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Study of self-crimp polyester fibers.

INTRODUCTION

Self-crimp fibers behave like natural wool with a textured appearance. The crimps are born from a composite of two parallel but attached fibers with differing shrinkage or expansion properties. Since the crimp is naturally born, the false twist or air-texturing involved in typical fiber processing for synthetic fibers becomes unnecessary and can be eliminated. The elimination of this element can significantly cut the manufacturing costs, creating a "dream product" for a synthetic-fiber producer. Another important characteristic of the selfcrimp yarn is that the crimp is not imposed on the fiber from the outside, but rather results from the rearrangement of the internal molecular structure of the fiber material. Usually, the crimp generated by either false twist or air-texturing is imposed on the fiber via mechanical deformation of the fiber as a 2D zig-zag crimp [1-4]. In some applications the crimped fiber must be extremely resilient, for example, in fiber filling for pillows, furniture, and so on. In such cases a mechanical 2D crimp is insufficient, and instead, a latent helical "self crimping" of the fiber is necessary.

Nevertheless, spinning of conjugated fiber represents the greatest challenge for the fiber industry [5-7]. The technical difficulties include: 1) the melt instability between the two ingredients; 2) the need to instantly adjust the throughput ratio; and 3) the complex design of the conjugated spinnerette. DuPont (Wilmington, DE) started to study the first self-crimp yarn (PP) in the early 1960s. Recently, the newly commercialized self-crimp products of DuPont, polyester T-400 and nylon T-800, have become very popular in the market. Following the success of DuPont, Unitica (Hyogo, Japan) also commercialized the self-crimp yarns, Z-10 and S-10. Furthermore, a nylon/polyurethane bicomponent filament, Sideria, developed by Kanebo (Japan), can adapt heat treatment to self-crimp itself to an appropriate degree. Self-crimp yarns have apparently become conspicuous in the fiber industry [8-12]; however, few researchers have investigated the crimp mechanisms and the effect of fiber-processing parameters on the "crimp potential". This study therefore attempts to produce and elucidate a new direction for self-crimp fiber development.

This study presents the results of an investigation on crimp formation that considers several parameters, including: 1) various cross-sectional geometries such as circles and triangles; 2) a combination of various polyester materials, for example, PET (polyethylene terephthalate), CD (Cation Dyeable PET), PTT (polytrimethylene terephthalate), and PBT (polybutylene terephthalate), in various ratios; and 3) the fiber-processing parameters (spinning temperature, heat treatment, and so on). Prior to studying the crimp, this study first examines the optimum rheological condition for fiber spinning. The results of this study can provide further insight into the curling mechanism of self-crimp yarn. Moreover, a crimp model is assessed, a method for characterizing crimp is established, and finally but most important, the optimum conditions for producing a high-quality self-crimp yarn are identified.

[FIGURE 1 OMITTED]

THEORETICAL REVIEW

The basic driver of self-crimping is a shrinkage differential within the fiber. Early theories to study the crimp mechanism were based on mechanical models of bimetallic strips. During the early 1980s, Denton [13] developed an advanced equation (Eq. 1) that used a geometrical and mechanical approach to describe this effect. Equation 1 has been proven practical when applied to most fibers with regular cross-sections (Fig. 1):

1/R = ([A.sub.1]*[[mu].sub.1]*[DELTA])/[I.sub.0] (1)

where 1/R denotes the crimp curvature; [A.sub.1] represents the area of either component; [[mu].sub.1] the first moment of [A.sub.1] is defined as the line from the center of [A.sub.1] to the bulk center of the composite; [DELTA] denotes the fractional differential shrinkage between the components; and [I.sub.0], the second moment of area of the whole fiber cross-section, is given by summing the increments of area times the squares of their distances from the axis of reference.

Denton [13] reached three important conclusions based on Eq. 1: 1) Fibers with a single interface exhibit the best crimp potential; 2) The crimp potential is maximized in a skein with a straight interface passing through the center of the cross-sectional area of a conjugated fiber; and 3) The crimp potential is zero for any cross-section with a center of symmetry (such as centric core/sheath).

