Research Article, J Fashion Technol Textile Eng S Vol: 0 Issue: 1

Analysis of Anisotropic Behaviour of Polyurethane Coated Knitted Fabric

Vesna Marija Poto�?i�?�? Matkovi�?�?^* and Zenun Skenderi

Faculty of Textile Technology, University of Zagreb, Prilaz b. Filipovi�?�?a 28a, Zagreb

Corresponding author :Vesna Marija Poto�?i�?�? Matkovi�?�?
Department of Design and Management of Textiles, Faculty of Textile Technology, University of Zagreb, Prilaz b. Filipovi�?�?a 28a, 10 000 Zagreb, Croatia
Tel: +38513712573; Fax: + 38513712573
E-mail: marija.potocic@ttf.hr

Received: Januray 27, 2015 Accepted: June 11, 2015 Published: June 14, 2015

Citation: Matkovi�?�? VMP, Skenderi Z (2015) Analysis of Anisotropic Behaviour of Polyurethane Coated Knitted Fabric. J Fashion Technol Textile Eng S1:007. doi:10.4172/2329-9568.S1-007

Abstract

Keywords: Anisotropy; Coated knitted fabric; Polyurethane; Elongation; Breaking force

Introduction

Although previous studies suggest that coating reduces the anisotropy of textile materials, this study shows that the polyurethane coating of knitted fabrics increase the anisotropic behaviour of knitted fabrics.

The advantages of the polyurethane layer, as compared to other polymers, are increased resistance to abrasion, tearing, they are water repellent and permeable to water vapour [1,2]. Knitted fabric, as the substrate of the composite, can be designed to meet different physical and mechanical requirements. Regardless of the chosen knitted structure, knitted substrate is always different in a direction of courses, than in a direction of wales; it is impossible to avoid this. This is a foundation for anisotropy. Accordingly, anisotropy cannot be avoided in coated knitted fabrics. It is a great elongation and unpredictable behaviour of coated knitting fabrics under the influence of a force, which rejects producers and users of the coated knitted fabrics, and it seems important to explore this field.

Mechanical properties of coated knitted fabrics are studied [3]. Well researched is woven fabric anisotropy [4,5] and coated woven fabrics anisotropy [6,7]. Studies of the mechanical properties of coated knitted fabrics in different directions are generally limited to multiaxial warp knitted fabrics [8,9]. It has been found that such coated knitted fabrics exhibit anisotropic properties under uniaxial loading. Yet the same knitted fabrics exhibit isotropic behaviour under multiaxial loading. In addition, from the literature [10] is known the anisotropic behaviour of coated single jersey knitted fabric. The anisotropy was very noticeable, maximum measured force was in the direction of the courses, but maximum elongation was in the direction of wales.

Experimental

For this study seven knitted fabrics (warp knitted simplex, charmeuse, power-net, voile and another power-net, and weft knitted single jersey and interlock) were chosen to cover a wide range of properties. The mass of the knitted fabrics is in the range from 47 to 184 g/m2, and thickness is from 0,26 to 0,74 mm. Knitted fabrics were coated with with three different polyurethane coatings (designated PU1 - PU3)under the same conditions, on the same laboratory coating line. The first type of the polyurethane coating (PU1) was prepared from the base of hydrophilic polyester which is used to obtain polyurethane coatings which let water vapor pass through. The second type (PU2) on the polyester base that is used to obtain a coating that lets a moderate amount of water vapor pass through, and the third type of coating (PU3) was obtained from the copolymer of polyester and polyether providing an impermeable compact polyurethane coating. The polyurethane coatings have nearly equal average thicknesses and weights (Table 3). One component polyurethanes were used. Polyurethane was coated on a knitted substrate using transfer process.The polyurethane paste was applied to the backing paper using the pump. Coatings are adjusted with the distance between the knife on which the polyurethane paste is applied and the roller under the knife. The applied polyurethane paste with the paper passes through the dryer where the temperature was adjusted to 80 °C. The other coating of the same polyurethane paste is applied in the same way. The final coating was a polyurethane binding agent on which the knitted fabric was laminated. In the dryer at 160°C the binder was cross linked.

As described above, the total number of coated knitted fabrics is 21 (7 knitted fabrics x 3 polyurethane coatings).

