Plasma treatments are able to introduce polar oxygen groups even if carried out in inert gases. The polymer radicals formed during the treatments are long-lived and typically react with water or oxygen from the gas background. An adhesion improvement by plasma treatments has been attributed variously to improved wettability, to surface cross-linking, or to interfacial diffusion.
The degradation of polymers caused by environmental UV radiation is the cause of yellowing of many polymer materials. In particular, polycarbonates are prone to yellowing, whereas polyacrylates are more stable to environmental radiation. The discoloration is due to photo-oxidation reactions.
Polycarbonate
Electromagnetic radiation at wavelength below 200 nm has enough energy to break polymer bond, and the photon energy of wavelength below 120 nm is sufficient to ionize most organic materials. Important for coating processes is VUV radiation at wavelength below 180 nm. Argon in a plasma shows primarily VUV emissions below 160 nm due to energy transfer from excited-state argon metastables to hydrogen from water and organic materials. Studies show that cross-linking reactions during low-pressure plasma treatments are mainly initiated by VUV radiation. Cross-linking reactions dominate on polymers containing at least one hydrogen atom bonded to adjoining carbon atoms. This is find in the thermoplastics like polyethylene, polypropylene, polystyrene, and polyamide. For PMMA and other acrylics, scission of bonds and the formation of weakly bonded layers is found after different plasma treatments.
Acrylate polymer - PMMA
Antireflective (AR) coating design for plastics
Special requirements have to be taken into account in the design of AR coatings for plastics:
- The layer materials have to be selected in order to achieve radiation protection of interfaces or of the bulk polymer dependent on the polymer substrate type.
- The layer materials and thicknesses have to be adjusted considering the mechanical and thermal film stresses and the total heat development during the deposition process.
- Designs should be suitable for integrating additional functions like improved scratch resistance, barrier function, antistatic function, or hydrophobic properties.
- Sometimes changed optical properties of interfacial zones due to plasma treatments have to be taken into account for design calculations.
Plastic eyeglasses have to be pass rigorous environmental and abrasion tests. AR layers which is preferred in quarter-half-quarter design, and hydrophobic top coatings are arranged on top of hard coatings several micrometers thick. Usually, the hard coatings are lacquers based on silanols, or organic-modified silica layers applied by vacuum deposition processes. These hard coated surfaces act as the substrate so that the vacuum deposition of the AR coating does not have to be done on the more sensitive polymer itself. AR coatings produced using wet-chemical processes comprise mainly single-layer and two-layer systems.
