Research Article, J Fashion Technol Textile Eng Vol: 5 Issue: 3

Characterization of Wool Fabric Surface in Terms of Transient Heat Transfer

Chendi Tu and Sachiko Sukigara^*

Department of Advanced Fibro-Science, Kyoto Institute of Technology, Kyoto, 606- 8585 Japan

*Corresponding Author : Sachiko Sukigara
Department of Advanced Fibro- Science, Kyoto Institute of Technology, Kyoto, 606-8585 Japan
Tel & Fax: +81-75-724-7365
E-mail: sukigara@kit.ac.jp

Received: May 26, 2017 Accepted: June 13, 2017 Published: June 17, 2017

Citation: Tu C, Sukigara S (2017) Characterization of Wool Fabric Surface in Terms of Transient Heat Transfer. J Fashion Technol Textile Eng 5:3. doi:10.4172/2329-9568.1000154

Abstract

Keywords: Thermal; Moisture; Wool; Fabric surface; qmax

Introduction

Garments that produce continuous uncomfortable stimuli on the skin are unwearable. Therefore, fabric with comfortable tactile qualities is important for garments. Many researchers have studied wool fabric hand to generate suitable surface properties for the enduse garments. Naylor [1] demonstrated that prickliness arises from coarse fiber on the fabric surface. Many studies have investigated improving wool fabric hand by making the wool fabric surface smoother and softer [2-4].

Fabric perception and hand can be characterized subjectively by the softness, smoothness, dampness and warmth/coolness. These subjective characteristics correspond to the physical properties of fabrics. In particular, surface fuzz, friction, and roughness of the wool fabric surface are strongly related; thus, precise characterization is needed. We expected that transient heat transfer from skin to fabric would be an important factor, because it is affected by the fabric surface. The characteristic value q_max was proposed by Kawabata and Yoneda [5,6] and has been shown to estimate the temperature perception of the fabric. This value is the maximum rate of heat flux from a heated plate to the fabric and is convenient to measure. q_max values are also used in the fabric hand values for knitted underwear [7]. q_max is used to describe the thermal contact properties, but it can also be used with other fabric mechanical and surface properties to design fabrics.

In this study, wool fabrics with different fiber diameters, fabric densities, and moisture regain were used to examine how effective q_max is for wool fabric surface characterization.

Experiment

Samples

The fiber diameter, fabric density, and moisture regain were chosen as factors related to q_max. Three groups of fabric samples were used to examine each effect.

I: Effect of fiber diameter (Sample Group I)

Three yarn samples (25 tex) were spun from wool fibers with three mean fiber diameters (MFD; 18.5, 21.5, and 24.5 μm). Both 1 × 1 rib stitch and plain knitted fabrics were produced from the yarns in two stitch densities. Table 1 lists the sample details.

Table 1: Characteristics of Sample Group I.

II: Effect of fabric moisture regains (Sample Group II)

In our previous work, a wool fabric sample (F3 in Table 1) was treated with enzymes [8]. In Table 2, samples E1 and E3 were treated with keratinase, E5 with proteinase, and E7 with both keratinase and proteinase. Samples E2, E4, E6, and E8 were the corresponding controls, respectively. Sample E9 was made by rinsing sample F3 with ultrasonication. These fabric samples had different moisture regain values at the same temperature and humidity [8].

Table 2: Characteristics of Sample Group II, 1x1 stitch.

III: Conventional samples

Samples in Groups I and II were produced for this study. For comparison, 19 conventional samples (18 wool and one wool blend) were collected (Table 3). Samples C1 to C11 were collected according to mean fiber diameter and were knitted fabrics, except C6. Samples S1 to S4 are 2 × 2 twill fabrics with the same yarn count (16.6 × 2 tex, MFD 20.5 μm) but with different fabric finishes to the surface. Samples S5 to S8 also have different fabric finishes. The surfaces of samples S5 to S7 have raised fibers.

Table 3: Characteristics of Sample Group III.

Measurements

q_max measurements: q_max is usually measured with the KES-F7 Thermolab II tester (Kato Tech Co., Ltd., Japan). The sample sizes of Groups I and II were too small for KES-F7; thus, we used the finger sensor unit (Kato Tech Co., Ltd.) of a finger robot developed by Kawabata et al. [9,10]. The finger sensor unit was connected to a compression tester (KES-G5, Kato Tech Co., Ltd.) to measure q_max. Samples were conditioned at 20.0 °C and 65% or 90% relative humidity (RH) for 4 h, and then placed on the temperature-controlled stage at 20 °C. The initial temperature of the contactor (0.785 cm²) was 30.0 °C and it compressed the sample at a speed of 1 mm/s up to 12.48 kPa.

