Thermal comfort properties of Viloft/cotton and Viloft/polyester blended knitted fabrics

Abstract

Viloft is a special regenerated cellulosic fiber with a flat cross-section and crenulated surface that maintains air gaps in the yarns that help to improve the thermal properties of the fabrics. This fiber is mainly used for underwear, socks and sportswear fabrics and blends of Viloft with polyester or cotton are commonly preferred in the market. In this study, thermal-related characteristics, such as the thermal conductivity, thermal diffusivity, thermal absorptivity, thermal resistance, moisture and air permeability, of Viloft/cotton and Viloft/polyester blended knitted fabrics were investigated. For this purpose, 100%–0%, 67%–33%, 50%–50%, 33%–67% and 0%–100% blends of Viloft/cotton and Viloft/polyester slivers were produced and spun as 19.7 tex on a ring spinning system. In addition, single-jersey and 1 × 1 rib fabrics were produced and the comfort properties of these fabrics were measured using the Alambeta, sweating guarded hotplate, Permetest and air permeability testing devices. A simplex lattice design for the blended fabric properties was also developed and statistical analyses were carried out. According to the results, Viloft-rich blends, in general, improved the thermal properties of the fabrics. However, the relative water vapor permeability of Viloft/polyester blended fabrics was found not to be significant and only small significances were present for cotton blended ones, statistically.

Keywords

Viloft cotton polyester comfort thermal conductivity thermal diffusivity thermal absorptivity thermal resistance water vapor and air permeability knitted fabrics

Clothing comfort or the sense of coolness/warmth is one of the important parameters for consumers. Comfort in fabrics is related to three main factors, namely thermo-physiological, sensorial and physiological. Thermo-physiological comfort is a general expression of factors such as the thermal properties, water vapor transmission, sweat absorption and drying ability of fabrics.¹ In recent years, the studies on comfort characteristics regarding the thermal behavior, air and moisture permeability of textile fabrics have increased. The structure of knitted fabrics is an important parameter for determining the thermal and moisture management properties.¹ The thermal comfort properties of single-jersey, 1 × 1 rib and interlock fabrics were investigated by Oglakcioglu and Marmarali.² They found that 1 × 1 rib and interlock fabrics have high thermal conductivity and thermal resistance values, whereas single-jersey fabrics have high relative water vapor permeability values. In another study, Ucar and Yilmaz³ investigated the thermal properties of 1 × 1, 2 × 2 and 3 × 3 rib fabrics. Circular knitted spacer fabrics were investigated in terms of heat, air and water vapor transfer properties by Ertekin and Marmarali⁴ and they concluded that the comfort properties are affected by dial height and the type of spacer yarns. The effect of fiber cross-section and fiber structure on the comfort properties of different knitted and woven fabrics has been investigated by several researchers.^5–8 They found that special fibers improved the thermal characteristics of fabrics. The effect of yarn properties on the thermal behavior of fabrics has also been investigated in previous studies.^9,10 Different fibers can be blended in order to improve the thermal characteristics of fabrics. Cotton–acrylic blended knitted fabrics were investigated by Cil et al.¹¹ and they found that increasing acrylic in the blend yields an increase in the longitudinal and transfer wicking abilities of the fabrics. The thermal comfort properties of Angora rabbit/cotton blended knitted fabrics were investigated by Oglakcioglu et al.¹² The moisture permeability of the polyester/viscose blended fabrics were also studied by Das et al.¹³ Kaplan and Okur¹⁴ developed a dynamic sweating hotplate system and investigated the steady-state and dynamic thermal comfort measurements. In addition, in some studies artificial neural network, statistical, mathematical and theoretical models were developed to determine the thermo-physiological properties of fabrics.^15–17

Recently, the inventions in artificial fibers in order to improve the comfort properties of fabrics have also been carried out. The comfort properties of cellulosic fibers have been investigated by several researchers^18–22 and some of these studies concentrated on a novel fiber: bamboo. Another recently invented regenerated cellulosic fiber with a special cross-section and surface characteristics that provides air chambers in the yarn and also in the fabric is Viloft®. It is mainly used for underwear, sportswear, socks, shirts etc.²³ The production of Viloft fiber is similar to that of viscose fiber; only the profile of the spinneret used is different. This fiber is also called as “thermal viscose” by the producer.

