Atmospheric pressure plasma activation of plastics and composites for improved adhesion
Summary form only given. Atmospheric pressure plasma treatment is a key process for surface preparation prior to adhesive bonding of plastics and composites for use in the aerospace, automotive, medical device, and electronics industries. A review is presented on methods for surface preparation of polymers and composites using low-temperature, atmospheric pressure radio frequency helium and oxygen plasmas. This is a promising technique for replacing traditional methods of surface preparation such as peel ply and abrasion. The plasma source generates an afterglow containing ∼1×1017 cm−3 oxygen atoms, ∼1×1017 cm−3 metastable oxygen molecules (1Δ g ), and ∼4×1014 cm−3 ozone molecules. With sufficient exposure to the afterglow, polymer and composite surfaces are fully activated such that when bonded and cured with epoxy adhesives, they undergo 100% cohesive failure. Depending on the material, lap shear strength and delamination resistance can be increased over 50% from that activated by solvent wiping or abrasion. Surface analysis by water contact angle and AFM indicates that there is no correlation between bond strength and surface energy or surface roughness. Instead, bond strength correlates with the fraction of the polymer surface sites that are oxidized and converted into carboxylic acid groups, as determined by x-ray photoemission and infrared spectroscopy. Polymers with aromatic groups in the backbone are especially suitable for activation with the atmospheric pressure plasma. The activation mechanism, surface characterization results, and bonding strength improvement are discussed in detail.