Page 272 - Handbook of Adhesives and Sealants

240 Chapter Six

surface. Molecular chain scission and crosslinking also occur depend-
ing on the polymer and on the nature of the flame treatment. This
process is widely used to prepare polyolefin surfaces for painting,
printing, or adhesive bonding. Flame treatment can be used for both
film and shaped products and can be used for both continuous and
bulk processing.
In the flame treating process gas burners are fed from the facility’s
mains (chief component methane) or bottled gas (propane or butane).
Of importance in the operation of a flame treater is the gas/air mix
ratio. Depending on the level of gas in the mix, the flame can have
substantially different characteristics. A slight excess of oxygen over
that required for complete combustion is recommended. This treating
method increases the resulting adhesive strength of joints made with
polyolefins and other low energy plastics. The flame oxidizes the sur-
face, resulting in an increase in critical surface energy. The time that
the flame is applied and its nearness to the surface are also important
quality control factors. A surface is typically exposed to the flame re-
gion just above the blue cone until it becomes glossy. It is important
not to overexpose the plastic because warping or other damage of the
part may result.
Automated flame processing equipment are available, similar to the
corona treaters. Hand-held equipment such as a torch or Bunsen
burner can also be used, although uniform treatment is more difficult.
The effect of polymer additives on the treatment efficiency are not as
great as with corona treatment.

6.5.2.3 Plasma treatment. A gas plasma treating process has been de-
veloped for surface treatment of many polymeric materials. It is a dry
process that is becoming a common method of treating many different
engineering plastics when maximum joint strength is required. Low
energy materials, such as polyolefins, polytetrafluoroethylene, poly-
ethylene terephthalate, nylon, silicone rubber, etc. are readily treated
with gas plasma. Relative bond strength improvements of ten to sev-
eral hundred times are possible depending on the substrate and gas
plasma. Plasma treatment has become a very popular treatment for
small to medium sized parts that can fit into a vacuum chamber and
for production volumes that are amenable to a batch type process.
Operationally, a plasma differs from corona and flame treatment in
that the process is completed at less than atmospheric pressure and
with gases other than air. Because of the necessity for partial vacuum,
plasma treatment is essentially a batch process. The type of plasma
gas can be selected to initiate a wide assortment of chemical reactions,
including: