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.

Biaxially oriented polyethylene terephthalate (PET) films can be bonded directly by oxygen plasma irradiation and low temperature heat press around 100°C. The irradiated films were kept in the atmosphere for six years, yet they can be bonded tightly as well. Dry-and wetpeel tests indicate that two bonding elements can be suggested, hydrogen bonding and chemical bonding. The films are bonded by these two elements at lower temperatures, but by the pure chemical bonding at higher temperatures. 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 hydrogen and chemical bondings. Dehydrated condensation reaction is proposed for the chemical bonding. It is briefly mentioned on two origins for the long lifetime of irradiated active surface.

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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To clarify the bonding mechanism for the plasma-irradiated PET films and the origin of the long lifetime of the bonding ability, we soaked the irradiated films in various types of liquid reagent, then tried to bond them by heat-pressing. The irradiated films do not lose the bonding ability after soaking in water, acid, alkali, and organic solvents, whereas they lose it after soaking in ionic solutions of AlCl 3 and FeCl 3 . The COOH is created on the surface by the irradiation, but it is consumed by the chemical reaction with Al and Fe ions. This is the origin of the inactivation of the irradiated surface and the disappearance of the bonding ability after the soaking. The result supports our proposal of "dehydrated condensation reaction" concerning COOH as the bonding mechanism. The irradiated films should be stable in a normal atmosphere for a long time because they are not subjected to water, acid, alkali, and organic solvents.