Crepe Yarn. 2. Experimental Methods of Strain Set and Determination of Experimental Conditions

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1 Crepe Yarn Part 8 : Effects of Strain Setting Process on the Tensile Properties of Some Textile Fibers. By Ichiro Genma and Yasuji Fukushima, Members, TMSJ Industrial Research Institute of Kanagawa Prefecture Abstract Part 8 concludes (1) that the contraction force fl develops in set fibers immersed in water and is in proportion to the strain set previously, fl as a general form being expressible as f1=e1 (e-s,), where E1 is a material constant, e is the strain set previously and e, is a limit strain beyond which fl shows; (2) that the amount of yarn contraction also increases linearly with the strain set previously, (3) and that initial Young's modulus E2 of set fibers, obtained by further stretching water after fl has developed, increases linearly with strain s, and E2 for viscose yarn, acetate and silk is expressed as E2=K+ks where K and k are material constants. Introduction The stress developed in fibers by stretching relaxes in time, and some part of it is set after a long period of time. Generally, however, it is difficult to set the entire strain under normal conditions. If a fiber is wetted during stretching and dried under constant strain, most of the strain is set. This is called cohesive set, and is explained as due to the formation of numerous weak bonds of the hydrogen bond type[51 in a stretched fiber. These bonds are not very stable that when the fiber is immersed in water again they break easily and the strain recovers partially. This phenomena is called swelling recovery by P. H. Hermans.[61 Now, for twisted yarns. Twisted yarns are usually treated by steam to stabilize their liveliness. This treatment is called twist-set and the mechanism of stabilization is to set the strain of each constituent fiber. Therefore, the strain of each constituent fiber recovers when yarn is wetted. The deformation of crepe yarn in water is thought to be decided by this kind of recovery phenomenon of strain which in composed fibers. Thus, to analyze the behaviour of twisted yarn when it is wetted, we have to know the mechanism of the set and the recovery phenomenon of set fibers when they are wetted. Strain set and the stability of set fibers are a problem not only of theoretical but also practical interest, but experimental information on this problem Vol. 10, No. 2 (1964) seems to be scarce. Explanations by P. H. Hermans about the swelling recovery of viscose rayon and a few reports by some other experts on the heat-set of nylon and Terylene[7) are just about the only published information available. Study of this problem, naturally, can extend over a wide field depending on how experimental conditions are set up. In our own study, we made experiments on a limited scale with the manufacture and use of crepe yarns borne in mind. We have tried to express the results in simple empirical formulas which can be easily used for calculation in future analyses. 2. Experimental 2-1. Methods of Strain Set and Determination of Experimental Conditions The recovery phenomenon of set strain by swelling depends on the setting conditions. For example, with synthetic fibers, the higher the setting temperature, the more stable the set is. With cellulosic fibers, the higher the temperature of steam, the larger the decrease in the swelling property and, consequently, the heavier the decline of the recovering ability.[81 Judging from these facts, the higher the steam temperature for twist-set, the more stable twist becomes. However, a high temperature can lower the deformability of yarn when it is wetted. 72

2 It is not easy, therefore, to choose a suitable condition for twist-set. We used a hot wet treatment under 100 C as a condition of experiment, because service crepe yarns made of various fibers are traditionally set in steam under 100 C. Clamp a sample to the jaws of a tensile tester and stretch the sample to a definite length. Three minutes later, lift the cylinder for a wet experiment that is attached to the tester until the sample gets completely into the cylinder. Pour hot water into the cylinder and leave the sample immersed in the hot water for three minutes. Then dry it in an atmosphere of 20 C, 65% R. H. for three hours. During the drying, stress is seen to reach a nearly constant value. The change of stress through these processes is shown in Fig. 1 and indicated by these symbols: 61: stress when the fiber is extended to a certain strain. 62. relaxed stress three minutes later. 68: stress three minutes after immersion in hot water. 64 : stress after three-hour drying under standard condition. Even this wet treatment does not diminish stress. Consequently, an unrestricted fiber shrinks instantaneously to 64 =0. This instantaneous shrinkage el is obtained from the length of the movement of the lower jaw until 64 gets to 0. In visco-elastic materials, such as textile fibers, delayed shrinkage develops with time, and stress c shows slowly. We put this delayed time at five minutes. This delayed shrinkage e2 is also obtained by the same way as el is obtained. Since the purpose of set is to relax stress positively and to set strain as much as possible, we may say that the smaller the value of c/o, 31,32, more complete the set. Therefore, we can judge the effect of set by comparing these values one another. C4/61, 31,32 obtained on viscose rayon, acetate and silk under different conditions are shown in Tables 1, 2 and 3. Our experiment has shown that the wet treatment is far more useful for setting strain than free relaxation in dry condition. It has shown also that the higher the temperature, the greater the set effect. In viscose and silk, however, the set effect is not influenced as much by the water temperature as in acetate. In other words, the values 64/61 and g1+g2 are nearly the same if the temperature is above 60 C for viscose and above 80 C for silk. These values for acetate are influenced notably by the water temperature and show the characteristic of tharmnnb tirity Fig. 1 Change of yarn stress throughout the setting process. Table 1 Effects of Setting Condition on Set of Viscose Rayon. r=10% Table 2 Effects E=10 Jo Table 3 Effects e=10% of Setting Condition on Set of Silk. of Setting Condition on Set of Acetate. 73 Journal of The Textile Machinery Society of Japan

