The use of a polytrimethylene terephthalate/polyester bi-component filament for the development of seamless garments

Abstract

Seamless garments are popular knitted products nowadays. They are widely used as power stretchable underwear, outerwear and sportswear for which the use of high elastic yarns is indispensable . For seamless garments knitted with circular seamless machines, polyurethane (PU)/polyamide (PA) core-spun yarn is commonly used as the base yarn in a plating fabric to enhance the elasticity of the garment. However, the use of PU/PA core-spun yarn has encountered some problems, such as the difficulty in size control and the ageing of PU elastic yarn. In this study, the polytrimethylene terephthalate (PTT)/polyester (PET) bi-component filament was used to replace the PU/PA core-spun yarn for the development of seamless garments due to its excellent extensibility and elastic recovery property. The manufacturing processes, including knitting, pre-treatment, dyeing and finishing, were developed, and the end-use performance of the fabrics, which were respectively produced with the PTT/PET bi-component filament and PU/PA core-spun yarn as the base yarn and PA fiber as the face yarn for seamless garments, was compared based on the five different mechanical and physical properties. The results show that the fabric made of the PTT/PET bi-component filament has much better dimensional stability, much better elastic recovery property and much better wrinkle resistance than those of the fabric made of PU/PA core-spun yarn. It is expected that this study could help us to further exploit the use of the PTT/PET bi-component filament in the development of seamless garments in the future.

Keywords

polytrimethylene terephthalate/polyester bi-component filament seamless garment elastic recovery end-use performance

Seamless garments are a special kind of knitted product produced without sewing stitches along the neck, waist or hip lines. As “one-step-molding garments”, seamless garments are widely used as power stretchable underwear, outerwear and sportswear. Fully fashionable seamless garments are full of comfort and softness and give wearers a sense of fitting, vigor, grace, fashion and diversity. As high-end knitted products, seamless garments have entered the mainstream knitwear market. To date, special weft and warp knitting technologies have been developed to produce seamless garments that can meet different consumers’ requirements.^1,2

Among many requirements by consumers, comfort, fitness and extensibility are the most important properties for seamless garments, particularly for seamless underwear and sportswear. In order to meet these requirements, the use of high elastic yarns is indispensable. For seamless garments knitted with circular seamless machines, polyurethane (PU)/polyamide (PA) core-spun yarn is commonly used as a base yarn in the plating structure to enhance the elasticity of the garment. However, the use of PU/PA core-spun yarns in the manufacture of seamless garments will bring some problems. The most important problems are the size control and the ageing of PU elastic yarn. Because of the difference in elasticity of PU/PA core-spun yarn, the size control becomes very difficult during the knitting process. Even for the PU/PA core-spun yarns from the same batch number but with different colors, their shrinkage can be different from each other. This makes the knitting process very difficult to control. In addition, the ageing of PU elastic yarn can result in the reduction of elasticity of the garment during use. In order to solve these problems, the polytrimethylene terephthalate (PTT)/polyester (PET) bi-component filament was proposed to replace PU/PA core-spun yarn for producing seamless garments in this study.

PTT is a new kind of polymer material made by the poly-condensation reaction of purified terephthalic acid (PTA) and 1,3-propanaediol (PDO). As a kind of elastic fiber and a member of the PET family, it has better cost effectiveness than PU elastic yarn and better dyeing ability than PET.³ PTT also has the best elastic recovery and anti-fatigue properties among all fibers. These special properties make PTT fiber an ideal substituent for PU elastic fibers used in many kinds of textile products where high elasticity is required. Some research work has been done in China for this purpose. Yuan and Wang⁴ studied the elasticity of PTT warp knitted elastic fabric and found that PTT warp knitted fabric has excellent resilience, which makes it very suitable for swimsuit use. Li and Wang⁵ compared the elasticity between PTT and PU/PA core-spun yarn woven fabrics and found that PTT fabrics have a better tight fitting effect with the body than the PU/PA core-spun yarn fabrics. Liu et al.⁶ studied the effect of PTT fiber on the elasticity of knitted fabric and found that the elastic elongations of all kinds of PTT fabrics are higher than those of common chemical fiber fabrics, but lower than those of PU elastic fiber fabrics. They also found that under the low load condition, the PTT fabrics exhibit a higher recovery and a lower plastic deformation than the commonly used chemical fiber fabrics. Zhao and Hu⁷ used fuzzy-integrative judgment to assess the end-use performance of five knitted fabrics made with different PTT blended yarns, and found that the fabrics knitted with PTT/wool/Tencel and PTT/bamboo blended yarns have the best end-use performance for sweater applications.

