The effect of aging and first heater temperature on the physical properties of polybutylene terephthalate textured yarn

2.1 Materials and Machines

PBT chips having an intrinsic viscosity of 0.87 dL/g were dried at 140°C for 4 hours. The Barmag Extruder (Germany) spinning machine with a 48-hole spinneret, a spinning temperature of 257°C, and a winding speed of 2850 m/min was used. An industrial Barmag FK6V-1000 false-twist texturing machine (Germany) was also used for producing textured yarns. The process speed was 550 m/min. Also, five different texturing temperatures of 160°C, 170°C, 180°C, 190°C, and 200°C were investigated. Other texturing conditions were maintained constant. The draw ratio and D/Y were 1.59 and 1.9, respectively. Disk-friction type texturing unit was used. The disk configuration was 1 + 5 + 1.

3.1 Linear density

Linear density of all samples was determined by measuring the weight of 90 m length of yarns.

3.2 Mechanical properties

Breaking elongation and tenacity of the yarn was measured using SIGMA500 machine. The gauge length for as spun yarns was 25 cm and for textured yarns was 4°m with 500 mm/min cross head speed.

3.3 Shrinkage

Shrinkage of yarns was measured using DIN 53840 standard. Samples were heated in an oven at 130°C for 10 minutes and then were cooled at room temperature for 60 minutes. Shrinkage (S) was calculated from following equation:

(1)

where L₁ and L₂ are yarn length before and after heating, respectively.

3.4 Crimp properties

The crimp properties of textured yarns including crimp contraction, crimp modulus, and crimp stability were determined based on DIN 53840 part 1 using Textechno Texturmat ME instrument. A yarn skein of 2500 dtex was heated at 120°C for 10 minutes and was then cooled at room temperature before length measurements. Crimp properties were calculated through the following equations:

(2)

(3)

(4)

in which L_g is the skein length after loading 500 cN for 10 seconds, L_z is the skein length after reducing the load to 2.5 cN for 10 minutes, L_f is the length after loading 25 cN for 10 seconds, and L_b is the length after loading first 2500 cN for 10 seconds and then measurement of skein length to 2.5 cN for 10 minutes.

3.5 Differential scanning calorimetry

Thermal analysis were conducted via a Mettler-Toledo DSC-1 instrument to find the crystallinity. The temperature was increased from 25°C to 250°C with a heating rate of 10°C/min. The crystallinity of samples (X) was calculated via below [Equation (5)]:

(5)

where ΔH is the melting enthalpy of sample and ΔH* is the enthalpy of fusion of a completely crystalline sample, that for PBT is equal to 145 J/g.14

3.6 X-ray diffraction

X-ray diffraction (XRD) analysis was carried out using Philips X'Pert-MPD instrument with CuKα radiation (λ = 1.54 A°) which operated at voltage of 40 Kv and current of 40 mA with angle diffraction of (2θ) between 2 and 70°.

The crystallite size was measured using the Scherrer equation15:

(6)

where K is the Scherrer constant (0.9), λ is the wavelength of CuKα X-ray (1.54 A), β is FWHM (full width at half maximum), and θ is the Bragg angle.

The crystalline orientation function (f_c) was measured using the equation:

(7)

3.7 Density

The density of the samples was measured by the flotation method at 20 ± 0.05°C. Weight fraction of crystallinity (X_c) was calculated by the following equation:

(8)

where ρ_c (1.396 g/cm³), ρ_a (1.281 g/cm³), and ρ are the crystal, amorphous, and sample densities, respectively.16

3.8 Birefringence

Birefringence was measured on a Ziess polarizing microscope (Germany) with a 30th order tilting compensator.

3.9 Statistical analysis

Data from each test was reported as mean values ± SD. Meanwhile, statistical analysis of data was done using two-way ANOVA and independent samples t test. A P-value of <.05 was defined for significant difference between the various samples.

3.10 Instantaneous elastic recovery

Tests for the elongation of the filaments were performed with the Shirley Micro-50 Machine. The samples were subjected at tension with initial length of 30 mm under 0.5 cN/tex force and at crosshead speed of 13 mm/min in different elongations (5%, 10%, 20%, and 25%). Then the crosshead with the same speed immediately returns to the first state. The instantaneous elastic recovery for the first and fifth cycles was calculated according to the following formula (15):

(9)

where L₀ is the initial length of the filament, L is the irreversible length of the filament after the head was returned to its initial position.

4.1 Thermal analysis

Table 1 shows the thermal behavior of samples, and Figure 1 presents the differential scanning calorimetry thermographs.

