BOPET film, full name Biaxially Oriented Polyethylene Terephthalate, is a film material made from polyester resin obtained through the reaction of terephthalic acid and ethylene glycol.

Properties and Applications

BOPET film possesses the following characteristics:

  • Optical Performance: Transparent texture with high optical clarity and a high refractive index.
  • Mechanical Performance: Excellent strength, high toughness, and impact resistance.
  • Thermal Stability: Outstanding resistance to high-temperature heat sealing and low-temperature freezing.
  • Chemical Resistance: Strong resistance to acid and alkali corrosion.
  • Barrier Properties: Excellent shielding performance against oxygen, water vapor, and external pollutants.

It is widely used in the printing, electronics, film/television, and photovoltaic industries. The production involves two main steps: stretching and orientation, where PET raw material is melted into a sheet and then uniformly stretched longitudinally and transversely.

I. Raw Material Chips for BOPET

BOPET production uses a mix of two or more types of chips depending on the film variety:

  1. Anti-blocking Masterbatch Chips: Resins with high additive concentrations used to improve performance; typically accounts for 10%–30% by mass.
  2. Super-bright Chips (White Chips): Pure chips without additives; typically accounts for 40%–90% by mass.
  3. Recycled Chips: Made from waste film; used at 0%–45%. High-performance films (like capacitor films) do not use recycled material.
  4. Special Functional Masterbatches: Used for specific needs, such as anti-static, wear-resistant, or anti-UV properties.

II. Anti-blocking Masterbatch Chips

To prevent “blocking” (sticking) during production and winding—which becomes more severe as the film gets thinner—anti-blocking agents are added. These agents can be inorganic, organic, thermosetting organic polymers, or silicone resins. For films requiring high optical performance, the refractive index of the anti-blocking agent must be close to PET’s refractive index of 1.64 (at 25°C) to minimize haze.

III. Base Material Chip Specifications

The performance of the base chips (white or masterbatch) directly affects the film’s haze, tensile strength, and surface properties. Key indicators include:

Indicator Impact/Description Standard Range
Intrinsic Viscosity [η] Determines mechanical strength and extrusion flow. Low viscosity results in poor strength; high viscosity increases energy consumption. 0.60–0.68 dl/g (up to 0.8 for special apps
Melting Point (Tm) Low melting points indicate impurities or uneven molecular weight. Used to set extrusion temperatures. 258–261°C
Carboxyl End Group [COOH] Indicates the degree of thermal or oxidative degradation. Affects melt flow and insulation properties. 25–40 mol/t
Diethylene Glycol (DEG) Formed during synthesis. Controls flexibility and crystallization rate. Too much lowers thermal stability. < 1.5% (Insulation film < 1.0%)
Ash Content Includes catalyst residues and impurities. High ash can cause the film to break during stretching. N/A
Water Content High moisture causes hydrolysis, bubbles, and yellowing. Must be removed via drying before extrusion. < 0.4%

Particles and Impurities

  • Aggregated Particles: Non-polyester particles >10μm (usually poorly dispersed additives). They clog melt filters and should be < 1.0 per mg.
  • Impurity Particles: Includes gels, black spots, and white spots. Gels are cross-linked resins that do not melt and can seriously damage film quality.