Although PTT fiber is a member of the PET family, its price is much higher than that of PET fiber. In addition, pure PTT fiber is too soft and the texture of fabrics knitted with pure PTT fiber is not as good as that with PET fiber. For this reason, the PTT/PET bi-component filament should be a better choice than pure PTT fiber for garment product development. Sun et al.⁸ investigated the properties of wool-like fabrics made with the PTT/PET bi-component filament and found that the PTT/PET wool-like fabrics have a similar property in the softness, but better elastic recovery and wrinkle recovery properties than pure wool fabrics. Li et al.⁹ tested and analyzed the tensile properties of 33 types of woven fabrics made with the PTT/PET bi-component filament and found that PTT/PET fabrics have high breaking elongations but low tensile strengths. Under the same condition, increasing the content of the PTT/PET filament in the weft yarns decreases the tensile strength but increases the breaking elongation. They also found that the twist of the PTT/PET filament has a significant effect on the tensile properties of the fabric under low load. Although many studies have been done for PTT/PET bi-component filament fabrics, the use of the PTT/PET bi-component filament in seamless product development has not been found in the literature.

This paper presents a study of using the PTT/PET bi-component filament in the development of seamless underwear products to replace the commonly used PU/PA core-spun yarn. The tensile properties of the PTT/PET bi-component filament are firstly discussed and compared with those of the PU/PA core-spun yarn. Then the knitting and dying processes are presented. Finally, the end-use performance in terms of dimensional stability, elastic recovery property and other properties is compared between the fabrics respectively knitted with the PTT/PET bi-component filament and PU/PA core-spun yarn as the base yarn in a plating structure. It is expected that this study can provide some useful information for the further exploitation of this new kind of fiber material in seamless garment use.

The polytrimethylene terephthalate/polyester bi-component filament and its tensile properties

Structure

The PTT/PET bi-component filament is produced with both PTT and PET disposed side by side during a spinning process. The ratio of PTT and PET can be varied according to the requirement. The commonly used ratio is 50/50. A cross-sectional microscopic picture of a PTT/PET bi-component filament dyed at 100℃ is shown in Figure 1, from which the disposition of PTT and PET can be clearly observed. As the glass temperature of PTT is lower than that of PET, PTT is easier to dye at a lower temperature than PET. In Figure 1, the red part is PTT and the white part is PET. In this way, the PTT/PET bi-component filament can be easily identified from PET.

The first PTT/PET bi-component filament is T400, which was commercialized by the DuPont Company. Other PTT/PET bi-component filaments can also be found in the market, such as PTT/PET bi-component filaments with fineness from 40 to 600 D produced by Haining Xingao Fibers Co., Ltd in China. The trademark of their products is CM800. In this study, 50 D PTT/PET bi-component filament is selected to replace the commonly used PU/PA core-spun yarn (20/30 D) for the development of seamless underwear. Because of this, when discussing the tensile properties of the 50 D PTT/PET bi-component filament, a comparison with PU/PA core-spun yarn (20/30 D) is also necessary.

Extensibility and elastic recovery property

Figure 2 shows the tensile curves of the 50 D PTT/PET bi-component filament and PU/PA core-spun yarn (20/30 D) before and after the treatment with boiling water for 30 minutes, hereinafter referred as heat treatment. The tensile test was carried out on a YG061 Single Fiber Electronic Tensile Strength Tester according to Chinese Standard FZ/T 01034. It can be seen that for the PU/PA core-spun yarn, the effect of the heat treatment on the tensile property is very small, as the curves before and after heat treatment are very close. However, for the PTT/PET bi-component filament, the effect of the heat treatment is very significant. After the heat treatment, the strain of the PTT/PET bi-component filament at break is obviously increased. At the same time, the capacity to withstand the tensile load is reduced. This implicates that the heat treatment makes PTT/PET bi-component filament easier to extend and more stretchable. The reason for this is that the PTT/PET bi-component filament becomes more crimped after the heat treatment. As shown in Figure 3, when the PTT/PET bi-component filament is treated with boiling water, the difference in PTT and PET heat shrinkage will cause the curing of the filament and, therefore, the crimps and coils are formed along the filament. This phenomenon, which is referred to as self-crimping, is determined by natural formation rather than mechanical action as PU/PA core-spun yarn.

