(210f) 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.