The traditional bonding process for non-woven fabrics uses chemical adhesives, which can lead to environmental pollution and pose health risks to production workers and consumers. In 1977, Japan’s Chisso Corporation leveraged the low melting point properties of polyethylene (PE) to achieve the industrial production of PE/polypropylene (PP) thermal bonding composite fibers without using chemical adhesives, relying on the fibers’ inherent bonding capabilities.

With the rapid advancement of polyester non-woven fabrics, many foreign companies have developed low melting point polyester (LMPET) fiber products to address the issues of compatibility and adhesion between PE and PET. LMPET is a collective term for a series of modified copolyesters with melting points lower than conventional polyesters, with a melting point range between 90 and 240°C. LMPET products with melting points below 200°C can manufacture skin-core composite staple fibers and filaments, which find applications in filter media, clothing, sleeping bags, sofas, bedding, soft cushions, healthcare products, and medical supplies. Blended nonwovens can also be used as thermal insulation in high-end wool, cotton quilts, and premium cotton clothing. LMPET products with melting points above 200°C typically produce various polyester color masterbatches.

Currently, domestic production technology for PE/PP skin-core composite fibers is relatively mature, while products from South Korea and Japan mainly dominate the market for LMPET/PET composite fibers. Since the 1990s, organizations such as the Dalian Synthetic Fiber Research Institute, Sinopec Yizheng Chemical Fiber Co., Ltd., Beijing Institute of Fashion Technology, Sinopec Tianjin Branch, Sinopec Beijing Yanshan Branch, Sinopec Liaoyang Petrochemical Branch, Luoyang Nanfeng Polyester Co., Ltd., and Shantou Ocean Polyester Film Co., Ltd. have conducted extensive research on the synthesis of LMPET. In 2003, Sinopec Shanghai Petrochemical Company (hereafter referred to as Shanghai Petrochemical) initiated industrial research on LMPET/PET skin-core composite staple fibers and has developed LMPET filaments with a melting point of 110°C, as well as LMPET/PET skin-core composite filaments. These are applied in high-end inner bonding core yarns, sofa fabrics, anti-fuzzing base yarns for synthetic fur, seamless garment stitching, and air conditioning filter nets, among other products.

1 Experimental Section

1.1 Main Raw Materials
PTA (Purified Terephthalic Acid) from Shanghai Petrochemical; EG (Ethylene Glycol) from Shanghai Petrochemical; IPA (Isophthalic Acid) from Yanshan Petrochemical; AA (Adipic Acid) from Liaoyang Petrochemical; DEG (Diethylene Glycol) from Shanghai Petrochemical; LMPET chips (melting point: 110°C) from Hubei Decon’s factory. For more details about our low-melt PET chips, please refer to https://www.polyestermfg.com/low-melting-polyester-chip/.

1.2 Experimental Equipment and Analytical Instruments
Experimental Equipment: 80L stainless steel polymerization kettle, including an esterification kettle and a condensation kettle; VC406 type screw extruder from Shanghai Second Textile Machinery; JWM-65 screw extruder from Shanghai Jinwei Chemical Fiber Machinery; AW909 type winder from Teijin, Japan.

Analytical Instruments: Microscope melting point apparatus from Leica, Germany; differential scanning calorimeter from Mettler Toledo, Switzerland.

1.3 Experimental Method
Synthesis of LMPET: PET, EG, modified monomers, catalysts, and colorants are added to the 80L esterification kettle in a specific ratio, with the esterification temperature set at 200–235°C. The esterification process is terminated when the distillate reaches 95% of the theoretical water yield. A stabilizer is then added, and the material is transferred to the condensation kettle, where it undergoes pre-condensation for about 1 hour at a low vacuum of 0–0.1 MPa and a temperature of 260–270°C, followed by condensation under a vacuum of less than 60 Pa at a temperature of 270–280°C. Once the stirring power reaches the required level, the material is discharged, cooled, and pelletized to obtain LMPET chips.

2 Results and Discussion

2.1 Effect of IPA Addition on the Melting Point Range of LMPET

By partially replacing PTA with IPA, the co-polymerization alters the regularity of the polyester chain segments, increasing the distance between macromolecular chains and thus lowering the melting point. Literature has already explored the relationship between the amount of IPA added and the melting point of LMPET, finding that generally, as the amount of IPA increases, the melting point of LMPET shifts to lower temperatures.
The relationship between the amount of IPA added (the mole fraction of IPA in the total amount of PTA and IPA) and the melting point of LMPET is shown in Figure 1 (the initial melting point is the temperature at which LMPET begins to melt, while the final melting point is the temperature at which LMPET is completely melted).
FIG.1-Influence-of-IPA-addition-on-melting-point-range-of-LMPET
From Figure 1, it can be seen that as the amount of IPA added increases within the range of 0–50%, the melting point of LMPET gradually decreases. The amount of IPA added shows a linear correlation with both the initial and final melting points of LMPET to some extent.