From Figure 2, it can also be seen that the tensile curves of the PU/PA core-spun yarn are almost linear from the initial point to the breaking point. However, the tensile curves of the PTT/PET bi-component filament are obviously non-linear. The slopes of the curves become smoother after about 2% elongation, particularly for the PTT/PET bi-component filament after the heat treatment. This implies that the increase of the tensile loads becomes very slow with the increase of strain, which makes the garment more comfortable under extension. From Figure 2, it also can be found that the strains at break of the PTT/PET bi-component filament before and after heat treatment are about 28.5% and 32%, respectively, much higher than those of PU/PA core-spun yarn (23%).

Besides the extensibility, the elastic recovery property under the repeating uses is also a very important requirement for seamless underwear. For this reason, the cycling tensile tests at a constant elongation, which simulated the wearing condition in a daily life, were also conducted to compare the elastic recovery property of the PTT/PET bi-component filament and PU/PA core-spun yarn. The tests were carried out according to Chinese Standard FZ/T 01034 with the following testing conditions: pre-tension: 1 gram; applied elongation: 12%; clamping length: 250 mm; speed of the jaw: 1000 mm/min; repeating cycles: 150.

The repeating testing results are shown in Figure 4. It can be seen that the PTT/PET bi-component filament has much better elastic recovery capacity than that of PU/PA core-spun yarn. Particularly after heat treatment, the elastic recovery capacity of the PTT/PET bi-component filament is significantly improved and its elastic recovery rate becomes almost constant after 85 cycles of testing. This is due to the special macromolecular structure of PTT. Although PTT and PET belong to the same family, their macromolecular configurations are different. While the macromolecular structure of PET has almost linear configuration, the macromolecular chains of PTT are in zigzag form, which makes PTT function like a spring. After the heat treatment, the coiled trimethylene unit in the crystalline part will be released and the internal stress is eliminated. At the same time, the unevenness that occurred during spinning will also be cleaned away. Most importantly, the zigzag molecular chains of PTT make the PTT/PET bi-component filament endure the repeating stretching actions.

The above analysis shows that the PTT/PET bi-component filament can be an ideal fiber material to replace the commonly used PU/PA core-spun yarn for seamless underwear due to its high extensibility and excellent elastic recovery capacity.

Knitting process

Knitting machine

The seamless underwear developed was knitted on a single circular SANTONI SM8 TOP 2 seamless machine equipped with eight knitting systems. This type of machine is widely used in China for producing seamless products. The machine gauge selected was E28.

Fabric structure

The commonly used knitted fabric structures for seamless underwear include plain stitch, pile stitch, plating stitch and mock rib stitch. As TOP 2 is a single face machine, the real rib fabric structure cannot be knitted on it. So mock rib stitches with a similar appearance to real rib are usually used in the areas of the waist, bottom hem, shoulder or chest. The mock rib stitches are formed by incorporating elastic yarns in the plain knitted fabric structure along the course direction. They can be 1 ×1, 2 × 1, 3 × 1, 3 × 2 etc., where the former number refers to the number of working needles and the latter number refers to the number of non-working needles. All the fabrics are plated to form the loops, no matter plain stitch or mock rib stitch. In this study, plating fabric based on plain knitted structure was used, as shown in Figure 5.

For the seamless underwear, PA yarn or PU/PA core-spun yarn are commonly used as the base yarn, and PA, cotton, Tencel, Modal, etc. are commonly used as the face yarn. As two kinds of yarns are knitted in the same plating fabric, the “two-face effect” is obtained. In this study, the face yarn used was the PA filament and the base yarn used was the PTT/PET bi-component filament. For the comparison, the PU/PA core-spun yarn was also used as the base yarn. Thus, two kinds of fabric were knitted with the same face yarn and different base yarns, as listed in Table 1.

Table 1.

Yarns used

Fabric	Face yarn	Base yarn
Fabric 1	PA filament (70 D)	PTT/PET bi-component filament (50 D)
Fabric 2	PA filament (70 D)	PU/PA core-spun yarn (20/30 D)

PA: polyamide; PTT: polytrimethylene terephthalate; PET: polyester; PU: polyurethane.