Mechanical and surface properties

Surface test: A surface tester (KES-SE-STP, Kato Tech Co., Ltd.) was used to measure the fabric surface friction. When the stage was moved 5 cm at a speed of 1 mm/s under a force of 0.49 N over the fabric, the friction between the contactor and fabric sample was recorded and the friction coefficient was calculated. The average friction coefficient (MIU) and the average deviation of MIU (MMD) were obtained for effective displacement intervals of 0.5 to 4.5 cm [11]. The means of three replicates were reported.

A surface tester (KES-SE, Kato Tech Co., Ltd.) was used to measure the fabric surface geometrical roughness (SMD) [11]. A single piano wire 0.5 cm in length (φ=0.5mm) was moved at 1 mm/s under a force of 0.098 N over the fabric. The concave–convex thickness change in the fabric was measured. The displacement of the piano wire was 3.0 cm and the effective distance was 2.0 cm. The means of three replicates were reported.

Bending test: For yarn consisting of a number of fibers, N_o, with a mean fiber diameter, d, the yarn bending rigidity, B_Y [12], is

(1)

where E is the Young’s modulus.

When two yarns (Y1, Y2) with the same yarn linear density (T) are compared,

(2)

where ρ is the density of wool. Thus,

(3)

The ratio of the bending rigidity of two yarns is given by

(4)

Equation (4) suggests that, for wool yarns with the same linear density, a larger MFD leads to a higher bending rigidity.

The bending rigidity (B) of a fabric was measured by a pure bending tester (KES-F2, Kato Tech Co., Ltd.). The clamped sample was bent at a maximum curvature of ±2.5 cm^-1 at a constant rate. The sample size was 2.5 × 2.5 cm². The moment–curvature curves were obtained to calculate B.

Three-dimensional surface measurement: The surface fuzz of the fabrics was observed by a three-dimensional measuring macroscope (VR-3100, Keyence Corporation, Japan).

Results and Discussion

Effect of MFD on the fabric bending and surface properties (Sample Group I)

In Figure 1, the bending rigidity (B) is plotted against MFD. The B values of samples F3 and F6 were higher than those of samples F1 and F4 in both the wale and course directions. The ratio of B for two fabrics was not equal to the corresponding ratio of the fiber diameter, d². The MFD dominates the bending rigidity of wool yarn and fabric, as these results showed. Samples with a higher course density showed a larger bending rigidity for the yarn with the same fiber diameter [12-15]. According to equation (1), B_Y is proportional to the number of wool fibers. A higher stitch density means that there are more fibers in the yarn, which increases B_Y, and explains why fabric with a higher stitch density was stiffer than that with a lower stitch density.

Figure 1: Effect of fiber diameter on the fabric bending rigidity, B (1 × 1 rib).

The surface properties (MIU and MMD) are shown in Figure 2. The effect of MFD on MIU was observed for sample F3 in the highdensity wale direction. The value of MIU for F3 was larger than that for F1 and F2, owing to the large MFD of F3. The higher stitch density in the rib stitch fabric resulted in a coarser surface, for example, the higher MIU value for sample F3 than for sample F6 along the wale and course directions. The difference in stitch density for MIU and MMD was greater in 1 × 1 rib stitch fabric than the plain knitted fabrics.

Figure 2: Effect of fiber diameter on the MIU and MMD for (left) rib stitch and (right) plain knitted fabrics.

Macroscope images of fuzz on the fabric surface are shown in Figure 3. The amount of fuzz on the fabric surface was greater for samples with MFD of 24.5 μm. Samples with a higher stitch density had more fuzz than samples with a lower stitch density. According to Euler’s equation, the threshold force required to buckle the fuzz is proportional to four times the fiber diameter. The MMD values reflected the amount of coarse and unbent fuzz on the surface [16].

Figure 3: Macroscope images of fuzz on the fabric surface.

SMD describes the knitted structure, and the SMD values are listed in Table 4. The effect of MFD on SMD is small, although the stitch density is reflected in the SMD values. The rib stitch fabric with a high stitch density showed a larger SMD in the wale and course directions than the low stitch-density fabric. In the plain knitted samples, the SMD of all samples were small so that the effect of both the MFD and stitch density on the SMD was marginal.