The thermo-physiological characteristics of fabrics containing Viloft are yet to be investigated thoroughly. In addition, this fiber is commonly used as blends of polyester and cotton in the market. For this purpose we intended to investigate the thermo-physiological properties of Viloft/cotton and Viloft/polyester blended knitted fabrics in this study. We produced 100%–0%, 67%–33%, 50%–50%, 33%–67% and 0%–100% blends of Viloft/cotton and Viloft/polyester slivers and spun as 19.7 tex on a ring spinning system. In addition, single-jersey and 1 × 1 rib fabrics were produced using these blended yarns and thermo-physiological comfort properties were measured using the Alambeta, sweating guarded hotplate, Permetest, thickness measurement and air permeability testing devices.

Materials and methods

Viloft, polyester and cotton were selected as yarn components in this study, since Viloft is commonly used with the blends of polyester or cotton in the market. The cross-section of polyester is round and the cotton is bean-like; however, Viloft is flat and has a crenulated surface (Figure 1). The physical properties of the fibers, such as linear density and staple length, are shown in Table 1.

Figure 1.

Cross-section and longitudinal scanning electron microscope images of Viloft fiber.

Table 1.

Properties of Viloft, polyester and cotton fibers

Property	Viloft	Polyester	Cotton (HVI)
Linear density	1.9 dtex	1.6 dtex	4.47 mic.
Staple length (mm)	38	38	30.1

HVI: high-volume instrument.

Viloft/polyester and Viloft/cotton blends were prepared as 100%–0%, 67%–33%, 50%–50%, 33%–67% and 0%–100%. The blended slivers were produced using a Trützschler BOA 046 opener, a DX 903 blowroom and carding machines. A two-passage drawing was applied using a Rieter SB-D40 drawframe. A Zinser 660 roving frame was used for producing rovings of 579 tex. Blended yarns were spun as 19.7 tex on an Edera spinning machine by determining the twist coefficient as α _e = 3.7. Single-jersey fabrics were produced using a Terrot circular knitting machine on 16 gauges and 30” diameter. In addition, 1 × 1 rib fabrics were also produced using a Mayer&CIE circular knitting machine on 28 gauges and 30” diameter. The stitch length of the fabrics was determined as 0.28 cm for all the fabrics produced. In addition, caustic soda treatment was applied to the cotton blended fabrics, while polyester blended fabrics and 100% Viloft fabrics were washed and dried.

An Alambeta testing device was used for measuring the thermal properties of the fabrics, such as thermal conductivity, thermal diffusivity and thermal absorptivity. Thermal resistance was measured using an SDL-sweating guarded hotplate according to TS EN 31092. Permetest, which works on the principle of heat flux sensing, is used for measuring relative water vapor permeability according to ISO 11092. Air permeability tests were carried out using the SDL Atlas air permeability testing device according to TS 391 EN ISO 9237 by keeping the air pressure at 50 Pa. Replications were carried out for the tests applied according to the related standards for different properties of the fabrics, and the averages of these measurements were calculated. In addition, the hairiness values of the blended yarns used in the study were also measured using an Uster Tester 5 S800. All of the average measurements and coefficients of variation (CV%) of each fabric are given in Tables 2 and 3 for single-jersey and 1 × 1 rib fabrics, respectively. In these tables, air permeability is given as the volume of the air (cm³) passing through a unit area (cm²) at a unit time (s). In addition, porosity values of the fabrics are calculated as⁹

P = (1 - m / ρ h) 100

(1)

where P is the porosity, m is the fabric weight, ρ is the fiber density in g/cm³ and h is the fabric thickness in mm. The fiber density value in the equation is calculated by taking the individual fiber densities and blend ratios into consideration. The structures of the selected single-jersey and 1 × 1 rib fabrics produced are displayed in Figure 2.

Figure 2.

100% Viloft fabric structures. (a) single jersey. (b) 1 × 1 rib.

Table 2.

