Atmospheric Pressure Radio Frequency Plasma Activation of Composites for Adhesive Bonding

Atmospheric pressure radio frequency plasma activation of composites for adhesive bonding

Thomas S. Williams, Hang Yu and Robert F. Hicks

University of California at Los Angeles, Department of Chemical and Biomolecular Engineering,

Los Angeles, California 90095-1592

The adhesion of bonded composite joints is of great concern to the aerospace, automotive, medical device, and electronics industries.  Recently, our group has developed 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 by abrasion.  The plasma source generates an afterglow containing ~1x10¹⁷ cm^-3 oxygen atoms, ~1x10¹⁷ cm^-3 metastable oxygen molecules (¹Δ_g), and ~4x10¹⁴ 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.  Bond strength does not correlate with water contact angle or surface roughness.  Instead it 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 will be discussed in detail at the conference.