3 In our experiment on nylon and Terylene, we used a small drawing apparatus shown in Fig. 2. Sample s was clamped to jaws C and C, attached to arms A and B, B being movable along connecting rod G by turning screw rod D. The jaws are easily removable by loosening screws E and F, one of them being used as weight (about 2 grams) during the measurement of the yarn length. After the sample was extended to a fixed length, the apparatus with the yarn in it was left in the setting medium for a fixed time and then dried in a room of 20 C, 65% R. H. for 24 hours. Then the jaws with sample on them were removed from the apparatus by loosening the screws E and F, and the yarn length was measured with a cathetometer immediately after the jaws were removed and also five minutes thereafter. With the yarn length in various stages denoted as follows: lo; length before extension (10 cm.); li; length after extension (in this experiment e is 10%); 12, length immediately after the jaws were removed from the apparatus; l3; length 5 min, thereafter. we get ei=(11-12)/10 si+s2 (11-13)/10 The results obtained under different set conditions are given in Tables 4 and 5. 64/61 could not be measured in our experiment. As shown in Tables 4 and 5, the values of Ei and 62 are affected by the temperature of the set medium and exhibits the thermoplastic characteristic. On the whole, however, Bi and e2 are much greater than those shown in Tables 1, 2 and 3. These experiments show that a setting condition under 100 C, though inadequate for setting nylon or Terylene, does set most of the strain of viscose, acetate and silk. As set conditions for a further experiment, we chose treatment in water 80 C for viscose, 95 C for silk and 60 C for acetate. By these setting conditions, the model twisted yarn of these materials was stabilized sufficiently for practical use. According to a patent we have for the manufacture of crepe fabric from nylon, the best result is obtained when crepe yarn is twist-set in steam under 100 C.[9] Accordingly, we chose treatment in water 95 C as the set condition for nylon and Terylene. We will call these setting conditions twist-set conditions. Fig. 2 Drawing apparatus used in experiment. Table 4 Effects of Setting Conditions on Set of Nylon. ~=10% Table 5 Effects 8=10% of Setting Conditions on Set of Terylene. Vol. 10, No. 2 (1964) 74

4 The strains of various fibers were set by the twist-set conditions, and E~ and 64 were measured. 1 and 64 for different initial strains are shown in Figs. 3, 4, 5, 6 and 7. As shown in the graphs, ei and 64 for viscose, acetate and silk are proportional to initial strain E, the value of e being extremely small. For nylon and Terylene, el and 64 are considerable, thus indicating the imperf ectness of set. Fig. 6 Residual stress Nylon 70d. and shrinkage after setting. Fig. 3 Residual stress and shrinkage after setting. Viscose rayon 75d. Fig. 7 Residual Terylene stress 40d. and shrinkage after setting Mechanism of Twist-Set Fig. 4 Residual stress and shrinkage after setting. Acetate 55d. Fig. 5 Residual Silk 21d. stress and shrinkage after setting. The experimental results have shown that fibers are not completely free from stress, even though the most of their strains are set by hot water treatment. It necessarily follows, then, that complete stabilization in the twisted yarn is impossible by the twist-set conditions. Yet service twisted yarns are stabilized completely by these conditions. This contradiction can be explained as follows. The cross-sectional area of viscose rayon or acetate twisted yarn swells in the twist-set process. The area decreases when yarn is transferred to a drying cycle. At the same time, some free space develops between fibers, because fibers emit moisture and become thinner Presumably, a small fiber shrinkage, a maximum of about 0.5%, can take place in these free spaces inside the structure without distorting the arrangement of fibers. Thus, the twisted yarn becomes stable in regard to the residual stress. 75 Journal of The Textile Machinery Society of Japan