2.2 Effect of AA Addition on the Melting Point Range of LMPET

Although the addition of IPA lowers the melting point of LMPET, it also reduces the crystallinity of LMPET. To improve the crystallization performance of LMPET, it is necessary to reduce the rigidity of the macromolecular chains to enhance their flexibility. The addition of AA disrupts the ordered structure of the PET molecular chains while simultaneously increasing the flexibility of the macromolecular chains, thus improving the crystallization performance of LMPET chips.

Based on quantitatively adding IPA to partially replace PTA, AA is further added to replace a portion of PTA. The relationship between the amount of AA added (the mole fraction of AA in the total amounts of PTA, IPA, and AA) and the melting point of LMPET is shown in Fig. 2.
Fig 2 Influence of AA addition on melting point range of LMPET
From Figure 2, it can be observed that as the amount of AA added increases within the range of 0–20%, the melting point of the copolyester gradually decreases. The amount of AA added shows a linear correlation with both the initial and final melting points of LMPET to some extent.

2.3 Effect of DEG Addition on the Melting Point Range of LMPET

The addition of DEG aims to increase the flexibility of the macromolecular chains in the copolyester, thereby improving the crystallization performance of LMPET chips.

Building on the quantitative addition of IPA to partially replace PTA, DEG is further added to replace a portion of EG. The relationship between the melting point range of LMPET and the amount of DEG added (the mole fraction of DEG in the total amounts of EG and DEG) is shown in Figure 3.
FIG.3 Influence of DEG addition on melting point range of LMPET
From Figure 3, we can see that as the amount of DEG added increases from 0 to 10%, the initial melting point does not change significantly, while the final melting point shows a certain degree of linear decline. The amount of DEG added exhibits a linear correlation with the final melting point of LMPET to some extent.

2.4 Comparison of Melting Point Ranges Between LMPET and Regular PET

LMPET chips with a melting point of 110°C were synthesized with 30% to 50% IPA addition, 0% to 10% AA addition (partially replace PTA), and 2% to 8% DEG addition (partially replace EG). Multiple samples of both regular PET and the aforementioned LMPET were analyzed, and their melting photos are shown in Figures 4 and 5.
FIG.4 REGULAR PET CHIP MELTING PIC
FIG.5 110C LMPET MELTING PIC
From Figure 4, it can be seen that when the temperature reaches 259°C, the edges of the chips begin to melt, indicating that the chip sample has started to melt; when the temperature reaches 261°C, the chip sample is completely melted, demonstrating that the melting point range of PET is relatively narrow, only 2°C.
From Figure 5, it can be observed that within the temperature range of 90–95°C, the striped texture in the chip sample begins to deform, the shadows of the stripes start to fade, and the edges begin to melt; when the temperature rises to 130–135°C, the striped texture completely disappears, and the chip sample becomes fully transparent and melted, indicating that the melting point range of LMPET is broader, reaching around 40°C.

2.5 Drying Process of LMPET

LMPET with a melting point of 110°C is classified as an amorphous polymer, which cannot enhance its softening point through crystallization during drying. Additionally, due to its low initial melting point, the sample remains in a highly elastic state during drying, making it prone to deformation due to extrusion, especially when using vacuum drum drying, where collisions and pressure can lead to further deformation and adhesion. Therefore, the drying temperature must be low, and the loading amount during drying should be less than that of conventional chip drying.

Generally, vacuum drum drying and filling drying methods are employed to dry LMPET chips with a melting point of 110°C.

The softening point of LMPET chips determines the drying temperature. A lower softening point results in a lower drying temperature. With lower drying temperatures, the vapor pressure of free moisture on the surface and the equilibrium moisture inside the chips is also lower, especially the equilibrium moisture inside, which slows the diffusion of moisture to the surface. Thus, the drying time must be significantly increased to remove moisture from the LMPET chips effectively. The vacuum level of the drum determines the moisture content in the air inside the drum; the higher the vacuum, the lower the moisture content, which is more favorable for moisture diffusion in the chips. Filling drying is mainly to prevent LMPET from absorbing moisture during use. Therefore, the drying of LMPET chips should primarily rely on vacuum drying, with filling drying as a supplement.

The drying process for LMPET chips with a melting point of 110°C is as follows:

  • Vacuum drum volume: 6m³
  • Loading: 2t
  • Vacuum level: -0.08MPa
  • Drying temperature: Room temperature to 55°C, with a constant temperature of 55°C for 48 hours
  • Filling drying temperature: 55–60°C, with dry air dew point temperature less than -30°C
  • The final moisture content of LMPET chips can be controlled below 100μg/g.

