Page 273 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 241

1. Surface crosslinking
2. Surface oxidation or reduction
3. Grafting of active radicals to material surfaces
4. Halogenation of surfaces
5. Deposition of inorganic and organic films

Gases or mixtures of gases used for plasma treatment of polymers
include nitrogen, argon, oxygen, nitrous oxide, helium, tetrafluoro-
methane, water, and ammonia. Each gas produces a unique surface
treatment process. It should be noted that surface chemistry modifi-
cation by plasma treatment can make polymer surfaces totally wet-
table or non-wettable. Non-wettable plasma treatments generally in-
volve the deposition of fluorine containing chemical groups to produce
medical products.
Gas plasma is an extremely reactive ionized gas. The main differ-
ence between plasma surface treatment and corona surface treat-
ments is the nature of the plasma (specialty gas vs. air) and the op-
erating pressure of the plasma (0.1 to 10 torr vs. 760 torr). With the
plasma treatment technique, a low-pressure inert gas is activated by
an electrodeless radio-frequency discharge or microwave excitation to
produce metastable excited species that react with the polymeric sur-
face. The plasma treatment produces changes only to the depth of
several molecular layers. Generally, only very short treating times
(secs to mins) are necessary. Commercial instruments are available
from several manufacturers to plasma treat parts prior to bonding.
Because of the necessity for very low pressures, a glass or ceramic
vacuum container is generally used. Thus, plasma treatment is gen-
erally thought of as a batch type process for parts of up to moderate
size. Continuous plasma treaters have been developed for processing
film and fiber and large volume chambers have been built for treating
large parts. However, capital expense has limited these applications
to specialty markets.
It is generally believed that the plasma treating process provides
surfaces with greater stability than chemical etch, corona, flame, or
other common polymeric treatment processes. Plasma treated parts
can be stored for weeks or longer in a clean, dry storage area. Expo-
sure to temperatures near the polymer’s glass transition temperature
will deteriorate the surface treatment. Once well bonded, the surfaces
of treated polymers are relatively stable excluding the effects of out-
side environmental influences.
With plasma treatment, surface wettability can be readily induced
on a variety of normally non-wettable materials as shown in Table