5 As for silk, the sericin sorounding the silk fibroin filaments swells and softens during twist-set and fills the space between filaments. Therefore, after drying, the filaments are put together by sericin, thus stabilizing the twisted yarn. In nylon and Terylene, both el and i are great, and these fibers do not swell at all during twist-set. They have no sizing materials around the filaments. Therefore, we cannot even hope to stabilize them by the method mentioned in the preceding paragraph. Nylon and Terylene being thermoplastic, yarns can probably be stabilized if a temperature exceeding the softening point of these fibers is used for twistset. The catch is, however, that the constituent fibers of yarns thus stabilized would refuse to shrink, thus making the yarns useless for manufacture into crepe fabrics. The only way to stabilize yarn liveliness while maintaining the shrinkability of the constituent fibers, is to agglutinate the fibers with sizing materials. In our experiment, we sized yarn with P.V.A before twisting and set it by steam at about 90 C after twisting to crepe yarn. Viscose rayon, acetate and silk swell longitudinally, the rate being about 4% for viscose rayon, % for acetate and % for silk. Therefore, within a certain limit of strain, strain recovery is concealed in the longitudinal swelling, and the actual fiber length increases substantially. Thus fi does not show until strain exceeds a certain limit. Fig. 8 shows that.fl for various fibers, except Terylene, is proportional to initial strain, and that fl as a general form can be expressed thus: f1=e1(e--e,~) where E1 is material constant and e~ is the limit beyond which fl develops. El and e,~ calculated from the graphs in Fig. 8 Table 6 Setting Conditions and Temperature of Water in Which fi is Measured 2-3. Swelling Recovery and Contraction Force When fibers with their strains set under certain conditions are immersed in hot water, the strains recover partially. Therefore, their contraction force can be measured by keeping them from shrinking. The contraction force, which we denote this force as f 1, can be measured when set fibers with their residual stress el+e2 (see Fig. 1) removed by lifting the lower jaw slightly, are immersed again in hot water in the same way as for strain setting. The force considered here is that which initially shows in the fibers distributed in the outer layer when the yarn absorbs water. fl for various fibers are shown in Fig. 8 in relation to the initial strain e. The set conditions and temperature of water in which fl was measured are given in Table 6. As we see from Fig. 8, the strain values at which fl starts vary with materials. f i for nylon and Terylene starts at negative strain values. fl for viscose rayon, acetate and silk starts at positive strain values. Nylon and Terylene have originally great residual shrinkages, i.e., 12% and 7%, respectively. Therefore, with imperfect set, this residual shrinkage contributes greatly to fiber shrinkage, even if they have not been strained. This is the reason why fl for nylon and Terylene starts at a negative strain value. Fig. 8 Relation fiber in between contraction force water and initial strain. appeared in set Vol. 10, No. 2 (1964) 76

6 give following empirical formulas: For viscose rayon f i =1.04 ( ) g/d. For acetate fx=0.53(e-0.01) g/d. For silk f i==3.34 g/d. For nylon f i = ( e) g/d. For Terylene fi= ( e) g/d. fore=0 to fl= { (e-0.04)} g/d. for e over The amout of strain recovery can be obtained by lifting the lower jaw of the tensile tester until fl gets to zero. However, the value thus obtained is not the true recovery, because it includes the longitudinal swelling of viscose rayon, acetate and silk, or the inherrent residual shrinkage of nylon and Terylene. The value may, therefore, be properly called yarn contraction, rather than strain recovery. Fig. 9 gives the results of measurement and shows that the amount of contraction is proportional to initial strain. Nylon and Terylene contract even when e is 0, because of the residual shrinkage mentioned above. Viscose rayon, and acetate stretch because of longitudinal swelling when e is within a certain limit, which is for viscose rayon and 0.01 for acetate. The effect of the longitudinal swelling did not show in silk in this experiment. The contraction of strain-set fibers is closely related to the deformation of crepe yarn in water. Therefore, we can expect to make crepe fabrics from nylon, Terylene, viscose rayon and silk. It seems extremely difficult to make crepe from acetate Young's Tensile Modulus of Strain-set Fibers When Stretched Further in Water. The force considered so far is that which accompanies strain recovery in water. As we have already said, when the twisted yarn absorbs water, this force develops at once in the fibers which have been stretched during twisting and set by the twist set treatment. Consider a case where yarn diameter swells. In this case, the fiber arranged in the outer layer of the yarn get stretched further. Therefore as shown in Fig. 10, development of additional force f2 in the fibers accompanies this stretching. We assume that f~ can be expressed as follows: f2=e2de where de is the amount of the fiber stretch induced by the lateral swelling of the yarn diameter and E2 is Young's Tensile modulus of set fibers in water. By after fi Fig. 9 Contraction of set fibers in water Fig. 10 Tension appears in fiber when the yarn diameter swells in water. Fig. 11 Load-extension diagram of stretching further in water Viscose rayon 75 d. stretching develops, set fiber obtained by after f 1 has developed. strain-set fibers further in water we can obtain load-extension dia- 77 Journal of The Textile Machinery Society of Japan

