In this study, the color variation of spun-dyed polyester filaments was systematically investigated using various processing technology parameters including linear density, velocity of cross-flow air, different processing ways and different textured technology parameters. The results showed that these parameters had varying degrees of influence on the color of spun-dyed polyester filament in processing. The degrees of color variation were expressed using the CIE1976 L*a*b* system. As a result, it provided an important theoretical guidance for working out actual production technology and controlling color quality of spundyed polyester filaments based on actual industrial equipment and technological conditions.
Key words spun-dyed polyester filament, color, value, chroma, hue-angle
As a modern alternative approach to conventional wet-dyeing methods, spun-dyeing of polyester filaments is becoming increasingly widespread. It overcomes the disadvantages of conventional methods and provides its final products with excellent characteristics, such as uniform coloration, level shade, a high degree of light fastness, relatively low cost and cleanliness. Therefore, it has been widely applied in many fields, such as military uniform, sports uniform and other special professional uniforms as well as automotive textiles, etc [1-8]. However, spun-dyed polyester filament is produced by incorporating colorant into polymer during polymerization and the process is long and complex. Thus, the spun-dyed process often has the problem of color variation [3,5,9,10].
Since the 1950s, there have been a number of studies on this problem. Beevers [11] briefly analyzed the influence on the appearance of colored fibers in yarns by factors including quality of materials and twist as well as count. Morgenstern et al. [3] proposed several methods to improve the levelness of spun-dyed colors. Tang et al. [12] investigated the production technology and methods to control the properties of spun-dyed, partially oriented polyester yarn (POY). Song [9] and Liu [10] have both sketchily analyzed the reasons for color variation of spun-dyed polyester fiber during processing and tried to find methods to control it. In addition, Yin [13] carried out an initial study on color variation of spun-dyed polyester fiber during the finishing process based on experimental instruments. However, color variation of spun-dyed polyester fibers during processing has never been investigated systematically, especially that based on actual industrial equipment.
In the present study the effects of several technological parameters on the color variation of spun-dyed polyester filaments during processing were investigated systematically. It involved linear density, cross-section shape, different processing methods and textured technology parameters. In addition, the authors theoretically analyzed the reasons for in color variation of spun-dyed polyester filaments.
Experimental Materials and Sample Preparation
Green-brown colorants with the two batch numbers YZ01 and YZ02 were prepared using the following ingredients obtained from Shanghai Yuchengcx Plastics Co., Ltd, China: black carbon, orange-red pigment, yellow pigment and polyethylene terephthalate (PET) chips. The PET chips were supplied by Yizheng Chemical Fiber Co., Ltd, China and had the following characteristics: intrinsic viscosity 0.644, melting point 261C, semi-bright; and 0.30% content of TiO^sub 2^.
Spun-dyed polyester POY filaments with different linear densities were produced using spinning equipment made by Noyvallesina Engineering Co., Italy, combined with a colorant injector made by Swiss Colortronic System Pte. Ltd under the following conditions: spinnerette shape, round/pentalobal; melt temperature, 290C; cool cross-flow air velocity, 0.45 m/s (adjusted to 0.25, 0.35, 0.45 and 0.55 m/s in experiment 2); cross-flow temperature, 20 0.5C; and relative humidity, 75%.
The spun-dyed polyester drawn yarn (DY) filaments were produced by an Isikawa drafting twist machine (modified to have the twisting and winding style as a parallel winding one) under the following conditions: temperature of hot disc and hot plate, 90 and 190C, respectively; draw ratios DR^sub 1^ and DR^sub 2^, 1.024 and 1.581, respectively.
The spun-dyed polyester drawn textured yarn (DTY) filaments were produced by a Barmag FK6-900 draw-texturing machine under the following conditions: draw ratio, 1.620; primary heater temperature, 180C; secondary heater temperature, 135C; value of D/Y (ratio of rotating linear velocity of twisting disc and linear delivery velocity of the filament), 1.612 (adjusted to 1.612,1.662,1.712 and 1.812 in experiment 4), secondary heater overfeed, 6.00% (adjusted to 4.00,5.00,6.00 and 6.94% in experiment 4).
The colorant addition of all spun-dyed filament samples used in the study was 4.76% and the colorant batch number was YZOl, except for experiment 2 which used YZ02.