Viloft/polyester and Viloft/cotton blended single-jersey fabric properties

Blend type (%)	Statistics	Thermal conductivity (W/mK)	Thermal diffusivity (10^–6) (m²/s)	Thermal absorptivity (Ws^0.⁵/m²K)	Thermal resistance (m²K/W)	Relative water vap. permeab. (%)	Air permeability (cm³/cm²s)	Fabric weight (g/m²)	Fabric thickness (mm)	Uster yarn hairiness (H)	Porosity (%)	Wales- courses /cm
100% polyester	Mean	0.0397	0.1720	97.13	0.0351	39.5	83.33	125.2	1.31	6.63	93.07	13.0-18.0
100% polyester	CV (%)	1.13	3.63	5.44	2.67	3.7	0	0.87	5.31	3.2	93.07	13.0-18.0
33% Viloft 67% polyester	Mean	0.0417	0.1703	100.97	0.0298	42.0	83.33	133.2	1.21	6.61	92.28	13.8-20.0
33% Viloft 67% polyester	CV (%)	0.85	1.21	0.85	5.14	3.3	0	1.62	5.58	5.5	92.28	13.8-20.0
50% Viloft 50% polyester	Mean	0.0424	0.1536	108	0.0362	38.5	82.66	135.6	1.19	7.07	92.14	13.4-19.0
50% Viloft 50% polyester	CV (%)	0.45	1.34	0	3.3	2.3	1.61	1.12	1.65	3.6	92.14	13.4-19.0
67% Viloft 33% polyester	Mean	0.0421	0.1196	121.67	0.0286	40.6	83.33	136	1.06	7.95	91.29	13.4-20.0
67% Viloft 33% polyester	CV (%)	2.34	2.76	1.4	3.11	3.2	0	1.55	7.45	2.3	91.29	13.4-20.0
100% Viloft	Mean	0.0433	0.1180	125.33	0.0382	41.1	83.33	143	1.12	8.20	91.60	13.0-20.0
100% Viloft	CV (%)	0.87	5.27	3.98	3.55	2.4	0	2.75	5.41	5.1	91.60	13.0-20.0
100% cotton	Mean	0.0498	0.1553	126.34	0.0316	39.6	83.33	125	1.16	5.15	92.91	12.2-21.0
100% cotton	CV (%)	1.4	5.67	1.97	1.83	1.6	0	2.93	4.26	3.2	92.91	12.2-21.0
33% Viloft 67% cotton	Mean	0.0494	0.1340	135	0.0328	39.6	78.99	131.4	1.16	6.16	92.55	12.2-19.0
33% Viloft 67% cotton	CV (%)	1.29	3.39	2.77	3.74	0.6	2.7	0.86	2.27	3.06	92.55	12.2-19.0
50% Viloft 50% cotton	Mean	0.0489	0.1283	136.34	0.0343	40.5	80.99	133.2	1.08	6.94	91.89	12.6-20.0
50% Viloft 50% cotton	CV (%)	0.75	2.94	0.91	2.81	4.9	3.21	1.86	4.1	3.05	91.89	12.6-20.0
67% Viloft 33% cotton	Mean	0.0489	0.1153	144.34	0.0380	39.9	78.99	139.2	1.14	7.14	91.97	12.6-20.0
67% Viloft 33% cotton	CV (%)	1.67	5.12	4.28	2.74	0.6	3.79	1.98	4.04	3.5	91.97	12.6-20.0
100% Viloft	Mean	0.0433	0.1180	125.33	0.0382	41.1	83.33	143	1.12	8.20	91.60	13.0-20.0
100% Viloft	CV (%)	0.87	5.27	3.98	3.55	2.4	0	2.75	5.41	5.1	91.60	13.0-20.0

Table 3.