As it can be seen in Table 1, by increasing first heater temperature from 160°C to 200°C, the percentage of crystallinity of textured yarn was reduced. The crystallization of textured yarn at temperatures of 160°C and 170°C is more than POY yarn. However, the crystallinity of textured yarn in 180°C, 190°C, and 200°C was slightly less than POY yarn. In the case of aged and then textured POY yarn, it seems that the percentage of crystallinity is almost equal to yarn without aging and textured at 190°C. POY is subjected to externally applied forces and internal stress during draw texturizing process. The externally applied forces are tensile force and yarn twisting forces mean torsional and bending that these forces will not allow crystallinity to develop the same as the case of heat stability of filaments. This is because simple tension will assist in the formation of a lattice, while bending and torsion will tend to distort the lattice.7

4.2 X-ray diffraction

X-ray diffraction patterns of samples are illustrated in Figure 2. The reflections observed at 2θ = 9.2°,17.2°, 23.25°, and 25.2° are corresponded to the reflections from the (001), (010), (100), and (1^-11) planes, for the α form, respectively.17, 18 The XRD results of the samples are listed in Table 2. As it can be observed, there is a decrease in percentage of crystallinity, crystallite size, and crystalline orientation function ƒ_c in (010) and (100) planes after texturizing of POY yarn with aging and without aging.

By comparing the aged and then textured POY yarn at 190°C and nonaged and then textured POY yarn at 190°C, it is clear that the percentage of crystallinity, crystallite size, and crytstalline orientation function are not significantly changed.

The PBT crystallization behavior is slightly different from the behavior reported for PET and nylon. The X-ray diffraction studies reported about PET showed that the crystal orientation function and size of crystal increased with increasing temperature of the yarn (155°C-185°C). Caracas et al.8 reported that the crystalline orientation factor (f_c) and the crystallite size of the nylon 66 yarn increases with an increase in the temperature of the texturizing up to 200°C, and then decreases at temperatures of 210°C and 220°C. In fact, with the increase in temperature, the forces of the intermolecular will be weakened, so the flexibility of the macromolecules and the movement of the structural elements increases. As the hardness of filament decreases, the regularization of the crystals becomes easier.

On the other hand, one of the factors affecting the quality textured filament yarn is the degree of crystallization of feed yarn.

In general, orientation and crystallization depend on the previous history of the yarn. If the crystallization or orientation is high enough, they will not increase in the textured and may even decrease. Because perpendicular force on the yarn axis can decrease the crystallization due to the presence of excessive force.19 The suitable temperature for crystallization is T_c, while the stabilization temperature (first heater temperature) is usually 10-15° below the softening temperature (T_S). At the stabilization temperature, the instability of the secondary bonds is higher than formation of a crystalline orientation. Therefore, considering that the crystallization rate of this polymer is high and POY yarn has a high crystallinity (33.55) and sufficiently crystallized before it is textured, and in texturizing in the case of under super-temperature conditions (higher than T_C) and under tension, the crystallization can not increase and it has been decreasing.

4.3 Density

The obtained results illustrated that POY fibers and textured yarns had a density of 1.31 and 1.32 g/cm³, respectively. They also had crystallinity of 28% and 35%, respectively. No accurate data were obtained for the textured yarn at different temperatures at the first heater, due to the low precision of the method and the low difference between the density results.

4.4 Orientation

Karakas et al.8 implied that the birefringence is a criterion of orientation and increases with increasing the temperature of heater. The POY and textured fibers had a diameter of about 0.021 mm and 0.015 mm, respectively, therefore the diameter of the POY fiber was more than that of textured fibers. POY fibers had a birefringence of 0.06. The textured fibers had birefringence of 0.08°C, 0.084°C, 0.088°C, 0.09°C, and 0.093 at 160°C, 170°C, 180°C, 190°C, and 200°C, respectively.

4.5 Effect of aging and first heater temperature on physical properties of textured PBT yarn

The physical properties of POY and textured samples are listed in Table 3.

The tenacity of textured yarns at various temperatures of first heater for 80 days is depicted in Figure 3. The tenacity increases by increasing the first heater temperature. Two-way ANOVA analysis showed that an increase in temperature had a significant effect on the yarn tenacity.

The variation of breaking-elongation in textured yarns at various temperatures of first heater during aging is shown in Figure 4. It can be seen that the breaking-elongation decreases significantly by increasing the temperature of first heater from 160°C to 200°C. In addition, the two-way ANOVA showed that an increment in temperature had significant effect on the breaking elongation.