2.6 Spinning Process of LMPET

The melting point of conventional PET is 259–261°C. To ensure complete melting of the crystallization formed during the drying process, the spinning temperature should be higher than the polymerization temperature, typically between 285–295°C.

Literature suggests that the flow properties of LMPET at 250°C are comparable to those of PET at 290°C (or 295°C), especially in the high shear rate region, where both exhibit similar flow characteristics. Other studies indicate that the flow curve of LMPET at 240°C closely aligns with that of PET at 280°C, meaning the melt viscosity of LMPET in the range of 240–250°C is comparable to that of PET at 280–290°C, allowing for similar flow performance during spinning.

In the experiment, LMPET chips with a melting point of 110°C were used to produce fully oriented yarn (FDY) with a specification of 27 dtex/3f. The spinning process conditions were as follows:

Screw temperature: 110–235°C
Box temperature: 230–235°C
Side-blowing air temperature: 20–22°C
Side-blowing air speed: 0.3m/s
Winding speed: 2780–2800m/min

As an amorphous polymer, the melting process of LMPET involves a gradual softening and deformation until it melts. Due to its low softening point and broad melting point range, it is essential to maintain the temperature in the screw cooling zone below the softening point of LMPET to avoid premature softening and adhesion, which can lead to material bridging and slippage in the feeding section of the screw. The temperature in the screwⅠzone should not be too high to ensure that the chips are conveyed in a solid particle state. The temperature in the screw Ⅱ and Ⅲ zones should be significantly raised to ensure sufficient melting of the chips. While the temperatures in the screw IV and V zones should be set to ensure that the LMPET melt has flow properties comparable to those of PET during spinning.

The first and second stretching roller temperatures for PET are generally set at around 90°C and 120°C, respectively. However, due to the low initial melting point of LMPET, higher temperatures can lead to fiber adhesion or even breakage; thus, the temperatures of the first and second stretching rollers for LMPET should be appropriately lowered.

The quality index of LMPET FDY is shown in Table 1.
Table 1

2.7 LMPET/PET Core-Sheath Composite Spinning Process

For the melt composite spinning of two components, the primary condition is that the apparent viscosities of the two components in the molten state should be relatively close. If the melt viscosities of the two components exceed a ratio of 1.5 during compounding, spinning becomes very difficult. When using LMPET and PET with a melting point difference of up to 140K for core-sheath composite spinning, it is essential to select an appropriate spinning temperature to ensure that the apparent viscosities of the two components are similar, thereby achieving a stable spinning state.

When LMPET and PET enter the spinning component through their respective metering pumps, heat exchange occurs between the two melts and the spinning barrel. The temperature difference between the melts gradually decreases, increasing the temperature and a decrease in the viscosity of the LMPET melt, while the temperature of the PET melt decreases and its viscosity increases. Therefore, LMPET should adopt a low-temperature melting and high-temperature spinning approach, while PET should use a high-temperature melting and low-temperature spinning method to ensure that the melt viscosities of the LMPET and PET components are similar during compounding. This ensures that the flow rates of the two components through the same spinneret are comparable.

In the experiment, LMPET with a melting point of 110°C and PET were used for core-sheath composite spinning at a blend ratio of 60:40. The spinning process conditions were as follows: LMPET screw temperature 110–220°C, PET screw temperature 280–295°C, barrel temperature 280–285°C, side-blowing air temperature 20–22°C, side-blowing airspeed 0.35 m/s, and winding speed 2780–2820 m/min. The temperatures of the first and second stretching rollers were set at 55°C and 75°C, respectively. The quality indicators of the finished product are shown in Table 2.

Table 2 Quality specifications of LMPET/PET-FDY-75dtex/24f

Table 2

Additionally, to ensure that the core layer of PET achieves a certain degree of stretching, the stretching temperature should not be lower than the glass transition temperature of PET.

3. Conclusion

  1. There exists a certain linear relationship between the addition amounts of IPA (0–50%), AA (0–20%), and DEG (0–10%) and the initial and final melting points of LMPET.
  2. The melting point range of LMPET, with a melting point of 110°C, is between 90–135°C, with IPA addition amounts between 30%–50%, AA addition amounts between 0%–10%, and DEG addition amounts between 2%–8%.
  3. The drying of LMPET should primarily use vacuum drum drying, supplemented by filling drying.
  4. LMPET’s spinning temperature should be lower than that of PET, and a low-temperature spinning process is recommended.
  5. The core-sheath composite spinning of LMPET/PET should utilize the process of low-temperature melting and high-temperature spinning for LMPET, and high-temperature melting and low-temperature spinning for PET.