Dimensional Stability of Men s Socks

Transcription

1 Dimensional Stability of Men s Socks Saber Ben Abdessalem 1*, Fatma Abidi 2, Sofiene Mokhtar 1 and Saber Elmarzougui 1 1 Technology High School of Ksar Hellal, Textile Research Unit, Tunisia, 2 National Engineering School of Monastir, Textile Department, Tunisia. ABSTRACT Dimensional stability is one of the main quality problems of men s sock manufacturing. Major shrinkage can be found after washing, undermining socks usability. Today, greater demand for quality is required as customers become more aware of shrinkage from laundering. In order to investigate the effect of pressing and repeated launderings on dimensional stability of men s socks, three commonly produced socks were subject to pressing at different temperatures and for different lengths of duration by an industrial sock pressing machine. The study demonstrates that pressing causes major shrinkage in the body and sole zones. The shrinkage is inversely proportional to temperature and duration of the pressing process. After the first laundering, major shrinkage was found. The second laundering had no significant effect on dimensional variations since the sock had already taken up its fully relaxed dimensions. The results can help knitters predict sock dimensions after thermal and humid treatments. Keywords: Socks, Dimensional Stability, Pressing, Sock Finishing, Heat Setting 1. Introduction Men s socks are specially weft-knitted garments that can have serious quality problems linked essentially to dimensional stability and comfort. Socks are not of a regular tubular structure. They are composed of different kinds of yarn with specially shaped knitting zones in the heel and toes. Body and sole zones can undergo major shrinkage after washing that can undermine socks usability because of size-mismatching problems. The particularity of sock stability compared with other weft-knitted fabrics or garments is linked to the fact that socks are subject to major deformation by everyday wear that can affect its dimensional stability. In addition, various materials with different physical properties, such as wool, polyamide and elastomer, can coexist in a sock, but dimensional stability is provided by simple pressing with a "sock press". In the literature, there were studies evaluating the comfort of socks and determining subjective evaluations and laboratory measurements that predict the level of comfort best (Morris et al., 1984; Nishimatsu et al., 1994; Tsujisaka et al., 2004). Baird et al. (1971) tested 80/20 wool/nylon socks and found that their wear life was about three times the wear life of similar pure wool socks. They noticed that socks which lost least fibres during wear and retained the best appearance were those which had been treated with resin after manufacture. However, studies concerning sock dimensional stability are very few. Weft-knitted fabrics are prone to undergo major changes in dimensions. Theses changes are often caused by distortion after repeated launderings (Anand et al., 2002). Many factors linked to knitting and finishing are mainly responsible for causing undesirable effects on knitted fabrics. Today, consumers are increasingly aware of garment quality and they expect higher standards of performance than before. Textile fabrics made of natural and synthetic fibres have to be fixed before use. A thermal treatment called heat setting is applied to fabrics before use to ensure stability after repeated launderings and it causes only insignificant shrinkage. This heat treatment aims to thermofix * Corresponding author. Tel.: (216) ; Fax: (216) address: saber_ba@yahoo.fr 61

2 the synthetic fibers in the socks such as polyamide and elastomer. Knitted fabrics allow garments to fit closely and snugly, making them ideal for next-to-skin wear (Morris et al., 1984). They are subject to stretching and mechanical deformation (Nishimatsu et al., 1994) since the yarn is put under high stress in knitting and finishing processes. This stress generates internal tension inside yarn and this tension remains when the fabric is taken off the machine or left in a highly distorted state (Tsujisaka et al., 2004). Knitted fabrics never fully recover from internal tensions and have to withstand considerable wear due to everyday use and laundering (Baird et al., 1971). Today, greater demand for quality is required as customers become more aware of shrinkage from laundering. Therefore, if consumers continue to expect higher quality and dimensionally stable socks, the actual production processes will need to be investigated. In an ordinary production process, socks are knitted and then toes are sewed. Afterwards, socks undergo a thermal treatment composed of steaming and pressing operations in order to stabilize the article dimensions. Knitted fabrics are inherently difficult to stabilise, as the construction allows for contraction up to 40% (Baird et al., 1971). The prediction of washing performance is therefore an enormous task. Commercial knitters will therefore benefit from research into the dimensional behaviour and techniques for predicting stable dimensions. finishing conditions and not in laboratory ones. Different studies emphasize the importance of working in commercially produced and finished specimens for reliably predicting the distortion and dimensional properties of knitted fabrics (Heap et al., 1983; Heap et al., 1985). The sock samples were placed by an operator on flat templates in the shape of a foot (Figure 1) and then stabilised by the industrial pressing machine sock press (Figure 2). First, the samples were sprayed with softened water and steamed in a steaming chamber, and then aligned in order to be ready for the pressing operation. The steaming (saturated vapour at 3.17 kg/m 3, 6 bars) aimed to introduce humidity to fabrics and prepare socks for heat setting. Its duration was the same as the pressing operation. After pressing, the samples were cooled and removed from the flat templates with automatic grips. In the pressing chamber (Figure 3), the heating plates exerted three consecutive pressures on the socks. These heating plates were metallic and covered with special fabric in order to reduce contact temperature with socks. Heat was transferred from plates to socks by conduction through the covering fabric. The temperature of the heating plates was displayed on a machine screen. It was approximately 35 C higher than contact temperature with socks during pressing. Treatment with flat templates was essential to ensure pressing with heating plates but it involves elongation that can cause shrinkage. The size of templates depended on that of socks in order to reduce the effect of elongation. The aims of the research are to study the effect of principal pressing parameters of dimensional stability and distortion of socks; and to study the effect of repeated launderings on sock shrinkage. The impact of heat setting on sock dimensional stability is also discussed. 2. Material and Methods The thermal treatment was performed in real Fig. 1. Flat template supporting socks 62

