DLC coatings on inner walls of PET bottles by a simplified PBII technique

Some research has been carried out on DLC coating of inner walls of PET bottles because the DLC coating can reduce the oxygen transmission rate through the wall of the bottles drastically. Existing techniques for DLC formation on the inner wall of PET bottles require rather complicated coating systems. In this study, we propose a simple way to coat DLC on the inner walls using a modified PBII technique. The configuration is that the outer wall is covered by an outer electrode connected to a grounded vacuum chamber and an inner electrode, which is connected to a positive high voltage pulsed power supply, is inserted inside the bottle. In the coating process, the bottle is filled with C2H2 gas at a suitable pressure. Using this approach we have succeeded in coating DLC on the inner walls of bottles. Oxygen and nitrogen gas transmission rates were examined for 20-s, 1-min and 5-min coating times on 50-μm thick PET films. We found that the DLC coatings reduce the rates drastically, and a 1-min coating time is enough to form a gas barrier. The results of micro-scratch tests for the DLC-coated PET films show good adhesion of the DLC coating and reduction of friction. The Meyer hardness and Young's modulus of the DLC-coated PET films were also estimated from the penetration depth–load relationship obtained by a depth sensing indenter with a diamond spherical indenter 200 μm in radius. It is revealed that for the 5-min coated film the apparent Meyer hardness at 5-mN load and Young's modulus were about 1.4 and 1.2 times larger than those of the uncoated film, respectively.