Viloft/polyester and Viloft/cotton 1 × 1 rib fabric properties

Blend type (%)	Statistics	Thermal conductivity (W/mK)	Thermal diffusivity (10^–6) (m²/s)	Thermal absorptivity (Ws^0.⁵/m²K)	Thermal resistance (m²K/W)	Relative water vap. permeab. (%)	Air permeability (cm³/cm²s)	Fabric weight (g/m²)	Fabric thickness (mm)	Uster yarn hairiness (H)	Porosity (%)	Wales- courses /cm
100% polyester	Mean	0.0439	0.1700	106.67	0.0236	40.0	83.33	168.6	1.48	6.63	91.75	9.4-18.0
100% polyester	CV (%)	1.27	3.36	2.69	4.42	2.2	0	3.52	5.69	3.2	91.75	9.4-18.0
33% Viloft 67% polyester	Mean	0.0475	0.1500	122.67	0.0267	41.8	81.33	167.4	1.28	6.61	90.83	8.7-18.0
33% Viloft 67% polyester	CV (%)	0.86	3.57	2.52	2.44	2.4	3.28	4.21	7.01	5.5	90.83	8.7-18.0
50% Viloft 50% polyester	Mean	0.0475	0.1360	128.67	0.0289	41.7	81.66	174	1.33	7.07	90.98	9.4-18.0
50% Viloft 50% polyester	CV (%)	0.52	5.91	2.64	5.64	7.4	2.04	3.06	3.77	3.6	90.98	9.4-18.0
67% Viloft 33% polyester	Mean	0.0470	0.1280	129	0.0306	40.2	79.33	167.4	1.35	7.95	91.58	9.9-19.0
67% Viloft 33% polyester	CV (%)	0.61	4.82	4.43	2.98	1.7	2.52	2.48	3.98	2.3	91.58	9.9-19.0
100% Viloft	Mean	0.0473	0.1220	135.34	0.0315	41.7	61.66	161.2	1.35	8.20	92.14	8.9-19.0
100% Viloft	CV (%)	0.36	4.69	2.72	3.35	2.4	6.04	3.08	4.27	5.1	92.14	8.9-19.0
100% cotton	Mean	0.0587	0.1677	143.34	0.0288	40.2	83.33	161	1.38	5.15	92.32	9.9-18.0
100% cotton	CV (%)	1.1	2.85	2.3	3.78	6.05	0	2.36	4.67	3.2	92.32	9.9-18.0
33% Viloft 67% cotton	Mean	0.0558	0.1723	134.34	0.0285	39.3	80.33	164.8	1.36	6.16	92.03	9.1-18.0
33% Viloft 67% cotton	CV (%)	1.22	2.61	0.7	2.14	1.3	2.23	2.36	1.66	3.06	92.03	9.1-18.0
50% Viloft 50% cotton	Mean	0.0549	0.1463	143.67	0.0275	37.9	75.99	165	1.29	6.94	91.59	9.1-18.0
50% Viloft 50% cotton	CV (%)	1.36	4.04	3.65	3.97	2.1	3.28	3.92	3.96	3.05	91.59	9.1-18.0
67% Viloft 33% cotton	Mean	0.0541	0.1510	139.33	0.0295	38.6	73.66	168.2	1.36	7.14	91.86	9.8-19.0
67% Viloft 33% cotton	CV (%)	1.17	4.62	1.88	1.68	2.8	3.75	2.86	1.7	3.5	91.86	9.8-19.0
100% Viloft	Mean	0.0473	0.1223	135.34	0.0315	41.7	61.66	161.2	1.35	8.20	92.14	8.3-19.0
100% Viloft	CV (%)	0.36	4.69	2.72	3.35	2.4	6.04	3.08	4.27	5.1	92.14	8.3-19.0

Design Expert 6.02 software was used for analyzing the results. Since the factors are the components of the ingredients of the mixture (blend), the statistical analysis is carried out by considering a simplex lattice design. Simplex designs are used to study the effects of mixture components on the response variable.²⁴ A simplex lattice design was developed by determining the Viloft/polyester and Viloft/cotton percentages in the fabric as mixture components by considering the replications of the tests carried out. Let “p” be the component number and “x” the proportion of the component; p = 2 in this study since we have investigated two-component mixtures, also 0 ≤ x_i ≤ 1; i = 1,2 and x₁ + x₂ = 1. A {p,m} simplex lattice design for p components has m + 1 spaced values from 0 to 1. The number of design points for {p,m} simplex lattice design can be calculated as follows:

N = \frac{(p + m - 1)!}{m! (p - 1)!}

(2)

Considering these restrictions, the simplex lattice mixture model for {2,4} can be illustrated as in Figure 3. Analyses of variance (ANOVAs) were carried out at α = 0.05 significance for both single-jersey and 1 × 1 rib knitted structures.

Figure 3.

Constrained factor space and design points for two-component simplex lattice design.

Results and discussion

The summarized ANOVA table for each property is displayed in Table 4. Here, p-values lower than 0.05 are considered as significant, which indicates that the effect of blend ratio is important on the related fabric property. The blend ratio is significant for most of the properties of the single-jersey and 1 × 1 rib fabrics; however, four of them are not significant, as seen in the table. The yarn hairiness and fabric properties are investigated below successively, in detail.

Table 4.

