Effects of pre-strain applied at a polyethylene terephthalate substrate before the coating of TiO2 film on the coating film quality and optical performance

A mold was designed to create various strains in polyethylene terephthalate (PET) substrates before the deposition of TiO2 film to simulate deposition process on a cylindrical drum. The residual stress of the PET substrate with TiO2 film significantly increased with increasing strain, decreasing the radius of curvature. Compared to the as-received PET substrate, there was a noticeable increase in the surface roughness in the PET/TiO2 specimens when a large strain was applied. The formation of voids or cavities in the TiO2 layer significantly increased the roughness of the specimen. The mean cavity size and depth increased with increasing strain. For strains ≦ 4%, the specimen's hardness and Young's modulus factored by the voids/cavities increased with increasing surface roughness. The optical absorption increased with increasing surface roughness before becoming asymptotic to a constant value. The strain applied to the PET substrate before TiO2 deposition greatly affects the optical reflection, transmittance, and absorption.