Equation 1 demonstrates that the amount of the differential shrinkage is the determinant of crimp curvature for a given cross-section. However, to obtain sufficient crimp the differential shrinkage must exceed a certain value. The differential shrinkage can be obtained in different ways and thermal shrinkage without steam was used in this work.

EXPERIMENTAL

Spinning Setup and Fiber Drawing

Two polyester resins, comprising PET as a fixed ingredient and another conjugated part selected from PBT, CD, or PTT, were spun simultaneously using a bicomponent spinning set. PET resin with IV-0.64 (dissolved in a solvent of phenol/tetrachloroethane 6/4 at room temperature) and CD with IV-0.45 were supplied by Hsing-Kuang (Taiwan). PBT resin with IV-0.9 was obtained from Chung Chun Petrochemical (Taiwan). PTT resin with IV-0.95 was supplied from Nan Ya Plastics (Taiwan). Notably, all the resins used were bright grade except for PET resin (delustered grade with Ti[O.sub.2] 0.32%) to obtain a high contrast image in analyzing the cross section of fiber. Before melt spinning, the test polymers were dried to maintain a moisture level below 100 ppm.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Prior to the melt spin, the melt-viscosity was measured via a continuous rheometry (Haake Polylab System) to determine the optimal spinning temperature. In this investigation, two individual single-screw extruders extruded PET and its conjugated part separately into the conjugated spin-pack. The spin-pack was a side-by-side device designed by HILLS Co. Moreover, a Barmag take-up winder (BARMAG DR4) was employed for collecting the yarns with a winding speed of 3,500 m/min. A 30-min filament package with a denier of 150 denier / 24 filaments was gathered for each experimental condition.

Crimp Characterization

Calculate the crimp potential, crimp shrinkage, and crimp index of each skein, using the following equation [14]:

CP = [([L.sub.2] - [L.sub.1])/[L.sub.1]]*100% (2)

where CP denotes the crimp potential %; [L.sub.1] represents the final skein length with 2.5 g load (crimped length following treatment); and [L.sub.2] is the final skein length with 700 g load (uncrimped length after treatment).

The elastic recovery test was conducted based on the German Standard Test Procedure DIN53835. A tensile-recovery instrument (Textechno Statimat ME, Germany) was employed with pretension of 0.5 cN/tex, tensile rate of 200 mm/min, and recovery period of 5 min. The elastic recovery (ER) was calculated according to Eq. 3 shown as follows [15]:

ER = [(2[L.sub.0] - [L.sub.1])/[L.sub.0]]*100% (3)

where [L.sub.0] denotes the initial length of skein under pretension; [L.sub.1] represents the final skein length following elongating to a certain degree and then recovering for 5 min.

RESULTS AND DISCUSSION

Melt Spinning Stability

The difference in viscosity between two individual polymers for producing a bicomponent fiber can cause serious migration and deform the interface [16, 17]. When two components, with equal throughput, are extruded using the same spinnerette, the higher-viscosity component inclines to occupy over half of the cross-sectional area to equalize the pressure drop. Consequently, dog-legging, or even worse, a broken filament occurs. The search for processing conditions under which viscosities of the two components are identical or very close is thus critical to conjugated spinning, especially for side-by-side products.

Figure 2a,d illustrates the dependence of viscosity on shear rate for processing PET and CD, PBT, and PTT, respectively, at various temperatures. The processing temperature of PET must be 20-30[degrees] higher than that of PTT and PBT to achieve the same melt viscosity. Notably, this temperature difference might be offset as two polymers merge together at the final section of the spinnerette. Since the residence time via the merged section is too short to even up the temperature differences, the merged section was assumed not to cause further viscosity difference. Figure 3a-d shows a series of PET/PBT conjugated spinning under different spinning conditions, where the spinning temperature was maintained at 290[degrees]C for PET but was varied from 250-265[degrees]C for PBT. The curved interface occurred in all the conditions except for Fig. 3c. This result agrees well with Fig. 2a,c, which illustrates a minimum difference in viscosity using the combination of PET/PBT 290[degrees]C/255[degrees]C (Fig. 4). The curved interface indicates an unbalanced pressure inside the spinnerette, the low-viscosity melt was pushed to occupy the high shear rate zone around the capillary wall. Generally, the formation of crimps in a skein is more difficult for a curved interface than a straight interface. Moreover, and more seriously, a curved interface implies the occurrence of a serious dog-legging problem which would deter and even stop the spinning. To prevent the formation of a curved interface, melt spinning for each condition here was performed to minimize the difference between the melt viscosities of two sides via temperature adjustment.

