December 20, 2007
Moisture Absorption and Release of Profiled Polyester and Cotton Composite Knitted Fabrics
By Su, Ching-Iuan Fang, Jun-Xian; Chen, Xin-Hong; Wu, Wen-Yean
Abstract Composite yarns were spun using profiled polyester fibers and nature cotton fibers at different blend ratios. The water absorption capacity, diffusion rate, and drying rate of knitted fabrics made from the three composite yarns were examined to shed light on their moisture absorption and release performance. Experimental results reveal that the diffusion rate and drying rate become better with decreasing cotton content. However, knitted fabrics made of profiled polyester alone showed the worst water absorption ability, which can be improved with the addition of cotton fiber. For core and cover yarns, the addition of profiled polyester filaments can enhance their performance in moisture absorption and release. In summary, textiles products made from core and cover yarns of T^sub f^/T^sub s^/C 42/46/12 and 42/35/23 can not only provide a better sense of touch but also comfort when worn with efficient moisture absorption and release.Key words profiled polyester fiber, composite yarn, core yarn, cover yarn, moisture absorption and release
In recent years, with rising living standards, people's needs and expectations of clothing and textile products have also become different. Products of ordinary quality face the fate of being eliminated from the market. As a result, blend yarns are made from different functional polyester fibers to meet the changing demand of the textile markets and to maintain a competitive edge in the industry [1-3]. To many, the textile industry seems to be doomed to extinction. However, the focus on improving the functions of textile products and clothing technology has injected new life into the industry and offered new direction for development. Goals for making textiles that can keep rain, heat, and moisture away from the body, provide a good sense of touch, and offer comfort when worn have been pursued by manufacturers [4-6].
Good moisture absorption and release can be found in fibers with greater specific surface area. The micro goves on the fiber surface foster capillary absorbency, siphoning moisture, which then diffuses and spreads over the fiber surface to be dissipated. Such rapid transportation of moisture or diffusion of sweat in the form of steam from the body towards the outside enables good moisture absorption and release, thus maintaining dryness and comfort. The performance in moisture absorption and release depends primarily on the fiber properties. Natural fibers such as cotton and wool are hydrophilic, meaning that their surface has bonding sites for water molecules. Therefore, water tends to be retained in the hydrophilic fibers, which have poor moisture transportation and release. On the other hand, synthetic fibers such as polyester are hydrophobic, meaning that their surface has few bonding sites for water molecules. Hence, they tend not to get wet and have good moisture transportation and release. Neither natural nor synthetic fibers can perform well in both moisture absorption and release at the same time. To achieve such would require moisture absorption and release finishing through which the structural design and quality of fibers are modified so that the textile products thus manufactured can have good performance in absorbing, transporting, and dissipating moisture.
Fabrics with good moisture absorption and release that have been developed include profiled polyester fibers (with cross, Y, W, and U profiles), and hollowed and microporous fibers (Wellkey(R) of Teijin, Japan and HyPore(R) of Union Chemical Laboratories, Taiwan). They are usually of multilayer structure with two or three alternating layers of hydrophilic fiber and hydrophobic fiber. In some cases, the fiber surface is treated with a hydrophilic agent that modifies the surface quality. Most profiled polyester fibers are developed to achieve moisture absorption and release. In this study, profiled polyester fibers with crossed goves are used as raw materials for spinning the yarns (and fabrics). Their good performance in moisture absorption and transportation through capillary siphoning, which results in good wearing, comfort have been reported in previous studies [7-9].
Most textile products currently available on the market are manufactured using polyester filaments (Tf) with 100% moisture absorption while efforts have also been made to develop polyester stable fiber (Ts) with good moisture absorption. In view of the drawback of polyester filaments being harder and rougher, appropriate amounts of polyester stable fiber and natural cotton (C) are added for better sense of touch. However, the moisture regain ratio of cotton fiber is as high as 8.5%. Too high a proportion of cotton in the blend yarn will affect moisture release. Hence, the optimal blend ratio of profiled polyester fiber and natural cotton for yarn spinning to achieve efficient moisture release and good sense of touch merits further investigation.
In this study, profiled polyester stable fibers and natural cotton were spun at different blending ratios to yield Ts/C blend yarn . With the addition of polyester filaments, core yarn and cover yarn were obtained. Experiments were conducted on the composite knitted fabrics to examine their water absorption capacity, diffusion rate, and drying rate. Results thus obtained can shed light on the effect of T^sub f^, T^sub s^, and C contents on moisture absorption and release, which can serve as useful references for the design of knitted fabrics with good performance in moisture absorption and release and comfortable sense of touch.
