0.42 (0.015)

Int 1 × 1-Slack 431.33 (26.558) 0.068 (0.001) 10.130 (0.95) 35.93 (2.24) 0.41 (0.036)

Int 1 × 1-Tight 266.33 (19.655) 0.062 (0.006) 9.67 (0.55) 37.43 (1.25) 0.39 (0.026)

The influences of the knit structure and tightness on these characteristics were evaluated with a two-way ANOVA, as shown in Table 5. It was observed that only the knit structure is influential on the water vapor resistance and water vapor permeability.
Table 5.
The results of the two-way analysis of variance for water vapor resistance, water vapor permeability and the water permeability index

Parameters and variables Df F Sig

Air permeability

Knit structure 3 578.03 0.000

Tightness 1 474.852 0.000

Knit structure * tightness 3 55.902 0.000

Water vapor resistance

Knit structure 3 16.286 0.000

Tightness 1 0.696 0.417

Knit structure * tightness 3 1.129 0.367

Water vapor permeability index

Knit structure 3 1.416 0.275

Tightness 1 1.532 0.234

Knit structure * tightness 3 0.645 0.597

The air permeability values for different fabric specimens are shown in Figure 6. The interlock fabric structures in both tight and slack form were found to have the lowest air permeability values. Single jersey fabrics have the highest air permeability values in the tight form, whose values are followed with rib 2 × 2 and rib 1 × 1 fabrics. Nonetheless, in slack form, the air permeability values become highest for rib 2 × 2 followed with rib 1 × 1 and then single jersey fabric. Confirmed statistically and in parallel with the previous studies,^33,40 these results are due to the influence of both knit type and tightness on the air permeability. Other than these, putting forward the combined influence of knit structure and tightness, it is observed that the difference between the air permeability values in single jersey and interlock structures is lower than the difference between the tight and slack forms of rib 1 × 1 and rib 2 × 2 structures. Nonetheless, the rib and single jersey structures should be preferred to improve the breathability characteristics of the fabrics.
Figure 6.
Air permeability values of knitted structures.

The water vapor resistance is demonstrated in Figure 7. According to the figure, water vapor resistance is highest for interlock and lowest for single jersey fabrics. Rib structures, on the other hand, had values between single jersey and interlock structures. This result is in parallel with the previous findings.²⁶ The ranking of the results are vice versa for relative water vapor permeability values, as shown in Table 4, since water vapor resistance is indirectly related to relative water vapor permeability. The results revealed that single jersey fabrics are better than the interlock structures in transmitting vapor from the body, enabling them to be more breathable. Considering the usage of fabrics as winter wear, it can be stated that the comfort characteristics worsen for the interlock fabrics if sweating occurs. Regarding the influence of tightness, it can be stated that although it was not confirmed statistically, as was done by Ozdil et al.,¹³ the difference between slack and tight structures in both 1 × 1 and 2 × 2 rib structures is higher than that in single jersey and interlock fabrics.
Figure 7.
Water vapor resistance values of knitted structures.

The water vapor permeability index i_m values of the knitted structures, which were calculated using the thermal resistance and water vapor resistance values, are given in Figure 8. Actually, i_m is dimensionless and has values between 0 and 1. A fabric with a value of 1 has both the thermal resistance and water vapor resistance of an air layer of the same thickness.⁹ Thus for winter clothing, fabrics with higher water vapor permeability index values are recommended. Although no significant relationship was found between the water permeability index i_m and the fabric parameters of knit structure and tightness, Figure 8 shows that rib 2 × 2 structures have higher index values and interlock structures have lower index values compared with the other ones. Since i_m is the ratio of the thermal resistance values to the water vapor resistance values, the index values for the rib and interlock structures can be explained with the fact that rib 2 × 2 structures have higher thermal resistance values but lower water vapor resistance values. Thus, the rib 2 × 2 structure provides better thermo-physiological comfort, followed by rib 1 × 1 and single jersey structures. Despite the fact that interlock structures provide thermal resistance, they are not suitable thermo-physiologically because of having higher water resistance.
Figure 8.
Water vapor permeability index i_m of knitted structures.