[FIGURE 6 OMITTED]

Crimp Investigation

Figure 5 represents the effect of the cross-sectional shape on crimp potential. The results indicate that, in the same cross-sectional area, the triangular shapes are superior to the round cross section. The calculation, derived from classical mechanics [18], reveals that the first moment [[mu].sub.1] and second moment [I.sub.0] for round shape and triangular shape are 0.405 r, 0.39 [r.sup.4], and 0.483 r, 0.235 [r.sup.4], respectively. Based on Eq. 1, the crimp curvature of triangular shape is twice that of round shape. This theoretical calculation agrees well with the experimental results. Furthermore, Fig. 5 demonstrates that the one in a triangular shape with a higher shrinkage component on the top, denoted the "regular triangle," exhibits greater crimp curvature than the other triangle with reversed order, denoted the "reversed triangle".

[FIGURE 7 OMITTED]

Figure 6 illustrates the influence of the volume ratio of two components on the crimp curvature. The optimal CP% generally occurred with a ratio of 50/50. It can be concluded that the optimal self-crimp can be achieved at the ratio around 50/50 and is independent of the combination of different components. Figure 7 illustrates a series of photos of the cross section for PET/PBT bicomponent fibers in various volume ratios. Notably, Denton [13] predicted that the optimal curvature for a round yarn would occur in a 50/50 combination for a straight interface and a 60/40 combination for a curved interface. From Figs. 6 and 7, even when both conditions are optimized, the 50/50 with straight interface outperformed the 60/40 with a curved interface. In short, both theoretical and experimental results indicate that the ratio for creating a self-crimp bicomponent yarn is 50/50 by volume.

[FIGURE 8 OMITTED]

Figure 8 depicts the dependence of crimp potential on fiber diameter. The result indicates the fiber with a diameter of 8 den (8 denier of PET is ~28 [micro]m in diameter) exhibits the optimal crimp potential. Increased diameter can increase both the first and second moments of a curved fiber; also, the cross sectional area is increased, as indicated in Eq. 1. Besides, the chain-orientation along the spun direction, which is critical for fiber shrinkage after heat treatment, is strongly influenced by fiber diameter owing to the temperature gradient during spinning [19-22]. An optimal diameter therefore exists for producing a self-crimp yarn. Notably, the optimal diameter of 8 dpf (denier per filament) is based on a winding speed of 3,500 m/min in this experiment. If the winding speed increases, then due to the increased heterogeneity of chain-orientation by faster solidification, the maximum diameter for crimp potential is reduced to less than 8 dpf.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

The temperature of thermal treatment following spinning is another key influence on fiber crimp curvature. Figure 9 demonstrates that the optimal crimp potential can be obtained when the ambient air temperature is around 100[degrees]C. This temperature range is ~20-30[degrees] higher than the [T.sub.g] of PET, which acts as the low shrinkage material on one side of the fiber. Equation 1 reveals that the self-crimp is caused by the difference in shrinkage ratios between the two sides of the fiber. However, the shrinkage difference is strongly affected by the ambient temperature in the thermal treatment process. Generally, the temperature must be higher than the [T.sub.g] of both sides to allow chain-relaxation from the oriented state but still must be maintained low enough to avoid the increase of shrinkage on both sides to reduce the shrinkage difference. A temperature range ~20-30[degrees] higher than the [T.sub.g] of the harder side within the fiber is identified as the optimum condition.