Knitting process control

Compared with the PU/PA core-spun yarn, knitting with the PTT/PET bi-component filament is easier. This is because special yarn tension control devices are not needed. However, the effect of heat shrinkage on the garment size and tightness should be taken into consideration when selecting fabric density and loop length. In this study, the same loop length, that is, 3.45 mm, which was used for producing commercial seamless garment, was selected at the knitting stage for both the PTT/PET bi-component filament and PU/PA core-spun yarn fabrics after a series of knitting tests. However, due to different elastic recovery properties of these two yarns, their loop lengths would be changed differently after the knitting process. After dry-relaxing for 24 hours, the loop length of the PTT/PET bi-component filament fabric is nearly the same, but the loop length of the PU/PA core-spun yarn fabric was reduced to 2.95 mm, which corresponded to a variation of about 14.5%. However, after the heat treatment, the variation of the loop length of the PTT/PET bi-component filament fabric was higher than that of the PU/PA core-spun fabric because the PTT/PET bi-component filament had a higher shrinkage effect than the PU/PA core-spun yarn. This was true because the loop length of the PTT/PET bi-component filament fabric was changed to 2.35 mm, which corresponded to a variation of 31.9%, higher than the variation of the loop length in the PU/PA core-spun yarn fabric, which was 23.2%. The different change in loop length would cause a different change in fabric density, garment size and tightness Therefore, the correct selection of loop length to obtain required fabric density, garment size and tightness is very important at the knitting stage.

The common problem that occurs during knitting plating stitch is “yarn skip”, that is, the base yarn skipping on the fabric face, as shown in Figure 6. The yarn skip can damage the appearance of the fabric, particularly in the case where the color and characteristics of the face and base yarns are different. To avoid this problem, the following knitting process controls were adopted in this study.

The first way was to control the yarn feeding position. As mentioned before, there are eight knitting systems on TOP 2 seamless machine, and each knitting system has eight yarn feeders. While feeders No. 1–3 are tipping yarn feeders, feeders No. 4–8 are flat yarn feeders. Usually, feeders No. 1 and 2 are used for the base yarn, and No. 4–6 are used for the face yarn. The correct position of the face yarn and base yarn during the knitting process is shown in Figure 7. In order to keep this correct position, both the horizontal and vertical feeding angles of the face yarn should be kept smaller than those of the base yarn by correct yarn feeder adjustment. As shown in Figure 8, feeder No. 6 was used for the face yarn, whose vertical feeding angle was 15°, and feeder No. 2 was used for the base yarn, whose vertical feeding angle was 25°. In this way, the correct positions of both face and base yarns could be assured.

The second way was to control the yarn feeding tension. According to the previous study,¹⁰ the tension of the base yarn should be adjusted to be higher than that of the face yarn. Since many factors, such as the yarn friction coefficient and rigidity, can affect yarn input tension, the correct yarn tension adjustment is not easy. In this study, a KTF device was used to adjust the yarn tension with correct values. The face yarn was PA (70 D), and its yarn input tension adjusted was 1.47 cN. The base yarn was PU/PA core-spun yarn (20/30 D) and PTT/PET bi-component filament (50 D), respectively. According to Zhang,¹⁰ their tension should be adjusted to be higher than that of PA. For the PU/PA core-spun yarn, its yarn input tension adjusted was about 2.94 cN. However, for the PTT/PET bi-component filament, the situation was different. Since the elastic recovery capacity of the PTT/PET bi-component filament could only be brought into play after the heat treatment, its yarn input tension could not be adjusted to a higher value. Otherwise, higher yarn tension during the knitting could cause a higher shrinkage of the fabric and make the fabric too thick and harsh. After a series of tests, a suitable yarn input tension was obtained for the PTT/PET bi-component filament, which was 1.76 cN. With the correct controls of the yarn feeding position and yarn input tension, the yarn skip problem was avoided during the knitting process. Therefore, the quality of the fabric was assured.

Pre-treatment, dyeing and finishing processes

Pre-treatment

The above knitted fabrics for seamless underwear were first subjected to a pre-treatment with the following condition in washing. The process is schematically shown in Figure 9:

W100 (detergent): 5 ml (water 10 L);

temperature: 80–90℃;

time: 30 min;

overflow rinse at 80℃;

one rinse cycle at 60℃.

Then, a pre-heat setting was performed at 150℃ for 30 s to make the fabrics have sufficient stability during the dyeing process.

Dyeing and heat-setting

The two-baths-two-steps method was used to dye the fabric made of the PTT/PET bi-component filament as the base yarn and the PA filament as the face yarn. The procedure includes the following steps: (1) dyeing of PTT/PET bi-component filament; (2) reduced washing; (3) dyeing of PA fiber; (4) heat setting.