3 1: Water spraying, 2: Steaming, 3: Sock aligning, 4: Heating press, 5: Cooling, 6: Sock reception Fig. 2. General structure of a sock pressing machine order to study the effect of theses two parameters of sock dimensional stability. All samples were relaxed in a flat position during the 24 hours from knitting to pressing. 3 men s socks commonly produced in the sock industry were knitted. A single cylinder machine was used (LONATI, Swisse gauge = 24, diameter = 3.5 inches, 120 needles). The socks bodies and soles had a plain-knitted structure. Socks are composed of main yarn that is generally cotton or wool, auxiliary yarn such as polyamide yarn that gives resistance, and elastic yarn such as Lycra that gives elasticity to the socks. 1, 1 : Heating plates, 2: Covering fabric, 3: Templates position, 4: Plates parallelism control Fig. 3. Structure of pressing mechanism There is no rigorous thermodynamic law that enables precise determination of necessary energy or heat needed by the fibre to rearrange and stabilize. This energy depends mainly on the nature of fibre. The duration of sock pressing is the time in which heating plates remain closed at only one pressing. This time is necessary so that the sock treatment temperature is reached on surface and also thickness. The length of duration is also difficult to determine. Thus, pressing temperatures and durations vary in These types of yarn were plated together during knitting by specific yarn guides. Polyamide yarn was textured in order to give thickness to the knitted structure. Information about sock components and pressing conditions is shown in Table 1. The heat setting conditions covered a large range of heating plates temperatures and treatment durations. The temperature level was dependent on the nature of fibres and their proportions in the socks. Socks containing a high proportion of thermoplastic fibres (polyamide, Lycra ) were treated at high temperature. Wool is sensitive to temperature and can be affected if treated at high temperature. Therefore, wool socks are less susceptible to pressing temperature,than other socks. 63

4 Sock Components Sock 1 : 70% cotton 30% polyamide Sock 2 : 79% cotton 20% polyamide 1% Lycra Sock 3 : 66% wool 32% polyamide 2% Lycra Table 1. Sock components and pressing conditions Heating plates temperature Pressing duration ( C) (s) 140 ; 150 ; 160 ; 170 ; ; 6 ; 8 ; 10 (Pressing duration = 4 s) (Heating plate temperature = 140 C) 130 ; 140 ; 150 ; 160 (Pressing duration = 4 s) 110 ; 120 ; 130 ; 140 (Pressing duration = 4 s) 4 ; 6 ; 8 ; 10 ; 12 (Heating plate temperature = 130 C) 4 ; 6 ; 8 ; 10 (Heating plate temperature = 110 C) Since there is not any standard or official procedure specifically for sock dimensional stability, the standard NF G entitled «Measuring method for the determination of dimensions variations", normally used for ordinary knitted fabric, was adopted to determine sock dimensional variations after pressing and three consecutive washings. All samples were washed in the following conditions: Washing machine capacity: 5 kg Washing temperature: 40 C Washing duration: 30 minutes Spin-drying speed: 500 round/minute Drying was performed at ambient air temperature in 24 hours. For preparation of the specimen, the sock samples were marked with a 5x5cm square at the body and sole zones before pressing by loose seams. Sewing was performed with dimensionally stable yarn and in the direction of the socks wales and courses. After pressing and washing, longitudinal DV L and transversal DV T dimensional variations were calculated: L L0 DV L (%) =.100 ; L W W DV T (%) = W L 0 : marked square length before treatment L: marked square length after treatment W 0 : marked square width before treatment W: marked square width after treatment Fabrics were fully conditioned for 24 hours after pressing and drying in a standard atmosphere of 21 ± 2 C and 65 ± 2% relative humidity. No tension was applied to the samples during square dimensions measurement. Seven specimens of each sample were tested and the mean dimensional variations were calculated. 3. Results Figures 4, 5 and 6 show plot graphs of average longitudinal and transversal dimensional variations versus heating plate temperatures. A positive dimensional variation indicates an increase in length or width. A negative value represents a decrease and hence shrinkage. It appears that pressing operation causes shrinkage in the body and sole zones of all socks. The shrinkage is inversely proportional to the pressing temperature. After the first laundering, the body and sole zones underwent shrinkage, which was approximately as serious as the one found after pressing. However, the second and third launderings caused 64