Profiled polyester stable fibers (T^sub s^, 1.55 dTex x 38 mm) and cotton fibers (C, 1.55 dTex x 28 mm) from Far Eastern Tex tile Ltd were spun into 20 tex T^sub s^/C blend yarn . Profiled polyester filaments (T^sub f^, 75d/48f) from Far Eastern Textile Ltd were positioned on an additional rack and blended with staple fibers behind the front roller of a spinning frame, thus yielding 20 tex core yarn and 20 tex cover yarn .
The yarns were then knitted into single jersey fabrics using a circular knitting machine, followed by scouring, bleaching, and stabilizing finishing. Experiments were then conducted under temperature of 20 +- 2[degrees]C and a relative humidity of 65 +- 2% to evaluate the moisture absorption and release performance of the knitted fabrics.
The spinning conditions were as follows:
1. T^sub s^/C roving count (Tex): 540;
2. Blend ratio of roving (TJC): 100/0, 80/20, 60/40, 40/ 60, 20/ 80, and 0/100;
3. Break draft: 1.21 (for cotton-rich yarn) and 1.30 (for polyester-rich yarn);
4. Twist factor (TM for tex system): 32;
5. Spindle speed (RPM): 12,000;
6. Yarn count (tex): 20;
7. T^sub s^/C ratio of blend yarn: 100/0, 80/20, 60/40, 40/60, 20/ 80, and 0/100;
8. T^sub f^/T^sub s^/C ratio of core and cover yarn: 42/58/0, 42/ 46/ 12,42/35/23, 42/23/35, 42/12/46, and 42/0/58;
1. The scouring and bleaching conditions were as follows:
a) Temperature: 98[degrees]C x 30 min;
b) Weight ratio of fabrics to water: 1 : 12;
c) Formula of chemicals: as shown in Table 1.
2. The stabilizing finishing condition was: 160[degrees]C x 60 s.
The following methods were used.
1. Water absorption capacity . We dip the samples into the water at 50 mm depth and measured the absorbed water height (mm) after 30 minutes for five different samples (25 mm x 250 mm) and took the average to indicate the water absorption capacity of the fabrics (as shown in Figure 1, according to Byreck method of JIS L 1097-1994 ).
2. Diffusion capacity . The sample fabrics were placed flat on a hydrophobic board with the outer surface facing up. The area (mm2) was measured with water allowed to diffuse at 30 second after dripping 0.2 ml of water using a precise dropper whose tip was 10 mm above the fabric surface. The measurement was repeated at five different points and the average of the diffusion area (mm2) taken to indicate the diffusion capacity of the fabrics (according to Dropping method of JIS L 1097-1994 ).
3. Drying rate . Referring to the Dropping method mentioned above and modifying it, we cut the sample fabrics into squares of 50 mm x 50 mm and recorded its dry weight (W^sub f^). 0.2 ml of water was dropped onto it using a precise dropper whose tip was 10 mm above the fabric surface and recorded its wet weight (W^sub 0^) at the initial stage. The change in weight (W^sub i^) was measured at 10-minute intervals. The remaining water ratio (%) was calculated at each interval using the formula (W^sub i^ - W^sub f^)/(W^sub 0^ - W^sub f^) x 100%. The remaining water ratios within the 1-hour experimental duration will shed light on the drying rate of the fabrics.
Results and Discussion
Water absorption capacity
Figure 2 shows the water absorption capacity of the three composite yarns obtained at different blend ratios. As can be seen, yarns containing no cotton at all including pure polyester yarn (T^sub s^/C: 100/0), core yarn (T^sub f^/T^sub s^/C: 42/58/0) and cover (T^sub f^/T^sub s^/C: 42/58/0) have the lowest water absorption capacity. Although capillary absorption occurs in profiled polyester fibers, they are hydrophobic by nature and thus have poor moisture absorption compared with cotton fibers, which are hydrophilic. Hence, composite yarns without cotton content have poor water absorption performance. With the capacity to regain moisture (with moisture regain ratio of 8.5%), cotton fibers enhance the capillary absorption effect. As a result, the height of water absorbed increases with increasing cotton content. As seen in Figure 3, the blend yarn with 40% cotton content as well as core and cover yarns (T^sub f^/T^sub s^/C : 42/35/23) reach the greatest absorbed water heights of 14.95, 22.46, and 21.50 cm, respectively. However, increasing cotton content at the expense of polyester fibers will undermine the capillary absorption of the latter. Hence, the height of water absorbed decreases with increasing cotton ratio in the yarn. As can be seen, blend yarn with 80% cotton content as well as core and cover yarns (T^sub f^/T^sub s^/C : 42/12/46) have relatively lower absorbed water height. In short, the addition of cotton at the optimal ratio can enhance the water absorption by capillary action of polyester fibers.
