Optical-grade BOPET base film, supplied as grade OB-100 in thicknesses from 23 µm to 188 µm, is a biaxially oriented polyethylene terephthalate substrate intended for precision optical coating, transparent conductive film deposition, polarizer base applications, optical clear adhesive carriers, and release liners. The material is produced on sequential biaxial stretching lines with machine-direction draw ratios in the range of 3.0–3.5 and transverse-direction draw ratios of 3.5–4.0, followed by heat-setting at 200–230 °C to stabilize shrinkage. Luminous transmittance is ≥ 89 % as measured according to ASTM D1003; spectral transmittance at 550 nm is ≥ 90 %; haze is ≤ 1.0 % for thicknesses up to 100 µm and ≤ 1.5 % for 188 µm. Coating-side surface roughness Ra is held below 30 nm. The film is available with one or both sides chemically treated to wetting tension levels of ≥ 56 mN/m per ASTM D2578, and with optional slip-modified skin layers for unwind stability. Core-layer resin is selected for low cyclic oligomer extractables because elevated cyclic trimer content transfers to coating rolls and creates haze after drying. Thickness uniformity across a 1500 mm web is controlled by on-line beta gauge scanning with ± 2 % typical deviation.

What Separates Optical-Grade BOPET Base Film from General-Purpose PET Film?

The principal differences are surface cleanliness, oligomer control, thickness uniformity, and haze stability after downstream coating. General-purpose BOPET is often specified for packaging and lamination where haze of 3–5 % and surface roughness Ra above 50 nm are acceptable; optical-grade material restricts both parameters to narrow windows. The comparative data in the table below represent typical production values for grade OB-100 and a standard packaging-grade film; values are not to be read as contractual limits unless fixed in a purchase specification.

The optical-grade film also differs in the design of skin layers. Slip particles are necessarily present for winding and handling, but their concentration and size distribution are shifted toward lower values; typical silica particle sizes are below 1 µm, and the resulting surface haze is minimized. A packaging-grade film may employ larger inorganic anti-blocking agents at higher loadings, which can generate visible scattering when the substrate is used under bright-edge-lit display panels. For applications requiring immediate adhesion of waterborne or solvent-based hard coats, the optical-grade substrate is supplied corona-treated to a wetting tension of at least 56 mN/m; the treatment level decays at a controlled rate and should be verified by ASTM D2578 before coating.

Surface Roughness, Coating Adhesion, and Dimensional Stability in Roll-to-Roll Processing

Roll-to-roll coating on production lines with web speeds above 30 m/min imposes a narrow process window for optical-grade BOPET. If surface roughness Ra exceeds 35 nm, the wetting layer of a 2–5 µm thick UV-curable hard coat may show thickness variation that is visible as orange-peel after curing. If roughness is below 5 nm, adhesion may become dependent on surface chemistry alone, and blocking during winding at tensions above 300 N/m can occur. Therefore, OB-100 is supplied with an Ra target of 15–25 nm on the coating side and 20–30 nm on the back side; the back-side roughness is generated by a low coefficient of friction treatment sufficient to permit stable slit-roll unwinding at 0.2–0.4 dynamic coefficient of friction measured by ASTM D1894.

Thermal shrinkage is controlled because downstream processes include sputtering of indium tin oxide, annealing at 130–150 °C, and hot-air drying of optical clear adhesives. At 150 °C for 30 min, machine-direction shrinkage above 1.0 % can produce registration error in patterned conductive layers; therefore, the optical-grade specification sets ≤ 0.8 % for MD and ≤ 0.4 % for TD at 150 °C/30 min. For thinner films below 50 µm, tension control is more critical: a 100 µm web can tolerate a cumulative tension spike of 400 N/m without permanent deformation, while a 25 µm web may show edge buckling at temporary tension above 150 N/m. Qualification on the actual coating line is required because published data for this specific configuration is limited.

Oligomer migration is a further distinction. During heat-setting and post-annealing, cyclic trimer from PET can bloom to the film surface and form a white deposit on coating rollers. Optical-grade BOPET reduces extractables below 0.5 % through resin selection and online cleaning, while general-purpose material may exceed 1.0 %. For transparent conductive coatings, upstream film cleaning with a web cleaner and ionizing bar is still recommended before sputtering because surface particle counts above 0.5 particles/cm² at sizes larger than 5 µm can cause pinholes. The base film itself is specified to have no visible gel particles larger than 200 µm; optical-grade material is filtered through 20 µm melt filters during extrusion.

If the Film Is Used as a Polarizer Base or Transparent Conductive Substrate

When OB-100 replaces triacetyl cellulose or cycloolefin polymer in polarizer and display applications, the design trade-off is dominated by birefringence, moisture uptake, and stiffness. BOPET has a water absorption of approximately 0.3–0.5 % by mass after 24 h immersion at 23 °C, compared with 2.0–3.0 % for TAC, and its humidity expansion coefficient is lower; this improves dimensional stability in high-humidity environments. However, the high stress-induced birefringence of biaxially oriented PET produces retardation that must be controlled when the film is placed between crossed polarizers. Low-retardation grades are produced by adjusting draw ratios and heat-setting to reduce in-plane retardation below 300 nm; standard optical-grade BOPET may have retardation above 1000 nm and is not suitable for polarizer-protective layers without further processing.

Compared with cycloolefin polymer substrates, BOPET provides higher tensile modulus and lower creep at elevated temperature, but higher optical anisotropy and higher density. The tensile modulus of OB-100 is approximately 4.0 GPa in the machine direction and 4.5 GPa in the transverse direction when measured by ASTM D882; this allows the film to bear higher coating tension without necking. Cycloolefin polymer may be specified where near-zero in-plane birefringence is required, but its solvent resistance and heat resistance are lower than those of BOPET.

For transparent conductive film, the base substrate must tolerate sputtering temperatures and DC magnetron power without distortion. The film is dimensionally stable up to 150 °C for 30 min; continuous exposure above 180 °C is not recommended because the material approaches its heat-set temperature and may lose the shrinkage stabilization imparted during processing. In roll-to-roll sputtering, the substrate is typically pre-dried at 80 °C for 2 h if stored at relative humidity above 70 %, because absorbed moisture can evolve in the vacuum chamber and raise chamber pressure above 1 × 10⁻⁴ Pa during deposition.

Storage and handling conditions impose further operational boundaries. The film should be kept in the original moisture-barrier packaging until use and conditioned at 23 °C ± 2 °C and 50 % ± 5 % RH for at least 24 h before slitting to prevent static charge accumulation and dimensional drift. Contact with high-concentration ketonic solvents during cleaning can promote surface crystallization or microcrazing in unstabilized PET; not all optical-grade BOPET grades are compatible with all hard-coat formulations. Coatings containing high-amine curing agents can attack the polyethylene terephthalate surface under hot-air drying above 120 °C, causing haze; this is a documented limitation for certain UV-curable resin systems. For sputtering and vacuum coating, the roll must be wound at controlled taper tension, typically 100–250 N/m depending on web width and thickness. In edge-lit backlight units, the film contributes to brightness uniformity only when the base substrate haze is below 1.0 % and the thickness variation across a 10 cm lateral repeat is below 0.5 µm; these values are verified by scanned haze mapping and cross-web caliper data. The product is supplied with a certificate of analysis that reports lot-specific values for ASTM D1003 haze, ASTM D1204 shrinkage, ASTM D882 tensile strength, and ASTM D2578 wetting tension.