5 shrinkage much less significantly than the first laundering. After pressing and the first laundering, the main shrinkage was found in the longitudinal direction. The extent of shrinkage after pressing and the first laundering was different for the three sock samples. Fig. 4. Effect of temperature on dimensional variations (Sock 1) Fig. 5. Effect of temperature on dimensional variations (Sock 2) 65

6 Fig. 6. Effect of temperature on dimensional variations (Sock 3) Figures 7, 8 and 9 show plot graphs of average longitudinal and transversal dimensional variations versus pressing duration. Pressing duration had a similar effect on socks dimensional variations as well as heating plate temperature. Fig. 7. Effect of pressing duration on dimensional variations (Sock 1) 66

7 Fig. 8. Effect of pressing duration on dimensional variations (Sock 2) Fig. 9. Effect of pressing duration on dimensional variations (Sock 3) 67

8 The measured shrinkage was inversely proportional to the pressing duration and the main shrinkage was measured in a longitudinal direction. The maximum shrinkage took place after the first wash cycle, and there was virtually no change in dimensions upon further washing treatments. At pressing durations of 12 seconds, yellowing was observed in Sock 1. This long pressing duration at 140 C damaged the cotton fibres of the sock. 4. Discussion The primary purpose of this study is to look into the effect of pressing temperature and duration on the dimensional stability of socks. Sock pressing is a heat setting process aiming to stabilize garment dimensions. It is usually carried out at temperatures higher than the glass transition temperature of the thermoplastic fibres composing the sock. The glass transition temperature (Tg) marks the onset of segmental mobility of the polymer. It is the temperature below which the polymer segments do not have sufficient energy to move past one another. If the temperature is above Tg the segments rearrange to relieve an externally applied stress which results in heat flow. This rearrangement due to the mobility of polymer macromolecular chains allows yarn relaxation and fabric stabilisation and shrinkage. Vasanthan (Vasanthan, 2004) demonstrated that polyamide yarn relaxation depends on heat setting temperature. It is shown that yarn is stabilized as heat setting temperature is high. In a previous study (Ben Abdessalem & al., 2005), we have shown that heat setting causes stabilisation of dimensional properties of synthetic fibres due to polymer mobility. The experimental results show that sock stabilisation is as significant as pressing temperature. Little shrinkage was found after laundering when high pressing temperature was used. This is because fabric relaxation continues after pressing (dimensions measurement is performed 24 hours after pressing), and low pressing temperature leads to incomplete heat setting and stabilisation. The same explanation also goes for the effect of pressing duration on shrinkage after pressing. When pressing treatment is fast, the provided energy for heat setting is low and major shrinkage is thus found after relaxation. Pressing socks at high temperature and during a long time seems to be beneficial for stabilisation, but the degradation of cotton and wool fibres has to be taken into account. The results of the wash cycles show that all socks display major shrinkage in longitudinal and transversal directions particularly after the first wash cycle. This is because relaxation accelerates when humid conditions, and agitation in the washing machine make yarn easy to relax. Very similar observations were formulated by Anand et al. (Anand et al., 2002) who tested the effect of successive launderings on ordinary rib and single jersey fabrics. Knapton et al.(knapton et al., 1970) showed that shrinkage of wool-knitted fabrics from any stage in processing to the fully-relaxed state depends on loop length, and shrinkage increases with the number of wash cycles. Chen et al. (Chen et al., 2002) evaluated the dimensional changes of steam-ironed plain knitted wool fabrics after washing and conditioning at 20 ± 2 C and 65 ± 2% RH for more than 24 hours. They noticed that steam-ironed fabrics exhibit further relaxation when agitated in water. After the first washing, socks are almost completely relaxed and very little shrinkage was found after the subsequent wash cycles. Therefore, washing socks systematically after pressing seems viable for dimensional stability but also very costly since a second pressing after washing will be necessary to eliminate sock wrinkling. The extent of shrinkage after pressing and the first washing is different for the three socks. Shrinkage for the cotton-polyamide sock was less significant compared to that of cotton-polamide-lycra socks since Lycra gives the fabric higher elasticity and promotes its aptitude to shrink. The wool sock exhibited major shrinkage after pressing (22%) and the first washing (16%) due to a significant proportion of elastic fiber. Lycra is generally introduced to the sock structure for comfort linked to its high extensibility about 600%. But its presence in the sock caused serious dimensional changes. These significant cases of shrinkage correspond to a high reduction of the sock size and have to be taken into account before sock knitting in order to avoid mismatching problems. 68

