Vacuum ultraviolet-induced surface modification of polyoxymethylene plates for photo-activation bonding

The surface photoactivation technique using vacuum ultra-violet (VUV) light at 172 nm in wavelength has been applied for adhesive-free bonding of polyoxymethylene (POM) plates. The POM surfaces were photochemically modified via VUV light irradiation in dry air conditions, resulting in the formation of oxygen-containing functionalities. The POM plates were successfully bonded with a practical bonding strength below the melting point. The maximum bonding strength was obtained at an irradiation time of 60 min, while a longer irradiation time resulted in lower bonding strength. Surface morphologies of the photoactivated and fractured POM surfaces were observed by a laser scanning microscope. Due to the VUV irradiation, numerous cracks were formed on the POM surfaces. The fractured surface shows that a cohesive failure occurs at the bonding interface. Atomic force microscopy was used to observe the cross-sectional images of the bonding samples. Around 10 μm width, significant phase contrast between the bonding interface and bulk POM indicates the different mechanical properties, due to VUV irradiation. The modified POM surfaces adhered through thermocompression, and the mechanical interlocking and entanglement of POM molecules can result in successful bonding.