The shrinkage difference shown in Eq. 1 is another key factor requiring detailed investigation. Figure 10 indicates that the combination of PET/PTT outperforms that of PET/CD and PET/PBT in crimp potential measurement. Moreover, Table 1 reveals that the thermal shrinkage differs among individual polyesters, with the maximum difference in shrinkage occurring for the combination of PET/PTT. We believe that the high thermal shrinkage of PTT results not only from the relaxation of oriented amorphous chains, but also from the relaxation of the planar zig-zag crystalline structure that is uniquely built by PTT [23, 24]. Figure 11 shows photos of the curved skein in various polyester combinations. The PET/PTT clearly exhibits the highest curvature and has the smallest crimp diameter. Meanwhile, the PET/CD illustrates the lowest curvature, which is consistent with Table 1 and Fig. 10.

Figure 12 shows the crimp stability for various polyester combinations. The skein here experienced five consecutive CP tests. The stability results reveal that PET/CD completely lost its crimp potential. In contrast, PET/PTT still exhibits a good CP ratio. As mentioned earlier, the self-crimp displays a more deformation resistant helical structure than a 2D zig-zag crimp because the helix was formed from the bending of the inner structure rather than from the outside twist as the zig-zag crimp was imposed. However, PET/CD has the lowest shrinkage difference (see Table 1), which leads to a significant reduction in its crimp stability following a harsh deformation test.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

Figure 13 illustrates the elastic recovery of a curved skein made from conjugated spinning. Straight skeins of individual pure polyesters were also included as references. The curved skein was first leveled off using pretension of 0.5 cN/tex, and then was elongated to various degrees to observe its recovery behavior. Figure 13 shows that the elastic recovery was primarily dominated by the composed material itself. PTT alone exhibits a super recovery behavior for increasing the elongation by up to 30% of its original length. Unsurprisingly, PTT has been confirmed as a good elastic fiber by other researchers [25, 26]. However, the PET/PTT skein, which has only half of PTT compared to a pure PTT skein, displays a close recovery result. This result indicates that the crimp created by the conjugating mechanism should contribute to the elastic recovery of the skein to compensate for the elasticity loss from the not participating part of PTT in a PET/PTT curved yarn.

CONCLUSIONS

In this study, self-crimp polyester yarns were manufactured using a conjugated spinning process. A theoretical model proposed by Denton [13] proved to be very useful for predicting crimp potential. Maintaining identical or very similar melt viscosities of the two components was demonstrated to be very critical for obtaining a straight interface and eliminating the dog-legging problem. Regarding the thermal treatment following melt spinning, a temperature range of ~20-30[degrees] higher than the [T.sub.g] of the harder side within the fiber is identified as the optimum condition.

The crimp tests illustrate that the triangular shapes are found to be superior to the round cross section. Furthermore, a triangular shape with a higher shrinkage component on the top, denoted the "regular triangle," exhibits greater crimp curvature than the other triangle with reversed order, denoted the "reversed triangle". The optimum volume ratio for making a self-crimp bicomponent skein is 50/50. Moreover, the optimal fiber thickness is 8 denier per filament. Finally, this study found that the combination of PET/PTT outperformed that of PET/PBT and PET/CD in terms of crimp potential, crimp stability, and elastic recovery. This phenomenon is primarily attributed to the markedly different thermal shrinkages of PET and PTT.
TABLE 1. Thermal shrinkage (%) for various polyester FOY (hot air at
120[degrees]C).

PET CD PBT PTT

6.06 6.76 8.03 10.43


Contract grant sponsor: National Science Council of the Republic of China; contract grant number: NSC 93-2216-E-027-003.

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S.P. Rwei

Institute of Organic and Polymeric Materials, National Taipei University of Technology, #1, Sec 3, Chung-Hsiao E. Rd., Taipei, Taiwan 106, R.O.C.

Y.T. Lin

Taiwan Textile Research Institute, Tu-chen City, Taiwan 236, R.O.C.

Y.Y. Su

Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan 106, R.O.C.

Correspondence to: Syang-Peng Rwei; e-mail: f10714@ntut.edu.tw
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Author:Rwei, S.P.; Lin, Y.T.; Su, Y.Y.
Publication:Polymer Engineering and Science
Geographic Code:1USA
Date:Jun 1, 2005
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