9 5. Conclusion The results obtained in the study indicate that industrial pressing parameters have a significant effect on the dimensional stability of socks. Heating plate temperature and pressing duration modulate the extent of shrinkage after pressing and repeated launderings. The first wash cycle causes major shrinkage mainly in the longitudinal direction. This causes serious size-mismatching, which is one of the main quality problems of sock production. The subsequent wash cycles have no significant effect on dimensional variations since socks have already been fully relaxed. These data present a great interest to sock manufacturers who can make better choices concerning pressing conditions. The relation between shrinkage and final sock dimensions has to be rigorously taken into account by fabric manufacturers before knitting to avoid mismatching problems. High temperature and long pressing duration seem to have a positive effect on sock dimensional stability but long exposure to high temperature can cause serious damage to textile fibre such as cotton yellowing. Pressing at low temperature preserves textile fibre but leads to incomplete relaxation and stabilisation of the sock. A simple pressing is not sufficient to obtain a completely relaxed sock. The pressing process of socks is relatively short compared to fine gauge fabrics finishing and does not involve humid treatments such as washing, dyeing or bleaching. These treatments are particularly beneficial to fabric relaxation because, in a humid environment, fibres move more easily in the yarn and fabric reaches a more relaxed state. Washing socks systematically after pressing is obviously the best solution to dimensional stability, but the high cost incurred will make this solution less viable when a second pressing after washing is necessary to eliminate sock wrinkling. Future work will focus on the effect of severe pressing conditions on the comfort of socks and mechanical performances such as tensile strength and elasticity. This will permit better finishing conditions for socks. REFERENCES [1] Anand, S.C., Brown, K.S.M, Higgins, L.G., Holmes, D.A., Hall M.E. & Conrad D. 2002, Effect of Laundering on the Dimensional Stability and Distortion of Knitted Fabrics, Autex Res. J., vol. 2, pp [2] Baird, K. & David, H.G , Wearing and Washing Performance of Wool/Nylon Blend Socks, Textile Res. J., vol. 41, pp [3] Ben Abdessalem, S., Jedda, H., Skhiri, S., Karray, S., Dahmen, J. & Boughamoura, H. 2005, Influence of thermofixation on artificial ACL ligament dimensional and mechanical properties, Eur. Phys. J. Appl. Phys., vol. 32, pp [4] Chen, Q.H., Au, K.F., Yuen C.W.M. & Yeung, K.W. 2002, Relaxation Shrinkage Characteristics of Steam-Ironed Plain Knitted Wool Fabrics, Textile Res. J., vol. 72, pp [5] Heap, S.A., Greenwood, P.F., Leah, R.D., Eaton, J.T., Stevens, J.C. & Keher, P. 1983, Prediction of Finished Weight and Shrinkage of Cotton Knits The Starfish Project, Part I: Introduction and General Overview, Textile Res. J., vol. 53, pp [6] Heap, S.A., Greenwood, P.F., Leah, R.D., Eaton, J.T., Stevens, J.C. & Keher P. 1985, Prediction of Finished Weight and Shrinkage of Cotton Knits The Starfish Project, Part II: Shrinkage and the Reference State, Textile Res. J., vol. 55, pp [7] Knapton, J.J.F., Richards, S. & Fong, S. 1970, The Dimensional Properties of Knitted Wool Fabrics, Textile Res. J., vol. 40, pp [8] Morris, M.A., Prato, H.H. & White, N.L. 1984, Relationship of Fiber Content and Fabric Properties to Comfort of Socks, Cloth. and Textiles Res. J., vol. 3, pp [9] Nishimatsu, T., Ohmura, K., Sekiguchi, S., Toba, E. & Shoh, K. 1998, Comfort Pressure Evaluation of Men's Socks Using an Elastic Optical Fiber, Textile Res. J., vol. 68, pp [10] Tsujisaka, T., Azuma, Y., Matsumoto, Y.I. & Morooka, H. 2004, Comfort Pressure of the Top Part of Men's Socks, Textile Res. J., vol. 74, pp [11] Vasanthan, N. 2004, Effect of Heat Setting Temperatures on Tensile Mechanical Properties of Polyamide Fibers, Textile Res. J., vol. 74, pp