Project Description

The methods for producing ultrafine fibers include direct spinning, composite spinning, and blend spinning. Among them, the focus of research and development is the method of using COPET to produce ultrafine fibers by alkali dissolution and stripping. Alkali-soluble polyester (COPET) is commonly used as the sea component, and regular PET is used as the island component for composite spinning. This article will discuss how to produce alkali-soluble sea-island polyester fiber from COPET (alkali-soluble PET Chip). High-quality 129dtex/36f POY sea-island fibers can be produced by selecting suitable process conditions.

1. Test
1.1 Raw Material
COPET is supplied by Hubei Decon company and regular PET is from Zhejiang Hengli company.

Intrinsic Viscosity dl/g 0.535 0.676
Melting Point 240 261
Color Value b 5.0 2.1
L 58.19 85.1
COOH Content Mol/t 6.02 31.6
DEG Content % 4.35 1.28
Moisture Content % 0.2 0.01

1.2 Production Process

Fig 1 Diagram of POY and DTY Process

Fig 1 Diagram of POY and DTY Process

1.3 Main Production

Equipment FBM330-02 Crystallization Drying Unit: Zhengzhou Zhongyuan Drying Technology Co., Ltd.                                            JWM60/25C Extruder: Shanghai Jinwei Machinery Manufacturing Co., Ltd.
Filament components: Carson Company, Japan
Winding Machine: Murata 768, Japan                                                                                                                                                                            Elasticity Adding Machine: FK6-1000V, Germany

1.4 Testing Method

Differential Scanning Calorimetry (DSC): Mettler DSC12E Differential Scanning Calorimeter
Intrinsic Viscosity: benzene-tetrachloroethane solution (volume ratio 1:1) with a Ubbelohde viscometer
Tensile Strength: Udter Tensorapid-3 Tensile Testing Machine
Evenness of Strips: Uster Tester-3 Strip Dryness Analyzer

2. Results and Discussion

2.1 Thermal Performance Analysis of COPET Chips

As shown in Figure 2, the melting point and glass transition temperature of COPET chips differ significantly from those of ordinary PET chips. This is due to the incorporation of the third monomer, sodium 5-sulfoisophthalic acid diethyl ester (SIPE), and the fourth monomer, polyethylene glycol (PEG), during the polymerization process. SIPE contains polar sulfonic acid salt groups, which have an electron-withdrawing effect and provide the possibility of polymer dissolution. PEG belongs to the flexible chain segment, improves the rigidity of the macromolecule, and regulates the viscosity of the system. The addition of these two monomers disrupts the regularity of the macromolecular chain, reducing the crystallinity of the polymer and lowering the melting point of COPET. Additionally, the addition of ether bonds on the macromolecular chain makes it susceptible to oxidation, leading to the generation of peroxides and decomposition into free radicals. These free radicals further accelerate the oxidation of the macromolecule, resulting in a decrease in the oxidative decomposition temperature and a reduction in the thermal stability of COPET chips.

Fig.2 DSC Thermograms of COPET Chip

Fig.2 DSC Thermograms of COPET Chip

2.2 Pre-crystallization Drying Process

To prevent the occurrence of filament floating and breakage, it is necessary to optimize the pre-crystallization drying process to ensure that the moisture content in COPET chips is below 30 μg/g. However, due to the poor thermal stability of COPET chips, excessively high pre-crystallization and drying temperatures can lead to chip degradation and reduced viscosity. Therefore, when selecting process conditions, it is important to employ low temperatures for prolonged drying periods to ensure that the chips do not experience significant viscosity reduction (less than 0.02dL/g) after drying. During production, the pre-crystallization temperature is set at 128-130℃ for 20-30 minutes, while the drying temperature ranges from 115 to 125℃ for 12 to 18 hours, with a dry air dew point of less than -70℃.

2.3 Composite Ratio

The composite ratio is an essential parameter for the formation of sea-island fibers, and the selection of an appropriate composite ratio primarily depends on whether the interface between the sea phase and the island phase of the POY fibers is clear. In production, the general choice for the composite ratio is 3/7 (sea/island). In actual production, the production cost should also be taken into consideration, allowing for the use of a smaller composite ratio. The choice of ratio is related to the quality of COPET, POY equipment conditions, and other factors. The composite ratio also affects other fiber properties, such as splitting time and dyeing.

2.4 Spinning Process

2.4.1 Spinning Components

The Japanese Carson components are employed, including the housing and two sets of melt channels. The melt channels consist of a sand bed and a distribution plate, with two parallel sand cavities in the sand bed. The melt distribution system consists of five distribution plates, all of which are of the surface-sealing type, and are locked together with bolts along with the spinneret plate. Metal sand is selected as the filtering medium, with an optional addition of an appropriate amount of glass beads. Because the COPET component pressure rises rapidly, the initial pressure of the components should be low, and assembly methods with slow pressure rise should be chosen wherever possible to extend the component’s lifespan. The sand-filling process generally involves using PET 40-60 mesh and COPET 30-40 mesh, with initial pressures for PET ranging from 10 to 11 MPa and for COPET ranging from 9 to 10 MPa.

