Mechanism of surface roughness development in melt spinning of blend fibers for artificial hair

Materials

The materials applied in this research were PA6 (NOVAMID 1020, DSM Japan Engineering Plastics), PET (PETMAX RE530A, TOYOBO Co., Ltd), amorphous co-PCT (AN004, Eastman Co., Ltd), amorphous co-polyamide (co-PA; NOVAMID X21, DSM Japan Engineering Plastics), poly(butylene terephthalate) (PBT; PBT 500LP, Polyplastics Co., Ltd), PBT copolymer (co-PBT; PBT 600FP, Polyplastics Co., Ltd) and polypropylene (PP; Y2000GP, Idemitsu Kosan Co., Ltd). According to the supplier data sheet, the chemical structure formulas of co-PA and co-PCT are represented as follows:

The chemical structure of the co-PBT was not disclosed by the producer. However, it was revealed that the melting temperature of the co-PBT is around 170°C, which is significantly lower than that of pure PBT, at 223°C. Combinations of major and minor components applied for the melt-spinning experiments are summarized in Table 1. Composition of major/minor components was fixed at 80/20 wt%. It should be noted that co-PA and co-PCT are amorphous polymers with their glass transition temperatures of 125 and 75°C, respectively.

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Table 1.

Conditions for melt spinning of blend fibers

Exp. No.	Major component (80 wt%)	Melting temperature (°C)	Minor component (20 wt%)	Melting temperature (°C)	Barrel temperatures				Nozzle diameter (mm)	Extrusion temperature (°C)
					C1 (°C)	C2 (°C)	C3 (°C)	C4 (°C)
1	PA6	220	PET	257	160	260	235	235	2.0	250–265
	PA6	220	co-PET	▵	160	260	235	235	2.0	250–265
2	PA6	220	PET	257	160	260	235	235	1.0	235–255
	co-PA	▵	PET	257	160	260	235	235	1.0	235–255
3	PP	165	PBT	223	160	260	235	235	1.0	210–240
	PP	165	co-PBT	170	160	260	235	235	1.0	210–240

Melt spinning of blend fibers

Dry-blended polymer pellets were melted and extruded through a twin screw extruder equipped with a metering pump and a spinneret with a single-hole spinning nozzle. The extruded filament was taken up using a winder placed at 2.0 m below the spinneret. Conditions for melt spinning are also shown in Table 1. In this table, barrel temperatures C1–C4 correspond to the zone temperatures from upstream to downstream in the extruder. In all cases, the highest temperature in the twin screw extruder was set to be higher than the melting temperatures of both components, whereas the extrusion temperature was adjusted in the regions of the metering pump and the spinneret. It should be noted that the highest barrel temperature of 260°C at zone C2 is only 3 K higher than the melting temperature of PET; however, the actual temperature of polymers in the extruder was supposed to be higher than the setting temperature because of the viscous heating of melted polymer blends. The throughput rate was controlled to 5.8 g/min. The nozzle diameter was 1.0 or 2.0 mm, and the take-up velocity was varied from 124 to 500 m/min. In cases of PP/PBT and PP/co-PBT combinations, winding of the fiber was impossible. Accordingly, only the free-fall samples were collected.

Analyses of as-spun blend fibers

Surfaces of as-spun fibers were observed by a scanning electron microscope (SEM; TOPCON Co., Ltd). The amount of crystalline phase in the as-spun fibers was analyzed through wide-angle X-ray diffraction (WAXD) measurement. The two-dimensional WAXD intensity distribution measurement for the fiber bundles was performed using a nickel-filtered CuKα radiation source generated at 60 kV-45 mA and a Mercury charge-coupled device (CCD) X-ray detector (Rigaku Co., Ltd) at a camera length of 35 mm. For each measurement, the exposure time of 10 s was repeated 5 times. Thermal analysis of the as-spun blend fibers was conducted with differential scanning calorimetry (DSC; TA Instruments.) at the heating and cooling rates of 10 K/min and 40 K/min, respectively.

Melt spinning of polymer blends of various combinations of polymers

Melt spinning of blend fibers of various combinations of polymers was carried out with the aim of elucidating the rule for the development of surface roughness. The crystallizability and melting temperature of each component were the factors of interest.