Method used

Measurements were carried out according to EN ISO 1421 Rubber or plastic coated fabrics - Determination of tensile strength and elongation at break, method 1 – strip test. According to this method, 10 test sampleswere measured for each specimen, 5 cut in a direction of courses and 5 in a direction of wales. Each test sample is 50 mm wide. Due to the great elongation of test samples, and in accordance of aforementioned standard, the distance between the claws was set to 100 mm. Preload was 2N. The mean value of force and elongation for each specimen was calculated. Mass of coated fabrics was obtained according to EN ISO 2286-2:2008 for coated surface fabrics, in which it is determined how to separate the coating from the substrate and to weigh 5 samples with a mass per unit area of 100 cm2. Thickness was measured according to EN ISO 5084: 1996i.e. 10 measurements for each fabric type.

Results

In the Table 1 the properties of knitted fabrics (designated S1-S7) as well as their construction are given. Table 2 presents properties of three different polyurethane coatings (designated PU1-PU3).

Table 1: Properties of knitted fabrics (S1-S7).

Table 2: Properties of polyurethane coating (PU1-PU3).

In the Table 3 elongation properties of all abovementioned 21 type of coated fabric (7 knitted substrates x 3 types of PU) is given. However, the t-test shows that there is no statistically considerable difference between the mean values of elongation of the 3 types of PU (p=0.92) or they can be observed as the same type of the polyester coating for the elongation.The t-test shows again that there is no statistically significant difference between the breaking forces of the three tested polyurethane-coated groups of the samples (p=0.93). Considering that Tab. 4presents average properties of 7 types of polyurethane-coated fabrics with different knitted substrate (designated Sc1-Sc7).

Table 3: Elongation properties of all 21 type of coated fabric (7 knitted substrates x 3 types of PU).

The differences in average elongation in the direction of courses and wales of knitted and coated knitted fabrics are shown in Figures 1 and 2.

The differences in average breaking forces in the direction of courses and wales of knitted and coated knitted fabrics are shown in Figures 3 and 4.

Discussion

After coating, knitted fabrics increased in elongation in both directions. Elongation of coated knitted fabrics in the direction of wales increased by an average of 16%, compared to knitted fabric. It is significant that, at the same time, elongation in the direction of courses increased after coating on average 102%. Comparing Fig. 1 and 2 it can be perceived that the elongation in a direction of wales after coating increases very predictably (in average 16%), while the mechanism of increase of elongation in a direction of courses is more complex and far larger (102%) [3]. Precisely because of the large increase in elongation of knitted fabric in the direction of courses, elongation anisotropy of coated knitted fabrics averagely increases, rather than decreases (Figures 1 and 2).

The average percentage difference in the elongation of samples of knitted fabrics in a direction of courses and wales was 48%; after coating elongation increased to 91% (Tables 1 and 4). It is interesting that in samples S1 and S2, which showed the lowest anisotropy, anisotropy increased, due to increased elongation of samples in the direction of courses, after coating. Actually, all the samples which have similar elongation by wale and course (S1 and S2), or higher elongation in a direction of course (S6, S7), after coating anisotropy is higher than before. Anisotropy was reduced only in samples which showed significantly lower elongation in the direction of courses of knitted fabric, than in the direction of the wales (samples S4 and S5).

Table 4: Average properties of polyurethane-coated knitted fabrics (S1c-S7c).

These results suggest that if one wishes to avoid the different elongation of coated knitted fabrics in the direction of courses and wales, should not be guided with the same logic as while coating woven textiles. When coating the woven fabric one would normally choose the fabric showing the smallest anisotropy and after coating the anisotropy will be further reduced. When coating knitted fabrics it is necessary to choose knitted fabric that initially shows a large anisotropy and lower elongation in the direction of courses, because elongation will increase after coating in this direction, and thereby the anisotropy will be reduced.

All the tested knitted fabrics shows relatively high anisotropy of breaking forces in a direction of wales and courses – it is 31% in average (Table 1). Coating increases breaking forces in a direction of wales averagely 24%, and in the direction of courses averagely 27%.Anisotropy of breaking forces after coating is not lower, but higher. It increases from 31% to 38% (Tables 1 and 4).

The results (Figures 3 and 4) show that the breaking forces in both direction increases relatively accurate but similarly. That is why it is difficult to predict whether anisotropy will after coating reduce or increase for the specific sample.

Conclusion

After coating, fabrics increased elongation in the direction of wales and courses. Elongation of coated knitted fabrics in the direction of wales was increased by an average of 16% compared to elongation of knitted fabric. It is significant that elongation in the direction of courses was increased after coating by an average of 102%. Because of the large increase in the elongation of fabrics in the direction of courses after coating, anisotropy of the elongation of coated knitted fabrics on average increases, and not decreases as previously reported.

After coating, breaking forces in the direction of wales increases in average 24%, and in the direction courses approximately 27%. By coating, the average anisotropy of breaking forces is not reduced, but the difference of the breaking forces in the direction of the courses and wales increases from 31% to 38%.

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