[14a-512-1~11] 3.7 Laser processing

[14a-512-10] Crack suppression of silica glass film formed by F₂ laser induced photochemical surface modification of hard silicone coating film on polycarbonate (2)

〇Hidetoshi Nojiri^1,2, Masayuki Okoshi¹ (1.National defence Academy, 2.Renias)

Keywords:Fluorine Laser, Silicone, Polycarbonate

Light-weighting of vehicle is now strongly required for reducing gasoline consumption and CO₂ emission. Transparent polycarbonate with hard silicone coating film has been used as a low density material of automotive windows. But more hardened surface with scratch resistant has been desired. Previously, we have found that the surface of silicone protective film coated on a polycarbonate resin could be photo-chemically modified into carbon-free silica (SiO₂) by irradiation of a 157 nm F₂ laser. Moreover, crack resistance was improved using mesh mask in the F₂ laser irradiation. In this study, crack suppression method for the thermal durability was considered. In experimentation, photo-chemically modified SiO₂ samples were prepared by F₂ laser irradiation to the surface of hard silicone film coated by dip coating method onto the film of acrylic resin on a polycarbonate substrate. The thickness of silicone resin was approximately 4 µm. F₂ laser was irradiated using one of 50 µm to 1 mm of aperture mesh mask, the single pulse fluence was varied from 4 to 14 mJ/cm², pulse repetition frequency was set to 10 Hz, and irradiation time was changed from 15 to 90 s. N₂ gas was filled around the laser beam and the surface of the sample. Then the shrinkage and the thickness of the SiO₂ modified layer were measured by a stylus-type surface profilometer. Namely, SiO₂ modified layer was etched by HF 1% diluted solution and the etched depth was measured by that profilometer. As a result, shrinkage and modified thickness did not depend on the aperture size of mesh mask, but only irradiation time.
Next, thermal test at 100 or 120℃ for 3 h was executed on those samples to investigate crack resistance in detail. Then many cracks were observed on the boundary between irradiation and non-irradiation area. It was thought that crack was easily occurred by elongation of silicone at the high temperature because of strong tensile stress generated at the boundary surface. The concave shape caused by shrinkage due to photochemical modification is corrected by rigidity of the silicone resin and that strong tensile occurs. We tried to rubbing the modified surface 300 times at the 1 N/cm² of load by steel wool. Then no crack was obtained at all after thermal test at 120℃ 3h. The tensile stress in the modified surface was thought to be eased according to steel wool rubbing.