Poly(ethylene terephthalate) (PET) films can be bonded directly by oxygen plasma irradiation and heat press at low temperatures of 100–160 °C. The irradiated films were kept in the atmosphere for six years, yet they can be bonded tightly. The irradiated surface is extremely active just after the irradiation, and it is considerably active after five years. Dry- and wet-peel tests suggest hydrogen bonding and chemical bonding. The films are bonded by these two elements at lower press temperatures, while by the pure chemical bonding at higher temperatures. Fourier transform infrared spectroscopy (FTIR) results on the non-irradiated, irradiated and bonded samples indicate that OH and COOH groups are created at the surface, they are responsible for the both bondings. Dehydrated condensation reaction is proposed for the chemical bonding. The hydrogen bonding is broken by water penetration, causing smaller peel strength under the wet-peel test. Cross-linking layer may be the origin for the long lifetime.

A plasma-irradiated poly(ethylene terephthalate) (PET) film has a long lifetime of bonding capability. To clarify its origin, a PET film was irradiated with oxygen plasma. It was then exposed to normal atmosphere including water vapor. FTIR absorption on the irradiated and non-irradiated films was measured at different times after the start of evacuation. The irradiated film has a larger amount of OH than the non irradiated film, and OH is generated on the film surface. The irradiated film has a larger amount of adsorbed water, because the surface is activated by the created OH. The adsorbed water is desorbed rapidly with increasing evacuation time in the non irradiated film, but it is desorbed more gradually in the irradiated film. Water has hydrogen bonds with OH; thus, the water desorption is suppressed. The OH absorption band is shifted to the lower wave number side owing to the hydrogen bonds. The irradiated surface may be protected by the water from the atmosphere.

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