Significance results of the Viloft blended fabric properties

Property	Fabric type	Blend type	p-value	Significance
Thermal conductivity	Single-jersey	Polyester blend	<0.0001	Significant
	Single-jersey	Cotton blend	<0.0001	Significant
	1 × 1 rib	Polyester blend	<0.0001	Significant
	1 × 1 rib	Cotton blend	<0.0001	Significant
Thermal diffusivity	Single-jersey	Polyester blend	0.0006	Significant
	Cotton blend	0.0003	Significant
	1 × 1 rib	Polyester blend	<0.0001	Significant
	Cotton blend	<0.0001	Significant
Thermal absorptivity	Single-jersey	Polyester blend	<0.0001	Significant
	Cotton blend	0.0074	Significant
	1 × 1 rib	Polyester blend	<0.0001	Significant
	Cotton blend	0.2057	Not Significant
Thermal resistance	Single-jersey	Polyester blend	0.002	Significant
	Cotton blend	<0.0001	Significant
	1 × 1 rib	Polyester blend	<0.0001	Significant
	Cotton blend	<0.0001	Significant
Relative water vapor permeability	Single-jersey	Polyester blend	0.5415	Not Significant
	Cotton blend	0.0304	Significant
	1 × 1 rib	Polyester blend	0.091	Not Significant
	Cotton blend	0.014	Significant
Air permeability	Single-jersey	Polyester blend	0.38	Not Significant
	Cotton blend	<0.0001	Significant
	1 × 1 rib	Polyester blend	<0.0001	Significant
	Cotton blend	<0.0001	Significant
Uster yarn hairiness	Single-jersey, 1 × 1 rib	Polyester blend	<0.0001	Significant
Uster yarn hairiness	Single-jersey, 1 × 1 rib	Cotton blend	<0.0001	Significant

Hairiness of the blended yarns

The hairiness of yarns is important to determine the fabric structure. Hairy yarns result in air gaps on the fabric and affect the comfort properties of fabrics. The hairiness results of Viloft/polyester and Viloft/cotton blended yarns used for both single-jersey and 1 × 1 rib fabrics are given in Figure 4. Here, the Viloft proportion in the blend is increased from 0% to 100%; however, the polyester or cotton blend in the yarn is decreased from 100% to 0% on the x-axis of the figure. Although the staple length of polyester is higher than that of cotton, as seen in Table 1, it is clear from the figure that Viloft/polyester blended yarns show higher hairiness than that of cotton blended ones. This situation may be related with the surface characteristics of the fibers used in that the higher fiber-to-fiber friction or increased cohesion forces, which maintain the fibers being kept in the yarn structure and avoid hairiness, could be obtained for viloft/cotton blends. Since, the surface area of viloft and cotton fibers is higher than that of round polyester, the cohesion forces between viloft-cotton are higher than viloft-polyester. In addition, increasing Viloft proportion in the yarn structure results in more hairiness for both cotton and polyester blends. Maximum hairiness was obtained for pure Viloft. This may be related to the cross-section of Viloft fiber, since it has a flat form. Hence, the rigidity of the fibers will increase and holding the fiber in the yarn structure will be difficult. The Viloft fibers tend to protrude from the main body of the yarn and hairiness occurs. Viloft fiber enhances not only air gaps in the yarn structure by using polyester or cotton in the blend, but also creates air gaps on the fabric surface due to increased yarn hairiness. This property is important for analyzing the thermal characteristics of the fabrics produced.

Figure 4.

Hairiness variaton of the Viloft/cotton and Viloft/polyester blended yarns.

Thermal conductivity

Thermal conductivity is defined as the heat transmitted through a unit area at a temperature gradient per unit length. Thermal conductivity (λ) can be expressed as

λ = q . h / Δ t (W / mK)

(3)

where q is heat flow density (W/m²), h is thickness in meters and Δt is the temperature difference in Kelvin. An important parameter for the thermal properties of fabrics is the air amount inside the fabric, and textile fabrics contain both fibers and air. In general, the thermal conductivity value of textile structures ranges between 0.033 and 0.01 W/mK. While air has the lowest thermal conductivity value compared to all fibers (λ_air = 0.025 W/mK), the thermal conductivity value of water is the highest with λ_water = 0.6 W/mK.⁷ Hence, air inside in a fabric is desirable; however, water inside in a fabric is not desirable from a thermal conductivity point of view.

The significance test (Table 4) indicated that thermal conductivity is significant for all the blended fabrics. The thermal conductivity variation of Viloft/polyester and Viloft/cotton blended single-jersey and 1 × 1 rib fabrics can be seen in Figure 5. The thermal conductivity of 1 × 1 rib fabrics for either polyester or cotton blends are higher than those of single-jersey fabrics, which may be related to the air amount inside the fabrics. The fiber/air proportion in the fabric increases with increasing thickness or weight of the fabrics. In other words, the air proportion of in the fabric decreases with increasing of both thickness and weight (see Tables 2 and 3). Since the surface structure and cross-section of Viloft fibers increase the air gaps in the yarn and the increased hairiness of Viloft blended fabrics results in an increase in the air amount on the fabric, the thermal conductivity values decrease with increasing Viloft proportion, particularly in cotton blended single-jersey and 1 × 1 rib fabrics. However, the thermal conductivity of polyester blended fabrics only slightly increases with increasing Viloft. One of the main reasons may be the material-related property that polyester has a lower thermal conductivity value at 0.141 W/mK than regenerated cellulosic fiber (viscose), which has a value of 0.289 W/mK.⁷ Hence, thermal conductivity increases with increasing Viloft proportion in polyester blended fabrics.