The relations between the fabric structural parameters and the breathability and thermo-physiological parameters were analyzed by correlation analyses and the approximation of these relations were given with the diagrams and a trend line, using the average values for different knitted structures, as shown in Figure 9.
Figure 9.
The relation between breathability properties and the fabric parameters of different knitted structures: (a) the relationship between water vapor resistance and thickness; (b) the relationship between air permeability and thickness; (c) the relationship between the water vapor permeability index and air permeability.

The Pearson correlation coefficient between water vapor resistance and thickness of the knitted structures was found to be 0.859 (p = 0.006, n = 24). Thickness increases the water vapor resistance, which leads to the conclusion that transportation of water vapor through a thin fabric will be easier, providing higher breathability to the fabric.^1,12,20,34 This is because, beside the lower values of mass per square meter and hairiness, lower thickness provides easy passage of water vapor through the fabric.²⁶ The Pearson correlation coefficient between air permeability and thickness was found to be –0.568 (p = 0.004, n = 24). The air permeability decreases with the thickness of the fabric, as was confirmed by the previous findings,^7,38,42 which state that lower thickness facilitated the passage of air through the fabric, decreasing the amount of time required to pass through the fabric. Finally, the Pearson correlation coefficient between the water vapor permeability index i_m and air permeability was found to be significant, obtaining the value of 0.793 (p = 0.019, n = 24), as shown in Figure 9. Being dependent upon the thermal resistance and water vapor resistance, the water permeability index values increase as the air permeability values decrease . Since air permeability has an indirect relationship with the water vapor resistance,^20,42 as the air permeability increases the water vapor resistance decreases. Thermal resistance also increases with an increase in air permeability, as there is a negative correlation between thermal conductivity and air permeability, as stated above. Thus, increasing the air permeability increases the thermal resistance but decreases the water vapor resistance, causing the water vapor permeability index to have higher values. Since air permeability is an indicator for the porosity of the fabric, this means that the water vapor permeability index increases as the porosity of the fabric structures increases. In fact, together with thickness permeability, it is stated to determine water vapor transmission, which is controlled by the diffusion process.^12,20

Regarding the breathability and thermo-physiological characteristics, it was found that single jersey and rib structures provide better characteristics in the usage as winter wear. Nonetheless, decreasing the thickness and increasing the porosity makes the user feel more comfortable.

Conclusions

This study examined the thermoregulation, breathability and thermo-physiological properties of acrylic-based single jersey, 1 × 1 rib, 2 × 2 rib and interlock flat knitted structures that are suitable for use as winter wear.

The experimental results revealed that the parameters of thermal comfort are significantly affected by the knit structure. Furthermore, it was found that tightness of the knitted structures has an influence on thermal conductivity, thermal absorptivity and air permeability, while it had no significant impact on water vapor resistance and the water vapor permeability index. Thickness was also found to have a statistically significant influence on providing thermoregulation properties and water vapor resistance, whereas air permeability as an indicator of porosity is influential on thermal conductivity and thermal absorptivity, in terms of thermoregulation characteristics, and influential on the water vapor permeability index, in terms of thermo-physiological characteristics. In addition, it directly increases the breathability of the fabric.

Regarding the flat knitted fabric based on acrylic for winter wear, it can be stated that rib 2 × 2 and interlock fabrics are the most suitable knit structures in terms of providing thermal resistance. Nonetheless, rib 2 × 2 makes the individual feel better because of providing warmth and allowing water vapor transmission and breathing when sweating occurs. Although the tightness is also important for some of the parameters that were covered in this study, the knit structure is found to be a more dominant factor than the tightness, based on the two-way ANOVA test results. The thickness and porosity are the two other parameters that should be controlled when fabric selection is made for winter wear, since increasing the thickness improves the thermal insulation of the fabric but deteriorates the breathability and thermo-physiological comfort in contrast to porosity of the fabric.

Analyzing the flat knitted fabrics as winter wear, this study contributes to the studies regarding acrylic fiber, which are few, and extends the properties that were covered within the analyses, allowing one to identify the dominant factors. The study can provide a useful base for future studies regarding fabrics for winter wear for which the fabrics made from different fiber blends, such as acrylic-wool and acrylic-PET fibers.