Test Instruments and Methods
The spectral reflectance of the spun-dyed polyester filaments are carried out on a Datacolor Spectraflash 450 supplied with color quality control software based on the CIE1976 L*a*b* system, under the following conditions: specular and UV included model, D^sub 65^ light source; CIE 1964 10 standard observer; large measuring aperture (LAV), intervals, 10 nm; wavelength resolution 3 nm; maximum reproducibility, E
The spun-dyed polyester filaments were wound in parallel and well distributed on a white paperboard until the background could not be seen. Each kind of spun-dyed polyester filament was tested four times at different angles. By taking the average as the final spectral reflectance of the spun-dyed polyester filaments, the color measurements could be obtained from the color quality control software.
The visual evaluation of color difference was carried out according to ASTM D2616 (1996). The crimp ratio was measured according to the Standard GB/T 6506-2001 (China).
Experiment 1: Effect of linear density on color variation
Table 1 lists the specifications of the spun-dyed polyester DY samples. The spectral reflectance curves of spun-dyed polyester filaments with different linear densities are shown in Figure 1. It can be seen that the spectral reflectance reduced with increasing linear density of a single filament.
Table 2 shows the CIE L*a*b* parameters of a spundyed single filament, which indicate that the value (L) and chroma (C) of a spun-dyed filament decreased when the linear density of the fiber increased.
By fitting the above data using linear and negative exponent relations respectively, the regression models of value (L) and chroma (C) with the square root of linear density of a single filament were obtained as equations (1), (2), (3) and (4).
The data were a better fit to a straight line than to a negative exponent by the evaluation of correlation coefficient. However, according to Lambert-Beer's law, the absorption of a fiber to visible light correlates with the light path (that is the diameter of a fiber) in a negative exponent relationship [14], and the diameter of a fiber correlates with the square root of the fiber's linear density in a linear relation [15]. So the intensity of reflected light on a fiber correlates with the square root of a fiber's linear density in a negative exponent relation. Consequently, the theoretical regression models of value (L) and chroma (C) with the square root of linear density of a single filament were expressed as equations (3) and (4).
The reason for the decreasing trend in the value (L) of a spun-dyed polyester filament with higher linear density is that the higher the linear density of a fiber, the smaller is the specific area of the fiber, and the less is the specular reflection on the surface of the fiber, which leads to a decreasing trend in the value (L) of a spun-dyed polyester filament.
The reasons for chroma (C) variation appear to be complicated. The object in this study was a deep green-brown in color, mixed by black carbon, orange-red pigment and yellow pigment. Therefore, the reflectance of spun-dyed filaments to visible light was very low. According to the chromatics theory, chroma (C) mainly correlates with the intensity of reflected light refracting from the interior of a fiber after selective absorption [16,17]. Figure 1 shows that with the decrease of linear density, the total reflectance increased, being made up of two parts: one being the increase of specular reflectance and the other the increase of the reflectance refracting from the interior of a fiber after selective absorption. However, the reflectance of light as the wavelength ranged from 520 to 640 nm increased remarkably.
Figures 2 and 3 compare the theoretical regression with the experiment, and they also illustrate that the simulations agree well with the experimental results.
Moreover, from Table 2 one can see that with the variation of linear density of a single filament, its hue-angle (H) basically remained unchangeable.
Experiment 2. Effect of cross-section shape on color variation
The shape of the cross-section of a spun-dyed polyester filament is also one of the important factors that affect the specular reflection to visible light. Furthermore, by designing a certain shape of spinnerette to obtain an abnormal cross-section, the spinning technology parameters, and particularly the velocity of cool cross-flow become the most important factors that affect the shape of the cross-section of a filament. Therefore, the effects of different cross-flow velocities on the color variation of a spun-dyed POY were investigated in this study.
The spectral reflectance curves of a spun-dyed POY with pentalobal cross-section produced under different velocities of cool cross-flow are shown in Figure 4. Note that the spectral reflectance reduced slightly with increasing cross-flow velocity. Table 3 shows the CIE color measurement values of a spun-dyed POY produced under different velocities of cool cross-flow, and furthermore that, with increasing crossflow velocity, the value for a spun-dyed POY with pentalobal cross-section decreased. This is because the variation of cross-flow velocity could affect the shape of the cross-section of POY, and the higher the cross-flow velocity, the bigger will be the modification rate of the pentalobal on the crosssection of the filament. This implies that the fluting depth on the surface of the filament would increase when the cross-flow velocity increased and furthermore that the reflection property on the surface of the filament will also vary with it.
In addition, it was found that with the variation of velocity of cool cross-flow, the chroma (C) and the hue-angle (H) of a spun-dyed POY with pentalobal cross-section basically remain unchangeable, i.e. the hue of the filament has no variation.