Figure 5.

Thermal conductivity variation of Viloft/polyester and Viloft/cotton blended single-jersey and 1 × 1 rib fabrics.

Thermal diffusivity

Thermal diffusivity (a) is the velocity of propagation of thermal impulse through a material. It is expressed as

a = λ / ρ c (m^{2} / s)

(4)

where c is the specific heat of the fabric (J/kgK). The significance test (Table 4) indicated that thermal diffusivity is significant for all the blended fabrics. Figure 6 demonstrates the thermal diffusivity variation of single-jersey and 1 × 1 rib polyester or cotton blended Viloft fabrics. Increasing the Viloft proportion in the blend decreases the thermal diffusivity values for both polyester and cotton blended single-jersey and 1 × 1 rib fabrics. This situation could be related to the structure of Viloft fiber in that the flat cross-section and crenulated surface of Viloft fiber increase the surface area and result in a decrease in the velocity of propagation of thermal impulse.

Figure 6.

Thermal diffusivity variation of Viloft/polyester and Viloft/cotton blended single-jersey and 1 × 1 rib fabrics.

Thermal absorptivity

Thermal absorptivity is an objective measurement of the warm–cool feeling of fabrics, introduced by Hes²⁵ to characterize the thermal feeling during short contact of the human skin with the fabric surface. In other words, it is the first feeling of heat exchange between fabric and human skin. Thermal absorptivity (b) can be expressed as

b = (λ ρ c) 1 / 2 (Wm - 2 s 1 / 2 K - 1)

(5)

where ρ is fabric density and c is the specific heat of the fabric. Low absorptivity values indicate a warm feeling, which is desired for winter fabrics, whereas high absorptivity values indicate cool feeling, which is desired for summer fabrics.

The thermal absorptivity of the blended fabrics is significant, except for Viloft/cotton blended 1 × 1 rib fabric (Table 4). The thermal absorptivity variation of Viloft/polyester and Viloft/cotton blended single-jersey and 1 × 1 rib fabrics can be seen in Figure 7. Here, the thermal absorptivity of 1 × 1 rib fabrics is, in general, slightly higher than that of single-jersey fabrics, which can be related with the fabric weight or thickness. Thicker fabrics with higher weight values have a high amount of fiber, and the thermal absorption or heat flux from the skin to the garment increases, as expected. Due to having higher absorption values, it can be said that 1 × 1 rib fabrics have a cooler feeling than those of single-jersey fabrics because of the definition of the thermal absorptivity. In addition, increasing Viloft proportion in the blend increases the absorptivity values of Viloft/cotton or Viloft/polyester single-jersey fabrics. For 1 × 1 rib fabrics, also absorptivity values of Viloft/polyester blended fabrics increase with increasing Viloft proportion. However, Viloft/cotton blended 1 × 1 rib fabric shows a variation for the thermal absorption values. Cotton-rich fabrics show higher thermal absorption values.

Figure 7.

Thermal absorptivity variation of Viloft/polyester and Viloft/cotton blended single-jersey and 1 × 1 rib fabrics.

Thermal resistance (insulation)

Thermal resistance (R_ct) of the fabrics was measured using the SDL-Atlas sweating guarded hotplate testing device under steady-state condition according to TS EN 31092 by setting the air temperature at 20℃, the relative humidity at 65% and the air flow at 1 m/s. The temperature of the square porous plate is also set to 35℃, which simulates the body skin. When the system reaches steady-state condition without a sample, R_ct₀ is recorded under these conditions. A fabric sample with a size of 30 × 30 cm² is mounted on the square porous plate (35℃) and the thermal resistance of the fabric (R_ct) is determined by

R_{ct} = \frac{A (T_{s} - T_{a})}{H} - R_{ct 0}

(6)

where T_s and T_a are the temperature (℃) values of the plate surface and ambient air, respectively; A is the area of the test section (m²) and H is the electrical power (W) necessary to keep the plate at 35℃.

