The dyeing properties and mechanical properties of DTY are not only related to POY post-processing conditions, but also It is related to the quality of POY raw silk. POY with potential hidden dangers and poor fiber structure cannot be overcome during the process of processing into DTY, and these defects can only be concealed to a minimum. Therefore, timely and correct evaluation of the quality of POY can, on the one hand, adjust the spinning process conditions in time to prevent the generation of POY with a large number of defects, which will affect the quality of the final product DTY; on the other hand, the POY that has already formed defects can Choose appropriate processing conditions when processing DTY, try to cover up the shortcomings of POY, and ensure the quality of the finished yarn. The following indicators of POY can better reflect its quality.
1. Evenness
1. Determination of evenness
Unevenness indicates the degree of uneven thickness in the length direction of the filament. For the detection of evenness, most of the Uster I-C, Uster II-C, and Uster III-c types produced by Uster in Switzerland are currently used at home and abroad. The measurement principle is to indirectly measure changes in the thickness of the filament based on changes in capacitance. When the wire passes through an air capacitor composed of two parallel metal plates at the detection point, when the quality of the wire per unit length changes, it will cause a change in capacitance. The rate of change in capacitance is related to the change in the quality of the wire between the plates of the detection capacitor. The rate has a linear relationship, and is converted into the average difference coefficient U% or variation coefficient CV% through an automatic integrator, and the spectrum curve and evenness curve are drawn during the test. When the mass of the wire is fixed and the dielectric constant changes, the U value will also change. The dielectric constant of the filament is related to the oil content and moisture content of the polyester fiber. Therefore, to ensure accurate Uster value measurement, the oil rate on the filament must be uniform and the humidity of the test room must be constant. In addition, when dust in the air contaminates the yarn, it will also cause changes in the quality of the yarn, causing the measured Uster value to be inaccurate. Therefore, when using a Uster instrument to measure evenness, it must be carried out in a clean environment with constant temperature and humidity.
When measuring the Uster value of evenness, it can be expressed by three U% values or CV% values: normal test, inhibition test and semi-inhibition test. The so-called normal test means that the signal from the material being tested is based on the length of the measurement field used, and the measurement results include unevenness in short segments and long segments. The suppression test results reflect unevenness in long segments, while unevenness in short segments is suppressed with the help of filters. The measurement of the semi-suppression value means that when only the unevenness of the middle segment is expected to appear on the unevenness curve, the suppression test can be used, and the appropriate paper feeding speed and sample speed are selected to make the unevenness of the middle segment appear clearly, and Short and long segment unevenness is suppressed. It can be seen that when yarns with the same evenness are measured, the obtained values are the smallest inhibition value, followed by the semi-inhibition value, and the highest normal test value.
(1) Average difference coefficient U%. It can be described in Figure 1-14, expressed as a percentage, and its calculation formula is:
( 2) Coefficient of variation CV%. When the mirage is evenly distributed, the change in quality can be considered to be close to a normal distribution: the coefficient of variation CV% is the standard deviation S divided by the mean, and the calculation formula is as follows:
If the changes in the measured yarn quality are normally distributed, the relationship between the average difference coefficient and the coefficient of variation is:
CV =1.25U
2. POY evenness curve and spectrum chart
If the evenness of POY is very good, then U% Very small, the unevenness curve is almost a straight line, and its spectrum is a flatter spectral line with lower amplitude, as shown in Figure 10-15(a). When the evenness of POY is large, a steamed bun-shaped peak or a chimney-shaped protruding peak will appear on the spectrum, as shown in Figure 10-15(b). The former is a continuous periodic wave, which is caused by process characteristics. Caused by differences in factors; the latter is a discontinuous periodic wave, generally caused by mechanical defects.
According to the literature, when CV% ≥ 1.2%, the POY has a greater degree of uneven thickness and false twist deformation. There are many ends broken; when CV% ≤ 0.9%, POY has uniform thickness, stable texturing, high dyeing rate when processed into DTY, and less stiff and wooly fibers. Therefore, it is generally believed that CV% ≤ 0.9% is a normal spectrum, and CV% ≤ 0.8% is an ideal POY stem spectrum.
Although the evenness U% or CV% value can reflect the evenness uniformity of POY to a certain extent, it cannot be used to identify the occurrence of unevenness.��The dyeing properties of the product. Teijin Corporation of Japan applies POY’s structural integrity parameter (ε0.2) to evaluate the quality of POY.
4. Cold tensile stress-strain curve of POY
POY has a typical stress-strain curve during static stretching (Figure 10-19). There are obvious yield points, yield zones, natural tensile zones, creep zones and fracture points on this curve, from which the intrinsic quality of POY can be evaluated.
1. The fluctuation value of tensile force at 60% elongation
If the strength and elongation unevenness of the yarn are large, then Fiber performance is poor. For POY, it is best to use the tensile force fluctuation value when the elongation is 60% to measure its quality. Because when POY is stretched to 60%, its natural stretching zone ends. This point is the turning point for the fiber to switch from thin-neck stretching to uniform stretching. If the tensile force at this point is uneven, it means that the fiber structure is uneven, which will cause fluctuations during post-processing, resulting in poor quality of the finished product. It is generally believed that the CV% value of its fluctuation should be less than 3%.
2. Selection of POY post-stretch ratio
The choice of POY post-stretch ratio has a great influence on the performance of post-processed finished products. If the stretching ratio is too high, the finished product will easily produce lint due to factors such as high processing tension; if the stretching ratio is too low, the post-processing process will be unstable and stiff fibers will easily occur, resulting in poor dyeing. Therefore, an appropriate stretch ratio must be selected. The post-stretch ratio can be estimated based on the cold tensile stress-strain curve; the post-stretch ratio can also be taken as the post-stretch ratio when POY is elongated by 90% (just the midpoint between the end of the yield zone and the break point on the stress-strain curve) , the stress value and fluctuation rate at 90% elongation are also extremely important indicators for measuring the quality of POY. In addition, the stress at 20% elongation is called the yield zone stress of POY, which is an indicator of the degree of POY pre-orientation.
The reproducibility of the stress-strain curve between different lengths of each wire package and between each wire package should be good. If the reproducibility is poor, especially between the strength at 60% elongation and the strength at maximum elongation, it will be difficult to select the most appropriate post-POY stretch ratio. This is also the reason why the optimal stretching zone range in DTY processing is narrow.
5. Unevenness
The unevenness of the strength, elongation and linear density of POY has a negative impact on the quality of the post-processed finished product. , weaving and use performance have a great impact. POY with high unevenness will not only have large unevenness in DTY after post-processing, which will affect the weaving and wearing properties of the fabric, but also will easily produce wool, stiff yarn and false twist unevenness during processing into DTY. , and even increase the breakage rate, affecting the stability of post-processing.
Among the three unevenness rates of POY’s strength, elongation, and linear density, the most important unevenness that affects product quality is linear density unevenness, followed by strength unevenness. rate and elongation unevenness. Because fibers with large linear density unevenness tend to increase their strength and elongation unevenness, and can easily cause fluctuations in false twist and false twist tension during DTY processing, thus affecting the quality of DTY.
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