Carboxyl End Group (-COOH) content is a pivotal quality parameter in Polyester (PET/PES) chips, directly impacting processing performance and final product properties. How does Carboxyl Influence Spinning and Film Production? Understanding its influence is essential for achieving optimal results in fiber/yarn spinning and film manufacturing.

Impact on Fiber Spinning Performance

1. Spinnability Optimization:

Optimal COOH (30-35 mol/t): Moderately promoting controlled thermal degradation, enhancing melt fluidity, and reducing fiber breakage.
For example, in industrial yarn production, maintaining COOH within 30-35 mol/t minimizes melt viscosity fluctuations caused by high-temperature degradation, significantly improving draw stability and process continuity.

However, certain industrial monofilament applications demand high-viscosity, low-carboxyl polyester chips. Key examples include sewing threads, filter meshes, and fishing lines. These uses place stringent requirements on strength, heat resistance, and dimensional stability.

Regular polyester chips, due to their higher carboxyl content, are prone to hydrolysis, which can trigger a decline in strength. In contrast, low-carboxyl chips maintain intact molecular chains, delivering superior hydrolysis resistance and significantly extended service life, particularly under demanding conditions.

Low COOH (<30 mol/t): Results in an excessively narrow molecular weight distribution and overly uniform melt structure. This hinders molecular orientation during spinning, leading to increased fiber breaks and reduced productivity.

2. Thermal Stability & Degradation Control:

COOH Value is a sensitive indicator of thermal degradation susceptibility.

High COOH (>35 mol/t): Accelerates thermal degradation in the melt phase. This leads to reduced fiber tenacity (strength) and uneven dye uptake.
For example, the carboxyl groups in textile-grade PET chips need to be controlled below 30 mol/t to avoid excessive degradation of the melt in the spinning box.

Impact on PET Film Production

1. Melt Stability & Film Forming

Excessive Carboxyl End Groups: Accelerating thermal degradation of the melt, increasing the risk of film breakage during production. Residual hydrogen ions can also impair the film’s insulating properties.
For example, film-grade PET chips require carboxyl end groups to be controlled within 20-30 mol/t to balance crystallization rate and melt strength.

Cooperatively controlling DEG Content: Carboxyl end groups and diethylene glycol (DEG) jointly influence molecular weight distribution. While elevated DEG content lowers the polymer’s melting point, excessively high carboxyl end groups counteract its plasticizing effect, resulting in increased film brittleness.

2. Optical & Thermal Properties:

High COOH content contributes to undesirable yellowish/greyish tint, increased haze (reduced clarity), and diminished thermal stability. This negatively impacts critical film properties like heat-seal performance and long-term aging resistance.

For example, high-quality BOPET film production generally mandates COOH levels ≤ 35 mol/t (often targeting lower) to guarantee consistent optical properties and thermal endurance.

Production Control Recommendations

  • For Fiber-Grade PET: Maintain carboxyl end groups within 30-35 mol/t. Achieve this by optimizing esterification yield, polycondensation vacuum level, and melt transfer temperature.

  • For Film-Grade PET: Limit carboxyl end groups to ≤30 mol/t. Implement coordinated control with DEG content (1.0-1.4%) and ash content (≤0.025%) to balance melt fluidity and film-forming performance.

In summary, carboxyl end group levels must be tailored to specific application requirements. This dynamic adjustment mitigates degradation risks from excessive levels while preventing melt structure rigidity at insufficient levels, thereby ensuring stable fiber spinning/film production and guaranteed end-product quality.