The blend ratios are obtained as significant for all the blended fabric types in Table 4. In addition, thermal resistance variations are also demonstrated in Figure 8. Although 1 × 1 rib fabrics are thicker than single-jersey fabrics, the obtained thermal resistances of 1 × 1 rib fabrics are lower than those of single-jersey fabrics, which is not an expected result. In general, thicker fabrics give high thermal resistance values in conductive measurement systems, where there are two plates on the front and back sides of the fabric surfaces and the fabric thickness becomes more important in order to determine the thermal resistance. However, we have used a convective measurement system where there is only one plate, and one side of fabric is laid on it. There is also air flow inside the equipment and convection is maintained. The lower thermal resistance of 1 × 1 rib fabrics compared to single jersey may be related to the fabric structures and the principle of testing device used. As seen in Figure 2, all the courses of the single-jersey fabrics are on the identical side of the fabric; however, for 1 × 1 rib fabrics, one course is on the one side of fabric (front side), while the successive course is on the other side (back side) of the fabric. Since the thermal contact between the fabric and hotplate occurs on the surface of the hotplate, one of the courses of the fabric is subjected to direct heat; however, the successive course is exposed to the air flow inside, and the heat loss from this side of fabric will also be higher than that of single-jersey because of the higher surface area of the back side of 1 × 1 rib fabric compared to single-jersey (see Figure 2). When the heat loss increases, the electricity power (H) in Equation (6), which is necessary to keep the plate at 35℃, will also increase and the obtained thermal resistance will decrease. In general, increasing the Viloft proportion in the blend increases the thermal resistance values of the fabrics. However, some variations occurred for the single-jersey fabrics. The highest thermal insulation is achieved using pure Viloft for both single-jersey and 1 × 1 rib fabrics. However, the 50%/50% blends of both cotton or polyester fabrics have values close to those of pure Viloft single-jersey and 1 × 1 rib fabrics.

Figure 8.

Thermal resistance variation of Viloft/polyester and Viloft/cotton blended single-jersey and 1 × 1 rib fabrics.

Relative water vapor permeability

Water vapor permeability (p%) is the ability of fabrics to transmit water vapor from one side to the other side. It is one of the most important comfort properties of the fabrics due to the need to prevent an uncomfortable sensation by transmitting moisture from human skin. It can be expressed as

p % = 1 00 \times q_{s} / q_{o}

(7)

where q_s and q_o are the heat flow values with and without a sample, respectively.³ The cross-section and surface characteristics of the fibers play an important role in this property. As is known, using polyester fibers with increased fiber surfaces (or irregular cross-sections) improves the water vapor permeability in fabrics. However, using Viloft in the blend shows little effect on the relative water vapor transmission of the single-jersey and 1 × 1 rib fabrics, as can be seen in Figure 9. In general, the water vapor permeability values of single-jersey and 1 × 1 rib fabrics are similar and close to each other. Moreover, the blend ratio is not significant for polyester blended fabrics, either in single-jersey or in 1x1 rib fabrics (Table 4). The blend ratio does not explain the relative water vapor permeability for the polyester blended fabrics. In addition, the p-values of cotton blended fabrics are also close to 0.05, which means poor significances are present. We can conclude that varying the Viloft amount in the blend has little effect on water vapor permeability, which is not significant or slightly significant, statistically.

Figure 9.

Relative water vapor permeability variation of Viloft/polyester and Viloft/cotton blended single jersey and 1 × 1 rib fabrics.

Air permeability

Air permeability is the air flow passing through a fabric under a given air pressure. The air permeability tests were carried out under 50 Pa pressure in this study. As shown in Table 4, the air permeability values are significant, except for polyester blended single-jersey fabric. Figure 10 demonstrates the air permeability variations of the fabrics used in the study. Increasing Viloft proportion in the blend decreases the air permeability for 1 × 1 rib fabrics, which may be related to the surface characteristics and cross-section of Viloft fiber that resist air flow due to increased surface area. In addition, the cotton blended single-jersey fabrics have lower air permeability compared to polyester blended ones. This is again related to the cross-sections of cotton and polyester in that cotton has a higher surface area than polyester. The air permeability of 1 × 1 rib fabrics is lower than that of single-jersey fabrics. This may be related to the higher fabric thickness, lower porosity and higher weight values of 1 × 1 rib compared to single-jersey, shown in Tables 2 and 3. In single-jersey fabrics, small variations occurred for the air permeability of the cotton and polyester blended fabrics.

Figure 10.

Air permeability variation of Viloft/polyester and Viloft/cotton blended single-jersey and 1 × 1 rib fabrics.

