The surface modifications produced by treatment of a synthetic sulfur
vulcanized styrene-butadiene rubber with oxidizing (oxygen, air, carbon
dioxide) and non oxidizing (nitrogen, argon) RF low pressure plasmas, and by
treatment with atmospheric plasma torch have been assessed by ATR-IR and XPS
spectroscopy, SEM, and contact angle measurements. The effectiveness of the
low pressure plasma treatment depended on the gas atmosphere used to
generate the plasma. A lack of relationship between surface polarity and
wettability, and peel strength values was obtained, likely due to the
cohesive failure in the rubber obtained in the adhesive joints. In general,
acceptable adhesion values of plasma treated rubber were obtained for all
plasmas, except for nitrogen plasma treatment during 15 minutes due to the
creation of low molecular weight moieties on the outermost rubber layer. A
toluene wiping of the N2 plasma treated rubber surface for 15 min
removed those moieties and increased adhesion was obtained.
On the other hand, the treatment of the rubber with atmospheric pressure by
means of a plasma torch was proposed. The wettability of the rubber was
improved by decreasing the rubber-plasma torch distance and by increasing
the duration because a partial removal of paraffin wax from the rubber
surface was produced. The rubber surface was oxidized by the plasma torch
treatment, and the longer the duration of the plasma torch treatment, the
higher the degree of surface oxidation (mainly creation of C–O moieties).
However, although the rubber surface was effectively modified by the plasma
torch treatment, the adhesion was not greatly improved, due to the migration
of paraffin wax to the treated rubber-polyurethane adhesive interface once
the adhesive joint was produced. On the other hand, the extended treatment
with plasma torch facilitated the migration of zinc stearate to the
rubber-adhesive interface, also contributing to deteriorate the adhesion in
greater extent. Finally, it has been found that cleaning of SBS rubber in an
ultrasonic bath prior to plasma torch treatment produced a partial removal
of paraffin waxes from the surface, and thus improved adhesion was obtained.