1) Dyeing of the PTT/PET bi-component filament

Based on the previous study,¹¹ disperse dye ANOCRON Red S2B, which was specially developed for PTT fiber and its blends by Shanghai ANOKY Textile Chemicals Co. Ltd, was selected to dye the PTT/PET bi-component filament. Compared to other disperse dyes, ANOCRON has an outstanding dyeing property, excellent washing fastness and compatibility, good sample repeatability and a low shade band. The dyeing curve for the PTT/PET bi-component filament part is shown in Figure 10. The recipe used is as follows: ANOCRON Red S2B (L): 2%; chelating agent: 1-2 g/L; disperse agent 509 A: 0.4 g/L; buffer adjustment pH: 4.5–5. The results have shown that the PTT/PET bi-component filament dyed at 120℃ with ANOCRON Red S2B disperse dyes has a color fastness above level 4. A good solid effect could also be obtained at 110–120℃ with low loss of elasticity.¹²

2) Reduced washing

A reduced washing should be undergone after dyeing of the PTT/PET bi-component filament. On the one hand, the color loss on the fabric surface will affect the hand feel. It makes the fabric harsh and stiff. On the other hand, PA fiber is also dyed with disperse dye, but with a light shade and poor colorfastness. A thorough reduced washing will remove some dyestuff and prepare for the burling bath of PA fiber to makes sure that the color can meet the requirements of customers. The curve of the reduced washing and rinsing cycle is shown in Figure 11. The recipe used for the reduced washing is: Na₂S₂O₄: 2 g/L; NaOH: 2 g/L; liquor ratio: 1:30. The recipe for rinsing is: soda: 2 g/L; Na₂CO₃: 2 g/L; liquor ratio: 1:50.

3) Dyeing of the PA fiber part

PA fiber is one of the most-used counterpart fibers for seamless products. Its dyeing temperature is lower than 100℃. Acid dye was used to dye PA fiber in this study. The dye curve of the PA fiber part is shown in Figure 12. The recipe used is as follows: weak acid dye Red 10B: 2%; pH: 5–6: leveling agent: 0.5 g/L.

4) Heat setting

After dyeing, all the fabrics were subjected to a heat setting at 150℃ for 1 minute and a softening treatment at 150℃ for 30 minutes.

Comparison of end-use performance

In order to assess the end-use performance of the finished fabrics produced, the following five physical and mechanical properties, which can affect the final uses of seamless garments, were tested and compared.

Dimensional stability in washing

The dimensional stability is a very important property for knitted products, particularly for seamless garments. In order to assess the dimensional stability of the finished fabrics respectively made of the PTT/PET bi-component filament and PU/PA core-spun yarn as the face yarn, a washing test was carried out on a fabric shrinkage tester, FOM71MP-LAB, according to ISO 6330 2000. The sample size was 20 cm × 20 cm, and the washing temperature and time were 40℃ and 8 minutes, respectively. Ten times of washing were carried out. After washing, the fabrics were dried at 60℃ in an oven.

Figure 13 shows the size change of the fabrics in both the crosswise and lengthwise directions. It can be seen that the size of the PTT/PET bi-component filament fabric finished after the heat treatment is almost unchanged in both crosswise and lengthwise directions from the first washing, while for the PU/PA core-spun yarn fabric, its size only becomes stable after five times of washing. This implicates that the dimensional stability of the PTT/PET bi-component filament fabric is better than that of the PU/PA core-spun yarn fabric.

Elastic recovery property

As mentioned before, the elastic recovery property is a very important property for seamless garments. In this study, the elastic recovery property of two finished fabrics was assessed under two conditions, that is, before and after chlorine treatment.

Elastic recovery property before chlorine treatment

Table 2 shows the elastic recovery rates of these two fabrics after 200 times of repeating tensile tests using a YG061-1500 tester according to Chinese Standard FZ/T 01034. The testing conditions were as follows: elongation: 30%; speed of jaw: 1000 mm/min; clamping length: 50 mm; pre-tension applied: 3 g. Five samples without chlorine treatment were used for each kind of fabric.

Table 2.

Elastic recovery rates of fabrics after 200 times of repeating tensile tests

Fabric	Elastic recovery rate (%)
Fabric	Sample 1	Sample 2	Sample 3	Sample 4	Sample 5
Fabric 1 (X1)	54.3	50.5	63.0	58.8	57.5
Fabric 2 (X2)	46.5	44.4	45.4	44.6	48.7

The results in Table 2 show that the PTT/PET bi-component filament fabric (Fabric 1) has a better elastic recovery property than the PU/PA core-spun yarn fabric (Fabric 2). In order to confirm this, a statistical analysis was conducted. The analysis includes two steps. The first step is to test if the distributions of the data obtained obey a normal distribution, and the second step is the significance test using the T t(n − 1) method.

