Experimental Analysis on Comfort Characteristics of Polyester/Nylon Warp Knitted Spacer Fabric for Shoe Insole Application

Alemu Aduna^*, Sakthival Santhnam and Meseret Bogale
^*Correspondence: Alemu Aduna, Department of Textile Technology Faculty of Textile and Apparel Fashion Technology, Ethiopia, Email:

Keywords

Comfort • Warp knitting • Spacer • Insoles

Introduction

Technical textile products have a great role in many applications. One of the most widely used textiles is spacer fabric products, and methods for producing spacer textile materials. Spacer textile materials naturally comprise two spaced and separated parts of fabric joined by a monofilament spacer yarns that extend within the layers of fabric. This research discusses the development and analyzes the characteristics to rise the comfort for shoe insole application [1]. Made of double layers, top and bottom and interlaced by yarns oriented in the third dimension, the spacer is a real 3D fibrous structure [2]. The fabric layers can be manufactured using various types of materials and structures. The top and bottom fabric layers are connected to each other by a monofilament yarns called spacer yarn. This type of structure with late air and water vapor transfer makes the fabric lighter and this helps to be selected for shoe insole. Mainly, spacer fabrics can be 1–15 mm thicker. Spacer fabrics are produced on double-bed Rachel machines by knitting the top and bottom layer concurrently on each needle bed [3]. The most important properties of spacer fabric lightness, flexibility, porosity, and high stiffness [4] Shoe insoles produced from warp-knitted spacer fabrics are comfortable for the wearer. They are mainly produced from polyuratine foam and other materials previously. [5,6] specially warp knitted spacer fabric applications like shoe insoles and others with the advancement of comfort ability [7-16]. The influencing factors such as thickness, density, air permeability, porosity, and thermal conductivity were measured according to the ASTM Standard for the purpose of shoe insole applications.

Experimental

Materials and Methods

Materials

Raw materials for the study were 100% polyester, 80/20% polyester/ nylon, and 70/30 polyester/ nylon filaments. The face and back layers of the spacer fabrics are formed with 100% polyester multifilament and are connected by the central layer made of monofilament (100% polyester), 80/20% polyester/nylon multifilament and is connected by the internal layer made of monofilament 80/20% polyester/nylon) and 70/30 polyester/ nylon multifilament and is connected by the middle layer made of monofilament (70/30 polyester/ nylon) filaments (Tables 1-3).

Samples preparation

The fabric manufacturing technique and the different types of fabrics produced for the study are explained in this section. The WKSF was produced by Rachel double needle bed warp knitting machine with six guide bars, a gauge of 28E of 170 inches width. Machine speed 1500 rpm. The WKSF development consists of six samples, namely, three hexagonal structures of 100% polyester, 80/20% polyester/Nylon, 70/30% polyester/Nylon with thickness of 2, 2.5 and 3 mm respectively, and the other three were lockknit structures of 100% polyester, rhombic mesh 80/20% polyester/Nylon, hexagonal 70/30% polyester/Nylon with constant thickness of 2.8 mm. Maintaining bottom surface layer as plain structure for all samples. Figure 1 shows the sample preparation. The structure of the face surface layers was selected as hexagonal &rhombic mesh and for the bottom surface layer the structure is maintained as a plain knit structure as shown in (Figure 8), (Figures 1 and 2).

Testing methods

The spacer fabrics are tested according to the ASTM standard. The test includes thickness, density, and air permeability; thermal conductivity and water vapor permeability properties. All tests are carried out under standard atmospheric conditions of 25 ± 2°C temperatures and 65 ± 2% relative humidity.

Thickness

The thickness tester is a specialized equipment to determine the thickness of spacer fabrics. ASTM D 5729 standard is referred.

Areal Density

Areal density of the spacer fabric is an important factor that should be considered and the cost is also directly related to density. Mass per unit area of the spacer fabric will be measured using a weighing balance (ASTM D 3776 - 07) and the areal density of the fabric will be calculated using the following formula

Areal density (g/m²) = (WPcm x CPcm -x-SL x 39.37x D) / (1000x 9000)             (1)

Where wpcm:-wales per centimeter, cpcm: - coarse per centimeter, SL: - stitch length, and D: - denier (count).