Firstly, SEM photographs of the PA6/PET and PA6/co-PCT blend fibers are compared in Figure 1. It should be noted that PET is a crystalline semi-aromatic polyester, whereas co-PCT is an amorphous semi-aromatic co-PCT. Both PET and co-PCT have similar glass transition temperatures of around 75°C. The SEM photographs show that these blend fibers exhibited smooth surface at a high extrusion temperature of 265°C. When the extrusion temperature was lowered to 255°C, which is lower than the melting temperature of PET, surface roughness was developed for the PA6/PET blend fiber, whereas the surface of the PA6/co-PCT fiber kept its smoothness. From these results, it would be reasonable to surmise that the crystallizability of the minor component is necessary for the development of surface roughness. OPEN IN VIEWER

Secondly, the effect of the crystallizability of the major component was investigated by using the amorphous co-PA as the major component. As shown in Figure 2, at the extrusion temperature of 255°C, both PA6/PET and co-PA/PET blend fibers exhibited a smooth surface, whereas roughness appeared on the surfaces of both blend fibers when the extrusion temperature was at and lower than 245°C. These results indicate that the crystallizability of the major component is not necessarily required for the development of surface roughness. OPEN IN VIEWER

Furthermore, comparison of the results for PA6/PET fibers in Figures 1 and 2 indicates that, at the same extrusion temperature, roughness development was suppressed with the change of nozzle diameter from 2.0 to 1.0 mm. On the other hand, because the attainable lowest extrusion temperature was lowered, eventually fibers with more enhanced roughness were obtained in the case of the smaller nozzle diameter of 1.0 mm. The influence of nozzle diameter on the roughness development will be discussed in detail in a forthcoming paper.

Thirdly, the effect of the relation between the melting temperatures of major and minor components was investigated by comparing the results of the melt spinning of PP/PBT and PP/co-PBT blends. The melting temperatures of PP, PBT and co-PBT are about 165, 223 and 170°C, respectively. The SEM photographs of PP/PBT and PP/co-PBT fibers are shown in Figure 3. There was a development of surface roughness for the PP/PBT blend at the extrusion temperature of 210°C, which was lower than the melting temperature of PBT. On the contrary, fibers with surface roughness could not be produced in the case of the PP/co-PBT blend. It should be noted that the melting temperature of co-PBT is similar to that of PP and, therefore, the extrusion temperature could not be lowered below the melting temperature of the minor component, co-PBT. OPEN IN VIEWER

The results of a series of experiments described above are summarized in Table 2. According to this table, the conditions necessary for the development of surface roughness are elucidated as follows: (1) the minor component needs to be crystallizable; (2) the major component can be either amorphous or crystalline polymer, but needs to be melt processable at a temperature lower than the melting temperature of the minor component; and (3) the extrusion temperature needs to be lower than the melting temperature of the minor component. It was also revealed that under the conditions for the appearance of surface roughness, roughness development is enhanced by lowering the extrusion temperature.

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Table 2.

Results of surface roughness development

Exp. No.1	Nozzle diameter (mm)	Extrusion temperatures (°C)						Take-up velocity (m/min)
		250	255			260	265
PA6/PET	2.0	○	○			▵	×	270
PA6/co-PCT	2.0	×	×			×	×	500
		Extrusion temperatures (°C)
Exp. No.2	Nozzle diameter (mm)	235	240		245	250	255	Take-up velocity (m/min)
PA6/PET	1.0	○	○		○	▵	×	124
co-PA/PET	1.0	○	○		○	▵	×	124
		Extrusion temperatures (°C)
Exp. No.3	Nozzle diameter (mm)	210	220			230	240	Take-up velocity (m/min)
PP/PBT	1.0	○	○			▵	×	*
PP/co-PBT	1.0	×	×			×	×	*

Analysis on the crystallization behavior of the minor component of blend fibers

The analyses described above suggested that the crystallizability of the minor component is necessary for the development of surface roughness. From the view point of the crystallization kinetics of the minor component, however, crystallization of the minor component PET was unlikely to take place within a short period in the extrusion and spinning processes. This is because, even though the extrusion temperature was lowered to a temperature slightly below its melting point, the crystallization rate is estimated to be extremely low. ⁷ Considering that the crystallization of the minor component can be detected more easily if the major component does not crystallize, detailed structural analyses of the co-PA/PET fibers prepared with the extrusion temperatures of 235, 245 and 255°C were carried out. As shown previously in Figure 2, the fibers prepared at the extrusion temperatures of 235 and 245°C exhibited a rough surface, whereas the fiber prepared at 255°C had a smooth surface.