Step 1: David testing for normal distribution

Hypotheses: H₀: samples obey the normal distribution; H₁: samples do not obey the normal distribution.

The calculated values of some statistical parameters required for the analysis are listed in Table 3.

Table 3.

Calculated values of some statistical parameters

Fabric	Average	Maximum value	Minimum value	Range (R)	Standard deviation (S)	R/S
Fabric 1	56.8%	63.0%	50.5%	12.5	4.71	2.65
Fabric 2	45.9%	48.7%	44.4%	4.3	1.76	2.44

At the significance level α = 0.05, when sample size N = 5, the upper and lower limits to the critical values are as below:

D_{1, 0 . 0 5} [5] = 2.15 D_{2, 0 . 0 5} [5] = 2.753

Since the calculation values are between the upper and lower limits to the critical values, the testing data obtained obey the normal distribution.

Step 2: Significance testing

The significance testing was carried out under the same condition for two kinds of fabrics using T − t(n − 1) method for the average value μ with an unknown standard deviation.

The testing values were coupled to be analyzed as below:

make ζ = ξ - η, than ζ - N (μ_{1} - μ_{2}, σ_{1}^{2} + σ_{2}^{2}); Z_{i} = X 1 - X 2 (i = 1, 2, 3, 4, 5) is the group testing values of ζ; then Z_{1} = - 7.8, Z_{2} = - 6.1, Z_{3} = - 17.6, Z_{4} = - 14.2, Z_{5} = - 8.8 .

The average value of Z_i : Z =10.9

Hypothesis: HO: μ = μ _O = 0 H1:μ ≠ μ _O = 0, when σ² is unknown.

The modified standard deviation of samples is S² = 23.31:

\frac{Z - μ_{0}}{S / \sqrt{n}} = \frac{10.9 - 0}{23.31 \sqrt{5}} = - 5.06

At the significance level α = 0.05, the value from the T-distribution table is as below:

T_{1 - α / 2} (5 - 1) = T_{1 - 0 . 0 25} (4) = 2.77

Since the absolute value of −0.56 is bigger than 2.77, hypothesis H₀ is rejected. This means that the elastic recovery rates of these two materials are significantly different. Since the average of the elastic recovery rate of the PTT/PET bi-component filament fabric is higher than that of PU/PA core-spun yarn fabric, the elastic recovery property of PTT/PET bi-component filament fabric is better than that of PU/PA core-spun yarn fabric.

Elastic recovery property after chlorine treatment

Seamless garments used as underwear, swimwear and Yoga suits are widely required by the market nowadays. In particular, swimwear made of PTT fiber can be used to replace swimwear made of PU fiber to improve the resistance to chlorine, because PU fiber is prone to fatigue and age under the presence of chlorine. On the other hand, elastic recovery is a very important required property for seamless garments. Under these considerations, the elastic recovery property of fabrics made of the PTT/PET bi-component filament and PU/PA core-spun yarn as the base yarn was also assessed after a chlorine treatment with a 2% sodium hypochlorite solution for 2, 4 and 6 days at room temperature. The treated fabrics were subjected to a cycling tensile test at a constant elongation according to Chinese Standard FZ/T 01034. The testing conditions were set as follows: repeating cycles: 150 times; elongation: 20%; tensile tester: YG061-1500; speed of jaw: 1000 mm/min; clamping length: 50 mm; pretension: 3 g. Considering the wearing condition of a one-piece swimming suit, under which the fabric undergoes more repeating stretch actions in the longitudinal direction than in the crosswise direction, the cycling tensile test was only carried out in the longitudinal direction.

The testing results are shown in Figure 14. It can be seen that the fabric made of the PTT/PET bi-component filament as the base yarn has much better elastic recovery property than that of the fabric made of the PU/PA core-spun yarn as the base yarn. After six days of the chlorine treatment, the elastic recovery rate of the PTT/PET bi-component filament fabric after 150 cycles of testing is still maintained at about 70%, much higher than that of the PU/PA core-spun yarn fabric, which is 42%. From Figure 14, it can also be seen that the reduction speed of the elastic recovery capacity of the PU/PA core-spun yarn fabric with repeating cycles is much faster than that of the PTT/PET bi-component filament fabric. This implicates that the PTT/PET bi-component filament fabric has better resistance to chlorine and is more suitable for use as swimwear.

