Effect of Twist Coefficient and Thermal Treatment Temperature on the
Properties of PET Yarn
1, 2, 3
Jia-Horng Lin
, Po-Ching Lu1, Jin-Jia Hu4, b, Yueh-Sheng Chen5 and
Ching-Wen Lou6, a
1
Laboratory of Fiber Application and Manufacturing, Department of Fiber and Composite Materials,
Feng Chia University, Taichung City 407, Taiwan, R.O.C.
2
School of Chinese Medicine, China Medical University, Taichung 40402, Taiwan, R.O.C.
3
Department of Biotechnology, Asia University, Taichung 41354, Taiwan, R.O.C.
4
Department of Biomedical Engineering, National Cheng Kung University, Taiwan, R.O.C.
5
Department of Biomedical Imaging and Radiological Science, China Medical University, Taichung
40402, Taiwan, R.O.C.
6
Institute of Biomedical Engineering and Material Science, Central Taiwan University of Science
and Technology, Taichung 406, Taiwan, R.O.C.
Corresponding email: acwlou@ctust.edu.tw, bjjhu@mail.ncku.edu.tw
Keywords: Twisting, polyethylene terephthalate, twist coefficient, thermal treatment
Abstract Twisting has an influence on not only the strength, luster, softness, and elasticity of the
yarns, but also the properties of the resulting fabrics. This study aims to evaluate how the rotor
twister speed and the thermal treatment temperature influence the properties of the polyethylene
terephthalate (PET) plied yarns and wrap yarns. Both yarn types are made with a twist coefficient of
1, 1.5, or 2, and then thermally treated in an oven at 70, 110, and 150 ℃. The resulting plied yarns
are wrap yarns are tested for tensile strength and elongation, and their surface is also observed by a
stereomicroscope to determine the influences of the two parameters. The results of the experiment
show that twist coefficient does not influence the tensile strength nor the elongation. The tensile
strength of various plied yarns and wrap yarns is approximately 4.4cN/dtex; the variation in the
twist coefficients is too small and thus hardly causes any significant differences in the yarn’s
properties.
Introduction
Polyethylene terephthalate (PET) is made of terephthalic acid and ethylene glycol. Due to its
good mechanical and thermal properties, PET fiber is the most commonly used synthetic fiber, such
as in the clothing, geotextile, construction, auto interior and medical fields [1-4]. Twisting, an
essential process of the production of yarn, gives the yarn good abrasion force and friction, thus
creating good strength in yarn. Twisting can change the properties of yarn, such as texture, strength,
elongation, luster, and crimp. Fabrics made of varying twist coefficients also possess different
properties of strength, elongation, elasticity, smoothness, and luster [5]. As a result, the variation in
twist coefficient has an influence on the properties of the yarn and fabrics. This study uses tensile
strength test, elongation test and stereomicroscopic observation to determine the effects of the rotor
twister speed and the thermal treatment temperature on the PET yarn.
Experimental
This study combines 75D PET yarns (Far Eastern New Century, Taiwan) to create yarns with a
greater fineness by twisting and plying. The plied yarns (150, 225) are made by plying, and then
twisted with a twist coefficient of 1, 1.5, or 2. The wrap yarns (75/75, 75/150, 150/75) are made by
twisting with a twist coefficient of 1, 1.5, or 2. After being thermally treated at 70, 110, and 150 ℃,
both yarn types are tested for tensile strength and elongation and observed by a stereomicroscope.
Tests
Tensile Strength and Elongation
This test follows ASTM 2256. The tensile strength and elongation of various plied yarns and
wrap yarns are tested by a Textechno Statimat (Germany). The distance between the machine’s
fixtures is 250 mm, and the tensile speed is 300 mm/min.
Stereomicroscopic Observation
The plied yarn that has the optimum tensile strength and elongation is then observed by a
stereomicroscope to determine its thickness. Magnification is 20 x.
Results And Discussion
Influence of Twist Coefficient on the Tensile Strength of the Plied Yarns and Wrap Yarns
Figure 1. The tensile strength of the plied yarns and wrap yarns as related to twist coefficients.