Sample code	Courses per cm	Wales per cm	Loop density (loops/cm²)	Weight (g/m²)	Thickness (mm)	Loop length l (cm)	Air permeability (mm/s)
SJ-Slack	5.00	4.30	21.50	294.4	1.58	0.87	753.33
SJ-Tight	6.00	4.30	28.50	349.1	1.64	0.81	735.33
Rib 1 × 1-Slack	8.00	10.70	82.40	326.6	1.67	0.72	810.67
Rib 1 × 1-Tight	10.70	10.30	112.89	383.8	1.71	0.46	509.33
Rib 2 × 2-Slack	7.30	11.70	78.11	422.2	2.03	0.62	841.00
Rib 2 × 2-Tight	9.00	12.70	105.30	453.5	2.12	0.53	644.33
Int 1 × 1-Slack	8.00	11.70	47.20	431.4	2.25	0.68	431.33
Int 1 × 1-Tight	9.70	12.00	57.72	514.1	2.25	0.60	266.33

Sample code	Thermal conductivity (W/mK)	Thermal resistance (m² mK/W)	Thermal absorptivity (Ws^1/2/ m² K)
SJ-Slack	0.048 (0.0005)	0.040 (0.001)	119.00 (6.24)
SJ-Tight	0.049 (0.0002)	0.039 (0.002)	110.53 (13.06)
Rib 1 × 1-Slack	0.050 (0.0005)	0.043 (0.001)	111.67 (3.05)
Rib 1 × 1-Tight	0.053 (0.0004)	0.038 (0.001)	123.67 (4.73)
Rib 2 × 2-Slack	0.049 (0.0006)	0.056 (0.001)	97.63 (7.59)
Rib 2 × 2-Tight	0.051 (0.0006)	0.054 (0.001)	107.80 (8.30)
Int 1 × 1-Slack	0.052 (0.0007)	0.051 (0.001)	112.00 (13.45)
Int 1 × 1-Tight	0.054 (0.0003)	0.051 (0.001)	132.00 (10.58)

Parameters and variables	Df	F	Sig
Thermal conductivity
Knit structure	3	92.735	0.000
Tightness	1	122.567	0.000
Knit structure * tightness	3	5.465	0.009
Thermal resistance
Knit structure	3	13.965	0.000
Tightness	1	4.091	0.060
Knit structure * tightness	3	2.458	0.100
Thermal absorptivity
Knit structure	3	410.030	0.013
Tightness	1	425.884	0.037
Knit structure * tightness	3	217.562	0.085

Sample code	Air permeability (mm/s)	Thermal resistance, R_ct (m²mK/W)	Water vapor resistance, R_t (Pa m²/W)	Relative water vapor permeability (%)	Water vapor permeability index i_m
SJ-Slack	753.33 (11.504)	0.051 (0.005)	7.50 (0.35)	43.53 (1.10)	0.42 (0.042)
SJ-Tight	735.33 (6.807)	0.055 (0.003)	7.77 (0.59)	42.40 (1.97)	0.43 (0.053)
Rib 1 × 1-Slack	810.67 (11.547)	0.058 (0.004)	8.30 (0.44)	40.83 (1.27)	0.42 (0.015)
Rib 1 × 1-Tight	509.33 (5.132)	0.046 (0.004)	7.33 (0.70)	43.60 (2.76)	0.38 (0.03)
Rib 2 × 2-Slack	841.00 (34.395)	0.068 (0.009)	9.27 (0.35)	38.30 (1.14)	0.45 (0.058)
Rib 2 × 2-Tight	644.33 (15.560)	0.066 (0.006)	8.20 (1.14)	40.90 (2.97)	0.42 (0.015)
Int 1 × 1-Slack	431.33 (26.558)	0.068 (0.001)	10.130 (0.95)	35.93 (2.24)	0.41 (0.036)
Int 1 × 1-Tight	266.33 (19.655)	0.062 (0.006)	9.67 (0.55)	37.43 (1.25)	0.39 (0.026)

Parameters and variables	Df	F	Sig
Air permeability
Knit structure	3	578.03	0.000
Tightness	1	474.852	0.000
Knit structure * tightness	3	55.902	0.000
Water vapor resistance
Knit structure	3	16.286	0.000
Tightness	1	0.696	0.417
Knit structure * tightness	3	1.129	0.367
Water vapor permeability index
Knit structure	3	1.416	0.275
Tightness	1	1.532	0.234
Knit structure * tightness	3	0.645	0.597

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 received no financial support for the research, authorship and/or publication of this article.

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