Fabric surface friction properties

Fabric surface friction properties are related to the handle of the garment. The complex concept of handle may be analyzed as the interaction between simple attributes of fabric quality, such as firmness, fullness, crispness and hardness, smoothness or sleekness. These properties are mainly determined by the surface friction properties of fabric.

There are two measuring heads in the Kawabata Evaluating System (KES-G5) to test the surface property of a fabric. The first measuring head is called the resistance head. It imitates human fingerprints and is composed of 10 fine steel wires in a size of 0.5 mm to form a plane. While testing, the plane slides on the fabric surface under a specified pressure to obtain the coefficient of kinetic friction of the fabric. The second measuring head is a straight-flanked ring. While testing, the straight-flanked ring contacts the fabric, and has a relative movement with the fabric on the lengthwise direction. Due to the rugged fabric surface, the ring will reciprocate upward and downward; the displacement represents the change of the fabric thickness.

In general, there are three indexes to represent the surface friction properties, as shown in Equations (1)–(3):

Coefficient of friction (MIU) : MIU = \frac{1}{x} \int_{0}^{x} μ dx

(1)

Mean deviation of MIU (MMD) : MMD = \frac{1}{x} \int_{0}^{x} | μ - MIU | dx

(2)

Geometrical roughness (SMD) : SMD = \frac{1}{x} \int_{0}^{x} | T - \bar{T} | dx

(3)

Two kinds of finished fabrics were tested for 10 times with the KES; the results are listed in Table 4. It can be seen that the coefficient of friction of two kinds of fabrics is almost the same. This is because the face yarn of these two fabrics is the same, that is, PA fiber. However, as the base yarns are different, the geometrical roughness varies. The PTT/PET bi-component filament fabric is rougher than the PU/PA core-spun yarn fabric. This originates from high crimping of the PTT/PET bi-component filament after the heat treatment, and the difference lies in the irregular spire structure along the molecular chain of the PTT/PET bi-component yarn. After heating, the yarn contracts, and its surface becomes uneven and rough, which makes uneven loops on the fabric surface and results in a poor appearance of the PTT/PET bi-component filament fabric.

Table 4.

Surface friction properties of the fabrics developed

Yarns used	Coefficient of friction (MIU)	Frictional roughness (MMD)	Geometrical roughness (SMD)
PA + PTT/PET bi-component filament	3.18	2.33	3.62
PA + PU/PA core-spun yarn	3.17	1.70	1.41

PA: polyamide; PTT: polytrimethylene terephthalate; PET: polyester; PU: polyurethane.

Thermal resistance and water-vapor resistance

In order to evaluate the comfort of the fabrics and imitate the sense of human skin touching the fabric, thermal resistance and water-vapor resistance properties were tested using a YG606G tester fabricated by Ningbo Textile Instrument Factory according to Standard ISO11092 1993: Textiles – Physiological effects – Measurement of thermal and water-vapor resistance under steady-state conditions (sweating guarded-hotplate test). The sample size used was 350 × 350 mm² and the central area of the test plate was 200 × 200 mm². While the temperatures in the upward and downward sides of the tester were the same, say, 35℃, the relative humidity of the upward and downward sides of the tester was different. The upward side was 40%, while the downward side was 100%. This difference formed a barometer difference and thus the water-vapor resistance could be measured.

Thermal resistance (R_ct) is a heat property and a measure of the temperature difference by which an object or material resists a heat flow under the condition of stable temperature gradient (heat per time unit or thermal resistance). The testing results of the thermal resistance for the PTT/PET bi-component filament fabric and PU/PA core-spun yarn fabric are 13.92 and 15.09 (10^–3 M²K/W), respectively. Compared with the thermal resistance of other knitted fabrics, 29–36 (10^–3 M²K/W) for the ordinary rib fabric made of 75 D PET + 20 D PU and 81–88 (10^–3 M²K/W) for the fleece fabric made of 75 D PET, the thermal resistance of the two fabrics developed in this study is much lower. This means that these fabrics are more suitable for cool seamless garments, particularly for spring and summer garments.

Physical exercises make one sweat. The lower the water-vapor resistance value, the better the respiratory property of fabric. Water-vapor resistance (R_et) is the ratio of moisture vapor pressure differences and the moisture vapor transmission quantity of per unit area of both sides of the fabric sample. Usually, the R_et value is between 148.7 and 3.9 (M²Pa/W). When the R_et value of a garment is smaller than 6 (M²Pa/W), its moisture vapor transmission is excellent, and the garment is suitable for extreme sports. When the value of a garment is larger than 30 (M²Pa/W), the micro-environment between the skin and the garment is airtight or badly ventilated.