Conclusions

In this study, we investigated the comfort properties of single-jersey and 1 × 1 rib fabrics, which are composed of polyester or cotton blends of a novel fiber called Viloft that has a flat cross-section and crenulated surface and is mainly used for its improved thermal properties. As a result of this statistical and experimental study, the following conclusions can be drawn.

Increasing Viloft proportion in the yarn structure resulted in more hairiness for both cotton and polyester blended yarns; however, Viloft–polyester blended yarns have higher hairiness than cotton blended ones. The Viloft fibers tend to protrude from the main body of the yarn, since it has a flat cross-section, and hairiness occurs.

The thermal conductivity of 1 × 1 rib fabrics for either polyester or cotton blends was higher than that of single-jersey fabrics, which may be related to the air amount inside the fabrics. The fiber/air proportion in the fabric increases with increasing the thickness and weight of the fabrics. The thermal conductivity values decreased with increasing Viloft proportion, particularly in cotton blended single-jersey and 1 × 1 rib fabrics; however, the thermal conductivity values of polyester blended fabrics slightly increased with increasing Viloft.

Increasing Viloft proportion in the blend decreased the thermal diffusivity values for both polyester and cotton blended single-jersey and 1 × 1 rib fabrics.

Thermal absorptivity of 1 × 1 rib fabrics was slightly higher than that for single-jersey fabrics, since thicker fabrics have a high amount of fibers and the thermal absorption or heat flux from the skin to the garment increases, as expected. Increasing Viloft proportion in the blend increased the thermal absorptivity values of Viloft/cotton or Viloft/polyester single-jersey fabrics.

Although 1 × 1 rib fabrics were thicker than single-jersey fabrics, the obtained thermal resistance of 1 × 1 rib fabrics was lower than that of single-jersey fabrics. This situation was related to the structures of the fabrics and the principle of testing device. For 1 × 1 rib fabrics, one course is on the front side of fabric; the successive course is on the back side of the fabric. However, all the courses of the single-jersey fabrics are on the identical side of the fabric. Since the thermal contact between the fabric and hotplate occurs on the surface of the hotplate, one of the courses of the fabric is subject to direct heat; however, the successive courses are exposed to the air flow inside, and the heat loss from this side of fabric will also be higher than that of single-jersey because of the higher surface area of the back side of 1 × 1 rib fabric compared to single-jersey. When the heat loss increases, the electricity power that is necessary to keep the plate at 35℃ will also increase and the obtained thermal resistance will decrease. In general, increasing Viloft proportion in the blend increased the thermal resistance values of the fabrics.

The blend ratio was not significant for the relative water vapor permeability of polyester blended fabrics both in single-jersey and 1 × 1 rib fabrics. In addition, the p-values of cotton blended fabrics were close to 0.05, which means poor significances are present. In general, the water vapor permeability values of single-jersey and 1 × 1 rib fabrics are similar and close to each other.

The air permeability of 1 × 1 rib fabrics was lower than that of single-jersey fabrics, which is related to the higher fabric thickness, lower porosity and higher weight values of 1 × 1 rib fabrics compared to single-jersey fabrics. Increasing Viloft proportion in the blend decreased the air permeability for 1 × 1 rib fabrics.

Viloft fiber enhances air gaps in the yarn due to its flat cross-section. This fiber structure also causes hairiness on the yarn structure that results in air gaps on the fabrics, as well. Hence, Viloft-rich fabrics have more air gaps in the fabrics, which is an important property for determining the thermal characteristics of the fabrics, since air has the lowest thermal conductivity value compared to all fibers. In general, we found that Viloft-rich fibers improved the thermal properties of the fabrics. The flat cross-section and crenulated surface of this fiber is also important for air permeability. However, the relative water vapor permeability of Viloft/polyester blended fabrics was found not to be significant and small significances were present for cotton blended ones. Using Viloft was found to have minor effects for the relative water vapor permeability of the fabrics produced in this study. Due to increased thermal properties, the Viloft blended fabrics could be used in particular for underwear, socks and sportswear, in order of priority. From the thermal resistance point of view, which is one of the most important thermal characteristics of fabrics, we can conclude that 100% Viloft fabrics maintain the maximum isolation. However, considering other parameters such as price, 50%/50% polyester or cotton blended single-jersey fabrics may be preferred by consumers.

Footnotes

Acknowledgment

We appreciate the contributions of KARSU Tekstil Sanayi ve Ticaret A.Ş Kayseri-Turkey and Prof. Lubos HES from the Technical University of Liberec-Czech Republic for the use of the Alambeta testing device.

Funding

This work was supported by the Research Fund of Erciyes University (project number FBY-12-3963).

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