PET Transparency: Metrics & Optimization

PET Transparency Core Indicators and Benchmark Requirements

Basic Light Transmittance: In its unmodified state, virgin PET must achieve a light transmittance of 85%-90% (test standard: GB/T 2410-2008, test wavelength 550nm); high-end applications (such as medical packaging and electronic films) require ≥90%.

Stability Requirements: After 12 months in an environment with a temperature range of -10°C to 40°C and a relative humidity of 30%-70%, the light transmittance must decrease by ≤3%, with no noticeable yellowing (yellowness index ΔYI ≤ 2).

Industry-Specific Benchmarks:

Food and Beverage Packaging: Light Transmittance ≥85%, Bubble Diameter ≤0.1mm, No Visible Impurities;

Medical Packaging: Light Transmittance ≥90%, Catalyst Residue ≤30ppm, Biocompatibility Compliant;

Electronic Film: Light Transmittance ≥90%, Haze ≤2%, Thickness Uniformity ±5%.​

Quantitative Analysis of Factors Affecting PET Transparency

(I) Molecular Structure and Crystallization Control Parameters

Crystallinity and Grain Size:

Critical Value: When the crystallinity is 15%-25% and the grain size is <0.5μm, the transmittance meets the standard.

Risk Range: When the crystallinity is >30% or the grain size is >1μm, the transmittance decreases by ≥10%, resulting in a milky white appearance.

Orientation Control: During the biaxial stretching process, a longitudinal orientation of ≥0.8 and a transverse orientation of ≥0.75 can reduce the scattering rate by 2%-3%.

Molecular Chain Parameters: The intrinsic viscosity (IV) should be controlled between 0.7-0.8dL/g. When the IV fluctuates by ±0.02dL/g, the transmittance fluctuates by ≤1%. When the IV is <0.65dL/g, transparency is likely to decrease due to molecular chain breakage. (II) Raw Material Purity Control Standards
Impurity Limits:

Unreacted monomers (terephthalic acid + ethylene glycol) ≤ 0.5% (gas chromatography);

Antimony catalyst residue ≤ 30 ppm (ICP-MS). Above 50 ppm, transmittance decreases by 5%-8% and the yellowness index increases by 3-5.

Moisture content: ≤ 20 ppm before processing (Karl Fischer method). Above 50 ppm, bubble generation rate ≥ 90% and localized transmittance decreases by 15%-20%.

(III) Key Processing Parameter Thresholds
During the drying process, key parameters are temperature, time, and moisture content. The acceptable range is 160-180°C, 4-6 hours, and moisture ≤ 20 ppm. If the material falls within the risk range (temperature < 150°C, time < 3 hours, moisture > 50 ppm), bubbles will form and transmittance will decrease by 15%-20%.

The melting process focuses on barrel temperature, which should be adjusted gradually (260°C → 275°C → 285°C), maintaining an overall temperature range of 270-285°C. Temperatures below 260°C will cause uneven melting, while temperatures above 290°C will cause thermal degradation. Both conditions will result in a 5%-10% decrease in light transmittance.

During the blow molding process, attention should be paid to the blow-up ratio, stretch ratio, and cooling air temperature. The recommended ranges are 2.5-3.5:1 for the blow-up ratio, 3-4:1 for the stretch ratio, and 15-25°C for the cooling air temperature. A blow-up ratio below 2:1 or above 4:1, a stretch ratio below 2:1 or above 5:1, and a cooling air temperature above 30°C can lead to uneven orientation (abnormal proportions) or grain growth (excessive air temperature), resulting in haze on the finished product. The key parameters in the film stretching process are the longitudinal draw ratio, transverse draw ratio, and heat-setting temperature. The acceptable ranges are a longitudinal draw ratio of 3-4:1, a transverse draw ratio of 3.5-4.5:1, and a heat-setting temperature of 120-140°C. If the longitudinal draw ratio is less than 2.5:1 or greater than 4.5:1, the transverse draw ratio is less than 3:1 or greater than 5:1, and the heat-setting temperature is less than 110°C or greater than 150°C, haze may increase by 3%-5% due to increased crystallinity (abnormal setting temperature).

PET Transparency Enhancement Process Optimization Steps

(I) Raw Material Pretreatment Process (Requires 3 Key Steps)

Raw Material Screening: Select chips with an IV value of 0.7-0.8 dL/g. Test for monomer residues of ≤0.5% and catalyst residues of ≤30 ppm. Unqualified raw materials must be discarded or undergo secondary purification (purification process: ethanol extraction followed by vacuum drying, with monomer residues ≤0.3% after purification). Drying: Use a "hot air drying + vacuum-assisted" process. Parameter settings: temperature 160-180°C, vacuum -0.085-0.095 MPa, drying time 4-6 hours. Moisture levels should be checked every hour until the content is ≤20 ppm.

Premixing: If a modifier is required, mix it with the dried chips in a high-speed mixer at a speed of 800-1000 rpm for 5-8 minutes to ensure uniform dispersion (dispersion test: no agglomerated particles under a microscope, particle size ≤5 μm).

(II) Key Process Control Points (Step-by-Step Operation)

Melt Extrusion Control:

Barrel temperature settings: Zone 1: 260°C, Zone 2: 275°C, Zone 3: 285°C; die head temperature: 280°C; screw speed: 30-50 rpm; back pressure: 5-8 MPa;

Equipped with a 10-20 μm precision filter element. Check the filter pressure differential every 2 hours and replace it if the pressure differential exceeds 0.5 MPa to prevent impurity accumulation.

Molding Process Optimization:

Blow molding: Utilize the "pre-blow (0.2-0.3 MPa, 0.5-1 s) → stretch (50-100 mm/s) → final blow (0.6-0.8 MPa, 1-2 s)" process. Cooling air is provided through a circular duct at a speed of 0.8-1.2 m/s to ensure uniform cooling of the bottle.

Film molding: Lengthwise stretching temperature: 80-90°C, widthwise stretching temperature: 90-100°C. After heat setting, utilize dual cooling using air and water, with a cooling rate ≥5°C/s, to inhibit grain growth.

(III) Solutions for Common Problems

If the "light transmittance meets the standard but yellowing occurs," the likely cause is excessive catalyst residue or thermal degradation. The corresponding solutions are: 1. Replace low-residue raw materials (catalyst residue ≤ 30 ppm); 2. Reduce the melt temperature by 5-10°C and add 0.1% antioxidant. If a product exhibits "localized fogging," it's often due to uneven cooling or an abnormal stretch ratio.

This can be addressed by adjusting the cooling duct to ensure uniform airflow and adjusting the stretch ratio to the standard range (3-4:1).
For "excessive bubbles," investigate whether insufficient drying or a low melt temperature are the cause. Solutions include: 1. Extend the drying time by 1-2 hours to ensure the moisture content is ≤20ppm; 2. Increase the melt temperature by 5-8°C to improve the melt state.

Testing and Quality Control Process

Online Testing:

Extrusion: Samples are taken every 30 minutes and the grain size is measured using a laser particle size analyzer to ensure it is <0.5μm.

Molding: An online transmittance meter (accuracy ±0.5%) is used for real-time monitoring. An alarm is triggered if the transmittance falls below the standard value by 1%. Offline Testing:

Finished Product Sampling: Sampling is conducted in accordance with GB/T 2828.1-2012, AQL 1.0. Testing includes light transmittance (GB/T 2410), yellowness index (GB/T 2409), and impurity content (microscope, 40x magnification).

Stability Testing: Samples are placed in a 40°C, 70% RH environment. Light transmittance is tested monthly for 6 months, and trends are recorded.

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