The testing results of the water-vapor resistance for the PTT/PET bi-component filament fabric and PU/PA core-spun yarn fabric were 1.79 and 1.43 (M²Pa/W), respectively. As the values of the two fabrics are very low, they are very suitable for sportswear.

Wrinkle resistance

Wrinkle resistance refers to the ability of the fabric to recover from wrinkles. The wrinkle resistance of the fabrics developed was tested on a LFY-1B crease recovery angle tester fabricated by Ningbo Textile Instrument Factory in China according to standard ISO2313. Ten tests (five for the warp direction and five for the weft direction) were performed for each kind of fabric sample. The overall crease recovery angle in degree was the sum of both the warp and weft crease recovery angles. The results are listed in Table 5. It can be seen that both kinds of fabrics have better wrinkle resistance in the warp direction than in the weft direction. This is because two kinds of fabrics were knitted with the same structure. However, the wrinkle resistance of the fabric made of the PTT/PET bi-component filament is much better than that of fabric made of PU/PA core-spun yarn due to higher crease recovery angles in both weft and warp directions. The reason for this is that the PTT/PET bi-component filament has a better elastic recovery property than the PU/PA core-spun yarn.

Table 5.

Wrinkle resistance of the fabrics developed

Yarns used	Crease recovery angle in the weft direction (degree)	Crease recovery angle in the warp direction (degree)	Overall crease recovery angle (degree)
PA + PTT/PET bi-component filament	69.62	156.26	225.88
PA + PU/PA core-spun yarn	57.53	113.20	170.73

PA: polyamide; PTT: polytrimethylene terephthalate; PET: polyester; PU: polyurethane.

Conclusions

The PTT/PET bi-component filament was selected to replace the PU/PA core-spun yarn for the development of seamless garments. The tensile and elastic recovery properties of two kinds of yarns were firstly tested and compared. Then, the manufacturing processes, including knitting, pre-treatment, dyeing and finishing, were developed for the production of plating knitted fabrics for seamless garment use. The end-use performance of the fabrics produced was finally compared based on five different mechanical and physical properties. According to the study, the following conclusions could be drawn.

The PTT/PET bi-component filament has an excellent extensibility and elastic recovery property. It can be used to replace the widely used PU/PA core-spun yarn for the development of seamless garments.

The knitting process of the PTT/PET bi-component filament is much easier controlled than that of the PU/PA core-spun yarn due to easier yarn tension control. However, as the elastic recovery and extensibility of the PTT/PET bi-component filament are only released after treatment with heat and moisture, the high shrinkage effect of the filament should be taken into consideration at the knitting stage to make sure that the finished product has the correct size and weight. A method that can be used is to adjust the loop length and to keep yarn tension as low as possible.

The two-baths-two-steps dyeing process can be used to dye the plating fabric made of the PTT/PET bi-component filament as the base yarn and the PA filament as the face yarn. While the PTT/PET bi-component filament of the fabric can be dyed with specially developed ANOCRON disperse dye at 110–120℃, PA fiber can be dyed with an acid dye under a lower temperature of less than 100℃. In order to gain the required color, the PTT/PET bi-component filament should be dyed first. To avoid staining and to enhance hand feel, a reduced washing treatment of the fabric under 60℃ is necessary after dyeing the PTT/PET bi-component filament.

The fabric made of the PTT/PET bi-component filament has much better dimensional stability, much better elastic recovery property after chlorine treatment and much better wrinkle resistance than the fabric made of the PU/PA core-spun yarn. However, its thermal resistance and water-vapor resistance are a little higher than those of the fabric made of the PU/PA core-spun yarn, but it is still suitable for sportswear and summer wear.

The fabric made of the PTT/PET bi-component filament has an uneven fabric surface due to different shrinkages of the PTT fiber and PET fiber after the heat treatment. Therefore, the PTT/PET bi-component filament is not suitable to be used as the face yarn in a plating fabric structure.

Footnotes

Acknowledgments

The authors would like to thank Shell Chemical for providing the PTT polymer, and Haining Xingao Fibers Co., Ltd for providing the PTT fiber and PTT/PET bi-component filament. The authors also express their appreciation to Zhejiang Huinongsi Knitting Underwear Co., Ltd for knitting the fabrics and seamless samples.

Funding

This work was supported by Shell Chemical Haining Xingao Fibers Co., Ltd.

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