Experiment 3: Effect of different processing methods on color variation
The effects of different processing methods on the color variation of a spun-dyed polyester filament with linear density of 156.4 dtex/72f were studied further. Figure 5 shows the spectral reflectance curves of spun-dyed filaments with round and pentalobal cross-sections produced by different processing methods. Table 4 shows the comparison of color measurements of spun-dyed filaments produced by different processing methods.
It is clear that the value (L) and chroma (C) of spundyed DTY with round cross-section increased remarkably in comparison with those of DY, whereas the hue-angle of DTY was smaller than that of DY. The color difference between DY and DTY was shown to be two to three grades by visual evaluation according to ASTM D2616. This could be due to the structure of DTY not only being bulky but also having a regular crimp on each single DTY, compared with DY. The contact area between adjacent fibers decreased, which made the intensity of light being transmitted directly between adjacent fibers decrease, and the boundary reflection increase. Therefore, the internal absorption of fibers to visible light decreased, and the proportion of the external reflected light increased. In addition to this, Table 4 also shows that the value (L) and chroma (C) of spun-dyed DTY with a pentalobal cross-section increased slightly compared with DY, whereas the hue-angle of DTY was smaller than that of DY. On the whole, the color variation of a spun-dyed filament with pentalobal cross-section was different from that of filaments with a round cross-section, produced by different processing methods.
Experiment 4: Effect of different textured technology parameters on color variation
Different texture technologies usually lead to morphological changes of a filament, and the reflection of the filament to visible light will change, thus causing color variation in the filament. Many factors control the effect of texture, including false twist, twisting tension, detwisting tension, secondary heater overfeed, the third overfeed and temperature, etc. In this study, different texture effects were obtained by adjusting the value of D/Y and the secondary heater overfeed.
The spectral reflectance curves of spun-dyed polyester DTY samples with round cross-sections but different D/Y values and different secondary heater overfeed are shown in Figures 6 and 7, respectively. Tables 5 and 6 show the effect of different D/Y values and different secondary heater overfeed on the color variation of a spun-dyed polyester DTY, respectively.
In general it can be seen from Tables 5 and 6 that with the increase of D/Y value and secondary overfeed, the crimp ratio of a spun-dyed DTY increased and the value (L) of a spun-dyed DTY increased slightly. However, the chroma (C) only varied a little, and the hue-angle (H) basically remained stable. By comparison, it was also found that the variation in the D/Y value had a slightly more significant effect on the color variation of a spun-dyed DTY than the variation of secondary overfeed. In short, with the improvement of crimp ratio, the color of a spun-dyed DTY became lighter and the hue of the filament showed little variation.
Conclusions
The processing technology parameters had different levels of influence on the color variation of spun-dyed polyester filaments. On the basis of the authors studies, one can draw the following conclusions.
1. With the increase of the linear density of a spun-dyed polyester DY, its value (L) and chroma (C) decreased, and value (L) and chroma (C) of a spundyed filament theoretically correlated with the square root of the fiber's linear density in a negative exponent relation. However, its hue-angle basically remained unchangeable.
2. The value (L) of a spun-dyed polyester POY with pentalobal cross-section decreased when the cool cross-flow air velocity increased. However, its chroma (C) and hue-angle basically remained unchanged.
3. The color variation of spun-dyed polyester filaments with pentalobal cross-section was different from that of filaments with a round cross-section, made by different processing methods. The value (L) and chroma (C) of DTY with a round cross-section were remarkably higher than that of DY, whereas the value (L) and chroma (C) of DTY with pentalobal cross-section were slightly higher than that of DY.
4. With the increase of crimp ratio of a spun-dyed polyester DTY, the value (L) of DTY increased slightly. However, the chroma (C) varied only slightly and the hue-angle basically remained stable.
Acknowledgements
The authors would like to thank Hainan Haihong Chemical Fiber Industry Co., Ltd. and The Key Laboratory of Functional Fabric of Shaanxi Province as well as the Academic Research Center of Xi'an University of Engineering, Science and Technology for providing the spinning machines and test instruments to accomplish this study.
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About the author:
Mei-yu Chen1. Kan Lai and Run-jun Sun
College of Textile and Material, Xi'an University of Engineering Science and Technologym Xi'an 710048. PR China
1 Corresponding author: e-mail: yuanshijidi@163.com
Copyright Textile Research Institute Jun 2006
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