With the addition of polyester filaments, both core and cover yarns have a greater content of polyester fibers, thus enhancing the capillary absorbency. Despite both being profiled polyester fibers, polyester filaments have far greater siphoning capacity than polyester stable fibers. Hence, as seen in Figure 2, core and cover yarns containing polyester filaments have higher absorbed water height than blend yarn comprising only polyester stable fibers and cotton. The difference in water absorption efficiency can be attributed to the different structural design in core and cover yarns. In core yarn, polyester filaments are lined up vertically in the core, while they are lined spirally around the body of the cover yarn. This accounts for the better moisture absorption capacity in core yarns than in cover yarns.
Figure 3 displays the diffusion rate of the three composite yarns obtained at different blend ratios. As can be seen, yarns containing no cotton at all including pure polyester yarn (T^sub s^/C : 100/ 0), core yarn (T^sub f^ /T^sub s^/C : 42/58/0), and cover yarn (T^sub f^/T^sub s^/C : 42/58/0) show the highest diffusion rate and have the largest area diffused with water, the areas being 1808,3410, and 3500 mm^sup 2^, respectively. On the other hand, pure cotton fabrics as well as core and cover yarns (T^sub f^/T^sub s^/C : 42/0/58) have diffusion areas of 1384, 2200, and 2751 mm2, respectively, when diffused with water, indicating a comparatively lower diffusion rate. This can be attributed to the hydrophilic nature of cotton fibers, which tends to absorb the water dropped onto the fabric surface. When wet with water, the cotton fibers expand, thus slowing down the diffusion rate. In short, the diffusion rate is dependent on the cotton or polyester content. The higher the cotton content, the lower the diffusion rate will be. In contrast, the higher the polyester content, the higher the diffusion rate will be.
As seen in Figure 3, the diffusion areas of T^sub s^/C yarns that contain no polyester filaments were all less than 2000 mm^sup 2^, while those of core and cover yarns that comprise polyester filaments all exceeded 2000 mm^sup 2^. This reveals that the addition of polyester filaments enhances the diffusion rate of yarn. Between the two, cover yarns with polyester filaments lined in spirals outside the polyester stable fibers show better diffusion rates than core yarns having polyester filaments lined at the center.
Figure 4 shows the drying rate of the three composite yarns obtained at different blend ratios. As can be seen, pure polyester yarn (T^sub s^/C : 100/0), which contains no cotton at all, has the lowest remaining water ratio of 0.57%, implying the fastest drying rate. In contrast, pure cotton yarn (T^sub s^/C : 0/100) without any polyester fiber has the highest remaining water ratio of 40.1%, indicating the slowest drying rate. This can be attributed to the difference in remaining water ratio between polyester and cotton fibers. Polyester fibers have a low remaining water ratio of 0.4% while that of cotton fibers is as high as 8.5%.
The same trend can be seen in fabrics made of core and cover yarns. Fabrics knitted with yarns without cotton (T^sub f^/T^sub s^/ C: 42/58/0) show a low remaining water ratio of 0.023%. However, with increasing proportion of cotton fibers, the remaining water ratio increases and reaches 6.493% for yarns of T^sub f^/T^sub s^/C : 42/0/58.
The addition of polyester filaments to the yarns increases the content of polyester fibers, which are hydrophobic by nature. Moisture is thus not retained in the fabrics as evidenced by the much lower remaining water ratio compared with that of blend yarn containing no polyester filaments.
Contrary to the difference in water absorption and diffusion capacity attributed to the structural difference between core and cover yarns, there is no significant variation in drying rate between them. In other words, the addition of polyester filaments helps enhance drying rate regardless of whether they are in the core or the outer surface of the yarn.
The experimental results of this study reveal that the use of profiled polyester fibers can enhance the diffusion rate and drying rate, leading to good moisture release. However, polyester fibers are hydrophobic and thus have poor water absorption capacity. This drawback can be overcome by adding cotton fibers, which are hydrophilic. Core and cover yarns containing profiled polyester filaments perform better in moisture absorption and release compared with T/C blend yarn. Nevertheless, profiled polyester filaments feel rough. Hence, textile products made from core and cover yarns with higher cotton content such as T^sub f^/T^sub s^/ C: 42/46/12 or 42/ 35/23 can not only provide a better sense of touch but also comfort when worn with efficient moisture absorption and release.
This research was supported by the National Science Council (NSC 92-2216-E-011-025). The authors are grateful to Far Eastern Textile Ltd for their assistance with this experiment.
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Ching-luan Su1, Jun-Xian Fang and Xin-Hong Chen
Department of Polymer Engineering. National Taiwan
University of Science and Technology, ROC
Department of Fashion Merchandising Management,
Shih Chien University
1 Corresponding author: Department of Polymer Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, section 4, Xaipei, Xaiwan, 106, ROC. Xel.: 8862 2737 6531; fax: 8862 2737 6544; email: email@example.com
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