Page 267 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 235

ment. Certain treatments can be removed by rubbing or scuffing or by
exposure of the part to elevated temperatures before the adhesive is
applied. It is best to bond polymeric parts as soon as possible after
they are treated. The actual ‘‘shelf life’’ of treated polymeric parts will
depend on the nature of the part, the handling and storage conditions,
and the type of surface treatment that was administered.
Liquid etchants can be used for chemical modification or dissolving
surface contamination. Etchants effectively treat irregularly shaped
objects that are difficult to treat by corona or flame treatments. A
number of etching solutions and procedures have been developed for
specific polymeric surfaces. The choice of the liquid etchant depends
on the polymer. Polyolefins are usually treated by oxidizing acids such
as chromic, sulfuric, nitric, or mixtures of these. Fluorocarbons are
usually treated by sodium-napthalene etching solution.
To facilitate the etching process, removal of organic surface contam-
inants must first be accomplished by passive chemical cleaning. Some-
times solvent degreasing is used prior to etching to gently soften and
swell the polymer surface. Chemical etching solutions for polymeric
substrates can be recipes that are mixed from raw materials or they
can be proprietary, commercial solutions. The temperature of these
chemical etching treatments is generally above 122–140 C to ensure
fast processing; however, effective treatments can also be carried out
at room temperature. These acids chemically alter the polymer surface
as well as produce roughened or even porous-like surfaces. The nature
of the surface modification is dependent on the polymer and the etch-
ant used. Chromic acid, for example, will oxidize polyolefin surfaces.
Whereas, sodium naphthalene etching processes will remove the sur-
face fluorine atoms from a fluorocarbon surface. Extensive etching of
the surface can lead to a cohesively weakened surface structure that
is undesirable for bonding purposes. As a result, surface treating pro-
cesses must be followed explicitly. Complete removal of the etching
chemicals by a rinse step is essential. Highly reactive etchants can
continue reacting with the surface after application of the adhesive
and, thereby, degrade the chemical and physical stability of the sur-
face.
Several novel active chemical treating processes have been recently
developed for polymeric substrates. Much of this development is due
to the increased usage of engineering plastics in certain industries and
concerns over environmental and safety issues with conventional
chemical processes. The newer processes include fluorine surface
treatment, grafting, and adhesive abrasion.
The fluorine treatment of plastics for improved adhesion is a rela-
tively new process. Through this treatment a new surface is applied
to the polymer. The new surface that will eventually be in contact with