PET pellets (Polyethylene Terephthalate) serve as the raw material for BOPET films. They are also the base material for polyester clothing and most plastic bottles for bottled water and beverages. PET gains wide popularity thanks to outstanding mechanical and optical properties.
PET has different specifications for diverse end uses. Based on three major processing routes: fiber spinning, bottle blow molding and film biaxial stretching, PET chips are classified into three grades: **fiber grade, bottle grade and film grade**.

Differences in Synthesis Processes of Three PET Grades

Fiber-grade PET

**Core target**:

Meet spinning requirements with good spinnability, tensile strength and thermal stability.

Process flow

1. Esterification: PTA (Purified Terephthalic Acid) reacts with EG (Ethylene Glycol) at 250~260℃ to form BHET.
2. Catalyst: Antimony acetate or titanium-based catalyst.
3. Melt polycondensation: Conducted at 270~290℃ under high vacuum (<100 Pa) to produce medium-molecular-weight PET with intrinsic viscosity IV ≈0.60~0.65 dL/g.
4. Pelletizing & drying: Melt is underwater cut and dried into fiber-grade PET chips.
5. Melt spinning: PET chips are melted and extruded through spinnerets, then drafted and wound to make polyester fibers.

Process features

Moderate molecular weight, no solid-state polycondensation (SSP) required. Matting agents such as TiO₂ can be added to produce semi-dull or full-dull fibers.

Bottle-grade PET

**Core target**

Improve molecular weight, transparency and gas barrier property, comply with food contact safety requirements.

Process flow

1. Esterification and melt polycondensation: Similar to fiber-grade PET, but adopts higher-purity PTA and EG to reduce acetaldehyde generation. IV reaches 0.70~0.75 dL/g after melt polycondensation and further viscosity build-up is needed.
2. Solid-state polycondensation (SSP): Chips react under nitrogen protection at 200~220℃, lifting IV to 0.80~0.85 dL/g.
Key function: Reduce acetaldehyde content below 1 ppm and increase crystallinity.
3. Injection & blow molding: Bottle-grade pellets are injection molded into preforms and then blow molded into finished bottles.

Process features

SSP is mandatory to raise molecular weight and improve mechanical strength and heat resistance. Toners such as blue masterbatch and UV stabilizers can be added as required.

Film-grade PET (raw material for BOPET)

Core target

High transparency, low impurities and narrow molecular weight distribution for biaxial stretching.

Process flow

1. High-purity polycondensation: Strict raw material purity requirement, metal ion content <10 ppm to avoid film defects. IV ≈0.62~0.68 dL/g after melt polycondensation.
2. Melt extrusion & casting: PET melt is extruded via T-die and cooled into thick sheets (100~300 μm).
3. Biaxial stretching (BOPET process)
- Machine Direction (MD): Stretched 3~4 times at 80~100℃
- Transverse Direction (TD): Stretched 2.5~3.5 times at 100~120℃
4. Heat setting: Treated at 200~230℃ to stabilize dimension and lower thermal shrinkage.

Process features

Narrow molecular weight distribution guarantees uniform stretching. SSP is generally not applied.

Performance Comparison of Three PET Grades

- Molecular weight & IV: Bottle-grade PET has the highest molecular weight (via SSP); film-grade PET has the narrowest molecular weight distribution; fiber-grade PET features moderate molecular weight.
- Crystallization: Bottle-grade PET has high crystallinity and good heat resistance; film-grade PET has low crystallinity for easier stretching.
- Additive system: Fiber grade focuses on spinnability; bottle grade prioritizes hygiene and transparency; film grade targets optical uniformity.
- Core technology: Fiber grade — melt spinning; Bottle grade — SSP + blow molding; Film grade — biaxial stretching.

Why PET Grades Differ in Applications

- Fiber-grade PET: Moderate molecular weight suitable for melt spinning, with relatively loose requirements on transparency and purity.
- Bottle-grade PET: High IV and low acetaldehyde are critical; SSP ensures food contact safety.
- Film-grade PET: Uniform molecular weight distribution determines stretching stability and optical performance of films.

Summary

Differences in synthesis processes and performance of PET chips are driven by end-product demands.
- Fiber-grade PET: Balances spinnability and cost for mass textile production.
- Bottle-grade PET: High purity, transparency and heat resistance, widely used in food packaging.
- Film-grade PET: Makes high-performance films through precise biaxial stretching, commonly used in electronics and optical industries.