The most significant factor for determining the fibers tenacity is orientation which can be obtained by birefringence.20

Caracas8 showed that, the breaking elongation of the yarn produced from the nylon 66 fibers was decreased and the tenacity was increased, by increasing the temperature of first heater to 200°C. However, at the temperatures of 210°C and 220°C the yarn tenacity was decreased.

Canoglu,6 implied that the temperature of first heater in the false twist texturing had a slight effect on the tenacity and elongation of the yarn. He also showed that by increasing temperature of first heater, the tenacity of the polyester yarn increases, which is due to increased crystallinity in the yarn. On the other hand, the elongation was decreased due to the dryness of the polyester yarn.

Texturizing the POY yarn and increasing the first heater temperature lead to an increment in the birefringence or orientation of yarn, which results in higher tenacity and lower breaking elongation of the textured yarn at five different temperatures than POY yarns. Also by increasing the temperature of first heater, the tenacity was increased, however the elongation was decreased.

In addition, two-way analysis of ANOVA showed that the aging had no significant effect on the tenacity and braking elongation of the yarn.

Popovic study13 showed that the tenacity and breaking elongation of the textured nylon yarn of 66 even after 12 months of maintenance were in the allowed range. Abbasi5 implied that the tensile properties of nylon 6 were relatively constant.

The effect of aging of the textured yarns at different temperatures of the heater, on yarn shrinkage, is shown in Figure 5. The shrinkage of the yarn was decreased by the temperature of first heater. The results of two-way ANOVA showed that the effect of temperature on yarn shrinkage was significant. The temperature of 160°C had the highest value (12.86) of shrinkage while the temperature of 200°C had the lowest (10.41). The shrinkage depends on the residual stress in the fibers. Therefore, by decreasing these tensions, the residual shrinkage in the fibers decreases. As the temperature increases, the internal stress of the fiber is released and the relaxation of the fiber would be better. As a result, there would be no tension inside the fiber to be released so that it can shrink it. Also, due to the fact that the orientation of the entire structure has increased by increasing the temperature of the texturizing operation, the shrinkage of the fiber has also decreased. The shrinkage of textured yarn is also higher than the POY fibers (Table 3), which indicates an increase in orientation in noncrystalline regions. Canoglu's studies6 also showed that by increasing the temperature of first heater, the amount of PET yarn shrinkage decreases.

In addition, two-way ANOVA analysis showed that aging had a significant effect on yarn shrinkage. Over time, the shrinkage was decreased and the maximum changes were about 4%, and the reduction was not important for applications. Popovic studies13 showed that there was no change in the shrinkage of nylon 66 after 12 months of maintenance, indicating that the yarn structure was stable after texturizing.

The crimp properties of the textured yarns at various temperatures of first heater and aging time of 80 days are depicted in Figures 6-8. An increment in heater temperature significantly increased the crimp contraction, crimp modulus, and crimp stability. The effect of aging on the variations of in crimp properties was significant.

Creating a deformation in the false twist texturing results in secondary bonds to be in an unstable state. The stabilization is in fact the sustainability of secondary bonds in a new position. Heating the polymer leads to more mobility of the chains in the molecular structure, and hence the stabilization would be faster and easier.21 Studies reported on PET6 and nylon8 also showed that the crimp properties of a textured yarn increases by increasing the temperature of the first heater.

It was also observed that the crimp and the crimp modulus had a decreasing trend over time. The maximum changes were about 4%, which does not matter in practical terms. The aging had no significant effect on wave stability. In the case of polypropylene (PP)11 and PET,12 the aging of the textured yarns results in a significant reduction in crimp contraction and crimp moduluse, while in the polyamide,5 the crimp properties remain relatively constant during the time.

Also, the t test showed that there was a no significant difference between the mechanical and crimp properties of the POY yarn while the textured yarn at 190°C and POY without time and then texture at 190°C.

4.6 Instantaneous elastic recovery

As it is obvious in Table 4 and Figure 9, by increasing the elongation, the instantaneous elastic recovery of the PBT filament was decreased in the first and fifth cycles. As the elongation increased higher than 25%, the fibers were broken. There are four flexible methylene groups in the PBT macromolecular unit chain which has a zigzagged molecular chain. Therefore a PBT filament is more flexible than PET and hence has a better elastic recovery than PET.16 Accordingly, no significant difference was observed in the obtained results, by comparing the aged-texturized and nonaged-texturized POY yarn in temperature of 190°C.