Polyester monofilament yarn is used in many industries, and its quality affects the final product’s performance and lifespan. To produce high-quality mono-filament yarn, manufacturers follow advanced processes to turn raw chips into top-notch yarn. This article explains the process and the relationship between various indices of chips.
1. Solid-State Polymerization
To make high-quality mono-filament yarn, manufacturers start with solid-state polymerization (SSP). SSP increases the molecular weight of polyester chips by heating them in a nitrogen atmosphere. This process raises the chips’ viscosity from 0.65 dL/g to over 1.0 dL/g. Higher viscosity improves the yarn’s strength, especially important when using single-step stretching spinning methods.
In SSP, the molecular chain growth involves ester exchange, esterification, and end-group condensation reactions. High viscosity chips are essential because they ensure strong yarn. If the chips have high carboxyl groups, they can degrade when melted, affecting the yarn’s quality. Reducing water content in the chips is also crucial to prevent degradation and improve yarn performance.
There are two main types of SSP equipment: batch and continuous. Both types start by crystallizing the chips, then heating them before the polymerization stage.
Batch SSP removes small by-products through vacuum, which helps increase molecular weight. This method is suitable for small-scale production and is easier to maintain, but it has lower efficiency and can result in inconsistent product quality.
Continuous SSP equipment produces chips in a continuous flow. It uses lower temperatures (200-220°C) and pressures (15-25 kPa) to reduce side reactions. This method is more efficient but complex and harder to control. In practice, continuous SSP is used primarily, with batch SSP as a supplementary process.
These SSP methods effectively increase polyester chips’ molecular weight and viscosity, leading to better yarn strength and stability.
2. Melting Extrusion
Mono-filament yarn is made using melting extrusion. In this process, the chips are melted and spun into yarn. Maintaining a stable melt is crucial for high-quality yarn. The process includes a screw extruder, spinning box, and spinning components.
First, the high-viscosity chips are melted in a screw extruder. Consistent melt flow is essential. As the chips move through the extruder, they heat up from external heaters and mechanical friction, turning from solid to liquid. This change happens because the heat gives energy to the polyester molecules, allowing them to move and flow smoothly.
During melting, two main heat sources are used: external heaters and shear heat from the melt flow. The melting process is divided into three stages: feeding, compression, and homogenization. In feeding, chips are moved into the extruder and compressed. In compression, chips form a solid bed and melt. In homogenization, the melt becomes uniform and flowable.
During extrusion, side reactions like hydrolysis, thermal degradation, and oxidation can break down the polyester chains, affecting the yarn’s quality. Factors such as chip carboxyl content, moisture level, and extrusion temperature influence these reactions.
Optimizing melting extrusion improves yarn quality, ensuring strong and stable yarn.
3. Stretching and Stabilization Process
After the molten polymer is extruded through a screw, it cools and reaches the spinneret. The fibers formed at this stage are called “as-spun fibers.” These fibers have low molecular alignment and are unstable. This makes them weak and stretchy, so they cannot be used directly. To improve their properties, we use a stretching and stabilization process. This aligns the polymer chains in the direction of stress.
During this process, three things happen: alignment, de-alignment, and crystallization. They compete with each other. By adjusting the process, we can change the fiber’s structure and its performance.
The reasons for improved fiber strength are:
- Chain Alignment: Stretching aligns the chains along the fiber’s length, making the fiber stronger.
- Ordered Structure: Alignment makes the fiber’s structure more orderly, which increases strength.
- Crack Prevention: Stretching helps align molecules around cracks, reducing further damage and increasing strength.
For making fibers, after choosing the raw material, we control stress, temperature, and time during spinning. Methods to increase strength include:
- Increasing Spinning Speed:
High-speed spinning (2500-5000 meters per minute) improves fiber strength by increasing orientation. To avoid quality issues like filament breaks, companies use cooling devices to reduce internal stress.
- Increasing Spinning Stress:
Celanese’s method uses heated liquid to uniformly heat the filaments, improving stress regulation.
- Optimizing Cooling Air Speed:
Adjusting the air speed helps stabilize the fiber structure and improve quality.
These methods enhance the physical properties of mono-filament yarn. It ensures good performance and stability.
4. Basic Properties of Viscosity-Increased Chips
Solid-state polymerization improves PET chips by lowering water and end-carboxyl group content, which increases the molecular weight. Table 2.3 shows that the viscosity of PET chips increased from 0.671 dL/g to 1.04-1.13 dL/g after treatment, meeting high-viscosity requirements.
End-carboxyl content decreased from 25.81 mol/t to 9.65-10.83 mol/t. Lower end-carboxyl content improves thermal stability and product performance.
Solid-state polymerization also reduces water content, which is crucial for smooth melting spinning. High water content speeds up hydrolysis and thermal degradation. By using heated nitrogen, water content dropped from 0.04% to about 0.0012%, making the chips easier to spin.
Glycol content remained around 0.85%, indicating minimal impact from polymerization. Color values are measured with the Hunter colorimeter method, where L indicates brightness, a shows red-green balance, and b shows blue-yellow balance. Stable b values are important for consistent dyeing.
Table 1: Changes in PET Properties Before and After Solid-State Polymerization
| Item | Intrinsic Viscosity dL/g | -COOH mol/t | DEG % | Color Value b | Melting Point ℃ |
| Wet Chips | 0.675 | 25.81 | 0.86 | 1.84 | 255.55 |
| Viscosity-Increased Chips 1 | 1.04 | 10.83 | 0.83 | 8.14 | 256.84 |
| Viscosity-Increased Chips 2 | 1.10 | 10.35 | 0.85 | 7.83 | 255.19 |
| Viscosity-Increased Chips 3 | 1.13 | 9.65 | 0.81 | 7.72 | 254.87 |
Figure 1 compares DSC results of chips before and after solid-state polymerization. Wet chips show a recrystallization peak at 145°C, while viscosity-increased chips do not. This shows that solid-state polymerization improves crystallization, resulting in sharper melting peaks.
Figure 1 DSC results of chips
In summary, these processes work together to produce high-quality mono-filament yarn by enhancing chip properties and optimizing spinning conditions.