Figure 1 shows that the denier of yarn, twist coefficient, or combining method does not
significantly influence the tensile strength. Different combining methods may influence the friction
and crimp of the plied yarns and wrap yarns; however, both yarns undergo a twisting process with
twist coefficient of 1, 1.5, or 2. The small variation in twist coefficient results in a similar tensile
strength of various yarns, between 4.3 and 4.5 cN/dtex.
Influence of Twist Coefficients on the Elongation of the Plied Yarns and Wrap Yarns
Figure 2. The elongation of the plied yarns and wrap yarns as related to coefficients of twisting.
Figure 2 shows that plied yarns have a lower elongation than wrap yarns. Lacking crimps in their
core, the wrap yarns have a small tensile deformation, and thus, a limited elongation. By contrast,
the elongation of the plied yarns increases with an increase in twist coefficient. In addition, the
elongation of the wrap yarns and plied yarns increases with the twist coefficient. The higher the
twist coefficient, the greater the amount of twists; this gives both yarn types a greater tensile
deformation.
Influence of Temperature of Thermal Treatment on the Tensile Strength of the 250D Plied
Yarns
Figure 3. The tensile strength of 225D as related to various temperatures of thermal treatment and
twist coefficients.
According to the result of elongation test, the 225 D plied yarn has the optimum elongation, and
thus, this plied yarn is chosen for the following three discussions. Thermal treatment, an important
factor to prevent the yarns from untwisting, should be performed at a temperature that between the
glass transition temperature (Tg) and the melting point temperature (Tm). As PET yarns have a Tg
of 75-80 ℃ and a Tm of 250 ℃, the temperature for thermal treatment is set to 70, 110, and 150
℃. Figure 3 shows that the tensile strength of 225D increases with the twist coefficient and thermal
treatment temperature. The thermally-treated plied yarns hardly untwist, and the level of untwisting
increases when the thermal treatment temperature increases. As a result, with a greater twist
coefficient and a higher temperature for thermal treatment, 225 D has a greater tensile strength.
Influence of Temperature of Thermal Treatment on the Elongation of 250D Plied Yarns
Figure 4. The elongation of 225D plied yarns as related to various temperatures of thermal
treatment and twist coefficients.
Figure 4 shows that the elongation of the 225D increases with the thermal treatment temperature
and twist coefficient. After being thermally treated, the plied yarns do not untwist; the higher the
temperature, the lower the level of untwisting of the plied yarns. Without untwisting or with a
greater twist coefficient, the plied yarns have a greater amount of crimps. These crimps can be
extended when the plied yarns are stretched, thus increasing the yarns’ elongation.
Influence of the Temperature of Thermal Treatment Thickness of 250D Plied Yarns
Figure 5. Thickness of 250 D plied yarns as related to various temperatures of thermal treatment
and twist coefficients.
Figure 5 shows the thickness of the 225 D plied yarns which are made with twist coefficients of
1, 1.5, and 2, and thermally treated at 70, 110, and 150 ℃. The thickness of 225 D decreases with
an increase in twist coefficient. The twisting force that is applied to the plied yarns increases as a
result of the ever-increasing twist coefficient, thus decreasing the thickness of the resulting plied
yarns. With the same twist coefficient, the higher the thermal treatment temperature, the lower the
thickness of the plied yarns. Thermal treatment prevents the untwisting of the plied yarns;
increasing the temperature can result in better-shaped plied yarn.
Conclusion
This study successfully evaluates the influences of rotor twister speeds and thermal treatment
temperature on the properties of the resulting yarns. Neither the combining method nor the rotor
speed significantly influences the tensile strength of the plied yarns and wrap yarns. The plied yarns
have a greater elongation than the wrap yarn. When thermally treated at an increasing temperature,
the tensile strength and elongation of the 225D plied yarns increases but the thickness of the 225D
plied yarns decrease.
Acknowledgements
This work would especially like to thank National Science Council of the Republic of China,
Taiwan, for financially supporting this research under Contract NSC 100-2622-E-166-001-cc3.
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