Bulk Density

The fabric mass density or fabric bulk density (g/m³) depends on both fabric weight and fabric thickness. The specimen with 50 cm² will be cut out randomly and weighted. Average of 10 observations will be taken for the sample. The bulk density of the fabric will be calculated using the following formula:

Bulk density (g/m³)=Areal Density (g/m²)/Thickness (m)              (2)

Porosity

The number, size, and type of pores are an important factor that should be considered for shoe sole materials in a porous material. The space of course and wale increase the size of pores which in turn increases the airflow. While increasing the fabric loop length, the air permeability also increases since the fabric porosity is increased [8]. WKSF porosity was calculated by using the density of nylon (1.35 g/m³) and polyester filament (1.38 g/m³). The standard test ASTM E 1294-89 for Porosity determination.

Porosity (%) = [1 - Fabric Density (g/m³) / Filament Density (g/m³)] X 100             (3)

Air permeability

This testing method includes the measurement of the air permeability of WKSF. Air permeability is essential parameter for shoe insole application, because it remarkably influences the comfort. It is the degree of air flow passing perpendicularly through a known area under a prescribed air pressure differential between the two surfaces of a material [9]. The air permeability of the spacer fabric will be evaluated based on ASTM Test Method D 737at 10 cm water head. The unit is cc/sec/cm².

Thermal properties

Thermal conductivity is a property of the material that articulates the flow of heat through the material [10,11], the test will be according to Lee's disk instrument (ASTM-D 570). The average of 10 measurements was taken to minimize the possible errors.

Thermal conductivity (λ) Wm^-1 K^-1 = (MSR d (2h+r)) / (A (T 1-T 2) (2h+2r))               (4)

Where,

M = Mass of brass disc (C) in kgs, S = Specific heat of the material of the disc (370 JKg^-1 K^-1)

R = rate of fall of temperature (dT/dt), h = thickness of brass disc in mm, r = radius of the brass disc in mm, d = thickness of the sample in mm, A = area of cross section of a sample in mm². Thermal resistivity is also conducted accordingly.

Water vapor permeability

The water vapor permeability is the amount of vapor transfer although a unit area of a fabric in unit time [12], the following formula is used to get WVP.

WVP (g/m²/24h) = (24xM) / (A x t)                       (5)

Where, M = loss of mass in gm; t = the time between weighing hrs. And A = internal area of the dish in m². The test is conducted with BS 7209 and ISO Standard 11092.

Conclusion

The comfort properties of warp-knitted spacer fabrics made from polyester/ nylon filaments were studied in this paper. The warp knitted spacer fabrics were applicable for shoe insoles to make comfortable than the existing PU foam. The spacer fabrics were produced from different materials with different thickness. Six samples, namely, three hexagonal structures of 100% polyester, 80/20% polyester/Nylon,70/30% polyester/Nylon with thickness of 2, 2.5, and 3mm respectively, and the other three were lock-knit structures of 100% polyester, rhombic mesh 80/20% polyester/Nylon,hexagonal 70/30% polyester/Nylon with constant thickness of 2.8 mm. Maintaining bottom surface layer as plain structure for all samples. The results discovered that the fabric thickness and porosity have a significant impact on air and water vapor permeability. Fabric porosity is the key aspect for permeability, thermal conductivity, and resistivity of warp-knitted spacer fabrics. The spacer fabric with higher thickness has higher thermal resistivity with lower thermal conductivity and lower air permeability. The fabric structure with more pores results in high porosity, good water vapor permeability. Spacer fabric with thickness 2.5 mm proves the best level of thermal conductivity with good air and water vapor permeability. Relating to the fabric face layer structure, more open or closed structures decide on the horizontal pore size of spacer fabrics. The open mesh hexagonal net structure shows good porosity than the other two structures. The results clearly demonstrate that the open structure with long loop and moderate moving number of filaments tests had good air and water vapor permeability. The SAS software shows the thickness and structure affecting the spacer fabric properties. Generally, spacer fabrics are more comfortable, recyclable, and environmentally friend when compared to the existing PU foam.

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