Wide-angle X-ray intensity distribution patterns of the co-PA/PET blend fibers prepared at extrusion temperatures of 235, 245 and 255°C are compared in Figure 4. Along with the amorphous halo, a circular crystalline reflection was observed only for the fibers spun at 235 and 245°C. It is well known that amorphous PET fibers can be obtained in the ordinary melt-spinning process because of their low crystallization rate, and a well-developed crystalline structure can be formed only with high take-up velocities, where the concept of orientation-induced crystallization is applicable. ^{8 – 10} The intensity distribution was averaged along the azimuthal angle and plotted against the diffraction angle, as shown in Figure 5. The crystalline reflection at the diffraction angle of 26.1 degrees, which was assigned to the (100) reflection from the triclinic crystal of PET, ¹¹ was observed only for the fibers prepared at the extrusion temperatures of 235 and 245°C. This result clearly indicates that the crystallization of PET occurred in the melt spinning of the co-PA/PET blend when the surface roughness is developed because of the low extrusion temperature. OPEN IN VIEWER

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Figure 5.

Wide-angle X-ray diffraction (WAXD) intensity distribution curves for co-polyamide (co-PA)/poly(ethylene terephthalate) (PET) fibers melt spun with different extrusion temperatures of 235, 245 and 255°C, with a throughput rate of 5.8 g/min, nozzle diameter of 1.0 mm and take-up velocity of 124 m/min.

The crystalline state of PET was also analyzed by DSC. In the measurement, the fiber samples were heated at a heating rate of 10 K/min up to 300°C, and started to be cooled immediately after reaching the maximum temperature at a cooling rate of 40 K/min. DSC thermograms are shown in Figure 6. In the heating process, although there was an influence of water adsorbed to co-PA, the clear cold crystallization peak of PET appeared at around 110°C for smooth surface fiber spun with an extrusion temperature of 255°C. Meanwhile, the melting peak of the PET component was observed at around 255°C for the three samples. The melting peak temperature of the fiber prepared at 255°C was slightly lower than the other two samples. Consulting the results of the WAXD measurement shown in Figures 4 and 5, it is considered that the melting peak of the fiber spun at 255°C contains the melting of crystals formed by the cold crystallization at around 110°C during the heating process, whereas at least a part of the crystals corresponding to the melting peak of fibers prepared at 235 and 245°C is formed during the spinning processes. If the cooling process started without holding time after the heating of samples up to 300°C, clear exothermic peaks appeared in the DSC thermogram for the samples with low extrusion temperatures. This result indicates that high crystallizability was introduced to the PET component in the spinning process with low extrusion temperatures, and such high crystallizability remained even after its melting. To analyze the thermal stability of high crystallizability, the effects of maximum temperature and holding time in the DSC measurement on the crystallization behavior of the PET component in the cooling process were investigated. The DSC thermograms for the fibers prepared at 245°C are summarized in Figure 7. It was found that the crystallization during the cooling process in the DSC measurement became less distinct with the increases of maximum temperature and holding time. These results suggest that (1) a ‘structure’ of long relaxation time was developed in the PET component in the co-PA/PET blend in the spinning process, (2) development of such ‘structure’ caused the enhancement of the crystallization of the PET component, and (3) the crystallization of PET eventually led to the development of surface roughness on the produced fibers. It should be noted that such mechanism for the formation of fibers with surface roughness is applicable to various combinations of polymer blends, as described in the first section of results and discussion in this paper. A strong memory effect of polymer melt after shear flow near or above the nominal melting temperature has been reported by Matsuba et al. ¹² OPEN IN VIEWER

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Figure 7.

Differential scanning calorimetry (DSC) thermograms obtained during the cooling process of co-polyamide (co-PA)/poly(ethylene terephthalate) (PET) (80/20 wt%) fibers melt spun with an extrusion temperatures of 245°C, with a throughput rate of 5.8 g/min, nozzle diameter of 1.0 mm and take-up velocity of 124 m/min. Different maximum temperatures and holding times that were applied to the sample before the start of measurements are indicated in the figure.