A novel process to form a silica-like thin layer on polyethylene terephthalate film and its application for gas barrier

A novel process for polyethylene terephthalate (PET) surface modification with a silica-like thin layer is proposed. 3-Aminopropyltrimethoxysilane was employed to react with acetone to form dimethyliminopropyltrimethoxysilane (DIPTMS) after aging for 10 days at room temperature. After hydrolysis of alkoxy groups in DIPTMS, dimethylimine-modified silica clusters occurred resulting in an increase of the solution viscosity. Consequently, a dense and homogeneous thin layer was easily dip-coated onto a PET film. After heat treatment at 150 °C, a smooth, flexible and transparent silica-like film (about 70 nm thick) was formed via dehydration and condensation. The surface of the PET dramatically changed from hydrophobic (water contact angle: 70±2°) to hydrophilic (45±3°). As an attempt for application, a dense film of lithium metasilicate (Li₂O·2SiO₂·nH₂O) was successfully prepared by dip coating on the modified PET film, which appeared very low oxygen permeability of about 0.17 cm³/m² day atm at 23 °C and 85% humidity.