Page 269 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 237

that is favorable for adhesion. Rather than chemical solutions, these
surface modifications are usually made by physical means such as
flame, electric discharge, UV light, or laser.

6.5.2.1 Corona discharge. Corona discharge treatment is a popular
method of dry surface preparation of polymer films. The purpose of
the treatment is to make the polymer surface more receptive to inks
or coatings; however, it has also been used effectively as a pretreat-
ment for adhesives. The treatment is believed to oxidize the surface
of the polymer so that the ink, coating, or adhesive can permeate the
roughness of the thin oxidized layer. The most common methods of
treating surfaces by oxidizing are corona treatment and flame treat-
ment (see next section). For film, corona treatment is the preferred
method of the two.
The schematic in Fig. 6.10 illustrates two basic arrangements for
treating film and coated paper with corona treating equipment. The
electric equipment consists basically of a high frequency generator
(10–20 kHz), a stationary electrode electrically connected to the gen-
erator, and a dielectrically covered treater roll that serves as the
grounded electrode. The material to be treated is carried over this
roller for continuous processing. A suitable voltage, typically 20 kV, is
developed between the electrodes. This produces a spark or corona
discharge by ionizing the air in the gap between the electrodes. The
ionized particles in the air gap bombard and penetrate into the mo-
lecular structure of the substrate. Free electrons and ions impact the
substrate with energies sufficient to break the molecular bonds on the
surface of most polymeric substrates. This creates free radicals that
react rapidly with oxygen to form polar chemical groups on the sub-
strate surface and increase the surface energy to a point where many
adhesives, paints, and coatings can wet the substrate.
Corona treating equipment is inexpensive, clean, and easily adapted
to in-line operations. Parameters that positively affect the efficiency
of the treating process include power input and temperature.
Whereas, line speed and humidity negatively affect the efficiency.
Stored film is generally considered more difficult to treat than film
that has just been processed (e.g., exiting an extruder). The opportu-
nity of low molecular weight additives and contaminants to migrate
to the surface is greater for stored film. These surface contaminants
impede the treating efficiency of the corona.
Corona treatment is mainly suitable for films although thin con-
tainers have been treated by nesting them on a shaped electrode and
rotating the part adjacent to a high voltage electrode. Approximately
25 mils (0.025 in) is the maximum thickness of sheet that can be
treated by the corona discharge method. However, high frequency arc