7 grams, as shown in Fig. 11. E2 in these diagrams is given by the slope in the first part of each curve. Fig. 12 shows the relation between E2 thus obtained and the initial strain E which has already been set. The set conditions and water temperature in which E2 was measured are the same as shown in Table 6. Nylon and Terylene were excluded from this experiment. Fig. 12 Initial Young's stretching the has developed As shown in Fig. 12, E2 for each fiber increases linearly with initial strain E, and E2 as a general form can be expressed by the equation E2=K+kE, where K and k are constants. K and k calculated from each diagram of Fig. 12 make these empirical formulas: For viscose rayon E2=40 e g/d. For acetate P22= ( e ) g/d. For silk E2=( E) g/d. Therefore, force f2 corresponding to small stretch de is given as follows: For viscose rayon f2 =40. E. de. g/d. For acetate f2=( ~) de. g/d. For silk f2 = ( E) de g/d. 3. Conclusions modulus set fiber of set further fiber obtained by in water after f i Taking viscose rayon, acetate, silk, nylon and Terylene, we have studied experimentally on (1) set of strain by hot wet treatment and (2) contraction force due to strain recovery in water. We have also studied initial Young's modulus, in water, of these fibers pre-elongated and set by conditions equivalent to the twist-set conditions for these crepe yarns. Results : (1) When stretched fibers are dried in an extended length after hot wet treatment, their tensile strains are partially set. In this case the higher the temperature of the treatment, the larger the proportion of strain which can be set and the more stable the effect of set. (2) When set fibers are immersed again in hot water, the bonds linked to the set are partially broken and swelling contraction develops in fibers. In this case, contraction force fi increases linearly with initial strain e. f~ as a general form can be expressed as follows: fi=e2(e-~n) where is a limit strain beyond which contraction occurs, the limit being >0 for viscose rayon and acetate, =0 for silk and <0 for nylon and Terylene. El is the material constant. (3) The amount of contraction also increases with initial strain. (4) Initial young's modulus E2 of set fibers obtained when they are stretched further in hot water after fi has developed, increases linearly with strain e. E2 as a general form can be expressed as follows: for viscose rayon, acetate and silk E2= K-1-ks where K and k are material constants. The force f 2 corresponding to this stretch is, then, as follows: f2 = (K+k-)4~. where d~ is amount of stretch. Acknowledgement g/d. We thank Prof. K. Fujino of Kyoto University and Mr. T. Mitsui, chief of the Textile Division, Industrial Research Institute of Kanagawa Prefecture, for their constant and valuable help in this study. We are indebted also to Mr. Y. Hara, Y. Kawai and Miss H. Sugimoto for their assistance in our experiments. Literature cited [1] G. E. Collins; J. Text. Inst., 30, P46 (1939) [2] I. Gemma and Y. Fukushima; J. Text. Mach. Soc., Japanese edition, Vol. 13, No. 6, 1960 [3] E. R. Kaswell; Textile Fibers, Yarns and Fabrics, p 98. Reihold Publishing Corp. N. Y [4] ditto (2) [5] R. Meredith; The mechanical Properties of Textile Vol. 10, No. 2 (1964) 78

8 Fibers, p 202. North-Holland Publishing Co. [6] P. H. Hermans; Physics and Chemistry of Cellulose Fibers, p. 445 [7] Inter alia; S. Ueda and T. Kimura; Bulletin of the Textile Research Institute. Japanese edition No. 30, D. N. Marvin; J. Soc. Dyers Col., [8] S. Okajima and Y. Kobayashi; J. Japan. Japanese edition, Vol. 9, [9] J. P Vol. Soc. No. 70, No. 16, Text. Cell. Ind. 5. May Journal of The Textile Machinery Society of Japan