To ensure the formation of sea-island fibers, the pressure difference between the sea phase and the island phase should not be too high, ideally staying below 5 MPa. Otherwise, the island phase is prone to uneven distribution, which can affect subsequent processing and spinning performance.

2.4.2 Spinning Temperature

Due to the significant differences in the properties of the sea phase and the island phase, when determining the spinning temperature, it is necessary to consider not only the melting points and rheological properties of the two types of slices but also the spinnability of COPET slices and the compatibility of the two melts. This ensures that the cross-section of the sea-island fibers remains sufficiently smooth, and there is a clear interface between the sea phase and the island phase, preventing any fibrillation.

Both PET and COPET melt undergo a certain degree of degradation during the process from the screw extruder to the spinneret plate. Adjustments in spinning temperature can be made based on the magnitude of the decrease in the viscosity of the oil-free filament. The viscosity reduction for PET should be controlled within 0.02 dL/g, while for COPET, due to slightly poorer thermal stability, the viscosity reduction should be kept within 0.07 dL/g. To ensure some level of compatibility between PET and COPET, it is necessary to adjust the temperature to make the viscosities of the two melts closer, preventing the occurrence of fibrillation. After experiments, the spinning temperature was determined to be 280℃, with temperatures for different sections of the screw as shown in Table below.

Components                                                        Screw Temperature/℃
PET 275 280 286 292 292
COPET 260 265 270 273 275

2.4.3 Cooling Conditions

The spinning of sea-island fibers requires relatively strict cooling and shaping conditions, as the quality of the formation directly affects the tensile performance and subsequent fiber opening results. Appropriate cooling conditions favor the reasonable and uniform distribution of the sea phase and the island phase within the fiber. The cooling air speed should be selected carefully, as excessive speed can generate turbulence, leading to significant filament oscillation and adversely affecting the evenness of the strips. This, in turn, can result in difficulties during subsequent processing and dyeing issues. On the other hand, too low of a cooling speed leads to poor heat dissipation and susceptibility to external disturbances. Through experimentation, it was determined that controlling the side blowing air speed at 0.4 m/s, with a temperature of 23℃ and a humidity of 65%, yields favorable shaping results and a low strip unevenness rate (1.3%).

2.4.4 Winding Speed

Excessive spinning speeds can cause excessive tension in the spinning process, leading to an increase in strip unevenness and unfavorable results in spinning sea-island fibers and winding formation. High-speed operation also results in significant equipment damage. Typically, the winding tension ranges from 0.11 to 0.22 cN/dtex. Through experimentation, it was found that a winding speed of 3000 m/min, a winding overfeed of 0.6%, and a winding tension of 0.2 cN/dtex result in excellent product quality, with a very clear interface between the sea phase and the island phase of the polyester sea-island fiber POY. The quality indicators of the sea-island fiber POY are outlined in the Table below.

Items Specs Items Specs
Types/(dtex/f) 129/36 Breaking Strength
CV (%)
Yarn Density/dtex 129 Yarn Unevenness, % 1.3
Elongation for Breaking, % 127 Oil Content, % 0.5

2.5 Elasticity Adding Process

The stretching and deformation process of sea-island fibers does not differ significantly from that of ordinary fibers. The single filament fineness of sea-island fibers before opening is not particularly fine. When selecting the elasticity-adding process for sea-island yarn, the typical POY processing method can be used as a reference. However, it is essential to consider that the thermal stability of COPET on the fiber surface is not ideal, making it unsuitable for stretching at excessively high temperatures. This is to avoid excessive fiber damage, hence the overall process should be relatively gentle.
The quality indicators for DTY are provided in the Table below.

Items Specs Items Specs
Yarn Density/dtex 82.9 Crimp Shrinkage Rate 14
Elongation for Breaking, % 129 Crimp Stability
Elongation for Breaking, % 127  

2.6 Alkali Dissolution Experiment

The method for opening sea-island fibers is carried out in an aqueous solution of alkali. In order to achieve fiber opening without damaging the PET, the alkali concentration is kept very low (no more than 2%). Generally, it is heated to boiling under atmospheric pressure, and the dissolution time is controlled appropriately. A certain amount of sea-island fiber sample (composite ratio 3/7) is boiled in a 2.4% NaOH solution with a liquor ratio of 1/20 for 30 minutes. The alkali reduction rate is calculated to confirm that the fiber-opening alkali reduction reaches 28%.

3 Conclusion

3.1 Choosing appropriate process conditions can yield high-quality 129/36f POY sea-island fibers. The sea phase process conditions include a pre-crystallization temperature of 130℃ for 30 minutes, a drying temperature of 120℃ for 17 hours, and a component pressure of 9 MPa, with a spinning speed of 3000 m/min.

3.2 The principles for selecting the elasticity-adding process for sea-island fiber DTY are quite similar to those for ordinary DTY, and the obtained DTY exhibits good fiber opening and peeling effects, meeting the requirements for ultrafine fibers.