Table 4: Surface roughness (SMD, μm) of wool samples F1–F12.

q_max

Effect of fabric moisture regain on q_max: The heat flux (q_max) of Group I samples was plotted against MFD for two ambient humidities (Figure 4). q_max was higher for samples conditioned at 90% RH than that at 65% RH. The average moisture regains of rib stitch fabrics were 12.6% (standard deviation [SD] 0.24%) at 65% RH and 19.2% (SD 0.28%) at 90% RH. For the plain weave fabrics, the moisture regain was 13.4% (SD 0.13%) at 65% RH and 20.4% (SD 0.41%) at 90% RH. The wool conditioned at 90% RH had a higher moisture regain than that conditioned at 65% RH. The increase in the moisture regain was approximately 7.0% for both structures. The thermal conductivities of water and dry wool were 0.580 and 0.0464 W/(cmãÂ?»K), respectively. The results suggested that the ambient humidity affected the moisture regain of wool, which affected the wool’s thermal properties, and higher moisture regain resulted in a larger q_max. Fabric with larger fiber diameters showed lower q_max values for rib stitch fabric with high density. MFD did not affect q_max in plain knitted samples.

Figure 4: Effect of mean fiber diameter on q_max for fabrics conditioned at 65% RH and 90% RH (20°C).

A further study was performed for enzyme-treated fabrics with various moisture regain values. The effect of moisture regain on q_max for Group II samples is shown in Figure 5. In samples conditioned at 90% RH, enzyme-treated samples showed a higher moisture regain than their controls. q_max increased with the increase in moisture regain. The results in Figures 4 and 5 suggest that the moisture in wool had a dominant effect on q_max. Schneider et al. [17] found that for moisture regain of 0–15%, water is strongly bound to the internal structure of wool fiber, and free water is present when the moisture regain exceeds 15%. Moisture regain measured for Sample Group II was 13.5–15% at 65% RH, and 20–22.5% at 90% RH at 20.0°C. The increase of q_max with moisture at 90% RH compared to 60% RH arose from the free water in the fabric, which was confirmed by the results in Figure 4.

Figure 5: Relationship between q_max and moisture regain for enzyme treated fabrics conditioned at 65% RH or 90% RH (20°C).

Surface fuzz and fiber density: q_max is affected by the surface roughness because the heat transfer depends on the contact area. The surface roughness parameter, SMD, indicates contact between the contactor and fabric surface [18]. If the fabric surface layer contains more air for low fiber densities, this should affect the q_max values. In Figure 5, q_max of sample F3 (rib stitch samples with larger stitch density) was lower than for the other samples.

The relationship among MFD, fuzz, and MMD suggests that fabric with a larger MFD has more unbent surface fuzz, which could be characterized by MMD. For fabrics with larger MFD and a larger amount of surface fuzz, the contact area may be smaller, reducing q_max.

Using q_max in fabric design

We used conventional wool fabrics (Sample Group III in Table 3) to investigate how to use q_max for characterization.

The relationship between q_max and thickness is shown in Figure 6 for all samples. The S samples (S1–S8) were finished with a different method to change the amount of fuzz on the surface. Samples S1– S4 was made from the same yarn with the same structure but four different finishes. The smoothest finished sample (S2), which had the least fuzz, showed the largest q_max value. The surfaces of samples S5 and S6 were covered with fibers, resulting in a low q_max. Thickness and surface fibers affect the q_max values, which indicate the warm/cool perception of the fabric.

Figure 6: Relationship between q_max and thickness.

In Figure 7, the q_max values for the fabrics conditioned at 65% RH are plotted against those for fabrics conditioned at 90% RH. A linear correlation (r²=0.858) was observed.

Figure 7: Effect of moisture on q_max (r²=0.858).

Conclusion

The surfaces of wool fabrics are covered by fuzz, which produces a warm feeling or sometimes a cooler feeling. This study focused on the fabric surface and heat transfer through a fabric. Surface characterization was carried out by using a KES surface tester. The physical value, q_max, was used to predict the warmth/coolness of the fabric.

Fabric made from thicker fibers produced a higher MMD. In this fabric, unbent stiff fuzz reduced the contact area with the heated plate and decreased q_max. SMD characterized the fabric structure, such as the stitch density. q_max increased as SMD decreased, indicating that the surface was smoother with less fuzz. Moisture in the fabric increased the q_max value. The q_max value of the sample conditioned at 90% RH was larger than that conditioned at 65% RH. The q_max value for wool fabrics was affected by moisture as well as surface fuzz.

Our results suggest that q_max could be used to characterize the fabric surface finishing effect, especially for fabric that contains moisture, as a fast and sensitive measurement.

Acknowledgement

Chendi Tu thanks China Scholarship Council for the scholarship (File No. 201608050085).

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